The NEURONS and
NEURAL
SYSTEM:
st
a 21 CENTURY PARADIGM
This material is excerpted from the full β-version of the text. The final
printed version will be more concise due to further editing and
economical constraints.
A Table of Contents and an index are located at the end of this
paper.
A few citations have yet to be defined and are indicated by “xxx.”
James T. Fulton
Neural Concepts
[email protected]
August 1, 2016
Copyright 2011 James T. Fulton
1
[xxx equatorial and axial are introduced in definitions on page 30.]
[xxx consolidate on angstrom or on nm throughout ]
[xxx the choice of C(arboxylic)-path is more definitive than A(cidic)-path in differentiating from
the H-best condition. Alternately, the label A(cetate)-path is more indicative of the actual
(predominant situation. This path includes a large number of esters ending in -ate as well as the
organic acids ending in -ic See Section 8.5.xxx.]
[xxx G-Path clearly avoids duplication between the old and new set of path labels ]
[xxx use the A-, G-, N - and P- path labels from now on, exccept in quotations. ]
THIS CHAPTER IS PRESENTED IN DRAFT FORM AT THE CURRENT TIME BECAUSE OF THE NEED FOR THE
INFORMATION BY THE GUSTATORY COMMUNITY.
8 Stage 1 & 2, Signal Generating & Processing
Neurons1
“Science is made up with facts as a house is made from stones. But a collection of facts is no
more a science than a pile of stones is a house.”
—Poincare' , Hypotheses in Physics (1952)
“In order to understand any part of nature, one must have both experimental data and a theory
for interpreting the data and predicting new data.”
– Shepherd, Outline of a Theory of Olfaction, 2005
This Part provides an in depth discussion of stage 1
operation (sensing) of the Gustatory Modality
8.5 The gustatory modality
Excepting the group at the University of Wisconsin, there has not been a large amount of
exploratory research into gustation with an academic focus since the 1980's. Most of the work
continues to relate to product development in the food industry, generally under the rubric
flavor rather than taste.
Quoting van der Heijden in 1993, “Humans can perceive four tastes, of which sweet and bitter
have received the most attention from scientists.” This will become evident when the sparse
information concerning the elicitation of the acidic and salty sensations are reviewed. The
literature fails to note the acidic sensation is based primarily on the sensing of organic acids. The
sensation of salty is elicited almost exclusively by the hydrated sodium ion.
While based on very fragmentary data, Boudreau has provided the most complete conceptual
1
August 1, 2016
Signal Generation & Processing 8- 3
description of the overall gustatory modality2. Inconveniently, he uses the term ganglion in place
of the more conventional nerve when speaking of the major nerves serving the oral cavity.
The following discussion will repeatedly encounter conflicts between two different modalities of
the neural system, the gustatory and the nocent modalities. While the gustatory modality is
reasonably well understood at the concept level, its differentiation from the less well understood
nocent (pain reporting) modality suffers. Many inorganic materials discussed in empirical
gustation investigations (such as HCl and CaCl2 ) are actually nocentaphores. The
nocentaphores will be discussed in Section 8.7.
8.5.1 Background for and summary--the gustatory modality hypothesis
8.5.1.1 Background
8.5.1.1.1 Historical documentation
Cagan & Kare edited a comprehensive volume with a focus on olfactory transduction in 19813.
The individual paper authors explored a broad range of potential transduction mechanisms.
Some focused on the role of proteins on the surface of the sensory neuron membrane. Others
focused on the potential for the lipids of the membrane to be involved in the primary
mechanism. No conclusions were drawn. Kurihara, Miyake & Yoshii explored the work of Kamo
et al4 in 1980. Kamo et al. attempted to mimic the sensory response of the gustatory neurons
much as Hodgkin did for the visual modality sensory neurons (pages 249-286). Their equation 2
expresses the two-way operation of the proposed fundamental chemical reaction of the
transduction process in abstract form. They then formulated a set of second order differential
equations based on the Law of Mass Action (which requires a totally reversible reaction in
solution). Unfortunately, there are a vast array of possible mechanisms that are satisfied by a
second order differential equation. These include the quantum-mechanical mechanisms such
as the production of nuclear isotopes.
In their analysis, they ignored the rapidly rising attack transient and concentrated on the
decreasing transient and the steady state value (prior to cessation of stimulation). However,
their boundary conditions necessarily included the rapidly rising attack transient in their ultimate
solution. While their particular solution of the differential equations is complete and includes the
pulse condition (as opposed to just the impulse condition), there are two problems with their
interpretation of it. First, they apparently did not treat the off-response properly by applying new
boundary conditions. The off response is a direct and independent measurement of the second
exponential in equation 2 and in the E/D equation of this work. Second, Kamo et al. did not
treat the singularity within their equations properly. Their particular solution does not apply at
w1 = w2. Removal of the singularity results in a different mathematical form that this author has
labeled the Hodgkin condition (Section xxx). They noted Kashiwagura et al contained
additional material on their investigation5.
2
Boudreau, J. (1989) Neurophysiology and stimulus chemistry of mammalian taste systems In Teranishi, R.
Buttery, R. & Shahidi, F. eds. Flavor Chemistry: Trends and Developments. Washington, DC: American
Chemical Society Chapter 10
3
Cagan, R. & Kare, M. ed. (1981) Biochemistry of taste and olfaction. NY: Academic Press Parts II & III
4
Kamo, N. Kashiwagura, T. Kurihara, K. & Kobatake, Y. (1980) A Theory of dynamic and steady responses
in chemoreception J theor Biol vol 83, pp 111-130
5
Kashiwagura, T. Kamo, N. Kurihara, K. & Kobatake, Y. (1980) Interpretation by theoretical model of dynamic
and steady components in frog gustatory response Am J Physiol (gastrointest.) Vol. 238, pp G445-G452
4 Neurons & the Nervous System
Scott & Mark explored coding within the taste system in 19876. Their analysis relied upon the
chemical theory of the neuron. Their abstract opens with;
“Attempts to define the organization of the taste system in terms of the physical
characteristics of stimuli have been largely unsuccessful.”
They noted specifically, “Molecular weight and pH did not relate to the total organizaton of the
system. . . “ Their work, employing multidimensional scaling (MDS)will be addressed below.
Fisher & Scott reviewed the subject of food flavours in 19977. The work is entirely conceptual,
with few substantive sketches, and is based on behavioral studies. It makes the conventional
assumptions that taste involves either ion channels passing molecules through the sensory lemma
or proteins on the surface of the sensory lemma. It does not demonstrate either of these
concepts is correct, nor does it provide primary data relative to these concepts. Their table 3.1,
from Kinnamon & Getchell, illustrates the continuing conceptual character of their thesis. Pages
85–87 provide a comprehensive list of conceptual gustatory features that suggest four basic
taste sensory channels.
Although dated, the discussion of the gustatory modality in Noback remains useful8. The
description of the taste buds as bowl shaped features located behind pores in the lingual
epithelium and containing on the order of 25 or more distinct gustatory sensory neurons is clear.
He also notes the seldom reported high turnover rate among the sensory neurons, as opposed
to just the sensory hair of each cell. “Each mature sensory neuron is replaced every 200 to 300
hours.” This turnover rate is similar to that of just the outer disks of the visual sensory neurons. It
is also similar to the turnover rate of the piezoelectric proteins within the auditory sensory neurons.
Like the retina of the visual system, the taste buds contain “sustentacular cells” that develop and
replace the older sensory neurons. This replacement is associated with a transfer of the synapse
with the orthodromic neurons from the old to the new sensory neuron.
Noback also notes the neurons immediately orthodromic to the sensory neurons emerge from
the taste bud and become myelinated immediately (therefore stage 3 neurons). As in the case
of the auditory neurons, this suggests the encoding of the analog signals occurs at the first Node
of Ranvier and not within the soma of these stage 3 neurons. This node of Ranvier occurs in what
is conventionally described as the dendrite of the signal projection neuron. However, the
myelinated neuron component directly after the encoding node of Ranvier is an axon segment.
The number of gustatory sensory neurons is small, estimated at about 10,000 in human babies
and the sensitivity of the modality is low relative to the olfactory modality (as many as 20,000
times the number of molecules are required for gustation as olfaction). However, it is probably
the method of delivery more than the sensitivity of the receptors that is limiting. The quantummechanical character of the response of the gustatory sensory neurons suggest they are highly
efficient at sensing molecules delivered to their immediate vicinity.
The subject of taste modifiers (additional chemicals applied with the stimuli) and enhanced
stimuli (stimuli with an additional ligand not found naturally) have not been introduced into a
comprehensive theory of gustatory sensing. However, a few enhanced stimuli have been
documented having perceived intensities as much as 30 to 10,000 times greater than the
strongest natural materials. Most of the stimuli with an artificial gustaphore have been centered
on the attempts to find artificial super-sweeteners, frequently with an auxiliary goal that they be
non-caloric from a nutrition perspective.
Glendinning et al. summarized the conventional wisdom concerning gustation in 2000. They
6
Scott, T. & Mark, G. (1987) The taste system encodes stimulus toxicity Brain Res vol 414, pp 197-203
7
Fisher, C. & Scott, T. (1997) Food Flavours: Biology and Chemistry. Cambridge, UK: The Royal Society of
Chemistry.
8
Noback, C. (1967) The Human Nervous System. NY: McGraw-Hill pp 118 &136-139
Signal Generation & Processing 8- 5
noted,
“Among the sensory systems, the taste system is unusual in its capacity to respond to a
large number of stimuli that vary greatly in molecular size, lipophilicity, and pH (e.g., salts),
amino acids, sugars, acids, vitamins, fatty acids, and many toxic compounds). To
accommodate this structural diversity, taste cells utilize a diverse array of transduction
mechanisms. However, only a fraction of these mechanism occurs within any given taste
cell, different subsets of taste cells appear to express different transduction mechanisms,
and hence different molecular receptive rages.”
All of Glendinning et al’s. discussion is dependent on the chemical theory of the neuron, and the
concept of pores through the dendrolemma of the sensory neurons or the presence of proteins
inserted into and traversing the dendrolemma (the conventional G-protein hypothesis). Their
figures are limited to simple conceptual sketches of a dendrolemma. No differentiation of
dendrolemma into special classes is considered. The figures depend on the porosity of the
hydraulic barrier (tight junction) between the sensory neurons for the sensing of simple positive
ions (illustrated as Na+ & H+). They note, the hydraulic barrier is known to be impermeable to
amiloride in vertebrates.
In summary, the Glendinning et al. material is an excellent source of laboratory results but
provides no detailed theory of gustatory neuron operation. They do not address the time
duration of the sensory mechanisms or explain the form of the C/D waveform. The internal
conflicts within their models are characterized by the discussion of G-protein versus cAMP
involvement in “sweet” sensing on page 325.
There is virtually no detailed discussions of the precise role of proteins in the gustatory modality,
other than the assertion that such a role must exist and subsequent Bayesian inferences derived
from analyses of the genetic code. McManus et al. made the strongest presentation in a two
page communication in 19819. They asserted the most likely site along a protein to act as a
sensory receptor was where proline residues disrupted the normal helical structure and exposed
two hydroxyl groups to provide an AH,B bonding site. Their subsequent paper focused on
polyphenols, rather than sugars, interacting with proteins.
Smith and Davis made an important assertion in 2000 (page 362) that they refuted almost
immediately,
“A universal characteristic of mammalian gustatory neurons is their responsiveness to
stimuli representing more than one of the classic four taste qualities.”
This statement suggests the major perceptions of taste do not correspond to any prior
categorization of individual stimuli into classes. It also suggests that, like in the case of yellow in
vision, the perception of a prominent taste may not be related to an individual sensory channel.
Such a perception may be the result of signal manipulation within stage 4. The many
multidimensional analyses of taste perceptions appears to support this assertion. However, Smith
and Davis follow immediately with the statement conflicting with their above view (page 363).
Speaking of the moth, Bombyx mori, they say,
“The most sharply tuned fiber is depolarized exclusively by specific isomers of the sugar
alcohol, inositol, the other three fibers are less sharply tuned . . .”
This work would recommend the term sugar alcohol be replaced by glyco-alcohol (from the
perspective of gustation) since the chemical is perceived as sweet but the glycophore
stimulating the receptor channel is not attached to a heterocyclic saccharide. Pure saturated
aliphatic alcohols are intrinsically tasteless. However, two points are significant. First, saturated
aliphatic alcohols (and aldehydes) rapidly form azeotropes in water–based solutions. Second,
alcohols containing significant low levels of impurities have been used to rate alcohols on a
perception scale in many pedagogical experiments.
9
McManus, J. Davis, K. Lilley, T. & Haslam, E. (xxx) xxx J Chem Soc Chem Commun vol. 7, pp 309-311
6 Neurons & the Nervous System
A similar recommendation can be made regarding the sugar acids, such as ascorbic acid, that
do not contain any carboxylic acid group (indicative of an organic acid) or any CH2OH group
(indicative of a saccharide). They are predominantly sweet alcohols based on gustatory
perception..
The common protocol of recording the number of action potentials in a five second interval
obliterates any information regarding the shape of the excitation/de-excitation function in
gustation. It is known that most animals can discriminate or identify a gustaphore in less than
one second.
---Doty edited a handbook of olfaction and gustation in 2003 that provides both academic and
clinical material10. It offers considerably more detailed information than Finger et al11. It focuses
on anatomy and the chemicals of gustation but does not address the neurophysiology of
olfaction or gustation at a significant level. The authors writing in Doty review many of the earlier
theories of olfaction and gustation and generally finds them wanting. On the other hand, they
do not converge on one specific theory or model that is able to describe chemical sensing. The
handbook is very useful for data mining but offers no significant theory of chemical sensing. See
Section 8.4.
---Spector & Travers (page 173) note the low likelihood of a chemotopic organization in the
gustatory modality based on considerable investigation by the community. Travers et al. have
provided a paper showing how complex the gustatory signaling paths are and the difficulty of
preparing a set of stimulants and a protocol for evaluating this modality unambiguously12. Their
figure 3 (lower right) is difficult to interpret, however it may show cases of a nominal pulse rate
being reduced in the presence of a second stimulant, a feature of a differencing channel (at
the M neurons in the figure presented below). It is common to find examples of summing (at the
P neurons of the following figure) among at least two channels in the neural paths of the chorda
tympani. Pfaffmann said as an example, “Three types of fiber were found : those stimulated (1)
by acids, (2) by acid and by sodium chloride, and (3) by acid and by quinine13.” He also noted,
“Since some of the fibers respond to such a surprisingly diverse group of stimuli, one is reminded
that the single fiber of these experiments is inferred from the appearance of the potential
record.”
8.5.1.1.2 Major problems with the RSC Jmol & JSmol Libraries
Molecular modeling and x-ray crystallography play a critical role in understanding the critical
role of transduction in the gustatory modality. As noted in Section 8.4.1.2.3, the field of molecular
modeling has developed unevenly and with little sophistication.
This work has relied upon the 3D Jmol and JSmol files in the archive provided by the Royal
Society of Chemistry. The reliance has been primarily for determining the distance between
various atoms within a molecule in 3D space. The Jmol archive has been evolving for the last
decade but exhibits some significant shortcomings, particularly the ability of anyone to submit
10
Doty, R. ed. (2003) Handbook of Olfaction and Gustation, 2nd revised and expanded edition. NY: Marcel
Dekker
11
Finger, T. Silver, W. & Restrepo, D. eds. (2000) The Neurobiology of taste and smell, 2nd Ed.. NY: WileyLiss
12
Travers, S. Pfaffmann, C. & Norgren, R. (1986) Convergence of Lingual and Palatal Gustatory Neural
Activity in the Nucleus of the Solitary Tract Brain Research, vol 365, pp 305-320
13
Pfaffmann, C. (1941) Gustatory Afferent Impulses J Cell Comp Physiol vol 17, pp243+
Signal Generation & Processing 8- 7
files that have not been curated by the RSC before deposition in the database. The JSmol
database was implemented a short time ago to complement the Jmol database that exhibits
a variety of additional shortcomings (See Sections 8.4.1.2.3 & 8.6.1.6.3).
News flash: The Jmol files are no longer available in 3D based on the cancellation of their
internet security certification. This in turn was based on the “Cessation of Activity” as of
15 October 2015. It appears these files are being supplanted by the JSmol files curated
by the same RSC. However, the JSmol 3D database was taken off the internet for an
unspecified period as of 19 Nov 2015 (as was the ability to contact the curator via the
website). While the JSmol files examined frequently have more header information than
the Jmol files, the information is frequently disguised with a dummy author’s name (Marvin)
appearing on large numbers of JSmol files.
This work will continue to rely upon the Jmol and JSmol files previously downloaded to the
computer files of this investigator in spite of their shortcomings until the RSC resolves its internal
problems. It is hoped that the actual d-values are at least proportional, if not precise, to those
given in ChemSpider.
8.5.1.1.3 How have the taste sensations been defined?
The chemical-sensing oriented portion of the psychophysical community has long speculated
on the functional organization of the gustatory and olfactory systems. A major problem has
been defining what is sensed by the gustatory and olfactory modalities. Initially, only the
perceived responses of humans, as computed in stage 5 and expressed using stage 6 of the
neural system, could be documented. More recently, electronic techniques have allowed
recording sensations, in stages 1 through 4 within the neural system. However, no rational
description of the ordering of gustatory or olfactory responses has been defined. Nor has the
number of “dimensions” associated with that ordering been established. Squire et al. (pages
642-645) have provided a discussion of this situation.
Pfaffmann et al. have discussed the delineation of the gustatory responses to different stimuli into
five classes described as;
S group–sweet or sugary
N group– salty, and specifically “salty based on the presence of Na+ ion.”
Q group– bitter, letter derived from the relevance of quinine
H group– acidic.
U group–umami, a potential taste apparently unique to monosodium glutamate in some
investigator’s eyes.
Johnson (page 753) has provided a discussion of these properties. Such properties are always
discussed in narrative form and no two authors agree at the detail level.
Some investigators have attempted to define a unique channel, the umami channel, associated
with mono-sodium glutamate. However, this compound appears to be a typical gustant
involving a sodium-based member of the N group, a moiety affecting the H group and a moiety
affecting the S group (See Section 8.5.1.6.2 & 3). An explicit unique definition of the perception
of umami has not been found in the literature.
In 2015, Running et al. have attempted to identify a sixth gustatory channel that they describe
as unique to the “fats” as defined in the food science field. As shown in Section 8.5.4.10, they
are in fact discussing the fatty acids that primarily stimulate the acidic channel, labeled G1 in
this work, but if unsaturated can stimulate other channels as well.
Squire et al. note (2003, page 613), “Most taste stimuli are hydrophilic molecules, including Na+
salt (salty), divalent salts, and KCl (salty and bitter), acids (sour), sugars (sweet), amino acids
(sweet, bitter, and umami), and proteins (sweet and bitter). Some taste stimuli are lipophilic,
including the bitter-tasting alkaloids and many synthetic sweeteners.”
8 Neurons & the Nervous System
Smith & Davis have provided a different behavioral perspective14, “These qualities and the
behavior associated with them provide the means by which an animal makes ingestive
decisions. Through these qualities, taste help to ensure the animals’ energy supply (sweet),
maintain the proper electrolyte balance (salt), and avoid the ingestion of toxic substances (sour,
bitter)”
A major area of study has been limited to studying the ionic portion of the chemical spectrum.
Another major area has been focused on the study of organic chemicals, primarily but not
exclusively the simple sugars.
While Spector and Travers have conceptually recognized two broad classes of receptors, these
two areas of study have yet to converge on a comprehensive functional model at a detailed
level. They noted in 2005 (page 177), “For all of the behavioral and electrophysiological work
that has been conducted to date, it is revealing that definitive evidence distinguishing various
modes of neural coding of taste quality remains to be seen.” Many anecdotal remarks also
appear in the literature but these have not generally been incorporated into a comprehensive
functional model either. On the same page, they note, “An examination of the literature leads
one to speculate that some of this complexity might be due to experimental factors that
potentially obscure the discovery of orderly principles. Most investigations have not employed
statistically adequate numbers of stimulants and receptors to support drawing precise
conclusions from the data.
Hudspeth & Tanaka provided a brief review that only touched on the chemical senses, taste and
olfaction15. They note the classical concept suggests the taste sensations of sweetness, sourness,
saltyness and bitterness. They quickly note the sensitivity of the human system to fat as well.
Many alternates to this brief listing have been proposed. Nearly a century ago, Ikeda suggested
an additional axis to the taste map involving glutamate (or more specifically monosodium
glutamate)16. He named the sensation associated with glutamate umami17. Sugimoto &
Ninomiya have provided a review of more recent work related to umami. Axel & Buck
suggested a very large number of independent olfactory sensor types. They inferred several
relationships between the olfactory system and over 1000 genes.
xxx add reference and brief comments re: sensing organic chemicals xxx
8.5.1.2 Anatomy of the peripheral portion of the gustatory modality
A problem related to taste is the variation in sensitivity to different stimuli with location on the
tongue. Age is known to play a significant role in taste, with many senior citizens experiencing
a loss in taste sensitivity. Children are believed to exhibit an enhanced taste sensitivity possibly
related to sensory cells covering a larger areas of the oral cavity. Eyzaguirre & Fidone have
described the sensitivity of the human tongue using Figure 8.5.1-1 18. The sour taste is located
at both edges of the tongue, salt and sweet at the tip of the tongue and bitter at the base of
14
Smith, D. & Davis, B. (2000) Neural representation of taste In Finger, T. Silver, W. & Restrepo, D. eds.
(2000) The Neurobiology of taste and smell, 2nd Ed.. NY: Wiley-Liss Chapter 14
15
Hudspeth, A. & Tanaka, K. (2000) Sensory systems: editorial overview Curr Opin Neurobiol vol xxx, pp 443446
16
Ikeda, K. (2002) New Seasonings Chem Senses vol 27, pp 847-849 A reprint of original
17
Sugimoto, K. & Ninomiya, Y. (2005) Introductory remarks on umami research: candidate receptors and signal
transduction mechanisms on umami Chem Senses vol 30(suppl 1) pp i21-i22
18
Eyzguirre, C. & Fidone, S. (1975) Physiology of the Nervous System, 2nd Ed. Chicago, Il: Yearbook
Publishers page 146
Signal Generation & Processing 8- 9
the tongue. Squire et al. (2003, pages 632-635) provide additional detail concerning the location
of the gustatory sensory neurons. Johnson has provided more details of the human tongue and
its connection to the main nerves. Johnson also provides a comprehensive posterior view of the
palo- and neo-cortex that places the various neural paths in the brain associated with the
gustatory modality in perspective.
Bartoshuk has questioned diagrams such as
this based on its probable origination in
1901 and on their experiments19. He noted
the presentation suggests an all-or-nothing
response for different areas. He also notes
that was not the intent of the original
author. “If taste qualities were arranged
chemotopically on the tongue such that
sweet receptors were found on the front,
then damage to the chorda tympani nerve
would selectively impair one’s ability to
taste sweet. Not only does this not occur,
damage to the chorda tympani often
produces virtually no change in the
subjective taste world of the patient.” He
provided an alternate figure describing the
sensitivity of the tongue showing the high
degree of overlap between the sensations
throughout the surface of the tongue.
More recent terminology for the types of
taste buds found on the human tongue
include;
1. fungiform papillae (found on the body or
anterior 2/3 of the tongue).
2. filiform papillae.
3. foliate papillae (found on the base or
posterior 1/3 of the tongue).
4. circumvallate papillae (found at the
base of the tongue arranged in a V-shpae).
Figure 8.5.1-1 Semi-schematic representation of
the tongue that is archaic (see text). The location
of different sensory modalities and the areas
innervated by different cranial nerves (V, VII & IX)
are shown. The vallate papillae are located in the
region marked by the dashed lines.
From
Eyzaguirre & Fidone, 1975.
The terms do not describe the character or
content of the individual taste bud. While specific data is unavailable, it appears each taste
bud incorporates a small number of sensory receptor cells of multiple types (probably all types
but in different proportions or exposures depending on location)
----The rough character of the tongue is due to the multiple papillae covering the forward sector
of the tongue. Each papilla contains about 250 individual taste buds. Each taste bud forms a
cavity containing a number of individual gustatory sensory neurons. The sensory neurons in each
bud may sense different stimulants.
- - -Mistretta studied the effects of age on the morphology of gustation and the number of gustatory
19
Bartoxhuk, L. (1993) Genetic and pathological taste variation: what can we learn from animal models and
human disease? In Margolis, F. ed. The Molecular Basis of Smell and Taste Transduction. NY: John Wiley
& Sons pp 251-267
10 Neurons & the Nervous System
sensory neurons in humans and other mammals20. She concluded, “The recent data from
quantitative studies of taste buds in old humans, rhesus monkeys, and rats complement
neurophysiological data on taste responses from aged rats and lead to the general conclusion
that the peripheral taste system is maintained structurally and functionally across the life span.
The number of the taste buds varies immensely with species. Kare has compiled a simple table
illustrating this variation21.
8.5.1.2.1 Morphology of the gustatory modality
Figure 8.5.1-2 is a schematic/block diagram of the proposed gustatory system. It shows the taste
bud schematically followed by a preliminary block diagram of the modality through stage 4.
The synapses within the taste bud are not shown in detail. However, the neurons leaving the
taste bud are known to become myelinated immediately. Lacking any data showing analog
signal differencing of the initial gustatory signals, no connections are shown to the potential
midget ganglion (difference) encoding neuron. The exemplar signal path is shown proceeding
to a parasol ganglion neuron wherein the first Node of Ranvier performs the encoding
(modulation) function. The nucleus solitarius is the “1st relay” and the parabrachial nucleus is the
“2nd relay” in the gustatory signaling chain. Signals from these elements divide and proceed to
both the ventral posteromedial thalamus (PVM) and to the amygdala/hypothalamus of the
limbic system. The signals from the PVM are known to proceed to the region of the operculum
of the parietal lobe (BA 2), the adjacent insula and inner surface of the temporal lobe
(potentially BA 27). Aggleton & Passingham have provided more specific information on these
locations in the macaque22. Rolls & Baylis have provided some data on the nearly parallel
olfactory and gustatory tracts in the primate orbiotfrontal cortex23. Signals from the
amygdala/hypothalamus are known to connect to the orbitofrontal cortex area of what this
work calls the prefrontal cortex.
20
Mistretta, C. (1989) Anatomy and Neurophysiology of the Taste System in Aged Animals Annal NY Acad
Sci vol 561, pp 277-290
21
Kare, M. (1971) Comparative study of taste In Beidler, L. ed. Taste: Handbook of Sensory Physiology, Vol
IV, Part 2 Chap 13
22
Aggleton, J. & Passingham, R. (1981) Stereotaxic surgery under x-ray guidance in the rhesus monkey Exp
Brain Res vol 44, pp271-276
23
Rolls, E. & Baylis, L. (1994) gustatory, olfactory and visual convergence within the primate orbitofrontal
cortex J Neurosci vol 14, pp 5437-5452
Signal Generation & Processing 8- 11
Figure 8.5.1-2 Proposed schematic of the gustatory system. The nucleus solitarius and
parabrachial nucleus may be the same element as defined by different investigators.
Complications arise in evaluating the gustatory and nociceptor modalities due to their sharing
certain nerves directed to the stage 2 signal processing engines. These engines appear to
contain multiple summation channels (P) and differencing channels (M) delivering signals to a
variety of higher level engines (not shown in detail). See text.
Each sensory neuron exhibits a small number of microvilli emanating from the neuron and into
the cup formed by the taste pore in the lingual epithelium. This is illustrated more clearly in
[Figure xxx ]
The figure shows the gustatory neurons delivering signals to nerves V (trigeminal), VII (facial), IX
(glassopharyngeal) and X (vagus) depending on their function and location within the oral
cavity. The routing is not widely accepted and many neurons appear to be routed via the
nerve V, the trigeminal nerve, which serves large areas of the mucosal membrane within the oral
and nasal cavities as well as lining the eye sockets. These nerves all enter the brain stem at the
interface between the pons and the medulla.
A major problem in evaluating the gustatory modality, illustrated in the previous and
following figures, is the mixture of sensory signals that arrive at the Nuclei of Solitarius (NoS).
In general, electrophysiological laboratory investigations must insure the data is
unadulterated and applies specifically to gustaphore receptor channels. Specifically, by
the time the signals reach the NoS, they cannot easily be distinguished between their
origin within the nocent or gustatory channels. Figures 788 & 912 in a recent reprinting of
an undefined edition of Gray’s Anatomy (widely circulated in Wikipedia, etc.) clearly
illustrates the problem. Figure 788 shows the chorda tympani as a branch of nerve IX
12 Neurons & the Nervous System
rather than nerve VII as shown in Mistretta (1990). Figure 912 uses the less definitive label
“facial nerve” more in agreement with Mistretta.
It is important to acquire gustatory signals from within the chorda tympani or other
branches of the main nerves before they are adulterated.
The afferent neurons within the taste bud frequently connect to multiple sensory neurons.
However, this may be to support the rapid turnover of the sensory neurons with the afferent
neurons connecting to only one functional neuron at a time. The literature does not suggest any
differencing of neural signals occurs within the taste bud. Therefore, the figure shows a neuron
from the taste bud connecting to a parasol (summing) ganglion neuron, P. The potential for a
midget (differencing) ganglion neuron, M, has been retained for discussion purposes only..
The neurons emanating from the taste bud become myelinated immediately and prior to
reaching the soma of the neurons. This myelination suggests the sensory neurons of the gustatory
modality are much like those of the auditory modality in that the first Node of Ranvier appears
prior to the soma and acts as the encoding element within this stage 3 neuron.
The figure illustrates the difficulty of isolating the gustatory neural signals from the nocent neuron
signals due to their sharing the nerves projecting both signal types to the nuclei solitarius.
The nucleus of solitarius is known to sum signals from multiple sensory neurons but whether on a
spatial basis or a stimulus basis remains unclear. This nucleus does not show an organization
similar to a glomeruli or any recognizable chemotopic organization. Figure 8.5.1-3 is an
schematic of the neural paths of mammals based specifically on the anatomy of sheep. The
figure offers considerable definition of the separate neural paths and way points. It also shows
the lack of a common focus of the gustatory signals as they pass through the rostral brainstem.
Signal Generation & Processing 8- 13
Figure 8.5.1-3 Schematic of mammalian neural paths in gustation. Based on the sheep but
applicable to any mammal. Mistretta noted, the nasoincisor duct with taste buds, schematically
indicated on the anterior hard palate and innervated by the greater palatine nerve, has not yet
been reported in the sheep. From Mistretta, 1990.
The role of the PBN remains unclear. Some authors suggest it is missing, or inconsequential in
humans24. Others sketch it in to conceptual drawings25.
Whether the nucleus solitarius and parabrachial nucleus should be considered stage 2 signal
processing elements or merely relay points is yet to be resolved. Their forward connections
suggest they should be considered elements of stage 4, signal manipulation.
24
Mistretta, C. (1990) Taste Development In Coleman, J. ed. Development of Sensory Systems in Mammals.
NY: John Wiley & Sons Chapter 14
25
Rolls, E. & Scott, T. (2003) Central taste anatomy and neurophysiology In Doty, R. ed. Handbook of
Olfaction and Gustation, 2nd Ed. NY: Marcel Dekker page 680
14 Neurons & the Nervous System
Many signal paths emanate from the nucleus solatarius and the parbrachial nucleus, with the
majority believed to connect to the ventral posteromedial thalamus (VPM) and the
amygdala/hypothalamus. Beyond those elements, the neural pathway of the gustatory
modality is poorly documented. The VPM appears to connect to an area of the cerebral cortex
known as the insula with spillover onto the operculum of the parietal lobe (BA 2) and the upper
medial surface of the temporal lobe (?BA 27). This region may constitute a portion of the
saliency map devoted to gustatory and olfactory representations available for review by the
cognitive functions of stage 5. The amygdala/hypothalamus appears to connect to the
orbitofrontal portion of the prefrontal cortex.
Anatomical data on the tongue of many species appears in Bradley26. Unfortunately, many
textbooks have adopted a simple map from Boring (1942) redrawn from some very early work
of Hanig (1901). The best description of the sensor location on the human tongue is shown in
Figure 8.5.1-4 from Yanagisawa et al27.
based on the studies of Collings28 [xxx add
words ]
Figure 8.5.1-4 Relative magnitude estimates for
locations on the human tongue ADD for the
historical gustaphores. From Collings, 1974.
26
Bradley, R. (1971) Tongue topography In Beidler, L. ed. Taste: Handbook of Sensory Physiology, Vol IV,
Part 2, Chap 1
27
Yanagisawa, K Bartoshuk, L. Karrer, T. et al. (1992) Anesthesia of the chorda tympani nerve:insights into
a source of sygeusia Chem Sneses vol. 17, p 724 (abstract).
28
Collings, V. (1974) Human taste response as a function of locus of stimulation on the tongue and soft palate
Percept Psychophys vol 16, pp 169-174
Signal Generation & Processing 8- 15
The unique labels in this figure are described by Mistretta in Figure 8.5.1-5
Figure 8.5.1-5 Diagram of types of lingual gustatory papillae, the fungiform, foliate, and
circumvallate, and of gustatory epithelium representative of that on the soft palate and
epiglottis. The black dots represent individual taste buds. From Mistretta, 1989.
---Pfaffman et al. provided some early gustatory information using squirrel monkeys29. They
recorded action potentials from the chorda tympani, a branch of the VII facial nerve. They
introduced the concept of labeled lines with little detail. They did note, “Two-thirds of our
sample of taste units fall readily into one of the four classic taste categories with a peak at one
basic taste stimulus. ‘Side bands’ around such peaks produce a certain degree of multiple
sensitivity. One-third of the responsive fibers, however, cannot be classified by a single ‘best
stimulus’ but appear to have broad multiple sensitivity. All of their reported waveforms suggest
parasol type encoding channels.
Recently Mattes has introduced data showing the gustatory modality of humans is also sensitive
to a series of free fatty acids30. [xxx add words ] Lim & Lawless explored the taste of a variety of
metal salts31. [xxx add words ] Rogers et al. have offered a limited tree focused on bitter taste
relationships32. [xxx add words ] Lundy & Norgren experiments suggesting feedback of signals
from the amygdala/hypothalamus and gustatory cortex to the parabrachial nucleus. However,
while they located their stimulation and recording sites adequately for anatomical purposes,
they did not identify the precise location of their sites with respect to the inputs and outputs of
various elements containing millions of individual neurons.
29
Pfaffman, C. Frank, M. Bartoshuk, L. & Snell, T. (1976) Coding gustatory information in squirrel monkey
chorda tympani In Sprague, J. & Epstein, A. eds. Progress in Psychobiology and Physiological Psychology.
NY: Academic Press. vol 6, pp 1-27
30
Mattes, R. (2009) Oral Detection of Short-, Medium-, and Long-Chain Free Fatty Acids in Humans Chem
Senses vol 34(2), pp 145-150
31
Lim, J. & Lawless, H. (2005) Qualitative Differences of Divalent Salts: Multidimensional Scaling and Cluster
Analysis Chem Senses vol 30(9), pp 719-726
32
Rodgers, S. Busch, J. Peters, H. & Christ-Hazelhof, E. (2005) Building a Tree of Knowledge: Analysis of
Bitter Molecules Chem Senses vol 30, pp 547–557
16 Neurons & the Nervous System
Citing Gilbertson33, Hudspeth & Tanaka have asserted that “mammals respond strongly to the
taste of fat34.” As discussed in this Part and in Part 2A supporting olfaction, fats and fatty acids
exhibiting a pair of double covalent bonds at the appropriate spacing are entirely capable of
stimulating various GR’s and OR’s. Alternately, fats and fatty acids have many opportunities to
mediate the chemical sensing of other gustants and olfactants.
---The mechanism of gustatory transduction has long been an unsolved problem. The likelihood
of four distinct perceived tastes, sweet, sour, salty, bitter & have been proposes since ancient
times. Umami became a potential fifth flavor during the early 1990's. Many, generally
unsuccessful, efforts have been made to explain these perceived tastes based on the chemical
structure of the stimulants35.
----
8.5.1.2.2 The morphology of the taste bud & sensory neuron
Figure 8.5.1-6 shows a modified figure from Mistretta (page 288), who modified a figure from
Murray based on his photomicrographs36. Mistretta eliminated the myelination of the stage 3
neurons and labeled the hydraulic seals as tight junctions (which could be misinterpreted). She
also annotated the Murray figure. The figure has been further annotated to show the presence
of a somatosensory neuron as added to the Murray figure by Finger & Simon37. The figure as
reproduced here is at a magnification of about 2100x.
The typical taste bud contains 5-50 sensory neurons and a number of supporting cells. It is
believed that some of these supporting cells are responsible for the phagocytosis of nonfunctional sensory neurons at the end of their life expectancy (nominally 200 hours)38.
Murray began the practice of numbering the sensory neurons based on their reflected
appearance after uranyl acetate staining; some stained cells appeared darker than others,
Mistretta chose not to adopt this notation.
33
Gilbertson, T. (1998) Gustatory mechanisms for the detection of fat Curr Opin Neurobiol vol 8, pp 447-452
34
Hudspeth, A. & Tanaka, K. (1998) Sensory systems: editorial overview curr Opin Neurobiol vol 8, pp 443446
35
Shallenberger, R. & Acree. T. (1967) Molecular theory of sweet taste Nature vol 216, pp 480-482
36
Murray, R. (1973) The ultrastructure of taste buds In Friedmann, I. ed. The Ultrastructure of Sensory Organs.
NY: American Elsevier Chapter 1
37
Finger, T. & Simon, S. (2000) Cell biology of taste epithelium In Finger, T. Silver, W. & Restrepo, D. eds.
(2000) The Neurobiology of taste and smell. NY: Wiley-Liss Chapter 12, page 293
38
Suzucki, Y. Takeda, M. Obara, N. & Nagai,Y.(1996) Phagocytic cells in the taste buds of rat circumvallate
papillae after denervation Chem Senses vol 21, pp 467-476
Signal Generation & Processing 8- 17
Figure 8.5.1-6 Morphological features of the mammalian taste bud. The analog waveforms from
the sensory neurons are converted to stage 3 projection waveforms at the first node of Ranvier
where the myelination begins. A second node of Ranvier is shown schematically as labeled.
Note the hydraulic seals that protect the neural system from potential contamination. Note also
the proximity of a somatosensory neuron. See text. Modified from an original in Murray, 1973.
Murray has provided a variety of actual micrographs of mammalian sensory neurons. However,
they are difficult to interpret without considerable experience. A Figure 8.5.1-7 from Finger &
Simon39, modified from Singh40, shows the same cytology of a gustatory sensory neuron but from
a fly Drosophila. The importance of this figure is that it shows internal lemma as expected to
39
Finger, T. & Simon, S. (2000) Cell biology of taste epithelium In Finger, T. Silver, W. & restrepo, D. eds. The
Neurobiology of Taste and Smell. NY: Wiley-Liss Chapter 12, page 290
40
Singh, R. (1997) Neurobiology of the gustatory systems of Drosophila and some terrestrial insects Micro Res
Tech vol 39(6), pp 547-563
18 Neurons & the Nervous System
support the electrolytic circuit of the neuron proposed by the Electrolytic Theory of the Neuron.
It also shows the interstitial cells supporting homeostasis, and probably phagocytosis of the no
longer functional sensory neurons.
The “outer dendritic segment” is not shown
in enough detail in the figure for the
purpose of this discussion. Figure 8.5.1-8
provides a more detailed view of mature
mammalian dendritic segments in
perspective view from Murray (page 55).
Each dendritic segment ends in one or
more structures called a colax. Each colax
contains a group of 9(2) + 2 microvilli that
emanate from its terminal surface just as
they do in photoreceptor sensory neurons.
The two in parenthesis indicates an
individual pair of clsoely spaced cylindrical
structures.
The nine represents the
arrangements of these cylindrical pairs
around the center pair. Murray shows
actual electron micrographs of these
structures at 40,000x (in plan view).
The size of the microvilli remains open to
discussion because of the difficulty in
measuring them precisely. Murray (page 8)
suggests they are about 100-200
millimicrons (nanometers) in width and 1-2
microns long. He notes others have set a
minimum diameter of 50 millimicrons.
Molecular structures do not exhibit hard
edges at these scales.
Murray has
estimated the total number of microvilli at
30-40 with another investigator suggesting
5-9 microvilli from each colax. Murray notes
Sclazi (1967) reported the microvilli were
coated with a material identified as
polysacchride in nature. More recently, the
coating has been described as present in
“rafts” on the surface.
Beets (1973, pg 229) has given similar
dimensions for the microvilli, 0.12 microns
wide and 2 microns long projecting into the
pore.
Figure 8.5.1-7 Tracing of a gustatory sensory
neuron showing internal lemma. Cell is from the
fly, Drosophila. See text. From Finger & Simon,
2000.
The axons of the sensory receptor cells are
even more difficult to measure, as many
have diameters at or below the resolution
limit of light microscopy. Many originating at sensory cells have been measured with diameters
in the 0.05 to 0.5 micron diameter range. An average diameter is about 0.2 microns.
Signal Generation & Processing 8- 19
8.5.1.2.3 The structure of the “sweet”
lemma of the microvilli
Glendinning et al. have discussed the
sweeteners (page 322), “These taste stimuli
include sugars, sugar alcohols, D-amino
acids, and some proteins.” They also note,
“There are no generally agreed upon
molecular features that are essential for a
compound to elicit sweet taste.” This work
takes a different view based primarily on
the detailed analyses of Shallenberger &
Acree and of Kier.
Beidler has provided a highly conceptual
view of the surface of a microvilli and a
proposed mechanism of transduction41.
The microvilli, or “curly hairs” of the sensory
neurons is approximately xxx in diameter
and xxx microns long. Slade has recently
provided real time images of the microvilli
on what appear to be chemosensory
neurons using atomic force microscopy
(AFM) (see Section 8.4.1.2.1). She notes the
microvilli are typically straight and
perpendicular to the dendritic surface
unless knocked over during the
investigation process.
At the Angstrom level, the wall of the
microvilli can be considered a flat surface.
The microvilli wall is specialized and consists Figure 8.5.1-8 Two views of the colax and microvilli
of a sandwich of the outer or of a gustatory sensory neuron of rabbit. The
plasmalemma and the reticulolemma of individual microvilli emanate from individual colax
the typical dendrite in close proximity. The structures in groups of up to nine. See text. From
total wall thickness is on the order of 160 Murray, 1973.
Angstrom. Figure 8.5.1-9 shows the cross
section of the proposed microvilli lemma of the gustatory sensory neuron. Dowhan & Bogdanov
reproduce a figure remarkably similar to the right half of this figure42.
The polar heads of the globosides are the presumed receptors of the S-best sensory neurons.
They are known to concentrate in “rafts” on the surface of the outer lemma. A better analogy
is a “bunch of grass sprouts” emanating from the space between the bilayers of the lemma.
Each exposed blade of grass consists of the polar head of the globoside molecule of the outer
bi-leaf of the plasmalemma. The globosides consist of a pair of lipid chains occupying a cylinder
about 9.7 Angstrom in diameter. The polar heads are much larger due to the array of sugars
they contain. The diameter of the heads are unknown until their detailed structural orientation
is determined. The polar heads are shown piercing the nominal surface of the microvilli into the
pore space and consisting of three galactose sugars. Each galactose sugar provides one AH,B
coordination site (shown as a white disk). The precise stereographic arrangement of the polar
heads of the globoside molecules relative to the lipid body is unknown at this time. However,
41
Beidler, L. (1971) Taste receptor stimulation with salts and acids In Beidler, L. ed. Handbook of Sensory
Physiology, Vol 4/1: Olfactory Sensing. Chap 11
Dowhan, W. & Bogdanov, M. (2002) Functional roles of lipids in membranes In Vance, D. & Vance,
J. eds. Biochemistry of Lipids, Lipoproteins and Membranes. 4th Ed. NY: Elsevier Chapter 1
42
20 Neurons & the Nervous System
the polar heads are known to form “rafts” on the external surface of the outer bilayer of the
lemma. The glycophores are negatively charged at pH 7.0 and attract positively charged
ligands in solution. The definition of a raft used in this work differs from that in Vance & Vance
(2002, page 29). The raft of interest here consists of only the phosphatidyl molecules forming the
sensory receptors of the various sensory channels that are embedded in the outer bilayer of type
4 lemma associated with the microvilli of the sensory neurons.
The globosides are also known to have surfactant properties (Merck Index), although this may
not be relevant when the nonpolar portion of the molecule is embedded in a liquid crystalline
leaf of the microvilli lemma.
As noted, the globosides are a major constituent of the type 4 lemma area of sensory neurons.
The type 2 lemma area shown is capped by the electrostenolytic area providing power to this
portion of the sensory neuron. It is intimately associated with an Activa formed within the type
4 area. This Activa is in intimate electrical contact with the type 2 area via the liquid crystalline
water shown shaded. The electrolytic aspects of these structures will be discussed in more detail
below.
The microvilli of the gustatory sensory neurons, like the microvilli and cilia of all sensory neurons
do not contain any pores for the passage of positive alkali or alkali ions. Nor do they contain any
GPCR’s involved in the transduction process. However, they do incorporate what the
community has struggled to define as “membrane rafts” supporting specialized, generally lipid
based sections of outer bilayer lemma43. The term membrane raft has not gained wide adoption
within the community since that time. “The definition adopted by the group was as follows:
‘Membrane rafts are small (10–200 nm), heterogeneous, highly dynamic, sterol- and
sphingolipid-enriched domains that compartmentalize cellular processes.’” The
electrophysiological processes associated with these areas are more important than their
biochemical structures suggest. Their role will be addressed again in Chapters 2 & 3. [xxx put
citation into 2 or 3 ]
43
Pike, L. (2006) Rafts defined: a report on the Keystone symposium on lipid rafts and cell function J Lipid
Res vol 47, pp 1597-1598
Signal Generation & Processing 8- 21
Figure 8.5.1-9 shows the electrophysiology of the “sweet” lemma shown above in greater detail.
The microvilli lemma leaf closest to the dendroplasm is proposed to be primarily insulating except
for the region of type 2 lemma where it is acts as an electrolytic diode. The leaf contacting the
mucosa of the lingual pore is insulating in the type 1 areas and acts as an electrolytic diode in
the type 2 and 4 areas. The bilayer acts as an active electrolytic device, an Activa, in the type
2 region when biased by the electrostenolytic process shown at the top of the type 2 column.
The current through this Activa is controlled by the electrical potential of the hydronium of the
type 2 area via the horizontal shaded area from the type 4 lemma column. In this instance,
hydronium refers to the liquid crystalline form of hydrogen-bonded water (H20)n, and not the
simpler ionic form of hydrated hydrogen (H3O)+ discussed in many introductory textbooks. The
current that passes through the diode of the outer leaf is determined by the chemical change
in the polar head of the globoside (or cerboside) forming the leaf in this region. In the absence
of any attack by a stimulus, the surface of the globoside in this area is polarized by the electrical
potential of the hydronium at -24 mV and the potential of the polar head of the globoside
combined. This net potential may have a significant impact on the stimulus/globoside
interaction. The high charge gain of this configuration will be developed in the following section.
Figure 8.5.1-9 The electrophysiology of the gustatory “sweet” microvilli MOD UPPER RIGHT. This
is the same histological and electrophysiology diagram as used for the olfactory sensory
neurons. The shaded area on the left shows the electrical circuit configuration of the shaded
histological area on the right. The C/D impulse response of this structure is shown at lower left
(without considering any other circuit elements. See text.
[xxx the diagram assumes capacitive coupling instead of a dipole potential as source of drive
to Activa. ]
22 Neurons & the Nervous System
The histological arrangement in the above figure is reminiscent of the simpler triune receptor
concept of Hucho44, which was presented to, but has not gained traction in the pharmacology
community. It differs from his type–I receptors involving a single protein bridging the lemma and
his type–II receptors which involve three proteins, an ‘R’ on the external surface of the lemma,
an ‘E’ on the inner surface and a ‘T’ connecting the two and defined as a coupling or G-protein.
In this case, his concept is redefined as no proteins are involved.
An important fact has emerged in conjunction with the above two figures and Section
3.2.4. It appears that the same carboxylic receptor, phosphatidylserine is used to both
sense carboxylic acid gustaphores within the oral cavity and to capture glutamate within
the fluid milieu of the body in order to power the neurons. The first mechanism (illustrated
on the right of both figures) is not selective with respect to the acidophores. However, the
second mechanism (on the left) requires only properly oriented glutamate be captured.
The mechanism illustrated on the left employs an enzyme of the mGluR family of proteins
to insure this selection process.
8.5.1.3 Electrophysiology of the sensory neurons
Teeter et al. have provided an interspecies review of the sensations generated in the gustatory
modality45. However, their interpretations are all based on an undefined linear circuit network
for the sensory receptors. No circuits or equations were presented. They generally assumed a
change in plasma membrane impedance resulted from stimulation.
8.5.1.3.1 The electrophysiology of the “sweet” gustatory sensory neuron
The physical similarity of all of the sensory neurons is impressive. The major morphological
difference is whether they have extended length axons (like the visual and olfactory sensory
neurons) or whether the pedicle of the sensory neurons is on the surface of the soma (like the
auditory and gustatory sensory neurons). Based on these cytological similarities, Figure 8.5.1-10
shows the proposed electrophysiology of the gustatory sensory neuron. It is virtually identical to
the auditory and olfaction sensory neurons and varies only in the basic transduction area from
the visual sensory neurons.
44
Hucho, F. (1993) Transmitter receptors–general principles and nomenclature In Hucho, F. ed.
Neurotransmitter Receptors. NY: Elsevier page 7
45
Teeter, J. Funakoshi, M. et al. (1987) Generation of the taste cell potential Chem Senses vol 12(2), pp 217-234
Signal Generation & Processing 8- 23
Figure 8.5.1-10 Candidate cytology & electrophysiology schematics of the gustatory sensory
neuron DUMMY ADD. The similarity to the other sensory neurons should be clear.
----
24 Neurons & the Nervous System
.
Earlier sections, including Section 3.2.4 provides additional details related to this figure.
8.5.1.4 Initial block diagram of the modality
Schifferstein has discussed the steps in gustation from a food technology perspective46. His
totallly psychophysical steps appear remote from the physiological steps identified in this work.
Figure 8.5.1-11 shows a provisional block diagram of the gustatory modality. Frame A shows a
simple linear representation of the gustatory modality that parallels that of xxx. However, the
functional descriptions at the gaps in the line segments have been changed.
Frame B shows a first alternate of greater complexity.
Frame C shows a potential signaling architecture analogous with that of the visual modality of
Section 8.2. and/or Chapter 17, Section 17.1.4 in “Processes in Biological Vision.” This frame
separates the qualities associated with the individual channels of gustation from the overall
intensity sensation processed through a separate “summation channel.” This summation channel
will be given the labe, the R-channel in analogy to that designation in the visual modality. Thus,
the top level architecture of the gustatory modality will be described as consisting of the A-Path,
the G-Path, the N-Path, the P-Path and the R-Path at the stage 1 and stage 2 level. Whether
there are difference channels between the signals generated by the stage 1 receptors will be
developed below.
46
Schifferstein, H. (2003) Human perception of taste mixtures In Doty, R. ed. (2003) Handbook of Olfaction
and Gustation, 2nd revised and expanded edition. NY: Marcel Dekker Chap 38
Signal Generation & Processing 8- 25
Figure 8.5.1-11 Provisional block diagram of the gustatory modality ADD. C; a generalized
architecture for gustation analogous to that described within the vision modality. See text.
Frame B expands the conceptual character of Frame A to illustrate xxx.
26 Neurons & the Nervous System
8.5.1.4.1 A proposed top level architecture of the gustatory modality
Frame C of the previous figure shows the probable character of the gustatory modality based
on its close analogy to the visual modality. The analogy has been proposed based on the
detailed analyses of the gustatory modality developed in this chapter. From a global
perspective, it appears very likely the architecture is analogous to that of the visual modality but
with the signal processing in stage 2 adapted to the specific needs of gustation.
A common question is whether the neural signals recorded at the output of the sensory neurons
correlate well with the perceived tastes of the subject. Frank has provided a comparison of the
sensed and perceived responses using factor analysis that shows this is a close correlation in
hamsters47. Her imposing a spherical overlay on her data leads one to believe the distance from
the axes of various stimulants is more limited as their appearance on the graph moves toward
the perimeter. A more open three-dimensional taste sensation space is suggested in this work
(Section 8.5.3 xxx).
Frank’s sensory recordings were at the chorda tympani nerve (generally action potential pulses
within stage 3). The conclusion can be drawn that the stage 2 through stage 4 activity is
designed to project and optimize the presentation of sensory neuron output data to be
analyzed by the prefrontal cortex of stage 5.
As noted in Section xxx, the stage 2 signal processing within gustation may be limited. It appears
any stage 2 signal processing does not occur in the taste buds or the oral cavity. Some signal
processing may occur at the immediate output of the stage 1 sensory neurons. It is more likely
this signal processing occurs in the nucleus solitarius tract (NST) or possibly the parabrachial
nucleus (Sections 8.5.2 & 8.5.8).
There are suggestions of a differencing function between the stage 1 output signals within stage
2 based on several discussions in the MDS literature of gustation. However, it is not well isolated
from the potential stage 4 information extraction function employed in developing the saliency
map of gustation. The proposed saliency map architecture is strongly dependent on the basis
functions of the MDS analyses presented in the literature (Section 8.5.2.4).
A differencing function within stage 2 is also strongly suggested by the apparent broad tuning
of the individual sensory receptors of stage 1. Efficient information extraction in stage 4 would
suggest that some pattern of direct projection of the A–, G–, N – & P– signals accompanied by
at least a partial set of differencing signals is utilized (Section 8.5.5). [xxx confirm this relationship
]
No data has been found in the literature describing the signals from the temporal lobe of
cerebral cortex describing the form of the data presented to the saliency map.
8.5.1.5 The chemistry most important to gustation
47
Frank, M. (2000) Neuron types, receptors, behavior, and taste quality Physiol Behav vol 69, pp 53–62
Signal Generation & Processing 8- 27
8.5.1.5.1 The chemical families of carbohydrates involved in gustation
The complexity of carbohydrate chemistry is truly mind-boggling. The smaller portion of this class
related to gustation remains in this category. The name carbohydrate was assigned based on
a major misunderstanding that they were hydrates of carbon. A modern definition of a
carbohydrate is given by Robyt48;
“a carbohydrate is a polyhydroxy aldehyde or ketone, or a compound that can be
derived from them by any of several means including,
1. reduction to give sugar alcohols,
2. oxidation to give sugar acids,
3. substitution of one or more of the hydroxl groups by various chemical groups, for
example hydrogen may be substituted to give deoxysugars and amino group [NH2 or
acetyl-NH] may be substituted to give amino sugars,
4. derivatization of the hydroxyl groups by various moieties, for example, phosphoric acid
to give phospho sugars, or sulfuric acid to give sulfo sugars, or reaction of the hydroxyl
groups with alcohols to give saccharides, oligosaccharides and polysacchaired.”
Lindhorst gives the clearest and most extensive description of the terminology used to describe
this very large region of organic chemistry49. He stresses the conformational chemistry of the
family. The conformation of the molecules is critically important to the role of the carbohydrates
in gustation, to the extent that Haworth projections, and other projections obfuscate the
required structural elements of the molecules.
8.5.1.5.2 Chemical families pertinent to gustatory receptor identification
The following discussions will focus on modifications to the outer bilayer of the cilia and villi of
sensory neurons as leading to the likely receptors of gustation. The chemistry in this area can be
extremely complex, and involve mechanisms not familiar to the majority of researchers.
Several authors of texts on carbohydrates make gratuitous statements about the role of
protein-based receptors in gustation and olfaction (without justification or citations). It will
become clear in this work that proteins are not required in the process of chemical sensing
by the neural system.
The complexity in the literature of potential lemma chemistry is partially due to the different time
periods in which various investigators have worked. It is also due to the difficulty in controlling
and identifying and avoiding uncontrolled chemical reactions involving constituents present in
very complex molecules.
When discussing complex organic chemistry, it is common in textbooks and journal articles to
address the degree of saturation in the bond structure in a cavalier manner. A more refined
approach is needed in this work. Figure 8.5.1-12(A) discusses the major possibilities. The case
of a saturated hydrocarbon is a boundary condition. The molecule contains no double or triple
bonds. However, an unsaturated hydrocarbon remains largely undefined. An unsaturated
hydrocarbon with only one double bond among many double bonds appears to retain most
of the properties of its fully saturated sibling. However, molecules with alternating single and
double bond patterns can exhibit significantly different electronic properties. The brain and
neural system is known to contain many molecules exhibiting a repeating isoprene structure
where carbon-carbon double bonds are separated by two carbon-carbon single bonds, a
structure associated with the squalenes Alternately, .visual sensory organs of the neural system
are known to contain large numbers of molecules exhibiting a repeating isoprene structure
where carbon-carbon double bonds are separated by one carbon-carbon single bond. Such
48
Robyt, J. (1998) Essentials of Carbohydrate Chemistry. NY Springer page 2
49
Lindehorst, T. (2003) Essentials of Carbohydrate Chemistry and Biochemistry, 2nd Ed. NY Wiley-VCH Chap
2
28 Neurons & the Nervous System
molecules are known as terepenes and are referred to as conjugated hydrocarbons over a
significant and defined portion of the entire molecules. They include the very important retinal
derivatives, the rhodonines, which exhibit significant intra– and inter–molecular electronic
conductivity when present in the liquid crystalline state. In the case of the rhodonines, the
conjugation even extends to the O– and OH– ligands of the structure and explains the multiband
spectral performance of the visual system.
Figure 8.5.1-12(B) attempts to normalize the terminology used to describe the chemistry involved
in sensory neuron receptor identification. The goal is to define traceable names to the chemical
receptors providing transduction of the presence of stimulants into graded electrical signals. To
this end, the figure attempts to rationalize the words and figures of Lehninger (1970) with those
of Benjamins et al. (2006). It employs an indenting style used to save space when using long
descriptors. The chemicals of interest are at the convergence of two very large general families
known as the phospholipids (phosphoglycerides) and the sphingolipids (containing no glycerol).
They both contain two long chain hydrocarbons and a polar head group. The polar head
groups typically exhibit a dipole potential and dipole moment because of their electrical
asymmetry at the atomic constituent level. As normally conceived, the hydrocarbon chains of
both families are nonpolar and electrically insulating. However, a thesis of the Electrolytic Theory
of the Neuron is that at least one of the chains in the above phospholipids and sphingolipids can
be present in a conducting (or semiconducting, i.e. directionally conducting) form. In this form,
a dipole potential can be exhibited between the two ends of the overall molecule (particularly
when the molecules are present in the highly oriented liquid crystalline state found in lemma
bilayers).
Signal Generation & Processing 8- 29
Figure 8.5.1-12 A normalized etiology of chemicals relating to proposed gustatory sensory
receptors.
Electrically conductive hydrocarbon chains in the liquid crystalline state are well known, and are
widely used in this age of the liquid crystal display technology. The conductivity of hydrocarbon
chains is primarily a function of the degree of saturation (unsaturation) of each chain. Fully
saturated and minimally unsaturated chains are electrical insulators. However, unsaturated
chains, particularly of the fully conjugated form, exhibit significant conductivity because of their
sharing of excited electron π-bonds along the length of their chains. In an appropriate
electrolytic circuit configuration, these conductive chains are able to transfer a potential at their
interface with their polar head to the extreme end of their hydrocarbon chain. It is proposed
that the value of this potential is used within the neural system to identify and quantify the
stimulants used in gustation.
30 Neurons & the Nervous System
Textbooks tend to show all phospholipids and sphingolipids as consisting of fully saturated fatty
acid chains as a matter of simplicity and convenience. Benjamins et al. in particular show a
range of saturated and unsaturated fatty acids of neurochemical interest. Their tabulation
focuses on products of the isoprene rule, the squalenes that exhibit conjugate carbon-carbon
bonds at every third carbon. A similar product of the isoprene rule, the terpenes exhibit
conjugate bonds at every other carbon-carbon bond. The terpenes are known to be
conductive and their may be other configurations providing conductivity, possibly involving
cross-linking between the two parallel long chains.
Four conductive phosphoglycerides have been identified as likely receptors in gustation. They
have been identified on the right as being the receptors in of the four primary sensory channels
of gustation.
Frame C: The literature contains discussions of cerebrosides, of both the glycophospholipid and
glycosphingolipid (defined as containing no glycerol) types containing one sugar ligand.
Cerebrosides containing oligosaccharide complexes have been named gangliosides and
globosides (the former containing sialic acid while the latter does not.
As one travels down this figure, it is not always clear how precisely and exclusively the terms have
been defined. The term gangliosides in particular seems to be used to describe both
galtanophospholipids and galtanosphingolipids in the journal literature. Benjamins et al. have
noted “galactocerebroside, in which galactose is β-glycosidically linked to cerebroside,
constitutes about 16% of total adult human brain lipids.” Such a high concentration suggests it
is a structural component rather than a sensory or signaling component. They note the
galactocerebroside appears primarily in the white matter, especially in myelin, a passive
electrically insulating material.
The routinely reported presence of the cerebrosides and gangliosides at high concentrations
within the brain does not necessarily indicate they are functional within the neural signaling
system. There is considerable potential for these molecules to participate as structural
components of the neuron walls. The reduced presence of oxygen in the case of the
sphingolipids also suggests they are not as electrically important as the phospholipids.
Benjamins et al. have provided a set of shorthand labels for many of the chemicals that will be
discussed in detail later in this chapter50. Most of the phosphoglycerides will be described using
their more specific labels; the carboxyphosphoglyceride of interest is described as phosphatidyl
serine or PtdSer.
The aliphatic alcohols are missing from the above table. It is generally believed that these
molecules do not stimulate the gustatory sensory receptors of humans. However, they
apparently do stimulate the olfactory sensory receptors. This situation suggests frequent
experimental difficulties in isolating the sensory neurons of the two modalities in the laboratory.
The difficulties frequently lead to confusion in the literature over the role of these molecules in
gustation.
8.5.1.5.3 The special case of the saturated aliphatic alcohols and aldehydes
The literature is conflicted concerning the perceived taste of the saturated aliphatic alcohols
and aldehydes. With respect to gustation, the question is how do these molecules stimulate a
GR in the absence of any intrinsic pair of atoms/electrophobic elements capable of
participating in a DACB. The answer is unusual, the lower alcohols and aldehydes are
appreciably soluble in water. This solubility is accounted for by the fact these chemicals
become hydrogen bonded to the water (including that of the saliva) immediately. The distance
between the two oxygen atoms of the hydrogen bond is 2.703 Angstrom. These two oxygen
atoms are capable of forming a DACB with GR 2. The result is a perceived sweet odor for the
solution as most frequently reported.
50
Benjamins, J. Hajra, A. & Agranoff, B. (2006) In Siegel, G. et al. eds. Basic Neurochemistry, 7th Ed. NY:
Elsevier Chapter 3
Signal Generation & Processing 8- 31
It is critically important when discussing the alcohols, aldehydes and organic acids (all
polar chemicals containing oxygen atoms with incomplete orbital shells) to recognize that
when they are placed in solution the oxygen atoms will form hydrogen bonds with water.
Thus the active species within the saliva is not the molecule typically shown in text books
but the hydrated form exhibiting two oxygen atoms separated by 2.703 Angstrom and
capable of forming a DACB with OR 2 of the gustatory modality. The form could be
described as R–O- -HO or as an organic hydroxyl ether where the O- -H—O structure is a
hydrogen (London) bond.
There is also a problem of obtaining alcohols and aldehydes with an impurity level below a few
parts per billion. Ho, Johnson & Leon documented this problem in 200651. They showed that
reagents available from reputable suppliers can not be relied upon in gustatory research due
to undocumented impurities.
8.5.1.5.4 The unique role of the hydrated sodium ion in gustation
As will be seen below, the sensing of sodium and its salts play a minimal if any role in fish
gustation. However, as the vertebrates moved onto land, the search for salt became a major
nutritional requirement. It is quite possible, the minor change in the G-path “sweet” receptor to
make it sensitive to the hydrated sodium ion was the best and easiest evolutionary solution to
this problem.
Sensing the inorganic sodium ion by the new N-path GR of the gustatory modality depends
entirely on the high degree of hydration immediately assumed by the sodium ion when it is
formed in solution. This hydrated sodium ion looks to the gustatory receptors like a typical
organic molecule with a specific distance (the d-value defined below) between the oxygen
orbitals of the water molecules enclosing the sodium ion. See Section 8.5.4.4.
8.5.1.5.5 Inorganic acids and astringents excite the nocent (pain) modality
Activity related to the gustatory modality began in antiquity. During the 19th and 20th Centuries,
that activity has been primarily within the psychophysical and food science communities. No
significant theory of gustation appeared. As a result, the current terminology has evolved from
the earliest psychophysical observations. The concepts of sweet, sour, salty and bitter evolved
into a formalism of S(sweet)-best, H(acid)-best, N(salty)-best and Q(bitter)-best sensory channels.
Unfortunately, this formalism does not correspond to the operation of the neural system.
Specifically, the gustatory modality of the neural system does not sense the hydrogen ion level
present in the saliva ( a Lowry-Bronsted acid concept, 1923), it senses the presence of the
carboxyl ligand of organic compounds according to the Lewis (1916) definition of an acid, The
situation is more complex, the sensing involves two coordinate bonds, each involving the sharing
of an electron pair, but no net charge transfer between the stimulant and the receptor. Thus,
no chemical reaction is involved. The coordinate bonds are fragile and temporary in character.
The actual operating mode of the gustatory modality suggests a redefinition of the four channels
described in this paragraph (See Section 8.5.2.6 xxx).
The role of inorganic (Lowry-Bronsted) acids and a variety of astringents is to stimulate the pain
sensing portion of the nocent (nociceptor) modality of the neural system. The nocent modality
is addressed in Section 8.7 following the olfactory modality of Section 8.6.
Some organic compounds also affect the nocent modality. Carbolic acid (phenol), although
called an acid since ancient times, is actually a very strong organic base. It is so caustic that is
is highly poisonous (in fact, destructive of organic tissue).
8.5.1.5.6 Definition of specific stereo–molecular structures
Understanding the gustatory modality involves considering polyfunctional compounds,
molecules with multiple properties normally associated with separate individual molecular
51
Ho, S. Johnson, B. & Leon, M. (2006) Long hydrocarbon chains serve as unique molecular features
recognized by ventral glomeruli of the rat olfactory bulb J Comp Neurol vol 498, pp 16–30
32 Neurons & the Nervous System
families in textbooks. A major difference in properties is associated with the cis– versus trans–
characterization. This characterization is further defined when used in conjunction with cyclic
compounds.
[xxx check if following their convention ]
Spanning a long time, the literature is inconsistent when naming “sweet” stimulants. This
work will show 1,2 cis-glycol or a partially dehydrogenated derivative is the primary
glycophore of gustation. When present in a sugar, the oxygen atoms of this gustaphore
are generally present as the OH–3 and OH-4 groups with one of the hydrogens removed.
Glycophore is the term used by Shallenberger and his associates for the “sweet”
gustaphore. This work will follow their convention. This allows the 1,2 cis-glycol to be
differentiated from the 1,2 trans-glycol; the latter is employed in a glycolipid susceptible
to the hydrated sodium ion natrophore of this work.
In the older carbohydrate literature, cis– and trans– are used to describe the direction
relevant hydroxyl ligands are pointing, rather than the side of the molecule they are on
relative to each other. The precision with regard to the direction they are pointing is not
quantified. The precise direction they are pointing is a critical factor in gustation.
Therefore, a more modern and precise description is required. In this work, a diol
(including a hydrated carboxyl) incorporated into a ring structure can have its relevant
hydroxyl groups pointing perpendicular to the plane of the ring (axially) and be described
as either axial-trans- or axial-cis- relative to that plane. similarly, the diol incorporated into
a ring structure can have its relevant hydroxyl groups pointing generally outward
(equatorially) in the plane of the ring. In this case, they can be pointing marginally above
or marginally below the plane of the ring by up to a nominal 19 degrees in some sugars,
even though most Newman diagrams suggest +/–30 degrees. If the relevant hydroxyls are
both pointing in the same direction relative to the plane of the ring, they are described
as equatorial-cis-. If one is pointing above the ring and one below the ring, they are
described as equatorial-trans- .
The actual total dihedral angle in 3D space between the adjacent equatorial-trans
Oxygen atoms in the saccharides is given as 64.9 degrees in most Jmol renderings of this
molecule.
Figure 8.5.1-13 summarizes the notation used in this work and required to interpret chemical
sensing adequately. The dihedral angles associated with the Newman representation are
discussed in greater detail in Glusker et al52. Glusker et al. are the authority with respect to
stereographic representation of molecules. They specifically note, “There is considerable misuse
in the literature of the term ‘dihedral angle’ (used where the term ‘torsion angle’ is intended).
Their figure 12.6 demonstrates that the Newman Diagram is a projection of the location of the
atoms and groups of atoms onto a 2D surface drawn perpendicular to a particular axis drawn
between two anchor atoms. The defined torsion angle (θ)of this projection is typically smaller
than the dihedral angle (φ) in 3D space relative to the same two anchor atoms. However, the
definition of the two angles results in the equation; dihedral angle = 180 – torsion angle (figure
12.11.
As noted in Section 8.4.1, the Newman Representation is not adequate for describing the ligand
conformations of interest in chemical sensing. Precise three-dimensional distances between the
major orbitals must be determined. No label could be found in the pedagogical literature for
the 120 degree rotation between the two methyl groups in n-butane or other simple molecular
forms.
52
Glusker, J. Lewis, M. & Rossi, M. (1994) Crystal Structure Analysis for Chemists and Biologists. NY: WileyVCH Page 462-480
Signal Generation & Processing 8- 33
Figure 8.5.1-13 Conformational notation applied to a Newman representation. As noted earlier,
the Newman representations are 2D projections of 3D conformations. While the equat-cis
conformation appears similar to the eclipsed conformation, the 3D distance between the methyl
groups are different. Similarly, the 3D distances between the methyl groups in the equat-trans
conformation are different than in the gauche conformations. Expanded from Morrison & Boyd,
1971.
While the term glucophore is frequently used in place of glycophore, one should not draw the
conclusion that the gustaphore is based exclusively on glucose.
8.5.1.5.7 Equilibrium in the context of gustation–a brief review
The subjects of equilibrium, of equilibrium states, the release/absorption of free energy as a result
of chemical activity and changes in the concentration or reactants plays an important but
poorly delineated role in gustation. The concepts are difficult to comprehend by many
investigators during their first encounter. In the case of gustation, the very low free energies
involved may require the introduction of thermal noise related to the average energy state of
any molecule(s) during the short term.
The subject of equilibria in chemistry and changes in free energy associated therewith
have not spawned a single amorphous community of investigators. Studying the literature
is complicated by the fact that the symbology used when discussing changes in the free
energy have used the labels, ΔF, ΔG and ΔH for the same quantity over the years. In
addition, as noted by xxx, certain disciplines have used a negative value for each of these
quantities to indicate the release of energy while others have omitted the minus sign.
34 Neurons & the Nervous System
Adamson has provided definitions for some of these terms and a summary (page 218)
“E= 3/2CRT, the root mean squared (RMS) velocity of a gas molecule (page 55).” It is equal
to the internal energy of a definite portion of matter (page 121). Adamson uses a capital
and italic E when referring to a molar basis.
“H, a molar quantity called the enthalpy, is equal to E + PV (Page 130).” ΔH describes the
total work, both PV and chemical, involved in the system.
“G = H– TS where ΔG is the maximum chemical work involved in H (page 214).” T is
absolute temperature and S is the entropy of the system. G is also known as the Gibb’s
free energy. G0 is generally defined as the molar free energy of a gas in its standard or
reference state of unit pressure ordinarily taken to be one atmosphere (page 256).”
Both E and H are described as state functions where P, V & T of the gas equation are also
described as state functions (page 130). This label us used to describe a change that is
only defined by the function values at two end points of some change that is unaffected
by the path of integration used to obtain these values.
The term just-noticeable difference (JND) when used in gustation and olfaction is frequently tied
closely to the ratio of the change in stimulus intensity to the RMS (root-mean-squared) change
in the noise level due to thermal agitation at the temperature of the experiments. The RMS
concept will not be addressed here but it is widely used in engineering and physics. The major
question becomes whether the noise limit of a subject in gustation is due to the quantity, E
related to the vibration of the basic molecules involved or is due to excess noise contributed
by the sensory neural system?
Introductory college chemistry texts typically stress the dynamics of molecules at temperatures
above absolute zero and define several types of equilibrium,
1. Physical equilibrium between molecules at the border between different states of mater or
in the undissolved and dissolved states.
2. Ionic equilibrium between the non-ionized and ionized forms of an ionic molecule (typically
while in solution).
3. Chemical equilibrium between a chemical and its constituent ionizable constituents or
between a mixture of two or more ionizable chemicals and the resulting mix of ionized
components and unionized chemicals.
To satisfy the needs of biological chemical sensing, it is probably necessary to define additional
equilibrium conditions related to coordinate chemistry and present in isolation from the
associated equilibrium conditions involving some of the individual participating constituents such
as,
4. Coordinate equilibrium between ions, hydrated ions, and non-electrolytes, and their
coordinate chemical bonding to the receptor material of sensory neuron receptors.
When speaking of coordinate equilibrium, the energy levels involved are so low that the
quantum mechanical variations in the energies of the various components at a given moment
may be important. The result is a variation in the instantaneous state of equilibrium on very short
time scales compared to the time scales employed in most gustatory (and olfactory)
investigation protocols.
Equilibrium involves three major characteristics;
C There are always two opposing tendencies.
C Although the two opposing tendencies neutralize each other at equilibrium, both tendencies
are still in full operation.
C A slight change in the conditions produces, a corresponding small and reversible change in
the state of the system. This is the chief characteristic of equilibrium.
Signal Generation & Processing 8- 35
In the context of gustation, equilibrium may be separated into multiple stages; equilibrium
between solid and liquid phases, equilibrium between solvated and hydrated molecules within
a liquid environment, equilibrium between molecules in solution and those in a captured context
relative to the sensory neuron receptors, and equilibrium between molecules in solution and the
same molecules present in the adjacent vapor state.
Equilibrium reactions may be self terminating and run to conclusion when one of the products
formed is insoluble, when one of the products is volatile or when water is one of the products
formed. For most practical situations, water can be considered un-ionizable (typically 2 parts
in 109 and giving rise to the concept of pH).
Chemical equilibrium is typically influenced by changes in temperature, changes in pressure or
changes in the concentration.
The Law of Mass Action (or Law of Molecular Concentration) may be stated as “The speed of
a reaction is proportional to the product of the molecular concentrations of the reacting
substances.”
An important parameter for the equilibrium at a given temperature, pressure and concentration
is the equilibrium constant, K, given as the ratio of the speed of the reaction proceeding to the
right divided by the speed of the reaction proceeding to the left in an equilibrium reaction
written with a double arrow , W , over or in place of the equality sign. K is typically defined as
the equilibrium value after a sufficient time interval to establish equilibrium. Pressure is not an
important factor for equilibria occurring within the fluid state of matter.
In the case of ionizable molecule, the term ionization constant is frequently encountered instead
of equilibrium constant. There are significant variations in the value of the ionization constant
for special chemical situations. Two important situations are the polyprotic acids, sulfuric acid
and phosphoric acid. These acids ionize in steps with each step in their ionization exhibiting a
different ionization constant.
A search of the literature for the description of equilibria in the context of coordinate chemistry
has not provided any guidance.
The mathematical definition of K follows the pattern shown in Figure 8.5.1-14 for simple molecules
in solution. The top equation shows the simplest case of a molecule ionizing. It employs a
double arrow indicating the reaction is reversible but will attain an equilibrium for a given
temperature and total concentration. The reaction proceeds to the left based on the product
of the concentrations shown on the right and equal to KR. The reaction will proceed to the right
based on the concentration of the concentration of the complete molecule and equal to KL.
The equilibrium constant for the reaction is given by K = KR/KL
Row II shows a reaction consisting of two reactants on the left and three on the right. The
equilibrium constant reflects the concentrations of each of the reactants. The expression for K
in the right column also indicates that; for the reaction to proceed to the right, all of the terms
on the left, A and B, must colllide with each other. For the reaction to proceed to the left, all of
the terms in the numerator on the right, C, D & E must simultaneously collide with each other.
This condition holds true for all of the forms shown in the figure.
Row III shows a more complex reaction involving more than single units of each reactant.
Following the style of the second row, the equilibrium constant of this type reaction involves
recognizing the amount of each reactant present. The product of two identical terms can be
shown as the square of the individual term as shown at far right.
The value of the equilibrium constant indicate how fast the reaction reaches equilibrium. A value
of K =100, 1000 or more indicates the reaction progresses rapidly to the right. If K < 1, the
reaction proceeds to the left.
36 Neurons & the Nervous System
Row IV shows a more
complex situation involving
a polyprotic acid.
Phosphoric acid ionizes in
multiple steps. As a result,
each step exhibits its own
equilibrium constant. The
nominal equilibrium
constants for the three
reactions are; 7.5 x 10–3, 6.3
x 10–8 & 3.6 x 10–13.
The equilibrium constants
usually represent the
concentrations of the
reactants at the beginning
of a reaction, time / zero.
The characteristic of a Figure 8.5.1-14 The equilibrium constant in different contexts ADD
system in chemical
equilibrium is that the
speeds of the forward and reverse reactions have become equal and K = 1.00 during this period.
The question of major interest here are;
C How are the equilibrium constants of coordinate chemistry calculated?
C How is the equilibrium constant for a DACB calculated?
C Can it be assumed the two hydrogen bonds are formed simultaneously?
The question arises because of form III and IV in the above figure. Form III calls for all of the
constituents of a reaction proceeding in one direction to collide with each other simultaneously.
Form IV shows that a reaction requiring the presence of a component of the ionization of a
polyprotic acid (or similar structure like a disaccharide becoming hydrolyzed) may be delayed
until the necessary reactant is available.
The calculation of equilibrium constants for the step-wise dissociations of weak acids and bases
becomes very unwieldy and graphic methods are usually employed53.
8.5.1.5.8 The change in free energy associated with a DACB –a brief review
If it can be determined how the association and dissociation of a DACB progresses, it is possible
to consider how that association or dissociation might be impacted by other sources of energy
that might drive the reaction(s). The formation of Each hydrogen bond of a DACB would be
expected to involve a change of energy of less than 5 kcal/mole. This is the value typically
given for the hydrogen bonds associated with water. Adamson54 has given that value when
associated with water. He has also given a higher value of about 7 kcal/mole(0.3 eV) for each
of the two hydrogen bonds in the dimer of acetic acid and suggested a net change of 14
kcal/mole (0.61 eV) for dissociation of the dimer. These are remarkably low energies compared
to the conventional chemical bond between hydrogen and oxygen of 119 kcal/mole (
5.1electron volts @1eV = 23,060 kcal/mole).
Such small energies are typically associated with changes in conformation within a molecule as
illustrated in Section 8.5.1.5.6. A few molecule form or decompose in these energy ranges.
53
Adamson, A. (1973) A Textbook of Physical Chemistry. NY Academic Press page 551
54
Adamson, A. (1973) A Textbook of Physical Chemistry. NY Academic Press page 298
Signal Generation & Processing 8- 37
Hydrogen iodide has a standard enthalpy of formation, ΔH0 of only 6.2 kcal/mole at 298Kelvin.
xxx gives the best value as ΔG = – xxx at xxxC. This is an exceedingly small value relative to the
energy fluctuations of a molecule at room or endothermic animal body temperature.
.
8.5.1.5.9 The primary question regarding the transduction mechanism of
gustation
The primary question regarding the mechanism supporting excitation and de-excitation process
of a chemical sensory neuron receptor in gustation and olfaction is; How is the DACB initially
formed and how and why is the DACB broken?.
The answer is most likely given in terms of the molecular quantum mechanics described in the
previous two subsections as they apply to solutions. At the detailed level, this description may
be quite complex. However, the first order mechanism may be tractable. This subject will be
pursued in Section 8.5.3.3.
8.5.1.6 Summary of the gustatory modality hypothesis
The following material will develop the theory of the gustatory modality as an explanation of the
empirical data in the literature based on the Electrolytic Theory of the Neuron and transduction
via a coordinate chemistry mechanism for each of the sensory channels of taste. The theory is
comprehensive. It encompasses the sensing of all four classical sensations of taste. It will be
labeled the Electrolytic Coordinate Chemistry Theory of Gustation.
The theory of gustatory sensing recognizes a three dimensional taste space with the four classical
sensations occupying four of the available six vertices. The space is expandable in either of two
ways, if additional data appears supporting a fifth channel related to the label “umami.” This
sensation could occupy an available vertex in the three dimensional space or it could require
expanding the space to a fourth dimension (as is the case in the visual modality when the
sensing of ultraviolet light is recognized– Section xxx).
Moving from a conceptual understanding of gustation to a more formalized theory has provided
new insights. These insights can be grouped as follows.
[xxx subdivide the following into groups with rational names. ]
Group 1 Gustatory Transduction, global aspects
C Gustatory transduction involves a coordination chemical process of very low energy that is
totally reversible. No chemical “reaction” is involved in the transduction process.
C Transduction occurs totally external to the sensory neuron lemma. No stimulant components
pass through the cell wall.
C The transduction mechanism uses a set of phospholipids, widely recognized as being present
in neural tissue but of previously unknown utility, as the gustatory sensory receptors (GR’s).
C The transduction mechanism employs a coordinate chemical union between the sensory
receptors and the stimulant that is stereo-chemically specific.
Group 2–Gustatory Transduction, Details
• The phospholipids forming a specific active region of a GR are present as a liquid crystal
portion of the dendritic tree, known cytologically as the cilia.
• There are four specific types of GR in gustation that selectively isolate the individual
gustaphores found among a wide range of stimulants.
• The individual gustaphore is associated with the appropriate GR through a dual anti-parallel
coordinate bond (DACB) arrangement.
38 Neurons & the Nervous System
• The perception of a taste is the result of a 2-step transduction process involving initially the
isolation of gustaphores affecting a given GR type and subsequently the measurement of the
dipole potential of the individual stimulant when bonded to the GR.
• The net dipole potential change due to a specific stimulant may be influenced by the specific
electrostatic field presented to the GR, resulting in the perception of super-sweet and superbitter qualities.
Group 3 Additional details of transduction
C The transduction mechanism results in a change in dipole potential of the phospholipids that
are known to be polar. This change in dipole potential is sensed by the first Activa of the sensory
neuron.
• The net change in dipole potential of the individual phospholipid GR is highly sensitive to
nearby electrostatic fields, particularly those of the associated gustaphore.
C The histology and physiology of the sensory neurons following the transduction mechanism is
the same as that of other sensory neurons. They lack an external axon like the auditory sensory
neurons and share a microvilli structure similar to the visual sensory neurons.
C The sensory channels of the gustatory system exhibit the same excitation/de-excitation function
and the same adaptation functions as other sensory modalities. Only the time constants vary
to accommodate anatomical and vascular conditions.
C The gustatory modality exhibits the same neural architecture as all other sensory modalities but
the stage 2 signal processing occurring outside the cranium appears to be minimal.
Group 4 The Perception of stimulants within the neural system
C The gustatory modality uses a small group of sensory receptors (nominally four) to sense a wide
variety of chemical structures (in the thousands).
C The small number of sensory channels operate independently and can therefore be
represented as statistically independent and therefore orthogonal, resulting in a threedimensional taste space within the neural system. This space can be unfolded into a single
dimensional space equivalent to the spectra of the visual and auditory modalities.
C The unfolded representation of the taste space highlights the common feature wherein many
stimuli are able to stimulate multiple sensory channels.
C The “salty” sensory receptor is in fact only sensitive to the fully hydrated sodium ion; the chlorine
ion plays no role in the transduction mechanism.
C The “sour” sensory channel is primarily sensitive to the carboxyl group of organic (Lewis) acids.
C The response of the gustatory modality to inorganic acids and various astringents (alkalineearth salts) appears to be primarily via nociceptors.
C The “sweet” sensory channel is primarily sensitive to a group, consisting mostly of either oxygen
or hydroxyl oxygen at positions 1,2 of a cis-glycol, generally in a cyclic structure. [See a more
precise definition of the 1,2 equatorial-trans-glycol given below]
C The “sweet” sensory channel exhibits an overlay mode of operation, involving a three-point
stereochemistry, that is supersensitive to man-made sweeteners (super-sweet glycophores.
C The “bitter” sensory channel is primarily sensitive to organics with two orbitals capable of
sharing an electron-pair and separated by a distance of 4.746 Angstrom.
C The “bitter” sensory channel also exhibits an overlay mode of operation, involving a three-point
Signal Generation & Processing 8- 39
stereochemistry, that is supersensitive to primarily man-made stimulants (super-bitter
picrophores).
C The frequently proposed independent “umami” sensory channel cannot be confirmed based
on the available empirical data and rigorous multidimensional analysis procedures. The
dominant chemical associated with this designation, monosodium glutamate, is unusual in that
it exhibits multiple gustaphores, hence its ability to stimulate the salty, sweet and acid channels
of gustation simultaneously.
Eliel et al. have discussed the different kinds of molecular models available in both physical and
computerized form55. No effort will be made here to standardize on one type of diagram or
physical representation. [xxx see final version of why four GR’s paper for broader discussion)
Simple line diagrams of chemicals leading up to include Haworth models will be used,
augmented by computer models of specific chemicals prepared using Molecules-3D Pro, v2.10
from Molecular Arts. The specific geometric relationships between stimulants and receptors may
not be shown faithfully because of the author’s limited familiarity with the program. However,
the critical dimensions will be specified as clearly as possible.
Figure 8.5.1-15 describes the sensory receptors related to taste. They are all phosphatidyl lipids
derived from the normal lipids forming the outer bilayer of the microvilli of the sensory neurons.
They differ primarily in the spacing between certain atoms in the structures that allow coordinate
bonding with a variety of stimuli in the AH,B or AH,B,X configuration (Section 8.5.3).
The figure is complex. However, by following the logic of Dowhan, it provides a variety of
answers. It begins at upper left describing the chemical structure of a typical phospholipid and
its ability to bond with a variety of terminal groups through esterification. The diagram has been
annotated here to define one of the lipid molecules to be electrically conductive, most likely
through complete conjugation of its fatty acid chain, like the isoprene chain of Vitamin A. The
ligand in the box is labeled on the right with the trivial name choline, or formally
phosphatidylcholine (PtdCho). This ligand plus the next one below it, ethanolamine, are the
principle phospholipids of the lemma of all cells. The examples of ligands below these two are
those commonly found in the lemma of neurons and particularly sensory neurons of the
gustatory system. There are four of these ligands, each supporting a separate sensory channel
of gustation leading to the sensation of sourness (presence of an organic acid or acidophore),
sweetness (presence of a glycophore), saltiness (presence of a hydrated ion of sodium or
natrophore) and bitterness (presence of a picrophore).
Each of the ligands exhibits a distinctive spacing between two or more orbitals susceptible to
coordinate bonding with stimulants exhibiting a similar spacing between elements supportive
of coordinate bonding. In the case of the sourness channel, the most important orbitals of
phosphatidylserine (PtdSer) exhibit a spacing of 2.34 Angstrom when nonresonant (2.07 Angstrom
when resonant). This is the same spacing as the two oxygen atoms of the carboxylic acid family.
Thus, any carboxylic acid is amenable to coordinate bonding to PtdSer and eliciting a sensation
of sourness. The proposed location of this bonding is shown by the small rectangular bracket
next to the two oxygen atoms. PtdSer is therefore defined as the receptor of the sensory neuron
initiating the sourness, or acid, channel of gustation.
Carboxylic acid is only mildly acidic, indicating it is not highly ionized in water. The
dominant species is the nonresonant carboxylic acid form with a complete hydroxyl
group. The spacing between the oxygen atoms in this configuration is 2.34 Angstrom.
Phosphatidylgalactose (PtdGal), also known as galactoceramide (CerGal) exhibits a spacing
of 2.86 Angstrom between OH–3 and OH–4. This spacing is compatible with a very large group
of sweeteners containing a 1,2 cis–glycol group that can dual-coordinate bond with this
receptor and elicit a sensation of sweetness.
[xxx may duplicate previous section 8.5.1.xxx ]
Spanning a long time, the literature is inconsistent when naming “sweet” stimulants. This
55
Eliel, E. Wilen, S. & Doyle, M. (2001) Basic Organic Stereochemistry. NY: Wiley Interscience pg 25
40 Neurons & the Nervous System
work will show 1,2 cis-glycol or a partially dehydrogenated derivative is the primary
glycophore of gustation. When present in a sugar, the oxygen atoms of this gustaphore
are generally present as the OH–3 and OH-4 groups with one of the hydrogens removed.
Glycophore is the term used by Shallenberger and his associates for the “sweet”
gustaphore. This work will follow their convention. This allows the 1,2 cis-glycol to be
differentiated from the 1,2 trans-glycol; the latter is employed in a glycolipid susceptible
to the hydrated sodium ion natrophore of this work.
In the older carbohydrate literatue, cis– and trans– are used to describe the direction
relevant hydroxyl ligands are pointing, rather than the side of the molecule they are on
relative to each other. The precision with regard to the direction they are pointing is not
quantified. The precise direction they are pointing is a critical factor in gustation.
Therefore, a more modern and precise description is required. In this work, a diol
(including a hydrated carboxyl) incorporated into a ring structure can have its relevant
hydroxyl groups pointing perpendicular to the plane of the ring (axially) and be described
as either axial-trans- or axial-cis- relative to that plane. similarly, the diol incorporated into
a ring structure can have its relevant hydroxyl groups pointing generally outward
(equatorially) in the plane of the ring. In this case, they can be pointing marginally above
or marginally below the plane of the ring by up to a nominal 19 degrees in some sugars,
even though most Newman diagrams suggest +/–30 degrees. If the relevant hydroxyls are
both pointing in the same direction relative to the plane of the ring, they are described
as equatorial-cis-. If one is pointing above the ring and one below the ring, they are
described as equatorial-trans- .
PtdGal is defined as the receptor of the sensory neuron initiating the sweetness channel of
gustation. The ligand is shown in its standard notation that places the atoms of interest at the
lower left. A bracket is shown to represent the stimulant bonding with the specified oxygen
atoms.
The PtdGal receptor is shown twice in a dashed box because of an additional
phenomenon that has been documented. Besides the common two-point coordinate
bonding arrangement illustrated with a spacing of 2.6 Angstrom in the top half of the box,
it was found that an enhanced sensation of sweetness could be elicited by some
stimulants if they employed a three-point rather than two-point bonding strategy. This
super sweet stimulant situation is shown in the lower half of the box.
Phosphatidylinositol (PtdIns) exhibits a spacing of 3.3 Angstrom between the two rightmost
hydroxyl groups in the figure. This spacing is uniquely matched to the spacing between the
oxygen atoms of adjacent water molecules hydrating with the sodium ion. Thus, this
phospholipid is specifically attractive to coordinate bonding to the hydrated sodium ion and
eliciting the sensation of saltiness. Note that this natrophore relates only to the sodium ion. The
Signal Generation & Processing 8- 41
Figure 8.5.1-15 Summary: sensory receptors of gustatory modality based on the Electrolytic
Theory of the Neuron and a coordinate chemistry mechanism. The glycol receptor is able to
sense a variety of man-made “super sugars” using a tripartite coordinate union when they
exhibit an electron rich area a prescribed distance from the two oxygen atoms of their AH,B
structure. The bitter (quinine) receptor may also operate in a tripartite mode to sense a variety
of “super bitter” stimuli. See text. Built using the style of Dowhan, 2002.
42 Neurons & the Nervous System
anion of the salt plays no role in eliciting the salty sensation. PtdIns is defined as the receptor of
the sensory neuron initiating the salty channel of gustation. Inositol monophosphate_19951069
demonstrates the esterification of inositol to the intrinsic Ptd.
Phosphatidyl 3'-O-aminoacyl glycerol (Ptd3'Og) is the fourth unique phospholipid of the gustatory
modality. It exhibits a spacing of 4.2 Angstrom between the doubly bonded oxygen and the
amine group of this amino acid based ligand. This spacing is compatible with a very large group
of chemical that can coordinate bond with it and elicit a sensation of bitterness. Ptd3'Og is
defined as the receptor of the sensory neuron initiating the bitter channel of gustation.
Based on the similarity in the stereochemistry of the bitter receptor and the sweetness
receptor, the bitter receptor has been reproduced twice within a dashed box. It appears
likely that some of the extremely bitter stimulants that have been found may employ a
three-point coordinate bonding arrangement like the super sweeteners do. This
potentiality is shown in the lower half of the box using the dashed triangle to suggest the
bonding arrangement..
The coordinate bonding between the various stimulants and receptors on the external lemma
of the sensory neurons involves a very low energy (5kcal/mole) and does not constitute a
chemical reaction in the conventional sense. No reaction products are formed and the original
species reappear when the hydrogen bonds are disrupted. The stimulants do not pass through
the lemma of the sensory neuron.
The major difference between the sensory receptor for sweetness and saltiness involves the steric
crowding of the hydroxyl ligands of the polar molecular structure. The galactose molecule
exhibits considerably more steric crowding than does the highly symmetric inositol molecule. This
results in a smaller dimension, d, between positions 3 and 4 of the galactose molecule than in
inositol.
The phosphatidyls of the gustatory sensory receptors are specialized forms of type 4 lemma.
They can be tabulated under simpler names indicative of their specific character, Figure 8.5.116. The new laboratory channel designations in the right hand column are developed in the
following paragraphs.
Figure 8.5.1-16 Lemma sub-type designations for the sensory receptors of gustation ADD. The
active groups are attached to a generic phosphatidyl molecule (Ptd) that provides the
electrical conductivity through the outer bilayer of the neuron dendrite. The label C–Best is used
to designate the organic acid sensitive channel, that excludes sensitivity to inorganic acids.
Moulton speculated on the location of the sensory receptors on the surface of the sensory
Signal Generation & Processing 8- 43
olfactory neurons in 1970 but there has been little discussion of the subject since56. The discussion
was exploratory and drew no conclusions. He did note the vast surface area of the cilia of the
sensory neurons compared to the estimated 3 sq. cm. of the olfactory epithelium in humans.
Figure 8.5.1-17 summarizes the performance of the gustatory modality and presents a rectilinear
graph for positioning stimulants in sensation space for the first time. As noted in Section 8.5.xxx,
the signals passed to the brain by the sensory channels are treated as independent, and
therefore orthogonal, by the brain. This orthogonal feature supports the presentation of the
sensory data in a three-dimensional format, as suggested by the “basis factors” of multidimensional scaling applied to the gustatory modality.
A d-value of 4.746 Angstrom is shown for the bitter sensory receptor. Some papers have
suggested a value of 4.36 Angstrom derived from features of several astringents (primarily alkaliearth salts).
Figure 8.5.1-17 UPDATE XXX’s Summary: performance of the gustatory modality based on the
Electrolytic Theory of the Neuron and a coordination chemistry mechanism. Top; stereogeometry of the transduction process. Middle; a tabulation of pertinent labels and parameters.
Bottom; a rectilinear sensation vs stimulus space describing the first order operation of the
gustatory modality. Mean values are calculated or reported. Distributions are conceptual. [xxx
Dashed line shows potential alternate bitter channel at d = 4.36 Angstrom]. See text.
56
Moulton, D. (1970) Detection and recognition of odor molecules In Ohloff, G. & Thomas, A. eds. Gustation
and Olfaction. NY: Academic Press pp 1-27
44 Neurons & the Nervous System
The discovery of a large number of man-made sweeteners, and obscure natural sweeteners,
with immensely more perceived sweetness than the natural reference, sucrose, has changed
the character of the search to understand the S-best sensory channel of gustation. van der Wel
has provided a comprehensive list of these sweeteners57. He also noted the propensity of these
sweeteners to exhibit multiple AH,B sites and potentially multiple AH,B,X sites (possibly with
multiple X dimensions)..
Figure 8.5.1-18 shows the first-order form of the gustaphores of taste. The titles are arbitrary but
designed to emphasize the character of the gustaphore. Acidophore suggests the Lewis acid
character of the acidophore, as opposed to a simple hydrogen ion. The first order acidophore
is invariably derived from a carboxyl group and could be labeled a carboxylophore or an
acetateophore (See Section 8.5.1.6.2). The glycophore suggests a sweetness, but could just as
readily be labeled an ethylophore to indicate the two carbon backbone. The term glycophore
is infrequently used and does not describe the general case. In the extended world of
sweetness, at least one oxygen orbital can be replaced by another electron-pair sharing
element.
In gustation, the term orbital will be used to designate oxygen, nitrogen or sulfur, and
potentially phosphorus, atoms with their outer electrons in hybridized form and exhibiting
shareable electron-pairs. [xxx add C=C double bond and resonant benzyl ring? ]
In the more general case, the oxygen can be replaced by nitrogen, sulfur or an electronegative
feature such as an unsaturated carbon bond or the π-bonding system of the benzene ring. In
the more limited case, the term glycol is also appropriate as it describes two hydroxyl groups
separated by two carbons. The controlling feature is the dimension, d, between the two orbitals.
Trans– glycol configurations, do not exhibit the required d-value of about 2.6 Angstrom. The
natrophore describes the configuration of the hydrated sodium ion giving what is commonly
called the salty sensation. The anion of the salt plays no role in this sensation. The natrophore
exhibits a 90 degree angle between the two orbitals due to the octahedral form of the hydrated
sodium ion. The gustaphore contributing to the bitter taste has been labeled a picrophore to
suggest its bitter taste. It could be described as the propophore to suggest the three carbon
backbone of the structure.
57
Van der Wel, H. (The role of organic chemistry in taste perception In van der Starre, H. ed. Proceedings of
7th ISOT London: IRL Press pp 13-19
Signal Generation & Processing 8- 45
An electronegative feature also appears to
be important in a special situation leading
to super sweetness, and potentially super
bitterness. An electronegative feature a
specific distance from the AH,B group
appears to bond to a site on the receptor.
This tripartite AH,B,X arrangement results in
a sweetness sensation orders of magnitude
(reported as great as 9500:1) greater than
that of the dipartite arrangement (Sections
8.5.3 & 8.5.10).
After the presence of two electron-pair
sharing orbitals and the hydrogen bond
associated with one of them, it is the
distance, d, between the orbitals
supporting the AH,B, or AH,B,X coordinate
union that is the important feature of the
gustaphores. The distances shown in the
figure are nominal as little statistically
precise data is available. There has been
confusion previously in the case of the
glycophore as to whether the distance
measured was between the orbitals or the
greater diagonal distance between one
orbital and a hydrogen.
8.5.1.6.1 Defining
perception space
the
gustatory
Three facts have emerged in the above
discussion;
• the d-values of the gustaphores in a
stimulant are key to interpreting gustation
• the d-value continuum represents the
fundamental dimension of gustation
• the four gustatory paths within the neural
system are treated as orthogonal
• the four gustatory neural paths represent
four “labeled-lines” and
• Each of the labeled lines may consist of
multiple individual neurons in that path.
Figure 8.5.1-18 The first-order gustaphores of taste.
The spacing, d, between the oxygen atoms (or
alternate orbitals) is the critical dimension in
determining the effectiveness of the gustaphore.
See text.
Based on these individual corollaries to the
hypothesis, the following assertion is important;
• the labeled line represent orthogonal paths that can be considered nodes along the
fundamental dimension.
The question becomes how is the information delivered to the higher information extraction
engines of stage 4. Furthermore, how can the information be represented to match the way the
stage 5 cognitive engines perceive the information.
Figure 8.5.1-19 provides the first calibrated graph of the gustatory perception space. The
vertical lines represent the nominal d-values of the sensory neuron receptors. The distributions
about these vertical lines represent the probability that a given gustaphore can interact with the
gustatory receptor (GR) associated with that d-value. Currently the widths of these distributions
are not known. A subsequent paper will show this horizontal number line can be folded at each
of the nominal d-values to form a three dimensional taste perception space.
46 Neurons & the Nervous System
Figure 8.5.1-19 The one-dimensional effectivity graph of gustatory performance based only on
the steric properties of the receptors and gustaphores. The term ring in the graph refers to a
cyclic compound. Centroid values of each distribution are at nominals of 2.276, 2.82, 3.243 &
4.746 Angstrom.
This graph can be considered analogous to a spectral plot of the visual modality based on a
constant amplitude narrow band spectral source traversing the wavelength spectrum. In that
analogy, the effectivity shown here is similar to the probability of excitation of an individual
chromophore of vision as a function of wavelength. By folding the visual modality
representation at the centroids of the chromophore peaks, the familiar 3D and 2D perceived
color spaces of vision are obtained.
Each response function is illustrated here with an arbitrary tuning width of ±5% . This tuning width
is associated primarily with the individual GR and will be more completely evaluated later. This
use of the term tuning is different from that commonly used within the psychology community.
See section 8.5.2.3.5.
It is proposed that the same folding procedure used to create the visual color solid of Munsell,
etc. can be applied to gustation. The result is a 3D volume with the centroids of the various
gustatory receptors (GR) at the nodes of the volume and the distances between the nodes
represented by the d-value scale.
The term “folding” used here is totally different from the “unfolding” term used to
differentiate the data points of one subject from a larger set of subjects in a MDS
representation. The concept is discussed in Chapter 7 of Cox & Cox. It does not appear
to be widely used..
8.5.1.6.2 A 3D olfactory perception space with calibrated scales
Signal Generation & Processing 8- 47
This paragraph will develop the MDS space with calibrated scales before introducing both a
quantitiative MDS space for gustation and a quantitative MDS-like space incorporating intensity
information.
When folding a continuous 1D fundamental dimension into a more useful 3D space several
options arise;
• Whether to employ a linear or logarithmic scale.
• Whether to adopt a left-hand or right-hand 3D space configuration.
For the purposes of this discussion, the lengths of the individual vectors associated with either the
linear or logarithmic scales will be ignored in favor of a set of equal length vectors (resulting in
a cubic perception space for illustration).
It is also important to recognize that the higher information extraction engines of the brain can
only add and subtract the amplitudes of the vectorial signals they receive over the
mathematically independent channels of the gustatory modality, always staying within the
boundary formed by the nodes at the corners of the perceptual space. [xxx how to explain this
in more precise, but not necessarily more difficult language ]
•Each gustaphore channel projects an analog intensity value to the information extraction
engines. While this intensity value may be propagated as a pulse signal within stage 3 circuits,
it is recovered at the output of stage 3 and processed within stage 4 as an analog signal.
While gustaphores with d-values marginally less than 2.26 and marginally greater than 4.74
Angstrom can form a DACB with a GR, the GR channels only report the signal intensities
associated with the labeled lines arriving at the information extraction engines of the brain.
These labeled lines represent the cardinal d-values associated with the GR’s (2.26, 2.82, 3.24 &
4.74 Angstrom). The extent of the stimulus application space extends only marginally beyond the
values of the extreme nodes. The axes of the 3D representation interconnecting these nodes
and representing the gustatory perceptual space do not extend beyond those extreme nodes.
Like the visual modality, the spacing between the nodes of the 1D gustatory dimension appear
to be independent of any underlying logarithmic relationship. Therefore, a linear dimension shall
be employed here.
Like the visual modality, the peaks in the sensitivity profile of gustation can profitably be
considered nodes in a 3D representation of the perceived gustatory space.
Figure 8.5.1-20 shows two arbitrary foldings of the linear parameter, the d-value, into three
dimensional form using the peak sensitivities of the GR’s in the modality as nodes of the
perceived space. Frame A repeats figure 1. Frame B shows the intensity values reported to the
stage 4 information extraction engines. While frame A indicates an effectivity profile for each
gustatory channel, frame B does not. The signals delivered to the stage 4 information extraction
engines arrive over “labeled lines.” There is no equivocation about their source. Frame B also
indicated the vectorial length of each segment of the fundamental dimension between the
nodes of gustation.
To maintain the integrity of the 3D representation, it is critically important that the head-totail character between the individual vectorial segments of the fundamental dimension
be maintained. If not required in setting the parameters for the computer solution to the
MDS, the program will select its own relationship by default. This may not maintain the
desired continuity of the fundamental dimension in the resulting representation.
Frame C shows the nodes of the perceived space labeled per their d-values using a Left-handrule for folding. Frame D shows the same framework as frame C but using a Right-hand-rule for
folding. In both cases, the heavy line demonstrates the folding of the fundamental dimension.
Dimension 1 represents the value +Δx, dimension 2 represents the value +Δy and dimension 3 represents the
value +Δz.
48 Neurons & the Nervous System
The signs of the scales assigned automatically by the MDS program are irrelevant. It is
important that the various nodes of the gustatory modality paths be located as shown in
framed D. It is critically important that the location of these nodes be shown in a
“standard representation” so that the representations from various investigators can be
compared. Such a comparison has been labeled a “Procrustes analysis” in some
documents (Cox & Cox, chapter 5). It requires employing rotation, mirroring and scaling
until congruent representations are obtained. Then, a calculation of similarity can be
performed if desired. Otherwise, a visual comparison can be made.
Dunn-Rankin et al. have developed a
complete procedure for performing the
above standardization procedures
under t he heading “Principal
Component Analysis (PCA) in their
application workbook for Windows
based computers58. They note, “Usually
the general factor solution that emerges
(from MDS) before rotation is not as
interpretable as a solution that can be
obtained by rotating the axes in order to
contrast the factor loadings more
effectively.” They provide the matrix for
rotating the axes in the clockwise
direction. They also use a routine called
Varimax to determine the optimum
angle of rotation. Abdi has expanded
on the Varimax procedure59.
As is immediately apparent, these two
frames are not superimposable (using
the chemists terminology), but the
A–G–N and G–N–P planes are identical
in both views except for a direction sign
change. Thus, the two volumes are
stackable, frame D on to frame C. There
is an intensity value from frame B
associated with each of the nodal
positions in both cases. This intensity
value constitutes a fourth dimension
within the complete data set.
If only one gustaphore is present during
an experiment, the gustaphore will be
represented by an analog value
associated with its node in either frame
C or D.
This is an important situation. The MDS
technique is qualitative. The typical
MDS analyses should only report
individual gustaphores at nodal
locations. If all of the gustaphores
Figure 8.5.1-20 Alternate representations of a 1D
parameter in a 3D perception space. A; the 1D
dimension, d-values. B; the intensity values
delivered to the stage 4 engines. C; the 3D
perception space using the Left-hand-rule. D; the
3D space using the Right-hand-rule. E; the
perceived location of mixed gustaphores. See
text.
58
Dunn-Rankin, P. Knezek, G. Wallace, S. & Zhang, S. (2004) Scaling Methods, 2nd Ed. Mahwah, NJ: Erlbaum
Associates Chapter 12
59
Abdi, H. (2003) Factor Rotations in Factor Analyses. In Lewis-Beck M., Bryman, A., Futing T. Eds.
Encyclopedia of Social Sciences Research Methods. Thousand Oaks (CA): Sage.
http://www.utdallas.edu/~herve/Abdi-rotations-pretty.pdf
Signal Generation & Processing 8- 49
exciting a single GR are not shown congruent at the nodal position, the precision of the
data is not statistically significant. The diameter of the cluster of values obtained by
repeating the experiment is inversely proportional to the precision of the results.
If two gustaphores are present during an experiment, the perceived stimulant will appear as a
point value between the nodes of the gustaphores at a position determined by the relative
intensities (available from the data for the fourth dimension) of the gustaphores sensed by the
GR’s.
• Location 1 represents an intensity in the G–channel twice that in the C– channel.
• Location 2 represents an intensity in the G–channel twice that in the N–channel.
• Location 3 represents an intensity in the P–channel equal to that in the N–channel.
• Location 4 represents an intensity in the N–channel twice that in the C–channel.
• Location 5 represents an intensity in the P–channel twice that in the C–channel.
Locations 4 & 5 are obviously not along the fundamental dimension of the d-value parameter.
They are similar to the magentas of color vision. They represent the simultaneous presence of
two gustaphores that are stimulating non-adjacent GR’s along the d-value dimension.
Monosodium glutamate is a stimulant exhibiting three gustaphores. If they each excited an
equal intensity signal in the N–, G– & A– channels. The perceived sensation would be in the
center of the A–G–N triangle. For different perceived intensities for the individual gustaphores,
the perception would move about within the plane of the A–G–N triangle.
As a corollary to the single gustaphore situation above, in MDS analyses involving multiple
gustaphores, the reported position in perceptual space is only accurate if the same
experimental protocol reports the individual gustaphores at their correct nodal positions.
When a series of stimulants containing the same glycophore are shown as a cluster in an
MDS representation, the diameter of the cluster is inversely proportional to the precision
(adequacy) of the experimental protocol. This matter will be addressed further below.
The choice of the left-hand rule or right-hand rule coordinate systems shown above is arbitrary.
Figure 8.5.1-21 shows a three-dimensional taste space based on the above and additional
considerations based on Section 8.5. It is rotated from the right-hand rule coordinate system
above. The figure shows the four basic taste sensations at four of the eight vertices of a cubic
space. The folded segments of the fundamental dimension are shown as dimension 1 (A–G), 2
(G–N) and 3 (N–P).
50 Neurons & the Nervous System
Figure 8.5.1-21 Potential taste sensation space for a mammalian species exhibiting four primary
taste sensation. The lower frame shows the nominal capture efficiency for a gustaphore of
different d-value than the related GR. See text.
[xxx check if this is useful ]
Signal Generation & Processing 8- 51
In the orthogonal coordinate system shown, the G–Path node occurs at the null value, 0,0,0.
However, this system leaves the C–P and G–P axes as non-orthogonal with respect to the other
axes. An alternate representation would shift the 0,0,0 value to the center of the volume and
consider the middle of dimensions 1, 2 & 3 as representing the zero value for that dimension..
---[xxx duplicated using different names after next figure ]
The potential for a fifth basic taste, umami, to be present at one of the remaining vertices
remains present. However, the subsequent analyses will show the major ligands of mono sodium
glutamate are;
• a carboxylic acid channel (A-path) gustaphore (limited to Lewis acids and their esters)
• a 1,2 equatorial-trans-glycol channel (G-path) gustaphore. and
• a hydrated sodium ion channel (N-path) gustaphore interacting with a 1,2 axial-trans inositol
GR.
Thus, mono-sodium glutamate, and in fact all putative stimulants perceived as umami-like are
found to consist of multiple gustaphores with their combined perception located in the C–S–N
plane.
It should be noted that the amplitude of the stimulus passed to the brain is not represented in
this figure. Such an amplitude parameter exists in a separate dimension from those shown as will
be developed in Section 8.5.2.3 and later.
8.5.1.6.3 The proposed qualitative 3D gustatory perception space
Figure 8.5.1-22 shows an alternate MDS space using a more relevant labeling and a right hand
rule widely used in engineering. It is recommended as a template for future research. Note the
labeled nodes do not incorporate any perceived intensity information. To use this presentation
format, there must be an attempt to use gustants of equal gustatory efficacy. The preferred
protocol is to use a series of single gustaphore gustants (SGG) analogous to the SOO of olfaction
(Section 8.6.xxx). If some of the stimulants are multiple gustaphore gustants (MGG), their
location in the perceived taste MDS representation will be at locations remote from the nodes
dependent on the character and relative efficacy of the individual gustaphores. The precise
locations will be determined by vector-based calculations. The representation of such MGGs
can be expected to appear in one of the planes annotated on the figure (or within the overall
volume if gustaphores exciting all four channels are present) if sufficient statistical precision is
available from the test protocols.
The template has the option of;
C stretching the cube shown to a rectangular solid and making the scales uniform in accordance
with the d-values at the nodes or,
C demarcating each dimension with scales of uniform, but different, spacing from the adjacent
dimensions.
If the data offers sufficient precision and indicates the nodes are misplaced, a generic rotation
of axes associated with the data is indicated as discussed in Section 8.5.2.3.1.
52 Neurons & the Nervous System
Figure 8.5.1-22 Preferred MDS space based on a right-hand rule more commonly used in
gustatory research. The heavy line in the upper frame is a folded version of the abscissa of the
lower frame. Available MDS software programs can reverse individual scales to arrive at this
right-hand rule space configuration. See text.
The potential for a fifth basic taste, umami, to be present at one of the remaining vertices
remains present. However, the subsequent analyses will show the major ligands of mono sodium
Signal Generation & Processing 8- 53
glutamate are;
• a carboxylic acid channel(A-path) gustaphore (limited to Lewis acids and their acetates)
• a 1,2 equatorial-trans-glycol channel (G-path) gustaphore. and
• a hydrated sodium ion channel (N-path) gustaphore interacting with a 1,2 axial-trans inositol
GR.
Thus, mono-sodium glutamate, and in fact all putative stimulants perceived as umami-like are
found to consist of multiple gustaphore gustants with their combined perception located in the
A–G–N plane. The perception of umami has occasionally been likened to “meat-like,” an
obvious mixture of individual perceptions.
Other MGGs absent any organic acids or acetates may be represented in the G-N-P plane. For
MGGs including derivatives of all four types of SGGs, their representation will appear within the
volume of the MDS space (with the continued assumption that all gustaphores present are of
equal efficacy). The net efficacy relative to a single node may be affected by the presence of
multiple gustaphores with the same nominal d-value.
It is stressed that the amplitude of the signals passed to the brain is not represented in this figure
and no intensity of the resultant perceived taste is present in the MDS space. Such an amplitude
parameter exists in a separate dimension from those shown as will be developed in Section
8.5.1.6.4 and later in Section 8.5.2.3 .
----When employing the MDS analytical technique, it is not usual to describe the axes (or
dimensions) specifically. Figure 8.5.1-23 presents a table that will be useful later in comparing
the data from various investigators employing the MDS analytical technique. As it stands, the
table is not useful. Every investigator used different dimension labels and mixed sub-sets. and
generally inadequately populated sub-sets.
Figure 8.5.1-23 Citations & parameters of recent MDS investigations ADD DATA to Smith. The
dimensional data is based on best estimates by this investigator. CT; chordata tympani. NG;
glossopharyngeal nerve.
54 Neurons & the Nervous System
Most of the available data sets ( Hellekant et al., 1997) are based on stimulants and not single
gustaphore gustants. As a result, the electrophysiological data is not of nominal amplitude when
collected within the neural system.
Many of the data sets available have not populated the perceived gustatory space adequately
to define their axes explicitly (Smith et al., 1979: Giza et al., 1991; ).
Smith & Travers have investigated the efficacy response function for a variety of functions based
on a largely conceptual model of the gustatory modality60. They sought to use many of the
probability equations of information theory, particularly relative to entropy) in a deterministic
context.
“The entropy measure, as applied here, varies continuously from 0.0 for a unit that
responds exclusively to one stimulus (i.e., narrowly tuned) to 1.0 for a cell that responds
equivalently to all four of the basic compounds (i.e., broadly tuned). Subtle variations in
the neural response profile of a cell, such as those produced by changes in stimulus
concentration, are reflected in this measure. Thus, the use of the equation for entropy to
describe the responsiveness of gustatory neurons provides a quantitative measure of their
breadth of tuning that can be meaningfully applied to the problems of gustatory quality
coding.”
“Investigations of both peripheral and central gustatory neurons in a variety of mammalian
species have clearly demonstrated that these cells are typically broadly responsive to
stimuli representing the four basic taste qualities (with many archaic citations) . This lack
of stimulus specificity first led Pfaffmann (1955, 1959) to propose that taste quality is coded
by the pattern of activity across a population of broadly tuned afferents, a concept which
has been further elaborated and given a quantitative basis by Erickson (1963, 1967, 1968,
1974; Erickson et al., 1965). This across-fiber pattern theory accounts for the ability of a
discrete number of neurons to code the tastes of the many thousands of potential
gustatory stimuli and has received support from a number of behavioral studies of taste
discrimination.”
“Although this pattern theory of quality coding can account well for the broad sensitivity
of gustatory neurons, it has recently been proposed that taste quality may be represented
in the nervous system in a more specific fashion (Nowlis & Frank, 1977; Pfaffmann, 1974;
Pfaffmann, Frank, Bartoshuk & Snell, 1976). For example, if a neuron responds better to
sucrose than to NaCl, HC1 or quinine, it may be carrying information only about
"sweetness", the lesser response to these other stimuli being background noise. This
labelled-line view of taste quality coding suggests that there are four separate neural
channels in the mammalian gustatory system that carry information about the four distinct
experiences of salty, sour, sweet and bitter (Nowlis & Frank, 1977). Since responses of these
differentially but broadly tuned neurons can be handled well by either theoretical
approach, the nature of the neural code for taste quality is a topic of much current
interest.”
These remarks are largely consistent with this work but for different underlying reasons, specifically
related to the interpretation of multiple channel stimulation as a noise-related phenomenon.
In 1979, Smith & Travers did not consider that their individual gustants )such as quinine HCl and
sucrose) might contain multiple gustaphores or that HCl might be primarily a nocent and not a
gustant. After discussing a variety of approaches to interpreting the results of others, they note,
“As a consequence of these various approaches to describing response breadth,
conclusions about the specificity of populations of taste-responsive neurons are sometimes
contradictory (Pfaffmann et al., 1979). For example, most cortical cells in the rabbit have
been reported to respond to three or four of the basic taste stimuli (Yamamoto &
Kawamura, 1975), whereas the typical cortical neuron in the dog and rat responds to only
one or two stimuli (Funakoshi et al., 1972). Either there are species differences in the
60
Smith, D. & Travers, J. (1979 A metric for the breadth of tuning of gustatory neurons Chem Sense Flav vol
4(3), pp 215-229
Signal Generation & Processing 8- 55
specificity of these central gustatory neurons or the various definitions of responsiveness
and breadth of tuning are creating an apparent discrepancy in the interpretation of these
data.”
They describe their approach to the tuning breadth problem,
“What is required to resolve this kind of problem is a metric for the breadth of
responsiveness of gustatory neurons that will be sensitive to subtle distinctions in neural
response functions rather than simply dichotomize units as either broadly or narrowly
tuned. In a recent analysis of the response properties of gustatory neurons-in the hamster
medulla, Travers & Smith (1979) introduced such a measure of response breadth based on
the equation for entropy from information theory (Shannon & Weaver, 1949). This
approach takes into account the relative magnitudes of the responses to each of the four
basic stimuli and provides a continuous scale for quantifying the breadth of a neuron's
sensitivities. The purpose of the present paper is to further elaborate this concept in order
to provide a more quantitative approach to the problem of the breadth of tuning of
gustatory neurons.” They note, “The application of the entropy equation as a measure of
response breadth in the present situation is distinctly different than its use in the context of
information theory, in which estimates of p, are based on response probabilities (Shannon
& Weaver, 1949).”
If the investigation was repeated using SGGs, the results would be distinctly different and more
repeatable.
The 1979 Smith & Travers paper did not report data from an MDS analysis; however, the 1983
paper did61.
“The present investigation is designed to examine the relationship between the taste neuron
types in the hamster brain stem suggested by the preceding paper and these proposed neural
codes for taste quality. Multidimensional scaling of the stimulus relationships within and across
the neuron groups is employed in an attempt to understand the role of these various classes of
neurons in the coding of taste quality.”
They use the –best labeling of their four coordinates (based on the stimulus used) rather that the
–path designations more directly related to the sensory neurons. They do note that the so-called
H– neuron group is dominated by the response to organic acids and not inorganic acids
exhibiting an H+ ion. They also make the important observation, “The exclusion of any one class
of neurons from the population results in a dispersal of its most effective stimuli within a
multidimensional ‘stimulus space’.” After a few more introductory remarks, they also note, “Thus,
the neural distinction between many behaviorally discriminable stimuli depends on the
simultaneous activity in different classes of neurons.” Concluding their Summary & Conclusions,
they note, “Particular sets of neurons are critical for coding the tastes of particular classes of
stimuli, but activity in more than one group of neurons is necessary for the unambiguous coding
of taste quality.” These assertions are all compatible with the hypothesis of this work.
Their introduction began with, “It is almost without exception that mammalian taste neurons
respond to stimuli representing more than one of the classical four taste qualities (many
citations). It was the multiple sensitivity of fibers in the chorda tympani nerve that first led
Pfaffmann (25-27) to propose that taste quality is coded by the relative amount of activity across
a number of taste afferents. Since the response of most taste fibers can be modulated by both
stimulus quality and intensity, the response of any particular cell is typically ambiguous with
respect to either parameter.” They then state their goal, “The purpose of the present paper is
to examine the relationship between the gustatory neuron types in the hamster brain stem and
the population response of these neurons to a variety of taste stimuli.” They study focused on
neuron types and not stimulus types! They use a variety of inorganic and organic molecules to
determine the sensitivity of four different classes of neurons within the hamster parabrachial
nuclei (PbN) neurons. “These data were chosen because the numbers of cells in each of the
neuron classes was nearly equal in the PbN data. This was important since one of the goals of
61
Smith, D. Van Buskirk, R. Travers, J. & Bieber S. (1983) Coding of taste stimuli by hamster brain stem
neurons J Neurophys vol 50(2), pp 541-558
56 Neurons & the Nervous System
the present analysis was to examine the stimulus relationships within each of the neuron groups
separately.” Under methods, “Identification and quantification of neural responses were the
same as reported in the preceding paper (35). The 5-s response measures were put into the
same multivariate matrix (m neurons x yt stimuli), from which the subsequent analyses
proceeded. From this matrix of 3 1 neurons x 18 stimuli, an across-neuron correlation matrix was
generated, which was used as the input matrix to a multidimensional scaling program (KYST, Bell
Telephone Laboratories). This program yielded a “stimulus space,” depicting the similarities and
differences in the across-neuron correlational relationships.” They went on, “The number of
dimensions necessary to represent these relationships adequately was chosen on the basis of
the “stress” in the solution and the interpretability of the space. Once the “stimulus space,”
defined by the responses of all the neurons, was created, additional multidimensional scaling
analyses were conducted on subsets of the neurons, defined by the cluster solution in the
preceding paper which suggested three neuron groups (S-, H-, and N-neurons) on the basis of
similarities among their response profiles..” They did not report on Q–Best (P–Path) neurons
although they did show two P–Path molecules in their three dimensional MDS.
Smith et al. report their results in human perceptual space although the data was collected by
interrogating hamster stage 3 projection neurons. Their figure 8 clearly shows separate groupings
for the stimuli focused on the A–Path, G–Path, N–Path and P-Path. Their use of four different
symbols to relate the stimulants affecting a particular neural channel maximally in their MDS
analysis is in excellent agreement with the hypothesis of this work (with spreads primarily due to
the limited number of test sequences performed and a lack of accounting for the use of MGGs.
Their node at 0,0,0 is clearly related to the A–Path, their node I is related to the G–Path, their
node II is related to the N–Path Node III locates the P–Path. The location of the data groups
suggests a rotation of the coordinate system by about 10 degrees is appropriate.
Their paper goes on to form a set of two-dimensional MDS representations based on collapsing
their 3D data set (with the confabulation of the results cautioned about in Section 8.5.xxx.
Recognizing this confabulation, it is clear from their figures that removing one set of related
stimulants does provide a multi-peaked response function for each set of stimulants.
They conclude in their Discussion that, “The present analyses have demonstrated the relationship
between the “across-fiber pattern” and labeled-line approaches to the analysis of neural
responses in the gustatory system. Although these two approaches are describing the same
phenomena from different perspectives, their implications for the coding of taste quality are
quite distinct. The results of the present analyses can be interpreted within either theoretical
framework and, taken alone, cannot argue for one of these alternatives over the other. To the
extent that particular classes of neurons are critical for establishing particular across-neuron
patterns, the same cells are important for coding quality in both theoretical approaches.
However, the implication for the roles of these cells in coding taste quality is quite different in the
labeled-line and the across-fiber pattern hypotheses.” They go on, “Whether one chooses to
accept a labeled-line or an across-fiber pattern interpretation of neural coding depends on
whether one believes that activity in a single neuron represents, in and of itself, a single taste
quality. Unless this point is experimentally testable, the distinction between these two theoretical
approaches is primarily a philosophical one.” Fortunately, a clearer understanding of the signal
processing context available leads to a testable situation.
The data and discussion of the Smith et al. paper is quite supportive of the hypothesis of this work
when one modification to their concept is introduced. Their discussion addresses the differences
between the above two theoretical approaches without incorporating the concept that the
signal paths are statistically independent (although their stimulating gustants may contain
statistically related gustaphores) and therefore orthogonal. Considering the vectorial addition
of the responses in the orthogonal representation of the perceptual space leads to the stage
4 information extraction neurons providing a composite signal to a lookup table that defines the
overall gustatory perception stored in the appropriate region f the saliency map and available
to the stage 5 cognitive neurons.
The Smith and the Danilova papers have begun providing a measure of the quality of their MDS
representations using the Kruskal stress parameter. The Danilova paper provides more visibility
into this parameter as a function of the number of dimensions set as a parameter in their MDS
calculations. This parameter will be addressed further in Section 8.5.2.
Signal Generation & Processing 8- 57
---Danilova et al. provided gustatory data in an MDS format as a result of their investigations with
the marmoset62,63. “Callithrix jacchus jacchus is a small New World monkey belonging to the
infraorder platyrrhina, which shared ancestry some 38 millions years ago with the catarrhina
group to which humans, apes and Old World monkeys belong.” The investigator is cautioned
to be aware of the findings in Section 8.5.1.6.8.
For reasons not clearly stated, the MDS representation in the 2002 paper did not include data
related to the sodium channel (N–Path). It did include HCl as a stimulus along with a group of
Lewis acids. It did include a variety of very complex gustant molecules (Table 1, example:
QHCl). They stated, “In our recent studies, we have strived to use a large number of
compounds. The main reason for this is that we wanted to address the question of quality and
coding in taste not the particular taste of a given compound.” This approach overlooks the fact
that many of their gustants incorporated many diverse gustaphores. The paper is discussed and
the representation is shown in Section 8.5.4.5.
As an aside relative to the Danilova & colleagues papers, data from the chorda tympani
(CT)should not be combined with data from the glossopharyngeal nerve (NG) other
nerves, and other orthodromic locations within the neural system without justification.
They began their analyses with a series of cluster analyses and noted,
“The hierarchical cluster analysis distinguished three major clusters in both CT and
NG(separately): S, Q, and H. The SCT fibers, 38% of all CT fibers, responded only to
sweeteners. The SCT fibers did not respond during stimulation with salts, acids, and bitter
compounds but exhibited OFF responses after citric and ascorbic acids, quinine
hydrochloride (QHCl), and salts (in 80% of SCT fibers). SNG fibers, 50% of all NG fibers, also
responded to sweeteners but not to stimuli of other taste qualities (except for citric acid,
which stimulated 70% of the SNGfibers). Some sweeteners, including natural (the sweet
proteins brazzein, monellin) and artificial [cyclamate, neohesperidin dihydrochalcone
(NHDHC), N-3,5-dichlorophenyl-N’-(S)-α-methylbenzylguanidineacetate (DMGA),
N-4-cyanophenylcarbamoyl-(R,S)-3-amino-3-(3,4-methylenedioxyphenyl) propionic acid
(CAMPA)] did not elicit responses in the S fibers.”
These findings appear to conflict with the virtually universal reports that individual sensory path
receptors exhibit broad tuning that assures they show some response to stimuli aimed at other,
or at points along the d-value line between both, sensory paths.
The above paragraph was followed by an apparently contradictory paragraph,
“In general, the response profiles of the SCT and SNG clusters were very similar, the
correlation coefficient between the responses to sweeteners in these clusters was 0.94.
Both the QCT and the QNG fibers (40 and 46% of all fibers) were predominantly responsive
to bitter compounds, although their responses to the same set of bitter compounds were
quite different. Sweeteners with sweet/bitter taste for humans also stimulated the Q
clusters. The H clusters (22 and 3% of all fibers) were predominantly responsive to acids and
did not respond to stimuli of other taste qualities. However, bitter stimuli, mainly QHCl,
inhibited activity in 70% of HCT fibers. Among a total of 90 fibers from both nerves there was
only 1 NaCl-best fiber in CT. We found, however, that 35% of the CT fibers reacted to salts
with inhibition of activity during stimulation, followed by an OFF response. This OFF response
was diminished or eliminated by amiloride.”
62
Danilova, V. Danilov, Y. Roberts, T. Tinti, J-M. & Hellekant, G. (2002) Sense of taste in a new world
monkey, the common marmoset: Recordings from the chorda tympani and glossopharyngeal nerves J
Neurophysiol vol 88, pp 579–594
63
Danilova, V. & Hellekant, G. (2003) Comparison of the responses of the chorda tympani and
glossopharyngeal nerves to taste stimuli in C57BL/6J mice BMC Neurosci vol 4,
http://www.biomedcentral.com/1471-2202/4/5
58 Neurons & the Nervous System
The data in the Danilova & Hellikant(2002) paper have reported negative pulse rate data from
neurons of the chorda tympani, when Chapter 9 will show the pulse rates are necessarily zero
or positive. The negative values may be entirely due to their mathematical manipulations
(example, subtracting a quiescent pulse rate from a gross pulse rate during stimulation). Their
comments regarding the suppression of responses suggests their electronic probes were
frequently interrogating stage 3 signal projection neurons of the differential type rather than of
the summation type (see Chapter 9).
----All of the phospholipids defined above are known to be associated with the sensory neurons of
gustation. They all exhibit a dipole moment that is a function of their state of coordination.
When present with one conjugated lipid in the liquid crystalline form, as found in a bilayer, they
exhibit a dipole potential that can be measured. This potential is also a function of the state of
coordination of the molecule.
----Similarly, the system does not sense the presence of a salt, or even the presence of sodium per
se. It senses the presence of the specifically hydrated form, Na:(H20)6+ of the sodium ion only.
It specifically does not sense the presence of the chlorine ion in solution. The operation of the
modality explains a critical aspect related to mono-sodium glutamate. This compound interacts
with the hydrated sodium receptor and potentially both the glycophore and organic acid
receptors. It does not interact with an independent umami sensory receptor.
Figure 8.5.1-24 provides a necessary conversion process if one is to transition from a behavioral
to a more fundamental understanding of the gustatory modality. No chemical reactions are
involved in gustation; no residues are formed. Only temporary coordinate chemical bonds are
involved. The “acid” sensory channel does not sense a proton; it operates only in the Lewis acid
sense in order to sense a carboxyl ligand.
Figure 8.5.1-24 The transition from behavioral to fundamental perspectives in gustation. All the
gustaphores exhibit an AH,B molecular configuration and suitable stereographic configuration
capable of forming a dual hydrogen bond with a receptor. Note: “picric acid” is not a Lewis
acid. It is the historical name for 2,4,6-trinitrophenol_6688.
To be clear, the hydrogen ion, H+, is not sensed by the gustatory modality. The gustatory
modality senses an AH,B stereochemical configuration capable of forming a DACB with
a d = 2.276 Angstrom dimension. This is the nominal “natural” configuration of the carboxyl
group, i.e., the organic (Lewis) acids and the esters of such acids, the esters.
Similarly, the “sweet” sensory channel does not sense the chemical groups typically associated
with a sugar; it senses the 1,2 equatorial-trans-glycol overlay group associated with a wide
variety of organic compounds. In research, the preferred SGG is the monosaccharide, glucose,
rather than the disaccharide, sucrose. The “salty” sensory receptor does not sense a salt; it
senses only the fully hydrated sodium ion. The “bitter” sensory receptor does not sense any
Signal Generation & Processing 8- 59
feature or chemical group uniquely related to the quinine molecule; it senses the picrophore,
a pair of oxygen atoms separated by a three-carbon chain with a specific structural relationship.
The simplest example of a picrophore is (2R, 4R)-pentanediol_2005883. There is no “umami”
sensory receptor, the typical umami stimulant, stimulates more than one of the above sensory
receptor channels.
The gustatory section concludes with:
The realization that the gustatory receptors depend on specific steric distances between orbitals
capable of sharing electron-pairs and not upon the functional groups within a molecule or the
interconnection of those groups.
The hydrated state of any cation is critically important to the gustatory transduction process.
When monosodium glutamate is ionized and hydrated in solution, it is capable of stimulating
three of the four sensory receptors, the acid, the sweet and the sodium receptors. There is no
evidence for a separate and unique umami sensory channel. It has been reported by Beets that
sodium benzoate can elicit sensations of sweet, bitter, sour, salty and/or tasteless64. However,
the elicited sensations appear to vary significantly among individuals. The following discussion
does not surface how sodium benzoate could elicit a bitter or tasteless response. See Section
8.5.4.5.1.
8.5.1.6.4 An equivalent quantitative 3D olfactory perception space EMPTY
The qualitative MDS representation proposed above can be used for more quantitative
investigations. These investigations typically employing electrophysiological interrogation of one
or more stage 3 signal projection neurons associated with the gustatory (and not nocent)
signaling channels. There are several obstacles to the use of a 3D MDS format;
1. The major obstacle is that the 3D MDS format cannot represent four independent
(orthogonal) sensory channels and, in addition, an amplitude parameter.
2. A second major obstacle is accommodating within the protocol adequate control of the
state of adaptation of all of the sensory receptors stimulated by the gustants.
3. The pulse code used among the stage 3 channels must be understood and evaluated
properly to avoid to preserve the timing of the first pulse and recognizing the variation in pulseto-pulse interval intrinsic to this coding.
The first obstacle indicates the test protocol must be restricted to stimulating no more than three
of the independent sensory channel GRs (bordering the A-G-N or G-N-P planes) in order to
accommodate intensity information along the third axis.
Without detailed knowledge of the efficacy profile of each of the GRs, it is difficult to insure or
compensate for the state of adaptation of the GRs of an individual sensory channel.
The pulse code used in gustation appears to be the same code used throughout the peripheral
neural system (See Chapter 9). Because of the relatively crude method of stimulation used in
both gustatory and olfaction research, it is typically best to make measurements related to the
stage 3 pulse trains after they have stabilized, and typically after several seconds from onset.
8.5.1.6.5 The gustatory response versus molarity of stimuli
The output as a function of input intensity has proven very important in understanding the
operation of the visual modality.
Zotterman has provided data on the gustatory response of humans based on recordings of taste
64
Beets, M. (1978) Op. Cit. pg 175
60 Neurons & the Nervous System
nerves65 with most of the data attributed to Borg et al. (1967). They confirmed their integrated
responses from the chorda tympani relative to the molarity of citric acid exhibited the same
slope on log-log paper as psychophysical responses, 0.5. They then showed the slope of N-best
channels exhibited a slope of 1.0 while the slope of H-best channels showed a slope of nominally
0.67 psychophysically for 14 students. In parallel experiments, the G-Path channels showed a
slope near 1.0 and higher than that of the H-best channel. This data is very similar to that
provided by Sato for the rat66. Sato more carefully recorded the saturation affects encountered
at molarities near and above 0.1 M. Several of the figures in Sato and in Borg et al. show
significant saturation that cannot be interpreted using a straight line as an overlay. No clear
threshold levels were documented. The size of the database suggests more experiments are
needed and saturation must be accounted for in reducing the data.
xxx in his Fermich Award Address has provided a mixed list of stimulant to the gustatory (and
probably the nociceptor modalities (page 41) with references.
8.5.1.6.6 The structural constraints on tasting sweet
Shallenberger & Acree presented a brief theory of sweetness in 1967 based on the presence of
a glycol group in the stimulus67. It stated the initial interaction “is neither a proton transfer nor an
electrostatic interaction, but probably involves London dispersion, the principle element of
hydrogen bonds.” However, it is important to note the in vitro experiment involved neutral
chemicals dispersed in water and not attached to any neural system. Much more
comprehensive papers appeared in 197068 and 197169. The 1970 paper by Shallenberger
reviewed the various previous theories of compounds that were perceived as sweet. None of
these theories has survived their time. He also addressed the question of why some molecules
tasted sweet while their conformal partners did not, and proposed it was due to stereographic
hindrances at the GR site. He also demonstrated that the previous idea that L–sugars were
tasteless while the D–sugars exhibited taste was a myth.
The 1970 paper addressed the question of why lead acetate and beryllium chloride are
reported to taste sweet (page 133). He noted that these materials do not disassociate in
the usual manner but form hydrates (much like the saturated alcohols are perceived as
sweet due to their forming hydrates (a.k.a. azeotropes).
The 1970 paper also addressed two medical patients being treated for
hypoparathyroidism. Simultaneously, they reported an anomaly in their gustatory modality
wherein they could not perceive the sensation of sweetness (aglycogeusia). Based on this
work, the condition appears to be genetic. The paper uses the terms sour and bitter rather
loosely and a more stringent protocol may be needed to evaluate this condition. It is
possible these patients were not forming the GR 2 receptor (proposed here to be PTDGal).
Later, a paper by Kier, Shallenberger & Lindley discussed a number of secondary structural
65
Zotterman, Y. (1971) The recording of the electrical response from human taste nerves In Beidler, L. ed.
Handbook of Sensory Physiology, Vol 4/1: Olfactory Sensing. Chap 6
66
Sato, M. (1971) Neural coding in taste as seen from recordings from peripheral receptors and nerves In
Beidler, L. ed. Handbook of Sensory Physiology, Vol 4/1: Olfactory Sensing. Chap 7
67
Shallenberger, R. & Acree, T. (1967) Molecular theory of sweet taste Nature vol 216, pp 480-482
68
Shallenberger, R. (1970) Molecular structure and taste In Ohloff, G. & Thomas, A. eds. Gustation and
Olfaction. NY: Academic Press pp 126-133
69
Shallenberger, R. & Acree, T. (1971) Chemical structure of compounds and their sweet and bitter taste In
Beidler, L. ed. Taste: Handbook of Sensory Physiology, Vol IV, Part 2, Chap 12
Signal Generation & Processing 8- 61
features that affect the intensity of the sensation of sweetness70. The paper considered many
alternatives and is very well structured. It also provided some information on the other basic
tastes. Shallenberger presented a very comprehensive discussion of the structural aspects of
sweetness in 1982 that appears to support the electrostatic premise71. In 1996, he provided a
synopsis of his work based on an electrostatic assumption, i.e., that no reaction between the
stimulus and the receptor occurred72. In 2000, Eggers, Acree & Shallenberger provided a review
of their work over a 30 year span73. The critical nature of the specific conformation of a odorant
was highlighted by Shallenberger. The Anti-anisaldehyde oxime is very sweet, yet its fraternal
twin, Syn-anisaldehyde oxime, is tasteless.
8.5.1.6.7 Extending the chemoreception concepts of Shallenberger, Kier and
Beets
[xxx incorporate Shallenberger in Ohloff & Thomas, page 126 ]
Evans provided some early material related to understanding the transduction of the sweet
sensation74 that appears to have led to the important documentation of Shallenberger and
Acree in 1967. Evans used the terminology of the day to suggest transduction “involved a
physical rather than a chemical reaction, but of course a very specific one.” He also discussed
the properties of an unspecified stereoisomer of inositol as a stimulant. After Shallenberger et
al. and Kier explored the coordinate bond pair concept in the context of the elicitation of a
sweet sensation, Beets (1978, pg 188) suggested this concept could be extended to include all
of the organic taste stimulants. This work will go farther and show that the concept also explains
the major inorganic stimulant class, those eliciting a “salty” sensation. It will show the salty
sensation is actually the sensation elicited by the hydrated form of the sodium ion acting as a
stimulant. The salty sensation is actually elicited by both bases and salts of the sodium ion in
hydration. However the activity of most bases containing sodium are too active chemically to
be used in gustation.
In section 8.6, it will be shown that the same concepts apply to the modality of olfaction, with
similar constraints related to the chemical activity of the phenols. While the phenols can cause
injury if employed in olfaction experiments, their derivatives play an important role in the field of
perfumery.
In 1990, a major symposium was held reviewing the work of many investigators during the 1980's.
The record of that meeting appeared in 199175. The use of computerized molecular modeling
was in its infancy at that time. However, the importance of 3D modeling was noted (page xxx)
as well as the fact that the response of the sensory neurons to sugars followed the E/D
mechanism described in detail in this work (page 295). Figure 8.5.1-25 shows Hellekant et al.’s
interpretation of the E/D mechanism (Section 8.7). It clearly defines a finite delay time (less than
2 sec), a rise time, which should not include the delay time, steep enough to obscure its
exponential character (time constant less than 0.25 seconds). See Section 8.5.6.2 for a discussion
of this figure.
70
Shallenberger, R. & Lindley, M. (1977) A lipophilic-hydrophobic attribute and component in the
stereochemistry of sweetness Food Chem vol 77(2), pp 145-153
71
Shallenberger, R. (1982) Advanced Sugar Chemistry. Westport, CT: AVI Publishing Chapter 10
72
Shallenberger, R. (1996) The AH,B glycophore and general taste chemistry Food Chem
vol 56(3), pp 209-214
73
Eggers, S. Acree, T. & Shallenberger, R. (2000) Sweetness chemoreception theory and sweetness transduction
Food Chem vol 68(1), pp 45-49
74
Evans, D. (1963) Chemical structure and stimulation by carbohydrates In Zotterman, Y. ed. Olfaction and
Taste. NY: Pergamon Press page 165+
75
ACS Symposium (1991) Sweeteners: Discovery, Molecular Design , and Chemoreception. Washington, DC:
ACS volume 450
62 Neurons & the Nervous System
An extension of the Shallenberger & Acree
and Kier concepts of multiple bonding was
also extended, at least conceptually by
Tinti & Nofre (page 206). They noted the
expression AH,B,X used in this work had
been standardized by that time as AH,B,G.
They proposed an expanded set of points
of importance; AH, B, G, D, Y, XH, E1 & E2
without describing each in detail. The
additional points were based on their Table
I that summarized the presence of
chemical groups that played some role in
various sweeteners (and the sweetness of
the resulting compound). Figure 8.5.1-26
shows their more complex situation. They
did not develop what that role was. They
noted, “To elicit sweetness, the
simultaneous binding of the eight sites
assumed to be involved in the sweetenerreceptor interaction is not a prerequisite.”
They did provide distances between some
selected orbitals present in the various
chemical groups. In other cases, they gave Figure 8.5.1-25 A representative summated
the dimensions relative to an arbitrary recording from the chorda tympani nerve of a
arrangement of orbitals. Their work did not rhesus monkey during a 10 s e c stimulation with a
provide or involve the d-value between the sweetener. See text. Modified from Hellekant et
complete family of orbitals defined in this al., 1991.
work and drawn from the earlier proposals
of Shallenberger & Acree, Kier and Beets.
The original Tinti & Nofre work was presented in German and has not been translated or widely
adopted by the English speaking research community. Their D group is dominated by the
cyanides. While they gave dimensions between the various ligands in their figure, they did not
provide angles
They did not identify any sensory receptor
that could be impacted by their multiple
varieties of gustants (or gustaphores) such
as AH,Y, AH,B, XH,Y, XH,E1 or XH,E2.
The identification of one or more individual
gustaphores based on an overlay group
within a complex molecule provides a
totally different and simpler solution than
the model of Tinti & Nofre.
Culbertson & Walters provided a paper in
the same volume focused on a 3D model
of the sweet taste receptor. Their model
was based on the molecular design
techniques available in that era and relied
upon known structure activity relationships
(SAR) as they were known at that time.
However, they did not identify any
receptor and all of their drawings involved
two dimensional representations.
Figure 8.5.1-26 Caricature of extended multipoint
model of sugar-receptor coupling. G is shown
large primarily because it involves very large
molecules relative to the other chemical groups.
No bond lengths between these groups were
described. From Tinti & Nofre, 1991.
Rohse & Belitz (chapter 13) provided an
early report on computer modeling to
discover the underlying rules of step one
gustation. Again, there was no default model and they used many very complex molecules
(exhibiting many individual gustaphores based on this work) in their analyses .They generally built
Signal Generation & Processing 8- 63
on the ideas of Shallenberger & Acree and of Kier. Their framework provided more precision by
defining an electrophilic/nucleophilic (e/n) system. Their description of their recognized e/n
systems appears internally contradictory compared to similar categorization in this work (see
Section 8.5.8.2). They also provided their finding of the perceived taste of these chemicals.
However, the allowed tastes appear to have been limited to the dichotomy of sweet versus
bitter (Tables II & IV). As part of their explorations, they did define what are called d-values in this
work (the distance between pairs of electrophilic and neurophilic sites within an individual
molecule). Unfortunately, they did not define the multitude of these values present in their
typical MGG molecule. They did define a threshold for sweetness in many cases.
Simon wrote on the mechanisms of sweet taste transduction (page237). However, the paper
primarily addressed the conceptual possibilities rather than documenting the actual situation.
Lindley addressed the subtle differences between similar chemicals that act as either sweetness
inhibitors or non-inhibitors (page 251) but without explaining their significance. This material can
be reinterpreted using the hypotheses of this work.
Most recently, Erickson has written on the core ideas of taste76 and has drawn many comments.
The paper was largely philosophical, contained no diagrams and provided little new information
on the physiology of the gustatory modality.
8.5.1.6.8 Proteins as stimulants and/or gustaphores
The biological community has long asserted a role for proteins in gustation, however there has
been a lack of definitive evidence. The assertion has been primarily associated with the
recognition that a very few identified proteins are perceived as sweet. Their sweetness ranges
from one hundred to three thousand times that of sucrose on a weight/weight basis. As noted
by Caldwell et al.77,“Only six proteins — brazzein, curculin, mabinlin, monellin, pentadin, and
thaumatin — have been identified that elicit a sweet taste response in humans.” Pentadin
appears to be a di-brazzein resulting from food preparation.
The 1991 ACS symposium addressed above contained only one paper focused on the role of
proteins in the perception of sweetness (page 28) along with a paper related to simple peptides
(page 41). Several authors noted the continued inability of the community to identify any
protein based sensory receptor. As noted by Lancet on page 234, “A most awaited
development is the future identification, isolation and characterization of the protein receptors
themselves.” This is clearly a Bayesian approach to science, anticipating a desired solution to
a problem.
Section 8.5.9.1 will develop the role of “sweet proteins” in greater detail and include a
theoretical explanation for their performance
8.5.1.6.9 Tests of the Electrolytic hypothesis of gustation
The bulk of the tests of the hypothesis are embedded in Section 8.5 itself. The successful reinterpretations of much of the previous literature is proof of the hypothesis beyond a reasonable
doubt. However, some unique and important tests can be developed.
1. An important test of the hypothesis is whether the simpler acetates, absent any other
odorophores, are perceived by the human as acidic. If so, they are clearly stimulating the APath of the gustatory modality. signaling plan as predicted by the hypothesis.
8.5.1.7 A dichotomy: the labeled-line and across-neuron-pattern theories
76
Erickson, R. (2008) A study of the science of taste: On the origins and influence of the core ideas Behav Brain
Sci vol 31, pp 59–105
77
Caldwell, J. Abildgaard, F. Džakula, Z. Ming, D. Hellekant, G. & Markley, J. (1998) Solution structure of
the thermostable sweet-tasting protein brazzein Nat Struct Mol Biol vol 5, pp 427 - 431
64 Neurons & the Nervous System
How the different stimuli are initially sensed in the peripheral portion of the gustatory system has
evolved along two separate paths, the labeled-line theory and the across-neuron-pattern
theory. These two approaches are seldom precisely defined and may not be totally distinct.
Several authors offer different interpretations of their meaning.
Christensen & White provided a comprehensive overview of this subject78 from its introduction
by Dethier in 1976. They note, “In the ‘labeled-line’ model, the sensory axons carrying information
to the CNS are ‘absolutely restricted’ with respect to selectivity, whereas in an ‘across-fiber’
code, ‘each stimulus would produce a different and characteristic total response profile’ across
the entire population of sensory neurons.
[xxx add and reference olfactory portion, Section 8.6.6 & 7 ]
Data is now available to resolve the labeled-line versus across-neuron-pattern dichotomy with
respect to gustation. Data such as Figure 8.5.1-27 from Pfaffmann et al., and similar individual
neurons make it very difficult to support the labeled-line concept in the gustatory modality
organization79. They also highlight the analog character of the initial waveforms at the receptor
neurons and within that modality. These individual neurons are responsive to a wide variety of
stimuli and may contribute to an across-neural-pattern theory. Alternately, they may contribute
to a simpler difference channel approach as used in vision.
78
Christensen, T. & White, J. (2000) Representation of olfactory information in the brain In Finger, T. Silver,
W. & Restrepo, D. eds. The Neurobiology of Taste and Smell, 2nd Ed. NY: Wiley-Liss
79
Pfaffmann, C. Frank, M. Bartoshuk, L. & Snell, T. (1976) Coding gustatory information in the squirrel
monkey Chorda Tympani, In Sprague, J. & Epstein, A. eds. Progress in Psychobiology and Physiological
Psychology. NY: Academic Press. vol 6, pp 1-27
Signal Generation & Processing 8- 65
Figure 8.5.1-27 Records of total chorda tympani responses to water and taste stimuli applied to
the anterior tongue. The first two deflections of the marker at the lower edge signal the flow of
distilled water, the third deflection signals duration of stimulus flow, the fourth indicates rinse with
distilled water. I division = 5 seconds. From Pfaffmann et al., 1976
66 Neurons & the Nervous System
8.5.1.8 Initial identification of human genetic differences
Bartoshuk has discussed the apparently genetic differences between, non–tasters, tasters and
super–tasters with reference to specific individual gustaphores80. These differences and the
inherent difficulty of comparing the perceived sensations among individual makes for interesting
conversations among gustatory researchers. The discussion following the 1993 paper is
particularly interesting to an outside observer.
More recently, Drewnowski has addresses the genetics of human taste perception briefly and
from a narrow perspective81. He notes, “The study of human taste genetics is largely the study
of bitter taste.” He does note the important distinction that bitter tastes have a threshold at
micro-molar levels, apparently to prevent ingested toxicity while the sweet tastes have a
threshold at milli-molar or higher levels. He lists a remarkably wide list of chemicals exhibiting
many different functional groups that all exhibit a bitter taste. Based on this list, and no
underlying discussion of the gustatory sensing mechanism, he asserts, “That would suggest that
bitterness is perceived through a variety of receptors and multiple transduction mechanisms.”
Such a conclusion appears unrealistic based on a deeper understanding of the mechanisms
involved.
8.5.1.9 Renewal of the gustatory sensory neurons
Van der Heijden has noted, “The taste cells are renewed continuously; the renewal cycle occurs
in 10 days to 2 weeks82.” This is the same renewal cycle as found in the visual and auditory
sensory neurons.
8.5.2 Analysis of perceived gustatory sensations–MDS and other techniques
Three tools have emerged as the principle means of understanding the taste performance of
the gustatory modality. As noted by many, taste is a perceived sensation that may not be due
purely to gustatory sensory neurons. In many cases, the olfactory sensory neurons appear to
contribute a major portion of the overall taste experience. The somatosensory neurons of the
oral cavity appear to also play a significant role.
In common with other sensory modalities, ethical and cultural restrictions have limited the
collection of data from the human species related to gustation, other than that from
psychophysical experiments. As a result, the major data bases are based on behavioral data
from the hamster, rat and occasional monkey.
Table 16.1 by Breslin in Finger, Silver & Restrepo provides a list of thresholds for a variety of
gustants and individual gustaphores collected from a variety of sources. No tolerances are
provided and the mixed sources make it difficult to correlate the data.
The first goal of any gustatory analysis is to determine the minimum number of distinct sensory
channel types required for a species to achieve its desired performance. This requires the
collection of data using a variety of stimulus types. To interpret the available data, dendrograms
and both two and three dimensional histograms have proven the most useful.
80
Bartoshuk, L. (1993) Genetic and pathological taste variation: what can we learn from animal models and
human disease? In Margolis, F. ed. The Molecular Basis of Smell and Taste Transduction: Ciba Foundation
Symposium 179. NY John Wiley pp 251–267
81
Drewnowski, A. (2003) Genetics of human taste perception In Doty, R. ed. Handbook of Olfaction and
Gustation. NY: Marcel Dekker Chapter 40
82
Van der Heijden, A. (1993) Sweet and bitter tastes In Acree, T. & Teranishi, R. eds. Flavor Science.
Washington, DC: American Chemical Society Chapter 3
Signal Generation & Processing 8- 67
The recent availability of SYSTAT, a suite of programs that can be run on a nominal Apple
computer system (and after version 13 on a Windows system, has led several investigators to
adopt it. In its current form, it is designed to use the modern graphical interfaces of these two
computer operating systems. It is basically a display program for relatively simple problems in
statistics. It does not specifically address cluster analysis, multidimensional scaling or the
optimization procedures related to these types of analyses. The program shares its name with
a variety of system level commands and macros within the Linux and Unix environments.
At this time the cluster analysis program in the BMDP2M software package is the preferred tool.
For MDS analysis, the techniques derived from the original KYST program is preferred.
8.5.2.1 Dendrographic representation
It is important to differentiate between dendrograms based on data acquired at the
sensory neurons, later in the neural signaling chain, or ultimately based on psycho-physical
perceptions. The goal of stage 2 and stage 4 signal processing is to extract specific
focused information from a complex signal environment. The different dendrograms are
indicative of the progress in this process. They can be significantly different.
When recording stage 3 signals, it is common to report the average pulse rate for an
interval on the order of five seconds. This type of report obscures any transient
performance and adaptation associated with the gustatory modality. Chemical sensing
neurons typically exhibit an initial time constant of less than one second.
[xxx edit following about adjacent figures consider moving part to section 8.5.8.1]
Figure 8.5.2-1 shows a typical dendrogram for the hamster from Smith et al83,84. Their work was
clearly exploratory. Their dendrograms do not reflect sensory receptor responses (stage 1), but
later responses (probably at the output of stage 2) after undefined neural signal processing. The
figure represents signals from an unspecified location within (on the surface of) the parabrachial
nuclei (PbN) which they differentiated from the nucleus tract of solitarius (NTS). They record
extracellular signals and then infer a signal came from a specific cell by the pulse heights of the
action potential streams. Additionally, they make a variety of Bayesian assumptions. They
assume there are only four gustatory sensory channels. They make the conventional assumption
that HCl is a gustatory stimulant without examining the question of whether it is a nociceptor
stimulant. They also restrict their stimulation to the anterior portion of the tongue of the hamster,
thus significantly skewing their results away from the contribution by the P-channel neurons.
They included several neurons exhibiting negligible response to any of the stimulants in their MDS
analyses. They restricted their MDS analyses to 2D (which would be appropriate in the absence
of P-channel stimulants and receptors. They only employ 30 or 31 neurons depending on the
portion of the brainstem examined and only 18 stimulants (only one of which was associated
with the P-Path channel of the gustatory modality. Their 18 stimulants were dominated by a
large variety of inorganic molecules, numbering 6. With these numbers, little statistical value can
be associated with the results. In fact, they noted a significant difference in their results
between their best-stimulus classification and their cluster analyses (footnote to Table I, pg 536
and the asterisks in the figure reproduced here).
The figure is more complete than most and includes the designations on the right applicable to
individual neurons in the set. However, it is less complete than some in only showing the
clustering for three situations, S or sweet-best, N or NaCl-best and H or HCl-best. They reported
only one neuron maximally sensitive to the Q-best channel (and then only minimally). Note the
asterisks denoting the perceived best response of the individual neurons listed on the right does
not always correspond to the clustering on the left. This is a problem in experimental protocol
development and may also be confounded by the lack of a sufficiently broad range of stimuli
to uncover the true best response of specific neurons. Unless a very large set of neurons is used,
the statistical independence of the individual clusters is not assured as the right side of the figure
83
Smith, D. van Buskirk, R. Travers, J. & Bieber, S. (1983) Gustatory neuron types in hamster brain stem J
Neurophysiol vol 50(2), pp 522-540
84
Smith, D. van Buskirk, R. Travers, J. & Bieber, S. (1983) Coding of taste stimuli by hamster brain stem
neurons J Neurophysiol vol 50(2), pp 541-558
68 Neurons & the Nervous System
is approached. In discussing this and similar figures (page xxx, Smith & Davis employ three best
conditions in this figure, four best conditions in immediately adjacent figures (adding quinone
or Q-best) and at least provide marginal support for a fifth best condition (adding U or umamibest). They also note, citing several references, “Individual gustatory neurons, both peripheral
and central, typically respond to stimuli representing several different taste qualities.” The
abscissa is scaled using values from a fundamentally abstract analytical routine, which makes
it difficult to compare, and account for the significant differences in the work of different
investigators. The cluster diagram does not provide any left terminus. The midpoint of the leftmost vertical line can be assumed to represent the sum of the entire ensemble.
A similar, but more recent, dendrogram for
a primate from Scott & Plata-Salaman omits
the labeling of the best performance of the
individual neurons but provides a different
orthogonal scale from Smith. The scale
remains abstract with both positive and
negative values. It includes, S-best, N-best,
Q-best and H-best categories. They have
also added heavy lines attempting to show
the level at which the different clusters
achieve statistical independence. Their
analysis shows that ~73% of the primate
neural responses are to sensations
described as sweet or salty. Those tuned to
quinine constituted ~22% and only ~5%
were oriented toward the detection of
acids.
Brining et al85. examined the sensitivity of
hamsters and describe the distance
between their clusters in more detail than
Smith et al. did.
8.5.2.2 ROC analysis
Lemon & Smith have recently consolidated
their database and applied the “receiver
operating characteristic (ROC) analysis
technique of Green & Swets (1966) to Figure 8.5.2-1 Cluster dendrogram of 31 hamster
action potential recordings from 162 (stage PbN neurons based on similarities in their neural
3) neurons at undefined locations on/within response profiles across 18 gustatory stimuli. The
the NST of rats86. Their results included neuron numbers and their best-stimulus
measured and standard deviation values. designations (S, H, or N) are shown on the right.
Their reported spike rates were from a few The asterisks indicate stimulants that are out of
to less than 75 pps. They used an AC place based on the best-stimulus criteria used by
coupled oscilloscope that degraded the the authors. The distance between neurons or
form of their action potentials significantly. cluster of neurons joined at each step is shown
They applied their stimuli to the apical along the abscissa. S, H, & N indicate the three
tongue at concentrations chosen to major clusters. From Smith et al., 1983.
provide half-maximum response in the
chorda tympani nerve. In applying the
ROC procedure, Lemon & Smith did not report any neurons where the action pulses exhibited
85
Brining, S. Belecky, T. & Smith, D. (1991) Taste Reactivity in the Hamster Physiol Behav vol 49, pp.
1265-1272
86
Lemon, C. & Smith, D. (2006) Influence of response variability on the coding performance of central
gustatory neurons J Neurophysiol vol. 26(28), pp 7433-7443
Signal Generation & Processing 8- 69
a high spontaneous discharge rate or where the action pulse rate decreased with stimulation
of the sensory neurons. These facts suggest the neurons probed were either at the entrance to
the NST or were at the exit and did not display any differential encoding. Their discussion
suggests they did encounter variation in sensory neuron sensitivity or application protocol
inconsistency that led them to assert, “Spike rate was found to be an unreliable predictor of
stimulus quality for each neuron tested.” They also concluded, “Results revealed poor
classification performance in some cases attributable to wide variations in the sensitivities of
neurons that compose a cell type.” These findings are not unexpected based on their protocols,
the very great adaptation capability of typical sensory neurons and the poor repeatability of
stimulation of a specific neuron within a taste bud. In their discussion, they considered a number
of stage 4 computational strategies without presenting any schematic of the appropriate
sections of the neural system (or even a semantic model).
MacMillan & Creelman87 have recently published “Detection theory: A User’s Guide.” for those
interested in exploring the ROC technique in detail.
8.5.2.3 Employing MDS techniques in understanding gustation
An important mathematical technique originally developed to address non-quantitative
phenomena in the social sciences has been applied to the sensory system of biology in recent
years. It is a very powerful program that generally must be employed in a reiterative manner in
order to process the data optimally. During the initial employment of the program, it provides
estimates of the adequacy of its performance for a given set of orthogonal axes in interpreting
the data set.
Kruskal & Wish were early pioneers in the development of MDS88. Their 1978 book is an excellent
primer on the subject. This section will excerpt and quote liberally from that book. It includes
citations to about a dozen MDS programs in use at that time, generally requiring large scale
computers of that era. There are at least a dozen current programs available to perform MDS
analyses, many in desktop computer environments. Detailing the differences between these is
well beyond the scope of this introduction. A Google search is suggested if a specific program
has not already been adopted by your institution. Two recent texts on MDS are by Borg89 and
by Cox & Cox90
8.5.2.3.1 Background relative to the MDS technique–map ,making
Typically, a Highway atlas provides a map of a region and an associated tabulation of the
distances between the major cities. Assume no information is given on the size of the cities or
their elevation above ground. But consider the problem of establishing the correct relative
locations of the cities if one is only given the distances between them in a matrix such as in
Figure 8.5.2-2. In this case, the matrix has a null diagonal. From this matrix, it is readily assumed
that the cities are all located on a two-dimensional surface. It may also be assumed they are
all the same size. However, these assumptions may not be correct. The MDS program is
designed to enable a researcher to uncover the “hidden structure” from a set of proximities such
as the data in the matrix. This hidden structure, or geometric configuration of points, is the 2D
map we are all familiar with. The points in frame (b) describe the geometric configuration of
points for the cities in the matrix. The relative position of the cities looks familiar but not quite
correct. Notice the program establishes two orthogonal axes (dimensions) with only relative
scales. The axes are determined by the distribution of the data points. If that distribution
changes (by adding or subtracting data points from the data set), a different set of axes are
liable to be chosen.
87
MacMillan, N. & Creelman, C. (1991) Detection Theory: A User’s Guide. NY: Cambridge Univ Press
88
Kruskal, J. & Wish, M. (1978) Multidimensional Scaling. Sage University Paper series on Quantitative
Applications in the Social Sciences, 07-011Beverly Hills, CA & London: Sage Publications
89
Borg, I. & Groenen, P. (2005) Modern Multidimensional Scaling: Theory and Applications. NY: Springer
90
Cox, T. & Cox, M. (2001) Multidimensional Scaling, Second Edition. Boca Raton, Fl: Chapman & Hall/CRC
70 Neurons & the Nervous System
Figure 8.5.2-2 Creation of a 2D MDS representation based on a matrix tabulation. Top; airline
distances between ten United States cities. Note the null diagonal. Bottom; the initial geometric
configuration of the cities based on applying MDS to the airline distances and the specification
that the points all fall within a 2D plane. Rhumb line at ~+38.5 degrees N. latitude. Modified from
Kruskal & Wish, 1978.
If other information is available, such as the latitude and longitude of any two cities, a rhumb line
can be drawn between them and (assuming a Mercator type projection) the rhumb line can
be considered the hypotenuse of a triangle with the differences in longitude and latitude
forming the absolute dimensions related to the data set. In this situation (San Francisco &
Washington are on the same latitude, ± 0.5 degrees), the data set can be rotated clockwise by
44 degrees to establish the new intrinsic dimensions 1 and 2 as the actual axes of the data set
as in Figure 8.5.2-3. Cox & Cox showed a similar situation for a group of English cities (page 2)
but after rotation, the cities remained a mirror image of their correct locations. They introduced
a cosine matrix that can be used to rotate the 2D MDS output file before putting the data into
the graphics display program. Rotation is about the vertical axis perpendicular to the 2D data
Signal Generation & Processing 8- 71
set. The 2D matrix can be easily expanded to cause a left to right reversal of the data set and
resulting imagery. The matrix can also be easily modified to introduce absolute scales by adding
a multiplier with or without a displacement factor to each dimension. The matrix can also be
expanded to process multi-D data sets based on the same procedures. It is easiest if the
rotations about the two principle planes of a 3D data set are treated separately. The result is a
properly oriented graphic based on absolute positioning, such as the d-value fundamental
dimension or in the example longitude and latitude .
Figure 8.5.2-3 Creation of a 2D MDS with absolute scales based on knowledge from an
additional source. Same ten cities as in previous figure. Note the intrinsic longitude scale is
reversed relative to the relational scale provided by the MDS program. The two latitude scales
are not correlated with each other or the longitude scales. See text. Modified from Kruskal &
Wish, 1978.
Once the rotation is completed to align the data with a recognized set of coordinates, the
scales of the two dimensions can be adjusted (independently) based on the known latitude and
longitude of at least two cities.
Note the significantly different representations between these two figures if an attempt is made
to collapse the data points onto either axis. The resulting 1D representation is essentially
meaningless in the first case. After the rotation into the intrinsic dimension space, the 1D
representation describes the latitude or longitude of the cities properly in relative (and absolute)
dimension.
The geographic representations of Kruskal & Wish can be extended into 3D space to indicate
the height above sea level of the cities, or the height of the tallest structures in each city. The
folding of such data into one dimensional representations must be done carefully to maintain
72 Neurons & the Nervous System
legitimacy. If both the height above sea level and the height of the highest structure are
introduced into the database, the data can no longer be represented using a 3D presentation
space. One dimension must be selected for elimination from the representation (and note of
that data deletion should be included in the appropriate text or caption).
It is therefore important to remember;
C the order of the representation space must be appropriate for the number of dimensions
of the database to be represented.
• the initial dimensions generated by the MDS program are always arbitrary and have no
intrinsic meaning.
• the solutions to ordinary MDS are always subject to rotation and mirroring (which is most
easily achieved in 2D representations.
• when rotation is performed based on other information, the intrinsic dimensions may
reveal significant relationships.
---The extraction of true axes from initial MDS analyses has become a high art in recent times.
Several software packages are available under the labels promax rotation, varimax
rotation, etc. In the current situation only extraction techniques maintaining orthogonality
is useful. An introductory discussion by J. D. Brown of the University of Hawai’i at Manoa
appears particularly helpful91. A lecture by A. Ainsworth of California State University
(Northridge) is very useful92. See also Section 8.6.6.2.2.
In the general case, a linear regression analysis is often required after running an MDS program
in order to surface the underlying (intrinsic) axes. In many cases, such regression analyses may
generate non-orthogonal axes. The analyses may also suggest more dimensions than originally
assigned to the data. Additional techniques drawn from factor analysis can highlight both local
groupings as well as large mean distances between groupings.
Note the increase in longitude with direction opposite to the positive going direction of the
relative dimension suggested by the MDS program. The directionality of the dimensions
indicated by the relative scales suggested by the MDS program when in tabular form are
arbitrary. They do not suggest a connectivity between these orthogonal dimensions when
assembled into a multidimensional representation space.
If additional data is available in matrix form describing the altitude above sea level is available
for the ten cities, the MDS program can combine the two matrices and generate a three
dimensional map (geometric distribution of points). If an additional matrix is available describing
the population of each city, MDS can include this data and develop a 4D geometric distribution
of points, ad infinitum. The challenge becomes how to represent this multidimensional data set
in the most useful form.
---Determining the dimensionality of the complete data set is an important task in MDS. It is the
primary reason the program is run iteratively. The criteria is the goodness-of-fit of the results to
the original data set. Sometimes the goodness-of-fit is described as the badness-of-fit on
philosophical grounds. In either case, the measure is a stress defined as the square root of a
normalized “residual sum of squares.”
The calculation of stress is a critically important element of an MDS program. Because of the
complexities involved, different MDS programs use different optimization programs, and
frequently give different results. A reported “stress” is MDS program specific. A suspiciously high
stress may come about because of a failure of the routine used in a specific MDS program to
91
Brown, J. (2009) Choosing the Right Type of Rotation in PCA and EFA In Shiken: JALT Test Eval SIG
Newsletter vol 13(3), pp 20-25 http://jalt.org/test/PDF/Brown31.pdf
Ainsworth, A. (xxx) Factor Analysis www.csun.edu/.../Psy524%20lecture 21FA cont.ppt
92
Signal Generation & Processing 8- 73
converge. Most programs limit the number of iterations in the stress calculation to avoid
meaningless results and print out an appropriate termination statement. Whenever a stress
below 0.01 is found, the possibility of a fully or partially degenerate solution should be
investigated.
The calculation of stress as a function of dimensionality is based on probability calculations so
the results are not static between iterations and can vary dramatically in some cases. To
alleviate this problem several different “levels” of stress analysis have been defined and
implemented within MDS programs.
Because of the variation in macro’s used to calculate the stress, it is common to define the stress
level using a program particular label such as the “Kruskal stress value.”
It has become common to present the stress level for a set of data by providing the calculated
stress for a 1D, 2D, 3D etc. MDS analysis, such as xxx, or as a single value for the MDS analysis
chosen for presentation and discussion such as 0.048 for a Kruskal stress value for figure 6 of
Hellekant et al., 1997.
In gustation investigations, the stress levels are relatively high, even after assuming a 3D data
space. This if because the data set is inherently 4D or higher, with the fourth dimension being the
intensity of the signal produced by the stage 1 sensory neurons (hopefully in the absence of
adaptation). Uneven exposure to the various stimulants can introduce a fifth dimension into the
data set. To eliminate the fourth dimension from the data set, some investigators have adjusted
the molarity of their test stimulants to generate a constant analog signal amplitude at the output
of the sensory neurons or a nominal action potential pulse count on stage 3 neurons orthodromic
to the sensory neurons. Adjusting the molarity in this way can ameliorate any difference in the
solubility, ionizability or hydration efficiency associated with a stimulant. Adjusting the molarity
in this way also accounts for multiple gustaphores of the same type within one stimulant. For
stimulants incorporating multiple gustaphores of different type, a more complex procedure is
required.
Hellekant et al. (1997) used a set of stimuli varying in molarity over a factor of 10,000:1 to
achieve a nominal action potential pulse rate. Oddly, all of their stimulants except quinine
were dissolved in an artificial saliva containing significant amount of the sodium ion. The
presence of this ion would effectively bias nearly all of their results.
Failing to eliminate the signal intensity variation among the stimulants or gustaphores, and any
variation in the stimulant application process, may distort the MDS representation significantly.
It may result in the third dimension incorporating the components of both the third and all higher
dimensions.
The investigator will greatly simplify his work if he eliminates any higher order dimensions
in his data set if possible before running any MDS program.
It is not clear how the information present in the higher level dimensions is distributed when the
number of dimensions is reduced below the optimum or necessary number. The information is
probably distributed in a structured manner. This assumption implies it is not treated randomly
and that it can not be compensated for by employing more samples and relying upon the
Central Limit Theorem.
The example above relied upon quantitative proximities in miles. This is not the general case in
most applications of MDS. The proximities are usually arrived at qualitatively, by asking subjects
to estimate the similarity/differences between different objects or perceived taste. Such
estimates are necessarily noisy, the subject will give different answers if the same samples are
presented a second time, in either the same or different order. Thus, it is important to collect the
estimated relationships between data points in a data set multiple times in order to obtain
optimal results. If the estimates associated with different times are labeled, the MDS will show
the degree of clustering of similar responses. This clustering can also be illustrated using a scatter
diagram (sometimes labeled a Shepard Diagram) as in Figure 8.5.2-4. The scatter diagram
typically shows the percentage of “same” judgements versus the inter-point distances. As with
any noisy process, the optimum relationship between the data points will begin to appear as
more data sets are added in accordance with the Central Limit Theorem applied to statistical
processes. As the number of data points are accumulated in the scatter diagram, it becomes
possible to discern the underlying relationship and consider writing an equation passing through
74 Neurons & the Nervous System
the centroid of each set of points.
Figure 8.5.2-4 Scatter diagrams and their application to gustation. Left; typical scatter diagram
with an estimate of the final noise free result (thin line). Right; a noise free data set and human
gustation showing variations due to the collection probability width of the GR’s and the ultimate
degenerate condition with constant collection probabilities over a specific width (horizontal line
segments). To maintain similarity, right frame indicates 20% of the potential gustaphores were
not captured by the defined GR’s. Left frame from Kruskal & Wish, 1978.
A step-like scatter diagram is indicative of the level of degeneracy in the data set, i.e., its lack
of continuity in the underlying functional relationship between the points in the data set.
Recognizing the fundamental dimension of gustation before folding, it is possible to
present a single scatter diagram (right frame) representing gustation in a species or higher
designation of animal(s). For a comprehensive data set with enough data points, discrete
step heights will be found at the nominal d-values of each GR present.
Thus a continuous scatter diagram supports the earlier Buck-like assumption of a large number
of gustaphores supporting a large number of stimulants on a one-to-one basis. A highly
discontinuous (degenerate) scatter diagram supports the combinatorial hypothesis presented
here. Buck has recently changed her position to a combinatorial approach.
Degeneracy points directly to the d–values of the underlying individual gustaphores in taste. The
Kruskal stress value approaches 0.0 with degeneracy.
---The terms metric and non-metric are used in a unique context in MDS. The term metric is used
to describe a data set that can be described by a specific equation where the dependent
variable is scalable with respect to the independent variable. The term non-metric is used to
describe the situation where the dependent variable rises or falls relative to the independent
variable but not in a scalable manner. Both the scatter diagram and a multidimensional
representation of the complete data set may show a clumping of data points into one or more
groups. This is defined as degeneracy in MDS. It is most apparent as a step-like pattern in the
scatter diagram. This degeneracy is critically important to finding new relationships about the
underlying phenomena (more specifically about finding new combinations of gustaphores in the
taste modality).
The quality of an MDS analysis is highly dependent on the number of data points in the original
set and their distribution over the underlying multidimensional space. A rough rule has evolved
that there should be at least twice as many stimulus pairs as parameters to be estimated, to
Signal Generation & Processing 8- 75
assure an adequate degree of statistical stability. This can be expressed as the number of stimuli
minus one should exceed four times the number of dimensions involved. In gustation, this
appears to give a bare minimum requirement unless the stimulants are distributed approximately
equally between the four sensory paths, A–, G–, N – & P–.
8.5.2.3.2 Dimensionality–selecting the number of dimensions
Kruskal & Wish discussed the appropriateness of selecting a given number of dimensions to
represent a data set (pages 48-58 and Appendix C). They used both real data and random
number theory to describe their findings. In general, what has come to be known as the Kruskal
stress index is developed. A series of values for this index are frequently given for an ascending
number of dimensions in the solution such as 0.246, 0.069, 0.035, and 0.018 for R equal 1, 2, 3 and
4 dimensions. As they note, a stress level of 0.02 represents a very good fit between the MDS
representation and the underlying data set. Stress levels above 0.100 for a given dimensionality
generally indicate the need to rerun the program using more dimensions or to collect more data
or refine the data set. A stress level below 0.005 is indicative of a degenerate data set. Figure
8.5.2-5 shows what has come to be known as the Kruskal stress index as used by Cox & Cox
(page 70) in one set of experiments.
Figure 8.5.2-5 The Kruskal stress index as a percentage. To operate at a number of dimensions
below the first inflection point, and typically below 5%, which ever occurs first is a nominal goal
in statistics. Five dimensions are the suggested complexity to be used for the underlying dataset.
Many investigators will develop MDS representations for both 4 & 5 dimensions to determine
which provides the best representation. From Cox & Cox, 1994.
Dropping from a two dimensional representation to a one dimensional representation is
equivalent to projecting the 2D data set onto one of the axes (after appropriate rotation if
necessary). Dropping the third coordinate from a three dimension representation to a twodimension representation is the same as projecting that dimension perpendicularly onto the
plane formed by the remaining two dimension. The effect of reducing a five-dimension
representation to a four dimension representation is straight forward from a mathematical
perspective but difficult to comprehend otherwise.
76 Neurons & the Nervous System
8.5.2.3.3 Multidimensional scaling applied to gustation
An adequate multidimensional scaling analysis of the taste system can provide strong evidence
for resolving the question of the number of independent sensory channels. A problem involves
the extreme sensitivity of the scaling procedure to the scope of the stimuli set. In 1963,
Gesteland et al. distinguished eight types of neuroreceptors based on one set of criteria93. Holley
et al in 1974 seven types of very selective cells and nine “categories” of less selective ones94. The
multidimensional scaling technique has matured in recent years. Revial et al. used the
technique to describe their data set obtained from the frog95. This set included the camphors
and the benzenes and at least one thiophene (a class of sulfur compounds). “Data analysis
revealed five factors that described 71% of the total variance, with one factor markedly
prevailing over the other four.” “In other words, the olfactory space is multi-dimensional but, in
the case of this particular set of compounds, one dimension is dominating.”
[xxx this paragraph needs integration to previously numbered section ]
More recently still, Smith et al. have provided a broad investigation into the gustatory modality
in the hamster96. The more limited scope of the taste sensation was characterized using only
three dimensions, except for one outlier representing quinine hydrochloride. By stretching the
QHCl criteria, they placed it in the acid (H) response class. Using this criteria, multidimensional
scaling suggested the taste sensations formed into three groups when observed in elements of
the brainstem, sugar sensitive S-neurons, salt sensitive N-neurons and acid sensitive H-neurons
that could be displayed in a two-dimensional space.
These results support the conclusion that the answers investigators obtain in chemoreceptor
research generally depend on what sample set they used.
Figure 8.5.2-6 shows the result of such a multidimensional scaling analysis of glossopharyngeal
nerve (NG) fiber of the rhesus monkey data by Hellekant, et al97. This analysis asserts there are
three “dimensions” to the stimulus sensing mechanism in the mammalian gustatory system. Each
of these dimensions constitutes a value continuum that is orthogonal to the other two
dimensions. The challenge is to define these continua using sufficiently precise terminology. The
figure also highlights the fact that these dimensions are continua in character and not binary.
Similar recordings from many others98,99,100 have used a three-dimensional space to describe the
perceived chemoreceptor space, at least for taste. However, both groups omitted several
93
Gesteland, R. Lettvin, J. Pitts, W. & Rojas, A. (1963) Odour specificities of the frog’s olfactory receptors in
Zotterman, ed. Olfaction and Taste I. London: Pergamon
94
Holley, A. Duchamp, A. Revial, M. Juge, A. & Macleod, P. (1974) Qualitative and quantitative discrimination
in the frog olfactory receptors An N. Y. Acad Sci vol 237, pp 102-114
95
Revial, M. Duchamp, A. Holley, A. & Cacleod, P. (1978) Odour discrimination by from olfactory receptors;
a second study Chem Sens Flav vol 3, pp 7-22 & 23-33
96
Smith, D. & Davis, B. (2000) Neural representation of taste In Finger, T. Silver, W. & Restrepo, D. eds.
(2000) The Neurobiology of taste and smell, 2nd Ed.. NY: Wiley-Liss Chapter 14
97
Hellekant, G. Danilova, V. & Ninomiya, Y. (1997) Primate sense of taste: behavioral and single chorda
tympani and glossopharyngeal nerve fiber recordings in the Rhesus Monkey, Macaca mulatta. J Neurophysiol
vol 77, pp 978-993
98
Squire, L. et al. (2003) Op. Cit. pg 644
99
Smith, D. Van Buskirk, R. Travers, J. & Bieber, S. (1983a) Gustatory neuron types in hamster brain stem J
Neurophysiol vol 50(2), pp 522-540
100
Smith, D. Van Buskirk, R. Travers, J. & Bieber, S. (1983b) Coding of taste stimuli by hamster brain stem
neurons J Neurophysiol vol 50(2), pp 541-558
Signal Generation & Processing 8- 77
significant groups of stimulants. Using these additional groups may require additional dimensions
to provide an appropriate framework for describing this wider set of stimulants.
Hellekant has written extensively on monkeys beginning in 1975.
Figure 8.5.2-6 Multidimensional scaling of gustatory stimulants in monkey ADD. From figure 11
of Hellekant et al., 1997.
An important feature of the multidimensional scaling procedure is to achieve optimal alignment
of the coordinate axes of the presentation. The procedure provides great flexibility in this area.
For a review of the technique and references, see Shepard, et. al101. A figure of merit is used in
101
Shepard, R. Romney, A. & Nerlove, S. (1972) Multidimensional scaling; theory and applications in the
behavioral sciences. NY: Seminar Press
78 Neurons & the Nervous System
these analyses to estimate the broadness of sensitivity of a specific sensory neuron channel. The
figure used is an entropy given by the equation, H = .K(n)AΣpiAlogpi where the coefficient K(n) depends
on the number, n, of stimulants in the set. Smith & Scott have given a brief discussion of this figure of merit102. The
value of K(n) is adjusted to insure a maximum value for this function of 1.0.
A problem not addressed in the presentations based on multidimensional scaling relates to the
underlying functions generating the three axes. The three-dimensional character of the
multidimensional scaling is strikingly similar to that of the visual system103. The optimized
dimensions in the visual case relate to the luminance channel and the two major chrominance
difference channels of human vision104. These neurophysical channels describe the perceived
scene as processed by stages 1 & 2 within the retina.
In the case of vision, the two chrominance difference channels result in two of the three
dimensions exhibiting zero’s or nulls along their axes. As a result, it is clear the chrominance
outputs of the stage 2 signal processing are electrically bipolar signals. Because of the
commonality of the spectral channels participation in the differencing, it is even possible to
specify the relative polarity of the regions of these signals.
If the signal processing in stage 2 of the chemoreceptor channels mimic that found in most other
sensory channels, the knobs atop the vertical stems would point both up and down in [Figure
8.5-1], the presentation would exhibit bipolar data sets. However the CNS does not provide
bipolar perceptions. It assigns names, e. g., salty, sugary etc., to perceptions relating to the
individual relative peaks in its perceptual space. Whether they were perceive based on bipolar
data presented in the saliency map of the parietal lobe or not is irrelevant.
Hellekant et al. made only a brief attempt to define the value continua in this figure. They
described dimension 1 as extending from a group of sweeteners in the background of the left
side and a group of bitter compound in the foreground. They described dimension 2 as isolating
a group consisting of citric and aspartic acid, NaCl, sodium cyclamate and MSG but described
no counter to that group. No discussion of the third dimension was offered.
Smith & Scott have provided a similar three-dimensional figure (from Giza & Scott, 1991105)
showing the effect of amiloride on taste perceptions or rat for a variety of stimulants106.
8.5.2.3.4 Strange representations due to dimension reduction
Since gustation in mammals involves four distinct signaling channels at stage 1, the qualitative
performance of the modality can be represented effectively in a 3-dimensional space. Such
qualitative evaluations have generally sought to employ stimulants of equal effectiveness. As
a result, the intensity of the perceived taste have not been considered. If variable
concentration stimulants are employed (described as quantitative data in this discussion), an
additional dimension is introduced that must be accommodated in selecting the number of
dimensions employed in the MDS analysis and in the choice of graphic representations. Similarly
when discussing the dimensionality of olfaction, it must be recognized that olfaction employs a
9-dimension qualitative space and a 10-dimension quantitative space (Section 8.6.2.9.7 &
102
Smith, D. & Scott, T. (2003) Gustatory neural coding In Doty, R. ed. (2003) Handbook of Olfaction and
Gustation, 2nd revised and expanded edition. NY: Marcel Dekker Chapter 35
103
Romney, K. & Indow. T. (2002) A model for the simultaneous analysis of reflectance spectra and basis
factors of Munsell color samples under D65. . . Proc Natl Acad Sci USA vol 99, no 17, pp 11543-11546
104
Fulton, J. (2005) Processes in Biological Vision. www.sightresearch.net Chapter 17 Part 1b
105
Giza, B. & Scott, T. (1991) The effect of amiloride on taste-evoked activity in the nucleus tractus
solitarius of the rat Brain Res vol 550, pp 247-256
106
Smith, D. & Scott, T. (2003) Gustatory neural coding In Doty, R. ed. Handbook of Olfaction and Gustation.
NY: Marcel Dekker, Chap 35 page 752
Signal Generation & Processing 8- 79
8.6.7.4).
Representing gustation in less than three dimensions leads to unusual representations that are
difficult to interpret. Scott and colleagues have demonstrated this difficulty most clearly in
1989107. Their figures 5 and 8B can be assembled into one representation even though some of
the molecules in the 3-dimensional representation are not present in the 2 and 1-dimensional
representations.
8.5.2.3.5 2D MDS representation applied to gustation
A two-dimensional presentation can be obtained based on multidimensional analysis. This form
provides the clearest separation of the perceived responses into distinct groups. Figure 8.5.2-7
shows such a presentation from Brining. The precise definition of position on this presentation is
complex. The presentation shows there are two distinct perpendicular “dimensions” to the data.
However, there is no defined orientation of the axes in this mode of analysis. The axes shown are
arranged arbitrarily. The range of the data is scaled by the mathematical tools with the circle
shown with a unit radius. It is interesting to note the location of the chlorine radical appears to
be independent of these two dimensions. The sugars form a tight group isolated from the other
less tightly grouped stimuli. The hydrogen ion present in the quinine hydrochloride sample
appears to play no functional role in that stimulus.
Smith & Scott (Doty, page 733) have noted
the stimuli would be shown on the unit
circle if their characteristics were
determined exclusively by the two
“dimensions.”
Clearly, the N-Path
representations include an additional
dimension while the other selected stimuli
provide responses dominated by the two
dimensions shown. This is a serious limitation
of a two dimensional representation of
data containing additional dimensions. This
limitation clearly indicates the data needs
to be replotted.
8.5.2.3.6 Initial considerations
related to entropy in gustation
Several of the papers discussed below
include a discussion of a parameter,
“entropy,” based on the information
content of a message as first developed by
Shannon in the 1940's.
In its true Figure 8.5.2-7 Two-dimensional histogram of
mathematical form, an entropy of 1.0 is hamster taste preferences for 12 different stimuli.
indicative of a totally stochastic process of The data needs to be replotted in three
multiple dimensions. In the context used in dimensions. See text. From Brining et al., 1991.
these papers related to gustation, it is
based on the equal stimulation of a large number of poorly defined putative stimulants as
developed by Smith, Travers & van Buskirk108. Several authors have suggested this interpretation
of entropy is analogous to the concept of white light in a colloquial or informal sense. In fact,
white light is not a good analogy to this situation. The recognition of the combinatorial
character of the stimulant identification process and the presence of an adaptation mechanism
within gustation requires a much more substantive formula for entropy in gustation.
Hellekant et al. (1997) have addressed the entropy of individual gustatory pathways at locations
107
Scott, T. & Yaxley, S. (1989) Interaction of taste and ingestion In Cagan, R. ed. Neural Mechanisms in Taste
Boca Raton, FL: CRC Press Chap. 7
Smith, D. Travers, J. & van Buskirk, R. (1979) Brainstem correlates of gustatory similarity in the
hamster Brain Res vol 4 pp 359-372.
108
80 Neurons & the Nervous System
along the neural system, at the chorda tympani and at the glossopharyngeal nuclei. Such an
entropy, different on a pathway basis at these two locations) is arguably different from the
entropy associated with the total gustatory perception within stage 5 cognition. An overall
entropy might be defined for that related to cognitive perception as separated from .a local
entropy related to an individual pathway.
In their brief discussion, Hellekant et al. used the term citric acid-best fibers in their table
5, but HCl-best fibers in table 4 to allow comparison between their work and an earlier
study. They used citric-acid best because HCl played a negligible role in their study. As
noted here, the more appropriate designation is the carboxylic (Lewis) acid-best pathway.
Spector & Travis use tuning as a synonym for an entropy value (ranging from 0 to 1.0, page 151)
they calculate based on a standardized set of four stimulants (that apparently does not
recognize the concentration of gustaphores may differ from the concentration of stimulants
containing those gustaphores). They do note, “Very critically, response breadth is affected
profoundly by the compounds and concentrations chosen (page 151).
Smith & Travers also used an entropy value approach to calculate a breadth of tuning on the
assumption that there was only one gustatory sensory channel sensitive to all four of the historical
gustatory stimulants109. Alternately, it could be asserted they were evaluating the cumulative
gustatory perception to stimulation by the four stimulants.
8.5.2.4 Changes in multidimensional presentation EMPTY
[xxx ad introductory material. Cite one of Kruskal or other authors in support. ]
8.5.2.4.1 Rotation and displacement of MDS axes
Scott and colleagues presented a series of paper utilizing MDS techniques in 1984 through 1991.
They did not totally recognize the underlying framework and limitations of the technique that
was introduced by Kruskal beginning in 1978. There are substantive conditions on how the axes
resulting from MDS analysis can be interpreted and either rotated or displaced ( Section
8.5.2.6.1). These papers are reviewed and their problems discussed in Section 8.5.8 and its
subsections.
8.5.2.4.2 Conclusions from analysis of the data
The collected papers of Schiffman provide the most taste data in one place, even though it has
not all been collected and presented in a single multi-dimensional space. Some of the
discussion accompanying this data may not be relevant in 2010. Figure 2 of her 2000 paper
exhibits some strange artifacts of the graphic arts (example, the leading edge of the E/D
response for NaCl leans to the left as it rises). The introduction of that paper also includes several
descriptions that may be overly limiting. The individual data sets do not represent a portion of
the overall taste sensation space. They are the result of individual MDS analyses on incomplete
datasets.
Lemon & Smith have provided data recorded at the NST of 45 adult male Sprague Dawley rats
for a limited group of primarily bitter stimuli110. Their protocol did not employ quinine HCl as a
stimulus and they grouped acid/bitter in their dendrogram and 3D histogram. Lacking a quinine
HCl-like stimulus, their 3D representation is probably distorted based on applying MDS techniques
109
Smith, D. & Travers, J. (1979) A metric for the breadth of tuning of gustatory neurons Chem Senses Flav
vol 4(1), pp 215-229
110
Lemon, C. & Smith, D. (2005) Neural representation of bitter taste in the nucleus of the solitary tract J
Neurophysiol vol 94, pp 3719–3729,
Signal Generation & Processing 8- 81
to an incomplete dataset.
The conclusion of Giza & Scott appears useful but incomplete in the absence of a group of
organic acid stimulants and a larger subject sample size. “The results imply that specific
receptors are responsible for the recognition and transduction of sodium salts and that this
specificity in the peripheral taste nerves to be manifested in the NTS.
Rohse & Belitz introduced computer modeling, basically using comparison techniques, to the
search for the underlying structure of molecules perceived as “sweet.” They did not pursue the
search into other channels of the gustatory modality. See Section 8.5.8.2. The methods of multidimensional analysis are quite sophisticated and do not lend themselves to total
computerization using a cook-book program. Beginning with Coombs in 1950, great progress
has been made in ordering psychological test data using methods that uncover the underlying
“dimensionality” of the data. The work of Shepard in the 1960's led to a technique labeled
multidimensional analysis111. It could determine the number of relevant underlying dimensions
and the character of the monotonic function relating the data points. A feature of Shepards
1962 paper was his analysis of when the technique would fail (p. 240). A more sophisticated
technique labeled single value decomposition (SVD) was developing in parallel with Shepard’s
version of multidimensional analysis. The SVD method was described in detail, with examples,
in Weller & Romney112. A broader review of these and other techniques can be found in
Shepard, Romney & Nerlove113.
Shepard asserted in 1962 that these techniques were not practical before the emergence of the
high capacity digital computer because of the intense mathematical manipulations required
(he used an IBM 7090 which was new in his time period). He could not have known that the
fundamental requirements underlying these techniques would be frequently overlooked when
it only required pressing a single key to invoke these powerful statistical techniques on a desktop
computer.
The histograms plotted using olfactory and gustatory data have used several, but not all of the
basis functions resulting from computerized analyses of the above types. In the case of vision,
a complete SVD analysis provides four basis functions; a brightness or R–function versus spectral
wavelength, and three basis functions (O–, P– and Q–functions) describing three chromatic
difference functions versus spectral wavelength. It has been common in the past to ignore the
O–function (the ultraviolet component) in vision resulting in an imprecise Q–function. In
gustation, it can be expected that omission of the acids as a group in a test protocol will distort
the resultant basis functions used to create the above histograms.
111
Shepard, R. (1962) The analysis of proximities: multidimensional scaling with an unknown distance function
Psychometrika vol27(2), pp 125-139 & vol 27(3), pp 219-246
112
Weller, S. & Romney, A. (1990) Metric Scaling: Correspondence Analysis. Series #07-075 Newbury Park,
Ca: Sage Publications
113
Shepard, R. Romney, A. & Nerlove, S. (1972) Multidimensional scaling; theory and applications in the
behavioral sciences. NY: Seminar Press
82 Neurons & the Nervous System
Figure 8.5.2-8 shows the underlying theory of the complete three-dimensional Munsell Color
Space114. In practice, the lens of the eye truncates the effective spectral range at 400 nm and
the two-dimensional Munsell Color Space results. Note carefully that the white point is not along
the spectral locus forming the axes of the figure. Similarly, yellow at a nominal wavelength of
572 nm is not a node of the color space; although yellow is frequently perceived as a dominant
color at higher cortical levels by combining brightness and chrominance information. A similar
taste space is sought from the experimental data base using SVD.
Without a complete statistically relevant
SVD analysis of the taste space in hand, a
complete taste space can only be
suggested.
Figure 8.5.2-9 shows this
suggested taste space. This space shows a
primary locus of sensory neurons for organic
acids at the zero coordinate point. It shows
a second primary locus of sensory neurons
for sugars (the G-Path) along one axis of the
three-dimensional space, another primary
locus of sensory neurons for sodium ions (the
N-Path) along a second orthogonal axis
and a final locus of primary sensory neurons
for bitter substances (the P-Path) along the
third orthogonal axis. From a behavioral
perspective, the acid sugar axis might be
considered primary, indicating a likely food
at its most positive extent and an
undesirable acidic condition at its negative
extension. HCL and the alkaline and
alkaline earth salts are considered nocents
in this work and not properly presented in
the gustatory perception space of the
overall saliency map of the neural system,
even though the inorganic acid, HCl, has
been traditionally considered a gustant.
The HCl in QHCl is employed to achieve
greater solubility for the overall molecule.
The quinine moiety contains the P-Path
gustaphore.
114
Figure 8.5.2-8 The foundation for the chromaticity
diagram of tetrachromatic vision. W marks the
location of tetrachromatic “white” in the
diagram. W’ shows the location of “white” for a
long wavelength trichromat. W” shows the
location of “white” for a theoretical short
wavelength trichromat.
Fulton, J. (2005) Processes in Biological Vision. http://neuronresearch.net/vision/pdf/17Performance1a.pdf
Section 17.3.3.1
Signal Generation & Processing 8- 83
Figure 8.5.2-9 A taste sensation space based on an incomplete experimental database paths
and overlaid by a rotated right-hand rle coordinate space from Section 8.5.1.6.2. With a larger
sample set, the values shown near each “Path” node would converge on, and define more
precisely, each node. The dashed lines define the total 3-D taste sensation space and a right
tetrahedron satisfying Henning’s description. HCl and the alkaline and alkaline earth molecules
are not proper elements of this 3D perception space. They belong in a fourth dimension
associated with a different modality. See text. Data points from Smith et al., 1983.
Hydrochloric acid is not properly represented in this figure. In dilute solution, this molecule is
always dissociated and each ion is hydrated. The hydrogen ion is actually present as either
H3O+, H5O2+, - - - - . H3O+ is frequently given the label “hydronium ion.” See Section 8.5.4.3.5 for
a broader discussion. The chlorine ion is similarly hydrated. When fully hydrated, its arrangement
is similar to that of sodium ion (Section 8.5.4.4) and it is surrounded by six water molecules. The
hydration of the hydrogen ion leads to a potential O–H- - O configuration, a hydrogen bond with
a d-value of 2.708 Angstrom (Table in Section 8.6.1.8). A DACB with this d-value can form with
the GR 2 receptor and be perceived as sweet. In the more general case the chlorine ion is a
strong astringent and will act as such and affect the nociceptors of the nocent modality located
84 Neurons & the Nervous System
in the mouth (Section 8.7.3.1). The other monovalent and divalent alkali salts are also considered
nociceptors rather than gustants. For hydrogen chloride and the alkali salts to be properly
shown, a 4-dimensional (4D) MDS analysis would be appropriate.
Multidimensional analysis of a complete data set shows that Henning’s tetrahedron (1916) is
inappropriate. Henning described the four tastes as located at corners of a equilateral
tetrahedron, much as Young had done earlier for the perception of color. While conceptually
simple, such a representation does not meet the requirements of orthogonality to avoid cross
contamination. The use of selected corners of a cube does provide such independence in both
areas of perception, taste and color vision. A right tetrahedron (all face angles at one vertex
are 90 degrees) can represent the taste space more appropriately.
Note a right tetrahedron drawn within the orthogonal space shown (with its 90 degree vertices
at A-Path) occupies only one third of the available taste space proposed here. While sodium
saccharide and sucrose fall outside of the tetrahedron shown, they would fall within the
orthogonal volume. Other basic tastes could occupy other vertices of a three dimensional cube
(See analysis of Schiffman’s paper below).
The proposed coordinate system has been overlaid with the data points from Smith et al. (1983b,
figure 8). The P-Path/N-Path plane has been rotated by 10 degrees from Smith et al. This is a
normal procedure and totally permissible in multidimensional analysis. It is assumed that with a
larger and more statistically defendable data set, the data points clustered around each axis
would converge on a single value for each class of stimulant thereby defining precisely the locus
of the “Best” point for each class of stimulants. The resultant best points would define the peak
sensitivity of the underlying sensory neuron types.
[xxx careful with MDS versus SVD ]
Smith et al.(1983a) used a very small stimulus data set in their multi-dimensional scaling
(MDS) process, including only one stimulant clearly associated with the bitter channel.
they used six inorganic compounds that are not relevant to this study. Their response data
set only included 31 neurons divided into three groups. It is important to note the data
in figure 1 of this paper. The data is from the parabrachial nucleus and not from the
actual sensory neurons as might be acquired from nerves VII, IX or X. The distribution of
sensitivity among their S–neurons and H–neurons suggest these neurons exhibit the result
of a summation process and are not typical distributions of sensory neurons. The power
of MDS developed clusters along the expected primary axes, figure 8 of Smith et al.
(1983b), but the groupings may be less tight than would be expected of sensory neurons
sensing only the organic compounds plus the sodium compounds. As a side note, it is of
interest that sodium saccharine did not appear to ionize in solution and thereby influence
the N-neurons.
The dimensionality of this figure is arbitrary. However, a convention is preferred. The axes shown
are those commonly found in three-dimensional presentations following the right-hand rule. The
scales of the individual basis functions have been linearly [xxx vectorially ] modified so that the
mean of A-Path occurs at zero and all other values are positive in all of these functions.
This approach surfaces a critical thesis concerning gustation. The precise location and the
degree of tuning associated with each descriptor, N-Path, P-Path etc. are properties of the
sensory neurons. In a statistically adequate group of stimuli. and target systems, these properties
are independent of the stimuli employed.
While the precise chemistry and the neurological circuitry associated with these scales is
unknown, it can be assumed the neurological circuitry at the level of the NST forms differences
between the signals from pairs of these nodes (probably limited to pairs along the individual
axes). These pairs can be labeled much as they are in the neurological circuitry of color vision
space. In the gustatory case, the axes can be associated with;
[xxx rewrite text to support the labels below and insure agreement with earlier dimensional
spaces. ]
Dimension
nodes
axes in Smith
preferred axes
Signal Generation & Processing 8- 85
of this work
Dimension 1 = A – S (x-axis)
Dimension 2 = S – N (y-axis)
Dimension 3 = N –P (z-axis)
A - G (x-axis)
G - N (y-axis)
N - P (x-axis)
In this figure, the mean of the A-path locus is taken as the common junction defined by the basis
functions of a new SVD analysis based on a larger, statistically adequate, data set. As noted
above, the salts of the alkali earth metals, Calcium, and Magnesium, generally appear along
the axis between the A-Path and N-path loci, although they belong to a separate node in a 4dimensional analysis.
This analysis suggests, or supports the hypothesis that, there are four primary types of sensory
neurons in gustation; G-path, A-path, N-path and P-path. All gustatory stimulants excite one or
more of these types. It may be useful to note that quinine hydrochloride contains the
complicated organic molecule, quinine, which is considered an alkaloid (alkali-like) in many
pharmaceutical applications. It does not appear to act as an alkaloid in gustation. Urea is a
much simpler carbamide H2NCONH2 and is weakly basic.
The data base is not large enough to address the question of a distinct umami sensitive sensory
neuron. To expand the theory to include a fifth primary sensory neuron type would require;
C development of a statistically relevant data base
C a database in which all of the potential sensory neuron types were stimulated and
C a SVD analysis to develop all of the relevant basis functions.
Every indication from this work and the literature is that the umami perception is a composite
resulting from the stimulation of the A-path, G-path and N-path simultaneously. See Section
8.5.4.9.
8.5.2.4.3 The basis functions of gustation
The basis functions, included as a step in the preparation of a histogram resulting from a
multidimensional analysis or more complex single value decomposition (SVD), can provide a
numerical scale for describing the tuning of individual sensory neurons. The SVD method is
described in detail, with examples, in Weller & Romney115. A broader review of these and other
techniques can be found in Shepard, Romney & Nerlove116. The work of Shepard in the 1960's
led to a technique labeled multidimensional analysis117. It could determine the number of
relevant underlying dimensions and the character of the monotonic function relating the data
points. A feature of Shepards 1962 paper was his analysis of when the technique would fail (p.
240).
While many three-dimensional histograms of gustation have appeared in the literature, the
authors have not generally presented the basis functions used to define their orthogonal axes.
In the case of vision, where it is easy to provide a constant amplitude stimulus across the visual
spectrum, the basis functions are found to describe the mean location and tuning of the visual
sensory neurons precisely (Section 17.3.5.2.4 of “Processes in Biological Vision”).
Scott & Giza have collected their data and organized it to suggest sensory profiles in their figure
4. However, the relationship to the basis functions is tenuous. This paper also includes a broad
discussion highlighting some of the problems in defining the neural coding of gustation.
115
Weller, S. & Romney, A. (1990) Metric Scaling: Correspondence Analysis. Series #07-075 Newbury Park,
Ca: Sage Publications
116
Shepard, R. Romney, A. & Nerlove, S. (1972) Multidimensional scaling; theory and applications in the
behavioral sciences. NY: Seminar Press
117
Shepard, R. (1962) The analysis of proximites: multidimensional scaling with an unknown distance function
Psychometrika vol27(2), pp 125-139 & vol 27(3), pp 219-246
86 Neurons & the Nervous System
MDS is a very powerful analytical tool in the hands of an expert118. It involves considerable
routine calculation that is amenable to and essentially requires computer support. Unfortunately,
the use of a canned computer program in the hands of the naive can lead to beautiful but
erroneous conclusions. The figures on pages 347 & 349 of Getchell et al119. (reproduced from
Scott & Mark, 1987) can be used as an example. The data is plotted in two dimensions, but as
the captions note, 5 to 6% of the data is unaccounted for using only two dimensions. The
phenomena being analyzed involves at least a three-dimensional space. By eliminating one of
the degrees of freedom in the computer routine, suitable presentations are obtained but they
force the data points into a two dimensional space. The rotation of MDS axes is an additional
experimental tool in MDS. The axes selected by the computer routine may not be the primal
axes of the data. The choice is determined by factors unrelated to the fundamental axes of the
data set. This is particularly true if the statistical sample size and experimental error associated
with individual odorophores is inadequate.
[xxx edit to agree with MDS paper ]
An argument can be made that the primal axes of the figure on page 347 (credited to Scott &
Mark120) should pass through NaCl with one axis extending to Fructose/Sucrose and the other
perpendicular to that axis as shown in Figure 8.5.2-10. Using the proposed axes, several
important points can be made.
C Sucrose is not a suitable sample for these experiments, since it is broken down into glucose and
fructose by hydrolyzation in the process of salivation.
C Both fructose and sucrose contain multiple odorophores based on their ring structure and their
CH2OH groups. Thus, these materials are shown farther along the axis than appropriate due to
their higher odorophore concentration than expected by the investigators.
C The group of chemicals clustered along the axes sloping to the upper right can be separated
into the organic acids in the plane of the paper and the picro-receptor stimulants such as
quinine, brucine and strychnine out of the plane of the paper..
C The constituents shown on the lower one-dimensional scale can be replotted relative to the
new axes and separated into in-plane and out-of-plane components.
C The role of the inorganic molecules in gustation (except those containing sodium) have not
been addressed in this work. An exception is the role of hydrogen sulfide in both gustation and
olfaction. See Section 8.6.3.5.2 for hydrogen sulfide’s role in olfaction. This gas, when hydrated,
also plays a role in gustation where it stimulates the sensory neurons of the picric path.
118
Weller, S. & Romney, A. (1990) Metric Scaling: correspondence analysis. Newbury Park, California: Sage
University Press
119
Getchell, T. Doty, R. Bartoshuk, L. & Snow, J. (1991) Smell and Taste in Health and Disease. NY: Raven
Press
120
Scott, T. & Mark, P. (1987). The taste system encodes stimulus toxicity. Brain Res vol 414(1), pp 197-203.
Signal Generation & Processing 8- 87
Schiffman has presented a recent paper
that provides data on a wider range of
stimulants than usual in an attempt to
demonstrate more than four basic tastes.
She also attempts to show the equilateral
tetrahedron of Henning is inadequate. It is
interesting that Schiffman attempts to draw
small equilateral tetrahedrons within the
boundaries of her orthogonal threedimensional taste space resulting from a
conventional MDS analysis. It is argued
here the orthogonal three-dimensional
taste space represents a more modern
theoretical right tetrahedron concept than
did Henning’s equilateral tetrahedron.
Schiffman presents a series of threedimensional spaces derived from MDS for
various limited ranges of stimulants. These
spaces are necessarily incomplete.
Schiffman & Dackis presented an earlier
paper that clearly recognized the
multidimensional character of MDS Figure 8.5.2-10 REWRITE A two-dimensional taste
It contains very useful space with alternate axes applied. The responses
analyses121.
information on a range of stimuli. They used are described as taste qualities as determined by
an early MDS analysis program by activity profiles across neurons. Dimension 1
Guttman122. It appeared in the same accounts for 91% of the data variance in rats.
journal as the previous paper by Shephard Dimension 2 accounts for 4% of the variance. Five
mentioned above. They chose to use a percent of the variance is unaccounted for by
three-dimensional space after considering these two dimensions. Modified from Scott &
up to five dimensions. In that paper Mark, 1987.
examining the role of amino acids, vitamins
and fatty acids in taste, they noted, “Since the four- and five-dimensional solutions do not
appreciably decrease the error and since they do not reveal new relations among the stimuli,
. . . a three-dimensional solution was considered appropriate for the data.” They gave the errors
as a function of dimension number in the paper; 1D-36%, 2D-21%, 3D-14%, 4D-10% and 5D-7%.
Stopping at 3D with a 14% error is considerably poorer than in vision where the 3D error was
below 5% in the Indow & Romney dataset.
It appears to be Schiffman”s challenge to show that her additional stimulants fall outside of the
three-dimensional space of the four basic tastes. If they do, it is important to incorporate all of
the taste sensations, including her metallic tastes in a new multi-dimensional analysis. That
analysis should show the existence of more than three significant orthogonal basis functions
(beside that for intensity alone). The results would be expected to define a four dimensional
histogram which may be difficult to draw on paper, but nevertheless would exist.
Schiffman’s interpretation of Henning’s and probably all other tetrahedrons as hollow with all
tastes represented on its surface is probably inappropriate. A more likely interpretation is that
the tetrahedron is a right tetrahedron where all tastes fall within its volume. Alternately, the three
dimensional taste space has several corners (nodes) not presently occupied by any “basic
taste.” By abandoning the tetrahedron concept altogether and allow the taste space to
expand to a cubic volume, additional possibilities appear. Until proven otherwise, it can be
assumed her metallic taste occupies one of the corners (Xmax, Ymax, 0 or Xmax, 0, Zmax or Xmax, Ymax,
Zmax) in the taste space proposed here and shown above. [ xxx check it is still above ]
121
Schiffman, S. & Dackis, C. (1975) Taste of nutrients: amino acids, vitamins, and fatty acids Percept
Psychophys vol 17(2), pp 140-146
122
Guttman, L. (1968) A general nonmetric technique for finding the smallest coordinate space for a
configuration of points Psychometricka, vol 33, pp 469-506
88 Neurons & the Nervous System
It is appropriate to consider the proposal of the earlier Henning in 1916 as archaic and the use
of MDS in the 1970's as primitive, not well understood and investigational.
8.5.2.4.4 The proposed human sensory space of gustation
Comparison between the multidimensional space of gustation and vision suggest a functional
analogy. This analogy can be exploited based on the Electrolytic Theory of the Neuron.
The analogy to be presented here is based on a number of hypotheses concerning gustation;
C MDS analysis of a sufficiently large set of stimuli and large set of species specific subjects.
C To exhibit effective gustaphores, the stimulant must be soluble in saliva.
C The sensory region of the microvilli lemma exhibits a negative potential due to both the
polarization of the sensory receptor molecules and the type 4 lemma of the dendroplasm.
C The negative field in the region of the type 4 lemma is attractive to positively polarized ligands
of stimuli molecules.
C Transduction involves a coordination chemistry process (a narrow type of stereochemical
process).
C Transduction involves the change in the base potential of the Activa formed within the type
4 lemma of the microvilli, and the resulting change in current flow into the dendroplasm of the
sensory neuron.
C Transduction probability is the product of a sensory receptor capture cross-section for the
gustaphore of interest times the correlation process reactivity coefficient for the sensory receptor
type of interest.
C The capture cross-section of the sensory receptor is the product of the number of sensory
molecules within the area of the type 4 lemma times the number of reaction sites per sensory
molecule.
C The reactivity coefficient of a stimulus toward a specific sensory receptor is the product of the
molarity of the stimulus in solution times the number of reactivity sites per molecule of the
stimulus.
Figure 8.5.2-11 shows the proposed taste sensation space for gustation involving four primary
taste sensations. Like in vision, signals from the individual sensory channels of gustation are
treated as independent within the CNS. Thus, the primary taste receptors can be shown as
orthogonal and located at the corners of a three-dimensional space. They are shown here with
a color scale drawn from The Electrolytic Theory of Color Vision for orientation and comparison).
The scale intervals are all equal in this figure, and equal to 0.10 Angstrom.
The figure shows the location of the four recognized sensory channels of the gustatory modality;
the A-path or acido-receptor at 2.276 Angstrom, the G-path or glyco-receptor at 2.82 Angstrom,
the N-path or natro-receptor at 3.243 Angstrom and the P-path or picro-receptor at 4.746
Angstrom.
The taste modality sensory neuron receptors have a narrow stereochemical capture range of
about ±0.10 Angstrom, narrow band receptors compared to the wide band receptors of vision.
Thus, an individual gustaphore must exhibit a d-value very similar to that of the receptor to
stimulate the system. Many stimulants consist of multiple gustaphores. As a result, an effective
d-value for the stimulant may lie anywhere along the d-value axis based on the relative
effectiveness of the individual gustaphores.
An assertion by Smith & Davis (page362) can be restated in the context of this work. A universal
characteristic of mammalian gustatory neurons is the simultaneous response of distinct sensory
channels to a single stimuli (consisting of multiple gustaphores).
Signal Generation & Processing 8- 89
The potential for a stimulus to be perceived
and described as exhibiting all four primary
tastes is finite. Under this proposal, a great
many active stimuli can be described as
exhibiting multiple primary tastes
simultaneously. By its name, it should be
obvious that mono-sodium glutamate in
solution is capable of stimulating both the
gluco-receptor and the natro-receptor
channels.
The nomenclature of the
chemical demonstrates why a separate
channel for a umami gustaphore is not
needed.
The adaptation mechanism associated
with each sensory neuron can result in the
perceived taste of any stimulus to vary as a
function of time, both short term and long
term. The importance of this is the complex
procedure used in formal wine-tasting.
8.5.2.4.5 The family
Response Functions
of
Figure 8.5.2-11 Proposed taste sensation space for
mammalian species exhibiting four primary taste
Neural a
sensations. The sensations are generated by the
acido-receptors, the gluco-receptors, the natroreceptors and the picro-receptors. Less specific
identifiers are shown in parentheses. Based on the
d-value parameter of the Electrolytic Theory of
Taste & Smell. See text.
The concept of a neural response function
(NRF) has appeared a number of times in
the literature. It is simple to define a neural
response function as the response of a
specific region of the oral cavity to a range
of selected stimuli (using only an arbitrary sequence of the stimuli rather than a calibrated
horizontal axis). However, a more detailed definition of such a function, or functions, has been
missing. Multidimensional analysis clearly shows the basic taste space is three dimensional and
each “best” sensory channel exhibits a sombrero hat shaped neural response function in this
space. These hats are the appropriate descriptions of the individual neural response functions.
Woolston & Erickson have developed their concept of an NRF from a totally philosophical and
psychophysical perspective123. Their data set includes a variety of stimuli, including the stimuli
normally associated with the four best channels. However, they chose to limit their analysis to
a two-dimensional taste space. Their conceptual NRF’s are derived from their two-dimensional
taste sensation space determined from the NST of 26 Sprague-Hawley rats. They used the
Guttman-Lingoes MDS program and show the percent error as a function of the number of
independent dimensions used using two different criteria. Frame A of their figure 8 is unlikely for
several reasons. First, it does not represent a section through the taste sensation space as
described above. Second, it appears the three absorption probabilities shown appear to have
been individually normalized to a common height. The actual situation for a given subject and
a real stimuli is not properly represented by their caricature.
Based on the taste sensation space hypothesized here, a number of more specific NRF’s can be
detailed. The goal is to have one or more two-dimensional graphical NRF’s with one axis
representing a calibrated variable and the second axis representing the response function. The
problem is the three-dimensional character of the taste sensation space. It is possible to define
a variety of two-dimensional NRF’s within this space.
123
Woolston, D. & Erickson, R. (1979) Concept of Neuron Types in Gustation in the Rat J Neurophysiol
vol 42(5), pp 1390-1406
90 Neurons & the Nervous System
MDS provides an arbitrary calibration for each axis of the taste sensation space. These
calibrations are stable as long as the stimuli set and subject set are broad enough to encompass
the entire taste sensation space. While the calibration is arbitrary, the scale is linear and the
distance between any pair of “best” loci is therefore linear. A percentage scale can be
calculated using this distance between pairs of “best”loci as the denominator. Figure 8.5.2-12
shows a nominal NRF between A-path and N-path along the X-axis. If all of the scales of the
taste sensation space have been adjusted to form spherical shells in the previous figure, the
plane of this presentation is rotationally symmetrical about the X-axis.
Figure 8.5.2-12 Proposed 2-D neural response function of the molecule X, NRFX.. The horizontal
scale is relative. It can be described in terms of the absolute d-value scale. The individual
responses are not normalized to the same level. The peak amplitude of each depends on the
precise definition of NRF used. The relative neural responses for stimulus B are equal at a relative
scale other than 50% because of the unequal heights of the individual responses. In practice,
the individual responses of the sensory receptors are unknown. The dashed lines represent
individual responses presented on a logarithmic vertical scale. Many stimulants incorporate
multiple gustaphores in order to excite separate sensory receptor channels. See text.
The individual “best” response functions can represent several different combinations of factors.
They can represent the relative density of sensory receptor molecules on the surface of the
dendrolemma for a given “best” locus. They can represent the product of the relative density
and the number of potential binding sites per molecule. They can represent the unbound sites
remaining after previous exposure of the sites to stimuli. Since it is also possible for the stimuli to
reflect different reactivity to the different “best” sites, it is possible to plot the graph to show the
unbound sites for a given “best” locus times the reactivity of a given stimuli to that locus.
As shown below, the typical gustatory or olfactory receptor)shows a very narrow response
function (dashed lines in the figure and there is little overlap between the individual response
functions. This situation is compatible with the “quantized” character of the gustaphores and
odorophores of chemical sensing. Because of the available molecular geometries, only a
limited set of d-values are available to stimulate the chemical sensing system. On the other
hand, many stimulants incorporate multiple gustaphores or odorophores targeting multiple
sensory receptor channels.
When fully annotated according to the above possibilities, the figure should prove more useful
than the conceptual figure 8(A) of Woolston & Erickson and result in more useful scatter plots
than the ones they conceptualize..
In the context of gustation, the molar concentration of a stimulus does not necessarily indicate
its total reactivity. That parameter is indicated by the molarity of the stimulus times the number
Signal Generation & Processing 8- 91
of reaction sites associated with each molecule.
[xxx edit ]
Since a majority of the stimuli do not appear along one of the axes of the taste sensation space,
NRFX, NRFY and NRFZ are of limited utility. An alternate approach is to calculate the likelihood of
an individual stimulus’ interaction with each of the “best” sensory channels based on the above
choice of parameters. The stimulus can then be described in terms of its relative participation
with each of the best channels as [NaGlutamate]x,y,z or NaGlutamate:x,y,z where x,y,z are
relative values indicating the salty, bitter and sweet sensations relative to the sour sensation
respectively. Using A-path as the 0,0,0 point of the taste sensation space suppresses the role of
A-path in the analysis. Since the space is linear, the zero point can be moved to anywhere in
the space without introducing distortion. By moving the zero point to correspond to the zero
point defined by the underlying MDS dimensions, the x,y,z values become more symmetrical
about zero and the four “best” loci are treated more equally. Experience will demonstrate the
best method of describing these stimulus values in a practical manner.
8.5.2.5 Conclusions from analysis of the MDS technique & examples EMPTY
The MDS technique is a powerful method of organizing qualitative data (as typically found in the
social sciences. Dendrographic techniques provide a similar means to organize primarily
qualitative data. Both techniques have been extended to introduce quantitative values for
relating the representations of inherently qualitative data.
All of the techniques related to MDS are statistically based and highly dependent on the number
of samples in the database. The goal is to use enough independent samples related to each
independent sensory channel to achieve a true mean of the total sample (The Central Limit
Theorem of the Mean). In this process, all of the samples related to a specific independent
channel will congregate ever closer to the actual mean of that channel, ie., the central value
of the specific independent channel. In the case of gustation, these central values will be the
peak sensitivity of each sensory receptor, the d-value of that channel. With sufficient sample
size, the perceived taste will be given precisely, again by the Central Limit Theorem, by the
Convolution integral of the individual probability distributions124. Failure to achieve an adequate
sample size leads to misplaced elements in a dendrogram and significant spread in the locations
of individual samples presented in an MDS representation.
In both dendrographic and MDS analyses, it is important to determine the number of
independent variables present in the data set. Failing this determination, the resulting
representations of the dataset can not be successfully defended. Kruskal & Wish have
developed a stress parameter applicable to either of these techniques. As shown in an earlier
figure, the degenerate condition is the ideal. In practice, the Kruskal stress value (or parameter
or index) at less than 0.048 (~5%) for the number of “dimensions” chosen is a satisfactory criteria.
Data sets not reaching this criteria cannot be relied upon as accurate for research purposes.
8.5.2.5.1 The representation of an MDS dataset in the preferred form
The orientation of the dimension axes developed by current software packages will always be
orthogonal but their orientation is entirely arbitrary. It is up to the investigator to determine the
absolute orientation and scales of the data as presented based on the availability of additional
data. This is a trivial problem if the data set is degenerate; the dataset will be presented with
clusters of data points corresponding to the independent channels of the gustatory modality.
Under less than degenerate conditions, some estimates must be used.
The data points related to one of the expected nodes can be used to rotate the entire data
set so the data points relating to that node lie along (or parallel to) the selected axes (but not
necessarily at the node). The data points can then be rotated as a group about the selected
axes until the other clusters align with their expected axes. If the software did not make the
appropriate selection of dimension scales, some mirroring of the data set may be required to
arrive at a congruence with the preferred right-hand rule geometry in the final representation.
124
Panter, P. (1965) Modulation, Noise and Spectral Analysis. NY:McGraw-Hill pp 124-141 or many statistics
texts
92 Neurons & the Nervous System
Once the data set is aligned with the desired axes, the absolute scales appropriate to the
individual “dimensions” can be matched to the relative scales provided by the software
package. The preferred coordinate system would use the d-values developed in the above
preferred representation (or more precise d-values for the individual channels if developed in
the future).
8.5.2.5.2Failure to accommodate hidden variables in MDS representations
Historically, investigators in gustation and olfaction have relied upon the use of standardized
concentrations of their stimulants in the development of their dendrographic and MDS
representations. These solutions have generally been based on molar measures based on; 1)
the chemical formula of the stimulants rather than the presence of multiple gustaphores or
odorophores (that are the effective stimulants), and 2) without due attention to the solubility of
the molecules in the saliva and/or mucosa of the modality.
Shepherd established that the concentrations he used did not result in equal intensity stimulation
of the olfactory modality (Section 8.6.3.10.1). Any college level textbook would show that the
solubility of any homologous chemical family varies significantly (and frequently becomes
negligible for members of the family that are solids at the test temperature).
The preparation of a series solutions of molecules for use in gustation or olfaction experiments
must accommodate the solubility of the molecules in the saliva and/or mucosa if meaningful
dendrographic or MDS representations are to be achieved (unless an added dimension relating
to the intensity of the neural signals is included in the overall data analysis.
8.5.3 The 2-step hypothesis of gustatory transduction
The literature of taste based on the chemical theory of the neuron contains a large number of
almost completely conceptual descriptions of gustatory transduction. One of the most recent
is presented by Gilbertson & Margolskee in Doty125. They describe all of their gustatory
transduction mechanisms as dependent on a rise in intracellular Ca++ followed by the release
of a chemical neurotransmitter at the pedicle of the sensory neuron. While many labeled
materials and presumed operations are shown in their block diagram, no schematic or
explanation of the steps in their operations are provided. They show the sensing of amino acids
as entirely independent from the sensing of the sugars. Both are shown as requiring undefined
pores through the microvilli. Their concepts do not account for the quiescent or dynamic
potential at the pedicle of the sensory neuron, the adaptation characteristic of the sensory
response, or the neural response function for any of the sensory channels.
[xxx use a different term for dendroplasm and dendroplasm in the following sections, cilia or
microvilli lemma and plasma? See page 48 of Cagan & Kare first. they use microvilli lemma ]
[xxx may need pages 213 to 246 in Cagan & Kare ]
After reviewing the literature of gustation from the perspective of the Electrolytic Theory of the
Neuron, what conclusions can be drawn about the transduction mechanism of gustation?
C The rapid replacement of the complete sensory neuron (~200 hrs) suggests transduction
involves a substantive change in the structure of each sensory neuron.
C The similarity of the gustatory pulse response to that of the visual, auditory and olfaction
modalities suggests a quantum-mechanical mechanism is involved, probably involving a
chemical reaction involving the outer lemma of the neurons.
C The only detailed hypothesis relating to gustatory transduction in the literature relies upon the
Law of Mass Action to describe a process described using a second order differential equation.
Gilbertson, T. & Margolskee, R. (2003) Molecular physiology of gustatory transduction In Doty, R. ed.
(2003) Handbook of Olfaction and Gustation, 2nd revised and expanded edition. NY: Marcel Dekker Chap 34
125
Signal Generation & Processing 8- 93
C All previously discussed transduction mechanisms in this and related works have involved a
change in state of one of the reactants/participants, thus ruling out the Law of Mass Action as
an underlying premise.
C The great variety of lipids found in the lemma of neurons suggest a sufficient variation in lipid
reaction with stimuli to define a set of independent variables in gustation and explain the
groupings of stimulus effects in many multidimensional analyses of stimuli.
Breslin’s work has suggested the phosphatidyl fatty acid of P-Path transduction.
A globoside appears a candidate for the phosphoglyceride of G-Pathtransduction.
C The complex grouping of stimuli in various multidimensional analyses based on the stimuli
suggest the chemical structures of the stimuli are not an independent variable in gustation.
C As demonstrated by Kashiwagura et al., preadaptation with NaCl affects the transduction of
CaCl, a compound devoid of sodium.
The following proposals will follow the fundamental chemistry found to be operative in the
discussion of olfaction, coordination chemistry between very complex molecular ligands
associated with specialized (type 4) regions of the lemma of microvilli emanating from individual
dendrites.
The challenge is to determine the structure of these complex ligands and their electronic state
prior to coordination with a variety of potential stimuli.
The stimuli themselves range from the nonpolar, non-ionizing sugars and sugar derivatives to the
highly ionic H+ and Na+ which may be present in solution in more complex coordinate forms.
---As a general introduction, the sensory neurons of the gustatory modality employ specialized
(type 4) lemma on the microvilli emanating from their dendritic terminal. The specialized lemma
consist of four different phospholipids known collectively as globosides, in the outer leaf of their
bilayer structure. Mixed collections of these sensory neurons are grouped in taste buds
embedded behind pores primarily in the epithelium of the tongue and oral cavity surfaces. In
vivo, the globosides exhibit a negatively polarized molecular surface to the fluid environment
of the oral cavity that offers a coordination chemistry association to molecules exhibiting a
positive hydrogen ion or a positive sodium ion on their surface, and two more complex
molecular arrangements, developed further below, commonly found in the sugars and in
quinine hydrochloride. The possibility of additional sensory neuron types sensitive to the
monosodium glutamate and various “metallic” salts cannot be dismissed at this time. Negative
ions, such as the chlorine ion, are not attracted to the globosides. The coordination chemistry
involved appears to generate a stable state that is long lasting. This change of chemical state
may contribute to the need to replace the sensory neurons on a 200 hour cycle time.
A conducting form of PtdIns is also a candidate for inclusion in the globosides used as receptors
of the gustatory modality. Inositol is considered a sugar derivative although it lacks an oxygen
within the ring structure of this cyclohexane. Inositol occurs as nine isomers. The inositols,
particularly the epi- and myo- isomers, are known to participate in a variety of coordinate
relationships with the alkali and alkali-earth metal ions126. PtdIns is a known phospholipid of the
cell lemma. The inositol group of the phospholipid could participate in the necessary coordinate
chemistry and are prime candidates for the N-Path sensory neuron receptor. Additional
background on the inositols is available127.
Lehninger fortuitously identified the four major globosides of gustation in 1970 based on their
126
Williams, R. & Atalla, R. (1981) Interactions of group II cations and borate ions with nonionic saccharides
In Brant, D. ed. Solution Properties of Polysaccharides Washington, DC: American Chemical Soc. Chapter
22
127
Cerny, M. Kocourek, J. & Pacak, J. (1963) The Monosaccharides. NY: Academic Press Chapter 35
94 Neurons & the Nervous System
presence in neural tissue128. He also described their biological formation.
---Many multidimensional analyses and cluster analyses have shown multiple inconsistencies when
interpreted in terms of a reversible chemical reaction leading to transduction. As in olfaction,
the conflicts are sufficient to spread doubt that the underlying mechanism involves reversible
chemistry. In all other excitation/de-excitation mechanisms studied, a quantum-mechanical
mechanism was involved that was not reversible. Thus, it is useful to consider, non reversible
mechanisms for gustation, particularly with the close association of gustation and olfaction.
A leading candidate for explaining transduction in gustation is the proliferation of different lipids
found in specialized areas of the bilayer lemma of the sensory neurons. These lipids have polar
terminals that are sugars, alcohols, amines, cholines, and many other complex chemistries. It is
possible that the major stimulants attach stereographically or attack these lipids, causing a
reaction and generating a free electron that is transferred through the nonpolar lipid tail to the
plasma inside the neuron. The reaction also changes the chemical constituency of the lipid
involved. As a result of the reaction, the particular lipid molecule is no longer viable. With time,
the viability of the entire neuron is degraded in this way and it must be replaced in an orderly
process to maintain nominal gustatory sensitivity. Failure to perform this continual replacement,
due to age or otherwise, leads to a loss of gustatory sensitivity.
Based on this hypothesis, the challenge is to identify the most likely lipid species able to react
with the appropriate group of stimuli.
The many multidimensional analyses in the gustation literature invariable display a two
dimensional plane with the stimulants grouped in not more than four major locations. The various
cluster analyses also show a few major branches with the various minor branches splitting from
these. See also Wright & Michels for the similar situation in olfaction.
It appears reasonable to hypothesize the gustatory sensory neurons operate in a mode very
similar to the olfactory transducers and employ specialized regions of lemma encasing the
protruding hair (villi) consisting of a few
different types of lipid molecules. Under this
hypothesis, the transduction function is
primarily dependent on the type of lipid
present and not on the precise nature of
the stimulant. Thus, one type of lipid reacts
to produce a signal that is interpreted as
sweet, a second type as sour, a third type
as xxx etc. irrespective of the chemical
formula of the stimulant. A single stimulant
could affect more than one type of
receptor. The signals from these sensory
neurons could be differenced higher up the
neural pathway to provide finer bipolar
signals describing the individual stimulants.
Following the proposed hypothesis, the
typical multidimensional analysis can be
modified slightly in its support. Figure 8.5.3-1
shows a diagram from xxx with the added
scales.
Figure 8.5.3-1 XXX modified multidimensional
8.5.3.1 Previous theories of gustatory analysis based on the polar head of lipids EMPTY.
128
Lehninger, A. (1970) Biochemistry NY: Worth Publishing pp 196-200 & 522-526
Signal Generation & Processing 8- 95
transduction
All previous theories of gustatory transduction have been based on the chemical theory of the
neuron and have been necessarily largely conceptual. Scott & Mark described the state of
understanding of the transduction mechanism in the gustatory modality in 1987, “Attempts to
define the organization of the taste system in terms of the physical characteristics of stimuli have
been largely unsuccessful.” Interestingly, after annotating reviews of gustatory transduction up
through 1991, Getchell et al. note on page 153, “To date, there have been no examples of 2nd
messenger modulating ion channels directly in taste cells.” They also note that in the case of at
least one bitter compound, denatonium, no membrane conductance change is thought to
occur at all; . . .” They conclude, “Since the final pathway converges and most taste cells
respond to more than one taste modality, it is not yet clear how taste qualities can be
discriminated.”
This work will propose an entirely different theory of transduction based on the Electrolytic Theory
of the Neuron. The theory eliminates the ambiguities encountered by earlier theories and
demonstrates that individual receptors respond to only one specific stereochemical structure
and measure the dipole potential of the gustaphore. It is this measurement that is passed to the
CNS along a conventional sensory neural circuit.
8.5.3.2 The AH,B & AH,B,X coordination chemistry of the gustatory channel
The conditions required in coordination chemistry are quite complex. Shallenberger has
provided a glossary addressing these conditions and their definition (pages 297-301)
The globoside receptors of the desirable/”sweet” gustatory channel are subject to attack by a
variety of chemicals. Shallenberger has explored this possibility extensively129. Jakinovich has
also offered ideas in this area130. Figure 8.5.3-2 shows the basic coordination chemistry he
proposes as the mechanism resulting in excitation of the G-Path sensory neuron. At the
minimum, AH represents a hydroxyl proton and B represents a neighboring hydroxyl oxygen
atom. While the configuration of these two units need not be identical on the two ligands, the
situation strongly suggests a stereochemical relationship, at least in the local area of each
ligand. The AH moiety may be OH, NH, NH2 or even CH in halogenated compounds. The B
moiety may be O, N, an unsaturated center, or even the π-bonding cloud of the benzene ring.
A unique feature of the hydroxyl group was highlighted by DuBois et al131. The hydroxyl
group can act as either the AH or the B element in the AH,B coordinate chemical bond.
A single hydroxyl group can act as either a hydrogen bond donor (via the hydrogen
atom) or acceptor (via the unpaired electrons of the oxygen atom). Thus two hydroxyl
groups of a molecule can provide both the AH structure and the B structure, as they
frequently do in the sugars.
Shallenberger & Acree noted a second stereochemical requirement can be used to describe
the difference in sweetness between D-leucine and L-leucine. The latter is not sweet to the
taste.
129
Shallenberger, R. (1982) Op Cit. Chap 10, Sweetness: A stereochemical attribute.
130
Jakinovich, W. (1981) Comparative study of sweet taste specificity In Cagan, R. & Kare eds Op. Cit. page
132
131
Dubois, G. Walters, D. Schiffman, S. et al. (1991) Concentration–Response relationships of sweeteners In
Walters, D. Orthoefer, F. & DuBois , G. eds. Sweeteners. Wash. DC: American Chemical Society Chapter
20, page 275
96 Neurons & the Nervous System
Figure 8.5.3-2 Proposed coordination chemistry of the G-Path sensory neurons clarifying the
condition described by Shallenberger & Acree. In the simplest case, all A’s & B’s are hydroxyl
oxygen and H’s are hydroxyl hydrogen. Their original text did not differentiate between the
distance between AH and B. His later writings referred to the H,B distance as 3 Angstrom. Both
numerics are ±7%. Modified from Shallenberger, 1971.
When originally investigating in 1967, Shallenberger & Acree asserted, the initial interaction “is
neither a proton transfer nor an electrostatic interaction, but probably involves London
dispersion, the principle element of hydrogen bonds.” This comment may be subject to
modification where more is known of the environment involved. The biochemistry field has
generally adopted the term Van der Waals forces as a synonym for the London forces of
“London dispersion.” Wikipedia offers a good overview in this area. Culberson & Lee describe
the multitude of potential gustatory sites based on Van der Waals forces (page 219) without
producing a practical model of gustation132. They employed a large number of MGGs that did
not allow reducing their data adequately. The use of multiple SGG’s (Section 8.5.1.6.3) would
have led to more appropriate results.
In 1970, Birch et al. initiated a study to determine if there was any reaction chemistry involved
132
Culberson, J. & Walters, D.(1991) Three-dimensional model of the sweet taste receptor In Walters, D.
Orthoefer, F. & DuBois , G. eds. Sweeteners. Wash. DC: American Chemical Society Chap. 16
Signal Generation & Processing 8- 97
133,134
. They concluded that, “there is a chemical reaction basis
in the AH,B relationship
(stoichiometric basis) for the initial chemistry of the sweetness phenomenon.” While
stoichiometry can be expected in coordinate bond arrangements, reaction chemistry exhibits
a more demanding requirement, residues. No residues of such a reaction haveever been
identified.
Shallenberger & Acree (page 263) also established the distance between the AH and the B
moiety had to be 3 Angstrom (~2.86–3.2 Angstrom or ±7%). This parameter allowed them to
define the specific members of various sugars and other molecules that could participate in the
AH,B relationship. In the case of glucose, they showed that only OH-4 and OH-3 were the logical
choice for a primary AH,B relationship. Using a galactose, they were able to establish that OH-4
was AH and the only remaining possibility for B is O-3 (Shallenberger, 1982, pp 265-275).
After discussing some very sophisticated principles of advanced sugar chemistry, Shallenberger
continues on page 275-276 to show that the receptor site structure is disymmetric, but not
asymmetric. “This planar structure is capable of resolving the chiral recognition problem
presented by the fact that the enantiomeric amino acids possess different taste, but the
enantiomeric sugars do not.”
Based on the overall analysis, Shallenberger & Acree are able to account for virtually all of the
variation in sweetness among the simple sugars, amino acids and other natural chemicals
exhibiting the appropriate AH,B relationship. They have also described a mechanism for various
hydrolyzed alkali and alkali earth ions to cause a perception of sweetness, particularly at low
molar concentrations (page 269). After initially abandoning the tetrahedron concept of taste
space in favor of an orthogonal space, Shiffman et al. have provided similar data on amino
acids versus their acetylized relatives135.
Glutamic acid and aspartic acid, being the only two negatively charged amino acids,
would not be expected to taste sweet. Shallenberger & Acree confirmed this observation
for glutamic acid (page 246). They also noted that γ-aminobutyric acid (GABA) is not
sweet because of the distance between the amine and carboxyl group is too large. This
is due to the loss of the closer COO– group, for material within the neural system, by
electrostenolysis.
---Working in the same time period as Shallenberger, Kier has extended the idea of the AH,B
coordination bond to include a tripartite or “three-point” coordination model136. This model
supports additional discrimination between similar stimuli and also explains the “super sugar”
qualities of many man-made sugar substitutes. Kier labeled the third point, X, while
Shallenberger chose to use the label γ for “greasy.” Later, the third point became known as the
dispersion point. Shallenberger & Lindley (1977) re-examined the work of these two groups as
reported by Shallenberger (1982, page 269) and surfaced the fact they were measuring
distances from different references, Figure 8.5.3-3. Quoting Shallenberger, “In the former (Kier),
the A to B distance is estimated to be 2.6 Angstrom. In the latter (Shallenberger et al.), the AH
proton to B orbital distance is estimated to be about 3 Angstrom.” Differences between these
two geometries may be significant, but cannot be adequately resolved without numerics of
three digit precision. In this work, the best available d-value between A and B is taken as 2.82
Angstrom for the sweet sensitive, G-path, GR.
133
Birch, G. Cowell, N. & Eyton, D. (1970A) A quantitative investigation of Shallenberger’s sweetness
hypothesis. J Food Tech vol 5, pp 277-280
134
Birch, G. Lee, C. & Rolfe, E. (1970B) Organoleptic effect in sugar structures J Sci Food Agric vol 21, pp
650-653
135
Schiffman, S. Moroch, K. & Dunbar, J. (1975) Taste of acetylated amino acids Chem Sens Flav vol 1, pp
387-401.
136
Kier, L. (1972) A molecular theory of sweet taste J Pharm Sci vol 61(9), pp 1394-1397
98 Neurons & the Nervous System
Definitions:
1. As this discussion progresses, it will
become appropriate to modify the
definition of dispersion point. There are
two locations to be discussed, one on
the GR and one or more on the
gustaphore(s).
These locations are
actually determined by the electrostatic
fields of the specific moiety. While a
centroid of an electrostatic field contour
can be defined as the dispersion point,
this point may vary with the contour
chosen. The term dispersion centroid
(DC) may be more appropriate.
Figure 8.5.3-3 Comparison of AH,B,X geometries.
Left, geometry reported by Kier. Right geometry
reported by Shallenberger group. See text.
2. It is not clear yet that the dispersion
centroid of a GR must be aligned precisely with a similar dispersion centroid on a
gustaphore (like applying a finger axially to a door bell). It may be that the action of the
dispersion point, or points, on a gustaphore is to change the location of the dispersion
centroid of the GR resulting in a change in the dipole potential presented to the 1st
amplifier of the sensory neuron. In this case, the location of the centroid of the
electrostatic field of the gustaphore may be lateral to the centroid of the GR. This case
also allows multiple locations for gustaphore centroids relative to the location of the GR
centroid.
3. The lateral action of a gustaphore electrostatic field on the electrostatic field of the GR
is compatible with multiple dispersion centroids located around the dispersion centroid of
the GR.
4. In general, a point described as a dispersion point in the literature is more appropriately
defined as the centroid of an electrostatic field subject to dislocation resulting in a change
in the dipole potential presented to the 1st amplifier (signal detector) within the sensory
neuron. One form of dislocation is dispersion.
With regard to the fourth point, Immel, writing in 1995, asserted that the dispersion point may not
be a precisely located point (as quantified differently by Kier and the Shallenberger group). He
noted the potential electrostatic fields associated with a wide group of super-sweet compounds
were changing slowly over a significant portion of the surface of the molecules.
---The coordination chemistry relationships developed by Shallenberger et al. and by Kier, and the
Electrolytic Theory of the Neuron provides a comprehensive description of the sweet sensory
neuron that does not involve any GPCR proteins or any movement of sodium ions through the
wall of the cell. The described performance of these sensory neurons is much more complete
and detailed than that of any other hypothesis.
---The primary structure of a stimulant that is perceived as sweet can be defined as its glycophore.
Shallenberger ((1982, page 265) defined the glycophore of the majority of sugars and many
sugar derivatives as consisting of the hydroxyls at position 3 and 4. OH-4 acts as AH and O-3 acts
as B. In other chemicals, the glycophore is represented differently but still exhibits the nominal
3 Angstrom spacing between AH and B.
---A more complete discussion of the AH,B,X relationship and its affect on perception appears in
Section 8.5.10.
Signal Generation & Processing 8- 99
---Figure 8.5.3-4 shows several examples of chemicals considered to taste sweet but do not
contain the structure of a sugar. Early investigators were unable to explain the reason for their
perceived sweetness. Based on this hypothesis and the advances in computational chemistry,
most of the gustaphores of these chemicals can be identified as shown by the brackets marked
AH,B. The specifics of these chemical structures will be addressed in later sections.
Preview: In the case of α-anisaldehyde oxime (now known as anisaldoxime, CAS 3717-2240), the chemical is quick to rearrange. As shown, it exhibits a hand-calculated d-value
between the oxygen closest to the ring and the centroid of the ring equal to 2.82
Angstrom. This is the nominal d-value for a G-path glycophore. In the case of saccharin
(planar except for the paired oxygen atoms), the hand-calculated d-value of 3.21
Angstrom between the sulfur and the centroid of the phenol ring is 14 % longer than ideal
but potentially acceptable for a non-caloric glycophore. The d-value between either
oxygen attached to the sulfur and the nitrogen is 2.5 Angstrom, 11% low compared to an
ideal glycophore. In essence, this chemical exhibits three marginal glycophores. Each of
the chlorine pairs in chloroform are separated by a d-value of 2.94 Angstrom as measured
using the program Jmol (4% above the nominal 2.62 Angstrom). In this case, the chlorine
atoms are the active orbital pairs in the AH,B relationship.
100 Neurons & the Nervous System
Figure 8.5.3-4 Sweet tasting non-sugars and the their AH,B relationships, with fructose as a
reference. In the unsaturated alcohol, the B may consist of the dual bond between two
carbons. Similarly, the B of α-anisaldehyde oxime appears to be the centroid of the nucleophilic
phenol ring. See text. From Shallenberger & Acree, 1971.
Kier extended the stereochemical requirement on an effective sweetener based on a study of
the amino acids and the growing number of man-made sweeteners (typically including amino
acid ligands)137.
[xxx edit ]
Kier developed a three point union involving the AH,B coordination relationship (with a nominal
spacing of 2.5 Angstrom) plus an additional bonding of undefined type about 3.5 Angstrom from
the A entity and 5.5 Angstrom from the B entity. It is not clear if the difference between the
nominal value of 2.5 Angstrom of Kier and the 3.0 Angstrom of Shallenberger & Acree is
137
Kier, L. (1972) A molecular theory of sweet taste J Pharm Sci vol 61(9), pp 1394-1397
Signal Generation & Processing 8- 101
significant. The value of 2.5 Angstrom reported by Kier appears to involve more complex AH,B
configurations than the simple carboxylic arrangement found in most Shallenberger & Acree
formulas. This may suggest, the 2.5 Angstrom value is off-peak compared to the 3.0 Angstrom
nominal.
Kier conjectured the third union involved “an electron-rich position capable of undergoing
electrophilic attack, engaging in localized charge transfer, or capable of participating in some
type of bonding involving the electron component such as a dispersion interaction.” Chemicals
able to satisfy this three point union exhibit sweetnesses 10's to xxx,000 times greater than
fructose, suggesting a significantly larger change in the dipole potential of the receptor per mass
unit of sweetener.
Eggers, Acree & Shallenberger (2000) have discussed and developed the three-point hypothesis
farther using the symbol γ to replace Kier’s X.
----
102 Neurons & the Nervous System
This sensitivity to attack and the resultant transduction signal intensity can be portrayed more
graphically as seen in Figure 8.5.3-5.
8.5.3.3 Proposed transduction
mechanism in gustation
The section is presented as provisional.
There remain no known texts with a
focus on coordinate chemistry and most
of the work reported in the journal
literature is based on metal-based
coordinate chemistry. The description of
organic coordinate chemistry (lacking
any metallic ions) is rudimentary. No
material has been located describing
equilibrium reactions associated with
hydrogen (London) bonds, much less on
the DACB type of such bonds.
The mechanism of transduction proposed in
this work involves a two-step process. The
same mechanism is employed in all four of
the sensory channels and receptors. This
situation is different than in the visual
modality where a different mechanism is
used in the long-wavelength channel
receptor.
Figure 8.5.3-5 Proposed sensitivity of the
desirable/”sweet” sensory channel ADD using the
multidimensional graphic of xxx in overlay as an
example.
The first step is a selection process wherein
various gustaphores form a dual coordinate
chemical bond with one of four types of sensory receptors. The second step involves a net
change in the dipole electrostatic potential of the gustaphore/receptor complex compared to
the previous dipole electrostatic potential of the sensory receptor alone that is presented to the
first Activa of the sensory neuron. The first step provides a gross determination of the character
of the gustaphore and the second provides a vernier measurement of the intensity of the
gustaphore as a stimulus.
A temporal aspect can be associated with the second step. Figure 1 of Jin et al illustrates
this aspect.
Immel has provided considerable information on the molecular electrostatic potential
(MEP) of sugars (Section 8.5.5.1.3). The calculations would need to be extended to give
the net dipole potential of a molecule when participating in a DACB. Immel did not
discuss the specific location of the X feature on the receptors when participating in an
AH,B,X (super-sweet) relationship. In fact, he did not consider the specific location of the
X feature on the stimulants. He noted in his abstract to chapter 4, “Most informative in
regard to the placement of the tripartite AH-B-X glucophore are the hydrophobicity
distributions, which show the lipophilic X-part to be an entire, obviously quite flexible,
region rather than a specific corner of the ‘sweetness triangle’.”
----
Signal Generation & Processing 8- 103
The first two steps will initially be discussed jointly. Section xxx will concentrate on the vernier
aspects of perception associated with the ΔDEP.
---Hellekant et al. provided a conceptual drawing of the excitation/de-excitation process based
on their data in 1991. It is shown in Section 8.5.1.6.7 and again in Section 8.5.6.6 where the
modified time line at the bottom of the figure is discussed. It discusses the delay time due to the
introduction of the stimulant and the beginning of the recovery time that includes the delay
internal to the sensory neuron involved. The mechanisms underlying the drawing and associated
uniquely with the Excitation-De-excitation Equations of Section xxx can now be discussed in
greater detail based on the background provided in Section 8.5.1.6.7.
C The total delay time consists of the time required by the protocol for the stimulant to reach the
sensory neuron receptors plus the intrinsic electrolytic delay within the neuron before the emitter
of the Activa achieves the threshol potential provided to minimize internal noise from being
treated as authentic signal.
C The response time includes the precise response waveform beginning with the end of the delay
time measurable at the collector (or nearby axon pedicle) of the neuron.
C The decay time represents the actual decay characteristic of the sensory neuron as the
stimulating molecules are released from the binding site of the neuron. The waveform is a true
exponential curve with a finite time constant characteristic of this part of the mechanism.
The premise adopted here is that the combining and releasing of the stimulant and phospholipid
ester of the receptor can be described as an equilibrium reaction within the coordinate
chemical sphere (rather than the conventional chemistry sphere). Taking the A-Path as an
exemplar, the equilibrium equation for a DACB is shown, using form II of Section8.5.1.6.7 as,
----PtdSer + odorophore º PtdSer:odorophore
Where the odorophore presents with O
and OH orbitals of a carboxylic group
As a result, the above equation can be written in an expanded form as,
PtdSer + O + OH º PtdSer (DACB, or :) odorophore
The equilibrium constant for this situation can be written as,
K = KR/KL =[PtdSer:odorophore]/[PtdSer] C [OH] C [OH]
----For a a molecule perceived as sweet, the equations change to reflect the proper receptor and
stimulant structure.
PtdTyr + odorophore º PtdTyr:odorophore
Where the odorophore presents with two
independent OH groups in an equat.–
trans–diol configuration.
As a result, the above equation can be written in an expanded form as,
PtdTyr + OH + OH º PtdTyr (DACB, or :) odorophore
The equilibrium constant for this situation can be written as,
K = KR/KL =[PtdTyr:odorophore]/[PtdTyr] C [OH] C [OH]
These equations assume the formation of the DACB occurs with the simultaneous presentation
104 Neurons & the Nervous System
of the two orbitals of the stimulant to the two orbitals of the receptor. They also assume very little
delay in the transit of the saliva by the stimulant compared to the measurable time delay of the
test instrumentation.
As noted in Section 8.5..1.6.7, there remains a question of
Signal Generation & Processing 8- 105
8.5.3.4 Proposed selection mechanism for “desireable/sweet” RENAME
It is important to have an understanding of the numbering system used in sugar chemistry and
the possibility of sugars with the hydroxyl groups pointing in different directions. Figure 8.5.3-6
shows these relationships. Current convention calls for numbering to be clockwise beginning
with the first carbon to the right of the oxygen in all hexose and pentose sugars. Note in the
pentoses that the first carbon is taken as that in the methyl alcohol. Rotation of the axis of the
moieties associated with a given carbon can affect the taste sensation significantly. For
instance, α-D-mannose is sweet, but β-D-mannose, which is very similar in structure, is distinctly
bitter according to Shallenberger & Acree (1971, pg 222)
106 Neurons & the Nervous System
[xxx rewrite the following to separate
gangliosides from globosides and address
third view of simpler receptor ]
Using the description of the gustatory
sensory neuron above, the question is what
is the structure of the type xxx neurolemma
covering the outer surface of the sensory
hair(s). The exterior bilayer of the lemma of
non-neural cells is known to be populated
primarily by phosphatidyl choline (lecithin).
However, in neural tissue, the exterior
bilayer is known to consist of 20-25% of
ganglioside, a very large family of
phosphoglycerides138. Figure 8.5.3-7 shows
the structure of two gangliosides. Only a
few of the available configurations have
been investigated in the laboratory139,140.
The top frame shows a well studied
ganglioside known as ganglioside GM2.
Ferrier & Collins have provided an alternate
Figure 8.5.3-6 The numbering system of the simple
sugars. Even the pentoses have six carbons. The
axial elements associated with each carbon can
invert structurally resulting in significantly different
chemical properties.
138
Christie, W. (2010) Gangliosides: structure,
http://lipidlibrary.aocs.org/Lipids/gang/index.htm
occurrence,
biology
and
analysis
139
Ledeen, R. ed. (1984) Ganglioside structure, function, and biomedical potential. NY: Plenum Press
140
Porcellati, G. Ceccarelli, B. & Tettamanti, G. (1975) Ganglioside function: biochemical and pharmacological
implications. NY: Plenum Press, c1976
Signal Generation & Processing 8- 107
141
Haworth Diagram of this ganglioside . They describe one of the R moieties as ceramide, a
saturated lipid. Similar forms of gangliosides are commonly found in the myelin sheaths of stage
3 neurons. The center of the frame shows a ganglioside with the complicated series of amino
sugars replaced with a chain of D-galactose moieties bridged in the α(1–>3) arrangement. This
arrangement provides three separate locations where a coordinate AH,B bond can occur
between a stimulus and this molecule. Only the potential AH,B bond at O-4 and O-6 of
galactose(I) is shown explicitly. One of the R moieties is shown fully saturated in order to provide
conductivity along the length of the molecule. This modification may not be necessary as
Hauser, and Sherer & Seelig have shown operation of the molecule as a molecular electrometer
is possible without it (Section 8.xxx). This moiety is a polyethine, a member of the terpene family.
Many variations are available within the polar head and the lipid tail of the gangliosides.
141
Ferrier, R. & Collins, P. (1972) Monosaccharide Chemistry. NY: Penguin Books page 255
108 Neurons & the Nervous System
Figure 8.5.3-7 Two gangliosides associated with the neural system. Top; the ganglioside GM2
isolated from a Tay-Sach brain. Center; proposed ganglioside supporting the G-Path channel
of gustation. The ganglioside contains three galactose ligands chained at the α(1–>3) sites and
a polyethine (fully conjugated) lipid for one of the R moieties. This structure provides multiple
glycophore binding sites using O-4 & O-6. An optional sialic acid moiety is shown as A at the
lower left. Bottom; a simpler ganglioside utilizing only one galactose moiety if AH,B coordinate
bonding is only possible using O-3 & O-4.
Lehninger has shown a ganglioside with two galactose moieties that could support two AH,B
bonds142. The number of chained sugars can reach ten in some oligosaccharides. The first of
three possible AH,B bonds is shown between the hydroxyl hydrogen at C-4 and a hydroxyl
oxygen of the stimulus, and a hydroxyl oxygen at C-6 and a hydroxyl hydrogen of the stimulus
separated by 2.6 Angstrom. The lower frame shows a simpler potential receptor with only one
galactose moiety if the only acceptable AH,B coordinate bonding must use O-3 and O-4 as
suggested by Shallenberger & Acree.
Gennis has described a digalactosyl diglyceride using a α(1–>6) linkage arrangement that can
be extended indefinitely while preserving the capability of O-3 & O-4 to support coordinate
142
Lehninger, A. (1970) Biochemistry NY: Worth Publishing page 201
Signal Generation & Processing 8- 109
143
bond pairing . The chain of galactose sugars (potentially labeled a galtan) can be extended
indefinitely but the effect on the conductivity of the resulting structure is unknown.
This family of phosphoglycerides is known to exhibit a negative polar head. In the case of the
gustatory sensory neurons, this polarity is enhanced by the negative polarity of the interstitial
liquid crystalline water of the type 4 microvilli bilayer. Thus, the in-vivo sensory ganglioside is
highly attractive to positive ions introduced into the saliva in significant concentration.
Shallenberger has described a series of n-galactose configurations employing different chaining
arrangements (pages 224-230). Only two of the six arrangements provide the maximum
opportunities for AH,B bonding.
---Concatenation of the galactose moieties can cause an increased capture area for the sugars
but potential interference with the L-configuraton sugars. The result would be an increased
sensitivity of the channel to the D-configurations at the expense of the L-configurations.
---As noted in Chapter xxx, a bilayer of sphingomyelin (of the same super family of
glycosphingolipids as ganglioside) is known to form an electrical diode, i.e. the bilayer is able to
conduct electrical charges asymmetrically from one polar head to the other polar head, after
surface treatment of one of the electrolytes with 10– 7 g/ml of alamethicin by Mueller & Rudin144.
Alamethicin is a cyclopeptide antibiotic containing a variety of amino-acids and at least one
carboxyl group. The precise role of alamethicin remains unknown.
The presence of a semiconducting region of type 2 lemma employing a ganglioside in its outer
layer introduces the immense possibilities of polysaccharide (sugar) chemistry to the gustatory
transduction function. Each of the four saccharide units of the ganglioside exhibit slightly
different arrangements and the sugar moiety is subject to attack in a variety of ways. Such
polysaccharides are known to be subject to decomposition by dilute bases and dilute acids.
They are heteropolysaccharides. They contain both α(1–>4) and α(1–>6) linkages. The presence
of a simple saccharide as a stimulus may result in the chaining of the simple saccharide to the
heteropolysaccharides of the type 4 membrane.
Any addition, decomposition or
rearrangement that causes a significant change in current flow into the plasma of the neuron,
via the first Activa of the sensory neuron, will constitute the fundamental step in transduction.
The typical lemma does not have two bilayers of ganglioside. It is more likely to have an outer
layer of ganglioside and an inner layer of phosphatidyl ethanolamine (PtdEtn). The important
feature is that the bilayer form a electrolytic diode that can transport an electron into the
dendroplasm of the sensory neuron when such an electron is released from the polar head of
the ganglioside.
With the presence of a diode accounted for, any significant rearrangement, fragmentation or
polymerization of the polar head could inject the negative charge associated with that head
into the plasma. of the sensory neuron. In the laboratory, the sphingolipids as a family are known
to be easily hydrolyzed (Lehninger, page 199).
[xxx edit ]
The proposed transduction mechanism of the gustatory modality involves the disruption of the
chemical structure of the ganglioside bilayer of the type xxx portion of the microvilli lemma and
143
Gennis, R. (1989) Biomembranes. NY: Springer-Verlag pg 29
144
Mueller, P. & Rudin, D. (1968) Action potentials induced in biomolecular lipid membranes Nature vol. 217,
pp 713-719
110 Neurons & the Nervous System
the injection of a negative charge (an electron) into the dendroplasm of the sensory neuron.
The presence of the electron is sensed by the same electrolytic structure as employed in the
olfactory modality. Most of the electrolytic structure is also shared with the visual and auditory
modalities, however, the free electron is generated by different means.
----
8.4.4.6xxx Sugars as potential GR’s in gustation
[xxx move or renumber ]
The significant presence of ganglioside or globoside attached to the surface of the
dendrolemma of gustatory microvilli introduce the potential for these phosphoglyceride -sugars
to provide the sensory substrate for gustation.
Drefus et al. have defined the gangliosides as “a complex group of glycosphingolipids
which contain one or more molecules of sialic acid145.” However, as Schauer et al. noted,
“Sialic acids are derivatives of neuraminic acid with either an acetyl or glycolyl residue at
the amino function and frequently one or more O-acetyl group(s) at C-4, C-7, C-8 and
preferably at C-9146.” Thus the gangliosides are a very large family that has been only
superficially explored in the literature. Merrill & Sandhoff, as an example, only discuss the
saturated sphingolipids as a matter of expediency147. In the same volume, Cook &
McMaster provide a set of nomenclature for the broader varieties of saturated and
unsaturated fatty acids in phospholipids148. The gangliosides are widely associated with
neuron tissue. The distribution of the gangliosides appears focused in the gray matter, the
unmyelinated portions of neurons, but this remains controversial. Yamakawa writing in the
same volume, noted the high concentration of at least a close relative, galactosyl
ceramide, in the white matter of the brain149. Cook & McMaster discuss the wide spectrum
of fatty acids needed within a mammalian species and note (page 187), “Unsaturated
fatty acids also must be synthesized in cells, supplemented by essential fatty acids in the
diet.” They also assert that trans-unsaturated fatty acids are not produced by mammalian
enzymes. . .” Their statements about cis-unsaturated fatty acids may be too narrow to
include the very small relative numbers of these acids needed in the body.
Galactose, which plays a major role in the following discussion was known as cerebrose
in earlier times.
Benjamins et al. have discussed the cerebrosides in the context of their occurrence within the
brain150. They distinguish between sphingolipids containing sialic acid (gangliosides) and those
that do not (globosides). They hypothesize the creation of these chemicals within the chemical
milieu of the brain. They also provide additional information about the long chain fatty acids of
the sphingolipids. The material does not address the operation of the sensory neurons or their
145
Dreyfus, H Urban, P. Harth, S. et al. (1976) Ritinal gangliosides: composition, evolution with age In
Porcellati, G. ed. Ganglioside Function. NY: Plenum Press
146
Schauer, R. Schroder, C. & Shukla, A. (1984) New techniques for the investigation of structure and
metabolism of sialic acids In Ledeen, R. et al. eds. ganglioside structure, function, and biomedical potential NY:
Plenum Press
147
Merrill, A. & Sandhoff, K. (2002) Sphingolipids: metabolism and cell signaling In Vance, D. & Vance, J.
eds. Biochemistry of Lipids, Lipoproteins and Membranes. 4th Ed. NY: Elsevier Chapter 14
148
Cook, H. & McMaster, C. (2002) Fatty acid desaturation and chain elongation in eukaryotes In Vance, D.
& Vance, J. eds. Biochemistry of Lipids, Lipoproteins and Membranes. 4th Ed. NY: Elsevier Chapter 7
149
Yamakawa, T. (1984) Wonders in glycolipids-a historical review In Ledeen, R. et al. eds. ganglioside
structure, function, and biomedical potential NY: Plenum Press
150
Benjamins, J. Hajra, A. & Agranoff, B. (2006) In Siegel, G. et al. eds. Basic Neurochemistry, 7th Ed. NY:
Elsevier Chapter 3
Signal Generation & Processing 8- 111
receptors. They do describe galactocerebroside (CerGal) as associated with their description
of the long chain fatty acid, sphingosine.
Crescenzi et al. have described the interaction of the Na+ ion with iso-carrageenan in water at
25°C151. Pass & Hales have explored the changes in enthalpy and entropy when the alkali metal
ions interact with polysaccharides152.
`Williams & Atalla have discussed the interaction of hexose sugars complexing with group II
cations such as calcium and magnesium153. They noted that either 3 or 4 coordinate bonds are
formed between the cation and one or two sugar residues. Going beyond the simplest
examples is difficult because of the immense number of isomers associated with a specific
formula polysaccharide.
They note that inositol, the polar head of a candidate
phosphoglyceride, forms nine isomers. A three ring polysaccharide, as found in ganglioside, can
exhibit up to 56 isomers154. However, the stereo preferences of these sugars may limit the useful
number of these configurations. Williams & Atalla noted the exceptional effectiveness of the
calcium ion in coordinating with these sugar alcohols in aqueous solution. They speculate this
may be due to the specific parameters of the orbital electrons in this species. Sodium is also
known to complex with inositol.
8.5.4 The initial selection operation of the gustatory sensory receptors
The overall operation of the gustatory sensory neurons are analogous to the operation of all
other sensory neurons studied. For more details of its operation, see Section xxx. Each sensory
channel employs a different sensory receptor based on the phospholipid forming the type 4
region of its villi.
This section will develop the stimulant/receptor relationship used by each of the sensory
channels of the gustatory modality. Based on the data in the previous section, there are only
four distinct channels, a considerably simpler situation than in olfaction.
--[xxx edit this down ]
Shallenberger (1996) has summarized his thesis on the operation of each of the chemoreceptor
types. The terminology is that of the advanced chemist and will not be reported here. In
essence, there are at least four types of gustatory sensory neurons capable of sensing each
historical class of gustatory sensations. The sensory neurons all rely upon coordination chemistry
concepts for transduction and no reaction chemistry result is involved in the transduction
process. The features of molecular symmetry/dissymmetry and chirality/pseudo-chirality play a
major role in distinguishing between the various stimuli. As a general rule, the transduction
process involves the generation of an electrical signal as the result of a pseudo-acid/base
titration phenomenon. Shallenberger’s thesis relegates the topology of the chemical structures
to a secondary role in transduction. His abstract summarizes his position;
“When considered jointly, all tastes (sweet, salt, bitter, sour) are variations on a common
electrostatic mechanism, and the primary distinction among them can be traced to the
symmetrical nature of the interaction between the substance and the taste receptor.
Sourness is a dissymmetric interaction between the hydronium ion (an acidophore) and
151
Crescenzi, V. Dentini, M. & Rizzo, R. (1981) Polyelectrolytic behavior of ionic polysaccharides In Brant,
D. ed. Solution Properties of Polysaccharides. Washington, DC: American Chemical Society Chapter 23
152
Pass, G. & Hales, P. (1981) Interaction between metal cations and anionic polysaccharides In Brant, D. ed.
Solution Properties of Polysaccharides. Washington, DC: American Chemical Society Chapter 24
153
Williams, R. & Atalla, R. (1981) Interactions of Group II cations and borate anions with nonionic saccharides
In Brant, D. ed. Solution Properties of Polysaccharides. Washington, DC: American Chemical Society Chapter
22
154
Hough, L. (1977) Selective substitution of hydroxyl groups in sucrose In Hickson, J. ed. Sucrochemistry.
Washington, DC: American Chemical Society
112 Neurons & the Nervous System
the taste receptor, whereas saltiness is a concerted symmetrical electrostatic interaction
between the Na+ and Cl– ions (the halophore) and the receptor. Sweetness is elicited
through a bilaterally symmetrical and concerted dipolar interaction between a
glycophore and the receptor, while bitterness can be traced to either dissymmetric ionic
or dipolar interactions between a picrophore and the receptor. As no products are ever
formed, taste phenomena are collectively grouped as being due to electrostatic
recognition interactions.”
Shallenberger expresses each of these relationships using an equation based on the notation
of Belitz et al. (1981). In all cases, transduction occurs via a nucleophilic/electrophilic (n/e) taste
receptor on the surface of the microvilli emanating from the dendrolemma of the gustatory
sensory neuron.
Shallenberger concludes, “In taste chemistry, the substrate is not transformed, nor is there a
direct physiological effect.” This statement is too simple. There is clearly a physiological effect
in that the organism perceives a different state. More specifically, there is no physiological effect
(that is physical transformation) recognized by the chemical theory of the neuron. There is a
profound physiological effect associated with the electrophysiology of the sensory neuron that
is reported to the CNS. The Electrolytic Theory of the Neuron recognizes this effect. A change
in the dipole moment of the phospholipid receptor at bilayer interface results in a change in the
potential at the base of the first Activa within the sensory neuron. This potential changes by a
few hundreds of microvolts to a few millivolts, and results in a change in the electrical potential
at the pedicle of the sensory neuron. See Section 8.5.10 xxx.
Shallenberger briefly mentions a “miracle fruit extract,” from the berry of the tree known as
Synsepalum dulcificum. Mixed with a sour stimulus, this extract is known to turn the perception
of many sour stimuli remarkably sweet. This material is now known as miraculin. The language
of Shallenberger can be easily misinterpreted. In ChemSpider, miraculin is a synonym for
triacontanol_62194 a totally saturated straight chain aliphatic. This material would contribute
to a sensation of sweetness when it formed an azeotrope in solution.
8.5.4.1 Operation of the “sweet” gustatory sensory neuron
Birch has provided an overview of the sugar sensing situation based on the SAR approach155.
The focus is on the degree of sweetness of several families of chemicals and the effect of
substitutions within these chemicals. He does assert the removal of the anomeric hydroxyl group
in sugars does not affect their degree of sweetness. He also reports on the removal of one
oxygen at a time at each of the five possible positions in pyranose structures. His findings suggest
some alternate alignments between a given sugar and a receptor may be possible or multiple
sugar receptors are present that operate according to different rules
Van der Heijden el al. have tabulated the relative sweetness of a large variety of
sweeteners156,157. They have also provided chemical diagrams for each of them showing multiple
sites of AH,B interaction with the sensory receptors in many cases. Their discussion is the most
recent advocating a third feature associated with each stimulant. They described the AH,B
distance as ranging from 0.314 nm (3.14 Angstrom) to 0.333 nm (3.33 Angstrom) in these
materials.
The attack upon the globoside embedded in the microvilli lemma of the neuron causes a
change in the potential of the base electrode of the Activa buried in the type 2 lemma of the
microvilli. The globoside is no longer functional and it is replaced as part of the 200 hour renewal
155
Birch, G. (1976) Structure-activity relationships in the taste of sugar molecules In Benz, G. ed. StructureActivity Relationships in Chemoreception London: Information Retrieval Limited pp 111-118
156
van der Heijden, A. van der Wei, H. & Peer, H. (1985) Structure-activity relationships in sweeteners. I.
Nitroanilines, sulphamates, oximes, isocoumarins and dipeptides Chem senses vol 10(1), pp 57-72
157
van der Heijden, A. van der Wei, H. & Peer, H. (1985) Structure-activity relationships in sweeteners. II.
Saccharins, acesulfames, chlorosugars, tryptophans and ureasChem senses vol 10(1), pp 73-88
Signal Generation & Processing 8- 113
cycle of these sensory neurons. The injected electron is stored on the distributed capacitance
of the plasma and results in a change in the base to emitter potential of the sensory Activa. This
causes a modulation of the current through the collector-emitter circuit of the sensory Activa.
capacitance. The sensory Activa and the distribution Activa are wired in a common emitter
circuit. The change in the current through the first Activa causes a mirror change in current
through the emitter-base circuit of the distribution Activa. This change results in a significant
change in the collector potential (axon potential) of the distribution Activa of the neuron.
The base-emitter circuit of the sensory Activa is normally biased just above cutoff and a
quiescent current is typically present. This current is due to the presence of agents in the saliva
that continually attack the globosides of the outer lemma, releasing a small continual electron
current.
As in the case of the visual modality, the rate of free electron generation within the base of the
sensory Activa and the impedance of the collector current supply source are critical in the
overall response of the sensory neuron. They control both the short term transient performance,
the excitation/de-excitation (E/D) characteristic, and the long term adaptation properties of the
neuron.
Shallenberger & Acree (page 244) have reproduced a graph from Cameron, 1947, showing the
sweetness as a function of concentration on a log-log graph. The slope is very nearly 1.0 for the
simple sugars at concentrations up to 2.0%.
Shallenberger has addressed the Lemieux Effect as additional confirmation of the AH,B theory
of G-Path channel sensory operation (page 267).
Van der Heijden (1993, page 108) addressed the potential for multiple sensory neuron receptors
to account for the large range of chemical structures contributing to the sensation. Lindley
(page 131), in the following article relating to sweetness antagonists reaches the opposite
conclusion, “Currently available evidence is consistent with the conclusion that these sweetness
inhibitors are competitive antagonists of sweet taste acting at a single receptor structure.” The
limited set of stimulants used in prior multi-dimensional analyses cannot totally rule out this option.
However, it appears unlikely after exploring the architecture of the neural system. Lindley (page
131), in an article following that of Van der Heijden and relating to sweetness antagonists
reaches the opposite conclusion, “Currently available evidence is consistent with the conclusion
that these sweetness inhibitors are competitive antagonists of sweet taste acting at a single
receptor structure.”
The analysis of the models of taste cells available to Van der Heijden led him to draw much the
same conclusions as in this work. [xxx tabulate ]
8.5.4.1.1 Review of the historical database BRIEF
The database of the sugars is extremely large and cannot be reviewed here in a reasonable
number of pages. See xxx for an extensive review. A more limited tabulation is provided in the
van der Heijden papers of 1985. Walters, Orthoefer & DuBois edited a large compendium of
papers on Sweeteners in 1991158. In summary, that volume described a variety of attempts to
come up with a practical theory of gustation related to both natural and artificial sweeteners.
No satisfactory theory evolved from that work.
Shallenberger (1993, section10.7.3) provided calculated values of the equilibrium constant for
sucrose, galactose and an unspecified enzyme acting as the gustatory receptor based on the
data of McBride and of Maes. He did note that minor changes in the amino acids of the
assumed enzymes could be significant in his calculations. He offered no data supporting the
presumed participation of enzymes or proteins as receptors in the gustatory process
8.5.4.1.2 Chemical identification of large classes of sugars (saccharides)
158
Walters, D. Orthoefer, F. & DuBois , G. eds. (1991) Sweeteners: Discovery, molecular design and
chemoreception Washington, DC: American Chemical Society ACS symposium #450
114 Neurons & the Nervous System
The sugars (saccharides) are invariably soluble and polarize electronically but do not ionize.
Therefore, they are not described as electrolytes.
To simplify the following initial discussion, only stimulants belonging to the polyhydroxyl aldehyde
or ketone sugars will be considered. A broader discussion will follow this section. Any
polysaccharides (particularly sucrose) applied to the chemical sensing modalities will be
assumed to be hydrolyzed into monosaccharides by the mucosa.
The resulting
monosaccharides will be considered non-electrolytes. Jakinovich has noted a requirement that
the three hydroxyl groups of the monosaccharide must be in the equatorial positions for the
sugar to be sensed by flies159. His graphics also suggest the critical steric feature of the
monosaccharides is the presence of an oxygen in the ring at position 6 adjacent to a carbon
at position 5 that is supporting a CH2OH group. While on the right track, these assumptions are
modified below.
---All monosaccharides are reducing sugars. They will reduce Fehling’s, Benedict’s, Nylander’s or
Tollens’ reagents. Fehling’s reagent involves a Cupric ion complexed with tartrate ions.
Benedicts’s reagent involves a Cupric ion complexed with a citrate ion. Nylander’s reagent
involves bismuth complexed with a tartrate. Tollen’s reagent contains the diamminesilver(I) ion,
[Ag(NH3)2]+. These reactions produce products that are frequently highly colored. This
coloration may play a part in explaining the coloration of the mucus associated with the
olfactory sensory neurons.
Wikipedia provides the following description of a reducing sugar.
A reducing sugar is any sugar that, in basic solution, forms some aldehyde or ketone. This
allows the sugar to act as a reducing agent, for example in Benedict's reaction.
Reducing sugars include glucose, fructose, glyceraldehyde, lactose, arabinose and
maltose. Significantly, sucrose and trehalose are not reducing sugars.
Benedict's reagent is used to determine if a reducing sugar is present. If it is a reducing
sugar, the mixture will turn brick red. Fehling's solution can also be used for the same
purpose, as both contain copper (II) ions, which are reduced to a brick red precipitate of
copper (I) oxide when the solution is heated. Copper (II) forms a distorted tetrahedral
coordination geometry in organic molecules160 that is not directly related to its
conventional inorganic structures based on its valence of Cu2+. Cu(I) and Cu(II) both
exhibit a conventional valence of 2+ even though they coordinate with four other atoms
or ligands (Spiro, pg 114). Electron parametric resonance (EPR) measurements clearly
identify three types of Copper in these molecules, Cu(I), Cu(II) and Cu(III) that are
illustrated by Spiro (page 122). The complexity of the coordination of copper in a protein
is illustrated on pages 35 and 310 of Spiro.
A reducing sugar occurs when its anomeric carbon is in the free form. Since sugars occur
in a chain as well as a ring structure, it is possible to have an equilibrium between these
two forms. When the hemi-acetal or ketal hydroxyl group is free, i.e. it is not locked, not
linked to another (sugar)molecule, the aldehyde (or keto-) form (i.e. the chain-form) is
available for reducing copper (II) ions. When a sugar is oxidized, its carbonyl group (i.e.
aldehyde or ketone group) is converted to a carboxyl group. [xxx This last sentence can
only be interpreted for sugars described using a straight chain chemical representation.
All monosaccharides present in solution are properly represented using a ring
representation (Lehninger, pg 220)]
----
159
Jakinovich, W. (1981) Comparative study of sweet taste specificity In Cagan, R. & Kare, M. eds.
Biochemistry of Taste and Olfaction. NY: Academic Press pg 119
160
Spiro, T. ed. (1981) Copper proteins. NY: John Wiley & Sons pg 6
Signal Generation & Processing 8- 115
As noted above, there appears to be a major problem with consistency among texts, journal
articles and Jmol files of the last quarter century attributed to a specific molecule perceived as
sweet. This is particularly apparent among the sugars where multiple representations are
available for a variety of the most studied sugars. The problem is exacerbated by the multiple
configurations, conformations and anomers of the sugars and the continual revisions of the
IUPAC naming rules during the 1980's.
+, – describes the rotation of polarized light by the sugar of interest (page 74+ in Morrison & Boyd,
2nd Ed, 10th Printing, 1971).
D, L– describe the configuration of the molecule about the lowest asymmetric carbon atom, the
carbonyl atom being at the top of the Fischer Diagram (regardless of the direction they rotate
polarized light (M & B, page 1000).
α−, β– describes the configuration of the two enantiomers of a given sugar when in their cyclic
form (when they are in solution) based on their degree of rotation of polarized light. The α
version causes greater rotation than the β version. The difference is related to the phenomenon
of mutarotation involving the orientation of the H and OH groups about the C1 carbon (pages
1002-04). The α– and β– forms of otherwise identical sugars are diastereomers differing only in
their configuration relative to the C1 and are therefore defined as anomers (pages 1002-06).
After a period of time in solution, a fixed temperature sensitive ratio between the α– and β–
components is achieved.
Morrison & Boyd also highlight the problem in illustrating the cyclic sugars using either the Fischer
Diagram or the Haworth Diagram when discussing the above conditions (page 1003). Only the
conformational representations properly illustrate the position of the CH2OH moiety of the sugar
properly. These moieties are almost universally in the equatorial position relative to the ring
structure rather than perpendicular to it as suggested by the Haworth Diagram. However, while
in the equatorial position, it is important to point out these moieties can either point up or down
(by about 19 degrees) relative to the position of the OH groups associated with the ring of a
particular sugar.
Variations in the above parameters are very important in determining the precise d-value of a
specific sugar.
8.5.4.1.3 The unique “sugar alcohols” or glyco-alcohols
The name sugar alcohols, although of long-standing, appears inappropriate in the context of
gustation. Many of these molecules lack the heterocyclic structure of the saccharides and
frequently do not exhibit a ring structure at all (ex. mannitol_6015). Mannitol and many other
molecules bearing this label do exhibit the glyco-alcohol structure required to be perceived as
sweet (two hydroxyl groups separated by two carbons, C–C, that continue to assume an open
but cyclic stereo structure..
Inositol is described above as an important “sugar alcohol involved in forming the sensory
receptors. This sugar alcohol does exhibit a cyclohexane ring structure.
This work would recommend the term sugar alcohol be replaced by glyco-alcohol (from the
perspective of gustation) since the chemical is perceived as sweet but the glycophore
stimulating the receptor channel is not attached to a heterocyclic saccharide.
8.5.4.1.4 The unique “sugar acids” or glyco-acids
A class of chemicals perceived primarily as sweet are the sugar acids, named primarily by their
method of fabrication rather than their chemical structure. They are similar to the sugar alcohols
in this respect (Section 8.5.4.1.3). See also Section 8.5.8.3). There are three common groups of
these molecules, the aldonic, aldaric and uronic acids. As in the case of gluconic acid_10240,
they exhibit an even less complete ring structure while incorporating a carboxylic acid group.
[xxx may want to eliminate d-values between pairs of C=C bonds. ]
The JSmol file for gluconic acid, when viewed via the DS 4.1 representation, exhibits d-values of
2.075, 2.076, 2.704, 2.722, 2.803, 2.819, 2.934 Angstrom and many higher values. The hypothesis
of this work indicates this molecule is very effective in exciting both the GR 1 and GR 2 channels
116 Neurons & the Nervous System
of gustation via multiple DACBs and the potential for marginally exciting the GR 3 channels.
However, the molecule is not a saccharide or a cyclohexane.
8.5.4.1.5 Sweetness antagonists (inhibitors)
Lindley provided a chapter in Acree & Teranishi (1993) on molecules that antagonized the
perception of sweetness. Many of his sweeteners belonged to the class of “super sweet”
molecules discussed in Section 8.5.4.2. His review of the state of the art at that time is particularly
useful, although many of the open questions can be resolved based on this work (specifically
the fact that the signals of the chorda tympani are orthodromic to stage 2 signal processing in
gustation. These signals are frequently based on taking the difference between signals
produced by stage 1 sensory receptors supporting different signal paths.
Shallenberger (1993, section 10.8) discussed the selective and non-selective inhibition of the
sensory receptors. It is also appropriate to discuss the inhibition achieved by interfering with the
gustaphores, by either binding to them or by affecting their solubility in the mucosa.
Shallenberger suggested the gymnemic acids may impact the receptors by indicating they had
to be applied to the tongue before other stimulants. He also noted that several materials,
including the gymnemic acids were “surface active” tast modifiers. Ziziphin_390256, is a very
complex triterpene glycoside that is considered a taste-modifiers. It could form a DACB with a
variety of gustatory receptors. After chewing leaves from the ziziphins, solutions sweetened with
sucrose taste like water. The anti-sweet activity is reversible, but sweetness recovery on the
tongue can take more than 10 minutes. However, it has no effect on the perception of the
other tastes, bitterness, sourness and saltiness161. These characteristics suggest the ziziphins are
selective gustatory receptor (GR 2) inhibitors.
8.5.4.2 Operation of the “super sweet” sensory neuron & AH,B,X
Beginning in the 1970's, several sweeteners became prominent that offered a sensation of
sweetness far exceeding the previous natural sweeteners. Kier (1972), followed by Shallenberger
& Lindley (1977) explored the potential chemical structures that could account for this
circumstance. Both groups defined slightly different mechanisms that involved a third element
of a tripartite arrangement based on the previous bipartite, AH,B, arrangement. The 1977 paper
discusses distances with reference to the two orbitals without considering the location of the
hydrogen atom. They described this feature as hydrophobic in nature and capable of
participating in a “dispersion” bond (a form of van der Waal bond) with an appropriate
hydrophobic bond on the sensory receptor. Figure 8.5.4-1 shows the arrangement of the
glycophores of two common sugars based on this tripartite concept.
161
Kurihara, Y. (1992) Characteristics of antisweet substances, sweet proteins, and sweetness-inducing proteins
Crit. Rev. Food Sci. Nutr. 32 (3): 231–252. doi:10.1080/10408399209527598
Signal Generation & Processing 8- 117
Figure 8.5.4-1 The “super sweet” tripartite glycophores of two sugars. The glycophores of the
stimulants are descibed by the shaded areas and the subscripts C. Left; the C-6 methyl carbon
acts as the hydrophobic feature. Right; the C-6 methylene carbon acts as the hydrophobic
feature. The term γ of Shallenberger & Lindley is interchangeable with the term X of Kier. From
Shallenberger & Lindley, 1977
Shallenberger & Lindley also described the glycophores of β-D-glucose and β-L-glucose using
the Haworth notation to show how they interacted with the same undefined sensory receptor
in a tripartite bond. These sugars show the same degree of sweetness.
The Kier paper discusses several more exotic sweeteners, involving nitrogen and sulfur chemical
elements.
Immel made an important assertion in the abstract to his chapter 4 regarding the MEP of the
super sweeteners, “Most informative in regard to the placement of the tripartite AH-B-X
glucophore are the hydrophobicity distributions, which show the lipophilic X-part to be an entire,
obviously quite flexible, region rather than a specific corner of the ‘sweetness triangle’.” See
Section 8.5.5.1.3.
118 Neurons & the Nervous System
[xxx need to reference or consolidate parallel molecule structure in Figure 8.5.-51 ]
See Section 8.5.10.3 for additional information related to the operation of super sweeteners.
8.5.4.2.1 The unique non– saccharide sweeteners EMPTY
Van der Heijden and colleagues provided an extensive report in 1985 on the SAR properties of
two groups of sweeteners that included the saccharins for completeness162,163.
The
nomenclature in the second paper, involving four parameters, differs from the three parameters
used in this work. [xxx expand ]
8.5.4.3 Operation of the “acidic” gustatory sensory neuron
While an acid sensing channel has been associated with the gustatory system since historic
times, reports on the related transduction process are virtually nonexistent in the scientific
literature. It is easy to assume any acid transduction would involve a hydrogen ion, this may be
an oversimplification. Referring to both recent history and his limited range of subject species,
Boudreau has noted164, “The system was labeled ‘acid’ because the most stimulating
compounds were Brknsted acids and the least stimulating were Brknsted bases. The most
excitatory compounds were carboxylic acids for all species. Also stimulating, but at a variable
rate, were phosphoric acids and a small number of nitrogen compounds functioning as Brknsted
acids.”
This work will show that earlier studies, including that of Boudreau were erroneous. The acidic
gustatory sensor neurons of animals are sensitive to and based on organic (Lewis) acids. They
do not involve any activity related to an isolated hydrogen ion. It will also be shown that the
phosphoric acids and nitrogen compounds function as normal Lewis acids and not as some
unique form of Brknsted acids.
8.5.4.3.1 Background
Boudreau’s comments suggest the subtleties between the classic acids, Lewis acids and Brknsted
acids need to be reviewed before attempting to understand the acidic sensory receptors.
A Brknsted acid is a proton donor and a base is a proton acceptor. The classic (or Brknsted) acid
ionizes to form a simple proton that may or may not be hydrated, H[H20]n+. Pitzer165 has discussed
the hydration of the hydrogen ion in water at biological temperatures and pressures is typically
H[H20)1+.
From a practical perspective, Lehninger has also noted, “the formalism introduced by J. N.
Brknsted and T. M. Lowry is more useful than that of Lewis in describing acid-base reactions in
dilute aqueous systems.” An acid-base reaction always involves a conjugate acid-base pair,
made up of a proton donor and the correspond proton acceptor.” In transduction, no acidbase reaction occurs but dilute aqueous systems are generally involved.
The more recent development of the Lewis Acid concept appears crucial to transduction.
162
Van der Heijden, A. van der Wel, H. & Peer, H. (1985) Structure-activity relationships in sweeteners. I.
Nitroanilines, sulphamates, oximes, isocoumarins and dipeptides Chem Senses vol 10(1) pp 57-72
163
Van der Heijden, A. van der Wel, H. & Peer, H. (1985) Structure-activity relationships in sweeteners. II.
Saccharins, acesulfames, chlorosugars, tryptophans and ureas Chem Senses vol 10(1) pp 73-88
164
Boudreau, J. (1989) Neurophysiology and stimulus chemistry of mammalian taste systems In Teranishi, R.
Buttery, R. & Shahidi, F. eds. Flavor Chemistry: Trends and Developments. Washington, DC: American
Chemical Society Chapter 10
165
Pitzer, K. (1982) Self-ionization of water at high temperature and the thermodynamic properties of the ions
J Phys Chem vol 86, pp 4704-4708
Signal Generation & Processing 8- 119
Lehninger (page 46) has noted, “A Lewis acid is defined as a potential electron-pair acceptor,
and a Lewis base is a potential electron-pair donor. The Lewis acid concept has led to the
description of a broad range of covalent and coordinate chemistry bonds in organic chemistry.
In acid channel transduction, the use of the Lewis concept appears to offer compatibility with
the other coordinate chemistry concepts applicable to the other gustatory channels. This is
particularly appropriate where oxygen is involved and it contributes a pair of electrons in a
coordinate covalent bond (as in the AH,B coordinate arrangement).
According to Morrison & Boyd (page 29), the Lewis acid concept is the most fundamental of the
acid concepts. They also note, “To be acidic in the Lowry-Brknsted sense, a molecule must, of
course, contain hydrogen.” This is not a requirement with regard to Lewis acids. However, there
may be additional requirements on the stimulus, as noted above, besides presenting an
available hydrogen ion.
Ohloff has presented a theory of A-path sensory receptor operation involving a specific
configuration of sensory receptor capable of hydrogen bonding166. It appears to represent a
hybrid of the sensory mechanism proposed below. Basically, the sensory receptor is capable of
closing a ring by employing a hydrogen bond contributed by the stimulus. There is no DACB
relationship. He does not indicate how his proposal accommodates a Lewis acid.
It is useful to note that in biochemistry, an apparent disassociation constant, K’, is used based on
the analytical conditions of the investigator rather than the more precise disassociation constant,
K, of thermodynamics.
It is useful here to consider the inorganic acids as largely outside the realm of natural foods and
the sharing of pairs of covalent bonds as the typical situation in gustatory transduction involving
organic acids. This formalism allows the ammonium radical, NH4+ to be treated as an organic
acid. It is also compatible with treating the carboxyl group as the primary component of most
of the organic acids.
A variety of phospholipids are known to be present on the outer surface lemma of the dendritic
surfaces of gustatory sensory neurons. Specific molecules from these families, called
plasmalogens (or phosphatidal derivatives) by some investigators, with one of the lipid chains
unsaturated, are potential candidates for the sensory receptor role in the A-path sensory
channel. A slightly more complex arrangement involves the presence of the same
plasmalogens in a liquid crystalline surface structure exhibiting electrically conductive features.
Such structures have become very significant in the display industry because of their remarkable
electrical properties.
8.5.4.3.2 Proposed acidic channel sensory neuron receptor
Based on this discussion and those of the previous sections, the sensory receptor of the A-path
(organic acid sensitive) sensory channel is likely to be a structure that can form a coordinate
structure with the carboxylic acids. A likely candidate would be PtdSer, a phospholipid known
to be present in the lemma of neurons and to contain a carboxylic acid group as shown in
Figure 8.5.4-2.
166
Ohloff, G. (1981) Bifunctional unit concept in flavour chemistry In Schreier, P. ed. Flavour ‘81. Berlin:
Walter de Gruyter pp 757-770
120 Neurons & the Nervous System
Figure 8.5.4-2 Proposed phosphatidylserine shown in polar form This molecule is proposed as the
active component of the GR 1 sensory receptor of gustation. It is commonly found on the
surface lemma of the dendritic structure of gustatory sensory neurons.
In another depiction, the serine ligand can be described as in Figure 8.5.4-3. This ligand offers
a variety of interpretations supporting an AH,B type coordinating situation. The two oxygen
atoms shown on the right provide a natural receptor for any other carboxylic acid group seeking
to form a DACB with a sensory receptor. The oxygen on the left is expected to readily form an
ester with phosphatidic acid. The presence of the amino nitrogen makes it possible for this
molecule to form DACBs with other than the carboxylic acids but is believed to be inhibited from
this action by the stero-chemistry of the overall phosphatidyl serine and its similar molecules
forming a liquid crystal on the surface of the outer lemma.
Discussions of carboxylic acid chemistry
frequently begin with a dimer as shown in
Figure 8.5.4-4. While typically shown with R
and R’ identical in initial discussions, this is
not a criteria. The structure shown is planar.
R and R’ may be quite different without
causing major differences in the coordinate
bond relationship. Where the differences
Figure 8.5.4-3 Serine as the potential sensory
receptor ligand. The oxygen at upper left forms
the ester with the phospholipid. The carboxyl
group at right forms the sensory receptor site of
the acid channel sensory neuron.
introduce strains in the bond configuration, the
Signal Generation & Processing 8- 121
transduction process may be less effective and the sensation of“acidity” reduced for the
stimulus. Almenningen et al. have provided detailed dimensions for the gas phase monomer
and the dimer of formic acid167. They provide statistical limits on their parameters after reviewing
the earlier work of others. Their statistics may be influenced by the presence of the hydrate of
formic acid, methanediol. Interestingly, the so-called hydrogen bond is measured from the
centers of the two oxygen atoms.
It is proposed that this
formation of a dimer
between the serine of
PtdSer and a carboxylic
acid stimulus constitutes the
major transduction process
in the (organic) acid-best
channel of the gustatory
modality.
The
transduction
requirement is that the
stimulus form a coordinate
Figure 8.5.4-4 A gas-phase carboxylic acid dimer EDIT. The bond with the carboxyl
dimensions are shown for the carboxyl group of the serine portion group of the serine, thereby
of phosphatidyl serine acting as the (organic) acid receptor of changing the dipole
the gustatory modality.
potential at the base of the
first Activa formed by the
asymmetrical bilayer lemma of the sensory neuron microvilli. Any stimulus capable of forming
a coordinate bond with the serine ligand will elicit a sensation of acidity within the neural system.
The hydrogen bond in each leg of such a dimer typically has an energy of 2-10 kCal/mole. This
low energy illustrates the ease with which the bond can be broken following the transduction
process.
As suggested by the difference in bond lengths relating to the left carbon, resonance does not
appear to play a role in this structure. However, more analysis may be warranted. Valla has
described nine different coordinate bonding arrangements for the carboxylic acid group, three
that involve two bonding paths168.
Because of the carboxyl group in their intrinsic structure, all amino acids can coordinate with the
proposed acid sensory receptor and stimulate the acid sensory channel to a degree. Boudreau
(page 130) reviewed the relative intensity of these sensations in several species.
---The d-value for phosphatidyl serine present as a liquid crystal embedded in the outer lemma of
the cilia of a sensory neuron is desired. At the present time, only a variety of values determined
under different conditions are available, Figure 8.5.4-5. The top two values are shown for
reference when considering whether the organic acid receptor might be sensitive to other
stimulants. The bottom value for the resonant carboxylic group is also shown for reference.
Eliminating these values, the remaining values range from 2.223 to 2.34 Angstrom. with the only
measured value at 2.268 Angstrom. All other values are calculated from poorly documented
models and bond lengths. [It must be noted that AminoAcidsGuide.com on the internet offers
no references to its authenticity. Its source is hidden behind a privacy shield.} Based on the Jmol
file from ChemSpider, the d-value for the carboxyl group of phosphatidyl serine in liquid
crystalline form on the surface of the receptor neuron lemma will be taken as 2.276 Angstrom.
167
Almenningen, A. Bastiansen, O. & Motzfeldt, T. (1969) A reinvestigation of the structure of monomer and
dimer formic acid by gas electron diffraction technique Acta Chem Scand vol 23(8), pp 2848-2864
168
Valla, V. & Bakola-Christianopoulou, M. (2007) Chemical aspects of organotin derivatives of beta-diketones,
quinonoids, steroids and some currently used drugs: A review of the literature with emphasis on the medicinal
potential of organotins Synth React Inorganic, Metal-Organic, and Nano-Metal Chem vol 37, pp 507–525
122 Neurons & the Nervous System
8.5.4.3.3 The polarized
forms of carboxylic
acids
Serine is typically described
as an amino acid with an
uncharged polar group (but
with a polarized amino
group NH2+CCOO –. The
form of the proposed
organic acid receptor is
based on the calculated dvalue of the neutral, nonFigure 8.5.4-5 Potential d-values for phosphatidyl serine. The top resonant carboxylic acid
The biochemistry
two rows and the bottom row are shown for reference. The only form.
measured d-value is 2.268 Angstrom for a dimer of Formic acid as l i t e r a t u r e f r e q u e n t l y
a gas by Almenningen et al. This value is also near the median for represents the carboxylic
acid group as COO– or
the group of O, OH distances. See text.
alternately as a hydrated
acid of the form COHOH
with a net positive charge. Either of these configurations would suggest different bond lengths
and angles resulting in a different d-values. It is necessary to determine what is the structure of
the carboxyl group when it is part of the complete phosphatidyl serine molecule with the
carboxyl group exposed to the solution of the saliva.
Looking at formic acid for a moment, it is a polar compound that is freely soluble in water in any
proportion and are thus completely soluble in water169.
Akiya & Savage have addressed the form of formic acid in solution.170 They showed that formic
acid decomposed in the presence of water but all of their results were for a temperature of 700
Kelvin and the necessary multiple atmosphere pressure. They appear irrelevant to the biological
conditions. Under biological conditions, it appears the relevant influences are the potential
polarization of the parent phosphatidic acid and the pH of the saliva.
Aloisio et al. addressed hydrogen bonding between formic acid and water at laboratory
temperatures in the formation of aerosols171. Zhou et al. also addressed hydrogen bonding
between water and formic acid in solution using some of the same graphics172. The graphics did
not differentiate between single and double bonds to oxygen. Both studies involved
computational chemistry. Many of the configurations studied would preclude the carboxyl
group acting as a receptor for other than water in the absence of some agent to prevent this
type of hydrogen bonding with water. It is possible that it is necessary to incorporate a more
realistic saliva in the environment of the formic acid.
Zhou et al. did provide a discussion of dipole moments and bond lengths under different
conditions.
The dipole moments of the complex in solution and in the gas phase are 1.78 and 5.22 D,
respectively. Such a large variation in the dipole moment produces a large change in the
169
Xxx see but do not cite my copy of a discussion in ref files/Formic acid in water
170
Akiya, N. & Savage, P. (1998) Role of Water in Formic Acid Decomposition AIChE J vol 44(2), pp 405-415
171
Aloisio, S. Hintze, P. & Vaida, V. (2002) The Hydration of Formic Acid J Phys Chem A vol 106(2), pp
363-370
172
Zhou, Z. Shi, Y. & Zhou, X. (2004) Theoretical Studies on the Hydrogen Bonding Interaction of Complexes
of Formic Acid with Water J Phys Chem A vol 108(5), pp 813-822
Signal Generation & Processing 8- 123
solute-solvent interaction energy that favors the aligned configuration. For the R2, it is
shortening by 0.042 Å in solution compared to the gas-phase value. The angle C–O - - -H
varied from 103.9/ to 147.7/, while O- - -H–O changed from 147.9/ to 160.9/. Differences in
other bond lengths between the gas-phase and solution-phase results are less than 0.020
Å, and differences in other angles are less than 4/.
These changes in dipole moment and angle appear to be significant.
Lehninger (page 196) shows PtdSer exhibits a complex polarization showing a negative value
near the terminal carboxyl group, a positive value near the amine group and another negative
value near the point of esterification. How, these values change with incorporation into the
liquid crystal of the lemma and connection to the first Activa of the neural system is not clear at
this time.
Lehninger (pages 46-49) has discussed the effect of pH on the “apparent disassociation”
constant, K’, of formic and acetic acid as a function of pH at 25o C. The apparent disassociation
constant of the biochemist differs from the disassociation constant, K, of the physical chemist.
At pH below 5.0, the dominant acetic acid species is the un-disassociated species. Formic acid
is more easily disassociated with disassociation dominant above 4.0 pH. Both chemicals exhibit
considerable buffering capacity over several whole number values of pH. Saliva pH is typically
6.35–6.85.
The tendency of a polar molecule organic to form a shell of water molecules isolating it from the
surrounding water would suggest a layer of water hydrogen bonded to the receptor that must
be discouraged by some mechanism (possibly catalytic or enzymatic) to support efficient
gustatory sensing.
Further discussion of this situation will require experimentation. For the present the best empirical
estimate of the d-value of PtdSer appears to remain the value of 2.268. However, the best
calculated value for PtdSer appears to be 2.276 from the Jmol file of ChemSpider (provided by
the Royal Society of Chemistry).
8.5.4.3.4 The perception of carboxylic acid derivatives as acids
The range of carboxylic acid derivatives is quite large in organic chemistry. Many of these are
formed through esterification of the hydroxyl group. The result is a very large number of
molecules (the acetates) that still exhibit a d-value near 2.276 Angstrom due to the continued
presence of two oxygen atoms capable of sharing their electron-pairs in a DACB. These
acetates, if soluble, stimulate the GR 1 channel and are perceived as acidic.
124 Neurons & the Nervous System
8.5.4.3.5 The perception of inorganic acids as nocents–HCl
Only a few inorganic acids have played a historical role in gustation, primarily HCl which has
frequently been employed as a standard absent any theoretical foundation. Most inorganic
acids are strong relative to organic acids and considered toxic to the biological system. In the
terms of chemical sensing, they are generally considered nocents at low concentrations and
capable of serious structural damage (including destruction of tissue) at high concentrations.
Most inorganic acids are totally ionized in solution and it becomes the hydrogen ion that is
actually of interest. This ion does not exist in an isolated form except in elementary chemistry
books. It is invariably in a coordinate chemical arrangement of the form H+:(H2O)n. Similarly,
most of the negative ions of inorganic acids do not exist in their elementary form in solution.
Quoting Powell et al.“ the number of water molecules in the first coordination shell of Cl– (the
hydration number) is #6 and depends on both the ionic strength and the type of cation173.”
Inorganic acids do not participate in coordinate bonding in their nominal formula state. Neither
the proton or most of the residues other than the proton exist in their simple ionized configuration
when in solution. Rather, they are typically hydrated. In the case of hydrochloric acid_307, the
bond length of the gas is 1.327 Angstrom. In solution, the proton is generally hydrated in a
structure consisting multiple water molecules, typically H5O2+, H7O3+ or H9O4+.. The simplest of
these hydrates exhibits two O- -H bonds. Assuming these bonds are in line, the oxygen atoms
could participate in a DACB with an appropriate receptor of the gustatory modality. This
hydrate exhibits a d-value of 3.48 Angstrom and results in the stimulation of GR 3 and the
perception of a salty material, not an acidic material.
Pauling & Marsh addressed the hydration of chlorine gas in 1952174. Their paper is very
informative and suggests it is the hydrate of chloride that is important in gustation related to
hydrochloric acid. They predict the chlorine is encased in a shell of water (6Cl2C46H2O) with the
oxygen atoms forming much of the outer surface. The result is a chathrate, a compound in
which molecules of one component are physically trapped within the crystal structure of
another. Bieze et al. have more recently studied the hydration of the chloride ion175. Their
conclusion was that Cl– was coordinately bound with six molecules of water resulting in a d-value
between the oxygen atoms of 4.44 to 5.45 Angstrom. This range would be ineffective with
respect to the primary gustatory receptors. However, it would be relevant to the olfactory
modality (Section 8.6.xxx). Foresman & Brooks have reported on the hydration of the chloride
ion in solution based on ab initio calculations176. Their analyses converge in general agreement
with the conclusion of Bieze et al.
These levels of hydration suggest the inorganic acids will disturb gustation by seeking to acquire
water molecules from the mucosa, saliva or cells of the body, rather than by forming a dual
coordinate bond (DCB) with the receptors.
Picric acid is an interesting case and discussed in more detail in Section 8.5.4.5. Its name is
ancient, suggesting its bitter taste was often confused with the sour taste of organic acids. It is
an organic carbohydrate compound but does not contain a carboxyl group. It can become
a Lewis acid through the loss of the hydrogen of the hydroxyl group or be considered a negative
173
Powell, D. Barnes, A. Enderby, J. et al. (1988) The hydration structure around chloride ions in aqueous solution
Faraday Discuss Chem Soc vol 85, pp 137-146 DOI: 10.1039/DC9888500137
174
Pauling, L. & Marsh, R. (1952) The Structure of Chlorine Hydrate PNAS vol 38, pp 112-118
175
Bieze, T. Tromp, R. van der Maarel, J. et al (1994) Hydration of Chloride Ions in a Polyelectrolyte Solution
Studied with Neutron Diffraction J Phys Chem vol 98 (16), pp 4454–4458
176
Foresman, J. & Brooks, C. (1987) An ab initio study of hydrated chloride ion complexes: Evidence of
polarization effects and nonadditivity Chem. Phys. vol 87(10), pp 5892-5895
Signal Generation & Processing 8- 125
ion by internal charge rearrangement. However, its major role in gustation is as a tastant
containing three identical gustaphores with d-values of 4.746 Angstrom that stimulate the
distinctly separate picric pathway significantly.
8.5.4.4 Operation of the “alkaline” gustatory sensory neuron
The alkali metals and the alkali earth metals are distinctly different and should not be lumped
together when considering their theoretical taste performance. The alkali metals (Lithium,
Sodium etc.) are singularly ionized while the alkali earth metals (Calcium, Magnesium etc.) are
typically doubly ionized. Of greater importance is their different coordinate bonding potential.
It was found earlier that the alkali metal ions were perceived as sweet at low concentrations and
this is believed to be due to their hydration resulting in an AH,B coordination capability with the
G-Path receptors.
Avenet & Lindemann (1989) have noted177, “Of the diversity of tastes induced by inorganic salts,
that of NaC1 has found primary attention. For man, NaC1 is the only substance which has a pure
salty taste, and the sodium ion appears to be more important for this taste than the anion.”
Quist & Marshall178,179 have described the hydration of NaCl and Na+ & Cl–. They note the nearly
constant hydration, Na[H2O]6+, regardless of temperature for concentrations above 1.2
moles/liter and NaCl[H2O]20 below 1.2 moles/liter. Zumdahl has provided the hydration energy
of the sodium ion, presumably for the fully hydrated case, as –402 kJ/mol180. He also provided
the first ionization energy of sodium as +495 kJ/mol.
Marshall provided the ionic radii for Li, Na & K as 0.6, 0.98 & 1.33 Angstrom respecively181. He also
provided the atom radii for these materials as 1.56, 1.86 & 2.23 Angstrom.
Shallenberger (1996) noted a general consensus that only NaCl elicited a true salt taste. His
words may be too casual. If the coordinate chemistry of the sodium ion is the principle
contributor to the N-Path channel’s response, it should be clear that it is the sodium ion (possibly
at a specific level of hydration) that elicits the maximum (or “true”) N-Path channel sensation.
The presence of the chlorine ion is largely irrelevant.
Figure 8.5.4-6 illustrates the two most common states of hydration of the sodium ion. In dilute
solution, it is believed to form Na(H20)2+. The distance between the oxygen atoms of this hydrate
is 4.7 Angstrom. In more concentrated solutions, it is believed to form Na(H20)6+ with the water
molecules arranged at the vertices of an octahedron. The distance between the water
molecules, that can act as AH,B coordinate structures, are nominally 3.3 Angstrom
177
Avenet, P. & Lindemann, B. (1989) Perspectives of Taste Reception J Membrane Biol vol 112, pp 1-8
178
Quist, A. & Marshall, W. (1968) The independence of isothermal equilibria in electrolyte solutions
on changes in dielectric constant J Phys Chem vol 72(6), pp 1536-1544
179
Quist, A. & Marshall, W. (1968) Electrical conductances of aqueous sodium chloride solutions from
0 to 800" and at pressures to 4000 bars J Phys Chem vol 72(2), pp 684-703
180
Zumdahl, S. (1993) Chemistry, 3rd Ed. Lexington, MA: D. C. Heath page 329
181
Marshall, S. (1956) Sodium: its manufacture, properties, and uses. NY: Reinhold
126 Neurons & the Nervous System
Figure 8.5.4-6 The sodium ion at hydration levels of 2 and 6. Only one of the three pairs of water
molecules are shown on the right. See text.
The unique features of the sodium ion are two. First, the sodium ion is very small, nominally 0.95
Angstrom in diameter. Second, the ion is normally highly hydrated in aqueous solution. The
hydration number is usually six (with the 2p shell fully involved), although it has been reported to
Signal Generation & Processing 8- 127
be four, two or one in various special cases. With a hydration number of six, the ensemble
exhibits a face centered cubic arrangement, with the water molecules located at the vertices
of an octahedron. At a hydration number of six, the locations of the water molecules are at a
spacing that is ideally arranged for coordination with a sensory receptor such as the oxygen rich
PtdIns. The fully hydrated sodium ion exhibits three separate odorophores with a d-value of 3.3
Angstrom. This raises the probability that a given hydrate will be successful in coordinate
bonding to a suitable sensory receptor.
Figure 8.5.4-7 presents a potential hydrated sodium “dimer” similar to that shown for the carboxyl
group. The dimensions are taken from Hille182 who credits them to Pauling. This figure shows the
hydrogens of the water molecules are out of plane relative to the sodium and oxygen
molecules. This suggests that one of the hydrogens can be moved into the plane of the
molecules and form a hydrogen bond with a pair of electrons of the oxygen of the other
hydrate.
Figure 8.5.4-7 A potential hydrated sodium “dimer.” Each of the units displayed consist of one
sodium ion fully hydrated with six water (H2O)molecules, only two of which are shown. The short
solid lines represent the O–H bonds. The heavy dashed lines represent the H- -O (hydrogen)
bonds. The slight dog-leg between the O–H bonds and the H- -O bonds are typically ignored.
The distance between the hydrogen bonds is taken to be 3.3 Angstrom.
As in the case of the carboxyl dimer, this figure suggests the spacing between the AH,B group
of the sodium sensitive sensory receptor should optimally have a spacing near 3.3 Angstrom.
182
Hille, B. (1971) The Hydration of sodium ions crossing the nerve membrane PNAS vol 68(2), pp 280-282
128 Neurons & the Nervous System
This small size is optimally matched to the coordination geometry of PtdIns. This makes the
sensitivity of the N-Path receptor maximally sensitive to the sodium ensemble.
---[xxx merge with above or below ]
No Protein Data Bank (pdb) file has been located, that have been curated by a prestigious
source, describing the hydrated form of the sodium ion in solution. However, Megyes et al. have
recently studied the hydration of the sodium ion in considerable detail and note several
differences from the previously reported data183. The study focused on very caustic and viscous
solutions incompatible with the gustatory modality. Their results do point toward further analysis
but seem to favor a coordination number of six in cases of interest. They differentiate between
the distance between the oxygen atoms of two water molecules in a high molar NaOH solution,
using the notation OwC C COw. from two adjacent oxygen atoms associated with water molecules
in the hydration sphere of sodium ions using the notation, OC C CO They note on page 5,
“The OwC C COw distance was found to be between 2.80 and 2.85 Å. A drastic decrease of
water-water coordination number can be observed with increase in concentration,
showing that the hydrogen bonded structure of the bulk water is gradually destroyed and
it completely disappears at the highest solute concentration. On the other hand, other OC
C CO interactions appear with values in the range of 3.2–3.8 Å, arising from the interaction
of the oxygen atoms in the hydration sphere of the sodium ions. Unfortunately it is not
possible to describe quantitatively these interactions on the basis of x-ray diffraction
measurements because the corresponding peak is rather broad and blurred.”
The best available bond lengths (C–O = 1.43 Angstrom and C–C = 1.53 Angstrom) and angles
for the distance between pairs of adjacent opposed axial oxygen atoms for muco-inositol as
3.2435 Angstrom. This is a precise value of undefined accuracy as are other values below. No
measured data has been located relative to these materials.
8.5.4.4.1 The details/confusion related toPtdIns
The chemistry of PtdIns is poorly documented because of the potential complexity of the family;
the essentially unknown glycerides forming the sn-1 and sn-2 elements of triglyceride in a given
situation, and the inositol can be drawn from a family of at least 9 isomers. Even the combining
of these two moieties can occur under a variety of conditions involving the phosphate portion
of the triglyceride. Text books generally show sn-1 and sn-2 as straight unsaturated stearic acid.
However, most researchers show various other acids depending on the results of their research.
The two glycerides are seldom straight. The conformation of these molecules also depends on
the environment in which they are formed. The electrical conductivity of the glyceride chains
has not been studied in any detail.
Parthasarathy & Eisenberg reviewed the subject in 1991, noting a major change in nomenclature
in 1968. They also make confusing statements about the isomers of inositol. “Myo-inositol is the
only stereoisomer to occur in phospholipids (pg 6)” and “This finding suggests that myo-inositol
is not the only stereoisomer found in inositol phospholipids (pg 14).” Their figure 6 requires careful
study because the hydroxyl groups are omitted from the chair conformation diagrams they
present. A new set of rules was being prepared during preparation of their published article184.
These rules (labeled recommendations) surfaced several critically important facts. To this day,
• the recommendations only apply specifically to myo-inositol as indicated by their title,
• the recommendations are provided with loopholes in order to serve different communities and
• the rules applicable to a specific moiety change when it is associated with a second moiety.
Several of these situations are explicitly stated in the recommendation.
183
Megyes, T. Bálint, S. Grósz T. et al. (2008) The structure of aqueous sodium hydroxide solutions: a combined
solution x-ray diffraction and simulation study J Chem Phys vol 128 044501
184
NC-IUB (Moss, G. ed.) (1988) Numbering
http://www.chem.qmul.ac.uk/iupac/cyclitol/myo.html
of
atoms
in
myo-inositol
Signal Generation & Processing 8- 129
A notable fact is that the computational chemistry community has dispensed with using
the numbering rules of formal chemistry as defined by the IUPAC/IUBMB when developing
.mol and .pdb files. They have begun numbering the atoms sequentially according to
their own arbitrary scheme. See Section 8.6.10.4. In the case of standalone muco-inositol,
they have chosen an oxygen as atom number 1 and assigned #2 to its associated carbon
and #3 to its associated hydrogen, then proceeded clockwise to the next carbon which
becomes atom #4...
[xxx probably condense to one sentence in favor of the following material based on
Molecular Arts ]
[xxx edit to show the axial-trans– muco-inositol ]
Generic inositol, or cyclohexane-1,2,3,4,5,6-hexol is a chemical compound with formula
C6H12O6 or (-CHOH-)6, a sixfold alcohol (polyol) of cyclohexane with all of the hydroxyl
groups in the equatorial plane of the hexane ring (± 19 degrees).
Myo-inositol has been the most studied because of its occurrence in muscle tissue. Of
most importance in this work is muco-inositol. The study of myo-inositol has resulted in a
specific effort to standardize its terminology by the IUPAC/IUBMB, also known as the NCIUB, cited above. As of the 1988 report, several options were still allowed to support
different research purposes. The committee has generally avoided using Fischer Diagrams
in their most recent work. It has stressed the need to use at least the Haworth Diagram
and more satisfactorily, the chair or wire-frame presentation. With the more recent
evolution of the computer-aided 3D representations based on .mol, .mol2 and .pdb files,
they will undoubtedly be recommended by the committee in the future.
In its most stable conformational geometry, the myo-inositol isomer assumes the chair
conformation, which puts the maximum number of hydroxyls to the equatorial position,
where they are farthest apart from each other. In this conformation the natural myo isomer
has a structure in which five of the six hydroxyls (the 1st, 3rd, 4th, 5th, and 6th are
equatorial, whereas the 2nd hydroxyl group is axial. Figure 8.5.4-8 shows this configuration
combined with a partial view of hydrated sodium. The chair diagram is described as
muco-inositol 3-phosphate by Cosgrove in 1980185. However, the 1988 recommendations
of the NC-IUBMB would assign the name
muco-inositol phosphate based on their
recommended renumbering after
phosphorylation. The carbon associated
with the phosphorous atom is assigned #1.
Cosgrove spent less than two pages on
muco-inositol in his book.
The carboncarbon bond lengths are 1.53 Angstrom.
The carbon-oxygen bond lengths are 1.43
Angstrom. The net (out-of-plane) bond
length between the OH-3 and OH-4 have
been estimated to be about 3.3 Angstrom
without considering the puckering of the
ring.
It is described as muco
inositol_16736990 in the JSmol files of the
RSC. However, the JSmol protocols are
under review (see above & Section
8.5.1.1.2) No experimental value has been
Figure 8.5.4-8 Muco-inositol phosphate and a fully located. The current best estimate based
hydrated sodium. The muco-inositol is combined on the JSmol file and the DS 4.1 visualizer of
with the phosphate portion of a phosphatidyl computational chemistry is 3.648 Angstrom.
moiety and acting as the Na-channel receptor
through dual coordinate bonds with a dimension, The figure is approximately to scale but
d, of nominally 3.3 Angstrom. See text.
185
Cosgrove, D. (1980) Inositol Phosphates: Their Chemistry, Biochemistry and Physiology. NY: Elsevier pg
82
130 Neurons & the Nervous System
employs different display techniques to allow representation of the details related to the mucoinositol. In this representation, the two hydrogen bonds are antiparallel, one originating at the
sodium ensemble and the other at the hydroxyl group.
In 2012, the Molecular Arts software package prepared an alternate representation of inositol
without identifying the specific conformation.
8.5.4.4.2 The details/confusion related to muco-inositol
The name inositol comes from the Greek word for “substance isolated from muscle.” Whereas
myo-inositol has a long history in the food field, the role of the other stereo-isomers is largely
unknown. The prefix myo is clearly redundant but found useful to help organize the nine stereoisomers of inositol. The prefix muco- is suggestive of the presence of this variant in the mucus of
the nose and potentially the saliva of the mouth. Muco-inositol has been recognized since at
least the 1970's due to its role primarily relating to the external lemma of neurons.
The nomenclature of the very large number of isomers in the inositol family is confused,
partly due to a major change in 1968. Cosgrove described the history of the inositols up
to 1980 using a now archaic nomenclature based on the 1988 recommendations186.
Citing Brownstein, he did point out the minimum energy required to convert the generic
chemical between chair conformations via a boat conformation (pages 41 & 82).
Parthasarathy & Eisenberg have described the subsequent history and nomenclature up
to 1991 and describe it as “in statu nascendi”187. Even while they were writing their paper,
a subsequent change (in which they participated) was being negotiated in 1988. As a
result, any representation of inositol in the literature, such as that in the special issue of the
Biochemical Journal of 1986, must be questioned and placed in historical context188.
Glusker et al. described six different conformations for derivatives of cyclohexane and
more complex molecules in 1994 (top)along with a more definitive set of labels for the
bonds extending away from the basic ring, based on conventional drawing practice)
(bottom) in Figure 8.5.4-9 .
186
D. (1980) Inositol Phosphates: Their Chemistry, Biochemistry and Physiology. NY: Elsevier Chap. 1 & 2
187
Parthasarathy, R. & Eisenberg, F. (1991) Bio-, stereochemistry, and nomenclature In Reitz, A. ed. Inositol
Phosphates and Derivatives. Washington, DC: American Chemical Society
188
Special Issue (1986) The phosphates of phosphatidylinositol in blood Biochem J vol 238(2)
Signal Generation & Processing 8- 131
Figure 8.5.4-9 Representations of a six-member ring from Glusker et al., 1994. Top; conformations
of six-member rings. Bottom; dispositions of bonds in cyclohexene, a six-membered ring with one
double bond. From Glusker et al., 1994.
Majumder & Biswas, writing in 2006, have addressed the variation in the rules over the
years in detail189. Figure 8.5.4-10 reproduces their interpretation of the 1989 IUB rules
applied to the conformations of the inositols.
Wikipedia has presented a pair of representations, a modified Fischer and a chair that do
not appear to agree with each other even though the Merck Index is cited. Later on the
page, a set of isomers are presented that do not appear to agree with Majumder &
Biswas. The citation is to a 2006 thesis from the Imperial College London. A title but no
author or more detailed source is provided. The thesis was neither published or archived.
The stereochemistry associated with coordinate chemistry requires detailed knowledge of the
conformation of the requisite chemicals. Discussions of gustation based on Haworth, Mills,
zig-zag or Fischer projections190 can only be used where the underlying conformation has already
been described. Cosgrove has provided both Haworth and chair diagrams (also known as
Johnson/Tate diagrams) of the inositols that can be used to compare their utility (pages 4-6).
189
Majumder, A. & Biswas, B. (2006) Biology of inositols and phosphoinositides. NY: Springer Chap. 1
190
Lindehorst, T. (2003) Essentials of Carbohydrate Chemistry and Biochemistry. NY: Wiley pg 9
132 Neurons & the Nervous System
The Haworth diagrams do not indicate the equatorial character of the hydroxyls at all and the
early chair diagrams indicate the difference between the adjacent equatorial hydroxyls poorly.
Figure 8.5.4-10 Stereo-isomers of inositol. The major interest of this work is in muco-inositol and the
enantomiers, D-chiro- and L-chiro-inositol. The latter probably exist as a 50/50 mixture at body
temperature. Muco-inositol potentially offers two receptor sites meeting the d-value criteria for
a hydrated sodium sensory receptor. From Majumder & Biswas, 2006.
The muco-inositol of Majumder & Biswas exhibits a clear symmetry about the hydroxyl up group
between the two down hydroxyl groups and the opposite side of the molecule. In Figure 8.5.411, Shaughnessy has documented two conformations of muco-inositol in the answers to a final
exam at the University of Alabama. His two forms are consistent with that of Majumder & Biswas.
These presentations do not provide a numbering system for muco-inositol. To avoid introducing
additional numbers to this discussion, muco-inositol will be described using the 1C4 notation with
the equatorial hydroxyl at position 1 being assumed to esterify with the phospholipid to form the
gustatory sensory receptor or GR. The axial oxygen at O-4 can participate in a dual coordinate
bond with a gustaphore with either the adjacent axial-trans- O-3 or O-5. Each of these pairs
represents a natraphore with a d = 3.3 Angstrom.
Whichever oxygen participates in the phosphorylation, the carbon associated with that oxygen
becomes C1 based on the current IUPAC/IUBMB nomenclature cited above.
Signal Generation & Processing 8- 133
Figure 8.5.4-11 Chair conformations of muco-inositol. Left; two axial down and one axial up
providing two potential natrophores. The three equatorial hydroxyl groups provide two pairs of
glycophores. Right; same molecule observed from a different position. From Shaughnessy, 2011.
Brownstein studied the confirmation of the inositols using low frequency nuclear magnetic
The data is limited. He noted the inter-convertibility of the two chair
resonance191.
conformations with the axial groups becoming equatorial and vice versa.
Figure 8.5.4-12 shows the muco-inositol_16736990 clearly, except for the chair aspects of the 1C4
conformation. For discussion, the carbon at the bottom of the figure is defined as C-1. The
upper axial oxygen, O-4, can participate in a dual coordinate bond with either or both of the
axial-trans- hydroxyl groups at positions 3 or 5 (hidden by the ring carbons). The molecule
exhibits a mirror symmetry about the
vertical axis in this representation.
Dowd, French & Reilly have given energies
for a variety of inositol conformations and
noted in a scatter diagram the distance
between ab initio calculations and
Figure 8.5.4-12 Muco-inositol conformation. The
suggested numbering proceeds clockwise from
the bottom carbon in this figure. Any of the three
equatorial hydroxyl groups at the bottom of the
figure could have esterified with the phospholipids
of the sensory neuron lemma.
191
Brownstein, S. (1959) Shifts in Nuclear Magnetic Resonance Absorption Due to Steric Effects. II.
Polysubstituted Cyclohexanes J Am Chem Soc vol 81(7), pp 1606–1608
134 Neurons & the Nervous System
measured values for inositol as of 1996192. Simperler et al193. have provided extensive data on
the melting point and dimer formations of the isomers of inositol. The report includes a dimer of
muco-inositol defined by a double H-bond of the antiparallel type between one equatorial and
one axial hydroxyl group of each dimer. Muco-inositol only forms one dimer of this type. This
dimer configuration is not used in the gustatory modality. Due to its symmetry, muco-inositol is
optically inactive. Its melting point is 290 Celsius.
Simperler et al. note the fact that muco-inositol can form ring crystalline motifs where each
OH is involved in two distinct hydrogen bond links. This suggests each phospholipid sensory
receptor of muco-inositol can form an axial-trans- dual coordinate bond with two
gustaphores at once by having the middle axial-trans- hydroxyl participate into two
pairings (See Section xxx).
8.5.4.4.3 The gustaphores of the inositol ion
The inositol ion (inosinate–) contains considerable oxygen. As a result, it exhibits two calculated
glycophores with d-value near 2.82 Angstrom and two natrophores with d-value near 3.66
Angstrom. The d=3.66 natrophores are likely to form a dimer with the sodium channel receptor.
The d=2.82 glycophores may be less optimum and thus less effective in stimulating the
G–channel receptor.
The fact that some of the inosinates are perceived as salty, in the absence of any sodium
or inorganic salts, provides strong support for the hypothesis of this work, that the GR 3
channel sensory receptor is PtdIns. PtdIns can easily form a dimer with these inosinates.
Additional confirmation of the hypothesis are presented in Section 8.5.15.
8.5.4.4.4 The perception of sodium as sweet at low concentrations
[xxx see also comments re Kcl and LiCl in Dzendolet67.pdf ]
Shallenberger & Acree have noted the perception of sweetness associated with salt in solution
at low concentrations. The trait appears common among the low atomic weight alkali metal
salts, Li, Na & K.
The literature related to the hydration of the alkali ions at low concentrations is extremely thin.
The terminology is also contradictory due to age. An anhydrous compound, HNaO, is described
by the CAS Register as sodium hydrate. In solution however, the sodium ion is known to exhibit
a variety of states of hydration. It appears sodium can support a coordinate number of 1, 2, 4
or 6 depending on the circumstances, and particularly the concentration of the ion in water.
Shallenberger & Acree have suggested that Na[4], with a tetrahedral form, can create a
coordinate bond with the G-Path sensory receptors because of the spacing between the pairs
of hydrates, one acting as an electron acceptor and the other as an electron donor. Beginning
in the same era, Marshall has indicated the ion is generally fully hydrated, Na[VI]+1, at low
(gustatory) concentrations194. See Section 8.5.3XXX.4
The hydration of sodium at low concentrations explains the perception of a sweet taste to
sodium salts. The hydration level would also suggest that the sodium or N-Path sensory channel
is also a coordination chemistry based sensory channel that senses the sodium ion (without
sensing the associated negative ion).
192
Dowd, M. French, A. & Reilly, P. (1996) Analysis of Inositol Ring Puckering Austral J Chem vol 49(3), pp
327-335
193
Simperler, A. Watt, S. Bonnet, P.. Jones, W. & Motherwell, W. (2006) Correlation of melting points of
inositols with hydrogen bonding patterns Cryst Eng Comm vol 8, pp 589–600
194
Marshall, W. (2008) Aqueous electrolyte ionization over extreme ranges as simple fundamental relation with
density and believed universal; sodium chloride ionization from 0o to 1000oC and to 1000 mPa (10000 atm.)
Nature Precedings : hdl:10101/npre.2008.2476.1
Signal Generation & Processing 8- 135
----
8.5.4.5 Operation of the “bitter” gustatory sensory neuron
Bitter tastes are often associated with harmful substances like alkaloids and glucosides, but many
other groups elicit a bitter sensation.
The operation of the bitter sensory channel is not well documented. Belitz et al195. provided a
theory of bitter taste related to the early work of Shallenberger. They suggest a topological
requirement, involving a hydrophobic pocket, is involved in bitter or P-Path channel sensing. The
concept appears to correlate well with experience involving a wide range of amides, amines
and amino acids. However, Shallenberger & Acree noted in a larger context in 1971, “Only
limited correlations between bitter taste and molecular structure can be found.” In 2000, Eggers,
Acree & Shallenberger took a different position after reviewing the state of the art and noted,
“These results suggest that both sweet and bitter reception share the same transduction
components and that the non-sugar sweet receptor system is related to the bitter receptor
system if it is not in fact the same.”
Kumazawa provided a discussion of the varied chemistry of bitter stimulants and potential bitter
channel sensor operation but was unable to identify any receptor or provide any description of
the pertinent sensory neuron196.
[xxx merge with above and below
]
8.6.10.3 The detailed nomenclature of the picric channel stimulants
8.6.10.3.1 Review of the historical database
Data showing the relative perception of bitterness in humans is rare. Figure 8.5.4-13 shows a
table from Van der Heijden from 1993. Note that nearly all of the references are to one source,
the 1985 paper by Belitz & Wieser in the first issue of an obscure journal197. The other references
are primarily in German.
195
Belitz, H. Chen, W. Jugel, H. et al. (1981) Structural requirements for sweet and bitter taste In Schreier, P.
ed. Flavour ‘81. Berlin: Walter de Gruyter pp 741-755
196
Kumazawa, T. Nomura, T. & Kurihara, K. (1988) Liposomes as model for taste cells: receptor sites for bitter
substances including n-c=s substances and mechanism of membrane potential changes Biochem vol 27, pp 1239
197
Belitz, H. & Wieser, H. (1985) Bitter compounds: occurrence and structure-activity relationships Food
Reviews Internat vol 1(1), pp271+
136 Neurons & the Nervous System
There are a number of questions about this tabulation. As noted in this work, listing molecules by
their functional group or similar chemical categorization is not meaningful in gustation. The
presence of a specific ligand, as defined in this work, is the proper criteria. The presence of that
ligand in a chemical does not make it a good representative of the chemical category listed.
Figure 8.5.4-13 Most potent of various classes of bitter compounds. The boxed values at upper
right appear to be a drafting error. The most bitter Picric acid should be shown following the
triterpenes (some being very bitter) at lower left. See text. Modified from Van der Heijden, 1993.
This fact is illustrated by the assertion by Van der Heijden page 81) relating to aspartame, “The
LL optical isomer is sweet, and the DD, DL and LD isomers are not.” The title of the figure suggests
that picric acid is one of the most potent of bitter stimulants. Yet the label potency is
accompanied by a foot-note saying the column represents threshold values in nanomoles per
liter unless otherwise noted. This statement would suggest picric acid has one of the highest
thresholds in the list. The drastic difference in threshold between the ureas and above and the
“heated protein extracts” below, or between the amides and the plant constituents below,
Signal Generation & Processing 8- 137
would suggest the likelihood of a separation between the bitter and the super bitter
compounds.
Acree addressed what he described as the “flavor activity” in the same work as Van der
Heijden. Acree noted,
“It is important not to confuse the definition of flavor activity with the detailed relationship
between stimulus and response describe by psychophysical or dose-response functions
for flavor. As is shown below, activity is usually defined at specific values of these
functions. Typical flavor activity measured in flavor units is,
flavor activity = (1/threshold) x concentration”
His text is careful to delineate between flavor activity and the psychological response
labeled flavor response. He goes on, “The concentration factor is the amount of
compound detected in or added to a food. The inverse threshold factor is somewhat
complicated conceptually and is sometimes difficult to measure. In simple terms, the
threshold is the concentration of a compound in a particular food system below which the
food system has no flavor. Thus compounds with high inverse threshold factors are
flavorful even at low concentrations.”
The above formula can be rewritten as flavor activity = concentration/threshold. In this
formulation, the threshold corresponds to the noise floor or other minimum level required for the
system to respond, and the concentration describes the magnitude of the signal relative to that
threshold. This is the context in which Acree discusses the function in his figure 2 of 1993
addressed below. In this context, the flavor activity is the intensity of the stimulus as measured
at the input to the adaptation amplifier of the sensory neuron. The flavor response is the
perceived stimulus level at stage 5, information extraction, of the neural system. The threshold
level corresponds to the cutoff potential of the input amplifier (adaptation amplifier or 1st Activa)
within the sensory neuron itself.
If the Acree formulation is appropriate, there are three problems with the above Van der
Heijden figure; the column labeled potency is truly the threshold value for most of the
values in the column and the values given for the phenols should be at the bottom of the
list, as having a nominal threshold similar or below that of the triterpenes (and
consequently requiring only a minimal concentration to elicit a psychological response.
Finally, sucrose octaacetate should not be considered a typical sugar; it contains a
powerful picrophore not found among the simple sugars. As a result, the most potent
gustaphores (picric acid, the triterpenes, amarogentin and artabsin) are at the bottom of
the list, in agreement with most of the literature.
In the case of the phenols, their threshold value is not near 1.0 nanomoles per liter. It
appears Van der Heijden encountered an error in taking the reciprocal of a very small
number.
Acree addressed the dynamic range of the gustatory modality in 1993. His figure 2 suggest an
instantaneous dynamic range with regard to a specific stimulant of about 100:1, which is similar
to the more easily determined 200:1 in the visual and hearing modalities. His figure 1 suggests
a overall dynamic range, resulting from adaptation, on the order of six orders of magnitude or
more, also compatible with the measured ranges of vision and hearing.
8.6.10.3.2 Summarizing the picrophores of taste
Van der Heijden (page 95) noted the claim of Kubota & Kubo (1969) “that bitterness could be
explained by using the AH-B concept, which had also been used for elucidating sweetness.
However the AH-B distance for bitterness is 0.15 nm (1.5 Angstrom) [xxx check ]compared to 0.30
nm for sweetness.” He appears to be confusing acidity for bitterness in this case.
The first property to note in this paragraph is that picric acid (CAS 88-89-1) is not an organic
(carboxylic) acid. The name is the common one for a NO2 substituted phenol, 2,4,6-trinitrophenol
of the formula, C6H2(NO2)3OH. It is frequently shown as an inorganic anion due to the loss of a
138 Neurons & the Nervous System
hydrogen. Figure 8.5.4-14 illustrates the complete picric acid molecule. It is proposed that this
picrophore has a d-value optimized for stimulating the picric channel sensory neuron that
employs PtdAsn as its receptor.
Taking picric acid as the premier member
of the bitter tasting chemical family, there
are two potential ligands of interest. They
appear simultaneously and in multiple
instances.
The first ligand of interest is O=N-C-=C-=CN=O and it is planar. Note the backbone
includes two nitrogen atoms instead of an
all carbon backbone. The notation C-=C is
used to indicate a resonant linkage
between the two carbons due to the
parent ring structure.
This gustaphore
occurs three times in this (gustant) molecule
In each case, the best available d-value is
4.746 Angstrom.
The second ligand of interest is the N-C-=C=C-N sequence alone. The nitrogen atoms
as configured in picric acid continue to
have an unshared pair of electrons
capable of supporting a dual antiparallel
coordinate bond but would need to
acquire a hydrogen through
rearrangement of from the solvent. The dvalue for these pairs remains 4.746
Angstrom.
Figure 8.5.4-14 The premier gustant of the bitter or
P-channel of gustation. Distances in Angstrom.
See text. From a PDB file on the Boston University
web site, 2012.
There is a potential third ligand in interest. It would originate at an nitrogen and terminate at an
oxygen, or vice versa, O=N-C-=C-=C-N . The d-value of these ligands is only slightly higher at
4.948 Angstrom.
Each of these potential ligands occurs multiple times in each molecule providing a rich source
of potential gustaphores and resulting in a very high effective concentration for this molecule..
The symmetry of the molecule suggests it has a dipole potential of near zero for all of the
odorophores discussed. The literature shows a wide variety of ionic structures for picric acid.
While of no concern with regard to coordinate bonding, they could affect the measurement of
the dipole potential of the molecule. However, it is never shown as an organic acid. Figure
8.5.4-15 shows it in a triply ionized state198 but without the removal of the hydrogen from the
hydroxyl group at top center. The noteworthy fact is the d-value of 4.746 Angstrom does not
change for the noted oxygen pairs. It is likely that different dipole potentials will result from the
various ionic forms.
198
http://www.chemspider.com/Chemical-Structure.6688.html
Signal Generation & Processing 8- 139
It is noteworthy that ChemSpider
differentiates between the average mass
of 229.103897 Da and the monoisotopic
mass of: 228.997101 Da typically reported.
Under a heading, “Names and Synonyms,”
it also offers a validation citation, along with
a variety of synonyms. The site makes a
significant effort to provide traceability with
its property assertions.
8.6.10.3.3 Amarogentin,
and quinine
artabsin
Amarogentin (CAS 21018-84-8) is a complex
molecule of 586 Dalton that the Merck
index suggests is one of the bitterest of
compounds. It hydrolyzes into three parts in
water, a sugar with a glucophore,
O24–O26, with d-value of 2.754 Angstrom
and two multiple ring structures. The one
ring structure exhibits a picrophore,
O28–O34, with a d-value of 4.648 Angstrom.
Thus amarogentin contains as a minimum
two different types of gustaphore. Its
psychophysical evaluation is
difficult
because of this feature.
Figure 8.5.4-15 Ionic forms of picric acid from the
literature. Neither the formula, or the coordinate
bond capability, of the molecule changes due to
the ionization shown. See text. Example from
ChemSpider, 2012.
Quinine is a complex molecule with a two
phenol ring structure that supports a picrophore between two oxygen atoms with a d-value of
4.801 Angstrom. The molecule also exhibits two phenol rings that can also act as an electron-pair
donor. These features offer additional sources of picrophores in conjunction with the other
oxygen and nitrogen atoms present.
Artabsin (HMDB36641) has only three oxygen and zero nitrogen or other orbital atoms. The most
relevant pairs in this molecule have d-values of 2.420 and 5.829 Angstrom. It does exhibit one
penta hetrocyclic ring, one penta cyclic ring and one 7-point ring. Further analysis will be
required to uncover the source of its picrophore(s).
140 Neurons & the Nervous System
Quinine provides a slightly more complex gustaphore relying on an out-of-plane condition. The
ligand can be described as O-C-=C-=C-=C-O. The backbone is all carbon but the geometry is
slightly more convoluted. The upper oxygen is in the plane of the two rings. However, the lower
oxygen is out of the plane resulting in a longer distance between the oxygen atoms, a d-value
of 4.801 Angstrom, as shown in Figure 8.5.4-16. The complex moiety on the right plays no
significant role in the gustaphore selection process, although it may play a role in the dipole
potential of the molecule.
The slightly different d-value in the third
significant digit may indicate the width of
the sensory receptor acceptance range, or
it may indicate a difference in optimization
procedure used to optimize the molecule
structure before defining this d-value.
Artabsin (HMDB36641) has been described
as a potent bitter stimulant. It has only
three oxygen and zero nitrogen or other
orbital atoms. The most relevant pairs in this
molecule have d-values of 2.420 and 5.829
Angstrom.
It does exhibit one penta
hetrocyclic ring, one penta cyclic ring and
one 7-point ring. Further analysis will be
required to uncover the source of its
picrophore(s).
8.6.10.3.4 The triterpenes
Figure 8.5.4-16 Quinine as a stimulant of the
Triterpenes are terpenes consisting of six P–channel of gustation. Distance is in Angstrom.
isoprene units and have the molecular See text. A PDB file provided by David Woodcock
formula C30H48.
The simple terpene of
consists of two isoprene units. Terpenes Okanaga Univ. Coll., 2012.
typically exhibit five complete or partial ring
structures. They are a large and varied class of hydrocarbons199, produced primarily by a wide
variety of plants, particularly the gums and resins of the conifers, though also by some insects
such as termites or swallowtail butterflies, which emit terpenes from their osmeterium. Triterpenes
are precursors to steroids in both plants and animals. They generally consist of more than three
ring structures. The simplest triterpenes exhibit little oxygen and no other orbitals involved in
taste. Many triterpenes containing more oxygen are known and they offer many opportunities
to exhibit picrophores.
199
https://www.google.com/search?q=triterpenes&hl=en&tbo=d&rls=com.microsoft:en-us:IE-SearchBox&r
lz=1I7GGLD_en&tbm=isch&tbs=simg:CAESEglApfbYCUQYkiE5VFo1_1Dz7jg&dur=4297&biw=995&
bih=552&sei=WIqiUMyLOumY2wWP7ICIBA#q=triterpenes&hl=en&tbo=d&rls=com.microsoft:en-us:IESearchBox&rlz=1I7GGLD_en&tbs=simg:CAESEglApfbYCUQYkiE5VFo1_1Dz7jg&tbm=isch&bav=on.2
,or.r_gc.r_pw.r_qf.&fp=d1a4c9382e039dc&bpcl=38093640&biw=995&bih=552
Signal Generation & Processing 8- 141
Figure 8.5.4-17 shows a typical triterpene associated with a sugar as well as one containing
many more oxygen atoms and hydroxyl groups. The upper triterpene after hydrolization is
tasteless while the lower triterpene is very likely to taste bitter due to its potential to form multiple
gustaphores capable of forming a dual antiparallel coordinate bond with a d-value of near
4.746 Angstrom.
8.5.4.5.1 Review of diverse bitter
gustants and gustaphores
The list of compounds historically presumed
to be bitter tasting is quite long and
involves many chemical structures as shown
by Spielman et al200. in Figure 8.5.4-18. Van
der Heijden has given a similar list of the
major bitter stimulants that includes their
relative potency. One of the goals of this
work is to more clearly identify the role of
these compounds. To this end, it is noted
that the role of the astringents formed by
the salts of the alkali earths are actually
nocent chemicals and not bitter
gustaphores in the context of the gustatory
modality (Section 8.5.1.5.4). However, time
will not allow the identification of all of
these chemical groups. Many of the listed
chemicals are now considered, and can be
shown, to be stimulants containing multiple
gustaphores exciting various neural
pathways.
Van der Heijden listed picric acid as a
reference gustant, but noted it was more
than six orders of magnitude less bitter than
the triterpenes and two plant constituents,
amarogentin and artabsin.
Van der
Heijden has also provided chemical
structures for each of these materials.
Unfortunately, these structures are twodimensional Fischer (stick) diagrams. These
diagrams illustrate the historic difficulty of
finding a common structural feature to
account for the bitter sensation.
Figure 8.5.4-17 A typical triterpene linked to a
sugar at lower left. The molecule exhibits little
oxygen but the capability of many points of
oxygen addition.
Below; a more highly
oxygenated triterpene. See text.
When noting large ranges in the
perceived bitterness of materials, it becomes necessary to consider the possibility of a
super-bitter category analogous to the super-sweet category in the glycophores of the
sugars. This is done in Section 8.5.4.6.
A 3d representation of amarogentin, CAS #21018-84-8, clearly shows it exhibits a glycophore with
d-value = 2.754 Angstrom (O24–C–C–O26) as well as a picrophore with d-value = 4.648 Angstrom
(O28–C–C–O34). Its complexity suggests it is also capable of participating in a super-bitter
perception such as that developed in the next major section.
Caffeine is a simpler molecule than amarogentin, similar to the complexity of some of the man-
200
Spielman, A. Huque, T. Whitney, G. & Brand, J. (1992) The diversity of bitter taste signal transduction
mechanisms In Corey, D. & Roper, S. eds. Sensory Transduction. Woods Hole, MA: Marine Biological
Laboratory Chapter 20 .
142 Neurons & the Nervous System
made super-sweet molecules. It exhibits a d-value = 4.7 Angstrom (only one percent below the
nominal 4.746 Angstrom of the proposed P-path GR). The AH,B path within caffeine is O4=C–N
–C–N8. Further review and comparison of this and other presumed super-bitter molecules will
be required to determine any presumed AH,B,X relationship that might affect the P-path GR
significantly.
Figure 8.5.4-18 The diversity of historically bitter compounds EDIT or MERGE with van der Heijden,
1993 of figure 8.5.4-27 by adding threshold column a/o d-value column. Right column; proposed
alternate designations. See text. Significantly modified from Spielman et al., 1991.
Spielman et al., writing in a Society of General Physiology symposium record, speculate on
whether a single sensory neuron could possibly accommodate the detection of this broad array
of chemical structures, or whether multiple sensory neurons are required. They offer only working
hypotheses on the character of the sensory neurons based conceptually on the chemical theory
of the neuron. Their assumptions are;
1. there is no one mechanism that would apply to all bitter stimuli,
2. there must be several signal transduction processes attuned to the broader chemical
categories of bitter compounds,
3. one group of bitter compound can be detected by several different mechanisms, and
4. there are still bitter compounds to be discovered or synthesized.
Van der Heijden reviewed the structural formulas for a variety of his bitter stimulants (page 95103) and concluded they did not meet the criteria for a suitable AH,B coordination bond
because the structures did not meet his criteria for bitterness of 0.15 nm (1.5 Angstrom) [xxx chk
]. His model of a sweet-bitter has not drawn favor in the taste research community. As shown
in this work, the AH,B spacing criteria to coordinate with the bitter sensory receptor is 0.42 nm (4.2
Angstrom) as developed below. [xxx 4.7xxx ]
The ability of the AH,B coordination structure to explain the operation of the G-Path channel
suggests a similar structure might be important in the P-Path channel. To date, only limited
Signal Generation & Processing 8- 143
insight has been obtained into the operation of the P-Path receptor mechanism. Shallenberger
& Acree have noted, “limited correlations between bitter taste and molecular structure exist
because this taste sensation is not one that is enthusiastically evaluated.” More recently (2003),
Drewnowski has noted201, “the biology of bitter taste perception is poorly understood.” and “The
study of human taste genetics is largely the study of bitter taste.”
The range of chemical structures stimulating the P-Path channel is wide. It includes many, but
not all, alkaloids, tannins, methylxanthines, peptides, isoprenoids and many non-sodium salts
(Section 8.xxx). Care must be taken to use the complete chemical name of a bitter stimulant,
the common names used in pharmacology and the food technologies are frequently less than
specific.
---A major problem for previous investigators has been their lack of any guidance on what to look
for. With the exception of the comment about three electronegative oxygen atoms lying in a
single plane by Shallenberger & Acree, the typical investigator has been looking at structures
involving less distance between the specific atoms. The rectilinear sensation space of this work
shows a quinine best receptor must involve A B spacings greater than 3.7 Angstrom to be
independent of the other sensory channels.
Denatonium, Figure 8.5.4-19, usually available as denatonium benzoate and as denatonium
saccharide, is the most bitter chemical compound known according to Wikipedia and Aversion
Technologies, Inc... Threshold for the benzoate is approx. 0.05 ppm. Dilutions of as little as 10
ppm are unbearably bitter to most humans. 30 ppm is claimed to be undrinkable. Denatonium
is a quaternary ammonium cation. It is a compound of a salt with an inert anion like benzoate
or saccharide. The structure of denatonium benzoate, C28H34N2O3 and molecular weight of
446.58, is related to the local anesthetic lidocaine, differing only by the addition of a benzyl
group to the amino nitrogen. Denatonium saccharide (meaning the salt of saccharin, a benzoic
sulfilimine and not of a simple sugar) has the formula, C28H33N3O4S & molecular weight of
507.46..
Figure 8.5.4-19 Denatonium benzoate & saccharide, the most bitter compounds ADD & EDIT
known. Labels don’t appear to match figure and difference between two upper forms is not
justified. Top; benzoic acid. Bottom; denatonium. Et; ethyl ligand. Mt; methyl ligand. PH;
phenyl ligand.
Denatonium is frequently used as an alcohol denaturant, along with brucine and quassin. These
201
Drewnowski, A. (2003) Genetics of human taste perception In Doty, R. ed. (2003) Handbook of Olfaction
and Gustation, 2nd revised and expanded edition. NY: Marcel Dekker Chap 40
144 Neurons & the Nervous System
chemicals are frequently shown as in the figure. Even the Merck Index shows each of them as
a pair of ions with the positive charge on the nitrogen of the denatonium. The applicable Jmol
files also show two ions. No information on the solubility or ionization ability of these chemicals
is given in the Index. Their actual role as picrophores can not be determined without showing
their actual structure in three dimensions. They obviously contain a large number of potential
orbitals and aromatic rings that could provide the appropriate d-values for stimulating the bitter
channel.
The complexity of the denatonium salts suggests they could stimulate almost any receptor of the
gustatory modality depending on the degree of ionization of these salts. Figure 8.5.4-20 shows
a 3D representation of denatonium benzoate.
Based on the proposed hypothesis, the
oxygen of the denatonium ion would not
result in a taste perception when
associated with either of the two rings (dvalues of approx. 3.72 & 5.96 Angstrom.
The upper nitrogen and ring exhibit a dvalue of 2.785 Angstrom and could be
perceived as sweet. The lower nitrogen
and ring exhibit a d-value of approximately
3.69 Angstrom and could be perceived as
salty. The benzoate ion is a simple organic
acid and would be expected to contribute
an acid taste to the solution (d-value of
about 2.268 Angstrom). These values
support the observation by others in Figure 8.5.4-20 Denatonium benzoate from Jmol.
Section 8.5.1. The ability of the benzoate The view shows the fact the two rings of
to also taste salty is in question because of denatonium are not in the same plane or parallel
the accuracy of the oxygen to ring d- to the aliphatic chain. See text.
values based on the Jmol of December
2012. It shows the distances between the
two oxygen orbitals and the centroid of the ring as equal, which would require the two oxygen
atoms to be in resonance.
Saroli has provided a major table of bitterness thresholds among a large group of denatonium
derivatives associated with various halogens202. His brief discussion of the potential AH,B
relationships for these molecules is confounded by the unusual dimensions he gives for the
distances between the relevant orbitals. His 3.2 Angstrom between the electrophilic ammonium
nitrogen atom and the nucleophilic oxygen of the carbonyl group differs markedly from the 1.43
Angstrom found using Discovery Studio 3.5 (DS3.5, 2012)from Accelyrs . However, hand
calculations would suggest his value of 3.2 Angstrom is in error. Saroli also uses a different variant
of the denatonium structure than CAS 1674-99-3. As noted earlier, the state of 3D computational
chemistry remains primitive. Significantly different values are given using the same Jmol file
found in the on-line Chemical Book (CAS 1674-99-3) when using the DS3.5 and the Argus Labs
4.0.1 (2004) visualizers due to the different degrees of planarity found. Both visualizers show four
bonds to the carbonyl oxygen without any graphic identification or other explanation. It can
be assumed the extra bonds, one to each of the nitrogen atoms are coordinate bonds rather
than valence bonds. The Jmol vers 13.0.8 (2012) visualizer gives the same planar configuration
as the Argus representation.
202
Saroli, A. (1985) Interaction of denatonium chloride with the bitter taste receptors Z Lebensm Unters Forsch
vol 180, pp 227-229
Signal Generation & Processing 8- 145
[xxx deprecate these paragraphs in favor of Sec 8.4.xxx in complete chap 8 printed
version ]
A problem at this time is the various visualization programs provide strikingly different
representations. Compare the above to Figure 8.5.4-21. The upper 3D representation
shows a totally planar structure for denatonium while the lower representation shows a
non-planar structure more complex than that in the upper frame and previous figure.
Note the four bonds to O13. This representation differs significantly from the Fischer
diagram in the center of the figure. DS3.5 describes the bond between C7 and N12 as
“aromatic.” It does not describe the bond between O13 and N1.
The distances between the atoms of
these three denatonium moiety
representations vary drastically.
Until the actual structure of the various
denatonium compounds can be
determined when in solution, the
hypothesis of this work can not be
tested or extended to include this family
of super-bitter picrophores.
Care must also be taken to identify the
visualizer used in a specific case. They do
not assign the same atom numbers to the
same atoms within a compound.
No dependable (and defendable)
conclusions can be drawn from these
representations. Any attempt to extend
the super bitter representation to the AH,B,X
relationship is detered by the problems with
these visualizers.
The only potential
picrophores of denatonium chloride
identified to date are;
• the centroid of the benzyl ring farthest
from the ammonium nitrogen and that
nitrogen with a d-value using the Argus
Lab’s visualizer of 4.72 Angstrom (6.794
using the DS3.5 visualizer)
•the centroid of the benzyl ring farthest
from the ammonium nitrogen and the
closer quadratic nitrogen with a d-value =
4.337 Angstrom using the DS3.5 visualizer
(much shorter using the Argus visualizer).
Figure 8.5.4-21 Alternate structures for the
denatonium family given by different visualizer
programs. The programs are unable to provide
consistent representations of this family, including
the specific CAS 1674-99-3. See text.
Only the 4.72 Angstrom d-value meets the
criteria of this hypothesis well. Denatonium
is the first example of the nucleophilic character of a ring structure serving as an orbital in
gustation discussed in this work..
---Figure 8.5.4-22 shows the chemical structure of quinine. Quinine hydrochloride is frequently used
in the laboratory as the quintessential bitter stimulant. Its complex structure makes it difficult to
analyze without a definitive set of requirements. However, based on the theory developed here,
the bipartite AH,B coordinate bond with the P-Path sensory receptor is clear as illustrated. The
spacing between the two oxygen atoms is estimated at 4.2 Angstrom after accounting for the
two-dimensional projection of this three-dimensional molecule. If the actual spacing is not close
to this value, an alternate AH,B coordinate bond will be required based on the presence of
Nitrogen at several points or the presence of the ring structures.
146 Neurons & the Nervous System
While nitrogen plays a major role in many
bitter stimulants, it is not a necessary
component of a P-Path stimulus. The
tannins (tannic acids) are a complex
family203 of (a) so-called condensed tannins
and (b) hydrolyzable tannins. The latter are
sugar esters with one or more
trihydroxybenzenecarboxylic acids as
shown in Figure 8.5.4-23 for corilagin (CAS
23094-69-1. The names based on their
structure are largely irrelevant in gustation.
The proliferation of hydroxyl radicals in
these compounds make the potential for
an AH,B coordinate bond of the required
spacing quite likely. While shown in Fischer
diagram form, and represented as planar
using the Argus and Jmol visualizers, the
molecule is in fact non-planar. To achieve
planarity, both involve a forced closure of
one outsized bond and very many
foreshortened bond lengths. The DS3.5
visualizer shows a more rational 3D
representation of the molecule. In that
representation, the molecule exhibits
hydroxyl pairs with a variety of d-values.
Several (at least six) are in the range
suggestive of stimulating the picric channel
receptor, from 4.65 to 4.93 Angstrom.
Figure 8.5.4-22 Quinine, the preferred bitter taste
in the laboratory.
The d-value shown was
obtained with DS3.5 visualizer, as opposed to a
value of 4.2 Angstrom obtained with a Jmol
visualizer and a Jmol file from kaist_ac_kr
(Woodcock).
The Argus visualizer provides a stunning electrostatic surface
profile for corilagin. However, it cannot be relied upon due to
the foreshortened dimensions also displayed. [xxx corilagin ESP
23094-69-1 ] [xxx corilagin DS3.5_23094-69-1.wpg ]
Lawless and Lee (Figure 6), following McManus et al204.,
described the potential for two adjacent oxygen atoms
separated by two carbons in a resonant 6-member ring to form
a dual antiparallel coordinate bond with an unspecified
peptide chain. They did not discuss any requirement that the
distance between the two oxygen atoms of the tannin match
the distance between the oxygen and nitrogen of the peptide
chain or any requirement that the lengths of the hydrogen
bonds meet nominal requirements.
Lawless & Lee have used the terms astringent as well as bitter
in their discussions of the tannins.
Figure 8.5.4-23 Corilagin, a
tannic acid & a complex
sugar ester, CAS 23094-69-1.
The four rings, including the
one on the right which is
heterocyclic, are found in
distinctly separate planes.
203
Lawless, H. & Lee, C. (1993) Common chemical sense In Acree, T. & Teranishi, R. eds. Flavor Science.
Washington, DC: American Chemical Society Chapter 2
204
McManus, J. Davis, K. Lilley, T. & Haslam, E. (1981) The association of proteins with polyphenols
J Chem Soc Chem Commun pp 309b-311
Signal Generation & Processing 8- 147
Figure 8.5.4-24 shows the chemical structure of caffeine, another bitter stimulus containing
significant nitrogen but little oxygen, and its parent xanthine on the right. Analysis using the Jmol
program, shows the d = 4.70 Angstrom between .N9 and O2 in both molecules. Such a value
is compatible with binding of these molecules to the picric channel receptor with a nominal d
= 4.827 Angstrom. The disparity of 2.7% is consistent with the weakly bitter perception of this
chemical by many humans. [xxx check with DS3.5 visualizer ] [xxx alt use 4.746 and a new
percentage ]
Quoting Glendinning et al.,
“Many different classes of compounds
elicit bitter taste in humans and inhibit
feeding in animals. These compounds
include alkaloids, tannins, methylxanthines, peptides, isoprenoids, and
many non-sodium salts.
Because
virtually all naturally occurring poisons
taste bitter to humans, this taste quality
is thought to have evolved as a
mechanism for avoiding toxic
substances.”
Figure 8.5.4-24 Caffeine, 1,3,7-trimethylxanthine
[xxx rewrite next two paragraphs consider and its parent xanthine on the right. Both exhibit
moving all or part to the thiol section. ]]
a d-value of 4.7 between the atoms shown in
Breslin may have uncovered the key to the both the DS3.5 and Jmol visualizers, although the
operation of the P-Path sensory neuron atom numbering schemes used differ.
(Section 8.xxx). This may be an excellent
clue to the operation of the P-Path sensory
channel since phosphatidic acid is a potential transducer in the type 4 lemma of the P-Path
sensory neuron microvilli. If the phosphatidic acid is reacting with the quinine hydrochloride with
the precipitation of quinine, the phosphatidic acid may become chlorinated and become
negatively charged. This action could mirror the nominal reaction between the phosphatidyl
fatty acid of type 4 lemma and quinine hydrochloride, resulting in the precipitation of the
quinine, permanent alteration of the phosphatidyl fatty acid and the injection of an electron into
the plasma of the microvilli. A comparison of the state of polarization of phosphatidic acid and
both PtdCho and PtdSer would appear to be useful. PtdSer is very similar structurally to
phosphatidic acid and shows a positive polarization, just the opposite of the more common
PtdCho forming a majority of the outer leaf of plasma lemma.
Breslin (page 440) has noted the genetics-based work of Kalmus205. Kalmus found a N–C=S bond
in a set of anti-thyroid drugs that may be significant. Individual humans exhibit considerable
difference in sensitivity to these two compounds. As many as 25% of the population may be
taste blind to the bitterness of these materials, suggesting a “spectral blindness” similar to
protanopia in vision. Since these taste-blind subjects are not blind to other quinine related
stimuli, it has been suggested there may be more than one P-Path receptor types. See Section
8.5.4.7.
205
Kalmus, H. (1971) The genetics of taste In Beidler, L. ed. Handbook of Sensory Physiology: Vol. IV,
Chemical Senses NY: Springer pp 165-179
148 Neurons & the Nervous System
Shallenberger & Acree have also reported the work of Kubota & Kubo206 suggesting an AH,B
relationship with a 1.5 Angstrom spacing might be important in bitter channel transduction.
However, they also suggest three electronegative oxygen atoms lying in a single plane may be
a characteristic of quinine class stimuli, such as picric acid with three NO2 ligands.
Figure 8.5.4-25 shows a pair of molecules that Shallenberger and Acree described as “most
intriguing variations in sweet taste between two closely related compounds.” They noted the
α– conformation was sweet tasting but the β– conformation of identical chemical structure was
tasteless. Based on the double antiparallel coordinate bond (DACB) theory of this work, the
difference is quite understandable; it is not the fact that the same groups are present in both
cases, it is the fact that the distance between the electron-rich aromatic ring and the hydroxyl
group varies so significantly. The sweet tasting α– or anti form has a d-value near 2.74 Angstrom
while the β– or syn oxime has a much larger d-value of about 4.8 Angstrom because of the
physical location of the hydroxyl group. Contrary to being tasteless, the β– form should stimulate
the P-path and be perceived as bitter (possibly at a different concentration level). The relevant
situation is shown in the lower half of the figure. It is critically important to discriminate between
these two conformation when discussing taste. These two substances are given a wide variety
of numbers on various databases. The closest source of consistent numbers given on one page
appears to be from the NCBI of NLM of the NIH207. The α– conformation is given the CID of
5371924 (items 27, 28 & 29, with a minor variation in graphic for 28 & 29). The β– conformation
is given the CID of 5371961 (items 1-26). It is the distance between the unshared pairs of orbital
electrons that is the significant feature of the two conformations. In this case one of the
unshared groups is provided by the
aromatic ring. The only d-value of interest
in this figure is that between the aromatic
ring and the most remote oxygen. The ring
and the remote oxygen can bind to the
sensory receptor of the G-path.
The sharing of a pair of electrons from an
aromatic ring will be found to be a
common and critical feature in the
olfactory modality discussed in Section 8.6.
The geometry of the electronic cloud
associated with the aromatic ring is poorly
defined with relation to its participation in a
coordinate bond situation. Initially, it will be
assumed the centroid of the cloud is
congruent with the centroid of the ring of
atoms forming the ring.
While the
electronic cloud may extend above and
below the plane of the ring, it will be
assumed the cloud is dumbbell shaped
and the two sections of the cloud are
cylindrically symmetrical.
Hence, the
distance between an orbital atom and the
geometric center of the ring will be taken
as the pertinent distance in any DACB
binding relationship.
At a more
sophisticated level, it may be shown that
the pertinent centroid varies with auxiliary
asymmetric moieties associated with the
ring just as the dipole moment of the overall
molecule varies with the associated
moieties.
206
Figure 8.5.4-25 Two “most intriguing” examples of
anisaldoxime Top row; representation from
Shallenberger & Acree with dimensions added.
Bottom row; 2D representations from a 3D
construction using Jmol with additional electron
pairs annotated.
See text.
Modified and
extended from Shallenberger & Acree, 1971.
Kubota, T. & Kubo, I. (1969) Bitterness and chemical structure Nature vol 223, pp 97-99
207
http://www.ncbi.nlm.nih.gov/sites/entrez
Signal Generation & Processing 8- 149
Kubota & Kubo examined the diterpenes from a specific species of plants, Isodon, extensively.
Using slightly different terminology than Shallenberger & Acree, possibly mis-translating their
concept and relying on the Brknsted acid concept of acids and bases, they assert that a proton
donor group (DH) and a proton acceptor group (A) must be within 1.5 Angstrom and form an
intramolecular bond in order to form a “bitter unit.” They do not address the stimulus/receptor
relationship.
[xxx re-examine d-values
The assertion of a distance between coordinate bonds of 1.5 Angstrom for bitter compounds has
not appeared in the recent literature, except in its restatement by Kubo in his 1994 doctoral
dissertation208. His determination of 1.5 Angstrom based on X-ray investigations may well refer
to the length of the O–H- -O hydrogen bond (nominally 1.70 Angstrom) rather than the spacing
between the two orbitals of the stimulant or GR. Kubo repeated the common phrase,
“However, the rationalization of bitterness in terms of chemical structure has been a difficult and
long standing problem.” His paper did not change that situation. He did provide a Fischer
diagram for a wide range of bitter stimulants, 30, but did not specifically identify the picrophore
on any of them. The picrophore associated with many of them is identified in this work. A paper
by Yamada et al209. in 1999 obtained detailed information on the structure of several more
diterpenes from Isodon and explored their bitterness relative to quinone. However, they did not
explore the transduction process.
8.5.4.5.2 Potential picric channel receptors
The earlier discussion has suggested a family of receptors based on simple chemicals esterfied
to phosphatidic acid. The same table in Leninger and in Yudkin & Offord of the 1980's suggest
two possibilities for the bitter phospholipid
receptor, the direct esterification of a
simple molecule to the phosphatidic acid
or the esterification of a simple molecule to
phosphatidyl glycerol at the 3' OH of the
glycerol. Figure 8.5.4-26 shows the simplest
case of aspartic acid esterified to either of
these potential moieties. The molecular
structure is very complex in 3D space.
However, the d-value between its various
pairs of oxygen orbitals are interesting;
3.105, 3.273, 4.375 and 4.827 Angstrom.
Some of these d-values are lost during the
esterification process. Either of the oxygen
orbitals shown closest together to the right
of this two-dimensional representation and
between the open boxes could be the
location of esterification. The value of 4.827
is very close to the 4.746 value for the three
odorophores of picric acid (with picric acid
being one of the most bitter common
chemicals and taken as the major bitter
gustant). The other d-values do not suggest Figure 8.5.4-26 Potential phosphatidyl aspartic
stimulation of any of the other channel acid receptor and picric acid as gustant. Open
receptors. .
boxes represent locations of antiparallel
coordinate bonds. Note the three picrophores of
In this proposed configuration, the precise each molecule of the gustant. See text.
208
Kubo, I. (1994) Structural Basis for Bitterness Based on Rabdosia Diterpenes Physiol Behav vol 56(6), pp.
1203-1207,
209
Yamada, Y. Sako, N. Ando, E. et al. (1999) New bitter diterpenes: Rabdosianone I & II: Biosci Biotechnol
Biochem vol 63(3), pp 524-529
150 Neurons & the Nervous System
formation of the one hydrogen bond, shown by the lower open box, appears clear. The
formation of the second hydrogen bond, the upper open box, is less clear when using the
standard form of aspartic acid generated by the Jmol and similar modeling programs.
Interchanging the hydroxyl group at upper center of the picric acid with the carboxyl oxygen
at upper left would support formation of the desired hydrogen bond within the upper open box.
Taking phosphatidyl aspartic acid (PtdAsp) as the reference picric channel receptor, its d-value
in the absence of any crowding becomes the reference value of d = 4.827 Angstrom. The ability
of picric acid, with a d-value only 1.7% different, to bind in a DACB appears well within the
acceptance range of the receptor.
The configuration shown suggests many of the ring based bitter gustants; quinine (d = 4.801),
caffeine (d = 4.700), xanthine (d = 4.700), xxx & xxx can bind to the proposed PtdAsp receptor
without difficulty.
An alternate to PtdAsp would be its close relative, phosphatidyl asparagine (PtdAsn), with NH2
replacing the hydroxyl group farthest from the amine carbon. However, its promising bond
lengths are slightly longer than for PtdAsp, 4.89 and 5.52 Angstrom.
8.5.4.5.3 Hydrated organic molecules as picric channel gustaphore
As noted in the discussion related to the hydrated sodium ion, many of the other bitter organic
gustants may only be stimulants when in a hydrated state.
8.5.4.5.4 hydrated hydrogen sulfide as an inorganic picric channel gustaphore
Xxx add here
It appears that hydrogen sulfide is also a bitter tasting gustant when in the hydrated state.
Signal Generation & Processing 8- 151
8.5.4.5.6 OBSOLETE MATERIAL ON PICROPHORE/RECEPTOR MATCH
[xxx edit or drop. must redraw figure to show d-value from Jmol etc. ]
The most probable phospholipid of the outer bilayer membrane of the microvilli capable of
operating as the sensory receptor is a form of Ptd3'Og with one unsaturated fatty acid providing
electrical conductivity along the length of the molecule. Figure 8.5.4-27 shows the structure of
this candidate along with its predicted sensitivity in taste sensation space. As shown by the
unspecified group, R, Ptd3'Og.is actually a
large family. In their experiments, Silvius et
al. found R to be mostly in the form of
alanine210. In this figure, an acyl substitution
has been introduced as an alternate to the
alanine. Any number of R ligands could be
introduced to provide an additional
carbon and either a hydroxyl group or
carboxyl oxygen.
The active portion of the overall
phosphatidyl molecule is shown at upper
left beginning with the ester to phosphoric
acid on the left. The pertinent feature is the
distance of about 4.7 Angstrom between
the oxygen atom, B, and the amine, AH.
This broad spacing suggests this sensory
receptor is positioned as shown in the lower
frame of the figure, achieving a sensation
space with minimum overlap with the other
sensory channels. As noted earlier, the
mean values of each distribution is
calculable but the distributions are shown
only conceptually.
XXX CAN PROBABLY ELIMINATE R’ AND THE
DOUBLE BOND BY PLACING H2 ON THE
LOWER CARBON AS IN ASPARTIC ACID.
Figure 8.5.4-27 Candidate sensory receptor
performance for the “bitter” channel. Upper left,
active ligand of bitter sensory receptor. Upper
right; quassin shown oriented so as to form an
AH,B coordinate bond with the sensory receptor.
Bottom; rectilinear taste space showing the
quinine channel centered at an A B spacing of
4.7.46 Angstrom. See text.
The upper right of the figure shows a very
complex molecule known for its bitterness,
quassin (or Surinam quassia in commerce).
It has a bitterness threshold of 1:60,000.
Note the symmetry of this molecule
suggesting it could form the AH,B bond in multiple ways. A similar graphic can be prepared
showing coordinate bonding with brucine, another important bitter stimulant.
210
Silvius, J. Mak, N. & McElhaney, R. (1980) Lipid and protein composition and thermotropic lipid phase
transitions in fatty acid-homogeneous membranes of Acholeplasma laidlawii b Biochim Biophysica vol 597(2),
pp 199-215
152 Neurons & the Nervous System
Figure 8.5.4-28 shows the proposed sensory receptor coordinate bonding to the common
laboratory stimulant quinine. The nominal spacing between the coordinate bonding orbitals of
quinine is 4.801 Angstrom, approximately one percent greater than the nominal bonding orbital
spacing of picric acid, 4.746 Angstrom. The receptor orbitals are oxygen and nitrogen while the
stimulant orbitals are both oxygen. R is shown as carboxylic acid in the figure. The left-most
oxygen forms the ester with the conductive phosphatidyl group.
The potential for an enhanced bitter
sensation resulting from a third site forming
an AH,B,X bond like that of the sugars
should not be dismissed. If the bitter
sensory receptor is in fact 3'-O-aminoacyl
glycerol, it exhibits a potential X site at
either of the two oxygen atoms associated
with the glycerol moiety and at either of
the two oxygen atoms of the carboxyl
group.
Figure 8.5.4-28 Candidate picric sensory receptor
and quinine coordinate bonding RESCALE. SHOW
left-most O as ester. 3D representation for quinine
from David Woodcock, http://Kaist.ac.kr.edu.
Signal Generation & Processing 8- 153
Figure 8.5.4-29 shows a key feature of the candidate P-path picrophores and GR’s. It is
particularly interesting because of the range of chemicals that can satisfy the nominal
picrophore requirement within an acceptable tolerance. The nominal situation based on picric
acid is a d-value equal to 4.746 Angstrom. This value is determined by the five carbon chain
immersed in a fusion of homocyclic six carbon rings. The rings may be incomplete, as long as
the orbitals occupy the positions commensurate with complete rings. The minimalist condition
can be labeled a (2R,4R)-pentanediol_2005883 with a d-value of 4.871. Note the bonds
extending beyond the ends of the upper carbon chain. The pentanediol exhibits methyl groups
on the two extreme carbons of the lower frame. A functional alternative is a 1,3 propanediol
immersed in a structure of one or more rings if its d-value is close enough to nominal to support
a P-path DACB.
Generically, 1,3 propanediol is described as
sweet in Merck Index. However, when in
the restricted configuration required here,
the resulting d-value prescribes a bitter
perception.
In the configurations shown, any of the
oxygen orbitals can be replaced by a
nitrogen or other orbital. In the bottom
structure, exchanging one or more oxygen
atoms for nitrogen has only a nominal
effect on the net d-value.
The upper frame shows the nominal
picrophore based on five carbons
associated with multiple ring structures. In
this configuration, the d-value of 4.746
Angstrom is independent of the particular
orbital (shown as oxygen in this example).
Exchanging either or both orbitals has no
effect on the d-value of the structure. This
configuration offers immense flexibility in
orbital selection.
The lower frame shows potential alternate
picrophores. Note the open bonds
indicative of the presence of additional
structure meeting the above criteria. In this
configuration, the d-value of the immersed
ligand varies with the length of the
particular orbital-carbon bond length.
For example,
Terminal bond d-value
type
Both O–C
1 O–C & 1 N–C
Both N–C
4.83 Angstrom
4.75
4.66
Figure 8.5.4-29 Potential P-path picrophores and
receptors. The candidates are symmetrical as
shown. However, each oxygen can be replaced
with another type orbital. Top; the nominal
candidate where the choice of orbital does not
change the d-value. Bottom; an alternate where
the d-value varies nominally with orbital. See text.
Deviation
From 4.746 A
+1.7%
0
–1.8%
8.5.4.6 Operation of the “super-bitter” sensory neurons
Figure 8.5.4-30 from van der Heijden is presented to describe the potential families of super-bitter
picrophores. It is likely that the chemicals with threshold sensitivities at concentrations at or
below 0.001 nanomoles per liter are employing a different or augmented mechanism for exciting
154 Neurons & the Nervous System
the P-path GR’s. It will be initially proposed that this mechanism is of the AH,B,X type so
successfully applied to the super-sweet glycophores. Van der Heijden provided potential AH,B,X
dimensions for his review of potential super-sweeteners but he did not extend the concept to
these potential super-bitter compounds.
Figure 8.5.4-30 The bitter and super-bitter stimulants of van der Heijden EDIT. CHG sugars to sugar
derivatives. As footnoted, potency refers to threshold sensitivity concentrations. Abbreviated
from van der Heijden, 1993.
The table includes a triterpene labeled lucidenic acid D1 and cites Nishitoba, 1989 in S Agric Biol
Chem vol 52 pp 1791+.. Figure 8.5.4-31 illustrates two stick figure representation of this chemical
under the nomenclature, HMDB: 36856 (Lucidenic acid D1) and HMDB 38199 of the Canadian
Human metabolome data base. While these representations suggest a complex non-planar
structure for this chemical, the 3D Jmol representation confirms it is a nearly planar as shown in
the next figure. It is very difficult to ascertain the picrophore in the upper representation.
However, in the lower representation, the candidates are quite obvious and the actual
picrophore involves the bracketed oxygen orbitals as will be confirmed using the next figure.
Signal Generation & Processing 8- 155
Figure 8.5.4-31 2D representations of Lucidenic
acid D1. Note the bracket at lower left indicating
the pair of orbitals forming the picrophore in this
chemical, d-value = 4.769 Angstrom.
Figure 8.5.4-32 displays a 3D representation of Lucidenic acid D1 (HMDB 28199) using Jmol. The
hypothesis of this work calls for a DACB with an optimum d-value of 4.746 Angstrom. The
measured value based on the Jmol model deviates from this prediction by only one part in onethousand, a very good confirmation of the hypothesis of this work. The relative simplicity of this
molecule makes it useful in searching for the dispersion centroid supporting the super-bitter
character of this molecule. Two most likely dispersion centroids for an AH,B,X model of a superbitter material are shown, X1 will be assumed to be the actual point until additional evidence
from other chemicals is gathered.
156 Neurons & the Nervous System
Figure 8.5.4-32 Lucidenic acid D1 (HMDB 38199) from Jmol. The baseline picrophore measures
4.769 Angstrom. The candidate dispersion centroid, X1, is 8.013 Angstrom from one orbital and
5.749 Angstrom from the other. The alternate dispersion centroid, X2, is at 9.655 & 6.310 Angstrom
with an acute angle of 120.62 degrees. The inset at upper right shows the molecule to be nonplanar.
The fact that the d-value of lucidenic acid D1 was within one part in one thousand of the
expected value based on the gustatory portion of the Electrolytic Theory of the Neuron is strong
vindication of this theory. However, the application of the AH,B,X concept will require better
understanding of the quantum-mechanical-(electrical) characteristics of the molecule.
Lucidenic acid is a general label for a very large family of triterpenes. Several lucidenic acids
are shown in Figure 8.5.4-33 from Guoa et al211. These materials exhibit a striking resemblance
to picric acid. One of these has been bracketed to indicate its picrophore. The rest of the
molecules on this page have had their structure changed to the point they exhibit no
picrophore and would not expect to be perceived as bitter.
211
Xiao-Yu Guoa, X-Y. Hana, J. Yea, M. et al. (2112) Identification of major compounds in rat bile after oral
administration of total triterpenoids of Ganoderma lucidum by high-performance liquid chromatography with
electrospray ionization tandem mass spectrometry J Pharm Biomed Anal vol 63, pp 29–39
Signal Generation & Processing 8- 157
Figure 8.5.4-33 Variants of lucidenic acid ADD. Only the derivative at lower left still exhibits the
pair of orbitals at the correct d-value to qualify as a picrophore. It can be expected to be bitter
and probably super-bitter. From Guoa et al., 2112.
158 Neurons & the Nervous System
Figure 8.5.4-34 shows a 2D view of amarogentin (CAS #21018-84-8) and a similar view of artabsin.
Unfortunately, the CAS # for artabsin is difficult to define and varies with databank. The
Brookhaven PDB databank code of 1A31 appears more reliable. This version of artabsin includes
a seven-carbon ring and two five sided rings (one homocyclic and one heterocyclic with the
inclusion of an oxygen.
The 3d representation of amarogentin, CAS #21018-84-8, clearly shows it exhibits a glycophore
with d-value = 2.754 Angstrom (O24–C–C–O26) as well as a picrophore with d-value = 4.648
Angstrom (O28–C–C–O34). It would therefore qualify as a stimulant exhibiting both a A-path
gustaphore and a P-path gustophore. Its complexity suggests it is also capable of participating
in a super-bitter perception such as that developed in the next major section.
A distinctly different variant of amarogentin labeled 692114 in the “Darwin” library of the GNUDarwin project is very complex as displayed in 3D. Lacking additional support, this
representation is impossible to analyze.
The structure of artabsin is considerably simpler but exhibits no obvious picrophore involving only
atoms for orbitals. Van der Heijden shows an artabsin with an eight-carbon ring and two fivesided rings (including an oxygen in one heterocyclic ring) citing Brieskorn, 1979, in German.
However, other Jmol representations show a seven-carbon ring. Both arrangements strongly
suggests the involvement of the ring structures in any functional AH,B picrophore and probably
any AH,B,X picrophore. The only potential picrophore that has been developed using DS3.5
visualizer is that from the upper oxygen and the centroid of the seven-sided ring, a d-value of
4.953 Angstrom. The lower oxygen and the seven-sided ring qualifies as a A-Path acidophore
with d = 2.860 Angstrom.
Figure 8.5.4-34 Potential AH,B,X geometries for amarogentin and artabsin. The artabsin is based
on a seven-sided ring. Amarogentin exhibits at least one picrophore with a d-value of 4.648
Angstrom (2% below the nominal 4.746 Angstrom). Artabsin exhibits one picrophore between
the upper oxygen orbital and the seven-sided ring with a d-value = 4.953 Angstrom (4% above
the nominal 4.746 Angstrom). Both may also support a dispersion centroid qualifying them as
super-bitter.
8.5.4.7 Summary of the proposed receptor d-values CONSOLIDATE
Signal Generation & Processing 8- 159
Figure 8.5.4-35 shows a summary of the d-values for the proposed four sensory receptors of
gustation and a provisional estimate of the width of each sensitivity. The sensitivity widths are
taken as 5% of the center value of the path based on the family of bitter (P-path) picrophores
discussed in earlier sections.
Figure 8.5.4-35 Proposed summary d-values for the gustatory receptors with an estimate of their
sensitivity profiles.
Each of the gustatory receptors is functionally a diol esterified to a phosphatidyl lipid. The
controlling feature of each GR is the d-value between the two oxygen orbitals of the diol. Immel
has discussed a variety of sugars using the diol extension (Section 8.5.5.1.3).
8.5.4.8 Other gustaphores
There are a number of organic and inorganic gustaphores described in the literature that can
be interpreted in the context of the hypothesis of this work.
Bryant & Mezine presented an extensive family of complex organic molecules in an investigation
of nocent stimulation of the trigeminal nerve of the rat. Probably all of the molecules quality as
gustants that are relevant to this work. However, the molecules contain large numbers of
individual gustaphores and odorophores that make their value in sensory research limited. See
Section 8.7.3.1.2.
8.5.4.8.1 CaCl2 & MgCl2 as gustaphores or nocents
The sensory significant of CaCl2 and MgCl2 are unclear from the literature. They are frequently
reported as being perceived as bitter as well as or rather than salty. Alternately, they are
described as astringents within the general class of nocents. The reasons for this will be
developed below.
Both CaCl2 and MgCl2. are described as deliquescent, forming crystals including six molecules
of water. In the biological context, they are so deliquescent that they are, like alum, astringent
in the presence of biological tissue. The anhydrous calcium salt is deliquescent; it can
160 Neurons & the Nervous System
accumulate enough water in its crystal lattice to form a solution. When calcium chloride is
added to water (solubility–74.5 gr/100 ml), much heat is liberated. Magnesium chloride acts
similarly. Its solubility at biological temperatures is 60 gr/100ml. While these materials completely
ionize in water, it is the coordination chemistry of the hydrated cations that is significant in
gustation. The anion plays no active role. Magnesium sulphate (milk of magnesia) is also
perceived as both bitter and salty.
When in solution, the cations of these chemicals form coordinate bonds with the water
molecules. Calcium generally bonds to six or seven water molecules, occasionally eight or less
than six. For six ligands, the structure is octahedral. For eight ligands, the structure is hexagonal
bipyramidal. Other bonding levels typically employ one of these forms with one or more
coordinate locations empty (although some sources report a pentagonal bipyramidal structure
for this salt). At biological temperatures one calcium is predominantly bound to six water
molecules. Calcium forms complexes with many molecules. The ligand distances vary between
2.1 and 2.6 Angstrom. When coordinate bonded to water, the ligand distance is normally 2.431
Angstrom.
For a coordination number of eight, the hexagonal bipyramidal structure exhibits angles of 60
degrees between the six oxygen ions in the horizontal plane and 90 degrees between the
oxygen ions in that plane and the two oxygen ions at the points of the pyramids. Figure 8.5.436 shows the medial plane of the octahedral structure of the calcium ion fully coordinated
with six water molecules after CaCl2 is dissolved in water. The ability of the oxygen atoms to
participate in paired coordinate bonds with a sensory receptor is obvious. The nominal
calcium to oxygen spacing is from Kim et al212. As shown, the Ca(H2O)6+2 complex could
exhibit gustaphores with d-values of 3.437, 4.210 and 4.862 Angstroms. The smaller Mg(H2O)6+2
exhibits equivalent values of 2.9, 3.585 and 4.14 Angstrom Glusker et al., 1999). Bock et al.
(1994) have noted that for MgCl2, the hydrogen atoms are at an angle of 120-127 degrees
from the Mg–O bond. This angle would be 127.7 degrees if the pHOH was the nominal 104.5
degrees
Figure 8.5.4-36 Calcium cation fully coordinated
with water when in solution. Coordinate bonds
are shown dashed.
The cation retains its
conventional valence of +2. Dimensions in
Angstrom. See text.
212
Kim, S. Gregor, W. Peloquin, J. Brynda, M. & Britt, R. (2004) Investigation of the Calcium-Binding Site
of the Oxygen Evolving Complex of Photosystem II Using 87Sr ESEEM Spectroscopy J Am Chem Soc vol 126,
pp 7228-7237
Signal Generation & Processing 8- 161
Yang et al. discuss variations in the angles between the oxygen ions in stressed complexes213.
Magnesium complexes in the same manner as calcium and with a similar variation in
coordinate levels (with six being the predominant value 79% of the time214). The oxygen to
oxygen spacing in the octahedron form is 2.9 Angstrom. The spacing between opposed
oxygen ions is 4.14 Angstrom. [xxx duplicates above ]
[xxx Is either form of calcium or magnesium good matches for the values of a picrophore,
d=4.2 Angstrom, or Natrophore, d=3.3 ] The d-value of 4.14 Angstrom for the opposing oxygen
ions of the hexagonal Magnesium complex makes this structure a potential picrophore if it is
able to satisfy the stereo-chemical requirements and reach the appropriate sensory receptor
site.
Figure 8.5.4-37 shows the relationship between the these “salts” and the receptive range of the
gustatory sensory receptors. It clearly shows why these salts are perceived as bitter. Both have
d-values matching the values of the bitter sensory channel. CaCl2 and MgCl2 are both
picrophores. Both exhibit d-values that could qualify them as natrophores also, depending on
the sensitivity range of the sensory receptors..
[xxx edit ]
The d-values of these “salts” aid in
determining the receptive range of the
sensory receptors. In the case of the
picrophores, they exhibit calculated-values
of 4.14 (Mg(H2O)6+2) to 4.22 (Quinine xxx).
These values suggest a receptive range of at
least 0.08 around a center value of 4.18. On
the other hand, the gustaphores of these
salts at 3.437 and 3.585 are less effective in
stimulating the sodium sensitive channels
and help define the sensitivity range of the
sodium sensitive channel. The sodium
sensitive channel can be considered to be
centered at d = 3.30 with a range at most
only marginally higher than 3.437. These
values suggest a center value of 3.30 with a
range on the order of 0.28 for the sodium
sensitive channel.
This analysis suggests the picrophores of
Magnesium salts are formed by the oxygen
atoms separated by 120 degrees about the
cation while the picrophores of calcium salts
are formed by the oxygen atoms on
opposite sides of the cation. In both cases,
these hydrated ion structures exhibit multiple
picrophores associated with each cation,
raising the probability that these structures
will form dual coordinated bonds with the
bitter sensory receptors. The nearest oxygen
atom pairs of these structures do not act as
effective gustaphores.
8.5.4.8.2 The
stimulants
thio
moieties
as
Figure 8.5.4-37 The potential gustaphores of CaCl2
and MgCl2 ADD. Both salts exhibit significant
picrophores, at 4.14 and 4.21 Angstrom. The
calcium salt may also exhibit a natrophore at
3.437 Angstrom, depending on the receptive
range of the sensory neuron receptor.
213
Yang, W. Lee, H-W Hellinga, H & Yang, J. (2002) Structural analysis, identification, and design of calciumbinding sites in proteins Proteins struct funct genet vol 47, pp 344-356
214
Glusker, J. Katz, A. & Bock, C. (1999) Metal ions in biological systems Rigaku J pp 8-16
162 Neurons & the Nervous System
A variety of thiol compounds, R:SH (mercaptan) and R:SR’ (a thioether), appear as gustants in the
literature. The lack of sufficient oxygen in some molecules has hampered their interpretation as
gustaphores. However, the formula shows the Sulphur in these moieties exhibits unpaired
electrons that can act as orbitals in a DACB arrangement. The amino acid, cysteine, is a member
of this group. It is frequently perceived as “sulphurous” in both its D– and L– isomer forms.
Sulphurous is a term outside the normal organic acid, sweet, sodium ion or picric channels of
gustation. Engel has provided simple Fischer diagrams for an assortment of sulphur bearing
compounds (including some homologs) providing a perception of sweetness215.
Figure 8.5.4-38 shows the distances between the relevant orbitals in cysteine. It should be obvious
that the sulphurous perception of cysteine is due to its ability to stimulate the acid receptor with
its d-value of 2.223 Angstrom, the sodium channel receptor with its d-value of 3.472 Angstrom and
the sweet receptor with its d-value of 2.785 Angstrom (and probably its d-value of 2.979
Angstrom). This situation is characteristic of many thiols. As will be shown when discussing the
perception of gustaphores, the thiols do not appear at the nodes of a multidimensional space.
Their perception results from stimulation of multiple sensory receptors simultaneously.
Based on the above analysis, cysteine may
not exhibit the identical set of gustaphores in
its two isomeric forms. [xxx ]
Figure 8.5.4-38 The d-values associated with
orbital pairs in the amino acid, cysteine. The
multiple pairs of orbitals provide multiple d-values
able to stimulate multiple gustatory receptors.
See text.
215
Engel, K-H. (1999) The importance of sulfur-containing compounds in fruit flavors In Teranishi, R. Wick,
E. & Hornstein, I. eds Flavor Chemistry: Thirty Years of Progress pg 267
Signal Generation & Processing 8- 163
Shallenberger & Acree have provided some information concerning a possible structural
arrangements important in understanding the role of thiols in gustation. Figure 8.5.4-39 shows this
molecule. The complete structural configuration elicits a bitter sensation but the abbreviated
form they presented in 1971 could not account for it. The thio-carbamide was originally found
in phenylthiocarbamide (PTC) by Fox (1932). The Jmol representation of this complete molecule
suggests it can be perceived as bitter based on the distance between the extreme nitrogen and
the aromatic ring acting as an electron donor. The aliphatic portion is planar but slightly out of
the plane of the ring. The distance from the extreme nitrogen to the center of the planar ring is
4.94 Angstrom. The effective location of the charge associated with the ring has not been
determined.
Steudel & Steudel have presented some
useful data on the bond lengths and angles
associated with a variety of sulphur based
compounds216.
8.5.4.8.3 The “water”
sensory response
gustatory
Several investigators have reported a
“water” response for their sensory neurons.
With an understanding of the concept of
pre-adaptation, this response should not be
surprising. A water flush after stimulation
with any stimulant or even normal saliva
can be expected to constitute a change in
the taste bud environment. This previous
exposure to a stimulant or saliva can be
considered a pre-adaptation step with the
water wash a subsequent stimulus test
interval. The sensory neurons can be
expected to report this change. Pages 262268 in Cagan & Kare discuss their
experiments.
Figure 8.5.4-39 Phenythiocarbamide as presented
in stick and 3D form. Stick form was shown for
discussion by Shallenberger & Acree in 1971. 3D
form created using Jmol. The more complete
structure suggests a perceived bitterness based
on the phenyl ring acting as an electron donor.
xxx has described the water taste in his
Firmench Award Address217. “Concentrations of salt that are weaker than the levels in saliva may
give rise to bitter tastes, which may in fact be water tastes. Water tastes occur when the mouth
is completely or nearly-completely adapted to any suprathreshold concentration of a compound
and then water is sampled.”
8.5.4.8.4 The “browned flavors” sensory response
Hodge, followed by Ohloff218, have studied the characteristics of a large range of molecules
associated with the sensations resulting from the browning of food products during cooking.
Hodge analyzed the chemical structure of a wide variety of molecules he associated with the
browned flavor sensation219. He described these sensations as;
216
Steudel, R. & Steudel, Y. (2009) Sulfur Dioxide and Water: Structures and Energies of the Hydrated Species
SO2AnH2O, [HSO3]–AnH2O, [SO3H]–AnH2O, and H2SO3AnH2O (n = 0–8) Eur J Inorg Chem pp 1393–1405
217
Xxx (2001) xxx In Spanier, A. et al. eds. Food Flavors and Chemistry. Royal Soc Chem pg 43
218
Ohloff, G. (1981) Bifunctional unit concept in flavour chemistry In Schreier, P. ed. Flavour ‘81. Berlin:
Walter de Gruyter pp 757-770
219
Hodge, J. Mills, F. & Fisher, B. (1972) Compounds of browned flavor derived from sugar-amine reactions
Wash, DC: Agricultural Research Service, U.S. Department of Agriculture.
164 Neurons & the Nervous System
“They are essential for the recognition and acceptance by taste of many processed foods,
especially cereal-derived foods. Browned flavors include caramelized sugar aromas (1); food
aromas that have been described variously as toasted, baked, nutty, or roasted (2); corny and
amine-like aromas from cooked grains and meals; and both the desirable and undesirable burnt
aromas and bitterish tastes of roasted malt, nuts, coffee, chicory, cocoa, meats, fruits, and
vegetables (2). The objectionable burnt and bitter flavors of overheated or long-stored
dehydrated foods also fall within this class.”
Whereas Hodge noted the ability of many of these molecules to form an intramolecular hydrogen
bond between a divalent oxygen and an adjacent hydroxyl moiety, this relationship did not
suggest any arrangement between these molecules and a sensory receptor. Ohloff, in the same
time period as Shallenberger & Acree were working on the sugars, proposed the “browned
flavor” stimulants employed the same AH,B coordinate bonding to a sensory receptor as the
sugars (with a spacing of less than 3 Angstrom) but with an additional relationship. He suggested
this relationship involved the hydrophobic part of the molecule. He was unable to specify the
precise relationship between this apolar portion of the molecule and the receptor as a substrate.
No multidimensional analyses involving these flavors could be located. It appears the “browned
flavor” stimulants could support a dual channel sensation involving the G-Path channel and one
or more other best channels.
8.5.4.8.5 The role of amines & amino acids in the taste sensation TIE 8.6.2.6.6
[ rewrite and broaden to cover both categories ]
Belitz et al. have provided a simple table showing the critical role of the carboxyl group and a
ligand containing some form of nitrogen in giving the amino acids a sweet taste220. More
recently, Van der Heijden (page 104) has described the relative sweetness and bitterness of the
amino acids based on their steric form. Of those amino acids addressed, the D– forms are
invariably sweet and the L– forms invariably bitter. Van der Heijden provides a brief list of
properties associated with amino acids and peptides and the sensation of bitterness. [xxx list ]
Kier221, has provided a description of the sensations elicited by the amino acids that differs
significantly from that of Van der Heijden.
Kato et al. have provided considerable background on the gustatory properties of the amino
acids222. The discussion makes it very clear that it focuses on the requirements of the sensory
receptors and not the obvious properties of the stimulants that determine the gustatory
sensations.
Because the amino acids exhibit such a variation between sweetness, non-sweetness and
bitterness, they appear to offer a chance to develop a better understanding of the structural
relationships required to elicit sweet and bitter tastes in general. Unfortunately, the various
authors provide different findings with regard to the perceived taste of each amino acid. Kato’s
citations are primarily to investigators working in the 19th Century.
8.5.4.8.6 The phenols and aliphatic-aromatics
Phenol (C6H5OH) is the first member of a large family generally described as aromatic alcohols,
ArOH, where Ar is phenyl, substituted phenyl or one of the other aryl groups. Hydrogen bonding
plays an important role in both the intermolecular and intramolecular chemistry of the aromatics.
220
Belitz, H. Chen, W. Jugel, H. et al. (1979) In Bourdreau, J. ed. Food Taste Chemistry. Washington, DC:
American Chemical Society Chapter 4
221
Kier, L. (1972) A molecular theory of sweet taste J Pharm Sci vol 61(9), pp 1394-1397
222
Kato, H. Rhue, M. & Nishimura, T. (1989) Role of free amino acids and peptides in food taste In Teranishi,
R. Buttery, R. & Shahidi, F. eds. Flavor Chemistry: Trends and Developments. Washington, DC: American
Chemical Society Chapter 13
Signal Generation & Processing 8- 165
Phenol is generally known as carbolic acid and is both poisonous and caustic. However, its
compounds play a major role in both gustation and, as the next major section, 8.6, will show in
olfaction. While the name carbolic acid suggests an organic acid, is is actually a highly basic
aromatic alcohol.
The shared charges of the aromatic chemical structure is an electron donor. As such, it is able
to perform as an “orbital” in the AH,B DACB mechanism. This capability provides many potential
gustaphores when an aromatic ring is associated with one or more aliphatic side chains. When
in an aliphatic-aromatic relationship, the electrostatic potential of the aromatic may become
asymmetrical relative to the plane of the aromatic moiety. Jmol and the other 3D
representations are then needed to calculate the distances between the center(s) of charge of
the ring and the orbitals of the aliphatic structures.
The halogens play a major role in determining the properties of a stimulant as a gustaphore.
Such role is not apparent in the typical 2Drepresentations of molecules.
The halogens can play an important role in the gustatory properties of the aromatics. They can
effectively withdraw an electron from the charge cloud of the aromatic, disrupting its ability to
act as a negative charge source in an AH,B bonding relationship. Many other ligands can also
affect the ability of the aromatic to act as an effective orbital (Morrison & Boyd, 1971, pp 822840).
8.5.4.8.7 The non-hydroxyl guanidines
The role of the quanidine moiety frequently appears in the gustation literature. Quanidine (not
to be confused with quinidine) is an excellent example supporting the hypotheses presented
here. It is a simple molecule containing no oxygen or hydroxyl groups. Its orbitals are all NH or
NH2 groups as shown in Figure 8.5.4-40. The material is typically described as a base based on
its pH value although its structure shows it readily forms DACB’s with the A-Path GR’s leading to
a perception of acidity.
Two of the orbital are connected to the third via a carbon atom with a d-value of 2.32 Angstrom.
In addition, the two orbitals are connected to each other via a carbon with a d-value of 2.38
Angstrom. These values suggest this chemical should form a DACB with the A-Path GR (nominal
d=2.276 Angstrom) quite effectively and probably act as a multi-acidophore stimulant.
166 Neurons & the Nervous System
Figure 8.5.4-40 Quanidine, a stimulant with three gustaphores. The molecule is planar. Each of
the orbital pairs represents a gustaphore exciting the A-Path sensory receptor in accordance with
the Electrolytic Theory of the Neuron and the hypothesis presented here.
When combined with a simple phenol, or other phenol variants, through dehydration, the resulting
derivatives are noted for their perception of
sweetness.
The phenol guanidine
combination at the nitrogen double bonded
to the rest of the guanidine exhibits a dvalue of 2.71 Angstrom which is well within
the 5% tolerance on the DACB range of the
G-path GR. [SC-45647 used by Hellekant et
al. 1997 in art file from PubChem 3D viewer
]
When the guanidine moiety is combined
with a carboxyl group, that group is capable
of providing an additional C-path
gustaphore to the overall structure. The
combination is also capable of acting as a
super sweetener of the AH,B,X variant as
suggested in Figure 8.5.4-41. The structural
requirements on an AH,B,X super sweetener
are developed in Section 8.5.3.2. No Jmol
3D representation has been found that
supported precise distance measurements
for this molecule. There is a potential X site
nominally 3-3.5 Angstrom from the AH site
and nominally 3.5-4.0 Angstrom from the B
site.
Figure 8.5.4-41 The guanidine derivative, SC-45647
as a super-sweetener. The distance between the
AH,B pair and the additional phenol ring results in
an AH,B,X configuration. No Jmol model of this
molecule has been located. See text.
Signal Generation & Processing 8- 167
Hellekant et al. have studied several derivatives of the quanidine molecule (Section 8.5.8.3) in
detail, especially when combined with a phenol by dehydration as well as a carboxyl group via
a carbon.
8.5.4.8.8 Procaine and other local anesthetics
Procaine is a topical anesthetic frequently experienced by people during dental maintenance
and generally perceived as bitter. Its structure is shown in Figure 8.5.4-42. The spacings relative
to its gustatory performance is shown based on a Jmol representation of the Jmol file (CAS 59-461 described as verified on Wikipedia). The value of 2.91 is shown as 3.729 using DS3.5. The value
of 4.75 is shown as 4.876 using DS3.5. These differences are major and need to be resolved. The
pair of larger values both support the perception of bitterness via the P-path. However, the pair
of smaller values differ significantly.
8.5.4.8.9 Nutmeg and Mace
Nutmeg and mace (the spice and not the
riot control gas) are two ingredients
frequently used in cooking both for their
color and delicate flavor. According to the
Merck Index, XII Ed., 1996, the essential oil
obtained by steam distillation of ground
nutmeg is used widely in the perfumery and
pharmaceutical industries. This volatile
fraction typically contains 60-80%
d-camphene by weight, as well as quantities
of d-pinene, limonene, d-borneol, l-terpineol,
geraniol, safrol, and myristicin.
Figure 8.5.4-42 Structure of procaine with d-values
as represented using the Jmol visualizer. The
values using DS3.5 are significantly different for the
both values but particularly for the larger
distance..
D-camphene (ChemSpider 83259) is a very
complex bicyclo-heptane hydrocarbon
containing none of the orbitals normally
associated with either olfaction or gustation
except the C=C bond associated with a methylene group. A further extension of the hypothesis
of this work will be required to address these chemicals as gustants. Some of them are volatile
enough to be important in olfaction. Borneol is an alcohol with the hydroxyl group replacing the
methylene group of camphene. Chrysantheonone is the aldehyde associated with borneol. The
acetate is even more complex and contains two oxygen orbitals but no simple aromatic ring
structures or covalent carbon bonds. See Section 8.6.2.7.4.
8.5.4.8.10 Heterocyclics–caramel and butterscotch
The perception of caramel/butterscotch is based on maltol_8066 (a.k.a., veltol), a heterocyclic
of oxygen with both a carbonyl and hydroxyl attached directly to the ring. The resulting d-values
to the hetercyclic oxygen are 3.629 and 3.974 Angstrom. If the ring is still able to support
dislocated electrons, the d-values of 2.573 for the carbonyl oxygen and 2.597 for the hydroxyl
oxygen should be considered. Caramel and butterscotch are categorized as gustants rather
than olfactants due to their low vapor pressure. Quoting the Perfume Shrine;
“You can create the scent of caramel with 3-hydroxy-4,5-dimethyl-2(5H)-furanone. If you take
that molecule and add a small amount of ethyl butyrate, ethyl valerate, and phenethyl acetate,
you get a nice fresh garden berry that would work great in an Escada launch. God forbid the
public knew it.”
Sotolon_56569, a.k.a., 3-Hydroxy-4,5-dimethyl-2(5H)-furanone is very similar to maltol, except it
includes two additional methane groups attached to the main heterocyclic ring. These
additional features probably contribute to the commercial utility of the material.
8.5.4.8.11 Heterocyclics–the pyridines
168 Neurons & the Nervous System
Ache & Carr have discussed a wide range of pyridines and their role in the gustation of crayfish223.
They provide individual formulas and structures that can be analyzed with respect to their
stimulation of various OR channels. They provide considerable material on the “tuning” of the
crayfish receptors in terms of these compounds. Of specific interest is their figure 5 showing the
linearity of the stage 3 pulse rate versus the stimulus concentration applied to the stage one GR’s
when plotted using a linear ordinate and logarithmic abscissa.
xxx, writing in Cagan, (page 112 have described the major pyridines using Figure 8.5.4-43 It
illustrates a variety of pyridine molecules known to affect the chemoreceptors of crayfish224.
Figure 8.5.4-43 Effectiveness of 12 pyridines in stimulating the chemoreceptors of crayfish. The
effectiveness of the substanced decreases from left to right in each row. See text. From Hatt &
Schmiedel-Jacob, 1984.
8.5.4.9 The putative “umami” sensory response
The Japanese, beginning in 1908, have suggested a fifth fundamental taste sensation called
umami. The term appears to have been derived from their word, umami (delicious). The
designation appears to have a strong representation in their culture. Whether this sensation is the
result of a mixture of the other four sensations, an enhancement of the overall taste sensation or
a fifth sensation is yet to be resolved. Physiology and Behavior published a special issue devoted
to Umami in 1991225. While useful data came out of that conference little information appeared
supporting umami as a unique sensation of gustation. Yamaguch & Ninomiya have recently
asserted, “Umami makes a variety of food palatable, although it is not palatable by itself226.” In
the same article, they quote Ikeda as saying while “umami cannot be produced by any
combination of taste qualities, many researchers believed that it could be duplicated by the four
conventional tastes.” This work will show the latter is the case.
8.5.4.9.1 History of umami
223
Ache, B. & Carr, W. (1989) Chemorecetion in aquatic invertebrates In Cagan, R. ed. Neural Mechanisms
in Taste. Boca Raton, FL: CRC Press Chap. 5
224
Hatt, H. & Schmiedel-Jacob, I. (1984) xxx J Comp Physiol vol 154, pp 855+
225
- - - - (1991) Special Issue Physiol Behav vol 49(5)
226
Yamaguchi, S. & Ninomiya, K. (1999) Umami and food palatability In Teranishi, R. Wick, E. & Hornstein,
I. eds. Flavor Chemistry: Thirty Years of Progress. NY: Kluwer Academic/Plenum Chapter 36
Signal Generation & Processing 8- 169
Yamaguchi has been the leading investigator of umami. He has prepared the most extensive
study of this putative sensation, including extensive lists of chemicals believed to involve the
umami sensation227. More recently, he has presented some results of multi-dimensional analyses
and asserted that umami is represented by a different dimension than the other four historical
sensations228. However, the printed record of this poster presentation does not include any
graphical material and speaks of dimensions and vertices of the taste sensation space that may
suffer in translation. It does reference, but not cite in the paper, a larger paper of ten years
earlier229. This paper did not include a full multi-dimensional analysis. It specifically did not include
the basis factors demonstrating a four dimensional taste space. It leaves the question of whether
umami is a taste enhancer or is sensed by a distinct sensory channel unanswered. It is yet to be
resolved whether their interpretation introduces a fourth dimension in taste space or merely
occupies another vertex of the three dimensional space.
8.5.4.9.2 Recent literature on umami
At this time, it is hypothesized that umami is represented in a 3D taste space as distinct from
any of the four nodes but not orthogonal to them. A perception of umami is the result of
simultaneous stimulation by a natrophore, an acidophore and a glycophore, the latter two
due to the glutamic acid component of the glutamate ligand. As a result, the sensation
of umami does not fall along any of the axes of the 3D taste space but within the volume
of that space.
The principal stimulant associated with umami is mono-sodium glutamate, an unusual chemical
that includes the primary N-Path channel stimulant (hydrated sodium) and a well studied
stimulant, glutamate. The glutamate ligand exhibits the ability to coordinate with the A-Path
sensory receptor via its carboxyl group(s), and to potentially coordinate with the G-path sensory
receptor via its spacing of an NH and O combination providing an AH,B union. The d-value of
these atoms is 2.66 Angstrom versus the nominal 2.82 Angstrom for the G-path GR. The naturally
occurring L-glutamate is reported to be tasteless while D-glutamate is sometimes reported to be
sweet. The sensations elicited by mono-sodium glutamate are primarily those of salty, with a
slightly acidic, and to some a slightly sweet taste. These conclusions are consistent with Belitz et
al (1979).
The glutamate anion when present in the fluid milieu of the organism (not the oral cavity) also
plays a critical role in powering the sensory neurons. In this role, it is frequently described as a
neuro-facilitator. However, in excess, it is known to cause excessive neural performance. In this
case, glutamate can increase the intensity of the taste sensation due to its increasing the static
potential at the collector of the first Activa formed within the microvilli. The resulting increase in
sensory neuron sensitivity could be reflected in the sweet channels and potentially in the acid
and sodium channels as well.
Several sources have made comments to the affect that inosine and guanosine compounds may
elicit a umami perception, but without data explaining how or why. These very complex
compounds are likely to excite a variety of the GR’s other than the organic acid GR. Kurihara &
Kashiwayanagi addressed umami in 1998 and described the principle chemicals associated with
it as mono-sodium glutamate, disodium inosinate and disodium guanylate230. Disodium guanylate
incorporates both a sodium ion and a glycophore. Disodium inosinate is unique in that the
sodium excites the N-Path receptor, PtdIns and the inosinate is a derivative of the family forming
227
Yamaguchi, S. (1979) The umami taste In Boudreau, J. ed Food Taste Chemistry. Washington, DC:
American Chemical Soc Chapter 2
228
Yamaguchi, S. & Komata, Y. (1987) Independence and primacy of umami as compared with the four basic
tastes Annals NY Acad Sci vol 510, pp 725-726 A special issue of this journal
229
Yamaguchi, S. (1979) The umami taste In Bourdreau, J. ed. Food Taste Chemistry. Washington, DC:
American Chemical Society Chapter 2.
Kurihara, K. & Kashiwayanagi, M. (1998) Introductory remarks on umami taste In Murphy,
C. ed. Olfaction and Taste, XII. NY: New York Academy of Sciences pp 393-397
230
170 Neurons & the Nervous System
the N-Path receptor, PtdIns. There is little evidence these chemicals excite an independent
sensory channel.
The inosinate forms a dual hydrogen bond dimer with the inositol of the PtdIns according
to this theory. The dual hydrogen bond spacing is obviously the ideal 3.3 Angstrom.
In 1991, Yamaguchi reported a large scale test comparing the sensations of Orientals and
Caucasians of European origin231. No significant differences were found.
Also in 1991, as part of an extensive study of the chimpanzee, Hellekant & Ninomiya specifically
sought to understand the relevance of umami232. They asked the question and then responded
as follows; “Is There an Umami Taste Quality? Unfortunately, we have not yet acquired enough
data to answer this question. We have no fibers that responded exclusively to the umami
compounds.” Continuing; “We can state that the present study has laid the foundation for an
answer to this question, by creating an overview of the fiber types that exist in the chimpanzee,
but does not answer the question.” Their subsequent study did not provide a clear answer to the
above question233. Their results indicated MSG was only sensed by the N-path GR’s. They did note,
“These results suggest that the bitter and sweet tastes are conveyed in specific and separate
groups of nerve fibers in the chimpanzee.”
8.5.4.9.3 The underlying mechanism–the perception of umami
Figure 8.5.4-44 shows a selected list of the broad range of chemicals claimed to be associated
with the umami sensation based on purely psychophysical tests234. Note the difficulty of
determining any specific structural commonality between these chemicals using only twodimensional “stick figure” diagrams (frequently with the positive ion omitted). However, note the
commonality of the carboxyl group in a majority of the chemicals. The presence of this group
suggests at least one of the sensations elicited by this family is the acidic sensation due to
coordination with the serine sensory receptor (representing the H–best channel). The pair of
adjacent OH groups in the pentane ring may be separated sufficiently to be able to coordinate
with the AH,B receptor site of serine. The hydrated sodium ion of MSG is obviously able to elicit
the salty sensation simultaneously.
231
Yamaguchi, S. (1991) Basic properties of umami and effects on humans Physiol Behav vol 49, pp. 833-841
232
Hellekant, G. & Ninomiya, Y. (1991) On the Taste of Umami in Chimpanzee Physiol Behav, vol 49, pp.
927-934
233
Hellekant, G. and Ninomiya, Y. (1994) Bitter taste in single chorda tympani taste fibers from chimpanzee.
Physiol Behav vol 56(6), pp 1185-1188,
234
Schlichtherle-Cerny, Affolter, M. & Cerny, C. (2004) Taste-Active glycoconjugates of glutamate: new umami
compounds In Hoffman, T. Ho, C-T & Pickenhagen, W. eds. Challenges in Taste Chemistry and Biology.
Washington, DC: American Chemical Society Chapter 14
Signal Generation & Processing 8- 171
Figure 8.5.4-44 A selected group of stimulants represented as “umami” type in the literature. Note
the presence of a carboxyl group in nearly every chemical (some shown more explicitly than
others. See text. From Schlichtherle-Cerny et al., 2004
Thus, it is conceivable that mono-sodium glutamate represents a group of chemicals that
stimulate more than one of the basic sensory channels. However, there are other chemicals in
the group that do not contain either sodium or glutamate, including a long list of inosinates. The
inosinates may be particularly liable to associate with the active portion of the sensory receptor
of the N-Path channel which is believed to be an inositol ligand. Two organic acids are said to
be 5-30 times more effective than monosodium L-glutamate in eliciting the umami response.
In the case of lactic acid, note the obvious presence of two gustaphores, the A-Path gustaphore
(carboxylic acid) and the G-path gustaphore (HO–C–C–OH).
Figure 8.5.4-45 indicates the character of most stimulants described previously as perceived as
umami. They are in fact gustants containing multiple gustaphores. The more complex gustants
in the previous figure are not shown to avoid complexity. All of the gustants shown exhibit two
or more gustaphores. The resulting perception is similar to magenta in vision; it requires the
stimulation of multiple gustaphores of well known characteristics. All of the gustants shown exhibit
an acidophore and all exhibit a glycophore. Only MSG and aspartic acid exhibit a natrophore.
The hydrated sodium ion of MSG theoretically exhibits six natrophores. The nucleotides at upper
right in the previous figure have not been analysed with respect to their gustaphores. Selected
hand calculations suggest they are not important gustaphores. Most of them have d-values
interdigitated with the GR’s of gustation.
172 Neurons & the Nervous System
Nucleotides are three component
systems consisting of (1) a nitrogenous
base, (2) a five carbon sugar, and (3)
phosphoric acid. If the phosphoric acid is
removed by partial hydrolysis, the
remaining components are known as
nucleosides.
While ubiquitous in the discussions of this
work, the phosphate group does not
participate in gustation. The d-values
between singly bonded oxygen orbitals is
2.63 Angstrom. The d-value between a
doubly bonded oxygen and a singly
bonded oxygen is 2.52 Angstrom. These
values appear in the notch between the
A-Path and G-path GR’s. Their limited
perceived taste helps define the
acceptability profile of these two GR’s.
The d-values associated with the fivecarbon sugar are also interdigitated
between the d-values of the A-Path and
G-path and the N-path and P-path.
Exploring the analogy with the visual system,
two situations arise; the first where the
gustant exhibits gustaphores affecting
adjacent GR’s on the d-value line. This
situation is analogous to the perception of
yellow caused by the presence of a red and
a green spectral component simultaneously.
Lactic acid is a clear example of this
situation. The second situation arises when
the gustant exhibits two gustaphores that are
not stimulating adjacent GR’s. The result is
analogous to the perception of magenta
caused by the presence of a red and a blue
spectral component simultaneously.
No
example of this situation appears in the
previous two figures.
It is noteworthy that aspartic acid contains
four distinct gustaphore types, one type
stimulating each of the fundamental GR’s.
It in fact has two copies of the acidophore,
carboxylic acid..
Figure 8.5.4-45Stimulants associated with umami
due to their multiple gustaphores EDIT TOP LINE of
the four fundamental types. Frame A; the
acceptance range of the four fundamental GR’s
of taste. Other frames as labeled. The sodium ion
is highly hydrated in the example of frame B. The
number next to some columns indicate the
potential number of gustaphores of that type
present. All gustaphores are shown at unity
relative effectivity.
Danilova et al. (2001, page 984) identified a unique M-cluster that was most closely associated
with MSG but included other stimulants containing at least two gustaphores associated with the
N-path, two with the A-Path and one with the G-path. It was clearly not due to a single new and
unique gustaphore. The M-cluster was analogous to white in color vision. It is perceived as off
of the d-value line and it involved stimulation of multiple GR’s simultaneously.
8.5.4.10 The putative “non esterified fat” sensory response
Signal Generation & Processing 8- 173
In 2015, Running et al. provided a paper suggesting that fats might be a separate class of
gustants235. Their Abstract opens with,
“Considerable mechanistic data indicate there may be a sixth basic taste: fat. However,
evidence demonstrating that the sensation of nonesterified fatty acids (NEFA, the proposed
stimuli for “fat taste”) differs qualitatively from other tastes is lacking. Using perceptual
mapping, we demonstrate that medium and long-chain NEFA have a taste sensation that
is distinct from other basic tastes (sweet, sour, salty, and bitter). Although some overlap was
observed between these NEFA and umami taste, this overlap is likely due to unfamiliarity
with umami sensations rather than true similarity.”
Running et al. state in their introduction,
“However, documentation that oral NEFA exposure elicits a perceptible and unique taste
sensation, in addition to their olfactory and somatosensory sensations, is weak overall and
absent in humans. Studies in rodent models indicate that taste aversions to nutritive oil and
long chain fatty acids do not generalize to other taste sensations or to textural qualities
(Pittman 2010), suggesting the sensation is unique in this species.”
Their conclusion is even more damaging to their null hypothesis. “Notably, the taste sensation
elicited by long chain fatty acids is not wholly consistent with the expectations of “fattiness.”
Given the clear unpleasantness of the sensation in isolation, and the incongruity with the term
“fatty,” which has strong textural context, we propose a new term to describe the taste of long
chain NEFA.”
The nomenclature between the food community and the general organic chemistry community
must be clarified. Their Table 1 lists a broad range of fatty acids, sugars and other test materials
accompanied by the concentrations of each. The table does not list any “fats,” fatty acids
esterified to glycerol 236. Their fatty acids are typically considered derivatives of the carboxylic
acid family in the larger field of organic chemistry. The derivatives listed are generally
unsaturated fatty acids.
Lenninger defines the major component of depot, or storage, fats in plant and animal cells (1972,
page 192) as a neutral fat, or glyceride, i.e., a fatty acid ester of the alcohol glycerol. Running
et al. define their “fats” as the term is used in the food science and food industry, specifically as
non-esterified fatty acids. Thus, they are actually talking specifically about the fatty acids derived
from the carboxylic acid family and not about fats per se. The individual fatty acids can be
obtained from fats by hydrolysis. Whether hydrolysis occurs in a given situation depends very
greatly on the solubility of the glyceride ester (fat) and the temperature (and duration of the
temperature profile). Cooking has a major impact on the solubility and digestibility of fat. To the
extent the fat is hydrolyzed, the investigator is no longer discussing a fat. He/She is discussing a
fatty acid, a totally different chemical substance. When hydrolyzed, the resultant fatty acid is a
NEFA by definition. The more important question is whether it is saturated or unsaturated.
The fatty acids are discussed in Section 8.5.4.3. Fully saturated carboxylic acid derivatives exhibit
only one gustaphore, that of the carboxylic group with a nominal d-value of 2.268 Angstrom.
They are perceived as acidic under the hypothesis of this work. If the derivative is not saturated,
the molecule will exhibit additional gustaphores based on the distance between the C=C double
bond(s) and the orbitals of the carboxylic group. As an example, oleic acid_393217 contains one
double bond and exhibits d-values of 10.759, 11.735 and 2.079 Angstrom.. Linoleic acid_4444105
exhibits two double bonds (separated by three trans- carbon atoms) and its d-values are 10.739,
11.711, 2.078, 11.777, 10.892 and 3.257 Angstrom. As developed in Section 8.5.1.6, d-values
greater than about 6 Angstrom are not effective in stimulating the gustatory modality but Section
8.6 will show they are significant in stimulating the olfactory and/or oskonatory modalities. Thus,
both oleic and linoleic acids will produce a taste perception of acidic and linoleic will also
produce a taste perception of salty. The relative intensity of the acidic and salty perceptions is
not documented. An unsturated fatty acid with more than three carbons between any pair of
235
Running, C. Craig, B. & Mattes, R. (2015) Oleogustus: The Unique Taste of Fat Chem Senses
doi:10.1093/chemse/bjv036
236
Gunstone, F. (1955) The component acids of chimpanzee fat Biochem J. vol 59, pp 454-457
174 Neurons & the Nervous System
orbitals may cause an olfactory perception but will not cause a significant gustatory perception.
Depending on the volatility of the material, it may be necessary to consider stimulation of the
olfactory epithelium via retrograde nasal entry of the material.
There is no reason to believe Running et al. have defined or isolated a new gustatory channel or
mechanism.
8.5.4.11 The mints as nocents instead of gustants
There has always been a debate concerning whether the mints constitute gustants or are in fact
nocents.
Mentha (also known as mint, from Greek míntha, is a genus of plants in the family Lamiaceae.
The essential oils in the leaves of these plants are from 40 to 90% menthol with the remainder
dominated by carvone, limonene and other biologically sensitive chemicals. The family consists
of at least 20 members and at least 10 hybrids of commercial interest.
Spearmint essential oil includes naturally occurring carvone and limonene. In the last lesson
related to the R,S system in a discussion of chirality, the Khan Academy notes that ( R) carvone
smells like spearmint while (S) carvone smells like caraway seed. (R)-(-)carvone_388655 exhibits
d-values of 4.943, 4.461 & 2.951 Angstrom. (+)-(S) carvone exhibits d–values of 5.347, 4.278 & 2.950
Angstrom. The 3D representation of these isomers demonstrates they are distinctly different
odorophores from the olfactory perspective.
Menthol is not currently addressed as an odorant in the theory of this work (see Section 8.6.3.3.1).
It is grouped with camphor and eucalyptol as more complex molecular shapes not amenable
with the DACB theory of odorants.
This investigator had a recent encounter with a commercial product known as Altoids and sold
in the candy section of most food stores. Altoids mints are currently available in four flavors:
peppermint, wintergreen, spearmint, and cinnamon. The variant of this product labeled
peppermint contained sorbitol and natural flavors as their principle ingredients according to its
label. To this investigator, this product was tasteless and odor-free but had a significant nocent
properties when placed on the tongue. According to Wikipedia, sorbitol_5576, also known as
glucitol, is a sugar alcohol with a sweet taste which the human body metabolizes slowly. It is
frequently considered a non-caloric sweetener. Mannitol_6015 is a closely related sugar alcohol.
I infer that the natural flavors were dominated by menthol and that menthol was perceived as
very cold but not perceptually sweet.
See Section 8.6.6.3 for a more global discussion of the role of the mints and menthol. Menthol
exists in a variety of isomers (Section 8.6.13.1).
8.5.5 The vernier (intensity) operation of the gustatory modality
This section will only discuss the output of individual stage 1 sensory neurons as a result of
gustatory stimulation. Under a variety of conditions, it is necessary to consider both the area
and accessibility of the stage 0 stimulus presentation (effective capture area of each type of
GR) and the stage 2 signal processing (potential forms of signal summation) before attempting
to relate perceived tastes to real stimulants. Both of these aspects are discussed in the next
chapter of this work. [xxx temporarily housed in Section 8.10 below the end mark of pt 2 for this
chapter]
It will also be shown in that later material that the long-standing debate relative to the
labeled-line versus the across-neuron (nerve) pattern can be satisfactorily resolved based on
additional clarity in the details being discussed.
8.5.5.1 Background
The head groups of the phospholipids forming the sensory receptors are known to be highly
polarized, thereby presenting a large dipole electrostatic potential (DEP) to the input terminal of
the first Activa within each sensory neuron relative to the potential of the gustatory cavity. The
Signal Generation & Processing 8- 175
dual coordinate chemical bond formed between the individual gustaphore and the sensory
receptor causes a significant change (and sometimes a drastic change in the case of the “super”
stimulants) in this potential. After this introductory material, the abbreviation DEP will be shortened
to DP.
This area of chemistry has not been widely studied until recently. The current method of study is
primarily one of mathematical analysis of the molecular structure of selected chemicals. The
process is very computationally demanding even on modern computers. The computational
cost of these calculations limit their pursuit. Venanzi & Venanzi and colleagues have been a
principle group pursuing the calculations of the more general molecular electrostatic potentials
(MEP) of selected tastants and gustaphores237,238,239,240,241. Their analyses related to amiloride
contribute greatly to the hypotheses presented here both for the basic vernier change in the DEP
generated by a typical tastant, and also an understanding of how the “super” tastants influence
the sensory receptors.
Amiloride has been implicated in both sodium transport through the cell wall in the
chemical theory of the neuron, and as an antagonist of the N-Path sensory receptor. Only
the role of amiloride as an antagonist in gustation is of interest here.
The 1988a paper provides the geometry and potentials related to acesulfame, a relatively potent
sweetner structurally related to saccharine and containing the same SO2 and N–C=O
arrangements. In fact these structures are present in a six sided ring in acesulfame and a five
sided ring in saccharin. . The 1988b paper explores a group of analogues of perillartine, many
containing a cyclohexadiene ring. Many MEP maps are provided. Their discussion opens with;
“The electrostatic potential maps for the perillartine analogues, especially the most potent
ones, yield information on the electronic features that determine the recognition pattern
of these molecules. This recognition pattern can be used for understanding not only the
activity of the analogues in this study but also the activity of other structurally similar
analogues. In addition, from an analysis of all the maps at the two different distances, some
conclusions can be drawn about the preliminary stages of the receptor-analogue
interaction.”
From an analysis of the electrostatic potential contour maps of the perillartine analogues,
the importance of two negative electrostatic potential regions emerges: (1) a broad region
near the nitrogen and oxygen atoms of the oxime group, which reflects the effect of the
lone pairs on each of these atoms and does not generally vary in its position or depth from
analogue to analogue, and (2) a second region in the hydrocarbon section of the
analogue; a region that not only varies in its depth, shape, and extension but also in its
orientation with respect to the region identified with the oxime moiety. The depth and
extent of this negative electrostatic region as well as its orientation with respect to the
potential region around the oxime moiety appear to be crucial for receptor recognition
and, thus, are significant indicators of taste potency. This result emphasizes the importance
of the hydrocarbon domain: a point recently underscored by the structure-activity data
and first made, in a different context, by Kier.
237
Venanzi, T. & Venanzi, C. (1988a) A molecular electrostatic-potential study of acesulfame Analytica
Chimica Acta vol 210 pp 213-218
238
Venanzi, C. & Venanzi, T. (1988b) Ab initio molecular electrostatic potentials of perillartine analogues:
Implications for sweet-taste receptor recognition J Med Chem vol 31(10) pp1879-1885
239
Venanzi, C. & Venanzi, T. (1992) Molecular recognition of amiloride analogs: a molecular electrostatic
potential analysis. 1. Pyrazine ring modifications J Med Chem vol 35 pp 1643-1649
240
Venanzi, C. & Venanzi, T. (1993) MEP of side chains In Simon, S. & Roper, S. eds. Mechanism of Taste
Transduction. Boca Raton, FL: CRC Press pp 427-462.Chap. 15
241
Buono, R. Venanzi, T. Zauhar et al. (1994) Molecular dynamics and static solvation studies of amiloride J
xxx pp xxx
176 Neurons & the Nervous System
They go on later;
“Although the recognition model is used here as an indicator of biological activity, a
complete description of the taste potency must involve other factors among which are
conformational dynamics, polarization, steric, and hydrophobic effects.”
The 1992 paper shows the dual coordinate chemical bond between several of their molecules.
However, these bonds relate to an artificial formic acid with d-value= 1.8Angstrom and do not
relate to the d-value of 3.3 required to bond to the N-Path receptor of this work. The paper
contains extensive citations. The dual bond suggested by the artwork in this paper is a parallel
rather than antiparallel bond. Closer 3D examination of the many electron pairs on nitrogen and
oxygen atoms in amiloride, Figure 8.5.5-1, may uncover a potential coordinate bond of the
appropriate spacing to block the N-Path receptors. While largely planar, amiloride is not planar
in the important areas associated with blocking the N-Path receptor.
Using the Jmol representation in
ChemSpider, amiloride exhibits d-values of
2.196, 2.956, 3.586 and 3.601 Angstrom
among others. The d-values when in solution
have not been found in the literature.
Their 1993 paper addressed the effect of
side chains in the MEP of a tastant. Their
focus on amiloride in the 1994 paper is not
easily
extended
to
other
stimulants/depressants of interest with the
needed precision. Like the 1992 paper, the
figures provide only Fischer (planar)
representations of very complex molecules.
This paper is focused on the MEP of their
molecules in solution. Overall, these papers
lack a framework pointing to a specific null
hypothesis underlying their exploratory
research.
Figure 8.5.5-1 The many lone pairs of electrons in
amiloride available for dual coordinate bonding.
The auxiliary lines indicate the distinct planar
structures in the figure.
Culbertson & Walters (1991) discussed a 3D
model of a sweet channel receptor that was
deduced from the effectiveness of a variety
of artificial sweeteners. They described their
model primarily from a stereographic perspective using a very conceptual caricature.
8.5.5.1.1 Dipole potential and related dipole moment
A critical aspect of intensity sensing in gustation involves measuring the dipole potential of a
stimulant when in a DACB relationship with the appropriate GR. While the dipole potential of
interest is related to the dipole moment of a stimulant in free space, the value of interest here is
an integrated value when in solution and relative to the DACB relationship. It is the dipole
potential presented to the GR at the DACB while the stimulant is in solution that is of fundamental
interest.
Narasimhan has presented some basic research on the dipole moments of a variety of saturated
dicarboxylic (Lewis) acids of interest in gustation242. He focused considerable attention on the
variation in dipole moments associated with various rotations around single carbon-coarbon
bonds. He used dioxan as an aid to disassociation of these molecules in dilute solution.
242
Narasimhan, P. (1957) Dipole moments of saturated dicarboxylic acids
Signal Generation & Processing 8- 177
Venanzi, Buono et al. looked at additional aspects of the MEP of amiloride (a complex molecule
“known to be a potent inhibitor of sodium transport in a variety of cellular and epithelial transport
systems”) in 1995243. Figure 8.5.5-2 shows a rearrangement of a Venanzi & Buono et al. figure. The
left frame shows amiloride in an uncombined configuration as exists in in the gas phase. The field
lines are relatively conforming to the shape of the molecule. The conventional dipole moment,
and dipole potential are derived from this configuration. The right frame shows the same
molecule in a potential “encounter” with acetic acid. These two entities are not likely to form a
DACB bond due to the difference in d-values of their relevant orbitals. Their paper did not
address how amiloride might couple with sodium or a putative sodium receptor.
Figure 8.5.5-2 Computed molecular electrostatic potential maps of amiloride. Left; amiloride “in
vacuum.” Right; amiloride in an encounter with acetic acid. The encounter involves the
guanidine group of amiloride. Neither Venanzi & Buono or this work indicates an actual
coordinate bond or bonds between the amiloride and acetic acid. Contours in units of kcal/mol.
See text and original captions. Rearrangement of Venanzi, Buono et al., 1995.
A zero potential line is shown passing through the space occupied by the hydrogen bonds. Not
shown is the presence of water surrounding these two structures in an overall 3D configuration.
The presence of water would cause the zero potential line to curve around the amiloride in a
closed 3D envelope. The parameter of interest here is the dipole potential (DP) between the
midpoint of the DACB and the water environment surrounding the amiloride. It is proposed that
this dipole potential is sensed by the sensory receptor in step 2 of the transduction mechanism
of chemical sensing.
In their amiloride forms 1, 18 & 19, similar to the case illustrated above, amiloride exhibits d-values
of 3.586 Angstrom between their N1 and N12 and their N1 and N9. These gustaphores are more
likely to couple to the sensory receptor of the N-Path and thereby block hydrated sodium from
stimulating this receptor. Their chlorination of these forms may bring the d-values even closer to
the 3.3 Angstrom value. Alternately, they could be coupling to a hydrated sodium entity and
thereby blocking one of the potential DACB opportunities of the hydrated sodium to couple to
the sensory receptor. Both of these possibilities are discussed in the Venanzi and colleagues
papers. Since, hydrated sodium exhibits several sites of DACB coupling, it is suggested that the
Venanzi, C. Buono, R. et al. (1995) From Maps to Models: A concerted computational approach to
analysis of the structure-activity relationships of amiloride analogues In Reynolds, C., et al.; Computer-Aided
Molecular Design. Washington, DC: American Chemical Society ACS Symposium Series, vol 589
243
178 Neurons & the Nervous System
concentration of amiloride in a solution would play an important role in the ability of hydrated
sodium to stimulate a sensory receptor.
Venanzi, Buono et al. developed a “pharmacophore hypothesis” to identify a molecule that
could form a stable blocking complex with an undefined sodium channel, based on certain
assumptions concerning the structure of amiloride. Their focus was not on the DACB coupling but
on substitutions in the the pyrazine ring. They note, “Although this approach has some features
in common with the Active Analogue Approach of Maisha, it is, in contrast, not a traditional QSAR
analysis.” Their pharmacophore does not relate to amiloride as a gustant with an appropriate
gustaphore (the strongly basic guanidine group) as defined in this work.
Venanzi, Buono et al. have noted two relevant forms of amiloride, “The Al free base conformer
(OCCN=180/) was found to be more stable than the A4 (OCCN=0/) in solution. This sheds light on
earlier NMR studies which were unable to distinguish between the two conformers in solution.”
Venanzi, Buono et al. noted, “In order to investigate these issues further, in the absence of
explicit knowledge of the molecular structure of the ion channel, we are using the
technique of stereolithography to build plastic models of the global energy minimum
conformers of amiloride analogues and of their complementary molecular shapes.”
It will be proposed below that the effective dipole potential of the combination of the
GR/gustaphore combination can present a very significantly different DP to the 1st amplifier of the
sensory neuron as a result of the dispersion process associated with the AH,B,X relationship.
The dipole potential of the homologous members of a stimulant family may vary significantly
being the dipole moment (measured in Debye and taken as the most easily measured
parameter) is the product of the dipole potential (measured in electrostatic volts) and the size
of the stimulant molecule (measured in an effective length between the charges required to
account for the net dipole potential). It can also vary significantly among closely related
derivatives. Methanol CH3OH, or CH4O, has a significant dipole moment of 1.71 Debye,
Methanal (formaldehyde), CH2O has a value of 2.33. CH3NO2 has a large value of 3.5 Debye
8.5.5.1.2 Dipole potential calculation
The calculation of dipole potentials based on computerization is in a rapidly expanding, but still
primitive state. The available calculations are primarily designed for stand-alone representations
and do not normally calculate the dipole potential between one protected area on the surface
of the molecule and the integrated surface of the remainder of the molecule immersed in a fluid
(typically mucos, saliva or water).
The computational field appears to be centered on a program called PyMOL and an add-on
called APBS244. PyMOL is a program based on the Python language and copyright 2004;
“PyMOL is one lone scientist's answer to the frustration he encountered with existing
visualization and modeling software as a practicing computational scientist.”
APBS -- Adaptive Poisson-Boltzmann Solver.
Quoting from the APBS User Guide/package-overview;
“The APBS sub-directory examples contains several test systems which show how to use
APBS for binding energy, solvation energy, and force calculations. The file
examples/README.html contains descriptions of the test cases and links to anticipated
results. Examples can be run and compared to expected results by running make test in
each example directory.
Additional examples are provided as part of the APBS tutorial (doc/html/tutorial/),
described in more detail in the Documentation section.”
244
http://www.poissonboltzmann.org/apbs/user-guide
Signal Generation & Processing 8- 179
The APBS program is moving rapidly toward a Windows compatible graphic interface but its utility
is still beyond this author’s capability at this time (11/2013). Much of its utility relies upon
manipulating binary code packages and preparing large data files in specific formats. It can
accept .PDB files from the Protein Data Bank. The PDB is focused on much larger molecules that
of interest in biological chemical sensing. The PDB data file must be converted to a PQR file using
the translator PDB2PQR, with several specified caveats.
8.5.5.1.3 Molecular electrostatic potential profiles
Immel245 has provided a dissertation on the molecular electrostatic potential (MEP) of a variety
of sugars. The dissertation is extensive, as indicated in the citation, and can contribute to the
hypothesis of this work. Only chapters 2, 3 & 4 out of 12 will be discussed here; the later chapters
deal with ever more complex sugars. The material is important because it calculates and displays
a 3D MEP that could be instrumental in calculating the dipole potential in this work. The dipole
potential results from the integration of the MEP of the molecule with respect to the location of
the DACB employed during coupling to the phospholipid of the sensory receptor.
Chapter 2 is extensive and develops the detailed 3D structure of a variety of saccharides along
with their MEPs. At one point (page 41), he notes, “Semiempirical calculations of other
conformations of 2 [ß-D-fructose, ed.] are encumbered with the fact that the minimum energy
geometries generated represent the state in vacuo, which may substantially be altered on
solvation with water. This applies to the conformations emerging from very elaborate ab initio
calculations and AM1-based semiempirical investigations, as well as to those emanating from the
more simple PIMM88 force field methodology.” He concludes with, “In summary, much remains
to be learned about the intricacies of the mechanism(s) involved in activation of sweet-sensitive
cells, and direct solid evidence is urgently required.”
Chapter 3 of Immel opens with an important observation, “Sucrose and other sugars are
over-functionalized with hydroxyl groups of almost identical properties, such that predictions of
their relative chemical reactivities are highly speculative. Especially difficult is the assignment
of relevant local electrostatic and hydrophobic molecular properties which determine their
chemical and biological behavior. In view of the availability of modern molecular modeling
techniques, the MOLCAD-program in particular, it deemed opportune to probe this methodology
with a structure as complex
as sucrose. In this chapter, the computational basics for
generating molecular electrostatic potential profiles and lipophilicity patterns (MEP's and MLP's)
are detailed and the far-reaching chemical implications and biological significance is discussed
for sucrose with the aim to gain further insights into hydroxyl group reactivities and the mechanism
by which sweetness is elicited.”
Within the context of gustation, the sugars are not over-functionalized because only
specific conformational pairs of hydroxyl groups can participate in DACB with the GR 2 (GPath) receptors.
His figure 3-1 stresses the importance of employing 3D representations of the sugars rather than
the more convenient Haworth representations. It also shows the location of several hydrogen
bonds in sucrose coupling the fructose and glucose moieties and preventing the participation
of these hydroxyl groups in forming DACB with the sensory receptors. He has also analyzed the
conformation of sucrose in other solvents than water. In this chapter, Immel makes the
assumption that sucrose does not hydrolyze into fructose and glucose. Therefore, he focuses on
how sucrose might bind to a receptor on strictly stereographic grounds using caricatures.
Immel’s conclusions relative to sucrose sensing are modest and do not consider the electrostatic
potentials of the combined phospholipid and monosaccharide structure resulting from the DACB.
245
Immel, S. (1995) Computer Simulation of Chemical and Biological Properties of Saccharides: Sucrose,
Fructose, Cyclodextrins, and Starch
http://csi.chemie.tu-darmstadt.de/ak/immel/script/redirect.cgi?filename=http://csi.chemie.tu-darmstadt.de/ak
/immel/publications/phdthesis/ Full text and index. Chapter 4;
http://csi.chemie.tu-darmstadt.de/ak/immel/publications/phdthesis/chapter04.pdf
180 Neurons & the Nervous System
Chapter 4 is devoted to the calculation of molecular electrostatic potentials (MEP) of various
sugars and assumes the AH-B-X concept. He notes, “The first rationalization of structure-sweetness
relationships by Shallenberger and Kier presumes the existence of a common AH-B-X glucophore
in all sweet substances, eliciting the sweet response via interaction with a complementary
hydrogen bond donor and acceptor functionality and a hydrophobic site in the taste receptor.
This very simple theory, also termed the "sweetness triangle",appears much too simple to explain
all of the observations at the present state of knowledge, particularly when bearing in mind that
sweet taste chemoreception is mediated by a cascade of complex biochemical processes that
are little understood at the cellular and molecular level.” This work has expanded on the work
of Shallenberger and Kier to overcome the “simplicity” objection, specifically as it relates to their
2D calculations and molecular representations.
At the time of his dissertation, Immel noted, “It is unlikely that progress can be made on this rather
speculative level by further reflections on the data presently available. If at all, it is clear that the
three-dimensional molecular shape of the sweet substrates and their respective physico-chemical
properties must be taken into consideration to mold a more precise overall picture of the
substrate-receptor-interactions involved in sweetness. Starting from these realizations the results
obtained from calculation of the molecular electrostatic potential profiles and hydrophobicity
patterns of the ß-pyranoid form of D-fructose should be discussed in the sequel.”
Immel provides little tabular results in Chapter 4. However, he does provide a series of Haworth
diagrams with specific comments about the sweetness of various similar monosaccharide forms.
The comments have not been fully correlated with this work to date. However, this can be done
by calculating the d-values for the various conformations.
Immel also notes, “The most striking shortcomings of the proposals (i) and (ii) in Fig. 2-14 for
the1,2-glucophore is seemingly the dramatic difference between fructose and sorbose, being
initiated by simple inversion of the remote 5-OH group. The proposed hydrogen bond releasing
effect of 5-OH, liberating the anomeric hydroxyl group 2-OH for interaction with the receptor by
competitive intramolecular hydrogen bonding with the ring oxygen – being possible in fructose,
but not in sorbose – was used to explain the taste difference of these two sugars. Due to the
nearly unaltered sweetness of the 5-deoxyderivative 8 in relation to fructose, this explanation
seems untenable.” The mere inversion of the 5-OH group is key to the sweetness of the
monosaccharide in the context of this work! He continues, “The taste properties of sorbose are
more readily rationalized, assuming the validity of Birch's assignment (iii, Fig. 2-14): a change in the
steric and electronic characteristics in direct vicinity of the 3,4-diol system would change the
sweetness quality substantially. [Fig. 2-14 describes conformations of fructose using Haworth
diagrams, ed.] The same effect is observed in the case of sucrose – in which the 2,3-diol unit was
assigned as the glucophoric moiety – when inverting the configuration at C-4 to
"galacto-sucrose", sweetness is almost completely lost. Similarly, going from glucose (in which the
3- and 4-OH groups seem to represent the AH-B unit) to the 2-epimeric mannose, sweetness
changes substantially.” Immel concludes his discussion with, “Although further evidence is
required to settle this question unequivocally, as of now, major significance is attributed to the
MLP's obtained for the two fructose conformers likely to prevail in solution: these(Fig. 4-3) clearly
favor Birch's[101]proposal (iii in Fig. 2-14), which places the AH-B couple of the glucophore into
the 3,4-diol grouping of fructose.” Note the use of the terms 1,2-glucophore and 3,4-diol in the
above discussion. He notes in Chapter 2, “ the hydrophilic portions of these sweeteners are more
compact, invariably located opposite to the hydrophobic region, and appear to contain the
AH-B couple of the glucophore: the glucosyl-2- and 3-OH group in sucrose and sucralose, versus
the 3,4-diol grouping in fructose.”
It appears further study of the Immel dissertation is warranted if further confirmation of the
hypothesis of this work is required. This would involve obtaining and familiarization with some of
the software packages he used, and specifically the writing of routines for obtaining the dipole
moment from spherical representations of the MEP shown in his dissertation. Immel has gone on
to publish a long list of follow-up papers based on his dissertation that are listed on the dissertation
website.
8.5.5.2 Analog intensity variation due to gustaphores
After selective dual coordinate binding of a gustaphore to the appropriate sensory receptor,
there remains the task of perceiving the difference in efficacy between tastants involving the
Signal Generation & Processing 8- 181
same gustaphore. This perception is supported by the electrostatic potential changes to the
sensory receptor brought on by the presence of the bound gustaphore as described in Figure
8.5.5-3. More detailed circuit descriptions will be presented after some introductory material.
182 Neurons & the Nervous System
Figure 8.5.5-3 The mechanism(s) of intensity determination (including dispersion) in gustation. Top;
circuit schematic of transduction elements. Bottom; caricatures expanded to show the proposed
layout of the receptors and tastants from an electrostatic potential perspective. The single plate
of a capacitor associated with each receptor is discussed in the text. Bottom left; quiescent state
of sensory receptor. Bottom middle; state of circuit when stimulated by a nominal tastant.
Bottom right; state of circuit when stimulated by a “super tastant,” a molecule exhibiting a single
gustaphore and an exposed region of high electrostatic potential appropriately positioned to
satisfy the AH,B,X criteria. See text.
Signal Generation & Processing 8- 183
The mechanism relies upon the analog signal sensitivity of the first (adaptation) Activa of each
sensory neuron. This mechanism has been described in detail in Chapter 2, Sections 2.2.3 & 2.2.4
and is also discussed in Section 8.5.1.3.
The upper left panel shows the basic circuit of a gustaphore specific path beginning with the
binding of a stimulant to a receptor ligand by way of the gustaphore/sensory receptor bond. This
bond causes a change in the electrostatic potential at the base terminal (B) of the 1st
(adaptation) Activa of the sensory neuron from its quiescent potential. This change is amplified
by the 1st Activa and results in a current flowing through the emitter terminal (E, arrow on lead at
lower left of upper frame). The lower three frames illustrate the individual states of the circuit as
indicated.
The phospholipid body shown in the upper panel is a very high impedance circuit resulting from
a reverse biased diode formed by the tail of the phospholipid. Because of its impedance, very
little current is required to flow through this element in order to generate a significant change in
potential at the base terminal (B).
The head group of the phospholipids identified as the sensory receptors in gustation are known
to exhibit a very large dipole potential between their bonding terminal and their connection to
the tail of the phospholipid. As a general rule, the change in the electrostatic potential presented
at (B) is small relative to this large dipole potential. However, in the case of super sweeteners and
super picric gustaphores, the change in the dipole potential of the receptor ligand is much larger
(accounting for the high efficacy of these materials). See Section 8.5.10.
The lower left panel illustrates the quiescent state of the sensory receptor and 1st amplifier in the
absence of any selected gustaphore. The receptor ligand exhibits its nominal large dipole
potential between its exposed terminal in the lingual cavity and its termination at the
phospholipid tail structure. This potential is passed to the other end of the tail structure and results
in an electrostatic potential, labeled the quiescent voltage (Vq) measurable as shown with
adequate instrumentation. This potential causes only a small current to flow through the 1st Activa
and into the dendroplasm of the sensory neuron.
The lower middle panel illustrates the situation when a specific gustaphore of a tastant becomes
bound to the sensory receptor. This tastant exhibits a “dipole potential” described as the
potential of its AB, H structure relative to the integrated MEP for the rest of the tastant exposed
to the saliva.
The distributed charge spread over the surface of the three-dimensional tastant molecule,
its molecular electrostatic potential, MEP, makes it awkward to speak of its dipole potential.
However, its effective dipole potential is the integrated potential of the entire surface
exposed to the saliva relative to the potential of its region involved in coordinate bonding
to the receptor.
The dipole potential of the tastant is usually small relative to the potential provided by the
receptor ligand. It causes the potential at terminal B to rise, causing greater current flow through
the 1st Activa into the dendroplasm. This increased current is amplified further in the 2nd Activa
of the sensory receptor and generates a change in potential at the pedicle of the neuron axon.
There are typically a large number of 1st Activa associated with an individual 2nd Activa. As a
result, many small currents are summed to create the signal used to drive the 2nd Activa. This
summation of currents causes a larger change in the pedicle potential of the 2nd Activa. It is this
change in potential at the pedicle of an individual sensory neuron that is summed with the signal
from other sensory receptors of the same class at higher levels within the neural system in order
to create a perception of the intensity of a specific tastant.
Calibrating the currents and potentials associated with a specific tastant and/or
gustaphore is particularly difficult because of the difficulty of controlling the density per unit
area of the tastant applied to a specific sensory neuron sensitive to that specific tastant.
Certain tastants (particularly man-made tastants) are known to have very high efficacy relative
to a specific gustaphore path (sometimes 10,000 times more effective than a simple gustaphore
of the same class. The mechanism employed relies upon a secondary electrostatic potential on
the surface of the tastant at a specific location unique to the receptor for that gustaphore path.
This secondary electrostatic potential is able to cause a major change in the intrinsically high
dipole potential of the receptor ligand. The result is a very large change in the base terminal (B)
184 Neurons & the Nervous System
potential compared to that created by the direct binding of the gustaphore to the receptor. This
mechanism is illustrated in the lower right panel. In sequence, the tastant must be selected due
to its gustaphore forming a dual coordinate bond with the receptor, a nominal change in the
base terminal potential is caused by the presence of the tastant in the signal path, and a much
larger change in the intrinsic dipole potential of the receptor ligand is caused by the close
proximity of the secondary electrostatic potential on the tastant to a similar (or complimentary)
potential feature on the receptor. See Section 8.5.4.2 for a broader discussion.
[xxx odorophore in this paragraph?? ]
How to precisely model the actions involved with the “super tastants” is unclear. A model based
strictly on electrostatics appears useful. In that case, the tastant can be modeled as a
conventional single gustaphore of dipole potential, Vg, supported by an auxiliary potential, Vsg,
presenting a charged area on the surface of the tastant as shown; and the sensory receptor can
be modeled as two large capacity Leyden Jars connected in opposition and resulting in a small
quiescent potential, Vq. The connection between the two electrostatic sources is supported by
the plate of a capacitor as shown on the right. When the odorophore binds with the receptor,
its small dipole potential causes the net potential, Vn, to become slightly more positive. However,
the introduction of the additional charge associated with the super tastant in the vicinity of the
capacitor plate of the receptor and at a small distance, X, introduces the possibility of changing
the dipole potential of one half of the receptor significantly. The resulting small percentage
change in the dipole potential of that half of the receptor can generate a very large percentage
change in the net potential, Vs, because of the small change introduced into the difference
between two large quantities. The result is a very large change, Vs – Vq, for the application of only
a small amount of tastant, the hallmark of a super tastant.
8.5.5.2.1 Two distinct response–concentration characteristics for sweeteners
DuBois et al. have addressed the perceptual response of humans to variations in concentration
of gustants consisting of both simple sugars and artificial sweeteners246. They find that these two
classes exhibit different response vs concentration characteristics. The implication is that they
employ different means of creating either the AH,B or AH,B,X relationship associated with their
DACB. The difference appears to highlight the potential for aftertastes among the artificial
sweeteners. It also appears likely, as they conclude, “Thus it is tentatively concluded that at least
two routes to receptor cell activation ust exist.” Recognizing that some of their simple sweeteners
are mono–, di– and poly–saccharides may be significant when analyzing this data. They discuss
the Beidler equation in multiple forms. Changing from % concentration to ppm concentration
complicates the comparisons.
The application of the Beidler equation involves a number of variables besides those explicitly
included in the formula.
8.5.5.2.2 Structure of simple artificial and Anti-sweeteners
Piisola has provided an overview of non-sucrose based sweeteners for the student versed in
organic chemistry structures247.
Figure 8.5.5-4 shows the dimensions of xylitol, one of the simplest of the sugar-alcohols. [xxx no
figure in aliphatic or ring form. In ring form it will probably resemble inositol. ]
246
Dubois, G. Walters, D. Schiffman, S. et al. (1991) Concentration–Response relationships of sweeteners In
Walters, D. Orthoefer, F. & DuBois , G. eds. Sweeteners. Wash. DC: American Chemical Society Chapter
20
247
Pissola, A.
(20014)Sugar Substitutes: The Chemistry of Synthetic Alternatives to Sucrose
https://www.jyu.fi/kemia/tutkimus/orgaaninen/en/research/Pihko/gm/Piisola_Sugar_Substitutes_2014.pdf
Signal Generation & Processing 8- 185
Sugar-alcohol is largely a historical term. Xylitol contains no saccharide or otherwise sugarlike structure. It is a highly hydroxylated, although not saturated, pentanol. In its aliphatic
form it contains three glycophores with d = 2.87 Angstrom (0.287 nm) giving it a perceived
sweetness but remaining non-caloric from
a nutrition perspective.
Xylitol is commonly thought to form a sugar
like ring in solution. It is approximately as
sweet as sucrose. This level of sweetness
suggests it does not exhibit any super-sweet
AH,B,X glycophores. See table in Section
8.5.9.6 for a comparison with other
glycophores.
8.5.5.2.3 Structure of super
sweeteners–acesulfame &
saccharine
[xxx check use of odorophore in text below
here. Some material copied from 8.6 ]
Mining the above papers and various
textbooks provides additional information on
the role of acesulfame and saccharine as
artificial sweeteners and “super” sweeteners.
It is generally reported that acesulfame, and Figure 8.5.5-4 The structural form of xylitol, a sweet
presumably saccharine oxidize slowly in air. alcohol EMPTY.
It is also reported that acesulfame combines
with a hydroxyl group when solvated. It is
not clear whether the reaction involves hydrogen bonding between the pyridine nitrogen and
the oxygen or whether the bonding is
directly between the nitrogen and the
oxygen.
Figure 8.5.5-5 suggests the
bonding is direct.
This figure provides a totally different
representation of these materials as
sweeteners. The upper frame shows the
materials as described based on dry
crystallographic observation. The lower
frame shows the same materials in their
proposed solvated forms. In the solvated
forms, the two chemicals exhibit a potential
gustaphore, O=C–N–O very similar to the
glycophores affecting the G– sensory path.
The open question is the precise d-value for
the two oxygen atoms s of these two forms.
A d-value of 2.6 ± a yet to be determined
tolerance would confirm their efficacy as
sweeteners. Note the potentially different
angles between the relevant atoms in
these six member and five member ring
molecules. The structure of the acesulfame
is basically planar except for the ring
oxygen which is out of plane by 22
degrees. Venanzi & Venanzi (1988a), Fig. 1,
have provided detailed MEP maps for the
crystal form of acesulfame computed for
planes parallel to but offset from the
specific structures in (1). The offset was a
Figure 8.5.5-5 Acesulfame and saccharine in dry
and solvated forms. Acesulfames on the left,
saccharines on the right. Upper row; dry form
determined by crystallography.
Lower row;
proposed solvated form.
186 Neurons & the Nervous System
requirement for the computational tools available in their time. Unfortunately, their attention was
centered on the oxygen atoms surrounding the sulfur atom rather than the glycophore proposed
here.
----[xxx discuss DEP in more detail. ]
In the context of this work, the important DEP is that between the baseline of the AH,B structure
and the portion of the molecule perpendicular to that baseline (for the simple sweetener) and
for the DEP of the X atom relative to the AH,B baseline for the super sweetener.
----
Signal Generation & Processing 8- 187
Figure 8.5.5-6 shows a potential configuration of acesulfame acting as a conventional sweetener
and as an enhanced super sweetener. The representation on the left shows the molecule
associated with a potassium ion, as usually found to avoid introduction of a natrophore if sodium
were used. The chemistry of this molecule is quite complex. However, the hetrocyclic nitrogen
can be considered a pyridine moiety with a free pair of electrons as shown. The pyridine nitrogen
is considered a tertiary amine since it lacks any associated hydrogen. This structure is known to
be sensitive to oxidation. It is commonly found to form pyridine N-oxide (M & B, pg 1088) or as
shown on the right in this figure to combine with a hydroxyl group from the solute (M & B, pg 721).
A mixed form of representation is used to stress the covalent form of the solvated acesulfame and
its ability to support the anticipated dual coordinate bonding to a G–path sensory receptor. If
the distance between the pyridine oxygen and the adjacent single covalently bonded oxygen
is approximately 2.6 Angstrom, the material will qualify as a glycophore.
The angles involving the pyridine nitrogen
are remarkably similar to those for a carbon
in the same position (M & B, pg 723).
The potential of the solvated acesulfame to
act as a super sweetener is clear. The
potential for the molecule to exhibit a
spacing between the AH,B baseline and
one of the three oxygen atoms at upper left
in the molecule, in accordance with the
AH,B,X geometries of Section 8.5.3, is
substantial. The required distances are 5.5
Angstrom from the more distance oxygen
and 3.5 Angstrom from the closer oxygen.
Figure 8.5.5-6 Acesulfame as a conventional and
super sweetener.
Left; crystalline form with
potassium as a ligand to avoid sodium that would
introduce a natraphore when hydrated. Right;
the proposed solvated ligand See text.
A similiar analysis can be applied to the
saccharine molecule to explain its role as a
sweetener and more specifically a super sweetener. In saccharine, the five member heterocyclic
ring, a pyrrole, has similar properties to the pyridine ring and the distances to the two oxygen
atoms bound to the sulfur atom are similar.
The artificial sweeteners–Saccharin and Aspartame
[xxx must edit the following paragraphs significantly and introduce aspartame 27_45 and or
aspartame 32_88.wpg showing a 2.91 glycophore and possibly still a picrophore. ]
Figure 8.5.5-7 shows the structure of aspartame in its role as a super-sweetener. Aspartame is
considered 150-200 times sweeter than sucrose. This is probably due to both its molecular
structure and the change it causes in the dipole potential of the sweetness sensory receptor. The
figure shows the AH,B,X group as formed of NH,O,O where the O at X is covalent oxygen An
alternate AH,B,X group is formed of NH,O,O with the O at X is singly bonded to a carbon and a
nitrogen at the lower portion of the figure. The configurations are compared in detail in Section
8.5.9.3 [xxx this call may change ].
188 Neurons & the Nervous System
Figure 8.5.5-7 Aspartame as a super-sweetener with an angle of 27.45 degrees at X. It can also
support an angle of 32.88 degrees. See text.
Aspartame has been found to play an important but controversial role in the sweetening of
foods. Its perceived side effects have spawned a cottage industry of de-toxifying programs. The
perceived problem is its breakdown into individual amino acids, and potentially methanol,
following ingestion after performing its desired gustatory task.
Figure 8.5.5-8 shows its principle “supersweet” glycophore is probably formed by the two
nitrogens, able to form a pair of AH,B coordinate bond with a distance between them of 2.6
Angstrom and the oxygen shown acting as X. The structure is nearly identical to the O-3, O-4 and
O-2 arrangement of galactose in the sweet channel receptor.
Signal Generation & Processing 8- 189
Figure 8.5.5-9 shows the reason aspartame
can taste bitter. It contains a picrophore
that is nearly a perfect structural match to
the sensory receptor, 3'-O’aminoacyl
glycerol. The reason is both are based on
the same ligand, aspartic amino acid. The
chemical can also elicit a sour taste due to
its carboxyl group.
Figure 8.5.5-8 Aspartame showing the axis of its
AH,B,X “supersweet” glycophore, the two nitrogen
atoms, and the location of the oxygen at the X
position. See text.
Aspartame incorporates three distinct gustophores.
----
Figure 8.5.5-9 Aspartame bonding to the picric
sensory receptor ADD HYDROGEN BONDS and
spacing of 4.2 Angstrom..
The structure of saccharin is relatively simple. However, its role as a sweetener probably
introduces known but new chemistry to this discussion. Figure 8.5.5-10 shows the structure of
saccharin. Deprotonation of the nitrogen leads to the normal anion of saccharin. Sulfur exhibits
5 shareable electron pairs with four involved in oxygen bonding in this example.
190 Neurons & the Nervous System
The bond lengths and angles relating to saccharin are well documented248,249. Shang et al. also
describe the distortions of the molecule relative to common descriptive drawing practice. Like
other chemicals, the individual bond lengths can vary by up to ten percent from standard bond
lengths in a specific moiety. The spacing between the two oxygen atoms associated with sulfur
is within the 7% tolerance (2.42-2.78 Angstrom) of the spacing originally derived from
Shallenberger & Acree for sweet channel stimulation. The third oxygen appears too far from the
initial pair to represent the X of the AH,B,X relationship. However, the phenol ring appears well
placed to satisfy this requirement. The shared electrons of the π bonds are known to provide the
necessary field potential. However, the elevation view of the molecule shows the symmetrical
arrangement of the AH & B oxygen atoms. As a result the triangle formed between these atoms
and the X location is isosceles.
The bitter after-taste associated with
Figure 8.5.5-10 The structure of saccharin using
standard bond lengths (see text). The spacing
between the two oxygen atoms is within the
tolerance for a sweet odorophore based on
standard bond distances. The triangle involving
the phenol as X is also isosceles based on the
elevation view of the molecule.
248
Zhang, Y. Wang, Y & Yu, H. (1995) Structural properties of o-sulphobenzoimide in complex crystals Cryst
Res Technol vol 30(6), pp 831-835
249
Matos, M. Miranda, M. Morais, V. & Liebman, J. (2005) Saccharin: a combined experimental and
computational thermochemical investigation of a sweetener and sulfonamide Molec Phys vol 103(2–3),
221–228
Signal Generation & Processing 8- 191
saccharin appears to be due to the odorophore formed between the isolated oxygen atom and
the phenol with a d-value of about 4.2 Angstrom (Section 8.5.xxx).
8.5.5.2.4 Potential dispersion centroids of super sweeteners
---Figure 8.5.5-11 shows a dimer situation with α-D-galactose representing both the sensory receptor
and the stimulant. O-3 and O-4 of each molecule are forming the AH,B coordinate bonds and
the four atoms define a plane (like that found in Ferrier).
Figure 8.5.5-11 ED A dimer situation with galactose as both sensory receptor and stimulant. The
radii associated with the X element intersect at O-2 of the stimulant but do not intersect at any
element of the receptor. Exchanging the centers of the rings results in an intersection near O-6
(or possibly C-6). For illustration only, the distances must be calculated in three-dimensional
space. See text.
192 Neurons & the Nervous System
Circles are overlaid on the coordinate oxygen molecules of the stimulant to locate X with radii
as estimated by Kier. Their intersection occurs at the nominal location of O-2 but do not intersect
near any element associated with the receptor. The circles do not intersect at the methylene C-2
of the stimulant in this projection. By switching the centers of the circles, their intersection occurs
near C-6 and O-6 of the stimulant.
If O-2, or C-6 or O-6, are significant in the sensation of super sweetness, the coordination
mechanism must bring one of these elements into some form of bond with the receptor, or an
adjacent receptor molecule of the sensory receptor complex. Figure 8.5.5-12 conceptualizes
an alternate coordinate bonding arrangement where the galactose receptor is bonding to an
unspecified glycophore. In the case of the glycophore, the electron-pair orbitals O-2, O-3 and
O-4 need not be oxygen-based. They can be any of the atoms or other species identified earlier
in Section 8.5.3.1.
-----
8.5.5.2.5 The super-bitter (picric)
stimulants EMPTY
[xxx show or discuss similar graphics for these
materials.
8.5.5.3 Defining the gustatory
receptor (standing alone)
Significant information is now available
concerning the steric geometry required of
the sensory receptors of gustation. The
challenge is to ascertain the most likely
organic molecules used within the microvilli
of the sensory neurons that can provide a
voltage change required by the Electrolytic
Theory of the Neuron to elicit a gustatory
sensation. The most specific information
relates to the distance, d, between the two
electron-pair sharing atoms found to be
present in all known gustatory stimulants.
Figure 8.5.5-12 Concept: A stacked situation with
the sensory receptor galactose interfacing with
the glycophore of an undefined stimulant. A third
(but strained) coordinate bond is shown between
the O-2 atoms, completing a tripartite union
(triangle).
The analyses performed here suggest the simplest steric forms of the gustatory receptor are
molecules containing ligands that are dimers of the ligands of the gustaphores, except probably
in the case of the sodium sensory receptor. True dimers tend to be planar structures. However,
the tripartite form of some glycophores suggest the interaction between the stimulant and the
receptor is probably not planar but involves a parallelism between the rings of the stimulant and
the receptor to support the juxtaposition of the two X features in the AH,B,X coordinate bonding
arrangement. This possibility introduces the question of the stimulant and the receptor being
enantiomers, either totally or at least with respect to the elements related to the AH,B,X
coordinate bonding. Because of the wide range of stimulants known to elicit a response via a
single receptor, it appears likely that only a degree of enantiomerism is required to satisfy the
coordinate bonding requirement.
Ferrier has described the unit cell of β-D-glucose as consisting of four molecules in an
orthorhombic configuration250. The molecules exhibited a complex orientation between each
other with hydrogen bonding involving the ring oxygen & O-6 bonded to the adjacent O-3 & O-2
Ferrier, W. (1960) The crystal structure of β-D-glucose Acta Cryst vol 13, pp678
250
Signal Generation & Processing 8- 193
respectively. The hydrogen atoms are not shown explicitly. This bonding was planar in character.
The O-1 and O-4 oxygen play no role in coordinate bonding within the cell.
The dimeric relationships assures an ability of the receptor to provide the necessary steric match
to the major stimulants and provides a gradation in the sensation intensity as a function of the
dimensional mismatch between the receptor and the less than ideal stimulant geometries. The
phospholipids known to be associated with the sensory neurons, if not the actual receptor areas
of those neurons, appear to provide this general capability. When present in the form containing
one conductive fatty acid chain (probably conjugated), these phospholipids provide a generic
receptor capability for each of the major taste sensations and an electrolytic path to the space
between the bilayers of the respective lemmas.
The literature of fatty acids originally assumed all fatty acid chains in lemma were fully saturated,
then were present with a single conjugated link near the middle of the chain and more recently
were present with a conjugated bond at every third carbon as in 4,7,10,13,16,19-docosahexenoic
acid (Siegel, 1999, pg 49; 2006, pg 35). The structure on page 34 is alleged to be the isoprene
subunit in natural polymerization. This material can be polymerized by two paths. One path
employs addition polymerization that leads to the squalenes (double bonds at every third
carbon). Cholesterol is the most common squalene. The second approach employs
condensation polymerization and leads to the terpenes (double bonds at every other carbon)
with the release of water. Vitamin A is the quintessential example of a fully conjugated terpene.
Additional research is likely to uncover very small amounts of the fully conjugated fatty acids (a
double bond at every other carbon according to the isoprene rule) in type 4 lemma dedicated
to only the sensory receptor function.
C Carboxylophore receptor– PtdSer provides a carboxyl ligand ideally located relative to the
surface of the microvilli lemma. The two oxygen atoms of the nonresonant carboxyl group exhibit
precisely the same dimension, d = 2.34 Angstrom, as do the acidophores associated with this
sensory channel.
The ability of a single sensory receptor to coordinate with both organic acids and amino acids
is illustrated in Figure 8.5.5-13.
The serine ligand of PtdSer is able to coordinate bond with other organic acids as well as with the
amino acids.
C Natrophore receptor– PtdIns provides a
ligand ideally located relative to the surface
of the microvilli lemma and is known to
associate with sodium in other processes. Its
cyclic structure provides the opportunity to
provide two electron-sharing atoms
positioned to match the required dimension,
d = 3.3 Angstrom, closely.
C Glycophore receptor– The variety of cyclic
ligands capable of forming phospholipids
and also coordinate bonding with the
potential sweet and super sweet stimulants
is very large. However, only a few of these
ligands have been reported as present in
sensory neuron tissue.
[xxx edit this after extending above
paragraph ]
Phosphatidyl xxx provides a ring structure
capable of coordinate bonding with the Figure 8.5.5-13 The serine ligand coordinate
principle tripartite glycophores of the bonding with organic acids and amino acids.
sweetness stimulants (and the bipartite
glycophores of a much wider range of sweetness stimulants).
194 Neurons & the Nervous System
----Based on its presence in globosides, it is likely the sensory receptor ligand is a form of galactose.
Such a ligand could coordinate bond with a galactose molecule acting as a stimulant in a planar
dimer (like found in the natural crystal of glucose) or it could coordinate with a galactose
molecule in a more complex three dimensional configuration.
Figure 8.5.5-14 shows a dimer situation with α-D-galactose representing both the sensory receptor
and the stimulant. O-3 and O-4 of each molecule are forming the AH,B coordinate bonds and
the four atoms define a plane (like that found in Ferrier).
Figure 8.5.5-14 EDIT A dimer situation with galactose as both sensory receptor and stimulant. The
radii associated with the X element intersect at O-2 of the stimulant but do not intersect at any
element of the receptor. Exchanging the centers of the rings results in an intersection near O-6
(or possibly C-6). See text.
Signal Generation & Processing 8- 195
C Picrophore receptor– Ptd3'Og provides a long ligand terminated by oxygen and the amine of
nitrogen that is ideally located parallel to the surface of the microvilli lemma to coordinate with
the picrophores. The dimension, d = 4.2 Angstrom.
Van der Heijden addressed the chemical structures related to bitterness (pages 94– 108).
However, the treatment is briefer and some confusion appears between sourness and bitterness
when discussing the AH,B dimension, d. He asserts the dimension is 0.15 nm [xxx chk ] for
bitterness (compared to 0.14 nm for the acidophores) instead of the value of 0.42 nm (4.2
Angstrom) [xxx chk ] for the picrophores developed above. His table VI describes picric acid as
the most potent of the picrophores.
He summarizes his findings in 1993 as, “Although some progress has been made upon more
precise characterization of bitter-receptor sites, the specifications among the various classes of
bitter principles are lacking. Even the existence of a bitterness receptor is rather speculative. The
relationship between the bitter taste and the shape of molecules is more complicated.” The last
sentence indicates the criteria for a bitter structure remained unknown at that time.
196 Neurons & the Nervous System
8.5.5.3.1 The nominal gustatory receptor EDIT
Figure 8.5.5-15 places the galactose-based sensory receptor in context with the sensory neuron.
The sensory receptor is esterfied to the electrolytically conductive fatty acid portion of the
phospholipid. The molecule normally exhibits a dipole potential. This dipole potential is a function
of the overall configuration of the combination of the sensory receptor and any coordinate
bonded stimulus. The resulting dipole potential at the bilayer membrane interface can change
by δV based on changes in the bond state of the sensory receptor. The smallest change in δV is
associated with the bipartite bonding via O-3 and O-4. This bonding is associated with a majority
of the natural sweeteners. A super-sweet sensation (per molar concentration of sweetener) is
associated with either tripartite bond, O-2, O-3 & O-4 or O-3, O-4 & O-6.
Signal Generation & Processing 8- 197
The galactose-based sensory receptor offers
the possibility of a “super-super-sweet”
sensation resulting from the coordinate
bonding to all four of its identified
coordinate sites (O-3, O-4, O-2 & O-6) at
once. Van der Heijden has supported this
assertion (1993, page 79).
Van der Heijden (pages 77–91) has
attempted to move the understanding of
enhanced sweeteners forward based on
two papers published in 1985251, 252. He
provided a series of figures describing
potential sites he labels δ for enhanced
sweetness relative to the nominal AH,B sites
of sweeteners. He did note the importance
of the distance A B rather than H B in
describing the baseline feature associated
with sweetness. His calculations, based on a
modified computer program, STERIMOL,
assume the broadest possible definition of
the AH,B condition and all possible auxiliary
sites within a given sweetener. As a result,
many potential auxiliary sites are defined
that may or may not contribute to
perceived sweetness. He did not address
the structural arrangements required to
bond maximally with the potential sensory
receptors. However, he did note the most
effective sweeteners typically had an angle
of 125 degrees at the A vertex.
Figure 8.5.5-15 The galactose-based receptor in
8.5.5.3.2 The electrolytic properties of context with the sensory microvilli. A bipartite
coordinate bond (d = 2.6 Angstrom) is the minimal
the receptors
requirement for eliciting a sweet sensation.
Extension of the bonding to either tripartite
configuration elicits a super- sweet sensation. A
super-super-sweet sensation is possible using a
quadripartite bond (O-2, O-3, O-4 & O-6). See
text.
The phosphatidyl moieties defined above,
when present in liquid crystalline form
constitute the receptor areas of the sensory
neuron villi. Excellent, although limited data
is available on the electrolytic (particularly
the surface potentials and the electron
density profiles) characteristics of the materials of interest here. Nearly all of the available data
relates to the two insulating and electrically inert, type 1, phospholipids, PtdCho and PtdEtn.
However, much of the orientation data related to these moieties should be useful. Hauser253 and
Hitchcock et al254. have provided some very early but invaluable information about these two
251
Van der Heijden, A. van der Wei, H. & Peer, H. (1985) Structure-activity relationships in sweeteners. I.
Chem Sens vol 10(1) pp 57-72
252
Van der Heijden, A. van der Wei, H. & Peer, H. (1985) Structure-activity relationships in sweeteners. II.
Chem Sens vol 10(1) pp.73-88
253
Hauser, H. (1976) Phospholipid model membranes: Demonstration of a structure-activity relationship In
Benz, G. ed. Structure-Activity Relationships in Chemoreception London: Information Retrieval Limited pp
13-24
254
Hitchcock, P. Mason, R. Thomas, K. & Shipley, G. 1974) Structural chemistry of 1,2
dilauroyl-DL-phosphatidylethanolamine: Molecular conformation and intermolecular packing of phospholipids
Proc. Nat. Acad. Sci. USA vol 71(8), pp 3036-3040
198 Neurons & the Nervous System
moieties. The 1981 Hauser et al. paper added considerable information about the packing of the
phospholipids when in both crystalline and liquid crystalline form255. Hitchcock et al. examined
phosphatidyl xxx. Many have built on their initial findings. Weiner & White provided more
information on a bilayer of phosphatidyl choline, PtdCho (which they label DOPC) in 1992256.
Flewelling & Hubbell describe the high internal potential of a typical lemma bilayer and
tentatively describe the source of the potential as the dipole moments of the phosphatidyl
moieties forming the bilayer257. Davis has provided data, contradicting Hitchcock et al. to a
degree. Davis found the individual phosphatidyl xxx molecules rotate about their long axis when
in the liquid crystalline state at biological temperatures. Seddon & Marsh have provided X-Ray
data showing additional detail regarding the organization of liquid crystalline films of two PtdEtn’s
of different chain lengths258. Sherer & Seelig have explored the orientation of phosphatidyl
choline polar heads and their dipole moment when subjected to the presence of strong
electrolytes259. Significantly, they note, “In spite of intensive research, the specific functional roles
of these headgroups are still, by and large, unknown. Also of interest, they used the term
molecular electrometer (a scalar device) to describe the quite accurate response of the
phospholipid to the electric charge at the membrane surface. It is the scalar dipole potential
that is the proposed result of initial transduction in this theory. Sherer & Seelig attempt to
rationalize the nomenclature of phospholipid chemistry and explore both the crystalline and liquid
crystalline form of these materials. White & Wimley have explored the presence of simple
peptides with phosphatidyl choline260. They address five fundamental questions. They also note
the dipeptides remain associated with the polar head. Marsh has reported on the water content
of the interior of bilayer membranes of PtdCho under various conditions261.
PtdCho and PtdEtn are particularly simple phospholipids that do not offer the requisite oxygen
rich binding sites required of receptors. All of the candidate type 4, semiconducting,
phospholipids exhibit significant oxygen atoms arranged to support coordinate bonding.
Figure 8.5.5-16 to show the operation of the microtubules (villi) at the molecular level. Lowe &
Gold have shown that odorant sensitivity and the odorant-evoked inward transduction current
are uniformly distributed along the cilia262. Based on this premise, Gold has asserted, “Thus, all
components of the transduction mechanism must be present in the cilia263.” Lowe & Gold also
showed that the latency of the transduction current is independent of the region of the cilia that
are stimulated: This implies that current is generated at the site of odorant binding.
255
Hauser, H. Pascher, I. Pearson, R. & Sundell, S. (1981) Preferred conformation and molecular packing of
phosphatidylethanolamine and phosphatidylcholine Bioehim Biophys Acta, vol 650, pp 21-51
256
Wiener, M. & White, S. (1992) Structure of a fluid dioleoylphosphatidylcholine bilayer determined by joint
refinement of x-ray and neutron diffraction data: III. Complete structure Biophys J vol 61(2) pp 434-447
257
Flewelling, R. & Hubbell, W. (1986) The membrane dipole potential in a total membrane potential model:
Applications to hydrophobic ion interactions with membranes Biophys j vol 49, pp 541-552
258
Seddon, J. Cevc, G. Kaye, R. & Marsh, D. (1984) X-ray diffraction study of the polymorphism of hydrated
diacyl and dialkylphosphatidylethanolamines Biochem vol 23(12), pp 2634–2644
259
Scherer, P. & Seelig, J. (1989) Electric charge effects on phospholipid headgroups. Phosphatidylcholine in
mixtures with cationic and anionic amphiphiles Biochem vol 28(19), pp 7720–7728
260
White, S. & Wimley, W. (1994) Peptides in lipid bilayers: structural and thermodynamic basis for
partitioning and folding Cur Opin Struct Biol vol 4, pp 79-86
261
Marsh, D. (2001) Polarity and permeation profiles in lipid membranes PNAS vol 98(14), pp 7777–7782
262
Lowe, G, Gold, G. 1991. The spatial distributions of odorant sensitivity and odorant induced currents in
salamander olfactory receptor cells. J Physiol vol 442, pp 147–68
263
Gold, G. (1999) Controversial issues in vertebrate olfactory transduction Annu. Rev. Physiol vol 61, pp
857–71
Signal Generation & Processing 8- 199
The figure starts with the two bilayer membranes separated by only a thin layer of water
crystallized into a hydronium liquid crystal by the confined quarters. The two bilayers are both
present as liquid crystals. They are formed of insulating lipids except in the shaded areas. The
insulating lipids are phosphoglycerides with fully saturated nonpolar tails (typically PtdEtn). In the
shaded areas, at least one of the tails is not saturated and is conductive like the polar head.
These phosphoglycerides are typically from the family of globosides.
They are typically
associated with neurons and exhibit more complex polar heads and unsaturated tails. It is
proposed these globosides constitute semiconducting electrolytic materials. The complex heads
provide the steric arrangement required to accommodate either the electrostenolytic process
or the OR mechanism shown. The more detailed discussion of these materials appears in [Section
5.2.4.1 ]
The shaded area on the left constitutes a first Activa. The semiconducting lipid of the
dendrolemma constitutes a region of P-type electronic material. The semiconducting lipid of the
reticulum also constitutes a region of P-type material. The region of hydronium between them
is an N-type electronic material. The sandwich forms an active electrolytic PNP transistor device,
an Activa. The polar head of the dendrolemma facing the mucosa provides a steric site suitable
for accommodating the electrostenolytic power supply providing –150 mV to the Activa collector
as shown (See Section xxx). The polar head of the reticulum facing the dendroplasm passes the
electron current directly into the dendroplasm as shown by the dashed arrow. The magnitude
of this current is controlled by the bulk potential of the hydronium N-type material.
Figure 8.5.5-16 The proposed molecular operation of the microtubules in GRN’s based on the
Electrolytic Theory of the Neuron. A portion of the surface of a microtubule extending from the
dendritic knob is shown. The portion is typically replicated many times. This proposal can be
compared to the equivalent conceptual transduction mechanisms of the chemical theory of the
neuron, such as Ache, 1994. See text.
The N-type material of the Activa is connected to an adjacent P-type region on the right, again
formed of a semiconducting globoside. The junction of the P and N materials forms a PN diode
junction and an electrical connection to the stimulus sensing complex, the GR. The diode is
forward biased and acts like a “low” impedance electrical conductor. The polar head of the
globoside forming the semiconducting dendrolemma is sterically selected to support a steric
200 Neurons & the Nervous System
coupling with the GR in liquid crystalline form. The polar head also constitutes one plate of a
capacitor, CT, with the GR as the dielectric. The other plate is formed by the interface between
the GR and the mucosa.
The GR may or may not exhibit a dipole moment in the absence of a odor stimulant (horizontal
bar inside left half of GR). However, in the presence of a sterically appropriate odor stimulant, it
is a zwitterion and will exhibit a dipole moment as shown by the inclined bar on the right. This
second dipole moment is due to a quantum-mechanical change in the configuration of the GR
molecule(s). This change in dipole moment between the absence and presence of a stimulant
causes a change in the dielectric properties and the resultant voltage on the capacitor, CT. The
change in voltage is passed through the forward biased PN junction to the base of the Activa.
This voltage controls the amplitude of the current passing through the Activa.
The change in the dipole moment of the GR acts to repel the stimulant from its steric coupling
with the GR. Thus, the presence of the stimulant in a steric relationship is only temporary. The
resultant voltage change at the Activa is also temporary. The stimulant molecule is free to go
without encountering any chemical change in character.
The waveform at lower left shows the current waveform entering the dendroplasm resulting from
a group of stimulant molecules coupling and uncoupling from the GR within a time period short
with respect to the time constant of the overall quantum-mechanical-electrolytic circuit. The
mathematical form of this waveform, including its latency, δ, are developed in Section xxx.
Hauser has described the stereographic structure of PtdCho (he uses the common name lecithin)
and PtdEtn and demonstrated how the dipole moment of these two materials is quite different.
The dipole moment of PtdCho is aligned with the long axis of the molecule and perpendicular
to the surface of the membrane. The dipole moment of PtdEtn is aligned nearly perpendicular
to the long axis of the molecule. This would suggest the surface potential of these materials when
formed into a liquid crystalline monolayer would be quite different. However, when so aligned,
it is not the vector quantity, the dipole moment that is important by the scalar dipole potential.
Vogel et al. have demonstrated the conductivity of the typical phospholipid is sufficient to allow
the measurement of a finite potential between its two ends when present as a monolayer film264.
They have measured the potential across phosphatidyl choline and phosphatidylethanolamine
when in the single layer liquid crystalline state. These potentials are found when the liquid
crystalline film has reached an equivalent molecular cross-sectional area of 40 sq Angstrom for
these materials. The potential is given as 555 mV for PtdEtn and 669 mV for PtdCho at 18-19C with
the hydrophobic surface negative. These are large voltages relative to the input dynamic range
of the first Activa of the sensory neuron (see the following sections).
8.5.5.3.3 Description of the operation of the sensory neuron in gustation
In the absence of adaptation phenomena (addressed below), the description of gustatory
intensity mechanism appears to be straight forward. This operation is illustrated conceptually in
Figure 8.5.5-17 using a lower range of dipole potentials than cited above. The dipole potentials
are closer to those reported by electro-physiologists in Section 8.5.10 below. The label PtdGR is
meant to represent a specific GR related to one of the sensory paths. The figure supports the
earlier schematic of [Figure 8.5.5-2 ]
In the left frame, the absolute difference between the putative dipole potential of PtdGR and
of PtdCho is shown as +20 mV resulting in a net potential, vq, at the base of the 1st amplifier of the
GRN of –20 mV. This is at the bottom of the operating range of the 1st amplifier. In the center
frame, the action of a nominal AH,B type of DACB between the gustaphore and the GR is
described. A gustaphore is inserted between the saliva and the GR with a dipole potential of +
20 mV. This action shifts the net potential, vg, by +20 mV to zero mV. Zero mV is the beginning of
the saturation condition of the amplifier. An analog intensity difference will be reported by the
264
Vogel, V. & Mobius, D. (1988) Local surface potentials and electric dipole moments of lipid monolayers:
contributions of the water/lipid and the lipid/air interfaces J Colloid Interface Sci vol 126(2), pp 408-420
Signal Generation & Processing 8- 201
GRN and perceived as representing the change in the dipole potential of any gustaphore with
a dipole potential between zero and +20 mV.
In the right frame, a gustaphore is introduced that causes a change in the dipole potential at the
base of the 1st amplifier of only one mV due to the gustaphore. However, the gustaphore does
cause a change in the dipole potential of the phospholipid/GR combination of 10 percent of its
quiescent value. This change is represented by a 26 degree change in the dipole potential
vector associated with the GR. If its quiescent value was 300 mV, this change alone causes the
net potential at the base of the 1st amplifier to change by 30 mV, driving the amplifier 10 mV into
saturation. To compensate for this change, the concentration of the gustaphore would need to
be reduced by 30:1 to be perceived appropriately by the system. If the dipole change due to
the gustaphore was only 0.1 mV, but the change in the GR remained at 30 mV, the concentration
of the gustaphore would need to be reduced by a factor of 300:1 to be represented
appropriately, etc. Achieving increases of perceived intensity on the order of several thousand
to one are reasonably achieved by this approach.
Figure 8.5.5-17 Effective potential at the base of the 1st amplifier (Activa) of the GRN. Illustrative
only. Down arrows are the effective dipole potential of the phospholipid/GR combination. Up
arrows are the dipole potential of the PtdCho bilayer of the outer lemma of the sensory neuron.
In the absence of any stimulant, the queiscent potential, vq, at the base of the first amplifier is –20
mV, i.e., cutoff. In the simple AH,B case, a stimulus with a dipole potential of +20 mV will raise the
base potential, vn, to 0 mV, nominal saturation (maximum processable signal amplitude). In the
AH,B,X case. a gustaphore introducing no change in dipole potential but causing a 10% change
in the dipole potential of the phospholipid/GR combination will drive the base potential xxx mV
beyond saturation. See text.
It is proposed that the net surface potential between the mucosa and the interior layer of the
type 4 bilayer membrane consists of three scalar components, the intrinsic dipole potential of the
phospholipid, the contribution to the total dipole potential due to the coordinate bond pairing
with the odorophore (AH,B), and any additional change in dipole potential caused by the thirdpoint association (X) between the odorophore and the receptor molecule. The intrinsic dipole
can be considered the quiescent contribution of the phospholipid to the biasing of the first Activa
of the sensory neuron with the sum of the contribution to the dipole potential from the AH,B
pairing and the X association constituting the signal element in the total electrical potential. With
the nominal gain of 200x for the Activa and a sensitivity of about 0.1 to 1.0 mV for the following
202 Neurons & the Nervous System
circuitry, a change in the quiescent potential of the phosphatidyl moiety of the sensory receptor
of less than five microvolts would constitute an adequate signal for eliciting a sensation within the
CNS. Such a small change is not currently measurable in the laboratory, but the presence of such
changes are indicated by the psychophysical data of Imamura et al. and of Randebrock for
changes in the potential due to coordinate coupling of the receptors with alcohols and
carboxylic acids of different chain lengths and internal bond arrangements (Section 8.6.2.3).
While Hauser has also suggested that the molecules of a monolayer membrane of PtdEtn are
interlocked by hydrogen bonds between the nitrogen and the adjacent oxygen of the
phosphorous oxide group, the more recent and comprehensive data of Davis suggest the
individual molecules are rotating rapidly about their long axis at biological temperatures. Such
rotation would appear to aid the contact between the receptor molecules and the stimulant
molecules in the mucosa.
Many empiricists have attempted to graphically conceptualize the transduction mechanism of
olfaction. See Ache265 reproduced in Schild & Restrepo (page 455), as an example. The
chemical neuron approach is much more complex, usually employs sweeping arrows and
symbolism, many question marks and has not shown itself amenable to verification or
deterministic calculations.
The above figure provides a closed form, verifiable and deterministic model of olfactory
transduction. It employs no putative pores or channels passing complex ions under the control
of undefined protein processes.
8.5.5.4 The perception versus stimulus intensity function
Green, Shaffer & Gilmore have presented data for sucrose and for ethanol asserting a
semilogarithmic perception based on largely psychophysical experiments266. The perception of
sucrose is labeled “sweet.” The perception of ethanol is labeled “irritation.” A function of
perception versus temperature is also presented.
Ossebaard & Smith have provided good data showing the semi-logarithmic transfer function of
the hydrated sodium-path of the gustatory modality for stimuli above a minimum
concentration267. They specifically note the difference in the sensory mechanisms related to their
presumed Na+ of NaCl and K+ of KCl.
8.5.6 Electrophysiology of gustation–the Excitation/De-excitation equation
[xxx consolidate adaptation material between 8.5.6 and end of 8.5.7 ]
The performance of the gustatory modality can be described using two different test protocols;
the first involving a short pulse lasting less than 1/3 of the duration of the shortest time constant
in the stage 1 sensory neuron (an impulse) and the second a long square pulse lasting longer than
the longest time constant in the stage 1 neuron (frequently a square wave). The first protocol
surfaces the quantum-mechanical character of the intensity aspect of the transduction process.
The second surfaces the adaptation aspects of the circuits within sensory neuron itself. The
impulse response can only be effectively measured by probing the analog output of the stage
265
Ache, B. (1994) Towards a common strategy for transducing olfactory information Semin Cell Biol vol 5,
pp 55-63
266
Green, B. Shaffer, G. & Gilmore, M. (1993) Derivation and evaluation of a semantic scale of oral
sensation magnitude with apparent ratio properties Chem Senses vol 18(6), pp 683-702
267
Ossebaard, C. & Smith, D. (1995) Effect of Amiloride on the Taste of NaCI, Na-gluconate and KCI in
Humans: Implications for Na+ Receptor Mechanisms Chem Senses vol 20(1), pp 37-46
Signal Generation & Processing 8- 203
1 neuron. The square wave response is also best measured at the axoplasma of the stage 1
neuron but crude data can be acquired by probing the stage 3 action potential generating
neurons. This method suffers from the incorporation of any stage 2 signal processing in the
measured data unbeknownst to the investigator.
[xxx Show or cite examples of the impulse response adjacent to the square pulse response ]
[xxx use a variant of Fulton Squire03pg611.wpg found in fig 8.7.1-2, Figure 8.5.6-1 ]
8.5.6.1 The impulse response of the
stage 1 neuron
8.5.6.1.1 Character of the DACB
phenomenon
The literature has long struggled with the
character of the “coupling” between the
gustaphores and the GR’s of gustation.
Conventional chemical reaction theory calls
for the generation of reaction products that
should be detectable. There are none.
Conventional chemical theory would also
suggest a slow reversible process reaching
some form of equilibrium. None has been
documented.
Figure 8.5.6-1 Framework for impulse response
versus square pulse analyses ADD. Modified from
Squire, 2003.
The actual process involves the DACB coordinate bond relationship. The single coordinate bond
known as the hydrogen bond is exceedingly weak, typically described as of 5 kCal or less energy
per bond. Such a bond is near the thermal threshold for stability at biological temperatures. The
requirement that pairs of these bonds be formed and maintained for a finite period is even more
demanding. In general, such DACB relationship can only be maintained for a short interval
before it is broken. The process of formation and destruction is repeatable until one of the
constituents is removed from the area. However, this repetitive cycle changes the character of
the transduction process. Each DACB becomes a quantum-mechanical process definable by
one or more time constants.
With the realization that the transduction process is a quantum-mechanical process, it shows a
very close analogy to that used in other stage 1 transduction neurons (particularly those of vision).
In fact, the entire transduction–sensory signal amplification mechanisms appear virtually identical
in all sensory neurons of the neural system (with the exception of the initial transduction step
tailored to the specific modality).
8.5.6.1.2 Circuit description of the gustatory sensory neuron with GR REFOCUS EDIT
[ refocus on gustatory neuron ]
With the exception of one simple two-terminal circuit applicable to chemoreception in general
proposed by Kurihara et al., no circuit diagram of the olfactory sensory neuron could be found
in the literature268. Their circuit diagram contains no capacitors or active elements and does not
address the detailed operation of the olfactory sensory neuron. Figure 8.5.6-2 shows the
proposed sensory neuron of olfaction based on the common functional circuit elements found
in other sensory modalities. The circuit is amenable to direct quantum-mechanical stimulation
268
Kurihara, K. Miyake, M. & Yoshii, K. (1981) Molecular mechanisms of transduction in chemoreception In
Cagan, R. & Kare, M. eds. Biochemistry of taste and olfaction. NY: Academic Press Chap 13
204 Neurons & the Nervous System
of the base region of the first Activa (as in photoreception) or to the application of an electrical
potential to the base electrode (as in phonoreception). Because of the expected low energy
of stimulation, the figure adopts the configuration used in hearing.
Figure 8.5.6-2 Candidate circuitry of the gustatory sensory neurons. All stage 1 and 2 elements
operate in the analog domain. The steric feature of interest in gustation is the d-value of the
DACB coupling. The Activa of the afferent synapse operates as an “active diode” with a bypass
capacitor. The electrolytic power supply forming the battery associated with impedance (1) in
the Activa collector circuit is shown in its hydraulic analog at the top of the figure. The dendrite
of the stage 3 ganglion typically functions as the action potential encoding device.
In the case of phonoreception, the first Activa is biased into the operating range required by a
conventional amplifier. The circuit is capable of accepting electrical signals in the tenths of a
millivolt to ten millivolt range. Such a range appears compatible with the energy levels involved
in the odor reception scheme suggested by Turin.
In hearing an AC electrical potential is created between the plates of a capacitor by piezoelectric action. In the case of olfaction, it is proposed a similar AC electrical potential is created
between the plates of a capacitor by dielectric polarization due to an electronic rearrangement
of the dielectric. the dielectric is formed by a stimulant sensing complex, GR. This rearrangement
is temporary and caused by the temporary presence of a stimulant in steric coupling with the GR.
The proposed configuration creates a quantum-mechanical shift in the electronic state of the GR
every time an appropriate stimulant couples with the GR. This coupling at the molecular level
would be a low probability event unless the GR was present in a liquid crystalline form on the
surface of the input structure of the sensory neuron. It is proposed that the GR is present in such
a form and is located on the surface of a specialized section of the dendroplasm of the sensory
neuron. It is this specialized section that creates the Activa just as it does in other modalities.
[xxx eliminate the capacitor part ]
It is further proposed that the GR is present in a liquid crystalline form where part of the crystalline
configuration forms the dielectric of a capacitor between the electrolyte of the mucosa and the
base region of the Activa within the sensory neuron.
Signal Generation & Processing 8- 205
Indicator dyes are characterized by their multiple electronic configurations. Each of these
configurations exhibits a moment. The change in configuration can be described by a transition
dipole moment. A non-zero transition dipole moment change in a material forming the dielectric
of a capacitor will result in a change in the potential between the electrodes of that capacitor.
The concept is discussed briefly in Adamson269. Maron & Lando go into the subject of dipole
moments, and the various spectra related to molecular structure270. They provide a pedagogical
energy diagram. For molecules of more that three atoms, the complexity of th spectra becomes
quite high. However, the change in polarization associated with the vibration-rotation spectra
are of interest here. The energies of interest are in the millivolt range and the associated
wavelengths are in the near infra-red range.
Petersen & Cone have provided an excellent paper discussing many aspects of the dipole
moment of proteins, particularly rhodopsin, the disk protein associated with vision271. It forms a
good tutorial as well as providing valuable protocols and experimental data. They provide a
footnote; “the dipole moment, μ, is in Debyes (D), where 1 D = 10–18 statcoulomb-cm. Thus an
electron and a proton separated by 1 Angstrom produce a dipole moment of 4.8 D. In other
words, 1 charge-D = 4.8 D.” They show that most small proteins separate into two classes based
on their dipole moment and their molecular weight, those with values of μ/W of 5 x 10–3 D/Dalton
and a second group with μ/W clustered about 20 x 10–3 D/Dalton. Rhodopsin with a dipole
moment of 720 Debye and a molecular weight of 35,000 ± 2000 Daltons, has a μ/W value of 20
x 10–3. Gerber et al. note that the proteins with the larger dipole moments all tend to aggregate
and suggest that the dipole moments aid in the aggregation process. This ability of the protein
to aggregate is crucially important in the formation of the outer disks of visual sensory neurons.
Aggregation may play a similar role in forming the olfactory receptors (OR’s). The change in the
dipole moment of rhodopsin upon saturating illumination is only 25 D (5 charge-D) or about 3%.
Petersen & Cone give precise data on the dielectric constant of rhodopsin solutions and its
change with applied frequency in a standard test cell. In the in-vivo situation, a value for the
aggregated material is believed to be necessary.
[xxx only paragraph in this part mentioning organo-metallics ]
Many complex organo-metallic compounds exhibit a dipole moment in the 2-5 Debye range272.
More comprehensive general lists are available but they tend to be old273. Following further
isolation of the chemistry of interest, a more detailed search may uncover the desired values for
their dipole moments. The lower molecular weight of these organo-metallic materials (the
molecular weight of copper tartrate is only 329 Daltons, give them much higher μ/W values than
those of proteins. Nominal static μ/W values for the organo-metallics are in the 2–6 x 10-3 range.
The transition dipole moment may be much larger relatively because of the small size of the
molecules.
Takashima has provided excellent background on the dipole moment274. An on-line calculator
for calculating the dipole moment of arbitrary proteins is available275. However, the structure of
the protein must be known in detail.
269
Adamson, A. (1973) A Textbook of Physical Chemistry. NY: Academic Press pp 911-913
270
Maron, S. & Lando, J. (1974) Fundamentals of Physical Chemistry. NY: Macmillan Chapter 5
271
Petersen, D. & Cone, R. (1975) The electric dipole moment of rhodopsin solubilized in Triton x-100 Biophys
J vol 15, pp 1181-1200
272
CRC (1975) Handbook of Chemistry and Physics. Cleveland, OH: CRC Press pg E-66
273
Smyth, C. (1941) Dipole moment and bond character in organometallic compounds J. Org. Chem vol 06(3),
pp 421–426
274
Takashima, S. (1999) Computation of the dipole moment of protein molecules using protein databases
Colloids and Surfaces A vol 148, pp 95-106
275
Felder, C. Prilusky, J. Silman, I. & Sussman, J. (2007) A server and database for dipole moments of proteins
Nucleic Acids Research vol 35, special Web Servers Issue.
206 Neurons & the Nervous System
Two technical challenges arise in attempting to define a steric mechanism of transduction. First,
the capture cross-section of the transduction mechanism must be as large as possible. Second,
the transition dipole moment must be as large as possible and it must be optimally sensed.
Forming the GR’s into a liquid crystalline structure is one method of increasing capture crosssection and potentially increasing the efficiency of sensing the net transition dipole moment.
---Not shown explicitly in these diagrams are the large dipole potentials introduced by the inner and
---Figure 8.5.6-3 shows the circuit along with its cytological equivalent. Frame A shows the
cytological layout of the proposed typical olfactory sensory neuron. All plasmalemma are
electrically insulating bilayer membranes, except for specialized sections that are
semiconducting. These specialized sections are active participants in forming active
semiconductor devices, Activa, or in supporting the electrostenolytic process electrically
powering the cell. All of the sensory neuron except for its axon is located within the olfactory
epithelium and its mucosa. The body of the sensory neuron peripheral to the soma is arbitrarily
defined as the dendrite portion. That portion between the soma and the olfactory bulb is
arbitrarily described as the axon.
Signal Generation & Processing 8- 207
The bulbous extreme end of the dendrite is labeled the knob. Its internal structure is not reported
in the literature. Multiple individual microtubules emanate from the knob. These function as
dendritic spines with multiple sensitive areas typically associated with synapses in non sensory
Figure 8.5.6-3 Proposed cytological and electrolytic description of the olfactory sensory neuron
ADD & MODIFY. MODIFY frame A to show stimulation. DEFINE SBC.
208 Neurons & the Nervous System
neurons. In the sensory neurons, these areas are modified to support transduction.
Only the microtubules (cilia) lie in the mucosa. The number of microtubules emanating from the
dendritic knob varies from species to species, from as few as four to as many as thirty. The typical
length of the microtubules is about 50 microns in mammals276. The microtubules are splayed into
a planar surface within the xxx thick mucosa.
The three horizontal arrows in the dendroplasm indicate where the multilayer sandwich to their
left actually extend up into the individual cilia of the neuron. The character of these layers
changes from insulating to semiconducting at multiple points along each cilia just as they do in
the microtubules of the visual sensory neurons. The two insets, A1 & A2, illustrate how the
transduction mechanism may appear at one point along the length of the cilia. Inset A1 shows
an Activa (black rectangle) supported by its electrostenolytic supply (E.S (1)) and the transduction
element consisting of an GR forming the dielectric of a capacitor, CT, with its outer conducting
surface formed by the mucosa. Inset A2 shows an alternate configuration where an auxiliary
electrostenolytic source (E.S. (0)) appears on the outer surface of the capacitor. This source
provides an electrical bias to the capacitor that may provide a higher sensitivity to the overall
circuit as discussed in Section xxx.
The dashed arrows associated with each electrostenolytic supply show the direction of electron
charge flowing into or out of the respective plasmas. The “conventional charge” defined
erroneously by Benjamin Franklin flows in the opposite direction.
Charges from each of the Activa in each cilia are introduced into the dendroplasm and
generate a potential between the dendroplasm and the podaplasm. This causes the Activa
shown between the dendroplasm and the axoplasm to transfer approximately 200 times as much
charge to the axoplasm. This additional charge passes through impedance associated with E.S.
(4) and generates the output voltage at the synapses shown.
The sensory neuron delivers a tonic generator waveform at the remote glomeruli by diffusion.
Because of the length of the axon required to pass through the cribriform plate and the low rate
of signal propagation by diffusion, the bandwidth of the olfactory sensory neurons is necessarily
low.
Frame B shows the same circuit configuration as in Frame A. [xxx Expand ]
Frame C shows the same circuit configuration as in Frame B but redrawn to illustrate the
commonness of the circuit. It is described as an PNP type asymmetric differential pair in
conventional electronic engineering terms. The left (or first) Activa is base driven and the right (or
distribution) Activa is emitter driven. The left Activa provides a high impedance input and a gain
of nominally 200:1. The right Activa provides a low impedance output at 1:1 gain suitable for
driving multiple synapses. [xxx Expand ]
Frame A1 has been expanded further and discussed in detail in Section 8.5.5.1.
Figure 8.5.6-4 reproduces a set of gustatory path waveforms from Danilova et al277. These are
summated waveforms from two locations along the gustatory neural pathway. Their analog
character is reconstructed from the action potential pulse streams recorded at these locations.
They noted the recordings were from the whole nerve. They also noted, “All stimuli elicited a
response except NaCl in the NG.” It is also noteworthy that they did not include HCl in their data
set. HCl, an inorganic acid, elicited negligible response in their broad set of experiments involving
these two nerves. Similar waveforms recorded at the chorda tympani were presented by
Pfaffmann in 1976.
276
Menco, B. (1983) The ultrastructure of olfactory and nasal respiratory epithelium surfaces In Reznick, G. &
Stinson, S. eds. Nasal Tumors in Animals and Man . . . Vol 1. Boca Raton, Fl: CRC Press pp 45-102
277
Danilova, V. Danilov,Y. Roberts, T. Tinti, J-M. Nofre, C. & Hellekant, G. (2002) Sense of Taste in a New
World Monkey, the Common Marmoset: Recordings From the Chorda Tympani and Glossopharyngeal Nerves
J Neurophysiol vol 88, pp 579–594
Signal Generation & Processing 8- 209
By comparing these responses with those obtained within the visual278 and auditory279 modalities,
it is possible to recognize several phenomena. First, the analysis will be limited by the lack of a
vertical scale for these waveforms. The paper did not say the waveforms were presented using
the same vertical scale. Second, the concentration of the stimulant varied dramatically between
these waveforms. [xxx include a simple version of formula and graph here if none in Section 8.2
or 8.3)
The waveform for sucralose on the left (a low caloric artificial sweetener of the AH,B,X type
with formula C12H19Cl3O8 ) has been overlaid with a nominal excitation/de-excitation
response to aid this discussion. Several features are noteworthy;
• The overall sucralose response does not show any sign of saturation or other distortion.
• The delay between points A and B is due partly to the physical diffusion of the stimulant
to the point of transduction at the GR on the surface of individual cilia of sensory neurons.
A second part is due to a delay within the transduction mechanism of the sensory neuron
that is stimulus intensity related.
• The rise time constant, τrise, associated with the response starting at point B is a function
of the stimulus intensity rather than a fixed value.
• The decay time constant, τdecay, has a fixed value determined by the internal electrolytic
components of the transduction mechanism.
• The net response waveform following the beginning of stimulation is the difference
between the exponential rising response and the exponential falling response (that both
start at the same time, B.
• The decay portion of the net response waveform is not related to and is not calculated
from the peak of the net response.
• The delay between the end of stimulation, C, and the discontinuity in the net response,
D, is the same as the part of the delay between points A and B due to phenomenon within
the transduction mechanism.
• The precipitous drop at point D is a real part of the overall excitation/de-excitation
function.
• The delay following point D is a true exponential function equal precisely to the decay
time constant, τdecay.
8.5.6.1.3 Analog waveforms generated by stage 1 neurons EMPTY REFOCUS
[ Refocus on gustatory modality ]
The literature provides many analog generator waveforms from olfactory receptor neurons (Schild
& Restrepo, page 443-444). These waveforms exhibit a latency as a function of stimulus intensity
that is not compatible with experiments involving the putative photoexcitation of cAMP. This
latency as a function of intensity is characteristic of the excitation/de-excitation mechanism
proposed in this work. Farbman has provided a very clear patch-clamp recording from the
olfactory cilium of a frog (page 110). The change in amplitude of up to 20 mV suggests the
recording was from the axoplasm of the sensory neuron. The variation in the amplitude of the
signal over time suggests some logarithmic compression due to the current to voltage conversion
but no sign of hard saturation or of action potential generation.
278
Fulton, J. (2004) Processes in
http://neuronresearch.net/vision/pdf/7Dynamics.pdf
Biological
279
Fulton, J. (2008) Processes in Biological
http://neuronresearch.net/hearing/pdf/5Generation.pdf
Vision.
Hearing.
Section
Section
7.2.4
5.4
210 Neurons & the Nervous System
Figure 8.5.6-4 Summated chorda tympani and glossopharyngeal nerves during taste stimulation
of the tongue in a marmoset. The thich horizontal line at the bottom of each recording indicates
the onset and end of stimulation using a computer controlled open flow system, the TasteO_Matic. NO vertical scales were given. See text. From Danilova et al., 2002.
Signal Generation & Processing 8- 211
Based on the overlay, it is clear that the summated sucralose waveform at the chorda tympani
was the result of rapid diffusion of the stimulant to the point of transduction, the transduction
process exhibited a very fast rise time indicating that the stimulus intensity was near the maximum
acceptable by the relevant GR’s, the decay characteristic (although noisy) was precisely as
expected and the decay time constant was nominally 2 seconds (subject to refining later).
Looking only at the left panel of the figure, the waveform for sucrose shows a similar shape to that
of sucralose except for the limited amplitude due to saturation in the transduction mechanism.
The decay time constant was also apparent and had a value of approximately 2 seconds
(compared to 12.5 msec in hearing). A similar degree of saturation appears to have occurred
in the waveforms for QHCl and SC-45647. The waveforms for NaCl and citric acid are
anachronistic during the washout interval following stimulation. The waveform for ethanol may
reflect the presence of another gustaphore within the stimulant sample.
Looking at the right panel, the lack of a response to NaCl was a common feature of their neural
bundle accessed within the glossopharyngeal nerve. The citric acid response was similar to that
from probing the CT (including the anachronistic response during the washout period. QHCl
exhibited a nearly ideal response relative to the ideal excitation/de-excitation equation. The
sucrose and SC-45647 responses show saturation within the transduction mechanism. The
sucralose response was as expected. The ethanol response was closer to what would be
expected by a typically tasteless aliphatic alcohol.
The minimum delay between points A and B is about 0.5 seconds for citric acid (as measured at
the chorda tympani).
8.5.6.1.4 The generic Excitation/De-excitation equation applied to gustation
EMPTY
The actual excitation/de-excitation equation is quite complex and can be found, including
several specific ramifications in the citations provided above. The equation is associated with a
quantum mechanical mechanism such as the forming and disbanding of DACB’s as hypothesized
here. The relevance of the equation to and association with a gated channel phenomenon,
such as proposed in the chemical theory of the neuron, has not been demonstrated.
8.5.6.1.5 Circuit parameters of gustatory transduction EDIT
[ Refocus to gustatory modality transduction ]
Schild & Restrepo have included considerable parametric data (with citations) on the axonal
portion of the olfactory sensory neurons. Statements like “whole cell capacitance” presume a two
terminal structure for the neurons. Under the Electrolytic Theory of the Neuron, this label should
be modified to the whole axoplasm capacitance as it was obtained by injecting charge into the
axon and measuring the resulting voltage rise. Values in the 2-10 pF range are consistent with the
sensory neurons of other modalities. They report a wide range of axon resting potentials,
undoubtedly because of the lack of control of the dendrite environment while investigators
made these measurements. A value of –70 mV would be expected and falls near the middle of
the range quoted. They report an average time constant of the axoplasm of ~ 60 ms, giving a
first order RC filter corner frequency of ~16 Hz. While lower than in other modalities, these values
are consistent with the extended length of the axon required to pass through the cribriform bone.
Measured time constants in some species have been as long as 100 ms. They did note an
important point, the delay displayed in their generator potentials of figure 4 are proportional to
the amplitude of the stimulus (page 433). Unfortunately, they have continued to repeat the
reports based on conclusions improperly drawn the work of Gesteland in 1971280. That assertion
was that sensory neurons exhibit action potentials (beginning on page 434). Schild & Restrepo
280
Gesteland, R. (1971) Neural coding in olfactory receptor cells In Beidler, L. ed. Handbook of Sensory
Physiology, Vol 4, Part 1: Chemical Senses. Berlin: Springer-Verlag pp 132-150
212 Neurons & the Nervous System
note, “These reports were discussed controversially, see Getchell281.” Gesteland was very clear
that his measurements were extracellularly, and in fact of the crudest kind. His measurements
were made between the two sides of the olfactory epithelium. Because of his technique (his
figure 1), he actually measured generator waveforms of opposite polarity to that at the axon of
a cell “and with action potentials spikes superimposed” (page 144). He was measuring the
complement of the generator waveform at the collector of the first Activa instead of at the
collector of the second Activa (the normal axon potential (See Section xxx). It is now known his
current path in figure 1 requires changing to reflect the documented role of the poditic
terminal.(Section xxx).
Gesteland does provide several parameters supporting the neural model developed here. [xxx
review gesteland 1971 ]
Additional data showing such action potentials has not appeared subsequently. It appears
Gesteland relied upon data collected extracellulary (intercellularly). It exhibits low amplitude
action potentials riding on top of generator waveforms. The combined waveform is easily
explained as due to capacitive coupling under the Electrolytic Theory of the Neuron. Schild &
Restrepo show generator waveforms on page 444 that are free of action potential features.
[xxx applies to an olfactory neuron ]
Schild & Restrepo reproduce a figure from Kurahashi282 that may represent the diode
characteristic of the axon load, Figure 8.5.6-5. If so, the load diode had a reverse cutoff current
(I0) of –25 pA. Unfortunately as noted in the original caption, the current traces shown on the left
were arbitrarily shifted. The nominal horizontal axis of each waveform does not correspond to
zero current. The holding voltage would have been accompanied by a holding current that is
not shown explicitly, but can be seen on the right.
281
Getchell, T. (1986) Functional properties of vertebrate olfactory receptor neurons Physiol Rev vol 66, pp 772818
282
Kurahashi, T. (1989) Activation by odorants of cation-selective conductance in the olfactory receptor cell
isolated for the newt J Physiol (Lond.) Vol 419, pp 177-192
Signal Generation & Processing 8- 213
Figure 8.5.6-5 Odorant induced currents at various holding potentials from a newt, Cynops
pyrrhogaster ADD. Current traces were arbitrarily shifted. See text. Modified from Kurahashi, 1989
214 Neurons & the Nervous System
8.5.6.2 The square pulse response of the stage 1 neuron
[xxx figure duplicates Figure -26 in the summary above ]
Hellekant et al. have provided the clearest example of the E/D response of a gustatory neuron
based on their empirical investigations283 in Figure 8.5.6-6. It is virtually identical to the theoretical
response described here based on the Electrolytic Theory of the Neuron. A minor exception
involves their labeling of the rise time as starting before the delay time has elapsed. The rise time
as defined in this work involves the time from when the response deviates significantly from the
baseline to the peak amplitude of the waveform. The deviation begins when the analog
electrical signal intensity from the dendritic structure is first sensed at the emitter of the Activa
within the neuron. Shallenberger (1993, section 10.7.1)was probably referring to this revised rise
time when he noted casually “the rapid nature of taste transactions, which is usually about 50
microseconds.”
The interval labeled the “resume time” by Hellekant et al. is modified to extend the actual
response time and more properly label the decay time in accordance with this work. The decay
time is an exponential explicitly describing the decay time constant of the neuron. It constitutes
the time following the cessation of stimulation as perceived at the emitter of the Activa and not
at the end of the stimulation time. Extensive data is provided in the Hellekant et al. paper but
many of the time constants and other values require re-interpretation in the light of the more
highly defined terms of the E/D equation of sensory neurons.
8.5.6.2.1 Circuit parameters
gustatory adaptation
of
In the above figure, the decay time
constant of the nominal gustatory neuron is
approximately two seconds. The corrected
rise time constant associated with the same
neuron (difference between the beginning
of the leading edge and the maximum
amplitude) is on the order of 0.6 seconds.
The delay time shown may include a
diffusion time for the stimulant to reach the
dendrite and may be dependent on the
test protocol used. It is also dependent on
the intensity of the stimulus. The rise time is
also a function of the intensity of the stimulus.
The variation in these values are illustrated in
Table 2 of Hellekant et al.
Figure 8.5.6-6 The characteristics of the E/D
response in gustation according to Hellekant et al.
The lower scale has been expanded to delineate
between the stimulation time and the response
time. See text. Modified from Hellekant et al.,
1991.
283
Hellekant, G. Walters, D. Culberson, J. et al. (1991) Electrophysiological evaluation of sweeteners In
Walters, D. Orthoefer, F. & DuBois , G. eds. Sweeteners. Wash. DC: American Chemical Society Chap. 22
Signal Generation & Processing 8- 215
8.5.6.3 Chemical kinetics at the receptor/gustaphore interface
Some of the papers cited below do not clearly indicate how they calculated their
concentrations. Unless noted otherwise, this section will speak in terms of a gram molecular
weight of solute dissolved in 100 ml of solvent. This is more important than might be
expected for two reasons; first some of the materials are complex sugars that are
hydrolyzed as they dissolve, second, many of the molecules considered are gustants
containing multiple gustaphores. The effective concentration may not be the same as the
concentration defined above and used by the authors.
The E/D equation describes the physiological transduction process at the sensory receptors in
precise and considerable detail. Because of its temperature term, it is even applicable to
exothermic animals with poor physiological temperature control. The C/D equation also provides
insight as to the chemical kinetics at the receptor/gustaphore interface on a global scale. It does
not, however, describe the kinetics of the transduction process in detail. This section will address
these kinetics.
The chemical kinetics of gustation have been studied since the 1960's. However, lacking an
adequate model of the transduction mechanism, these studies have tended to fall back on the
investigators concepts of the fundamentals of chemistry. Thus, the laws of chemical equilibrium
related to solutions and the Law of Mass Action have played a significant role in these
investigations. Much of the analyses have revolved around the perceptions of human subjects.
These investigations have led to many approximations to overcome the difficulties with the
models assumed and the mathematics involved. The work of Hill ( xxx, cited in DuBois), Beidler
(1954 & 1961), Stone & Oliver (1966), Dzendolet (1967), Dubois et al. (1991), Shallenberger (1993)
and a variety of others will be explicitly discussed here.
Most of the above investigators rearranged an equation first offered by Beidler in the 1950's in
order to evaluate the constants applicable to the equation. They had little interest in using the
equation in an operational context. The goal of this discussion is to gain insight into the chemical
kinetics of the transduction process supporting the Excitation/De-excitation Equation of the
previous section. This will further the overall understanding of the gustatory modality and
probably other sensory modalities as well (such as olfaction, Section 8.6)
An initial global conclusion based on the composite data from the above authors is that it is
possible to correlate the human perceived responses to gustatory stimulations as a function of
a linear transfer function over a significant instantaneous range of stimulation. This perceived
response also correlates well with the stage 3 pulse rates along various nerves serving the
gustatory modality. This reinforces the assumption that psychophysical intensity profiles do linearly
match similar electrophysiological intensity profiles.
The following discussions are based primarily on poorly defined conceptual models of gustatory
transduction assuming a conventional chemical reaction in some state of equilibrium.
Determining whether an equilibrium condition is ever achieved, or even applies to gustatory
transduction is a goal of this discussion.
Shallenberger reviewed much of the same material as reviewed below in 1993 in a text284. Little
new material appeared in this volume relative to the present discussion. Several parameters were
provided however.
8.5.6.3.1 The Beidler equation of chemical kinetics in transduction
284
Shallenberger, R. (1993) Taste Chemistry. Glasgow, Scotland: Chapman & Hall
216 Neurons & the Nervous System
Beidler derived an equation for the concentration over perceived response (C/R) as a function
of the stimulus concentration (See figure in following discussion of the Dzendolet “correction”).285
The derivation was based on his interpretation of the Mass Action Law. K was the conventional
equilibrium constant for the reactants involved, the sensory receptor reaction sites and the
molecules of odorant in solution in the saliva. Beidler’s analysis relied upon the following
assumptions,
“(I) The reactions involved in stimulation are in a time-independent state, very likely in
thermodynamic equilibrium, since the response to 0.I M NaCI was shown to remain constant
during I0 minutes of continued salt stimulation. The magnitude of this response is the same no
matter whether immediately preceded by higher or lower concentrations of the salt.
(2) Stimulation is very rapid as shown by the fact that a response may be recorded within 50
msec. after 0.2 K NaCl is applied to the surface of the tongue.
(3) The responses are completely reversible.
(4) Both the cations and anions enter the reaction although the magnitude of response is primarily
determined by the presence of the cation.
(5) As the strength of the stimulus is increased, a level of response is reached at which a further
increase in stimulus does not result in an appreciable increase in response.
(6) The receptors of the tongue respond to a large number of different substances and over a
wide range of concentrations.”
The Beidler paper draws a variety of conclusions based on these assumptions. This work would
question a variety of these assumptions.
(Item 1) His use of average pulse rates at the stage 3 chorda tympani implies steady-state
conditions. However, the perception of saltiness is known to be stimulus intensity sensitive. His
results would be quite different if he reported on the pulse rate during the first few seconds of the
response to stimulation (as suggested in item 2..
(Item 3) The responses may not be revesable under conditions of adaptation typically associated
with stimuli of high concentration over extended periods, or the presence of other molecular
stimulations to the system.
(Item 4) Beidler did not demonstrate that the anion played any role in his reported investigation.
The role of the chlorine ion is quite independent of the role of the sodium ion with respect to the
N-Path, the hydrated sodium ion sensitive channel of gustation)
(Item 5) This item remains true and is discussed with respect to DuBois et al. below.
(Item 6) This statement remains correct but can be made considerably more precise based on
this work. Each type of receptor, GR 1, GR 2, GR 3 or GR 4 responds to a large number of
stimulants belonging to one of the four stimulant classes, acidic, dulcal, hydrated sodium-like or
picric.
Beidler self proclaimed his equation (shown in the following figure), “This is the fundamental
equation relating the magnitude of response to the concentration of the applied chemical
stimulus.” It is clearly not fundamental but derived and does not apply at R=0 where C/R is
undefined. In his “Application to Electrophysiological Data” section, he notes, his fundamental
equation “does not necessarily prove that the chosen theory of stimulation is a correct one.”
Beidler also notes, “low values of the change in free energy(which he explored) should be
expected for all species of mammals if the mechanism of taste stimulation as outlined in this
paper is a general one.” The low values reported are noted, but the mechanism of state
stimulation is disputed by this work. With regard to his analyses involving NACl stimulation, he
notes, “clearly indicates that the ions of chemical stimulus are loosely bound to some substance
of the taste receptor.” It is proposed here that this observation can be extended to a variety of
non-ionic molecules capable of forming a DACB as well.
Beidler concluded his 1954 paper with the provocative assertion, “The recently proposed (Baradi,
285
Beidler, L. (1954) A theory of taste stimulation J Gen Physiol pp 133-139
Signal Generation & Processing 8- 217
A. F., and Bourne, G. H., Nature, 1951,168, 977. ) enzymatic reactions for chemoreceptors do not
appear plausible for sodium salt stimulation of the taste receptors of the rat.”
DuBois et al. offered a rearrangement of the Beidler equation in a more application oriented form
where the output signal (response, R) is a function of the input stimulus (concentration, C) that is
also shown in the following figure. This form describes the function, f( ) as linear in C for C below
a critical point where it becomes asymptotic to a constant value, Rm. As noted below, diffeerent
investigators have chosen to use a variety of forms of the Beidler equation that makes retrieval
of the constants involved easier.
---Dzendolet expanded on what had become the baseline Beidler Equation in 1967. He showed
that the assumptions made by Beidler resulted in his equation being limited to regions of high
stimulus concentration applied to receptors exhibiting minimal quiescent stimulation levels. Such
a situation is frequently described in engineering as the small signal condition (applicable to only
a limited extent of the dynamic range available). Dzendolet sought to provide a broader (large
signal condition) equation by removing some of the simplifications.
Beidler defined Z as the number of sites which combined with the stimulus at the concentration
C. Dzendolet indicated this expression was correct only for very high concentrations of the
stimulus when Z is negligible. For the more general condition, he asserted that the mass action
equation without Beidler’s assumption would replace C by C–Z. Based on this change, he
defined the magnitude of the perceived response as R =aZ, where a is a constant and Z equals
the number of filled receptor sites. Rm is defined as the perceived response when all available
receptor sites are filled, aN. Dzendolet asserts his variant “has the advantage of being exact and
applicable at all stimulus concentrations.” Figure 8.5.6-7 compares the Dzendolet equation
compared to the equation of Beidler. Dzendolet chose to put all of the constants on the right
hand side of the equation and leave the expression Rm - R as a term on the left. To express his
equation in the form of Beidler, that term should be made a denominator under all of the right
hand terms.
Dzendolet compared his equation to the
earlier data of Beidler from fig. 5 of a 1953
paper. The data was collected from the
stage 3 chorda tympani nerve of a rat to
various concentrations of NaCl.
He
calculated Rm as equal to 29.0 and noted
the sensitivity of the graphic to the value of
Rm. For other experiments, Rm may be
arbitrarily assigned the value of unity, and
other responses treated as fractional values
of Rm.
Dzendolet spoke in terms of the Law of Mass
Action and solution-based equilibria. They
also assumed a reversible bond between
the receptor and the gustaphore. This led
him to speak of molecular associations and
dissociations of reaction chemistry rather
than the DACB’s of coordinate chemistry.
Figure 8.5.6-7 Comparison of the Beidler and
Dzendolet equations describing the perceived
response reflecting a stimulus at a concentration
C where a and K are constants and Rm is the
maximum perceived response based on
calibration experiments. See text.
Dzendolet provided a replot of the Beidler
data, Figure 8.5.6-8, that does not relate to
the output as a function of input
representation in gustation but is of the
parametric type. It attempts to describe a more precise relationship between his stimulus
concentration to response ratio, C(Rm–R)/R versus the perceived response (R) of the applicable
chemical kinetics. The figure is provocative but must be discussed in its parametric context. As
R approaches Rm, the abscissa must approach zero regardless of the value of C. Because of this
feature, he related the curve to two distinct phases of transduction, an interval of chemical
disassociation between the receptors and a gustaphore and a related interval of association
between he same elements. He noted, the left-hand portion of the waveform is straight with a
218 Neurons & the Nervous System
positive slope. However, the form of the graph cannot be linear on the right where it exhibits a
lower negative slope. Dzendolet chose to sketch in a transition from the rising to the falling portion
of the waveform and provide two linear dashed lines for reference.
Signal Generation & Processing 8- 219
Based on his interpretation of this curve, the
left portion describes the disassociation of
the receptor/stimulus into a free receptor
site and a free stimulus molecule (or ion of
the molecule). Conversely, the right portion
represents the association of receptor sites
and stimulus molecules (or ions thereof). He
concludes, “In other words, a response is
initiated when a dissociation rather than an
association reaction occurs at the receptor.”
No physiological model supporting this
mathematical interpretation was provided.
It is difficult to accept the interpretation of
Dzendolet when in the context of chemical
equilibrium [xxx see Smith’s College Chem,
page 233 ] since it implies the sensory
receptors were initially associated with the
stimulant before the stimulant was applied
to the sensory organ itself. Beidler plotted
C/R as a function of the independent
variable, C, where C/R is clearly undefined
at C = 0. It would be preferred if they
focuses on either the response or the ratio of
response to concentration as a function of
concentration rather than the reciprocal
forms. A value of C/R approaching zero is
equivalent to a value of R/C approaching
infinity (neither of which is a realistic
situation).
Figure 8.5.6-8 Parametric graph of the kinetics of
gustation by Dzendolet ADD. Note, the ordinate
does not relate to an independent variable of
excitation. The value of R is relative to Rmax = 1.0.
Dzendolet attributed different operating
environments to the positive and negative slope
regions (which appear inappropriate). See text.
The dashed lines were his contributions to data
from Beidler, 1953. From Dzendolet, 1967.
DuBois et al. cited the Beidler equation in a different form than as published in Beidler (1954).
However, it is in a more conventional output as a function of input form. In this form, the equation
can be graphed more conventionally. For values of C less than 1/K, the equation rises linearly
with concentration. Significantly above C = 1/K, the equation is asymptotic to Rm. There is no
reversal in the slope of R as a function of C. This form can be interpreted as; the number of
occupied sensory receptor sites is increasing with the concentration of the applied stimulus, and
as the number of available receptor sites becomes more limited, the effectivity of the stimulus is
reduced as would be expected. This interpretation avoids the problem with the dissociation issue
with the analysis of Dzendolet. It will also be shown to tie together the analyses of DuBois et al.
addressed below.
By rearranging the equation of Dzendelot as DuBois did for the Beidler equation, a very similar
output-input function to that of DuBois is obtained, R= f(C) is linear for (Rm–R) n a/K. (If Rm-R) o a/K,
R becomes asymptotic to “a”. Dzendolet defined “a” as an arbitrary constant equal to Rm/N
when all available receptor sites were filled.
It appears the mass action equation of chemistry should be approached in differential form when
used in calculations related to gustation and other transduction mechanisms.
8.5.6.3.2 Transduction kinetics of the amino acids
The amino acids play a major role in the transduction mechanism associated with gustation.
Stone & Oliver employed two forms of the Beidler equation to investigate the human gustatory
220 Neurons & the Nervous System
responses of a group of amino acids286. One employed linear scales to plot C/R= f( C). The other
used logarithmic scales to illustrate a rearranged Beidler equation given by R/Rm = CK/(CK+1)
and designed to show all of the data fitted a linear response proportional to the auxiliary function
CK/(CK+1). The response, R, is clearly not linear with respect to the underlying concentration, C,
by inspection. The authors employed the auxiliary function to avoid needing to account for how
their data did not fit a single straight line over a significant range of concentrations. C
represented molar concentrations in their stimuli.
Figure 8.5.6-9 reproduces their figure 1. Note the scales are both linear and the ordinates are
different for the two frames. most of the best estimate line for each amino acid is estimated from
only a few data points. The slope of each line is an estimate of 1/Rm and the intercept with the
vertical axis is an estimate of 1/KRm for that amino acid. Stone and Oliver asserted, “The data
indicated that the evaluators rated the intensity of the stimuli in the expected manner; response
rose rapidly and stabilized as concentration was maximized.” They also noted, “The evaluators
also indicated their hedonic impressions and the taste qualities perceived.” However, this
information was not included in this paper. A problem was also noted, “For some stimuli there was
good agreement between evaluators (e.g., aspartic and glutamic acids, phenylalanine and
arginine; for others there appeared to be an almost complete reversal, primarily at the lower
concentrations.” Only five evaluators is a very small number for obtaining statistically significant
data points. However, estimates were made. The rating system was crude, from 0 to 1.0;
examples 0–no taste, 0.5– moderate intensity, 1.0– extremely intense following presentation of an
identified standard with a rating of 0.50. A total of 20 responses was obtained from each
evaluator for each concentration of each stimulus, but only the final 15 scores were retained for
analysis. The amino acids were obtained from commercial sources (typically $98% purity) without
any additional purification.
It is noteworthy that Shallenberger (1993, section 10.7.2) changed the modified form of R/Rm
suggested by Stone & Oliver by omitting the term CK in the numerator on the right. He then
proceeded to describe the resulting equation conceptually as a sigmoidal form. His analysis
omitted the fact that the response, R/Rm is proportional to C below a transition point defined by
KC = 1 and asymptotic to Rm for concentrations above this point. He also failed to describe the
toe of his sigmoidal function as due to noise in the measurements. There is no real relationship
between gustatory transduction and a sigmoidal function. The noise floor has been investigated
by McBride287. His paper “demonstrates that the Beidler equation also provides a good
description of the human taste response, as obtained by two psychophysical methods (JND
cumulation, category rating).” It also addressed all four of the gustatory Paths defined in this
work. The McBride paper relies upon a paper by Maes288, One of the major assertions of Maes
after discussing the modifications introduced to simplify the plotting of the Beidler equation is that,
“the linearizing plots should not be used at all for the quantitative evaluation of data. Direct,
numerical iterative curve-fitting methods seem to give more reliable results.” He supports his
position by noting, “even modest amounts of response variability may interfere badly with the
evaluation of these plots.” He provides massive support for his arguements. No one has
demonstrated the lower regions of the graphs of these modifications actually relate to the
mechanisms of gustation. This work suggests these lower regions are noise limited regions just as
they are in the visual and auditory modalities. See also Section 8.5.1.5.7 regarding justnoticeable-differences. Shallenberger (1993, section 10.7.3) also noted the danger of using socalled rating data when the data was plotted on an equal-interval(linear) scale with an arbitrary
zero point.
286
Stone, H. & Oliver, S. (1966) Beidler’s theory and human taste stimulation Percept Psychophysics vol 1, pp
358-360
287
McBride, R. (1987) Taste psychophysics and the Beidler equation Chem Senses vol 12 (2), pp 323-332. doi:
10.1093/chemse/12.2.323
288
Maes, F. (1985) Response noise affects the graphical evaluation of response versus concentration curves
Chem Senses vol10 (1), pp 23-34. doi: 10.1093/chemse/10.1.23
Signal Generation & Processing 8- 221
The final estimates are shown in Figure 8.5.610 along with estimates of the free energy
change parameter for a presumed reaction
where ΔF = –RTlnK. The values exhibit
interesting groupings. The first six rows relate
to on-polar amino acids. The last two rows
relate to acidic amino acids believed to be
critical to the electrostenolytic process
providing power to the neural system
(Section 3.2). Arginine is the only basic
amino acid in the table. The last three are
obviously distinct from the earlier amino
acids in the table. The slopes of these last
three amino acids are significantly different
from the non-polar amino acids. ΔF for the
non-polar amino acids is very low,
suggesting a non-ionic bond with the
sensory neuron receptor. Although not
discussed in detail, Stone & Oliver suggest
“the initial step is most likely adsorption, in
agreement with Beidler’s conclusion (1954).”
This work hypothesizes the first step involves
a DACB between the stimulus and the
neural receptor. The three substantially
higher values for the bottom three rows of
the table probably relate to a totally
different mode of operation or the limited
statistical precision of the raw data.
Figure 8.5.6-9 Beidler equation plotting data for
amino acids, C/R versus C. Note the linear scales
and different ordinate scales. The poins derived
from the pooled data from five subjects. The
method of least squares was employed to
determine the equation for each line. From Stone
& Oliver, 1966.
Figure 8.5.6-10 Amino acid slope values, maximum responses & equilibrium constants. The free
energy is calculated from th equilibrium constant using the equation under the table. See text.
The bottom pair of amino acids are acidic and involved in the electrostenolytic process powering
the neural system. Arginine is the only basic amino acid in the table. The others are described
as non-polar. From Stone & Oliver, 1966.
Shallenberger (1993, section 10.7.2) gave the parameter K a value of 9.8 (7–15) for NaCl
stimulating rats when measured neurophysiologically, with the value depending on the protocol
employed. Shallenberger (1993, section 10.7.2) gave the free energy change for NaCl binding
222 Neurons & the Nervous System
to the N-Path receptor as –1.37 kJ/mol. and described this value as “reasonable value for an
inonic interaction.” He did not indicate whether this value applied to NaCl before solvation or
when in its fully hydrated ionic form. He did suggest the change could be attributed largely to
a change in entropy. He cited the work of Simon that will be discussed in Section 8.5.6.3.5.
If the Stone & Oliver data was replotted to display R or R/Rm = f( C), the resulting curves exhibit a
linear relationship below a breakpoint. For concentrations above this breakpoint, the response
approaches Rm asymptotically. This is the typical saturation condition expected in the
transduction process (Section xxx showing saturation in the reported P/D equation of vision ].
The Stone & Oliver representations do not support the analysis of Dzendolet, nor do the analyses
of DuBois et al. discussed in the next subsection. However, Stone & Oliver did not discuss the
equilibrium equation that they assume is the source of their data. The very low ΔF values suggest
the DACB associations and dissociations involved occur at energy levels quite near the thermal
noise level. A very sophisticated analysis of this situation may be required.
8.5.6.3.3 Comparing the human perceived response to simple sugars
Dubois et al. provided data in 1991 that has been redrawn in Figure 8.5.6-11 to show that using
the molarity of molecules that hydrolyze upon going into solution is inappropriate. Fructose (mol.
wt. = 180) is generally accepted to be the most sweet of the simple sugars. The ordinate scale
is in percent concentration based on equal molarities of the dry sugars. However, the
saccharides totally hydrolyze on going into solution. Thus the figure actually shows the responses
of a variety of simple sugars on a scale of questionable concentration precision.
The best fit regression line for sucrose (a disaccharide subject to hydrolysis) shown in their figure
2 exhibits an intercept with the abscissa at 1.4% which is of doubtful theoretical importance and
probably results from the difficulties of calibrating human perceptual responses. As a result of the
intercept, he indicates a slope of the response to sucrose versus the stimulus to be 0.94. Except
for this intercept, his regression line for sucrose overlays their fructose data very well. The slope of
the regression line for fructose is given as 1.27. The slope for glucose is given as 0.60. D-Glucose
is a monosaccHaride of the hexose family with a 6 carbon ring whereas D-sucrose (cane sugar)
is a disaccharide containing one residue of D-glucose (blood sugar or corn sugar) and one of Dfructose (fruit sugar, also a hexose but with a 5 carbon ring). Fructose is generally found to exist
in both furanose and pyranose forms (with about 20% furanose at 20C). DuBois et al defined their
fructo-oligosaccharide sweetener as a footnote to their Table I. They gave the slope for the data
shown as 0.27. DuBois et al., while discussing these slopes asserted that fructose exhibited a
higher potency compared to that of sucrose. However, the slopes are nominally equal after
considering the intercept problem in the sucrose data. There appears to be an uncontrolled
variable present in this data. Each of the simple sugars of DuBois et al. is a gustant containing one
or more gustaphores stimulating the G-Path via the GR 2 receptor of this work. To stimulate this
sensory receptor, the requirement is that each gustant molecule exhibit one or more gustaphores
of the two carbon equatorial-trans-diol form (See Section 8.5.1.6) Several of the gustants used
occur in multiple forms in solution. α-D-glucopyranose (using the current systemic name) exhibits
two gustaphores meeting this requirement. . β-D-glucopyranose (with the same molecular
weight of 180) exhibits 3 and can be expected to be perceived as sweeter on a equal dry weight
molar basis. Similarly, α-D-fructosefuranose exhibits two equat-trans-diol gustaphores while β-Dfructosefuranose exhibits only one equat-trans-diol gustaphore. Sucrose ( α-D-glucopyranosyl-βD-fructosefuranoside) may hydrolyze in solution to exhibit two appropriate gustaphores due to
the glucose residue and only one due to the fructose residue. Clearly these variables must be
accounted for in any repetition of the DuBois et al. experiments involving simple sugars. The use
of uncharacterized oligosaccharides are obviously not recommended as stimulants in such
experiments. Single gustaophore gustants (SGG) or at least gustants with an equal numbe of
gustaphores per equal molar quantity are preferred.
Signal Generation & Processing 8- 223
Figure 8.5.6-11 The perceived response of humans to polysaccharides of nominal concentration
based on their dry weight. The sucrose data from Dubois et al., figure 2 overlays the fructose
curve but shows an intercept at a response of 1.4%. The alternate line with dog leg drawn
through the glucose data is suggestive of a concentration near 25% was equal to the reciprocal
value of the equilibrium constant (K) for this situation. See text. Composite from Dubois et al.,
1991.
Taking the change in molar concentrations following hydrolyzation of the selected saccharide,
the figure can be re-interpreted to indicate the average number of gustaphores present for each
labeled sugar is important and the average number of gustaphores in the oligosaccharides
(containng two to ten simple sugars) after hydrolyzation is typically lower than the mixture of
fructoses and/or glucoses.
Based on the figure, the concentrations used by DuBois et al. apparently did not exceed the
threshold where C = 1/K except there is an indication that the glucose data is approaching that
threshold.
DuBois et al. also provided similar data for three very complex sugar alcohols (containing a
heterocyclic structure of oxygen). While exhibiting an abundance of hydroxyl groups, they only
exhibited a very few equat-trans-diol groups. Their perceived response versus concentration
appears to reflect their very large molecular weights relative to their number of gustaphores. The
concentrations had to be about twice as high to achieve a response comparable to that of the
simple sugars.
DuBois et al. considered a “Hill equation” as an alternative to the variant of the Beidler equation
they presented. The applicability of the Hill equation of 1910 to various problems was reviewed
by Weiss in 1997289. In general, the Hill equation is applicable to situations like the association of
multiple atoms of oxygen to hemoglobin before the hemoglobin becomes active. He noted,
“The Hill coefficient is best thought of as an interaction coefficient, reflecting the extent of positive
cooperativity among multiple ligand binding sites.” He also makes the assertion, “Despite its
appealing simplicity, the Hill equation is not a physically realistic reaction scheme, raising the
question of whether it should be abandoned in favor of realistic schemes.” There is no known
reason to consider the Hill equation with regard to individual receptor/stimulant binding or the
binding of thousands, if not millions, of receptor sites on the dendritic structure of a given sensory
neuron. It has been demonstrated on multiple occasions that the sensory receptors of the neural
system are capable of reporting single events, be they individual visual photons, individual
289
Weiss, J. (1997) The Hill equation revisited: uses and misuses FASEB J vol 11, pp 835-841
224 Neurons & the Nervous System
audible phonons or minuscule stimulations applied to somatosensory receptors. In practice, such
sensing is frequently obscured by noise either accompanying the stimulant (ex., photon noise)
arising in the sensory receptor neuron. There does not appear to be any reason to consider the
Hill equation in the context of gustation. The Hill equation reduces to the Beidler equation if no
cooperation is required between multiple ligands of stimulant in order to achieve signal
generation by the individual receptor neuron.
The responses reported by DuBois et al. for both simple sugars and sugar alcohols appeared linear
with respect to concentration up to the beginning of saturation in the number of empty and
available receptor sites.
8.5.6.3.4 Comparing the human perceived response between sweeteners
DuBois et al. also provided human perceived responses to a list of artificial sweeteners of their
time period (and including glycine for comparative purposes). There graphical data invariable
exhibits a linear rise in the response as a function of concentration up to an inflection point where
they became asymptotic to a constant response level. DuBois et al. used concentrations given
in parts per million (ppm) rather than in molar units. The inflection points are given in ppm in Table
II under the title, 1/K. These ppm values appear to track the increased effectiveness of these
sweeteners (Section 8.5.xxx)
It can be argued that the Hill equation used to fit the data for three of the sweeteners can be
replaced by the Beidler equation without seriously compromising the fit of the equation to the
statistical error bars provided for the individual gustants.
While the response functions shown in figures 5 through 8 initially appear distinctly different from
those for the simple sugars and sugar alcohols of figures 2 thorough 4, they are merely
representations stressing different regions of the same underlying function. Note the significantly
different scales for the ordinate axis among these figures. The different scales reflect two
conditions; first, the significantly different sensitivity of the receptors to stimulants in an AH,B,X
relationship to the less effective simple stimulants involving an AH,B relationship, and to the
inflection point located at 1/K. The three functions shown in Figure 5 for alitame, aspartame and
sucralose exhibit an intercept on the ordinate axis that does not go through zero concentration.
The later figures do not show such an intercept. A major assertion of the DuBois et al. paper is,
“it is tentatively concluded that at least two routes to receptor cell activation must exist.” The
AH,B and the AH,B,X mechanisms appear to support this assertion.
8.5.6.3.5 Mechanisms of sweet taste transduction from Simon
Simon has provided a very comprehensive proposal related to the sweet transduction
mechanism based primarily on his background and conceptualization290. It is sophisticated but
does not provide a physiological model of the process other than the conventional sketches of
neurons with ions and messengers traveling in diverse directions. He proposes to consider
saccharides alone as well as with or without accompanying salts and with transduction only on
the surface of the dendrites or also by passing through tight junctions in order to stimulate the
basolateral membrane of sensory neurons. He then limits his investigation to only glucose and
sucrose in the presence or absence of salts (apparently only NaCl). As of 1991, he also asserts,
“Since receptors for naturally occurring saccharides have not yet been isolated rom lingual
epithelia, there is no direct evidence for their existence.” He also asserts, “One reason for
questioning the existence of receptors is that D– and L–glucose are equally sweet.” This is hardly
a viable reason for doubting the existence of a type of receptor. It is a better reason for doubting
his concepts of transduction.He does provide strong evidence that saccharides do not bind
effectively to proteins, based on his presumption that proteins are an element involved in
transduction.
290
Simon, S. (1991) Mechanisms of sweet taste transduction In Walters, D. Orthoefer, F. & DuBois , G. eds.
Sweeteners. Wash. DC: American Chemical Society Chapter 18
Signal Generation & Processing 8- 225
While his page 238 can be largely discounted due to limitation in his model(s), selected portions
of the data related to thermodynamics is quite valueable in supporting the hypothesis of this
work. His comments concerning the effects of amiloride on the transduction of saccharides will
be addressed in Section 8.5.7.3.
Simon proposed two distinct conceptual models;
In the first model, the binding of saccharides to receptors opens an amiloride-inhibitable
channel permitting small cations to enter and depolarize the taste cells. In the second
model, the binding of ssaccharides to receptors activates a second messenger cascade
that either closes potassium channels and /or opens chloride channels. In the second
model, taste transduction can occur inthe absence of salts.”
Neither of these models employs the DACB relationship of Shallenberger and this work; neither
the AH,B or AH,B,X configurations were considered. Neither of these models appears viable
based on this work; however, much of the information in Simon is applicable in support of the
hypothesis of this work. This assertion is based on his basic premise concerning the stimulation of
sensory receptor neurons,
“It was demonstrated that the interaction of saccharides with taste cells depolarise them.
The depolarization of taste cells implies that ions must flow across their cell membranes.
Hence ions (salts) are involved in the response to saccharides (emphasis added).”
The implication is not the only option. Further more ions are not salts, they are ions that may have
originated in dry salts before solvation. Simon clearly does not appreciate the options under the
Electrolytic Theory of the Neuron. Holes (the absence of electrons) in a structured membrane can
imitate positive ions but are identifiable by differences in speed of transport. Unhydrated Ions
generally can not cell membranes. They must first be combined with other lipids or proteins.
Hydrated sodium salts exibit unique d-values that allow them to form DACB bonds with the
organic ligands of the sensory neuron receptors (Section 8.5.xxx). Simon then asserts that “the
remainder of this manuscript will deal with the ion currents that are induced by the interaction
of saccharides with taste cells.” This Bayesian assumption negates the value of this paper.
Simon illustrates six distinct conceptual models of how the sensory receptor neurons might
operate based on the currently discussed chemical theory of the neuron. The figure was
attributed to J. Verbrugge. He then discusses each one determining the difficulties associated
with each variant. Multiple ions are entering or leaving each caricature at various undefined
times and at various undefined rates. He also notes, “Taste cells also contain transport pathways
not mentioned above.” As a result, the models are less than satisfying to a biophysicist.
8.5.6.3.6 Solubility of the natural sugars from Andersen et al.
Figure 8.5.6-12 reproduces the data of Andersen et al291. on the solubility of various sugars as a
function of temperature. The data was collected from a variety of sources and more recent data
may be available292. They showed significant differences exist at all temperatures of biological
interest. In a separate table they showed the relative solubility of these natural sugars as
measured by multiple investigators. The sequence of solubilities was routinely given as Dfructose>sucrose>D-glucose>maltose>D-galactose>lactose. The slopes of the solubilities are
uniform with the exception of D-galactose and with some proclivity for D-glucose and maltose
to show a higher slope at higher temperatures. A noted feature is the uniformity of both the
monosaccharide and disaccharide members of this family.
291
Andersen, H. Funakoshi, M. & Zotterman, Y. (1963) Electrophysiological responses to sugars and their
depression by salt In Zotterman, Y. ed. Olfaction and Taste. NY:
292
Yalkowsky, S. He, Y. & Jain, P. eds. (2010) Handbook of aqueous solubility data Boca Raton, FL: CRC
Press Also a 2003 edition.
226 Neurons & the Nervous System
Andersen et al. asserted, “These results indicate that the disaccharides are not split on the tongue
or at the receptor sites when the stimulation of the proper receptors takes place.” This assertion
appears to be incorrect and based on the observation that the molarity is changed when the
disaccharides are hydrolyzed but the sugars as gustaphores are not. Based on this work,
hydrolyzation leads to both a change in the molarity of the solution and an equal or similar
creation of more gustaphores due to the creation of more equat-trans-diol gustaphore sites on
the increased number of molecules (Section 8.5.6.3.3). The net effect is no significant change in
activity level with hydrolyzation of most polysaccharides.
The massive compilation of Yalkowsky et al. suggests that the solubility of the sugars is still not a
settled science. Variations of up to 5:1 between values at a given temperature fora single sugar
are not uncommon, even when measured by the same investigator during the same year (ex.,
fructose).
Signal Generation & Processing 8- 227
Figure 8.5.6-12 Solubility of the natural saccharides versus temperature. A collage from multiple
sources. From Andersen et al., 1963
8.5.7 Antagonists (blocking agents) & adaptation in the gustatory modality
Based on the previous analyses, it is apparent that there are a variety of means of interfering with
the normal operation of the gustatory modality. These include,
C chemically capturing a gustant prior to its effective stimulation of the sensory receptors.
228 Neurons & the Nervous System
C mechanically interfering with a gustant physically reaching the sensor receptors.
C physically isolating the sensory receptors by causing a closure of the entrance to the taste buds.
C chemically occupying the receptor sites on one or more types of receptors for an interval of
time.
C reducing the sensitivity of the sensory neurons by interfering with their electrostenolytic
mechanisms electrically powering the neurons.
In the temporal domain, it is also possible to depress the sensitivity of a given gustatory channel
through the process of adaptation. High intensity stimulation of the channel for only a short
period will cause a major reduction in its sensitivity for a prolonged period, based on the attack
and decay time constants of the sensory receptor neurons.
A variety of terms have been associated with the above mechanisms, mediation, amelioration,
suppression, blocking, etc. One chemical playing a major role in this area is amiloride, most
probably named by association with amelioration.
To avoid interfering with operation of the gustatory modality, the investigator should be
careful not to employ nocents (such as inorganic acids, including HCl) or astringents (such
as the alkali earth salts) in their protocols. See Section 8.5.4.3.5.
As in the process of powering the individual neurons, there are a series of antagonists that
operate to impede the stimulation by various gustants and gustaphores. As dopamine is an
antagonist to the neuro-facilitator glutamic acid, it has been asserted that amiloride is an
antagonist to the sodium ions in exciting the sodium (N–) channel of gustation. There are other
antagonists to the gustatory modality that will be addressed below.
Shallenberger (1993, section 10.8) has provided a valuable review of potential antagonists and
their point(s) of action.
Recent computer generated representations of amiloride suggest it may not be an
antagonist to natrophores but is in fact a strong picrophore that may distract the individual
from perceiving the intensity of the natrophore. This will be discussed further in Section
8.5.7.2.
The mechanism is slightly different in gustation compared to neural fueling. But the result is largely
the same. The antagonist is able to form a dual antiparallel coordinate bond with the sensory
receptor site and prevent the appropriate gustaphore from reaching the receptor.
The ability of amiloride to interfere with the hydrated sodium ion is interesting because amiloride
contains no sodium. It is a totally organic molecule that is able to bond to the totally organic
sensory receptor site. Amiloride (DB00594) contains one oxygen and seven nitrogen orbitals. It
offers an abundant number of orbital pairs that could provide a d-value near that of the
hydrated sodium receptor at 3.24 Angstrom.
Breslin, in his Firmenich Award Address has provided some data on the properties of sucrose and
urea when mixed and when impacted by the presence of 0.3M sodium acetate293. When mixed,
they tend to suppress the perception of each other. When sodium acetate was added, the
perceived response of sucrose was nearly restored while the perception of urea was further
suppressed.
8.5.7.1 Generic blocking agents
293
Breslin, P. (2001) Human gustation and flavor In Spanier, A. et al. eds. Food Flavors and Chemistry. Royal
Soc Chem pg 47 Also published in Flavor & Fragrance J.
Signal Generation & Processing 8- 229
Andersen et al. reported on the suppression of the sweetness perception by salt294. They
experimented on dogs under the assumption that all gustatory signals passed along the chorda
tympani.
Breslin has addressed the question of blockade of gustatory stimulants295. He also discusses
enhancement, synergy, suppression and masking by multiple agents simultaneously. Although
discussing, chlorhexidine, gymnema sylvestre, lactisole and a combination labeled PALG, the
emphasis is on amiloride. It is important to determine what mechanism from those listed above
is being considered by the following investigators. Are they blocking access to the receptors,
blocking the formation of a DACB between gustant and receptor, or interfering with the X site in
an AH,B,X relationship to overcome the capability of the super-sweeteners?
Breslin describes two known sweetness suppressants. He notes that gymnema applied to the oral
cavity prior to application of a sweet tasting substance results in suppression of the perception
of sweetness. He also notes lactisole is a fast-acting competitive antagonist to almost any
compound tasting sweet.
Lindley has written on the chemistry of anti-sweeteners in 1991296 and 1993297. His 1991 introduction
discussed several molecules and their potential means of interfering with gustation, even
addressing the subject of detergents applied to the sensory receptors. Unfortunately, the
common names he used for his molecules are not in common use now and the gustants were
diluted in 5% or 10%(w/v) sucrose. His procedure employed SAR techniques. His conclusions are
well stated and suggest his inhibitors are competing for the same receptor locations as the
sweeteners being inhibited. He did note that in the absence of an AH group within an AH,B
structure, the abilitiy of his inhibitors was eliminated. Based on his conclusions,”then it is a logical
extension to conclude there is a single receptor structure that ‘codes’ for sweetness.”
In 1993, he summarized as follows, “Currently available evidence is consistent with the conclusion
that these sweetness inhibitors are competitive antagonists of sweet taste acting at a single
receptor structure.”
Breslin asserts the sodium ion is the primary known blocker of the bitter taste sensation. He offers
no description of the mechanism causing this action. Blocking in this manner could be related
to a neural differencing in either the dimension 1 = Q – H or dimension 2 = N – H channels
described in the multidimensional analyses above. The importance of the differencing might be
less than that due to the summation of the various stimuli joining in forming the overall taste
sensation.
He discusses PALG, a protein (especially lactoglobulin) bound to phosphatidic acid, as a good
blocker of bitter tasting compounds, especially quinine which it precipitates. His terminology is
that of a pharmacist or specialists in food product development. The designation phosphatidic
acid describes a fatty acid lipid that is a precursor for many of the potential fatty acids in the
sensory neuron lemma. Quinine in soluble form generally relates to quinine hydrochloride. This
may be an excellent clue to the operation of the P-Path sensory channel since phosphatidic acid
is a potential transducer in the type 4 lemma of the P-Path sensory neuron microvilli. If the
phosphatidic acid is reacting with the quinine hydrochloride with the precipitation of quinine, the
phosphatidic acid may become chlorinated and become negatively charged. This action could
294
Andersen, H. Funakoshi, M. & Zotterman, Y. (1963) Electrophysiological responses to sugars and their
depression by salt In Zotterman, Y. ed. Olfaction and Taste. NY:
295
Breslin, P. (2000) Human gustation In Finger, T. Silver, W. & Restrepo, D. eds. The Neurobiology of taste
and smell, 2nd Ed.. NY: Wiley-Liss Chapter 16
296
Lindley, M. (1991) Phenoxyalkanoic acid sweentess inhibitors In Walters, D. Orthoefer, F. & DuBois , G.
eds. Sweeteners. Wash. DC: American Chemical Society Chap 19
297
Lindley, M. (1993) Sweetness antagonists In Acree, T. & Teranishi, R. eds. Flavor Science. Washington,
DC: American Chemical Society Chapter 4
230 Neurons & the Nervous System
mirror the nominal reaction between the phosphatidyl fatty acid of type 4 lemma and quinine
hydrochloride, resulting in the precipitation of the quinine, permanent alteration of the
phosphatidyl fatty acid and a significant change in the flow of current into the plasma of the
microvilli. A comparison of the state of polarization of phosphatidic acid and both PtdCho and
PtdSer would appear to be useful.
As Breslin noted (page 437), “. . .a potent selective blocker of salty taste in humans remains to be
discovered.” He also noted (page 438), “There are no known blocking agents for acid sourness
or umami in humans at present (2000).” The theory of this work offers a variety of new
approaches to discovering, or designing, such gustaphores xxx.
In 2001, Breslin & Tharp presented a paper on the potential for blocking the N-Path and the P-Path
by a complex nitrogen molecule, chlorhexidine298. They note, “Psychophysical studies using
pharmacological blockers of specific taste qualities (e.g. bitterness or sweetness) hold the
potential to provide insight into both the type and number of transduction mechanisms. For this
technique to highlight a particular component of taste physiology, two prerequisites must be met.
First, the agent must specifically block a taste quality or qualities and not taste in general.
Second, the pharmacological agent must have a known biochemical action that could be
effective on taste physiology.” xxx
8.5.7.2 Gymnemic acids as G-path blockers
Quoting Wikipedia, “Gymnemic acids are Glycosides isolated from the leaves of Gymnema
sylvestre (Asclepiadaceae). Gymnemic acids like ziziphin and hodulcine are anti-sweet
compounds, or sweetness inhibitors. After chewing the leaves, solutions sweetened with sucrose
taste like water.
More than 20 homologues of gymnemic acid are found in the leaves. Gymnemic acid 1 has the
highest anti-sweet properties. It suppresses the sweetness of most of the sweeteners including
intense artificial sweeteners such as aspartame and natural sweeteners such as thaumatin, a
sweet protein. The anti-sweet activity is reversible, but sweetness recovery on the tongue can
take more than 10 minutes.”
The gymnemic acids appear to occupy the G-path GR sites much like Glycine and the
catacholines (Sec 3.5.5.3 & 3.5.5.4) do in occupying the glutamate receptor sites in the
electrostenolytic process fueling the neurons themselves. It is generally claimed that these
chemicals as a group do not affect the other pathways of gustation. Shallenberger (1993, section
10.8) has reviewed the sketchy data available on whether gymnemic acid operates as a
competitive or non-competitive inhibitor. Since it must be applied to the sensory receptors before
the stimulant of interest, it appears it occupies the sensory neuron receptor sites prior to the
stimulus attempting to occupy those same sites. The fact that washing out the impact of
gymnemic acid on the sensory receptors may require time periods of up to ten minutes also
suggests the acid has captured the receptor site.
The ability of lactisole, a dual gustaphore (triple if the sodium ion is present and hydrated)
gymnetic acid is shown in Figure 8.5.7-1, to block normal sugar sensitivity at the GR is obvious from
its configuration. It is claimed by some ( ? A dubious claim at best) that this chemical has no
affect on the other gustatory pathways.
298
Breslin, P. & Tharp, C. (2001) Reduction of Saltiness and Bitterness After a Chlorhexidine Rinse Chem
Senses vol 26, pp 105–116
Signal Generation & Processing 8- 231
Hellekant et al. (1985) have described the
ability of miraculin to overcome and/or
alleviate the effects of the gymnemic
acids299. Quoting Wikipedia, “Miraculin is a
natural sugar substitute, a glycoprotein
extracted from the fruit of Synsepalum
dulcificum. The berry, which contains active
polyphenols, was first documented by
explorer Chevalier des Marchais, who
searched for many different fruits during a
1725 excursion to his native West Africa.
Miraculin itself is not sweet. However, after
the taste buds are exposed to miraculin,
ordinarily sour foods, such as citrus, are
perceived as sweet. This effect lasts up to an
hour.
Figure 8.5.7-1 Lactisole, a sweetness blocker. The
O-C-C=O structure is a glycophore that can form
a DACB with the GR but it does not readily release
from this bonding like other glycophores, thereby
preventing its replacement by other molecules.
Miraculin works by binding to the sweet receptors on the tongue. Miraculin's effect lasts as long
as the protein is bound to the tongue, which can be up to an hour. It makes most acidic foods
taste sweet, but does not improve the taste of bitter things.”
8.5.7.3 Affect of amiloride on the monkey & other species
Hellekant et al. (1988) addressed the affect of amiloride on the perception of sweetness to a
variety of stimulants in the monkey, Macaca mulatto, at the chorda tympani of the monkey
specifically300. As noted earlier, there may be a question as to whether the amiloride blocked the
natrophore at the GR of stage 1 or actually overwhelmed the perception of saltiness due to the
intensity of the amiloride acting as a picrophore at the output of a stage 2 neuron. Additional
analysis of the protocol used by Hellekant et al. may be required to determine the character of
the nerve they interrogated.
Avenet & Lindeman have provided additional information from the laboratory on the role of
amiloride in taste related to several species301. Their temporal response measurements and
current/voltage plots are highly supportive of the electrolytic theory of the neuron. and show the
activity resulting from changes in the presence of amiloride clearly. The effect of amiloride on
the voltage/current characteristic is quite clear.
[xxx see Simon, pg 239+ in Walters with regard to amiloride and the sugars and salts. ]
Simon has noted several variants in the response to amiloride among the mammals (page 239)
that make dependence on generalizations in the literature difficult. Later on page 248, in
discussion of his Direct Activation Model concept and his figure 1a, Simon asserts, they “suggest
there are two saccharide-stimulated chorda tympani (CT) pathways that can be distinguished
by their amiloride sensitivity (emphasis in the original). Thus it appears that the amiloride-sensitive
component is salt dependent and the amiloride-insensitive is salt (mucosal) independent.”
8.5.7.3.1 Current problem related to the available archives and visualizers
299
Hellekant, G. Segerstad, H. Roberts, T. van der Wel, H et al. (1985) Effects of gymnemic acid on the chorda
tympani proper nerve responses to sweet, sour, salty and bitter taste stimuli in the chimpanzee Acta Physiol
Scand, vol 124(39), pp ~ 408+
300
Hellekant, G. DuBois, G. Roberts, T. & van der Wei, H. (1988) On the gustatory effect of amiloride in the
monkey (Macaca mulatto) Chem Senses Vol.13(l) pp.89-93
301
Avenet, P. & Lindemann, B. (1988) Amiloride-Blockable Sodium Currents in Isolated Taste Receptor Cells
J Membrane Biol vol 105, pp 245-255
232 Neurons & the Nervous System
Figure 8.5.7-2 provides two totally different representations of amiloride (DB00594 from the
Canadian Data Bank), both showing the molecule as planar. The left frame using the Jmol
visualizer suggests the lower two outer nitrogen atoms can act as a acidophore with d-value =
2.86 Angstrom but no pair capable of interfering with a natrophore was found. No atom pair
offering a d-value suitable for interfering with a natrophore was identified. The right frame using
the DS3.5 visualizer shows the same molecule, from the same Jmol file. In this case, the two lower
outer nitrogen atoms have a d-value of 4.723 Angstrom that is better centered on the P-path
channel of d-value = 4.746 Angstrom than its only potential natrophore of d-value = 3.599
Angstrom is centered on the N-path channel d=value of 3.3 Angstrom. Simple hand calculations
using accepted bond lengths strongly suggests the Jmol visualizer should not be relied upon even
though the underlying file may be correct, as represented by the DS3.5 visualizer. Using DS4.1 to
visualize the similar Jmol file from Chemspider for amiloride_15403, gives a totally different
conformation and significantly different d-values than either of the above sources.
The Protein Data Bank (pdb) files related to muco-inositol, like other specific conformations of
multi-conformational molecules, are also difficult to find in the literature, and many of those on
prestigous sites, including the NIH, are corrupted (referring to a different stereoisomer than mucoinositol) as of November, 2012. An edited conversation with one of these curators said;
’‘In fact, practically all of the contents in our database is not generated by ourselves but
aggregated from other databases. By the same token as above, errors/weaknesses in
these databases therefore unavoidably find their way into our services until someone points
them out to us.
If you search (
) with "inositol", you'll find exactly the same situation:
"Inositol - Compound Summary (CID 892) Also known as: myo-inositol, meso-Inositol,
Scyllo-inositol, Muco-Inositol, Allo-inositol, i-Inositol, Myoinositol, epi-Inositol, mesoinositol"
Most likely, therefore, we got the data in ( ) from ( ). ( (
) does distinguish between
stereoisomers of inositol; though we have found many other errors in (
) for other
molecules - no one service/site is perfect in this regard.)
One little trick to test what we have before downloading an SD file (especially with 3D
coordinates) is to first just request a (2D) drawing of a molecule. You'll see that all six
stereocenters are undefined. What this means is that the 3D mol/SDF generator will simply
*create* some default stereochemistry and geometry.”
In the majority of this work, Jmol files downloaded from ChemSpider will be relied upon.
ChemSpider is the website of the Royal Society of Chemistry (RSC) in Great Britain. The files will
be visualized using the Design Studio visualizer, DS3.5 (up until July 2015 and DS4.1 thereafter), as
indicated earlier (Section 8.5.4). As of 1 December, 2015, the RSC has taken down their 3D
representation of amiloride_15403 in their JSmol archive entirely along with probably all other
JSmol representations of molecules.
See Section 8.5.1.1.2 for additional important information regarding the RSC files.
Signal Generation & Processing 8- 233
Figure 8.5.7-2 Amiloride (DB00594) as represented by a Jmol file from the Canadian Data Bank
using two different visualizers. Note the totally different distances between the same atoms. Left;
using the Jmol visualizer. The lower two outer nitrogen atoms exhibit a d-value = 2.86 which is
suggestive of a acidophore. Right; using the DS3.5 visualizer. The pair of nitrogen atoms with a
d-value = 4.723 Angstrom is a nearly ideal picrophore. The other two pairs shown are relatively
ineffective, but could achieve a DACB with the N-path GR. See text.
8.5.7.4 Adaptation and/or suppression by antagonists in gustation EMPTY
xxx in his Firmenich Award Address (page 44) has addressed the definitions of adaptation and
cross-adaptation in the context of gustation as used in the food industry.
[xxx significance of adaptation of individual sensory paths and the resultant after-taste, colors,
and other terminology.
Evaluation by human perception of monosodium glutamate and other gustants containing
three distinct gustaphores becomes extremely difficult. The requirement to control the
state of adaptation of all of the affected channels before they are stimulated is a difficult
one. The requirement to avoid desensitization of any of the sensory receptors associated
with these channels by antagonists may be even more difficult.
8.5.8 Analysis of the literature based on the hypothesis
The empirical literature seeking an understanding of gustation is immense. However, much of that
reviewed below emanated from a group centered at Pennsylvania State University (Pfaffmann
and then Travers) and at the University of Wisconsin (Danilova and Hellekant) over a period of
many years. Most papers totally ignore any theoretical framework for explaining the collected
data. This section will review several groups of papers prepared by teams over a period of years.
As time has gone by, it is easy to see the improvement in their test protocols, their data
presentation and graphics. representations
The framework developed in this work and confirmed by the recent empirical record shows
that hydrochloric acid, HCl, does not play a role in the gustatory modality, except in its fully
hydrated form (Section 8.5.4.3.5). The gustatory modality does not sense HCl and it is not
reported in signals from the chorda tympani. HCl is an inorganic acid that attacks the
neural system and attempts to digest biological tissue. Like capsaicin, it is classified as an
234 Neurons & the Nervous System
irritant. Both are sensed by the nociceptor modality. While HCl is also found in the digestive
tract and is neutralized before reaching the large intestine, capsaicin is know to irritate the
entire alimentary canal.
On the other hand, the empirical investigations have lacked a theoretical framework. A
particular problem has been the fact that the gustatory modality involves four orthogonal
dimensions, the three dimensions supporting the selection mechanism and the one dimension
describing the intensity of the total perceived sensation at the output of the saliency map.
Assuming the modality only involved three dimensions has left all of the collected data in disarray,
particularly with regard to MDS representations but also with regard to what stimuli were included
in their stimulant sets. The MDS representations in the literature cannot be effectively compared
because they have employed arbitrary “dimensions” selected by the computer programs and
have distributed the values associated with the fourth dimensions among the values presented
within the assumed three dimensional perception space.
In future experimentation, it is necessary to allow for four dimensions in the MDS program resolving
the collected data. The three dimensions containing the four nodes of the signal pathways can
then be transferred to the presentation program of choice. At this point, any rotation, reflection
and scaling can be performed to optimize the presentation. These procedures must be applied
to the four dimensional data set. After these procedures have been completed, the values
associated with the fourth dimension can then be displayed as a one-dimensional plot
representing the perceived intensity for each of the stimulants. The one-dimensional plot as a
function of the fundamental dimension (the d-value line) will be similar to figure 5 in in Hellekant
et al. (1997) except the stimulants will be clumped around their appropriate node along the dvalue line.
The added definition of the gustatory neural system and the definitized hypothesis based on the
Electrolytic Theory of the Neuron presented above, requires a review, and reinterpretation, of the
past literature. This is particularly true based on the recognition that;
• there are a minimal number of gustatory channels within the neural system (4)
• that individual stimulants may exhibit multiple gustaphores of the same or mixed types and
• that a combinatorial mechanism is employed to support a multitude of gustatory stimulants.
The demonstration of a long sought fundamental dimension ordering the effectiveness of the
gustatory modality is also critically important in the reinterpretation of the literature.
An additional, critically important, factor is where the data was collected. The optimum data
would be collected at the output of the stage 1 sensory neurons. However, this is extremely
difficult while maintaining the integrity of the cells to normal stimulation. The next best data is
obtained from the stage 3 neurons emanating from the taste buds and typically traveling along
the chorda tympani before it is merged into the facial nerve (n. VII or IX depending on
investigator). This data is not contaminated by any known stage 2 signal processing. It is in pulse
coded format. Data may also be acquired from the glossopharyngeal nerve before it is
delivered to the brain stem. However the opportunity for contamination of the signals is present
because this nerve carries so many other signals (both afferent and efferent). Examining signals
at the nuclei tract of solitarius (NTS) or the parabrachial xxx
8.5.8.1 Reinterpretation of Smith et al. of 1983 using hamsters
Smith, van Buskirk, Travers & Bieber issued two papers in 1983 that can be reinterpreted based on
the hypothesis and supporting material presented here. A major problem with interpreting their
data is the lack of statistical error bars associated with their raw data. Much of the data involves
very small signals that cannot be interpreted with precision (parts of figures 3, 4, 6 & 7). As a result,
the dendrograms and the 3D representation of their data does not always agree as indicated
in the footnote to table 1 of the first paper. All of their data was acquired at either the nucleus
tractus solitarius (NTS) or at the parabrachial nuclei (PbN), locations within the brainstem. No
attempt was made to determine whether the collected signals were at the input to or at the
output of these stage 2 signal processing engines. Nor was their any assertion that the signals
were not corrupted at the input or only involved gustatory signals at the output of these engines.
Signal Generation & Processing 8- 235
It is difficult to demonstrate the data is not corrupted from data supplied from nociceptors and
other types of sensory neurons at these locations within the brainstem. Stimuli were only provided
to the anterior portion of the tongue. They employ the historically recognized terms for H-best,
Na-best, sucrose-best and quinine-best neurons. As they note, “Such an approach may well
overlook other potentially important characteristics of a neuron.” In fact it also overlooks
important characteristics of the stimulants as well.
Their selection of stimulants shows a distinctly Bayesian approach to their experiments. The
stimulants, and the molarity of each, were selected based on their expectations of what the
results would look like.
Their cluster analyses were performed using the BMDP2M software package and their MDS
analyses employed the KYST package without any description of the default, and/or established,
parameters for these programs.
Their dendrograms included a number of anachronistic inclusions. For the NTS neurons, they only
identified three major clusters, H–, S– and N –. The N – cluster was subdivided into three subclusters. Six neurons were included within these N – subclusters even though they were identified
as either H-best neurons or S-best neurons. Their raw data in figure 4 shows distinctly that the four
neurons labeled H-best were in fact not H-best. #23 was maximally sensitive to sucrose, the
hydrated sodium ion and to HCl. #24 showed similar sensitivity. #26 & #27 exhibited very low
signal intensities with peak sensitivities to sodium and HCl in the first case and HCl alone in the
second case.
The sensitivity to HCl suggests these neurons are responding to nociceptors (see the comparable
data of Hellekant et al. in the next section where HCl played a negligible role in data collected
earlier in the signaling channels). Neuron #23 strongly suggests it is reproducing signals at the
output of the NTS engine.
For the PbN dendrogram, a similar situation is found. Only three of the four historical clusters were
found using their stimulants and software. One ostensibly Q-best neuron (#31) was placed in an
H-cluster. According to figure 6, this neuron exhibited marginal sensitivity with some sensitivities
registered as negative relative to the quiescent neuron output. A negative output is normally
associated with the output of a differencing circuit within a stage 2 engine rather than the
unidirectional signals associated with a stage 1 sensory neuron. #20 and #23 of the PbN neurons
exhibited very minimal responses and are difficult to categorize authoritatively. Both #16 and #17
exhibit very broad sensitivities to sugars, salts, HCl and urea in the first case and to all but the
sugars and quinine in the second case. #24 although described as H-best but found in the Ncluster, shows significant sensitivity to the hydrated sodium ion (nominally as great as to HCl). #30
shows negligible response. The response to HCl is only marginally higher than its response to
hydrated sodium. These responses as a group suggest, the signals are recorded at the output of
the NTS and PbN engines where the signals would be at maximum strength as they are encoded
in stage 3 pulse form for projection to other engines farther up the brainstem.
The first paper does not include any 3D representations based on MDS analyses. It does present
two 2D representations. However the distance between the clusters is of the same magnitude
as the diameter of the clusters with the shape of some clusters significantly impacted by one
neuron at a considerable distance from the other neurons in that cluster. While they only found
one Q-best neuron, which they attribute to only stimulating the tip of the hamster tongue, the
following Hellekant et al. paper found multiple Q-best (or P–channel) neurons in the tip of the
monkey tongue.
Smith et al. generally support the labeled-line presumption relative to gustation but do not make
any strong assertion.
In the second paper, the authors make a strong statement in their introduction that, “It is almost
without exception that mammalian taste neurons respond to stimuli representing more than one
of the classical four taste qualities.” They accompany this statement with a list of citations but no
detailed discussion. This position is not supported by this work or by the following paper by
Hellekant et al. They do make the important assertion that, “Since the response of most taste
fibers can be modulated by both stimulus quality and intensity, the response of any particular cell
is typically ambiguous with respect to either parameter.” This position is supported here and is the
236 Neurons & the Nervous System
main reason investigators attempt the standardize the molarity of their stimulants relative to the
pulse rate they encounter for specific neurons. Most of the data in their figure 1 & 2 reflect very
broad sensitivity for individual neurons. They did provide one 3D representation of an MDS
analysis reproduced here as Figure 8.5.8-1. Lacking a larger data set, the dashed circles can be
used to indicate the mean location of the particular odorophores affecting each sensory
receptor channel. The data set can be rotated to bring the vertical axis parallel to the vertical
dashed line. Then the X,Y plane can be rotated to bring it into registration with the heavy dashed
lines of the X and Y axes. Alternately, the heavy dashed lines can be considered the
fundamental axes of the perceptual gustatory space for the data points shown. If HCl remains
in this group, it must be recognized that its gustaphore results from its hydration (becoming an
azeotrope in order to be sensed by the gustatory modality).
Signal Generation & Processing 8- 237
Figure 8.5.8-1 Three-dimensional space showing the location of 18 stimuli. Note the location of
the sugars compared to the organic acids. Note the separation between the ionizable salts of
ionic sodium at lower right and Na-saccharin at upper center. Urea and QHCl occupy a distinctly
separate area. The dashed circles suggest the mean location of the particular group of
odorophores. The heavy dashed lines suggest the degree of rotation of the axes required to
bring the data into alignment with the preferred coordinates of Section 8.5.1. Data and original
coordinates From Smith et al. 1979.
8.5.8.2 Reinterpretation/expansion of Rohse & Belitz of 1991
238 Neurons & the Nervous System
The text of the paper by Rohse & Belitz302 was brief and only examined similarities between very
complex molecules perceived as sweet or bitter without any discussion of underlying theoretical
principles. They did note the importance of an “e/n system” relationship implying the importance
of an electrophilic and nucleophilic components in the transduction process. They described a
number of candidate e/n systems including,
1. OH/OH in sugars,
2. NH3+/COO– in amino acids and peptides,
3. NH/SO2 in oxathiazinone,
4. OH/CO in β-hydroxyketones,
5. CONH2/COO– in carboxyalkyl benzamides,
6. CONH2/OR in alkoxyphenyl ureas and
7. NH2/CO in ureas.
They did provide distances between these different group pairings. However, they did not
indicate the points from which their distances were measured.
Furthermore, they did not recognize the minimal properties required for sweetness within these
molecules. Nor did they speak in terms of an overlay group structure associated with their
pairings. Such an overlay structure can be critical because it can affect the net distance in 3D
space between the actual orbitals. They also failed to address the participation of resonant
structures in their computer modeling.
A revised (condensed) and expanded (to include the resonant structures) description of the
overlay groups required for the perception of sweetness is suggested below,
[xxx edit is the H in NH groups really necessary? ]
Electrophilic structure
Nucleophilic structure
OH
NH3+
NH2
NH
OH
OH
OH
(ex., 1, 4 above)
O
O (of carbonal group)
O (of either sulfxxx group)
O (of carbonal group)
Shorthand e/n notation
OH/O (using AH,B concept)
OH/O
Any C=C double bond
OH/C=C
Any Huckel structure (resonant) OH/Ar ring
The OH group is susceptible of forming a hydrogen bond with a nucleophilic element of a
separate molecule, resulting in a London bond. In a London bond, the hydrogen is held to one
oxygen by a covalent bond and the other oxygen atom by purely electrostatic forces. The net
bond strength is on the order of 5 kcal/mole compared to 50-100 kcal/mole for most covalent
bonds (M&B, 1971, section 15.5). The distance between the two oxygen atoms, that are
individually capable of participating in additional London bonds. is typically 2.703 Angstrom. As
noted below, the ability of many saturated alcohols and aldehydes to form azeotropes in this
manner accounts for their typically mild sweet taste.
The chemistries of nitrogen and sulfur become very complex compared to that of oxygen in their
roles in gustation. Their chemistries must be analyzed in greater detail than can be addressed
here (See M&B, 1971, sections 11.15-18)
The role of sulfur in gustation , and particularly the complex chemistry associated with sulfur
bonded to oxygen within a sulfonyl group), has not been studied in this work.
There are a variety of ways to satisfy the Huckel rule (4n + 2, n=0,1,2,3,etc.) within a molecule. The
six sided benzyl ring is by far the most common.
302
Rohse, H. Belitz, H. (1991) Shape of sweet receptors studied by computer modeling [1991] xxx
Signal Generation & Processing 8- 239
The above listing does not include a pairing of two C=C double bonds in a DACB relationship and
caution should be employed when determining the d-values of many compounds.
The Rohse & Belitz paper is a transitional step between the work of Shallenberger & Acree and
of Kier and this work. It only addresses a small group of large molecules perceived as sweet plus
a few similar molecules perceived as bitter (without providing any definition of these terms. The
paper is compatible with the more detailed hypothesis of this work.
8.5.8.3 Reinterpretation of Hellekant et al. of 1997 using M. mulatta
The paper of Hellekant et al. is very well constructed, provides statistical error bars on its basic
data and interprets that data with great care. However, it lacked the fundamental dimension
provided by the d-values of the DACB for their stimulants. It also overlooked the fact that one
of their stimulants, sucrose is actually a disaccharide and its gustaphore molarity after hydrolysis
in water is twice the molarity of sucrose. This fact is clearly reflected in their data in figures 5D and
10C. To compensate for the effect on molarity of hydrolysis, hydrolysis also has the potential for
increasing the number of gustaphores in the solution by a factor of two. They collected data at
both the chorda tympani (CT) supporting the anterior part of the tongue and the
glossopharyngeal nerve (NG or xxx) supporting the posterior portion of the tongue.
Hellekant et al. used several guanidine derivatives in parts of his investigation but does nor show
them in his cluster analyses or MDS results. These derivatives were assigned numbers rather than
names. Unfortunately, the NC-00174 and NC-00351 numbers are also used for other chemicals
in the general field of chemistry. The chemical SC-45647 (C18H26N4O2, mol wt. 330.4) is shown as
such in the literature but the term NC– in their figure 1 should probably be written as H2NC– as
shown in its 3D representations to avoid confusion with cyanide groups. These chemicals are
perceived as super sweet due to their AH,B,X arrangement with aromatic–N group acting as the
AH,B structure with a d-value of 2.71 Angstrom.
Like Smith et al., Hellekant et al. employed both multidimensional scaling and dendrographic
clustering techniques to organize their data. Unlike Smith et al., they also provided a large three
dimensional table describing the performance of each of neuron interrogated.
They calculated entropy values for their two sets of data using the formula proposed by Smith &
Travers, 1979. This analysis will show these calculations need to be repeated using different
neuron groupings.
Hellekant et al. used an artificial saliva as their solvent rather than distilled water. To the extent
this saliva mimics that of their subjects, it appears to remove another variable from their
experiments. However, in some cases, it may introduce the hydrated sodium ion into stimulant
samples where it is normally absent.
Their data from the chorda tympani is particularly clear. It forms four distinct clusters in their figure
4 using the historical labels, N –, H–, S– & Q–. The analysis to follow appears to justify the use of the
new labels proposed in this work of N –, A–, S– and P–. This is particularly true because the
hydrogen ion plays virtually no role in gustation and the compound quinine hydrochloride is not
a good example of the bitter tasting chemicals generally related to, and displaying the
complexity of picric acid.
Only five cells appear misplaced based on their analyses.
• RH95D13G is described as an H-neuron in the S-cluster based on its response to a variety of
artificial sweeteners and the two organic acids. All of the artificial sweeteners include at least
one carboxyl gustaphore. Therefore,
• their S-cluster should be subdivided into those stimulant containing only a glycophore
and those containing multiple gustaphores, typically a glycophore, a acidophore and
frequently a natrophore.
• this neuron is primarily sensitive to the organic acids and belongs in the A-Path of this
work.
240 Neurons & the Nervous System
• RH92A23B is described as an H-neuron because of its sensitivity to the carboxyl gustaphore
(C–path) but is placed in the S-cluster based on its strong response to xylitol. Xylitol is a sugar
alcohol typically shown in aliphatic form with three glycophores with d-values of 2.87 Angstrom..
Typically sugar alcohols form an open ring-like structure when in solution. No purity of the xylitol
was provided in the paper. This neuron also showed the highest sensitivity of any to HCl.
Therefore, [xxx conclusion] See Section 8.5.4.1.4.
• RH92U23P, RH92U24D & RH92A23H are described as H-neurons within the N-cluster based on
their response to several glutamates and citric and aspartic acids. However, all four of the
chemicals they responded to contain the acidophore (the carboxylic acid group). Therefore,
• Their presence in the N-cluster is only partially justified based on their actual primary
sensitivity to an acidophore
• No data was provided concerning the purity of the citric and aspartic acids or any
effort expended to avoid their contamination by sodium salts.
• Neuron susceptible to one of the gustaphores in a complex stimulant cannot be
classified based on their anecdotal (or undetailed) characteristics.
It is concluded that the four H-neurons listed above within the N-cluster and one H-neuron listed
within the S-cluster of the dendrogram of figure 4 are in fact sensitive to the acidophore present
in all of the complex stimulants described as belonging to the N-cluster. These neurons show
marginal to negligible sensitivity to NaCl in figure 3. MSG & MSG + GMP, are multiple gustaphore
stimulants that should not be assigned to any exclusive N-channel of gustation.
It should be noted that only two neurons in the set from the chorda tympani showed significant
sensitivity to HCl.
Figure 8.5.8-2 reproduces their figure 6 showing the data from their MDS analysis of the neurons
within the chorda tympani without change. However, an overlay has been added describing
the folded 3D representation of the fundamental dimension of gustation. Five facts are notable.
• First, their 3D space uses a set of perspective axes rather than truly orthogonal axes.
• Second, the location of the G– node is only established approximately because the distance
between the disaccharide sucrose and the monosaccharide fructose do not clearly establish the
correct position of the glycophore node.
• Third, as shown in their figure 5, the data set contains considerable variation in their average
action potential counts. This variation constitutes a fourth dimension not accounted for in their
analyses, except by the high Kruskal stress index of 0.048. The variation exceeds 10:1.
• Fourth, some of their average counts in figure 5 are negative, suggesting some stage 2 signal
processing prior to the creation of the action potential pulses in the stage 3 chorda tympani.
• Fifth, their dimension 1 is essentially parallel to the G–N axis, their dimension 2 axis nominally
parallel to the N–P axis and their dimension 3 axis is nominally parallel to the G–C axis of the
proposed new 3D space based on the fundamental dimension. Only citric acid, aspartic acid
are grossly misrepresented with respect to the new coordinate system. The one HCl data point
is extraneous in this representation.
Signal Generation & Processing 8- 241
Figure 8.5.8-2 Distribution of 23 stimuli in a 3D space based on data from 47 CT fibers. Kruskal stress
level: 0.048. After accounting for the perspective coordinates used by Hellekant et al., their
dimensions (axes) essentially parallel the folded segments of the fundamental dimension based
on the d-values of the underlying gustaphores. See text. Modified from figure 6 of Hellekant et
al., 1997.
RH92U27J is classified as a Q-neuron based on very marginal performance when stimulated by
QHCl and no neuron responded significantly to denatonium benzoate at the molarity of the
stimulants employed to excite the neurons within the chorda tympani.
Their data from the glossopharyngeal nerve can be readily interpreted based on the above
discussion. Only two neurons responded marginally to denatonium benzoate. Only one of their
neurons responded significantly to HCl They did not show the denatonium in their graphic and
the HCl location appears anachronistic compared to the location of citric acid and aspartic acid
(both organic acids). Their representation of their 3D MDS analysis used the same perspective
coordinate system as their figure 6 instead of a truly orthogonal system. The overlay does show
the continuity of the d-value line (A–G–N–P) as developed in Section 8.5.1. With this continuity,
the dimension scales could be augmented with the addition of explicit d-value scales.
Their Kruskal stress value was marginally higher at 0.067. The folded fundamental dimension of this
work best fits their data if rotated by 10 degrees clockwise in the plane of the paper relative to
242 Neurons & the Nervous System
the arbitrary dimensions (axes) assigned by the MDS program. The location of the C–node is not
established definitively based on the data available. All of the artificial sweeteners containing
an acidophore as well as a glycophore are shown appropriately along the G–C dimension. The
two stimulants exhibiting only a single acidophore, citric acid and aspartic acid, are shown
anachronistically among the natrophores. No data was provided concerning the purity of these
two acids or any effort expended to avoid their contamination by sodium salts.
When examining the response of the neurons to a stimulant perceived as bitter by humans,
sucrose octa-acetate (SOA) was added to the list of stimulants. This material has little chemical
resemblance to sucrose. While it maintains a heterocyclic ring containing one oxygen, in other
respects it resembles picric acid in the number of oxygen atoms exposed along its periphery. It
was not included in most of their analyses. Caffeine is shown in a logical, if not demonstrated,
position along the P–G axis. Sodium cyclamate is shown in a position dominated by its
natrophore. NaCl + amiloride is shown near the P–node along the N–P dimension much as in
their figure 6.
In their supporting dendrogram (fig. 9), they define an M-cluster and no H- or N-cluster. The cluster
is dominated by the stimulants containing multiple gustaphores, and in particular a natrophore
and an acidophore. As discussed with respect to the CT neurons, the M-cluster designation
should be abandoned and the neurons reassigned based on their dominant response to an Ncluster and a C-cluster. Specifically;
RH94E21D is strongly stimulated by NaCl, monosodium glutamate and both citric and aspartic
acid.
Figure 8.5.8-3 reproduces their figure 11 showing the data from their MDS analysis of the neurons
within the glossopharyngeal nerve without change. However, an overlay has been added
describing the folded 3D representation of the fundamental dimension of gustation. [xxx change
C– to A– ]
Signal Generation & Processing 8- 243
Figure 8.5.8-3 Distribution of 18 stimuli in a 3D space based on data from 33 NG fibers. Kruskal
stress level: 0.067. After accounting for the perspective coordinates used by Hellekant et al., their
dimensions (axes) can be overlaid by a folded form of the fundamental dimension based on the
d-value parameter developed in this work. The P-path node appears poorly defined by only two
data points. The best fit appears to call for the fundamental axes to be rotated 10 degrees
counterclockwise in the plane of the paper. See text. Modified from figure 11 of Hellekant et al.,
1997.
As in the previous figure, several facts are notable.
• First, their 3D space uses a set of perspective axes rather than truly orthogonal axes.
• Second, as shown in their figure 10, the data set contains considerable variation in their
average action potential counts. This variation constitutes a fourth dimension not accounted for
in their analyses, except by the high Kruskal stress index of 0.067. The variation exceeds 10:1
• Third, the location of the G– node is only established approximately because the distance
between the disaccharide sucrose and the monosaccharide fructose do not clearly establish the
correct position of the glycophore node.
.• Fourth, the location of the C–node cannot be adequately defined because of the
anachronistic comingling of the citric acid and aspartic acid acidophores with the natrophores,
probably due to the above variation.
244 Neurons & the Nervous System
• Fifth, some of their average counts in figure 10 are negative, suggesting some stage 2 signal
processing prior to the creation of the action potential pulses they measured at the NG.
• Sixth, their dimension 1 axis is nominally parallel to a G–P diagonal of the proposed new 3D
space based on the fundamental dimension. Their dimension 2 is nominally parallel to the G–N
axis.
• Seventh, citric acid and aspartic acid are grossly misrepresented with respect to the new
coordinate system. The one HCl data point is extraneous in this representation.
• Eighth, the P-node is not well represented in this figure because of the presence of only two Ppath picrophores in the figure and the significant distortion of the figure due to the comingling
of the fourth dimension amplitude information into the other values in the data set.. The third
picrophore, SOA was omitted from this figure for some reason.
• Ninth, the combination of sodium chloride and amiloride should be shown next to NaCl in the
figure but with its amplitude contribution, in the fourth dimension, greatly reduced.
All of the artificial sweeteners, except aspartame, are properly located along the C–G axis
associated with the dimension 3. The position of aspartane is appropriate if the stimulant contains
a natrophore as well as its recognized glycophore and acidophore.
They did introduce a series of stimulants known to be perceived as sweet as other artificial
sweeteners by humans with only numbers for labels. The stimulants were all quanidine derivatives
incorporating a phenol ring. Although outside the scope of this section, these can be identified
as glycophores exhibiting a d-value of 2.71 Angstrom but absent a conventional glycol group.
They form glycophores because of the d-value between the phenol ring and the nitrogen to
which it is attached. Each of the quanidines also contained an acidophore. For unexplained
reasons, these guanidines were reported in some of their experiments but not others. Their
molarity was 10,000:1 lower than their reference sucrose, indicating they are acting as supersweeteners. Their SC-45647 at 0.04mM was statistically sweeter than the other two numbered
stimulants at 0.03mM.
While the code number SC-45647 is respected in the literature, The code numbers
beginning with NC– are not unique in the literature and represent different chemicals in
different catalogs within the chemical literature.
The Hellekant et al. paper includes many more pieces of information but they must all be
reinterpreted within the hypothesis of gustation developed here before reiterating any
conclusions drawn from them. They also introduced the fact that they have been publishing a
set of similar papers related to the chimpanzee. These also require reinterpretation based on the
current very successful hypothesis.
As an example their fig. 2 shows the variation in their “bar code” representations of the action
potentials for different chemicals. These variations are due to the output of the stage 1 sensory
neurons exhibiting considerable variation during this interval as shown by Dailova in the next
section (her figs 1, 2, 7 & 8). The variation in pulse rate indicates clearly that counting the pulses
during a five second interval and computing a mean rate is at best an exploratory technique.
Interestingly, they close with the description of ethanol as an “enigmatic taste” wherein here the
alcohols are recognized as tasteless with regard to gustation but possibly irritants in higher
concentrations affecting the nocicepters.
8.5.8.4 The investigations of Hellekant et al. of using chimpanzees
During an extended period beginning in 1984, the Hellekant team explored the gustatory
modality of the chimpanzee on the assumption that this species was the closest to humans.
Some of their results have been cited under appropriate sections above. They generally
addressed the effect of a set of common examples of each type of gustatory stimulant in
Signal Generation & Processing 8- 245
separate papers303,304. In the 1996 paper, they compared the responses at their “proper” chorda
tympani among a variety of species to stimulants frequently used in human taste perception
experiments305. In a 1997 paper they addressed the subject of the efficacy of ethanol to elicit a
They quote Hallenberg (1914), the weakest
gustatory response in various species306.
concentrations give neither a sweet, sour, salty nor bitter sensation). Nevertheless, the opinion of
Wilson et al. (1973), that low concentrations taste
mainly sweet, seems to be the prevailing one. The closing sentence of their opening paragraph
notes, “There is, however, less agreement on the taste of higher concentrations, especially if one
disregards a burning sensation as a taste quality. In their discussion, they note, “However, as will
be described in a following study, ethanol is a powerful stimulus of many fibers in the lingual
proper nerve. Because this nerve lacks taste fibers, its activity must give rise to a different
sensation. This may underlie and cause the tactile sensation of astringency often confused with
bitterness according to Amerine and Roessler.”
They concluded;
“On the Question if Ethanol Has a Taste. It is evident that pure ethanol has a taste to the
rhesus monkey. This conclusion rests on the fact that ethanol elicited a response in taste
fibers conducting taste. The question then is does ethanol taste to humans? It may seem
redundant to ask this question here, but many sources suggest that ethanol itself has no
taste [e.g., (26)]. Recordings from the CT of chimpanzees yielded responses to stimulation
with ethanol that were similar to those described here (unpublished data). Thus, ethanol
elicits a taste not only in old-world monkeys, including the rhesus monkey, but also in a
species that from a human standpoint is evolutionary closer. Further, the human CT
recording published by Diamant et al. (5,6) shows a taste response to ethanol.
Consequently, ethanol tastes to humans. It seems that the idea that ethanol in itself has
no taste to humans should be put to rest.”
Their set of experiments required multi-molar concentrations of the alcohols to elicit significant
responses within the chorda tympani. They did not demonstrate a unique fiber that responded
to alcohol. In their MDS analyses using a small set of stimulants that provided a Kruskal stress level
below 0.003, ethanol at very high molarities was grouped with the sugars. They also performed
an MDS analysis on a set of mixtures of conventional stimulants and alcohol. It was accompanied
by a discussion of the role of alcohol in wines. These results suggest the alcohol may be acting
as an agonist to these other stimulants instead of a gustaphore. The rest of their paper fails to
provide any description of the mechanism of taste related to gustation and this work will continue
to consider the aliphatic alcohols as nociceptive channels rather than the gustatory channels.
Their conclusion that alcohol is a stimulant of the gustatory modality remains questionable in the
context of this work.
They continued their study of the chimpanzee in 1997 in a different journal307. That study did not
address the role of alcohol. It did however present a series of MDS analyses for the less
controversial stimulants as well as further discussions of the role of amiloride and the potential for
303
Hellekant, G. & Ninomiya, Y. (1994) Bitter Taste in Single Chorda Tympani Taste Fibers From Chimpanzee
Physiol Behav vol 56(6), pp 1185-1188
304
Hellekant, G. Ninomiya, Y. Dubois, G. Danilova, V. & Roberts, T. (1996) Taste in Chimpanzee: I. The
Summated Response to Sweeteners and the Effect of Gymnemic Acid Physiol Behav vol 60(2), 469-479
305
Hellekant, G. & Danilova, V. (1996) Species differences toward sweeteners Food Chem vol 56(3), pp.
323-328
306
Hellekant, G. Danilova, V. Roberts, T. & Ninomiya, Y. (1997) The Taste of Ethanol in a Primate Model:
I. Chorda Tympani Nerve Response in Macaca mulatta. Alcohol vol 14(5), pp 473-484
307
Hellekant, G. Ninomiya, Y. & Danilova, V. (1997) Taste in Chimpanzees II: Single Chorda Tympani Fibers
Physiol Behav vol 61(6), pp. 829–841
246 Neurons & the Nervous System
a distinct umami perception. Their study did not identify a unique umami pathway. They did
identify several sub-clusters under the N-path cluster in their dendrograms. Their third paper on
the chimpanzee focused on the gymnemic acids and miraculin308. The paper also provided more
support for the labeled-line interpretation of the gustatory modality. They did not resolve their
issue completely,
“In conclusion, the result of the discussion on labeled-line or across-fiber pattern coding
depends on what level of taste these two coding mechanisms are considered; if a change
of activity in all taste fibers is a prerequisite for the perception of each taste quality, then
the results here clearly do not favor the across-fiber pattern for coding of taste in
chimpanzees; on the other hand, the labeled-line theory does not explain the ability to
distinguish compounds within the same taste quality and with the same temporal pattern.”
The third paper included a broader set of conclusions attempting to summarize the full set of
papers related to the chimpanzee
Their conclusion was the perceptions of taste in chimpanzee were indeed very similar to that of
humans. However, many of their occasional putative discussions of the theoretical operation of
the gustatory modality are not supported here.
In 2003, the team presented another paper constituting an overview of their work of recent years
focused on the evolution of the primates309. They presented a set of cladograms and
dendrograms comparing several species among the primates. The diagrams did not exhibit any
umami pathway. The presentation is interesting but involves considerable speculation about the
motivation behind variances (if any) in the gustatory modalities of primates.
8.5.8.5 Reinterpretation of the 2002 paper of Danilova et al.
Danilova, working with the Hellekant team has provided a data filled paper on the Marmoset,
a small New World monkey. However, lacking a theoretical model, the discussion involves many
assumptions that are difficult to defend.
• The transient data of figure xxx shows the difficulty of using an average number of action
potential pulses within a 5 sec. interval as a criteria. The actual pulse rate varies widely over a
range of at least xxx to one within this interval. The recovered analog waveforms associated with
stage 1 transduction exhibit significantly different rise and fall times that are also impacted by the
solubility and transport velocity within the saliva of the individual stimulants.
• The data of figure xxx shows the significantly different intensity of the signals recorded at both
the chorda tympani and the glossopharyngeal nerve. Such a variation is not compatible with
the use of MDS techniques limited to 3 dimensions (a situation that requires the intensity of the
molarities of the stimulants (actually gustaphores) to be adjusted to generate similar intensity
levels within the neural system. Otherwise, a four dimensional MDS is required to account for the
variables present.
• While presenting a 3D MDS representation, the stimulant set used was grossly deficient in
natrophores and acidophores of appropriate molarity. The result is a data set that falls primarily
along the glycophore-picrophore axis with virtually no representation at the acidophore node
or the natrophore node.
• The resulting 1D representation is distorted by the presence of the unrecognized variable
related to the intensity variation among the signaling channels. In this case, it can be argued that
the second and third dimensions are more related to the intensity parameter than to the P–N
dimension and the C–G dimensions of the folded fundamental dimension.
308
Hellekant, G. Ninomiya, Y. & Danilova, V. (1998) Taste in Chimpanzees. III: Labeled-line Coding in Sweet
Taste Physiol Behav vol 65(2), pp 191–200
309
Hladik, C-M. Pasquet, P. Danilova, V. & Hellekant, G. (2003) The evolution of taste perception:
psychophysics and taste nerves tell the same story in human and non-human primates C. R. Palevol vol 2, pp
281–287
Signal Generation & Processing 8- 247
The team has provided an interesting 3D representation of the perceptions of the marmoset, a
new world monkey. Their 3D representation is so compressed in dimension 3 as to suggest the
data set was prepared as a 2D representation and was then plotted in a 3D coordinate system.
However, this could be a scaling problem since no absolute scales were introduced. The
stimulant set used is nearly devoid of simple acidophores and only included one hydrated sodium
ion natrophore, NaCl. The result was a nearly 2D presentation exhibiting a high Kruskal stress
index of 0.14, nearly three times the average value for other Hellekant team representations.
Figure 8.5.8-4 reproduces her figure with an attempt to overlay the coordinate system with the
folded fundamental dimension of this work.
Figure 8.5.8-4 A presentation based on a 3D MDS analysis with a limited gustaphore set shown in
perspective rather than orthogonally. The red line (dog leg) can be considered to be in the
G–N–P plane of the preferred gustatory perception space. Note the nocent, HCl in the gustatory
MDS space. Replacing it with a Lewis acid would identify the node of the A(cidic)-Path more
accurately. It is suggested that many of the gustants were not of equal stimulant intensity and
a 4-dimensional MDS analysis may give a more interpretable perception space for this stimulant
set. See text. Modified from Danilova et al., 2002.
They did not provide a significant analysis of the representation of their MDS program results.
Many of the artificial sweeteners involved large and very complex molecular structures that are
amenable to generating multiple picrophores as well as one or more of the wanted glycophores.
They noted that many of the artificial sweeteners were indeed perceived as bitter by humans.
248 Neurons & the Nervous System
The Danilova team introduced a chemical, sucrose octa-acetate(SOA) into their experiments.
The name is archaic and misleading. SOA is in fact a picrophore formed by replacing all of the
hydroxyl groups (6) of sucrose with acetate groups (8) in a very complex geometry. While it has
been described as a sugar acetate in earlier literature, it does not exhibit any chemical structure
associated with a perception of sweetness (whether a glycol group or the quanidine derivative
discussed above) and it is not perceived as sweet. Its perception is that of a bitter product due
to the proliferation of hydroxyl groups on its periphery exhibiting d-values on the order of 4.74
Angstrom.
[xxx reword this paragraph ]
Her 3D data table shows a predominance of G–path and P– path stimulants to the virtual
exclusion of N–path and C–path stimulants. Thus, the data set is more realistically a one
dimensional data set along the G–P dimension with the data points projected onto a diagonal
surface and then rotated into the dimension 1–dimension 2 plane to the essential elimination of
data in dimension 3.
The team made a throw-away reference to the H-best receptors in their study. However, the
response of their inorganic acids was generally below threshold and not relevant. Of the 49
chorda tympani neurons interrogated by the Danilova team, not one showed significant
sensitivity to HCl. The same can be said for the 41 glossopharyngeal neurons. As a result, that
team used the terminology citrus acid-best rather than H-best in their discussion and figure
captions. The only gustaphore of citrus acid is of course, its carboxylic acid group that leads to
the designation of the A(cidic)–path in this work.
8.5.8.6 Reinterpretation of the Giza & Scott 1991 paper
Giza & Scott have extended the multidimensional gustatory presentation to account for
differences in protocol for the same materials310. Figure 8.5.8-5 shows the effect of introducing
amiloride into the protocol as a pre-wash before repeating the same stimulant protocol.
Amiloride is a nitrogen and amine heavy cyclic hydrocarbon containing no alkali metal311. In its
pharmaceutical form, it is combined with HCl and water, C6H8ClN7O•HCl•2H2O, to form a
potassium-conserving antikaliuretic-diuretic agent, Whether its activity in gustation is also distinct
with respect to potassium is unclear.
This figure was not dimensioned. The “dimensions” shown have been added for discussion. They
approximate those in the above figure with dimension 1 being a Quinine-sugar axis and
dimension 2 a sodium-sugar axis. The third dimension has been labeled the quinine-polycose axis
for discussion. “Acid,” particularly organic acid, stimuli appear under represented in the figure.
It is also clear that the correct MDS analysis would have employed four dimensions in order to
account for the differences in intensity of the N-Path signals caused by the difference in
concentration of the four NaCl solution stimuli.
Giza & Scott did not dwell on their term polycose except to indicate it was perceived as
a unique taste they described only as starchy. This term is nebulous at best. It could
describe a perception attributed more to an astringent, a nocent rather than a gustant.
The popular literature describes starch as a polysaccharide, a long chain of glucose
molecules linked at the α(1¸4) and branched at the α(1¸6) positions, and known as
amylose when helically coiled.
Amylose is not truly soluble in water but forms hydrated micelles It is considered largely
tasteless but known to begin to taste sweet in the mouth due to the breakdown of the
amylose into glucose and maltose due to enzymatic action attacking amylase. Prior to
such breakdown, it may be perceived primarily by its texture via the somatosensory
modality.
310
Giza, B. & Scott, T. (1991) The effect of amiloride on taste-evoked activity in the nucleus tractus
solitarius of the rat Brain Res vol 550, pp 247-256
311
http://www.drugbank.ca/drugs/DB00594/structure?dim=3d A Jmol representation of amiloride
Signal Generation & Processing 8- 249
The change of their polycose to glucose as a function of time and amylase concentration
makes their results highly dependent on the timing of their protocols.
Smith & Scott have reviewed this figure in Doty (page 751). They assert, “taste offers no known
analogy to the pathology of color blindness.” Actually, it appears to offer a significant analogy
to conventional color vision when interpreted as in this work (See Section 8.5.2.4.2). Color vision
can be explained fully using a three-dimensional color space. More approximately, color vision
can be explained using an incomplete two-dimensional color space known as the Munsell Color
Space. This is similar to the situation in the previous figure. One Smith & Scott comment may need
modification. They note, “Blocking the NaCl response of the N-best neurons with amiloride
resulted in the inability of the remaining cells to discriminate between sodium and non-sodium
salts, . . .” Giza & Smith were more specific, “Responses to all 7 stimuli that contained Na+ or Li+
were suppressed by amiloride.” Even this statement in their summary appears subject to
modification as monosodium glutamate was unaffected by the amiloride treatment. In the
original paper, Giza & Scott noted, “The most striking findings of this study of NTS taste cells are (1)
the effects of amiloride on responsiveness to NaCl and LIC1 are either profound or minimal,
depending on the cell, . . . .” A larger statistical sample is needed to resolve the effect on the
potassium (another alkali metal) salt data point.
It is important to note the work of Ossebaard & Smith regarding amiloride in humans312. They
suggest several subtleties indicating a lower effectiveness in humans than in some other species.
312
Ossebaard, C. & Smith, D. (1995) Effect of amiloride on the taste of NaCl, Na-gluconate and KCl in humans:
implications for Na+ receptor mechanisms Chem Senses vol 20, pp 37-46
250 Neurons & the Nervous System
Figure 8.5.8-5 Changes in three-dimensional taste spaces of one rat caused by amiloride as
recorded at the NST. 15 stimuli were used in a before (A) and after (B) experiment where the
tongue and palate were washed with amiloride. After amiloride application, all the Na+-bearing
stimuli, except MSG, and the lithium salt migrate to the positions of the one organic acid, the nonsodium salts, and quinine. N1-N4, 0.01-0.3 M NaCl; Li, LiCl;.S; sucrose; Sa, Sodium saccharin; G,
glucose; H, HCl; Ci, citric acid; Q, quinine hydrocholoride; K, KCl; Ca, CaCl2; M, MSG; P, polycose.
From Giza & Scott, 1991.
----
Signal Generation & Processing 8- 251
Giza & Scott presented a 3D representation of the effect of amiloride on a variety of stimulants313.
The before and after 3D plots did not label their axes, nor did they discuss the differences
between the scales supporting the axes. Their block diagram of the gustatory modality generally
follows an expanded version proposed in this work. Their discussion of entropy as applied to
gustation deserves additional analysis. To be useful, the definition of entropy should be explored
as it is found at the output of the saliency map for gustation.
Dimensions 1, 2 & 3 were labeled arbitrarily in this work to aid discussion. Based on the hypothesis
of this work, the stimulant set included a variety of non gustatory (primarily alkaline earth salts).
Because of the limited range of the data set (no acidophores and only one picrophore) it is
difficult to determine the character of the axes. It is reasonable to conclude the automatically
attached scales of the before and after views are different due to the movement of the points
within the data set that the MDS program manipulated. It can also be deduced;
• G remained unchanged between the two views and S and SA only moved marginally.
• Q moved significantly, suggesting the large movement of the N’s caused a re-scaling in
dimension 1.
• The large movement of all of the N’s in dimension 1 suggests dimension 1 is more concerned
with the intensity dimension than the spatial dimensions associated with the d-value dimensions.
• The components of each stimulant in dimension 3 remained largely unchanged for all of the
stimulants suggesting dimension 3 was not associated with the perceived intensity of the
stimulants.
• The components of each stimulant in dimension 2 remained largely at the same locations
except for the N’s.
It can be concluded that;
• At least the dimension 1 scales are different between the before and after plots
• The few gustaphores employed do not allow for the delineation of the the complete perceived
gustatory space
• The consistency of the positions of the sugars and quinine suggest dimensions 2 and 3 are
related to the G–P axis of gustation.
• The large component changes associated with dimension 1 suggest it represents or is highly
influenced by the intensity dimension of perception. This finding suggests amiloride is blocking the
selective DACB of the N–stimulants with the N-path GR’s.
• Amiloride had negligible impact on the other stimulants of the gustatory and nociceptive
modalities of the neural system.
8.5.8.7 Reinterpretation of the review by Spector & Travers of 2005
Spector & Travers lamented the lack of a fundamental dimension in gustation in 2005. As a result
of this work, their review can be considered largely archaic. Their observation that many
experimental protocols lacked sufficient samples to be statistically relevant is confirmed by the
variability observed in most of the reports of laboratory investigations reviewed above. Without
providing error bars to quantify the utility of their data, the conclusions drawn by most
investigators must be considered only exploratory in character. They do note their entropy varied
substantially based on the stimulants employed in their experiments.
8.5.8.8 Reinterpretation of the nociceptor data from Kashiwagura et al. 1980
Placed in the context of this work, the investigations of Kamo et al. addressed in Section 8.5.1
applied to the chemoreception system as stated, while the investigations of Kashiwagura can be
clearly related to the nociceptor modality of the neural system. Kashiwagura et al314. show the
313
Giza, B. & Scott, T. (1991) The effect of amiloride on taste-evoked activity in the nucleus tractus
solitarius of the rat Brain Res vol 550, pp 247-256
314
Kashiwagura, T. Kamo, N. Kurihara, K. & Kobatake, Y. (1980) Interpretation by theoretical model of
dynamic and steady components in frog gustatory response Am J Physiol (gastrointest.) Vol. 238, pp G445
252 Neurons & the Nervous System
measured responses at the glossopharyngeal nerve of the bullfrog, Rana catesbeiana, for
comparison with the mathematical analysis of Kamo et al. They varied both the flow rates of
the stimulants and the temperature of the experiment. The experiments were carried out in-vivo
at 18°C except when exploring the affect of temperature between 2.5°C and 20 oC, They noted
the dynamic portions of their waveforms were sensitive to flow rate but the steady state values
generally were not.
[xxx condense next two paragraphs ]
Kashiwagura et al. arbitrarily separated the pedestal (steady state) portion of their responses from
the transient (or dynamic) portion without providing any physiological mechanism justifying such
treatment. In fact, the pedestal portion represented the response of their test configuration under
steady state conditions (after all transients had died away). Generally, their stimulus was
removed far before this condition was realized. As a result, their pedestal height was generally
taken as equal to the height of the response at termination of stimulation. Their page G447 states,
“The theoretical definition of the steady state response is the response at t = 4, but it is not
available experimentally. As an experimental approximation, the magnitude of the response at
4.5 min after the onset of stimulation is taken as the magnitude of the steady component.” This
assumption assumes the stimulation is a step instead of a finite duration pulse as shown for all of
their waveforms. In many cases their responses are not nearly horizontal after 4.5 min.
While not clear, Kashiwagura et al. appear to use the expression dynamic component to
refer to the overall waveform less a constant value present throughout the duration of the
stimulus. They associate the fast component and the attack time constant of their
discussion. The slow component is associated with a decay time constant. However, a
more meaningful decay time constant for their waveforms is obtained by evaluating the
decay transient after cessation of stimulation. This decay characteristic repeatedly
suggests a true decay time constant of less than one second. If correct, the majority of
their slow component is due to adaptation within the first amplifier of the sensory neuron.
Their focus on the application of alkali earth salts, acting generally as astringents, is a clear
indication that their stage 3 signals from the glossopharyngeal nerve originated with the
nociceptor neurons and not from the gustatory sensory neurons that are insensitive to the alkali
earth salts.
---Gustatory experiments are traditionally carried out under conditions convenient to the
investigator. Stimuli are applied by pouring or brushing the material onto areas of the tongue that
are only coarsely delineated. Typical time intervals are measured in tens of seconds with initial
application frequently reported using voice records from the investigator. The records are
generally action potential streams taken at the stage 3 neurons orthodromic to the stage 1
sensory neurons, amplified and integrated. The integrated records faithfully represent the original
analog waveforms of the sensory neurons prior to encoding by the stage 3 neurons. However,
the rapidly rising attack portion of the waveform is frequently poorly reproduced. This makes
determination of the precise shape of the leading edge of the responses difficult to determine.
---Figure 8.5.8-6(top) provides a block diagram supportive of the exploratory investigations of
Kashiwagura et al. In the context of the Electrolytic Theory of the Neuron, Kashiwagura et al.
explored the operation of the non-neural stage 0 of the nociceptor modality and both stage 1A,
transduction and stage 1B, adaptation of stage 1, sensory signal development. They did not
address any potential signal processing within stage 2. They did record their signals via a probing
of the GP nerve at the point indicated following pulse signal encoding (action potential
generation) in stage 3A. Unfortunately, the data is highly conflated. However, using the model
of this work, the material can be teased apart with some success. The following discussion will
examine portions of such an operation.
Signal Generation & Processing 8- 253
Figure 8.5.8-6 Integral of stage 3 action potential recovered from the glossopharyngeal nerve of
the bullfrog, Rana catesbeiana. Top; block diagram supporting the Kashiwagura et al.
experiments. Bottom; waveforms from their paper with additional annotation. 1–2; pure delay
interval. 2–3; attack interval. 3–4; adaptation interval. 4–5; actual decay interval after cessation
of stimulation. See text. Lower frame modified from Kashiwagura, 1980.
Kashiwagura et al. introduced several equations based on their interpretation of the above block
diagram as consisting of a single black box. This led to some confusion in their discussion.
Omitting the details here, they observed on page G451, “As the flow rate becomes larger, the
value of [their time constant becomes smaller]. An analytic solution of [the overall process from
stage 0 to stage 3A], P, however, is not obtainable when the above equation is introduced into
the kinetic equations set up for Equation 1. Use of an analog computer enables simulation for the
time course of the response as shown below.” In fact, a suitable overall (analytical) equation is
easily arrived at based on the block diagram proposed here.
They noted the character of their stimulation by defining the stimulation function of stage 0 as a
square pulse with an exponential leading edge where the time constant of the exponential could
be controlled. They then described a kinetic equation (typical of common chemistry) that did
not consider the trailing portion of their waveforms following cessation of stimulation. They
ascribed the leading edge of their waveform to one portion of the kinetic equation and the noisy
decay portion of their waveforms to a second. The true decay characteristic of their kinetic
equation is actually the decay characteristic after stimulation withdrawal. They provided a set
of recorded waveforms illustrating a wide range of operating conditions (without vertical scale
calibrations).
Their figures have appeared in textbooks in several variants but unfortunately the captions
are usually misleading as to the origin of the major decay in signal amplitude with time. This
major decay is due to adaptation within the sensory neuron and not to the fundamental
excitation/de-excitation mechanism of transduction.
254 Neurons & the Nervous System
Figure 8.5.8-7(bottom) provides a set of temperature related waveforms with waveform D further
annotated. Based on the Electrolytic Theory of the Neuron presented here, these waveforms can
be interpreted in considerably greater detail. They provided no vertical scale in this figure and
it is quite likely that the individual waveforms exhibited considerably different amplitudes. Note
the frames A & B were acquired at the same temperature. As noted by Kashiwagura et al., frame
A shows the time constant related to the flow rate of the stimulant and not the leading edge of
the excitation/de-excitation (E/D) mechanism of transduction. The performance in frame A is
dominated by the time constant of stage 0.
It is proposed here without further justification that the nociceptor transduction process
utilizes the same excitation/de-excitation mechanism as all other sensory modalities of the
sensory neurons (Section 8.5.6 xxx). This mechanism, and the associated adaptation
mechanism employs a temperature parameter that is biologically limited and does not
employ absolute zero Kelvin as its lower limit. The relationship with regard to temperature
is quite complex but analytical.
Their solution, P, for the overall response of the frog to stimulation did not include any
temperature term and cannot be used to describe their measured waveforms where
temperature is a variable.
Frames B through E show that the leading edge of the E/D mechanism is quite sharp, with total
rise times (interval 2–3) of less than a few seconds following an absolute delay (interval 1–2) of a
few seconds or less depending on temperature. The decay time constant of the E/D mechanism
can be determined from the interval 4–5. However the scale is marginal for reading values off
of this figure. The interval is about 12 seconds, suggesting a decay time constant of about 3
seconds or less at 12o C. The adaptation mechanism associated with the first amplifier (Activa)
of the transduction process is quite prominent in interval 3–5. This parameter changes
significantly with temperature and intensity of the stimulus. Since no vertical scale was provided
with these waveforms, the effect of temperature and stimulus intensity cannot be de-convolved
in these images. Based on frames D and E, it appears the adaptation time constant is about 100
seconds at 12–18o C. It may be considerably longer in frames B and C, as expected for an
exothermic vertebrate.
If the waveforms of B through E are in-fact plotted to the same vertical scale, the change in the
overall response with temperature is significant, although little if any change in the rate of rise and
the decay characteristic after stimulation ends are apparent. This situation would suggest a
significant change in the gain of the 1st amplifier and its adaptation time constant (both related
to its chemical source of power) with temperature.
8.5.8.9 The conditioned taste aversion MDS data of Chang & Scott–1984
Chang & Scott have highlighted the effect on MDS results of varying the physiological state of
rats in the collection of data (Section 8.5.8.9). In summary, based on the hypothesis of this work,
their results are very difficult to interpret. First, their conditioning stimulus was not a SGG; it was a
MGG containing at least two gustaphores closely related to many of their other stimulants.
Second, they included the nocent, HCl, in their gustatory stimulant set. Third, they did not attempt
to control the concentration of their gustants to achieve a near constant perceived sensation
level. This mixing of stimulants from different modalities along with a mix of concentrations and
the use of MGGs makes deconflating their data (specifically rotating the axes of their work into
a standardized 3D space) very awkward. They did not describe the three dimensions of their
data space. The data set they attempted to display in a 3D space requires at least 5 dimensions
to properly analyze.
Chang & Scott do not appear to have appreciated the state of the art of MDS analysis at the
time of their work. They attempted to use a technique designed to address qualitative
differences and include quantitative differences in their stimulant set.
----
Signal Generation & Processing 8- 255
Chang & Scott have highlighted the effect on MDS results of varying the physiological state of
rats in the collection of data. Unfortunately, they did not employ a model of the gustatory
modality before developing their protocol. They employed single neuron recording from a
location within the Nucleus Tractus Solitarius. The selected neurons had to be sensitive to stimuli
representing the four signaling channels the investigators were accustomed to, salty, inorganic
acid, sweet and bitter. No organic acids were included in their basic stimulus set, but citric acid
was added in a followup experiment. No other inorganic acids were included except HCl. They
did include two different concentrations of sodium saccharin (0.25 and 0.0025 M), suggesting an
additional dimension related to perceived intensity was present in their database.
In summary, based on the hypothesis of this work, their results are very difficult to interpret. First,
their conditioning stimulus was not a SGG; it was a MGG containing at least two gustaphores
closely related to many of their other stimulants. Second, they included the nocent, HCl, in their
stimulant set. Third, they did not attempt to control the concentration of their gustants to achieve
a near constant perceived sensation level. This mixing of stimulants from different modalities
along with a mix of concentrations and the use of MGGs makes deconflating their data
(specifically rotating the axes of their work into a standardized 3D space) very awkward. They
did not describe the three dimensions of their data space. The data set they attempted to display
in a 3D sapce requires at least 5 dimensions to properly analyze.
Figure 8.5.8-7 shows their results for a group of rats that had been conditioned to be wary of
NaSaccharin. The paper is extensive and data-filled. They note,
(Previous) “Explorations of the neural substrates of conditioned taste aversions (CTAs) have
focused principally on diencephalic and telencephalic structures. The nucleus tractus
solitarius (NTS) is the initial gustatory relay in the rat’s hindbrain. It is worthy of investigation
for its part in mediating CTAs in that it is sensitive to several physiological conditions which
affect feeding while also being a site of anatomical convergence for vagal afferents from
the viscera and centrifugal projections from areas (hypothalamus, amygdala) implicated
in emotions and hedonics.”
“We compared single neuron responses from NTS to several taste stimuli in three groups of
rats: (1) those receiving exposure to 0.0025 M sodium saccharin without physiological
consequences; (2) those made ill through intraperitoneal injections of LiCl but having no
obvious gustatory referent for their malaise (sensitization-pseudoconditioning controls); (3)
those in which exposure to 0.0025 M sodium saccharin (the conditioned stimulus, CS) was
paired with LiCl-induced poisoning (the unconditioned stimulus, US), creating a pronounced
aversion to the saccharin.”
According to response profiles, NTS neurons in all three groups could be divided into subsets
of about 30%, which showed a sweet-sensitive profile, and 70%, which were primarily
sensitive to nonsweet qualities. The major effect of the conditioning procedure was to
increase responsiveness to the saccharin CS only among the sweet-sensitive subset.”
“The axes of the spaces are unlabeled, for the attributes they represent-which should
underlie taste just as frequency underlies pitch and wavelength, color-are not yet
determined.”
“The significance of the results is that: (1) CTAs affect sensory activity at a lower order level
than had heretofore been demonstrated; (2) NTS shows sensitivity to yet another
physiological condition, reinforcing the involvement of the hindbrain in hedonics and
sophisticated taste-related processes; (3) there is a subset of taste neurons, rather distinct
according to its sensitivity profile, which is also functionally unique in its response to
conditioning by a sweet CS.”
256 Neurons & the Nervous System
Figure 8.5.8-7 Three dimensional MDS spaces representing a CTA group and a control group.
NaSaccharine, CS, is the same coordinates in each space. See text. From Chang & Scott, 1984.
Note the presence of HCl and citric acid in their control space. The major problem with the
paper and the above graphic is that they did not rely upon any theory of gustation and they
subsequently did not rotate and rectify their MDS data to represent a three-dimensional
perceived gustatory space. They did adjust their dimensionless representations so that
NaSaccharie occurred at the same coordinates in each frame (2.5 units to the right and 2.0 units
up in the checkerboard of both frames. However, this forced retention of one molecular location
was not justified by any axiom of MDS analysis. The third dimension appears to be about 2,5 units
above the plane of the checkerboard. However, it is not clear this is an important feature of their
analysis. A more important shortcoming is the lack of any definition of the dimensions of each
axis and any calibration of these axes. They described sodium saccharin as “a novel taste
stimulus that served as the conditioned stimulus (CS).” The use of sodium saccharin (NaSac) as
a reference is unfortunate since it is obviously a gustant containing at least two gustaphores.
8.5.9 Extending the hypothesis to include the sweet proteins
Only six proteins are known that are perceived as sweet by humans, and apparently old world
monkeys. The success of the hypothesis presented here suggests it can probably be extended
to account for this situation. The following paragraphs address this extension. While less well
demonstrated by empirical investigations than the basic hypothesis presented above, there is
adequate data to form such an extension.
The extended hypothesis will include the following corrolary;
Signal Generation & Processing 8- 257
• The six known sweet proteins (many of their derivatives and potentially other proteins) can form
DACB’s with the G-path GR’s of this work by modifying specific amino acid residues located
along gradual turns at the periphery of the space occupied by the proteins.
The restriction to the gradual turns, such as the β-turn, eliminates those amino acids found within
τηε α−helix and sheet portions of the protein. The requirement for a modified amino acid is
consistent with our knowledge of a long list of rare amino acids as well as the recognition and
good documentation of one such modified amino acid in brazzein. Brazzein is the smallest known
(6.5 kDa) and one of the most well studied sweet proteins.
The following discussion will limit the modifications to specific amino acids of the sweet proteins
to the first order case; i. e., the introduction of an orbital (either oxygen or nitrogen associated
with one or more hydrogens to support structural rules and/or support the requisite hydrogen
bonding. The potential introduction of more uncommon orbitals, including the reliance on a
phenol ring structure (such as in a modified tyrosine or phenylalanine) will be left to future study.
8.5.9.1 The study of protein mutations in human & primate gustation
[xxx edit text above and below the - - - marks into one text. Currently have two 8.5.9.1s ]
This paragraph will provide a more specific framework leading to clarification of the role of the
proteins as gustatory stimulants versus true gustaphores and the various situations in which proteins
might exhibit a distinct taste.
Currently, proteins exist in three distinct structural forms;
• native or those with a highly distinct (and generally unique) entwined ligand form,
• man-made and either reconstructed or fabricated structures assumed to contain ligands in the
same concatenation and be entwined (folded) identically to the native form and
• denatured or proteins with an alternate (frequently simpler and unfolded) structural
arrangement as a result of heating or specific chemical treatment.
Based on the hypothesis of this work, a protein can be considered a stimulant containing one or
more individual gustaphores or be itself a gustaphore. A criteria for any gustaphore is that it be
able to form a DACB with a GR. In the case of a typically structurally complex protein, it is
necessary that any gustaphore be accessible at the proteins surface in order to form hydrogen
bonds with a GR that is no more than 2.7 Angstrom distant. Thus, the configuration of the protein
is critically important when exploring its gustatory properties.
While textbooks have frequently indicated that the individual amino acids forming a
protein have no intrinsic biological or toxic effects, this statement needs qualification to
separate gustation from other biological processes.
Jin and others have investigated the structure of proteins in the context of their gustatory
properties. They note the relevance of the N – and C–terminals of the protein as well as a variety
of surface ligands that could relate to individual gustaphores. Along the backbone of the
protein, the side-chains, known as R-groups are particularly relevant.
The C–terminal of a protein consists of a carboxylic acid group and is by definition a carboxylic
acid gustaphore or acidophore under the hypothesis of this work if it satisfies the 2.7 Angstrom
stereo-chemical criteria defined above. Frequently, this terminal is not readily available as an
acidophore.
The N –terminal of a protein consists of only an NH2 group and does not constitute a gustaphore
according to this hypothesis.
Many of the amino acids forming a protein exhibit side chains when linked known as R-groups.
Glutamic acid and aspartic acid both contain carboxylic acid in their R-group that can
potentially represent an acidophore along the backbone of the protein. They both exhibit ideal
d-values of 2.7 Angstrom. Glutamine and asparagine have R-groups containing an O=C-NH2
structure that could act as an acidophore. They both exhibit a d-value of 2.22 Angstrom (18.5%
low compared to the ideal 2.7 Angstrom for an acidophore). [xxx edit above values ]
258 Neurons & the Nervous System
Jin et al. have studied 25 derivatives (they describe them as mutations) of a simple wild protein,
brazzein (6.5 kDa)315. They showed this protein was tasteless in its wild form but was perceived as
sweet when an amino acid without an acidophore R-group was replaced by glutamic acid with
its acidophore R-group (Section 8.5.9.1) The Jin et al. study did much to correlate the human
perception of taste with the recordings from G-path neurons in the proper CT neurons (where
care was taken to interrogate the chorda tympani before it merged with the lingual nerve.
xxx316, writing in Cagan [xxx page 91], discusses the artificial sweeteners monellin and thaumatin
(E957 in the sweetener index). He notes thaumatin includes 11 lysine residues. He also notes the
stepwise acetylization of the lysine residues leads to a progressive loss in the sweeteners
capability. Lysine exhibits a d-value of 2.605 Angstrom when incorporated on the outer surface
of a protein and thus, each lysine residue is capable of forming a DACB with the GR 2 sensory
receptor of gustation. See also Section 8.5.9.1. “Acetylization of more than four lysines residues
renders it tasteless.” This assertion suggests the other lysine residues may be located internally in
thaumatin. xxx notes, “The effect of thaumatin stimulation of the tongue has been recorded in
several mammals.” They also assert it only occurs in catarrhine primates. “No neural response has
been obtained in all other prosimian and platyrrhine primates tested.” This assertion follows the
Thaumatin column in Table 1 of Hellekant et al317. in 1991.
Monellin exhibits similar properties to thaumatin. With a few exceptions, its stimulation is also
limited to the cattrhine animals. The exceptons suggest further investigation is needed with
respect to both thaumatin and monellin.
It is generally recognized that solvent molecules do not penetrate the interior of the volume
occupied by the proteins. For purposes of DACB bonding, the proposed gustaphores need to
be located near the exterior surface of the protein volume. [xxx edit? ]
Figure 8.5.9-1 summarizes the characteristics of the known proteins perceived as sweet.
315
Jin, Z. Danilova, V. Assadi-Porter, F. Markley, J. & Hellekant, G. (2003) Monkey Electrophysiological and
Human Psychophysical Responses to Mutants of the Sweet Protein Brazzein: Delineating Brazzein Sweetness
Chem Senses vol 28, pp 491–498
316
Scott, T. & Yaxley, S. ( 1989) Interaction of taste and ingestion In Cagan, R. ed. Neural Mechanisms in
Taste Boca Raton, FL: CRC Press Chap. 7
317
Hellekant, G. Walters, D. Culberson, J. et al. (1991) Electrophysiological evaluation of sweeteners In
Walters, D. Orthoefer, F. & DuBois , G. eds. Sweeteners. Wash. DC: American Chemical Society Chap. 22
Signal Generation & Processing 8- 259
Figure 8.5.9-1 Characteristics of proteins perceived as sweet by humans ADD Thautadin is more
recently known as thaumatin (E957). See text.
----There are currently six proteins known to be perceived as sweet by humans. There are indications
this perception may be limited to humans, apes and old world monkeys318. These proteins
generally satisfy the definition of a super sweet stimulant because of their high degree of
sweetness compares to sucrose, typically in the 500:1 to 100,000:1 range on a molarity basis. The
ratios are typically much lower on a weight/weight basis. These ratios and how they degrade on
molecular substitution suggest the AH,B,X model of the Shellenberger team can be used to
explain the gustatory operation of these proteins. The protein brazzein has been a popular
subject because of its high temperature of denaturing, ~85 C. Quoting Jin et al, who cite
Caldwell et al., “Fruit brazzein consist of two forms, one with pGlu at its N-terminus and one
without (des pGlu) which is twice as sweet as the first form. The latter form is nominally 500 times
sweeter than a 10% sucrose solution and up to 2000 times sweeter that a 2% sucrose solution on
a weight basis.” Assadi-Porter et al give a different set of numbers without attribution, “Brazzein,
as isolated from the fruit, is 500 times sweeter than sucrose on a weight basis (9500 times sweeter
on a per molecule basis).
The search for the mechanism by which “sweet proteins” interact with the sweet GR is a very
active area. Most of the work remains conceptual and involving the structural docking of these
proteins with a very complex putative T1R1 receptor derived from the genetic code. No
mechanism of interaction between these elements has been offered.
318
Assadi-Porter, F. Aceti, D. Cheng,H. & Markley, J. (2000a) Efficient Production of Recombinant Brazzein,
a Small, Heat-Stable, Sweet-Tasting Protein of Plant Origin Arch Biochem Biophys vol. 376(2), pp 252-258
260 Neurons & the Nervous System
Some of the concepts used on searching for the fundamental gustatory mechanism involving the
sweet proteins are quite colorful, such as involving a “venus fly trap model” (VFTM). Others have
been seeking to understand the relationship between the sweet proteins and the GR using amino
acid sequencing and structural analysis of the complete protein.
Searching the amino acid sequence of these small proteins, 54 to 202 amino acid residues,
appears to have been most successful. However, this path has yet to uncover the mechanism
involved.
8.5.9.1.1 The folding of proteins
It has long been known that protein folding was dominated by hydrogen bonding between
various amino acid residues frequently separated by multiple intermediate residues. Lehninger
(1972, pg 112) provided an excellent illustration of this condition. The ultimate folding of a given
protein is a direct result of these hydrogen bonds and an indirect result of the genetic code itself
for that protein, since the code defines the sequence of the individual residues and thereby their
initial spatial relationships.
Lattman & Rose made some important observations about protein folding under a provocative
title in a significant early paper319;
“The folding reactions of many small, globular proteins exhibit two-state kinetics, in which
the folded and unfolded states interconvert readily without observable intermediates.
Typically, the free energy difference, AG, between the native and denatured states of such
a protein is quite small, lying in the range of approximately -5 to -15 kcal/mol. We point out
that, under these circumstances, a population of native-like molecules will persist, even in
the presence of mutations sufficiently destabilizing to change the sign of AG. Therefore, it
is not energy per se that determines conformation. A corollary to this argument is that
specificity-not stability-would be the more informative focus in future folding studies.”
They went on; “A protein molecule adopts its unique, three-dimensional equilibrium structure
spontaneously under physiological conditions in many, if not all, cases. This native structure can
be denatured readily by elevated temperatures or chemical perturbations, both of which induce
chain disorder but leave covalent bonds intact. . . It is well established that the information
necessary to drive this reversible disorder + order transition is encrypted solely within the linear
amino acid sequence; hence, the structure of a protein is implicit in the gene that encodes it.”
Later they note, “There is a profound organizing aspect of two-state behavior that is often
overlooked: proteins are either folded in a native-like manner or not folded at all.”
“What is the origin of conformational specificity? One attractive candidate has been internal
packing. Globular proteins are known to have mean packing densities reminiscent of solids, a
consequence of the exquisite complementarity between interior side chains, which fit together
like
pieces of a three-dimensional jigsaw puzzle (27). This experimental fact can be interpreted to
mean that protein conformation is linked tightly to internal packing.”
Finally, “In sum, we propose that the derivation of a reliable strategy to predict structure from
sequence will depend critically upon elucidation of the stereochemical code that underlies
conformational specificity.”
319
Lattman, E. and Rose, G. (1993) Protein folding-what's the question? Proc Nati Acad Sci USA
vol 90, pp 439-441
Signal Generation & Processing 8- 261
Thirteen years later, Rose et al. provided a major article on folding320. They provided a Part 5 as
follows;
Part 5. A Backbone-Based Theory of Folding This perspective has described 10 seemingly
disparate aspects of protein folding. In particular:
1. The native fold is unique. The folding reaction is U º N, not U º N1 N2 . . .
Ni.
2. Folding is reversible.
3. No covalent bonds are made or broken in the folding reaction, U º N. Only weak bonds are
involved.
4. Folding conditions and unfolding conditions are similar, respectively, for most mesophilic
proteins, regardless of sequence.
5. The U º N reaction is highly cooperative. Most single-domain proteins fold in an all-or-none
manner.
6. The fold is built on a scaffold of hydrogen-bonded -helices and -strands.
7. The number of stable domains is limited to a few thousand.
8. Proteins typically avoid metastable kinetic traps under native folding conditions.
9. Protecting denaturing osmolytes fold unfold proteins by operating predominantly on the
backbone in the unfolded state, dialing folding up/down but leaving the fold itself unaltered.
10. Stability and conformation are not synonymous. The native conformation can still be attained
under grossly destabilizing conditions. Such conditions shift the U º N equilibrium toward either N
or U, but not toward N* (i.e., an alternative folded state)..
In 2012, Porter & Rose provided additional perspective on folding with extremely colorful and
complex figures321. Their approach was based on thermodynamic assumptions. Their principle
contribution was a “Domain Identification Algorithm. Domains were identified in solved protein
structures by using our structure-energy equivalence of domains (SEED) algorithm. The minimum
size of a domain was fixed at 25 residues, approximating the size of a unit of supersecondary
structure and the minimum chain length needed to attain a protein-like surface/volume ratio. No
fixed limit was imposed on the maximum size. The algorithm identifies an optimal set of non
overlapping units that maximizes both collective QRs and chain coverage.” They did not address
sweet proteins that were near the shorter sequence end of their algorithm space. Their focus on
long peptide chains is beyond the scope of that needed here.
8.5.9.1 Searching glycophore location based on complex protein theory
de Vos et al. have provided the schematic of amino acid folding for thaumatin 1 at 3.1 Angstrom
resolution322. Thaumatin consists of a single polypeptide chain of 207 amino acids, has a
molecular weight of 22.2 kDa (Iyengar et al., 1979). The amino acid sequence (Iyengar et al.,
1979) and 3-dimensional structure (De Vos et al., 1985) are known. The taste of thaumatin and
that of another sweet protein, xxx Speaking of both thaumatin and monellin they noted, “The
sweet taste of the proteins can be registered at a very low concentration (10' M), comparable
to those in hormone-hormone receptor interaction. These proteins are about 100,000 times
sweeter than sucrose on a molar basis and several thousand times sweeter on a weight basis. In
fact, these two proteins are the two sweetest compounds known to man.” They also note, “five
tripeptides in monellin have their homologous counterparts in thaumatin at residues 94-96,
100-102, 101-103, 118-120, and 128-130. All five regions are well exposed. It is likely that some of
these regions
are responsible for the immunological cross-reactivity between monellin and thaumatin, and their
conformation may be important for sweet receptor binding.” They conclude without any
discussion related to the theory of gustation, “Recently the gene for thaumatin II has been cloned
and expressed (16). Thaumatin II is as intensely sweet as thaumatin I but is different from it at five
320
Rose, G. Fleming, P. Banavar, J. & Maritan, A. (2006) A backbone-based theory of protein folding PNAS
vol 103(45), pp 16623–16633
321
Porter, L. & Rose, G. (2012) A thermodynamic definition of protein domains PNAS vol 109(24), pp
9420–9425
322
De Vos, A. Hatadat, M. Van Der Wel, H. et al. (1985) Three-dimensional structure of thaumatin I, an
intensely sweet protein Natl. Acad. Sci. USA vol 82, pp 1406-1409
262 Neurons & the Nervous System
positions [residues 46, 63, 67, 76, and 113 (4)]. It is almost certain that thaumatin II has the same
three-dimensional structure as thaumatin I described here.”
Tancredi et al. provided a comparative study of three of the sweet proteins in 2004323.
Schematics of each are shown in their figure 1. Their lack of commonality in structure, except for
the various loops, is immediately obvious. As they note in the opening to their abstract, “The
mechanism of interaction of sweet proteins with the [putative] T1R2-T1R3 sweet taste receptor has
not yet been elucidated.” They synthesized a group of peptide sequences representing the
“hairpin structures” thought to be representative of these structures. “However, none of the
peptides has a sweet
taste.” They noted in 2004, “In spite of several attempts, the glucophores of sweet proteins have
not yet been identified with certainty, even for brazzein, monellin and thaumatin, the only sweet
proteins of known structure.” They provide considerable data on their peptides.
Their introduction ends with, “Here we present an extensive search for ‘sweet fingers’ on
the surface of monellin, thaumatin and brazzein, the three sweet proteins best characterized from
a structural point of view. As a result of the search three potential ‘sweet finger’ peptides were
designed, synthesized and studied from a conformational point of view.”
Their conclusions end with, “In summary, we can conclude that the data presented in this paper
do not support the idea of a localized ‘sweet finger’ site that could explain the properties of
sweet proteins according to the rules established for small sweet molecules. Alternative
explanations that could better explain the experimental observations, such as the ‘wedge model’
recently proposed by us [40], should therefore be explored.” In essence, they did not find any
trace of the actual glycophore associated with these three sweet proteins. Only detailed
examination of the 3D representations of their “sequences” could determine if they contained
a glycophore (using the requirements based on the hypothesis presented here).
Temussi discussed his wedge model in 2002 under the heading, hypothesis324. After a discussion,
he suggests a future plan of investigation.
Temussi provided an overview of the search for the gustatory sites of the sweet proteins in 2006,
based primarily on the field of genetics and genetic code expression325. While citing the work of
Shallenberger et al and Kier, he dismissed their work in terms of its applicability to the sweet
proteins. He developed his wedge model introduced above by Tancredi et al.
8.5.9.2 Amino acid sequencing of the sweet proteins
In a 2003 study, the team of Jin - - - Hellekant examined the role of more than 25 derivatives of
the wild protein brazzein (which they described as mutants) in human gustation326. Their
conclusions do not appear surprising within the context of the hypothesis presented here,
“The effects of charge and side chain size were examined at two locations, namely
positions 29 and 36. The findings indicate that charge is important for eliciting sweetness,
323
Tancredi, T. Pastore, A. Salvadori, S. Esposito, V. & Temussi, P. (2004) Interaction of sweet proteins with
their receptor A conformational study of peptides corresponding to loops of brazzein, monellin and thaumatin
Eur. J. Biochem. v 271, pp 2231–2240
324
Temussi, P.A. (2002) Why are sweet proteins sweet? Interaction of brazzein, monellin and thaumatin with
the T1R2–T1R3 receptor. FEBSLett vol 526, pp 1–3
325
Temussi, xxx (2006) xxx
326
Jin, Z. Danilova, V. Assadi-Porter, F. Markley, J. & Hellekant, G. (2003) Monkey Electrophysiological and
Human Psychophysical Responses to Mutants of the Sweet Protein Brazzein: Delineating Brazzein Sweetness
Chem Senses vol 28, pp 491–498
Signal Generation & Processing 8- 263
whereas the length of the side-chain plays a lesser role. We also found that the N- and
C-termini are important for the sweetness of brazzein. The close correlation (r = 0.78)
between the results of the above two methods corroborates our hypothesis that S fibers
convey sweet taste in primates.”
They also draw other conclusions, “Fruit brazzein consists of two forms, one with pGlu at its N
terminus and one without (des-pGlu) which is twice as sweet as the first form. The latter form is 500
times sweeter than a 10% sucrose solution and up to 2000 times sweeter than a 2% sucrose
solution on a weight basis. It has the smallest molecular size among sweet proteins (6.5 kDa) and
high solubility in aqueous solution.” des-brazzein has a weight of 6.353 kDa.
This work would suggest that charge and side chain length play different roles. Side chain length
is critical when attempting to meet the d-value criteria for a glycophore within a stimulant,
whereas, the charge is largely related to the perceived intensity of a given stimulant.
Jin et al. also provided a set of “bar code” recordings from the chorda tympani suggestive of the
rate of response of the taste system following initial application of their stimulants.
Caldwell et al. have indicated there are a total of six known simple proteins (as opposed to
glycoproteins) perceived as sweet. Their figure 3 shows the ball-and-stick versions of brazzein,
thaumatin and monellin obtained using MolScript. The general view is they exhibit no common
structural features other than individual curved strands forming the external envelope of each
molecule. Brazzein is much the simpler of the three.
Brazzein and the other five proteins qualify as a super-sweetener and an effort should be made
to determine how it satisfies the proposed AH,B,X criteria for such sweeteners.
Figure 8.5.9-2 reproduces their protein and indicates locations of principle attention.
264 Neurons & the Nervous System
Figure 8.5.9-2 Diagram showing the position of mutations on the brazzein molecule and the
corresponding changes in taste (red, increased sweetness, black, the same, light blue, decreased
sweetness in comaprison with WT brazzein, dark blue, scored as water). Intramolecular disulfide
bonds are shown as yellow lines. Three potential glycophores are shown by black dots at
locations 14, 34 and 35. See text. From Jin et al., 2003.
Assadi-Porter et al. (2000, pg 259) has provided a figure equivalent to that of Jin et al. that is
slightly easier to interpret. Their paper also focused on two different locations, pGlu1 and His31.
Caldwell et al. provided an additional schematic of brazzein with some additional specifics. They
asserted brazzein contains
• a single α-helix (residues 21-29)
• a three-stranded anti-parallel β-sheet (strand I, residues 5-7)
• strand II (residues 44-50
• strand III (residues 34-39)
• a poorly defined stretch of random coil between (residues 9-20)
• a β-turn between strands II and III (residues 40-43)
• a poorly defined N-terminus (residues 1-4)
• a poorly defined C-terminus (residues 52-53.
These descriptions were based on the overlay of a series of conformers showing the configuration
of the molecule at individual times. They also provided additional detail regarding the disulfide
bonds of the molecule. Their analyses were based primarily on examining conventional structural
Signal Generation & Processing 8- 265
details. They did note a disulfide bridge between the N-terminus and the C-terminus. Their
analysis also focuses attention on Arg43 and on His31 as important to sweetness. Their figure 2b
shows some additional structural details concerning the residues along the brazzein chain.
A comparison of the findings of Jin et al, Caldwell et al. and Assadi-Porter et al. will be made in
Section 8.5.9.6.
Brazzein has been found to exist in two very similar forms. The first is 54 residues long and has a
“non-amino acid” group at its N-terminus called pGlu or pyroglutamic acid. The alternate variant
is 53 residues long and is without this unique group (des-pGlu1). The numbering in both cases
begins with the pGlu group even if absent. Assadi-Porter et al. (2000, pg 252) asserts the des- form
is perceived as 2:1 sweeter than the pGlu form by humans. Figure 8.5.9-3 reproduces the amino
acid sequence for brazzein from Caldwell et al.327. The figure includes three arrows pointing to
residues of particular focus. These may also be modified and therefore “non amino acids.” The
focus is partly due to the location of the residues #38-#45 near the exterior envelope of the
molecule and on a curve that may contribute to a larger than typical d-value between some
of the orbitals in the side chains of the residues.
Pyroglutamic acid (pGlu, pE and a.k.a. pidolic acid) forming the N-terminal of the brazzein amino
acid sequence has lost its carboxylic group during peptide bond formation, thereby losing its
potential to act as an acidophore within brazzein. However Glu41 and Asp29 retain the ability
to act as acidophores if adequately exposed to the GR’s for the G-path.
Figure 8.5.9-3 The amino acid sequence of brazzein with three positions marked. The potential
for a modified serine residue at location 14 or 34 and a modified glycine at location 35 is worthy
of further study. See text. From Caldwell et al., 1998.
---[xxx figure and text somewhat duplicates figure and text later re b-turn ]
A fully functional 3D representation of brazzein is available from the PDB bank of Europe. That
bank supports a viewer called Astex. Like other 3d viewers encountered in 2013, they are not fully
implemented. Astex 3.0 is provided on an “as-is” basis under a GNU license with a latest
copyright date of 2007 and at best a sketchy manual (click on user commands on left navigation
panel). Figure 8.5.9-4 shows the complexity of brazzein in the vicinity of the β-turn using this
viewer. Astex operates, and shares many commands, with Jmol and more specifically Discovery
Studio vers. 3.5. Measuring the d-values of the various orbital pairs available in this area leads to
the conclusion that the sharp curvature in this turn and the general location of Glu41 and Arg43
on the outside of the curve leads to spacings higher than those that can support a glycophore
in this region. The curvature and location of the residues in the range of residues 30 to 33 may
offer higher probability of locating the glycophore(s).
327
Caldwell, J. Abildgaard, F. Dzakula, Z. et al. (1998) Solution structure of the thermostable sweet-tasting
protein brazzein Nat Struct Biol vol 5(6), pp 427-431
266 Neurons & the Nervous System
Figure 8.5.9-4 A partial 3D ball and stick representation of the β-turn in brazzein EDIT. The area to
the left of the broad yellow sheet is the β-turn of interest. Residue lys42 is at lower left, residue
Glu41 is center left and residue Arg43 is at upper left. The interesting possibility that one of the
carboxylic oxygen atoms of Glu41 and the oxygen associated with the peptide bond leading to
Lys42 might form an AH,B group with a d-value near 2.8 Angstrom and the location of the
guanidinium group of Arg43 might form the dispersion centroid, X, needs additional examination.
---Assadi-Porter et al. (2000) designed a series of 15 synthetic genes that when placed in e-coli
could lead to expression of a series of brazzein derivatives328. The experiments were quite
successful. All but one of their derivatives appears to have folded properly into their native state
and were perceived as super sweet by humans. Whether their synthetic genes were able to
encode processes necessary to create non-amino-acid inserts into the amino acid chain is not
clear. They did note the N-terminal and C-terminal of their derivatives were physically quite close
together. They focused on residues near Arg43 on a portion of a flexible loop as particularly
pertinent to sweetness. When unfolded, all of their derivatives became tasteless. They also noted
that recreating their derivatives using only D-amino acids led to tasteless products. While possibly
not adequately worded, they assert that Arg43 is necessary for sweetness in brazzein.
[xxx some duplication of words re glycine and serine ]
Glycophores discussed in this work so far have all been found quite widely and involve well
documented chemistry. For one of the above six proteins perceived as sweet to satisfy the
requirements of containing at least one glycophore, they need to exhibit at least one structure
easily accessible from outside of the protein periphery. This glycophore would consist of an HO-CC-O structure with a d-value between the two oxygen atoms of nominally 2.82 Angstrom with a
potential range of ±6% or more. This requirement suggests a trans- arrangement along a curved
portion of the protein similar structurally to a simple phenol ring. Such a structure could be
obtained from a serine absent its normal methylene group (CH2) or a hydroxylated glycine
incorporated into a polypeptide chain. While formation of such an amino acid based on the
genetic code is unlikely (no triplet code is recognized for them, they could be formed after
insertion of the parent amino acid into the polypeptide chain (Lehninger, 1970, pp 72). Thus the
glycine residue at locations 35 could be oxygenated (or involve a hydroxyl substitution for the
terminal hydrogen) or the methylene group could be removed from the serine residue at either
location 14 or 34. It is not obvious the current methods of protein decomposition and
328
Assadi-Porter, xxx (2000) pp 259-xxx
Signal Generation & Processing 8- 267
identification using the conventional amino acid analyzer329 would recognize such a “rare amino
acid.” However, Caldwell et al. and others have recognized pGlu in a similar situation as
discussed above.
---[xxx change to reflect above possibilities for modified amino acids ]
A new characterization of some of the observations of Jin et al. regarding brazzein are of interest.
“Two features are noteworthy. One is that addition of an N-terminal Ala or mutation of the side
chain from Asp to Asn did not change the sweetness. The other is that mutations of residues close
to one another in the protein sequence led to remarkable differences in sweetness. For example,
mutation at position 29 (changing Asp29 to Ala, Lys or Asn) made the molecule much sweeter
than WT (the wild type of Brazzein), while mutations at positions 30 or 33 (Lys30Asp or Arg33Ala)
removed all sweetness. The same pattern occurred again at the $-turn region, where Glu41Lys
gave the highest sweetness score among the mutants tested, whereas a mutation two residues
distant (Arg43Ala) abolished the sweetness.” Their studies showed that it was not the proteins
themselves that were gustaphores but that the amino acid residues at specific locations were
gustaphores. “Specifically, our findings suggest that residues 29–33 and residues 39–43, plus
residue 36 connecting these stretches, as well as the N- and C-termini determine the sweetness
of brazzein. As a consequence of these studies, brazzein variants identified to have enhanced
sweet qualities could become candidates for a new generation of low-caloric natural
sweeteners.” Such residues all contain the glycophore found in any amino acid. They would be
expected to be functional gustaphores based on the hypothesis presented here if they were
physically exposed sufficiently to form a DACB with the G-path GR’s.
They noted the importance of changing the net charge on the protein by substitution of a
residue.
The role of charge in their discussion of the charge on the molecule differs significantly from the
dipole potential of the molecule to be discussed in a following paper. That paper shows that the
dipole potential of a molecule when bound to a GR is measured by the sensory neuron amplifier
following the transduction mechanism at the surface of the GR.
---[xxx edit paragraph and indent ]
Their focus on locations 29 and 41 is interesting. Both of these locations were initially suitable as
acidophore sites according to this hypothesis. The glutamic acid residue at location 41 appears
to be the primary glycophore for brazzein at this time with changes at location 29 significantly
impacting the total charge on the molecule when in the DACB relationship with the G-path GR..
Note, when discussing the effect of changing the residue at location 29, the protein is still
acting as a glycophore during such experiments. This is a strong indication that the
effective glycophore of brazzein is the glutamic acid residue at location 41. This residue
appears to be on the exposed surface of the molecule near the β-turn nominally at
location 43.
---In 2003, Assadi-Porter et al. reported on a study of the hydrogen bonds within the folded forms
of brazzein in conjunction with their sequence-function investigations330. The study centered on
the hydorgen bonds interconnecting residues within brazzein. They examined five single point
mutants using isotope labeled atoms by high resolution NMR. This is basically a shotgun approach
as no theory was offered as to how such hydrogen bond changes should affect the sweetness
of these variants of their recombinant des-brazzein. They repeated their previous assertion that,
329
Wade, L. (xxx) Organic Chemistry, 7th Ed.
NY: Pearson Chapter 24, section 24.9B
http://www.pearsonhighered.com/showtell/wade_032159231X/wade_032159231X.html
330
Assadi-Porter, F. Abildgaard, F. Blad, H. & Markley, J. (2003) Correlation of the Sweetness of Variants of
the Protein Brazzein with Patterns of Hydrogen Bonds Detected by NMR Spectroscopy J Biol Chem vol
278(33), pp 31331–31339
268 Neurons & the Nervous System
“The characteristics that make a compound taste sweet to humans are not well understood.”
They did offer a table of sweetness threshold and sweetness intensity as a function of their
mutations. They also offered an extensive table of apparent lengths of the hydrogen bonds they
encountered based on an empirical equation from Cornilescu et al (1999). The values clustered
around 2.91 Angstrom.
8.5.9.3 The high electrical charge of the peptides
Understanding the dipole potential of the sweet proteins when in their DACB state with a GR
could be extremely valuable information. This potential plays a major role in the perception of
the intensity of all gustaphores.
As specifically noted by Assadi-Porter et al. (2000, pg 252), “Brazzein is highly charged and polar.
. .Charged and polar residues also appear to be critical for the sweetness of thaumatin and
monellin Their figure 1 has provided data on “Protein elution patterns on a preparative Vydac
reversed phase HPLC column for brazzein isolated from fruit, recombinant wild-type
(des-pGlu1-brazzein), and selected mutant brazzeins. “ They also provided considerable onedimensional 1H-NMR data at 600 and 750 MHz. Their conclusions are noteworthy, particularly their
assertion that the presence of Arg43 is essential for sweetness and that the area near Arg43 is
critically involved in the perceived sweetness of these materials. They provide an explanation for
the small extraneous peaks around 25 minutes in their HPLC results that may be subject to
additional discussion.
Jin has studied the difference in gustatory performance between brazzein and a variety of its
derivatives. These derivatives remained in the appropriate condition to act as glycophores
during their evaluation. An important fact can be noted about locations 29 and 41. Both are
occupied by the only two common acidic amino acids (negatively charged). Changing the
ligand at these locations can be expected to, and did result in, major changes in the perceived
sweetness of these molecules.
Xue has provided some charge data for monelllin331. Quoting from their abstract;
“A small number of proteins have the unusual property of tasting intensely sweet. Despite
many studies aimed at identifying their sweet taste determinants, the molecular basis of
protein sweetness is not fully understood. Recent mutational studies of monellin have
implicated positively charged residues in sweetness. In the present work, the effect of
overall net charge was investigated using the complementary approach of negative
charge alterations. Multiple substitutions of Asp/Asn and Glu/Gln residues radically altered
the surface charge of single-chain monellin by removing six negative charges or adding
four negative charges. . . Despite different sizes and non-homologous primary, secondary,
and tertiary structures (where known: brazzein [9]; monellin [10]; neoculin [11]; thaumatin
[12]), these proteins share not only intense sweetness that develops more slowly than that
of sugars, but an aftertaste that can persist for hours ([8,13] and data not shown).”
Xue et al. also note, “Despite different sizes and non-homologous primary, secondary, and tertiary
structures, the sweet proteins share not only intense sweetness that develops more slowly than
that of sugars, but an aftertaste that can persist for hours.” These characteristics are almost
certainly related to the size of the molecules and their more limited Brownian motion when in the
presence of predominantly smaller molecules.
Their discussion opens with “Many years of study have eliminated numerous proposals about
the molecular basis of sweet protein taste.” They did not offer any substantial change in this
situation. They do dismiss the importance of local pockets on the surface of the GB’s and the socalled “sweet finger” of the protein wedging into such pockets. Xue et al. offer no theoretical
framework or model to ground their primarily conceptual discussion. Rather than explore specific
binding between stimulants and GR’s, they discuss the electrostatic properties of their proteins
331
Xue, W.-F. Szczepankiewicz, O. Thulin, E. Linse, S. & Carey, J. (2009)
in monellin sweetness Biochim Biophys Acta vol 1794, pp 410–420
Role of protein surface charge
Signal Generation & Processing 8- 269
(native and derivative) in terms of their coulombic properties contributing to the selection and
binding mechanisms. They also consider their coulombic properties as rate determining in
diffusion rather than as properties measured by the stage 1 transduction process and reported
as intensity information.
Xue et al. do note the perceived sweetness of the sweet proteins does appear to increase as the
charge on the stimulant becomes more positive, up to a saturation point. Such a polarity ad such
a saturation are totally compatible with the electrophysiology of the stage 1 sensory neurons
(Section 8.xxx)
---Comparing the electrolytic properties of sucrose (which immediately hydrolyzes to glucose in
solution) with those of the six sweet proteins (and especially brazzein) by chromatography and
electrophoresis could provide important information about both their absolute and relative
electrical properties. Performing similar comparisons with derivatives of brazzein prepared by Jin
et al. would also provide excellent information. However, note the qualifications concerning
chromatography in Section 8.5.9.5.
8.5.9.4 Studies related to monellin
Hellenkant & van der Wei have studied two of the best known of maybe a dozen “sweet”
proteins, Monellin and thaumatin332. Monellin is a protein from Discoreophyllm cumminsii Diels and
thaumatin is a protein from Thaumatococcus daniellii Benth. They note the extreme sweetness
of these two proteins compared to sucrose, 60,000 to 100,000 times sweeter. Based on this work,
they employ an AH,B,X coupling to the sensory receptors of OR 2. They provide several additional
clues to how these proteins achieve the perception of sweetness and note their performance is
largely limited to catarrhine primates. They elicit little perception of sweetness in non-catarrhine
primates and all non-primates tested. In the future, this information might help determine the
difference(s) between the OR 2 receptors of these distinct groups. The proteins are soluble in
water at low concentrations and have a long lasting aftertaste suggesting their low volatility. The
aftertaste is not easily affected by washout procedures.
Kohmura et al. have undertaken a decades long study of provided papers on monellin beginning
in a journal no longer published. However the papers are available from the National Institute
of Informatics of Japan333. The large number of papers are highly redundant with regard to the
overall properties of monellin but provide useful highlights of their various experiments
The first abstract follows334,
“Monellin, a sweet protein, consists of two non covalently associated polypeptide chains, the A
chain of 44 amino acid residues and the B chain of 50 residues. Two different primary structures
have been reported for each of the A and B chains. The A and B chains corresponding to one
of the reported monellin structures were synthesized by the stepwise solid-phase method using
the Fmoc strategy in overall yields of 14.1% and 5.6%, respectively. The characterization of the
synthetic peptides by HPLC, FAB-MS, amino acid analysis and sequencing fully supported the
expected structures. The individual synthetic A and B chains were not sweet. Combination of the
two chains, and subsequent HPLC purification gave monellin in a yield of 53.9%. The synthetic
monellin had a distinct, lingering sweet taste (4000 times sweeter than sucrose) and was
crystallized by a vapor diffusion method. The synthetic product was identical to natural monellin
by HPLC, but not by tryptic mapping. These results indicate that the reported structure for monellin
differs slightly from that of natural monellin.
332
Hellenkant, G. & van der Wei, H. (1989) Taste modifiers and sweet proteins In Cagan, R. ed. Neural
Mechanisms in Taste Boca Raton, FL: CRC Press Chap. 4
333
http://ci.nii.ac.jp/info/en/articles/quickguide.html
334
Kohmura, M. Nio, N. Ariyoshi, Y. (1990a) Solid-phase synthesis and crystallization of monellin, an intensely
sweet protein Agri Biol Chem vol 54(6), pp 1521-1530
270 Neurons & the Nervous System
The fact that HPLC and tryptic mapping gave different results with regard to the natural and
synthetic monellin may be of great importance.
The second paper includes a complete amino acid sequence for monellin as shown in Figure
8.5.9-5335. Kohmura et al found that monellin consisted of two distinct peptide chains, A with 44
residues and B with 50 residues, that were not covalently bonded. They noted that two different
primary structures had been reported for each of these chains with differences at several
locations. After furhter investigation, the questionable locations were assigned the values of
Asp22, Glu25 & Asp26 for the A chain and Glu49 & Asn 50 for the B chain. It is noteworthy that
fusion of the two chains at the C-terminus of the B chain and the N-terminus of the A chain did
produce a perception of sweetness for the result. Additional HPLC and tryptic data is found in
the paper.
335
Kohmura, M. Nio, N. Ariyoshi, Y. (1990b) Complete amino acid sequence of the sweet protein monellin
Agri Biol Chem vol 54(9): pp 2219-2224
Signal Generation & Processing 8- 271
Figure 8.5.9-5 Complete amino acid chain for monellin. Top; A-chain of nomellin. Approximately
10% of the material carries an extra Phe at the N-terminus. Bottom, B-chain. Approximately 19%
of which carries an extra Thr at the N-terminus and approximately 24% lacks the N-terminal Gly.
From Kohmura et al., 1990b.
272 Neurons & the Nervous System
The third paper focused on derivatives of monellin336. They sought to determine the mode of
interaction with the GR of the G-path by various sweet peptides but noted the difficulty of
studying their peptides because of their ability to assume a variety of conformations in water.
They then reverted to the study of the complete proteins. They continued to explore their
materials without a clear theoretical framework. The paper did not provide any significant
conclusions.
The xxx paper was focused on the solid crystalline state337. It contains significant information that
has not been reviewed in detail. Kohmura and team is continuing to study monellin. Search
scholar for more recent articles..
In a 1994 paper338, they continued to seek to understand the selection and transduction of
monellin by replacing various amino acids while maintaining an active perception of sweetness.
As for other investigators, this negative approach does not lead to an explanation of transduction
mechanism, only to the determination of what other residues increase or decrease perceived
sweetness.
No schematic could be found among their work showing the folding of monellin like the
schematics of Jin et al. and Assadi-Porter et al. for brazzein.
8.5.9.5 Recent experiments to characterize rare amino acids
Chaudhuri & Yeates have recently described the fact that some proteins are produced from
alternate readings of accepted stop codons of DNA and tRNA339. They note, “The difficulty arises
from the distinction that, unlike other amino acids, rare amino acids are not coded for by
dedicated codons. Instead, they are incorporated in special circumstances by the UGA (opal;
selenocysteine) and the UAG (amber; pyrrolysine) codons, which are ordinarily interpreted as stop
signals to terminate translation.” Their may be other situations yet to be discovered. This subject
matter is very much on the forefront of biological research340.
Shen et al. have provided an excellent paper on identification and molecular cloning of the
protein required for generating the rare protein selenocysteine341. They identified this specific
protein by way of an unusual presence (a mobility shift) in an RNA electrophoretic assay.
However, they did not identify, or discuss the identification of, the actual amino acid.
336
Kohmura, M. Nio, N. Ariyoshi, Y. (1990c) Solid-phase synthesis and crystallization of [Asn22, Gln25,
Asn26]-A-chain-[Asn49, Glu50]-B-chain-monellin, an analogue of the sweet protein monellin Agri Biol Chem
vol 54(12), pp3157-62
337
Kohmura, M. Nio, N. Ariyoshi, Y. (1991) Solid-phase synthesis of crystalline monellin, a sweet protein Agri
Biol Chem vol 55(2), pp 539-45
338
Kohmura, M. Nio, N. Ariyoshi, Y. (1994) Solid-phase synthesis and structure-taste relationships of analogs
of the sweet protein monellin Biosci Biotech Biochem vol 58(8), pp 1522-1524
339
Ghaudhuri, B. & Yeates, T. (2005) A computational method to predict genetically encoded rare amino
acids in proteins Genome Biol vol 6, pg:R79
340
Wang, Q. Parrish, A. & Wang L (March 2009). "Expanding the genetic code for biological studies". Chem.
Biol vol 16(3), pp 323–36. doi:10.1016/j.chembiol.2009.03.001
341
Shen, Q. Wu, R. Leonard, J. & Newburger, P. (1998) Identification and Molecular Cloning of a Human
Selenocysteine Insertion Sequence-binding Protein J Biol Chem vol 273(10), pp. 5443–5446,
Signal Generation & Processing 8- 273
Goodsell described the more advanced genetic coding used to identify and form
selenocysteine342. “Quite surprisingly, cells modify their genetic code to add selenocysteine into
their proteins. The basic genetic code used by all organisms on Earth specifies twenty amino
acids, along with a few stop codons. In order to add a 21st amino acid to this code, cells
reinterpret UGA stop codons in special cases. But this causes a problem: how does a cell know
when to read UGA as "stop" and when to read it as "selenocysteine"? This is done by using a
special signal sequence that is located after the UGA codon. In bacteria, this signal is
immediately after the UGA codon, in the coding portion of the messenger RNA. In our cells, the
signal is much further away at the end of the coding sequence. As shown on the next page, this
sequence is recognized by a special elongation factor that delivers the selenocysteine tRNA to
the ribosome at just the right moment.” This mechanism is called translational recoding. The
description of this mechanism is not new343. Goodsell goes on to note, “About 15 selenoproteins
have been found in human cells. These include several deiodinase enzymes that are essential for
the generation of thyroid hormones, several glutathione peroxidases that are important for the
conversion and detoxification of compounds with reactive oxygen atoms, and several proteins
of unknown function.”
Gromer et al. have provided more information on these selenium containing proteins344. “
Xu et al. have described an entire seleno protein family of rare amino acids in considerable
detail, including locating some of these rare amino acids in relation to their common brethren345.
Their figure 2 shows a common binding site (lysine) across a wide variety of selenium protein
derivatives. Their figure 6 shows a potential transition from serine to selenocysteine (Sec). Their
31
P-NMR spectra show the location of one of these unusual amino acids in the context of other
amino acids, Figure 8.5.9-6. The presence of monoselenophosphate (SeP) is clearly displayed in
the left-most three frames and it is absent from the right-most. The complete meaning of this
display can be found in the paper. The point of interest here is that the yield in the processes
leading to this display was not high, but the ability of NMR to identify the product clearly was
excellent. Three digit accuracy was obtained in labeling the SeP at +23.2 ppm. The height of the
material in the upper global views could easily be missed, but the signal to noise ratio in the lower
views is quite adequate.
342
Goodsell, D. (2008) Selenocysteine Synthase PDB protein data bank doi: 10.2210/rcsb_pdb/mom_2008_8
ePub Version
343
Böck A. and Stadtman T. (1988) Selenocysteine, a highly specifi c component of certain enzymes, is
incorporated by a UGA-directed co-translational mechanism. Biofactors vol 1, pp 245–250
344
Gromer, S. Eubel, J. Lee, B. & Jacob, J. (2005) Human selenoproteins at a glance. Cell Molec Life Sci vol
62, pp 2414-2437
345
Xu, X.-M. Carlson, B. Mix, H. et al. (2007) Biosynthesis of Selenocysteine on Its tRNA in Eukaryotes PloS
vol 5(1), pp 0096-0104
274 Neurons & the Nervous System
8.5.9.6 Proposed mechanism of sweet protein perception
Figure 8.5.9-6 In vitro NMR spectroscopic analysis showing SeP presence. Top row; conventional
31
P-NMR of four samples prepared by different paths. Lower row; expanded views showing
excellent signal-to-noise. No SeP was created by the reaction leading to the right-most frames.
See original article for details. From Xu et al., 2007.
Although not as well supported as the main hypothesis of gustation, primarily because of
limitations in analytic instrumentation, the bonding of the sweet proteins to the nominal G-path
GR appears quite defendable. This is particularly true if the laboratory record is broadened to
allow for the presence of rare amino acids in brazzein, and presumably all sweet proteins, rather
than just the 20 common amino acids identified by most present laboratory instrumentation.
The following discussion is meant to be global, but where necessary it may be restricted to the
perception of sweetness by humans and old world monkeys. In general, no change in perceived
taste quality was found with changes in concentration of brazzein or its derivatives.
Signal Generation & Processing 8- 275
As noted above, the present state of the art is quite limited in this leading edge research area.
it is proposed that the perceived sweetness of certain proteins is due to three features of their
interaction with the G-path GR’s of the system;
• the sweet proteins exhibit glycophores located along the external periphery of their molecular
space, generally described as a point along a β-turn of a flexible loop,
• these glycophores participate in an AH,B,X relationship with the GR’s,
• the AH,B,X relationship involves a DACB between two orbitals of the glycophore with a d-value
approximating 2.82 Angstrom and the dispersion centroid, X, is separated from the AH,B orbitals
by the same distance as for other super-sweet glycophores,
• the very high dipole potential of these proteins is the major contributor to the great perceived
sweetness of these proteins and the high signal response of the stage 1 sensory neurons.
---The work of Goodsell and others at the beginning of the 21st Century to identify the 21st (common)
amino acid has gone a long way toward rewriting how the genetic code is actually read, to
include reinterpreting the stop codes as conditional, a mechanism known as translational
recoding.
However, this area remains provisional. Assadi-Porter et al. (2000a) have asserted that brazzein
can be synthesized in the laboratory with good yield using totally conventional techniques
(without introducing a
translational recoding ) and
a codon of ATG for the
modified amino acid, pGlu
or (pidolic acid) as shown in
Figure 8.5.9-7.
The
conv entional glutamic
amino acid is formed using
the codon GAA.
----
Figure 8.5.9-7 Amino acid sequence of brazzein and DNA
sequence of synthetic gene. A; top line, the pGLU (pyroglutamic
acid) residue is shown at position 1 of the complete brazzein
protein. Second line, brazzein absent pGlu (des-pGlu1). The
bridges represent four disulfide bond pairings. C; the DNA
sequence of the synthetic gene used for heterologous
production of brazzein in E. coli. The stop code is TAG. Excerpt
from Asadi-Porter et al., 2000a.
276 Neurons & the Nervous System
[xxx provide a discussion here.
The challenge is to locate and document the necessary orbital configuration(s) along the amino
acid sequence. In the case of brazzein, there is general agreement that the glycophore is
located along the β-turn of the flexible loop between locations 34 and 45 or an equivalent
location between residues 30 and 35 (as suggested by the arrows in the above figure from Jin.
The amino chains in these regions, (SGECFYDEKRNL and KHARSG) offer a variety of side chains
which could support the required d-value and dispersion centroid distance. The simplest
configurations would be represented by modifications to glycine (G) through β-turn of a flexible
loop hydroxylation and serine (S) through demethylenation that provided an oxygen orbital
attached to the second carbon resulting in a glycophore of HO-C-C-O and a third orbital at the
dispersion centroid. In this case, the residues would be modified similarly to the pGlu (pE) residue
at the N-terminus. The X orbital is likely to be an N associated with the a distant peptide bond.
Additional analysis will be required to identify this orbital since the distance to the dispersion
centroid will strongly depend on the local curvature of the amino acid chain. Figure 8.5.9-8
shows the situation under discussion. The left frame shows the nominal schematic of a peptide
chain. Each shaded group is planar and includes a peptide bond. This bond has considerable
similarity to a double bond and does not allow free rotation. However, as noted, the bonds
outside of the planar groups are free to rotate. As a result, the ultimate bond angles along the
chain are subject to other constraints as noted below for the α-helix. The oxygen-carbon bond
shown within each planar group is also more constrained than indicated, suggestive of a dual
bond. In this application, it is suggested the left-most R-group is another oxygen atom with
possibly an additional ligand. The two left-most oxygen atoms are at the appropriate distance
to form a glycophore of the AH,B type. It is proposed that a “super” glycophore of the AH,B,X
type can be formed in conjunction with a nitrogen located farther to the right along the chain.
The distance between the AH,B orbitals and the X orbital is highly dependent on the angles of
rotation about the non-planar carbon.
Figure 8.5.9-8 Options available for the dispersion centroid supporting a glycophore in an α-helix.
Left, a schematic of the peptide chain focusing on the individual planar groups (shown shaded),
the out-of-plane R-groups and the points of rotation between the planar groups. Right, the
nominal spacing of the same structure in its minimal energy α-helix arrangement. The residue
side-chains are not shown. See text. A collage modified from Lehninger, 1972.
Signal Generation & Processing 8- 277
The right frame shows the minimum energy configuration of the schematic peptide structure, the
α-helix. The frame from Lehninger has been modified to include the rigidized oxygen atom of the
planar structure on the left and the out-of-plane oxygen on the right. Using the nominal spacings
shown, it is clear there are a number of nitrogen orbitals that could support a dispersion centroid
at a distance from the AH,B orbitals that satisfies the requirements the Shallenberger team and
Kier discussed in Section 8.5.3. It is not likely that the appropriate distance to X can be achieved
within a minimum energy α-helix. Under relaxed conditions of a less curved, peptide chain,
virtually any required dispersion distance necessary to support the AH,B,X configuration could be
provided. The forming of hydrogen bonds over extended intra-chain distances is well recognized
in protein chemistry.
Based on the above structural considerations, the challenge is to find an amino acid structure
that can provide an oxygen orbital attached to the left-most (out-of-plane) carbon of the left
frame. None of the normally discussed 20 amino acids forming proteins meet this requirement.
However there are other common amino acids as well as several rare amino acids that might
provide the necessary orbital, or like in the case of pGlu, an amino acid might be modified within
the protein after its expression following its DNA code using the mechanism of translational
recoding.
It is possible that threonine (T) could provide the oxygen orbital via its hydroxyl group, proline (P)
could provide the necessary orbital via the nitrogen in its ring structure or alanine could provide
the desired structure via a conceptually simple methyl-hydroxyl transferase. However, the simpler
solution would be to modify either glycine (G) by hydroxylation or serine (S) by demethylenation.
While no threonine is found within the brazzein peptide chain, the other three candidates do
occur within the zones expected to include the glycophore of brazzein (P12, S14, S34 or G35). The
latter three candidates have been marked in the above brazzein peptide chain from Caldwell
et al.
---A set of images of brazzein are available from the European PDB bank under code 1BRZ. A 3D
projection of brazzein is available at
http://www.ebi.ac.uk/pdbe-apps/nmrviewer/cgi-bin/index.py?page=main&pdbId=1brz&wiz=
cluster&type=2
It provides an amazing level of detail with the oxygen and other atoms shown in color. The ball
and stick model suggests the various cartoon models only show a computed backbone, not
necessarily the actual location of the amino acid residues.
---It is not obvious to this investigator whether the conventional laboratory identification procedures,
which are generally based on relative diffusion rates for the individual amino acids would indicate
clearly the presence of a rare amino acid in place of a glycine or serine. The description of rare
amino acids by Lehninger in 1972 or earlier (page 72) appears applicable at this point. “It is likely
that other rare amino acids of proteins will be discovered. . . that they will be derivatives of
presently known common amino acids and that they will be limited in occurrence to single
proteins or to a class of proteins.” The six sweet proteins could easily be such a class. The material
of Shen et al. and Xu et al. in the previous section support these earlier observations. Most of the
rare forms described by Lehninger involve chemical additions. Lehninger describes an azaserine
on page 73 approaching the desired amino acid form but not normally found in proteins.
Derivatives of nearly all the well-known 20 amino acids have been isolated and described. There
are at least 220 (some say 500346) rare amino acids, mostly found in plants. At least 25 human
proteins include selenocysteine (Sec), a rare amino acid, in their primary structure347.
346
Wagner, I. & Musso, H. (1983) New Naturally Occurring Amino Acids. Angew. Chem Int. Ed. Engl. 22 (22):
816–828. doi:10.1002/anie.198308161*
347
Kryukov GV, Castellano S, Novoselov SV, Lobanov AV, Zehtab O, Guigo R, et al. (2003) Characterization
of mammalian selenoproteomes. Science vol 300, pp1439–1443
278 Neurons & the Nervous System
Kryukov et al have been very specific concerning identifying the rare amino acids. “In the
universal genetic code, 61 codons encode 20 amino acids, and 3 codons are terminators.
However, the UGA codon has a dual function in that it signals both the termination of protein
synthesis and incorporation of the amino acid selenocysteine (Sec) (1–3). Available
computational tools lack the ability to correctly assign UGA function. Consequently, there are
numerous examples of misinterpretations of UGA codons as both Sec codons (4) and terminators
(5, 6), including annotations of the human genome (7, 8), where no selenoproteins have been
correctly predicted. With 18 human selenoprotein genes previously discovered (3), the estimates
of the actual number of such genes vary greatly (9). All previously characterized selenoproteins
except selenoprotein P (10) contain single Sec residues that are located in enzyme-active sites
and are essential for their activity. Thus, misidentification of UGA codons leads to a loss of crucial
biological and functional information. Sec is cotranslationally incorporated into nascent
polypeptides in response to UGA codons when a specific stem-loop structure, designated the Sec
insertion sequence (SECIS) element, is present in the 3 untranslated regions (UTRs) in eukaryotes.”
They have provided a tabulation in their figure 2 of false positive reporting at locations occupied
by a selenoprotein.
Analysis of proteins by chromatography is probably the ultimate example of overlooking
the facts using Bayesian experimentation. The nominal chromatograph protocol assumes
there are only 20 amino acids in the material under examination and assigns names to
each residue based on its relative position with this restricted order of 20 amino acids. The
conventional chromatography technique offers no opportunity to uncover the presence of
rare amino acids.
Figure 8.5.9-9 shows the output of a typical automatic recording chromatograph. Note the
presence of ammonia as a reference among the other 17 residues labeled by their conventional
amino acid names. Where do the other 200-500 rare amino acics appear in this representation.
Figure 8.5.9-9 The relative positions of the amino acid residues in chromatography Where does
selenocysteine (Sec) appear in this presentation? Where do the other rare amino acids appear?
The area under each peak is proportional to the amount of each amino acid in the mixture. See
text. From Lehninger, 1972.
Based on recent activity in a broad group of laboratories (only addressed briefly above),
it appears quite likely that at least one rare amino acid has been formed and inserted into
a β-turn (or equivalent) of the six known sweet proteins thereby allowing their participation
in transduction by DACB at the conventional GR of the G-pathway.
The following two sections will focus on the requirements for DACB between a sweet protein and
a GR, and the requirements/affects of the location and charge associated with a specific residue
Signal Generation & Processing 8- 279
on the preceived intensity of the stimulant. It will be assumed in those sections that each sweet
protein only supports one glycophore. The initial discussion will bound the problem of DACB
binding and the identification of the residue forming the dispersion centroid, X. The following
discussion will bound the potential for changing the dipole potential of the stimulant and the
resulting perceived sweetness intensity.
A limitation of prior analyses has been the Bayesian assumption that proteins only contain a
mixture of the 20 common amino acids, and the position enunciated by Caldwell (1998, 1st
paragraph) that the sweet proteins contain no carbohydrates.
A limitation of the current analysis is its dependence on the frequently qualitative statements
concerning the replacement of one residue by another in experiments. Caldwell et al. stated the
generality, supported by citations, that “Chemical modification of the N-terminus, cysteine,
hisitdine, arginine, lysine, tyrosine, and glutamate or aspartate residues of brazzein have been
found to reduce or abolish its sweet taste.” This statement appears counter to that of Jin based
on neuron recordings, “Asp29Ala, Asp29Asn, and Glu41Lys elicited strong responses with fast onset
and with a temporal profile reminding of sucrose rather than WT brazzein, while neither mutant
Glu36Ala or His31Ala gave a response.” Ratios are more useful than comments that one is
sweeter (or less sweet) than the other variant.
It is postulated that a change from an AH,B,X configuration to an AH,B configuration may cause
a major change in perceived sweetness (by as much as 10,000:1), but not necessarily a total loss
of perceived sweetness. Thus separating the change in the bonding mechanism should be kept
separate from the perception mechanism.
Assadi (2000, pp 259-) Most of the mutants in the flexible loop region around Arg43 were found
to decrease, but not completely eliminate, the sweetness of the protein.
Jin also noted “mutations of residues close to one another in the protein sequence led to
remarkable differences in sweetness.”
Jin and other investigators have reported multiple situations where modification of WT brazzein
led to higher perceived sweetness; a result suggesting the precise form of WT brazzein may not
be optimized for sweetness alone.
Following the earlier discussions, the transduction mechanism will be divided into two steps; the
mechanism of gustaphore/GR selection leading to signaling along the G-pathway, and the
mechanism of dipole potential measurement by the first amplifier of individual G-pathway stage
1 sensory neurons.
8.5.9.6.1 Details of proposed sweet protein selection
This section will focus on a comparison of the findings of Jin et al, Caldwell et al. and Assadi-Porter
et al. within a broader realm consistent with the inclusion of rare amino acids in the sweet
proteins.
The first order conclusions are that;
• the AH,B glycophore of brazzein is found between residues 40 and 43, most likely at residue 41
• a dispersion center, X, is also located in this range and most probably found at location 43
(probably involving Arg43, an amino acid residue incorporating a highly charged guanidinium
group).
• the slow rise and fall in the perception of sweetness from the sweet proteins is a result of their
large size and relatively low average diffusion velocity in the face of Brownian motion by the
smaller constituents of saliva.
• a rare amino acid is found at location 43 (not generally identified by conventional–and
automated–chromatography techniques).
• the highly polar character of brazzein makes almost any modification in the amino acid
sequence outside of locations 40 and 43 significant in modifying the dipole potential of the
molecule and thereby its perceived sweetness (as discussed in the next section).
•
280 Neurons & the Nervous System
The conventional amino acid sequence of interest for brazzein is Phe38, Tyr39 Asp40, Glu41, Lys42,
Arg43, Asn44 with the two residues in italic shown to expand the margins of the poorly defined
set. However, it is possible that any of these amino acids may in fact be a rare amino acid not
found within our past Bayesian assumption.
----Summary positions related to brazzein
Assadi: two regions of the protein are critical for the sweetness of brazzein; a region that includes
the N – and C–termini, which are located close to one another, and a region that includes the
flexible loop around Arg43.
Jin: residues 29-33 and residues 39-43, plus residue 36 connecting these stretches as well as the
N – and C–terminals determine the sweetness of brazzein.
Jin: WT brazzein has no bitter after taste or any other secondary taste quality
Jin: the taste quality associated with brazzein did not change specificity with concentration.
Caldwell: One site is near the N-terminus and contains athe unique histidine, His31, which is within
12 Angstrom of Arg33 and near examples of other residues of the types impoicates as critical for
sweet taste, including tyrosines (residues 8, 24, and 51), lysines (residues 3, 5, 6, 27 and 30),
glutamates (residues 9 and 53), and aspartates (residues 2 and 50). The second candidate site
centers around the other arginine residue, Arg43, . .
It is generally agreed that sweetness of the sweet proteins is lost when unfolded.
---Relevance of location 4
Assadi; disruption of one disulfide bridge by mutation of Cys4 is sufficient to abolish the sweetness.
---Relevance of location 29
Jin: mutation of Asp29 to Asp29Ala, Asp29Asn or Asp29Lys markedly increased sweetness over WT.
The notation describes substitution of the second residue in place of the first.
---Relevance of location 30
Jin: mutation of Lys30 to Lys30Asp removed all sweetness at the concentration used.
---Relevance of location 31
Jin: a mutation from His31 to Ala31 introduced a significant loss in sweetness (although a previous
experiment had found an increase in sweetness threshold by this change).
---Relevance of location 33
Signal Generation & Processing 8- 281
Jin: mutation of Arg33 to Arg33Ala removed all sweetness at the concentration used.
---Relevance of location 36
Jin: mutations from Glu36 to Glu36Ala, Glu36Gln or Glu36Lys destroyed the perception of
sweetness.
---Relevance of location 41
Caldwell: did not address the perceptual significance of replacing Glu41.
Jin changes at location 41 from Glu41 to Glu41Lys could dramatically increase sweetness.
--Relevance of location 43
Jin noted a mutation at location 43 from Arg43 to Arg43Ala abolished the sweetness (?or just
downgraded 1t to regular sweetness?)
-- -Jin: a mutation at location 29 increased the sweetness significantly over brazzein.
Jin: mutations at locations 30 or 33 removed all sweetness.
---Relevance of mutations to the N-terminus
Jin: omitting pGlu1 increased the perceived sweetness of brazzein by 2:1.
Jin: addition of Ala to the N-terminus or mutation of the side chain from Asp to Asn did not
change the sweetness.
Assadi: replacement of pGlu1 with Ala1 increases the sweetness of the protein by a factor of 4.
---Relevance of mutations to the C-terminus
Assadi: the deletion of the normal C-terminal residue or the addition of residues at the C-terminal
is detrimental to the sweetness of brazzein.
----Concluding analysis
The fabrication of what appears to be a fully functional brazzein from a synthetic gene code
using a TAG stop codon supports the conclusion that only the 20 common amino acids plus pGlu
are required to create brazzein. How the pGlu was created from the amino acid gene triplets
during synthetic gene preparation and expression in E. coli is not clear. Assadi-Porter et al. use
the codon ATG for this residue.
282 Neurons & the Nervous System
The above assertions are in general agreement. Disruption of the N – and C– termini, or possibly
the bridge between them reduces the perceived sweetness significantly but does not abolish it.
However some augmentations of the N –terminal actually raises the perceived sweetness by a
small factor.
Disruption of the Arg43 site also reduces perceived sweetness significantly but does not abolish
it.
Only disruption of the Arg43 is reported to abolish sweetness.
[xxx edit ]
It is proposed that such hydrogen bonding as found between residues of helical and sheet forms
of the protein can form equally easily between the residues of the various single strands and a
distinct G-path GR. Depending on the curvature of the chain, Any of the nitrogen orbitals could
represent the X element in the AH,B,X relationship. For a configuration such as the β-turn of
brazzein and potentially other sweet proteins, involving less tight twisting, the distance to an
appropriate nitrogen orbital is quite likely.
8.5.9.6.2 The AH,B,X relationship in sweet proteins
At least one glycophore has been identified in brazzein that is compatible with its super sweetness
in the context of the hypothesis of this work. Because of the complexity of the protein, there may
be more.
The introduction of the sweet proteins into the repertoire of glycophores requires modification of
the previous hypothesis related to their structural geometry. For the first time, although also
observed among the organic picrophores, the structural requirement does not include the
presence of only two carbon atoms between the AH,B orbitals, nor does it require they maintain
a nominal equat-trans relationship within a single ligand. The requirement is loosened to only
require the two orbitals to be on the external surface of a complex structure and maintain an
average distance between the orbitals of 2.82 Angstrom plus or minus a value to be determined,
but at least ±5% at the half amplitude of the efficacy function.
This relaxation of the requirement makes it even more difficult to recognize the glycophores
based on any schematic description (Fischer diagram, etc.) or even 2D ball and stick structure.
The glycophores in complex molecules such as the sweet proteins can only be visualized using
3D graphic representations. See Sections 8.5.3.2 & 8.5.3.3.
Figure 8.5.9-10 shows the only glycophore of brazzein identified to date in a space-filled 3D
representation to illustrate its surface location and the location of the key orbitals. Several other
features are identified primarily for orientation purposes. Leu28 is to the immediate upper right
of Asp29.
Note the relatively small area involved in this glycophore relative to the total volume of this
“small” sweet protein. While possible to locate the glycophore in a ball and stick representation
of this molecule, it is a visually demanding task and very difficult to display for pedagogical
purposes.
Signal Generation & Processing 8- 283
Figure 8.5.9-10 Location of the glycophore of brazzein on an annotated space-filled model. The
oxygen orbitals of both Asp25 & Asp29 are assumed to be from two separate resonant carboxylic
ligands in the Astex representation. The dispersion centroid can be assumed to be either the
hydroxyl group of Tyr24 or the phenol ring of that same residue. See text.
The potential orbitals of the β-turn (such as in Glu41, Lys42, Glu41, Arg43 etc) are on the outside
of the turn and too widely spaced to be glycophores. However, they may contribute significantly
to the dipole potential of the stimulant discussed in a following section (particularly the
guanidinium group in Arg43).
Figure 8.5.9-11 shows the area of the glycophore described above in a schematic format. The
defined glycophore is not represented by a simple ligand, or even a contiguous small group of
orbitals and carbon atoms. It is actually formed by elements of three different residues operating
in electrolytic consort. The two closest oxygen orbitals of Asp25 and Asp29 act as the AH,B group
while the electrostatic field of either the hydroxyl group or the phenol ring of the Tyr24 residue acts
as the dispersion centroid, X, exciting the sensitive area of the G-path GR. See Section xxx.
The Astex viewer assumes the resonant condition for the carboxyl group when preparing its
representation. If a polar representation was assumed, the d-value calculated between the two
oxygen orbitals shown might be marginally different.
284 Neurons & the Nervous System
Figure 8.5.9-11 Schematic of glycophore involving Tyr24, Asp25 & Asp29. Astex represents the
carboxylic acid groups of Asp25 & Asp29 as in resonance. If they are not, the dimensions of the
AH,B,X triangle could change marginally. The two oxygen orbitals closest to each other are used
in defining AH,B. The oxygen orbital of the hydroxyl group in Tyr24 is shown as the dispersion
centroid, X.in this figure. The phenol ring of Tyr24 may be or contribute to the overall dispersion
centroid. The shaded areas represent planar structures associated with the peptide bonds. See
text.
It is important to note that any structural modification to the molecule that changes the AH,X or
B,X dimensions of a super-sweet glycophore will reduce its role to that of a common sweet
glycophore. Any structural modification leading to a change in the baseline, AH,B is likely to
cause a loss in the DACB to the GR and complete extermination of the perception of sweetness.
Thus a reversion of an AH,B,X form of sweetener may cause its reversion to an AH,B sweetener with
a loss of perceived sweetness of as much as 2000:1 but without total extermination of perceived
sweetness. Loss of the AH,B relationship leads to total abolition of the perceived sweetness
associated with that glycophore. A casual reference in the literature to the total loss of
perceived sweetness may only apply super-sweetness at the concentration used in the tests, and
not a truly total loss of sweetness at concentrations compatible with other normal sweeteners.
Jin et al. have highlighted the variability of the super-sweetness of brazzein with respect to Asp29
and Lys30. Their assertion that substitution of the residue asparagine (Asn) for Asp29 leads to
increased sweetness is compatible with replacing one oxygen of the carboxylic acid group with
an amide group. An increase in sweetness after substitution of Ala for Asp29 suggests there may
be more than one glycophore present in brazzein. Substitution of Lys for Asp29 introduces the
possibility of destroying the initial glycophore and creating a replacement.
It is quite possible that other glycophores are present in the super-sweet stimulant brazzein.
However, the area around His31 (residues 29 to 33) was explored and showed little promise.
Locations within the α-helix, within strand 2 of the molecule and along the β-turn are not promising
on structural grounds. The required dimensions are difficult to achieve within these regions.
Residues extending outward from the mean path shown in cartoons of brazzein tend to have
orbitals too widely space to form the necessary geometric AH,B,X relationship.
8.5.9.6.3 Quantifying the AH,B,X dimensions in proteins.
Figure 8.5.9-12 from Caldwell et al. illustrates a problem in describing the precise locations and
dimensions features within complex molecules such as proteins. Thermal forces cause a constant
Signal Generation & Processing 8- 285
“dancing” within the molecule, not unlike the Brownian motion associated with individual
molecules in a solution or gel. It is typical for the various 3D representation programs to present
a “cartoon” representing the average of typically 43 or more conformers (conformations at
individual moments in time) describing a given protein. In an accompanying “stick” version of
such a cartoon, Caldwell et al. suggest a possible glycophore of brazzein may be found near the
Arg33 and Arg43 residues at lower left and upper right in this figure. It is not likely these residues
are involved in the glycophore DACB process but they may contribute considerably to the dipole
potential of this molecule.
Figure 8.5.9-12 43 superimposed conformers of brazzein in solution.
Caldwell et al., 1998.
From
286 Neurons & the Nervous System
Figure 8.5.9-13 compares the available parameters for AH,B,X for various glycophores.
Figure 8.5.9-13 Expanded table of AH,B,X parameters for the glycophores of taste. Paired gustaphores are
shown where more than one AH,B,X conformation are of potential interest. Many potential AH,B,X
configurations are omitted here. The boxed values are the most likely GR values for the G-path. All of the A–B
values shown are within the estimated ranges of Kier and Shallenberger from the 1960's. The values for
sweetness relative to sucrose are very rough and usually provided in semantic rather than tabular form.
The table shows two potential GR’s for the G-path, both phosphatidylgalactose (PtdGal) . The
first involves A=O3 and B=O4 as the primary DACB participant as suggested by Kier and by
Shallenberger et al. in the 1960's (Section 8.5.5). Jmol suggests this configuration has a d = 2.8
Angstrom. The second involves A = O2 and B=O3 as the primary DACB participant with a d=2.86
Angstrom. Both values are bracketed by the values originally proposed by Kier and by
Shallenberger et al. and both are within 5% of the value of 2.92 Angstrom exhibited by many
artificial sweeteners. The first candidate GR appears more compatible with the AH,B,X
configuration of most artificial sweeteners with an angle A near 120 degrees. This nominal G-path
gustatory receptor (GR) is shown in Figure 8.5.5-9
Galactose and the artificial and protein sweeteners can exhibit multiple AH,B,X configurations.
Only those configurations closest to the values of Kier and Shallenberger et al. are shown here for
comparison. Aspartame provides two different potential AH,B.X glycophores as noted in the
figure. Aspartame and brazzein using the OH group as X show a good match for the A–B and B–X
distances and the angle, pX. The pB is different when X = OH in brazzein. Similarly, the A–B
distance to the centroid of the phenol ring of Brazzein is the same as the A–B distances for
aspartame and brazzein to the OH group. These ranges may help determine the effective
diameter of the dispersion centroid(s).
Signal Generation & Processing 8- 287
8.5.10 The dipole potential (DP) in the perceived intensity of gustation
Once a stimulant has become associated with the appropriate gustatory path through a DACB,
with a GR, the response of the sensory neuron associated with that GR becomes important. As
discussed in Section xxx, the response involves an electrolytic amplifier within the stage 1 sensory
neuron that measures a change in potential at its input relative to the surrounding saliva. This
change in potential is caused by the dipole potential of the stimulant molecule. The dipole
potential of the stimulant molecule is described in terms of the electrostatic potential of the
molecule in the vicinity of its AH,B relative to the integrated potential of the rest of the molecule
in contact with the saliva (resulting from its aggregate charge distribution).
Every sensory neuron contains two electrolytic amplifiers or Activa. Typically, the Activa forming
the first amplifier is in electrical contact with its GR and exhibits an input range of about xxx 15
milli-Volts and an input threshold of tens of microvolts. The resulting usable dynamic signal range,
in the absence of adaptation, is typically on the order of 200:1.
The typical natural glycophore (of the AH,B type) introduces a dipole potential less than the
available 15 mV range. However, some glycophores of the AH,B,X type can introduce a larger
potential change that results in saturation of the first amplifier. This may occur by two distinct
mechanisms; first, the dipole potential of the stimulant may be quite large and/or second, the
stimulant may place a charged region in the vicinity of the dispersion centroid of the GR that
results in a large change in the dipole potential of the GR itself. The combination of these two
mechanisms can easily cause a saturation in the input Activa.
The dipole potential of a molecule has not played a major role in chemical research prior to the
1990's. However, a related dipole moment is commonly addressed and it is frequently
demonstrated in undergraduate chemistry programs. Commonly, the moment arm of such a
dipole moment is described relative to an arbitrary geometrical axis. The lack of a dipole
moment under conditions of molecular symmetry are also commonly addressed. Here, the
moment arm of the moment is not of interest, only the aggregate charge described above and
associated with that moment arm is of interest. The separation of the charge by a distance equal
to the moment arm can be expressed as a potential.
Beginning in the 1990's, the significance of the dipole potential of biological membranes has
become of considerable interest. In the following, both the composite dipole potential of the
bilayer membrane and the dipole potential of only the outer layer are critically important.
One can conceive of a gustaphore bonding to a specific GR but not introducing any net
change in the dipole potential presented to the input of the 1st Activa of the sensory
neuron. In this case, no signal would be generated at the output of the Activa and there
would be no perception of taste. This situation appears to be common among the “antisweeteners” and potentially other gustatory channel blockers.
The fact that a phospholipid of the outer bilayer exhibits a measurable dipole potential implies
that it exhibits a finite (although quite high) resistance value.
In the following discussions, it is critically important that the specific character of a given molecule
be appreciated. As an example, the description of a molecule as a glyco-lipid is entirely
inadequate. In the case of the outer layer of the external lemma of a sensory neuron, it will
typically consist of a phospholipid such as phosphatidylserine (PtdSer), acting as a GR, that is
susceptible to DACB linking to an organic acid. The change in the dipole potential from that of
PtdSer alone and when stimulated by the organic acid is of critical importance in perceiveing
the intensity of the stimulant.
8.5.10.1 Background
The study of the dipole potential of biological membranes experienced a renaissance in
the mid 1990's. Significant changes occurred in the area of computational chemistry which
allowed for much more clarity in interpreting the results of Langmuir Trough Experiments.
288 Neurons & the Nervous System
Brockman provided an excellent introduction appropriate to the time period348. His
Abstract highlights many of the conflicting concepts proposed in that era. In discussing
some of these, he does review the status of phosphatidylcholine (PtdCho),
phosphatidylinositol (PtdIns) and other phospholipid of significance in gustation. In a
section labeled Perspective, he notes, “Measurements of ΔV in monolayers have been
made for over 60 years and in bilayer membranes for over 20 years. However, the use of
these measurements for probing the regulation of biological phenomena has been
minimal.”
Maggio gave an even more extensive introduction relevant to gustation at nearly the
same time349. He opened with the observation concerning his subject, glycosphingolipids
(GSL), “A hallmark of the structure of GSLs is the extraordinary variety given by the different
number and type of carbohydrate residues that, joined by glycosidic linkages, are linked
to the hydroxyl group in carbon 1' of the ceramide moiety. While providing some valuable
background, much of the terminology used in these papers has been replaced in the
interim. There is considerable graphical data but a shortage of detailed schematics and
relationships.
Much of this early work involved less than ideally formed bilayer membranes. Transport of
heavy ions through less than perfect artificial biological membranes provides virtually
useless information.
Data on the dipole potential of fully elaborated gustaphores in solution is quite rare if not nonexistent. No concerted effort has been found to record this parameter as it relates to gustation.
Such measurements are commonly made on other organic molecules, particularly due to the
recent focus on liquid crystal display chemistry and the electrolytic character of the biological
membrane.
It may become necessary to adopt a more explicit name for the dipole potential
associated with a stimulant when in a conductive environment such as saliva. While the
dipole potential of the phospholipids of the GR sensed by the first Activa are constrained
by their liquid crystalline structure, that of the stimulant is not. The stimulant is nearly
surrounded by a conductive fluid. The dipole potential of interest here is that scalar
potential of the stimulant presented at the AH,B interface with the GR with respect to the
saliva surround.
Beitinger et al. have reported the dipole characteristics of a variety of gangliosides, some of
which may also be applicable to the globosides350. Their measurements were made using an
upper air interface and a lower water interface by floating their material on a water pool. They
note,
“Gangliosides are characteristic glycosphingolipids containing different numbers of
negatively charged sialic acids. These molecules being particularly: abundant in nerve cell
membranes of vertebrates are components of the outer bilayer leaflet and might be
intimately involved in various cellular biological events.”
“More than 2000 articles on gangliosides were published during the past decade, a lot of them with special
regard to the large hydrophilic sugar moiety and its great potential for hydrogen-bonding. These reports give
evidence that the polar headgroup determines the physicochemical properties of these molecules. These
reports give evidence that the polar headgroup determines the physicochemical properties of these molecules.
348
Brockman, H. (1994) Dipole potential of lipid membranes Chem Phys Lipids vol 73, pp 57-79
349
Maggio, B. (1994) The Surface Behavior of Glycosphingolipids in Biomembranes: A New Frontier of
Molecular Ecology Prog Biophys Molec Biol vol. 62, pp. 55-117
350
Beitinger, H. Vogel, V. Mobius, D. & Rahmann, H. (1989) Surface potentials and electric dipole moments
of ganglioside and phospholipid monolayers: contribution of the polar headgroup at the water/lipid interface
Biochim Biophys Acta, vol 984, pp 293-300
Signal Generation & Processing 8- 289
Several studies (t3C-NMR, ESR, X-ray diffraction) on the orientation of the sugar headgroup indicate that
the polar headgroup is relatively rigid, and in mixed phospholipid/ganglioside bilayers fully extended and
approximately perpendicular to the interface.”
“The formation of a monolayer can be measured as a change in surface pressure and
surface potential. The surface potential is proportional to the change of the normal
component of the dipole density with regard to the pure water surface. Numerous reports
exist of measurements with respect to the total surface potential and the overall dipole
moment, respectively, of simple giycosphingolipids and gangliosides.”
“The phospholipids used for the experiments have been obtained with a degree of purity
higher than 99%. The probes showed in each case a single spot in HPTLC and were used
without further purification.”
“In summary, we have given the sign and the magnitude of the potential drop across the
Water/lipid interface for different ganglioside, phospholipid and sulfatide monolayers. The
apparent dipole moment per ganglioside headgroup differs from other lipids. The most
remarkable result is that among gangliosides the size and the number of charges do not
cause large changes in the potential drop. Our potential data imply that the main purpose
of nature to vary in biomembranes the number of charges per ganglioside headgroup,
e.g., from one to three, is not to change the apparent potential drop across the headgroup
region.”
“The headgroup potential of the pure ganglioside monolayers reaches minus several
hundred millivolts which can influence the electrostatic properties of neuronal bilayer
surfaces strongly.”
While the Beitinger et al. data is not directly
applicable to gustation, it provides a very good
scenario of the applicability of their techniques to
such research. In some cases, their surface potential
may correspond to the relevant dipole potential of
the AH,B configuration gustaphores.
Their demonstration that many of the fatty acid side
chains of the phospholipids of interest are
conductive, and in fact exhibit a dipole potential is
of critical importance. Repeating some of their
experiments with dioleoylphosphatidylcholine
(DOPC), dipalmitoylphosphatidylethanolamine
(DPPE), dipalmitoylphosphatidylserine (DPPS),
dioleoylphosphatidic acid (DOPA), but using the
phospholipids defined in this work should be very
rewarding. Their values for DPPS may be directly
applicable to the PtdSer (the GR of the A-Path) of
this work.
Figure 8.5.10-1 Abridged Table II of
Beitinger et al., surface potentials of a
variety of relevant chemicals. See text for
abbreviations. See their extensive caption
for more information. From Beitinger et al.,
1989.
Figure 8.5.10-1 shows a portion of their Table II for a
pH of 7.4 and 20 ± 0.5 C. Note the sharp division
between the surface potential values associated
with their gangliosides and the phospholipids alone
(dashed line). The data at pH 7.4 is particularly
relevant to the gustatory modality. What they
describe as gangliosides appear to be a
combination of a phospholipid and a sugar as
opposed to their simple phospholipids (DPPE and
DPPS). GM1; monosialic ganglioside. GD1a and GD1b;
disialic gangliosides. GT1b; trisialic ganglioside. GMix;
a mixture of sialic gangliosides. C18; stearic acid.
Note the dipole potential order of the gangliosides
at their concentrations; GD1b>GM1>GD1a>GT1b.
290 Neurons & the Nervous System
The values given for the phospholipids are for single layers. When arranged in a bilayer, the
potentials oppose each other resulting in a significantly lower net value, i.e., a DPPE/DPPS bilayer
exhibits a net dipole potential on the order of 270 mV at a pressure of 30 milliNewtons/meter
(mN/m). A bilayer of exclusively DPPE gives a net dipole potential nominally equal to zero. The
value for C18, an 18 carbon atom fatty acid is included to illustrate how much of the dipole
potential of a single phospholipid can be attributed to the tail group as opposed to the head
group. Assuming the DPPE and DPPS employ the same tail group, the head group of the
phospholipid is the controlling factor in the net dipole potential of the monolayer.
8.5.10.1.1 Background related to the phospholipid structure of the GR
The Beitinger et al paper has been widely cited and spawned considerable research of interest
here.
Zheng & Vanderkooi provided an early introduction to the study of lipid bilayers in 1992351. They
open with a particularly important comment, “It has been known for several years that the
passive permeability of lipid bilayers is considerably greater for anions than for cations.” This
expresses a conventional chemical viewpoint and is based on the conventional wisdom drawn
from experiments with tetraphenylborate in two forms. They do note, “The bilayer translocation
rate and partition coefficient are both several orders of magnitude larger for the anion
tetraphenylboron (TPB-) than they are for the structurally similar tetraphenylphosphonium (TPP+)
cation.” Importantly, Zheng & Vanderkooi did not cite the theoretical and experimental work of
Anderson & Fuchw of 1975 related to the use of tetraphenylborate in their experiments and
discussed below. It should be made clearer that the transport of positive ions and negative ions
through a uniform bilayer membrane is virtually impossible while the transport of electrons alone
through such a membrane is a quite straight forward electrolytic process. The difference in the
rate of transmission of electrons in the two directions is due to the underlying mechanisms of
transport, leading to the well established concept of electrons and “holes.”
They have described their method of calculating the dipole potential of a bilayer of either
PtdCho or PtdEtn. They have provided a skeletal model of the molecules they are talking about
and the important statement, “The P-N vector in the polar head group does not lie parallel to the
bilayer plane as has often been claimed, but makes an angle of 150 with the bilayer plane in
DLPE (PtdEtn), and 90 and 250, respectively, in the two independent DMPC (PtdCho) molecules.
This gives the result that the z component of the head group dipole is nonzero with the negative
end of the dipole pointing toward the bilayer, thereby causing the head group contribution
to the internal potential to be negative.” Their model appears to omit the water molecules found
between the two bilayers but does include the water on the outer surfaces of their bilayers. See
Section xxx. Their figures 2 & 3 and table 1 describes their calculated potential fields within the
bilayer and these are similar to the measured values [xxx be specific and cite sources ]. Their
figures 1, 2 & 3 have been combined in Figure 8.5.10-2. The left frame illustrates their interpretation
of their 1,2-dimyristoyl-sn-glycero-3-phosphocholine (or PtdCho as abbreviated in this work). “The
P-N vector in the polar head group does not lie parallel to the bilayer plane as has often been
claimed, but makes an angle of 15 degrees with the bilayer plane in DLPE (τ in the Fig.), and 9
degrees and 25 degrees, respectively, in the two independent DMPC molecules. This gives the
result that the z component of the head group dipole is nonzero with the negative end of the
dipole pointing toward the bilayer, thereby causing the head group contribution to the internal
potential to be negative. The acyl esters make a positive contribution to the internal potential,
but the magnitude of this contribution is only one quarter to one third that of the polar head
group, as can be seen from Table 1.”
351
Zheng, C. & Vanderkooi, G. (1992) Molecular origin of the internal dipole potential in lipid bilayers:
calculation of the electrostatic potential Biophys. J Biophys Soc vol 63, pp 935-941
Signal Generation & Processing 8- 291
Figure 8.5.10-2 Lipid and bilayer chains from Zheng & Vanderkooi. Left; a nominally standard
representation of a phospholipid (they labeled DMPC) showing two different glyceride chains
and two distinct angles of interest. Center; their representation of a bilayer of DMPC:2H2O and
the electrostatic potentials associated with that bilayer. Right; their representation of DLPE:HAc
and the electrostatic potential associated with that bilayer. Note the variation in scales. See text.
From Zheng & Vanderkooi, 1992.
The center frame shows a bilayer of their DMPC hydrated on its external surfaces but not showing
the water normally found at the interface between the two DMPC layers in biological specimens
(Section xxx). Above the schematic are the potential plots for a unit charge brought to a position
adjacent to or interior to the bilayer. The left scales provide energy values typically used by
physicists and the right scales provide voltage potentials of more immediate interest here. The
right frame shows similar information for 1,2-dilauroyl-DL-phosphatidylethanolamine:acetic acid
(DLPE:HAc). Again, it does not show the water expected to be found at the interface between
the two bilayers. Note the different fatty acids used in their DLPE. The figure at lower right is
considerably more complex than that at lower center, suggesting a more complex mixture of
fatty acids associated with a less pure sample of DLPE. Zheng & Vanderkooi address this
complication in their methods and results sections.
Figure 8.5.10-3 reproduces their Table 1 for purposes of later comparison. While their calculated
values are not precisely applicable to the biological situation, they do describe relative values
that may be useful in the discussions in the following sections. The calculated values for just the
phospholipid groups are nearly an order of magnitude lower than that of other theoretical and
laboratory investigators. Their conclusions must be interpreted in the context of other
experimental results.
292 Neurons & the Nervous System
Majewski et al. have provided significant
information on the dipole potentials of a
single layer of a bilayer membrane based
largely on X-ray experiments352. Their figures
1 & 6 are reproduced here as Figure 8.5.104. Note the angle of the head group of the
PtdEtn in agreement with the comments of
Sheng & Vanderkooi. Their abstract notes,
at the concentrations and conditions
specified, “the two components do not
phase-separate and no evidence for
domain formation was observed. X-ray Figure 8.5.10-3 Atypical calculated components
scattering measurements reveal that GM1 is of the electrostatic potential at the bilayer
accommodated within the host DPPE midplane. See text. From Zheng & Vanderkooi,
monolayer and does not distort the 1992.
hexagonal in-plane unit cell or out-of-plane
two-dimensional (2-D) packing compared
with a pure DPPE monolayer. The oligosaccharide headgroups were found to extend normally
from the monolayer surface, and the incorporation of these glycolipids into DPPE monolayers did
not affect hydrocarbon tail packing (fluidization or condensation of the hydrocarbon region). This
is in contrast to previous investigations of lipopolymer-lipid mixtures, where the packing structure
of phospholipid monolayers was greatly altered by the inclusion of lipids bearing hydrophilic
polymer groups.”
Note carefully; the group labeled GM1 is distinctly different than the group labeled DPPE.
C The upper fatty acid of GM1 terminates with an NH ligand while the upper fatty acid of DPPE
terminates with an Oxygen atom.
C The lower fatty acids are significantly different in the two cases.
C The ganglioside of GM1 has replaced a very critical ligand incorporating the PO3 group and the
NH3 group, both highly polar in character.
C In their lower frame, a molecular schematic is shown containing a five-sided ring that is not
present in their upper schematics or discussed in their text. [xxx check ]
C The schematic in the lower frame shows a mixture of two diglyceride materials in a ratio of 4:2.
As a result, what they describe as a ganglioside is actually a glycan/diglyceride, a member of the
cerebroside family and not generally derived from a phospholipid, whereas their DPPE is a
phospholipid containing different glycerides. Their paper includes much other data. However,
when discussing gustation, these differences make comparing their data with other data in the
literature difficult.
352
Majewski, J. Kuhl, T. Kjaer, K. & Smith, G. (2001) Packing of Ganglioside-Phospholipid Monolayers: An
X-Ray Diffraction and Reflectivity Study Biophys J vol 81, pp 2707–2715
Signal Generation & Processing 8- 293
Figure 8.5.10-4 Chemical structure of DPPE & GM1 (a specific PtdEtn & a “ganglioside”). Top; the
chemical structure of the two materials. Bottom, the schematic of the mixed materials “DPPE”
and “GM1.” in a ratio of 4:2 at an air-water interface. The “gangliosides” incorporate a five-sided
ring not present in the upper schematics. See text. From Majewski et al., 2001.
Peterson et al provided information on the dipole potential of a variety of phospholipids
incorporating various fatty acid chains, including their test configuration and specific information
294 Neurons & the Nervous System
regarding substitutions of sulfur along the fatty acid chains of DPPC353. Their focus was on the
electrolytic properties of a single monolayer as they might affect protein binding to the material.
Their actual results are of little apparent value here. Figure 8.5.10-5 reproduces their Table 1. “The
absolute dipole potential of unlabeled and sulfur-containing DPPC membranes, φd, was
measured by the CR method (Pickar and Benz, 1978). The values recorded for pure lipids (DPPC,
DHPC, GMO; Table 1) were found to be in good agreement with values reported earlier (Pickar
and Benz, 1978; Gawrisch et al., 1992). φd of DPhPC was estimated to be (228± 5) mV.” The top
two entries in this table indicate the range of the dipole potential achieved by merely varying the
character of the fatty acids forming the diglyceride tail. Both of these entries would be labeled
phosphatidylcholine (PtdCho) in most academic journal papers.
Shamberger & Clarke provided
additional information concerning
the dipole potential of a single
phospholipid layer and its use in a
bilayer similar to the biological
equivalent354.
They addressed
several questions and inconsistencies
regarding previous theory of,and
laboratory values for dipole
potentials related to a membrane.
They conclude with a very important
statement, “The electric field
produced by the dipole potential in
the membrane interface is extremely Figure 8.5.10-5 The dipole potential of different lipids
large, i.e., 108 – 109 V/m (Brockman measured with the CR method See text. From Peterson et
1994; Cafiso, 1995; Clarke, 2001), al., 2002.
which is significantly larger
than that caused by a typical total
membrane potential (e.g., a 100-mV membrane potential produces an electric field stength of
~2.5 x 107 V/m).” Restated, the internal dipole potentials related to a biological bilayer are far
larger than the net dipole potential of the package.
Their conclusions in 2002 are important. After discussing the use of tetraphenylarsonium (TPA ) and
tetraphenylborate (TPB) as potentially positive and negative ions capable of penetrating
biological membranes, they noted “The dipole potential values determined in this way are
generally 100 to 200 mV higher (Hladky and Haydon, 1973; Beitinger et al., 1989; Smaby and
Brockman, 1990) than those previously determined using hydrophobic ions on lipid bilayers. For
dioleoylphosphatidylcholine (DOPC), for example, Beitinger et al. (1989) have determined values
of 420 and 431 mV at pH 7.4 using two different buffer systems. For egg yolk lecithin (predominant
component DOPC) Hladky and Haydon (1973) determined a value of 441 mV. The conductance
measurements of Pickar and Benz (1978) using hydrophobic ion yielded, on the other hand, a
value of 224 mV for DOPC (Table 3). This discrepancy between bilayer and monolayer values of
the dipole potential has been known for many years, but as yet no generally accepted
explanation for it has been found.”
They conclude relatively optomistically, “Although the absolute value of the dipole potential can
still not be precisely defined, due to the uncertainty in the calculated values of the hydration
energies of the hydrophobic ions, the calculations carried out here demonstrate that relatively
small differences in the hydration energies of TPB , TPP , and TPA can easily account for the
353
Peterson, U. Mannock, D. Lewis, R. Pohl, P. McElhaney, R. & Pohl, E. (2002) Origin of membrane dipole
potential: Contribution of the phospholipid fatty acid chains Chem Phys Lipids vol 117, pp 19–27
354
Schamberger, J. & Clarke, R. (2002) Hydrophobic Ion Hydration and the Magnitude of the Dipole Potential
Biophys J vol 82, pp 3081–3088
Signal Generation & Processing 8- 295
differences between dipole potential values previously reported from monolayer and bilayer
measurements.” Their discussion did not demonstrate the validity of this statement.
TPA amd TPB are very large ions, typically 10 Angstrom diameter based on Jmol 3D
representations as opposed to the calculated sizes they provided in their Table 1. They did
attempt to rationalize the 25% difference in their values calculated using their volume data versus
their area data. No physical evidence of pores on the order of 10 Angstrom have been reported
in the biological literature. In the absence of identifiable pores, the likelihood of their penetration
of a natural, or properly prepared analog of a, bilayer membrane is extremely unlikely. Anderson
& Fuchs have performed a much more comprehensive study of tetraphenylborate in lipid
bilayers355. Their experiments focus on the transient “transport” of charges through lipid bilayers.
They did not demonstrate any net flow of these materials all of the way through the membrane.
In 2005, Elana Pohl appears to have repeated much of the material in the 2002 Peterson et al.
paper as part of a review356. The paper appeared in an obscure publication that is difficult to
locate. The Google Scholar leads to a publisher’s expanded Abstract that includes a figure 3
requiring an unidentified pore to explain any transport of potassium through the membrane.
The above summaries show the complexity and the exploratory character of recent work in this
area. No substantial results have been shown indicating that physical ions, particularly of the
heavy alkali or alkali metal atoms, are able to pass through the lemma of a sensory neuron as
part of the gustatory process.
8.5.10.1.2 Background related to the DP of a stimulant in saliva
With the recent appearance of high quality 3D molecule modeling, many of the better programs
are now capable of presenting electrostatic fields created by a stimulant. These fields are
generally limited to the free-space condition and are not necessarily indicative of the molecule
when present in solution. While saliva and water may not normally be considered good electrical
conductors, they are at the impedance level associated with the neural system in general, and
the gustatory modality in particular.
Thus, the dipole potential for a stimulant is that potential presented at the AH,B interface with the
appropriate GR when surrounded by saliva acting as the local electrical ground environment.
A conceptually similar interface is shown in figure 2a of Venanzi & Venanzi (1992) for amiloride.
The electrostatic fields (of given strength) predicted by these computational chemistry programs
generally look like balloons slightly larger than but generally following the contours of the atoms
of the stimulant molecule as in Figure 8.5.10-6 for aspartame. With distance (and resulting lower
field strength), the balloons become spherical. While aspartame is a man-made sweetener, it is
not considered a super-sweetener. The dimensions in this figure are A–B = 2.91 Angstrom, B–X =
5.359 Angstrom, A–X = 4.426 Angstrom and angle A = ~90 degrees. It should be noted that the
charge pattern is not defined by points but by loci. However, the stimulant will not form the
required DACB relationship unless its actual AH,B spacing is within the tolerance required for this
parameter by the GR. The tolerance on the locus of the dispersion centroid, C, has not been
determined but in the case of aspartame, its C is probably outside of the acceptable locus for
the sweet GR.
355
Andersen, O. & Fuchs, M. (1975) Potential Energy Barriers to Ion Transport Within Lipid Bilayers Studies
with Tetraphenylborate Biophys J vol 15, pp 795-830
356
Pohl, E. (2005) Dipole Potential of Bilayer Membranes Advances Planar Lipid Bilayers Liposomes
vol 1, pp 77–100
296 Neurons & the Nervous System
Signal Generation & Processing 8- 297
298 Neurons & the Nervous System
Figure 8.5.10-6 Electrostatic field of aspartame showing the AH,B,X relationship in this man-made
sweetener. Free space representation. See text for dimensions. The charge cloud associated
with the hexane ring is shown in a light pink in the lower frame. Drawn using DS3.5 & aspartame
from http://kaist.ac.uk .
Signal Generation & Processing 8- 299
In the presence of a conducting surround, the electrostatic field external to the molecule is
forced to conform to the geometrical shape of the solvent/molecule interface except where the
stimulant molecule is in close proximity to a relevant GR or other non-solvent structure.
8.5.10.1.3 Background related to the dispersion centroid relative to AH,B,X ADD
Van der Heijden and his team recognized early in their work that the location of the dispersion
centroid, X in the AH,B,X relationship between a stimulant and its appropriate GR was not likely
to be a distinct point357. “To describe stereochemical requirements more precisely, new
conceptual
parameters were introduced, namely α, δ and ω (minimum, optimum and maximum distances
between these third binding sites and the atoms A, H and B of the AH-B moieties respectively,
especially appropriate for homologous series) and the S value (shortest distance between the
position of an atom and the plane formed by the atoms A, H and B of the AH-B moiety).
On page 58 of the paper (1985a), the van der Heijden team had stressed that their set of
sweeteners was not :pure;
“The literature on the five series of sweeteners (nitroanilines, sulphamates, oximes,
isocoumarins and dipeptides) was searched for sweetness values and for details as to
whether these substances are denoted as tasteless or bitter, because in the latter situation
their possible sweetness potency may be masked by the bitter taste.”
Many of these chemical classes also exhibit other types of gustaphores.
357
Van der Heijden, A. van der Wei, H. & Peer, H. (1985a) Structure-activity relationships in sweeteners. I.
Nitroanilines, sulphamates, oximes, isocoumarins and dipeptides Chem Senses vol 10(1), pp.57-72
300 Neurons & the Nervous System
[xxx following para need editing for continuity and overlap ]
Van der Heijden et al, 1985a, provided a 3D histogram for a group of sweeteners that was further
perfected in the 1985b paper and reproduced there and in the 1993 paper. It was supported
by considerable calculations based on their exploratory investigations. Figure 8.5.10-7 shows
possible alternate interpretations of their 1993 histogram. Focusing only on the glycophores; the
principle axis of interest is drawn through the points A and B, and the median of the distance
between point A and point B is taken as 0.282 nm (2.82 Angstrom). Using the old axes for
simplicity, instead of performing a rotation of the data set (Section 8.5.2.3 xxx on MDS), it appears
there is an association between the data points and a plane near 45 degrees to the original axes.
It is proposed the angle should be defined relative to the plane of the DACB relationship between
the two pairs of orbitals shown. The long alternate axis and the four orbitals remain in the x-y
plane of the original axes (but are rotated by about 23 degrees). This interpretation is similar to
that illustrated in [Figure 8.5.5-8 of Section 8.5.5.1]. That figure suggests the four orbitals in the
DACB relationship are generally perpendicular to the plane of the glycophore and the GR. The
distance and angle to the point X then suggests the loci of the most sensitive area of the GR to
dipole potential modification.
The specific stimulants included in this data
set are not material here but are detailed in
the 1985a and 1993 papers. In general, they
are “1, sulphamates (site 1); 2, isocoumarins
(site 2); 3, oximes + nitroanilines (site 3); 4,
dipeptides (site 3).
As noted by the van der Heijden team, the
location of the dispersion point is best
described by a loci centered on the
locations in their 3D histograms. They did not
indicate that their specified points were
statistically precise with small standard
deviations.
The 1985b paper extended their
investigation358; “The previously introduced
conceptual parameters (α, δ, ω and S) to
describe the stereochemical requirements
for organic compounds to taste sweet, were
now applied to another series of sweeteners
and to some well-known potent
substances.” They also provided a selection
of “recalculated” dispersion point locations
for gustaphores in general ( page 79). Their
abstract closes with the interesting assertion,
“It is remarkable that the average δ positions
belonging to sweeteners with similar AH-B
moieties are located very close to each
other.”
Figure 8.5.10-7 3D histogram of centroid of X for a
group of stimulants. Note potential alternate axes
and nominal 45o angle of the plane containing a
majority of the points (loci) to the notional plane
of the DACB relationship. The distance between
A and B’ in this relationship is 0.27 nm. All
distances in nm. See text. Modified from van der
Heijden et al., 1993
In 1987, the Van der Heijden team provided
a major review of their earlier work359. Their studies are now dated but are clearly suggestive of
and compatible with the concept of a locus rather than a point describing the sensitivity of a GR
to a charge associated with a gustaphore. They did not clearly differentiate between stimulants
358
Van der Heijden, A. van der Wei, H. & Peer, H. (1985b) Structure-activity relationships in sweeteners. II.
Saccharins, acesulfames, chlorosugars, tryptophans and ureas Chem Senses vol 10(1), pp 73-88
359
Van der Heijden, H. van der Heijden, A. Peer, H. (1987) Sweeteners Food Rev Internat vol 3(3), pp 193-268
Signal Generation & Processing 8- 301
to the four distinct GR’s and resultant gustatory paths. They also defined a plane determined by
the three atoms A, H & B and provided a distance S for their dispersion points from that plane.
Their concept requires more study as to whether their concept applied to a stimulant when in an
AH,B,X relationship with a GR involving a DACB with the appropriate GR or whether it applied to
only the stimulant. It is not clear there is any relevance of the location of the H in determining a
relevant plane, and even where this atom is at a given instant.
It may require further analysis to confirm or deny the following points;
• The pair of orbitals labeled A & B probably relate to the glycophore but might relate to the GR.
• The loci described in the above discussion may relate to an electron-rich or electrophobic area,
such as a dual bond carbon or a ring structure, rather than a single atom.
• The loci described for the glycophore may not correspond to the location of the feature of the
corresponding GR. The loci may describe a location that is pushing the charge pattern of the GR
from a location near its side.
8.5.10.2 Results of the net change of a DP due to a AH,B type stimulant
[
Section 8.5.1 presented both a physical and electrolytic schematic of the first amplifier (Activa)
within a stage 1 sensory neuron dedicated to gustation. Section 8.5.5.1 included a more detailed
circuit schematic highlighting an “capacitative” relationship between the gustaphore and the
GR.
Figure 8.5.10-8 expands on the electrolytic schematic to show the proposed stimulation by a AH,B
type stimulant, applicable to all four of the gustatory paths. The right-most molecular group of the
type 4 outer lemma and the AH,B type stimulant have been expanded to allow a more detailed
discussion.
First, note the net potential between the saliva and the hydronium channel between the bilayers
is quite small (a nominal 24 mV). This small potential is due primarily to the opposing potentials of
the two phospholipids forming the outer layer of the bilayer lemma. The precise potential requires
detailed knowledge of these two phospholipids and the characteristics of the receptor ligand
(or gustatory receptor, GR). As noted by xxx, merely describing these molecules as phospholipids
is inadequate. The precise character of the glycerides in their tail structure must be known if the
precise dipole of this structure is to be specified.
The net dipole potential is also dependent on any hydrated water forming a boundary layer on
the active surface of the GR.
It is suggested here without justification that one of the purposes of the saliva is to act as
a wetting agent for food brought into the oral cavity. In this role, it would also act as a
wetting agent to the tissue of the oral cavity and effectively eliminate any hydrated
boundary layer in the position indicated by the dotted box at upper right. If the above
suggestion is accepted, this box and any contribution to the net dipole potential due to
water associated with the boundary layer adjacent to the GR surface can be eliminated
from further discussion.
Note the phospholipid facing the saliva is a phospholipid esterified to the GR. It is not a
ganglioside as investigated by Beitinger et al. and some later investigators. The presence of the
phosphate moiety probably plays a major role in determining the dipole potential of the outer
layer and insuring the inner layer and outer layer together exhibit a small net dipole potential.
If the outer phospholipid is a mirror image of the inner phospholipid, and there is no boundary
layer at the active surface of the GR, the small net potential of 24 mV as shown could be due
entirely to the dipole potential of the GR when esterified to the phospholipid. In the case of the
A-Path, this would be the dipole potential of serine (Ser) when esterified to Ptd.
Assuming the quiescent potential of –24 mV is due primarily to the dipole potential of the GR, it
becomes clear that upon forming a DACB with a stimulant as indicated by the O–H- - O bonds
at upper right, the net dipole potential of the pair would be reflected in a change in the
quiescent potential at the base of the Activa. If the net change was to a more positive potential,
302 Neurons & the Nervous System
the current through the Activa would be increased as indicated in the input characteristic of the
Activa shown at lower left. This would result in an increased current through the type 2 lemma
forming the Activa as represented in the lower right output characteristic.
Figure 8.5.10-8 Detailed electrolytic sensory neuron operation in AH.B situation. The last
phospholipid/GR/stimulant on the rigth in the type 4 region has been expanded to show the
details required. The characteristics at lower left show the operating performance of the Activa
shown at upper left. See Text.
This operating methodology is easily defended using the Electrolytic Theory of the Neuron and
is far more parsimonious than any equivalent chemical theory of gustation. No unknown, or undemonstrable chemical reactions are required. In addition as noted above, the temporal
response of the excitation/de-excitation process from a kinetics perspective is limited to the
formation and breaking of a coordinate chemistry bond. Such bonding and unbonding are
quantum-mechanical events requiring a higher level of differential equation, than a first order
kinetic equation, to describe their temporal characteristics.
Note that a very highly polar stimulant at high concentration can cause a saturation within the
affected gustatory pathways. The subject will not be able to perceive the quality of such a
stimulant compared to similar stimulants until its concentration is reduced and the signal pathway
returns to its normal operating regime.
Signal Generation & Processing 8- 303
To appreciate the perceived difference between a family of similar stimulants, the use of a
glycan similar to that shown in [Figure 8.5.10-4] that does not hydrolyze when introduced into the
saliva is particularly attractive. A glycan can exhibit only a single glycophore while simultaneously
exhibiting a dipole potential determined by the number of sugar residues present in its overall
structure and their arrangement. As Majewski et al. note, the sialic acid sugars are highly polar.
8.5.10.3 Details of proposed net change in DP by AH,B,X stimulants
It has long been known that both complex glycophores and complex picrophores represent
super gustaphores. In particular, the artificial sweeteners represent glycophores that are
extremely effective in stimulating the G-path GR’s. It has been established by Kier and by
Shallenberger et al. that these sweeteners exhibit an electrostatic potential at a location about
5 Angstrom away from the AH,B group and at an angle to the perpendicular line bisecting the
distance between the two orbitals. It has been proposed that this electrostatic potential can
cause a change in the electrostatic potential within the GR when brought into close alignment
to the GR as a result of DACB. As noted above, the challenge is to locate the position of the
“charge” near the surface of the glycophore and how it impacts the GR. It appears the super
sweeteners can utilize an electrostatic charge that can vary witin a range not yet determined.
However, the range encountered in the known super-sweeteners suggests the location of the
“dispersion point” centroid is within the GR and not more remote as in the phosphate group of
the phospholipid esterified to the GR.
[xxx combine next two paragraphs ]
Noting the gross arrangement of the gustaphore and phospholipid in [Figure 8.5.5-2] and the
potential impact of the gustaphore on the electrostatic potential of the phospholipid in [Figure
8.5.10-2]. the tail-to-tail arrangement of the two phospholipids of a bilayer lemma leads to a
bucking circuit where the two phospholipids oppose each other. . Each lipid may exhibit a
potential on the order of 250 mV but the net potential across them applied to the base of the 1st
Activa in the absence of stimulation may be on the order of 24 mV. As noted above, stimulants
typically cause a positive change in the net potential at the base of the 1st Activa. If the stimulant
should also cause a significant positive going change in the dipole potential of the outer bilayer,
the total net change at the base of the 1st Activa would be considerably enhanced.
The conclusion can be drawn that the electrostatic potential of the super-sweet glycophore can
cause a significant change in the dipole potential of the GR alone and the resultant net change
in the dipole potential presented to the base of the Activa is a combination of the change in
dipole potential represented by the glycophore alone and the change in dipole potential of the
GR caused by a change in the charge density near the dispersion centroid. This change is
illustrated conceptually in Figure 8.5.10-9. The labels on the left apply equally to the features on
the right. The change in the potential applied to the Activa base in the AH,B configuration is due
entirely to the change in dipole potential (a vectorial sum) due to the coupling of the GR to the
potential of the stimulant, VGS1. In the AH,B,X configuration, the change in the net potential can
involve a change in the dipole potential of the GR, ΔVGR and the dipole potential of the stimulant,
VGS2 added vectorially. This change can be so large that the concentration of the stimulant must
be reduced to maintain operation of the gustatory modality in the linear range.
304 Neurons & the Nervous System
----[xxx add introductory sentence ]
Sugars have many polar hydroxyl groups
(–OH) and are overall highly polar. One of
the –OH groups is usually found in a –CH2OH
group. Calculation of the overall dipole
potential of the sugars is complicated. If a
significantly polar group of another
molecule is brought close to one of the
hydroxyl groups, the electrical charge
distribution near the hydroxyl, and therefore
the dipole potential of the sugar, can be
changed significantly. This appears to be
phenomenon brought into play by the
“super sweeteners.” The optimum sugar
moiety for use in the receptor will have its
various hydroxyl groups in optimum location
relative to the AH,B group. Determining this
degree of optimization requires careful
examination of a 3-D representation of the
sugar. If located appropriately, the moiety
can be optimally esterified with
phosphatidic acid to provide the desired
optimum receptor. [xxx cite or consolidate
with the figure “Concept: a stacked
situation. . . “
Figure 8.5.10-9 The schematic difference between
the AH,B and AH,B,X configurations during DACB
binding. Left; AH,B stimulation as illustrated in the
previous figure. Right’ the enhanced stimulation
associated with the AH,B,X configuration. The
proximity of the locus of charge concentration of
the gustaphore to the dispersion centroid of the
GR is a critical parameter.
The most likely sweetness receptor appears
to be phosphatidyl galactose based on its
known presence in many situations involving
lemma. Figure 8.5.10-10 shows galactose
projected on a plane including the axis
between the presumed AH,B participants,
O-3 and O-4. It shows two triangles that are
not in this plane. There dimensions are
therefore distorted in the figure. [xxx need to
give distances for these two triangles. ] To
be effective in the AH,B,X relationship, the
three points of each triangle must be
accessible by a potential stimulant (no
significant other structure interfering in the
plane of the triangle). It appears galactose
can accommodate two different AH,B,X
relationships within the present tolerances on
the required dimensions of the triangles.
Both the ring oxygen and O-2 are
susceptible to an applied electronic force
from the stimulant, thereby changing the
dipole potential of the overall phosphatidyl
galactose molecule. This makes galactose
a preferred candidate for the super sweet
(as well as the sweet) receptor.
Sucralose, a super sweetener (600:1 relative
to glucose), is a disaccharide wherein the
most likely glycophore involves the O–2 and
Figure 8.5.10-10 Galactose in the plane of O-3 &
O-4. Two potential AH,B,X triangles are shown. O1 is the normal site for esterification to the
phosphatidic acid. See text.
Signal Generation & Processing 8- 305
O–3 atoms since O–4 has been replaced by chlorine.
Van der Heijden addressed the question of the AH,B,X configuration in considerable detail in
1985360,361. He continued to use the line between A and H as his baseline rather than the A,B
baseline preferred here. (see figure 8.5.3-3) His figure 2 in the first paper may be important. He
shows the location of the field of interest at distance X may lie at a distinct distance from any
specific atom, suggesting it may be a specific location in the molecular electrostatic potential
(MEP) that is critically important. The papers are discussed in more detail in Section 8.6.10.
In using the A,H baseline, Van der Heijden addressed several cases where there was no second,
B, orbital present. This introduces an entirely different framework from that of Shellenberger and
colleagues and the extension of their concept pursued here.
8.5.10.4 Details of proposed of sweet protein DP measurements
The sweet proteins are known to be highly charged and polar. In the case of brazzein, it is likely
that many of the substitutions that have been performed in the laboratory have cause significant
changes in the dipole potential of these stimulants; where the dipole potential is the electrolytic
potential introduced at the GR caused by the potential difference between the region of the
glycophore orbitals and the rest of the stimulant relative to the solvent.
[xxx cover adjustments due to location and specific residue selection along remainder of chain.
[xxx review Assadi papers on charge ]
[xxx address glu36 here
8.5.11 Correlating genes to gustatory receptors
Dahanukar et al. have made some important observations concerning the gustatory genes of
Drosophila in an extensive paper362. They have associated Gr5a with the sensing of a
disaccharide sugar, trehalose. Such disaccharides are typically broken down before sensing in
gustation. They also noted, “Gr5a-labeled neurons are responsive not only to trehalose, but to
sucrose and other sugars.”
Subsequently, they find that, “Gr5a is required for detection of a small subset of sugars including
trehalose. We generate deletion mutants lacking Gr64a and find that it is required for response
to a complementary subset of sugars. Strikingly, flies lacking both Gr5a and Gr64a do not show
electrophysiological or behavioral responses to any tested sugar. These results demonstrate that
the sugars divide into two classes that are dependent either on Gr5a or on Gr64a for their
responses.” They go on, “Classic physiological and biochemical studies led to the proposal of a
‘‘fructose’’ site in sugar-sensing neurons. Our studies provide a molecular and genetic identity
to this site: fructose response is completely abolished by loss of Gr64a and is completely restored
by the addition of a Gr64a transgene.”
Whether humans involve two separate sweetness
receptors is not yet clear. Either or both of these genes can be considered directly associated
with the proposed human sweetness receptor, proposed here to be PtdGal, phosphatidyl
galactose.
They have associated gene Gr66a with the bitter sensory channel. This gene can be directly
associated with the proposed bitter receptor, Ptd3'Og, phosphatidyl 3'-O-aminoacyl glycerol.
360
Van der Heijden, A. van der Wei, H. & Peer, H. (1985) Structure-activity relationships in sweeteners. I.
Chem Sens vol 10(1) pp 57-72
361
Van der Heijden, A. van der Wei, H. & Peer, H. (1985) Structure-activity relationships in sweeteners. II.
Chem Sens vol 10(1) pp 73-88
362
Dahanukar, A. Lei, Y-T. Kwon, J. & Carlson, J. (2007) Two Gr genes underlie sugar reception in drosophila
Neuron vol 56, pp 503–516
306 Neurons & the Nervous System
8.5.11.1 Rationalization of the lipid versus protein versus sugar debate in gustation
EMPTY
8.5.12 Clinical (medical) disorders affecting taste
Bromley and Doty, writing in Doty, have illustrated the limited understanding of the taste modality
from a clinical perspective363. They list over a dozen medical specialties involved in trying to
understand taste disorders. They list nearly a dozen clinically recognized subdisorders and about
forty medical conditions contributing to or mediating taste disorders. They offer no matrix or other
technique leading to the underlying causes of these disorders.
Costanzo et al., in the following chapter, discuss head injuries and taste, concluding that the
predominant reports of loss of taste through head injury actually involve the more easily
explained loss of olfactory sensation.
Shallenberger described two patients being treated for hypoparathyroidism that were unable
to perceive the sensation of sweetness (Section 8.5. 1.1.6). Assuming the glandular diagnosis was
correct, the lack of ability to form PtdGal may account for a common problem among the
geriatric population, particularly women. Functional loss of the GR 2 receptor could result in the
sugars being perceived as sour, bitter, and potentially salty as well; it would depend on the width
of the efficacy function of these other receptor channels.
8.5.13 Man-made taste sensors EMPTY
Julia Tsitron et al364.have recently reported on a Bayesian model of a man-made gustatory sensor
focused on the reception of only four chemicals related to glucose. [See reference papers xxx.
]
8.5.14 Overview of the complete gustatory modality–stage 2 and higher ADD
8.5.14.1 Gross gustatory signal paths within the human cerebrum
Yamamato has provided good material on the gross organization of the stage 4 signal paths
within the human brain, and associated paths in several mammals365. The material is extensive
and will not be reviewed here. He writes using the global expression cortical gustatory area
(CGA).
Figure 8.5.14-1 shows the general areas associated with gustation in the cerebrum. It is not clear
whether these are the initial stage 4 information extraction areas or later association areas. The
mappings are old, dating from the 1930-40's.
363
Bromley, S. & Doty, R. (2003) Clinical disorders affecting taste: evaluation and management In Doty, R.
ed. Handbook of Olfaction and Gustation. NY: Marcel Dekker Chapter 44
364
Tsitron, J. Ault, A. Broach, J. & Morozov, A. (2011) Decoding Complex Chemical Mixtures with a Physical
Model of a Sensor Array PLoS Comput Biol vol 7(10): e1002224. doi:10.1371/journal.pcbi.1002224
365
Yamamato, T. (1989) Role of the cortical gustatory area in taste discrimination In Cagan, R. ed. Neural
Mechanisms in Taste Boca Raton, FL: CRC Press Chap. 9
Signal Generation & Processing 8- 307
“A; Cortical areas eliciting orolingual sensations following electrical stimulation. Solid circles
indicate the point where electrical stimulation elicited taste sensations. Encircled area with
stripes, tongue sensory area; area within dashed line, mouth sensory area. B; Three clinical cases
of brain damage showing taste impairment: (1) bullet wound which elicited ageusia to four basic
taste qualities, (2) bullet wound which elicited hypogeusia, mainly to sweet and sour tastes, (3)
hematoma which elicited hypogeusia, mainly to salty and bitter tastes.”
His figure 4 conceptual representation does not recognize the role of Lewis acids in gustation and
follows the conventional view of the time in using the inorganic nocent, HCl, as the reference for
the acid channel of gustation.
8.5.15 Confirmation of the hypothesis
EMPTY
8.5.15.1 Role of inositol as an organic
taste enhancer
Stone & Oliver has noted the remarkable
ability of the organic molecule, inositol, as a
“taste enhancer.366” This observation is
strong confirmation of the choice within this
theory to adopt muco-inositol as esterified to
phosphatidic acid as the primary receptor
of the N-Path sensory neurons.
This places inositol, as a gustaphore, in the
optimum position to form a dimer with the
sensory receptor.
This dimer exhibits
minimum stress in its d-value relative to the
optimum d-value of the receptor and insures
maximum transduction efficacy for this
gustaphore.
[xxx 8.5.1.1.1, 8.5.1.6.7, 8.5.3, 8.5.4.4 and Fig
8.5.4-8 before condensation ]
End Section 8.5
Figure 8.5.14-1 Gross stage 4 gustatory information
extraction areas in humans ADD. See text. CS;
central sulcus.
SS; Sylvian sulcus.
From
Yamamato, 1989.
366
Stone, H. & Oliver, S. (1966) Beidler’s theory and human taste stimulation Percept Psychophysics vol 1, pp
358-360
308 Neurons & the Nervous System
Table of Contents 1 August 2016
8 Stage 1 & 2, Signal Generating & Processing Neurons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
8.5 The gustatory modality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
8.5.1 Background for and summary--the gustatory modality hypothesis . . . . . . 3
8.5.1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
8.5.1.1.1 Historical documentation . . . . . . . . . . . . . . . . . . . . . . . . . 3
8.5.1.1.2 Major problems with the RSC Jmol & JSmol Libraries . . 6
8.5.1.1.3 How have the taste sensations been defined? . . . . . . . 7
8.5.1.2 Anatomy of the peripheral portion of the gustatory modality . . 8
8.5.1.2.1 Morphology of the gustatory modality . . . . . . . . . . . . . 10
8.5.1.2.2 The morphology of the taste bud & sensory neuron
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
8.5.1.2.3 The structure of the “sweet” lemma of the microvilli
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.5.1.3 Electrophysiology of the sensory neurons . . . . . . . . . . . . . . . . . . 22
8.5.1.3.1 The electrophysiology of the “sweet” gustatory sensory
neuron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
8.5.1.4 Initial block diagram of the modality . . . . . . . . . . . . . . . . . . . . . . 24
8.5.1.4.1 A proposed top level architecture of the gustatory
modality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
8.5.1.5 The chemistry most important to gustation . . . . . . . . . . . . . . . . . 26
8.5.1.5.1 The chemical families of carbohydrates involved in
gustation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
8.5.1.5.2 Chemical families pertinent to gustatory receptor
identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
8.5.1.5.3 The special case of the saturated aliphatic alcohols and
aldehydes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
8.5.1.5.4 The unique role of the hydrated sodium ion in gustation
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
8.5.1.5.5 Inorganic acids and astringents excite the nocent (pain)
modality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
8.5.1.5.6 Definition of specific stereo–molecular structures . . . . 31
8.5.1.5.7 Equilibrium in the context of gustation–a brief review
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
8.5.1.5.8 The change in free energy associated with a DACB –a
brief review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
8.5.1.5.9 The primary question regarding the transduction
mechanism of gustation . . . . . . . . . . . . . . . . . . . . . . . . . . 37
8.5.1.6 Summary of the gustatory modality hypothesis . . . . . . . . . . . . . . 37
8.5.1.6.1 Defining the gustatory perception space . . . . . . . . . . 45
8.5.1.6.2 A 3D olfactory perception space with calibrated scales
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
8.5.1.6.3 The proposed qualitative 3D gustatory perception space
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
8.5.1.6.4 An equivalent quantitative 3D olfactory perception
space EMPTY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
8.5.1.6.5 The gustatory response versus molarity of stimuli . . . . 59
8.5.1.6.6 The structural constraints on tasting sweet . . . . . . . . . . 60
8.5.1.6.7 Extending the chemoreception concepts of
Shallenberger, Kier and Beets . . . . . . . . . . . . . . . . . . . . . 61
8.5.1.6.8 Proteins as stimulants and/or gustaphores . . . . . . . . . . 63
8.5.1.6.9 Tests of the Electrolytic hypothesis of gustation . . . . . . 63
8.5.1.7 A dichotomy: the labeled-line and across-neuron-pattern theories
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
8.5.1.8 Initial identification of human genetic differences . . . . . . . . . . . 66
8.5.1.9 Renewal of the gustatory sensory neurons . . . . . . . . . . . . . . . . . . 66
8.5.2 Analysis of perceived gustatory sensations–MDS and other techniques
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
Signal Generation & Processing 8- 309
8.5.2.1 Dendrographic representation . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
8.5.2.2 ROC analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
8.5.2.3 Employing MDS techniques in understanding gustation . . . . . . 69
8.5.2.3.1 Background relative to the MDS technique–map ,making
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
8.5.2.3.2 Dimensionality–selecting the number of dimensions
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
8.5.2.3.3 Multidimensional scaling applied to gustation . . . . . . 76
8.5.2.3.4 Strange representations due to dimension reduction
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
8.5.2.3.5 2D MDS representation applied to gustation . . . . . . . 79
8.5.2.3.6 Initial considerations related to entropy in gustation
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
8.5.2.4 Changes in multidimensional presentation EMPTY . . . . . . . . . . . 80
8.5.2.4.1 Rotation and displacement of MDS axes . . . . . . . . . . . 80
8.5.2.4.2 Conclusions from analysis of the data . . . . . . . . . . . . . 80
8.5.2.4.3 The basis functions of gustation . . . . . . . . . . . . . . . . . . . 85
8.5.2.4.4 The proposed human sensory space of gustation . . . 88
8.5.2.4.5 The family of Neural Response Functions . . . . . . . . . . . 89
8.5.2.5 Conclusions from analysis of the MDS technique & examples EMPTY
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
8.5.2.5.1 The representation of an MDS dataset in the preferred
form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
8.5.2.5.2Failure to accommodate hidden variables in MDS
representations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
8.5.3 The 2-step hypothesis of gustatory transduction . . . . . . . . . . . . . . . . . . . . . 92
8.5.3.1 Previous theories of gustatory transduction . . . . . . . . . . 95
8.5.3.2 The AH,B & AH,B,X coordination chemistry of the gustatory
channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
8.5.3.3 Proposed transduction mechanism in gustation . . . . . 102
8.5.3.4 Proposed selection mechanism for “desireable/sweet”
RENAME . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
8.4.4.6xxx Sugars as potential GR’s in gustation . . . . . . . . . . . . . . . . . . . 110
8.5.4 The initial selection operation of the gustatory sensory receptors . . . . . 111
8.5.4.1 Operation of the “sweet” gustatory sensory neuron . . . . . . . . 112
8.5.4.1.1 Review of the historical database BRIEF . . . . . . . . . . . 113
8.5.4.1.2 Chemical identification of large classes of sugars
(saccharides) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
8.5.4.1.3 The unique “sugar alcohols” or glyco-alcohols . . . . . 115
8.5.4.1.4 The unique “sugar acids” or glyco-acids . . . . . . . . . . 115
8.5.4.1.5 Sweetness antagonists (inhibitors) . . . . . . . . . . . . . . . . 116
8.5.4.2 Operation of the “super sweet” sensory neuron & AH,B,X . . . . 116
8.5.4.2.1 The unique non– saccharide sweeteners EMPTY . . . 118
8.5.4.3 Operation of the “acidic” gustatory sensory neuron . . . . . . . . 118
8.5.4.3.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
8.5.4.3.2 Proposed acidic channel sensory neuron receptor
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
8.5.4.3.3 The polarized forms of carboxylic acids . . . . . . . . . . . 122
8.5.4.3.4 The perception of carboxylic acid derivatives as acids
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
8.5.4.3.5 The perception of inorganic acids as nocents–HCl
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
8.5.4.4 Operation of the “alkaline” gustatory sensory neuron . . . . . . . 125
8.5.4.4.1 The details/confusion related toPtdIns . . . . . . . . . . . . 128
8.5.4.4.2 The details/confusion related to muco-inositol . . . . . 130
8.5.4.4.3 The gustaphores of the inositol ion . . . . . . . . . . . . . . . 134
8.5.4.4.4 The perception of sodium as sweet at low concentrations
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
8.5.4.5 Operation of the “bitter” gustatory sensory neuron . . . . . . . . . 135
8.6.10.3 The detailed nomenclature of the picric channel stimulants
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
8.6.10.3.1 Review of the historical database . . . . . . . . . . . . . . . 135
310 Neurons & the Nervous System
8.6.10.3.2 Summarizing the picrophores of taste . . . . . . . . . . . 137
8.6.10.3.3 Amarogentin, artabsin and quinine . . . . . . . . . . . . . 139
8.6.10.3.4 The triterpenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
8.5.4.5.1 Review of diverse bitter gustants and gustaphores
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
8.5.4.5.2 Potential picric channel receptors . . . . . . . . . . . . . . . 149
8.5.4.5.3 Hydrated organic molecules as picric channel
gustaphore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
8.5.4.5.4 hydrated hydrogen sulfide as an inorganic picric channel
gustaphore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
8.5.4.5.6 OBSOLETE MATERIAL ON PICROPHORE/RECEPTOR MATCH
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
8.5.4.6 Operation of the “super-bitter” sensory neurons . . . . . . . . . . . . 153
8.5.4.7 Summary of the proposed receptor d-values CONSOLIDATE
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
8.5.4.8 Other gustaphores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
8.5.4.8.1 CaCl2 & MgCl2 as gustaphores or nocents . . . . . . . . 159
8.5.4.8.2 The thio moieties as stimulants . . . . . . . . . . . . . . . . . . . 161
8.5.4.8.3 The “water” gustatory sensory response . . . . . . . . . . . 163
8.5.4.8.4 The “browned flavors” sensory response . . . . . . . . . . 163
8.5.4.8.5 The role of amines & amino acids in the taste sensation TIE
8.6.2.6.6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
8.5.4.8.6 The phenols and aliphatic-aromatics . . . . . . . . . . . . . 164
8.5.4.8.7 The non-hydroxyl guanidines . . . . . . . . . . . . . . . . . . . . 165
8.5.4.8.8 Procaine and other local anesthetics . . . . . . . . . . . . 167
8.5.4.8.9 Nutmeg and Mace . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
8.5.4.8.10 Heterocyclics–caramel and butterscotch . . . . . . . . 167
8.5.4.8.11 Heterocyclics–the pyridines . . . . . . . . . . . . . . . . . . . . 167
8.5.4.9 The putative “umami” sensory response . . . . . . . . . . . . . . . . . . 168
8.5.4.9.1 History of umami . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
8.5.4.9.2 Recent literature on umami . . . . . . . . . . . . . . . . . . . . . 169
8.5.4.9.3 The underlying mechanism–the perception of umami
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
8.5.4.10 The putative “non esterified fat” sensory response . . . . . . . . . 172
8.5.4.11 The mints as nocents instead of gustants . . . . . . . . . . . . . . . . . 173
8.5.5 The vernier (intensity) operation of the gustatory modality . . . . . . . . . . . 174
8.5.5.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174
8.5.5.1.1 Dipole potential and related dipole moment . . . . . . 176
8.5.5.1.2 Dipole potential calculation . . . . . . . . . . . . . . . . . . . . 178
8.5.5.1.3 Molecular electrostatic potential profiles . . . . . . . . . 179
8.5.5.2 Analog intensity variation due to gustaphores . . . . . . . . . . . . . 181
8.5.5.2.1 Two distinct response–concentration characteristics for
sweeteners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
8.5.5.2.2 Structure of simple artificial and Anti-sweeteners . . . 183
8.5.5.2.3 Structure of super sweeteners–acesulfame & saccharine
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184
8.5.5.2.4 Potential dispersion centroids of super sweeteners
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
8.5.5.2.5 The super-bitter (picric) stimulants EMPTY . . . . . . . . . . 190
8.5.5.3 Defining the gustatory receptor (standing alone) . . . . . . . . . . 190
8.5.5.3.1 The nominal gustatory receptor EDIT . . . . . . . . . . . . . 193
8.5.5.3.2 The electrolytic properties of the receptors . . . . . . . . 194
8.5.5.3.3 Description of the operation of the sensory neuron in
gustation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
8.5.5.4 The perception versus stimulus intensity function . . . . . . . . . . . 199
8.5.6 Electrophysiology of gustation–the Excitation/De-excitation equation
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
8.5.6.1 The impulse response of the stage 1 neuron . . . . . . . . . . . . . . . 200
8.5.6.1.1 Character of the DACB phenomenon . . . . . . . . . . . . 200
Signal Generation & Processing 8- 311
8.5.6.1.2 Circuit description of the gustatory sensory neuron with
GR REFOCUS EDIT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200
8.5.6.1.3 Analog waveforms generated by stage 1 neurons EMPTY
REFOCUS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206
8.5.6.1.4 The generic Excitation/De-excitation equation applied to
gustation EMPTY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208
8.5.6.1.5 Circuit parameters of gustatory transduction EDIT . . 208
8.5.6.2 The square pulse response of the stage 1 neuron . . . . . . . . . . . 210
8.5.6.2.1 Circuit parameters of gustatory adaptation . . . . . . . 210
8.5.6.3 Chemical kinetics at the receptor/gustaphore interface . . . . 211
8.5.6.3.1 The Beidler equation of chemical kinetics in transduction
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
8.5.6.3.2 Transduction kinetics of the amino acids . . . . . . . . . . 215
8.5.6.3.3 Comparing the human perceived response to simple
sugars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218
8.5.6.3.4 Comparing the human perceived response between
sweeteners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220
8.5.6.3.5 Mechanisms of sweet taste transduction from Simon
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220
8.5.6.3.6 Solubility of the natural sugars from Andersen et al.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221
8.5.7 Antagonists (blocking agents) & adaptation in the gustatory modality
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223
8.5.7.1 Generic blocking agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224
8.5.7.2 Gymnemic acids as G-path blockers . . . . . . . . . . . . . . . . . . . . . 225
8.5.7.3 Affect of amiloride on the monkey & other species . . . . . . . . . 226
8.5.7.3.1 Current problem related to the available archives and
visualizers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227
8.5.7.4 Adaptation and/or suppression by antagonists in gustation EMPTY
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228
8.5.8 Analysis of the literature based on the hypothesis . . . . . . . . . . . . . . . . . . 228
8.5.8.1 Reinterpretation of Smith et al. of 1983 using hamsters . . . . . . 229
8.5.8.2 Reinterpretation/expansion of Rohse & Belitz of 1991 . . . . . . . 232
8.5.8.3 Reinterpretation of Hellekant et al. of 1997 using M. mulatta
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233
8.5.8.4 The investigations of Hellekant et al. of using chimpanzees . . 239
8.5.8.5 Reinterpretation of the 2002 paper of Danilova et al. . . . . . . 240
8.5.8.6 Reinterpretation of the Giza & Scott 1991 paper . . . . . . . . . . 242
8.5.8.7 Reinterpretation of the review by Spector & Travers of 2005
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245
8.5.8.8 Reinterpretation of the nociceptor data from Kashiwagura et al.
1980 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245
8.5.8.9 The conditioned taste aversion MDS data of Chang & Scott–1984
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248
8.5.9 Extending the hypothesis to include the sweet proteins . . . . . . . . . . . . . 250
8.5.9.1 The study of protein mutations in human & primate gustation
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251
8.5.9.1.1 The folding of proteins . . . . . . . . . . . . . . . . . . . . . . . . . . 254
8.5.9.1 Searching glycophore location based on complex protein theory
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255
8.5.9.2 Amino acid sequencing of the sweet proteins . . . . . . . . . . . . . 256
8.5.9.3 The high electrical charge of the peptides . . . . . . . . . . . . . . . . 262
8.5.9.4 Studies related to monellin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263
8.5.9.5 Recent experiments to characterize rare amino acids . . . . . . 266
8.5.9.6 Proposed mechanism of sweet protein perception . . . . . . . . . 267
8.5.9.6.1 Details of proposed sweet protein selection . . . . . . . 274
8.5.9.6.2 The AH,B,X relationship in sweet proteins . . . . . . . . . . 277
8.5.9.6.3 Quantifying the AH,B,X dimensions in proteins. . . . . . 280
8.5.10 The dipole potential (DP) in the perceived intensity of gustation . . . . . 282
8.5.10.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282
8.5.10.1.1 Background related to the phospholipid structure of the
GR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285
312 Neurons & the Nervous System
8.5.10.1.2 Background related to the DP of a stimulant in saliva
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290
8.5.10.1.3 Background related to the dispersion centroid relative to
AH,B,X ADD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291
8.5.10.2 Results of the net change of a DP due to a AH,B type stimulant
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294
8.5.10.3 Details of proposed net change in DP by AH,B,X stimulants
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295
8.5.10.4 Details of proposed of sweet protein DP measurements . . . . 297
8.5.11 Correlating genes to gustatory receptors . . . . . . . . . . . . . . . . . . . . . . . . 298
8.5.11.1 Rationalization of the lipid versus protein versus sugar debate in
gustation EMPTY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298
8.5.12 Clinical (medical) disorders affecting taste . . . . . . . . . . . . . . . . . . . . . . . 298
8.5.13 Man-made taste sensors EMPTY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299
8.5.14 Overview of the complete gustatory modality–stage 2 and higher ADD
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299
8.5.14.1 Gross gustatory signal paths within the human cerebrum . . . 299
8.5.15 Confirmation of the hypothesis EMPTY . . . . . . . . . . . . . . . . . . . . . . . . . . . 299
8.5.15.1 Role of inositol as an organic taste enhancer . . . . . . . . . . . . . 299
End Section 8.5
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300
Signal Generation & Processing 8- 313
Chapter 8 List of Figures 8/1/16
Figure 8.5.1-1 Semi-schematic representation of the tongue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Figure 8.5.1-2 Proposed schematic of the gustatory system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Figure 8.5.1-3 Schematic of mammalian neural paths in gustation . . . . . . . . . . . . . . . . . . . . . . . 13
Figure 8.5.1-4 Relative magnitude estimates for locations on the human tongue ADD . . . . . . 14
Figure 8.5.1-5 Diagram of types of lingual gustatory papillae . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Figure 8.5.1-6 Morphological features of the mammalian taste bud . . . . . . . . . . . . . . . . . . . . . 17
Figure 8.5.1-7 Tracing of a gustatory sensory neuron showing internal lemma . . . . . . . . . . . . . 18
Figure 8.5.1-8 Two views of the colax and microvilli of a gustatory sensory neuron . . . . . . . . . 19
Figure 8.5.1-9 The electrophysiology of the gustatory “sweet” microvilli MOD . . . . . . . . . . . . . 21
Figure 8.5.1-10 Candidate cytology & electrophysiology schematics of the gustatory sensory
neuron DUMMY ADD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Figure 8.5.1-11 Provisional block diagram of the gustatory modality ADD . . . . . . . . . . . . . . . . . 25
Figure 8.5.1-12 A normalized etiology of chemicals relating to proposed gustatory sensory
receptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Figure 8.5.1-13 Conformational notation applied to a Newman representation . . . . . . . . . . . 33
Figure 8.5.1-14 The equilibrium constant in different contexts ADD . . . . . . . . . . . . . . . . . . . . . . . 35
Figure 8.5.1-15 Summary: sensory receptors of gustatory modality . . . . . . . . . . . . . . . . . . . . . . . 41
Figure 8.5.1-16 Lemma sub-type designations for the sensory receptors of gustation ADD . . . 42
Figure 8.5.1-17 UPDATE XXX’s Summary: performance of the gustatory modality . . . . . . . . . . . 43
Figure 8.5.1-18 The first-order gustaphores of taste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Figure 8.5.1-19 The one-dimensional effectivity graph of gustatory performance . . . . . . . . . . 46
Figure 8.5.1-20 Alternate representations of a 1D parameter in a 3D perception space . . . . 48
Figure 8.5.1-21 Potential taste sensation space for a mammalian species . . . . . . . . . . . . . . . . 50
Figure 8.5.1-22 Preferred MDS space based on a right-hand rule . . . . . . . . . . . . . . . . . . . . . . . . 52
Figure 8.5.1-23 Citations & parameters of recent MDS investigations ADD DATA to Smith . . . 53
Figure 8.5.1-24 The transition from behavioral to fundamental perspectives in gustation . . . . 58
Figure 8.5.1-25 A representative summated recording from the chorda tympani nerve . . . . . 61
Figure 8.5.1-26 Caricature of extended multipoint model of sugar-receptor coupling . . . . . . 62
Figure 8.5.1-27 Records of total chorda tympani responses to water and taste stimuli . . . . . . 65
Figure 8.5.2-1 Cluster dendrogram of 31 hamster PbN neurons . . . . . . . . . . . . . . . . . . . . . . . . . . 68
Figure 8.5.2-2 Creation of a 2D MDS representation based on a matrix tabulation . . . . . . . . . 70
Figure 8.5.2-3 Creation of a 2D MDS with absolute scales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
Figure 8.5.2-4 Scatter diagrams and their application to gustation . . . . . . . . . . . . . . . . . . . . . . 74
Figure 8.5.2-5 The Kruskal stress index as a percentage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
Figure 8.5.2-6 Multidimensional scaling of gustatory stimulants in monkey ADD . . . . . . . . . . . 77
Figure 8.5.2-7 Two-dimensional histogram of hamster taste preferences . . . . . . . . . . . . . . . . . . 79
Figure 8.5.2-8 The foundation for the chromaticity diagram of tetrachromatic vision . . . . . . . 82
Figure 8.5.2-9 A taste sensation space based on an incomplete experimental database paths
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
Figure 8.5.2-10 REWRITE A two-dimensional taste space with alternate axes applied . . . . . . . 86
Figure 8.5.2-11 Proposed taste sensation space for a mammalian species . . . . . . . . . . . . . . . . 88
Figure 8.5.2-12 Proposed 2-D neural response function of the molecule X, NRFX. . . . . . . . . . . . 90
Figure 8.5.3-1 XXX modified multidimensional analysis based on the polar head of lipids EMPTY
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
Figure 8.5.3-2 Proposed coordination chemistry of the G-Path sensory neurons . . . . . . . . . . . . 96
Figure 8.5.3-3 Comparison of AH,B,X geometries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
Figure 8.5.3-4 Sweet tasting non-sugars and the their AH,B relationships . . . . . . . . . . . . . . . . . 100
Figure 8.5.3-5 Proposed sensitivity of the desirable/”sweet” sensory channel ADD . . . . . . . . 102
Figure 8.5.3-6 The numbering system of the simple sugars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
Figure 8.5.3-7 Two gangliosides associated with the neural system . . . . . . . . . . . . . . . . . . . . . . 108
Figure 8.5.4-1 The “super sweet” tripartite glycophores of two sugars . . . . . . . . . . . . . . . . . . . 117
Figure 8.5.4-2 Proposed phosphatidylserine shown in polar form . . . . . . . . . . . . . . . . . . . . . . . . 120
Figure 8.5.4-3 Serine as the potential sensory receptor ligand . . . . . . . . . . . . . . . . . . . . . . . . . . 120
Figure 8.5.4-4 A gas-phase carboxylic acid dimer EDIT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
Figure 8.5.4-5 Potential d-values for phosphatidyl serine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
Figure 8.5.4-6 The sodium ion at hydration levels of 2 and 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
Figure 8.5.4-7 A potential hydrated sodium “dimer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
Figure 8.5.4-8 Muco-inositol phosphate and a fully hydrated sodium . . . . . . . . . . . . . . . . . . . . 129
Figure 8.5.4-9 Representations of a six-member ring from Glusker et al., 1994 . . . . . . . . . . . . . 131
314 Neurons & the Nervous System
Figure 8.5.4-10 Stereo-isomers of inositol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
Figure 8.5.4-11 Chair conformations of muco-inositol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
Figure 8.5.4-12 Muco-inositol conformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
Figure 8.5.4-13 Most potent of various classes of bitter compounds . . . . . . . . . . . . . . . . . . . . . 136
Figure 8.5.4-14 The premier gustant of the bitter or P-channel of gustation . . . . . . . . . . . . . . . 138
Figure 8.5.4-15 Ionic forms of picric acid from the literature. . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
Figure 8.5.4-16 Quinine as a stimulant of the P–channel of gustation . . . . . . . . . . . . . . . . . . . . 140
Figure 8.5.4-17 A typical triterpene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
Figure 8.5.4-18 The diversity of historically bitter compounds EDIT . . . . . . . . . . . . . . . . . . . . . . . 142
Figure 8.5.4-19 Denatonium benzoate & saccharide, the most bitter compounds ADD & EDIT
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
Figure 8.5.4-20 Denatonium benzoate from Jmol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
Figure 8.5.4-21 Alternate structures for the denatonium family . . . . . . . . . . . . . . . . . . . . . . . . . 145
Figure 8.5.4-22 Quinine, the preferred bitter taste in the laboratory . . . . . . . . . . . . . . . . . . . . . 146
Figure 8.5.4-23 Corilagin, a tannic acid & a complex sugar ester . . . . . . . . . . . . . . . . . . . . . . . 146
Figure 8.5.4-24 Caffeine, 1,3,7-trimethylxanthine and its parent . . . . . . . . . . . . . . . . . . . . . . . . . 147
Figure 8.5.4-25 Two “most intriguing” examples of anisaldoxime . . . . . . . . . . . . . . . . . . . . . . . . 148
Figure 8.5.4-26 Potential phosphatidyl aspartic acid receptor and picric acid . . . . . . . . . . . . 149
Figure 8.5.4-27 Candidate sensory receptor performance for the “bitter” channel . . . . . . . . 151
Figure 8.5.4-28 Candidate picric sensory receptor and quinine coordinate bonding RESCALE
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
Figure 8.5.4-29 Potential P-path picrophores and receptors. . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
Figure 8.5.4-30 The bitter and super-bitter stimulants of van der Heijden EDIT . . . . . . . . . . . . . 154
Figure 8.5.4-31 2D representations of Lucidenic acid D1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
Figure 8.5.4-32 Lucidenic acid D1 (HMDB 38199) from Jmol . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
Figure 8.5.4-33 Variants of lucidenic acid ADD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
Figure 8.5.4-34 Potential AH,B,X geometries for amarogentin and artabsin . . . . . . . . . . . . . . . 158
Figure 8.5.4-35 Proposed summary d-values for the gustatory receptors . . . . . . . . . . . . . . . . . 159
Figure 8.5.4-36 Calcium cation fully coordinated with water . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
Figure 8.5.4-37 The potential gustaphores of CaCl2 and MgCl2 ADD . . . . . . . . . . . . . . . . . . . . 161
Figure 8.5.4-38 The d-values associated with orbital pairs in the amino acid, cysteine . . . . . 162
Figure 8.5.4-39 Phenythiocarbamide as presented in stick and 3D form . . . . . . . . . . . . . . . . . 163
Figure 8.5.4-40 Quanidine, a stimulant with three gustaphores . . . . . . . . . . . . . . . . . . . . . . . . . 166
Figure 8.5.4-41 The guanidine derivative, SC-45647 as a super-sweetener . . . . . . . . . . . . . . . 166
Figure 8.5.4-42 Structure of procaine with d-values as represented using the Jmol visualizer
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
Figure 8.5.4-43 Effectiveness of 12 pyridines in stimulating the chemoreceptors of crayfish
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
Figure 8.5.4-44 A selected group of stimulants represented as “umami” type . . . . . . . . . . . . 171
Figure 8.5.4-45Stimulants associated with umami due to their multiple gustaphores EDIT . . . 171
Figure 8.5.5-1 The many lone pairs of electrons in amiloride available for dual coordinate bonding
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
Figure 8.5.5-2 Computed molecular electrostatic potential maps of amiloride . . . . . . . . . . . 177
Figure 8.5.5-3 The mechanism(s) of intensity determination (including dispersion) in gustation
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
Figure 8.5.5-4 The structural form of xylitol, a sweet alcohol EMPTY . . . . . . . . . . . . . . . . . . . . . . 183
Figure 8.5.5-5 Acesulfame and saccharine in dry and solvated forms . . . . . . . . . . . . . . . . . . . 184
Figure 8.5.5-6 Acesulfame as a conventional and super sweetener . . . . . . . . . . . . . . . . . . . . . 185
Figure 8.5.5-7 Aspartame as a super-sweetener . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
Figure 8.5.5-8 Aspartame showing the axis of its AH,B,X “supersweet” glycophore . . . . . . . . 186
Figure 8.5.5-9 Aspartame bonding to the picric sensory receptor ADD . . . . . . . . . . . . . . . . . . 187
Figure 8.5.5-10 The structure of saccharin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188
Figure 8.5.5-11 ED A dimer situation with galactose as both sensory receptor and stimulant
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
Figure 8.5.5-12 Concept: A stacked situation with the sensory receptor galactose interfacing with
the glycophore of an undefined stimulant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190
Figure 8.5.5-13 The serine ligand coordinate bonding with organic acids and amino acids
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
Figure 8.5.5-14 EDIT A dimer situation with galactose as both sensory receptor and stimulant
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192
Signal Generation & Processing 8- 315
Figure 8.5.5-15 The galactose-based receptor in context with the sensory microvilli . . . . . . . 194
Figure 8.5.5-16 The proposed molecular operation of the microtubules in GRN’s . . . . . . . . . 196
Figure 8.5.5-17 Effective potential at the base of the 1st amplifier (Activa) of the GRN . . . . . 198
Figure 8.5.6-1 Framework for impulse response versus square pulse analyses ADD . . . . . . . . . 200
Figure 8.5.6-2 Candidate circuitry of the gustatory sensory neurons . . . . . . . . . . . . . . . . . . . . . 201
Figure 8.5.6-3 Proposed cytological and electrolytic description of the olfactory sensory neuron
ADD & MODIFY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
Figure 8.5.6-4 Summated chorda tympani and glossopharyngeal nerves during taste stimulation
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
Figure 8.5.6-5 Odorant induced currents at various holding potentials from a newt, Cynops
pyrrhogaster ADD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209
Figure 8.5.6-6 The characteristics of the E/D response in gustation . . . . . . . . . . . . . . . . . . . . . . 210
Figure 8.5.6-7 Comparison of the Beidler and Dzendolet equations . . . . . . . . . . . . . . . . . . . . . 213
Figure 8.5.6-8 Parametric graph of the kinetics of gustation by Dzendolet ADD . . . . . . . . . . . 215
Figure 8.5.6-9 Beidler equation plotting data for amino acids, C/R versus C . . . . . . . . . . . . . . 216
Figure 8.5.6-10 Amino acid slope values, maximum responses & equilibrium constants . . . . 217
Figure 8.5.6-11 The perceived response of humans to polysaccharides of nominal concentration
based on their dry weight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219
Figure 8.5.6-12 Solubility of the natural saccharides versus temperature . . . . . . . . . . . . . . . . . 222
Figure 8.5.7-1 Lactisole, a sweetness blocker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226
Figure 8.5.7-2 Amiloride (DB00594) as represented by a Jmol file . . . . . . . . . . . . . . . . . . . . . . . 228
Figure 8.5.8-1 Three-dimensional space showing the location of 18 stimuli. . . . . . . . . . . . . . . . 232
Figure 8.5.8-2 Distribution of 23 stimuli in a 3D space based on data from 47 CT fibers . . . . . 236
Figure 8.5.8-3 Distribution of 18 stimuli in a 3D space based on data from 33 NG fibers . . . . 237
Figure 8.5.8-4 A presentation based on a 3D MDS analysis with a limited gustaphore set . . . 241
Figure 8.5.8-5 Changes in three-dimensional taste spaces of one rat caused by amiloride
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244
Figure 8.5.8-6 Integral of stage 3 action potential recovered from the glossopharyngeal nerve of
the bullfrog . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
Figure 8.5.8-7 Three dimensional MDS spaces representing a CTA group and a control group.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250
Figure 8.5.9-1 Characteristics of proteins perceived as sweet by humans ADD . . . . . . . . . . . 253
Figure 8.5.9-2 Diagram showing the position of mutations on the brazzein molecule . . . . . . 258
Figure 8.5.9-3 The amino acid sequence of brazzein with three positions marked . . . . . . . . . 259
Figure 8.5.9-4 A partial 3D ball and stick representation of the b-turn in brazzein EDIT . . . . . . 260
Figure 8.5.9-5 Complete amino acid chain for monellin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265
Figure 8.5.9-6 In vitro NMR spectroscopic analysis showing SeP presence . . . . . . . . . . . . . . . . 267
Figure 8.5.9-7 Amino acid sequence of brazzein and DNA sequence of synthetic gene . . . 268
Figure 8.5.9-8 Options available for the dispersion centroid supporting a glycophore in an a-helix
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270
Figure 8.5.9-9 The relative positions of the amino acid residues in chromatography . . . . . . . 273
Figure 8.5.9-10 Location of the glycophore of brazzein on an annotated space-filled model
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278
Figure 8.5.9-11 Schematic of glycophore involving Tyr24, Asp25 & Asp29 . . . . . . . . . . . . . . . . 279
Figure 8.5.9-12 43 superimposed conformers of brazzein in solution . . . . . . . . . . . . . . . . . . . . . 280
Figure 8.5.9-13 Expanded table of AH,B,X parameters for the glycophores of taste . . . . . . . 281
Figure 8.5.10-1 Abridged Table II of Beitinger et al., surface potentials . . . . . . . . . . . . . . . . . . . 284
Figure 8.5.10-2 Lipid and bilayer chains from Zheng & Vanderkooi . . . . . . . . . . . . . . . . . . . . . . 286
Figure 8.5.10-3 Atypical calculated components of the electrostatic potential at the bilayer
midplane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286
Figure 8.5.10-4 Chemical structure of DPPE & GM1 (a specific PtdEtn & a “ganglioside”) . . . 288
Figure 8.5.10-5 The dipole potential of different lipids measured with the CR method . . . . . 288
Figure 8.5.10-6 Electrostatic field of aspartame showing the AH,B,X relationship . . . . . . . . . . 291
Figure 8.5.10-7 3D histogram of centroid of X for a group of stimulants . . . . . . . . . . . . . . . . . . 293
Figure 8.5.10-8 Detailed electrolytic sensory neuron operation in AH.B situation . . . . . . . . . . 295
Figure 8.5.10-9 The schematic difference between the AH,B and AH,B,X configurations during
DACB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296
Figure 8.5.10-10 Galactose in the plane of O-3 & O-4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297
Figure 8.5.14-1 Gross stage 4 gustatory information extraction areas in humans ADD . . . . . . 299
316 Neurons & the Nervous System
(Active)
SUBJECT INDEX (using advanced indexing option)
3D . 6, 7, 32, 33, 39, 46-48, 51, 56, 59, 61, 62, 71-73, 80, 81, 83, 87, 129, 141, 144-146, 148, 149, 152,
154, 155, 158, 163, 165, 166, 169, 174, 176, 177, 179, 227, 230, 231, 233-238, 241,
242, 244, 248, 249, 256, 259, 260, 271, 277, 280, 289, 290, 293
3-D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83, 297
9(2) + 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
99% . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284
acidophore . . . . . . 39, 44, 111, 158, 165, 169, 171, 172, 227, 228, 234-238, 241, 251, 252, 259, 262
across-neuron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64, 174
action potential . . . . . . . . . . . . . . . . . 26, 67, 68, 73, 200, 201, 205, 206, 208, 235, 238, 240, 246, 247
Activa . 20, 21, 38, 88, 102, 103, 109, 112, 113, 121, 123, 137, 169, 174, 181, 182, 196-199, 201-205,
208, 210, 248, 282, 283, 294-296
active diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201
adaptation . . . . 38, 59, 67, 69, 73, 79, 89, 92, 113, 137, 163, 181, 197, 199, 210, 212, 223, 228, 246248, 282
adaptation amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
AH,B . . . 5, 19, 39, 41, 44, 45, 58, 61, 62, 95, 97-102, 107-109, 112, 113, 116, 119, 120, 125, 127, 142,
144-146, 148, 151, 152, 154-156, 158, 164-166, 169, 170, 178, 181, 183-187, 189-192,
194, 197, 198, 205, 220, 221, 224, 233, 234, 253, 257, 260, 263, 268, 269, 271, 274,
277-284, 290, 291, 293-297
AH,B,G . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
AH,B,X . . . 39, 44, 45, 62, 95, 98, 102, 116, 142, 145, 152, 154-156, 158, 166, 178, 181, 183-187, 190,
198, 205, 220, 221, 224, 234, 253, 257, 263, 268, 269, 271, 274, 277, 279-282, 291,
293, 295-297
aliphatic-aromatics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
amiloride . . . . . . . . . . . . 5, 58, 78, 175-178, 199, 221, 223, 224, 226-228, 237, 238, 240, 242-245, 290
ammonia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272
amplification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200
amygdala . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 14, 15, 249
anomeric carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
arginine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216, 217, 273, 275
ascorbic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
association areas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299
atomic force microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
attention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2, 92, 125, 176, 184, 257, 258
axon segment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
axoplasm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205, 206, 208
azeotrope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112, 231
bar code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238, 257
Bayesian . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5, 63, 67, 221, 230, 272-274, 299
bilayer . . . . 20, 21, 27, 39, 42, 58, 93, 94, 106, 109, 112, 121, 151, 193, 195, 198, 203, 282-287, 289,
290, 294, 296
bilayer membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151, 193, 198, 282, 285, 287, 289
Bombyx mori . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
boundary layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294
brazzein . . . . . . . . . . . . . . . . . . . . . . . . . . . 57, 63, 251-253, 256-263, 266, 268, 269, 271, 273-281, 297
Brownian motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263, 274, 280
C/D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5, 21, 211
calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90, 213, 250
cAMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5, 206
camphor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174
capsaicin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
Central Limit Theorem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73, 91
cerebrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299
chirality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111, 174
cis- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32, 38, 40, 110
Signal Generation & Processing 8- 317
computation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
computational . . . . . . . . . . . . . . . . 69, 99, 122, 129, 144, 174, 177-179, 184, 187, 266, 272, 282, 290
computational chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99, 122, 129, 144, 282, 290
confirmation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113, 133, 134, 155, 180, 299, 300
continuum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45, 76
coordinate bond . . . . 38, 39, 42, 61, 97, 109, 120, 121, 124, 132-134, 138, 141, 145, 146, 148, 151,
176, 177, 182, 186, 190, 191, 194, 198, 200, 223
coordinate chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34-37, 41, 93, 102, 119, 125, 131, 213, 295
cribriform plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
cross section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
cross-section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88, 203
Cu(I) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
Cu(II) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
DACB . . . . . 30, 31, 36-38, 47, 58, 83, 102-104, 116, 119, 120, 123, 124, 148, 150, 153, 155, 162, 165,
166, 174, 176-180, 183, 197, 200, 201, 212, 217, 218, 221, 224, 226, 228, 233, 245,
251, 252, 261, 262, 268, 273, 279-282, 290, 293, 294, 296
data base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82, 85, 154
database . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7, 60, 68, 72, 83, 85, 91, 113, 135, 203, 227, 249
Debye . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178, 202
denatonium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95, 143-145, 236
dendrolemma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5, 90, 110, 112, 196
determinants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263
DG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33, 34, 37
dihedral . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21, 109, 182, 196, 201, 209
diol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32, 40, 159, 180, 218, 219, 222
dipole moment . . . . . . . . . . . 28, 58, 112, 122, 123, 148, 176-178, 180, 195, 197, 202, 203, 282, 284
dipole potential . . 21, 28, 38, 58, 95, 98, 101, 102, 121, 138, 140, 176-179, 182, 183, 185, 193, 195,
197, 198, 261, 262, 268, 273, 274, 278, 280, 282-285, 288-290, 293-297
disparity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
dispersion point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97, 98, 293, 296
DNA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266, 268, 271
dopamine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223
dulcal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212
dynamic range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137, 197, 213
E/D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3, 61, 80, 113, 210, 211, 248
electrometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107, 195
electrostenolytic process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21, 196, 203, 217, 225
enantiomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115, 190
entropy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34, 54, 55, 78-80, 111, 218, 234, 244, 245
epiglottis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
equilibrium . . . . . . . . . . . . . . . . . . . 33-36, 102, 103, 113, 114, 200, 211, 212, 215, 217-219, 254, 255
equilibrium constant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35, 36, 103, 113, 212, 217, 219
exothermic animals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
expanded . . 6, 24, 33, 48, 62, 71, 78, 92, 103, 143, 180, 181, 205, 210, 213, 233, 244, 267, 281, 290,
294, 295
feedback . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Fehling’s reagent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
free energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33, 34, 36, 212, 217, 218, 254
GABA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
ganglion neuron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 12
ganglioside . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106-111, 283, 284, 287, 288, 294
genetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66, 143, 147, 256
genome . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266, 272
globoside . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19, 21, 93, 95, 110, 112, 196
glomeruli . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12, 31, 205
gluconic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
glucophore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33, 102, 117, 139, 180
glutamate . . . . . . . . . . . . . . . 7, 8, 22, 39, 49, 51, 53, 58, 59, 89, 93, 169-171, 225, 228, 237, 243, 273
glycol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32, 38, 40, 41, 44, 51, 53, 58, 60, 238, 242
318 Neurons & the Nervous System
glycophore . . 5, 32, 33, 39, 40, 44, 45, 49, 58, 61, 98, 99, 108, 112, 115, 141, 158, 169, 171, 184-186,
189-191, 226, 234-236, 238, 241, 255-257, 260-262, 268-271, 273, 274, 277-280, 282,
293-297
GPCR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
guanidine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166, 177, 178, 234
gustophore . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
g-protein . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5, 22
Hodgkin Condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
hole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
homogeneous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
homologs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162
hormone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255
Huckel Rule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233
hydrogen bond . . . 30, 36, 45, 58, 83, 95, 119, 121, 127, 134, 149, 150, 164, 170, 180, 200, 233, 262
hydrogen sulfide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86, 150
hydronium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21, 83, 111, 195, 196, 294
hydronium liquid crystal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195
hypothalamus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 14, 15, 249
h-neurons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76, 234, 235
inositol . . . . . . . . . . . . 5, 42, 51, 53, 61, 93, 111, 115, 128-130, 132-134, 170, 171, 183, 227, 299, 300
intramolecular hydrogen bond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
inverting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180
IUB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128, 129, 131
IUBMB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129, 132
IUPAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115, 128, 129, 132
just-noticeable difference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Kruskal stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56, 73-75, 91, 235-239, 241
labeled line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
labeled-line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64, 174, 231, 240
Langmuir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282
latency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195, 197, 206
limbic system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
London dispersion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60, 96
LOT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283
lysine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252, 267, 273
major nerves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
marker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
masking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224
MDS . . . . 4, 26, 46-49, 51-53, 55-57, 59, 67, 69-75, 78-81, 84, 86-92, 229-231, 234-237, 239-242, 245,
248-250, 293
microtubule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
microvilli . . . . . . . 11, 18-21, 38, 39, 88, 92, 93, 109, 110, 112, 121, 147, 151, 169, 190-192, 194, 225
miraculin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112, 226, 240
modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 91, 113
multi-dimensional . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43, 76, 80, 81, 84, 87, 113, 169
myelin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30, 107
myelinated . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4, 10, 12
Myelination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12, 16, 17
N1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145, 177, 244, 255
N2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255
narrow band . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46, 88
natrophore . . . . . . . . . . . . . 32, 39, 40, 44, 161, 169, 171, 185, 191, 223, 226, 227, 234, 237, 238, 241
navigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 260
neural coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8, 56, 60, 78, 85, 208
neural response function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89, 90, 92
neurotransmitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22, 92
neuro-facilitator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169, 223
nocent modality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31, 83
Node of Ranvier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4, 10, 12, 17
noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33, 34, 54, 74, 91, 103, 137, 216, 218, 220, 267
Signal Generation & Processing 8- 319
n-neurons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76, 84
n-type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
orbital . 31, 44, 45, 97, 99, 111, 139, 140, 145, 148, 152, 153, 156, 158, 162, 165, 166, 223, 251, 260,
269, 271, 277, 279, 297
orbitals . . . . . 31, 32, 38, 39, 44, 45, 59, 62, 103, 104, 116, 140, 144, 149, 152-155, 157-159, 162, 165,
167, 172, 173, 177, 189, 223, 233, 251, 259, 268, 269, 271, 277-280, 293, 294, 296,
297
orbitofrontal cortex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
oskonatory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
O–H- -O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
P/D equation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218
pain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3, 31
parametric . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114, 208, 213, 215
parietal lobe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 14, 78
patch-clamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206
pedestal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245, 246
percept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14, 87, 216, 300
perceptual space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47, 49, 56, 78
picrophore . . . . 39, 44, 59, 112, 137-141, 149, 151, 153-158, 161, 185, 187, 192, 223, 226, 228, 238,
241, 242, 245
piezoelectric . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
pnp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196, 205
poditic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208
polyprotic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35, 36
pons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
protocol . . . . . . . . . . . . . . . . . 6, 49, 51, 59, 60, 67, 69, 80, 81, 103, 199, 210, 218, 226, 242, 249, 272
pulse-to-pulse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
pyrazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175, 178
pyridine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168, 184, 185
p-type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
quantum-mechanical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3, 4, 92, 94, 156, 197, 199-201, 295
rafts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18-20
reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248
residue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110, 218, 252, 259-262, 268-270, 272-276, 278, 279, 297
resonance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114, 121, 133, 144, 279
saliency map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14, 26, 56, 78, 82, 229, 244
SAR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62, 112, 118, 224
second messenger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221
signal-to-noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267
stage 0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174, 246-248
stage 1 . . 2, 24, 26, 67, 73, 78, 116, 174, 199-201, 206, 210, 226, 229, 230, 238, 240, 246, 263, 268,
274, 282, 294
stage 1A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
stage 1B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
stage 2 . . . . . . . . . . . . . . . . . . . 11, 13, 24, 26, 38, 67, 78, 116, 174, 200, 226, 229, 230, 235, 238, 246
stage 3 . 4, 12, 16, 17, 26, 47, 56, 58, 59, 67, 68, 73, 107, 168, 200, 201, 211-213, 229, 230, 235, 246,
247
stage 3A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
stage 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5, 10, 13, 26, 45, 47, 48, 56, 67, 69, 299
stage 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7, 14, 26, 45, 56, 80, 137
stage 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34, 56, 72-75, 91, 185, 235-239, 241, 300
structural chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194
sugar alcohol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5, 115, 174, 234
synapse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4, 201
SYSTAT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
s-neurons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
temporal lobe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 14, 26
tests of the hypothesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
tetrahedron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83, 84, 87, 97
thalamus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10, 14
threshold . . . . . . . . . . . . . . 60, 63, 66, 136, 137, 142, 143, 151, 153, 154, 200, 219, 242, 262, 275, 282
320 Neurons & the Nervous System
topography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
torsion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
transduction . . 3, 5, 6, 8, 9, 16, 19, 20, 22, 28, 37, 38, 43, 59, 61, 63, 66, 81, 88, 92-95, 102, 109, 111,
112, 118, 119, 121, 135, 141, 142, 148, 149, 175, 177, 181, 195, 196, 199, 200, 203,
204, 206-208, 211, 213, 215, 216, 218, 220, 221, 225, 232, 240, 246-248, 261, 263,
266, 273, 274, 300
transition dipole moment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202, 203
translation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169, 266
trans- . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32, 38, 40, 51, 53, 58, 110, 132-134, 173, 218, 219, 222, 261
trigeminal nerve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11, 159
type 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21, 194
type 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20, 21, 109, 112, 295
type 4 . . . . . . . . . . . . . . . . . . . . . . . . . . 20, 21, 42, 88, 93, 109, 111, 147, 191, 195, 198, 225, 294, 295
umami . . . . . . . . . . . . . . . . . . . . . . . . . . . 7, 8, 16, 37, 39, 51-53, 58, 59, 68, 85, 89, 168-172, 225, 240
V2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Van der Waals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
verification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
white matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30, 110
Wikipedia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11, 96, 114, 131, 143, 167, 174, 225, 226
xxx . . 1-3, 5, 8, 11, 19, 24-26, 31, 33, 37, 39, 43, 51, 56, 60, 61, 68, 72, 73, 87, 94, 101-103, 106, 107,
109, 111-113, 122, 124, 134, 143, 145, 147, 150, 151, 161, 163, 168, 175, 189, 191,
194-198, 204, 205, 208, 211, 218, 220, 221, 225, 228, 229, 232, 234, 240, 248, 252,
255, 256, 260, 261, 263, 265, 278, 282, 285, 286, 293, 294, 299
xylitol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183, 184, 234
[xxx . . . 2, 14, 15, 20, 21, 26, 32, 37, 39, 43, 44, 47, 50, 51, 61, 64, 67, 76, 80, 84, 86, 91, 92, 100, 106,
109-111, 113-115, 118, 128, 129, 134, 135, 137, 142, 145-147, 149, 151, 161, 162,
164, 174, 182-185, 190-192, 199-202, 205, 208, 210, 215, 226, 228, 233, 234, 237,
242, 245, 251, 252, 259, 261, 269, 277, 285, 287, 293, 296-298, 300