Absolute levels of some free ammo acids in
normal and biologically fractionated retinas
Adolph I. Cohen,* Michael McDaniel,** and Harry Orr***
Using isolated (1) normal retinas, (2) receptorless retinas, and (3) retinas with markedly reduced
inner layers, respectively, derived from 90-day-old pigmented mice which were normal, or
possessed of a genetic receptor dystrophy, or treated postnatally with monosodium glutamate,
extracts were assayed with an amino acid analyzer and/or by fiuorometric ultramicro techniques. In millimoles per kilogram protein, the glycine, ananine, and GABA levels were,
respectively, 17.0, 4.7, and 18.7 in (1), 38.0, 8.7, and 26.0 in (2), and 9.0, 2.9, and 6.0 in (3).
These data suggest a relative concentration of these amino acids in the inner retina with
glycine and GABA levels in (2) matching or exceeding published values for any central
nervous system region. Taurine, however, had concentrations of 410, 150, and 500 in (1), (2),
and (3), respectively, thus suggesting high levels everywhere and a relative concentration in
the outer retina. In some groups, glutamate, GABA glutamine, or aspartate levels were lower
in receptor-containing retinas from dark-adapted as compared to light-adapted animals. Glutamate concentrations were similar in (1), (2), and (3) and aspartate and glutamine plus serine
somewhat elevated in (2). Relative neurophysiologic inactivity and/or the different lesion
mechanisms in the txoo abnormal retinas must affect these data. However, when large concentration differences between (1) and (2) and between (1) and (3) are in opposing directions,
true distribution differences in (1) are suggested. The data may bear on amino acids as possible neural transmitters and/or on local metabolic specializations.
From the Departments of Ophthalmology and
Pharmacology, Washington University School of
Medicine, St. Louis, Mo. 63110.
"Research funded by Grant EY-00258-10 of the
National Institutes of Health. Recipient of
Career Development Award EY-03170-09 of the
National Institutes of Health.
0
"Research funded by Grant NS-05221 of the
National Institutes of Health and by Grant BC-4
of the American Cancer Society.
00
"Trainee under Grant NS-05613-05 of the National Institutes of Health.
Manuscript submitted for publication Feb. 28,
1973; manuscript accepted April 10, 1973.
Reprint requests: Dr. Adolph I. Cohen, Ophthalmology Department, Washington University
School of Medicine, 660 S. Euclid Ave., St.
Louis, Mo. 63110.
.he stratification of the neurons and
synaptic zones of the vertebrate retina has
long made it an object of choice in studying histochemical distributions. Most of the
early studies were carried out on enzymes,1
but more recent studies have involved substrates.2 While such studies have usually
aimed at substrates related to the vegetative metabolism of the tissue, molecules
having possible relations to the neural activities of the retina may also be studied
and neural activity can be modulated with
light. The current report deals with concentrations of certain free amino acids.
Some of these molecules may be involved
686
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Volume 12
Number 9
in neural activity apart from their general
role in synthetic activities.
The ideal approach'2 includes rapidly arresting retinal metabolism by freezing the
eye, preparing frozen-dried slices taken in
planes tangential to the retina such that
these represent known strata of the retina,
and subjecting these to biochemical analyses. Unfortunately, for the above sample
sizes, appropriate analytic methods do not
yet exist for a number of molecules such
as glycine and taurine which are of possible importance in retinal metabolism.
However, a possible indirect means of
both screening a large number of molecules
for possible distributional asymmetries in
the retina and circumventing the current
analytic limitations has been proposed by
Cohen.3 This involves chemical comparisons of whole retinas of normal mice with
retinas of the same species which have
either lost their receptors as a result of a
genetic dystrophy and retinas which have
lost a major portion of the inner retina as
a result of 10 days of postnatal treatment
of newborn mice with monosodium glutamate. Such comparative studies not only
extend the previous descriptions of the
abnormal retinas, but may, if cautiously
interpreted, suggest distributions in normal retinas of the amino acids under investigation.
Methods
A. Isolation of whole-mouse retinas. Retinas of
90-day-old C57 and C3H mice were employed in
these experiments. The latter strain is homozygous
for a recessive gene which causes receptor degeneration prior to 60 postnatal days. Normal
retinas of C57 (bl. 6) mice served as control
samples, and other retinas were obtained from
mice treated with monosodium glutamate according to the schedule of Potts, Modrell, and Kingsbury1 but limited to 10 postnatal days. These
retinas showed a massive although subtotal destruction of the inner retina.
Animals were dark adapted for at least 90
minutes and their retinas removed under deep red
illumination. Retinas from light-adapted animals
were isolated under white illumination. Animals
were killed by decapitation and the eyes removed
by evulsion. The two eyes were quickly dip-
Free amino acids in retinas 687
rinsed and placed in a dish containing 50 ml. of
ice-cold physiologic saline. The cornea was then
stabbed and a fold of the cornea undercut with a
DeWecker iris scissors. The lens was removed and
the two margins of the corneal slash were grasped
with watchmaker forceps. The eye was torn apart.
The retina usually separated cleanly from the
pigment epithelium although some small strips of
loosely adherent pigmented tissue were occasionally present at points of the extreme retinal margins. These were readily picked off with watchmaker forceps. However, a few small patches of
pigmented tissue often adhered to the C3H
retinas. These usually proved to be removable by
gently abrading them with one prong of a forceps.
Each isolated retina was rinsed in a fresh bath
of 50 ml. of ice-cold physiologic saline, and placed
on the inner surface of a test tube which was
corked and immersed in liquid nitrogen. The tubes
were transported under frozen CO= and stored at
-70° C. Because of the variable amounts of adherent transfer fluid, wet weight values proved
unreliable. Following two rinsings, a volume of
the rinsing solution equal to that of a control
retina contained no detectable amino acids.
The time from decapitation of the animal to
immersion of both eyes in ice-cold saline was less
than 20 seconds, that until freezing of both retinas
averaged 5.5 minutes for two control eyes. With
deliberate delays of retinal removals from evulsed
but unopened eyes to prolong the evulsion-tofreezing interval, a 10 per cent loss of some amino
acids was observed between the isolation of the
two retinas. As retinas were pooled, this could
contribute to the variance about the mean assay
value.
B. Chemical methodology for whole-mouse
retinas.
Extraction. At -20° C , two control or four experimental retinas were pooled, weighed, and
placed in homogenizers. Twenty-eight microliters
of 0.1 N HC1-99 per cent methaiiol were added
for 25 minutes at -20° C. in order to extract the
ice/1 The now softened retinas were partially
homogenized and brought to 0° C. where 195 /A
of 0.3 N HC10, in 1 mM. EDTA was added.
Homogenization was then completed. The tubes
were centrifuged at 10,000 g for 35 minutes at
0° C. and the HClOi precipitate was assayed for
protein by the method of Lowry and co-workers,(!
using bovine serum albumin standards. Supernatant fluid (205 jtl) was transferred to 7 by 70
mm. tubes and neutralized with 28 fA of 1.45
N K;CO:i at 0° C. The tubes were recentrifuged
at 10,000 g for 25 minutes at 0° C. to remove a
precipitate. The supernatant was stored at -70°
C. until analyzed.
Analytic methods.
FLUOROMETRIC. Glutamate was measured on all
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688 Cohen, McDaniel, and Orr
Investigative Ophthalmology
September 1973
Fig. I. A montage of light micrographs from glutaraldehyde-osmium-fixed isolated retinas of
mice of 90 days of age (x690). The retina of the C3H mouse is above, a control retina is in
the center, and a glutamate-treated retina is below. Outer segments, when present, are to the
left, the vitreal faces of the retinas are to the right. The retinas are aligned by the outer
aspects of their inner nuclear layers.
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Free amino acids in retinas 689
Volume 12
Number 9
extracts by the method of Lowry and Passonneau.7
GABA levels were determined with "GABase"
(Sigma Chemical Co. St. Louis, Mo.) by a procedure of Berger, Lowry, and Carter (unpublished).
AM iNO
ACID
ANALYZER.
Except
for
GABA,
amino acid levels were determined using a Beckman Model 120C Amino Acid Analyzer. Extracts
to be analyzed were diluted 23x with 0.2 N Na+
citrate buffer, pH 2.3. A 69 by 0.9 cm. column
packed with Beckman Type UR-30 resin was used.
The samples were eluted with 0.2 M Na+ citrate
buffer, pH 3.25 at a temperature of 55° C, at a
flow rate of 102 ml. per hour. While Y-aminobutyric acid (GABA) levels were also determined
on some extracts using a basic amino acid column
set-up, generally GABA assays employed the
fluorometric method. Glutamine was also measured fluorometricallys on some samples. It was
concluded that not less than 90 per cent of the
glutamine plus serine levels from the analyzer
represented glutamine.
Results
A. Description of normal and abnormal
mouse retinas at 90 days. Fig. 1 shows a
montage of light micrographs of the three
90 day retinas employed in this study to
facilitate their comparison. The normal
mouse eye at this age has a diameter of
3 mm. The normal, unfixed retina is about
0.35 mm. in thickness. We estimate its volume at 4.7 mm.3 and its protein at 0.32
mg. The C3H retina was found by electron
microscopy to have no detectable inner or
outer segments of the photoreceptors, but
rare receptor terminals were present. This
confirms the observation of Karli, Stoekel,
and Porte" who still found rare receptor
terminals in retinas from one-year-old mice
with receptor dystrophy. The inner plexiform layer in our C3H mice was slightly
reduced in thickness (ca. 0.9 of control
mice) and electron microscopy indicated
some atrophy of bipolar terminals which
were condensed and somewhat more
sparse. This may point to some loss of
bipolar cells and/or reduced branching and
size of their terminals. The electron microscopic appearance of other cells and synapses were quite normal. However, Grafstein, Murray, and Ingoglia,10 who studied
protein synthesis and axonal transport in
retinas of mice with receptor dystrophy,
found that their 3- to 6-month-old mice had
20 per cent fewer ganglion cells and the
remaining ganglion cells were somewhat
smaller. We have no reason to believe our
animals differ in this regard. The C3H eye
has normal dimensions. We estimate the
retinal volume at 2.8 mm.3 and its protein
at 0.14 mg.
The retinas of the glutamate-treated mice
have been described by Cohen.11 These
possess a quasi-normal concentration of
receptors but the size of the whole eye is
reduced to 1.8 mm. in diameter as compared to diameters of 3.0 mm. for both
control and C3H mice. The reduction in
retinal surface area reduces the absolute
number of receptors. We estimate the retinal volume at 3.2 mm.3 and protein at 0.18
mg. The receptor terminals appear to have
the usual concentrations of synaptic vesicles and receive processes in apparent
synapsis. The inner plexiform layer and
inner nuclear layers are markedly reduced
in thickness (0.3 of control eyes) but some
examples of all known cell types and synapses survive. Many nuclei in the inner
nuclear layer must belong to surviving
Miiller cells and it is likely that ganglion
cells and amacrine cells are proportionally
more depleted than bipolars or horizontal
cells. Some preliminary electroretinographic studies on glutamate-treated animals performed in cooperation with Dr. Y. Honda
showed an absence of the "b" wave and an
evocable "a" wave.
B. Chemical data from mouse retinas.
Table I shows the results of the chemical
assays. The outstanding differences seen
relate to glycine, alanine, GABA, and
taurine. The first three exhibited their highest concentrations in the receptorless C3H
retinas and their lowest concentrations in
the glutamate-treated retinas. Taurine, on
the other hand, showed its highest concentration in the glutamate-treated retinas and
its markedly lowest concentrations in the
receptorless C3H retinas. Glutamine plus
serine (not less than 90 per cent gluta-
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690 Cohen, McDaniel, and On
Investigative Ophthalmology
September 1973
Table I
Amino
acid"
Glycine
Alanine
CABAf
Taurine
Clutamate
Aspartate
Glutamine
+ serine
Control
Light
Dark
Light
16.9 ± 0.5 17.4 ± 0.8 37.8 ± 1.5
4.3 ± 0.3
5.1+ 0.4
8.6 ± 0.3
16.7 + 0.7 13.6+ 0.3 26.6+ 1.0
417 + 19
409 + 15
145 +14
45.7+ 1.3 37.2+ 1.2 40.1+ 1.6
13.8+ 1.0 12.0+ 0.3 21.6+ 0.8
15.2+ 0.8
Dark
Per
cent
control
38.0+1.8
8.8 + 0.5
26.6 ±0.4
160 +9
40.2 + 0.8
18.5 ±0.7
22.9+1.2
C3H
13.5+ 0.3
23.5+ 0.7
Light
Dark
Per
cent
control
221
185
175
37
97
153
8.8 + 0.4
3.3 ± 0.5
5.0 ±0.9
483+9
45.0 + 1.6
18.2 ±1.6
8.6 ± 0.8
2.5+ 0.3
3.6+ 0.4
525 + 20
36.5± 1.2
11.2+ 0.6
51
62
28
123
98
113
160
18.2 ±1.6
10.5+ 0.8
100
Glutamatiz treated
"Each value is an average of separate assays on six different groups of retinas. Light- and dark-adapted values are averaged
in calculating per cent control values. Values in mmoles per kilogram of protein ± S.E.M.
fGABA levels determined by enzymatic assay, remaining amino acid levels determined by amino acid analyzer.
mine) and aspartate showed their highest
concentrations in the C3H retinas and the
concentration of glutamate was similar in
the three retinas studied. Some groups of
dark adapted, receptor-containing retinas
showed somewhat lower concentrations of
glutamate, aspartate, GABA, and glutamine
than light-adapted retinas. Although some
of these differences were significant at the
< 0.001 or < 0.005 level by Student's t
test), the latter differences will not be considered further at this time.
Discussion
Certain problems in interpreting these
data must be emphasized. Despite the
small size of the mouse ocular sphere, the
superficial position of the retina, and the
rapid chilling in ice-cold saline (<20 seconds for two eyes), metabolism is not as
rapidly nor as completely arrested as in
a frozen eye. Thus significant changes in
the level of certain amino acids may occur.
Moreover, the C3H retinas take somewhat
more time (ca. 30 seconds per retina) to
isolate than other retinas.
As outer segments contain a lesser ratio
of cytoplasm to cell membrane than most
retinal regions, and as data are referred
to protein, this biases concentrations when
outer segments are included in the sample,
but similar problems are general to all comparisons of regional, chemical assays of
brain referred to protein or dry weight.
More important is the question of
whether large changes in the content of
amino acids in the residual retina are generated by the genetic lesion or glutamate
treatment directly, or by changes in the
level of neurologic activity consequent to
the lesion. Some significant degree of distortion of this type must be present. Do
such considerations render the data useless
for obtaining suggestions of retinal localizations? How "normal" are the modified
retinas?
We note that there is some persistent
capacity for light-evoked neurophysiologic
activity in both abnormal retinas. Glutamate-treated mice are said4 to have an
electroretinogram with a persistent "a"
wave, a sign of the electrical activity of
photoreceptors,12 and such strains of mice
with receptor dystrophy as have been studied show some light perception at high
threshold. This perception is mediated by
the retina as evidenced by its abolition
when the optic nerve is sectioned13'14 and
possesses an action spectrum compatible
with the rhodopsin absorption spectrum.15
Both Karli, Stoekel, and Porte9 and one of
us (A. I. Cohen, unpublished observations)
have failed to discern by electron microscopy any residue of surviving outer segments in the dystrophic retinas we studied,
but there were very sparse receptor terminals and presumably receptor somata which
could contain rhodopsin. The phenomenon
suggests some persistent functional channels for exciting some retinal ganglion cells.
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Volume VZ
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That the comparative studies have some
validity in suggesting distribution differences is indicated by the fact that the genetic receptor dystrophy and inner retinal
destruction by glutamate must occur
through quite different mechanisms, yet
in almost all cases an inner retinal concentration of some entity as predicted by
a large concentration increase in the absence of the receptors was verified by a
decrease in its concentration with reduction of the mass of the inner retina. Conversely, the one entity which showed a
large decrease in concentration in the absence of the receptors, showed an increase
in concentration with reduction of the inner
retina. This is more likely to reflect localizations than result from coincidental alterations of metabolism in opposing directions. Moreover, there is an astonishing
agreement between our inferred localizations and localizations reported in the literature for the same molecules as deduced
from direct chemical assays of retinal layers
or from other techniques.
The relative concentrations of taurine
and other free amino acids of the normal
rat retina as reported by Pasantes-Morales
and co-workers1" generally agree well with
our data on control retinas of the mouse,
but their absolute values are lower. The
amino acid concentrations of the bovine
retina as reported by Yamamoto and coworkers17 are highly variable but of similar magnitudes to those in this report.
Taurine was first identified as a major
free amino acid of the retina by Kubicek
and Dolenekis and this was confirmed by
Brotherton.1!l Of previous studies on free
amino acids in retinas which could pertain
to localizations, Brotherton's19 findings are
potentially of considerable interest as she
studied rats with receptor dystrophy. However, while her conclusions report losses
in taurine and elevations in glycine and
aspartic acid in rat retinas with receptor
dystrophy, her text speaks of taurine increases in affected animals. Attempts to
contact Dr. Brotherton were unsuccessful.
Preliminary radioautographic studies of
Free amino acids in retinas 691
retinas employing labeled taurine have
been carried out in the laboratory of R.
Young (personal communication) and
show heaviest labeling over receptors and
pigment epithelium in the rat and frog.
As the details of the receptor dystrophy
in the mouse and rat are not identical, the
taurine decrease might be associated with
the fact of receptor loss rather than with
the mechanism of the loss. The somewhat
increased taurine concentration following
massive loss of the inner retina tends to
support this view. The question is whether
the taurine loss is mainly due to the receptors being relatively rich in taurine or
whether the absence of the receptors also
turns off a process that accumulates taurine
or one of its precursors in other retinal regions. Complicating our thinking on this
matter are some preliminary measurements
showing similar taurine levels in normal
retinas of newborn mice, 9-day-old mice,
and 90-day-old mice. There are no signs
of receptor terminals or inner and outer
segments in newborn mice.
Starr and Voaden20 studied the metabolism and release of taurine in the rat retina.
They found that the bulk of the amino
acid was tightly bound, its metabolism
was slow, and a comparison of taurine's
maximum rate of uptake and affinity for
membrane carrier seemed to preclude its
rapid reuptake by cells unless it was present in high extracellular concentrations. In
their view the preceding made it unlikely
to be a transmitter. However, PasanteMorales and co-workers21 report a lightinduced efflux of taurine from the chick
retina, rapid uptake of taurine by the frog
retina,22 and suppression of the "b" wave
of the electroretinogram23 by taurine.
With reference to other amino acids, our
findings exhibit an almost total consistency
with findings from radioautographic studies by Bruun and Ehinger,24 and Ehinger.25
These authors found localized uptakes of
glycine, alanine, and GABA in the inner
retina of the rabbit, but glutamate and
aspartate to be generally distributed. With
reference to a postulated role for glutamate
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Investigative Ophthalmology
September 1973
692 Cohen, McDaniel, and Orr
as a retinal transmitter, we agree with
Ehinger that this might only mean that
glial cells of Miiller effectively sweep up
extracellular glutamate which might still
be functioning as a transmitter at some
synapse. Ehinger found that extracts of
the labeled retinas largely contained the
label in the same molecules in which it
was administered. The only disparity between the two investigations lies in the
somewhat elevated aspartate concentration
seen in our receptorless retinas.
Kuriyama, Roberts, and Kakefuda20
found the enzymes necessary for the synthesis of GABA to be mainly localized in
the inner retina of the rabbit. Lam and
Steinman27 found an uptake of radiolabeled
GABA in both horizontal cells and cells
of the inner retina of the goldfish, but only
in the former was the uptake influenced
by light modulation. Graham2S-20 found
most of the retinal GABA to occur at the
level of the amacrine cells, but some to be
present in horizontal cells. A study, highly
similar to ours but restricted to GABA was
reported by Macaione30 while our studies
were in progress. This investigator compared the GABA content of normal rat
retinas either with those whose inner retina
had been largely destroyed by postnatal
glutamate treatment or those whose receptors had been destroyed by iodoacetate
plus malate treatment. Our results match
Macaione's to the extent that glutamate
treatment markedly diminished the GABA
concentration of the retinas but are in conflict with his since he saw no differences
between his iodoacetate plus malatetreated and control animals.
Partly because of the observations by
Curtis81 that acidic amino acids (such as
glutamate, aspartate, cysteine, etc.) depolarize and glycine and GABA hyperpolarize certain nerve cells when delivered
iontophoretically in their vicinity, much
attention has been paid to amino acids as
possible neurotransmitters. Moreover, most
of the initial speculations bearing on
amino acids acting as neurotransmitters
in particular brain regions stem from ob-
servations of high relative concentrations
of particular amino acids in these regions,
and our values for glycine, GABA, and
taurine tend to match or exceed the highest local values reported for central nervous system.3231 But although a store of
transmitter might favor a high local concentration, it does not follow from a high
local concentration of an amino acid that
it is being used as a transmitter. Obviously for an agent to be considered as a transmitter requires an overall evaluation based
in part on its presence, storage, release,
postsynaptic action, and inactivation, and
such studies have not been completed
for any amino acid in the retina. Except
for two recent studies of GABA,3r>> 3r> most
of the earlier physiologic and pharmacologic literature on the possible role of
amino acids as retinal transmitters has been
reviewed by Ehinger,25 and will not be discussed here.
Thus, of the findings at hand, perhaps
that of most immediate interest is the confirmation of a high level of retinal taurine
and its marked depletion with receptor
dystrophy. As animals with receptor dystrophy often serve as models for retinitis
pigmentosa in humans, a thoroughgoing
investigation of retinal taurine is indicated.
The authors wish to thank Drs. Ralph Bradshaw
and Blake Moore for the use of their amino acid
analyzers, Dr. O. H. Lowry for critical reading of
the manuscript, and Shirley Freeman and Z. Jean
Cohen for expert technical assistance.
REFERENCES
1. Lowry, O. H., Roberts, N. R., and Schultz,
D. W.: Quantitative histochemistry of retina.
II. Enzymes of glucose metabolism, J. Biol.
Chem. 236: 2813, 1961.
2. Matschinsky, F. M., Passonneau, J. V., and
Lowry, O. H.: Quantitative histochemical
analysis of glycolytic intermediates and cofactors with an oil well technique, J. Histochem. Cytochem. 16: 29, 1968.
3. Cohen, A. I.: Rods and cones and the problem of visual excitation. In: The Retina.
Morphology, function, and clinical characteristics (Jules Stein Symposium, 1966). Allen,
R., and Hall, M., Editors. Los Angeles, 1969,
UCLA Forum in Medical Science, p. 31
4. Potts, A. M., Modrell, R. W., and Kingsbury,
C : Permanent fractionation of the electro-
Downloaded From: http://iovs.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/933603/ on 06/14/2017
Volume 12
Number 9
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
retinogram by sodium glutamate, Am. J.
Ophthalmol. 50: 900, 1960.
Nelson, S., Lowry, O. H., and Passonneau,
J. V.: Changes in energy reserves in mouse
brain associated with compressive head injury. In: Head Injury Conference Proceedings.
Caveness, W. F., and Walker, A. E., Editors.
Philadelphia, 1966, Lippincott, p. 444.
Lowry, O. H., Rosebrough, N. J., Farr, A. L.,
et al.: Protein measurement with the Folin
phenol reagent, J. Biol. Chem. 193: 265, 1951.
Lowry, O. H., and Passonneau, J. V.: A flexible system of enzymatic analysis. New York,
1972, Academic Press, Inc.
Curthoys, N. P., and Lowry, O. H.: Am. J.
Physiol. (In press.) 1973.
Karli, P., Stoekel, M. E., and Porte, A.:
Degenerescence des cellules visuelles photoreceptrices et persistence d'une sensibilite de
la retine a la stimulation-photique. Observations au microscope electronique, Z. Zellforsch. 65: 236, 1965.
Grafstein, B., Murray, M., and Ingoglia, N.
A.: Protein synthesis and axonal transport in
retinal ganglion cells of mice lacking visual
receptors, Brain Res. 44: 37, 1972.
Cohen, A. I.: An electron microscopic study
of the modification by monosodium glutamate
of the retinas of normal and "rodless" mice,
Am. J. Anat. 120: 319, 1967.
Brown, K. T.: The electroretinogram. Its
components and their origins, Vis. Res. 8:
633, 1968.
Karli, P.: Retines sans cellules visuelles.
Recherches morphologiques, physiologiques,
et physiopathologiques chez les rongeurs,
Arch. Anat. Histol. Embryol. 35: 1, 1952.
Karli, P.: fitude de la valeur fonctionelle
d'une retine depourvue de cellules visuelles
photoreceptrices, Arch. Sci. Physiol. 8: 305,
1954.
Bonaventure, N., and Karli, P.: Sensibite
visuelle spectrale chez des souris a retine
entierement depourvue de cellules visuelle
photoreceptrice, C. R. Soc. Biol. 155: 2015,
1961.
Pasantes-Morales, H., Klethi, J., Ledig, M., et
al.: Free amino acids of chicken and rat
retina, Brain Res. 41: 494, 1972.
Yamamoto, K., Yoshitani, Y., Fujiwara, H.,
et al.: Study on free amino acids in the retina,
Acta. Soc. Ophthalmol. Jap. 74: 1561, 1970.
Kubicek, R., and Dolenek, A.: Taurine et
acides amines dans la retine des animaux, J.
Chromatogr. 1: 266, 1958.
Brotherton, J.: Studies on the metabolism of
the rat retina with special reference to
retinitis pigmentosa. II. Amino acid content
as shown by chromatography, Exp. Eye Res.
1: 246, 1962.
Free amino acids in retinas 693
20. Starr, M. S., and Voaden, M. J.: The uptake,
metabolism, and release of 1JC-taurine by rat
retina in vitro, Vis. Res. 12: 1261, 1972.
21. Pasantes-Morales, H., Urban, P. F., Klethi,
J., et al.: Light-stimulated release of [;sr'S]
taurine from chicken retina, Brain Res. 51:
375, 1973.
22. Pasantes-Morales, H., Klethi, J., Urban, P. F.,
et al.: The physiological role of taurine in
retina uptake and effect on electroretinogram,
Physiol. Chem. Phys. 4: 339, 1972.
23. Pasantes-Morales, H., Klethi, J., Urban, P. F.,
et al.: Etude de l'effet de la taurine sur
l'electroretinogramme de la retine en perfusion, C. R. Acad. Sci., Series D, 275: 699,
1972.
24. Bruun, A., and Ehinger, B.: Uptake of the
putative neurotransmitter, glycine, into the
rabbit retina, INVEST. OPHTHALMOL. 11: 191,
1972.
25. Ehinger, B.: Cellular location of the uptake
of some amino acids into the rabbit retina,
Brain Res. 46: 293, 1972.
26. Kuriyama, K., Roberts, E., and Kakefuda, T.:
Association of the y-aminobutyric acid system with a synaptic vesicle fraction from
mouse brain, Brain Res. 8: 132, 1968.
27. Lam, D. M. K., and Steinman, L.: The uptake of (y-H 3 ) aminobutyric acid in the
goldfish retina, Proc. Nat. Acad. Sci. 68:
2777, 1971.
28. Graham, L. T., Jr.: Intraretinal distribution
of GABA, Trans. Am. Soc. Neurochem. 1: 46,
1970.
29. Graham, L. T., Jr.: Intraretinal distribution
of GABA content and GAD activity, Brain
Res. 36: 476, 1972.
30. Macaione, S.: Localization of GABA system in
rat retina, J. Neurochem 19: 1397, 1972.
31. Curtis, D. R.: Amino acid transmitters in the
mammalian central nervous system, Proc.
Fourth Int. Cong. Pharmacol. 1: 9, 1969.
32. Aprison, M. H., and Werman, R.: The distribution of glycine in cat spinal cord, Life
Sci. 4: 2075, 1965.
33. Graham, L. T., Jr., Shank, R. P., Werman,
R., et al.: Distribution of some synaptic transmitter suspects in cat spinal cord: glutamic
acid, aspartic acid, y-aminobutyric acid, glycine, and glutamine, J. Neurochem. 14: 465,
1967.
34. Guidotti, A., Badiani, G., and Pepeu, G.:
Taurine distribution in cat brain, J, Neurochem. 19: 431, 1972.
35. Starr, M. S., and Voaden, M. J.: The uptake
of ( 14 C) y-aminobutyric acid by the isolated
retina of the rat, Vis. Res. 12: 549, 1972.
36. Voaden, M. J., and Starr, M. S.: The efflux
of radioactive GABA from rat retina in vitro,
Vis. Res. 12: 559, 1972.
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