AMER. ZOOL., 33:200-211 (1993)
The Sites and Consequences of Melatonin Binding in Mammals12
ERIC L. BITTMAN
Department of Zoology and Program in Neuroscience and Behavior, University of Massachusetts,
Amherst, Massachusetts 01003
SYNOPSIS. Melatonin, a hormone of the pineal gland, exerts multiple
effects upon the brain-pituitary axis of vertebrates. Among mammals, the
best documented physiological roles of melatonin involve the photoperiodic induction of reproductive and other seasonal adjustments. Daylength regulates the effects of gonadal steroids upon gonadotropin secretion and sexual behavior as well as the frequency of a neural generator
of GnRH pulses. In hamsters, these effects are paralleled by changes in
GnRH, AVP and beta-endorphin immunoreactivity, and in opiate receptor density in the medial amygdala. Autoradiographic studies indicate a
high concentration of 2[125I]-iodomelatonin binding sites in the suprachiasmatic nuclei of some photoperiodic mammals but not in others. In
contrast, such binding sites have been found in the pars tuberalis of all
seasonally breeding mammals studied to date.
changes in cell metabolism so that adaptive
responses result. Removal of the pineal gland
of various vertebrates interferes with a range
of functions, many of which can be restored
by replacement of melatonin.
Among the vertebrates, the principal
source of circulating melatonin is the pineal
gland. In many non-mammals, the pineal
synthesizes and secretes melatonin in a circadian fashion after its isolation from the
organism (Underwood, 1989). Light suppresses the synthesis of melatonin and sets
the phase of (entrains) endogenous rhythms
of melatonin synthesis and secretion. While
the pineal is directly photosensitive and
contains rod-like cells in many vertebrates,
accessory eyes and extraretinal photoreceptors influence pineal metabolism in most
fishes, amphibians, and reptiles. In nonmammals, sympathetic projections to the
pineal are of uncertain physiological importance and may modulate the more direct
effects of light upon melatonin synthesis. In
contrast, adult mammals have dispensed
with direct influences of light and rely exclusively on retinally regulated sympathetic
input for endogenous rhythmicity and pho1
From the Symposium Neural Aspects of Reprotic
control of pineal melatonin synthesis.
ductive Endocrinology: Old Questions, New Approaches
presented at the Annual Meeting of the American SociIntegumentary actions of melatonin may
ety of Zoologists, 27-30 December 1990 at San Antobe
ancestral: pinealectomy eliminates the
nio, Texas.
pronounced
daily rhythm of color change
- This paper is dedicated to Howard A. Bern, who
in lampreys, and transplantation of the pisparked my interest in comparative endocrinology.
INTRODUCTION
An old adage holds that comparative
endocrinology deals not so much with
changes in hormones themselves as in the
uses to which they are put. The evolution
of the pineal hormone, melatonin, supports
this statement as well as any and better than
most. Melatonin may have pre-dated the
appearance of the chordates (Vivien-Roels
et ah, 1984), and may be synthesized by all
vertebrate photoreceptors. This indoleamine exerts local influences on retinal disk
shedding, pigment aggregation, and calcium-activated dopamine release (Weichmann, 1986). Gern and Karn (1983) suggest
that melatonin originally acted within photoreceptors to regulate their sensitivity to
light, and that its export for clearance made
available to the rest of the organism a potential endocrine signal which could apprise
other tissues of the light: dark environment.
This implies the expression of receptor molecules for melatonin in distant target sites,
whose occupation triggers appropriate
200
MELATONIN BINDING IN MAMMALS
neal gland results in local blanching of the
skin (Young, 1935; Eddy and Stratum, 1968).
Indeed, the ability of melatonin to promote
melanosome aggregation in ranid tadpole
skin has served as a sensitive bioassay. The
physiological roles of melatonin, however,
are by no means restricted to the action for
which it was named. When brook lampreys
are pinealectomized during the spring,
metamorphosis fails to occur in late summer (Eddy, 1969). Melatonin may also
mediate the influence of photoperiod on
amphibian metamorphosis (Delgado et al,
1987). In tiger salamanders, pinealectomy
interferes with celestial orientation and
melatonin injections shift the direction in
which animals swim when allowed access
to a light source (Adler and Taylor, 1980).
In some fish, lizards and birds, melatonin
secretion by the pineal gland acts to regulate
multiple circadian rhythms, influencing
period and integrity of" these endogenous
oscillations (Kavaliers, 1989; Underwood,
1989).
201
docrine function as the seasons progress or
upon manipulation of photoperiod in the
laboratory. These effects have been most
thoroughly explored in hamsters and ewes,
in which the influence of daylength is
blocked by pinealectomy and replicated by
melatonin administration (Fig. 1). During
the breeding season, testosterone and estradiol exert relatively weak negative feedback
influences on LH secretion (Fig 1A). Upon
exposure to daylengths typical of the nonbreeding season, however, these hormones
become markedly more effective in restraining GnRH, and hence gonadotropin, secretion. Since this change occurs in the absence
of a marked alteration in hypophyseal
responsiveness to GnRH, photoperiodic
influences must involve a neural site of
action. The important effects which gonadal
steroid hormones exert upon sexual behavior also vary seasonally in both sexes (Fig.
1B). During the breeding season, lower doses
of exogenous estradiol are required to prime
ovariectomized animals for behavioral
estrus, and the latency for testosteroneinduced restoration of intromissions is
THE ROLE OF MELATONIN IN MAMMALS
In contrast, pinealectomy has little effect shorter in orchidectomized males (Goodman etal, 1982; Lincoln etal, 1972; Mieron mammalian locomotor rhythms (Kind nicki et al, 1990; Bittman et al, 1990).
et al, 1970; Finkelstein et al, 1978; Cheung Finally, pulsatile secretion of LH in casand McCormack, 1982). While administra- trates proceeds at a brisk pace during the
tion of exogenous melatonin can entrain breeding season, reflecting the unchecked
such rhythms, the effects of physiological operation of a neural generator of GnRH
doses are ambiguous (Darrow and Doyle, pulses (Fig 1C). Inhibitory photoperiods
1989). Melatonin's effects are unusual slow LH pulses and reduce the abundance
among entraining agents in that they are of hypophyseal prolactin and beta-LH
restricted to the end of the subjective day. mRNAs in castrates (Goodman et al, 1982;
Since serum melatonin concentrations are Bittman et al, 1990).
near their minimum at this time, the sigDaylength acts through the circadian sysnificance of this effect is questionable. Furthermore, evidence has not been presented tem to determine the duration of the nocto rule out the possibility that melatonin turnal rise in serum melatonin concentraentrains locomotor rhythms through actions tions, and this parameter carries critical
upon the retina (see Armstrong, 1989, for information to the brain. Pinealectomized
sheep and hamsters respond to a long-duraan alternative point of view).
In contrast, the mammalian pineal gland tion melatonin signal by entering neuroengoverns seasonal adaptations of mammals docrine state appropriate to winter, and are
including reproduction, thermogenesis, driven into the summer condition by
molt, metabolism, torpor, and associated repeated nightly infusions of melatonin
behaviors (see Karsch et al, 1984; Under- which mimic the long day secretory pattern,
wood and Goldman, 1987; Glass 1988, for regardless of the ambient daylength or phase
reviews). Radical changes occur in respon- of the infusion (Karsch et al, 1984; Undersiveness to gonadal steroid hormones and wood and Goldman, 1987; Bartness and
in steroid-independent aspects of neuroen- Goldman, 1988; Wayne et al, 1988).
202
ERIC L. BITTMAN
A
800
_
E
600
X
400
f
- 14L:10D
"10L:14D
SERUM TESTOSTERONE (ngfml)
D
14L:10D
•
!0l:14D
DISTRIBUTION OF MELATONIN
BINDING SITES
% E2 In capauto
long day
short day
Photoperiodic vertebrates utilize endogenous circadian clocks for the measurement
of photoperiod (Underwood and Goldman,
1987). Destruction of the suprachiasmatic
nucleus of the mammalian hypothalamus
(SCN), which compromises endogenous circadian rhythms including that of pineal
melatonin synthesis, render seasonal breeders incapable of discriminating between long
and short photoperiods. When changes in
photoperiod are eliminated in the laboratory, however, some species repeatedly leave
and re-enter breeding condition at intervals
of approximately one year even though the
pattern of melatonin secretion may remain
constant (Karsch et al., 1989). Fluctuations
in photoperiod, as measured by the circadian system and transduced into an endocrine message by the pineal gland, may set
the phase of such circannual oscillators in
ground squirrels, sheep, and perhaps other
species.
short day plnx
FIG. 1. Seasonal changes in neuroendocrine function
in seasonal breeders. A. Photoperiodic regulation of
the negative feedback potency of testosterone, assessed
by measurement of serum LH concentrations, in castrated male golden hamsters. Physiological doses of
testosterone inhibit LH secretion more effectively in
winter daylengths (*, P < 0.05; data of Bittman and
Krey, 1988.) B. An example of photoperiodic regulation of steroid-induced sexual behavior. Short days
(filled bars) reduce the effectiveness of estradiol in
priming ovariectomized golden hamsters for the
expression of sexual receptivity following a fixed dose
ofprogesterone(dataofBittman«a/., 1990). C. Effects
of season which occur in the absence of gonadal steroid
hormones in ewes. In long days, LH pulses are infrequent as indicated by a long interpulse interval (IPI,
shaded bars). Amplitude of LH pulses is high (open
bars). Upon exposure to short days, both the interval
While autoradiographic approaches to the
identification of putative receptors are not
new, localization of melatonin binding sites
proceeded slowly until the introduction of
an analog which can be radiolabeled to high
specific activity (see Weaver et al, 1991 for
review). 2-[ 125 I]-iodomelatonin (IMEL)
binds saturably, specifically, reversibly, and
with high affinity (Kj of 25-175 pM) to cell
membranes in the nervous system of
numerous vertebrates. It is argued that low
affinity binding sites also exist (Krause et
al., 1990). Several lines of evidence establish that the high affinity melatonin binding
sites are coupled to G proteins and fulfil
criteria of bona fide receptor molecules.
First, analogs of guanosine triphosphate
which interfere with regeneration of G r coupled receptors decrease the affinity, and
sometimes the capacity, of IMEL binding
between LH pulses and the amplitude of each pulse
decrease. Pinealectomized animals given the same short
day challenge fail to experience an increase in LH pulse
frequency or a decrease in pulse amplitude. (Data of
Bittman et al., 1985.)
MELATONIN BINDING IN MAMMALS
in lizard, chick, sheep, hamster and rat tissues (Weaver et ai, 1991). Second, pertussis
toxin, which inhibits recycling of G, proteins, blocks melatonin actions on Xenopus
melanophores (White et ai, 1987). Third,
physiological doses of melatonin inhibit
fbrskolin-activated cAMP accumulation in
mammalian pars tuberalis culture and
explant systems (Morgan et ai, 1989a;
Weaver et ai, 1991). Although the description of the molecular characteristics of
melatonin binding sites has only just begun
(Rivkees et ai, 1990; Pickering et ai, 1990),
standard strategies for cloning of G-coupled
receptors may soon be applied to this problem. Sequence analysis may provide an
understanding of the evolution of melatonin receptors and the intracellular sequelae
of melatonin binding.
In most autoradiographic studies of IMEL
binding, film images have been generated
in unfixed frozen tissues. While high concentrations of binding sites are obvious and
quantitation can be achieved by such methods, there is a risk that dispersed but physiologically important sites might be missed.
For example, avid binding to GnRH cells
would be difficult to detect since such neurons are generally not clustered in a single
brain nucleus. Furthermore, it has not yet
proven possible to define the characteristics
{e.g., connectivity, peptide phenotype) of
cells or processes which bind IMEL. Finally,
IMEL binding may fluctuate with time of
day or endocrine state to the extent that
reports of a paucity of IMEL binding sites
in a particular area should be regarded with
caution. Nevertheless, these methods have
allowed us to gain new insights into the
physiological basis of melatonin's effects.
Among vertebrates studied to date, a total
absence of specific IMEL binding sites has
been found only in the Atlantic hagfish,
Myxine glutinosa (Vernadakis et ai, 1991).
Binding sites are found in the tectum, telencephalon, and diencephalon of the river
lamprey {Petromyzon marinus), hedgehog
skate {Raja erinacea), rainbow trout {Salmo
gardineri), and clawed toad {Xenopus laevis;
Vernadakis et ai, 1991; Norgren and Bittman, unpublished). While high concentrations of IMEL binding sites exist in lizard
203
brain, their anatomical distribution has not
been described (Rivkees et ai, 1990; Cassone, 1990). Extensive binding is found in
the brains of several orders of birds, with
high concentrations of binding sites in retinorecipient areas, limbic structures, and in
mesencephalic and thalamic nuclei (Rivkees et ai, 1989; Cassone, 1990).
A surprising degree of diversity exists in
the anatomical distribution of IMEL binding sites among mammals (Figs. 2, 3). In
myomorph rodents, the highest concentration of IMEL binding sites is found in the
pars tuberalis of the pituitary (PT; Fig. 3 A).
Within the brain, intense binding is largely
restricted to the suprachiasmatic nucleus of
the hypothalamus, the paraventricular and
reuniens nuclei of the thalamus, and the area
postrema of rats, house mice, and Syrian
and Siberian hamsters (Fig. 2A). No marked
differences were found between the former
two species, in which daylength and pinealectomy have little influence on reproduction, and the latter, which are highly photoperiodic. In the white footed mouse
{Peromyscus leucopus), however, a much
wider distribution of IMEL binding exists,
with concentrations in the preoptic area,
hypothalamus, amygdala, and thalamic
regions. No obvious quantitative differences were apparent between photosensitive and -insensitive races of mice (Weaver
et ai, 1990). A similar pattern has been
found in the 13-lined ground squirrel, Citellus tridecemlineatus (Thomas, Zucker and
Bittman, unpublished).
Further work described the pattern of
IMEL binding in one species of each of seven
other mammalian orders (Table 1). Binding
is found throughout the rabbit anterior
hypothalamus, including the suprachiasmatic region, as well as the preoptic area,
hippocampus, and cortex (Fig 2B). Again,
PT contains a high concentration of binding
sites (Fig 3B). In the little brown bat, binding is found mostly in the preoptic area,
suprachiasmatic region, and mediobasal
hypothalamus, including the arcuate nucleus
and the median eminence/PT region (Bittman, unpublished data). This is of interest
in light of evidence that photoperiod and
melatonin regulate testis weight in vesper-
204
ERIC L. BITTMAN
FIG. 2. Autoradiographic images illustrating total binding of 2-[l25I]iodomelatonin in the anterior hypothalamus
of (A) a Siberian hamster, (B) a European rabbit, (C) a little brown bat, (D) a Suffolk ewe, (E) a musk shrew,
and (F) a 9-banded armadillo. Nonspecific binding is negligible in all cases (data not shown), so these images
are representative of specific binding. Notice that intense binding is evident in the suprachiasmatic nuclei of
the hamster and bat, but absent in that of the ewe and shrew, despite the fact that all are photoperiodic seasonal
breeders. Calibration bars: A, C, E, 200 microns; B, F, 500 microns; D, 2,000 microns.
tilionid bats (Beasley et al, 1984). While no
primate has been studied in detail, the
human SCN contains a high concentration
of binding sites (Reppert et al, 1988). Evidence for IMEL binding in the primate PT
is lacking.
In other species, however, IMEL binding
sites are not found in the SCN. One example
is the sheep, in which binding sites are concentrated in the PT, in septal and preoptic
areas, in the inner and outer molecular layers of the hippocampus and the stratum
lacunosum, the entorhinal cortex, subiculum, and apical interpeduncular nucleus
(Figs. 2D and 3D; de Reviers, 1989; Bittman and Weaver, 1990). IMEL binding is
almost completely restricted to the PT in
the mustelids studied to date. While specific
binding is detectable in pineal of skunks and
pars distalis of ferrets, IMEL binding sites
have not been detected in the brain of either
species (Duncan et al, 1990; Weaver and
Reppert, 1990). Patterns of IMEL binding
in non-mustelid carnivores have not been
reported. IMEL binding in the musk shrew
{Suncus murinus), a photoperiodic insectivore of the tropics (Rissman, 1987) displays
yet a different pattern: specific binding is
MELATONIN BINDING IN MAMMALS
205
'•'4
FIG. 3. Total binding of 2-[l25I]iodomelatonin in the medial basal hypothalamus and pars tuberalis of (A) a
Siberian hamster, (B) a European rabbit, (C) a little brown bat, (D) a Suffolk ewe, (E) a musk shrew, and (F) a
9-banded armadillo. Binding in the pars tuberalis/median eminence region is evident in A-E; in the bat, binding
is also found in the arcuate nucleus. Calibration bars as in Figure 2.
prominent in the PT, detectable in hippocampus, and sparse elsewhere (Figs. 2E, 3E).
No binding is evident in the SCN.
What role does melatonin play at each of
these sites? In light of the role of the SCN
as a master pacemaker (Ralph et ai, 1990),
it has been suggested that melatonin acts in
this area to entrain or coordinate circadian
rhythms. Melatonin can shift the phase of
in vitro electrical activity in the rat SCN
(McArthur etai, 1989; Mason and Brooks,
1989) and synchronize the developing cir-
cadian clocks of hamsters in utero (Davis
and Mannion, 1988). Nevertheless, evidence that melatonin exerts physiologically
important effects on circadian rhythmicity
in adult mammals is less than compelling
for reasons outlined above. Indeed, melatonin injections entrain locomotor rhythms
only at pharmacological doses and only
when given at the nadir of the diurnal
rhythm of melatonin binding (Armstrong,
1989; Laitenen et al, 1989). An alternative
possibility is that SCN melatonin binding
206
ERIC L. BITTMAN
TABLE 1. Summary ofsites ofspecific 2-['"l]iodomelatonin binding among eutherian mammals studied to date. *
Order
Edentata
Pholidota
Carnivora
Tubilidentata
Insectivora
Primates
Scandentia
Dermoptera
Chiroptera
Macroscelidea
Lagomorpha
Rodentia
Myomorphs
Sciurids
Hystrichomorphs
Artiodactyla
Perrisodactyla
Cetacea
Hyracoidea
Sirenia
Proboscidea
Species
PT
SCN
HPC
Other
9-banded armadillo
Ferret
Spotted skunk
Mink
Musk shrew
Man
Little brown bat
European rabbit
Siberian hamster
Syrian hamster
Norwegian rat
White-footed mouse
Golden-mantled
ground squirrel
Guinea pig
Sheep
* Higher taxonomic relationships devised by Novacek (1990). With the exception of the rodents, only one
species has been studied in each order. + indicates high concentrations of binding sites; - indicates low or
negligible binding. No data are available for orders in which no symbols are given. Abbreviations: SCN,
suprachiasmatic nucleus, ME/PT, median eminence/pars tuberalis, hpc, hippocampus.
participates in seasonal reproductive and responses to melatonin and short days (Hasother photoperiodic responses. Administra- tings et ai, 1985; Bonnefond et ai, 1989).
tion of melatonin to the vicinity of th SCN Where SCN lesions have altered responses
can either mimic or block photoperiodic to melatonin, these effects might reflect disresponses in Syrian and Siberian hamsters, inhibition of gonadotropin secretion rather
white footed mice, sheep and skunks (Berria than destruction of a site containing melaet ai, 1989; Glass, 1988). Interpretation of tonin receptors (Bartness et al., 1991). At
such experiments is clouded, however, by the very least, the absence of notable IMEL
the issue of diffusion from the site of appli- binding in the SCN of sheep, skunks and
cation. Infusions of melatonin are generally ferrets suggests that melatonin action in this
effective over a considerable range of dien- area cannot be a universal feature of species
cephalic sites (Badura et ai, 1990; Hastings in which the pineal melatonin secretion
et al., 1985; Lincoln, 1989; Glass, 1988). mediates photoperiodic control of reproMost important, SCN lesions fail to elimi- duction.
nate gonadal responses of pinealectomized
IMEL binding in the PT is a more wideSyrian hamsters or skunks to exogenous spread, if not a universal, characteristic of
melatonin (Berria et al., 1988; Bittman et mammals. Photoperiodic responses in musai, 1988; Maywood et al., 1990). In Syrian telids may be subserved by melatonin action
hamsters, lesions of a region of the anterior at this site alone. The role of the PT in reprohypothalamus dorsal to the SCN eliminates ductive function is poorly understood, and
207
MELATONIN BINDING IN MAMMALS
B
goratdotroplns -
gonadotropins -
FIG. 4. Two alternative explanations for the modulation of LH secretion by melatonin, based on autoradiographic studies of 2-['"I]iodomelatonin binding. Left, melatonin is proposed to act upon cells in the preoptic
region or anterior hypothalamus. Targets may be GnRH neurons themselves or interneurons which regulate
them by synaptic input. Melatonin target cells may also respond directly to estradiol. Right, pars tuberalis is
postulated to be the sole important target of melatonin. In response to an appropriate melatonin signal, pars
tuberalis is hypothesized to secrete a hormone which is as yet unidentified and which in turn regulates the
ensemble of neurons which control the secretion of GnRH.
evidence for a route of communication with
the brain is fragmentary. While it is clear
from ultrastructural studies that the PT contains secretory cells, the nature of their
product and its neuroendocrine role remain
elusive. Thyrotrophs and gonadotrophs are
present in the PT of Siberian hamsters and
sheep respectively, and seasonal fluctuations occur in PT ultrastructure and immunoreactivity in hamsters (Wittowski et ai,
1988). Nevertheless, the majority of PT cells
remain uncharacterized. Furthermore, it is
not clear how melatonin action at the level
of the PT could explain photoperiodinduced changes in the feedback or behavioral potency of gonadal steroid hormones
or the frequency of the hypothalamicpreoptic GnRH pulse generator (Fig. 1C).
It has been suggested that melatonin elicits
secretion of PT hormones which influence
GnRH release, but this idea is purely speculative (Fig. 4).
PHOTOPERIODIC REGULATION OF
NEUROENDOCRINE FUNCTION
Although the sites at which melatonin acts
to regulate reproductive function remain
unclear, the nature of the response to changes
in daylength or administration of melatonin
suggests neurotransmitter systems which
might be seasonally modulated. One strategy is to search for changes in the concentration, distribution, and characteristics of
neuropeptides and their receptors which
regulate steroid hormone feedback, GnRH
pulses and sexual behavior when seasonal
breeders are transferred between stimulatory and inhibitory photoperiods.
One might imagine that daylength regulates the affinity, abundance, and/or distribution of neural receptors for gonadal steroid hormones. There are almost no data to
support this idea, and considerable evidence to refute it. For example, exposure of
hamsters or ewes to inhibitory photoperiods had little influence on hypothalamic or
preoptic androgen or estrogen receptor
occupancy (Bittman and Blaustein, 1990;
Bittman and Krey, 1988;Prins^a/., 1990).
Furthermore, the ability of estradiol to
induce the appearance of progestin receptors is little changed by photoperiod even
when feedback and behavioral responses are
markedly altered (Bittman and Blaustein,
208
ERIC L. BITTMAN
and whether other GnRH terminal fields or
receptor populations, such as those in the
central gray which regulate sexual behavior
5~
600Si S
are under similar control.
1
D 14L:10D
Vasopressin fiber staining is also selec•
10L:14D
0
10L:14D, SCGi
tively regulated by photoperiod and by testosterone in at least two hamster species
(Buijs et al., 1986; Bittman et al., 1990).
Short days reduce fiber densities or numbers
IBNST MPOA
SCN
of immunoreactive cells in the BNST and
the medial amygdala, areas critical for hamster sexual behavior, and in the lateral sepFIG. 5. Effects of 10 weeks of short day exposure on
tum, in which vasopressin acts to inhibit
specific opiate binding in the selected brain regions of
gonad-intact male golden hamsters. Shaded bars, short
hibernation (Hermes et al, 1989).
days; open bars, long days. Hatched bars indicate effects
Endogenous opiates mediate negative
of short day exposure in hamsters subjected to superior
feedback
influences of testosterone upon LH
cervical ganglionectomy (SCGx), which denervates the
secretion, slow GnRH pulses, and inhibit
pineal gland. Abbreviations: MA, medial amygdala;
IM, intercalated cell masses of amygdala; mBNST, bed
male sexual behavior. Thus they are logical
nucleus of stria terminalis, medial portion; MPOA,
candidates for regulators of photoperiodic
medial preoptic area; SCN, suprachiasmatic nucleus.
influences. Short days elevate hypothalamic
*P < 0.05.
and preoptic beta endorphin content and
reduce responsiveness to castration and tes1990; Bittman et al, 1990). Unfortunately, tosterone in Syrian hamsters (Roberts et al.,
such research has employed homogeniza- 1985). Such photoperiods increase the numtion and in vitro assay of receptor popula- ber of immunopositive neurons within the
tions. Dramatic changes in small subpopu- arcuate nucleus of white-footed mice at parlations of steroid receptor-positive neurons ticular times of day (Glass et al., 1988). In
would be missed, and alternative approaches contrast, short days decrease the number of
should be taken to this problem (Blaustein, immunopositive beta endorphin neurons in
1993). Nevertheless, responses of target cells the arcuate nucleus of Siberian hamsters
to steroid hormones can be altered by fluc- (Bittman et al., 1991).
tuations in immediate-early gene expresPhotoperiod-induced changes in opision without changes in steroid receptors atergic pathways include modulation of not
(Diamond et al, 1990), and melatonin may only the endogenous ligands, but also recepact through such a mechanism.
tor populations. The effects of opiate recepImmunocytochemical studies have sug- tor antagonists on LH secretion depend upon
gested a range of photoperiodic influences photoperiod in the ram (Lincoln et al., 1987)
on neuropeptides in seasonal breeders. as well as in the Syrian hamster (Roberts et
Influences of photoperiod and melatonin on al., 1985). Furthermore, short days amplify
classical neurotransmitters were reviewed the inhibitory influences of methadone, an
relatively recently (Glass, 1988) and will not opiate receptor agonist, on chemoinvestibe discussed here. Inhibitory photoperiods gatory and sexual behavior of male golden
elevate the concentration of GnRH in the hamsters (Tubbiola and Bittman, unpubMBH or the number of immunoreactive lished). Significantly, photoperiodic influcells or fibers in the anterior hypothalamus ences on responsiveness to opiate agonists
and preoptic area of some species (Syrian and antagonists persist when castrated long
hamsters, white-footed mice, and sheep), are and short day hamsters are treated with
without effect in others (ferrets and mink), equivalent doses of testosterone.
and decrease them in Siberian hamsters (see
One interpretation of these observations
Glass, 1988 and Bittman et al., 1991 for is that daylength regulates opiate receptor
reviews). It remains to be determined concentration or affinity. Study of specific
whether photoperiod and testosterone [3H]-naloxone binding by in vitro autorainteract to regulate GnRH staining patterns diography indicates that this is the case (Fig.
I
MELATONIN BINDING IN MAMMALS
5). Exposure of hamsters to inhibitory photoperiods reduces naloxone binding within
the medial nucleus of the amygdala, a region
necessary for the expression of male sexual
behavior (Tubbiola etai, 1989). Castration
elevates amygdaloid [3H]-naloxone binding
and eliminates the effect of photoperiod.
Photoperiod regulates the ability of androgen to suppress opiate binding in this critical
brain region: testosterone maintenance from
the time of castration significantly lowers
opiate binding only in long day hamsters.
One major direction for future work is
the description of photoperiod-induced
changes in neurotransmitter systems.
Beyond this task lays the challenge of working out mechanisms which underlie the
interaction between gonadal steroids and
melatonin. We still must draw in the links
between the melatonin binding sites and the
effector systems responsible for the feedback, behavioral and other responses which
are so dramatically regulated by daylength.
ACKNOWLEDGMENTS
The original research described in this
paper was supported by NSF grant BNS8616935 and NIH grant RO1-MH44132. I
thank Maria Al-Shamma, George Drake,
Richard Hurlbut and Beth Meyer for
invaluable technical assistance.
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