The photoperiodic response in Syrian hamster depends upon a melatonin-driven circadian rhythm of sensitivity to melatonin

Melatonin and Seasonal Synchrony in Mammals

In mammals, seasonal opportunities and challenges are anticipated through programmed changes in physiology and behavior. Appropriate anticipatory timing depends on synchronization to the external solar year, achieved through the use of day length (photoperiod) as a synchronizing signal. In mammals, nocturnal production of melatonin by the pineal gland is the key hormonal mediator of photoperiodic change, exerting its effects via the hypothalamopituitary axis. In this review/perspective, we consider the key developments during the history of research into the seasonal synchronizer effect of melatonin, highlighting the role that the pars tuberalis-tanycyte module plays in this process. We go on to consider downstream pathways, which include discrete hypothalamic neuronal populations. Neurons that express the neuropeptides kisspeptin and (Arg)(Phe)-related peptide-3 (RFRP-3) govern seasonal reproductive function while neurons that express somatostatin may be involved in seasonal metabolic adaptations. Finally, we identify several outstanding questions, which need to be addressed to provide a much thorough understanding of the deep impact of melatonin upon seasonal synchronization.

Melatonin Administration Methods for Research in Mammals and Birds

Endocrine research in animals often entails exogenous hormone administration. Special issues arise when developing administration protocols for hormones with circadian and seasonal periodicity. This article reviews various methods for the exogenous administration of hormones with such periodicities by focusing on melatonin. We discuss that methodological variations across studies can affect experimental results. Melatonin administration techniques used in vertebrates includes infusion pumps, beeswax pellets, oral administration, injections, SILASTIC capsules, osmotic pumps, transdermal delivery, beads, and sponges.

Melatonin-induced changes in the expression of thyroid hormone-converting enzymes in hypothalamus depend on the timing of melatonin injections and genetic background in mice

Recent studies have identified TSHB, Dio2, and Dio3 as key genes for the photoperiodic regulation of gonads. In mammals, the expression of these genes is controlled by melatonin. Surprisingly, this effect of melatonin was shown to be conserved in several reproductively non-photoperiodic laboratory mouse strains that have thus become a valuable model to decipher the mechanisms through which melatonin controls the expression of TSHB, Dio2, and Dio3. In this study, we assessed the effects of intraperitoneal melatonin injections and of their timing on the expression of TSHB, TSHR, Dio2, and Dio3 in the hypothalamo-hypophysial systems of melatonin-proficient CBA/N and melatonin-deficient C57BL/6J mice kept under long-day conditions. In CBA/N mice, Dio3 expression was induced by a daily melatonin injection at ZT14 only, whereas in C57BL/6J mice, a daily melatonin injection induced Dio3 expression at all time points investigated (ZT8, 14, and 20) without changes in TSHB expression in both strains. Dio2 expression was suppressed by a daily melatonin injection only in C57BL/6J mice and only at ZT8. Effect of a daily melatonin injection on TSHR expression was strain- and region- specific. Melatonin levels elevated in plasma and hypothalamus after intraperitoneal injections of melatonin at ZT8 for 7days in C57BL/6J returned to basal levels within 1h after the final injection, while in CBA/N mice melatonin levels in hypothalamus remained high for at least 1h. These data suggest that Dio2 and Dio3 expression in the hypothalamus is differentially regulated by the timing of melatonin injections through strain-specific mechanisms.

Melatonin

Melatonin (MEL) is a hormone synthesized and secreted by the pineal gland deep within the brain in response to photoperiodic cues relayed from the retina via an endogenous circadian oscillator within the suprachiasmatic nucleus in the hypothalamus. The circadian rhythm of melatonin production and release, characterized by nocturnal activity and daytime quiescence, is an important temporal signal to the body structures that can read it. Melatonin acts through high-affinity receptors located centrally and in numerous peripheral organs. Different receptor subtypes have been cloned and characterized: MT(1) and MT(2) (transmembrane G-protein-coupled receptors), and MT(3). However, their physiological role remains unelucidated, although livestock management applications already include the control of seasonal breeding and milk production. As for potential therapeutic applications, exogenous melatonin or a melatonin agonist and selective 5-hydroxytrypiamine receptor (5-HT(2c)) antagonist, eg, S 20098, can be used to manipulate circadian processes such as the sleep-vake cycle, which are frequently disrupted in many conditions, most notably seasonal affective disorder.

Melatonin in animal models

Melatonin is a hormone synthesized and secreted during the night by the pineal gland. Its production is mainly driven by the Orcadian clock, which, in mammals, is situated in the suprachiasmatic nucleus of the hypothalamus. The melatonin production and release displays characteristic daily (nocturnal) and seasonal patterns (changes in duration proportional to the length of the night) of secretion. These rhythms in circulating melatonin are strong synchronizers for the expression of numerous physiological processes. In mammals, the role of melatonin in the control of seasonality is well documented, and the sites and mechanisms of action involved are beginning to be identified. The exact role of the hormone in the diurnal (Orcadian) timing system remains to be determined. However, exogenous melatonin has been shown to affect the circadian clock. The molecular and cellular mechanisms involved in this well-characterized “chronobiotic” effect have also begun to be characterized. The circadian clock itself appears to be an important site for the entrapment effect of melatonin and the presence of melatonin receptors appears to be a prerequisite. A better understanding of such “chronobiotic” effects of melatonin will allow clarification of the role of endogenous melatonin in circadian organization.

Melatonin: both master clock output and internal time-giver in the circadian clocks network

Daily rhythms in physiological and behavioral processes are controlled by a network of circadian clocks, reset by inputs and delivering circadian signals to the brain and peripheral organs. In mammals, at the top of the network is a master clock located in the suprachiasmatic nuclei (SCN) of the hypothalamus, mainly reset by ambient light. The nocturnal synthesis and release of melatonin by the pineal gland are tightly controlled by the SCN clock and inhibited by light exposure. Several roles of melatonin in the circadian system have been identified. As a major hormonal output, melatonin distributes temporal cues generated by the SCN to the multitude of tissue targets expressing melatonin receptors. In some target structures, like the Pars tuberalis of the adenohypophysis, these melatonin signals can drive daily rhythmicity that would otherwise be lacking. In other target structures, melatonin signals are used for the synchronization (i.e., adjustment of the timing of existing oscillations) of peripheral oscillators, such as the fetal adrenal gland. Due to the expression of melatonin receptors in the SCN, endogenous melatonin is also able to feedback onto the master clock, although its physiological significance needs further characterization. Of note, pharmacological treatment with exogenous melatonin can synchronize the SCN clock. From a clinical point of view, provided that the subject is not exposed to light at night, the daily profile of circulating melatonin provides a reliable estimate of the timing of the human SCN. During the past decade, a number of melatonin agonists have been developed for treating circadian, psychiatric and sleep disorders. These drugs may target the SCN for improving circadian timing or act indirectly at some downstream level of the circadian network to restore proper internal synchronization.

An (n-3) polyunsaturated fatty acid-deficient diet disturbs daily locomotor activity, melatonin rhythm, and striatal dopamine in Syrian hamsters

Several studies suggest that (n-3) PUFA may play a role in the regulation of cognitive functions, locomotor and exploratory activity, and affective disorders. Additionally, (n-3) PUFA affect pineal function, which is implicated in the sleep-wake rhythm. However, no studies to our knowledge have explored the role of PUFA on the circadian system. We investigated the effect of an (n-3) PUFA-deficient diet on locomotor and pineal melatonin rhythms in Syrian hamsters used as model species in circadian rhythm research. To assess the possible relationship between voluntary wheel running activity and dopaminergic neurotransmission, we also measured endogenous monoamine concentrations in the striatum. Two-month-old male hamsters, fed either an (n-3) PUFA-deficient or an (n-3) PUFA-adequate diet, were housed individually in cages equipped with run wheels. At 3 mo, cerebral structures were extracted for biochemical and cellular analysis. In (n-3) PUFA-deficient hamsters, the induced changes in the pineal PUFA membrane phospholipid composition were associated with a reduction in the nocturnal peak level of melatonin that was 52% lower than in control hamsters (P < 0.001). The (n-3) PUFA-deficient hamsters also had higher diurnal (P < 0.01) and nocturnal (P = 0.001) locomotor activity than the control hamsters, in parallel with activation of striatal dopaminergic function (P < 0.05). The (n-3) PUFA-deficient hamsters exhibited several symptoms: chronic locomotor hyperactivity, disturbance in melatonin rhythm, and striatal hyperdopaminergia. We suggest that an (n-3) PUFA-deficient diet lessens the melatonin rhythm, weakens endogenous functioning of the circadian clock, and plays a role in nocturnal sleep disturbances as described in attention deficit/hyperactivity disorder.

Seasonal photoperiodism in vertebrates: from coincidence to amplitude

In vertebrates living in regions that range from tropical to polar zones, the day length (photoperiod) is a powerful synchronizer of seasonal changes in endocrine and metabolic physiology. This seasonal photoperiodism depends on the responses of internal circadian clocks to changing patterns of light-dark exposure, which can be conceptualized in the form of "coincidence-timing" models. The structural basis for this timing function is formed by a specialized "photoperiodic axis" that links light reception to the neuroendocrine system. In this review we describe the essential elements of this axis in mammals and birds, and discuss recent progress in understanding the cellular and molecular mechanisms through which this axis transduces photoperiodic change into altered endocrine output.

Role of the pineal gland and melatonin in the photoperiodic control of hypothalamic gonadotropin-releasing hormone in the male jerboa (Jaculus orientalis), a desert rodent

The neuroendocrine mechanism underlying seasonal changes in gonadal activity of the jerboa, a desert hibernating rodent adapted to harsh climatic conditions, are poorly understood. We investigated the role of the pineal gland and melatonin in the photoperiodic control of hypothalamic gonadotropin-releasing hormone (GnRH). Intact and pinealectomized male jerboas were subjected to short photoperiod, while others were kept under long photoperiod and injected daily with melatonin or vehicle. Testes activity was monitored by evaluating the testes volume during 10 weeks. GnRH immunoreactivity was investigated quantitatively with image analysis. Following melatonin administration, the hormone peaked in plasma after 30 min, with return to control levels 2.5 h later. Exposure to short photoperiod and melatonin resulted in marked increase in the number of GnRH-containing cells in the preoptic area and mediobasal hypothalamus, whereas GnRH immunoreactivity of fibers and terminals in the median eminence decreased under these conditions. The findings indicate that in the jerboa short photoperiod induces testicular regression by prolonging the duration of melatonin as an endocrine signal. This mechanism probably involves inhibition of GnRH release in the median eminence, with consequent accumulation of GnRH in perikarya of the preoptic area and mediobasal hypothalamus. Interestingly, GnRH cells of the median eminence did not appear to be influenced by the photoperiod and pineal melatonin, whereas their number was increased by exogenous melatonin. The latter data suggest for the first time the involvement of an extrapineal melatonin, whose origin remains to be identified, in the modulation of the GnRH regulatory system in rodents.

Basic aspects of melatonin action

Melatonin is synthesized and secreted during the dark period of the light-dark cycle. Thus, melatonin has an obvious association with sleep, at least in diurnal animals. Rhythmic nocturnal melatonin secretion is directly generated by the circadian clock, located in mammals within the suprachiasmatic nuclei (SCN), and is entrained to a 24-h period by the light-dark cycle. The periodic secretion of melatonin may be used as a circadian mediator to any system than can "read" the message. In addition, direct effects of the hormone on the SCN could explain some of melatonin effects on the circadian system. Duration of melatonin nocturnal secretion is directly proportional to the length of the night and it has been demonstrated experimentally to be the critical parameter for photoperiod integration. The two main hypotheses to explain the action of melatonin are the duration hypothesis (supporting that night length is coded by the duration of the melatonin secretory phase) and the coincidence hypothesis (holding that physiological responses are linked to the existence of a diurnal rhythm in sensitivity to melatonin). The sites and mechanisms of action of melatonin for circadian and photoperiodic responses are far from being elucidated, but action through specific membrane receptor sites is well documented. In view of melatonin s lipophilic nature, interactions with specific intracellular proteins like calmodulin or tubulin, or with nuclear receptor sites, have also been considered, whereas the physiological significance of the documented antioxidant effect of melatonin remains to be settled. Melatonin seems to act as an "arm" of the circadian clock, giving a time-related signal to a number of body functions; one of these, the circadian organization of an organism's defence, is discussed in some detail as an example.

Molecular characterization of the long-day response in the Soay sheep, a seasonal mammal

In mammals, seasonal timekeeping depends on the generation of a nocturnal melatonin signal that reflects nightlength/daylength. To understand the mechanisms by which the melatonin signal is decoded, we studied the photoperiodic control of prolactin secretion in Soay sheep, which is mediated via melatonin responsive cells in the pars tuberalis of the pituitary. We demonstrate that the phases of peak expression of the clock genes Cryptochrome1 (Cry1), Period1 (Per1), and RevErbalpha respond acutely to altered melatonin secretion after a switch from short to long days. Cry1 is activated by melatonin onset, forming the dusk component of the molecular decoder, while Per1 expression at dawn reflects the offset of melatonin secretion. The Cry1-Per1 interval immediately adjusts to the melatonin signal on the first long day, and this is followed within 24 hr by an increase in prolactin secretion. The timing of peak RevErbalpha expression also responds to a switch to long days due to altered melatonin secretion but does not immediately reset to an entrained long-day state. These data suggest that effects of melatonin on clock gene expression are pivotal events in the neuroendocrine response and that pars tuberalis cells can act as molecular calendars, carrying a form of "photoperiodic memory."

Independence of circadian entrainment state and responses to melatonin in male Siberian hamsters

Background

Seasonal fluctuations in physiology and behavior depend on the duration of nocturnal melatonin secretion programmed by the circadian system. A melatonin signal of a given duration, however, can elicit different responses depending on whether an animal was previously exposed to longer or shorter photoperiod signals (i.e., its photoperiodic history). This report examined in male Siberian hamsters which of two aspects of photoperiod history – prior melatonin exposure or entrainment state of the circadian system – is critical for generating contingent responses to a common photoperiodic signal.

Results

In Experiment #1, daily melatonin infusions of 5 or 10 h duration stimulated or inhibited gonadal growth, respectively, but had no effect on entrainment of the locomotor activity rhythm to long or short daylengths, thereby demonstrating that melatonin history and entrainment status could be experimentally dissociated. These manipulations were repeated in Experiment #2, and animals were subsequently exposed to a 12 week regimen of naturalistic melatonin signals shown in previous experiments to reveal photoperiodic history effects. Gonadal responses differed as a function of prior melatonin exposure but were unaffected by the circadian entrainment state. Experiment #3 demonstrated that a new photoperiodic history could be imparted during four weeks of exposure to long photoperiods. This effect, moreover, was blocked in animals treated concurrently with constant release melatonin capsules that obscured the endogenous melatonin signal: Following removal of the implants, the gonadal response depended not on the immediately antecedent circadian entrainment state, but on the more remote photoperiodic conditions prior to the melatonin implant.

Conclusions

The interpretation of photoperiodic signals as a function of prior conditions depends specifically on the history of melatonin exposure. The photoperiodic regulation of circadian entrainment state contributes minimally to the interpretation of melatonin signals.

Generation of the melatonin endocrine message in mammals: a review of the complex regulation of melatonin synthesis by norepinephrine, peptides, and other pineal transmitters

Melatonin, the major hormone produced by the pineal gland, displays characteristic daily and seasonal patterns of secretion. These robust and predictable rhythms in circulating melatonin are strong synchronizers for the expression of numerous physiological processes in photoperiodic species. In mammals, the nighttime production of melatonin is mainly driven by the circadian clock, situated in the suprachiasmatic nucleus of the hypothalamus, which controls the release of norepinephrine from the dense pineal sympathetic afferents. The pivotal role of norepinephrine in the nocturnal stimulation of melatonin synthesis has been extensively dissected at the cellular and molecular levels. Besides the noradrenergic input, the presence of numerous other transmitters originating from various sources has been reported in the pineal gland. Many of these are neuropeptides and appear to contribute to the regulation of melatonin synthesis by modulating the effects of norepinephrine on pineal biochemistry. The aim of this review is firstly to update our knowledge of the cellular and molecular events underlying the noradrenergic control of melatonin synthesis; and secondly to gather together early and recent data on the effects of the nonadrenergic transmitters on modulation of melatonin synthesis. This information reveals the variety of inputs that can be integrated by the pineal gland; what elements are crucial to deliver the very precise timing information to the organism. This also clarifies the role of these various inputs in the seasonal variation of melatonin synthesis and their subsequent physiological function.

Seasonal aspects of sleep in the Djungarian hamster

BACKGROUND

Changes in photoperiod and ambient temperature trigger seasonal adaptations in the physiology and behaviour of many species, including the Djungarian hamster. Exposure of the hamsters to a short photoperiod and low ambient temperature leads to a reduction of the polyphasic distribution of sleep and waking over the light and dark period. In contrast, a long photoperiod enhances the daily sleep-wake amplitude leading to a decline of slow-wave activity in NREM sleep within the light period. It is unknown whether these changes can be attributed specifically to photoperiod and/or ambient temperature, or whether endogenous components are contributing factors. The influence of endogenous factors was investigated by recording sleep in Djungarian hamsters invariably maintained at a low ambient temperature and fully adapted to a short photoperiod. The second recording was performed when they had returned to summer physiology, despite the maintenance of the 'winter' conditions.

RESULTS

Clear winter-summer differences were seen in sleep distribution, while total sleep time was unchanged. A significantly higher light-dark cycle modulation in NREM sleep, REM sleep and waking was observed in hamsters in the summer physiological state compared to those in the winter state. Moreover, only in summer, REM sleep episodes were longer and waking bouts were shorter during the light period compared to the dark period. EEG power in the slow-wave range (0.75-4.0 Hz) in both NREM sleep and REM sleep was higher in animals in the summer physiological state than in those in the 'winter' state. In winter SWA in NREM sleep was evenly distributed over the 24 h, while in summer it decreased during the light period and increased during the dark period.

CONCLUSION

Endogenous changes in the organism underlie the differences in sleep-wake redistribution we have observed previously in hamsters recorded in a short and long photoperiod.

Melatonin: from seasonal to circadian signal

In mammals, the role of melatonin in the control of seasonality is well documented, and the sites and mechanisms of action involved are beginning to be identified. The exact role of the hormone in the circadian timing system remains to be determined. However, exogenous melatonin has been shown to affect the circadian clock. Identification of the molecular and cellular mechanisms involved in this well characterized chronobiotic effect will allow clarification of the role of endogenous melatonin in circadian organization.

Decoding photoperiodic time and melatonin in mammals: what can we learn from the pars tuberalis?

The cellular and molecular mechanisms through which the melatonin signal is decoded to drive/synchronize photoperiodic responses remain unclear. Much of our current understanding of the processes involved in this readout derives from studies of melatonin action in the pars tuberalis of the anterior pituitary. Here, the authors review current knowledge and highlight critical gaps in our present understanding.

Testicular development in Siberian hamsters depends on frequency and pattern of melatonin signals

We investigated the impact of frequency and pattern of melatonin signals on reproductive development in Siberian hamsters. Juvenile males gestated in short day lengths and housed in constant illumination to suppress melatonin secretion were infused with melatonin for 5 h either once or twice per day for 20 days. Melatonin infusions at either frequency produced equivalent increases in testes and body weights that exceeded those of animals infused with saline but were indistinguishable from those of hamsters transferred to long day lengths. The reproductive system appears to be maximally stimulated by a single short melatonin signal each day. Other animals kept from birth in a short photoperiod were treated 6 h after onset of darkness with the beta-adrenergic receptor antagonist DL-propranolol to shorten melatonin secretion on the night of injection but not on subsequent nights. This permitted interpolation of short nightly melatonin signals of 4-5 h duration against a background of long melatonin signals of 10-12 h duration on other nights. Treatment regimes that maintained a 1:1 ratio of short to long melatonin signals for 8 wk stimulated reproductive development; a 1:2 signal ratio, in each of three different patterns, was uniformly ineffective. The number of successive short melatonin signals had little influence on the interval across which successive melatonin signals were summated to influence photoperiodic traits. The neuroendocrine axis appears more responsive to short melatonin signal frequency than pattern for development of the summer phenotype.

Effects of photoperiod reduction on rat circadian rhythms of BP, heart rate, and locomotor activity

The effects of a photoperiod reduction in the entrainment of circadian rhythms of systolic blood pressure (SBP), diastolic blood pressure (DBP), heart rate (HR), and spontaneous locomotor activity (SLA) were determined in conscious Wistar rats by using radiotelemetry. Two groups of seven rats were maintained in a 12:12-h light-dark (12L/12D) photoperiod for 11 wk and then placed in a reduced photoperiod of 8:16-h light-dark (8L/16D) by advancing a 4-h darkness or by advancing and delaying a 2-h darkness for 6 wk. Finally, they were resynchronized to 12L/12D. Advancing a 4-h dark phase induced a 1-h advance of acrophase for SBP, DBP, and HR, but not for SLA. The percent rhythm, amplitude, and the 12-h mean values of all parameters were significantly decreased by the photoperiod reduction. When symmetrically advancing and delaying a 2-h dark phase, a 1 h 20 min delay of acrophases and a decrease in percent rhythms and amplitudes of SBP, DBP, HR, and SLA were observed. Only the 12-h mean values of HR and SLA were decreased. Our findings show that the cardiovascular parameters differ from SLA in phase-shift response to photoperiod reduction and that the adjustment of circadian rhythms to change from 12L/12D to 8L/16D photoperiod depends on the direction of the extension of the dark period.

Organization of rat circadian rhythms during daily infusion of melatonin or S20098, a melatonin agonist

Daily administration of melatonin or S20098, a melatonin agonist, is known to entrain the free-running circadian rhythms of rats. The effects of the duration of administration on entrainment were studied. The animals demonstrated free-running circadian rhythms (running-wheel activity, body temperature, general activity) in constant darkness. Daily infusions of melatonin or S20098 for 1, 8, or 16 h entrained the circadian rhythms to 24 h. Two daily infusions of 1 h (separated by 8 h) entrained the activity peak within the shorter time interval. The entraining properties of melatonin and S20098 were similar and were affected neither by pinealectomy nor by infusion of 1- or 8-h duration. However, with 16-h infusion, less than half of the animals became entrained. Once entrained, the phase angle between the onset of infusion and the rhythms (onset of activity or acrophase of body temperature) increased with the duration of infusion. Before entrainment, the free-running period increased with the duration of infusion, an effect that was not predictable from the phase response curve.

Long-term exposure of hypothalamic explants to melatonin alters the release of gonadotrophin releasing hormone and the density of melatonin binding sites in the pars tuberalis of the male mink (Mustela vison)

To investigate the action of melatonin on the reproductive system, the effect of prolonged versus short-term exposure to melatonin on the release of gonadotrophin releasing hormone (GnRH) was examined in hypothalamic explants of male mink sacrificed in July, September or November. Mediobasal hypothalamic (MBH) explants including the pars tuberalis (PT) were incubated for 1 night with or without melatonin (10(-8) M) for 8 hr or 16 hr and the release of GnRH was then measured. The next day, the explants were incubated further but in a melatonin free buffer, and the release of GnRH was measured with increasing time. Half of the July and September explants had melatonin binding sites quantified by autoradiography. In November, a 16-hr exposure to melatonin induced a significant increase in the release of GnRH during the night, compared with control or 8-hr melatonin exposure. This increase persisted for at least 45 min after the withdrawal of melatonin, suggesting a stimulatory effect of melatonin on the synthesis of GnRH; this effect was apparent in July, September and November. In September, the density of melatonin binding in the PT was significantly lower in the explants incubated for 16 hr with melatonin, compared with those incubated for 8 hr. Thus, in vitro, a long exposure to melatonin, mimicking a single long night, stimulates the release and synthesis of GnRH in parallel with a decrease in the density of melatonin binding in the PT. These effects seem to depend heavily on the duration of exposure to melatonin.

Environmental control of the seasonal variations in the daily pattern of melatonin synthesis in the European hamster, Cricetus cricetus

Nocturnal patterns of pineal melatonin concentrations were measured at hourly intervals in the European hamster, Cricetus cricetus, maintained under different natural or experimental environmental conditions. There were pronounced variations in the night peak of pineal melatonin both in the duration and the amplitude of the melatonin peak and in the onset and decline of melatonin synthesis. The duration of the melatonin peak increased proportionally with increased dark period. The amplitude increased abruptly from LD 16/8 to LD 15/9 and remained constant in all other photoperiods. The onset of synthesis started 6:00 hours after the onset of darkness in LD 16/8, 15/9, and 14/10, while it started 4:00 hours after dark onset in shorter photoperiods (LD 12/12 and 10/14). This result is opposite to that observed in the rat. The decline of synthesis was delayed as darkness increased and was directly related to lights on in long photoperiods, while it was endogenous in short photoperiods. Temperature, under a long photoperiod, also seems to be implicated in the regulation of the amplitude of the melatonin peak.

Melatonin secretion in rams maintained in constant darkness depends on the timing of a single 1-hour light pulse given the previous night

Previous data have demonstrated that a single 1-hr light pulse at night affects the secretion of melatonin in the ram if it was given at the appropriate time. This paper reports the effect on melatonin secretion of a 1-hr light pulse given at two different times at night to two groups of rams kept in constant darkness the day following light application. It appears that the animals were able to remember the light pulse if it was given 12 hr but not 9 hr after the lights were turned off. This memory could possibly be stored in the suprachiasmatic nucleus as reported recently in the rat.