Melatonin,immunity and cost of reproductive state in male European starlings.

The effects of reproductive condition and exogenous melatonin on immune function were investigated in castrated European starlings, Sturnus vulgaris. Photorefractory and photostimulated starlings exposed to long days were implanted with melatonin or with blank capsules. Photostimulated starlings with blank capsules exhibited reduced splenocyte proliferation in response to the T-cell mitogen, concanavalin A, compared with the other long-day birds. Exogenous melatonin prevented the suppression of immune function by photostimulation. Photorefractory starlings, with or without melatonin implants, exhibited enhanced immune function compared with photostimulated starlings implanted with blanks. This enhancement was not mediated by endogenous melatonin, but appeared to be related to changes in reproductive state. In addition to the traditional costs of reproduction in birds (e.g. raising of young), there may be a cost of the reproductive state of starlings (i.e. whether they are photorefractory or photostimulated). These data are, we believe, the first to indicate a direct effect of reproductive state on immune function that is independent of both photoperiod (i.e., changes in the duration of melatonin secretion) and gonadal steroids.     

Importance of duration of nocturnal melatonin secretion in determining the reproductive response to inductive photoperiod in the ewe

The pineal gland, through its nocturnal melatonin secretion, mediates the effects of inhibitory (long) and stimulatory (short) photoperiod on reproduction in female sheep. Earlier studies revealed that duration of the nighttime melatonin rise is important in determining the inhibitory effect of day length on reproduction in the ewe. The present study tested whether the duration is also important in mediating the inductive response to short days. Pinealectomized ewes, housed under long days, received a short-day melatonin infusion (16-h duration) for 90 days. Reproductive status was monitored from the response to estradiol negative feedback of luteinizing hormone (LH) secretion. This short-day melatonin pattern led to unambiguous reproductive induction, despite the exposure to inhibitory long days. The increase in serum LH was comparable, in terms of latency and magnitude, to that in pinealectomized controls receiving the same short-day melatonin pattern under short days, and in pineal-intact controls transferred from long to short days. Since the reproductive status conformed to the length of time that melatonin was elevated each day rather than to photoperiod, these results support the conclusion that duration of the nighttime melatonin rise mediates the reproductive response of the ewe to an inductive photoperiod. In all, the melatonin rhythm is considered an integral component of the physiologic mechanism measuring day length; through duration of its nocturnal secretion, melatonin encodes both inhibitory and stimulatory photoperiods.     

Endogenous circannual cycles of ovarian activity and changes in prolactin and melatonin secretion in wild and domestic female sheep maintained under a long-day photoperiod

The present study examines the ovulatory activity of wild and domesticated ewes subjected to either a constant photoperiod of long days (16L:8D) or natural changes in daily photoperiod for 16 mo. The aim was to determine whether an endogenous reproductive rhythm controls seasonal reproductive activity in these sheep, and how the photoperiod might affect this. The effects of long-day photoperiods on long-term changes in prolactin and melatonin secretion were also evaluated. The two species showed changes in reproductive activity under the constant photoperiod conditions, suggesting the existence of an endogenous rhythm of reproduction. This rhythm was differently expressed in the two types of ewe (P < 0.05), with the domestic animals exhibiting much greater sensitivity to the effects of long days. A circannual rhythm of plasma prolactin concentration was also seen in both species and under both photoperiod conditions, although in both species the amplitude was always lower in the long-day animals (P < 0.01). The duration of the nocturnal melatonin plasma concentrations reflected the duration of darkness in both species and treatments. The peak melatonin concentration did not differ between seasons either under natural or long-day photoperiods.     

Effects of melatonin feeding on smoltification in masu salmon (Oncorhynchus masou)

Influences of photoperiod on plasma melatonin profiles and effects of melatonin administration on long-day-induced smoltification in masu salmon (Oncorhynchus masou) were investigated in order to reveal the roles of melatonin in the regulation of smoltification in salmonids. Under light-dark (LD) cycles, plasma melatonin levels exhibited daily variation, with higher values during the dark phase than during the light phase. The duration of nocturnal elevation under short photoperiod (LD 8:16) was longer than that under long photoperiod (LD 16:8). Melatonin feeding (0.01, 0.1 and 1 mg/kg body weight) elevated plasma levels of melatonin in a dose-dependent manner for at least 7 h but not for 24 h. When masu salmon reared under short photoperiod were exposed to long photoperiod (LD 16:8) and fed melatonin (1 mg/kg body weight) 7 hours before the onset of darkness, a significantly smaller proportion of smolts appeared in the melatonin-fed group after 32 days than in the control group. However, after 59 days of the treatment, there was no difference in the proportion of smolts between the control and melatonin-treated groups. Thus, melatonin feeding mimicked the effects of short photoperiod, which delays but does not completely suppress smoltification. These results indicate that the day length is transduced into changes in the duration of nocturnal elevation in plasma melatonin levels, and that artificial modification of the plasma melatonin pattern possibly delays the physiological processes of smoltification induced by long-day photoperiodic treatment.     

Short days and exogenous melatonin increase aggression of male Syrian hamsters (Mesocricetus auratus)

Many nontropical rodent species rely on photoperiod as a primary cue to coordinate seasonally appropriate changes in physiology and behavior. Among these changes, some species of rodents demonstrate increased aggression in short, "winter-like" compared with long "summer-like" day lengths. The precise neuroendocrine mechanisms mediating changes in aggression, however, remain largely unknown. The goal of the present study was to examine the effects of photoperiod and exogenous melatonin on resident-intruder aggression in male Syrian hamsters (Mesocricetus auratus). In Experiment 1, male Syrian hamsters were housed in long (LD 14:10) or short (LD 10:14) days for 10 weeks. In Experiment 2, hamsters were housed in long days and half of the animals were given daily subcutaneous melatonin injections (15 microg/day in 0.1 ml saline) 2 h before lights out for 10 consecutive days to simulate a short-day pattern of melatonin secretion, while the remaining animals received injections of the vehicle alone. Animals in both experiments were then tested using a resident-intruder model of aggression and the number of attacks, duration of attacks, and latency to initial attack were recorded. In Experiment 1, short-day hamsters underwent gonadal regression and displayed increased aggression compared with long-day animals. In Experiment 2, melatonin treatment also increased aggression compared with control hamsters without affecting circulating testosterone. Collectively, the results of the present study demonstrate that exposure to short days or short day-like patterns of melatonin increase aggression in male Syrian hamsters. In addition, these results suggest that photoperiodic changes in aggression provide an important, ecologically relevant model with which to study the neuroendocrine mechanisms underlying aggression in rodents.     

Effects of constant darkness and constant light on circadian organization and reproductive responses in the ram

The relationship between circadian rhythms in the blood plasma concentrations of melatonin and rhythms in locomotor activity was studied in adult male sheep (Soay rams) exposed to 16-week periods of short days (8 hr of light and 16 hr of darkness; LD 8:16) or long days (LD 16:8) followed by 16-week periods of constant darkness (dim red light; DD) or constant light (LL). Under both LD 8:16 and LD 16:8, there was a clearly defined 24-hr rhythm in plasma concentrations of melatonin, with high levels throughout the dark phase. Periodogram analysis revealed a 24-hr rhythm in locomotor activity under LD 8:16 and LD 16:8. The main bouts of activity occurred during the light phase. A change from LD 8:16 to LD 16:8 resulted in a decrease in the duration of elevated melatonin secretion (melatonin peak) and an increase in the duration of activity corresponding to the changes in the ratio of light to darkness. In all rams, a significant circadian rhythm of activity persisted over the first 2 weeks following transfer from an entraining photoperiod to DD, with a mean period of 23.77 hr. However, the activity rhythms subsequently became disorganized, as did the 24-hr melatonin rhythms. The introduction of a 1-hr light pulse every 24 hr (LD 1:23) for 2 weeks after 8 weeks under DD reinduced a rhythm in both melatonin secretion and activity: the end of the 1-hr light period acted as the dusk signal, producing a normal temporal association of the two rhythms. Under LL, the 24-hr melatonin rhythms were disrupted, though several rams still showed periods of elevated melatonin secretion. Significant activity rhythms were either absent or a weak component occurred with a period of 24 hr. The introduction of a 1-hr dark period every 24 hr for 2 weeks after 8 weeks under LL (LD 23:1) failed to induce or entrain rhythms in either of the parameters. The occurrence of 24-hr activity rhythm in some rams under LL may indicate nonphotoperiodic entrainment signals in our experimental facility. Reproductive responses to the changes in photoperiod were also monitored. After pretreatment with LD 8:16, the rams were sexually active; exposure to LD 16:8, DD, or LL resulted in a decline in all measures of reproductive function. The decline was slower under DD than LD 16:8 or LL.(ABSTRACT TRUNCATED AT 400 WORDS)     

Spontaneous c-Fos rhythm in the rat suprachiasmatic nucleus: location and effect of photoperiod

A recently reported circadian rhythm in the spontaneous c-Fos immunoreactivity in the rat suprachiasmatic nucleus (SCN) is expressed mostly in the dorsomedial (dm) SCN, where vasopressinergic cells are located. The aim of the present study is to find out whether day length, i.e., photoperiod, affects the dm-SCN rhythm and, if so, how the rhythm adjusts to a change from a long to a short photoperiod. In addition, a question of whether the spontaneous c-Fos production is localized in vasopressin- producing cells or in other cells is also studied to characterize further the dm-SCN rhythmicity. Combined immunostaining for c-Fos and arginine vasopressin (AVP) revealed that most of c-Fos immunopositive cells were devoid of AVP; the results suggest that c-Fos-producing cells in the dm-SCN are mostly not identical with those producing AVP. In rats maintained under a long photoperiod with 16:8-h light-dark cycle (LD 16:8) daily and then released into darkness, the time of the afternoon and evening decline of the spontaneous c-Fos immunoreactivity in the dm-SCN differed just slightly from the time in rats maintained originally under a short LD 8:16 photoperiod; however, the morning c-Fos rise occurred about 4 h earlier under the long than under the short photoperiod. After a change from a long to a short photoperiod, a rough but not yet a fine adjustment of the morning c-Fos rise to the change was accomplished within 3-6 days. The results show that similar to the recently reported ventrolateral SCN rhythmicity, the intrinsic dm-SCN rhythmicity is also affected by the photoperiod and suggest that the whole SCN state is photoperiod dependent.     

The effect of pinealectomy on circadian plasma melatonin levels in house sparrows and European starlings.

We determined 24-hr plasma melatonin profiles in intact, sham-pinealectomized, and pinealectomized European starlings (Sturnus vulgaris) and house sparrows (Passer domesticus) in a light-dark (LD) cycle and in constant darkness (DD). In the intact and sham-pinealectomized birds of both species, a melatonin rhythm was found, with low levels during the day and high levels during the night. Pinealectomy abolished the nighttime peak of melatonin in both species; hence, levels were low at all times sampled. This uniform response of plasma melatonin to pinealectomy contrasts with the differential response of circadian activity rhythms to pinealectomy for these two species. In DD, locomotor activity in pinealectomized house sparrows is usually arrhythmic, whereas in starlings a rhythm usually persists. This suggests that in the latter species free-running circadian rhythms are not necessarily dependent on a rhythm in plasma melatonin. The same is true for the synchronized activity rhythm observed in pinealectomized birds of both species in LD, as well as for the damped rhythm that persists in pinealectomized house sparrows following an LD-to-DD transfer. The results are consistent with the hypothesis that the pineal and its periodic output of melatonin constitute only one component in a system of at least two coupled pacemakers. They also suggest that there are species differences in the relative role played by the pineal and other pacemakers in controlling circadian rhythms in behavior.     

Phase shifts in the circadian rhythm in plasma concentrations of melatonin in rams induced by a 1-hour light pulse

The effect of a 1-hr light pulse, given at night, on the timing of the circadian rhythm in the plasma concentration of melatonin was examined in Soay rams to investigate the mechanisms involved in determining the duration of the nocturnal peak in melatonin secretion. Animals (n = 8) were housed under short days (LD 8:16) or long days (LD 16:8) and received a light pulse at various times of night. They were released into constant dim red light (DD) on day 1. Blood samples were collected hourly for 30 hr from 1000 hr on day 3, and the plasma concentration of melatonin was determined by radioimmunoassay to assess the timing of the melatonin peak. Control animals (n = 8) were maintained under the same conditions but received no light pulse. Under short days, a light pulse given early in the night caused a phase delay in the melatonin peak, and a light pulse given in the late night caused a phase advance. The mean duration of the melatonin peak was slightly reduced following a light pulse in the early or late night, and slightly increased following a pulse given near the middle of the night. Under long days, both light-pulse treatments given at night caused a phase delay in the melatonin peak, but there was no significant change in duration of the melatonin peak. The duration of the melatonin peak at day 3 under DD in the control animals was similar for all treatments, regardless of the previous entraining photoperiod (mean duration: 12.6-14.8 hr) and was similar to that under short days (14.6 hr), but was significantly longer than that under long days (8.2 hr). Information on the phase response curve in the Soay ram and on the period of the circadian oscillator governing the melatonin rhythm (c 23.0 hr under DD) predicts a close phase relationship between the end of the light phase and the onset of the melatonin peak as observed under normal 24-hr LD cycles. The current results also indicate that light acts to entrain the circadian rhythm influencing the onset and offset of melatonin secretion, and thus dictates the duration of the melatonin peak.     

Changes in the daily rhythm of plasma corticosterone concentration related to seasonal conditions in the white-throated sparrow, Zonotrichia albicollis.

Circadian variations in concentrations of plasma corticosterone were investigated in the white-throated sparrow maintained on short (10-hr) or long (16-hr) daily photoperiods. In addition, the plasma concentrations of corticosterone were determined throughout a day in birds that were in the reproductively photosensitive spring migratory condition, the reproductively photorefractory post nuptial molt condition, and the fall migratory condition. Distinct unimodal rhythms were found in photosensitive birds. The daily rise occurred 12 hr after the offset of light in birds kept on both the short and the long photoperiodic regimens. There was no discernible daily variation in photorefractory birds kept on a 16 hr daily photoperiod and there was a bimodal rhythm in the birds that were in the fall migratory condition. The results are consistent with an hypothesis that assigns an important role to the circadian rhythm of corticosteroid concentration in the photoperiodic mechanism controlling seasonal reproductive and migratory conditions in the white-throated sparrow.     

What photoperiodic signal is provided by a continuous-release melatonin implant?

The purpose of this study was to evaluate whether the insertion of a continuous-release melatonin implant into ewes provides a short-day photoperiodic signal or acts as a functional pinealectomy (provides no specific photoperiodic signal but renders ewes incapable of responding to changes in photoperiod). Ewes primed with 60 long days (18L:6D) during the spring were moved to intermediate day length (13L:11D) for 66 days and then given one of five treatments: 1) short-day control, second drop in photoperiod to 8L:16D; 2) intermediate-photoperiod control, kept on 13L:11D; 3) pinealectomy and kept on 13L:11D; 4) melatonin implant and kept on 13L:11D; 5) melatonin implant and moved to 8L:16D. Mean number of estrous cycles per group and total duration of reproductive activity were determined. Ewes in all groups began to exhibit estrous cycles after the initial reduction in photoperiod. The number of estrous cycles and duration of reproductive activity differed among groups. The number of estrous cycles and duration of reproductive activity was extended in ewes receiving the second drop in photoperiod compared to that of the intermediate-photoperiod controls. Pinealectomized ewes had a number of estrous cycles and duration of reproductive activity similar to those of ewes maintained on the intermediate photoperiod. Melatonin implants increased the number of estrous cycles and prolonged reproductive activity in ewes maintained on the intermediate photoperiod; melatonin implants did not prevent the extension of reproductive activity in ewes receiving the second photoperiodic drop to the short daylength.(ABSTRACT TRUNCATED AT 250 WORDS)     

Circadian behavioral and melatonin rhythms in the European starling under light-dark cycles with steadily changing periods: evidence for close mutual coupling?

In European starlings exposed to constant conditions, circadian rhythms in locomotion and feeding can occasionally exhibit complete dissociation from each other. Whether such occasional dissociation between two behavioral rhythms reflects on the strength of the mutual coupling of their internal oscillators has not been investigated. To examine this, as well as to elucidate the role of melatonin in this system, we simultaneously measured the rhythms of locomotion, feeding and melatonin secretion in starlings exposed to light-dark (LD) cycles of low intensity with steadily changing periods (T). In birds initially entrained to T 24 LD cycles (12L:12D, 10:0.2 lx), beginning on day 15, T was either lengthened to 26.5 h (experiment 1) or shortened to T 21.5 h (experiment 2) by changing the daily dark period 4 min each day. After 18 and 19 cycles of T 26.5 and T 21.5, respectively, birds were released into constant dim light conditions (LL(dim); 0.2 lx) for about 2 weeks. Locomotor and feeding rhythms were continuously recorded. Plasma melatonin levels were measured at three times: in T 24, when T equaled 26 or 22 h and at the end of T 26.5 or T 21.5 exposure. The results show that, contrary to our expectations, the three rhythms were not dissociated. Rather they remained synchronized and changed their phase angle difference with the light zeitgeber concomitantly and at the same rate. The melatonin rhythm stayed in synchrony with the behavioral rhythms and as a consequence, peaked either during day or at night, depending on the phase relationship between the activity rhythm and the zeitgeber cycle.     

Photoperiod influences the annual reproductive cycle of the male pallid bat (Antrozous pallidus)

Adult pallids bats collected in April or May, were maintained in short or long photoperiods (10 or 14 h light/day) for 3-6 months. In August, the short-day bats had regressed testes, epididymal spermatozoa and fully developed accessory sex glands, corresponding to the autumnal reproductive condition of field animals; long-day bats had testes undergoing spermatogenesis, few epididymal spermatozoa and undeveloped accessory sex glands (summer reproductive condition). Bats in each photoperiod manifested the expected autumnal reproductive pattern in October. We suggest that photoperiod influences the reproductive physiology of male pallid bats by affecting an endogenous circannual reproductive rhythm.     

Effects of cold,short day and melatonin on thermogenesis,body weight and reproductive organs in Alaskan red-backed voles

Summary This study examined whether cold, short day or melatonin causes reproductive regression and stimulates nonshivering thermogenesis in a subarctic rodentClethrionomys rutilus. Red-backed voles born and raised at 23°C and 22 h light per day (LD 22: 2) at Fairbanks, Alaska (65°N) were exposed in one of six groups to: 1) long day (LD 22:2), 23°C, injected daily with melatonin or saline 2 h before lights out, 2) long day, 3°C, injected daily with melatonin or saline, 3) short day (LD 8:16), 23°C or 3°C. Voles were tested for nonshivering thermogenesis (NST) prior to and after 8 wk exposure. Body weight, testes weight and female reproductive tract weight were assessed after 8 wk in long day and 12 wk in short day.NST was not altered by short day or melatonin but cold (3°C) caused an increase in NST which was similar in long day and short day.Body weight of males and females was not affected by short day but was decreased by melatonin.Short day did not alter mean testes weight (about 20% voles regressed) but reduced mean female reproductive tract weight (more than 40% voles regressed). Melatonin reduced testes weight and female reproductive tract weight (more than 50% of voles of both sexes regressed).The results suggest that in northern red-backed voles: 1) the pineal does not mediate seasonal changes in thermogenic capacity, 2) the pineal may mediate reduction of body weight and regression of reproductive organs but, in addition to daylength, other cues or factors may be important, 3) populations may exhibit variability in sensitivity of reproduction to photoperiod which could allow for opportunistic breeding.Abbreviations NST nonshivering thermogenesis - NE norepinephrine - RMR resting metabolic rate     

Maternal transfer of photoperiodic information in Siberian hamsters. III. Melatonin injections program postnatal reproductive development expressed in constant light

In Siberian hamsters, transference of photoperiodic information from dam to fetus influences pubertal testicular development of the young when reared either in constant light (LL) or postnatal photoperiods of intermediate length (i.e. 14L:10D). The effects of short photoperiods during gestation can be mimicked by administering melatonin to pregnant females. This experiment examined whether there exists a daily pattern of sensitivity to melatonin when it is administered to pineal-intact pregnant females housed on a long photoperiod. Groups of pregnant and lactating females received melatonin at each hour of the day. The young were not treated with exogenous melatonin. At the approximate time of maturation of their endogenous pineal melatonin rhythm (Day 15), the young were placed in LL to suppress pineal melatonin secretion. Young males were killed at 28 days of age. Afternoon (1200 h-2000 h) and late night (0400 h) injections of melatonin into females caused their male young to develop as though gestation occurred on a short photoperiod. Melatonin injections at other times were ineffective. The daily pattern of effectiveness of exogenous melatonin administration to pregnant females resembles that observed in adult males of this and other hamster species and is consistent with the hypothesis that a daily rhythm in sensitivity to melatonin is involved in the transduction of photoperiodic signals.     

Nightly duration of pineal melatonin secretion determines the reproductive response to inhibitory day length in the ewe

The pineal controls the reproductive response of ewes to both stimulatory (short) and inhibitory (long) day lengths. Melatonin, a pineal hormone whose nocturnal secretion is entrained by photoperiod, mediates the effect of stimulatory photoperiod. We now report that melatonin also mediates the effect of inhibitory day length, monitored as response to estradiol negative feedback on luteinizing hormone (LH) secretion. Ovariectomized, estradiol-implanted ewes were pinealectomized and intravenously infused with melatonin to restore the nightly melatonin rise. Following transfer from short to long days, and a concurrent switch from short- to long-day melatonin patterns, LH dropped precipitously in pinealectomized ewes, matching the photoinhibitory response of pineal intact controls. LH dropped similarly in pinealectomized ewes when long-day melatonin was infused under short days. Pinealectomized ewes transferred from long to short days displayed a marked LH rise, provided melatonin was also switched to the short-day pattern. LH remained suppressed if long-day melatonin was infused following transfer to short days. These data indicate the nighttime melatonin rise mediates reproductive responses to inhibitory, as well as stimulatory photoperiods; they further suggest the duration of this rise controls suppression of LH under long days. Rather than being strictly pro- or antigonadal, the pineal participates in measuring day length.     

Entrainment of the melatonin rhythms in early postnatal lambs and their mothers

Although the developing sheep can produce an appropriately timed melatonin rhythm as early as 1 week after birth, it is not known whether the lamb is able to adjust its melatonin rhythm to a change in daylength. The ability of the young lamb to entrain its pattern of melatonin secretion to a new photoperiod was determined in the present study. Eight female lambs and their mothers were raised in long days (LD 16:8) beginning 2 weeks postpartum. At 7 weeks of age, the time of lights-off was advanced 8 hr, the short-day photoperiod then being LD 8:16; the time of lights-on remained unchanged. Concentrations of melatonin were measured in blood samples collected hourly on days - 1, 0, 2, 4, 6, and 13 relative to the light change. On day 0, all mothers and daughters had advanced the onset of melatonin secretion by at least 1 hr, and by day 13, 12 of 16 had completely entrained to the new photoperiod. The rate of entrainment among individuals varied; the mean rate for lambs and mothers did not differ. This study provides evidence that the melatonin-rhythm-generating system matures shortly after birth.     

Role of melatonin in photoperiodic time measurement in the migratory redheaded bunting (Emberiza bruniceps) and the nonmigratory Indian weaver bird (Ploceus philippinus)

In the present study, we asked the question whether physiological responses to day length of migratory redheaded bunting (Emberiza bruniceps) and nonmigratory Indian weaver bird (Ploceus philippinus) are mediated by the daily rhythm of melatonin. Melatonin was given either by injection at certain times of the day or as an implant. In series I experiments on the redheaded bunting, melatonin was administered by subcutaneous injections daily at zeitgeber time (ZT) 4 (morning) or ZT10 (evening) and by silastic capsules in photosensitive unstimulated buntings that were held in natural day lengths (NDL) at 27 degrees N beginning from mid February, and in artificial day lengths (ADL, 12L:12D and 14L:10D). Melatonin did not affect the photoperiod-induced cycles of gain and loss in body mass and testicular growth-involution, but there was an effect on temporal phasing of the growth-involution cycle of testes in some groups. For example, the rate of testicular growth and development was faster in birds that received melatonin injection at ZT4 in NDL, and was slower in birds that carried melatonin implants both in NDL and ADL. In series II experiments on Indian weaver birds, melatonin was given in silastic capsules in the first week of September when they still had large gonads. Birds were exposed for 12 weeks to short day length (8L:16D; group 1), to long day length (eight weeks of 16L:8D and four weeks of 18L:6D; group 2), or to both short and long day lengths (four weeks each of 8L:16D, 16L:8D, and 18L:6D; groups 3 and 4). Whereas groups 1 to 3 carried melatonin or empty implant from the beginning, group 4 received one after four weeks. All birds underwent testicular regression during the first four weeks irrespective of the photoperiod they were exposed to or the implant they carried in, and there was a slight re-initiation of testis growth in some birds during the next eight weeks of long day lengths. However, with the exception of group 2, there was no difference in mean testis volume during the period of experiment between the melatonin- and empty-implant birds. The data on androgen-dependent beak color also supported the observations on testes. Together, these results do not support the idea that the daily rhythm of melatonin is involved in the photoperiodic time measurement in birds. However, there may still be a role of melatonin in temporal phasing of the annual reproductive cycle in birds.     

Dynamics of the adjustment of clock gene expression in the rat suprachiasmatic nucleus to an asymmetrical change from a long to a short photoperiod

The molecular clockwork of the rat suprachiasmatic nucleus, the site of the circadian clock, is affected by the photoperiod (Sumová et al., 2003). The aim of the present study was to partly elucidate the dynamics of the adjustment of the clockwork to a change from a long to a short photoperiod accomplished by an asymmetrical prolongation of the dark period into the morning hours. Rats maintained under a regime with 16 h of light and 8 h of darkness per day (LD 16:8) were transferred to LD 8:16, and after 2, 3, and 13 days, daily profiles of Per1, Per2, Bmal1, and Cry1 mRNA were assessed by in situ hybridization. The rhythms of Per1, Per2, and Bmal1 expression adjusted to the change from a long to a short photoperiod with larger phase delays of the morning Per mRNA rise and Bmal1 mRNA decline than of the evening and nighttime Per mRNA decline and Bmal1 mRNA rise. The rhythm of Cry1 expression adjusted to the change by parallel delays of the Cry1 mRNA rise and decline. Adjustment of the Cry1 mRNA rhythm to short days was almost accomplished within 13 days, whereas adjustment of the Per1 and Bmal1 mRNA rhythms took longer. Different dynamics of the adjustment of rhythms in clock gene expression to a change from a long to a short photoperiod suggests complex resetting effects of the photoperiod change.     

Role of the pituitary gland in the development of photorefractoriness and generation of long-term changes in prolactin secretion in rams

Hypothalamo-pituitary disconnected Soay rams were exposed to two photoperiodic treatments: 1) constant long days (16L:8D) for 48 wk after pretreatment under short days (LD group), and 2) constant short days (8L:16D) for 48 wk after pretreatment under long days (SD group). In the LD group, plasma prolactin (PRL) concentrations increased from 0 to 8 wk (maximum: 143.3 +/- 8.4 microg/l; 8.8 +/- 1. 2 wk), decreased from 9 to 34 wk (minimum: 15.6 +/- 1.6 microg/l; 34. 5 +/- 1.5 wk), and finally increased again under the constant conditions, with a similar cyclical pattern for all individuals. In the SD group, PRL concentrations showed an inverse pattern (minimum: 8.6 +/- 2.6 microg/l; 17.1 +/- 2.0 wk; maximum: 46.4 +/- 5.5 microg/l; 30.2 +/- 3.2 wk), with more variability. Plasma concentrations of FSH were basal in both groups. The duration of the daily nocturnal melatonin peak (measured at 10, 24, and 44 wk) remained close to 8 h under long days (high-fidelity melatonin signal) but decreased significantly (13.8 h to 9.3 h) under short days (low-fidelity melatonin signal). The results support the conclusion that the melatonin signal encoding photoperiod acts within the pituitary gland to induce both acute (inductive) and chronic (refractory) effects photoperiod on PRL secretion.