Photoperiodic time measurement in seasonal reproduction of the weaver bird (Ploceus philippinus)

Use of a circadian clock in photoperiodic time measurement is demonstrated in the tropical photoperiodic weaver bird with the help of resonance, ahemeral, and asymmetrical skeleton photoperiods. Different asymmetrical skeleton photoperiods and seasonal scotophase scans indicate (1) that light entrains endogenous circadian rhythms (ECR) of photosensitivity and the position of the photoinducible phase shifts according to the length of the basic photoperiod, (2) a seasonal variation in response to asymmetrical skeleton photoperiods, and (3) dissociation in the two gonadotrophins LH and FSH and a possibility of two distinct ECRs of photosensitivity for LH and FSH. Annual phasing of the ECRs of photosensitivity of the two gonadotrophins and/or interaction of hormones might be involved in the seasonal reproduction and photosensitivity of this bird.     

Plasma follicle-stimulating hormone during photoperiodically induced sexual maturation in male Japanese quail.

Follicle-stimulating hormone (FSH) was measured in Japanese quail using a heterologous radioimmunoassay, the specificty of which was confirmed by its cross-reactions with purified chicken FSH and luteinizing hormone (LH). Plasma concentrations of FSH, LH and testosterone were determined in quail during the testicular growth and sexual maturation which follows their transfer from short to long daylengths. All three hormones could be detected in short-day birds but their concentrations were greatly increased following photostimulation. Plasma FSH increased 12-fold during the first 9 long days, remained at this level for a week, and then declined steadily so that by the time the birds were sexually mature the level of FSH had decreased to one-third of the maximum level. LH reached a high level (five times the short day level) after 4 long days. Thereafter two patterns of LH secretion could be distinguished. In one experiment the high level of LH was maintained unchanged throughout sexual maturation while in another experiment LH secretion decreased significantly between days 11 and 28 of photostimulation. A strong correlation existed between testicular growth and the plasma FSH concentration. It was maximal during the phase of rapid testicular growth and decreased as spermiogenesis began. The pituitary FSH content increased during photostimulation. Castration caused a 20-fold rise in plasma FSH compared with that in intact quail. The change in LH concentration after castration was about eightfold. The changes in hormone secretion were strikingly similar to those found during sexual development and puberty in the rat.     

Seasonal change in luteinizing hormone subunit mRNA in Japanese quail and effects of short daylength and low temperature

Changes in pituitary mRNA levels of LHbeta-subunit (LHbeta) and glycoprotein hormone alpha-subunit (common alpha) were investigated in male Japanese quail under natural and laboratory conditions to clarify the mechanisms of seasonal regulation of luteinizing hormone (LH) secretion. In Experiment 1, birds were kept in outdoor cages under natural conditions from August for 12 months. Both LHbeta and common alpha mRNA levels decreased rapidly from August to September, and after a period of low levels from October through January, they began to increase in February and continued to increase until July. There were more pronounced seasonal changes in testicular weight and cloacal protrusion width with large decreases from August to September and increases from March to May. In Experiment 2, birds were kept on laboratory conditions and transferred from long to short daylengths at 20 or 9 degrees C and held for 14 days. Although common alpha mRNA levels, plasma LH concentrations, testicular weight, and cloacal protrusion area decreased on short days without low temperatures, levels of LHbeta mRNA did not change. Short daylengths combined with low temperatures induced testicular regression and caused decrease in all the parameters measured. Low temperatures under long days did not induce any change in the parameters significantly. These results suggest that (1) synthesis as well as secretion of LH is regulated seasonally, (2) short daylength does not suppress LH synthesis completely unless combined with low ambient temperature, and (3) the effect of photoperiod on the endocrine system regulating LH secretion is predominant over the effect of ambient temperature but ambient temperature acts as an environmental cue to terminate reproductive activities at late summer to early autumn in Japanese quail.     

Circadian rhythm of melatonin in the pineal gland of the Japanese quail (Coturnix coturnix japonica)

Melatonin was measured by radioimmunoassay in homogenates of pineal glands from quail (Coturnix coturnix japonica) kept under different photoperiods and in darkness. Under 8-, 12- and 16-h daylengths melatonin levels were increased during the dark period, the duration of the increase depending on the duration of the dark period. As the daylength was increased the peak occurred closer to lights-off, reflecting the more rapid melatonin rise under the longer photoperiods. The pineal melatonin rhythm continued in darkness with an amplitude relative to that seen under a light/dark cycle of slightly less than one-half after 2 days in darkness and one-third after 6 days in darkness. The corresponding average periods of the rhythm were 25.5 h and 25.7 h. These results show that there is a circadian rhythm of melatonin in the pineal gland of the quail which is entrained by light/dark cycles and which continues in darkness.     

The drive on luteinizing hormone secretion in castrated tree sparrows (Spizella arborea) exposed to short days is daylength independent

For many, if not most, photoperiodic species of birds, short days are nongonadostimulatory. The tacit assumption that short days are also nonphotostimulatory was tested by determining whether plasma concentrations of luteinizing hormone (LH) in castrated tree sparrows exposed to short daily photoperiods (8 hr or less) are daylength dependent or independent. Castration of tree sparrows held on an 8-hr daily photoperiod evoked, within 2 weeks, a fivefold elevation in plasma LH concentration. Over the next 24 weeks, plasma LH concentrations of castrated birds were resistant, first to a stepwise reduction in daylength from 8 to 2 hr and later to a 2-, 4-, or 6-hr increase therein. At no time did LH concentrations differ among castrated birds held on the same or different short-day photoregimes. Conversely, at all times, LH concentrations of castrated birds, regardless of photoperiodic history, exceeded those of intact males held on an 8-hr daily photoperiod and sampled at the beginning of the experiment. These data, which argue that the drive on LH secretion in castrated tree sparrows exposed to short days is daylength independent and, therefore, likely intrinsic, verify the tacit assumption that short days are nonphotostimulatory.     

Quantitative bioassay of luteinizing hormone releasing activity in the quail hypothalamus during photostimulated sexual development.

A quantitative bioassay using dispersed chicken pituitary cells was employed to measure luteinizing hormone releasing activity in pooled hypothalamic extracts from groups of quail during photostimulated sexual development. Levels of releasing activity with 95% confidence limits were expressed in terms of microlitres of a standard hypothalamic extract (CS).LH releasing activity was measurable in the hypothalamus of male and female quail reared under both short and long photoperiods. Using an approximate conversion factor from CS, levels ranged between 5.1 and 13.3 ng synthetic LH-RH/mg basal hypothalamic tissue.Dramatic increases in LH secretion seen after transfer to photostimulatory daylengths were not associated with any large changes in hypothalamic LH releasing activity. Several significant fluctuations in levels of releasing activity were observed but these could not readily be correlated with changes in plasma LH levels. Combined data from four experiments show that the mean concentration of releasing activity was just significantly higher in groups of birds receiving 7 or more long days than in short day quail and birds receiving fewer than 7 stimulatory photoperiods (P < 0.05). This difference represents a 30% increase in releasing activity in long-term photostimulated quail.Castration did not alter LH releasing activity during one 5-week experiment, although a group of castrated birds maintained on long days for 1 year showed the highest level of releasing activity found in any quail.     

A comparison of the responses of captive willow ptarmigan (Lagopus lagopus lagopus), red grouse (Lagopus lagopus scoticus), and hybrids to increasing daylengths with observations on the modifying effects of nutrition and crowding in red grouse

The photoperiodic responses of two races of Lagopus lagopus from different latitudes and of hybrids between them were compared under the same lighting conditions. The captive birds were descended from northern Norwegian willow ptarmigan (Lagopus lagopus lagopus) and Scottish red grouse (Lagopus lagopus scoticus). Under natural conditions, the red grouse begin to lay eggs 3 weeks earlier than the willow ptarmigan. Photosensitive birds were transferred from an 8-hr daylength to a lighting regime in which the daylength was increased by 1 hr per week for 11 weeks. At the start of the study, the red grouse had greater LH levels and comb sizes than the wilow ptarmigan or the hybrids. The critical daylength needed to stimulate LH secretion was less than 12 hr in both races but it was not clear whether it was greater in the willow ptarmigan than in the red grouse. However, the races differed quantitatively: the willow ptarmigan had lower LH levels at 12, 13, and 14-hr photoperiods than did the red grouse or the hybrids. In all the males, comb heights increased significantly (P < 0.05) after the photoperiod was increased to 12 hr and reached their maximum in the red grouse, hybrids, and willow ptarmigan at photoperiods of 13, 14, and 16 hr, respectively. The female red grouse, hybrids, and willow ptarmigan laid their first eggs after the photoperiod was increased to 15, 17, and 19 hr, respectively. At the end of the study the red grouse but not the hybrids or willow ptarmigan were becoming photorefractory. A low-protein diet did not alter the timing of the onset of seasonal breeding in captive female red grouse although it did cause a reduction in the rate of lay and egg weight. The onset of seasonal breeding was delayed, however, when the birds were crowded. It was concluded that in L. lagopus, the photoperiodic response seems to be determined by inherited factors. Further, the absence of a clear difference between the critical daylengths in red grouse and willow ptarmigan raises the possibility that differences in the timing of the onset of seasonal breeding in these races may be caused by modifications in the neural or endocrine pathways “downstream” from the biological clock.     

Pineal melatonin mediates photoperiodic control of pulsatile luteinizing hormone secretion in the ewe

Seasonal breeding in the ewe is regulated by photoperiod through a pineal-dependent mechanism. Changes in the ability of estradiol to inhibit tonic LH secretion are critical. During anestrus, this ovarian steroid gains the ability to slow the frequency of pulsatile LH secretion through an action on the brain. Exposure of ovariectomized, estradiol-implanted ewes to short photoperiods during summer anestrus revealed that daylength can control LH pulse frequency. After removal of estradiol, LH pulse frequency still differed between long- and short-day ewes, suggesting photoperiodic modulation of LH and presumably GnRH secretion independent of gonadal steroids. Significantly, the effects of daylength expressed both in the presence and the absence of estradiol failed to occur in pinealectomized ewes. Long-term infusions of melatonin, given in physiological patterns to pinealectomized ewes, mimicked the effects of photoperiod on pineal-intact ewes. Specifically, a pattern of melatonin characteristic of that in short days (16-hour night-time rise) led to an increase in LH pulse frequency to a breeding season rate. Conversely, melatonin infusions typifying a long-day pattern (8-hour night-time rise) produced an anestrous pulse pattern. Pituitary sensitivity to GnRH was not reduced in sheep which were reproductively suppressed by photoperiod or melatonin treatments. These observations support the conclusion that day-length acts through pineal melatonin secretion to regulate a neural LH pulse generator which, by changing the frequency of GnRH pulses, determines the ewe's seasonal reproductive state.     

Effects of androgens on the testes of intact and hypophysectomized Japanese quail

The effects of testosterone propionate administered via Silastic capsules on avian spermatogenesis were investigated in intact and hypophysectomized quail with the following results. (1) In intact sexually mature quail low doses of testosterone inhibited luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion and led to testicular regression. Large doses also blocked gonadotrophin secretion but maintained the testes in a spermatogenetically active condition. Testis size was decreased but not seminiferous tubule diameter. A combination of testosterone plus ovine LH or FSH was more effective in maintaining testicular weight. (2) Following complete hypophysectomy of mature quail large doses of androgens were unable to maintain spermatogenesis. Treatment of hypophysectomized birds for 14 days only retarded the rate of testicular regression. (3) Testosterone induced slight testicular growth in intact sexually immature quail maintained on short daylengths, but this spermatokinetic action was lost if the birds were hypophysectomized prior to treatment. (4) In immature quail which were hypophysectomized and treated with testosterone, testosterone plus ovine FSH, or testosterone plus ovine LH, 1 week later, testicular growth was greatest with the testosterone/FSH combination. The data are consistent with the view that androgens play an important role in avian spermatogenesis, as they do in mammals. However, pituitary factors, probably FSH, are also essential for full spermatogenesis and testicular growth to occur.     

The pineal gland and the photoperiodic control of luteinizing hormone secretion in intact and castrated Japanese quail

The effects of pinealectomy on a range of photoperiodic responses were investigated in male Japanese quail by measuring plasma LH concentrations in intact, sham-operated and pinealectomized birds in the following four experiments: (1) transfer of sexually quiescent birds from a short photoperiod of 8 h light: 16 h darkness (8L:16D) to a photostimulatory daylength of 16L:8D; (2) transfer of sexually mature birds from 16L:8D to 8L:16D; (3) castration in 16L:8D and exposure to 13L:11D; (4) castration in 8L:16D and exposure to 13L:11D. There was no evidence of effects of the pineal gland on the photoperiodically induced changes in LH secretion, the quantitative relationship between LH secretion and photoperiod in intact and castrated birds, or the induction of relative photorefractoriness by prolonged exposure to 16L:8D. This suggests that there is no pineal influence on the photoperiodic clock or its effectors in this bird.     

Photoinducible phase-specific light induction of Cry1 gene in the pars tuberalis of Japanese quail

Prolactin (PRL) secretion is regulated by photoperiod in mammals and birds. In mammals, the pars tuberalis (PT) in the pituitary is involved in the regulation of photoperiodic regulation of PRL secretion. In birds, however, hypothalamic vasoactive intestinal peptide is implicated in PRL secretion, and physiological roles of the avian PT remain unknown. In the present study, we show that PRL secretion increases under long days and short days with a night interruptive schedule, both of which also cause gonadal growth in Japanese quail. We have also found Cry1 gene expression in the PT of Japanese quail. Cry1 expression was rhythmic under long and short photoperiods in the PT, and the peak was phase delayed under a lengthened photoperiod. Moreover, expression of Cry1 gene was induced by a light pulse but only when given during the photoinducible phase. In our previous study, we have shown rhythmic Per2 gene expression with a peak in the PT during the early day under various photoperiods. When taken together with the results from the present study, different phase relationships between Per2 and Cry1 in the Japanese quail PT under different photoperiods may decode photoperiodic information and regulate photoperiodic PRL secretion in a manner similar to that of mammals.     

A possible role for the eyes in the photoperiodic response of quail

The effects of blinding on the photoperiodic responses of male Japanese quail were investigated by measuring plasma luteinizing-hormone (LH) concentrations in intact and castrated birds. Blinded birds were still able to respond to short and long days with appropriate changes in LH levels, suggesting that the basic photoperiodic mechanisms do not require retinal photoreception. However, there were clear-cut differences between blinded and sighted birds with the LH levels being higher in blinded quail. This difference between blinded and sighted was greater in short than in long days, and was also enhanced by castration. In conclusion we propose that short day information transmitted by the eyes has an inhibitory effect for LH secretion independent from sex steroid negative feedback effects in quail.     

Photoperiodic induction in vitro: the dynamics of gonadotropin-releasing hormone release from hypothalamic explants of the Japanese quail.

The Japanese quail is a photoperiodic animal that under certain experimental conditions can respond to a single long day with a wave of LH secretion. Such a system offers an opportunity to analyze the photoneuroendocrine changes as they occur in real time, especially as all of the neural machinery (photoreceptor, clock, and GnRH system) is believed to lie within the hypothalamus. The first detectable rise in LH occurs at about hour 23 of the long day, and this single inductive event leads to prolonged LH secretion lasting for up to 2 weeks and peaking 2-4 days after the dawn of the long day. The size of the quail's hypothalamus is such that the entire structure, including both the GnRH cell bodies and the median eminence, can be cultured for some hours, and the rates of GnRH release measured therefrom. The present experiments used hypothalamic explants from quail at different times throughout the photoperiodic response, superfused them for up to 7.5 h in vitro, and measured the dynamics of GnRH release. A significant step increase of 80% in GnRH release occurred between hours 22.5 and 23 in quail that had been exposed to a long day: an equivalent change was not found in hypothalami taken from quail maintained only under short day lengths. In explants taken from quail at the peak of LH secretion (53 h after dawn of the long day), the rates of GnRH release were double those found in control quail not exposed to the long day. Explants taken 14 days after the long day, when LH secretion had subsided fully, showed no difference in GnRH release between photo-stimulated and control quail. These results suggest that photoperiodic induction involves a timed increase in GnRH release, and the rise at hour 23 is believed to represent photoperiodic induction actually taking place within the brain in vitro. They also suggest that the wave of LH secretion triggered by the single long day is, at least in part, a neuroendocrine or neural phenomenon; this confirms earlier indirect evidence to this effect.     

The endocrine control by luteinizing hormone of testosterone secretion from the testis of the Japanese quail

Injections of chicken or ovine luteinizing hormone (LH) into sexually mature male Japanese quail greatly increased plasma levels of testosterone. Maximal responses were obtained within 15 min of an iv injection and between 1 and 2 hr following sc or im injections. Saline treatment had no effect on plasma testosterone. In chronically castrated quail LH was not effective in altering androgen levels. The responses to LH were dose related, significant increases being obtained following sc injections of 5 μg of chicken LH (fraction AE1) or 10 μg of ovine LH (NIH-LH-S19). Chicken LH (AE1) was appropriately 1.8 times as potent as NIH-LH-S19. Ovine FSH (NIH-FSH-S10) stimulated testosterone release in very large doses (1 mg) but was at least 100 times less active than LH-S19. An iv injection of an antiserum raised against chicken LH into mature male quail caused a rapid decrease in plasma testosterone levels. Treatment with FSH-S-10 for up to 1 week failed to facilitate the subsequent response to an injection of LH. The responsiveness of the testis to exogenous LH was tested at various times during a photoinduced gonadal growth cycle. Sexually immature quail showed only a marginal response to an sc injection of 20 μg of NIH-LH-S19. A marked increase in responsiveness occurred after 6 long days. This coincides with the time when plasma testosterone levels increase naturally after transfer to long daylengths and with the period when Leydig cell maturation becomes complete. These in vivo results add further weight to the belief that, in birds, or at least in the quail, peripheral androgens are controlled by pituitary LH and that FSH plays no significant role in the acute release of testosterone from the mature testis.     

Effects of gonadectomy and steroid treatment on plasma gonadotropins and the response of superfused pituitaries to gonadotropin-releasing hormone in the turtle Sternotherus odoratus

Gonadectomized (gonadex) turtles, Sternotherus odoratus, had significantly elevated plasma FSH, but LH was less consistently affected. Estradiol (E2)-implants suppressed plasma FSH in gonadex females but not in males: testosterone (T) partially suppressed FSH in males. In contrast, E2-treatment markedly suppressed pituitary LH content and in vitro LH secretion in gonadex and intact turtles (inhibitory effects of E2 were less in intact than ovariectomized females). These steroid effects were relatively specific for gonadotropin; pituitary TSH content was not altered. In vitro, pituitary LH secretion responded to doses of GnRH greater than or equal to 1 ng/ml and LH output remained elevated for at least 3 hr of continuous superfusion with gonadotropin-releasing hormone (GnRH). In general, gonadectomy elevated pituitary responsiveness to GnRH while E2 and T suppressed this responsiveness; the effects of E2 are greater in gonadectomized than intact turtles. Thus, negative gonadal feedback appears to be involved in the secretion of gonadotropins in turtles, and steroidal actions may be partly due to suppression of pituitary hormone content and responsiveness to GnRH.     

Changes in gonadotrophin-releasing hormone and LH in Japanese quail during the first few days of photostimulation

Castrated Japanese quail responded to a single long day with a five- to eightfold increase in plasma LH levels. A rise in LH secretion appeared 19-24 h after dawn and LH levels were still increased 3 days later, despite the fact that the birds had been returned to a short daylength. Pituitary LH content decreased, reflecting these changes in secretion, although significant falls in content were only found 36-96 h after dawn, when LH secretion was maximal. Hypothalamic gonadotrophin-releasing hormone content was not altered. One interpretation of this is that increased synthesis of the peptide compensates fully for the increased secretion.     

A circadian rhythm of serum follicle-stimulating hormone in women.

While a nocturnal decline in serum LH levels in the early follicular phase of the menstrual cycle has been well established, a diurnal variation in serum FSH levels in women has not been demonstrated. We addressed this issue by determining serum LH and FSH levels at 15-min intervals for 24 h in the early follicular phase (EFP; n = 16) and late follicular phase (LFP; n = 10) of the menstrual cycle and in postmenopausal women (PMW; n = 10). Serum estradiol was simultaneously measured at hourly intervals. As expected, EFP, but not LFP and PMW, women had a 15% nocturnal decline (P less than 0.01) in transverse mean LH levels compared to values in the daytime hours. In contrast, nocturnal FSH transverse mean values were significantly lower than daytime values in all groups studied, demonstrating an 18% decline in EFP (P less than 0.001), a 17% decline in LFP (P less than 0.00001), and a 4.3% decline in PMW (P less than 0.01). Cosinor analysis revealed a circadian rhythm for FSH, with acrophases in the afternoon and nadirs at night in all three groups. The circadian amplitudes were 1.43 +/- 0.22, 1.02 +/- 0.16, and 8.42 +/- 1.31 IU/L for EFP, LFP, and PMW, respectively. The EFP nocturnal decline in LH did not conform to a cosine rhythm. A diurnal variation in estradiol was not present in any of the groups of women. These data constitute the first demonstration of a robust circadian rhythm of serum FSH in women. The comparable timing of the acrophase in all groups of subjects and its presence in the postmenopausal years suggest a central, rather than peripheral, feedback mechanism(s) for the circadian rhythmicity. This observation provides strong evidence for a dissociation in the hypothalamic regulation of pituitary LH and FSH secretion in women. The circadian peak and nadir of circulating FSH may prove to be determining for appropriate follicular development.     

Neuroendocrine regulation of gonadotrophin-releasing hormone secretion in the Japanese quail

Following the transfer of quail to long days, an increase in plasma and pituitary levels of LH and FSH is observed. The integrity of both the preoptic region (POR) and the posterodorsal part of the infundibular nuclear complex (PD-INC) appears to be essential for mediating this photoperiodic response. The INC may, however, operate autonomously in being able to regulate the low levels of LH-RH release of nonphotostimulated immature birds. By combining the techniques of electrical stimulation and electrolytic lesioning it can be demonstrated that stimulation of the POR can increase LH-RH secretion in quail where the photoinduced release has been blocked by a PD-INC lesion. Electrical stimulation of the preoptic region in immature birds similarly stimulates the secretion of LH-RH. These results suggest a direct link, perhaps neurosecretory, between the preoptic region and the median eminence.     

Photoperiodic differences in in vitro pituitary gonadotropin basal secretion and gonadotropin-releasing hormone responsiveness in the golden hamster

In order to examine pituitary gonadotropin secretion and responsiveness to GnRH after photic-induced changes in reproductive condition, an in vitro pituitary perifusion system was established for male golden hamster tissue. Anterior pituitaries from adult males which had been maintained on 14 h light:10 h dark (long days) or 6 h light:18 h dark (short days) for 10 weeks were perifused using an Acusyst perifusion system. Perfusates from unstimulated tissue (basal secretion) and from tissue stimulated with hourly pulses of GnRH (25, 50, or 100 ng/ml) were assayed for LH and FSH by RIA. Tissue from short-day animals had lower basal LH secretion than tissue from long day animals, but there were no significant photoperiodic differences for GnRH-stimulated LH secretion. In contrast, there were no photoperiodic differences in basal FSH secretion, but tissue from short-day animals secreted more FSH than tissue from long-day animals when stimulated with GnRH. Bioactivity of a small number of perfusate samples was assessed using in vitro rat granulosa cell and mouse Leydig cell assays for FSH and LH, respectively, and did not show any photoperiodic differences in LH or FSH bioactivity for GnRH-stimulated tissue. These studies indicate that the pituitaries of gonadally regressed hamsters are capable in vitro of responding to GnRH with similar or greater levels of gonadotropin release compared to pituitaries from animals with functional gonads. Therefore, it appears that the lowered serum gonadotropin levels seen in vivo in gonadally regressed animals are not due to a reduction in intrinsic pituitary sensitivity to GnRH.     

Hormonal regulation of the annual reproductive cycle of golden hamsters

The seasonally breeding golden hamster, Mesocricetus auratus, is a photoperiodic species. Reproduction is confined to the spring and summer months and is curtailed by the short days of fall and winter. The photoperiodic response is dependent on an endogenous circadian sensitivity to light. Reacquisition of reproductive activity occurs spontaneously, even in the complete absence of light. These animals are refractory to the inhibitory effects of short days. Refractoriness is terminated by exposure to long days for a period of 11 or more weeks, after which the animals are once again sensitive to short days. The short-day response in the male is characterized by testicular regression induced by a decline in pituitary and circulating titers of LH, FSH, and PRL. These animals demonstrate an increased sensitivity to negative-feedback effects of exogenous (and, presumably, endogenous) testosterone that is associated with a decreased castration response. The reproductively inactive female fails to ovulate. Estrous cycles are absent, supplanted by a daily surge of LH and FSH. Spontaneous recovery of testicular function is associated with an increase in circulating titers of LH, FSH, PRL, and testosterone, and a resumption of spermatogenic activity. In the female the daily surge of LH and FSH is replaced by the 4-day estrous cycle. The pineal gland is essential for the occurrence of photoperiodically induced changes in the hamster's reproductive cycle. The pineal produces the indole, melatonin, with peak synthesis and release occurring during the subjective night. Administration of melatonin, by injection, at the appropriate time of day, will induce gonadal regression in both intact and pinealectomized hamsters maintained on normally stimulatory photoperiods. Subcutaneous melatonin implants, however, prevent gonadal regression induced by short photoperiods or melatonin injections. Melatonin implants also prevent photoinduced gonadal recrudescence. Constant release melatonin implants may function by interfering with target tissue sensitivity to endogenous melatonin. The target tissue for melatonin and the mechanism by which the pineal is involved in the photoperiodic response remain unknown.