A melatonin-independent seasonal timer induces neuroendocrine refractoriness to short day lengths.

The duration of nocturnal pineal melatonin secretion transduces effects of day length (DL) on the neuroendocrine axis of photoperiodic rodents. Long DLs support reproduction, and short DLs induce testicular regression, followed several months later by spontaneous recrudescence; gonadal regrowth is thought to reflect development of tissue refractoriness to melatonin. In most photoperiodic species, pinealectomy does not diminish reproductive competence in long DLs. Turkish hamsters (Mesocricetus brandti) deviate from this norm: elimination of melatonin secretion in long-day males by pinealectomy or constant light treatment induces testicular regression and subsequently recrudescence; the time course of these gonadal transitions is similar to that observed in males transferred from long to short DLs. In the present study, long-day Turkish hamsters that underwent testicular regression and recrudescence in constant light subsequently were completely unresponsive to the antigonadal effects of short DLs. Other hamsters that manifested testicular regression and recrudescence in short DLs were unresponsive to the antigonadal effects of pinealectomy or constant light. Long-term suppression of melatonin secretion induces a physiological state in Turkish hamsters similar or identical to the neuroendocrine refractoriness produced by short-day melatonin signals (i.e., neural refractoriness to melatonin develops in the absence of circulating melatonin secretion). A melatonin-independent interval timer, which would remain operative in the absence of melatonin during hibernation, may determine the onset of testicular recrudescence in the spring. In this respect, Turkish hamsters differ from most other photoperiodic rodents.     

Low ambient temperature accelerates short-day responses in Siberian hamsters by altering responsiveness to melatonin

Exposure to low ambient temperatures (Ta) accelerates appearance of the winter phenotype in Siberian hamsters transferred from long to short day lengths. Because melatonin transduces the effects of day length on the neuroendocrine axis, the authors assessed whether low Ta promotes the transition to winterlike traits by accelerating the onset of increased nocturnal melatonin secretion or by enhancing responsiveness to melatonin in short day lengths. Male hamsters were transferred from 16L (16 h light/day) to 8L (8 h light/day) photoperiods and held at 5 degrees C or 22 degrees C. Locomotor activity was recorded continuously, and body mass, testis size, and pelage color were determined biweekly for 8 weeks. The duration of nocturnal locomotion (alpha), a reliable indicator of the duration of nocturnal melatonin secretion, lengthened significantly earlier in hamsters exposed to a Ta of 5 degrees C than 22 degrees C. Cold exposure increased the proportion of hamsters that were photoresponsive: gonadal regression in short days increased from 44% at 22 degrees C to 81% at 5 degrees C (p < 0.05); low Ta did not, however, accelerate testicular regression in animals that were photoresponsive. Nonphotoresponsive animals at 5 degrees C temporarily had longer alphas during the first 4 weeks in short days and significant decreases in body mass and testicular size that were reversed during the ensuing weeks when alpha decreased. In a 2nd experiment, pinealectomized male hamsters infused for 10 h/day with melatonin for 2 weeks had significantly lower body and testes masses when maintained at 5 degrees C but not 22 degrees C. Low-ambient temperature appears to accelerate the appearance of the winter phenotype primarily by increasing target tissue responsiveness to melatonin and to a lesser extent by augmenting the rate at which the duration of nocturnal melatonin secretion increases in short day lengths.     

Elimination of short-day melatonin signaling accelerates gonadal recrudescence but does not break refractoriness in male Turkish hamsters

Long days stimulate and short days (SDs) inhibit the reproductive axis of photoperiodic rodents. In long-day Turkish hamsters, unlike most other rodents, elimination of pineal melatonin secretion by constant light or pinealectomy initiates a cycle of gonadal involution and recrudescence outwardly similar to that induced by short days. The present study assessed whether short days and constant light induce the seasonal reproductive cycle via common or different interval timing mechanisms. Male hamsters that had undergone gonadal involution in SDs for 8 or 14 weeks were treated with LL for 14 and 8 weeks, respectively. If SDs and LL act via independent mechanisms, then gonadal quiescence of SD-regressed males, which normally lasts 10 weeks, might be extended by LL treatment; alternatively, if SDs and LL act on the same timer, or the timer cannot be retriggered, then LL will not extend the duration of reproductive quiescence. Neither of these outcomes materialized. Instead, male hamsters exposed to LL while reproductively quiescent exhibited accelerated gonadal recrudescence. Extended LL treatment did not restore responsiveness to SDs in photorefractory hamsters. In Turkish hamsters, photoperiodic history determines whether constant light inhibits or stimulates the hypothalamic-pituitary-testicular axis.     

Exogenous T3 mimics long day lengths in Siberian hamsters

Siberian hamsters (Phodopus sungorus) exhibit seasonal cycles of reproduction driven by changes in day length. Day length is encoded endogenously by the duration of nocturnal melatonin (Mel) secretion from the pineal gland. Short-duration Mel signals stimulate reproduction and long-duration signals inhibit reproduction. The mechanism by which Mel signals are decoded at the level of neural target tissues remains uncharacterized. In Siberian hamsters, exposure to short day lengths or injections of Mel in long days results in a decrease in hypothalamic expression of type 2 iodothyronine deiodinase (Dio2) mRNA. Dio2 catalyzes the conversion of the thyroid hormone thyroxine to triiodothyronine (T3). Thus exposure to short and long day lengths should decrease and increase hypothalamic T3 concentrations, respectively. We tested the hypothesis that exogenous T3 administered to short-day hamsters would mimic exposure to long day lengths with respect to gonadal stimulation. Hamsters gestated and raised in short day lengths that exhibited photoinhibition of the testes were given daily subutaneous injections of T3 or saline vehicle for 4 wk beginning at week 12 of life. The results indicate that exogenous T3 induced gonadal growth in short-day hamsters and delayed spontaneous gonadal development by an interval equal to the number of weeks during which T3 was administered. T3 injections delayed gonadal regression if given coincident with the transfer of hamsters from long to short day lengths. These results suggest that T3 mimics long day exposure in Siberian hamsters and may serve as an intermediate step between the Mel rhythm and the reproductive response.     

Short day lengths affect perinatal development of the male reproductive system in the Siberian hamster, Phodopus sungorus.

The Siberian hamster, Phodopus sungorus, breeds seasonally. In the laboratory, seasonal breeding can be controlled by photoperiod, which affects the duration of nightly melatonin secretion. Winterlike, short day lengths induce gonadal regression in adult animals, and pups born and maintained in short days undergo pubertal gonadal development later than animals born into long days. However, to date there have been no reports of gestational photoperiod affecting fetal development of reproductive systems. The spinal nucleus of the bulbocavernosus (SNB) and its target muscles, the bulbocavernosus (BC) and levator ani (LA), compose a sexually dimorphic, androgen-sensitive neuromuscular system involved in male reproduction. The SNB neuromuscular system was studied in male Siberian hamsters maintained from conception in short-day (8 h light, 16 h dark; 8L:16D) versus long-day (16L:8D) conditions. On the day of birth, and at postnatal (PN) days 2 and 18, the BC/LA muscles of hamsters gestated and raised in the short photoperiod were significantly reduced relative to those of their long-day counterparts. Testes weights were not significantly different between groups until day 18. Thus, photoperiod exposure during gestation and after birth affects perinatal development of the SNB system in this species, and these effects can be seen as early as the day of birth. Because photoperiod did not significantly affect testes weights until PN18, these results suggest that either perinatal photoperiod affects fetal androgen production without affecting testes weight or it influences BC/LA development independently from androgen.     

Simulated natural day lengths synchronize seasonal rhythms of asynchronously born male Siberian hamsters

Photoperiodism research has relied on static day lengths and abrupt transitions between long and short days to characterize the signals that drive seasonal rhythms. To identify ecologically relevant critical day lengths and to test the extent to which naturally changing day lengths synchronize important developmental events, we monitored nine cohorts of male Siberian hamsters (Phodopus sungorus) born every 2 wk from 4 wk before to 12 wk after the summer solstice in a simulated natural photoperiod (SNP). SNP hamsters born from 4 wk before to 2 wk after the solstice underwent rapid somatic and gonadal growth; among those born 4-6 wk after the solstice, some delayed puberty by many weeks, whereas others manifested early puberty. Hamsters born eight or more weeks after the solstice failed to undergo early testicular development. The transition to delayed development occurred at long day lengths, which induce early puberty when presented as static photoperiods. The first animals to delay puberty may do so predominantly on the basis of postnatal decreases in day length, whereas in later cohorts, a comparison of postnatal day length to gestational day length may contribute to arrested development. Despite differences in timing of birth and timing of puberty, autumn gonadal regression and spring gonadal and somatic growth occurred at similar calendar dates in all cohorts. Incrementally changing photoperiods exert a strong organizing effect on seasonal rhythms by providing hamsters with a richer source of environmental timing cues than are available in simple static day lengths.     

Testicular recrudescence in intermediate day lengths reflects loss of photoperiodic memory in Siberian hamsters

Siberian hamsters transferred from a long (16 h light/day [16 L]) to an intermediate (13.5 L) day length (DL) undergo testicular regression within 2 months followed approximately 2 months later by "spontaneous" testicular recrudescence. Recovery of gonadal function after prolonged exposure to intermediate DLs is thought to reflect development of neuroendocrine refractoriness to intermediate-duration melatonin signals. The authors tested the alternative hypothesis that testicular recrudescence in 13.5 L occurs when the "memory" for the 16-L photoperiod fades and hamsters can no longer compare the 13.5-L to the prior 16-L day length. Adult hamsters transferred from 16 L to 13.5 L that underwent testicular involution were either maintained continuously in 13.5 L for 41 weeks or given a supplementary 2-week treatment of 16 L before being returned to 13.5 L. The supplementary treatment was administered either after hamsters had been in 13.5 L for 10 weeks and had involuted testes, or after 24 weeks, when the gonads had undergone recrudescence. The authors found that 16 L treatment administered at week 10 delayed final gonadal recrudescence by approximately 12 weeks; similar 16-L treatment at week 24 induced a second gonadal regression when animals were returned to 13.5 L. The most parsimonious hypothesis to account for these findings is that gonadal recrudescence in intermediate DLs reflects fading of the "memory" for prior long DLs rather than induction of refractoriness to melatonin signals generated in intermediate DLs.     

Pineal-independent regulation of photo-nonresponsiveness in the Siberian hamster (Phodopus sungorus)

The pineal hormone melatonin influences circadian rhythms and also mediates reproductive responses to photoperiod. The authors tested whether pinealectomy influences circadian oscillators responsible for induction of nonresponsiveness to short day lengths by preventing normal short-day patterns of circadian entrainment. Adult male Siberian hamsters were pinealectomized or sham operated, maintained in either 18 h light per day (18L) or 15L for 10 weeks, and then tested for responsiveness to 10L. Because pinealectomized hamsters do not show gonadal regression in short day lengths, responsiveness was assessed by measuring phase angle of entrainment and the length of the nightly activity period following transfer to 10L. The incidence of nonresponsiveness was significantly higher in 18L hamsters than in 15L hamsters but was unaffected by pineal status. Fully 88% of 18L hamsters failed to entrain to 10L in the normal short-day manner; the duration of nightly activity remained compressed, and the phase angle of entrainment was large and negative relative to lights off. The 15L hamsters entrained normally to 10L. Exposure to constant light after 10L treatment was equally effective in inducing arrhythmicity in pinealectomized and intact hamsters. Changes in the period of morning and evening circadian oscillators subsequent to 18L treatment did not predict circadian responsiveness to short photoperiod. Long-day induction of photo-nonresponsiveness, which prevents winter responses to short day lengths, occurs independently of pineal melatonin feedback on the circadian system.     

Short day lengths delay development of the SNB neuromuscular system in the Siberian hamster,Phodopus sungorus

The Siberian hamster, Phodopus sungorus, breeds seasonally. In the laboratory, the seasonal breeding can be controlled by photoperiod, which affects the durations of nightly melatonin secretions. Winterlike short day lengths induce gonadal regression in adult animals, and pups born and maintained in short days undergo gonadal development much later than animals born into long days. The spinal nucleus of the bulbocavernosus (SNB) and its target muscles, the bulbocavernosus (BC) and levator ani (LA), comprise a sexually dimorphic, androgen-sensitive neuromuscular system involved in male reproduction. The SNB neuromuscular system was studied in male Siberian hamsters maintained from conception in short-day (8:16 h light/dark cycle) versus long-day (16:8 h light/dark cycle) conditions. At 40–47 days of age, development of three components of the SNB neuromuscular system were all significantly delayed in hamsters raised in the short photoperiod: BC/LA muscle weight, the size of SNB motoneuronal somata, and the area of the neuromuscular junctions at the BC/LA muscles of short-day hamsters were each significantly reduced relative to those of long-day counterparts. Thus, development of the SNB reproductive system is delayed under short day lengths in this species. © 1998 John Wiley & Sons, Inc. J Neurobiol 35: 355–360, 1998     

Photoperiodic regulation of circulating leukocytes in juvenile Siberian hamsters: mediation by melatonin and testosterone

The reproductive system of Siberian hamsters (Phodopus sungorus) undergoes rapid phenotypic responses to changes in day length that occur around the time of weaning. The present experiments tested whether the immune system of Siberian hamsters is similarly photoperiodic early in life and whether photoperiodic changes in melatonin or gonadal hormone secretions mediate any such responses to day length. Circulating blood leukocyte concentrations (WBC) were measured in juvenile male Siberian hamsters that were gestated in long-days (LD), transferred to short-days (SD) on the day of birth, and subsequently either remained in SD or were transferred from SD to LD at 18 days of age (day 18). WBC values were comparable between LD and SD hamsters on day 18. Between day 18 and day 32, SD hamsters exhibited a 3-fold increase in WBC, whereas LD hamsters failed to undergo a significant increase in WBC during this interval. WBC of LD hamsters was significantly lower than that of SD hamsters on day 25 and on day 32. In LD housed males, peripheral injections of melatonin delivered so as to extend the nocturnal duration of elevated endogenous melatonin secretion (i.e., provided in late afternoon) on days 18-31 increased WBC as measured on day 32. Peripubertal (day 17) gonadectomy abolished the immunosuppressive effect of LD exposure on WBC, and treatment with silastic implants containing testosterone suppressed WBC independent of photoperiod treatment. These data indicate that juvenile Siberian hamsters are immunologically responsive to photoperiod and that the leukocyte responses to day length are the result of melatonin-mediated effects of photoperiod on testicular hormone secretion.     

Temperature dependence of gonadal regression in Syrian hamsters exposed to short day lengths

We sought to determine whether ambient temperature (T(a)) affects gonadal function by altering the rate at which circadian rhythms entrain to short day lengths. Syrian hamsters were housed in cages where they received 14 h of light per day ("long days," 14L) at 22 degrees C. Hamsters were then transferred to cages to receive 10 h of light per day ("short days," 10L) and kept at 5, 22, or 28 degrees C or were maintained in 14L at 22 degrees C. Body mass and estimated testis volume as well as duration of nocturnal locomotor activity (alpha), previously established as a reliable indicator of the duration of nocturnal melatonin secretion, were determined over the course of 24 wk. Testicular regression in short days was accelerated by 4 wk at 5 degrees C and delayed by 3 wk at 28 degrees C relative to 22 degrees C. The interval between alpha-expansion and initiation of testicular regression was markedly affected by T(a) with delays of 0, 3, and 6 wk at 5, 22, and 28 degrees C, respectively. All hamsters held at 5 and 22 degrees C underwent testicular regression, but 25% of those maintained at 28 degrees C failed to do so. We suggest that T(a) modulates testicular regression primarily by affecting responsiveness of neuroendocrine target tissues to long melatonin signals.     

Pineal-dependent and -independent effects of photoperiod on immune function in Siberian hamsters (Phodopus sungorus)

Siberian hamsters (Phodopus sungorus) exhibit reproductive and immunological responses to photoperiod. Short (<10-h light/day) days induce gonadal atrophy, increase leukocyte concentrations, and attenuate thermoregulatory and behavioral responses to infection. Whereas hamster reproductive responses to photoperiod are dependent on pineal melatonin secretion, the role of the pineal in short-day induced changes in immune function is not fully understood. To examine this, adult hamsters were pinealectomized (PINx) or sham-PINx, and transferred to short days (9-h light/day; SD) or kept in their natal long-day (15-h light/day; LD) photoperiod. Intact and PINx hamsters housed in LD maintained large testes over the next 12 weeks; sham-PINx hamsters exhibited gonadal regression in SD, and PINx abolished this effect. Among pineal-intact hamsters, blood samples revealed increases in leukocyte, lymphocyte, CD62L+ lymphocyte, and T cell counts in SD relative to LD; PINx did not affect leukocyte numbers in LD hamsters, but abolished the SD increase in these measures. Hamsters were then treated with bacterial lipopolysaccharide (LPS), which induced thermoregulatory (fever), behavioral (anorexia, reductions in nest building), and somatic (weight loss) sickness responses in all groups. Among pineal-intact hamsters, febrile and behavioral responses to LPS were attenuated in SD relative to LD. PINx did not affect sickness responses to LPS in LD hamsters, but abolished the ameliorating effects of SD on behavioral responses to LPS. Surprisingly, PINx failed to abolish the effect of SD on fever. In common with the reproductive system, PINx induces the LD phenotype in most aspects of the immune system. The pineal gland is required for photoperiodic regulation of circulating leukocytes and neural-immune interactions that mediate select aspects of sickness behaviors.     

Timing of testicular recrudescence in siberian hamsters is unaffected by pinealectomy or long-day photoperiod after 9 weeks in short days

In this study, the authors asked whether pinealectomy or temporary exposure to a stimulatory photoperiod affects the timing of spontaneous testicular recrudescence in adult Siberian hamsters chronically exposed to short days (9:15 light:dark). In Experiment 1, hamsters were pinealectomized after 6, 9, or 12 weeks in short days. Pinealectomy after 9 or 12 weeks did not affect the timing of spontaneous gonadal growth (27.7 +/- 1.9 and 25.4 +/- 1.3 weeks, respectively) compared to sham-operated controls (28.6 +/- 0.9 weeks). Enlarged testes occurred earlier in animals that were pinealectomized after 6 weeks in short days (21.8 +/- 2.1 weeks). In Experiment 2, adult hamsters were exposed to short days for 9 weeks, transferred to long days (16:8 light:dark) for 4 weeks, and then returned to short days for 23 additional weeks. Although long-day interruption caused gonadal growth in 15 out of 19 hamsters, the temporary long-day exposure did not affect the timing of spontaneous gonadal growth following return to short days (28.2 +/- 0.9 weeks) in 10 of the 15, relative to the timing observed in control hamsters continuously maintained in short days (28.2 +/- 1.1 weeks). Four out of 19 hamsters did not show gonadal growth following long-day exposure. Spontaneous gonadal growth in these hamsters (28.0 +/- 1.4 weeks) also occurred at the same time as controls. The remaining 5 hamsters exhibited enlarged testes following long-day exposure (12.0 +/- 0.0 weeks) but were refractory to the second short-day exposure. All hamsters exhibited entrainment of wheel-running activity following the change in photoperiod. A final group of 13 animals were pinealectomized before long-day transfer. They exhibited gonadal growth (at 17.2 +/- 0.8 weeks) but failed to regress a second time when returned to short days. The timing of gonadal growth in these animals was delayed relative to the sham-operated hamsters temporarily transferred to long days (Experiment 2) but accelerated relative to the hamsters pinealectomized at 9 weeks, which remained continuously in short days (Experiment 1). The results of both experiments suggest that a pineal-independent process mediates the timing of spontaneous gonadal growth in Siberian hamsters chronically exposed to a short-day photoperiod.     

Illuminance threshold for maintenance of testes in Syrian hamsters (Mesocricetus auratus) is higher in continuous light than in long photoperiods

The illuminance threshold for maintenance of testicular function was found to be considerably higher in Syrian hamsters kept in continuous light (LL) than in hamsters on long-day (14-hr) photoperiods (LD 14:10), or in a similar-length skeleton photoperiod (LDSK); the threshold lay between 3 and 30 lux in LL and at approximately 0.3 lux in LD 14:10 or LDSK. The threshold for testicular maintenance in LL was related to the capacity of LL to suppress nocturnal melatonin secretion: 400 lux totally suppressed, 30 or 3 lux partially suppressed, and 0.3 lux failed to suppress melatonin secretion. Hamsters in the LD and LDSK groups, whose locomotion was entrained into a pattern characteristic of long-day exposure, maintained full testicular function; those whose locomotion free-ran or assumed a pattern of entrainment characteristic of short-day exposure underwent testicular regression. These results suggest that light signals entrain the circadian rhythms of locomotion and melatonin secretion in a similar manner, and that LL is less effective than LD or LDSK in shortening the duration of melatonin secretion. For hamsters in LL, a direct relationship was seen between the free-running period (tau) of locomotion and log10 illuminance at 0.3, 3.0, and 30 lux, but tau at 400 lux was no longer than tau at 30 lux. Splitting of locomotion did not occur at 0.3 or 3.0 lux, and occurred in 43% and 62% of hamsters in 30 and 400 lux, respectively.     

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.     

Ventromedial hypothalamic mediation of photoperiodic gonadal responses in male Syrian hamsters.

Short day lengths induce testicular regression in seasonally breeding Syrian hamsters. To test whether the ventromedial hypothalamus is necessary to maintain reproductive quiescence once testicular regression has been achieved, photoregressed male hamsters were subjected to lesions of the ventromedial hypothalamus (VMHx), pinealectomy (Pinx), or sham operation (Sham). VMHx hamsters underwent accelerated gonadal recrudescence compared to Pinx and Sham hamsters. Recovery of prolactin concentrations (PRL) to values characteristic of long-day hamsters was hastened in the VMHx animals compared to Sham hamsters. Concentrations of follicle stimulating hormone (FSH) increased prematurely in both the VMHx and Pinx animals, beginning a few weeks after surgery. By the time the gonads had undergone recrudescence and the hamsters were refractory to melatonin, PRL and FSH concentrations had returned to baseline long-day values in all groups; there was no evidence of hypersecretion of either hormone in any of the animals with lesions. Melatonin concentrations of VMHx hamsters did not differ from those of sham-operated animals, but because only a single determination was made, it remains possible that VMH damage altered the duration of nightly melatonin secretion. An intact VMH appears to be essential for the continued maintenance of reproductive suppression induced by exposure to short day lengths; these and earlier findings suggest that the VMH-dorsomedial hypothalamic complex mediates regression of the reproductive apparatus during decreasing day lengths of late summer and early autumn and also is necessary to sustain regression during the winter months.     

Maternal photoperiodic history affects offspring development in Syrian hamsters

During the first 7 weeks of postnatal life, short day lengths inhibit the onset of puberty in many photoperiodic rodents, but not in Syrian hamsters. In this species, timing of puberty and fecundity are independent of the early postnatal photoperiod. Gestational day length affects postnatal reproductive development in several rodents; its role in Syrian hamsters has not been assessed. We tested the hypothesis that cumulative effects of pre- and postnatal short day lengths would restrain gonadal development in male Syrian hamsters. Males with prenatal short day exposure were generated by dams transferred to short day lengths 6 weeks, 3 weeks, and 0 weeks prior to mating. Additional groups were gestated in long day lengths and transferred to short days at birth, at 4 weeks of age, or not transferred (control hamsters). In pups of dams exposed to short day treatment throughout gestation, decreased testis growth was apparent by 3 weeks and persisted through 9 weeks of age, at which time maximum testis size was attained. A subset of males (14%), whose dams had been in short days for 3 to 6 weeks prior to mating displayed pronounced delays in testicular development, similar to those of other photoperiodic rodents. This treatment also increased the percentage of male offspring that underwent little or no gonadal regression postnatally (39%). By 19 weeks of age, males housed in short days completed spontaneous gonadal development. After prolonged long day treatment to break refractoriness, hamsters that initially were classified as nonregressors underwent testicular regression in response to a 2nd sequence of short day lengths. The combined action of prenatal and early postnatal short day lengths diminishes testicular growth of prepubertal Syrian hamsters no later than the 3rd week of postnatal life, albeit to a lesser extent than in other photoperiodic rodents.     

Amplitude modulation of the nightly melatonin rise in the neonatal lamb and the subsequent timing of puberty

Spring-born female lambs require a decrease in day length for the normal timing of puberty the following autumn. If this decrease occurs early in postnatal life (i.e. 0-10 weeks), puberty is delayed. This study tested the hypothesis that failure of the neonatal lamb to respond to the critical long-day to short-day signal is due to inadequate nocturnal melatonin secretion. The approach was to artificially increase, to adult levels, the low nighttime rises of melatonin during the early postnatal period. Eight female lambs served as controls; they were raised on short days until 17 wk of age, and then exposed to 5 wk of long days, after which they were returned to short days. This alternating sequence of photoperiods during mid-development would be expected to induce normal puberty. Sixteen experimental females were exposed to the critical block of long days much earlier; they were placed in long days between 2 and 7 wk of age and in short days thereafter. Half (n = 8) received no further treatment. The other half (n = 8) were infused nightly with melatonin during the 8-h dark phase of the 5-wk, long-day photoperiod. This increased the amplitude of the natural nighttime melatonin rises 3- to 4-fold, well into the adult range.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

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.