Gonadal growth and hormone concentrations in photoregressed Siberian hamsters: pinealectomy versus photostimulation

Proliferation, apoptosis, and number of Sertoli cells in the Syrian hamster during recrudescence after exposure to short photoperiod†‡

The Sertoli cell (Sc) has been described as a quiescent cell once the animal has reached sexual maturity. Syrian hamster is an animal that displays testicular regression due to short photoperiod, during which process germ cells and Sc are removed through apoptosis. The aim of this work was to investigate histochemically whether the spontaneous testicular recrudescence processes after exposure to a short photoperiod lead to an increase in Sc proliferative activity in order to restore the normal population. Three spontaneous recrudescence groups were established: initial (IR), advanced (AR), and total (TR) recrudescence, which were compared with animal undergoing the regression process (mild: MRg, strong: SRg, and total: TRg) and animals in long photoperiod (Controls). Histological sections were submitted to histochemical techniques for detecting apoptotic and proliferative Sc with bright-field and fluorescence microscopy. For each group, the proliferative Sc index (PScI) and apoptotic Sc index (AScI), and the total number of Sc were obtained. The results revealed the existence of Vimentin+/TUNEL+ as well as Vimentin+/PCNA+ cells. The PScI was significantly higher in TRg and IR than in the other groups. The AScI was only significantly higher in MRg and SRg with respect to the other groups. The total number of Sc increased among TRg, IR, and AR, reaching values similar to those of the Controls. In conclusion, the increase in Sc proliferation from final regression and recrudescence, accompanied by a similar rate of apoptosis to the Control group, is the cause of the restoration of the Sc population during spontaneous recrudescence.

Pleiotropic effects of proopiomelanocortin and VGF nerve growth factor inducible neuropeptides for the long-term regulation of energy balance

Seasonal rhythms in energy balance are well documented across temperate and equatorial zones animals. The long-term regulated changes in seasonal physiology consists of a rheostatic system that is essential to successful time annual cycles in reproduction, hibernation, torpor, and migration. Most animals use the annual change in photoperiod as a reliable and robust environmental cue to entrain endogenous (i.e. circannual) rhythms. Research over the past few decades has predominantly examined the role of first order neuroendocrine peptides for the rheostatic changes in energy balance. These anorexigenic and orexigenic neuropeptides in the arcuate nucleus include neuropeptide y (Npy), agouti-related peptide (Agrp), cocaine and amphetamine related transcript (Cart) and pro-opiomelanocortin (Pomc). Recent studies also indicate that VGF nerve growth factor inducible (Vgf) in the arcuate nucleus is involved in the seasonal regulation of energy balance. In situ hybridization, qPCR and RNA-sequencing studies have identified that Pomc expression across fish, avian and mammalian species, is a neuroendocrine marker that reflects seasonal energetic states. Here we highlight that long-term changes in arcuate Pomc and Vgf expression is conserved across species and may provide rheostatic regulation of seasonal energy balance.

Lectin-binding pattern of glycoconjugates during spontaneous testicular recrudescence in Syrian hamster (Mesocricetus auratus) after exposure to short photoperiod

Lectin histochemistry was used to characterise glycoconjugates and cellular apoptosis in the seminiferous epithelium and interstitium of hamster testis during spontaneous recrudescence. An increase in the LTA lectin affinity was observed in spermatids in the Golgi phase. An increase in labelling of PNA and Con-A lectin in acrosome of spermatids (acrosome phase) as well as increased labelling with Con-A in spermatids (cap phase) was observed. Spermatocytes showed decreased affinity with PNA and AAA lectins and an increase in positivity for LTA and GNA lectins. Spermatogonia showed a slight decrease in positivity to WGA and an increase in labelling with Con-A and a decreased affinity for the AAA lectin. At the end of recrudescence, all these germinal cells showed a similar pattern to the control. The Sertoli cells showed a gradual decrease in labelling with the GNA lectin and the Leydig cells an increase in labelling with Con-A and GNA. Particularly unusual was the observation of apoptotic spermatocytes and spermatids positive for PNA, GNA, AAA and Con-A, together with spermatocytes positive to LTA. In conclusion, the normal lectin pattern is recovered during testis recrudescence and germ cell apoptotic activity is low, as is observed by specific lectins for germ cells in apoptosis.

Interval Timer Control of Puberty in Photoinhibited Siberian Hamsters

Puberty, which is markedly delayed in male Siberian hamsters ( Phodopus sungorus) born into short day lengths, is controlled by an interval timer regulated by the duration of nocturnal melatonin secretion. Properties of the interval timer were assessed by perturbing normal patterns of melatonin secretion in males gestated and maintained thereafter in 1 of 2 short day lengths, 10 h light/day (10L) or 12L. Melatonin secretion of short-day hamsters was suppressed by constant light treatment or modified by daily injection of propranolol to mimic nocturnal melatonin durations typical of long-day hamsters. Constant light treatment during weeks 3 to 5 induced early incomplete gonadal growth in 12L but not 10L hamsters but did not affect late onset of gonadal development indicative of puberty in either photoperiod. Propranolol treatment during postnatal weeks 3 to 5 induced transient growth of the testes and ultimately delayed the timing of puberty by 3 weeks. Similar treatments between weeks 5 and 7 or on alternate weeks for 24 weeks did not affect the interval timer. The first 2 weeks after weaning may constitute a critical period during which the interval timer is highly responsive to photoperiod. Alternatively, the hamsters' photoperiodic history rather than age or developmental stage may be the critical variable. The interpolation of long-day melatonin signals at the time of weaning does not appear to reset the interval timer to its zero position but may reduce timer responsiveness to long-day melatonin signals several weeks later.

Effects of Pinealectomy and Short Day Lengths on Reproduction and Neuronal RFRP-3, Kisspeptin, and GnRH in Female Turkish Hamsters

Long days (LDs) stimulate and short days (SDs) inhibit reproduction in photoperiodic rodents by modifying nocturnal pineal melatonin secretion. In LD Turkish hamsters, unlike other rodents, pinealectomy induces reproductive quiescence comparable to that produced by SDs. We assessed whether SDs and pinealectomy induce similar or different patterns of kisspeptin and gonadotropin-inhibitory hormone (also known as RFamide-related peptide-3 [RFRP-3] in mammals) expression, important mediators of seasonal reproductive changes in other species. Brains were harvested from sham-operated female Turkish hamsters maintained in LDs and SDs and LD-pinealectomized (pinx) females, all housed in their respective photoperiods for 12 weeks. Uterine weights were substantially higher in LD-sham than in LD-pinx and SD-sham females. RFRP-3-immunoreactive(-ir) cells in the dorsomedial hypothalamic nucleus were greater in number and size in the reproductively competent LD-sham hamsters than in both reproductively suppressed SD-sham and LD-pinx hamsters. LD-sham hamsters had more kisspeptin-ir cells in the anteroventral periventricular nucleus than did LD-pinx hamsters. Reproductive quiescence, whether induced by short-day lengths or pinealectomy, was generally accompanied by comparable changes in RFRP-3 and kisspeptin, suggesting that long-duration melatonin signaling and withdrawal of melatonin by pinealectomy may act through the same neural substrates to induce gonadal quiescence.

The death of sertoli cells and the capacity to phagocytize elongated spermatids during testicular regression due to short photoperiod in Syrian hamster (Mesocricetus auratus)

In the Syrian hamster (Mesocricetus auratus), an animal that displays testicular regression due to short photoperiod, germ cells are removed by apoptosis during this process and the apoptotic remains are phagocytized by Sertoli cells. The aim of this work was to investigate morphologically whether the testicular regression process due to short photoperiod leads to the apoptosis of Sertoli cells, and whether, during testicular regression, the elongated spermatids are eliminated through phagocytosis by Sertoli cells. To this end, we studied testis sections during testicular regression in Syrian hamster subjected to short photoperiod by means of several morphological techniques using conventional light microscopy (hematoxylin and eosin [H&E], semi-thin section vimentin, immunohistochemistry, SBA lectin, and TUNEL staining), fluorescence microscopy, and transmission electron microscopy (TEM). H&E and semi-thin sections identified Sertoli cells with a degenerated morphology. Greater portion of Sertoli cells that were positive for TUNEL staining were observed especially during the mild regression (MR) and strong regression (SR) phases. In addition, TEM identified the characteristic apoptotic changes in the nucleus and cytoplasm of Sertoli cells. Moreover, during testicular regression and using light microscopy, some elongated spermatids were seen in basal position next to the Sertoli cell nucleus. This Sertoli phagocytic activity was higher in MR and SR phases. TEM confirmed this to be the result of the phagocytic activity of Sertoli cells. In conclusion, during testicular regression in Syrian hamster due to short photoperiod, when germ cells are known to be lost through apoptosis, there is morphological evidences that Sertoli cells are also lost through apoptosis, while some elongated spermatids are phagocytized and eliminated by the Sertoli cells.

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.

Seasonal regulation of reproduction: altered role of melatonin under naturalistic conditions in hamsters

The seasonal reproductive cycle of photoperiodic rodents is conceptualized as a series of discrete melatonin-dependent neuroendocrine transitions. Least understood is the springtime restoration of responsiveness to winter-like melatonin signals (breaking of refractoriness) that enables animals to once again respond appropriately to winter photoperiods the following year. This has been posited to require many weeks of long days based on studies employing static photoperiods instead of the annual pattern of continually changing photoperiods under which these mechanisms evolved. Maintaining Siberian hamsters under simulated natural photoperiods, we demonstrate that winter refractoriness is broken within six weeks after the spring equinox. We then test whether a history of natural photoperiod exposure can eliminate the requirement for long-day melatonin signalling. Hamsters pinealectomized at the spring equinox and challenged 10 weeks later with winter melatonin infusions exhibited gonadal regression, indicating that refractoriness was broken. A photostimulatory effect on body weight is first observed in the last four weeks of winter. Thus, the seasonal transition to the summer photosensitive phenotype is triggered prior to the equinox without exposure to long days and is thereafter melatonin-independent. Distinctions between photoperiodic and circannual seasonal organization erode with the incorporation in the laboratory of ecologically relevant day length conditions.

Testicular dynamics in Syrian hamsters exposed to both short photoperiod and low ambient temperature

The object of this study was to determine the details of morphological dynamics of spermatogenesis in Syrian hamsters exposed to both short photoperiod and low ambient temperature. Eight-week-old male hamsters, kept in a long photoperiod (14 h L, 10 h D), were transferred to a short photoperiod (6 h L, 18 h D) and kept there for 13 weeks to induce testicular regression. Some hamsters were then transferred from the room at 23 degrees C to that at 5 degrees C (5 degrees C group). Remaining hamsters were continuously kept at 23 degrees C (23 degrees C group). Thereafter, the morphology was examined. As a result, it took only 8 weeks until spermatogenesis recovered in the 23 degrees C group. However, it was not until 20 weeks that spermatogenesis was recognized in the 5 degrees C group. As the regulation of seasonal testicular activity is characterized by coordinated shifts in the relationships among mitosis, meiosis, and apoptosis, the changes in the proliferative and apoptotic activities were examined. Although no significant difference in proliferative activity of spermatogonia between the 5 degrees C and the 23 degrees C groups was confirmed, a notable increase in the rate of apoptosis was observed in the 5 degrees C group. Furthermore, this increase was more salient during the hibernation period. These findings suggest that both cold ambient temperature and hibernation caused the delay of testicular recrudescence and this delay arose from the increase of apoptotic activity but not the change in proliferative activity in spermatogonia in the 5 degrees C group.

Seasonal plasticity in the copulatory neuromuscular system of green anole lizards: a role for testosterone in muscle but not motoneuron morphology

The copulatory system of green anoles is highly sexually dimorphic. Males possess bilateral copulatory organs called hemipenes, each independently controlled by two muscles: the transversus penis (TPN) and retractor penis magnus (RPM). The TPN everts the hemipene through the cloaca and the RPM retracts it. Adult females do not possess hemipenes or either of these two muscles. The spinal nucleus projecting to the TPN and RPM contains more and larger motoneurons in males than females. Because anoles breed seasonally, two experiments were designed to test whether adult copulatory morphology varies with environmental condition, and if so, whether the effect is mediated by testicular androgens. Three groups of adult males were used in each experiment: males from breeding environmental conditions with reproductive testes (BS); males in breeding conditions with regressed testes (BS-X); and males in nonbreeding conditions with regressed testes (NBS). Experiment 1 compared gonadally intact males and Experiment 2 compared castrated males treated with either testosterone (T) or an empty implant. In both experiments, copulatory and control motoneurons appeared smaller in NBS males, but T did not affect their size. In contrast, while hemipene and RPM muscle fiber size were not plastic across season in gonadally intact males, T in castrated males significantly increased both measures under BS and BS-X, but not NBS, conditions. These results demonstrate that neuron soma size might change on a general level and environmental cues can mediate T-induced changes in peripheral structures, suggesting that plasticity across copulatory system components is regulated by different mechanisms.

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.

Histochemical study of glycoconjugates in active and photoperiodically-regressed testis of hamster (Mesocricetus auratus)

The objective of the present study was to characterize glycoconjugates of hamster testis in gonadally-active and -inactive states by lectin histochemical methods. Thirteen HRP- or digoxigenin-labeled lectins were used in samples obtained from fertile and photoinhibited hamsters. In gonadally-active hamsters, spermatozoa tails were stained with Con-A, HPA, PNA, UEA-I, LTA, AAA, WGA and LFA and weakly with GNA and RCA-I. Spermatozoa acrosomes were labeled with HPA, SBA, WGA and PNA. Spermatid acrosomes were labeled with SBA, RCA-I, PNA, and WGA. Staining with GNA and Con-A was found in the Golgi phase and HPA staining was found in the Golgi phase and maturated spermatids. Cytoplasm of spermatocytes was labeled with Con-A, GNA, LTA, AAA, RCA-I, HPA, WGA and LFA, whereas spermatocyte membranes were stained with Con-A, LTA and AAA. Spermatogonia were strongly labeled with Con-A and moderately labeled with AAA, WGA and LFA. Sertoli cells were positive after staining with Con-A, AAA, WGA, and LFA. The lamina propria was positive after staining with UEA-I, LTA, AAA and LFA. Leydig cells showed strong labeling with SBA, Con-A, GNA, SNA and MAA, moderate labeling with WGA, weak labeling with RCA-I, AAA and LFA. In gonadally-inactive hamsters, spermatocytes showed increased staining with HPA, PNA and AAA, whereas staining with Con-A, GNA and LTA had disappeared. Spermatogonia showed an increased labeling with AAA and WGA, but labeling with Con-A and LFA had disappeared. Sertoli cells were strongly labeled with GNA. Con-A and GNA staining was decreased in Leydig cells of gonadally-inactive hamsters but PNA and HPA staining was increased. The lamina propria in regressed testes showed intense labeling with PNA. These results suggest that histological, morphological and hormonal changes occurring in hamster testis during exposure to a short photoperiod are reflected in altered patterns of expression and distribution of N- and O-linked glycans.

Temporal integration of melatonin infusion duration: signal averaging versus frequency dependence

Day length affects somatic and reproductive physiology of Siberian hamsters via regulation of the duration of nocturnal pineal melatonin secretion. Nightly 'long' (e.g. 12 hr) or 'short' (e.g. 6 hr) melatonin signals inhibit or stimulate gonadal growth, respectively. When long and short signals are presented in combination, however, neuroendocrine mechanisms exhibit a frequency-dependent response, stimulating gonadal growth only if short signals are presented every second night or more frequently. The present experiments further assessed formal models for the temporal integration of melatonin signals changing abruptly in duration from night to night. Photo-inhibited Siberian hamsters were housed in constant light and infused subcutaneously with various combinations of nightly short or long melatonin signals according to one of the several regimes that varied the frequency of short melatonin signal occurrence, average duration of the nightly melatonin signal, or both. Six weeks of nightly alternating short and long signals yielded different gonadal responses depending on the average melatonin signal duration. Moreover, when average melatonin signal duration was held constant between groups, gonadal stimulation was independent of the frequency of the constituent melatonin signals except when the duration of the short signal was reduced to 3 hr. Thus, neuroendocrine mechanisms do not solely categorize melatonin signals as either long or short but attend also to the duration of each component signal. In the majority (six of seven) of infusion regimes, reproductive responses to chimeric patterns of long and short melatonin signals were compatible with a simple signal-averaging mechanism.

Proliferation and apoptosis in the seminiferous epithelium of photoinhibited Syrian hamsters (Mesocricetus auratus)1

In the hamster, male reproductive quiescence is accomplished via testicular atrophy and the germinal epithelium is regressed to spermatogonia and spermatocytes after 8-14 weeks of short photoperiods. However, the cellular mechanisms involved in this process have not been elucidated. As it is suggested that the regulation of seasonal testicular activity is characterized by coordinated shifts in the relationships between mitosis, meiosis and apoptosis, the changes in the proliferative and apoptotic activity in the seminiferous epithelium of photoinhibited Syrian hamster were examined and compared with those maintained in natural photoperiod. The proliferative activity was studied using BrdU immunostaining, and germ cell apoptosis was assessed by in situ TUNEL labelling and transmission electron microscopy. A significant increase in the rate of apoptosis (percentage of TUNEL-positive spermatogonia + spermatocytes) was observed in photoinhibited animals (2.84 +/- 0.16) compared with those exposed to natural photoperiod (0.77 +/- 0.03, p < 0.05). The majority of apoptotic germ cells were spermatocytes and in some occasions spermatogonia. Germ cell apoptosis was confirmed by morphological characteristics: condensation of the chromatin and nuclear fragmentation. The rate of proliferation (percentage of BrdU-positive spermatogonia + preleptotene spermatocytes) was significantly higher in photoinhibited hamsters (42.7 +/- 2.6) compared with animals exposed to natural photoperiod (31.1 +/- 1.6, p < 0.05). After the exposure to a short photoperiod the apoptotic index positively correlated with the proliferative index (r = 0.8150, p < 0.05). In conclusion, the seminiferous epithelium of photoinhibited Syrian hamsters is characterized by an increased rate of apoptosis associated to an enhanced rate of proliferation.

Photoperiod and androgens act independently to induce spinal nucleus of the bulbocavernosus neuromuscular plasticity in the Siberian hamster, Phodopus sungorus

In the Siberian hamster, Phodopus sungorus, short-day photoperiods induce the winter phenotype, which in males includes a decrease in the production of androgens and changes in physiology to inhibit reproduction. Motoneurones of the spinal nucleus of the bulbocavernosus (SNB) and their target muscles, the bulbocavernosus and the levator ani, a neuromuscular system involved in male copulation, also display seasonal plasticity in P. sungorus. It is not known whether the plasticity seen in the SNB system of gonadally intact hamsters is due to the effects of photoperiod per se, or to the photoperiod-induced changes in androgen production. To answer this question, we castrated adult male hamsters from long days and then implanted them with capsules containing either testosterone or blanks. Half of the hamsters from each hormone condition were moved into short photoperiod (8 : 16 h light/dark cycle) while the rest were maintained under long-day conditions (15 : 9 h light/dark cycle). After 15 weeks, many measures of the SNB system, such as somata size and weight of target muscles, responded only to androgen, not to photoperiod. However, there were effects of photoperiod on the neuromuscular junctions (NMJs) that were independent of androgen status. For example, the number of synaptic zones per NMJ and the area of the NMJs were significantly increased by short days and/or testosterone treatment. The two factors exerted an additive, rather than an interactive, effect on these measures. Another striated muscle, the extensor digitorum longus, which is present in both sexes and plays no specialized role in reproduction, displayed neither an effect of androgen nor of photoperiod on fibre size or NMJ structure. These results suggest that, in addition to androgenic effects on SNB plasticity, there is also an androgen-independent effect of photoperiod on the SNB neuromuscular system.

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.

Photic entrainment of circannual rhythms in golden-mantled ground squirrels: role of the pineal gland

Entrainment of circannual rhythms of body mass and reproduction was monitored for 3 years in female golden-mantled ground squirrels maintained in a simulated natural photoperiod. Both pinealectomized and pineal-intact squirrels generated circannual rhythms of body mass and estrus, but only the intact animals entrained these rhythms to a period of 365 days. In the second and third years after treatment, the period of the body mass rhythm was significantly shorter than 365 days for pinealectomized squirrels, and variance in tau among these animals was significantly greater than for intact squirrels. A similar pattern was evident in the rhythm of reproduction, which was phase-disrupted in pinealectomized squirrels but entrained in intacts. Seasonal changes in duration of nocturnal melatonin secretion by the pineal appear to be necessary to produce phase-delays required to entrain the circannual clock to a period of 12 months.

Triggering of neuroendocrine refractoriness to short-day patterns of melatonin in Siberian hamsters

Short day lengths induce gonadal regression within 6 weeks in Siberian hamsters, but this inhibitory effect eventually wanes and reproductive competence is regained after 4-6 months in short days. These experiments were undertaken to determine whether continuous exposure to short days for several months is necessary to induce neuroendocrine refractoriness, or if a few weeks of short days are sufficient to trigger processes that culminate in refractoriness several months later. Adult male Siberian hamsters transferred from their natal long-day photoperiod of 15L (15 h light/day) to a short-day photoperiod of 10L were pinealectomized (PINx) after 0, 6, 12, 21 or 40 weeks of exposure to 10L. Intact hamsters kept in 10L manifested gonadal regression by week 6 and completed gonadal recrudescence by week 40, at which time they too were PINx. Beginning at week 40 all hamsters were infused s.c. with melatonin for 10 h/day for 6 consecutive weeks. This treatment induces gonadal regression in photosensitive hamsters. At the end of melatonin treatment, maximal gonadal regression was recorded for hamsters PINx at week 0 and those PINx after 40 weeks in long days. Hamsters PINx after 40 week of short day treatment were completely unresponsive to melatonin whereas those PINx after 6 and 12 weeks had intermediate responses. The percentage of hamsters whose reproductive apparatus was refractory to melatonin at week 40 increased with increasing duration of exposure to short days prior to PINx. Refractoriness was induced by relatively few weeks of short days in some hamsters, whereas others required much more extensive exposure. Induction of refractoriness is triggered by a fraction of the short days that hamsters experience in nature but may not be manifested until many weeks later when it coincides with gonadal recrudescence. In nature all hamsters are exposed to short days for at least 25 weeks, ensuring refractoriness in most individuals.

Frequency coding of melatonin signals sufficient to induce testicular growth in photoregressed Siberian hamsters

Integration of melatonin signals over the course of several weeks was studied in male Siberian hamsters maintained in a short day length (10 h light/day) from birth (d 0). On d 35, hamsters with undeveloped testes were housed in constant light and received 5 h s.c. infusions of melatonin (MEL; 100 ng/infusion) or saline (SAL) over the next 30 days. Infusions were provided either every day (30 MEL), every other day (1 MEL/1 OFF), for 2 consecutive days followed by 2 days with no infusion (2 MEL/2 OFF), for 5 consecutive days followed by 5 days with no infusion (5 MEL/5 OFF), for 15 consecutive days followed by 15 days with no infusion (15 MEL/15 OFF), or saline every day (30 SAL). On d 65 testes weights of 30 MEL hamsters were greater than those from 30 SAL, 1 MEL/1 OFF, or 2 MEL/2 OFF groups, but did not differ significantly from those of 5 MEL/5 OFF and 15 MEL/15 OFF animals. Serum FSH concentrations of 30 MEL hamsters exceeded those of all other groups which did not differ among each other. Between 15 and 30 consecutive daily melatonin signals are necessary and sufficient to initiate and sustain maximal gonadotropic activity in juvenile male hamsters kept in constant light. The neuroendocrine system responsive to melatonin does not bridge intervals of more than a day in any of several combinations and apparently is frequency-coded for maximal responsiveness to daily signals.

Seasonal plasticity of neuromuscular junctions in adult male Siberian hamsters (Phodopus sungorus)

Transfer of adult Siberian hamsters, Phodopus sungorus, from long day (16 h light and 8 h dark; 16L:8D) to short day (8L:16D) photoperiods induces an involution of the gonads and a cessation of reproductive behavior 8-10 weeks later. The motoneurons of the spinal nucleus of the bulbocavernosus and their target muscles, the bulbocavernosus and the levator ani, are sexually dimorphic and are necessary for successful reproduction by male mammals. We demonstrate that after transfer of adult male Siberian hamsters to short photoperiods, the bulbocavernosus motoneurons, their target muscles and neuromuscular junctions are all significantly smaller than those of males that remain under long day conditions. Photoperiod also affected the number of active zones within each neuromuscular junction, an apparent remodeling of these synapses. Thus, this neuromuscular system of adult Siberian hamsters demonstrates considerable seasonal plasticity in response to changes in photoperiod.

Melatonin chimeras alter reproductive development and photorefractoriness in Siberian hamsters

Nightly melatonin (MEL) durations > 8 h provoke gonadal regression and decreases in body mass, whereas signals < 7 h stimulate gonadal and somatic growth in male Siberian hamsters. The authors sought to determine the minimum frequency of short MEL signals sufficient to induce the long-day phenotype in several photoperiodic traits. D,L-propranolol (hereafter propranolol) injections shortened MEL signals on the night of treatment without altering MEL on the subsequent night; this permitted interpolation of short MEL signals at variable frequencies against a background of long MEL signals (chimeras). Hamsters kept in short days (10 h light/day, 10L) were injected with propranolol 6 h after dark onset for 28 consecutive weeks beginning at 30 days of age (Week 0) either every other day or once every 3, 6, or 9 days. Control animals were injected with saline or with propranolol during the light phase or were transferred to long days (16L) at Week 0. Hamsters in 16L underwent rapid gonadal development and increases in body mass and displayed summer pelage color, as did hamsters treated with propranolol every other day. Animals treated with propranolol less frequently than every other day uniformly maintained undeveloped gonads and winter-like body weights, but pelage color became proportionately darker with increased frequency of propranolol treatments. The onset of spontaneous testicular development in 10L was unaffected by propranolol injections. After termination of injections at Week 28, testicular regression was not observed in most 10L animals that previously had undergone spontaneous testicular development; however, 40% of hamsters that had been injected with propranolol every 3rd night did manifest the winter phenotype after Week 28. In an alternating sequence, short MEL signals completely override long signals and induce the summer phenotype. Threshold frequencies differ for MEL stimulation of long-day pelage and gonadal phenotypes. The timing and development of refractoriness to MEL does not depend in any simple manner on the number of long MEL signals or on the accumulation of a reaction product produced by long, and depleted by short, MEL signals.

Morphological and histochemical changes in the epididymis of hamsters (Mesocricetus auratus) subjected to short photoperiod

The morphological involution and histochemical changes of the Syrian hamster (Mesocricetus auratus) epididymis induced by a short light period were investigated. Under short-day conditions, the epididymis showed marked morphological changes including a decrease in luminal diameter, disappearance of spermatozoa, increase of interductal tissue, increase of intraepithelial lipofuscin deposits, the presence of phagolysosomes in the principal cells and macrophage-like cells, and a considerable modification of most clear cells. With lectin histochemistry changes were found in the glycoconjugates of principal cells of the regressed epididymis, either a decrease (PNA, WGA, HPA and DBA) or an increase (MAA) in the affinity of lectins to the Golgi area, or a decrease (HPA) or an increase (PNA) in lectin binding to stereocilia. Both morphological and histochemical results showed that, under this light condition, the cauda epididymidis presented the most prominent alterations, and that the epididymis showed increased absorptive activity and a decreased synthesis of glycoproteins. All these changes are probably due to the decrease in testosterone levels.

Effects of aging and melatonin administration on gonadotropin-releasing hormones (GnRH) gene expression in the male and female rat

It is well documented that in the rat of both sexes aging is associated with a decline in reproductive functions. We have recently shown that melatonin exerts a positive influence on GnRH gene expression in the adult male rats. In order to evaluate the effect of aging as well as melatonin on GnRH mRNA levels, we have studied the effect of 2.5-day administration of melatonin to young (50-55 day of age) and aged (18 month of age) rats of both sexes. In the young males melatonin induced a 11% increase in the hybridization signal. In the aged males, the GnRH mRNA levels were 13% lower than those observed in the young animals. Melatonin administration to aged animals completely restored GnRH mRNA levels when compared to those observed in the young untreated male rats. In contrast, melatonin did not modify the hybridization signal in young female rats, while aging induced a 20% decrease in mRNA levels. Melatonin administration to aged female induced a 18% increase in GnRH mRNA levels, thus completely reversing the influence of aging. These results indicate that the decrease in GnRH gene expression which is likely involved in the decline of reproductive functions in aging can be totally reversed by a short term administration of melatonin, then suggesting that the pineal hormone may be involved in the decrease of GnRH neuronal activity during aging.

Effects of pinealectomy and melatonin on gonadotropin-releasing hormone (GnRH) gene expression in the male rat brain

Melatonin, a pineal hormone, is known to be an important neurohormonal factor involved in the timing of reproductive events which occur seasonally in various mammalian species. In order to evaluate the influence of melatonin on neurons which are producing gonadotropin-releasing hormone (GnRH), we studied the effect of light-dark cycle as well as pinealectomy and melatonin administration on GnRH gene expression in the adult male rat medial preoptic area (MPOA) using quantitativein situ hybridization. The animals were kept under artificial light (light on 6:00 h-20:00 h). In animals which were sacrificed at 24:00 h (when endogenous melatonin levels are high), the hybridization signal was higher than that detected in animals sacrificed at 20:00 h (before the onset of darkness). Administration of melatonin during the light period (16:00 h) induced a 15% increase in the amount of GnRH mRNA after 4 h. Three weeks after pinealectomy mRNA levels were decreased by 35%. Injection of melatonin to pinealectomized rats 4 h before sacrifice increase the amount of GnRH mRNA, completely reversing the decrease in mRNA induced by pinealectomy. These results strongly suggest that melatonin produced by the pineal gland exerts a positive influence on GnRH neuronal activity in the male rat.

Suprachiasmatic nucleus and photoperiodic regulation of gonadal development in the Siberian hamster, Phodopus sungorus

Photoregressed Siberian hamsters exposed at 19 days of age to constant light for 24 h manifested increased testicular development 16 days later. Hamsters that sustained lesions of the suprachiasmatic nucleus (SCN) 3 days after the light pulse had significantly heavier gonads than did sham-operated animals or hamsters pinealectomized at 19 days of age. Enhanced reproductive development after SCN ablation is not due solely to lesion-induced elimination of melatonin secretion. SCN neural activity beginning 3 days after the light pulse is not necessary for light-induced gonadal growth.