Effect of photoperiod on the size of the Leydig cell population and the rate of recruitment of Leydig cells in adult Syrian hamsters

The number of Leydig cells was determined by stereologic procedures in adult Syrian hamsters housed in long days (14L:10D) to maintain testicular activity (active), in short days (5L:19D) for 12-13 wk to induce testicular regression (photoperiod-induced regressed), or in short days for a period of 21 wk or more to allow spontaneous gonadal recrudescence (spontaneously recrudesced). Testes were removed, sliced, fixed, embedded in Epon 812, and observed by bright-field microscopy. Testicular and seminal vesicle weights, plasma testosterone concentration, total Leydig cell volume per testis, and volume of single Leydig cell were greater (p less than 0.01) in active and recrudesced animals than in regressed animals. The density of Leydig cells was greater in the regressed testes, but the total number per testis was not influenced by photoperiod. In Experiment 2, the rate of recruitment of Leydig cells was determined in 5 adult hamsters exposed to long days (active) or 5 hamsters whose testes were regressed by exposure of animals to short days for 13 wk followed by long-day exposure to initiate testicular growth (photoperiod-induced recrudescing). Hamsters were injected for 3 days/wk for 3 wk with tritiated thymidine, 0.5 or 1 microCi/g body weight. Testes were fixed and tissues prepared, as above, and processed for autoradiography. Again, the photoperiod did not influence the number of Leydig cells per testis. Labeling of Leydig cell nuclei revealed that recruitment of new Leydig cells occurred at approximately 1.3% per day in recrudescing testes but also occurred at approximately 0.6% per day in active testes. Without change in the total number of Leydig cells, new Leydig cells were added continually to the existing population in adult hamsters with either recrudescing or active testes.     

Photoperiodic variation of Leydig cell numbers in the testis of the golden hamster: a possible mechanism for their renewal during recrudescence

Golden hamster testes regress after short day exposure. The present study asks: 1) are Leydig cell numbers depleted during short days, and 2) if so, how are they replenished during recrudescence. Control hamsters were shown 14 h of light and 10 h of dark (LD 14:10) for 10 weeks (n = 12). Testicular regression was induced by LD 6:18 for 10 weeks (n = 4), and recrudescence by switching regressed hamsters to LD 14:10 for 3 and 5 weeks (n = 8 for each group). All hamsters were injected with [3H]thymidine [3 microCi/gm body wt., intraperitoneally (i.p.)] 1 h or 2 weeks before sacrifice. Leydig cell number per testis was determined by stereological analysis of sections of perfusion-fixed testes, and labeling indices were determined by autoradiography. Leydig cell numbers were reduced significantly from 18.2 X 10(6) in control to 9.0 X 10(6) in regressed testes (p less than 0.05); then increased to 14.0 X 10(6) and 17.9 X 10(6) in 3- and 5-week recrudesced hamsters. The labeling index was nondetectable (n.d.) for regressed hamsters. In control and recrudescing hamsters the labeling index was measured at two times (t1 = 1 h vs. t2 = 2 weeks post-injection): in controls, t1 = 0.22 +/- 0.15% (mean +/- SEM) vs. t2 = 0.28 +/- 0.22%; in 1 week recrudesced, n.d. vs. 1.92 +/- 0.77% (p less than 0.05); at 3 wk, n.d. vs. 4.58 +/- 1.74% (p less than 0.05); at 5 weeks, 1.92 +/- 0.61% vs. 2.25 +/- 0.59%.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Morphometric studies on hamster testes in gonadally active and inactive states: light microscope findings

This study provides quantitative information on the testes of seasonally breeding golden hamsters during active and regressed states of gonadal activity. Seminiferous tubules occupied 92.5% of testis volume in adult gonadally active animals. Leydig cells constituted 1.4% of the testicular volume. The mean volume of an individual Leydig cell was 1092 microns 3, and each testis contained about 25.4 million Leydig cells. The volume of an average Sertoli cell nucleus during stage VII-VIII of the cycle was 502 microns 3. A gram of hamster testis during the active state of gonadal activity contained 44.5 million Sertoli cells, and the entire testis contained approximately 73.8 million Sertoli cells. Testes of the hamsters exposed to short photoperiods for 12-13 wk displayed a 90% reduction in testis volume that was associated with a decrease in the volume of seminiferous tubules (90.8% reduction), tubular lumena (98.8%), interstitium (72.7%), Leydig cell compartment (79.3%), individual Leydig cells (69.7%), Leydig cell nuclei (50.0%), blood vessels (85.5%), macrophages (68.9%), and Sertoli cell nuclei (34.1%). The diameter (61.1%) and the length (36.8%) of the seminiferous tubules were also decreased. Although the number of Leydig cells per testis was significantly lower (p less than 0.02) after short-photoperiod exposure, the number of Sertoli cells per testis remained unchanged. The individual Sertoli cell in gonadally active hamsters accommodated, on the average, 2.27 pre-leptotene spermatocytes, 2.46 pachytene spermatocytes, and 8.17 round spermatids; the corresponding numbers in the regressed testes were 0.96, 0.20, and 0.04, respectively. The striking differences in the testicular structure between the active and regressed states of gonadal activity follow photoperiod-induced changes in endocrine function and suggest that the golden hamster may be used as a model to study structure-function relationships in the testis.     

Estrogen modifies the circadian timing and amplitude of the luteinizing hormone surge in female hamsters exposed to short photoperiods

LH surges occur 3 h later in intact anovulatory hamsters exposed to nonstimulatory photoperiods (6L:18D) for 8 wk than the proestrous LH surges from the same hamsters housed in 6L:18D for 3 weeks. In ovariectomized hamsters housed in 6L:18D for 3 wk, the LH surge was observed at the same time of day as in intact anovulatory hamsters at 8 wk. Implanting Silastic capsules containing estradiol benzoate (EB) advanced the timing of the daily surge of LH in ovariectomized hamsters housed in 6L:18D for 8 wk. EB also affected the magnitude of the LH surge in hamsters housed in 6L:18D for 8 wk. Two days after receiving EB implants, daily LH surges in anovulatory hamsters were suppressed by 75% and in ovariectomized "regressed" hamsters by 37%. This difference between groups was probably due to ovarian progesterone in intact animals. Estrogen is not required for LH surges in anovulatory hamsters but suppresses LH release when administered exogenously. The delay in the timing of the LH surge in anovulatory hamsters may result from the decline in estrogen resulting from short photoperiod exposure.     

Termination of gonadal refractoriness in Turkish hamsters, Mesocricetus brandti

The Turkish hamster (Mesocricetus brandti) is a photoperiodic species. In this investigation, we characterized the photoperiodic requirements for termination of gonadal refractoriness, defined as the inability of the animal to respond to short-day treatment with gonadal regression. Paired testes weights were reduced to less than 20% of their original weight by 10 wk of 12L:12D treatment. This was followed by spontaneous testicular recrudescence (completed by Week 25 of 12L:12D treatment), the overt indication of refractoriness to short photoperiods. Next, the period of long-day exposure sufficient for termination of refractoriness was determined. Refractory males were exposed to 16L:8D for 5 to 20 wk. Ten weeks of 16L:8D treatment was enough for the animals to regain the sensitivity to a second challenge of 12L:12D treatment. Fifteen weeks of 20L:4D or 16L:8D terminated refractoriness in female Turkish hamsters; 20L:4D therefore was not interpreted as a short day by refractory hamsters. This was unexpected because in photosensitive animals this photoperiod acts like a short day, causing gonadal regression. These results suggest that Turkish hamsters are similar to Syrian hamsters in that both species require two or more months of long days in summer to recover sensitivity to the short days of the following fall.     

Effects of photoperiod and hCG on the regulation of testicular LH/hCG receptors in Syrian hamsters (Mesocricetus auratus)

The regulation of testicular LH/hCG receptors was studied in Syrian (golden) hamsters with testicular atrophy induced by exposure to short photoperiod (5L:19D) and in gonadally active hamsters kept in a long photoperiod (14L:10D). By 24 h after injection of hCG, long-photoperiod hamsters showed a dose-related decrease in the number of testicular LH/hCG receptors. At 48 and 72 h, there was a recovery from this 'down-regulation'. The recovery was much faster than has been reported for the rat and mouse, and it resulted in elevation of testicular LH/hCG receptor concentrations above basal values. Hamsters with short photoperiod-induced testicular atrophy showed an increase in testicular LH/hCG receptors after injection of hCG, except for animals injected with a very high dose. The hCG-induced increase in testicular LH/hCG binding in these animals was associated with reappearance of testosterone responses to subsequent hCG stimulation. Response of testicular LH/hCG receptors to hCG in prepubertal hamsters resembled that measured in animals with short photoperiod-induced gonadal atrophy.     

Functional maturation of the gonads of Turkish hamsters under various photoperiods

The golden hamster, Mesocricetus auratus, is the only photoperiodic rodent to date that has been shown to fail to respond to inhibitory (i.e., short, less than 12.5 h/day) photoperiods until after pubertal onset. In other photoperiodic hamsters, mice, and voles, short photoperiods greatly retard gonadal maturation. The Turkish hamster, Mesocricetus brandti, is a photoperiodic rodent that as an adult is reproductively competent only on photoperiods of 15-17 h of light per day; photoperiods of less than 15 or greater than 17 h of light promote gonadal regression. In this report we addressed two questions: a) are prepubertal M. brandti photoperiodic, and b) if so, is gonadal maturation enhanced or suppressed by exposure to photoperiods of greater than 17 h of light per day? Turkish hamsters were raised on photoperiods of 12, 16, 20, or 24 (= LL) h of light per day. Testicular growth was retarded for 16 wk by 12L:12D. Very long days, 20L:4D, or LL did not retard testicular development. In females, pubertal onset, as indicated by first vaginal estrus, was delayed in young raised on 12L:12D and in 2 of 18 and 4 of 19 young raised on 20L:4D and LL, respectively. These results demonstrate that prepubertal Turkish hamsters are photoperiodic, but respond differently from adults to photoperiods greater than 17 h of light per day.     

Melatonin mediates alternation of seasonality in Syrian hamsters

The annual cycle of reproductive activity in the Syrian hamster, Mesocricetus auratus, is the result of interaction between seasonal changes in daylength (photoperiodism) and seasonal changes in responsiveness to daylength (seasonality). The present experiment was designed to investigate the role of the pineal gland and its hormone, melatonin, in the alternation of seasonality (scotosensitivity and scotorefractoriness). Male hamsters were maintained on short daylengths (10L:14D) to establish scotorefractoriness, and then they were transferred to long daylengths (14L:10D) for conversion to scotosensitivity (sensitive to short daylengths). Before transfer to long daylengths, some of the hamsters were pinealectomized and others were sham-operated or unoperated. Some of the pinealectomized hamsters received single daily melatonin or saline injections while on long daylengths. After 14 wk on long daylengths, the hamsters were transferred to short daylengths for 10 wk to test for conversion to scotosensitivity. Pinealectomized hamsters were given three daily melatonin injections while on short daylengths. Such treatment is known to promote gonadal regression in scotosensitive but not in scotorefractory hamsters. Examination of testes after the short daylength interval revealed that exposure of nonpinealectomized hamsters to long daylengths had reestablished scotosensitivity (regressed testes). Pinealectomized hamsters that received no melatonin replacement while on long daylengths remained scotorefractory (enlarged testes), whereas those that received single daily injections of melatonin during long daylengths were found to be scotosensitive. These results indicate that a daily pulse of melatonin during expsoure to long daylengths has an important role in reestablishing responsiveness (scotosensitivity) to short daylengths.     

Sources of variation in preweaning weight of mice.

The effects of body weight of dam after parturition (D), litter size at birth (N), and birth weight (B) on the weight of mice at 12 da of age (M) were examined in 3 experiments under different conditions: Experiment 1, cohabitating of breeders at 3 wk of age, a sexually immature stage; Experiment 2, cohabiting of breeders at 9 wk of age, a mature stage; and Experiment 3, cohabiting of breeders at 9 wk of age plus cross-fostering at birth. In all experiments, litter size was standardized to 8 at birth. Standard partial regression coefficients of M on D, N, and B were .325 +/- .036, .298 +/- .038, and .483 +/- .034 in Experiment 1; .204 +/- .045, .161 +/- .044, and .370 +/- .043 in Experiment 2; and .375 +/- .098, .074 +/- .097, and --.137 +/- .287 in Experiment 3. The variation in M attributable jointly to all independent variables (D, N, and B) was 49, 22, and 17% in Experiments 1, 2, and 3, respectively. It was concluded that when sexually mature mice at nearly the same age are cohabited for reproduction, the effects of D, N, and B on an indirect measure of milk production (M) are not large. The potential for genetically improving milk production under the condition of Experiment 3 was discussed.     

Gonadal regression despite light pulses coincident with locomotor activity in the Syrian hamster

Feedback lighting (LDFB), which illuminates an animal cage in response to active wheel running, exposes only the photosensitive portion of the phase-response curve to light. In the hamster, the photoinducible zone of the circadian rhythm of photoperiodic photosensitivity occurs during the interval of active wheel running. Since LDFB exposes the photoinducible zone almost as much as constant light (LL), we predicted that LDFB would maintain gonadal function just as LL does. Surprisingly, 10 male hamsters exposed to 1-sec pulses of LDFB for 8 wk had regressed testes similar to those of hamsters in continuous darkness (DD) and significantly smaller than hamsters exposed to LL (P less than 0.01). Two of 5 male hamsters exposed to 2-min pulses of LDFB underwent complete testicular regression and two had partially regressed testes. All females exposed to LDFB or to DD ceased showing cyclic signs of ovulation within 20 days, whereas most hamsters exposed to LL continued to show signs of cyclic ovulation. Six of the 8 hamsters exposed to LL had ova in their oviducts at autopsy, and also had significantly larger uteri (P less than 0.01) than hamsters exposed to DD or LDFB. None of the latter two groups (n = 6 and 9, respectively) had oviductal ova at autopsy. These results demonstrate that considerable exposure of the photoinducible zone to light does not necessarily maintain gonadal function. Light delivered to the photoinducible zone by LDFB may disrupt the normal alignment (internal coincidence) of circadian rhythms, thereby causing gonadal regression. Gonadal induction can occur when the photoinducible zone is exposed to light; however, it may not be the light itself, but rather the action of the light to alter the phase relationships of several oscillators, that causes induction and maintenance of the gonads.     

Periventricular and suprachiasmatic lesion effects on photoperiodic responses of the hamster hypophyseal-gonadal axis

We examined the involvement of neural mechanisms within the suprachiasmatic nucleus (SCN) and periventricular area (PVA), and the role of prolactin (Prl) in control of endocrine function in short day-exposed Syrian hamsters. Hamsters bearing lesions of the SCN or PVA, hamsters implanted with an anterior pituitary under the kidney capsule to provide sustained Prl levels, and sham-operated hamsters were exposed to either 14L:10D or 8L:16D. After 9 wk, hamsters were sacrificed, and their testes and pituitaries were studied in vitro to assess their secretory capacity. SCN lesions and large periventricular lesions impinging on the paraventricular nucleus prevented testicular regression during short-day exposure. Small periventricular lesions and pituitary implants did not prevent gonadal regression in hamsters exposed to short days. Testis weights were positively correlated with basal and luteinizing hormone (LH)-stimulated androgen production in the control and lesioned groups; pituitary implants prevented the decline in androgen production in vitro in gonadally regressed animals. The relative in vitro pituitary response to gonadotropin-releasing hormone (GnRH) stimulation in control and lesioned groups was not reduced by short-day exposure. These data indicate that either axons coursing dorsally from the SCN or extra-SCN structures in the periventricular/paraventricular area are necessary for testicular regression in short photoperiods.     

Effect of various photoperiods on testicular weight, weekly sperm output and plasma levels of LH and testosterone over the reproductive season in male turkeys

The effects of duration and variation in photoperiod on testis weight, testicular sperm production, semen output, and hormone status over the reproductive season in male turkeys were investigated. In Experiment 1, four groups of males raised from 17 to 23 wk of age under a constant short photoperiod were subjected to a constant short (Group 1: 7L:17D; Group 2: 10.5L:13.5D), constant long (Group 3: 14L:10D) or progressively increasing photoperiod (Group 4: 7L:17D to 14L:10D) up to 60 wk of age. In Experiment 2, four groups of males first raised as in Experiment 1 up to 23 wk of age were placed under a constant short (Group 5: 10.5L:13.5D), constant long (Group 6: 14L:10D), or night-interrupted photoperiod (Group 7: 6L:2.5D:1L:14.5D, referred to as subjective 9.5L:14.5D; Group 8: 6L:3.5D:1L:13.5D), referred to as subjective 10.5L:13.5D) up to 60 wk of age. Males in Groups 2-4 had similar reproductive characteristics, whereas sexual maturity was delayed from 29 to 49 wk in males from Group 1. In Experiment 2, males in Groups 5 and 8 had similar reproductive characteristics, whereas sexual maturity was delayed in males in Group 7 in a manner similar to that observed in Group 1. In both experiments, plasma LH and testosterone concentrations were poor indicators of testis development and semen production, irrespective of age and photoperiod. We conclude that a moderately short photoperiod such as 10.5L:13.5D or subjective 10.5L:13.5D may stimulate reproductive characteristics of male turkeys in a manner comparable to constant long or increasing photoperiods. We inferred the existence of a threshold of photosensitivity in male turkeys for photoperiods longer than 9.5L:14.5D, but shorter than or equal to 10.5L:13.5D.     

Decreasing photoperiods and the testicular cycle of a subtropical bird (spotted Munia), Lonchura punctulata

Abstract

The testicular cycle of subtropical Spotted Munia was studied over a period of 11 months in the following schedules: L/D: 0.25/23.75; L/D: 1/23; L/D: 3/21; L/D: 5/19; L/D: 8/16 and near‐complete darkness. All schedules completely abolished annual gonadal regression with the exception of L/D: 8/16 and near‐complete darkness. In the former, gonads regressed but entered the next reproductive cycle 4 months earlier than the normal scheduled time. In the latter also the cycle was completed though it was sharp and short. It was observed that the greater the duration of the dark period the better the gonadal response is. Sensitivity to short photoperiods/long dark periods seems to be characteristic of the low‐latitude birds; what relevance it has to the normal reproductive cycle of munias in nature is not known.     

Effects of photoperiod, hypophysectomy, and follicle-stimulating hormone on testicular follicle-stimulating hormone binding sites in golden hamsters

Experiments were conducted to partially characterize and to examine the regulation of unoccupied testicular follicle-stimulating hormone (FSH) binding sites in adult golden hamsters. Testicular FSH binding sites were measured in the 1800 X gav fraction of whole testicular homogenates using iodinated bovine FSH. Binding of FSH was highly specific for FSH, located primarily in the testes, was time- and temperature-dependent, initially reversible, saturable, and consistent with a model consisting of a single class of high-affinity binding sites (range of equilibrium association constants (Ka) 2-12 X 10(10) M-1). Exposure of hamsters to a short photoperiod consisting of 5L:19D was associated with an increase in concentration (fmol/mg protein), but a reduction in total content (fmol/testes) of testicular FSH binding sites. There was no appreciable 5L:19D-associated alteration in receptor affinity (average Ka = 7.83 X 10(10) M-1). Injections of ovine prolactin (oPRL), ovine luteinizing hormone (oLH), or ovine FSH (oFSH) for 3 days into hamsters housed in 5L:19D for 12 wk had no effect on photoperiod-induced changes in testicular FSH binding sites. On Days 5 and 6 post hypophysectomy, a dramatic increase in FSH binding site concentration occurred, with but marginal effects on binding site affinity. Injections of 5 micrograms oFSH on Days 2, 3, and 4 after hypophysectomy prevented the increase in binding site concentrations measured on Day 5. Injection of a combination of 5 micrograms oFSH, 50 micrograms oPRL, and 25 micrograms oLH also reduced testicular FSH binding site concentrations in hypophysectomized hamsters, but oPRL or oLH by themselves were ineffective. The data indicate a homologous down-regulation of testicular FSH binding sites, but do not exclude the involvement of other hormones.     

Catecholamine effects on testicular testosterone production in the gonadally active and the gonadally regressed adult golden hamster

Several lines of evidence support a role of testicular innervation and peripheral catecholamines in the control of male gonadal function, particularly before puberty. It was therefore of interest to compare the effects of catecholamines on androgen production during the periods of gonadal activity and quiescence in a seasonally breeding species. We have examined direct effects of epinephrine (EPI), norepinephrine (NE), the beta-adrenergic agonist isoproterenol (ISO), and the alpha-adrenergic agonist phenylephrine (PHE) on testicular testosterone (T) production in hamsters with gonadal regression induced by 12 wk exposure to short photoperiod (SD) and in gonadally active hamsters maintained in long photoperiod (LD). Fragments of decapsulated testes were incubated with various combinations of these catecholamines (10(-5)-10(-9) M), human chorionic gonadotropin (hCG; 3.1 mIU/ml), the beta-receptor antagonist propranolol (10(-5) M) and the alpha-l-receptor antagonist prazosin (10(-5) M), for 6 h. In the incubations of testes from LD hamsters, the accumulation of T in the medium was stimulated by hCG but not affected by either catecholamine. However, EPI, NE, and PHE at 10(-5) M, but not ISO, augmented the stimulation of T by hCG. In sharp contrast to these findings, T production by the regressed testes of SD animals was stimulated by EPI (at 10(-8)-10(-5) M), NE (at 10(-6)-10(-5) M), and PHE (at 10(-6)-10(-5) M) in a dose-related manner, but unaffected by ISO. These stimulatory effects were prevented by prazosin, but not by propranolol. Moreover, 10(-5) M of EPI, NE, and PHE augmented the stimulatory effect of hCG on T production. We conclude that the seasonal transition from gonadal activity to quiescence in the adult golden hamster is accompanied by a major increase in the responsiveness of testicular steroidogenesis to catecholamines acting via the alpha-1-adrenoreceptor and that catecholamines can modulate Leydig cell response to gonadotropins in this species. These findings could be related to up-regulation of the alpha-1-receptor in the testis of the SD animal and suggest that catecholamines may be involved in the regulation of the testis during physiological suppression of gonadotropin release and during stress.     

Evidence of an annual rhythm in a small proportion of Siberian hamsters exposed to chronic short days

Siberian hamsters are photoperiodic seasonally breeding rodents. To date, there has been no evidence that Siberian hamsters exhibit an annual rhythm in reproductive, thermoregulatory, molt, or body mass changes. However, given that the termination of their winter cycle is under endogenous control, the authors thought it possible that under particular conditions, other aspects of their seasonal cycle may be endogenously mediated. Consequently, the authors monitored the reproductive condition, body mass, and molt of hamsters chronically exposed to short days (LD 9:15) for up to 2 years. All animals were taken from previous experiments and had exhibited gonadal regression, followed by refractoriness to shorts days and spontaneous gonadal growth, as well as a complete cycle of body mass and molt changes. Although some hamsters died during the study, the authors continued to monitor some animals for up to 106 weeks of short-day exposure. Of the 57 animals monitored, 4 (7.02%) exhibited a second cycle of reproductive regression and recrudescence. Furthermore, the timing of the second regression was similar in the 4 animals, occurring about 1 year after the initial short-day exposure (50.5 +/- 1.71 weeks). However, the timing of the second bout of recrudescence was more variable, occurring at about week 80 (79.5 +/- 8.01 weeks). Hamsters exhibited regressed gonads for about 7 months (29.0 +/- 8.02 weeks). Although the body mass of the 4 hamsters declined at the time of the second cycle of gonadal regression, it never recovered. No hamster exhibited a second molt cycle. These observations indicate that a small percentage of Siberian hamsters chronically exposed to short days can exhibit an annual cycle of reproduction.     

Gonadal hormone-dependent and -independent regulation of immune function by photoperiod in Siberian hamsters

Siberian hamsters (Phodopus sungorus) exhibit changes in reproductive and immune function in response to seasonal variations in day length. Exposure to short days induces gonadal regression and inhibits testosterone secretion. In parallel, short days enhance immune function: increasing leukocyte numbers and attenuating cytokine and behavioral responses to infection. We examined whether photoperiodic changes in leukocyte phenotypes and sickness behaviors are dependent on concurrent photoperiodic changes in gonadal function. Male hamsters were gonadectomized or sham-gonadectomized and either exposed to short days (9 h light/day; SD) or kept in their natal long-day (15 h light/day; LD) photoperiod for 10-13 wk. Blood samples were obtained for leukocyte enumeration, and hamsters were challenged with bacterial LPS, which induced behavioral (anorexia, reductions in nest building) and somatic (weight loss) sickness responses. Among gonad-intact hamsters, exposure to SD increased total and CD62L+ lymphocytes and CD3+ T lymphocytes in blood and significantly attenuated LPS-induced sickness responses. Independent of photoperiod, castration alone increased total and CD62L+ lymphocyte and CD3+ T lymphocyte numbers and attenuated somatic and anorexic sickness responses. Among castrated hamsters, SD exposure increased lymphocyte numbers and suppressed sickness behaviors. In castrated hamsters, the magnitude of most immunological effects of SD were diminished relative to those evident in gonad-intact hamsters. The SD phenotype in several measures of immunity can be instated via elimination of gonadal hormones alone; however, photoperiodic effects on immune function persist even in castrated hamsters. Thus, photoperiod affects the immune system and neural-immune interactions underlying sickness behaviors via gonadal hormone-dependent and -independent mechanisms.     

Gonado-modulatory effects of high humidity is pineal dependent in Indian palm squirrel Funambulus pennanti (Wroughton).

F. pennanti presented a clear biphasic pineal mediated seasonal sexual cycle. This sexual cycle was essentially characterised by a very short period of sexual quiescence with an arrest of spermatogenesis during October-November. A small but clear decrease in sexual activity was also observed during March-April. This decrease in sexual activity, however, had no quantifiable effect on spermatogenesis. Sexual recrudescence was observed from December-January. The testes remained sexually active from January till September. Almost an inverse relationship was observed between pineal and testicular weight. Pinealectomy, however, prevented naturally induced gonadal regression during both the periods, i.e. September-November and February-April. Exposure of animals to high RH (80 +/- 4%) during sexually active phase induced a steep regression in testicular weight of sham-operated animals even in the presence of gonad stimulatory long photoperiod (16L:8D) and high temperature (40 degrees +/- 5 degrees C) while exposure of animals to moderate RH (65 +/- 5%) during sexual regression phase partially prevented testicular regression even in presence of inhibitory short photoperiod (11L:13D) and normal environmental temperature (30 degrees +/- 5 degrees C). Pinealectomized animals, neither exhibited testicular regression in February-March nor had involuted testes in September-October, thus, suggesting that the effect of humidity is mediated via the pineal gland.     

Neuroendocrine changes in male hamsters following photostimulation

Transfer of gonadally regressed male golden hamsters from a short (5 L:19 D) to a stimulatory (14 L:10 D) photoperiod elicits, within 24 hr, significant changes in hypothalamic dopamine, serotonin, and possibly norepinephrine metabolism. Hypothalamic LHRH content was significantly elevated in short-photoperiod animals, but within 24 hr of transfer to a 14:10 photoperiod, LHRH declined to levels not different from those in hamsters maintained continuously in a long photoperiod. Plasma FSH levels were also significantly elevated within 24 hr of transfer, but increases in plasma LH were somewhat slower. Chronic treatment with the tyrosine hydroxylase inhibitor, alpha-methyl tyrosine (alpha MPT), which inhibits catecholamine synthesis, blocked the effect of a stimulatory photoperiod on plasma FSH levels, while treatment of 5:19 hamsters with the catecholamine precursor, L-dopa, mimicked the effects of photostimulation on plasma FSH levels. Testicular weights were not affected by alpha MPT or L-dopa treatment for 1 week. From these data, it appears that endocrine events associated with photoperiod-induced testicular recrudescence are under the control of hypothalamic neurotransmitters.