Neuroendocrine regulation of seasonal reproductive activity in the male golden hamster

Entrainment of Syrian hamsters to short photoperiod T-cycles does not reverse the inhibitory nature of feedback lighting on the photoperiodic response

Experiments examining the photoperiodic effects of various light-dark cycles unexpectedly demonstrated that light during the subjective night is not necessarily photoinductive to the reproductive system of long day breeders. Feedback lighting (illumination in response to locomotor activity) and a high-frequency light-dark cycle with 1 minute (m) of light followed with 1 minute of dark (LD1m:1m) failed to maintain reproductive function in Syrian hamsters despite exposing the animal to 5-6 hours of light per cycle during the subjective night. In an effort to determine why feedback lighting and LD1m:1m do not maintain reproductive function, male Syrian hamsters were exposed to one of the following lighting conditions for ten weeks: a light-dark cycle with 14 hours of light followed by a 10 hour light-dark cycle of 1 minute of light followed by 1 minute of dark ¿LD14:10(1m:1m)¿; a cycle of a 14 hour light-dark cycle of 1 minute of light followed by 1 minute of dark, followed by 10 hours of dark ¿LD14(1m:1m):10)¿; a light-dark cycle of 2 hours of light and 21 hours of dark (LD2:21); a light-dark cycle of 2 hours of light and 22 hours of dark (LD2:22); LD2:21 in conjunction with feedback lighting (LD2:21/FB); LD2:22 in conjunction with feedback lighting (LD2:22/FB); constant dark (DD) or constant light (LL). All hamsters exposed to LD14(1m:1m):10 entrained. Only 7 of 12 hamsters exposed to LD14:10(1m:1m) entrained. This supports previous findings, which suggest that the circadian system has difficulty differentiating between LD1m:1m and LL. Exposure to LD2:22 and DD induced testicular regression, while LL, LD2:21 and LD14(1m:1m):10 maintained reproductive function, as expected. The addition of approximately 5 hours of nocturnal illumination by feedback lighting, however, not only failed to prevent reproductive regression in LD2:22, but attenuated the stimulatory nature of LD2:21. This suggests that feedback lighting is not only not stimulatory, but may be actively inhibitory. Perhaps this is due to a photoinhibitory effect of multitransitional nocturnal illumination. This hypothesis is supported by the finding that LD14:10(1m:1m) allows regression, while LD14(1m:1m):10 is stimulatory. These findings also imply that the state of the circadian system is instrumental in the reproductive system's interpretation of a light signal.

Effects of photoperiod and melatonin on the development of growth hormone cells and the pituitary-adrenal axis in the Djungarian hamster, Phodopus sungorus

The development of GH cells and the pituitary-adrenal axis was morphologically examined in male Djungarian hamsters (Phodopus sungorus) exposed to short days and those kept under long days and receiving daily afternoon injections of melatonin, from the time of weaning (20 days) until 100 days of age. The postnatal increase in area of ACTH cells under long days was inhibited in short-day-exposed or melatonin-treated animals. It was suggested that a short photoperiod may suppress, via melatonin, the development of ACTH cells. GH cells were not affected by age, photoperiod or exogenous melatonin. Under long days, the zona fasciculata decreased in volume with age, while the zona reticularis increased. Such changes in the volumes of these adrenocortical zones were depressed under short days. In addition, the volumes of the zona fasciculata and zona reticularis in long-day-housed animals became respectively larger and smaller subsequent to orchidectomy and melatonin administration. These results suggest that fasciculata cells in deeper levels become progressively differentiated into reticularis cells, that short photoperiod inhibits development of both zonae, and that such an inhibition is caused mainly by the decreased secretion of androgens.

Photoperiodic modulation of GnRH mRNA in the male Syrian hamster

Male Syrian hamsters (Mesocricetus auratus) are seasonal breeders. They show marked testicular regression when exposed to short autumnal photoperiods, and then remain sexually quiescent for several months. By mid-winter, however, they show a loss in responsiveness to the inhibitory influence of short photoperiods and their testes begin to recrudesce. To shed light on the neuroendocrine mechanism responsible for mediating these reproductive changes, we examined the influence of photoperiod on the expression of GnRH mRNA in the hamster forebrain. Adult males were either exposed to short photoperiods (6L:18D) for 16 weeks or were maintained under long photoperiods (14L:10D); additional animals were exposed to short or long photoperiods for 22 weeks. As expected, exposure to short photoperiods for 12 weeks resulted in a marked decrease (P<0.01) in testicular mass and serum testosterone levels, but after 22 weeks these reproductive parameters were once again significantly elevated (P<0.01). In contrast, quantitative in situ hybridization histochemistry revealed no difference (P>0.05) between the GnRH mRNA levels of the short-photoperiod hamsters and their aged-matched long-photoperiod controls, although an age-related decrease (P<0.05) was evident in both photoperiod-treatment groups. These data emphasize that GnRH mRNA is highly expressed in hamsters even when their reproductive axis has been rendered sexually quiescent by exposure to short photoperiods, and that photoperiod-induced changes in GnRH secretion, rather than synthesis, are more likely to regulate the timing of the breeding season. On the other hand, the data indicate that GnRH mRNA levels show an aging-related decrease, regardless of photoperiod, suggesting that in the long term a decrease in GnRH gene expression may contribute to the reduced fertility of old hamsters.

Plasma catecholamines and plasma corticosterone following restraint stress in juvenile alligators

Ten juvenile alligators, mean body mass 793 g, hatched from artificially incubated eggs and raised under controlled conditions, were held out of water with their jaws held closed for 48 hr. An initial blood sample was taken and further samples collected at 1, 2, 4, 8, 24, and 48 hr. Epinephrine, norepinephrine, and dopamine were measured in plasma aliquots of 1.5 ml using high pressure liquid chromatography with electrochemical detection. Corticosterone was measured by radioimmunoassay. Plasma glucose was measured using the Trinder method and plasma calcium, cholesterol, and triglycerides were measured in an autoanalyzer. Epinephrine was about 4 ng/ml at the initial bleed, but declined steadily to < 0.4 ng/ml by 24 hr. Norepinephrine was also about 4 ng/ml at the initial bleed, but rose to over 8 ng/ml at 1 hr, and then declined to < 0.2 ng/ml at 24 hr. A second, but smaller increase in plasma norepinephrine was seen at 48 hr. Plasma dopamine was low at the initial bleed (< 0.7 ng/ml), rose to over 8 ng/ml at 1 hr, then declined to < 0.2 ng/ml. Plasma corticosterone rose progressively for the first 4 hr, declined at 8 hr and 24 hr, then rose again at 48 hr. Plasma glucose rose significantly by 24 hr and remained elevated for 48 hr. Plasma calcium increased at 1, 2, and 4 hr then returned to levels not significantly different from the initial sample at 24 and 48 hr. The white blood cells showed changes indicating immune system suppression. By the end of the treatment the hetorophil/lymphocyte ratio increased to 4.7. These results suggest that handling alligators, taking multiple blood samples, and keeping them restrained for more than 8 hr is a severe stress to the animals.

An essential role for ovarian inhibin in pineal gland-mediated anestrus in Syrian hamsters

When transferred from long photoperiod (LP) to short photoperiod (SP), female Syrian hamsters exhibit depressions of follicle stimulating hormone (FSH) and follicular development, cessation of ovulation, and marked ovarian interstitial tissue hyperplasia. Pinealectomy prevents these effects of SP. The object of this study was to determine the role of inhibin in the regulation of FSH during SP-induced anestrus. Adult LSH/SsLak hamsters maintained in LP (LD 14:10 hr) were transferred to SP (LD 8:16 hr) on the day of estrus, and groups of animals killed at either 16.00 hr on proestrus or 08.00 hr on estrus during each of five consecutive 4-day estrus cycles after transfer. Groups of females that became anestrus in SP were killed either at 08.00 or 16.00 hr, 12 days after the last observed estrus discharge. Compared to LP controls, serum FSH levels on estrus increased significantly (P<0.01) during the first two cycles in SP before declining to concentrations that were significantly lower than control values (P<0.01). Serum inhibin levels increased significantly by the third, fourth, and five days of estrus in SP. Regression analysis revealed a significant inverse correlation between serum inhibin and FSH levels on estrus during SP exposure (P=0.021) but not on proestrus. Relative levels of inhibin alpha- and betaA-subunit mRNAs were lower in ovaries from SP proestrus and anestrus females killed at 16.00 hr as compared to those from proestrus LP controls; they were elevated in ovaries from SP estrus and anestrus females killed at 08.00 hr compared to those from estrus LP controls. The absence of antral follicles on estrus in the last cycles of SP and anestrus suggests that the increase in circulating inhibin and inhibin mRNAs may be derived from hyperplastic interstitium. These observations suggest that inhibin may play an essential role in suppressing FSH secretion during pineal gland-mediated anestrus in Syrian hamsters.

Effect of photoperiod on neural estrogen and progestin receptor immunoreactivity in female Syrian hamsters

This study explored the possibility that reduced behavioral responsiveness to estradiol and progesterone in female Syrian hamsters exposed to a short photoperiod is associated with a reduction in the concentration of neural steroid receptors. The effects of long and short photoperiod (LP; SP) exposure on steroid receptor immunoreactivity were examined in the ventromedial hypothalamus (VMH), medial tuberal region (mTu), medial preoptic area (mPOA), medial nucleus of the amygdala (mAMYG), and the arcuate nucleus (ARC) of ovariectomized hamsters. In Experiment 1, exposure to SP for ten weeks attenuated the lordosis response following sequential treatment with estradiol and progesterone. In a separate group of animals not given hormones, SP decreased the staining intensity of estrogen receptor immunoreactive (ERIR) cells in the mPOA while increasing the number of detectable ERIR cells in part of the mAMYG. In Experiment 2, SP diminished the lordosis response as it did in Experiment 1. One week later, the same females were administered estradiol systemically to induce progestin receptors (PR). Animals housed in SP showed significantly reduced progestin receptor immunoreactivity (PRIR) in the VMH, mTu, mPOA, mAMYG, and ARC. Experiment 3 examined whether the results of Experiment 2 might have been influenced by photoperiodic effects on peripheral metabolism of estradiol. Among hamsters housed in LP or SP, PRs were induced by estradiol implanted unilaterally in the medial basal hypothalamus, thus bypassing possible photoperiodic effects on peripheral estradiol availability. This treatment resulted in significantly fewer cells with detectable PRIR in the VMH and mPOA of SP females, suggesting that the photoperiodic influences on PR induction observed in Experiment 2 do not depend on alterations in the peripheral availability of estradiol.

Distribution of lectin binding in spermatogonia of Syrian hamsters in gonadally active and inactive states

Lectin binding patterns in spermatogonia of Syrian hamsters in gonadally active and inactive states were examined by light and electron microscopy. After exposure to a short day cycle (SD), the testis weight and the diameter of seminiferous tubules decreased, reaching the minimum at 13 weeks. At that time, spermatogenesis was severely disrupted. In the animals kept exposed to an SD, spermatogenesis reinitiated spontaneously after 23 weeks. In the animals transferred to a long day cycle (LD) after exposure to an SD for 13 weeks, spermatogenesis reinitiated only 4 weeks later. As to the lectin binding, Dolichos biflorus agglutinin (DBA) bound specifically to spermatogonia. The number of DBA-positive spermatogonia per one seminiferous tubule increased until 13 weeks after exposure to an SD, and then gradually decreased. DBA bound to only type A spermatogonia in active testes, whereas it bound to type A, intermediate and type B spermatogonia in inactive testes. Moreover, SDS-PAGE and Western blot analysis of testes in active and inactive states indicated that DBA-binding bands (115 kDa, 76 kDa) in inactive testes were intense compared with those in active testes. The 82 kDa band was detected only in inactive testes. These results supported the finding obtained from lectin histochemistry. DBA-binding glycoprotein was detected in all types of spermatogonia in inactive testes, suggesting that this glycoprotein way concern the active/inactive state of spermatogenesis. The present study also indicated that DBA is a useful marker for spermatogonia in inactive testes of Syrian hamster.

Rapid photoperiod-induced increase in detectable GnRH mRNA-containing cells in Siberian hamster

To determine whether changes in gonadotropin-releasing hormone (GnRH) neurons are early indicators of photostimulation, Siberian hamsters were placed in short days (6:18-h light-dark) at 3 (experiment 1) or 6 (experiment 2) wk of age where they were held for 3 (experiment 1) or 4 (experiment 2) wk. Hamsters were then moved to long photoperiod (16:8-h light-dark). In experiment 1, brains were collected 1-21 days after transfer from short to long days. In experiment 2, brains were collected only on the second morning of long day exposure. Long and short day controls were included in both experiments. Cells containing GnRH mRNA, as visualized by in situ hybridization, were counted. As expected, there were no differences in the number of detectable GnRH mRNA-containing cells among animals chronically exposed to long or short photoperiods. However, on the second morning after transfer from short to long photoperiod, a positive shift in the distribution of GnRH mRNA-containing cells occurred relative to the respective controls in the two experiments. Increases in follicle-stimulating hormone secretion and gonadal growth occurred days later. In conclusion, a rapid but transient increase in the distribution of detectable GnRH mRNA-containing cells is an early step in the photostimulation of the hypothalamic-pituitary-gonadal axis.

Melatonin increases the in situ activity of tyrosine hydroxylase in the mediobasal hypothalamus of male Syrian hamsters

The effects of daily late afternoon injections of melatonin on the in situ activity of tyrosine hydroxylase (TH) were examined in the median eminence/arcuate region of the mediobasal hypothalamus (MBH) and the neurointermediate lobe (NIL) of the male Syrian hamster. TH activity was determined in tissue extracts by measuring the accumulation of L-DOPA following administration of the dopa decarboxylase inhibitor, NSD-1015. After 9 weeks of melatonin treatment, highly significant increases in the activity of MBH TH were demonstrated over a 24 hr period, compared to saline-treated controls. Melatonin-induced elevations in TH occurred concomitantly with decreases in tuberoinfundibular dopamine (TIDA) and tuberohypophyseal dopamine (THDA) concentrations. Similar findings were observed in castrated hamsters, indicating that the melatonin-induced increase in TH was not secondary to melatonin-induced changes in circulating levels of gonadal hormones. These data led to the interpretation that melatonin treatment elevated TIDA synthesis either through a direct action on the arcuate nuclei or on neurons impinging on these nuclei.

A mutation of the circadian timing system (tau gene) in the seasonally breeding Syrian hamster alters the reproductive response to photoperiod change

The tau mutation is a semi-dominant autosomal mutation which, in homozygotes, accelerates the period of the circadian activity cycle by approximately 4 h. In mammals, the circadian system contributes to seasonal photoperiodic time measurement by generating a repeated daily melatonin signal during the hours of darkness. Our earlier studies suggest an altered response to the melatonin signal in tau mutants. This study investigated whether tau and wild-type hamsters exhibit a differential response to photoperiod change. Reproductively active animals were maintained on stimulatory photoperiods of 16 h light (16L) per 24 h (wild-type) or 12L per 20 h (tau) before being exposed to an increase in night-length to 9, 10, 11, 12 or 14 h for 84 cycles. Wild-types exhibited testicular atrophy at 13L:11Dark (13L:11D), with full regression at photoperiods of 12L:12D. Taus exhibited complete regression at photoschedules comprising 10 h darkness or more per 20-h cycle. Plasma prolactin concentrations were decreased following exposure to at least 9 and 10 h darkness in taus and wild-types, respectively. Thus, the tau genotype may exhibit a different critical night-length with respect to both the gonadal and prolactin axes, of approximately 1-2 h shorter than wild-type genotypes. These data support the hypothesis that the circadian tau mutation has altered the basis of photoperiodic time measurement, perhaps by altering the generation and/or interpretation of the melatonin signal.

Effects of delta 9-tetrahydrocannabinol on copulatory behavior and neuroendocrine responses of male rats to female conspecifics

Male rats exposed to sexually receptive females, exhibit a rapid increase in plasma levels of luteinizing hormone (LH) and prolactin, and concomitant increases in noradrenergic activity in the medial basal hypothalamus (MBH) and median eminence (ME) as well as in dopaminergic activity in the MBH. Delta-9-tetrahydrocannabinol (THC; 5 mg/kg b.wt., PO), the chief psychoactive constituent of marijuana, blocked the MBH and ME noradrenergic response and the dopaminergic response in the MBH in male rats exposed for 20 min to sexually receptive females, and suppressed the expected increases in plasma LH and prolactin levels. Moreover, THC treatment decreased the percentage of animals exhibiting copulatory behavior and increased the latency periods to mount and intromit. These findings indicate that THC interferes with the neuroendocrine and behavioral responses of male rats to the presence of a receptive female.

Photoperiodic regulation of substance P immunoreactivity in the mating behavior pathway of the male golden hamster

Mating behavior in the male golden hamster is regulated by both gonadal steroids and photoperiod. Gonadal steroids may regulate mating behavior by actions on the medial nucleus of the amygdala, bed nucleus of the stria terminalis, and medial preoptic area. Neurons in these areas actively accumulate gonadal steroids and lesions of these nuclei disrupt mating behavior in male hamsters. Photoperiodic regulation of mating behavior is regulated, at least in part, by decreased responsiveness to gonadal steroids. Therefore, we sought to determine if the changes induced by changes in gonadal steroids would mimic those induced by changes in photoperiod. The number of substance P-containing neurons in these areas decrease following castration and are restored with testosterone treatment suggesting that this peptide may mediate steroidal regulation of male mating behavior. To determine the effect of photoperiod on substance P, peptide containing neurons were counted in (1) enucleates (n = 6), (2) enucleated castrates treated with testosterone (n = 6), (3) castrates treated with testosterone (n = 4), and (4) intact controls (n = 6). Bilateral enucleation caused a decrease in the number of substance P neurons in the medial nucleus, bed nucleus of the stria terminalis, and medial preoptic area (P less than 0.05). Testosterone treatment prevented this decrease (P less than 0.05). Thus, a decrease in daylength causes a decrease in substance P in the medial nucleus of the amygdala, the medial bed nucleus of the stria terminalis and the medial preoptic area that is mediated by changes in testosterone levels.

Demonstration of a direct projection from the retina to the hypothalamic supraoptic nucleus of the hamster

Hamsters were injected in the left eye with unconjugated cholera toxin subunit B (CHB) and the tracer was localized using immunohistochemistry. A large number of immunoreactive fibers was found in the suprachiasmatic nuclei and the contralateral lateral hypothalamic area. Labeled fibers coursed around the supraoptic nucleus and nerve terminals accumulated in a zone dorsally and laterally to the nucleus. Single fibers from this plexus penetrated into the supraoptic nucleus, where few fibers arborized into delicate immunoreactive profiles possessing varicosities. Labeled fibers were identified only in the dorsal and lateral parts of the nucleus, and mostly at the caudal level of the optic chiasm. These results show a direct retinal innervation of the magnocellular neurons of the supraoptic nucleus, and indicate a direct photic influence on the hypothalamo-neurohypophysial system.

Chronic effects of marihuana smoking on luteinizing hormone, follicle-stimulating hormone and prolactin levels in human males

The purpose of this investigation was to assess the chronic effects of marihuana smoking on the basal and stimulated secretion of the pituitary hormones luteinizing hormone (LH), follicle-stimulating (FSH) and prolactin (PRL). Ten male chronic marihuana users and 10 age- and sex-matched healthy volunteers were studied by measuring hormone levels before and after i.v. administration of thyrotropin releasing hormone (TRH) and gonadotropin releasing hormone (GnRH). The basal and stimulated levels of LH were reduced in marihuana users, whereas FSH and PRL levels and responses were not different from the control subjects. The chronic use of marihuana may selectively impair the hypothalamic control mechanisms regulating LH secretion.

Reinitiation of spermatogenesis by exogenous gonadotropins in a seasonal breeder, the woodchuck (Marmota monax), during gonadal inactivity

The present study was undertaken (1) to document structural and functional changes in the testes of seasonally breeding woodchuck during active and inactive states of spermatogenesis and (2) to evaluate the ability of exogenous gonadotropins to reinitiate spermatogenesis outside the breeding season. During seasonal gonadal inactivity, there were significant (P less than 0.05) reductions in volumes of several testicular features (testis, seminiferous tubules, tubular lumen, interstitial tissue, individual Leydig cells, Leydig cell nuclei, and Leydig cell cytoplasm) as compared with gonadally active animals. The diameter of the seminiferous tubules was decreased by 26%, and Leydig cell numbers also declined in the regressed testes. These changes were accompanied by a decline in testosterone (T) levels in both plasma and testis, and reduction in epithelial height of accessory reproductive organs. A hormonal regimen was developed that would reinitiate spermatogenesis in captive, sexually quiescent woodchucks. A combination of PMSG and hCG markedly stimulated testicular growth and function and restored spermatogenesis qualitatively. Quantitatively normal spermatogenesis was restored in 2 of 6 treated males. Morphometric analyses revealed substantial increases in seminiferous tubular diameter and in the volume of seminiferous tubules, tubular lumen, total Leydig cells, and individual Leydig cells in the hormone-treated animals. These increased values corresponded to 99, 75, 68, 51, and 200%, respectively, of the values measured in naturally active woodchucks. Leydig cell numbers, however, remained unchanged and approximated only 31% of the number found in naturally active testes. Hormonal stimulation also resulted in a significant rise in serum T as well as in the total content of testicular T, and a marked increase in epithelial height in various accessory reproductive glands. The most effective hormonal protocol for stimulating spermatogenesis was treatment with 12.5 IU of PMSG twice a week for 4 weeks followed by 12.5 IU of PMSG + 25 IU of hCG twice a week for 4 weeks.

The effects of short day exposure on seasonal and circadian reproductive rhythms of female golden hamsters

Four groups of female golden hamsters were exposed to short photoperiods (SP, LD 10:14) for 4, 14, 20, or 27 weeks and tested for physiological markers (uterine weight and estrous cycles) and behavioral (lordosis, approach and aggressive behaviors) measures while in contact with a stud male. After behavioral testing, females were ovariectomized and, during the next 2 weeks, were tested twice more (with a stud male) after replacement with 0.33 microgram (low dose) and 1.0 microgram (high dose) EB plus progesterone (500 micrograms). Results show that, after 14 weeks of SP conditions, uterine weights and percentage of females showing normal estrous cycles are at a minimum. This is mirrored by minimal levels of lordosis and maximal levels of aggressive and approach behavior at week 14. Physiological measures did not fully recover (to preregression levels) until week 27; however, behavioral measures show an earlier recovery by week 20. SP exposure also affects the circadian patterning of behaviors: Females that show lordosis at week 14 did so later in the day than did females tested at other weeks. Females in the regressed state also fail to show a significant decrease in approach behaviors (and a significant increase in receptive behaviors) over the course of the circadian day, a pattern seen in nonregressed females. Following hormone replacement with the low EB (+P) dose, females do not become receptive; however, at the higher dose, all but the week 14 group show increased receptivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of daylength on male hamster sexual behavior: masking effects of testosterone

Exposure of male hamsters to short photoperiods for 6-8 weeks cause deficits in sexual behavior with receptive females. The present experiment tested the hypothesis that short photoperiodic effects on behavior could be masked in the presence of chronic and stable levels of testosterone. Males were castrated and administered Silastic capsules of testosterone while housed in long (16L:8D) or short (8L:16D) photoperiodic conditions for 7 weeks. Sexual behavior tests at this time indicated that the short photoperiod males copulated less well, but group differences were not robust. Testosterone capsules were then removed and half the animals in both 16L:8D and 8L:16D were transferred to the opposite photoperiod. Sexual behavior was tested 18 days later as the effects of this functional castration developed. These tests indicate that photoperiodic effects were much more obvious in the absence of testosterone than they were during week 7 tests when testosterone was still present. The behavior of the males that were transferred from one photoperiod to the other demonstrated that exposure to the short photoperiod for only 18 days was not sufficient to generate short photoperiod-like sexual behavior deficits. In contrast, exposure to the long photoperiod for 18 days was sufficient to reverse short photoperiodic effects that had already developed.

The effects of short day exposure on seasonal and circadian reproductive rhythms in male golden hamsters

Male golden hamsters were exposed to long (LD 14:10) or short (LD 10:14) photoperiods (Groups LP and SP, respectively) and tested 4 times over a 30-week period. At each test time, animals were tested twice, once in their subjective night and once in their subjective day. During each test animals were observed for approach and copulatory behaviors in response to hormonally primed receptive females. To determine gonadal condition, changes in testes size and sperm production over weeks in SP or LP condition were also measured. Results show that SP conditions induce gonadal regression (at 9 weeks) and recrudescence (at 17 weeks); gonadal function was fully restored by 21 weeks in SP. This pattern is mirrored by a decline in copulatory behaviors during regression, followed by a resumption of sexual behaviors during gonadal recrudescence. In contrast, approach measures showed an inverse pattern; males showed the highest level of approach behaviors at week 13, during gonadal quiescence. Short day conditions also induce changes in the circadian patterning of copulatory behaviors: whereas LP animals always showed more mounts, intromissions and ejaculations in the dark than in the light, between 1 to 13 weeks in short day conditions. SP animals show equal levels of copulatory behavior in the dark and in the light. At the time of gonadal recrudescence, SP animals start to show circadian patterns of sexual behaviors comparable to LP animals. These differences were not found for approach behaviors. The relationship between the different behavioral systems and physiological measures, and the effects on these of short day exposure are discussed.

The olfactory bulbs enhance reproductive hormone secretion in male rats on long or short photoperiod

Abstract Unlike seasonally breeding species such as the Syrian hamster, Sprague-Dawley laboratory rats do not normally respond to short photoperiod (6L18D) with reproductive regression. However, removal of the olfactory bulbs (BX) unmasks a photoperiodic response in pre-pubertal rats so that blinding or short photoperiod results in an inhibition of reproductive hormone secretion and/or a delay in pubertal development. This is apparently mediated by pineal melatonin which inhibits gonadotropin and/or prolactin secretion, but the mechanism by which BX facilitates the response to photoperiod is not clear. Experiment I was performed to determine serum levels of reproductive hormones at frequent intervals following BX and/or maintenance on short days. Twenty-three-day old male rats were BX or underwent sham BX (SH). They were thereafter maintained on a 14L:10D (long photoperiod) or 6L:18D photoperiod for the duration of the study. At 6 weeks following surgery, BX rats on either photoperiod had smaller testes than the SH groups. At week 8, the BX group on 6L:18D had smaller testes than the other three groups. There were transient reductions in serum luteinizing hormone and follicle-stimulating hormone in the BX rats on short photoperiod, but there were prolonged effects of BX decreasing prolactin levels in rats on long or short photoperiod. In SH rats, testosterone was elevated for weeks 6 through 10 of the study, and BX blocked this increase. Experiment II was performed to determine whether BX alters testosterone feedback on gonadotropin secretion. Twenty-three-day old male rats were BX or underwent SH operation and were then returned to a room on 14L10D. Six to 8 weeks later, all animals were castrated and placed on 6L18D or returned to 14L:10D. Eight weeks following castration, the rats were implanted with Silastic capsules containing 0, 10, 20 or 40mm testosterone. The post-castration increase in serum luteinizing hormone and follicle-stimulating hormone was lower in the BX than SH rats. In long photoperiod, serum luteinizing hormone and follicle-stimulating hormone were often lower in the BX rats receiving no testosterone replacement or lower doses of testosterone than in the SH group receiving similar capsules. Maintenance on short photoperiod increased the responsiveness to testosterone so that even the rats receiving low doses of testosterone had very low luteinizing hormone and follicle-stimulating hormone levels whether they were SH or BX. In summary, BX rats on long or short photoperiod had lower serum prolactin and testosterone levels than the comparable SH group and BX inhibited the post-castration increase in gonadotropin secretion. The data therefore suggest that the olfactory bulbs tonically enhance reproductive hormone secretion (especially around the time of pubertal development).

Effects of psychoactive and nonpsychoactive cannabinoids on the hypothalamic-pituitary axis of the adult male rat

The acute dose-response effects of delta-9-tetrahydrocannabinol (THC), cannabinol (CBN) and cannabidiol (CBD) on gonadotropin and testosterone (T) secretion and on hypothalamic norepinephrine (NE) metabolism were tested in adult male rats. THC and CBN both produced an acute suppression of plasma-luteinizing hormone (LH) and T levels and median eminence NE turnover although a dose-response relationship could not be demonstrated. CBD had no significant effect on any of these parameters and none of these cannabinoids had any effect on plasma follicle-stimulating hormone levels or median eminence LH-releasing hormone (LHRH) content. Except for the highest dose of CBN, none of the in vivo cannabinoid treatments significantly altered in vitro LH secretion although there was a trend towards decreased LH secretion. These results suggest that the decrease in LH secretion in THC- or CBN-treated rats is due to reductions in NE stimulation of LHRH release rather than to changes in LHRH synthesis or pituitary LHRH response.

Pituitary hormone gene expression in male golden hamsters: interactions between photoperiod and testosterone

Abstract Daylength regulates neuroendocrine function in male golden hamsters. Exposure to short days triggers gonadal regression and decreases serum luteinizing hormone (LH), prolactin and testosterone concentrations. Inhibitory photoperiods also amplify the negative feedback actions of androgens upon gonadotropin secretion. To examine whether these changes arise from altered adenohypophyseal gene expression, we measured the abundance of the messenger ribonucleic acids (mRNAs) encoding beta-LH, prolactin and proopiomelanocortin in anterior pituitaries of male golden hamsters which were either left intact, castrated, castrated and implanted with testosterone, or pinealectomized and maintained in either long (14 h light/10 h dark) or short (5 h light/19 h dark) days. Short days caused testicular atrophy in intact male hamsters and reduced serum LH in intact and castrated, testosterone-replaced hamsters. The relative abundance of beta-LH mRNA was suppressed by exposure to short days only in castrated hamsters. Serum prolactin was decreased by short days regardless of circulating testosterone concentrations. Prolactin mRNA abundance was decreased by short days in all pineal-intact groups. Castration reduced proopiomelanocortin mRNA abundance in long days and testosterone replacement reversed this effect. In the presence of testosterone, photoperiod influenced serum LH concentrations without altering hypophyseal abundance of beta-LH mRNA. In contrast, photoperiodic influences on prolactin secretion were correlated with alterations in steady-state mRNA abundance.

Alterations in hypothalamic serotonin metabolism in male hamsters with photoperiod-induced testicular regression

Transfer of adult male golden hamsters to a short photoperiod (SD; 8L:16D) resulted in a significant decrease in testicular weight and plasma luteinizing hormone (LH) and prolactin (Prl) levels as compared to animals maintained in a long photoperiod (LD; 16L:8D). These changes were accompanied by significant increases in serotonin (5-HT) synthesis in the medial basal hypothalamus (MBH) as determined by measuring 5-hydroxytryptophan accumulation after inhibition of aromatic amino acid decarboxylase with NSD-1015. Serotonin synthesis in the median eminence, anterior hypothalamus and olfactory bulbs was not different in SD vs LD animals. Transfer of SD gonadally regressed hamsters to LD restored MBH 5-HT synthesis to control levels.

Nocturnal illumination does not necessarily stimulate the photoperiodic response, despite mimicking the effects of constant light on the circadian system in the male Syrian hamster

In an effort to determine the inductive component(s) of photic input in long day seasonal breeders, adult male Syrian hamsters (Mesocricetus auratus) were exposed to one of nine lighting conditions for a duration of 10 weeks: a light-dark cycle of 14 hours of light followed by 10 hours of dark (LD 14:10, a long photoperiod); LD 10:14 (a short photoperiod); a high frequency light-dark cycle of 1 hour of light and 1 hour of dark (LD 1:1); a higher frequency light-dark cycle of 1 minute of light and 1 minute of dark (LD 1m:1m); constant light (LL); constant dark (DD); feedback lighting (LDFB; a condition that illuminates the cage in response to locomotor activity); a feedback lighting neighbor control (LDFB NC; the animal receives the same light pattern as a paired animal in LDFB, but has no control over it); or reverse feedback lighting (rLDFB; a condition that darkens an illuminated cage in response to locomotor activity). Exposure to LL, LD 1:1, LD 1m:1m, LDFB and rLDFB significantly and similarly lengthened the free-running period of the locomotor rhythm when compared to the period of animals in DD. The paired tests and accessory reproductive glands weights, spermiogenesis, seminiferous tubule diameter and serum concentrations of testosterone, prolactin, LH and FSH, suggest that LD 14:10, LL, LD 1:1, rLDFB and LDFB NC maintain reproductive function in the Syrian hamster, while LD 10:14, DD, LD 1m:1m and LDFB do not. It is known that as little as two 1-second pulses of light are stimulatory if coincident with the subjective night (17.22). Thus, it is not surprising that LD 1:1 is stimulatory. LD 1m:1m is not stimulatory, however, despite an identical quanta of light per 24 hours and similar phase relationship. It appears that mere light exposure during the subjective night is not necessarily reproductively inductive. It would also appear that behaviorally generated light-dark cycles can be (i.e., LDFB), but are not necessarily (i.e., rLDFB) inhibitory to the maintenance of the reproductive system in long day breeders. Furthermore, the lighting pattern derived from LDFB is stimulatory if given exogenously (i.e., LDFB NC). Although it is not understood why light exposure that is coincident with the subjective night (i.e., LD 1m:1m and LDFB) is not stimulatory in long day breeders, a possible hypothesis is that an internal coincidence model is involved in the photoperiodic response and that multiple transitions during the subjective night may cause a dissociation of internal oscillations which must be in phase for light to be stimulatory.

Pinealectomy prevents short photoperiod inhibition of male hamster sexual behavior

The role of the pineal gland in mediating photoperiodic influences on copulatory behavior (CB) of male hamsters (Mesocricetus auratus) was assessed in the presence and absence of testosterone (T). The results demonstrate that the pineal gland is necessary for short photoperiod exposure to alter CB. Sexually experienced males were exposed to either long (14L:10D; LP) or short (8L:16D; SP) photoperiods for 13 weeks; after the first 2 weeks of exposure, all animals were castrated and then either pinealectomized (PINX) or sham operated (SHAM PINX). CB tests over an 8-week period following surgery indicated that copulatory impairments developed in all animals, but deficits occurred more rapidly among short photoperiod males with intact pineal glands (SP-SHAM PINX), compared to pinealectomized males housed in either the long (LP-PINX) or short photoperiod (SP-PINX). LP-PINX and SP-PINX animals were not statistically different on any of the CB measures examined. Nine weeks after castration (11 weeks of photoperiod exposure), all hamsters were given a T-filled Silastic capsule to restore CB. Restoration of sexual behavior was less rapid and less complete among SP-SHAM PINX hamsters. Additionally, males in this group took longer to initiate copulation relative to the pinealectomized hamsters. These findings are compared to other reports suggesting that photoperiodic effects on the sexual behavior of female hamsters do not require an intact pineal gland.

Short photoperiods affect male hamster sociosexual behaviors in the presence and absence of testosterone

Male hamsters were exposed to long (LD 14:10) or short (LD 8:16) photoperiods (LP; SP) to evaluate the effects of these environmental conditions on sociosexual behaviors. In Experiment 1, gonadally intact males in SP exhibited deficits in sexual behavior, reflected both in performance as well as initiation measures. Some aspects of the males' chemoinvestigation of females or their odors were also significantly different between LP and SP hamsters. In Experiment 2, castration resulted in the development of copulatory impairments, but they occurred more rapidly among males in SP conditions. Subsequent testosterone (T) replacement restored mounts, intromissions and ejaculations on tests given 2 and 4 weeks after T, but this happened more quickly in the LP group. SP males were still slower than LP males to initiate mounts and intromissions on their second test. These influences of photoperiod are discussed in the context of steroid-independent and steroid-dependent effects on behavior and the role of impaired processing of chemosensory information is evaluated.

The ontogenesis of skin and organ characteristics in the Syrian golden hamster. II. Body and organ weights as well as blood glucose and plasma insulin levels

The ontogenesis of the organ weights of the liver, kidneys, adrenals, testes and ovaries as well as the blood glucose and plasma insulin levels were studied in a total of 464 golden hamsters of both sexes of the acromelanic white inbred strain Bio 1.5 and agouti coloured outbred strain Han:AURA. Familial and seasonal influences were excluded by means of randomisation (25 groups from 1-365 days of life). Body weight development was found to be sigmoid and showed significant differences in terms of age, sex, and strain. Liver and kidney weights developed in parallel. Here, too, strain differences (agoutis greater than acromelanics) were seen, and sex differences were observed in the case of the kidneys (females greater than males). The increased adrenal weight in male hamsters compared to females was in accordance with species- but not rodent-typical behaviour. Both the testes and ovarian weights varied considerably. Age as well as, in part, strain differences were seen in the case of the blood glucose and insulin levels. The continual development of body and organ weight could be explained in terms of physiological changes.

Stimulation of testicular function during the nonbreeding season in the woodchuck ()

Treatment of male woodchucks with a series of s.c. injections of pregnant mare serum gonadotropin (PMSG) and human chorionic gonadotropin (hCG) induced testicular growth, sperm production and marked increase in serum testosterone levels in the fall, approximately 4 mo before the expected spontaneous onset of testicular activity. These results suggest that it may be possible to induce and/or maintain reproductive competence in this species outside of its normally brief breeding season.

Photoperiodic influences on sexual behavior in male Syrian hamsters

The effect of photoperiodic conditions on sexual behavior was investigated in male Syrian hamsters that were either gonadally intact, or castrated and treated with low doses of testosterone throughout the experiment. Hamsters were exposed to long (LD 16:8) or short (LD 8:16) days for 7 weeks; for the next 8 weeks, either they were exposed to an intermediate daylength (LD 12:12), or daylength conditions remained unchanged. Sexual behavior was affected by photoperiod conditions in both gonadally intact animals and testosterone-treated castrates, but to different degrees. Intact males exposed to short days for 15 weeks exhibited gonadal regression, and their copulatory performance was impaired. The percentage of animals that intromitted or ejaculated was significantly reduced. Additional measures of sexual performance among the copulating males were also affected. In contrast, among the castrates with testosterone clamped at low but stable levels, the proportion of males that mounted, intromitted, or ejaculated was not affected by photoperiod. However, among the males that continued to copulate, sexual performance changes were present in the short-day castrates that resembled those displayed by the intact males. We infer that these behavioral effects in both hormonal conditions reflect primarily a difficulty in the attainment of intromission. Gonadal regression alone cannot easily account for the behavioral deficits of the intact males, because circulating testosterone levels at the end of the experiment were not significantly different between the gonadally intact hamsters and the castrated, testosterone-treated hamsters exposed continuously to short days. Males transferred from either long or short days to the intermediate-daylength condition responded behaviorally to this photoperiod as if it were a short day, that is, their ejaculatory frequency declined. We conclude that male hamsters exposed to photoinhibitory daylengths exhibit deficits in their sexual behavior, not only because endogenous levels of testosterone decrease, but also because the substrates on which this hormone acts become less responsive. We hypothesize that under physiological conditions, the episodic secretion of testosterone imposes constraints on the maintenance or restoration of copulation, and that the potent behavioral effects achieved by constant-release implants of testosterone may mask the presence of photoperiodically induced alterations in the hamster's sensitivity to this gonadal hormone.

Effects of daily afternoon melatonin administration on body weight and thyroid hormones in female hamsters

Female hamsters were treated each afternoon for 8 weeks with subcutaneous injections of 25 micrograms of either melatonin or vehicle solution. Animals were sacrificed in either the morning or afternoon of diestrus and proestrus, along with their respective melatonin-induced acyclic pairs. Melatonin-treated hamsters had significantly greater mean body weights than did the vehicle-treated hamsters (P less than 0.05). Terminal mean serum T4 levels and free thyroxine index (FT41) were significantly reduced (P less than 0.05) in melatonin-treated hamsters compared with values obtained from animals subjected to the vehicle alone. In addition, T4 levels, FT41, T3 levels, and FT31 were significantly influenced by day and time of sacrifice independently of the melatonin treatment. T3 uptake was significantly lower on the morning of proestrus in melatonin-treated animals. These results demonstrate that chronic afternoon melatonin administration in female hamsters results in the loss of estrous cyclicity, a significant gain in body weight, and the reduction of T4 levels and T3 uptake. Changes in serum T3 levels are a function of the time of sample collection and are not influenced by melatonin treatment. In addition, these data indicate that thyroid function in general changes during the estrous cycle in these animals.

Evidence that olfactory bulbectomy does not influence testicular regression in golden hamsters on short photoperiod by altering pineal melatonin production

A recent study has shown that olfactory bulbectomy (BX) will prevent reproductive regression associated with short photoperiod in male golden hamsters. The results of experiments reported in this paper show that bulbectomized hamsters on long or short photoperiod still show a large nocturnal elevation in pineal melatonin production and that BX inhibits the reproductive regression induced by exogenous melatonin in pinealectomized hamsters. The data therefore indicate that BX does not inhibit short photoperiod induced testicular regression by altering melatonin secretion.

Are the short-photoperiod-induced decreases in serum prolactin responsible for the seasonal changes in energy balance in Syrian and Siberian hamsters?

Serum prolactin (PRL) decreases in Syrian (Mesocricetus auratus) and Siberian (Phodopus sungorus sungorus) hamsters following short-photoperiod exposure. Both species also exhibit short-photoperiod-induced changes in body and lipid mass, but in opposite directions; Syrian hamsters increase and Siberian hamsters decrease their body weight, changes reflected nearly exclusively in their carcass lipid content. The purpose of these experiments was to determine whether the photoperiod-induced changes in PRL were responsible for the seasonal changes in energy balance in Syrian and Siberian hamsters by using the strategy of experimentally producing serum PRL levels opposite to those normally associated with the photoperiod in which the animals were housed. In long photoperiods serum PRL was reduced to short-day levels by subcutaneous (s.c.) CB-154 (bromoergocryptine, a dopamine agonist) injections. In short photoperiods, serum PRL was elevated to long-day levels in Syrian hamsters by ectopic pituitary explants, and in Siberian hamsters, serum PRL was elevated by chronic s.c. infusions of ovine PRL (oPRL). In both species, manipulations of serum PRL did not affect food intake, carcass composition, or the wet weight of various white and brown adipose tissue pads (WAT and BAT, respectively). Body weight increased in CB-154-treated Syrian hamsters and decreased in Siberian hamsters, an effect partially reversed by coadministration of oPRL in Syrian, but not Siberian, hamsters. Thus, lowering serum PRL to short-day levels in long-day-housed hamsters of both species mimicked the directional change in body weight appropriate for each species when they are exposed to short days. This effect of CB-154 on body weight may be a result of some as yet unidentified effect of dopaminergic stimulation on overall growth since 1) these changes in body weight were not reflected as changes in lipid mass, as occurs naturally following short-day exposure for each species, and 2) neither species exhibited a reciprocal change in body weight when serum PRL was experimentally elevated in short days. Alternatively, it may be that once the energetic responses to short-day exposure have been fully expressed, the ability of PRL to stimulate the target sites of action for PRL for these responses may be decreased. BAT protein content, cytochrome oxidase activity (measures of metabolic growth of this tissue), and retroperitoneal total and specific lipoprotein lipase (LPL) activities were increased by CB-154 treatment in Syrian hamsters.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential distributions of cholecystokinin in hamster and rat forebrain

Rats and golden hamsters show a differential feeding response to intracranial injections of cholecystokinin (CCK). Rats, but not hamsters reduce food intake after CCK injections into the hypothalamic paraventricular nucleus. In view of this species difference, we undertook an immunohistochemical study of the distribution of CCK-immunoreactivity in the hamster hypothalamus and remaining forebrain. CCK-immunoreactive perikarya were abundant in the neocortex, claustrum, hippocampal formation, amygdaloid complex, bed nucleus of the stria terminalis, nucleus of the lateral olfactory tract and in the magnocellular basal nucleus. CCK-immunoreactive neurons had a more restricted distribution in the diencephalon and were relatively rare in the preoptic area-hypothalamus. The only exception was the suprachiasmatic nucleus and adjacent medial anterior hypothalamus, in which CCK-immunoreactive neurons were numerous. CCK-containing perikarya were not observed in the hamster hypothalamic paraventricular and supraoptic nuclei, where they have been reported to occur in the rat. Groups of CCK-positive perikarya were also noted in the hamster thalamic paratenial and parafascicular nuclei. CCK-immunoreactive fibers/terminals were localized in the caudate and putamen, periventricular zones, dorsolateral geniculate, thalamic reticular nucleus and the superficial layer of the optic tectum. Fiber/terminal labeling was also present in those regions associated with CCK-immunoreactive perikarya. Our results indicate that the telencephalic distribution of CCK-containing neurons in the hamster appears to be similar to that reported in the rat. However, several differences occur in the diencephalon. Perhaps the most striking is that the hamster differs from the rat in having a large group of CCK-containing neurons in the suprachiasmatic nucleus, and in lacking the CCK-containing perikarya observed in the rat paraventricular and supraoptic nuclei. These differences may underly species differences in feeding responses to intracranial CCK injections and gonadal responses to short photoperiods. Our data further suggest that the distribution of neuropeptides and other neuroactive substances may not always be conserved during evolution.

Seasonal differences in testicular receptors and steroidogenesis

Gonadal function in most animal species exhibits considerable annual fluctuations, with gametogenesis and fertility often being confined to a short and rigidly controlled breeding season. In males, production of androgenic steroids by the testis is usually maximal immediately before and during the breeding season. In the golden hamster, seasonal regression of the testes is associated with decrease in the total content of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and prolactin (PRL) receptors, and similar findings have been reported for other mammalian species. However, the concentration of LH and FSH receptors per unit of testis weight is typically elevated rather than suppressed during testicular regression. Reduction in the number of testicular LH and PRL receptors in adult golden hamsters exposed to short photoperiod is due primarily to suppression of pituitary PRL release under these circumstances. Regulation of seasonal changes in testicular FSH binding, as well as regulation of the levels of LH, PRL and FSH receptors in other seasonally breeding species remain to be elucidated. Reduction in the content of LH receptors in the testes is accompanied by reduced capacity to produce androgens in response to LH stimulation. Although these events are likely to be causally related, other mechanisms are also involved. In particular, seasonal regression is accompanied by reduced capacity of the testes to convert C21 steroid precursors into biologically active androgens. Seasonal loss of FSH receptors was reported to be accompanied by increased rather than reduced responsiveness of the Sertoli cells to FSH, thus resembling the situation in immature animals. It can be concluded that alterations in the ability of the testes to bind pituitary gonadotropins and to respond to gonadotropic stimulation are among the mechanisms responsible for seasonal shifts between gonadal activity and quiescence.

The effect of photoperiod on development of sexual behavior and fertility in golden hamsters

Male hamsters were raised on short (SD) or long (LD) photoperiods from birth, and the expression of sexual behavior at 6, 7, 10, and 14 weeks was compared. Neither mounting, intromissions, ejaculations, nor ultrasonic vocalizations differed in the two groups until 14 weeks of age, when these behaviors were performed more often by LD males. Sperm appeared in daily penile smears at an earlier age in SD than LD males, but LD males had larger flank glands and testes beginning at 6 and 7 weeks, respectively. Female siblings of the males in this study were mated around 7 weeks of age to adult males. Almost all females on both photoperiods conceived normally, and greater than 90% of the offspring delivered survived to weaning in both groups. Together, these results show that hamsters reared on SD are behaviorally and physiologically capable of reproducing for a period of time after puberty. The possibility that social influences or environmental factors other than photoperiod act on the juvenile hamster to retard reproductive development is discussed.

Short photoperiods increase ultrasonic vocalization rates among male Syrian hamsters

Two experiments investigated the effects of daylength on the emission of 35 kHz ultrasonic (US) calls among male hamsters. In Experiment 1, castrated males received Silastic implants subcutaneously that contained either low doses of testosterone in oil or oil alone; US calls were recorded when these males were paired with receptive females. Males exposed to eight hours of light per day (short photoperiod) called more often than males exposed to fourteen hours of light per day (long photoperiod). This was true whether or not they received testosterone. In Experiment 2, a similar testing and photoperiod exposure paradigm was used, but the subjects were gonadally intact. Among males exposed to short photoperiods, US call rates increased while endogenous testosterone levels decreased. In contrast, hamsters exposed to long photoperiods maintained stable calling rates and testosterone levels. These findings are related to recent studies concerning the neural mechanisms that regulate ultrasonic vocalizations and to the possible role of photoperiod in modulating conspecific aggression.

Effects of inhibitory and stimulatory photoperiods and sexual maturation on the ability of hamster testes to respond to hCG in vitro

The decrease in gonadal weight produced in adult golden hamsters by exposure to short photoperiods was accompanied by a marked reduction in the ability of the testes to produce testosterone from endogenous precursors in vitro, both without and with hCG stimulation. These changes were significant after 4-7 weeks in short photoperiod (5L: 19D) and were even more pronounced after 17-20 weeks. Production of testosterone in vitro by testes of immature hamsters was comparable to values obtained in adult animals with short photoperiod-induced gonadal atrophy. Delay of sexual maturation induced by daily injections of bromocriptine was accompanied by a further decrease in testicular testosterone production in vitro. Exposure of gonadally-regressed adult hamsters to a long, stimulatory photoperiod (14L: 10D) produced a rapid and marked increase in testicular testosterone production, which was coincident with the previously demonstrated increase in serum gonadotrophin levels after 1-5 days of photostimulation. Furthermore, testosterone production in vitro by regressed testes of animals exposed to short photoperiod was increased significantly by one large dose of hCG administered 26 h before killing the animals. It is concluded that the suppressive effects of short photoperiods on the ability of the hamster testis to produce testosterone and to respond to hCG stimulation are due to reductions in endogenous LH, FSH and prolactin release, with a consequent loss of testicular LH/hCG receptors and decreased activity of enzymes involved in the biosynthesis of testosterone.