Sex differences in the onset of seasonal reproductive quiescence in hamsters

Day length is the primary cue used by many mammals to restrict reproduction to favourable spring and summer months, but it is unknown for any mammal whether the seasonal loss of fertility begins at the same time and occurs at the same rate in females and males; nor it established whether the termination of mating behaviour in males and females coincides with the loss of fertility. We speculated that females, owing to their greater energetic investment in reproduction, are the limiting sex in terminating offspring production in short days (SDs). Oestrous cycles and production of young were monitored in Syrian hamsters (Mesocricetus auratus) transferred from long days (LDs) to SDs. Females were mated to LD males after three to eight weeks of SD treatment; in a parallel experiment, males housed in SDs were mated to LD females. After five and eight weeks in SDs, at least twice as many males as females were fertile. Both males and females continued to copulate for several weeks after becoming infertile. The onset of seasonal infertility occurs earlier in females than males and the decline in fertility precedes the seasonal loss of mating behaviour in both sexes.     

Asymmetric learning to avoid heterospecific males in Mesocricetus hamsters

If a female mates with a male of a closely related species, her fitness is likely to decline. Consequently, females may develop behavioral mechanisms to avoid mating with heterospecific males. In some species, one such mechanism is for adult females to learn to discriminate against heterospecific males after exposure to such males. We have previously shown that adult, female Syrian hamsters (Mesocricetus auratus) learn to discriminate against male Turkish hamsters (Mesocricetus brandti) after exposure to a single heterospecific male during 8 days across a wire-mesh barrier. Here we repeated that experiment but this time we exposed female Turkish hamsters to a male Syrian hamster for 8 days and then measured sexual and aggressive behaviors towards that heterospecific male and towards a conspecific male. In contrast to female Syrian hamsters, female Turkish hamsters did not differ in their latency to go into lordosis or in any measure of aggression towards either type of male. Female Turkish hamsters spent less time in lordosis with the heterospecific male, but the percentage of trials in which females copulated with conspecific and heterospecific males did not differ. When comparing females from both species that had been exposed to a heterospecific male for 8days, female Syrian hamsters copulated less and were more aggressive towards the heterospecific male compared to the behavior of female Turkish hamsters. We discuss how this asymmetric response between females of the two species may be due to the much larger geographical range of Turkish hamsters compared to Syrian hamsters.     

Androgenic control of immunoreactive somatostatin in the Harderian gland of the Syrian hamster

Harderian glands of Syrian hamsters contained measurable levels of immunoreactive somatostatin. After an extraction procedure, serial dilutions of tissue were assayed and showed parallelism in the displacement curve with dilutions of purified somatostatin standard in the radioimmunoassay. Somatostatin concentrations were higher in female hamsters (10.0 +/- 2.1 ng/mg protein) than in males (2.6 +/- 0.4 ng/mg protein). Castrated males had somatostatin values in the range of females (12.4 +/- 2.3 ng/mg protein) at 1 month after gonadectomy. Testosterone implants prevented the rise of Harderian gland somatostatin in castrated males. Gonadectomized males had lower somatostatin content in the gland than did control males (1.0 +/- 0.2 ng/mg protein) at 2 months after castration. Somatostatin values in females were unaffected by gonadectomy, but there were variations during the oestrous cycle, with a nadir detected at dioestrus-1 and maximal values coincident with the day of the ovulation.     

Testosterone and chemosensory detection in male Syrian hamster

Gonadal steroids stimulate both sexual motivation and performance. However, steroid facilitation of appetitive sexual behavior is poorly understood. The present study determined if castration impairs chemosensory detection in male hamsters. Chemosensory cues are the principal sensory modality to initiate mating in this species. We compared LiCl-induced conditioned taste avoidance to female hamster vaginal secretion (FHVS) in gonad-intact and castrated males. Following overnight water deprivation, males received FHVS for 15 min, followed by LiCl (2 ml of 0.15 M) or saline ip. The next day, fluid consumption in a two-bottle choice test was recorded for 5.5 h. Pairings were repeated 4×. Initially, discrimination of FHVS from estrous females (10 or 100 μg/ml) was compared with plain water. Subsequently, we determined if males could distinguish FHVS from Syrian vs. Djungarian females or from estrous vs. anestrous females. When 100 μg/ml FHVS was paired with saline, all gonad-intact and 86% of castrated males preferred FHVS over water. However, when 100 μg/ml FHVS was paired with LiCl, the preference was reversed: 12.5% of intact males and 25% of castrates preferred FHVS (P < 0.05 vs. saline pairing). When exposed to 10 μg/ml FHVS, neither gonad-intact nor castrated males expressed conditioned taste avoidance, suggesting that 10 μg/ml FHVS is below the threshold for detection. Comparing discrimination of FHVS from Syrian and Djungarian females, only castrated males developed a significant conditioned taste avoidance to Syrian FHVS paired with LiCl. While 71% of castrated males preferred Syrian FHVS after saline pairing, only 12.5% of castrates preferred Syrian FHVS after pairing with LiCl (P < 0.05). In gonad-intact males, 57% preferred Syrian FHVS after saline pairing, while 14% preferred Syrian FHVS following LiCl pairing (P > 0.05). Neither gonad-intact nor castrated males successfully discriminated between FHVS from estrous and anestrous females. These data demonstrate that castrated males perform as well as gonad-intact males in a test of LiCl-induced conditioned taste avoidance. Therefore, it is unlikely that steroids enhance detection of sexually relevant chemosensory cues.     

Adult female hamsters require long and sustained exposures to heterospecific males to avoid interspecific mating

Interspecific mating normally decreases female fitness. In many species, females avoid heterospecific males innately or by imprinting on their parents. Alternatively, adult females could learn to discriminate against heterospecific males after exposure to such males. For example, Syrian hamster (Mesocricetus auratus) females learn to discriminate between conspecific males and Turkish hamster (M. brandti) males during adulthood by exposure to males of both species. Adult females not previously exposed to Turkish hamster males will mate similarly with conspecific and heterospecific males. However, in a previous study we showed that exposure to a heterospecific male and a conspecific male for 8 days led to mating avoidance and aggression towards the heterospecific male. Here we conducted two experiments to investigate how much exposure to the heterospecific male was required for females to avoid mating with the heterospecific male (Experiment 1) and how long that avoidance lasted in the absence of continuous exposure to heterospecific stimuli (Experiment 2). Fast and durable learning would indicate the evolution of an efficient avoidance response. In Experiment 1, females were exposed to a heterospecific male for 1, 4 h, 4 or 8 days and then paired with that male. We found more avoidance of interspecific mating after 4 or 8 days of exposure than after 1 or 4 h of exposure. In Experiment 2, females were exposed to a heterospecific male for 8 days and then paired with that male either 10 min later or 8 days later. We found that after an 8-day delay females were highly sexually receptive to the heterospecific male. Additionally, a comparison between the current experiments and a previous study indicates that female Syrian hamsters do not require concurrent exposure to a conspecific male and a heterospecific male to learn to avoid interspecific mating; exposure to a heterospecific male is sufficient.     

Can captivity lead to inter-species mating in two Mesocricetus hamster species?

In two closely related species, females generally prefer conspecific males over heterospecific males. We found that estrous (but not diestrous) female Syrian hamsters Mesocricetus auratus prefer the odors of conspecific males to odors of Turkish hamsters Mesocricetus brandti . However, female Syrian hamsters are not aggressive toward male Turkish hamsters and will readily mate with them. We hypothesize that many generations in captivity led to a reduction in females' ability to avoid inter-species mating, possibly related to the heightened sexual receptivity observed in Mesocricetus hamsters in captivity. To test this hypothesis, we replicated a study carried out with female Turkish hamsters soon after the current laboratory stock of this species was established. In that study, female Turkish hamsters showed lordosis toward male Syrian hamsters in only 20% of interactions and attacked heterospecific males in 80% of the pairings. Using animals descended from that original colony (after many generations in captivity and certain episodes of inbreeding), 100% of female Turkish hamsters mated with heterospecific males and none showed aggression toward heterospecific males. Thus female avoidance of inter-specific mating may be affected by captive rearing conditions.     

Effects of prepubertal manipulation with androgens on the development of sexual differences in the harderian glands of Syrian hamsters.

The onset of sexual differences in the metabolism of porphyrins and melatonin in the Harderian glands of Syrian hamsters was studied. Three weeks after birth, the porphyrin concentrations were already higher in glands of females than in those of males. Castration of 22-day-old male hamsters led to an increase in Harderian porphyrin concentrations, although the levels of intact females were not reached. The administration of testosterone to 22-day-old female hamsters resulted in a marked decrease in porphyrin concentrations. Study of the development of sexual differences in the enzymes involved in melatonin synthesis, N-acetyltransferase (NAT) and hydroxyindole-O-methyltransferase (HIOMT) indicated that not all the sexual differences observed in these glands begin at the same time. Thus, while differences in NAT activity were detected after the age of 3 weeks, male-female differences in HIOMT activity were only observed after 7 weeks. Castration of prepubertal male hamsters lowered NAT but not HIOMT activities. The administration of testosterone to prepubertal female hamsters led to male activity levels in both enzymes. Although circulating androgens seem to have a crucial role in maintaining sexual differences, other hormones including those from the pituitary and thyroid glands are probably also important for generating these sexual differences.     

Comparative genetics of hamster amylases

Syrian (Mesocricetus auratus) and Chinese (Cricetulus griseus) hamsters were phenotyped by electrophoresis for salivary and pancreatic amylases. Syrian hamsters possess two salivary amylase electromorphs, the more anodal (fast) being invariant in 250 outbred and 17 representatives of 5 highly inbred lines. The slow electromorph had activity equal to that of the fast amylase (heavy), or had distinctly less activity (light), or was absent (null). The slow electromorph is inherited as an autosomal semidominant trait with two alleles, Amy ^s and Amy ^o . Amy ^s homozygotes produce heavy, Amy ^o homozygotes null, and heterozygotes light phenotypes, respectively. Five inbred strains of hamsters were homozygous Amy ^o . Pancreatic amylase was monomorphic. Eight outbred Chinese hamsters showed no salivary amylase activity with electrophoresis, but slight activity with long incubation on starch-agar plates. However, pancreatic amylase activity in the Chinese hamster exceeded that in Syrian hamsters. Site duplication and apparent null alleles for amylase genes occur in muroid rodents. The evolutionary implications are discussed.George H. Bunch Chair Professor of Biology     

Naloxone antagonism of diazepam-induced feeding in the Syrian hamster

Three experiments investigated the effects of the intragastric administration of the benzodiazepine diazepam on feeding in non-deprived Syrian hamsters ($\text{mesocricetus auratus}$). In the first experiment diazepam (0, 0.5, 1.0, 2.0, and 4.0 mg/kg) produced dose dependant increases in feeding. 4.0 mg/kg of diazepam produced significantly more feeding than the other doses tested and the lowest dose tested (0.5 mg/kg) produced a significant increase in feeding. In the second experiment naloxone (10 mg/kg) partially antagonized the effect of 4 mg/kg of diazepam on feeding. In the third experiment the ability of naloxone (0.1, 1.0, 5.0, 10.0 or 20 mg/kg) to reduce feeding produced by either 4 mg/kg or 2 mg/kg of diazepam was tested. Naloxone partially antagonized the effects of 4 mg/kg of diazepam on feeding in a dose dependant manner. While 2 mg/kg of diazepam produced significantly less feeding than 4 mg/kg, naloxone did not antagonize the effect of 2 mg/kg on feeding. The results suggest that two mechanisms are involved in diazepam-induced feeding in hamsters. The high dose of diazepam may produce increased feeding by activating the endorphin system while the low dose of diazepam produces increased feeding via a naloxone insensitive mechanism.     

Conditioned defeat in male and female Syrian hamsters

A brief exposure to social defeat in male Syrian hamsters (Mesocricetus auratus) leads to profound changes in the subsequent agonistic behavior exhibited by the defeated animals. Following defeat in the home cage of an aggressive conspecific, male hamsters will subsequently fail to defend their home territory even if the intruder is a smaller, nonaggressive male. This phenomenon has been called conditioned defeat. In Experiment 1, we examined the duration of conditioned defeat by repeatedly testing (every 3-5 days) defeated hamsters with a nonaggressive intruder. We found that conditioned defeat occurs in all defeated male hamsters and persists for a prolonged period of time (at least 33 days) in the majority of male hamsters tested despite the fact that these animals are never attacked by the nonaggressive intruders. In Experiment 2, we examined whether conditioned defeat could be induced in female Syrian hamsters. While conditioned defeat occurred in some females, they displayed only low levels of submissive/defensive behavior and, in contrast to males, the conditioned defeat response did not persist beyond the first test. These results suggest that in male hamsters conditioned defeat is a profound, persistent behavioral change characterized by a total absence of territorial aggression and by the frequent display of submissive and defensive behaviors. Conversely, social defeat in female hamsters does not appear to induce long-term behavioral changes. Finally, in Experiment 3, we determined that plasma adrenocorticotropin-like immunoreactivity increases in females following social defeat in a manner similar to that seen in males, suggesting that the disparate behavioral reactions of males and females are not due to sex differences in the release of, or response to, plasma adrenocorticotropin.     

Maternal photoperiodic history affects offspring development in Syrian hamsters

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

Enhanced longevity in tau mutant Syrian hamsters,Mesocricetus auratus

The single-gene mutation tau in the Syrian hamster shortens the circadian period by about 20% in the homozygous mutant and simultaneously increases the mass-specific metabolic rate by about 20%. Both effects might be expected to lead to a change in longevity. To test such expectations, the life span of male and female hamsters from three genotypes (wild-type, heterozygous, and homozygous tau mutants, all derived from heterozygote crosses to randomize the genetic background) was recorded in constant darkness. Male hamsters lived significantly longer than females: the overall average life span was 96.9 weeks (SE = 2.5, n = 118) for males and 82.0 weeks (SE = 2.1, n = 99) for females. To our surprise, male and female homozygous mutant hamsters lived significantly longer rather than shorter compared to wild-types. For males, the difference between the two genotypes was on average 14%; for females, the difference was 16%. The mortality rate of wild-type males was significantly different from that of homozygous tau males but not different from that of heterozygotes. Overall, survival of wild-type females was statistically distinguishable from both heterozygous and homozygous mutant females. Male and female wild-type hamsters were heavier than homozygote mutants throughout the entire life span, and heterozygous mutants had intermediate weights. There was no correlation between body mass and life span, and the causes of the extended life span in tau mutant hamsters remain unresolved.     

Breeding history and olfactory discrimination in the Syrian hamster

The ability of outbred female Syrian hamsters to differentially respond to urine samples obtained from either inbred or outbred Syrian males was examined. The odors obtained from the males were introduced into a Y-maze olfactometer. On the basis of time spent in proximity to the stimuli, estrous females evidenced a preference for the urine of the outbred males. The same females when diestrus exhibited no apparent preferences. It is possible that the ability of females to make such subtle discriminations may impact on mate selection leading to positive assortment.     

ELECTRON MICROSCOPIC RADIOAUTOGRAPHIC STUDIES OF THE CAROTID BODY FOLLOWING INJECTIONS OF LABELED BIOGENIC AMINE PRECURSORS

Adult Syrian hamsters were given a subcutaneous injection of reserpine 3 days before an intraperitoneal injection of ³H-3,4 dihydroxyphenylalanine or ³H-5, hydroxytryptophan and the carotid bodies were subsequently prepared for electron microscopic radioautography. Other Syrian hamsters were given a subcutaneous injection of reserpine and the carotid bodies were subjected to a sensitive cytochemical test for the detection of unsubstituted amines. These studies were made to determine whether the labeled amine precursors were incorporated into the cells and to see whether the parenchymal cells were affected by reserpine treatment. Material from hamsters treated first with reserpine and subsequently injected with ³H-3,4 dihydroxyphenylalanine or ³H-5, hydroxytryptophan exhibited reduced grains of silver over the cells which were associated mainly with the dense cores of the cytoplasmic granules. These studies offer evidence that the granules of the carotid body incorporate catecholamine and indolamine precursors. Material from hamsters incubated for the presence of unsubstituted amines gave a positive reaction (opaque cytoplasmic granules) for catecholamines but not for indolamines. The latter substances may not be present in quantities sufficient to register a positive reaction in the cytochemical test. The opaque granules, indicative of the presence of catecholamines, decreased in density after reserpine treatment. 5 days after one reserpine injection the granules had regained opacity and were comparable to those seen in the control cells.     

Gender differences and time course of castration-induced changes in porphyrins, indoles, and proteins in the Harderian glands of the Syrian hamster.

Sexual differences and the effects of orchidectomy were determined for porphyrin and melatonin concentrations and for the activities of the enzymes N-acetyltransferase and hydroxyindole-O-methyltransferase, which synthesize melatonin from serotonin, in the Harderian glands of the Syrian hamster. Porphyrin concentrations in intact males were about 1/400th those of intact females. Castration for 1 week increased male Harderian porphyrin concentrations 10-fold; by 3 weeks, castrated male porphyrin levels were 140 times those of control values. N-Acetyltransferase activity in intact male Harderian glands was about 4 times that of females. Castration led to a drop in N-acetyltransferase activity to female levels within 2 weeks. Hydroxyindole-O-methyltransferase activity was 7 times higher in females than in males and castration had no effect on male Harderian hydroxyindole-O-methyltransferase activity. Neither gender nor castration influenced Harderian melatonin concentrations. Soluble proteins in Harderian glands from male and female hamsters and from male hamsters castrated for 1 and 4 weeks were examined by sodium dodecyl sulfate--polyacrylamide gel electrophoresis. The gel profiles revealed several differences among the protein distribution in male and female gland lysates. Orchidectomy led to a female protein pattern within 4 weeks.     

Photoperiodic and hormonal influences on fur density and regrowth in two hamster species

Temperate and boreal mammals undergo seasonal changes in pelage that facilitate thermoregulation in winter and summer. We investigated photoperiodic influences on pelage characteristics of male Siberian and Syrian hamsters. Fur density (mg fur/cm2 skin) was measured by weighing the shavings of fur patches removed from the dorsal and ventral surfaces of hamsters maintained in long days (LDs) or transferred to short days (SDs). Patches were reshaved 3 wk later to assess fur regrowth (mg regrown fur/cm2 skin). Fur density was greater in SD than in LD Siberian hamsters after 11 wk of differential phototreatment. The onset of increased fur density in SDs was accompanied by a transient increase in fur regrowth (11-14 wk on the dorsal surface and 7-10 and 11-14 wk on the ventral surface), suggestive of a seasonal molting process. Fur density, body mass, and pelage color of Siberian hamsters returned to values characteristic of LD males after a similar duration of prolonged (>27 wk) SD treatment and appear to be regulated by a similar or common interval-timing mechanism. In Syrian hamsters, dorsal fur density, fur regrowth, and hair lengths were greater in SD than in LD males. Castration increased and testosterone (T) treatment decreased dorsal and ventral fur regrowth in LD and SD hamsters, but the effects of T manipulations on fur density were limited to a decrease in dorsal fur density after T treatment. Decreased circulating T in SDs likely contributes to the seasonal molt of male hamsters by increasing the rate of fur growth during the transition to the winter pelage.     

Splenic hypertrophy and extramedullary hematopoiesis induced in male Syrian hamsters by short photoperiod or melatonin injections and reversed by melatonin pellets or pinealectomy

Adult male Syrian hamsters either placed in a short photoperiod alone or kept in a long photoperiod and given daily afternoon injections of the pineal indole melatonin (25 micrograms) exhibited splenic hypertrophy and extramedullary hematopoiesis in addition to a marked regression in testicular weight. The testicular regression as well as the changes in spleen weight and histology could be prevented if the animals in short photoperiod were either pinealectomized or implanted subcutaneously with a pellet containing 1 mg melatonin. Female Syrian hamsters given afternoon injections of melatonin for 7 or 12 weeks had ovaries devoid of corpora lutea; additionally, these animals had reduced relative spleen weights compared to the control animals. In conclusion, it is apparent that spleen weight varies with the functional status of the gonads. Splenic hypertrophy accompanied by pineal-induced testicular regression in males may be related to splenic extramedullary hematopoiesis.     

The persistence of DNA damage in the pancreas of Syrian golden hamsters treated with N-nitrosobis(2-oxopropyl)amine

DNA damage was estimated in the liver, pancreas and salivary gland of Syrian hamsters given N-nitrosobis(2-oxopropyl)amine (BOP) by alkaline sucrose gradient centrifugation. A single BOP dose (10 mg/kg) produced in all 3 tissues extensive DNA damage that was largely repaired in the salivary gland by 4 weeks, while in the liver and pancreas, some DNA damage persisted until 4 weeks. When higher BOP doses (20 and 40 mg/kg) were used, considerable DNA damage was still evident in the pancreas, but not in the liver at 6 weeks. Greater damage persisted in hamsters given 40 mg/kg, compared with those administered 20 mg/kg.     

Food hoarding is increased by food deprivation and decreased by leptin treatment in Syrian hamsters

Compensatory increases in food intake are commonly observed after a period of food deprivation in many species, including laboratory rats and mice. Thus it is interesting that Syrian hamsters fail to increase food intake after a period of food deprivation, despite a fall in plasma leptin concentrations similar to those seen in food-deprived rats and mice. In previous laboratory studies, food-deprived Syrian hamsters increased the amount of food hoarded. We hypothesized that leptin treatment during food deprivation would attenuate food-deprivation-induced increases in hoarding. Baseline levels of hoarding were bimodally distributed, with no hamsters showing intermediate levels of hoarding. Both high (HH) and low hoarding (LH) hamsters were included in each experimental group. Fifty-six male hamsters were either food deprived or given ad libitum access to food for 48 h. One-half of each group received intraperitoneal injections of leptin (4 mg/kg) or vehicle every 12 h during the food-deprivation period. Within the HH group, the hoarding score increased significantly in food-deprived but not fed hamsters (P < 0.05). Leptin treatment significantly decreased hoarding in the food-deprived HH hamsters (P < 0.05). The LH hamsters did not increase hoarding regardless of whether they were food deprived or had ad libitum access to food. These results are consistent with the idea that HH hamsters respond to energetic challenges at least in part by changing their hoarding behavior and that leptin might be one factor that mediates this response.     

Leptin inhibits the reproductive axis in adult male Syrian hamsters exposed to long and short photoperiod

The aim of the study was to investigate the effects of acute leptin treatment of adult Syrian hamsters exposed to a long (LP, eugonadal males) and short photoperiod (SP, hypogonadal males). Animals were exposed to LP (L:D 14:10) or SP (L:D 10:14) for 10 weeks. Afterwards, both LP and SP hamsters were allocated to a control (SP-C, LP-C) or leptin-treated group (SP 3, SP 10, SP 30 or LP3, LP 10, LP 30). One hour before sacrifice, a single dose of leptin (3, 10 or 30 μg/kg) or vehicle was administered (i.p.) to the males. Testis weight, serum and pituitary luteinizing hormone (LH) concentrations, as well as the hypothalamic concentration of gonadotropin-releasing hormone (GnRH) were recorded. Histological analysis of the testis was performed and GnRH concentration in the culture medium of hypothalamic explants was examined. A dramatic regression of testicular weight and histological atrophy of seminiferous tubules, as well as a decrease in serum and pituitary LH concentrations were found in SP males. All doses of leptin significantly reduced serum LH levels and medium GnRH concentrations in both photoperiod groups. Pituitary LH and hypothalamic GnRH concentrations were not affected by leptin. In conclusion, we demonstrated that leptin inhibited the reproductive axis of Syrian male hamsters exposed to LP and SP and fed ad libitum.