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.     

Rapid cessation of estrous cyclicity and depressed castration response in short photoperiod-treated, inbred LSH/SsLaK hamsters

This study examined the effects of transfer from long photoperiod (LP) to short photoperiod (SP) on the cessation of ovarian cyclicity and the castration response in inbred LSH/SsLak golden Syrian hamsters. Forty-six 8 to 10-wk-old female hamsters were acclimatized in LP (14L:10D; lights on at 0600 h) during which time animals were monitored for regular ovarian cyclicity. Twenty-six animals were transferred to SP (8L:16D; lights on at 0600 h) and examined daily for vaginal discharges. One day after the day of the first missed ovulation, individual SP-exposed animals were bilaterally ovariectomized; concomitantly, an LP control animal in diestrus I underwent the same procedure. Thirty days after ovariectomy, the hamsters were fitted with intra-atrial silastic cannulae. On the following two postoperative days, 0.6 ml blood samples were collected at 0700, 1200, 1400, and 1600 h for SP animals and at 0700, 1400, 1600 and 1800 h for LP controls. On the third day, the animals were decapitated and sera and pituitaries saved for determination of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and prolactin (PRL) by radioimmunoassay (RIA). All SP-exposed animals displayed their last estrous discharge 14-34 days after transfer to SP (mean = 23.0 +/- 0.8 days). Their ovaries were characterized by the absence of corpora lutea, the presence of large atretic antral follicles, few growing follicles, and interstitium that was stimulated to varying degrees. Total and adjusted pituitary weights were decreased by SP exposure (p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)     

Melatonin decreases estrogen receptor expression in the medial preoptic area of inbred (LSH/SsLak) golden hamsters.

Daily late afternoon injections of melatonin (25 micrograms/day s.c.) were found to reduce the number of cells expressing estrogen receptor immunoreactivity in the medial preoptic area of ovariectomized inbred (LSH/SsLak) golden hamsters. Employing immunocytochemical analysis with the H222 monoclonal antibody to the human estrogen receptor, we examined the effects of melatonin on estrogen receptor expression in the hypothalamus, particularly the medial preoptic area, of ovariectomized virgin female hamsters. Analysis of the results showed that melatonin administration induced a 50-70% decrease in numbers of estrogen receptor-immunoreactive neurons in the medial preoptic area of ovariectomized female hamsters. Furthermore, an overall qualitative decrease in the intensity of estrogen receptor immunoreactivity was observed. In intact regularly cycling female hamsters used to monitor the efficacy of melatonin treatment, there were significant reductions in the serum levels of FSH, LH, and prolactin as measured by radioimmunoassay and in uterine and pituitary weights after 8 wk of melatonin treatment. These results suggest that melatonin may exert its anti-reproductive effects in hamsters by modulating estrogen receptor levels in medial preoptic area neurons, thus influencing steroid feedback mechanisms.     

Maturation of the hypothalamo-pituitary-gonadal axis of male Syrian hamsters.

Light-microscope immunocytochemistry (ICC) was used to investigate postnatal changes in the morphology of LHRH neurons in the brains of male Syrian hamsters and to relate these changes to more overt maturational developments within the hypothalamo-pituitary-gonadal axis. The animals were maintained under long-day photoperiods (14L:10D), and groups of 6-7 were killed at 10-day intervals from Day 15 to Day 65. Their brains were fixed with 4% paraformaldehyde, sectioned sagittally with a vibratome (75 microns), and processed for ICC using monoclonal LHRH antibody HU4H. Throughout the study period, the hamsters showed a progressive increase in plasma gonadotropin levels, closely followed by an increase in testicular weight and plasma testosterone levels. Histology of the testes revealed that spermatogenesis was already qualitatively completed by Day 35 and quantitative aspects were established by Day 45. Within the brain, LHRH neuronal perikarya were distributed primarily in the medial septal-preoptic area and the diagonal band of Broca; morphologically, these immunopositive neurons were either monopolar or bipolar. The total number of LHRH neurons detected in the areas examined was approximately 440 throughout the developmental period, and the relative proportions of monopolar and bipolar subtypes (86% and 14%, respectively) remained unchanged. In contrast, the area of the perikarya, as determined by autoimage analysis, showed a highly significant age-related increase, both for the monopolar and bipolar neurons. It is suggested that these developmental changes in the LHRH neurons reflect an increase in LHRH synthesis and may, therefore, provide a neuroendocrine trigger for the onset of puberty.     

Influence of ovaries and photoperiod on reproductive function in the mare.

A 16 h daily photoperiod hastened the onset of the ovulatory season (first ovulation); gonadotrophin and follicular changes prior to the onset were similar in intact light-treated and control mares. A preovulatory decline in FSH concentrations before the onset of the ovulatory season preceded the decrease in number of follicles (15--25 mm) and the rise in LH concentrations which was temporally associated with the growth of an ovulatory follicle. Seasonal changes of FSH and LH concentrations were found in ovariectomized mares and were influenced by photoperiod. During the anovulatory season, there was no ovarian influence on gonadotrophin concentrations. However, during the ovulatory season the ovaries exerted a positive influence on seasonally elevated LH concentrations during oestrus and a negative influence during dioestrus. The ovaries exerted a negative influence on seasonally elevated FSH concentrations throughout the oestrous cycle. The onset of the ovulatory season occurred at the time of the first sustained increase in LH concentrations resulting from positive seasonal (increasing photoperiod) and ovarian influences.     

Differential alteration of the reproductive axis by testosterone and estrogen in peripubertal and adult male Siberian hamsters (Phodopus sungorus)

In male Siberian hamsters, administration of adult physiological levels of testosterone (T) and estrogen (E2) to juveniles inhibited pubertal onset by distinct pathways. It is presently unclear if T and E2 also exert an inhibitory effect on the reproductive function of sexually mature and sexually maturing hamsters. This study aims to determine if there is an age-dependent decline in the sensitivity of the hypothalamic-pituitary-gonadal (HPG) axis to these inhibitory steroids and if their actions remain distinct. Peripubertal and adult male Siberian hamsters were implanted with a silastic capsule containing T, E2, or cholesterol (Ch, control). Testosterone treatment significantly reduced testes mass and length and impaired spermatogenesis in both ages. In contrast, E2 treatment reduced testes mass only in peripubertal, but not adult, animals. In fact, E2 treatment significantly increased testes mass in adults without altering spermatogenesis. In addition, circulating E2 is very high immediately prior to pubertal onset and declines thereafter. Our results showed the inhibitory effects of T persist into adulthood whereas those of E2 subside as the animals become sexually mature. The decreased sensitivity of the HPG axis to the inhibitory effects of E2 in adult animals and the high level of circulating E2 immediately prior to pubertal onset suggest E2 may play an important role in the regulation of puberty in this species.     

Female reproductive responses to photoperiod and male odours in the subterranean rodentCtenomys talarum

The tuco-tucoCtenomys talarum Thomas, 1898 is a solitary subterranean rodent that breeds seasonally in coastal sand-dune grasslands from Argentina. We assessed whether short-day photoperiod determines the onset of reproduction by an increase in female’s receptivity and whether male odour interacts with photoperiod as a cue by producing a greater response in receptivity. Receptivity was measured by an increase in the percentage of vaginal epithelial cells, particularly cornified cells. Females were trapped during their non-reproductive season (summer-long days) and randomly assigned to different day-length experimental groups: constant summer (CS), early winter (EW), and natural (NA) photoperiod. We examined the vaginal epithelium using vaginal smears during the first experimental phase (120 days) to test the effect of photoperiod. In the second experimental phase (15 days), we presented the females with male-soiled shavings to test the effect of photoperiod and male odor.Ctenomys talarum females showed some evidence of reproductive responsiveness to photoperiodic cuing. The proportion of epithelial cells peaked earlier in females when winter day-length was advanced than in females under natural photoperiod. Contrary to that expected, male odours did not stimulate female receptivity. These results suggest that, while photoperiod may participate in regulating reproduction, the integration of other environmental and social factors, as well as the identification of bodily conditions that lead to differences in individual response needs further consideration.     

Growth and reproductive development of male Microtus montanus is affected by the prenatal photoperiod

Rates of growth and sexual maturation of microtine rodents vary in response to photoperiod. Previous work with Microtus montanus has shown that the photoperiod present prior to weaning influences how voles will respond to photoperiods seen following weaning. The data presented demonstrate that information about the photoperiod seen by the mother during pregnancy influences the postweaning development of male M. montanus. Adult M. montanus were paired in photoperiods consisting of 8, 14 or 16 h light/day. Their litters were conceived and born in these photoperiods. On the day of birth the litters were recorded and retained in the gestation photoperiod (Groups C8, C14 and C16) or transferred to the 14-h photoperiod (Groups E8 and E16). The growth of males was followed from weaning until 74 days of age, at which time the voles were sacrificed and their reproductive organs weighed. There were no differences in body weight or length between groups at 18 days of age. At 74 days of age the development of the voles could be ranked in the following sequence: C8 less than E16 less than C14 less than E8 less than C16.     

Phase shift of the circannual reproductive rhythm in European hamsters by 2 days of long photoperiod

OBJECTIVEs AND DESIGN: In European hamsters a circannual clock drives the seasonal changes in the reproductive state. Its resetting by photoperiod is clearly phase dependent. In mid subjective winter a 1-month pulse of long photoperiod (LP) advances the onset of the reproductive phase of animals maintained in constant short photoperiod (SP) by up to 1.5 months. The present study investigated whether shorter pulses, i.e. 8, 4 or 2 days LP-pulses are still effective to phase shift the circannual rhythm. MAIN FINDINGS: All pulses induced gonadal development after a similar time relative to the offset of the pulse and earlier than in the control group. Thus, they all shared a similar effectiveness. CONCLUSIONS: In European hamsters a very brief LP-pulse can phase shift the reproductive rhythm but its strength is not determined by its duration at least not in the tested range.     

Influence of photoperiod and temperature on reproductive mode in the Brine shrimp, Artemia franciscana

Brine shrimp, Artemia, exhibit two modes of reproduction: oviparity (diapause cyst production) and ovoviviparity (live larvae release). Environmental conditions determining these developmental routes are poorly understood, so we investigated the effects of photoperiod and temperature on reproductive mode. Nauplii of A. franciscana were hatched from cysts produced in the Great Salt Lake, Utah, and raised in 2% natural sea salt water under photoperiods of 24, 14, 12, or 10 h at 28 degrees or 20 degrees C. Mating pairs of mature shrimp were isolated and reared continuously under those conditions. The mode of reproduction shown by each pair was determined daily throughout their life span, and found to be greatly affected by photoperiod, and less influenced by temperature. The relative degree of oviparity increased as the photoperiod became shorter at both temperatures. In contrast, the degree of ovoviviparity was higher as the photoperiod became longer at both temperatures. The critical photoperiod appears to be between 12 and 14 h. For all photoperiods examined, the degree of oviparity was higher at 28 degrees C than at 20 degrees C, whereas the degree of ovoviviparity was greater at 20 degrees C than at 28 degrees C.     

The effects of photoperiod on androgen-induced reproductive behavior in male ring doves, Streptopelia risoria

The effect of photoperiod on reproductive behavior in male ring doves (Streptopelia risoria) was studied in androgen-injected castrates maintained under long (16L:8D) or short (86:16D) photoperiods. Behavioral recordings were made over a 2-week period during which each male was paired with a female for 6.5 hr/day. There was some indication that males held on long days display higher levels of courtship activity during the initial period following pairing, but the evidence was not conclusive. Day length had no effect upon copulatory behavior. Long-day males exhibited consistently higher levels of nest building than short-day males, indicating that photoperiod affects building through some mechanism other than changes in endogenous androgen levels.     

Stimulatory effects on the reproductive axis in female songbirds by conspecific and heterospecific male song

Courtship vocalizations of male songbirds can profoundly enhance the reproductive physiology and behavior of conspecific females. However, no study has fully investigated the selectivity of conspecific song effects on reproductive development in birds. We studied the effects of conspecific and heterospecific song on reproductive development in domesticated (canaries) and wild songbirds (song sparrows). As expected, conspecific song enhanced follicular development. Unexpectedly, however, birds exposed to heterospecific song also underwent enhanced follicular development (compared to birds exposed to no song); conspecific and heterospecific songs were equally effective in enhancing ovarian development. In canaries exposed to 18L:6D, conspecific song induced oviposition earlier and at a greater frequency than in heterospecific and no song groups, with the fewest eggs being laid in the no song group. These results indicate that conspecific and heterospecific male song can enhance reproductive activity in female songbirds. Whether or not activation of the reproductive axis in female songbirds by heterospecific song occurs in the wild remains unclear. It is also unclear as to whether the ability of the reproductive axis to respond to heterospecific song performs a specific function, or whether it is simply a consequence of greater selection pressure acting upon behavioral responses to song.     

The influence of artificial photoperiod on the growth,appetite and reproductive status of male red deer and sheep

Young male red deer and Suffolk X (Finn X Dorset) sheep were kept on an artificial photoperiod such that two cycles of daylength occurred during one calendar year. They were penned separately, fed to appetite, weighed weekly and measured tri-weekly.Both species showed two cycles of intake, growth and gonadal activity in response to the daylength when only one would have been shown on natural photoperiod, although in the sheep these cycles were of lower amplitude than in the deer. The deer grew two sets of antlers during the study. A lag of some 3–4 months occurred between an event such as peak food intake and the time it would have been expected to occur relative to the daylength cycle. It is considered that although daylength controls these cycles, there is an endogenous rhythm which photoperiod cannot completely suppress.     

Effect of 6-methoxybenzoxazolinone on sex ratio and breeding performance in Microtus montanus

The plant derivative, 6-mathoxybenzoxazolinone (6-MBOA), which has been demonstrated to initiate reproduction in field populations of the montane meadow vole (Microtus montanus), was administered via feeding or Silastic capsule implants to mated pairs of laboratory bred M. montanus. The animals remained paired for 120 days, and the number, size, and sex ratios of the resulting litters were recorded. Both the size and frequency of litters were significantly greater in 6-MBOA-treated pairs than in controls. By using implants, it was possible to treat one or both sexes in a mated pair. The positive effects of this compound on litter size and number of litters occurred when the females received implants, which indicates that the male has no influence on these parameters. The most unusual result of these experiments was that 6-MBOA has a significant effect on the sex ratio of the litters. Animals receiving 6-MBOA produced significantly more females than did control pairs. This result occurred regardless of the method of administration, and in the case of the implant studies, regardless of which sex received the active implants. These findings are discussed in relation to the ecology and life history strategy of Microtus montanus.     

Low dose of bisphenol A impairs the reproductive axis of prepuberal male rats

The objective of the present work was to study the effect of a low dose of bisphenol A (BPA), on the reproductive axis of prepuberal male rats exposed to the endocrine disruptor (ED) during gestation and lactation period. Wistar-mated rats were treated with either 0.1 % ethanol or BPA in their drinking water until their offspring were weaned at the age of 21 days. The estimated average dose of exposure to dams was approximately 3 μg/kg/day of BPA. The pups were sacrificed on the 35th day of life. Body weight was measured during the development and at the moment of the sacrifice; testicular and seminal vesicles weight and their respective relative weights were also measured. LH, FSH and testosterone were determined and histological studies of testicular tissue were also performed. Body weight at the moment of the sacrifice was significantly higher in the group exposed to BPA; testicular weight decreased significantly; seminal vesicles weight and relative weights of testes and seminal vesicles were not modified by treatment. LH and FSH serum levels increased significantly after treatment, meanwhile testosterone showed no significant changes. Histological studies showed the lumen of seminal tubes reduced by the presence of immature cells of the spermatic lineage. Our results suggest that pre- and early postnatal exposure to a low dose of BPA disrupts the normal function of the reproductive axis in prepuberal male rats. The effects of the ED may be exerted at different levels of the axis and may be dependent on the dose, manner of administration, and the moment of exposure to the disruptor.     

Influence of high ambient temperature on the reproductive function of the male rat

An attempt was made to determine the extent to which an inhibition of testicular androgen production by high ambient temperature may explain the damaging effect of heat on spermatogenesis, using male albino rats. Spermatogenesis and androgen synthesis were affected by high heat in the intact rat and in the hypophysectomized rat maintained on pituitary gonadotrophins or testosterone propionate. Heat treatment decreased the weight of the ventral prostate to half that of the control. High ambient temperatures did not severely damage testosterone-maintained spermatogenesis in hypophysectomized rats. Heat treatment did not lead to remarkable changes in the ultrastructure of the gonadotrophic cells of the anterior pituitary gland although there was a generalized and partial degranulation of anterior pituitary cells.