Search

In Siberian hamsters, the rate of testicular maturation during juvenile life can be influenced by both the prenatal photoperiod and the day length experienced postnatally. In this report, potential postnatal photoperiodic mechanisms modified by prenatal photoperiod were investigated in this species. The study examined the effect of prenatal photoperiodic history on the postnatal pineal melatonin (MEL) rhythm and on postnatal secretion of FSH and PRL. In the first study, the pineal MEL content of hamsters, gestated in either 16 h of light and 8 h of darkness/day (16L) or 10L and raised postnatally in 14L, was monitored at various times of the day and night at 18 days of age. We found that prenatal photoperiod did influence the postnatal pineal MEL rhythm in 18-day-old males, but a similar effect was not evident in females. For males, the durations of the nocturnal elevation of pineal MEL were 8.5 and 7 h in 16L and 10L gestated hamsters, respectively. However, MEL rhythms were similar to each other in the corresponding groups of females (8.5- and 9-h durations of elevated pineal MEL in 16L and 10L prenatal photoperiod groups, respectively). In a subsequent study using the same photoperiod paradigm, FSH and PRL concentrations were examined in both genders at 3- to 10-day intervals between 18-62 days of age. The serum PRL (day 22) and FSH (days 18 and 22) concentrations in males were significantly affected by prenatal photoperiod. Specifically, circulating serum PRL (on day 22) and FSH (on days 18 and 22) concentrations were increased substantially in 10L gestated, compared to 16L gestated, males raised in 14L after birth. In contrast, serum FSH concentrations in female hamsters were not different between 16L and 10L gestated groups at these times. In another study using the same experimental design, the pattern of testicular development was explored in males. Hamsters that had experienced a 10L photoperiod prenatally and were raised in 14L exhibited rapid testicular growth from 27-52 days of age compared to hamsters that had experienced a 16L prenatal photoperiod and were reared in 14L. These results support the hypothesis that in juvenile male hamsters exposed to 14L postnatally, endogenous MEL production and serum FSH concentrations are influenced by photoperiodic information received during fetal life. In addition, these findings help to explain why males gestated in 10L and raised in 14L exhibit accelerated testicular development in the first 2 months of life compared to males gestated in 16L and transferred to 14L after birth.(ABSTRACT TRUNCATED AT 400 WORDS)

Prepubertal reproductive development in juvenile male Siberian hamsters can be strongly influenced by photoperiod information received during gestation. Information transmitted from the mother hamster to her fetuses appears to modify the photoperiodic mechanism of the developing hamsters so that they may respond differently to certain intermediate day lengths [i.e. 14 h of light and 10 h of darkness/day (14L)] experienced after birth depending on whether gestation occurred in longer or shorter day lengths. In adult and juvenile hamsters, the duration of the nocturnal elevation of pineal and serum melatonin (MEL) is an important component of the photoperiodic system coding for day length. In the present study, we investigated whether the photoperiod in effect during gestation could influence the responsiveness of developing male hamsters to daily MEL infusions of fixed durations administered after weaning. The results indicated that hamsters gestated under 16L or 10L did not differ from each other with respect to testicular growth in response to any of the fixed duration MEL infusions. Thus, target tissue responsiveness to fixed duration MEL infusions (over a range of 6-10 h) was the same regardless of the gestation photoperiod to which the animals had been exposed.

A comprehensive developmental study of serum LH, FSH and androgen concentrations was carried out in male mice of two inbred strains. Gonadotropic hormone (GTH) levels rose 10-15 days prior to the pubertal increase in serum androgen with FSH preceding LH in this regard. In both strains GTH titer rose to a peak at 30 or 35 days and then steadily declined to adult levels which were strain-specific. Serum androgen was detected at low, relatively steady levels until the pubertal increase between 30 and 50 days postpartum. The results are interpreted as supporting the hypothesis that a shift in feedback sensitivity, occurring at about 20 days of age, may be involved in the onset of puberty in the male mouse.

Transfer of adult male hamsters from a long to a short photoperiod causes testicular atrophy, which is accompanied by decreases in testicular LH receptors and in plasma PRL and testosterone levels. A decrease in plasma gonadotropin levels is also frequently observed. When the decline in peripheral PRL concentration is prevented by transplantation of homologous pituitary(ies) under the kidney capsule, testicular atrophy is delayed and incomplete. This appears to be due to the maintenance of testicular LH receptors by PRL secreted from the grafts and probably also to the stimulation of GSH release from the in situ pituitary. In hamsters maintained in a long photoperiod, ectopic pituitary homografts can increase testicular LH receptor levels, concentrations of testosterone and FSH in the plasma, and the weights of the testes and the seminal vesicles. In contrast to the findings obtained in rats and mice, chronic hyperprolactinemia in the male hamster does not inhibit gonadotropin release or interfere with copulatory behavior. These findings are consistent with our earlier suggestion that PRL plays an important part in the regulation of gonadal function in the male hamster but indicate that testicular atrophy in a short photoperiod cannot be explained solely by a reduction in PRL release.

Prepubertal and young adult male rats were castrated and continuous intravenous infusions of testosterone were administered for 2–3 days. Blood samples were obtained at various intervals and serum concentrations of FSH and LH were determined. In 43-day-old males a dose of approximately 57 μg/day was required to suppress serum LH to levels observed in intact controls, and FSH was also suppressed at this dose. Within one day after termination of infusions serum LH and FSH concentrations had returned to (or exceeded) the levels observed in castrates infused with vehicle only. This was true even after a very large dose of testosterone had been administered (918 μg/day). In 59-day-old males serum LH was suppressed somewhat by 40 μg testosterone/ day and further by 200 μg/day. FSH did not appear to be suppressed as readily in these rats as compared to the younger animals. Further experiments in which castrated rats were injected with testosterone or testosterone propionate revealed marked differences in the mag...

Adult male golden hamsters maintained in a long photoperiod (14 h of light and 10 h of darkness) or in a short photoperiod (5 h of light and 19 h of darkness for 7 weeks) were castrated and either given one anterior pituitary transplant under the kidney capsule or sham-operated. Additional animals were castrated and grafted or sham-grafted at the time of transfer to the short photoperiod. Starting 2 weeks after castration, all animals were injected three times a week with 20 micrograms testosterone propionate (TP). After 3 weeks, the dose of TP was increased to 80 micrograms and, after an additional 2 weeks, to 320 micrograms per injection. Blood samples were collected 2 weeks after castration and 1 day after the last injection of 20, 80, and 320 micrograms TP. Short photoperiod reduced and pituitary grafts increased plasma PRL levels. Plasma testosterone levels were related to the dose of injected TP, but were not influenced by photoperiod or pituitary transplants. Before the onset of TP injections, plasma LH and FSH levels in grafted and sham-grafted hamsters did not differ. In each of the three photoperiod conditions, injections of TP were consistently less effective in suppressing plasma gonadotropin levels in pituitary-grafted animals than in sham-grafted controls. These results indicate that PRL modulates the effects of exogenous testosterone on LH and FSH release in adult castrated male golden hamsters, this effect of PRL is due to reducing the sensitivity of the hypothalamic-pituitary system to feedback inhibition by testosterone, and suppression of pituitary PRL release in short photoperiod may be partially responsible for the concomitant increase in the sensitivity of LH and FSH release to inhibition by testosterone.

The ontogeny of the pineal melatonin rhythm was determined in three rodent species. Pineal glands were obtained either during the day or during the expected peak in melatonin at night. In all species the rhythm was first detected during the second week of life. Investigations of the photic regulation of pineal melatonin revealed that light could inhibit the nocturnal increase in melatonin in all species by the end of the second week of life. These studies indicate that for these species the development of the rhythm in pineal melatonin and the development of the mechanism involved in the photic suppression of the nocturnal increase in melatonin are independent of the role the pineal gland plays in regulating reproductive function in response to changes in environmental lighting.

Hamster serum gonadotropins were measured by RIA at 4 h intervals during the estrous cycle. On the afternoon of proestrus both LH and FSH exhibited a surge, but unlike the situation in the rat and mouse FSH returned to “baseline” with LH by early evening of proestrus. Shortly following this return FSH concentrations increased and reached a second peak by noon on estrus which was equal in magnitude and longer in duration than that occurring on proestrus. FSH fell to its lowest levels on diestrus 2 (D2) and early proestrus. Serum gonadotropins were measured by RIA 6 h following unilateral ovariectomy on D2. A slight elevation of LH resulted while FSH increased to a level equal in magnitude to that found during the proestrous surge. In intact females administration of a total of 45 μg FSH in 5 injections on D2 resulted in ovulation of twice the normal number of eggs. The t½ of this rat FSH in the male hamster was found to be 122 minutes. The low levels of FSH during the cycle between D2 and proestrus, the la...

The Djungarian or Siberian hamster (Phodopus sungorus) is a seasonally breeding rodent in which maturation of gonadal function depends upon the photoperiod during rearing. It was hypothesized that the ability of short days to block testicular growth resulted from insufficient gonadotropin secretion during critical stages of development. This question was studied by measurement of serum concentrations of FSH, LH, PRL, and androgens by RIA systems validated for use in this species. Males reared from birth in long (16 h of light, 8 h of darkness) or short (10 h of light, 14 h of darkness) photoperiods were killed at 5- to 10-day intervals between 5 and 60 days of age. Regardless of photoperiod before 15 days of age, body and testes weights similarly increased. Serum concentrations of FSH and PRL gradually increased during this age period, although PRL concentrations were statistically higher in males under long days compared to those under short days. Circulating serum LH and androgen levels were high before 10 days of age, but decreased by 15-20 days of age in both photoperiods. Under long days, the period between 15 and 30 days of age was characterized by rapid body (1 g/day) and testicular (10-38 mg/day) growth, peak serum FSH concentrations (20-25 days), sustained elevation in serum LH and androgen concentrations, and further increases in serum PRL values. After 30 days of age, a reduced growth rate for body and testes occurred; serum FSH levels declined, while adult serum concentrations of LH, PRL, and androgens were attained. In contrast, hamsters exposed to short days from birth exhibited a slower rate of body and testicular growth by 20 days of age. Short days blocked peak FSH secretion and suppressed serum concentrations of LH and androgens after 20 days of age. PRL titers were significantly lower in short day compared to those in long day housed hamsters at all ages. These results support the hypothesis that the short day-induced suppression of gonadotropins and PRL secretion during development blocked gonadal maturation.

In the Syrian hamster, a sharp peak in pineal melatonin occurs toward the end of the dark period. In the present communication, we describe characteristics of this rhythm. First, the time of the initial increase in pineal melatonin is not altered by daily melatonin injections that induce gonadal regression. Second, the rapid decrease of pineal melatonin in the morning is not prevented by acutely extending the dark period. Third, a consistent nocturnal increase in pineal melatonin is not observed in a 20-h light, 4-h darkness lighting schedule. However, acute exposure to a longer dark period, at the appropriate time of day, allows expression of the pineal melatonin rhythm in these animals. Finally, the rhythm in pineal melatonin appears to be truly circadian and is tightly coupled to the circadian rhythm in running activity.

Male hamsters were maintained on long (14L:10D) or short (10L:14D) photoperiods. The serum concentrations of LH and FSH were reduced in the animals kept on the short photoperiod, and these animals had atrophied testes and sex accessories. Serum gonadotropin concentrations increased following castration in both long and short photoperiods, but gonadotropin secretion was inhibited by much smaller doses of testosterone in the males maintained on the short photoperiod as comapred to males kept on a long photoperiod. The levels of testosterone required to suppress serum gonadotropin levels in castrated animals corresponded reasonably well with the serum androgen levels observed in intact males, suggesting that serum androgen is a major regulator of gonadotropin secretion in the male hamster.

In confirmation of earlier work, daily melatonin infusions of 9- or 12-h duration inhibited testicular development in pinealectomized juvenile Djungarian hamsters, while daily infusions of 6-h duration did not prevent gonadal growth. Two other methoxyindoles (5-methoxytryptophol and 5-methoxytryptamine) had less than 4% of the activity shown by melatonin. When pinealectomized hamsters received two short duration (i.e. 3, 5, or 6 h) melatonin infusions each day, gonadal development was not inhibited if the two daily infusions were separated by a period of 2 or 3 h without melatonin treatment. Thus, the effects of separate pulses of melatonin were not additive in this paradigm. When single, short duration daily infusions of melatonin (5-h) or isoproterenol (6-h) were administered to pineal-intact hamsters, gonadal inhibition occurred only when the infusions were given at times that would lead to an expected overlap with and extension of the endogenous nocturnal melatonin peak. Significant inhibition of testicular growth could be obtained with as few as 2-4 days of melatonin treatment. These observations further support the importance of melatonin peak duration and also indicate other interesting temporal aspects of melatonin action in the Djungarian hamster.

Hamsters were maintained on a long photoperiod (14L:10D) and were injected once daily with melatonin (10-25 mug) or sesame oil. Males which received melatonin during the afternoon (e.g., 6.5-13.75 h after lights-on) showed regressed testes and decreased levels of serum LH and FSH after several weeks of treatment. Injections of the oil vehicle or injections of melatonin given in the morning (3 h after lights-on) had no detectable effect on testicular size or on serum gonadotropins. Females which received melatonin during the afternoon became acyclic after several weeks of treatment and showed a diurnal pattern of LH secretion. The acyclic females required 4-6 weeks to resume estrous cyclicity following termination of the melatonin injections. The effects of melatonin on gonadal function and on serum gonadotropin concentrations in both sexes were similar to the previously observed effects of prolonged exposure to short photoperiods. These results indicate that chronic daily injections of melatonin can depress reproductive function in hamsters and that the effectiveness of the injections is dependent upon the time of day at which they are administered.

Male rats were injected with gonadotropin antiserum (GTH A/S) or normal rabbit serum on days 1, 3, and 5 of life. The former animals had normal testis growth and spermatic development at maturity; however, they failed to impregnate normal females with which they cohabited. The reproductive failure of these rats may have been largely a result of an incomplete pattern of masculine sexual behavior. It was found that the experimental males achieved very few intromissions and ejaculations in sex behavior tests when compared to control males injected with normal rabbit serum. Since the phallus was underdeveloped in GTH A/S treated males, these data do not distinguish between a peripheral or central neural cause for the behavioral deficit. However, the same males showed high levels of feminine sexual behavior after estrogen-progesterone priming, suggesting that behavioral centers in the CNS had not been “masculinized” during neonatal life. Nevertheless, masculinization of the GTH release mechanism(s) apparently ...

Castrated neonatal male rats (6–9 days old) were treated with testosterone propionate and estradiol benzoate. Both steroids were capable of suppressing the increases in serum LH and FSH concentrations which normally follow castration. Serum levels of both gonadotropins were decreased by 8–9 hr after the injection of either 3 μg testosterone propionate or 0.2 μg estradiol benzoate. A study was also made of the effects of exogenous steroids in intact female neonates (5–10 days old). FSH levels could be decreased by either androgen or estrogen; however, the large degree of variability in serum LH concentrations observed in control females made it difficult to draw any firm conclusion regarding the effect of steroids on LH secretion in females. (Endocrinology 89:112,1971)

Measurements of pituitary gonadotropins in adult female rats showed that a sharp drop in hypophyseal FSH occurs at approximately the same time as the proestrous drop in LH. Between the morning and late afternoon of proestrus pituitary LH and FSH both decreased by approximately 50%. A similar decrease in hypophyseal LH and FSH was found to occur in pubertal rats on the day preceding their first ovulation. In the pubertal rats FSH and LH levels continued to fall, reaching their lowest levels at estrus and subsequently rising again. The data suggest that an acute release of pituitary FSH occui’s approximately concurrently with the ovulatory surge of LH. In further experiments follicular development was initiated i n immature rats by PMS treatment and ovuia-tion was obtained in response to injections of FSH given 2 days after the PMS. It was demonstrated that the ovulation could not have been solely due to LH contamination in the injected FSH. Also, the amount of FSH required for ovulation in the PMS-primed r...

The rate of secretion of progesterone and 20α-hydroxypregn-4-en-3-one (20α-OHP) into the ovarian vein, the concentration of luteinizing hormone (LH) in serum and the time of ovulation were determined in mature, cycling rats. Each progestin was determined by a double isotope method, and LH was measured by means of a radioimmunoassay. The secretory rates of both progestins at 2 pm on proestrus were 50% greater than during late diestrus. These increases occurred at least 2 hr before the occurrence of the LH surge. Thus, both progestins could have been involved in triggering the release of LH. Throughout most of the estrous cycle, serum LH concentration was very low; but during proestrus, between 4 and 6pm, the serum concentration rose many-fold, after which the concentration of serum LHdecreased rapidly. In contrast to LH, the secretion of the progestins, with the exception of the secretory rates at 4 am, continued to increase steadily after the LH surge until 8 am on estrus. After 8 am on estrus, progestin ...

Antiserum prepared against ovine LH neutralized both LH and FSH of ovine origin. Cross-reaction with LH and FSH of hamster origin was also observed. When the crude antiserum was injected in hamsters shortly prior to the proestrous critical period for gonadotropin release, ovulation was blocked. Following removal of most of the FSH-antibodies by adsorption with FSH, the ovulation-blocking potency of the antiserum was greatly reduced. No significant loss of anti-LH activity had occurred during adsorption. Thus, the results suggest a prominent role of FSH during the proestrous critical period for ovulation in the hamster. (Endocrinology 84: 236, 1969)

Serum concentrations of LH, FSH and prolactin were measured in neonatal rats. Intact male pups consistently showed low levels of FSH and LH, but serum levels of both these hormones were increased several-fold one day after castration. The response to castration was greater in 5- to 10-day-old males as compared to younger animals. Indeed, the gonadotropin levels in 5- to 10-day-old castrates were about as high as those seen in adult males which had been castrated for one day. Female pups displayed variable serum LH levels, but both LH and FSH levels were considerably higher than those observed either in males of the same ages or in adult diestrous females. Serum prolactin levels were low in pups of both sexes as compared to adult rats. (Endocrinology 88: 771, 1971)

Application of an anodal DC electrochemical stimulus to the ventral hippocampus (VHPC) produced changes in the multiple unit activity (MUA) of the arcuate (ARH) and ventromedial hypothalamic nuclei and in tonic and cyclic mechanisms controlling release of pituitary LH in female rats. In ovariectomized estrogen-primed rats, both hypothalamic nuclei responded to VHPC stimulation in less than 6 min with increases in MUA which persisted for hours. The effect was apparently due to the deposition of iron since stainless steel electrodes were effective while platinum electrodes were not. This response was dependent on the presence of an intact medial corticohypothalamic tract, and it did not occur in the dorsomedial hypothalamus, zona incerta or median eminence. Stimulation of the VHPC region which increased ARH neuronal activity also resulted in a significant decrease in plasma LH within 3 hr after its application. Stimulation of the caudate nucleus or mere insertion of electrodes into the VHPC did not lower pl...

The involvement of the ovaries during late pregnancy in the rat upon serum prolactin was investigated. Ovariectomy on day 17 or day 21 of pregnancy prevented the dramatic rise of prolactin found in sham-ovariectomized animals between days 21 and 23 of pregnancy. While animals ovariectomized on day 17 of pregnancy failed to carry viable fetuses beyond day 19 of pregnancy, rats ovariectomized on day 21 of pregnancy had viable pups in utero on day 23 of pregnancy. These data suggest that the rise in serum prolactin during late pregnancy is stimulated by ovarian factors.

The involvement of the ovaries during late pregnancy in the rat upon serum prolactin was investigated. Ovariectomy on day 17 or day 21 of pregnancy prevented the dramatic rise of prolactin found in sham-ovariectomized animals between days 21 and 23 of pregnancy. While animals ovariectomized on day 17 of pregnancy failed to carry viable fetuses beyond day 19 of pregnancy, rats ovariectomized on day 21 of pregnancy had viable pups in utero on day 23 of pregnancy. These data suggest that the rise in serum prolactin during late pregnancy is stimulated by ovarian factors.

A progonadal role of the pineal in both adult and juvenile male Djungarian hamsters has been demonstrated which confirms and extends previous reports. In short day-raised hamsters, stimulation of testicular development was achieved by transfer to long days. This stimulatory effect of long days was blocked by pinealectomy. Gonadal stimulation could be elicited in pinealectomized juveniles by timed daily infusions of melatonin. When given for 4 or 6 h daily at a dose rate of 0.83 or 83 ng/h, melatonin infusions stimulated testicular development and body growth. Measurement of plasma FSH and PRL indicated that this pubertal response resulted at least in part from a stimulatory effect of melatonin on the secretion of these hormones. Longer duration infusions (8 or 12 h) did not stimulate testicular development or body growth, and plasma FSH and PRL levels remained low. The efficacy of the short duration infusions did not appear to depend upon the time of day of administration. These findings are in close agreement with those of a previous study which investigated the antigonadal effect of long duration melatonin infusions. The present results demonstrate that melatonin administration can mimic the progonadal effects of the pineal in this species and add further support to the hypothesis that the pineal influences the reproductive system through changes in the duration of daily melatonin production.

Testicular function and pelage color are regulated by photoperiod in the Djungarian hamster. To investigate the critical daylengths of these functions, adult male hamsters were exposed to one of four photoperiods: 16 h of light, 8 h of darkness (16L:8D), 14L:10D, 12L:12D, or 10L:14D. 10L:14D and 12L:12D induced the winter molt and testicular regression, in contrast to 14L:10D which induced only the latter response, and 16L:8D which maintained the summer pelage and large testes. Melatonin injections administered 4, 2, or 0 h before lights-off to hamsters exposed to 16L:8D mimicked the effects in hamsters exposed to 10:14D, 12L:12D or 14L:10D, respectively, on pelage color and testicular weight. Based on previous observations, the elevated circulating melatonin levels resulting from these injections were expected to extend the endogenous melatonin peak. Thus, this finding suggests that the duration of circadian melatonin elevation is the critical parameter determining its effect not only on the gonads, but also on the pelage. Since 14L:10D induced testicular regression but not the winter molt, this study also investigated whether circulating FSH levels, known to affect testicular function, and PRL levels, which have been shown to affect pelage color, might be affected differently by 14L:10D. Both FSH and PRL levels were found to be suppressed in 14L:10D hamsters compared to those in 16L:8D hamsters, although the interval between the initial decrease and eventual recovery was less than that in 10L:14D hamsters. Thus, the differential responses of the pelage and gonads to 14L:10D do not appear to be based on selective suppression of FSH in this photoperiod. However, different responses to 14L:10D compared to 10L:14D may be related to the shorter period of suppression of both PRL and FSH by the 14L:10D daylengths.

Hamsters were maintained on a long photoperiod (14L:10D) and were injected once daily with melatonin (10-25 mug) or sesame oil. Males which received melatonin during the afternoon (e.g., 6.5-13.75 h after lights-on) showed regressed testes and decreased levels of serum LH and FSH after several weeks of treatment. Injections of the oil vehicle or injections of melatonin given in the morning (3 h after lights-on) had no detectable effect on testicular size or on serum gonadotropins. Females which received melatonin during the afternoon became acyclic after several weeks of treatment and showed a diurnal pattern of LH secretion. The acyclic females required 4-6 weeks to resume estrous cyclicity following termination of the melatonin injections. The effects of melatonin on gonadal function and on serum gonadotropin concentrations in both sexes were similar to the previously observed effects of prolonged exposure to short photoperiods. These results indicate that chronic daily injections of melatonin can depress reproductive function in hamsters and that the effectiveness of the injections is dependent upon the time of day at which they are administered.

In adult male hamsters, 2 months of exposure to a short photoperiod (5 h of light:19 h of darkness) caused testicular regression and a precipitous decline in plasma PRL, in agreement with earlier reports from other laboratories. Depressed release of PRL cannot be explained by a reduction in testicular steroidogenesis, because castration of males kept in a long photoperiod did not reduce PRL levels and administration of testosterone to males kept in a short photoperiod failed to reverse the decline in plasma PRL concentration. Treatment of such "regressed" animals with PRL, GH, or ectopic pituitary transplants stimulated growth of the testes and the accessory reproductive glands, increased the concentration of LH receptors in the testes, and elevated plasma testosterone levels. A single injection of 250 microgram PRL was sufficient to increase testicular LH binding, and chronic treatment with pituitary grafts completely reversed testicular regression. The effectiveness of exogenous PRL in stimulating testicular growth and LH receptors was significantly influenced by the timing of the injection. In some experiments, gonadotropin levels appeared elevated in animals injected with PRL, but these differences were not statistically significant. In hamsters with gonadal regression induced by exposure to a short photoperiod, daily administration of 20 microgram H and/or 150 microgram FSH had no apparent effect on testicular function. However, treatment with large doses of hCG and/or PMS gonadotropin resulted in significant stimulation of testicular growth and steroidogenesis. Chronic treatment of males maintained in a long photoperiod (14 h of light:10 h of darkness) with an inhibitor of PRL release, 2-Br-alpha-ergocryptine, resulted in a decreased weight of the testes and seminal vesicles. Administration of this inhibitor for a longer period (2 months) produced a significant increase in body weight but had little effect on testicular function. These results indicate that changes in the release of PRL (and possibly also GH) may plan an important role in mediating the effects of the photoperiod on testicular function in the golden hamster.

Hereditary dwarf mice (Snell dwarf, dw/dw and Ames dwarf, dfldf) are genetically prolactin-deficient and sterile. Treatment with prolactin (PRL) stimulates testicular growth and activity, and induces fertility in these animals. To elucidate the mechanism of this action of PRL, and to identify the possible physiological functions of this hormone during sexual maturation, we have examined the effects of treating male dwarf mice with PRL on plasma and pituitary levels of LH and FSH, on the concentration of testosterone (T) in testes and plasma, and on T production in response to hCG in vitro. Both Snell and Ames male dwarf mice had a significantly lower concentration of FSH in the pituitary and the peripheral plasma, and of LH in the pituitary, than did the normal adult males of the same strain. Treatment with ovine PRL, or with a PRL-producing ectopic homograft of the anterior pituitary from a normal animal, produced a significant elevation in plasma FSH levels in male dwarf mice of both strains. The same i...

The pineal melatonin rhythm has been proposed to represent a physiological signal which mediates the effects of photoperiod in a variety of seasonal breeding mammals. Studies in other species have led to the hypothesis that an endogenous circadian mechanism controls pineal melatonin content and can adapt the melatonin rhythm to reflect shifts in the prevailing light-dark schedule. In this report, photoperiodic regulation of the pineal melatonin content was investigated in the male Djungarian hamster, a species that is a long day breeder and photo-periodic from early life. In the first study, a daily pineal melatonin rhythm persisted after 6-8 weeks of constant dark conditions in a specific phase relationship to the onset of wheel running activity. Pineal melatonin levels increased around the time of activity onset, and high values persisted for 14-17 h. In subsequent studies, adult males [previously maintained under long days (16 h of light and 8 h of darkness)] were subjected to one of three conditions: 1) a 3-h advance in the onset of darkness, 2) a delay in the time of light onset, or 3) both alterations in the photoperiod. The acute 3-h advance in onset of the dark period caused an earlier rise in pineal melatonin production by approximately 1.5 h compared to that in controls. In contrast, the falling phase of the pineal melatonin rhythm was not altered by extension of the dark period. Therefore, aspects of the pineal melatonin pattern, i.e. the rising phase and peak duration, reflected changes in the photoperiod, while the falling phase anticipated the onset of the light period predicted by the previous light-dark schedule. These observations support the hypothesis that melatonin production by the pineal is an endogenous circadian rhythm and, as a physiological signal, may convey information about the daylength.

Hamster serum gonadotropins were measured by RIA at 4 h intervals during the estrous cycle. On the afternoon of proestrus both LH and FSH exhibited a surge, but unlike the situation in the rat and mouse FSH returned to "baseline" with LH by early evening of proestrus. Shortly following this return FSH concentrations increased and reached a second peak by noon on estrus which was equal in magnitude and longer in duration than that occurring on proestrus. FSH fell to its lowest levels on diestrus 2 (D2) and early proestrus. Serum gonadotropins were measured by RIA 6 h following unilateral ovariectomy on D2. A slight elevation of LH resulted while FSH increased to a level equal in magnitude to that found during the proestrous surge. In intact females administration of a total of 45 mug FSH in 5 injections on D2 resulted in ovulation of twice the normal number of eggs. The t1/2 of this rat FSH in the male hamster was found to be 122 minutes. The low levels of FSH during the cycle between D2 and proestrus, the large increase in serum FSH following unilateral ovariectomy, and the "doubled" ovulation in intact hamsters following the administration of FSH on D2, suggest that the serum FSH concentration on D2 is critical in determining the number of follicles which will be available for the subsequent ovulation.

A comprehensive developmental study of serum LH, FSH and androgen concentrations was carried out in male mice of two inbred strains. Gonadotropic hormone (GTH) levels rose 10-15 days prior to the pubertal increase in serum androgen with FSH preceding LH in this regard. In both strains GTH titer rose to a peak at 30 or 35 days and then steadily declined to adult levels which were strain-specific. Serum androgen was detected at low, relatively steady levels until the pubertal increase between 30 and 50 days postpartum. The results are interpreted as supporting the hypothesis that a shift in feedback sensitivity, occurring at about 20 days of age, may be involved in the onset of puberty in the male mouse.

To determine which parameter of the day/night pattern of pineal melatonin secretion is the critical component signaling daylength information in the Djungarian hamster, we have developed a method for giving timed sc melatonin infusions in pinealectomized juvenile males. When given for 12 h daily, as little as 10 ng melatonin (14 pg/min) consistently induced testicular regression within 12 days. However, 10 ng melatonin infused for 4 or 6 h daily did not inhibit gonadal development. The effects of these infusions on the reproductive system did not depend on the time of day at which melatonin was administered. In complementary experiments, the minimal daily infusion duration and the critical daylength for induction of testicular regression were determined. The critical length of infusion (7-8 h) was in close agreement with the estimated duration of melatonin secretion during the critical scotophase. These findings support the hypothesis that melatonin mediates the pineal-antigonadal effects of short day exposure in the Djungarian hamster. Furthermore, the data strongly suggest that duration is the feature of nighttime melatonin release that is most important for photoperiodic time measurement in this species.

Male hamsters were maintained on long (14L:10D) or short (10L:14D) photoperiods. The serum concentrations of LH and FSH were reduced in the animals kept on the short photoperiod, and these animals had atrophied testes and sex accessories. Serum gonadotropin concentrations increased following castration in both long and short photoperiods, but gonadotropin secretion was inhibited by much smaller doses of testosterone in the males maintained on the short photoperiod as compared to males kept on a long photoperiod. The levels of testosterone required to suppress serum gonadotropin levels in castrated animals corresponded reasonably well with the serum androgen levels observed in intact males, suggesting that serum androgen is a major regulator of gonadotropin secretion in the male hamster.

Prepubertal and young adult male rats were castrated and continuous intravenous infusions of testosterone were administered for 2-3 days. Blood samples were obtained at various intervals and serum concentrations of FSH and LH were determined. In 43-day-old males a dose of approximately 57 mug/day was required to suppress serum LH to levels observed in intact controls, and FSH was also suppressed at this dose. Within one day after termination of infusions serum LH and FSH concentration had returned to (or exceeded) the levels observed in castrates infused with vehicle only. This was true even after a very large dose of testosterone had been administered (918 mug/day). In 59-day-old males serum LH was suppressed somewhat by 40 mug testosterone/day and further by 200 mug/day. FSH did not appear to be suppressed as readily in these rats as compared to the younger animals. Further experiments in which castrated rats were injected with testosterone or testosterone propionate revealed marked differences in the magnitude and time course of the action of these compounds upon LH release. It is suggested that the rate of metabolism of these steroids is important in limiting their effects, especially when administered by single daily injections. Furthermore, it appears that "recovery" of the hypothalamic-pituitary system from exposure to high concentrations of testosterone is rapid following clearance of the steroid from the blood.

We have examined the effects of PRL-producing ectopic pituitary grafts on plasma FSH and LH levels in male golden hamsters with anterior (AHD) and anteolateral hypothalamic deafferentation (ALHD). After hypothalamic deafferentation or sham deafferentation, hamsters received pituitary transplants under the kidney capsule. In the first experiment, the animals were unilaterally nephrectomized 5-6 weeks after transplantation to remove the ectopic pituitaries. In a second experiment the ectopic pituitaries remained under the kidney capsule for the duration of the experiment. Serial blood samples were collected before and after pituitary transplantation and after transplant removal. In the first experiment, ALHD resulted in a significant (P less than 0.01) suppression of plasma LH levels that lasted through the entire study, but had no significant effect on plasma FSH levels or on testicular and seminal vesicle weight. The presence of pituitary grafts resulted in a significant (P less than 0.01) elevation in plasma FSH levels in the sham-deafferented hamsters but had no effect on plasma FSH levels in those with ALHD. After transplant removal there was a significant (P less than 0.01) drop in plasma FSH levels in the hamsters that had been sham deafferented but not in those with a deafferented hypothalamus. Plasma LH levels were not affected by the pituitary transplants or their removal in either group. In the second experiment, ALHD but not AHD significantly (P less than 0.05) reduced testicular and seminal vesicle weights. Transplantation of an anterior pituitary elevated plasma FSH levels in the control group but not in the AHD and ALHD groups. Both types of deafferentations suppressed plasma LH levels below the limit of sensitivity of the assay for the duration of the experiment. Plasma PRL levels were markedly elevated only in the groups with an ectopic pituitary transplant. These results suggest that the anterior connections of the medial basal hypothalamus are necessary in order for PRL-producing pituitary transplants to influence FSH secretion from the in situ pituitary.

The effect of pinealectomy on reproductive function was examined in the Turkish hamster, Mesocricetus brandti. Pinealectomy resulted in testicular regression in this species. This result was unexpected since pinealectomy prevents short day-induced regression of the gonads in the closely related Syrian hamster, Mesocricetus auratus. Decentralization of the superior cervical ganglia and exposure to continuous illumination also caused testicular regression in the Turkish hamster. These manipulations are believed to block pineal melatonin synthesis. In each case (i.e. pinealectomy, decentralization of the superior cervical ganglia, exposure to continuous illumination), the testes regressed after approximately 3--9 weeks and underwent recrudescence after approximately 16--28 weeks. This cycle of testicular regression and recrudescence was similar to that observed in Turkish hamsters exposed to a short day photoperiod. In further experiments, the effects of exogenous melatonin were studied in Turkish and Syrian hamsters. The results of these studies suggest that, in Turkish hamsters, pineal melatonin may be involved in both the maintenance of testis function during exposure to a long day photoperiod and also in the suppression of reproductive function in short days. This is in contrast to the Syrian hamster, in which melatonin appears to be important only for inhibition of gonadal function in short days.

Syrian hamsters were maintained on a long photoperiod (lights on 0600-2000 h) and injected daily with melatonin (10 microgram per injection) for 6-7 weeks. Females injected at 2000 h or 0400 h ceased estrous cyclicity, and males treated similarly at these times had reduced testis weights by the sixth week of treatment. Daily administration of the same dosage of melatonin at 2400 and 0100 h failed to interrupt estrous cyclicity or reduce testis weight. These observations suggest that exogenous melatonin administered at 2000 h or 0400 h may be temporally adding to endogenous circulating melatonin, inducing gonadal quiescence.

The significance of ovarian estradiol (E2) and progesterone secretion in the regulation of pituitary LH and FSH secretion and pituitary responses to LHRH was investigated in the hamster. Cycling females showed increased LH and FSH responses to LHRH on the morning of proestrus as compared to the responses observed on diestrus day 2. Pituitary responsiveness to LHRH declined on the evening of proestrus, after the preovulatory LH/FSH release. The secondary increase in serum FSH concentration on the morning of estrus was accompanied by a selective increase in the pituitary FHS response to exogenous LHRH. Hamsters ovariectomized (ovx) on diestrus day 2 exhibited daily afternoon LH surges but not FSH surges for at least 10 days after ovx. The magnitude of the LH surges in ovx hamsters was approximately 30-50% of that observed in proestrous females. The pituitary LH response to exogenous LHRH in ovx animals was about 25% as great as in proestrus hamsters. Serum FSH concentrations in ovx females increased by only 30% after LHRH injection, while similar treatment with LHRH resulted in 3- to 4-fold increments in serum FSH in proestrous hamsters. Implantation of E2 capsules in ovx hamsters resulted in increased gonadotropin responses to exogenous LHRH. Serum LH concentrations in the E2-implanted, LHRH-injected animals were as great as those observed after LHRH injection in proestrous females. Administration of LHRH, LH, or progesterone on the morning of proestrus failed to detectably alter the timing or magnitude of the proestrus afternoon FSH surge. The present results suggest that the increasing serum titers of estrogen on diestrus and early proestrus result in increased pituitary sensitivity to LHRH, and this increased sensitivity probably contributes to the magnitude of the preovulatory LH surge. The increases in LH and progesterone which occur during the afternoon do not seem to be responsible for triggering the proestrous FSH surge.