Milton H. Stetson

Hormonal regulation of the annual reproductive cycle of golden hamsters.

Abstract The seasonally breeding golden hamster, Mesocricetus auratus , is a photoperiodic species. Reproduction is confined to the spring and summer months and is curtailed by the short days of fall and winter. The photoperiodic response is dependent on an endogenous circadian sensitivity to light. Reacquisition of reproductive activity occurs spontaneously, even in the complete absence of light. These animals are refractory to the inhibitory effects of short days. Refractoriness is terminated by exposure to long days for a period of 11 or more weeks, after which the animals are once again sensitive to short days. The short-day response in the male is characterized by testicular regression induced by a decline in pituitary and circulating titers of LH, FSH, and PRL. These animals demonstrate an increased sensitivity to negative-feedback effects of exogenous (and, presumably, endogenous) testosterone that is associated with a decreased castration response. The reproductively inactive female fails to ovulate. Estrous cycles are absent, supplanted by a daily surge of LH and FSH. Spontaneous recovery of testicular function is associated with an increase in circulating titers of LH, FSH, PRL, and testosterone, and a resumption of spermatogenic activity. In the female the daily surge of LH and FSH is replaced by the 4-day estrous cycle. The pineal gland is essential for the occurrence of photoperiodically induced changes in the hamsters reproductive cycle. The pineal produces the indole, melatonin, with peak synthesis and release occurring during the subjective night. Administration of melatonin, by injection, at the appropriate time of day, will induce gonadal regression in both intact and pinealectomized hamsters maintained on normally stimulatory photoperiods. Subcutaneous melatonin implants, however, prevent gonadal regression induced by short photoperiods or melatonin injections. Melatonin implants also prevent photoinduced gonadal recrudescence. Constant release melatonin implants may function by interfering with target tissue sensitivity to endogenous melatonin. The target tissue for melatonin and the mechanism by which the pineal is involved in the photoperiodic response remain unknown.

Detailed Diurnal Rhythm of Sensitivity to Melatonin Injections in Turkish Hamsters, Mesocricetus brandti

A diurnal rhythm of sensitivity to exogenous melatonin was defined in adult male Turkish hamsters, Mesocricetus brandti. Melatonin was administered daily by subcutaneous injections (15 μg in 0.1 ml 10% ethanolic saline) for 10 weeks in animals exposed to 16 L:8 D. As in golden and Djungarian hamsters, two periods of melatonin sensitivity were identified. The first, in late afternoon, persisted for 6 hr, from 7 hr to 1 hr before lights off. The second period was briefer, of only 2 hr duration in the late night, terminating at the time of lights on. Melatonin injections given during these sensitive periods promoted testicular regression in most animals; melatonin administered at other times of the day was without effect on testicular function in most animals of these groups. Gonadal regression induced by properly timed melatonin injections was rapid, in many groups nearly complete in 6 to 7 weeks. The results are discussed in relation to the function of pineal melatonin in photoperiodic time measurement in hamsters.

Reproductive Refractoriness in Hamsters: Environmental and Endocrine Etiologies

The golden hamster, Mesocricetus auratus, is typical of temperate-latitude rodents in its annual reproductive cycle. When housed under natural photoperiods, adult hamsters are reproductively active in the spring and summer months but inactive during the fall and winter (Vendrely et al. 1971a,b, 1972; see Figure 11-1 A). Laboratory experiments have demonstrated that the most important Zeitgeber for the annual reproductive cycle is the yearly variation in photoperiod: the entire breeding cycle can be reproduced under artificial daylengths when temperature and nutritional variables are held constant (Reiter 1968a). The critical daylength for the golden hamster is 12.5 hours of light per day: light cycles containing more than 12.5 hours of light are perceived as long days and are favorable to gonadal growth and maintenance, while those shorter than 12.5 hours of light result in gonadal regression (Gaston and Menaker 1967). The reproductive changes brought about by varying the daylength are mediated by changes in gonadotropin release (Berndtson and Desjardins 1974).

Growth hormone is thyrotropic in Fundulus heteroclitus.

Abstract Ovine growth hormone elevates serum T 4 in both intact and hypophysectomized Fundulus heteroclitus . This activity appears to be an intrinsic property of oGH and is not due to contamination by either oTSH or other glycoprotein hormones. Moreover, while hypophysectomy prevents the oTSH-induced increase in serum T 4 found in intact fish, simultaneous treatment with oGH fully restores the normal response. Unlike oTSH, oGH lacks the ability to elevate thyroidal radioiodine uptake in hypophysectomized Fundulus . Thus, oGH appears to act at a level beyond the iodine trap stimulating thyroid function.

Pituitary autotransplants in Fundulus heteroclitus: Effect on thyroid function

Serum thyroxin (T4) levels are reported for Fundulus heteroclitus undergoing hypophysectomy and pituitary autotransplantation. Hypophysectomy significantly lowered serum T4 compared to levels found in sham-operated (P < 0.01) and unoperated controls (P < 0.01). In fish receiving pituitary autografts serum T4 levels were significantly enhanced over either control group (P < 0.01; P < 0.001). These results strongly suggest that: (a) pituitary stimulation is necessary for maintenance of normal T4 levels in this species, and (b) pituitary thyrotropin release is under inhibitory hypothalamic control.

A test of the coincidence and duration models of melatonin action in Siberian hamsters: the effects of 1-hr melatonin infusions on testicular development in intact and pinealectomized prepubertal Phodopus sungorus.

The pineal hormone melatonin is known to play an important role in mediating photoperiodic messages to the reproductive system in seasonal breeding animals. Our goal was to test, in a single experimental paradigm, two hypotheses that have been forwarded to describe how the circadian rhythm of pineal melatonin transmits photoperiodic information to the reproductive system: 1) induction, i.e., a short‐day effect, occurs when secreted melatonin and a circadian rhythm of sensitivity to melatonin coincide in time; 2) induction occurs following exposure to elevated circulating melatonin levels for a prescribed duration. In order to determine the relative validity of these hypotheses, we investigated the testicular maturation response to 1‐hr daily infusions of 10, 25, and 50 ng of melatonin in pinealectomized intact and prepubertal Siberian hamsters (Phodopus sungorus). Animals received, beginning on day 15 of life, programmed subcutaneous infusions of melatonin or vehicle at one of five time points (19:00–20:00, 20:00–21:00, 21:00–22:00, 24:00–01:00, and 03:00–04:00 hr) for 15 days. In animals gestated and raised in a long photoperiod (LD16:8=16L, where L is the duration of light in hours, and D that of dark), melatonin infusion right after lights off (20:00–21:00 hr) significantly retarded gonadal maturation; this dose was ineffective at other times tested. Doses of 10 and 25 ng melatonin were ineffective at all time points. Identical results were obtained in prepubertal hamsters gestated in a short photoperiod (LD10:14=10L) and raised in 16L; these results were independent of the presence or absence of the pineal gland. In animals gestated and raised in 10L, melatonin infusions failed to suppress testicular development beyond that induced by the photoperiod; testicular development was maximally suppressed in all groups. The results of these investigations are best explained under the experimental conditions employed here: 1) the photoperiodic gonadal response in juvenile Siberian hamsters is regulated by the coincidence in time of exogenously administered melatonin with an intrinsic rhythm of sensitivity to melatonin, which, under the constraints imposed by our experimental design, occurred at 20:00–21:00 hr; and 2) the duration of the melatonin signal alone, equal in all groups, cannot explain the results.

Age, Photoperiodic Responses, and Pineal Function in Meadow Voles, Microtus pennsylvanicus

We tested whether juvenile males of Microtus pennsylvanicus were more sensitive than adults to the suppressive effects of short photoperiods. Voles were transferred to short photoperiods (10L:14D) at 20 or 80 d of age, and 60 d later (i.e., at 80 or 140 d) the animals were killed at intervals throughout the day and night. Pineal glands were collected for measurement of melatonin, and the testes were weighed. There were no differences in paired testicular weights of 80 and 140 d old animals held on long days (median testicular weights: 1,953 and 1,843 mg). In contrast, median testicular weights of voles held on short days were 504 and 1,112 mg, respectively, at 80 and 140 d of age; the testicular weights of both groups were significantly different from their age‐matched controls (P .001, two‐sample t‐tests on log transformed data). The responses of the two age groups were compared by normalizing the individual values by the mean and variance of the respective long‐day controls. This comparison suggests that the responsiveness to photoperiod decreases as the animals age (t‐test, P= .01). Duration and amplitude of the nocturnal rise in pineal melatonin content were similar in differently aged animals. In two experiments, voles were injected daily with melatonin from 20 to 80 or 80 to 140 d of age. Melatonin‐injected animals had smaller testes than did saline‐injected controls (ANOVA: P= .01), and injections were more effective in the afternoon than in the morning (P= .01). Comparison of the effectiveness of short day and melatonin injections in juvenile and adult voles suggests that while short days inhibited testicular development of young animals more than it induced regression of adults, this decrease in responsiveness may involve factors other than alterations in the nocturnal pattern of melatonin production.

Maternal transfer of photoperiodic information in rodents

Abstract In many photoperiodic rodents the maternal system transmits photoperiodic information to the young in utero. The young then use this information in an integrated reproductive response to prevailing photoperiodic conditions at the time they leave the nest. Thus it is quite common for weaned young of the same species to respond differently to the same photoperiod, the response dependent on the photoperiod perceived by the dam during gestation. The precise mechanism by which transfer of photoperiodic information from mother to young is accomplished is not entirely known. Central to successful transmission of photoperiodic information is the maternal pineal gland and its hormone, melatonin; in the absence of these no information is transferred to the young, while daily treatment of the pregnant pinealectomized mother with exogenous melatonin has led to successful transmission from mother to young of short-day, but not long-day, information. Reproductive development of newly weaned young, rendered incapable (by pinealectomy) of responding directly to ambient photoperiod, or functionally pinealectomized by exposure to continuous illumination (LL), directly reflects gestational photoperiod seen by the mother; pinealectomized or LL-exposed weanlings gestated on short (inhibitory) days experience a slow rate of reproductive development while similarly treated weanlings that were gestated on long (stimulatory) days undergo rapid gonadal and sex accessory gland growth. Many questions remain about the mechanism of maternal transfer of photoperiodic information to her young. Answers to these questions can only be determined after attaining a thorough knowledge of the basic properties of the photoperiodic time measurement system of the species being investigated.

The effects of prolactin and TSH on thyroid function in Fundulus heteroclitus

Treatment with ovine prolactin significantly reduces serum T4 in Fundulus heteroclitus, both in control (P < 0.05) and TSH-stimulated fish (P < 0.01), without affecting thyroidal radioiodine uptake. Prolactin also reduces the loss of radioiodine from F. heteroclitus (P < 0.001). This loss is markedly increased by ovine TSH (P < 0.001), but only in the absence of prolactin. Although other explanations are possible, prolactin is probably goiterogenic in F. heteroclitus, lowering thyroidal secretion of T4 as well as inhibiting the peripheral effects of this hormone.

A test of the coincidence and duration models of melatonin action in Siberian hamsters. II. The effects of 4- and 8-hr melatonin infusions on testicular development of pinealectomized juvenile Siberian hamsters (Phodopus sungorus).

In a previous paper we demonstrated that properly timed 1‐hr infusions of 50 ng melatonin effectively suppressed testicular development in juvenile Siberian hamsters. Only melatonin infused between 20:00 and 21:00 hr was effective in animals exposed to 16L (lights off 20:00 hr). In this paper we further investigate the importance of the coincidence and duration hypotheses of daily exposure of melatonin. Prepubertal Siberian hamsters received either 4‐ or 8‐hr melatonin infusions at various times either on long photoperiod (LD 16:8=16L) or on short photoperiod (LD 10:14=10L). Daily 8‐hr melatonin infusions suppressed testicular development in both photoperiods. Daily 4‐hr, 50 ng/hr, melatonin infusions at 17:00–21:00 hr inhibited testicular growth in 16L and daily 4‐hr melatonin infusions (either 50 ng/h or 50 ng/day) inhibited testicular growth at 17:00–21:00 hr in 10L. We also tested the efficacy of an interrupted melatonin infusion of long duration (8 hr). Pinealectomized prepubertal male Siberian hamsters, born on 16L, were infused with two signals of 4 hr separated by an interval of 2 hr. Melatonin‐infused groups had significantly inhibited testicular growth compared to vehicle‐infused animals. Testicular development was maximally inhibited only in those groups in which the period of melatonin sensitivity identified in the previous paper (20:00–21:00 hr) overlapped or immediately followed a period of melatonin infusion. Considering the restrictions of the experimental design employed in these studies, the results are best explained by the hypothesis that the photoperiodic gonadal response in juvenile Siberian hamsters is regulated by the coincidence in time of exogenously administered melatonin with an intrinsic rhythm of sensitivity to melatonin, which occurred at 20:00–21:00 hr. The duration of the melatonin signal alone can not explain the results.