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M H Stetson

Publications and source records attributed to M H Stetson.

At least 73 records · Page 4Linked to original sources
PubMed1986

Naloxone administration to female hamsters advances puberty by enhancing luteinizing hormone release.

In the female hamster, a daily rhythm of gonadotropin release begins almost 3 weeks prior to the initiation of 4-day estrous cycles. A temporal relationship exists between the onset of this cyclic release of gonadotropin and age at puberty. We hypothesized that since opiate agonists depress circulating gonadotropins and antagonists increase them in both adult and immature rodents, endogenous opiates may influence the mechanism controlling cyclical gonadotropin release in the prepubertal female hamster and thus affect rate of sexual maturation and hence the age at puberty. This proposal was tested by chronic administration of naloxone (NAL), an opiate receptor antagonist. We predicted that NAL might induce the early initiation of daily surges of luteinizing hormone (LH) if endogenous opiates inhibit sexual maturation. Naloxone was injected daily (50 mg/kg body wt) at about 1300 hr from Days 1 through 17 of age. The NAL injections increased serum LH and significantly advanced the age at which first estrus vaginal discharge was observed (32 vs 38 days for saline-injected controls in Experiment I and 31 vs 37 days in Experiment II). However, the NAL injections did not correspondingly advance the age of initiation of endogenously generated daily cycles of circulating LH. We conclude that blockade of opiate receptors accelerates sexual maturation by directly inducing the release of LH and not by advancing the age of initiation of endogenous gonadotropin surges.

Age Factors↗
PubMed1986

Effects of exogenous and endogenous melatonin on gonadal function in hamsters.

In this article we review the functions of the pineal gland and of pineal melatonin in regulating gonadal activity in three species of hamster, the golden (Syrian) hamster (Mesocricetus auratus), the Turkish (Brandt's) hamster (Mesocricetus brandti), and the Djungarian (Siberian, or hairy-footed) hamster (Phodopus sungorus sungorus). Some experimental treatments elicit the same response in all three species; this is especially true for the regulation of the nocturnal elevation of pineal melatonin content and release and for the gonadal response to timed melatonin injections in intact animals throughout the day. In other experimental paradigms, species differences are paramount; pinealectomy or subcutaneous implants of melatonin in intact individuals have different effects on the photoperiodic gonadal response in all species. Yet in all hamster species investigated, the pineal gland and its hormone melatonin constitute part of the central mechanism whereby environmental information (photoperiod) is transduced to neuroendocrine signals responsible for the functional integrity of the reproductive system.

Animals↗
PubMed1985

A mathematical method for estimating paired testes weight from in situ testicular measurements in three species of hamster.

Linear regression analysis was used to determine the correlation between testicular volume (calculated from measurements of length and width) and weights of paired testes. Line equations that can be used to predict weights of paired testes from the volume of one or both testes are given for three species of hamster (Mesocricetus auratus, Mesocricetus brandti, and Phodopus sungorus sungorus). Because these parameters are highly correlated (r greater than or equal to 0.97), the line equations provide a highly reliable means of estimating weights of paired testes from in situ surgical measurements. Weights of testes predicted in this manner are highly correlated with actual weights of paired testes (r greater than or equal to 0.96). The surgical method for measurement of the gonads is also described. This technique for estimating paired testicular weight allows considerable reduction in the number of animals required for long-term experiments and also permits longitudinal studies of individual animals.

Animals↗
PubMed1985

Photoperiodic regulation of thyroid responsiveness to TSH in Fundulus heteroclitus.

Treatment of the killifish, Fundulus heteroclitus, with ovine thyrotropin (oTSH) produced elevations in serum thyroxin (T4) that varied seasonally with a strong inverse correlation to environmental photoperiod (P less than 0.001). At 20 degrees C, oTSH (0.2 IU) increased serum T4 to levels ranging from 1.5 micrograms/100 ml in midsummer to 7.5 micrograms/100 ml in midwinter despite relatively stable resting levels throughout the two years of study. Similar rates of clearance of T4 in summer and winter suggest that an alteration in thyroid response to oTSH accounts for this change. The serum T4 response to oTSH is both photoperiod- and temperature-dependent. In laboratory studies, fish exposed to a short artificial photoperiod (LD 8:16) for one month responded to oTSH with T4 levels about twice as high as those in fish exposed to long days (LD 14:10). On the other hand the T4 response to oTSH was sharply reduced at 5 degrees C from that seen at 20 degrees C. This suggests that, in nature, seasonal changes in photoperiod and temperature have opposite effects on thyroidal responsiveness to TSH. Overall, the annual variation in the T4 response to oTSH appears to be driven by photoperiod, inasmuch as the alterations preceded major temperature changes in the wild and can be shown to occur at constant temperature in captivity. Photoperiodic induction of changes in thyroid sensitivity may aid in the maintenance of basal T4 levels under changing thermal conditions.

Animals↗
PubMed1985

Photoperiod-dependent negative feedback effects of thyroid hormones in Fundulus heteroclitus.

In Fundulus heteroclitus, an annual cycle in the response of the thyroid to ovine thyroid-stimulating hormone (oTSH) is characterized by maximal thyroxin (T4) secretion in mid-winter and minimal T4 secretion in summer. Four daily injections of oTSH, given in winter caused serum T4 to plateau at elevated levels for several days, while in summer fish similar treatment resulted in far more fluctuating titers of serum T4; maximum levels were similar in both groups. The difference in sustenance rather than magnitude of Peak T4 led to an examination of the negative feedback effects of thyroid hormones as they might relate to these seasonal changes. Radioiodine uptake by thyroid follicles served as a simple, but effective bioassay for endogenous TSH. Fish collected in summer were more sensitive to negative feedback of T3 than those collected in winter; feedback effects of T4 in the two groups were not significantly different. The effects of specific photoperiods on negative feedback sensitivity to T3 and T4 were also tested. Exposure of winter fish for one month to long days (LD 14:10) enhanced the degree of reduction of iodine uptake caused by T4 in the aquarium water (10 micrograms/100 ml). Negative feedback in short-day (LD 8:16) winter fish was not demonstrated. It is concluded that long days increase and short days diminish the negative feedback sensitivity of the hypothalamus-pituitary axis to thyroid hormones in F. heteroclitus. Such photoperiodically induced changes may act to aid in the year-round maintenance of T4 levels necessary for seasonal adaptation and survival.

Animals↗
PubMed1985

Functional specificity of gonadotropin and thyrotropin in Fundulus heteroclitus.

Gonadotropic hormones (GTHs) and thyrotropic hormones (TSHs) reportedly bear close evolutionary and structural relationships, and the thyroid appears to be active in reproduction in some fish species. We tested the sensitivity of the thyroid of Fundulus heteroclitus to glycoprotein hormones from mammalian and piscine sources. Six mammalian glycoprotein hormones, including four gonadotropins and two thyrotropins, produced dose-dependent elevations in serum thyroxin. A release of endogenous gonadotropins was elicited by injecting GnRH. This resulted in gonadal stimulation, with no alteration in circulating thyroxin levels and the rate of radioiodine uptake. We also treated fish with partially purified salmon gonadotropin (SG-G100). The gonadotropic actions of this extract were confirmed by steroid elevations, and again T4 and 125I uptake remained at resting levels. The lack of response of the thyroid gland to fish gonadotropins suggests that TSH receptors in Fundulus heteroclitus can differentiate between endogenous thyrotropin and gonadotropin(s), even though most heterologous glycoprotein hormones are thyrotropic.

Animals↗
PubMed1985

Effects of ovariectomy on clock-timed daily gonadotropin rhythms in prepubertal golden hamsters.

Daily rhythms of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are measurable in the serum of prepubertal female golden hamsters by 17 days after birth. These rhythms, which are characterized by peak levels at 1700 h, persist until they are replaced by a 4-day rhythm as ovulatory cycles begin, approximately 3 wk later. We have tested the proposition that the ovaries are required for the onset and maintenance of clock-timed gonadotropin release by removing the ovaries and measuring the levels of LH and FSH in prepubertal hamsters. Ovariectomy was performed both before and after the onset of the rhythm and the effect of removal was determined by subsequent collection of blood samples during the mid- to late-prepubertal period. Ovariectomy on 7, 10 or 13 days after birth results in tonic levels of LH and FSH in blood samples collected at 1400, 1700 and 2000 h on Days 17 through 29. Sham-operated or intact controls had significantly elevated levels of these hormones at 1700 h. Ovariectomy on Day 21 and killing on Day 25 at the same times of day abolished the rhythm of serum LH measured in sham-ovariectomized controls. Ovariectomy on Day 21 and killing on Days 26, 28 or 30 at hourly intervals resulted in variable but nonrhythmic patterns of circulating LH. Thus, ovariectomy before the initiation of clock-timed gonadotropin release prevented its initiation; ovariectomy after its initiation abolished the rhythm. These results show that the ovary provides an essential "message" to the brain-pituitary axis for the initiation and maintenance of clock-timed gonadotropin release in prepubertal females.

Animals↗
PubMed1985

Testicular function and pelage color have different critical daylengths in the Djungarian hamster, Phodopus sungorus sungorus.

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.

Animals↗
PubMed1985

Neonatal androgen abolishes clock-timed gonadotrophin release in prepubertal and adult female hamsters.

Testosterone propionate (100 micrograms) or oil was injected within 24 h of birth. At 25 days of age blood samples were obtained at 14:00, 17:00 and 20:00 h. There was a significant increase in serum LH, FSH and progesterone concentrations between 14:00 and 17:00 h in the controls, followed by a decrease at 20:00 h. These rhythms were absent in testosterone propionate-injected animals. Ovariectomy of adults was followed by similar increases of LH and FSH in androgenized and oil-injected females (gonadectomy response) but the large surge of gonadotrophins observed in controls 1 day after implantation of an oestradiol-containing capsule (positive feedback) was not detectable in androgenized females. These results show that the initial effects of neonatal androgenization on cyclic gonadotrophin release in the female are present before puberty and are separable from effects on steroid modulation of gonadotrophin secretion.

Animals↗
PubMed1984

Prolactin profiles during estrous cycle and pregnancy in hamster as measured by homologous RIA.

The development of a homologous radioimmunoassay (RIA) for the measurement of serum hamster prolactin (PRL) has facilitated our studies on monitoring the secretion patterns of this hormone during different states of reproduction. Four salient findings emerge from the present studies. First, rapid cardiac puncture without anesthesia does not affect serum PRL levels during selected times in the estrous cycle or during pregnancy. Second, serum PRL levels during the estrous cycle describe a daily rhythm of PRL release with maximum serum concentrations each day occurring in the afternoon and the highest of these occurring on proestrus (day 4 of cycle). Third, PRL release during pregnancy is characterized by a single surge each day; although the time of maximum PRL release varies, levels increase during the hours of darkness and decline to basal during the light. Fourth, the amount of PRL released during pregnancy diminishes with time; on day 15 the total amount of PRL in the serum is less than 10% of that measured on day 5.

Animals↗
PubMed1984

Twenty four-hour rhythm of gonadotropin release induces cyclic progesterone secretion by the ovary of prepubertal and adult golden hamsters.

Cyclic release of gonadotropin (GTH) begins on day 16 or 17 of age, about 2 weeks before ovulatory estrous cycles are initiated in female golden hamsters (Mesocricetus auratus). The daily cycles of LH and FSH are characterized by surges that peak at about 1700 h. The timing of this daily surge is similar to that observed in adult photoperiod-induced anovulatory or lactating females and to that of ovulatory females on the afternoon of proestrus. The purpose of this investigation was to determine if initiation of cyclic GTH release during the prepubertal period results in significant changes in ovarian function. A second objective was to examine the similarity between cyclic secretion of GTH in the prepubertal animal and that in the short day anovulatory adult. Groups of prepubertal females were killed throughout a 24-h period on days 13-14, 19-20, and 27-28, i.e. both before and after the expected onset of cyclic GTH release. LH did not vary significantly on days 13-14, but thereafter showed a significant rhythm, with a maximum at 1700 h. Levels of progesterone (P) were low and nonvarying on days 13-14, but a significant rhythm was measured in samples collected on days 27-28. When samples were collected from photoperiod-induced anovulatory adults, the rhythm of circulating LH and P was similar to that in the late prepubertal female. Ovariectomy of 22-day-old females resulted in low and tonic levels of P when the animals were killed 3 days later at 1400, 1700, and 2000 h, showing that the cyclical rhythm of P measured in these samples results from ovarian, not adrenal, secretion. The response of the daily cycle of LH to phenobarbital (PB) blockade in prepubertal females was identical to that in the adult: the LH surge did not occur on the afternoon of PB injection, but reappeared 24 h later at the expected time. The late afternoon surge of P was also blocked by PB administration. These results show 1) that the initiation of cyclic GTH secretion by the prepubertal female hamster influences ovarian maturation as reflected by changes in the levels and patterns of secretion of P; 2) that the rhythms of LH, FSH, and P secretion in the prepubertal female are similar to those in the photoperiod-induced anovulatory adult; and 3) that the response of the prepubertal female to barbiturate blockade is identical to that of the adult.(ABSTRACT TRUNCATED AT 400 WORDS)

Aging↗
PubMed1983

Prolactin--thyroid interaction in Fundulus heteroclitus.

Injections of adult Fundulus heteroclitus with 0.2 IU of oTSH over 4 days caused an increase in serum T4, with no effect on serum T3. Administration of 12 IU of oPRL in conjunction with the same dosage of TSH prevented the TSH-induced rise in T4, without affecting serum T3 levels. The failure of TSH to cause T4 levels to rise in the presence of PRL may result from either inhibition of release or acceleration of metabolic clearance of T4. We therefore conducted three experiments to examine potential effects of PRL on the kinetics of peripheral clearance of thyroid hormones. 125I T4 was cleared from serum in a biphasic pattern that was unaltered by PRL. Clearance of labeled T3 followed a similar pattern that was also not influenced by PRL treatment. Generation of labeled T3 by deiodination of a dose of 125I T4 was quantified over a 24-h period. Again, PRL-treated fish showed no significant differences. Since PRL is without effects on thyroid hormone clearance patterns or deiodination rate, we conclude that PRL prevents TSH-induced increases in serum T4 in this species by directly affecting thyroid function.

Animals↗
PubMed1983

Time course of sensitivity of golden hamsters to melatonin injections throughout the day.

Twenty-four groups of adult male golden hamsters, housed on 14 h of light, 14L:10D, received daily injections of melatonin, each at a different hour of the day. Injections (15 micrograms melatonin/0.1 ml ethanol:saline, 1:10, s.c.) persisted for 27 weeks. At the beginning of this investigation and every 2-4 weeks thereafter, laparotomies were performed on a few animals of each group and the testes measured to assess the efficacy of the melatonin injections in causing testicular regression. Two periods of melatonin sensitivity were identified. The first was brief, of 1-h duration, immediately prior to lights on (0500-0600 h). The second period extended for 6 h throughout late afternoon and early evening (1500-2100 h; lights out 2000 h). Regression was complete in all these groups except that injected at 1500 h in which the testes regressed to only 950 mg before recrudescence occurred. In all other affected groups, testes regressed to less than 400 mg. In those groups where melatonin did not cause full testicular regression, daily injections resulted in partial regression, usually to approximately 2000 mg prior to regrowth.

Animals↗
PubMed1983

Role of the pineal and its hormone melatonin in the termination of photorefractoriness in golden hamsters.

Continuous exposure of male hamsters to short day lengths induces testicular regression. This is followed many weeks later by spontaneous recrudescence of the testes with reinitiation of spermatogenesis and function of the accessory sexual glands. Hamsters at this stage of the annual reproductive cycle are refractory to short photoperiods--even continuous darkness will not induce another bout of testicular regression. Animals refractory to short days are also refractory to the pineal hormone melatonin and a number of investigators attribute spontaneous recrudescence and photo and melatonin refractoriness to a developed target cell insensitivity to endogenous melatonin from the pineal. Refractoriness is terminated by exposure to long days for at least 11 weeks. The pineal gland is reported to be essential for this process. We report here the effects of pinealectomy, daily melatonin injections, and constant-release melatonin implants on the ability of male hamsters to recover from the refractory state. In the absence of the pineal gland, refractory male hamsters did not discriminate (count?) 15 weeks of long days to terminate refractoriness. Daily melatonin injections at 1900 h, but not at 1200 h (lights 0600-2000 h) during the 15 weeks of long-day exposure blocked the recovery from refractoriness. Constant-release melatonin implants abolished the animals ability to measure 12 and 15 weeks of long days to terminate refractoriness. These results demonstrate that general target tissue insensitivity to melatonin cannot account for the refractory state in hamsters, that a multiplicity of target tissues may exist for melatonin to account for its varied roles throughout the annual reproductive cycle in hamsters, and that the pineal gland is intimately involved in the animals' ability to measure a prescribed duration of long days to terminate refractoriness.

Animals↗
PubMed1983

Simulation of peak pineal melatonin release restores sensitivity to evening melatonin injections in pinealectomized hamsters.

In intact hamsters held on LD 14:10, pineal melatonin production and release peaks late in the evening. If these animals receive a daily injection of melatonin approximately 8 h before the endogenous peak (at the time of lights out), they respond with testicular regression. Pinealectomized hamsters receiving only this evening injection do not respond. The hypothesis tested here is very simple: replacement of the pineal melatonin rhythm in pinealectomized hamsters with a daily melatonin injection at the time of peak melatonin release in intact hamsters should reestablish sensitivity to evening (lights out) injections of melatonin. Conversely, melatonin replacement at times other than this should be ineffective. Pinealectomized hamsters maintained on LD 14:10 (lights 0600 - 2000 h) were injected with melatonin (15 microgram) at the time of the endogenous melatonin peak (2 h prior to lights on) and in the evening (0.5 h prior to lights out). This injection paradigm produced a rapid testicular regression that appears to be dependent on the timing of melatonin injections with respect to the animal's circadian system. Two daily injections given with the same frequency (8.5 h apart) but during a different time of the day were not effective. The results suggest that rhythmic sensitivity to melatonin is not affected by removal of the pineal, and that this gland serves as the source of endogenous melatonin, which must be present at the proper time for exogenous injections of melatonin to produce testicular regression.

Animals↗
PubMed1983

The effect of daily injections and constant release implants of melatonin on the endogenous pineal melatonin rhythm in golden hamsters.

In this study we tested the hypothesis that exogenous melatonin exerts its effects on the reproductive system of hamsters by directly or indirectly altering the endogenous rhythm of melatonin production and release. Melatonin was injected in male hamsters housed on LD 14:10 (lights 0600-2000 hr) either at 1200 or 1900 hr (15 micrograms in 0.1 ml ethanol:saline 1:10) daily for 12 weeks. Testicular regression occurred in all animals of the 1900-hr injection group, while melatonin injected at noon was without effect. A third group of animals received small implants of melatonin subcutaneously at 0, 4, and 8 weeks. Implants were 4 mm in length and contained a melatonin:beeswax mixture (1:25) drawn up into polyethylene tubing (2.2 mm i.d.). These implants release approximately 10-15 micrograms melatonin/day, and had no effect on testicular size, as these animals also remained on LD 14:10. After 12 weeks the animals of each group were sacrificed at 1- or 2-hr intervals around the clock. Pineals were saved and assayed for melatonin content. In each group the nocturnal rhythm of pineal melatonin was similar; peak melatonin levels were achieved 6 hr after lights out (0200 hr) and levels remained elevated for approximately 4 hr. These results exclude a mode of action of exogenous melatonin on the pineal melatonin rhythm as a basis for the testicular response to melatonin in hamsters. They also pose some interesting questions of feedback regulation by melatonin on its own production and release.

Animals↗
PubMed1982

Ontogeny of the gonadal response of golden hamsters to short photoperiod, blinding, and melatonin.

The age dependency of the time delay preceding a gonadal response to short photoperiod, blinding, and daily evening melatonin injection in young male and female hamsters has been examined to determine why the reproductive system of prepubertal golden hamsters is apparently unaffected by short-day treatment. Anestrous occurred in 50% of the female hamsters during the 10th week after blinding whether or not blinding occurred at 3, 5, 7, or 9 weeks of age. A similar latency was found following initiation of melatonin injections although the variance was greater (50% became anestrous during the 8th, 11th, 9th, and 8th week after initiation at ages of 3, 5, 7, and 9 weeks, respectively). After transfer to short days, however, females were 18 to 22 weeks old before 50% had become anestrous and the latency intervals were 19, 13, 11, and 10 weeks for transfer at 3, 5, 7, and 9 weeks of age, respectively. In males, mean testicular weights decreased to 50% of control values during the 9th and 10th week following blinding regardless of the age at which blinding occurred; the latency following initiation of melatonin injection or transfer to short days ranged between 8 and 11 weeks. These data support the conclusion that prepubertal male and female golden hamsters 3 weeks of age and older respond to light deprivation and melatonin treatment in the same manner as adults, but that the latency of the response is such that sexual maturity is attained before regression occurs. The response of 3-week-old female hamsters to short photoperiods, however, is apparently confounded by factors not governed by the pineal gland.

Aging↗