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Biomedical subjects

M H Stetson

Publications and source records attributed to M H Stetson.

At least 91 records · Page 5Linked to original sources
PubMed1982

Regulation of tonic gonadotropin release in prepubertal female hamsters.

Basal serum gonadotropin levels were monitored weekly in female hamsters from birth to 10 weeks of age. Hamsters raised on three different photoperiods presented uniform pre- and postpubertal patterns of serum LH and FSH, suggesting that gonadotropin release in the young hamster occurs independently of ambient photoperiod. In all groups, serum LH levels increased gradually in animals up to 4 weeks of age, after which levels plateaued at 50--100 ng/ml. Serum FSH was markedly elevated in 2- and 3-week-old hamsters (800--1200 ng/ml), but remained at 200--400 ng/ml in all other groups. We next examined the change in the responsiveness of the pituitary to exogenous gonadotropin-releasing hormone (GnRH) challenge. Female hamsters 2 days of age failed to respond to any dose (0.025--1000 ng) of GnRH, while 10-day old females responded in typical dose-dependent fashion. GnRH-stimulated LH release first occurred in 6-day-old hamsters and was maximal by day 9, whereas FSH release first occurred on day 8 and was maximal by day 9. The prepubertal pattern of gonadotropin release can, in part, be explained on the basis of the development of pituitary GnRH sensitivity, which occurs independently of photoperiod.

Animals↗
PubMed1981

The anovulatory hamster: a comparison of the effects of short photoperiod and daily melatonin injections on the induction and termination of ovarian acyclicity.

Cyclic female hamsters were rendered anovulatory by daily subcutaneous melatonin injections (25 microgram/0.1 ml oil) in 29 days or by transfer to a short light cycle, LD 6:18 (lights 1000-1600 hrs) in 33 days. Estrous cyclicity was reinitiated in these animals in 44 or 45 days following cessation of melatonin injections or transfer to long light cycles (LD 14:10, lights 0600-2000 hrs), respectively. Exposure of both groups to LD 6:18 after reinitiation of estrous cyclicity caused a second cessation of ovulation in 75 (melatonin group) or 61 (short light cycle group) days. Thus, although both treatments disrupted estrous cyclicity for nearly 6 weeks, this was not sufficient to induce photorefractoriness (failure to respond to short light cycles with continued estrous cyclicity). Rather, every animal responded to LD 6:18 and ceased ovulating. Melatonin-induced anovulatory hamsters showed daily gonadotropin release patterns identical to those reported in hamsters in other anovulatory states (lactating, prepubertal, and photoinduced anovulatory hamsters); that is, peak LH and FSH release at 1700 hrs daily.

Animals↗
PubMed1981

Correlative changes in the response to castration and the onset of refractoriness in male golden hamsters.

This investigation was designed to determine if a correlation exists between a change in the response of the hypothalamo-hypophysial axis to castration and the onset of refractoriness in male hamsters exposed to short days (in this case LD 6:18). The castration response, here defined as a significant increase in serum gonadotropins over levels in intact males, in males exposed to short days for 2 weeks was no different from that observed in long day males. On weeks 4 and 6 the response was greatly attenuated, and on weeks 8 and 10 no castration response was observed. Refractoriness was first observed in a few animals exposed to LD 6:18 for 8 weeks, and in increasing numbers of animals on weeks 10, 12, 13 and 14 of short-day treatment. Thus, short day exposure results in a simultaneous loss of response of the hypothalamo-hypophysial axis to castration and initiation of refractoriness.

Animals↗
PubMed1980

Circadian pacemaker times gonadotropin release in free-running female hamsters.

Female golden hamsters (Mesocricetus auratus) were housed individually on a photoperiod of LD 6:18 (lights 1000-1600 h). Estrous cyclicity was interrupted for an average of 16 wk after which cycles resumed spontaneously. Such animals are photorefractory, remaining cyclic on normally nonstimulatory photoperiods. Photorefractory females were exposed to continuous darkness in which estrous cyclicity in the population rapidly became asynchronous as each hamster's "day" assumed the endogenous periodicity (tau) of her circadian clock. Tau usually approximated but rarely equaled 24 h. Successive estrous cycles in each animal possessed a periodicity of 4 tau. Preovulatory luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release occurred at a specific time of the animal's circadian day, bearing a discrete phase relationship of 2 to 3 h to the circadian rhythm of locomotor activity. Thus, although seemingly asynchronous with respect to laboratory time, peak preovulatory gonadotropin release on proestrus in free-running hamsters was synchronized with respsect to circadian time. We conclude that a circadian pacemaker times preovulatory gonadotropin release in hamsters.

Animals↗
PubMed1980

Maturation of the clock-timed gonadotropin release mechanism in hamsters: a key event in the pubertal process?

Gonadotropin secretion patterns were monitored in 10, 14 to 21-, and 35-day-old prepubertal female hamsters by RIA. Blood samples were obtained between 1500-1900 h to coincide with the time when the adult female hamster experiences a clock-timed surge in gonadotropin release. Before 16 days of age, the female hamster appears to exhibit a tonic pattern of gonadotropin release. On day 16 and thereafter, the female hamster exhibits a daily surge of gonadotropin release. Around the clock sampling (every 3 h) has shown that this cyclic pattern of gonadotropin release is not present at any time studied in female hamsters younger than 16 days of age, appears to be clock timed, and occurs at the same time of day as the proestrus surge in cyclic adults. The maturation of the clock timed gonadotropin release mechanism may indicate maturation of the reproductive control system, a key event in the pubertal process.

Animals↗
PubMed1977

The estrous cycle in golden hamsters: a circadian pacemaker times preovulatory gonadotropin release.

In a population of cycling female hamsters entrained to an LD 6:18 light cycle (lights 1000-1600 hours), preovulatory release of luteinizing hormone and follicle-stimulating hormone occurred in some animals at 1300-1400 hours and in others at 1900 hours. In every case peak release was phase-locked (2-3-hour positive phase angle) to the circadian rhythm of locomotor activity. The pattern of entrainment of gonadotropin release on LD 6:18 is fully explicable in terms of the hamster's phase response curve to light. We conclude that periodic gonadotropin release in cycling females is timed by a circadian oscillator (biological clock) that is probably the same oscillator driving the circadian rhythm of locomotor activity.

Animals↗