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Biomedical subjects

M Menaker

Publications and source records attributed to M Menaker.

At least 127 records · Page 7Linked to original sources
PubMed1975

The interaction of castration and photoperiod in the regulation of hypophyseal and serum gonadotropin levels in male golden hamsters.

Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) were measured in intact and castrate adult male hamsters maintained on photostimulatory (LD 14:10) and non-photostimulatory (LD 6:18) light:dark cycles to assess the interaction of photic stimuli and gonadal hormones on pituitary gonadotropin release. Immunoreactive serum LH and FSH levels increased 1.6- and 8-fold respectively, within 3 days after photostimulated hamsters were castrated. In contrast, castration failed to alter serum LH concentration and had only a slight, if any, effect on FSH concentration in hamsters exposed to nonstimulatory photoperiods that induced testicular atrophy. In a second experiment, male hamsters previously maintained on LD 14:10 were castrated, transferred with intact animals to LD 6:18, and killed periodically over 60 days. In intact animals, pituitary content and serum levels of LH and FSH declined substantially during exposure to the non-stimulatory LD 6:18 cycle. In castrated animals, serum LH and FSH levels which had increased 2- and 8-fold in response to the castration eventually declined to about the levels found in the intact initial control animals. In contrast to serum gonadotropins, the increased hypophyseal content of LH and FSH following castration was not reduced during exposure to LD 6:18. Exposure to nonstimulatory photoperiods does not alter the increased hypophyseal LH and FSH content observed after castration. However, our results indicate that exposure to short days renders the hypothalamic-hypophyseal neuroendocrine system governing gonadotropin release relatively insensitive to gonadal steroid hormone feedback.

Animals↗
PubMed1972

Regulation of testis function in golden hamsters: a circadian clock measures photoperiodic time.

The photoperiodic testicular response of adult golden hamsters was examined by the use of a 6-hour light period coupled with dark periods of 18, 30, 42, and 54 hours. Cycle lengths of 24 and 48 hours resulted in testicular regression, whereas testicular weight was maintained by cycle lengths of 36 and 60 hours. Our data demonstrate a circadian rhythm of sensitivity to the effects of light on the photoperiodic testicular response of the hamster. The position of light relative to the circadian system (as measured by the locomotor rhythm) is critical in the response.

Animals↗
PubMed1971

Pineal function in sparrows: circadian rhythms and body temperature.

Deep body temperature of the house sparrow, Passer domesticus, was monitored continuously by radio telemetry. Pinealectomy abolished the normal circadian rhythm of body temperature in constant darkness, and significantly altered the amplitude of body temperature rhythms entrained to light cycles. The body temperature minima of pinealectomized birds never fell as low as those of unoperated birds regardless of the light conditions; the temperature maxima of both normal and pinealectomized birds were higher in light than in darkness. In sparrows the pineal organ is essential to the normal function of the biological clock controlling both activity and body temperature rhythms and may be directly involved in thermoregulation.

Animals↗
PubMed1970

Photoperiodically significant photoreception in sparrows: is the retina involved?

Blind house sparrows (150 birds) and normal ones (199 birds) were subjected to various photoperiodic treatments consisting of cycles of 12 hours light, 12 hours dark and 16 hours light, 8 hours dark at intensities ranging from 20 to 500 lux. The testicular, response of the blind birds was found to be indistinguishable from the response of normal birds under these conditions. The data show that a functional extraretinal photoreceptor exists in this species which is fully capable of mediating the gonadal response to photoperiodic stimuli.

Animals↗
PubMed1970

Extraretinal light perception in the sparrow. 3. The eyes do not participate in photoperiodic photoreception.

Photoperiodic control of testis growth in Passer domesticus (house sparrow) is mediated entirely by extraretinal photoreceptors in the brain. The eyes do not participate in photoperiodically significant photoreception. Removal of the pineal organ does not affect either the response to light or, to a first approximation, the process of recrudescence. The intensity of light reaching the retina and that reaching the extraretinal photoreceptor were varied independently. This technique will make it possible to study brain photoreception in species of birds that will not tolerate blinding. Extreme caution is necessary in the interpretation of brain lesion experiments in which reproductive function is modified, since photoreception by brain receptors of unknown anatomical location affects testicular state.

Animals↗
PubMed1968

Pineal function: the biological clock in the sparrow?

The pineal organ of the house sparrow, Passer domesticus, is essential for persistence of the circadian locomotor rhythm in constant conditions. Upon removal of the pineal body, activity becomes arrhythmic. However, pinealectomy does not abolish the rhythm of locomotor activity in birds exposed to light-dark cycles. Pinealectomized birds are entrained by light cycles in much the same manner as are normal birds. Our data demonstrate that the pineal organ is a crucial component of the endogenous time-measuring system of the sparrow.

Animals↗
PubMed1967

Photoperiodic control of hamster testis.

The response of the testes of juvenile and adult hamsters to various photoperiods was examined. The testes of juvenile animals reached maturity regardless of the light cycle on which the animals were raised. However, the testes of adult hamsters required at least 12.5 hours of light per day to maintain spermatogenesis and prevent degeneration. This is one of the few demonstrations of a response suitable for study in investigations of the photoperiodic control of testicular function in a laboratory mammal.

Animals↗
PubMed1967

Circadian clock in photoperiodic time measurement: a test of the Bünning hypothesis.

A technique has been developed for effectively separating the direct inductive effect of a light signal from its effect on the phase of the rhythm of sensitivity to photoperiodic induction. With this technique it has been shown that a 75-minute pulse of light per day, when appropriately positioned with respect to the circadian activity cycle of the sparrow Passer domesticus, is sufficient to produce a response normally produced only by long days. The results cannot be interpreted in terms of a requirement of an absolute amount of either darkness or light and offer strong confirmation of Bünning's hypothesis concerning the mechanism of photoperiodic time measurement.

Animals↗
PubMed1966

Entrainment of circadian rhythms by sound in Passer domesticus.

The circadian locomotor rhythm of house sparrows was entrained by a sound stimulus. The birds were maintained at a constant temperature in, dim green light. The entraining agent was 4 (1/2) 12 hours of tape-recorded bird song ,played each day. Variations in the response to this stimulus have been correlated with individual variations in free-running period. This is the first clear demonstration that a biological clock can be influenced by sound stimuli.

Animals↗