Twenty Four-Hour Rhythm of Gonadotropin Release Induces Cyclic Progesterone Secretion by the Ovary of Prepubertal and Adult Golden Hamsters*

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Abstract
Cyclic release of gonadotropin (GTH) begins on day 16 or 17 of age, about 2 wk before ovulatory estrous cycles are initiated in female golden hamsters (M. auratus). The daily cycles of LH [luteinizing hormone] 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. It was determined if initiation of cyclic GTH release during the prepubertal period results in significant changes in ovarian function. A 2nd 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 ovaraian, 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. 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; the rhythms of LH, FSH and P secretion in the prepubertal female are similar to those in the photoperiod-induced anovulatory adult; and the response of the prepubertal female to barbiturate blockade is identical to that of the adult. The initiation of cyclic GTH secretion at the midpoint of the prepubertal period represents the final maturation of neuronal oscillator systems that will function in the adult.