Timing of Gonadotrophin and Ovarian Steroid Secretion at Diestrus in the Rat

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Abstract
The ovarian steroids responsible for the pro-estrous rise in uterine weight and release of an ovulatory surge of gonadotrophin and for estrous vaginal cornification appear to be secreted primarily on the day before proestrus (diestrus). To investigate the timing of the gonadotrophin release responsible for this ovarian secretion and for the resultant uterine and vaginal smear changes, rats exposed to light from 5 AM to 7 PM and running 3 consecutive 4-day cycles were hypophysectomized at 9 AM, 1 PM, 3 PM, or 5 PM on the day of diestrus, and were autopsied at proestrus or estrus. The characteristic rise in uterine weight at proestrus was noted in 0/6 and 1/8 rats hypophysectomized at 9 AM and 1 PM, respectively, but in 6/8 and 7/8 rats hypophysectomized at 3 PM and 5 PM. Similarly, hypo-physectomy blocked the estrous vaginal cornification when performed at 9 AM (0/6) or at 1 PM (0/8), whereas vaginal cornification occurred in 4/6 and 7/7 animals when hypophysectomy was delayed until 3 PM or 5 PM, respectively. Thus, sufficient amounts of gonadotrophin have been discharged by 3 PM on the day of diestrus to stimulate secretion of the ovarian hormones responsible for the cyclic changes seen at proestrus and estrus. Sham hypophysectomy did not inhibit the gonadotrophin or steroid secretion required for the uterine and vaginal smear changes or for ovulation. Furthermore, the uterine and vaginal smear changes were not inhibited by barbiturate treatment, suggesting that the neural mechanism regulating this basal gonadotrophin secretion, in contrast to that regulating the cyclic ovulatory surge of LH [luteinizinghormone], is not susceptible to the suppressive action of barbiturates.