Ovarian Regulation of Postcoital Gonadotropin Release in the Rabbit: Reexamination of a Functional Role for 20 αDihydroprogesterone1

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Abstract
In the rabbit, it has been proposed that an ovarian progestin, 20 .alpha.-dihydroprogesterone (20.alpha.P), released at mating, is essential for a normal postcoital LH [luteinizing hormone] surge. Plasma levels of LH and 20.alpha.P were measured after mating in rabbits, and it was observed that the frequency, magnitude and time-course of changes in circulating levels of 20.alpha.P seemed inappropriate to account for the rapid and major surge of LH secretion. The role of the ovary in regulating post-coital LH secretion was re-evaluated. In chronically ovariectomized (> 30 days) does pretreated with estrogen, mating induced a normal LH surge in only 1 of 10 animals, indicating that an ovarian product in addition to estrogen is required for a normal postcoital LH surge. When 20.alpha.P was injected soon after mating in chronically ovariectomized does pretreated with estrogen, only 2 of 9 displayed normal LH surges; this proportion is not different from that (1/10) observed with estrogen treatment alone. To demonstrate that the estrogen treatment, which produced supraphysiologic plasma estradiol levels, did not block LH release, 6 intact anestrus females were treated with the same estrogen regimen. Estrus was induced in 5, and each displayed a large post-coital LH surge and ovulated. As a final test of the 20.alpha.P hypothesis, 5 spontaneously estrous does were ovariectomized within 15 min post coitum to abolish acute increases in circulating ovarian hormones. Three animals released LH in amounts and temporal pattern indistinguishable from intact estrous does. A 4th released smaller amounts of LH. Two of 4 sham-operated does also had normal LH surges. Ovarian hormones are probably required before mating to support the capacity of the LH secretory mechanism to respond to coitus. Chronic alteration in the hormonal milieu by ovariectomy appeared to produce a change in the hypothalamo-hypophysial complex that is not reversed by estrogen alone. Neither 20.alpha.P nor any other ovarian hormone is required post coitum, at least after 15 min, for normal LH release.