Regulation of the preovulatory gonadotropin surge by endogenous steroids

Estradiol secreted by growing ovarian follicle(s) has been considered class ically to be the neural trigger for the preovulatory surge of gonadotropins . The observation that the estradiol-induced gonadotropin surge in ovariect omized rats is of lesser magnitude and duration than that found in the cycl ing rat at proestrus has resulted in a search for other steroid regulators. Progesterone is a major regulator of the preovulatory gonadotropin surge. It can only act in the presence of an estrogen background, which is necessa ry for the synthesis of progesterone receptors. In the estrogen-primed ovar iectomized rat, progesterone is able to initiate and enhance the gonadotrop in surge to the magnitude observed on the day of proestrus and limit it to 1 day. The physiological role of progesterone in the induction of the preov ulatory gonadotropin surge hs been demonstrated by the attenuation of the p rogesterone-induced surge and the endogenous proestrus surge by progesteron e receptor antagonist RU486 and the progesterone synthesis inhibitor trilos tane. The promoter region of the follicle-stimulating hormone (FSH)-beta ge ne contains multiple progesterone response elements and progesterone brings about FSH release as well. The reduction of progesterone of the 5 alpha-po sition appears to be important for the regulation of progesterone secretion . Corticosteroids appear to play a significant role in the secondary FSH su rge on late proestrus and early estrus. (Steroids 63:616-629, 1998). (C) 19 98 by Elsevier Science Inc.