Regulation of the preovulatory gonadotropin surge by endogenous steroids

Estradiol secreted by growing ovarian follicle(s) has been considered classically to be the neural trigger for the preovulatory surge of gonadotropins. The observation that the estradiol-induced gonadotropin surge in ovariectomized rats is of lesser magnitude and duration than that found in the cycling 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 necessary for the synthesis of progesterone receptors. In the estrogen-primed ovariectomized rat, progesterone is able to initiate and enhance the gonadotropin surge to the magnitude observed on the day of proestrus and limit it to 1 day. The physiological role of progresterone in the induction of the preovulatory gonadotropin surge has been demonstrated by the attenuation of the progesterone-induced surge and the endogenous proestrus surge by progesterone receptor antagonist RU486 and the progesterone synthesis inhibitor trilostane. The promoter region of the follicle-stimulating hormone (FHS)-beta gene contains multiple progesterone response elements and progesterone brings about FSH release as well. The reduction of progesterone in the 5 alpha-position appears to be important for the regulation of progesterone secretion. Corticosteroids appear to play a significant role in the secondary FSH surge on late proestrus and early estrus.