Antiestrogen inhibits the induction of progestin receptors by estradiol in the hypothalamus-preoptic area and pituitary.

Estradiol treatment of ovariectomized-adrenalectomized rats produced an increase in cytosol progestin binding in the hypothalamus-preoptic area (HPOA), pituitary, and uterus. In the HPOA and pituitary, this induction of progestin receptors by estradiol was inhibited by the antiestrogen CI-628 under a variety of dose and time conditions. In the uterus, inhibition of the full effect of estradiol on progestin binding was observed after 3 days of injection of antiestrogen and estradiol. In the absence of estradiol, the antiestrogen produced a slight induction of progestin receptors in the HPOA and pituitary and a substantial induction of progestin receptors in the uterus. The results suggest that the induction of progestin receptors could be a key intermediate step in some behavioral and neuroendocrine actions of estradiol.