Sequential inhibition of sexual behavior by progesterone in female rats: comparison with a synthetic antiestrogen.

When a large dose of progesterone was administered to ovariectomized rats 24 hr after a 2-microgram injection of estradiol benzoate (EB), sexual receptivity was inhibited at 54 hr (sequential inhibition). Larger doses of progesterone (1 mg) were required to inhibit the induction of sexual receptivity when tested at 54 hr than were necessary to facilitate at 30 hr. This inhibition was not due to copulatory stimuli from the first test, because inhibition occurred even when the first test was omitted. The inhibition was dose dependent on estradiol; increasing the EB priming dose offset the inhibition caused by 1 mg of progesterone. The results of an experiment that dissociated behaviorally the antiestrogenic action of progesterone from that of a synthetic antiestrogen, CI-628, are consistent with the notion that progesterone and synthetic antiestrogens inhibit the neural effects of estradiol by separate mechanisms of action.