On the site of action of progesterone in the blockade of the estradiol-induced gonadotropin discharge in the rhesus monkey.

The site of action of progesterone (P) in the blockade of estradiol-induced gonadotropin discharges was examined in rhesus monkeys bearing hypothalamic lesions which abolish endogenous GnRH production. Normal ovulatory menstrual cycles were re-established in these animals by the pulsatile, hourly administration of GnRH. In the follicular phase of these induced menstrual cycles, P-containing Silastic capsules were implanted sc yielding luteal phase plasma P concentrations which normally block estradiol-induced gonadotropin surges. P failed to block estradiol-induced surges in lesioned animals on GnRH replacement. In such animals, however, P advanced the initiation of these surges. While estradiol acts on the pituitary to cause gonadotropin discharges, P appears to block this effect by acting on the central nervous system. On the other hand, the results also suggest that the site of facilitatory action of P on gonadotropin release is at the level of the pituitary gland.