Mode of action of progesterone in the blockade of gonadotropin surges in the rhesus monkey.

The action of progesterone (P) to block estradiol-induced gonadotropin surges in the rhesus monkey requires an intact hypothalamus. The present study was performed to determine whether the blocking action of P is the result of an inhibition in hypothalamic GnRH secretion or of some other mechanism. Ovulatory menstrual cycles were initiated in acyclic rhesus monkeys with intact nervous systems by a pulsatile GnRH replacement regimen previously shown to be effective in sustaining normal ovarian cycles in monkeys with hypothalamic lesions. Whereas unambiguous surges of LH and FSH were elicited by estradiol benzoate during the follicular phase of these cycles, no such gonadotropin discharges were obtained in the presence of luteal phase levels of P generated by sc P-containing Silastic capsules. Because exogenous GnRH replacement was provided throughout these experiments, it is concluded that P blockade of estradiol-induced gonadotropin surges is not the consequence of an interruption in GnRH release. The results suggest, rather, that P may cause the production of an inhibitory factor from the central nervous system which blocks the positive feedback action of estradiol on the pituitary gland.