The stimulatory effect on gonadotropin release of implants of estradiol or progesterone in certain sites in the central nervous system.

The sites of stimulatory feedback of ovarian steroids on gonadotropin release were examined. In estrogen-primed ovariectomized rats, estradiol or progesterone was implanted stereotaxically into various hypothalamic and extrahypothalamic areas. Both steroids elevated plasma LH and FSH levels at 6 h when implanted unilaterally into the preoptic-anterior hypothalamic area. Bilateral implants at this site were ineffective; presumably, this was due to the destructive lesion produced by the cannulae since in sham-implanted or anteriorly deafferented rats systemic estrogen was also ineffective. Unilateral estradiol implants in the medial basal hypothalamus elevated LH and FSH at 30 but not at 6 h. Maximum increases of plasma gonadotropins occurred 30 h after implantation of estradiol in the medial amygdaloid nuclei; progesterone implants at this site elevated only FSH at 6 h. Dissociation in the release of LH and FSH was also observed following implantation of estradiol into the hippocampus. Plasma FSH levels were dramatically reduced at 30 h while LH was slightly elevated. Progesterone implants in the hippocampus or medial basal hypothalamus did not alter plasma gonadotropin levels.