Progestins modulate the action of estrogen on gonadotropin-releasing hormone, luteinizing hormone and prolactin in the rat.

The aim of the present study was to evaluate the effect of progesterone and of various synthetic progestins on hypothalamic gonadotropin-releasing hormone (GnRH) and on pituitary and plasma LH and prolactin (Prl) concentrations in ovariectomized rats. Groups of 6 rats were treated for 2 weeks with a pharmacological dose of progesterone, desogestrel, medroxyprogesterone acetate or norethisterone enanthate (NET). The same treatment was also repeated in association with estradiol benzoate (EB). Groups of ovariectomized rats were also treated with EB only or vehicle. Ovariectomized rats showed hypothalamic concentrations of GnRH significantly higher than control rats. Progesterone and NET significantly increased GnRH concentrations and reversed the EB-induced changes. Rats treated with progesterone and NET also showed reduced pituitary LH concentration, with high plasma LH levels. All progestins blocked the increase of pituitary LH induced by EB, but were inactive in influencing the inhibitory effect of EB on plasma LH levels. The administration of progesterone and progestins did not induce significant changes of pituitary or plasma Prl, but reversed the EB-induced increase. These results showed that progesterone and progestins have a marked effect on hypothalamic and pituitary hormone secretion and that they modulate estrogen-induced effects.