Effects of progesterone and estradiol on sexual attractivity of female rhesus monkeys

Abstract

The mechanism whereby progesterone (P) reduces sexual attractivity was studied in female rhesus monkeys following ovariectomy and SC implantation of silastic capsules containing estradiol (E 2). Daily IM administration of 10 mg P to such females caused a significant decline in mounting rate and ejaculation by male partners. Females' willingness to accept male mounts (receptivity) was not affected, and the display of sexual invitations by females (proceptivity) actually increased after treatment with P, which produced plasma levels of P similar to those present during the luteal phase of the menstrual cycle and in pregnancy. Similar behavioral effects were obtained by instilling 250 μg P daily directly into the vagina, which did not significantly increase plasma P. Daily IM treatment with 250 μg P caused a significant increase in plasma P without affecting sexual interaction. Removal of E 2 capsules caused a significant decline in mounting rate and ejaculation without affecting the females' receptivity or proceptivity. This effect of E 2 withdrawal was not accentuated by perineal application just prior to testing of vaginal secretions collected from donor females treated with E 2 and P. It is concluded that P probably disrupts sexual interaction in monkeys by antagonizing the facilitatory effect of E 2 on production of a vaginal factor which otherwise enhances sexual attractivity. Ovariectomized females displayed significantly more sexual invitations following combined treatment with E 2 and P than with P alone, suggesting that E 2 may directly affect neural tissues controlling proceptivity.