PROGESTERONE-FACILITATED LORDOSIS IN MEDIAL PREOPTIC AREA-LESIONED, JUVENILE GUINEA-PIGS

Juvenile female guinea pigs rarely display lordosis in response to est radiol and progesterone treatments that elicit sexual receptivity in a dults. To test the hypothesis that the medial preoptic area (MPOA) ton ically inhibits the display of steroid-induced lordosis in juveniles, 11-day-old guinea pigs were ovariectomized (OVX) and received bilatera l, sham, or electrolytic lesions aimed at the MPOA 3-4 days later. At 20-22 days of age, these females were tested for the expression of sex ual receptivity following injections of estradiol benzoate (EB, 10 mu g sc) and progesterone (0.5 mg sc, 40 h after EB). The lesions damaged portions of the MPOA, the nucleus of the diagonal band of Broca, the lateral aspect of the medial preoptic nucleus, the medial part of the preventricular portion of the periventricular nucleus, and the anterio r commissure. The lesions did not alter the display of estradiol-induc ed lordosis. However, after treatment with EB plus progesterone, 20% o f the sham-lesioned females displayed lordosis, as compared to 80% of the MPOA-lesioned animals. These data are consistent with the hypothes is that neurons originating in and/or traversing the MPOA tonically su ppress the display of progesterone-facilitated lordosis in juvenile gu inea pigs. Removal of this inhibitory input allows prepubertal females to respond behaviorally to estradiol and progesterone in an adult-typ ical fashion. (C) 1995 Academic Press, Inc.