SITE-SPECIFIC OPIOID RECEPTOR BLOCKADE ALLOWS PREPUBERTAL GUINEA-PIGSTO DISPLAY PROGESTERONE-FACILITATED LORDOSIS

Ovariectomized (OVX) juvenile guinea pigs (similar to 3 weeks old) rar ely display steroid-induced sexual receptivity. Systemic administratio n of the opioid receptor antagonist naloxone enhances the display of p rogesterone-facilitated lordosis in prepubertal females, suggesting th at endogenous opioids tonically inhibit the expression of sexual recep tivity at this age. This study was designed to ascertain the neural si te(s) at which naloxone injection would stimulate lordosis in juvenile guinea pigs. Hartley guinea pigs were OVX at 10-11 days of age and 2- 6 days later implanted with bilateral cannulae aimed at the medial pre optic area/anterior hypothalamus (MPOA/AH), ventrolateral hypothalamus /ventromedial hypothalamus (VLH/VMH), or mesencephalic central gray (M CG). At 21-23 days of age, following administration of estradiol benzo ate (10 mu g) and progesterone (0.5 mg), naloxone (100 ng/side) or 0.9 % saline was injected through the cannulae and the guinea pigs were te sted for the display of lordosis. The MPOA/AH was the only site at whi ch application of naloxone reliably elicited lordosis (87% positive re sponse vs 12% for saline). Few females (<17%) displayed lordosis follo wing injections of naloxone or saline into the VLH/VMH or MCG. a secon d experiment demonstrated that the stimulation of lordosis following M POA/AH naloxone application was prevented by prior injection of the op ioid agonist morphine (500 ng/side) at the same site. These data suppo rt the hypothesis that endogenous opioids acting in the MPOA/AH, but n ot the VLH/VMH or MCG, tonically inhibit the display of progesterone-f acilitated lordosis in prepubertal guinea pigs. (C) 1998 Academic Pres s.