IBOTENIC ACID-INDUCED LESIONS OF THE MEDIAL PREOPTIC AREA ANTERIOR HYPOTHALAMUS ENHANCE THE DISPLAY OF PROGESTERONE-FACILITATED LORDOSIS INMALE-RATS

Electrical lesions of the medial preoptic area/anterior hypothalamus ( MPOA/AH) have been reported to enhance the display of steroid-induced lordosis in castrated male rats. This study employed the cell body-spe cific neurotoxin, ibotenic acid, to ascertain whether neurons originat ing in this region (as opposed to axons of passage) tonically inhibit steroid-induced lordosis in adult male rats. Castrated, adult Long-Eva ns males received bilateral electrical lesions or injections of iboten ic acid or vehicle aimed at the MPOA/AH. Following administration of e stradiol benzoate (EB) and progesterone, lordosis quotients (LQs) and lordosis ratings (LRs) were significantly higher in groups of rats wit h electrical lesions (LQ = 62.2 +/- 15.1; LR = 1.22 +/- 0.34) and ibot enic acid-induced lesions (LQ = 58.1 +/- 12.2; LR = 0.99 +/- 0.24) tha n in the control group (LQ = 12.8 +/- 7.3; LR = 0.22 +/- 0.13). To det ermine whether this enhancement of receptive behavior in MPOA/AH-lesio ned males was an effect on estradiol-induced, as compared to progester one-facilitated lordosis, groups of castrated rats in a second experim ent received bilateral injections of ibotenic acid or vehicle aimed at the MPOA/AH and were tested for lordosis after administration of EB a lone and again after injection of progesterone. Following treatment wi th EB alone, rats with ibotenic acid-induced MPOA/AH lesions tended to be slightly less receptive than control animals. However, following i njections of progesterone, LQs and LRs were higher in the MPOA/AH-lesi oned group than in the control animals, as had been observed in the fi rst experiment. These data are consistent with the hypothesis that cel l bodies, rather than axons of passage, originating in the MPOA/AH exe rt tonic inhibitory control over the display of progesterone-facilitat ed lordosis in adult male rats.