Recovery of mating behavior in the female rat following VMH lesions

On the day of proestrus, female rats were given large electrolytic lesions aimed at the ventromedial nucleus of the hypothalamus (VMH). Following a postoperative period of extended diestrus the vaginal smears showed irregular periods of vaginal cornification with a tendency toward prolonged periods of cornified smears. Sexual receptivity, measured in terms of the lordosis-to-mount ratio (L% = L/M X 100) was low on the evening of the first postsurgical proestrus, but improved markedly in subsequent mating sessions. Although the lordosis response was present, the intromission frequency remained below that observed during mating sessions with control females. Findings at autopsy together with the prolonged periods of vaginal cornification suggest that VMH lesions result in blocked or delayed ovulation. The behavioral data contradict previous reports of blocked mating behavior in VMH females based upon indirect measures of receptivity, i.e., the presence of vaginal plugs or sperm on the morning following overnight caging with sexually active males.