The importance of the peripeduncular nucleus in the neuroendocrine control of sexual behavior and milk ejection in the rat.

Electrocoagulations situated in the lateral midbrain tegmentum cause severe deficits in sexual behavior and lactational performance of rats. In this study we determined the extent to which these effects could be reproduced by axon-sparing lesions using the excitotoxin, ibotenic acid; in another group of rats, 6-OHDA was infused in the same area to degenerate the mesencephalic catecholamine neuronal elements affected by the electrocoagulations. It was found that ibotenic acid, but not 6-OHDA, reproduced most of the effects produced by electrolytic lesions. Thus, female rats bearing ibotenic acid lesions showed little sexual receptivity and proceptivity in response to estrogen and progestin treatment, and the milk ejection reflex appeared nonfunctional following the lesion. Ibotenic acid-infused male rats failed to ejaculate on most postoperative observations, though they continued to mount he estrous female. Examination of the lesions with immunohistochemical visualization of tyrosine-hydroxylase- and substance P-positive neurons, and thionine staining, revealed that the neurotoxins exhibited the intended selectivity, though the ibotenic acid lesions were associated with loss of substance P-immunoreactive nerve terminals in the substantia nigra and the peripeduncular region. It is suggested that the peripeduncular nucleus plays an important role in the neuroendocrine control of male and female copulatory behavior, as well as in the regulation of the milk ejection reflex.