The luteinizing hormone-releasing hormone (LH-RH) neuronal networks of the guinea pig brain. II. The regulation on gonadotropin secretion and the origin of terminals in the median eminence.

Cell bodies synthesizing LH-RH are located throughout the central nervous system including the hypothalamic arcuate nucleus, the medial preoptic area and medial septal nucleus. The contribution of each of these cell groups to the LH-RH terminals in the median eminence was assessed by immunocytochemistry following placement of radiofrequency lesions in male guinea pigs. Lesions in the arcuate nucleus resulted in an almost complete absence of LH-RH fibers in the median eminence. Lesions in the medial preoptic area or suprachiasmatic nucleus produced a decrease in the amount of immunoreactivity throughout the median eminence; but there was only a small decrease in the numbers of labeled fibers, which was localized to the internal zone of the median eminence and the external zone on the ventral and lateral surfaces of the infundibular stalk. Lesions of the mammillary bodies, medial amygdaloid nucleus, septal nucleus and fornix had no effect. The effects of these lesions on pituitary gonadotropin secretion was also assessed. Only lesions in the arcuate nucleus diminished plasma concentrations of luteinizing hormone (LH) and testosterone in the male guinea pigs. Similary only arcuate lesions prevented the postcastration rise in plasma LH in ovariectomized female guinea pigs. These data strongly suggest that the arcuate nucleus is the major but not the sole source of LH-RH terminals in the median eminence, and these are responsible for the neural regulation of tonic gonadotropin secretion.