Location of the neuroendocrine gonadotropin-releasing hormone neurons in the monkey hypothalamus by retrograde tracing and immunostaining*,**.

Abstract In order to localize neuroendocrine gonadotropin-releasing hormone (GnRH) neurons in the monkey hypothalamus, four juvenile cynomolgus macaques (one female, three males) were each given two or three microinjections (0.2 to 0.3 mul per site) of the retrograde tracer wheat germ agglutinin-apoHorseradish peroxidase-10 nm colloidal gold into the superficial, median eminence region of the infundibular stalk. Five to 15 days following surgery, the brains were fixed by perfusion and vibratomed at 40 mum in the frontal plane. Every 12th section was immunostained with rabbit anti-GnRH using the peroxidase anti-peroxidase technique with diaminobenzidine as the chromogen. Neuroendocrine GnRH neurons were easily identified in tissue sections as brown, immunostained cell bodies containing more than three distinct, dark blue, tracer-filled lysosomes. Neuronal counts from each complete series of sections were compiled by anatomical region, and the percentages of GnRH and neuroendocrine GnRH neurons determined. The highest proportion of neuroendocrine GnRH neurons (with projections to the median eminence) occurred in the ventral hypothalamic tract, especially in its medial third (71%), and in the supraoptic decussation just anterior to it. Proportions decreased moving laterally into the middle third (58%) and lateral third (25%) of the ventral hypothalamic tract. Further anterior and lateral, progressively smaller but significant neuroendocrine GnRH contributions were found in the supraoptic nucleus (57%) and lateral hypothalamus (33%), and in the medial preoptic area (26%). Although the medial preoptic area contained a greater percentage of the total GnRH-immunoreactive cell bodies (36%) than the ventral hypothalamic tract (27%), as a whole, the ventral hypothalamic tract contained 60% of the neuroendocrine GnRH neurons compared to only 25% from the medial preoptic area. Large numbers of GnRH cell bodies found in the diagonal band of Broca near the organum vasculosum of the lamina terminalis were not retrogradely labeled. GnRH neurons were not observed in the arcuate nucleus, the few in the paraventricular nucleus were not neuroendocrine, and the contribution from the periventricular zone was negligible. Our results here are the first to identify the neurons giving rise to the neuroendocrine GnRH system in juvenile monkeys. The data indicate that more GnRH neurons close to the infundibulum serve a neuroendocrine (perhaps hypophysiotropic) role than do those in more anterior areas. Furthermore, they suggest that the ventral hypothalamic tract is the most important, and perhaps most influential, neuroendocrine GnRH cell group in primates. The data substantiate the observed autonomy of the medial basal hypothalamus in controlling gonadotropin secretion and menstrual cyclicity in these animals. However, they also infer that perhaps 60% of the GnRH neurons do not project to the primate median eminence, and thus may serve other non-neuroendocrine functions.