ABSTRACT
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.
ABSTRACT
Abstract In order to localize neuroendocrine dopamine neurons in the monkey hypothalamus, one female and three male juvenile cynomolgus macaques 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 pm in the frontal plane. Every 12th section was immunostained with rabbit anti-tyrosine hydroxylase using the peroxidase anti-peroxidase technique with diaminobenzidine as the chromogen. Neuroendocrine, immunoreactive neurons were easily recognized as brown, immunopositive cell bodies containing more than three distinct dark blue granules, confirmed by electron microscopy to be tracer-filled lysosomes. Neuronal counts from each complete series of sections were compiled by anatomical region, and the percentages of tyrosine hydroxylase-immunoreactive and neuroendocrine, immunoreactive neurons determined. Although regional and interanimal variations were observed, we estimated that 5,400 of the total 19,000 tyrosine hydroxylaseimmunoreactive neurons in the juvenile macaque hypothalamus were neuroendocrine. When averaged by anatomical region, the suprachiasmatic nuclear groups contained 7% of all immunoreactive neurons (50% were neuroendocrine) and 15% of all neuroendocrine, immunoreactive neurons in these animals. The combined periventricular zones contained 20% of all immunoreactive neurons (more than 50% of ventral and 38% of dorsal were neuroendocrine) and 58% of all neuroendocrine, immunoreactive neurons. The paraventricular nucleus included 50% of all immunoreactive neurons, more than any other nucleus, (3% were neuroendocrine) and 11% of the total neuroendocrine, immunoreactive neurons. The ventral paraventricular nucleus contained only 2% of all immunoreactive neurons (13% were neuroendocrine) and 3% of the total neuroendocrine group. The zona incerta contained 15% of all immunoreactive neurons (0% were retrogradely labeled) but 0% of the neuroendocrine cells. The arcuate nucleus subdivisions contained about 5% of all immunoreactive neurons (more than 60% were neuroendocrine) and 8% of the neuroendocrine population. The ventral hypothalamic tract contained about 1% of all immunoreactive neurons (medially, 63%, and further laterally, 25% were neuroendocrine) and 5% of all neuroendocrine, immunoreactive neurons in these animals. The presence of the retrograde tracer from the median eminence in tyrosine hydroxylase-immunoreactive neurons, combined with knowledge of the location of dopaminergic cell groups, permitted assessment of the A11-A14 dopaminergic neurons which project to the primate infundibulum. Neuroendocrine dopamine neurons occurred predominantly in the All periventricular zones (65% of the total), being greatest around the ventral aspect of the entire third ventricle. They were less numerous in more dorsal regions of All extending up to the level of the paraventricular nucleus. The A12 arcuate (tuberoinfundibular) projection (15% of the total) was not nearly as prominent as All in primates, in contrast to rodents. None of the A13 incertohypothalamic dopamine neurons (0%) projected to the median eminence. The A14 anterior-ventral periventricular region, including the suprachiasmatic nuclear groups, provided the substantial remainder (20%) of all neuroendocrine dopamine neurons. In summary, our results suggest the involvement of a regionally specific dopaminergic system in the hypothalamic control of anterior pituitary hormone secretion in primates. The data also indicate that 75% of all tyrosine hydroxylase-immunopositive neurons do not project to the median eminence, and probably serve other functions. Although the retrograde tracer may not have labeled all neuroendocrine dopamine neurons, it may have identified some dopamine neurons which only interact with other median eminence nerve terminals, or other types of tyrosine hydroxylase-containing, neuropeptidergic neurons which project to the infundibulum. However, considering the known locations of dopaminergic neurons and the large numbers of labeled cells, the results here are a reliable indication of the diverse origins of median eminence-directed dopamine neurons in the juvenile primate.
