The long term goals of this research are to determine how GnRH neuronal secretory activity is integrated in the primate and how that integration might be altered by the steroid milieu. In mammals, the hormone is manufactured by a population of broadly scattered neurons whose embryonic origin is the olfactory placode. There is evidence in rodents that it is a subpopulation of these which subserves the gonadotrophic release function through secretion into the hypophysial portal vasculature. In primates the distribution of GnRH elements differs from rodents in that there is an additional cell cluster in the medial basal hypothalamus and a large projection to the posterior pituitary. The present studies will trace the neuronal origin of GnRH projections to the median eminence and to the posterior lobe in male and female macaques that differ in their endocrine status. A lipophilic dye (DiI) will be used as a retrograde tracer and applied to these sites in fixed tissue. GnRH neurons are then identified immunocytochemically. The mechanism underlying the release of the hormone and the coordination of its pulsatility will also be studied through ultrastructural analysis employing single and double label immunocytochemical techniques to identify presynaptic elements and glial processes. Not only do GnRH neurons receive diverse synaptic input but in the rhesus, unlike the rodent, GnRH cells are also wrapped in glial sheaths. Our preliminary data suggest that the degree of ensheathment may differ between intact and ovariectomized animals. We will pursue the possibility of plasticity in the organization of GnRH neurons and the role of steroids in modulating these changes by studying intact cycling or amenorrheic females and animals castrated for varying lengths of time. Reversibility of castration-induced phenomena will be studied with replacement protocols. Part of this analysis will include an examination of the nature of the synaptic input to GnRH cells. Some of the major transmitters known to modulate GnRH activity in the primate are beta endorphin, corticotrophin releasing factor and norepinephrine. The contribution of these transmitters to the synaptic input to GnRH neurons will be quantified using computerized morphometry. Finally, GnRH neurons and their processes will be analyzed for synaptic interactions which might underlie their episodic secretion and for the route by which the hormone is secreted into the ventricular space. These studies will speak to the issues of plasticity and redundancy of GnRH neurons and the secretory pathways that regulate the reproductive function.
