Our studies will focus on the chemical and physiological factors which control the secretion of vasopressin by magnocellular neurons in the hypothalamic supraoptic nucleus. These factors will be identified, characterized and an attempt will be made to relate these to the osmotic regulation of vasopressin secretion. The use of an in vitro approach with the hypothalamo-neurohypophysial complex (HNC) will give the degree of control necessary to achieve these goals. The HNC contains the entire supraoptic-neuroendocrine final common pathway including the two intact supraoptic nuclei (NSO), the supraoptico-neurohypophysial tract (SOHT) and the neurosecretory terminals of the neural lobe (NL). Every major site suspected of being involved in the osmotic regulation of vasopressin secretion is present in the HNC. The HNC explant retains osmosensitivity in vitro and can approach in vivo responsiveness. Thus, the entire functional pathway is present for the exploration of the functional integration of osmotic stimuli and the transduction of this information at the level of the supraoptic nucleus. We will characterize the release of vasopressin and establish the conditions which allow precise temporal and semi-quantitative analys of responsiveness via the SOHT. Effects of chemical and osmotic stimuli and their interactions will be characterized and will include screening for the effects of various putative neurotransmitters on the release of vasopressin (cholinergic, angiotensin II, norepinephrine, enkephalin, dopamine). In particular, we will focus on the specific contribution of the NSO with particular emphasis on the nicotinic cholinergic system which has been strongly implicated in the control of vasopressin secretion. The localization of putative nicotinic receptors on vasopressin-containing magnocellular neurons in NSO will be examined. Parallel experiments will be run to assess specific properties of these receptors which may contribute to osmotic sensitivity of the NSO. Investigation of the electrophysiological properties of identified cells in the NSO during chemical and osmotic challenges will provide detailed temporal comparison of the physiological responses with the release of vasopressin. Concomitant double- and triple-labeling procedures on dye-marked neurons will be used for cytoarchitectonic reconstruction and electron microscopic analysis of synaptic inputs on immunocytochemically-identified neurons in the NSO. Thus, the HNC explant represents a simple, viable, highly controlled mammalian neural system in which unknown physiological processes express their effects on a well defined final pathway. Ideal neuroendocrine control system.
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