Neurotransmitter release occurs as a result of axonal spikes invading nerve terminals. Although there is considerable evidence that depolarization of neuronal somata leads to the entry of Ca2+ and to the subsequent secretion of neurotransmitters and/or neurohormones, the molecular details of how ionic currents control the release of neuroactive substances from nerve terminals remain undetermined. This proposal takes advantage of a mammalian system in which these questions can be directly addressed. Much is already known about the electrical activity of the rat neurohypophysis and the subsequent release of its peptide hormones has been well characterized. It is now uniquely possible to prepare isolated nerve terminals from this neuroendocrine structure which respond to depolarization by releasing identified peptides via Ca2+ -dependent exocytosis. Recent studies have shown that release from the neurohypophysis is regulated by specific patterns of electrical activity. Thus these nerve terminals have all the properties necessary to analyze in detail the process of depolarization-secretion coupling. We propose to study, using patch-clamp and biochemical methodology, the electrophysiology of identified, in terms of peptide secreted, nerve terminals in conjunction with their differential release of neurohormones. In particular we propose to characterize in detail the Ca2+ channels found at these nerve terminals and to determine which are important for the release of vasopressin vs oxytocin. Furthermore, we will look for possible intraterminal sites of Ca2+ action leading to the exocytotic secretion of these neuroactive substances. Elucidation of the molecular mechanisms underlying such interactions would represent a major advance in the understanding of how neuronal communication is regulated.
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