Although there is considerable evidence that the electrical activity of neuronal somata leads to the entry of Ca2+ and to the subsequent secretion of transmitters (i.e., Depolarization-secretion coupling), the molecular details of how ionic currents control the release of specific neuroactive substances from nerve terminals remain undetermined. Vasopressin (AVP) and oxytocin (OT) have crucial roles in fluid homeostatic mechanisms and in various reproductive functions. Both hormones may also be central neurotransmitters and have been implicated in stress, learning, and memory processes, as well as the development and maintenance of tolerance to ethanol. These peptides are synthesized by magnocellular neurons (MCN) of the hypothalamus and secreted from neurohypophysial (NH) terminals. OT neurons are characterized by a high frequency discharge during suckling which leads to the pulsatile release of OT. AVP neurons are characterized by their asynchronous phasic activity (bursting) during maintained AVP release. In both cases, it is the clustering, with different time courses for each peptide, of spikes which facilitates hormone release. We hypothesize that autocrine/paracrine feedback effects might help determine efficacy of bursting patterns of electrical activity to facilitate release of AVP vs. OT. ATP is thought to be co-released with the HNS peptides. Purines, such as ATP and adenosine, interact with specific receptors on neurons and glia, leading to a variety of effects. It is not known, however, whether these effects are at somata and/or synapses in the central nervous system (CNS). Although the electrical and secretory activities of the HNS are affected by purines, their specific effects on membrane ionic conductances in these CNS neurons and their nerve terminals are not well understood. The HNS affords the unique opportunity of unraveling the complicated effects of purines in the CNS by comparing such effects on different neuronal compartments. The goal of the research proposed here is to determine membrane mechanisms that mediate purinergic-induced modifications of neurohormone secretion resulting from electrical stimulation. To achieve these objectives, perforated-patch recordings of Ca2+ currents will be made from identified, isolated nerve terminals obtained from the HNS of adult rats. Effects on release will be compared between the HNS and NH terminals by the use of radioimmunoassays and capacitance measurements. Patch-clamp recordings from nerve terminals in the intact HNS will be exploited in order to analyze how bursting electrical activity regulates peptide release in the complete system. These studies will provide a unique opportunity to determine if purinergic feedback regulation occurs at the terminals of CNS neurons.
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