Depolarization-secretion coupling is thought to occur via electrical activity leading to the entry of calcium and the subsequent secretion of transmitters. The molecular details of how such patterns of activity control the release of neuropeptides from nerve terminals in the intact central nervous system (CNS), however, remain undetermined. Vasopressin (AVP) and oxytocin (OT) are synthesized by magnocellular neurons of the hypothalamus and secreted from neurohypophysial (NH) terminals; together they comprise the Hypothalamic-Neurohypophysial System (HNS). OT neurons are characterized by a high frequency discharge during suckling or parturition which leads to the pulsatile release of OT. AVP neurons are characterized by their asynchronous phasic activity during maintained AVP release. In both cases, it is the clustering of spikes (bursting) which facilitates neuropeptid release. We have discovered that there are different calcium-channel subtypes in AVP vs. OT terminals, but that their biophysical properties alone cannot explain the differential facilitationof release by such burst patterns. Conversely, we have demonstrated that autocrine/paracrine purinergic feedback effects help determine the efficacy of different bursting patterns of electrica activity in facilitating release of AVP vs. OT. Along with ATP, Zinc is also co-released with the HNS peptides. Zn2+ interacts with many effectors on neurons, leading to a variety of effects. It is not known, however, at what specific sites these effects occur at synapses in the CNS. The HNS now affords the unique opportunity of unraveling the complicated effects of endogenous Zinc in the CNS by comparing such effects on isolated terminals vs. the intact, whole system. The goal of the research proposed here is to determine molecular mechanisms that mediate endogenous Zinc-induced facilitation of neuropeptide secretion during physiological patterns of electrical stimulation. To achieve these objectives, recordings of ATP- and calcium-currents will be made from identified (as AVP vs. OT), isolated nerve terminals vs. intact preparations of the HNS of adult rats and mice. Selective fluorescent-indicator dyes will monitor intracellular Ca2+ and Zn2+ changes in NH terminals. Effects on neuropeptide release will be compared between the intact HNS and isolated NH terminals by the use of well-defined in vitro perfusion assays and capacitance measurements. This proposal takes advantages of newly available genetic tools that facilitate the elucidation of the function of novel Zinc receptors with greater specificty than is possible with traditional antagonist drugs. These receptor knockout studies will provide a unique opportunity to determine if endogenous Zinc feedback regulation occurs at the terminals of CNS neurons. Furthermore, since synaptic vesicles/neurosecretory granules appear to contain ATP and Zn2+, these feedback mechanisms could be physiologically important at many other synapses in the CNS.
Purines and Zinc mediate effects that include modulation of neurotransmission, synaptic plasticity, drug addiction, aggressive behavior, memory loss, and have possible roles in Alzheimer's disease. Furthermore, oxytocin has an important role in social behaviors and in Autism. This proposal takes advantages of newly available genetic tools that facilitate the elucidation of the function of these different transmitters and gives hope for he determination of such feedback effects at CNS synapses, only then can therapeutic drugs be targeted to alleviate such diseases.
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