Molecular Mechanisms of Neuropeptide Secretion from Neurohypophysial Terminals: a Dissertation

Abstract

A clear definition of the mechanisms involved in synaptic transmission is of paramount importance for the understanding of the processes governing synaptic efficacy. Despite decades of intense study, these mechanisms remain poorly understood. The work contained in this thesis examines several such mechanisms using the hypothalamic-neurohypophysial system (HNS), a classical preparation for the study of Ca-dependent neuropeptide release. The first portion of this thesis is comprised of my efforts to define the cellular machinery essential for the exocytosis of secretory granules isolated from peptidergic neurohypophysial terminals of the HNS. Here, using the planar lipid bilayer model system, I have been able to show that syntaxin alone in the target membrane is sufficient to elicit fusion of modified neurohypophysial secretory granules. Surprisingly, SNAP-25 does not appear to be necessary for this process. This suggests that syntaxin may be able to substitute for SNAP-25 to form functional non-cognate fusion complexes. Additionally, the coupling of amperometric detection with the planar lipid bilayer system has allowed me to confirm these results using native, unmodified secretory granules, and also provides some insight into the kinetics of release in this reconstituted system. This model system should provide a convenient means for the study of additional regulatory factors believed to be involved in secretory vesicle exocytosis. The second and third sections of this thesis involve my examination of the role of presynaptic Ca stores in neuropeptide secretion from isolated peptidergic neurohypophysial terminals (NHT). I initially examined the source of recently discovered ryanodine-sensitive Ca stores in this system. Using Immuno-electron microscopy I have found that ryanodine receptor (RyR) labeling appears to co-localize with large dense core granules. Additionally, I have shown that a large conductance cation channel, with similarities to the RyR, found in the membrane of these granules has the same characteristic response to pharmacological agents specific for the RyR. Further, application of RyR agonists modulates basal neuropeptide release from NHT. These results