Efferent function of vestibular afferent endings? Similar localization of N-type calcium channels, synaptic vesicle and synaptic membrane-associated proteins

We investigated the distribution of N-type voltage-dependent calcium channe ls that mediate Ca2+ entry initiating transmitter release in the rat vestib ular sensory epithelium. We used confocal microscopy to assess the in vitro labeling by fluorescent specific ligand binding, omega-conotoxin-GVIA and also the immunolabeling of presynaptic soluble N-ethylmaleimide-sensitive f usion factor attachment protein receptor (SNARE) proteins, syntaxin, 25,000 mel, wt synaptosome-associated protein and synaptotagmin: components of th e neurotransmitter exocytosis machinery. We found that there was a close an atomical association between the voltage-gated calcium channels, the synapt ic vesicle and synaptic membrane-associated proteins on the efferent nerve calyces and probably afferent boutons, which are postsynaptic compartments. Our data suggest that these peripheral afferent endings possess the presyna ptic Ca2+ channels and the components of the prosynaptic SNARE proteins inv olved in synaptic vesicle docking and calcium-dependent exocytosis. They pr ovide additional evidence Fur a secretory function and efferent role of the se endings in hair cell neurotransmission. (C) 2000 IBRO. Published by Else vier Science Ltd. All rights reserved.