Nicotinic acetylcholine receptors that bind alpha-bungarotoxin on neurons.

摘要

Neuronal nicotinic acetylcholine receptors (nAChRs) that bind {dollar}alpha{dollar}-bungarotoxin ({dollar}alpha{dollar}Bgt) are widely distributed in both the central and peripheral nervous systems. Despite their abundance, only recently has a physiological response been demonstrated for a native receptor of this class ({dollar}alpha{dollar}Bgt-AChRs). The chick ciliary ganglion (CG) provides an excellent system for the study of such receptors, as they are abundant, developmentally regulated, and diffuse throughout the plasma membrane. Their function is unknown; synaptic transmission through the ganglion is mediated by a second, distinct class of nAChRs (mAb 35-AChRs).; Both nAChR classes exhibit cholinergic pharmacology as determined by competition binding studies. {dollar}alpha{dollar}Bgt-AChRs exhibit higher affinity for ligands such as {dollar}alpha{dollar}Bgt, {dollar}alpha{dollar}-Cobrotoxin, methyllycaconitine (MLA), and nicotine, while mAb 35-AChRs exhibit higher affinity for L-lobeline. A novel class of {dollar}alpha{dollar}Bgt-binding nAChRs found in embryonic chick muscle and containing the {dollar}alpha{dollar}7 gene product shows a similar pharmacological profile to its neuronal counterpart.; Recent studies showing that {dollar}alpha{dollar}Bgt-AChRs are ligand-gated ion channels whose activation leads to an elevation of intracellular calcium levels led to consideration of the cellular and developmental events that might be influenced through these receptors. One candidate for such an action is the regulation of neuronal morphology, as neurite initiation and outgrowth have previously been shown to be influenced by intracellular calcium levels. In this dissertation, it is shown that applications of nicotine and ACh can cause retraction of neurites on CG neurons in a calcium-dependent manner and that this retraction is apparently mediated through {dollar}alpha{dollar}Bgt-AChR activation of voltage-dependent calcium channels.; A sensitive solid-phase immunoprecipitation assay has revealed a novel minor population of receptors that bind both {dollar}alpha{dollar}Bgt and mAb 35. The putative receptors do not appear to contain any of the known neuronal nAChR gene products nor the one muscle AChR gene product that could account simultaneously for the {dollar}alpha{dollar}Bgt and mAb 35 binding properties. A newly cloned rat gene ({dollar}alpha{dollar}9) also does not appear to provide a candidate. The results suggest that additional nAChR genes may remain to be cloned and that individual neurons may have as many as four classes of nAChRs with different subunit compositions.