Binding of Alpha-Bungarotoxin to Single Identified Neurons of 'Aplysia' which have Different Ionic Responses to Acetylcholine,

Abstract : Identifiable Aplysia neurons have one or more of three different ionic responses to acetylcholine, due to Na, Cl, and K conductance increases, respectively. The nature of the acetylcholine receptor mediating these three responses was studied using alpha-bungarotoxin. All three physiologic responses are blocked by alpha-bungarotoxin and I(125) alpha-bungarotoxin binds saturably to single neurons dissected from the ganglia. The apparent dissociation constant for I(125) alpha-bungarotoxin binding is not significantly different in neurons with different ionic response to acetylcholine. Most Na neurons, however, have a greater density of acetylcholine receptors. When studied electrophysiologically only the Na responses are blocked by hexamethonium. These observations are consistent with the view that Na, Cl, and K responses to acetylcholine are activated through a single class of acetylcholine receptors. The efficacy of hexamethonium in blocking toxin binding to all types of neurons suggests that it has a common binding site on all Aplysia acetylcholine receptors. Thus the inhibition of the Na response by hexamethonium may be a result of the binding to a site which prevent the conductance change rather than preventing acetylcholine from binding to its receptor.