Analysis of alpha-bungarotoxin binding in the goldfish central nervous system

We report here the equilibrium, kinetic, and pharmacological analysis of alpha-125I-bungarotoxin (alpha-125I-Bgt) binding to a Triton X-100-solubilized goldfish brain synaptosomal fraction. In addition, a refined analysis of equilibrium binding to a particulate synaptosomal fraction is presented. Equilibrium binding from both particulate and soluble fractions revealed an apparent heterogeneity of binding sites. Kinetic analysis of the soluble receptor revealed linear association kinetics and nonlinear dissociation kinetics. The dissociation curve suggested the presence of at least two rate constants. Potential sources of the binding heterogeneity found in both the equilibrium binding and dissociation kinetics experiments are (1) multiple receptor species, (2) multiple ligand species, and (3) different, possibly interconvertible, states of a single receptor type. No evidence for the first two alternatives was found. Support for the third alternative was obtained by observing the effect of cholinergic ligands on alpha-125I-Bgt dissociation. Carbamylcholine and d-tubocurarine increased the apparent proportion of rapidly dissociating sites, suggesting that the two binding affinities can be interconverted and may arise from a single receptor type. Evidence concerning the identity of the alpha-Bgt binding protein as a nicotinic acetylcholine receptor is discussed.