On the specificity of 125-I-alpha-bungarotoxin binding to axonal membranes.

125I-alpha-Bungarotoxin (alpha-BGT) was used to characterize the binding sites for cholinergic ligands in lobster walking leg nerve membranes. The toxin binding component has been visualized histochemically on the external surfaces of intact axons and isolated axonal membrane fragments. Binding of alpha-BGT to nerve membrane preparations was demonstrated to be saturable and highly reversible (KappD congruent to 1.7 +/- 0.32 X 10(-7) M; Bmax congruent to 249 +/- 46 pmol/mg protein) at pH 7.8, 10 mM-Tris buffer. Binding showed a marked sensitivity to ionic strength that was attributable to the competitive effects of inorganic cations (particularly Ca2+ and Mg2+) in the medium. 125I-alpha-BGT binding could be inhibited by cholinergic drugs (atropine congruent to d-tubocurarine greater than nicotine greater than carbamylcholine congruent to choline) and local anesthetics (procaine greater that tetracaine congruent to lidocaine), but was unaffected by other neuroactive compounds tested (e.g., tetrodotoxin, 4-aminopyridine, quinuclidinyl benzilate, octopamine, bicuculline, haloperidol, ouabain). The pharmacological sensitivity of toxin binding resembles that of nicotine binding to axonal membranes, but differs significantly from nicotinic cholinergic receptors described in neuromuscular junctions, fish electric organs, sympathetic ganglia, and the CNS. The possible physiological relevance of the axonal cholinergic binding component and its relationship to alpha-BGT binding sites in other tissues are discussed.