Alteration of Acetylcholine Penetration into, and Effects on, Venom-Treated Squid Axons by Physostigmine and Related Compounds

Abstract

Choline and neostigmine markedly antagonize the effect of acetylcholine (ACh) on the action potential of the venom-treated squid axon, although they themselves have no effect on conduction. Physostigmine also antagonizes the blocking action of ACh at a concentration well below that which has any effect on conduction. In contrast, d-tubocurarine (curare) increases the effect of ACh on the action potential. Choline, neostigmine, and physostigmine markedly decrease the penetration of C14-1abeled ACh into the axoplasm of the squid axon. Curare, in contrast, increases the penetration of ACh, whereas dimethylcurare gives variable results. The results provide an explanation why physostigmine and neostigrnine do not influence the action of ACh on axonal conduction in a way similar to that observed at the junction. The additive effect of curare and ACh on the action potential may be due either to the greater rate of penetration of ACh or to an additive effect of the two compounds on the receptor, or to a combination of both factors. I N T R O D U C T I O NThe electrical activity of the squid giant axon is normally unaffected by the external application of even very high concentrations of acetylcholine (ACh), neosdgmine, d-tubocurarine (curare), and other lipid-insoluble quaternary nitrogen compounds. Nachmansohn attributed this failure to the existence of permeability barriers for these lipid-insoluble compounds. He and his associates have shown that these compounds do not penetrate into the axoplasm of the squid axon in contrast to the lipid-soluble compounds which affect electrical activity (1-3). If ACh has an essential role in axonal conduction as postulated by Nachmansohn (4), a reduction of the barriers surrounding the squid axon may allow the demonstration of an effect of ACh. An action of ACh and curare on the electrical activity of the squid axon has indeed been demonstrated by pretreatment of the axon with certain snake venoms I I I 7