Inhibition of choline acetyltransferase activity in squid giant axon

Abstract

Using a sensitive radiometric assay the choline acetyltransferase (acetyl-CoA: choline O-acetyltransferase, EC 2.3.1.6) activities of the squid fin nerve, giant axon, axoplasm, and axon envelope were determined. Hexamethylene-1-[4(1-naphthylethenyl)pyridinium]-6-trimethyl ammonium bromide (Compound I) inhibited 94 and 88% of choline acetyltransferase in the intact giant axon at 5 · 10 −3 and 5 · 10 −4 M, respectively, when experiments were conducted in the dark. Exposure of solutions of the inhibitor to light caused inactivation (5 · 10 −3 M gave only 33% inhibition). Compound I (5 · 10 −3 M) depolarized the axon and blocked conduction equally well in the light and dark. Another choline acetyltransferase inhibitor 4-(1-naphthylethynyl)pyridine methiodide (Compound III) had similar effects as described for Compound I except that its potency as a choline acetyltransferase inhibitor was not inactivated by light. Pretreatment of the axon with 25 μg/ml of cottonmouth moccasin venom caused a 5-fold potentiation of the ability of Compounds I and III to block conduction, whereas the venom did not affect the choline acetyltransferase inhibition produced by these compounds. Compounds I and III (1 · 10 −3 M) did not alter the maximal number of stimuli the squid axon could conduct when stimulated at 100 per second or when repetitively firing due to decreasing divalent cation content of media. It is concluded that block of conduction and inhibition of choline acetyltransferase, by these compounds, are separate unrelated effects.