Eserine and Other Tertiary Amine Interactions withTorpedoAcetylcholine Receptor Postsynaptic Membrane Vesicles

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Abstract
Interaction of the tertiary amines, arecolone, eserine (physostigmine), (+)-epibatidine, and (±)-epibatidine, with Torpedo nicotinic acetylcholine receptor-enriched membrane vesicles was investigated to characterize their action on the receptor, using stopped-flow thallium (I)-flux spectrofluorimetry. Arecolone, (+)-epibatidine, and (±)-epibatidine were agonists with activation constants of 390, 19, and 39 μM, respectively. Eserine was not an agonist but rather an antagonist for agonist-induced activation of the receptor with an inhibition constant of ∼150 μM. The choice of the fluorescent dye used (entrapped within the membrane vesicles) was critical for interpretation of the effects of eserine. With 1,3,6,8-pyrene tetrasulfate (PTS), eserine appeared to act as an agonist. However, it was shown that such an effect was caused by rapid diffusion of the uncharged form of the amine across the membrane followed by direct interaction with PTS rather than eserine-induced cation transport. The use of a different fluorescent dye, 8-aminonaphthaline-1,3,6-trisulfate, with which eserine does not interact allowed demonstration of the action of eserine as an antagonist rather than as an agonist.