Eserine and other tertiary amine interactions with Torpedo acetylcholine receptor postsynaptic membrane vesicles

Interaction of the tertiary amines, arecolone, eserine (physostigmine), (+) -epibatidine, and (+/-)-epibatidine. with Torpedo nicotinic acetylcholine r eceptor-enriched membrane vesicles was investigated to characterize their a ction on the receptor, using stopped-flow thallium (I)-flux spectrofluorime try. Arecolone, (+)-epibatidine, and (+/-)-epibatidine were agonists with a ctivation constants of 390, 19, and 39 mu M, respectively. Eserine was not an agonist but rather an antagonist for agonist-induced activation of the r eceptor with an inhibition constant of similar to 150 mu M. The choice of t he fluorescent dye used (entrapped within the membrane vesicles) was critic al for interpretation of the effects of eserine. With 1,3,6,8-pyrene tetras ulfate (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 rathe r than eserine-induced cation transport. The use of a different fluorescent dye, 8-aminonaphthaline- 1,3,6-trisulfate, with which eserine does not int eract allowed demonstration of the action of eserine as an antagonist rathe r than as an agonist.