Beschreibung:
<jats:p>We have studied the interaction of the reversible acetylcholine esterase inhibitor (—)physostigimine (<jats:sc>d</jats:sc>‐eserine) with the nicotinic acetylcholine receptor (nAChR) from <jats:italic>Torpedo marmorata</jats:italic> electric tissue by means of ligandinduced ion flux into nAChR‐rich membrane vesicles and of equilibrium binding. We find that (—)physostigmine induces cation flux (and also binds to the receptor) even in the presence of saturating concentrations of antagonists of acetylcholine, such as <jats:sc>d</jats:sc>‐tubocurarine, α‐bungarotoxin or antibody WF6. The direct action on the acetylcholine receptor is not affected by removal of the methylcarbamate function from the drug and thus is not due to carbamylation of the receptor. Antibodies FK1 and benzoquinonium antagonize channel activation (and binding) of eserine, suggesting that the eserine binding site(s) is separate from, but adjacent to, the acetylcholine binding site at the receptor. In addition to the channel activating site(s) with an affinity of binding in the 50 μM range, there exists a further class of low‐affinity (K<jats:sub>d</jats:sub>˜ mM) sites from which eserine acts as a direct blocker of the acetylcholine‐activated channel. Our results suggest the existence of a second pathway of activation of the nAChR channel.</jats:p>