Eur J Pharmacol 1998 Dec 18;3632-3:197-202. doi: 10.1016/s0014-2999(98)00793-6.

Tacrine and physostigmine block nicotinic receptors in Xenopus oocytes injected with Torpedo electroplaque membranes.

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Tacrine and physostigmine were tested for direct nicotinic actions on Xenopus oocytes microinjected with Torpedo electroplaque membranes. In this preparation, responses to acetylcholine arise 6-8 h after microinjection, due to the incorporation of nicotinic receptors into the plasma membrane by a process not involving protein synthesis. Currents elicited by acetylcholine (100-1000 microM) were recorded by two-electrode voltage clamping. Tacrine (1-1000 microM) and physostigmine (1-100 microM) exerted a potent, reversible block of the nicotinic receptors. The concentration-dependence curves fitted simple hyperbolas, suggesting a stoichiometry of 1:1 in the drug-channel interactions. Currents elicited by the highest acetylcholine concentration were inhibited by tacrine with maximal affinity, indicating an action at a site other than the ligand-binding domain. Inhibition was reduced at depolarising potentials, which is consistent with a preferential interaction with the ligand-bound form of the receptor. Blockade by tacrine or physostigmine was accompanied by a concentration-dependent slowing of the desensitisation, resembling the action of local anaesthetics. These results could indicate a modulatory effect of these drugs on neurosecretion through nicotinic receptors.

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