Cholinergic drugs potentiate human nicotinic α4β2 acetylcholine receptors by a competitive mechanism

Effects of cholinergic drugs on human α4β2 nicotinic acetylcholine receptors expressed in Xenopus oocytes have been investigated in electrophysiological and ligand binding experiments. Atropine, scopolamine, physostigmine, and tacrine combine potentiation of ion current induced by low concentrations of acetylcholine with inhibition of ion current evoked by high concentrations of acetylcholine. Rivastigmine, galanthamine, and dichlorvos cause only inhibition of ion current evoked by low concentrations of acetylcholine. Binding experiments show that the potentiating cholinergic drugs atropine, scopolamine, and physostigmine are competitive ligands of human α4β2 nicotinic acetylcholine receptors. Conversely, the inhibitory cholinergic drugs galanthamine and rivastigmine are non-competitive. The non-competitive drugs are not allosteric, since they do not affect the saturation curve of the radioligand [3H]cytisine. Effects of potentiating cholinergic drugs on nicotinic acetylcholine receptors are consistent with and predicted by a model comprising competitive drug effects at two equivalent agonist recognition sites on the nicotinic acetylcholine receptor combined with non-competitive ion channel block.