PHYSOSTIGMINE, GALANTHAMINE AND CODEINE ACT AS NONCOMPETITIVE NICOTINIC RECEPTOR AGONISTS ON CLONAL RAT PHEOCHROMOCYTOMA CELLS

The acetylcholine esterase inhibitor (-)-physostigmine has been shown to act as agonist on nicotinic acetylcholine receptors from muscle and brain, by binding to sites on the a-polypeptide that are distinct fro m those for the natural transmitter acetylcholine (Schroder et al., 19 94). In the present report we show that (-)-physostigmine, galanthamin e, and the morphine derivative codeine activate single-channel current s in outside-out patches excised from clonal rat pheochromocytoma (PC1 2) cells. Although several lines of evidence demonstrate that the thre e alkaloids act on the same channels as acetylcholine, the competitive nicotinic antagonist methyllycaconitine only inhibited channel activa tion by acetylcholine but not by (-)-physostigmine, galanthamine or co deine. In contrast, the monoclonal antibody FK1, which competitively i nhibits (-)-physostigmine binding to nicotinic acetylcholine receptors , did not affect channel activation by acetylcholine but inhibited act ivation by (-)-physostigmine, galanthamine and codeine. The three alka loids therefore act via binding sites distinct from those for acetylch oline, in a 'noncompetitive' fashion. The potency of (-)-physostigmine and related compounds to act as a noncompetitive agonist is unrelated to the level of acetylcholine esterase inhibition induced by these dr ugs. (-)-Physostigmine, galanthamine and codeine do not evoke sizable whole-cell currents, which is due to the combined effects of low open- channel probability, slow onset and slow inactivation of response. In contrast, they sensitize PC12 cell nicotinic receptors in their submax imal response to acetylcholine. While the abundance of nicotinic acety lcholine receptor isoforms expressed in PC12 cells excludes identifica tion of specific nicotinic acetylcholine receptor subtypes that intera ct with noncompetitive agonists, the identical patterns of single-chan nel current amplitudes observed with acetylcholine and with noncompeti tive agonists suggested that all PC12 cell nicotinic acetylcholine rec eptor subtypes that respond to acetylcholine also respond to noncompet itive agonist. The action of noncompetitive agonists therefore seems t o be highly conserved between nicotinic acetylcholine receptor subtype s, in agreement with the high level of structural conservation in the sequence region harboring major elements of this site.