An electrophysiological characterization of naturally occurring tobacco alkaloids and their action on human α4β2 and α7 nicotinic acetylcholine receptors

Nicotinic acetylcholine receptor (nAChR) subtype-selective pharmacological profiles of tobacco alkaloids are essential for understanding the physiological effects of tobacco products. In this study, automated electrophysiology was used to functionally characterize the effects of distinct groups of tobacco alkaloids on human alpha 4 beta 2 and alpha 7 nAChRs. We found that, in tobacco alkaloids, pyridine as a hydrogen bond acceptor and a basic nitrogen atom at a distance of 4-7 angstrom are pharmacophoric elements necessary for molecular recognition by alpha 4 beta 2 and a7 nAChRs with various degrees of selectivity, potency, and efficacy. While four alkaloids-nicotine, nornicotine, anabasine and R-anatabine-potently activated alpha 4 beta 2, they were also weak agonists of alpha 7 nAChRs. Nicotine was the most potent agonist of alpha 4 beta 2, while anabasine elicited the highest activation of alpha 7. None of the tobacco alkaloids enhanced nAChR activity elicited by the endogenous ligand acetylcholine; therefore, none was considered to be a positive allosteric modulator (PAM) of either alpha 4 beta 2 or alpha 7 nAChRs. In contrast, we identified tobacco alkaloids, such as the tryptophan metabolite 6-hydroxykynurenic acid, that decreased the activity of both alpha 4 beta 2 and alpha 7 nAChRs. Our study identified a class of alkaloids with positive and negative effects against human alpha 4 beta 2 and alpha 7 nAChRs. It also revealed human alpha 4 beta 2 to be the principal receptor for sensing the most abundant alkaloids in tobacco leaves.