Nicotinic Acetylcholine Receptor Accessory Subunits Determine the Activity Profile of Epibatidine Derivatives.

Epibatidine is a potent analgetic agent with very high affinity for brain nicotinic acetylcholine receptors (nAChR). We determined the activity profiles of three epibatidine derivatives, RTI-36, RTI-76, and RTI-102, which have affinity for brain nAChR equivalent to that of epibatidine but reduced analgetic activity. RNAs coding for nAChR monomeric subunits and/or concatamers were injected into Xenopus oocytes to obtain receptors of defined subunit composition and stoichiometry. The epibatidine analogs produced protracted activation of high sensitivity (HS) α 4- and α 2-containing receptors with the stoichiometry of 2alpha:3beta subunits but not low sensitivity (LS) receptors with the reverse ratio of alpha and beta subunits. Although not strongly activated by the epibatidine analogs, LS α 4- and α 2-containing receptors were potently desensitized by the epibatidine analogs. In general, the responses of α 4(2) β 2(2) α 5 and β 3 α 4 β 2 α 6 β 2 receptors were similar to those of the HS α 4 β 2 receptors. RTI-36, the analog closest in structure to epibatidine, was the most efficacious of the three compounds, also effectively activating α 7 and α 3 β 4 receptors, albeit with lower potency and less desensitizing effect. Although not the most efficacious agonist, RTI-76 was the most potent desensitizer of α 4- and α 2-containing receptors. RTI-102, a strong partial agonist for HS α 4 β 2 receptors, was effectively an antagonist for LS α 4 β 2 receptors. Our results highlight the importance of subunit stoichiometry and the presence or absence of specific accessory subunits for determining the activity of these drugs on brain nAChR, affecting the interpretation of in vivo studies since in most cases these structural details are not known. SIGNIFICANCE STATEMENT: Epibatidine and related compounds are potent ligands for the high-affinity nicotine receptors of the brain, which are therapeutic targets and mediators of nicotine addiction. Far from being a homogeneous population, these receptors are diverse in subunit composition and vary in subunit stoichiometry. We show the importance of these structural details for drug activity profiles, which present a challenge for the interpretation of in vivo experiments since conventional methods, such as in situ hybridization and immunohistochemistry, cannot illuminate these details.
(Copyright © 2020 by The American Society for Pharmacology and Experimental Therapeutics.)