Functions and pharmacology of α2β2 nicotinic acetylcholine receptors; in and out of the shadow of α4β2 nicotinic acetylcholine receptors

Abstract

Although α2 was the first neuronal nicotinic acetylcholine receptor (nAChR) receptor subunit to be cloned, due to its low level of expression in rodent brain, its study has largely been neglected. This study provides a comparison of the α2 and α4 structures and their functional similarities, especially in regard to the existence of low and high sensitivity forms based on subunit stoichiometry. We show that the pharmacological profiles of the low and high sensitivity forms of α2β2 and α4β2 receptors are very similar in their responses to nicotine, with high sensitivity receptors showing protracted responses. Sazetidine A, an agonist that is selective for the high sensitivity α4 receptors also selectively activates high sensitivity α2 receptors. Likewise, α2 receptors have similar responses as α4 receptors to the positive allosteric modulators (PAMs) desformylflustrabromine (dFBr) and NS9283. We show that the partial agonists for α4β2 receptors, cytisine and varenicline are also partial agonists for α2β2 receptors. Studies have shown that levels of α2 expression may be much higher in the brains of primates than those of rodents, suggesting a potential importance for human therapeutics. High-affinity nAChR have been studied in humans with PET ligands such as flubatine. We show that flubatine has similar activity with α2β2 and α4β2 receptors so that α2 receptors will also be detected in PET studies that have previously presumed to selectively detect α4β2 receptors. Therefore, α2 receptors need more consideration in the development of therapeutics to manage nicotine addiction and declining cholinergic function in age and disease.