Untranslated region-dependent exclusive expression of high-sensitivity subforms of α4β2 and α3β2 nicotinic acetylcholine receptors

alpha 4 beta 2 nicotinic acetylcholine receptors (nAChRs) are recognized as the principal nicotine binding site in brain. Recombinant alpha 4 beta 2 nAChR demonstrate biphasic concentration-response relationships with low- and high-EC50 components. This study shows that untranslated regions (UTR) can influence expression of high-sensitivity subforms of alpha 4 beta 2 and alpha 3 beta 2 nAChR. Oocytes injected with alpha 4 and beta 2 RNA lacking UTR expressed biphasic concentration-response relationships for acetylcholine with high-sensitivity EC50 values of 0.5 to 2.5 mu M (14-24% of the population) and low- sensitivity EC50 values of 110 to 180 mu M (76-86%). In contrast, message with UTR expressed exclusively the high-sensitivity alpha 4 beta 2 nAChR subform with an acetylcholine EC50 value of 2.2 mu M. Additional studies revealed pharmacological differences between high- and low-sensitivity alpha 4 beta 2 subforms. Whereas the antagonists dihydro-beta-erythroidine (IC50 of 3-6 nM) and methyllycaconitine IC50 of 40-135 nM) were not selective between high- and low-sensitivity alpha 4 beta 2, chlorisondamine, mecamylamine, and d-tubocurarine were, respectively, 100-, 8-, and 5-fold selective for the alpha 4 beta 2 subform with low sensitivity to acetylcholine. Conversely, agonists that selectively activated the high- sensitivity alpha 4 beta 2 subform with respect to efficacy as well as potency were identified. Furthermore, two of these agonists were shown to activate mouse brain alpha 4 beta 2 as well as the ferret high- sensitivity alpha 4 beta 2 expressed in Xenopus laevis oocytes. With the use of UTR-containing RNA, exclusive expression of a novel high-sensitivity alpha 3 beta 2 nAChR was also achieved. These studies 1) provide further evidence for the existence of multiple subforms of alpha 4 beta 2 nAChR, 2) extend that to alpha 3 beta 2 nAChR, 3) demonstrate UTR influence on beta 2-containing nAChR properties, and 4) reveal compounds that interact with alpha 4 beta 2 in a subformselective manner.