Four pharmacologically distinct subtypes of alpha 4 beta 2 nicotinic acetylcholine receptor expressed in Xenopus laevis oocytes

Nicotinic receptors generally are presumed to consist of two alpha and thre e non-alpha subunits. We varied the relative levels of expression of the ne uronal nicotinic alpha 4 and beta 2 receptor subunits in Xenopus laevis ooc ytes by nuclear injection of cDNAs coding for these subunits in alpha:beta ratios of 9:1, 1:1, and 1:9. The sensitivities of the receptors to acetylch oline and d-tubocurarine were investigated in voltage-clamp experiments. Fo r receptors expressed at the 9:1 and 1:1 alpha:beta ratios, the EC50 value of acetylcholine is similar to 60 mu M. For the majority of the receptors e xpressed at the 1:9 alpha:beta ratio, the sensitivity to acetylcholine is e nhanced 30-fold. No evidence for more than one type of acetylcholine bindin g site in a single receptor is obtained. The sensitivity to d-tubocurarine decreases with decreasing alpha:beta ratio. IC50 values of d-tubocurarine a re 0.2, 0.5, and 2 mu M for the 9:1, 1:1, and 1:9 alpha:beta ratios, respec tively. At the 1:9 alpha:beta ratio, additional receptors with an IC50 valu e of 163 mu M d-tubocurarine are expressed. At least two components with di stinct sensitivities to d-tubocurarine are required to account for the shif t in IC50. The combined agonist and antagonist effects reveal four distinct subtypes of alpha 4 beta 2 nicotinic receptors. The results imply that the subunit stoichiometry of heteromeric alpha 4 beta 2 acetylcholine receptor s is not restricted to 2 alpha:3 beta.