Four pharmacologically distinct subtypes of alpha4beta2 nicotinic acetylcholine receptor expressed in Xenopus laevis oocytes.

Nicotinic receptors generally are presumed to consist of two alpha and three non-alpha subunits. We varied the relative levels of expression of the neuronal nicotinic alpha4 and beta2 receptor subunits in Xenopus laevis oocytes 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 acetylcholine and d-tubocurarine were investigated in voltage-clamp experiments. For receptors expressed at the 9:1 and 1:1 alpha:beta ratios, the EC50 value of acetylcholine is approximately 60 microM. For the majority of the receptors expressed at the 1:9 alpha:beta ratio, the sensitivity to acetylcholine is enhanced 30-fold. No evidence for more than one type of acetylcholine binding site in a single receptor is obtained. The sensitivity to d-tubocurarine decreases with decreasing alpha:beta ratio. IC50 values of d-tubocurarine are 0.2, 0.5, and 2 microM for the 9:1, 1:1, and 1:9 alpha:beta ratios, respectively. At the 1:9 alpha:beta ratio, additional receptors with an IC50 value of 163 microM d-tubocurarine are expressed. At least two components with distinct sensitivities to d-tubocurarine are required to account for the shift in IC50. The combined agonist and antagonist effects reveal four distinct subtypes of alpha4beta2 nicotinic receptors. The results imply that the subunit stoichiometry of heteromeric alpha4beta2 acetylcholine receptors is not restricted to 2alpha:3beta.