Both CY- and ,&subunits Contribute to the Agonist Sensitivity of Neuronal Nicotinic Acetylcholine Receptors

A family of genes has been identified that encodes subunits of nicotinic acetylcholine receptors (nAChRs) and is expressed in the nervous system. Functional neuronal nAChRs can be expressed in Xenopus oocytes by injection of RNA encoding 1 of 2 different &subunits (/32,84) in pairwise combination with RNA encoding 1 of 3 different a-subunits (a2, (~3, a4). We examined the sensitivity of these 6 different aB-subunit combinations to the nicotinic agonists ACh, nicotine, cytisine, and 1,l -dimethyl-4-phenylpiperazinium (DMPP). Each subunit combination displayed a distinct pattern of sensitivity to these 4 agonists. The a2B2 combination was B-fold more sensitive to nicotine than to acetylcholine, while the a382 combination was 17-fold less sensitive to nicotine than to ACh, and the a384 combination was equally sensitive to both nicotine and ACh. nAChRs composed of ~2, a3, or a4 in combination with /32 were 14-lOO-fold less sensitive to cytisine than to ACh. In contrast, nAChRs composed of a2, a3, or a4 in combination with /34 were 3-17fold more sensitive to cytisine than to ACh. The a2B2, a3/32, and a384 combinations were each equally sensitive to DMPP and ACh, while the a2/34, a4/32, and a484 combinations were 4-24-fold less sensitive to DMPP than to ACh. We also demonstrated that these differences are neither a consequence of variation in the relative amounts of RNA injected nor an artifact of oocyte expression. The oocyte system can accurately express ligand-gated ion channels because mouse muscle nAChRs expressed in oocytes display pharmacological properties similar to those reported for these receptors expressed on BC3H-1 cells. We conclude that both the aand the &subunits contribute to the pharmacological characteristics of neuronal nAChRs.