''Orphan'' alpha 6 nicotinic AChR subunit can form a functional heteromeric acetylcholine receptor

Previously, a rat brain cDNA was reported that was designated alpha 6 because of its homology with nicotinic acetylcholine receptor (AChR) alpha subunits, being especially similar to alpha 3, but no acetylcholine-gated cation channels were detected when it was expressed in Xenopus laevis oocytes alone or in combination with other known rat AChR subunits. We cloned chicken alpha 6 and human beta 4 AChR subunits and tested for acetylcholine-gated cation channels with alpha 6 by expression in X. laevis oocytes alone or in pairwise combination with chicken alpha 3, beta 2, or beta 4 or with human alpha 3, beta 2, or beta 4 AChR subunits. Chicken alpha 6 formed detectable functional AChRs only when expressed together with the human beta 4 subunit. The alpha 6 beta 4 AChR-mediated currents show strong inward rectification and dependence on extracellular Ca2+. It exhibited a distinct pharmacological profile with an EC(50) value of 28 mu M for acetylcholine, 24 nM for (+)-epibatidine, 6.6 mu M cytisine, and 15 mu M 1,1-dimethyl-4-phenylpiperazinium. Both cytisine and 1,1-dimethyl-4-phenylpiperazinium behaved as partial (similar to 30%) agonists. Remarkably, nicotine (EC(50) = 22 mu M) was an even weaker partial agonist (similar to 18%) and had a relatively long-lasting inhibitory effect. Coexpression of the previously cloned rat alpha 6 subunit with the human the beta 4 subunit also resulted in functional alpha 6 beta 4 AChRs with properties resembling those of the chicken/human alpha 6 beta 4 AChRs, Therefore, alpha 6 can function as part of AChRs with unusual pharmacological properties.