Abstract

Two functional types of nicotinic acetylcholine receptors (nAChRs) are expressed when human embryonic kidney cells are permanently transfected with equal amounts of human α4 and β2 subunit cDNAs. Most (82%) of these nAChRs exhibit an EC₅₀of 74 ± 6 μM for ACh, a much lower sensitivity than the remaining fraction (EC₅₀ of 0.7 ± 0.4 μM) or than expected from expression of equal amounts of α4 and β2 mRNAs inXenopus laevis oocytes. We have found three conditions that can increase the number of nAChRs with high sensitivity to activation. These are: 1) transient transfection with additional β2 subunits, 2) overnight incubation in nicotine, or 3) overnight culture at 29°C. Using metabolic labeling with [³⁵S]methionine to measure subunit stoichiometry, we found that the majority of nAChRs had a stoichiometry of (α4)₃(β2)₂. Overnight treatment with nicotine increased the number of nAChRs and increased the proportion of the (α4)₂(β2)₃stoichiometry. Alternate α4β2 nAChR stoichiometries with distinct functional properties raise the possibility for an interesting mode of synaptic regulation for nicotinic signaling in the mammalian brain.