First and Second Transmembrane Segments of α3, α4, β2, and β4 Nicotinic Acetylcholine Receptor Subunits Influence the Efficacy and Potency of Nicotine

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Abstract
The first three transmembrane segments (M1-M3) of human nicotinic acetylcholine receptors (nAChRs) have been implicated in determining the efficacy of nicotine by studies of alpha3/alpha4 subunit chimeras. Nicotine has full efficacy on the alpha4beta2 nAChR and partial efficacy on the alpha3beta2 nAChR. Now, we have exchanged individually three amino acids between the alpha4 and the alpha3 subunits at positions 226(M1), 258(M2), and 262(M2). Also, similar exchanges were made in the beta2 and beta4 subunits at positions 224(M1), 226(M1), and 254(M2) (using alpha subunit numbering). Expression of these mutated nAChRs in Xenopus laevis oocytes showed that the mutated M1 amino acids were important in influencing the potency of ACh and nicotine. It is hypothesized that these M1 amino acids affect the stability between the resting and activated states of the nAChR. M2 amino acids altered the efficacy of nicotine, usually without altering its potency. When the residue located at position 258 in the M2 region of the alpha subunit was valine (as in the alpha3 subunit), the resulting nAChR exhibited partial efficacy for nicotine that was voltage-dependent. Therefore, we believe that these M2 amino acids contribute to the formation of a binding site for nicotine in the alpha3beta2 nAChR channel, which results in a low-affinity channel block, causing the lower efficacy of nicotine on this nAChR.