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 α3/α4 subunit chimeras (Kuryatov et al., 2000a). Nicotine has full efficacy on the α4β2 nAChR and partial efficacy on the α3β2 nAChR. Now, we have exchanged individually three amino acids between the α4 and the α3 subunits at positions 226(M1), 258(M2), and 262(M2). Also, similar exchanges were made in the β2 and β4 subunits at positions 224(M1), 226(M1), and 254(M2) (using α 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 α subunit was valine (as in the α3 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 α3β2 nAChR channel, which results in a low-affinity channel block, causing the lower efficacy of nicotine on this nAChR.