Vol. 61, Issue 6, 1416-1422, June 2002

First and Second Transmembrane Segments of 3, 4, 2, and 4 Nicotinic Acetylcholine Receptor Subunits Influence the Efficacy and Potency of Nicotine

Ray Rush, Alexander Kuryatov, Mark E. Nelson, and Jon Lindstrom

Department of Neuroscience, University of Pennsylvania Medical School, Philadelphia, Pennsylvania

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 3/4 subunit chimeras (Kuryatov et al., 2000a). Nicotine has full efficacy on the 42 nAChR and partial efficacy on the 32 nAChR. Now, we have exchanged individually three amino acids between the 4 and the 3 subunits at positions 226(M1), 258(M2), and 262(M2). Also, similar exchanges were made in the 2 and 4 subunits at positions 224(M1), 226(M1), and 254(M2) (using  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  subunit was valine (as in the 3 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 32 nAChR channel, which results in a low-affinity channel block, causing the lower efficacy of nicotine on this nAChR.