Functional contribution of α3L8′ to the neuronal nicotinic α3 receptor

The role of position L8′, located in transmembrane domain 1 of the neuronal nicotinic α3 subunit, was characterized by using two‐electrode voltage clamp in Xenopus oocytes. Four amino acids (Ala, Ser, Phe, and Tyr) were inserted at this conserved position, and the mutant subunit was coexpressed with either wild‐type β2 or β4 subunits. These substitutions led to significant alterations in the pharmacodynamic parameters of cholinergic agents, resulting in loss of function. Ala and Ser substitutions resulted in losses in agonist (ACh, nicotine, and DMPP) potency and intrinsic activity at both α3β2 and α3β4 receptors. Similarly, significant changes in antagonist potency were produced by the Ala and Ser substitutions. Phe and Tyr mutations did not alter the receptor's EC50 for ACh or nicotine but reduced the EC50 for DMPP at both receptors. The Phe mutation also reduced the intrinsic activity of all agonists tested at both receptors. The Tyr mutation, though, led to a decrease in intrinsic activity for all agonists at the α3β2 receptor, yet resulted in no changes for DMPP, a decrease for nicotine, and an increase for ACh at the α3β4 receptor. The most dramatic changes in the receptor's functional properties were produced by substitutions that introduced the largest changes in amino acid volume. Additional replacements (Gly, Thr, and Val) suggested an inverse correlation between amino acid volume at position α3L8′ and EC50 for α3β4 nAChRs; however, α3β2 nAChRs displayed a nonlinear correlation. These data demonstrate that structural alterations at position α3L8′ could propagate to the agonist‐binding site. © 2008 Wiley‐Liss, Inc.