Extending the analysis of nicotinic receptor antagonists with the study of α6 nicotinic receptor subunit chimeras

Heterologous expression systems have increased the feasibility of developing selective ligands to target nicotinic acetylcholine receptor (nAChR) subtypes. However, the α6 subunit, a component in nAChRs that mediates some of the reinforcing effects of nicotine, is not easily expressed in systems such as the Xenopus oocyte. Certain aspects of α6-containing receptor pharmacology have been studied by using chimeric subunits containing the α6 ligand-binding domain. However, these chimeras would not be sensitive to an α6-selective channel blocker; therefore we developed an α6 chimera (α4/6) that has the transmembrane and intracellular domains of α6 and the extracellular domain of α4. We examined the pharmacological properties of α4/6-containing receptors and other important nAChR subtypes, including α7, α4β2, α4β4, α3β4, α3β2, and α3β2β3, as well as receptors containing α6/3 and α6/4 chimeras. Our data show that the absence or presence of the β4 subunit is an important factor for sensitivity to the ganglionic blocker mecamylamine, and that dihydro-β-erythroidine is most effective on subtypes containing the α4 subunit extracellular domain. Receptors containing the α6/4 subunit are sensitive to α-conotoxin PIA, while receptors containing the reciprocal α4/6 chimera are insensitive. In experiments with novel antagonists of nicotine-evoked dopamine release, the α4/6 chimera indicated that structural rigidity was a key element of compounds that could result in selectivity for noncompetitive inhibition of α6-containing receptors. Our data extend the information available on prototypical nAChR antagonists, and establish the α4/6 chimera as a useful new tool for screening drugs as selective nAChR antagonists.