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 alpha 6 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 alpha 6-containing receptor pharmacology have been studied by using chimeric subunits containing the alpha 6 ligand-binding domain. However, these chimeras would not be sensitive to an alpha 6-selective channel blocker; therefore we developed an alpha 6 chimera (alpha 4/6) that has the transmembrane and intracellular domains of alpha 3 and the extracellular domain of alpha 4. We examined the pharmacological properties of alpha 4/6-containing receptors and other important nAChR subtypes, including alpha 7, alpha 4 beta 2, alpha 4 beta 4, alpha 3 beta 4, alpha 3 beta 2, and alpha 3 beta 2 beta 3, as well as receptors containing alpha 6/3 and alpha 6/4 chimeras. Our data show that the absence or presence of the beta 4 subunit is an important factor for sensitivity to the ganglionic blocker mecamylamine, and that dihydro-beta-erythroidine is most effective on subtypes containing the alpha 4 subunit extracellular domain. Receptors containing the alpha 6/4 subunit are sensitive to alpha-conotoxin PIA, while receptors containing the reciprocal alpha 4/6 chimera are insensitive. In experiments with novel antagonists of nicotine-evoked dopamine release, the alpha 4/6 chimera indicated that structural rigidity was a key element of compounds that could result in selectivity for noncompetitive inhibition of alpha 6 containing receptors. Our data extend the information available on prototypical nAChR antagonists, and establish the alpha 4/6 chimera as a useful new tool for screening drugs as selective nAChR antagonists. (c) 2008 Elsevier Ltd. All rights reserved.