Cesa_2012_Mol.Pharmacol_81_239

We are interested in the allosteric modulation of neuronal nicotinic acetylcholine receptors (nAChRs). We have postulated that the anthelmintic morantel (Mor) positively modulates (potentiates) rat alpha3beta2 receptors through a site located at the beta(+)/alpha(-) interface that is homologous to the canonical agonist site (J Neurosci 29:8734-8742, 2009). On this basis, we aimed to determine the site specificity by studying differences in modulation between alpha3beta2 and alpha4beta2 receptors. We also compared modulation by Mor with that of the related compound oxantel (Oxa). Whereas Mor and Oxa each potentiated alpha3beta2 receptors 2-fold at saturating acetylcholine (ACh) concentrations, Mor had no effect on alpha4beta2 receptors, and Oxa inhibited ACh-evoked responses. The inhibition was noncompetitive, but not due to open channel block. Furthermore, the nature and extent of modulation did not depend on subunit stoichiometry. We studied six positions at the alpha(-) interface that differ between alpha3 and alpha4. Two positions (alpha3Ile57 and alpha3Thr115) help mediate the effects of the modulators but do not seem to contribute to specificity. Mutations in two others (alpha3Leu107 and alpha3Ile117) yielded receptors with appreciable alpha4-character; that is, Mor potentiation was reduced compared with wild-type alpha3beta2 control and Oxa inhibition was evident. A fifth position (alpha3Glu113) was unique in that it discriminated between the two compounds, showing no change in Mor potentiation from control but substantial Oxa inhibition. Our work has implications for rational drug design for nicotinic receptors and sheds light on mechanisms of allosteric modulation in nAChRs, especially the subtle differences between potentiation and inhibition.