The Role of the Fifth Subunit on Sensitivity of α4β2-Containing Nicotinic Acetylcholine Receptors to Allosteric Modulators

Abstract

α4β2 nicotinic acetylcholine receptors (nAChRs) belong to the family of pentameric ligand gated ion channels, which includes muscle nAChRs, 5-HT3, GABA receptors type A and C, and glycine receptors. α4β2 nAChRs are the predominant heteromeric nAChR in the brain. The α4β2 nAChR is the predominant nAChR subtype in the brain. Here, it constitutes one of the most important modulatory receptor systems influencing activities such as cognition, mood, consciousness and nociception and converging evidence indicates that this type is a key mediator of the rewarding and reinforcing effects of nicotine. α4 and β2 nAChR subunits assemble into alternate stoichiometries (α4β2)2α4 and (α4β2)2β2 that display remarkable differing sensitivity to activation by agonists and allosteric modulators (Moroni et al., 2006; Carbone et al., 2009). Positive allosteric modulators of α4β2 nAChRs have attracted considerable interest as potential tools for the treatment of cognitive impairment, chronic pain and depression, and as an aid for smoking cessation. However, despite the therapeutic potential of these types of compounds, it is not yet know the structural elements determining stoichiometry-specific sensitivity to α4β2 nAChR allosteric modulators. Here, we have used fully concatenated (α4β2)2α4, (α4β2)2β2 and (α4β2)2α5 nAChRs in combination with chimeric receptors, single point mutations and homology models of the alternate α4β2 nAChRs to determine the effects of desformylflustrabromine (dFBr), a selective positive allosteric modulator of α4β2 nAChR, on α4β2nAChRs and structural elements responsible for those effects. We show that dFBr effects are receptor stoichiometry-dependent and that the fifth subunits is a major determinant of the potentiating effects of dFBr.