Presence of multiple binding sites on α9α1 0 nAChR receptors alludes to stoichiometric-dependent action of the α-conotoxin, Vc1.1

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels involved in fast synaptic transmission. nAChRs are pentameric receptors formed from a combination of different or similar subunits to produce heteromeric or homomeric channels. The heteromeric, alpha 9 alpha 10 nAChR subtype is well-known for its role in the auditory system, being expressed in cochlear hair cells. These nAChRs have also been shown to be involved in immune-modulation. Antagonists of a alpha 9 alpha 10 nAChRs, like the alpha-conotoxin Vc1.1, have analgesic effects in neuropathic pain. Unlike other nAChR subtypes there is no evidence that functional receptor stoichiometries of alpha 9 alpha 10 exist. By using 2-electrode voltage clamp methods and maintaining a constant intracellular Ca2+ concentration, we observed a biphasic activation curve for ACh that is dependent on receptor stoichiometry.Vc1.1, but not the alpha 9 alpha 10 antagonists RgIA or atropine, inhibits ACh-evoked currents in a biphasic manner. Characteristics of the ACh and Vc1.1 activation and inhibition curves can be altered by varying the ratio of alpha 9 and alpha 10 mRNA injected into oocytes, changing the curves from biphasic to monophasic when an excess of alpha 10 mRNA is used. These results highlight the difference in the pharmacological profiles of at least two different alpha 9 alpha 10 nAChR stoichiometries, possibly (alpha 9)(3)(alpha 10)(2) and (alpha 9)(2)(alpha 10)(3). As a result, we infer that there is an additional binding site for ACh and Vc1.1 at the alpha 9-alpha 9 interface on the hypothesized (alpha 9)(3)(alpha 10)(2) nAChR, in addition to the alpha 10-alpha 9 and or alpha 9-alpha 10 interfaces that are common to both stoichiometries. This study provides further evidence that receptor stoichiometry contributes another layer of complexity in understanding Cys-loop receptors. (C) 2014 Elsevier Inc. All rights reserved.