Unraveling the High- and Low-Sensitivity Agonist Responses of Nicotinic Acetylcholine Receptors

The neuronal alpha 4 beta 2 nicotinic acetylcholine receptors exist as two distinct subtypes, (alpha 4)(2)(beta 2)(3) and (alpha 4)(3)(beta 2)(2), and biphasic responses to acetylcholine and other agonists have been ascribed previously to coexistence of these two receptor subtypes. We offer a novel and radical explanation for the observation of two distinct agonist sensitivities. Using different expression ratios of mammalian alpha 4 and beta 2 subunits and concatenated constructs, we demonstrate that a biphasic response is an intrinsic functional property of the (alpha 4)(3)(beta 2)(2) receptor. In addition to two high-sensitivity sites at alpha 4 beta 2 interfaces, the (alpha 4)(3)(beta 2)(2) receptor contains a third low-sensitivity agonist binding site in the alpha 4 alpha 4 interface. Occupation of this site is required for full activation and is responsible for the widened dynamic response range of this receptor subtype. By site-directed mutagenesis, we show that three residues, which differ between the alpha 4 beta 2 and alpha 4 alpha 4 sites, control agonist sensitivity. The results presented here provide a basic insight into the function of pentameric ligand-gated ion channels, which enables modulation of the receptors with hitherto unseen precision; it becomes possible to rationally design therapeutics targeting subpopulations of specific receptor subtypes.