The additional ACh binding site at the α4(+)/α4(-) interface of the (α4β2)2α4 nicotinic ACh receptor contributes to desensitization

BACKGROUND AND PURPOSE Nicotinic ACh (alpha 4 beta 2)(2)alpha 4 receptors are highly prone to desensitization by prolonged exposure to low concentrations of agonist. Here, we report on the sensitivity of the three agonist sites of the (alpha 4 beta 2)(2)alpha 4 to desensitization induced by prolonged exposure to ACh. We present electrophysiological data that show that the agonist sites of the (alpha 4 beta 2)(2)alpha 4 receptor have different sensitivity to desensitization and that full receptor occupation decreases sensitivity to desensitization. EXPERIMENTAL APPROACH Two-electrode voltage-clamp electrophysiology was used to study the desensitization of concatenated (alpha 4 beta 2)(2)alpha 4 receptors expressed heterologously in Xenopus oocytes. Desensitization was assessed by measuring the degree of functional inhibition caused by prolonged exposure to ACh, as measured under equilibrium conditions. We used the single-point mutation 4W182A to measure the contribution of individual agonist sites to desensitization. KEY RESULTS (alpha 4 beta 2)(2)alpha 4 receptors are less sensitive to activation and desensitization by ACh than (alpha 4 beta 2)(2)beta 2 receptors. Incorporation of 4W182A into any of the agonist sites of concatenated (alpha 4 beta 2)(2)alpha 4 receptors decreased sensitivity to activation and desensitization but the effects were more pronounced when the mutation was introduced into the 4(+)/4(-) interface. CONCLUSIONS AND IMPLICATIONS The findings suggest that the agonist sites in (alpha 4 beta 2)(2)alpha 4 receptors are not functionally equivalent. The agonist site at the alpha 4(+)/alpha 4(-) interface defines the sensitivity of (alpha 4 beta 2)(2)alpha 4 receptors to agonist-induced activation and desensitization. Functional differences between (alpha 4 beta 2)(2)alpha 4 and (alpha 4 beta 2)(2)beta 4 receptors might shape the physiological and behavioural responses to nicotinic ligands when the receptors are exposed to nicotinic ligands for prolonged periods of times.