BACKGROUND AND PURPOSE: Nicotinic ACh (á4â2)2á4 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 (á4â2)2á4 to desensitization induced by prolonged exposure to ACh. We present electrophysiological data that show that the agonist sites of the (á4â2)2á4 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 (á4â2)2á4 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 á4W182A to measure the contribution of individual agonist sites to desensitization.KEY RESULTS: (á4â2)2á4 receptors are less sensitive to activation and desensitization by ACh than (á4â2)2â2 receptors. Incorporation of á4W182A into any of the agonist sites of concatenated (á4â2)2á4 receptors decreased sensitivity to activation and desensitization but the effects were more pronounced when the mutation was introduced into the á4(+)/á4(-) interface.CONCLUSIONS AND IMPLICATIONS: The findings suggest that the agonist sites in (á4â2)2á4 receptors are not functionally equivalent. The agonist site at the á4(+)/á4(-) interface defines the sensitivity of (á4â2)2á4 receptors to agonist-induced activation and desensitization. Functional differences between (á4â2)2á4 and (á4â2)2â2 receptors might shape the physiological and behavioural responses to nicotinic ligands when the receptors are exposed to nicotinic ligands for prolonged periods of times.
Acetylcholine Animals