Nicotine-induced up-regulation and desensitization of α4β2 neuronal nicotinic receptors depend on subunit ratio

Abstract

Desensitization induced by chronic nicotine exposure has been hypothesized to trigger the up-regulation of the α4β2 neuronal nicotinic acetylcholine receptor (nAChR) in the central nervous system. We studied the effect of acute and chronic nicotine exposure on the desensitization and up-regulation of different α4β2 subunit ratios (1α:4β, 2α:3β, and 4α:1β) expressed in Xenopus oocytes. The presence of α4 subunit in the oocyte plasmatic membrane increased linearly with the amount of α4 mRNA injected. nAChR function and expression were assessed during acute and after chronic nicotine exposure using a two-electrode voltage clamp and whole-mount immunofluorescence assay along with confocal imaging for the detection of the α4 subunit. The 2α4:3β2 subunit ratio displayed the highest ACh sensitivity. Nicotine dose-response curves for the 1α4:4β2 and 2α4:3β2 subunit ratios displayed a biphasic behavior at concentrations ranging from 0.1 to 300 μM. A biphasic curve for 4α4:1β2 was obtained at nicotine concentrations higher than 300 μM. The 1α4:4β2 subunit ratio exhibited the lowest ACh- and nicotine-induced macroscopic current, whereas 4α4:1β2 presented the largest currents at all agonist concentrations tested. Desensitization by acute nicotine exposure was more evident as the ratio of β2:α4 subunits increased. All three α4β2 subunit ratios displayed a reduced state of activation after chronic nicotine exposure. Chronic nicotine-induced up-regulation was obvious only for the 2α4: 3β2 subunit ratio. Our data suggest that the subunit ratio of α4β2 determines the functional state of activation, desensitization, and up-regulation of this neuronal nAChR. We propose that independent structural sites regulate α4β2 receptor activation and desensitization.