Chronic Exposure to Nicotine Upregulates the Human α4β2 Nicotinic Acetylcholine Receptor Function

Abstract

Widely expressed in the brain, the α4β2 nicotinic acetylcholine receptor (nAChR) is proposed to play a major role in the mechanisms that lead to and maintain nicotine addiction. Using the patch-clamp technique and pharmacological protocols, we examined the consequences of long-term exposure to 0.1–10 μm nicotine in K-177 cells expressing the major human brain α4β2 receptor. The acetylcholine dose–response curves are biphasic and revealed both a high- and a low-affinity component with apparent EC₅₀values of 1.6 and 62 μm. Ratios of receptors in the high- and low-affinity components are 25 and 75%, respectively. Chronic exposure to nicotine or nicotinic antagonists [dihydro-β-erytroidine (DHβE) or methyllycaconitine (MLA)] increases the fraction of high-affinity receptors up to 70%. Upregulated acetylcholine-evoked currents increase by twofold or more and are less sensitive to desensitization. Functional upregulation is independent of protein synthesis as shown by the lack of effect of 20 μmcycloheximide. Single-channel currents recorded with 100 nmacetylcholine show predominantly high conductances (38.8 and 43.4 pS), whereas additional smaller conductances (16.7 and 23.5 pS) were observed with 30 μm acetylcholine. In addition, long-term exposure to dihydro-β-erytroidine increases up to three times the frequency of channel openings. These data indicate, in contrast to previous studies, that human α4β2 nAChRs are functionally upregulated by chronic nicotine exposure.