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

Abstract

Widely expressed in the brain, the ␣4␤2 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 M nicotine in K-177 cells expressing the major human brain ␣4␤2 receptor. The acetylcholine dose-response curves are biphasic and revealed both a high-and a low-affinity component with apparent EC 50 values of 1.6 and 62 M. Ratios of receptors in the high-and low-affinity components are 25 and 75%, respectively. Chronic exposure to nicotine or nicotinic antagonists [dihydro-␤-erytroidine (DH␤E) 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 M cycloheximide. Single-channel currents recorded with 100 nM acetylcholine show predominantly high conductances (38.8 and 43.4 pS), whereas additional smaller conductances (16.7 and 23.5 pS) were observed with 30 M acetylcholine. In addition, long-term exposure to dihydro-␤-erytroidine increases up to three times the frequency of channel openings. These data indicate, in contrast to previous studies, that human ␣4␤2 nAChRs are functionally upregulated by chronic nicotine exposure.

Cellular and synaptic mechanisms of nicotine addiction

“…Physiologically relevant nicotine concentrations have been shown to upregulate ␣4␤2-containing receptors (e.g., Flores et al, 1991;Buisson and Bertrand, 2001). Upregulation of other receptor subtypes can occur with higher nicotine concentrations in some cells (e.g., Schwartz and Kenneth, 1985;Rogers and Wonnacott, 1997;Molinari et al, 1998).…”

“…In association with the upregulation of ligand binding following nicotine preexposure, some laboratories report increases in nicotinic responses (Ksir et al, 1987;Clarke et al, 1988;Rowell and Wonnacott 1990;Yu and Wecker 1994;Buisson et al, 2000;Buisson and Bertrand, 2001), while others have found decreases in function (Marks et al, 1985(Marks et al, , 1993Lapchak et al, 1989). Differences in assays and treatment paradigms may explain some of this variability, but these mixed effects complicate the formulation of a reasonable prediction of the effects of nicotine selfadministration on nAChR responses.…”

Modeling nicotinic neuromodulation from global functional and network levels to nAChR based mechanisms

“…For simplicity, we collapse rapidly and slowly desensitized states into one sate. Using known activation and desensitization parameters for human α4β2 and α7 nAChRs, we verify that, despite the simplifications, the nAChR model reproduces experimental whole-cell current recordings (eg from oocytes, human embryonic kidney 293 cells, neurons) in response to ACh and Ni (Figure 3 [68,69,42,70,71,72] ). Note that this parameterization of the nAChR model allows us to account for subtype-and agonist specific receptor responses.…”

Molecular basis of the selective binding of MDMA enantiomers to the alpha4beta2 nicotinic receptor subtype: Synthesis, pharmacological evaluation and mechanistic studies

“…When NGF-differentiated PC12 cells were pretreated with (R)-MDMA, (S)-MDMA or the racemic mixture for 24 h, at a concentration of 100 μM, a significant increase in binding of nAChR. It is known that this up-regulation is independent of de novo protein synthesis [47] but it is consistent with a higher ratio of the high-affinity component of [ 3 H]epibatidine binding [17], as suggested by previous studies on chronic exposure to nicotine [28] . This effect was significantly higher for (R)-MDMA than for the racemic mixture (see Figure2).…”