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 ␣4␤2 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 ␣4␤2 subunit ratios (1␣:4␤, 2␣:3␤, and 4␣:1␤) expressed in Xenopus oocytes. The presence of ␣4 subunit in the oocyte plasmatic membrane increased linearly with the amount of ␣4 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 ␣4 subunit. The 2␣4:3␤2 subunit ratio displayed the highest ACh sensitivity. Nicotine doseresponse curves for the 1␣4:4␤2 and 2␣4:3␤2 subunit ratios displayed a biphasic behavior at concentrations ranging from 0.1 to 300 M. A biphasic curve for 4␣4:1␤2 was obtained at nicotine concentrations higher than 300 M. The 1␣4:4␤2 subunit ratio exhibited the lowest ACh-and nicotine-induced macroscopic current, whereas 4␣4:1␤2 presented the largest currents at all agonist concentrations tested. Desensitization by acute nicotine exposure was more evident as the ratio of ␤2:␣4 subunits increased. All three ␣4␤2 subunit ratios displayed a reduced state of activation after chronic nicotine exposure. Chronic nicotine-induced up-regulation was obvious only for the 2␣4: 3␤2 subunit ratio. Our data suggest that the subunit ratio of ␣4␤2 determines the functional state of activation, desensitization, and up-regulation of this neuronal nAChR. We propose that independent structural sites regulate ␣4␤2 receptor activation and desensitization. Neuronal nicotinic acetylcholine receptors (nAChRs)1 belong to a superfamily of ligand-gated ion channels (e.g. ␥-aminobutyric acid, glutamate, 5-hydroxytryptamine, among others) and play an important role in modulating neurotransmitter release in distinct areas of the central and peripheral nervous system (1-5). Nicotine is the active ingredient of tobacco and specifically binds to nAChRs in the brain (3). One of the most remarkable effects of chronic nicotine exposure is the up-regulation of the ␣4␤2 subtype in the central nervous system (6 -10). Another important effect of chronic nicotine exposure is the long lasting functional deactivation of nAChR receptor (11-17). Chronic nicotine exposure produces a loss of nicotinic functional activity as a result of rapid and persistent desensitization (11, 18 -20). Desensitization induced by chronic exposure to nicotine has been hypothesized to trigger the up-regulation of the ␣4␤2 nAChR (3, 6, 21-23). The effect of chronic nicotine exposure on the activity of nAChR subtypes may be related to symptoms associated with nicotine addiction (3, 24, 25) such as tolerance, dependence, and withdrawal. In contrast to the aforementioned studies, a recent work suggests that the ␣4␤2 subtype expressed in the stable cell line K-177 functionally up-reg...

Gene regulation of α4β2 nicotinic receptors: microarray analysis of nicotine‐induced receptor up‐regulation and anti‐inflammatory effects

“…1b). These data are consistent with (Zhang and Steinbach 2003) showing that 3 [H]-cytisine does not significantly label internal receptors in intact cells, and with reports showing nicotine-induced up-regulation increases surface a4b2 receptors (Lopez-Hernandez et al 2004;Kuryatov et al 2005), but see (Vallejo et al 2005).…”

“…The heteromeric a4b2 nAChRs exist in two stoichiometries: (a4)2(b2)3 and (a4)3(b2)2(34) and the stoichiometry may play a role in receptor up-regulation (Lopez-Hernandez et al 2004). Folding, assembly, and glycosylation of these receptors occurs in the ER, and properly folded receptors are trafficked to the cell surface after passage through the Golgi.…”

Introduction of unsaturation into theN-n-alkyl chain of the nicotinic receptor antagonists, NONI and NDNI: Effect on affinity and selectivity

“…Moreover, the function (ie, sensitivity to receptor activation, time to desensitization, and upregulation) of these different subtype classes is also dependent on subunit ratio. 94,95 Furthermore, exposure to drug (eg, nicotine) also influences nAChR subtype stoichiometry and function in recombinant receptor systems. 95,96 Evidence has also been obtained that suggests that different DA neurons in the substantia nigra can be categorized based on the expression of the specific subtype composition.…”

Discovery of a novel nicotinic receptor antagonist for the treatment of nicotine addiction: 1-(3-Picolinium)-12-triethylammonium-dodecane dibromide (TMPD)

“…To further complicate the association of a particular response with a specific nAChR subtype, studies using recombinant receptors report that when the ratio of subunit pairs is varied, different subtype classes are formed and the function of these different subtype classes is dependent on subunit ratio [44]. Exposure to drug also influences nAChR subtype stoichiometry and function in recombinant receptor systems [44][45][46].…”

“…Exposure to drug also influences nAChR subtype stoichiometry and function in recombinant receptor systems [44][45][46]. Different DA neurons can be categorized based upon the expression of the specific subtype composition [27].…”