Selective down-regulation of [(125)I]Y0-alpha-conotoxin MII binding in rat mesostriatal dopamine pathway following continuous infusion of nicotine

Chronic Nicotine Differentially Regulates α6- and β3-Containing Nicotinic Cholinergic Receptors in Rat Brain

We investigated the effects of chronic nicotine on alpha6- and beta3-containing nicotinic acetylcholine receptors (nAChRs) in two rat brain regions using three methodological approaches: radioligand binding, immunoprecipitation, and nicotine-stimulated synaptosomal release of dopamine. Nicotine was administered by osmotic minipumps for 2 weeks. Quantitative autoradiography with [(125)I]alpha-conotoxin MII to selectively label alpha6(*) nAChRs showed a 28% decrease in binding in the striatum but no change in the superior colliculus. Immunoprecipitation of nAChRs labeled by [(3)H]epibatidine in these two regions showed that chronic nicotine increased alpha4- and beta2-containing nAChRs by 39 to 67%. In contrast, chronic nicotine caused a 39% decrease in alpha6-containing nAChRs in striatum but no change in superior colliculus. No changes in beta3-containing nAChRs were seen in either region after chronic nicotine. The decreased expression of alpha6-containing nAChRs persisted for at least 3 days, recovering to baseline by 7 days after removal of the pumps. There was a small but significant decrease in total nicotine-stimulated dopamine release in striatal synaptosomes after nicotine exposure. However, the component of dopamine release that was resistant to alpha-conotoxin MII blockade was unaffected, whereas dopamine release that was sensitive to blockade by alpha-conotoxin MII was decreased by 56%. These findings indicate that the alpha6(*) nAChR is regulated differently from other nAChR subtypes, and they suggest that the inclusion of a beta3 subunit with alpha6 may serve to inhibit nicotine-induced down-regulation of these receptors.

Partial recovery of striatal nicotinic receptors in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys with chronic oral nicotine

Recent studies in nonhuman primates show that chronic nicotine treatment protects against nigrostriatal degeneration, with a partial restoration of neurochemical and functional measures in the striatum. The present studies were done to determine whether long-term nicotine treatment also protected against striatal nicotinic receptor (nAChR) losses after nigrostriatal damage. Monkeys were administered nicotine in the drinking water for 6 months and subsequently lesioned with the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) over several months while nicotine was continued. (125)I-Epibatidine, [(125)I]5-[(125)I]iodo-3(2(S)-azetidinylmethoxy)-pyridine (A85380), and (125)I-alpha-conotoxinMII autoradiography was performed to evaluate changes in alpha4beta2* and alpha3/alpha6beta2* nAChRs, the major striatal subtypes. Nicotine treatment increased alpha4beta2* nAChRs by > or =50% in striatum of both unlesioned and lesioned animals. This increase in alpha4beta2* nAChRs was significantly greater in lesioned compared with unlesioned monkey striatum. Chronic nicotine treatment led to a small decrease in alpha3/alpha6beta2* nAChR subtypes. The decline in alpha3/alpha6beta2* subtypes, defined using alpha-conotoxinMII-sensitive (125)I-epibatidine or [(125)I]A85380 binding, was significantly smaller in striatum of nicotine-treated lesioned monkeys compared with unlesioned monkeys. This difference was not observed for alpha3/alpha6beta2* nAChRs identified using (125)I-alpha-conotoxinMII. These data suggest that there are at least two striatal alpha3/alpha6beta2* subtypes that are differentially affected by chronic nicotine treatment in lesioned animals. In addition, the results showing an improvement in striatal alpha4beta2* and select alpha3/alpha6beta2* nAChR subtypes, combined with previous work, demonstrate that chronic nicotine treatment restores and/or protects against the loss of multiple molecular markers after nigrostriatal damage. Such findings suggest that nicotine or nicotinic agonists may be of therapeutic value in Parkinson's disease.

Beta3 subunits promote expression and nicotine-induced up-regulation of human nicotinic alpha6* nicotinic acetylcholine receptors expressed in transfected cell lines

Nicotinic acetylcholine receptors (AChRs) containing alpha6 subunits are typically found at aminergic nerve endings where they play important roles in nicotine addiction and Parkinson's disease. alpha6* AChRs usually contain beta3 subunits. beta3 subunits are presumed to assemble only in the accessory subunit position within AChRs where they do not participate in forming acetylcholine binding sites. Assembly of subunits in the accessory position may be a critical final step in assembly of mature AChRs. Human alpha6 AChRs subtypes were permanently transfected into human tsA201 human embryonic kidney (HEK) cell lines. alpha6beta2beta3 and alpha6beta4beta3 cell lines were found to express much larger amounts of AChRs and were more sensitive to nicotine-induced increase in the amount of AChRs than were alpha6beta2 or alpha6beta4 cell lines. The increased sensitivity to nicotine-induced up-regulation was due not to a beta3-induced increase in affinity for nicotine but probably to a direct effect on assembly of AChR subunits. HEK cells express only a small amount of mature alpha6beta2 AChRs, but many of these subunits are on the cell surface. This contrasts with Xenopus laevis oocytes, which express a large amount of incorrectly assembled alpha6beta2 subunits that bind cholinergic ligands but form large amorphous intracellular aggregates. Monoclonal antibodies (mAbs) were made to the alpha6 and beta3 subunits to aid in the characterization of these AChRs. The alpha6 mAbs bind to epitopes C-terminal of the extracellular domain. These data demonstrate that both cell type and the accessory subunit beta3 can play important roles in alpha6* AChR expression, stability, and up-regulation by nicotine.

An autoradiographic analysis of [125I]alpha-bungarotoxin binding in rat brain after chronic nicotine exposure

Chronic exposure to nicotine has been shown to increase binding to high affinity nicotinic cholinergic receptors in rat brain, but the effect of this treatment on the low affinity alpha7 nicotinic receptors has been less well characterized. Male Sprague-Dawley rats were treated with saline or nicotine (6 mg/kg/day, by osmotic minipump) for 14 days. Frozen brain sections were then prepared and processed for quantitative autoradiography using [(125)I]alpha-bungarotoxin to measure the effect of this treatment on low affinity nicotinic receptors. Nicotine exposure increased [(125)I]alpha-bungarotoxin binding in 26 of 52 brain regions analyzed; increases ranged from 10 to 70% over saline controls. Increases were seen in all areas of the brain, but were more prominent in forebrain areas, and especially in cerebral cortex. These data demonstrate that low affinity alpha7 nicotinic receptors are also up-regulated by chronic nicotine. This phenomenon may be relevant to the heavy use of tobacco products in diseases like schizophrenia, and needs to be considered in the design of pharmaceuticals directed at this receptor system.

Differential regulation of mesolimbic alpha 3/alpha 6 beta 2 and alpha 4 beta 2 nicotinic acetylcholine receptor sites and function after long-term oral nicotine to monkeys

Because the mesolimbic dopamine system plays a critical role in nicotine addiction/reinforcement and because nicotinic receptors regulate dopamine release, we initiated a study to evaluate the long-term effects of nicotine (>6 months at the final dose) on nicotinic acetylcholine receptor (nAChR) sites and function in the nucleus accumbens of nonhuman primates. Nicotine was given in the drinking water as this mode of administration is long-term but intermittent, thus resembling smoking in this aspect. We determined the effects of nicotine treatment on function and binding of the alpha3/alpha6beta2* and alpha4beta2* nAChRs subtypes in nucleus accumbens, a region directly implicated in the addictive effects of nicotine. To evaluate function, we measured nicotine and K+-evoked [3H]dopamine release from nucleus accumbens synaptosomes. Changes in alpha4beta2* and alpha3/alpha6beta2* nAChRs were measured using 125I-epibatidine, [125I]A85380 [5-[125I]iodo-3(2(S)-azetidinylmethoxy) pyridine] and 125I-alpha-conotoxin MII autoradiography. Chronic nicotine treatment, which led to plasma nicotine levels in the range of smokers, significantly increased nucleus accumbens alpha4beta2* nAChR sites and function compared with control. By contrast, this treatment did not significantly change alpha3/alpha6beta2* nAChR sites or evoked dopamine release in this region compared with control. Thus, these data are distinct from previous results in striatum in which the same nicotine treatment paradigm decreased striatal alpha3/alpha6beta2* nAChR sites and function. The finding that long-term nicotine treatment selectively modulates alpha4beta2* and not alpha3/alpha6beta2* nAChR expression in primate nucleus accumbens is consistent with the results of studies in nicotinic receptor mutant mice implicating the alpha4beta2* nAChR subtype in nicotine-mediated addiction.