Chronic nicotine differentially regulates α6-and β3-containing nicotinic cholinergic receptors in rat brain

We investigated the effects of chronic nicotine on alpha 6- and beta 3-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 [I-125]alpha-conotoxin MII to selectively label alpha 6* nAChRs showed a 28% decrease in binding in the striatum but no change in the superior colliculus. Immunoprecipitation of nAChRs labeled by [H-3] epibatidine in these two regions showed that chronic nicotine increased alpha 4- and beta 2- containing nAChRs by 39 to 67%. In contrast, chronic nicotine caused a 39% decrease in alpha 6- containing nAChRs in striatum but no change in superior colliculus. No changes in beta 3-containing nAChRs were seen in either region after chronic nicotine. The decreased expression of alpha 6- 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 6-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 alpha 6* nAChR is regulated differently from other nAChR subtypes, and they suggest that the inclusion of a beta 3 subunit with alpha 6 may serve to inhibit nicotineinduced down- regulation of these receptors.