Effects of chronic nicotine on heteromeric neuronal nicotinic receptors expressed in rat primary cultured neurons

Nicotine increases the number of neuronal nicotinic receptors (nAChRs) in brain. This study investigated the effects of chronic nicotine treatment on nAChRs expressed in primary cultured neurons. In particular, we studied the chronic effects of nicotine exposure on the total density, surface expression and turnover rate of heteromeric nAChRs. The receptor density was measured by [ 125 I]epibatidine ([ 125 I]EB) binding. Untreated and nicotine-treated neurons were compared from several regions of embryonic (E19) rat brain. Twelve days of treatment with 10 µM nicotine produced a two-fold up-regulation of nAChRs. Biotinylation and whole-cell binding studies indicated that up-regulation resulted from an increase in the number of cell surface receptors as well as intracellular receptors. nAChR subunit composition in cortical and hippocampal neurons was assessed by immunoprecipitation with subunit-selective antibodies. These neurons contain predominantly α4, β2 and α5 subunits, but α2, α3, α6 and β4 subunits were also detected. Chronic nicotine exposure yielded a two-fold increase in the β2-containing receptors and a smaller up-regulation in the α4-containing nAChRs. To explore the mechanisms of up-regulation we investigated the effects of nicotine on the receptor turnover rate. We found that the turnover rate of surface receptors was > two weeks and chronic nicotine exposure had no effect on this rate.