Subsets of acetylcholine-stimulated 86Rb+ efflux and [125I]-epibatidine binding sites in C57BL/6 mouse brain are differentially affected by chronic nicotine treatment

Abstract

Nicotinic cholinergic receptor (nAChR) sites that bind nicotine with high affinity (likely α4β2-nAChR) increase following chronic nicotine treatment. Effects of chronic treatment on other nAChR binding sites and functional responses of nAChRs are less well studied. Therefore, C57BL/6 mice were intravenously infused for 10 days with saline or nicotine (five doses, 0.25–4.0 mg/kg/h) and nAChR function and three different nicotinic binding sites in 12 brain regions were assessed. Plasma nicotine and cotinine increased linearly with dose. 86Rb+ efflux with higher sensitivity to acetylcholine tended to decrease with increasing dose, whereas efflux with lower sensitivity to acetylcholine tended to increase. As anticipated, likely α4β2-nAChR [125I]-epibatidine binding sites increased with treatment (estimated dosage for one-half maximal increase was 0.44 mg/kg/h, plasma nicotine ≈20 ng/ml). 86Rb+ efflux with higher sensitivity to acetylcholine and cytisine-sensitive [125I]-epibatidine binding are predominantly α4β2-nAChR. A high correlation between these parameters was observed across brain regions and slopes of these regression lines decreased with treatment dose, suggesting a decrease in function per unit receptor. Likely α3β4-nAChR binding sites were unaffected even at the highest dose (4.0 mg/kg/h, ≈210 ng/ml). A third set of diverse nAChR binding sites increased in some brain regions, but only after high-dose treatment.