Nicotine binding and nicotinic receptor subunit RNA after chronic nicotine treatment

DBA mice were chronically treated with nicotine by continuous intravenous infusion of 4.0 mg/kg/hr for 10 d. Drug-treated mice were tolerant to the acute effects of nicotine on locomotor activity and body temperature. The effect of chronic treatment on the amount of L-³H-nicotine binding and RNA encoding for α₄, the most widely expressed nicotinic α-subunit, was measured in three brain regions. Chronic treatment increased L-³H-nicotine binding in cortex and midbrain but had no effect in cerebellum. In contrast, chronic treatment had no effect on the levels of mRNA encoding for α₄ in any of the three brain regions. Subsequently brains were sectioned and L-³H- nicotine binding was measured using quantitative autoradiographic methods. In addition, the relative amounts of mRNA for the major nicotinic receptor subunits (α₄ and β₂), as well as for three additional minor subunits (α₂, α₃, and α₅), were determined by in situ hybridization histochemistry followed by quantitation of image intensity. Chronic nicotine treatment resulted in increases in the amount of L-³H-nicotine binding in many but not all brain areas measured. In contrast, chronic treatment had little effect on the intensity of the hybridization signal for the nicotinic subunit mRNA. The results suggest that chronic treatment with nicotine under conditions resulting in maximal steady-state increases in L-³H-nicotine binding has little effect on RNA levels encoding any of four nicotinic α- subunits and the β₂-subunit.