Menthol enhances nicotine-induced upregulation. (a) Representative coronal slice (bregma −3.1) with α4-mCherryα6-GFP-labeled neurons in the VTA, SNc, and SNr. Bar, 100 μm. (b1) Potential assemblies of midbrain nAChRs revealed by GFP and mCherry fluorescence. The subunits indicated by * are uncertain (α4, α6, β2, or β3). (b2) Representative α4-mCherryα6-GFP* neurons. (c1–3) α4-mCherry*, α6-GFP*, or α4α6* nAChR-integrated density. The data are mean±SEM, normalized to vehicle. (c1–3) Chronic treatments were 10 days: vehicle, 2 mg/kg/h nicotine, or 2 mg/kg/h nicotine with 2 mg/kg/h menthol. (d1–3) Mice were given drug dosing identical to CPP assays. Number of mice for each treatment is indicated in parenthesis. *p<0.05, one-way ANOVA, post hoc Tukey.