For self-administration experiments in mice and rats, (−)-nicotine hydrogen tartrate salt (Sigma Chemical Co., St. Louis, MO) was dissolved in 0.9% sterile saline. All doses of nicotine refer to the free-base form. The NMDA antagonist LY235959 (Tocris, Ellisville, MO) or lidocaine (2%, Sigma Chemical Co.) were microinjected at a volume of 0.5 μl for over 1 min, and the injector remained in place for an additional 2 min to allow for diffusion. The pH of solutions was adjusted to ~7.4.
