into the intravenous catheter by a Razel syringe pump (Med Associates). Each nicotine self-administration session was performed using 2 retractable levers (1 active, 1 inactive) that extend 1 cm into the chamber. Completion of the response criteria on the active lever resulted in the delivery of an intravenous nicotine infusion (0.03 ml infusion volume for mice; 0.1 ml for rats). Responses on the inactive lever were recorded but had no scheduled consequences. For dose-response studies (fixed and progressive ratio schedules), animals were presented with each dose of nicotine for at least 5 days (mice) or 3 days (rats); the mean intake over the last 3 (mice) or 2 (rats) sessions for each dose was calculated and used for statistical analysis. Nicotine doses were presented according to a within-subjects Latin square design. In between each dose, subjects were placed back on the training dose for at least 2 days or until their intake returned to baseline levels before being tested on the next dose in the Latin-square design.
