Mice and rats were mildly food restricted to 85–90% of their free-feeding body weight and trained to press a lever in an operant chamber (Med Associates, St. Albans, VT) for food pellets (20 mg pellets mice; 45 mg food pellets rats; TestDiet, Richmond, IN) under a fixed-ratio 5, time out 20 sec (FR5TO20 sec) schedule of reinforcement prior to catheter implantation. Once stable responding was achieved (>30 pellets per session in mice; >90 pellets per session in rats), subjects were catheterized as described above. The animals were allowed at least 48 h to recover from surgery, then permitted to respond for food reinforcement again under the FR5TO20 sec schedule. Once food responding criteria was reestablished, subjects were permitted to acquire intravenous nicotine self-administration by autoshaping during 1 h daily sessions, 7 days per week. Nicotine was delivered through the tubing 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
