General procedure and apparatus described previously (24,25). At the start of the session, the houselight was extinguished and a green stimulus light was presented. In the presence of the green light, 10 responses on the lever (FR10) produced a 0.2-s, 0.2-ml, 30 μg/kg injection of nicotine, extinguished the green light and illuminated the amber stimulus light for 2 s. Each reinforcement was followed by a 60-s timeout period, during which the chamber was dark and lever presses had no programmed consequences. One-hour sessions were conducted five days/week.
