One week after catheter implantation, animals were trained to self-administer cocaine during daily 2 h sessions conducted in a 43 × 43 × 53 cm Plexiglas chamber (Med. Associates, Inc., St Albans, VT). The beginning of the session was signaled by the onset of a cue light positioned above a lever and lever extension (the lever remained extended throughout the session). Each lever press (fixed ratio 1, FR1 schedule) initiated intravenous cocaine delivery (0.33 mg/infusion, over 6 s) controlled by a syringe pump (Model PHM-100, Med Associates, Inc., St. Albans, VT), termination of the cue light and simultaneous onset of a 20 s tone (67 db, 1 kHz) and houselight (25 W) stimulus; lever presses during stimulus presentation had no programmed consequences. Subsequent trials were initiated by the animals and typically occurred with inter-trial intervals of approximately 5–6 minutes. Training was complete when stable responding was established (i.e., <10% variability in press number during a consecutive 2–3 day period; total training period of 8–12 days).
