One 3-h self-administration session occurred every day for 7 consecutive days. Rats were transferred from their home cages to the operant chambers and their cannulas attached to infusion lines. Each session began with the activation of the house light, the entry of the retractable lever and the illumination of the light cue for 30 s. If the rat pressed the active lever during this initial presentation of the light cue, it received an infusion of heroin (0.05 mg/kg/infusion) followed by termination of the light. Subsequently, active lever-presses led to heroin infusions, and activation of the light (conditioning stimulus), according to a schedule of continuous reinforcement. Drug was infused in a volume of 150 µl over a 5-s period, and responses during this period were recorded, but did not lead to additional infusion.
