Twenty-six Plexiglas operant chambers (model ENV-008CT, Med Associates, Georgia, VT) were each enclosed in larger sound-attenuating plywood chambers (model ENV-018M, Med Associates). Each operant box had a house light (28 V) and two levers, one retractable and one stationary, located 10 cm apart and 8 cm above the floor of the box. The retractable lever (active lever) was connected to an infusion pump positioned outside the sound-insulating chamber for the delivery of drugs (Razel Scientific Instruments, Stamford, CT). The stationary lever served to control for baseline, non-reinforced operant behavior; pressing this lever had no consequence (inactive lever), but all presses were recorded. A white light (28V) stimulus located 3 cm above the active lever was illuminated for 30 s at the beginning of the session, and for the duration of each drug infusion (5 s), serving as a discrete cue or conditioned stimulus for drug delivery. Each self-administration chamber was fitted to deliver constant-current, intermittent, inescapable, electric foot shock through a scrambler (model ENV-414, Med Associates) to the grid floor.
