Mouse and rat operant chambers (Med-Associates, St. Albans, VT), housed within light- and sound-attenuating cubicles, were used for behavioral training and testing in all of the drug discrimination studies. In the first mouse discrimination experiment (Experiment 1), each inner chamber contained two response levers and a house light. A recessed well centered between the two levers contained a liquid dipper that delivered 0.02 ml of sweetened-condensed milk (by volume: one part condensed milk, one part sugar, and two parts water) as reinforcement. In the mouse chambers used for the second mouse discrimination experiment (Experiment 2), the inner chambers contained two nose poke apertures. A food dispenser delivered 14-mg food pellets (Bioserv Inc., Frenchtown, NJ) to a food cup centered between the two nose poke apertures. The rat chambers contained a food dispenser that delivered 45-mg food pellets (Bioserv) to a food cup located between two response levers. For all discrimination experiments, illumination of lights, delivery of food pellets, and recording of lever presses were controlled by a computer-based system (MED-PC IV, MED Associates Inc., St. Albans, VT).
