Drug discrimination training was conducted in operant chambers (Med-Associates, Georgia, VT, USA) enclosed in ventilated sound-attenuating cubicles. The chambers were equipped with two retractable levers, an amber stimulus light above each lever, and a white houselight located on the opposite wall. The reinforcer was sweetened condensed milk (Törsleffs, Haugen –Gruppen Oy, Vantaa, Finland) diluted 1∶2 in water and delivered at a volume of 25 µl in a drinking cup located between the levers.
