Mice were food restricted to an average 2.6 g of chow per day resulting to 85% of free feeding body weight for the duration of the experiment. Mice were trained to discriminate 5 mg/kg morphine from saline administered 20 min before start of the discrimination session. Twenty-min sessions were conducted Monday through Friday during the light period of the light-dark cycle, one session per day. Mice were initially trained to lever press for sweetened condensed milk after administration of 0 or 1 mg/kg morphine under a fixed ratio (FR) schedule 1. The morphine dose was gradually increased to 5 mg/kg and the response requirement to FR 5. This training procedure was adopted because of problems with nonexistent responding when training was started with the 5 mg/kg dose and FR 5 requirement. Morphine and saline were administered in a pseudorandom order and the positions of morphine and saline levers were counterbalanced among mice. Each training session started with the illumination of the houselight and the stimulus lights above the levers. Upon completion of the response requirement on the correct lever, the
