Discrimination training between 0 and 5 mg/kg morphine under FR 5 schedule was considered stable when the following criteria were met for at least four out of five consecutive sessions: 1) ≥10 reinforcers earned per session, 2) first completed response requirement on the correct lever, and 3) ≥80% correct total responses. After these criteria were met, mice were tested with 0, 2.5, 5, and 10 mg/kg of morphine to generate a dose-response function. The morphine doses were administered in a within-subjects Latin square design. Each test session was preceded by two training sessions (one morphine, one saline session) satisfying the criteria stated above. If the criteria were not satisfied during these two sessions, animals were required to meet the criteria above for 4 out of 5 consecutive sessions before testing continued.
