Training Procedures

Preliminary training and discrimination training were identical to procedures described previously (Harvey and Baker, 2016). Briefly, subjects were initially trained to lever press under a fixed ratio (FR) schedule of reinforcement that was gradually increased from FR1 to FR 20. Six errorless sessions were conducted to establish schedule control on each lever separately. Subjects were administered a 0.9% bacteriostatic saline solution (vehicle, V) or the training drug of 1 or 3 mg/kg 4-MMC (D) for a total of six 20 min training sessions in the following order: V, V, D, D, V, D. The lever assigned to each stimulus condition was counterbalanced among animals in each training group.

Discrimination training proceeded in a pseudorandom order, with no more than two consecutive drug or saline sessions. Correct responses on the drug or vehicle lever were reinforced on a FR 20 resetting schedule. Criteria for stimulus control included a minimum of 8 out of 10 consecutive training sessions in which the percentage of responses during the first FR and the percentage of responses for the total session were at least 80% on the injection-appropriate lever.