“…The last 2-3 days of cocaine self-administration data at each dose were averaged and used to compare dose-response performance between WT and KO mice.After the completion of the above cocaine dose-response experiment, the animals were switched to cocaine self-administration under PR reinforcement schedule. During PR conditions, the work requirement (lever presses) needed to receive a cocaine infusion was raised progressively within each test session according to the following PR series:1,2,4,6,9,12,15,20,25,32,40,50, 62, 77, 95, 118, 145, 178, 219, 268, 328, 402, 492, and 603 until the break point was reached.The break point was defined as the maximal workload (i.e., number of lever presses) completed for the last cocaine infusion prior to a 1-h period during which no infusions were obtained by the animal. Animals were tested for cocaine self-administration under PR reinforcement at three doses (starting at 0.25, then 1 and then 0.5 mg/kg/infusion) from days 30 to 38.After the completion of the PR experiments, the same groups of animals continued for cocaine extinction and reinstatement tests.…”