Effect of aripiprazole on the dose–response function of methamphetamine self-administration under a PR schedule. Test sessions under a PR schedule ended when rats did not achieve reinforcement within an hour. The top panel is the dose–response function of methamphetamine determined after 15 escalation sessions. The middle panel is the data from ShA rats. The bottom panel is the data from LgA rats. Data are expressed as the number of injections/session on the left axis and the ratio per injection on the right axis. Error bars are SEM values. Filled symbols indicate the data of LgA rats, and open symbols indicate the data of ShA rats. Circles indicate the vehicle treatment; squares, 1 mg/kg of aripiprazole; triangles, 3 mg/kg of aripiprazole; inverted triangles, 10 mg/kg of aripiprazole. Aripiprazole was subcutaneuously injected 1 h before test sessions. S: saline. #p<0.05, ##p<0.01 compared with responding by ShA rats at the same dose of methamphetamine. *p<0.05, **p<0.01, ***p<0.001 compared with the vehicle treatment at the same dose of methamphetamine.