Figure 2.
Effect of previous repeated treatment (twice daily, 15 d) with saline, nicotine (0.35 mg/kg), MDMA (2.5 mg/kg), cocaine (15 mg/kg), or amphetamine (2.5 mg/kg) on instrumental responding on a progressive ratio schedule of reinforcement. Data are represented as means ± SEM. ***p < 0.001; **p < 0.01; *p < 0.05. Sal, Saline; Nic, nicotine; Coc, cocaine; Amph, amphetamine; PR, progressive ratio.