Fig. 1.

Effects of AM4113 or SR141716A on heroin self-administration under a fixed-ratio 2 (FR2) reinforcement schedule. Administration of AM4113 (3 and 10 mg/kg, i.p.) significantly decreased the number of heroin infusions (a) and active lever presses (b), but had no effect on inactive lever presses (c). Administration of SR141716A (3 and 10 mg/kg, i.p.) dose-dependently decreased the number of heroin infusions (d), but did not alter active lever presses (e) or inactive lever presses (f). Data are presented as the mean ± SEM. ^**P < 0.01 and ^***P < 0.001 vs. vehicle (0 mg/kg, i.p.)