Fig. 1.
ocaine self-administration is blocked by intra-accumbens infusion of a melanocortin antagonist. The effect of direct infusions of SHU-9119 into the nucleus accumbens on lever pressing for cocaine was determined. (A) Rats were trained to lever press for cocaine (0.50 mg/kg/injection) and tested with different doses of cocaine on test day 1 as indicated. Subjects were then tested in the presence of a single dose of SHU-9119 (0.25 μg/side) on test day 2 at the same dose of cocaine from the previous day. The results are expressed as the number of lever presses and are the means ± SEM (n = 3 for the 0.125 dose of cocaine and n = 6 animals for each of the two higher doses of cocaine, 0.25 and 0.50). Repeated-measures anova shows a significant interaction between treatment groups (vehicle vs. SHU-9119) and dose of cocaine (F2,12 = 13.99). Post-hoc analysis demonstrates that there are significant differences at the two lower doses of cocaine (*P < 0.01). (B) The results are expressed as the total amount of cocaine self-administered at each dose in the absence or presence of SHU-9119.