Microdialysis experiments in rats. Values represent mean dopamine concentrations as a percentage of baseline ± SEM (average of 5 samples before MOR agonist administration). The lined and white rectangles in the x axis indicate the period of MOR agonist perfusion and M40 infusion, respectively; the arrows in D and F indicate the time point of systemic administration. (A and B) Effect of intra-VTA morphine (1–10 μM) or methadone (10–300 μM) on VTA dopamine. *P < 0.05 and ***P < 0.001 versus 1 μM morphine (A); *P < 0.05 and **P < 0.01 versus 10 μM methadone (B); 1-way ANOVA with Dunnett’s multiple comparisons, comparing the average of 8 samples after MOR agonist administration (n = 7–8 animals/group). (C) Effect of intra-NAc morphine (10 μM) or methadone (300 μM) on NAc dopamine. Results were nonsignificant in both cases; paired, 2-tailed t test, comparing the average of 8 samples after MOR agonist administration versus baseline values (n = 8 animals/group). (D) Effect of systemic administration (1 mg/kg, i.p.) of morphine or methadone on VTA and NAc dopamine in the VTA and contralateral NAc. *P < 0.05; paired, 2-tailed t test, comparing the average of 5 samples after MOR administration versus baseline values (n = 6–7 animals/group). (E) Effect of intra-VTA DAMGO (10 μM) or fentanyl (10 μM) on VTA dopamine, respectively (n = 7 and 9 animals/group). **P < 0.01; paired, 2-tailed t test, comparing the average of 8 samples after MOR agonist administration versus baseline values. (F) Effect of systemic administration (0.03 mg/kg, i.p.) of fentanyl on VTA and NAc dopamine in the VTA and contralateral NAc. **P < 0.01; paired, 2-tailed t test, comparing the average of 8 samples after MOR administration versus baseline values (n = 7 animals/group). DA, dopamine.