Effects of chronic treatment of baclofen on morphine challenge-induced release of dopamine in the NAc in rat. Drug treatments are as described previously. After 4-day repeated morphine administration, implantation of the microdialysis probes was performed during the 3-day morphine withdrawal period. On day 8, after basal levels of DA and became stable (a), all rats were subjected to an acute challenge of morphine (3 mg/kg, s.c.).The dialysates were monitored for 3 h. DA release was expressed as average increase of% basal level over 3 h (b) and percentages of basal mean levels per 20-min intervals (c). ## P < 0.01, compared to saline/morphine control group; * P < 0.05 and ** P < 0.01, compared to the morphine/saline treatment group; n = 7–8 rats per group.