Effects of acute 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 on day 5. 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 compared to the morphine/saline treatment group; n = 8 per group.