Effects of acute treatment of baclofen on the expression of morphine-induced sensitization. Four groups of rats were exposed to morphine and one group of rats was given saline for 4 consecutive days as described previously. After a 3-day withdrawal period, all five groups of animals were subjected to an acute challenge of morphine (3 mg/kg, s.c.) on day 8. Four morphine-treated groups were pretreated with saline or baclofen (1.25, 2.5, 5 mg/kg, s.c.) 15 min prior to the morphine challenge (b). The locomotor activity was monitored for 3 h in the chambers. Each bar represents the total distance traveled for the 3 h test period. a: the saline/morphine group and morphine/saline group. Data are shown as means ± S.E.M (n = 7–8 per group). ## P < 0.01 compared to saline/morphine group; * P < 0.05 compared to morphine/saline group.