Fig. 2.
(+)-Morphine pretreatment given into the posterior nucleus accumbens shell attenuates the conditioned place preference produced by (−)-morphine from the posterior nucleus accumbens shell. After completion of the pre-conditioning measurement on the 1st day, groups of rats were pretreated with different doses (0.1 to 1000 pg) of (+)morphine or vehicle for 45 min and were place conditioned after microinjection of (−)morphine (5 μg) or vehicle given into the posterior nucleus accumbens shell twice a day for three days. The post-conditioning was measured on the 5th day. Each column represents the mean of conditioned place preference score and the vertical bar represents the S.E.M.; n = 6–17; Paired t test was used to compare production of the conditioned place preference of individual dose: For the group of rats pretreated with vehicle followed by vehicle or (−)-morphine challenge, t = 0.6 and 6.4 and df = 12 and 9, respectively. For the group of the rats pretreated with different dose of (+)-morphine (0.1, 0.3, 1, 3, 10, 30, 100 or 1000 pg) followed by (−)-morphine challenge, t = 3.7, 2.9, 6.4, 0.8, 0.7, 4.4, 4.2 and 7.2 and df = 7, 5, 8, 6, 12, 5, 12 and 17, respectively. # P < 0.05, ## P < 0.01, ### P < 0.001. One-way ANOVA followed by Dunnett’s post-test was used to test difference between groups, F(8, 91) = 3.02; * P < 0.05, ** P < 0.01.