Gene expression changes induced by morphine treatment in the striatum of the four inbred strains of mice. The four strains were C57BL/6J, DBA/2J, SWR/J, and 129P3/J. (a) Clustering image of genes whose expression was the most significantly altered, according to microarray analysis (SAL, control group; MOR, acute [ACU] and chronic [CHR] morphine groups). Colored rectangles represent expression levels of the gene indicated by the probe set on the left. Intensity of the color is proportional to the fold change, as indicated on the bar below the cluster image. Hierarchical clustering was performed with the dChip software using Euclidean distance and average linkage method. (b) Validation of morphine-induced regulation of expression of the selected genes by quantitative real-time reverse transcription polymerase chain reaction (qPCR). Results are presented as fold change over control group with standard error (n = 6 to 9). Significant main effects from multivariate analysis of variance for morphine treatment (***P < 0.001) and interaction ($P < 0.05) are indicated. Difference between morphine-treated and control groups was analyzed using Bonferroni post hoc test (#P < 0.05).