Comparison of the number of genes with expression altered by genotype and morphine treatment. (a) In all, 178 probe sets were shared between genes with different levels across the four inbred strains (red circle) and morphine-responsive genes (yellow circle). (b) Approximately three-quarters of genes upregulated by chronic treatment (green circles) were also altered by acute morphine treatment (blue circles). On the other hand, 29 genes with a decreased level of mRNA after acute treatment were found in the list of 145 downregulated genes after repeated morphine administration. The list of probe sets with greater than 1.2-fold change over control was analyzed.