Relationship between increases in neuropeptide mRNA expression in the striatum of CB1−/− mutants and CB1 receptor expression in wild-type mice (CB1+/+). Dynorphin (A), substance P (B), enkephalin (C), and GAD 67 mRNA levels (D) were measured in 11 striatal regions of CB1−/− mice (n = 7) and expressed as percentage of levels in CB1+/+ mice (n = 7). These data were correlated with CB1 mRNA levels in CB1+/+ animals. Significant positive correlations were found for increases in dynorphin and substance P expression (both in striatonigral neurons), but not for enkephalin (in striatopallidal neurons) or GAD 67 expression (in both neuronal types). ∗∗, P < 0.01; ∗, P < 0.05.