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Cannabinoid CB 1 receptors are densely expressed on striatal projection neurons expressing dopamine D 1 or D 2 receptors. However, the specific neuronal distribution of CB 1 receptors within the striatum is not known. Previous research has established that the endocannabinoid system controls facilitation of behavior by dopamine D 2 receptors, but it is not clear if endocannabinoids also modulate D 1 receptor-mediated motor behavior. In the present study, we show that cannabinoid CB 1 receptor mRNA is present in striatonigral neurons expressing substance P and dopamine D 1 receptors, as well as in striatopallidal neurons expressing enkephalin and dopamine D 2 receptors. We explored the functional relevance of the interaction between dopamine D 1 and D 2 receptors and cannabinoid CB 1 receptors with behavioral pharmacology experiments. Potentiation of endogenous cannabinoid signaling by the …