Crosstalk between Dopamine D2 receptors and cannabinoid CB1 receptors regulates CNR1 promoter activity via ERK1/2 signaling

Previously, we found that chronic methamphetamine treatment altered cannabinoid type 1 receptor (CB1R)-dependent cAMP/PKA/dopamine and cAMP-regulated phosphoprotein of Mr 32000 (DARPP-32)/T34/PP2B signaling and decreased levels of CB1R protein and mRNA in the nucleus accumbens. These findings suggested the existence of signaling interplay between mesolimbic dopamine and CB1R. In this study, we further investigate interactions between CB1R and dopamine D2 receptor (D2R) signaling. Activation of either CB1R or D2R increased extracellular signal-regulated kinases 1 and 2 (ERK1/2) phosphorylation, while co-stimulation of CB1R and D2R evoked an additive effect on the phospho-ERK1/2 signal. This effect was mediated through a pertussis toxin-sensitive Gi/o pathway in primary striatal cells. Furthermore, the mRNA level of CB1R was increased via dopamine D2 receptor short form (D2SR) by treatment with D2R agonist quinpirole in D2SR/C6 glioma cells. This effect could be suppressed by co-treatment with the ERK1/2 inhibitor U0126. To test if D2SR could transcriptionally regulate CB1R, the 5-untranslated region (5-UTR) of the cannabinoid receptor 1 (CNR1) gene was sequenced from rat brain. Results showed that the CNR1 gene includes two exons, which contain 375bp of 5-UTR and are separated by a 17-kb intron. A luciferase reporter assay showed that the maximal D2SR-responsive promoter activity is located in the -1 to -222 region of CNR1 promoter. Overall, we demonstrate previously unidentified crosstalk between D2R and CB1R via ERK1/2 signaling that enhances the expression of CB1R by modulating its promoter activity.