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 32,000 (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 D2 R evoked an additive effect on the phospho-ERK1/2 signal. This effect was mediated through a pertussis toxin-sensitive Gαi/o pathway in primary striatal cells. Furthermore, the mRNA level of CB₁R was increased via dopamine D2 receptor short form (D(2S)R) by treatment with D₂R agonist quinpirole in D(2S)R/C6 glioma cells. This effect could be suppressed by co-treatment with the ERK1/2 inhibitor U0126. To test if D(2S)R 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 375 bp of 5'-UTR and are separated by a 17-kb intron. A luciferase reporter assay showed that the maximal D(2S)R-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.