Forskolin induces preproenkephalin and preprodynorphin mRNA in rat striatum as demonstrated by in situ hybridization histochemistry.

Cyclase response elements (CREs) are located in the promoter regions of several neuropeptide and immediate early genes. Activation of the adenylate cylase/cAMP second messenger cascade leads to phosphorylation of CRE-binding proteins (P-CREBs) which bind to CREs in the promoter regions of these genes and alter their rate of transcription. We have previously reported an increase in striatal immunoreactivity for P-CREB (phosphorylated on Ser-133) and Fos following intracerebroventricular (ICV) injection of H2O-soluble forskolin, a direct activator of adenylate cyclase. Because CREs are located in the promoter regions of the opioid peptide genes, preproenkephalin (PPE) and preprodynorphin (PPD), we investigated what effect continuous ICV infusion of H2O-soluble forskolin has on striatal PPE and PPD mRNA levels. Quantitative in situ hybridization histochemistry demonstrated that continuous activation of the adenylate cyclase/cAMP second messenger cascade results in a significant induction of striatal PPE and PPD mRNA at 6, 24, and 72 h. The sustained induction of striatal PPE and PPD mRNA indicates that pro-opioid gene transcription is not desensitized following 72 h of continuous adenylate cyclase activation. Continuous ICV infusion of 1, 9-dideoxyforskolin, a forskolin analog which does not activate adenylate cyclase, did not induce striatal PPE and PPD mRNA. These data are consistent with cAMP-dependent protein kinase-induced phosphorylation and binding of CREBs to CREs in the promoter regions of pro-opioid genes during sustained activation of adenylate cyclase.