REGULATION OF DELTA-OPIOID RECEPTOR MESSENGER-RNA LEVELS BY RECEPTOR-MEDIATED AND DIRECT ACTIVATION OF THE ADENYLYL CYCLASE-PROTEIN KINASE-A PATHWAY

The effects of activation of the adenylyl cyclase-protein kinase A pat hway on the expression of delta-opioid receptor mRNA in the NG108-15 n euroblastoma x glioma cell line has been investigated. Activation of p rostaglandin E(1) (PGE(1)) receptors, which are positively coupled to adenylyl cyclase, resulted in a reduction in S-receptor messenger RNA levels. Direct stimulation of adenylyl cyclase by forskolin or treatme nt of cells with the cyclic AMP analogue dibutyryl cycle AMP (db-cAMP) mimicked the effect of PGE,. Down-regulation in receptor protein leve ls, as measured by loss of radioligand binding sites, was also observe d and its extent correlated well with the decrease in the amount of de lta-opioid receptor trans scripts. D-Ser(2)-Leu-enkephalin-Thr(6) (DSL ET) inhibition of adenylyl cyclase activity was also diminished after db-cAMP treatment, Inhibitors of protein kinase A (PKA) partially reve rsed the PGE(1)- and db-cAMP-mediated repression of the delta-opioid r eceptor mRNA levels. The rate of degradation of delta-opioid receptor mRNA in the presence of actinomycin D was not altered in response to d b-cAMP, suggesting that mRNA stability is not reduced by PKA action. T he regulation of delta-opioid receptor mRNA levels by db-cAMP was not sensitive to the protein synthesis inhibitor cycloheximide, suggesting that de novo protein synthesis is not required in this process.