Endogenous δ-opioid and ORL₁ receptors couple to phosphorylation and activation of p38 MAPK in NG108-15 cells and this is regulated by protein kinase A and protein kinase C

The p38 mitogen-activated protein kinase (MAPK) cascade transduces multiple extracellular signals from cell surface to nucleus and is employed in cellular responses to cellular stresses and apoptotic regulation. The involvement of the p38 MAPK cascade in opioid- and opioid receptor-like receptor-1 (ORL₁) receptor-mediated signal transduction was examined in NG108-15 neuroblastoma x glioma hybrid cells. Stimulation of endogenous δ- opioid receptor (DOR) or ORL₁ resulted in activation of p38 MAPK. It also reduced the activation of extracellular signal-regulated kinases (ERKs), another member of the MAPK family, with slower kinetics. Activation of p38 MAPK was abolished by selective antagonists of DOR or ORL₁, pretreatment with pertussis toxin, or SB203580, a specific inhibitor of p38 MAPK. Inhibition of p38 MAPK had no significant effect on opioid-induced ERK activation, indicating that p38 MAPK activity was not required for ERK activation, though its stimulation preceded ERK activation. Inhibition of protein kinase A (PKA) strongly diminished p38 activation mediated by DOR or ORL₁ but had no significant effect on ERK activation, and protein kinase C (PKC) inhibitors potentiated stimulation of p38 while inhibiting activation of ERKs. Taken together, our results provide the first evidence for coupling of DOR and ORL₁ to the p38 MAPK cascade and clearly demonstrate that receptor-mediated activation of p38 MAPK both revolves PKA and is negatively regulated by PKC.