p38<i>β</i> Mitogen-Activated Protein Kinase Signaling Mediates Exenatide-Stimulated Microglial <i>β</i>-Endorphin Expression.

Recent discoveries established that activation of glucagon-like peptide-1 receptors (GLP-1Rs) mediates neuroprotection and antinociception through microglial -endorphin expression. This study aimed to explore the underlying signaling mechanisms of microglial -endorphin. GLP-1Rs and -endorphin were coexpressed in primary cultures of microglia. Treatment with the GLP-1R agonist exenatide concentration-dependently stimulated microglial expression of the -endorphin precursor gene proopiomelanocortin (POMC) and peptides, with EC values of 4.1 and 7.5 nM, respectively. Exenatide also significantly increased intracellular cAMP levels and expression of -protein kinase A (PKA), -p38, and -cAMP response element binding protein (CREB) in cultured primary microglia. Furthermore, exenatide-induced microglial expression of POMC was completely blocked by reagents that specifically inhibit adenylyl cyclase and activation of PKA, p38, and CREB. In addition, knockdown of p38 (but not p38) using short interfering RNA (siRNA) eliminated exenatide-induced microglial p38 phosphorylation and POMC expression. In contrast, lipopolysaccharide increased microglial activation of p38, and knockdown of p38 (but not p38) partially suppressed expression of proinflammatory factors (including tumor necrosis factor-, interleukin-1, and interleukin-6). Exenatide-induced phosphorylation of p38 and CREB was also totally blocked by the PKA inhibitor and siRNA/p38, but not by siRNA/p38 Seven-day intrathecal injections of siRNA/p38 (but not siRNA/p38) completely blocked exenatide-induced spinal p38 activation, -endorphin expression, and mechanical antiallodynia in rats with established neuropathy, although siRNA/p38 and siRNA/p38 were not antiallodynic. To our knowledge, our results are the first to show a causal relationship between the PKA-dependent p38 mitogen-activated protein kinase/CREB signal cascade and GLP-1R agonism-mediated microglial -endorphin expression. The differential role of p38 and p38 activation in inflammation and nociception was also highlighted.