p38 β Mitogen-Activated Protein Kinase Signaling Mediates Exenatide-Stimulated Microglial β -Endorphin Expression.

Recent discoveries established that activation of glucagon-like peptide-1 receptors (GLP-1Rs) mediates neuroprotection and antinociception through microglial β -endorphin expression. This study aimed to explore the underlying signaling mechanisms of microglial β -endorphin. GLP-1Rs and β -endorphin were coexpressed in primary cultures of microglia. Treatment with the GLP-1R agonist exenatide concentration-dependently stimulated microglial expression of the β -endorphin precursor gene proopiomelanocortin (POMC) and peptides, with EC50 values of 4.1 and 7.5 nM, respectively. Exenatide also significantly increased intracellular cAMP levels and expression of p -protein kinase A (PKA), p -p38, and p -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 β (but not p38 α ) 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 α (but not p38 β ) partially suppressed expression of proinflammatory factors (including tumor necrosis factor- α , interleukin-1 β , and interleukin-6). Exenatide-induced phosphorylation of p38 and CREB was also totally blocked by the PKA inhibitor and siRNA/p38 β , but not by siRNA/p38 α Seven-day intrathecal injections of siRNA/p38 β (but not siRNA/p38 α ) completely blocked exenatide-induced spinal p38 activation, β -endorphin expression, and mechanical antiallodynia in rats with established neuropathy, although siRNA/p38 β and siRNA/p38 α were not antiallodynic. To our knowledge, our results are the first to show a causal relationship between the PKA-dependent p38 β mitogen-activated protein kinase/CREB signal cascade and GLP-1R agonism-mediated microglial β -endorphin expression. The differential role of p38 α and p38 β activation in inflammation and nociception was also highlighted.