p38b Mitogen-Activated Protein Kinase Signaling Mediates Exenatide-Stimulated Microglial b-Endorphin Expression

Recent discoveries established that activation of glucagon-like peptide-1 receptors (GLP-1Rs) mediates neuroprotection and antinociception through microglial b-endorphin expression. This study aimed to explore the underlying signaling mechanisms of microglial b-endorphin. GLP-1Rs and b-endorphin were coexpressed in primary cultures of microglia. Treatment with the GLP-1R agonist exenatide concentration-dependently stimulated microglial expression of the b-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 pprotein kinase A (PKA), p-p38, and p-cAMP response element binding protein (CREB) in cultured primary microglia. Furthermore, exenatide-induced microglial expression of POMCwas completely blocked by reagents that specifically inhibit adenylyl cyclase and activation of PKA, p38, and CREB. In addition, knockdown of p38b (but not p38a) 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 p38a (but not p38b) partially suppressed expression of proinflammatory factors (including tumor necrosis factor-a, interleukin-1b, and interleukin-6). Exenatideinduced phosphorylation of p38 and CREB was also totally blocked by the PKA inhibitor and siRNA/p38b, but not by siRNA/p38a. Seven-day intrathecal injections of siRNA/p38b (but not siRNA/p38a) completely blocked exenatide-induced spinal p38 activation, b-endorphin expression, and mechanical antiallodynia in rats with established neuropathy, although siRNA/p38b and siRNA/p38a were not antiallodynic. To our knowledge, our results are the first to show a causal relationship between the PKA-dependent p38b mitogen-activated protein kinase/CREB signal cascade and GLP-1R agonism– mediated microglial b-endorphin expression. The differential role of p38a and p38b activation in inflammation and nociception was also highlighted.