J. Neurochem. (2010) 114, 810–819.
Abstract
Microglia plays an important role in many neurodegenerative conditions. ATP leaked or released by damaged cells triggers microglial activation through P2 receptors, and stimulates the release of oxygen radicals, proinflammatory cytokines and chemokines from activated microglia. However, little is known about mechanisms underlying ATP‐induced chemokine release from microglia. In this study, we found that a high concentration of ATP induces the mRNA expression and release of CXCL2 from microglia. A similar effect was observed following treatment of microglia with a P2X7 receptor (P2X7R) agonist, 2′‐and 3′‐O‐(4‐benzoylbenzoyl) ATP, and this was inhibited by pre‐treatment with a P2X7R antagonist, Brilliant Blue G. ATP induced both activation of nuclear factor of activated T cells (NFAT) and MAPKs (p38, ERK, and JNK) through P2X7R. ATP‐induced mRNA expression of CXCL2 was inhibited by INCA‐6 (an NFAT inhibitor), SB203580 (a p38 inhibitor), U0126 (a MEK‐ERK inhibitor) and JNK inhibitor II (a JNK inhibitor). However, MAPK inhibitors did not inhibit activation of NFAT. In addition, protein kinase C inhibitors suppressed ATP‐induced ERK and JNK activation, and also inhibited ATP‐induced CXCL2 expression in microglia. These results suggest that ATP increased CXCL2 production via both NFAT and protein kinase C/MAPK signaling pathways through P2X7 receptor stimulation in microglia.
BackgroundNeuropathic pain is a highly debilitating chronic pain following damage to peripheral sensory neurons and is often resistant to all treatments currently available, including opioids. We have previously shown that peripheral nerve injury induces activation of cytosolic phospholipase A2 (cPLA2) in injured dorsal root ganglion (DRG) neurons that contribute to tactile allodynia, a hallmark of neuropathic pain. However, lipid mediators downstream of cPLA2 activation to produce tactile allodynia remain to be determined.Principal FindingsHere we provide evidence that platelet-activating factor (PAF) is a potential candidate. Pharmacological blockade of PAF receptors (PAFRs) reduced the development and expression of tactile allodynia following nerve injury. The expression of PAFR mRNA was increased in the DRG ipsilateral to nerve injury, which was seen mainly in macrophages. Furthermore, mice lacking PAFRs showed a reduction of nerve injury-induced tactile allodynia and, interestingly, a marked suppression of upregulation of tumor necrosis factor α (TNFα) and interleukin-1β (IL-1β) expression in the injured DRG, crucial proinflammatory cytokines involved in pain hypersensitivity. Conversely, a single injection of PAF near the DRG of naïve rats caused a decrease in the paw withdrawal threshold to mechanical stimulation in a dose-dependent manner and an increase in the expression of mRNAs for TNFα and IL-1β, both of which were inhibited by pretreatment with a PAFR antagonist.ConclusionsOur results indicate that the PAF/PAFR system has an important role in production of TNFα and IL-1β in the DRG and tactile allodynia following peripheral nerve injury and suggest that blocking PAFRs may be a viable therapeutic strategy for treating neuropathic pain.
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