Electroacupuncture Relieves Nerve Injury-Induced Pain Hypersensitivity via the Inhibition of Spinal P2X7 Receptor-Positive Microglia

BACKGROUND: Electroacupuncture (EA) has therapeutic effects on neuropathic pain induced by nerve injury; however, the underlying mechanisms remain unclear. In this study, we examined whether EA treatment relieves pain hypersensitivity via the down-regulation of spinal P2X7 receptor-positive (P2X7R(+)) microglia-mediated overexpression of interleukin (IL)-1 beta and/or IL-18. METHODS: Male Sprague-Dawley rats underwent chronic constriction injury (CCI) or 3 '-O-(4-benzoylbenzoyl) adenosine 5 '-triphosphate (BzATP) intrathecal injection. Von Frey and Hargreaves tests were performed to evaluate the effect of EA on pain hypersensitivity. The spinal P2X7R, IL-1 beta, and IL-18 expression levels were determined by real-time polymerase chain reaction, Western blot analysis, immunofluorescence staining, and enzyme-linked immunosorbent assay. The selective P2X7R antagonist A-438079 was used to examine the P2X7R(+) microglia-dependent release of IL-1 beta and IL-18. Primary cultures were subsequently used to assess the P2X7R(+) microglia-induced IL-1 beta and IL-18 release. RESULTS: EA treatment significantly improved the pain thresholds and inhibited spinal P2X7R(+) microglia activation induced by CCI or BzATP administration, which was accompanied by the suppression of spinal IL-1 beta and IL-18 overexpression. Moreover, A-438079 also improved pain thresholds and suppressed overexpression of IL-1 beta in the CCI- and BzATP-injected rats. The analysis of cultured microglia further demonstrated that A-438079 markedly decreased BzATP-induced IL-1 beta release. CONCLUSIONS: EA treatment relieves nerve injury-induced tactile allodynia and thermal hyperalgesia via the inhibition of P2X7R(+) microglia-mediated IL-1 beta overexpression.