Electroacupuncture inhibits excessive interferon-γ evoked up-regulation of P2X4 receptor in spinal microglia in a CCI rat model for neuropathic pain

Although electroacupuncture (EA) is effective in the relief of neuropathic pain, the underlying mechanisms remain unclear. Previous studies have reported immunomodulatory effects of EA in rats. Since excessive release of interferon-gamma (IFN-gamma) after nerve injury transforms quiescent spinal microglia into an activated state with more neuropathic pain, associated with purinergic receptor P2X4 expression, it is possible that EA may mediate its analgesic effect by attenuating IFN-gamma release and subsequent generation of P2X4R(+) microglia. Male rats underwent chronic constriction injury (CCI) or IFN-gamma intrathecal injection and von Frey tests were performed to evaluate the effect of EA on pain thresholds. Spinal IFN-gamma and P2X4R expression levels were measured by immunohistochemistry, real-time PCR, enzyme immunoassay, and/or western blots. In vitro primary cultures of microglia were used to examine IFN-gamma activation of P2X4R(+) cells. In CCI rats, EA treatment significantly increased paw withdrawal threshold relative to control. IFN-gamma facilitated P2X4R(+) microglia activation both in vitro and in vivo. EA also down-regulated both P2X4R and IFN-gamma expression in the spinal cord after CCI. However, EA did not exert the same analgesic effect after intrathecal IFN-gamma injection. EA ameliorated tactile allodynia after peripheral nerve injury by down-regulating excessive expression of IFN-gamma in the spinal cord and subsequently reducing expression of P2X4R.