Low frequency electroacupuncture alleviates neuropathic pain by activation of spinal microglial IL-10/β-endorphin pathway

Electroacupuncture produces analgesia in chronic pain patients and animal models of pain hypersensitivity. The current study aims to illustrate the mechanisms underlying electroacupuncture-attenuated neuropathic pain. Neuropathic rats, induced by tight ligation of L5/L6 spinal nerves, markedly reduced mechanical thresholds in the ipsilateral hindpaws relative to the contralateral hindpaws. Low frequency (2 Hz) electroacupuncture stimulation for a period of 20 min alleviated neuropathic pain in the ipsilateral hindpaws of neuropathic rats in a time-dependent manner. The same electroacupuncture treatment also stimulated spinal gene and protein expression of IL-10 and β-endorphin but not dynorphin A, measured by real-time quantitative PCR and ELISA kits. Intrathecal injection of the specific IL-10 antibody in neuropathic rats completely blocked electroacupuncture-increased spinal expression of β-endorphin, but the β-endorphin antibody failed to alter electroacupuncture-stimulated spinal IL-10 expression. Using a double fluorescence immunostaining technique, we observed that electroacupuncture stimulated spinal IL-10 and β-endorphin expression in microglia but not in neurons or astrocytes in the spinal dorsal horn of neuropathic rats. Pretreatment with intrathecal injection of the microglial inhibitor minocycline, specific IL-10 antibody and β-endorphin antiserum (but not the dynorphin A antibody), or selective μ-opioid receptor antagonist CTAP (but not κ- or δ-opioid receptor antagonist) completely blocked electroacupuncture-induced attenuation of neuropathic pain. These results suggest that low frequency electroacupuncture alleviates neuropathic pain through stimulation of the spinal microglial expression of IL-10 and subsequent expression of β-endorphin.