Go-sha-jinki-Gan (GJG) ameliorates allodynia in chronic constriction injury model mice via suppression of TNF-α expression in the spinal cord

BACKGROUND Alternative medicine is noted for its clinical effect and minimal invasiveness in the treatment of neuropathic pain. Go-sha-jinki-Gan, a traditional Japanese herbal medicine, has been used for meralgia and numbness in elderly patients. However, the exact mechanism of GJG is unclear. This study aimed to investigate the molecular mechanism of the analgesic effect of GJG in a chronic constriction injury model. RESULTS GJG significantly reduced allodynia and hyperalgesia from the early phase (von Frey test, p<0.0001; cold-plate test, p<0.0001; hot-plate test p¼0.011; two-way repeated measures ANOVA). Immunohistochemistry and Western blot analysis revealed that GJG decreased the expression of Iba1 and tumor necrosis factor-a in the spinal cord. Double staining immunohistochemistry showed that most of the tumor necrosis factor-a was co-expressed in Iba1-positive cells at day 3 post-operation. GJG decreased the phosphorylation of p38 in the ipsilateral dorsal horn. Moreover, intrathecal injection of tumor necrosis factor-a opposed the anti-allodynic effect of GJG in the cold-plate test. CONCLUSIONS Our data suggest that GJG ameliorates allodynia in chronic constriction injury model mice via suppression of tumor necrosis factor-a expression derived from activated microglia. GJG is a promising drug for the treatment of neuropathic pain induced by neuro-inflammation.