Interleukin-6-mediated hyperalgesia/allodynia and increased spinal IL-6 expression in a rat mononeuropathy model

It has been suggested that neuroimmunologic mechanisms may be involved in the development and maintenance of neuropathic pain. To further address this concept, the immunoreactive spinal expression of the pro-inflammatory cytokine, interleukin-6 (IL-6), was determined in the mononeuropathy model in the rat, sciatic cryoneurolysis (SCN). This well-established animal model expresses behaviors suggestive of neuropathic pain in humans. Immunohistochemical localization in the spinal cord was determined at 3, 7, 14, 21, 35, and 120 days after SCN (n = 6 per time point). Immunoreactive IL-6 increased incrementally in the substantia gelatinosa and motoneurons over time following SCN as compared with normal rats. In an additional study, recombinant human IL-6 was administered intrathecally to normal and previously SCN-lesioned rats. Intrathecal IL-6 produced touch-evoked allodynia (increased sensitivity to a nonnoxious stimulus) in normal rats and thermal hyperalgesia (increased sensitivity to a noxious stimulus) in previously lesioned SCN rats. These results provide evidence that IL-6 may be involved in the cascade of events leading to the development and maintenance of behaviors suggestive of neuropathic pain following peripheral nerve injury.