Inflammation confers dual effects on nociceptive processing in chronic neuropathic pain model

Although inflammation induces pain, immune cells also produce mediators that can effectively counteract it. To further elucidate the role of the immune response, we analyzed the relationship of pain behavior, several inflammatory signals, and opioid peptides using partial sciatic nerve ligation in mice at different levels of immunocompromise.

Sciatic nerves of C57BL/6C, nonobese diabetic (NOD), or nonobese diabetic-severe combined immune deficiency (NOD-SCID) mice were partially ligated. Responses to mechanical and radiant heat stimuli were observed. Inflammation was detected by immunohistochemistry and flow cytometry. Inflammatory cytokines and opioid peptides were analyzed using real-time polymerase chain reaction and enzyme-linked immunosorbent assay or immunostaining.

Inflammation in immunocompromised mice was subordinate when compared with that seen in C57BL/6C mice. In addition, immunocompromised mice had less pain hypersensitivity at early stages. Whereas proinflammatory tumor necrosis factor-α (TNF-α), interleukin 1β (IL-1β), interleukin 6 (IL-6), and interferon-γ (IFN-γ), as well as antiinflammatory interleukin 1 receptor antagonist (IL-1Ra), interleukin 4 (IL-4), interleukin 10 (IL-10), and interleukin 13 (IL-13) cytokine expression and protein were increased in C57BL/6C mice, they were lower in immunocompromised mice. Although enkephalin, dynorphin, and β-endorphin messenger RNA expression also increased in C57BL/6C mice, peaking on day 14, this result was not observed in immunocompromised mice.

The contribution of inflammation to nerve injury is complex with biphasic modulation. During the early phase, a wide range of proinflammatory cytokines are released, leading to enhanced pain. In contrast, the analgesic effect of opioid peptides and antiinflammatory cytokines was more predominate in the later phases of injury, leading to attenuated pain responses.