The journal of pain : official journal of the American Pain Society
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Insights into mechanisms governing resolution of inflammatory pain are of great importance for many chronic pain-associated diseases. Here we investigate the role of macrophages/monocytes and the anti-inflammatory cytokine interleukin-10 (IL-10) in the resolution of transient inflammatory pain. Depletion of mice from peripheral monocytes/macrophages delayed resolution of intraplantar IL-1β- and carrageenan-induced inflammatory hyperalgesia from 1 to 3 days to >1 week. Intrathecal administration of a neutralizing IL-10 antibody also markedly delayed resolution of IL-1β- and carrageenan-induced inflammatory hyperalgesia. Recently, we showed that IL-1β- and carrageenan-induced hyperalgesia is significantly prolonged in LysM-GRK2(+/-) mice, which have reduced levels of G-protein-coupled receptor kinase 2 (GRK2) in LysM(+) myeloid cells. Here we show that adoptive transfer of wild-type, but not of GRK2(+/-), bone marrow-derived monocytes normalizes the resolution of IL-1β-induced hyperalgesia in LysM-GRK2(+/-) mice. Adoptive transfer of IL-10(-/-) bone marrow-derived monocytes failed to normalize the duration of IL-1β-induced hyperalgesia in LysM-GRK2(+/-) mice. Mechanistically, we show that GRK2(+/-) macrophages produce less IL-10 in vitro. In addition, intrathecal IL-10 administration attenuated IL-1β-induced hyperalgesia in LysM-GRK2(+/-) mice, whereas it had no effect in wild-type mice. Our data uncover a key role for monocytes/macrophages in promoting resolution of inflammatory hyperalgesia via a mechanism dependent on IL-10 signaling in dorsal root ganglia. ⋯ We show that IL-10-producing monocytes/macrophages promote resolution of transient inflammatory hyperalgesia. Additionally, we show that reduced monocyte/macrophage GRK2 impairs resolution of hyperalgesia and reduces IL-10 production. We propose that low GRK2 expression and/or impaired IL-10 production by monocytes/macrophages represent peripheral biomarkers for the risk of developing chronic pain after inflammation.
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Lumbar disc herniation (LDH) is a major cause of sciatica, but the underlying mechanisms are not well understood. Chemokine CCL2 has been implicated to play a vital role in the neuroinflammation and central sensitization after spinal nerve ligation. Here we investigated the expression and the role of CCL2 and its receptor CCR2 in LDH-induced pain. Implantation of autologous nucleus pulposus induced persistent pain hypersensitivity, associated with increased mRNA expression of CCL2 and CCR2 in the dorsal root ganglion and spinal cord. Interestingly, CCL2 was increased in neurons and CCR2 was mainly increased in macrophages in the dorsal root ganglion, whereas CCL2 and CCR2 were increased in astrocytes and neurons, respectively, in the spinal cord. Intrathecal injection of CCR2 antagonist RS504393 at 3 days or 10 days significantly attenuated nucleus pulposus-induced mechanical allodynia. The results suggest that CCL2/CCR2 in the dorsal root ganglion and spinal cord is involved in the maintenance of LDH-induced pain. Targeting CCL2/CCR2 signaling may be a potential treatment for chronic radicular neuropathic pain. ⋯ These results suggest that CCL2/CCR2 signaling in the dorsal root ganglion and spinal cord is involved in LDH-induced pain via distinct mechanisms. These findings provide evidence of the antinociceptive effect of CCR2 antagonist on radicular neuropathic pain.