TRAF6 Contributes to CFA-Induced Spinal Microglial Activation and Chronic Inflammatory Pain in Mice.

Tumor necrosis factor receptor-associated factor 6 (TRAF6) has been reported to be expressed in spinal astrocytes and is involved in neuropathic pain. In this study, we investigated the role and mechanism of TRAF6 in complete Freund's adjuvant (CFA)-evoked chronic inflammatory hypersensitivity and the effect of docosahexaenoic acid (DHA) on TRAF6 expression and inflammatory pain. We found that TRAF6 was dominantly increased in microglia at the spinal level after intraplantar injection of CFA. Intrathecal TRAF6 siRNA alleviated CFA-triggered allodynia and reversed the upregulation of IBA-1 (microglia marker). In addition, intrathecal administration of DHA inhibited CFA-induced upregulation of TRAF6 and IBA-1 in the spinal cord and attenuated CFA-evoked mechanical allodynia. Furthermore, DHA prevented lipopolysaccharide (LPS)-caused increase of TRAF6 and IBA-1 in both BV2 cell line and primary cultured microglia. Finally, intrathecal DHA reduced LPS-induced upregulation of spinal TRAF6 and IBA-1, and alleviated LPS-induced mechanical allodynia. Our findings indicate that TRAF6 contributes to pain hypersensitivity via regulating microglial activation in the spinal dorsal horn. Direct inhibition of TRAF6 by siRNA or indirect inhibition by DHA may have therapeutic effects on chronic inflammatory pain.

Intrathecal curcumin attenuates pain hypersensitivity and decreases spinal neuroinflammation in rat model of monoarthritis.

Curcumin is a major component of turmeric and reportedly has anti-inflammatory and anti-oxidant effects. Neuroinflammation has been recognized to play an important role in the pathogenesis of various diseases in the central nervous system. Here we investigated the anti-nociceptive and anti-neuroinflammatory effect of curcumin on arthritic pain in rats. We found that repeated oral treatment with curcumin, either before or after complete Freund's adjuvant (CFA) injection, dose-dependently attenuated CFA-induced mechanical allodynia and thermal hyperalgesia, but had no effect on joint edema. Repeated intrathecal injection of curcumin reversed CFA-induced pain hypersensitivity. Furthermore, such a curcumin treatment reduced CFA-induced activation of glial cells and production of inflammatory mediators [interleukin-1ß (IL-1ß), monocyte chemoattractant protein-1 (MCP-1), and monocyte inflammatory protein-1 (MIP-1α)] in the spinal cord. Curcumin also decreased lipopolysaccharide-induced production of IL-1ß, tumor necrosis factor-α, MCP-1, and MIP-1α in cultured astrocytes and microglia. Our results suggest that intrathecal curcumin attenuates arthritic pain by inhibiting glial activation and the production of inflammatory mediators in the spinal cord, suggesting a new application of curcumin for the treatment of arthritic pain.

Ligustilide inhibits microglia-mediated proinflammatory cytokines production and inflammatory pain.

Ligustilide is the main component of Danggui essential oil, and recently reported to have anti-inflammatory and neuroprotective effect. Increasing evidence suggests that glia-mediated neuroinflammation in the spinal cord plays a vital role in the pathogenesis of chronic pain. In the present study, we investigated the anti-inflammatory and anti-nociceptive effect of ligustilide both in vitro and in vivo. In microglial cell line BV2 cells, lipopolysaccharide (LPS) time-dependently increased the mRNA expression of proinflammatory cytokines (TNF-α, IL-1ß, and IL-6), which was decreased by pretreatment with ligustilide in a dose-dependent manner. Ligustilide also decreased LPS-induced proinflammatory cytokines production in primary cultured microglia. In vivo, intrathecal injection of LPS induced mechanical allodynia in mice. Intravenous injection of ligustilide prevented LPS-induced mechanical allodynia, and decreased LPS-induced TNF-α, IL-1ß, and IL-6 up-regulation in the spinal cord. In addition, repetitive intravenous injection of ligustilide attenuated intraplantar injection of complete Freund's adjuvant (CFA)-induced mechanical allodynia and thermal hyperalgesia. The same treatment of ligustilide also inhibited CFA-induced TNF-α, IL-1ß, and IL-6 up-regulation and microglial activation in the spinal cord. Taken together, our data suggest that ligustilide can alleviate inflammatory pain partly through inhibition of microglial activation and proinflammatory cytokines production, which indicates a possible benefit from the use of ligustilide in the treatment of inflammatory pain and neuroinflammation-associated disorders.

Repetitive hyperbaric oxygen treatment attenuates complete Freund's adjuvant-induced pain and reduces glia-mediated neuroinflammation in the spinal cord.

UNLABELLED: Hyperbaric oxygen (HBO) therapy is reported to attenuate pain in both clinical pain conditions and animal pain models, but the underlying mechanism remains to be investigated. Here, we show that 7 daily 60-minute HBO (100% oxygen, 2 atmosphere absolute) treatments effectively and persistently inhibited heat hyperalgesia, mechanical allodynia, and paw edema induced by peripheral injection of complete Freund's adjuvant (CFA). Five daily 60-minute HBO treatments also produced a prolonged reversal effect of the ongoing inflammatory pain. Furthermore, such an HBO treatment reduced CFA-induced activation of glial cells, phosphorylation of mitogen-activated protein kinases, and production of a variety of proinflammatory cytokines (tumor necrosis factor alpha [TNF-α], interleukin-1 beta [IL-1ß], and interleukin-6 [IL-6]) and chemokines (monocyte chemoattractant protein-1 [MCP-1], keratinocyte-derived chemokine [KC], and IFN-gamma-inducible protein 10 [IP-10]) in the spinal cord. HBO treatment also decreased lipopolysaccharide-induced mRNA expression of these cytokines and chemokines in primary cultures of astrocytes and microglia. In addition, the mRNA expressions of IL-1ß, IL-6, MCP-1, KC, and IP-10 in the inflamed paw skin were decreased by HBO. Taken together, these data suggest that HBO treatment is an effective therapy for inflammatory pain in animals. The inhibition of the neuroinflammation that is mediated by glial cells and inflammatory mediators may, at least in part, contribute to the antinociceptive effect of HBO therapy. PERSPECTIVE: Our results suggest that repetitive HBO treatment attenuates CFA-induced pain and reduces glial activation and inflammatory mediators' production. These findings provide evidence of the antinociception effect of HBO on inflammatory pain and characterize some of the underlying mechanisms.