TLR8 in the Trigeminal Ganglion Contributes to the Maintenance of Trigeminal Neuropathic Pain in Mice.

Trigeminal neuropathic pain (TNP) is a significant health problem but the involved mechanism has not been completely elucidated. Toll-like receptors (TLRs) have recently been demonstrated to be expressed in the dorsal root ganglion and involved in chronic pain. Here, we show that TLR8 was persistently increased in the trigeminal ganglion (TG) neurons in model of TNP induced by partial infraorbital nerve ligation (pIONL). In addition, deletion or knockdown of Tlr8 in the TG attenuated pIONL-induced mechanical allodynia, reduced the activation of ERK and p38-MAPK, and decreased the expression of pro-inflammatory cytokines in the TG. Furthermore, intra-TG injection of the TLR8 agonist VTX-2337 induced pain hypersensitivity. VTX-2337 also increased the intracellular Ca2+ concentration, induced the activation of ERK and p38, and increased the expression of pro-inflammatory cytokines in the TG. These data indicate that TLR8 contributes to the maintenance of TNP through increasing MAPK-mediated neuroinflammation. Targeting TLR8 signaling may be effective for the treatment of TNP.

TLR8 and its endogenous ligand miR-21 contribute to neuropathic pain in murine DRG.

Toll-like receptors (TLRs) are nucleic acid-sensing receptors and have been implicated in mediating pain and itch. Here we report that Tlr8 -/- mice show normal itch behaviors, but have defects in neuropathic pain induced by spinal nerve ligation (SNL) in mice. SNL increased TLR8 expression in small-diameter IB4+ DRG neurons. Inhibition of TLR8 in the DRG attenuated SNL-induced pain hypersensitivity. Conversely, intrathecal or intradermal injection of TLR8 agonist, VTX-2337, induced TLR8-dependent pain hypersensitivity. Mechanistically, TLR8, localizing in the endosomes and lysosomes, mediated ERK activation, inflammatory mediators' production, and neuronal hyperexcitability after SNL. Notably, miR-21 was increased in DRG neurons after SNL. Intrathecal injection of miR-21 showed the similar effects as VTX-2337 and inhibition of miR-21 in the DRG attenuated neuropathic pain. The present study reveals a previously unknown role of TLR8 in the maintenance of neuropathic pain, suggesting that miR-21-TLR8 signaling may be potential new targets for drug development against this type of chronic pain.

Spinal CXCL9 and CXCL11 are not involved in neuropathic pain despite an upregulation in the spinal cord following spinal nerve injury.

Chemokines-mediated neuroinflammation in the spinal cord plays a critical role in the pathogenesis of neuropathic pain. Chemokine CXCL9, CXCL10, and CXCL11 have been identified as a same subfamily chemokine which bind to CXC chemokine receptor 3 to exert functions. Our recent work found that CXCL10 is upregulated in spinal astrocytes after spinal nerve ligation (SNL) and acts on chemokine receptor CXCR3 on neurons to contribute to central sensitization and neuropathic pain, but less is known about CXCL9 and CXCL11 in the maintenance of neuropathic pain. Here, we report that CXCL9 and CXCL11, same as CXCL10, were increased in spinal astrocytes after SNL. Surprisingly, inhibition of CXCL9 or CXCL11 by spinal injection of shRNA lentivirus did not attenuate SNL-induced neuropathic pain. In addition, intrathecal injection of CXCL9 and CXCL11 did not produce hyperalgesia or allodynia behaviors, and neither of them induced ERK activation, a marker of central sensitization. Whole-cell patch clamp recording on spinal neurons showed that CXCL9 and CXCL11 enhanced both excitatory synaptic transmission and inhibitory synaptic transmission, whereas CXCL10 only produced an increase in excitatory synaptic transmission. These results suggest that, although the expression of CXCL9 and CXCL11 are increased after SNL, they may not contribute to the maintenance of neuropathic pain.

Chemokine Receptor CXCR3 in the Spinal Cord Contributes to Chronic Itch in Mice.

Recent studies have shown that the chemokine receptor CXCR3 and its ligand CXCL10 in the dorsal root ganglion mediate itch in experimental allergic contact dermatitis (ACD). CXCR3 in the spinal cord also contributes to the maintenance of neuropathic pain. However, whether spinal CXCR3 is involved in acute or chronic itch remains unclear. Here, we report that Cxcr3 -/- mice showed normal scratching in acute itch models but reduced scratching in chronic itch models of dry skin and ACD. In contrast, both formalin-induced acute pain and complete Freund's adjuvant-induced chronic inflammatory pain were reduced in Cxcr3 -/- mice. In addition, the expression of CXCR3 and CXCL10 was increased in the spinal cord in the dry skin model induced by acetone and diethyl ether followed by water (AEW). Intrathecal injection of a CXCR3 antagonist alleviated AEW-induced itch. Furthermore, touch-elicited itch (alloknesis) after compound 48/80 or AEW treatment was suppressed in Cxcr3 -/- mice. Finally, AEW-induced astrocyte activation was inhibited in Cxcr3 -/- mice. Taken together, these data suggest that spinal CXCR3 mediates chronic itch and alloknesis, and targeting CXCR3 may provide effective treatment for chronic pruritus.

Chemokine CXCL13 activates p38 MAPK in the trigeminal ganglion after infraorbital nerve injury.

Recent data demonstrated that chemokine CXCL13 mediates neuroinflammation and contributes to the maintenance of neuropathic pain after nerve injury in the spinal cord. Pro-nociceptive chemokines activate mitogen-activated protein kinases (MAPKs) which are potential signaling pathways contributing to the nociceptive behavior in inflammatory or neuropathic pain. However, whether activation of p38 and JNK MAPK signaling pathway in the trigeminal ganglion (TG) are involved in CXCL13 and its receptor CXCR5-mediated orofacial pain has not yet been clarified. Here, we show that the unilateral partial infraorbital nerve ligation (pIONL) induced a profound orofacial pain in wild-type (WT) mice. Western blot results showed that pIONL induced p38 but not JNK activation in the TG of WT mice. However, the orofacial pain induced by pIONL was alleviated in Cxcr5 -/- mice, and the activation of p38 was also abrogated in Cxcr5 -/- mice. Furthermore, intra-TG injection of CXCL13 evoked mechanical hypersensitivity and increased p-p38 expression in WT mice. But CXCL13 had no effect on pain behavior or p-p38 expression in Cxcr5 -/- mice. Finally, pretreatment with p38 inhibitor, SB203580, attenuated the pIONL-induced mechanical allodynia and decreased the mRNA expression of pro-inflammatory cytokines including tumor necrosis factor-α (TNF-α) and interleukin-1ß (IL-1ß) in the TG. Taken together, our data suggest that CXCL13 acts on CXCR5 to increase p38 activation and further contributes to the pathogenesis of orofacial neuropathic pain.