Upregulated TLR3 Promotes Neuropathic Pain by Regulating Autophagy in Rat With L5 Spinal Nerve Ligation Model

Abstract

Microglia, rapidly activated following peripheral nerve injury (PNI), accumulate within the spinal cord and adopt inflammation that contributes to development and maintenance of neuropathic pain. Microglia express functional Toll-like receptors (TLRs), which play pivotal roles in regulating inflammatory processes. However, little is known about the role of TLR3 in regulating neuropathic pain after PNI. Here TLR3 expression and autophagy activation was assayed in dorsal root ganglions and in microglia following PNI by using realtime PCR, western blot and immunohistochemistry. The role of TLR3/autophagy signaling in regulating tactile allodynia was evaluated by assaying paw mechanical withdrawal threshold and cold allodynia after intrathecal administration of Poly (I:C) and 3-methyladenine (3-MA). We found that L5 spinal nerve ligation (SNL) induces the expression of TLR3 in dorsal root ganglions and in primary rat microglia at the mRNA and protein level. Meanwhile, L5 SNL results in an increased activation of autophagy, which contributes to microglial activation and subsequent inflammatory response. Intrathecal administration of Poly (I:C), a TLR3 agonist, significantly increases the activation of microglial autophagy, whereas TLR3 knockdown markedly inhibits L5 SNL-induced microglial autophagy. Poly (I:C) treatment promotes the expression of proinflammatory mediators, whereas 3-MA (a specific inhibitor of autophagy) suppresses Poly (I:C)-induced secretion of proinflammatory cytokines. Autophagy inhibition further inhibits TLR3-mediated mechanical and cold hypersensitivity following SNL. These results suggest that inhibition of TLR3/autophagy signaling contributes to alleviate neurophathic pain triggered by SNL.

MiR-206-3p alleviates chronic constriction injury-induced neuropathic pain through targeting HDAC4

“…Patients with diabetes, cancer, and AIDS also experience with neuropathic pain. Several lines of evidence indicate that expression changes of mRNA and protein in the DRG are related to neuropathic pain [5, 33]. It is well known that miRNAs are involved in the regulation of neuropathic pain [1, 3].…”

Chronic constriction injury of sciatic nerve changes circular RNA expression in rat spinal dorsal horn

“…GO enrichment analysis revealed that thousands of target genes were involved in the GO term: biological processes, cellular component, and molecular function, which indicates that these differential circRNAs may influence genes of varied biological function. KEGG analysis showed that there were 112 pathways related to the 100 circRNAs, the top three were reported to be related to neuropathic pain: the Hippo signaling pathway,40 MAPK signaling pathway,41–43 and endocytosis 44,45…”

Toll-like receptors and their role in persistent pain

“…Beyond the previous observations in cultured DRGs, there are few investigations into the relationship between TLR3 and pain. A recent investigation identified elevated TLR3 mRNA and protein in the rat spinal cord following nerve injury, along with increased activation of microglial autophagy in response to nerve injury or application of TLR3 agonists (Chen & Lu, 2017). In a separate model of chronic pancreatitis, intrathecal infusion of TLR3 antisense oligodeoxynucleotide to knockdown TLR3 expression resulted in modest relief from mechanical allodynia (Qian, et al, 2011), suggesting TLR3 mechanisms participate under certain conditions of inflammatory pain.…”