The CNS role of Toll-like receptor 4 in innate neuroimmunity and painful neuropathy

Abstract

Neuropathic pain remains a prevalent and persistent clinical problem because of our incomplete understanding of its pathogenesis. This study demonstrates for the first time, to our knowledge, a critical role for CNS innate immunity by means of microglial Toll-like receptor 4 (TLR4) in the induction phase of behavioral hypersensitivity in a mouse and rat model of neuropathy. We hypothesized that after L5 nerve transection, CNS neuroimmune activation and subsequent cytokine expression are triggered by the stimulation of microglial membrane-bound TLR4. To test this hypothesis, experiments were undertaken to assess tactile and thermal hypersensitivity in genetically altered (i.e., TLR4 knockout and point-mutant) mice after L5 nerve transection. In a complementary study, TLR4 antisense oligodeoxynucleotide (ODN) was administered intrathecally to L5 spinal nerve injured rats to reduce the expression of spinal TLR4. Both the genetically altered mice and the rats treated with TLR4 antisense ODN displayed significantly attenuated behavioral hypersensitivity and decreased expression of spinal microglial markers and proinflammatory cytokines as compared with their respective control groups. This finding shows that TLR4 contributes to the initiation of CNS neuroimmune activation after L5 nerve transection. Further understanding of this early, specific, innate CNS͞microglial response and how it leads to sustained glial͞neuronal hypersensitivity may point to new therapies for the prevention and treatment of neuropathic pain syndromes.astrocytes ͉ behavioral hypersensitivity ͉ microglial activation ͉ nerve injury ͉ neuropathic pain N europathic pain remains a prevalent, persistent, and debilitating problem. Attempts to elucidate its mechanisms have focused principally on peripheral nerves, dorsal root ganglion, and CNS neurons. Recently, however, research has expanded into the burgeoning field of glial͞neuronal transmission and CNS immunologic responses to nerve injury. CNS glia display immune cell functions in both normal and pathologic conditions, and there is increasing evidence that neuropathic pain arising from nerve injury has a CNS neuroimmune component (1, 2). Spinal glial activation triggers rapid, graded CNS expression of proinflammatory cytokines (including TNF-␣, IL-1␤, and IL-6) that contributes to the initiation and maintenance of behavioral hypersensitivity after L5 nerve transection (1, 3, 4). The onset of proinflammatory cytokine expression correlates with microglial activation and the initiation of behavioral hypersensitivity (5-8), and neuroimmune activation in painful neuropathy has been established (3, 9, 10). However, the mechanistic links between L5 nerve transection, microglial activation, and the genesis of behavioral hypersensitivity have remained unknown.Cells of the innate immune system, including monocytes͞ macrophages, natural killer cells, neutrophils, and microglia recognize invariant molecular structures of pathogens (termed pathogen-associated molecular patterns, PAMP) by means of stable, gene...

Glia in pathological pain: A role for fractalkine

“…However, TLRs are also stimulated by several non-pathogen factors, for example, the cell stress proteins, heat-shock proteins (Guo and Schluesener, 2007). Indeed, without CNS pathogen stimulation, the role for TLR4 was identified in initiating spinal nerve traumainduced neuropathic pain because mice lacking the TLR4 receptor did not develop neuropathic pain nor activation of spinal innate immune cells or glial activation (Tanga et al, 2005). Further, blocking spinal cord TLR4 production in neuropathic rats also blocked pathological pain and microglial/macrophage activation (Tanga et al, 2005), supporting that TLR4 signaling in the spinal cord is important for mediating neuropathic pain.…”

Microglia in the spinal cord and neuropathic pain

“…Mice lacking either toll‐like receptor 2, 3 or 4 show impaired microglial activation in the dorsal horn after PNI42, 43, 44.…”

CNS‐infiltrating CD4+ T lymphocytes contribute to murine spinal nerve transection‐induced neuropathic pain

“…Surgery was conducted following a previously published procedure [33]. Briefly, mice were anesthetized by inhalation of halothane (4% for induction and 2% for maintenance) in 100% O 2 .…”