Microglia, the intrinsic macrophages of the central nervous system, have previously been shown to be activated in the spinal cord in several rat mononeuropathy models. Activation of microglia and subsequent release of proinflammatory cytokines are known to play a role in inducing a behavioral hypersensitive state (hyperalgesia and allodynia) in these animals. The present study was undertaken to determine whether minocycline, an inhibitor of microglial activation, could attenuate both the development and existing mechanical allodynia and hyperalgesia in an L5 spinal nerve transection model of neuropathic pain. In a preventive paradigm (to study the effect on the development of hypersensitive behaviors), minocycline (10, 20, or 40 mg/kg intraperitoneally) was administered daily, beginning 1 h before nerve transection. This regimen produced a decrease in mechanical hyperalgesia and allodynia, with a maximum inhibitory effect observed at the dose of 20 and 40 mg/kg. The attenuation of the development of hyperalgesia and allodynia by minocycline was associated with an inhibitory action on microglial activation and suppression of proinflammatory cytokines at the L5 lumbar spinal cord of the nerveinjured animals. The effect of minocycline on existing allodynia was examined after its intraperitoneal administration initiated on day 5 post-L5 nerve transection. Although the postinjury administration of minocycline significantly inhibited microglial activation in neuropathic rats, it failed to attenuate existing hyperalgesia and allodynia. These data demonstrate that inhibition of microglial activation attenuated the development of behavioral hypersensitivity in a rat model of neuropathic pain but had no effect on the treatment of existing mechanical allodynia and hyperalgesia.

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...

Peripheral inflammation induces central sensitization characterized by the development of allodynia and hyperalgesia to mechanical and thermal stimuli. Recent evidence suggests that activation of glial cells and a subsequent increase in proinflammatory cytokines contribute to the development of behavioral hypersensitivity after nerve injury or peripheral inflammation. In the present study, we examined mRNA and protein expression of glial markers and proinflammatory cytokines at the lumbar spinal cord, brainstem and forebrain following intraplantar administration of complete Freunds adjuvant (CFA) in rats. Gene expression studied by real-time reverse transcriptase-polymerase chain reaction (RT-PCR) for microglial markers (Mac-1, TLR4 and CD14) showed a significant increase in their expression during all phases (acute, subacute and chronic) of inflammation. Conversely, up-regulation of astroglial markers [glial fibrillary acidic protein (GFAP) and S100B] was observed only at the subacute and chronic phases of inflammation. Increased immunoreactivity for OX-42 (CR3/CD11b) and GFAP at various brain regions was also observed after the acute and subacute phases of the inflammation, respectively. Quantification of proinflammatory cytokines (IL-1beta, IL-6 and TNF-alpha) at the mRNA (by real-time RT-PCR) and protein level (by ELISA) revealed enhanced expression during the acute, subacute and chronic phases of CFA-induced peripheral inflammation. This study demonstrates that CFA-induced peripheral inflammation induces robust glial activation and proinflammatory cytokines both spinally and supraspinally. In addition, similar to nerve injury-induced behavioral hypersensitivity microglial activation preceded astrocytic activation following CFA-induced peripheral inflammation, supporting a role of microglia in the initiation phase and astrocytes in maintaining hypersensitivity. These findings further support a unifying theory that glial activation and enhanced cytokine expression at the CNS have a role in eliciting behavioral hypersensitivity.

No abstract