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Cytokines and Chemokines at the Crossroads of Neuroinflammation, Neurodegeneration, and Neuropathic Pain

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      Abstract

      Cytokines and chemokines are proteins that coordinate the immune response throughout the body. The dysregulation of cytokines and chemokines is a central feature in the development of neuroinflammation, neurodegeneration, and demyelination both in the central and peripheral nervous systems and in conditions of neuropathic pain. Pathological states within the nervous system can lead to activation of microglia. The latter may mediate neuronal and glial cell injury and death through production of proinflammatory factors such as cytokines and chemokines. These then help to mobilize the adaptive immune response. Although inflammation may induce beneficial effects such as pathogen clearance and phagocytosis of apoptotic cells, uncontrolled inflammation can result in detrimental outcomes via the production of neurotoxic factors that exacerbate neurodegenerative pathology. In states of prolonged inflammation, continual activation and recruitment of effector cells can establish a feedback loop that perpetuates inflammation and ultimately results in neuronal injury. A critical balance between repair and proinflammatory factors determines the outcome of a neurodegenerative process. This review will focus on how cytokines and chemokines affect neuroinflammation and disease pathogenesis in bacterial meningitis and brain abscesses, Lyme neuroborreliosis, human immunodeficiency virus encephalitis, and neuropathic pain.

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      Mechanisms underlying inflammation in neurodegeneration.

      Inflammation is associated with many neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and multiple sclerosis. In this Review, we discuss inducers, sensors, transducers, and effectors of neuroinflammation that contribute to neuronal dysfunction and death. Although inducers of inflammation may be generated in a disease-specific manner, there is evidence for a remarkable convergence in the mechanisms responsible for the sensing, transduction, and amplification of inflammatory processes that result in the production of neurotoxic mediators. A major unanswered question is whether pharmacological inhibition of inflammation pathways will be able to safely reverse or slow the course of disease. 2010 Elsevier Inc. All rights reserved.
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        BDNF from microglia causes the shift in neuronal anion gradient underlying neuropathic pain.

        Neuropathic pain that occurs after peripheral nerve injury depends on the hyperexcitability of neurons in the dorsal horn of the spinal cord. Spinal microglia stimulated by ATP contribute to tactile allodynia, a highly debilitating symptom of pain induced by nerve injury. Signalling between microglia and neurons is therefore an essential link in neuropathic pain transmission, but how this signalling occurs is unknown. Here we show that ATP-stimulated microglia cause a depolarizing shift in the anion reversal potential (E(anion)) in spinal lamina I neurons. This shift inverts the polarity of currents activated by GABA (gamma-amino butyric acid), as has been shown to occur after peripheral nerve injury. Applying brain-derived neurotrophic factor (BDNF) mimics the alteration in E(anion). Blocking signalling between BDNF and the receptor TrkB reverses the allodynia and the E(anion) shift that follows both nerve injury and administration of ATP-stimulated microglia. ATP stimulation evokes the release of BDNF from microglia. Preventing BDNF release from microglia by pretreating them with interfering RNA directed against BDNF before ATP stimulation also inhibits the effects of these cells on the withdrawal threshold and E(anion). Our results show that ATP-stimulated microglia signal to lamina I neurons, causing a collapse of their transmembrane anion gradient, and that BDNF is a crucial signalling molecule between microglia and neurons. Blocking this microglia-neuron signalling pathway may represent a therapeutic strategy for treating neuropathic pain.
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          The neuropathic pain triad: neurons, immune cells and glia.

          Nociceptive pain results from the detection of intense or noxious stimuli by specialized high-threshold sensory neurons (nociceptors), a transfer of action potentials to the spinal cord, and onward transmission of the warning signal to the brain. In contrast, clinical pain such as pain after nerve injury (neuropathic pain) is characterized by pain in the absence of a stimulus and reduced nociceptive thresholds so that normally innocuous stimuli produce pain. The development of neuropathic pain involves not only neuronal pathways, but also Schwann cells, satellite cells in the dorsal root ganglia, components of the peripheral immune system, spinal microglia and astrocytes. As we increasingly appreciate that neuropathic pain has many features of a neuroimmune disorder, immunosuppression and blockade of the reciprocal signaling pathways between neuronal and non-neuronal cells offer new opportunities for disease modification and more successful management of pain.
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            Author and article information

            Affiliations
            1Division of Bacteriology and Parasitology, Tulane National Primate Research Center, Tulane University, 18703 Three Rivers Road, Covington, LA 70433, USA
            2Division of Comparative Pathology, Tulane National Primate Research Center, Tulane University, 18703 Three Rivers Road, Covington, LA 70433, USA
            Author notes

            Academic Editor: Luc Vallières

            Journal
            Mediators Inflamm
            Mediators Inflamm
            MI
            Mediators of Inflammation
            Hindawi Publishing Corporation
            0962-9351
            1466-1861
            2013
            12 August 2013
            : 2013
            23997430 3753746 10.1155/2013/480739
            Copyright © 2013 Geeta Ramesh et al.

            This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

            Funding
            Funded by: http://dx.doi.org/10.13039/100000002 National Institutes of Health
            Award ID: 51OD011104/P51RR000164
            Funded by: http://dx.doi.org/10.13039/100000065 National Institute of Neurological Disorders and Stroke
            Award ID: NS048952
            Funded by: http://dx.doi.org/10.13039/100000025 National Institute of Mental Health
            Award ID: MH077544
            Categories
            Review Article

            Immunology

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