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Fine PG. Long-Term Consequences of Chronic Pain: Mounting Evidence for Pain as a Neurological Disease and Parallels with Other Chronic Disease States. PAIN MEDICINE 2011; 12:996-1004. [DOI: 10.1111/j.1526-4637.2011.01187.x] [Citation(s) in RCA: 174] [Impact Index Per Article: 13.4] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
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Zheng FY, Xiao WH, Bennett GJ. The response of spinal microglia to chemotherapy-evoked painful peripheral neuropathies is distinct from that evoked by traumatic nerve injuries. Neuroscience 2011; 176:447-54. [PMID: 21195745 PMCID: PMC3040270 DOI: 10.1016/j.neuroscience.2010.12.052] [Citation(s) in RCA: 85] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/17/2010] [Revised: 12/23/2010] [Accepted: 12/24/2010] [Indexed: 01/22/2023]
Abstract
Painful peripheral neuropathies produced by nerve trauma are accompanied by substantial axonal degeneration and by a response in spinal cord microglia that is characterized by hypertrophy and increased expression of several intracellular and cell-surface markers, including ionizing calcium-binding adapter molecule 1 (Iba1) and Cd11b (a complement receptor 3 antigen recognized by the OX42 antibody). The microglia response has been hypothesized to be essential for the pathogenesis of the neuropathic pain state. In contrast, the painful peripheral neuropathies produced by low doses of cancer chemotherapeutics do not produce degeneration of axons in the peripheral nerve, although they do cause partial degeneration of the sensory axons' distal-most tips, that is the intraepidermal nerve fibers that form the axons' terminal receptor arbors. The question thus arises as to whether the relatively minor and distal axonal injury characterizing the chemotherapy-evoked neuropathies is sufficient to evoke the microglial response that is seen after traumatic nerve injury. We examined the lumbar spinal cord of rats with painful peripheral neuropathies due to the anti-neoplastic agents, paclitaxel, vincristine, and oxaliplatin, and the anti-retroviral agent, 2',3'-dideoxycytidine (ddC), and compared them to rats with a complete sciatic nerve transection and the partial sciatic nerve injury produced in the chronic constriction injury model (CCI). As expected, microglia hypertrophy and increased expression of Iba1 were pronounced in the nerve transection and CCI animals. However, there was no microglia hypertrophy or increased Iba1 staining in the animals treated with paclitaxel, vincristine, oxaliplatin, or ddC. These results suggest that the mechanisms that produce neuropathic pain after exposure to chemotherapeutics may be fundamentally different than those operating after nerve trauma.
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Affiliation(s)
- F. Y. Zheng
- Department of Anesthesia, McGill University, Montréal, Québec, Canada
| | - W.-H. Xiao
- Department of Anesthesia, McGill University, Montréal, Québec, Canada
- The Alan Edwards Centre for Research on Pain, McGill University, Montréal, Québec, Canada
| | - G. J. Bennett
- Department of Anesthesia, McGill University, Montréal, Québec, Canada
- The Alan Edwards Centre for Research on Pain, McGill University, Montréal, Québec, Canada
- Faculty of Dentistry, McGill University, Montréal, Québec, Canada
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Abstract
The transition from acute to chronic pain appears to occur in discrete pathophysiological and histopathological steps. Stimuli initiating a nociceptive response vary, but receptors and endogenous defence mechanisms in the periphery interact in a similar manner regardless of the insult. Chemical, mechanical, and thermal receptors, along with leucocytes and macrophages, determine the intensity, location, and duration of noxious events. Noxious stimuli are transduced to the dorsal horn of the spinal cord, where amino acid and peptide transmitters activate second-order neurones. Spinal neurones then transmit signals to the brain. The resultant actions by the individual involve sensory-discriminative, motivational-affective, and modulatory processes in an attempt to limit or stop the painful process. Under normal conditions, noxious stimuli diminish as healing progresses and pain sensation lessens until minimal or no pain is detected. Persistent, intense pain, however, activates secondary mechanisms both at the periphery and within the central nervous system that cause allodynia, hyperalgesia, and hyperpathia that can diminish normal functioning. These changes begin in the periphery with upregulation of cyclo-oxygenase-2 and interleukin-1β-sensitizing first-order neurones, which eventually sensitize second-order spinal neurones by activating N-methyl-d-aspartic acid channels and signalling microglia to alter neuronal cytoarchitecture. Throughout these processes, prostaglandins, endocannabinoids, ion-specific channels, and scavenger cells all play a key role in the transformation of acute to chronic pain. A better understanding of the interplay among these substances will assist in the development of agents designed to ameliorate or reverse chronic pain.
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Affiliation(s)
- C Voscopoulos
- Department of Anesthesiology, Critical Care, and Pain Medicine, University at Buffalo, Buffalo, NY, USA
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Toda S, Sakai A, Ikeda Y, Sakamoto A, Suzuki H. A local anesthetic, ropivacaine, suppresses activated microglia via a nerve growth factor-dependent mechanism and astrocytes via a nerve growth factor-independent mechanism in neuropathic pain. Mol Pain 2011; 7:2. [PMID: 21211063 PMCID: PMC3022746 DOI: 10.1186/1744-8069-7-2] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/19/2010] [Accepted: 01/07/2011] [Indexed: 12/13/2022] Open
Abstract
Background Local anesthetics alleviate neuropathic pain in some cases in clinical practice, and exhibit longer durations of action than those predicted on the basis of the pharmacokinetics of their blocking effects on voltage-dependent sodium channels. Therefore, local anesthetics may contribute to additional mechanisms for reversal of the sensitization of nociceptive pathways that occurs in the neuropathic pain state. In recent years, spinal glial cells, microglia and astrocytes, have been shown to play critical roles in neuropathic pain, but their participation in the analgesic effects of local anesthetics remains largely unknown. Results Repetitive epidural administration of ropivacaine reduced the hyperalgesia induced by chronic constrictive injury of the sciatic nerve. Concomitantly with this analgesia, ropivacaine suppressed the increases in the immunoreactivities of CD11b and glial fibrillary acidic protein in the dorsal spinal cord, as markers of activated microglia and astrocytes, respectively. In addition, epidural administration of a TrkA-IgG fusion protein that blocks the action of nerve growth factor (NGF), which was upregulated by ropivacaine in the dorsal root ganglion, prevented the inhibitory effect of ropivacaine on microglia, but not astrocytes. The blockade of NGF action also abolished the analgesic effect of ropivacaine on neuropathic pain. Conclusions Ropivacaine provides prolonged analgesia possibly by suppressing microglial activation in an NGF-dependent manner and astrocyte activation in an NGF-independent manner in the dorsal spinal cord. Local anesthetics, including ropivacaine, may represent a new approach for glial cell inhibition and, therefore, therapeutic strategies for neuropathic pain.
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Affiliation(s)
- Shigeru Toda
- Department of Pharmacology, Nippon Medical School, 1-1-5 Sendagi, Bunkyo-ku, Tokyo, Japan
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Sweitzer S, De Leo J. Propentofylline: glial modulation, neuroprotection, and alleviation of chronic pain. Handb Exp Pharmacol 2011:235-50. [PMID: 20859798 DOI: 10.1007/978-3-642-13443-2_8] [Citation(s) in RCA: 49] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
Propentofylline is a unique methylxanthine with clear cyclic AMP, phosphodiesterase, and adenosine actions, including enhanced synaptic adenosine signaling. Both in vitro and in vivo studies have demonstrated profound neuroprotective, antiproliferative, and anti-inflammatory effects of propentofylline. Propentofylline has shown efficacy in preclinical models of stroke, opioid tolerance, and acute and chronic pain. Clinically, propentofylline has shown efficacy in degenerative and vascular dementia, and as a potential adjuvant treatment for schizophrenia and multiple sclerosis. Possible mechanisms of action include a direct glial modulation to decrease a reactive phenotype, decrease glial production and release of damaging proinflammatory factors, and enhancement of astrocyte-mediated glutamate clearance. This chapter reviews the literature that supports a myriad of protective actions of this small molecule and implicates propentofylline as a potential therapeutic for the treatment of chronic pain. From these studies, we propose a CNS multipartite synaptic action of propentofylline that includes modulation of pre- and postsynaptic neurons, astrocytes, and microglia in the treatment of chronic pain syndromes, including, but not limited to, neuropathic pain.
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Affiliation(s)
- Sarah Sweitzer
- Department of Pharmacology, University of South Carolina, USC School of Medicine, Columbia, SC 29208, USA
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Activation of astrocytes in the spinal cord contributes to the development of bilateral allodynia after peripheral nerve injury in rats. Brain Res 2010; 1363:72-80. [DOI: 10.1016/j.brainres.2010.09.105] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/06/2010] [Revised: 08/23/2010] [Accepted: 09/29/2010] [Indexed: 01/13/2023]
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Ma L, Nagai J, Ueda H. Microglial activation mediates de novo lysophosphatidic acid production in a model of neuropathic pain. J Neurochem 2010; 115:643-53. [PMID: 20722972 DOI: 10.1111/j.1471-4159.2010.06955.x] [Citation(s) in RCA: 58] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
We recently demonstrated that de novo lysophosphatidic acid (LPA) production in the spinal cord occurs in the early phase after nerve injury or LPA injection, and underlies the peripheral mechanisms of neuropathic pain. In this study, we examined the possible involvement of spinal cord microglia in such LPA-mediated functions. Intrathecal LPA injection rapidly increased the gene expression of CD11b and protein expression of phosphor-p38, accompanied by a morphological change of microglia from a ramified to amoeboid shape. Although early treatment with minocycline significantly inhibited LPA-induced neuropathic pain-like behavior and microglial activation, late treatment did not. Early treatment with minocycline also blocked LPA-evoked de novo LPA production and the increased activation of cytosolic phospholipase A(2), an LPA synthesis-related enzyme. Similar results were observed when the sciatic nerve was partially injured: early, but not late, treatment with minocycline significantly inhibited the injury-induced neuropathic pain, microglial activation, de novo LPA production and the underlying increased activation of cytosolic phospholipase A(2) as well as calcium-independent phospholipase A(2), another LPA synthesis-related enzyme. These findings suggest that the early phase of microglial activation is involved in de novo LPA production, and that this underlies the initial mechanisms of nerve injury-induced neuropathic pain.
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Affiliation(s)
- Lin Ma
- Division of Molecular Pharmacology and Neuroscience, Nagasaki University Graduate School of Biomedical Sciences, Bunkyo-machi, Nagasaki, Japan
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Horvath RJ, Romero-Sandoval AE, De Leo JA. Inhibition of microglial P2X4 receptors attenuates morphine tolerance, Iba1, GFAP and mu opioid receptor protein expression while enhancing perivascular microglial ED2. Pain 2010; 150:401-413. [PMID: 20573450 DOI: 10.1016/j.pain.2010.02.042] [Citation(s) in RCA: 104] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/12/2009] [Revised: 02/24/2010] [Accepted: 02/26/2010] [Indexed: 02/04/2023]
Abstract
Anti-nociceptive tolerance to opioids is a well-described phenomenon, which severely limits the clinical efficacy of opioids for the treatment of chronic pain syndromes. The mechanisms that drive anti-nociceptive tolerance, however, are less well understood. We have previously shown that glia have a central role in the development of morphine tolerance and that administration of a glial modulating agent attenuated tolerance formation. Recently, we have demonstrated that morphine enhances microglial Iba1 expression and P2X4 receptor-mediated microglial migration via direct mu opioid receptor signaling in in vitro microglial cultures. We hypothesize that P2X4 receptors drive morphine tolerance and modulate morphine-induced spinal glial reactivity. Additionally, we hypothesize that perivascular microglia play a role in morphine tolerance and that P2X4 receptor expression regulates perivascular microglia ED2 expression. To test these hypotheses, rats were implanted with osmotic minipumps releasing morphine or saline subcutaneously for seven days. Beginning three days prior to morphine treatment, P2X4 receptor antisense oligonucleotide (asODN) was injected intrathecally daily, to selectively inhibit P2X4 receptor expression. P2X4 receptor asODN treatment inhibited morphine-induced P2X4 receptor expression and blocked anti-nociceptive tolerance to systemically administered morphine. P2X4 receptor asODN treatment also attenuated the morphine-dependent increase of spinal ionized calcium binding protein (Iba1), glial fibrillary acidic protein (GFAP) and mu opioid receptor protein expression. Chronic morphine also decreased perivascular microglial ED2 expression, which was reversed by P2X4 receptor asODN. Together, these data suggest that the modulation of P2X4 receptor expression on microglia and perivascular microglia may prove an attractive target for adjuvant therapy to attenuate opioid-induced anti-nociceptive tolerance.
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MESH Headings
- Analysis of Variance
- Animals
- Antigens, CD/metabolism
- Antigens, Differentiation, Myelomonocytic/metabolism
- Behavior, Animal/drug effects
- CD11b Antigen/metabolism
- Calcium-Binding Proteins/metabolism
- Disease Models, Animal
- Drug Interactions
- Drug Tolerance/physiology
- Gene Expression Regulation/drug effects
- Gene Expression Regulation/physiology
- Glial Fibrillary Acidic Protein/metabolism
- Hyperalgesia/drug therapy
- Male
- Microfilament Proteins
- Microglia/drug effects
- Microglia/metabolism
- Morphine/administration & dosage
- Naloxone/pharmacology
- Narcotic Antagonists/pharmacology
- Nerve Tissue Proteins/metabolism
- Oligodeoxyribonucleotides, Antisense/pharmacology
- Pain Measurement
- Random Allocation
- Rats
- Rats, Sprague-Dawley
- Receptors, Cell Surface/metabolism
- Receptors, Opioid/metabolism
- Receptors, Purinergic P2X4/genetics
- Receptors, Purinergic P2X4/metabolism
- Signal Transduction
- Spinal Cord/pathology
- Time Factors
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Affiliation(s)
- Ryan J Horvath
- Department of Pharmacology, Dartmouth Medical School, Hanover, NH 03755, USA Neuroscience Center at Dartmouth, Lebanon, NH 03756, USA Department of Anesthesiology, Dartmouth-Hitchcock Medical Center, Lebanon, NH 03756, USA
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Alkaitis MS, Solorzano C, Landry RP, Piomelli D, DeLeo JA, Romero-Sandoval EA. Evidence for a role of endocannabinoids, astrocytes and p38 phosphorylation in the resolution of postoperative pain. PLoS One 2010; 5:e10891. [PMID: 20531936 PMCID: PMC2878341 DOI: 10.1371/journal.pone.0010891] [Citation(s) in RCA: 52] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/07/2010] [Accepted: 05/05/2010] [Indexed: 12/15/2022] Open
Abstract
BACKGROUND An alarming portion of patients develop persistent or chronic pain following surgical procedures, but the mechanisms underlying the transition from acute to chronic pain states are not fully understood. In general, endocannabinoids (ECBs) inhibit nociceptive processing by stimulating cannabinoid receptors type 1 (CB(1)) and type 2 (CB(2)). We have previously shown that intrathecal administration of a CB(2) receptor agonist reverses both surgical incision-induced behavioral hypersensitivity and associated over-expression of spinal glial markers. We therefore hypothesized that endocannabinoid signaling promotes the resolution of acute postoperative pain by modulating pro-inflammatory signaling in spinal cord glial cells. METHODOLOGY/PRINCIPAL FINDINGS To test this hypothesis, rats receiving paw incision surgery were used as a model of acute postoperative pain that spontaneously resolves. We first characterized the concentration of ECBs and localization of CB(1) and CB(2) receptors in the spinal cord following paw incision. We then administered concomitant CB(1) and CB(2) receptor antagonists/inverse agonists (AM281 and AM630, 1 mg x kg(-1) each, i.p.) during the acute phase of paw incision-induced mechanical allodynia and evaluated the expression of glial cell markers and phosphorylated p38 (a MAPK associated with inflammation) in the lumbar dorsal horn. Dual blockade of CB(1) and CB(2) receptor signaling prevented the resolution of postoperative allodynia and resulted in persistent over-expression of spinal Glial Fibrillary Acidic Protein (GFAP, an astrocytic marker) and phospho-p38 in astrocytes. We provide evidence for the functional significance of these astrocytic changes by demonstrating that intrathecal administration of propentofylline (50 microg, i.t.) attenuated both persistent behavioral hypersensitivity and over-expression of GFAP and phospho-p38 in antagonist-treated animals. CONCLUSIONS/SIGNIFICANCE Our results demonstrate that endocannabinoid signaling via CB(1) and CB(2) receptors is necessary for the resolution of paw incision-induced behavioral hypersensitivity and for the limitation of pro-inflammatory signaling in astrocytes following surgical insult. Our findings suggest that therapeutic strategies designed to enhance endocannabinoid signaling may prevent patients from developing persistent or chronic pain states following surgery.
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MESH Headings
- Animals
- Astrocytes/drug effects
- Astrocytes/metabolism
- Astrocytes/pathology
- Behavior, Animal/drug effects
- Biomarkers/metabolism
- Calcium-Binding Proteins/metabolism
- Cannabinoid Receptor Modulators/metabolism
- Endocannabinoids
- Glial Fibrillary Acidic Protein/metabolism
- Male
- Microfilament Proteins
- Neuroglia/drug effects
- Neuroglia/metabolism
- Pain, Postoperative/enzymology
- Pain, Postoperative/pathology
- Phosphorylation/drug effects
- Rats
- Rats, Sprague-Dawley
- Receptor, Cannabinoid, CB1/antagonists & inhibitors
- Receptor, Cannabinoid, CB1/metabolism
- Receptor, Cannabinoid, CB2/antagonists & inhibitors
- Receptor, Cannabinoid, CB2/metabolism
- Spinal Cord/metabolism
- Spinal Cord/pathology
- Xanthines/pharmacology
- p38 Mitogen-Activated Protein Kinases/metabolism
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Affiliation(s)
- Matthew S. Alkaitis
- Neuroscience Center at Dartmouth, Dartmouth Medical School, Lebanon, New Hampshire, United States of America
- Nuffield Department of Clinical Laboratory Sciences, University of Oxford, Oxford, United Kingdom
| | - Carlos Solorzano
- Departments of Pharmacology and Biological Chemistry, University of California Irvine, Irvine, California, United States of America
| | - Russell P. Landry
- Neuroscience Center at Dartmouth, Dartmouth Medical School, Lebanon, New Hampshire, United States of America
- Department of Anesthesiology, Dartmouth Medical School, Lebanon, New Hampshire, United States of America
| | - Daniele Piomelli
- Departments of Pharmacology and Biological Chemistry, University of California Irvine, Irvine, California, United States of America
| | - Joyce A. DeLeo
- Neuroscience Center at Dartmouth, Dartmouth Medical School, Lebanon, New Hampshire, United States of America
- Department of Anesthesiology, Dartmouth Medical School, Lebanon, New Hampshire, United States of America
- Department of Pharmacology and Toxicology, Dartmouth Medical School, Lebanon, New Hampshire, United States of America
| | - E. Alfonso Romero-Sandoval
- Neuroscience Center at Dartmouth, Dartmouth Medical School, Lebanon, New Hampshire, United States of America
- Department of Anesthesiology, Dartmouth Medical School, Lebanon, New Hampshire, United States of America
- Department of Pharmacology and Toxicology, Dartmouth Medical School, Lebanon, New Hampshire, United States of America
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Whitehead KJ, Smith CGS, Delaney SA, Curnow SJ, Salmon M, Hughes JP, Chessell IP. Dynamic regulation of spinal pro-inflammatory cytokine release in the rat in vivo following peripheral nerve injury. Brain Behav Immun 2010; 24:569-76. [PMID: 20035858 DOI: 10.1016/j.bbi.2009.12.007] [Citation(s) in RCA: 54] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 10/24/2009] [Revised: 12/16/2009] [Accepted: 12/16/2009] [Indexed: 01/02/2023] Open
Abstract
Spinal release of cytokines may play a critical role in the maladapted nociceptive signaling underlying chronic pain states. In order to investigate this biology, we have developed a novel 'high flux' intrathecal microdialysis approach in combination with multiplex bead-based immunoassay technology to concurrently monitor the spinal release of interleukin (IL)-1beta, IL-6 and tumour necrosis factor (TNF)alpha in rats with unilateral sciatic nerve chronic constriction injury (CCI). Intrathecal microdialysis was performed under isoflurane/N(2)O anaesthesia in rats with confirmed mechanical hypersensitivity. In a first study, C-fiber strength electrical stimulation of the operated nerve in neuropathic rats was found to evoke a dramatic increase in IL-1beta efflux ( approximately 15-fold) that was significantly greater than that observed in the sham-operated group. Spinal IL-6 efflux was also responsive to primary afferent stimulation, whereas TNFalpha was not. In a second study, treatment with the glial inhibitor propentofylline for 7days normalized CCI-induced mechanical hypersensitivity. In the same animals, this treatment also significantly reduced intrathecal IL-1beta, IL-6 and TNFalpha and prevented afferent stimulation-evoked cytokine release of both IL-1beta and IL-6. These results provide support for glia as the source of the majority of intrathecal IL-1beta, IL-6 and TNFalpha that accompanies mechanical hypersensitivity in the CCI rat. Moreover, our studies demonstrate the ability of a neurone-glia signaling mechanism to dynamically modulate this release and support a role of spinal IL-1beta in the phasic transmission of abnormal pain signals.
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Affiliation(s)
- K J Whitehead
- Pain Signalling Group, Neuropharmacology and Neurobiology Section, School of Clinical and Experimental Medicine, College of Medical and Dental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK.
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Leung L, Cahill CM. TNF-alpha and neuropathic pain--a review. J Neuroinflammation 2010; 7:27. [PMID: 20398373 PMCID: PMC2861665 DOI: 10.1186/1742-2094-7-27] [Citation(s) in RCA: 429] [Impact Index Per Article: 30.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/20/2009] [Accepted: 04/16/2010] [Indexed: 12/12/2022] Open
Abstract
Tumor necrosis factor alpha (TNF-α) was discovered more than a century ago, and its known roles have extended from within the immune system to include a neuro-inflammatory domain in the nervous system. Neuropathic pain is a recognized type of pathological pain where nociceptive responses persist beyond the resolution of damage to the nerve or its surrounding tissue. Very often, neuropathic pain is disproportionately enhanced in intensity (hyperalgesia) or altered in modality (hyperpathia or allodynia) in relation to the stimuli. At time of this writing, there is as yet no common consensus about the etiology of neuropathic pain - possible mechanisms can be categorized into peripheral sensitization and central sensitization of the nervous system in response to the nociceptive stimuli. Animal models of neuropathic pain based on various types of nerve injuries (peripheral versus spinal nerve, ligation versus chronic constrictive injury) have persistently implicated a pivotal role for TNF-α at both peripheral and central levels of sensitization. Despite a lack of success in clinical trials of anti-TNF-α therapy in alleviating the sciatic type of neuropathic pain, the intricate link of TNF-α with other neuro-inflammatory signaling systems (e.g., chemokines and p38 MAPK) has indeed inspired a systems approach perspective for future drug development in treating neuropathic pain.
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Affiliation(s)
- Lawrence Leung
- Centre for Neurosciences Studies, 18, Stuart Street, Queen's University, Kingston, Ontario K7L 3N6, Canada.
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Toth CC, Jedrzejewski NM, Ellis CL, Frey WH. Cannabinoid-mediated modulation of neuropathic pain and microglial accumulation in a model of murine type I diabetic peripheral neuropathic pain. Mol Pain 2010; 6:16. [PMID: 20236533 PMCID: PMC2845559 DOI: 10.1186/1744-8069-6-16] [Citation(s) in RCA: 96] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/28/2009] [Accepted: 03/17/2010] [Indexed: 12/13/2022] Open
Abstract
BACKGROUND Despite the frequency of diabetes mellitus and its relationship to diabetic peripheral neuropathy (DPN) and neuropathic pain (NeP), our understanding of underlying mechanisms leading to chronic pain in diabetes remains poor. Recent evidence has demonstated a prominent role of microglial cells in neuropathic pain states. One potential therapeutic option gaining clinical acceptance is the cannabinoids, for which cannabinoid receptors (CB) are expressed on neurons and microglia. We studied the accumulation and activation of spinal and thalamic microglia in streptozotocin (STZ)-diabetic CD1 mice and the impact of cannabinoid receptor agonism/antagonism during the development of a chronic NeP state. We provided either intranasal or intraperitoneal cannabinoid agonists/antagonists at multiple doses both at the initiation of diabetes as well as after establishment of diabetes and its related NeP state. RESULTS Tactile allodynia and thermal hypersensitivity were observed over 8 months in diabetic mice without intervention. Microglial density increases were seen in the dorsal spinal cord and in thalamic nuclei and were accompanied by elevation of phosphorylated p38 MAPK, a marker of microglial activation. When initiated coincidentally with diabetes, moderate-high doses of intranasal cannabidiol (cannaboid receptor 2 agonist) and intraperitoneal cannabidiol attenuated the development of an NeP state, even after their discontinuation and without modification of the diabetic state. Cannabidiol was also associated with restriction in elevation of microglial density in the dorsal spinal cord and elevation in phosphorylated p38 MAPK. When initiated in an established DPN NeP state, both CB1 and CB2 agonists demonstrated an antinociceptive effect until their discontinuation. There were no pronociceptive effects demonstated for either CB1 or CB2 antagonists. CONCLUSIONS The prevention of microglial accumulation and activation in the dorsal spinal cord was associated with limited development of a neuropathic pain state. Cannabinoids demonstrated antinociceptive effects in this mouse model of DPN. These results suggest that such interventions may also benefit humans with DPN, and their early introduction may also modify the development of the NeP state.
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Affiliation(s)
- Cory C Toth
- Department of Clinical Neurosciences and the Hotchkiss Brain Institute, University of Calgary, Calgary, Alberta, Canada.
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Abstract
Recent studies suggest that astroglia, a major non-neuronal cell type in the central nervous system, actively participate in synaptic activity and potentially contribute to neurological disorders including chronic pain. Astroglia exhibit a hyperactive phenotype, also referred to as reactive astrocytosis, in response to peripheral injury. This process is often referred to as astroglial activation. By immunostaining against glial fibrillary acidic proteins, an intermediate cytoskeleton filament protein selectively localized to matured astrocytes, hypertrophy of astrocytes are clearly visible in the spinal cord dorsal horn and spinal trigeminal nucleus following a variety of injuries. Injury-related astroglial activation correlates with behavioral hyperalgesia and conversely, astroglial inhibition attenuates pain hypersensitivity. Astrocytes have a close anatomical relationship with neurons. Interactions between astrocytes and neurons contribute to the mechanisms of chronic pain. Astroglial activation is accompanied by initiation of cellular signal transduction pathways that lead to transcriptional gene regulation and release of a variety of chemical mediators or gliotransmitters, down-regulation of glutamate transporter activity that directly affects synaptic transmission, changes in gap junction proteins by which calcium waves spread, and alterations of the blood brain barrier. These coordinated changes in astroglial functions in turn modulate neuronal activity and facilitate pain transmission.
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Affiliation(s)
- Ke Ren
- Department of Neural and Pain Sciences, Dental School; & Program in Neuroscience, University of Maryland, Baltimore, MD 21201-1586, USA
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64
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Gao YJ, Ji RR. Chemokines, neuronal-glial interactions, and central processing of neuropathic pain. Pharmacol Ther 2010; 126:56-68. [PMID: 20117131 DOI: 10.1016/j.pharmthera.2010.01.002] [Citation(s) in RCA: 456] [Impact Index Per Article: 32.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/14/2010] [Accepted: 01/15/2010] [Indexed: 12/23/2022]
Abstract
Millions of people worldwide suffer from neuropathic pain as a result of damage to or dysfunction of the nervous system under various disease conditions. Development of effective therapeutic strategies requires a better understanding of molecular and cellular mechanisms underlying the pathogenesis of neuropathic pain. It has been increasingly recognized that spinal cord glial cells such as microglia and astrocytes play a critical role in the induction and maintenance of neuropathic pain by releasing powerful neuromodulators such as proinflammatory cytokines and chemokines. Recent evidence reveals chemokines as new players in pain control. In this article, we review evidence for chemokine modulation of pain via neuronal-glial interactions by focusing on the central role of two chemokines, CX3CL1 (fractalkine) and CCL2 (MCP-1), because they differentially regulate neuronal-glial interactions. Release of CX3CL1 from neurons is ideal to mediate neuronal-to-microglial signaling, since the sole receptor of this chemokine, CX3CR1, is expressed in spinal microglia and activation of the receptor leads to phosphorylation of p38 MAP kinase in microglia. Although CCL2 was implicated in neuronal-to-microglial signaling, a recent study shows a novel role of CCL2 in astroglial-to-neuronal signaling after nerve injury. In particular, CCL2 rapidly induces central sensitization by increasing the activity of NMDA receptors in dorsal horn neurons. Insights into the role of chemokines in neuronal-glial interactions after nerve injury will identify new targets for therapeutic intervention of neuropathic pain.
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Affiliation(s)
- Yong-Jing Gao
- Pain Research Center, Department of Anesthesiology, Brigham and Women's Hospital and Harvard Medical School, Boston, MA 02115, United States.
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65
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Nakagawa T, Kaneko S. Spinal Astrocytes as Therapeutic Targets for Pathological Pain. J Pharmacol Sci 2010; 114:347-53. [DOI: 10.1254/jphs.10r04cp] [Citation(s) in RCA: 70] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022] Open
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A novel method for modeling facial allodynia associated with migraine in awake and freely moving rats. J Neurosci Methods 2009; 185:236-45. [PMID: 19837113 DOI: 10.1016/j.jneumeth.2009.10.006] [Citation(s) in RCA: 51] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/16/2009] [Revised: 10/08/2009] [Accepted: 10/08/2009] [Indexed: 12/25/2022]
Abstract
Migraine is a neurovascular disorder that induces debilitating headaches associated with multiple symptoms including facial allodynia, characterized by heightened responsivity to normally innocuous mechanical stimuli. It is now well accepted that immune activation and immune-derived inflammatory mediators enhance pain responsivity, including the trigeminal system. Nociceptive ("pain" responsive) trigeminal nerves densely innervate the cranial meninges. We have recently proposed that the meninges may serve as a previously unidentified, key interface between the peripheral immune system and the CNS with potential implications for understanding underlying migraine mechanisms. Our focus here is the development of a model for facial allodynia associated with migraine. We developed a model wherein an indwelling catheter is placed between the skull and dura, allowing immunogenic stimuli to be administered over the dura in awake and freely moving rats. Since the catheter does not contact the brain itself, any proinflammatory cytokines induced following manipulation derive from resident or recruited meningeal immune cells. While surgery alone does not alter immune activation markers, TNF or IL6 mRNA and/or protein, it does decrease gene expression and increase protein expression of IL-1 at 4 days after surgery. Using this model we show the induction of facial allodynia in response to supradural administration of either the HIV glycoprotein gp120 or inflammatory soup (bradykinin, histamine, serotonin, and prostaglandin E2), and the induction of hindpaw allodynia in our model after inflammatory soup. This model allows time- and dose-dependent assessment of the relationship between changes in meningeal inflammation and corresponding exaggerated pain behaviors.
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Abstract
Pain normally subserves a vital role in the survival of the organism, prompting the avoidance of situations associated with tissue damage. However, the sensation of pain can become dissociated from its normal physiological role. In conditions of neuropathic pain, spontaneous or hypersensitive pain behavior occurs in the absence of the appropriate stimuli. Our incomplete understanding of the mechanisms underlying chronic pain hypersensitivity accounts for the general ineffectiveness of currently available options for the treatment of chronic pain syndromes. Despite its complex pathophysiological nature, it is clear that neuropathic pain is associated with short- and long-term changes in the excitability of sensory neurons in the dorsal root ganglia (DRG) as well as their central connections. Recent evidence suggests that the upregulated expression of inflammatory cytokines in association with tissue damage or infection triggers the observed hyperexcitability of pain sensory neurons. The actions of inflammatory cytokines synthesized by DRG neurons and associated glial cells, as well as by astrocytes and microglia in the spinal cord, can produce changes in the excitability of nociceptive sensory neurons. These changes include rapid alterations in the properties of ion channels expressed by these neurons, as well as longer-term changes resulting from new gene transcription. In this chapter we review the diverse changes produced by inflammatory cytokines in the behavior of sensory neurons in the context of chronic pain syndromes.
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Watkins LR, Hutchinson MR, Rice KC, Maier SF. The "toll" of opioid-induced glial activation: improving the clinical efficacy of opioids by targeting glia. Trends Pharmacol Sci 2009; 30:581-91. [PMID: 19762094 DOI: 10.1016/j.tips.2009.08.002] [Citation(s) in RCA: 284] [Impact Index Per Article: 18.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/01/2009] [Revised: 07/27/2009] [Accepted: 08/03/2009] [Indexed: 12/24/2022]
Abstract
Glial activation participates in the mediation of pain including neuropathic pain, due to release of neuroexcitatory, proinflammatory products. Glial activation is now known to occur in response to opioids as well. Opioid-induced glial activation opposes opioid analgesia and enhances opioid tolerance, dependence, reward and respiratory depression. Such effects can occur, not via classical opioid receptors, but rather via non-stereoselective activation of toll-like receptor 4 (TLR4), a recently recognized key glial receptor participating in neuropathic pain as well. This discovery identifies a means for separating the beneficial actions of opioids (opioid receptor mediated) from the unwanted side-effects (TLR4/glial mediated) by pharmacologically targeting TLR4. Such a drug should be a stand-alone therapeutic for treating neuropathic pain as well. Excitingly, with newly-established clinical trials of two glial modulators for treating neuropathic pain and improving the utility of opioids, translation from rats-to-humans now begins with the promise of improved clinical pain control.
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Affiliation(s)
- Linda R Watkins
- Department of Psychology & Neuroscience and The Center for Neuroscience, University of Colorado at Boulder, Boulder, Colorado USA.
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Gwak YS, Unabia GC, Hulsebosch CE. Activation of p-38alpha MAPK contributes to neuronal hyperexcitability in caudal regions remote from spinal cord injury. Exp Neurol 2009; 220:154-61. [PMID: 19699199 DOI: 10.1016/j.expneurol.2009.08.012] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/22/2009] [Revised: 08/10/2009] [Accepted: 08/13/2009] [Indexed: 12/24/2022]
Abstract
In the present study, we examined whether activation of p-38alpha MAPK modulates mechanical allodynia and neuronal hyperexcitability, and if propentofylline (PPF, a glial modulator) modulates specifically localized activated p-38alpha MAPK expression in caudal regions remote from a low thoracic hemisection injury in rats. T13 spinal hemisection produces bilateral mechanical allodynia in hindpaws with evoked (in response to mechanical stimuli) neuronal hyperexcitability in lumbar spinal wide dynamic range (WDR) neurons compared to sham controls. The mechanical allodynia and the evoked activity of WDR neurons is attenuated by intrathecal and topical administration of SB203580, an inhibitor of p-38 MAPK activation, dose dependently (p<0.05); however, the spontaneous activity showed no significant differences compared to sham controls. After T13 spinal hemisection, significantly increased phosphorylated (activated form) p-38alpha MAPK expression was present in both superficial and deep dorsal horn neurons as well as in microglia, but not in astrocytes, in the lumbar spinal cord compared to sham controls (p<0.05). Intrathecal application of PPF significantly attenuated the expression of phosphorylated p-38alpha MAPK in superficial dorsal horn neurons (10 mM) and in microglia (1 and 10 mM) in the lumbar spinal cord compared to the hemisection group (p<0.05). In conclusion, our present data demonstrate that activated neuronal and microglial, but not astrocytic, p-38alpha MAPK contributes to the maintenance of neuronal hyperexcitability in caudal regions following spinal cord injury.
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Affiliation(s)
- Young S Gwak
- Department of Neuroscience, University of Texas Medical Branch, 301 University Boulevard, Galveston, TX 77555-1043, USA.
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Constandil L, Hernández A, Pelissier T, Arriagada O, Espinoza K, Burgos H, Laurido C. Effect of interleukin-1beta on spinal cord nociceptive transmission of normal and monoarthritic rats after disruption of glial function. Arthritis Res Ther 2009; 11:R105. [PMID: 19586548 PMCID: PMC2745785 DOI: 10.1186/ar2756] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/13/2009] [Revised: 06/09/2009] [Accepted: 07/08/2009] [Indexed: 12/02/2022] Open
Abstract
Introduction Cytokines produced by spinal cord glia after peripheral injuries have a relevant role in the maintenance of pain states. Thus, while IL-1β is overexpressed in the spinal cords of animals submitted to experimental arthritis and other chronic pain models, intrathecal administration of IL-1β to healthy animals induces hyperalgesia and allodynia and enhances wind-up activity in dorsal horn neurons. Methods To investigate the functional contribution of glial cells in the spinal cord nociceptive transmission, the effect of intrathecally administered IL-1β was studied in both normal and adjuvant-induced arthritic rats with or without glial inhibition. Four weeks after induction of monoarthritis, rats were treated with the glial cell inhibitor propentofylline (10 μg i.t. daily during 10 days) and submitted to a C-fiber-mediated reflex paradigm evoked by single and repetitive (wind-up) electric stimulation. Results Both the propentofylline treatment and the monoarthritic condition modified the stimulating current required for threshold activation of C reflex responses. Intrathecal IL-1β increased spinal cord wind-up activity in normal and monoarthritic rats without propentofylline pre-treatment, but resulted in decreased wind-up activity in normal and monoarthritic propentofylline-treated animals. Intrathecal saline did not produce any effect. Thus, glial inactivation reverted into inhibition the excitatory effect of IL-1β on spinal cord wind-up, irrespective of the normal or monoarthritic condition of rats. Conclusions The results suggest that the excitatory effect of nanomolar doses of IL-1β on spinal wind-up in healthy rats is produced by an unidentified glial mediator, while the inhibitory effects of IL-1β on wind-up activity in animals with inactivated glia resulted from a direct effect of the cytokine on dorsal horn neurons. The present study failed to demonstrate a differential sensitivity of normal and monoarthritic rats to IL-1β administration into the spinal cord and to disruption of β glial function, as both normal and monoarthritic animals changes wind-up activity in the same direction after propentofylline treatment, suggesting that after glial inhibition normal and monoarthritic animals behave similarly relative to the capability of dorsal horn neurons to generate wind-up activity when repeatedly stimulated by C-fibers.
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Affiliation(s)
- Luis Constandil
- Laboratory of Neurobiology, Department of Biology, Faculty of Chemistry and Biology, University of Santiago of Chile, Ave. Libertador B. O'Higgins 3363, Casilla 40 Correo 33, Santiago, Chile.
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Abstract
Glia have emerged as key contributors to pathological and chronic pain mechanisms. On activation, both astrocytes and microglia respond to and release a number of signalling molecules, which have protective and/or pathological functions. Here we review the current understanding of the contribution of glia to pathological pain and neuroprotection, and how the protective, anti-inflammatory actions of glia are being harnessed to develop new drug targets for neuropathic pain control. Given the prevalence of chronic pain and the partial efficacy of current drugs, which exclusively target neuronal mechanisms, new strategies to manipulate neuron-glia interactions in pain processing hold considerable promise.
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Jo D, Chapman CR, Light AR. Glial Mechanisms of Neuropathic Pain and Emerging Interventions. Korean J Pain 2009. [DOI: 10.3344/kjp.2009.22.1.1] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022] Open
Affiliation(s)
- Daehyun Jo
- Pain Research Center, Department of Anesthesiology, School of Medicine, University of Utah, Salt Lake City, UT 84109 USA
| | - C. Richard Chapman
- Pain Research Center, Department of Anesthesiology, School of Medicine, University of Utah, Salt Lake City, UT 84109 USA
| | - Alan R. Light
- Pain Research Center, Department of Anesthesiology, School of Medicine, University of Utah, Salt Lake City, UT 84109 USA
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Mika J, Wawrzczak-Bargiela A, Osikowicz M, Makuch W, Przewlocka B. Attenuation of morphine tolerance by minocycline and pentoxifylline in naive and neuropathic mice. Brain Behav Immun 2009; 23:75-84. [PMID: 18684397 DOI: 10.1016/j.bbi.2008.07.005] [Citation(s) in RCA: 132] [Impact Index Per Article: 8.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/02/2008] [Revised: 07/14/2008] [Accepted: 07/14/2008] [Indexed: 12/29/2022] Open
Abstract
We have previously demonstrated that glial inhibitors reduce the development of allodynia and hyperalgesia, potentiating the effect of a single morphine dose in a neuropathic pain model. This study explores the effects of two glial activation inhibitors, minocycline and pentoxifylline, on the development of tolerance to morphine in naive and chronic constriction injury (CCI)-exposed mice. Administration of morphine to naive (20 mg/kg; i.p.) and CCI-exposed mice (40 mg/kg; i.p.) twice daily resulted in tolerance to its anti-nociceptive effect after 6 days. Injections of morphine were combined with minocycline (30 mg/kg, i.p.) or pentoxifylline (20 mg/kg, i.p.) administered as two preemptive doses before first morphine administration in naive or pre-injury in CCI-exposed mice, and repeated twice daily 30 min before each morphine administration. With treatment, development of morphine tolerance was delayed by 5 days (from 6 to 11 days), as measured by the tail-flick test in naive and by tail-flick, von Frey, and cold plate tests in CCI-exposed mice. Western blot analysis of CD11b/c and GFAP protein demonstrated that minocycline and pentoxifylline, at doses delaying development of tolerance to morphine analgesia, significantly diminished the morphine-induced increase in CD11b/c protein level. We found that repeated systemic administration of glial inhibitors significantly delays development of morphine tolerance by attenuating the level of this microglial marker under normal and neuropathic pain conditions. Our results support the idea that targeting microglial activation during morphine therapy/treatment is a novel and clinically promising method for enhancing morphine's analgesic effects, especially in neuropathic pain.
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Affiliation(s)
- Joanna Mika
- Department of Pain Pharmacology, Institute of Pharmacology, Polish Academy of Sciences, 12 Smetna Street, 31-343 Krakow, Poland
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76
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Hulsebosch CE, Hains BC, Crown ED, Carlton SM. Mechanisms of chronic central neuropathic pain after spinal cord injury. ACTA ACUST UNITED AC 2008; 60:202-13. [PMID: 19154757 DOI: 10.1016/j.brainresrev.2008.12.010] [Citation(s) in RCA: 225] [Impact Index Per Article: 14.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 12/29/2008] [Indexed: 12/25/2022]
Abstract
Not all spinal contusions result in mechanical allodynia, in which non-noxious stimuli become noxious. The studies presented use the NYU impactor at 12.5 mm drop or the Infinite Horizons Impactor (150 kdyn, 1 s dwell) devices to model spinal cord injury (SCI). Both of these devices and injury parameters, if done correctly, will result in animals with above level (forelimb), at level (trunk) and below level (hindlimb) mechanical allodynia that model the changes in evoked somatosensation experienced by the majority of people with SCI. The sections are as follows: 1) Mechanisms of remote microglial activation and pain signaling in "below-level" central pain 2) Intracellular signaling mechanisms in central sensitization in "at-level" pain 3) Peripheral sensitization contributes to "above level" injury pain following spinal cord injury and 4) Role of reactive oxygen species in central sensitization in regional neuropathic pain following SCI. To summarize, differential regional mechanisms contribute to the regional chronic pain states. We propose the importance of understanding the mechanisms in the differential regional pain syndromes after SCI in the chronic condition. Targeting regional mechanisms will be of enormous benefit to the SCI population that suffer chronic pain, and will contribute to better treatment strategies for other chronic pain syndromes.
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Affiliation(s)
- Claire E Hulsebosch
- Department of Neuroscience and Cell Biology, University of Texas Medical Branch, 301 University Boulevard, Galveston, TX 77555-1043, USA.
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77
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Current World Literature. Curr Opin Anaesthesiol 2008; 21:684-93. [DOI: 10.1097/aco.0b013e328312c01b] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
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78
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Abstract
PURPOSE OF REVIEW Recent studies show that peripheral injury activates both neuronal and nonneuronal or glial components of the peripheral and central cellular circuitry. The subsequent neuron-glia interactions contribute to pain hypersensitivity. This review will briefly discuss novel findings that have shed light on the cellular mechanisms of neuron-glia interactions in persistent pain. RECENT FINDINGS Two fundamental questions related to neuron-glia interactions in pain mechanisms have been addressed: what are the signals that lead to central glial activation after injury and how do glial cells affect central nervous system neuronal activity and promote hyperalgesia? SUMMARY Evidence indicates that central glial activation depends on nerve inputs from the site of injury and release of chemical mediators. Hematogenous immune cells may migrate to/infiltrate the brain and circulating inflammatory mediators may penetrate the blood-brain barrier to participate in central glial responses to injury. Inflammatory cytokines such as interleukin-1beta released from glia may facilitate pain transmission through its coupling to neuronal glutamate receptors. This bidirectional neuron-glia signaling plays a key role in glial activation, cytokine production and the initiation and maintenance of hyperalgesia. Recognition of the contribution of the mutual neuron-glia interactions to central sensitization and hyperalgesia prompts new treatment for chronic pain.
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Affiliation(s)
- Ke Ren
- Department of Neural and Pain Sciences, Dental School and Program in Neuroscience, University of Maryland, Baltimore, Maryland 21201-1586, USA
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79
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Gliopathy ensures persistent inflammation and chronic pain after spinal cord injury. Exp Neurol 2008; 214:6-9. [PMID: 18708053 DOI: 10.1016/j.expneurol.2008.07.016] [Citation(s) in RCA: 95] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/10/2008] [Accepted: 07/23/2008] [Indexed: 12/12/2022]
Abstract
Research focused on improving recovery of function, including the reduction of central neuropathic pain (CNP) after spinal cord injury (SCI) is essential. After SCI, regional neuropathic pain syndromes above, at and below the level or spinal injury develop and are thought to have different mechanisms, but may share common dysfunctional glial mechanisms. Detloff et al., [Detloff, M.R., Fisher, L.C., McGaughy, V., Longbrake, E.E., Popovich, P.G., Basso, D.M., Remote activation of microglia and pro-inflammatory cytokines predict the onset and severity of below-level neuropathic pain after spinal cord injury in rats. Exp. Neurol. (2008), doi: 10.1016/j.expneurol.2008.04.009.] describe events in the lumbar region of the spinal cord after a midthoracic SCI injury, the so called "below-level" pain and compares the findings to peripheral nerve lesion findings. This commentary briefly reviews glial contributions and intracellular signaling mechanisms, both neuronal and glial, that provide the substrate for CNP after SCI, including the persistent glial production of factors that can maintain sensitization of dorsal horn neurons in segments remote from the spinal injury; ie. dorsal horn hyperexcitability to formerly non noxious stimuli that become noxious after SCI resulting in allodynia. The term "gliopathy" is proposed to describe the dysfunctional and maladaptive response of glial cells, specifically astrocytes and microglia, to neural injury that is initiated by the sudden injury induced increase in extracellular concentrations of glutamate and concomitant production of several proinflammatory molecules. It is important to understand the roles that different glia play in "gliopathy", a condition that appears to persist after SCI. Furthermore, targeted treatment of gliopathy will attenuate mechanical allodynia in both central and peripheral neuropathic pain syndromes.
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A comparison of spinal Iba1 and GFAP expression in rodent models of acute and chronic pain. Brain Res 2008; 1219:116-26. [PMID: 18538310 DOI: 10.1016/j.brainres.2008.05.004] [Citation(s) in RCA: 152] [Impact Index Per Article: 9.5] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/25/2008] [Revised: 04/29/2008] [Accepted: 05/01/2008] [Indexed: 12/26/2022]
Abstract
The treatment of acute and chronic pain is still deficient. The modulation of glial cells may provide novel targets to treat pain. We hypothesize that astrocytes and microglia participate in the initiation and maintenance of both, acute surgical and chronic neuropathic pain. Rats underwent paw incision, L5 nerve exposure or L5 nerve transection surgery. Behavioral mechanical allodynia was assessed using von Frey filaments. Immunohistochemistry was performed using anti-ionized calcium binding adaptor protein, Iba-1 (microglia), and anti-Glial Fibrillary Acidic Protein, GFAP (astrocytes) on day 1, 4 and 7 after surgery. Following paw incision and at spinal L5 segment GFAP expression was increased in laminae I-II and Iba1 in deep laminae on day 1, in the entire dorsal horn on day 4 and dissipated on day 7 after paw incision in parallel with the allodynia. L5 nerve transection induced mechanical allodynia from day 1 to 7 which correlated with Iba-1 increases on day 1, 4 (entire dorsal horn) and day 7 after nerve injury (deep laminae of the dorsal horn) at spinal L5 segment. Conversely, GFAP increased at later time points from day 4 (deep laminae) and on day 7 (entire dorsal horn). Our data demonstrates that astrocytes (GFAP expression) play a role in the initiation of acute pain and the maintenance of chronic pain while Iba-1 increases closely correlated with the early phase of neuropathic pain. Iba1 and GFAP increased rostrally, at L3 segment, after paw incision (day 4) and only Iba1 increased following L5 nerve transection (day 7).
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81
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Chronic constriction injury of the infraorbital nerve in the rat using modified syringe needle. J Neurosci Methods 2008; 172:43-7. [PMID: 18501433 DOI: 10.1016/j.jneumeth.2008.04.013] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/22/2007] [Revised: 04/08/2008] [Accepted: 04/09/2008] [Indexed: 12/24/2022]
Abstract
Here we report a method for performing a chronic constriction injury (CCI) of the infraorbital nerve (ION) in the rat as a component of a chronic pain model. The surgical approach to the ION is described together with the use of a modified dental syringe needle that simplifies placing two chromic gut ligatures around the ION. This method makes the surgical procedure easier, the nerve injury more consistent across animals and reduces secondary damage to the ION and surrounding tissue. Pain behavior testing together with immunostaining for markers of nerve injury in the spinal trigeminal nucleus show the suitability of this procedure as a model of orofacial pain.
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Detloff MR, Fisher LC, McGaughy V, Longbrake EE, Popovich PG, Basso DM. Remote activation of microglia and pro-inflammatory cytokines predict the onset and severity of below-level neuropathic pain after spinal cord injury in rats. Exp Neurol 2008; 212:337-47. [PMID: 18511041 DOI: 10.1016/j.expneurol.2008.04.009] [Citation(s) in RCA: 204] [Impact Index Per Article: 12.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/17/2007] [Revised: 03/19/2008] [Accepted: 04/08/2008] [Indexed: 12/29/2022]
Abstract
Spinal cord injury (SCI) impairs sensory systems causing chronic allodynia. Mechanisms underlying neuropathic pain have been more extensively studied following peripheral nerve injury (PNI) than after central trauma. Microglial activation, pro-inflammatory cytokine production and activation of p38 MAP kinase pathways may induce at-level allodynia following PNI. We investigated whether midthoracic SCI elicits similar behavioral and cellular responses below the level of injury (lumbar spinal cord; L5). Importantly, we show that anatomical connections between L5 and supraspinal centers remain intact after moderate SCI allowing direct comparison to a well-established model of peripheral nerve injury. We found that SCI elicits below-level allodynia of similar magnitude to at-level pain caused by a peripheral nerve injury. Moreover, the presence of robust microglial activation in L5 cord predicted allodynia in 86% of rats. Also increased phosphorylation of p38 MAP kinase occurred in the L5 dorsal horn of allodynic rats. For below-level allodynia after SCI, TNF-alpha and IL-1beta increased in the L5 dorsal horn by 7 dpo and returned to baseline by 35 dpo. Interestingly, IL-6 remains at normal levels early after SCI and increases at chronic time points. Increased levels of pro-inflammatory cytokines also occurred in the thalamus after SCI-induced allodynia. These data suggest that remote microglial activation is pivotal in the development and maintenance of below-level allodynia after SCI. Fractalkine, a known activator of microglia, and astrocytes were not primary modulators of below-level pain. Although the mechanisms of remote microglial activation are unknown, this response may be a viable target for limiting or preventing neuropathic pain after SCI in humans.
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Affiliation(s)
- Megan Ryan Detloff
- Center for Brain and Spinal Cord Repair, Neuroscience Graduate Studies Program, The Ohio State University 43210, USA
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83
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Cao H, Zhang YQ. Spinal glial activation contributes to pathological pain states. Neurosci Biobehav Rev 2008; 32:972-83. [PMID: 18471878 DOI: 10.1016/j.neubiorev.2008.03.009] [Citation(s) in RCA: 181] [Impact Index Per Article: 11.3] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/03/2007] [Revised: 02/28/2008] [Accepted: 03/13/2008] [Indexed: 12/21/2022]
Abstract
Chronic pain, a pathological state, affects millions of people worldwide. Despite decades of study on the neuronal processing of pain, mechanisms underlying the creation and maintenance of enhanced pain states after injury or inflammation remain far from clear. In the last decade, however, the discovery that glial activation amplifies pain has challenged classic neuronal views of "pain". This review focuses on recent developments in understanding that spinal cord glia are involved in pathological pain. We overview the action of spinal glia (both microglia and astrocytes) in several persistent pain models, and provide new evidence that spinal glia activation contributes to the development and maintenance of arthritic pain facilitation. We also attempt to discuss some critical questions, such as how signals are conveyed from primary afferents to spinal glia following peripheral nerve injury and inflammation. What causes glia to become activated after peripheral/central injury/inflammation? And how the activated glia alter neuronal sensitivity and pain processing? Answers to these questions might open a new approach for treatment of pathological pain.
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Affiliation(s)
- Hong Cao
- Institute of Neurobiology, Institutes of Brain Science and State Key Laboratory of Medical Neurobiology, Fudan University, 138 Yixueyuan Road, Shanghai 200032, China
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84
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Gwak YS, Crown ED, Unabia GC, Hulsebosch CE. Propentofylline attenuates allodynia, glial activation and modulates GABAergic tone after spinal cord injury in the rat. Pain 2008; 138:410-422. [PMID: 18353556 DOI: 10.1016/j.pain.2008.01.021] [Citation(s) in RCA: 96] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/07/2007] [Revised: 12/19/2007] [Accepted: 01/22/2008] [Indexed: 01/05/2023]
Abstract
In this study, we evaluated whether propentofylline, a methylxanthine derivative, modulates spinal glial activation and GABAergic inhibitory tone by modulation of glutamic acid decarboxylase (GAD)(65), the GABA synthase enzyme, in the spinal dorsal horn following spinal cord injury (SCI). Sprague-Dawley rats (225-250 g) were given a unilateral spinal transverse injury, from dorsal to ventral, at the T13 spinal segment. Unilateral spinal injured rats developed robust bilateral hindlimb mechanical allodynia and hyperexcitability of spinal wide dynamic range (WDR) neurons in the lumbar enlargement (L4-L5) compared to sham controls, which was attenuated by intrathecal (i.t.) administration of GABA, dose-dependently (0.01, 0.1, 0.5 microg). Western blotting and immunohistochemical data demonstrated that the expression level of GAD(65) protein significantly decreased on both sides of the lumbar dorsal horn (L4/5) after SCI (p<0.05). In addition, astrocytes and microglia showed soma hypertrophy as determined by increased soma area and increased GFAP and CD11b on both sides of the lumbar dorsal horn compared to sham controls, respectively (p<0.05). Intrathecal treatment with propentofylline (PPF 10 mM) significantly attenuated the astrocytic and microglial soma hypertrophy and mechanical allodynia (p<0.05). Additionally, the Western blotting and immunohistochemistry data demonstrated that i.t. treatment of PPF significantly prevented the decrease of GAD(65) expression in both sides of the lumbar dorsal horn following SCI (p<0.05). In conclusion, our present data demonstrate that propentofylline modulates glia activation and GABAergic inhibitory tone by modulation of GAD(65) protein expression following spinal cord injury.
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Affiliation(s)
- Young Seob Gwak
- Department of Neuroscience & Cell Biology, University of Texas Medical Branch, 301 University Boulevard, Galveston, TX 77555-1043, USA
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85
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Scholz J, Woolf CJ. The neuropathic pain triad: neurons, immune cells and glia. Nat Neurosci 2008; 10:1361-8. [PMID: 17965656 DOI: 10.1038/nn1992] [Citation(s) in RCA: 1302] [Impact Index Per Article: 81.4] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Abstract
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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Affiliation(s)
- Joachim Scholz
- Neural Plasticity Research Group, Department of Anesthesia and Critical Care, Massachusetts General Hospital and Harvard Medical School, Charlestown, Massachusetts 02129, USA.
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Liu J, Li W, Zhu J, Zhang J, Feng X, Guan R, Xu J. The Effect of Pentoxifylline on Existing Hypersensitivity in a Rat Model of Neuropathy. Anesth Analg 2008; 106:650-3, table of contents. [DOI: 10.1213/ane.0b013e31815efaba] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/20/2023]
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Abstract
Currently the clinical needs for pain and headache management are not met. Despite the numerous and exciting recent advances in understanding the molecular and cellular mechanisms that originate pain, we cannot yet fully explain the mechanism underlying the biology of chronic pain. Pain is a natural mechanism preserving our species survival; however, when the protective quality is lost, physiologic changes to the peripheral and central nervous systems result in the formation of chronic pain states. Once we understand how this chronic pain state is created, either through genetic, environmental, therapeutic, or other triggers we may be able to enhance our species existence, limiting maladaptive pain and suffering. The future therapeutic targets will need to address the genetics, neurophysiologic changes of the neurons and brain as well as help control immune systems including the glia. The key to successful headache and pain therapy is research aimed at prevention and minimizing the plastic changes triggering chronic pain.
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88
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White FA, Jung H, Miller RJ. Chemokines and the pathophysiology of neuropathic pain. Proc Natl Acad Sci U S A 2007; 104:20151-8. [PMID: 18083844 PMCID: PMC2154400 DOI: 10.1073/pnas.0709250104] [Citation(s) in RCA: 266] [Impact Index Per Article: 15.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/28/2007] [Indexed: 11/18/2022] Open
Abstract
Chemokines and chemokine receptors are widely expressed by cells of the immune and nervous systems. This review focuses on our current knowledge concerning the role of chemokines in the pathophysiology of chronic pain syndromes. Injury- or disease-induced changes in the expression of diverse chemokines and their receptors have been demonstrated in the neural and nonneural elements of pain pathways. Under these circumstances, chemokines have been shown to modulate the electrical activity of neurons by multiple regulatory pathways including increases in neurotransmitter release through Ca-dependent mechanisms and transactivation of transient receptor channels. Either of these mechanisms alone, or in combination, may contribute to sustained excitability of primary afferent and secondary neurons within spinal pain pathways. Another manner in which chemokines may influence sustained neuronal excitability may be their ability to function as excitatory neurotransmitters within the peripheral and central nervous system. As is the case for traditional neurotransmitters, injury-induced up-regulated chemokines are found within synaptic vesicles. Chemokines released after depolarization of the cell membrane can then act on other chemokine receptor-bearing neurons, glia, or immune cells. Because up-regulation of chemokines and their receptors may be one of the mechanisms that directly or indirectly contribute to the development and maintenance of chronic pain, these molecules may then represent novel targets for therapeutic intervention in chronic pain states.
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Affiliation(s)
- Fletcher A. White
- *Departments of Cell Biology, Neurobiology and Anatomy, and Anesthesiology, Loyola University Chicago, Maywood, IL 60626; and
| | - Hosung Jung
- Molecular Pharmacology and Structural Biochemistry, Northwestern University, Chicago, IL 60611
| | - Richard J. Miller
- Molecular Pharmacology and Structural Biochemistry, Northwestern University, Chicago, IL 60611
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89
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Spinal astrocyte glutamate receptor 1 overexpression after ischemic insult facilitates behavioral signs of spasticity and rigidity. J Neurosci 2007; 27:11179-91. [PMID: 17942713 DOI: 10.1523/jneurosci.0989-07.2007] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Using a rat model of ischemic paraplegia, we examined the expression of spinal AMPA receptors and their role in mediating spasticity and rigidity. Spinal ischemia was induced by transient occlusion of the descending aorta combined with systemic hypotension. Spasticity/rigidity were identified by simultaneous measurements of peripheral muscle resistance (PMR) and electromyography (EMG) before and during ankle flexion. In addition, Hoffman reflex (H-reflex) and motor evoked potentials (MEPs) were recorded from the gastrocnemius muscle. Animals were implanted with intrathecal catheters for drug delivery and injected with the AMPA receptor antagonist NGX424 (tezampanel), glutamate receptor 1 (GluR1) antisense, or vehicle. Where intrathecal vehicle had no effect, intrathecal NGX424 produced a dose-dependent suppression of PMR [ED50 of 0.44 microg (0.33-0.58)], as well as tonic and ankle flexion-evoked EMG activity. Similar suppression of MEP and H-reflex were also seen. Western blot analyses of lumbar spinal cord tissue from spastic animals showed a significant increase in GluR1 but decreased GluR2 and GluR4 proteins. Confocal and electron microscopic analyses of spinal cord sections from spastic animals revealed increased GluR1 immunoreactivity in reactive astrocytes. Selective GluR1 knockdown by intrathecal antisense treatment resulted in a potent reduction of spasticiy and rigidity and concurrent downregulation of neuronal/astrocytic GluR1 in the lumbar spinal cord. Treatment of rat astrocyte cultures with AMPA led to dose-dependent glutamate release, an effect blocked by NGX424. These data suggest that an AMPA/kainate receptor antagonist can represent a novel therapy in modulating spasticity/rigidity of spinal origin and that astrocytes may be a potential target for such treatment.
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90
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Abstract
Damage to nerves at various levels of the peripheral and central nervous systems will lead to sensory loss, but in a significant number of patients this is accompanied by a series of distressing painful signs and symptoms. Although animal models and clinical studies have shed much needed light on the underlying mechanisms that produce this maladaptive plasticity, the presently available drugs do not always fully control the pain. This review covers some of the important mechanisms that include ion channels, central processing through excitatory amino acid and neuropeptide receptors and, finally, the role of monoamine systems that originate in the brain and descend to alter spinal events. The targets for presently licensed and potential novel drugs are covered in this context, as are perspectives on future research priorities.
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Affiliation(s)
- Lucy A Bee
- University College London, Department of Pharmacology, University College London Gower Street, London, WC1E 6BT, UK
| | - Anthony H Dickenson
- University College London, Department of Pharmacology, Gower Street, London, WC1E 6BT, UK
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91
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Ji RR, Suter MR. p38 MAPK, microglial signaling, and neuropathic pain. Mol Pain 2007; 3:33. [PMID: 17974036 PMCID: PMC2186318 DOI: 10.1186/1744-8069-3-33] [Citation(s) in RCA: 443] [Impact Index Per Article: 26.1] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/24/2007] [Accepted: 11/01/2007] [Indexed: 12/26/2022] Open
Abstract
Accumulating evidence over last several years indicates an important role of microglial cells in the pathogenesis of neuropathic pain. Signal transduction in microglia under chronic pain states has begun to be revealed. We will review the evidence that p38 MAPK is activated in spinal microglia after nerve injury and contributes importantly to neuropathic pain development and maintenance. We will discuss the upstream mechanisms causing p38 activation in spinal microglia after nerve injury. We will also discuss the downstream mechanisms by which p38 produces inflammatory mediators. Taken together, current data suggest that p38 plays a critical role in microglial signaling under neuropathic pain conditions and represents a valuable therapeutic target for neuropathic pain management.
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Affiliation(s)
- Ru-Rong Ji
- Pain Research Center, Department of Anesthesiology, Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts, USA.
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92
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Griffin RS, Costigan M, Brenner GJ, Him Eddie Ma C, Scholz J, Moss A, Allchorne AJ, Stahl GL, Woolf CJ. Complement induction in spinal cord microglia results in anaphylatoxin C5a-mediated pain hypersensitivity. J Neurosci 2007; 27:8699-708. [PMID: 17687047 PMCID: PMC6672952 DOI: 10.1523/jneurosci.2018-07.2007] [Citation(s) in RCA: 188] [Impact Index Per Article: 11.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022] Open
Abstract
Microarray expression profiles reveal substantial changes in gene expression in the ipsilateral dorsal horn of the spinal cord in response to three peripheral nerve injury models of neuropathic pain. However, only 54 of the 612 regulated genes are commonly expressed across all the neuropathic pain models. Many of the commonly regulated transcripts are immune related and include the complement components C1q, C3, and C4, which we find are expressed only by microglia. C1q and C4 are, moreover, the most strongly regulated of all 612 regulated genes. In addition, we find that the terminal complement component C5 and the C5a receptor (C5aR) are upregulated in spinal microglia after peripheral nerve injury. Mice null for C5 had reduced neuropathic pain sensitivity, excluding C3a as a pain effector. C6-deficient rats, which cannot form the membrane attack complex, have a normal neuropathic pain phenotype. However, C5a applied intrathecally produces a dose-dependent, slow-onset cold pain in naive animals. Furthermore, a C5aR peptide antagonist reduces cold allodynia in neuropathic pain models. We conclude that induction of the complement cascade in spinal cord microglia after peripheral nerve injury contributes to neuropathic pain through the release and action of the C5a anaphylatoxin peptide.
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Affiliation(s)
- Robert S. Griffin
- Neural Plasticity Research Group, Department of Anesthesia and Critical Care, Massachusetts General Hospital and Harvard Medical School, Charlestown, Massachusetts 02129, and
| | - Michael Costigan
- Neural Plasticity Research Group, Department of Anesthesia and Critical Care, Massachusetts General Hospital and Harvard Medical School, Charlestown, Massachusetts 02129, and
| | - Gary J. Brenner
- Neural Plasticity Research Group, Department of Anesthesia and Critical Care, Massachusetts General Hospital and Harvard Medical School, Charlestown, Massachusetts 02129, and
| | - Chi Him Eddie Ma
- Neural Plasticity Research Group, Department of Anesthesia and Critical Care, Massachusetts General Hospital and Harvard Medical School, Charlestown, Massachusetts 02129, and
| | - Joachim Scholz
- Neural Plasticity Research Group, Department of Anesthesia and Critical Care, Massachusetts General Hospital and Harvard Medical School, Charlestown, Massachusetts 02129, and
| | - Andrew Moss
- Neural Plasticity Research Group, Department of Anesthesia and Critical Care, Massachusetts General Hospital and Harvard Medical School, Charlestown, Massachusetts 02129, and
| | - Andrew J. Allchorne
- Neural Plasticity Research Group, Department of Anesthesia and Critical Care, Massachusetts General Hospital and Harvard Medical School, Charlestown, Massachusetts 02129, and
| | - Gregory L. Stahl
- Center for Experimental Therapeutics and Reperfusion Injury, Department of Anesthesiology, Perioperative and Pain Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts 02115
| | - Clifford J. Woolf
- Neural Plasticity Research Group, Department of Anesthesia and Critical Care, Massachusetts General Hospital and Harvard Medical School, Charlestown, Massachusetts 02129, and
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93
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Abstract
The treatment of neuropathic pain is a major unresolved medical challenge. Present pharmacotherapies only have modest efficacy and numerous side effects. The use of opioid analgesics is additionally coupled with dependence and withdrawal syndromes. Ibudilast (AV-411) is a non-selective phosphodiesterase inhibitor that is also known to suppress glial cell activation. It has been used clinically for other indications with a good safety profile. As glial cell activation is considered to crucially contribute to neuropathic pain as well as opioid dependence and withdrawal, the authors conceived that ibudilast may be useful for treating these conditions. Preclinical data indicate that ibudilast crosses the blood-brain barrier, is well tolerated, is active on oral administration, reduces glial activation and attenuates pain symptoms in diverse rat models of neuropathic pain. In addition, it enhances acute morphine analgesia and attenuates morphine tolerance and withdrawal. Thus ibudilast may improve opioid efficacy and is a promising therapeutic candidate for neuropathic pain, with a novel mechanism of action.
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Affiliation(s)
- Annemarie Ledeboer
- Avigen, Inc., Department of Preclinical Development, Alameda, CA 94502, USA.
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94
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Watkins LR, Hutchinson MR, Ledeboer A, Wieseler-Frank J, Milligan ED, Maier SF. Norman Cousins Lecture. Glia as the "bad guys": implications for improving clinical pain control and the clinical utility of opioids. Brain Behav Immun 2007; 21:131-46. [PMID: 17175134 PMCID: PMC1857294 DOI: 10.1016/j.bbi.2006.10.011] [Citation(s) in RCA: 248] [Impact Index Per Article: 14.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 08/16/2006] [Revised: 10/03/2006] [Accepted: 10/04/2006] [Indexed: 12/19/2022] Open
Abstract
Within the past decade, there has been increasing recognition that glia are far more than simply "housekeepers" for neurons. This review explores two recently recognized roles of glia (microglia and astrocytes) in: (a) creating and maintaining enhanced pain states such as neuropathic pain, and (b) compromising the efficacy of morphine and other opioids for pain control. While glia have little-to-no role in pain under basal conditions, pain is amplified when glia become activated, inducing the release of proinflammatory products, especially proinflammatory cytokines. How glia are triggered to become activated is a key issue, and appears to involve a number of neuron-to-glia signals including neuronal chemokines, neurotransmitters, and substances released by damaged, dying and dead neurons. In addition, glia become increasingly activated in response to repeated administration of opioids. Products of activated glia increase neuronal excitability via numerous mechanisms, including direct receptor-mediated actions, upregulation of excitatory amino acid receptor function, downregulation of GABA receptor function, and so on. These downstream effects of glial activation amplify pain, suppress acute opioid analgesia, contribute to the apparent loss of opioid analgesia upon repeated opioid administration (tolerance), and contribute to the development of opioid dependence. The potential implications of such glial regulation of pain and opioid actions are vast, suggestive that targeting glia and their proinflammatory products may provide a novel and effective therapy for controlling clinical pain syndromes and increasing the clinical utility of analgesic drugs.
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Affiliation(s)
- Linda R Watkins
- Department of Psychology and the Center for Neuroscience, Muenzinger D-244, Campus Box 345, University of Colorado at Boulder, Boulder, CO 80309-0345, USA.
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