Hyperalgesia due to nerve injury-role of peroxynitrite

Preconditioning by voluntary wheel running attenuates later neuropathic pain via nuclear factor E2-related factor 2 antioxidant signaling in rats

ABSTRACT

Animal and human studies have shown that exercise prior to nerve injury prevents later chronic pain, but the mechanisms of such preconditioning remain elusive. Given that exercise acutely increases the formation of free radicals, triggering antioxidant compensation, we hypothesized that voluntary running preconditioning would attenuate neuropathic pain by supporting redox homeostasis after sciatic nerve injury in male and female rats. We show that 6 weeks of voluntary wheel running suppresses neuropathic pain development induced by chronic constriction injury across both sexes. This attenuation was associated with reduced nitrotyrosine immunoreactivity-a marker for peroxynitrite-at the sciatic nerve injury site. Our data suggest that prior voluntary wheel running does not reduce the production of peroxynitrite precursors, as expression levels of inducible nitric oxide synthase and NADPH oxidase 2 were unchanged. Instead, voluntary wheel running increased superoxide scavenging by elevating expression of superoxide dismutases 1 and 2. Prevention of neuropathic pain was further associated with the activation of the master transcriptional regulator of the antioxidant response, nuclear factor E2-related factor 2 (Nrf2). Six weeks of prior voluntary wheel running increased Nrf2 nuclear translocation at the sciatic nerve injury site; in contrast, 3 weeks of prior wheel running, which failed to prevent neuropathic pain, had no effect on Nrf2 nuclear translocation. The protective effects of prior voluntary wheel running were mediated by Nrf2, as suppression was abolished across both sexes when Nrf2 activation was blocked during the 6-week running phase. This study provides insight into the mechanisms by which physical activity may prevent neuropathic pain. Preconditioning by voluntary wheel running, terminated prior to nerve injury, suppresses later neuropathic pain in both sexes, and it is modulated through the activation of Nrf2-antioxidant signaling.

The Role of Endothelial Dysfunction in Peripheral Blood Nerve Barrier: Molecular Mechanisms and Pathophysiological Implications

The exchange of solutes between the blood and the nerve tissue is mediated by specific and high selective barriers in order to ensure the integrity of the different compartments of the nervous system. At peripheral level, this function is maintained by the Blood Nerve Barrier (BNB) that, in the presence, of specific stressor stimuli can be damaged causing the onset of neurodegenerative processes. An essential component of BNB is represented by the endothelial cells surrounding the sub-structures of peripheral nerves and increasing evidence suggests that endothelial dysfunction can be considered a leading cause of the nerve degeneration. The purpose of this review is to highlight the main mechanisms involved in the impairment of endothelial cells in specific diseases associated with peripheral nerve damage, such as diabetic neuropathy, erectile dysfunction and inflammation of the sciatic nerve.

Effects of Glutathione on Mechanical Allodynia and Central Sensitization in Chronic Postischemic Pain Rats

Background

The chronic postischemia pain (CPIP) model is an animal model using ischemia/reperfusion injury that mimics the symptoms of complex regional pain syndrome type I. Glutathione (GSH) prevents ischemia/reperfusion injury by scavenging free radicals. We conducted this study to investigate the protective effect of GSH in CPIP rats via changes of mechanical allodynia and phospholyration of the N-methyl-D-aspartate receptor subunit GluN1.

Methods

We divided 45 rats into 5 groups: sham, CPIP, CPIP + GSH 100 mg/kg, CPIP + GSH 200 mg/kg, and CPIP + GSH 500 mg/kg. Rats in the sham and CPIP groups received normal saline and rats in the other groups received GSH at the designated doses thirty minutes prior to reperfusion. Withdrawal thresholds were evaluated before sugery as well as 1, 3, and 7 days after surgery. pGluN1 level in the spinal cord was also measured.

Results

GSH treated rats show a significant increase in the withdrawal thresholds of both hind paws as compared with the CPIP group dose-dependently. The expression of pGluN1 in the GSH treated rats significantly decreased as compared to the CPIP group (all P < 0.05).

Conclusion

These findings suggest that GSH inhibited the development of mechanical allodynia and central sensitization in CPIP rats.

Role of NMDA Receptor-Mediated Glutamatergic Signaling in Chronic and Acute Neuropathologies

N-Methyl-D-aspartate receptors (NMDARs) have two opposing roles in the brain. On the one hand, NMDARs control critical events in the formation and development of synaptic organization and synaptic plasticity. On the other hand, the overactivation of NMDARs can promote neuronal death in neuropathological conditions. Ca(2+) influx acts as a primary modulator after NMDAR channel activation. An imbalance in Ca(2+) homeostasis is associated with several neurological diseases including schizophrenia, Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis. These chronic conditions have a lengthy progression depending on internal and external factors. External factors such as acute episodes of brain damage are associated with an earlier onset of several of these chronic mental conditions. Here, we will review some of the current evidence of how traumatic brain injury can hasten the onset of several neurological conditions, focusing on the role of NMDAR distribution and the functional consequences in calcium homeostasis associated with synaptic dysfunction and neuronal death present in this group of chronic diseases.

A peroxynitrite decomposition catalyst prevents mechanical allodynia and NMDA receptor activation in the hind-paw ischemia reperfusion injury rats

The contributions of superoxide and nitric oxide to ischemia/reperfusion (I/R)-induced neuropathic pain have previously been demonstrated in an animal model that mimics the symptoms of complex regional pain syndrome type I (CRPS I). Targeting peroxynitrite, which is the product of their interaction, may provide effective treatments for I/R-induced neuropathic pain. In this study, the effect of the peroxynitrite decomposition catalyst FeTMPyP [5,10,15,20-tetrakis (N-methyl-4′-pyridyl)porphyrinato iron (III)], administered at doses of 1, 3 and 10 mg/kg via intraperitoneal injection 30 min prior to reperfusion, was evaluated in rats with chronic post-ischemic pain. The pain behavior of the rats was tested with a von Frey filament. Phosphorylation of N-methyl-D-aspartate (NMDA) receptors in the L4/6 section of the spinal cord was measured on the third day following reperfusion by western blotting. The rats treated with 3 or 10 mg/kg FeTMPyP demonstrated significant increases in their paw withdrawal thresholds and decreased levels of phosphorylated NMDA receptor subunit 1 compared with those of the vehicle group (all P<0.001). These findings suggest that nitrosative stress, specifically that associated with peroxynitrite, may be involved in the mechanical allodynia and central sensitization that are associated with CRPS I and may provide a rationale for CRPS I treatment strategies using peroxynitrite decomposition catalysts.

Sensory and signaling mechanisms of bradykinin, eicosanoids, platelet-activating factor, and nitric oxide in peripheral nociceptors

Peripheral mediators can contribute to the development and maintenance of inflammatory and neuropathic pain and its concomitants (hyperalgesia and allodynia) via two mechanisms. Activation or excitation by these substances of nociceptive nerve endings or fibers implicates generation of action potentials which then travel to the central nervous system and may induce pain sensation. Sensitization of nociceptors refers to their increased responsiveness to either thermal, mechanical, or chemical stimuli that may be translated to corresponding hyperalgesias. This review aims to give an account of the excitatory and sensitizing actions of inflammatory mediators including bradykinin, prostaglandins, thromboxanes, leukotrienes, platelet-activating factor, and nitric oxide on nociceptive primary afferent neurons. Manifestations, receptor molecules, and intracellular signaling mechanisms of the effects of these mediators are discussed in detail. With regard to signaling, most data reported have been obtained from transfected nonneuronal cells and somata of cultured sensory neurons as these structures are more accessible to direct study of sensory and signal transduction. The peripheral processes of sensory neurons, where painful stimuli actually affect the nociceptors in vivo, show marked differences with respect to biophysics, ultrastructure, and equipment with receptors and ion channels compared with cellular models. Therefore, an effort was made to highlight signaling mechanisms for which supporting data from molecular, cellular, and behavioral models are consistent with findings that reflect properties of peripheral nociceptive nerve endings. Identified molecular elements of these signaling pathways may serve as validated targets for development of novel types of analgesic drugs.

Reactive nitroxidative species and nociceptive processing: determining the roles for nitric oxide, superoxide, and peroxynitrite in pain

Pain is a multidimensional perception and is modified at distinct regions of the neuroaxis. During enhanced pain, neuroplastic changes occur in the spinal and supraspinal nociceptive modulating centers and may result in a hypersensitive state termed central sensitization, which is thought to contribute to chronic pain states. Central sensitization culminates in hyperexcitability of dorsal horn nociceptive neurons resulting in increased nociceptive transmission and pain perception. This state is associated with enhanced nociceptive signaling, spinal glutamate-mediated N-methyl-D: -aspartate receptor activation, neuroimmune activation, nitroxidative stress, and supraspinal descending facilitation. The nitroxidative species considered for their role in nociception and central sensitization include nitric oxide (NO), superoxide ([Formula: see text]), and peroxynitrite (ONOO(-)). Nitroxidative species are implicated during persistent but not normal nociceptive processing. This review examines the role of nitroxidative species in pain through a discussion of their contributions to central sensitization and the underlying mechanisms. Future directions for nitroxidative pain research are also addressed. As more selective pharmacologic agents are developed to target nitroxidative species, the exact role of nitroxidative species in pain states will be better characterized and should offer promising alternatives to available pain management options.

Nitric oxide and pain: 'Something old, something new'

Challenges have emerged following the revival of nitric oxide (NO) from 'something old', a simple gas derived from nitrogen and oxygen with a role in the early stages of evolution, into 'something new', an endogenously formed biological mediator regulating a wide variety of physiological functions. Although pain is a common sensation, it encompasses multiple neurobiologic components, of which NO is only one. In pain research, the study of NO is complicated by convoluted problems related mostly to the effects of NO, which are pro- or anti-nociceptive depending on the circumstances. This dual function reflects the multi-faceted roles of the NO molecule described in physiology. This review covers current information about NO and its implications in pain mechanisms. In addition, it follows the pain pathways, demonstrating the role of NO in peripheral nociceptive transmission as well in central sensitization. This knowledge may provide the scientific basis for developing new drugs that are indicated for different types of pain, drugs that may be related to the chemical links of NO. A comprehensive approach to understanding the effects of NO will help clinicians identify novel agents that combine the pharmacological profile of native drugs with a controllable manner of NO release. Inhibitors of NO synthesis may have analgesic effects and would be of interest for treating inflammatory and neuropathic pain. Unfortunately, only a few of these compounds have reached the stage of clinical pain trials.

"Listening" and "talking" to neurons: implications of immune activation for pain control and increasing the efficacy of opioids

It is recently become clear that activated immune cells and immune-like glial cells can dramatically alter neuronal function. By increasing neuronal excitability, these non-neuronal cells are now implicated in the creation and maintenance of pathological pain, such as occurs in response to peripheral nerve injury. Such effects are exerted at multiple sites along the pain pathway, including at peripheral nerves, dorsal root ganglia, and spinal cord. In addition, activated glial cells are now recognized as disrupting the pain suppressive effects of opioid drugs and contributing to opioid tolerance and opioid dependence/withdrawal. While this review focuses on regulation of pain and opioid actions, such immune-neuronal interactions are broad in their implications. Such changes in neuronal function would be expected to occur wherever immune-derived substances come in close contact with neurons.

Bridges between nervous and immune systems: their disconnection and clinical consequences

Nervous and immune systems are connected by several mutual links, thus constituting a diffuse functional network in the body. In particular, neurohormones, neuropeptides, and cytokines represent the major mediators of the so-called psychoneuroendocrinoimmune axis. In this review, special emphasis is placed on certain pathologies characterized by a disconnection of the existing bridges between nervous and immune systems. For instance, spinal cord injury (SCI) is a clinical condition in which loss of neurons and very poor axon growth represent the main features. The role played by infiltrating and resident immunocompetent cells is still debated in SCI. However, to enhance axon growth in SCI, current therapeutic attempts are based on the stimulation of the immune response within the central nervous system, thus triggering either cell-mediated or humoral immune responsiveness.

Neutrophils-derived peroxynitrite contributes to acute hyperalgesia and cell influx in zymosan arthritis

We investigated the contribution of neutrophils to joint hyperalgesia and peroxynitrite formation in zymosan arthritis. Rats received 1 mg zymosan intra-articular, and joint hyperalgesia was measured using the rat knee-joint articular incapacitation test. After 6 h, joint exudates were collected by aspiration for the assessment of cell influx, myeloperoxidase activity, and nitrite (as an index of nitric oxide formation) levels. Nitrotyrosine content, used as an index of peroxynitrite formation, was measured in joint exudates, using enzyme-linked immunosorbent assay. A group of rats was rendered neutropenic through the administration of a rabbit anti-rat neutrophil antibody (2 ml kg(-1), i.p.) 30 min before injection of 1 mg zymosan intra-articular. Other groups received uric acid (100 or 250 mg kg(-1), i.p.), the peroxynitrite scavenger, 30 min before 1 mg zymosan intra-articular. Controls received the vehicle. The significant inhibition of joint hyperalgesia in neutropenic animals was associated to significantly decreased cell influx, myeloperoxidase activity, nitric oxide, and nitrotyrosine levels in the joint exudates, as compared to naive rats. Uric acid administration inhibited both hyperalgesia and cell influx, as compared to controls. Neutrophils are involved in both nitric oxide and peroxynitrite formation in zymosan arthritis, thereby contributing to acute joint hyperalgesia. Scavenging of reactive nitrogen species (e.g. peroxynitrite) inhibits neutrophil migration and joint hyperalgesia in the acute phase of zymosan arthritis in rats.

Polymer-based biodegradable drug delivery systems in pain management

Pain is an unpleasant sensory experience commonly produced by damage to bodily tissues and it is one of the most significant public health problems, because 21.5% of the world population is estimated to suffer from pain. It results in a total loss of more than 165 billion US dollars each year in the United States alone. Pain reflects a mixture of various pathophysiologic, psychologic, and genetic contributions. When undertreated, pain usually results in serious immune and metabolic upset. Therefore, it requires wide understanding and intensive effort for a better management. Currently, pain control is limited by the modest efficiency of the used drugs, the serious side effects of these drugs, and the inefficacy of conventional drug administration. By the introduction of the technology of biodegradable controlled-release devices into clinical practice, pain control not only benefits from these novel methods for a better delivery of various drugs, but the side effects of the drugs are reduced because use of the devices improves patient compliance. Biodegradable controlled-release devices are polymer-based devices that are designed to deliver drugs locally in a predesigned manner. Recently, there was a high interest in developing these devices for the delivery of different drugs used for pain control. This paper first highlights the dimensions and basics of the problem of pain. Then, it presents an overview of the biodegradable polymers that are used in drug delivery systems and summarizes the studies carried out on these systems in the field of pain management. We refer to our experience in developing a device for multimodal drug delivery, including the use of nanotechnology. Future perspectives are also presented.

Immune and inflammatory mechanisms in neuropathic pain

Tissue damage, inflammation or injury of the nervous system may result in chronic neuropathic pain characterised by increased sensitivity to painful stimuli (hyperalgesia), the perception of innocuous stimuli as painful (allodynia) and spontaneous pain. Neuropathic pain has been described in about 1% of the US population, is often severely debilitating and largely resistant to treatment. Animal models of peripheral neuropathic pain are now available in which the mechanisms underlying hyperalgesia and allodynia due to nerve injury or nerve inflammation can be analysed. Recently, it has become clear that inflammatory and immune mechanisms both in the periphery and the central nervous system play an important role in neuropathic pain. Infiltration of inflammatory cells, as well as activation of resident immune cells in response to nervous system damage, leads to subsequent production and secretion of various inflammatory mediators. These mediators promote neuroimmune activation and can sensitise primary afferent neurones and contribute to pain hypersensitivity. Inflammatory cells such as mast cells, neutrophils, macrophages and T lymphocytes have all been implicated, as have immune-like glial cells such as microglia and astrocytes. In addition, the immune response plays an important role in demyelinating neuropathies such as multiple sclerosis (MS), in which pain is a common symptom, and an animal model of MS-related pain has recently been demonstrated. Here, we will briefly review some of the milestones in research that have led to an increased awareness of the contribution of immune and inflammatory systems to neuropathic pain and then review in more detail the role of immune cells and inflammatory mediators.

GW274150, a novel and highly selective inhibitor of the inducible isoform of nitric oxide synthase (iNOS), shows analgesic effects in rat models of inflammatory and neuropathic pain

Nitric oxide (NO), synthesised by different isoforms of nitric oxide synthase (NOS), has been linked with the development and maintenance of nociception. We studied the role of the inducible isoform, iNOS, in two different rat pain models with an inflammatory component. iNOS was immunohistochemically detected locally in the paw 6h after Freund's Complete Adjuvant (FCA) injection, showing a plateau at 24-72 h and falling slowly in the following weeks. This correlated with the late phase of the hypersensitivity to pain revealed in the behavioural tests. A highly selective iNOS inhibitor GW274150 (1-30 mg/kg orally, 24h after FCA) suppressed the accumulation of nitrite in the inflamed paw indicating substantial iNOS inhibition. At the same time it partially reversed FCA-induced hypersensitivity to pain and edema in a dose-dependent manner. After Chronic Constriction Injury (CCI) surgery to the sciatic nerve, iNOS presence was only detected locally in the region of the nerve (inflammatory cells). GW274150 (3-30 mg/kg orally, 21 days after surgery) also reversed significantly the CCI-associated hypersensitivity to pain. No iNOS was detectable in dorsal root ganglia, spinal cord or brain in either model. This study demonstrates a role for peripherally-expressed iNOS in pain conditions with an inflammatory component and the potential value of iNOS inhibitors in such conditions.

Aging, peripheral nerve injury and nociception: effects of the antioxidant 16-desmethyltirilazad

Peripheral neuropathies increase with aging, and reactive oxygen species contribute to the symptomatology of neuropathic pain disorders. In this study, we examined age-related differences in the therapeutic efficacy of pre- or post-treatments of the amino-steroidal antioxidant 16-desmethyltirilazad in delaying the onset and/or limiting the duration of tactile-evoked allodynia following the induction of peripheral mononeuropathies in rats. Two different models of nerve injury were utilized to induce allodynia in young and aged rats: (1) the chronic constriction injury (CCI) model of Bennett and Xie [Bennett GJ, Xie Y-K. A peripheral mononeuropathy in rat that produces disorders of pain sensation like those seen in man. Pain 1988;33:87-107]; (2) the partial sciatic nerve ligation (PSNL) model of Seltzer et al. [Seltzer Z, Dubner R, Shir YA. Novel behavioral model of neuropathic pain disorders produced in rats by partial sciatic nerve injury. Pain 1990;43:205-18]. Calibrated von Frey filaments were used to examine changes in paw withdrawal threshold values. The results demonstrated that pre-treating young and aged rats with 16-desmethyltirilazad prior to the induction of peripheral mononeuropathies prevented the onset of neuropathic pain. However, once post-operative tactile allodynia was established, post-treatment therapy was ineffective at reversing the symptoms. These findings support the mediatory role of reactive oxygen species in neuropathic pain disorders, and suggest that the antiallodynic efficacy of antioxidant intervention is dependent on the time course of administration.

Immune regulation of central nervous system functions: from sickness responses to pathological pain

Classically, the central nervous system (CNS) and the immune system are thought to operate independently of each other. This simplistic view has been corrected in recent years, first with the recognition that the brain dynamically modulates the immune system, and later with the reverse; that is, that the immune system modulates the CNS as well. The evidence that the immune system regulates CNS functions is first reviewed. This immune-to-brain communication pathway triggers the production of a constellation of CNS-mediated phenomena, collectively referred to as 'sickness responses'. These sickness responses are created by immune-to-brain signals activating CNS glia to release glial proinflammatory cytokines. The most recently recognized member of this constellation of changes is enhanced pain responsivity. The hypothesis is then developed that pathological, chronic pain may result from 'tapping into' this ancient survival-oriented circuitry, including the activation of immune and glial cells and the release of immune/glial proinflammatory cytokines. This can occur at the level of peripheral nerves, dorsal root ganglia, spinal cord, and likely at higher brain areas. The implications of this model for human chronic pain syndromes and clinical resolution of these chronic pain states are then discussed.

Peri-sciatic proinflammatory cytokines, reactive oxygen species, and complement induce mirror-image neuropathic pain in rats

In inflammatory neuropathy, immune activation near intact peripheral nerves induces mechanical allodynia. The identity of the peripheral immune product(s) that lead to these changes in pain behavior is unknown. The present series of studies utilized the sciatic inflammatory neuropathy (SIN) model to examine this question. Here, inflammatory neuropathy is created by injecting an immune activator (zymosan) around one sciatic nerve via an indwelling catheter. Our prior studies demonstrated that peri-sciatic zymosan activated macrophages and neutrophils to release proinflammatory cytokines and reactive oxygen species (ROS). In addition, zymosan is a classical activator of the complement cascade. Thus the present series of experiments examined whether any of these inflammatory mediators are involved in the initial induction of SIN-induced ipsilateral or bilateral allodynias. Peri-sciatic injection of selective inhibitors/antagonists revealed that a number of immune products are early mediators of the resultant allodynias, including proinflammatory cytokines (tumor necrosis factor, interleukin-1, and interleukin-6), ROS, and complement. Thus these immune-derived substances can markedly alter sensory nerve function at mid-axon.

Evidence that exogenous and endogenous fractalkine can induce spinal nociceptive facilitation in rats

Recent evidence suggests that spinal cord glia can contribute to enhanced nociceptive responses. However, the signals that cause glial activation are unknown. Fractalkine (CX3C ligand-1; CX3CL1) is a unique chemokine expressed on the extracellular surface of spinal neurons and spinal sensory afferents. In the dorsal spinal cord, fractalkine receptors are primarily expressed by microglia. As fractalkine can be released from neurons upon strong activation, it has previously been suggested to be a neuron-to-glial signal that induces glial activation. The present series of experiments provide an initial investigation of the spinal pain modulatory effects of fractalkine. Intrathecal fractalkine produced dose-dependent mechanical allodynia and thermal hyperalgesia. In addition, a single injection of fractalkine receptor antagonist (neutralizing antibody against rat CX3C receptor-1; CX3CR1) delayed the development of mechanical allodynia and/or thermal hyperalgesia in two neuropathic pain models: chronic constriction injury (CCI) and sciatic inflammatory neuropathy. Intriguingly, anti-CX3CR1 reduced nociceptive responses when administered 5-7 days after CCI, suggesting that prolonged release of fractalkine may contribute to the maintenance of neuropathic pain. Taken together, these initial investigations of spinal fractalkine effects suggest that exogenous and endogenous fractalkine are involved in spinal sensitization, including that induced by peripheral neuropathy.

Snake venom phospholipase A2s (Asp49 and Lys49) induce mechanical allodynia upon peri-sciatic administration: involvement of spinal cord glia, proinflammatory cytokines and nitric oxide

Snakebites constitute a serious public health problem in Central and South America, where species of the lancehead pit vipers (genus Bothrops) cause the majority of accidents. Bothrops envenomations are very painful, and this effect is not neutralized by antivenom treatment. Two variants of secretory phospholipases A2 (sPLA2), corresponding to Asp49 and Lys49 PLA2s, have been isolated from Bothrops asper venom. These sPLA2s induce hyperalgesia in rats following subcutaneous injection. However, venom in natural Bothrops bites is frequently delivered intramuscularly, thereby potentially reaching peripheral nerve bundles. Thus, the present series of experiments tested whether these sPLA2s could exert pain-enhancing effects following administration around healthy sciatic nerve. Both were found to produce mechanical allodynia ipsilateral to the injection site; no thermal hyperalgesia was observed. As no prior study has examined potential spinal mechanisms underlying sPLA2 actions, a series of anatomical and pharmacological studies were performed. These demonstrated that both sPLA2s produce activation of dorsal horn astrocytes and microglia that is more prominent ipsilateral to the site of injection. As proinflammatory cytokines and nitric oxide have each been previously implicated in spinally mediated pain facilitation, the effect of pharmacological blockade of these substances was tested. The results demonstrate that mechanical allodynia induced by both sPLA2s is blocked by interleukin-1 receptor antagonist, anti-rat interleukin-6 neutralizing antibody, the anti-inflammatory cytokine interleukin-10, and a nitric oxide synthesis inhibitor (L-NAME). As a variety of immune cells also produce and release sPLA2s during inflammatory states, the data may have general implications for the understanding of inflammatory pain.

Inflammation and hyperalgesia induced by nerve injury in the rat: a key role of mast cells

Inflammatory cells and their mediators are known to contribute to neuropathic pain following nerve injury. Mast cells play a key role in non-neural models of inflammation and we propose that mast cells and their mediators (in particular histamine) are important in the development of neuropathic pain. In rats, where the sciatic nerve was partially ligated, we showed that stabilisation of mast cells with sodium cromoglycate reduced the recruitment of neutrophils and monocytes to the injured nerve and suppressed the development of hyperalgesia. Treatment with histamine receptor antagonists suppressed the development of hyperalgesia following nerve injury and alleviated hyperalgesia once it was established. These results suggest that mast cell mediators such as histamine released within hours of nerve injury contribute to the recruitment of leukocytes and the development of hyperalgesia.

Beyond neurons: evidence that immune and glial cells contribute to pathological pain states

Chronic pain can occur after peripheral nerve injury, infection, or inflammation. Under such neuropathic pain conditions, sensory processing in the affected body region becomes grossly abnormal. Despite decades of research, currently available drugs largely fail to control such pain. This review explores the possibility that the reason for this failure lies in the fact that such drugs were designed to target neurons rather than immune or glial cells. It describes how immune cells are a natural and inextricable part of skin, peripheral nerves, dorsal root ganglia, and spinal cord. It then examines how immune and glial activation may participate in the etiology and symptomatology of diverse pathological pain states in both humans and laboratory animals. Of the variety of substances released by activated immune and glial cells, proinflammatory cytokines (tumor necrosis factor, interleukin-1, interleukin-6) appear to be of special importance in the creation of peripheral nerve and neuronal hyperexcitability. Although this review focuses on immune modulation of pain, the implications are pervasive. Indeed, all nerves and neurons regardless of modality or function are likely affected by immune and glial activation in the ways described for pain.

Sciatic inflammatory neuritis (SIN): behavioral allodynia is paralleled by peri-sciatic proinflammatory cytokine and superoxide production

We have recently developed a model of sciatic inflammatory neuritis (SIN) to assess how immune activation near peripheral nerves influences somatosensory processing. Administration of zymosan (yeast cell walls) around a single sciatic nerve produces dose-dependent low-threshold mechanical allodynia without thermal hyperalgesia. Low (4 microg) doses produce both territorial and extraterritorial allodynia restricted to the injected hindleg. In contrast, higher (40 microg) doses produce territorial and extraterritorial allodynias of both hindlegs, an effect not accounted for by systemic spread of the zymosan. The aim of these experiments was to determine whether these behavioral allodynias were correlated with immunological and/or anatomical changes in or around the sciatic nerve. These experiments reveal that zymosan-induced bilateral allodynia was associated with the following: (a) increased release of both interleukin-1beta and tumor necrosis factor-alpha from peri-sciatic immune cells; (b) increased release of reactive oxygen species from perisciatic immune cells; (c) no change in circulating levels of proinflammatory cytokine; (d) no apparent zymosan-induced influx of immune cells into the sciatic nerve from the endoneurial blood vessels; (e) mild edema of the sciatic, which was predominantly restricted to superficial regions closest to the peri-sciatic immune cells; and (f) no anatomic evidence of changes in either the ipsilateral saphenous nerve or contralateral sciatic nerve that could account for the appearance of extraterritorial or contralateral ("mirror") allodynia, respectively. No reliable differences were found when the low-dose zymosan was compared with vehicle controls. Taken together, these data suggest that substances released by peri-sciatic immune cells may induce changes in the sciatic nerve, leading to the appearance of bilateral allodynia.

Tyrosine nitration: localisation, quantification, consequences for protein function and signal transduction

The nitration of free tyrosine or protein tyrosine residues generates 3-nitrotyrosine the detection of which has been utilised as a footprint for the in vivo formation of peroxynitrite and other reactive nitrogen species. The detection of 3-nitrotyrosine by analytical and immunological techniques has established that tyrosine nitration occurs under physiological conditions and levels increase in most disease states. This review provides an updated, comprehensive and detailed summary of the tissue, cellular and specific protein localisation of 3-nitrotyrosine and its quantification. The potential consequences of nitration to protein function and the pathogenesis of disease are also examined together with the possible effects of protein nitration on signal transduction pathways and on the metabolism of proteins.

Gout: diagnosis, pathogenesis, and clinical manifestations

Gout is a common form of arthritis, in which many of the risk factors, pathogenetic mechanisms, and clinical features have been recognized for years. Nevertheless, new information has become available regarding the normal physiologic role of uric acid as an antioxidant, and greater insight has been obtained regarding the inflammatory process in acute gout. New studies have improved our understanding of the role of genetic and environmental factors responsible for hyperuricemia, and we know more about the significance of the association of hyperuricemia with other diseases. Clinically, rare complications and disease manifestations in new populations continue to be discussed, and diagnostic methods continue to be refined.

Hyperalgesia due to nerve injury: role of neutrophils

The hypothesis that the early inflammatory cell, the neutrophil, contributes to the hyperalgesia resulting from peripheral nerve injury was tested in rats in which the sciatic nerve was partially transected on one side. The extent and time-course of neutrophilic infiltration of the sciatic nerve and innervated paw skin after partial nerve damage was characterized using immunocytochemistry. The number of endoneurial neutrophils was significantly elevated in sections of operated nerve compared to sections of sham-operated nerve for the entire period studied, i.e. up to seven days post-surgery. This considerable elevation in endoneurial neutrophil numbers was only observed at the site of nerve injury. Depletion of circulating neutrophils at the time of nerve injury significantly attenuated the induction of hyperalgesia. However, depletion of circulating neutrophils at day 8 post-injury did not alleviate hyperalgesia after its normal induction. It is concluded that endoneurial accumulation of neutrophils at the site of peripheral nerve injury is important in the early genesis of the resultant hyperalgesia. The findings support the notion that a neuroimmune interaction occurs as a result of peripheral nerve injury and is important in the subsequent development of neuropathic pain.