Reduced hyperalgesia in nerve-injured WLD mice: relationship to nerve fiber phagocytosis, axonal degeneration, and regeneration in normal mice

Neurobiology of pain

The neurobiology of pain had a notable interest in research focused on the study of neuronal plasticity development, nociceptors, molecular identity, signaling mechanism, ionic channels involved in the generation, modulation and propagation of action potential in all type of excitable cells. All the findings open the possibility for developing new therapeutic treatment. Nociceptive/inflammatory pain and neuropathic pain represent two different kinds of persistent chronic pain. We have reviewed the different mechanism suggested for the maintenance of pain, like descending nociceptive mechanism and their changes after tissue damage, including suppression and facilitation of defence behavior during pain. The role of these changes in inducing NMDA and AMPA receptors gene expression, after prolonged inflammation is emphasized by several authors. Furthermore, a relation between a persistent pain and amygdale has been shown. Molecular biology is the new frontier in the study of neurobiology of pain. Since the entire genome has been studied, we will able to find new genes involved in specific condition such as pain, because an altered gene expression can regulate neuronal activity after inflammation or tissue damage.

Reduced nerve injury-induced neuropathic pain in kinin B1 receptor knock-out mice

Injury to peripheral nerves often results in a persistent neuropathic pain condition that is characterized by spontaneous pain, allodynia, and hyperalgesia. Nerve injury is accompanied by a local inflammatory reaction in which nerve-associated and immune cells release several pronociceptive mediators. Kinin B1 receptors are rarely expressed in nontraumatized tissues, but they can be expressed after tissue injury. Because B1 receptors mediate chronic inflammatory painful processes, we studied their participation in neuropathic pain using receptor gene-deleted mice. In the absence of neuropathy, we found no difference in the paw-withdrawal responses to thermal or mechanical stimulation between B1 receptor knock-out mice and 129/J wild-type mice. Partial ligation of the sciatic nerve in the wild-type mouse produced a profound and long-lasting decrease in thermal and mechanical thresholds in the paw ipsilateral to nerve lesion. Threshold changed neither in the sham-operated animals nor in the paw contralateral to lesion. Ablation of the gene for the B1 receptor resulted in a significant reduction in early stages of mechanical allodynia and thermal hyperalgesia. Furthermore, systemic treatment with the B1 selective receptor antagonist des-Arg9-[Leu8]-bradykinin reduced the established mechanical allodynia observed 7-28 d after nerve lesion in wild-type mice. Partial sciatic nerve ligation induced an upregulation in B1 receptor mRNA in ipsilateral paw, sciatic nerve, and spinal cord of wild-type mice. Together, kinin B1 receptor activation seems to be essential to neuropathic pain development, suggesting that an oral-selective B1 receptor antagonist might have therapeutic potential in the management of chronic pain.

Antisense knock down of TRPA1, but not TRPM8, alleviates cold hyperalgesia after spinal nerve ligation in rats

Patients with neuropathic pain frequently experience hypersensitivity to cold stimulation. However, the underlying mechanisms of this enhanced sensitivity to cold are not well understood. After partial nerve injury, the transient receptor potential ion channel TRPV1 increases in the intact small dorsal root ganglion (DRG) neurons in several neuropathic pain models. In the present study, we precisely examined the incidence of cold hyperalgesia and the changes of TRPA1 and TRPM8 expression in the L4 and L5 DRG following L5 spinal nerve ligation (SNL), because it is likely that the activation of two distinct populations of TRPA1- and TRPM8-expressing small neurons underlie the sensation of cold. We first confirmed that L5 SNL rats developed cold hyperalgesia for more than 14 days after surgery. In the nearby uninjured L4 DRG, TRPA1 mRNA expression increased in trkA-expressing small-to-medium diameter neurons from the 1st to 14th day after the L5 SNL. This upregulation corresponded well with the development and maintenance of nerve injury-induced cold hyperalgesia of the hind paw. In contrast, there was no change in the expression of the TRPM8 mRNA/protein in the L4 DRG throughout the 2-week time course of the experiment. In the injured L5 DRG, on the other hand, both TRPA1 and TRPM8 expression decreased over 2 weeks after ligation. Furthermore, intrathecal administration of TRPA1, but not TRPM8, antisense oligodeoxynucleotide suppressed the L5 SNL-induced cold hyperalgesia. Our data suggest that increased TRPA1 in uninjured primary afferent neurons may contribute to the exaggerated response to cold observed in the neuropathic pain model.

The role of neuroinflammation in neuropathic pain: mechanisms and therapeutic targets

Neuroinflammation is a proinflammatory cytokine-mediated process that can be provoked by systemic tissue injury but it is most often associated with direct injury to the nervous system. It involves neural-immune interactions that activate immune cells, glial cells and neurons and can lead to the debilitating pain state known as neuropathic pain. It occurs most commonly with injury to peripheral nerves and involves axonal injury with Wallerian degeneration mediated by hematogenous macrophages. Therapy is problematic but new trials with anti-cytokine agents, cytokine receptor antibodies, cytokine-signaling inhibitors, and glial and neuron stabilizers provide hope for future success in treating neuropathic pain.

BDNF in sensory neurons and chronic pain

Neurotrophic factors, which support neuronal survival and growth during development of the nervous system, have been shown to play significant roles in the transmission of physiologic and pathologic pain. Brain-derived neurotrophic factor (BDNF), synthesized in the primary sensory neurons, is anterogradely transported to the central terminals of the primary afferents in the spinal dorsal horn, where it is involved in the modulation of painful stimuli. In models of inflammatory and neuropathic pain, BDNF synthesis is greatly increased in different populations of dorsal root ganglion (DRG) neurons. Furthermore, it is now known that the activation of mitogen-activated protein kinases occurs in these sensory neurons and contributes to persistent inflammatory and neuropathic pain by regulating BDNF expression. The recent discovery that BDNF upregulation in the DRG and spinal cord contributes to chronic pain hypersensitivity indicates that blocking BDNF in sensory neurons could provide a fruitful strategy for the development of novel analgesics.

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.

The effect of site and type of nerve injury on the expression of brain-derived neurotrophic factor in the dorsal root ganglion and on neuropathic pain behavior

A number of rat neuropathy models have been developed to simulate human neuropathic pain conditions, such as spontaneous pain, hyperalgesia, and allodynia. In the present study, to determine the relative importance of injury site (proximal or distal to the primary afferent neurons) and injury type (motor or sensory), we examined pain-related behaviors and changes of brain-derived neurotrophic factor expression in the dorsal root ganglion in sham-operated rats, and in the L5 dorsal rhizotomy, L5 ventral rhizotomy, L5 dorsal rhizotomy+ventral rhizotomy, and L5 spinal nerve transection models. L5 ventral rhizotomy and spinal nerve transection produced not only mechanical and heat hypersensitivity, but also an increase in brain-derived neurotrophic factor mRNA/protein in the L5 dorsal root ganglion at 7 days after surgery. In contrast, rats in the L5 dorsal rhizotomy and dorsal rhizotomy+ventral rhizotomy groups did not show both pain behaviors at 7 days after surgery, despite brain-derived neurotrophic factor upregulation in medium- and large-size neurons in the L5 dorsal root ganglion. On the other hand, L5 spinal nerve transection, but not dorsal rhizotomy, dorsal rhizotomy+ventral rhizotomy or ventral rhizotomy, increased the expression of brain-derived neurotrophic factor in the L4 dorsal root ganglion at 7 days after surgery. Taken together, these findings suggest that the upregulation of brain-derived neurotrophic factor expression in the L4 and L5 dorsal root ganglion neurons may be, at least in part, involved in the pathophysiological mechanisms of neuropathic pain and that the selective nerve root injury models may be useful for studying the underlying mechanisms of chronic pain after nerve injury.

TNFalpha-induced MMP-9 promotes macrophage recruitment into injured peripheral nerve

Matrix metalloproteinase-9 (MMP-9) is an extracellular protease that is induced hours after injury to peripheral nerve. This study shows that MMP-9 gene deletion and neutralization with MMP-9 antibody reduce macrophage content in injured wild-type nerves. In mice with delayed Wallerian degeneration (WldS), MMP-9 and tumor necrosis factor alpha (TNFalpha) decline in association with the reduced macrophage recruitment to injured nerve that characterizes this strain of mice. We further determined that TNFalpha acts as an MMP-9 inducer by establishing increased MMP-9 levels after TNFalpha injection in rat sciatic nerve in vivo and primary Schwann cells in vitro. We found reduced MMP-9 expression in crushed TNFalpha knockout nerves that was rescued with exogenous TNFalpha. Finally, local application of MMP-9 on TNFalpha-/- nerves increased macrophage recruitment to the lesion. These data suggest that TNFalpha lies upstream of MMP-9 in the pathway of macrophage recruitment to injured peripheral nerve.

Role of the immune system in chronic pain

During the past two decades, an important focus of pain research has been the study of chronic pain mechanisms, particularly the processes that lead to the abnormal sensitivity - spontaneous pain and hyperalgesia - that is associated with these states. For some time it has been recognized that inflammatory mediators released from immune cells can contribute to these persistent pain states. However, it has only recently become clear that immune cell products might have a crucial role not just in inflammatory pain, but also in neuropathic pain caused by damage to peripheral nerves or to the CNS.

Role of mitogen-activated protein kinase activation in injured and intact primary afferent neurons for mechanical and heat hypersensitivity after spinal nerve ligation

To investigate whether activation of mitogen-activated protein kinase (MAPK) in damaged and/or undamaged primary afferents participates in neuropathic pain after partial nerve injury, we examined the phosphorylation of extracellular signal-regulated protein kinase (ERK), p38 MAPK, and c-Jun N-terminal kinase (JNK) in the L4 and L5 dorsal root ganglion (DRG) in the L5 spinal nerve ligation (SNL) model. We first confirmed, using activating transcription factor 3 and neuropeptide Y immunoreactivity, that virtually all L4 DRG neurons are spared from axotomy in this model. In the injured L5 DRG, the L5 SNL induced the activation of ERK, p38, and JNK in different populations of DRG neurons. In contrast, in the uninjured L4 DRG, the L5 SNL induced only p38 activation in tyrosine kinase A-expressing small- to medium-diameter neurons. Intrathecal ERK, p38, and JNK inhibitor infusions reversed SNL-induced mechanical allodynia, whereas only p38 inhibitor application attenuated SNL-induced thermal hyperalgesia. Furthermore, the L5 dorsal rhizotomy did not prevent SNL-induced thermal hyperalgesia. We therefore hypothesized that p38 activation in the uninjured L4 DRG might be involved in the development of heat hypersensitivity in the L5 SNL model. In fact, the treatment of the p38 inhibitor and also anti-nerve growth factor reduced SNL-induced upregulation of brain-derived neurotrophic factor and transient receptor potential vanilloid type 1 expression in the L4 DRG. Together, our results demonstrate that the L5 SNL induces differential activation of MAPK in injured and uninjured DRG neurons and, furthermore, that MAPK activation in the primary afferents may participate in generating pain hypersensitivity after partial nerve injury.

Immune and glial cell factors as pain mediators and modulators

A decade ago the attention of pain scientists was focused on a small number of molecules such as prostaglandin and bradykinin as peripheral pain mediators or modulators. These factors were known to be produced by tissue damage or inflammation, and considered responsible for the activation and sensitization of peripheral pain signaling sensory neurons. A small number of molecules were also identified as central pain mediators, most notably glutamate and substance P released from central nociceptive nerve terminals, and, starting at that time, appreciation that nitric oxide might be produced by dorsal horn neurons and act as a diffusible transmitter to increase excitability of central pain circuits. During the last decade evidence has emerged for many novel pain mediators. The old ones have not disappeared, although their roles have been redefined in some cases. Prostaglandin E2 (PGE2), for instance, is now recognized as playing a prominent role in CNS as well as peripheral tissues. The newly identified mediators include a variety of factors produced and released from nonneuronal cells-predominantly immune and glial cells. The evidence is now growing apace that these are important mediators of persistent pain states and can act at a number of loci. Here we review the actions of several of these factors-the pro-inflammatory cytokines, some chemokines, and some neurotrophic factors, which, in addition to their traditionally recognized roles, are all capable of changing the response properties of peripheral and central pain signaling neurons. We review these actions, first in periphery, where a substantial literature has accumulated, and then in spinal cord, where the role of factors from nonneuronal cells has only recently been identified as of considerable importance.

Anti-NGF therapy profoundly reduces bone cancer pain and the accompanying increase in markers of peripheral and central sensitization

Bone cancer pain can be difficult to control, as it appears to be driven simultaneously by inflammatory, neuropathic and tumorigenic mechanisms. As nerve growth factor (NGF) has been shown to modulate inflammatory and neuropathic pain states, we focused on a novel NGF sequestering antibody and demonstrated that two administrations of this therapy in a mouse model of bone cancer pain produces a profound reduction in both ongoing and movement-evoked bone cancer pain-related behaviors that was greater than that achieved with acute administration of 10 or 30 mg/kg of morphine. This therapy also reduced several neurochemical changes associated with peripheral and central sensitization in the dorsal root ganglion and spinal cord, whereas the therapy did not influence disease progression or markers of sensory or sympathetic innervation in the skin or bone. Mechanistically, the great majority of sensory fibers that innervate the bone are CGRP/TrkA expressing fibers, and if the sensitization and activation of these fibers is blocked by anti-NGF therapy there would not be another population of nociceptors, such as the non-peptidergic IB4/RET-IR nerve fibers, to take their place in signaling nociceptive events.

Tumor-induced injury of primary afferent sensory nerve fibers in bone cancer pain

Bone is the most common site of chronic pain in patients with metastatic cancer. What remains unclear are the mechanisms that generate this pain and why bone cancer pain can be so severe and refractory to treatment with opioids. Here we show that following injection and confinement of NCTC 2472 osteolytic tumor cells within the mouse femur, tumor cells sensitize and injure the unmyelinated and myelinated sensory fibers that innervate the marrow and mineralized bone. This tumor-induced injury of sensory nerve fibers is accompanied by an increase in ongoing and movement-evoked pain behaviors, an upregulation of activating transcription factor 3 (ATF3) and galanin by sensory neurons that innervate the tumor-bearing femur, upregulation of glial fibrillary acidic protein (GFAP) and hypertrophy of satellite cells surrounding sensory neuron cell bodies within the ipsilateral dorsal root ganglia (DRG), and macrophage infiltration of the DRG ipsilateral to the tumor-bearing femur. Similar neurochemical changes have been described following peripheral nerve injury and in other non-cancerous neuropathic pain states. Chronic treatment with gabapentin did not influence tumor growth, tumor-induced bone destruction or the tumor-induced neurochemical reorganization that occurs in sensory neurons or the spinal cord, but it did attenuate both ongoing and movement-evoked bone cancer-related pain behaviors. These results suggest that even when the tumor is confined within the bone, a component of bone cancer pain is due to tumor-induced injury to primary afferent nerve fibers that innervate the tumor-bearing bone. Tumor-derived, inflammatory, and neuropathic mechanisms may therefore be simultaneously driving this chronic pain state.

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.

Differential activation of MAPK in injured and uninjured DRG neurons following chronic constriction injury of the sciatic nerve in rats

To investigate the intracellular signal transduction pathways involved in the pathophysiological mechanisms of neuropathic pain after partial nerve injury, we examined the activation of extracellular signal-regulated protein kinase (ERK) and p38 mitogen-activated protein kinase (MAPK) in the dorsal root ganglion (DRG) in the chronic constriction injury (CCI) model. The CCI induced an increase in the phosphorylation of ERK in predominantly injured medium-sized and large-sized DRG neurons and in satellite glial cells. Treatment with the MAPK kinase 1/2 inhibitor, U0126, suppressed CCI-induced mechanical allodynia and partially reversed the increase in neuropeptide Y (NPY) expression in damaged DRG neurons. In contrast, the CCI induced the activation of p38, mainly in uninjured small-to-medium-diameter DRG neurons and in satellite glial cells. The p38 inhibitor, SB203580, reversed the CCI-induced heat hyperalgesia and also the increase in brain-derived neurotrophic factor (BDNF) expression in intact DRG neurons. On the other hand, the nerve growth factor (NGF)-induced increase in BDNF expression in small-to-medium-diameter neurons was reversed by SB203580, whereas the anti-NGF-induced increase in NPY in medium-sized and large-sized neurons was partially blocked by U0126. Taken together, our results demonstrate that the activation of ERK and p38 and also the changes in NPY and BDNF expression may occur in different populations of DRG neurons after CCI, partially through alterations in the target-derived NGF. These changes in injured and intact primary afferents are likely to have a substantial role in pathological states, and MAPK pathways in nociceptors may be potential targets for the development of novel analgesics.

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.

Contribution of degeneration of motor and sensory fibers to pain behavior and the changes in neurotrophic factors in rat dorsal root ganglion

To elucidate the role of the degeneration of motor and sensory fibers in neuropathic pain, we examined the pain-related behaviors and the changes of brain-derived neurotrophic factor (BDNF) in the L4/5 dorsal root ganglion (DRG) and the spinal cord after L5 ventral rhizotomy. L5 ventral rhizotomy, producing a selective lesion of motor fibers, produced thermal hyperalgesia and increased BDNF expression in tyrosine kinase A-containing small- and medium-sized neurons in the L5 DRG and their central terminations within the spinal cord, but not in the L4 DRG. Furthermore, L5 ventral rhizotomy up-regulated nerve growth factor (NGF) protein in small to medium diameter neurons in the L5 DRG and also in ED-1-positive cells in the L5 spinal nerve, suggesting that NGF synthesized in the degenerative fibers is transported to the L5 DRG and increases BDNF synthesis. On the other hand, L5 ganglionectomy, producing a selective lesion of sensory fibers, produced heat hypersensitivity and an increase in BDNF and NGF in the L4 DRG. These data indicate that degeneration of L5 sensory fibers distal to the DRG, but not motor fibers, might influence the neighboring L4 nerve fibers and induce neurotrophin changes in the L4 DRG. We suggest that these changes of neurotrophins in the intact primary afferents of neighboring nerves may be one of many complex mechanisms, which can explain the abnormal pain behaviors after nerve injury. The ventral rhizotomy and ganglionectomy models may be useful to investigate the pathophysiological mechanisms of neuropathic pain after Wallerian degeneration in motor or sensory or mixed nerve.

T lymphocytes play a role in neuropathic pain following peripheral nerve injury in rats

A catastrophic consequence of peripheral nerve injury is the development of abnormal, chronic neuropathic pain. The inflammatory response at the injury site is believed to contribute to the generation and maintenance of such persistent pain. However, the physiological significance and potential contribution of T cells to neuropathic pain remains unclear. Here we show that T cells infiltrate injured sciatic nerves following chronic constriction injury (CCI), but not uninjured nerves. Congenitally athymic nude rats, which lack mature T cells, developed a significantly reduced mechanical allodynia and thermal hyperalgesia following CCI, compared with their heterozygous littermates. To understand further the role played by different T-cell subsets, we generated polarized populations of type 1 and type 2 T cells, with different cytokine secretion profiles, from spleens of sciatic nerve-injured heterozygous rats. Passive transfer of type 1 T cells, which produce proinflammatory cytokines, into nude rats enhanced the recipients' pain hypersensitivity to a level similar to that of heterozygous donor rats. In contrast, passive transfer of polarized type 2 T cells, which produce anti-inflammatory cytokines, into heterozygous rats modestly though significantly attenuated their pain hypersensitivity. Thus, injection of type 1 and type 2 T-cell subsets produces opposing effects on neuropathic pain. These findings suggest the modulation of the T-cell immune response as a potential target for the treatment of neuropathic pain.

Impaired neuropathic pain responses in mice lacking the chemokine receptor CCR2

Mice lacking the chemokine receptor chemotactic cytokine receptor 2 (CCR2) have a marked attenuation of monocyte recruitment in response to various inflammatory stimuli and a reduction of inflammatory lesions in models of demyelinating disease. In the present study, we compared nociceptive responses in inflammatory and neuropathic models of pain in CCR2 knockout and wild-type mice. In acute pain tests, responses were equivalent in CCR2 knockout and wild-type mice. In models of inflammatory pain, CCR2 knockout mice showed a 70% reduction in phase 2 of the intraplantar formalin-evoked pain response but only a modest (20-30%) and nonsignificant reduction of mechanical allodynia after intraplantar Freund's adjuvant (CFA). In a model of neuropathic pain, the development of mechanical allodynia was totally abrogated in CCR2 knockout mice. CFA administration induced marked up-regulation of CCR2 mRNA in the skin and a moderate increase in the sciatic nerve and dorsal root ganglia (DRG). In response to nerve ligation, persistent and marked up-regulation of CCR2 mRNA was evident in the nerve and DRG. Disruption of Schwann cells in response to nerve lesion resulted in infiltration of CCR2-positive monocytes/macrophages not only to the neuroma but also to the DRG. Chronic pain also resulted in the appearance of activated CCR2-positive microglia in the spinal cord. Collectively, these data suggest that the recruitment and activation of macrophages and microglia peripherally and in neural tissue may contribute to both inflammatory and neuropathic pain states. Accordingly, blockade of the CCR2 receptor may provide a novel therapeutic modality for the treatment of chronic pain.

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.

Degeneration of myelinated efferent fibers induces spontaneous activity in uninjured C-fiber afferents

We demonstrated recently that uninjured C-fiber nociceptors in the L4 spinal nerve develop spontaneous activity after transection of the L5 spinal nerve. We postulated that Wallerian degeneration leads to an alteration in the properties of the neighboring, uninjured afferents from adjacent spinal nerves. To explore the role of degeneration of myelinated versus unmyelinated fibers, we investigated the effects of an L5 ventral rhizotomy in rat. This lesion leads to degeneration predominantly in myelinated fibers. Mechanical paw-withdrawal thresholds were assessed with von Frey hairs, and teased-fiber techniques were used to record from single C-fiber afferents in the L4 spinal nerve. Behavioral and electrophysiological data were collected in a blinded manner. Seven days after surgery, a marked decrease in withdrawal thresholds was observed after the ventral rhizotomy but not after the sham operation. Single fiber recordings revealed low-frequency spontaneous activity in 25% of the C-fiber afferents 8-10 d after the lesion compared with only 11% after sham operation. Paw-withdrawal thresholds were inversely correlated with the incidence of spontaneous activity in high-threshold C-fiber afferents. In normal animals, low-frequency electrocutaneous stimulation at C-fiber, but not A-fiber, strength produced behavioral signs of secondary mechanical hyperalgesia on the paw. These results suggest that degeneration in myelinated efferent fibers is sufficient to induce spontaneous activity in C-fiber afferents and behavioral signs of mechanical hyperalgesia. Ectopic spontaneous activity from injured afferents was not required for the development of the neuropathic pain behavior. These results provide additional evidence for a role of Wallerian degeneration in neuropathic pain.

Cellular pathology changes in rat skin following intradermal injection of nerve growth factor: neutrophil-dependent and -independent events

Nerve growth factor (NGF) regulates the survival and development of specific populations of neurones and is involved in wound healing. A further area of study relating to the role of neurotrophins in the mature animal has concerned the possibility that NGF may be a pivotal mediator of inflammation and pain. It has previously been shown that injection of intradermal NGF can result in a neutrophil-dependent hyperalgesia in the rat. The purpose of the present study was to examine the pathological consequence of NGF injected intradermally into mature rat skin and to examine further the role of neutrophils. Standard histopathology techniques (H & E) were employed to determine inflammatory cell counts. Circulating neutrophils were depleted using an anti-rat neutrophil antiserum and results were compared to treatment with vehicle controls. Saline-pretreated rats exhibited normal circulating neutrophil numbers and the dorsal skin showed a significant increase of neutrophil and macrophages at 3 and 5 h and lymphocytes at 5 h after NGF treatment. By comparison, skin sites from neutrophil-depleted rats did not demonstrate a significant increase in neutrophil and macrophage accumulation after NGF administration. All NGF-treated sites, independent of pretreatment, demonstrated abnormal muscle fibre morphology and proliferation of the muscle sarcolemmal nuclei after NGF injection, indicative of tissue injury. In addition, oedema and some fibroplasia were also noted. Furthermore, fibrin production was increased at 3 and 5 h after NGF administration. It is suggested that NGF has a damaging effect on rat muscle which is independent of accumulating neutrophil and other inflammatory cells. In conclusion, the findings indicate a link between NGF-induced neutrophil and macrophage accumulation, as the increase in dermal macrophages was not observed in neutrophil-depleted rats. The results also suggest that NGF can have a profound effect on rat muscle and that this effect may be related to muscle regeneration.

Effect of lumbar 5 ventral root transection on pain behaviors: a novel rat model for neuropathic pain without axotomy of primary sensory neurons

A peripheral nerve injury often causes neuropathic pain but the underlying mechanisms remain obscure. Several established animal models of peripheral neuropathic pain have greatly advanced our understanding of the diverse mechanisms of neuropathic pain. A common feature of these models is primary sensory neuron injury and the commingle of intact axons with degenerating axons in the sciatic nerve. Here we investigated whether neuropathic pain could be induced without sensory neuron injury following exposure of their peripheral axons to the milieu of Wallerian degeneration. We developed a unilateral lumbar 5 ventral root transection (L5 VRT) model in adult rats, in which L5 ventral root fibers entering the sciatic nerve were sectioned in the spinal canal. This model differs from previous ones in that DRG neurons and their afferents are kept uninjured and intact afferents expose to products of degenerating efferent ventral root fibers in the sciatic nerve and the denervated muscles. We found that the L5 VRT produced rapid (24 h after transection), robust and prolonged (56 days) bilateral mechanical allodynia, to a similar extent to that in rats with L5 spinal nerve transection (L5 SNT), cold allodynia and short-term thermal hyperalgesia (14 days). Furthermore, L5 VRT led to significant inflammation as demonstrated by infiltration of ED-1-positive monocytes/macrophages in the DRG, sciatic nerve and muscle fibers. These findings demonstrated that L5 VRT produced behavioral signs of neuropathic pain with high mechanical sensitivity and thermal responsiveness, and suggested that neuropathic pain can be induced without damage to sensory neurons. We propose that neuropathic pain in this model may be mediated by primed intact sensory neurons, which run through the milieu of Wallerian degeneration and inflammation after nerve injury. The L5 VRT model manifests the complex regional pain syndrome in some human patients, and it may provide an additional dimension to dissect out the mechanisms underlying neuropathic pain.

The cytokine network of Wallerian degeneration: tumor necrosis factor-alpha, interleukin-1alpha, and interleukin-1beta

Wallerian degeneration (WD) is the inflammatory response of the nervous system to axonal injury, primarily attributable to the production of cytokines, the mediator molecules of inflammation. We presently document the involvement of the inflammatory cytokines TNFalpha, interleukin (IL)-1alpha, and IL-1beta in peripheral nerve (PNS) injury in C57/BL/6NHSD (C57/BL) mice that display the normal rapid progression of WD (rapid-WD) and C57/BL/6-WLD/OLA/NHSD mice that display abnormal slow progression of WD (slow-WD). TNFalpha and IL-1alpha mRNAs were expressed, whereas TNFalpha but not IL-1alpha protein was synthesized in intact PNS of C57/BL mice. TNFalpha and IL-1alpha protein synthesis and secretion were rapidly upregulated during rapid-WD in Schwann cells. IL-1beta mRNA expression and protein synthesis and secretion were induced sequentially in Schwann cells with a delay after injury. Thereafter, recruited macrophages contributed to the production of TNFalpha, IL-1alpha, and IL-1beta, which in turn augmented myelin phagocytosis by macrophages. Observations suggest that TNFalpha and IL-1alpha are the first cytokines with protein production that is upregulated during rapid-WD. TNFalpha and IL-1alpha may initiate, therefore, molecular and cellular events in rapid-WD (e.g., the production of additional cytokines and NGF). TNFalpha, IL-1alpha, and IL-1beta may further regulate, indirectly, macrophage recruitment, myelin removal, regeneration, and neuropathic pain. In contrast to rapid-WD, the production of TNFalpha, IL-1alpha, and IL-1beta protein was deficient in slow-WD, although their mRNAs were expressed. mRNA expression and protein production of TNFalpha, IL-1alpha, and IL-1beta were differentially regulated during rapid-WD and slow-WD, suggesting that mRNA expression, by itself, is no indication of the functional involvement of cytokines in WD.

The cytokine network of Wallerian degeneration: tumor necrosis factor-alpha, interleukin-1alpha, and interleukin-1beta

Wallerian degeneration (WD) is the inflammatory response of the nervous system to axonal injury, primarily attributable to the production of cytokines, the mediator molecules of inflammation. We presently document the involvement of the inflammatory cytokines TNFalpha, interleukin (IL)-1alpha, and IL-1beta in peripheral nerve (PNS) injury in C57/BL/6NHSD (C57/BL) mice that display the normal rapid progression of WD (rapid-WD) and C57/BL/6-WLD/OLA/NHSD mice that display abnormal slow progression of WD (slow-WD). TNFalpha and IL-1alpha mRNAs were expressed, whereas TNFalpha but not IL-1alpha protein was synthesized in intact PNS of C57/BL mice. TNFalpha and IL-1alpha protein synthesis and secretion were rapidly upregulated during rapid-WD in Schwann cells. IL-1beta mRNA expression and protein synthesis and secretion were induced sequentially in Schwann cells with a delay after injury. Thereafter, recruited macrophages contributed to the production of TNFalpha, IL-1alpha, and IL-1beta, which in turn augmented myelin phagocytosis by macrophages. Observations suggest that TNFalpha and IL-1alpha are the first cytokines with protein production that is upregulated during rapid-WD. TNFalpha and IL-1alpha may initiate, therefore, molecular and cellular events in rapid-WD (e.g., the production of additional cytokines and NGF). TNFalpha, IL-1alpha, and IL-1beta may further regulate, indirectly, macrophage recruitment, myelin removal, regeneration, and neuropathic pain. In contrast to rapid-WD, the production of TNFalpha, IL-1alpha, and IL-1beta protein was deficient in slow-WD, although their mRNAs were expressed. mRNA expression and protein production of TNFalpha, IL-1alpha, and IL-1beta were differentially regulated during rapid-WD and slow-WD, suggesting that mRNA expression, by itself, is no indication of the functional involvement of cytokines in WD.

Mechanical hyperalgesia after an L5 ventral rhizotomy or an L5 ganglionectomy in the rat

An L5 spinal nerve ligation (SNL) in the rat leads to behavioral signs of mechanical hyperalgesia. Our recent finding that an L5 dorsal root rhizotomy did not alter the mechanical hyperalgesia following an L5 SNL suggests that signals originating from the proximal stump of the injured nerve are not essential. We postulate that Wallerian degeneration of L5 nerve fibers leads to altered properties of adjacent intact nociceptive afferents. To investigate the role of degeneration in sensory versus motor fibers, five injury models were examined concurrently in a blinded fashion. An L5 ganglionectomy produced a selective lesion of sensory fibers. An L5 ventral root rhizotomy produced a selective lesion of motor fibers. The three control lesions included: (1) SNL with L5 dorsal root rhizotomy; (2) L5 dorsal root rhizotomy; and (3) exposure of the L5 roots without transection (sham). Paw withdrawal thresholds to mechanical stimuli were measured at three sites in the rat hindpaw corresponding to the L3, L4, and L5 dermatomes. Both the ganglionectomy and the ventral rhizotomy produced a significant, lasting (>or=20 d) decrease of mechanical withdrawal thresholds that was comparable to that produced by the SNL lesion. The L5 dorsal rhizotomy, by itself, produced a short lasting (<or=6 d) decrease in thresholds, whereas the sham procedure did not produce a significant change. We propose that interactions between degenerating motor and sensory fibers of the injured nerve and intact afferent fibers of neighboring nerves play a critical role for both initiation and maintenance of mechanical hyperalgesia in neuropathic pain.

A new model of sciatic inflammatory neuritis (SIN): induction of unilateral and bilateral mechanical allodynia following acute unilateral peri-sciatic immune activation in rats

Immune activation near healthy peripheral nerves may have a greater role in creating pathological pain than previously recognized. We have developed a new model of sciatic inflammatory neuritis to assess how such immune activation may influence somatosensory processing. The present series of experiments reveal that zymosan (yeast cell walls) acutely injected around the sciatic nerve of awake unrestrained rats rapidly (within 3h) produces low threshold mechanical allodynia in the absence of thermal hyperalgesia. Low (4 microg) doses of zymosan produce both territorial and extra-territorial allodynia restricted to the ipsilateral hindpaw. Higher (40-400 microg) doses of zymosan again produce both territorial and extra-territorial allodynia. However, allodynia is now expressed both in the ipsilateral as well as contralateral hindpaws. Several lines of evidence are provided that the appearance of this contralateral ('mirror') allodynia reflects local actions of zymosan on the sciatic nerve rather than spread of this immune activator to the general circulation. Since many clinical neuropathies result from inflammation/infection of peripheral nerves rather than frank physical trauma, understanding how immune activation alters pain processing may suggest novel approaches to pain control.

Delayed peripheral nerve degeneration, regeneration, and pain in mice lacking inducible nitric oxide synthase

Inducible nitric oxide synthase (iNOS) may be a critical factor in the repair of injured tissues. In mice lacking iNOS we observed abnormalities in how the peripheral nerve responds to each of 3 fundamental types of injury: chronic constriction partial nerve injury (a model of neuropathic pain), nerve crush, and nerve transection. In each type of injury, mice lacking iNOS had evidence of a regenerative delay, preceded by slowing of myelinated fiber Wallerian degeneration (WD). In wild-type mice, iNOS immunoreactivity and the presence and upregulation of its mRNA were demonstrated distal to injury, but neither was observed in the knockout mice. Slowed WD was suggested by the abnormal persistence of apparent myelinated fiber profiles distal to the injury zones in mice lacking iNOS compared to wild-type controls. In mice lacking iNOS there were fewer regenerating myelinated fibers, smaller caliber regenerating fibers, and slowed reinnervation of muscle endplates distal to the injury zone. Slowed degeneration was also associated with normal initiation but delayed expression of neuropathic pain. Our findings highlight important relationships among nitric oxide, WD, neuropathic pain, and axon regeneration.

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.

2000 Volvo Award winner in basic science studies: Exogenous tumor necrosis factor-alpha mimics nucleus pulposus-induced neuropathology. Molecular, histologic, and behavioral comparisons in rats

STUDY DESIGN

This study tested the hypothesis that the 17-kDa form of tumor necrosis factor-alpha is the pathophysiologic agent expressed by herniated nucleus pulposus in vivo that is primarily responsible for the histologic and behavioral manifestations of experimental sciatica associated with herniated lumbar discs.

OBJECTIVE

The authors determined the molecular weight and concentration of active tumor necrosis factor-alpha in rat herniated disc and used exogenous tumor necrosis factor-alpha at the same molecular weight to study its neuropathologic effect on rat nerve root and dorsal root ganglion preparations in vivo.

SUMMARY OF BACKGROUND DATA

Expressed by herniated nucleus pulposus in culture, tumor necrosis factor-alpha causes neuropathologic injury in nerve roots and neuropathic pain states in which mechanical allodynia is seen in response to peripheral stimuli.

METHODS

Western blotting was used to identify the molecular weight of the operative tumor necrosis factor-alpha protein form, and measures of optical density were used for semiquantitative determination of concentration. Plastic-embedded nerve roots and dorsal root ganglion were used for neuropathologic evaluation, and von Frey stimulation was used to quantify mechanical allodynia.

RESULTS

The 17-kDa form of tumor necrosis factor-alpha is expressed by herniated nucleus pulposus at a concentration of approximately 0.48 ng per herniated rat lumbar disc. Exogenous tumor necrosis factor-alpha applied in vivo to rat nerve roots produced neuropathologic changes and behavior deficits that mimicked experimental studies with herniated nucleus pulposus applied to nerve roots.

CONCLUSIONS

The data reinforce other evidence that tumor necrosis factor-alpha is involved in mechanisms of neuropathic pain.

Hyperalgesia due to nerve injury-role of peroxynitrite

We carried out a partial ligation of the sciatic nerve in rats to induce nerve injury and neuropathic hyperalgesia. We showed that nitrotyrosine, a marker of peroxynitrite activity, was formed after partial nerve injury. Double-labelling immunohistochemistry showed that nitrotyrosine-immunoreactive cells were mainly macrophages and Schwann cells. Daily treatment with uric acid, a scavenger of peroxynitrite, decreased nitrotyrosine formation in the injured sciatic nerve, and produced concomitant alleviation of thermal hyperalgesia and Wallerian degeneration. These results provide the first evidence that peroxynitrite is formed after partial nerve injury, and contributes to the initiation of thermal hyperalgesia and Wallerian degeneration. We hypothesize that uric acid alleviates hyperalgesia and Wallerian degeneration by inhibiting oxidative damage caused by peroxynitrite and possibly also by decreasing the production of other inflammatory mediators such as prostaglandins.

Transient action of the endothelial constitutive nitric oxide synthase (ecNOS) mediates the development of thermal hypersensitivity following peripheral nerve injury

Neuropathic pain is a disabling feature of peripheral nerve injury. Following injury, local inflammation and the release of mediators may contribute to ectopic mechanosensitivity of the nerve-trunk and pain hypersensitivity. In the present study we investigated whether nitric oxide (NO) action and local nitric oxide synthase (NOS) expression play a role in pain hypersensitivity and A fibre-mediated ectopic hyperexcitability following a chronic constriction injury to a rat sciatic nerve. Using immunohistochemical methods we provide evidence for a unique endothelial constitutive nitric oxide synthase (ecNOS) immunoreactivity localized in early axonal endbulb-like structures of injured peripheral nerve axons. Moreover, we show that following nerve injury there is increased ecNOS-mRNA expression within the lumbar sympathetic ganglia, and that axoplasmic transport in sympathetic and other axons rather than local non-neural synthesis accounts for its accumulation in nerve fibres. We also demonstrate here that local inhibition of NOS action with the broad-spectrum inhibitor NG-nitro-L-arginine-methyl ester (L-NAME), but not more specific inhibitors of other NOS isoforms, has stereospecific, dose- and time-dependent analgesic effects that were reversed by local administration of L-arginine, the natural precursor of NO. In further work, using a teased fibre preparation, we show that administration of L-NAME, but not D-NAME, to the injury site also blocks ectopic mechanosensitivity of injured A-fibres. Our results indicate that an early and transient local ecNOS expression within early axonal endbulb-like structures, some arising from sympathetic axons, plays a critical role in the development of neuropathic pain.

Epineurial application of TNF elicits an acute mechanical hyperalgesia in the awake rat

Tumor necrosis factor alpha (TNF) injected into the sciatic nerve and neutralizing antibodies to its receptor injected around the nerve are respectively associated with inducing and blocking pain behavior beginning 1 to 3 days post-injection. This study examined the acute effects of TNF applied around the nerve trunk on the mechanical threshold (determined with von Frey hairs) and withdrawal latency to radiant heat. TNF (0.9 and 7.7 ng in 90 microL) injected onto the nerve via an indwelling catheter elicited a decrease in mechanical threshold. Following the low dose of TNF, no change in thermal latency was observed; after the 7.7 ng dose, thermal thresholds decreased and returned to baseline multiple times within the 3-hour observation period. Identical doses of TNF injected near, but not on the nerve, 90 ng of TNF injected on the nerve, and vehicle were without effect on either modality. These data indicate that effects of acutely administered TNF to the nerve trunk are capable of producing modality specific pain behavior. These changes may represent a first step in TNF-induced neuropathic pain.

Depletion of macrophages reduces axonal degeneration and hyperalgesia following nerve injury

Inflammatory mechanisms are believed to play an important role in hyperalgesia resulting from nerve injury. Hyperalgesia following nerve injury is temporally linked with Wallerian degeneration and macrophage recruitment, and is reduced in WLD mice, in which Wallerian degeneration is delayed. We sought more direct evidence that macrophages contribute to hyperalgesia and Wallerian degeneration by depleting macrophages with liposomes loaded with dichloromethylene diphosphonate (clodronate, Cl(2)MDP). Rats were subjected to partial ligation of the sciatic nerve. Intravenous injection of liposome-encapsulated clodronate reduced the number of macrophages in the injured nerve, alleviated thermal hyperalgesia and protected both myelinated and unmyelinated fibres against degeneration. The results confirm the role of circulating monocytes/macrophages in the development of neuropathic hyperalgesia and Wallerian degeneration due to partial nerve injury. Macrophage depletion immediately after nerve injury could have some clinical potential in prevention of neuropathic pain.

Mechanical hyperalgesia after an L5 spinal nerve lesion in the rat is not dependent on input from injured nerve fibers

An injury to a peripheral nerve in animals often leads to signs of neuropathic pain including hyperalgesia to heat, cold and mechanical stimuli. The role of injured and intact nerve fibers in mechanical hyperalgesia was evaluated in rats subjected to an L5 spinal nerve ligation-and-cut ('modified SNL lesion'). To assess the contribution of injured afferents, an L5 dorsal rhizotomy was performed immediately before, or 7 days after the modified SNL lesion. To study the role of adjacent intact spinal nerves, an L4 dorsal rhizotomy was performed 7 days after the modified SNL lesion. The up-down method of Dixon (Dixon WJ, Annu Rev Pharmacol Toxicol 1980;20:441-462) was used to measure the paw withdrawal threshold to mechanical stimuli at three sites on the rat hindpaw corresponding to the L3, L4, and L5 dermatomes. We found that the modified SNL lesion produced a significant, lasting (20 days) decrease of the mechanical withdrawal threshold. The severity and duration of mechanical hyperalgesia varied across testing sites. The L5 and L4 dermatome test sites developed the most severe and lasting mechanical hyperalgesia. In contrast, the L3 testing site developed significantly less severe and shorter lasting mechanical hyperalgesia. L5 dorsal rhizotomy, by itself, produced a transient decrease in mechanical withdrawal thresholds. L5 dorsal rhizotomy performed before, or 7 days after, the modified SNL lesion did not prevent or resolve the observed decrease in mechanical withdrawal thresholds. L4 dorsal rhizotomy performed 7 days after the modified SNL lesion resulted in an immediate reversal of mechanical withdrawal thresholds back to baseline values. These results suggest that, after L5 spinal nerve ligation-and-cut, mechanical hyperalgesia develops and persists independent of input from injured afferents. We propose that the Wallerian degeneration that develops after a nerve injury leads to interactions between the degenerating fibers of the injured spinal nerve and the intact fibers of adjacent spinal nerves. This leads to changes in the intact fibers that play a critical role for both initiation and maintenance of mechanical hyperalgesia.

Upregulation and interaction of TNFalpha and gelatinases A and B in painful peripheral nerve injury

Chronic constriction injury (CCI) to peripheral nerve causes a painful neuropathy in association with a process of axonal degeneration and endoneural remodeling that involves macrophage recruitment and local increase in extracellular proteases and tumor necrosis factor alpha (TNF-alpha). Cell surface activation of TNF-alpha from its transmembrane precursor, as well as sequestration of TNF-alpha receptors II and I, is performed by the zinc-dependent endopeptidase family of matrix metalloproteinases (MMPs). Among TNF-alpha-converting MMPs, basal lamina degrading gelatinases are thought to play a role in sciatic nerve injury. In the present study, we determined the forms of TNF-alpha involved in the development of CCI neuropathy in rats, using Western blot analysis, and the temporal correlation of TNF-alpha and TNFRI protein profiles with gelatinases activity at the site of peripheral nerve injury. We observed two peaks in TNF-alpha protein during the first week of CCI that correspond to previously reported peaks in painful behavior. We propose that the first peak at 6 h post-CCI is due to the local expression of the cytotoxic transmembrane 26 kDa TNF-alpha protein released by resident Schwann cells, mast cells and macrophages. This peak in TNF-alpha protein expression corresponds to an increase in gelatinase B (MMP-9) activity, which is greatly upregulated as early as 3 h following CCI to rat sciatic nerve. The second peak occurs at 5 days post-CCI, and may represent TNF-alpha protein released by hematogenously recruited macrophages. This peak is marked by the increase in active soluble 17 kDa TNF-alpha and by gelatinase A (MMP-2) upregulation. These observations suggest that there is a pathogenic role for the TNF-alpha-converting function of MMP-2 in painful CCI neuropathy. We conclude that severe nerve injury induces MMPs, TNF-alpha and TNFRI, which interactively control the privileged endoneurial environment and the pathogenesis of the painful neuropathies associated with the macrophage-dependent processes of Wallerian degeneration.

Strain differences in neuropathic hyperalgesia

The purpose of this study was to investigate strain-related differences in the onset and maintenance of thermal hyperalgesia following the induction of peripheral nerve injury in two inbred strains of rats (Fischer 344 and Lewis) and two outbred strains of rats (Sprague-Dawley and Wistar). Neuropathic pain was induced via unilateral ligation of the left sciatic nerve with chromic gut sutures. A plantar analgesia meter was used to measure paw-withdrawal latency from the ligated vs. unligated hind paws of inbred vs. outbred strains of rats to investigate strain-related differences in nerve injury-induced thermal hyperalgesia. The results demonstrated no significant effects of animal strain on presurgical paw-withdrawal latency values. Following the sciatic nerve ligation (SNL) surgery, a significant hyperalgesic response was elicited from the Sprague-Dawley and Wistar rats (outbred strains) for at least 28 days. Conversely, data analyses from the inbred strains failed to demonstrate significant hyperalgesic responses to peripheral nerve injury, with the exception of postsurgical day 10. These data emphasize the importance of considering the strain of the rat being investigated before extrapolating the results from animals experiments to treatment strategies for humans with chronic neuropathic pain.

Nerve injury, axonal degeneration and neural regeneration: basic insights

Axotomy or crush of a peripheral nerve leads to degeneration of the distal nerve stump referred to as Wallerian degeneration (WD). During WD a microenvironment is created that allows successful regrowth of nerve fibres from the proximal nerve segment. Schwann cells respond to loss of axons by extrusion of their myelin sheaths, downregulation of myelin genes, dedifferentiation and proliferation. They finally aline in tubes (Büngner bands) and express surface molecules that guide regenerating fibres. Hematogenous macrophages are rapidly recruited to the distal stump and remove the vast majority of myelin debris. Molecular changes in the distal stump include upregulation of neurotrophins, neural cell adhesion molecules, cytokines and other soluble factors and their corresponding receptors. Axonal injury not only induces muscle weakness and loss of sensation but also leads to adaptive responses and neuropathic pain. Regrowth of nerve fibres occurs with high specificity with formerly motor fibres preferentially reinnervating muscle. This involves recognition molecules of the L2/HNK-1 family. Nerve regeneration occurs at a rate of 3-4 mm/day after crush and 2-3 mm/day after sectioning a nerve. Nerve regeneration can be fostered pharmacologically. Upon reestablishment of axonal contact Schwann cells remyelinate nerve sprouts and downregulate surface molecules characteristic for precursor/premyelinating or nonmyelinating Schwann cells. At present it is unclear whether axonal regeneration after nerve injury is impeded in neuropathies.

Local expression of inducible nitric oxide synthase in an animal model of neuropathic pain

Peripheral nerve injury is associated with local inflammation and neuropathic pain. In this study we investigated the local expression of the inducible isoform of nitric oxide synthase (iNOS) following a chronic constriction injury (CCI) to the sciatic nerve, a rat model of neuropathic pain. Western blot analysis and immunohistochemical co-localization methods were used to identify temporal and spatial expression of iNOS and its cells of origin. Changes in mRNA were analyzed by RT-PCR and iNOS specific primers. We report that CCI injury induced local iNOS expression in both macrophages and Schwann cells within and distal to the injury site. The local increase in iNOS mRNA expression paralleled both the temporal and spatial protein expression. This study supports the hypothesis that CCI is associated with a local inflammatory reaction mediated at least in part by iNOS. Local activation of the iNOS-NO system may play an important role in the pathogenesis of peripheral nerve injury and neuropathic pain.

Expression of three forms of nitric oxide synthase in peripheral nerve regeneration

Nitric oxide (NO) is a short-lived molecule with messenger and cytotoxic functions in nervous, cardiovascular, and immune systems. Nitric oxide synthase (NOS), the enzyme responsible for NO synthesis, exists in three different forms: the neuronal (nNOS), present in discrete neuronal populations; the endothelial (eNOS), present in vascular endotheliun, and the inducible isoform (iNOS), expressed in various cell types when activated, including macrophages and glial cells. In this study, we have investigated the possible involvement of NO in Wallerian degeneration and the subsequent regeneration occurring after sciatic nerve ligature, using histochemistry and immunocytochemistry for the three NOS isoforms, at different postinjury periods. Two days after lesion, the three NOS isoforms are overexpressed, reaching their greatest expression during the second week. nNOS is upregulated in dorsal root ganglion neurons, centrifugally transported and accumulated in growing axons. eNOS is overexpressed in vasa nervorum of the distal stump and around ligature, and iNOS is induced in recruited macrophages. These findings indicate that different cellular sources contribute to maintain high levels of NO at the lesion site. The parallelism between NOS inductions and well-known repair phenomena suggests that NO, acting in different ways, may exert a beneficial effect on nerve regeneration.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Nerve injury and regeneration: basic insights and therapeutic interventions

Recent observations have provided new insight into neuronal responses to axotomy, signalling of the Schwann cell switch from 'operating' to 'proliferation' mode and temporal molecular changes in the responsiveness of Schwann cells to neuronal signals, as well as into the role of macrophages in Wallerian degeneration, nerve repair and neuropathic pain. Furthermore, promising therapeutic interventions have been developed to promote axon regeneration and to attenuate axotomy-induced neuronal cell death by means of pharmacological treatment or application of neurotrophic proteins using various strategies and routes of delivery.

Differences in sympathetic innervation of mouse DRG following proximal or distal nerve lesions

Nerve injury leads to novel sympathetic innervation of the dorsal root ganglion (DRG). We have hypothesized previously that the degenerating nerve increases the sympathetic sprouting in the DRG and pain after chronic sciatic constriction injury (CCI) by virtue of its influence on sensory and sympathetic axons spared by the injury. However, L5 spinal nerve ligation and transection (SNL) results in the complete isolation of the L5 DRG from the degenerating stump, yet sympathetic axons invade the ganglion, and sympathetically dependent pain develops. We investigated the role of Wallerian degeneration in both sympathetic sprouting and neuropathic pain in these two models of painful peripheral neuropathy by comparing responses of normal C57B1/6J and C57B1/Wlds mice in which degeneration is impaired. After CCI, Wlds mice, unlike 6J mice, did not develop thermal or mechanoallodynia or sympathetic innervation of the L5 DRG. After SNL, both strains developed mechanoallodynia and sympathetic sprouts in L5, but only 6J mice developed thermal allodynia. Observation of the origins of the invading sympathetic axons revealed that after CCI, sympathetics innervating blood vessels and dura (probably intact) sprouted into the ganglion, but after SNL sympathetics (probably axotomized) invaded from the injured spinal nerve. Based on these findings, we hypothesize that there are two mechanisms for sympathetic sprouting into DRG, differentially dependent on Wallerian degeneration. Analysis of pain behavior in these animals reveals that (i) mechanoallodynia and sympathetic innervation of the DRG tend to coincide and (ii) thermal allodynia and Wallerian degeneration, but not sympathetic innervation of the DRG tend to coincide.

Painful mononeuropathy in C57BL/Wld mice with delayed wallerian degeneration: differential effects of cytokine production and nerve regeneration on thermal and mechanical hypersensitivity

Wallerian degeneration with macrophage influx and production of proinflammatory cytokines is a critical factor in the development of hyperalgesia in animal models of neuropathic pain. We hypothesized that in the mouse strain with delayed Wallerian degeneration, the C57BL/Wld mouse, the temporal course of mechanical allodynia and thermal hyperalgesia as well as the temporal profile of cytokine expression after nerve injury would differ from normal mice. Here we used the model of chronic constriction injury of the sciatic nerve (CCI) to study the correlation of pain related behavior with peripheral nerve de- and regeneration and concomitant cytokine production. Indeed, after CCI, C57BL/Wld mice showed markedly reduced thermal hyperalgesia compared to normal C57BL/6 mice, temporally related to the delayed recruitment of hematogeneous macrophages to the injured nerve. Endoneurial tumor necrosis factor-alpha (TNF)-like immunoreactivity increased rapidly in normal mice but did so with a delayed time course in C57BL/Wld mice. In addition, the duration of mechanical allodynia was significantly prolonged in C57BL/Wld mice as compared to C57BL/6 mice, in accordance with the delay in regeneration of sensory nerve fibers in these mice. These results suggest that macrophage invasion and production of TNF may influence the development of thermal hyperalgesia and that regenerative activity is linked to mechanical allodynia in peripheral mononeuropathy.

Dissociation of microglial activation and neuropathic pain behaviors following peripheral nerve injury in the rat

Peripheral nerve injury commonly leads to neuropathic pain states fostered, in part, by neuroimmunologic events. We used two models of neuropathic pain (L5 spinal nerve cryoneurolysis (SPCN) and chronic constriction injury (CCI)) to assess the role of spinal glial activation responses in producing pain behaviors. Scoring of glial responses subjectively encompassed changes in cell morphology, cell density and intensity of immunoreactivity with specific activation markers (OX-42 and anti-glial fibrillary acidic protein (GFAP) for microglia and astrocytes, respectively). Glial responses were compared with tactile sensitivity (mechanical allodynia) at 1, 3 or 10 days following SPCN and with thermal hyperalgesia at 10 days in the CCI group. Neuropathic pain behaviors preceded and did not closely correlate with microglial responses in either model. Perineural application of bupivacaine prior to SPCN prevented spinal microglial responses but not pain behaviors. Spinal astrocytic responses to SPCN were early, robust and not altered by bupivacaine. The current findings support the use of bupivacaine as a tool to suppress microglial activation and challenge the putative role of microglia in initiating or potentiating pain behaviors which result from nerve injury.

Tumour necrosis factor-alpha induces ectopic activity in nociceptive primary afferent fibres

Tumour necrosis factor-alpha, a pro-inflammatory cytokine, is expressed endoneurially following a variety of local and systemic pathophysiological insults which give rise to pain. We administered tumour necrosis factor-alpha to pentobarbital-anaesthetized rats, either topically along a restricted portion of the sciatic nerve or injected subcutaneously within the distribution of the sural nerve. Single nociceptive primary afferent fibres were assessed for ectopic discharge and receptor sensitization. Low concentrations (0.001-0.01 ng/ml) of tumour necrosis factor-alpha applied along the nerve elicited a dose-dependent, rapid onset (1-3 min) increase in discharge; higher concentrations led to reduced firing rates. C-fibres developed higher mean firing frequencies than A delta-fibres. Bursting frequency in both fibre types reached several (6) Hz. No change in mechanical threshold was observed. Intradermal injection (50 pg in 50 microliters) led to ectopic discharge and a decrease in mechanical threshold; these effects developed at different rates, suggesting multiple actions of the cytokine. Our data suggest that acute application of tumour necrosis factor-alpha to the axon can lead to aberrant electrophysiologic activity independent of peripheral receptor involvement. This low level of ectopic firing of nociceptive axons may produce wind-up in dorsal horn neurons or may, by itself, be interpreted as pain.