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

Pro-inflammatory cytokines and leukocyte integrins associated with chronic neuropathic pain in traumatic and inflammatory neuropathies: Initial observations and hypotheses

Leukocyte infiltration and persistence within peripheral nerves have been implicated in chronic nociception pathogenesis in murine peripheral neuropathy models. Endoneurial cytokine and chemokine expression contribute to leukocyte infiltration and maintenance of a pro-inflammatory state that delays peripheral nerve recovery and promotes chronic pain behaviors in these mice. However, there has been a failure to translate murine model data into safe and effective treatments for chronic neuropathic pain in peripheral neuropathy patients, or develop reliable biomarkers that may help diagnose or determine treatment responses in affected patients. Initial work showed that persistent sciatic nerve CD11b+ CD45+ leukocyte infiltration was associated with disease severity in three mouse models of inflammatory and traumatic peripheral neuropathies, implying a direct contributing role in disease pathogenesis. In support of this, CD11b+ leukocytes were also seen in the sural nerve biopsies of chronic neuropathic pain patients with three different peripheral neuropathies. Systemic CD11b antagonism using a validated function-neutralizing monoclonal antibody effectively treated chronic nociception following unilateral sciatic nerve crush injury (a representative traumatic neuropathy model associated with axonal degeneration and increased blood-nerve barrier permeability) and does not cause drug addiction behaviors in adult mice. These data suggest that CD11b could be an effective molecular target for chronic neuropathic pain treatment in inflammatory and traumatic peripheral neuropathies. Despite known murine peripheral neuropathy model limitations, our initial work suggests that early expression of pro-inflammatory cytokines, such as tissue inhibitor of metalloproteinases-1 may predict subsequent chronic nociception development following unilateral sciatic nerve crush injury. Studies aligning animal model investigation with observational data from well-characterized human peripheral neuropathies, including transcriptomics and proteomics, as well as animal model studies using a human clinical trial design should foster the identification of clinically relevant biomarkers and effective targeted treatments with limited addiction potential for chronic neuropathic pain in peripheral neuropathy patients.

Intravenous administration of human mesenchymal stem cells derived from adipose tissue and umbilical cord improves neuropathic pain via suppression of neuronal damage and anti-inflammatory actions in rats

Mesenchymal stem cells (MSCs), which are isolated from adipose tissue (AD-MSCs), umbilical cord (UC-MSCs), or bone marrow, have therapeutic potential including anti-inflammatory and immunomodulatory activities. It was recently reported that MSCs are also effective as a therapeutic treatment for neuropathic pain, although the underlying mechanisms have yet to be resolved. Therefore, in this study, we investigated the effects of human AD- and UC-MSCs on neuropathic pain and its mechanisms using rat models of partial sciatic nerve ligation (PSNL). AD- or UC-MSCs were intravenously administered 4 days after PSNL. Antinociceptive effects were then evaluated using the von Frey and weight-bearing tests. We found that, 3–9 days after the administration of AD- or UC-MSCs to PSNL-exposed rats, both the mechanical threshold and differences in weight-bearing of the right and left hind paws were significantly improved. To reveal the potential underlying antinociceptive mechanisms of MSCs, the levels of activation transcription factor 3- and ionized calcium-binding adapter molecule 1-positive cells were measured by immunohistochemical analysis. AD- and UC-MSCs significantly decreased the levels of these proteins that were induced by PSNL in the dorsal root ganglia. Additionally, UC-MSC significantly improved the PSNL-induced decrease in the myelin basic protein level in the sciatic nerve, indicating that UC-MSC reversed demyelination of the sciatic nerve produced by PSNL. These data suggest that AD- and UC-MSCs may help in the recovery of neuropathic pain via the different regulation; AD-MSCs exhibited their effects via suppressed neuronal damage and anti-inflammatory actions, while UC-MSCs exhibited their effects via suppressed neuronal damage, anti-inflammatory actions and remyelination.

Nuclear Factor κB-COX2 Pathway Activation in Non-myelinating Schwann Cells Is Necessary for the Maintenance of Neuropathic Pain in vivo

Chronic neuropathic pain leads to long-term changes in the sensitivity of both peripheral and central nociceptive neurons. Glial fibrillary acidic protein (GFAP)-positive glial cells are closely associated with the nociceptive neurons including astrocytes in the central nervous system (CNS), satellite glial cells (SGCs) in the sensory ganglia, and non-myelinating Schwann cells (NMSCs) in the peripheral nerves. Central and peripheral GFAP-positive cells are involved in the maintenance of chronic pain through a host of inflammatory cytokines, many of which are under control of the transcription factor nuclear factor κB (NFκB) and the enzyme cyclooxygenase 2 (COX2). To test the hypothesis that inhibiting GFAP-positive glial signaling alleviates chronic pain, we used (1) a conditional knockout (cKO) mouse expressing Cre recombinase under the hGFAP promoter and a floxed COX2 gene to inactivate the COX2 gene specifically in GFAP-positive cells; and (2) a tet-Off tetracycline transactivator system to suppress NFκB activation in GFAP-positive cells. We found that neuropathic pain behavior following spared nerve injury (SNI) significantly decreased in COX2 cKO mice as well as in mice with decreased glial NFκB signaling. Additionally, experiments were performed to determine whether central or peripheral glial NFκB signaling contributes to the maintenance of chronic pain behavior following nerve injury. Oxytetracycline (Oxy), a blood-brain barrier impermeable analog of doxycycline was employed to restrict transgene expression to CNS glia only, leaving peripheral glial signaling intact. Signaling inactivation in central GFAP-positive glia alone failed to exhibit the same analgesic effects as previously observed in animals with both central and peripheral glial signaling inhibition. These data suggest that the NFκB-COX2 signaling pathway in NMSCs is necessary for the maintenance of neuropathic pain in vivo.

Protection against oxaliplatin-induced mechanical and thermal hypersensitivity in Sarm1-/- mice

Chemotherapy-induced peripheral neuropathy (CIPN) is a common dose-limiting side effect of cancer treatment, often associated with degeneration of sensory axons or their terminal regions. Presence of the slow Wallerian degeneration protein (WLD^S), or genetic deletion of sterile alpha and TIR motif containing protein 1 (SARM1), which strongly protect axons from degeneration after injury or axonal transport block, alleviate pain in several CIPN models. However, oxaliplatin can cause an acute pain response, suggesting a different mechanism of pain generation. Here, we tested whether the presence of WLD^S or absence of SARM1 protects against acute oxaliplatin-induced pain in mice after a single oxaliplatin injection. In BL/6 and Wld^S mice, oxaliplatin induced significant mechanical and cold hypersensitivities which were absent in Sarm1^-/- mice. Despite the presence of hypersensitivity there was no significant loss of intraepidermal nerve fibers (IENFs) in the footpads of any mice after oxaliplatin treatment, suggesting that early stages of pain hypersensitivity could be independent of axon degeneration. To identify other changes that could underlie the pain response, RNA sequencing was carried out in DRGs from treated and control mice of each genotype. Sarm1^-/- mice had fewer gene expression changes than either BL/6 or Wld^S mice. This is consistent with the pain measurements in demonstrating that Sarm1^-/- DRGs remain relatively unchanged after oxaliplatin treatment, unlike those in BL/6 and Wld^S mice. Changes in levels of four transcripts - Alas2, Hba-a1, Hba-a2, and Tfrc - correlated with oxaliplatin-induced pain, or absence thereof, across the three genotypes. Our findings suggest that targeting SARM1 could be a viable therapeutic approach to prevent oxaliplatin-induced acute neuropathic pain.

Deletion of Acid-Sensing Ion Channel 3 Relieves the Late Phase of Neuropathic Pain by Preventing Neuron Degeneration and Promoting Neuron Repair

Neuropathic pain is one type of chronic pain that occurs as a result of a lesion or disease to the somatosensory nervous system. Chronic excessive inflammatory response after nerve injury may contribute to the maintenance of persistent pain. Although the role of inflammatory mediators and cytokines in mediating allodynia and hyperalgesia has been extensively studied, the detailed mechanisms of persistent pain or whether the interactions between neurons, glia and immune cells are essential for maintenance of the chronic state have not been completely elucidated. ASIC3, a voltage-insensitive, proton-gated cation channel, is the most essential pH sensor for pain perception. ASIC3 gene expression is increased in dorsal root ganglion neurons after inflammation and nerve injury and ASIC3 is involved in macrophage maturation. ASIC currents are increased after nerve injury. However, whether prolonged hyperalgesia induced by the nerve injury requires ASIC3 and whether ASIC3 regulates neurons, immune cells or glial cells to modulate neuropathic pain remains unknown. We established a model of chronic constriction injury of the sciatic nerve (CCI) in mice. CCI mice showed long-lasting mechanical allodynia and thermal hyperalgesia. CCI also caused long-term inflammation at the sciatic nerve and primary sensory neuron degeneration as well as increased satellite glial expression and ATF3 expression. ASIC3 deficiency shortened mechanical allodynia and attenuated thermal hyperalgesia. ASIC3 gene deletion shifted ATF3 expression from large to small neurons and altered the M1/M2 macrophage ratio, thereby preventing small neuron degeneration and relieved pain.

Mitochondrial bioenergetics, glial reactivity, and pain-related behavior can be restored by dichloroacetate treatment in rodent pain models

Glial reactivity in the dorsal horn of the spinal cord is a hallmark in most chronic pain conditions. Neuroinflammation-associated reactive glia, in particular astrocytes, have been shown to exhibit reduced mitochondrial respiratory function. Here, we studied the mitochondrial function at the lumbar spinal cord tissue from complete Freund's adjuvant-induced inflammatory pain rat and chronic constriction injury mouse models by high-resolution respirometry. A significant decrease in mitochondrial bioenergetic parameters at the injury-related spinal cord level coincided with highest astrocytosis. Oral administration of dichloroacetate (DCA) significantly increased mitochondrial respiratory function by inhibiting pyruvate dehydrogenase kinase and decreased glial fibrillary acidic protein and Iba-1 immunoreactivity in spinal cord. Importantly, DCA treatment significantly reduced the ipsilateral pain-related behavior without affecting contralateral sensitivity in both pain models. Our results indicate that mitochondrial metabolic modulation with DCA may offer an alternative therapeutic strategy to alleviate chronic and persistent inflammatory pain.

Growth and elongation of axons through mechanical tension mediated by fluorescent-magnetic bifunctional Fe3O4·Rhodamine 6G@PDA superparticles

Background

The primary strategy to repair peripheral nerve injuries is to bridge the lesions by promoting axon regeneration. Thus, the ability to direct and manipulate neuronal cell axon regeneration has been one of the top priorities in the field of neuroscience. A recent innovative approach for remotely guiding neuronal regeneration is to incorporate magnetic nanoparticles (MNPs) into cells and transfer the resulting MNP-loaded cells into a magnetically sensitive environment to respond to an external magnetic field. To realize this intention, the synthesis and preparation of ideal MNPs is an important challenge to overcome.

Results

In this study, we designed and prepared novel fluorescent-magnetic bifunctional Fe₃O₄·Rhodamine 6G@polydopamine superparticles (FMSPs) as neural regeneration therapeutics. With the help of their excellent biocompatibility and ability to interact with neural cells, our in-house fabricated FMSPs can be endocytosed into cells, transported along the axons, and then aggregated in the growth cones. As a result, the mechanical forces generated by FMSPs can promote the growth and elongation of axons and stimulate gene expression associated with neuron growth under external magnetic fields.

Conclusions

Our work demonstrates that FMSPs can be used as a novel stimulator to promote noninvasive neural regeneration through cell magnetic actuation.

Spinal microglia contribute to cancer-induced pain through system xC --mediated glutamate release

Introduction:

Microglial cells, the resident macrophages of the central nervous system, are a key contributor to the generation and maintenance of cancer-induced pain (CIP). In healthy organisms, activated microglia promote recovery through the release of trophic and anti-inflammatory factors to clear toxins and pathogens and support neuronal survival. Chronically activated microglia, however, release toxic substances, including excess glutamate, causing cytotoxicity. Accordingly, rising attention is given to microglia for their role in abnormal physiology and in mediating neurotoxicity.

Objectives:

To examine the nociceptive relationship between peripherally-released glutamate and microglial xCT.

Methods:

A validated murine model of 4T1 carcinoma cell–induced nociception was used to assess the effect of peripheral tumour on spinal microglial activation and xCT expression. Coculture systems were then used to investigate the direct effect of glutamate released by wildtype and xCT knockdown MDA-MB-231 carcinoma cells on microglial activation, functional system x_C⁻ activity, and protein levels of interferon regulatory factor 8 (IRF8), a transcription factor implicated in microglia-mediated nociception.

Results:

Blockade of system x_C⁻ with sulfasalazine (SSZ) in vivo attenuated nociception in a 4T1 murine model of CIP and attenuates tumour-induced microglial activation in the dorsal horn of the spinal cord. Furthermore, knockdown of xCT in MDA-MB-231 cells mitigated tumour cell–induced microglial activation and functional system x_C⁻ activity in vitro.

Conclusions:

These data collectively demonstrate that the system xCT antiporter is functionally implicated in CIP and may be particularly relevant to pain progression through microglia. Upregulated xCT in chronically activated spinal microglia may be one pathway to central glutamate cytotoxicity. Microglial xCT may therefore be a valuable target for mitigating CIP.

Origins of antidromic activity in sensory afferent fibers and neurogenic inflammation

Neurogenic inflammation results from the release of biologically active agents from the peripheral primary afferent terminal. This release reflects the presence of releasable pools of active product and depolarization-exocytotic coupling mechanisms in the distal afferent terminal and serves to alter the physiologic function of innervated organ systems ranging from the skin and meninges to muscle, bone, and viscera. Aside from direct stimulation, this biologically important release from the peripheral afferent terminal can be initiated by antidromic activity arising from five anatomically distinct points of origin: (i) afferent collaterals at the peripheral-target organ level, (ii) afferent collaterals arising proximal to the target organ, (iii) from mid-axon where afferents lacking myelin sheaths (C fibers and others following demyelinating injuries) may display crosstalk and respond to local irritation, (iv) the dorsal root ganglion itself, and (v) the central terminals of the afferent in the dorsal horn where local circuits and bulbospinal projections can initiate the so-called dorsal root reflexes, i.e., antidromic traffic in the sensory afferent.

Schwann cells regulate sensory neuron gene expression before and after peripheral nerve injury

Sensory neurons in the PNS demonstrate substantial capacity for regeneration following injury. Recent studies have identified changes in the transcriptome of sensory neurons, which are instrumental for axon regeneration. The role of Schwann cells (SCs) in mediating these changes remains undefined. We tested the hypothesis that SCs regulate expression of genes in sensory neurons before and after PNS injury by comparing mice in which LDL Receptor-related Protein-1 (LRP1) is deleted in SCs (scLRP1^-/- mice) with wild-type (scLRP1^+/+ ) littermates. LRP1 is an endocytic and cell-signaling receptor that is necessary for normal SC function and the SC response to nerve injury. scLRP1^-/- mice represent a characterized model in which the SC response to nerve injury is abnormal. Adult DRG neurons, isolated from scLRP1^-/- mice, with or without a conditioning nerve lesion, demonstrated increased neurite outgrowth when cultured ex vivo, compared with neurons from wild-type mice. Following sciatic nerve crush injury, nerve regeneration was accelerated in vivo in scLRP1^-/- mice. These results were explained by transcriptional activation of RAGs in DRG neurons in scLRP1^-/- mice prior to nerve injury. Although the presence of abnormal SCs in scLRP1^-/- mice primed DRG neurons for repair, nerve regeneration in scLRP1^-/- mice resulted in abnormalities in ultrastructure, principally in Remak bundles, and with the onset of neuropathic pain. These results demonstrate the importance of SCs in controlling RAG expression by neurons and the potential for this process to cause chronic pain when abnormal. The SC may represent an important target for preventing pain following PNS injury.

Sustained Local Release of Methylprednisolone From a Thiol-Acrylate Poly(Ethylene Glycol) Hydrogel for Treating Chronic Compressive Radicular Pain

STUDY DESIGN

A preclinical animal model of chronic ligation of the sciatic nerve was used to compare the effectiveness of a slow-release hydrogel carrying methylprednisolone to methylprednisolone injection alone, which simulates the current standard of care for chronic compressive radiculopathy (CR).

OBJECTIVE

To extend the short-term benefits of steroid injections by using a nonswelling, biodegradable hydrogel as carrier to locally release methylprednisolone in a regulated and sustained way at the site of nerve compression.

SUMMARY OF BACKGROUND DATA

CR affects millions worldwide annually, and is a cause of costly disability with significant societal impact. Currently, a leading nonsurgical therapy involves epidural injection of steroids to temporarily alleviate the pain associated with CR. However, an effective way to extend the short-term effect of steroid treatment to address the chronic component of CR does not exist.

METHODS

We induced chronic compression injury of the sciatic nerves of rats by permanent ligation. Forty-eight hours later we injected our methylprednisolone infused hydrogel and assessed the effectiveness of our treatment for 4 weeks. We quantified mechanical hyperalgesia using a Dynamic Plantar Aesthesiometer (Ugo Basile, Stoelting Co., IL, USA), whereas gait analysis was conducted using the Catwalk automated gait analysis platform (Noldus, Leesburg, VA, USA). Macrophage staining was performed with immunohistochemistry and quantification of monocyte chemoattractant protein-1 in sciatic nerve lysates was performed with multiplex immunoassay using a SECTOR Imager 2400A (Meso Scale Discovery, Rockville, MA, USA).

RESULTS

We demonstrate that using the hydrogel to deliver methylprednisolone results in significant (P < 0.05) reduction of hyperalgesia and improvement in the gait pattern of animals with chronic lesions as compared with animals treated with steroid alone. In addition, animals treated with hydrogel plus steroid showed significant reduction in the number of infiltrating macrophages at the sciatic nerve and reduced expression of the neuroinflammatory chemokine monocyte chemoattractant protein-1 (P < 0.05).

CONCLUSION

Use of hydrogels as carriers for sustained local release of steroids provides significantly better control of pain in an animal model of chronic CR. Our steroid-infused hydrogel could be an effective extender of the short-term benefits of epidural steroid injections for patients with chronic compression-induced radicular pain.

LEVEL OF EVIDENCE

N/A.

Preventive Effects of Bee Venom Derived Phospholipase A₂ on Oxaliplatin-Induced Neuropathic Pain in Mice

Oxaliplatin, a chemotherapy drug used to treat colorectal cancer, induces specific sensory neurotoxicity signs that are aggravated by cold and mechanical stimuli. Here we examined the preventive effects of Bee Venom (BV) derived phospholipase A₂ (bvPLA₂) on oxaliplatin-induced neuropathic pain in mice and its immunological mechanism. The cold and mechanical allodynia signs were evaluated by acetone and von Frey hair test on the hind paw, respectively. The most significant allodynia signs were observed at three days after an injection of oxaliplatin (6 mg/kg, i.p.) and then decreased gradually to a normal level on days 7–9. The oxaliplatin injection also induced infiltration of macrophages and upregulated levels of the pro-inflammatory cytokine interleukin (IL)-1β in the lumbar dorsal root ganglia (DRG). Daily treatment with bvPLA₂ (0.2 mg/kg, i.p.) for five consecutive days prior to the oxaliplatin injection markedly inhibited the development of cold and mechanical allodynia, and suppressed infiltration of macrophages and the increase of IL-1β level in the DRG. Such preventive effects of bvPLA₂ were completely blocked by depleting regulatory T cells (Tregs) with CD25 antibody pre-treatments. These results suggest that bvPLA₂ may prevent oxaliplatin-induced neuropathic pain by suppressing immune responses in the DRG by Tregs.

CD11b+Ly6G- myeloid cells mediate mechanical inflammatory pain hypersensitivity

Pain hypersensitivity at the site of inflammation as a result of chronic immune diseases, pathogenic infection, and tissue injury is a common medical condition. However, the specific contributions of the innate and adaptive immune system to the generation of pain during inflammation have not been systematically elucidated. We therefore set out to characterize the cellular and molecular immune response in two widely used preclinical models of inflammatory pain: (i) intraplantar injection of complete Freund's adjuvant (CFA) as a model of adjuvant- and pathogen-based inflammation and (ii) a plantar incisional wound as a model of tissue injury-based inflammation. Our findings reveal differences in temporal patterns of immune cell recruitment and activation states, cytokine production, and pain in these two models, with CFA causing a nonresolving granulomatous inflammatory response whereas tissue incision induced resolving immune and pain responses. These findings highlight the significant differences and potential clinical relevance of the incisional wound model compared with the CFA model. By using various cell-depletion strategies, we find that, whereas lymphocyte antigen 6 complex locus G (Ly)6G(+)CD11b(+) neutrophils and T-cell receptor (TCR) β(+) T cells do not contribute to the development of thermal or mechanical pain hypersensitivity in either model, proliferating CD11b(+)Ly6G(-) myeloid cells were necessary for mechanical hypersensitivity during incisional pain, and, to a lesser extent, CFA-induced inflammation. However, inflammatory (CCR2(+)Ly6C(hi)) monocytes were not responsible for these effects. The finding that a population of proliferating CD11b(+)Ly6G(-) myeloid cells contribute to mechanical inflammatory pain provides a potential cellular target for its treatment in wound inflammation.

Role of macrophages in Wallerian degeneration and axonal regeneration after peripheral nerve injury

The peripheral nervous system (PNS) has remarkable regenerative abilities after injury. Successful PNS regeneration relies on both injured axons and non-neuronal cells, including Schwann cells and immune cells. Macrophages are the most notable immune cells that play key roles in PNS injury and repair. Upon peripheral nerve injury, a large number of macrophages are accumulated at the injury sites, where they not only contribute to Wallerian degeneration, but also are educated by the local microenvironment and polarized to an anti-inflammatory phenotype (M2), thus contributing to axonal regeneration. Significant progress has been made in understanding how macrophages are educated and polarized in the injured microenvironment as well as how they contribute to axonal regeneration. Following the discussion on the main properties of macrophages and their phenotypes, in this review, we will summarize the current knowledge regarding the mechanisms of macrophage infiltration after PNS injury. Moreover, we will discuss the recent findings elucidating how macrophages are polarized to M2 phenotype in the injured PNS microenvironment, as well as the role and underlying mechanisms of macrophages in peripheral nerve injury, Wallerian degeneration and regeneration. Furthermore, we will highlight the potential application by targeting macrophages in treating peripheral nerve injury and peripheral neuropathies.

The neuroimmunology of degeneration and regeneration in the peripheral nervous system

Peripheral nerves regenerate following injury due to the effective activation of the intrinsic growth capacity of the neurons and the formation of a permissive pathway for outgrowth due to Wallerian degeneration (WD). WD and subsequent regeneration are significantly influenced by various immune cells and the cytokines they secrete. Although macrophages have long been known to play a vital role in the degenerative process, recent work has pointed to their importance in influencing the regenerative capacity of peripheral neurons. In this review, we focus on the various immune cells, cytokines, and chemokines that make regeneration possible in the peripheral nervous system, with specific attention placed on the role macrophages play in this process.

The role of G-protein receptor 84 in experimental neuropathic pain

G-protein receptor 84 (GPR84) is an orphan receptor that is induced markedly in monocytes/macrophages and microglia during inflammation, but its pathophysiological function is unknown. Here, we investigate the role of GPR84 in a murine model of traumatic nerve injury. Naive GPR84 knock-out (KO) mice exhibited normal behavioral responses to acute noxious stimuli, but subsequent to partial sciatic nerve ligation (PNL), KOs did not develop mechanical or thermal hypersensitivity, in contrast to wild-type (WT) littermates. Nerve injury increased ionized calcium binding adapter molecule 1 (Iba1) and phosphorylated p38 MAPK immunoreactivity in the dorsal horn and Iba1 and cluster of differentiation 45 expression in the sciatic nerve, with no difference between genotypes. PCR array analysis revealed that Gpr84 expression was upregulated in the spinal cord and sciatic nerve of WT mice. In addition, the expression of arginase-1, a marker for anti-inflammatory macrophages, was upregulated in KO sciatic nerve. Based on this evidence, we investigated whether peripheral macrophages behave differently in the absence of GPR84. We found that lipopolysaccharide-stimulated KO macrophages exhibited attenuated expression of several proinflammatory mediators, including IL-1β, IL-6, and TNF-α. Forskolin-stimulated KO macrophages also showed greater cAMP induction, a second messenger associated with immunosuppression. In summary, our results demonstrate that GPR84 is a proinflammatory receptor that contributes to nociceptive signaling via the modulation of macrophages, whereas in its absence the response of these cells to an inflammatory insult is impaired.

Stromal cell-derived CCL2 drives neuropathic pain states through myeloid cell infiltration in injured nerve

Neuropathic pain resulting from peripheral nerve injury involves many persistent neuroinflammatory processes including inflammatory chemokines that control leukocyte trafficking and activate resident cells. Several studies have shown that CCL2 chemokine, a potent attractant of monocytes, and its cognate receptor, CCR2, play a critical role in regulating nociceptive processes during neuropathic pain. However, the role of CCL2 in peripheral leukocyte infiltration-associated neuropathic pain remains poorly understood. In particular, the contribution of individual CCL2-expressing cell populations (i.e. stromal and leukocytes) to immune cell recruitment into the injured nerve has not been established. Here, in preclinical model of peripheral neuropathic pain (i.e. chronic constriction injury of the sciatic nerve), we have demonstrated that, CCL2 content was increased specifically in nerve fibers. This upregulation of CCL2 correlated with local monocyte/macrophage infiltration and pain processing. Furthermore, sciatic intraneural microinjection of CCL2 in naïve animals triggered long-lasting pain behavior associated with local monocyte/macrophage recruitment. Using a specific CCR2 antagonist and mice with a CCL2 genetic deletion, we have also established that the CCL2/CCR2 axis drives monocyte/macrophage infiltration and pain hypersensitivity in the CCI model. Finally, specific deletion of CCL2 in stromal or immune cells respectively using irradiated bone marrow-chimeric CCI mice demonstrated that stromal cell-derived CCL2 (in contrast to CCL2 immune cell-derived) tightly controls monocyte/macrophage recruitment into the lesion and plays a major role in the development of neuropathic pain. These findings demonstrate that in chronic pain states, CCL2 expressed by sciatic nerve cells predominantly drove local neuro-immune interactions and pain-related behavior through CCR2 signaling.

Attenuation of inflammatory and neuropathic pain behaviors in mice through activation of free fatty acid receptor GPR40

Background

The G-protein-coupled receptor 40 (GPR40) is suggested to function as a transmembrane receptor for medium- to long-chain free fatty acids and is implicated to play a role in free fatty acids-mediated enhancement of glucose-stimulated insulin secretion from pancreas. However, the functional role of GPR40 in nervous system including somatosensory pain signaling has not been fully examined yet.

Results

Intrathecal injection of GPR40 agonist (MEDICA16 or GW9508) dose-dependently reduced ipsilateral mechanical allodynia in CFA and SNL models and thermal hyperalgesia in carrageenan model. These anti-allodynic and anti-hyperalgesic effects were almost completely reversed by a GPR40 antagonist, GW1100. Immunohistochemical analysis revealed that GPR40 is expressed in spinal dorsal horn and dorsal root ganglion neurons, and immunoblot analysis showed that carrageenan or CFA inflammation or spinal nerve injury resulted in increased expression of GPR40 in these areas. Patch-clamp recordings from spinal cord slices exhibited that bath-application of either MEDICA16 or GW9508 significantly decreased the frequency of spontaneous excitatory postsynaptic currents in the substantia gelatinosa neurons of the three pain models.

Conclusions

Our results indicate that GPR40 signaling pathway plays an important suppressive role in spinal nociceptive processing after inflammation or nerve injury, and that GPR40 agonists might serve as a new class of analgesics for treating inflammatory and neuropathic pain.

Electronic supplementary material

The online version of this article (doi:10.1186/s12990-015-0003-8) contains supplementary material, which is available to authorized users.

[Study on novel mechanism underlying analgesia targeting TRPV1]

Transient receptor potential protein (TRP) channels are distributed in pain pathways including primary afferent neurons and function as transduction of various noxious stimuli to innocuous stimuli. TRP channels are considered as molecular basis of chronic pain. Targeting TRPs may lead to novel class of analgesics, and so drug-discovery efforts are focused on TRP agonists and its antagonists. Few products have, however, been placed on the market, because most of candidates have adverse effects. A lesion or disease of the somatosensory nervous system causes neuropathic pain, a type of chronic pain. Neuropathic pain is intolerable and obstinate and therefore, debilitates the affected patients. A great deal of effort has been made to develop medicine targeting molecules involved in neuropathic pain, whereby the promising therapeutically targeted molecules have been identified. Neuroinflammation, based on pathological alteration in crosstalk between nervous system and immune system, has been a focus of attention as pathological mechanism involved in development of neuropathic pain. Recently, we used an animal model for neuropathic pain to find the possibility that neuropathic pain was exacerbated by adipokines derived from perineural adipocytes distributed in injured peripheral neurons. A working hypothesis is therefore proposed that the perineural adipocytes interacts with the immune cells, which also have TRPV1, in injured peripheral nerve, followed by a paracrine loop involving proinflammatory cytokines, chemokines and adipokines derived from them which aggravates and prolongs pain. Here, we overview the developmental status in TRPV1-targetting analgesics and illustrate our recent findings in terms of neuroinflammation.

Imaging neuroinflammation in vivo in a neuropathic pain rat model with near-infrared fluorescence and ¹⁹F magnetic resonance

Chronic neuropathic pain following surgery represents a serious worldwide health problem leading to life-long treatment and the possibility of significant disability. In this study, neuropathic pain was modeled using the chronic constriction injury (CCI). The CCI rats exhibit mechanical hypersensitivity (typical neuropathic pain symptom) to mechanical stimulation of the affected paw 11 days post surgery, at a time when sham surgery animals do not exhibit hypersensitivity. Following a similar time course, TRPV1 gene expression appears to rise with the hypersensitivity to mechanical stimulation. Recent studies have shown that immune cells play a role in the development of neuropathic pain. To further explore the relationship between neuropathic pain and immune cells, we hypothesize that the infiltration of immune cells into the affected sciatic nerve can be monitored in vivo by molecular imaging. To test this hypothesis, an intravenous injection of a novel perfluorocarbon (PFC) nanoemulsion, which is phagocytosed by inflammatory cells (e.g. monocytes and macrophages), was used in a rat CCI model. The nanoemulsion carries two distinct imaging agents, a near-infrared (NIR) lipophilic fluorescence reporter (DiR) and a ¹⁹F MRI (magnetic resonance imaging) tracer, PFC. We demonstrate that in live rats, NIR fluorescence is concentrated in the area of the affected sciatic nerve. Furthermore, the ¹⁹FF MRI signal was observed on the sciatic nerve. Histological examination of the CCI sciatic nerve reveals significant infiltration of CD68 positive macrophages. These results demonstrate that the infiltration of immune cells into the sciatic nerve can be visualized in live animals using these methods.

Comprehensive analysis of neurobehavior associated with histomorphological alterations in a chronic constrictive nerve injury model through use of the CatWalk XT system

Object

Neuropathic pain is debilitating, and when chronic, it significantly affects the patient physically, psychologically, and socially. The neurobehavior of animals used as a model for chronic constriction injury seems analogous to the neurobehavior of humans with neuropathic pain. However, no data depicting the severity of histomorphological alterations of the nervous system associated with graded changes in neurobehavior are available. To determine the severity of histomorphological alteration related to neurobehavior, the authors created a model of chronic constrictive injury of varying intensity in rats and used the CatWalk XT system to evaluate neurobehavior.

Methods

A total of 60 Sprague-Dawley rats, weighing 250–300 g each, were randomly assigned to 1 of 5 groups that would receive sham surgery or 1, 2, 3, or 4 ligatures of 3-0 chromic gut loosely ligated around the left sciatic nerve. Neurobehavior was assessed by CatWalk XT, thermal hyperalgesia, and mechanic allodynia before injury and periodically after injury. The nerve tissue from skin to dorsal spinal cord was obtained for histomorphological analysis 1 week after injury, and brain evoked potentials were analyzed 4 weeks after injury.

Results.

Significant differences in expression of nerve growth factor existed in skin, and the differences were associated with the intensity of nerve injury. After injury, expression of cluster of differentiation 68 and tumor necrosis factor–α was increased, and expression of S100 protein in the middle of the injured nerve was decreased. Increased expression of synaptophysin in the dorsal root ganglion and dorsal spinal cord correlated with the intensity of injury. The amplitude of sensory evoked potential increased with greater severity of nerve damage. Mechanical allodynia and thermal hyperalgesia did not differ significantly among treatment groups at various time points. CatWalk XT gait analysis indicated significant differences for print areas, maximum contact maximum intensity, stand phase, swing phase, single stance, and regular index, with sham and/or intragroup comparisons.

Conclusions.

Histomorphological and electrophysiological alterations were associated with severity of nerve damage. Subtle neurobehavioral differences were detected by the CatWalk XT system but not by mechanical allodynia or thermal hyperalgesia. Thus, the CatWalk XT system should be a useful tool for monitoring changes in neuropathic pain, especially subtle alterations.

Gamma knife irradiation of injured sciatic nerve induces histological and behavioral improvement in the rat neuropathic pain model

We examined the effects of gamma knife (GK) irradiation on injured nerves using a rat partial sciatic nerve ligation (PSL) model. GK irradiation was performed at one week after ligation and nerve preparations were made three weeks after ligation. GK irradiation is known to induce immune responses such as glial cell activation in the central nervous system. Thus, we determined the effects of GK irradiation on macrophages using immunoblot and histochemical analyses. Expression of Iba-1 protein, a macrophage marker, was further increased in GK-treated injured nerves as compared with non-irradiated injured nerves. Immunohistochemical study of Iba-1 in GK-irradiated injured sciatic nerves demonstrated Iba-1 positive macrophage accumulation to be enhanced in areas distal to the ligation point. In the same area, myelin debris was also more efficiently removed by GK-irradiation. Myelin debris clearance by macrophages is thought to contribute to a permissive environment for axon growth. In the immunoblot study, GK irradiation significantly increased expressions of βIII-tubulin protein and myelin protein zero, which are markers of axon regeneration and re-myelination, respectively. Toluidine blue staining revealed the re-myelinated fiber diameter to be larger at proximal sites and that the re-myelinated fiber number was increased at distal sites in GK-irradiated injured nerves as compared with non-irradiated injured nerves. These results suggest that GK irradiation of injured nerves facilitates regeneration and re-myelination. In a behavior study, early alleviation of allodynia was observed with GK irradiation in PSL rats. When GK-induced alleviation of allodynia was initially detected, the expression of glial cell line-derived neurotrophic factor (GDNF), a potent analgesic factor, was significantly increased by GK irradiation. These results suggested that GK irradiation alleviates allodynia via increased GDNF. This study provides novel evidence that GK irradiation of injured peripheral nerves may have beneficial effects.

Genes, molecules and patients--emerging topics to guide clinical pain research

This review selectively explores some areas of pain research that, until recently, have been poorly understood. We have chosen four topics that relate to clinical pain and we discuss the underlying mechanisms and related pathophysiologies contributing to these pain states. A key issue in pain medicine involves crucial events and mediators that contribute to normal and abnormal pain signaling, but remain unseen without genetic, biomarker or imaging analysis. Here we consider how the altered genetic make-up of familial pains reveals the human importance of channels discovered by preclinical research, followed by the contribution of receptors as stimulus transducers in cold sensing and cold pain. Finally we review recent data on the neuro-immune interactions in chronic pain and the potential targets for treatment in cancer-induced bone pain.

Nerve Injury-related Autoimmunity Activation Leads to Chronic Inflammation and Chronic Neuropathic Pain

Background:

Peripheral nerve injuries that provoke neuropathic pain are associated with chronic inflammation and nervous lesions. The authors hypothesized that chronic neuropathic pain might be caused by chronic inflammation resulting from a nervous autoimmune reaction triggered by nerve injury.

Methods:

The authors observed chronic inflammation and neuropathic behaviors for up to 12 weeks after nerve injury in T lymphocyte-deficient nude mice and their heterozygous littermates. Lymphocyte proliferation and Schwann cell apoptosis were examined after coculture of each population with various neural tissues from normal rats and those with nerve injury.

Result:

Nude mice recovered faster and exhibited less thermal hyperalgesia after nerve injury compared to their heterozygous littermates. A large number of IL-17+ cells indicative of lymphocyte activation were found in the injured sciatic nerve and spinal cord (L4-6) of heterozygous littermates, but far fewer of these populations were found in nude mice. In vitro lymphocyte proliferation was enhanced after coculture with nerve tissues from normal rats compared to nerve tissue-free phosphate-buffered saline controls. In particular, coculture with sciatic nerve tissue enhanced proliferation by 80%, dorsal root ganglion by 46%, and spinal cord by 14%. Moreover, neural tissues from rats with nerve injury markedly increased the lymphocyte proliferation compared to coculture with tissues from corresponding normal rats. Schwann cell apoptosis was triggered in vitro when cocultured with lymphocytes from neuropathic rats.

Conclusion:

Our study suggests that chronic neuropathic pain might be caused by chronic inflammation resulting from a nervous autoimmune reaction triggered by nerve injury.

Selectively reducing cytokine/chemokine expressing macrophages in injured nerves impairs the development of neuropathic pain

It has been well documented that Wallerian degeneration following nerve injury is associated with inflammatory reaction. Such local inflammation contributes to the development of chronic neuropathic pain. Macrophages are one of the major players in the process of either or both degeneration/regeneration and hypersensitivity. To elucidate whether cellular and molecular changes involved in Wallerian degeneration are simultaneously involved in the induction and maintenance of neuropathic pain, and to identify which subpopulation of macrophages can be responsible for the chronic pain following nerve injury, we investigated the peripheral effects of an anti-inflammatory cytokine TGF-β1 in neuropathic pain. Rat sciatic nerves were partially ligated. Macrophages accumulated in injured sciatic nerves displayed heterogeneity with two distinctive functional phenotypes. While MAC1(+) macrophages were able to express IL-6 and MIP-1α, ED1(+) macrophages were always devoid of signals of inflammatory mediators. Intraneural injection of TGF-β1 resulted in delayed and attenuated neuropathic pain behaviour. In parallel, we observed that exposure of the nerve to TGF-β1 dramatically reduced the number of MAC1(+) macrophages. Consequently, the expression of IL-6 and MIP-1α decreased in the injured nerve. Very interestingly, local TGF-β1 treatment had no effect on the population of ED1(+) phagocytic macrophages. In addition to its effect on selective subsets of macrophages, TGF-β1 also reduced T-lymphocyte infiltration. Our results revealed the critical roles of cytokine/chemokine secreting MAC1(+) macrophages in the development of neuropathic pain, and highlighted the needs and benefits of targeting specific populations of macrophages in alleviating neuropathic pain without delaying nerve regeneration.

The role of the immune system in the generation of neuropathic pain

Persistent pain is a sequela of several neurological conditions with a primary immune basis, such as Guillain-Barré syndrome and multiple sclerosis. Additionally, diverse forms of injury to the peripheral or the central nervous systems--whether traumatic, metabolic, or toxic--result in substantial recruitment and activation of immune cells. This response involves the innate immune system, but evidence also exists of T-lymphocyte recruitment, and in some patient cohorts antibodies to neuronal antigens have been reported. Mediators released by immune cells, such as cytokines, sensitise nociceptive signalling in the peripheral and central nervous systems. Preclinical data suggest an immune pathogenesis of neuropathic pain, but clinical evidence of a central role of the immune system is less clear. An important challenge for the future is to establish to what extent this immune response initiates or maintains neuropathic pain in patients and thus whether it is amenable to therapy.

Heterogeneity of macrophages in injured trigeminal nerves: cytokine/chemokine expressing vs. phagocytic macrophages

BACKGROUND

Macrophages are important immune effector cells in both innate and adaptive immune responses. Injury to peripheral nerves triggers activation of resident macrophages and infiltration of haematogenous macrophages, which they play critical roles in Wallerian degeneration and neuropathic pain. As macrophages are able to change their phenotypes in response to environment cues, we attempt to identify distinct phenotypes of macrophages in injured nerves and to understand the potential contribution of each macrophage subpopulation to the genesis of neuropathic pain associated with nerve injury.

METHODS

Rat mental nerves (terminal branches of trigeminal nerve) were loosely ligated. Sensitivity to mechanical stimuli at the lower lip area was monitored using calibrated von Frey Hairs. We examined the expression pattern of Iba-1, MAC1 and ED1 which allow us to reveal the immunophenotypes of macrophages at different time points post-injury. Functional status of each macrophage subpopulation was further investigated by colocalization with cytokines/chemokines, myelin basic protein and MHC II antigen, which reflect respectively secretory, phagocytic and antigen presentation properties of activated macrophages.

RESULTS

Following nerve injury, a burst of Iba-1(+) macrophages was found in injured mental nerves. Among them, we detected two major immunophenotypes: MAC1(+) cytokines/chemokines secreting macrophages and ED1(+) phagocytic macrophages. Small, round shaped MAC1(+) macrophages were distributed essentially around the lesion site and existed only at early time points. Large, irregular and foamy ED1(+) macrophages were found among damaged nerve fibers and they persisted for at least 3 months post-injury. Although ED1(+) macrophages did not secrete inflammatory mediators, they were able to express neurotransmitter CGRP and MHC II at later time points. In parallel, we observed that mechanical allodynia developed after the nerve ligation was at its lowest level within 1 month. Although slightly increased afterwards, the head escape threshold maintained significantly lower than before injury until 3 months. We suggest that MAC1(+) macrophages contribute to the initiation of neuropathic pain by releasing cytokines/chemokines, and ED1(+) macrophages may contribute in maintaining the hypersensitivity under other mechanisms.

CONCLUSION

Our results highlighted the heterogeneity and the plasticity of macrophages in response to the injury and provided further information on their potential involvement in neuropathic pain. Exploring the full spectrum of macrophage phenotypes in injured nerve is necessary. Individual macrophage population may be selectively targeted by cell-specific intervention for an effective treatment of neuropathic pain.

The ology of neuropathy: an integrative review of the role of neuroinflammation and TNF-α axonal transport in neuropathic pain

This 2011 Peripheral Nerve Society plenary lecture reviews the role of axonal transport in neuroimmune communication following peripheral nerve injury, linking focal changes in Schwann cell activation and release of the proinflammatory cytokine tumor necrosis factor-alpha (TNF-α) with subsequent activation and sensitization of ascending sensory neurons and glia which culminate in the neuropathic pain state. New data demonstrate that axonally transported (biotinylated) TNF-α activates and localizes with dorsal horn astrocytes within 96 h after injection into sciatic nerve, and that glial fibrillary acidic protein (GFAP) activation in these glial cells is diminished in TNF receptor 1 knockout mice. The pathophysiology, neuropathology and molecular biology of Wallerian degeneration are also reviewed from a perspective that links it to upregulation of proinflammatory cytokines and the development of neuropathic pain states. Finally, insights into neuroimmune communication provide rationale for new therapy based on interference with the processes of Wallerian degeneration, cytokine signaling and TNF-α protein sequestration.

CXCL5 mediates UVB irradiation-induced pain

Many persistent pain states (pain lasting for hours, days, or longer) are poorly treated because of the limitations of existing therapies. Analgesics such as nonsteroidal anti-inflammatory drugs and opioids often provide incomplete pain relief and prolonged use results in the development of severe side effects. Identification of the key mediators of various types of pain could improve such therapies. Here, we tested the hypothesis that hitherto unrecognized cytokines and chemokines might act as mediators in inflammatory pain. We used ultraviolet B (UVB) irradiation to induce persistent, abnormal sensitivity to pain in humans and rats. The expression of more than 90 different inflammatory mediators was measured in treated skin at the peak of UVB-induced hypersensitivity with custom-made polymerase chain reaction arrays. There was a significant positive correlation in the overall expression profiles between the two species. The expression of several genes [interleukin-1β (IL-1β), IL-6, and cyclooxygenase-2 (COX-2)], previously shown to contribute to pain hypersensitivity, was significantly increased after UVB exposure, and there was dysregulation of several chemokines (CCL2, CCL3, CCL4, CCL7, CCL11, CXCL1, CXCL2, CXCL4, CXCL7, and CXCL8). Among the genes measured, CXCL5 was induced to the greatest extent by UVB treatment in human skin; when injected into the skin of rats, CXCL5 recapitulated the mechanical hypersensitivity caused by UVB irradiation. This hypersensitivity was associated with the infiltration of neutrophils and macrophages into the dermis, and neutralizing the effects of CXCL5 attenuated the abnormal pain-like behavior. Our findings demonstrate that the chemokine CXCL5 is a peripheral mediator of UVB-induced inflammatory pain, likely in humans as well as rats.

Wallerian degeneration: the innate-immune response to traumatic nerve injury

Traumatic injury to peripheral nerves results in the loss of neural functions. Recovery by regeneration depends on the cellular and molecular events of Wallerian degeneration that injury induces distal to the lesion site, the domain through which severed axons regenerate back to their target tissues. Innate-immunity is central to Wallerian degeneration since innate-immune cells, functions and molecules that are produced by immune and non-immune cells are involved. The innate-immune response helps to turn the peripheral nerve tissue into an environment that supports regeneration by removing inhibitory myelin and by upregulating neurotrophic properties. The characteristics of an efficient innate-immune response are rapid onset and conclusion, and the orchestrated interplay between Schwann cells, fibroblasts, macrophages, endothelial cells, and molecules they produce. Wallerian degeneration serves as a prelude for successful repair when these requirements are met. In contrast, functional recovery is poor when injury fails to produce the efficient innate-immune response of Wallerian degeneration.

Etanercept attenuates pain-related behavior following compression of the dorsal root ganglion in the rat

PURPOSE

TNFα is an inflammatory mediator related to neuropathic pain including sciatica. Much basic research suggests that anti-TNFα therapy may be useful for the treatment of sciatica. The purpose of this study was to clarify the effects of etanercept in a dorsal root ganglion (DRG) compression model.

METHODS

Adult male Sprague-Dawley rats (200-250 g, n = 60) were used. An L-shaped stainless rod was used to compress the left L5 DRG in the saline and etanercept groups. No rod was used in the sham group. In the etanercept group, 1 mg of etanercept was applied locally onto the DRG at the end of surgery. Saline was applied in the saline and sham groups. On day 3 and day 7 after surgery, the number of ED1-immunoreactive (IR) cells (macrophages) in the DRG was calculated by immunohistochemical methods (n = 6). In addition, double-immunofluorescence labeling for ED1 and TNFα was performed. Behavioral testing with von Frey filaments and a heat stimulator was performed (n = 12).

RESULTS

ED1-IR cells in the DRG significantly increased in the control group compared with the sham group (p < 0.05). Some ED1-IR cells were co-labeled for TNFα. In the etanercept group, decrease in mechanical threshold was significantly inhibited compared with the saline group (p < 0.05). Thermal hyperalgesia was observed in the control group, but in neither the sham nor etanercept group (p < 0.05).

CONCLUSION

Etanercept attenuated the pain-related behavior induced by DRG compression. These findings suggest that mechanical effects on the DRG might be reduced by etanercept in addition to the effects on nucleus pulposus in lumbar disc herniation.

Wallerian degeneration and peripheral nerve conditions for both axonal regeneration and neuropathic pain induction

Wallerian degeneration is a cascade of stereotypical events in reaction to injury of nerve fibres. These events consist of cellular and molecular alterations, including macrophage invasion, activation of Schwann cells, as well as neurotrophin and cytokine upregulation. This review focuses on cellular and molecular changes distal to various types of peripheral nerve injury which simultaneously contribute to axonal regeneration and neuropathic pain induction. In addition to the stereotypical events of Wallerian degeneration, various types of nerve damage provide different conditions for both axonal regeneration and neuropathic pain induction. Wallerian degeneration of injured peripheral nerve is associated with an inflammatory response including rapid upregulation of the immune signal molecules like cytokines, chemokines and transcription factors with both beneficial and detrimental effects on nerve regeneration or neuropathic pain induction. A better understanding of the molecular interactions between the immune system and peripheral nerve injury would open the possibility for targeting these inflammatory mediators in therapeutic interventions. Understanding the pleiotropic effects of cytokines/chemokines, however, requires investigating their highly specific pathways and precise points of action.

Behavioral models of pain states evoked by physical injury to the peripheral nerve

Physical injury or compression of the root, dorsal root ganglion, or peripheral sensory axon leads to well-defined changes in biology and function. Behaviorally, humans report ongoing painful dysesthesias and aberrations in function, such that an otherwise innocuous stimulus will yield a pain report. These behavioral reports are believed to reflect the underlying changes in nerve function after injury, wherein increased spontaneous activity arises from the neuroma and dorsal root ganglion and spinal changes increase the response of spinal projection neurons. These pain states are distinct from those associated with tissue injury and pose particular problems in management. To provide for developing an understanding of the underlying mechanisms of these pain states and to promote development of therapeutic agents, preclinical models involving section, compression, and constriction of the peripheral nerve or compression of the dorsal root ganglion have been developed. These models give rise to behaviors, which parallel those observed in the human after nerve injury. The present review considers these models and their application.

Leptin derived from adipocytes in injured peripheral nerves facilitates development of neuropathic pain via macrophage stimulation

Nerve injury may result in neuropathic pain, characterized by allodynia and hyperalgesia. Accumulating evidence suggests the existence of a molecular substrate for neuropathic pain produced by neurons, glia, and immune cells. Here, we show that leptin, an adipokine exclusively produced by adipocytes, is critical for the development of tactile allodynia through macrophage activation in mice with partial sciatic nerve ligation (PSL). PSL increased leptin expression in adipocytes distributed at the epineurium of the injured sciatic nerve (SCN). Leptin-deficient animals, ob/ob mice, showed an absence of PSL-induced tactile allodynia, which was reversed by the administration of leptin to the injured SCN. Perineural injection of a neutralizing antibody against leptin reproduced this attenuation. Macrophages recruited to the perineurium of the SCN expressed the leptin receptor and phosphorylated signal transducer and activator of transcription 3 (pSTAT3), a transcription factor downstream of leptin. PSL also up-regulated the accepted mediators of neuropathic pain--namely, cyclooxygenase-2, inducible nitric oxide synthase, and matrix metalloprotease-9--in the injured SCN, with transcriptional activation of their gene promoters by pSTAT3. This up-regulation was partly reproduced in a macrophage cell line treated with leptin. Administration of peritoneal macrophages treated with leptin to the injured SCN reversed the failure of ob/ob mice to develop PSL-induced tactile allodynia. We suggest that leptin induces recruited macrophages to produce pronociceptive mediators for the development of tactile allodynia. This report shows that adipocytes associated with primary afferent neurons may be involved in the development of neuropathic pain through adipokine secretion.

Future treatment strategies for neuropathic pain

The prevalence of people suffering from chronic pain is extremely high and pain affects millions of people worldwide. As such, persistent pain represents a major health problem and an unmet clinical need. The reason for the high incidence of chronic pain patients is in a large part due to a paucity of effective pain control. An important reason for poor pain control is undoubtedly a deficit in our understanding of the underlying causes of chronic pain and as a consequence our arsenal of analgesic therapies is limited. However, there is considerable hope for the development of new classes of analgesic drugs by targeting novel processes contributing to clinically relevant pain. In this chapter we highlight a number of molecular species which are potential therapeutic targets for future neuropathic pain treatments. In particular, the roles of voltage-gated ion channels, neuroinflammation, protein kinases and neurotrophins are discussed in relation to the generation of neuropathic pain and how by targeting these molecules it may be possible to provide better pain control than is currently available.

Dorsal root compression produces myelinated axonal degeneration near the biomechanical thresholds for mechanical behavioral hypersensitivity

Increased sensitivity to mechanical stimuli produced by transient cervical nerve root compression is dependent on the severity of applied load. In addition, trauma in the nervous system induces local inflammation, Wallerian degeneration, and a host of other degenerative processes leading to axonal dysfunction. Here, axonal degeneration and inflammation were assessed following transient dorsal root compression to establish a relationship between conditions for dorsal root axonal changes and those previously established for the onset and maintenance of mechanical behavioral hypersensitivity (26.3 mN and 38.2 mN, respectively). Compression loads were applied over a range (0-110 mN) known to produce sustained behavioral hypersensitivity. CD68- and NF200-immunoreactivity, as well as axonal pathological changes, were assessed in the dorsal root to investigate the load thresholds requisite for inducing macrophage infiltration and axonal degeneration relative to those thresholds for producing the onset and persistence of behavioral hypersensitivity. Neurofilament accumulation and the depletion of NF200-immunoreactivity in the region of compressed tissue were produced for loads that produce mechanical behavioral hypersensitivity. A 50th-percentile load threshold was determined (31.6 mN) governing the onset of NF200 depletion. However, CD68-immunoreactivity was increased for nearly all loads, suggesting that macrophage recruitment may not be directly related to nerve root-mediated behavioral hypersensitivity. This study provides new evidence for threshold-mediated degenerative changes in the context of behavioral hypersensitivity following nerve root compression.

"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.

Pathophysiology of peripheral neuropathic pain: immune cells and molecules

Damage to the peripheral nervous system often leads to chronic neuropathic pain characterized by spontaneous pain and an exaggerated response to painful and/or innocuous stimuli. This pain condition is extremely debilitating and usually difficult to treat. Although inflammatory and neuropathic pain syndromes are often considered distinct entities, emerging evidence belies this strict dichotomy. Inflammation is a well-characterized phenomenon, which involves a cascade of different immune cell types, such as mast cells, neutrophils, macrophages, and T lymphocytes. In addition, these cells release numerous compounds that contribute to pain. Recent evidence suggests that immune cells play a role in neuropathic pain in the periphery. In this review we identify the different immune cell types that contribute to neuropathic pain in the periphery and release factors that are crucial in this particular condition.

Rho kinase inhibitor improves motor dysfunction and hypoalgesia in a rat model of lumbar spinal canal stenosis

STUDY DESIGN

Immunohistochemical and behavioral study using a rat cauda equina compression model.

OBJECTIVE

To investigate, after cauda equina compression by spinal canal stenosis (SCS), Rho activation in the spinal cord and cauda equina, and the effect of intrathecal administration of a Rho kinase inhibitor on hypoalgesia and motor dysfunction.

SUMMARY OF BACKGROUND DATA

Compression of the cauda equina caused by SCS is a common clinical disorder associated with sensory disturbance and intermittent claudication. Cauda equina compression is thought to reduce blood flow and result in nerve degeneration caused by various cytokines. Rho, a member of the small GTPases, is a signal transmitter. It promotes Wallerian degeneration, decreases blood flow in the spinal cord and brain, and increases expression of several cytokines. Currently, Rho kinase inhibitor is used clinically to treat progressive nerve damage due to cerebrovascular disorders. However, its effect for SCS has not been evaluated.

METHODS

Forty-two 6-week-old male Sprague-Dawley rats (200-250 g) were used. For the SCS model (n = 27), a small piece of silicon was placed under the lamina of the fourth lumbar vertebra. In the sham-operated group, laminectomies were performed at L5 only (n = 15). We examined mechanical sensitivity and motor function using von Frey hairs and a treadmill, and immunohistochemically localized Rho in the spinal ventral neurons, axons, and Schwann cells in the cauda equina. We also examined the effects of intrathecally administered Rho kinase inhibitor for hypoalgesia or motor dysfunction caused by SCS.

RESULTS

We observed motor dysfunction and hypoalgesia and activated Rho-immunoreactive cells in spinal ventral neuroreported to induce neurite and axonal outgrowth in the spinal cord and brain after nervous system injury. In addition, 1 report showed that Rho kinase was involved in Wallerian degeneration that was rescued by Rho kinase inhibitor. Furthermore, it is thought that Rho is involved in TNF-alpha and interleukin (IL) production in the central nervous system, and the production was inhibited by administering Rho kinase inhibitor in the central nervous system. Regardns, axons, and Schwann cells in the cauda equina. Intrathecal administration of Rho kinase inhibitor improved mechanical hypoalgesia and motor dysfunction caused by SCS.

CONCLUSION

Activated Rho may play an important role in nerve damage in the cauda equina in SCS. Rho kinase inhibitor may be a useful tool in determining the pathomechanism of cauda equina syndrome caused by SCS.

An evolving cellular pathology occurs in dorsal root ganglia, peripheral nerve and spinal cord following intravenous administration of paclitaxel in the rat

Paclitaxel (Taxol) is a frontline antineoplastic agent used to treat a variety of solid tumors including breast, ovarian, or lung cancer. The major dose limiting side effect of paclitaxel is a peripheral sensory neuropathy that can last days to a lifetime. To begin to understand the cellular events that contribute to this neuropathy, we examined a marker of cell injury/regeneration (activating transcription factor 3; ATF3), macrophage hyperplasia/hypertrophy; satellite cell hypertrophy in the dorsal root ganglia (DRG) and sciatic nerve as well as astrocyte and microglial activation within the spinal cord at 1, 4, 6 and 10 days following intravenous infusion of therapeutically relevant doses of paclitaxel. At day 1 post-infusion, there was an up-regulation of ATF3 in a subpopulation of large and small DRG neurons and this up-regulation was present through day 10. In contrast, hypertrophy of DRG satellite cells, hypertrophy and hyperplasia of CD68(+) macrophages in the DRG and sciatic nerve, ATF3 expression in S100beta(+) Schwann cells and increased expression of the microglial marker (CD11b) and the astrocyte marker glial fibrillary acidic protein (GFAP) in the spinal cord were not observed until day 6 post-infusion. The present results demonstrate that using the time points and markers examined, DRG neurons show the first sign of injury which is followed days later by other neuropathological changes in the DRG, peripheral nerve and dorsal horn of the spinal cord. Understanding the cellular changes that generate and maintain this neuropathy may allow the development of mechanism-based therapies to attenuate or block this frequently painful and debilitating condition.

NO pain: potential roles of nitric oxide in neuropathic pain

The disabling human syndrome of "neuropathic pain" is an intractable complication of peripheral nerve injury or degeneration. A complex interaction between injured peripheral axons, sensory neurons and central nervous system signaling is thought to account for it. In this brief review, we present evidence that the free radical signaling molecule, nitric oxide (NO) may act at several levels of the nervous system during the development of experimental neuropathic pain. For example, NO may directly influence injured axons in the periphery, may indirectly influence pain by its role in the process of Wallerian degeneration, and may signal in the dorsal horn of the spinal cord. While it is premature to argue for therapeutic approaches that alter NO actions, it may be an important player in the cascade of events that generate neuropathic pain.

Pharmacological, behavioural and mechanistic analysis of HIV-1 gp120 induced painful neuropathy

A painful neuropathy is frequently observed in people living with human immunodeficiency virus type 1 (HIV-1). The HIV coat protein, glycoprotein 120 (gp120), implicated in the pathogenesis of neurological disorders associated with HIV, is capable of initiating neurotoxic cascades via an interaction with the CXCR4 and/or CCR5 chemokine receptors, which may underlie the pathogenesis of HIV-associated peripheral neuropathic pain. In order to elucidate the mechanisms underlying HIV-induced painful peripheral neuropathy, we have characterised pathological events in the peripheral and central nervous system following application of HIV-1 gp120 to the rat sciatic nerve. Perineural HIV-1 gp120 treatment induced a persistent mechanical hypersensitivity (44% decrease from baseline), but no alterations in sensitivity to thermal or cold stimuli, and thigmotactic (anxiety-like) behaviour in the open field. The mechanical hypersensitivity was sensitive to systemic treatment with gabapentin, morphine and the cannabinoid WIN 55,212-2, but not with amitriptyline. Immunohistochemical studies reveal: decreased intraepidermal nerve fibre density, macrophage infiltration into the peripheral nerve at the site of perineural HIV-1 gp120; changes in sensory neuron phenotype including expression of activating transcription factor 3 (ATF3) in 27% of cells, caspase-3 in 25% of cells, neuropeptide Y (NPY) in 12% of cells and galanin in 13% of cells and a spinal gliosis. These novel findings suggest that this model is not only useful for the elucidation of mechanisms underlying HIV-1-related peripheral neuropathy but may prove useful for preclinical assessment of drugs for the treatment of HIV-1 related peripheral neuropathic pain.

Altered pain behavior and regeneration after nerve injury in TNF receptor deficient mice

The pro-inflammatory cytokine tumor necrosis factor (TNF)-alpha is an important mediator in hyperalgesia, nerve injury, and regeneration. Here, we used mice deficient of TNF receptor (TNFR) 1 or 2 to investigate the role of TNF signaling via receptor in each pain behavior and nerve de- and regeneration after chronic constriction injury (CCI) of the sciatic nerve. We found an absence of thermal hyperalgesia in mice deficient of TNFR1 and a reduction in mechanical and cold allodynia in mice deficient of TNFR1 or TNFR2 compared with wild-type mice. Nerve conduction studies and nerve pathology did not reveal major differences between genotypes in the temporal course of de- and regeneration of the nerve. We propose that the functional effects of the TNFRs on pain symptoms are independent of effects on nerve regeneration. Furthermore, the differential action of TNF via each of its receptors should be taken into account when considering clinical trials with TNF inhibitors for pain.