Distinct functional types of macrophage in dorsal root ganglia and spinal nerves proximal to sciatic and spinal nerve transections in the rat

Autophagy-targeting modulation to promote peripheral nerve regeneration

Nerve regeneration following traumatic peripheral nerve injuries and neuropathies is a complex process modulated by diverse factors and intricate molecular mechanisms. Past studies have focused on factors that stimulate axonal outgrowth and myelin regeneration. However, recent studies have highlighted the pivotal role of autophagy in peripheral nerve regeneration, particularly in the context of traumatic injuries. Consequently, autophagy-targeting modulation has emerged as a promising therapeutic approach to enhancing peripheral nerve regeneration. Our current understanding suggests that activating autophagy facilitates the rapid clearance of damaged axons and myelin sheaths, thereby enhancing neuronal survival and mitigating injury-induced oxidative stress and inflammation. These actions collectively contribute to creating a favorable microenvironment for structural and functional nerve regeneration. A range of autophagy-inducing drugs and interventions have demonstrated beneficial effects in alleviating peripheral neuropathy and promoting nerve regeneration in preclinical models of traumatic peripheral nerve injuries. This review delves into the regulation of autophagy in cell types involved in peripheral nerve regeneration, summarizing the potential drugs and interventions that can be harnessed to promote this process. We hope that our review will offer novel insights and perspectives on the exploitation of autophagy pathways in the treatment of peripheral nerve injuries and neuropathies.

Harmonizing neuropathic pain research: outcomes of the London consensus meeting on peripheral tissue studies

ABSTRACT

Neuropathic pain remains difficult to treat, with drug development hampered by an incomplete understanding of the pathogenesis of the condition, as well as a lack of biomarkers. The problem is compounded by the scarcity of relevant human peripheral tissues, including skin, nerves, and dorsal root ganglia. Efforts to obtain such samples are accelerating, increasing the need for standardisation across laboratories. In this white paper, we report on a consensus meeting attended by neuropathic pain experts, designed to accelerate protocol alignment and harmonization of studies involving relevant peripheral tissues. The meeting was held in London in March 2024 and attended by 28 networking partners, including industry and patient representatives. We achieved consensus on minimal recommended phenotyping, harmonised wet laboratory protocols, statistical design, reporting, and data sharing. Here, we also share a variety of relevant standard operating procedures as supplementary protocols. We envision that our recommendations will help unify human tissue research in the field and accelerate our understanding of how abnormal interactions between sensory neurons and their local peripheral environment contribute towards neuropathic pain.

Infraorbital nerve injury triggers sex-specific neuroimmune responses in the peripheral trigeminal pathway and common pain behaviours

Trigeminal neuropathic pain is emotionally distressing and disabling. It presents with allodynia, hyperalgesia and dysaesthesia. In preclinical models it has been assumed that cephalic nerve constriction injury shows identical molecular, cellular, and sex dependent neuroimmune changes as observed in extra-cephalic injury models. This study sought empirical evidence for such assumptions using the infraorbital nerve chronic constriction model (ION-CCI). We compared the behavioural consequences of nerve constriction with: (i) the temporal patterns of recruitment of macrophages and T-lymphocytes at the site of nerve injury and in the trigeminal ganglion; and (ii) the degree of demyelination and axonal reorganisation in the injured nerve. Our data demonstrated that simply testing for allodynia and hyperalgesia as is done in extra-cephalic neuropathic pain models does not provide access to the range of injury-specific nociceptive responses and behaviours reflective of the experience of trigeminal neuropathic pain. Similarly, trigeminal neuroimmune changes evoked by nerve injury are not the same as those identified in models of extra-cephalic neuropathy. Specifically, the timing, magnitude, and pattern of ION-CCI evoked macrophage and T-lymphocyte activity differs between the sexes.

HK2 in microglia and macrophages contribute to the development of neuropathic pain

Neuropathic pain is a complex pain condition accompanied by prominent neuroinflammation involving activation of both central and peripheral immune cells. Metabolic switch to glycolysis is an important feature of activated immune cells. Hexokinase 2 (HK2), a key glycolytic enzyme enriched in microglia, has recently been shown important in regulating microglial functions. Whether and how HK2 is involved in neuropathic pain-related neuroinflammation remains unknown. Using a HK2-tdTomato reporter line, we found that HK2 was prominently elevated in spinal microglia. Pharmacological inhibition of HK2 effectively alleviated nerve injury-induced acute mechanical pain. However, selective ablation of Hk2 in microglia reduced microgliosis in the spinal dorsal horn (SDH) with little analgesic effects. Further analyses showed that nerve injury also significantly induced HK2 expression in dorsal root ganglion (DRG) macrophages. Deletion of Hk2 in myeloid cells, including both DRG macrophages and spinal microglia, led to the alleviation of mechanical pain during the first week after injury, along with attenuated microgliosis in the ipsilateral SDH, macrophage proliferation in DRGs, and suppressed inflammatory responses in DRGs. These data suggest that HK2 plays an important role in regulating neuropathic pain-related immune cell responses at acute phase and that HK2 contributes to neuropathic pain onset primarily through peripheral monocytes and DRG macrophages rather than spinal microglia.

The role of neuroinflammation in the transition of acute to chronic pain and the opioid-induced hyperalgesia and tolerance

Current evidence suggests that activation of glial and immune cells leads to increased production of proinflammatory mediators, creating a neuroinflammatory state. Neuroinflammation has been proven to be a fundamental mechanism in the genesis of acute pain and its transition to neuropathic and chronic pain. A noxious event that stimulates peripheral afferent nerve fibers may also activate pronociceptive receptors situated at the dorsal root ganglion and dorsal horn of the spinal cord, as well as peripheral glial cells, setting off the so-called peripheral sensitization and spreading neuroinflammation to the brain. Once activated, microglia produce cytokines, chemokines, and neuropeptides that can increase the sensitivity and firing properties of second-order neurons, upregulating the signaling of nociceptive information to the cerebral cortex. This process, known as central sensitization, is crucial for chronification of acute pain. Immune-neuronal interactions are also implicated in the lesser-known complex regulatory relationship between pain and opioids. Current evidence suggests that activated immune and glial cells can alter neuronal function, induce, and maintain pathological pain, and disrupt the analgesic effects of opioid drugs by contributing to the development of tolerance and dependence, even causing paradoxical hyperalgesia. Such alterations may occur when the neuronal environment is impacted by trauma, inflammation, and immune-derived molecules, or when opioids induce proinflammatory glial activation. Hence, understanding these intricate interactions may help in managing pain signaling and opioid efficacy beyond the classical pharmacological approach.

Transiliac Endoscopic-Assisted L5S1 Intraforaminal Lumbar Interbody Fusion: Technical Considerations and Potential Complications

OBJECTIVE

We sought to determine the clinical outcomes, complications, and fusion rates in transiliac endoscopic-assisted L5S1 intraforaminal lumbar interbody fusion (iLIF).

METHODS

Between September 2020 and September 2021, patients with L5S1 degenerative disk disease were enrolled in a prospective study on transiliac L5S1 iLIF and followed for a minimum of 12 months. Conflict of the preoperative planned approach with the ilium was mandatory. The primary outcome measures were the Oswestry Disability Index, the visual analog scale (VAS) score for low back pain (VAS back) and leg pain (VAS leg), and the modified MacNab criteria. The secondary outcomes were complications and fusion rates.

RESULTS

Five consecutive patients were enrolled: 2 males and 3 females with a mean age of 50 ± 12.9. All had 12 months' follow-up. The mean improvement in the Oswestry Disability Index, VAS back, and VAS leg (44 ± 11.75, 6.6 ± 1.7, and 4.7 ± 4.2, respectively) was more than 3 times the minimum clinically important difference. The modified MacNab criteria were good or excellent in 80% of cases at all endpoints. Three patients had ipsilateral lower limb dysesthesia. One patient had revision surgery for foraminal bone fragment removal. All patients achieved fusion.

CONCLUSIONS

The transiliac iLIF is a feasible but demanding surgical technique that allows overcoming cases in which the ilium prevents endoscopic transforaminal access to L5S1. Our preliminary results had good clinical outcomes and high fusion rates. The main complication was late-onset dysesthesia of the ipsilateral lower limb, 10 to 14 days after surgery. Special care must be taken to prevent L5 dorsal root ganglion injury.

Integrated analysis reveals Atf3 promotes neuropathic pain via orchestrating JunB mediated release of inflammatory cytokines in DRG macrophage

The dorsal root ganglion (DRG) is actively involved in the development of neuropathic pain (NP), serving as an intermediate station for pain signals from the peripheral nervous system to the central nervous system. The mechanism by which DRG is involved in NP regulation is not fully understood. The immune system plays a pivotal role in the physiological and pathological states of the human body. In recent years, the immune system has been thought to play an increasingly important role in the pathogenesis of NP. The immune system plays a key role in pain through specific immune cells and their immune-related genes (IRGs). However, the mechanism by which IRGs of DRG regulate NP action has not been fully elucidated. Here, we performed Gene Ontology (GO) and the Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses of IRGs in DRG bulk-RNA sequencing data from spared nerve injury (SNI) model mice and found that their IRGs were enriched in many pathways, especially in the immune response pathway. Subsequently, we analyzed single-cell RNA sequencing (scRNA-seq) data from DRGs extracted from the SNI model and identified eight cell populations. Among them, the highest IRG activity was presented in macrophages. Next, we analyzed the scRNA and bulk-sequencing data and deduced five common transcription factors (TFs) from differentially expressed genes (DEGs). The protein-protein interaction (PPI) network suggested that Atf3 and JunB are closely related. In vitro experiments, we verified that the protein and mRNA expressions of Atf3 and JunB were up-regulated in macrophages after lipopolysaccharide (LPS) stimulation. Moreover, the down-regulation of Atf3 reduced the release of inflammatory cytokines and decreased the protein and mRNA expression levels of JunB. The down-regulation of JunB also reduced the release of inflammatory cytokines. Furthermore, overexpression of JunB attenuated the effect of Atf3 down-regulation in reducing the release of inflammatory cytokines. Therefore, we speculated that Atf3 might promote NP through JunB-mediated release of inflammatory factors in DRG macrophages.

Age-related neuroimmune signatures in dorsal root ganglia of a Fabry disease mouse model

Pain in Fabry disease (FD) is generally accepted to result from neuronal damage in the peripheral nervous system as a consequence of excess lipid storage caused by alpha-galactosidase A (α-Gal A) deficiency. Signatures of pain arising from nerve injuries are generally associated with changes of number, location and phenotypes of immune cells within dorsal root ganglia (DRG). However, the neuroimmune processes in the DRG linked to accumulating glycosphingolipids in Fabry disease are insufficiently understood.Therefore, using indirect immune fluorescence microscopy, transmigration assays and FACS together with transcriptomic signatures associated with immune processes, we assessed age-dependent neuroimmune alterations in DRG obtained from mice with a global depletion of α-Gal A as a valid mouse model for FD. Macrophage numbers in the DRG of FD mice were unaltered, and BV-2 cells as a model for monocytic cells did not show augmented migratory reactions to glycosphingolipids exposure suggesting that these do not act as chemoattractants in FD. However, we found pronounced alterations of lysosomal signatures in sensory neurons and of macrophage morphology and phenotypes in FD DRG. Macrophages exhibited reduced morphological complexity indicated by a smaller number of ramifications and more rounded shape, which were age dependent and indicative of premature monocytic aging together with upregulated expression of markers CD68 and CD163.In our FD mouse model, the observed phenotypic changes in myeloid cell populations of the DRG suggest enhanced phagocytic and unaltered proliferative capacity of macrophages as compared to wildtype control mice. We suggest that macrophages may participate in FD pathogenesis and targeting macrophages at an early stage of FD may offer new treatment options other than enzyme replacement therapy.

DRGquant: A new modular AI-based pipeline for 3D analysis of the DRG

BACKGROUND

The dorsal root ganglion (DRG) is structurally complex and pivotal to systems processing nociception. Whole mount analysis allows examination of intricate microarchitectural and cellular relationships of the DRG in three-dimensional (3D) space.

NEW METHOD

We present DRGquant a set of tools and techniques optimized as a pipeline for automated image analysis and reconstruction of cells/structures within the DRG. We have developed an open source software pipeline that utilizes machine learning to identify substructures within the DRG and reliably classify and quantify them.

RESULTS

Our methods were sufficiently sensitive to isolate, analyze, and classify individual DRG substructures including macrophages. The activation of macrophages was visualized and quantified in the DRG following intrathecal injection of lipopolysaccharide, and in a model of chemotherapy induced peripheral neuropathy. The percent volume of infiltrating macrophages was similar to a commercial source in quantification. Circulating fluorescent dextran was visualized within DRG macrophages using whole mount preparations, which enabled 3D reconstruction of the DRG and DRGquant demonstrated subcellular spatial resolution within individual macrophages.

COMPARISON WITH EXISTING METHOD(S)

Here we describe a reliable and efficient methodologic pipeline to prepare cleared and whole mount DRG tissue. DRGquant allows automated image analysis without tedious manual gating to reduce bias. The quantitation of DRG macrophages was superior to commercial solutions.

CONCLUSIONS

Using machine learning to separate signal from noise and identify individual cells, DRGquant enabled us to isolate individual structures or areas of interest within the DRG for a more granular and fine-tuned analysis. Using these 3D techniques, we are better able to appreciate the biology of the DRG under experimental inflammatory conditions.

LXR agonist improves peripheral neuropathy and modifies PNS immune cells in aged mice

Background

Peripheral neuropathy is a common and progressive disorder in the elderly that interferes with daily activities. It is of importance to find efficient treatments to treat or delay this age-related neurodegeneration. Silencing macrophages by reducing foamy macrophages showed significant improvement of age-related degenerative changes in peripheral nerves of aged mice. We previously demonstrated that activation of the cholesterol sensor Liver X receptor (LXR) with the potent agonist, GW3965, alleviates pain in a diet-induced obesity model. We sought to test whether LXR activation may improve neuropathy in aged mice.

Methods

21-month-old mice were treated with GW3965 (25 mg/Kg body weight) for 3 months while testing for mechanical allodynia and thermal hyperalgesia. At termination, flow cytometry was used to profile dorsal root ganglia and sciatic nerve cells. Immune cells were sorted and analyzed for cholesterol and gene expression. Nerve fibers of the skin from the paws were analyzed. Some human sural nerves were also evaluated. Comparisons were made using either t test or one-way ANOVA.

Results

Treatment with GW3965 prevented the development of mechanical hypersensitivity and thermal hyperalgesia over time in aged mice. We also observed change in polarization and cholesterol content of sciatic nerve macrophages accompanied by a significant increase in nerve fibers of the skin.

Conclusions

These results suggest that activation of the LXR may delay the PNS aging by modifying nerve-immune cell lipid content. Our study provides new potential targets to treat or delay neuropathy during aging.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02423-z.

Sympathectomy decreases pain behaviors and nerve regeneration by downregulating monocyte chemokine CCL2 in dorsal root ganglia in the rat tibial nerve crush model

ABSTRACT

Peripheral nerve regeneration is associated with pain in several preclinical models of neuropathic pain. Some neuropathic pain conditions and preclinical neuropathic pain behaviors are improved by sympathetic blockade. In this study, we examined the effect of a localized "microsympathectomy," ie, cutting the gray rami containing sympathetic postganglionic axons where they enter the L4 and L5 spinal nerves, which is more analogous to clinically used sympathetic blockade compared with chemical or surgical sympathectomy. We also examined manipulations of CCL2 (monocyte chemoattractant protein 1), a key player in both regeneration and pain. We used rat tibial nerve crush as a neuropathic pain model in which peripheral nerve regeneration can occur successfully. CCL2 in the sensory ganglia was increased by tibial nerve crush and reduced by microsympathectomy. Microsympathectomy and localized siRNA-mediated knockdown of CCL2 in the lumbar dorsal root ganglion had very similar effects: partial improvement of mechanical hypersensitivity and guarding behavior, reduction of regeneration markers growth-associated protein 43 and activating transcription factor 3, and reduction of macrophage density in the sensory ganglia and regenerating nerve. Microsympathectomy reduced functional regeneration as measured by myelinated action potential propagation through the injury site and denervation-induced atrophy of the tibial-innervated gastrocnemius muscle at day 10. Microsympathectomy plus CCL2 knockdown had behavioral effects similar to microsympathectomy alone. The results show that local sympathetic effects on neuropathic pain may be mediated in a large part by the effects on expression of CCL2, which in turn regulates the regeneration process.

Molecular Changes in the Dorsal Root Ganglion during the Late Phase of Peripheral Nerve Injury-induced Pain in Rodents: A Systematic Review

The dorsal root ganglion is widely recognized as a potential target to treat chronic pain. A fundamental understanding of quantitative molecular and genomic changes during the late phase of pain is therefore indispensable. The authors performed a systematic literature review on injury-induced pain in rodent dorsal root ganglions at minimally 3 weeks after injury. So far, slightly more than 300 molecules were quantified on the protein or messenger RNA level, of which about 60 were in more than one study. Only nine individual sequencing studies were performed in which the most up- or downregulated genes varied due to heterogeneity in study design. Neuropeptide Y and galanin were found to be consistently upregulated on both the gene and protein levels. The current knowledge regarding molecular changes in the dorsal root ganglion during the late phase of pain is limited. General conclusions are difficult to draw, making it hard to select specific molecules as a focus for treatment.

Increased Expression of Thymic Stromal Lymphopoietin in Chronic Constriction Injury of Rat Nerve

Thymic stromal lymphopoietin (TSLP) is a well-known cytokine for T helper 2 inflammatory responses. A nerve injury activates the neuroinflammation cascade and neuron-glia interaction in dorsal root ganglions (DRG)s, leading to neuropathic pain. Therefore, this study was to investigate the role of TSLP after nerve injury. Male Sprague-Dawley rats were divided as an experimental group with chronic constriction injury (CCI) to the sciatic nerve and a control group. The mechanical pain threshold response was determined by calibration forceps. After assessment of mechanical allodynia, the ipsilateral spinal cord, DRG, sciatic nerve and skin were harvested. Immunofluorescence staining was performed to identify cell types with various markers. Western blot analyses were performed to evaluate protein expressions. Mechanical allodynia developed after CCI and persisted for the next 14 days. Astrocyte reactions occurred and continued until day 14, too. After CCI, DRG and the sciatic nerve also had significantly increased expressions of TSLP/TSLP-R/STAT5. The TSLPR was localized to sensory neuronal endings innervating the skin. This study is the first to demonstrate that the TSLP complex and the STAT5 pathway in nerve are potential therapeutic targets because of their roles in pain regulation after nerve injury.

Can FDA-Approved Immunomodulatory Drugs be Repurposed/Repositioned to Alleviate Chronic Pain?

Pain is among the most widespread chronic health condition confronting society today and our inability to manage chronic pain contributes to the opioid abuse epidemic in America. The immune system is known to contribute to acute and chronic pain, but only limited therapeutic treatments such as non-steroid anti-inflammatory drugs have resulted from this knowledge. The last decade has shed light on neuro-immune interactions mediating the development, maintenance, and resolution of chronic pain. Here, we do not aim to perform a comprehensive review of all immune mechanisms involved in chronic pain, but to briefly review the contribution of the main cytokines and immune cells (macrophages, microglia, mast cells and T cells) to chronic pain. Given the urgent need to address the Pain crisis, we provocatively propose to repurpose/reposition FDA-approved immunomodulatory drugs for their potential to alleviate chronic pain. Repositioning or repurposing offers an attractive way to accelerate the arrival of new analgesics.

Contribution of colony-stimulating factor 1 to neuropathic pain

Molecular and cellular interactions among spinal dorsal horn neurons and microglia, the resident macrophages of the central nervous system, contribute to the induction and maintenance of neuropathic pain after peripheral nerve injury. Emerging evidence also demonstrates that reciprocal interactions between macrophages and nociceptive sensory neurons in the dorsal root ganglion contribute to the initiation and persistence of nerve injury-induced mechanical hypersensitivity (allodynia). We previously reported that sensory neuron-derived colony-stimulating factor 1 (CSF1), by engaging the CSF1 receptor (CSF1R) that is expressed by both microglia and macrophages, triggers the nerve injury-induced expansion of both resident microglia in the spinal cord and macrophages in the dorsal root ganglion and induces their respective contributions to the neuropathic pain phenotype. Here, we review recent research and discuss unanswered questions regarding CSF1/CSF1R-mediated microglial and macrophage signaling in the generation of neuropathic pain.

Sensory neuron-associated macrophages as novel modulators of neuropathic pain

The peripheral nervous system comprises an infinity of neural networks that act in the communication between the central nervous system and the most diverse tissues of the body. Along with the extension of the primary sensory neurons (axons and cell bodies), a population of resident macrophages has been described. These newly called sensory neuron-associated macrophages (sNAMs) seem to play an essential role in physiological and pathophysiological processes, including infection, autoimmunity, nerve degeneration/regeneration, and chronic neuropathic pain. After different types of peripheral nerve injury, there is an increase in the number and activation of sNAMs in the sciatic nerve and sensory ganglia. The activation of sNAMs and their participation in neuropathic pain development depends on the stimulation of pattern recognition receptors such as Toll-like receptors and Nod-like receptors, chemokines/cytokines, and microRNAs. On activation, sNAMs trigger the production of critical inflammatory mediators such as proinflammatory cytokines (eg, TNF and IL-1β) and reactive oxygen species that can act in the amplification of primary sensory neurons sensitization. On the other hand, there is evidence that sNAMs can produce antinociceptive mediators (eg, IL-10) that counteract neuropathic pain development. This review will present the cellular and molecular mechanisms behind the participation of sNAMs in peripheral nerve injury-induced neuropathic pain development. Understanding how sNAMs are activated and responding to nerve injury can help set novel targets for the control of neuropathic pain.

Analysis of the immune response to sciatic nerve injury identifies efferocytosis as a key mechanism of nerve debridement

Sciatic nerve crush injury triggers sterile inflammation within the distal nerve and axotomized dorsal root ganglia (DRGs). Granulocytes and pro-inflammatory Ly6C^high monocytes infiltrate the nerve first and rapidly give way to Ly6C^negative inflammation-resolving macrophages. In axotomized DRGs, few hematogenous leukocytes are detected and resident macrophages acquire a ramified morphology. Single-cell RNA-sequencing of injured sciatic nerve identifies five macrophage subpopulations, repair Schwann cells, and mesenchymal precursor cells. Macrophages at the nerve crush site are molecularly distinct from macrophages associated with Wallerian degeneration. In the injured nerve, macrophages ‘eat’ apoptotic leukocytes, a process called efferocytosis, and thereby promote an anti-inflammatory milieu. Myeloid cells in the injured nerve, but not axotomized DRGs, strongly express receptors for the cytokine GM-CSF. In GM-CSF-deficient (Csf2^-/-) mice, inflammation resolution is delayed and conditioning-lesion-induced regeneration of DRG neuron central axons is abolished. Thus, carefully orchestrated inflammation resolution in the nerve is required for conditioning-lesion-induced neurorepair.

Origin and Differentiation of Nerve-Associated Macrophages

The mature peripheral nervous system is a steady network structure yet shows remarkable regenerative properties. The interaction of peripheral nerves with myeloid cells has largely been investigated in the context of damage, following trauma or infection. Recently, specific macrophages dedicated to homeostatic peripheral nerves have come into focus. These macrophages are defined by tissue and nerve type, are seeded in part prenatally, and self-maintain via proliferation. Thus, they are markedly distinct from monocyte-derived macrophages invading after local disturbance of nerve integrity. The phenotypic and transcriptional adaptation of macrophages to the discrete nervous niche may exert axon guidance and nerve regeneration and thus contribute to the stability of the peripheral nervous network. Deciphering these conserved macrophage-nerve interactions offers new translational perspectives for chronic diseases of the peripheral nervous system, such as diabetic neuropathy and pain.

Unique Characteristics of the Dorsal Root Ganglion as a Target for Neuromodulation

Objective

The dorsal root ganglion (DRG) is a novel target for neuromodulation, and DRG stimulation is proving to be a viable option in the treatment of chronic intractable neuropathic pain. Although the overall principle of conventional spinal cord stimulation (SCS) and DRG stimulation—in which an electric field is applied to a neural target with the intent of affecting neural pathways to decrease pain perception—is similar, there are significant differences in the anatomy and physiology of the DRG that make it an ideal target for neuromodulation and may account for the superior outcomes observed in the treatment of certain chronic neuropathic pain states. This review highlights the anatomy of the DRG, its function in maintaining homeostasis and its role in neuropathic pain, and the unique value of DRG as a target in neuromodulation for pain.

Methods

A narrative literature review was performed.

Results

Overall, the DRG is a critical structure in sensory transduction and modulation, including pain transmission and the maintenance of persistent neuropathic pain states. Unique characteristics including selective somatic organization, specialized membrane characteristics, and accessible and consistent location make the DRG an ideal target for neuromodulation. Because DRG stimulation directly recruits the somata of primary sensory neurons and harnesses the filtering capacity of the pseudounipolar neural architecture, it is differentiated from SCS, peripheral nerve stimulation, and other neuromodulation options.

Conclusions

There are several advantages to targeting the DRG, including lower energy usage, more focused and posture-independent stimulation, reduced paresthesia, and improved clinical outcomes.

Subarachnoid Hemorrhage Induces Dynamic Immune Cell Reactions in the Choroid Plexus

Subarachnoid hemorrhage (SAH) is a specific form of hemorrhagic stroke that frequently causes intracranial hypertension. The choroid plexus (CP) of the brain ventricles is responsible for producing cerebrospinal fluid and forms the blood - cerebrospinal fluid barrier. The aim of the current study was to determine whether SAH induces an immune cell reaction in the CP and whether the resulting increase in intracranial pressure (ICP) itself can lead to cellular changes in the CP. SAH was induced by injecting non-heparinized autologous blood to the cisterna magna. Artificial cerebrospinal fluid (ACSF) instead of blood was used to assess influence of increased ICP alone. SAH and ACSF animals were left to survive for 1, 3, and 7 days. SAH induced significantly increased numbers of M1 (ED1+, CCR7+) and M2 (ED2+, CD206+) macrophages as well as MHC-II+ antigen presenting cells (APC) compared to naïve and ACSF animals. Increased numbers of ED1+ macrophages and APC were found in the CP only 3 and 7 days after ACSF injection, while ED2+ macrophage number did not increase. CD3+ T cells were not found in any of the animals. Following SAH, proliferation activity in the CP gradually increased over time while ACSF application induced higher cellular proliferation only 1 and 3 days after injection. Our results show that SAH induces an immune reaction in the CP resulting in an increase in the number of several macrophage types in the epiplexus position. Moreover, we also found that increased ICP due to ACSF application induced both an immune reaction and increased proliferation of epiplexus cells in the CP. These findings indicate that increased ICP, and not just blood, contributes to cellular changes in the CP following SAH.

Granulocyte-Macrophage Colony Stimulating Factor As an Indirect Mediator of Nociceptor Activation and Pain

The interaction between the immune system and the nervous system has been at the center of multiple research studies in recent years. Whereas the role played by cytokines as neuronal mediators is no longer contested, the mechanisms by which cytokines modulate pain processing remain to be elucidated. In this study, we have analyzed the involvement of granulocyte-macrophage colony stimulating factor (GM-CSF) in nociceptor activation in male and female mice. Previous studies have suggested GM-CSF might directly activate neurons. However, here we established the absence of a functional GM-CSF receptor in murine nociceptors, and suggest an indirect mechanism of action, via immune cells. We report that GM-CSF applied directly to magnetically purified nociceptors does not induce any transcriptional changes in nociceptive genes. In contrast, conditioned medium from GM-CSF-treated murine macrophages was able to drive nociceptor transcription. We also found that conditioned medium from nociceptors treated with the well established pain mediator, nerve growth factor, could also modify macrophage gene transcription, providing further evidence for a bidirectional crosstalk.SIGNIFICANCE STATEMENT The interaction of the immune system and the nervous system is known to play an important role in the development and maintenance of chronic pain disorders. Elucidating the mechanisms of these interactions is an important step toward understanding, and therefore treating, chronic pain disorders. This study provides evidence for a two-way crosstalk between macrophages and nociceptors in the peripheral nervous system, which may contribute to the sensitization of nociceptors by cytokines in pain development.

Dorsal root ganglion macrophages contribute to both the initiation and persistence of neuropathic pain

Paralleling the activation of dorsal horn microglia after peripheral nerve injury is a significant expansion and proliferation of macrophages around injured sensory neurons in dorsal root ganglia (DRG). Here we demonstrate a critical contribution of DRG macrophages, but not those at the nerve injury site, to both the initiation and maintenance of the mechanical hypersensitivity that characterizes the neuropathic pain phenotype. In contrast to the reported sexual dimorphism in the microglial contribution to neuropathic pain, depletion of DRG macrophages reduces nerve injury-induced mechanical hypersensitivity and expansion of DRG macrophages in both male and female mice. However, fewer macrophages are induced in the female mice and deletion of colony-stimulating factor 1 from sensory neurons, which prevents nerve injury-induced microglial activation and proliferation, only reduces macrophage expansion in male mice. Finally, we demonstrate molecular cross-talk between axotomized sensory neurons and macrophages, revealing potential peripheral DRG targets for neuropathic pain management.

Interactions among spinal dorsal horn neurons and microglia contribute to the induction and maintenance of neuropathic pain after peripheral nerve injury. The authors show that depletion of macrophages in the dorsal root ganglia prevents and reverses ongoing nerve injury-induced hypersensitivity.

An Intimate Role for Adult Dorsal Root Ganglia Resident Cycling Cells in the Generation of Local Macrophages and Satellite Glial Cells

The intricate interactions between neurons, glial, and inflammatory cells within peripheral ganglia are physiologically important, but not well explored. Here, we show that adult dorsal root ganglia (DRG) contain populations of self-renewing cells, collectively referred as DRG resident cycling cells (DRCCs), that are active not only in "quiescent" ganglia but also accelerate their turnover in response to distal axotomy. An unexpected proportion of DRCCs were resident macrophages. These cells closely accompanied, and aligned with recycling satellite glial cells (SGCs) that were juxtaposed to sensory neurons and possessed stem cell-like properties. Selective inhibition of colony stimulating factor 1 receptor prevented the local proliferation of macrophages. Interestingly, DRCC turnover was accompanied by apoptosis at later intervals indicating a balanced cellular milieu in the DRGs. These findings identify a complex interactive multicellular DRG microenvironment supporting self-renewal of both macrophages and SGCs and its potential implications in the overall response of adult DRGs to injury.

Toll-like receptor 2 plays an essential role in electroacupuncture analgesia in a mouse model of inflammatory pain

BACKGROUND

Inflammatory pain occurs when local tissue injury activates macrophages and neutrophils, hence increasing pro-inflammatory cytokine and chemokine levels. Toll-like receptor 2 (TLR2) antagonism reportedly suppresses neuropathic and inflammatory pain.

AIMS

In the present study, we investigated the effect of electroacupuncture (EA) on TLR2 and related signalling molecules in a complete Freund's adjuvant (CFA)-induced mouse model of inflammatory pain to determine whether EA can attenuate inflammatory pain via the TLR2 signalling pathway.

METHODS

EA significantly reduced mechanical and thermal hyperalgesia in the animal model. A similar effect was produced by TLR2 antagonism induced by CU-CPT22 injection.

RESULTS

TLR2 expression in the dorsal root ganglia, spinal cord and thalamus increased following induction of inflammation. Expression levels of downstream molecules such as pPI3K, pAkt and pmTOR also increased, as did those of MAPK subfamily members such as pERK, pp38 and pJNK. Transcription factors (pCREB and pNFκB) and nociceptive ion channels (Nav1.7 and Nav1.8) were also involved.

CONCLUSION

Increased expression of the above molecules was attenuated by both EA and TLR2 antagonism. Our results show that EA attenuates inflammatory pain via TLR2 signalling.

Rapid Isolation of Dorsal Root Ganglion Macrophages

There are growing interests to study the molecular and cellular interactions among immune cells and sensory neurons in the dorsal root ganglia after peripheral nerve injury. Peripheral monocytic cells, including macrophages, are known to respond to a tissue injury through phagocytosis, antigen presentation, and cytokine release. Emerging evidence has implicated the contribution of dorsal root ganglia macrophages to neuropathic pain development and axonal repair in the context of nerve injury. Rapidly phenotyping (or "rapid isolation of") the response of dorsal root ganglia macrophages in the context of nerve injury is desired to identify the unknown neuroimmune factors. Here we demonstrate how our lab rapidly and effectively isolates macrophages from the dorsal root ganglia using an enzyme-free mechanical dissociation protocol. The samples are kept on ice throughout to limit cellular stress. This protocol is far less time consuming compared to the standard enzymatic protocol and has been routinely used for our Fluorescence-activated Cell Sorting analysis.

Sensory profiles are comparable in patients with distal and proximal entrapment neuropathies, while the pain experience differs

OBJECTIVE

Distal and proximal entrapment neuropathies such as carpal tunnel syndrome (CTS) and cervical radiculopathy (CR) share similar etiologies. Experimental models suggest that, despite comparable etiology, pathomechanisms associated with injuries of the peripheral and central axon branches are distinct. This study therefore compared self-reported and elicited sensory profiles in patients with distal and proximal entrapment neuropathies.

METHODS

Patients with electrodiagnostically confirmed CTS (n = 103) and patients with CR (n = 23) were included in this study. A group of healthy participants served as controls (n = 39). Symptoms and sensory profiles were evaluated using quantitative sensory testing (QST) and a self-reported neuropathic pain questionnaire (painDETECT).

RESULTS

Both patient groups were characterized by a loss of function in thermal and mechanical detection in the main pain area and dermatome compared to healthy reference data (p < .001). There was no significant difference between patients with CTS and CR in pain and detection thresholds except for reduced vibration sense in the main pain area (p < .001) and reduced pressure pain sensitivity in the dermatome in patients with CR (p < .001). However, patients with CR reported higher pain intensities (p = .008), more severe pain attacks (p = .009) and evoked pain by light pressure (p = .002) compared to patients with CTS.

CONCLUSION

While QST profiles were similar between patients with CTS and CR, self-reported pain profiles differed and may suggest distinct underlying mechanisms in these patient cohorts.

Primary Cultures from Rat Dorsal Root Ganglia: Responses of Neurons and Glial Cells to Somatosensory or Inflammatory Stimulation

Primary cultures of rat dorsal root ganglia (DRG) consist of neurons, satellite glial cells and a moderate number of macrophages. Measurements of increased intracellular calcium [Ca²⁺]_i induced by stimuli, have revealed that about 70% of DRG neurons are capsaicin-responsive nociceptors, while 10% responded to cooling and or menthol (putative cold sensors). Cultivation of DRG in the presence of a moderate dose of lipopolysaccharide (LPS, 1 µg/ml) enhanced capsaicin-induced Ca²⁺ signals. We therefore investigated further properties of DRG primary cultures stimulated with 10 µg/ml LPS for a short period. Exposure to LPS for 2 h resulted in pronounced release of tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) into the supernatants of DRG cultures, increased expression of both cytokines in the DRG cells and increased TNF immunoreactivity predominantly in macrophages. We further observed an accumulation of the inflammatory transcription factors NF-IL6 and STAT3 in the nuclei of LPS-exposed DRG neurons and macrophages. In the presence of the cytotoxic agent cisplatin (5 or 10 µg/ml), the number of macrophages was decreased significantly, the growth of satellite glial cells was markedly suppressed, but the vitality and stimulus-induced Ca²⁺ signals of DRG neurons were not impaired. Under these conditions the LPS-induced production and expression of TNF-α and IL-6 were blunted. Our data suggest a potential role for macrophages and satellite glial cells in the initiation of inflammatory processes that develop in sensory ganglia upon injury or exposure to pathogens.

Molecular and cellular identification of the immune response in peripheral ganglia following nerve injury

BACKGROUND

Neuroinflammation accompanies neural trauma and most neurological diseases. Axotomy in the peripheral nervous system (PNS) leads to dramatic changes in the injured neuron: the cell body expresses a distinct set of genes known as regeneration-associated genes, the distal axonal segment degenerates and its debris is cleared, and the axons in the proximal segment form growth cones and extend neurites. These processes are orchestrated in part by immune and other non-neuronal cells. Macrophages in ganglia play an integral role in supporting regeneration. Here, we explore further the molecular and cellular components of the injury-induced immune response within peripheral ganglia.

METHODS

Adult male wild-type (WT) and Ccr2 ^-/- mice were subjected to a unilateral transection of the sciatic nerve and axotomy of the superior cervical ganglion (SCG). Antibody arrays were used to determine the expression of chemokines and cytokines in the dorsal root ganglion (DRG) and SCG. Flow cytometry and immunohistochemistry were utilized to identify the cellular composition of the injury-induced immune response within ganglia.

RESULTS

Chemokine expression in the ganglia differed 48 h after nerve injury with a large increase in macrophage inflammatory protein-1γ in the SCG but not in the DRG, while C-C class chemokine ligand 2 was highly expressed in both ganglia. Differences between WT and Ccr2 ^-/- mice were also observed with increased C-C class chemokine ligand 6/C10 expression in the WT DRG compared to C-C class chemokine receptor 2 (CCR2)^-/- DRG and increased CXCL5 expression in CCR2^-/- SCG compared to WT. Diminished macrophage accumulation in the DRG and SCG of Ccr2 ^-/- mice was found compared to WT ganglia 7 days after nerve injury. Interestingly, neutrophils were found in the SCG but not in the DRG. Cytokine expression, measured 7 days after injury, differed between ganglion type and genotype. Macrophage activation was assayed by colabeling ganglia with the anti-inflammatory marker CD206 and the macrophage marker CD68, and an almost complete colocalization of the two markers was found in both ganglia.

CONCLUSIONS

This study demonstrates both molecular and cellular differences in the nerve injury-induced immune response between DRG and SCG and between WT and Ccr2 ^-/- mice.

The Glutamatergic System in Primary Somatosensory Neurons and Its Involvement in Sensory Input-Dependent Plasticity

Glutamate is the most common neurotransmitter in both the central and the peripheral nervous system. Glutamate is present in all types of neurons in sensory ganglia, and is released not only from their peripheral and central axon terminals but also from their cell bodies. Consistently, these neurons express ionotropic and metabotropic receptors, as well as other molecules involved in the synthesis, transport and release of the neurotransmitter. Primary sensory neurons are the first neurons in the sensory channels, which receive information from the periphery, and are thus key players in the sensory transduction and in the transmission of this information to higher centers in the pathway. These neurons are tightly enclosed by satellite glial cells, which also express several ionotropic and metabotropic glutamate receptors, and display increases in intracellular calcium accompanying the release of glutamate. One of the main interests in our group has been the study of the implication of the peripheral nervous system in sensory-dependent plasticity. Recently, we have provided novel evidence in favor of morphological changes in first- and second-order neurons of the trigeminal system after sustained alterations of the sensory input. Moreover, these anatomical changes are paralleled by several molecular changes, among which those related to glutamatergic neurotransmission are particularly relevant. In this review, we will describe the state of the art of the glutamatergic system in sensory ganglia and its involvement in input-dependent plasticity, a fundamental ground for advancing our knowledge of the neural mechanisms of learning and adaptation, reaction to injury, and chronic pain.

Interleukin-8 levels in rat models of nerve damage and neuropathic pain

Interleukin-8 (IL-8) is a pro-inflammatory cytokine that has been shown to play a role in inflammatory and autoimmune disorders. The objective of the present study was to assess the levels of IL-8 in rat serum, dorsal root ganglion (DRG) and the sciatic nerve following four different forms of sciatic nerve injury. The models used to induce the injury included partial sciatic ligation (PSL), chronic constriction injury (CCI), perineural inflammation (neuritis) and complete sciatic transection (CST). Mechanical and thermal hyperalgesia were detected by measuring withdrawal responses from a mechanical stimulus and withdrawal latency from thermal stimulation. Enzyme-linked immunosorbent assays (ELISA) was used to assess the IL-8 levels in the affected and contralateral sciatic nerves. Rats exposed to PSL and neuritis developed significant nociceptive response (mechanical and thermal hyperalgesia) in the affected side at three days post-surgery whereas the CCI group at eight days post-surgery. No mechanical or thermal hyperalgesia was observed in rats exposed to CST at either three or eight days postsurgery. Additionally, IL-8 levels were significantly increased in the injured sciatic nerve at 3 and 8days following PSL and neuritis as well as at 8days following CCI when compared to naïve animals. A significant up regulation of IL-8 levels was observed in the ipsilateral DRG at 3 and 8days following CST compared to naïve animals. The serum IL-8 levels remained unchanged in all models of nerve damage. The results of this study suggest that IL-8's role in the neuropathic pain etiology may be specific to nerve injury type.

Inflammatory Changes in Paravertebral Sympathetic Ganglia in Two Rat Pain Models

Injury to peripheral nerves can lead to neuropathic pain, along with well-studied effects on sensory neurons, including hyperexcitability, abnormal spontaneous activity, and neuroinflammation in the sensory ganglia. Neuropathic pain can be enhanced by sympathetic activity. Peripheral nerve injury may also damage sympathetic axons or expose them to an inflammatory environment. In this study, we examined the lumbar sympathetic ganglion responses to two rat pain models: ligation of the L5 spinal nerve, and local inflammation of the L5 dorsal root ganglion (DRG), which does not involve axotomy. Both models resulted in neuroinflammatory changes in the sympathetic ganglia, as indicated by macrophage responses, satellite glia activation, and increased numbers of T cells, along with very modest increases in sympathetic neuron excitability (but not spontaneous activity) measured in ex vivo recordings. The spinal nerve ligation model generally caused larger responses than DRG inflammation. Plasticity of the sympathetic system should be recognized in studies of sympathetic effects on pain.

Cellular reactions of the choroid plexus induced by peripheral nerve injury

The choroid plexus (CP) of brain ventricles forms the blood-cerebrospinal fluid (blood-CSF) barrier that is involved in many diseases affecting the central nervous system (CNS). We used ED1 and ED2 immunostaining to investigate epiplexus cell changes in rat CP after chronic constriction injury (CCI). In contrast to naïve CP, the CP of sham-operated rats showed an increase in the number of ED1+ cells of a similar magnitude during all periods of survival up to 3 weeks, while the number of ED2+ increased only at 3 days from operation. In comparison to naïve and sham-operated animals, the number of ED1+ and ED2+ cells in the epiplexus position increased with the duration of nerve compression. We detected no or negligible cell proliferation in the CP after sham- or CCI-operation. This suggests that increased number of ED1+ and ED2+ cells in the epiplexus position of the CP is derived from peripheral monocytes passing through altered blood-CSF barrier. The changes in epiplexus cells indicate that the CP reacts to tissue injury after the surgical approach itself and that the response to peripheral nerve lesion is greater. This suggests a role for an altered blood-CSF barrier allowing for propagation of signal molecules from damaged tissue and nerve to the CNS.

Effects of differently activated rodent macrophages on sensory neurons: implications for arthritis pain

OBJECTIVE

In arthritis, macrophages invade the affected joint. Experimental arthritis models have shown that macrophages also invade the dorsal root ganglia (DRGs) of the inflamed segments in which the perikarya of sensory neurons are located. It is unclear whether this macrophage invasion contributes to arthritis pain and/or furthers neuronal damage. The present study was undertaken to investigate how differently activated macrophages affect DRG neurons.

METHODS

We determined the phenotype of macrophages in the DRGs of rats with antigen-induced arthritis (AIA). In a DRG neuron-macrophage coculture system, we investigated whether differently activated macrophages (stimulated with either lipopolysaccharide [LPS]/interferon-γ [IFNγ], tumor necrosis factor [TNF], or interleukin-4) damage DRG neurons and/or stimulate them to release the mediator calcitonin gene-related peptide (CGRP), which promotes pain and neurogenic inflammation.

RESULTS

Macrophages in the DRGs of rats with AIA showed the phenotype of TNF-stimulated macrophages but did not express inducible nitric oxide synthase, which was found in cultured macrophages only after LPS/IFNγ activation. In neuron-macrophage cocultures, activation of macrophages stimulated DRG neurons to release CGRP within 1 hour, indicating neuronal activation by macrophages. Only 48-hour activation of macrophages with LPS/IFNγ increased the neuronal cell death rate in culture, provided that the macrophages were in direct contact with DRG neurons. This effect was dependent on nitric oxide.

CONCLUSION

Macrophages have the potential to stimulate sensory neurons in the DRGs, and this may contribute to arthritis pain. If they are classically activated, such as after LPS/IFNγ stimulation, this may also further neuronal cell death. This is not the case in AIA but may occur in models involving damage of sensory neurons.

Induction of a reactive state in perineuronal satellite glial cells akin to that produced by nerve injury is linked to the level of p75NTR expression in adult sensory neurons

Satellite glial cells (SGCs) surrounding primary sensory neurons are similar to astrocytes of the central nervous system in that they buffer the extracellular environment via potassium and calcium channels and express the intermediate filament glial fibrillary acidic protein (GFAP). Peripheral nerve injury induces a reactive state in SGCs that includes SGC proliferation, increased SGC/SGC coupling via gap junctions, decreased inward rectifying potassium channel 4.1 (Kir 4.1) expression and increased expression of GFAP and the common neurotrophin receptor, p75NTR. In contrast, neuronal p75NTR expression, normally detected in ∼80% of adult rat sensory neurons, decreases in response to peripheral axotomy. Given the differential regulation of p75NTR expression in neurons versus SGCs with injury, we hypothesized that reduced signaling via neuronal p75NTR contributes to the induction of a reactive state in SGCs. We found that reducing neuronal p75NTR protein expression in uninjured sensory neurons by intrathecal subarachnoid infusion of p75NTR-selective anti-sense oligodeoxynucleotides for one week was sufficient to induce a "reactive-like" state in the perineuronal SGCs akin to that normally observed following peripheral nerve injury. This reactive state included significantly increased SGC p75NTR, GFAP and gap junction protein connexin-43 protein expression, increased numbers of SGCs surrounding individual sensory neurons and decreased SGC Kir 4.1 channel expression. Collectively, this supports the tenet that reductions in target-derived trophic support leading to, or as a consequence of, reduced neuronal p75NTR expression plays a critical role in switching the SGC to a reactive state.

Genome wide expression profiling during spinal cord regeneration identifies comprehensive cellular responses in zebrafish

BACKGROUND

Among the vertebrates, teleost and urodele amphibians are capable of regenerating their central nervous system. We have used zebrafish as a model to study spinal cord injury and regeneration. Relatively little is known about the molecular mechanisms underlying spinal cord regeneration and information based on high density oligonucleotide microarray was not available. We have used a high density microarray to profile the temporal transcriptome dynamics during the entire phenomenon.

RESULTS

A total of 3842 genes expressed differentially with significant fold changes during spinal cord regeneration. Cluster analysis revealed event specific dynamic expression of genes related to inflammation, cell death, cell migration, cell proliferation, neurogenesis, neural patterning and axonal regrowth. Spatio-temporal analysis of stat3 expression suggested its possible function in controlling inflammation and cell proliferation. Genes involved in neurogenesis and their dorso-ventral patterning (sox2 and dbx2) are differentially expressed. Injury induced cell proliferation is controlled by many cell cycle regulators and some are commonly expressed in regenerating fin, heart and retina. Expression pattern of certain pathway genes are identified for the first time during regeneration of spinal cord. Several genes involved in PNS regeneration in mammals like stat3, socs3, atf3, mmp9 and sox11 are upregulated in zebrafish SCI thus creating PNS like environment after injury.

CONCLUSION

Our study provides a comprehensive genetic blue print of diverse cellular response(s) during regeneration of zebrafish spinal cord. The data highlights the importance of different event specific gene expression that could be better understood and manipulated further to induce successful regeneration in mammals.

Inflammation in dorsal root ganglia after peripheral nerve injury: effects of the sympathetic innervation

Following a peripheral nerve injury, a sterile inflammation develops in sympathetic and dorsal root ganglia (DRGs) with axons that project in the damaged nerve trunk. Macrophages and T-lymphocytes invade these ganglia where they are believed to release cytokines that lead to hyperexcitability and ectopic discharge, possibly contributing to neuropathic pain. Here, we examined the role of the sympathetic innervation in the inflammation of L5 DRGs of Wistar rats following transection of the sciatic nerve, comparing the effects of specific surgical interventions 10-14 days prior to the nerve lesion with those of chronic administration of adrenoceptor antagonists. Immunohistochemistry was used to define the invading immune cell populations 7 days after sciatic transection. Removal of sympathetic activity in the hind limb by transecting the preganglionic input to the relevant lumbar sympathetic ganglia (ipsi- or bilateral decentralization) or by ipsilateral removal of these ganglia with degeneration of postganglionic axons (denervation), caused less DRG inflammation than occurred after a sham sympathectomy. By contrast, denervation of the lymph node draining the lesion site potentiated T-cell influx. Systemic treatment with antagonists of α1-adrenoceptors (prazosin) or β-adrenoceptors (propranolol) led to opposite but unexpected effects on infiltration of DRGs after sciatic transection. Prazosin potentiated the influx of macrophages and CD4(+) T-lymphocytes whereas propranolol tended to reduce immune cell invasion. These data are hard to reconcile with many in vitro studies in which catecholamines acting mainly via β2-adrenoceptors have inhibited the activation and proliferation of immune cells following an inflammatory challenge.

Tumor necrosis factor-α-dependent infiltration of macrophages into the dorsal root ganglion in a rat disc herniation model

STUDY DESIGN

A prospective molecular mechanism of macrophages infiltration in experimental disc herniation.

OBJECTIVE

To investigate the mechanisms of macrophages infiltration into the dorsal root ganglion (DRG) in a rat model of disc herniation.

SUMMARY OF BACKGROUND DATA

Macrophages infiltrate the DRG after application of nucleus pulposus (NP) on the DRG, and may play an important role in radiculopathy. However, the mechanisms of macrophages infiltration after NP application remain poorly understood.

METHODS

After experimental disc herniation in this study, we investigated changes in the expression of ED1 (a marker of macrophages) and vascular cell adhesion molecule-1 (VCAM-1) in DRG using immunofluorescence. We also investigated the expression of ED1 and VCAM-1 in DRG by treatment with tumor necrosis factor-α (TNF-α) inhibitor at the time of surgery.

RESULTS

We found a massive ED1-positive macrophages infiltrated the DRG, and VCAM-1-like immunoreactivity vessels became evident after NP application. Furthermore, both macrophage infiltration and VCAM-1 expression were prevented by treatment with TNF-α inhibitor at the time of surgery.

CONCLUSION

These findings indicated that macrophages infiltration into the DRG was TNF-α-dependent, and might be partly mediated by VCAM-1 in the early stage of experimental lumbar disc herination. Taken together, this study provides important preliminary data suggesting that TNF-α plays an important role in the macrophage infiltration.

LEVEL OF EVIDENCE

N/A.

Satellite glial cell proliferation in the trigeminal ganglia after chronic constriction injury of the infraorbital nerve

We have examined satellite glial cell (SGC) proliferation in trigeminal ganglia following chronic constriction injury of the infraorbital nerve. Using BrdU labeling combined with immunohistochemistry for SGC specific proteins we positively confirmed proliferating cells to be SGCs. Proliferation peaks at approximately 4 days after injury and dividing SGCs are preferentially located around neurons that are immunopositive for ATF-3, a marker of nerve injury. After nerve injury there is an increase GFAP expression in SGCs associated with both ATF-3 immunopositive and immunonegative neurons throughout the ganglia. SGCs also express the non-glial proteins, CD45 and CD163, which label resident macrophages and circulating leukocytes, respectively. In addition to SGCs, we found some Schwann cells, endothelial cells, resident macrophages, and circulating leukocytes were BrdU immunopositive.

Activation profile of dorsal root ganglia Iba-1 (+) macrophages varies with the type of lesion in rats

The interactions between neurons, immune and immune-like glial cells can initiate the abnormal processes that underlie neuropathic pain. In the peripheral nervous system the resident macrophages may play an important role. In this study we investigated in experimental adult Sprague-Dawley rats how Iba-1 (ionized calcium binding adaptor molecule 1) (+) resident macrophages in the dorsal root ganglion (DRG) are activated after a spinal nerve ligation (SNL) or streptozotocin (STZ)-induced diabetes. The activation profile was defined by comparing the responses of resident macrophages against microglia in the spinal cord as they share a common origin. After SNL, the Iba-1 (+) macrophages in L5 DRG reached their activation peak 5 days later, clustered as satellite cells around large A-neurons, expressed the MHC-II marker, but did not show p-p38 and p-ERK1/2 activation and did not secrete IL-18. After STZ-induced diabetes, the Iba-1 (+) macrophages reached their activation peak 1 week later in L4 and L5 DRG, remained scattered between neurons, expressed the MHC-II marker only in L5 DRG, did not show p-p38 and p-ERK1/2 activation and did not secrete any of the investigated cytokines/chemokines. These responses suggest that depending on the type of lesion DRG Iba-1 (+) resident macrophages have different activation mechanisms, which are dissimilar to those in microglia.

Local and remote immune-mediated inflammation after mild peripheral nerve compression in rats

After experimental nerve injuries that extensively disrupt axons, such as chronic constriction injury, immune cells invade the nerve, related dorsal root ganglia (DRGs), and spinal cord, leading to hyperexcitability, raised sensitivity, and pain. Entrapment neuropathies, such as carpal tunnel syndrome, involve minimal axon damage, but patients often report widespread symptoms. To understand the underlying pathology, a tube was placed around the sciatic nerve in 8-week-old rats, leading to progressive mild compression as the animals grew. Immunofluorescence was used to examine myelin and axonal integrity, glia, macrophages, and T lymphocytes in the nerve, L5 DRGs, and spinal cord after 12 weeks. Tubes that did not constrict the nerve when applied caused extensive and ongoing loss of myelin, together with compromise of small-, but not large-, diameter axons. Macrophages and T lymphocytes infiltrated the nerve and DRGs. Activated glia proliferated in DRGs but not in spinal cord. Histologic findings were supported by clinical hyperalgesia to blunt pressure and cold allodynia. Tubes that did not compress the nerve induced only minor local inflammation. Thus, progressive mild nerve compression resulted in chronic local and remote immune-mediated inflammation depending on the degree of compression. Such neuroinflammation may explain the widespread symptoms in patients with entrapment neuropathies.

Bilateral changes of cannabinoid receptor type 2 protein and mRNA in the dorsal root ganglia of a rat neuropathic pain model

Cannabinoid receptor type 2 (CB2R) plays a critical role in nociception. In contrast to cannabinoid receptor type 1 ligands, CB2R agonists do not produce undesirable central nervous system effects and thus promise to treat neuropathic pain that is often resistant to medical therapy. In the study presented here, we evaluated the bilateral distribution of the CB2R protein and messenger RNA (mRNA) in rat dorsal root ganglia (DRG) after unilateral peripheral nerve injury using immunohistochemistry, western blot, and in situ hybridization analysis. Unilateral chronic constriction injury (CCI) of the sciatic nerve induced neuropathic pain behavior and bilateral elevation of both CB2R protein and mRNA in lumbar L4-L5 as well as cervical C7-C8 DRG when compared with naive animals. CB2R protein and mRNA were increased not only in DRG neurons but also in satellite glial cells. The fact that changes appear bilaterally and (albeit at a lower level) even in the remote cervical DRG can be related to propagation of neuroinflammation alongside the neuraxis and to the neuroprotective effects of CB2R.

Segmental spinal root avulsion in the adult rat: a model to study avulsion injury pain

Road traffic accidents are the most common cause of avulsion injury, in which spinal roots are torn from the spinal cord. Patients suffer from a loss of sensorimotor function, intractable spontaneous pain, and border-zone hypersensitivity. The neuropathic pains are particularly difficult to treat because the lack of a well-established animal model of avulsion injury prevents identifying the underlying mechanisms and hinders the development of efficacious drugs. This article describes a hindlimb model of avulsion injury in adult rats where the L5 dorsal and ventral spinal root are unilaterally avulsed (spinal root avulsion [SRA]), leaving the adjacent L4 spinal root intact. SRA produced a significant ipsilateral hypersensitivity to mechanical and thermal stimulation by 5 days compared with sham-operated or naïve rats. This hypersensitivity is maintained for up to 60 days. No autotomy was observed and locomotor deficits were minimal. The hypersensitivity to peripheral stimuli could be temporarily ameliorated by administration of amitriptyline and carbamazepine, drugs that are currently prescribed to avulsion patients. Histological assessment of the L4 ganglion cells revealed no significant alterations in calcitonin gene-related peptide (CGRP), IB4, transient receptor potential cation channel subfamily V member 1 (TrpV1), or N52 staining across groups. Immunohistochemistry of the spinal cord revealed a localized glial response, phagocyte infiltration, and neuronal loss within the ipsilateral avulsed segment. A comparable response from glia and phagocytes was also found in the intact L4 spinal cord, supporting the role for central mechanisms within the L4-5 spinal cord in contributing to the generation of the pain-related behavior. The SRA model provides a platform to investigate possible new pharmacological treatments for avulsion injuries.

Causes and prevention of chronic postsurgical pain

PURPOSE OF REVIEW

Surgical incision invariably causes some measure of nerve damage and inflammatory response that, in most cases, heals quickly without long-term negative consequence. However, a subset of these patients go on to develop lasting neuropathic pain that is difficult to treat and, in many cases, prevents the return to normal activities of life. It remains unknown why two patients with identical surgical interventions may go on to develop completely divergent pain phenotypes or no pain at all. Aggressive, early analgesic therapy has been shown to reduce the incidence of chronic postsurgical pain (CPSP), but no specific regional anesthetic technique or systemic pharmacologic therapy has been shown to prevent CPSP.

RECENT FINDINGS

Inflammation and glial cell activation have recently been shown to be just as important in the transition from normal acute pain to pathologic chronic pain as nerve injury itself and that central sensitization may not be solely due to repetitive nociceptive firing at the time of nerve injury. This has opened a number of new therapeutic possibilities for prevention of CPSP.

SUMMARY

Here, we discuss the causes of CPSP and current useful preventive strategies in the perioperative period. We also discuss future potential disease-modifying treatments of CPSP.

Site-specific subtypes of macrophages recruited after peripheral nerve injury

After partial ligation of mouse sciatic nerve, the subtypes of macrophages were examined in the injured nerve and dorsal root ganglia (DRGs). Many M1 macrophages, which were inducible nitric oxide synthase (iNOS)-positive and arginase-1 (Arg-1)-negative, and neutrophils infiltrated the injured nerve. In contrast, almost all macrophages infiltrating the ipsilateral side of DRGs after the nerve injury were iNOS⁻/Arg-1⁺, M2 type. The infiltration of M1 and M2 macrophages was first observed in the injured nerve and ipsilateral DRGs on days 1 and 2, respectively. In addition, the macrophage infiltration preceded the activation of microglia in the ipsilateral dorsal horn of spinal cord. Thus, infiltrating macrophages after peripheral nerve injury may play unique roles dependent on the location in the development of neuropathic pain.