Up-regulation of acid-sensing ion channel 3 in dorsal root ganglion neurons following application of nucleus pulposus on nerve root in rats

Transcriptional reprogramming post-peripheral nerve injury: A systematic review

Neuropathic pain is characterised by periodic or continuous hyperalgesia, numbness, or allodynia, and results from insults to the somatosensory nervous system. Peripheral nerve injury induces transcriptional reprogramming in peripheral sensory neurons, contributing to increased spinal nociceptive input and the development of neuropathic pain. Effective treatment for neuropathic pain remains an unmet medical need as current therapeutics offer limited effectiveness and have undesirable effects. Understanding transcriptional changes in peripheral nerve injury-induced neuropathy might offer a path for novel analgesics. Our literature search identified 65 papers exploring transcriptomic changes post-peripheral nerve injury, many of which were conducted in animal models. We scrutinize their transcriptional changes data and conduct gene ontology enrichment analysis to reveal their common functional profile. Focusing on genes involved in 'sensory perception of pain' (GO:0019233), we identified transcriptional changes for different ion channels, receptors, and neurotransmitters, shedding light on its role in nociception. Examining peripheral sensory neurons subtype-specific transcriptional reprograming and regeneration-associated genes, we delved into downstream regulation of hypersensitivity. Identifying the temporal program of transcription regulatory mechanisms might help develop better therapeutics to target them effectively and selectively, thus preventing the development of neuropathic pain without affecting other physiological functions.

Pyroptosis: A spoiler of peaceful coexistence between cells in degenerative bone and joint diseases

BACKGROUND

As people age, degenerative bone and joint diseases (DBJDs) become more prevalent. When middle-aged and elderly people are diagnosed with one or more disorders such as osteoporosis (OP), osteoarthritis (OA), and intervertebral disc degeneration (IVDD), it often signals the onset of prolonged pain and reduced functionality. Chronic inflammation has been identified as the underlying cause of various degenerative diseases, including DBJDs. Recently, excessive activation of pyroptosis, a form of programed cell death (PCD) mediated by inflammasomes, has emerged as a primary driver of harmful chronic inflammation. Consequently, pyroptosis has become a potential target for preventing and treating DBJDs.

AIM OF REVIEW

This review explored the physiological and pathological roles of the pyroptosis pathway in bone and joint development and its relation to DBJDs. Meanwhile, it elaborated the molecular mechanisms of pyroptosis within individual cell types in the bone marrow and joints, as well as the interplay among different cell types in the context of DBJDs. Furthermore, this review presented the latest compelling evidence supporting the idea of regulating the pyroptosis pathway for DBJDs treatment, and discussed the potential, limitations, and challenges of various therapeutic strategies involving pyroptosis regulation.

KEY SCIENTIFIC CONCEPTS OF REVIEW

In summary, an interesting identity for the unregulated pyroptosis pathway in the context of DBJDs was proposed in this review, which was undertaken as a spoiler of peaceful coexistence between cells in a degenerative environment. Over the extended course of DBJDs, pyroptosis pathway perpetuated its activity through crosstalk among pyroptosis cascades in different cell types, thus exacerbating the inflammatory environment throughout the entire bone marrow and joint degeneration environment. Correspondingly, pyroptosis regulation therapy emerged as a promising option for clinical treatment of DBJDs.

Functional Stimulation and Imaging to Predict Neuromodulation of Chronic Low Back Pain

Back pain is one of the most common aversive sensations in human experience. Pain is not limited to the sensory transduction of tissue damage; rather, it encompasses a range of nervous system activities including lateral modulation, long-distance transmission, encoding, and decoding. Although spine surgery may address peripheral pain generators directly, aberrant signals along canonical aversive pathways and maladaptive influence of affective and cognitive states can result in persistent subjective pain refractory to classical surgical intervention. The clinical identification of who will benefit from surgery-and who will not-is increasingly grounded in neurophysiology.

Changes in ion-channels in the dorsal root ganglion after exposure to autologous nucleus pulposus and TNF. A rat experimental study

Purpose

It is known that contact of nucleus pulposus with the dorsal root ganglion may induce changes in nerve conduction and pain behavior. It has also been suggested that the behavioristic changes are caused by changes in voltage-gated ion channels, which in turn have been upregulated by TNF. Such upregulations have previously been shown for NaV 1.8 and NaV 1.9. In this investigation, we expanded the number of studied ion channels after the application of nucleus pulposus or TNF.

Methods

Following removal of the left L4-5 fact joint, a disc puncture was performed and the dorsal root ganglion was exposed to nucleus pulposus (n = 5) and TNF (n = 5). Operated rats without disc puncture served as sham (n = 5) and 5 non-operated (naïve) rats were included. After 24 h, the DRGs were harvested and analyzed by quantitative PCR on validated pre-spotted primer plates displaying genes for 90 voltage-gated ion channels.

Results

It was evident that the changes in operated animals were separate from the naïve rats. It was also apparent that gene expression changes in rats with nucleus pulposus or TNF application showed similar trends and were also separated from sham-operated animals.

Conclusion

The application of nucleus pulposus and TNF onto the DRG in rats induces comparable changes in gene expression of several ion channels. Since the changes induced by TNF and NP are similar, one might also suspect that TNF mediates the NP-induced changes. However, such a mechanism needs further investigation.

Multiplex epigenome editing of ion channel expression in nociceptive neurons abolished degenerative IVD-conditioned media-induced mechanical sensitivity

Background

Low back pain is a major contributor to disability worldwide and generates a tremendous socioeconomic impact. The degenerative intervertebral disc (IVD) has been hypothesized to contribute to discogenic pain by sensitizing nociceptive neurons innervating the disc to stimuli that is nonpainful in healthy patients. Previously, we demonstrated the ability of degenerative IVDs to sensitize neurons to mechanical stimuli; however, elucidation of degenerative IVDs discogenic pain mechanisms is required to develop therapeutic strategies that directly target these mechanisms.

Aims

In this study, we utilized CRISPR epigenome editing of nociceptive neurons to identify mechanisms of degenerative IVD-induced changes to mechanical nociception and demonstrated the ability of multiplex CRISPR epigenome editing of nociceptive neurons to modulate inflammation-induced mechanical nociception.

Methods and Results

Utilizing an in vitro model, we demonstrated degenerative IVD-produced IL-6-induced increases in nociceptive neuron activity in response to mechanical stimuli, mediated by TRPA1, ASIC3, and Piezo2 ion channel activity. Once these ion channels were identified as mediators of degenerative IVD-induced mechanical nociception, we developed singleplex and multiplex CRISPR epigenome editing vectors that modulate endogenous expression of TRPA1, ASIC3, and Piezo2 via targeted gene promoter histone methylation. When delivered to nociceptive neurons, the multiplex CRISPR epigenome editing vectors abolished degenerative IVD-induced mechanical nociception while preserving nonpathologic neuron activity.

Conclusion

This work demonstrates the potential of multiplex CRISPR epigenome editing as a highly targeted gene-based neuromodulation strategy for the treatment of discogenic pain, specifically; and, for the treatment of inflammatory chronic pain conditions, more broadly.

Do Inflammatory Cytokines Affect Patient Outcomes After ACDF?

STUDY DESIGN

Prospective cohort study.

OBJECTIVE

The aim was to determine the relationship between serum inflammatory mediators, preoperative cervical spine disease severity, and clinical outcomes after anterior cervical discectomy and fusion (ACDF).

SUMMARY OF BACKGROUND DATA

Given the role of the inflammatory cascade in spinal degenerative disease, it has been hypothesized that inflammatory markers may serve as a predictor of patient outcomes after surgery.

MATERIALS AND METHODS

All patients over age 18 who underwent ACDF for cervical spondylosis with associated radiculopathy and/or myelopathy between 2015 and 2017 from a single institution were prospectively recruited. Preoperative serum inflammatory markers including interleukin (IL)-6, IL-8, tumor necrosis factor-α, high-mobility group box-1 (HMGB1), and white blood cells were measured and correlated to patient demographics, surgical characteristics, duration of symptoms, previous opioid use, and preoperative and 1-year postoperative patient-reported outcomes measures (PROMs) including the neck disability index (NDI), visual analog scale neck pain, visual analog scale arm pain, and Physical and Mental Component Scores of the Short Form-12 (PCS and MCS, respectively) using spearman's rho coefficient.

RESULTS

A total of 77 patients were enrolled with follow-up PROMs available for 62% (n=48) of patients at a minimum of 1-year after ACDF. The absolute concentrations of IL-6 and tumor necrosis factor-α were found to be weakly correlated with one another (ρ=0.479). Preoperative symptoms lasting <1-year were weakly correlated with elevation in HMGB1 (ρ=0.421). All other patient demographics exhibited negligible correlation with the preoperative inflammatory markers. Lower preoperative PCS (ρ=0.355) and higher preoperative NDI (ρ=0.336) were weakly correlated with elevated HMGB1. Lower MCS (ρ=0.395) and higher NDI (ρ=0.317) preoperatively were weakly correlated with elevated white blood cells. Postoperative improvement in MCS (ρ=0.306) and MCS recovery ratio (ρ=0.321) exhibited a weakly positive correlation with IL-6.

CONCLUSION

Preoperative cytokine levels demonstrated minimal correlation with preoperative symptoms or clinical improvement, suggesting that profiling of patient cytokines has limited utility in predicting outcomes after ACDF.

LEVEL OF EVIDENCE

Level III.

Assessment of the duration and effectiveness of intra-articular lidocaine injections for groin pain in patients with labral tears involving early osteoarthritis

Introduction: Intra-articular lidocaine injections have been used to confirm the hip pathology and may predict the efficacy of arthroscopic surgery. We have routinely performed the injections as a surgical indicator. The aim of this study was to assess the duration and effectiveness of these diagnostic intra-articular lidocaine injections on groin pain in patients with labral tears involving early osteoarthritis. Methods: A total of 113 patients were included in this study. All patients received one injection of 10 ml of 1% lidocaine into the hip joint under fluoroscopy. The duration and effectiveness of the injection were assessed 2 weeks after the injection and at a minimum of 1 year of follow-up. The effect of the injection was graded as 0: unchanged or worse; 1: an effect only on the day of injection; 2: the effect lasted a few days; 3: the effect lasted about a week; and 4: symptom remission. In addition, we recorded whether hip arthroscopic surgery was eventually performed. Results: The effect was rated as 0 in 19 patients (16.8%), as 1 in 30 patients (26.5%), as 2 in 38 patients (33.6%), as 3 in 13 patients (11.5%), and as 4 in 13 patients (11.5%). Seventy-two patients (63.7%) underwent hip arthroscopic surgery. No relationship with patients’ characteristics was found. Conclusion: In total, 83% of patients experienced some effect of the lidocaine injection. Furthermore, 11.5% of patients experienced complete remission of their symptoms.

The role of structure and function changes of sensory nervous system in intervertebral disc-related low back pain

Low back pain (LBP) is a common musculoskeletal symptom, which can be developed in multiple clinical diseases. It is widely recognized that intervertebral disc (IVD) degeneration (IVDD) is one of the leading causes of LBP. However, the pathogenesis of IVD-related LBP is still controversial, and the treatment means are also insufficient to date. In recent decades, the role of structure and function changes of sensory nervous system in the induction and the maintenance of LBP is drawing more and more attention. With the progress of IVDD, IVD cell exhaustion and extracellular matrix degradation result in IVD structural damage, while neovascularization, innervation and inflammatory activation further deteriorate the microenvironment of IVD. New nerve ingrowth into degenerated IVD amplifies the impacts of IVD-derived nociceptive molecules on sensory endings. Moreover, IVDD is usually accompanied with disc herniation, which could injure and inflame affected nerves. Under mechanical and pro-inflammatory stimulation, the pain-transmitting pathway exhibits a sensitized function state and ultimately leads to LBP. Hence, relevant pathogenic factors, such as neurotrophins, ion channels, inflammatory factors, etc., are supposed to serve as promising therapeutic targets for LBP. The purpose of this review is to comprehensively summarize the current evidence on 1) the pathological changes of sensory nervous system during IVDD and their association with LBP, and 2) potential therapeutic strategies for LBP targeting relevant pathogenic factors.

In vitro model of distinct catabolic and inflammatory response patterns of endothelial cells to intervertebral disc cell degeneration

To evaluate dominant cell-to-cell paracrine interactions, including those of human annulus fibrosus (AF), nucleus pulposus (NP), and endothelial cells (ECs), in the production of inflammatory mediators and catabolic enzymes, ECs was cultured in soluble factors derived from AF or NP cells (AFCM or NPCM, respectively) and vice versa. We analysed IL-6 and -8, vascular endothelial growth factor (VEGF), matrix metalloproteinase (MMP)-1 and -3, nerve growth factor (NGF)-β, and brain-derived neurotrophic factors (BDNFs) with qRT-PCR and ELISA. We implement a microfluidic platform to analyse migration properties of AF and NP cells and ECs in 3D cultures. Our results show that IL-1β-stimulated AF cells produced significantly higher levels of IL-6 and -8, VEGF, and MMP-1 than IL-1β-stimulated NP cells. However, production of IL-6 and -8, VEGF, and MMP-3 was significantly higher in NP cells than in AF cells, under the presence of ECs conditioned medium. We observed considerable migration of NP cells co-cultured with ECs through the microfluidic platform. These results suggest that AF cells may play a major role in the initial degeneration of intervertebral disc. Furthermore, it was found that interactions between NP cells and ECs may play a significant role in the development or progression of diseases.

Effects of photobiomodulation on annulus fibrosus cells derived from degenerative disc disease patients exposed to microvascular endothelial cells conditioned medium

Intervertebral disc (IVD) degeneration with chronic low back pain is associated with neo-vascularisation into the deeper IVD regions. During this process, endothelial cells (ECs), which are primarily responsible for angiogenesis, interact with the adjacent annulus fibrosus (AF) cells, which are the first line of defence against the invasion of vascular structures into deeper IVD regions. However, the accumulation of inflammatory and catabolic enzymes that results from this interaction promotes matrix degradation and an inflammatory response. Thus, regulating the production of these mediators and catabolic enzymes could ameliorate IVD degeneration. Photobiomodulation (PBM) therapy is a non-invasive stimulation known to have biologically beneficial effects on wound healing, tissue repair, and inflammation. Here, we examined the effects of PBM, administered at various wavelengths (645, 525, and 465 nm) and doses (16, 32, and 64 J/cm²), on EC-stimulated human AF cells. Our results show that PBM selectively inhibited the EC-mediated production of inflammatory mediators, catabolic enzymes, and neurotrophins by human AF cells in a dose- and wavelength-dependent manner. These results suggest that PBM could be a superior and advanced treatment strategy for IVD degeneration.

Differential Expression of Acid - Sensing Ion Channels in Mouse Primary Afferents in Naïve and Injured Conditions

Injury and inflammation cause tissue acidosis, which is a common feature of various painful conditions. Acid-Sensing Ion channels (ASICs) are amongst the main excitatory channels activated by extracellular protons and expressed in the nervous system. Six transcripts of ASIC subunits including ASIC1a, ASIC1b, ASIC2a, ASIC2b, ASIC3, and ASIC4 are encoded by four genes (Asic1–4) and have been identified in rodents. Most ASIC subunits are present at substantial levels in primary sensory neurons of dorsal root ganglia (DRG) except for ASIC4. However, their expression pattern in DRG neurons remains largely unclear, mainly due to the lack of antibodies with appropriate specificity. In this study, we examined in detail the expression pattern of ASIC1-3 subunits, including splice variants, in different populations of DRG neurons in adult mice using an in situ hybridization technique (RNAscope) with high sensitivity and specificity. We found that in naïve condition, all five subunits examined were expressed in the majority of myelinated, NF200-immunoreactive, DRG neurons (NF200⁺). However, ASIC subunits showed a very different expression pattern among non-myelinated DRG neuronal subpopulations: ASIC1 and ASIC3 were only expressed in CGRP-immunoreactive neurons (CGRP⁺), ASIC2a was mostly expressed in the majority of IB4-binding neurons (IB4⁺), while ASIC2b was expressed in almost all non-myelinated DRG neurons. We also found that at least half of sensory neurons expressed multiple types of ASIC subunits, indicating prevalence of heteromeric channels. In mice with peripheral nerve injury, the expression level of ASIC1a and ASIC1b in L4 DRG and ASIC3 in L5 DRG were altered in CGRP⁺ neurons, but not in IB4⁺ neurons. Furthermore, the pattern of change varied among DRGs depending on their segmental level, which pointed to differential regulatory mechanisms between afferent types and anatomical location. The distinct expression pattern of ASIC transcripts in naïve condition, and the differential regulation of ASIC subunits after peripheral nerve injury, suggest that ASIC subunits are involved in separate sensory modalities.

Interaction between Mesenchymal Stem Cells and Intervertebral Disc Microenvironment: From Cell Therapy to Tissue Engineering

Low back pain (LBP) in one of the most disabling symptoms affecting nearly 80% of the population worldwide. Its primary cause seems to be intervertebral disc degeneration (IDD): a chronic and progressive process characterized by loss of viable cells and extracellular matrix (ECM) breakdown within the intervertebral disc (IVD) especially in its inner region, the nucleus pulposus (NP). Over the last decades, innovative biological treatments have been investigated in order to restore the original healthy IVD environment and achieve disc regeneration. Mesenchymal stem cells (MSCs) have been widely exploited in regenerative medicine for their capacity to be easily harvested and be able to differentiate along the osteogenic, chondrogenic, and adipogenic lineages and to secrete a wide range of trophic factors that promote tissue homeostasis along with immunomodulation and anti-inflammation. Several in vitro and preclinical studies have demonstrated that MSCs are able to acquire a NP cell-like phenotype and to synthesize structural components of the ECM as well as trophic and anti-inflammatory mediators that may support resident cell activity. However, due to its unique anatomical location and function, the IVD presents distinctive features: avascularity, hypoxia, low glucose concentration, low pH, hyperosmolarity, and mechanical loading. Such conditions establish a hostile microenvironment for both resident and exogenously administered cells, which limited the efficacy of intradiscal cell therapy in diverse investigations. This review is aimed at describing the characteristics of the healthy and degenerated IVD microenvironment and how such features influence both resident cells and MSC viability and biological activity. Furthermore, we focused on how recent research has tried to overcome the obstacles coming from the IVD microenvironment by developing innovative cell therapies and functionalized bioscaffolds.

Molecular pain markers correlate with pH-sensitive MRI signal in a pig model of disc degeneration

Intervertebral disc (IVD) degeneration is a leading cause of chronic low back pain that affects millions of people every year. Yet identification of the specific IVD causing this pain is based on qualitative visual interpretation rather than objective findings. One possible approach to diagnosing pain-associated IVD could be to identify acidic IVDs, as decreased pH within an IVD has been postulated to mediate discogenic pain. We hypothesized that quantitative chemical exchange saturation transfer (qCEST) MRI could detect pH changes in IVDs, and thence be used to diagnose pathologically painful IVDs objectively and noninvasively. To test this hypothesis, a surgical model of IVD degeneration in Yucatan minipigs was used. Direct measurement of pH inside the degenerated IVDs revealed a significant drop in pH after degeneration, which correlated with a significant increase in the qCEST signal. Gene analysis of harvested degenerated IVDs revealed significant upregulation of pain-, nerve- and inflammatory-related markers after IVD degeneration. A strong positive correlation was observed between the expression of pain markers and the increase in the qCEST signal. Collectively, these findings suggest that this approach might be used to identify which IVD is causing low back pain, thereby providing valuable guidance for pain and surgical management.

Immunomodulation of mesenchymal stem cells in discogenic pain

BACKGROUND CONTEXT

Back pain is a highly prevalent health problem in the world today and has a great economic impact on health-care budgets. Intervertebral disc (IVD) degeneration has been identified as a main cause of back pain. Inflammatory cytokines produced by macrophages or disc cells in an inflammatory environment play an important role in painful progressive degeneration of IVD. Mesenchymal stem cells (MSCs) have shown to have immunosuppressive and anti-inflammatory properties. Mesenchymal stem cells express a variety of chemokines and cytokines receptors having tropism to inflammation sites.

PURPOSE

This study aimed to develop an in vitro controlled and standardized model of inflammation and degeneration of IVD with rat cells and to evaluate the protective and immunomodulatory effect of conditioned medium (CM) from the culture of MSCs to improve the conditions presented in herniated disc and discogenic pain processes.

STUDY DESIGN

This is an experimental study.

METHODS

In this study, an in vitro model of inflammation and degeneration of IVD has been developed, as well as the effectiveness of CM from the culture of MSCs.

RESULTS

Conditioned medium from MSCs downregulated the expression of various proinflammatory cytokines produced in the pathogenesis of discogenic pain such as interleukin (IL)-1β, IL-6, IL-17, and tumor necrosis factor (TNF).

CONCLUSION

Mesenchymal stem cells represent a promising alternative strategy in the treatment of IVD degeneration inasmuch as there is currently no treatment which leads to a complete remission of long-term pain in the absence of drugs.

Effect of an Acid-sensing Ion Channels Inhibitor on Pain-related Behavior by Nucleus Pulposus Applied on the Nerve Root in Rats

STUDY DESIGN

Controlled, interventional animal study.

OBJECTIVE

To examine the effect of an inhibitor of acid-sensing ion channel 3 (ASIC3) on pain-related behavior induced by application of the nucleus pulposus (NP) onto the dorsal root ganglion (DRG) in rats.

SUMMARY OF BACKGROUND DATA

ASIC3 is associated with acidosis pain in inflamed or ischemic tissues and is expressed in sensory neurons and NP cells. The ASIC3 inhibitor, APETx2, increases the mechanical threshold of pain in models of knee osteoarthritis or postoperative pain. However, the efficacy of APETx2 for pain relief in the NP application model remains unknown.

METHODS

Autologous NP was applied to the left L5 nerve root of 183 adult female Sprague-Dawley rats. The DRGs were treated with NP plus one of the following four treatments: saline solution (SM), low (0.01 μg: LD), medium (0.1 μg: MD), or high dose (1.0 μg: HD) of APETx2. Behavioral testing was performed to investigate the mechanical withdrawal threshold using von Frey hairs. Expression of nerve growth factor, hypoxia-inducible factor-1α (HIF1α), activating transcription factor-3, and ionized calcium-binding adaptor molecule-1 was evaluated using immunohistochemistry. Statistical differences among multiple groups were assessed using the Steel test, the Tukey-Kramer test, and the Dunnett test. P < 0.05 were considered significant.

RESULTS

The thresholds in the HD group were higher than those in the SM group at Days 14 and 21 (P < 0.05). In the MD group, the threshold was higher than in the SM group at Day 14 (P < 0.05). High doses of APETx2 reduced the expression of HIF1α after Day 14 compared with the SM group (P < 0.05).

CONCLUSION

APETx2 significantly improved pain-related behavior in a dose-dependent manner. APETx2 may inhibit ASIC3 and partly inhibit Nav1.8 channels. This ASIC3 channel inhibitor may be a potential therapeutic agent in early-stage lumbar disc herniation.

LEVEL OF EVIDENCE

N/A.

The Roles of Acidosis in Osteoclast Biology

The adverse effect of acidosis on the skeletal system has been recognized for almost a century. Although the underlying mechanism has not been fully elucidated, it appears that acidosis acts as a general stimulator of osteoclasts derived from bone marrow precursors cells and enhances osteoclastic resorption. Prior work suggests that acidosis plays a significant role in osteoclasts formation and activation via up-regulating various genes responsible for its adhesion, migration, survival and bone matrix degradation. Understanding the role of acidosis in osteoclast biology may lead to development of novel therapeutic approaches for the treatment of diseases related to low bone mass. In this review, we aim to discuss the recent investigations into the effects of acidosis in osteoclast biology and the acid-sensing molecular mechanism.

Changes in the Expressions of Iba1 and Calcitonin Gene-Related Peptide in Adjacent Lumbar Spinal Segments after Lumbar Disc Herniation in a Rat Model

Lumbar disc herniation is commonly encountered in clinical practice and can induce sciatica due to mechanical and/or chemical irritation and the release of proinflammatory cytokines. However, symptoms are not confined to the affected spinal cord segment. The purpose of this study was to determine whether multisegmental molecular changes exist between adjacent lumbar spinal segments using a rat model of lumbar disc herniation. Twenty-nine male Sprague-Dawley rats were randomly assigned to either a sham-operated group (n=10) or a nucleus pulposus (NP)-exposed group (n=19). Rats in the NP-exposed group were further subdivided into a significant pain subgroup (n=12) and a no significant pain subgroup (n=7) using mechanical pain thresholds determined von Frey filaments. Immunohistochemical stainings of microglia (ionized calcium-binding adapter molecule 1; Iba1), astrocytes (glial fibrillary acidic protein; GFAP), calcitonin gene-related peptide (CGRP), and transient receptor potential vanilloid 1 (TRPV1) was performed in spinal dorsal horns and dorsal root ganglions (DRGs) at 10 days after surgery. It was found immunoreactivity for Iba1-positive microglia was higher in the L5 (P=0.004) dorsal horn and in the ipsilateral L4 (P=0.009), L6 (P=0.002), and S1 (P=0.002) dorsal horns in the NP-exposed group than in the sham-operated group. The expression of CGRP was also significantly higher in ipsilateral L3, L4, L6, and S1 segments and in L5 DRGs at 10 days after surgery in the NP-exposed group than in the sham-operated group (P<0.001). Our results indicate that lumbar disc herniation upregulates microglial activity and CGRP expression in many adjacent and ipsilateral lumbar spinal segments.

Stem Cell Approaches to Intervertebral Disc Regeneration: Obstacles from the Disc Microenvironment

Intervertebral disc (IVD) degeneration results in segmental instability and irritates neural compressive symptoms, such as low back pain and motor deficiency. The transplanting of stem cell into degenerative discs has attracted increasing clinical attention, as a new and proven approach to alleviating disc degeneration and to relieving discogenic pains. Aside from supplementation with stem cells, the IVD itself already contains a pool of stem and progenitor cells. Since the resident disc stem cells are incapable of reversing the pathologic changes that occur during aging and disc degeneration, it has been debated as to whether transplanted stem cells are capable of providing an efficient and durable therapeutic effect, even though there have been positive outcomes in both animal models and in clinical trials. This review aims to decipher the interactions between the stem cell and the disc microenvironment. Within their new niches in the IVD, the exogenous stem cell shows metabolic adaptation to the low-glucose supply, hypoxia, and compressive loadings, but demonstrates little tolerance to the disc-like acidity and hypertonicity. Similarly, the survival of endogenous stem cells is threatened as well by the harsh disc microenvironment, which may exhaust the stem cell resources and restrict the self-repair capacity of a degenerating IVD. To eliminate the intrinsic obstacles within the stressful disc niches, stem cells should be delivered with an injectable scaffold that provides both survival and mechanical support. Quick healing or concretion of the injection injuries, which minimizes stem cell leakage and disturbance to disc homeostasis, is of equal importance toward achieving efficient stem cell-based disc regeneration.

Up-Regulation of Pain Behavior and Glial Activity in the Spinal Cord after Compression and Application of Nucleus Pulposus onto the Sciatic Nerve in Rats

Study Design

Experimental animal study.

Purpose

To evaluate pain-related behavior and changes in glial activity in the spinal dorsal horn after combined sciatic nerve compression and nucleus pulposus (NP) application in rats.

Overview of Literature

Mechanical compression and inflammation caused by prostaglandins and cytokines at disc herniation sites induce pain. Structural changes and pain-associated cytokines in the dorsal root ganglia and spinal dorsal horn contribute to prolonged pain. Glial cells in the spinal dorsal horn may also function in pain transmission.

Methods

The sciatic nerve was compressed with NP for 2 seconds using forceps in the NP+nerve compression group; the sham-operated group received neither compression nor NP; and the control group received no operation. Mechanical hyperalgesia was measured for 3 weeks using von Frey filaments. Glial activity in the spinal dorsal horn was examined 7 days and 14 days postsurgery using anti-glial fibrillary acidic protein and anti-Ionized calcium binding adaptor molecule-1 antibodies to detect astrocytes and microglia, respectively.

Results

Mechanical hyperalgesia was detected throughout the 14-day observation in the NP+nerve compression group, but not in control or sham-operated groups (p<0.05). Both astrocytes and microglia were significantly increased in the spinal dorsal horn of the NP+nerve compression group compared to control and sham groups on days 7 and 14 (p<0.05).

Conclusions

Nerve compression with NP application produces pain-related behavior, and up-regulates astrocytes and microglia in the spinal dorsal horn, suggesting that these glia may be related to pain transmission.

Lumbar radiculopathy and its neurobiological basis

Lumbar radiculopathy, a group of diseases in which the dorsal root ganglia (DRG) or dorsal roots are adversely affected by herniated discs or spinal stenosis, are clinically characterized by spontaneous and evoked types of pain. The pain is underpinned by various distinct pathophysiological mechanisms in the peripheral and central nervous systems. However, the diagnosis of lumbar radiculopathy is still unsatisfactory, because the association of the pain with the neurobiological basis of radiculopathy is largely unknown. Several animal models used to explore the underlying neurobiological basis of lumbar radiculopathy could be classified as mechanical, chemical, or both based on the component of injury. Mechanical injury elevates the intraneural pressure, reduces blood flow, and eventually establishes ischemia in the dorsal root and the DRG. Ischemia may induce ischemic pain and cause nerve damage or death, and the subsequent nerve damage or death may induce neuropathic pain. Chemical injury predominately induces inflammation surrounding the dorsal roots or DRG and consequent inflammatory mediators cause inflammatory pain. Furthermore, DRG neurons sensitized by inflammatory mediators are hypersensitive to innocuous mechanical force (stretch or compression) and responsible for mechanical allodynia in radiculopathy. As well, central sensitization in the spinal cord may play an important role in pain generation in lumbar radiculopathy. Increasing knowledge of pain-generating mechanisms and their translation into clinical symptoms and signs might allow for dissecting the mechanisms that operate in each patient. With precise clinical phenotypic characterization of lumbar radiculopathy and its connection to a specific underlying mechanism, we should be able to design optimal treatments for individuals. This review discusses the present knowledge of lumbar radiculopathy and proposes a novel mechanism-based classification.

Evaluation of pain behavior and calcitonin gene-related peptide immunoreactive sensory nerve fibers in the spinal dorsal horn after sciatic nerve compression and application of nucleus pulposus in rats

STUDY DESIGN

Animal study.

OBJECTIVE

To evaluate pain behavior and neuropeptide changes in the spinal dorsal horn after sciatic nerve compression and application of nucleus pulposus (NP) in rats.

SUMMARY OF BACKGROUND DATA

The pathomechanisms of lumbar disc herniation pain have not been fully elucidated. Pain-associated neuropeptides, including substance P and calcitonin gene-related peptide (CGRP), are produced in dorsal root ganglion neurons and transported to spinal dorsal horn nerve terminals where they function in pain transmission. However, changes in CGRP-immunoreactive (IR) sensory nerve terminals have not been reported in models of disc herniation. This study evaluated pain-related behavior and changes in CGRP-IR terminals in the spinal dorsal horn after combined sciatic nerve compression and NP application.

METHODS

Five groups of rats underwent either sciatic nerve compression with NP (n = 20), application of NP only (n = 20), nerve compression only (n = 20), and sham operation with neither compression nor NP (n = 20) or no operation (controls, n = 20). Mechanical hyperalgesia was measured every second day for 3 weeks. CGRP-IR terminals in each spinal dorsal horn lamina were examined 7 and 14 days postsurgery. Pain behavior and CGRP immunoreactivity were compared among the 5 groups.

RESULTS

Mechanical hyperalgesia was found in the NP only, nerve compression only, and the NP with nerve compression groups (P ≤ 0.05). CGRP-IR nerve terminals in the superficial laminae (I and II) and the deep laminae (III-VI) significantly increased in the NP only, nerve compression only, and NP with nerve compression groups compared with control and sham groups (P ≤ 0.05). Significant mechanical hyperalgesia and increased CGRP-IR nerve terminals were found in the NP with nerve compression group compared with the NP only and nerve compression only groups (P ≤ 0.05).

CONCLUSION

Our results indicate that nerve compression plus NP application produces the most pain-related behavior. CGRP-IR nerve terminals increased in laminae I and II that transmit pain and in laminae III to VI that transmit proprioception. Findings suggest that nerve compression plus NP application induces changes in CGRP expression in the superficial and deep laminae, and these changes are partly responsible for disc herniation pain.

Effect of deep tissue incision on pH responses of afferent fibers and dorsal root ganglia innervating muscle

BACKGROUND

Understanding the mechanisms underlying deep tissue pain in the postoperative period is critical to improve therapies. Using the in vitro plantar flexor digitorum brevis muscle-nerve preparation and patch clamp recordings from cultured dorsal root ganglia neurons innervating incised and unincised muscle, the authors investigated responses to various pH changes.

METHODS

Incision including the plantar flexor digitorum brevis muscle or sham operation was made in the rat hind paw. On postoperative day 1, in vitro single-fiber recording was undertaken. On the basis of previous studies, the authors recorded from at least 40 fibers per group. Also DiI-labeled dorsal root ganglia innervating muscle from rats undergoing incision and a sham operation were cultured and tested for acid responses, using whole cell patch clamp recordings.

RESULTS

The prevalence of responsive group IV afferents to lactic acid pH 6.5 in the incision group (15 of 67; 22.3%) was greater than that in the control group (2 of 35; 5.7%; P=0.022). In dorsal root ganglia neurons innervating muscle, incision increased mean current amplitudes of acid-evoked currents; the acid-sensing ion channel blocker, amiloride 300 μM, inhibited more than 75% of the acid-evoked current, whereas, the transient receptor vanilloid receptor 1 blocker (AMG9810 1 μM) did not cause significant inhibition.

CONCLUSION

The authors' experiments demonstrated that incision increases the responses of flexor digitorum brevis muscle afferent fibers to weak acid solutions, and increased acid-evoked currents in dorsal root ganglia innervating muscle. The authors' data suggest that up-regulation of acid-sensing ion channels might underlie this increased chemosensitivity caused by surgery.

Role of cytokines in intervertebral disc degeneration: pain and disc content

Degeneration of the intervertebral discs (IVDs) is a major contributor to back, neck and radicular pain. IVD degeneration is characterized by increases in levels of the proinflammatory cytokines TNF, IL-1α, IL-1β, IL-6 and IL-17 secreted by the IVD cells; these cytokines promote extracellular matrix degradation, chemokine production and changes in IVD cell phenotype. The resulting imbalance in catabolic and anabolic responses leads to the degeneration of IVD tissues, as well as disc herniation and radicular pain. The release of chemokines from degenerating discs promotes the infiltration and activation of immune cells, further amplifying the inflammatory cascade. Leukocyte migration into the IVD is accompanied by the appearance of microvasculature tissue and nerve fibres. Furthermore, neurogenic factors, generated by both disc and immune cells, induce expression of pain-associated cation channels in the dorsal root ganglion. Depolarization of these ion channels is likely to promote discogenic and radicular pain, and reinforce the cytokine-mediated degenerative cascade. Taken together, an enhanced understanding of the contribution of cytokines and immune cells to these catabolic, angiogenic and nociceptive processes could provide new targets for the treatment of symptomatic disc disease. In this Review, the role of key inflammatory cytokines during each of the individual phases of degenerative disc disease, as well as the outcomes of major clinical studies aimed at blocking cytokine function, are discussed.

Acid-sensing ion channel 3 deficiency increases inflammation but decreases pain behavior in murine arthritis

OBJECTIVE

Through its location on nociceptors, acid-sensing ion channel 3 (ASIC-3) is activated by decreases in pH and plays a significant role in musculoskeletal pain. We recently showed that decreases in pH activate ASIC-3 located on fibroblast-like synoviocytes (FLS), which are key cells in the inflammatory process. The purpose of this study was to test whether ASIC-3-deficient mice with arthritis have altered inflammation and pain relative to controls.

METHODS

Collagen antibody-induced arthritis (CAIA) was generated by injection of an anti-type II collagen antibody cocktail. Inflammation and pain parameters in ASIC-3(-/-) and ASIC-3(+/+) mice were assessed. Disease severity was assessed by determining clinical arthritis scores, measuring joint diameters, analyzing joint histology, and assessing synovial gene expression by quantitative polymerase chain reaction analysis. Cell death was assessed with a Live/Dead assay of FLS in response to decreases in pH. Pain behaviors in the mice were measured by examining withdrawal thresholds in the joints and paws and by measuring their physical activity levels.

RESULTS

Surprisingly, ASIC-3(-/-) mice with CAIA demonstrated significantly increased joint inflammation, joint destruction, and expression of interleukin-6 (IL-6), matrix metalloproteinase 3 (MMP-3), and MMP-13 in joint tissue as compared to ASIC-3(+/+) mice. ASIC-3(+/+) FLS showed enhanced cell death when exposed to pH 6.0 in the presence of IL-1β, which was abolished in ASIC-3(-/-) FLS. Despite enhanced disease severity, ASIC-3(-/-) mice did not develop mechanical hypersensitivity of the paw and showed greater levels of physical activity.

CONCLUSION

Our findings are consistent with the hypothesis that ASIC-3 plays a protective role in the inflammatory arthritides by limiting inflammation through enhanced synoviocyte cell death, which reduces disease severity, and through the production of pain, which reduces joint use.

Mechanical compression and nucleus pulposus application on dorsal root Ganglia differentially modify evoked neuronal activity in the thalamus

A combination of mechanical compression caused by a protruding disc and leakage of nucleus pulposus (NP) from the disc core is presumed to contribute to intervertebral disc hernia-related pain. Experimental models of disc hernia including both components have resulted in changes in neuronal activity at the level of the dorsal root ganglion (DRG) and spinal cord, but changes within the brain have been less well studied. However, acute application of NP to a DRG without mechanical compression rapidly increases neuronal activity in the thalamus, a major brain relay nucleus processing information from sensory pathways including ascending nociceptive tracts. The combination of mechanical compression and NP might therefore result in further increases in central neuronal activity. Using an experimental disc herniation rat model including both mechanical compression and NP the present study aimed to investigate changes in neuronal activity in the contralateral thalamic ventral posterior lateral nucleus in vivo. Measurements were obtained while electrically stimulating the ipsilateral sciatic nerve at Aδ fiber intensities. The L4 DRG was subjected to light mechanical compression and NP exposure, and acute changes in evoked thalamic responses were recorded for up to 40 min. In order to compare effects in naïve animals with effects following a longer period of NP exposure, animals that were either disc-punctured or sham-operated 24 h previously were also included. In all animals, light mechanical compression of the DRG depressed the number of evoked neuronal responses. Prior NP exposure resulted in less potent changes following mechanical compression (80% of baseline) than that observed in naïve animals (50%). During the subsequent NP application, the number of evoked responses compared to baseline increased in pre-exposed animals (to 87%) as well as in naïve animals (72%) in which the removal of the mechanical compression resulted in a further increase (106%). The contribution of acute DRG compression and disc material leakage to changes in transmission in central neuronal networks is likely to be complex and to involve both short-term and long-term effects. Since a light mechanical compression may reduce transmission in nociceptive pathways, it is possible that the presence or absence of NP is crucial for pain development in the acute phase of disc herniation.

Serotonin facilitates peripheral pain sensitivity in a manner that depends on the nonproton ligand sensing domain of ASIC3 channel

Tissue acidosis and inflammatory mediators play critical roles in inflammatory pain. Extracellular acidosis activates acid-sensing ion channels (ASICs), which have emerged as key sensors for extracellular protons in the central and peripheral nervous systems and play key roles in pain sensation and transmission. Additionally, inflammatory mediators, such as serotonin (5-HT), are known to enhance pain sensation. However, functional interactions among protons, inflammatory mediators, and ASICs in pain sensation are poorly understood. In the present study, we show that 5-HT, a classical pro-inflammatory mediator, specifically enhances the proton-evoked sustained, but not transient, currents mediated by homomeric ASIC3 channels and heteromeric ASIC3/1a and ASIC3/1b channels. Unexpectedly, the effect of 5-HT on ASIC3 channels does not involve activation of 5-HT receptors, but is mediated via a functional interaction between 5-HT and ASIC3 channels. We further show that the effect of 5-HT on ASIC3 channels depends on the newly identified nonproton ligand sensing domain. Finally, coapplication of 5-HT and acid significantly increased pain-related behaviors as assayed by the paw-licking test in mice, which was largely attenuated in ASIC3 knock-out mice, and inhibited by the nonselective ASIC inhibitor amiloride. Together, these data identify ASIC3 channels as an unexpected molecular target for acute actions of 5-HT in inflammatory pain sensation and reveal an important role of ASIC3 channels in regulating inflammatory pain via coincident detection of extracellular protons and inflammatory mediators.

Osthole, a herbal compound, alleviates nucleus pulposus-evoked nociceptive responses through the suppression of overexpression of acid-sensing ion channel 3 (ASIC3) in rat dorsal root ganglion

BACKGROUND

Osthole (Ost), a natural coumarin derivative, has been shown to inhibit many pro-inflammatory mediators and block voltage-gated Na+ channels. During inflammation, acidosis is an important pain inducer which activates nociceptors by gating depolarizing cationic channels, such as acid-sensing ion channel 3 (ASIC3). The aim of this study was to examine the effects of Ost on nucleus pulposus-evoked nociceptive responses and ASIC3 over-expression in the rat dorsal root ganglion, and to investigate the possible mechanism.

MATERIAL/METHODS

Radicular pain was generated with application of nucleus pulposus (NP) to nerve root. Mechanical allodynia was evaluated using von Frey filaments with logarithmically incremental rigidity to calculate the 50% probability thresholds for mechanical paw withdrawal. ASIC3 protein expression in dorsal root ganglions (DRGs) was assessed with Western blot and immunohistochemistry. Membrane potential (MP) shift of DRG neurons induced by ASIC3-sensitive acid (pH6.5) was determined by DiBAC(4) (3) fluorescence intensity (F.I.).

RESULTS

The NP-evoked mechanical hyperalgesia model showed allodynia for 3 weeks, and ASIC3 expression was up-regulated in DRG neurons, reaching peak on Day 7. Epidural administration of Ost induced a remarkable and prolonged antinociceptive effect, accompanied by an inhibition of over-expressed ASIC3 protein and of abnormal shift of MP. Amiloride (Ami), an antagonist of ASIC3, strengthened the antinociceptive effect of Ost.

CONCLUSIONS

Up-regulation of ASIC3 expression may be associated with NP-evoked mechanical hyperalgesia. A single epidural injection of Ost decreased ASIC3 expression in DGR neurons and the pain in the NP-evoked mechanical hyperalgesia model. Osthole may be of great benefit for preventing chronic pain status often seen in lumbar disc herniation (LDH).

Modification of rat model of sciatica induced by lumber disc herniation and the anti-inflammatory effect of osthole given by epidural catheterization

One of the most treatable causes of lower back pain and associated sciatica is lumbar disc herniation (LDH), which is characterized by rupture of the hard outer wall (annulus fibrosis) in a lumbar intervertebral disc. In the current study, we aimed to: (1) develop and characterize a rat model of sciatica induced by LDH, while introducing a novel method of epidural catheterization; (2) use this model to evaluate the effect of osthole on pain due to LDH, and (3) gain insight into the mechanisms through which osthole affects sciatica induced by LDH. The results indicate that our newly developed rat model maintained mechanical allodynia for 28 days without reduction. Moreover, cyclooxygenase-2 (COX-2) and nitric oxide synthase (NOS) were overexpressed in the associated inflammatory response, which is consistent with clinical manifestations of the disease. We then used this model to study the effect and mechanisms through which osthole affected pain due to LDH. Our study suggests that osthole is capable of reversing hyperalgesia due to LDH, potentially through modulation of activity of COX-2 and NOS, two important proteins for the exacerbation of pain due to LDH. Finally, a molecular modeling simulation showed that osthole has unique binding capabilities to both NOS and COX-2. As the model-induced mechanical hyperalgesia response was consistent, and the position of the catheter tip and the extension/spreading of the drug in the epidural space were reliable, this study developed an improved model to study remedies for sciatic pain. Moreover, our studies demonstrate that osthole may be a feasible treatment for the reduction of pain due to hyperalgesia.

Local ASIC3 modulates pain and disease progression in a rat model of osteoarthritis

Background

Recent data have suggested a relationship between acute arthritic pain and acid sensing ion channel 3 (ASIC3) on primary afferent fibers innervating joints. The purpose of this study was to clarify the role of ASIC3 in a rat model of osteoarthritis (OA) which is considered a degenerative rather than an inflammatory disease.

Methods

We induced OA via intra-articular mono-iodoacetate (MIA) injection, and evaluated pain-related behaviors including weight bearing measured with an incapacitance tester and paw withdrawal threshold in a von Frey hair test, histology of affected knee joint, and immunohistochemistry of knee joint afferents. We also assessed the effect of ASIC3 selective peptide blocker (APETx2) on pain behavior, disease progression, and ASIC3 expression in knee joint afferents.

Results

OA rats showed not only weight-bearing pain but also mechanical hyperalgesia outside the knee joint (secondary hyperalgesia). ASIC3 expression in knee joint afferents was significantly upregulated approximately twofold at Day 14. Continuous intra-articular injections of APETx2 inhibited weight distribution asymmetry and secondary hyperalgesia by attenuating ASIC3 upregulation in knee joint afferents. Histology of ipsilateral knee joint showed APETx2 worked chondroprotectively if administered in the early, but not late phase.

Conclusions

Local ASIC3 immunoreactive nerve is strongly associated with weight-bearing pain and secondary hyperalgesia in MIA-induced OA model. APETx2 inhibited ASIC3 upregulation in knee joint afferents regardless of the time-point of administration. Furthermore, early administration of APETx2 prevented cartilage damage. APETx2 is a novel, promising drug for OA by relieving pain and inhibiting disease progression.

ASICs Do Not Play a Role in Maintaining Hyperalgesia Induced by Repeated Intramuscular Acid Injections

Repeated intramuscular acid injections produce long-lasting mechanical hyperalgesia that depends on activation of ASICs. The present study investigated if pH-activated currents in sensory neurons innervating muscle were altered in response to repeated acid injections, and if blockade of ASICs reverses existing hyperalgesia. In muscle sensory neurons, the mean acid-evoked current amplitudes and the biophysical properties of the ASIC-like currents were unchanged following acidic saline injections when compared to neutral pH saline injections or uninjected controls. Moreover, increased mechanical sensitivity of the muscle and paw after the second acid injection was unaffected by local blockade of ASICs (A-317567) in the muscle. As a control, electron microscopic analysis showed that the tibial nerve was undamaged after acid injections. Our previous studies demonstrated that ASICs are important in the development of hyperalgesia to repeated acid injections. However, the current data suggest that ASICs are not involved in maintaining hyperalgesia to repeated intramuscular acid injections.

Increased response of muscle sensory neurons to decreases in pH after muscle inflammation

Acid sensing ion channels (ASIC) are found in sensory neurons, including those that innervate muscle tissue. After peripheral inflammation there is an increase in proton concentration in the inflamed tissue, which likely activates ASICs. Previous studies from our laboratory in an animal model of muscle inflammation show that hyperalgesia does not occur in ASIC3 and ASIC1 knockout mice. Therefore, in the present study we investigated if pH activated currents in sensory neurons innervating muscle are altered after induction of muscle inflammation. Sensory neurons innervating mouse (C57/Bl6) muscle were retrogradely labeled with 1,1-dioctadecyl-3,3,3,3 tetramethylindocarbocyanine perchlorate (DiI). Two weeks after injection of DiI, mice were injected with 3% carrageenan to induce inflammation (n=8; 74 neurons) or pH 7.2 saline (n=5; 40 neurons, control) into the gastrocnemius muscle. 24 h later sensory neurons from L4-L6 dorsal root ganglia (DRG) were isolated and cultured. The following day the DRG neuron cultures were tested for responses to pH by whole-cell patch-clamp technique. Approximately 40% of neurons responded to pH 5 with an inward rapidly desensitizing current consistent with ASIC channels in both groups. The mean pH-evoked current amplitudes were significantly increased in muscle sensory neurons from inflamed mice (pH 5.0, 3602 ± 470 pA) in comparison to the controls (pH 7.4, 1964 ± 370 pA). In addition, the biophysical properties of ASIC-like currents were altered after inflammation. Changes in ASIC channels result in enhanced responsiveness to decreases in pH, and likely contribute to the increased hyperalgesia observed after muscle inflammation.

Acid-Sensing Ion Channels and Pain

Pathophysiological conditions such as inflammation, ischemia, infection and tissue injury can all evoke pain, and each is accompanied by local acidosis. Acid sensing ion channels (ASICs) are proton-gated cation channels expressed in both central and peripheral nervous systems. Increasing evidence suggests that ASICs represent essential sensors for tissue acidosis-related pain. This review provides an update on the role of ASICs in pain sensation and discusses their therapeutic potential for pain management.

ASIC1 and ASIC3 play different roles in the development of Hyperalgesia after inflammatory muscle injury

Acid-sensing ion channels (ASICs) respond to acidosis that normally occurs after inflammation. We examined the expression of ASIC1, ASIC2, and ASIC3 mRNAs in lumbar dorsal root ganglion neurons before and 24 hours after carrageenan-induced muscle inflammation. Muscle inflammation causes bilateral increases of ASIC2 and ASIC3 but not ASIC1 (neither ASIC1a nor ASIC1b) mRNA, suggesting differential regulation of ASIC1 versus ASIC2 and ASIC3 mRNA. Similar mRNA increases were observed after inflammation in knockout mice: ASIC2 mRNA increases in ASIC3-/- mice; ASIC2 and ASIC3 mRNAs increase in ASIC1-/- mice. Prior behavioral studies in ASIC3-/- mice showed deficits in secondary hyperalgesia (increased response to noxious stimuli outside the site of injury) but not primary hyperalgesia (increased response to noxious stimuli at the site of injury). In this study, we show that ASIC1-/- mice do not develop primary muscle hyperalgesia but develop secondary paw hyperalgesia. In contrast, and as expected, ASIC3-/- mice develop primary muscle hyperalgesia but do not develop secondary paw hyperalgesia. The pharmacological utility of the nonselective ASIC inhibitor A-317567, given locally, was tested. A-317567 reverses both the primary and the secondary hyperalgesia induced by carrageenan muscle inflammation. Thus, peripherally located ASIC1 and ASIC3 play different roles in the development of hyperalgesia after muscle inflammation.

PERSPECTIVE

This study shows changes in ASIC mRNA expression and behavioral hyperalgesia of C57Bl/6 (wild type), ASIC1-/-, and ASIC3-/- mice before and after the induction of muscle inflammation. A-317567 was effective in reversing hyperalgesia in these animals, suggesting the potential of ASICs as therapeutic targets for muscle inflammatory pain.

ASIC1 and ASIC3 play different roles in the development of Hyperalgesia after inflammatory muscle injury

Acid-sensing ion channels (ASICs) respond to acidosis that normally occurs after inflammation. We examined the expression of ASIC1, ASIC2, and ASIC3 mRNAs in lumbar dorsal root ganglion neurons before and 24 hours after carrageenan-induced muscle inflammation. Muscle inflammation causes bilateral increases of ASIC2 and ASIC3 but not ASIC1 (neither ASIC1a nor ASIC1b) mRNA, suggesting differential regulation of ASIC1 versus ASIC2 and ASIC3 mRNA. Similar mRNA increases were observed after inflammation in knockout mice: ASIC2 mRNA increases in ASIC3-/- mice; ASIC2 and ASIC3 mRNAs increase in ASIC1-/- mice. Prior behavioral studies in ASIC3-/- mice showed deficits in secondary hyperalgesia (increased response to noxious stimuli outside the site of injury) but not primary hyperalgesia (increased response to noxious stimuli at the site of injury). In this study, we show that ASIC1-/- mice do not develop primary muscle hyperalgesia but develop secondary paw hyperalgesia. In contrast, and as expected, ASIC3-/- mice develop primary muscle hyperalgesia but do not develop secondary paw hyperalgesia. The pharmacological utility of the nonselective ASIC inhibitor A-317567, given locally, was tested. A-317567 reverses both the primary and the secondary hyperalgesia induced by carrageenan muscle inflammation. Thus, peripherally located ASIC1 and ASIC3 play different roles in the development of hyperalgesia after muscle inflammation.

PERSPECTIVE

This study shows changes in ASIC mRNA expression and behavioral hyperalgesia of C57Bl/6 (wild type), ASIC1-/-, and ASIC3-/- mice before and after the induction of muscle inflammation. A-317567 was effective in reversing hyperalgesia in these animals, suggesting the potential of ASICs as therapeutic targets for muscle inflammatory pain.

Acid-sensing ion channel 3 expression in mouse knee joint afferents and effects of carrageenan-induced arthritis

Arthritis is associated with decreases in local pH. Of the acid-sensing ion channels (ASIC), ASIC3 is most sensitive to such a pH change, abundantly expressed in dorsal root ganglion (DRG), and critical for the development of secondary hyperalgesia. The purpose of this study was to investigate the upregulation of ASIC3, using an acute arthritic pain model in mice. We examined ASIC3 expression in DRG neurons innervating the knee joint with and without carrageenan-induced arthritis by means of retrograde labeling and immunohistochemistry. We also examined the difference of DRG phenotype between ASIC3+/+ and ASIC3-/- mice. ASIC3 immunoreactivity was present in 31% of knee joint afferents and dominantly in small cells. After joint inflammation, ASIC3-immunoreactive neurons significantly increased in number by 50%. Calcitonin gene-related peptide (CGRP) increased similarly in both ASIC3+/+ and ASIC3-/- mice. Soma size distribution of ASIC3-immunoreactive neurons without CGRP expression was shifted to smaller-diameter neurons. Our results suggest that ASIC3 plays an important role in acute arthritic pain. Specifically, we propose that ASIC3 upregulation along with CGRP and phenotypic change in ASIC3-immunoreactive neurons without CGRP are responsible for the development of secondary hyperalgesia after carrageenan-induced arthritis.

PERSPECTIVE

This article shows that ASIC3 is upregulated along with CGRP in knee joint afferents and that there is a phenotypic change in ASIC3-immunoreactive nonpeptidergic neurons in an animal model of acute arthritis. Understanding the basic neurobiology after acute arthritis could lead to future new pharmacological management of arthritis.

Acid-sensitive ion channels and receptors

Acidosis is a noxious condition associated with inflammation, ischaemia or defective acid containment. As a consequence, acid sensing has evolved as an important property of afferent neurons with unmyelinated and thinly myelinated nerve fibres. Protons evoke multiple currents in primary afferent neurons, which are carried by several acid-sensitive ion channels. Among these, acid-sensing ion channels (ASICs) and transient receptor potential (TRP) vanilloid-1 (TRPV1) ion channels have been most thoroughly studied. ASICs survey moderate decreases in extracellular pH, whereas TRPV1 is activated only by severe acidosis resulting in pH values below 6. Two-pore-domain K(+) (K(2P)) channels are differentially regulated by small deviations of extra- or intracellular pH from physiological levels. Other acid-sensitive channels include TRPV4, TRPC4, TRPC5, TRPP2 (PKD2L1), ionotropic purinoceptors (P2X), inward rectifier K(+) channels, voltage-activated K(+) channels, L-type Ca(2+) channels, hyperpolarization-activated cyclic nucleotide gated channels, gap junction channels, and Cl(-) channels. In addition, acid-sensitive G protein coupled receptors have also been identified. Most of these molecular acid sensors are expressed by primary sensory neurons, although to different degrees and in various combinations. Emerging evidence indicates that many of the acid-sensitive ion channels and receptors play a role in acid sensing, acid-induced pain and acid-evoked feedback regulation of homeostatic reactions. The existence and apparent redundancy of multiple pH surveillance systems attests to the concept that acid-base regulation is a vital issue for cell and tissue homeostasis. Since upregulation and overactivity of acid sensors appear to contribute to various forms of chronic pain, acid-sensitive ion channels and receptors are considered as targets for novel analgesic drugs. This approach will only be successful if the pathological implications of acid sensors can be differentiated pharmacologically from their physiological function.

Do corticosteroids produce additional benefit in nerve root infiltration for lumbar disc herniation?

STUDY DESIGN

Experimental animal study.

OBJECTIVE

To determine whether corticosteroids produce additional benefit to nerve root infiltration (NRI) for experimental lumbar disc herniation.

SUMMARY OF BACKGROUND DATA

NRI is used for nonsurgical treatment of radicular symptoms caused by lumbar disc herniation or lumbar spinal canal stenosis. Various studies have shown that NRI using local anesthetic or combinations of local anesthetic and corticosteroid can provide both short- and long-term pain relief. However, whether corticosteroids produce additional benefit to NRI remains controversial.

METHODS

A total of 174 adult female Sprague-Dawley rats were used in this study. The left L5 nerve root and dorsal root ganglion (DRG) were exposed. For the nontreatment group, autologous nucleus pulposus was harvested from the tail and applied to the DRG. For treatment groups, 1% lidocaine (Lido group), 0.4% dexamethasone (Dexa group), 1% lidocaine + 0.4% dexamethasone (Lido + Dexa group), or saline (Saline group) was injected into the underlayer of epineurium just distal to the nucleus pulposus. At 2, 7, 14, and 21 days after surgery, withdrawal threshold was determined using the von Frey test for mechanical allodynia. Expression of tumor necrosis factor (TNF)-alpha in the DRG was examined by immunohistochemical analyses and immunoblotting.

RESULTS

Withdrawal threshold decreased in the nontreatment group from day 2 to day 14. Conversely, Lido, Dexa, and Lido + Dexa groups showed no decreases in withdrawal thresholds, and no significant differences were observed among these 3 groups. Immunohistochemical analyses showed that TNF-alpha was localized in DRG neurons in all groups. Immunoblotting showed that expression of TNF-alpha in the DRG was lower in Lido, Dexa, and Lido + Dexa groups than in the nontreatment group. No significant differences were observed among these 3 groups.

CONCLUSION

NRI prevented mechanical allodynia. However, no additional benefit from using corticosteroid was identified, suggesting that corticosteroid may be unnecessary for NRI.

Role of ASIC3 in the primary and secondary hyperalgesia produced by joint inflammation in mice

The acid sensing ion channel 3 (ASIC3) is critical for the development of secondary hyperalgesia as measured by mechanical stimulation of the paw following muscle insult. We designed experiments to test whether ASIC3 was necessary for the development of both primary and secondary mechanical hyperalgesia that develops after joint inflammation. We used ASIC3 -/- mice and examined the primary (response to tweezers) and secondary hyperalgesia (von-Frey filaments) that develops after joint inflammation comparing to ASIC3 +/+ mice. We also examined the localization of ASIC3 to the knee joint afferents innervating the synovium using immunohistochemical techniques before and after joint inflammation. We show that secondary mechanical hyperalgesia does not develop in ASIC3 -/- mice. However, the primary mechanical hyperalgesia of the inflamed knee joint still develops in ASIC3 -/- mice and is similar to ASIC3 +/+ mice. In knee joint synovium from ASIC3 +/+ mice without joint inflammation, ASIC3 was not localized to joint afferents that were stained with an antibody to protein gene product (PGP) 9.5 or calcitonin gene-related peptide (CGRP). ASIC3 was found, however, in synoviocytes of the knee joint of uninflamed mice. In ASIC3 +/+ mice with joint inflammation, ASIC3 co-localized with PGP 9.5 or CGRP in joint afferents innervating the synovium. We conclude that the decreased pH that occurs after inflammation would activate ASIC3 on primary afferent fibers innervating the knee joint, increasing the input to the spinal cord resulting in central sensitization manifested behaviorally as secondary hyperalgesia of the paw.

Thalamic activation in a disc herniation model

STUDY DESIGN

A novel approach combining a rodent disc herniation model with electrophysiologic recordings of thalamic evoked responses.

OBJECTIVE

To assess short-term effects of nucleus pulposus (NP) application on dorsal root ganglions (DRG) on high threshold afferent fiber evoked activation in the thalamus.

SUMMARY OF BACKGROUND DATA

Epidural application of NP in combination with mechanical compression induces pain related behavior in rats associated with enhanced activity of pain-processing neurons in the dorsal horn of the spinal cord. However, possible effects on neuronal activity in the pain processing ventral posterior lateral (VPL) thalamic nucleus following NP application on DRG have not been investigated.

METHODS

Responses in the contralateral VPL evoked by electrical stimulation of the sciatic nerve and of the fourth lumbar (L4) DRG were recorded in adult Sprague-Dawley rats. Records were obtained before and during application (5, 10, and 20 minutes) of NP or of adipose tissue (AT) to the L4 DRG. AT was used as control for mechanical effects of NP application.

RESULTS

Application of NP resulted in an increase of evoked thalamic responses to 138% +/- 10% of control after 20 minutes (P < 0.01), whereas AT application for 20 minutes resulted in a reduction of evoked responses to 77% +/- 4% (P < 0.05). Recordings in control animals (i.e., with no application) demonstrated stable evoked neuronal thalamic activity for up to 40 minutes.

CONCLUSION

The study demonstrates that NP application onto DRG increases afferent fiber evoked responses in the thalamus and in view of the opposite effects of AT application suggests that these effects may be specific for NP. The results show that NP affects sensory transmitting pathways within a few minutes, possibly due to rapid and reversible alterations in the neuronal excitability. The study thus introduces a rodent model for studying sensory afferent evoked thalamic activity related to DRG injury which may be used to evaluate analgesics and anti-inflammatory drugs used for pain relief in disc herniation and neuropathic pain patients.