Expression and co-expression of VR1, CGRP, and IB4-binding glycoprotein in dorsal root ganglion neurons in rats: differences between the disc afferents and the cutaneous afferents

The influence of intervertebral disc overloading on nociceptor calcium flickering

Introduction

Mechanical overloading can trigger a degenerative-like cascade in an organ culture of intervertebral disc (IVD). Whether the overloaded IVD can influence the activation of nociceptors (i.e., the damage sensing neurons) remains unknown. The study aims to investigate the influence of overloaded IVD conditioned medium (CM) on the activation of nociceptors.

Methods

In the static loading regime, force-controlled loading of 0.2 MPa for 20 h/day representing "long-term sitting and standing" was compared with a displacement-controlled loading maintaining original IVD height. In the dynamic loading regime, high-frequency-intensity loading representing degenerative "wear and tear" was compared with a lower-frequency-intensity loading. CM of differently loaded IVDs were collected to stimulate the primary bovine dorsal root ganglion (DRG) cultures. Calcium imaging (Fluo-4) and calcitonin gene-related peptide (CGRP) immunofluorescent labeling were jointly used to record the calcium flickering in CGRP(+) nociceptors.

Results

Force-controlled loading led to a higher IVD cell death compared to displacement-controlled loading. Both static and dynamic overloading (force-controlled and high-frequency-intensity loadings) elevated the frequency of calcium flickering in the subsurface space of CGRP(+) nociceptors compared to their mild loading counterparts.

Conclusion

In the organ culture system, IVD overloading mediated an altered IVD-nociceptor communication suggesting a biological mechanism associated with discogenic pain.

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.

Intervertebral disc degeneration and how it leads to low back pain

The purpose of this review was to evaluate data generated by animal models of intervertebral disc (IVD) degeneration published in the last decade and show how this has made invaluable contributions to the identification of molecular events occurring in and contributing to pain generation. IVD degeneration and associated spinal pain is a complex multifactorial process, its complexity poses difficulties in the selection of the most appropriate therapeutic target to focus on of many potential candidates in the formulation of strategies to alleviate pain perception and to effect disc repair and regeneration and the prevention of associated neuropathic and nociceptive pain. Nerve ingrowth and increased numbers of nociceptors and mechanoreceptors in the degenerate IVD are mechanically stimulated in the biomechanically incompetent abnormally loaded degenerate IVD leading to increased generation of low back pain. Maintenance of a healthy IVD is, thus, an important preventative measure that warrants further investigation to preclude the generation of low back pain. Recent studies with growth and differentiation factor 6 in IVD puncture and multi-level IVD degeneration models and a rat xenograft radiculopathy pain model have shown it has considerable potential in the prevention of further deterioration in degenerate IVDs, has regenerative properties that promote recovery of normal IVD architectural functional organization and inhibits the generation of inflammatory mediators that lead to disc degeneration and the generation of low back pain. Human clinical trials are warranted and eagerly anticipated with this compound to assess its efficacy in the treatment of IVD degeneration and the prevention of the generation of low back pain.

Netrin-1 mediates nerve innervation and angiogenesis leading to discogenic pain

Objective

Discogenic low back pain (LBP) is associated with nociceptive nerve fibers that grow into degenerated intervertebral discs (IVD) but the etiopathogenesis of disease is not fully understood. The purpose of this study was to clarify the role of Netrin-1 in causing discogenic LBP.

Methods

The level of nociceptive nerve innervation was examined in disc degenerative patients and rat needle-punctured models by immunohistochemistry. Nucleus pulposus (NP) cells were isolated from IVD tissues of rats and induced degeneration by interleukin-1β (IL-1β) or tumor necrosis factor α (TNFα). The candidate genes related to neuron outgrowth and migration were selected by Next-generation sequencing (NGS). CRISPR/Cas9 was used to knockdown Netrin-1 in NP cells. The impact of Netrin-1 on nerve innervation were evaluated with P2X2、NF200 staining and microfluidics assay. Meanwhile the CD31 staining and transwell assay were used to evaluate the impact of Netrin-1 in angiogenesis. The proteins and RNA extracted from NP cells related to catabolism and anabolism were examined by western blot assay and RT-qPCR experiment. ChIP and luciferase experiments were used to assess the intracellular transcriptional regulation of Netrin-1. Further, a needle-punctured rat model followed by histomorphometry and immunofluorescence histochemistry was used to explore the potential effect of Netrin-1 on LBP in vivo.

Results

The level of nerve innervation was increased in severe disc degenerative patients while the expression of Netrin-1 was upregulated. The supernatants of NP cells stimulated with IL-1β or TNFα containing more Netrin-1 could promote axon growth and vascular endothelial cells migration. Knocking down Netrin-1 or overexpressing transcription factor TCF3 as a negative regulator of Netrin-1 attenuated this effect. The needle-punctured rat model brought significant spinal hypersensitivity, nerve innervation and angiogenesis, nevertheless knocking down Netrin-1 effectively prevented disc degeneration-induced adverse impacts.

Conclusion

Discogenic LBP was induced by Netrin-1, which mediated nerve innervation and angiogenesis in disc degeneration. Knocking down Netrin-1 by CRISPR/Cas9 or negatively regulating Netrin1 by transcription factor TCF3 could alleviate spinal hypersensitivity.

The translational potential of this article

This study on Netrin-1 could provide a new target and theoretical basis for the prevention and treatment for discogenic back pain.

TRPV1 in dorsal root ganglion contributed to bone cancer pain

Tumor growth in situ or bone metastases in cancer patients all can induce bone cancer pain. It is frequently occurred in patients with breast, prostate, and lung cancer. Because of the lack of effective treatment, bone cancer pain causes depression, anxiety, fatigue, and sleep disturbances in cancer patients, disrupts the daily quality of life, and results in huge economic and psychological burden. Over the past years, transient receptor potential channels (TRPs), especially TRP vanilloid 1 (TRPV1) in dorsal root ganglion (DRG), have been considered to be involved in bone cancer pain. The characteristic of TRPV1 had been well studied. The mechanisms under TRPV1 regulation in DRG with bone cancer pain are complex, including inflammatory mediators, endogenous formaldehyde, and other mechanisms. In the present review, we summarize the role and potential mechanism of TRPV1 in DRG in bone cancer pain. As the primary sensory neurons, targeting the TRPV1 channel in DRG, might have fewer side effects than in central. We hope systematically understand of TRPV1 modulation in DRG will bring more effective strategy.

Transient receptor potential melastatin-7 in the rat dorsal root ganglion

AIMS

Transient receptor potential melastatin-7 (TRPM7) is a selective cation permeable channel which plays important roles in cellular and developmental biology such as cell proliferation, survival, differentiation and migration. This channel is also known to be necessary for transmitter release in the peripheral nervous system. In this study, immunohistochemistry for TRPM7 was conducted in the rat lumbar dorsal root ganglion (DRG).

METHODS

Triple immunofluorescence methods were used to demonstrate distribution of TRPM7 and its relationship to other TRP channels in the DRG. Retrograde tracing and double immunofluorescence methods were also performed to know peripheral targets of DRG neurons containing TRPM7 and TRP vanilloid 1 (TRPV1). In addition, transection of the sciatic nerve was conducted to demonstrate an effect of the nerve injury on TRPM7expression in the DRG.

RESULTS

TRPM7-immunoreactivity was expressed by 53.9% of sensory neurons in the 4th lumbar DRG. TRPM7-immunoreactive (-IR) DRG neurons mostly had small (<600 µm²) and medium-sized (600-1200 µm²) cell bodies. By triple and double immunofluorescence methods, approximately 70% of TRPM7-IR DRG neurons contained TRPV1-immunoreactivity. Although the number of DRG neurons co-expressing TRPM7 and TRPM8 was small in the DRG, almost all of TRPM8-IR DRG neurons co-expressed TRPM7-immunoreactivity. By combination of retrograde tracing method and immunohistochemistry, TRPM7 was expressed by half of DRG neurons innervating the plantar skin (61.9%) and gastrocnemius muscle (51.2%), and 79.6% of DRG neurons innervating the periosteum. Co-expression of TRPM7 and TRPV1 among periosteum DRG neurons (75.7%) was more abundant than among cutaneous (53.2%) and muscular (40.4%) DRG neurons. DRG neurons which co-expressed these ion channels in the periosteum had smaller cell bodies compared to the skin and muscle. In addition, the sciatic nerve transection decreased the number of TRPM7-IR neurons in the DRG (approximately 60% reduction). The RT-qPCR analysis also demonstrated reduction of TRPM7 mRNA in the injured DRG.

CONCLUSION

The present study suggests that TRPM7 is mainly located in small nociceptors in the DRG. The content of TRPM7 in DRG neurons is probably different among their peripheral targets. TRPM7 in DRG neurons may be able to respond to noxious stimulation from their peripheral tissues. The nerve injury can decrease the level of TRPM7 mRNA and protein in DRG neurons.

In vivo Mouse Intervertebral Disc Degeneration Models and Their Utility as Translational Models of Clinical Discogenic Back Pain: A Comparative Review

Low back pain is a leading cause of disability worldwide and studies have demonstrated intervertebral disc (IVD) degeneration as a major risk factor. While many in vitro models have been developed and used to study IVD pathophysiology and therapeutic strategies, the etiology of IVD degeneration is a complex multifactorial process involving crosstalk of nearby tissues and systemic effects. Thus, the use of appropriate in vivo models is necessary to fully understand the associated molecular, structural, and functional changes and how they relate to pain. Mouse models have been widely adopted due to accessibility and ease of genetic manipulation compared to other animal models. Despite their small size, mice lumbar discs demonstrate significant similarities to the human IVD in terms of geometry, structure, and mechanical properties. While several different mouse models of IVD degeneration exist, greater standardization of the methods for inducing degeneration and the development of a consistent set of output measurements could allow mouse models to become a stronger tool for clinical translation. This article reviews current mouse models of IVD degeneration in the context of clinical translation and highlights a critical set of output measurements for studying disease pathology or screening regenerative therapies with an emphasis on pain phenotyping. First, we summarized and categorized these models into genetic, age-related, and mechanically induced. Then, the outcome parameters assessed in these models are compared including, molecular, cellular, functional/structural, and pain assessments for both evoked and spontaneous pain. These comparisons highlight a set of potential key parameters that can be used to validate the model and inform its utility to screen potential therapies for IVD degeneration and their translation to the human condition. As treatment of symptomatic pain is important, this review provides an emphasis on critical pain-like behavior assessments in mice and explores current behavioral assessments relevant to discogenic back pain. Overall, the specific research question was determined to be essential to identify the relevant model with histological staining, imaging, extracellular matrix composition, mechanics, and pain as critical parameters for assessing degeneration and regenerative strategies.

Degenerative IVD conditioned media and acidic pH sensitize sensory neurons to cyclic tensile strain

Low back pain is among the leading causes of disability worldwide. The degenerative intervertebral disc (IVD) environment contains pathologically high levels of inflammatory cytokines and acidic pH hypothesized to contribute to back pain by sensitizing nociceptive neurons to stimuli that would not be painful in healthy patients. We hypothesized that the degenerative IVD environment drives discogenic pain by sensitizing nociceptive neurons to mechanical loading. To test this hypothesis, we developed an in vitro model that facilitated the investigation of interactions between the degenerative IVD environment, nociceptive neurons innervating the IVD and mechanical loading of the disc; and, the identification of the underlying mechanism of degenerative IVD induced nociceptive neuron sensitization. In our model, rat dorsal root ganglia (DRG) neurons were seeding onto bovine annulus fibrosus tissue, exposed to degenerative IVD conditioned media and/or acidic pH, and subjected to cyclic tensile strain (1 Hz; 1%-6% strain) during measurement of DRG sensory neuron activity via calcium imaging. Using this model, we demonstrated that both degenerative IVD conditioned media and degenerative IVD acidic pH levels induced elevated nociceptive neuron activation in response to physiologic levels of mechanical strain. In addition, interleukin 6 (IL-6) was demonstrated to mediate degenerative IVD conditioned media induced elevated nociceptive neuron activation. These results demonstrate IL-6 mediates degenerative IVD induced neuron sensitization to mechanical loading and further establishes IL-6 as a potential therapeutic target for the treatment of discogenic pain. Data further suggests the degenerative IVD environment contains multiple neuron sensitization pathways (IL-6, pH) that may contribute to discogenic pain.

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.

Disc degeneration induces a mechano-sensitization of disc afferent nerve fibers that associates with low back pain

OBJECTIVE

We aimed to investigate mechano-sensitivity at the afferent nerve fibers projecting to degenerated intervertebral disc (IVD) and nociceptive behaviour in a rat model of low back pain (LBP).

DESIGN

Animal model with LBP was established by lumbar 4/5 IVD puncture and nucleus pulposus aspiration. In vivo single nerve recordings (n = 121) were introduced to measure discharge frequency at the afferent nerve fiber innervating the IVD during mechanical stimulations (von Frey filament or intradiscal pressure). Nerve growth factor (NGF) expression levels in the IVD (n = 20) were assessed by Western blot. LBP-related behaviour (n = 22) was assessed by measuring changes in rearing, mechanical paw-withdrawal threshold, and dynamic weight bearing in a freely walking rat. Inhibitory effect of morphine on the neuronal excitability (n = 19) and painful behaviour (n = 28) was also assessed.

RESULTS

Compared to those with sham or naïve IVD, animal group with degenerated IVD displayed the sensitized neuronal responses and painful behaviour, with hyperexcitability of the afferent nerve fibers in any range of mechanical stimulations (von Frey filament stimulation; 1, 2, and 26 g; intradiscal pressure, 1,500-3,000 mm Hg), strong upregulation of NGF (200-250 % increase), and LBP-like behaviour such as failure of rearing, front limbs-dependent walking pattern, and hypersensitivity in hind-paws. However, the neuronal hyperexcitability and pain behaviour were attenuated after local (30 μM) or systemic (3 mg kg^-1) morphine administration.

CONCLUSIONS

Our study suggests that enhanced mechano-sensitivity at the afferent nerve fiber innervating degenerated IVD is deeply correlated with LBP development, which supports the hypothesis that hyperexcited responses at the nerve fibers represent a decisive source of LBP.

Multiplex Epigenome Editing of Dorsal Root Ganglion Neuron Receptors Abolishes Redundant Interleukin 6, Tumor Necrosis Factor Alpha, and Interleukin 1β Signaling by the Degenerative Intervertebral Disc

Back pain is the leading cause of disability worldwide and contributes to significant socioeconomic impacts. It has been hypothesized that the degenerative intervertebral disc (IVD) contributes to back pain by sensitizing nociceptive neurons innervating the IVD to stimuli that would not be painful to healthy patients. However, the inflammatory signaling networks mediating this sensitization remain poorly understood. A better understanding of the underlying mechanisms of degenerative IVD-induced changes in nociception is required to improve the understanding and treatment of back pain. Toward these ends, a novel in vitro model was developed to investigate degenerative IVD-induced changes in dorsal root ganglion (DRG) neuron activation by measuring DRG neuron activity following neuron seeding on human degenerative IVD tissue collected from patients undergoing surgical treatment for back pain. Lentiviral clustered regularly interspaced palindromic repeat (CRISPR) epigenome editing vectors were built to downregulate the inflammatory receptors TNFR1, IL1R1, and IL6st in DRG neurons in single- and multiplex. Multiplex CRISPR epigenome editing of inflammatory receptors demonstrated that degenerative IVD tissue drives thermal sensitization through the simultaneous and redundant signaling of interleukin (IL)-6, tumor necrosis factor alpha (TNF-α), and IL-1β. This work elucidates redundant signaling pathways in neuron interactions with the degenerative IVD and suggests the need for multiplex targeting of IL-6, TNF-α, and IL-1β for pain modulation in the degenerative IVD.

Differential responses of neurons in the rat caudal ventrolateral medulla to visceral and somatic noxious stimuli and their alterations in colitis

Visceral and somatic types of pain have been reported to manifest crucial differences not only in the experience, but also in their peripheral and central processing. However, the precise neuronal mechanisms that responsible for the modality-specific transmission of pain signals, especially at the supraspinal level, remain unclear. Very little is known also about the potential involvement of such mechanisms in the development of viscero-somatic hyperalgesia. Therefore, in the present study performed on urethane-anesthetized adult male Wistar rats we examined responses of neurons in the caudal ventrolateral medulla (CVLM)-the first site for supraspinal processing of both internal and external pain signals-to visceral (colorectal distension, CRD) and somatic (squeezing of the tail) noxious stimulations and evaluated alterations in response properties of these cells after the induction of colitis. It has been found out that the CVLM of healthy control rats, along with harboring of cells excited by both stimulations (23.7%), contained neurons that were activated by either visceral (31.9%) or somatic noxious stimuli (44.4%). In inflamed animals, the percentages of the visceral and somatic nociceptive cells were decreased (to 18.3% and 34.3%, correspondingly) and the number of bimodal neurons was increased (up to 47.4%); these alterations were associated with substantially enhanced responses of both the modality-specific and convergent CVLM neurons not only to CRD, but also to squeezing of the tail. Under these conditions, visceral and somatic pain stimuli induced similar changes in arterial blood pressure and respiratory rate, whereas in the absence of intestinal inflammation noxious CRD and tail stimulation evoked predominantly divergent autonomic reactions. The data obtained can benefit to a deeper understanding of the neuronal mechanisms that underlie differential supraspinal processing of visceral and somatic noxious stimuli and can potentially contribute to the realization of specific cardiovascular and respiratory accompaniments inherent to a particular type of pain. Therewith, results of the study elucidate colitis-induced alterations in these mechanisms, which may be responsible for the combined development of visceral hypersensitivity and somatic hyperalgesia.

Neurochemical difference between somato- and viscero-projecting sensory neurons in the pig

The chemical coding of porcine somato (skin)- and viscero (urinary bladder)-projecting sensory neurons have been studied and compared using immunohistochemistry. Cell bodies of skin and bladder afferents were identified following Fast Blue injections into the skin of the hind leg as well as into wall of the urinary bladder, respectively. Immunohistochemistry revealed that small and medium-sized neurons projecting to both skin and bladder contained all of the studied substances i.e. substance P (SP), calcitonin gene-related pepide (CGRP), transient receptor potential vanilloid (TRPV1), lectin from Bandeiraea simplicifolia - Griffonia simplicifolia isolectin B4 (IB4) and galanin (GAL). Moreover, small-sized sensory neurons projecting to the bladder and skin of hind leg showed predominantly immunoreactivity to SP and TRPV1 and CGRP, as well as to CGRP and TRPV1 and IB4. It is worth stressing that the subset of sensory neurons innervating the skin exhibited these substances more often than bladder-projecting neurons. In addition, medium-sized skin-projecting neurons contained SP/GAL; SP/CGRP and CGRP/IB4 much more often than their bladder counterparts. On the other hand, small-sized perikarya that innervate the skin were less frequently expressed TRPV1, CGRP and GAL than the bladder-projecting neurons. In conclusion, the present report describes, for the first time, significant differences in the chemical coding between somato- and viscero-projecting sensory neurons in dorsal root ganglia. Moreover, these results provide morphological basis for further functional studies, which may explain the exact roles played by various subpopulations of somato- and viscero-projecting sensory neurons.

TRPV1-Like Immunoreactivity in the Human Locus K, a Distinct Subregion of the Cuneate Nucleus

The presence of transient receptor potential vanilloid type-1 receptor (TRPV1)-like immunoreactivity (LI), in the form of nerve fibres and terminals, is shown in a set of discrete gray matter subregions placed in the territory of the human cuneate nucleus. We showed previously that those subregions share neurochemical and structural features with the protopathic nuclei and, after the ancient name of our town, collectively call them Locus Karalis, and briefly Locus K. TRPV1-LI in the Locus K is codistributed, though not perfectly overlapped, with that of the neuropeptides calcitonin gene-related peptide and substance P, the topography of the elements immunoreactive to the three markers, in relation to each other, reflecting that previously described in the caudal spinal trigeminal nucleus. Myelin stainings show that myelinated fibres, abundant in the cuneate, gracile and trigeminal magnocellular nuclei, are scarce in the Locus K as in the trigeminal substantia gelatinosa. Morphometric analysis shows that cell size and density of Locus K neurons are consistent with those of the trigeminal substantia gelatinosa and significantly different from those of the magnocellular trigeminal, solitary and dorsal column nuclei. We propose that Locus K is a special component of the human dorsal column nuclei. Its functional role remains to be determined, but TRPV1 appears to play a part in it.

CRISPR Epigenome Editing of AKAP150 in DRG Neurons Abolishes Degenerative IVD-Induced Neuronal Activation

Back pain is a major contributor to disability and has significant socioeconomic impacts worldwide. The degenerative intervertebral disc (IVD) has been hypothesized to contribute to back pain, but a better understanding of the interactions between the degenerative IVD and nociceptive neurons innervating the disc and treatment strategies that directly target these interactions is needed to improve our understanding and treatment of back pain. We investigated degenerative IVD-induced changes to dorsal root ganglion (DRG) neuron activity and utilized CRISPR epigenome editing as a neuromodulation strategy. By exposing DRG neurons to degenerative IVD-conditioned media under both normal and pathological IVD pH levels, we demonstrate that degenerative IVDs trigger interleukin (IL)-6-induced increases in neuron activity to thermal stimuli, which is directly mediated by AKAP and enhanced by acidic pH. Utilizing this novel information on AKAP-mediated increases in nociceptive neuron activity, we developed lentiviral CRISPR epigenome editing vectors that modulate endogenous expression of AKAP150 by targeted promoter histone methylation. When delivered to DRG neurons, these epigenome-modifying vectors abolished degenerative IVD-induced DRG-elevated neuron activity while preserving non-pathologic neuron activity. This work elucidates the potential for CRISPR epigenome editing as a targeted gene-based pain neuromodulation strategy.

Mediation of Movement-Induced Breakthrough Cancer Pain by IB4-Binding Nociceptors in Rats

Cancer-induced bone pain is characterized by moderate to severe ongoing pain that commonly requires the use of opiates. Even when ongoing pain is well controlled, patients can suffer breakthrough pain (BTP), episodic severe pain that "breaks through" the medication. We developed a novel model of cancer-induced BTP using female rats with mammary adenocarcinoma cells sealed within the tibia. We demonstrated previously that rats with bone cancer learn to prefer a context paired with saphenous nerve block to elicit pain relief (i.e., conditioned place preference, CPP), revealing the presence of ongoing pain. Treatment with systemic morphine abolished CPP to saphenous nerve block, demonstrating control of ongoing pain. Here, we show that pairing BTP induced by experimenter-induced movement of the tumor-bearing hindlimb with a context produces conditioned place avoidance (CPA) in rats treated with morphine to control ongoing pain, consistent with clinical observation of BTP. Preventing movement-induced afferent input by saphenous nerve block before, but not after, hindlimb movement blocked movement-induced BTP. Ablation of isolectin B4 (IB4)-binding, but not TRPV1+, sensory afferents eliminated movement-induced BTP, suggesting that input from IB4-binding fibers mediates BTP. Identification of potential molecular targets specific to this population of fibers may allow for the development of peripherally restricted analgesics that control BTP and improve quality of life in patients with skeletal metastases.SIGNIFICANCE STATEMENT We present a novel preclinical measure of movement-induced breakthrough pain (BTP) that is observed in the presence of morphine controlling ongoing pain. Blockade of sensory input before movement prevented BTP, whereas nerve block after movement failed to reverse BTP. These observations indicate that blocking peripheral sensory input may prevent BTP and targeting central sites may be required for pain relief once BTP has been initiated. Preventing sensory input from TRPV1-expressing fibers failed to alter movement-induced BTP. In contrast, preventing sensory input from isolectin B4 (IB4)-binding fibers blocked movement-induced BTP. Therefore, examining molecular targets on this population of nociceptive fibers may prove useful for developing an improved strategy for preventing BTP in cancer patients with skeletal metastases.

Dissociation between the relief of skeletal pain behaviors and skin hypersensitivity in a model of bone cancer pain

Recent studies have suggested that in humans and animals with significant skeletal pain, changes in the mechanical hypersensitivity of the skin can be detected. However, whether measuring changes in skin hypersensitivity can be a reliable surrogate for measuring skeletal pain itself remains unclear. To explore this question, we generated skeletal pain by injecting and confining GFP-transfected NCTC 2472 osteosarcoma cells unilaterally to the femur of C3H male mice. Beginning at day 7 post-tumor injection, animals were administered vehicle, an antibody to the P2X3 receptor (anti-P2X3) or anti-NGF antibody. Pain and analgesic efficacy were then measured on days 21, 28, and 35 post-tumor injection using a battery of skeletal pain-related behaviors and von Frey assessment of mechanical hypersensitivity on the plantar surface of the hind paw. Animals with bone cancer pain treated with anti-P2X3 showed a reduction in skin hypersensitivity but no attenuation of skeletal pain behaviors, whereas animals with bone cancer pain treated with anti-NGF showed a reduction in both skin hypersensitivity and skeletal pain behaviors. These results suggest that although bone cancer can induce significant skeletal pain-related behaviors and hypersensitivity of the skin, relief of hypersensitivity of the skin is not always accompanied by attenuation of skeletal pain. Understanding the relationship between skeletal and skin pain may provide insight into how pain is processed and integrated and help define the preclinical measures of skeletal pain that are predictive end points for clinical trials.

Distribution and chemical coding of sensory neurons innervating the skin of the porcine hindlimb

The aim of the present study was to establish the origin and chemical phenotyping of neurons involved in skin innervation of the porcine hind leg. The dorsal root ganglia (DRGs) of the lumbar (L₄-L₆) and sacral (S₁-S₃) spinal nerves were visualized using the fluorescent tracer Fast Blue (FB). The morphometric analysis of FB-positive (FB+)neurons showed that in the L₄, L₅, S₁ and S₂ DRGs, the small-sized perikarya constituted the major population, whereas in the L₆ and S₃ DRGs the medium-sized cells made up the major population. In all these ganglia, large-sized FB+ perikarya constituted only a small percentage of all FB+ neurons. Immunohistochemistry revealed that small- and medium-sized FB+ perikarya contained sensory markers such as: substance P (SP), calcitonin gene related peptide (CGRP) and galanin (GAL); as well as various other factors such as somatostatin (SOM), calbindin-D28k (CB), pituitary adenylate cyclase-activating polypeptide (PACAP) and neuronal nitric oxide synthase (nNOS). Meanwhile large-sized FB+ perikarya usually expressed SP, CGRP or PACAP. In the lumbar DRGs, some large cells also contained SOM and CB. Double-labeling immunohistochemistry showed that SP-positive neurons co-expressed CGRP, GAL or PACAP; while PACAP-positive cells co-expressed GAL or nNOS. Neurons stained for SOM were also immunoreactive for CB or GAL, while neurons stained for nNOS were also immunoreactive for GAL. In conclusion, the present data has indicated that the distribution and chemical phenotyping of the porcine skin-projecting neurons are different within DRGs of the lumbar (forming a femoral nerve) and sacral (forming a sciatic nerve) spinal nerves.

TRPV1 receptor in the human trigeminal ganglion and spinal nucleus: immunohistochemical localization and comparison with the neuropeptides CGRP and SP

This work presents new data concerning the immunohistochemical occurrence of the transient receptor potential vanilloid type-1 (TRPV1) receptor in the human trigeminal ganglion (TG) and spinal nucleus of subjects at different ontogenetic stages, from prenatal life to postnatal old age. Comparisons are made with the sensory neuropeptides calcitonin gene-related peptide (CGRP) and substance P (SP). TRPV1-like immunoreactive (LI) material was detected by western blot in homogenates of TG and medulla oblongata of subjects at prenatal and adult stages of life. Immunohistochemistry showed that expression of the TRPV1 receptor is mostly restricted to the small- and medium-sized TG neurons and to the caudal subdivision of the spinal trigeminal nucleus (Sp5C). The extent of the TRPV1-LI TG neuronal subpopulation was greater in subjects at early perinatal age than at late perinatal age and in postnatal life. Centrally, the TRPV1 receptor localized to fibre tracts and punctate elements, which were mainly distributed in the spinal tract, lamina I and inner lamina II of the Sp5C, whereas stained cells were rare. The TRPV1 receptor colocalized partially with CGRP and SP in the TG, and was incompletely codistributed with both neuropeptides in the spinal tract and in the superficial laminae of the Sp5C. Substantial differences were noted with respect to the distribution of the TRPV1-LI structures described in the rat Sp5C and with respect to the temporal expression of the receptor during the development of the rat spinal dorsal horn. The distinctive localization of TRPV1-LI material supports the concept of the involvement of TRPV1 receptor in the functional activity of the protopathic compartment of the human trigeminal sensory system, i.e. the processing and neurotransmission of thermal and pain stimuli.

Characterization of cutaneous and articular sensory neurons

BACKGROUND

A wide range of stimuli can activate sensory neurons and neurons innervating specific tissues often have distinct properties. Here, we used retrograde tracing to identify sensory neurons innervating the hind paw skin (cutaneous) and ankle/knee joints (articular), and combined immunohistochemistry and electrophysiology analysis to determine the neurochemical phenotype of cutaneous and articular neurons, as well as their electrical and chemical excitability.

RESULTS

Immunohistochemistry analysis using RetroBeads as a retrograde tracer confirmed previous data that cutaneous and articular neurons are a mixture of myelinated and unmyelinated neurons, and the majority of both populations are peptidergic. In whole-cell patch-clamp recordings from cultured dorsal root ganglion neurons, voltage-gated inward currents and action potential parameters were largely similar between articular and cutaneous neurons, although cutaneous neuron action potentials had a longer half-peak duration (HPD). An assessment of chemical sensitivity showed that all neurons responded to a pH 5.0 solution, but that acid-sensing ion channel (ASIC) currents, determined by inhibition with the nonselective acid-sensing ion channel antagonist benzamil, were of a greater magnitude in cutaneous compared to articular neurons. Forty to fifty percent of cutaneous and articular neurons responded to capsaicin, cinnamaldehyde, and menthol, indicating similar expression levels of transient receptor potential vanilloid 1 (TRPV1), transient receptor potential ankyrin 1 (TRPA1), and transient receptor potential melastatin 8 (TRPM8), respectively. By contrast, significantly more articular neurons responded to ATP than cutaneous neurons.

CONCLUSION

This work makes a detailed characterization of cutaneous and articular sensory neurons and highlights the importance of making recordings from identified neuronal populations: sensory neurons innervating different tissues have subtly different properties, possibly reflecting different functions.

Interleukin-6 and interleukin-6 receptor expression, localization, and involvement in pain-sensing neuron activation in a mouse intervertebral disc injury model

The pathological mechanism of intractable low back pain is unclear. However, intervertebral disc (IVD) degeneration is a primary cause of low back pain, and pain-related mediators, such as interleukin-6 (IL-6), have been correlated with discogenic pain. The objective of this study is to elucidate the mechanism of local IL-6 and IL-6 receptor (IL-6R) expression after IVD injury as well as determine the involvement of IL-6/IL-6 signaling in discogenic pain. To do this, quantitative and immunohistological analyses in a mouse model of IVD injury were performed. Firstly, we measured the local expression levels of IL-6 and IL-6R in IVDs by enzyme-linked immunosorbent assay (ELISA). Secondly, we immunohistochemically confirmed their localization in injured IVDs. Lastly, we evaluated the effects of intradiscal injection of an IL-6 inhibitor by evaluating pain-related protein, calcitonin gene-related peptide (CGRP), expression in dorsal root ganglia (DRG) neurons that innervate IVDs. Injured IVDs showed increased production of IL-6 and IL-6R. IL-6 and IL-6R expression in the injured IVD were predominantly localized in the annulus fibrosus and endplate, and intradiscal injection of the IL-6 inhibitor suppressed CGRP expression in the DRG neurons. These results show that IL-6 and IL-6R expression levels are responsive to IVD injury and that inhibition of IL-6/IL-6R signaling may be a promising analgesic treatment for degenerative disc diseases.

New insights of nociceptor sensitization in bone cancer pain

INTRODUCTION

Numerous studies have shown that an intact CNS is required for the conscious perception of cancer-induced bone pain (CIBP) and that changes in the CNS are clearly evident. Accordingly, the blockage of nociceptive stimulus into the CNS can effectively relieve or markedly attenuate CIBP, revealing the clinical implication of the blockage of ongoing peripheral inputs for the control of CIBP.

AREAS COVERED

In this review, the heterogeneity and excitability of nociceptors in bone are covered. Furthermore, their role in initiating and maintaining CIBP is also described.

EXPERT OPINION

Developing mechanistic therapies to treat CIBP is a challenge, but they have the potential to fundamentally change our ability to effectively block/relieve CIBP and increase the functional status and quality of life of patients with bone metastasis. Further studies are desperately needed at both the preclinical and clinical levels to determine whether the targets as mentioned in this review are viable and feasible for patient populations.

Sensory Neurons, Ion Channels, Inflammation and the Onset of Neuropathic Pain

Neuropathic pain often fails to respond to conventional pain management procedures. here we review the aetiology of neuropathic pain as would result from peripheral neuropathy or injury. We show that inflammatory mediators released from damaged nerves and tissue are responsible for triggering ectopic activity in primary afferents and that this, in turn, provokes increased spinal cord activity and the development of ‘central sensitization’. Although evidence is mounting to support the role of interleukin-1β, prostaglandins and other cytokines in the onset of neuropathic pain, the clinical efficacy of drugs which antagonize or prevent the actions of these mediators is yet to be determined. basic science findings do, however, support the use of pre-emptive analgesia during procedures which involve nerve manipulation and the use of anti-inflammatory steroids as soon as possible following traumatic nerve injury.

Efficacy of anti-nerve growth factor therapy for discogenic neck pain in rats

STUDY DESIGN

Immunohistological analysis of the cervical dorsal root ganglia (DRG).

OBJECTIVE

To investigate immunohistologically in rats whether intradiscal administration of anti-nerve growth factor (NGF) antibody in injured cervical intervertebral discs (IVDs) suppresses pain-related peptide expression in DRG neurons.

SUMMARY OF BACKGROUND DATA

Neck pain can involve the entire neck and become chronic and intractable. Cervical disc degeneration is a primary cause of neck pain, and pain-related mediators, such as NGF, have been correlated with discogenic pain.

METHODS

We examined Sprague-Dawley rats that received 10 punctures in the C5-C6 IVD, and were treated with saline (puncture group) or an anti-NGF antibody (anti-NGF group). The retrograde neurotracer Fluoro-Gold (FG) was then injected into the C5-C6 IVD. In addition, we examined a sham group that did not receive punctures (disc nonpuncture). The C2-C7 DRG were harvested 1 week after surgery and immunostained for calcitonin gene-related peptide (CGRP), a marker for peptide-containing neurons. We determined for each group the percentages of FG-labeled DRG neurons that were CGRP-immunoreactive (CGRP-ir).

RESULTS

FG-labeled neurons innervating the C5-C6 IVD were found in all C2-C7 DRG examined. The percentage of FG-labeled CGRP-ir DRG neurons in the puncture group was significantly higher than that observed in the sham (P < 0.001) and anti-NGF groups (P < 0.001), but there was no significant difference between the sham and anti-NGF groups (P > 0.05). Therefore, intradiscal administration of anti-NGF antibody suppressed CGRP expression the cervical DRG.

CONCLUSION

Neurons located in the C2-C7 DRG innervated the C5-C6 IVD. These findings indicate that neck pain may be derived from degenerated IVDs. Furthermore, intradiscal administration of anti-NGF antibody suppressed CGRP expression in the cervical DRG innervating the injured IVD. Therefore, inhibiting NGF upregulation in the cervical IVD may be an efficient treatment for discogenic neck pain.

LEVEL OF EVIDENCE

N/A.

The neurobiology of skeletal pain

Disorders of the skeleton are one of the most common causes of chronic pain and long-term physical disability in the world. Chronic skeletal pain is caused by a remarkably diverse group of conditions including trauma-induced fracture, osteoarthritis, osteoporosis, low back pain, orthopedic procedures, celiac disease, sickle cell disease and bone cancer. While these disorders are diverse, what they share in common is that when chronic skeletal pain occurs in these disorders, there are currently few therapies that can fully control the pain without significant unwanted side effects. In this review we focus on recent advances in our knowledge concerning the unique population of primary afferent sensory nerve fibers that innervate the skeleton, the nociceptive and neuropathic mechanisms that are involved in driving skeletal pain, and the neurochemical and structural changes that can occur in sensory and sympathetic nerve fibers and the CNS in chronic skeletal pain. We also discuss therapies targeting nerve growth factor or sclerostin for treating skeletal pain. These therapies have provided unique insight into the factors that drive skeletal pain and the structural decline that occurs in the aging skeleton. We conclude by discussing how these advances have changed our understanding and potentially the therapeutic options for treating and/or preventing chronic pain in the injured, diseased and aged skeleton.

IB4-binding sensory neurons in the adult rat express a novel 3' UTR-extended isoform of CaMK4 that is associated with its localization to axons

Calcium/calmodulin-dependent protein kinase 4 (gene and transcript: CaMK4; protein: CaMKIV) is the nuclear effector of the Ca(2+) /calmodulin kinase (CaMK) pathway where it coordinates transcriptional responses. However, CaMKIV is present in the cytoplasm and axons of subpopulations of neurons, including some sensory neurons of the dorsal root ganglia (DRG), suggesting an extranuclear role for this protein. We observed that CaMKIV was expressed strongly in the cytoplasm and axons of a subpopulation of small-diameter DRG neurons, most likely cutaneous nociceptors by virtue of their binding the isolectin IB4. In IB4+ spinal nerve axons, 20% of CaMKIV was colocalized with the endocytic marker Rab7 in axons that highly expressed CAM-kinase-kinase (CAMKK), an upstream activator of CaMKIV, suggesting a role for CaMKIV in signaling though signaling endosomes. Using fluorescent in situ hybridization (FISH) with riboprobes, we also observed that small-diameter neurons expressed high levels of a novel 3' untranslated region (UTR) variant of CaMK4 mRNA. Using rapid amplification of cDNA ends (RACE), reverse-transcription polymerase chain reaction (RT-PCR) with gene-specific primers, and cDNA sequencing analyses we determined that the novel transcript contains an additional 10 kb beyond the annotated gene terminus to a highly conserved alternate polyadenylation site. Quantitative PCR (qPCR) analyses of fluorescent-activated cell sorted (FACS) DRG neurons confirmed that this 3'-UTR-extended variant was preferentially expressed in IB4-binding neurons. Computational analyses of the 3'-UTR sequence predict that UTR-extension introduces consensus sites for RNA-binding proteins (RBPs) including the embryonic lethal abnormal vision (ELAV)/Hu family proteins. We consider the possible implications of axonal CaMKIV in the context of the unique properties of IB4-binding DRG neurons.

Expression of background potassium channels in rat DRG is cell-specific and down-regulated in a neuropathic pain model

Neuropathic pain is associated with hyperexcitability of DRG neurons. Despite the importance of leakage potassium channels for neuronal excitability, little is known about their cell-specific expression in DRGs and possible modulation in neuropathic pain. Multiple leakage channels are expressed in DRG neurons, including TASK1, TASK3, TRESK, TRAAK, TWIK1, TREK1 and TREK2 but little is known about their distribution among different cell types. Our immunohistochemical studies show robust TWIK1 expression in large and medium size neurons, without overlap with TRPV1 or IB4 staining. TASK1 and TASK3, on the contrary, are selectively expressed in small cells; TASK1 expression closely overlaps TRPV1-positive cells, while TASK3 is expressed in TRPV1- and IB4-negative cells. We also studied mRNA expression of these channels in L4-L5 DRGs in control conditions and up to 4 weeks after spared nerve injury lesion. We found that TWIK1 expression is much higher than TASK1 and TASK3 and is strongly decreased 1, 2 and 4 weeks after neuropathic injury. TASK3 expression, on the other hand, decreases 1 week after surgery but reverts to baseline by 2weeks; TASK1 shows no significant change at any time point. These data suggest an involvement of TWIK1 in the maintenance of the pain condition.

Platelet-rich plasma combined with hydroxyapatite for lumbar interbody fusion promoted bone formation and decreased an inflammatory pain neuropeptide in rats

STUDY DESIGN

A prospective interventional trial, using a rat model of lumbar interbody fusion.

OBJECTIVE

To examine the potential efficacy of platelet-rich plasma (PRP) for lumbar interbody fusion, using hydroxyapatite (HA).

SUMMARY OF BACKGROUND DATA

PRP is an autologous product containing a high concentration of platelets in a small volume of plasma and has osteoinductive effects. HA has osteoconductive ability and has been used in combination with autogenous bone for spine fusion. However, reports using PRP with HA for spine fusion are very few. The purpose of this study was to examine the efficacy of PRP with HA for spinal interbody fusion and at the same time to estimate the change in immunoreactivity of the inflammatory neuropeptide, calcitonin gene-related peptide (CGRP), in dorsal root ganglion (DRG) neurons innervating spinal discs.

METHODS

A total of 35 Sprague-Dawley rats were used in this study. Twenty-one rats were used for conducting interbody fusion experiments, 7 rats were used as immunostaining controls, and 7 other rats were used as blood donors for making PRP. L5-L6 interbody fusion was performed on 21 rats using HA + PRP (n = 7), HA + platelet-poor plasma (n = 7), or HA + saline (n = 7). Simultaneously, Fluoro-Gold neurotracer was applied to the intervertebral space to detect DRG neurons innervating the discs. L5-L6 lumbar radiographs were obtained and lumbar DRGs were immunostained for CGRP. The rate of bone union and the change in CGRP immunoreactive DRG neurons innervating the discs were evaluated and compared among groups.

RESULTS

All L5-L6 lumbar discs were fused in the PRP + HA group (fused 7/total 7), whereas only 1 case was fused in the platelet-poor plasma group (1 of 7) and no cases in the HA-only group (0 of 7), which was a significant difference. Upon immunohistochemical analysis, CGRP-positive neurons innervated L5-L6 intervertebral discs in nonunion cases, and these were significantly increased compared with those in union cases.

CONCLUSION

Our study suggests that using PRP with HA was beneficial for spine fusion. This combination may promote bone union and also decrease inflammatory neuropeptide in sensory neurons innervating the discs.

Neuroplasticity of sensory and sympathetic nerve fibers in a mouse model of a painful arthritic joint

OBJECTIVE

Many forms of arthritis are accompanied by significant chronic joint pain. This study was undertaken to investigate whether there is significant sprouting of sensory and sympathetic nerve fibers in the painful arthritic knee joint and whether nerve growth factor (NGF) drives this pathologic reorganization.

METHODS

A painful arthritic knee joint was produced by injection of Freund's complete adjuvant (CFA) into the knee joint of young adult mice. CFA-injected mice were then treated systemically with vehicle or anti-NGF antibody. Pain behaviors were assessed, and at 28 days following the initial CFA injection, the knee joints were processed for immunohistochemistry analysis using antibodies against calcitonin gene-related peptide (CGRP; sensory nerve fibers), neurofilament 200 kd (NF200; sensory nerve fibers), growth-associated protein 43 (GAP-43; sprouted nerve fibers), tyrosine hydroxylase (TH; sympathetic nerve fibers), CD31 (endothelial cells), or CD68 (monocyte/macrophages).

RESULTS

In CFA-injected mice, there was a significant increase in the density of CD68+ macrophages, CD31+ blood vessels, and CGRP+, NF200+, GAP-43+, and TH+ nerve fibers in the synovium, as well as a significant increase in joint pain-related behaviors. None of these findings were observed in sham-injected mice. Administration of anti-NGF reduced these pain-related behaviors and the ectopic sprouting of nerve fibers, but had no significant effect on the increase in density of CD31+ blood vessels or CD68+ macrophages.

CONCLUSION

These findings demonstrate that ectopic sprouting of sensory and sympathetic nerve fibers occurs in the painful arthritic joint and may be involved in the generation and maintenance of arthritic pain.

What is different about spinal pain?

BACKGROUND

The mechanisms subserving deep spinal pain have not been studied as well as those related to the skin and to deep pain in peripheral limb structures. The clinical phenomenology of deep spinal pain presents unique features which call for investigations which can explain these at a mechanistic level.

METHODS

Targeted searches of the literature were conducted and the relevant materials reviewed for applicability to the thesis that deep spinal pain is distinctive from deep pain in the peripheral limb structures. Topics related to the neuroanatomy and neurophysiology of deep spinal pain were organized in a hierarchical format for content review.

RESULTS

Since the 1980's the innervation characteristics of the spinal joints and deep muscles have been elucidated. Afferent connections subserving pain have been identified in a distinctive somatotopic organization within the spinal cord whereby afferents from deep spinal tissues terminate primarily in the lateral dorsal horn while those from deep peripheral tissues terminate primarily in the medial dorsal horn. Mechanisms underlying the clinical phenomena of referred pain from the spine, poor localization of spinal pain and chronicity of spine pain have emerged from the literature and are reviewed here, especially emphasizing the somatotopic organization and hyperconvergence of dorsal horn "low back (spinal) neurons". Taken together, these findings provide preliminary support for the hypothesis that deep spine pain is different from deep pain arising from peripheral limb structures.

CONCLUSIONS

This thesis addressed the question "what is different about spine pain?" Neuroanatomic and neurophysiologic findings from studies in the last twenty years provide preliminary support for the thesis that deep spine pain is different from deep pain arising from peripheral limb structures.

Octreotide inhibits capsaicin-induced activation of C and Aδ afferent fibres in rat hairy skin in vivo

1. The present study investigated whether the somatostatin receptor (SSTR) agonist, octreotide, could inhibit the activation of dorsal skin afferent fibres induced by local injection of capsaicin in the rat. 2. Single unit activity from Aδ mechano-heat sensitive (AMH; n = 41) and C mechano-heat sensitive (CMH; n = 30) afferents was recorded after their isolation in thin filaments from the dorsal cutaneous nerve branches. The effect of subcutaneous octreotide injection on the change in discharge rate and mechanical threshold induced by capsaicin was determined. 3. Capsaicin (0.05%) injection into the edge of the receptive field of both AMH and CMH units increased their discharge rate and decreased their mechanical threshold. Pre-injection of octreotide inhibited these responses, and co-application of SSTR antagonist, cyclosomatostatin, reversed the inhibitory effect of octreotide. 4. The present study provides electrophysiological evidence that the signal evoked by the somatostatin receptor inhibits the activation and mechanical sensitization evoked by capsaicin in the terminals in small-diameter sensory neurons.

Antagonism of nerve growth factor-TrkA signaling and the relief of pain

Nerve growth factor (NGF) was originally discovered as a neurotrophic factor essential for the survival of sensory and sympathetic neurons during development. However, in the adult NGF has been found to play an important role in nociceptor sensitization after tissue injury. The authors outline mechanisms by which NGF activation of its cognate receptor, tropomyosin-related kinase A receptor, regulates a host of ion channels, receptors, and signaling molecules to enhance acute and chronic pain. The authors also document that peripherally restricted antagonism of NGF-tropomyosin-related kinase A receptor signaling is effective for controlling human pain while appearing to maintain normal nociceptor function. Understanding whether there are any unexpected adverse events and how humans may change their behavior and use of the injured/degenerating tissue after significant pain relief without sedation will be required to fully appreciate the patient populations that may benefit from these therapies targeting NGF.

The majority of myelinated and unmyelinated sensory nerve fibers that innervate bone express the tropomyosin receptor kinase A

Although skeletal pain is a leading cause of chronic pain and disability, relatively little is known about the specific populations of nerve fibers that innervate the skeleton. Recent studies have reported that therapies blocking nerve growth factor (NGF) or its cognate receptor, tropomyosin receptor kinase A (TrkA) are efficacious in attenuating skeletal pain. A potential factor to consider when assessing the analgesic efficacy of targeting NGF-TrkA signaling in a pain state is the fraction of NGF-responsive TrkA+ nociceptors that innervate the tissue from which the pain is arising, as this innervation and the analgesic efficacy of targeting NGF-TrkA signaling may vary considerably from tissue to tissue. To explore this in the skeleton, tissue slices and whole mount preparations of the normal, adult mouse femur were analyzed using immunohistochemistry and confocal microscopy. Analysis of these preparations revealed that 80% of the unmyelinated/thinly myelinated sensory nerve fibers that express calcitonin gene-related peptide (CGRP) and innervate the periosteum, mineralized bone and bone marrow also express TrkA. Similarly, the majority of myelinated sensory nerve fibers that express neurofilament 200 kDa (NF200) which innervate the periosteum, mineralized bone and bone marrow also co-express TrkA. In the normal femur, the relative density of CGRP+, NF200+ and TrkA+ sensory nerve fibers per unit volume is: periosteum>bone marrow>mineralized bone>cartilage with the respective relative densities being 100:2:0.1:0. The observation that the majority of sensory nerve fibers innervating the skeleton express TrkA+, may in part explain why therapies that block NGF/TrkA pathway are highly efficacious in attenuating skeletal pain.

Direct application of the tumor necrosis factor-α inhibitor, etanercept, into a punctured intervertebral disc decreases calcitonin gene-related peptide expression in rat dorsal root ganglion neurons

STUDY DESIGN

retrograde neurotracing and immunohistochemistry were used to investigate the effect of the tumor necrosis factor (TNF)-α inhibitor, etanercept, on calcitonin gene-related peptide (CGRP) expression in dorsal root ganglion (DRG) neurons innervating intervertebral discs in rats.

OBJECTIVE

to clarify the action of a TNF-α inhibitor on a sensory neuropeptide in DRG neurons innervating intervertebral discs.

SUMMARY OF BACKGROUND DATA

degeneration of lumbar intervertebral discs is a cause of low back pain. TNF-α in the intervertebral disc is a major contributor to discogenie pain. Effects of TNF-α inhibition on CGRP expression in DRG neurons were evaluated.

METHODS

the neurotracer FluoroGold was applied to the surfaces of L4/5 discs to label their innervating DRG neurons (n = 30). Of 30 rats, 10 were in a nonpunctured disc sham surgery control group, whereas the other 20 were in experimental groups in which intervertebral discs were punctured with a 23-gauge needle. Etanercept or saline was applied into the punctured discs (n = 10 each treatment). After 14 days of surgery, DRGs from L1 to L6 were harvested, sectioned, and immunostained for CGRP. The proportion of FluoroGold-labeled CGRP-immunoreactive DRG neurons was evaluated in all groups.

RESULTS

FluoroGold-labeled neurons innervating the L4/5 disc were distributed throughout L1-L6 DRGs in all groups. Of the FluoroGold-labeled neurons, the proportion of CGRP-immunoreactive neurons was 21% ± 4% in the sham surgery control group, 32% ± 7% in the puncture + saline group, and 23% ± 4% in the puncture + etanercept group. The proportion of CGRP-immunoreactive neurons was significantly greater in the puncture + saline group compared with the sham control and puncture + etanercept groups (P < 0.01).

CONCLUSION

in this model, CGRP was upregulated in DRG neurons innervating damaged discs. However, direct intradiscal application of etanercept immediately after disc puncture suppressed CGRP expression in DRG neurons innervating injured discs. This finding may further elucidate the mechanism for the effectiveness of etanercept in upregulation of neuropeptide in DRG neurons innervating intervertebral discs.

Expression of α1-adrenoceptors on peripheral nociceptive neurons

The purpose of this study was to determine whether α(1)-adrenoceptors are expressed on primary nociceptive afferents that innervate healthy skin. Skin and dorsal root ganglia were collected from adult male Wistar rats and assessed using fluorescence immunohistochemistry with antibodies directed against α(1)-adrenoceptors alone or in combination with specific labels including myelin basic protein and neurofilament 200 (markers of myelinated nerve fibres), protein gene product 9.5 (a pan-neuronal marker), tyrosine hydroxylase (sympathetic neurons), isolectin B(4) (IB(4): non-peptidergic sensory neurons), calcitonin gene related peptide (CGRP) and transient receptor potential vanilloid receptor 1 (TRPV1) (peptidergic sensory neurons). Double labelling in dorsal root ganglia confirmed the expression of α(1)-adrenoceptors within sub-populations of CGRP, IB(4) and TRPV1 immunoreactive neurons. Myelinated and unmyelinated sensory nerve fibres in the skin expressed α(1)-adrenoceptors whereas sympathetic nerve fibres did not. The expression of α(1)-adrenoceptors on C- and A-delta nociceptive afferent fibres provides a histochemical substrate for direct excitation of these fibres by adrenergic agonists. This may help to explain the mechanism of sensory-sympathetic coupling that sometimes develops on surviving primary nociceptive afferents in neuropathic pain states.

Intervertebral disc, sensory nerves and neurotrophins: who is who in discogenic pain?

The normal intervertebral disc (IVD) is a poorly innervated organ supplied only by sensory (mainly nociceptive) and postganglionic sympathetic (vasomotor efferents) nerve fibers. Interestingly, upon degeneration, the IVD becomes densely innervated even in regions that in normal conditions lack innervation. This increased innervation has been associated with pain of IVD origin. The mechanisms responsible for nerve growth and hyperinnervation of pathological IVDs have not been fully elucidated. Among the molecules that are presumably involved in this process are some members of the family of neurotrophins (NTs), which are known to have both neurotrophic and neurotropic properties and regulate the density and distribution of nerve fibers in peripheral tissues. NTs and their receptors are expressed in healthy IVDs but much higher levels have been observed in pathological IVDs, thus suggesting a correlation between levels of expression of NTs and density of innervation in IVDs. In addition, NTs also play a role in inflammatory responses and pain transmission by increasing the expression of pain-related peptides and modulating synapses of nociceptive neurons at the spinal cord. This article reviews current knowledge about the innervation of IVDs, NTs and NT receptors, expression of NTs and their receptors in IVDs as well as in the sensory neurons innervating the IVDs, the proinflammatory role of NTs, NTs as nociception regulators, and the potential network of discogenic pain involving NTs.

A phenotypically restricted set of primary afferent nerve fibers innervate the bone versus skin: therapeutic opportunity for treating skeletal pain

Although musculoskeletal pain is one of the most common causes of chronic pain and physical disability in both developing and developed countries, relatively little is known about the nerve fibers and mechanisms that drive skeletal pain. Small diameter sensory nerve fibers, most of which are C-fiber nociceptors, can be separated into two broad populations: the peptide-rich and peptide-poor nerve fibers. Peptide-rich nerve fibers express substance P (SP) and calcitonin gene-related peptide (CGRP). In contrast, the peptide-poor nerve fibers bind to isolectin B4 (IB(4)) and express the purinergic receptor P(2)X(3) and Mas-related G protein-coupled receptor member d (Mrgprd). In the present report, we used mice in which the Mrgprd(+) nerve fibers express genetically encoded axonal tracers to determine the peptide-rich and peptide-poor sensory nerve fibers that innervate the glabrous skin of the hindpaw as compared to the bone marrow, mineralized bone and periosteum of the femur. Whereas the skin is richly innervated by CGRP(+), SP(+), P(2)X(3)(+) and Mrgprd(+) sensory nerve fibers, the bone marrow, mineralized bone and periosteum receive a significant innervation by SP(+) and CGRP(+), but not Mrgprd(+) and P(2)X(3)(+) nerve fibers. This lack of redundancy in the populations of C-fibers that innervate the bone may present a unique therapeutic opportunity for targeting skeletal pain as the peptide-rich and peptide-poor sensory nerve fibers generally express a different repertoire of receptors and channels to detect noxious stimuli. Thus, therapies that target the specific types of C-nerve fibers that innervate the bone may be uniquely effective in attenuating skeletal pain as compared to skin pain.

The effects of sympathetic outflow on upregulation of vanilloid receptors TRPV(1) in primary afferent neurons evoked by intradermal capsaicin

The vanilloid receptor TRPV(1) is a key nociceptive molecule located in primary afferent nociceptive neurons in dorsal root ganglia (DRG) for initiating neurogenic inflammation and pain. Our recent study demonstrates that up-regulation of TRPV(1) receptors by intradermal injection of capsaicin is modulated by activation of the protein kinase C (PKC) cascade. Neurogenic inflammation and pain resulting from capsaicin injection are sympathetically dependent, responding to norepinephrine, adenosine 5'-triphosphate (ATP) and/or neuropeptide Y released from sympathetic efferents. In a rat model of acute neurogenic inflammatory pain produced by capsaicin injection, we used immunofluorescence and Western blots combined with pharmacology and surgical sympathectomies to analyze whether the capsaicin-evoked up-regulation of TRPV(1) in DRG neurons is affected by sympathetic outflow by way of activating the PKC cascade. Sympathetic denervation reduced significantly the capsaicin-evoked expressions of TRPV(1), calcitonin gene-related peptide and/or phosphorylated PKC and their co-expression. These reductions could be restored by exogenous pretreatment with an analog of ATP, alpha,beta-methylene ATP. Inhibition of PKC with chelerythrine chloride prevented the ATP effect. Consistent results were obtained from experiments in which capsaicin-evoked changes in cutaneous inflammation (vasodilation and edema) were examined after sympathetic denervation, and the effects of the above pharmacological manipulations were evaluated. Our findings suggest that the capsaicin-evoked up-regulation of TRPV(1) receptors in DRG neurons is modulated sympathetically by the action of ATP released from sympathetic efferents to activate the PKC cascade. Thus, this study proposes a potential new mechanism of sympathetic modulation of neurogenic inflammation.

Direct single injection of p38 mitogen-activated protein kinase inhibitor does not affect calcitonin gene-related peptide expression in dorsal root ganglion neurons innervating punctured discs in rats

STUDY DESIGN.: Immunohistological analysis of punctured disc after application of a p38 MAP kinase inhibitor. OBJECTIVE.: To examine effect of direct application on dorsal root ganglion (DRG) neurons innervating damaged rat discs. SUMMARY OF BACKGROUND DATA.: Degeneration of lumbar discs is one cause of low back pain. Pathogenesis may involve sensory nerve ingrowth into disc inner layers; tumor necrosis factor-alpha (TNF-alpha) is thought to be a major inducer of ingrowth. Because p38 mitogen-activated protein kinase (p38) upregulates TNF-alpha expression and may play a crucial role in pain sensation, we investigated the effect of one injection of inhibitor on expression of the pain-related neuropeptide calcitonin gene-related peptide (CGRP). METHODS.: The neuro-tracer fluoro-gold was applied to the surfaces of L4/5 discs to label the innervating DRG neurons (n = 30). Of 30 rats, 10 were controls, whereas the other 20 were the experimental model (i.e., discs were punctured with 23-gauge needle). P38 specific inhibitor or saline was applied simultaneously (n = 10 each, Puncture + inhibitor and puncture + saline groups). Fourteen days postsurgery, DRGs from L1 to L6 were harvested, sectioned, and immunostained for CGRP. Proportion of CGRP-immunoreactive DRG neurons was evaluated in all groups. RESULTS.: Fluoro-gold-labeled neurons innervating the L4/5 disc were distributed throughout L1 to L6 DRGs in all groups. Proportions of labeled neurons positive for CGRP were 15.2% +/- 8% (controls), 27.2% +/- 10% (puncture + saline), and 25.2% +/- 8% (puncture + inhibitor). Proportion of immunoreactive neurons was significantly increased in the puncture groups compared with controls. However, there was no significant difference between the 2 puncture groups (P > 0.1). CONCLUSION.: In this model, CGRP was upregulated in DRG neurons innervating the damaged disc. However, a direct single application of p38 inhibitor did not suppress CGRP expression in innervating DRG neurons. Future research with p38 inhibitor in this model should evaluate multiple or systemic administration of inhibitor.

Sustained morphine treatment augments capsaicin-evoked calcitonin gene-related peptide release from primary sensory neurons in a protein kinase A- and Raf-1-dependent manner

Studies have shown that long-term (5alpha,6alpha)-7,8-didehydro-4,5-epoxy-17-methylmorphinan-3,6-diol (morphine) treatment increases the sensitivity to painful heat stimuli (thermal hyperalgesia). The cellular adaptations contributing to sustained morphine-mediated pain sensitization are not fully understood. It was shown previously (J Neurosci 22:6747-6755, 2002) that sustained morphine exposure augments pain neurotransmitter [such as calcitonin gene-related peptide (CGRP)] release in the dorsal horn of the spinal cord in response to the heat-sensing transient receptor potential vanilloid 1 receptor agonist 8-methyl-N-vanillyl-6-nonenamide (capsaicin). In the present study, we demonstrate that sustained morphine-mediated augmentation of CGRP release from isolated primary sensory dorsal root ganglion neurons is dependent on protein kinase A and Raf-1 kinase. Our data indicate that, in addition to neural system adaptations, sustained opioid agonist treatment also produces intracellular compensatory adaptations in primary sensory neurons, leading to augmentation of evoked pain neurotransmitter release from these cells.

Capsaicin-sensitive sensory nerve fibers contribute to the generation and maintenance of skeletal fracture pain

Although skeletal pain can have a marked impact on a patient's functional status and quality of life, relatively little is known about the specific populations of peripheral nerve fibers that drive non-malignant bone pain. In the present report, neonatal male Sprague-Dawley rats were treated with capsaicin or vehicle and femoral fracture was produced when the animals were young adults (15-16 weeks old). Capsaicin treatment, but not vehicle, resulted in a significant (>70%) depletion in the density of calcitonin-gene related peptide positive (CGRP(+)) sensory nerve fibers, but not 200 kDa neurofilament H positive (NF200(+)) sensory nerve fibers in the periosteum. The periosteum is a thin, cellular and fibrous tissue that tightly adheres to the outer surface of all but the articulated surface of bone and appears to play a pivotal role in driving fracture pain. In animals treated with capsaicin, but not vehicle, there was a 50% reduction in the severity, but no change in the time course, of fracture-induced skeletal pain-related behaviors as measured by spontaneous flinching, guarding and weight bearing. These results suggest that both capsaicin-sensitive (primarily CGRP(+) C-fibers) and capsaicin-insensitive (primarily NF200(+) A-delta fibers) sensory nerve fibers participate in driving skeletal fracture pain. Skeletal pain can be a significant impediment to functional recovery following trauma-induced fracture, osteoporosis-induced fracture and orthopedic surgery procedures such as knee and hip replacement. Understanding the specific populations of sensory nerve fibers that need to be targeted to inhibit the generation and maintenance of skeletal pain may allow the development of more specific mechanism-based therapies that can effectively attenuate acute and chronic skeletal pain.

Increases in transient receptor potential vanilloid-1 mRNA and protein in primary afferent neurons stimulated by protein kinase C and their possible role in neurogenic inflammation

A recent study by our group demonstrates pharmacologically that the transient receptor potential vanilloid-1 (TRPV(1)) is activated by intradermal injection of capsaicin to initiate neurogenic inflammation by the release of neuropeptides in the periphery. In this study, expression of TRPV(1), phosphorylated protein kinase C (p-PKC), and calcitonin gene-related peptide (CGRP) in dorsal root ganglion (DRG) neurons was visualized by using immunofluorescence, real-time PCR, and Western blots to examine whether increases in TRPV(1) mRNA and protein levels evoked by capsaicin injection are subject to modulation by the activation of PKC and to analyze the role of this process in the pathogenesis of neurogenic inflammation. Capsaicin injection into the hindpaw skin of anesthetized rats evoked increases in the expression of TRPV(1), CGRP and p-PKC in mRNA and/or protein levels and in the number of single labeled TRPV(1), p-PKC, and CGRP neurons in ipsilateral L4-5 DRGs. Coexpressions of TRPV(1) with p-PKC and/or CGRP in DRG neurons were also significantly increased after CAP injection. These evoked expressions at both molecular and cellular levels were significantly inhibited after TRPV(1) receptors were blocked by 5'-iodoresiniferatoxin (5 microg) or PKC was inhibited by chelerythrine chloride (5 microg). Taken together, these results provide evidence that up-regulation of TRPV(1) mRNA and protein levels under inflammatory conditions evoked by capsaicin injection is subject to modulation by the PKC cascade in which increased CGRP level in DRG neurons may be related to the initiation of neurogenic inflammation. Thus, up-regulation of TRPV(1) receptors in DRG neurons seems critical for initiating acute neurogenic inflammation.

Glial cell line-derived neurotrophic factor family ligands enhance capsaicin-stimulated release of calcitonin gene-related peptide from sensory neurons

The glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) are a group of peptides that have been implicated as important factors in inflammation, since they are released in increased amounts during inflammation and induce thermal hyperalgesia upon injection. Mouse isolated sensory neurons in culture and freshly dissociated spinal cord slices were used to examine the enhancement in stimulated-release of the neuropeptide, calcitonin gene-related peptide (CGRP), as a measure of sensitization. Exposure of isolated sensory neurons in culture to GDNF, neurturin, and artemin enhanced the capsaicin-stimulated release of immunoreactive calcitonin gene-related peptide (iCGRP) two- to threefold, but did not increase potassium-stimulated release of iCGRP. A similar profile of sensitization was observed in freshly dissociated spinal cord slices. Persephin, another member of the GFL family thought to be important in development, was unable to induce an enhancement in the release of iCGRP. These results demonstrate that specific GFLs are important mediators affecting sensory neuronal sensitivity, likely through modulation of the capsaicin receptor. The sensitization of sensory neurons during inflammation, and the pain and neurogenic inflammation resulting from this sensitization, may be due in part to the effects of these selected GFLs.

Inside information: the unique features of visceral sensation

Most of what is written and believed about pain and nociceptors originates from studies of the "somatic" (non-visceral) sensory system. As a result, the unique features of visceral pain are often overlooked. In the clinic, the management of visceral pain is typically poor, and drugs that are used with some efficacy to treat somatic pain often present unwanted effects on the viscera. For these reasons, a better understanding of visceral sensory neurons-particularly visceral nociceptors-is required. This review provides evidence of functional, morphological, and biochemical differences between visceral and non-visceral afferents, with a focus on potential nociceptive roles, and also considers some of the potential mechanisms of visceral mechanosensation.