Nociceptive characteristics of tumor necrosis factor-α in naive and tumor-bearing mice

Novel therapies for cancer-induced bone pain

Cancer pain is a growing problem, especially with the substantial increase in cancer survival. Reports indicate that bone metastasis, whose primary symptom is bone pain, occurs in 65-75% of patients with advanced breast or prostate cancer. We optimized a preclinical in vivo model of cancer-induced bone pain (CIBP) involving the injection of Lewis Lung Carcinoma cells into the intramedullary space of the femur of C57BL/6 mice or transgenic mice on a C57BL/6 background. Mice gradually reduce the use of the affected limb, leading to altered weight bearing. Symptoms of secondary cutaneous heat sensitivity also manifest themselves. Following optimization, three potential analgesic treatments were assessed; 1) single ion channel targets (targeting the voltage-gated sodium channels NaV1.7, NaV1.8, or acid-sensing ion channels), 2) silencing µ-opioid receptor-expressing neurons by modified botulinum compounds, and 3) targeting two inflammatory mediators simultaneously (nerve growth factor (NGF) and tumor necrosis factor (TNF)). Unlike global NaV1.8 knockout mice which do not show any reduction in CIBP-related behavior, embryonic conditional NaV1.7 knockout mice in sensory neurons exhibit a mild reduction in CIBP-linked behavior. Modified botulinum compounds also failed to cause a detectable analgesic effect. In contrast, inhibition of NGF and/or TNF resulted in a significant reduction in CIBP-driven weight-bearing alterations and prevented the development of secondary cutaneous heat hyperalgesia. Our results support the inhibition of these inflammatory mediators, and more strongly their dual inhibition to treat CIBP, given the superiority of combination therapies in extending the time needed to reach limb use score zero in our CIBP model.

Metabolic underpinnings of cancer-related fatigue

Cancer-related fatigue (CRF) is one of the most prevalent and detrimental complications of cancer. Emerging evidence suggests that obesity and insulin resistance are associated with CRF occurrence and severity in cancer patients and survivors. In this narrative review, we analyzed recent studies including both preclinical and clinical research on the relationship between obesity and/or insulin resistance and CRF. We also describe potential mechanisms for these relationships, though with the caveat that because the mechanisms underlying CRF are incompletely understood, the mechanisms mediating the association between obesity/insulin resistance and CRF are similarly incompletely delineated. The data suggest that, in addition to their effects to worsen CRF by directly promoting tumor growth and metastasis, obesity and insulin resistance may also contribute to CRF by inducing chronic inflammation, neuroendocrinological disturbance, and metabolic alterations. Furthermore, studies suggest that patients with obesity and insulin resistance experience more cancer-induced pain and are at more risk of emotional and behavioral disruptions correlated with CRF. However, other studies implied a potentially paradoxical impact of obesity and insulin resistance to reduce CRF symptoms. Despite the need for further investigation utilizing interventions to directly elucidate the mechanisms of cancer-related fatigue, current evidence demonstrates a correlation between obesity and/or insulin resistance and CRF, and suggests potential therapeutics for CRF by targeting obesity and/or obesity-related mediators.

Mechanisms of cancer pain

Personalised and targeted interventions have revolutionised cancer treatment and dramatically improved survival rates in recent decades. Nonetheless, effective pain management remains a problem for patients diagnosed with cancer, who continue to suffer from the painful side effects of cancer itself, as well as treatments for the disease. This problem of cancer pain will continue to grow with an ageing population and the rapid advent of more effective therapeutics to treat the disease. Current pain management guidelines from the World Health Organisation are generalised for different pain severities, but fail to address the heterogeneity of mechanisms in patients with varying cancer types, stages of disease and treatment plans. Pain is the most common complaint leading to emergency unit visits by patients with cancer and over one-third of patients that have been diagnosed with cancer will experience under-treated pain. This review summarises preclinical models of cancer pain states, with a particular focus on cancer-induced bone pain and chemotherapy-associated pain. We provide an overview of how preclinical models can recapitulate aspects of pain and sensory dysfunction that is observed in patients with persistent cancer-induced bone pain or neuropathic pain following chemotherapy. Peripheral and central nervous system mechanisms of cancer pain are discussed, along with key cellular and molecular mediators that have been highlighted in animal models of cancer pain. These include interactions between neuronal cells, cancer cells and non-neuronal cells in the tumour microenvironment. Therapeutic targets beyond opioid-based management are reviewed for the treatment of cancer pain.

Differences and Similarities in Spontaneous Activity Between Animal Models of Cancer-Induced Pain and Neuropathic Pain

Background

Clinical data on cancer-induced pain (CIP) demonstrate widespread changes in sensory function. It is characterized in humans not only by stimulus-invoked pain, but also by spontaneous pain. In our previous studies in an animal model of CIP, we observed changes in intrinsic membrane properties and excitability of dorsal root ganglion (DRG) sensory neurons corresponding to mechanical allodynia and hyperalgesia, of which abnormal activities of Aβ-fiber sensory neurons are consistent in a rat model of peripheral neuropathic pain (NEP).

Objective

To investigate whether there are related peripheral neural mechanisms between the CIP and NEP models of spontaneous pain, we compared the electrophysiological properties of DRG sensory neurons at 2–3 weeks after CIP and NEP model induction.

Methods

CIP models were induced with metastasis tumour-1 rat breast cancer cells implanted into the distal epiphysis of the femur. NEP models were induced with a polyethylene cuff implanted around the sciatic nerve. Spontaneous pain in animals is measured by spontaneous foot lifting (SFL). After measurement of SFL, the animals were prepared for electrophysiological recordings of spontaneous activity (SA) in DRG neurons in vivo.

Results

Our data showed that SFL and SA occurred in both models. The proportion of SFL and SA of C-fiber sensory neurons in CIP was more significantly increased than in NEP models. There was no difference in duration of SFL and the rate of SA between the two models. The duration of SFL is related to the rate of SA in C-fiber in both models.

Conclusion

Thus, SFL may result from SA activity in C-fiber neurons in CIP and NEP rats. The differences and similarities in spontaneous pain between CIP and NEP rats is related to the proportion and rate of SA in C-fibers, respectively.

γ-Terpinene complexed with β-cyclodextrin attenuates spinal neuroactivity in animals with cancer pain by Ca2+ channel block

OBJECTIVES

Considering that γ-terpinene (γ-TPN) is a monoterpene found in Cannabis oil, with high lipophilicity and limited pharmacokinetics, our objective was to evaluate whether its complexation in β-cyclodextrin (γ-TPN/β-CD) could improve its physicochemical properties and action on cancer pain, as well as verify the mechanisms of action involved.

METHODS

The γ-TPN/β-CD was prepared and submitted to physicochemical characterization. Animals with sarcoma 180 were treated (vehicle, γ-TPN 50 mg/kg, γ-TPN/β-CD 5 mg/kg or morphine) and assessed for hyperalgesia, TNF-α and IL-1β levels, iNOS and c-Fos activity. The effects of γ-TPN on calcium channels were studied by patch-clamp and molecular docking.

RESULTS

β-CD improved the physicochemical properties and prolonged the anti-hyperalgesic effect of γ-TPN. This compound also reduced the levels of IL-1β, TNF-α and iNOS in the tumour, and c-Fos protein in the spinal cord. In addition, it reduced Ca2+ current, presenting favourable chemical interactions with different voltage-dependent calcium channels.

CONCLUSION

These results indicate that the complexation of γ-TPN into β-CD increases its stability and time effect, reducing spinal neuroactivity and inflammation by blocking calcium channels.

Acute P38-Mediated Enhancement of P2X3 Receptor Currents by TNF-α in Rat Dorsal Root Ganglion Neurons

Purpose

Tumor necrosis factor-α (TNF-α) is a pro-inflammatory cytokine and involves in a variety of pain conditions. Some findings suggest that TNF-α may act directly on primary afferent neurons to induce acute pain hypersensitivity through non-transcriptional regulation. This study investigated whether TNF-α had an effect on functional activity of P2X3 receptors in primary sensory neurons. Herein, we report that a brief (5 min) application of TNF-α rapidly enhanced the electrophysiological activity of P2X3 receptors in rat dorsal root ganglia (DRG) neurons.

Methods

Electrophysiological recordings were carried out on rat DRG neurons, and nociceptive behavior was quantified in rats.

Results

A brief (5 min) exposure of TNF-α rapidly increased P2X3 receptor-mediated and α,β-methylene-ATP (α,β-meATP)-evoked inward currents in a dose-dependent manner. The potentiation of P2X3 receptor-mediated ATP currents by TNF-α was voltage-independent. TNF-α shifted the concentration-response curve for α,β-meATP upwards, with an increase of 31.57 ± 6.81% in the maximal current response to α,β-meATP. This acute potentiation of ATP currents by TNF-α was blocked by p38 mitogen-activated protein kinase (MAPK) inhibitor SB202190, but not by non-selective cyclooxygenase inhibitor indomethacin, suggesting involvement of p38 MAPK, but not cyclooxygenase. Moreover, intraplantar injection of TNF-α and α,β-meATP produced a synergistic effect on mechanical allodynia in rats. TNF-α-induced mechanical allodynia was also alleviated after local P2X3 receptors were blocked.

Conclusion

These results suggested that TNF-α rapidly sensitized P2X3 receptors in primary sensory neurons via a p38 MAPK dependent pathway, which revealed a novel peripheral mechanism underlying acute mechanical hypersensitivity by peripheral administration of TNF-α.

TNF-α acutely enhances acid-sensing ion channel currents in rat dorsal root ganglion neurons via a p38 MAPK pathway

Background

Tumor necrosis factor-α (TNF-α) is a pro-inflammatory cytokine involved in pain processing and hypersensitivity. It regulates not only the expression of a variety of inflammatory mediators but also the functional activity of some ion channels. Acid-sensing ion channels (ASICs), as key sensors for extracellular protons, are expressed in nociceptive sensory neurons and contribute to pain signaling caused by tissue acidosis. It is still unclear whether TNF-α has an effect on functional activity of ASICs. Herein, we reported that a brief exposure of TNF-α acutely sensitized ASICs in rat dorsal root ganglion (DRG) neurons.

Methods

Electrophysiological experiments on rat DRG neurons were performed in vitro and acetic acid induced nociceptive behavior quantified in vitro.

Results

A brief (5min) application of TNF-α rapidly enhanced ASIC-mediated currents in rat DRG neurons. TNF-α (0.1-10 ng/ml) dose-dependently increased the proton-evoked ASIC currents with an EC₅₀ value of 0.12 ± 0.01 nM. TNF-α shifted the concentration-response curve of proton upwards with a maximal current response increase of 42.34 ± 7.89%. In current-clamp recording, an acute application of TNF-α also significantly increased acid-evoked firing in rat DRG neurons. The rapid enhancement of ASIC-mediated electrophysiological activity by TNF-α was prevented by p38 mitogen-activated protein kinase (MAPK) inhibitor SB202190, but not by non-selective cyclooxygenase inhibitor indomethacin, suggesting that p38 MAPK is necessary for this enhancement. Behaviorally, TNF-α exacerbated acid-induced nociceptive behaviors in rats via activation of local p38 MAPK pathway.

Conclusions

These results suggest that TNF-α rapidly enhanced ASIC-mediated functional activity via a p38 MAPK pathway, which revealed a novel peripheral mechanism underlying TNF-α involvement in rapid hyperalgesia by sensitizing ASICs in primary sensory neurons.

Animal Models of Cancer-Related Pain: Current Perspectives in Translation

The incidence of pain in cancer patients during diagnosis and treatment is exceedingly high. Although advances in cancer detection and therapy have improved patient prognosis, cancer and its treatment-associated pain have gained clinical prominence. The biological mechanisms involved in cancer-related pain are multifactorial; different processes for pain may be responsible depending on the type and anatomic location of cancer. Animal models of cancer-related pain have provided mechanistic insights into the development and process of pain under a dynamic molecular environment. However, while cancer-evoked nociceptive responses in animals reflect some of the patients’ symptoms, the current models have failed to address the complexity of interactions within the natural disease state. Although there has been a recent convergence of the investigation of carcinogenesis and pain neurobiology, identification of new targets for novel therapies to treat cancer-related pain requires standardization of methodologies within the cancer pain field as well as across disciplines. Limited success of translation from preclinical studies to the clinic may be due to our poor understanding of the crosstalk between cancer cells and their microenvironment (e.g., sensory neurons, infiltrating immune cells, stromal cells etc.). This relatively new line of inquiry also highlights the broader limitations in translatability and interpretation of basic cancer pain research. The goal of this review is to summarize recent findings in cancer pain based on preclinical animal models, discuss the translational benefit of these discoveries, and propose considerations for future translational models of cancer pain.

Tumor necrosis factor alpha secreted from oral squamous cell carcinoma contributes to cancer pain and associated inflammation

Patients with oral cancer report severe pain during function. Inflammation plays a role in the oral cancer microenvironment; however, the role of immune cells and associated secretion of inflammatory mediators in oral cancer pain has not been well defined. In this study, we used 2 oral cancer mouse models: a cell line supernatant injection model and the 4-nitroquinoline-1-oxide (4NQO) chemical carcinogenesis model. We used the 2 models to study changes in immune cell infiltrate and orofacial nociception associated with oral squamous cell carcinoma (oSCC). Oral cancer cell line supernatant inoculation and 4NQO-induced oSCC resulted in functional allodynia and neuronal sensitization of trigeminal tongue afferent neurons. Although the infiltration of immune cells is a prominent component of both oral cancer models, our use of immune-deficient mice demonstrated that oral cancer-induced nociception was not dependent on the inflammatory component. Furthermore, the inflammatory cytokine, tumor necrosis factor alpha (TNFα), was identified in high concentration in oral cancer cell line supernatant and in the tongue tissue of 4NQO-treated mice with oSCC. Inhibition of TNFα signaling abolished oral cancer cell line supernatant-evoked functional allodynia and disrupted T-cell infiltration. With these data, we identified TNFα as a prominent mediator in oral cancer-induced nociception and inflammation, highlighting the need for further investigation in neural-immune communication in cancer pain.

Neuron-restrictive silencer factor-mediated downregulation of μ-opioid receptor contributes to the reduced morphine analgesia in bone cancer pain

Neuron-restrictive silencer factor–induced downregulation of μ-opioid receptor is involved in the reduction of morphine analgesia in sarcoma-induced bone cancer pain.

Bone cancer pain has been reported to have unique mechanisms and is resistant to morphine treatment. Recent studies have indicated that neuron-restrictive silencer factor (NRSF) plays a crucial role in modulating the expression of the μ-opioid receptor (MOR) gene. The present study elucidates the regulatory mechanisms of MOR and its ability to affect bone cancer pain. Using a sarcoma-inoculated murine model, pain behaviors that represent continuous or breakthrough pain were evaluated. Expression of NRSF in the dorsal root ganglion (DRG) and spinal dorsal horn was quantified at the transcriptional and translational levels, respectively. Additionally, chromatin immunoprecipitation assays were used to detect NRSF binding to the promoter of MOR. Furthermore, NRSF was genetically knocked out by antisense oligodeoxynucleotide, and the expression of MOR and the effect of morphine were subsequently analyzed. Our results indicated that in a sarcoma murine model, NRSF expression is upregulated in dorsal root ganglion neurons, and the expression of NRSF mRNA is significantly negatively correlated with MOR mRNA expression. Additionally, chromatin immunoprecipitation analysis revealed that NRSF binding to the neuron-restrictive silencer element within the promoter area of the MOR gene is promoted with a hypoacetylation state of histone H3 and H4. Furthermore, genetically knocking down NRSF with antisense oligodeoxynucleotide rescued the expression of MOR and potentiated the systemic morphine analgesia. The present results suggest that in sarcoma-induced bone cancer pain, NRSF-induced downregulation of MOR is involved in the reduction of morphine analgesia. Epigenetically, up-regulation of MOR could substantially improve the effect of system delivery of morphine.

Emerging factors in the progression of cancer-related pain

Most cancer patients experience severe pain during their disease course, and the management of cancer pain is a major challenge for patients and the healthcare team. Many diverse translational models of cancer pain in recent years have improved our understanding of cancer-related pain. Cancer and associated cells in the cancer microenvironment may release various peripheral mediators, including ATP, formaldehyde, protons, proteases, endothelin, bradykinin, TNF and NGF, that result in the activation and/or sensitization of peripheral and central neurons, that contribute to the clinical manifestations of cancer-related pain. Identification of these mediators and the peripheral and central mechanisms by which they contribute to cancer-related pain may provide novel therapeutic targets to alleviate cancer patient suffering.

Targeting glia for bone cancer pain

INTRODUCTION

Bone cancer pain (BCP) remains to be a clinical challenge with limited pharmaceutical interventions. Therefore, novel therapeutic targets for the management of BCP are in desperate need. Recently, a growing body of evidence has suggested that glial cells may play a pivotal role in the pathogenesis of BCP. Areas covered: This review summarizes the recent progress in the understanding of glia in BCP and reveals the potential therapeutic targets in glia for BCP treatment. Expert opinion: Pharmacological interventions inhibiting the activation of glial cells, suppressing glia-derived proinflammatory cytokines, cell surface receptors, and the intracellular signaling pathways may be beneficial for the pain management of advanced cancer patients. However, these pharmacological interventions should not disrupt the normal function of glia cells since they play a vital supportive and protective role in the central nervous system.

Epigallocatechin-3-gallate attenuates bone cancer pain involving decreasing spinal Tumor Necrosis Factor-α expression in a mouse model

Tumor metastasis to bone often elicits a wide array of symptoms, in which pain is a significant factor in catastrophic complications of bone cancer. The complete understanding of bone cancer-related pain is still unknown, while several pathophysiological components have been suggested, from tumor-stimulated osteolysis, nerve compression, stimulations of ion channels, and locally generated inflammatory cytokines. In particular, it has been shown that pro-inflammatory cytokine TNFα-mediated actions are necessary for the development of bone cancer pain. As a member of catechin family in green tea extracts, EGCG (Epigallocatechin-3-gallate) can reduce excess free radicals and attenuate overactive inflammatory signaling including TNFα. In addition, EGCG or its related molecules have been used to control neuropathic pain in various preclinical settings. However, its potential use in bone cancer-caused pain has not yet been reported. Here we show that treating a mouse model of bone cancer by EGCG, results in a dramatic reduction in pain behavior and a significant decrease of TNFα expression within the spinal cord of tumor-bearing mice. Thus, this study reveals an anti-nociceptive role for EGCG in the progression of pain caused by tumor bone metastasis, and highlights a potential scheme by using anti-TNFα as a therapeutic option for osteolytic pain.

Use of Animal Models in Understanding Cancer-induced Bone Pain

Many common cancers have a propensity to metastasize to bone. Although malignancies often go undetected in their native tissues, bone metastases produce excruciating pain that severely compromises patient quality of life. Cancer-induced bone pain (CIBP) is poorly managed with existing medications, and its multifaceted etiology remains to be fully elucidated. Novel analgesic targets arise as more is learned about this complex and distinct pain state. Over the past two decades, multiple animal models have been developed to study CIBP’s unique pathology and identify therapeutic targets. Here, we review animal models of CIBP and the mechanistic insights gained as these models evolve. Findings from immunocompromised and immunocompetent host systems are discussed separately to highlight the effect of model choice on outcome. Gaining an understanding of the unique neuromolecular profile of cancer pain through the use of appropriate animal models will aid in the development of more effective therapeutics for CIBP.

Quercetin reduces Ehrlich tumor-induced cancer pain in mice

Cancer pain directly affects the patient's quality of life. We have previously demonstrated that the subcutaneous administration of the mammary adenocarcinoma known as Ehrlich tumor induces pain in mice. Several studies have shown that the flavonoid quercetin presents important biological effects, including anti-inflammatory, antioxidant, analgesic, and antitumor activity. Therefore, the analgesic effect and mechanisms of quercetin were evaluated in Ehrlich tumor-induced cancer pain in mice. Intraperitoneal (i.p.) treatments with quercetin reduced Ehrlich tumor-induced mechanical and thermal hyperalgesia, but not paw thickness or histological alterations, indicating an analgesic effect without affecting tumor growth. Regarding the analgesic mechanisms of quercetin, it inhibited the production of hyperalgesic cytokines IL-1β and TNFα and decreased neutrophil recruitment (myeloperoxidase activity) and oxidative stress. Naloxone (opioid receptor antagonist) inhibited quercetin analgesia without interfering with neutrophil recruitment, cytokine production, and oxidative stress. Importantly, cotreatment with morphine and quercetin at doses that were ineffective as single treatment reduced the nociceptive responses. Concluding, quercetin reduces the Ehrlich tumor-induced cancer pain by reducing the production of hyperalgesic cytokines, neutrophil recruitment, and oxidative stress as well as by activating an opioid-dependent analgesic pathway and potentiation of morphine analgesia. Thus, quercetin treatment seems a suitable therapeutic approach for cancer pain that merits further investigation.

Neurobiological studies of chronic pain and analgesia: Rationale and refinements

Chronic pain is a complex condition for which the need for specialized research and therapies has been recognized internationally. This review summarizes the context for the international call for expansion of pain research to improve our understanding of the mechanisms underlying pain in order to achieve improvements in pain management. The methods for conducting sensory assessment in animal models are discussed and the development of animal models of chronic pain is specifically reviewed, with an emphasis on ongoing refinements to more closely mimic a variety of human pain conditions. Pharmacological correspondences between pre-clinical pain models and the human clinical experience are noted. A discussion of the 3Rs Framework (Replacement, Reduction, Refinement) and how each may be considered in pain research is featured. Finally, suggestions are provided for engaging principal investigators, IACUC reviewers, and institutions in the development of strong partnerships to simultaneously expand our knowledge of the mechanisms underlying pain and analgesia while ensuring the humane use of animals in research.

Pain in the cancer patient: different pain characteristics CHANGE pharmacological treatment requirements

Twenty years ago, the main barriers to successful cancer pain management were poor assessment by physicians, and patients' reluctance to report pain and take opioids. Those barriers are almost exactly the same today. Cancer pain remains under-treated; in Europe, almost three-quarters of cancer patients experience pain, and almost a quarter of those with moderate to severe pain do not receive any analgesic medication. Yet it has been suggested that pain management could be improved simply by ensuring that every consultation includes the patient's rating of pain, that the physician pays attention to this rating, and a plan is agreed to increase analgesia when it is inadequate. After outlining current concepts of carcinogenesis in some detail, this paper describes different methods of classifying and diagnosing cancer pain and the extent of current under-treatment. Key points are made regarding cancer pain management. Firstly, the pain may be caused by multiple different mechanisms and therapy should reflect those underlying mechanisms - rather than being simply based on pain intensity as recommended by the WHO three-step ladder. Secondly, a multidisciplinary approach is required which combines both pharmacological and non-pharmacological treatment, such as psychotherapy, exercise therapy and electrostimulation. The choice of analgesic agent and its route of administration are considered, along with various interventional procedures and the requirements of palliative care. Special attention is paid to the treatment of breakthrough pain (particularly with fast-acting fentanyl formulations, which have pharmacokinetic profiles that closely match those of breakthrough pain episodes) and chemotherapy-induced neuropathic pain, which affects around one third of patients who receive chemotherapy. Finally, the point is made that medical education should place a greater emphasis on pain therapy, both at undergraduate and postgraduate level.

Biomarkers: symptoms, survivorship, and quality of life

OBJECTIVES

To review the evidence on a number of biomarkers that show potential clinical utility in the prediction of and treatment responsiveness for the four most common symptoms associated with cancer and its treatment (ie, pain, fatigue, sleep disturbance, depression).

DATA SOURCES

Review and synthesis of review articles and data-based publications.

CONCLUSION

A growing body of evidence suggests that sensitive and specific biomarkers will be available to assist clinicians with the assessment and management of symptoms.

IMPLICATIONS FOR NURSING PRACTICE

Nurses will play a critical role in educating patients about their risk for specific symptoms based on an evaluation of specific biomarkers. Nurses will be involved in using biomarker data to titrate medications based on patient's responses to symptom management interventions.

Minocycline blocks lipopolysaccharide induced hyperalgesia by suppression of microglia but not astrocytes

Systemic injection of lipopolysaccharide (LPS) induces a robust immune response as well as thermal and mechanical hyperalgesia. Spinal and peripheral glial cells have been implicated as important mediators in this hyperalgesia but the specific contributions of microglia versus astrocytes are not entirely clear. To better define these mechanisms, this study examined the febrile response, nociceptive sensitivity, glial cell reactivity and cytokine production in the dorsal root ganglion (DRG) and spinal cord in rats following systemic treatment with LPS and the effects of minocycline in countering these responses. Intraperitoneal LPS injection resulted in an increase in core body temperature and produced hyperalgesia to heat and mechanical stimuli. Western blot studies revealed increased expression of microgial cell, macrophage and satellite cell markers in DRG and microglial and astrocyte markers in spinal cord following LPS treatment. Real-time RT-PCR indicated that LPS treatment increased cytokine mRNA expression levels in both the DRG and the spinal cord. Minocycline suppressed all LPS-induced behavioral effects but not the febrile response. Moreover, minocycline prevented LPS-induced microglia/macrophage activation and cytokine responses in spinal cord and DRG, but did not affect the activation of astrocytes/satellite cells. These data demonstrate that LPS-induced changes in nociceptive sensitivity are likely mediated by activation of microglial cells and/or macrophages in the spinal cord and DRG.

Acute inhibition of signalling phenotype of spinal GABAergic neurons by tumour necrosis factor-alpha

Spinal application of TNFα induces both allodynia and hyperalgesia, and at least part of the pronociceptive effects of TNFα have been suggested as due to the impaired function of spinal inhibitory neurons (disinhibition). The present study explores the effects of TNFα on the signalling phenotype of spinal GABAergic neurons identified in transgenic mice expressing green fluorescent protein at the glutamic acid decarboxylase 67 (GAD67) promoter. Acute application of TNFα directly inhibits the excitability of a subset of GAD67(+) spinal neurons. TNFα-induced inhibition was dependent on the activation of p38 mitogen-activated protein kinase (MAPK) within these GAD67(+) neurons. TNFα receptor 1 (TNFR1) but not receptor 2 (TNFR2) was identified on spinal GAD67(+) neurons, suggesting that TNFα signals through TNFR1. Voltage-clamp recordings of GAD67(+) neurons indicated that the inhibitory effect of TNFα was through suppression of the hyperpolarization-activated cation current (I(h)). This study defines a novel mechanism of spinal disinhibition mediated by a TNFα-TNFR1-p38 pathway within GABAergic inhibitory interneurons.

The intrathecally administered kappa-2 opioid agonist GR89696 and interleukin-10 attenuate bone cancer-induced pain through synergistic interaction

BACKGROUND

Although bone cancer-related pain is one of the most disruptive symptoms in patients with advanced cancer, patients are often refractory to pharmacological treatments; thus, more effective treatments for bone cancer pain are needed. We evaluated the analgesic efficacy of and interaction between intrathecal GR89696, a κ(2)-opioid receptor agonist, and interleukin (IL)-10 in a rat model of bone cancer pain.

METHODS

The rat model of bone cancer pain was produced by right tibia intramedullary injection of rat breast cancer cells, and an intrathecal catheterization was performed. Ten days later, a paw-withdrawal threshold to mechanical stimulus by von Frey hairs was measured using the up-down method, after intrathecal administration of GR89696 and IL-10. The interaction between the 2 drugs was also evaluated using an isobolographic analysis.

RESULTS

Intrathecal GR89696 and IL-10 significantly increased the paw withdrawal threshold of the cancer cell-implanted rat, in a dose-dependent manner, with 50% effective dose values (95% confidence interval) of 50.78 μg (31.80-80.07μg) and 0.83 μg (0.59-1.15 μg), respectively. Isobolographic analysis revealed a synergistic interaction between intrathecal GR89696 and IL-10.

CONCLUSIONS

Intrathecally administered GR89696 and IL-10 attenuated bone cancer-induced pain, and the 2 drugs interacted synergistically in the spinal cord. These results raise the intriguing possibility of κ(2)-opioid receptor agonists and IL-10 as a new therapeutic approach for the management of bone cancer-associated pain.

Increased local concentration of complement C5a contributes to incisional pain in mice

BACKGROUND

In our previous study, we demonstrated that local injection of complement C5a and C3a produce mechanical and heat hyperalgesia, and that C5a and C3a activate and sensitize cutaneous nociceptors in normal skin, suggesting a contribution of complement fragments to acute pain. Other studies also have shown that the complement system can be activated by surgical incision, and the systemic blockade of C5a receptor (C5aR) reduces incision-induced pain and inflammation. In this study, we further examined the possible contribution of wound area C5a to incisional pain.

METHODS

Using of a hind paw incisional model, the effects of a selective C5aR antagonist, PMX53, on nociceptive behaviors were measured after incision in vivo. mRNA levels of C5 and C5aR in skin, dorsal root ganglia (DRG) and spinal cord, and C5a protein levels in the skin were quantified after incision. The responses of nociceptors to C5a were also evaluated using the in vitro skin-nerve preparation.

RESULTS

Local administration of PMX53 suppressed heat hyperalgesia and mechanical allodynia induced by C5a injection or after hind paw incision in vivo. mRNA levels of C5 and C5aR in the skin, but not DRG and spinal cord, were dramatically increased after incision. C5a protein in the skin was also increased after incision. In vitro C5a did not increase the prevalence of fibers with ongoing activity in afferents from incised versus control, unincised skin. C5a sensitized C-fiber afferent responses to heat; however, this was less evident in afferents adjacent to the incision. PMX53 blocked sensitization of C-fiber afferents to heat by C5a but did not by itself influence ongoing activity or heat sensitivity in afferents innervating control or incised skin. The magnitude of mechanical responses was also not affected by C5a in any nociceptive fibers innervating incised or unincised skin.

CONCLUSIONS

This study demonstrates that high locally generated C5a levels are present in wounds for at least 72 hours after incision. In skin, C5a contributes to hypersensitivity after incision, but increased responsiveness of cutaneous nociceptors to C5a was not evident in incised skin. Thus, high local concentrations of C5a produced in wounds likely contribute to postoperative pain.

Hematopoietic colony-stimulating factors: new players in tumor-nerve interactions

A variety of cancers are accompanied by debilitating pain, which constitutes the primary reason for poor quality of life in cancer patients. There is an urgent demand for the development of specific mechanism-based therapies against cancer pain. Recently, important advances have been made in mechanisms contributing to cancer pain. A notable finding was that the tumor-derived hematopoietic growth factors, granulocyte- and granulocyte-macrophage-colony-stimulating factors (G-CSF/GM-CSF), subserve important functions in the generation of pain hypersensitivity in tumor-affected regions. In this context, their receptors were unexpectedly found on pain-sensing nerves and were observed to be functionally linked to nociceptive sensitization and tumor-induced pain. Here, we review evidence supporting a role for G-/GM-CSF in sensitization of pain-sensing nerves, the underlying signaling pathways and the cross-talk with other pronociceptive cytokines, peptides and modulators derived from immune cells, osteoclasts and tumor cells. These findings hold implications in the therapy of pain in disease states, such as cancer and rheumatoid arthritis.

Involvement of peripheral adenosine 5'-triphosphate and P2X purinoceptor in pain-related behavior produced by orthotopic melanoma inoculation in mice

Adenosine 5'-triphosphate (ATP) plays an important role in nociceptive processing. We used a mouse model of skin cancer pain to investigate the role of ATP in cancer pain. Orthotopic inoculation of B16-BL6 melanoma cells into the hind paw produced spontaneous licking of the tumor-bearing paw. Intraperitoneal injection of the P2 purinoceptor antagonist suramin suppressed spontaneous licking dose-dependently. Two P2X purinoceptor antagonists also suppressed spontaneous licking. An intraplantar injection of ATP, which did not induce licking in the healthy paw, increased licking of the tumor-bearing paw. Spontaneous firing of the tibial nerve was significantly increased in tumor-bearing mice and was inhibited by suramin. Extracellular concentration of ATP was significantly increased in the tumor-bearing paw than in the normal paw. ATP is concentrated in the culture medium of melanoma, lung cancer and breast cancer cells, but not fibroblasts. The P2X(3) receptor was expressed in about 40% of peripherin-positive small and medium-sized neurons in the dorsal root ganglia. P2X(3)-positive neurons were significantly increased in melanoma-bearing mice. These results suggest that ATP and P2X, especially P2X(3), receptors are involved in skin cancer pain, due to the increased release of ATP and increased expression of P2X(3) receptors in the sensory neurons.

Intraperitoneal injection of thalidomide attenuates bone cancer pain and decreases spinal tumor necrosis factor-α expression in a mouse model

Background

Tumor necrosis factor α (TNF-α) may have a pivotal role in the genesis of mechanical allodynia and thermal hyperalgesia during inflammatory and neuropathic pain. Thalidomide has been shown to selectively inhibit TNF-α production. Previous studies have suggested that thalidomide exerts anti-nociceptive effects in various pain models, but its effects on bone cancer pain have not previously been studied. Therefore, in the present study, we investigated the effect of thalidomide on bone cancer-induced hyperalgesia and up-regulated expression of spinal TNF-α in a mouse model.

Results

Osteosarcoma NCTC 2472 cells were implanted into the intramedullary space of the right femurs of C3H/HeJ mice to induce ongoing bone cancer related pain behaviors. At day 5, 7, 10 and 14 after operation, the expression of TNF-α in the spinal cord was higher in tumor-bearing mice compared to the sham mice. Intraperitoneal injection of thalidomide (50 mg/kg), started at day 1 after surgery and once daily thereafter until day 7, attenuated bone cancer-evoked mechanical allodynia and thermal hyperalgesia as well as the up-regulation of TNF-α in the spinal cord.

Conclusions

These results suggest that thalidomide can efficiently alleviate bone cancer pain and it may be a useful alternative or adjunct therapy for bone cancer pain. Our data also suggest a role of spinal TNF-α in the development of bone cancer pain.

A p38 mitogen-activated protein kinase-dependent mechanism of disinhibition in spinal synaptic transmission induced by tumor necrosis factor-alpha

Tumor necrosis factor-α (TNFα) is a proinflammatory cytokine that contributes to inflammatory and neuropathic pain. The mechanism by which TNFα modulates synaptic transmission in mouse substantia gelatinosa was studied using whole-cell patch clamp and immunohistochemistry. TNFα was confirmed to significantly increase the frequency of spontaneous EPSCs (sEPSCs) in spinal neurons and to also produce a robust decrease in the frequency of spontaneous IPSCs (sIPSCs). The enhancement of excitatory synaptic transmission by TNFα is in fact observed to be dependent on the suppression of sIPSCs, or disinhibition, in that blockade of inhibitory synaptic transmission prevents the effect of TNFα on sEPSCs but not vice versa. TNFα-induced inhibition of sIPSCs was blocked by neutralizing antibodies to TNF receptor 1 (TNFR1) but not to TNFR2 and was abolished by the p38 mitogen-activated protein kinase inhibitor SB202190 [4-(4-fluorophenyl)-2-(4-hydroxyphenyl)-5-(4-pyridyl)1H-imidazole]. TNFα rapidly inhibited spontaneous action potentials in GABAergic neurons identified in transgenic mice expressing enhanced green fluorescent protein controlled by the GAD67 promoter. This inhibitory effect was also blocked by intracellular delivery of SB202190 to the targeted cells. The inhibition of spontaneous activity in GABAergic neurons by TNFα is shown as mediated by a reduction in the hyperpolarization-activated cation current (Ih). These results suggest a novel TNFα-TNFR1-p38 pathway in spinal GABAergic neurons that may contribute to the development of neuropathic and inflammatory pain by TNFα.

Mechanism of cancer pain

Ongoing and breakthrough pain is a primary concern for the cancer patient. Although the etiology of cancer pain remains unclear, animal models of cancer pain have allowed investigators to unravel some of the cancer-induced neuropathologic processes that occur in the region of tumor growth and in the dorsal horn of the spinal cord. Within the cancer microenvironment, cancer and immune cells produce and secrete mediators that activate and sensitize primary afferent nociceptors. Pursuant to these peripheral changes, nociceptive secondary neurons in spinal cord exhibit increased spontaneous activity and enhanced responsiveness to three modes of noxious stimulation: heat, cold, and mechanical stimuli. As our understanding of the peripheral and central mechanisms that underlie cancer pain improves, targeted analgesics for the cancer patient will likely follow.

Evoked pain behavior and spinal glia activation is dependent on tumor necrosis factor receptor 1 and 2 in a mouse model of bone cancer pain

Bone-cancer-related pain is one of the most disabling factors in patients suffering from primary bone cancer or bone metastases. Recent studies point toward an important role of proinflammatory cytokines, example tumor necrosis factor-alpha (TNF), for tumor growth and bone-cancer-associated pain. Mechanisms by which TNF, through its receptor subtypes, TNF receptor 1 (TNFR1) and -2 (TNFR2), elicits altered sensation and pain behavior, are still incompletely understood. To look for a potential role of TNF in bone cancer pain, cancer-related pain was analyzed in fibrosarcoma-bearing C57Bl/6J wild type mice after systemic antagonism of TNF. To further clarify the role of TNF receptor (TNFR) in bone-cancer pain, naive and fibrosarcoma-bearing C57Bl/ 6J wild type and transgenic mice with a deficiency of TNFR1 (TNFR1ko), TNFR2 (TNFR2ko), and TNFR1+2 (TNFR1+2ko) were compared regarding cancer-related pain and hyperalgesia, tumor growth, osteoclast activation, and spinal astrogliosis. Systemic antagonism of TNF significantly alleviated tactile hypersensitivity and spontaneous bone-cancer-related pain behavior. Most interestingly, combined deletion of the TNFR1 and TNFR2, but not of either gene alone, almost completely inhibited the development of tactile hypersensitivity, whereas spontaneous pain behavior was transiently increased. Accordingly, spinal astrogliosis was markedly reduced, whereas tumor growth was significantly increased in TNFR1+2ko mice. In contrast, deletion of the TNFR1 or TNFR2 gene alone did not change tumor growth or spinal astrogliosis. Our findings suggest that the combined absence of TNFR1 and TNFR2 is necessary for the attenuation of cancer-related tactile hypersensitivity and concomitant spinal astrogliosis, whereas tumor growth seems to be inhibited by combined TNFR activation. These findings support the hypothesis of cytokine-dependent pain development in cancer pain. Differential targeting of TNFR activation could be an interesting strategy in bone-cancer-related pain conditions.

Nociceptive sensitization by complement C5a and C3a in mouse

Activation of the complement system by injury increases inflammation by producing complement fragments C5a and C3a which are able to recruit and activate immune cells. Complement activation may contribute to pain after inflammation and injury. In this study, we examined whether C5a and C3a elicit nociception when injected into mouse hind paws in vivo, and whether C5a and C3a activate and/or sensitize mechanosensitive nociceptors when applied on peripheral terminals in vitro. We also examined the dorsal root ganglia (DRG) for C5a receptor (C5aR) mRNA and effects of C5a and C3a on intracellular Ca(2+) concentration ([Ca(2+)](i)) using Ca(2+) imaging. Heat hyperalgesia was elicited by intraplantar injection of C5a, and mechanical hyperalgesia by C5a and C3a. After exposure to either C5a or C3a, C-nociceptors were sensitized to heat as evidenced by an increased proportion of heat responsive fibers, lowered response threshold to heat and increased action potentials during and after heat stimulation. A-nociceptors were activated by complement. However, no change was observed in mechanical responses of A- and C-nociceptors after C5a and C3a application. The presence of C5aR mRNA was detected in DRG. C5a and C3a application elevated [Ca(2+)](i) and facilitated capsaicin-induced [Ca(2+)](i) responses in DRG neurons. The results suggest a potential role for complement fragments C5a and C3a in nociception by activating and sensitizing cutaneous nociceptors.

A decrease in anandamide signaling contributes to the maintenance of cutaneous mechanical hyperalgesia in a model of bone cancer pain

Tumors in bone are associated with pain in humans. Data generated in a murine model of bone cancer pain suggest that a disturbance of local endocannabinoid signaling contributes to the pain. When tumors formed after injection of osteolytic fibrosarcoma cells into the calcaneus bone of mice, cutaneous mechanical hyperalgesia was associated with a decrease in the level of anandamide (AEA) in plantar paw skin ipsilateral to tumors. The decrease in AEA occurred in conjunction with increased degradation of AEA by fatty acid amide hydrolase (FAAH). Intraplantar injection of AEA reduced the hyperalgesia, and intraplantar injection of URB597, an inhibitor of FAAH, increased the local level of AEA and also reduced hyperalgesia. An increase in FAAH mRNA and enzyme activity in dorsal root ganglia (DRG) L3-L5 ipsilateral to the affected paw suggests DRG neurons contribute to the increased FAAH activity in skin in tumor-bearing mice. Importantly, the anti-hyperalgesic effects of AEA and URB597 were blocked by a CB1 receptor antagonist. Increased expression of CB1 receptors by DRG neurons ipsilateral to tumor-bearing limbs may contribute to the anti-hyperalgesic effect of elevated AEA levels. Furthermore, CB1 receptor protein-immunoreactivity as well as inhibitory effects of AEA and URB597 on the depolarization-evoked Ca(2+) transient were increased in small DRG neurons cocultured with fibrosarcoma cells indicating that fibrosarcoma cells are sufficient to evoke phenotypic changes in AEA signaling in DRG neurons. Together, the data provide evidence that manipulation of peripheral endocannabinoid signaling is a promising strategy for the management of bone cancer pain.

Contributions of central and peripheral TRPV1 receptors to mechanically evoked and spontaneous firing of spinal neurons in inflamed rats

TRPV1 receptors are activated and/or modulated by noxious heat, capsaicin, protons and other endogenous agents released following tissue injury. There is a growing appreciation that this molecular integrator may also have a role in mechanosensation. To further understand this role, we investigated the systemic and site-specific effects of a selective TRPV1 receptor antagonist, A-889425, on low-intensity mechanical stimulation in inflamed rats. Systemic administration of A-889425 (30 and 100 micromol/kg po) reduced mechanical allodynia in complete Freund's adjuvant (CFA)-inflamed rats. Systemic A-889425 (3 and 10 micromol/kg iv) also decreased the responses of spinal wide dynamic range (WDR) neurons to low-intensity mechanical stimulation in CFA-inflamed but not uninjured rats. This effect of A-889425 was likely mediated via multiple sites since local injection of A-889425 into the spinal cord (1-3 nmol), ipsilateral hindpaw (200 nmol), and cerebral ventricles (30-300 nmol) all attenuated WDR responses to low-intensity mechanical stimulation. In addition to an effect on mechanotransmission, systemic administration of A-889425 reduced the spontaneous firing of WDR neurons in inflamed but not uninjured rats. Spontaneous firing is elevated after injury and may reflect ongoing pain in the animal. Local injection experiments indicated that this effect of A-889425 on spontaneous firing was mainly mediated via TRPV1 receptors in the spinal cord. Thus the current data demonstrate that TRPV1 receptors have an enhanced role after an inflammatory injury, impacting both low-intensity mechanotransmission and possibly spontaneous pain. Furthermore this study delineates the differential contribution of central and peripheral TRPV1 receptors to affect spontaneous or mechanically evoked firing of WDR neurons.

Rapid improvement in verbal fluency and aphasia following perispinal etanercept in Alzheimer's disease

Background

Recent clinical studies point to rapid and sustained clinical, cognitive, and behavioral improvement in both Alzheimer's disease and primary progressive aphasia following weekly perispinal administration of etanercept, a TNF-alpha inhibitor that acts by blocking the binding of this cytokine to its receptors. This outcome is concordant with recent basic science studies suggesting that TNF-alpha functions in vivo as a gliotransmitter that regulates synaptic function in the brain. We hypothesized that perispinal etanercept had the potential to improve verbal function in Alzheimer's disease, so we included several standarized measures of verbal ability to evaluate language skills in a clinical trial of perispinal etanercept for Alzheimer's disease.

Methods

This was a prospective, single-center, open-label, pilot study, in which 12 patients with mild-to-severe Alzheimer's disease were administered etanercept, 25–50 mg, weekly by perispinal administration for six months. Two additional case studies are presented.

Results

Two-tailed, paired t-tests were conducted comparing baseline performance to 6-month performance on all neuropsychological measures. Test batteries included the California Verbal Learning Test-Second Edition, Adult Version; Logical Memory I and II(WMS-LM-II) from the Wechsler Memory Scale-Abbreviated; the Comprehensive Trail Making Test (TMT); Boston Naming Test; and letter(FAS) and category verbal fluency. All measures revealed a significant effect except for the Boston Naming Test and the TMT-4, with WMS-LM-II being marginally significant at p = .05. The FAS test for letter fluency was most highly significant with a p < 0.0007. In addition, rapid improvement in verbal fluency and aphasia in two patients with dementia, beginning minutes after perispinal etanercept administration, is documented.

Conclusion

In combination with the previously reported results of perispinal etanercept in Alzheimer's disease and primary progressive aphasia, these results further argue that larger scale studies of this therapeutic intervention, including Phase 3 trials, are warranted in dementias. In addition, these results may provide insight into the basic pathophysiologic mechanisms underlying Alzheimer's disease and related forms of dementia, and suggest the existence of novel, rapidly reversible, TNF-mediated pathophysiologic mechanisms in Alzheimer's disease which are worthy of further investigation.

Endogenous tumor necrosis factor alpha (TNFalpha) requires TNF receptor type 2 to generate heat hyperalgesia in a mouse cancer model

To provide a tool to investigate the mechanisms inducing and maintaining cancer-related pain and hyperalgesia, a soft tissue tumor/metastasis model was developed that is applicable in C57BL/6J wild-type and transgenic mice. We show that the experimental tumor-induced heat hyperalgesia and nociceptor sensitization were prevented by systemic treatment with the tumor necrosis factor alpha (TNFalpha) antagonist etanercept. In naive mice, exogenous TNFalpha evoked heat hyperalgesia in vivo and sensitized nociceptive nerve fibers to heat in vitro. TNFalpha enhanced the expression of the nociceptor-specific heat transducer ion channel transient receptor potential vanilloid 1 (TRPV1) and increased the amplitudes of capsaicin and heat-activated ionic currents via p38/MAP (mitogen-activated protein) kinase and PKC (protein kinase C). Deletion of the tumor necrosis factor receptor type 2 (TNFR2) gene attenuated heat hyperalgesia and prevented TRPV1 upregulation in tumor-bearing mice, whereas TNFR1 gene deletion played a minor role. We propose endogenous TNFalpha as a key player in cancer-related heat hyperalgesia and nociceptor sensitization that generates TRPV1 upregulation and sensitization via TNFR2.

Peripheral inflammation increases Scya2 expression in sensory ganglia and cytokine and endothelial related gene expression in inflamed tissue

The sensation of pain (nociception) is a critical factor in host defense during tissue injury and inflammation and is initiated at the site of injury by activation of primary afferent C-fiber and A- partial differential nerve endings. Inflammation induces tissue alterations that sensitize these nociceptive nerve terminals, contributing to persistent pain. To understand this 'algesic tissue environment' and peripheral nervous signaling to the CNS and immune system, we examined cytokine and endothelial-related gene expression profiles in inflamed rat tissues and corresponding dorsal root ganglia (DRG) by microarray and RT-PCR following hind paw injection of carrageenan. In inflamed tissue, forty-two cytokine and endothelial-related genes exhibited elevated expression. In contrast, in DRG, only Scya2 (chemokine C-C motif ligand 2) mRNA was up-regulated, leading to an increase in its gene product monocyte chemoattractant protein-1. Scya2 mRNA was localized by in situ hybridization-immunocytochemical double-labeling to a subpopulation of vanilloid receptor-1 (transient receptor potential vanilloid subtype 1) containing neurons, and its expression was increased by direct transient receptor potential vanilloid subtype 1 stimulation with the vanilloid agonist resiniferatoxin, indicating sensitivity to nociceptive afferent activity. Our results are consistent with the idea that monocyte chemoattractant protein-1 at the site of peripheral injury and/or in DRG is involved in inflammatory hyperalgesia.

Antihyperalgesic effects induced by the IL-1 receptor antagonist anakinra and increased IL-1β levels in inflamed and osteosarcoma-bearing mice

Based on the well established involvement of IL-1beta in inflammatory hyperalgesia, we have assessed the possible role played by IL-1beta in a murine model of bone cancer-induced pain. With this aim, we measured IL-1beta levels at the region of the tibia and the spinal cord in mice bearing a tibial osteosarcoma induced by the inoculation of NCTC 2472 cells, and we tested whether the IL-1 receptor antagonist, anakinra, inhibits some hypernociceptive reactions evoked by the neoplastic injury. Parallel experiments were performed in mice with a chronic inflammatory process (intraplantar injection of complete Freund's adjuvant, CFA). IL-1beta levels were increased in the tibial region of osteosarcoma-bearing mice and in the paws of inflamed mice. To a lesser extent, the content of IL-1beta in the spinal cord was also augmented in both situations. Osteosarcoma-induced thermal hyperalgesia was inhibited by 30 and 100 mg/kg of systemic anakinra, but only 300 mg/kg prevented inflammatory thermal hyperalgesia. Mechanical hyperalgesia induced by the osteosarcoma was blocked by 100 and 300 mg/kg of anakinra, whereas a partial reversion of inflammatory mechanical hyperalgesia was induced by 300 mg/kg. Anakinra, intrathecally administered (1 and 10 microg) did not modify hyperalgesia of either origin. Besides, both tumoral and inflammatory mechanical allodynia remained unaltered after the administration of anakinra. In conclusion, some hyperalgesic symptoms observed in this model of bone cancer are mediated by the peripheral release of IL-1beta and may be inhibited by antagonists of type I IL-1 receptors with a similar or greater potency than symptoms produced by inflammation.

Acute p38-mediated modulation of tetrodotoxin-resistant sodium channels in mouse sensory neurons by tumor necrosis factor-alpha

Tumor necrosis factor-alpha (TNFalpha) is a proinflammatory cytokine involved in the development and maintenance of inflammatory and neuropathic pain conditions. TNFalpha can have long-lasting effects by regulating the expression of a variety of inflammatory mediators, including other cytokines and TNFalpha itself. However, the speed with which TNFalpha induces tactile and thermal hypersensitivity suggests that transcriptional regulation cannot fully account for its sensitizing effects, and some recent findings suggest that TNFalpha may act directly on primary afferent neurons to induce pain hypersensitivity. In the present study, we show that peripheral administration of TNFalpha induces thermal hypersensitivity in wild-type mice but not in transient receptor potential vanilloid receptor TRPV1(-/-) mice. In contrast, TNFalpha produced equivalent mechanical hypersensitivity in TRPV1(-/-) mice and wild-type littermates, suggesting a role for TRPV1 in TNFalpha-induced thermal, but not mechanical, hypersensitivity. Because tetrodotoxin (TTX)-resistant Na+ channels are a critical site of modulation underlying mechanical hypersensitivity in inflammatory and neuropathic pain conditions, we tested the effects of TNFalpha on these channels in isolated mouse dorsal root ganglion (DRG) neurons. We report that acute application of TNFalpha rapidly enhances TTX-resistant Na+ currents in isolated DRG neurons. This potentiation of TTX-resistant currents by TNFalpha is dramatically reduced in DRG neurons from TNF receptor 1 (TNFR1) knock-out mice and is blocked by the p38 mitogen-activated protein kinase inhibitor SB202190 [4-(4-fluorophenyl)-2-(4-hydroxyphenyl)-5-(4-pyridyl)1H-imidazole]. Mechanical hypersensitivity induced by peripherally applied TNFalpha is also significantly reduced by SB202190. These results suggest that TNFalpha may induce acute peripheral mechanical sensitization by acting directly on TNFR1 in primary afferent neurons, resulting in p38-dependent modulation of TTX-resistant Na+ channels.