TNF-alpha and neuropathic pain--a review

Transcriptional mechanisms underlying sensitization of peripheral sensory neurons by granulocyte-/granulocyte-macrophage colony stimulating factors

BACKGROUND

Cancer-associated pain is a major cause of poor quality of life in cancer patients and is frequently resistant to conventional therapy. Recent studies indicate that some hematopoietic growth factors, namely granulocyte macrophage colony stimulating factor (GMCSF) and granulocyte colony stimulating factor (GCSF), are abundantly released in the tumor microenvironment and play a key role in regulating tumor-nerve interactions and tumor-associated pain by activating receptors on dorsal root ganglion (DRG) neurons. Moreover, these hematopoietic factors have been highly implicated in postsurgical pain, inflammatory pain and osteoarthritic pain. However, the molecular mechanisms via which G-/GMCSF bring about nociceptive sensitization and elicit pain are not known.

RESULTS

In order to elucidate G-/GMCSF mediated transcriptional changes in the sensory neurons, we performed a comprehensive, genome-wide analysis of changes in the transcriptome of DRG neurons brought about by exposure to GMCSF or GCSF. We present complete information on regulated genes and validated profiling analyses and report novel regulatory networks and interaction maps revealed by detailed bioinformatics analyses. Amongst these, we validate calpain 2, matrix metalloproteinase 9 (MMP9) and a RhoGTPase Rac1 as well as Tumor necrosis factor alpha (TNFα) as transcriptional targets of G-/GMCSF and demonstrate the importance of MMP9 and Rac1 in GMCSF-induced nociceptor sensitization.

CONCLUSION

With integrative approach of bioinformatics, in vivo pharmacology and behavioral analyses, our results not only indicate that transcriptional control by G-/GMCSF signaling regulates a variety of established pain modulators, but also uncover a large number of novel targets, paving the way for translational analyses in the context of pain disorders.

Inflammation: therapeutic targets for diabetic neuropathy

There are still no approved treatments for the prevention or of cure of diabetic neuropathy, and only symptomatic pain therapies of variable efficacy are available. Inflammation is a cardinal pathogenic mechanism of diabetic neuropathy. The relationships between inflammation and the development of diabetic neuropathy involve complex molecular networks and processes. Herein, we review the key inflammatory molecules (inflammatory cytokines, adhesion molecules, chemokines) and pathways (nuclear factor kappa B, JUN N-terminal kinase) implicated in the development and progression of diabetic neuropathy. Advances in the understanding of the roles of these key inflammatory molecules and pathways in diabetic neuropathy will facilitate the discovery of the potential of anti-inflammatory approaches for the inhibition of the development of neuropathy. Specifically, many anti-inflammatory drugs significantly inhibit the development of different aspects of diabetic neuropathy in animal models and clinical trials.

Neuroinflammation and the generation of neuropathic pain

Inflammation is the process by which an organism responds to tissue injury involving both immune cell recruitment and mediator release. Diverse causes of neuropathic pain are associated with excessive inflammation in both the peripheral and central nervous system which may contribute to the initiation and maintenance of persistent pain. Chemical mediators, such as cytokines, chemokines, and lipid mediators, released during an inflammatory response have the undesired effect of sensitizing and stimulating nociceptors, their central synaptic targets or both. These changes can promote long-term maladaptive plasticity resulting in persistent neuropathic pain. This review aims to provide an overview of inflammatory mechanisms at differing levels of the sensory neuroaxis with a focus on neuropathic pain. We will compare and contrast neuropathic pain states such as traumatic nerve injury which is associated with a vigorous inflammatory response and chemotherapy induced pain in which the inflammatory response is much more modest. Targeting excessive inflammation in neuropathic pain provides potential therapeutic opportunities and we will discuss some of the opportunities but also the clinical challenges in such an approach.

Curcumin: a novel therapeutic for burn pain and wound healing

INTRODUCTION

Managing burn injury-associated pain and wounds is a major unresolved clinical problem. Opioids, nonsteroidal antiinflammatory drugs (NSAIDs), antidepressants and anticonvulsants remain the most common forms of analgesic therapy to treat burn patients. However, prolonged treatment with these drugs leads to dose escalation and serious side effects. Additionally, severe burn wounds cause scarring and are susceptible to infection. Recent encouraging findings demonstrate that curcumin, a major bioactive component found in turmeric, is a natural pharmacotherapeutic for controlling both severe burn pain and for improved wound healing.

AREAS COVERED

This article covers current pr-clinical and clinical studies on the analgesic and wound healing effects. Particular emphasis has been placed on studies aimed at developing improved curcumin delivery vehicles that increase its bioavailability. Based on the available evidence, a hypothesis is proposed that the dual beneficial effects of curcumin, analgesia and enhanced wound healing are mediated through common anti-inflammatory mechanisms.

EXPERT OPINION

Emerging studies have demonstrated that curcumin is a promising investigational drug to treat both pain and wounds. The adequate control of severe burn pain, particularly over the long courses required for healing, as well improvements in burn wound healing are unmet clinical needs.

TNFα secretion of monocytes exposed to pulsed radiofrequency treatment: a possible working mechanism of PRF chronic pain management

Pulsed radiofrequency treatment (PRF) is a promising new technique increasingly used in treatment of chronic pain. The molecular working mechanism of PRF is not exactly known and is currently being investigated. This study investigates a possible role of PRF-induced modulation of TNFα secretion by differentiated monocytes in chronic pain management. The results show no significant PRF-induced change in TNFα secretion of lipopolysaccharides (LPS)-stimulated monocytes. However, PRF does significantly increase TNFα secretion of differentiated monocytes that have not been stimulated with LPS. This may indicate a possible role of PRF treatment in increasing TNFα production of nonstimulated monocytes. More research is needed to determine whether this is truly a part of the working mechanism of PRF in chronic pain management and which other factors are involved.

Mechanisms of intervertebral disk degeneration/injury and pain: a review

Degeneration of the intervertebral disk and its treatments are currently intensely investigated topics. Back pain is a condition whose chronic and debilitating nature combined with its prevalence make it a major health issue of substantial socioeconomic importance. Although researchers, and even sometimes clinicians, focus on the degenerated disk as the problem, to most patients, pain is the factor that limits their function and impacts their well-being. The purpose of this review is to delineate the changes associated with disk degeneration and to outline mechanisms by which they could be the source of back pain. Although the healthy disk is only innervated in the external layer of its annulus fibrosus, adjacent structures are plentiful with nociceptive receptors. Stimulation of such structures as a consequence of processes initiated by disk degeneration is explored. The concept of discogenic pain and possible mechanisms such as neoinnervation and solute transport are discussed. Finally, how such pain mechanisms may relate to current and proposed treatment strategies is discussed.

Src/p38 MAPK pathway in spinal microglia is involved in mechanical allodynia induced by peri-sciatic administration of recombinant rat TNF-α

Our previous work has shown that peri-sciatic administration of recombinant rat TNF-α (rrTNF) induces mechanical allodynia and up-regulation of TNF-α in the spinal dorsal horn of rats; however, the underlying mechanisms remain unknown. In the current study, we found that the levels of phosphorylated Src-family kinases (p-SFKs) and phosphorylated p38 mitogen-activated protein kinase (p-p38 MAPK) were significantly increased in bilateral lumbar spinal dorsal horn on day 3 after rrTNF administration. Double immunofluorescence staining revealed that p-SFKs and p-p38 MAPK were nearly restricted to the microglia. Intrathecal delivery of SFKs inhibitor PP2 or p38 MAPK inhibitor SB203580, started 30 min before rrTNF administration and given once daily thereafter for 7 days, blocked mechanical allodynia in bilateral hind paws and increase of TNF-α expression in the spinal dorsal horn. Moreover, PP2 inhibited the up-regulation of p-p38 MAPK induced by rrTNF. We also found that intrathecal injection of TNF-α neutralization antibody alleviated mechanical allodynia in bilateral hind paws and suppressed up-regulation of p-SFKs and p-p38 MAPK. These results suggest that activation of the SFKs/p38 MAPK pathway in microglia and subsequent TNF-α expression in the spinal dorsal horn may contribute to the mechanical hyperalgesic state induced by peri-sciatic administered rrTNF.

The mechanism of functional up-regulation of P2X3 receptors of trigeminal sensory neurons in a genetic mouse model of familial hemiplegic migraine type 1 (FHM-1)

A knock-in (KI) mouse model of FHM-1 expressing the R192Q missense mutation of the Cacna1a gene coding for the α1 subunit of Ca_V2.1 channels shows, at the level of the trigeminal ganglion, selective functional up-regulation of ATP -gated P2X3 receptors of sensory neurons that convey nociceptive signals to the brainstem. Why P2X3 receptors are constitutively more responsive, however, remains unclear as their membrane expression and TRPV1 nociceptor activity are the same as in wildtype (WT) neurons. Using primary cultures of WT or KI trigeminal ganglia, we investigated whether soluble compounds that may contribute to initiating (or maintaining) migraine attacks, such as TNFα, CGRP, and BDNF, might be responsible for increasing P2X3 receptor responses. Exogenous application of TNFα potentiated P2X3 receptor-mediated currents of WT but not of KI neurons, most of which expressed both the P2X3 receptor and the TNFα receptor TNFR2. However, sustained TNFα neutralization failed to change WT or KI P2X3 receptor currents. This suggests that endogenous TNFα does not regulate P2X3 receptor responses. Nonetheless, on cultures made from both genotypes, exogenous TNFα enhanced TRPV1 receptor-mediated currents expressed by a few neurons, suggesting transient amplification of TRPV1 nociceptor responses. CGRP increased P2X3 receptor currents only in WT cultures, although prolonged CGRP receptor antagonism or BDNF neutralization reduced KI currents to WT levels. Our data suggest that, in KI trigeminal ganglion cultures, constitutive up-regulation of P2X3 receptors probably is already maximal and is apparently contributed by basal CGRP and BDNF levels, thereby rendering these neurons more responsive to extracellular ATP.

Targeting anti-inflammatory treatment can ameliorate injury-induced neuropathic pain

Tumor necrosis factor-α plays important roles in immune system development, immune response regulation, and T-cell-mediated tissue injury. The present study assessed the net value of anti-tumor necrosis factor-α treatment in terms of functional recovery and inhibition of hypersensitivity after peripheral nerve crush injury. We created a right sciatic nerve crush injury model using a Sugita aneurysm clip. Animals were separated into 3 groups: the first group received only a skin incision; the second group received nerve crush injury and intraperitoneal vehicle injection; and the third group received nerve crush injury and intraperitoneal etanercept (6 mg/kg). Etanercept treatment improved recovery of motor nerve conduction velocity, muscle weight loss, and sciatic functional index. Plantar thermal and von Frey mechanical withdrawal thresholds recovered faster in the etanercept group than in the control group. On day 7 after crush injury, the numbers of ED-1-positive cells in crushed nerves of the control and etanercept groups were increased compared to that in the sham-treated group. After 21 days, ED-1-positive cells had nearly disappeared from the etanercept group. Etanercept reduced expression of interleukin-6 and monocyte chemotactic and activating factor-1 at the crushed sciatic nerve. These findings demonstrate the utility of etanercept, in terms of both enhancing functional recovery and suppressing hypersensitivity after nerve crush. Etanercept does not impede the onset or progression of Wallerian degeneration, but optimizes the involvement of macrophages and the secretion of inflammatory mediators.

TNFα levels and macrophages expression reflect an inflammatory potential of trigeminal ganglia in a mouse model of familial hemiplegic migraine

Latent changes in trigeminal ganglion structure and function resembling inflammatory conditions may predispose to acute attacks of migraine pain. Here, we investigated whether, in trigeminal sensory ganglia, cytokines such as TNFα might contribute to a local inflammatory phenotype of a transgenic knock-in (KI) mouse model of familial hemiplegic migraine type-1 (FHM-1). To this end, macrophage occurrence and cytokine expression in trigeminal ganglia were compared between wild type (WT) and R192Q mutant Ca_V2.1 Ca²⁺ channel (R192Q KI) mice, a genetic model of FHM-1. Cellular and molecular characterization was performed using a combination of confocal immunohistochemistry and cytokine assays. With respect to WT, R192Q KI trigeminal ganglia were enriched in activated macrophages as suggested by their morphology and immunoreactivity to the markers Iba1, CD11b, and ED1. R192Q KI trigeminal ganglia constitutively expressed higher mRNA levels of IL1β, IL6, IL10 and TNFα cytokines and the MCP-1 chemokine. Consistent with the report that TNFα is a major factor to sensitize trigeminal ganglia, we observed that, following an inflammatory reaction evoked by LPS injection, TNFα expression and macrophage occurrence were significantly higher in R192Q KI ganglia with respect to WT ganglia. Our data suggest that, in KI trigeminal ganglia, the complex cellular and molecular environment could support a new tissue phenotype compatible with a neuroinflammatory profile. We propose that, in FHM patients, this condition might contribute to trigeminal pain pathophysiology through release of soluble mediators, including TNFα, that may modulate the crosstalk between sensory neurons and resident glia, underlying the process of neuronal sensitisation.

Curcumin attenuates diabetic neuropathic pain by downregulating TNF-α in a rat model

The mechanisms involved in diabetic neuropathic pain are complex and involve peripheral and central pathophysiological phenomena. Proinflammatory tumour necrosis factor α (TNF-α) and TNF-α receptor 1, which are markers of inflammation, contribute to neuropathic pain. The purpose of this experimental study was to evaluate the effect of curcumin on diabetic pain in rats. We tested 24 rats with diabetes induced by a single intraperitoneal injection of streptozotocin and 24 healthy control rats. Twelve rats in each group received 60 mg/kg oral curcumin daily for 28 days, and the other 12 received vehicle. On days 7, 14, 21, and 28, we tested mechanical allodynia with von Frey hairs and thermal hyperalgesia with radiant heat. Markers of inflammation in the spinal cord dorsal horn on day 28 were estimated with a commercial assay and Western blot analysis. Compared to control rats, diabetic rats exhibited increased mean plasma glucose concentration, decreased mean body weight, and significant pain hypersensitivity, as evidenced by decreased paw withdrawal threshold to von Frey hairs and decreased paw withdrawal latency to heat. Curcumin significantly attenuated the diabetes-induced allodynia and hyperalgesia and reduced the expression of both TNF-α and TNF-α receptor 1. Curcumin seems to relieve diabetic hyperalgesia, possibly through an inhibitory action on TNF-α and TNF-α receptor 1.

Neuropathic pain-attenuating potential of aliskiren in chronic constriction injury model in rats

The present study was designed to investigate the potential of aliskiren, a direct renin inhibitor, in chronic constriction injury (CCI)-induced neuropathic pain in rats. Neuropathic pain was induced by placing four loose ligatures around the sciatic nerve. Acetone drop, von Frey hair, pin-prick and hot plate tests were performed to assess cold allodynia, mechanical allodynia, mechanical and heat hyperalgesia, respectively. The levels of Tumor necrosis factor-alpha (TNF-α) were measured in the sciatic nerve as an inflammatory marker. CCI was associated with the development of cold allodynia, mechanical allodynia, mechanical and heat hyperalgesia along with a rise in the levels of Tumor necrosis factor-alpha (TNF-α). Administration of aliskiren (25 or 50 mg/kg intraperitoneal (i.p.)) for 14 days in CCI-subjected rats significantly attenuated CCI-induced pain-related behavior and rise in TNF-α level. It may be concluded that aliskiren-mediated anti-inflammatory actions may be responsible for its beneficial effects in neuropathic pain in rats.

Sensory neuropathy in paraneoplastic leucocytosis

Granulocyte colony-stimulating factor (G-CSF) and granulocyte-macrophage colony-stimulating factor (GM-CSF) sensitise nerves to mechanical stimuli experimentally and may induce bone and muscle pain when used as supportive drugs. G-CSF and GM-CSF produced endogenously by tumour cells can cause paraneoplastic leucocytosis. Whether paraneoplastic leucocytosis is associated with changes in pain sensitivity is not yet clear. We report on a patient with advanced-stage thyroid cancer who developed extreme leucocytosis within a period of 4 weeks (103 000 white blood cells/mm(3)), composed mostly of neutrophils and eosinophils. Parallel to this leukemoid reaction, allodynia and hyperalgesia developed in the absence of tissue inflammation. The course of disease of an elderly male with advanced stage metastatic thyroid cancer with new onset neuropathic pain followed by the development of extreme leucocytosis in a leukemoid reaction suggests paraneoplastic release of myeloid CSFs. The coincidence of pain sensitisation and extreme leucocytosis suggests a causal contribution of G-CSF and GM-CSF.

Alteration of neuropathic and visceral pain in female C57BL/6J mice lacking the PPAR-α gene

RATIONALE

Peroxisome proliferator-activated receptors (PPARs) participate in the control of chronic neuropathic and inflammatory pain, and these receptors could play a role on acute pain.

OBJECTIVES

We used null (PPAR-α -/-) and wild-type female mice and the PPAR-α blocker GW6471 to evaluate (1) the role of PPAR-α on neuropathic pain, (2) the involvement of PPAR-α on visceral and acute thermal nociception, and (3) tissue levels of pro-inflammatory factors.

METHODS

Neuropathic pain was induced by sciatic nerve ligature. Acute thermal nociception was evaluated through hot-plate, tail-immersion, and writhing tests. The pro-inflammatory factors nitric oxide, TNF-α, and interleukins-1β and -3 were measured.

RESULTS

Regarding neuropathic pain, higher sensitivity to thermal and mechanical non-noxious and noxious stimuli was observed in mice lacking PPAR-α. Cold and mechanical allodynia and heat hyperalgesia were augmented in null mice. With respect to visceral nociception, writhes after acetic acid were enhanced in mutant mice. Although basal thermal sensitivity was enhanced in PPAR-α -/- mice, cutaneous thermal nociception did not differ between genotypes. Blockade of PPAR-α was devoid of effects on acute thermal and writhing tests. Finally, nerve ligature enhanced pro-inflammatory factors in plantar tissue, levels being higher in null mice. No changes in pro-inflammatory factors were observed in the hot-plate test.

CONCLUSIONS

Genetic ablation of PPAR-α is involved in neuropathic and visceral nociception. Lack of PPAR-α is not involved in acute thermal pain, but it is involved in basal thermal reaction. Changes are biological adaptations to receptor deletion because blockade of PPAR-α does not affect inflammatory pain or thermal reactions.

Effects of LPS on P2X3 receptors of trigeminal sensory neurons and macrophages from mice expressing the R192Q Cacna1a gene mutation of familial hemiplegic migraine-1

A knockin (KI) mouse model with the R192Q missense mutation in the Cacna1a gene commonly detected in familial hemiplegic migraine was used to study whether trigeminal ganglia showed a basal inflammatory profile that could be further enhanced by the lipopolysaccharide (LPS) toxin. Adenosine-5'-triphosphate (ATP)-gated purinergic ionotropic receptor 3 (P2X3) currents expressed by the large majority of trigeminal sensory neurons were taken as functional readout. Cultured R192Q KI trigeminal ganglia showed higher number of active macrophages, basal release of tumor necrosis factor alpha (TNFα), and larger P2X3 receptor currents with respect to wild type (WT) cells. After 5 h application of LPS in vitro, both WT and R192Q KI cultures demonstrated significant increase in macrophage activation, very large rise in TNFα mRNA content, and ambient protein levels together with fall in TNFα precursor, suggesting potent release of this inflammatory mediator. Notwithstanding the unchanged expression of P2X3 receptor protein in WT or R192Q KI cultures, LPS evoked a large rise in WT neuronal currents that recovered faster from desensitization. Basal R192Q KI currents were larger than WT ones and could not be further augmented by LPS. These data suggest that KI cultures had a basal neuroinflammatory profile that might facilitate the release of endogenous mediators (including ATP) to activate constitutively hyperfunctional P2X3 receptors and amplify nociceptive signaling by trigeminal sensory neurons.

The Relationship Between Lifestyle, Metaflammation, and Chronic Pain

Chronic pain has a significant economic and social impact on the community. The most common medical treatments for it include paracetamol, anti-inflammatory agents, and opioid analgesics. However, many of these medications cause side effects, and their long-term effectiveness is questionable. The traditional alternative to the biomedical approach is cognitive behavioral therapy. However, this has also been shown in recent studies to have only modest benefit. It is becoming clear that the effective management of chronic pain requires a more holistic, systems-based approach, hence the emerging interest in the relationship between pain and lifestyle. The authors aim to review the literature regarding the relationship between comprehensive lifestyle changes, markers of systemic inflammation, and the perception of chronic pain. An extensive search of bibliographic databases, including MEDLINE, PubMed, Web of Science, and Cochrane Library databases was made. A total of 2197 articles were identified using the search strategy. Only 44 articles were retrieved for critical appraisal, of which only 2 studies met the prespecified primary inclusion criteria and were included in the final review. These data provided some evidence that a single lifestyle factor (sleep restriction or disturbance) can produce elevated levels of interleukin-6, which is associated with higher pain intensity ratings. However, this review has highlighted a paucity of research based around the relationship between lifestyle, metaflammation, and chronic pain. There is a clear need for well-designed trials examining comprehensive lifestyle interventions and their effect on both pain intensity and markers of metaflammation.

SNPs in PTGS2 and LTA predict pain and quality of life in long term lung cancer survivors

PURPOSE

Lung cancer survivors report the lowest quality of life relative to other cancer survivors. Pain is one of the most devastating, persistent, and incapacitating symptoms for lung cancer survivors. Prevalence rates vary with 80-100% of survivors experiencing cancer pain and healthcare costs are five times higher in cancer survivors with uncontrolled pain. Cancer pain often has a considerable impact on quality of life among cancer patients and cancer survivors. Therefore, early identification, and treatment is important. Although recent studies have suggested a relationship between single nucleotide polymorphisms (SNPs) in several cytokine and inflammation genes with cancer prognosis, associations with cancer pain are not clear. Therefore, the primary aim of this study was to identify SNPs related to pain in lung cancer survivors.

PATIENTS AND METHODS

Participants were enrolled in the Mayo Clinic Lung Cancer Cohort upon diagnosis of their lung cancer. 1149 Caucasian lung cancer survivors (440 surviving <3 years; 354 surviving 3-5 years; and 355 surviving >5 years) completed study questionnaires and had blood DNA samples available. Ten SNPS from PTGS2 and LTA genes were selected based on the serum-based studies in the literature. Outcomes included pain, and quality of life as measured by the SF-8.

RESULTS

Of the 10 SNPs evaluated in LTA and PTGS2 genes, 3 were associated with pain severity (rs5277; rs1799964), social function (rs5277) and mental health (rs5275). These results suggested both specificity and consistency of these inflammatory gene SNPs in predicting pain severity in lung cancer survivors.

CONCLUSION

These results provide support for genetic predisposition to pain severity and may aid in identification of lung cancer survivors at high risk for morbidity and poor QOL.

Pain mediators and wound healing--establishing the connection

Pain accompanies every disruption of the skin surface in a normal sensate individual. The intensity and duration of the pain varies depending on the nature of trauma, the healing trajectory and various host factors. Pain mediator release is the mechanism for pain perception following peripheral stimulus and central interpretation. The various mediators may have promoting effects on wound healing in the short term, but it appears that protracted release of these mediators may well have detrimental effects on wound healing. The exaggerated release of pain mediators may result in nociceptor hypersensitization, hyperinflammatory cellular and extracellular matrix (ECM) changes, and in some cases, the potential for a fibrotic healing pattern. This relates to an imbalance between mediators with differing healing characteristics arising in certain pathological conditions. In this respect, it may be worth examining pain mediator agonists or antagonists, not only on compassionate grounds of pain control, but relating to the potential effects on overall wound healing.

LipoxinA(4) induced antinociception and decreased expression of NF-κB and pro-inflammatory cytokines after chronic dorsal root ganglia compression in rats

Inflammatory and immune responses following nerve injury have been shown to play an important role in neuropathic pain. Lipoxins are endogenous lipoxygenase-derived eicosanoids performing protective roles in a range of pathophysiologic processes. Here, we examined the effects of intrathecal lipoxinA4 (LXA4) on NF-κB activation and pro-inflammatory cytokine (TNF-α, IL-1β and IL-6) expression in dorsal root ganglia (DRG) following chronic compression of DRG (CCD), a model of neuropathic pain. Daily intrathecal injection of vehicle or LXA4 (10 ng or 100 ng) was performed for three successive days post-CCD. CCD induced both mechanical allodynia and thermal hyperalgesia, and increased the expression of TNF-α, IL-1β, IL-6 and NF-κB. Intrathecal injection of LXA4 prevented the development of neuropathic pain and inhibited NF-κB activation and pro-inflammatory cytokine upregulation in a dose-dependent manner. In this study, we have shown the strong protective effect of intrathecal LXA4 on the development of nociceptive behaviors induced by CCD and that these effects might be associated with its anti-inflammatory and pro-resolution properties.

Adalimumab in acute sciatica reduces the long-term need for surgery: a 3-year follow-up of a randomised double-blind placebo-controlled trial

INTRODUCTION

Two subcutaneous injections of adalimumab in severe acute sciatica significantly reduced the number of back operations in a short-term randomised controlled clinical trial.

OBJECTIVE

To determine in a 3-year follow-up study whether the short-term benefit of adalimumab in sciatica is sustained over a longer period of time.

METHODS

The primary outcome of this analysis was incident discectomy. Three years after randomisation, information on surgery could be retrieved in 56/61 patients (92%).A multivariate Cox proportional hazard models, adjusted for potential confounders, was used to determine factors predisposing to surgery.

RESULTS

Twenty-three (41%) patients had back surgery within 3 years, 8/29 (28%) in the adalimumab group and 15/27 (56%) in the placebo group, p=0.04. Adalimumab injections reduced the need for back surgery by 61% (HR)=0.39 (95% CI 0.17 to 0.92). In a multivariate model, treatment with a tumour necrosis factor-α antagonist remained the strongest protective factor (HR=0.17, p=0.002). Other significant predictors of surgery were a good correlation between symptoms and MRI findings (HR=11.6, p=0.04), baseline intensity of leg pain (HR=1.3, p=0.06), intensity of back pain (HR=1.4, p=0.03) and duration of sickness leave (HR=1.01 per day, p=0.03).

CONCLUSION

A short course of adalimumab in patients with severe acute sciatica significantly reduces the need for back surgery.

COMP-angiopoietin-1 recovers molecular biomarkers of neuropathy and improves vascularisation in sciatic nerve of ob/ob mice

BACKGROUND

Leptin-deficient ob/ob mice are a model of type 2 diabetes induced peripheral neuropathy. Ob/ob mice exhibit obesity, insulin resistance, hyperglycaemia, and alterations of peripheral nerve fibres and endoneural microvessels. Here we test the hypothesis that cartilage oligomeric matrix protein (COMP)-Ang-1, a soluble and stabile form of Ang-1 which promotes angiogenesis and nerve growth, improves regeneration of nerve fibres and endoneural microvessels in ob/ob mice.

METHODS AND FINDINGS

COMP-Ang-1 (100 ng/ml) or NaCl were intraperitoneally (i.p.) injected into male (N = 184), 3-month old, ob/ob or ob/+ mice for 7 and 21 days. We measured expression of Nf68, GAP43, Cx32, Cx26, Cx43, and TNFα in sciatic nerves using Western blot analysis. To investigate the inflammation in sciatic nerves, numbers of macrophages and T-cells were counted after immunofluorescence staining. In ultrathin section, number of myelinated/non-mylinated nerve fibers, g-ratio, the thickness of Schwann cell basal lamina and microvessel endothelium were investigated. Endoneural microvessels were reconstructed with intracardial FITC injection. Treatment with COMP-Ang-1 over 21 days significantly reduced fasting blood glucose and plasma cholesterol concentrations compared to saline treated ob/ob mice. In addition, COMP-Ang-1 treatment: 1) up-regulated expression of Nf68 and GAP43; 2) improved expression of gap junction proteins including connexin 32 and 26; 3) suppressed the expression of TNFα and Cx43 and 4) led to decreased macrophage and T-cell infiltration in sciatic nerve of ob/ob mice. The significant changes of sciatic nerve ultrastructure were not observed after 21-day long COMP-Ang-1 treatment. COMP-Ang-1 treated ob/ob mice displayed regeneration of small-diameter endoneural microvessels. Effects of COMP-Ang-1 corresponded to increased phosphorylation of Akt and p38 MAPK upon Tie-2 receptor.

CONCLUSIONS

COMP-Ang-1 recovers molecular biomarkers of neuropathy, promotes angiogenesis and suppresses inflammation in sciatic nerves of ob/ob mice suggesting COMP-Ang-1 as novel treatment option to improve morphologic and protein expression changes associated with diabetic neuropathy.

Drosophila nociceptors mediate larval aversion to dry surface environments utilizing both the painless TRP channel and the DEG/ENaC subunit, PPK1

A subset of sensory neurons embedded within the Drosophila larval body wall have been characterized as high-threshold polymodal nociceptors capable of responding to noxious heat and noxious mechanical stimulation. They are also sensitized by UV-induced tissue damage leading to both thermal hyperalgesia and allodynia very similar to that observed in vertebrate nociceptors. We show that the class IV multiple-dendritic(mdIV) nociceptors are also required for a normal larval aversion to locomotion on to a dry surface environment. Drosophila melanogaster larvae are acutely susceptible to desiccation displaying a strong aversion to locomotion on dry surfaces severely limiting the distance of movement away from a moist food source. Transgenic inactivation of mdIV nociceptor neurons resulted in larvae moving inappropriately into regions of low humidity at the top of the vial reflected as an increased overall pupation height and larval desiccation. This larval lethal desiccation phenotype was not observed in wild-type controls and was completely suppressed by growth in conditions of high humidity. Transgenic hyperactivation of mdIV nociceptors caused a reciprocal hypersensitivity to dry surfaces resulting in drastically decreased pupation height but did not induce the writhing nocifensive response previously associated with mdIV nociceptor activation by noxious heat or harsh mechanical stimuli. Larvae carrying mutations in either the Drosophila TRP channel, Painless, or the degenerin/epithelial sodium channel subunit Pickpocket1(PPK1), both expressed in mdIV nociceptors, showed the same inappropriate increased pupation height and lethal desiccation observed with mdIV nociceptor inactivation. Larval aversion to dry surfaces appears to utilize the same or overlapping sensory transduction pathways activated by noxious heat and harsh mechanical stimulation but with strikingly different sensitivities and disparate physiological responses.

Deconstructing the neuropathic pain phenotype to reveal neural mechanisms

After nerve injury maladaptive changes can occur in injured sensory neurons and along the entire nociceptive pathway within the CNS, which may lead to spontaneous pain or pain hypersensitivity. The resulting neuropathic pain syndromes present as a complex combination of negative and positive symptoms, which vary enormously from individual to individual. This variation depends on a diversity of underlying pathophysiological changes resulting from the convergence of etiological, genotypic, and environmental factors. The pain phenotype can serve therefore, as a window on underlying pathophysiological neural mechanisms and as a guide for developing personalized pain medicine.

Tumour necrosis factor gene polymorphism and disease prevalence

Tumour necrosis factor (TNF), an important proinflammatory cytokine, plays a role in the regulation of cell differentiation, proliferation and death, as well as in inflammation, innate and adaptive immune responses, and also implicated in a wide variety of human diseases. The presence of DNA sequence variations in regulatory region might interfere with transcription of TNF gene, influencing the circulating level of TNF and thus increases the susceptibility to human diseases (infectious, cancer, autoimmune, neurodegenerative and other diseases). In this review, we have comprehensively analysed various published case-control studies of different types of human diseases, in which TNF gene polymorphism played a role, and computationally predicted several single nucleotide polymorphisms (SNPs) lie in transcription factor-binding sites (TFBS) of transcription factors (TFs). It has been observed that TNF enhancer polymorphism is implicated in several diseases, and TNF rs1800629 and rs361525 SNPs are the most important in human disease susceptibility as these might influence the transcription of TNF gene. Thirty-two SNPs lies in TFBS of 20 TFs have been detected in the TNF upstream region. It has been found that TNF enhancer polymorphism influences the serum level of TNF in different human diseases and thus affects the susceptibility to diseases. The presence of DNA sequence variation in TNF gene causes the modification of transcriptional regulation and thus responsible for association of susceptibility/resistance with human diseases.

Modulation of spinal cord synaptic activity by tumor necrosis factor α in a model of peripheral neuropathy

BACKGROUND

The cytokine tumor necrosis factor α (TNFα) is an established pain modulator in both the peripheral and central nervous systems. Modulation of nociceptive synaptic transmission in the spinal cord dorsal horn (DH) is thought to be involved in the development and maintenance of several pathological pain states. Increased levels of TNFα and its receptors (TNFR) in dorsal root ganglion (DRG) cells and in the spinal cord DH have been shown to play an essential role in neuropathic pain processing. In the present experiments the effect of TNFα incubation on modulation of primary afferent synaptic activity was investigated in a model of peripheral neuropathy.

METHODS

Spontaneous and miniature excitatory postsynaptic currents (sEPSC and mEPSCs) were recorded in superficial DH neurons in acute spinal cord slices prepared from animals 5 days after sciatic nerve transection and in controls.

RESULTS

In slices after axotomy the sEPSC frequency was 2.8 ± 0.8 Hz, while neurons recorded from slices after TNFα incubation had significantly higher sEPSC frequency (7.9 ± 2.2 Hz). The effect of TNFα treatment was smaller in the slices from the control animals, where sEPSC frequency was 1.2 ± 0.2 Hz in slices without and 2.0 ± 0.5 Hz with TNFα incubation. Tetrodotoxin (TTX) application in slices from axotomized animals and after TNFα incubation decreased the mEPSC frequency to only 37.4 ± 6.9% of the sEPSC frequency. This decrease was significantly higher than in the slices without the TNFα treatment (64.4 ± 6.4%). TTX application in the control slices reduced the sEPSC frequency to about 80% in both TNFα untreated and treated slices. Application of low concentration TRPV1 receptors endogenous agonist N-oleoyldopamine (OLDA, 0.2 μM) in slices after axotomy induced a significant increase in mEPSC frequency (175.9 ± 17.3%), similar to the group with TNFα pretreatment (158.1 ± 19.5%).

CONCLUSIONS

Our results indicate that TNFα may enhance spontaneous transmitter release from primary afferent fibres in the spinal cord DH by modulation of TTX-sensitive sodium channels following sciatic nerve transection. This nerve injury also leads to enhanced sensitivity of presynaptic TRPV1 receptors to endogenous agonist. Modulation of presynaptic receptor activity on primary sensory terminals by TNFα may play an important role in neuropathic pain development.

Long-term IL-1β exposure causes subpopulation-dependent alterations in rat dorsal root ganglion neuron excitability

The effect of interleukin-1β (IL-1β) on the electrical properties of sensory neurons was assessed at levels and exposure times comparable to those found in animal models of neuropathic pain. Experiments involved whole cell current-clamp recordings from rat dorsal root ganglion (DRG) neurons in defined-medium, neuron-enriched cultures. Five- to six-day exposure to 100 pM IL-1β produced subpopulation-dependent effects on DRG neurons. These included an increase in the excitability of medium-diameter and small-diameter isolectin B(4) (IB(4))-positive neurons that was comparable to that found after peripheral nerve injury. By contrast, a reduction in excitability was observed in large-diameter neurons, while no effect was found in small-diameter IB(4)-negative neurons. Further characterization of changes in medium and small IB(4)-positive neurons revealed that some, but not all, effects of IL-1β were mediated through its receptor, IL-1RI. Although the acute actions of IL-1β on sensory neurons have been well studied and related to acute and/or inflammatory pain, the present study shows how sensory neurons respond to long-term cytokine exposure. Such effects are relevant to understanding processes that contribute to the onset of neuropathic pain.

Antinociceptive activity of a synthetic curcuminoid analogue, 2,6-bis-(4-hydroxy-3-methoxybenzylidene)cyclohexanone, on nociception-induced models in mice

This study investigated the potential antinociceptive efficacy of a novel synthetic curcuminoid analogue, 2,6-bis-(4-hydroxy-3-methoxybenzylidene)cyclohexanone (BHMC), using chemical- and thermal-induced nociception test models in mice. BHMC (0.03, 0.1, 0.3 and 1.0 mg/kg) administered via intraperitoneal route (i.p.) produced significant dose-related inhibition in the acetic acid-induced abdominal constriction test in mice with an ID(50) of 0.15 (0.13-0.18) mg/kg. It was also demonstrated that BHMC produced significant inhibition in both neurogenic (first phase) and inflammatory phases (second phase) of the formalin-induced paw licking test with an ID(50) of 0.35 (0.27-0.46) mg/kg and 0.07 (0.06-0.08) mg/kg, respectively. Similarly, BHMC also exerted significant increase in the response latency period in the hot-plate test. Moreover, the antinociceptive effect of the BHMC in the formalin-induced paw licking test and the hot-plate test was antagonized by pre-treatment with the non-selective opioid receptor antagonist, naloxone. Together, these results indicate that the compound acts both centrally and peripherally. In addition, administration of BHMC exhibited significant inhibition of the neurogenic nociception induced by intraplantar injections of glutamate and capsaicin with ID(50) of 0.66 (0.41-1.07) mg/kg and 0.42 (0.38-0.51) mg/kg, respectively. Finally, it was also shown that BHMC-induced antinociception was devoid of toxic effects and its antinociceptive effect was associated with neither muscle relaxant nor sedative action. In conclusion, BHMC at all doses investigated did not cause any toxic and sedative effects and produced pronounced central and peripheral antinociceptive activities. The central antinociceptive activity of BHMC was possibly mediated through activation of the opioid system as well as inhibition of the glutamatergic system and TRPV1 receptors, while the peripheral antinociceptive activity was perhaps mediated through inhibition of various inflammatory mediators.

Pharmacological modulation of central nociception in the management of chronic musculoskeletal pain

SUMMARY Chronic musculoskeletal pain, defined as pain lasting beyond the usual healing time of 6 weeks to 3 months, is a very common condition. It adversely affects the quality of life of patients and has a significant economic impact on our society. There is an ever increasing understanding of the pathophysiology of chronic pain. This has resulted in the effective use of various medications aimed at modulating both central and peripheral sensitizations. There are also new agents being developed based on fundamental research. The pharmacological agents used in the modulation of central nociception in chronic musculoskeletal pain are reviewed in this article.

The antinociceptive effect of the asthma drug ibudilast in rat models of peripheral and central neuropathic pain

Ibudilast, an asthma drug, has demonstrated antinociceptive effects in several rat models of peripheral neuropathic pain, and a proposed mechanism of action is the inhibition of release of the cytokine tumor necrosis factor-α (TNF-α) from activated spinal microglia. Spinal glial activation has also been demonstrated in rat models of central neuropathic pain following spinal cord injury (SCI). The current study evaluated the effect of a short course of treatment with ibudilast on SCI-induced pain, and for comparison, following a chronic constriction injury (CCI; the Bennett model) of the sciatic nerve in rats. The effects of ibudilast treatment on spinal (SCI and CCI rats), and nerve tissue (CCI only) TNF-α content were also evaluated. Following an acute midthoracic SCI with a microvascular clip (20-g force), hindpaw withdrawal thresholds were significantly decreased, indicating below-level cutaneous tactile hypersensitivity. Likewise, unilateral loose ligation of the sciatic nerve led to a robust ipsilateral tactile hypersensitivity. Rats were treated with either ibudilast (10 mg/kg IP) or vehicle (2 mL/kg) during the period of robust and steady hindpaw hypersensitivity for each model--CCI rats were treated 14-16 days post-surgery, and SCI rats were treated 30-32 days post-surgery--and tested daily. Ibudilast ameliorated hindpaw hypersensitivity in both SCI and CCI rats, whereas vehicle treatment had no effect. Interestingly, repeated treatment led to increased baseline thresholds, beyond the duration of the drug half-life, suggesting persistent changes in neuropathic pain processing. In SCI rats, an increase in TNF-α content in spinal tissue rostral to the SCI was observed. Ibudilast treatment did not significantly alter this increase. In rats with a CCI, TNF-α content was markedly increased in the ipsilateral sciatic nerve and was partially reduced following ibudilast, but not vehicle, treatment. Ibudilast could be useful for the treatment of neuropathic pain of central as well as peripheral origin.

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

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

Blocking TNF-α with infliximab alleviates ovariectomy induced mechanical and thermal hyperalgesia in rats

Studies have proved an increased expression of tumor necrosis factor alpha (TNF-α) in estrogen deficiency animals, and TNF-α also plays a role in inflammation and neuropathic pain. This study aimed to explore the relationship between TNF-α and ovariectomy induced hyperalgesia. 36 female Sparague-Dawley were included, estrogen depletion models were established by ovariectomy. Then infliximab (a TNF-α blocker) was administrated to the ovariectomized rats for 8 weeks. Pain behavioral tests were performed once a week. The bone mineral density (BMD), serum estradiol and TNF-α level were determined at the 8th week after ovariectomy. The expression of TNF-α in lumbar 5 dorsal root ganglions (L5 DRGs) was examined by immunofluorescence method. Significant hyperalgesia to mechanical and thermal stimuli in groups Ovx-1 and Ovx-2 was observed 1 week after the operation. After treated with infliximab, the pain threshold of Ovx-2 was partially restored, although still lower than the Sham group. The serum TNF-α level of Ovx-1 was significantly higher than Sham and Ovx-2. TNF-α immunofluorescence indicated a significant increase in the expression of TNF-α at L5 DRGs in group Ovx-1 when compared with groups Sham and Ovx-2. The BMD of group Ovx-2 was significantly higher than group Ovx-1 and lower than group Sham. In conclusion, TNF-α plays an important role in estrogen deficiency induced mechanical and thermal hyperalgesia, and DRG may be one site on which TNF-α acts to cause hyperalgesia. Blocking the effect of TNF-α could partially alleviate the estrogen deficiency induced hyperalgesia in rats. Thus, TNF-α may contribute to chronic pain in postmenopausal women.

Different expression of tissue inhibitor of metalloproteinase family members in rat dorsal root ganglia and their changes after peripheral nerve injury

Matrix metalloproteinase 9 (MMP9) and MMP2 are important in the development and maintenance of neuropathic pain behavior induced by peripheral nerve injury. The enzymatic activity of MMP9 and MMP2 is balanced specifically by tissue inhibitor of metalloproteinase 1 (TIMP1) and TIMP2, respectively. In present study, we measured the effect of peripheral nerve injury on the expression of TIMP1 and TIMP2 in adult dorsal root ganglia (DRG). A dramatic increase of TIMP1 mRNA and a decrease of TIMP2 in DRG after sciatic nerve transection (SNT) were displayed through a real-time PCR method. Furthermore, data showed by in situ hybridization that TIMP1 mRNA was only localized in DRG satellite cells under normal conditions. TIMP1 mRNA was increased in satellite cells, and induced within sensory neurons after SNT. Analysis of neuronal profiles showed that induced TIMP1 mRNA was mainly contained in small and medium DRG neurons. Further study displayed that induced TIMP1 mRNA was predominantly present in activating transcription factor 3 (ATF3)-positive injured DRG neurons. Comparatively, TIMP2 mRNA was mostly contained within sensory neurons and the overall amount decreased at the late stage after nerve injury. These data showed different change of TIMPs in DRG after peripheral nerve injury.

Interleukin-13 reduces hyperalgesia and the level of interleukin-1β in BALB/c mice infected with Leishmania major with an up-regulation of interleukin-6

The anti-inflammatory cytokines interleukin-10 (IL-10) and interleukin-13 (IL-13) were shown to reduce hyperalgesia in some models such as rats exposed to UV rays. In addition, IL-10 was also shown to reduce hyperalgesia in high dose of Leishmania major-induced inflammation in BALB/c mice accompanied by a significant decrease in the levels of interleukin-1β (IL-1β) in the paws of infected mice, while no effect on the levels of IL-6 was observed. In this study, we injected BALB/c mice with a high dose of L. major and treated them with IL-13 (15 ng/animal) for twelve days (excluding the weekends) and hyperalgesia was assessed using thermal pain tests. Furthermore, the levels of IL-1β and IL-6 were also assessed at different post-infection days. Our results show that IL-6 and more importantly IL-1β don't play a direct role in the L. major-induced hyperalgesia and that IL-13 induces this hyperalgesia through the down-regulation of IL-1β and another proinflammatory cytokine (most probably TNF-α). Furthermore, our data show that IL-13 leads to the upregulation of the level IL-6 which initially seems to have no direct role in the induced hyperalgesia. Therefore, we suggest that the L. major-induced hyperalgesia is mainly mediated by the cytokine cascade leading to the production of sympathetic amines.

Anti-TNF therapy in the injured spinal cord

Spinal cord injury (SCI) has a significant impact on the quality and expectancy of life. It also carries a heavy economic burden, with considerable costs associated with primary care and loss of income. The normal architecture of the spinal cord is radically disrupted by injury. After the initial insult, structure and function are lost through active secondary processes that involve reactive astrocytes, glial progenitors, microglia, macrophages, fibroblasts and Schwann cells. These cells produce chemokines and cytokines such as tumor necrosis factor (TNF)-α and interleukin (IL)-1β, which mediate the recruitment of inflammatory cells to the injury site. Targeting of these cytokines represents a potential strategy to reduce the secondary damage in SCI. In this review, we focus on several emerging strategies to neutralize TNF-α, including antibodies, soluble receptors, recombinant TNF-binding proteins, TNF receptor fusion proteins, and non-specific agents (e.g. thalidomide) and discuss their potential as therapy for SCI.

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.

Tumor necrosis factor alpha sensitizes spinal cord TRPV1 receptors to the endogenous agonist N-oleoyldopamine

Modulation of synaptic transmission in the spinal cord dorsal horn is thought to be involved in the development and maintenance of different pathological pain states. The proinflamatory cytokine, tumor necrosis factor alpha (TNFalpha), is an established pain modulator in both the peripheral and the central nervous system. Up-regulation of TNFalpha and its receptors (TNFR) in dorsal root ganglion (DRG) cells and in the spinal cord has been shown to play an important role in neuropathic and inflammatory pain conditions. Transient receptor potential vanilloid 1 (TRPV1) receptors are known as molecular integrators of nociceptive stimuli in the periphery, but their role on the spinal endings of nociceptive DRG neurons is unclear. The endogenous TRPV1 receptor agonist N-oleoyldopamine (OLDA) was shown previously to activate spinal TRPV1 receptors. In our experiments the possible influence of TNFalpha on presynaptic spinal cord TRPV1 receptor function was investigated. Using the patch-clamp technique, miniature excitatory postsynaptic currents (mEPSCs) were recorded in superficial dorsal horn neurons in acute slices after incubation with 60 nM TNFalpha. A population of dorsal horn neurons with capsaicin sensitive primary afferent input recorded after the TNFalpha pretreatment had a basal mEPSC frequency of 1.35 +/- 0.20 Hz (n = 13), which was significantly higher when compared to a similar population of neurons in control slices (0.76 +/- 0.08 Hz; n = 53; P < 0.01). In control slices application of a low concentration of OLDA (0.2 uM) did not evoke any change in mEPSC frequency. After incubation with TNFalpha, OLDA (0.2 uM) application to slices induced a significant increase in mEPSC frequency (155.5 +/- 17.5%; P < 0.001; n = 10). Our results indicate that TNFalpha may have a significant impact on nociceptive signaling at the spinal cord level that could be mediated by increased responsiveness of presynaptic TRPV1 receptors to endogenous agonists. This could be of major importance, especially during pathological conditions, when increased levels of TNFalpha and TNFR are present in the spinal cord.