A Promising New Approach for the Treatment of Inflammatory Pain: Transfer of Stem Cell-Derived Tyrosine Hydroxylase-Positive Cells.

OBJECTIVES: The appearance of endogenous tyrosine hydroxylase-positive cells (TH+ cells) in collagen-induced arthritis was associated with an anti-inflammatory effect. Here we investigated putative anti-inflammatory and antinociceptive effects of the transfer of induced, bone marrow stem cell-derived TH+ cells (iTH+ cells) on murine antigen-induced arthritis (AIA). METHODS: Bone marrow-derived stem cells were differentiated into iTH+ cells. These cells were transferred to mice immunized against methylated bovine serum albumin (mBSA) 2 days before AIA was induced by injection of mBSA into one knee joint. In AIA control mice and iTH+-treated mice the severity of AIA, pain-related behavior, humoral and cellular responses, and the invasion of macrophages into the dorsal root ganglia were assessed. RESULTS: The intravenous transfer of iTH+ cells before AIA induction did not cause a sustained suppression of AIA severity but significantly reduced inflammation-evoked pain-related behavior. The iTH+ cells used for transfer exhibited enormous production of interleukin-4. A major difference between AIA control mice and iTH+-treated AIA mice was a massive invasion of the dorsal root ganglia by iNOS-negative, arginine 1-positive macrophages corresponding to an M2 phenotype. The differences in other cellular and humoral immune parameters such as release of cytokines from stimulated lymphocytes between AIA control mice and iTH+-treated mice were small. CONCLUSIONS: The transfer of iTH+ cells may cause a long-lasting reduction of arthritis-induced pain even if it does not ameliorate inflammation. The invasion of M2 macrophages into the dorsal root ganglia is likely to be an important mechanism of antinociception.

Neuronal IL-17 receptor upregulates TRPV4 but not TRPV1 receptors in DRG neurons and mediates mechanical but not thermal hyperalgesia.

In addition to the proinflammatory cytokines tumor necrosis factor-α, interleukin-6 and interleukin-1ß, the cytokine interleukin-17 (IL-17) is considered an important mediator of autoimmune diseases such as rheumatoid arthritis. Because tumor necrosis factor-α and interleukin-1ß have the potential to influence the expression of transduction molecules such as transient receptor potential vanilloid 1 (TRPV1) in dorsal root ganglion (DRG) neurons and thus to contribute to pain we explored in the present study whether IL-17A activates DRG neurons and influences the expression of TRPV1. The IL-17A receptor was visualized in most neurons in dorsal root ganglion (DRG) sections as well as in cultured DRG neurons. Upon long-term exposure to IL-17A, isolated and cultured rat DRG neurons showed a significant upregulation of extracellular-regulated kinase (ERK) and nuclear factor κB (NFκB). Long-term exposure of neurons to IL-17A did not upregulate the expression of TRPV1. However, we found a pronounced upregulation of transient receptor potential vanilloid 4 (TRPV4) which is considered a candidate transduction molecule for mechanical hyperalgesia. Upon the injection of zymosan into the paw, IL-17A-deficient mice showed less mechanical hyperalgesia than wild type mice but thermal hyperalgesia was not attenuated in IL-17A-deficient mice. These data show, therefore, a particular role of IL-17 in mechanical hyperalgesia, and they suggest that this effect is linked to an activation and upregulation of TRPV4.

Spinal interleukin-6 is an amplifier of arthritic pain in the rat.

OBJECTIVE: Significant joint pain is usually widespread beyond the affected joint, which results from the sensitization of nociceptive neurons in the central nervous system (central sensitization). This study was undertaken to explore whether the proinflammatory cytokine interleukin-6 (IL-6) in the joint induces central sensitization, whether joint inflammation causes the release of IL-6 from the spinal cord, and whether spinal IL-6 contributes to central sensitization. METHODS: In anesthetized rats, electrophysiologic recordings of spinal cord neurons with sensory input from the knee joint were made. Neuronal responses to mechanical stimulation of the rat knee and leg were monitored. IL-6 and soluble IL-6 receptor (sIL-6R) were applied to the knee joint or the spinal cord. Spinal release of IL-6 was measured by enzyme-linked immunosorbent assay. Soluble gp130, which neutralizes IL-6/sIL-6R, was spinally applied during the development of joint inflammation or during established inflammation. RESULTS: A single injection of IL-6/sIL-6R into the rat knee joint as well as application of IL-6/sIL-6R to the rat spinal cord significantly increased the responses of spinal neurons to mechanical stimulation of the knee and ankle joint, i.e., induced central sensitization. Application of soluble gp130 to the rat spinal cord attenuated this effect of IL-6. The development of knee inflammation in the rat caused spinal release of IL-6. Spinal application of soluble gp130 attenuated the development of inflammation-evoked central sensitization but did not reverse it. CONCLUSION: Our findings indicate that the generation of joint pain in the rat involves not only IL-6 in the joint but also IL-6 released from the spinal cord. Spinal IL-6 contributes to central sensitization and thus promotes the widespread hyperalgesia observed in the course of joint disease.

Pain-related behaviors associated with persistence of mechanical hyperalgesia after antigen-induced arthritis in rats.

Upon transient musculoskeletal diseases, some patients develop persistent pain while others recover from pain. Here, we studied whether such heterogeneity also occurs in rats after recovery from unilateral antigen-induced arthritis (AIA) in the knee joint, and which pain phenotype may predict the course of pain. Typically, inflammatory swelling lasts about 3 weeks. Pain-related behaviors were monitored for 84 days after AIA induction. Unbiased cluster analysis of intragroup differences at day 84 of AIA revealed that about one-third of the rats (cluster 1) showed persistent mechanical hyperalgesia at the injected knee joint, whereas the other rats (cluster 2) had recovered from pain. Retrograde analysis of pain-related behaviors revealed that cluster 1 rats exhibited more severe mechanical hyperalgesia at the injected knee from day 3 of AIA and mechanical hyperalgesia at the contralateral knee. Cluster 1 and 2 rats did not show different inflammatory swelling, secondary mechanical and thermal hyperalgesia at the ipsilateral hindpaw, guarding score, and asymmetry of weight bearing during AIA. Thus, in particular, early severe mechanical hyperalgesia in the inflamed joint and segmental contralateral mechanical hyperalgesia seem to be a risk factor for the development of persistent mechanical hyperalgesia pointing to the importance of spinal mechanisms. However, none of the rats showed an expression of ATF3 in dorsal root ganglion neurons, nor morphological spinal microglia activation thus not suggesting development of neuropathic pain. Both clusters showed a persistent upregulation of pCREB in dorsal root ganglion neurons, inversely correlated with mechanical hyperalgesia at the knee. The role of pCREB needs to be further explored.

Gain-of-function mutation in SCN11A causes itch and affects neurogenic inflammation and muscle function in Scn11a+/L799P mice.

Mutations in the genes encoding for voltage-gated sodium channels cause profound sensory disturbances and other symptoms dependent on the distribution of a particular channel subtype in different organs. Humans with the gain-of-function mutation p.Leu811Pro in SCN11A (encoding for the voltage-gated Nav1.9 channel) exhibit congenital insensitivity to pain, pruritus, self-inflicted injuries, slow healing wounds, muscle weakness, Charcot-like arthropathies, and intestinal dysmotility. As already shown, knock-in mice (Scn11a+/L799P) carrying the orthologous mutation p.Leu799Pro replicate reduced pain sensitivity and show frequent tissue lesions. In the present study we explored whether Scn11a+/L799P mice develop also pruritus, muscle weakness, and changes in gastrointestinal transit time. Furthermore, we analyzed morphological and functional differences in nerves, skeletal muscle, joints and small intestine from Scn11a+/L799P and Scn11a+/+ wild type mice. Compared to Scn11a+/+ mice, Scn11a+/L799P mice showed enhanced scratching bouts before skin lesions developed, indicating pruritus. Scn11a+/L799P mice exhibited reduced grip strength, but no disturbances in motor coordination. Skeletal muscle fiber types and joint architecture were unaltered in Scn11a+/L799P mice. Their gastrointestinal transit time was unaltered. The small intestine from Scn11a+/L799P showed a small shift towards less frequent peristaltic movements. Similar proportions of lumbar dorsal root ganglion neurons from Scn11a+/L799P and Scn11a+/+ mice were calcitonin gene-related peptide (CGRP-) positive, but isolated sciatic nerves from Scn11a+/L799P mice exhibited a significant reduction of the capsaicin-evoked release of CGRP indicating reduced neurogenic inflammation. These data indicate important Nav1.9 channel functions in several organs in both humans and mice. They support the pathophysiological relevance of increased basal activity of Nav1.9 channels for sensory abnormalities (pain and itch) and suggest resulting malfunctions of the motor system and of the gastrointestinal tract. Scn11a+/L799P mice are suitable to investigate the role of Nav1.9, and to explore the pathophysiological changes and mechanisms which develop as a consequence of Nav1.9 hyperactivity.

Joint pain.

Both inflammatory and degenerative diseases of joints are major causes of chronic pain. This overview addresses the clinical problem of joint pain, the nociceptive system of the joint, the mechanisms of peripheral and central sensitization during joint inflammation and long term changes during chronic joint inflammation. While the nature of inflammatory pain is obvious the nature and site of origin of osteoarthritic pain is less clear. However, in both pathological conditions mechanical hyperalgesia is the major pain problem, and indeed, both joint nociceptors and spinal nociceptive neurons with joint input show pronounced sensitization for mechanical stimulation. Molecular mechanisms of mechanical sensitization of joint nociceptors are addressed with an emphasis on cytokines, and molecular mechanisms of central sensitization include data on the role of excitatory amino acids, neuropeptides and spinal prostaglandins. The overview will also address long-term changes of pain-related behavior, response properties of neurons and receptor expression in chronic animal models of arthritis.

Transient Receptor Potential vanilloid 4 ion channel in C-fibres is involved in mechanonociception of the normal and inflamed joint.

The Transient Receptor Potential vanilloid 4 ion channel (TRPV4) is an important sensor for osmotic and mechanical stimuli in the musculoskeletal system, and it is also involved in processes of nociception. In this study we investigated the putative role of TRPV4 ion channels in joint pain. In anesthetized rats we recorded from mechanosensitive nociceptive A∂- and C-fibres supplying the medial aspect of the knee joint. The intraarticular injection of the TRPV4 antagonist RN-1734 into the knee joint reduced the responses of C-fibres of the normal joint to noxious mechanical stimulation and the responses of the sensitized C-fibres of the acutely inflamed joint to innocuous and noxious mechanical stimulation. The responses of nociceptive A∂-fibres were not significantly altered by RN-1734. The intraarticular application of the TRPV4 agonists 4αPDD, GSK 1016790 A, and RN-1747 did not consistently alter the responses of A∂- and C-fibres to mechanical stimulation of the joint nor did they induce ongoing activity. We conclude that TRPV4 ion channels are involved in the responses of C-fibres to noxious mechanical stimulation of the normal joint, and in the enhanced sensitivity of C-fibres to mechanical stimulation of the joint during inflammation of the joint.

Contribution of Inflammation and Bone Destruction to Pain in Arthritis: A Study in Murine Glucose-6-Phosphate Isomerase-Induced Arthritis.

OBJECTIVE: Arthritis is often characterized by inflammation and bone destruction. This study was undertaken to investigate the contribution of inflammation and bone destruction to pain. METHODS: Inflammation, bone resorption, pain-related behaviors, and molecular markers (activating transcription factor 3 [ATF-3], p-CREB, and transient receptor potential vanilloid channel 1) in sensory neurons were measured in murine glucose-6-phosphate isomerase (G6PI)-induced arthritis, a model of rheumatoid arthritis. Depletion of Treg cells before immunization changed self-limiting arthritis into nonremitting arthritis with pronounced bone destruction. Zoledronic acid (ZA) was administered to reduce bone resorption. RESULTS: Compared to nondepleted mice, Treg cell-depleted mice exhibited arthritis with more severe bone destruction and higher guarding scores (P < 0.05; n = 10 mice per group) as well as more persistent thermal hyperalgesia (P < 0.05), but displayed similar mechanical hyperalgesia at the hindpaws (n = 18-26 mice per group). These pain-related behaviors, as well as an up-regulation of the neuronal injury marker ATF-3 in sensory neurons (studied in 39 mice), appeared before the clinical score (inflammation) became positive and persisted in Treg cell-depleted and nondepleted mice. In the late stage of arthritis, Treg cell-depleted mice treated with ZA showed less bone resorption (<50%; P < 0.01) and less thermal hyperalgesia (P < 0.01) than Treg cell-depleted mice without ZA treatment (n = 15 mice per group), but ZA treatment did not reduce the clinical score and local mechanical hyperalgesia. CONCLUSION: Pain-related behaviors precede and outlast self-limiting arthritis. In nonremitting arthritis with enhanced bone destruction, mainly local thermal, but not local mechanical, hyperalgesia was aggravated. The up-regulation of ATF-3 indicates an early and persisting affection of sensory neurons by G6PI-induced arthritis.

Interleukin-17A is involved in mechanical hyperalgesia but not in the severity of murine antigen-induced arthritis.

Interleukin-17A (IL-17A) is considered an important pro-inflammatory cytokine but its importance in joint diseases such as rheumatoid arthritis (RA) is unclear. It has also been reported that IL-17A may induce pain but it is unclear whether pro-inflammatory and pro-nociceptive effects are linked. Here we studied in wild type (WT) and IL-17A knockout (IL-17AKO) mice inflammation and hyperalgesia in antigen-induced arthritis (AIA). We found that the severity and time course of AIA were indistinguishable in WT and IL-17AKO mice. Furthermore, the reduction of inflammation by sympathectomy, usually observed in WT mice, was preserved in IL-17AKO mice. Both findings suggest that IL-17A is redundant in AIA pathology. However, in the course of AIA IL-17AKO mice showed less mechanical hyperalgesia than WT mice indicating that IL-17A contributes to pain even if it is not crucial for arthritis pathology. In support for a role of IL-17A and other members of the IL-17 family in the generation of pain we found that sensory neurones in the dorsal root ganglia (DRG) express all IL-17 receptor subtypes. Furthermore, in isolated DRG neurones most IL-17 isoforms increased tetrodotoxin- (TTX-) resistant sodium currents which indicate a role of IL-17 members in inflammation-evoked sensitization of sensory nociceptive neurones.

Long-Lasting Activation of the Transcription Factor CREB in Sensory Neurons by Interleukin-1ß During Antigen-Induced Arthritis in Rats: A Mechanism of Persistent Arthritis Pain?

OBJECTIVE: In spite of successful treatment of immune-mediated arthritis, many patients still experience pain. We undertook this study to investigate whether antigen-induced arthritis (AIA) in rats triggers neuronal changes in sensory neurons that outlast the inflammatory process. METHODS: We induced unilateral AIA in the knee joint and assessed in sensory neurons the expression of CREB, a transcription factor that regulates genes involved in neuronal plasticity. We tested whether neutralization of the effects of tumor necrosis factor (TNF) by etanercept or infliximab or neutralization of the effects of interleukin-1ß (IL-1ß) by anakinra influences the up-regulation of phospho-CREB, and we studied the up-regulation of phospho-CREB by IL-1ß and TNF in cultured dorsal root ganglion (DRG) neurons. RESULTS: Unilateral AIA caused bilateral up-regulation of phospho-CREB in lumbar DRG neurons. While inflammation and pain subsided within 21 days, the up-regulation of phospho-CREB still persisted on day 42. At this time point mechanical hyperalgesia at the knee reappeared in the absence of swelling. TNF neutralization during AIA significantly reduced pain-related behavior but did not prevent phospho-CREB up-regulation. In contrast, anakinra, which only reduced thermal hyperalgesia, prevented phospho-CREB up-regulation, suggesting a role of IL-1ß in this process. In cultured DRG neurons the application of IL-1ß significantly enhanced phospho-CREB. CONCLUSION: Immune-mediated arthritis causes neuroplastic changes in sensory neurons that outlast the inflammatory phase. Such changes may facilitate the persistence or recurrence of pain after remission of arthritis. IL-1ß is an important trigger in this process, although its neutralization barely reduced mechanical hyperalgesia during inflammation.

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

OBJECTIVE: In arthritis, macrophages invade the affected joint. Experimental arthritis models have shown that macrophages also invade the dorsal root ganglia (DRGs) of the inflamed segments in which the perikarya of sensory neurons are located. It is unclear whether this macrophage invasion contributes to arthritis pain and/or furthers neuronal damage. The present study was undertaken to investigate how differently activated macrophages affect DRG neurons. METHODS: We determined the phenotype of macrophages in the DRGs of rats with antigen-induced arthritis (AIA). In a DRG neuron-macrophage coculture system, we investigated whether differently activated macrophages (stimulated with either lipopolysaccharide [LPS]/interferon-γ [IFNγ], tumor necrosis factor [TNF], or interleukin-4) damage DRG neurons and/or stimulate them to release the mediator calcitonin gene-related peptide (CGRP), which promotes pain and neurogenic inflammation. RESULTS: Macrophages in the DRGs of rats with AIA showed the phenotype of TNF-stimulated macrophages but did not express inducible nitric oxide synthase, which was found in cultured macrophages only after LPS/IFNγ activation. In neuron-macrophage cocultures, activation of macrophages stimulated DRG neurons to release CGRP within 1 hour, indicating neuronal activation by macrophages. Only 48-hour activation of macrophages with LPS/IFNγ increased the neuronal cell death rate in culture, provided that the macrophages were in direct contact with DRG neurons. This effect was dependent on nitric oxide. CONCLUSION: Macrophages have the potential to stimulate sensory neurons in the DRGs, and this may contribute to arthritis pain. If they are classically activated, such as after LPS/IFNγ stimulation, this may also further neuronal cell death. This is not the case in AIA but may occur in models involving damage of sensory neurons.

Interleukin-6-dependent influence of nociceptive sensory neurons on antigen-induced arthritis.

INTRODUCTION: Interleukin-6 (IL-6) is an important mediator of inflammation. In addition to cells involved in inflammation, sensory nociceptive neurons express the IL-6 signal-transducer glycoprotein 130 (gp130). These neurons are not only involved in pain generation but also produce neurogenic inflammation by release of neuropeptides such as calcitonin gene-related peptide (CGRP). Whether IL-6 activation of sensory neurons contributes to the induction of inflammation is unknown. This study explored whether the action of IL-6 on sensory neurons plays a role in the generation of neurogenic inflammation and arthritis induction. METHODS: In SNS-gp130(-/-) mice lacking gp130 selectively in sensory neurons and appropriate control littermates (SNS-gp130(flox/flox)), we induced antigen-induced arthritis (AIA), and assessed swelling, histopathological arthritis scores, pain scores, expression of CGRP in sensory neurons, serum concentrations of CGRP and cytokines, and the cytokine release from single cell suspensions from lymph nodes and spleens. In wild-type mice CGRP release was determined during development of AIA and, in cultured sensory neurons, upon IL-6 stimulation. RESULTS: Compared to SNS-gp130(flox/flox) mice SNS-gp130(-/-) mice showed significantly weaker initial swelling, reduced serum concentrations of CGRP, IL-6, and IL-2, no inflammation-evoked upregulation of CGRP in sensory neurons, but similar histopathological arthritis scores during AIA. During the initial swelling phase of AIA, CGRP was significantly increased in the serum, knee and spleen. In vitro, IL-6 augmented the release of CGRP from cultured sensory neurons. Upon antigen-specific restimulation lymphocytes from SNS-gp130(-/-) mice released more interleukin-17 and interferon-γ than lymphocytes from SNS-gp130(flox/flox) mice. In naive lymphocytes from SNS-gp130(flox/flox) and SNS-gp130(-/-) mice CGRP reduced the release of IL-2 (a cytokine which inhibits the release of interleukin-17 and interferon-γ). CONCLUSIONS: IL-6 signaling in sensory neurons plays a role in the expression of arthritis. Selective deletion of gp130 signaling in sensory neurons reduces the swelling of the joint (most likely by reducing neurogenic inflammation) but increases some proinflammatory systemic cellular responses such as the release of interleukin-17 and interferon-γ from lymphocytes upon antigen-specific restimulation. Thus IL-6 signaling in sensory neurons is not only involved in pain generation but also in the coordination of the inflammatory response.

Spatio-temporal pattern of induction of bradykinin receptors and inflammation in rat dorsal root ganglia after unilateral nerve ligation.

Expression of bradykinin receptors was analyzed in freshly isolated dorsal root ganglion neurons of the ipsi- and contralateral segments L4/L5, L2/L3, and T12/T13 two to twenty days after unilateral injury of the adult rat sciatic nerve using gold labeled bradykinin. The number of infiltrating leucocytes was investigated by flow cytometry. Sciatic nerve injury transiently increased the proportion of neurons expressing bradykinin receptors not only in the ipsilateral ganglia L4/L5, but also in the homonymous contralateral ganglia and also bilaterally in the adjacent ganglia L2/L3. Neurons of the ganglia T12/T13 were not affected. The time course of upregulation was different between neurons of the injured nerve and uninjured ones. Furthermore, the proportion of neurons expressing a high density of receptors increased also bilaterally in ganglia L4/L5 and L2/L3. As on the ipsilateral side, the increase in neurons expressing bradykinin receptors in the contralateral homonymous ganglia was due to an induction of the B1 receptor subtype and an upregulation of the B2 subtype. As a possible source for stimulating factors for induction of bradykinin receptors the number of macrophages and lymphocytes was investigated two to twenty days after nerve ligation. No increase was observed prior to day ten and only in ipsilateral ganglia L4/L5, not contralaterally and not in adjacent ganglia L2/L3 and T12/T13. The experiments show that the induction of bradykinin receptors following a unilateral nerve lesion is not restricted to neurons projecting into the damaged nerve but is (i) bilateral, (ii) different in time course between injured and uninjured neurons, and (iii) locally confined to neurons of the adjacent ganglia. Macrophages and lymphocytes are increased after ten day ligation only in the affected ganglia and are probably not involved in the induction of bradykinin receptors.

Prostaglandin E2 increases the expression of the neurokinin1 receptor in adult sensory neurones in culture: a novel role of prostaglandins.

(1) Peripheral inflammation causes an increase in the proportion of primary afferent neurones that express neurokinin(1) (NK(1)) receptors for substance P (SP). This upregulation may contribute to the neuronal mechanisms of inflammatory pain. The aim of this study was to identify endogenous mediators that stimulate upregulation of NK(1) receptors in dorsal root ganglion (DRG) neurones. Cultured DRG neurones from the adult normal rat were exposed for 2 days to media that contained specific mediators, namely potassium in high concentration, prostaglandin E(2) (PGE(2)), somatostatin (SRIF), and compounds influencing second messenger cascades. After fixation neurones were labelled with an NK(1) receptor antibody. (2) Repetitive addition of the inflammatory mediator PGE(2) or dibutyryl-cyclic adenosine 3',5' monophophate (db-cAMP) to the culture medium enhanced the proportion of neurones with NK(1) receptor-like immunoreactivity from about 12% up to 40%. PGE(2)-induced upregulation was prevented by coadministration of PGE(2) and a protein kinase A inhibitor or SRIF to the medium. High potassium concentration, protein kinase C inhibitors and omission of nerve growth factor from the medium had no effect. (3) In calcium-imaging experiments, bath application of SP evoked increases of the intracellular calcium concentration in about 20% of the neurones. This proportion increased to about 40% after PGE(2)-pretreatment, but the increase was prevented when PGE(2) and SRIF were coadministered to the medium. (4) These data show that the expression of NK(1) receptor-like immunoreactivity in DRG neurones is regulated by the inflammatory mediator PGE(2). This upregulation depends on the intracellular adenylyl cyclase-protein kinase A pathway.

Experimental arthritis causes tumor necrosis factor-alpha-dependent infiltration of macrophages into rat dorsal root ganglia which correlates with pain-related behavior.

After peripheral nerve damage macrophages infiltrate the dorsal root ganglia (DRG) in which cell bodies of lesioned neurons are located. However, infiltration of macrophages into the DRGs was also reported in complete Freund's adjuvant (CFA)-induced inflammation raising the question whether CFA inflammation induces nerve cell damage or whether peripheral inflammation may also trigger macrophage infiltration into DRGs. Related questions are, first, which signals trigger macrophage infiltration into DRGs and, second, is macrophage infiltration correlated with pain-related behavior. Using the rat model of unilateral antigen-induced arthritis (AIA) in the knee we found a massive infiltration of ED1(+) macrophages into the ipsi- and contralateral lumbar DRGs but not into thoracic DRGs. At no time point of AIA DRG neurons showed expression of activating transcription factor-3 (ATF3) indicating that macrophage infiltration is not explainable by nerve cell lesions in this model. During AIA, lumbar but not thoracic DRGs exhibited a bilateral de novo expression of vascular cell adhesion molecule-1 (VCAM-1) which is known to be involved in macrophage infiltration. Tumor necrosis factor-alpha (TNF-alpha) neutralization with etanercept or infliximab treatment after induction of AIA significantly reduced both macrophage infiltration and VCAM-1 expression. It also decreased mechanical hyperalgesia at the inflamed joint although the joint inflammation itself was barely attenuated, and it reduced mechanical hyperalgesia at the non-inflamed contralateral knee joint. Thus, bilateral segment-specific infiltration of macrophages into DRGs is part of an unilateral inflammatory process in peripheral tissue and it may be involved in the generation of hyperalgesia in particular on the non-inflamed side.

The cytokine TNFalpha increases the proportion of DRG neurones expressing the TRPV1 receptor via the TNFR1 receptor and ERK activation.

TNFalpha is involved in the generation of hyperalgesia in pathological states such as neuropathy and inflammation. The pronociceptive action of TNFalpha may be mediated at least in part by activation of the TRPV1 receptor which transduces heat stimuli in primary nociceptive afferents and mediates thermal hyperalgesia. In the present study, we investigated in cultured dorsal root ganglion (DRG) neurones, the somata of primary afferent fibres, whether TNFalpha increases TRPV1 receptor expression. We found that long-term exposure of DRG neurones of both rat and mouse to TNFalpha significantly increased the proportion of DRG neurones expressing TRPV1 receptor-like immunoreactivity. This TNFalpha effect was abolished in mice DRG neurones when DRG cultures were obtained from tnfr1/2-/- and tnfr1-/-, but not from tnfr2-/- mice. Furthermore, we found that activation of ERK but not of p38 kinase or cyclooxygenases is critically involved in the TNFalpha-induced increase of TRPV1 receptor expression.