Early induction of uncoupling protein-2 in pulmonary macrophages in hyperoxia-associated lung injury.

CONTEXT: High concentrations of inspired oxygen contribute to the pathogenesis of neonatal bronchopulmonary dysplasia and adult acute respiratory distress syndrome. Animal models of hyperoxia-associated lung injury (HALI) are characterized by enhanced generation of reactive oxygen species (ROS) and an adaptive antioxidant response. ROS contribute to pathogenesis, partly through enhancing pro-inflammatory activity in macrophages. Uncoupling protein-2 (UCP2) is an inner mitochondrial membrane protein whose expression lowers mitochondrial superoxide (O2ⁱ⁻) production. UCP2, therefore, has potential to contribute to antioxidant response. It is inducible in macrophages. OBJECTIVES AND METHODS: We hypothesized that induction of UCP2 occurred in response to pulmonary hyperoxia in vivo and that expression localized to pulmonary macrophages. We then investigated mechanisms of UCP2 regulation in hyperoxia-exposed macrophages in vitro and correlated changing UCP2 expression with mitochondrial membrane potential (Δψm) and O2ⁱ⁻ production. RESULTS: UCP2 is induced in lungs of mice within 1 h of hyperoxia exposure. Induction occurs in pulmonary alveolar macrophages in vivo, and can be replicated in vitro in isolated macrophages. UCP2 mRNA does not change. UCP2 increases quickly after the first hyperoxia-induced burst of mitochondrial O2ⁱ⁻ generation. Suppression of Δψm and mitochondrial O2ⁱ⁻ production follow and persist while UCP2 is elevated. DISCUSSION AND CONCLUSIONS: Induction of UCP2 is an early response to hyperoxia in pulmonary macrophages. The mechanism is post-transcriptional. UCP2 induction follows a transient rise in mitochondrial ROS generation. The subsequent falls in Δψm and mitochondrial O2ⁱ⁻ support the notion that regulable UCP2 expression in macrophages acts to contain mitochondrial ROS generation. That, in turn, may limit inappropriate pro-inflammatory activation in HALI.

IL-17 induces hyperalgesia via TNF-dependent neutrophil infiltration.

Interleukin-17 (IL-17) and tumour necrosis factor-α (TNF) are critical in the pathogenesis of arthritis but their relationship during inflammatory pain has received limited attention. We aimed to establish whether IL-17 can induce hyperalgesia in acute conditions, and investigated the role of TNF in mediating the pain response. Hyperalgesia was elicited in C57BL/6 mice by injection of recombinant IL-17, TNF or vehicle into the plantar tissue. Elevated pain was measured by the Hargreaves test for thermal hyperalgesia and Linton incapacitance tester for weight-bearing change. Cellular infiltration during hyperalgesia was determined by histological analysis and myeloperoxidase assay. IL-17 was found to induce hyperalgesia, but this was dependent on neutrophil migration and TNF binding to TNF receptor 1 (TNFR1). Because TNF-induced hyperalgesia was also dependent on neutrophil migration, the relationship between the resident fibroblasts, the cytokines and the migrating neutrophils was further investigated. By means of an air pouch model of cell migration, it was established that IL-17-induced neutrophil infiltration was dependent of TNF/TNFR1 as this interaction was required for the induction of the chemokine keratinocyte chemoattractant. These findings suggest that IL-17 causes acute hyperalgesia indirectly by inducing TNF from resident cells. The subsequent production of keratinocyte chemoattractant then triggers neutrophil chemotaxis to the plantar tissue, releasing algesic mediators locally to sensitise the nerve.

In vivo fluorescence imaging of E-selectin: quantitative detection of endothelial activation in a mouse model of arthritis.

OBJECTIVE: In vivo optical imaging can delineate at the macroscopic level processes that are occurring at the cellular and molecular levels. E-selectin, a leukocyte adhesion molecule expressed on endothelium, is induced by tumor necrosis factor α (TNFα) and other cytokines involved in the pathogenesis of rheumatoid arthritis (RA). Collagen-induced arthritis (CIA) in mice is widely used to study the disease mechanisms and identify new treatments for RA. The purpose of this study was to demonstrate E-selectin-targeted fluorescence imaging in vivo in a mouse model of paw edema generated by local injection of TNFα as well as in mice with CIA. METHODS: Animals with either CIA or TNFα-induced paw edema were injected with anti-E-selectin or control antibodies labeled with a DyLight 750-nm near-infrared (NIR) probe. In vivo imaging studies were undertaken using an NIR optical imaging system, and images were coregistered with plain radiographic images. RESULTS: The mean fluorescence intensity measured over the time-course of TNFα-induced edema demonstrated a 1.97-fold increase (P<0.001) in signal in inflamed paws at 8 hours following injection of anti-E-selectin antibody, as compared to that in the isotype control. In the CIA model, a 2.34-fold increase in E-selectin-targeted signal was demonstrated (P<0.01). Furthermore, significant E-selectin-targeted signal was observed in the paws of animals immunized with collagen that did not display overt signs of arthritis. CONCLUSION: E-selectin-targeted fluorescence in vivo imaging is a quantifiable method of detecting endothelial activation in arthritis and can potentially be applied to the quantification of disease and the investigation of the effects of new therapies. Importantly, this approach may also be useful for the detection of subclinical disease in RA.

Nonsteroidal anti-inflammatory drugs increase TNF production in rheumatoid synovial membrane cultures and whole blood.

Nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit cyclooxygenase activity and hence PG production. However, the ability of NSAIDs to ameliorate pain and tenderness does not prevent disease progression in rheumatoid arthritis, a disease whose pathogenesis is linked to the presence of proinflammatory cytokines, such as TNF-alpha. To understand this observation, we have examined the effect of NSAIDs on the production of clinically validated proinflammatory cytokines. We show that a variety of NSAIDs superinduce production of TNF from human peripheral blood monocytes and rheumatoid synovial membrane cultures. A randomized, double-blinded, crossover, placebo-controlled trial in healthy human volunteers also revealed that the NSAID drug celecoxib increased LPS-induced TNF production in whole blood. NSAID-mediated increases in TNF are reversed by either the addition of exogenous PGE(2) or by a PGE(2) EP2 receptor agonist, revealing that PGE(2) signaling via its EP2 receptor provides a valuable mechanism for controlling excess TNF production. Thus, by reducing the level of PGE(2), NSAIDs can increase TNF production and may exacerbate the proinflammatory environment both within the rheumatoid arthritis joint and the systemic environment.

Nimesulide improves the disease modifying anti-rheumatic profile of methotrexate in mice with collagen-induced arthritis.

Methotrexate is a disease modifying anti-rheumatic drug that is widely used for the treatment of rheumatoid arthritis. Nimesulide is a non-steroidal anti-inflammatory drug which is frequently used as adjuvant therapy for symptomatic alleviation of rheumatoid arthritis. In this study, we have evaluated the potential influence of nimesulide on the disease modifying anti-rheumatic properties of methotrexate using the collagen-induced arthritis model. Mice were immunized with collagen type II for the induction of arthritis and treated with methotrexate (2.5mg/kg) twice a week, nimesulide (20mg/kg) every other day or a combination of both drugs. Treatment started one week after the onset of arthritis until day 40. An arthritic index was used to compare the severity of arthritis between different treatments. In addition, articular hyperalgesia, joint stiffness, radiological deterioration and intra-articular leucocytic infiltration were evaluated. Methotrexate alone showed modest but significant analgesic and anti-inflammatory effects, and the effects of nimesulide were comparable. On the other hand, nimesulide significantly improved the disease modifying anti-rheumatic profile of methotrexate in terms of arthritic index and joint mobility. Furthermore, although nimesulide failed to show any radiological evidence of articular protection, it significantly improved methotrexate-induced joint protection as judged by X-ray analysis.

Apremilast, a novel PDE4 inhibitor, inhibits spontaneous production of tumour necrosis factor-alpha from human rheumatoid synovial cells and ameliorates experimental arthritis.

INTRODUCTION: Type 4 phosphodiesterases (PDE4) play an important role in immune cells through the hydrolysis of the second messenger, cAMP. Inhibition of PDE4 has previously been shown to suppress immune and inflammatory responses, demonstrating PDE4 to be a valid therapeutic target for immune-mediated pathologies. We assessed the anti-inflammatory effects of a novel PDE4 inhibitor, apremilast, in human synovial cells from rheumatoid arthritis (RA) patients, as well as two murine models of arthritis. METHODS: Cells liberated from tissue excised from arthritic joints of RA patients were cultured in the presence of increasing concentrations of apremilast for 48 hours and spontaneous tumour necrosis factor-alpha (TNFalpha) production was analysed in culture supernatants by ELISA. In addition, arthritis was induced in BALB/c and DBA/1 mice by passive transfer of anti-type II collagen mAb and immunisation with type II collagen, respectively. Mice with established arthritis received 5 or 25 mg/kg apremilast and disease severity was monitored relative to mice receiving vehicle alone. At the end of the study, paws were removed and processed for histopathological assessment. Behavioural effects of apremilast, relative to rolipram, were assessed in naïve DBA/1 mice using an automated activity monitor (LABORAS). RESULTS: Apremilast dose dependently inhibited spontaneous release of TNFalpha from human rheumatoid synovial membrane cultures. Furthermore, apremilast significantly reduced clinical score in both murine models of arthritis over a ten day treatment period and maintained a healthy joint architecture in a dose-dependent manner. Importantly, unlike rolipram, apremilast demonstrated no adverse behavioural effects in naïve mice. CONCLUSIONS: Apremilast is an orally available PDE4 inhibitor that reduces TNFalpha production from human synovial cells and significantly suppresses experimental arthritis. Apremilast appears to be a potential new agent for the treatment of rheumatoid arthritis.

Treatment of murine osteoarthritis with TrkAd5 reveals a pivotal role for nerve growth factor in non-inflammatory joint pain.

The origin of pain in osteoarthritis is poorly understood, but it is generally thought to arise from inflammation within the innervated structures of the joint, such as the synovium, capsule and bone. We investigated the role of nerve growth factor (NGF) in pain development in murine OA, and the analgesic efficacy of the soluble NGF receptor, TrkAD5. OA was induced in mice by destabilisation of the medial meniscus and pain was assessed by measuring hind-limb weight distribution. RNA was extracted from joints, and NGF and TNF expressions were quantified. The effect of tumour necrosis factor (TNF) and neutrophil blockade on NGF expression and pain were also assessed. NGF was induced in the joints during both post-operative (day 3) and OA (16weeks) pain, but not in the non-painful stage of disease (8weeks post-surgery). TrkAd5 was highly effective at suppressing pain in both phases. Induction of NGF in the post-operative phase of pain was TNF-dependent as anti-TNF reduced NGF expression in the joint and abrogated pain. However, TNF was not regulated in the late OA joints, and pain was not affected by anti-TNF therapy. Fucoidan, by suppressing cellular infiltration into the joint, was able to suppress post-operative, but not late OA pain. These results indicate that NGF is an important mediator of OA pain and that TrkAd5 represents a potent novel analgesic in this condition. They also suggest that, unlike post-operative pain, induction of pain in OA may not necessarily be driven by classical inflammatory processes.

ANTI-CD3 therapy expands the numbers of CD4+ and CD8+ Treg cells and induces sustained amelioration of collagen-induced arthritis.

OBJECTIVE: To assess the therapeutic potential of anti-CD3 monoclonal antibodies (mAb) for rheumatoid arthritis, using collagen-induced arthritis as an animal model. METHODS: Arthritis was induced in DBA/1 mice by immunization with type II collagen. After disease onset, a single injection of anti-CD3 mAb (20 microg/mouse) was administered, and arthritis severity was monitored over a 10-day period. RESULTS: Anti-CD3 mAb treatment resulted in a sustained reduction in disease activity, which was associated with an increase in the proportion of naturally occurring CD4+CD25+FoxP3+ regulatory T (Treg) cells and the generation of a population of CD8+CD25+FoxP3+ Treg cells. Anti-CD3 mAb treatment did not alter the capacity of CD4+ Treg cells to suppress effector T cell proliferation and interferon-gamma (IFNgamma) production in vitro. However, CD4+ Treg cells from both anti-CD3 mAb-treated and control mice were unable to suppress interleukin-17 (IL-17) production. In contrast, CD8+ Treg cells induced by anti-CD3 therapy suppressed IL-17 production as well as CD4+ T cell proliferation and IFNgamma production. CONCLUSION: These results show that anti-CD3 mAb treatment has important therapeutic potential for rheumatoid arthritis and has the capacity to generate antiarthritic CD8+ Treg cells and expand the relative numbers of CD4+ Treg cells.

Indoleamine 2,3 dioxygenase-mediated tryptophan catabolism regulates accumulation of Th1/Th17 cells in the joint in collagen-induced arthritis.

OBJECTIVE: Indoleamine 2,3 dioxygenase (IDO) is a catabolic enzyme that initiates the kynurenine pathway of tryptophan degradation and has immunomodulatory properties. The aim of this study was to investigate the regulation of collagen-induced arthritis by tryptophan catabolism mediated by IDO. METHODS: Arthritis was induced by immunization with type II collagen. After induction of arthritis, the expression of IDO was analyzed by quantitative reverse transcription-polymerase chain reaction. The effect of IDO deficiency on collagen-induced arthritis was assessed in vivo by administration of 1-methyltryptophan and clinical and histologic evaluation of IDO-deficient mice. The requirement for IDO activation was bypassed by administration of L-kynurenine. RESULTS: IDO was induced in lymph node dendritic cells after collagen immunization. Systemic inhibition of tryptophan catabolism during active arthritis increased disease severity. Conversely, bypassing the requirement for tryptophan degradation by the administration of L-kynurenine resulted in amelioration of arthritis. Furthermore, IDO-deficient mice showed a higher incidence of arthritis and exacerbated disease severity compared with IDO-competent mice. Such increased disease activity in IDO-deficient mice correlated early with increased production of the proinflammatory cytokines interferon-gamma and interleukin-17 by lymph node T cells and later with increased infiltration of Th1 and Th17 cells in the inflamed joints. CONCLUSION: Our data indicate that the induction of IDO controls the accumulation of Th1 and Th17 pathogenic T cells at the site of inflammation during collagen-induced arthritis. Therefore, manipulation of the kynurenine pathway of tryptophan degradation provides the potential for therapeutic intervention in rheumatoid arthritis.

New advances in musculoskeletal pain.

Non-malignant musculoskeletal pain is the most common clinical symptom that causes patients to seek medical attention and is a major cause of disability in the world. Musculoskeletal pain can arise from a variety of common conditions including osteoarthritis, rheumatoid arthritis, osteoporosis, surgery, low back pain and bone fracture. A major problem in designing new therapies to treat musculoskeletal pain is that the underlying mechanisms driving musculoskeletal pain are not well understood. This lack of knowledge is largely due to the scarcity of animal models that closely mirror the human condition which would allow the development of a mechanistic understanding and novel therapies to treat this pain. To begin to develop a mechanism-based understanding of the factors involved in generating musculoskeletal pain, in this review we present recent advances in preclinical models of osteoarthritis, post-surgical pain and bone fracture pain. The models discussed appear to offer an attractive platform for understanding the factors that drive this pain and the preclinical screening of novel therapies to treat musculoskeletal pain. Developing both an understanding of the mechanisms that drive persistent musculoskeletal pain and novel mechanism-based therapies to treat these unique pain states would address a major unmet clinical need and have significant clinical, economic and societal benefits.

Blockade of tumor necrosis factor in collagen-induced arthritis reveals a novel immunoregulatory pathway for Th1 and Th17 cells.

IL-17 is implicated in the pathogenesis of rheumatoid arthritis (RA) and has previously been shown to be induced by tumor necrosis factor (TNF) in vitro. The aim of this study was to assess the impact of TNF inhibition on IL-17 production in collagen-induced arthritis, a model of RA. TNF blockade using TNFR-Fc fusion protein or anti-TNF monoclonal antibody reduced arthritis severity but, unexpectedly, expanded populations of Th1 and Th17 cells, which were shown by adoptive transfer to be pathogenic. Th1 and Th17 cell populations were also expanded in collagen-immunized TNFR p55(-/-) but not p75(-/-) mice. The expression of IL-12/IL-23 p40 was up-regulated in lymph nodes (LN) from p55(-/-) mice, and the expansion of Th1/Th17 cells was abrogated by blockade of p40. Treatment of macrophages with rTNF also inhibited p40 production in vitro. These findings indicate that at least one of the ways in which TNF regulates Th1/Th17 responses in arthritis is by down-regulating the expression of p40. Finally, although TNF blockade increased numbers of Th1 and Th17 cells in LN, it inhibited their accumulation in the joint, thereby providing an explanation for the paradox that anti-TNF therapy ameliorates arthritis despite increasing numbers of pathogenic T cells.

Regulation of pain sensitivity in experimental osteoarthritis by the endogenous peripheral opioid system.

OBJECTIVE: OA is the most common joint disease, affecting 10-15% of people over 60 years of age. However, up to 40% of individuals with radiologic damage are asymptomatic. The purpose of this study was to assess the role of the endogenous opioid system in delaying the onset of pain in a murine model of osteoarthritis (OA). METHODS: Osteoarthritis was induced by transection of the medial meniscotibial ligament. Pain was assessed by monitoring weight distribution and activity. At various times postsurgery, the opioid receptor antagonists naloxone or peripherally restricted naloxone methiodide were administered, and pain was assessed. Levels of the micro-opioid receptor were assessed in the nerves innervating the joint by real-time reverse transcription-polymerase chain reaction analysis. RESULTS: As in human disease, significant joint damage occurred in mice before the onset of pain. To assess whether delayed pain was partly the result of increased endogenous opioid function, naloxone or naloxone methiodide was administered. Both opioid receptor antagonists led to pain onset 4 weeks earlier than in vehicle-treated mice, indicating a role of the peripheral opioid system in masking OA pain. The expression of the micro-opioid receptor in the peripheral nerves supplying the joint was transiently increased in naloxone-responsive mice. CONCLUSION: These findings indicate that a temporal induction of micro-opioid receptors in the early stages of OA delays the onset of pain. This is of clinical relevance and may contribute to the assessment of patients presenting with pain late in the disease. Furthermore, it may point to a mechanism by which the body blocks pain perception in moderate states of tissue damage, allowing an increased chance of survival.

Protocol for the induction of arthritis in C57BL/6 mice.

Collagen-induced arthritis is a well-validated, but strain-dependent mouse model of rheumatoid arthritis, with H-2(q) and H-2(r) strains showing the greatest degree of susceptibility. This protocol describes the induction of arthritis in the C57BL/6 strain (H-2(b)), which forms the genetic background of the majority of genetically modified strains. This protocol involves purification of type II collagen from chicken sternums, immunization of mice, clinical assessment of arthritis and analysis of T- and B-cell responses to type II collagen. Key aspects of the protocol are the need to use chicken collagen for immunization and the importance of avoiding aggressive behavior in males. The incidence of arthritis varies from 50 to 80% and is milder than the classical collagen-induced arthritis model. This procedure takes approximately 3 months to complete.

Collagen-induced arthritis as a model of hyperalgesia: functional and cellular analysis of the analgesic actions of tumor necrosis factor blockade.

OBJECTIVE: There is a disparity in the animal models used to study pain in rheumatoid arthritis (RA), which tends to be acute in nature, and models used to assess the pathogenesis of RA. The latter models, like human RA, are lymphocyte-driven and polyarthritic. We assessed pain behavior and mechanisms in collagen-induced arthritis (CIA), the model of preclinical arthritis used most commonly in the field of immunology. We then validated the model using anti-tumor necrosis factor (anti-TNF) therapy, which has analgesic effects in models of inflammation as well as in human RA. METHODS: CIA was induced in DBA/1 mice by immunization with type II collagen at the base of the tail. Swelling and mechanical and thermal hyperalgesia were assessed before and for 28 days after the onset of arthritis. Spontaneous behavior was assessed using an automated activity monitor. Glial activity was assessed by glial fibrillary acidic protein expression, and nerve damage was evaluated by activating transcription factor 3 expression. The actions of anti-TNF therapy on nociception were then evaluated. RESULTS: Arthritis resulted in a decrease in the threshold for thermal and mechanical stimuli, beginning on the day of onset. Decreased spontaneous activity was also observed. A significant increase in the number of hyperplasic spinal cord astrocytes was observed beginning 10 days after the onset of arthritis. Anti-TNF therapy was profoundly analgesic, with an efficacy similar to that of cyclooxygenase 2 inhibition, and reduced astrocyte activity in CIA. CONCLUSION: This study shows that the CIA model is suitable for testing not only antiinflammatory but also analgesic drugs for potential use in RA, and highlights the importance of using appropriate disease models to assess relevant pain pathways.

Collagen-induced arthritis in C57BL/6 mice is associated with a robust and sustained T-cell response to type II collagen.

Many genetically modified mouse strains are now available on a C57BL/6 (H-2b) background, a strain that is relatively resistant to collagen-induced arthritis. To facilitate the molecular understanding of autoimmune arthritis, we characterised the induction of arthritis in C57BL/6 mice and then validated the disease as a relevant pre-clinical model for rheumatoid arthritis. C57BL/6 mice were immunised with type II collagen using different protocols, and arthritis incidence, severity, and response to commonly used anti-arthritic drugs were assessed and compared with DBA/1 mice. We confirmed that C57BL/6 mice are susceptible to arthritis induced by immunisation with chicken type II collagen and develop strong and sustained T-cell responses to type II collagen. Arthritis was milder in C57BL/6 mice than DBA/1 mice and more closely resembled rheumatoid arthritis in its response to therapeutic intervention. Our findings show that C57BL/6 mice are susceptible to collagen-induced arthritis, providing a valuable model for assessing the role of specific genes involved in the induction and/or maintenance of arthritis and for evaluating the efficacy of novel drugs, particularly those targeted at T cells.

Analysing the effect of novel therapies on cytokine expression in experimental arthritis.

Type II collagen-induced arthritis (CIA) is an animal model of rheumatoid arthritis that has been used extensively to address questions of disease pathogenesis and to validate novel therapeutic targets. Susceptibility to CIA is strongly associated with major histocompatibility complex class II genes, and the development of arthritis is accompanied by a robust T- and B-cell response to type II collagen. The main pathological features of CIA include proliferative synovitis with infiltration of inflammatory cells, pannus formation, cartilage degradation, erosion of bone and fibrosis. Pro-inflammatory cytokines, such as tumour necrosis factor alpha and interleukin-1beta, are expressed in the arthritic joints in both murine CIA and human rheumatoid arthritis, and blockade of these molecules results in amelioration of disease. Hence, there is a great deal of interest in the development of small-molecular-weight inhibitors of pro-inflammatory cytokines. There is also interest in the development and testing of drugs with the capacity to modulate the immune pathways involved in driving the inflammatory response in arthritis. For these reasons, there is a need to monitor the effect of novel treatments on cytokine expression in vivo. In this review, we outline the various techniques used to detect cytokines in experimental arthritis and describe how these techniques have been used to quantify changes in cytokine expression following therapeutic intervention.

The differential contribution of tumour necrosis factor to thermal and mechanical hyperalgesia during chronic inflammation.

Therapies directed against tumour necrosis factor (TNF) are effective for the treatment of rheumatoid arthritis and reduce pain scores in this condition. In this study, we sought to explore mechanisms by which TNF contributes to inflammatory pain in an experimental model of arthritis. The effects of an anti-TNF agent, etanercept, on behavioural pain responses arising from rat monoarthritis induced by complete Freund's adjuvant were assessed and compared with expression of TNF receptors (TNFRs) by dorsal root ganglion (DRG) cells at corresponding time points. Etanercept had no effect on evoked pain responses in normal animals but exerted a differential effect on the thermal and mechanical hyperalgesia associated with rat arthritis induced by complete Freund's adjuvant (CFA). Joint inflammation was associated with increased TNFR1 and TNFR2 expression on DRG cells, which was maintained throughout the time course of the model. TNFR1 expression was increased in neuronal cells of the DRG bilaterally after arthritis induction. In contrast, TNFR2 expression occurred exclusively on non-neuronal cells of the macrophage-monocyte lineage, with cell numbers increasing in a TNF-dependent fashion during CFA-induced arthritis. A strong correlation was observed between numbers of macrophages and the development of mechanical hyperalgesia in CFA-induced arthritis. These results highlight the potential for TNF to play a vital role in inflammatory hyperalgesia, both by a direct action on neurons via TNFR1 and by facilitating the accumulation of macrophages in the DRG via a TNFR2-mediated pathway.

The role of inflammatory mediators on nociception and pain in arthritis.

Pain is the most common complaint of individuals with osteoarthritis but the cause of symptoms in this disorder remains unclear. Quantitative sensory testing reveals that in patients with chronic joint disease there is diffuse and persistent alteration of nociceptive (pain) pathways, irrespective of the level of activity of the underlying disease. Inflammatory mediators contribute to this plasticity either by directly activating high threshold receptors or more commonly by sensitizing nociceptive neurons to subsequent everyday stimuli. This involves early post-translational modification of receptors/ion channels and later, longer-lasting transcription-dependent mechanisms involving changes to the chemical phenotype of the neuron. Included amongst these changes are the increased production and release of various pro- and anti-inflammatory neuropeptides which have diverse actions on both circulating and resident cell populations. These neurally derived mediators act synergistically with cytokines and growth factors to contribute to ongoing tissue injury. It is becoming apparent that the interaction between a damaged joint and the sensory nervous system is far from straightforward and that activity arising from such interactions may produce not only pain but may also influence the subsequent course of the underlying disease.

Inhibition of inflammation and hyperalgesia in NK-1 receptor knock-out mice.

We sought to characterise the contribution of the neuropeptide substance P to the outcome of two models of footpad inflammation of differing severity. In an intense inflammatory model produced by intra-plantar Mycobacterium tuberculosus (10 mg/ml) substantial reductions in footpad swelling, histological outcome and mechanical hyperalgesia were observed from early time points in mice lacking the neurokin-1 receptor for substance P compared with wild-type controls. Conversely, in a less intense model (M. tuberculosus 1 mg/ml) no differences were observed other than for a reduction in mechanical hyperalgesia at later time points (day 9 onwards). The results point to a previously unrecognised influence of substance P on peripheral tissue injury and the maintenance of hyperalgesia during more severe or more chronic phases of inflammatory disease.