Relationship between neuropeptide immunoreactive nerves and inflammatory cells in adjuvant arthritic rats

CGRP and Painful Pathologies Other than Headache

CGRP has long been suspected as a mediator of arthritis pain, although evidence that CGRP directly mediates human musculoskeletal pain remains circumstantial. This chapter describes in depth the evidence surrounding CGRP's association with pain in musculoskeletal disorders and also summarises evidence for CGRP being a direct cause of pain in other conditions. CGRP-immunoreactive nerves are present in musculoskeletal tissues, and CGRP expression is altered in musculoskeletal pain. CGRP modulates musculoskeletal pain through actions both in the periphery and central nervous system. Human observational studies, research on animal arthritis models and the few reported randomised controlled trials in humans of treatments that target CGRP provide the context of CGRP as a possible pain biomarker or mediator in conditions other than migraine.

Environmental cold exposure increases blood flow and affects pain sensitivity in the knee joints of CFA-induced arthritic mice in a TRPA1-dependent manner

BACKGROUND

The effect of cold temperature on arthritis symptoms is unclear. The aim of this study was to investigate how environmental cold affects pain and blood flow in mono-arthritic mice, and examine a role for transient receptor potential ankyrin 1 (TRPA1), a ligand-gated cation channel that can act as a cold sensor.

METHODS

Mono-arthritis was induced by unilateral intra-articular injection of complete Freund's adjuvant (CFA) in CD1 mice, and in mice either lacking TRPA1 (TRPA1 KO) or respective wildtypes (WT). Two weeks later, nociception and joint blood flow were measured following exposure to 10 °C (1 h) or room temperature (RT). Primary mechanical hyperalgesia in the knee was measured by pressure application apparatus; secondary mechanical hyperalgesia by automated von Frey system; thermal hyperalgesia by Hargreaves technique, and weight bearing by the incapacitance test. Joint blood flow was recorded by full-field laser perfusion imager (FLPI) and using clearance of (99m)Technetium. Blood flow was assessed after pretreatment with antagonists of either TRPA1 (HC-030031), substance P neurokinin 1 (NK1) receptors (SR140333) or calcitonin gene-related peptide (CGRP) (CGRP8-37). TRPA1, TAC-1 and CGRP mRNA levels were examined in dorsal root ganglia, synovial membrane and patellar cartilage samples.

RESULTS

Cold exposure caused bilateral primary mechanical hyperalgesia 2 weeks after CFA injection, in a TRPA1-dependent manner. In animals maintained at RT, clearance techniques and FLPI showed that CFA-treated joints exhibited lower blood flow than saline-treated joints. In cold-exposed animals, this reduction in blood flow disappears, and increased blood flow in the CFA-treated joint is observed using FLPI. Cold-induced increased blood flow in CFA-treated joints was blocked by HC-030031 and not observed in TRPA1 KOs. Cold exposure increased TRPA1 mRNA levels in patellar cartilage, whilst reducing it in synovial membranes from CFA-treated joints.

CONCLUSIONS

We provide evidence that environmental cold exposure enhances pain and increases blood flow in a mono-arthritis model. These changes are dependent on TRPA1. Thus, TRPA1 may act locally within the joint to influence blood flow via sensory nerves, in addition to its established nociceptive actions.

Suppression of Peripheral Pain by Blockade of Voltage-Gated Calcium 2.2 Channels in Nociceptors Induces RANKL and Impairs Recovery From Inflammatory Arthritis in a Mouse Model

OBJECTIVE

A hallmark of rheumatoid arthritis (RA) is the chronic pain that accompanies inflammation and joint deformation. Patients with RA rate pain relief as the highest priority; however, few studies have addressed the efficacy and safety of therapies directed specifically toward pain pathways. The ω-conotoxin MVIIA (ziconotide) is used in humans to alleviate persistent pain syndromes, because it specifically blocks the voltage-gated calcium 2.2 (CaV 2.2) channel, which mediates the release of neurotransmitters and proinflammatory mediators from peripheral nociceptor nerve terminals. The aims of this study were to investigate whether blockade of CaV 2.2 can suppress arthritis pain, and to examine the progression of induced arthritis during persistent CaV 2.2 blockade.

METHODS

Transgenic mice expressing a membrane-tethered form of MVIIA under the control of a nociceptor-specific gene (MVIIA-transgenic mice) were used in the experiments. The mice were subjected to unilateral induction of joint inflammation using a combination of antigen and collagen.

RESULTS

CaV 2.2 blockade mediated by tethered MVIIA effectively suppressed arthritis-induced pain; however, in contrast to their wild-type littermates, which ultimately regained use of their injured joint as inflammation subsided, MVIIA-transgenic mice showed continued inflammation, with up-regulation of the osteoclast activator RANKL and concomitant joint and bone destruction.

CONCLUSION

Taken together, our results indicate that alleviation of peripheral pain by blockade of CaV 2.2- mediated calcium influx and signaling in nociceptor sensory neurons impairs recovery from induced arthritis and point to the potentially devastating effects of using CaV 2.2 channel blockers as analgesics during inflammation.

Effect of genetic deletion of the vanilloid receptor TRPV1 on the expression of Substance P in sensory neurons of mice with adjuvant-induced arthritis

The neuropeptide Substance P (SP), expressed by nociceptive sensory afferents in joints, plays an important role in the pathogenesis of arthritis. Capsaicin causes neurons in the dorsal root ganglia (DRG) to release SP from their central and peripheral axons, suggesting a functional link between SP and the capsaicin receptor, the transient receptor potential vanilloid 1 (TRPV1). The expression of both TRPV1 and SP have been reported to increase in several models of arthritis but the specific involvement of TRPV1-expressing articular afferents that can release SP is not completely understood. We here wanted to ascertain whether the increase in the number of SP-positive primary afferents in arthritis may be affected by genetic deletion of TRPV1. For this, we used immunohistochemistry to quantify the expression of SP in primary afferent neurons in wild-type mice (WT) vs. TRPV1-knockout (KO) mice with adjuvant-induced arthritis (AIA). We found that the expression of SP in DRG (1) increased significantly over naïve level in both WT and KO mice 3 weeks after AIA, (2) was significantly higher in KO mice than in WT mice in naïve mice and 2-3 weeks after AIA, (3) was significantly higher on the side of AIA than on the contralateral, vehicle-injected side at all time points in WT mice, but not in KO mice, and (4) increased predominantly in small-size neurons in KO mice and in small- and medium-size neurons in WT mice. Since the size distribution of SP-positive DRG neurons in arthritic TRPV1-KO mice was not significantly different from that in naïve mice, we speculate that the increased expression of SP is unlikely to reflect recruitment of A-fiber primary afferents and that the higher expression of SP in KO mice may represent a plastic change to compensate for the missing receptor in a major sensory circuit.

Sensory-nerve-derived neuropeptides: possible therapeutic targets

This review examines our developing understanding of the families and activities of some of the best known sensory-nerve-derived inflammatory neuropeptides, namely substance P, calcitonin gene-related peptide and galanin. Evidence to date shows involvement of these transmitters in a wide range of systems that includes roles as inflammatory modulators. There is an increasing understanding of the mechanisms involved in the release of the peptides from sensory nerves and these are key in understanding the potential of neuropeptides in modulating inflammatory responses and may also provide novel targets for anti-inflammatory therapy. The neuropeptides released act via specific G protein coupled receptors, most of which have now been cloned. There is knowledge of selective agonists and antagonists for many subtypes within these families. The study of neuropeptides in animal models has additionally revealed pathophysiological roles that in turn have led to the development of new drugs, based on selective receptor antagonism.

Autoimmune disease and innervation

In the decades before 1987, most of the research devoted to neuronal innervation was carried out in primary and secondary lymphoid organs at very different locations. This was an important period in order to understand hard-wiring of immune organs in physiology. Between 1988 and 1997, with the appearance of specific antibodies against neuronal markers, innervation was studied in inflamed tissue of patients and of animals with autoimmune diseases. This period clearly revealed that nerve fibers of, both, the sympathetic and sensory nervous system are altered, but only small amounts of tissue have been investigated by qualitative but not quantitative techniques. Between 1998 and 2007, with the understanding that sympathetic and sensory neurotransmitters might play opposite roles in inflammation, nerve fibers of the different nervous systems have been studied in parallel using quantitative techniques. These studies have been carried out in a large number of patients with long-standing autoimmune diseases. It turned out that sympathetic nerve fibers are lost in chronically inflamed tissue, while substance P-positive nerve fibers sprout into the inflamed area. This might be important because high concentrations of sympathetic neurotransmitters are antiinflammatory whereas substance P has a proinflammatory role. The first challenge for future research is the determination of innervation in the early human autoimmune disease. The second challenge is the identification of reasons for the differential loss of sympathetic in relation to sensory nerve fibers. It might well be that nerve repellent factors specific for the sympathetic nerve fiber might play an important role for the observed differential loss. Whether, or not, a therapy can be based on these findings remains to be established.

Levels of gastrin-releasing peptide and substance P in synovial fluid and serum correlate with levels of cytokines in rheumatoid arthritis

It is well known that cytokines are highly involved in the disease process of rheumatoid arthritis (RA). Recently, targeting of neuropeptides has been suggested to have potential therapeutic effects in RA. The aim of this study was to investigate possible interrelations between five neuropeptides (bombesin/gastrin-releasing peptide (BN/GRP), substance P (SP), vasoactive intestinal peptide, calcitonin-gene-related peptide, and neuropeptide Y) and the three cytokines tumour necrosis factor (TNF)-α, IL-6, and monocyte chemoattractant protein-1 in synovial fluid of patients with RA. We also investigated possible interrelations between these neuropeptides and soluble TNF receptor 1 in serum from RA patients. Synovial fluid and sera were collected and assayed with ELISA or RIA. The most interesting findings were correlations between BN/GRP and SP and the cytokines. Thus, in synovial fluid, the concentrations of BN/GRP and SP grouped together with IL-6, and SP also grouped together with TNF-α and monocyte chemoattractant protein-1. BN/GRP and SP concentrations in synovial fluid also grouped together with the erythrocyte sedimentation rate. In the sera, BN/GRP concentrations and soluble TNF receptor 1 concentrations were correlated. These results are of interest because blocking of SP effects has long been discussed in relation to RA treatment and because BN/GRP is known to have trophic and growth-promoting effects and to play a role in inflammation and wound healing. Furthermore, the observations strengthen a suggestion that combination treatment with agents interfering with neuropeptides and cytokines would be efficacious in the treatment of RA. In conclusion, BN/GRP and SP are involved together with cytokines in the neuroimmunomodulation that occurs in the arthritic joint.

Neurogenic Aspects of Inflammation

The relationship between the inflammatory process and the nervous system is twofold. The nervous system is activated by inflammation which causes inflammatory pain and impaired motor function. Conversely, the nervous system acts back on the peripheral process. This is achieved by output systems at different levels, including primary afferent fibers (neurogenic inflammation), spinal cord (reflexes), and the brain (eg, neuroendocrine functions). This article first addresses the activation of the nociceptive system by inflammation; the second part describes the effects of the nervous system on inflamed tissue.

A role for substance P in arthritis?

Substance P is a neuropeptide that is released from sensory nerves and which has a number of pro-inflammatory effects. In this article, we review the evidence for a role of substance P in arthritis, both in experimental animal models and rheumatoid arthritis patients. Substance P expression is altered in the joint and dorsal horn of arthritic animals, exogenous substance P and neurokinin 1 (NK(1)) receptor antagonists modulate responses in the joint, and there is some evidence for a role of substance P in human joint disease. However, the therapeutic potential of NK(1) receptor antagonists in the treatment of rheumatoid arthritis remains controversial.

Biochemical changes in arthritic rats: dehydroascorbic and ascorbic acid levels

The objective of this work was to evaluate the use of vitamin C as a biomarker in the inflammatory phase of the rat adjuvant arthritis and to correlate it with other parameters used for disease evaluation. Paw swelling was used for physical evaluation and the levels of ascorbate and dehydroascorbate in the serum of male rats, before and after adjuvant arthritis induction, were quantified by a high-performance liquid chromatography method (HPLC). The optimised HPLC assay enabled the quantification of both forms of the vitamin in rat sera, with the same extraction method and using different detectors, instead of obtaining dehydroascorbate by subtraction of the total ascorbate measurement. This method was used to follow the severity of adjuvant arthritis and the results were correlated with other already established disease activity parameters. A decrease of ascorbic acid and dehydroascorbic acid was observed with the increase of right paw circumference during the course of adjuvant arthritis. The disease associated changes in the serum concentrations of ascorbic acid, from biosynthesis and from recycling, can be evaluated by the direct quantification of dehydroascorbic acid. This provides some evidence for the potential of the quantification of these biomarkers to study the disease activity, and as a tool for the establishment of therapeutic protocols, to evaluate the anti-inflammatory effect of new drugs or formulations.

Immune regulation in adjuvant disease and other arthritis models: relevance to pathogenesis of chronic arthritis

Experimental models of arthritis and their human counterparts fall into three distinct classes: (a) responses of T cells to disseminated microbial antigens (Ags) as such; (b) responses of T cells to cartilage autoAgs; and (c) responses of T cells to major histocompatibility complex (HLA-B27, DRB1) or other membrane components (LFA-1) expressed on bone marrow-derived cells. The primary immune response is driven, in naturally occurring disease, by microbial infection, e.g. with streptococci, enteric gram-negative rods or spirochetes, or is experimentally induced with mycobacterial and other adjuvants. The response to cartilage components, such as collagen type-II and various proteoglycans, may be driven by cross-reactive microbial Ags, heat shock proteins (HSPs) in particular, or the adjuvant effect of intense primary joint inflammation, as in rheumatoid arthritis and the spondyloarthropathies. Adjuvant disease appears to be purely T-cell-mediated, whereas both T cells and antibody play a role in collagen and many other forms of arthritis. Experimental evidence suggests a pathogenetic role for T-cell receptor gammadelta T cells in some lesions. Arthritis may be regulated by microbial and tissue HSPs, when these are administered by a nonimmunizing route or as altered peptide ligands, by anti-idiotypic responses that block the action of effector T cells, and by competing Ags. Immune regulation involving natural killer (NK), NK T and certain subsets of gammadelta and alphabeta T cells, which may affect the occurrence, localization and character of this group of diseases, presents a challenge for further investigation.

Neurovascular plasticity in the knee joint of an arthritic mouse model

Lower numbers of neuropeptide-containing fibers in arthritic joints have been found as compared to control joints. This may be the result of fiber depletion, necrosis of fibers, or proliferation of soft tissues without neural sprouting. To discriminate between these possibilities, we studied the relationships between soft tissue proliferation, changes in vascularity of synovial tissues, and changes in joint innervation during arthritis. Arthritis was induced in the knee joint of mice by a single subpatellar injection of methylated bovine serum albumin after previous immunization. Antibodies to protein gene product 9.5, S-100, and growth-associated protein-43 (GAP-43) were used to study the general innervation pattern. Antibodies to calcitonin gene-related peptide (CGRP), vasointestinal polypeptide (VIP), substance P (SP), and tyrosine hydroxylase (TH) were used to localize sensory (SP, CGRP, VIP) and sympathetic (TH) fibers. Blood vessels of the joint were studied with ink perfusion, GAP-43, and a vascular marker (LF1). Directly after the induction of arthritis, the synovial cavity was enlarged and filled with leukocytes. From day 4 onward, small sprouting blood vessels penetrated the avascular mass of cells in the joint cavity. After 1 week, the vascular sprouting activity and GAP-43 immunoreactivity were maximal, and after 2 weeks, vascular sprouting activity diminished. In the subsequent period, the synovia slowly regained their prearthritic appearance and thickness. The most pronounced changes in the general staining pattern of CGRP, SP, VIP, and TH were found in the periosteum. From 2 days to 4 weeks after the induction of arthritis, the layer of SP, CGRP, and VIP fibers in the femoral periosteum was thicker and more irregular. GAP-43 staining showed many terminal varicosities, which suggested sprouting of nerve fibers. From 2 days to 2 weeks after the induction of arthritis, the SP and CGRP fibers in the periosteum showed gradual depletion. In the thickened subsynovial tissues that were revascularized, no ingrowth of neural elements was found. As the total number of nerve fibers in the synovial tissue did not change, large parts of the synovia directly facing the joint cavity were not innervated at 1 week after the induction of arthritis. These results strongly suggest that periosteal SP and CGRP fibers were depleted during arthritis. Synovial proliferation without concomitant fiber growth is the main cause of the reduced number of immunocytochemically detectable fibers in the mouse arthritic knee joint.

Autonomic neuropeptides in the interface membrane of aseptic loose hip prostheses

We analyzed the presence of autonomic nerve fibers in the interface membranes (n = 9) surrounding aseptic loosened hip prostheses by immunohistochemistry. The study focused on the autonomic messengers neuropeptide Y (NPY), tyrosine hydroxylase (TH), the rate-limiting enzyme in the synthesis of noradrenaline (NA), and vasoactive intestinal polypeptide (VIP). Protein gene product (PGP) 9.5, a general marker of peripheral nerve fibers, was also analyzed to establish the neuronal character of the immunoreactive structures. PGP 9.5-positive and NPY-positive nerve fibers were identified in all 9 samples, and VIP-immunoreactive and TH-immunoreactive fibers were found in 7. There was a difference in the distribution of nerve fibers both between and within the samples. Among the neuropeptides analyzed, NPY was most abundant. NPY-positive and TH-positive fibers were predominantly found around the blood vessel walls forming varicose nerve terminals. VIP-positive fibers were mainly observed as thin varicose nerve terminals with no relationship to blood vessels. Autonomic neuropeptides exert not only vasoactive and immunoregulatory effects, but also have been found to have direct effects on bone tissue. Moreover, the autonomic nervous system has been strongly implicated in nociception and inflammation. Neuronal NPY, TH, and VIP in the interface membrane may prove to contribute to the pathologic mechanisms leading to aseptic loosening of hip prostheses.

Interleukin-11 (IL-11) in aseptic loosening of total hip replacement (THR)

The chronic inflammatory response to abrasion particles from total hip replacement (THR) is believed to cause osteolysis and to contribute to prosthetic loosening. The expression of interleukin-11(IL-11) and its major cellular sources in the interface and pseudocapsular tissues obtained from total hip revisions performed for aseptic loosening were investigated. The avidin-biotin-peroxidase complex (ABC) and alkaline phosphatase-anti-alkaline phosphatase (APAAP) methods were used for staining and VIDAS image analysis for quantification. IL-11 was found in the interface and pseudocapsular tissues in the aseptic loosening of THR. IL-11 containing cells were more numerous in the interface (760 +/- 171 cells) and pseudocapsular tissues (684 +/- 171 cells) than in the control synovial tissue (235 +/- 68 cells). Because IL-11 is an important component of cytokine network mediating osteoblast-osteoclast communication in normal and pathological bone remodeling, the current findings suggest that IL-11 may contribute to periprosthetic osteolysis and to the loosening of THR.

Matrix metalloproteinase 13 (collagenase 3) in human rheumatoid synovium

OBJECTIVE

To show the eventual presence and extent of production of matrix metalloproteinase 13 (MMP-13, or collagenase 3) in rheumatoid synovial tissue samples and extracts, and to assess the inhibition characteristics of recombinant MMP-13.

METHODS

Immunohistochemical avidin-biotin-peroxidase complex staining/morphometry was used to analyze MMP-13-positive cells in situ. Neutral salt extraction of synovial tissue, electrophoresis of the extract in different buffer systems, and Western blotting were also used. The inhibitory properties of doxycycline, clodronate, pamidronate, and D-penicillamine for recombinant enzyme were determined with a soluble type II collagen assay.

RESULTS

MMP-13 was detected in fibroblast- and macrophage-like mononuclear cells in the synovial lining and stroma and in vascular endothelial cells. The overall expression of MMP-13 in these cells in the synovial stroma was high in rheumatoid arthritis (86 +/- 12%) compared with osteoarthritis (17 +/- 5%) patient samples (P = 0.0027). In a high-pH native electrophoresis gel, immunoreactivity to anti-MMP-1 and anti-MMP-13 were clearly separated, with anti-MMP-13-immunoreactive material migrating faster than anti-MMP-1-immunoreactive material. Finally, in contrast to MMP-1 and MMP-8, MMP-13 was found to be relatively resistant to the inhibitory effects of doxycycline and clodronate in vitro.

CONCLUSION

Due to its localization in synovial tissue, its substrate profile, increased expression, and relative resistance to known MMP inhibitors, MMP-13 is suggested to play a major role in the pathogenesis of tissue destruction in rheumatoid arthritis.

Microtopography of the autonomic nerves in the rat knee: a fluorescence microscopic study

BACKGROUND

The autonomic innervation of the joint is involved in different functions, such as sensory inputs, modulation of the function of immune cells, and trophic actions. To have a basis for further studies of the arthritic knee joint we have investigated the topographical distribution of different neuropeptides in knees of newborn and adult rats and in adult rats after arthritis induction.

METHODS

The distribution of the neuropeptides calcitonin gene-related peptide (CGRP), neurokinin A (NKA), substance P (SP), and neuropeptide Y (NPY) was analyzed using fluorescence immunohistochemistry. Samples were investigated after fixation by perfusion and decalcification by a special method which allows studies in bone tissue. Vascular structures were analyzed by scanning electron microscopy (SEM) of vascular resin casts.

RESULTS

In all tissues of the joint (synovial membrane, vessels, fibrous structures, bone, and cartilagineous tissues) CGRP and NKA are the most frequent neuropeptides. They are localized in free or perivascular fibers predominantly around arteries and arterioles. The NPY-ergic perivascular fibers even enter the vessel wall. Generally, SP-ergic fibers occur rarely. Free NKA- and CGRP-ergic nerve fibers spread out in the synovial lining layer reaching the synovial cavity and the outer layers of the articular and metaphyseal cartilage. In the cartilagineous tissue these nerves contact the chondrocytes. The density of NKA- and CGRP-immunoreactive fibers is lower in newborn rats than in adult rats. Six hours after arthritis induction SP-, NKA-, and CGRP-immunoreactivity is enhanced especially in perivascular fibers. The related vessels are dilated substantially.

CONCLUSIONS

The distribution pattern of the autonomic nerves found in this study might reflect the functions of these nerves: control of the microcirculation, sensory and even trophic functions. The new finding of CGRP- and NKA-ergic fibers in the outer layer of the cartilage can also have implications for the pathogenesis of rheumatoid arthritis.

Neural involvement in post-traumatic osteopenia: an experimental study in the rat

The effect of sciatic nerve resection on post-traumatic bone loss and mechanical strength of the ipsilateral (IL) and contralateral (CL) femoral shafts and necks was studied 25 days after a tibial fracture. We subjected 45 male rats to a standardized tibial fracture, stabilized it with a modular intramedullary nail and then randomly allocated the animals to two groups: right sciatic nerve resection (SNR group) or sham operation (sham group). All of the operated hindlimbs were further immobilized in a plaster cast to avoid unequal loadbearing between the two groups. After 25 days of healing, 85Sr incorporation in the IL femora was 10% lower in the SNR group compared to the sham group, indicating a lower bone mineralization after sciatic nerve resection. The bone mineral content was 15% higher in the SNR group ipsilaterally. Accordingly, the bending moment and energy absorption in the femoral midshaft were higher in the SNR group compared to the sham group. The sciatic nerve resection protected the femoral shaft against the normally occurring post-traumatic bone loss after a tibial fracture. This protective effect of the neurectomy also occurred in the femoral neck, but not to the same extent. A protective effect was also present in the CL femur, suggesting additional systemic effects of the sciatic nerve resection.

The neuronal regulation of fracture healing. Effects of sciatic nerve resection in rat tibia

The effect of sciatic nerve resection on tibial fracture healing was studied in rats 25 days post-trauma. To prevent differences in loading between sham-operated and nerve-resected animals the fractured limbs were cast-immobilized. On radiograms 8 of 11 fractures in the sham-operated animals showed very little callus formation in contrast to only 1 of 8 fractures in the group with nerve resection. Measured by single-photon absorptiometry, animals with sciatic nerve resection had a higher bone mineral content than the sham-operated animals. However, the mechanical strength in three-point cantilever bending was not better in the nerve-resected rats, implying a defective organization of the large callus. These results suggest neural regulation plays a role in the type of fracture healing, primary or secondary, and in the amount and quality of the callus.

The effect of intra-articular capsaicin on passive synovial anaphylaxis and blood flow in the rat knee joint

Normal rat and human synovium is innervated by small diameter, unmyelinated, peptide-containing nerves. A close anatomical association between these nerves and mast cells has been postulated23, although functional interactions have not been described. Capsaicin is frequently used to activate sensory nerves and we have examined both acute and long-term effects of capsaicin on passive synovial anaphylaxis (PSA) and blood flow in the rat knee joint. The acute injection of capsaicin into the synovial space (330 nmol, 30 min prior to antigen) significantly inhibited plasma extravasation into the joint tissues (measured by accumulation of [125I]-human serum albumin) following PSA, and produced vasoconstriction in normal joints (measured by 133Xe clearance). There was no effect on plasma extravasation when capsaicin was injected 3 h prior to antigen. Inhibition of the PSA response following acute intra-articular capsaicin was not reversed by pretreatment with the cyclo-oxygenase inhibitor indomethacin (to inhibit thromboxane generation) or in rats chronically treated with guanethidine (to deplete noradrenaline from post-ganglionic sympathetic fibres). Further, a longer term pre-treatment of the joints with a single intra-articular injection of capsaicin (3.3 mumol) also attenuated plasma extravasation following induction of PSA 7 days later, and was accompanied by a non-significant decrease in joint blood flow. Plasma extravasation in response to compound 48/80, a non-immunological mediator of mast-cell degranulation, was not affected in joints treated with capsaicin 7 days previously.(ABSTRACT TRUNCATED AT 250 WORDS)