Participation of NK1 receptors in nociceptin-induced modulation of rat knee joint mechanosensitivity

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.

Correlation between synovial fluid cytokine concentrations and limb function in normal dogs and in dogs with lameness from spontaneous osteoarthritis

OBJECTIVE

To determine the relationship between synovial biomarker concentrations and severity of lameness and to assess the ability to differentiate normal from osteoarthritic joints with synovial biomarker concentrations.

STUDY DESIGN

Prospective clinical study.

SAMPLE POPULATION

Twelve hounds with no evidence of osteoarthritis (OA) and 27 client-owned dogs with unilateral lameness and joint pain in a single joint from naturally occurring OA.

METHODS

Enrollment in the OA group required a history of lameness, radiographic evidence of OA on orthogonal joint radiographs, and ≥6% gait asymmetry between contralateral limbs. The concentrations of 14 synovial OA biomarkers in synovial samples obtained after gait analysis were measured with enzyme-linked immunosorbent assays and compared between normal and OA joints.

RESULTS

Concentrations of monocyte chemoattractant protein (MCP)-1, substance P, interleukin (IL)-6, IL-8, KC-like, matrix metalloproteinase (MMP)-1, and MMP-3 were greater (P ≤ .05) in OA than in normal joints. The concentrations of bradykinin and tissue inhibitors of metalloproteinase-4 were decreased in OA compared with normal joints. Monocyte chemoattractant protein 1 was identified as the most accurate marker to distinguish OA from normal joints. No correlation was detected between any OA biomarker concentration, individually or in combination, and severity of gait asymmetry at the walk.

CONCLUSION

Differences in proinflammatory and anti-inflammatory biomarkers were detected between OA and normal joints, but no relationship was identified between biomarker concentrations and gait asymmetry in dogs with OA.

CLINICAL IMPACT

This information will help guide future studies to elucidate how factors such as disease chronicity, severity, and etiology affect these relationships.

Endocannabinoids inhibit neurogenic inflammation in murine joints by a non-canonical cannabinoid receptor mechanism

Neurogenic inflammation is a local inflammatory response that is driven by the peripheral release of neuropeptides from small diameter afferents which occurs in many organs including joints. The knee joint has a rich endocannabinoid system which has been shown to decrease acute synovitis. The aim of this study was to investigate the influence of joint afferents on leukocyte-endothelial interactions within the synovial microcirculation of mice and determine the role of endocannabinoids on this inflammatory response. Electrical, antidromic stimulation of the saphenous nerve decreased leukocyte rolling at the lowest frequency tested (0.5Hz), while increasing leukocyte rolling at higher frequencies (2.0 and 5.0Hz). The leukocyte rolling effect of nerve stimulation was completely abolished by pre-treating the knee with the vasoactive intestinal peptide antagonist VIP_6-28; however, neither calcitonin gene related peptide nor substance P antagonism had an effect on this neurogenic inflammatory response. Treating knees with the endocannabinoid breakdown inhibitor URB597 completely blocked leukocyte rolling and this effect could be reversed with the non-canonical cannabinoid antagonist O-1918. These results provide evidence that antidromic stimulation of the mouse saphenous nerve promotes leukocyte rolling within the synovial microcirculation, and that endocannabinoids can attenuate this neurogenic inflammatory response.

Knee osteoarthritis pain following medial meniscectomy in the nonhuman primate

OBJECTIVE

A number of promising compounds developed for osteoarthritic pain have failed to demonstrate clinical efficacy. To enhance preclinical translational research for osteoarthritis, a model of knee osteoarthritis pain was developed in Macaca fascicularis and the effects of two distinct pharmacological classes of drugs were tested on pain-related behavior.

DESIGN

Behavioral assessments were developed specifically for the macaque. Baseline knee pressure threshold and weight bearing were assessed prior to a unilateral medial meniscectomy (MMx). Fifteen days following MMx, macaques underwent a once daily exercise regimen for 36 days. Sixty-seven days following MMx, macaques were assigned to one of three treatment groups (n = 3/group), either non-steroidal anti-inflammatory drug (NSAID) diclofenac, NK1 receptor antagonist aprepitant or vehicle, and treated for 5 days. Animals were tested 3-4 h after p.o. dosing and testing was performed blinded. Treatment utilized a crossover design-each animal received all treatments-and a 9-day washout period was utilized between treatments.

RESULTS

Vehicle-treated macaques consistently demonstrated decreased ipsilateral pressure threshold ("hyperalgesia") and decreased weight bearing. While diclofenac increased weight bearing and pressure threshold, full attenuation of pain was not obtained. No significant improvement of either knee pressure or weight bearing was observed with aprepitant.

CONCLUSIONS

Unilateral MMx in the macaque evoked pain-related behaviors and knee joint pathology reminiscent of osteoarthritis. The behavioral endpoints were sensitive to NSAID treatment but not sensitive to NK1 receptor block, which parallel clinical findings. The current macaque osteoarthritis model could be used to test potential treatments for osteoarthritis pain.

Role of capsaicin-sensitive nerves and tachykinins in mast cell tryptase-induced inflammation of murine knees

OBJECTIVE, DESIGN

Mast cell tryptase (MCT) is elevated in arthritic joints, but its direct effects are not known. Here, we investigated MCT-evoked acute inflammatory and nociceptive mechanisms with behavioural, in vivo imaging and immunological techniques.

MATERIAL AND SUBJECTS

Neurogenic inflammation involving capsaicin-sensitive afferents, transient receptor potential vanilloid 1 receptor (TRPV1), substance P (SP), neurokinin A (NKA) and their NK1 tachykinin receptor were studied using gene-deleted mice compared to C57Bl/6 wildtypes (n = 5-8/group).

TREATMENT

MCT was administered intraarticularly or topically (20 μl, 12 μg/ml). Capsaicin-sensitive afferents were defunctionalized with the TRPV1 agonist resiniferatoxin (RTX; 30-70-100 μg/kg s.c. pretreatment).

METHODS

Knee diameter was measured with a caliper, synovial perfusion with laser Doppler imaging, mechanonociception with aesthesiometry and weight distribution with incapacitance tester over 6 h. Cytokines and neuropeptides were determined with immunoassays.

RESULTS

MCT induced synovial vasodilatation, oedema, impaired weight distribution and mechanical hyperalgesia, but cytokine or neuropeptide levels were not altered at the 6-h timepoint. Hyperaemia was reduced in RTX-treated and TRPV1-deleted animals, and oedema was absent in NK1-deficient mice. Hyperalgesia was decreased in SP/NKA- and NK1-deficient mice, weight bearing impairment in RTX-pretreated, TRPV1- and NK1-deficient animals.

CONCLUSIONS

MCT evokes synovial hyperaemia, oedema, hyperalgesia and spontaneous pain. Capsaicin-sensitive afferents and TRPV1 receptors are essential for vasodilatation, while tachykinins mediate oedema and pain.

A commentary on modelling osteoarthritis pain in small animals

OBJECTIVE

To describe the currently used animal models for the study of osteoarthritis (OA) pain, with an emphasis on small animals (predominantly mice and rats).

OUTLINE

Narrative review summarizing the opportunities and limitations of the most commonly used small animal models for the study of pain and pain pathways associated with OA, and discussing currently used methods for pain assessment. Involvement of neural degeneration in OA is briefly discussed. A list of considerations when studying pain-related behaviours and pathways in animal models of OA is proposed.

CONCLUSIONS

Animal models offer great potential to unravel the complex pathophysiology of OA pain, its molecular and temporal regulation. They constitute a critical pathway for developing and testing disease-specific symptom-modifying therapeutic interventions. However, a number of issues remain to be resolved in order to standardize pre-clinical OA pain research and to optimize translation to clinical trials and patient therapies.

Activation of PAR(2) receptors sensitizes primary afferents and causes leukocyte rolling and adherence in the rat knee joint

BACKGROUND AND PURPOSE

The PAR(2) receptors are involved in chronic arthritis by mechanisms that are as yet unclear. Here, we examined PAR(2) activation in the rat knee joint.

EXPERIMENTAL APPROACH

PAR(2) in rat knee joint dorsal root ganglia (DRG) cells at L3-L5, retrogradely labelled with Fluoro-gold (FG) were demonstrated immunohistochemically. Electrophysiological recordings from knee joint nerve fibres in urethane anaesthetized Wistar rats assessed the effects of stimulating joint PAR(2) with its activating peptide, 2-furoyl-LIGRLO-NH(2) (1-100 nmol·100 μL(-1) , via close intra-arterial injection). Fibre firing rate was recorded during joint rotations before and 15 min after administration of PAR(2) activating peptide or control peptide. Leukocyte kinetics in the synovial vasculature upon PAR(2) activation were followed by intravital microscopy for 60 min after perfusion of 2-furoyl-LIGRLO-NH(2) or control peptide. Roles for transient receptor potential vanilloid-1 (TRPV1) or neurokinin-1 (NK(1) ) receptors in the PAR(2) responses were assessed using the selective antagonists, SB366791 and RP67580 respectively.

KEY RESULTS

PAR(2) were expressed in 59 ± 5% of FG-positive DRG cells; 100 nmol 2-furoyl-LIGRLO-NH(2) increased joint fibre firing rate during normal and noxious rotation, maximal at 3 min (normal; 110 ± 43%, noxious; 90 ± 31%). 2-Furoyl-LIGRLO-NH(2) also significantly increased leukocyte rolling and adhesion over 60 min. All these effects were blocked by pre-treatment with SB366791 and RP67580 (P < 0.05 compared with 2-furoyl-LIGRLO-NH(2) alone).

CONCLUSIONS AND IMPLICATIONS

PAR(2) receptors play an acute inflammatory role in the knee joint via TRPV1- and NK(1) -dependent mechanisms involving both PAR(2) -mediated neuronal sensitization and leukocyte trafficking.

Involvement of Nav 1.8 sodium ion channels in the transduction of mechanical pain in a rodent model of osteoarthritis

INTRODUCTION

A subgroup of voltage gated sodium channels including Nav1.8 are exclusively expressed on small diameter primary afferent neurons and are therefore believed to be integral to the neurotransmission of nociceptive pain. The present study examined whether local application of A-803467, a selective blocker of the Nav 1.8 sodium channel, can reduce nociceptive transmission from the joint in a rodent model of osteoarthritis (OA).

METHODS

OA-like changes were induced in male Wistar rats by an intra-articular injection of 3 mg sodium monoiodoacetate (MIA). Joint nociception was measured at day 14 by recording electrophysiologically from knee joint primary afferents in response to non-noxious and noxious rotation of the joint both before and following close intra-arterial injection of A-803467. The effect of Nav1.8 blockade on joint pain perception and secondary allodynia were determined in MIA treated animals by hindlimb incapacitance and von Frey hair algesiometry respectively.

RESULTS

A-803467 significantly reduced the firing rate of joint afferents during noxious rotation of the joint but had no effect during non-noxious rotation. In the pain studies, peripheral injection of A-803467 into OA knees attenuated hindlimb incapacitance and secondary allodynia.

CONCLUSIONS

These studies show for the first time that the Nav1.8 sodium channel is part of the molecular machinery involved in mechanotransduction of joint pain. Targeting the Nav1.8 sodium channel on joint nociceptors could therefore be useful for the treatment of OA pain, avoiding the unwanted side effects of non-selective nerve blocks.

Intra-articular administration of tachykinin NK₁ receptor antagonists reduces hyperalgesia and cartilage destruction in the inflammatory joint in rats with adjuvant-induced arthritis

Persistent pain associated with inflammatory arthritis is an aggravating factor that decreases patients' quality of life. Current therapies for joint pain have limited effectiveness and produce unwanted negative side effects. Although the involvement of substance P and its cognate tachykinin receptor, NK(1), in joint inflammation has been extensively documented through animal experiments, the development of oral tachykinin NK(1) receptor antagonists against arthritis-induced pain has been unsuccessful in humans to date. To explore the possibility of using tachykinin NK(1) receptor antagonists as local therapeutic agents for inflammatory arthritis, we examined the effects of tachykinin NK(1) receptor antagonists administered into the rat ankle joint on hyperalgesia in complete Freund's adjuvant (CFA)-induced inflammatory monoarthritis. Administration of the tachykinin NK(1) receptor antagonist WIN 51708 or GR 82334 into the affected ankle joint at day 3 following intra-articular CFA injection reduced the mechanical hyperalgesia 12 h after the tachykinin NK(1) receptor antagonist injection and their analgesic effects persisted for at least 2 days. Histological examinations revealed that intra-articular WIN 51708 reduced the CFA-induced destructive changes in the cartilage. These findings suggest that intra-articular injection of tachykinin NK(1) receptor antagonists is a promising strategy for relieving the hyperalgesia that occurs in inflammatory arthritis.

Cathepsin K inhibition reduces CTXII levels and joint pain in the guinea pig model of spontaneous osteoarthritis

Cathepsin K is a cysteine proteinase which is believed to contribute to osteoarthritis (OA) pathogenesis. This brief report evaluates the effect of the novel selective cathepsin K inhibitor AZ12606133 on cartilage metabolism in the Dunkin-Hartley guinea pig model of spontaneous OA. In parallel, electrophysiological studies were performed to determine whether acute and chronic treatment with the cathepsin K inhibitor could alter joint nociception. Acute treatment of OA knees with AZ12606133 had no effect on joint afferent nerve activity; however, prolonged (1 month) administration of the cathepsin K inhibitor delivered via a chronically implanted osmotic pump significantly reduced mechanosensitivity in response to both non-noxious and noxious joint movements. Urinal concentrations of the cartilage breakdown products cross-linked C-telopeptides of type II collagen (CTXII) were also reduced by chronic cathepsin K inhibition. These data suggest that prolonged AZ12606133 administration can reduce cartilage turnover and joint nociception in the Dunkin-Hartley guinea pig model of spontaneous OA.

Unravelling the relationship between age, nociception and joint destruction in naturally occurring osteoarthritis of Dunkin Hartley guinea pigs

Osteoarthritis (OA) is a debilitating and painful disease, the incidence of which increases with advancing age. One of the confounding aspects of OA is that there is a disconnect between the severity of joint degeneration and the intensity of pain reported. This study examined the relationship between age, joint nociception, and joint pathology in an animal model of naturally occurring OA. Dunkin Hartley guinea pigs were grouped according to age: young (2-5 months) and senescent (17-37 months). Joint nociception was objectively measured in these animals by recording electrophysiologically from knee joint primary afferents in response to non-noxious and noxious movements of the knee. Joint pathology in the same knees was then determined by histomorphology and micro-computerized tomography (micro-CT). A principal components analysis was carried out on the data to determine if any correlation exists between each of the measured variables. In aged guinea pigs, 33% of joint mechanosensory nerves were spontaneously active, whereas young animals showed no such neural activity at rest. The frequency of afferent firing evoked by noxious movements was greater in old guinea pigs. Micro-CT and histopathological determination of OA positively correlated with age; however, there was no significant correlation between the severity of joint degeneration and nociception. In the Dunkin Hartley model of inveterate OA, the level of joint pathology correlates well with increasing age. This study also provides the first objective evidence that there is no correlation between joint nociception and articular damage, thereby corroborating the clinical observation that pain is a poor predictor of OA severity.

Age alters the ability of substance P to sensitize joint nociceptors in guinea pigs

Pain perception is altered during senescence and it is thought that this could in part be due to changes in peripheral pain sensing processes. The present study examined the effect of substance P (SP) (10(-10)-10(-8) mol; 0.1 mL bolus close intraarterial) on knee joint afferent mechanosensitivity in young (2- to 5-mo-old) and aged (17- to 36-mo-old) Dunkin-Hartley guinea pigs. Single unit electrophysiological recordings were made from knee joint primary afferent nerves in response to normal (nonnoxious) and painful (noxious) rotation of the joint. In young and old animals, local application of SP had a sensitizing effect on joint afferents in response to movements made in the normal working range of the knee. With noxious hyper-rotation of the joint, SP was able to increase afferent firing rate in young but not in old animals. These data demonstrate a lack of SP-mediated sensitization of joint nociceptors during senescence and suggest a peripheral deficiency in joint nociception with respect to age.

Arthritis and pain. Neurogenic origin of joint pain

Arthritis pain affects millions of people worldwide yet we still have only a limited understanding of what makes our joints ache. This review examines the sensory innervation of diarthroidal joints and discusses the neurophysiological processes that lead to the generation of painful sensation. During inflammation, joint nerves become sensitized to mechanical stimuli through the actions of neuropeptides, eicosanoids, proteinase-activated receptors and ion channel ligands. The contribution of immunocytes to arthritis pain is also reviewed. Finally, the existence of an endogenous analgesic system in joints is considered and the reasons for its inability to control pain are postulated.

Electrophysiological evidence that the vasoactive intestinal peptide receptor antagonist VIP6-28 reduces nociception in an animal model of osteoarthritis

OBJECTIVE

The present study examined whether local administration of the neuropeptide vasoactive intestinal polypeptide (VIP) could modulate joint nociception in normal rat knee joints and if the VIP antagonist VIP(6-28) could ameliorate joint mechanosensitivity in an animal model of osteoarthritis (OA).

METHODS

OA was induced in male Wistar rats by intra-articular injection of 3mg sodium monoiodo-acetate with a recovery period of 14 days. Electrophysiological recordings were made from knee joint primary afferents in response to normal rotation and noxious hyper-rotation of the joint both before and following close intra-arterial injection of different doses of VIP and VIP(6-28).

RESULTS

Local application of VIP to normal knees caused afferent firing rate to be significantly enhanced during normal rotation (up to 180% P<0.01; n=17) and during hyper-rotation (up to 37% P<0.01; n=17) of the knee. VIP-induced sensitization was blocked by pre-administration of the VIP receptor antagonist VIP(6-28). In the OA group, application of VIP(6-28) caused afferent firing rate to be significantly reduced during normal rotation (up to 45% P<0.05; n=17) and during hyper-rotation (up to 34% P<0.01; n=15) of the knee joint.

CONCLUSION

These findings indicate that VIP is involved in peripheral sensitization of knee joint afferents especially in response to normal joint movements. OA-induced sensitization of knee joint afferents was inhibited by local administration of VIP(6-28), indicating that VIP is released into OA knee joints, potentially contributing to joint pain. As such, VIP(6-28) may prove to be a beneficial agent for the treatment of arthritis pain.

Stimulation of sensory neuropeptide release by nociceptin/orphanin FQ leads to hyperaemia in acutely inflamed rat knees

The peripheral effect of the 'opioid-like' peptide nociceptin/orphanin FQ (N/OFQ) on joint blood flow was investigated in acutely inflamed rats. Sensory neuropeptide release from capsaicin-sensitive nerves and the involvement of synovial mast cells and leukocytes on these vasomotor responses were also studied. Blood flow measurements of exposed knee joints were performed in urethane-anaesthetised rats (2 mg kg(-1) intraperitoneal) using laser Doppler perfusion imaging. Topical administration of N/OFQ (10(-13)-10(-8) mol) to acutely inflamed joints caused a dose-dependent increase in synovial perfusion with an ED(50) of 4.0 x 10(-10) mol. This vasodilatatory response was blocked by the selective NOP receptor antagonist [Phe(1)-(CH(2)-NH)-Gly(2)]-Nociceptin(1-13)-NH(2) (10(-9) mol) (P<0.0001).Co-administration of N/OFQ with the neurokinin-1 (NK(1)) receptor antagonist [D-Arg1,D-Phe5,D-Trp7,9,Leu11]-Substance P (10(-12) mol), the vasoactive intestinal peptide (VIP) receptor antagonist VIP(6-28) (10(-9) mol) or the calcitonin gene-related peptide (CGRP) receptor antagonist CGRP(8-37) (10(-9) mol) all blocked the hyperaemic effect of N/OFQ (P<0.0001). Treatment of acutely inflamed knees with capsaicin (8-methyl-N-vanillyl-6-noneamide) to destroy unmyelinated joint afferents also inhibited N/OFQ vasomotor activity. Stabilisation of synovial mast cells with disodium cromoglycate (cromolyn) ameliorated N/OFQ responses, whereas inactivation of circulating leukocytes with the pan-selectin inhibitor fucoidin completely blocked N/OFQ-induced hyperaemia in these joints. These experiments show that in acutely inflamed knee joints, N/OFQ acts on NOP receptors located on synovial mast cells and leukocytes leading to the secondary release of proinflammatory mediators into the joint. These agents subsequently stimulate sensory neuropeptide release from capsaicin-sensitive nerves culminating in vasodilatation and increased articular blood flow.

Nociceptin/orphanin FQ evokes knee joint pain in rats via a mast cell independent mechanism

Nociceptin/orphanin FQ (N/OFQ) is an opioid-like neuropeptide that has been shown to cause peripheral sensitization of knee joint afferents; however, the effect of the peptide on joint pain behaviour is unknown. In addition to having a direct effect on peripheral nerves, N/OFQ has also been shown to activate connective tissue mast cells causing the local release of potentially pain causing mediators. The present study tested the effect of peripherally administered N/OFQ on joint pain and examined whether synovial mast cells contribute to these responses. Hindlimb weight bearing and von Frey hair algesiometry were measured before and following a single injection of N/OFQ in the vicinity of the right knee of male Wistar rats. Compared to saline-treated controls, N/OFQ caused a conspicuous shift in hindlimb weight bearing in favour of the contralateral non-injected leg. Similarly, paw withdrawal threshold and latency were significantly reduced following N/OFQ administration indicative of secondary hyperalgesia. To test the involvement of synovial mast cells in these pain reactions, a separate group of rats were treated with the mast cell stabilizer cromolyn (20 mg/kg s.c.) 5 min prior to N/OFQ injection. Cromolyn treatment had no significant effect on N/OFQ-induced weight bearing deficit nor secondary hyperalgesic responses. In conclusion, these data support the premise that N/OFQ has a pain causing effect in the periphery which occurs independently of mast cell activation.

Vasoactive intestinal peptide (VIP) is a modulator of joint pain in a rat model of osteoarthritis

Osteoarthritis (OA) is a debilitating disease in which primarily weight-bearing joints undergo progressive degeneration. Despite the widespread prevalence of OA in the adult population, very little is known about the factors responsible for the generation and maintenance of OA pain. Vasoactive intestinal peptide (VIP) was identified in the synovial fluid of arthritis patients nearly 20 years ago and the aim of this study was to examine whether VIP could be involved in the generation of OA pain. Hindlimb weight bearing was used as a measure of joint pain, while von Frey hair algesiometry applied to the plantar surface of the ipsilateral hindpaw tested for secondary mechanical hyperalgesia. Intra-articular injection of VIP into normal rat knee joints caused a significant shift in weight bearing in favour of the contralateral non-injected hindlimb as well as causing a reduction in ipsilateral paw withdrawal threshold. These pain responses were blocked by co-administration of the VPAC receptor antagonist VIP6-28. Induction of OA by intra-articular sodium monoiodoacetate injection resulted in a reduction in weight bearing on the affected leg, but no evidence of secondary hyperalgesia in the paw. Treatment of OA knees with a single injection of VIP6-28 diminished hindlimb incapacitance while increasing paw withdrawal threshold. This study showed for the first time that peripheral application of VIP causes increased knee joint allodynia and secondary hyperalgesia. Furthermore, antagonists that inhibit VIP activity may prove beneficial in the alleviation of OA pain.

Chronic arthritis down-regulates peripheral mu-opioid receptor expression with concomitant loss of endomorphin 1 antinociception

OBJECTIVE

To determine whether peripheral administration of the endogenous mu-opioid peptide endomorphin 1 could reduce knee joint pain, using animal models of acute and chronic arthritis.

METHODS

Extracellular electrophysiologic recordings were made of rat knee joint primary afferent nerve activity in response to noxious hyperrotation of the joint. Neuronal activity was assessed before and following local injection of endomorphin 1. Comparisons were made between normal knees and knees with adjuvant-induced monarthritis, tested at 48 hours and 1 week posttreatment. Expression of mu-opioid receptors in the dorsal root ganglia ipsilateral to the chronically inflamed joints was determined by real-time polymerase chain reaction (PCR) and immunohistochemical analysis.

RESULTS

In normal knees, endomorphin 1 caused up to a 75% reduction in joint afferent nerve activity, which was blocked by the mu-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-amide. The inhibitory effect of endomorphin 1 was sustained in acutely inflamed knees. Conversely, in chronically inflamed joints, endomorphin 1 had no observable effect on the primary afferent nerve firing rate elicited by a noxious mechanical stimulus and, as such, was significantly different from the rate in normal joints. Immunohistochemical and real-time PCR analysis of the L3-L5 dorsal root ganglia ipsilateral to the chronic arthritis lesion revealed a reduction in mu-opioid receptor protein and gene expression compared with that in normal control animals.

CONCLUSION

Taken together, these results provide the first electrophysiologic evidence that selective activation of peripheral mu-opioid receptors reduces normal knee joint mechanosensitivity to a noxious stimulus. Furthermore, the analgesic effect of endomorphin 1 is lost during chronic inflammation due to down-regulation of mu-opioid receptor expression in afferent nerve cell bodies. These findings begin to explain the ambiguous efficacy of peripherally administered mu-opioid drugs in controlling chronic inflammatory joint pain.

Inhibitory effect of amiloride and gadolinium on fine afferent nerves in the rat knee: evidence of mechanogated ion channels in joints

Synovial joints are complex sensory organs which provide continuous feedback regarding position sense and degree of limb movement. The transduction mechanisms which convert mechanical forces acting on the joint into an electrochemical signal which can then be transmitted to the central nervous system are not well understood. The present investigation examined the effect of the mechanogated ion channel blockers amiloride and gadolinium on knee joint mechanosensitivity. In deeply anaesthetised rats (sodium thiopental: 100-120 mg/kg, i.p.), single unit extracellular recordings were made from knee joint group III (Adelta) and group IV (C) primary afferents in response to mechanical rotation of the joint. Afferent firing rate was measured before and after topical application of either amiloride (0.1 mM, 1 mM) or gadolinium (250 microM) onto the receptive field of the sensory unit and recording was continued every 10 min up to a total of 50 min. With normal rotation of the knee, joint mechanosensitivity was significantly reduced by both amiloride (P<0.0001; n=10-21) and gadolinium (P=0.001; n=12) and this effect was sustained throughout the recording period. This investigation provides the first in vivo electrophysiological evidence that joint mechanotransduction involves the activation of amiloride and gadolinium-sensitive mechanogated ion channels. Future studies to determine the mechanogated ion channel subtypes present in joints and the modulation of their gating properties during inflammation may yield novel approaches for the control of arthritis pain.

Endomorphin-1 causes synovial hypoaemia in rat knee joints via a capsaicin-sensitive neural pathway

In joints, synthetic mu-opioids reduce inflammatory changes such as protein extravasation and associated oedema formation. However, the effect of endogenous opioid peptides on other inflammatory processes such as altered tissue blood flow has not been investigated. The present study examined the peripheral effects of the endogenous mu-opioid ligand endomorphin-1 (EM-1) on rat knee joint blood flow using laser Doppler perfusion imaging. Topical application of EM-1 (10(-16)-10(-9) mol) to exposed rat knee joints resulted in a dose-dependent increase in synovial vascular resistance with a maximum rise of 56% occurring with the 10(-9) mol dose. Destruction of unmyelinated articular afferents by capsaicin treatment completely abolished the hypoaemic effects of EM-1. These findings suggest that EM-1 acts peripherally in knee joints to decrease synovial blood flow, and this hypoaemic response is dependent on the presence of capsaicin-sensitive nerves.

Involvement of sympathetic efferents but not capsaicin-sensitive afferents in nociceptin-mediated dual control of rat synovial blood flow

This study set out to examine the vasomotor effects of the opioid-like peptide nociceptin on knee joint capsular blood flow in urethane-anaesthetized rats. Topical application of nociceptin (10(-15)-10(-8) mol) caused a progressive fall in joint perfusion that was significantly inhibited by the specific nociceptin receptor antagonist [Phe(1)-(CH(2)-NH)-Gly(2)] Nociceptin(1-13)-NH(2) as well as the nonspecific opioid antagonist naloxone. To test whether this constrictor response was sympathetically mediated, we administered nociceptin in animals treated with guanethidine to produce sympathetic blockade or in the presence of the alpha-adrenoceptor antagonist phentolamine. Both guanethidine treatment and phentolamine coadministration attenuated the constrictor response to nociceptin. Inhibition of nociceptin-mediated vasoconstriction revealed a supplementary hyperemic response that persisted in animals whose knee joints were treated with 1% capsaicin to destroy the articular unmyelinated nerve supply. These results show that, in the rat knee, peripheral administration of nociceptin primarily causes a sympathetically mediated vasoconstriction. In addition, high-dose nociceptin produces a vasodilatatory response that is likely due to the direct action of nociceptin on vascular smooth muscle and not by a neurogenic mechanism.