Peripheral modulation of rat knee joint afferent mechanosensitivity by nociceptin/orphanin FQ

Analgesic effects of intraarterial injection of imipenem/cilastatin sodium in a rat model of knee osteoarthritis

Angiogenesis plays a role in the mechanism underlying musculoskeletal pain; thus, embolization of blood vessels may exert an analgesic effect. Recent clinical studies have reported promising therapeutic outcomes for arterial embolization in knee osteoarthritis (OA). However, the placebo effect in human studies cannot be ignored, underscoring the need for objective evidence to validate the analgesic effects. However, basic research data supporting this role are limited. Thus, we investigated the analgesic effect of intraarterial administration of imipenem/cilastatin sodium (IPM/CS) in a model of knee OA induced by monosodium iodoacetate (MIA) using Sprague-Dawley rats. First, we infused IPM/CS in the right femoral artery and investigated the knee joint mechanical pressure threshold using pressure application measurement (PAM). Next, the nociceptive signals originating from the knee were analyzed via the spontaneous excitatory postsynaptic current (sEPSC) recording within the neural cells in the dorsal spinal horn using the in vivo patch-clamp recording. In the model of knee OA, the mechanical thresholds at the damaged knee were decreased compared with those of the contralateral knee, whereas these thresholds remained stable in the sham group (p < 0.05). The pressure threshold of the model of knee OA was significantly increased following intraarterial infusion of IPM/CS but not saline (p < 0.05). A notable decrease in the average sEPSC frequency in the model of knee OA following intraarterial infusion of IPM/CS but not saline (p < 0.05). These results indicated that intraarterial infusion of IPM/CS attenuated nociception caused by knee OA.

Preclinical Assessment of Inflammatory Pain

While acute inflammation is a natural physiological response to tissue injury or infection, chronic inflammation is maladaptive and engenders a considerable amount of adverse pain. The chemical mediators responsible for tissue inflammation act on nociceptive nerve endings to lower neuronal excitation threshold and sensitize afferent firing rate leading to the development of allodynia and hyperalgesia, respectively. Animal models have aided in our understanding of the pathophysiological mechanisms responsible for the generation of chronic inflammatory pain and allowed us to identify and validate numerous analgesic drug candidates. Here we review some of the commonly used models of skin, joint, and gut inflammatory pain along with their relative benefits and limitations. In addition, we describe and discuss several behavioral and electrophysiological approaches used to assess the inflammatory pain in these preclinical models. Despite significant advances having been made in this area, a gap still exists between fundamental research and the implementation of these findings into a clinical setting. As such we need to characterize inherent pathophysiological pathways and develop new endpoints in these animal models to improve their predictive value of human inflammatory diseases in order to design safer and more effective analgesics.

Tapping into the endocannabinoid system to ameliorate acute inflammatory flares and associated pain in mouse knee joints

Introduction

During the progression of rheumatoid arthritis (RA), there are frequent but intermittent flares in which the joint becomes acutely inflamed and painful. Although a number of drug therapies are currently used to treat RA, their effectiveness is variable and side effects are common. Endocannabinoids have the potential to ameliorate joint pain and inflammation, but these beneficial effects are limited by their rapid degradation. One enzyme responsible for endocannabinoid breakdown is fatty acid amide hydrolase (FAAH). The present study examined whether URB597, a potent and selective FAAH inhibitor, could alter inflammation and pain in a mouse model of acute synovitis.

Methods

Acute joint inflammation was induced in male C57BL/6 mice by intra-articular injection of 2% kaolin/2% carrageenan. After 24 hr, articular leukocyte kinetics and blood flow were used as measures of inflammation, while hindlimb weight bearing and von Frey hair algesiometry were used as measures of joint pain. The effects of local URB597 administration were then determined in the presence or absence of either the cannabinoid (CB)1 receptor antagonist AM251, or the CB2 receptor antagonist AM630.

Results

URB597 decreased leukocyte rolling and adhesion, as well as inflammation-induced hyperaemia. However, these effects were only apparent at low doses and the effects of URB597 were absent at higher doses. In addition to the anti-inflammatory effects of URB597, fatty acid amide hydrolase (FAAH) inhibition improved both hindlimb weight bearing and von Frey hair withdrawal thresholds. The anti-inflammatory effects of URB597 on leukocyte rolling and vascular perfusion were blocked by both CB1 and CB2 antagonism, while the effect on leukocyte adherence was independent of cannabinoid receptor activation. The analgesic effects of URB597 were CB1 mediated.

Conclusions

These results suggest that the endocannabinoid system of the joint can be harnessed to decrease acute inflammatory reactions and the concomitant pain associated with these episodes.

Functional plasticity of the N/OFQ-NOP receptor system determines analgesic properties of NOP receptor agonists

Despite high sequence similarity between NOP (nociceptin/orphanin FQ opioid peptide) and opioid receptors, marked differences in endogenous ligand selectivity, signal transduction, phosphorylation, desensitization, internalization and trafficking have been identified; underscoring the evolutionary difference between NOP and opioid receptors. Activation of NOP receptors affects nociceptive transmission in a site-specific manner, with antinociceptive effects prevailing after peripheral and spinal activation, and pronociceptive effects after supraspinal activation in rodents. The net effect of systemically administered NOP receptor agonists on nociception is proposed to depend on the relative contribution of peripheral, spinal and supraspinal activation, and this may depend on experimental conditions. Functional expression and regulation of NOP receptors at peripheral and central sites of the nociceptive pathway exhibits a high degree of plasticity under conditions of neuropathic and inflammatory pain. In rodents, systemically administered NOP receptor agonists exerted antihypersensitive effects in models of neuropathic and inflammatory pain. However, they were largely ineffective in acute pain while concomitantly evoking severe motor side effects. In contrast, systemic administration of NOP receptor agonists to non-human primates (NHPs) exerted potent and efficacious antinociception in the absence of motor and sedative side effects. The reason for this species difference with respect to antinociceptive efficacy and tolerability is not clear. Moreover, co-activation of NOP and μ-opioid peptide (MOP) receptors synergistically produced antinociception in NHPs. Hence, both selective NOP receptor as well as NOP/MOP receptor agonists may hold potential for clinical use as analgesics effective in conditions of acute and chronic pain.

Useful animal models for the research of osteoarthritis

Osteoarthritis (OA) is a major cause of suffering for millions of people. Investigating the disease directly on humans may be challenging. The aim of the present study is to investigate the advantages and limitations of the animal models currently used in OA research. The animal models are divided into induced and spontaneous. Induced models are further subdivided into surgical and chemical models, according to the procedure used to induce OA. Surgical induction of OA is the most commonly used procedure, which alters the exerted strain on the joint and/or alter load bearing leading to instability of the joint and induction of OA. Chemical models are generated by intra-articular injection of modifying factors or by systemically administering noxious agents, such as quinolones. Spontaneous models include naturally occurring and genetic models. Naturally occurring OA is described in certain species, while genetic models are developed by gene manipulation. Overall, there is no single animal model that is ideal for studying degenerative OA. However, in the present review, an attempt is made to clarify the most appropriate use of each model.

The pronociceptive effect of proteinase-activated receptor-4 stimulation in rat knee joints is dependent on mast cell activation

Proteinase-activated receptor-4 (PAR(4)) is a G-protein-coupled receptor activated by serine proteinases released during tissue repair and inflammation. We have previously shown that PAR(4) activation sensitises articular primary afferents leading to joint pain. This study examined whether mast cells contribute to this PAR(4)-induced sensitisation and consequent heightened pain behaviour. The expression of PAR(4) on synovial mast cells was confirmed with immunofluorescent staining of rat knee joint sections. Electrophysiological recordings were made from joint primary afferents in male Wistar rats during both nonnoxious and noxious rotations of the knee. Afferent firing rate was recorded for 15 minutes after close intra-arterial injection of 10(-9) to 10(-5)mol of the PAR(4) activating peptide, AYPGKF-NH(2), or the inactive peptide, YAPGKF-NH(2) (100-μl bolus). Rats were either naive or pretreated with the mast cell stabilise, cromolyn (20mg/kg). Mechanical withdrawal thresholds were determined using a dynamic planter aesthesiometer and weight bearing determined using an incapacitance tester. These behavioural measurements were taken before and after intra-articular AYPGKF-NH(2), or the inactive peptide, YAPGKF-NH(2) (100μg). Local administration of AYPGKF-NH(2) caused a significant increase in joint primary afferent firing rate and pain behaviour compared with the control peptide YAPGKF-NH(2). These effects were blocked by pretreatment with cromolyn. These data reveal that PAR(4) is expressed on synovial mast cells and the activation of PAR(4) has a pronociceptive effect that is dependent on mast cell activation. Proteinase-activated receptor-4 is expressed on synovial mast cells, and the activation of Proteinase-activated receptor-4 has a pronociceptive effect that is dependent on mast cell activation.

The Rat Grimace Scale: a partially automated method for quantifying pain in the laboratory rat via facial expressions

We recently demonstrated the utility of quantifying spontaneous pain in mice via the blinded coding of facial expressions. As the majority of preclinical pain research is in fact performed in the laboratory rat, we attempted to modify the scale for use in this species. We present herein the Rat Grimace Scale, and show its reliability, accuracy, and ability to quantify the time course of spontaneous pain in the intraplantar complete Freund's adjuvant, intraarticular kaolin-carrageenan, and laparotomy (post-operative pain) assays. The scale's ability to demonstrate the dose-dependent analgesic efficacy of morphine is also shown. In addition, we have developed software, Rodent Face Finder^®, which successfully automates the most labor-intensive step in the process. Given the known mechanistic dissociations between spontaneous and evoked pain, and the primacy of the former as a clinical problem, we believe that widespread adoption of spontaneous pain measures such as the Rat Grimace Scale might lead to more successful translation of basic science findings into clinical application.

Paradoxical effects of the cannabinoid CB2 receptor agonist GW405833 on rat osteoarthritic knee joint pain

OBJECTIVE

The present study examined whether local administration of the cannabinoid-2 (CB(2)) receptor agonist GW405833 could modulate joint nociception in control rat knee joints and in an animal model of osteoarthritis (OA).

METHOD

OA was induced in male Wistar rats by intra-articular injection of sodium monoiodo-acetate with a recovery period of 14 days. Immunohistochemistry was used to evaluate the expression of CB(2) and transient receptor potential vanilloid channel-1 (TRPV1) receptors in the dorsal root ganglion (DRG) and synovial membrane of sham- and sodium mono-iodoacetate (MIA)-treated animals. Electrophysiological recordings were made from knee joint primary afferents in response to rotation of the joint both before and following close intra-arterial injection of different doses of GW405833. The effect of intra-articular GW405833 on joint pain perception was determined by hindlimb incapacitance. An in vitro neuronal release assay was used to see if GW405833 caused release of an inflammatory neuropeptide (calcitonin gene-related peptide - CGRP).

RESULTS

CB(2) and TRPV1 receptors were co-localized in DRG neurons and synoviocytes in both sham- and MIA-treated animals. Local application of the GW405833 significantly reduced joint afferent firing rate by up to 31% in control knees. In OA knee joints, however, GW405833 had a pronounced sensitising effect on joint mechanoreceptors. Co-administration of GW405833 with the CB(2) receptor antagonist AM630 or pre-administration of the TRPV1 ion channel antagonist SB366791 attenuated the sensitising effect of GW405833. In the pain studies, intra-articular injection of GW405833 into OA knees augmented hindlimb incapacitance, but had no effect on pain behaviour in saline-injected control joints. GW405833 evoked increased CGRP release via a TRPV1 channel-dependent mechanism.

CONCLUSION

These data indicate that GW405833 reduces the mechanosensitivity of afferent nerve fibres in control joints but causes nociceptive responses in OA joints. The observed pro-nociceptive effect of GW405833 appears to involve TRPV1 receptors.

Proteinase-Activated Receptor-4 (PAR4) Activation Leads to Sensitization of Rat Joint Primary Afferents Via a Bradykinin B2 Receptor-Dependent Mechanism

The G-protein-linked receptor, proteinase-activated receptor-4 (PAR4) is activated by proteinases released into the joint during inflammation. It is unclear whether PAR4 has a pro- or anti-nociceptive effect and whether it directly affects nerve activity. In this study, we examined the expression of PAR4 in joints and dorsal root ganglion (DRG) neurons and whether activation of PAR4 has an effect on nociception in normal rat knee joints. Electrophysiological recordings were made from joint primary afferents in male Wistar rats during both nonnoxious and noxious rotations of the knee. Afferent firing rate was recorded for 15 min post close intra-arterial injection of 10−9–10−5 mol of the PAR4 activating peptide, AYPGKF-NH2, or the inactive peptide, YAPGKF-NH2 (100 μl bolus). Rats were either naive or pretreated with the selective PAR4 antagonist, pepducin P4pal-10, the transient receptor potential vanilloid-1 (TRPV1) antagonist, SB366791, or the bradykinin B2 receptor antagonist, HOE140. Immunofluorescence experiments showed extensive PAR4 expression in the knee joint and in sensory neurons projecting from the joint. AYPGKF-NH2 significantly increased joint afferent firing during nonnoxious and noxious rotation of the knee. The inactive control peptide, YAPGKF-NH2 was without effect. Systemic pretreatment with the PAR4 antagonist, pepducin P4pal-10, inhibited the AYPGKF-NH2-induced increase in firing rate. Pretreatment with HOE140, but not SB366791, also blocked this increase in firing rate. These data reveal that in normal rat knee joints, PAR4 activation increases joint primary afferent activity in response to mechanical stimuli. This PAR4-induced sensitization is TRPV1-independent but involves B2 receptor activation, suggesting a role for kinins in this process.

Grading of monosodium iodoacetate-induced osteoarthritis reveals a concentration-dependent sensitization of nociceptors in the knee joint of the rat

Osteoarthritis (OA) is a degenerative joint disease characterized by joint pain for which there is currently no effective treatment. Previous studies have found that intra-articular injection of monosodium iodoacetate (MIA) caused a dose-dependent destruction of rat knees with concomitant increased pain. In this study, varying degrees of OA were induced by intra-articular injection of 0.1 mg, 0.3 mg and 3 mg MIA. Electrophysiological recordings were made from knee joint primary afferents in response to rotation of the joint and firing frequencies were determined and compared to saline-injected control joints. The analgesic effect of local application of the classic non-steroidal anti-inflammatory drug (NSAID) diclofenac (0.1 mg/0.1 ml bolus) was also determined in each group. Joint afferent firing frequency was significantly enhanced in OA knees compared to saline injected control joints and the magnitude of this sensitization showed a direct relationship with increasing dose of MIA. Diclofenac reduced nociception significantly in the 3 mg MIA treated joint, but had no effect on nerve mechanosensitivity in rats with milder OA. This study shows for the first time that MIA produces a graded sensitization of joint nociceptors making this a useful model for the study of pain mechanisms in joints with progressive OA severity. The anti-nociceptive effect of diclofenac further indicates that the MIA model offers an attractive means of objectively testing potential therapeutic agents.

Cannabinoid-mediated antinociception is enhanced in rat osteoarthritic knees

OBJECTIVE

To determine whether local administration of the cannabinoid 1 (CB(1)) receptor agonist arachidonyl-2-chloroethylamide (ACEA) can modulate joint nociception in control rat knee joints and in experimental osteoarthritis (OA).

METHODS

OA was induced in male Wistar rats by intraarticular injection of 3 mg of sodium mono-iodoacetate, with a recovery period of 14 days. Electrophysiologic recordings were made of knee joint primary afferent nerve fibers in response to normal rotation and noxious hyperrotation of the joint both before and after close intraarterial injection of different doses of ACEA.

RESULTS

Local application of the CB(1) agonist significantly reduced the firing rate of afferent nerve fibers by up to 50% in control knee joints (n=19) and up to 62% in OA knee joints (n=29; P<0.01). Coadministration of the CB(1) receptor antagonist AM251 or the transient receptor potential vanilloid 1 (TRPV-1) ion channel antagonist SB366791 significantly reduced the desensitizing effect of ACEA. The CB(1) receptor antagonist AM251 by itself had no effect in the control joint but significantly increased the firing rate of afferent nerve fibers in the OA joint.

CONCLUSION

These findings indicate that activation of peripheral CB(1) receptors reduces the mechanosensitivity of afferent nerve fibers in control and OA knee joints. Blockade of either the CB(1) receptor or the TRPV-1 channel significantly reduced the efficacy of ACEA, which suggests that both receptors are involved in cannabinoid-mediated antinociception. The increased nerve activity observed following CB(1) receptor antagonism suggests a tonic release of endocannabinoids during OA. As such, peripheral CB(1) receptors may be important targets in controlling OA pain.

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.

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.

Bi-directional modulation of 5-hydroxytryptamine-induced plasma extravasation in the rat knee joint by nociceptin

The role of nociceptin, the endogenous ligand for the opioid receptor-like (ORL1) receptor, in nociceptive processing is controversial. Most studies demonstrate hyperalgesia following supraspinal administration, analgesia following intrathecal and peripheral administration at higher doses, and hyperalgesia following intrathecal and peripheral application at lower doses. The present study investigates the effect of nociceptin on synovial plasma extravasation and its ability to modulate 5-hydroxytryptamine-induced synovial plasma extravasation using the rat knee joint model of inflammation. Nociceptin alone does not alter synovial plasma extravasation from baseline. Nociceptin at concentrations up to 1 nM enhances 5-hydroxytryptamine-induced synovial plasma extravasation (up to 50%) and nociceptin at concentrations above 100 nM inhibits 5-hydroxytryptamine-induced synovial plasma extravasation (down to 45%). The novel, selective ORL1 receptor antagonist J-113397 potently inhibits the pro-inflammatory effect of nociceptin, but only partly inhibits, at higher concentrations, the anti-inflammatory effects of nociceptin.These findings demonstrate a dose-dependent bi-directional effect of nociceptin on inflammatory processes and may indicate a target for novel therapeutics.