Human neutrophils as a source of nociceptin: a novel link between pain and inflammation

The orphanin FQ/nociceptin (OFQ/N) system

Orphanin FQ/nociceptin (OFQ/N) was the first novel neuropeptide discovered as the natural ligand of an orphan G protein-coupled receptor (GPCR). Orphan GPCRs are proteins classified as receptors on the basis of their sequence similarities to known GPCRs but that lack the ligands that activate them in vivo. One such orphan GPCR exhibited sequence similarities with the opioid receptors. OFQ/N was isolated as its natural ligand and shown to also share sequence similarities to the opioid peptides. This led to numerous studies attempting to find functional similarities and differences between the OFQ/N and opioid systems. This chapter will summarize our knowledge of the OFQ/N system and of its roles in the organism.

Opioid and nociceptin receptors regulate cytokine and cytokine receptor expression

Opioids were originally discovered because of their ability to induce analgesia, but further investigation has shown that the opioids regulate the function of cells involved in the immune response. We suggest that the regulation of cytokine, chemokine, and cytokine receptor expression is a critical component of the immunomodulatory activity of the opioids. In this paper we review the literature dealing with the regulation of cytokine and cytokine receptor expression by agonists for the three major opioid receptor types (mu, kappa, and delta), and nociceptin, the natural agonist for the orphanin FQ/nociceptin receptor. Although the opioid receptors share a high degree of sequence homology, opposing roles between the kappa opioid receptor (KOR) and the mu opioid receptor (MOR) have become apparent. We suggest that activation of the KOR induces an anti-inflammatory response through the down-regulation of cytokine, chemokine and chemokine receptor expression, while activation of the MOR favors a pro-inflammatory response. Investigation into the opioid receptor-like (ORL1)/nociceptin system also suggests a role for this receptor as a down-regulator of immune function. These effects suggest a broad role for opioids in the modulation of the function of the immune system, and suggest possible targets for the development of new therapeutics for inflammatory and infectious diseases.

Regulation of nociceptin/orphaninFQ secretion by immune cells and functional modulation of interleukin-2

Previous research has demonstrated that numerous populations of immune cell, including lymphocytes, synthesize nociceptin (N/OFQ) precursor mRNA although little is known regarding the immunological role of N/OFQ. In the present study we have demonstrated significant effects of mitogens, pro-inflammatory cytokines, cyclic AMP analogues, glucocorticoids and CRF on N/OFQ secretion by rat splenocytes in vitro. N/OFQ (10(-14) to 10(-10)M) was also shown to inhibit proliferation of Con A-activated splenocytes and production of IL-2 in vitro. In summary we have shown how a variety of stimuli relevant to inflammation can regulate endogenous N/OFQ secretion by splenocytes in vitro. We also suggest that N/OFQ may promote anti-inflammatory actions via suppression of IL-2 in vivo.

Proinflammatory and vasodilator effects of nociceptin/orphanin FQ in the rat mesenteric microcirculation are mediated by histamine

Nociceptin/orphanin FQ (N/OFQ) is the endogenous ligand for the N/OFQ peptide receptor (NOP). N/OFQ causes hypotension and vasodilation, and we aimed to determine the role of histamine in inflammatory microvascular responses to N/OFQ. Male Wistar rats (220-300 g, n = 72) were anesthetized with thiopental (30 mg/kg bolus, 40-90 mg x kg(-1) x h(-1) iv), and the mesentery was prepared for fluorescent intravital microscopy using fluorescein isothiocyanate-conjugated BSA (FITC-BSA, 0.25 ml/100 g iv) or 1 microm fluorescently labeled microspheres. N/OFQ (0.6-60 nmol/kg iv) caused hypotension (SAP, baseline: 154 +/- 11 mmHg, 15 nmol/kg N/OFQ: 112 +/- 10 mmHg, P = 0.009), vasodilation (venules: 23.9 +/- 1.2 microm, 26.7 +/- 1.2 microm, P = 0.006), macromolecular leak (interstitial gray level FITC-BSA: 103.7 +/- 3.4, 123.5 +/- 11.8, P = 0.009), and leukocyte adhesion (2.0 +/- 0.9, 15.2 +/- 0.9/100 microm, P = 0.036). Microsphere velocity also decreased (venules: 1,230 +/- 370 microm/s, P = 0.037), but there were no significant changes in blood flow. Flow cytometry measured a concurrent increase in neutrophil expression of cd11b with N/OFQ vs. controls (Geo mean fluorescence: 4.19 +/- 0.13 vs. 2.06 +/- 0.38, P < 0.05). The NOP antagonist [Nphe(1),Arg(14),Lys(15)]N/OFQ-NH(2) (UFP-101; 60 and 150 nmol/kg iv), H(1) and H(2)antagonists pyrilamine (mepyramine, 1 mg/kg iv) and ranitidine (1 mg/kg iv), and mast cell stabilizer cromolyn (1 mg x kg(-1) x min(-1)) also abolished vasodilation and macromolecular leak to N/OFQ in vivo (P < 0.05), but did not affect hypotension. Isolated mesenteric arteries (approximately 200 microm, n = 25) preconstricted with U-46619 were also mounted on a pressure myograph (60 mmHg), and both intraluminally and extraluminally administered N/OFQ (10(-5) M) caused dilation, inhibited by pyrilamine in the extraluminal but not the intraluminal (control: -6.9 +/- 3.8%; N/OFQ: 32.6 +/- 8.4%; pyrilamine: 31.5 +/- 6.8%, n = 18, P < 0.05) experiments. We conclude that, in vivo, mesenteric microvascular dilation and macromolecular leak occur via N/OFQ-NOP-mediated release of histamine from mast cells. Therefore, N/OFQ-NOP has an important role in microvascular inflammation, and this may be targeted during disease, particularly as we have proven that UFP-101 is an effective antagonist of microvascular responses in vivo.

Human peripheral blood mononuclear cells express nociceptin/orphanin FQ, but not mu, delta, or kappa opioid receptors

BACKGROUND

Expression of opioid receptors on peripheral blood mononuclear cells (PBMC) is controversial. These receptors are currently classified as classical (MOP/mu/mu, DOP/delta/delta and KOP/kappa/kappa) and nonclassical NOP (nociceptin/orphanin FQ; N/OFQ).

METHODS

In this volunteer study we probed for the expression of both classical and nonclassical opioid receptors using 1) radioligand binding, 2) specific antibody binding, and 3) polymerase chain reaction-based experimental paradigms.

RESULTS

Membranes prepared from PBMC from healthy volunteers did not bind either [3H]diprenorphine (a nonselective radioligand for classical opioid receptors) or [3H]N/OFQ. There was significant concentration-dependent binding of each radioligand to control tissues expressing recombinant MOP and NOP. In addition, using fluorescence-activated cell sorting paradigms, there was no binding of fluorescent naloxone or either of two MOP antibodies to whole PBMC, though fluorescent naloxone did bind to recombinant MOP (as a positive control). Using primers specific for classical and nonclassical opioid receptors, and RNA extracted from the PBMC of 10 healthy volunteers, we were also unable to detect MOP, DOP, and KOP transcripts. In contrast, NOP was detected in all samples.

CONCLUSIONS

Despite using several complementary experimental strategies, we failed to demonstrate protein for classical or nonclassical opioid receptors on PBMC from healthy volunteers. We detected NOP mRNA, suggesting low-density NOP expression on these immunocytes. It is possible that N/OFQ, produced by the PBMC itself, may be involved in the control of immune function.

Levels of neuropeptides nocistatin, nociceptin/orphanin FQ and their precursor protein in a rat neuropathic pain model

Neuropeptides nociceptin/orphanin FQ (N/OFQ) and nocistatin (NST) are related to pain modulation. The amounts of these peptides and their precursor protein, prepronociceptin (ppN/OFQ) in the brain, spinal cord and serum samples of rats with partial sciatic nerve ligation (PSNL) were compared with those in naïve rats using radioimmunoassay (RIA). There was a significant rise in the levels of ppN/OFQ, N/OFQ and NST in the brains of PSNL rats. Their spinal cords showed significantly increased ppN/OFQ and NST levels but no change in N/OFQ levels. The PSNL rats also had increased serum NST (statistically significant) and N/OFQ (statistically insignificant) with decreased ppN/OFQ suggesting important roles of these peptides in neuropathic pain mechanism.

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.

[Expression und function of the ORL-1 receptor on human leukocytes]

BACKGROUND

The ORL-1 receptor is expressed by human leukocytes. Limited knowledge exists about the function and interaction between the nervous and immune systems. The aim of our study was to investigate the expression of the nociceptin-ORL-1 receptor system on different leukocyte subsets and the influence of the ORL-1 receptor on the intracellular production of cytokines.

METHODS

Blood from healthy volunteers of different age and sex was analysed for the expression of the ORL-1 receptor by PCR and flow cytometry and the influence of nociceptin on the LPS-induced production of intracellular cytokines by flow cytometry.

RESULTS

The ORL-1 receptor mRNA is expressed by granulocytes, lymphocytes and monocytes. We could also show the expression of the ORL-1 receptor protein on the cell surface of all types of white blood cells. Nociceptin has no influence on LPS-induced cytokine production in human monocytes. There was neither a difference between young and old nor between male or female volunteers.

CONCLUSION

The ORL-1 receptor is expressed by all subtypes of leukocytes. The function of this receptor is not the modulation of cytokine production and requires further studies.

Nociceptin and its receptor in rat dorsal root ganglion neurons in neuropathic and inflammatory pain models: implications on pain processing

Nociceptin (NC), by activating its receptor, the opioid receptor-like 1 (ORL1) receptor, exerts an effect on a number of functions in the nervous system including locomotion, learning and memory, and processing of pain signals. Data on the expression of NC and ORL1 receptor in dorsal root ganglion (DRG) neurons and on its modulation after nerve injury and inflammation are controversial. We therefore sought to investigate the immunoreactivity (IR) of NC and ORL1 receptor in DRG neurons in two pain models, a pure neuropathic pain model, namely partial sciatic nerve transection (PST), and an inflammatory pain model, complete Freund's adjuvant (CFA) injection into the hindpaw. In intact DRG neurons, both NC and ORL1 receptor IR were present in mainly small- and medium-sized neurons, NC IR in 31% and ORL1 receptor IR in 33% of all neuronal profiles. Both NC and ORL1 receptor IR were upregulated 7 days after nerve injury (to 56 and 55%) and inflammation (to 53 and 48%), respectively. Activating transcription factor 3 (ATF3), a neuronal marker of nerve injury, was induced in DRG neurons 7 and 14 days after PST and 7 days after CFA injection. Double labeling with ATF3 revealed expression of NC and ORL1 receptor in intact as well as in injured primary afferent neurons. Thus, NC and the ORL1 receptor may be involved in the modulation of neuropathic and inflammatory pain at the level of the primary afferent neuron.

Low utility of plasma Nociceptin/orphanin FQ in the diagnosis of hepatocellular carcinoma

AIM

The utility of serum alpha-fetoprotein (alpha-FP) in the detection of hepatocellular carcinoma (HCC) is questionable. Very high circulating levels of nociceptin/orphanin FQ (N/OFQ), a ligand for a novel opioid receptor, have recently been reported in HCC. The aim of this study was to assess the role of plasma N/OFQ in the diagnosis of HCC arising in patients with liver cirrhosis.

METHODS

Plasma N/OFQ levels were measured by ELISA in 58 patients (28 HCC and 30 liver cirrhosis) and in 25 healthy controls. The values were correlated with clinical and laboratory features including alpha-FP. Spearman index, biserial correlation coefficient, non parametric combination (NPC) test and discriminant stepwise analysis were used for statistical evaluation of data.

RESULTS

The upper normal limit of nociceptin was 122 pg/mL. Plasma levels above this cut-off were found in 21.4% of patients with HCC, in 23.3% of those with cirrhosis and in 8% of healthy subjects. alpha-FP serum levels > 200 ng/mL were found in 46.4% of the patients with HCC and in none of those with cirrhosis. No correlation was found between N/OFQ levels and any of the clinical and laboratory features, including alpha-FP. By NPC test, HCC and cirrhotic patients were different with regard to alpha-FP (P = 0.000) but not in terms of nociceptin (P = 0.595). By point biserial correlation, HCC presence was positively correlated with alpha-FP (rpb = 0.52, P = 0.000) but not with N/OFQ (rpb = 0.16, P = 0.157). In a discriminant analysis, alpha-FP was significant in the Wilks test (Y = -0.709 + 0.03 alpha-FP) and properly classified 81% of all patients and 61% of HCC. N/OFQ had lower sensitivity, specificity and predictive values than alpha-FP.

CONCLUSION

Nociceptin is increased in patients with chronic liver disease, independently of the presence of HCC, although the underlying mechanism has yet to be clarified. We conclude it is not a useful marker for HCC.

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.

Orphanin FQ/nociceptin interactions with the immune system in vivo: gene expression changes in lymphoid organs and regulation of the cytokine response to staphylococcal enterotoxin A

Orphanin FQ/nociceptin (OFQ/N) is a neuropeptide implicated in immunomodulation. Here, we show that endogenous and exogenous OFQ/N modulated the cytokine response to the bacterial superantigen staphylococcal enterotoxin A (SEA). Specifically, OFQ/N enhanced TNFalpha and IFNgamma transcripts in the spleen when injected prior to SEA challenge. Moreover, mice lacking the OFQ/N precursor gene showed diminished TNFalpha and IFNgamma mRNA induction in the spleen following SEA challenge. In addition, mRNA levels of ppOFQ/N and the OFQ/N receptor, NOP, were altered in thymus and spleen after SEA challenge. Overall, this suggests that the OFQ/N system can influence immune function and is itself influenced by immune stimuli.

Involvement of nociceptin/orphanin FQ and its receptor in electroacupuncture-produced anti-hyperalgesia in rats with peripheral inflammation

The neuropeptide nociceptin/orphanin FQ (N/OFQ), the endogenous agonist of the N/OFQ peptide receptor (NOP receptor), has been demonstrated to be involved in many physiological and pathological functions including pain regulation. In the present study, the involvement of N/OFQ-NOP receptor system in electroacupuncture (EA)-produced anti-hyperalgesia was investigated in rats with peripheral inflammation. Intrathecal (i.t.) administration of N/OFQ (15 nmol) or EA at acupoints GB30 and GB34 could significantly attenuate hyperalgesia which was induced by subcutaneously injecting complete Freund's adjuvant (CFA) into one hindpaw of rats, manifesting as decreased paw withdrawal latency (PWL) to the noxious thermal stimulus. The anti-nociceptive effect of N/OFQ or EA was significantly blocked by intrathecal injection of [Nphe(1)]nociceptin(1-13)NH(2) (20 nmol), a selective antagonist of the NOP receptor, indicating the NOP-receptor-mediated mechanism. Additionally, the combination of N/OFQ injection with EA treatment could enhance anti-hyperalgesia compared to that produced by each component alone. These findings suggested that the spinal N/OFQ-NOP system might be involved in EA analgesia, which may be one of the mechanisms underlying the anti-nociceptive effect of EA in rat's peripheral inflammatory pain.

Identification of mature nocistatin and nociceptin in human brain and cerebrospinal fluid by mass spectrometry combined with affinity chromatography and HPLC

Nocistatin (NST) and nociceptin/orphanin FQ (NCP) are two important bio-peptides derived from the precursor protein prepronociceptin (ppNCP), involved in several central nervous system (CNS) functions including pain transmission. Since the actual form of human NST in CNS is not fully characterized, we studied the structure of NST from human brain tissue and cerebrospinal fluid (CSF) samples. NST and NCP were isolated from human brain and CSF samples by affinity chromatography combined with HPLC. Mass spectrometry was used for the identification and characterization of the peptides. The total NST immunoreactivity was detected as 11.5+/-2.3 pmol/g tissue for the brain and 0.44 pmol/ml for the pooled CSF sample after the HPLC purification by radioimmunoassay. The presence of two different forms of mature nocistatin (NST-17 and NST-30) and a possible N-terminal methionine cleaved NST-29 were confirmed by both radioimmunoassay and mass spectrometry. Affinity chromatography, HPLC and mass spectrometry methods used in this study were highly sensitive and suitable for identification of actual chemical structures and quantification of very small amounts of peptides in biological samples. The present findings may help further for search for new treatment of neuropathic pain, which is often poorly managed by current therapies.

Nociceptin/orphanin FQ stimulates human monocyte chemotaxis via NOP receptor activation

Nociceptin/orphanin FQ (N/OFQ) produces several biological actions by activating the N/OFQ peptide receptor (NOP). It has been previously shown that N/OFQ stimulates leukocyte chemotaxis both in vitro and in vivo. In the present study we investigated the ability of N/OFQ, in comparison with the proinflammatory peptide formyl-Met-Leu-Phe (fMLP), to stimulate human neutrophil and monocyte chemotaxis and the release of lysozyme and superoxide anion (O2-) production from neutrophils. fMLP stimulated all the leukocyte functions examined. N/OFQ stimulated monocyte (pEC50 12.15) but not neutrophil chemotaxis. The production of O2- from neutrophils was not affected by N/OFQ while the release of lysozyme was increased in a concentration dependent manner (pEC50 11.00) although the maximal effects evoked by N/OFQ were about half of those of fMLP. The NOP ligands [Arg14, Lys15]N/OFQ, N/OFQ(1-13)NH2, Ro 64-6198, UFP-101 and the opioid antagonist naloxone were used for pharmacologically characterizing the receptor involved in the monocyte chemoattractant action of N/OFQ. [Arg14, Lys15]N/OFQ, N/OFQ(1-13)NH2, and Ro 64-6198 mimicked the action of N/OFQ showing similar maximal effects and the following order of potency: [Arg14, Lys15]N/OFQ (pEC50 13.22)>Ro 64-6198 (pEC50 12.96)>N/OFQ(1-13)NH2 (pEC50 12.67)>N/OFQ (pEC50 12.15). Moreover, the monocyte chemoattractant action of N/OFQ was not modified by naloxone 1 microM while antagonized by UFP-101 10 microM (pA2 7.00). Thus, the order of potency of agonists and the antagonist selectivity demonstrated that N/OFQ stimulates human monocyte chemotaxis via NOP receptor activation.

[Nphe1,Arg14,Lys15]N/OFQ-NH2 is a competitive antagonist of NOP receptors in the periaqueductal gray

Nociceptin/orphanin FQ (N/OFQ) and N/OFQ peptide (NOP) receptors are implicated in many physiological functions including pain regulation. This study quantitatively investigated the interaction of a novel NOP receptor antagonist, UFP-101 ([Nphe1,Arg14,Lys15]N/OFQ-NH2), with N/OFQ in the ventrolateral periaqueductal gray, a crucial midbrain area for pain regulation. N/OFQ concentration-dependently activated G-protein coupled inwardly rectifying K+ (GIRK) channels in ventrolateral neurons of periaqueductal gray slices. UFP-101 antagonized N/OFQ-induced GIRK channel activation in a concentration-dependent manner and produced a parallel shift of the concentration-response curve of N/OFQ. The pA2 value estimated from Schild plot is 6.92+/-0.06. At concentrations up to 1 microM, UFP-101 had no effect on membrane current per se and did not affect the GIRK current activated by [d-Ala2, N-Me-Phe4, Gly-ol5]-enkephalin, a mu-opioid receptor agonist. It is concluded that UFP-101 is a potent and competitive peptide antagonist of NOP receptors that mediate GIRK channel activation in ventrolateral periaqueductal gray neurons.

Role of nociceptin/orphanin FQ (Noc/oFQ) in murine experimental colitis

Nociceptin/orphanin (Noc/oFQ), endogenous agonist for nociceptin receptor (NOR), is thought to be a stimulator of neurogenic inflammation. We investigated the possible role of Noc/oFQ in the development of colitis using NOR-deficient mice treated with dextran sulfate sodium (DSS). Colitis was significantly improved in NOR-deficient mice against wild-type mice. Expression level of mucosal addressin cell adhesion molecule-1 (MAdCAM-1) and infiltrating cells also significantly decreased in NOR-deficient mice against wild-type mice. Nociceptin expression increased in wild-type mice after DSS treatment. These results suggest stimulation by Noc/oFQ deteriorates colonic inflammation via up-regulation of adhesion molecule.

Ro 64-6198 [(1S,3aS)-8-(2,3,3a,4,5,6-Hexahydro-1H-phenalen-1-yl)-1-phenyl-1,3,8-triaza-spiro[4.5]decan-4-one] acts differently from nociceptin/orphanin FQ in rat periaqueductal gray slices

Ro 64-6198 [(1S,3aS)-8-(2,3,3a,4,5,6-hexahydro-1H-phenalen-1-yl)-1-phenyl-1,3,8-triaza-spiro[4.5]decan-4-one] was developed as a nonpeptide agonist of nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptors, using bioassays at cloned receptors expressed in cell cultures. We have investigated the actions of Ro 64-6198 at native NOP receptors of the ventrolateral periaqueductal gray (PAG), a crucial site for N/OFQ-induced reversal of opioid analgesia, using the patch-clamp recording technique in rat brain slices. Ro 64-6198, like N/OFQ, activated G protein-coupled inwardly rectifying K(+) channels (GIRK) in ventrolateral PAG neurons but displayed only 60% efficacy and 22% potency of N/OFQ. Unlike N/OFQ that activated GIRK through NOP receptors in almost all tested neurons, Ro 64-6198 affected only 62% (114/185) of the neurons recorded, among which 57% were sensitive to CompB (J-113397), a selective NOP receptor antagonist. The effect of Ro 64-6198 was not affected by naloxone (1 microM), sulpiride (10 microM), and [1-(2-methoxyphenyl)-4-[4-2-phthalimido)butyl]piperazine (NAN-190) (1 microM), respectively, the antagonist of opioid, dopamine D(2), and 5-HT(1A) receptors. In Ro 64-6198-unresponsive neurons, N/OFQ activated GIRK through NOP receptors. It is concluded that Ro 64-6198 is a weak agonist of NOP receptors both in terms of potency and efficacy in ventrolateral PAG neurons. Heterogeneity of NOP receptors has been proposed from binding studies and in vivo functional studies. The possibility was discussed that two subsets of NOP receptors exist in ventrolateral PAG neurons, and Ro 64-6198 activates only one subset but N/OFQ activates both of them.

Pathogenesis of Rheumatoid Arthritis

Although widely diverse mechanisms have been held responsible for tissue damage in rheumatoid arthritis (RA), it is likely that immune complexes are the underlying cause. Self-aggregating complexes of 7s rheumatoid factors in synovial fluid are a distinguishing feature of RA, whilst circulating complexes of 19s rheumatoid factor directed against the hinge region of 7s immunoglobulins are perhaps less specific. Other autoimmune complexes, such as those containing antibodies directed against citrullinated peptides, have been identified and may be more specific for RA, although the antigens against which these antibodies are directed have not been fully characterized. Together with phagocytic cells such as neutrophils, immune complexes are critical to the pathogenesis of RA; their effects are mediated by a complex cascade involving complement activation and stimulation of phagocytes via C5a and Fc receptors. These mechanisms result in a release of mediators of inflammation and joint destruction: cytokines, metalloproteinases, and reactive oxygen intermediates. This article will review recent, and some not too recent, progress made towards working out the pathogenesis of RA.