Proteinases and proteinase-activated receptor 2: a possible role to promote visceral hyperalgesia in rats

A pilot study of fecal serine-protease activity: a pathophysiologic factor in diarrhea-predominant irritable bowel syndrome

BACKGROUND & AIMS

The pathogenesis of irritable bowel syndrome (IBS) remains only partially understood, and no specific or universally effective patient management procedure has been developed to date. Our study was designed to evaluate if colonic luminal serine-proteases may be a relevant pathophysiologic marker of IBS.

METHODS

Fecal samples of 38 IBS patients, 15 patients with ulcerative colitis (UC), and 15 healthy controls were studied. Fecal serine-protease activity was determined photometrically by using azocasein as a proteolytic substrate; fecal pancreatic elastase-1 and mast cell tryptase content were measured by enzyme-linked immunosorbent assay. Fecal secretory leukocyte protease inhibitor concentration was determined by enzyme-linked immunosorbent assay in control subjects and in patients with diarrhea-predominant IBS.

RESULTS

Fecal serine-protease activity was 3-fold higher in patients with diarrhea-predominant IBS than in both controls and IBS patients with either constipation or alternating bowel habits. Fecal serine-protease activity was not correlated with the frequency of bowel movements in all groups. Increased serine-protease activity also was detected in stools of UC patients. No significant difference was observed in the fecal mast cell tryptase and pancreatic elastase concentrations between all groups, or in the fecal secretory leukocyte protease inhibitor concentration between controls and diarrhea-predominant IBS patients.

CONCLUSIONS

Fecal serine-protease activity is increased markedly in patients with diarrhea-predominant IBS. This increase, however, is not coupled with changes in either mast cell tryptase or pancreatic elastase concentrations. Thus, serine-protease activity in the colon may be a pathophysiologic factor in the development of diarrhea-predominant IBS.

Neural Upregulation in Interstitial Cystitis

Interstitial cystitis (IC) is a syndrome of bladder hypersensitivity with symptoms of urgency, frequency, and chronic pelvic pain. Although no consensus has been reached on the underlying cause of IC, several pathophysiologic mechanisms, including epithelial dysfunction, mast cell activation, and neurogenic inflammation, have been proposed. Despite multiple different causes of urinary cystitis, the bladder's response to cystitis is limited and typical. Animal experiments have shown upregulation of proteinase-activated receptors, tryptase, beta-nerve growth factor, inducible nitric oxide synthase, nuclear transcription factor-kappaB, c-Fos, phosphodiesterase 1C, cyclic adenosine monophosphate (cAMP)-dependent protein kinase, and proenkephalin B. After the noxious stimulus has abated, downregulation of genes appears to follow. Distention of the bladder results in the release of adenosine triphosphate (ATP) from urothelial cells, which activates purinergic P2X3 receptors. Activation by ATP of P2X3-expressing afferents is a fundamental signaling factor in bladder sensation and appears to play a role in bladder reflexes. Fos proteins present in spinal cord neurons have been shown to be upregulated in animals that have undergone cyclophosphamide-induced chemical cystitis. These and other findings suggest that neural upregulation occurs both peripherally and centrally in subjects with chronic cystitis. It is unclear whether neural mechanisms and inflammation are the cause of IC or the result of other initiating events. Neural upregulation is known to play a role in the chronicity of pain, urgency, and frequency and represents an exciting area of research that may lead to additional treatments and a better understanding of IC.

Role for protease activity in visceral pain in irritable bowel syndrome

Mediators involved in the generation of symptoms in patients with irritable bowel syndrome (IBS) are poorly understood. Here we show that colonic biopsy samples from IBS patients release increased levels of proteolytic activity (arginine cleavage) compared to asymptomatic controls. This was dependent on the activation of NF-kappaB. In addition, increased proteolytic activity was measured in vivo, in colonic washes from IBS compared with control patients. Trypsin and tryptase expression and release were increased in colonic biopsies from IBS patients compared with control subjects. Biopsies from IBS patients (but not controls) released mediators that sensitized murine sensory neurons in culture. Sensitization was prevented by a serine protease inhibitor and was absent in neurons lacking functional protease-activated receptor-2 (PAR2). Supernatants from colonic biopsies of IBS patients, but not controls, also caused somatic and visceral hyperalgesia and allodynia in mice, when administered into the colon. These pronociceptive effects were inhibited by serine protease inhibitors and a PAR2 antagonist and were absent in PAR2-deficient mice. Our study establishes that proteases are released in IBS and that they can directly stimulate sensory neurons and generate hypersensitivity symptoms through the activation of PAR2.

Proteinase-activated receptors in the lower urinary tract

Proteinase-activated receptors (PARs) are G-protein-coupled receptors that convert specific extracellular proteolytic activity into intracellular signals, and have been suggested to play diverse roles in the body. In this review, evidence for the roles of PARs in bladder contractility and inflammation are presented. The role of PARs in prostate cancer is also reviewed. The existing literature in this area can be difficult to interpret due to the many nonspecific actions of the pharmacological tools employed. Although there are reports that PAR activators can cause contraction of bladder smooth muscle, further pharmacological and molecular studies are required to define roles for these receptors in bladder contractility. While structural studies suggest that roles for PARs in bladder inflammation are likely, few functional investigations have been performed. The significance of the expression of PARs on sensory nerves innervating the bladder and changes in receptor expression in inflammatory disease models are fascinating areas for future research. Finally, it seems probable that PARs, particularly PAR1, may play important roles in the growth and metastasis of prostate cancers.

Protease-activated receptors: potential therapeutic targets in irritable bowel syndrome?

Protease-activated receptors (PARs) are a family of four G-protein-coupled receptors (PAR-1 to PAR-4) activated by the proteolytic cleavage of their N-terminal extracellular domain. This activation first involves the recognition of the extracellular domain by proteases, such as thrombin, but also trypsin or tryptase which are particularly abundant in the gastrointestinal tract, both under physiological circumstances and in several digestive diseases. Activation of PARs, particularly of PAR-1 and -2, modulates intestinal functions, such as gastrointestinal motility, visceral nociception, mucosal inflammatory response, and epithelial functions (intestinal secretion and permeability). As these physiological properties have been shown to be altered in various extents and combinations in different clinical presentations of irritable bowel syndrome, PARs appear as putative targets for future therapeutic intervention in these patients.

Colonic luminal proteases activate colonocyte proteinase-activated receptor-2 and regulate paracellular permeability in mice

Luminal activation of protease-activated receptors-2 (PAR(2)) on colonocytes by trypsin or PAR(2)-activating peptide increases colonic paracellular permeability (CPP). The aim of this study was to evaluate the role of proteases from endogenous and bacterial origin in the modulation of CPP and colonocyte PAR(2) expression in mice. CPP was assessed with (51)Cr-EDTA after intracolonic administration of different protease inhibitors. After 12 days of oral antibiotic treatment, measurements of colonic luminal serine protease activity (CLSPA), CPP, mucosal mouse mast cell proteinase-1 (MMCP-1) content, immunochemistry of PAR(2) and assessment of effects of PAR(2) agonist (SLIGRL) and mast cell degranulator (C48/80) on CPP in Ussing chambers were performed. Immunochemistry was repeated after intracolonic trypsin administration. Colonic infusion of protease inhibitors significantly reduced CPP. In antibiotic-treated mice, CLSPA was reduced coupled with a decrease in PAR(2) expression, but with no change in CPP and MMCP-1 content. Trypsin administration restored PAR(2) expression. The increase in CPP induced by SLIGRL and C48/80 was reduced after antibiotic treatment. Protease activity of colonic content plays an important role in the regulation of mucosal barrier through activation of PAR(2).

In vivo and transgenic animal models used to study visceral hypersensitivity

Measurement of visceral sensitivity in animals is mainly based on 'pseudoaffective' responses, which are brain stem reflexes. For example, in female, but not male rats, acute partial restraint stress induces hypersensitivity to colorectal distension. Mucosal mast cell density increases in rats after nematode infection or maternal deprivation, and both also induce colon hypersensitivity. Significantly, the proximity between nerves and mast cells has been found to be increased in adult rats submitted to maternal deprivation. Protease activation of the proteinase-activated receptor-2 also increases visceral nociception in rats, suggesting that an increase in paracellular permeability may be the primum movens in several animal models of visceral hypersensitivity. Accumulating evidence suggests that sensitization of visceral afferents is not restricted to the presumed nociceptor population, suggesting that most of the mechanosensitive afferent population can contribute to visceral discomfort and pain. Other inflammation-produced changes (e.g. subunit composition of purine-gated P2X channels) in visceral sensory neurones may also contribute to visceral hypersensitivity. This article discusses use of in vivo strategies (and transgenic mouse models) to reveal putative roles in mechanosensitivity and sensitization for molecules not previously considered to have mechanosensory functions.

Serotonergic and non-serotonergic targets in the pharmacotherapy of visceral hypersensitivity

Visceral hypersensitivity is considered a key mechanism in the pathogenesis of functional gastrointestinal (GI) disorders. Targeting visceral hypersensitivity seems an attractive approach to the development of drugs for functional GI disorders. This review summarizes current knowledge on targets for the treatment of visceral hypersensitivity, and the status of current and future drug and probiotic treatment development, and the role of pharmacogenomic factors.

Protease-activated receptor-4: a novel mechanism of inflammatory pain modulation

BACKGROUND AND PURPOSE

Protease-activated receptor-4 (PAR(4)), the most recently discovered member of the PARs family, is activated by thrombin, trypsin and cathepsin G, but can also be selectively activated by small synthetic peptides (PAR(4)-activating peptide, PAR(4)-AP). PAR(4) is considered a potent mediator of platelet activation and inflammation. As both PAR(1) and PAR(2) have been implicated in the modulation of nociceptive mechanisms, we investigated the expression of PAR(4) in sensory neurons and the effects of its selective activation on nociception.

EXPERIMENTAL APPROACH AND KEY RESULTS

We demonstrated the expression of PAR(4) in sensory neurons isolated from rat dorsal root ganglia by reverse transcription-polymerase chain reaction and immunofluorescence. We found that PAR(4) colocalized with calcitonin gene-related peptide and substance P. We also showed that a selective PAR(4)-AP was able to inhibit calcium mobilization evoked by KCl and capsaicin in rat sensory neurons. Moreover, the intraplantar injection of a PAR(4)-AP significantly increased nociceptive threshold in response to thermal and mechanical noxious stimuli, while a PAR(4) inactive control peptide had no effect. The anti-nociceptive effects of the PAR(4)-AP were dose-dependent and occurred at doses below the threshold needed to cause inflammation. Finally, co-injection of the PAR(4)-AP with carrageenan significantly reduced the carrageenan-induced inflammatory hyperalgesia and allodynia, but had no effect on inflammatory parameters such as oedema and granulocyte infiltration.

CONCLUSIONS AND IMPLICATIONS

Taken together, these results identified PAR(4) as a novel potential endogenous analgesic factor, which can modulate nociceptive responses in normal and inflammatory conditions.

The bidirectional communication between neurons and mast cells within the gastrointestinal tract

Normal or disordered behaviour of the gastrointestinal tract is determined by a complex interplay between the epithelial barrier, immune cells, blood vessels, smooth muscle and intramurally located nerve elements. Mucosal mast cells (MMCs), which are able to detect noxious and antigenic threats and to generate or amplify signals to the other cells, are assigned a rather central position in this complex network. Signal input from MMCs to intrinsic enteric neurons is particularly crucial, because the enteric nervous system fulfils a pivotal role in the control of gastrointestinal functions. Activated enteric neurons are able to generate an alarm program involving alterations in motility and secretion. MMC signalling to extrinsic nerve fibres takes part in pathways generating visceral pain or extrinsic reflexes contributing to the disturbed motor and secretory function. Morphological and functional studies, especially studies concerning physiological stress, have provided evidence that, apart from the interaction between the enteric nervous system and MMCs, there is also a functional communication between the central nervous system and these mast cells. Psychological factors trigger neuronal pathways, which directly or indirectly affect MMCs. Further basic and clinical research will be needed to clarify in more detail whether basic patterns of this type of interactions are conserved between species including humans.

Protease-activated receptor 2 sensitizes the transient receptor potential vanilloid 4 ion channel to cause mechanical hyperalgesia in mice

Exacerbated sensitivity to mechanical stimuli that are normally innocuous or mildly painful (mechanical allodynia and hyperalgesia) occurs during inflammation and underlies painful diseases. Proteases that are generated during inflammation and disease cleave protease-activated receptor 2 (PAR2) on afferent nerves to cause mechanical hyperalgesia in the skin and intestine by unknown mechanisms. We hypothesized that PAR2-mediated mechanical hyperalgesia requires sensitization of the ion channel transient receptor potential vanilloid 4 (TRPV4). Immunoreactive TRPV4 was coexpressed by rat dorsal root ganglia (DRG) neurons with PAR2, substance P (SP) and calcitonin gene-related peptide (CGRP), mediators of pain transmission. In PAR2-expressing cell lines that either naturally expressed TRPV4 (bronchial epithelial cells) or that were transfected to express TRPV4 (HEK cells), pretreatment with a PAR2 agonist enhanced Ca2+ and current responses to the TRPV4 agonists phorbol ester 4alpha-phorbol 12,13-didecanoate (4alphaPDD) and hypotonic solutions. PAR2-agonist similarly sensitized TRPV4 Ca2+ signals and currents in DRG neurons. Antagonists of phospholipase Cbeta and protein kinases A, C and D inhibited PAR2-induced sensitization of TRPV4 Ca2+ signals and currents. 4alphaPDD and hypotonic solutions stimulated SP and CGRP release from dorsal horn of rat spinal cord, and pretreatment with PAR2 agonist sensitized TRPV4-dependent peptide release. Intraplantar injection of PAR2 agonist caused mechanical hyperalgesia in mice and sensitized pain responses to the TRPV4 agonists 4alphaPDD and hypotonic solutions. Deletion of TRPV4 prevented PAR2 agonist-induced mechanical hyperalgesia and sensitization. This novel mechanism, by which PAR2 activates a second messenger to sensitize TRPV4-dependent release of nociceptive peptides and induce mechanical hyperalgesia, may underlie inflammatory hyperalgesia in diseases where proteases are activated and released.

Protease-activated receptor 2 sensitizes TRPV1 by protein kinase Cepsilon- and A-dependent mechanisms in rats and mice

Proteases that are released during inflammation and injury cleave protease-activated receptor 2 (PAR2) on primary afferent neurons to cause neurogenic inflammation and hyperalgesia. PAR2-induced thermal hyperalgesia depends on sensitization of transient receptor potential vanilloid receptor 1 (TRPV1), which is gated by capsaicin, protons and noxious heat. However, the signalling mechanisms by which PAR2 sensitizes TRPV1 are not fully characterized. Using immunofluorescence and confocal microscopy, we observed that PAR2 was colocalized with protein kinase (PK) Cepsilon and PKA in a subset of dorsal root ganglia neurons in rats, and that PAR2 agonists promoted translocation of PKCepsilon and PKA catalytic subunits from the cytosol to the plasma membrane of cultured neurons and HEK 293 cells. Subcellular fractionation and Western blotting confirmed this redistribution of kinases, which is indicative of activation. Although PAR2 couples to phospholipase Cbeta, leading to stimulation of PKC, we also observed that PAR2 agonists increased cAMP generation in neurons and HEK 293 cells, which would activate PKA. PAR2 agonists enhanced capsaicin-stimulated increases in [Ca2+]i and whole-cell currents in HEK 293 cells, indicating TRPV1 sensitization. The combined intraplantar injection of non-algesic doses of PAR2 agonist and capsaicin decreased the latency of paw withdrawal to radiant heat in mice, indicative of thermal hyperalgesia. Antagonists of PKCepsilon and PKA prevented sensitization of TRPV1 Ca2+ signals and currents in HEK 293 cells, and suppressed thermal hyperalgesia in mice. Thus, PAR2 activates PKCepsilon and PKA in sensory neurons, and thereby sensitizes TRPV1 to cause thermal hyperalgesia. These mechanisms may underlie inflammatory pain, where multiple proteases are generated and released.

The neurobiology of itch

The neurobiology of itch, which is formally known as pruritus, and its interaction with pain have been illustrated by the complexity of specific mediators, itch-related neuronal pathways and the central processing of itch. Scratch-induced pain can abolish itch, and analgesic opioids can generate itch, which indicates an antagonistic interaction. However, recent data suggest that there is a broad overlap between pain- and itch-related peripheral mediators and/or receptors, and there are astonishingly similar mechanisms of neuronal sensitization in the PNS and the CNS. The antagonistic interaction between pain and itch is already exploited in pruritus therapy, and current research concentrates on the identification of common targets for future analgesic and antipruritic therapy.

Colonic hyperalgesia triggered by proteinase-activated receptor-2 in mice: involvement of endogenous bradykinin

Intracolonic (i.col.) administration of the PAR2-activating peptide (PAR2AP) SLIGRL-NH2 slowly develops visceral hypersensitivity to i.col. capsaicin in ddY mice. Thus, we further analyzed roles of PAR2 in colonic hypersensitivity, using the novel potent PAR2AP, 2-furoyl-LIGRL-NH2 and PAR2-knockout (KO) mice. In ddY mice, i.col. 2-furoyl-LIGRL-NH2 produced delayed (6 h later) facilitation of capsaicin-evoked visceral nociception, an effect being much more potent than SLIGRL-NH2. Such effects were mimicked by i.col. trypsin. In wild-type (WT), but not PAR2-KO, mice of C57BL/6 background, i.col. PAR2 agonists caused delayed facilitation of sensitivity to capsaicin. The PAR2-triggered visceral hypersensitivity was abolished by a bradykinin B2 receptor antagonist, HOE-140. Our data thus provide ultimate evidence for role of PAR2 in colonic hypersensitivity, and suggest involvement of the bradykinin-B2 pathway.

Neurogenic components of trypsin- and thrombin-induced inflammation in rat skin, in vivo

Activation of protease-activated receptors (PAR) can induce vasodilation (VD) and increase of vascular permeability either directly by stimulating endothelial cells or indirectly via activation of nociceptors and subsequent release of neuropeptides (neurogenic inflammation). We aimed to estimate the relative contribution of the two pathways for stimulation with endogenous activators of PAR-2 (trypsin) and of PAR-1, 3 and 4 (thrombin) using in vivo dermal microdialysis in rats. Protein extravasation (PE) was assessed by increase of protein concentration in the dialysate, and VD was quantified by laser Doppler scanning. Both trypsin (10(-8)-10(-4) M) and thrombin (10(-6), 10(-5.5) and 10(-5) M) provoked PE and local VD in a dose-dependent manner. Trypsin (10(-4) M)-induced PE was inhibited by 87.2 +/- 21% due to the substance P (SP) NK1 receptor antagonist SR140333. VD was blocked by 58.15 +/- 10.1% in response to the calcitonin gene-related peptide (CGRP) receptor antagonist CGRP(8-37). By contrast, CGRP(8-37) did not affect thrombin-induced VD, while blockade of SP receptors prevented the PE elicited only by low doses of thrombin (10(-6) M), being ineffective at higher thrombin concentrations. In conclusion, intradermal trypsin elicits a neurogenic inflammation in rat, probably mediated via PAR-2 activation on nociceptors and subsequent SP and CGRP release. Thrombin-induced PE and VD are mediated mainly by a non-neurogenic mechanism.

Calcitonin gene-related peptide partially mediates nociception in acute experimental pancreatitis

BACKGROUND

The mechanism by which pancreatitis causes pain is unknown. The neuropeptide calcitonin gene-related peptide (CGRP) is released after sensory nerve activation and promotes nociceptive signaling in models of visceral pain. We hypothesized that acute pancreatitis leads to the activation of pancreatic sensory neurons that release CGRP in the dorsal horn of the spinal cord. This signal is ultimately transmitted to the brain, and pain is sensed.

METHODS

To induce pancreatitis, rats were injected with l-arginine (500 mg/kg) intraperitoneally or saline (control). Pancreatitis was confirmed by measuring serum amylase and evaluating pancreatic histology. Activation of nociceptive pathways was evaluated by counting Fos-like immunoreactive nuclei (FLI) in the dorsal horn of the spinal cord at T3-L1. Some animals received the CGRP antagonist CGRP(8-37) (50 microg intrathecally) 2 hours before perfusion. Animals were compared using a 2-tailed t test.

RESULTS

l-Arginine treatment induced acute necrotizing pancreatitis in the rat at 24 hours. l-Arginine (24 hours) increased FLI in the dorsal horn of the spinal cord, with a peak effect at L1. Intrathecal administration of CGRP(8-37) significantly decreased the number of FLI nuclei in the dorsal horn of the spinal cord in T11-L1.

CONCLUSIONS

Nociception in the l-arginine model of acute pancreatitis is partially mediated by the release of CGRP in the dorsal horn of the spinal cord. Antagonism of CGRP or its receptors may be useful in treating pain from acute pancreatitis.

Visceral sensation and colitis: inflammation and hypersensitivity do not always go hand in hand

Visceral sensitivity and inflammation are discussed in relation to novel observations in the current issue of the Journal by Larsson and her colleagues. This paper shows that neither acute nor chronic intestinal inflammation initiated by dextran sodium sulfate is associated with an increase in visceral sensitivity to balloon distension in mice. These findings are discussed in the context of recent work highlighting the role of mast cells, the induction of endogenous antinociceptive pathways and the role of altered bacterial flora in visceral hypersensitivity.

Characterization of proteinase-activated receptor 2 signalling and expression in rat hippocampal neurons and astrocytes

Proteinase-activated receptors (PARs1-4) have recently been identified as the molecular entity underlying the cellular effects of serine proteinases. In the present study we have investigated PAR2 signalling, expression and desensitization using cultured and acute slice preparations. Trypsin, SLIGRL and 2f-LIGKV-OH, agonists for PAR2, induced a transient increase in intracellular Ca(2+) levels in both neurons and astrocytes, via activation of the phospholipase C/IP(3) pathway. Furthermore, a single application of trypsin, but not SLIGRL nor 2f-LIGKV-OH, leads to prolonged desensitization of PAR2 responses. PAR2 immunoreactivity was observed in neurons (glutamatergic and GABAergic) and astrocytes within cultures and acute slices, with prominent labelling in neuronal somata and proximal dendrites. Functionally, cultured neurons which exhibited the highest levels of PAR2 labelling, also exhibited the largest Ca(2+) signals upon PAR2 activation. Given the importance of Ca(2+) signalling in hippocampal synaptic plasticity and neurodegeneration, PAR2 may play a key modulatory role in these processes.

Gastrointestinal pain in functional bowel disorders: sensory neurons as novel drug targets

Functional bowel disorders (FBDs) are defined by symptoms of gastrointestinal (GI) dysfunction, discomfort and pain in the absence of a demonstrable organic cause. Since the prevalence of FBDs, particularly functional dyspepsia and irritable bowel syndrome, can be as high as 20%, FBDs represent a significant burden in terms of direct healthcare and productivity costs. There is emerging evidence that the discomfort and pain experienced by many FBD patients is due to persistent hypersensitivity of primary afferent neurons, which may develop in response to infection, inflammation or other insults. This concept identifies vagal and spinal sensory neurons as important targets for novel therapies of GI hyperalgesia. Sensory neuron-specific targets can be grouped into three categories: receptors and sensors at the peripheral nerve terminals, ion channels relevant to nerve excitability and conduction and transmitter receptors. Particular therapeutic potential is attributed to targets that are selectively expressed by afferent neurons, such as the transient receptor potential channel TRPV1, acid-sensing ion channels and tetrodotoxin-resistant Na + channels.

Corticotropin-releasing hormone receptor 1 antagonist blocks brain-gut activation induced by colonic distention in rats

BACKGROUND & AIMS

The corticotropin-releasing hormone receptor 1 mediates stress-induced changes in colonic motor activity and emotion. We tested the hypothesis that pretreatment with JTC-017, a specific corticotropin-releasing hormone receptor 1 antagonist, blocks colorectal distention-induced hippocampal noradrenaline release and visceral perception in rats. We also investigated whether pretreatment with JTC-017 blocks acute or chronic colorectal distention-induced adrenocorticotropic hormone release, anxiety, and stress-induced changes in colonic motility.

METHODS

Rats were pretreated intrahippocampally with alpha-helical corticotropin-releasing hormone (1.25 microg/kg; vehicle), a nonspecific corticotropin-releasing hormone receptor antagonist, or intraperitoneally with JTC-017 (10 mg/kg). Hippocampal noradrenaline release after microdialysis and the frequency of abdominal contractions were measured in response to acute colorectal distention. Plasma adrenocorticotropic hormone levels, anxiety-related behavior, and stress-induced changes in colonic motility were evaluated after acute or chronic colorectal distention followed by exposure to an elevated plus maze.

RESULTS

Administration of alpha-helical corticotropin-releasing hormone or JTC-017 significantly attenuated hippocampal noradrenaline release and reduced the frequency of abdominal contractions induced by acute distention. In addition, JTC-017 significantly reduced plasma adrenocorticotropic hormone and anxiety after acute distention. After chronic distention, changes in plasma adrenocorticotropic hormone and anxiety were not significant because of habituation. In contrast, a significant increase in fecal pellet output during the elevated plus maze was observed after chronic distention. This increase in fecal pellet output was blocked by pretreatment with JTC-017.

CONCLUSIONS

Our results suggest that JTC-017, a specific corticotropin-releasing hormone receptor 1 antagonist, attenuates hippocampal noradrenaline release, visceral perception, adrenocorticotropic hormone release, and anxiety after acute colorectal distention in rats. In addition, JTC-017 blocks stress-induced changes in colonic motility after chronic colorectal distention in rats.

Repeated exposure to water avoidance stress in rats: a new model for sustained visceral hyperalgesia

Chronic stress plays an important role in the development and exacerbation of symptoms in functional gastrointestinal disorders. To better understand the mechanisms underlying this relationship, we aimed to characterize changes in visceral and somatic nociception, colonic motility, anxiety-related behavior, and mucosal immune activation in rats exposed to 10 days of chronic psychological stress. Male Wistar rats were submitted daily to either 1-h water avoidance (WA) stress or sham WA for 10 consecutive days. The visceromotor response to colorectal distension, thermal somatic nociception, and behavioral responses to an open field test were measured at baseline and after chronic WA. Fecal pellets were counted after each WA stress or sham WA session as a measure of stress-induced colonic motility. Colonic samples were collected from both groups and evaluated for structural changes and neutrophil infiltration, mast cell number by immunohistochemistry, and cytokine expression by quantitative RT-PCR. Rats exposed to chronic WA (but not sham stress) developed persistent visceral hyperalgesia, whereas only transient changes in somatic nociception were observed. Chronically stressed rats also exhibited anxiety-like behaviors, enhanced fecal pellet excretion, and small but significant increases in the mast cell numbers and the expression of IL-1beta and IFN-gamma. Visceral hyperalgesia following chronic stress persisted for at least a month. Chronic psychological stress in rats results in a robust and long-lasting alteration of visceral, but not somatic nociception. Visceral hyperalgesia is associated with other behavioral manifestations of stress sensitization but was only associated with minor colonic immune activation arguing against a primary role of mucosal immune activation in the maintenance of this phenomenon.

Epithelial effects of proteinase-activated receptors in the gastrointestinal tract

The intestinal epithelium plays a crucial role in providing a barrier between the external environment and the internal milieu of the body. A compromised mucosal barrier is characteristic of mucosal inflammation and is a key determinant of the development of intestinal diseases such as Crohn's disease and ulcerative colitis. The intestinal epithelium is regularly exposed to serine proteinases and this exposure is enhanced in numerous disease states. Thus, it is important to understand how proteinase-activated receptors (PARs), which are activated by serine proteinases, can affect intestinal epithelial function. This review surveys the data which demonstrate the wide distribution of PARs, particularly PAR-1 and PAR-2, in the gastrointestinal tract and accessory organs, focusing on the epithelium and those cells which communicate with the epithelium to affect its function. PARs have a role in regulating secretion by epithelia of the salivary glands, stomach, pancreas and intestine. In addition, PARs located on subepithelial nerves, fibroblasts and mast cells have important implications for epithelial function. Recent data outline the importance of the cellular site of PAR expression, as PARs expressed on epithelia may have effects that are countered by PARs expressed on other cell types. Finally, PARs and their ability to promote epithelial cell proliferation are discussed in terms of colon cancer.

The role of the vanilloid (capsaicin) receptor (TRPV1) in physiology and pathology

The cloning of the vanilloid receptor 1 opened a floodgate for discoveries regarding the function of this complex molecule. It has been found that, in addition to heat, protons and vanilloids, this receptor also responds to various endogenous ligands. Furthermore, it has been also emerged that, through associations with other molecules, the vanilloid receptor 1 plays an important role in the integration of various stimuli and modulation of cellular excitability. Although, originally, the vanilloid receptor 1 was associated with nociceptive primary afferent fibres, it has been gradually revealed that it is broadly expressed in the brain, epidermis and visceral cells. The expression pattern of the vanilloid receptor 1 indicates that it could be involved in various physiological functions and in the pathomechanisms of diverse diseases. Here, we summarise the molecular, pharmacological and physiological characteristics, and putative functions, of the vanilloid receptor 1, and discuss the therapeutic potential of this molecule.

Protease-activated receptors and inflammatory hyperalgesia

Recent advances in basic science pointed to a role for proteinases, through the activation of proteinase-activated receptors (PARs) in nociceptive mechanisms. Activation of PAR1, PAR2 and PAR4 either by proteinases or by selective agonists causes inflammation inducing most of the cardinal signs of inflammation: swelling, redness, and pain. Sub-inflammatory doses of PAR2 agonist still induced hyperalgesia and allodynia while PAR2 has been shown to be implicated in the generation of hyperalgesia in different inflammatory models. In contrast, sub-inflammatory doses of PAR1 increases nociceptive threshold, inhibiting inflammatory hyperalgesia, thereby acting as an analgesic agent. PARs are present and functional on sensory neurons, where they participate either directly or indirectly to the transmission and/or inhibition of nociceptive messages. Taken together, the results discussed in this review highlight proteinases as signaling molecules to sensory nerves. We need to consider proteinases and the receptors that are activated by proteinases as important potential targets for the development of analgesic drugs in the treatment of inflammatory pain.

2-Furoyl-LIGRL-NH2, a potent agonist for proteinase-activated receptor-2, as a gastric mucosal cytoprotective agent in mice

1. Proteinase-activated receptor-2 (PAR(2)), expressed in capsaicin-sensitive sensory neurons, plays a protective role in gastric mucosa. The present study evaluated gastric mucosal cytoprotective effect of 2-furoyl-LIGRL-NH(2), a novel highly potent PAR(2) agonist, in ddY mice and in wild-type and PAR(2)-knockout mice of C57BL/6 background. 2. Gastric mucosal injury was created by oral administration of HCl/ethanol solution in the mice. The native PAR(2)-activating peptide SLIGRL-NH(2), administered intraperitoneally (i.p.) at 0.3-1 micromol kg(-1) in combination with amastatin, an aminopeptidase inhibitor, but not alone, revealed gastric mucosal protection in ddY mice, which was abolished by ablation of capsaicin-sensitive sensory neurons. 3. I.p. administration of 2-furoyl-LIGRL-NH(2) at 0.1 micromol kg(-1), without combined treatment with amastatin, exhibited gastric mucosal cytoprotective activity in ddY mice, the potency being much greater than SLIGRL-NH(2) in combination with amastatin. This effect was also inhibited by capsaicin pretreatment. 4. Oral administration of 2-furoyl-LIGRL-NH(2) at 0.003-0.03 micromol kg(-1) also protected against gastric mucosal lesion in a capsaicin-reversible manner in ddY mice. 5. I.p. 2-furoyl-LIGRL-NH(2) at 0.1-0.3 micromol kg(-1) caused prompt salivation in anesthetized mice, whereas its oral administration at 0.003-1 micromol kg(-1) was incapable of eliciting salivation. 6. In wild-type, but not PAR(2)-knockout, mice of C57BL/6 background, i.p. administration of 2-furoyl-LIGRL-NH(2) caused gastric mucosal protection. 7. Thus, 2-furoyl-LIGRL-NH(2) is considered a potent and orally available gastric mucosal protective agent. Our data also substantiate a role for PAR(2) in gastric mucosal protection and the selective nature of 2-furoyl-LIGRL-NH(2).

Modulation of capsaicin-evoked visceral pain and referred hyperalgesia by protease-activated receptors 1 and 2

Protease-activated receptors (PARs) 1 and 2 are expressed in capsaicin-sensitive sensory neurons, being anti- and pro-nociceptive, respectively. Given the possible cross talk between PAR-2 and capsaicin receptors, we investigated if PAR-2 activation could facilitate capsaicin-evoked visceral pain and referred hyperalgesia in the mouse and also examined the effect of PAR-1 activation in this model. Intracolonic (i.col.) administration of capsaicin triggered visceral pain-related nociceptive behavior, followed by referred hyperalgesia. The capsaicin-evoked visceral nociception was suppressed by intraperitoneal (i.p.) TFLLR-NH2, a PAR-1-activating peptide, but not FTLLR-NH2, a control peptide, and unaffected by i.col. TFLLR-NH2. SLIGRL-NH2, a PAR-2-activating peptide, but not LRGILS-NH2, a control peptide, administered i.col., facilitated the capsaicin-evoked visceral nociception 6-18 h after administration, while i.p. SLIGRL-NH2 had no effect. The capsaicin-evoked referred hyperalgesia was augmented by i.col. SLIGRL-NH2, but not LRGILS-NH2, 6-18 h after administration, and unaffected by i.p. SLIGRL-NH2, and i.p. or i.col. TFLLR-NH2. Our data suggest that PAR-1 is antinociceptive in processing of visceral pain, whereas PAR-2 expressed in the colonic luminal surface, upon activation, produces delayed sensitization of capsaicin receptors, resulting in facilitation of visceral pain and referred hyperalgesia.

Role of protease-activated receptor-2 in inflammation, and its possible implications as a putative mediator of periodontitis

Proteinase-activated receptor-2 (PAR2) belongs to a novel subfamily of G-protein-coupled receptors with seven-transmembrane domains. This receptor is widely distributed throughout the body and seems to be importantly involved in inflammatory processes. PAR2 can be activated by serine proteases such as trypsin, mast cell tryptase, and bacterial proteases, such as gingipain produced by Porphyromonas gingivalis. This review describes the current stage of knowledge of the possible mechanisms that link PAR2 activation with periodontal disease, and proposes future therapeutic strategies to modulate the host response in the treatment of periodontitis.

Proteinase-activated receptors: transducers of proteinase-mediated signaling in inflammation and immune response

Serine proteinases such as thrombin, mast cell tryptase, trypsin, or cathepsin G, for example, are highly active mediators with diverse biological activities. So far, proteinases have been considered to act primarily as degradative enzymes in the extracellular space. However, their biological actions in tissues and cells suggest important roles as a part of the body's hormonal communication system during inflammation and immune response. These effects can be attributed to the activation of a new subfamily of G protein-coupled receptors, termed proteinase-activated receptors (PARs). Four members of the PAR family have been cloned so far. Thus, certain proteinases act as signaling molecules that specifically regulate cells by activating PARs. After stimulation, PARs couple to various G proteins and activate signal transduction pathways resulting in the rapid transcription of genes that are involved in inflammation. For example, PARs are widely expressed by cells involved in immune responses and inflammation, regulate endothelial-leukocyte interactions, and modulate the secretion of inflammatory mediators or neuropeptides. Together, the PAR family necessitates a paradigm shift in thinking about hormone action, to include proteinases as key modulators of biological function. Novel compounds that can modulate PAR function may be potent candidates for the treatment of inflammatory or immune diseases.

Protease-activated receptors: regulation of neuronal function

Certain serine proteases from the circulation (e.g., coagulation factors), inflammatory cells (e.g., mast-cell tryptase, neutrophil proteinase 3), and from many other cell types (e.g., trypsins) can specifically signal to cells by cleaving protease-activated receptors (PARs), a family of four G protein-coupled receptors. Proteases cleave PARs at specific sites within the extracellular amino-terminus to expose amino-terminal tethered ligand domains that bind to and activate the cleaved receptors. The proteases that activate PARs are often generated and released during injury and inflammation, and activated PARs orchestrate tissue responses to injury, including hemostasis, inflammation, pain, and repair. This review concerns protease and PAR signaling in the nervous system. Neurons of the central and peripheral nervous systems express all four PARs. Proteases that may derive from the circulation, inflammatory cells, or neural tissues can cleave PARs on neurons and thereby activate diverse signaling pathways that control survival, morphology, release of neurotransmitters, and activity of ion channels. In this manner proteases and PARs regulate neurodegeneration, neurogenic inflammation, and pain transmission. Thus, PARs may participate in disease states and PAR antagonists or agonists may be useful therapies for certain disorders.

Activation of trigeminal nociceptive neurons by parotid PAR-2 activation in rats

To clarify involvement of protease-activated receptor-2 (PAR-2) in parotid pain, we examined whether PAR-2 activation in the parotid gland could activate trigeminal nociceptive neurons in anesthetized rats, by analyzing immunoreactive Fos as a nociceptive marker. Either the PAR-2 agonist SLIGRL-NH2 or capsaicin, injected into the parotid duct, caused expression of Fos in the trigeminal subnucleus caudalis, although the PAR-2-inactive reversed peptide had no such effect. The Fos expression caused by PAR-2 activation was inhibited by ablation of capsaicin-sensitive sensory neurons. Intraductal SLIGRL-NH2 did not increase vascular permeability in the parotid gland. Our data thus reveal that activation of PAR-2 in the parotid gland can cause activation of trigeminal nociceptive neurons via capsaicin-sensitive sensory nerves most probably by a non-inflammatory mechanism.

Nerve growth factor mediates alterations of colonic sensitivity and mucosal barrier induced by neonatal stress in rats

BACKGROUND & AIMS

Maternal deprivation (MD) increases nerve growth factor (NGF) expression and colonic mast cell density and alters visceral sensitivity. This study aimed to establish whether NGF overexpression induced by neonatal stress is involved in altered visceral sensitivity and gut mucosal integrity in adult rats.

METHODS

Male Wistar rat pups were either submitted to MD and treated with anti-NGF antibodies or left with their dam and treated daily with NGF. All rats were tested 10 weeks later for visceral sensitivity and 12 weeks later for gut permeability, myeloperoxidase activity, and mast cell numbers. Colonic NGF and NGF receptor expression were determined at 14 days and 12 weeks of age. To determine the involvement of colonic NGF overexpression and mast cell hyperplasia in visceral hyperalgesia induced by MD, neonatally deprived adult rats received anti-NGF antibodies or doxantrazole.

RESULTS

MD increased visceral sensitivity to rectal distention, gut permeability, colonic myeloperoxidase activity, and mast cell density, and anti-NGF antibodies abolished these effects. Neonatal daily treatment with NGF mimicked the alterations induced by MD on both rectal sensitivity and mucosal barrier. In deprived compared with nondeprived rats, colonic NGF immunostaining and NGF messenger RNA were increased at 14 days and 12 weeks. Overexpression of NGF receptor messenger RNA, present at 14 days, was not observed later. Moreover, adult deprived rats treated with doxantrazole or anti-NGF antibodies exhibited normal gut permeability and visceral sensitivity to rectal distention.

CONCLUSIONS

These data indicate that NGF triggers and maintains long-term alterations of visceral sensitivity and gut mucosal integrity induced by MD.

Silent peptic ulcer disease: frequency, factors leading to "silence," and implications regarding the pathogenesis of visceral symptoms

BACKGROUND

Peptic ulcer disease may occur in the absence of dyspeptic symptoms. The pathogenesis of dyspepsia in peptic ulcer disease is unclear. Whether the presence of Helicobacter pylori infection or use of non-steroidal anti-inflammatory drugs affects dyspeptic symptoms in patients with peptic ulcer disease has not been determined. The aim of the study was to determine the frequency and risk factors for peptic ulcer disease in a cohort of asymptomatic, unselected patients undergoing routine screening EGD.

METHODS

This was a prospective study of a cohort of Chinese subjects undergoing screening EGD as part of a routine health maintenance program. Routine EGD screening was performed in 6457 consecutive subjects who underwent a self-paid, health evaluation. Those with endoscopy-confirmed peptic ulcer disease were enrolled to assess the risk factors that distinguish asymptomatic patients with peptic ulcer disease from patients with symptoms because of peptic ulcer disease.

RESULTS

A total of 704 (10.9%) patients were found to have peptic ulcer disease, of which two thirds (n=496) were asymptomatic. Both uni- and multivariate analysis showed that the asymptomatic patients tended to have a larger body mass index, to be habitual tea drinkers, and to have an ulcer that was less than 1 cm in diameter and in a healing stage. Gender, blood group, history of hypertension and/or diabetes, ulcer location, Helicobacter pylori status, use of non-steroidal anti-inflammatories or sedative medications, habitual coffee drinking, and habits with respect to smoking of tobacco or ingestion of alcohol, had no association with symptoms.

CONCLUSIONS

The results of this study suggest that silent peptic ulcer disease is common in Taiwan. Dyspeptic symptoms because of peptic ulcer disease may be influenced by intrinsic (body mass index and ulcer characters) and extrinsic (habitual tea drinking) factors. Non-steroidal anti-inflammatory drug use and Helicobacter pylori status had no significant effect on the symptomatology of peptic ulcer disease. These findings may contribute to the understanding of the pathogenesis in the visceral symptoms of peptic ulcer disease.

Role of capsaicin-sensitive afferent nerves in different models of gastric inflammation in rats

Capsaicin-sensitive afferent nerves are described as being protective against gastric inflammation; their destruction leads to an exacerbation of inflammatory processes. However, these nerves have been shown to exert a pro-inflammatory action on stress-induced gastritis in rats. Our study aimed to investigate the role of capsaicin-sensitive afferent nerves in different experimental models of gastritis in rats. Functional ablation of sensory nerves was achieved by systemic capsaicin treatment (100 mg/kg). Gastritis was induced by mild (iodoacetamide, diquat, surgical duodeno-gastric reflux [DGR]) and strong (70% ethanol, indomethacin) inflammatory agents. Antagonists of the CGRP1 and NK1 receptors, hCGRP8-37 and SR140333, were administered in rats treated with iodoacetamide and ethanol. Macroscopic damage scores (MDS), myeloperoxidase (MPO) activity and malondialdehyde (MDA) concentration were evaluated after sacrifice. Macroscopic lesions appeared only in ethanol and indomethacin gastritis and were enhanced by capsaicin treatment. Gastric MPO activity was significantly increased by all agents compared to controls. Capsaicin treatment did not have any effect on MPO activity in indomethacin-treated rats or in rats submitted to surgery for duodeno-gastric reflux. However, it abolished the increase in MPO induced by iodoacetamide and diquat, and significantly enhanced that induced by ethanol. hCGRP8-37 and SR140333 abolished the increase in MPO activity and MDA concentration in iodoacetamide treated rats. In ethanol-treated rats, SR140333 diminished MPO activity. These results indicate that, depending upon the nature and duration of the experimental inflammation, capsaicin-sensitive afferent nerves may act differently to control gastric inflammatory processes, suggesting the involvement of a neurogenic component in some forms of gastric inflammation.

Intrathecal Protease-Activated Receptor Stimulation Produces Thermal Hyperalgesia through Spinal Cyclooxygenase Activity

Activation of protease-activated receptors (PARs) in non-neural tissue results in prostaglandin production. Because PARs are found in the spinal cord and increased prostaglandin release in the spinal cord causes thermal hyperalgesia, we hypothesized that activation of these spinal PARs would stimulate prostaglandin production and cause a cyclooxygenase-dependent thermal hyperalgesia. PARs were activated using either thrombin or peptide agonists derived from the four PAR subtypes, delivered to the lumbar spinal cord. Dialysis experiments were conducted in conscious, unrestrained rats using loop microdialysis probes placed in the lumbar intrathecal space. Intrathecal thrombin stimulated release of prostaglandin E (PGE)(2) but not aspartate or glutamate. Intrathecal delivery of the PAR 1-derived peptide SFLLRN-NH(2) and the PAR 2-derived peptide SLIGRL both stimulated PGE(2) release; PAR 3-derived TFRGAP and PAR 4-derived GYPGQV were inactive. Intrathecal thrombin had no effect upon formalin-induced flinching or tactile sensitivity but resulted in a thermal hyperalgesia. Intrathecal SFLLRN-NH(2) and SLIGRL both produced thermal hyperalgesia. Consistent with their effects on spinal PGE(2), hyperalgesia from these peptides was blocked by pretreatment with the cyclooxygenase inhibitor ibuprofen. SLIGRL-induced hyperalgesia was also blocked by the selective inhibitors SC 58,560 [5-(4-fluorophenyl)-1-[4-(methylsulfonyl)phenyl]-3-(trifluoromethyl)-1H-pyrazole; cyclooxygenase (COX) 1] and SC 58,125 [5-(4-chlorophenyl)-1-(4-methoxyphenyl)-3-(trifluoromethyl)-1H-pyrazole; COX 2]. These data indicate that activation of spinal PAR 2 and possibly PAR 1 results in the stimulation of the spinal cyclooxygenase cascade and a prostaglandin-dependent thermal hyperalgesia.

Protease-activated receptor 2 sensitizes the capsaicin receptor transient receptor potential vanilloid receptor 1 to induce hyperalgesia

Inflammatory proteases (mast cell tryptase and trypsins) cleave protease-activated receptor 2 (PAR2) on spinal afferent neurons and cause persistent inflammation and hyperalgesia by unknown mechanisms. We determined whether transient receptor potential vanilloid receptor 1 (TRPV1), a cation channel activated by capsaicin, protons, and noxious heat, mediates PAR2-induced hyperalgesia. PAR2 was coexpressed with TRPV1 in small- to medium-diameter neurons of the dorsal root ganglia (DRG), as determined by immunofluorescence. PAR2 agonists increased intracellular [Ca2+] ([Ca2+]i) in these neurons in culture, and PAR2-responsive neurons also responded to the TRPV1 agonist capsaicin, confirming coexpression of PAR2 and TRPV1. PAR2 agonists potentiated capsaicin-induced increases in [Ca2+]i in TRPV1-transfected human embryonic kidney (HEK) cells and DRG neurons and potentiated capsaicin-induced currents in DRG neurons. Inhibitors of phospholipase C and protein kinase C (PKC) suppressed PAR2-induced sensitization of TRPV1-mediated changes in [Ca2+]i and TRPV1 currents. Activation of PAR2 or PKC induced phosphorylation of TRPV1 in HEK cells, suggesting a direct regulation of the channel. Intraplantar injection of a PAR2 agonist caused persistent thermal hyperalgesia that was prevented by antagonism or deletion of TRPV1. Coinjection of nonhyperalgesic doses of PAR2 agonist and capsaicin induced hyperalgesia that was inhibited by deletion of TRPV1 or antagonism of PKC. PAR2 activation also potentiated capsaicin-induced release of substance P and calcitonin gene-related peptide from superfused segments of the dorsal horn of the spinal cord, where they mediate hyperalgesia. We have identified a novel mechanism by which proteases that activate PAR2 sensitize TRPV1 through PKC. Antagonism of PAR2, TRPV1, or PKC may abrogate protease-induced thermal hyperalgesia.

Activation of nociceptive neurons in T9 and T10 in cerulein pancreatitis

Mechanisms of pain transduction in acute pancreatitis are poorly understood. Increased Fos expression in the spinal cord is a marker of activation of nociceptive neurons. We hypothesized that cerulein pancreatitis leads to increased Fos expression at T9 and T10, which receive sensory input from the pancreas. Rats were injected with cerulein (100 microg/kg, s.c.) or saline carrier (NS). Endpoints at 4, 6, and 10 h were serum amylase, myeloperoxidase activity (MPO), and spinal cord Fos expression (number of immunoreactive nuclei/section dorsal gray matter). Fos-like immunoreactivity (FLI) at T9-T10 was compared to internal controls (T6, T12). An average of 20 spinal cord histologic sections were evaluated per rat. Some animals were injected with the mu-opioid receptor agonist, buprenorphine (90 microg/kg, s.c.), 3 h after cerulein, and their endpoints were measured at 6 h. Analysis of variance and t tests were used for statistical analysis. Results are means +/- SEM. As expected, cerulein induced edematous pancreatitis, with a 4-fold increase in serum amylase at 6 h [cer (n = 8): 14,000 +/- 1,300 U/ml versus NS (n = 10): 3,700 +/- 300, P < 0.005)] and a 2-fold increase in MPO activity (0.25 +/- 0.05) activity units/dry wt versus 0.13 +/- 0.02, P < 0.05). Cerulein induced nearly a 2-fold increase in FLI at T9 and T10 [n = 10 (cer) and n = 13 (NS): T9, 14 +/- 1.5 versus 7.8 +/- 0.88; T10, 15 +/- 1.7 versus 8.3 +/- 0.70; P < 0.05]. Peak effects of cerulein on FLI occurred at 6 h and were greatest at T9/T10 with relative sparing of T6/T12. T6/T12 expression was similar in experimental and control groups. Buprenorphine significantly reduced both serum amylase and FLI and T9/T10. Cerulein-induced acute pancreatitis in rat increases visceral nociceptive signaling at spinal cord levels T9 and T10, with a peak at 6 h. Blockade of this effect by the mu-opioid receptor agonist buprenorphine could occur either by direct activation of central opioid receptors and/or an anti-inflammatory mechanism. FLI is a useful tool for studying the pathophysiology of pain in experimental acute pancreatitis.

Modulation of visceral pain and inflammation by protease-activated receptors

The gastrointestinal (GI) tract is exposed to a large array of proteases, under both physiological and pathophysiological conditions. The discovery of G protein-coupled receptors activated by proteases, the protease-activated receptors (PARs), has highlighted new signaling functions for proteases in the GI tract, particularly in the domains of inflammation and pain mechanisms. Activation of PARs by selective peptidic agonists in the intestine or the pancreas leads to inflammatory events and changes in visceral nociception, suggesting that PARs could be involved in the modulation of visceral pain and inflammation. PARs are present in most of the cells that are potentially actors in the generation of irritable bowel syndrome (IBS) symptoms. Activation of PARs interferes with several pathophysiological factors that are involved in the generation of IBS symptoms, such as altered motility patterns, inflammatory mediator release, altered epithelial functions (immune, permeability and secretory) and altered visceral nociceptive functions. Although definitive studies using genetically modified animals, and, when available, pharmacological tools, in different IBS and inflammatory models have not yet confirmed a role for PARs in those pathologies, PARs appear as promising targets for therapeutic intervention in visceral pain and inflammation processes.

Protease-activated receptors: contribution to physiology and disease

Proteases acting at the surface of cells generate and destroy receptor agonists and activate and inactivate receptors, thereby making a vitally important contribution to signal transduction. Certain serine proteases that derive from the circulation (e.g., coagulation factors), inflammatory cells (e.g., mast cell and neutrophil proteases), and from multiple other sources (e.g., epithelial cells, neurons, bacteria, fungi) can cleave protease-activated receptors (PARs), a family of four G protein-coupled receptors. Cleavage within the extracellular amino terminus exposes a tethered ligand domain, which binds to and activates the receptors to initiate multiple signaling cascades. Despite this irreversible mechanism of activation, signaling by PARs is efficiently terminated by receptor desensitization (receptor phosphorylation and uncoupling from G proteins) and downregulation (receptor degradation by cell-surface and lysosomal proteases). Protease signaling in tissues depends on the generation and release of proteases, availability of cofactors, presence of protease inhibitors, and activation and inactivation of PARs. Many proteases that activate PARs are produced during tissue damage, and PARs make important contributions to tissue responses to injury, including hemostasis, repair, cell survival, inflammation, and pain. Drugs that mimic or interfere with these processes are attractive therapies: selective agonists of PARs may facilitate healing, repair, and protection, whereas protease inhibitors and PAR antagonists can impede exacerbated inflammation and pain. Major future challenges will be to understand the role of proteases and PARs in physiological control mechanisms and human diseases and to develop selective agonists and antagonists that can be used to probe function and treat disease.

Activated mast cells in proximity to colonic nerves correlate with abdominal pain in irritable bowel syndrome

BACKGROUND & AIMS

The mechanisms underlying abdominal pain perception in irritable bowel syndrome (IBS) are poorly understood. Intestinal mast cell infiltration may perturb nerve function leading to symptom perception. We assessed colonic mast cell infiltration, mediator release, and spatial interactions with mucosal innervation and their correlation with abdominal pain in IBS patients.

METHODS

IBS patients were diagnosed according to Rome II criteria and abdominal pain quantified according to a validated questionnaire. Colonic mucosal mast cells were identified immunohistochemically and quantified with a computer-assisted counting method. Mast cell tryptase and histamine release were analyzed immunoenzymatically. Intestinal nerve to mast cell distance was assessed with electron microscopy.

RESULTS

Thirty-four out of 44 IBS patients (77%) showed an increased area of mucosa occupied by mast cells as compared with controls (9.2% +/- 2.5% vs. 3.3 +/- 0.8%, respectively; P < 0.001). There was a 150% increase in the number of degranulating mast cells (4.76 +/- 3.18/field vs. 2.42 +/- 2.26/field, respectively; P = 0.026). Mucosal content of tryptase was increased in IBS and mast cells spontaneously released more tryptase (3.22 +/- 3.48 pmol/min/mg vs. 0.87 +/- 0.65 pmol/min/mg, respectively; P = 0.015) and histamine (339.7 +/- 59.0 ng/g vs. 169.3 +/- 130.6 ng/g, respectively; P = 0.015). Mast cells located within 5 microm of nerve fibers were 7.14 +/- 3.87/field vs. 2.27 +/- 1.63/field in IBS vs. controls (P < 0.001). Only mast cells in close proximity to nerves were significantly correlated with severity and frequency of abdominal pain/discomfort (P < 0.001 and P = 0.003, respectively).

CONCLUSIONS

Colonic mast cell infiltration and mediator release in proximity to mucosal innervation may contribute to abdominal pain perception in IBS patients.

Up-regulation and activation of proteinase-activated receptor 2 in early and delayed radiation injury in the rat intestine: influence of biological activators of proteinase-activated receptor 2

Proteinase-activated receptor 2 (Par2, F2rl1, also designated PAR-2 or PAR2) is prominently expressed in the intestine and has been suggested as a mediator of inflammatory, mitogenic and fibrogenic responses to injury. Mast cell proteinases and pancreatic trypsin, both of which have been shown to affect the intestinal radiation response, are the major biological activators of Par2. Conventional Sprague-Dawley rats, mast cell-deficient rats, and rats in which pancreatic exocrine secretion was blocked pharmacologically by octreotide underwent localized irradiation of a 4-cm loop of small bowel. Radiation injury was assessed 2 weeks after irradiation (early, inflammatory phase) and 26 weeks after irradiation (chronic, fibrotic phase). Par2 expression and activation were assessed by in situ hybridization and immunohistochemistry, using antibodies that distinguished between total (preactivated and activated) Par2 and preactivated Par2. Compared to unirradiated intestine, irradiated intestine exhibited increased Par2 expression, particularly in areas of myofibroblast proliferation and collagen accumulation, after both single-dose and fractionated irradiation. The majority of Par2 expressed in fibrotic areas was activated. Postirradiation Par2 overexpression was greatly attenuated in both mast cell-deficient and octreotide-treated rats. The severity of acute mucosal injury did not affect postirradiation Par2 expression. Mast cells and pancreatic proteinases may exert their fibro-proliferative effects partly through activation of Par2. Par2 may be a potential target for modulating the intestinal radiation response, particularly delayed intestinal wall fibrosis.

A protective role of protease-activated receptor 1 in rat gastric mucosa

BACKGROUND AND AIMS

On activation, protease-activated receptor (PAR)-2 modulates multiple gastric functions and exerts mucosal protection via activation of sensory neurons. The role of PAR-1, a thrombin receptor, in the stomach remains unknown. We thus examined if the PAR-1 agonist could protect against gastric mucosal injury in rats.

METHODS

Gastric mucosal injury was created by oral administration of ethanol/HCl or absolute ethanol in conscious rats. Gastric mucosal blood flow and acid secretion were determined in anesthetized rats. Immunohistochemical analyses of PAR-1 and cyclooxygenase (COX)-1 were also performed in rat and human stomach.

RESULTS

The PAR-1 agonist TFLLR-NH(2), administered intravenously in combination with amastatin, protected against the gastric mucosal injury induced by ethanol/HCl or absolute ethanol. The protective effect of TFLLR-NH(2) was abolished by indomethacin or a COX-1 inhibitor but not by ablation of sensory neurons with capsaicin. TFLLR-NH(2) produced an NO-independent increase in gastric mucosal blood flow that was partially inhibited by blockade of the endothelium-derived hyperpolarizing factor pathway. This inhibitory effect was promoted by indomethacin. TFLLR-NH(2) suppressed carbachol-evoked acid secretion in an indomethacin-reversible manner. Immunoreactive PAR-1 and COX-1 were expressed abundantly in rat gastric muscularis mucosae and smooth muscle, and the former protein was also detectable in blood vessels. Similar staining was observed in human gastric muscularis mucosae.

CONCLUSIONS

The PAR-1 agonist, given systemically, protects against gastric mucosal injury via COX-1-dependent formation of prostanoids, modulating multiple gastric functions. Our data identify a novel protective role for PAR-1 in gastric mucosa, and the underlying mechanism is entirely different from that for PAR-2.

Stimulation of proteinase-activated receptor 2 excites jejunal afferent nerves in anaesthetised rats

Proteinase-activated receptor 2 (PAR2) is a receptor for mast cell tryptase and trypsins and might participate in brain-gut communication. However, evidence that PAR2 activation can lead to afferent impulse generation is lacking. To address this issue, we examined the sensitivity of jejunal afferent nerves to a hexapeptide agonist of PAR2, SLIGRL-NH2, and the modulation of the resulting response to treatment with drugs and vagotomy. Multiunit recordings of jejunal afferent activity were made using extracellular recording techniques in anaesthetised male rats. SLIGRL-NH2 (0.001-1 mg kg-1, I.V.) increased jejunal afferent firing and intrajejunal pressure. The reverse peptide sequence (1 mg kg-1, I.V.), which does not stimulate PAR2, was inactive. Naproxen (10 mg kg-1, I.V.), but not a cocktail of omega-conotoxins GVIA and SVIB (each at 25 mug kg-1, I.V.), curtailed both the afferent response and the intrajejunal pressure rise elicited by the PAR2 agonist. Although neither treatment modulated the peak magnitude of the afferent firing, they each altered the intestinal motor response, unmasking an initial inhibitory component. Nifedipine (1 mg kg-1, I.V.) reduced the peak magnitude of the afferent nerve discharge and abolished the initial rise in intrajejunal pressure produced by SLIGRL-NH2. Vagotomy did not significantly influence the magnitude of the afferent response to the PAR2 agonist, which involves a contribution from capsaicin-sensitive fibres. In conclusion, intravenous administration of SLIGRL-NH2 evokes complex activation of predominantly spinally projecting extrinsic intestinal afferent nerves, an effect that involves both direct and indirect mechanisms.