A role for spinal nitric oxide in mediating visceral hyperalgesia in the rat

Piezo2 channels expressed by colon-innervating TRPV1-lineage neurons mediate visceral mechanical hypersensitivity

Inflammatory and functional gastrointestinal disorders such as irritable bowel syndrome (IBS) and obstructive bowel disorder (OBD) underlie the most prevalent forms of visceral pain. Although visceral pain can be generally provoked by mechanical distension/stretch, the mechanisms that underlie visceral mechanosensitivity in colon-innervating visceral afferents remain elusive. Here, we show that virally mediated ablation of colon-innervating TRPV1-expressing nociceptors markedly reduces colorectal distention (CRD)-evoked visceromotor response (VMR) in mice. Selective ablation of the stretch-activated Piezo2 channels from TRPV1 lineage neurons substantially reduces mechanically evoked visceral afferent action potential firing and CRD-induced VMR under physiological conditions, as well as in mouse models of zymosan-induced IBS and partial colon obstruction (PCO). Collectively, our results demonstrate that mechanosensitive Piezo2 channels expressed by TRPV1-lineage nociceptors powerfully contribute to visceral mechanosensitivity and nociception under physiological conditions and visceral hypersensitivity under pathological conditions in mice, uncovering potential therapeutic targets for the treatment of visceral pain.

Alleviation of Irritable Bowel Syndrome-Like Symptoms and Control of Gut and Brain Responses with Oral Administration of Dolichos lablab L. in a Mouse Model

Irritable bowel syndrome (IBS) is a common gastrointestinal disorder manifesting as unexplained abdominal pain and bowel habit changes. The pathogenesis of post-infectious IBS is associated with gut–brain axis dysfunction, including low-grade colonic inflammation and anxiety-related long-term brain changes. This study analyzed the efficacy of a standardized extract of Dolichos lablab L. extract (DL), a bean species, in an IBS mouse model resembling post-infectious, diarrhea-dominant IBS. Using a zymosan-induced animal IBS model, we found that oral administration of DL significantly attenuated zymosan-induced increases in colonic macroscopic scores and minimized weight loss without affecting food intake. In the DL-treated mice, the mast cell count and tumor necrosis factor-α level in the colon markedly decreased, similar to results in sulfasalazine-treated mice and in mice with lipopolysaccharide-stimulated bone marrow-derived mast cells. The number of visceral pain-related behaviors was much lower in the DL-treated mice. Anxiety-like behaviors significantly improved, comparable to that after treatment with amitriptyline. The c-Fos expression level in the prefrontal cortex was significantly reduced. Our data suggest that DL could be beneficial for treating IBS by acting on the gut and brain.

Colonic mucosal N-methyl-D-aspartate receptor mediated visceral hypersensitivity in a mouse model of irritable bowel syndrome

OBJECTIVE

The aim of this study was to investigate whether colonic mucosal N-methyl-D-aspartate receptor (NMDAR) participates in visceral hypersensitivity in irritable bowel syndrome (IBS).

METHODS

C57BL/6 mice were administered intrarectally with trinitrobenzenesulfonic acid (TNBS) for the establishment of an IBS-like visceral hypersensitivity model. Those received an equivalent volume of 50% ethanol were regarded as the controls. Abdominal withdrawal reflex (AWR) scores in response to colorectal distention (CRD) were used to assess visceral sensitivity. NMDAR levels in the colonic mucosa were detected by both immunohistochemistry and Western blot. The concentrations of glutamate and ammonia in the feces of the mice were measured. Changes in visceral sensitivity after the intracolonic administration of ammonia or NMDAR antagonist were recorded.

RESULTS

The established IBS-like mouse model of visceral hypersensitivity showed no evident inflammation in the colon. NMDAR levels in the colonic mucosa of the IBS-like mice were significantly higher compared with the controls, and were positively associated with AWR scores. The glutamate level in the feces of the TNBS-treated mice was similar to that of the controls, although the ammonia level was significantly higher. Intracolonic administration of ammonia induced visceral hypersensitivity in mice, which was repressed by pretreatment with NMDAR antagonist MK801.

CONCLUSIONS

Overexpressed NMDAR in the colonic mucosa may participate in the pathogenesis of visceral hypersensitivity in IBS. Our study identifies the effect of ammonia in the colonic lumen on NMDAR in the colonic mucosa as a potential novel targeted mechanism for IBS treatment.

Complex regulation of the regulator of synaptic plasticity histone deacetylase 2 in the rodent dorsal horn after peripheral injury

Histone deacetylases (HDACs), HDAC2 in particular, have been shown to regulate various forms of learning and memory. Since cognitive processes share mechanisms with spinal nociceptive signalling, we decided to investigate the HDAC2 expression in the dorsal horn after peripheral injury. Using immunohistochemistry, we found that spinal HDAC2 was mainly seen in neurons and astrocytes, with neuronal expression in naïve tissue 2.6 times greater than that in astrocytes. Cysteine (S)‐nitrosylation of HDAC2 releases HDAC2 gene silencing and is controlled by nitric oxide (NO). A duration of 48 h after intraplantar injection of complete Freund's adjuvant, there was an ipsilateral increase in the most important NO‐producing enzyme in pain states, nitric oxide synthase (nNOS), accompanied by an increase in HDAC2 S‐nitrosylation. Moreover, a subset of nNOS‐positive neurons expressed cFos, a known target of HDAC2, suggesting that derepression of cFos expression following HDAC2 S‐nitrosylation might occur after noxious stimulation. We saw no change in global HDAC2 expression in both short‐ and long‐term pain states. However, HDAC2 was increased in astrocytes 7 days after neuropathic injury suggesting that HDAC2 might inhibit astrocytic gene expression in neuropathic pain states. All together, our results indicate that the epigenetic regulation of transcriptional programmes in the dorsal horn after injury is cell specific. Moreover, the prominent role of NO in persistent pain states suggests that HDAC2 S‐nitrosylation could play a crucial role in the regulation of gene expression leading to hypersensitivity.

Our manuscript describes for the first time the regulation of the memory regulator histone deacetylase 2 (HDAC2) in the superficial dorsal horn of adult rats following peripheral injury. Our cell‐specific approach has revealed a complex pattern of expression of spinal HDAC2 that depends on the injury and the cell type, suggesting a sophisticated regulation of gene expression by HDAC2.

A systematic review of the evidence for central nervous system plasticity in animal models of inflammatory-mediated gastrointestinal pain

BACKGROUND

Abdominal pain frequently accompanies inflammatory disorders of the gastrointestinal tract (GIT), and animal models of GIT inflammation have been developed to explore the role of the central nervous system (CNS) in this process. Here, we summarize the evidence from animal studies for CNS plasticity following GIT inflammation.

METHODS

A systematic review was conducted to identify studies that: (1) used inflammation of GIT organs, (2) assessed pain or visceral hypersensitivity, and (3) presented evidence of CNS involvement. Two hundred and eight articles were identified, and 79 were eligible for analysis.

RESULTS

Rats were most widely used (76%). Most studies used adult animals (42%) with a bias toward males (74%). Colitis was the most frequently used model (78%) and 2,4,6-trinitrobenzenesulfonic acid the preferred inflammatory agent (33%). Behavioral (58%), anatomical/molecular (44%), and physiological (24%) approaches were used alone or in combination to assess CNS involvement during or after GIT inflammation. Measurement times varied widely (<1 h-> 2 wk after inflammation). Blinded outcomes were used in 42% studies, randomization in 10%, and evidence of visceral inflammation in 54%. Only 3 studies fulfilled our criteria for high methodological quality, and no study reported sample size calculations.

CONCLUSIONS

The included studies provide strong evidence for CNS plasticity following GIT inflammation, specifically in the spinal cord dorsal horn. This evidence includes altered visceromotor responses and indices of referred pain, elevated neural activation and peptide content, and increased neuronal excitability. This evidence supports continued use of this approach for preclinical studies; however, there is substantial scope to improve study design.

Antinociceptive effect of berberine on visceral hypersensitivity in rats

AIM: To assess the protective effect of berberine administration and the role of nitric oxide (NO) in visceral hypersensitivity.

METHODS: Fifty male Sprague-Dawley rats were randomly assigned to five groups. An inflammatory bowel disease model was induced in rats by intracolonic instillation of 1 mL 4% acetic acid at 8 cm proximal to the anus for 30 s and restraint stress. After subsidence of inflammation on day 7 of the experiment, the rats were subjected to rectal distension, performed by a balloon (6-Fr, 2 mm external diameter, disposable silicon balloon-urethral catheter for pediatric use) which was rapidly inflated with increasing volumes of prewarmed (37 °C) water (0.1, 0.2, 0.3, 0.4, 0.6, 0.8 and 1 mL) for 30 s at four-minute intervals, and then the abdominal withdrawal reflex (AWR) and the level of fecal output were measured, respectively. AWR scores either 0, 1, 2, 3 or 4 were obtained by blinded observers. Rats had been pretreated with berberine or aminoguanidine (NO synthetase inhibitor) or berberine + aminoguanidine before measurement.

RESULTS: The rats in the placebo group showed a hypersensitive response to rectal distension (2.69 ± 0.08 vs 1.52 ± 0.08, P = 0.000) and defecated more frequently than those in the control group (5.0 ± 0.16 vs 0.44 ± 0.16, P = 0.000). Comparing the berberine with placebo group, the AWR scores were reduced for all distension volumes and were significant at 0.2-1 mL (1.90 ± 0.08 vs 2.69 ± 0.08, P = 0.000), while the numbers of hard pellets, soft pellets, formless stools, and total fecal output in the placebo group were significantly larger than in the berberine group (5.0 ± 0.16 vs 2.56 ± 0.16, P = 0.000). Administration of aminoguanidine or berberine + aminoguanidine before VH score measurement reversed the antinociceptive effect of berberine (2.52 ± 0.08 vs 1.90 ± 0.08, P = 0.000; 2.50 ± 0.08 vs 1.90 ± 0.08, P = 0.000). The numbers of hard pellets, soft pellets, formless stool, and total of fecal output in aminoguanidine group were significantly larger than the corresponding values in control group, berberine group, and berberine + aminoguanidine group (4.81 ± 0.16 vs 0.44 ± 0.16, P = 0.000; 4.81 ± 0.16 vs 2.56 ± 0.16, P = 0.000; 4.81 ± 0.16 vs 3.75 ± 0.16, P = 0.000). The berberine and berberine + aminoguanidine groups showed reduced defecation, but aminoguanidine alone did not reduce defecation (2.56 ± 0.16 vs 4.81 ± 0.16, P = 0.000; 3.75 ± 0.16 vs 4.81 ± 0.16, P = 0.000).

CONCLUSION: Berberine had an antinociceptive effect on visceral hypersensitivity, and NO might play a role in this effect.

Influence of nitric oxide synthase inhibition on the motility and sensitivity of distal colon in man

BACKGROUND

Alterations of nitrergic innervation have been implicated in the pathophysiology of motor-sensory abnormalities of post infectious functional dyspepsia and could be involved in the pathophysiology of post infectious irritable bowel syndrome. The role of nitrergic neurons in the control of distal colonic sensorimotor function in man is not known. The aim of this study is to evaluate the motility and sensitivity of distal colon in healthy subjects before and after a nitric oxide synthase inhibitor (L-NMMA).

METHODS

A 700-mL balloon connected with a barostat-manometry assembly was placed in the descending colon of 10 healthy subjects and distension (4 mmHg/2 min) was performed. Intra-balloon pressure was then set at minimal distending pressure + 2 mmHg for 30 min, placebo or L-NMMA (8 mg kg(-1) h(-1)) was administered i.v. in double-blind, randomized, cross-over design and distensions were repeated.

KEY RESULTS

Placebo and L-NMMA did not influence colonic compliance, motility index, and tone. Placebo did not affect thresholds for first perception and discomfort and the areas under the pressure-perception curve. L-NMMA did not alter thresholds for first perception, but significantly decreased the pressure thresholds for discomfort (P = 0.008) and increased the areas under the pressure-perception score (P = 0.01).

CONCLUSIONS & INFERENCES

In man, inhibition of nitric oxide synthase sensitizes the distal colon to distension. Impaired nitrergic innervation is a mechanism that may be involved in the pathogenesis of hypersensitivity to colonic distension.

Mast cells and the cyclooxygenase pathway mediate colonic afferent nerve sensitization in a murine colitis model

INTRODUCTION

Intestinal inflammation alters colonic afferent nerve sensitivity which may contribute to patients' perception of abdominal discomfort. We aimed to explore whether mast cells and the cyclooxygenase pathway are involved in altered afferent nerve sensitivity during colitis.

METHODS

C57Bl6 mice received 3% dextran-sulfate sodium (DSS) in drinking water for 7 days to induce colitis. Control animals received regular water. On day 8 inflammation was assessed in the proximal colon by morphology and histology. Extracellular afferent nerve discharge was recorded from the mesenteric nerve of a 2 cm colonic segment. Subgroups were treated in vitro with the mast cell stabilizer doxantrazole (10⁻⁴M) or the cyclooxygenase inhibitor naproxen (10⁻⁵M).

RESULTS

DSS colitis resulted in morphological and histological signs of inflammation. At baseline, peak firing was 11±2 imp s⁻¹ in colitis segments and 5±1 imp s⁻¹ in uninflamed control segments (p<0.05; mean ± SEM; each n=6). In colitis segments, afferent nerve discharge to bradykinin (0.5 μM) was increased to 47±7 compared to 23±6 imp s⁻¹ in recordings from non-inflamed control tissue (p<0.05). Mechanosensitivity during luminal ramp distension (0-80 cm H₂O) was increased reaching 24±5 imp s⁻¹ at 80 cm H₂O during colitis compared to 14±2 in non-inflamed controls (p<0.05). Doxantrazole or naproxen reduced afferent discharge to bradykinin and luminal ramp distension in colitis segments to control levels.

CONCLUSION

Intestinal inflammation sensitizes mesenteric afferent nerve fibers to bradykinin and mechanical stimuli. The underlying mechanism responsible for this sensitization seems to involve mast cells and prostaglandins.

Antinociceptive effect of VSL#3 on visceral hypersensitivity in a rat model of irritable bowel syndrome: a possible action through nitric oxide pathway and enhance barrier function

Irritable bowel syndrome (IBS) is a functional bowel disorder characterized by visceral hypersensitivity and altered bowel function. There are increasing evidences suggested that VSL#3 probiotics therapy has been recognized as an effective method to relieve IBS-induced symptoms. The aim of this study was to examine the effects of VSL#3 probiotics on visceral hypersensitivity (VH), nitric oxide (NO), fecal character, colonic epithelium permeability, and tight junction protein expression. IBS model was induced by intracolonic instillation of 4% acetic acid and restraint stress in rats. After subsidence of inflammation on the seventh experimental day, the rats were subjected to rectal distension, and then the abdominal withdrawal reflex and the number of fecal output were measured, respectively. Also, colonic permeability to Evans blue was measured in vivo, and tight junction protein expression was studied by immunohistochemistry and immunoblotting method. Rats had been pretreated with VSL#3 or aminoguanidine (NOS inhibitor) or VSL#3+ aminoguanidine before measurements. The rats at placebo group showed hypersensitive response to rectal distension (P < 0.05) and defecated more stools than control rats (P < 0.05), whereas VSL#3 treatment significantly attenuated VH and effectively reduced defecation. Aminoguanidine reduced the protective effects of VSL#3 on VH. A pronounced increase in epithelial permeability and decreased expression of tight junction proteins (occludin, ZO-1) in placebo group were prevented by VSL#3, but not aminoguanidine. VSL#3 treatment reduce the hypersensitivity, defecation, colonic permeability and increase the expression of tight junction proteins (occludin, ZO-1). As the part of this effect was lowered by NOS inhibitor, NO might play a role in the protective effect of VSL#3 to some extent.

Stress-induced hyperalgesia is associated with a reduced and delayed GABA inhibitory control that enhances post-synaptic NMDA receptor activation in the spinal cord

GABA and glutamate are both affected by stress and are involved in nociception. Thus, we determined whether stress-induced enhancement of inflammatory hyperalgesia is mediated by an imbalance between glutamate and GABA neurotransmission. Male rats were subjected daily to 10 to 20 minutes per day of either forced swimming (FS) or sham swimming for 3 consecutive days; nonconditioned rats served as controls. Some rats were treated i.p. with ketamine (5 mg/kg), diazepam (2 mg/kg), flumazenil (0.1 mg/kg), or vehicle (0.9% NaCl), 30 to 60 minutes before each conditioning session or nociception assessment. Pain behavior, spinal nociceptive neuronal activation and GABA and glutamate release were respectively evaluated by the formalin test, the expression of c-Fos and in vivo microdialysis of superficial laminae of the lumbar spinal cord, 48 hours after the last conditioning session. Nitric oxide metabolites (NO(x)) were determined as markers of post-synaptic NMDA receptor activation. FS stress enhanced formalin-induced hyperalgesia, increased pain-elicited c-Fos expression, decreased basal and delayed pain-induced GABA release, and increased basal and induced glutamate release. Hyperalgesia and c-Fos overexpression were blocked only by prestress treatment with diazepam and post-stress treatment with ketamine, whereas changes in GABA and glutamate release were reversed by prestress treatment with diazepam. Diazepam effects were blocked by flumazenil. NO(x) increased in lumbar spinal cord of FS rats by a mechanism antagonized by ketamine. Thus, stress-induced hyperalgesia is initiated by a decreased and delayed GABA release and GABA-A receptor activation, whereas it is maintained by increased glutamate release and NMDA glutamate receptor activation at the spinal level.

Antinociceptive effect of chronic lithium on visceral hypersensitivity in a rat model of diarrhea-predominant irritable bowel syndrome: The role of nitric oxide pathway

BACKGROUND AND AIM

Lithium, a widely used drug in bipolar-affective disorders, plays gastro-protective roles. The effects of lithium on several tissues are mediated through nitric oxide (NO), which regulates gastrointestinal motility and mucosal integrity. The aim of this study was to investigate the protective effect of chronic lithium administration on visceral hypersensitivity and to investigate the role of NO as a potential mechanism of lithium in a rat model of irritable bowel syndrome.

METHODS

Colitis was induced by the intracolonic administration of acetic acid. After subsidence of inflammation on the seventh experimental day, nociception and defecation parameters were measured. A subgroup of animals had been pretreated with lithium carbonate (600 mg/L) for 35 days. Thereafter, either a non-selective NO synthase (NOS) inhibitor (N-nitro-L-arginine methyl ester [L-NAME], 10 mg/kg), a selective NOS inhibitor (aminoguanidine, 100 mg/kg), or saline were administered intraperitoneally 1 h before measurements.

RESULTS

Chronic lithium attenuated the visceral hypersensitivity, increased the nociceptive threshold, and decreased stool frequency. L-NAME and aminoguanidine decreased the nociceptive threshold and reduced the protective effects of lithium on visceral hypersensitivity. Stool frequency was increased in both the lithium-treated and water-treated groups by L-NAME administration, but not aminoguanidine. The form of defecation in the lithium-treated rats shifted toward hard stools rather than being soft and formless, but NOS inhibitors did not change the stool consistency pattern.

CONCLUSION

The results indicate the antinociceptive property of chronic lithium on visceral hypersensitivity. As this effect was lowered by NOS inhibitors, NO might play a role in the protective effect of lithium to some extent.

Is irritable bowel syndrome an inflammatory disorder?

Histopathologic data demonstrate low-grade mucosal inflammation in a subset of patients with irritable bowel syndrome (IBS). This inflammatory infiltrate is mainly represented by increased numbers of T lymphocytes and mast cells lying in the lamina propria. The close apposition of immunocytes to gut nerves supplying the mucosa provides a basis for neuroimmune cross-talk, which may explain gut sensorimotor dysfunction and related symptoms in patients with IBS. A previous gastroenteritis (due to Campylobacter jejuni, Salmonella, Shigella, Escherichia coli, and, likely, viruses) is now an established etiologic factor for IBS (hence, postinfectious IBS). Other putative causes, such as undiagnosed food allergies, genetic abnormalities, stress, or bile acid malabsorption, may also promote and maintain a low-grade mucosal inflammation in IBS. The identification of mucosal inflammation in IBS has pathophysiologic implications and paves the way for novel therapeutic options.

Adenosine receptor agonists modulate visceral hyperalgesia in the rat

BACKGROUND/AIMS

Adenosine is an endogenous modulator of nociception. Its role in visceral nociception, particularly in visceral hyperalgesia, has not been studied. The aim of this study was to determine the effects of adenosine receptor agonists in a model of visceral hyperalgesia.

METHODS

The visceromotor response (VMR) in rats to colorectal distension (CRD; 80 mmHg, 20 seconds) was quantified by electromyographic recordings from the abdominal musculature. Three hours after the intracolonic administration of zymosan (25 mg/mL, 1 mL), VMRs to CRD were measured before and after either subcutaneous or intrathecal administration of an adenosine receptor agonist.

RESULTS

Subcutaneous injection of 5'-N-ethylcarboxyamidoadenosine (NECA; an A1 and A2 receptor agonist), R(-)-N6-(2-phenylisopropyl)-adenosine (R-PIA; a selective A1 receptor agonist), or CGS-21680 hydrochloride (a selective A2a receptor agonist) dose-dependently (10-100 mg/kg) attenuated the VMR to CRD, although hindlimb weakness occurred at the higher doses tested. Intrathecal administration of NECA or R-PIA dose-dependently (0.1-1.0 microg/kg) decreased the VMR, whereas CGS-21680 hydrochloride was ineffective over the same concentration range. Higher intrathecal doses of the A1/A2 receptor agonist NECA produced motor weakness.

CONCLUSIONS

Adenosine receptor agonists are antihyperalgesic, but also produce motor weakness at high doses. However, activation of the spinal A1 receptor significantly attenuates the VMR to CRD without producing motor weakness.

Short-term sensitization of colon mechanoreceptors is associated with long-term hypersensitivity to colon distention in the mouse

BACKGROUND & AIMS

Using a mouse model that reproduces major features of irritable bowel syndrome (long-lasting colon hypersensitivity without inflammation), we examined the contributions of 2 proteins, transient receptor potential vanilloid 1 (TRPV1) and acid-sensing ion channel 3 (ASIC3), on development of behavioral hypersensitivity and assessed the function of colon mechanoreceptors of hypersensitive mice.

METHODS

Visceral nociceptive behavior was measured as the visceromotor response (VMR) to colorectal distention (CRD) before and after intracolonic treatment with zymosan or saline. Colon pathology was assessed in parallel experiments by quantifying myeloperoxidase activity, intralumenal pH, and tissue histology. Electrophysiologic experiments were performed on naïve and zymosan-treated hypersensitive mice using an in vitro colon-pelvic nerve preparation.

RESULTS

Zymosan, but not saline, produced significant and persistent increases in the VMRs of control mice; zymosan produced nonsignificant increases in the VMRs in TRPV1 and ASIC3 knockout mice. Colon myeloperoxidase activity and pH were unaffected by either CRD or intracolonic treatments. Pelvic nerve mechanoreceptors recorded from zymosan-treated or naïve mice had similar sensitivity to stretch of the colon. When applied acutely, zymosan sensitized muscular/mucosal mechanoreceptors in both naïve and hypersensitive mice.

CONCLUSIONS

Zymosan produced sensitization of colon mechanoreceptors acutely in vitro and chronic (>or=7 weeks) behavioral hypersensitivity in the absence of inflammation. The behavioral hypersensitivity was partially dependent on both TRPV1 and ASIC3 because deletions of either of these genes blunted zymosan's effect, suggesting that these proteins may be important peripheral mediators for development of functional (ie, noninflammatory) visceral hypersensitivity.

Severity of mucosal inflammation as a predictor for alterations of visceral sensory function in a rat model

Transient inflammation is known to alter visceral sensory function and frequently precede the onset of symptoms in a subgroup of patients with irritable bowel syndrome (IBS). Duration and severity of the initial inflammatory stimulus appear to be risk factors for the manifestation of symptoms. Therefore, we aimed to characterize dose-dependent effects of trinitrobenzenesulfonic acid (TNBS)/ethanol on: (1) colonic mucosa, (2) cytokine release and (3) visceral sensory function in a rat model. Acute inflammation was induced in male Lewis rats by single administration of various doses of TNBS/ethanol (total of 0.8, 0.4 or 0.2 ml) in test animals or saline in controls. Assessment of visceromotor response (VMR) to colorectal distensions, histological evaluation of severity of inflammation, and measurement of pro-inflammatory cytokine levels (IL-2, IL-6) using enzyme-linked immunosorbent assay (ELISA) were performed 2h and 3, 14, 28, 31 and 42 days after induction. Increased serum IL-2 and IL-6 levels were evident prior to mucosal lesions 2h after induction of colitis and persist up to 14 days (p<0.05 vs. saline), although no histological signs of inflammation were detected at 14 days. In the acute phase, VMR was only significantly increased after 0.8 ml and 0.4 ml TNBS/ethanol (p<0.05 vs. saline). After 28 days, distension-evoked responses were persistently elevated (p<0.05 vs. saline) in 0.8 and 0.4 ml TNBS/ethanol-treated rats. In 0.2 ml TNBS/ethanol group, VMR was only enhanced after repeated visceral stimulation. Visceral hyperalgesia occurs after a transient colitis. However, even a mild acute but asymptomatic colitis can induce long-lasting visceral hyperalgesia in the presence of additional stimuli.

Possible role of nitric oxide in visceral hypersensitivity in patients with irritable bowel syndrome

Visceral hypersensitivity is a consistent finding in a considerable proportion of patients with irritable bowel syndrome (IBS), and may provide a physiological basis for the development of IBS symptoms. In this study, we aimed to confirm the hypothesis that nitric oxide (NO) is involved in maintaining visceral hypersensitivity in IBS. Ten healthy volunteers (HV) and 12 IBS patients with documented hypersensitivity to rectal distension underwent a rectal barostat study. The effect of placebo and the specific NO synthase inhibitor NG -monomethyl-L-arginine (L-NMMA) on resting volume, rectal sensitivity to distension and rectal compliance was evaluated in a double-blind, randomized, cross-over fashion. NG -monomethyl-L-arginine did not alter resting volumes in HV or IBS patients. In HV, l-NMMA did not alter rectal sensory thresholds compared to placebo (45 +/- 3 and 46 +/- 3 mmHg, respectively). In contrast, L-NMMA significantly increased the threshold for discomfort/pain in IBS patients (placebo: 18 +/- 2, l-NMMA: 21 +/- 3 mmHg, P < 0.05). Rectal compliance was not affected by L-NMMA. Although NO does not seem to play a major role in normal rectal sensation or tone, we provide evidence that NO may be involved in the pathophysiology of visceral hypersensitivity in IBS.

A combination of peppermint oil and caraway oil attenuates the post-inflammatory visceral hyperalgesia in a rat model

OBJECTIVE

Visceral hyperalgesia plays a pivotal role in manifestation of symptoms in patients with functional gastrointestinal disorders. In clinical studies combined treatment of peppermint- and caraway oil significantly reduced symptoms. Thus, the aim of this study was to characterize the effects of peppermint- and caraway oil, individually and in combination, on visceral nociception in a rat model of post-inflammatory visceral hyperalgesia.

MATERIAL AND METHODS

On day 28, male Lewis rats (n=80) were randomized to treatment with a rectal administration of trinitrobenzene sulphonic acid (TNBS)/ethanol or physiological saline solution. To quantify the visceromotor response to a standardized colorectal distension, bipolar electrodes were implanted into the external oblique musculature, just superior to the inguinal ligament for electromyographic recordings on day 3. On day 0, baseline measurement was performed. Thereafter, oral treatment with peppermint- or caraway oil or combination treatment was started and continued for 14 consecutive days. After 7 and 14 days of treatment a colorectal distension was performed. Colonic tissue samples were obtained on days 0, 7 and 14 to assess histological alterations due to the different treatment groups and the influence of different compounds.

RESULTS

After a single instillation of TNBS/ethanol persistent elevation of the visceromotor response at all different time-points was observed, although colonic mucosa was completely normal. After 14 days of combined treatment with peppermint- and caraway oil, a reduced visceromotor response of up to 50% compared to placebo was detected in TNBS/ethanol pretreated animals. In contrast, neither peppermint- nor caraway oil had a significant effect on post-inflammatory visceral hyperalgesia. In saline-treated controls there was no significant difference in the visceromotor response.

CONCLUSIONS

These data show that combined treatment with peppermint- and caraway oil modulates post-inflammatory visceral hyperalgesia synergistically. The exact mechanisms have to be further investigated.

Spinal cord stimulation attenuates visceromotor reflexes in a rat model of post-inflammatory colonic hypersensitivity

Spinal cord stimulation (SCS) has been found to relieve neuropathic and ischemic pain clinically and to attenuate a nociceptive reflex in an animal model of acute colonic hypersensitivity. The goal of the present study was to determine the effect of SCS in a rat model of post-inflammatory colonic hypersensitivity. Acute inflammation was induced in rats by a single enema of trinitrobenzenesulfonic acid (TNBS) (50 mg/kg, 0.5 ml, 25% EtOH). Control rats received a single saline enema. A visceromotor behavioral response (VMR), induced by innocuous colorectal distention (30 mm Hg, 10 min) was used to quantify the level of colonic sensitivity on day 3 and 30 post-enema. Prior to VMR testing, under general anesthesia, an electrode (cathode) was placed epidurally on the dorsal surface of the spinal cord at L1 with a paravertebral anode plate. Three to 7 days after implantation of the SCS electrode, the effect of SCS (50 Hz, 0.2 ms, amplitude 90% of motor threshold for 30 min) on colonic sensitivity was determined. On day 30, rats that had received a single TNBS enema were hypersensitive to innocuous colonic distention when compared to rats that received a saline enema (VMR/10 min: TNBS: 17.2+/-0.8 vs. Saline: 9.6+/-1.1, p<0.01). Spinal cord stimulation significantly reduced the VMR in the TNBS-enema group to a value that resembled the saline-enema group (VMR/10 min: TNBS: 11.2+/-1.2 vs. Saline: 10.0+/-1.0). This study provides the first evidence that SCS might be a potential therapeutic for the treatment of abdominal pain observed in patients with post-inflammatory irritable bowel syndrome.

Molecular basis of functional gastrointestinal disorders

There are a number of abnormalities of gastrointestinal function, including sensory and motor dysfunction, which are believed to play a role in the manifestation of symptoms in patients with functional gastrointestinal disorders (FGID). In addition, there is a remarkable psychiatric comorbidity. Family and twin studies have provided strong evidence for a clustering of FGID in families and an increased concordance in monozygotic compared to dizygotic twins. This points towards the role of one or more hereditary (genetic) factors. Considering these disorders of function and the psychiatric comorbidity, polymorphisms of adrenergic, opioidergic or serotonergic receptors as well as G-protein beta3 (GNB3) subunit gene polymorphisms (C825T) and polymorphisms of 5-HT transporter genes are suitable causes. In addition, mediators or regulators of mucosal inflammation may trigger events that ultimately result in the manifestation of FGID. Thus, relevant polymorphisms of genes with immunmodulating and/or neuromodulating features (OPRM1, IL-4, IL-4R, TNFalpha) may also play a role in the manifestation of FGIDs.

Role of spinal nitric oxide synthase-dependent processes in the initiation of the micturition hyperreflexia associated with cyclophosphamide-induced cystitis

The aim of this study was to examine the participation of nitrergic neurotransmission in the initiation of micturition hyperreflexia associated to cyclophosphamide (CP)-induced cystitis in rats. Micturition threshold volume was significantly reduced 4 h after CP administration (100 mg/kg, i.p.); this reduction was attenuated by intra-arterially injected N(G)-nitro-l-arginine-methyl ester (l-NAME), a non selective nitric oxide synthase (NOS) inhibitor, but not by intravesical infusion of S-methyl-l-thiocitrulline (l-SMTC), another structurally different NOS inhibitor. Interestingly, l-NAME failed to affect micturition threshold volume in normal rats. The magnitude of isolated detrusor strips contractions elicited by either carbachol or nerve activation was significantly reduced in CP-treated rats but was unaffected by the addition of N(G)-nitro-l-arginine (l-NOARG), a nonselective NOS inhibitor. In contrast, intrathecal l-NAME and l-SMTC but not N(G)-nitro-d-arginine-methyl ester (d-NAME) administration augmented the micturition threshold volume in CP-treated rats in an l-arginine preventable manner. As with the systemic injection, intrathecal l-NAME also did not affect the micturition threshold volume in normal rats. Four hours after CP injection, the number of neuronal NOS immunoreactive or nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) positive neurons in spinal lumbosacral segments (L6-S2) was not altered whereas the number of c-Fos immunoreactive neurons increased significantly in the dorsal gray commissural nucleus (DGC), the parasympathetic sacral nucleus (PSN) and lamina X of these segments. Ca(2+)-dependent, but not Ca(2+)-independent NOS activity increased significantly in spinal L6-S2 segments but not in thoracic segments of CP-treated rats. These data indicate that the micturition hyperreflexia observed in the initial hours of CP-induced cystitis is associated with an increase in Ca(2+)-dependent NOS activity in spinal L6-S2 segments suggesting an increased production of nitric oxide (NO). The increased production of NO in these spinal segments appears to be necessary for the initiation of the micturition hyperreflexia.

Effects of glutamate receptor antagonists on spinal dorsal horn neurons during zymosan-induced inflammation in rats

These experiments examined the effects of spinal administration of the N-methyl-D-aspartate (NMDA) receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV), the non-NMDA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX), or the metabotropic glutamate receptor antagonist DL-2-amino-3-phosphonoproprionic acid (AP3) on responses of spinal dorsal horn neurons evoked by thermal and mechanical stimuli applied to the rat hindpaw in either an inflamed or noninflamed state. Administration of APV, DNQX, or AP3 decreased heat-evoked neuronal discharges of wide dynamic range (WDR) neurons that were previously augmented by zymosan-induced inflammation. APV and DNQX also decreased heat-evoked discharges of WDR neurons that were previously unaffected by saline injection. Administration of either APV or DNQX, but not AP3, decreased heat-evoked neuronal discharges of nociceptive-specific (NS) neurons in both zymosan- and saline-injected rats. These data suggest that NMDA and non-NMDA receptors contribute to spinal processing of thermal stimuli in both the inflamed and noninflamed state, whereas metabotropic glutamate receptors might serve a role that is unique to WDR neurons in the inflamed state. Only DNQX consistently increased mechanical response thresholds and decreased slopes of the mechanical stimulus response functions (SRFs) of NS and WDR neurons, but this effect was observed in both inflamed and noninflamed states. These data suggest that spinal processing of mechanical stimuli is preferentially mediated by glutamate acting at non-NMDA receptors in either the inflamed or noninflamed state.

Quantitative assessment and characterization of visceral nociception and hyperalgesia in mice

Colorectal distension (CRD) is a well-characterized model of visceral nociception, which we adapted to the mouse. CRD reproducibly evoked contractions of the abdominal musculature [visceromotor response (VMR)], which was graded to stimulus intensity. The magnitude of VMR was greater in male C57BL6 and female 129S6 mice than in male 129S6 and B6.129 mice. In 129S6, C57BL6, and B6.129 mice strains, VMR was reduced dose dependently by morphine (1-10 mg/kg) and by the kappa-opioid agonist U-69593 (0.2-2 mg/kg), although U-69593 was significantly less potent in C57BL6 mice. In additional experiments, the VMR was recorded from adult male 129S6 mice before and after intracolonic administration of various irritants. Only 30% ethanol significantly enhanced responses to CRD. The colon hyperalgesia persisted for 14 days and was associated with a significant shift of the morphine dose-response function to the left. We believe this will be a useful model for study of visceral nociception and hyperalgesia, including studies of transgenic mice with mutations relevant to pain.

The role of nitric oxide in the phosphorylation of cyclic adenosine monophosphate—responsive element-binding protein in the spinal cord after intradermal injection of capsaicin

We investigated the involvement of nitric oxide (NO) in the phosphorylation of cyclic adenosine monophosphate-responsive element-binding protein (CREB) in the spinal cord of rats during central sensitization after intradermal capsaicin injection. CREB and phosphorylated CREB (p-CREB) were measured by immunoblotting. The level of p-CREB increased by 20 minutes, peaked between 20 and 60 minutes after capsaicin injection, and started to decrease after 150 minutes. CREB itself did not show an obvious change after capsaicin injection. The p-CREB expression on the ipsilateral side of the spinal dorsal horn, but not on the contralateral side, increased significantly after capsaicin injection. The increase in p-CREB induced by capsaicin injection was partially blocked by pretreatment with N(G)-nitro-L-arginine methyl ester (L-NAME), an NO synthase inhibitor, administered through a microdialysis fiber placed across the spinal cord. D-NAME, an inactive form of L-NAME, had no effect. CREB phosphorylation, not the level of CREB, was induced within 20 minutes by microdialysis administration of SIN-1, an NO donor. These results indicate that CREB phosphorylation in the spinal cord results from both endogenous and exogenous NO release and that p-CREB may play a role in central sensitization or in longer-term changes in gene expression induced by strong peripheral noxious stimulation.

Subcutaneous formalin and intraplantar carrageenan increase nitric oxide release as measured by in vivo voltammetry in the spinal cord

The paper describes in vivo voltammetric detection of nitric oxide with carbon fibre microelectrodes at the lumbar spinal dorsal horn level of decerebrated-spinalized rats during peripheral noxious inflammatory processes. At the lumbar (L3-L4) dorsal horn level, a nitric oxide dependent peak of oxidation current (650 mV), remaining stable for up to 4h ((92 +/- 5)% of control) could be detected indicating that significant amounts of nitric oxide are produced continuously. Following subcutaneous injection in the hindpaw of 50 microl of 0.5% formalin the oxidation current rapidly increased ((115 +/- 5)% of control at 25 min) and reached (120 +/- 6)% of control 1h later. Subsequently the voltammograms stabilized for up to 90 min and decreased ((107 +/- 4)% at 124 min). After an injection in the hindpaw of 150 microl of 4% carrageenan, the voltammograms remained at control level for 1h and then the oxidation current increased continuously for up to 4h ((145 +/- 16)% of control at 240 min); such an increase was reversed by ketamine. In these two models of inflammation, the delay in onset and the duration of the increases in NO release within the dorsal horn relate, to some extent, to the time course of the peripheral inflammatory processes, since they are shorter after formalin than after carrageenan. The results provide a direct in vivo demonstration that the intercellular messenger nitric oxide participates in the transmission of noxious afferent messages within the dorsal horn of the spinal cord following peripheral inflammation.

The role of CNS NMDA receptors and nitric oxide in visceral hyperalgesia

The studies summarized here document the role of NMDA receptors and nitric oxide in the lumbosacral spinal cord and rostral ventromedial medulla in the maintenance of visceral hyperalgesia. Experiments were conducted in rats in which drugs were administered into either the lumbosacral intrathecal space or directly into the rostral ventromedial medulla. The visceral stimulus was noxious colorectal distension, administered before and 3 h after intracolonic instillation of either saline or 25% zymosan. The visceromotor response to colonic distension was quantified and found to be significantly enhanced in rats in which the colon had previously been treated with zymosan. Enhanced responses to distension were attenuated dose-dependently by intrathecal administration of the NMDA receptor channel blocker MK-801 and by inhibition of the neuronal isoform of nitric oxide synthase (nNOS). In corresponding studies wherein drugs were administered directly into the rostral ventromedial medulla, NMDA receptor antagonism and NOS inhibition dose-dependently attenuated exaggerated responses to colonic distension. Taken together, these data suggest that zymosan-produced visceral hyperalgesia is influenced both at the level of the spinal cord and rostral ventromedial medulla, and that descending facilitatory influences from the rostral ventromedial medulla are important to the maintenance of visceral hyperalgesia.

Direct clinical evidence for spinal hyperalgesia in a patient with irritable bowel syndrome

OBJECTIVE

Our objective was to evaluate GI motor and sensory function and spinal cord testing in a patient with severe irritable bowel syndrome.

METHODS

A patient is described who underwent an extensive assessment of GI motor and sensory function including transit studies, colonic and rectal barostat studies, sensory and manometric studies of the small bowel, and colon and anorectal physiology testing. The patient also underwent testing with spinal cord stimulation and spinal drug delivery as part of a pain management assessment.

RESULTS

The viscerosomatic referral pain pattern resulting from rectal distention was consistent with spinal hyperalgesia. The patient underwent testing for spinal cord stimulation and spinal drug delivery.

CONCLUSION

This novel finding provides direct clinical evidence for the presence of spinal hyperalgesia in a patient with irritable bowel syndrome, consistent with the existing indirect clinical evidence and animal data.

Visceral pain

Visceral pain, although different from somatic pain in several important features, is not as widely researched and consequently our knowledge of neurophysiologic mechanisms as well as clinical management of visceral pain states remains unsatisfactory. Several recent studies have employed different visceral pain animal models to provide insight into the peripheral and central nervous system mechanisms underlying pain originating from the urinary bladder, ureter, and gastrointestinal tract. The effects of opioid and nonopioid drugs in these models have also been evaluated and are reviewed in this article. The importance of anatomic pathways relaying pain sensation in the central nervous system, particularly the newly described dorsal column pathway, is also discussed. In human subjects, new techniques like positron emission tomography are now being used to better understand visceral pain perception. Such findings deriving from basic animal research and human studies summarized in the present overview lead to a better understanding of visceral pain states and may be helpful in developing better treatment strategies to combat visceral pain states in the clinical setting.

Cooperation of NMDA and tachykinin NK(1) and NK(2) receptors in the medullary transmission of vagal afferent input from the acid-threatened rat stomach

Noxious challenge of the rat gastric mucosa by hydrochloric acid (HCl) is signaled to the nucleus tractus solitarii (NTS) and area postrema (AP). This study examined the participation of glutamate and tachykinins in the medullary transmission process. Activation of neurons was visualized by in situ hybridization autoradiography of c-fos messenger RNA (mRNA) 45 min after intragastric (IG) administration of 0.5 M HCl or saline. IG HCl caused many neurons in the NTS and some neurons in the AP to express c-fos mRNA. The NMDA glutamate receptor antagonist MK-801 (2 mg/kg), the NK(1) tachykinin receptor antagonist GR-205,171 (3 mg/kg) and the NK(2) receptor antagonist SR-144,190 (0.1 mg/kg) failed to significantly reduce the NTS response to IG HCl, whereas the triple combination of MK-801, GR-205,171 and SR-144,190 inhibited it by 45--50%. Only in rats that had been preexposed IG to HCl 48 h before the experiment was MK-801 alone able to depress the NTS response to IG HCl. In contrast, the c-fos mRNA response in the AP was significantly augmented by MK-801, an action that was prevented by coadministration of GR-205,171 plus SR-144,190. Inhibition of neuronal nitric oxide synthase with 7-nitroindazole (45 mg/kg) was without effect on the IG HCl-evoked c-fos mRNA expression in the NTS and AP. Our data show that glutamate acting via NMDA receptors and tachykinins acting via NK(1) and NK(2) receptors cooperate in the vagal afferent input from the acid-threatened stomach to the NTS and participate in the processing of afferent input to the AP in a different and complex manner. These opposing interactions in the AP and NTS and the increase in NMDA receptor function in the NTS after a gastric acid insult are likely to have a bearing on the neuropharmacology of dyspepsia.

S-nitroso-l-cysteine releases norepinephrine in rat spinal synaptosomes

Although nitric oxide (NO) participates in development of hypersensitivity states in the spinal cord thought to underlie chronic pain, it also participates in analgesia produced by various drugs. In rats with a hypersensitivity state following peripheral nerve injury, spinal administration of an NO donor or l-cysteine alone produced no effect, whereas their combination, which yields s-nitroso-l-cysteine (SNC) powerfully reduced hypersensitivity. In the current study, we examined the ability of SNC to stimulate release of a known spinal analgesic neurotransmitter, norepinephrine (NE), as a possible mechanism of analgesic action of NO in the spinal cord. SNC (but not the NO donor alone or decomposed SNC) produced a concentration-dependent release of NE from rat spinal cord synaptosomes. The d-isomer of SNC was less potent than the l-isomer, and the effect of SNC was partially blocked by l-, but not d-leucine, implicating an interaction with the l-amino acid transporter. SNC-induced NE release was partially Na(+) dependent, but largely Ca(2+) independent. NE uptake inhibitors partially antagonized the effect of SNC, but guanylate cyclase inhibitors were without effect. These data are therefore consistent with NO stimulating NE release in the spinal cord via reaction with thiol containing compounds, such as cysteine, entry into NE terminals via active transport, and production of both exocytotic and carrier mediated release.

New concepts of irritable bowel syndrome

Significant recent advances in basic and clinical science have improved our understanding of irritable bowel syndrome (IBS). Sensory abnormalities, particularly visceral hypersensitivity after sensitizing stimulation, indicate neural dysfunction in patients with IBS. This dysfunction could be mediated by N-methyl-D-aspartate or calcium gene-related peptide receptors in the spinal cord. The stress response in the gut is augmented in IBS, which may be related to hypothalamic release of corticotropin-releasing factor. Postinfectious IBS may be related to psychologic factors that allow persistent inflammation. Finally, functional brain imaging has shown augmented central nervous system responses to visceral pain in IBS, particularly in the prefrontal cortex. Low-dose tricyclic antidepressants are useful to control symptoms, and the new serotonin type 3 (5-HT3) receptor antagonists show promise for symptom control.