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

Sex differences in zymosan-induced behavioral visceral hypersensitivity and colorectal afferent sensitization

Sex differences in visceral nociception have been reported in clinical and preclinical studies, but the potential differences in sensory neural encoding of the colorectum between males and females are not well understood. In this study, we systematically assessed sex differences in colorectal neural encoding by conducting high-throughput optical recordings in intact dorsal root ganglia (DRGs) from control and visceral hypersensitive mice. We found an apparent sex difference in zymosan-induced behavioral visceral hypersensitivity: enhanced visceromotor responses to colorectal distension were observed only in male mice, not in female mice. In addition, a higher number of mechanosensitive colorectal afferents were identified per mouse in the zymosan-treated male group than in the saline-treated male group, whereas the mechanosensitive afferents identified per mouse were comparable between the zymosan- and saline-treated female groups. The increased number of identified afferents in zymosan-treated male mice was predominantly from thoracolumbar (TL) innervation, which agrees with the significant increase in the TL afferent proportion in the zymosan group as compared with the control group in male mice. In contrast, female mice showed no difference in the proportion of colorectal neurons between saline- and zymosan-treated groups. Our results revealed a significant sex difference in colorectal afferent innervation and sensitization in the context of behavioral visceral hypersensitivity, which could drive differential clinical symptoms in male and female patients.NEW & NOTEWORTHY We used high-throughput GCaMP6f recordings to study 2,275 mechanosensitive colorectal afferents in mice. Our results revealed significant sex differences in the zymosan-induced behavioral visceral hypersensitivity, which were present in male but not female mice. Male mice also showed sensitization of colorectal afferents in the thoracolumbar pathway, whereas female mice did not. These findings highlight sex differences in sensory neural anatomy and function of the colorectum, with implications for sex-specific therapies for treating visceral pain.

Maternal separation leads to dynamic changes of visceral hypersensitivity and fecal metabolomics from childhood to adulthood

We assessed dynamic changes in visceral hypersensitivity and fecal metabolomics through a mouse model of irritable bowel syndrome (IBS) from childhood to adulthood. A mouse model of IBS was constructed with maternal separation (MS) in early life. Male mice aged 25, 40, and 70 days were used. Visceral sensitivity was assessed by recording the reaction between the abdominal withdrawal reflex and colorectal distension. Metabolomics was identified and quantified by liquid chromatography-tandem mass spectrometry. The visceral sensitivity of the MS group was significantly higher than that of the non-separation (NS) group in the three age groups. The top four fecal differential metabolites in the different age groups were lipids, lipid molecules, organic heterocyclic compounds, organic acids and derivatives, and benzenoids. Five identical differential metabolites were detected in the feces and ileal contents of the MS and NS groups at different ages, namely, benzamide, taurine, acetyl-L-carnitine, indole, and ethylbenzene. Taurine and hypotaurine metabolism were the most relevant pathways at P25, whereas histidine metabolism was the most relevant pathway at P40 and P70. Visceral hypersensitivity in the MS group lasted from childhood to adulthood. The different metabolites and metabolic pathways detected in MS groups of different ages provide a theoretical basis for IBS pathogenesis.

Involvement of acid sensing ion channel (ASIC)-3 in an acute urinary bladder-colon cross sensitization model in rodent

Introduction

Irritable bowel syndrome and bladder pain syndrome are both characterized by pain in response to organ distension. Epidemiologic studies showed that these two syndromes are often overlapped. Such overlap may be due to sharing of common extrinsic innervations between the colorectum and the urinary bladder, where cross-sensitization of the urinary bladder and the colon would occur in response to mechanical distension of either organ. The aim of this project was to develop and characterize a rodent model of urinary bladder-colon sensitization and to assess the role of the acid sensing ion channel (ASIC)-3.

Methods

Double retrograde labelling was performed to identify extrinsic primary afferent neurons innervating both the colon (Fluororuby) and urinary bladder (Fluorogold) in the L6-S1 dorsal root ganglia (DRG) in Sprague Dawley rats. The phenotype of the colon/urinary bladder co-innervating primary afferent neurons was assessed using immunohistochemistry directed against ASIC-3. Cross-organ sensitization was induced in Sprague Dawley rats by using an echography-guided intravesical administration of acetic acid (0.75%) under brief isoflurane anesthesia. Colonic sensitivity was assessed in conscious rats by measuring abdominal contraction during isobaric colorectal distension (CRD). Measurement of urinary bladder and colonic paracellular permeabilities and tissue myeloperoxidase assay were performed. The involvement of ASIC-3 was assessed by use of S1 intrathecal administration of the ASIC-3 blocker, APETx2 (2.2 µM).

Results

Immunohistochemistry showed that 73.1% of extrinsic primary afferent neurons co-innervating the colon and the urinary bladder express ASIC-3. By contrast, extrinsic primary afferent neurons innervating the colon only or the urinary bladder only were positive for ASIC-3 in 39.3% and 42.6%, respectively. Echography-guided intravesical administration of acetic acid resulted in colonic hypersensitivity to colorectal distension. This effect started 1 h post-injection and lasted up to 24 h, and was not longer seen after 3 days after injection. No colonic hyperpermeability and no difference in urinary bladder and colon MPO activity was observed between control and acetic acid-treated rats. Colonic sensitization by intravesical acetic acid administration was prevented by S1 intrathecal administration of APETx2.

Conclusion

We developed an acute pelvic cross-organ sensitization model in conscious rat. In this model, cross-organ sensitization is likely to involve S1-L6 extrinsic primary afferents co-innervating the colon and urinary bladder through an ASIC-3 pathway.

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.

Effect of altered gut microbiota on visceral hypersensitivity of postinfectious irritable bowel syndrome mice

OBJECTIVE

Irritable bowel syndrome (IBS) is a common functional bowel disorder characterized with visceral hypersensitivity. Previous studies indicated gut microbiota alteration associated short-chain fatty acids (SCFAs) dysregulation is associated with IBS development. The aim of the study is to explore the potential role of microbiota dysbiosis mediated visceral hypersensitivity in postinfectious-IBS (PI-IBS) mouse model.

METHODS

Four-week-old NIH mice were randomly allocated into four groups: control mice, PI-IBS mice, PI-IBS mice co-housing with normal mice, and PI-IBS mice were administrated with a cocktail of antibiotics. Trichinella spiralis infection established PI-IBS mouse model. Microbiota in cecal contents and feces were analyzed by 16S rDNA sequencing. SCFAs were detected by gas chromatography. 5-hydroxytryptamine (5-HT) was evaluated by ELISA, and N-methyl-D-aspartate receptors (NMDARs) were examined by western blot. Visceral sensitivity was determined by abdominal withdrawal reflex in response to colorectal distention.

RESULTS

Increased SCFAs were observed in cecal contents and feces in PI-IBS mice accompanied with higher 5-HT and NMDAR subunits expressions in ileum and colon. Visceral hypersensitivity was observed in PI-IBS mice compared to control mice. When administrated with antibiotics cocktails and co-housing with normal mice, PI-IBS mice showed decreased SCFAs, 5-HT, NMDAR subunits expressions, and improved visceral hypersensitivity.

CONCLUSION

Gut microbiota alteration induced increased SCFAs, 5-HT and NMDAR subunits expressions were associated with visceral hypersensitivity in PI-IBS mice. The critical role of gut microbiota in improving visceral hypersensitivity was further identified by treatment of antibiotics cocktail and co-housing.

The association between dietary acid load and odds and severity of irritable bowel syndrome in adults

No study has been conducted to investigate the association between dietary acid load and irritable bowel syndrome (IBS). So, this cross-sectional study was performed to investigate the association between dietary acid load and odds of IBS, its severity, and IBS subtypes. A sample of 3362 Iranian subjects was selected from health centers in Isfahan province. A validated semi-quantitative food frequency questionnaire (DS-FFQ) was applied to estimate dietary intakes. The dietary acid load was measured using net endogenous acid production (NEAP), dietary acid load (DAL), and potential renal acid load (PRAL) scores. In crude models, the highest compared with the lowest category of the PRAL score was significantly associated with increased odds of IBS severity in participants with BMI ≥ 25 (kg/m²) (OR = 1.54; 95% CI = (1.03-2.32). Also, the results indicated a significant positive association between the PARL and odds of mixed subtype of IBS (OR = 1.74; 95% CI = (1.11-2.74); P trend = 0.02). In propensity score-adjusted model with potential confounders, only a positive association was found between PRAL and odds of mixed subtype of IBS (OR = 1.78; 95% CI = (1.05-3.00); P trend = 0.03). The DAL and NEAP scores tended to show non-significant similar findings. This study indicates that dietary acid load might be associated with odds of mixed type of IBS. However, further research is warranted to infer these findings.

The association between dietary acid load and odds and severity of irritable bowel syndrome in adults

No study has been conducted to investigate the association between dietary acid load and irritable bowel syndrome (IBS). So, this cross-sectional study was performed to investigate the association between dietary acid load and odds of IBS, its severity, and IBS subtypes. A sample of 3362 Iranian subjects was selected from health centers in Isfahan province. A validated semi-quantitative food frequency questionnaire (DS-FFQ) was applied to estimate dietary intakes. The dietary acid load was measured using net endogenous acid production (NEAP), dietary acid load (DAL), and potential renal acid load (PRAL) scores. In crude models, the highest compared with the lowest category of the PRAL score was significantly associated with increased odds of IBS severity in participants with BMI ≥ 25 (kg/m2) (OR = 1.54; 95% CI = (1.03-2.32). Also, the results indicated a significant positive association between the PARL and odds of mixed subtype of IBS (OR = 1.74; 95% CI = (1.11-2.74); P trend = 0.02). In propensity score-adjusted model with potential confounders, only a positive association was found between PRAL and odds of mixed subtype of IBS (OR = 1.78; 95% CI = (1.05-3.00); P trend = 0.03). The DAL and NEAP scores tended to show non-significant similar findings. This study indicates that dietary acid load might be associated with odds of mixed type of IBS. However, further research is warranted to infer these findings.

Gut Non-Bacterial Microbiota: Emerging Link to Irritable Bowel Syndrome

As a common functional gastrointestinal disorder, irritable bowel syndrome (IBS) significantly affects personal health and imposes a substantial economic burden on society, but the current understanding of its occurrence and treatment is still inadequate. Emerging evidence suggests that IBS is associated with gut microbial dysbiosis, but most studies focus on the bacteria and neglect other communities of the microbiota, including fungi, viruses, archaea, and other parasitic microorganisms. This review summarizes the latest findings that link the nonbacterial microbiota with IBS. IBS patients show less fungal and viral diversity but some alterations in mycobiome, virome, and archaeome, such as an increased abundance of Candida albicans. Moreover, fungi and methanogens can aid in diagnosis. Fungi are related to distinct IBS symptoms and induce immune responses, intestinal barrier disruption, and visceral hypersensitivity via specific receptors, cells, and metabolites. Novel therapeutic methods for IBS include fungicides, inhibitors targeting fungal pathogenic pathways, probiotic fungi, prebiotics, and fecal microbiota transplantation. Additionally, viruses, methanogens, and parasitic microorganisms are also involved in the pathophysiology and treatment. Therefore, the gut nonbacterial microbiota is involved in the pathogenesis of IBS, which provides a novel perspective on the noninvasive diagnosis and precise treatment of this disease.

Pathology and physiology of acid‑sensitive ion channels in the digestive system (Review)

As a major proton-gated cation channel, acid-sensitive ion channels (ASICs) can perceive large extracellular pH changes. ASICs play an important role in the occurrence and development of diseases of various organs and tissues including in the heart, brain, and gastrointestinal tract, as well as in tumor proliferation, invasion, and metastasis in acidosis and regulation of an acidic microenvironment. The permeability of ASICs to sodium and calcium ions is the basis of their physiological and pathological roles in the body. This review summarizes the physiological and pathological mechanisms of ASICs in digestive system diseases, which plays an important role in the early diagnosis, treatment, and prognosis of digestive system diseases related to ASIC expression.

Identification of a Pain-Specific Gene Expression Profile for Pediatric Recurrent Abdominal Pain

Objectives: Functional Abdominal Pain (FAP) and Irritable Bowel Syndrome (IBS) are common recurrent abdominal pain diagnoses with the hallmark, lack of inflammation. To identify a biological signature for IBS/FAP in the colon, this study used genetic profiling to uncover gene expression changes associated with IBS/FAP and abdominal pain. Methods: Patients (8 to 17 years) newly diagnosed with IBS or FAP were enrolled in the study. At diagnostic colonoscopy, three rectal biopsies were collected, and gene expression analysis was performed using a Qiagen PCR Array. Relative fold difference in gene expression for 84 pain-associated genes was calculated using the 2-ΔΔ Cq method compared with pain-free controls. Factors affecting pain burden (Pain Burden Interview; PBI) were analyzed, including age, sex, rectal inflammation, and gene expression. Data were analyzed using multiple stepwise linear regression and 2-tailed t tests (P ≤ 0.05). Results: Of the 22 total patients in the study, 19 were diagnosed with either IBS-Constipation (frequency of 5.26%), IBS-Diarrhea (47.37%), IBS-Mixed (10.53%), or FAP (36.84%). IBS/FAP patients reported significantly higher pain burden at the time of diagnosis compared to pain-free controls (p < 0.001), as well as significantly higher abdominal pain (p = 0.01). Of the 84 genes, expression of GRIN1 (p = 0.02), MAPK3 (p = 0.04), P2X4 (p = 0.04), and PTGES3 (p = 0.02) were all significantly associated with PBI score. Discussion: Abdominal pain associated with IBS/FAP in pediatric patients may be linked to the expression of GRIN1, MAPK3, P2X4, and PTGES3, pointing to potential novel therapeutic targets for management of recurring abdominal pain.

Acid-sensing ion channel 3: An analgesic target

Acid-sensing ion channel 3 (ASIC3) belongs to the epithelial sodium channel/degenerin (ENaC/DEG) superfamily. There are 7 different ASIC subunits encoded by 5 different genes. Most ASIC subunits form trimeric ion channels that upon activation by extracellular protons mediate a transient inward current inducing cellular excitability. ASIC subunits exhibit differential tissue expression and biophysical properties, and the ability of subunits to form homo- and heteromeric trimers further increases the complexity of currents measured and their pharmacological properties. ASIC3 is of particular interest, not only because it exhibits high expression in sensory neurones, but also because upon activation it does not fully inactivate: a transient current is followed by a sustained current that persists during a period of extracellular acidity, i.e. ASIC3 can encode prolonged acidosis as a nociceptive signal. Furthermore, certain mediators sensitize ASIC3 enabling smaller proton concentrations to activate it and other mediators can directly activate the channel at neutral pH. Moreover, there is a plethora of evidence using transgenic mouse models and pharmacology, which supports ASIC3 as being a potential target for development of analgesics. This review will focus on current understanding of ASIC3 function to provide an overview of how ASIC3 contributes to physiology and pathophysiology, examining the mechanisms by which it can be modulated, and highlighting gaps in current understanding and future research directions.

Integrative analysis of the gut microbiome and metabolome in a rat model with stress induced irritable bowel syndrome

Stress is one of the major causes of irritable bowel syndrome (IBS), which is well-known for perturbing the microbiome and exacerbating IBS-associated symptoms. However, changes in the gut microbiome and metabolome in response to colorectal distention (CRD), combined with restraint stress (RS) administration, remains unclear. In this study, CRD and RS stress were used to construct an IBS rat model. The 16S rRNA gene sequencing was used to characterize the microbiota in ileocecal contents. UHPLC-QTOF-MS/MS assay was used to characterize the metabolome of gut microbiota. As a result, significant gut microbial dysbiosis was observed in stress-induced IBS rats, with the obvious enrichment of three and depletion of 11 bacterial taxa in IBS rats, when compared with those in the control group (q < 0.05). Meanwhile, distinct changes in the fecal metabolic phenotype of stress-induced IBS rats were also found, including five increased and 19 decreased metabolites. Furthermore, phenylalanine, tyrosine and tryptophan biosynthesis were the main metabolic pathways induced by IBS stress. Moreover, the altered gut microbiota had a strong correlation with the changes in metabolism of stress-induced IBS rats. Prevotella bacteria are correlated with the metabolism of 1-Naphthol and Arg.Thr. In conclusion, the gut microbiome, metabolome and their interaction were altered. This may be critical for the development of stress-induced IBS.

Tetrahydrocannabinol (THC) Exacerbates Inflammatory Bowel Disease in Adolescent and Adult Female Rats

Inflammatory Bowel Disease (IBD) is a life-long disorder that often begins between the ages of 15 and 30. Anecdotal reports suggest cannabinoids may be an effective treatment. This study sought to determine whether home cage wheel running is an effective method to assess IBD, and whether Tetrahydrocannabinol (THC), the primary psychoactive compound in cannabis, can restore wheel running depressed by IBD. Adolescent and adult female Sprague-Dawley rats were individually housed in a cage with a running wheel. Rats were injected with trinitrobenzene sulphonic acid (TNBS) into the rectum to induce IBD-like symptoms. One day later, both vehicle and TNBS treated rats were injected with a low dose of THC (0.32 mg/kg, s.c.) or vehicle. Administration of TNBS depressed wheel running in adolescent and adult rats. No antinociceptive effect of THC was evident when administered 1 day after TNBS. In fact, administration of THC prolonged TNBS-induced depression of wheel running for over 5 days in adolescent and adult rats. These results show that home cage wheel running is depressed by TNBS-induced IBD, making it a useful tool to evaluate the behavioral consequences of IBD, and that administration of THC, instead of producing antinociception, exacerbates TNBS-induced IBD. PERSPECTIVE: This article advances research on inflammatory bowel disease in two important ways: 1) Home cage wheel running is a new and sensitive tool to assess the behavioral consequences of IBD in adolescent and adult rats; and 2) Administration of the cannabinoid THC exacerbates the negative behavioral effects of IBD.

Costunolide ameliorates intestinal dysfunction and depressive behaviour in mice with stress-induced irritable bowel syndrome via colonic mast cell activation and central 5-hydroxytryptamine metabolism

Irritable bowel syndrome (IBS) is a common chronic functional bowel disease, associated with a high risk of depression and anxiety. The brain-gut axis plays an important role in the pathophysiological changes involved in IBS; however, an effective treatment for the same is lacking. The natural compound costunolide (COS) has been shown to exert gastroprotective, enteroprotective, and neuroprotective effects, but its therapeutic effects in IBS are unclear. Our study explored the effect of COS on intestinal dysfunction and depressive behaviour in stress-induced IBS mice. Mice were subjected to chronic unpredictable mild stress to trigger IBS, and some were administered COS. Behavioural tests, histochemical assays, western blotting, and measurement of 5-hydroxytryptamine (5-HT) levels in the colon and hippocampus were applied to monitor the physiological and molecular consequences of COS treatment in IBS mice. COS administration relieved intestinal dysfunction and depression-like behaviours in IBS mice. Improvements in low-grade colon inflammation and intestinal mucosal permeability, inhibition of the activation of mast cells, upregulation of colonic Occludin expression, and downregulation of Claudin 2 expression were also observed. COS was also found to upregulate GluN2A, BDNF, p-ERK1/2, and p-CREB expression and 5-HT levels in hippocampal cells but inhibited 5-HT metabolism. Molecular docking showed that COS could form hydrogen bonds with the serotonin transporter (SERT) to affect the reuptake of 5-HT in the intercellular space. In conclusion, COS alleviates intestinal dysfunction and depressive behaviour in stress-induced IBS mice by inhibiting mast cell activation in the colon and regulating 5-HT metabolism in the central nervous system.

Possible implications of animal models for the assessment of visceral pain

Acute pain, provoked generally after the activation of peripheral nociceptors, is an adaptive sensory function that alerts the individual to avoid noxious stimuli. However, uncontrolled acute pain has a maladaptive role in sensory activity leading to development of a chronic pain state which persists even after the damage is resolved, or in some cases, in the absence of an initial local acute injury. Huge numbers of people suffer from visceral pain at least once during their life span, leading to substantial health care costs. Although studies reporting on the mechanism of visceral pain are accumulating, it is still not precisely understood. Therefore, this review aims to elucidate the mechanism of visceral pain through an evaluation of different animal models and their application to develop novel therapeutic approaches for treating visceral pain. To assess the nociceptive responses in viscera, several visceral pain models such as inflammatory, traction, stress and genetic models utilizing different methods of measurement have been devised. Among them, the inflammatory and traction models are widely used for studying the visceral pain mechanism of different disease conditions and post-operative surgery in humans and animals. A hapten, 2,4,6-trinitrobenzene sulfonic acid (TNBS), has been extensively used as an inflammatory agent to induce visceral pain. The traction model seems to cause a strong pain stimulation and autonomic reaction and could thus be the most appropriate model for studying the underlying visceral pain mechanism and for probing the therapeutic efficacies of various anesthetic and analgesics for the treatment of visceral pain and hyperalgesia.

Attenuation of Visceral and Somatic Nociception by Ghrelin Mimetics

Purpose

The anti-nociceptive properties of ghrelin have been demonstrated in alleviating inflammatory and neuropathic pain. Whether a ghrelin receptor-mediated mechanism attenuates visceral and somatic pain in the absence of active inflammation remains to be explored. Here, we investigate the efficacy of peripherally restricted (ipamorelin) and a globally active (HM01) selective ghrelin receptor agonist in an experimental model of non-inflammatory visceral hypersensitivity and somatic mechanical allodynia.

Materials and Methods

Visceral hypersensitivity was induced by dilute acetic acid (0.6%) infusion in the colon of rats in the absence of colonic epithelial inflammation. Ghrelin mimetics HM01 and ipamorelin were administered orally or intravenously, respectively. The ghrelin receptor antagonist H0900 was administered orally. Colonic sensitivity was assessed via a visceromotor behavioral response (VMR) quantified as the number of abdominal contractions in response to graded isobaric pressures (0-60 mmHg) of colorectal distension (CRD). Somatic mechanical allodynia was quantified by the number of ipsilateral paw withdrawals in response to a calibrated von Frey filament.

Results

Compared to vehicle controls, ghrelin mimetics HM01 and ipamorelin significantly attenuated colonic hypersensitivity and somatic allodynia. The anti-nociceptive effects of the ghrelin mimetics were blocked after administration of the ghrelin receptor antagonist H0900.

Conclusion

We have shown that ghrelin receptor-mediated mechanisms are involved in visceral and somatic hypersensitivity in the absence of active colonic inflammation. Furthermore, visceral and somatic hypersensitivity could be attenuated by a peripherally restricted ghrelin mimetic. These results highlight a potential novel approach for treating acute visceral and somatic pain by ghrelin mimetics.

Fructose and irritable bowel syndrome

Irritable bowel syndrome (IBS) is a chronic disorder characterised by recurrent abdominal pain or discomfort and transit disturbances with heterogeneous pathophysiological mechanisms. The link between food and gastrointestinal (GI) symptoms is often reported by patients with IBS and the role of fructose has recently been highlighted. Fructose malabsorption can easily be assessed by hydrogen and/or methane breath test in response to 25 g fructose; and its prevalence is about 22 % in patients with IBS. The mechanism of fructose-related symptoms is incompletely understood. Osmotic load, fermentation and visceral hypersensitivity are likely to participate in GI symptoms in the IBS population and may be triggered or worsened by fructose. A low-fructose diet could be integrated in the overall treatment strategy, but its role and implication in the improvement of IBS symptoms should be evaluated. In the present review, we discuss fructose malabsorption in adult patients with IBS and the interest of a low-fructose diet in order to underline the important role of fructose in IBS.

In silico screening of GMQ-like compounds reveals guanabenz and sephin1 as new allosteric modulators of acid-sensing ion channel 3

Acid-sensing ion channels (ASICs) are voltage-independent cation channels that detect decreases in extracellular pH. Dysregulation of ASICs underpins a number of pathologies. Of particular interest is ASIC3, which is recognised as a key sensor of acid-induced pain and is important in the establishment of pain arising from inflammatory conditions, such as rheumatoid arthritis. Thus, the identification of new ASIC3 modulators and the mechanistic understanding of how these compounds modulate ASIC3 could be important for the development of new strategies to counteract the detrimental effects of dysregulated ASIC3 activity in inflammation. Here, we report the identification of novel ASIC3 modulators based on the ASIC3 agonist, 2-guanidine-4-methylquinazoline (GMQ). Through a GMQ-guided in silico screening of Food and Drug administration (FDA)-approved drugs, 5 compounds were selected and tested for their modulation of rat ASIC3 (rASIC3) using whole-cell patch-clamp electrophysiology. Of the chosen drugs, guanabenz (GBZ), an α₂-adrenoceptor agonist, produced similar effects to GMQ on rASIC3, activating the channel at physiological pH (pH 7.4) and potentiating its response to mild acidic (pH 7) stimuli. Sephin1, a GBZ derivative that lacks α₂-adrenoceptor activity, has been proposed to act as a selective inhibitor of a regulatory subunit of the stress-induced protein phosphatase 1 (PPP1R15A) with promising therapeutic potential for the treatment of multiple sclerosis. However, we found that like GBZ, sephin1 activates rASIC3 at pH 7.4 and potentiates its response to acidic stimulation (pH 7), i.e. sephin1 is a novel modulator of rASIC3. Furthermore, docking experiments showed that, like GMQ, GBZ and sephin1 likely interact with the nonproton ligand sensor domain of rASIC3. Overall, these data demonstrate the utility of computational analysis for identifying novel ASIC3 modulators, which can be validated with electrophysiological analysis and may lead to the development of better compounds for targeting ASIC3 in the treatment of inflammatory conditions.

Animal Models for Functional Gastrointestinal Disorders

Functional gastrointestinal disorders (FGID), such as functional dyspepsia (FD) and irritable bowel syndrome (IBS) are characterized by chronic abdominal symptoms in the absence of an organic, metabolic or systemic cause that readily explains these complaints. Their pathophysiology is still not fully elucidated and animal models have been of great value to improve the understanding of the complex biological mechanisms. Over the last decades, many animal models have been developed to further unravel FGID pathophysiology and test drug efficacy. In the first part of this review, we focus on stress-related models, starting with the different perinatal stress models, including the stress of the dam, followed by a discussion on neonatal stress such as the maternal separation model. We also describe the most commonly used stress models in adult animals which brought valuable insights on the brain-gut axis in stress-related disorders. In the second part, we focus more on models studying peripheral, i.e., gastrointestinal, mechanisms, either induced by an infection or another inflammatory trigger. In this section, we also introduce more recent models developed around food-related metabolic disorders or food hypersensitivity and allergy. Finally, we introduce models mimicking FGID as a secondary effect of medical interventions and spontaneous models sharing characteristics of GI and anxiety-related disorders. The latter are powerful models for brain-gut axis dysfunction and bring new insights about FGID and their comorbidities such as anxiety and depression.

Hypothalamic paraventricular nucleus neurons activated by estrogen GPER1 receptors promote anti-inflammation effects in the early stage of colitis

The hypothalamus-pituitary-adrenal (HPA) axis is known to mediate gut-brain interaction, and the pathological inflammatory process in the intestine can induce HPA axis involved 'fight or flight' response to suppress or facilitate intestinal inflammation. Hypothalamic paraventricular nucleus (PVN) neurons are responsible for controlling the HPA axis activity, but their exact role in modulating intestinal inflammation remains unclear. In this study, we used the dextran sulfate sodium (DSS)-induced mice colitis model, gene editing, and RNA interference to determine the effects of PVN neurons on intestinal inflammation. We found that at the early stage (third day) after DSS treatment, there was a mild inflammation in the colorectal area and an increased neuron activation in the PVN but not in the adjacent area. At the same time, ~80% of activated PVN neurons also expressed novel estrogen GPER1 receptor. The colitis noticeably worsened in GPER1-knockout mice and local PVN GPER1-knockdown mice. These results indicated that PVN GPER1 positive neurons potentially have a protective function during the early stages of DSS-induced colitis, and this may be a mechanism by which the central nervous system attempts to suppress intestinal inflammation to achieve self-protection.

Visceral pain from colon and rectum: the mechanotransduction and biomechanics

Visceral pain is the cardinal symptom of functional gastrointestinal (GI) disorders such as the irritable bowel syndrome (IBS) and the leading cause of patients' visit to gastroenterologists. IBS-related visceral pain usually arises from the distal colon and rectum (colorectum), an intraluminal environment that differs greatly from environment outside the body in chemical, biological, thermal, and mechanical conditions. Accordingly, visceral pain is different from cutaneous pain in several key psychophysical characteristics, which likely underlies the unsatisfactory management of visceral pain by drugs developed for other types of pain. Colorectal visceral pain is usually elicited from mechanical distension/stretch, rather than from heating, cutting, pinching, or piercing that usually evoke pain from the skin. Thus, mechanotransduction, i.e., the encoding of colorectal mechanical stimuli by sensory afferents, is crucial to the underlying mechanisms of GI-related visceral pain. This review will focus on colorectal mechanotransduction, the process of converting colorectal mechanical stimuli into trains of action potentials by the sensory afferents to inform the central nervous system (CNS). We will summarize neurophysiological studies on afferent encoding of colorectal mechanical stimuli, highlight recent advances in our understanding of colorectal biomechanics that plays critical roles in mechanotransduction, and review studies on mechano-sensitive ion channels in colorectal afferents. This review calls for focused attention on targeting colorectal mechanotransduction as a new strategy for managing visceral pain, which can also have an added benefit of limited CNS side effects, because mechanotransduction arises from peripheral organs.

Marine Toxins and Nociception: Potential Therapeutic Use in the Treatment of Visceral Pain Associated with Gastrointestinal Disorders

Visceral pain, of which the pathogenic basis is currently largely unknown, is a hallmark symptom of both functional disorders, such as irritable bowel syndrome, and inflammatory bowel disease. Intrinsic sensory neurons in the enteric nervous system and afferent sensory neurons of the dorsal root ganglia, connecting with the central nervous system, represent the primary neuronal pathways transducing gut visceral pain. Current pharmacological therapies have several limitations, owing to their partial efficacy and the generation of severe adverse effects. Numerous cellular targets of visceral nociception have been recognized, including, among others, channels (i.e., voltage-gated sodium channels, VGSCs, voltage-gated calcium channels, VGCCs, Transient Receptor Potential, TRP, and Acid-sensing ion channels, ASICs) and neurotransmitter pathways (i.e., GABAergic pathways), which represent attractive targets for the discovery of novel drugs. Natural biologically active compounds, such as marine toxins, able to bind with high affinity and selectivity to different visceral pain molecular mediators, may represent a useful tool (1) to improve our knowledge of the physiological and pathological relevance of each nociceptive target, and (2) to discover therapeutically valuable molecules. In this review we report the most recent literature describing the effects of marine toxin on gastrointestinal visceral pain pathways and the possible clinical implications in the treatment of chronic pain associated with gut diseases.

Corticotropin-releasing factor induces inflammatory cytokines via the NLRP6-inflammatory cytokine axis in a murine model of irritable bowel syndrome

OBJECTIVE

This study aimed to determine the effect of corticotropin-releasing factor (CRF) on regulating the NOD-like receptor pyrin domain-containing protein 6 (NLRP6)-inflammatory cytokine axis in a murine model of irritable bowel syndrome (IBS).

METHODS

C57BL/6 mice were subjected to water avoidance stress (WAS) for 1 h per day for 10 days, and the abdominal withdrawal reflex (AWR) and colonic inflammation were assessed. We also measured the levels of CRF, NLRP6 inflammasome components, myeloperoxidase, D-lactate, interleukin (IL)-1β, and IL-18. In vitro experiments with Caco-2 cell line were also performed. In addition, we assessed the effect of Clostridium butyricum (C. butyricum) on IBS mice.

RESULTS

IBS mice exhibited visceral hypersensitivity and inflammation, accompanied by increases in CRF, myeloperoxidase, D-lactate, IL-1β, and IL-18 levels, but a decrease in NLRP6 expression. In vitro data showed that CRF suppressed NLRP6, but induced IL-1β and IL-18 levels, in Caco-2 cells. C. butyricum restored CRF levels and maintained the NLRP6-inflammatory cytokine axis in IBS mice.

CONCLUSIONS

CRF induces the NLRP6-inflammatory cytokine axis in IBS mice. C. butyricum could be beneficial in controlling IBS.

The Role of Toxins in the Pursuit for Novel Analgesics

Chronic pain is a major medical issue which reduces the quality of life of millions and inflicts a significant burden on health authorities worldwide. Currently, management of chronic pain includes first-line pharmacological therapies that are inadequately effective, as in just a portion of patients pain relief is obtained. Furthermore, most analgesics in use produce severe or intolerable adverse effects that impose dose restrictions and reduce compliance. As the majority of analgesic agents act on the central nervous system (CNS), it is possible that blocking pain at its source by targeting nociceptors would prove more efficient with minimal CNS-related side effects. The development of such analgesics requires the identification of appropriate molecular targets and thorough understanding of their structural and functional features. To this end, plant and animal toxins can be employed as they affect ion channels with high potency and selectivity. Moreover, elucidation of the toxin-bound ion channel structure could generate pharmacophores for rational drug design while favorable safety and analgesic profiles could highlight toxins as leads or even as valuable therapeutic compounds themselves. Here, we discuss the use of plant and animal toxins in the characterization of peripherally expressed ion channels which are implicated in pain.

Spinal Afferent Innervation of the Colon and Rectum

Despite their seemingly elementary roles, the colon and rectum undertake a variety of key processes to ensure our overall wellbeing. Such processes are coordinated by the transmission of sensory signals from the periphery to the central nervous system, allowing communication from the gut to the brain via the "gut-brain axis". These signals are transmitted from the peripheral terminals of extrinsic sensory nerve fibers, located within the wall of the colon or rectum, and via their axons within the spinal splanchnic and pelvic nerves to the spinal cord. Recent studies utilizing electrophysiological, anatomical and gene expression techniques indicate a surprisingly diverse set of distinct afferent subclasses, which innervate all layers of the colon and rectum. Combined these afferent sub-types allow the detection of luminal contents, low- and high-intensity stretch or contraction, in addition to the detection of inflammatory, immune, and microbial mediators. To add further complexity, the proportions of these afferents vary within splanchnic and pelvic pathways, whilst the density of the splanchnic and pelvic innervation also varies along the colon and rectum. In this review we traverse this complicated landscape to elucidate afferent function, structure, and nomenclature to provide insights into how the extrinsic sensory afferent innervation of the colon and rectum gives rise to physiological defecatory reflexes and sensations of discomfort, bloating, urgency, and pain.

MLCK-mediated intestinal permeability promotes immune activation and visceral hypersensitivity in PI-IBS mice

BACKGROUND

Alterations in intestinal permeability regulated by tight junctions (TJs) are associated with immune activation and visceral hypersensitivity in irritable bowel syndrome (IBS). Myosin light chain kinase (MLCK) is an important mediator of epithelial TJ. The aim of this study is to investigate the role of MLCK in the pathogenesis of IBS using a post infectious IBS (PI-IBS) mouse model.

METHODS

Trichinella spiralis-infected PI-IBS mouse model was used. Urine lactulose/mannitol ratio was measured to assess intestinal epithelial permeability. Western blotting was used to evaluate intestinal TJ protein (zonula occludens-1) and MLCK-associated protein expressions. Immune profile was assessed by measuring Th (T helper) 1/Th2 cytokine expression. Visceral sensitivity was determined by abdominal withdrawal reflex in response to colorectal distension.

RESULTS

Eight weeks after inoculation with T. spiralis, PI-IBS mice developed decreased pain and volume thresholds during colorectal distention, increased urine lactulose/mannitol ratio, elevated colonic Th1/Th2 cytokine ratio, and decreased zonula occludens-1 expression compared to the control mice. MLCK expression was dramatically elevated in the colonic mucosa of PI-IBS mice compared to the control mice, alongside increased pMLC/MLC and decreased MLCP expression. Administration of MLCK inhibitor and TJ blocker both reversed the increased intestinal permeability, visceral hypersensitivity, and Th1-dominant immune profile in PI-IBS mice.

CONCLUSION

MLCK is a pivotal step in inducing increased intestinal permeability promoting low-grade intestinal immune activation and visceral hypersensitivity in PI-IBS mice. MLCK inhibitor may provide a potential therapeutic option in the treatment of IBS.

Gas chromatography/mass spectrometry based metabolomic study in a murine model of irritable bowel syndrome

AIM

To study the role of microbial metabolites in the modulation of biochemical and physiological processes in irritable bowel syndrome (IBS).

METHODS

In the current study, using a metabolomic approach, we analyzed the key metabolites differentially excreted in the feces of control mice and mice with IBS, with or without Clostridium butyricum (C. butyricum) treatment. C57BL/6 mice were divided into control, IBS, and IBS + C. butyricum groups. In the IBS and IBS + C. butyricum groups, the mice were subjected to water avoidance stress (WAS) for 1 h/d for ten days. Gas chromatography/mass spectrometry (GC-MS) together with multivariate analysis was employed to compare the fecal samples between groups.

RESULTS

WAS exposure established an appropriate model of IBS in mice, with symptoms of visceral hyperalgesia and diarrhea. The differences in the metabolite profiles between the control group and IBS group significantly changed with the progression of IBS (days 0, 5, 10, and 17). A total of 14 differentially excreted metabolites were identified between the control and IBS groups, and phenylethylamine was a major metabolite induced by stress. In addition, phenylalanine metabolism was found to be the most relevant metabolic pathway. Between the IBS group and IBS + C. butyricum group, 10 differentially excreted metabolites were identified. Among these, pantothenate and coenzyme A (CoA) biosynthesis metabolites, as well as steroid hormone biosynthesis metabolites were identified as significantly relevant metabolic pathways.

CONCLUSION

The metabolic profile of IBS mice is significantly altered compared to control mice. Supplementation with C. butyricum to IBS mice may provide a considerable benefit by modulating host metabolism.

Beneficial effects of Rifaximin in post-infectious irritable bowel syndrome mouse model beyond gut microbiota

BACKGROUND AND AIMS

Rifaximin is a minimally absorbed antibiotic, which has shown efficacy in irritable bowel syndrome (IBS) patients. However, the mechanism on how it effects in IBS is still incompletely defined. In this study, Trichinella spiralis-infected post-infectious (PI) IBS mouse model was used, to assess the action of rifaximin on visceral hypersensitivity, barrier function, gut inflammation, and microbiota.

METHODS

Post-infectious IBS model was established by T. spiralis infection in mice. Rifaximin were administered to PI-IBS mice for seven consecutive days. The abdominal withdrawal reflex and threshold of colorectal distention were employed to evaluate visceral sensitivity. Smooth muscle contractile response was recorded in the organ bath. Intestinal permeability was measured by Ussing chamber. Expression of tight junction protein and cytokines were measured by Western blotting. Ilumina miseq platform was used to analyze bacterial 16S ribosomal RNA.

RESULTS

Post-infectious IBS mice treated with rifaximin exhibited decreased abdominal withdrawal reflex score, increased threshold, reduced contractile response, and intestinal permeability. Rifaximin also suppressed the expression of interleukin-12 and interleukin-17 and promoted the expression of the major tight junction protein occludin. Furthermore, rifaximin did not change the composition and diversity, and the study reavealed that rifaximin had a tiny effect on the relative abundance of Lactobacillus and Bifidobacterium in this PI-IBS model.

CONCLUSIONS

Rifaximin alleviated visceral hypersensitivity, recovered intestinal barrier function, and inhibited low-grade inflammation in colon and ileum of PI-IBS mouse model. Moreover, rifaximin exerts anti-inflammatory effects with only a minimal effect on the overall composition and diversity of the gut microbiota in this model.

Fructo-oligosaccharide intensifies visceral hypersensitivity and intestinal inflammation in a stress-induced irritable bowel syndrome mouse model

AIM

To determine whether fructo-oligosaccharide (FOS) affects visceral sensitivity, inflammation, and production of intestinal short-chain fatty acids (SCFA) in an irritable bowel syndrome (IBS) mouse model.

METHODS

Mice were randomly assigned to daily oral gavage of saline solution with or without FOS (8 g/kg body weight) for 14 d. Mice were further assigned to receive either daily one-hour water avoidance stress (WAS) or sham-WAS for the first 10 d. After 2 wk, visceral sensitivity was measured by abdominal withdrawal reflex in response to colorectal distension and mucosal inflammation was evaluated. Gas chromatography, real-time reverse transcription PCR, and immunohistochemistry assays were used to quantify cecal concentrations of SCFA, intestinal cytokine expression, and number of intestinal mast cells per high-power field (HPF), respectively.

RESULTS

Mice subjected to WAS exhibited visceral hypersensitivity and low-grade inflammation. Among mice subjected to WAS, FOS increased visceral hypersensitivity and led to higher cecal concentrations of acetic acid (2.49 ± 0.63 mmol/L vs 1.49 ± 0.72 mmol/L, P < 0.05), propionic acid (0.48 ± 0.09 mmol/L vs 0.36 ± 0.05 mmol/L, P < 0.01), butyric acid (0.28 ± 0.09 mmol/L vs 0.19 ± 0.003 mmol/L, P < 0.05), as well as total SCFA (3.62 ± 0.87 mmol/L vs 2.27 ± 0.75 mmol/L, P < 0.01) compared to saline administration. FOS also increased ileal interleukin (IL)-23 mRNA (4.71 ± 4.16 vs 1.00 ± 0.99, P < 0.05) and colonic IL-1β mRNA (2.15 ± 1.68 vs 0.88 ± 0.53, P < 0.05) expressions as well as increased mean mast cell counts in the ileum (12.3 ± 2.6 per HPF vs 8.3 ± 3.6 per HPF, P < 0.05) and colon (6.3 ± 3.2 per HPF vs 3.4 ± 1.2 per HPF, P < 0.05) compared to saline administration in mice subjected to WAS. No difference in visceral sensitivity, intestinal inflammation, or cecal SCFA levels was detected with or without FOS administration in mice subjected to sham-WAS.

CONCLUSION

FOS administration intensifies visceral hypersensitivity and gut inflammation in stress-induced IBS mice, but not in the control mice, and is also associated with increased intestinal SCFA production.

Reduced interstitial cells of Cajal and increased intraepithelial lymphocytes are associated with development of small intestinal bacterial overgrowth in post-infectious IBS mouse model

OBJECTIVE

Intestinal dysmotility and immune activation are likely involved in the pathogenesis of small intestinal bacteria overgrowth (SIBO) in irritable bowel syndrome (IBS). We aimed at investigating the role of interstitial cells of Cajal (ICC) and intestinal inflammation in the development of SIBO using a post-infectious IBS (PI-IBS) mouse model.

MATERIALS AND METHODS

NIH mice were randomly infected with Trichinella spiralis. Visceral sensitivity and stool pattern were assessed at 8-weeks post-infection (PI). Intestinal bacteria counts from jejunum and ileum were measured by quantitative real-time PCR to evaluate the presence of SIBO. ICC density, intraepithelial lymphocytes (IELs) counts, and intestinal cytokine levels (IL1-β, IL-6, toll-like receptor-4 (TLR-4), IL-10) in the ileum were examined.

RESULTS

PI-IBS mice demonstrated increased visceral sensitivity compared with the control group. One-third of the PI-IBS mice developed SIBO (SIBO+/PI-IBS) and was more likely to have abnormal stool form compared with SIBO negative PI-IBS (SIBO-/PI-IBS) mice but without difference in visceral sensitivity. SIBO+/PI-IBS mice had decreased ICC density and increased IELs counts in the ileum compared with SIBO-/PI-IBS mice. No difference in inflammatory cytokine expression levels were detected among the groups except for increased TLR-4 in PI-IBS mice compared with the control group.

CONCLUSIONS

Development of SIBO in PI-IBS mice was associated with reduced ICC density and increased IELs counts in the ileum. Our findings support the role of intestinal dysmotility and inflammation in the pathogenesis of SIBO in IBS and may provide potential therapeutic targets.

Neuroimmune factors in functional gastrointestinal disorders: A focus on irritable bowel syndrome

BACKGROUND

Abnormal abdominal pain perception is the most bothersome and difficult to treat symptom of functional gastrointestinal disorders (FGIDs). Visceral pain stimuli are perceived and transmitted by afferent neurons residing in the dorsal root ganglia that have sensory nerve endings in the gut wall and mesentery. Accumulating evidence indicates that peripheral activation and sensitization of these sensory nerve endings by bioactive mediators released by activated immune cells, in particular mast cells, can lead to aberrant neuroimmune interactions and the development and maintenance of visceral hypersensitivity. Besides direct neuronal activation, low concentrations of proteases, histamine, and serotonin can chronically sensitize nociceptors, such as TRP channels, leading to persistent aberrant pain perception.

PURPOSE

This review discusses the potential mechanisms underlying aberrant neuroimmune interactions in peripheral sensitization of sensory nerves. A better understanding of the cells, mediators, and molecular mechanisms triggering persistent aberrant neuroimmune interactions brings new insights into their contribution to the physiology and pathophysiology of visceral pain perception and provides novel opportunities for more efficient therapeutic treatments for these disorders.

Administration of Wasabia koreana Ameliorates Irritable Bowel Syndrome-Like Symptoms in a Zymosan-Induced Mouse Model

Irritable bowel syndrome (IBS) is a functional gastrointestinal disease with complex pathophysiology involving the brain-gut axis. To assess the effects of Wasabia koreana (WK) on IBS, we employed a mouse model of colonic zymosan injection presenting with diarrhea-predominant IBS-like symptoms. Oral WK administration significantly diminished stool score, suppressed colon length and weight change, and minimized body weight loss without affecting food intake. In WK-treated mice, the submucosal thickening and epithelial lining of the colon were inhibited and were similar to those of naïve mice. Infiltration of mast cells into the colon and serum tumor necrosis factor-α levels were markedly suppressed. These effects were comparable to those of sulfasalazine, an anti-inflammatory drug. Furthermore, the number of visceral pain-related behaviors was significantly decreased, and locomotion activities measured in the elevated plus maze and open field tests were significantly increased by WK in a dose-dependent manner compared with amitriptyline, an antidepressant. These changes were accompanied by reduced FosB2 expression in the brain. Taken together, these data suggest that WK may have potential as a medicinal food for IBS by acting on inflammatory diarrhea and neural activity.

Transfer of CD11c+ lamina propria mononuclear phagocytes from post-infectious irritable bowel syndrome causes mucosal barrier dysfunction and visceral hypersensitivity in recipient mice

The role of low-grade inflammation in the development of post‑infectious irritable bowel syndrome (PI‑IBS) has attracted increasing attention. Abnormal CD11c+ mononuclear phagocytes, such as dendritic cells (DCs), macrophages, and monocytes, are involved in the disruption of immune tolerance in organisms, which can lead to the development of chronic inflammatory diseases. The present study tested the hypothesis that CD11c+ lamina propria mononuclear phagocytes (CD11c+ LPMPs) contribute to increased mucosal permeability and visceral hypersensitivity in a PI‑IBS mouse model. CD11c+ LPMPs were isolated and purified via the digestion of intestinal tissues and magnetic‑activated cell sorting. We detected increased mucosal permeability, visceral hypersensitivity and intestinal inflammation during both the acute and chronic stages of Trichinella infection. Following the transfer of CD11c+ LPMPs from PI‑IBS mice into normal mice, low‑grade inflammation was detected, as demonstrated by increased IL‑4 expression in the ileum, as well as enhanced mucosal permeability, as indicated by decreased transepithelial electrical resistance and the pre-sence of ultrastructural alterations. More importantly, the mice that underwent adoptive transfer of CD11c+ LPMPs from the PI‑IBS mice also exhibited increased abdominal withdrawal reflex scores and a decreased threshold. Our data demonstrated that the CD11c+ LPMPs from this PI‑IBS mouse model were not only able to transfer enteric inflammation to the normal mice but also caused abnormal intestinal function, characterized by epithelial barrier disruption and visceral hyperalgesia.

Colonic N-methyl-d-aspartate receptor contributes to visceral hypersensitivity in irritable bowel syndrome

BACKGROUND AND AIMS

N-methyl-d-aspartate receptor (NMDAR) in brain, spinal cord, and enteric nervous system is involved in visceral hypersensitivity. This study aimed to reveal the functional expression of NMDAR on mucosal cells in colon and to investigate the downstream signal pathway from colonic NMDAR activation to visceral hypersensitivity in irritable bowel syndrome (IBS).

METHODS

The expression of mucosal NMDAR in IBS patients and healthy controls was assessed by immunohistochemistry and Western blot and correlated with abdominal pain/discomfort scores quantified by a validated questionnaire. Electromyography recording in response to colorectal distension was recorded to measure the colonic sensitivity of mice receiving NMDA administration intracolonically. Brain-derived neurotrophic factor (BDNF) expression and extracellular signal-regulated kinase (ERK) pathway activation were examined in human colonic epithelial HT29 cells after NMDA stimulation, with or without MK801 or U0126 pretreatment.

RESULTS

A significant upregulation of mucosal NMDAR was observed in IBS patients compared with controls, which was significantly correlated with abdominal pain/discomfort scores. Intracolonic administration of NMDA in normal mice produced increased colonic sensitivity to colorectal distension and elevated expression of BDNF and activation of ERK. Activation of NMDAR in colonic epithelial HT29 cells in vitro induced increased BDNF secretion in cell supernatants and higher BDNF expression in cells, as well as elevated phosphorylated ERK.

CONCLUSIONS

This study demonstrated that the activation of mucosal NMDAR in colon may contribute to the visceral hypersensitivity in IBS, by increasing production of BDNF in an ERK-dependent pathway.

TRPA1 in the spinal dorsal horn is involved in post-inflammatory visceral hypersensitivity: in vivo study using TNBS-treated rat model

Introduction

The transient receptor potential ankyrin-1 (TRPA1) channel, a pain transducer and amplifier, is drawing increasing attention in the field of visceral hypersensitivity, commonly seen in irritable bowel syndrome and inflammatory bowel disease. However, the role of TRPA1 in visceral nociception during post-inflammatory states is not well defined. Here, we explore the correlation between TRPA1 expression in the spinal dorsal horn (SDH) and persistent post-inflammatory visceral hypersensitivity.

Methods

We injected rats intracolonically with 2,4,6-trinitrobenzene sulfonic acid (TNBS) or vehicle (n=12 per group). Post-inflammatory visceral hypersensitivity was assessed by recording the electromyographic activity of the external oblique muscle in response to colorectal distension. TRPA1 expression and distribution in the spinal cord and colon were examined by Western blotting and immunohistochemistry.

Results

Animals exposed to TNBS had more abdominal contractions than vehicle-injected controls (P<0.05), which corresponded to a lower nociceptive threshold. Expression of TRPA1 in the SDH (especially in the substantia gelatinosa) and the colon was significantly greater in the TNBS-treated group than in controls (P<0.05). In the SDH, the number of TRPA1-immunopositive neurons was 25.75±5.12 in the control group and 34.25±7.89 in the TNBS-treated group (P=0.023), and integrated optical density values of TRPA1 in the control and TNBS-treated groups were 14,544.63±6,525.54 and 22,532.75±7,608.11, respectively (P=0.041).

Conclusion

Our results indicate that upregulation of TRPA1 expression in the SDH is associated with persistent post-inflammatory visceral hypersensitivity in the rat and provides insight into potential therapeutic targets for the control of persistent visceral hypersensitivity.

Nociceptive TRP Channels: Sensory Detectors and Transducers in Multiple Pain Pathologies

Specialized receptors belonging to the transient receptor potential (TRP) family of ligand-gated ion channels constitute the critical detectors and transducers of pain-causing stimuli. Nociceptive TRP channels are predominantly expressed by distinct subsets of sensory neurons of the peripheral nervous system. Several of these TRP channels are also expressed in neurons of the central nervous system, and in non-neuronal cells that communicate with sensory nerves. Nociceptive TRPs are activated by specific physico-chemical stimuli to provide the excitatory trigger in neurons. In addition, decades of research has identified a large number of immune and neuromodulators as mediators of nociceptive TRP channel activation during injury, inflammatory and other pathological conditions. These findings have led to aggressive targeting of TRP channels for the development of new-generation analgesics. This review summarizes the complex activation and/or modulation of nociceptive TRP channels under pathophysiological conditions, and how these changes underlie acute and chronic pain conditions. Furthermore, development of small-molecule antagonists for several TRP channels as analgesics, and the positive and negative outcomes of these drugs in clinical trials are discussed. Understanding the diverse functional and modulatory properties of nociceptive TRP channels is critical to function-based drug targeting for the development of evidence-based and efficacious new generation analgesics.

Dai-Kenchu-To, a Herbal Medicine, Attenuates Colorectal Distention-induced Visceromotor Responses in Rats

Background/Aims

Dai-kenchu-to (DKT), a traditional Japanese herbal medicine, is known to increase gastrointestinal motility and improve ileal function. We tested our hypotheses that (1) pretreatment with DKT would block the colorectal distention-induced visceromotor response in rats, and (2) pretreatment with DKT would attenuate colorectal distention-induced adrenocorticotropic hormone (ACTH) release and anxiety-related behavior.

Methods

Rats were pretreated with vehicle or DKT (300 mg/kg/5 mL, per os). Visceromotor responses were analyzed using electromyography in response to colorectal distention (10, 20, 40, 60, and 80 mmHg for 20 seconds at 3-minutes intervals). Anxiety-related behavior was measured during exposure to an elevated-plus maze after colorectal distention. Plasma ACTH and serum corticosterone levels were measured after exposure to the elevated-plus maze.

Results

Colorectal distention produced robust contractions of the abdominal musculature, graded according to stimulus intensity, in vehicle-treated rats. At 40, 60, and 80 mmHg of colorectal distention, the visceromotor responses of DKT-treated rats was significantly lower than that of vehicle-treated rats. At 80 mmHg, the amplitude was suppressed to approximately one-third in DKT-treated rats, compared with that in vehicle-treated rats. Smooth muscle compliance and the velocity of accommodation to 60 mmHg of stretching did not significantly differ between the vehicle-treated and DKT-treated rats. Similarly, the DKT did not influence colorectal distention-induced ACTH release, corticosterone levels, or anxiety-related behavior in rats.

Conclusions

Our results suggest that DKT attenuates the colorectal distention-induced visceromotor responses, without increasing smooth muscle compliance, ACTH release or anxiety-related behavior in rats.

Neonatal vaginal irritation results in long-term visceral and somatic hypersensitivity and increased hypothalamic-pituitary-adrenal axis output in female mice

Neonatal irritation of the vagina permanently sensitizes the vagina and distant somatic structures in a corticotrophin-releasing factor-dependent manner.

Experiencing early life stress or injury increases a woman's likelihood of developing vulvodynia and concomitant dysregulation of the hypothalamic–pituitary–adrenal (HPA) axis. To investigate the outcome of neonatal vaginal irritation (NVI), female mouse pups were administered intravaginal zymosan on postnatal days 8 and 10 and were assessed as adults for vaginal hypersensitivity by measuring the visceromotor response to vaginal balloon distension (VBD). Western blotting and calcium imaging were performed to measure transient receptor potential ankyrin 1 (TRPA1) and vanilloid 1 (TRPV1) in the vagina and innervating primary sensory neurons. Serum corticosterone (CORT), mast cell degranulation, and corticotropin-releasing factor receptor 1 (CRF₁) expression were measured as indicators of peripheral HPA axis activation. Colorectal and hind paw sensitivity were measured to determine cross-sensitization resulting from NVI. Adult NVI mice had significantly larger visceromotor response during VBD than naive mice. TRPA1 protein expression was significantly elevated in the vagina, and calcium transients evoked by mustard oil (TRPA1 ligand) or capsaicin (TRPV1 ligand) were significantly decreased in dorsal root ganglion from NVI mice, despite displaying increased depolarization-evoked calcium transients. Serum CORT, vaginal mast cell degranulation, and CRF₁ protein expression were all significantly increased in NVI mice, as were colorectal and hind paw mechanical and thermal sensitivity. Neonatal treatment with a CRF₁ antagonist, NBI 35965, immediately before zymosan administration largely attenuated many of the effects of NVI. These results suggest that NVI produces chronic hypersensitivity of the vagina, as well as of adjacent visceral and distant somatic structures, driven in part by increased HPA axis activation.

Irritable bowel syndrome: new insights into symptom mechanisms and advances in treatment

Despite being one of the most common conditions leading to gastroenterological referral, irritable bowel syndrome (IBS) is poorly understood. However, recent years have seen major advances. These include new understanding of the role of both inflammation and altered microbiota as well as the impact of dietary intolerances as illuminated by magnetic resonance imaging (MRI), which has thrown new light on IBS. This article will review new data on how excessive bile acid secretion mediates diarrhea and evidence from post infectious IBS which has shown how gut inflammation can alter gut microbiota and function. Studies of patients with inflammatory bowel disease (IBD) have also shown that even when inflammation is in remission, the altered enteric nerves and abnormal microbiota can generate IBS-like symptoms. The efficacy of the low FODMAP diet as a treatment for bloating, flatulence, and abdominal discomfort has been demonstrated by randomized controlled trials. MRI studies, which can quantify intestinal volumes, have provided new insights into how FODMAPs cause symptoms. This article will focus on these areas together with recent trials of new agents, which this author believes will alter clinical practice within the foreseeable future.

Altered Ion Channel/Receptor Expression and Function in Extrinsic Sensory Neurons: The Cause of and Solution to Chronic Visceral Pain?

The gastrointestinal tract is unique in that it is innervated by several distinct populations of neurons, whose cell bodies are either intrinsic (enteric, viscerofugal) or extrinsic (sympathetic, sensory afferents) to the wall of the gut. We are usually completely unaware of the continuous, complicated orchestra of functions that these neurons conduct. However, for patients with Inflammatory Bowel Disease (IBD) or functional gastrointestinal disorders, such as Functional Dyspepsia (FD) and Irritable Bowel Syndrome (IBS) altered gastrointestinal motility, discomfort and pain are common, debilitating symptoms. Whilst bouts of inflammation underlie the symptoms associated with IBD, over the past few years there is increased pre-clinical and clinical evidence that infection and inflammation are key risk factors for the development of several functional gastrointestinal disorders, in particular IBS. There is a strong correlation between prior exposure to gut infection and symptom occurrence; with the duration and severity of the initial illness the strongest associated risk factors. This review discusses the current body of evidence for neuroplasticity during inflammation and how in many cases fails to reset back to normal, long after healing of the damaged tissues. Recent evidence suggests that the altered expression and function of key ion channels and receptors within extrinsic sensory neurons play fundamental roles in the aberrant pain sensation associated with these gastrointestinal diseases and disorders.

TRP channel functions in the gastrointestinal tract

Transient receptor potential (TRP) channels are predominantly distributed in both somatic and visceral sensory nervous systems and play a crucial role in sensory transduction. As the largest visceral organ system, the gastrointestinal (GI) tract frequently accommodates external inputs, which stimulate sensory nerves to initiate and coordinate sensory and motor functions in order to digest and absorb nutrients. Meanwhile, the sensory nerves in the GI tract are also able to detect potential tissue damage by responding to noxious irritants. This nocifensive function is mediated through specific ion channels and receptors expressed in a subpopulation of spinal and vagal afferent nerve called nociceptor. In the last 18 years, our understanding of TRP channel expression and function in GI sensory nervous system has been continuously improved. In this review, we focus on the expressions and functions of TRPV1, TRPA1, and TRPM8 in primary extrinsic afferent nerves innervated in the esophagus, stomach, intestine, and colon and briefly discuss their potential roles in relevant GI disorders.

Differential Contribution of TRPA1, TRPV4 and TRPM8 to Colonic Nociception in Mice

Background

Various transient receptor potential (TRP) channels in sensory neurons contribute to the transduction of mechanical stimuli in the colon. Recently, even the cold-sensing menthol receptor TRPM(melastatin)8 was suggested to be involved in murine colonic mechano-nociception.

Methods

To analyze the roles of TRPM8, TRPA1 and TRPV4 in distension-induced colonic nociception and pain, TRP-deficient mice and selective pharmacological blockers in wild-type mice (WT) were used. Visceromotor responses (VMR) to colorectal distension (CRD) in vivo were recorded and distension/pressure-induced CGRP release from the isolated murine colon ex vivo was measured by EIA.

Results

Distension-induced colonic CGRP release was markedly reduced in TRPA1^-/- and TRPV4^-/- mice at 90/150 mmHg compared to WT. In TRPM8-deficient mice the reduction was only distinct at 150 mmHg. Exposure to selective pharmacological antagonists (HC030031, 100 μM; RN1734, 10 μM; AMTB, 10 μM) showed corresponding effects. The unselective TRP blocker ruthenium red (RR, 10 μM) was as efficient in inhibiting distension-induced CGRP release as the unselective antagonists of mechanogated DEG/ENaC (amiloride, 100 μM) and stretch-activated channels (gadolinium, 50 μM). VMR to CRD revealed prominent deficits over the whole pressure range (up to 90 mmHg) in TRPA1^-/- and TRPV4^-/- but not TRPM8^-/- mice; the drug effects of the TRP antagonists were again highly consistent with the results from mice lacking the respective TRP receptor gene.

Conclusions

TRPA1 and TRPV4 mediate colonic distension pain and CGRP release and appear to govern a wide and congruent dynamic range of distensions. The role of TRPM8 seems to be confined to signaling extreme noxious distension, at least in the healthy colon.

Acupoint Specificity on Colorectal Hypersensitivity Alleviated by Acupuncture and the Correlation with the Brain-Gut Axis

This project was focused on the study of the effect of the different acupoints on visceral hypersensitivity and the correlation with the brain-gut axis. By using a mouse model of zymosan-induced colorectal hypersensitivity, and observing the response of hypersensitivity model to colorectal distension stimulation in acupuncture at different acupoints, we selected the specific acupoints. With immunohistochemical staining method, we observed c-fos expression, distribution and changes after acupuncture on sensory pathway, including colorectum, spinal dorsal horn and different regions of brain center in the model with colorectal distension stimulation, and evaluated the acupuncture effect on brain-gut axis. The results revealed that the effectiveness of acupuncture for alleviating visceral hypersensitivity was different at individual acupoint, meaning Tianshu (ST25), Zusanli (ST36) and Shangjuxu (ST37) > Quchi (LI11) and Dachangshu (BL25) > Ciliao (BL32). C-fos expression was concentrated in anterior cingulate cortex, hypothalamus, spinal dorsal horn and colorectum in model of zymosan-induced colorectal hypersensitivity and it was down-regulated after acupuncture. The results demonstrates that the acupoint specificity presents in acupuncture for relieving visceral hypersensitivity and the effects are more predominated at the acupoints on stomach meridian innervated by the same or adjacent spinal ganglion segments. The model of zymosan-induced colorectal hypersensitivity can be the animal model simulating brain-gut interaction.

Role of the potassium chloride cotransporter isoform 2-mediated spinal chloride homeostasis in a rat model of visceral hypersensitivity

Visceral hypersensitivity represents an important hallmark in the pathophysiology of irritable bowel syndrome (IBS), of which the mechanisms remain elusive. The present study was designed to examine whether cation-chloride cotransporter (CCC)-mediated chloride (Cl(-)) homeostasis of the spinal cord is involved in chronic stress-induced visceral hypersensitivity. Chronic visceral hypersensitivity was induced by exposing male Wistar rats to water avoidance stress (WAS). RT-PCR, Western blotting, and immunohistochemistry were used to assess the expression of CCCs in the spinal cord. Patch-clamp recordings were performed on adult spinal cord slices to evaluate Cl(-) homeostasis and Cl(-) extrusion capacity of lamina I neurons. Visceral sensitivity was estimated by measuring the abdominal withdrawal reflex in response to colorectal distension (CRD). After 10 days of WAS exposure, levels of both total protein and the oligomeric form of the K(+)-Cl(-) cotransporter isoform 2 (KCC2), but not Na(+)-K(+)-2Cl(-) transporter isoform 1 (NKCC1), were significantly decreased in the dorsal horn of the lumbosacral spinal cord. The downregulation of KCC2 resulted in a depolarizing shifted equilibrium potential of GABAergic inhibitory postsynaptic current and impaired Cl(-) extrusion capacity in lamina I neurons of the lumbosacral spinal cord from WAS rats. Acute noxious CRD disrupted spinal KCC2 expression and function 2 h after the final distention in sham rats, but not in WAS rats. Pharmacological blockade of KCC2 activity by intrathecal injection of a KCC2 inhibitor [(dihydroindenyl)oxy] alkanoic acid enhanced visceral nociceptive sensitivity in sham rats, but not in WAS rats. These results suggest that KCC2 downregulation-mediated impairment of spinal cord Cl(-) homeostasis may play an important role in chronic stress-induced visceral hypersensitivity.

Sensory TRP channels: the key transducers of nociception and pain

Peripheral detection of nociceptive and painful stimuli by sensory neurons involves a complex repertoire of molecular detectors and/or transducers on distinct subsets of nerve fibers. The majority of such molecular detectors/transducers belong to the transient receptor potential (TRP) family of cation channels, which comprise both specific receptors for distinct nociceptive stimuli, as well as for multiple stimuli. This chapter discusses the classification, distribution, and functional properties of individual TRP channel types that have been implicated in various nociceptive and/or painful conditions.