Mechanical activation of vagal afferents involves opposing cation and TREK1 currents and NO regulation

Vagal afferents convey signals of mechanical stimulation in the gut to the brain, which is essential for the regulation of food intake. However, ion channels sensing mechanical stimuli are not fully understood. This study aimed to examine the ionic currents activated by mechanical stimulation and a possible neuro-modulatory role of nitric oxide on vagal afferents. Nodose neuronal currents and potentials, and intestinal afferent firing by mechanical stimulation were measured by whole-cell patch clamp, and in vitro afferent recording, respectively. Osmotically activated cation and two-pore domain K+ currents were identified in nodose neurons. The membrane potential displayed a biphasic change under hypotonic stimulation. Cation channel-mediated depolarization was followed by a hyperpolarization mediated by K+ channels. The latter was inhibited by l-methionine (TREK1 channel inhibitor) and l-NNA (nitric oxide synthase inhibitor). Correspondingly, mechanical stimulation activated opposing cation and TREK1 currents. NOS inhibition decreased TREK1 currents and potentiated jejunal afferent nerve firing induced by mechanical stimuli. This study suggested a novel activation mechanism of ion channels underlying adaptation under mechanical distension in vagal afferent neurons. The guts' ability to perceive mechanical stimuli is vital in determining how it responds to food intake. The mechanosensation through ion channels could initiate and control gut function.

A172 PSYCHOSOCIAL VARIABLES RELATED TO J-POUCH SURGERY FOR INFLAMMATORY BOWEL DISEASE: A SCOPING REVIEW

Background

Ulcerative Colitis (UC) and Crohn’s Disease (CD) are subtypes of Inflammatory Bowel Disease (IBD) which is a condition with an unclear etiology causing inflammation of the small and large intestine (Public Health Agency of Canada, 2021). IBD is treated by diet, medications, and/or surgeries, with the most common surgery recommended to UC patients being j-pouch surgery (Crohn’s and Colitis Canada, n.d.). J-pouch surgery is often accompanied by numerous side effects (e.g., leakage, pouchitis), but it has been shown to help manage symptoms and restore function (Crohn’s and Colitis Canada, n.d.). While noteworthy research has examined the functional and biological outcomes of j-pouch surgery (e.g., Kayal et al., 2020), far less has considered the perioperative psychosocial implications.

Aims

The aim of this scoping review is to assess the current literature concerning psychosocial factors related to j-pouch surgery for patients with IBD.

Methods

We used the PRISMA-ScR (Tricco et al., 2018) and JBI recommendations (Peters et al., 2020) as methodological guidelines for conducting this review. We conducted our search during the summer and fall of 2021 and searched the following sources: Medline, PsychInfo, CINAHL, EBM Reviews, ProQuest Dissertations and Theses Global, ResearchGate, Prospero, and PrePubMed. We included articles from 1980-September 13, 2021.

Results

Our initial search produced 718 articles. Of those that met our inclusion criteria, the majority discussed quality of life as their sole psychosocial variable. Among these studies, however, many used quality of life measures (e.g., IBDQ, SF-36) that prioritize health-related factors, with only a subset of questions directly addressing quality of life. The second most investigated psychosocial variables were those related to sexual health and functioning. When considering the levels of evidence, very few of our results were randomized control trials (RCT), while many were reviews (but not exclusively of RCT) and non-randomized controlled studies.

Conclusions

We have identified a substantial need for studies examining psychosocial implications of and for j-pouch surgery among patients with IBD. In our discussion, we identify common variables and outline the strongest articles in the current literature, investigate the disadvantages of currently used measures, and propose specific directions for future research.

Funding Agencies

None

A133 EVALUATION OF DYSPHAGIA IN AN ACADEMIC CENTRE

Background

Dysphagia is a common referral to gastroenterology with both benign and malignant etiologies. There is a lack of studies regarding its clinical assessment, complications and outcomes in Canada.

Aims

To evaluate the assessment and outcomes of secondary and tertiary level dysphagia referrals in adult patients.

Methods

We conducted a retrospective analysis of 98 consecutive adult patients who were initially referred for dysphagia to the Division of Gastroenterology at the Kingston Health Sciences Centre in 2015. Chart reviews were performed to assess demographics, characteristics of dysphagia, time for clinical assessment and outcomes.

Results

Of the referred patients, 19 did not present for their clinic appointment. The majority of patients (60%) were >60 yrs old with an equal distribution of male and female patients. In the assessed patients, reflux was the most common etiology (55.5%) followed by oropharyngeal causes (13.5%) and motility disorders (12%). Only 3 patients of the cohort were diagnosed with esophageal cancer, 2 of whom had metastasis at the time of presentation. Time from referral to clinical assessment was <4 weeks in just 30% and >12 weeks in 55%. For the patients diagnosed with malignancy, 2 were assessed within of 2 weeks while the third patient was assessed and endoscoped at 12 weeks. In patients sent for endoscopy, 28% had an endoscopy <4 weeks after assessment while 47% had endoscopy > 12 weeks after assessment. Manometry was performed in 18% of the patients assessed. Information was available on the course of the dysphagia in 65 patients. Dysphagia had resolved or improved in 83%, remained unchanged in 14% and worsened in 3%.

Conclusions

Reflux-related dysphagia was by far the most common cause of dysphagia in this referral cohort. Wait times for clinical assessment greatly exceeded current CAG recommendations in the majority. Given current resource constraints, this highlights the need for improved referral information and patient care pathways in the management of patients referred with dysphagia.

Funding Agencies

None

A80 CANADIAN NEUROGASTROENTEROLOGY NETWORK (CNN) SURVEY ON PH/MOTILITY TESTING IN CANADA

Background

Anecdotal reports suggest that access to pH/motility testing is problematic in Canada, but to date there is little data documenting this.

Aims

To assess the volume and accessibility of motility lab testing in Canada.

Methods

The CNN developed a questionnaire directed at the scope, volume and accessibility of pH/motility testing in Canadian labs. Fifty-three labs were identified using lists provided by companies that supply pH/motility recording equipment in Canada. Of these, 12 labs were excluded (10 had incorrect or absent contact information, 1 had recently closed and 1 had just opened). Questionnaires were sent in early 2020 to the remaining 41 labs, and respondents were asked to use data from their last fiscal year pre-pandemic.

Results

26 completed questionnaires were returned (i.e., 63% response rate, but representing ~ 51% of active labs): 23 adult units (7 community, 15 academic and 1 private) and 3 academic pediatric units. Of the adult units, 6 performed studies in children <12 yrs old. All 3 pediatric units provided both esophageal and anorectal high-resolution manometry (HRM) and pH/Impedance recording, with wait times of < 3 months. All 23 adult labs provided esophageal HRM, but just 50% performed anorectal manometry and only 3 anorectal manometry with biofeedback. Ambulatory pH/Impedance was performed in all but 1 adult unit. 15 of 23 adult centres reported access to colon transit studies and only one performed colonic manometry. No units performed antroduodenal manometry. Five units offered Bravo wireless pH recording and 4 performed ENDOFLIP. In adult units, the median number of procedures per year were as follows: esophageal HRMs - 278 (range: 50–1140); pH/impedance - 225 (range: 40–634); anorectal manometry - 90 (range: 10–450). Corresponding median wait times in months were as follows: esophageal HRM - 4 (range: 0.5–14); pH/Impedance - 4.5 (range: 0.5–14); anorectal manometry - 4.6 (range: 2–9). Only 6 of the 23 adult units met recommended wait time targets of <2 months. Testing was performed by a nurse in ~ 80% of centres, while testing was done by technicians in 2 units and physicians in 3 units. 8 units accepted referrals from primary care physicians, whereas the remainder only accepted specialist referrals. 50% screened referrals for appropriateness and restricted access accordingly.

Conclusions

The scope of motility and pH testing across Canada is variable, with lower GI testing lacking in many regions. Wait times vary significantly across labs and the majority of centres exceed recommended limits of 2 months. The reasons underlying the identified limitations to pH/motility testing access warrant further study.

Funding Agencies

None

Are psychological interventions effective in treating functional dyspepsia? A systematic review and meta-analysis

BACKGROUND AND AIM

Functional dyspepsia (FD) is a common gastrointestinal disorder, characterized primarily by postprandial fullness or early satiety and/or pain in the epigastrium with no endoscopic evidence of disease. Psychological therapies have been adapted to the treatment of disordered gut-brain interaction such as FD. We sought to determine if psychological interventions were efficacious in providing symptom management and improving health-related quality of life in patients suffering from FD.

METHODS

Data were sorted that belonged to Embase (1947 to January 2020), PsychINFO (1806 to January 2020), and Ovid MEDLINE (1946 to January 2020). Randomized controlled trials using a psychological intervention in adults meeting relevant diagnostic criteria for FD were included. Data including symptom scores and quality of life measures were extracted. A random-effect model meta-analysis with standardized mean differences was used.

RESULTS

Nine randomized controlled trials were identified that met our inclusion criteria. These were small, single-centered studies and used varying psychological therapies. Three studies had a sham treatment arm, leading to a high risk of bias in the remaining studies. All the studies reported beneficial effects of psychological treatment on patient's symptoms, some of which persisted up to 1 year. Psychological intervention was associated with an improvement in global FD symptom scores (standardized differences in means -1.33, 95% confidence interval -1.97 to -0.68).

CONCLUSIONS

Despite the limited data, the available evidence suggests that psychological therapy is beneficial in treating patients with FD and should be considered by treating physicians if available and patients are willing. Large well-designed, sham controlled trials are needed for this extremely common disorder.

A195 A RARE CAUSE OF SEVERE REFRACTORY DIARRHEA IN A PATIENT WITH COMMON VARIABLE IMMUNE DEFICIENCY ASSOCIATED INTESTINAL ENTEROPATHY

Background

CVID is the most common type of severe antibody deficiency. Gastrointestinal manifestations affect approximately 20–50% of patients. Boland et al. described in a case series that 2/3 CVID patients were able to achieve clinical and endoscopic remission with Vedolizumab. This α4β7 integrin antagonist inhibits intestinal T cell translocation by blocking integrin interactions with mucosal vascular addressin cell adhesion molecule 1, reducing lymphocyte mediated inflammation. However, despite its novel use for this indication, limited data is available on the consequences of this therapy in patients with CVID.

Aims

To report on a case assessing the efficacy and outcomes of Vedolizumab for the treatment of CVID associated autoimmune enteropathy.

Methods

We present the case of a 50-year-old male presenting with severe refractory diarrhea and malnutrition. A colonoscopy demonstrated patchy ulceration and biopsies revealed ulcerated active colitis, negative for CMV. He was treated with Vedolizumab and Total Parental Nutrition (TPN). His diarrhea resolved, he gained 20 kg and he was weaned off TPN. In 2019, he re-presented with severe diarrhea. Subsequently endoscopic evaluation revealed patchy edematous colonic mucosa and biopsies demonstrated minimally active colitis, negative for CMV. He again responded to Vedolizumab re-induction, however shortly after, his diarrhea returned aggressively. CT enterography demonstrated active jejunal inflammation. Subsequently, an EGD revealed multiple duodenal ulcers and luminal narrowing. Biopsies of the small bowel were sent to histopathology.

Results

CMV superinfection was diagnosed on pathology (image 1). This patient’s diarrhea completely resolved with IV Gancyclovir and he was discharged on maintenance treatment with oral Valganciclovir.

Conclusions

This represents the first reported case of CMV enteritis secondary to Vedolizumab for the treatment of CVID associated autoimmune enteropathy. In this case, clinical and endoscopic remission was observed with Vedolizumab, however subsequently hampered by CMV reactivation. Hommel et al., published a positive correlation in a single centre retrospective cohort study of CMV reactivation in patients with ulcerative colitis treated with Vedolizumab. A large retrospective review of data from a multicenter consortium database of over 1000 Vedolizumab treated IBD patients reported CMV colitis in only 4 patients. CMV reactivation appears to be an exceptionally rare but important event in patients treated with Vedolizumab. Based on this report, patients with CVID associated enteropathy and refractory diarrhea should be carefully screened for CMV when treated with Vedolizumab. Further prospective data assessing the incidence of CMV reactivation in patients with Vedolizumab therapy is required to further define these findings.

Funding Agencies

None

Examining Psychosocial Mechanisms of Pain-Related Disability in Inflammatory Bowel Disease

Disability in inflammatory bowel disease (IBD) is under-investigated. Models theorize that disability is the result of a disease and its related impairments, limitations, and restrictions. This disablement process can be affected by psychosocial factors. Pain, depression, catastrophizing, and social support are associated with IBD-disability outcomes, but no studies have examined these factors concurrently. This study examined the role of psychosocial factors in the process of IBD disablement within the context of pain. Depressive symptoms, pain catastrophizing, and perceived social support were proposed as mediators in the relationship between pain and pain-related disability in cross-sectional and longitudinal models. Cross-sectionally, the mediation effects of depressive symptoms and pain catastrophizing, but not perceived social support, were significant. Longitudinally, depression was a significant mediator. Depressive symptoms and pain catastrophizing have mechanistic roles in the relationship between IBD patients' pain and pain-related disability and should be targets for intervention.

Effect of high-fat diet on mechanosensitive transient receptor potential channel activation in vagal afferent neurons

Mechanical stimulation of the gastrointestinal tract is an important stimulus of satiety and can be transduced by transient receptor potential (TRP) channels. Several studies have revealed attenuated vagally-mediated satiety responses including mechanosensitivity in diet-induced obesity; however, ion channels underlying this hyposensitivity have not been fully understood. This study aimed to examine the effect of chronic high-fat diet on activation of selected mechanosensitive TRP channels in vagal afferents. C57/BL6 mice were fed on either a high-fat or low-fat diet for 6-8 weeks. An increase in the intracellular calcium to hypotonic solution and activators of TRPV1, TRPV4, and TRPA1 was measured in nodose neurons using Ca²⁺-imaging techniques. Jejunal afferent nerve firing induced by mechanical stimulation and TRP channel agonists was measured using in vitro extracellular multiunit afferent recording. In high-fat diet-fed mice, we observed reduced calcium influx and jejunal afferent response induced by mechanical stimuli and agonists of TRPV4 and TRPA1, but not TRPV1. Our data show diet-induced obesity disrupts the activation of TRPV4 and TRPA1, at both the cellular level and the level of nerve terminals in the small intestine, which may partly explain reduced mechanosensitivity of vagal afferents and may contribute to decreased gut-brain satiety signaling in obesity.

Chronic high fat diet impairs glucagon like peptide-1 sensitivity in vagal afferents

Dysfunction of the gut-brain axis is one of the potential contributors to the pathophysiology of obesity and is therefore a potential target for treatment. Vagal afferents innervating the gut play an important role in controlling energy homeostasis. There is an increasing evidence for the role of vagal afferents in mediating the anorexigenic effects of glucagon-like peptide-1 (GLP-1), an important satiety and incretin hormone. This study aimed to examine the effect of chronic high fat diet on GLP-1 sensitivity in vagal afferents. C57/BL6 mice were fed either a high-fat or low-fat diet for 6-8 weeks. To evaluate gastrointestinal afferent sensitivity and nodose neurons' response to GLP-1, extracellular afferent recordings and patch clamp were performed, respectively. Exendin-4 (Ex-4) was used as an agonist of the GLP-1 receptor. C-Fos Expression was examined as an indication of afferent input to the nucleus tractus solitarius (NTS). Food intake was monitored in real-time before and after Ex-4 treatment to monitor the consequence of the high fat diet on the satiating effect of GLP-1. In high fat fed (HFF) mice, GLP-1 caused lower activation of intestinal afferent nerves, and failed to potentiate mechanosensitive nerve responses compared to low fat fed (LFF). GLP-1 increased excitability in LFF and this effect was reduced in HFF neurons. Consistent with these findings on vagal afferent nerves, GLP-1 receptor stimulation given systemically, had a reduced satiating effect in HFF compared to LFF mice, and neuronal activation in the NTS was also reduced. The present study demonstrated chronic high fat diet impaired vagal afferent responses to GLP-1, resulting in impaired satiety signaling. GLP-1 sensitivity may account for the impairment of satiety signaling in obesity and thus a therapeutic target for obesity treatment.

Inducible nitric oxide synthase-derived nitric oxide reduces vagal satiety signalling in obese mice

KEY POINTS

Obesity is associated with disrupted satiety regulation. Mice with diet-induced obesity have reduced vagal afferent neuronal excitability and a decreased afferent response to satiety signals. A low grade inflammation occurs in obesity with increased expression of inducible nitric oxide synthase (iNOS). Inhibition of iNOS in diet-induced obese mice restored vagal afferent neuronal excitability, increased the afferent response to satiety mediators and distention of the gut, and reduced short-term energy intake. A prolonged inhibition of iNOS reduced energy intake and body weight gain during the first week, and reduced amounts of epididymal fat after 3 weeks. We identified a novel pathway underlying disrupted satiety regulation in obesity. Blocking of this pathway might be clinically useful for the management of obesity.

ABSTRACT

Vagal afferents regulate feeding by transmitting satiety signals to the brain. Mice with diet-induced obesity have reduced vagal afferent sensitivity to satiety signals. We investigated whether inducible nitric oxide synthase (iNOS)-derived NO contributed to this reduction. C57BL/6J mice were fed a high- or low-fat diet for 6-8 weeks. Nodose ganglia and jejunum were analysed by immunoblotting for iNOS expression; NO production was measured using a fluorometric assay. Nodose neuron excitability and intestinal afferent sensitivity were evaluated by whole-cell patch clamp and in vitro afferent recording, respectively. Expression of iNOS and production of NO were increased in nodose ganglia and the small intestine in obese mice. Inhibition of iNOS in obese mice by pre-treatment with an iNOS inhibitor increased nodose neuron excitability via 2-pore-domain K⁺ channel leak currents, restored afferent sensitivity to satiety signals and reduced short-term energy intake. Obese mice given the iNOS inhibitor daily for 3 weeks had reduced energy intake and decreased body weight gain during the first week, compared to mice given saline, and lower amounts of epididymal fat at the end of 3 weeks. Inhibition of iNOS or blocking the action of iNOS-derived NO on vagal afferent pathways might comprise therapeutic strategies for hyperphagia and obesity.

Increased TASK channel-mediated currents underlie high-fat diet induced vagal afferent dysfunction

We have previously demonstrated that satiety sensing vagal afferent neurons are less responsive to meal-related stimuli in obesity because of reduced electrical excitability. As leak K⁺ currents are key determinants of membrane excitability, we hypothesized that leak K⁺ currents are increased in vagal afferents during obesity. Diet-induced obesity was induced by feeding C57Bl/6J mice a high-fat diet (HFF) (60% energy from fat) for 8-10 wk. In vitro extracellular recordings were performed on jejunal afferent nerves. Whole cell patch-clamp recordings were performed on mouse nodose ganglion neurons. Leak K⁺ currents were isolated using ion substitution and pharmacological blockers. mRNA for TWIK-related acid-sensitive K⁺ (TASK) subunits was measured using quantitative real-time PCR. Intestinal afferent responses to nutrient (oleate) and non-nutrient (ATP) stimuli were significantly decreased in HFF mice. Voltage clamp experiments revealed the presence of a voltage-insensitive resting potassium conductance that was increased by external alkaline pH and halothane, known properties of TASK currents. In HFF neurons, leak K⁺ current was approximately doubled and was reduced by TASK1 and TASK3 inhibitors. The halothane sensitive current was similarly increased. Quantitative PCR revealed the presence of mRNA encoding TASK1 (KCNK3) and TASK3 (KCNK9) channels in nodose neurons. TASK3 transcript was significantly increased in HFF mice. The reduction in vagal afferent excitability in obesity is due in part to an increase of resting (leak) K⁺ conductance. TASK channels may account for the impairment of satiety signaling in diet-induced obesity and thus is a therapeutic target for obesity treatment. NEW & NOTEWORTHY This study characterized the electrophysiological properties and gene expression of the TWIK-related acid-sensitive K⁺ (TASK) channel in vagal afferent neurons. TASK conductance was increased and contributed to decreased excitability in diet-induced obesity. TASK channels may account for the impairment of satiety signaling in diet-induced obesity and thus is a promising therapeutic target.

Anti-Hu antibodies activate enteric and sensory neurons

IgG of type 1 anti-neuronal nuclear antibody (ANNA-1, anti-Hu) specificity is a serological marker of paraneoplastic neurological autoimmunity (including enteric/autonomic) usually related to small-cell lung carcinoma. We show here that IgG isolated from such sera and also affinity-purified anti-HuD label enteric neurons and cause an immediate spike discharge in enteric and visceral sensory neurons. Both labelling and activation of enteric neurons was prevented by preincubation with the HuD antigen. Activation of enteric neurons was inhibited by the nicotinic receptor antagonists hexamethonium and dihydro-β-erythroidine and reduced by the P2X antagonist pyridoxal phosphate-6-azo (benzene-2,4-disulfonic acid (PPADS) but not by the 5-HT₃ antagonist tropisetron or the N-type Ca-channel blocker ω-Conotoxin GVIA. Ca⁺⁺ imaging experiments confirmed activation of enteric neurons but not enteric glia. These findings demonstrate a direct excitatory action of ANNA-1, in particular anti-HuD, on visceral sensory and enteric neurons, which involves nicotinic and P2X receptors. The results provide evidence for a novel link between nerve activation and symptom generation in patients with antibody-mediated gut dysfunction.

Stress increases descending inhibition in mouse and human colon

BACKGROUND

A relationship between stress and the symptoms of irritable bowel syndrome (IBS) has been well established but the cellular mechanisms are poorly understood. Therefore, we investigated effects of stress and stress hormones on colonic descending inhibition and transit in mouse models and human tissues.

METHODS

Stress was applied using water avoidance stress (WAS) in the animal model or mimicked using stress hormones, adrenaline (5 nM), and corticosterone (1 μM). Intracellular recordings were obtained from colonic circular smooth muscle cells in isolated smooth muscle/myenteric plexus preparations and the inhibitory junction potential (IJP) was elicited by nerve stimulation or balloon distension oral to the site of recording.

KEY RESULTS

Water avoidance stress increased the number of fecal pellets compared to control (p < 0.05). WAS also caused a significant increase in IJP amplitude following balloon distension. Stress hormones also increased the IJP amplitude following nerve stimulation and balloon distension (p < 0.05) in control mice but had no effect in colons from stressed mice. No differences were observed with application of ATP between stress and control tissues, suggesting the actions of stress hormones were presynaptic. Stress hormones had a large effect in the nerve stimulated IJP in human colon (increased >50%). Immunohistochemical studies identified alpha and beta adrenergic receptor immunoreactivity on myenteric neurons in human colon.

CONCLUSIONS & INFERENCES

These studies suggest that WAS and stress hormones can signal via myenteric neurons to increase inhibitory neuromuscular transmission. This could lead to greater descending relaxation, decreased transit time, and subsequent diarrhea.

Effects of inflammation on the innervation of the colon

Inflammatory bowel diseases (IBD) such as ulcerative colitis and Crohn's disease lead to altered gastrointestinal (GI) function as a consequence of the effects of inflammation on the tissues that comprise the GI tract. Among these tissues are several types of neurons that detect the state of the GI tract, transmit pain, and regulate functions such as motility, secretion, and blood flow. This review article describes the structure and function of the enteric nervous system, which is embedded within the gut wall, the sympathetic motor innervation of the colon and the extrinsic afferent innervation of the colon, and considers the evidence that colitis alters these important sensory and motor systems. These alterations may contribute to the pain and altered bowel habits that accompany IBD.

The long chain fatty acid oleate activates mouse intestinal afferent nerves in vitro

Vagal afferents innervating the gastrointestinal tract serve an important nutrient-sensing function, and these signals contribute to satiety. Detection of nutrients occurs largely through the release of mediators from specialized enteroendocrine cells within the mucosa of the gastrointestinal tract. The signaling pathways leading to vagal afferent activation are not clear; however, previous in-vivo studies have implicated a role for cholecystokinin (CCK). We used an in vitro intestinal afferent extracellular recording preparation to study the effect of luminal perfusion of the long chain fatty acid oleate on mouse intestinal afferent activity. Oleate activated intestinal afferents in a concentration-dependent fashion, with an EC50 value of approximately 25 mmol/L. The L-type calcium channel blocker nicardipine attenuated the effect of oleate. Vagotomy resulted in a significant (>60%) reduction of the responses to both oleate and CCK. The CCK-1 receptor antagonist lorglumide nearly abolished responses to CCK and oleate. Our experiments therefore suggest that oleate activates intestinal afferents, with vagal afferents primarily involved; however, nonvagal fibres also contribute. The activation is dependent on CCK release, likely via activation of L-type channels on mucosal enteroendocrine cells, finally resulting in activation of CCK-1 receptors on the afferent terminals.

Impaired intestinal afferent nerve satiety signalling and vagal afferent excitability in diet induced obesity in the mouse

Gastrointestinal vagal afferents transmit satiety signals to the brain via both chemical and mechanical mechanisms. There is indirect evidence that these signals may be attenuated in obesity. We hypothesized that responses to satiety mediators and distension of the gut would be attenuated after induction of diet induced obesity. Obesity was induced by feeding a high fat diet (60% kcal from fat). Low fat fed mice (10% kcal from fat) served as a control. High fat fed mice were obese, with increased visceral fat, but were not hyperglycaemic. Recordings from jejunal afferents demonstrated attenuated responses to the satiety mediators cholecystokinin (CCK, 100 nm) and 5-hydroxytryptamine (5-HT, 10 μm), as was the response to low intensity jejunal distension, while responses to higher distension pressures were preserved. We performed whole cell patch clamp recordings on nodose ganglion neurons, both unlabelled, and those labelled by fast blue injection into the wall of the jejunum. The cell membrane of both labelled and unlabelled nodose ganglion neurons was less excitable in HFF mice, with an elevated rheobase and decreased number of action potentials at twice rheobase. Input resistance of HFF neurons was also significantly decreased. Calcium imaging experiments revealed reduced proportion of nodose ganglion neurons responding to CCK and 5-HT in obese mice. These results demonstrate a marked reduction in afferent sensitivity to satiety related stimuli after a chronic high fat diet. A major mechanism underlying this change is reduced excitability of the neuronal cell membrane. This may explain the development of hyperphagia when a high fat diet is consumed. Improving sensitivity of gastrointestinal afferent nerves may prove useful to limit food intake in obesity.

Glucagon-like peptide-1 inhibits voltage-gated potassium currents in mouse nodose ganglion neurons

BACKGROUND

Glucagon-like peptide-1 (GLP-1) is a major hormone known to regulate glucose homeostasis and gut function, and is an important satiety mediator. These actions are at least in part mediated via an action on vagal afferent neurons. However, the mechanism by which GLP-1 activates vagal afferents remains unknown. We hypothesized that GLP-1 acts on nodose ganglion neuron voltage-gated potassium (KV) channels, increasing membrane excitability.

METHODS

Employing perforated patch clamp recordings we examined the effects of GLP-1 on membrane properties as well as voltage-gated potassium currents. Extracellular recordings of jejunal afferents were performed to demonstrate the functional relevance of these effects at the nerve terminal.

KEY RESULTS

Glucagon-like peptide-1 depolarized a subpopulation of nodose neurons. This membrane depolarization was used to identify neurons containing functional GLP-1 receptors. In these neurons, GLP-1 decreased rheobase and broadened the action potential, and increased the number of action potentials elicited at twice rheobase. We identified a GLP-1 sensitive current whose reversal potential shifted in a depolarizing direction when extracellular potassium was increased. We identified two macroscopic K currents, IA, an inactivating current and IK a sustained current. GLP-1 caused inhibition of these currents, IK by 45%, P < 0.05 and IA currents by 52%P < 0.01, associated with a hyperpolarizing shift of steady-state inactivation curves for both currents. In extracellular recordings of jejunal afferents, GLP-1 increased firing rate, the effect blocked by the K(+) channel antagonist 4-AP.

CONCLUSIONS & INFERENCES

These experiments indicate that GLP-1 receptor activation results in vagal afferent excitation, due at least in part to inhibition of sustained and inactivating potassium currents. This mechanism may be important in satiety and glucose homeostatic signals arising from the gastrointestinal tract.

Visceral afferents - determinants and modulation of excitability

An essential property of visceral sensory afferents is to be able to alter their firing properties in response to changes in the microenvironment at the level of the sensory ending. Significant progress has been made in recent years in understanding the ionic mechanisms of the regulation of afferent neuronal excitability, and in identifying the mechanisms by which this can be altered. This article will review some of the recent developments in the state of knowledge regarding mechanisms of increased excitability after inflammation, and pharmacological modulation of excitability, concentrating on afferent nerves innervating the GI tract and urinary bladder.

Impairment of rectal afferent mechanosensitivity in experimental diabetes in the rat

Diabetes mellitus results in neuropathy of both somatic and visceral nerves. In diabetic patients with faecal incontinence, impaired rectal sensory function, manifested by a decreased sensitivity to balloon distention is common. This may contribute to unawareness of rectal filling and incontinence. There has been little study to date of visceral mechanosensation in experimental diabetes however. We hypothesized that experimental diabetes would impair mechanosensitivity in rectal afferent nerves. Diabetes was induced in rats by i.p. injection of streptozotocin. Controls were injected with citrate. In vitro recordings were performed from rectal afferent nerves innervating isolated segments of rectum. In control animals, three distinct populations of mechanosensitive fibres were identified. Low threshold fibres responded at low intensity stretch and reached a maximal firing rate at less than 10 g of stretch (11/24 units). Wide dynamic sensitivity units responded at low intensity stretch (<2 g) but encoded stimulus intensity in a linear fashion up to 20 g (12/24 units). High threshold units responded at greater than 5 g. In diabetic animals there was a near complete loss of LT units (1/19) and most (16/29) had properties similar to WD units. However, their response threshold was significantly increased. Firing rates in response to maximal distention did not change in diabetic animals. We conclude that experimental diabetes selectively affects the detection of low threshold 'physiologic' rectal distention, such as that which might occur during rectal filling, prior to defaecation.

The gaseous mediator, hydrogen sulphide, inhibits in vitro motor patterns in the human, rat and mouse colon and jejunum

Hydrogen sulphide (H2S) has been recently proposed as a transmitter in the brain and peripheral tissues. Its role in the gastrointestinal tract is still unknown despite some data which suggest an involvement mediating smooth muscle relaxation. The aim of this study was to investigate the effect of this gas on intestinal segments from mouse jejunum and colon, and muscular strips from the human and rat colon. In isolated segments of mouse colon and jejunum, bath applied sodium hydrogen sulphide (NaHS) (a H2S donor) caused a concentration-dependent inhibition of spontaneous motor complexes (MCs) (IC(50) 121 micromol L(-1) in the colon and 150 micromol L(-1) in the jejunum). This inhibitory effect of NaHS on MCs was (i) unaffected by tetrodotoxin (TTX), capsaicin, pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate and N-nitro-L-arginine suggesting a non-neural effect and (ii) significantly reduced by apamin 3 micromol L(-1). NaHS concentration-dependently inhibited the spontaneous motility in strips from human colon (IC(50) 261 micromol L(-1)) and rat colon (IC(50) 31 micromol L(-1)). The inhibitory effect of NaHS on colonic strips was (i) unaffected by the neural blocker TTX (1 micromol L(-1)) with IC(50) 183 micromol L(-1) for the human colon and of 26 micromol L(-1) for the rat colon and (ii) significantly reduced by glybenclamide (10 micromol L(-1)), apamin (3 micromol L(-1)) and TEA (10 mmol L(-1)) with IC(50) values of 2464, 1307 and 2421 micromol L(-1) for human strips, and 80, 167 and 674 micromol L(-1) for rat strips respectively. We conclude that H2S strongly inhibits in vitro intestinal and colonic motor patterns. This effect appears to be critically dependent on K channels particularly apamin-sensitive SK channels and glybenclamide-sensitive K (ATP) channels.

The gaseous mediator, hydrogen sulphide, inhibits in vitro motor patterns in the human, rat and mouse colon and jejunum

Hydrogen sulphide (H2S) has been recently proposed as a transmitter in the brain and peripheral tissues. Its role in the gastrointestinal tract is still unknown despite some data which suggest an involvement mediating smooth muscle relaxation. The aim of this study was to investigate the effect of this gas on intestinal segments from mouse jejunum and colon, and muscular strips from the human and rat colon. In isolated segments of mouse colon and jejunum, bath applied sodium hydrogen sulphide (NaHS) (a H2S donor) caused a concentration-dependent inhibition of spontaneous motor complexes (MCs) (IC(50) 121 micromol L(-1) in the colon and 150 micromol L(-1) in the jejunum). This inhibitory effect of NaHS on MCs was (i) unaffected by tetrodotoxin (TTX), capsaicin, pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate and N-nitro-L-arginine suggesting a non-neural effect and (ii) significantly reduced by apamin 3 micromol L(-1). NaHS concentration-dependently inhibited the spontaneous motility in strips from human colon (IC(50) 261 micromol L(-1)) and rat colon (IC(50) 31 micromol L(-1)). The inhibitory effect of NaHS on colonic strips was (i) unaffected by the neural blocker TTX (1 micromol L(-1)) with IC(50) 183 micromol L(-1) for the human colon and of 26 micromol L(-1) for the rat colon and (ii) significantly reduced by glybenclamide (10 micromol L(-1)), apamin (3 micromol L(-1)) and TEA (10 mmol L(-1)) with IC(50) values of 2464, 1307 and 2421 micromol L(-1) for human strips, and 80, 167 and 674 micromol L(-1) for rat strips respectively. We conclude that H2S strongly inhibits in vitro intestinal and colonic motor patterns. This effect appears to be critically dependent on K channels particularly apamin-sensitive SK channels and glybenclamide-sensitive K (ATP) channels.

Inflammation-induced hyperexcitability of nociceptive gastrointestinal DRG neurones: the role of voltage-gated ion channels

Gastrointestinal (GI) inflammation modulates the intrinsic properties of nociceptive dorsal root ganglia neurones, which innervate the GI tract and these changes are important in the genesis of abdominal pain and visceral hyperalgesia neurones exhibit hyperexcitability characterized by a decreased threshold for activation and increased firing rate, and changes in voltages-gated Na(+) and K(+) channels play a major role in this plasticity. This review highlights emerging evidence that specific subsets of channels and signalling pathways are involved and their potential to provide novel selective therapeutics targets for the treatment of abdominal pain.

Two TTX-resistant Na+ currents in mouse colonic dorsal root ganglia neurons and their role in colitis-induced hyperexcitability

The composition of Na+ currents in dorsal root ganglia (DRG) neurons depends on their neuronal phenotype and innervation target. Two TTX-resistant (TTX-R) Na+ currents [voltage-gated Na channels (Nav)] have been described in small DRG neurons; one with slow inactivation kinetics (Nav1.8) and the other with persistent kinetics (Nav1.9), and their modulation has been implicated in inflammatory pain. This has not been studied in neurons projecting to the colon. This study examined the relative importance of these currents in inflammation-induced changes in a mouse model of inflammatory bowel disease. Colonic sensory neurons were retrogradely labeled, and colitis was induced by instillation of trinitrobenzenesulfonic acid (TNBS) into the lumen of the distal colon. Seven to ten days later, immunohistochemical properties were characterized in controls, and whole cell recordings were obtained from small (<40 pF) labeled DRG neurons from control and TNBS animals. Most neurons exhibited both fast TTX-sensitive (TTX-S)- and slow TTX-R-inactivating Na+ currents, but persistent TTX-R currents were uncommon (<15%). Most labeled neurons were CGRP (79%), tyrosine kinase A (trkA) (84%) immunoreactive, but only a small minority bind IB4 (14%). TNBS-colitis caused ulceration, thickening of the colon and significantly increased neuronal excitability. The slow TTX-R-inactivating Na current density (Nav1.8) was significantly increased, but other Na currents were unaffected. Most small mouse colonic sensory neurons are CGRP, trkA immunoreactive, but not isolectin B4 reactive and exhibit fast TTX-S, slow TTX-R, but not persistent TTX-R Na+ currents. Colitis-induced hyperexcitability is associated with increased slow TTX-R (Nav1.8) Na+ current. Together, these findings suggest that colitis alters trkA-positive neurons to preferentially increase slow TTX-R Na+ (Nav1.8) currents.

Ileitis modulates potassium and sodium currents in guinea pig dorsal root ganglia sensory neurons

Intestinal inflammation induces hyperexcitability of dorsal root ganglia sensory neurons, which has been implicated in increased pain sensation. This study examined whether alteration of sodium (Na+) and/or potassium (K+) currents underlies this hyperexcitability. Ileitis was induced in guinea pig ileum with trinitrobenzene sulphonic acid (TBNS) and dorsal root ganglion neurons innervating the site of inflammation were identified by Fast Blue or DiI fluorescence labelling. Whole cell recordings were made from acutely dissociated small-sized neurons at 7-10 days. Neurons exhibited transient A-type and sustained outward rectifier K+ currents. Compared to control, both A-type and sustained K+ current densities were significantly reduced (42 and 34%, respectively; P < 0.05) in labelled neurons from the inflamed intestine but not in non-labelled neurons. A-type current voltage dependence of inactivation was negatively shifted in labelled inflamed intestine neurons. Neurons also exhibited tetrodotoxin-sensitive and resistant Na+ currents. Tetrodotoxin-resistant sodium currents were increased by 37% in labelled neurons from the inflamed intestine compared to control (P < 0.01), whereas unlabelled neurons were unaffected. The activation and inactivation curves of these currents were unchanged by inflammation. These data suggest ileitis increases excitability of intestinal sensory neurons by modulating multiple ionic channels. The lack of effect in non-labelled neurons suggests signalling originated at the nerve terminal rather than through circulating mediators and, given that Na+ currents are enhanced whereas K+ currents are suppressed, one or more signalling pathways may be involved.

Release of nitric oxide in the central nervous system mediates tonic and phasic contraction of the cat lower oesophageal sphincter

Nitric oxide (NO) in the brainstem is implicated in the control of swallowing and oesophageal peristalsis. This study examines the role of brainstem NO in the maintenance of lower oesophageal sphincter (LOS) tone, relaxation and contraction. In urethane-anaesthetized cats, oesophageal peristalsis and sphincter pressures were continuously monitored. Drugs were administered into the fourth ventricle. Oesophageal peristalsis and sphincter relaxation and contraction were induced by superior laryngeal nerve stimulation or intra-oesophageal balloon distention. Basal sphincter pressure was significantly reduced after the i.c.v. administration of the nitric oxide synthase (NOS) inhibitor, l-Ng-monomethyl arginine. The inhibitor's d-isomer had no significant effect on basal sphincter pressure, while l-arginine partially reversed the effect. The NOS inhibitor had no effect on sphincter relaxation, whereas the contraction of the sphincter following relaxation was significantly inhibited. Central nitric oxide synthase inhibition reduces basal LOS tone and contraction amplitude but has no effect on swallow or balloon distention induced sphincter relaxation. Therefore, central release of NO acts in the pathway to stimulate dorsal motor nucleus of the vagus neurones projecting to excitatory neurones in the sphincter. Inhibition of nitric oxide synthase in the CNS does not prevent relaxation of the LOS, suggesting that other pathways that do not utilize NO are important in the induction of LOS relaxation.

Central nervous system nitric oxide induces oropharyngeal swallowing and esophageal peristalsis in the cat

BACKGROUND & AIMS

The functional role of brainstem nitric oxide (NO) in swallowing and esophageal peristalsis remains unknown. We examined the effects of blockade of central nervous system (CNS) NO synthase (NOS) on swallowing and on primary and secondary peristalsis.

METHODS

(1) The effect of intravenous (IV) NOS inhibitor N(G)-nitro-L-arginine (L-NNA) on swallowing and swallowing-induced peristalsis was examined. (2) An NOS inhibitor (N(G)-monomethyl-L-arginine [L-NMMA]) was administered into the fourth ventricle intracerebroventricularly (ICV), and its effects on swallowing and primary and secondary peristalsis were examined.

RESULTS

(1) IV L-NNA significantly reduced the number of oropharyngeal swallows and the induction of primary peristalsis in the smooth muscle portion of the esophageal body; the change was not significant within the striated muscle portion. (2) L-NMMA given ICV significantly reduced the number of oropharyngeal swallows and the incidence of primary peristalsis in both smooth and striated muscle, but the reduction in amplitude was significant only for the smooth muscle contraction. There was a significant reduction in both the amplitude and incidence of secondary peristalsis, only in the smooth muscle portion.

CONCLUSIONS

CNS NO is an important neurotransmitter in the induction of oropharyngeal swallowing and esophageal peristalsis. The neural substrates mediating striated and smooth muscle peristalsis may be both anatomically and neurochemically distinct.

Superior laryngeal nerve stimulation in the cat: effect on oropharyngeal swallowing, oesophageal motility and lower oesophageal sphincter activity

Superior laryngeal nerve (SLN) stimulation can activate the brainstem swallowing mechanism to produce a complete swallowing sequence consisting of oropharyngeal, oesophageal and lower oesophageal sphincter (LOS) components. However, little is known of the effect of SLN stimulation (peripheral-sensory input from the pharynx) on the characteristics of oesophageal motor activity, especially in the smooth muscle portion. The present study examined the effect of varying stimulus train length and frequency on each of the three components of the reflex. Acute studies were performed in urethane anaesthetized cats. Oesophageal motility was monitored using conventional manometric techniques, and oropharyngeal swallowing by the mylohyoid electromyogram. SLN stimulus train length (1-10 sec) and frequency (5-30 Hz) were varied independently. Increased train length or frequency resulted in (1) an increase in oropharyngeal swallowing and incidence of the complete swallowing response, (2) an increase in latency to onset of the oesophageal peristaltic wave, (3) reduction of the amplitude of the evoked peristaltic contraction in the smooth muscle portion, without altering its velocity, (4) increased LOS relaxation, and increased LOS after-contraction. The LOS contraction was abolished by atropine (100 micrograms kg-1). Therefore, increased SLN stimulation not only results in excitation of the central swallowing program and the oropharyngeal stage of swallowing, but has major effects on the oesophageal and LOS stages of swallowing. Afferent SLN stimuli can impact on the control mechanisms for each stage, to inhibit or excite the stages in different ways.