Gary M. Mawe

Fundamentals of Neurogastroenterology: Basic Science

This review examines the fundamentals of neurogastroenterology that may underlie the pathophysiology of functional GI disorders (FGIDs). It was prepared by an invited committee of international experts and represents an abbreviated version of their consensus document that will be published in its entirety in the forthcoming book and online version entitled ROME IV. It emphasizes recent advances in our understanding of the enteric nervous system, sensory physiology underlying pain, and stress signaling pathways. There is also a focus on neuroimmmune signaling and intestinal barrier function, given the recent evidence implicating the microbiome, diet, and mucosal immune activation in FGIDs. Together, these advances provide a host of exciting new targets to identify and treat FGIDs and new areas for future research into their pathophysiology.

Activation of neuronal P2X7 receptor-pannexin-1 mediates death of enteric neurons during colitis

Inflammatory bowel diseases (IBDs) are chronic relapsing and remitting conditions associated with long-term gut dysfunction resulting from alterations to the enteric nervous system and a loss of enteric neurons. The mechanisms underlying inflammation-induced enteric neuron death are unknown. Here using in vivo models of experimental colitis we report that inflammation causes enteric neuron death by activating a neuronal signaling complex composed of P2X7 receptors (P2X7Rs), pannexin-1 (Panx1) channels, the Asc adaptor protein and caspases. Inhibition of P2X7R, Panx1, Asc or caspase activity prevented inflammation-induced neuron cell death. Preservation of enteric neurons by inhibiting Panx1 in vivo prevented the onset of inflammation-induced colonic motor dysfunction. Panx1 expression was reduced in Crohns disease but not ulcerative colitis. We conclude that activation of neuronal Panx1 underlies neuron death and the subsequent development of abnormal gut motility in IBD. Targeting Panx1 represents a new neuroprotective strategy to ameliorate the progression of IBD-associated dysmotility.

Serotonin signalling in the gut—functions, dysfunctions and therapeutic targets

Serotonin (5-HT) has been recognized for decades as an important signalling molecule in the gut, but it is still revealing its secrets. Novel gastrointestinal functions of 5-HT continue to be discovered, as well as distant actions of gut-derived 5-HT, and we are learning how 5-HT signalling is altered in gastrointestinal disorders. Conventional functions of 5-HT involving intrinsic reflexes include stimulation of propulsive and segmentation motility patterns, epithelial secretion and vasodilation. Activation of extrinsic vagal and spinal afferent fibres results in slowed gastric emptying, pancreatic secretion, satiation, pain and discomfort, as well as nausea and vomiting. Within the gut, 5-HT also exerts nonconventional actions such as promoting inflammation and serving as a trophic factor to promote the development and maintenance of neurons and interstitial cells of Cajal. Platelet 5-HT, originating in the gut, promotes haemostasis, influences bone development and serves many other functions. 5-HT3 receptor antagonists and 5-HT4 receptor agonists have been used to treat functional disorders with diarrhoea or constipation, respectively, and the synthetic enzyme tryptophan hydroxylase has also been targeted. Emerging evidence suggests that exploiting epithelial targets with nonabsorbable serotonergic agents could provide safe and effective therapies. We provide an overview of these serotonergic actions and treatment strategies.

Enhanced excitability of myenteric AH neurones in the inflamed guinea-pig distal colon

The electrical and synaptic properties of myenteric neurones in normal and inflamed guinea‐pig distal colons were evaluated by intracellular microelectrode recording. Chronic inflammation was established 6 days following administration of trinitrobenzene sulfonic acid (TNBS). In S neurones, inflammation only altered synaptic inputs as the amplitude of fast excitatory postsynaptic potentials were significantly larger (31 ± 2 mV compared to 20 ± 1 mV) and they were more likely to receive slow excitatory synaptic input (85% compared to 55%). AH neurones displayed altered electrical properties in colitis compared to control tissues: they generated more action potentials during a maximal depolarising current pulse (7 ± 1 compared to 1.6 ± 0.2); they had a smaller afterhyperpolarisation (9 ± 2 mV s compared to 20 ± 2 mV s); and they were more likely to receive fast excitatory synaptic input (74% compared to 17%), possess spontaneous activity (46% compared to 3%), and generate anodal break action potentials (58% compared to 19%). Although the resting membrane potential, input resistance and action potential characteristics were unaltered in AH neurones from inflamed tissues, they exhibited an enhanced Cs+‐sensitive rectification of the current–voltage relationship. This suggests that the increase in excitability of AH neurones may involve a colitis‐induced augmentation of the hyperpolarisation‐activated cation current (Ih) in these cells. An increased excitability, selectively in AH neurones, suggests that the afferent limb of intrinsic motor reflexes is disrupted in the inflamed colon and this may contribute to dysmotility associated with inflammatory diseases.

Enterochromaffin cell hyperplasia and decreased serotonin transporter in a mouse model of postinfectious bowel dysfunction.

Abstract  Patients with postinfective irritable bowel syndrome and Trichinella spiralis‐infected mice share many features including visceral hypersensitivity and disordered motility. We assessed enterochromaffin (EC) numbers and serotonin transporter (SERT) using National Institute of Health (NIH) female mice studied for up to 56 days post‐T. spiralis infection. The effects of steroid treatment and the T‐cell dependence of the observed responses were assessed by infection of hydrocortisone‐treated or T‐cell receptor knock out [TCR (β×δ) KO] animals. Enterochromaffin cell density in uninfected animals increased from duodenum 10.0 cells mm−2 (5.9–41.0) to colon 61.8. (46.3–162) cells mm−2P < 0.0001. Infection increased duodenal and jejunal counts which rose to 37.3 (22–57.7) cells mm−2 and 50.6 (7–110.8) cells mm−2, respectively, at day 14. Infection significantly reduced jejunal SERT expression, with luminance values falling from 61.0 (45.1–98.3) to a nadir of 11.6 (0–36.0) units at day 9, P < 0.001. Specific deficiencies in all T cells reduced EC hyperplasia and abrogated infection‐induced mastocytosis. Thus infection induced inflammation increases EC numbers, as has been reported in PI‐IBS, and reduces SERT. This may increase mucosal 5HT availability and contribute to the clinical presentation of PI‐IBS.

Review article: intestinal serotonin signalling in irritable bowel syndrome

Alterations in motility, secretion and visceral sensation are hallmarks of irritable bowel syndrome. As all of these aspects of gastrointestinal function involve serotonin signalling between enterochromaffin cells and sensory nerve fibres in the mucosal layer of the gut, potential alterations in mucosal serotonin signalling have been explored as a possible mechanism of altered function and sensation in irritable bowel syndrome. Literature related to intestinal serotonin signalling in normal and pathophysiological conditions has been searched and summarized.

Indiscriminate loss of myenteric neurones in the TNBS-inflamed guinea-pig distal colon.

Abstract  This investigation was conducted to establish whether guinea‐pig trinitrobenzene sulfonic acid (TNBS)‐colitis was associated with a change in the number of neurones of the myenteric plexus, and, if so, whether select subpopulations of neurones were affected. Total neurones were quantified with human (Hu) antiserum, and subpopulations were evaluated with antisera directed against choline acetyltransferase, nitric oxide synthase, calretinin, neuronal nuclear protein or vasoactive intestinal peptide (VIP). Colitis was associated with a loss of 20% of the myenteric neurones, most of which occurred during the first 12 h past‐TNBS administration. During this period, myenteric ganglia were infiltrated with neutrophils while lymphocytes appeared at a later time‐point. The neuronal loss persisted at a 56‐day time‐point, when inflammation had resolved. The decrease in myenteric neurones was not associated with a decrease in any given subpopulation of neurones, but the proportion of VIP‐immunoreactive neurones increased 6 days following TNBS administration and returned to the control range at the 56 days. These findings indicate that there is an indiscriminant loss of myenteric neurones that occurs during the onset of TNBS‐colitis, and the loss of neurones may be associated with the appearance of neutrophils in the region.

Activation of colonic mucosal 5-HT(4) receptors accelerates propulsive motility and inhibits visceral hypersensitivity.

BACKGROUND & AIMS 5-hydroxytryptamine receptor (5-HT(4)R) agonists promote gastrointestinal motility and attenuate visceral pain, but concerns about adverse reactions have restricted their availability. We tested the hypotheses that 5-HT(4) receptors are expressed in the colonic epithelium and that 5-HT(4)R agonists can act intraluminally to increase motility and reduce visceral hypersensitivity. METHODS Mucosal expression of the 5-HT(4)R was evaluated by reverse-transcriptase polymerase chain reaction and immunohistochemical analysis of tissues from 5-HT(4)R(BAC)-enhanced green fluorescent protein mice. Amperometry, histology, and short-circuit current measurements were used to study 5-HT, mucus, and Cl(-) secretion, respectively. Propulsive motility was measured in guinea pig distal colon, and visceromotor responses were recorded in a rat model of colonic hypersensitivity. 5-HT(4)R compounds included cisapride, tegaserod, naronapride, SB204070, and GR113808. RESULTS Mucosal 5-HT(4) receptors were present in the small and large intestines. In the distal colon, 5-HT(4) receptors were expressed by most epithelial cells, including enterochromaffin and goblet cells. Stimulation of 5-HT(4)Rs evoked mucosal 5-HT release, goblet cell degranulation, and Cl(-) secretion. Luminal administration of 5-HT(4)R agonists accelerated propulsive motility; a 5-HT(4)R antagonist blocked this effect. Bath application of 5-HT(4)R agonists did not affect motility. Oral or intracolonic administration of 5-HT(4)R agonists attenuated visceral hypersensitivity. Intracolonic administration was more potent than oral administration, and was inhibited by a 5-HT(4)R antagonist. CONCLUSIONS Mucosal 5-HT(4) receptor activation can mediate the prokinetic and antinociceptive actions of 5-HT(4)R agonists. Colon-targeted, intraluminal delivery of 5-HT(4)R agonists might be used to promote motility and alleviate visceral pain, while restricting systemic bioavailability and resulting adverse side effects.

Serotonin transporter function and expression are reduced in mice with TNBS-induced colitis.

Abstract  Regulated release of serotonin (5‐HT) from enterochromaffin (EC) cells activates neural reflexes that are involved in gut motility, secretion, vascular perfusion and sensation. The 5‐HT‐selective reuptake transporter (SERT) terminates serotonergic signalling in the intestinal mucosa. The aim of this investigation was to determine whether mucosal 5‐HT content, release, and/or reuptake are altered in a murine model of immune cell‐mediated colitis. Experiments were conducted 6 days after colitis was induced by 2,4,6‐trinitrobenzene sulfonic acid, a time point when macroscopic and histological damage scores indicated significant inflammation. During inflammation, SERT transcript levels and immunoreactivity were reduced, and the uptake of [3H] 5‐HT was impaired. Increases in mucosal 5‐HT content and the number of 5‐HT‐immunoreactive mast cells in the lamina propria were also detected in the inflamed region, whereas EC cell numbers did not change. Mucosal 5‐HT released under basal and stimulated conditions was unchanged in animals with colitis. These data suggest that murine colitis alters 5‐HT signalling by increasing 5‐HT availability through decreased 5‐HT uptake by mucosal epithelial cells. These findings support the concept that altered 5‐HT signalling could be a contributing factor in altered gut function and sensitivity in inflammatory bowel disease.

Gastrointestinal Conditions in Children With Autism Spectrum Disorder: Developing a Research Agenda

* Abbreviations: ASD — : autism spectrum disorder GI — : gastrointestinal 5-HT — : serotonin Autism spectrum disorders (ASDs) are a set of complex neurodevelopmental disorders defined behaviorally by impaired social interaction, delayed and disordered language, repetitive or stereotypic behavior, and a restricted range of interests. ASDs represent a significant public health issue with recent estimates indicating that as many as 1% of children in the United States are diagnosed with an ASD.1,2 Many individuals with ASDs have symptoms of associated medical conditions, including seizures, sleep problems, metabolic conditions, and gastrointestinal (GI) disorders, which have significant health, developmental, social, and educational impacts. Gastrointestinal complaints are a commonly reported concern for parents and may be related to problem behaviors and other medical issues such as dysregulated sleep (ATN Annual Registry Report, unpublished data, November 2009).3 Despite the magnitude of these issues, potential GI problems are not routinely considered in ASD evaluations. This likely reflects several factors, including variability in reported rates of GI disorders, controversies regarding the relationship between GI symptoms and the putative causes of autism, the limited verbal capacity of many ASD patients, and the lack of recognition by clinicians that certain behavioral manifestations in children with ASDs are indicators of GI problems (eg, pain, discomfort, or nausea).4–10 Whether GI issues in this population are directly related to the pathophysiology of autism, or are strictly a comorbid condition of ASD remains to be determined, but clinical practice and research to date indicate the important role of GI conditions in ASDs and their impact on children as well as their parents and clinicians.9 On November 15, 2009, a symposium addressing these issues was organized as an adjunct to the annual meeting of the North American Society for Pediatric Gastroenterology, Hepatology, and Nutrition. A … Address correspondence to Daniel L. Coury, MD, Professor of Pediatrics and Psychiatry, The Ohio State University, Chief, Developmental & Behavioral Pediatrics, Nationwide Childrens Hospital, 700 Childrens Dr, Timken G-350, Columbus OH 43205-2696