Properties of cholinergic and non-cholinergic submucosal neurons along the mouse colon

Helminth infection driven gastrointestinal hypermotility is independent of eosinophils and mediated by alterations in smooth muscle instead of enteric neurons

Intestinal helminth infection triggers a type 2 immune response that promotes a 'weep-and sweep' response characterised by increased mucus secretion and intestinal hypermotility, which function to dislodge the worm from its intestinal habitat. Recent studies have discovered that several other pathogens cause intestinal dysmotility through major alterations to the immune and enteric nervous systems (ENS), and their interactions, within the gastrointestinal tract. However, the involvement of these systems has not been investigated for helminth infections. Eosinophils represent a key cell type recruited by the type 2 immune response and alter intestinal motility under steady-state conditions. Our study aimed to investigate whether altered intestinal motility driven by the murine hookworm, Nippostrongylus brasiliensis, infection involves eosinophils and how the ENS and smooth muscles of the gut are impacted. Eosinophil deficiency did not influence helminth-induced intestinal hypermotility and hypermotility did not involve gross structural or functional changes to the ENS. Hypermotility was instead associated with a dramatic increase in smooth muscle thickness and contractility, an observation that extended to another rodent nematode, Heligmosomoides polygyrus. In summary our data indicate that, in contrast to other pathogens, helminth-induced intestinal hypermotility is driven by largely by myogenic, rather than neurogenic, alterations with such changes occurring independently of eosinophils. (<300 words).

The Imperative for Innovative Enteric Nervous System-Intestinal Organoid Co-Culture Models: Transforming GI Disease Modeling and Treatment

This review addresses the need for innovative co-culture systems integrating the enteric nervous system (ENS) with intestinal organoids. The breakthroughs achieved through these techniques will pave the way for a transformative era in gastrointestinal (GI) disease modeling and treatment strategies. This review serves as an introduction to the companion protocol paper featured in this journal. The protocol outlines the isolation and co-culture of myenteric and submucosal neurons with small intestinal organoids. This review provides an overview of the intestinal organoid culture field to establish a solid foundation for effective protocol application. Remarkably, the ENS surpasses the number of neurons in the spinal cord. Referred to as the "second brain", the ENS orchestrates pivotal roles in GI functions, including motility, blood flow, and secretion. The ENS is organized into myenteric and submucosal plexuses. These plexuses house diverse subtypes of neurons. Due to its proximity to the gut musculature and its cell type complexity, there are methodological intricacies in studying the ENS. Diverse approaches such as primary cell cultures, three-dimensional (3D) neurospheres, and induced ENS cells offer diverse insights into the multifaceted functionality of the ENS. The ENS exhibits dynamic interactions with the intestinal epithelium, the muscle layer, and the immune system, influencing epithelial physiology, motility, immune responses, and the microbiome. Neurotransmitters, including acetylcholine (ACh), serotonin (5-HT), and vasoactive intestinal peptide (VIP), play pivotal roles in these intricate interactions. Understanding these dynamics is imperative, as the ENS is implicated in various diseases, ranging from neuropathies to GI disorders and neurodegenerative diseases. The emergence of organoid technology presents an unprecedented opportunity to study ENS interactions within the complex milieu of the small and large intestines. This manuscript underscores the urgent need for standardized protocols and advanced techniques to unravel the complexities of the ENS and its dynamic relationship with the gut ecosystem. The insights gleaned from such endeavors hold the potential to revolutionize GI disease modeling and treatment paradigms.

Functional and Transcriptomic Characterization of Postnatal Maturation of ENS and SIP Syncytium in Mice Colon

The interplay of the enteric nervous system (ENS) and SIP syncytium (smooth muscle cells-interstitial cells of Cajal-PDGFRα+ cells) plays an important role in the regulation of gastrointestinal (GI) motility. This study aimed to investigate the dynamic regulatory mechanisms of the ENS-SIP system on colon motility during postnatal development. Colonic samples of postnatal 1-week-old (PW1), 3-week-old (PW3), and 5-week-old (PW5) mice were characterized by RNA sequencing, qPCR, Western blotting, isometric force recordings (IFR), and colonic motor complex (CMC) force measurements. Our study showed that the transcriptional expression of Pdgfrα, c-Kit, P2ry1, Nos1, and Slc18a3, and the protein expression of nNOS, c-Kit, and ANO1 significantly increased with age from PW1 to PW5. In PW1 and PW3 mice, colonic migrating movement was not fully developed. In PW5 mice, rhythmic CMCs were recorded, similar to the CMC pattern described previously in adult mice. The inhibition of nNOS revealed excitatory and non-propulsive responses which are normally suppressed due to ongoing nitrergic inhibition. During postnatal development, molecular data demonstrated the establishment and expansion of ICC and PDGFRα+ cells, along with nitrergic and cholinergic nerves and purinergic receptors. Our findings are important for understanding the role of the SIP syncytium in generating and establishing CMCs in postnatal, developing murine colons.

Site-specific pathophysiology in a neonatal mouse model of gastroparesis

BACKGROUND

Early-life events impact maturation of the gut microbiome, enteric nervous system, and gastrointestinal motility. We examined three regions of gastric tissue to determine how maternal separation and gut microbes influence the structure and motor function of specific regions of the neonatal mouse stomach.

METHODS

Germ-free and conventionally housed C57BL/6J mouse pups underwent timed maternal separation (TmSep) or nursed uninterrupted (controls) until 14 days of life. We assessed gastric emptying by quantifying the progression of gavaged fluorescein isothiocyanate (FITC)-dextran. With isolated rings of forestomach, corpus, and antrum, we measured tone and contractility by force transduction, gastric wall thickness by light microscopy, and myenteric plexus neurochemistry by whole-mount immunostaining.

KEY RESULTS

Regional gastric sampling revealed site-specific differences in contractile patterns and myenteric plexus structure. In neonatal mice, TmSep prolonged gastric emptying. In the forestomach, TmSep increased contractile responses to carbachol, decreased muscularis externa and mucosa thickness, and increased the relative proportion of myenteric plexus nNOS+ neurons. Germ-free conditions did not appreciably alter the structure or function of the neonatal mouse stomach and did not impact the changes caused by TmSep.

CONCLUSIONS AND INFERENCES

A regional sampling approach facilitates site-specific investigations of murine gastric motor physiology and histology to identify site-specific alterations that may impact gastrointestinal function. Delayed gastric emptying in TmSep is associated with a thinner muscle wall, exaggerated cholinergic contractile responses, and increased proportions of inhibitory myenteric plexus nNOS+ neurons in the forestomach. Gut microbes do not profoundly affect the development of the neonatal mouse stomach or the gastric pathophysiology that results from TmSep.

Postnatal development of the enteric neurons expressing neuronal nitric oxide synthase

Neurons, expressing neuronal nitric oxide synthase (nNOS) in the enteric ganglia are inhibitory motor neurons or interneurons. The aim of the study was to identify the percentage, cross-sectional area of nNOS-immunoreactive (IR) neurons and their colocalization with choline acetyltransferase (ChAT), vasoactive intestinal polypeptide (VIP), and neuropeptide Y in the intramural ganglia of the myenteric (MP) and submucous plexus (SP) of the small intestine (SI) and large intestine (LI) of rats of different age groups using immunohistochemical methods. In the intramural ganglia of the MP, the largest percentage of nNOS-IR neurons was detected in newborn rats in the LI (81 ± 0.9%) and SI (48 ± 4.1%). Subsequently, it decreased in ontogenesis up to 60 days of life (26 ± 0.9% LI, 29 ± 3.2% SI), and did not change until senescence. In the SP, abundant nNOS-IR neurons were also detected in newborns (82 ± 7.0% SI, 85 ± 3.2% LI), while their percentage decreased significantly in the next 20 days. Furthermore, a very small number of nNOS-IR neurons was detected in 30-day- and 2-month-old animals, but they again appeared in large numbers in aged rats. In the MP, the highest percentage of nNOS+/ChAT+ neurons was in 1-day-old, 10-day-old, and 2-year-old rats. In the SP, the largest number of nNOS-IR neurons colocalized ChAT regardless of age. In the MP of all rats, many nNOS-IR neurons colocalized VIP, and the maximal percentage of nNOS+/VIP+ neurons was found in 2-year-old rats, minimal-in newborns. In conclusion, nNOS expression in neurons of the gut is decreased in early postnatal ontogenesis and subsequently increased in aged rats.

Quantitative Spatial Analysis of Neuroligin-3 mRNA Expression in the Enteric Nervous System Reveals a Potential Role in Neuronal-Glial Synapses and Reduced Expression in Nlgn3R451C Mice

Mutations in the Neuroligin-3 (Nlgn3) gene are implicated in autism spectrum disorder (ASD) and gastrointestinal (GI) dysfunction, but cellular Nlgn3 expression in the enteric nervous system remains to be characterised. We combined RNAScope in situ hybridization and immunofluorescence to measure Nlgn3 mRNA expression in cholinergic and VIP-expressing submucosal neurons, nitrergic and calretinin-containing myenteric neurons and glial cells in both WT and Nlgn3^R451C mutant mice. We measured Nlgn3 mRNA neuronal and glial expression via quantitative three-dimensional image analysis. To validate dual RNAScope/immunofluorescence data, we interrogated available single-cell RNA sequencing (scRNASeq) data to assess for Nlgn3, Nlgn1, Nlgn2 and their binding partners, Nrxn1-3, MGDA1 and MGDA2, in enteric neural subsets. Most submucosal and myenteric neurons expressed Nlgn3 mRNA. In contrast to other Nlgns and binding partners, Nlgn3 was strongly expressed in enteric glia, suggesting a role for neuroligin-3 in mediating enteric neuron-glia interactions. The autism-associated R451C mutation reduces Nlgn3 mRNA expression in cholinergic but not in VIPergic submucosal neurons. In the myenteric plexus, Nlgn3 mRNA levels are reduced in calretinin, nNOS-labelled neurons and S100 β -labelled glia. We provide a comprehensive cellular profile for neuroligin-3 expression in ileal neuronal subpopulations of mice expressing the R451C autism-associated mutation in Nlgn3, which may contribute to the understanding of the pathophysiology of GI dysfunction in ASD.

The enteric nervous system

Of all the organ systems in the body, the gastrointestinal tract is the most complicated in terms of the numbers of structures involved, each with different functions, and the numbers and types of signaling molecules utilized. The digestion of food and absorption of nutrients, electrolytes, and water occurs in a hostile luminal environment that contains a large and diverse microbiota. At the core of regulatory control of the digestive and defensive functions of the gastrointestinal tract is the enteric nervous system (ENS), a complex system of neurons and glia in the gut wall. In this review, we discuss 1) the intrinsic neural control of gut functions involved in digestion and 2) how the ENS interacts with the immune system, gut microbiota, and epithelium to maintain mucosal defense and barrier function. We highlight developments that have revolutionized our understanding of the physiology and pathophysiology of enteric neural control. These include a new understanding of the molecular architecture of the ENS, the organization and function of enteric motor circuits, and the roles of enteric glia. We explore the transduction of luminal stimuli by enteroendocrine cells, the regulation of intestinal barrier function by enteric neurons and glia, local immune control by the ENS, and the role of the gut microbiota in regulating the structure and function of the ENS. Multifunctional enteric neurons work together with enteric glial cells, macrophages, interstitial cells, and enteroendocrine cells integrating an array of signals to initiate outputs that are precisely regulated in space and time to control digestion and intestinal homeostasis.

Neurochemical phenotypes of huntingtin-associated protein 1 in reference to secretomotor and vasodilator neurons in the submucosal plexuses of rodent small intestine

Huntingtin-associated protein 1(HAP1) is an immunohistochemical marker of the stigmoid body (STB). Brain and spinal cord regions with lack of STB/HAP1 immunoreactivity are always neurodegenerative targets, whereas STB/HAP1 abundant regions are usually spared from neurodegeneration. In addition to the brain and spinal cord, HAP1 is abundantly expressed in the excitatory and inhibitory motor neurons in myenteric plexuses of the enteric nervous system (ENS). However, the detailed expression of HAP1 and its neurochemical characterization in submucosal plexuses of ENS are still unknown. In this study, we aimed to clarify the expression and neurochemical characterization of HAP1 in the submucosal plexuses of the small intestine in adult mice and rats. HAP1 was highly expressed in the submucosal plexuses of both rodents. The percentage of HAP1-immunoreactive submucosal neurons was not significantly varied between the intestinal segments of these rodents. Double immunofluorescence results revealed that almost all the cholinergic secretomotor neurons containing ChAT/ CGRP/ somatostatin/ calretinin, non-cholinergic secretomotor neurons containing VIP/NOS/TH/calretinin, and vasodilator neurons containing VIP/calretinin expressed HAP1. Our current study is the first to clarify that STB/HAP1 is expressed in secretomotor and vasodilator neurons of submucosal plexuses, suggesting that STB/HAP1 might modulate or protect the secretomotor and vasodilator functions of submucosal neurons in ENS.

Neonatal antibiotics have long term sex-dependent effects on the enteric nervous system

Infants and young children receive the highest exposures to antibiotics globally. Although there is building evidence that early life exposure to antibiotics increases susceptibility to various diseases including gut disorders later in life, the lasting impact of early life antibiotics on the physiology of the gut and its enteric nervous system (ENS) remains unclear. We treated neonatal mice with the antibiotic vancomycin during their first 10 postnatal days, then examined potential lasting effects of the antibiotic treatment on their colons during young adulthood (6 weeks old). We found that neonatal vancomycin treatment disrupted the gut functions of young adult female and male mice differently. Antibiotic-exposed females had significantly longer whole gut transit while antibiotic-treated males had significantly lower faecal weights compared to controls. Both male and female antibiotic-treated mice had greater percentages of faecal water content. Neonatal vancomycin treatment also had sexually dimorphic impacts on the neurochemistry and Ca2+ activity of young adult myenteric and submucosal neurons. Myenteric neurons of male mice were more disrupted than those of females, while opposing changes in submucosal neurons were seen in each sex. Neonatal vancomycin also induced sustained changes in colonic microbiota and lasting depletion of mucosal serotonin (5-HT) levels. Antibiotic impacts on microbiota and mucosal 5-HT were not sex-dependent, but we propose that the responses of the host to these changes are sex-specific. This first demonstration of long-term impacts of neonatal antibiotics on the ENS, gut microbiota and mucosal 5-HT has important implications for gut function and other physiological systems of the host. KEY POINTS: Early life exposure to antibiotics can increase susceptibility to diseases including functional gastrointestinal (GI) disorders later in life. Yet, the lasting impact of this common therapy on the gut and its enteric nervous system (ENS) remains unclear. We investigated the long-term impact of neonatal antibiotic treatment by treating mice with the antibiotic vancomycin during their neonatal period, then examining their colons during young adulthood. Adolescent female mice given neonatal vancomycin treatment had significantly longer whole gut transit times, while adolescent male and female mice treated with neonatal antibiotics had significantly wetter stools. Effects of neonatal vancomycin treatment on the neurochemistry and Ca2+ activity of myenteric and submucosal neurons were sexually dimorphic. Neonatal vancomycin also had lasting effects on the colonic microbiome and mucosal serotonin biosynthesis that were not sex-dependent. Different male and female responses to antibiotic-induced disruptions of the ENS, microbiota and mucosal serotonin biosynthesis can lead to sex-specific impacts on gut function.

Computational simulations and Ca2+ imaging reveal that slow synaptic depolarizations (slow EPSPs) inhibit fast EPSP evoked action potentials for most of their time course in enteric neurons

Transmission between neurons in the extensive enteric neural networks of the gut involves synaptic potentials with vastly different time courses and underlying conductances. Most enteric neurons exhibit fast excitatory post-synaptic potentials (EPSPs) lasting 20–50 ms, but many also exhibit slow EPSPs that last up to 100 s. When large enough, slow EPSPs excite action potentials at the start of the slow depolarization, but how they affect action potentials evoked by fast EPSPs is unknown. Furthermore, two other sources of synaptic depolarization probably occur in enteric circuits, activated via GABA_A or GABA_C receptors; how these interact with other synaptic depolarizations is also unclear. We built a compartmental model of enteric neurons incorporating realistic voltage-dependent ion channels, then simulated fast EPSPs, slow EPSPs and GABA_A or GABA_C ligand-gated Cl^- channels to explore these interactions. Model predictions were tested by imaging Ca²⁺ transients in myenteric neurons ex vivo as an indicator of their activity during synaptic interactions. The model could mimic firing of myenteric neurons in mouse colon evoked by depolarizing current during intracellular recording and the fast and slow EPSPs in these neurons. Subthreshold fast EPSPs evoked spikes during the rising phase of a slow EPSP, but suprathreshold fast EPSPs could not evoke spikes later in a slow EPSP. This predicted inhibition was confirmed by Ca²⁺ imaging in which stimuli that evoke slow EPSPs suppressed activity evoked by fast EPSPs in many myenteric neurons. The model also predicted that synchronous activation of GABA_A receptors and fast EPSPs potentiated firing evoked by the latter, while synchronous activation of GABA_C receptors with fast EPSPs, potentiated firing and then suppressed it. The results reveal that so-called slow EPSPs have a biphasic effect being likely to suppress fast EPSP evoked firing over very long periods, perhaps accounting for prolonged quiescent periods seen in enteric motor patterns.

The gastrointestinal tract is the only organ with an extensive semi-autonomous nervous system that generates complex contraction patterns independently. Communication between neurons in this “enteric” nervous system is via depolarizing synaptic events with dramatically different time courses including fast synaptic potentials lasting around 20–50 ms and slow depolarizing synaptic potentials lasting for 10–120 s. Most neurons have both. We explored how slow synaptic depolarizations affect generation of action potentials by fast synaptic potentials using computational simulation of small networks of neurons implemented as compartmental models with realistic membrane ion channels. We found that slow synaptic depolarizations have biphasic effects; they initially make fast synaptic potentials more likely to trigger action potentials, but then actually prevent action potential generation by fast synaptic potentials with the inhibition lasting several 10s of seconds. We confirmed the inhibitory effects of the slow synaptic depolarizations using live Ca²⁺ imaging of enteric neurons from mouse colon in isolated tissue. Our results identify a novel form of synaptic inhibition in the enteric nervous system of the gut, which may account for the vastly differing time courses between signalling in individual gut neurons and rhythmic contractile patterns that often repeat at more than 60 s intervals.

Characterization of Neurochemical Signature Alterations in the Enteric Nervous System in Autoimmune Encephalomyelitis

To date, it has remained unclear whether gastrointestinal symptoms, which are frequently observed in patients with multiple sclerosis (MS), are accompanied by pathology of the enteric nervous system (ENS). Here, the neurotransmitter signature of ENS neurons and morphological alterations of interstitial cells of Cajal (ICCs) were studied in patients with MS and mice with experimental autoimmune encephalomyelitis (EAE), which is an animal model of MS. Immunohistochemical analysis was performed on colonic whole mounts from mice with EAE and on paraffin-embedded sections of intestinal tissue from patients with MS. Antibodies against neurotransmitters or their enzymes (including vasoactive intestinal peptide (VIP), neuronal nitric oxide synthase (nNOS), and choline acetyltransferase (ChAT)) were used in conjunction with pan-neuronal markers. In addition, the presence of anoctamin 1 (ANO1)-expressing ICCs was studied. ENS changes were observed in the myenteric plexus, but they were absent in the submucosal plexus of both EAE mice and patients with MS. There was a significant decrease in the percentage of ChAT-positive neurons in EAE mice as opposed to a trend toward an increase in patients with MS. Moreover, while ANO1 expression was decreased in EAE mice, patients with MS displayed a significant increase. Although additional studies are necessary to accomplish an in-depth characterization of ENS alterations in MS, our results imply that such alterations exist and may reveal novel insights into the pathophysiology of MS.

Chronic Morphine Induces IL-18 in Ileum Myenteric Plexus Neurons Through Mu-opioid Receptor Activation in Cholinergic and VIPergic Neurons

The gastrointestinal epithelium is critical for maintaining a symbiotic relationship with commensal microbiota. Chronic morphine exposure can compromise the gut epithelial barrier in mice and lead to dysbiosis. Recently, studies have implicated morphine-induced dysbiosis in the mechanism of antinociceptive tolerance and reward, suggesting the presence of a gut-brain axis in the pharmacological effects of morphine. However, the mechanism(s) underlying morphine-induced changes in the gut microbiome remains unclear. The pro-inflammatory cytokine, Interleukin-18 (IL-18), released by enteric neurons can modulate gut barrier function. Therefore, in the present study we investigated the effect of morphine on IL-18 expression in the mouse ileum. We observed that chronic morphine exposure in vivo induces IL-18 expression in the ileum myenteric plexus that is attenuated by naloxone. Given that mu-opioid receptors (MORs) are mainly expressed in enteric neurons, we also characterized morphine effects on the excitability of cholinergic (excitatory) and vasoactive intestinal peptide (VIP)-expressing (inhibitory) myenteric neurons. We found fundamental differences in the electrical properties of cholinergic and VIP neurons such that VIP neurons are more excitable than cholinergic neurons. Furthermore, MORs were primarily expressed in cholinergic neurons, although a subset of VIP neurons also expressed MORs and responded to morphine in electrophysiology experiments. In conclusion, these data show that morphine increases IL-18 in ileum myenteric plexus neurons via activation of MORs in a subset of cholinergic and VIP neurons. Thus, understanding the neurochemistry and electrophysiology of MOR-expressing enteric neurons can help to delineate mechanisms by which morphine perturbs the gut barrier.

Enteric neuroimmune interactions coordinate intestinal responses in health and disease

The enteric nervous system (ENS) of the gastrointestinal (GI) tract interacts with the local immune system bidirectionally. Recent publications have demonstrated that such interactions can maintain normal GI functions during homeostasis and contribute to pathological symptoms during infection and inflammation. Infection can also induce long-term changes of the ENS resulting in the development of post-infectious GI disturbances. In this review, we discuss how the ENS can regulate and be regulated by immune responses and how such interactions control whole tissue physiology. We also address the requirements for the proper regeneration of the ENS and restoration of GI function following the resolution of infection.

Divergent effects of exendin-4 and interleukin-6 on rat colonic secretory and contractile activity are associated with changes in regional vagal afferent signaling

BACKGROUND

The pro-inflammatory cytokine, interleukin (IL)-6 is elevated in individuals with the functional bowel disorder, irritable bowel syndrome (IBS). IL-6 can independently modify intestinal secreto-motor function, thereby contributing to IBS pathophysiology. Additionally, hormonal changes may underlie symptom flares. Post-prandial exacerbation of IBS symptoms has been linked to secretion of the incretin hormone, glucagon-like peptide-1 (GLP-1), which can also influence colonic secreto-motor activity. This study aimed to ascertain if the effects of GLP-1 on colonic secretory and contractile activity was impacted by elevated IL-6 levels and if sensory signals regarding such changes were reflected in altered vagal afferent activity.

METHODS

Colonic secretory currents and circular muscle contractile activity was investigated in Sprague Dawley rats using Ussing chamber and organ bath electrophysiology. Regional afferent signaling was assessed using extracellular electrophysiological recordings from colonic vagal afferents.

KEY RESULTS

Application of the GLP-1 receptor agonist, exendin-4 (Ex-4) in the presence of IL-6 potentiated colonic secretory currents and transepithelial resistance. Vagal afferent fibers originating in the submucosal layer exhibited larger responses to Ex-4 when IL-6 was also present. In contrast, co-application of Ex-4 and IL-6 to gut-bath chambers suppressed circular muscle contractile activity. The activity in extrinsic afferents originating in the colonic myenteric layer was similarly suppressed.

CONCLUSIONS & INFERENCES

Application of Ex-4 in the presence of IL-6 had divergent modulatory effects on colonic secretion and contractile activity. Similar patterns were observed in vagal afferent signaling originating in the submucosal and myenteric neuronal layers, indicating regional afferent activity reflected immune- and endocrine-mediated changes in colonic function.

Flupirtine enhances NHE-3-mediated Na+ absorption in rat colon via an ENS-dependent mechanism

Recent studies in our lab have shown that the KV7 channel activator, flupirtine, inhibits colonic epithelial Cl- secretion through effects on submucosal neurons of the enteric nervous system (ENS). We hypothesized that flupirtine would also stimulate Na+ absorption as a result of reduced secretory ENS input to the epithelium. To test this hypothesis, unidirectional 22Na+ fluxes were measured under voltage-clamped conditions. Pharmacological approaches using an Ussing-style recording chamber combined with immunofluorescence microscopy techniques were used to determine the effect of flupirtine on active Na+ transport in the rat colon. Flupirtine stimulated electroneutral Na+ absorption in partially seromuscular-stripped colonic tissues, while simultaneously inhibiting short-circuit current (ISC; i.e., Cl- secretion). Both of these effects were attenuated by pretreatment with the ENS inhibitor, tetrodotoxin. The Na+/H+ exchanger isoform 3 (NHE-3)-selective inhibitor, S3226, significantly inhibited flupirtine-stimulated Na+ absorption, whereas the NHE-2-selective inhibitor HOE-694 did not. NHE-3 localization near the apical membranes of surface epithelial cells was also more apparent in flupirtine-treated colon versus control. Flupirtine did not alter epithelial Na+ channel (ENaC)-mediated Na+ absorption in distal colonic tissues obtained from hyperaldosteronaemic rats and had no effect in the normal ileum but did stimulate Na+ absorption in the proximal colon. Finally, the parallel effects of flupirtine on ISC (Cl- secretion) and Na+ absorption were significantly correlated with each other. Together, these data indicate that flupirtine stimulates NHE-3-dependent Na+ absorption, likely as a result of reduced stimulatory input to the colonic epithelium by submucosal ENS neurons.NEW & NOTEWORTHY We present a novel mechanism regarding regulation of epithelial ion transport by enteric neurons. Activation of neuronal KV7 K+ channels markedly stimulates Na+ absorption and inhibits Cl- secretion across the colonic epithelium. This may be useful in developing new treatments for diarrheal disorders, such as irritable bowel syndrome with diarrhea (IBS-D).

Activation of KCNQ (KV7) K+ channels in enteric neurons inhibits epithelial Cl- secretion in mouse distal colon

Voltage-gated Kv7 (KCNQ family) K+ channels are expressed in many neuronal populations and play an important role in regulating membrane potential by generating a hyperpolarizing K+ current and decreasing cell excitability. However, the role of KV7 channels in the neural regulation of intestinal epithelial Cl- secretion is not known. Cl- secretion in mouse distal colon was measured as a function of short-circuit current (ISC), and pharmacological approaches were used to test the hypothesis that activation of KV7 channels in enteric neurons would inhibit epithelial Cl- secretion. Flupirtine, a nonselective KV7 activator, inhibited basal Cl- secretion in mouse distal colon and abolished or attenuated the effects of drugs that target various components of enteric neurotransmission, including tetrodotoxin (NaV channel blocker), veratridine (NaV channel activator), nicotine (nicotinic acetylcholine receptor agonist), and hexamethonium (nicotinic antagonist). In contrast, flupritine did not block the response to epithelium-targeted agents VIP (endogenous VPAC receptor ligand) or carbachol (nonselective cholinergic agonist). Flupirtine inhibited Cl- secretion in both full-thickness and seromuscular-stripped distal colon (containing the submucosal, but not myenteric plexus) but generated no response in epithelial T84 cell monolayers. KV7.2 and KV7.3 channel proteins were detected by immunofluorescence in whole mount preparations of the submucosa from mouse distal colon. ICA 110381 (KV7.2/7.3 specific activator) inhibited Cl- secretion comparably to flupirtine. We conclude that KV7 channel activators inhibit neurally driven Cl- secretion in the colonic epithelium and may therefore have therapeutic benefit in treating pathologies associated with hyperexcitable enteric nervous system, such as irritable bowel syndrome with diarrhea (IBS-D).

CD8 T cell-Derived Perforin and TNF-α Are Crucial Mediators of Neuronal Destruction in Experimental Autoimmune Enteric Ganglionitis

In neuron-specific ovalbumin-transgenic CKTAC mice, antigen-specific OT-I CD8 T cells home to the enteric nervous system, where they attack and destroy neurons of the myenteric and submucosal plexus. Clinically, experimental autoimmune enteric ganglionitis (EAEG) manifests with gastrointestinal dysmotility and rapidly progresses to lethal ileus. Although interferon-γ has been identified as capable of damaging neurons in EAEG, the role of perforin, Fas/FasL, and tumor necrosis factor-α (TNF-α) in this disease is still a matter of debate. Thus, CKTAC mice were adoptively transferred with either perforin^-/- or wild-type OT-I CD8 T cells. In addition, CKTAC mice that had received wild-type OT-I CD8 T cells were treated by either anti-TNF-α or anti-FasL. Furthermore, wild-type OT-I CD8 T cells were adoptively transferred into CKTAC mice with neuron-specific deletion of Fas. Although neither inactivation of enteric neuronal Fas nor anti-FasL treatment improved the disease, the absence of perforin from OT-I CD8 T cells and anti-TNF-α treatment significantly ameliorated EAEG and prevented lethal ileus by rescue of enteric neurons. Thus, these experiments identify perforin and TNF-α as important in the pathogenesis of EAEG.

Microbial Modulation of the Development and Physiology of the Enteric Nervous System

The gastrointestinal tract harbors an intrinsic neuronal network, the enteric nervous system (ENS). The ENS controls motility, fluid homeostasis, and blood flow, but also interacts with other components of the intestine such as epithelial and immune cells. Recent studies indicate that gut microbiota diversification, which occurs alongside postnatal ENS maturation, could be critical for the development and function of the ENS. Here we discuss the possibility that this functional relationship starts in utero, whereby the maternal microbiota would prime the developing ENS and shape its physiology. We review ENS/microbiota interactions and their modulation in physiological and pathophysiological contexts. While microbial modulation of the ENS physiology is now well established, further studies are required to understand the contribution of the gut microbiota to the development and pathology of the ENS and to reveal the precise mechanisms underlying microbiota-to-ENS communications.

Functional circuits and signal processing in the enteric nervous system

The enteric nervous system (ENS) is an extensive network comprising millions of neurons and glial cells contained within the wall of the gastrointestinal tract. The major functions of the ENS that have been most studied include the regulation of local gut motility, secretion, and blood flow. Other areas that have been gaining increased attention include its interaction with the immune system, with the gut microbiota and its involvement in the gut-brain axis, and neuro-epithelial interactions. Thus, the enteric circuitry plays a central role in intestinal homeostasis, and this becomes particularly evident when there are faults in its wiring such as in neurodevelopmental or neurodegenerative disorders. In this review, we first focus on the current knowledge on the cellular composition of enteric circuits. We then further discuss how enteric circuits detect and process external information, how these signals may be modulated by physiological and pathophysiological factors, and finally, how outputs are generated for integrated gut function.

Antibiotic exposure postweaning disrupts the neurochemistry and function of enteric neurons mediating colonic motor activity

The period during and immediately after weaning is an important developmental window when marked shifts in gut microbiota can regulate the maturation of the enteric nervous system (ENS). Because microbiota-derived signals that modulate ENS development are poorly understood, we examined the physiological impact of the broad spectrum of antibiotic, vancomycin-administered postweaning on colonic motility, neurochemistry of enteric neurons, and neuronal excitability. The functional impact of vancomycin on enteric neurons was investigated by Ca²⁺ imaging in Wnt1-Cre;R26R-GCaMP3 reporter mice to characterize alterations in the submucosal and the myenteric plexus, which contains the neuronal circuitry controlling gut motility. 16S rDNA sequencing of fecal specimens after oral vancomycin demonstrated significant deviations in microbiota abundance, diversity, and community composition. Vancomycin significantly increased the relative family rank abundance of Akkermansiaceae, Lactobacillaceae, and Enterobacteriaceae at the expense of Lachnospiraceae and Bacteroidaceae. In sharp contrast to neonatal vancomycin exposure, microbiota compositional shifts in weaned animals were associated with slower colonic migrating motor complexes (CMMCs) without mucosal serotonin biosynthesis being altered. The slowing of CMMCs is linked to disruptions in the neurochemistry of the underlying enteric circuitry. This included significant reductions in cholinergic and calbindin+ myenteric neurons, neuronal nitric oxide synthase+ submucosal neurons, neurofilament M+ enteric neurons, and increased proportions of cholinergic submucosal neurons. The antibiotic treatment also increased transmission and responsiveness in myenteric and submucosal neurons that may enhance inhibitory motor pathways, leading to slower CMMCs. Differential vancomycin responses during neonatal and weaning periods in mice highlight the developmental-specific impact of antibiotics on colonic enteric circuitry and motility.

Restraint stress induced gut dysmotility is diminished by a milk oligosaccharide (2'-fucosyllactose) in vitro

Background

Stress causes severe dysmotility in the mammalian gut. Almost all research done to date has concentrated on prevention of stress-induced altered gut motility but not on treatment. We had previously shown that intraluminal 2′FL could acutely moderate propulsive motility in isolated mouse colonic segments. Because 2′FL appeared to modulate enteric nervous system dependent motility, we wondered if the oligosaccharide could reverse the effects of prior restraint stress, ex vivo. We tested whether 2′FL could benefit the dysmotility of isolated jejunal and colonic segments from animals subjected to prior acute restraint stress.

Methods

Jejunal and colonic segments were obtained from male Swiss Webster mice that were untreated or subjected to 1 hour of acute restraint stress. Segments were perfused with Krebs buffer and propagating contractile clusters (PCC) digitally video recorded. 2′FL or β-lactose were added to the perfusate at a concentration of 1 mg/ml. Spatiotemporal maps were constructed from paired before and after treatment recordings, each consisting of 20 min duration and PCC analyzed for frequency, velocity and amplitude.

Key Results

Stress decreased propulsive motility in murine small intestine while increasing it in the colon. 2′FL in jejunum of previously stressed mice produced a 50% increase in PCC velocity (p = 0.0001), a 43% increase in frequency (p = 0.0002) and an insignificant decrease in peak amplitude. For stressed colon, 2′FL reduced the frequency by 23% (p = 0.017) and peak amplitude by 26% (p = 0.011), and was without effect on velocity. β-lactose had negligible or small treatment effects.

Conclusions & Inferences

We show that the prebiotic 2′FL may have potential as a treatment for acute stress-induced gut dysmotility, ex vivo, and that, as is the case for certain beneficial microbes, the mechanism occurs in the gut, likely via action on the enteric nervous system.

Down syndrome mouse models have an abnormal enteric nervous system

Children with trisomy 21 (Down syndrome [DS]) have a 130-fold increased incidence of Hirschsprung Disease (HSCR), a developmental defect where the enteric nervous system (ENS) is missing from distal bowel (i.e., distal bowel is aganglionic). Treatment for HSCR is surgical resection of aganglionic bowel, but many children have bowel problems after surgery. Post-surgical problems like enterocolitis and soiling are especially common in children with DS. To determine how trisomy 21 affects ENS development, we evaluated the ENS in two DS mouse models, Ts65Dn and Tc1. These mice are trisomic for many chromosome 21 homologous genes, including Dscam and Dyrk1a, which are hypothesized to contribute to HSCR risk. Ts65Dn and Tc1 mice have normal ENS precursor migration at E12.5 and almost normal myenteric plexus structure as adults. However, Ts65Dn and Tc1 mice have markedly reduced submucosal plexus neuron density throughout the bowel. Surprisingly, the submucosal neuron defect in Ts65Dn mice is not due to excess Dscam or Dyrk1a, since normalizing copy number for these genes does not rescue the defect. These findings suggest the possibility that the high frequency of bowel problems in children with DS and HSCR may occur because of additional unrecognized problems with ENS structure.

Neuronal Development and Onset of Electrical Activity in the Human Enteric Nervous System

BACKGROUND & AIMS

The enteric nervous system (ENS) is the largest branch of the peripheral nervous system, comprising complex networks of neurons and glia, which are present throughout the gastrointestinal tract. Although development of a fully functional ENS is required for gastrointestinal motility, little is known about the ontogeny of ENS function in humans. We studied the development of neuronal subtypes and the emergence of evoked electrical activity in the developing human ENS.

METHODS

Human fetal gut samples (obtained via the MRC-Wellcome Trust Human Developmental Biology Resource-UK) were characterized by immunohistochemistry, calcium imaging, RNA sequencing, and quantitative real-time polymerase chain reaction analyses.

RESULTS

Human fetal colon samples have dense neuronal networks at the level of the myenteric plexus by embryonic week (EW) 12, with expression of excitatory neurotransmitter and synaptic markers. By contrast, markers of inhibitory neurotransmitters were not observed until EW14. Electrical train stimulation of internodal strands did not evoke activity in the ENS of EW12 or EW14 tissues. However, compound calcium activation was observed at EW16, which was blocked by the addition of 1 μmol/L tetrodotoxin. Expression analyses showed that this activity was coincident with increases in expression of genes encoding proteins involved in neurotransmission and action potential generation.

CONCLUSIONS

In analyses of human fetal intestinal samples, we followed development of neuronal diversity, electrical excitability, and network formation in the ENS. These processes are required to establish the functional enteric circuitry. Further studies could increase our understanding of the pathogenesis of a range of congenital enteric neuropathies.

Acute Toxoplasma gondii infection alters the number of neurons and the proportion of enteric glial cells in the duodenum in Wistar rats

BACKGROUND

Toxoplasma gondii infection can occur through the ingestion of raw meat that contains tissue cysts or food that contains oocysts. Through the ingestion of oocysts, the parasite crosses the intestinal barrier, where the enteric nervous system is located. The objective was to investigate the kinetics of neuronal and glial responses during acute T. gondii infection.

METHODS

We used 45 Wistar rats that were divided into a control group and infected groups that were evaluated at 6, 12, 24, 48, 72 hours, 7 days, 10 days, and 15 days after infection. The rats received 5000 sporulated oocysts of the parasite orally. To detect neurons and enteric glia cells, the myenteric and submucosal plexuses of the duodenum underwent double-labeling immunohistochemical techniques to evaluate HuC/HuD and S100, HuC/HuD and ChAT, and HuC/HuD and nNOS.

KEY RESULTS

We observed a reduction of the total neuron population in the submucosal plexus 72 hours after infection. Cholinergic neurons decreased in the submucosal plexus 15 days after infection, and nitrergic neurons decreased in the myenteric plexus 72 hours after infection. A decrease in the number of glial cells was observed 7 days after infection in the submucosal plexus, and an increase in the enteric glial cell (EGC)/neuron ratio was found in both plexuses 48 hours after infection.

CONCLUSIONS AND INFERENCES

We found decrease of neurons and increase in the EGC/neuron ratio in both plexuses caused by acute T. gondii infection, with major alterations 72 hours after oral infection. The number of cholinergic neurons decreased in the submucosal plexus, and the number of nitrergic neurons decreased in the myenteric plexus. A decrease in the number of enteric glial cells was observed in the submucosal plexus, and an increase in the enteric glial cell/neuron ratio was observed in both ganglionate plexuses of the duodenum.

Regional complexity in enteric neuron wiring reflects diversity of motility patterns in the mouse large intestine

The enteric nervous system controls a variety of gastrointestinal functions including intestinal motility. The minimal neuronal circuit necessary to direct peristalsis is well-characterized but several intestinal regions display also other motility patterns for which the underlying circuits and connectivity schemes that coordinate the transition between those patterns are poorly understood. We investigated whether in regions with a richer palette of motility patterns, the underlying nerve circuits reflect this complexity. Using Ca²⁺ imaging, we determined the location and response fingerprint of large populations of enteric neurons upon focal network stimulation. Complemented by neuronal tracing and volumetric reconstructions of synaptic contacts, this shows that the multifunctional proximal colon requires specific additional circuit components as compared to the distal colon, where peristalsis is the predominant motility pattern. Our study reveals that motility control is hard-wired in the enteric neural networks and that circuit complexity matches the motor pattern portfolio of specific intestinal regions.

Intestinal Epithelial Wnt Signaling Mediates Acetylcholine-Triggered Host Defense against Infection

Regulated antimicrobial peptide expression in the intestinal epithelium is key to defense against infection and to microbiota homeostasis. Understanding the mechanisms that regulate such expression is necessary for understanding immune homeostasis and inflammatory disease and for developing safe and effective therapies. We used Caenorhabditis elegans in a preclinical approach to discover mechanisms of antimicrobial gene expression control in the intestinal epithelium. We found an unexpected role for the cholinergic nervous system. Infection-induced acetylcholine release from neurons stimulated muscarinic signaling in the epithelium, driving downstream induction of Wnt expression in the same tissue. Wnt induction activated the epithelial canonical Wnt pathway, resulting in the expression of C-type lectin and lysozyme genes that enhanced host defense. Furthermore, the muscarinic and Wnt pathways are linked by conserved transcription factors. These results reveal a tight connection between the nervous system and the intestinal epithelium, with important implications for host defense, immune homeostasis, and cancer.

Signaling of free fatty acid receptors 2 and 3 differs in colonic mucosa following selective agonism or coagonism by luminal propionate

BACKGROUND

Propionate exhibits affinity for free fatty acid receptor 2 (FFA2, formerly GPR43) and FFA3 (GPR41). These two G protein-coupled receptors (GPCRs) are expressed by enteroendocrine L cells that contain anorectic peptide YY (PYY) and glucagon-like peptide 1 (GLP-1), while FFA3 is also expressed by enteric neurons. Few studies have investigated the individual roles of FFA2 and FFA3 in propionate's gastrointestinal (GI) effects. Here, we compared FFA2, FFA3, and propionate mucosal responses utilizing selective ligands including an FFA3 antagonist, in mouse and human colonic mucosa.

METHODS

Vectorial ion transport was measured in native colonic preparations from normal mouse and human colon with intact submucosal innervation. Endogenous fecal pellet propulsion was monitored in colons isolated from wild-type (WT) and PYY-/- mice.

KEY RESULTS

FFA2 and FFA3 signaling differed significantly. FFA2 agonism involved endogenous L cell-derived PYY and was glucose dependent, while FFA3 agonism was independent of PYY and glucose, but required submucosal enteric neurons for activity. Tonic FFA3 activity was observed in mouse and human colon mucosa. Apical propionate responses were a combination of FFA2-PYY mediation and FFA3 neuronal GLP-1- and CGRP-dependent signaling in mouse ascending colon mucosa. Propionate also slowed WT and PYY-/- colonic transit, and this effect was blocked by a GLP-1 receptor antagonist.

CONCLUSIONS & INFERENCES

We conclude that luminal propionate costimulates FFA2 and FFA3 pathways, reducing anion secretion and slowing colonic motility; FFA2 via PYY mediation and FFA3 signaling by activation of enteric sensory neurons.

A new described mechanisms of intestinal glandular atrophy induced by vagal nerve/Auerbach network degeneration following subarachnoid hemorrhage: The first experimental study

Stress ulcers is a trouble complication of subarachnoid hemorrhage (SAH). Although gastrointestinal ulcerations may be attributed to increased HCL secretion in SAH; the exact mechanism of that complication has not been investigated definitively. We studied if vagal network degeneration may cause intestinal atrophy following SAH. Study was conducted on 25 rabbits, with 5 control group (Group-A), 5 SHAM group (Group-B), and 15 SAH group via injection of autologue blood to cisterna magna. Seven animals followed for seven days (Early Decapitated-Group-C) and eight animals followed 21 days (Late Decapitated-Group-D). The vagal nodosal ganglia (NGs), Auerbach plexuses and goblet cells of duodenums were examined by current stereological methods and compared statistically. The mean numbers of degenerated axon density/mm² of gastric branches of vagal nerves was 8 ± 2, 34 ± 11, 189 ± 49 and 322 ± 81 in the Group A, B, C, and D respectively. The mean numbers of degenerated neuron density/mm³ of NGs was 5 ± 2, 54 ± 7, 691 ± 87 and 2930 ± 410 in the Group A, B, C, and D respectively. The mean numbers of degenerated Auerbach neurons 2 ± 1, 4 ± 1, 12 ± 3 and 27 ± 5/mm³ in the Group A, B, C, and D respectively. The mean numbers of degenerated goblet cells/mm³ were 4.3 ± 1.02, 11.5 ± 0.26, 143 ± 26 and 937 ± 65 Group A, B, C, and D respectively. Statistical analysis showed that vagal network ischemia could cause intestinal bleeding and so atrophy in SAH progression. Statistical analyses of groups were; Group-D/Group-A < 0.001, Group-D/Group-B < 0.005, Group-C/Group-A < 0.005. Undiscovered effect of ischemic vagal network injuries should be regarded as a major cause of stress ulcerations following SAH which has not been mentioned in the literature.

The extracellular matrix glycoprotein tenascin-X regulates peripheral sensory and motor neurones

KEY POINTS

Tenascin-X (TNX) is an extracellular matrix glycoprotein with anti-adhesive properties in skin and joints. Here we report the novel finding that TNX is expressed in human and mouse gut tissue where it is exclusive to specific subpopulations of neurones. Our studies with TNX-deficient mice show impaired defecation and neural control of distal colonic motility that can be rescued with a 5-HT₄ receptor agonist. However, colonic secretion is unchanged. They are also susceptible to internal rectal intussusception. Colonic afferent sensitivity is increased in TNX-deficient mice. Correspondingly, there is increased density of and sensitivity of putative nociceptive fibres in TNX-deficient mucosa. A group of TNX-deficient patients report symptoms highly consistent with those in the mouse model. These findings suggest TNX plays entirely different roles in gut to non-visceral tissues - firstly a role in enteric motor neurones and secondly a role influencing nociceptive sensory neurones Studying further the mechanisms by which TNX influences neuronal function will lead to new targets for future treatment.

ABSTRACT

The extracellular matrix (ECM) is not only an integral structural molecule, but is also critical for a wide range of cellular functions. The glycoprotein tenascin-X (TNX) predominates in the ECM of tissues like skin and regulates tissue structure through anti-adhesive interactions with collagen. Monogenic TNX deficiency causes painful joint hypermobility and skin hyperelasticity, symptoms characteristic of hypermobility Ehlers Danlos syndrome (hEDS). hEDS patients also report consistently increased visceral pain and gastrointestinal (GI) dysfunction. We investigated whether there is a direct link between TNX deficiency and GI pain or motor dysfunction. We set out first to learn where TNX is expressed in human and mouse, then determine how GI function, specifically in the colon, is disordered in TNX-deficient mice and humans of either sex. In human and mouse tissue, TNX was predominantly associated with cholinergic colonic enteric neurones, which are involved in motor control. TNX was absent from extrinsic nociceptive peptidergic neurones. TNX-deficient mice had internal rectal prolapse and a loss of distal colonic contractility which could be rescued by prokinetic drug treatment. TNX-deficient patients reported increased sensory and motor GI symptoms including abdominal pain and constipation compared to controls. Despite absence of TNX from nociceptive colonic neurones, neuronal sprouting and hyper-responsiveness to colonic distension was observed in the TNX-deficient mice. We conclude that ECM molecules are not merely support structures but an integral part of the microenvironment particularly for specific populations of colonic motor neurones where TNX exerts functional influences.

Cholinergic Submucosal Neurons Display Increased Excitability Following in Vivo Cholera Toxin Exposure in Mouse Ileum

Cholera-induced hypersecretion causes dehydration and death if untreated. Cholera toxin (CT) partly acts via the enteric nervous system (ENS) and induces long-lasting changes to enteric neuronal excitability following initial exposure, but the specific circuitry involved remains unclear. We examined this by first incubating CT or saline (control) in mouse ileal loops in vivo for 3.5 h and then assessed neuronal excitability in vitro using Ca²⁺ imaging and immunolabeling for the activity-dependent markers cFos and pCREB. Mice from a C57BL6 background, including Wnt1-Cre;R26R-GCaMP3 mice which express the fluorescent Ca²⁺ indicator GCaMP3 in its ENS, were used. Ca²⁺-imaging using this mouse model is a robust, high-throughput method which allowed us to examine the activity of numerous enteric neurons simultaneously and post-hoc immunohistochemistry enabled the neurochemical identification of the active neurons. Together, this provided novel insight into the CT-affected circuitry that was previously impossible to attain at such an accelerated pace. Ussing chamber measurements of electrogenic ion secretion showed that CT-treated preparations had higher basal secretion than controls. Recordings of Ca²⁺ activity from the submucous plexus showed that increased numbers of neurons were spontaneously active in CT-incubated tissue (control: 4/149; CT: 32/160; Fisher's exact test, P < 0.0001) and that cholinergic neurons were more responsive to electrical (single pulse and train of 20 pulses) or nicotinic (1,1-dimethyl-4-phenylpiperazinium (DMPP; 10 μM) stimulation. Expression of the neuronal activity marker, pCREB, was also increased in the CT-treated submucous plexus neurons. c-Fos expression and spontaneous fast excitatory postsynaptic potentials (EPSPs), recorded by intracellular electrodes, were increased by CT exposure in a small subset of myenteric neurons. However, the effect of CT on the myenteric plexus is less clear as spontaneous Ca²⁺ activity and electrical- or nicotinic-evoked Ca²⁺ responses were reduced. Thus, in a model where CT exposure evokes hypersecretion, we observed sustained activation of cholinergic secretomotor neuron activity in the submucous plexus, pointing to involvement of these neurons in the overall response to CT.

Neurally Released GABA Acts via GABAC Receptors to Modulate Ca2+ Transients Evoked by Trains of Synaptic Inputs, but Not Responses Evoked by Single Stimuli, in Myenteric Neurons of Mouse Ileum

γ-Aminobutyric Acid (GABA) and its receptors, GABAA,B,C, are expressed in several locations along the gastrointestinal tract. Nevertheless, a role for GABA in enteric synaptic transmission remains elusive. In this study, we characterized the expression and function of GABA in the myenteric plexus of the mouse ileum. About 8% of all myenteric neurons were found to be GABA-immunoreactive (GABA+) including some Calretinin+ and some neuronal nitric oxide synthase (nNOS+) neurons. We used Wnt1-Cre;R26R-GCaMP3 mice, which express a genetically encoded fluorescent calcium indicator in all enteric neurons and glia. Exogenous GABA increased the intracellular calcium concentration, [Ca2+]i of some myenteric neurons including many that did not express GABA or nNOS (the majority), some GABA+, Calretinin+ or Neurofilament-M (NFM)+ but rarely nNOS+ neurons. GABA+ terminals contacted a significantly larger proportion of the cell body surface area of Calretinin+ neurons than of nNOS+ neurons. Numbers of neurons with GABA-induced [Ca2+]i transients were reduced by GABAA,B,C and nicotinic receptor blockade. Electrical stimulation of interganglionic fiber tracts was used to examine possible effects of endogenous GABA release. [Ca2+]i transients evoked by single pulses were unaffected by specific antagonists for each of the 3 GABA receptor subtypes. [Ca2+]i transients evoked by 20 pulse trains were significantly amplified by GABAC receptor blockade. These data suggest that GABAA and GABAB receptors are not involved in synaptic transmission, but suggest a novel role for GABAC receptors in modulating slow synaptic transmission, as indicated by changes in [Ca2+]i transients, within the ENS.

Treatment with low doses of aspirin during chronic phase of experimental Chagas' disease increases oesophageal nitrergic neuronal subpopulation in mice

Patients with Chagas' disease may develop dysfunctions of oesophageal and colonic motility resulting from the degeneration or loss of the myenteric neurons of the enteric nervous system. Studies have shown that the use of aspirin, also known as acetylsalicylic acid (ASA), influences the pathogenesis of the disease. However, this remains controversial. The aim of this study was to evaluate the consequences of treatment with low doses of aspirin during the chronic phase of Chagas' disease on oesophageal function. Twenty male Swiss mice, 60 days of age, were used. The animals were infected with Y strain of Trypanosoma cruzi, injected intraperitoneally. Aspirin was given at a dose of 50 mg/kg to some of the infected animals, from the 55th to 63rd day after inoculation on consecutive days, and from the 65th to 75th day on alternate days. We investigated food passage of time, wall structure and nitrergic neuronal population of the distal oesophagus. Our data revealed that the use of low doses of aspirin in chronic Chagas' disease caused an increase in the number of nitrergic neurons and partially prevented hypertrophy of the oesophagus. In addition, the aspirin administration impeded Chagas' diseases associated changes in intestinal transit time. Thus treatment with aspirin in the chronic phase of Chagas' disease changes the natural history of the disease and raises the possibility of using it as a new therapeutic approach to the treatment of this aspect of Chagas' disease pathology.

Mucosal glucagon-like peptide 1 (GLP-1) responses are mediated by calcitonin gene-related peptide (CGRP) in the mouse colon and both peptide responses are area-specific

BACKGROUND

Glucagon-like peptide (GLP)-1 is an incretin hormone and its mimetics are proven antidiabetic and antiobesity drugs. GLP-1 exerts antimotility and mucosal proliferative activities but its epithelial ion transport effects are uncharacterized and these may contribute to the gastrointestinal (GI) disturbance, i.e., diarrhea experienced with some GLP-1 mimetics. Our aim was to establish GLP-1 agonist mechanisms and identify potential mucosal mediator(s) in the colonic tissue from C57BL/6J mice.

METHODS

A tissue survey of GLP-1 responses (using exendin 4, Ex4) and α-calcitonin gene-related peptide (αCGRP) was undertaken, dividing the mouse colon into eight adjacent mucosal-submucosal preparations. Each preparation was voltage-clamped and changes in short-circuit current (Isc) measured. The involvement of submucosal neurons in GLP-1 agonism was tested using Ex(9-39) and tetrodotoxin (TTX), and CGRP receptors were blocked with BIBN4094.

KEY RESULTS

Ex4 responses along the length of the colon were inhibited by the GLP-1 antagonist, Ex(9-39) or TTX, indicating neural mediation in all colonic regions. In the ascending colon, Ex4 increased Isc levels that were abolished by 10 nM BIBN4096, while in the descending colon it reduced Isc levels that were again BIBN4096-sensitive, but at 1 μM. The latter αCGRP response was dependent on epithelial Cl^- conductance and Na⁺ /K⁺ -ATPase, and was partially (~25%) peptide YY-mediated, but was not nitrergic, somatostatin sst₂ , or α₂ -adrenoceptor-mediated.

CONCLUSIONS AND INFERENCES

GLP-1 modulates epithelial ion transport indirectly by activating CGRP-containing submucosal enteric neurons in the mouse colon. This GLP-1-CGRP response was area-specific and could potentially contribute to the diarrheal side effect of certain GLP-1R therapeutics.

VPAC Receptor Subtypes Tune Purinergic Neuron-to-Glia Communication in the Murine Submucosal Plexus

The enteric nervous system (ENS) situated within the gastrointestinal tract comprises an intricate network of neurons and glia which together regulate intestinal function. The exact neuro-glial circuitry and the signaling molecules involved are yet to be fully elucidated. Vasoactive intestinal peptide (VIP) is one of the main neurotransmitters in the gut, and is important for regulating intestinal secretion and motility. However, the role of VIP and its VPAC receptors within the enteric circuitry is not well understood. We investigated this in the submucosal plexus of mouse jejunum using calcium (Ca²⁺)-imaging. Local VIP application induced Ca²⁺-transients primarily in neurons and these were inhibited by VPAC1- and VPAC2-antagonists (PG 99-269 and PG 99-465 respectively). These VIP-evoked neural Ca²⁺-transients were also inhibited by tetrodotoxin (TTX), indicating that they were secondary to action potential generation. Surprisingly, VIP induced Ca²⁺-transients in glia in the presence of the VPAC2 antagonist. Further, selective VPAC1 receptor activation with the agonist ([K15, R16, L27]VIP(1-7)/GRF(8-27)) predominantly evoked glial responses. However, VPAC1-immunoreactivity did not colocalize with the glial marker glial fibrillary acidic protein (GFAP). Rather, VPAC1 expression was found on cholinergic submucosal neurons and nerve fibers. This suggests that glial responses observed were secondary to neuronal activation. Trains of electrical stimuli were applied to fiber tracts to induce endogenous VIP release. Delayed glial responses were evoked when the VPAC2 antagonist was present. These findings support the presence of an intrinsic VIP/VPAC-initiated neuron-to-glia signaling pathway. VPAC1 agonist-evoked glial responses were inhibited by purinergic antagonists (PPADS and MRS2179), thus demonstrating the involvement of P2Y₁ receptors. Collectively, we showed that neurally-released VIP can activate neurons expressing VPAC1 and/or VPAC2 receptors to modulate purine-release onto glia. Selective VPAC1 activation evokes a glial response, whereas VPAC2 receptors may act to inhibit this response. Thus, we identified a component of an enteric neuron-glia circuit that is fine-tuned by endogenous VIP acting through VPAC1- and VPAC2-mediated pathways.

Gastrointestinal dysfunction and enteric neurotoxicity following treatment with anticancer chemotherapeutic agent 5-fluorouracil

BACKGROUND

The use of the anticancer chemotherapeutic agent 5-fluorouracil (5-FU) is often limited by nausea, vomiting, constipation, and diarrhea; these side-effects persist long after treatment. The effects of 5-FU on enteric neurons have not been studied and may provide insight into the mechanisms underlying 5-FU-induced gastrointestinal dysfunction.

METHODS

Balb/c mice received intraperitoneal injections of 5-FU (23 mg/kg) 3 times/week for 14 days. Gastrointestinal transit was analysed in vivo prior to and following 3, 7, and 14 days of 5-FU treatment via serial x-ray imaging. Following 14 days of 5-FU administration, colons were collected for assessment of ex vivo colonic motility, gross morphological structure, and immunohistochemical analysis of myenteric neurons. Fecal lipocalin-2 and CD45⁺ leukocytes in the colon were analysed as markers of intestinal inflammation.

KEY RESULTS

Short-term administration of 5-FU (3 days) increased gastrointestinal transit, induced acute intestinal inflammation and reduced the proportion of neuronal nitric oxide synthase-immunoreactive neurons. Long-term treatment (7, 14 days) resulted in delayed gastrointestinal transit, inhibition of colonic migrating motor complexes, increased short and fragmented contractions, myenteric neuronal loss and a reduction in the number of ChAT-immunoreactive neurons after the inflammation was resolved. Gross morphological damage to the colon was observed following both short- and long-term 5-FU treatment.

CONCLUSIONS & INFERENCES

Our results indicate that 5-FU induces accelerated gastrointestinal transit associated with acute intestinal inflammation at day 3 after the start of treatment, which may have led to persistent changes in the ENS observed after days 7 and 14 of treatment contributing to delayed gastrointestinal transit and colonic dysmotility.

Reduction of epithelial secretion in male rat distal colonic mucosa by bile acid receptor TGR5 agonist, INT-777: role of submucosal neurons

BACKGROUND

Recent evidence from rat neuron-free mucosa study suggests that the membrane bile acid receptor TGR5 decreases colonic secretion under basal and stimulated conditions. As submucosal neurons are key players in secretory processes and highly express TGR5, we investigated their role in TGR5 agonist-induced inhibition of secretion and the pathways recruited.

METHODS

TGR5 expression and localization were assessed in rat proximal (pC) and distal (dC) colon by qPCR and immunohistochemistry with double labeling for cholinergic neurons in whole-mount preparations. The influence of a selective (INT-777) or weak (ursodeoxycholic acid, UDCA) TGR5 agonist on colonic secretion was assessed in Ussing chambers, in dC preparation removing seromuscular ± submucosal tissues, in the presence of different inhibitors of secretion pathways.

KEY RESULTS

TGR5 mRNA is expressed in full thickness dC and pC and immunoreactivity is located in colonocytes and pChAT-positive neurons. Addition of INT-777, and less potently UDCA, decreased colonic secretion in seromuscular stripped dC by -58.17± 2.6%. INT-777 effect on basal secretion was reduced in neuron-free and TTX-treated mucosal-submucosal preparations. Atropine, hexamethonium, indomethacin, and L-NAME all reduced significantly INT-777's inhibitory effect while the 5-HT₄ antagonist, RS-39604, and lidocaine abolished it. INT-777 inhibited stimulated colonic secretion induced by nicotine, but not cisapride, carbachol or PGE2.

CONCLUSIONS & INFERENCES

TGR5 activation inhibits basal and stimulated distal colonic secretion in rats by acting directly on epithelial cells and also inhibiting submucosal neurons. This could represent a counter-regulatory mechanism, at the submucosal level, of the known prosecretory effect of bile acids in the colon.

Generation of Human Embryonic Stem Cell Line Expressing zsGreen in Cholinergic Neurons Using CRISPR/Cas9 System

Lineage specific human embryonic stem cell (hESC) reporter cell line is a versatile tool for biological studies on real time monitoring of differentiation, physiological and biochemical features of special cell types and pathological mechanism of disease. Here we report the generation of ChAT-zsGreen reporter hESC line that express zsGreen under the control of the choline acetyltransferase (ChAT) promoter using CRISPR (Clustered Regularly Interspersed Short Palindromic Repeats)/Cas9 system. We show that the ChAT-zsGreen hESC reporter cell lines retain the features of undifferentiated hESC. After cholinergic neuronal differentiation, cholinergic neurons were clearly labeled with green fluorescence protein (zsGreen). The ChAT-zsGreen reporter hESC lines are invaluable not only for the monitoring cholinergic neuronal differentiation but also for study physiological and biochemical hallmarks of cholinergic neurons.

Neural FFA3 activation inversely regulates anion secretion evoked by nicotinic ACh receptor activation in rat proximal colon

KEY POINTS

Luminal short-chain fatty acids (SCFAs) influence gut physiological function via SCFA receptors and transporters. The contribution of an SCFA receptor, free fatty acid receptor (FFA)3, to the enteric nervous system is unknown. FFA3 is expressed in enteric cholinergic neurons. Activation of neural FFA3 suppresses Cl(-) secretion induced by nicotinic ACh receptor activation via a Gi/o pathway. Neural FFA3 may have an anti-secretory function by modulating cholinergic neural reflexes in the enteric nervous system.

ABSTRACT

The proximal colonic mucosa is constantly exposed to high concentrations of microbially-produced short-chain fatty acids (SCFAs). Although luminal SCFAs evoke electrogenic anion secretion and smooth muscle contractility via neural and non-neural cholinergic pathways in the colon, the involvement of the SCFA receptor free fatty acid receptor (FFA)3, one of the free fatty acid receptor family members, has not been clarified. We investigated the contribution of FFA3 to cholinergic-mediated secretory responses in rat proximal colon. FFA3 was immunolocalized to enteroendocrine cells and to the enteric neural plexuses. Most FFA3-immunoreactive nerve fibres and nerve endings were cholinergic, colocalized with protein gene product (PGP)9.5, the vesicular ACh transporter, and the high-affinity choline transporter CHT1. In Ussing chambered mucosa-submucosa preparations (including the submucosal plexus) of rat proximal colon, carbachol (CCh)-induced Cl(-) secretion was decreased by TTX, hexamethonium, and the serosal FFA3 agonists acetate or propionate, although not by an inactive analogue 3-chloropropionate. Serosal application of a selective FFA3 agonist (N-[2-methylphenyl]-[4-furan-3-yl]-2-methyl-5-oxo-1,4,5,6,7,8-hexahydro-quinoline-3-carboxamide; MQC) dose-dependently suppressed the response to CCh but not to forskolin, with an IC50 of 13 μm. Pretreatment with MQC inhibited nicotine-evoked but not bethanechol-evoked secretion. The inhibitory effect of MQC was reversed by pretreatment with pertussis toxin, indicating that FFA3 acts via the Gi/o pathway. Luminal propionate induced Cl(-) secretion via the cholinergic pathway, which was reduced by MQC, as well as by TTX, hexamethonium or removal of the submucosal plexus. These results suggest that the SCFA-FFA3 pathway has a novel anti-secretory function in that it inhibits cholinergic neural reflexes in the enteric nervous system.

Neurochemical features of endomorphin-2-containing neurons in the submucosal plexus of the rat colon

AIM: To investigate the distribution and neurochemical phenotype of endomorphin-2 (EM-2)-containing neurons in the submucosal plexus of the rat colon.

METHODS: The mid-colons between the right and left flexures were removed from rats, and transferred into Kreb’s solution. For whole-mount preparations, the mucosal, outer longitudinal muscle and inner circular muscle layers of the tissues were separated from the submucosal layer attached to the submucosal plexus. The whole-mount preparations from each rat mid-colon were mounted onto seven gelatin-coated glass slides, and processed for immunofluorescence histochemical double-staining of EM-2 with calcitonin gene-related peptide (CGRP), choline acetyltransferase (ChAT), nitric oxide synthetase (NOS), neuron-specific enolase (NSE), substance P (SP) and vasoactive intestinal peptide (VIP). After staining, all the fluorescence-labeled sections were observed with a confocal laser scanning microscope. To estimate the extent of the co-localization of EM-2 with CGRP, ChAT, NOS, NSE, SP and VIP, ganglia, which have a clear boundary and neuronal cell outline, were randomly selected from each specimen for this analysis.

RESULTS: In the submucosal plexus of the mid-colon, many EM-2-immunoreactive (IR) and NSE-IR neuronal cell bodies were found in the submucosal plexus of the rat mid-colon. Approximately 6 ± 4.2 EM-2-IR neurons aggregated within each ganglion and a few EM-2-IR neurons were also found outside the ganglia. The EM-2-IR neurons were also immunopositive for ChAT, SP, VIP or NOS. EM-2-IR nerve fibers coursed near ChAT-IR neurons, and some of these fibers were even distributed around ChAT-IR neuronal cell bodies. Some EM-2-IR neuronal cell bodies were surrounded by SP-IR nerve fibers, but many long processes connecting adjacent ganglia were negative for EM-2 immunostaining. Long VIP-IR processes with many branches coursed through the ganglia and surrounded the EM-2-IR neurons. The percentages of the EM-2-IR neurons that were also positive for ChAT, SP, VIP or NOS were approximately 91% ± 2.6%, 36% ± 2.4%, 44% ± 2.5% and 44% ± 4.7%, respectively, but EM-2 did not co-localize with CGRP.

CONCLUSION: EM-2-IR neurons are present in the submucosal plexus of the rat colon and express distinct neurochemical markers.

Inhibiting Inducible Nitric Oxide Synthase in Enteric Glia Restores Electrogenic Ion Transport in Mice With Colitis

BACKGROUND & AIMS

Disturbances in the control of ion transport lead to epithelial barrier dysfunction in patients with colitis. Enteric glia regulate intestinal barrier function and colonic ion transport. However, it is not clear whether enteric glia are involved in epithelial hyporesponsiveness. We investigated enteric glial regulation of ion transport in mice with trinitrobenzene sulfonic acid- or dextran sodium sulfate-induced colitis and in Il10(-/-) mice.

METHODS

Electrically evoked ion transport was measured in full-thickness segments of colon from CD1 and Il10(-/-) mice with or without colitis in Ussing chambers. Nitric oxide (NO) production was assessed using amperometry. Bacterial translocation was investigated in the liver, spleen, and blood of mice.

RESULTS

Electrical stimulation of the colon evoked a tetrodotoxin-sensitive chloride secretion. In mice with colitis, ion transport almost completely disappeared. Inhibiting inducible NO synthase (NOS2), but not neuronal NOS (NOS1), partially restored the evoked secretory response. Blocking glial function with fluoroacetate, which is not a NOS2 inhibitor, also partially restored ion transport. Combined NOS2 inhibition and fluoroacetate administration fully restored secretion. Epithelial responsiveness to vasoactive intestinal peptide was increased after enteric glial function was blocked in mice with colitis. In colons of mice without colitis, NO was produced in the myenteric plexus almost completely via NOS1. NO production was increased in mice with colitis, compared with mice without colitis; a substantial proportion of NOS2 was blocked by fluoroacetate administration. Inhibition of enteric glial function in vivo reduced the severity of trinitrobenzene sulfonic acid-induced colitis and associated bacterial translocation.

CONCLUSIONS

Increased production of NOS2 in enteric glia contributes to the dysregulation of intestinal ion transport in mice with colitis. Blocking enteric glial function in these mice restores epithelial barrier function and reduces bacterial translocation.

Changes in Nicotinic Neurotransmission during Enteric Nervous System Development

Acetylcholine-activating pentameric nicotinic receptors (nAChRs) are an essential mode of neurotransmission in the enteric nervous system (ENS). In this study, we examined the functional development of specific nAChR subtypes in myenteric neurons using Wnt1-Cre;R26R-GCaMP3 mice, where all enteric neurons and glia express the genetically encoded calcium indicator, GCaMP3. Transcripts encoding α3, α4, α7, β2, and β4 nAChR subunits were already expressed at low levels in the E11.5 gut and by E14.5 and, thereafter, α3 and β4 transcripts were the most abundant. The effect of specific nAChR subtype antagonists on evoked calcium activity in enteric neurons was investigated at different ages. Blockade of the α3β4 receptors reduced electrically and chemically evoked calcium responses at E12.5, E14.5, and P0. In addition to the α3β4 antagonist, antagonists to α3β2 and α4β2 also significantly reduced responses by P10-11 and in adult preparations. Therefore, there is an increase in the diversity of functional nAChRs during postnatal development. However, an α7 nAChR antagonist had no effect at any age. Furthermore, at E12.5 we found evidence for unconventional receptors that were responsive to the nAChR agonists 1-dimethyl-4-phenylpiperazinium and nicotine, but were insensitive to the general nicotinic blocker, hexamethonium. Migration, differentiation, and neuritogenesis assays did not reveal a role for nAChRs in these processes during embryonic development. In conclusion, there are significant changes in the contribution of different nAChR subunits to synaptic transmission during ENS development, even after birth. This is the first study to investigate the development of cholinergic transmission in the ENS.

Enteric plexuses of two choline-acetyltransferase transgenic mouse lines: chemical neuroanatomy of the fluorescent protein-expressing nerve cells

We studied cholinergic circuit elements in the enteric nervous system (ENS) of two distinct transgenic mouse lines in which fluorescent protein expression was driven by the choline-acetyltransferase (ChAT) promoter. In the first mouse line, green fluorescent protein was fused to the tau gene. This construct allowed the visualization of the fiber tracts and ganglia, however the nerve cells were poorly resolved. In the second mouse line (ChATcre-YFP), CRE/loxP recombination yielded cytosolic expression of yellow fluorescent protein (YFP). In these preparations the morphology of enteric neurons could be well studied. We also determined the neurochemical identity of ENS neurons in muscular and submucous layers using antibodies against YFP, calretinin (CALR), calbindin (CALB), and vasoactive intestinal peptide (VIP). Confocal microscopic imaging was used to visualize fluorescently-conjugated secondary antibodies. In ChATcre-YFP preparations, YFP was readily apparent in somatodendritic regions of ENS neurons. In the myenteric plexus, YFP/CALR/VIP staining revealed that 34% of cholinergic cells co-labeled with CALR. Few single-stained CR-positive cells were observed. Neither YFP nor CALR co-localized with VIP. In GFP/CALB/CALR staining, all co-localization combinations were represented. In the submucosal plexus, YFP/CALR/VIP staining revealed discrete neuronal populations. However, in separate preparations, double labeling was observed for YFP/CALR and CALR/VIP. In YFP/CALR/CALB staining, all combinations of double staining and triple labeling were verified. In conclusion, the neurochemical coding of ENS neurons in these mouse lines is consistent with many observations in non-transgenic animals. Thus, they provide useful tools for physiological and pharmacological studies on distinct neurochemical subtypes of ENS neurons.