Involvement of enteric nerves in permeability changes due to deoxycholic acid in rat jejunum in vivo

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.

Melatonin-Activated Receptor Signaling Pathways Mediate Protective Effects on Surfactant-Induced Increase in Jejunal Mucosal Permeability in Rats

A well-functional intestinal mucosal barrier can be compromised as a result of various diseases, chemotherapy, radiation, and chemical exposures including surfactants. Currently, there are no approved drugs targeting a dysfunctional intestinal barrier, which emphasizes a significant medical need. One candidate drug reported to regulate intestinal mucosal permeability is melatonin. However, it is still unclear if its effect is primarily receptor mediated or antioxidative, and if it is associated with enteric neural pathways. The aim of this rat intestinal perfusion study was to investigate the mechanisms of melatonin and nicotinic acetylcholine receptors on the increase in intestinal mucosal clearance of 51Cr-labeled ethylenediaminetetraacetate induced by 15 min luminal exposure to the anionic surfactant, sodium dodecyl sulfate. Our results show that melatonin abolished the surfactant-induced increase in intestinal permeability and that this effect was inhibited by luzindole, a melatonin receptor antagonist. In addition, mecamylamine, an antagonist of nicotinic acetylcholine receptors, reduced the surfactant-induced increase in mucosal permeability, using a signaling pathway not influenced by melatonin receptor activation. In conclusion, our results support melatonin as a potentially potent candidate for the oral treatment of a compromised intestinal mucosal barrier, and that its protective effect is primarily receptor-mediated.

Progress in understanding relationship between bile acid metabolic disorder and gut diseases

There are a large number of microorganisms in the human intestine, which rely on the nutrition in the digestive tract to survive. At the same time, they affect the intestinal neuro-immune function through the metabolism substances produced by themselves. The enteric neuro-immune system regulates the functions of digestion and absorption so as to maintain the homeostasis in the intestine. Intestinal bile acid metabolism disorder might induce gut dysfunction or intestinal immune imbalance. This review describes the effect of intestinal microbes on the enteric nervous system or other signal molecules of the bile acid pathway linked to some intestinal disorders, with an aim to provide a theoretical basis for clinical treatment of the related diseases.

Effect of increased bile acid synthesis or fecal excretion in irritable bowel syndrome-diarrhea

OBJECTIVES

Approximately 25% of patients with irritable bowel syndrome-diarrhea (IBS-D) have increased total fecal bile acids (BA) and serum C4 (surrogate for BA synthesis). BA synthesis-related genes (KLB and FGFR4) are associated with colonic transit (CT) in IBS-D. Our aims were: (i) to compare phenotype and pathophysiology in IBS-D patients with increased or normal fecal excretion or synthesis of BA; and (ii) to explore association of variations in two candidate bile-acid synthesis genes (KLB and FGFR4) in these two subgroups of IBS-D.

METHODS

A total of 64 IBS-D patients underwent on one occasion: fasting serum C4 and FGF19, total fecal fat and BA excretion, CT, intestinal and colonic permeability, and candidate genotyping (rs17618244 (KLB), rs351855 (FGFR4)). Colonic sensation and tone were measured in 47 of the IBS-D patients. IBS-D subgroups were identified by fecal BA >2,337 mM per 48 h or by serum C4 >47.1 ng/ml.

RESULTS

IBS-D patients with fecal BA >2,337 mM per 48 h (19/54) had significantly greater body mass index, fecal fat, percent chenodeoxycholic acid (CDCA) in feces, and intestinal permeability, and borderline increased CT (P=0.13). Those IBS-D patients with serum C4 >47.1 ng/ml (13/54) had increased total fecal BA excretion and borderline increased colonic permeability. Variants in genes involved in feedback regulation of BA synthesis (KLB, P=0.06 and FGFR4, P=0.09) were potentially associated with the subgroup with elevated serum C4.

CONCLUSIONS

IBS-D with increased BA excretion or synthesis is associated with significant pathophysiological changes relative to patients with normal BA profile. BA diarrhea is identified more effectively with total fecal BA than with serum C4.

Deoxycholic acid induced changes in electrophysiological parameters and macromolecular permeability in murine small intestine with and without functional enteric nervous system plexuses

BACKGROUND

We have previously shown in mice that the fecal proportion and concentration of the hydrophobic bile acid deoxycholic acid (DCA) is elevated with high-fat feeding and that these changes are able to disrupt the intestinal barrier function. The aim of this study was to investigate whether these changes are mediated by the enteric nervous system (ENS).

METHODS

The function of the ENS in the small intestinal tissues of mice was compromised by two different methods: by removing the seromuscular layer and by incubating the intact tissues with tetrodotoxin (TTX), a neural conduction blocker, before DCA treatment. Tissues with or without functional plexuses were mounted into a Ussing chamber system and treated with 3 mM DCA for 20 min. After DCA treatment, the intestinal permeability to fluorescein was assessed. Short-circuit current (Isc ) and transepithelial resistance (TER) were recorded throughout the experiment.

KEY RESULTS

DCA increased intestinal fluorescein permeability only in tissues where the seromuscular layer was removed. In tissues with intact seromuscular layer, DCA induced a significant increase in TER, which was attenuated by blocking of the neural function by TTX.

CONCLUSIONS & INFERENCES

The results of this study suggest that the DCA-induced increase observed in fluorescein permeability is not mediated through neural pathways, but more due to a direct effect on the epithelium. However, as TTX was able to attenuate the DCA-induced increase in TER, it can be speculated that DCA is also able to elicit responses through neural pathways.

Reprint of: Role of enteric neurotransmission in host defense and protection of the gastrointestinal tract

Host defense is a vital role played by the gastrointestinal tract. As host to an enormous and diverse microbiome, the gut has evolved an elaborate array of chemical and physicals barriers that allow the digestion and absorption of nutrients without compromising the mammalian host. The control of such barrier functions requires the integration of neural, humoral, paracrine and immune signaling, involving redundant and overlapping mechanisms to ensure, under most circumstances, the integrity of the gastrointestinal epithelial barrier. Here we focus on selected recent developments in the autonomic neural control of host defense functions used in the protection of the gut from luminal agents, and discuss how the microbiota may potentially play a role in enteric neurotransmission. Key recent findings include: the important role played by subepithelial enteric glia in modulating intestinal barrier function, identification of stress-induced mechanisms evoking barrier breakdown, neural regulation of epithelial cell proliferation, the role of afferent and efferent vagal pathways in regulating barrier function, direct evidence for bacterial communication to the enteric nervous system, and microbial sources of enteric neurotransmitters. We discuss these new and interesting developments in our understanding of the role of the autonomic nervous system in gastrointestinal host defense.

Role of enteric neurotransmission in host defense and protection of the gastrointestinal tract

Host defense is a vital role played by the gastrointestinal tract. As host to an enormous and diverse microbiome, the gut has evolved an elaborate array of chemical and physicals barriers that allow the digestion and absorption of nutrients without compromising the mammalian host. The control of such barrier functions requires the integration of neural, humoral, paracrine and immune signaling, involving redundant and overlapping mechanisms to ensure, under most circumstances, the integrity of the gastrointestinal epithelial barrier. Here we focus on selected recent developments in the autonomic neural control of host defense functions used in the protection of the gut from luminal agents, and discuss how the microbiota may potentially play a role in enteric neurotransmission. Key recent findings include: the important role played by subepithelial enteric glia in modulating intestinal barrier function, identification of stress-induced mechanisms evoking barrier breakdown, neural regulation of epithelial cell proliferation, the role of afferent and efferent vagal pathways in regulating barrier function, direct evidence for bacterial communication to the enteric nervous system, and microbial sources of enteric neurotransmitters. We discuss these new and interesting developments in our understanding of the role of the autonomic nervous system in gastrointestinal host defense.

The intestinal barrier and its regulation by neuroimmune factors

BACKGROUND

The ability to control uptake across the mucosa and protect from damage of harmful substances from the lumen is defined as intestinal barrier function. A disturbed barrier dysfunction has been described in many human diseases and animal models, for example, inflammatory bowel disease, irritable bowel syndrome, and intestinal hypersensitivity. In most diseases and models, alterations are seen both of the paracellular pathway, via the tight junctions, and of the transcellular routes, via different types of endocytosis. Recent studies of pathogenic mechanisms have demonstrated the important role of neuroimmune interaction with the epithelial cells in the regulation of barrier function. Neural impulses from extrinsic vagal and/or sympathetic efferent fibers or intrinsic enteric nerves influence mucosal barrier function via direct effects on epithelial cells or via interaction with immune cells. For example, by nerve-mediated activation by corticotropin-releasing hormone or cholinergic pathways, mucosal mast cells release a range of mediators with effects on transcellular, and/or paracellular permeability (for example, tryptase, TNF-alpha, nerve growth factor, and interleukins).

PURPOSE

In this review, we discuss current physiological and pathophysiological aspects of the intestinal barrier and, in particular, its regulation by neuroimmune factors.

Intestinal bile acid physiology and pathophysiology

Bile acids (BAs) have a long established role in fat digestion in the intestine by acting as tensioactives, due to their amphipathic characteristics. BAs are reabsorbed very efficiently by the intestinal epithelium and recycled back to the liver via transport mechanisms that have been largely elucidated. The transport and synthesis of BAs are tightly regulated in part by specific plasma membrane receptors and nuclear receptors. In addition to their primary effect, BAs have been claimed to play a role in gastrointestinal cancer, intestinal inflammation and intestinal ionic transport. BAs are not equivalent in any of these biological activities, and structural requirements have been generally identified. In particular, some BAs may be useful for cancer chemoprevention and perhaps in inflammatory bowel disease, although further research is necessary in this field. This review covers the most recent developments in these aspects of BA intestinal biology.

Neonatal maternal separation of rat pups results in abnormal cholinergic regulation of epithelial permeability

Neonatal maternal separation (MS) predisposes adult rats to develop stress-induced mucosal barrier dysfunction/visceral hypersensitivity and rat pups to develop colonic epithelial dysfunction. Our aim was to examine if enhanced epithelial permeability in such pups resulted from abnormal regulation by enteric nerves. Pups were separated from the dam for 3 h/day (days 4-20); nonseparated (NS) pups served as controls. On day 20, colonic tissues were removed and mounted in Ussing chambers. Horseradish peroxidase (HRP) flux was used to measure macromolecular permeability. HRP flux was increased in MS versus NS pups. The enhanced flux was inhibited by the cholinergic muscarinic antagonist atropine and the nicotinic antagonist hexamethonium. The cholinergic component was greater in tissues from MS versus NS pups, suggesting that increased cholinergic activity was responsible for the MS elevated permeability. Western blots and immunohistochemistry of colonic tissues demonstrated increased expression of choline acetyltransferase (ChAT) in MS pups, indicating greater synthesis of acetylcholine. Since a previous study indicated that corticotrophin-releasing factor (CRF) mediates barrier dysfunction in MS pups, we examined if the two pathways were linked. In MS tissues, nonselective CRF receptor antagonism inhibited the enhanced flux, and the addition of atropine did not produce further inhibition. Using selective receptor antagonists, we identified that CRF receptor 2 was involved in mediating this effect. These findings suggest that CRF, via CRF receptor 2, acts on cholinergic nerves to induce epithelial barrier dysfunction. Our study provides evidence that MS stimulates synthesis of acetylcholine, which, together with released CRF, creates a condition conducive to the development of epithelial barrier defects.

Bile salt diarrhea

Alterations in bile acid metabolism and in the enterohepatic circulation are often associated with chronic diarrhea and should be considered when more common causes of chronic diarrhea have been excluded. Bile acid diarrhea most often occurs in disease or resection of the terminal ileum, in which there is increased exposure of the colonic mucosa to bile salts with consequent activation of fluid and electrolyte secretion. Congenital or acquired defects in the enterohepatic circulation of bile acids also may lead to diarrhea. Although multiple diagnostic tests may be considered to confirm abnormal fecal bile acid losses, the most critical elements of the clinical evaluation of suspected bile acid diarrhea are a careful history to exclude more common causes of chronic diarrhea and a diagnostic trial of bile acid-binding resins.

Systemic phosphatidylcholine pretreatment protects canine esophageal mucosa during acute experimental biliary reflux

AIM: To characterize the consequences of short-term exposure to luminal bile on mucosal mast cell reactions in a canine model, and to determine the effects of systemic phosphatidylcholine pretreatment in this condition.

METHODS: Twenty mongrel dogs were used for experiments. Group 1 (n  = 5) served as a saline-treated control, while in group 2 (n = 5) the esophagus was exposed to bile for 3 h. In group 3 (n  = 5) the animals were pretreated with 7-nitroindazole to inhibit the neuronal isoform of nitric oxide synthase. In group 4 (n  = 5) phosphatidylcholine solution (50 mg/kg) was administered iv before the biliary challenge. Mucosal microcirculation was observed by intravital videomicroscopy. Myeloperoxidase and nitric oxide synthase activities, the degrees of mast cell degranulation and mucosal damage were evaluated via tissue biopsies.

RESULTS: Exposure to bile evoked significant mast cell degranulation and leukocyte accumulation. The red blood cell velocity and the diameter of the postcapillary venules increased significantly. The tissue ATP content and constitutive nitric oxide synthase activity decreased, while the inducible nitric oxide synthase activity increased significantly as compared to the control values. 7-nitroindazole treatment significantly exacerbated the mucosal mast cell degranulation and tissue damage. In contrast, phosphatidylcholine pretreatment prevented the bile-induced ATP depletion, the inducible nitric oxide synthase and myeloperoxidase activity and the mast cell degranulation increased.

CONCLUSION: The neuronal nitric oxide synthase - mast cell axis plays an important role in the esophageal mucosal defense system. Systemic phosphatidylcholine pretreatment affords effective protection through ameliorating the bile-induced ATP depletion and secondary inflammatory reaction.

Effects of nicotinic receptor blockade on the colonic mucosal response to luminal bile acids in anaesthetized rats

AIM

To test the hypothesis that nicotinic receptor mechanisms mediate the effects of bile acids on the colonic mucosa.

METHODS

The epithelial transport response to 4 mm deoxycholic acid (DCA) was studied in vitro and in vivo, in rat colon. In vitro, epithelial resistance was measured by square pulse analysis, and net membrane current was calculated from the transmucosal potential difference (PD) and resistance. In vivo, we measured PD and net fluid transport.

RESULTS

In vitro, DCA significantly increased membrane current and induced a progressive decrease in epithelial resistance, which in the distal colon eventually resulted in a significant PD reduction. This response was not significantly affected by hexamethonium. In vivo, DCA reduced PD with a significantly larger response in distal colon, but had no consistent effect on net fluid absorption. Nicotinic receptor blockade per se increased net fluid absorption and slightly reduced PD in proximal colon, and inhibited spontaneous net fluid secretion and markedly reduced PD in distal colon. Nicotinic receptor blockade significantly attenuated the bile-acid induced PD response.

CONCLUSION

The data do not support the theory that a bile acid-activated secretory reflex exists in rat colon. The reduced PD response after hexamethonium suggests that a mechanism involving nicotinic receptors may potentiate the permeability response to luminal bile acids.