Mucin and protein release in the rabbit jejunum: effects of bethanechol and vagal nerve stimulation

Guardians of the gut: influence of the enteric nervous system on the intestinal epithelial barrier

The intestinal mucosal surface forms one of the largest areas of the body, which is in direct contact with the environment. Co-ordinated sensory functions of immune, epithelial, and neuronal cells ensure the timely detection of noxious queues and potential pathogens and elicit proportional responses to mitigate the threats and maintain homeostasis. Such tuning and maintenance of the epithelial barrier is constantly ongoing during homeostasis and its derangement can become a gateway for systemic consequences. Although efforts in understanding the gatekeeping functions of immune cells have led the way, increasing number of studies point to a crucial role of the enteric nervous system in fine-tuning and maintaining this delicate homeostasis. The identification of immune regulatory functions of enteric neuropeptides and glial-derived factors is still in its infancy, but has already yielded several intriguing insights into their important contribution to the tight control of the mucosal barrier. In this review, we will first introduce the reader to the current understanding of the architecture of the enteric nervous system and the epithelial barrier. Next, we discuss the key discoveries and cellular pathways and mediators that have emerged as links between the enteric nervous, immune, and epithelial systems and how their coordinated actions defend against intestinal infectious and inflammatory diseases. Through this review, the readers will gain a sound understanding of the current neuro-immune-epithelial mechanisms ensuring intestinal barrier integrity and maintenance of intestinal homeostasis.

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.

Role of Muscarinic Acetylcholine Receptors in Intestinal Epithelial Homeostasis: Insights for the Treatment of Inflammatory Bowel Disease

Inflammatory bowel disease (IBD), which includes Crohn’s disease and ulcerative colitis, is an intestinal disorder that causes prolonged inflammation of the gastrointestinal tract. Currently, the etiology of IBD is not fully understood and treatments are insufficient to completely cure the disease. In addition to absorbing essential nutrients, intestinal epithelial cells prevent the entry of foreign antigens (micro-organisms and undigested food) through mucus secretion and epithelial barrier formation. Disruption of the intestinal epithelial homeostasis exacerbates inflammation. Thus, the maintenance and reinforcement of epithelial function may have therapeutic benefits in the treatment of IBD. Muscarinic acetylcholine receptors (mAChRs) are G protein-coupled receptors for acetylcholine that are expressed in intestinal epithelial cells. Recent studies have revealed the role of mAChRs in the maintenance of intestinal epithelial homeostasis. The importance of non-neuronal acetylcholine in mAChR activation in epithelial cells has also been recognized. This review aimed to summarize recent advances in research on mAChRs for intestinal epithelial homeostasis and the involvement of non-neuronal acetylcholine systems, and highlight their potential as targets for IBD therapy.

Altered sympathovagal balance and pain hypersensitivity in TNBS-induced colitis

INTRODUCTION

Pain hypersensitivity, abnormal motility and autonomic dysfunction contribute to functional symptoms of inflammatory bowel disease (IBD).

MATERIAL AND METHODS

The aim of this study was to assess: nociceptive thresholds for mechanical allodynia (MA) and thermal hyperalgesia (TH), intestinal motility (distal colonic transit and emptying), and cardiac autonomic neuropathy (indices of heart rate variability - HRV) in male Wistar rats with experimental trinitrobenzene sulfonic acid (TNBS) induced colitis. To identify a potential vagal contribution the bilateral subdiaphragmatic vagotomy (SDV) was performed.

RESULTS

Experimental colitis resulted in a significant decrease in pain threshold (MA 23.60 ±2.12, p < 0.001, TH 8.51 ±1.49, p < 0.001), reduced expulsion time (6.2 ±3.5, p < 0,01) and increase in the sympathetic autonomic activity (LFnu 32.54 ±21.16, p < 0.03). The animals with diminished vagal integrity presented with reduced gastrointestinal motility (39.8 ±25.1, p < 0.01) and a decrease in the parasympathetic high-frequency domain of HRV (HFnu 55.37 ±22.80, p < 0.002). The vagotomized rats with colitis showed the strongest nociceptive response (MA 22.46 ±3.02, p < 0.004; TH 7.99 ±1.12, p < 0.003) as well as significant changes in sympatho-vagal balance on HRV testing (LFnu 28.25 ±14.66, p < 0.04; HFnu 71.34 ±14.55, p < 0.04).

CONCLUSIONS

The relationship between the cardiovascular and gastrointestinal system is modulated by neural, hormonal and inflammatory factors. This leads to dysregulation of the brain-gut interactions in the course of IBD. Sensitization and visceral-somatic convergence trigger pain hypersensitivity and autonomic sympathovagal imbalance. While integral vagal innervation impacts analgesic mechanisms via modulation of the immune response, SDV raises sympathetic activity and induces excessive hyperalgesia.

Dietary choline deficiency and excess induced intestinal inflammation and alteration of intestinal tight junction protein transcription potentially by modulating NF-κB, STAT and p38 MAPK signaling molecules in juvenile Jian carp

This study investigated the effects of choline on intestinal mucosal immune and the possible mechanisms in fish by feeding juvenile Jian carp (Cyprinus carpio var. Jian) with graded levels of dietary choline (165-1820 mg/kg diet) for 65 days. The results firstly showed that choline deficiency induced inflammatory infiltration in the proximal intestine (PI), mid intestine (MI) and distal intestine (DI) of fish. Meanwhile, compared with the optimal choline group, choline deficiency decreased the activities of lysozyme and acid phosphatase, contents of complement 3 and IgM in the intestine, downregulated the mRNA levels of antimicrobial peptides (liver-expressed antimicrobial peptide (LEAP) 2A and defensin-3 in the PI and MI, LEAP-2B and hepcidin in the PI, MI and DI), anti-inflammatory cytokines (interleukin (IL) 10 and transforming growth factor β2 in the PI, MI and DI), and signaling molecule IκB in the PI, MI and DI; while upregulated the mRNA levels of pro-inflammatory cytokines (IL-6a and tumor necrosis factor α in the MI and DI, interferon γ2b in the PI and MI, IL-1β and IL-6b in the PI, MI and DI), and signaling molecules (Toll-like receptor 4 in the MI, myeloid differentiation primary response 88 in the PI and MI, Janus kinase 3 and tyrosine kinase 2 in the MI and DI, nuclear factor kappa B (NF-κB), signal transducers and activators of transcription (STAT) 4 and STAT5 in the PI, MI and DI) of juvenile Jian carp, further indicating that choline deficiency caused inflammation and immunity depression in the intestine of fish. But choline deficiency decreased the PI IL-6a mRNA level, and increased the DI LEAP-2A and defensin-3 mRNA levels with unknown reasons. Furthermore, dietary choline deficiency downregulated mRNA levels of tight junction (TJ) proteins (claudin 3c in the PI and MI, claudin 7, claudin 11 and occludin in the PI, MI and DI) and signaling molecule mitogen-activated protein kinases p38 in the PI, MI and DI of juvenile Jian carp, whereas upregulated the mRNA levels of claudin 3b in the MI and DI, and claudin 3c in the DI. Moreover, the excessive choline exhibited negative effects on intestinal immunity and TJ proteins that were similar to the choline deficiency. In summary, dietary choline deficiency or excess caused the depression of intestinal mucosal immune by inducing inflammation and dysfunction of the intestinal physical barrier, and regulating related signaling molecules of fish.

Cholinergic receptor activation on epithelia protects against cytokine-induced barrier dysfunction

AIM

Various types of cholinergic receptors are expressed on intestinal epithelia. Their function is not completely understood. We hypothesize that cholinergic receptor activation on epithelium may serve a protective function in cytokine-induced barrier dysfunction.

METHODS

The effect of cholinergic receptor activation on cellular barrier function in epithelial cells was assessed by measuring electrical impedance, and by determining para-cellular transport in transwell experiments. Cell lysates treated with cytokine and/or cholinergic agonists were analysed for cyto- and chemokine production, and tight junction (TJ) protein rearrangement was assessed. Primary colonic epithelial cells were isolated from surgically resected colon tissue of patients with inflammatory bowel disease.

RESULTS

IL-1β induced production of chemokines (CXCL-1, CXCL-10, IL-8, CCL-7) and led to a rearrangement of TJ proteins (occludin and ZO-1). This response was inhibited by pre-treatment with muscarinic, rather than nicotinic, acetylcholine receptor agonists. Treatment with IL-1β enhanced paracellular permeability (4kD dextran) and reduced impedance across the monolayer, which was counteracted by pre-incubation with acetylcholine, or muscarinic receptor agonist bethanechol. The protective effect of acetylcholine was antagonized by atropine, underscoring muscarinic receptor involvement. IL-1β induced transcription of myosin light chain kinase and phosphorylation of myosin light chain, and this cytokine-induced phosphorylation of MLC was inhibited by muscarinic receptor agonists. Furthermore, in epithelial cells from resection material of patients with Crohn's disease and ulcerative colitis, high expression of CXCL-8 was associated with a reduced choline acetyl transferase expression, suggesting an aberrant epithelial production of ACh in inflammatory context.

CONCLUSION

Acetylcholine acts on muscarinic receptors on epithelial cells to maintain epithelial barrier function under inflammatory conditions.

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 digestive neuronal-glial-epithelial unit: a new actor in gut health and disease

The monolayer of columnar epithelial cells lining the gastrointestinal tract--the intestinal epithelial barrier (IEB)--is the largest exchange surface between the body and the external environment. The permeability of the IEB has a central role in the regulation of fluid and nutrient intake as well as in the control of the passage of pathogens. The functions of the IEB are highly regulated by luminal as well as internal components, such as bacteria or immune cells, respectively. Evidence indicates that two cell types of the enteric nervous system (ENS), namely enteric neurons and enteric glial cells, are potent modulators of IEB functions, giving rise to the novel concept of a digestive 'neuronal-glial-epithelial unit' akin to the neuronal-glial-endothelial unit in the brain. In this Review, we summarize findings demonstrating that the ENS is a key regulator of IEB function and is actively involved in pathologies associated with altered barrier function.

Cholinergic signalling in gut immunity

The gut immune system shares many signalling molecules and receptors with the autonomic nervous system. A good example is the vagal neurotransmitter acetylcholine (ACh), for which many immune cell types express cholinergic receptors (AChR). In the last decade the vagal nerve has emerged as an integral part of an immune regulation network via its release of ACh; a system coined "the cholinergic anti-inflammatory reflex". The perspective of cholinergic immune regulation in the gut mucosa has been widened by the recent discovery of populations of ACh producing immune cells in the spleen and other organs. As such, ACh, classically referred to as neurotransmitter, may serve a much broader function as bi-directional signalling molecule between neurons and non-neuronal cell types of the immune system.

n-3 polyunsaturated fatty acids in the maternal diet modify the postnatal development of nervous regulation of intestinal permeability in piglets

The intestinal epithelial barrier (IEB) plays a key role in the maintenance of gut homeostasis and the development of the immune system in newborns. The enteric nervous system (ENS), a key regulator of gastrointestinal functions, has been shown to be modulated by nutritional factors. However, it remains currently unknown whether maternal diet, in particular n-3 polyunsaturated fatty acids (n-3PUFAs), can impact upon the IEB in newborn piglets and whether the ENS is involved in this effect. Sows received either a control diet (lard based) or an n-3PUFA diet (linseed oil based) during gestation and lactation. Intestinal paracellular permeability was assessed in Ussing chambers on piglets at birth, 3, 7, 14, 21 and 28 postnatal days (PND). Basal jejunal permeability increased significantly and similarly in both groups until PND14 and decreased thereafter. However, at PND28, permeability was higher in n-3PUFA animals as compared to controls. In addition, a vasoactive intestinal peptide (VIP) receptor antagonist increased paracellular permeability in controls but not in n-3PUFA piglets. Conversely, atropine and hexamethonium decreased paracellular permeability in the n-3PUFA group but not in the control group. Moreover, the n-3PUFA diet increased the proportion of choline acetyltransferase (ChAT)-immunoreactive (IR) neurons and decreased the proportion of VIP-IR neurons in the submucosal plexus of piglet jejunum compared to controls. In addition, in primary culture of rat ENS, we showed that 20:5n-3 but not 18:3n-3 increased the proportion of ChAT-IR neurons and decreased the proportion of VIP-IR neurons. In conclusion, supplementation of the maternal diet with n-3PUFAs modified intestinal permeability probably via diet-induced neuroplastic changes in the ENS of newborn piglets.

Vagus nerve activity augments intestinal macrophage phagocytosis via nicotinic acetylcholine receptor alpha4beta2

BACKGROUND & AIMS

The vagus nerve negatively regulates macrophage cytokine production via the release of acetylcholine (ACh) and activation of nicotinic acetylcholine receptors (nAChR). In various models of intestinal inflammation, vagus nerve efferent stimulation ameliorates disease. Given the actively constrained cytokine responses of intestinal macrophages, we explored the effect of nAChR activation on endocytosis and phagocytosis by macrophages residing in the peritoneal and mucosal compartment.

METHODS

The phagocytic uptake by intestinal and peritoneal macrophages was measured by fluorescence-activated cell sorter analysis, and the nAChR involved was determined by pharmacologic blockade, short hairpin RNA-assisted gene knockdown, and the use of specific nAChR knockout mice. The effect of electrical vagus nerve stimulation on epithelial translocation and macrophage uptake of luminal particles was studied in mice.

RESULTS

In isolated intestinal and peritoneal macrophages, nAChR activation enhanced endocytosis and phagocytosis. This effect was mediated via stimulated recruitment of GTPase Dynamin-2 to the forming phagocytic cup. These effects involve nAChR alpha4/beta2, rather than nAChR alpha7. Despite enhanced bacterial uptake, acetylcholine reduced NF-kappaB activation and pro-inflammatory cytokine production, while stimulating anti-inflammatory interleukin-10 production. Vagus nerve stimulation in mice altered mucosal immune responses by augmenting epithelial transport and uptake of luminal bacteria by lamina propria macrophages.

CONCLUSIONS

ACh enhances phagocytic potential while inhibiting immune reactivity via nAChR alpha4/beta2 in mouse macrophages. Hence, vagus nerve efferent activity may stimulate surveillance in the intestinal mucosa and peritoneal compartment.

The vagus nerve as a modulator of intestinal inflammation

The cholinergic nervous system attenuates the production of pro-inflammatory cytokines and inhibits inflammatory processes. Hence, in animal models of intestinal inflammation, such as postoperative ileus and dextran sulfate sodium-induced colitis, vagus nerve stimulation ameliorates disease activity. On the other hand, in infectious models of microbial peritonitis, vagus nerve activation seemingly acts counteractive; it impairs bacterial clearance and increases mortality. It is originally indicated that the key mediator of the cholinergic anti-inflammatory pathway, acetylcholine (ACh), inhibits cytokine release directly via the alpha7 nicotinic ACh receptor (nAChR) expressed on macrophages. However, more recent data also point towards the vagus nerve as an indirect modulator of innate inflammatory processes, exerting its anti-inflammatory effects via postganglionic modulation of immune cells in primary immune organs. This review discusses advances in the possible mechanisms by which the vagus nerve can mediate the immune response, and the role of nAChR activation and signalling on macrophages and other immune cells.

Neuro-glial crosstalk in inflammatory bowel disease

Inflammatory bowel disease (IBD) is a multifactorial disease in which environmental, immune and genetic factors are involved in the pathogenesis. Although biological therapies (antibodies anti-tumour necrosis factor-alpha or anti-integrin) have considerably improved the symptoms and quality of life of IBD patients, some drawbacks have emerged limiting their long-term use. In addition, prevention of relapses and treatment of resistant ulcers remains a clinical challenge. In this context, a better understanding of the pathophysiology of IBD and the development of novel therapeutic intervention would benefit from further basic and preclinical research into the role of the cellular microenvironment and the interaction between its cellular constituents. In this context, the role of the enteric nervous system (ENS) in the regulation of the intestinal epithelial barrier (IEB) and the gut immune response has fuelled an increased interest in the last few years. Recent advances, summarized in this review, have highlighted the ENS as playing a key role in the control of IEB functions and gut immune homeostasis, and that alterations of the ENS could be directly associated in the development of IBD and its associated symptoms.

The protective effect of the vagus nerve in a murine model of chronic relapsing colitis

The vagus nerve inhibits the response to systemic administration of endotoxin, and we have recently extended this observation to show that the vagus attenuates acute experimental colitis in mice. The purpose of the present study was to determine whether there is a tonic counterinflammatory influence of the vagus on colitis maintained over several weeks. We assessed disease activity index, macroscopic and histological damage, myeloperoxidase (MPO) activity, and Th1 and Th2 cytokine profiles in chronic colitis induced by administration of dextran sodium sulfate (DSS) in drinking water for three cycles during 5 days with 11 days of rest between each cycle (DSS 3, 2, 2%) in healthy and vagotomized C57BL/6 mice and in mice deficient in macrophage-colony stimulating factor (M-CSF). A pyloroplasty was performed in vagotomized mice. Vagotomy accelerated the onset and the severity of inflammation during the first and second but not the third cycle. Although macroscopic scores were not significantly changed, histological scores as well as MPO activity and colonic tissue levels of IL-1alpha, TNF-alpha, IFN-gamma, and IL-18 but not IL-4 were significantly increased in vagotomized mice compared with sham-operated mice that received DSS. In control mice (without colitis), vagotomy per se did not affect any inflammatory marker. Vagotomy had no effect on the colitis in M-CSF-derived macrophage-deficient mice. These results indicate that the vagus protects against acute relapses on a background of chronic inflammation. Identification of the molecular mechanisms underlying the protective role of parasympathetic nerves opens a new therapeutic avenue for the treatment of acute relapses of chronic inflammatory bowel disease.

Vagus nerve integrity and experimental colitis

Previous studies have identified a counterinflammatory vagal reflex in the context of endotoxic shock. We have extended this observation to show that the vagus confers protection against acute (5 days) colitis induced by dextran sodium sulfate (DSS) or by dinitrobenzene sulfonic acid (DNBS). We have shown that this is mediated via macrophages and involves the suppression of proinflammatory cytokines. In this study, we have examined whether the vagal integrity confers long-lasting protection by studying DNBS- and DSS-induced inflammatory responses in the colon at 9 to 61 days postvagotomy. The integrity of vagotomy was confirmed at all time points using CCK-induced satiety. As previously described in a DNBS and DSS model, vagotomy associated with the pyloroplasty increased all indices of inflammation. Vagotomy increased the disease activity index as well as the macroscopic and histological scores by 75 and 41%, respectively. In addition, myeloperoxidase (MPO) activity, serum levels of C-reactive protein (CRP), and colonic tissue levels of proinflammatory cytokine increased when colitis was induced 9 days postvagotomy. However, these increases in inflammatory indices were substantially diminished in mice with colitis induced 21, 33, and 61 days postvagotomy. This was accompanied by an increased production of interleukin-10, transforming growth factor-beta, Forkhead Box P3 (FOXP3) staining in colonic tissue, and serum corticosterone. These findings indicate that although vagal integrity is an important protective factor, other counterinflammatory mechanisms come into play if vagal integrity is compromised beyond 2 wk.

Muscarinic acetylcholine receptor activation increases transcellular transport of macromolecules across mouse and human intestinal epithelium in vitro

The intestinal epithelium acts as a barrier restricting uptake of luminal macromolecules such as dietary antigens and microbes. Here, we examined the role of cholinergic signalling in the regulation of permeability to macromolecules. Mouse jejunum was mounted in Ussing chambers and permeability was determined by measuring the flux of the antigen-sized protein, horseradish peroxidase (HRP), across the tissue. Baseline HRP permeability was significantly reduced by neural blockade with tetrodotoxin or cholinergic muscarinic antagonism with atropine, suggesting that ongoing release of endogenous acetylcholine from enteric nerves regulates barrier function. Exogenous addition of the muscarinic agonist bethanechol caused significant increases in both HRP flux and the area of HRP-containing endosomes in enterocytes. Bethanechol-enhanced HRP flux was abrogated by the M3 receptor antagonist, 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP), the phospholipase A(2) inhibitor quinacrine, and the cyclooxygenase inhibitor indomethacin. Complementary in vitro studies showed direct effects of bethanechol on T84 epithelial cells, where increased HRP uptake was associated with increased F-actin, and increased cytosolic phospholipase A(2) (cPLA(2)) phosphorylation. Taken together, these results provide evidence for cholinergic regulation of transepithelial transport of macromolecules, mainly mediated by activation of M3 receptors with subsequent involvement of phospholipase A(2) and cyclooxygenase products.

The vagus nerve: a tonic inhibitory influence associated with inflammatory bowel disease in a murine model

BACKGROUND & AIMS

The recently proposed Inflammatory Reflex describes an interaction between the vagus nerve and peripheral macrophages, resulting in attenuation of proinflammatory cytokine release in response to systemic exposure to bacterial endotoxin. The purpose of this study was to determine whether a similar vagus/macrophage axis modulates the inflammatory responses in the colon in mice.

METHODS

We assessed the Disease Activity Index (DAI), macroscopic and histologic damage, serum amyloid-P level, and myeloperoxidase activity in colitis induced by administration of dextran sodium sulfate (DSS) in healthy and vagotomized C57BL/6 and in mice deficient in macrophage-colony stimulating factor (M-CSF)-induced and in hapten-induced colitis. A pyloroplasty was performed in vagotomized mice.

RESULTS

DAI, macroscopic and histologic scores, myeloperoxidase activity, levels of serum amyloid-P, and colonic tissue levels of interleukin (IL)-1beta, IL-6, and tumor necrosis factor-alpha were increased significantly in vagotomized mice 5 days post-DSS and 3 days after hapten-induced colitis compared with sham-operated mice that received DSS or the hapten. Pretreatment with nicotine significantly decreased each of these markers in vagotomized mice with DSS colitis, and all markers except DAI and IL-6 in sham-operated DSS-treated mice. Conversely, hexamethonium treatment significantly increased each of these markers in the sham-operated DSS-treated mice. Vagotomy had no effect on the colitis in M-CSF-deficient mice.

CONCLUSIONS

The vagus nerve plays a counterinflammatory role in acute colitis via a macrophage-dependent mechanism, involving hexamethonium-sensitive nicotinic receptors. The identification of a counterinflammatory neural pathway would open new therapeutic avenues for treating acute exacerbations of inflammatory bowel disease.

Quantitative changes in gastric mucosal glycoproteins: effect of cholinergic agonist and vagal nerve stimulation in the rat

The role of the vagus nerve and cholinergic mechanisms in the control of the rat gastric mucin and protein (PROT) release in vivo was investigated. Under urethane anaesthesia (1.25 g kg(-1)), the rats had their gastric lumen perfused with saline. Mucus secretion was measured as a function of adherent mucus on the mucosa surface and the luminal content of sialic acids (SIA), galactose (GAL), pyruvate and PROT. Electrical stimulation of the vagi significantly increased the levels of mucus (3.23 +/- 025 microg g(-1) tissue, P < 0.05), free sialic acid (FS) (0.18 +/- 0.04 mg mL(-1), P < 0.05) and PROT (0.25 +/- 0.003 mg mL(-1), P < 0.05) when compared with control animals. Bilateral cervical vagotomy had no significant effect on adherent mucus or basal levels of PROT, SIA and GAL (P > 0.05) with respect to the control. In both vagotomized and vagal intact animals, the cholinergic agonist (carbachol, 200 mg kg(-1)) significantly increased PROT, adherent mucus and FS (P < 0.05) and decreased bound sialic acid (P > 0.05). There were no visible haemorrhagic streaks on the gastric mucosa of vagotomized, vagal intact and carbachol-treated animals. The results suggest that vagus nerve does not exert a tonic control on gastric glycoprotein secretion in vivo and that cholinergic effect on the mucus secreting cells may be implemented via the intrinsic nerves of the enteric nervous system.

The effect of Aloe vera A. Berger (Liliaceae) on gastric acid secretion and acute gastric mucosal injury in rats

The effect of varying doses of ethanol extract of Aloe vera (Liliaceae) on acute gastric mucosal lesions induced by 0.6 M HCl and acid output was studied in the pylorus ligated and lumen perfuse rats, respectively. Acid secretion was determined by titration of the collected gastric juice to pH 7.0. Intraperitoneal injection of Aloe vera, dose dependently inhibited gastric acid secretion. The plant was more active as a gastroprotective agent at lower concentration against mucosal injury induced by 0.6 M HCl. In conclusion, Aloe vera is endowed with gastric acid anti-secretory activity and could protect the gastric mucosa at low concentrations against injurious agents.

Secretion of the trefoil factor TFF3 from the isolated vascularly perfused rat colon

The trefoil factor TFF3 is a peptide predominantly produced by mucus-secreting cells in the small and large intestines. It has been implicated in intestinal protection and repair. The mechanisms that govern TFF3 secretion are poorly understood. The aim of this study was, therefore, to evaluate the influence of neurotransmitters, hormonal peptides and mediators of inflammation on the release of TFF3. For this purpose, an isolated vascularly perfused rat colon preparation was used. After a bolus administration of 1 ml isotonic saline into the lumen, TFF3 secretion was induced by a 30-min intra-arterial infusion of the compounds to be tested. TFF3 was evaluated in the luminal effluent using a newly developed radioimmunoassay. TFF3 was barely detected in crude luminal samples. In contrast, dithiothreitol (DTT) treatment of the effluent revealed TFF3 immunoreactivity, which amounted to about 0.3 pmol min(-1) cm(-1) in the basal state. Gel chromatography of DTT-treated luminal samples revealed a single peak that co-eluted with the monomeric form of TFF3. TFF3 was not detected in the portal effluent. Bethanechol (10(-6)-10(-4) M), vasoactive intestinal peptide (VIP, 10(-8)-10(-7) M) or bombesin (10(-8)-10(-7) M) induced a dose-dependent release of TFF3. In contrast, substance P evoked a modest release of TFF3, whereas calcitonin gene-related peptide (CGRP), somatostatin, neurotensin or peptide YY (PYY) did not modify TFF3 secretion. The degranulator compound bromolasalocid, 16,16-dimethyl PGE2 (dmPGE2) or interleukin-1-beta (IL-1-beta) also evoked a marked release of TFF3. In conclusion, TFF3 in the colonic effluent is present in a complex. This association presumably involves a disulfide bond. Additionally, the present results suggest a role for enteric nervous system and resident immune cells in mediation of colonic TFF3 secretion.

Stress-induced decrease of the intestinal barrier function. The role of muscarinic receptor activation

Recently the breakdown of the barrier function of the intestinal epithelium after application of an experimental psychological and physical stress protocol in rats has been observed. Not only did smaller molecules pass from the luminal to the serosal side, but so also did larger proteins with the dimensions of luminal antigens and toxins. The increased permeability for macromolecules is primarily due to a decrease of the tightness of the zonula occludens, but an increased endocytotic uptake indicates that transcytosis is increased also. From studies of model systems it can be concluded that activation of the intracellular protein kinase C route by muscarinic receptor activation or histamine receptor activation can be one of the underlying cellular pathways. The physical pathway relaying the stress from the brain to the intestinal tract appears to be the parasympathetic branch of the autonomic nervous system. The difference in reaction of different strains suggests that coping style is an important determinant of the response of the intestinal barrier to stress.

Neural control of intestinal ion transport and paracellular permeability is altered by nutritional status

This study examined the effect of fasting on the neural control of ion transport and paracellular permeability in piglet jejunum. Muscle-stripped tissues from fed or 48-h fasted piglets were mounted in Ussing chambers. Neural blockade with tetrodotoxin (TTX) or antagonists of muscarinic or nicotinic receptors caused reductions in basal short-circuit current that were approximately threefold greater in fasted piglets. The TTX-induced reduction in short-circuit current in fasted piglets was due to a decrease in residual ion flux and was abolished in the absence of HCO(-)(3). Intestinal paracellular permeability, as indicated by tissue conductance (G(t)) and fluxes of inulin and mannitol, was significantly increased by fasting. TTX increased inulin flux and G(t) in fed but not fasted piglets. In fasted piglets, carbachol reduced G(t) by 29% and mannitol flux by 27% but had no effect on these parameters in the fed state. We conclude that fasting enhances enteric neural control of basal ion transport and increases paracellular permeability in piglet jejunum. Tonic release of enteric neurotransmitters regulates paracellular permeability in the fed state, and cholinergic stimulation restores fasting-induced elevations in paracellular permeability to fed levels.

Declined human esophageal mucin secretion in patients with severe reflux esophagitis

It has been recently demonstrated that human esophageal submucosal mucous glands exhibit the ability to secrete copious amounts of mucin, well known within the gastrointestinal tract for its protective quality against hydrogen ion and pepsin. Since mucin may also play a protective role within the esophageal compartment, we have studied the rate of secretion of esophageal mucin in patients with RE. Mucin was assessed by periodic acid-Schiff methodology in esophageal secretion collected during continuous perfusion with saline (period I) followed by HCl (period II), HCl/pepsin (period III), and final saline (period IV), mimicking the natural gastroesophageal scenario. The basal rate of the luminal release of mucin in patients with grade II RE was 18% lower as compared with controls. During exposure of the esophageal mucosa to an HCl/pepsin solution, esophageal mucin output in the RE group was 52% lower than in the control group (0.154 +/- 0.027 vs 0.320 +/- 0.049 mg/cm2/min; P = 0.025). Furthermore, the rates of esophageal mucin output in patients with grade III RE during esophageal perfusion with saline and HCl/pepsin were 62% (0.090 +/- 0.021 vs 0.239 +/- 0.036 mg/cm2/min; P = 0.016) and 86% (0.048 +/- 0.010 vs 0.320 +/- 0.049 mg/cm2/min; P = 0.001) lower when compared with corresponding values in controls. After endoscopic healing of RE, the overall impairment in the rate of esophageal mucin secretion in patients with grade II improved from 31% to 17% at the end of therapy, whereas in patients with grade III the impairment in mucin secretion improved only marginally from 71% to 69%.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscarinic stimulation of gallbladder epithelium. III. Antagonism of cAMP-mediated effects

Elevation of adenosine 3',5'-cyclic monophosphate (cAMP) levels in Necturus gallbladder (NGB) epithelium activates an apical membrane Cl- conductance and decreases transepithelial fluid transport (Jv). Acetylcholine (ACh), which had no effects on Jv by itself, antagonized the electrophysiological effects of forskolin (FSK) and theophylline and the decrease in Jv produced by FSK. By itself, ACh had no effects on basal cAMP levels but antagonized the increases in cAMP induced by FSK and theophylline. ACh had no effect on phosphodiesterase activity and prevented both the electrophysiological response and the elevation in cAMP by theophylline. In conclusion, the effect of ACh is mediated by inhibition of adenylate cyclase. A pertussis toxin (PTX)-sensitive G protein may mediate inhibition of adenylate cyclase because pretreatment with PTX prevented the reversal of the electrophysiological effects of FSK by ACh, and PTX catalyzed the ribosylation of cell membranes from NGB epithelium. ACh could have a physiological role in modulating the effects of secretagogues that act via elevation of cAMP levels.

Human esophageal secretion: mucosal response to luminal acid and pepsin

BACKGROUND/AIMS

Although esophageal histology in humans reveals numerous submucosal mucous glands, their secretion has never been explored. Therefore, we have studied the chemical composition and physical characteristics of esophageal secretion under the impact of luminal saline, acid, and acid/pepsin solutions.

METHODS

The esophageal lumen in 21 healthy volunteers was continuously perfused with saline, HCI, or HCI/pepsin. Perfusates were assayed for mucin, protein, and viscosity. In addition, analysis of amino acid and sugar composition of purified esophageal mucin was performed.

RESULTS

Esophageal perfusion with saline resulted in luminal release of mucin at the rate of 0.23 +/- 0.03 mg.cm-2 x min-1. Acid/pepsin solution significantly enhanced luminal release of mucin (0.32 +/- 0.03 mg.cm-2 x min-1; P < 0.01). HCI/pepsin solution also significantly increased the luminal output of protein (P < 0.01) and significantly impaired the viscosity of the esophageal perfusate (P < 0.05). Threonine, serine, and proline were the major amino acids within the esophageal mucin, whereas galactose was the predominant carbohydrate.

CONCLUSIONS

Luminally released esophageal mucin, shown for the first time in humans, contributes significantly to maintaining the high viscosity of esophageal secretions. Significant increase in the luminal release of mucin under the impact of acid and pepsin, with subsequent decline of the perfusate viscosity, may indicate that mucin is the major target for gastric acid and pepsin, absorbing the deleterious impact of the gastroesophageal refluxate.