Alterations in melatonin and 5-HT signalling in the colonic mucosa of mice with dextran-sodium sulfate-induced colitis

Amperometry approach curve profiling to understand the regulatory mechanisms governing the concentration of intestinal extracellular serotonin

Enterochromaffin (EC) cells located within the intestinal mucosal epithelium release serotonin (5-HT) to regulate motility tones, barrier function and the immune system. Electroanalytical methodologies have been able to monitor steady state basal extracellular 5-HT levels but are unable to provide insight into how these levels are influenced by key regulatory processes such as release and uptake. We established a new measurement approach, amperometry approach curve profiling, which monitors the extracellular 5-HT level at different electrode-tissue (E-T) distances. Analysis of the current profile can provide information on contributions of regulatory components on the observed extracellular 5-HT level. Measurements were conducted from ex vivo murine ileum and colon using a boron-doped diamond (BDD) microelectrode. Amperometry approach curve profiling coupled with classical pharmacology demonstrated that extracellular 5-HT levels were significantly lower in the colon when compared to the ileum. This difference was due to a greater degree of activity of the 5-HT transporter (SERT) and a reduced amount of 5-HT released from colonic EC cells. The presence of an inhibitory 5-HT4 autoreceptor was observed in the colon, where a 40% increase in extracellular 5-HT was the half maximal inhibitory concentration for activation of the autoreceptor. This novel electroanalytical approach allows estimates of release and re-uptake and their contribution to 5-HT extracellular concentration from intestinal tissue be obtained from a single series of measurements.

Ginsenoside CK Alleviates DSS-Induced IBD in Mice by Regulating Tryptophan Metabolism and Activating Aryl Hydrocarbon Receptor via Gut Microbiota Modulation

Dysbiosis of gut microbiota is believed to be associated with inflammatory bowel disease (IBD). Ginsenoside compound K (CK), the main metabolite of Panax ginseng ginsenoside, has proven effective as an anti-inflammatory agent in IBD. However, the mechanisms by which CK modulates gut microbiota to ameliorate IBD remain poorly understood. Herein, CK demonstrated the potential to suppress the release of proinflammatory cytokines by gut microbiota modulation. Notably, supplementation with CK promoted the restoration of a harmonious balance in gut microbiota, primarily by enhancing the populations of Lactobacillus and Akkermansia. Furthermore, CK considerably elevated the concentrations of tryptophan metabolites derived from Lactobacillus that could activate the aryl hydrocarbon receptor. Overall, the promising alleviative efficacy of CK primarily stemmed from the promotion of Lactobacillus growth and production of tryptophan metabolites, suggesting that CK should be regarded as a prospective prebiotic agent for IBD in the future.

Therapeutic applications of melatonin in disorders related to the gastrointestinal tract and control of appetite

Most animals have large amounts of the special substance melatonin, which is controlled by the light/dark cycle in the suprachiasmatic nucleus. According to what is now understood, the gastrointestinal tract (GIT) and other areas of the body are sites of melatonin production. According to recent studies, the GIT and adjacent organs depend critically on a massive amount of melatonin. Not unexpectedly, melatonin's many biological properties, such as its antioxidant, anti-inflammatory, pro-apoptotic, anti-proliferative, anti-metastasis, and antiangiogenic properties, have drawn the attention of researchers more and more. Because melatonin is an antioxidant, it produces a lot of secretions in the GIT's mucus and saliva, which shields cells from damage and promotes the development of certain GIT-related disorders. Melatonin's ability to alter cellular behavior in the GIT and other associated organs, such as the liver and pancreas, is another way that it functions. This behavior alters the secretory and metabolic activities of these cells. In this review, we attempted to shed fresh light on the many roles that melatonin plays in the various regions of the gastrointestinal tract by focusing on its activities for the first time.

5-HT induces regulatory B cells in fighting against inflammation-driven ulcerative colitis

Serotonin (5-hydroxytryptamine or 5-HT) is a neuroendocrine peptide endowed with immunomodulatory functions. Regulatory B cells (Bregs) play an important role in maintaining intestinal immune homeostasis. We analyzed the differences of 5-HT and Bregs between peripheral blood of ulcerative colitis (UC) and healthy controls (HC). Besides, 5-HT-treated B cells were adoptively transferred into colitis mice to elucidate the role of 5-HT in regulating Bregs. The level of serum 5-HT and IL-10 in UC patients was lower and both were negatively correlated with disease activity. 5-HT7 receptor (5-HT7R) was higher expressed on Bregs in UC. 5-HT promoted IL-10 production in Bregs through the activation of STAT3. And adoptive transfer of 5-HT-treated B cells alleviated intestinal inflammation via inducing IL-10-producing B cells in mice. Our results suggest that 5-HT/5-HT7R signaling pathway facilitate functional Bregs in constraining inflammation in UC, which may be a new potential prospect in the treatment of UC.

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.

Prophylactic effect of Tongxieyaofang polysaccharide on depressive behavior in adolescent male mice with chronic unpredictable stress through the microbiome-gut-brain axis

Major depression disorder is more common among adolescents and is a primary reason for suicide in adolescents. Some antidepressants are ineffective and may possess side effects. Therefore, developing an adolescent antidepressant is the need of the hour. We designed the stress model of adolescent male mice induced by chronic unpredictable stress (CUS). The mice were treated using Tongxieyaofang neutral polysaccharide (TXYF-NP), Tongxieyaofang acidic polysaccharide (TXYF-AP), TXYF-AP + TXYF-NP and fructooligosaccharide + galactooligosaccharides to determine their body weight, behavior, and serum hormone levels. RT-qPCR was used to detect the gene expression of Crhr1, Nr3c1, and Nr3c2 in the hypothalamus and hippocampus and the gene expression of glutamic acid and γ-aminobutyric acid-related receptors in the hippocampus. RT-qPCR, Western blot, and ELISA detected tryptophan metabolism in the colon, serum, and hippocampus. 16s rDNA helped sequence colon microflora, and non-targeted metabolomics enabled the collection of metabolic profiles of colon microflora. In adolescent male mice, CUS induced depression-like behavior, hypothalamic-pituitary-adrenal axis hyperactivity, hippocampal tissue damage, abnormal expression of its related receptors, and dysregulation of tryptophan metabolism. The 16s rDNA and non-targeted metabolomics revealed that CUS led to colon microflora disorder and bile acid metabolism abnormality. Tongxieyaofang polysaccharide could improve the bacterial community and bile acid metabolism disorder by upregulating the relative abundance of Lactobacillus gasseri, Lachnospiraceae bacterium 28-4, Bacteroides and Ruminococcaceae UCG-014 while preventing CUS-induced changes. TXYF-P can inhibit depression-like behavior due to CUS by regulating colonic microflora and restoring bile acid metabolism disorder. Thus, based on the different comparisons, TXYF-NP possessed the best effect.

Administration of Bifidobacterium pseudolongum suppresses the increase of colonic serotonin and alleviates symptoms in dextran sodium sulfate-induced colitis in mice

Previous studies suggested that altered gut serotonin (5-HT) signaling is implicated in the pathophysiology of inflammatory bowel disease (IBD). Indeed, 5-HT administration reportedly exacerbated the severity of murine dextran sodium sulfate (DSS)-induced colitis that mimics human IBD. Our recent study suggested that Bifidobacterium pseudolongum, one of the most predominant bifidobacterial species in various mammals, reduces the colonic 5-HT content in mice. The present study thus tested whether the administration of B. pseudolongum prevents DSS-induced colitis in mice. Colitis was induced by administering 3% DSS in drinking water in female BALB/c mice, and B. pseudolongum (10⁹ CFU/day) or 5-aminosalicylic acid (5-ASA, 200 mg/kg body weight) was intragastrically administered once daily throughout the experimental period. B. pseudolongum administration reduced body weight loss, diarrhea, fecal bleeding, colon shortening, spleen enlargement, and colon tissue damage and increased colonic mRNA levels of cytokine genes (Il1b, Il6, Il10, and Tnf) almost to an extent similar to 5-ASA administration in DSS-treated mice. B. pseudolongum administration also reduced the increase of colonic 5-HT content, whereas it did not alter the colonic mRNA levels of genes that encode the 5-HT synthesizing enzyme, 5-HT reuptake transporter, 5-HT metabolizing enzyme, and tight junction-associated proteins. We propose that B. pseudolongum is as beneficial against murine DSS-induced colitis as the widely used anti-inflammatory agent 5-ASA. However, further studies are needed to clarify the causal relationship between the reduced colonic 5-HT content and reduced severity of DSS-induced colitis caused by B. pseudolongum administration.

Three-Dimensional-Printed Electrochemical Multiwell Plates for Monitoring Food Intolerance from Intestinal Organoids

Common symptoms of food intolerance are caused by chemical components within food that have a pharmacological activity to alter the motility of the gastrointestinal tract. Food intolerance is difficult to diagnose as it requires a long-term process of eliminating foods that are responsible for gastrointestinal symptoms. Enterochromaffin (EC) cells are key intestinal epithelium cells that respond to luminal chemical stimulants by releasing 5-HT. Changes in 5-HT levels have been shown to directly alter the motility of the intestinal tract. Therefore, a rapid approach for monitoring the impact of chemicals in food components on 5-HT levels can provide a personalized insight into food intolerance and help stratify diets. Within this study, we developed a three-dimensional (3D)-printed electrochemical multiwell plate to determine changes in 5-HT levels from intestinal organoids that were exposed to varying chemical components found in food. The carbon black/poly-lactic acid (CB/PLA) electrodes had a linear range in physiological concentrations of 5-HT (0.1-2 μM) with a limit of detection of 0.07 μM. The electrodes were stable for monitoring 5-HT overflow from intestinal organoids. Using the electrochemical multiwell plate containing intestinal organoids, increases in 5-HT were observed in the presence of 0.1 mM cinnamaldehyde and 10 mM quercetin but reduction in 5-HT levels was observed in 1 mM sorbitol when compared to control. These changes in the presence of chemicals commonly found in food were verified with ex vivo ileum tissue measurements using chromatography and amperometry with boron-doped diamond electrodes. Overall, our 3D electrochemical multiwell plate measurements with intestinal organoids highlight an approach that can be a high-throughput platform technology for rapid screening of food intolerance to provide personalized nutritional diet.

Monitoring Gut Epithelium Serotonin and Melatonin Overflow Provides Spatial Mapping of Inflammation

Electrochemical arrays were used to measure the overflow of serotonin (5-HT) and melatonin (MEL) from the entire colon of healthy mice and mice with chemical-induced inflammatory bowel disease (IBD), to understand the interplay between inflammation and colonic function. We show that 5-HT overflow is increased, whilst MEL levels are reduced, in inflamed tissues. The levels of MEL are increased at the interface between healthy and inflamed regions within the colon and may limit the spread of inflammation. Understanding the interplay between inflammation and mucosal epithelial signalling can provide key insight into colonic function and aid the development of effective therapeutic strategies to treat gastrointestinal diseases.

N-Acetylserotonin is an oxidation-responsive activator of Nrf2 ameliorating colitis in rats

N-Acetylserotonin (NAS) is an intermediate in the melatonin biosynthetic pathway. We investigated the anti-inflammatory activity of NAS by focusing on its chemical feature oxidizable to an electrophile. NAS was readily oxidized by reaction with HOCl, an oxidant produced in the inflammatory state. HOCl-reacted NAS (Oxi-NAS), but not NAS, activated the anti-inflammatory nuclear factor erythroid 2-related factor 2 (Nrf2)-heme oxygenase (HO)-1 pathway in cells. Chromatographic and mass analyses demonstrated that Oxi-NAS was the iminoquinone form of NAS and could react with N-acetylcysteine possessing a nucleophilic thiol to form a covalent adduct. Oxi-NAS bound to Kelch-like ECH-associated protein 1, resulting in Nrf2 dissociation. Moreover, rectally administered NAS increased the levels of nuclear Nrf2 and HO-1 proteins in the inflamed colon of rats. Simultaneously, NAS was converted to Oxi-NAS in the inflamed colon. Rectal NAS mitigated colonic damage and inflammation. The anticolitic effects were significantly compromised by the coadministration of an HO-1 inhibitor.

Melatonin Mitigates Oxazolone-Induced Colitis in Microbiota-Dependent Manner

Levels of type 2 cytokines are elevated in the blood and intestinal tissues of ulcerative colitis (UC) patients in the active phase; this phenomenon indicates the participation of type 2 immune response in UC progression. The beneficial effects of melatonin in dextran sodium sulfate (DSS) and 2,4,6-trinitrobenzene sulfonic acid (TNBS) colitis models have been illustrated, but its role in the oxazolone (Oxa)-induced colitis model (driven by type 2 immune response) remains relatively unknown. We investigated the relationship between melatonin concentration and the severity of UC, revealing a significantly negative correlation. Subsequently, we investigated the effects of melatonin in Oxa-induced colitis mice and the potential underlying mechanisms. Administration of melatonin significantly counteracted body weight loss, colon shortening, and neutrophil infiltration in Oxa-induced colitis mice. Melatonin treatment mitigated Oxa-induced colitis by suppressing type 2 immune response. In addition, melatonin attenuated intestinal permeability by enhancing the expression of ZO-1 and occludin in colitis mice. Interestingly, the protective effect of melatonin was abolished when the mice were co-housed, indicating that the regulation of gut microbiota by melatonin was critical in alleviating Oxa-induced colitis. Subsequently, 16S rRNA sequencing was performed to explore the microbiota composition. Decreased richness and diversity of intestinal microbiota at the operational taxonomic unit (OTU) level resulted from melatonin treatment. Melatonin also elevated the abundance of Bifidobacterium, a well-known probiotic, and reduced proportions of several harmful bacterial genera, such as Desulfovibrio, Peptococcaceae, and Lachnospiraceae. Fecal microbiota transplantation (FMT) was used to explore the role of microbiota in the function of melatonin in Oxa-induced colitis. Microbiota transplantation from melatonin-treated mice alleviated Oxa-induced colitis, suggesting that the microbiome participates in the relief of Oxa-induced colitis by melatonin. Our findings demonstrate that melatonin ameliorates Oxa-induced colitis in a microbiota-dependent manner, suggesting the therapeutic potential of melatonin in treating type 2 immunity-associated UC.

Disruption of autophagy by increased 5-HT alters gut microbiota and enhances susceptibility to experimental colitis and Crohn's disease

Autophagy, an essential intracellular recycling process, is linked to the pathogenesis of various diseases including Crohn’s disease (CD). Factors that lead to the development of impaired autophagy during intestinal inflammation remain largely unexplored. Here, we report the impact of the interaction between serotonin [5-hydroxytryptamine;(5-HT)] and autophagy in colitis in mouse and human studies. In mice, increased gut 5-HT inhibited autophagy and led to enhanced colitis susceptibility. Reciprocally, mice with reduced 5-HT exhibited up-regulated autophagy via the mammalian target of rapamycin pathway, which resulted in significantly decreased colitis. Deletion of autophagy gene, Atg7, in an epithelial-specific manner, in concert with reduced 5-HT, promoted the development of a colitogenic microbiota and abolished the protective effects conferred by reduced 5-HT. Notably, in control and patient peripheral blood mononuclear cells, we uncovered that 5-HT treatment inhibited autophagy. Our findings suggest 5-HT as a previously unidentified therapeutic target in intestinal inflammatory disorders such as CD that exhibits dysregulated autophagy.

Nano-analysis reveals high fraction of serotonin release during exocytosis from a gut epithelium model cell

Electrochemical methods were used to explore the exocytotic nature of serotonin (5‐HT) release in human carcinoid BON cells, an in vitro human enterochromaffin cell model, to understand the mechanisms operating the release of gut‐derived 5‐HT in the intestinal mucosal epithelium. We show that the fractional vesicular 5‐HT release in BON cells is 80 % compared to previous work in pancreatic beta cells (34 %). The fractional release increased from 80 % in control BON cells to 87 % with 5‐HT preincubation and nearly 100 % with the combination of 5‐HT and the 5‐HT₄ autoreceptor agonist, cisapride. Thus, partial release is the primary mechanism of exocytosis in BON cells, resulting in a variable amount of the vesicular content being released. Factors that control secretion of 5‐HT from enterochromaffin cells or BON cells are important as partial release provides a mechanism for development of effective therapeutic strategies to treat gastrointestinal diseases.

The Mechanism of Secretion and Metabolism of Gut-Derived 5-Hydroxytryptamine

Serotonin, also known as 5-hydroxytryptamine (5-HT), is a metabolite of tryptophan and is reported to modulate the development and neurogenesis of the enteric nervous system, gut motility, secretion, inflammation, sensation, and epithelial development. Approximately 95% of 5-HT in the body is synthesized and secreted by enterochromaffin (EC) cells, the most common type of neuroendocrine cells in the gastrointestinal (GI) tract, through sensing signals from the intestinal lumen and the circulatory system. Gut microbiota, nutrients, and hormones are the main factors that play a vital role in regulating 5-HT secretion by EC cells. Apart from being an important neurotransmitter and a paracrine signaling molecule in the gut, gut-derived 5-HT was also shown to exert other biological functions (in autism and depression) far beyond the gut. Moreover, studies conducted on the regulation of 5-HT in the immune system demonstrated that 5-HT exerts anti-inflammatory and proinflammatory effects on the gut by binding to different receptors under intestinal inflammatory conditions. Understanding the regulatory mechanisms through which 5-HT participates in cell metabolism and physiology can provide potential therapeutic strategies for treating intestinal diseases. Herein, we review recent evidence to recapitulate the mechanisms of synthesis, secretion, regulation, and biofunction of 5-HT to improve the nutrition and health of humans.

Overproduction of Gastrointestinal 5-HT Promotes Colitis-Associated Colorectal Cancer Progression via Enhancing NLRP3 Inflammasome Activation

Chronic inflammation is a key driver for colitis-associated colorectal cancer. 5-hydroxytryptamine (5-HT), a neurotransmitter, has been reported to promote inflammation in the gastrointestinal tract. However, the mechanism behind this remains unclear. In this study, we found that 5-HT levels, as well as the expression of tryptophan hydroxylase 1 (TPH1), the 5-HT biosynthesis rate-limiting enzyme, were significantly upregulated in colorectal tumor tissues from patients with colorectal cancer, colorectal cancer mouse models, and colorectal cancer cell lines when compared with normal colorectal tissues or epithelial cell lines. Colorectal cancer cell-originated 5-HT enhanced NLRP3 inflammasome activation in THP-1 cells and immortalized bone marrow-derived macrophages (iBMDM) via its ion channel receptor, HTR3A. Mechanistically, HTR3A activation led to Ca²⁺ influx, followed by CaMKIIα phosphorylation (Thr286) and activation, which then induced NLRP3 phosphorylation at Ser198 (mouse: Ser194) and inflammasome assembling. The NLRP3 inflammasome mediated IL1β maturation, and release upregulated 5-HT biosynthesis in colorectal cancer cells by inducing TPH1 transcription, revealing a positive feedback loop between 5-HT and NLRP3 signaling. Silencing TPH1 or HTR3A by short hairpin RNA slowed down tumor growth in an established CT26 and iBMDM coimplanted subcutaneous allograft colorectal cancer mouse model, whereas treatment with TPH1 inhibitor 4-chloro-DL-phenylalanine or HTR3A antagonist tropisetron alleviated tumor progression in an azoxymethane/dextran sodium sulfate-induced colorectal cancer mouse model. Addressing the positive feedback loop between 5-HT and NLRP3 signaling could provide potential therapeutic targets for colorectal cancer.

Berberine improves colitis by triggering AhR activation by microbial tryptophan catabolites

Inflammatory bowel diseases (IBD) are kind of recurrent inflammatory issues that occur in the gastrointestinal tract, and currently clinical treatment is still unideal due to the complex pathogenesis of IBD. Basically, gut barrier dysfunction is triggered by gut microbiota dysbiosis that is closely associated with the development of IBD, we thus investigated the therapeutic capacity of berberine (BBR) to improve the dysregulated gut microbiota, against IBD in rats, using a combinational strategy of targeted metabolomics and 16 s rDNA amplicon sequencing technology. Expectedly, our data revealed that BBR administration could greatly improve the pathological phenotype, gut barrier disruption, and the colon inflammation in rats with dextran sulfate sodium (DSS)-induced colitis. In addition, 16S rDNA-based microbiota analysis demonstrated that BBR could alleviate gut dysbiosis in rats. Furthermore, our targeted metabolomics analysis illustrated that the levels of microbial tryptophan catabolites in the gastrointestinal tract were significantly changed during the development of the colitis in rats, and BBR treatment can significantly restore such changes of the tryptophan catabolites accordingly. At last, our in vitro mechanism exploration was implemented with a Caco-2 cell monolayer model, which verified that the modulation of the dysregulated gut microbiota to change microbial metabolites coordinated the improvement effect of BBR on gut barrier disruption in the colitis, and we also confirmed that the activation of AhR induced by microbial metabolites is indispensable to the improvement of gut barrier disruption by BBR. Collectively, BBR has the capacity to treat DSS-induced colitis in rats through the regulation of gut microbiota associated tryptophan metabolite to activate AhR, which can greatly improve the disrupted gut barrier function. Importantly, our finding elucidated a novel mechanism of BBR to improve gut barrier function, which holds the expected capacity to promote the BBR derived drug discovery and development against the colitis in clinic setting.

Melatonin controls microbiota in colitis by goblet cell differentiation and antimicrobial peptide production through Toll-like receptor 4 signalling

Microbial dysbiosis has long been postulated to be associated with the pathogenesis of inflammatory bowel disease (IBD). Although evidence supporting the anti-colitic effects of melatonin have been accumulating, it is not clear how melatonin affects the microbiota. Herein, we investigated the effects of melatonin on the microbiome in colitis and identified involvement of Toll-like receptor (TLR) 4 signalling in the effects. Melatonin improved dextran sulfate sodium (DSS)-induced colitis and reverted microbial dysbiosis in wild-type (WT) mice but not in TLR4 knockout (KO) mice. Induction of goblet cells was observed with melatonin administration, which was accompanied by suppression of Il1b and Il17a and induction of melatonin receptor and Reg3β, an antimicrobial peptide (AMP) against Gram-negative bacteria. In vitro, melatonin treatment of HT-29 intestinal epithelial cells promotes mucin and wound healing and inhibits growth of Escherichia coli. Herein, we showed that melatonin significantly increases goblet cells, Reg3β, and the ratio of Firmicutes to Bacteriodetes by suppressing Gram-negative bacteria through TLR4 signalling. Our study suggests that sensing of bacteria through TLR4 and regulation of bacteria through altered goblet cells and AMPs is involved in the anti-colitic effects of melatonin. Melatonin may have use in therapeutics for IBD.

Tryptophan promoted β-defensin-2 expressionviathe mTOR pathway and its metabolites: kynurenine banding to aryl hydrocarbon receptor in rat intestine

In this study, we investigated the signalling pathways mediating tryptophan (Trp)-promoted β-defensin-2 (BD-2) expression in rat intestinal mucosa. Sprague Dawley rats were administered with l-Trp and treated with rapamycin (RAPA), 1-methyltryptophan (1-MT), or para-chlorophenyl-amine (PCPA) to inhibit mammalian target of rapamycin (mTOR), indoleamine-2,3-dioxygenase (IDO), or tryptophan hydroxylase (TPH), respectively. The mRNA and protein levels of BD-2 in the jejunal and ileal mucosa of rats increased with administration of l-Trp. Intraperitoneal injection of RAPA significantly decreased the mRNA level of BD-2 and the concentrations of p-mTORC1 and BD-2 in the jejunal and ileal mucosa of rats with administration of l-Trp (P < 0.05). Oral administration of 1-MT decreased the IDO activity and the mRNA and protein levels of BD-2, and increased the concentrations of tumour necrosis factor (TNF-α), interleukin (IL)-17, and IL-22 in the jejunal and ileal mucosa of rats with administration of l-Trp (P < 0.05). Intraperitoneal injection of PCPA decreased the TPH activity and increased the mRNA and protein levels of BD-2, but did not change the concentrations of TNF-α, IL-17, or IL-22 in the jejunal and ileal mucosa of rats with administration of l-Trp. The results indicate the Trp-promoted BD-2 expression in the jejunum and ileum via the mTOR pathway and its metabolites: kynurenine banding to aryl hydrocarbon receptor in rat intestine.

In this study, we investigated the signalling pathways mediating tryptophan (Trp)-promoted β-defensin-2 (BD-2) expression in rat intestinal mucosa.

Evaluation of the therapeutic activity of melatonin and resveratrol in Inflammatory Bowel Disease: A longitudinal PET/CT study in an animal model

Inflammatory Bowel Disease (IBD) is a group of chronic disorders of the gastrointestinal tract, which two main types are Crohn's disease and ulcerative colitis. Although conventional therapeutic strategies have demonstrated to be effective in the IBD treatment, it is necessary to incorporate novel therapeutic agents that target other mechanisms involved in the pathogenesis of the disease, such as oxidative stress. For this reason, the efficacy in vivo of two antioxidant compounds, melatonin and resveratrol, has been investigated in an animal model of TNBS (2, 4, 6-trinitrobenzenesulfonic acid) induced colitis. PET/CT (Positron emission tomography/Computer Tomography) scans were performed to assess disease activity and evaluate treatment response. SUV_max (Standardized Uptake Value) values, body weight changes and histological evaluation were used as inflammatory indices to measure the efficacy of both treatments. SUV_max values increased rapidly after induction of colitis, but after the beginning of the treatment (day 3) a statistically significant decrease was observed on days 7 and 10 in treated animals compared to the non-treated group. This remission of the disease was also confirmed by histological analysis of the colon tissue using the Nancy histological index (p value < 0.05 for differences between non-treated and both groups of treated animals). Moreover, statistical analysis showed a correlation (R² = 65.52%) between SUV_max values and weight changes throughout the treatment. Overall, this study demonstrates the potential of resveratrol, and melatonin in lower extent, as therapeutic agents in the IBD treatment.

Kynurenic acid/GPR35 axis restricts NLRP3 inflammasome activation and exacerbates colitis in mice with social stress

Psychological stress is well known to increase colitis susceptibility and promote relapse. Metabolic changes are commonly observed under psychological stress, but little is known how this relates to the progression of colitis. Here we show that kynurenic acid (KA) is an endogenous driver of social stress-exacerbated colitis via regulating the magnitude of NLRP3 inflammasome. Chronic social defeat stress (CSDS) in mice induced colonic accumulation of KA, and mice receiving KA during CSDS had defects in colonic NLRP3 inflammasome activation. Mechanistically, KA activated GPR35 signaling to induce autophagy-dependent degradation of NLRP3 in macrophages, thereby suppressing IL-1β production. Socially defeated mice with KA treatment displayed enhanced vulnerability to subsequent dextran sulphate sodium (DSS)-induced colonic injury and inflammatory disturbance, and this effect was reversed by autophagic inhibition that blocked the NLRP3-suppressive effect of KA. Thus, our research describes a mechanism by which KA/GPR35 signaling represses adaptive NLRP3 inflammasome activation to increase colitis susceptibility and suggests a potential metabolic target for the intervention of stress-related colonic disorder.

Tryptophan (Trp) modulates gut homeostasis via aryl hydrocarbon receptor (AhR)

The intestinal homeostasis is an orchestrated dynamic equilibrium state composed of the coexistence and interactions among the nutrients, microbial flora, and immune system. The intestinal balance disorder can trigger a series of diseases, such as inflammatory bowel disease (IBD). Many of tryptophan (Trp) metabolites, such as kynurenine and indole, generated under a series of endogenous enzymes or microbial metabolism, have been reported enable to bind and activate the aryl hydrocarbon receptor (AhR), this series of process is termed the Trp-AhR pathway. The activated Trp-AhR pathway can induce the expression of downstream cytokines such as interleukin-22 (IL-22) and interleukin-17 (IL-17), thereby regulating the intestinal homeostasis. This review highlights the advance of Trp-AhR pathway in the regulation of intestinal homeostasis and provides some insights for the clinical strategies that expect to effectively prevent and treat gut diseases via intervening the Trp-AhR pathway.

Real-Time Detection of Melatonin Using Fast-Scan Cyclic Voltammetry

Melatonin is an important hormone whose functions span from regulating circadian rhythm in the brain to providing anti-inflammatory properties in the immune system. Melatonin secretion from the pineal gland is known; however, the mechanism of melatonin signaling in the immune system is not well understood. The lymph node is the hub of the immune system, and melatonin secretion from lymphocytes was proposed to be an important source specifically for regulating cytokine secretion. Methods exist to quantify the concentration of melatonin within biological samples; however, they often suffer from either a lack of selectivity for melatonin over common biological interferences or temporal resolution, which is not amenable to measuring real-time signaling dynamics. Here, we have characterized an electrochemical method for optimal melatonin detection with subsecond resolution using fast-scan cyclic voltammetry at carbon-fiber microelectrodes. The oxidation peaks detected for melatonin were at 1.0, 1.1, and 0.6 V. Evidence for electrode fouling of the tertiary peak was present; therefore, an optimized waveform was developed scanning from 0.2 to 1.3 V at 600 V/s. The optimized waveform eliminated the detection of fouling products on the electrode with a 24 ± 10 nM limit of detection. Melatonin was distinguished between biological interferences, and codetection with the major synthetic precursor, serotonin, was possible. This method was used to detect melatonin in live lymph node slices and provides the first real-time measurements within the lymph node using FSCV. Real-time detection of melatonin dynamics could provide useful information on the mechanism of immunomodulation during inflammatory disease.

Alterations in melatonin and 5-HT signalling in the colonic mucosa of mice with dextran-sodium sulfate-induced colitis

BACKGROUND AND PURPOSE

Inflammatory bowel disease (IBD) is characterized by pain, bleeding, cramping and altered gastrointestinal (GI) function. Changes in mucosal 5-HT (serotonin) signalling occur in animal models of colitis and in humans suffering from IBD. Melatonin is co-released with 5-HT from the mucosa and has a wide variety of actions in the GI tract. Here, we examined how melatonin signalling is affected by colitis and determined how this relates to 5-HT signalling.

EXPERIMENTAL APPROACH

Using electroanalytical approaches, we investigated how 5-HT release, reuptake and availability as well as melatonin availability are altered in dextran sodium sulfate (DSS)-induced colitis in mice. Studies were conducted to explore if melatonin treatment during active colitis could reduce the severity of colitis.

KEY RESULTS

We observed an increase in 5-HT and a decrease in melatonin availability in DSS-induced colitis. A significant reduction in 5-HT reuptake was observed in DSS-induced colitis animals. A reduction in the content of 5-HT was observed, but no difference in tryptophan levels were observed. A reduction in deoxycholic acid-stimulated 5-HT availability and a significant reduction in mechanically-stimulated 5-HT and melatonin availability were observed in DSS-induced colitis. Orally or rectally administered melatonin once colitis was established did not significantly suppress inflammation.

CONCLUSION AND IMPLICATIONS

Our data suggest that DSS-induced colitis results in a reduction in melatonin availability and an increase in 5-HT availability, due to a reduction/loss of tryptophan hydroxylase 1 enzyme, 5-HT content and 5-HT transporters. Mechanosensory release was more susceptible to inflammation when compared with chemosensory release.