Electroanalytical approaches to study signaling mechanisms in the gastrointestinal tract

3D-Printed Microelectrodes for Biological Measurement

Microelectrodes are useful electrochemical sensors that can provide spatial biological monitoring. Carbon fiber has been by far the most widely used microelectrode; however, a vast number of different materials and modification strategies have been developed to broaden the scope of microelectrodes. Carbon composite electrodes provide a simple approach to making microelectrodes with a wide range of materials, but manufacturing strategies are complex. 3D printing can provide the ability to make microelectrodes with high precision. We used fused filament fabrication to print single strands of carbon black/polylactic acid (CB/PLA) and multiwall carbon nanotube/polylactic acid (MWCNT/PLA), which were then made into microelectrodes. Microelectrodes ranged from 70 μm in diameter to 400 μm in diameter and were assessed using standard redox probes. MWCNT/PLA electrodes exhibited greater sensitivity, a lower limit of detection, and stability for the measurement of serotonin (5-HT). Both CB/PLA and MWCNT/PLA microelectrodes were able to monitor 5-HT overflow from the ex vivo ileum tissue. MWCNT/PLA microelectrodes were utilized to show differences in 5-HT overflow from ex vivo ileum and colon following exposure to odorants present in spices. These findings highlight that any conductive thermoplastic material can be fabricated into a microelectrode. This simple strategy can utilize a wide range of materials to make 3D-printed microelectrodes for a diverse range of applications.

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.

Effects of secretin gene knockout on the diversity, composition, and function of gut microbiota in adult male mice

The gut microbiota plays a vital role in maintaining gastrointestinal homeostasis, however, whether it is influenced by gut hormones remains unknown. Secretin is a well-known gastrointestinal hormone produced by enteroendocrine S cells. This study utilized 16S rRNA amplicon sequencing to characterize the effect of SCT deficiency on the gut microbiota. Our results show that systemic SCT knockout alters the composition and abundance of the mouse gut microbiota but does not affect fecal short-chain fatty acids and lipids concentrations. At the genus level, the abundance of Turicibacter, Bacteroides, Ruminococcu, Romboutsia, Asaccharobacter, and Parasutterella increased in SCT-/- mice, whereas the abundance of Akkermansia and Escherichia decreased. Functional prediction results showed that lack of SCT reduced the abundance of carbohydrate metabolism-related pathways but increased the abundance of linoleic acid metabolism and branched-chain amino acid degradation. Overall, systemic SCT knockout had only minor effects on gut microbiota composition and function in adult male mice fed a standard chow diet.

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.

Progress on traditional Chinese medicine in treatment of ischemic stroke via the gut-brain axis

Ischemic stroke is a common issue that severely affects the human health. Between the central nervous system and the enteric system, the " Gut-Brain " axis, the bidirectional connection involved in the neuro-immuno-endocrine network, is crucial for the occurrence and development of ischemic stroke. Ischemic stroke can lead to change in the gut microbiota and gastrointestinal hormones, which will then reversely affect the disease development. Traditional Chinese Medicine (TCM) has unique advantages with reference to the treatment for ischemic stroke. The latest research revealed that a significant portion of medicines and prescriptions of TCM exert their therapeutic effects by improving the gut microbiota and regulating the secretion of gastrointestinal hormones. The present review summarized the Chinese medicines that play a therapeutic role in cerebral ischemia through regulating the "Gut-Brain" axis and described the corresponding mechanisms. This study attempts to provide reference for clinical selection of Chinese medicines and helps better understand the relevant mechanisms of action.

Characterization of Skeletal Phenotype and Associated Mechanisms With Chronic Intestinal Inflammation in the Winnie Mouse Model of Spontaneous Chronic Colitis

BACKGROUND

Osteoporosis is a common extraintestinal manifestation of inflammatory bowel disease (IBD). However, studies have been scarce, mainly because of the lack of an appropriate animal model of colitis-associated bone loss. In this study, we aimed to decipher skeletal manifestations in the Winnie mouse model of spontaneous chronic colitis, which carries a MUC2 gene mutation and closely replicates ulcerative colitis. In our study, Winnie mice, prior to the colitis onset at 6 weeks old and progression at 14 and 24 weeks old, were compared with age-matched C57BL/6 controls. We studied several possible mechanisms involved in colitis-associated bone loss.

METHODS

We assessed for bone quality (eg, microcomputed tomography [micro-CT], static and dynamic histomorphometry, 3-point bending, and ex vivo bone marrow analysis) and associated mechanisms (eg, electrochemical recordings for gut-derived serotonin levels, real-time polymerase chain reaction [qRT-PCR], double immunofluorescence microscopy, intestinal inflammation levels by lipocalin-2 assay, serum levels of calcium, phosphorus, and vitamin D) from Winnie (6-24 weeks) and age-matched C57BL6 mice.

RESULTS

Deterioration in trabecular and cortical bone microarchitecture, reductions in bone formation, mineral apposition rate, bone volume/total volume, osteoid volume/bone surface, and bone strength were observed in Winnie mice compared with controls. Decreased osteoblast and increased osteoclast numbers were prominent in Winnie mice compared with controls. Upregulation of 5-HTR1B gene and increased association of FOXO1 with ATF4 complex were identified as associated mechanisms concomitant to overt inflammation and high levels of gut-derived serotonin in 14-week and 24-week Winnie mice.

CONCLUSIONS

Skeletal phenotype of the Winnie mouse model of spontaneous chronic colitis closely represents manifestations of IBD-associated osteoporosis/osteopenia. The onset and progression of intestinal inflammation are associated with increased gut-derived serotonin level, increased bone resorption, and decreased bone formation.

Gut hormones in microbiota-gut-brain cross-talk

The homeostasis of the gut-brain axis has been shown to exert several effects on physiological and psychological health. The gut hormones released by enteroendocrine cells scattered throughout the gastrointestinal tract are important signaling molecules within the gut-brain axis. The interaction between gut microbiota and gut hormones has been greatly appreciated in gut-brain cross-talk. The microbiota plays an essential role in modulating many gut-brain axis-related diseases, ranging from gastrointestinal disorders to psychiatric diseases. Similarly, gut hormones also play pleiotropic and important roles in maintaining health, and are key signals involved in gut-brain axis. More importantly, gut microbiota can affect the release and functions of gut hormones. This review highlights the role of gut microbiota in the gut-brain axis and focuses on how microbiota-related gut hormones modulate various physiological functions. Future studies could target the microbiota-hormones-gut brain axis to develop novel therapeutics for different psychiatric and gastrointestinal disorders, such as obesity, anxiety, and depression.

The Microbiota-Gut-Brain Axis

The importance of the gut-brain axis in maintaining homeostasis has long been appreciated. However, the past 15 yr have seen the emergence of the microbiota (the trillions of microorganisms within and on our bodies) as one of the key regulators of gut-brain function and has led to the appreciation of the importance of a distinct microbiota-gut-brain axis. This axis is gaining ever more traction in fields investigating the biological and physiological basis of psychiatric, neurodevelopmental, age-related, and neurodegenerative disorders. The microbiota and the brain communicate with each other via various routes including the immune system, tryptophan metabolism, the vagus nerve and the enteric nervous system, involving microbial metabolites such as short-chain fatty acids, branched chain amino acids, and peptidoglycans. Many factors can influence microbiota composition in early life, including infection, mode of birth delivery, use of antibiotic medications, the nature of nutritional provision, environmental stressors, and host genetics. At the other extreme of life, microbial diversity diminishes with aging. Stress, in particular, can significantly impact the microbiota-gut-brain axis at all stages of life. Much recent work has implicated the gut microbiota in many conditions including autism, anxiety, obesity, schizophrenia, Parkinson’s disease, and Alzheimer’s disease. Animal models have been paramount in linking the regulation of fundamental neural processes, such as neurogenesis and myelination, to microbiome activation of microglia. Moreover, translational human studies are ongoing and will greatly enhance the field. Future studies will focus on understanding the mechanisms underlying the microbiota-gut-brain axis and attempt to elucidate microbial-based intervention and therapeutic strategies for neuropsychiatric disorders.

Region specific differences in the effect of propofol on the murine colon result in dysmotility

Propofol is the most widely used intravenous anaesthetic agent for maintenance of anaesthesia and sedation. Studies in varying regions of the bowel have shown conflicting differences on the effects of propofol on motility. There the aim of this study was to understand the influence of propofol on colonic function and explore by which mechanism any changes occur. Functional studies were conducted using isolated colonic tissue from C57BL6 mice which were exposed to 5 μM propofol. Faecal pellet motility, colonic migratory motor complexes (CMMCs) and functional bioassays were utilised to monitor colonic function and nitric oxide production was monitored by amperometry. There was a signficant reduction in amplitude of CMMCs in the distal colon in the presence of 5 μM propofol, however no difference was observed in the proximal colon. A signficant increase in the 5-HT evoked contractions were observed in distal colon in the presence of 5 μM propofol. Additionally, a reduction in the NO production in the presence of 5 μM propofol was only observed in the distal colon. As a result, in the presence of 5 μM propofol, faecal pellet transit was increased, and velocity was reduced. At clinically relevant doses, propofol was shown to reduce colonic motility by inhibiting nitric oxide synthase in only the distal region of the colon. Our findings indicate that propofol has a considerable influence on colonic signalling mechanisms and impairs colonic motility, which may have implications in its clinical use especially for maintenance.

Changes in murine anorectum signaling across the life course

BACKGROUND

Increasing age is associated with an increase in the incidence of chronic constipation and fecal impaction. The contribution of the natural aging process to these conditions is not fully understood. This study examined the effects of increasing age on the function of the murine anorectum.

METHODS

The effects of increasing age on cholinergic, nitrergic, and purinergic signaling pathways in the murine anorectum were examined using classical organ bath assays to examine tissue function and electrochemical sensing to determine age-related changes in nitric oxide and acetylcholine release.

KEY RESULTS

Nitrergic relaxation increased between 3 and 6 months, peaked at 12 months and declined in the 18 and 24 months groups. These changes were in part explained by an age-related decrease in nitric oxide (NO) release. Cholinergic signaling was maintained with age by an increase in acetylcholine (ACh) release and a compensatory decrease in cholinesterase activity. Age-related changes in purinergic relaxation were qualitatively similar to nitrergic relaxation although the relaxations were much smaller. Increasing age did not alter the response of the anorectum smooth muscle to exogenously applied ACh, ATP, sodium nitroprusside or KCl. Similarly, there was no change in basal tension developed by the anorectum.

CONCLUSIONS AND INFERENCES

The decrease in nitrergic signaling with increasing age may contribute to the age-related fecal impaction and constipation previously described in this model by partially obstructing defecation.

The Onset and Progression of Chronic Colitis Parallels Increased Mucosal Serotonin Release via Enterochromaffin Cell Hyperplasia and Downregulation of the Serotonin Reuptake Transporter

BACKGROUND

Serotonin (5-hydroxytryptamine, 5-HT) has been linked with several inflammation-associated intestinal diseases, including ulcerative colitis (UC). The largest pool of 5-HT in the body is in enterochromaffin (EC) cells located throughout the intestinal tract. EC cells are mechanosensitive and detect noxious stimuli, inducing secretion of 5-HT, which plays an important role in enteric reflexes and immunomodulation. In this study, we evaluated intestinal 5-HT levels in the Winnie mouse model of spontaneous chronic colitis, which closely replicates UC.

METHODS

Real-time electrochemical recordings of 5-HT oxidation currents were obtained from ex vivo preparations of jejunum, ileum, proximal, and distal colon from Winnie (5-25 weeks old) and age matched C57BL/6 mice. EC cells were examined by immunohistochemistry, and the gene expression of tryptophan hydroxylase 1 (5-HT synthesis) and the serotonin reuptake transporter (SERT) were determined by quantitative Real-Time Polymerase Chain Reaction (RT-qPCR).

RESULTS

Compression-evoked and basal 5-HT concentrations were elevated in the distal and proximal colon of Winnie mice. EC cell hyperplasia and downregulation of SERT on the transcriptional level were identified as mechanisms underlying increased levels of 5-HT. Increase in mucosal 5-HT release was observed at the onset of disease at 7-14 weeks, confirmed by disease activity scores. Furthermore, increases in 5-HT levels and progression of disease activity correlated linearly with age, but not sex.

CONCLUSIONS

Our findings in the Winnie mouse model of spontaneous chronic colitis demonstrate for the first time that the onset and progression of chronic UC-like intestinal inflammation is associated with increased 5-HT levels in the colonic mucosa.

Mucosal serotonin overflow is associated with colonic stretch rather than phasic contractions

BACKGROUND

Many studies have shown that mucosal serotonin (5-HT) is associated with motility, however, recently there have been some questions to the precise role of this transmitter. The majority of studies have focused on understanding the role of mucosal 5-HT on colonic migratory motor complexes, but very few studies have been carried out to understand how 5-HT release may be associated with other motility patterns.

METHODS

Using distal colon segments from C57BL/6J mice, mucosal 5-HT overflow was monitored using amperometry while applying tension in longitudinal or circular directions to stretch the tissue.

KEY RESULTS

Phasic and basal 5-HT levels were not associated with the strength of phasic contractions, while being altered using scopolamine and L-NNA. There was a significant increase in mucosal 5-HT with longitudinal and circular muscle stretch. A greater applied force was needed to activate 5-HT release in the circular muscle. In the longitudinal muscle, 5-HT levels increased with stretch until 3 mN, after which the levels returned back to baseline. This stretch-evoked 5-HT overflow was inhibited by transient receptor potential A1 (TRPA1) agonist, 30 μM ruthenium red in both circular and longitudinal muscle preparations. The decreased 5-HT overflow after 3 mN of tension was reversed using a 5-HT4 antagonist 100 nM GR113808.

CONCLUSIONS & INFERENCES

Our findings show a relationship between colonic stretch and mucosal 5-HT overflow, while no relationship is observed with phasic colonic contractions. Such findings provide more insight into the role of mucosal 5-HT in influencing the pattern of colonic motility to diversify fecal propulsion.

Electrochemical fecal pellet sensor for simultaneous real-time ex vivo detection of colonic serotonin signalling and motility

Various investigations have focused on understanding the relationship between mucosal serotonin (5-HT) and colonic motility, however contradictory studies have questioned the importance of this intestinal transmitter. Here we described the fabrication and use of a fecal pellet electrochemical sensor that can be used to simultaneously detect the release of luminal 5-HT and colonic motility. Fecal pellet sensor devices were fabricated using carbon nanotube composite electrodes that were housed in 3D printed components in order to generate a device that had shape and size that mimicked a natural fecal pellet. Devices were fabricated where varying regions of the pellet contained the electrode. Devices showed that they were stable and sensitive for ex vivo detection of 5-HT, and no differences in the fecal pellet velocity was observed when compared to natural fecal pellets. The onset of mucosal 5-HT was observed prior to the movement of the fecal pellet. The release of mucosal 5-HT occurred oral to the fecal pellet and was linked to the contraction of the bowel wall that drove pellet propulsion. Taken, together these findings provide new insights into the role of mucosal 5-HT and suggest that the transmitter acts as a key initiator of fecal pellet propulsion.

Escherichia coli Nissle 1917 enhances bioavailability of serotonin in gut tissues through modulation of synthesis and clearance

Accumulating evidence shows indigenous gut microbes can interact with the human host through modulation of serotonin (5-HT) signaling. Here we investigate the impact of the probiotic Escherichia coli Nissle 1917 (EcN) on 5-HT signalling in gut tissues. Ex-vivo mouse ileal tissue sections were treated with either EcN or the human gut commensal MG1655, and effects on levels of 5-HT, precursors, and metabolites, were evaluated using amperometry and high performance liquid chromatography with electrochemical detection (HPLC-EC). Exposure of tissue to EcN cells, but not MG1655 cells, was found to increase levels of extra-cellular 5-HT. These effects were not observed when tissues were treated with cell-free supernatant from bacterial cultures. In contrast, when supernatant recovered from untreated ileal tissue was pre-incubated with EcN, the derivative cell-free supernatant was able to elevate 5-HT overflow when used to treat fresh ileal tissue. Measurement of 5-HT precursors and metabolites indicated EcN also increases intracellular 5-HTP and reduces 5-HIAA. The former pointed to modulation of tryptophan hydroxylase-1 to enhance 5-HT synthesis, while the latter indicates an impact on clearance into enterocytes through SERT. Taken together, these findings show EcN is able to enhance 5-HT bioavailability in ileal tissues through interaction with compounds secreted from host tissues.

Mucosal adenosine triphosphate mediates serotonin release from ileal but not colonic guinea pig enterochromaffin cells

BACKGROUND

Mechanical stimulation of the mucosal epithelium results in increased serotonin (5-HT) release from enterochromaffin (EC) cells. Little is known about how this process varies in different regions of the intestinal tract; however, purines are felt to play a role. We studied the relationship between mechanical stimulation, adenosine triphosphate (ATP), and 5-HT release from ileal and colonic mucosal tissue.

METHODS

Amperometric recordings of ATP and 5-HT were carried out using an ATP biosensor and boron-doped diamond microelectrode. Levels of extracellular ATP and 5-HT were monitored using high performance liquid chromatography.

KEY RESULTS

Under basal conditions, 5-HT levels were significantly decreased in the ileum (p < 0.001) but not the colon in the presence of the P2 antagonist suramin (100 μM). Ecto-ATPase inhibitor ARL67156 (10 μM) elevated ATP levels in the ileum and colon (both p < 0.001), but only 5-HT levels in the ileum (p < 0.001). Exogenous ATP increased 5-HT release in the presence of tetrodotoxin in the ileum (p < 0.001), but had not effect in the colon. Mechanical stimulation increased levels of 5-HT in the ileum (p < 0.001) and colon (p < 0.01), but levels returned to baseline in the presence of suramin and MRS2179 in the ileum. The onset of 5-HT release was delayed following mechanical stimulation. The rise time of the ATP response was quicker than that of 5-HT during mechanical stimulation.

CONCLUSIONS & INFERENCES

During mechanical stimulation of the mucosal epithelium, ATP mediates 5-HT release from EC cells in the ileum, but not the colon. Mucosal 5-HT signaling following mechanical stimulation is varied in different regions of the intestinal tract.

Simultaneous detection of gastric acid and histamine release to unravel the regulation of acid secretion from the guinea pig stomach

Gastric acid secretion is regulated by three primary components that activate the parietal cell: histamine, gastrin, and acetylcholine (ACh). Although much is known about these regulatory components individually, little is known on the interplay of these multiple activators and the degree of regulation they pose on the gastric acid secretion mechanism. We utilized a novel dual-sensing approach, where an iridium oxide sensor was used to monitor pH and a boron-doped diamond electrode was used for the detection of histamine from in vitro guinea pig stomach mucosal sections. Under basal conditions, gastrin was shown to be the main regulatory component of the total acid secretion and directly activated the parietal cell rather than by mediating gastric acid secretion through the release of histamine from the enterochromaffin-like cell, although both pathways were active. Under stimulated conditions with ACh, the gastrin and histamine components of the total acid secretion were not altered compared with levels observed under basal conditions, suggestive that ACh had no direct effect on the enterochromaffin-like cell and G cell. These data identify a new unique approach to investigate the regulation pathways active during acid secretion and the degree that they are utilized to drive total gastric acid secretion. The findings of this study will enhance our understanding on how these signaling mechanisms vary under pathophysiology or therapeutic management.

Chromatographic analysis of age-related changes in mucosal serotonin transmission in the murine distal ileum

BACKGROUND

In the upper bowel, alterations in motility and absorption of key nutrients have been observed as part of the normal ageing process. Serotonin (5-HT) is a key signalling molecule in the gastrointestinal tract and is known to influence motility, however little is known of how the ageing process alters 5-HT signalling processes in the bowel.

RESULTS

An isocratic chromatographic method was able to detect all 5-HT precursors and metabolites. Using extracellular and intracellular sampling approaches, we were able to monitor all key parameters associated with the transmission process. There was no alteration in the levels of tryptophan and 5-HTP between 3 and 18 month old animals. There was a significant increase in the ratio of 5-HT:5-HTP and an increase in intracellular 5-HT between 3 and 18 month old animals suggesting an increase in 5-HT synthesis. There was also a significant increase in extracellular 5-HT with age, suggesting increased 5-HT release. There was an age-related decrease in the ratio of intracellular 5-HIAA:extracellular 5-HT, whilst the amount of 5-HIAA did not change with age. In the presence of an increase in extracellular 5-HT, the lack of an age-related change in 5-HIAA is suggestive of a decrease in re-uptake via the serotonin transporter (SERT).

CONCLUSIONS

We have used intracellular and extracellular sampling to provide more insight into alterations in the neurotransmission process of 5-HT during normal ageing. We observed elevated 5-HT synthesis and release and a possible decrease in the activity of SERT. Taken together these changes lead to increased 5-HT availability and may alter motility function and could lead to the changes in adsorption observed in the elderly.