Characteristics of mucosally projecting myenteric neurones in the guinea-pig proximal colon

Causal relationship between gut microbiota and constipation: a bidirectional Mendelian randomization study

Background

Constipation is a prevalent gastrointestinal disorder affecting approximately 15% of the global population, leading to significant healthcare burdens. Emerging evidence suggests that gut microbiota plays a pivotal role in the pathogenesis of constipation, although causality remains uncertain due to potential confounding factors in observational studies. This study aims to clarify the causal relationships between gut microbiota and constipation using a bidirectional Mendelian Randomization (MR) approach, which helps to overcome confounding issues and reverse causality.

Methods

Utilizing data from genome-wide association studies (GWAS) from the MiBioGen consortium and other sources, we identified genetic variants as instrumental variables (IVs) for 196 bacterial traits and constipation. These IVs were rigorously selected based on their association with the traits and absence of linkage with confounding factors. We applied several MR methods, including Inverse Variance Weighted (IVW), MR Egger, and MR-PRESSO, to examine the causal effects in both directions.

Results

Our analysis revealed a significant causal relationship where specific bacterial taxa such as Coprococcus1 (OR = 0.798, 95%CI: 0.711-0.896, p < 0.001), Coprococcus3 (OR = 0.851, 95%CI: 0.740-0.979, p = 0.024), Desulfovibrio (OR = 0.902, 95%CI: 0.817-0.996, p = 0.041), Flavonifractor (OR = 0.823, 95%CI: 0.708-0.957, p < 0.001), and Lachnospiraceae UCG004, whereas others including Ruminococcaceae UCG005 (OR = 1.127, 95%CI: 1.008-1.261, p = 0.036), Eubacterium nodatum group (OR = 1.080, 95%CI: 1.018-1.145, p = 0.025), Butyricimonas (OR = 1.118, 95%CI: 1.014-1.233, p = 0.002), and Bacteroidetes (OR = 1.274, 95%CI: 1.014-1.233, p < 0.001) increase constipation risk. In the reverse MR analysis, constipation was found to influence the abundance of certain taxa, including Family XIII, Porphyromonadaceae, Proteobacteria, Lentisphaeria, Veillonellaceae, Victivallaceae, Catenibacterium, Sellimonas, and Victivallales, indicating a bidirectional relationship. Sensitivity analyses confirmed the robustness of these findings, with no evidence of heterogeneity or horizontal pleiotropy.

Conclusion

The relationship between our study gut microbiota and constipation interacts at the genetic level, which gut microbiota can influence the onset of constipation, and constipation can alter the gut microbiota. Coprococcus1, Coprococcus3, Desulfovibrio, Flavonifractor and Lachnospiraceae UCG004 play a protective role against constipation, while Ruminococcaceae UCG005, Eubacterium nodatum group, Butyricimonas, and Bacteroidetes are associated with an increased risk. In addition, constipation correlates positively with the abundance of Family XIII, Porphyromonadaceae and Proteobacteria, while negatively with Lentisphaeria, Veillonellaceae, Victivallaceae, Catenibacterium, Sellimonas, and Victivallales.

Fecal supernatants from dogs with idiopathic epilepsy activate enteric neurons

Introduction

Alterations in the composition and function of the gut microbiome have been reported in idiopathic epilepsy (IE), however, interactions of gut microbes with the enteric nervous system (ENS) in this context require further study. This pilot study examined how gastrointestinal microbiota (GIM), their metabolites, and nutrients contained in intestinal contents communicate with the ENS.

Methods

Fecal supernatants (FS) from healthy dogs and dogs with IE, including drug-naïve, phenobarbital (PB) responsive, and PB non-responsive dogs, were applied to cultured myenteric neurons to test their activation using voltage-sensitive dye neuroimaging. Additionally, the concentrations of short-chain fatty acids (SCFAs) in the FS were quantified.

Results

Our findings indicate that FS from all examined groups elicited neuronal activation. Notably, FS from PB non-responsive dogs with IE induced action potential discharge in a higher proportion of enteric neurons compared to healthy controls, which exhibited the lowest burst frequency overall. Furthermore, the highest burst frequency in enteric neurons was observed upon exposure to FS from drug-naïve dogs with IE. This frequency was significantly higher compared to that observed in PB non-responsive dogs with IE and showed a tendency to surpass that of healthy controls.

Discussion

Although observed disparities in SCFA concentrations across the various FS samples might be associated with the induced neuronal activity, a direct correlation remains elusive at this point. The obtained results hint at an involvement of the ENS in canine IE and set the basis for future studies.

Distribution, quantification, and characterization of substance P enteric neurons in the submucosal and myenteric plexuses of the porcine colon

The pig is an important translational model for studying intestinal physiology and disorders for its many homologies with humans, including the organization of the enteric nervous system (ENS), the major regulator of gastrointestinal functions. This study focused on the quantification and neurochemical characterization of substance P (SP) neurons in the pig ascending (AC) and descending colon (DC) in wholemount preparations of the inner submucosal plexus (ISP), outer submucosal plexus (OSP), and myenteric plexus (MP). We used antibodies for the pan-neuronal marker HuCD, and choline acetyltransferase (ChAT) and neuronal nitric oxide synthase (nNOS), markers for excitatory and inhibitory transmitters, for multiple labeling immunofluorescence and high-resolution confocal microscopy. The highest density of SP immunoreactive (IR) neurons was in the ISP (222/mm2 in the AC, 166/mm2 in the DC), where they make up about a third of HuCD-IR neurons, compared to the OSP and MP (19-22% and 13-17%, respectively, P < 0.001-0.0001). HuCD/SP/ChAT-IR neurons (up to 23%) were overall more abundant than HuCD/SP/nNOS-IR neurons (< 10%). Most SP-IR neurons contained ChAT-IR (62-85%), whereas 18-38% contained nNOS-IR with the highest peak in the OSP. A subpopulation of SP-IR neurons contains both ChAT- and nNOS-IR with the highest peak in the OSP and ISP of DC (33-36%) and the lowest in the ISP of AC (< 10%, P < 0.001). SP-IR varicose fibers were abundant in the ganglia. This study shows that SP-IR neurons are functionally distinct with variable proportions in different plexuses in the AC and DC reflecting diverse functions of specific colonic regions.

Lactic acid bacteria in relieving constipation: mechanism, clinical application, challenge, and opportunity

Constipation is a prevalent gastrointestinal symptom that can considerably affect a patients' quality of life. Although several drugs have been used to treat constipation, they are associated with high costs, side effects, and low universality. Therefore, alternative intervention strategies are urgently needed. Traditional lactic acid bacteria (LAB), such as Bifidobacterium and Lactobacillus, play a vital role in regulating intestinal microecology and have demonstrated favorable effects in constipation; however, a comprehensive review of their constipation relief mechanisms is limited. This review summarizes the pathogenesis of constipation and the relationship between intestinal motility and gut microbiota, elucidates the possible mechanism by which LAB alleviates of constipation through a systematic summary of animal and clinical research, and highlights the challenges and applications of LAB in the treatment of constipation. Our review can improve our understanding of constipation, and advance targeted microecological therapeutic agents, such as LAB.

Interaction between the Gut Microbiota and Intestinal Motility

The gut microbiota is the largest symbiotic ecosystem with the host and has been proven to play an important role in maintaining the stability of the intestinal environment. The imbalance of the gut microbiota is caused by the imbalance between the symbiotic microbiota and the pathogenic microbiota. The commensal microbiome regulates intestinal motility, while the pathogenic microbiome causes intestinal motility disorder, resulting in disease development. Intestinal motility is a relatively general term, and its meaning may include intestinal muscle contraction, intestinal wall biomechanics, intestinal compliance, and transmission. The role of intestinal microecology and intestinal motility are interrelated, intestinal flora disorder mediates intestinal motility, and abnormal intestinal motility affects colonization of the intestinal flora. In this review, we briefly outlined the interaction between gut microbiota and intestinal motility and provided a reference for future studies.

High-fat diets on the enteric nervous system: Possible interactions and mechanisms underlying dysmotility

Obesity is a chronic disease that affects various physiological systems. Among them, the gastrointestinal tract appears to be a main target of this disease. High-fat diet (HFD) animal models can help recapitulate the classic signs of obesity and present a series of gastrointestinal alterations, mainly dysmotility. Because intestinal motility is governed by the enteric nervous system (ENS), enteric neurons, and glial cells have been studied in HFD models. Given the importance of the ENS in general gut physiology, this review aims to discuss the relationship between HFD-induced neuroplasticity and gut dysmotility observed in experimental models. Furthermore, we highlight components of the gut environment that might influence enteric neuroplasticity, including gut microbiota, enteric glio-epithelial unit, serotonin release, immune cells, and disturbances such as inflammation and oxidative stress.

Potential Roles of Enterochromaffin Cells in Early Life Stress-Induced Irritable Bowel Syndrome

Irritable bowel syndrome (IBS) is one of the most common functional gastrointestinal disorders, also known as disorders of the gut–brain interaction; however, the pathophysiology of IBS remains unclear. Early life stress (ELS) is one of the most common risk factors for IBS development. However, the molecular mechanisms by which ELS induces IBS remain unclear. Enterochromaffin cells (ECs), as a prime source of peripheral serotonin (5-HT), play a pivotal role in intestinal motility, secretion, proinflammatory and anti-inflammatory effects, and visceral sensation. ECs can sense various stimuli and microbiota metabolites such as short-chain fatty acids (SCFAs) and secondary bile acids. ECs can sense the luminal environment and transmit signals to the brain via exogenous vagal and spinal nerve afferents. Increasing evidence suggests that an ECs-5-HT signaling imbalance plays a crucial role in the pathogenesis of ELS-induced IBS. A recent study using a maternal separation (MS) animal model mimicking ELS showed that MS induced expansion of intestinal stem cells and their differentiation toward secretory lineages, including ECs, leading to ECs hyperplasia, increased 5-HT production, and visceral hyperalgesia. This suggests that ELS-induced IBS may be associated with increased ECs-5-HT signaling. Furthermore, ECs are closely related to corticotropin-releasing hormone, mast cells, neuron growth factor, bile acids, and SCFAs, all of which contribute to the pathogenesis of IBS. Collectively, ECs may play a role in the pathogenesis of ELS-induced IBS. Therefore, this review summarizes the physiological function of ECs and focuses on their potential role in the pathogenesis of IBS based on clinical and pre-clinical evidence.

Microbiota-gut-brain axis: enteroendocrine cells and the enteric nervous system form an interface between the microbiota and the central nervous system

The microbiota-gut-brain axis transmits bidirectional communication between the gut and the central nervous system and links the emotional and cognitive centers of the brain with peripheral gut functions. This communication occurs along the axis via local, paracrine, and endocrine mechanisms involving a variety of gut-derived peptide/amine produced by enteroendocrine cells. Neural networks, such as the enteric nervous system, and the central nervous system, including the autonomic nervous system, also transmit information through the microbiota-gut-brain axis. Recent advances in research have described the importance of the gut microbiota in influencing normal physiology and contributing to disease. We are only beginning to understand this bidirectional communication system. In this review, we summarize the available data supporting the existence of these interactions, highlighting data related to the contribution of enteroendocrine cells and the enteric nervous system as an interface between the gut microbiota and brain.

Pursuing Multiple Biomarkers for Early Idiopathic Parkinson's Disease Diagnosis

Parkinson's disease (PD) ranks first in the world as a neurodegenerative movement disorder and occurs most commonly in an idiopathic form. PD patients may have motor symptoms, non-motor symptoms, including cognitive and behavioral changes, and symptoms related to autonomic nervous system (ANS) failures, such as gastrointestinal, urinary, and cardiovascular symptoms. Unfortunately, the diagnostic accuracy of PD by general neurologists is relatively low. Currently, there is no objective molecular or biochemical test for PD; its diagnosis is based on clinical criteria, mainly by cardinal motor symptoms, which manifest when patients have lost about 60-80% of dopaminergic neurons. Therefore, it is urgent to establish a panel of biomarkers for the early and accurate diagnosis of PD. Once the disease is accurately diagnosed, it may be easier to unravel idiopathic PD's pathogenesis, and ultimately, finding a cure. This review discusses several biomarkers' potential to set a panel for early idiopathic PD diagnosis and future directions.

Characterization of gut contractility and microbiota in patients with severe chronic constipation

Chronic constipation (CC) is one of the most common gastrointestinal disorders worldwide. Its pathogenesis, however, remains largely unclear. The purpose of the present work was to gain an insight into the role of contractility and microbiota in the etiology of CC. To this end, we studied spontaneous and evoked contractile activity of descending colon segments from patients that have undergone surgery for refractory forms of CC. The juxta-mucosal microbiota of these colon samples were characterized with culture-based and 16S rRNA sequencing techniques. In patients with CC the spontaneous colonic motility remained unchanged compared to the control group without dysfunction of intestinal motility. Moreover, contractions induced by potassium chloride and carbachol were increased in both circular and longitudinal colonic muscle strips, thus indicating preservation of contractile apparatus and increased sensitivity to cholinergic nerve stimulation in the constipated intestine. In the test group, the gut microbiota composition was assessed as being typically human, with four dominant bacterial phyla, namely Firmicutes, Bacteroidetes, Proteobacteria, and Actinobacteria, as well as usual representation of the most prevalent gut bacterial genera. Yet, significant inter-individual differences were revealed. The phylogenetic diversity of gut microbiota was not affected by age, sex, or colonic anatomy (dolichocolon, megacolon). The abundance of butyrate-producing genera Roseburia, Coprococcus, and Faecalibacterium was low, whereas conventional probiotic genera Lactobacillus and Bifidobacteria were not decreased in the gut microbiomes of the constipated patients. As evidenced by our study, specific microbial biomarkers for constipation state are absent. The results point to a probable role played by the overall gut microbiota at the functional level. To our knowledge, this is the first comprehensive characterization of CC pathogenesis, finding lack of disruption of motor activity of colonic smooth muscle cells and insufficiency of particular members of gut microbiota usually implicated in CC.

Enteric nervous system: sensory transduction, neural circuits and gastrointestinal motility

The gastrointestinal tract is the only internal organ to have evolved with its own independent nervous system, known as the enteric nervous system (ENS). This Review provides an update on advances that have been made in our understanding of how neurons within the ENS coordinate sensory and motor functions. Understanding this function is critical for determining how deficits in neurogenic motor patterns arise. Knowledge of how distension or chemical stimulation of the bowel evokes sensory responses in the ENS and central nervous system have progressed, including critical elements that underlie the mechanotransduction of distension-evoked colonic peristalsis. Contrary to original thought, evidence suggests that mucosal serotonin is not required for peristalsis or colonic migrating motor complexes, although it can modulate their characteristics. Chemosensory stimuli applied to the lumen can release substances from enteroendocrine cells, which could subsequently modulate ENS activity. Advances have been made in optogenetic technologies, such that specific neurochemical classes of enteric neurons can be stimulated. A major focus of this Review will be the latest advances in our understanding of how intrinsic sensory neurons in the ENS detect and respond to sensory stimuli and how these mechanisms differ from extrinsic sensory nerve endings in the gut that underlie the gut-brain axis.

Intrinsic sensory neurons provide direct input to motor neurons and interneurons in mouse distal colon via varicose baskets

Connections from intrinsic primary afferent neurons (IPANs), to ascending motor and interneurons have been described in guinea pig colon. These mono- and polysynaptic circuits may underlie polarized motor reflexes evoked by local gut stimulation. There is a need to translate findings in guinea pig to mouse, a species increasingly used in enteric neuroscience. Here, mouse distal colon was immunolabeled for CGRP, a marker of putative IPANs. This revealed a combination of large, intensely immunofluorescent axons in myenteric plexus and circular muscle, and thinner varicose axons with less immunofluorescence. The latter formed dense, basket-like varicosity clusters (CGRP+ baskets) that enveloped myenteric nerve cell bodies. Immunolabeling after 4-5 days in organ culture caused loss of large CGRP+ axons, but not varicose CGRP+ fibers and CGRP+ baskets. Baskets were characterized further by triple labeling with CGRP, nitric oxide synthase (NOS) and calretinin (CALR) antibodies. Approximately half (48%) of nerve cell bodies inside CGRP+ baskets lacked both NOS and CALR, while two overlapping populations containing NOS and/or CALR comprised the remainder. Quantitative analysis revealed CGRP+ varicosities were most abundant in baskets, followed by CALR+ varicosities, with a high degree of colocalization between the two markers. Few NOS+ varicosities occurred in baskets. Significantly higher proportions of CALR+ and CGRP+ varicosities colocalized in baskets than in circular muscle. In conclusion, CGRP+ baskets in mouse colon are formed by intrinsic enteric neurons with a neurochemical profile consistent with IPANs and have direct connections to both excitatory and inhibitory neurons.

Morphological evidence for novel enteric neuronal circuitry in guinea pig distal colon

The gastrointestinal (GI) tract is unique compared to all other internal organs; it is the only organ with its own nervous system and its own population of intrinsic sensory neurons, known as intrinsic primary afferent neurons (IPANs). How these IPANs form neuronal circuits with other functional classes of neurons in the enteric nervous system (ENS) is incompletely understood. We used a combination of light microscopy, immunohistochemistry and confocal microscopy to examine the topographical distribution of specific classes of neurons in the myenteric plexus of guinea-pig colon, including putative IPANs, with other classes of enteric neurons. These findings were based on immunoreactivity to the neuronal markers, calbindin, calretinin and nitric oxide synthase. We then correlated the varicose outputs formed by putative IPANs with subclasses of excitatory interneurons and motor neurons. We revealed that calbindin-immunoreactive varicosities form specialized structures resembling 'baskets' within the majority of myenteric ganglia, which were arranged in clusters around calretinin-immunoreactive neurons. These calbindin baskets directly arose from projections of putative IPANs and represent morphological evidence of preferential input from sensory neurons directly to a select group of calretinin neurons. Our findings uncovered that these neurons are likely to be ascending excitatory interneurons and excitatory motor neurons. Our study reveals for the first time in the colon, a novel enteric neural circuit, whereby calbindin-immunoreactive putative sensory neurons form specialized varicose structures that likely direct synaptic outputs to excitatory interneurons and motor neurons. This circuit likely forms the basis of polarized neuronal pathways underlying motility.

Effects of heat-killed <i>Lactobacillus casei</i> subsp. <i>casei</i> 327 intake on defecation in healthy volunteers: a randomized, double-blind, placebo-controlled, parallel-group study

Lactobacillus casei subsp. casei 327 (L. casei 327) was isolated from brown rice. A preliminary study showed that intake of 50 mg of heat-killed L. casei 327 is effective in improving defecation. In this study, we conducted a randomized, double-blind, placebo-controlled, parallel-group trial to investigate the effect of intake of heat-killed L. casei 327 (25 mg: approximately 5 × 10¹⁰ bacteria) on defecation in healthy volunteers with relatively low defecation frequencies. We selected 104 healthy Japanese adults with relatively low defecation frequencies (approximately 3–5 times a week) by screening and pretrial tests. Subjects (n=52 in each group) were randomly given a tablet containing L. casei 327 (group A) or a placebo tablet (group P) daily for 2 weeks. After eliminating data for 9 subjects who met the exclusion criteria for efficacy analysis, data for 95 subjects were analyzed. The defecation frequency and number of defecation days during the intake period and their changes from the pretrial period were significantly higher in group A than in group P. The fecal volume during the intake period was higher in group A than group P, but the difference was not statistically significant. However, the change from the pretrial period was significantly higher in group A than in group P. There were no significant differences between groups in the values of fecal shape, color, odor, and feeling after defecation. These results suggested that intake of L. casei 327 improves defecation in healthy adults who have relatively low defecation frequencies.

Is There a Role for Endogenous 5-HT in Gastrointestinal Motility? How Recent Studies Have Changed Our Understanding

Over the past few years, there have been dramatic changes in our understanding of the role of endogenous 5-hydroxytryptamine (5-HT) in the generation of gastrointestinal (GI) motility patterns in the small and large intestine. The idea that endogenous 5-HT played a major role in the generation of peristalsis in the small intestine was first proposed in the mid 1950s, after it was discovered that endogenous 5-HT could be released from the mucosa at a similar time that peristalsis occurred; and that exogenous 5-HT could potently stimulate peristalsis. The fact that exogenous 5-HT stimulated peristalsis and that there was a similarity in timing between the release of 5-HT from the mucosa and the onset of peristalsis led investigators to propose that release of endogenous 5-HT from the mucosa was causally related to the generation of peristalsis. In further support of this, other studies showed that selective 5-HT antagonists could inhibit or block peristalsis, and other motor patterns, such as the migrating motor complex. Taken together, based on these findings, some laboratories believed that endogenous 5-HT (synthesized in the gut wall) was an important mediator, or initiator, of different propulsive motor patterns in the lower GI tract. This notion changed dramatically in the past few years, however, after it was discovered that removal of the mucosa abolished all cyclical release of endogenous 5-HT, but did not block peristalsis, nor the cyclical migrating complex. Furthermore, other laboratories revealed that genetic deletion of the gene tryptophan hydroxylase 1 (TPH-1) (that synthesizes endogenous 5-HT in the mucosa) actually had no inhibitory effect on transit of intestinal contents in live animals. Then, perhaps one of the most startling of all observations was the discovery that selective 5-HT receptor antagonists actually have the same inhibitory effects on peristalsis and the migrating complex in segments of intestine that had been depleted of all endogenous 5-HT. Taken together, these recent findings have led to a major revision in our understanding of the functional role of endogenous 5-HT in the generation of propulsive motor patterns in the lower GI tract. This review will focus on how our understanding of endogenous 5-HT in the GI tract has changed substantially in recent times.

The Enteric Network: Interactions between the Immune and Nervous Systems of the Gut

Interactions between the nervous and immune systems enable the gut to respond to the variety of dietary products that it absorbs, the broad spectrum of pathogens that it encounters, and the diverse microbiome that it harbors. The enteric nervous system (ENS) senses and reacts to the dynamic ecosystem of the gastrointestinal (GI) tract by translating chemical cues from the environment into neuronal impulses that propagate throughout the gut and into other organs in the body, including the central nervous system (CNS). This review will describe the current understanding of the anatomy and physiology of the GI tract by focusing on the ENS and the mucosal immune system. We highlight emerging literature that the ENS is essential for important aspects of microbe-induced immune responses in the gut. Although most basic and applied research in neuroscience has focused on the brain, the proximity of the ENS to the immune system and its interface with the external environment suggest that novel paradigms for nervous system function await discovery.

The Diverse Metabolic Roles of Peripheral Serotonin

Serotonin (5-hydroxytryptamine or 5-HT) is a multifunctional bioamine with important signaling roles in a range of physiological pathways. Almost all of the 5-HT in our bodies is synthesized in specialized enteroendocrine cells within the gastrointestinal (GI) mucosa called enterochromaffin (EC) cells. These cells provide all of our circulating 5-HT. We have long appreciated the important contributions of 5-HT within the gut, including its role in modulating GI motility. However, evidence of the physiological and clinical significance of gut-derived 5-HT outside of the gut has recently emerged, implicating 5-HT in regulation of glucose homeostasis, lipid metabolism, bone density, and diseases associated with metabolic syndrome, such as obesity and type 2 diabetes. Although a new picture has developed in the last decade regarding the various metabolic roles of peripheral serotonin, so too has our understanding of the physiology of EC cells. Given that they are scattered throughout the lining of the GI tract within the epithelial cell layer, these cells are typically difficult to study. Advances in isolation procedures now allow the study of pure EC-cell cultures and single cells, enabling studies of EC-cell physiology to occur. EC cells are sensory cells that are capable of integrating cues from ingested nutrients, the enteric nervous system, and the gut microbiome. Thus, levels of peripheral 5-HT can be modulated by a multitude of factors, resulting in both local and systemic effects for the regulation of a raft of physiological pathways related to metabolism and obesity.

Gene-environment interactions and the enteric nervous system: Neural plasticity and Hirschsprung disease prevention

Intestinal function is primarily controlled by an intrinsic nervous system of the bowel called the enteric nervous system (ENS). The cells of the ENS are neural crest derivatives that migrate into and through the bowel during early stages of organogenesis before differentiating into a wide variety of neurons and glia. Although genetic factors critically underlie ENS development, it is now clear that many non-genetic factors may influence the number of enteric neurons, types of enteric neurons, and ratio of neurons to glia. These non-genetic influences include dietary nutrients and medicines that may impact ENS structure and function before or after birth. This review summarizes current data about gene-environment interactions that affect ENS development and suggests that these factors may contribute to human intestinal motility disorders like Hirschsprung disease or irritable bowel syndrome.

Postnatal development of the myenteric glial network and its modulation by butyrate

The postnatal period is crucial for the development of gastrointestinal (GI) functions. The enteric nervous system is a key regulator of GI functions, and increasing evidences indicate that 1) postnatal maturation of enteric neurons affect the development of GI functions, and 2) microbiota-derived short-chain fatty acids can be involved in this maturation. Although enteric glial cells (EGC) are central regulators of GI functions, the postnatal evolution of their phenotype remains poorly defined. We thus characterized the postnatal evolution of EGC phenotype in the colon of rat pups and studied the effect of short-chain fatty acids on their maturation. We showed an increased expression of the glial markers GFAP and S100β during the first postnatal week. As demonstrated by immunohistochemistry, a structured myenteric glial network was observed at 36 days in the rat colons. Butyrate inhibited EGC proliferation in vivo and in vitro but had no effect on glial marker expression. These results indicate that the EGC myenteric network continues to develop after birth, and luminal factors such as butyrate endogenously produced in the colon may affect this development.

Relationship Between Microbiota of the Colonic Mucosa vs Feces and Symptoms, Colonic Transit, and Methane Production in Female Patients With Chronic Constipation

BACKGROUND & AIMS

In fecal samples from patients with chronic constipation, the microbiota differs from that of healthy subjects. However, the profiles of fecal microbiota only partially replicate those of the mucosal microbiota. It is not clear whether these differences are caused by variations in diet or colonic transit, or are associated with methane production (measured by breath tests). We compared the colonic mucosal and fecal microbiota in patients with chronic constipation and in healthy subjects to investigate the relationships between microbiota and other parameters.

METHODS

Sigmoid colonic mucosal and fecal microbiota samples were collected from 25 healthy women (controls) and 25 women with chronic constipation and evaluated by 16S ribosomal RNA gene sequencing (average, 49,186 reads/sample). We assessed associations between microbiota (overall composition and operational taxonomic units) and demographic variables, diet, constipation status, colonic transit, and methane production (measured in breath samples after oral lactulose intake).

RESULTS

Fourteen patients with chronic constipation had slow colonic transit. The profile of the colonic mucosal microbiota differed between constipated patients and controls (P < .05). The overall composition of the colonic mucosal microbiota was associated with constipation, independent of colonic transit (P < .05), and discriminated between patients with constipation and controls with 94% accuracy. Genera from Bacteroidetes were more abundant in the colonic mucosal microbiota of patients with constipation. The profile of the fecal microbiota was associated with colonic transit before adjusting for constipation, age, body mass index, and diet; genera from Firmicutes (Faecalibacterium, Lactococcus, and Roseburia) correlated with faster colonic transit. Methane production was associated with the composition of the fecal microbiota, but not with constipation or colonic transit.

CONCLUSIONS

After adjusting for diet and colonic transit, the profile of the microbiota in the colonic mucosa could discriminate patients with constipation from healthy individuals. The profile of the fecal microbiota was associated with colonic transit and methane production (measured in breath), but not constipation.

Inflammation-induced abnormalities in the subcellular localization and trafficking of the neurokinin 1 receptor in the enteric nervous system

Activated G protein-coupled receptors traffic to endosomes and are sorted to recycling or degradative pathways. Endosomes are also a site of receptor signaling of sustained and pathophysiologically important processes, including inflammation. However, the mechanisms of endosomal sorting of receptors and the impact of disease on trafficking have not been fully defined. We examined the effects of inflammation on the subcellular distribution and trafficking of the substance P (SP) neurokinin 1 receptor (NK1R) in enteric neurons. We studied NK1R trafficking in enteric neurons of the mouse colon using immunofluorescence and confocal microscopy. The impact of inflammation was studied in IL10(-/-)-piroxicam and trinitrobenzenesulfonic acid colitis models. NK1R was localized to the plasma membrane of myenteric and submucosal neurons of the uninflamed colon. SP evoked NK1R endocytosis and recycling. Deletion of β-arrestin2, which associates with the activated NK1R, accelerated recycling. Inhibition of endothelin-converting enzyme-1 (ECE-1), which degrades endosomal SP, prevented recycling. Inflammation was associated with NK1R endocytosis in myenteric but not submucosal neurons. Whereas the NK1R in uninflamed neurons recycled within 60 min, NK1R recycling in inflamed neurons was delayed for >120 min, suggesting defective recycling machinery. Inflammation was associated with β-arrestin2 upregulation and ECE-1 downregulation, which may contribute to the defective NK1R recycling. We conclude that inflammation evokes redistribution of NK1R from the plasma membrane to endosomes of myenteric neurons through enhanced SP release and defective NK1R recycling. Defective recycling may be secondary to upregulation of β-arrestin2 and downregulation of ECE-1. Internalized NK1R may generate sustained proinflammatory signals that disrupt normal neuronal functions.

Gut chemosensing mechanisms

The enteroendocrine system is the primary sensor of ingested nutrients and is responsible for secreting an array of gut hormones, which modulate multiple physiological responses including gastrointestinal motility and secretion, glucose homeostasis, and appetite. This Review provides an up-to-date synopsis of the molecular mechanisms underlying enteroendocrine nutrient sensing and highlights our current understanding of the neuro-hormonal regulation of gut hormone secretion, including the interaction between the enteroendocrine system and the enteric nervous system. It is hoped that a deeper understanding of how these systems collectively regulate postprandial physiology will further facilitate the development of novel therapeutic strategies.

Nutrient-induced changes in the phenotype and function of the enteric nervous system

The enteric nervous system (ENS) integrates numerous sensory signals in order to control and maintain normal gut functions. Nutrients are one of the prominent factors which determine the chemical milieu in the lumen and, after absorption, also within the gut wall. This review summarizes current knowledge on the impact of key nutrients on ENS functions and phenotype, covering their acute and long-term effects. Enteric neurones contain the molecular machinery to respond specifically to nutrients. These transporters and receptors are not expressed exclusively in the ENS but are also present in other cells such as enteroendocrine cells (EECs) and extrinsic sensory nerves, signalling satiety or hunger. Glucose, amino acids and fatty acids all activate enteric neurones, as suggested by enhanced c-Fos expression or spike discharge. These excitatory effects are the result of a direct neuronal activation but also involve the activation of EECs which, upon activation by luminal nutrients, release mediators such as ghrelin, cholecystokinin or serotonin. The presence or absence of nutrients in the intestinal lumen induces long-term changes in neurotransmitter expression, excitability, neuronal survival and ultimately impact upon gut motility, secretion or intestinal permeability. Together with EECs and vagal nerves, the ENS must be recognized as an important player initiating concerted responses to nutrients. It remains to be studied how, for instance, nutrient-induced changes in the ENS may influence additional gut functions such as intestinal barrier repair, intestinal epithelial stem cell proliferation/differentiation and also the signalling of extrinsic nerves to brain regions which control food intake.

Transmission to interneurons is via slow excitatory synaptic potentials mediated by P2Y(1) receptors during descending inhibition in guinea-pig ileum

Background

The nature of synaptic transmission at functionally distinct synapses in intestinal reflex pathways has not been fully identified. In this study, we investigated whether transmission between interneurons in the descending inhibitory pathway is mediated by a purine acting at P2Y receptors to produce slow excitatory synaptic potentials (EPSPs).

Methodology/Principal findings

Myenteric neurons from guinea-pig ileum in vitro were impaled with intracellular microelectrodes. Responses to distension 15 mm oral to the recording site, in a separately perfused stimulation chamber and to electrical stimulation of local nerve trunks were recorded. A subset of neurons, previously identified as nitric oxide synthase immunoreactive descending interneurons, responded to both stimuli with slow EPSPs that were reversibly abolished by a high concentration of PPADS (30 μM, P2 receptor antagonist). When added to the central chamber of a three chambered organ bath, PPADS concentration-dependently depressed transmission through that chamber of descending inhibitory reflexes, measured as inhibitory junction potentials in the circular muscle of the anal chamber. Reflexes evoked by distension in the central chamber were unaffected. A similar depression of transmission was seen when the specific P2Y₁ receptor antagonist MRS 2179 (10 μM) was in the central chamber. Blocking either nicotinic receptors (hexamethonium 200 μM) or 5-HT₃ receptors (granisetron 1 μM) together with P2 receptors had no greater effect than blocking P2 receptors alone.

Conclusions/Significance

Slow EPSPs mediated by P2Y₁ receptors, play a primary role in transmission between descending interneurons of the inhibitory reflexes in the guinea-pig ileum. This is the first demonstration for a primary role of excitatory metabotropic receptors in physiological transmission at a functionally identified synapse.

Identification and mechanosensitivity of viscerofugal neurons

Enteric viscerofugal neurons are interneurons with cell bodies in the gut wall; they project to prevertebral ganglia where they provide excitatory synaptic drive to sympathetic neurons which control intestinal motility and secretion. Here, we studied the mechanosensitivity and firing of single, identified viscerofugal neurons in guinea-pig distal colon. Flat sheet preparations of gut were set up in vitro and conventional extracellular recordings made from colonic nerve trunks. The nicotinic agonist, 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP) (1mM), was locally pressure ejected onto individual myenteric ganglia. In a few ganglia, DMPP promptly evoked firing in colonic nerves. Biotinamide filling of colonic nerves revealed that DMPP-responsive sites corresponded to viscerofugal nerve cell bodies. This provides a robust means to positively identify viscerofugal neuron firing. Of 15 single units identified in this way, none responded to locally-applied capsaicin (1 μM). Probing with von Frey hairs at DMPP-responsive sites reliably evoked firing in all identified viscerofugal neurons (18/18 units tested; 0.8-5 mN). Circumferential stretch of the preparation increased firing in all 14/14 units (1-5 g, p<0.05). Both stretch and von Frey hair responses persisted in Ca(2+)-free solution (6 mM Mg(2+), 1mM EDTA), indicating that viscerofugal neurons are directly mechanosensitive. To investigate their adequate stimulus, circular muscle tension and length were independently modulated (BAY K8644, 1 μM and 10 μM, respectively). Increases in intramural tension without changes in length did not affect firing. However, contraction-evoked shortening, under constant load, significantly decreased firing (p<0.001). In conclusion, viscerofugal neuron action potentials contribute to recordings from colonic nerve trunks, in vitro. They provide a significant primary afferent output from the colon, encoding circumferential length, largely independent of muscle tension. All viscerofugal neurons are directly mechanosensitive, although they have been reported to receive synaptic inputs. In short, viscerofugal neurons combine interneuronal function with length-sensitive mechanosensitivity.

A comparison between colonic submucosa and mucosa to detect Lewy pathology in Parkinson's disease

BACKGROUND

  Lewy bodies and neurites (LN), the two pathological hallmarks of Parkinson's disease (PD), are found in the enteric nervous system (ENS). Previously, we have shown that whole mounts of submucosa obtained after microdissection of colonic biopsies can be used for the detection of LN in the submucosal plexus (SMP) of PD patients. Recent reports suggest that Lewy pathology may extend beyond the submucosa to involve the digestive mucosa. The aim of the present research was to determine whether the analysis of the mucosa obtained after microdissection may help improve the sensitivity of colonic biopsies to detect Lewy pathology in the colon of PD patients.

METHODS

  Nine PD patients and 10 controls were included. Four biopsies were taken from the sigmoid/descending colon junction during the course of a rectosigmoidoscopy (short colonoscopy) in PD patients and during a total colonoscopy for colorectal screening in controls. Biopsies were microdissected, the mucosa was separated from the submucosa and both structures were analyzed by immunohistochemistry. Immunohistochemical analysis was performed using antibodies against phosphorylated alpha-synuclein to detect LN and neurofilaments NF200 kDa to label the neuronal structures.

KEY RESULTS

  Lewy neurites were present in the SMP of four patients and in the mucosa of three patients. Remarkably, among the patients who displayed LN within their mucosa, one was devoid of Lewy pathology in his SMP. No LN were observed in the mucosa and the SMP of controls.

CONCLUSIONS & INFERENCES

  The parallel analysis of colonic mucosa, along with the SMP, can help detect Lewy pathology in PD.

Peristalsis and fecal pellet propulsion do not require nicotinic, purinergic, 5-HT3, or NK3 receptors in isolated guinea pig distal colon

The neuronal mechanism by which distension of the colon triggers peristalsis and the propulsion of colonic contents is incompletely understood. In this study, we used video imaging and spatiotemporal mapping techniques to investigate the neuroneuronal mechanisms underlying peristalsis in isolated guinea pig distal colon. In direct contrast to previous studies, we found that hexamethonium (100 muM-1 mM) or mecamylamine (20 muM) never abolished peristalsis or fecal pellet propulsion, although a temporary blockade of peristalsis was common, giving the impression perhaps that peristalsis was blocked permanently. During the initiation of peristalsis, the intraluminal propulsive force applied to an inserted fecal pellet was significantly reduced by hexamethonium 100 muM, even though, once initiated, the propagation velocity of fecal pellets was never reduced by nicotinic antagonists. In the presence of hexamethonium or mecamylamine, further addition of PPADS (10 muM), ondansetron (1 muM), and SR 142801 (300 nM) had no inhibitory effect on the propagation velocity of fecal pellets. In these preparations, antagonists for nicotinic, purinergic (P2), serotonergic (5-HT3), or tachykinergic (NK3) receptors always abolished responses to the agonists for these receptors, confirming that when peristalsis occurred, nicotinic, P2, 5-HT3, and NK3 receptors were blocked. Tetrodotoxin abolished nonnicotinic peristalsis. In summary, nicotinic transmission contributes to excitatory neuroneuronal transmission underlying peristalsis and fecal pellet propulsion but is not required for peristalsis, nor fecal pellet propulsion, as once thought. These observations could be explained by an excitatory nonnicotinic neuroneuronal pathway that can generate peristalsis and induce normal fecal pellet propagation velocities but does not require nicotinic, P2, 5-HT3, or NK3 receptors.

Short-chain fatty acids regulate the enteric neurons and control gastrointestinal motility in rats

BACKGROUND & AIMS

Little is known about the environmental and nutritional regulation of the enteric nervous system (ENS), which controls gastrointestinal motility. Short-chain fatty acids (SCFAs) such as butyrate regulate colonic mucosa homeostasis and can modulate neuronal excitability. We investigated their effects on the ENS and colonic motility.

METHODS

Effects of butyrate on the ENS were studied in colons of rats given a resistant starch diet (RSD) or intracecal perfusion of SCFAs. Effects of butyrate were also studied in primary cultures of ENS. The neurochemical phenotype of the ENS was analyzed with antibodies against Hu, choline acetyltransferase (ChAT), and neuronal nitric oxide synthase (nNOS) and by quantitative polymerase chain reaction. Signaling pathways involved were analyzed by pharmacologic and molecular biology methods. Colonic motility was assessed in vivo and ex vivo.

RESULTS

In vivo and in vitro, RSD and butyrate significantly increased the proportion of ChAT- but not nNOS-immunoreactive myenteric neurons. Acetate and propionate did not reproduce the effects of butyrate. Enteric neurons expressed monocarboxylate transporter 2 (MCT2). Small interfering RNAs silenced MCT2 and prevented the increase in the proportion of ChAT- immunoreactive neurons induced by butyrate. Butyrate and trichostatin A increased histone H3 acetylation in enteric neurons. Effects of butyrate were prevented by inhibitors of the Src signaling pathway. RSD increased colonic transit, and butyrate increased the cholinergic-mediated colonic circular muscle contractile response ex vivo.

CONCLUSION

Butyrate or histone deacetylase inhibitors might be used, along with nutritional approaches, to treat various gastrointestinal motility disorders associated with inhibition of colonic transit.

Lactobacillus reuteri enhances excitability of colonic AH neurons by inhibiting calcium-dependent potassium channel opening

Probiotics are live non-pathogenic commensal organisms that exert therapeutic effects in travellers' diarrhea, irritable bowel syndrome and inflammatory bowel disease. Little is known about mechanisms of action of commensal bacteria on intestinal motility and motility-induced pain. It has been proposed that probiotics affect intestinal nerve function, but direct evidence for this has thus far been lacking. We hypothesized that probiotic effects might be mediated by actions on colonic intrinsic sensory neurons. We first determined whether sensory neurons were present in rat colon by their responses to chemical mucosal stimulation and identified them in terms of physiological phenotype and soma morphotype. Enteric neuron excitability and ion channel activity were measured using patch clamp recordings. We fed 10(9)Lactobacillus reuteri (LR) or vehicle control to rats for 9 days. LR ingestion increased excitability (threshold for evoking action potentials) and number of action potentials per depolarizing pulse, decreased calcium-dependent potassium channel (IK(Ca)) opening and decreased the slow afterhyperpolarization (sAHP) in sensory AH neurons, similar to the IK(Ca) antagonists Tram-34 and clotrimazole. LR did not affect threshold for action potential generation in S neurons. Our results demonstrate that LR targets an ion channel in enteric sensory nerves through which LR may affect gut motility and pain perception.

Intrinsic ruminal innervation in ruminants of different feeding types

According to their feeding habits, ruminants can be classified as grazers, concentrate selectors and those of intermediate type. The different feeding types are reflected in distinct anatomical properties of the forestomachs. The present study was designed to investigate whether the intrinsic innervation patterns of the rumen (the main part of the forestomach) differ between intermediate types and grazers. Myenteric plexus preparations from the rumen of goats (intermediate type), fallow deer (intermediate type), cattle (grazer) and sheep (grazer) were analysed by immunohistochemical detection of the following antigens: Hu-protein (HuC/D), choline acetyltransferase (ChAT), nitric oxide synthase (NOS), vasoactive intestinal peptide (VIP), neuropeptide Y (NPY), substance P (SP), calbindin (CALB) and somatostatin (SOM). Myenteric ganglia of cattle contained 73 +/- 6 neurons per ganglion, whereas the ganglia of sheep were significantly smaller (45 +/- 18 neurons per ganglion). The ganglion density of the myenteric plexus was highest in fallow deer (15 +/- 3 ganglia per cm(2)) and lowest in cattle (6 +/- 1 ganglia per cm(2)). All myenteric neurons were either ChAT or NOS positive. The proportion of NOS-positive neurons was significantly lower in sheep (29.5 +/- 8.2% of all neurons) than in goats (44.2 +/- 9.8%). In all species, additional analysis of the different neuropeptides revealed the following subpopulations in descending order of percentile appearance: ChAT/SP > NOS/VIP/NPY > ChAT/- > NOS/NPY. Expression of CALB was detected in a minority of the ChAT-positive neurons in all species. Somatostatin immunoreactive somata were found only in preparations obtained from fallow deer and sheep. These data suggest that the rumen of grazers is under stronger cholinergic control than the rumen of species belonging to the intermediate type, although most subpopulations of neurons are present in all species. However, whether the strong mixing patterns of low quality roughage during digestion are enabled by the prominent excitatory input of the rumen of grazers requires elucidation in further studies.

Purinergic mechanisms in the control of gastrointestinal motility

For many years, ATP and adenosine have been implicated in movement regulation of the gastrointestinal tract. They act through three major receptor subtypes: adenosine or P1 receptors, P2X receptors and P2Y receptors. Each of these major receptor types can be subdivided into several different classes and is widely distributed amongst various neurons, muscle types, glia and interstitial cells that regulate intestinal functions. Several key roles for the different receptors and their endogenous ligands have been identified in physiological and pharmacological studies. For example, adenosine acting at A(1) receptors appears to inhibit intestinal motility in various pathological conditions. Similarly, ATP acting at P2Y receptors is an important component of inhibitory neuromuscular transmission, acting as a cotransmitter with nitric oxide. ATP acting at P2X and P2Y(1) receptors is important for synaptic transmission in simple descending excitatory and inhibitory reflex pathways. Some P2Y receptor subtypes prefer uridine nucleotides over purine nucleotides. Thus, roles for UTP and UDP as enteric transmitters in place of ATP cannot be excluded. ATP also appears to be important for sensory transduction, especially in chemosensitive pathways that initiate local inhibitory reflexes. Despite this evidence, data are lacking about the roles of either adenosine or ATP in more complex motility patterns such as segmentation or the interdigestive migrating motor complex. Clarification of roles for purinergic transmission in these common, but understudied, motility patterns will depend on the use of subtype-specific antagonists that in some cases have not yet been developed.

Modulation of peristalsis by NK3receptor antagonism in guinea-pig isolated ileum is revealed as intraluminal pressure is raised

1. NK(3) tachykinin receptors mediate slow excitatory transmission in the enteric nervous system and play a role in reflexes induced by the intestinal stretch or mucosal compression. However, there is little evidence to suggest that these receptors are important in peristalsis. We have examined the effects of the NK(3) receptor antagonist, talnetant, on peristalsis in guinea-pig isolated ileum induced by optimal and by supra-maximal distension pressures. 2. At the guinea-pig NK(3) receptor, talnetant was shown to have high affinity (pK(B) 8.8) and selectivity over the guinea-pig NK(1) and NK(2) receptors. 3. Peristaltic waves in the ileum elicited by optimal distension pressures (1-3 cmH(2)O) were unaffected by talnetant at a supra-maximal concentration (250 nm). 4. Distension at a higher pressure (4 cmH(2)O) induced peristalsis in which there was incomplete closure of the lumen during each peristaltic wave and an increase in the periods of inactivity observed between bursts of peristaltic activity. The addition of talnetant (250 nm) increased the number of peristaltic events by reducing these periods of inactivity and thus, increased the productivity of the peristaltic reflex. 5. The data suggest that NK(3) receptors are not involved in the modulation of peristaltic movements by physiological stimuli, but they may have a role in modulation of reflexes in extreme or pathological conditions.

Sensory transmission in the gastrointestinal tract

The gastrointestinal (GI) tract must balance ostensibly opposite functions. On the one hand, it must undertake the process of digestion and absorption of nutrients. At the same time, the GI tract must protect itself from potential harmful antigenic and pathogenic material. Central to these processes is the ability to 'sense' the mechanical and chemical environment in the gut wall and lumen in order to orchestrate the appropriate response that facilitates nutrient assimilation or the rapid expulsion through diarrhoea and/or vomiting. In this respect, the GI tract is richly endowed with sensory elements that monitor the gut environment. Enteric neurones provide one source of such sensory innervation and are responsible for the ability of the decentralized gut to perform complex reflex functions. Extrinsic afferents not only contribute to this reflex control, but also contribute to homeostatic mechanisms and can give rise to sensations, under certain circumstances. The enteric and extrinsic sensory mechanisms share a number of common features but also some remarkably different properties. The purpose of this review is to summarize current views on sensory processing within both the enteric and extrinsic innervation and to specifically address the pharmacology of nociceptive extrinsic sensory pathways.

The relationship between the effects of short-chain fatty acids on intestinal motility in vitro and GPR43 receptor activation

The G protein-coupled receptors, GPR41 and GPR43, are activated by short-chain fatty acids (SCFAs), with distinct rank order potencies. This study investigated the possibility that SCFAs modulate intestinal motility via these receptors. Luminal SCFA concentrations within the rat intestine were greatest in the caecum (c. 115 mmol L(-1)) and proximal colon. Using similar concentrations (0.1-100 mmol L(-1)), SCFAs were found to inhibit electrically evoked, neuronally mediated contractions of rat distal colon, possibly via a prejunctional site of action; this activity was independent of the presence or absence of the mucosa. By contrast, SCFAs reduced the amplitude but also reduced the threshold and increased the frequency of peristaltic contractions in guinea-pig terminal ileum. In each model, the rank-order of activity was acetate (C2) approximately propionate (C3) approximately butyrate (C4) > pentanoate (C5) approximately formate (C1), consistent with activity at the GPR43 receptor. GPR43 mRNA was expressed throughout the rat gut, with highest levels in the colon. However, the ability of SCFAs to inhibit neuronally mediated contractions of the colon was similar in tissues from wild-type and GPR43 gene knockout mice, with identical rank-orders of potency. In conclusion, SCFAs can modulate intestinal motility, but these effects can be independent of the GPR43 receptor.

Defensive and pathological functions of the gastrointestinal NK3 receptor

In general, normal gut functions are unaffected by selective NK(3) receptor antagonists such as talnetant (SB-223412), osanetant (SR 142901) or SB-235375. However, NK(3) receptors may mediate certain defensive or pathological intestinal processes. The precise mechanisms, by which this role is achieved, are not fully understood. In summary, intense stimulation of the intrinsic primary afferent neurones (IPANs) of the enteric nervous system is thought to release tachykinins from these neurones, to induce slow excitation (slow EPSPs) of connecting IPANs. This is hypothesised to cause hypersensitivity and disrupt intestinal motility, at least partly explaining why NK(3) receptor antagonism can reduce the level of disruption caused by supramaximal distension pressures in vitro. Tachykinin release from IPANs may also increase C-fibre sensitivity, directly or indirectly. Thus, NK(3) receptor antagonists can inhibit nociception associated with intestinal distension, in normal animals or after pre-sensitisation by restraint stress. Importantly, such inhibition has been found with SB-235375, a peripherally restricted antagonist. SB-235375 can also reduce a visceromotor response to brief colorectal distension without affecting similar responses to skin pinch, providing additional evidence for intestinal-specific activity. NK(3) receptor biology is, therefore, revealing a novel pathway by which disruptions in intestinal motility and nociception can be induced.

Acidity induces c-Fos expression in a subpopulation of human colonic submucosal neurons

Enteric neurons responding to chemical challenge of the mucosa have been characterized in animal models mainly in the myenteric plexus. However, in humans, the existence of enteric neurons responding to chemical stimulation of the mucosa remains currently unknown. Therefore, the aim of our study was to identify and characterize human submucosal neurons activated by mucosal challenge with butyrate or hydrochloric acid. Segments of human colon were placed in a modified Ussing chamber and incubated on the mucosal side with butyric acid (20 mM, pH 6.5), sodium butyrate (20 mM, pH 7.5), hydrochloric acid (10 mM, pH 6.5) or culture medium (pH 7.5). After 90 min of culture, tissues were fixed and microdissected to obtain whole mount preparation of submucosa containing the Meissner's plexus. Neuron specific enolase (NSE), c-Fos, vasoactive intestinal peptide (VIP) and substance P (SP) were detected using immunohistochemical methods. Tetrodotoxin (TTX, 1 microM) was used to inhibit neuronal activity. After 90 min of culture, butyric acid induced a significant 5.6-fold increase in the proportion of c-Fos-immunoreactive neurons compared to control (19 +/- 4% versus 4 +/- 1%, respectively, p < 0.001). 41 +/- 5% of c-Fos-immunoreactive neurons were VIP-immunoreactive and 3 +/- 2% were SP-immunoreactive. Butyric acid did not modify the proportion of VIP-immunoreactive neurons. The increase in c-Fos-immunoreactive neurons induced by butyric acid was reproduced with hydrochloric acid at the same pH but not with sodium butyrate. Finally, preincubation of the tissue with TTX prevented the effect of butyric acid. In conclusion, our results demonstrate that acidic mucosal challenge induced the activation of a population of human submucosal neurons with a specific neurochemical coding.

Sensory elements within the circular muscle are essential for mechanotransduction of ongoing peristaltic reflex activity in guinea-pig distal colon

We have recently identified a population of mechanosensory myenteric S-interneurons in the distal colon of guinea-pigs. However, the role of the longitudinal (LM) and circular muscle (CM) in transducing these mechanosensory signals into enteric reflexes is unclear. In this study, we have investigated whether the LM or CM layer is necessary for activation of ascending excitatory and descending inhibitory neuronal pathways by static stretch of the paralysed isolated guinea-pig distal colon. Simultaneous intracellular recordings were made from pairs of CM cells at either end of isolated sheet preparations of distal colon that were devoid of mucosa and submucous plexus; and were maintained under circumferential stretch. In the presence of nifedipine (1 microm), an ongoing discharge of excitatory junction potentials (EJPs) and inhibitory junction potentials (IJPs) were recorded simultaneously at the oral and anal ends of the preparation. When the LM was sharp dissected off the myenteric plexus, the synchronized discharge of ascending EJPs and descending IJPs in the CM layer was unaffected. In contrast, when the majority of CM was sharp dissected off the myenteric plexus, ongoing neural activity was absent, or substantially decreased in both the LM and CM. In these preparations, immunohistochemical staining, together with transmural electrical stimuli confirmed that the myenteric plexus was always present and intact in these preparations. When full-thickness strips of CM were removed from progressively longer lengths of myenteric plexus, a graded reduction in the correlation of coordinated oral EJPs and anal IJPs occurred. However, removing approximately 40% of the thickness of CM layer from the entire preparation did not significantly disrupt, nor reduce the degree of correlation between oral EJPs and anal IJPs, suggesting that critical sensory elements did not lie adjacent to the submucosal plexus. It is concluded that mechanosensory transmission that underlies repetitive firing of ascending excitatory and descending inhibitory neuronal pathways is critically dependent upon sensory elements within the CM layer. These elements are likely to activate stretch-sensitive interneurons in the myenteric plexus. No evidence was found to suggest that the connectivity between the LM and the myenteric plexus was required for mechanotransduction.

Neural and non-neural mediation of propionate-induced contractile responses in the rat distal colon

Short-chain fatty acids (SCFAs), including propionate, butyrate and acetate, are fermentation products of carbohydrates in the colon. We investigated the contractile effects of SCFAs on the rat distal colon. Mechanical activity of the circular muscle in strip preparations was recorded in vitro. Propionate and butyrate concentration-dependently (10 micromol L(-1)-10 mmol L(-1)) induced rapid, large amplitude phasic contractions (the first phase) followed by tonic contractions (the second phase). Acetate itself had no effect on muscle activity, although preincubation with acetate attenuated both phases of the propionate-induced response. The propionate-induced phasic contraction was attenuated by atropine, tetrodotoxin and the 5-HT4 receptor antagonist SB-204070. The propionate-induced tonic contraction was attenuated by the cyclo-oxygenase inhibitor piroxicam. Antagonists of 5-HT1A, 5-HT2A and 5-HT3 receptors had no effect on the responses. Propionate-induced responses were not observed in mucosa-free preparations. These results suggest that propionate acts on receptors in the mucosa causing the release of 5-HT from enterochromaffin cells. 5-HT acts through 5-HT4 receptors on the endings of intrinsic primary afferent neurones that in turn activate cholinergic motor neurones that contract the circular muscle. Propionate also causes tonic contraction, via prostaglandin release, in the rat distal colon.

Serotonin excites neurons in the human submucous plexus via 5-HT3 receptors

BACKGROUND & AIMS

Serotonin (5-hydroxytryptamine [5-HT]) is a key signaling molecule in the gut. Recently, the neural 5-HT3 receptor received a lot of attention as a possible target in functional bowel diseases. Yet, the 5-HT3 receptor-mediated changes in properties of human enteric neurons is unknown.

METHODS

We used a fast imaging technique in combination with the potentiometric dye 1-(3-sulfonatopropyl)-4-[beta[2-(di-n-octylamino)-6-naphthyl]vinyl]pyridinium betaine to monitor directly the membrane potential changes in neurons of human submucous plexus from surgical specimens of 21 patients. An Ussing chamber technique was used to study 5-HT3 receptor involvement in chloride secretion.

RESULTS

Local microejection of 5-HT directly onto ganglion cells resulted in a transient excitation of enteric neurons characterized by increased spike discharge. This response was mimicked by the 5-HT3 receptor agonist, 2-methyl-5-HT, and blocked by the 5-HT3 receptor antagonist, tropisetron. The proportions of 5-HT-responsive nerve cells per ganglion ranged from 25.5% +/- 18.4% in the duodenum to 54.2% +/- 46.9% in the colon. Interestingly, 2-methyl-5-HT did not evoke chloride secretion in the human intestine but it did in the guinea-pig intestine. Specific 5-HT3A and 5-HT3B receptor subunit immunoreactivity as well as 5-HT3A and 5-HT3B receptor-specific messenger RNA were detected in the tissue samples. Based on co-labeling with the pan-neuronal marker HuC/D we conclude that submucous nerve cells potentially express heteromeric 5-HT3A/B receptors.

CONCLUSIONS

We show that 5-HT excited human enteric neurons via 5-HT3 receptors, which may comprise both 5-HT3A and 5-HT3B receptor subunits.

Bursts of recurrent excitation in the activation of intrinsic sensory neurons of the intestine

Sensory neurons intrinsic to the wall of the intestine receive input from stimuli in the lumen. These stimuli interact with the mucosal epithelium causing release of sensory mediators that depolarize the sensory nerve terminals. The depolarization and the subsequent pattern of action potential (AP) discharge controls the type and magnitude of the reflex evoked. The characteristics of this AP discharge were investigated in 60 intrinsic sensory neurons from the myenteric plexus of the guinea-pig ileum. Intracellular electrophysiological recordings were made from neurons near intact mucosa during electrical stimulation of the mucosa and/or neuronal soma. Most neurons (87%) responded to mucosal stimulation with a burst of 3.8+/-0.3 APs (average instantaneous frequency, fINT 39+/-4 Hz). In 38%, a somatically evoked AP triggered a similar burst of 2.9+/-0.3 APs (fINT 52+/-6 Hz) while in 50% of neurons, there was ongoing spontaneous bursting (3.8+/-0.2 APs, fINT 48+/-6 Hz). APs in all of these bursts had an inflection on the rising phase and they persisted during somatic hyperpolarization indicating they were generated in a distal process rather than the soma. Collision experiments confirmed this and suggested that bursts originated near the mucosal sensory nerve terminals. A reduction in membrane excitability reduced the number of APs in a burst suggesting a brief depolarizing event, such as a voltage- or ligand-gated ion channel, was responsible. Bursting behavior in the intrinsic sensory neurons is common for mucosal stimuli and may involve a novel transmitter acting at the sensory nerve terminal. Further, some bursting involves positive feedback between the nerve terminals and other elements in the epithelium. This is a novel and potentially important component of intestinal sensory transduction.

Mechanism of butyrate-induced hyperpolarization of cultured rat myenteric neurones

Short-chain fatty acids produced by the bacterial fermentation of carbohydrates are present in high concentrations within the colonic lumen and have been shown to alter the excitability of enteric neurones. The present study was designed to investigate the mechanisms of butyrate-induced changes in membrane potential of myenteric neurones. Myenteric neurones from 4-10-day-old rats were isolated from the small and large intestine by an enzymatic digestion with collagenase and kept in culture. Membrane potential was measured with the whole-cell patch-clamp technique and the intracellular Ca2+ concentration was measured with the fura-2 method. The short-chain fatty acid butyrate (10-100 mmol L(-1)) induced a reversible and concentration-dependent hyperpolarization of the membrane with a half-maximal effect at 30 mmol L(-1). The hyperpolarization evoked by butyrate (50 mmol L(-1)) was strongly inhibited by charybdotoxin (10(-7) mol L(-1)), a specific blocker of Ca2+ -dependent K+ channels. The butyrate-induced hyperpolarization was resistant against blockade of phospholipase C by U-73122 (10(-5) mol L(-1)), and resistant against inclusion of heparin (6 x 10(-6) mol L(-1)), an inositol-1,4,5-trisphosphate receptor antagonist, in the patch-pipette. In contrast, ruthenium red (3 x 10(-5) mol L(-1)), an inhibitor of ryanodine receptors, significantly reduced both the hyperpolarization of the membrane as well as the increase in the intracellular Ca2+ concentration evoked by butyrate. Even in neurones permeabilized with saponin (10 mg L(-1)), butyrate was able to stimulate a release of stored intracellular Ca2+ suggesting a direct action of the short-chain fatty acid at the stores without mediation of a soluble intracellular second messenger.

Mechanosensory S-neurons rather than AH-neurons appear to generate a rhythmic motor pattern in guinea-pig distal colon

Simultaneous intracellular recordings were made from myenteric neurons and circular muscle (CM) cells in isolated, stretched segments of guinea-pig distal colon. We have shown previously that maintained stretch generates a repetitive and coordinated discharge of ascending excitatory and descending inhibitory neuronal reflex pathways in the distal colon. In the presence of nifedipine (1-2 microm) to paralyse the muscle, simultaneous recordings were made from 25 pairs of AH (after-hyperpolarization)-neurons and CM cells separated by 100-500 microm. In all 25 AH-neurons, proximal process potentials (PPPs) were never recorded, even though at the same time, all recordings from neighbouring CM cells showed an ongoing discharge of inhibitory junction potentials (IJPs) anally, or excitatory junction potentials (EJPs) orally. In fact, 24 of 25 AH-neurons were totally silent, while in one AH-cell, some spontaneous fast excitatory postsynaptic potentials (FEPSPs) were recorded. All 10 electrically silent AH-cells that were injected with neurobiotin were found to be multipolar Dogiel type II neurons. In contrast, when recordings were made from myenteric S-neurons, two distinct electrical patterns of electrical activity were recorded. Recordings from 25 of 48 S-neurons showed spontaneous FEPSPs, the majority of which (22 of 25) showed periods when discrete clusters of FEPSPs (mean duration 88 ms) could be temporally correlated with the onset of EJPs or anal IJPs in the CM. Nine S-neurons were electrically quiescent. The second distinct electrical pattern in 14 S-neurons consisted of bursts, or prolonged trains of action potentials, which could be reduced to proximal process potentials (PPPs) in six of these 14 neurons during membrane hyperpolarization. Unlike FEPSPs, PPPs were resistant to a low Ca(2+)-high Mg(2+) solution and did not change in amplitude during hyperpolarizing pulses. Mechanosensory S-neurons were found to be uniaxonal or pseudounipolar filamentous neurons, with morphologies consistent with interneurons. No slow EPSPs were ever recorded from AH- or S-type neurons when IJPs or EJPs occurred in the CM. In summary, we have identified a population of mechanosensory S-neurons in the myenteric plexus of the distal colon which appear to be largely stretch sensitive, rather than muscle-tension sensitive, since they generate ongoing trains of action potentials in the presence of nifedipine. No evidence was found to suggest that in paralysed preparations, the repetitive firing in ascending excitatory or descending inhibitory nerve pathways was initiated by myenteric AH-neurons, or slow synaptic transmission.

Calbindin-immunoreactive neurones in the ovine rumen

In small laboratory animals, such as guinea pigs, immunoreactivity for the calcium-binding protein calbindin (CALB) can be used to distinguish functionally different classes of myenteric neurones. The rumen of sheep is a highly specialized gastrointestinal region, and the control of its functions requires specific intrinsic innervation patterns. The aim of this study was to neurochemically identify and characterize CALB-positive myenteric neurones of the ovine rumen. Therefore, we performed quadruple immunohistochemistry against CALB, substance P (SP), vasoactive intestinal peptide (VIP), and nitric oxide synthase (NOS) using whole-mount preparations of the ruminal myenteric plexus. On average, 3 +/- 2 and 1 +/- 0.4 myenteric neurones/ganglion were CALB-immunoreactive in suckling lambs and adult sheep, respectively. These neurones had Dogiel type-I morphology. Most of them (89.2% +/- 8.7% and 71.7% +/-44.8% in suckling lambs and adult sheep, respectively) did not colocalize any of the other antigenes. Since it has been shown in previous studies that ruminal myenteric neurones are immunoreactive for either choline acetyltransferase (ChAT) or NOS, we defined neurones which were CALB-positive and NOS-negative as CALB/ChAT. The other CALB-positive neurones were encoded CALB/NOS/+/-VIP (10.3% +/- 9.3% and 26.7% +/- 46.2% in suckling lambs and adult sheep, respectively) or CALB/ChAT/SP (0.5% +/- 1.0% and 1.7% +/- 1.9% in suckling lambs and adult sheep, respectively). We used cryostat sections of the ruminal wall to analyze the projections of the CALB-positive neurones. CALB-immunoreactive somata were exclusively located within the myenteric plexus. CALB-immunoreactive nerve fibers were found primarily in the lamina propria of the ruminal papillae. We conclude that CALB-positive myenteric neurones within the ovine rumen project to the epithelium; however, their functional role remains to be investigated.

Effects of the peripherally acting NK receptor antagonist, SB-235375, on intestinal and somatic nociceptive responses and on intestinal motility in anaesthetized rats

We investigated the effects of the selective NK(3) tachykinin receptor antagonist, SB-235375, on noxious signalling from gut and skin and on intestinal motility in anaesthetized rats. We also measured penetrance into brain and spinal cord. Nociceptive responses in reaction to colorectal distension and skin pinch were assessed by recording the electromyogram (EMG) from the external oblique muscle (a visceromotor response). Motility was measured by recording intraluminal pressure waves during changes in baseline pressure in the jejunum. Colorectal compliance was assessed by measuring luminal pressure change during isovolumic distension. SB-235375 (20 mg kg(-1), by i.v. bolus) reduced the EMG response to colorectal distension by over 90%. The reduction was slow at onset, peaked at about 60 min, and lasted for over 2 h. Responses to noxious skin pinch were unchanged. Amplitudes of propulsive waves in the jejunum were slightly reduced, but their frequency of occurrence was unchanged. SB-235375 decreased colorectal compliance by 5-10%. There was undetectable penetration of i.v. SB-235375 into brain or spinal cord. We conclude that SB-235375 acts peripherally to substantially reduce nociceptive signalling from colorectum without affecting noxious signalling from skin and with little effect on intestinal motility.

Neurokinin NK1 and NK3 receptors as targets for drugs to treat gastrointestinal motility disorders and pain

NK1 and NK3 receptors do not appear to play significant roles in normal GI functions, but both may be involved in defensive or pathological processes. NK1 receptor antagonists are antiemetic, operating via vagal sensory and motor systems, so there is a need to study their effects on other gastro-vagal functions thought to play roles in functional bowel disorders. Interactions between NK1 receptors and enteric nonadrenergic, noncholinergic motorneurones suggest a need to explore the role of this receptor in disrupted colonic motility. NK1 receptor antagonism does not exert consistent analgesic activity in humans, but similar studies have not been carried out against pain of GI origin, where NK1 receptors may have additional influences on mucosal inflammatory or "irritant" processes. NK3 receptors mediate certain disruptions of intestinal motility. The activity may be driven by tachykinins released from intrinsic primary afferent neurones (IPANs), which induce slow EPSP activity in connecting IPANs and hence, a degree of hypersensitivity within the enteric nervous system. The same process is also proposed to increase C-fibre sensitivity, either indirectly or directly. Thus, NK3 receptor antagonists inhibit intestinal nociception via a "peripheral" mechanism that may be intestine-specific. Studies with talnetant and other selective NK3 receptor antagonists are, therefore, revealing an exciting and novel pathway by which pathological changes in intestinal motility and nociception can be induced, suggesting a role for NK3 receptor antagonism in irritable bowel syndrome.

Cyclooxygenase-2 contributes to dysmotility and enhanced excitability of myenteric AH neurones in the inflamed guinea pig distal colon

We have previously demonstrated that trinitrobenzene sulphonic acid (TNBS)-induced colitis in guinea pig is associated with hyperexcitability of myenteric AH neurones, enhanced synaptic activity in the myenteric plexus, increased serotonin (5-HT) availability in the mucosa, and decreased propulsive motor activity. The current study tested the hypothesis that the activation of cyclooxygenase (COX) contributes to these alterations in bowel functions. DFU inhibition of COX-2, but not SC-560 inhibition of COX-1, restored to normal levels the electrical properties of myenteric AH neurones, the proportion of S neurones exhibiting slow EPSPs, and the rate of propulsive motor activity. Neither inhibitor was effective in altering the level of inflammation, the increased availability of mucosal 5-HT, or the enhanced fast EPSPs in myenteric AH and S neurones. COX-2 expression is enhanced in the myenteric plexus and cells within the smooth muscle layers during colitis, possibly reflecting the site at which COX-2 inhibition acts to allow recovery of motor function. In support of this concept, COX-1, but not COX-2, inhibition was effective in restoring normal mucosal prostaglandin levels. These results indicate that the various changes that occur in the motor neural pathways of the distal colon in TNBS-induced colitis do not involve a single neuroimmune mechanism. COX-2 activation is a critical step in the enhanced excitability of AH neurones as well as diminished propulsive motility in TNBS colitis, whereas other yet to be resolved pathways, that do not involve COX-1 or COX-2 activation, lead to altered 5-HT content in the mucosa and an augmentation of fast EPSPs.

Neurokinin-1 and -3 receptor blockade inhibits slow excitatory synaptic transmission in myenteric neurons and reveals slow inhibitory input

Recent studies have shown that tachykinins mediate slow synaptic transmission to myenteric AH (afterhyperpolarising) neurons via neurokinin-3 receptors (NK(3)R). This study investigated a similar role for neurokinin-1 receptors (NK(1)R) and compared the effect of selective receptor antagonists on non-cholinergic slow excitatory post-synaptic potentials (EPSPs) recorded in myenteric AH neurons of the guinea-pig ileum. Slow EPSPs evoked by electrical stimulation of circumferentially oriented presynaptic nerves were mimicked by application of senktide, an NK(3)R agonist. [Sar(9),Met(O(2))(11)]-substance P, an NK(1)R agonist, depolarised a smaller number of neurons. SR142801, a selective NK(3)R antagonist (100 nM), inhibited slow EPSPs and responses to senktide, but had no effect on depolarisations evoked by forskolin, an activator of adenylate cyclase. SR140333, a selective NK(1)R antagonist, inhibited slow EPSPs in a subset of neurons and blocked responses to [Sar(9),Met(O(2))(11)]-substance P, but not to senktide or forskolin. Slow EPSPs that were predominantly mediated by NK(1)R had significantly shorter latencies than those due to activation of NK(3)R. After blockade of slow EPSPs, slow hyperpolarizing responses to presynaptic nerve stimulation were revealed in one-third of neurons. These events, which were associated with a decrease in input resistance and blocked by tetrodotoxin, were equated with slow inhibitory postsynaptic potentials. They were abolished by the 5-hydroxytryptamine(1A) receptor antagonist 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]-piperazine (NAN-190), but unaffected by phentolamine, an alpha-adrenoceptor antagonist. In conclusion, these results provide the first direct evidence that NK(1)R mediate some slow excitatory synaptic input to myenteric AH neurons, and suggest that NK(1)R and NK(3)R activate distinct signal transduction pathways. These results also demonstrate that slow inhibitory synaptic transmission, which may be mediated by 5-hydroxytryptamine, is more prevalent in the myenteric plexus than previously indicated.