Neural components of distension-evoked secretory responses in the guinea-pig distal colon

The recruitment of mechanosensitive enteric neurons in the guinea pig gastric fundus is dependent on ganglionic stretch level

BACKGROUND

Serving as a reservoir, the gastric fundus can expand significantly, with an initial receptive and a following adaptive relaxation, controlled by extrinsic and intrinsic reflex circuits, respectively. We hypothesize that mechanosensitive enteric neurons (MEN) are involved in the adaptive relaxation, which is initiated when a particular gastric volume and a certain stretch of the stomach wall is reached. To investigate whether the responsiveness of MEN in the gastric fundus is dependent on tissue stretch, we performed mechanical stimulations in stretched versus ganglia "at rest".

METHODS

Responses of myenteric neurons in the guinea pig gastric fundus were recorded with membrane potential imaging using Di-8-ANEPPS. MEN were identified by small-volume intraganglionic injection in ganglia stretched to different degrees using a self-constructed stretching tool. Immunohistochemical staining identified the neurochemical phenotype of MEN. Hexamethonium and capsaicin were added to test their effect on recruited MEN.

KEY RESULTS

In stretched compared to "at rest" ganglia, significantly more MEN were activated. The change in the ganglionic area correlated significantly with the number of additional recruited MEN. The additional recruitment of MEN was independent from nicotinic transmission and the ratio of active MEN in stretched ganglia shifted towards a nitrergic phenotype.

CONCLUSION AND INFERENCES

The higher number of active MEN with increasing stretch of the ganglia and their greater share of nitrergic phenotype might indicate their contribution to the adaptive relaxation. Further experiments are necessary to address the receptors involved in mechanotransduction.

Intestinal barrier function in the naked mole-rat: an emergent model for gastrointestinal insights

The intestinal barrier plays a crucial role in homeostasis by both facilitating the absorption of nutrients and fluids and providing a tight shield to prevent the invasion by either pathogen or commensal microorganisms. Intestinal barrier malfunction is associated with systemic inflammation, oxidative stress, and decreased insulin sensitivity, which may lead to the dysregulation of other tissues. Therefore, a deeper understanding of physiological aspects related to an enhanced barrier function is of significant scientific and clinical relevance. The naked mole-rat has many unusual biological features, including attenuated colonic neuron sensitivity to acid and bradykinin and resistance to chemical-induced intestinal damage. However, insight into their intestinal barrier physiology is scarce. Here, we observed notable macroscopic and microscopic differences in intestinal tissue structure between naked mole-rats and mice. Moreover, naked mole-rats showed increased number of larger goblet cells and elevated mucus content. In measuring gut permeability, naked mole-rats showed reduced permeability compared with mice, measured as transepithelial electrical resistance, especially in ileum. Furthermore, intestinal ion secretion induced by serotonin, bradykinin, histamine, and capsaicin was significantly reduced in naked mole-rats compared with mice, despite the expression of receptors for all these agonists. In addition, naked mole-rats exhibited reduced prosecretory responses to the nonselective adenylate cyclase activator forskolin. Collectively, these findings indicate that naked mole-rats possess a robust and hard-to-penetrate gastrointestinal barrier that is resistant to environmental and endogenous irritants. Naked mole-rats may therefore provide valuable insights into the physiology of the intestinal barrier and set the stage for the development of innovative and effective therapies.NEW & NOTEWORTHY This is the first study to characterize the intestinal function of naked mole-rats. We found that these animals show a robust gut tissue structure, displaying thicker intestinal layers, longer villi, and larger crypts. Naked mole-rats showed more and larger goblet cells, with increased mucus content. Intestinal permeability, especially in the ileum, was substantially lower than that of mice. Finally, naked mole-rats showed reduced intestinal anion secretion in response to serotonin, bradykinin, histamine, capsaicin, and forskolin.

Mechanosensitive enteric neurons in the guinea pig gastric fundus and antrum

BACKGROUND

Coping with the ingested food, the gastric regions of fundus, corpus, and antrum display different motility patterns. Intrinsic components of such patterns involving mechanosensitive enteric neurons (MEN) have been described in the guinea pig gastric corpus but are poorly understood in the fundus and antrum.

METHODS

To elucidate mechanosensitive properties of myenteric neurons in the gastric fundus and antrum, membrane potential imaging using Di-8-ANEPPS was applied. A small-volume injection led to neuronal compression. We analyzed the number of MEN and their firing frequency in addition to the involvement of selected mechanoreceptors. To characterize the neurochemical phenotype of MEN, we performed immunohistochemistry.

KEY RESULTS

In the gastric fundus, 16% of the neurons reproducibly responded to mechanical stimulation and thus were MEN. Of those, 83% were cholinergic and 19% nitrergic. In the antrum, 6% of the neurons responded to the compression stimulus, equally distributed among cholinergic and nitrergic MEN. Defunctionalizing the sensory extrinsic afferents led to a significant drop in the number of MEN in both regions.

CONCLUSION

We provided evidence for MEN in the gastric fundus and antrum and further investigated mechanoreceptors. However, the proportions of the chemical phenotypes of the MEN differed significantly between both regions. Further investigations of synaptic connections of MEN are crucial to understand the hardwired neuronal circuits in the stomach.

The enteric nervous system

Of all the organ systems in the body, the gastrointestinal tract is the most complicated in terms of the numbers of structures involved, each with different functions, and the numbers and types of signaling molecules utilized. The digestion of food and absorption of nutrients, electrolytes, and water occurs in a hostile luminal environment that contains a large and diverse microbiota. At the core of regulatory control of the digestive and defensive functions of the gastrointestinal tract is the enteric nervous system (ENS), a complex system of neurons and glia in the gut wall. In this review, we discuss 1) the intrinsic neural control of gut functions involved in digestion and 2) how the ENS interacts with the immune system, gut microbiota, and epithelium to maintain mucosal defense and barrier function. We highlight developments that have revolutionized our understanding of the physiology and pathophysiology of enteric neural control. These include a new understanding of the molecular architecture of the ENS, the organization and function of enteric motor circuits, and the roles of enteric glia. We explore the transduction of luminal stimuli by enteroendocrine cells, the regulation of intestinal barrier function by enteric neurons and glia, local immune control by the ENS, and the role of the gut microbiota in regulating the structure and function of the ENS. Multifunctional enteric neurons work together with enteric glial cells, macrophages, interstitial cells, and enteroendocrine cells integrating an array of signals to initiate outputs that are precisely regulated in space and time to control digestion and intestinal homeostasis.

Short- and Long-Term Effects of Cocaine on Enteric Neuronal Functions

Cocaine is one of the most consumed illegal drugs among (young) adults in the European Union and it exerts various acute and chronic negative effects on psychical and physical health. The central mechanism through which cocaine initially leads to improved performance, followed by addictive behavior, has already been intensively studied and includes effects on the homeostasis of the neurotransmitters dopamine, partly mediated via nicotinic acetylcholine receptors, and serotonin. However, effects on the peripheral nervous system, including the enteric nervous system, are much less understood, though a correlation between cocaine consumption and gastrointestinal symptoms has been reported. The aim of the present study was to gain more information on the effects of cocaine on enteric neuronal functions and the underlying mechanisms. For this purpose, functional experiments using an organ bath, Ussing chamber and neuroimaging techniques were conducted on gastrointestinal tissues from guinea pigs. Key results obtained are that cocaine (1) exhibits a stimulating, non-neuronal effect on gastric antrum motility, (2) acutely (but not chronically) diminishes responses of primary cultured enteric neurons to nicotinic and serotonergic stimulation and (3) reversibly attenuates neuronal-mediated intestinal mucosal secretion. It can be concluded that cocaine, among its central effects, also alters enteric neuronal functions, providing potential explanations for the coexistence of cocaine abuse and gastrointestinal complaints.

Distension evoked mucosal secretion in human and porcine colon in vitro

It was suggested that intestinal mucosal secretion is enhanced during muscle relaxation and contraction. Mechanisms of mechanically induced secretion have been studied in rodent species. We used voltage clamp Ussing technique to investigate, in human and porcine colonic tissue, secretion evoked by serosal (Pser) or mucosal (Pmuc) pressure application (2-60 mmHg) to induce distension into the mucosal or serosal compartment, respectively. In both species, Pser or Pmuc caused secretion due to Cl- and, in human colon, also HCO3- fluxes. In the human colon, responses were larger in proximal than distal regions. In porcine colon, Pmuc evoked larger responses than Pser whereas the opposite was the case in human colon. In both species, piroxicam revealed a strong prostaglandin (PG) dependent component. Pser and Pmuc induced secretion was tetrodotoxin (TTX) sensitive in porcine colon. In human colon, a TTX sensitive component was only revealed after piroxicam. However, synaptic blockade by ω-conotoxin GVIA reduced the response to mechanical stimuli. Secretion was induced by tensile rather than compressive forces as preventing distension by a filter inhibited the secretion. In conclusion, in both species, distension induced secretion was predominantly mediated by PGs and a rather small nerve dependent response involving mechanosensitive somata and synapses.

Stimulation of extrinsic sympathetic nerves differentially affects neurogenic motor activity in guinea pig distal colon

The speed of pellet propulsion through the isolated guinea pig distal colon in vitro significantly exceeds in vivo measurements, suggesting a role for inhibitory mechanisms from sources outside the gut. The aim of this study was to investigate the effects of sympathetic nerve stimulation on three different neurogenic motor behaviors of the distal colon: transient neural events (TNEs), colonic motor complexes (CMCs), and pellet propulsion. To do this, segments of guinea pig distal colon with intact connections to the inferior mesenteric ganglion (IMG) were set up in organ baths allowing for simultaneous extracellular suction electrode recordings from smooth muscle, video recordings for diameter mapping, and intraluminal manometry. Electrical stimulation (1-20 Hz) of colonic nerves surrounding the inferior mesenteric artery caused a statistically significant, frequency-dependent inhibition of TNEs, as well as single pellet propulsion, from frequencies of 5 Hz and greater. Significant inhibition of CMCs required stimulation frequencies of 10 Hz and greater. Phentolamine (3.6 μM) abolished effects of colonic nerve stimulation, consistent with a sympathetic noradrenergic mechanism. Sympathetic inhibition was constrained to regions with intact extrinsic nerve pathways, allowing normal motor behaviors to continue without modulation in adjacent extrinsically denervated regions of the same colonic segments. The results demonstrate differential sensitivities to sympathetic input among distinct neurogenic motor behaviors of the colon. Together with findings indicating CMCs activate colo-colonic sympathetic reflexes through the IMG, these results raise the possibility that CMCs may paradoxically facilitate suppression of pellet movement in vivo, through peripheral sympathetic reflex circuits.

Mechanosensitive Enteric Neurons (MEN) at Work

In the last decade, we characterized an enteric neuronal subpopulation of multifunctional mechanosensitive enteric neurons (MEN) while studying the gastrointestinal peristalsis. MEN have been described in a variety of gastrointestinal regions and species. This chapter summarizes existing data on MEN, describing their proportions, firing behaviors, adaptation musters, and chemical phenotypes. We also discuss MEN sensitivity to different mechanical stimulus qualities such as compression and tension along with pharmacology of their responses.

Cinnamaldehyde Induces Release of Cholecystokinin and Glucagon-Like Peptide 1 by Interacting with Transient Receptor Potential Ankyrin 1 in a Porcine Ex-Vivo Intestinal Segment Model

Cinnamaldehyde and capsaicin have been reported to exert effects on the gastric function, mediated by the interaction with transient receptor potential ankyrin channel 1 (TRPA1) and transient receptor potential vanilloid channel 1 (TRPV1), respectively. This study examined whether these compounds could trigger the release of cholecystokinin (CCK) and/or glucagon-like peptide 1 (GLP-1) in the pig's gut in a porcine ex-vivo intestinal segment model. Furthermore, it was verified whether this response was mediated by TRPA1 or TRPV1 by using the channel's antagonist. These gut peptides play a key role in the "intestinal brake", a feedback mechanism that influences the function of proximal parts of the gut. Structural analogues of cinnamaldehyde were screened as well, to explore structure-dependent activation. Results showed a significant effect of capsaicin on GLP-1 release in the proximal small intestine, TRPV1 independent. TRPA1 showed to be strongly activated by cinnamaldehyde, both in proximal and distal small intestine, evidenced by the release of CCK and GLP-1, respectively. Out of all structural derivates, cinnamaldehyde showed the highest affinity for TRPA1, which elucidates the importance of the α,β-unsaturated aldehyde moiety. In conclusion, cinnamaldehyde as a TRPA1 agonist, is a promising candidate to modulate gastric function, by activating intestinal brake mechanisms.

Functional circuits and signal processing in the enteric nervous system

The enteric nervous system (ENS) is an extensive network comprising millions of neurons and glial cells contained within the wall of the gastrointestinal tract. The major functions of the ENS that have been most studied include the regulation of local gut motility, secretion, and blood flow. Other areas that have been gaining increased attention include its interaction with the immune system, with the gut microbiota and its involvement in the gut-brain axis, and neuro-epithelial interactions. Thus, the enteric circuitry plays a central role in intestinal homeostasis, and this becomes particularly evident when there are faults in its wiring such as in neurodevelopmental or neurodegenerative disorders. In this review, we first focus on the current knowledge on the cellular composition of enteric circuits. We then further discuss how enteric circuits detect and process external information, how these signals may be modulated by physiological and pathophysiological factors, and finally, how outputs are generated for integrated gut function.

Compression and stretch sensitive submucosal neurons of the porcine and human colon

The pig is commonly believed to be a relevant model for human gut functions-however, there are only a few comparative studies and none on neural control mechanisms. To address this lack we identified as one central aspect mechanosensitive enteric neurons (MEN) in porcine and human colon. We used neuroimaging techniques to record responses to tensile or compressive forces in submucous neurons. Compression and stretch caused Ca-transients and immediate spike discharge in 5-11% of porcine and 15-24% of human enteric neurons. The majority of these MEN exclusively responded to either stimulus quality but about 9% responded to both. Most of the MEN expressed choline acetyltransferase and substance P; nitric oxide synthase-positive MEN primarily occurred in distal colon. The findings reveal common features of MEN in human and pig colon which we interpret as a result of species-independent evolutionary conservation rather than a specific functional proximity between the two species.

Xenin-25 induces anion secretion by activating noncholinergic secretomotor neurons in the rat ileum

Xenin-25 is a neurotensin-like peptide that is secreted by enteroendocrine cells in the small intestine. Xenin-8 is reported to augment duodenal anion secretion by activating afferent neural pathways. The intrinsic neuronal circuits mediating the xenin-25-induced anion secretion were characterized using the Ussing-chambered, mucosa-submucosa preparation from the rat ileum. Serosal application of xenin-25 increased the short-circuit current in a concentration-dependent manner. The responses were abolished by the combination of Cl^--free and HCO3- -free solutions. The responses were almost completely blocked by TTX (10^-6 M) but not by atropine (10^-5 M) or hexamethonium (10^-4 M). The selective antagonists for neurotensin receptor 1 (NTSR1), neurokinin 1 (NK1), vasoactive intestinal polypeptide (VIP) receptors 1 and 2 (VPAC1 and VPAC2, respectively), and capsaicin, but not 5-hydroxyltryptamine receptors 3 and 4 (5-HT₃ and 5-HT₄), NTSR2, and A803467, inhibited the responses to xenin-25. The expression of VIP receptors (Vipr) in rat ileum was examined using RT-PCR. The Vipr1 PCR products were detected in the submucosal plexus and mucosa. Immunohistochemical staining showed the colocalization of NTSR1 and NK1 with substance P (SP)- and calbindin-immunoreactive neurons in the submucosal plexus, respectively. In addition, NK1 was colocalized with noncholinergic VIP secretomotor neurons. Based on the results from the present study, xenin-25-induced Cl^-/ HCO3- secretion is involved in NTSR1 activation on intrinsic and extrinsic afferent neurons, followed by the release of SP and subsequent activation of NK1 expressed on noncholinergic VIP secretomotor neurons. Finally, the secreted VIP may activate VPAC1 on epithelial cells to induce Cl^-/ HCO3- secretion in the rat ileum. Activation of noncholinergic VIP secretomotor neurons by intrinsic primary afferent neurons and extrinsic afferent neurons by postprandially released xenin-25 may account for most of the neurogenic secretory response induced by xenin-25. NEW & NOTEWORTHY This study is the first to investigate the intrinsic neuronal circuit responsible for xenin-25-induced anion secretion in the rat small intestine. We have found that nutrient-stimulated xenin-25 release may activate noncholinergic vasoactive intestinal polypeptide (VIP) secretomotor neurons to promote Cl^-/ HCO3- secretion through the activation of VIP receptor 1 on epithelial cells. Moreover, the xenin-25-induced secretory responses are mainly linked with intrinsic primary afferent neurons, which are involved in the activation of neurotensin receptor 1 and neurokinin 1 receptor.

Unexpected Roles for the Second Brain: Enteric Nervous System as Master Regulator of Bowel Function

At the most fundamental level, the bowel facilitates absorption of small molecules, regulates fluid and electrolyte flux, and eliminates waste. To successfully coordinate this complex array of functions, the bowel relies on the enteric nervous system (ENS), an intricate network of more than 500 million neurons and supporting glia that are organized into distinct layers or plexi within the bowel wall. Neuron and glial diversity, as well as neurotransmitter and receptor expression in the ENS, resembles that of the central nervous system. The most carefully studied ENS functions include control of bowel motility, epithelial secretion, and blood flow, but the ENS also interacts with enteroendocrine cells, influences epithelial proliferation and repair, modulates the intestinal immune system, and mediates extrinsic nerve input. Here, we review the many different cell types that communicate with the ENS, integrating data about ENS function into a broader view of human health and disease. In particular, we focus on exciting new literature highlighting relationships between the ENS and its lesser-known interacting partners.

Piezo proteins: incidence and abundance in the enteric nervous system. Is there a link with mechanosensitivity?

Piezo channels play fundamental roles in many physiological processes. Their presence and functional role in the enteric nervous system is still not known. We hypothesize that they play a role in mechanotransduction in enteric neurons. Our aims are to quantify the presence of both Piezo1 and 2 in enteric neurons throughout the gastrointestinal tract using immunohistochemistry and analyze their function(s) using neuroimaging techniques and pharmacological investigations. In order to perform a systematic and comparative study, we performed our experiments in gastrointestinal tissue from guinea pigs, mice and humans. Piezo1 (20-70%) is expressed by both enteric neuronal cell bodies and fibers in the myenteric and submucosal plexi of all the species investigated. Generally, Piezo1 expressing somata are more numerous in the submucosal plexus (50-80%) than in the myenteric plexus (15-35%) apart from the stomach where Piezo1 is expressed in up to 60% of cell bodies. Myenteric Piezo1 neurons mainly (60-100%) but not exclusively, also express nitric oxide synthase, a minority express choline acetyltransferase. In the submucosal plexus, Piezo1 neurons co-express vasoactive intestinal peptide (40-90%). Conversely, expression of Piezo2 is extremely rare in the somata of enteric neurons and is present in few neurites. In functional experiments, 38-76% of the mechanosensitive neurons expressed Piezo1 channels. Statistical analysis showed a positive significant correlation between mechanosensitive and Piezo1 positive neurons. However, pharmacological experiments using an activator and an inhibitor of Piezo channels did not demonstrate changes in mechanotransduction. A major role of Piezo1 in the mechanosensitivity of enteric neurons can be excluded.

Enteric glia regulate gut motility in health and disease

The enteric nervous system, often referred to as the second brain, is the largest assembly of neurons and glia outside the central nervous system. The enteric nervous system resides within the wall of the digestive tract and regulates local gut reflexes involved in gastrointestinal motility and fluid transport; these functions can be accomplished in the absence of the extrinsic innervation from the central nervous system. It is neurons and their circuitry within the enteric nervous system that govern the gut reflexes. However, it is becoming clear that enteric glial cells are also actively involved in this process through the bidirectional signaling with neurons and other cells in the gut wall. We synthesize the recently discovered modulatory roles of enteric gliotransmission in gut motility and provide our perspective for future lines of research.

Changes in Enteric Neurons of Small Intestine in a Rat Model of Irritable Bowel Syndrome with Diarrhea

Background/Aims

Physical and/or emotional stresses are important factors in the exacerbation of symptoms in irritable bowel syndrome (IBS). Several lines of evidence support that a major impact of stress on the gastrointestinal tract occurs via the enteric nervous system. We aimed to evaluate histological changes in the submucosal plexus (SMP) and myenteric plexus (MP) of the distal ileum in concert with the intestinal motor function in a rat model of IBS with diarrhea.

Methods

The rat model was induced by heterotypic chronic and acute stress (CAS). The intestinal transit was measured by administering powdered carbon by gastric gavage. Double immunohistochemical fluorescence staining with whole-mount preparations of SMP and MP of enteric nervous system was used to assess changes in expression of choline acetyltransferase, vasoactive intestinal peptide, or nitric oxide synthase in relation to the pan neuronal marker, anti-Hu.

Results

The intestinal transit ratio increased significantly from control values of 50.8% to 60.6% in the CAS group. The numbers of enteric ganglia and neurons in the SMP were increased in the CAS group. The proportions of choline acetyltransferase- and vasoactive intestinal peptide-immunoreactive neurons in the SMP were increased (82.1 ± 4.3% vs. 76.0 ± 5.0%, P = 0.021; 40.5 ± 5.9% vs 28.9 ± 3.7%, P = 0.001), while nitric oxide synthase-immunoreactive neurons in the MP were decreased compared with controls (23.3 ± 4.5% vs 32.4 ± 4.5%, P = 0.002).

Conclusions

These morphological changes in enteric neurons to CAS might contribute to the dysfunction in motility and secretion in IBS with diarrhea.

Chronic infection with Toxoplasma gondii induces death of submucosal enteric neurons and damage in the colonic mucosa of rats

Intestinal epithelial secretion is coordinated by the submucosal plexus (SMP). Chemical mediators from SMP regulate the immunobiological response and direct actions against infectious agents. Toxoplasma gondii is a worldwide parasite that causes toxoplasmosis. This study aimed to determine the effects of chronic infection with T. gondii on the morphometry of the mucosa and the submucosal enteric neurons in the proximal colon of rats. Male adult rats were distributed into a control group (n = 10) and an infected group (n = 10). Infected rats received orally 500 oocysts of T. gondii (ME-49). After 36 days, the rats were euthanized and samples of the proximal colon were processed for histology to evaluate mucosal thickness in sections. Whole mounts were stained with methylene blue and subjected to immunohistochemistry to detect vasoactive intestinal polypeptide. The total number of submucosal neurons decreased by 16.20%. Vasoactive intestinal polypeptide-immunoreactive neurons increased by 26.95%. Intraepithelial lymphocytes increased by 62.86% and sulfomucin-producing goblet cells decreased by 22.87%. Crypt depth was greater by 43.02%. It was concluded that chronic infection with T. gondii induced death and hypertrophy in the remaining submucosal enteric neurons and damage to the colonic mucosa of rats.

Mechanosensitive enteric neurons in the guinea pig gastric corpus

For long it was believed that a particular population of enteric neurons, referred to as intrinsic primary afferent neuron (IPAN)s, encodes mechanical stimulation. We recently proposed a new concept suggesting that there are in addition mechanosensitive enteric neurons (MEN) that are multifunctional. Based on firing pattern MEN behaved as rapidly, slowly, or ultra-slowly adapting RAMEN, SAMEN, or USAMEN, respectively. We aimed to validate this concept in the myenteric plexus of the gastric corpus, a region where IPANs were not identified and existence of enteric sensory neurons was even questioned. The gastric corpus is characterized by a particularly dense extrinsic sensory innervation. Neuronal activity was recorded with voltage sensitive dye imaging after deformation of ganglia by compression (intraganglionic volume injection or von Fry hair) or tension (ganglionic stretch). We demonstrated that 27% of the gastric neurons were MEN and responded to intraganglionic volume injection. Of these 73% were RAMEN, 25% SAMEN, and 2% USAMEN with a firing frequency of 1.7 (1.1/2.2), 5.1 (2.2/7.7), and of 5.4 (5.0/15.5) Hz, respectively. The responses were reproducible and stronger with increased stimulus strength. Even after adaptation another deformation evoked spike discharge again suggesting a resetting mode of the mechanoreceptors. All MEN received fast synaptic input. Fifty five percent of all MEN were cholinergic and 45% nitrergic. Responses in some MEN significantly decreased after perfusion of TTX, low Ca(++)/high Mg(++) Krebs solution, capsaicin induced nerve defunctionalization and capsazepine indicating the involvement of TRPV1 expressing extrinsic mechanosensitive nerves. Half of gastric MEN responded to intraganglionic volume injection as well as to ganglionic stretch and 23% responded to stretch only. Tension-sensitive MEN were to a large proportion USAMEN (44%). In summary, we demonstrated for the first time compression and tension-sensitive MEN in the stomach; many of them responded to one stimulus modality only. Their proportions and the basic properties were similar to MEN previously identified by us in other intestinal region and species. Unlike in the intestine, the responsiveness of some gastric MEN is enhanced by extrinsic TRPV1 expressing visceral afferents.

Mechanical stress activates neurites and somata of myenteric neurons

The particular location of myenteric neurons, sandwiched between the 2 muscle layers of the gut, implies that their somata and neurites undergo mechanical stress during gastrointestinal motility. Existence of mechanosensitive enteric neurons (MEN) is undoubted but many of their basic features remain to be studied. In this study, we used ultra-fast neuroimaging to record activity of primary cultured myenteric neurons of guinea pig and human intestine after von Frey hair evoked deformation of neurites and somata. Independent component analysis was applied to reconstruct neuronal morphology and follow neuronal signals. Of the cultured neurons 45% (114 out of 256, 30 guinea pigs) responded to neurite probing with a burst spike frequency of 13.4 Hz. Action potentials generated at the stimulation site invaded the soma and other neurites. Mechanosensitive sites were expressed across large areas of neurites. Many mechanosensitive neurites appeared to have afferent and efferent functions as those that responded to deformation also conducted spikes coming from the soma. Mechanosensitive neurites were also activated by nicotine application. This supported the concept of multifunctional MEN. 14% of the neurons (13 out of 96, 18 guinea pigs) responded to soma deformation with burst spike discharge of 17.9 Hz. Firing of MEN adapted rapidly (RAMEN), slowly (SAMEN), or ultra-slowly (USAMEN). The majority of MEN showed SAMEN behavior although significantly more RAMEN occurred after neurite probing. Cultured myenteric neurons from human intestine had similar properties. Compared to MEN, dorsal root ganglion neurons were activated by neurite but not by soma deformation with slow adaptation of firing. We demonstrated that MEN exhibit specific features very likely reflecting adaptation to their specialized functions in the gut.

Properties of submucosal venules in the rat distal colon

BACKGROUND AND PURPOSE

Venules within the gut wall may have intrinsic mechanisms for maintaining the circulation even upon the intestinal wall distension. We aimed to explore spontaneous and nerve-mediated contractile activity of colonic venules.

EXPERIMENTAL APPROACH

Changes in the diameter of submucosal venules of the rat distal colon were measured using video microscopy. The innervation of the microvasculature was investigated using fluorescence immunohistochemistry.

KEY RESULTS

Submucosal venules exhibited spontaneous constrictions that were abolished by blockers of L-type Ca(2+) channels (1 μM nicardipine), Ca(2+)-ATPase (10 μM cyclopiazonic acid), IP3 receptor (100 μM 2-APB), Ca(2+)-activated Cl(-) channels (100 μM DIDS) or store-operated Ca(2+) entry channels (10 μM SKF96365). Transmural nerve stimulation (TNS at 10 Hz) induced a phasic venular constriction that was blocked by phentolamine (1 μM, α-adrenoceptor antagonist) or sympathetic nerve depletion using guanethidine (10 μM). Stimulation of primary afferent nerves with TNS (at 20 Hz) or capsaicin (100 nM) evoked a sustained venular dilatation that was attenuated by calcitonin gene-related peptide (CGRP) 8-37 (2 μM), a CGRP receptor antagonist. Immunohistochemistry revealed sympathetic and primary afferent nerves running along submucosal venules.

CONCLUSIONS AND IMPLICATIONS

Submucosal venules of the rat distal colon exhibit spontaneous constrictions that appear to primarily rely on Ca(2+) release from sarcoplasmic reticulum and subsequent opening of Ca(2+)-activated Cl(-) channels that trigger Ca(2+) influx through L-type Ca(2+) channels. Venular contractility is modulated by sympathetic as well as CGRP-containing primary afferent nerves, suggesting that submucosal venules may play an active role in regulating the microcirculation of the digestive tract.

Substance P as a neuronal factor in the enteric nervous system of the porcine descending colon in physiological conditions and during selected pathogenic processes

The present investigation pertains to changes in substance P-like immunoreactive (SP-LI) nerve structures of the enteric nervous system (ENS) in the porcine descending colon, caused by chemically-induced inflammation and nerve injury (axotomy). The distribution pattern of SP-LI structures was studied using the double immunofluorescence technique in the myenteric (MP), outer submucous (OSP) and inner submucous (ISP) plexuses, as well as in the circular muscle and mucosal layers. Under physiological conditions, SP-LI neurons have been shown to constitute 4.13 ± 0.24%, 3.36 ± 0.26%, and 7.92 ± 0.16% in the MP, OSP, and ISP, respectively. Changes in SP-immunoreactivity depended on the pathological factor studied. The numbers of the SP-LI perikarya amounted to 7.89 ± 0.34, 5.56 ± 0.30, and 19.96 ± 0.57 in chemically-induced colitis, and 4.28 ± 0.13%, 7.18 ± 20%, and 11.62 ± 0.48% after axotomy in MP, OSP, and ISP, respectively. The both studied processes generally resulted in an increase in the number of SP-LI nerve fibers in the circular muscle and mucosal layers. The obtained results suggest that SP-LI nerve structures of the ENS may participate in various pathological processes in the porcine descending colon and exact functions of SP probably depend on the type of the pathological factor.

Neurochemistry of myenteric plexus neurons of bank vole (Myodes glareolus) ileum

The neurochemistry of enteric neurons differs among species of small laboratory rodents (guinea-pig, mouse, rat). In this study we characterized the phenotype of ileal myenteric plexus (MP) neuronal cells and fibers of the bank vole (Myodes glareolus), a common rodent living in Europe and in Northern Asia which is also employed in prion experimental transmission studies. Six neuronal markers were tested: choline acetyltransferase (ChAT), neuronal nitric oxide synthase (nNOS), calbindin (CALB), calcitonin gene-related peptide (CGRP) and substance P (SP), along with HuC/D as a pan-neuronal marker. Neurons expressing ChAT- and nNOS-immunoreactivity (IR) were 36 ± 12% and 24 ± 5%, respectively. Those expressing CGRP-, SP- and CALB-IR were 3 ± 3%, 21 ± 5% and 6 ± 2%, respectively. Therefore, bank vole MPs differ consistently from murine MPs in neurons expressing CGRP-, SP- and CALB-IR. These data may contribute to define the prion susceptibility of neuron cell populations residing within ileal MPs from bank voles, along with their morpho-functional alterations following oral experimental prion challenge.

Effect of hyoscine butylbromide (Buscopan®) on cholinergic pathways in the human intestine

BACKGROUND

Hyoscine butylbromide (HBB, Buscopan(®) ) is clinically used to treat intestinal cramps and visceral pain. Various studies, mainly on animal tissues, suggested that its antimuscarinic action is responsible for its spasmolytic effect. However, functional in vitro studies with human tissue have not been performed so far.

METHODS

We wanted to provide a comprehensive study on the mode of action of HBB in human intestinal samples and investigated HBB (1 nmol L(-1) -10 μmol L(-1)) effects on muscle activity with isometric force transducers and calcium imaging, on epithelial secretion with Ussing chamber technique and on enteric neurons using fast neuroimaging.

KEY RESULTS

Hyoscine butylbromide concentration dependently reduced muscle contractions, calcium mobilization, and epithelial secretion induced by the muscarinic agonist bethanechol with IC50 values of 429, 121, and 224 nmol L(-1), respectively. Forskolin-induced secretion was not altered by HBB. Cholinergic muscarinic muscle and epithelial responses evoked by electrical nerve stimulation were inhibited by 1-10 μmol L(-1) HBB. Moreover, HBB significantly reduced the bethanechol-induced action potential discharge in enteric neurons. Interestingly, we observed that high concentrations of HBB (10 μmol L(-1)) moderately decreased nicotinic receptor-mediated secretion, motility, and nerve activity.

CONCLUSIONS & INFERENCES

The results demonstrated the strong antimuscarinic action of HBB whereas the nicotinic antagonism at higher concentrations plays at most a moderate modulatory role. The muscle relaxing effect of HBB and its inhibition of muscarinic nerve activation likely explain its clinical use as an antispasmodic drug. Our results further highlight a so far unknown antisecretory action of HBB which warrants further clinical studies on its use in secretory disorders.

Intraepithelial lymphocytes, goblet cells and VIP-IR submucosal neurons of jejunum rats infected with Toxoplasma gondii

Toxoplasma gondii (T. gondii) crosses the intestinal barrier in oral infections and can lead to changes in different cell types, including the neurons located there. In the gastrointestinal system, the autonomous nervous system component that regulate blood flow and mucous secretion is the submucosal plexus. The aim of this study was to examine the effects of T. gondii infection on intraepithelial lymphocytes (IELs), goblet cells and submucosal neurons that are immunoreactive to vasoactive intestinal peptide (VIP-IR) of rat jejunum. Twenty male rats distributed as a control group (CG) and an infected group (IG), which received a suspension with 500 parasite oocysts (strain ME-49, genotype II) orally, were assessed. Routine histological sections were used to quantify IELs and to detect mucins secreted by goblet cells. Whole mounts including the submucosal layer were examined using immunofluorescence to detect the VIP neurotransmitter. Quantitative alterations in IELs were not observed. However, the reduction (P < 0.05) in the number of goblet cells that produce neutral mucins (PAS+) and sulphomucins (AB pH 1.0) and the maintenance of sialomucin-secreting cells (AB pH 2.5) resulting in a more fluid mucous were observed. Concerning the VIP-IR submucosal neurons, an increase in fluorescence on IG animals was observed. There was a reduction (P < 0.05) in the number of VIP-IR submucosal neurons and atrophy of their cell bodies in IG rats. Infection with T. gondii caused alterations in the chemical composition of the intestinal mucous and reduction in the neuron number and atrophy of the remaining neurons in this cell subpopulation.

Transient receptor potential (TRP) channels as drug targets for diseases of the digestive system

Approximately 20 of the 30 mammalian transient receptor potential (TRP) channel subunits are expressed by specific neurons and cells within the alimentary canal. They subserve important roles in taste, chemesthesis, mechanosensation, pain and hyperalgesia and contribute to the regulation of gastrointestinal motility, absorptive and secretory processes, blood flow, and mucosal homeostasis. In a cellular perspective, TRP channels operate either as primary detectors of chemical and physical stimuli, as secondary transducers of ionotropic or metabotropic receptors, or as ion transport channels. The polymodal sensory function of TRPA1, TRPM5, TRPM8, TRPP2, TRPV1, TRPV3 and TRPV4 enables the digestive system to survey its physical and chemical environment, which is relevant to all processes of digestion. TRPV5 and TRPV6 as well as TRPM6 and TRPM7 contribute to the absorption of Ca²⁺ and Mg²⁺, respectively. TRPM7 participates in intestinal pacemaker activity, and TRPC4 transduces muscarinic acetylcholine receptor activation to smooth muscle contraction. Changes in TRP channel expression or function are associated with a variety of diseases/disorders of the digestive system, notably gastro-esophageal reflux disease, inflammatory bowel disease, pain and hyperalgesia in heartburn, functional dyspepsia and irritable bowel syndrome, cholera, hypomagnesemia with secondary hypocalcemia, infantile hypertrophic pyloric stenosis, esophageal, gastrointestinal and pancreatic cancer, and polycystic liver disease. These implications identify TRP channels as promising drug targets for the management of a number of gastrointestinal pathologies. As a result, major efforts are put into the development of selective TRP channel agonists and antagonists and the assessment of their therapeutic potential.

Activation of neuronal SST₁ and SST₂ receptors decreases neurogenic secretion in the guinea-pig jejunum

BACKGROUND

Vasoactive intestinal peptide (VIP) submucosal neurons, the main regulators of gut secretion, display inhibitory postsynaptic potentials mediated by somatostatin (SOM) acting on SST(1) and SST(2) receptors (SSTR(1), SSTR(2)) in the guinea-pig small intestine. We investigated the implications of this for neurally-evoked mucosal secretion.

METHODS

Mucosal-submucosal preparations from guinea-pig jejunum were mounted in Ussing chambers to measure Cl(-) secretion, measured by short circuit current (I(sc)). All drugs were added serosally. Veratridine (1 μmol L(-1)) was used to stimulate neurons and provide a robust secretory response for pharmacological testing.5-hydroxytrptamine (5-HT, 300 nmol L(-1)) was used to specifically activate non-cholinergic secretomotor neurons, while 1,1-dimethyl-4-phenylpiperazinium (DMPP, 10 μmol L(-1)) was used to stimulate all secretomotor neurons.

KEY RESULTS

Somatostatin (50 nmol L(-1)) induced a tetrodotoxin (TTX, 1 μmol L(-1))-sensitive decrease in secretion. Somatostatin also reduced the veratridine-induced increase in I(sc). The effects of SOM were significantly reduced by blocking SSTR(1) and SSTR(2) individually or together. Blocking SSTR(1) abolished the inhibition produced by SOM. Quantitative PCR demonstrated that SSTR(1) and SSTR(2) were much more highly expressed in the submucosa than the mucosa. Submucosal SSTR(1) expression was several fold higher than SSTR(2). Responses to DMPP (biphasic) and 5-HT (monophasic) were TTX-sensitive. Somatostatin significantly reduced the 5-HT-induced increase in I(sc), and the second, more sustained phase evoked by DMPP.

CONCLUSIONS & INFERENCES

These data suggest that SOM exerts its antisecretory effects by suppressing firing of VIP secretomotor neurons, rather than via a direct action on mucosal enterocytes.

Prostaglandin I(2) sensory input into the enteric nervous system during distension-induced colonic chloride secretion in rat colon

AIM

Intestinal pressure differences or experimental distension induce ion secretion via the enteric nervous system, the sensorial origin of which is only poorly understood. This study aimed to investigate sensorial inputs and the role of afferent and interneurones in mechanically activated submucosal secretory reflex circuits.

METHODS

Distension-induced rheogenic chloride secretion was measured as increase in short-circuit current 10 min after distension (DeltaI(SC)(10); distension parameters +/- 100 microL, 2 Hz, 20 s) in partially stripped rat distal colon in the Ussing-chamber in vitro. PGE(2) and PGI(2) were measured by radioimmunoassay.

RESULTS

DeltaI(SC)(10) was 2.0 +/- 0.2 micromol h(-1) cm(-2) and could be attenuated by lobeline, mecamylamine and dimethylphenylpiperazine, indicating an influence of nicotinergic interneurones. Additionally, a contribution of afferent neurones was indicated from the short-term potentiation of DeltaI(SC)(10) by capsaicin (1 microm). As evidence for its initial event, indomethacin (1 microm) inhibited distension-induced secretion and the release of PGI(2) was directly detected after distension. Furthermore, serotoninergic mediation was confirmed by granisetron (100 microm) which was functionally localized distally to PGI(2) in this reflex circuit, as granisetron inhibited an iloprost-induced I(SC), while indomethacin did not affect serotonin-activated ion secretion.

CONCLUSIONS

Distension-induced active electrogenic chloride secretion in rat colon is mediated by a neuronal reflex circuit which includes afferent neurones and nicotinergic interneurones. It is initiated by distension-induced PGI(2) release from subepithelial cells triggering this reflex via serotoninergic 5-HT(3) receptor transmission. Functionally, this mechanism may help to protect against intestinal stasis but could also contribute to luminal fluid loss, e.g. during intestinal obstruction.

Immunohistochemical characteristics of submucosal Dogiel type II neurons in rat colon

Secretory and motility reflexes are evoked by physiological stimuli in the isolated rat distal colon, which is therefore expected to contain intrinsic primary afferent (sensory) neurons. Dogiel type II neurons (putative intrinsic primary afferent neurons) exhibit several long processes emerging from large oval or round cell bodies. This study has examined the immunohistochemical characteristics of type II neurons in the submucosal plexus of rat distal colons by using whole-mount preparations. Neuronal cell bodies positive for both substance P (SP) and calretinin have been observed in colchicine-treated rats. Neurofilament 200 immunostaining has confirmed the type II morphology of SP-positive neurons. Moreover, all submucosal type II neurons identified by neurofilament 200 immunoreactivity are positive for calretinin. Calcitonin gene-related peptide (CGRP)-positive neurons in the submucosal plexus are distinct from type II neurons because they are negative for calretinin and have smaller cell bodies than the SP-positive submucosal type II neurons. Most (73%) of the submucosal neurons including type II neurons exhibit immunoreactivity for the neurokinin-1 receptor (NK1R), a receptor for SP, on the surface of cell bodies. Immunoreactivity for the EP3 receptor (EP3R), a receptor for prostaglandin E2, has been detected in 51% of submucosal neurons including type II neurons. Thus, submucosal type II neurons in the rat distal colon are immunopositive for SP/calretinin but immunonegative for CGRP. SP released from submucosal type II neurons probably acts via NK1Rs on type II and non-type II submucosal neurons to mediate intrinsic reflexes. EP3R-positive submucosal type II neurons may be potential targets of prostaglandin E2.

Multifunctional rapidly adapting mechanosensitive enteric neurons (RAMEN) in the myenteric plexus of the guinea pig ileum

An important feature of the enteric nervous system (ENS) is its capability to respond to mechanical stimulation which, as currently suggested for the guinea-pig ileum, is encoded by specialized intrinsic primary afferent neurons (IPANs). We used von Frey hairs or intraganglionic volume injections to mimic ganglion deformation as observed in freely contracting preparations. Using fast voltage-sensitive dye imaging we identified rapidly adapting mechanosensitive enteric neurons (RAMEN, 25% of all neurons) in the myenteric plexus of the guinea pig ileum. RAMEN responded with phasic spike discharge to dynamic changes during ganglion deformation. This response was reproducible and increased with increasing forces. Deformation-evoked spike discharge was not changed by synaptic blockade with hexamethonium, omega-conotoxin or low Ca(2+)/high Mg(2+), defunctionalization of extrinsic afferents with capsaicin or muscle paralysis with nifedipine, suggesting direct activation of RAMEN. All RAMEN received hexamethonium-sensitive fast EPSPs, which were blocked by omega-conotoxin and low Ca(2+)/high Mg(2+). Seventy-two per cent of RAMEN were cholinergic, 22% nitrergic, and 44% were calbindin and NeuN negative, markers used to identify IPANs. Mechanosensitivity was observed in 31% and 47% of retrogradely traced interneurons and motor neurons, respectively. RAMEN belong to a new population of mechanosensitive neurons which differ from IPANs. We provided evidence for multifunctionality of RAMEN which may fulfil sensory, integrative and motor functions. In light of previously identified mechanosensitive neuron populations, mechanosensitivity appears to be a property of many more enteric neurons than generally assumed. The findings call for a revision of current concepts on sensory transmission within the ENS.

Alterations to enteric neural signaling underlie secretory abnormalities of the ileum in experimental colitis in the guinea pig

Inflammatory bowel diseases (IBD) can involve widespread gastrointestinal dysfunction, even in cases in which inflammation is localized to a single site. The underlying pathophysiology of dysfunction in noninflamed regions is unclear. We examined whether colitis is associated with altered electrogenic ion transport in the ileal mucosa and/or changes in the properties of ileal submucosal neurons. Colitis was induced by administration of trinitrobenzene sulfonic acid (TNBS), and the uninflamed ileum from animals was examined 3, 7, and 28 days later. Electrogenic ion transport was assessed in Ussing chambers. Intracellular microelectrode recordings were used to examine the neurophysiology of the submucosal plexus of the ileum in animals with colitis. Noncholinergic secretion was reduced by 33% in the ileum from animals 7 days after the induction of colitis. The epithelial response to vasoactive intestinal peptide (VIP) was unaltered in animals with colitis, but the response to carbachol was enhanced. Slow excitatory synaptic transmission was dramatically reduced in VIP-expressing, noncholinergic secretomotor neurons. This change was detected as early as 3 days following TNBS treatment. No changes to fast synaptic transmission or the number of VIP neurons were observed. In addition, cholinergic secretomotor neurons fired more action potentials during a given stimulus, and intrinsic primary afferent neurons had broader action potentials in animals with colitis. These findings implicate changes to enteric neural circuits as contributing factors in inflammation-induced secretory dysfunction at sites proximal to a localized inflammatory insult.

Purinergic receptors and gastrointestinal secretomotor function

Secretomotor reflexes in the gastrointestinal (GI) tract are important in the lubrication and movement of digested products, absorption of nutrients, or the diarrhea that occurs in diseases to flush out unwanted microbes. Mechanical or chemical stimulation of mucosal sensory enterochromaffin (EC) cells triggers release of serotonin (5-HT) (among other mediators) and initiates local reflexes by activating intrinsic primary afferent neurons of the submucous plexus. Signals are conveyed to interneurons or secretomotor neurons to stimulate chloride and fluid secretion. Inputs from myenteric neurons modulate secretory rates and reflexes, and special neural circuits exist to coordinate secretion with motility. Cellular components of secretomotor reflexes variably express purinergic receptors for adenosine (A1, A2a, A2b, or A3 receptors) or the nucleotides adenosine 5'-triphosphate (ATP), adenosine diphosphate (ADP), uridine 5'-triphosphate (UTP), or uridine diphosphate (UDP) (P2X(1-7), P2Y(2), P2Y(4), P2Y(6), P2Y(12) receptors). This review focuses on the emerging concepts in our understanding of purinergic regulation at these receptors, and in particular of mechanosensory reflexes. Purinergic inhibitory (A(1), A(3), P2Y(12)) or excitatory (A(2), P2Y(1)) receptors modulate mechanosensitive 5-HT release. Excitatory (P2Y(1), other P2Y, P2X) or inhibitory (A(1), A(3)) receptors are involved in mechanically evoked secretory reflexes or "neurogenic diarrhea." Distinct neural (pre- or postsynaptic) and non-neural distribution profiles of P2X(2), P2X(3), P2X(5), P2Y(1), P2Y(2), P2Y(4), P2Y(6), or P2Y(12) receptors, and for some their effects on neurotransmission, suggests their role in GI secretomotor function. Luminal A(2b), P2Y(2), P2Y(4), and P2Y(6) receptors are involved in fluid and Cl(-), HCO(3) (-), K(+), or mucin secretion. Abnormal receptor expression in GI diseases may be of clinical relevance. Adenosine A(2a) or A(3) receptors are emerging as therapeutic targets in inflammatory bowel diseases (IBD) and gastroprotection; they can also prevent purinergic receptor abnormalities and diarrhea. Purines are emerging as fundamental regulators of enteric secretomotor reflexes in health and disease.

Pharmacological analysis of components of the change in transmural potential difference evoked by distension of rat proximal small intestine in vivo

The reflex response to distension of the small intestine in vivo is complex and not well understood. The aim of this study was to characterize the neural mechanisms contributing to the complex time course of the intestinal secretory response to distension. Transmucosal potential difference (PD) was used as a marker for mucosal chloride secretion, which reflects the activity of the secretomotor neurons. Graded distensions (5, 10, and 20 mmHg) of distal rat duodenum with saline for 5 min induced a biphasic PD response with an initial peak (rapid response) followed by a plateau (sustained response). The rapid response was significantly reduced by the neural blockers tetrodotoxin and lidocaine (given serosally) and by intravenous (iv) administration of the ganglionic blocker hexamethonium and the NK(1) receptor antagonist SR-140333. Serosal TTX and iv SR-140333 significantly reduced the sustained response, which was also reduced by the NK(3) receptor antagonist talnetant and by the vasoactive intestinal polypeptide (VPAC) receptor antagonist [4Cl-d-Phe(6), Leu(17)]-VIP. Serosal lidocaine and iv hexamethonium had no significant effect on this component. Inhibition of nitric oxide synthase had no effect on any of the components of the PD response to distension. The PD response to distension thus seems to consist of two components, a rapidly activating and adapting component operating via nicotinic transmission and NK(1) receptors, and a slow component operating via VIP-ergic transmission and involving both NK(1) and NK(3) receptors.

Autonomic nervous system and secretion across the intestinal mucosal surface

Chloride secretion is important because it is the driving force for fluid movement into the intestinal lumen. The flow of accumulated fluid flushes out invading micro-organisms in defense of the host. Chloride secretion is regulated by neurons in the submucosal plexus of the enteric nervous system. Mechanosensitive enterochromaffin cells that release 5-hydroxytryptamine (5-HT) and activate intrinsic afferent neurons in the submucosal plexus and initiate chloride secretion. Mechanical stimulation by distention may also trigger reflexes by a direct action on intrinsic afferent neurons. Dysregulation of 5-HT release or altered activity of intrinsic afferents is likely to occur in states of inflammation and other disorders.

Mucosal stimulation activates secretomotor neurons via long myenteric pathways in guinea pig ileum

This study examined whether mucosal stimulation activates long secretomotor neural reflexes and, if so, how they are organized. The submucosa of in vitro full thickness guinea pig ileal preparations was exposed in the distal portion and intracellular recordings were obtained from electrophysiologically identified secretomotor neurons. Axons in the intact mucosa of the oral segment were stimulated by a large bipolar stimulating electrode. In control preparations, a single stimulus pulse evoked a fast excitatory postsynaptic potential (EPSP) in 86% of neurons located 0.7-1.0 cm anal to the stimulus site. A stimulus train evoked multiple fast EPSPs, but slow EPSPs were not observed. To examine whether mucosal stimulation specifically activated mucosal sensory nerve terminals, the mucosa/submucosa was severed from the underlying layers and repositioned. In these preparations, fast EPSPs could not be elicited in 89% of cells. Superfusion with phorbol dibutyrate enhanced excitability of sensory neurons and pressure-pulse application of serotonin to the mucosa increased the fast EPSPs evoked by mucosal stimulation, providing further evidence that sensory neurons were involved. To determine whether these reflexes projected through the myenteric plexus, this plexus was surgically lesioned between the stimulus site and the impaled neuron. No fast EPSPs were recorded in these preparations following mucosal stimulation whereas lesioning the submucosal plexus had no effect. These results demonstrate that mucosal stimulation triggers a long myenteric pathway that activates submucosal secretomotor neurons. This pathway projects in parallel with motor and vasodilator reflexes, and this common pathway may enable coordination of intestinal secretion, blood flow, and motility.

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.

Hydrogen sulfide is a novel prosecretory neuromodulator in the Guinea-pig and human colon

BACKGROUND & AIMS

Hydrogen sulfide (H(2)S) has been suggested as a novel gasomediator. We explored its unknown neuromodulatory role in human and guinea-pig colon.

METHODS

We used immunohistochemistry to detect H(2)S-producing enzymes cystathionine gamma-lyase (CSE) and cystathionine beta-synthase (CBS) in enteric neurons, Ussing chambers to measure mucosal ion secretion, and neuroimaging with voltage- and Ca(++)-sensitive dyes to record H(2)S effects on guinea-pig and human enteric neurons.

RESULTS

More than 90% of guinea-pig and human submucous and myenteric neurons were colabeled for CSE and CBS. Myenteric interstitial cells of Cajal were CSE-immunoreactive. The exogenous H(2)S donor NaHS (0.2-2.5 mmol/L) concentration-dependently increased chloride secretion in human and guinea-pig submucosa/mucosa preparations, but not in the colonic epithelial cell line T84. The secretory response was reduced significantly by tetrodotoxin (0.5 micromol/L), capsaicin desensitization (10 micromol/L), and the transient receptor potentials vanilloid receptor 1 antagonist capsazepine (10 micromol/L). The endogenous H(2)S donor L-cysteine also induced secretion that was diminished significantly by capsaicin desensitization, the CBS inhibitor amino-oxyacetic acid, and the CSE inhibitor propargylglycine. NaHS increased spike discharge in 23% of guinea-pig and 36% of human submucous neurons, but had no effect on Ca(++) mobilization in cultured guinea-pig enteric neurons. This excitatory response was reduced significantly by capsaicin desensitization and capsazepine, but not by glibenclamide (10 micromol/L).

CONCLUSIONS

The presence of H(2)S-producing enzymes in human and guinea-pig enteric neurons, the excitatory action on enteric neurons, and the prosecretory effects of NaHS suggest H(2)S as a novel gut-signaling molecule. Its action mainly involves transient receptor potentials vanilloid receptor 1 receptors on extrinsic afferent terminals, which in turn activate enteric neurons.

Effects of cholera toxin on the potential difference and motor responses induced by distension in the rat proximal small intestine in vivo

Cholera toxin (CT) may induce uncontrolled firing in recurrent networks of secretomotor neurons in the submucous plexus. This hypothesis was tested in chloralose-anesthetized rats in vivo. The secretory reflex response to graded intestinal distension was measured with or without prior exposure to luminal CT. The transmural potential difference (PD) was used as a marker for electrogenic chloride secretion. In controls, distension increased PD, and this response was reduced by the neural blocker tetrodotoxin given serosally and the vasoactive intestinal peptide (VIP) receptor antagonist [4Cl-d-Phe(6),Leu(17)]VIP (2 mug.min(-1).kg(-1) iv) but unaffected by the serotonin 5-HT(3) receptor antagonist granisetron, by the nicotinic receptor antagonist hexamethonium, by the muscarinic receptor antagonist atropine, or by the cyclooxygenase inhibitor indomethacin. Basal PD increased significantly with time in CT-exposed segments, an effect blocked by granisetron, by indomethacin, and by [4Cl-d-Phe(6),Leu(17)]VIP but not by hexamethonium or atropine. In contrast, once the increased basal PD produced by CT was established, [4Cl-d-Phe(6),Leu(17)]VIP and indomethacin had no significant effect, whereas granisetron and hexamethonium markedly depressed basal PD. CT significantly reduced the increase in PD produced by distension, an effect reversed by granisetron, indomethacin, and atropine. CT also activated a specific motility response to distension, repeated cluster contractions, but only in animals pretreated with granisetron, indomethacin, or atropine. These data are compatible with the hypothesis that CT induces uncontrolled activity in submucous secretory networks. Development of this state depends on 5-HT(3) receptors, VIP receptors, and prostaglandin synthesis, whereas its maintenance depends on 5-HT(3) and nicotinic receptors but not VIP receptors. The motility effects of CT (probably reflecting myenteric activity) are partially suppressed via a mechanism involving 5-HT(3) and muscarinic receptors and prostaglandin synthesis.

Novelty stress increases fecal pellet output in mongolian gerbils: effects of several drugs

Stress-induced colonic functional changes have been investigated mainly under conditions involving physical stress, like in the restraint stress model. In this study, we established a new stress-induced defecation model involving the placement of Mongolian gerbils in a novel environment (novelty stress) and determined the effects of several drugs on novelty stress-induced fecal pellet output. When animals kept in groups were placed individually in small cages, the fecal pellet output markedly increased, although the upper intestinal transit measured by charcoal method was not changed. The concentration of plasma adrenocorticotropic hormone was moderately but significantly increased by the novelty stress. Drugs reportedly effective for stress-induced defecation, like alosetron hydrochloride, atropine sulfate, and trimebutine maleate, inhibited both the novelty stress-induced increase in fecal pellet output and spontaneous defecation. In contrast, TAK-637, a tachykinin NK1-receptor antagonist, and diazepam inhibited the novelty stress induced defecation but did not inhibit spontaneous defecation. The present study indicated that novelty stress increases fecal pellet output without affecting the upper intestinal transit; this model may be useful for evaluating the effects of drugs on stress-stimulated colonic motility.

Synaptic facilitation and enhanced neuronal excitability in the submucosal plexus during experimental colitis in guinea-pig

Intestinal secretion is regulated by submucosal neurones of the enteric nervous system. Inflammation of the intestines leads to aberrant secretory activity; therefore we hypothesized that the synaptic and electrical behaviours of submucosal neurones are altered during colitis. To test this hypothesis, we used intracellular microelectrode recording to compare the excitability and synaptic properties of submucosal neurones from normal and trinitrobenzene sulphonic acid (TNBS)-inflamed guinea-pig colons. Inflammation differentially affected the electrophysiological characteristics of the two functional classes of submucosal neurones. AH neurones from inflamed colons were more excitable, had shorter action potential durations and reduced afterhyperpolarizations. Stimulus-evoked fast and slow excitatory postsynaptic potentials (EPSPs) in S neurones were larger during colitis, and the incidence of spontaneous fast EPSPs was increased. In control preparations, fast EPSPs were almost completely blocked by the nicotinic receptor antagonist hexamethonium, whereas fast EPSPs in inflamed S neurones were only partially inhibited by hexamethonium. In inflamed tissues, components of the fast EPSP in S neurones were sensitive to blockade of P2(X) and 5-HT(3) receptors while these antagonists had little effect in control preparations. Control and inflamed S neurones were equally sensitive to brief application of acetylcholine, ATP and 5-HT, suggesting that synaptic facilitation was due to a presynaptic mechanism. Immunoreactivity for 5-HT in the submucosal plexus was unchanged by inflammation; this indicates that altered synaptic transmission was not due to anatomical remodelling of submucosal nerve terminals. This is the first demonstration of alterations in synaptic pharmacology in the enteric nervous system during inflammation.

Inhibition of nitric oxide synthesis potentiates the colonic permeability increase triggered by luminal bile acids

AIM

Experiments were performed in anaesthetized rats to clarify the role of nitric oxide (NO) in the control of colonic permeability.

METHODS

Colonic luminal pressure, the transmucosal potential difference (PD) and the clearance of [3H] mannitol and [14C] urea from blood to lumen were measured. NO synthesis was blocked with Nomega-nitro-L-arginine (L-NNA) i.v. and mucosal permeability was increased by deoxycholic acid (DCA, 4 mm). The involvement of histamine in the response was studied by giving the histamine H1 receptor blocker pyrilamine.

RESULTS

In proximal colon, L-NNA per se increased luminal pressure and PD but had no significant effect on clearance. DCA per se increased luminal pressure, had no significant effect on PD, but increased mannitol and urea clearance and the clearance ratio. L-NNA and pyrilamine both blocked the luminal pressure effect of DCA but L-NNA had no significant effect on the clearance response to DCA. In distal colon, L-NNA per se had no significant effect on pressure and clearance, but increased PD like in proximal colon. DCA had no significant effect on luminal pressure, but markedly reduced PD and increased both clearance and clearance ratio. In this segment, L-NNA significantly potentiated the clearance response to DCA, and further increased clearance ratio to a value not significantly different from unity (1.00 +/- 0.05).

CONCLUSION

The data suggest that in vivo, moderate concentrations of bile acids increase colonic permeability in rats via a mechanism that is inhibited by NO in distal but not in proximal colon. In distal colon, NO may contribute to the maintenance of epithelial barrier function.

Effects of TAK-637 on NK(1) receptor-mediated mechanisms regulating colonic secretion

This study investigates the effect of a selective NK(1) receptor antagonist TAK-637 on enteric mechanisms involved in regulation of epithelial secretion in the colon. Mucosal sheets isolated from guinea-pig colon were placed in modified Ussing chambers and the net active transport of electrolytes was measured as short-circuit current (Isc). GR-73632, a selective NK(1) receptor agonist, induced an increase in basal Isc, which was inhibited by TAK-637 (IC(50) of 21 nM). The increase in Isc induced by GR-73632 was significantly attenuated by tetrodotoxin (TTX, 1 microM), indicating that TAK-637 inhibits neuronal NK(1) receptors. Moreover, TAK-637 reduced the TTX-resistant component of the response to GR-73632 suggesting that NK(1) receptors expressed by epithelial cells are inhibited by TAK-637. In separate experiments, TAK-637 partially inhibited the submaximal Isc induced by electrical field stimulation (EFS, 0.5 ms, 15 Hz) of enteric nerves or by activation of primary afferent fibers using capsaicin (50 microM). TAK-637 had no significant effect on the basal Isc or on responses induced by neurokinin A (NKA), senktide, or forskolin. The results imply that inhibition of peripheral NK(1) receptors may reduce autonomic epithelial secretion in response to activation of autonomic secretomotor pathways, while having no significant effect on basal epithelial transport.

Vasoactive intestinal peptide induces IL-8 production in human colonic epithelial cells via MAP kinase-dependent and PKA-independent pathways

Vasoactive intestinal peptide (VIP) has been shown to be a key regulator of intestinal epithelial functions such as mucus and chloride secretion, paracellular permeability, and cell proliferation. However, its regulatory role in intestinal epithelial chemokine production remains unknown. The aim of this study was (1) to determine whether VIP can modulate intestinal epithelial interleukin-8 (IL-8) production and (2) to identify intracellular mediators responsible for this effect. In the human colonic epithelial cell line HT29-Cl.16E, VIP stimulates IL-8 secretion dose-dependently and IL-8 mRNA level at 10(-9) M. The protein kinase A (PKA) inhibitor PKI did not abolish the effect of VIP. However, inhibition of the ERK1/2 and p38 MAPK pathways reduced the VIP-stimulated IL-8 secretion and mRNA level. Together, our results showed that VIP stimulates IL-8 production in intestinal epithelial cells via PKA-independent and MAPK-dependent pathways. These data suggest that VIPergic pathways can play an immunomodulatory role in intestinal epithelial cells, by regulating epithelial IL-8 secretion.

Distribution and function of the cannabinoid-1 receptor in the modulation of ion transport in the guinea pig ileum: relationship to capsaicin-sensitive nerves

We investigated the distribution and function of cannabinoid (CB)(1) receptors in the submucosal plexus of the guinea pig ileum. CB(1) receptors were found on both types of submucosal secretomotor neurons, colocalizing with VIP and neuropeptide Y (NPY), the noncholinergic and cholinergic secretomotor neurons, respectively. CB(1) receptors colocalized with transient receptor potential vanilloid-1 receptors on paravascular nerves and fibers in the submucosal plexus. In the submucosal ganglia, these nerves were preferentially localized at the periphery of the ganglia. In denervated ileal segments, CB(1) receptor immunoreactivity in submucosal neurons was not modified, but paravascular and intraganglionic fiber staining was absent. Short-circuit current (I(sc)) was measured as an indicator of net electrogenic ion transport in Ussing chambers. In the ion-transport studies, I(sc) responses to capsaicin, which activates extrinsic primary afferents, and to electrical field stimulation (EFS) were reduced by pretreatment with the muscarinic antagonist atropine, abolished by tetrodotoxin, but were unaffected by VIP receptor desensitization, hexamethonium, alpha-amino-3-hydroxy-5-methlisoxazole-4-proprionic acid, or N-methyl-d-aspartate glutamate receptor antagonists. The responses to capsaicin and EFS were reduced by 47 +/- 12 and 30 +/- 14%, respectively, by the CB(1) receptor agonist WIN 55,212-2. This inhibitory effect was blocked by the CB(1) receptor antagonist, SR 141716A. I(sc) responses to forskolin or carbachol, which act directly on the epithelium, were not affected by WIN 55,212-2. The inhibitory effect of WIN 55,212-2 on EFS-evoked secretion was not observed in extrinsically denervated segments of ileum. Taken together, these data show cannabinoids act at CB(1) receptors on extrinsic primary afferent nerves, inhibiting the release of transmitters that act on cholinergic secretomotor pathways.

Submucosal secretomotor and vasodilator reflexes

The presence of neuronal reflexes within the intestine that modulate mucosal ion secretion and blood flow have been recognized for many years, but the organization of these reflexes was unclear. This review highlights important findings from recent in vitro guinea-pig studies which have shown that both intrinsic primary afferent neurones (IPANs) and extrinsic primary afferent neurones (EPANs) can respond to chemical and/or mechanical stimuli to activate pathways, the afferent and efferent elements of which are confined to the walls of the intestine. Enteric neuronal pathways involve both myenteric and submucosal plexus neurones whereas capsaicin-sensitive afferent nerves evoke secretion by stimulating submucosal secretomotor neurones and vasodilation by direct actions on the submucosal arterioles. In this review, the cellular mechanisms involved in these pathways are described and the implications of these findings are discussed.

Regulation of intestinal secretion involved in the interaction between neurotransmitters and prostaglandin E2

In this short review, it will be described that neurotransmitter-induced secretion in the intestine may be influenced by the tissue level of prostaglandin E2 (PGE2). In the normal condition, vasoactive intestinal polypeptide (VIP) and acetylcholine (ACh) are the predominant neurotransmitters of secretomotor neurones. VIP and ACh activate distinct second messenger systems in epithelial cells, i.e. adenosine 3', 5'-cyclic monophosphate (cAMP) and calcium ion (Ca2+), respectively. An increase in intracellular cAMP induces a small amount of chloride (Cl-) secretion in epithelial cells, while simultaneous increases in intracellular Ca2+ and cAMP greatly enhances the cAMP-induced Cl- secretion. When the concentration of prostaglandins reaches a high level in the intestinal tissue substance P, which is a neurotransmitter of sensory neurones, can also induce a massive Cl- secretion by cross-potentiation of cAMP and Ca2+ in epithelial cells. In conclusion, it is considered that the concentration of tissue PGE2 may indicate tissue alert level, and when this level elevates, PGE2 enhances ACh and SP-induced Cl- secretion, thus mediating massive fluid secretion for host defence.

Mechanically evoked reflex electrogenic chloride secretion in rat distal colon is triggered by endogenous nucleotides acting at P2Y1, P2Y2, and P2Y4 receptors

Mechanical activation of the mucosal lining of the colon by brush stroking elicits an intestinal neural reflex and an increase in short circuit current (Isc) indicative of electrogenic chloride ion transport. We tested whether endogenous nucleotides are physiologic regulators of mucosal reflexes that control ion transport. The brush stroking-evoked Isc response in mucosa and submucosa preparations (M-SMP) of rat colon was reduced by the P2Y1 receptor (R) antagonist 2'deoxy-N6-methyl adenosine 3',5'-diphosphate diammonium salt (MRS 2179) and further blocked by tetrodotoxin (TTX). M-SMP Isc responses to serosal application of the P2Y1 R agonist 2-methylthioadenosine-diphosphate (2MeSADP) or the P2Y2/P2Y4 R agonist 5'uridine-triphosphate (UTP) were reduced but not abolished by TTX. The potency profile of nucleotides for increasing Isc was 5'adenosine-triphosphate (ATP; effective concentration at half maximal response [EC50] 0.65 x 10(4) M) congruent with UTP (EC50 1.0 x 10(-4) M) congruent with 2MeSADP (EC50 = 1.60 x 10(-4) M). Mucosal touch and distention-induced Ca2+ transients in submucous neurons were reduced by apyrase and prevented by blocking the P2Y1 R with MRS 2179 and TTX; denervation of the mucosa. It did not occur by touching a ganglion directly. 2MeSADP Ca2+ responses occurred in subsets of neurons with or without substance P (SP) responses. The potency profile of nucleotides on the neural Ca2+ response was 2MeSADP (5 x 10(-7) M) > UTP (6 x 10(-6) M) > ATP (9 x 10(-5) M). The expression of P2Y R immunoreactivity (ir) in nerve cell bodies was in the order of P2Y1 R > P2Y4 R >> P2Y2 R. P2Y1R ir occurred in the cell somas of more than 90% of neuronal nitric oxide synthase, vasoactive intestinal peptide (VIP), calretinin, or neuropeptide Y (NPY)-ir neurons, 78% of somatostatin neurons, but not in calbindin or SP neurons. P2Y2 R ir was expressed in a minority of SP, VIP, NPY, vesicular acetylcholine transporter, and calcitonin gene-related peptide-ir varicose fibers (5-20%) and those surrounding calbindin (5-20%) neurons. P2Y4 ir occurred mainly in the cell somas of 93% of NPY neurons. Reverse transcriptase polymerase chain reaction of the submucosa demonstrated mRNA for P2Y1R, P2Y2, P2Y4, P2Y6, and P2Y12 Rs. Expression of P2Y1, P2Y2, and P2Y4 protein was confirmed by western blots. In conclusion, endogenous nucleotides acting at P2YRs transduce mechanically evoked reflex chloride ion transport in rat distal colon. Nucleotides evoke reflexes by acting primarily at postsynaptic P2Y1 Rs and P2Y4 R on VIP+/NPY+ secretomotor neurons, at P2Y2 Rs on no more than 2% of VIP+ secretomotor neurons, and 2Y2 Rs mainly of extrinsic varicose fibers surrounding putative intrinsic primary afferent and secretomotor neurons. During mucosal mechanical reflexes, it is postulated that P2Y1 R, P2Y2 R, and P2Y4 R are activated by endogenous ATP, UTP, and 5'uridine-diphosphate.