Computational model of the migrating motor complex of the small intestine

Computational simulations and Ca2+ imaging reveal that slow synaptic depolarizations (slow EPSPs) inhibit fast EPSP evoked action potentials for most of their time course in enteric neurons

Transmission between neurons in the extensive enteric neural networks of the gut involves synaptic potentials with vastly different time courses and underlying conductances. Most enteric neurons exhibit fast excitatory post-synaptic potentials (EPSPs) lasting 20–50 ms, but many also exhibit slow EPSPs that last up to 100 s. When large enough, slow EPSPs excite action potentials at the start of the slow depolarization, but how they affect action potentials evoked by fast EPSPs is unknown. Furthermore, two other sources of synaptic depolarization probably occur in enteric circuits, activated via GABA_A or GABA_C receptors; how these interact with other synaptic depolarizations is also unclear. We built a compartmental model of enteric neurons incorporating realistic voltage-dependent ion channels, then simulated fast EPSPs, slow EPSPs and GABA_A or GABA_C ligand-gated Cl^- channels to explore these interactions. Model predictions were tested by imaging Ca²⁺ transients in myenteric neurons ex vivo as an indicator of their activity during synaptic interactions. The model could mimic firing of myenteric neurons in mouse colon evoked by depolarizing current during intracellular recording and the fast and slow EPSPs in these neurons. Subthreshold fast EPSPs evoked spikes during the rising phase of a slow EPSP, but suprathreshold fast EPSPs could not evoke spikes later in a slow EPSP. This predicted inhibition was confirmed by Ca²⁺ imaging in which stimuli that evoke slow EPSPs suppressed activity evoked by fast EPSPs in many myenteric neurons. The model also predicted that synchronous activation of GABA_A receptors and fast EPSPs potentiated firing evoked by the latter, while synchronous activation of GABA_C receptors with fast EPSPs, potentiated firing and then suppressed it. The results reveal that so-called slow EPSPs have a biphasic effect being likely to suppress fast EPSP evoked firing over very long periods, perhaps accounting for prolonged quiescent periods seen in enteric motor patterns.

The gastrointestinal tract is the only organ with an extensive semi-autonomous nervous system that generates complex contraction patterns independently. Communication between neurons in this “enteric” nervous system is via depolarizing synaptic events with dramatically different time courses including fast synaptic potentials lasting around 20–50 ms and slow depolarizing synaptic potentials lasting for 10–120 s. Most neurons have both. We explored how slow synaptic depolarizations affect generation of action potentials by fast synaptic potentials using computational simulation of small networks of neurons implemented as compartmental models with realistic membrane ion channels. We found that slow synaptic depolarizations have biphasic effects; they initially make fast synaptic potentials more likely to trigger action potentials, but then actually prevent action potential generation by fast synaptic potentials with the inhibition lasting several 10s of seconds. We confirmed the inhibitory effects of the slow synaptic depolarizations using live Ca²⁺ imaging of enteric neurons from mouse colon in isolated tissue. Our results identify a novel form of synaptic inhibition in the enteric nervous system of the gut, which may account for the vastly differing time courses between signalling in individual gut neurons and rhythmic contractile patterns that often repeat at more than 60 s intervals.

Squalamine Restores the Function of the Enteric Nervous System in Mouse Models of Parkinson's Disease

BACKGROUND

Parkinson's disease (PD) is a progressive neurodegenerative disorder thought to be caused by accumulation of α-synuclein (α-syn) within the brain, autonomic nerves, and the enteric nervous system (ENS). Involvement of the ENS in PD often precedes the onset of the classic motor signs of PD by many years at a time when severe constipation represents a major morbidity. Studies conducted in vitro and in vivo, have shown that squalamine, a zwitterionic amphipathic aminosterol, originally isolated from the liver of the dogfish shark, effectively displaces membrane-bound α-syn.

OBJECTIVE

Here we explore the electrophysiological effect of squalamine on the gastrointestinal (GI) tract of mouse models of PD engineered to express the highly aggregating A53T human α-syn mutant.

METHODS

GI motility and in vivo response to oral squalamine in PD model mice and controls were assessed using an in vitro tissue motility protocol and via fecal pellet output. Vagal afferent response to squalamine was measured using extracellular mesenteric nerve recordings from the jejunum. Whole cell patch clamp was performed to measure response to squalamine in the myenteric plexus.

RESULTS

Squalamine effectively restores disordered colonic motility in vivo and within minutes of local application to the bowel. We show that topical squalamine exposure to intrinsic primary afferent neurons (IPANs) of the ENS rapidly restores excitability.

CONCLUSION

These observations may help to explain how squalamine may promote gut propulsive activity through local effects on IPANs in the ENS, and further support its possible utility in the treatment of constipation in patients with PD.

Antibiotic exposure postweaning disrupts the neurochemistry and function of enteric neurons mediating colonic motor activity

The period during and immediately after weaning is an important developmental window when marked shifts in gut microbiota can regulate the maturation of the enteric nervous system (ENS). Because microbiota-derived signals that modulate ENS development are poorly understood, we examined the physiological impact of the broad spectrum of antibiotic, vancomycin-administered postweaning on colonic motility, neurochemistry of enteric neurons, and neuronal excitability. The functional impact of vancomycin on enteric neurons was investigated by Ca²⁺ imaging in Wnt1-Cre;R26R-GCaMP3 reporter mice to characterize alterations in the submucosal and the myenteric plexus, which contains the neuronal circuitry controlling gut motility. 16S rDNA sequencing of fecal specimens after oral vancomycin demonstrated significant deviations in microbiota abundance, diversity, and community composition. Vancomycin significantly increased the relative family rank abundance of Akkermansiaceae, Lactobacillaceae, and Enterobacteriaceae at the expense of Lachnospiraceae and Bacteroidaceae. In sharp contrast to neonatal vancomycin exposure, microbiota compositional shifts in weaned animals were associated with slower colonic migrating motor complexes (CMMCs) without mucosal serotonin biosynthesis being altered. The slowing of CMMCs is linked to disruptions in the neurochemistry of the underlying enteric circuitry. This included significant reductions in cholinergic and calbindin+ myenteric neurons, neuronal nitric oxide synthase+ submucosal neurons, neurofilament M+ enteric neurons, and increased proportions of cholinergic submucosal neurons. The antibiotic treatment also increased transmission and responsiveness in myenteric and submucosal neurons that may enhance inhibitory motor pathways, leading to slower CMMCs. Differential vancomycin responses during neonatal and weaning periods in mice highlight the developmental-specific impact of antibiotics on colonic enteric circuitry and motility.

Regional complexity in enteric neuron wiring reflects diversity of motility patterns in the mouse large intestine

The enteric nervous system controls a variety of gastrointestinal functions including intestinal motility. The minimal neuronal circuit necessary to direct peristalsis is well-characterized but several intestinal regions display also other motility patterns for which the underlying circuits and connectivity schemes that coordinate the transition between those patterns are poorly understood. We investigated whether in regions with a richer palette of motility patterns, the underlying nerve circuits reflect this complexity. Using Ca²⁺ imaging, we determined the location and response fingerprint of large populations of enteric neurons upon focal network stimulation. Complemented by neuronal tracing and volumetric reconstructions of synaptic contacts, this shows that the multifunctional proximal colon requires specific additional circuit components as compared to the distal colon, where peristalsis is the predominant motility pattern. Our study reveals that motility control is hard-wired in the enteric neural networks and that circuit complexity matches the motor pattern portfolio of specific intestinal regions.

Computational motility models of neurogastroenterology and neuromodulation

The success of neuromodulation therapies, particularly in the brain, spinal cord, and peripheral nerves, has been greatly aided by computational, biophysical models. However, treating gastrointestinal disorders with electrical stimulation has been much less explored, partly because the mode of action of such treatments is unclear, and selection of stimulation parameters is often empirical. Progress in gut neuromodulation is limited by the comparative lack of biophysical models capable of simulating neuromodulation of gastrointestinal function. Here, we review the recently developed biophysical models of electrically-active cells in the gastrointestinal system that contribute to motility. Biophysical models are replacing phenomenologically-defined models due to advancements in electrophysiological characterization of key players in the gut: enteric neurons, smooth muscle fibers, and interstitial cells of Cajal. In this review, we explore existing biophysically-defined cellular and network models that contribute to gastrointestinal motility. We focus on recent models that are laying the groundwork for modeling electrical stimulation of the gastrointestinal system. Developing models of gut neuromodulation will improve our mechanistic understanding of these treatments, leading to better parameterization, selectivity, and efficacy of neuromodulation to treat gastrointestinal disorders. Such models may have direct clinical translation to current neuromodulation therapies, such as sacral nerve stimulation.

Electrical stimulation of gut motility guided by an in silico model

OBJECTIVE

Neuromodulation of the central and peripheral nervous systems is becoming increasingly important for treating a diverse set of diseases-ranging from Parkinson's Disease and epilepsy to chronic pain. However, neuromodulation of the gastrointestinal (GI) tract has achieved relatively limited success in treating functional GI disorders, which affect a significant population, because the effects of stimulation on the enteric nervous system (ENS) and gut motility are not well understood. Here we develop an integrated neuromechanical model of the ENS and assess neurostimulation strategies for enhancing gut motility, validated by in vivo experiments.

APPROACH

The computational model included a network of enteric neurons, smooth muscle fibers, and interstitial cells of Cajal, which regulated propulsion of a virtual pellet in a model of gut motility.

MAIN RESULTS

Simulated extracellular stimulation of ENS-mediated motility revealed that sinusoidal current at 0.5 Hz was more effective at increasing intrinsic peristalsis and reducing colon transit time than conventional higher frequency rectangular current pulses, as commonly used for neuromodulation therapy. Further analysis of the model revealed that the 0.5 Hz sinusoidal currents were more effective at modulating the pacemaker frequency of interstitial cells of Cajal. To test the predictions of the model, we conducted in vivo electrical stimulation of the distal colon while measuring bead propulsion in awake rats. Experimental results confirmed that 0.5 Hz sinusoidal currents were more effective than higher frequency pulses at enhancing gut motility.

SIGNIFICANCE

This work demonstrates an in silico GI neuromuscular model to enable GI neuromodulation parameter optimization and suggests that low frequency sinusoidal currents may improve the efficacy of GI pacing.

Comparison of pH and motility of the small intestine of healthy subjects and patients with symptomatic constipation using the wireless motility capsule

Gastrointestinal luminal pH shows a rise from the duodenum to the terminal ileum in healthy individuals. Our objectives were to compare the pH in the proximal small intestine (SI) (first 60 min of small intestinal transit) lumen of human volunteers and patients with symptomatic constipation; to quantify contractile pressure profiles of the proximal SI, and to assess the relationship between luminally-recorded contractile pressure and small intestinal transit times (SITT) of a non-disintegrating capsule that measures pH and pressure activity (wireless motility capsule). We used previously acquired records from 39 healthy subjects and 41 patients with symptomatic constipation. Mean pH (±SD) of the proximal SI was similar in healthy subjects and patients with constipation at 6.2 (±0.6) and 6.3 (±0.4), respectively. In 13 of the healthy subjects, pH did not rise uniformly in the proximal SI though the pH_median was 6.0 (5th, 95th percentiles 3.09, 7.06) and the pH fluctuated over a mean period of 28 min. Large inter-individual variability in frequency of pressure activity (Ct) and area under pressure curve (AUC) were observed in the proximal SI of healthy subjects and patients with constipation. Median AUC was 3996 mmHg s^-1 (5th, 95th percentiles 948, 16866 mmHg s^-1) in these two populations combined. Ct and AUC showed a strong direct linear correlation at r = 0.91, p < 1 × 10^-6. An inverse correlation (suggesting longer SITT with lower pressure activity) was observed between Ct/AUC and SITT in both healthy subjects and patients with symptomatic constipation. The pooled results for both groups showed: AUC and SITT correlation at r = -0.49, p < 1 × 10^-6. We concluded that both the frequency and amplitude of contractions in the proximal SI are important for the propagation of non-disintegrating capsules. The observed pH fluctuations in the proximal SI may impact supersaturation and precipitation of weakly basic drugs.

Excitability and Synaptic Transmission in the Enteric Nervous System: Does Diet Play a Role?

Changes in diet are a challenge to the gastrointestinal tract which needs to alter its processing mechanisms to continue to process nutrients and maintain health. In particular, the enteric nervous system (ENS) needs to adapt its motor and secretory programs to deal with changes in nutrient type and load in order to optimise nutrient absorption.The nerve circuits in the gut are complex, and the numbers and types of neurons make recordings of specific cell types difficult, time-consuming, and prone to sampling errors. Nonetheless, traditional research methods like intracellular electrophysiological approaches have provided the basis for our understanding of the ENS circuitry. In particular, animal models of intestinal inflammation have shown us that we can document changes to neuronal excitability and synaptic transmission.Recent studies examining diet-induced changes to ENS programming have opted to use fast imaging techniques to reveal changes in neuron function. Advances in imaging techniques using voltage- or calcium-sensitive dyes to record neuronal activity promise to overcome many limitations inherent to electrophysiological approaches. Imaging techniques allow access to a wide range of ENS phenotypes and to the changes they undergo during dietary challenges. These sorts of studies have shown that dietary variation or obesity can change how the ENS processes information-in effect reprogramming the ENS. In this review, the data gathered from intracellular recordings will be compared with measurements made using imaging techniques in an effort to determine if the lessons learnt from inflammatory changes are relevant to the understanding of diet-induced reprogramming.

Use of Genetically Encoded Calcium Indicators (GECIs) Combined with Advanced Motion Tracking Techniques to Examine the Behavior of Neurons and Glia in the Enteric Nervous System of the Intact Murine Colon

Genetically encoded Ca²⁺ indicators (GECIs) have been used extensively in many body systems to detect Ca²⁺ transients associated with neuronal activity. Their adoption in enteric neurobiology has been slower, although they offer many advantages in terms of selectivity, signal-to-noise and non-invasiveness. Our aims were to utilize a number of cell-specific promoters to express the Ca²⁺ indicator GCaMP3 in different classes of neurons and glia to determine their effectiveness in measuring activity in enteric neural networks during colonic motor behaviors. We bred several GCaMP3 mice: (1) Wnt1-GCaMP3, all enteric neurons and glia; (2) GFAP-GCaMP3, enteric glia; (3) nNOS-GaMP3, enteric nitrergic neurons; and (4) ChAT-GCaMP3, enteric cholinergic neurons. These mice allowed us to study the behavior of the enteric neurons in the intact colon maintained at a physiological temperature, especially during the colonic migrating motor complex (CMMC), using low power Ca²⁺ imaging. In this preliminary study, we observed neuronal and glial cell Ca²⁺ transients in specific cells in both the myenteric and submucous plexus in all of the transgenic mice variants. The number of cells that could be simultaneously imaged at low power (100–1000 active cells) through the undissected gut required advanced motion tracking and analysis routines. The pattern of Ca²⁺ transients in myenteric neurons showed significant differences in response to spontaneous, oral or anal stimulation. Brief anal elongation or mucosal stimulation, which evokes a CMMC, were the most effective stimuli and elicited a powerful synchronized and prolonged burst of Ca²⁺ transients in many myenteric neurons, especially when compared with the same neurons during a spontaneous CMMC. In contrast, oral elongation, which normally inhibits CMMCs, appeared to suppress Ca²⁺ transients in some of the neurons active during a spontaneous or an anally evoked CMMC. The activity in glial networks appeared to follow neural activity but continued long after neural activity had waned. With these new tools an unprecedented level of detail can be recorded from the enteric nervous system (ENS) with minimal manipulation of tissue. These techniques can be extended in order to better understand the roles of particular enteric neurons and glia during normal and disordered motility.

Mathematical modelling of enteric neural motor patterns

1. The enteric nervous system modulates intestinal behaviours, such as motor patterns and secretion. Although much is known about different types of neurons and simple reflexes in the intestine, it remains unclear how complex behaviours are generated. 2. Mathematical modelling is an important tool for assisting the understanding of how the neurons and reflexes can be pieced together to generate intestinal behaviours. 3. Models have identified a functional role for slow excitatory post-synaptic potentials (EPSPs) by distinguishing between fast and slow EPSPs in the ascending excitation reflex. These models also discovered coordinated firing of similarly located neurons as emergent properties of feed-forward networks of interneurons in the intestine. A model of the recurrent network of intrinsic sensory neurons identified important control mechanisms to prevent uncontrolled firing due to positive feedback and that the interaction between these control mechanisms and slow EPSPs is necessary for the networks to encode ongoing sensory stimuli. This model also showed that such networks may mediate migrating motor complexes. 4. A network model of vasoactive intestinal peptide neurons in the submucosal plexus found this relatively sparse recurrent network could produce uncontrolled firing under conditions that appear to be related to cholera toxin-induced hypersecretion. 5. Abstract modelling of the intestinal fed-state motor patterns has identified how stationary contractions can arise from a polarized network. 6. These models have also helped predict and/or explained pharmacological evidence for two rhythm generators and the requirement of feedback from contractions in the circular muscle.

Colonic migrating motor complexes, high amplitude propagating contractions, neural reflexes and the importance of neuronal and mucosal serotonin

The colonic migrating motor complex (CMMC) is a critical neurally mediated rhythmic propulsive contraction observed in the large intestine of many mammals. It seems to be equivalent to the high amplitude propagating contractions (HAPCs) in humans. This review focuses on the probable neural mechanisms involved in producing the CMMC or HAPC, their likely dependence on mucosal and neuronal serotonin and pacemaker insterstitial cells of Cajal networks and how intrinsic neural reflexes affect them. Discussed is the possibility that myenteric 5-hydroxytryptamine (5-HT) neurons are not only involved in tonic inhibition of the colon, but are also involved in generating the CMMC and modulation of the entire enteric nervous system, including coupling motility to secretion and blood flow. Mucosal 5-HT appears to be important for the initiation and effective propagation of CMMCs, although this mechanism is a longstanding controversy since the 1950s, which we will address. We argue that the slow apparent propagation of the CMMC/HAPC down the colon is unlikely to result from a slowly conducting wave front of neural activity, but more likely because of an interaction between ascending excitatory and descending (serotonergic) inhibitory neural pathways interacting both within the myenteric plexus and at the level of the muscle. That is, CMMC/HAPC propagation appears to be similar to esophageal peristalsis. The suppression of inhibitory (neuronal nitric oxide synthase) motor neurons and mucosal 5-HT release by an upregulation of prostaglandins has important implications in a number of gastrointestinal disorders, especially slow transit constipation.

Both exogenous 5-HT and endogenous 5-HT, released by fluoxetine, enhance distension evoked propulsion in guinea-pig ileum in vitro

The roles of 5-HT3 and 5-HT4 receptors in the modulation of intestinal propulsion by luminal application of 5-HT and augmentation of endogenous 5-HT effects were studied in segments of guinea-pig ileum in vitro. Persistent propulsive contractions evoked by saline distension were examined using a modified Trendelenburg method. When 5-HT (30 nM), fluoxetine (selective serotonin reuptake inhibitor; 1 nM), 2-methyl-5-HT (5-HT3 receptor agonist; 1 mM), or RS 67506 (5-HT4 receptor agonist, 1 μM) was infused into the lumen, the pressure needed to initiate persistent propulsive activity fell significantly. A specific 5-HT4 receptor antagonist, SB 207266 (10 nM in lumen), abolished the effects of 5-HT, fluoxetine, and RS 67506, but not those of 2-methyl-5-HT. Granisetron (5-HT3 receptor antagonist; 1 μM in lumen) abolished the effect of 5-HT, fluoxetine, RS 67506, and 2-methyl-5-HT. The NK3 receptor antagonist SR 142801 (100 nM in lumen) blocked the effects of 5-HT, fluoxetine, and 2-methyl-5-HT. SB 207266, granisetron, and SR 142801 had no effect by themselves. Higher concentrations of fluoxetine (100 and 300 nM) and RS 67506 (3 and 10 μM) had no effect on the distension threshold for propulsive contractions. These results indicate that luminal application of exogenous 5-HT, or increased release of endogenous mucosal 5-HT above basal levels, acts to lower the threshold for propulsive contractions in the guinea-pig ileum via activation of 5-HT3 and 5-HT4 receptors and the release of tachykinins. The results further indicate that basal release of 5-HT is insufficient to alter the threshold for propulsive motor activity.

Bacteroides fragilis polysaccharide A is necessary and sufficient for acute activation of intestinal sensory neurons

Symbionts or probiotics are known to affect the nervous system. To understand the mechanisms involved, it is important to measure sensory neuron responses and identify molecules responsible for this interaction. Here we test the effects of adding Lactobacillus rhamnosus (JB-1) and Bacteroides fragilis to the epithelium while making voltage recordings from intestinal primary afferent neurons. Sensory responses are recorded within 8 s of applying JB-1 and excitability facilitated within 15 min. Bacteroides fragilis produces similar results, as does its isolated, capsular exopolysaccharide, polysaccharide A. Lipopolysaccharide-free polysaccharide A completely mimics the neuronal effects of the parent organism. Experiments with a mutant Bacteroides fragilis devoid of polysaccharide A shows that polysaccharide A is necessary and sufficient for the neuronal effects. Complex carbohydrates have not been reported before as candidates for such signalling between symbionts and the host. These observations indicate new neuronal targets and invite further study of bacterial carbohydrates as inter-kingdom signalling molecules between beneficial bacteria and sensory neurons.

Neurogenic and myogenic motor patterns of rabbit proximal, mid, and distal colon

The rabbit colon consists of four distinct regions. The motility of each region is controlled by myogenic and neurogenic mechanisms. Associating these mechanisms with specific motor patterns throughout all regions of the colon has not previously been achieved. Three sections of the colon (the proximal, mid, and distal colon) were removed from euthanized rabbits. The proximal colon consists of a triply teniated region and a single tenia region. Spatio-temporal maps were constructed from video recordings of colonic wall diameter, with associated intraluminal pressure recorded from the aboral end. Hexamethonium (100 μM) and tetrodotoxin (TTX; 0.6 μM) were used to inhibit neural activity. Four distinct patterns of motility were detected: 1 myogenic and 3 neurogenic. The myogenic activity consisted of circular muscle (CM) contractions (ripples) that occurred throughout the colon and propagated in both antegrade (anal) and retrograde (oral) directions. The neural activity of the proximal colon consisted of slowly (0.1 mm/s) propagating colonic migrating motor complexes, which were abolished by hexamethonium. These complexes were observed in the region of the proximal colon with a single band of tenia. In the distal colon, tetrodotoxin-sensitive, thus neurally mediated, but hexamethonium-resistant, peristaltic (anal) and antiperistaltic (oral) contractions were identified. The distinct patterns of neurogenic and myogenic motor activity recorded from isolated rabbit colon are specific to each anatomically distinct region. The regional specificity motor pattern is likely to facilitate orderly transit of colonic content from semi-liquid to solid composition of feces.

Comparative analysis of phase III migrating motor complexes in stomach and small bowel using wireless motility capsule and antroduodenal manometry

BACKGROUND

  Assessment of phase III MMC is often not performed due to the invasive nature of antroduodenal manometry used to detect it. The aim of the study was to evaluate the ability of wireless motility capsule (WMC) to detect phase III MMC and correlate it with the simultaneous measurements by antroduodenal manometry (ADM).

METHODS

  Eighteen patients underwent simultaneous ADM and WMC. MMCs were identified first on ADM and then correlated with WMC events occurring simultaneously. Frequency of contractions per min, AUC, MI, and criteria for amplitude thresholds of contractions representing MCCs on WMC tracings were defined.

KEY RESULTS

  In 18 patients, a total of 29 MMCs were recorded by ADM. WMC detected 86% of MMC events measured by ADM. Hundred percent (10/10) of MMCs in stomach were detected by WMC, whereas 79% (15/19) of MMCs were detected in SB. The sensitivity and specificity of WMC high amplitude contractions to represent phase III MMC were 90% and 71.8% in the stomach; 73.7% and 84.7% in SB, respectively, and negative predictive value was 99.9% in both regions.

CONCLUSIONS & INFERENCES

  Wireless motility capsule was able to detect the phase III MMCs as the high amplitude contractions with good fidelity. WMC does not detect the propagation of MMC. Using the pressure thresholds, WMC can detect high amplitude contraction representing phase III MMC with favorable sensitivity/specificity profile and 99.9% negative predictive value. This observation may have clinical significance, as the absence of high amplitude contractions recorded by WMC during fasting state suggests absence of MMCs.

Endogenous peptide YY and neuropeptide Y inhibit colonic ion transport, contractility and transit differentially via Y₁ and Y₂ receptors

BACKGROUND AND PURPOSE

Peptide YY (PYY) and neuropeptide Y (NPY) activate Y receptors, targets under consideration as treatments for diarrhoea and other intestinal disorders. We investigated the gastrointestinal consequences of selective PYY or NPY ablation on mucosal ion transport, smooth muscle activity and transit using wild-type, single and double peptide knockout mice, comparing mucosal responses with those from human colon.

EXPERIMENTAL APPROACH

Mucosae were pretreated with a Y₁ (BIBO3304) or Y₂ (BIIE0246) receptor antagonist and changes in short-circuit current recorded. Colonic transit and colonic migrating motor complexes (CMMCs) were assessed in vitro and upper gastrointestinal and colonic transit measured in vivo.

KEY RESULTS

Y receptor antagonists revealed tonic Y₁ and Y₂ receptor-mediated antisecretory effects in human and wild-type mouse colon mucosae. In both, Y₁ tone was epithelial while Y₂ tone was neuronal. Y₁ tone was reduced 90% in PYY⁻/⁻ mucosa but unchanged in NPY⁻/⁻ tissue. Y₂ tone was partially reduced in NPY⁻/⁻ or PYY⁻/⁻ mucosae and abolished in tetrodotoxin-pretreated PYY⁻/⁻ tissue. Y₁ and Y₂ tone were absent in NPYPYY⁻/⁻ tissue. Colonic transit was inhibited by Y₁ blockade and increased by Y₂ antagonism indicating tonic Y₁ excitation and Y₂ inhibition respectively. Upper GI transit was increased in PYY⁻/⁻ mice only. Y₂ blockade reduced CMMC frequency in isolated mouse colon.

CONCLUSIONS AND IMPLICATIONS

Endogenous PYY and NPY induced significant mucosal antisecretory tone mediated by Y₁ and Y₂ receptors, via similar mechanisms in human and mouse colon mucosa. Both peptides contributed to tonic Y₂-receptor-mediated inhibition of colonic transit in vitro but only PYY attenuated upper GI transit.

Meaningful or redundant complexity - mechanisms behind cyclic changes in gastroduodenal pH in the fasting state

The antroduodenal region is probably the site of the most common chronic infection of mankind, helicobacter-induced antral gastritis. After meals, the remaining gastric contents are evacuated by an interdigestive motor programme, the so-called migrating motor complex (MMC). The most characteristic feature of the MMC is phase III, a series of contractions at slow wave frequency (3 min⁻¹ in the stomach, approx. 12 min⁻¹ in the duodenum). Phase III is associated with complex changes in antroduodenal pH, the most prominent feature being a rapid alkalinization of the antral lumen immediately after the end of antral phase III. Before and during antral phase III (late phase II), gastric acid secretion increases and reflux of bile-containing fluid from the duodenum frequently occurs. At the start of duodenal phase III, the pacemaker driving the motor waves is located proximally in the contracting segment, and the motor waves are uniformly antegrade. After passing the papilla, the pacemaker which is now in the middle of the contracting segment stops its migration and waves passing the papilla hence become retrograde. Bile is diverted into the gall bladder. Duodenal phase III activates electrogenic chloride and bicarbonate secretion and release of secretory IgA. During the second half of phase III, there is accordingly reflux of bile-free fluid, bicarbonate and secretory IgA containing fluid from the duodenum into the stomach. Possible physiological and pathophysiological implications of this complex system, in particular the role of the gastric mucus layer in antral Helicobacter infection, will be discussed.

Synaptic transmission at functionally identified synapses in the enteric nervous system: roles for both ionotropic and metabotropic receptors

Digestion and absorption of nutrients and the secretion and reabsorption of fluid in the gastrointestinal tract are regulated by neurons of the enteric nervous system (ENS), the extensive peripheral nerve network contained within the intestinal wall. The ENS is an important physiological model for the study of neural networks since it is both complex and accessible. At least 20 different neurochemically and functionally distinct classes of enteric neurons have been identified in the guinea pig ileum. These neurons express a wide range of ionotropic and metabotropic receptors. Synaptic potentials mediated by ionotropic receptors such as the nicotinic acetylcholine receptor, P2X purinoceptors and 5-HT(3) receptors are seen in many enteric neurons. However, prominent synaptic potentials mediated by metabotropic receptors, like the P2Y(1) receptor and the NK(1) receptor, are also seen in these neurons. Studies of synaptic transmission between the different neuron classes within the enteric neural pathways have shown that both ionotropic and metabotropic synaptic potentials play major roles at distinct synapses within simple reflex pathways. However, there are still functional synapses at which no known transmitter or receptor has been identified. This review describes the identified roles for both ionotropic and metabotropic neurotransmission at functionally defined synapses within the guinea pig ileum ENS. It is concluded that metabotropic synaptic potentials act as primary transmitters at some synapses. It is suggested identification of the interactions between different synaptic potentials in the production of complex behaviours will require the use of well validated computer models of the enteric neural circuitry.

Calcium activity in different classes of myenteric neurons underlying the migrating motor complex in the murine colon

The spontaneous colonic migrating motor complex (CMMC) is a cyclical contractile and electrical event that is the primary motor pattern underlying fecal pellet propulsion along the murine colon. We have combined Ca(2+) imaging with immunohistochemistry to determine the role of different classes of myenteric neurons during the CMMC. Between CMMCs, myenteric neurons usually displayed ongoing but uncoordinated activity. Stroking the mucosa at the oral or anal end of the colon resulted in a CMMC (latency: 6 to 10 s; duration: 28 s) that consisted of prolonged increases in activity in many myenteric neurons that was correlated to Ca(2+) transients in and displacement of the muscle. These neurons were likely excitatory motor neurons. Activity in individual neurons during the CMMC was similar regardless of whether the CMMC occurred spontaneously or was evoked by anal or oral mucosal stimulation. This suggests that convergent interneuronal pathways exist which generate CMMCs. Interestingly, Ca(2+) transients in a subset of NOS +ve neurons were substantially reduced during the CMMC. These neurons are likely to be inhibitory motor neurons that reduce their activity during a complex (disinhibition) to allow full excitation of the muscle. Local stimulation of the mucosa evoked synchronized Ca(2+) transients in Dogiel Type II (mitotracker/calbindin-positive) neurons after a short delay (1-2 s), indicating they were the sensory neurons underlying the CMMC. These local responses were observed in hexamethonium, but were blocked by ondansetron (5-HT(3) antagonist), suggesting Dogiel Type II neurons were activated by 5-HT release from enterochromaffin cells in the mucosa. In fact, removal of the mucosa yielded no spontaneous CMMCs, although many neurons (NOS +ve and NOS ve) exhibited ongoing activity, including Dogiel Type II neurons. These results suggest that spontaneous or evoked 5-HT release from the mucosa is necessary for the activation of Dogiel Type II neurons that generate CMMCs.

Indirect evidence for increased mechanosensitivity of jejunal secretomotor neurones in patients with idiopathic bile acid malabsorption

AIM

The interdigestive motor rhythm, the migrating motor complex (MMC), is accompanied by active secretion of chloride during periods of distally propagating maximal motor activity (MMC phase III). We studied the behaviour of this system in bile acid malabsorption (BAM), a relative common cause of chronic diarrhoea. We measured motor activity and transmucosal potential difference (PD, reflecting active chloride secretion), in the proximal jejunum in healthy controls (n = 18) and in a group of patients with BAM (n = 11). The phase III-generated voltage was related to the degree of BAM quantified by the (75)SeHCAT test.

METHODS

We used a multi-channel intestinal infusion system to simultaneously measure jejunal pressure and PD. Saline passing calomel half-cells was infused into the jejunum and subcutaneously. Pressure and PD were recorded in the fasting state and after a test meal.

RESULTS

In the absence of motor activity, jejunal PD was not significantly different from zero in either group. During MMC phase III, PD reached significantly higher mean and peak levels in BAM patients. The product of MMC phase III length multiplied by voltage, over 3 h, was also significantly higher in BAM patients (controls: median 307 mV x cm, range 70-398; BAM: median 511, range 274-2271, P < 0.01). This value was also significantly correlated with the degree of BAM as reflected by the (75)SeHCAT test (P < 0.05).

CONCLUSION

Phase III induced jejunal secretion may be upregulated in BAM patients, resulting in overload of colonic reabsorption capacity.

Electrical stimulation of the mucosa evokes slow EPSPs mediated by NK1 tachykinin receptors and by P2Y1 purinoceptors in different myenteric neurons

Slow excitatory postsynaptic potentials (EPSPs) in enteric neurons arise from diverse sources, but which neurotransmitters mediate specific types of slow EPSPs is unclear. We investigated transmitters and receptors mediating slow EPSPs in myenteric neurons evoked by electrical stimulation of the mucosa in guinea pig small intestine. Segments of ileum or jejunum were dissected to allow access to the myenteric plexus adjacent to intact mucosa, in vitro. AH and S neurons were impaled with conventional intracellular electrodes. Trains of stimuli delivered to the mucosa evoked slow EPSPs in AH neurons that were blocked or depressed by the neurokinin-1 (NK1) tachykinin antagonist SR140333 (100 nM) in 10 of 11 neurons; the NK3 tachykinin receptor antagonist SR142801 (100 nM) had no effect on slow EPSPs in seven of nine AH neurons. Single pulses to the mucosa evoked fast EPSPs and slow depolarizations in S neurons. The depolarizations were divided into intermediate (durations 300-900 ms) or slow (durations 1.3-9 s) EPSPs. The slow EPSPs were blocked by pyridoxal phosphate-6-axophenyl-2-4-disulfonic acid (30 microM, N = 3) or the specific P2Y(1) antagonist MRS 2179 (10 microM, N = 6) and were predominantly in anally projecting S neurons that were immunoreactive for nitric oxide synthase (NOS). In contrast, intermediate EPSPs were predominantly evoked in NOS-negative neurons; these were abolished by MRS 2179 (N = 8). Thus activation of pathways running from the mucosa excites three different types of slow EPSP in myenteric neurons, which are mediated by either a tachykinin (NK1, AH neurons) or a purine nucleotide (P2Y(1), S neurons).

Synaptic plasticity: the new explanation of visceral hypersensitivity in rats with Trichinella spiralis infection?

BACKGROUND AND AIM

Synaptic plasticity plays an important role in affecting the intensity of visceral reflex. It may also be involved in the development of visceral hypersensitivity. The aim of this study was to investigate the role of synaptic plasticity on visceral hypersensitivity of rats infected by Trichinella spiralis.

METHODS

Thirty male Sprague-Dawley (SD) rats were randomly divided into control, acute, and chronic infection groups, and were investigated at 1 week after adaptive feeding and at 2 and 8 weeks post infection (PI) by oral administration of 1 ml phosphate-buffered saline (PBS) containing 8,000 Trichinella spiralis larvae. Visceral sensitivity was evaluated by electromyography (EMG) recording during colorectal distension. Intestinal inflammation was observed by hematoxylin-eosin (HE) staining. Synaptic ultrastructure parameters, such as postsynaptic density (PSD) length, synaptic cleft, and number of synaptic vesicles, were examined by transmission electron microscopy (TEM). The expression of protein associated with synaptic plasticity, including postsynaptic density-95 (PSD-95), synaptophysin, calbindin-28 K, N-methyl-D-aspartate receptor-1 (NMDA-R1), alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPA-R), and glial cell line-derived neurotrophic factor (GDNF), were analyzed by Western blot.

RESULTS

(1) Visceral hypersensitivity was noted in the chronic infection group, although the inflammation was nearly eliminated (P<0.05). Severe inflammation and downregulation of visceral sensitivity were observed in the acute infection group (P<0.05). (2) There were many more synaptic vesicles and longer PSD in the chronic infection group than in the control group (P<0.05, respectively). However, in comparison with control rats, disappearance of mitochondria cristae in the synapses, and decrease of synaptic vesicles and length of PSD were observed in the acute infection group. There was no significant difference in width of synaptic cleft among the three groups. (3) Compared with the control, the expression of proteins associated with synaptic plasticity was significantly upregulated during chronic infection phase (P<0.05), and downregulated during acute infection phase.

CONCLUSION

Synaptic plasticity was observed in SD rats infected by Trichinella spiralis and was associated with visceral sensitivity, which suggests that it may play an important role in the formation of visceral hypersensitivity.

Localized release of serotonin (5-hydroxytryptamine) by a fecal pellet regulates migrating motor complexes in murine colon

BACKGROUND & AIMS

The colonic migrating motor complex (CMMC) is a motor pattern that regulates the movement of fecal matter through a rhythmic sequence of electrical activity and/or contractions along the large bowel. CMMCs have largely been studied in empty preparations; we investigated whether local reflexes generated by a fecal pellet modify the CMMC to initiate propulsive activity.

METHODS

Recordings of CMMCs were made from the isolated murine large bowel, with or without a fecal pellet. Transducers were placed along the colon to record muscle tension and propulsive force on the pellet and microelectrodes were used to record electrical activity from either side of a fecal pellet, circular muscle cells oral and anal of a pellet, and in colons without the mucosa.

RESULTS

Spontaneous CMMCs propagated in both an oral or anal direction. When a pellet was inserted, CMMCs increased in frequency and propagated anally, exerting propulsive force on the pellet. The amplitude of slow waves increased during the CMMC. Localized mucosal stimulation/circumferential stretch evoked a CMMC, regardless of stimulus strength. The serotonin (5-hydroxytryptamine-3) receptor antagonist ondansetron reduced the amplitude of the CMMC, the propulsive force on the pellet, and the response to mucosal stroking, but increased the apparent conduction velocity of the CMMC. Removing the mucosa abolished spontaneous CMMCs, which still could be evoked by electrical stimulation.

CONCLUSIONS

The fecal pellet activates local mucosal reflexes, which release serotonin (5-hydroxytryptamine) from enterochromaffin cells, and stretch reflexes that determine the site of origin and propagation of the CMMC, facilitating propulsion.

The enteric nervous system: normal functions and enteric neuropathies

Most aspects of the normal organisation and functioning of the enteric nervous system have been resolved in recent years, especially for the small and large intestines, where the ENS has essential roles in controlling bowel movement and transmucosal fluid exchange. The roles of the ENS in the esophagus are not understood, and the relative roles of intrinsic reflexes in relation to extrinsic control of the stomach require clarification. In the small intestine and colon, it needs to be understood how neural activity is orchestrated to subserve different functional outcomes, for example propulsion, mixing and retrograde movement. However, the most important future challenges are to properly understand the molecular and cellular changes that underlie enteric neuropathies, to utilise knowledge of the normal neurochemistry, pharmacology and physiology of the ENS to devise strategies to treat disorders of motility and secretion, and to develop effective therapeutic compounds. It is suggested that ion channels of enteric neurons have been under-investigated as therapeutic targets. Other future challenges lie in the identification of biomarkers for functional bowel disorders and in the use of neural stem cells for restitution of ENS function.

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.

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.

Elevated motility-related transmucosal potential difference in the upper small intestine in the irritable bowel syndrome

The pathophysiology of irritable bowel syndrome (IBS) is complex and incompletely known. Very little has been studied regarding the role of submucous neuronal activity. We therefore measured small intestinal transmural potential difference (PD, reflecting mainly electrogenic chloride secretion), and its linkage with fasting motor activity [migrating motor complex (MMC)] in controls (n = 16) and patients with IBS [n = 23, 14 diarrhoea predominant (d-IBS) and nine constipation predominant (c-IBS)]. Transmural-PD and its relation to MMC phase III was measured by modified multilumen manometry for 3 h in the fasting state using one jejunal and one duodenal infusion line as flowing electrodes. The amplitude and duration of motor phase III was similar in controls and IBS patients, but the propagation speed of phase III was higher in IBS patients. In IBS patients, maximal PD during MMC phase III was significantly elevated in both the duodenum and jejunum (P < 0.05) and the PD decline after phase III was significantly prolonged in the jejunum (P < 0.01). The PD elevation was seen in both duodenum and jejunum in d-IBS patients, but only in the jejunum in the c-IBS patients. On the basis of previous modelling studies, we propose that the enhanced secretion may reflect disturbed enteric network behaviour in some patients with IBS.

Local inhibitory reflexes excited by mucosal application of nutrient amino acids in guinea pig jejunum

The motility of the gut depends on the chemicals contained in the lumen, but the stimuli that modify motility and their relationship to enteric neural pathways are unclear. This study examined local inhibitory reflexes activated by various chemical stimulants applied to the mucosa to characterize effective physiological stimuli and the pathways they excite. Segments of the jejunum were dissected to allow access to the circular muscle on one-half of the preparation while leaving the mucosa intact on the circumferentially adjacent half. Chemicals were transiently applied to the mucosa, and responses were recorded intracellularly in nearby circular muscle cells. The amino acids l-phenylalanine, l-alanine, or l-tryptophan (all 1 mM) evoked inhibitory junction potentials (IJPs; latency 150-300 ms, amplitude 3-8 mV, each n > 6) that were blocked by TTX and partially blocked by antagonists of P2X receptors and/or a combination of antagonists at 5-HT(3) and 5-HT(4) receptors. The putative mediators 5-HT (10 microM), ATP (1 mM), and CCK-8 (1-10 microM) elicited IJPs mediated via 5-HT(3), P2X, and CCK-B receptors, respectively. Responses were only partially reduced by the effective antagonists. IJPs evoked by electrically stimulating the mucosa were unaffected by antagonists that reduced chemically evoked responses. Both chemically and electrically evoked IJPs were resistant to nicotinic, NK(1), NK(3), alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid, N-methyl-d-aspartate, or CGRP receptor blockade. We conclude that mucosal stimulation by amino acids activates local neural pathways whose pharmacology depends on the nature of the stimulus. Transmitters involved at some synapses in these pathways remain to be identified.

Mechanisms underlying nutrient-induced segmentation in isolated guinea pig small intestine

Mechanisms underlying nutrient-induced segmentation within the gut are not well understood. We have shown that decanoic acid and some amino acids induce neurally dependent segmentation in guinea pig small intestine in vitro. This study examined the neural mechanisms underlying segmentation in the circular muscle and whether the timing of segmentation contractions also depends on slow waves. Decanoic acid (1 mM) was infused into the lumen of guinea pig duodenum and jejunum. Video imaging was used to monitor intestinal diameter as a function of both longitudinal position and time. Circular muscle electrical activity was recorded by using suction electrodes. Recordings from sites of segmenting contractions showed they are always associated with excitatory junction potentials leading to action potentials. Recordings from sites oral and anal to segmenting contractions revealed inhibitory junction potentials that were time locked to those contractions. Slow waves were never observed underlying segmenting contractions. In paralyzed preparations, intracellular recording revealed that slow-wave frequency was highly consistent at 19.5 (SD 1.4) cycles per minute (c/min) in duodenum and 16.6 (SD 1.1) c/min in jejunum. By contrast, the frequencies of segmenting contractions varied widely (duodenum: 3.6-28.8 c/min, median 10.8 c/min; jejunum: 3.0-27.0 c/min, median 7.8 c/min) and sometimes exceeded slow-wave frequencies for that region. Thus nutrient-induced segmentation contractions in guinea pig small intestine do not depend on slow-wave activity. Rather they result from a neural circuit producing rhythmic localized activity in excitatory motor neurons, while simultaneously activating surrounding inhibitory motor neurons.

Slow excitatory post-synaptic potentials in myenteric AH neurons of the guinea-pig ileum are reduced by the 5-hydroxytryptamine7 receptor antagonist SB 269970

Serotonin (5-HT) is a key modulator of neuronal excitability in the central and peripheral nervous system. In the enteric nervous system, 5-HT causes a slow depolarization in the intrinsic sensory neurons, but the receptor responsible for this has not been correlated with known gene products. The aim of this study was to determine whether the newly characterized 5-HT7 receptor may participate in the 5-HT-mediated depolarization of, and synaptic transmission to, the intrinsic sensory neurons of the guinea-pig ileum. Intracellular electrophysiological recordings were made from intrinsic sensory neurons identified as myenteric AH neurons from guinea-pig ileum. 5-HT (5 microM) applied to the cell body evoked both a fast depolarization (5-HT3 mediated) and/or a slow depolarization (5-HT1P-like). The 5-HT1/5/7 receptor agonist 5-carboxamidotryptamine (5-CT) (5 microM) evoked only a slow depolarization. When the fast depolarization evoked by 5-HT was blocked with granisetron (1 microM, 5-HT3 receptor antagonist), only a slow depolarization remained; this was abolished by the 5-HT7 receptor antagonist SB 269970 (1 microM, control: 14+/-2 mV, granisetron+SB 269970: -1+/-2 mV). The slow depolarization evoked by 5-CT was also significantly reduced by SB 269970 (control: 14+/-1 mV, SB 269970: 5+/-2 mV) suggesting a 5-HT7 receptor was activated by exogenous application of 5-CT and 5-HT. Slow excitatory postsynaptic potentials evoked by stimulating descending neural pathways (containing serotonergic fibers) were reduced by SB 269970 (control: 8+/-3 mV, SB 269970: 3+/-1 mV). However, SB 269970 had no effect on slow excitatory postsynaptic potentials evoked by stimulation of circumferential (tachykinergic) pathways (control: 7+/-1 mV, SB 269970: 6+/-1 mV). These data are consistent with the presence on enteric AH neurons of functional 5-HT7 receptors that participate in slow synaptic transmission.

Synaptic depression as a timing device

A depressing synapse transforms a time interval into a voltage amplitude. The effect of that transformation on the output of the neuron and network depends on the kinetics of synaptic depression and properties of the postsynaptic neuron and network. Using as examples neural circuits that incorporate depressing synapses, we show how short-term depression can contribute to a surprising variety of time-dependent computational and behavioral tasks.

Recurrent networks of submucous neurons controlling intestinal secretion: a modeling study

Secretomotor neurons, immunoreactive for vasoactive intestinal peptide (VIP), are important in controlling chloride secretion in the small intestine. These neurons form functional synapses with other submucosal VIP neurons and transmit via slow excitatory postsynaptic potentials (EPSPs). Thus they form a recurrent network with positive feedback. Intrinsic sensory neurons within the submucosa are also likely to form recurrent networks with positive feedback, provide substantial output to VIP neurons, and receive input from VIP neurons. If positive feedback within recurrent networks is sufficiently large, then neurons in the network respond to even small stimuli by firing at their maximum possible rate, even after the stimulus is removed. However, it is not clear whether such a mechanism operates within the recurrent networks of submucous neurons. We investigated this question by performing computer simulations of realistic models of VIP and intrinsic sensory neuron networks. In the expected range of electrophysiological properties, we found that activity in the VIP neuron network decayed slowly after cessation of a stimulus, indicating that positive feedback is not strong enough to support the uncontrolled firing state. The addition of intrinsic sensory neurons produced a low stable firing rate consistent with the common finding that basal secretory activity is, in part, neurogenic. Changing electrophysiological properties enables these recurrent networks to support the uncontrolled firing state, which may have implications with hypersecretion in the presence of enterotoxins such as cholera-toxin.

Nerves, reflexes, and the enteric nervous system: pathogenesis of the irritable bowel syndrome

The bowel exhibits reflexes in the absence of CNS input. To do so, epithelial sensory transducers, such as enterochromaffin (EC) cells, activate the mucosal processes of intrinsic (IPANs) and extrinsic primary afferent (sensory) neurons. EC cells secrete serotonin (5-HT) in response to mucosal stimuli. Submucosal IPANs, which secrete acetylcholine and calcitonin gene-related peptide, initiate peristaltic and secretory reflexes and are activated via "5-HT1P" receptors. Release of neurotransmitters is enhanced by 5-HT4 receptors, which are presynaptic and strengthen neurotransmission in prokinetic pathways. 5-HT3 receptors mediate signaling to the CNS and thus ameliorate cancer chemotherapy-associated nausea and the visceral hypersensitivity of diarrhea-predominant irritable bowel syndrome (IBS-D); however, because 5-HT3 receptors also mediate fast ENS neurotransmission and activate myenteric IPANs, they may be constipating. 5-HT4 agonists are prokinetic and relieve discomfort and constipation in IBS-C and chronic constipation. 5-HT4 agonists do not initiate peristaltic and secretory reflexes but strengthen pathways that are naturally activated. Serotonergic signaling in the mucosa and the ENS is terminated by a transmembrane 5-HT transporter, SERT. Mucosal SERT and tryptophan hydroxylase-1 expression are decreased in experimental inflammation, IBS-C, IBS-D, and ulcerative colitis. Potentiation of 5-HT due to the SERT decrease could account for the discomfort and diarrhea of IBS-D, while receptor desensitization may cause constipation. Similar symptoms are seen in transgenic mice that lack SERT. The loss of mucosal SERT may thus contribute to IBS pathogenesis.

Effect of bile acid enterohepatic circulation on interdigestive migrating motor complex

AIM: To investigate the effects of enterohepatic circulation of bile acid on interdigestive migrating motor complex (MMC).

METHODS: The dog models of extracorporeal circulation of bile acid were established firstly (n = 12). Then the records of MMC were observed with bile flow intact, during total external biliary diversion and after re-infusion of the bile into the dogs duodenum respectively. At the same time, the concentration of plasma motilin (MTL) was measured under different conditions.

RESULTS: The interdigestive MMC with bile flow intact included 3 phases. The concentration of plasma MTL cycled with MMC. The level of MTL in phase Ⅲ was much higher than that in phaseⅠ(433.8 ± 46.3 ng/L vs 112.6 ± 21.7 ng/L, P = 0.000 < 0.01). During total external biliary diversion, phase Ⅲ was not obvious. Compared with the normal period of MMC, the one during total external biliary divevsion was much longer (110.2 ± 13.6 min vs 99.1 ± 14.4 min, P = 0.001 < 0.01); the duration of phaseⅠwas shorter (31.0 ± 6.6 min vs 53.2 ± 9.2 min, P = 0.002 < 0.01) and that of phase Ⅱ was longer (79.2 ± 11.8 min vs 41.3 ± 9.9 min, P = 0.001 < 0.01). The dynamic index of last 30 min of phase Ⅱwas lower than that of normal (41.3 ± 6.8 mV/s vs 69.1 ± 9.2 mV/s, P = 0.001 < 0.01). At the same time, the concentrations of plasma MTL did not show significant differences with the changing of MMC. Phase Ⅲ was also not obvious after re-infusion of the bile. However, compared with pre-infusion, the characteristics of MMC changed and were more similar to normal MMC.

CONCLUSION: Enterohepatic circulation of bile acid plays an important role in the maintenance of interdigestive MMC. MTL is an important factor in the initiation of phase Ⅲ of MMC.

MULTIPLE LEVELS OF SENSORY INTEGRATION IN THE INTRINSIC SENSORY NEURONS OF THE ENTERIC NERVOUS SYSTEM

1. The enteric nervous system (ENS) is present in the wall of the gastrointestinal tract and contains all the functional classes of neuron required for complete reflex arcs. One of the most important and intriguing classes of neuron is that responsive to sensory stimuli: sensory neurons with cell bodies intrinsic to the ENS. 2. These neurons have three outstanding and interrelated features: (i) reciprocal connections with each other; (ii) a slow excitatory post-synaptic potential (EPSP) resulting from high-speed firing in other sensory neurons; and (iii) a large after-hyperpolarizing potential (AHP) at the soma. Slow EPSP depolarize the cell body, generate action potentials (APs) and reduce the AHP. Conversely, the AHP limits the firing rate and, hence, reduces transmission of slow EPSP. 3. Processing of sensory information starts at the input terminals as different patterns of APs depending on the sensory modality and recent sensory history. At the soma, the ability to fire APs and, hence, drive outputs is also strongly determined by the recent firing history of the neuron (through the AHP) and network activity (through the slow EPSP). Positive feedback within the population of intrinsic sensory neurons means that the network is able to drive outputs well beyond the duration of the stimuli that triggered them. 4. Thus, sensory input and subsequent reflex generation are integrated over several hierarchical levels within the network on intrinsic sensory neurons.