Neurogenic control of myoelectric complexes in the mouse isolated colon

Enteric Neuronal Substrates Underlying Spontaneous and Evoked Neurogenic Contractions in Mouse Colon

BACKGROUND & AIMS

Gastrointestinal motility persists when peripheral cholinergic signaling is blocked genetically or pharmacologically, and a recent study suggests nitric oxide drives propagating neurogenic contractions.

METHODS

To determine the neuronal substrates that underlie these contractions, we measured contractile-associated movements together with calcium responses of cholinergic or nitrergic myenteric neurons in unparalyzed ex vivo preparations of whole mouse colon. We chose to look at these 2 subpopulations because they encompass nearly all myenteric neurons.

RESULTS

Many, but not all, cholinergic neurons of the middle colon exhibited contractile-associated calcium responses with distinct features. By contrast, a large population of nitrergic neurons of the middle colon shut their activity off just before contraction onset, whereas another population of nitrergic neurons initiated a response just after contraction onset. When contractions were evoked by a variety of stimuli to the proximal and distal colon, the same neuronal subtypes exhibited the same activity patterns during the contraction. However, stimulation of proximal colon produced a transient, stimulation-locked response before the ensuing contraction in a subpopulation of cholinergic neurons and in nearly all nitrergic neurons, suggesting that distinct neuronal activity patterns underlie specific stimuli. Finally, although blockade of nitric oxide failed to arrest the generation or propagation of neurogenic contractions, chemogenetic elimination of nitrergic activity impaired their propagation to middle and distal colon.

CONCLUSIONS

Genetic approaches were used to study the activity patterns of enteric neurons underlying spontaneous and evoked neurogenic contractions in unparalyzed colon. These approaches can be combined with a variety of other approaches to identify the neuronal subtypes and subclasses that coordinate colonic motility.

Integrated responses of the SIP syncytium generate a major motility pattern in the colon

The peristaltic reflex has been a central concept in gastrointestinal motility; however, evidence was published recently suggesting that post-stimulus responses that follow inhibitory neural responses provide the main propulsive force in colonic motility. This new concept was based on experiments on proximal colon where enteric inhibitory neural inputs are mainly nitrergic. However, the nature of inhibitory neural inputs changes from proximal to distal colon where purinergic inhibitory regulation dominates. In spite of the transition from nitrergic to purinergic regulation, post-stimulus responses and propulsive contractions were both blocked by antagonists of a conductance (ANO1) exclusive to interstitial cells of Cajal (ICC). How purinergic neurotransmission, transduced by PDGFRα+ cells, can influence ANO1 in ICC is unknown. We compared neural responses in proximal and distal colon. Post-stimulus responses were blocked by inhibition of nitrergic neurotransmission in proximal colon, but P2Y1 receptor antagonists were more effective in distal colon. Ca2+ entry through voltage-dependent channels (CaV3) enhances Ca2+ release in ICC. Thus, we reasoned that hyperpolarization caused by purinergic responses in PDGFRα+ cells, which are electrically coupled to ICC, might decrease inactivation of CaV3 channels and activate Ca2+ entry into ICC via anode-break upon cessation of inhibitory responses. Post-stimulus responses in distal colon were blocked by MRS2500 (P2Y1 receptor antagonist), apamin (SK channel antagonist) and NNC55-0396 (CaV3 antagonist). These compounds also blocked propagating contractions in mid and distal colon. These data provide the first clear demonstration that integration of functions in the smooth muscle-ICC-PDGFRα+ cell (SIP) syncytium generates a major motility behaviour. KEY POINTS: Propagating propulsive contractions initiated by the enteric nervous system are a major motility behaviour in the colon. A major component of contractions, necessary for propulsive contractions, occurs at cessation of enteric inhibitory neurotransmission (post-stimulus response) and is generated by interstitial cells of Cajal (ICC), which are electrically coupled to smooth muscle cells. The nature of enteric inhibitory neurotransmission shifts from proximal colon, where it is predominantly due to nitric oxide, to distal colon, where it is predominantly due to purine neurotransmitters. Different cells transduce nitric oxide and purines in the colon. ICC transduce nitric oxide, but another type of interstitial cell, PDGFRα+ cells, transduces input from purinergic neurons. However, the post-stimulus responses in proximal and distal colon are still generated in ICC. This paper explores how integrated behaviours of ICC, PDGFRα+ cells and smooth muscle cells accomplish propulsive motility in the colon.

Autologous cell transplantation for treatment of colorectal aganglionosis in mice

Neurointestinal diseases cause significant morbidity and effective treatments are lacking. This study aimes to test the feasibility of transplanting autologous enteric neural stem cells (ENSCs) to rescue the enteric nervous system (ENS) in a model of colonic aganglionosis. ENSCs are isolated from a segment of small intestine from Wnt1::Cre;R26iDTR mice in which focal colonic aganglionosis is simultaneously created by diphtheria toxin injection. Autologous ENSCs are isolated, expanded, labeled with lentiviral-GFP, and transplanted into the aganglionic segment in vivo. ENSCs differentiate into neurons and glia, cluster to form neo-ganglia, and restore colonic contractile activity as shown by electrical field stimulation and optogenetics. Using a non-lethal model of colonic aganglionosis, our results demonstrate the potential of autologous ENSC therapy to improve functional outcomes in neurointestinal disease, laying the groundwork for clinical application of this regenerative cell-based approach.

Ca2+ dynamics in interstitial cells: foundational mechanisms for the motor patterns in the gastrointestinal tract

The gastrointestinal (GI) tract displays multiple motor patterns that move nutrients and wastes through the body. Smooth muscle cells (SMCs) provide the forces necessary for GI motility, but interstitial cells, electrically coupled to SMCs, tune SMC excitability, transduce inputs from enteric motor neurons, and generate pacemaker activity that underlies major motor patterns, such as peristalsis and segmentation. The interstitial cells regulating SMCs are interstitial cells of Cajal (ICC) and PDGF receptor (PDGFR)α+ cells. Together these cells form the SIP syncytium. ICC and PDGFRα+ cells express signature Ca2+-dependent conductances: ICC express Ca2+-activated Cl- channels, encoded by Ano1, that generate inward current, and PDGFRα+ cells express Ca2+-activated K+ channels, encoded by Kcnn3, that generate outward current. The open probabilities of interstitial cell conductances are controlled by Ca2+ release from the endoplasmic reticulum. The resulting Ca2+ transients occur spontaneously in a stochastic manner. Ca2+ transients in ICC induce spontaneous transient inward currents and spontaneous transient depolarizations (STDs). Neurotransmission increases or decreases Ca2+ transients, and the resulting depolarizing or hyperpolarizing responses conduct to other cells in the SIP syncytium. In pacemaker ICC, STDs activate voltage-dependent Ca2+ influx, which initiates a cluster of Ca2+ transients and sustains activation of ANO1 channels and depolarization during slow waves. Regulation of GI motility has traditionally been described as neurogenic and myogenic. Recent advances in understanding Ca2+ handling mechanisms in interstitial cells and how these mechanisms influence motor patterns of the GI tract suggest that the term "myogenic" should be replaced by the term "SIPgenic," as this review discusses.

GutMap: A New Interface for Analysing Regional Motility Patterns in ex vivo Mouse Gastrointestinal Preparations

Different regions of the gastrointestinal tract have specific functions and thus distinct motility patterns. Motility is primarily regulated by the enteric nervous system (ENS), an intrinsic network of neurons located within the gut wall. Under physiological conditions, the ENS is influenced by the central nervous system (CNS). However, by using ex vivo organ bath experiments, ENS regulation of gut motility can also be studied in the absence of CNS influences. The current technique enables the characterisation of small intestinal, caecal, and colonic motility patterns using an ex vivo organ bath and video imaging protocol. This approach is combined with the novel edge detection script GutMap, available in MATLAB, that functions across Windows and Mac platforms. Dissected intestinal segments are cannulated in an organ bath containing physiological saline with a camera mounted overhead. Video recordings of gut contractions are then converted to spatiotemporal heatmaps and analysed using the GutMap software interface. Using data analysed from the heatmaps, parameters of contractile patterns (including contraction propagation frequency and velocity as well as gut diameter) at baseline and in the presence of drugs/treatments/genetic mutations can be compared. Here, we studied motility patterns of female mice at baseline and in the presence of a nitric oxide synthase inhibitor (Nω-Nitro-L-arginine; NOLA) (nitric oxide being the main inhibitory neurotransmitter of gut motility) to showcase the application of GutMap. This technique is suitable for application to a broad range of animal models of clinical disorders to understand underlying biological pathways contributing to gastrointestinal dysfunction. Key features • Enhanced video imaging analysis of gut contractility in rodents using a novel software interface. • New edge detection algorithm to accurately contour curvatures of the gastrointestinal tract. • Allows for output of high-resolution spatiotemporal heatmaps across Windows and Mac platforms. • Edge detection and analysis method makes motility measurements accessible in different gut regions including the caecum and stomach.

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.

Insights on gastrointestinal motility through the use of optogenetic sensors and actuators

The muscularis of the gastrointestinal (GI) tract consists of smooth muscle cells (SMCs) and various populations of interstitial cells of Cajal (ICC), platelet-derived growth factor receptor α⁺ (PDGFRα⁺ ) cells, as well as excitatory and inhibitory enteric motor nerves. SMCs, ICC and PDGFRα⁺ cells form an electrically coupled syncytium, which together with inputs from the enteric nervous system (ENS) regulate GI motility. Early studies evaluating Ca²⁺ signalling behaviours in the GI tract relied upon indiscriminate loading of tissues with Ca²⁺ dyes. These methods lacked the means to study activity in specific cells of interest without encountering contamination from other cells within the preparation. Development of mice expressing optogenetic sensors (GCaMP, RCaMP) has allowed visualization of Ca²⁺ signalling behaviours in a cell specific manner. Additionally, availability of mice expressing optogenetic modulators (channelrhodopsins or halorhodospins) has allowed manipulation of specific signalling pathways using light. GCaMP expressing animals have been used to characterize Ca²⁺ signalling behaviours of distinct classes of ICC and SMCs throughout the GI musculature. These findings illustrate how Ca²⁺ signalling in ICC is fundamental in GI muscles, contributing to tone in sphincters, pacemaker activity in rhythmic muscles and relaying enteric signals to SMCs. Animals that express channelrhodopsin in specific neuronal populations have been used to map neural circuitry and to examine post junctional neural effects on GI motility. Thus, optogenetic approaches provide a novel means to examine the contribution of specific cell types to the regulation of motility patterns within complex multi-cellular systems. Abstract Figure Legends Optogenetic activators and sensors can be used to investigate the complex multi-cellular nature of the gastrointestinal (GI tract). Optogenetic activators that are activated by light such as channelrhodopsins (ChR2), OptoXR and halorhodopsinss (HR) proteins can be genetically encoded into specific cell types. This can be used to directly activate or silence specific GI cells such as various classes of enteric neurons, smooth muscle cells (SMC) or interstitial cells, such as interstitial cells of Cajal (ICC). Optogenetic sensors that are activated by different wavelengths of light such as green calmodulin fusion protein (GCaMP) and red CaMP (RCaMP) make high resolution of sub-cellular Ca²⁺ signalling possible within intact tissues of specific cell types. These tools can provide unparalleled insight into mechanisms underlying GI motility and innervation. This article is protected by copyright. All rights reserved.

Propulsive colonic contractions are mediated by inhibition-driven poststimulus responses that originate in interstitial cells of Cajal

The peristaltic reflex is a fundamental behavior of the gastrointestinal (GI) tract in which mucosal stimulation activates propulsive contractions. The reflex occurs by stimulation of intrinsic primary afferent neurons with cell bodies in the myenteric plexus and projections to the lamina propria, distribution of information by interneurons, and activation of muscle motor neurons. The current concept is that excitatory cholinergic motor neurons are activated proximal to and inhibitory neurons are activated distal to the stimulus site. We found that atropine reduced, but did not block, colonic migrating motor complexes (CMMCs) in mouse, monkey, and human colons, suggesting a mechanism other than one activated by cholinergic neurons is involved in the generation/propagation of CMMCs. CMMCs were activated after a period of nerve stimulation in colons of each species, suggesting that the propulsive contractions of CMMCs may be due to the poststimulus excitation that follows inhibitory neural responses. Blocking nitrergic neurotransmission inhibited poststimulus excitation in muscle strips and blocked CMMCs in intact colons. Our data demonstrate that poststimulus excitation is due to increased Ca2+ transients in colonic interstitial cells of Cajal (ICC) following cessation of nitrergic, cyclic guanosine monophosphate (cGMP)-dependent inhibitory responses. The increase in Ca2+ transients after nitrergic responses activates a Ca2+-activated Cl− conductance, encoded by Ano1, in ICC. Antagonists of ANO1 channels inhibit poststimulus depolarizations in colonic muscles and CMMCs in intact colons. The poststimulus excitatory responses in ICC are linked to cGMP-inhibited cyclic adenosine monophosphate (cAMP) phosphodiesterase 3a and cAMP-dependent effects. These data suggest alternative mechanisms for generation and propagation of CMMCs in the colon.

Activation of ENS Circuits in Mouse Colon: Coordination in the Mouse Colonic Motor Complex as a Robust, Distributed Control System

The characteristic motor patterns of the colon are coordinated by the enteric nervous system (ENS) and involve enterochromaffin (EC) cells, enteric glia, smooth muscle fibers, and interstitial cells. While the fundamental control mechanisms of colonic motor patterns are understood, greater complexity in the circuitry underlying motor patterns has been revealed by recent advances in the field. We review these recent advances and new findings from our laboratories that provide insights into how the ENS coordinates motor patterns in the isolated mouse colon. We contextualize these observations by describing the neuromuscular system underling the colonic motor complex (CMC) as a robust, distributed control system. Framing the colonic motor complex as a control system reveals a new perspective on the coordinated motor patterns in the colon. We test the control system by applying electrical stimulation in the isolated mouse colon to disrupt the coordination and propagation of the colonic motor complex.

Nitric Oxide Regulates Estrus Cycle Dependent Colonic Motility in Mice

Women are more susceptible to functional bowel disorders than men and the severity of their symptoms such as diarrhea, constipation, abdominal pain and bloating changes over the menstrual cycle, suggesting a role for sex hormones in gastrointestinal function. Nitric oxide (NO) is a major inhibitory neurotransmitter in the gut and blockade of nitric oxide synthase (NOS; responsible for NO synthesis) increases colonic motility in male mice ex vivo. We assessed the effects of NOS inhibition on colonic motility in female mice using video imaging analysis of colonic motor complexes (CMCs). To understand interactions between NO and estrogen in the gut, we also quantified neuronal NOS and estrogen receptor alpha (ERα)-expressing myenteric neurons in estrus and proestrus female mice using immunofluorescence. Mice in estrus had fewer CMCs under control conditions (6 ± 1 per 15 min, n = 22) compared to proestrus (8 ± 1 per 15 min, n = 22, One-way ANOVA, p = 0.041). During proestrus, the NOS antagonist N-nitro-L-arginine (NOLA) increased CMC numbers compared to controls (189 ± 46%). In contrast, NOLA had no significant effect on CMC numbers during estrus. During estrus, we observed more NOS-expressing myenteric neurons (48 ± 2%) than during proestrus (39 ± 1%, n = 3, p = 0.035). Increased nuclear expression of ERα was observed in estrus which coincided with an altered motility response to NOLA in contrast with proestrus when ERα was largely cytoplasmic. In conclusion, we confirm a cyclic and sexually dimorphic effect of NOS activity in female mouse colon, which could be due to genomic effects of estrogens via ERα.

Intrinsic cholinergic innervation in the human sigmoid colon revealed using CLARITY, three-dimensional (3D) imaging, and a novel anti-human peripheral choline acetyltransferase (hpChAT) antiserum

BACKGROUND

We previously reported the specificity of a novel anti-human peripheral choline acetyltransferase (hpChAT) antiserum for immunostaining of cholinergic neuronal cell bodies and fibers in the human colon. In this study, we investigate 3D architecture of intrinsic cholinergic innervation in the human sigmoid colon and the relationship with nitrergic neurons in the enteric plexus.

METHODS

We developed a modified CLARITY tissue technique applicable for clearing human sigmoid colon specimens and immunostaining with hpChAT antiserum and co-labeling with neuronal nitric oxide synthase (nNOS) antibody. The Z-stack confocal images were processed for 3D reconstruction/segmentation/digital tracing and computational quantitation by Imaris 9.2 and 9.5.

KEY RESULTS

In the mucosa, a local micro-neuronal network formed of hpChAT-ir fibers and a few neuronal cell bodies were digitally assembled. Three layers of submucosal plexuses were displayed in 3D structure that were interconnected by hpChAT-ir fiber bundles and hpChAT-ir neurons were rarely co-labeled by nNOS. In the myenteric plexus, 30.1% of hpChAT-ir somas including Dogiel type I and II were co-labeled by nNOS and 3 classes of hpChAT-ir nerve fiber strands were visualized in 3D images and videos. The density and intensity values of hpChAT-ir fibers in 3D structure were significantly higher in the circular than in the longitudinal layer.

CONCLUSIONS AND INFERENCES

The intrinsic cholinergic innervation in the human sigmoid colon was demonstrated layer by layer for the first time in 3D microstructures. This may open a new venue to assess the structure-function relationships and pathological alterations in colonic diseases.

Advances in colonic motor complexes in mice

The primary functions of the gastrointestinal (GI) tract are to absorb nutrients, water, and electrolytes that are essential for life. This is accompanied by the capability of the GI tract to mix ingested content to maximize absorption and effectively excrete waste material. There have been major advances in understanding intrinsic neural mechanisms involved in GI motility. This review highlights major advances over the past few decades in our understanding of colonic motor complexes (CMCs), the major intrinsic neural patterns that control GI motility. CMCs are generated by rhythmic coordinated firing of large populations of myenteric neurons. Initially, it was thought that serotonin release from the mucosa was required for CMC generation. However, careful experiments have now shown that neither the mucosa nor endogenous serotonin are required, although, evidence suggests enteroendocrine (EC) cells modulate CMCs. The frequency and extent of propagation of CMCs are highly dependent on mechanical stimuli (circumferential stretch). In summary, the isolated mouse colon emerges as a good model to investigate intrinsic mechanisms underlying colonic motility and provides an excellent preparation to explore potential therapeutic agents on colonic motility, in a highly controlled in vitro environment. In addition, during CMCs, the mouse colon facilitates investigations into the emergence of dynamic assemblies of extensive neural networks, applicable to the nervous system of different organisms.

A Novel Mode of Sympathetic Reflex Activation Mediated by the Enteric Nervous System

Enteric viscerofugal neurons provide a pathway by which the enteric nervous system (ENS), otherwise confined to the gut wall, can activate sympathetic neurons in prevertebral ganglia. Firing transmitted through these pathways is currently considered fundamentally mechanosensory. The mouse colon generates a cyclical pattern of neurogenic contractile activity, called the colonic motor complex (CMC). Motor complexes involve a highly coordinated firing pattern in myenteric neurons with a frequency of ∼2 Hz. However, it remains unknown how viscerofugal neurons are activated and communicate with the sympathetic nervous system during this naturally-occurring motor pattern. Here, viscerofugal neurons were recorded extracellularly from rectal nerve trunks in isolated tube and flat-sheet preparations of mouse colon held at fixed circumferential length. In freshly dissected preparations, motor complexes were associated with bursts of viscerofugal firing at 2 Hz that aligned with 2-Hz smooth muscle voltage oscillations. This behavior persisted during muscle paralysis with nicardipine. Identical recordings were made after a 4- to 5-d organotypic culture during which extrinsic nerves degenerated, confirming that recordings were from viscerofugal neurons. Single unit analysis revealed the burst firing pattern emerging from assemblies of viscerofugal neurons differed from individual neurons, which typically made partial contributions, highlighting the importance and extent of ENS-mediated synchronization. Finally, sympathetic neuron firing was recorded from the central nerve trunks emerging from the inferior mesenteric ganglion. Increased sympathetic neuron firing accompanied all motor complexes with a 2-Hz burst pattern similar to viscerofugal neurons. These data provide evidence for a novel mechanism of sympathetic reflex activation derived from synchronized firing output generated by the ENS.

Neural motor complexes propagate continuously along the full length of mouse small intestine and colon

Cyclical propagating waves of muscle contraction have been recorded in isolated small intestine or colon, referred to here as motor complexes (MCs). Small intestinal and colonic MCs are neurogenic, occur at similar frequencies, and propagate orally or aborally. Whether they can be coordinated between the different gut regions is unclear. Motor behavior of whole length mouse intestines, from duodenum to terminal rectum, was recorded by intraluminal multisensor catheter. Small intestinal MCs were recorded in 27/30 preparations, and colonic MCs were recorded in all preparations (n = 30) with similar frequencies (0.54 ± 0.03 and 0.58 ± 0.02 counts/min, respectively). MCs propagated across the ileo-colonic junction in 10/30 preparations, forming "full intestine" MCs. The cholinesterase inhibitor physostigmine increased the probability of a full intestine MC but had no significant effect on frequency, speed, or direction. Nitric oxide synthesis blockade by N^ω-nitro-l-arginine, after physostigmine, increased MC frequency in small intestine only. Hyoscine-resistant MCs were recorded in the colon but not small intestine (n = 5). All MCs were abolished by hexamethonium (n = 18) or tetrodotoxin (n = 2). The enteric neural mechanism required for motor complexes is present along the full length of both the small and large intestine. In some cases, colonic MCs can be initiated in the distal colon and propagate through the ileo-colonic junction, all the way to duodenum. In conclusion, the ileo-colonic junction provides functional neural continuity for propagating motor activity that originates in the small or large intestine.NEW & NOTEWORTHY Intraluminal manometric recordings revealed motor complexes can propagate antegradely or retrogradely across the ileo-colonic junction, spanning the entire small and large intestines. The fundamental enteric neural mechanism(s) underlying cyclic motor complexes exists throughout the length of the small and large intestine.

Tonic inhibition of murine proximal colon is due to nitrergic suppression of Ca2+ signaling in interstitial cells of Cajal

Spontaneous excitability and contractions of colonic smooth muscle cells (SMCs) are normally suppressed by inputs from inhibitory motor neurons, a behavior known as tonic inhibition. The post-junctional cell(s) mediating tonic inhibition have not been elucidated. We investigated the post-junctional cells mediating tonic inhibition in the proximal colon and whether tonic inhibition results from suppression of the activity of Ano1 channels, which are expressed exclusively in interstitial cells of Cajal (ICC). We found that tetrodotoxin (TTX), an inhibitor of nitric oxide (NO) synthesis, L-NNA, and an inhibitor of soluble guanylyl cyclase, ODQ, greatly enhanced colonic contractions. Ano1 antagonists, benzbromarone and Ani9 inhibited the effects of TTX, L-NNA and ODQ. Ano1 channels are activated by Ca²⁺ release from the endoplasmic reticulum (ER) in ICC, and blocking Ca²⁺ release with a SERCA inhibitor (thapsigargin) or a store-operated Ca²⁺ entry blocker (GSK 7975 A) reversed the effects of TTX, L-NNA and ODQ. Ca²⁺ imaging revealed that TTX, L-NNA and ODQ increased Ca²⁺ transient firing in colonic ICC. Our results suggest that tonic inhibition in the proximal colon occurs through suppression of Ca²⁺ release events in ICC. Suppression of Ca²⁺ release in ICC limits the open probability of Ano1 channels, reducing the excitability of electrically-coupled SMCs.

Identification of a Rhythmic Firing Pattern in the Enteric Nervous System That Generates Rhythmic Electrical Activity in Smooth Muscle

The enteric nervous system (ENS) contains millions of neurons essential for organization of motor behavior of the intestine. It is well established that the large intestine requires ENS activity to drive propulsive motor behaviors. However, the firing pattern of the ENS underlying propagating neurogenic contractions of the large intestine remains unknown. To identify this, we used high-resolution neuronal imaging with electrophysiology from neighboring smooth muscle. Myoelectric activity underlying propagating neurogenic contractions along murine large intestine [also referred to as colonic migrating motor complexes, (CMMCs)] consisted of prolonged bursts of rhythmic depolarizations at a frequency of ∼2 Hz. Temporal coordination of this activity in the smooth muscle over large spatial fields (∼7 mm, longitudinally) was dependent on the ENS. During quiescent periods between neurogenic contractions, recordings from large populations of enteric neurons, in mice of either sex, revealed ongoing activity. The onset of neurogenic contractions was characterized by the emergence of temporally synchronized activity across large populations of excitatory and inhibitory neurons. This neuronal firing pattern was rhythmic and temporally synchronized across large numbers of ganglia at ∼2 Hz. ENS activation preceded smooth muscle depolarization, indicating rhythmic depolarizations in smooth muscle were controlled by firing of enteric neurons. The cyclical emergence of temporally coordinated firing of large populations of enteric neurons represents a unique neural motor pattern outside the CNS. This is the first direct observation of rhythmic firing in the ENS underlying rhythmic electrical depolarizations in smooth muscle. The pattern of neuronal activity we identified underlies the generation of CMMCs.SIGNIFICANCE STATEMENT How the enteric nervous system (ENS) generates neurogenic contractions of smooth muscle in the gastrointestinal (GI) tract has been a long-standing mystery in vertebrates. It is well known that myogenic pacemaker cells exist in the GI tract [called interstitial cells of Cajal (ICCs)] that generate rhythmic myogenic contractions. However, the mechanisms underlying the generation of rhythmic neurogenic contractions of smooth muscle in the GI tract remains unknown. We developed a high-resolution neuronal imaging method with electrophysiology to address this issue. This technique revealed a novel pattern of rhythmic coordinated neuronal firing in the ENS that has never been identified. Rhythmic neuronal firing in the ENS was found to generate rhythmic neurogenic depolarizations in smooth muscle that underlie contraction of the GI tract.

Excitatory Neuronal Responses of Ca2+ Transients in Interstitial Cells of Cajal in the Small Intestine

Interstitial cells of Cajal (ICC) regulate smooth muscle excitability and motility in the gastrointestinal (GI) tract. ICC in the deep muscular plexus (ICC-DMP) of the small intestine are aligned closely with varicosities of enteric motor neurons and thought to transduce neural responses. ICC-DMP generate Ca²⁺ transients that activate Ca²⁺ activated Cl^- channels and generate electrophysiological responses. We tested the hypothesis that excitatory neurotransmitters regulate Ca²⁺ transients in ICC-DMP as a means of regulating intestinal muscles. High-resolution confocal microscopy was used to image Ca²⁺ transients in ICC-DMP within murine small intestinal muscles with cell-specific expression of GCaMP3. Intrinsic nerves were stimulated by electrical field stimulation (EFS). ICC-DMP exhibited ongoing Ca²⁺ transients before stimuli were applied. EFS caused initial suppression of Ca²⁺ transients, followed by escape during sustained stimulation, and large increases in Ca²⁺ transients after cessation of stimulation. Basal Ca²⁺ activity and the excitatory phases of Ca²⁺ responses to EFS were inhibited by atropine and neurokinin 1 receptor (NK1) antagonists, but not by NK2 receptor antagonists. Exogenous ACh and substance P (SP) increased Ca²⁺ transients, atropine and NK1 antagonists decreased Ca²⁺ transients. Neurokinins appear to be released spontaneously (tonic excitation) in small intestinal muscles and are the dominant excitatory neurotransmitters. Subcellular regulation of Ca²⁺ release events in ICC-DMP may be a means by which excitatory neurotransmission organizes intestinal motility patterns.

Neurogenic and myogenic patterns of electrical activity in isolated intact mouse colon

BACKGROUND

Relatively little is known about the electrical rhythmicity of the whole colon, where long neural pathways are preserved.

METHODS

Smooth muscle electrical activity was recorded extracellularly from the serosa of isolated flat-sheet preparations consisting of the whole mouse colon (n=31).

KEY RESULTS

Two distinct electrical patterns were observed. The first, long intense spike bursts, occurred every 349±256 seconds (0.2±0.2 cpm), firing action potentials for 31±11 seconds at 2.1±0.5 Hz. They were hexamethonium- and tetrodotoxin-sensitive, but persisted in nicardipine as 2 Hz electrical oscillations lacking action potentials. This pattern is called here neurogenic spike bursts. The second pattern, short spike bursts, occurred about every 30 seconds (2.0±0.6 cpm), with action potentials firing at about 1 Hz for 9 seconds (1.0±0.2 Hz, 9±4 seconds). Short spike bursts were hexamethonium- and tetrodotoxin-resistant but nicardipine-sensitive and thus called here myogenic spike bursts. Neurogenic spike bursts transiently delayed myogenic spike bursts, while blocking neurogenic activity enhanced myogenic spike burst durations. External stimuli significantly affected neurogenic but not myogenic spike bursts. Aboral electrical or mechanical stimuli evoked premature neurogenic spike bursts. Circumferential stretch significantly decreased intervals between neurogenic spike bursts. Lesioning the colon down to 10 mm segments significantly increased intervals or abolished neurogenic spike bursts, while myogenic spike bursts persisted.

CONCLUSIONS & INFERENCES

Distinct neurogenic and myogenic electrical patterns were recorded from mouse colonic muscularis externa. Neurogenic spike bursts likely correlate with neurogenic colonic migrating motor complexes (CMMC) and are highly sensitive to mechanical stimuli. Myogenic spike bursts may correspond to slow myogenic contractions, whose duration can be modulated by enteric neural activity.

Dialysis membrane-enforced microelectrode array measurement of diverse gut electrical activity

A variety of electrical activities occur depending on the functional state in each section of the gut, but the application of microelectrode array (MEA) is rather limited. We thus developed a dialysis membranes-enforced technique to investigate diverse and complex spatio-temporal electrical activity in the gut. Muscle sheets isolated from the gastrointestinal (GI) tract of mice along with a piece of dialysis membrane were woven over and under the strings to fix them to the anchor rig, and mounted on an 8×8 MEA (inter-electrode distance=150µm). Small molecules (molecular weight <12,000) were exchanged through the membrane, maintaining a physiological environment. Low impedance MEA was used to measure electrical signals in a wide frequency range. We demonstrated the following examples: 1) pacemaker activity-like potentials accompanied by bursting spike-like potentials in the ileum; 2) electrotonic potentials reflecting local neurotransmission in the ileum; 3) myoelectric complex-like potentials consisting of slow and rapid oscillations accompanied by spike potentials in the colon. Despite their limited spatial resolution, these recordings detected transient electric activities that optical probes followed with difficulty. In Addition, propagation of pacemaker-like potential was visualized in the stomach and ileum. These results indicate that the dialysis membrane-enforced technique largely extends the application of MEA, probably due to stabilisation of the access resistance between each sensing electrode and a reference electrode and improvement of electric separation between sensing electrodes. We anticipate that this technique will be utilized to characterise spatio-temporal electrical activities in the gut in health and disease.

Spontaneous Ca(2+) transients in interstitial cells of Cajal located within the deep muscular plexus of the murine small intestine

KEY POINTS

Interstitial cells of Cajal at the level of the deep muscular plexus (ICC-DMP) in the small intestine generate spontaneous Ca(2+) transients that consist of localized Ca(2+) events and limited propagating Ca(2+) waves. Ca(2+) transients in ICC-DMP display variable characteristics: from discrete, highly localized Ca(2+) transients to regionalized Ca(2+) waves with variable rates of occurrence, amplitude, duration and spatial spread. Ca(2+) transients fired stochastically, with no cellular or multicellular rhythmic activity being observed. No correlation was found between the firing sites in adjacent cells. Ca(2+) transients in ICC-DMP are suppressed by the ongoing release of inhibitory neurotransmitter(s). Functional intracellular Ca(2+) stores are essential for spontaneous Ca(2+) transients, and the sarco/endoplasmic reticulum Ca(2+) -ATPase (SERCA) pump is necessary for maintenance of spontaneity. Ca(2+) release mechanisms involve both ryanodine receptors (RyRs) and inositol triphosphate receptors (InsP3 Rs). Release from these channels is interdependent. ICC express transcripts of multiple RyRs and InsP3 Rs, with Itpr1 and Ryr2 subtypes displaying the highest expression.

ABSTRACT

Interstitial cells of Cajal in the deep muscular plexus of the small intestine (ICC-DMP) are closely associated with varicosities of enteric motor neurons and generate responses contributing to neural regulation of intestinal motility. Responses of ICC-DMP are mediated by activation of Ca(2+) -activated Cl(-) channels; thus, Ca(2+) signalling is central to the behaviours of these cells. Confocal imaging was used to characterize the nature and mechanisms of Ca(2+) transients in ICC-DMP within intact jejunal muscles expressing a genetically encoded Ca(2+) indicator (GCaMP3) selectively in ICC. ICC-DMP displayed spontaneous Ca(2+) transients that ranged from discrete, localized events to waves that propagated over variable distances. The occurrence of Ca(2+) transients was highly variable, and it was determined that firing was stochastic in nature. Ca(2+) transients were tabulated in multiple cells within fields of view, and no correlation was found between the events in adjacent cells. TTX (1 μm) significantly increased the occurrence of Ca(2+) transients, suggesting that ICC-DMP contributes to the tonic inhibition conveyed by ongoing activity of inhibitory motor neurons. Ca(2+) transients were minimally affected after 12 min in Ca(2+) free solution, indicating these events do not depend immediately upon Ca(2+) influx. However, inhibitors of sarco/endoplasmic reticulum Ca(2+) -ATPase (SERCA) pump and blockers of inositol triphosphate receptor (InsP3 R) and ryanodine receptor (RyR) channels blocked ICC Ca(2+) transients. These data suggest an interdependence between RyR and InsP3 R in the generation of Ca(2+) transients. Itpr1 and Ryr2 were the dominant transcripts expressed by ICC. These findings provide the first high-resolution recording of the subcellular Ca(2+) dynamics that control the behaviour of ICC-DMP in situ.

Neuronal Differentiation in Schwann Cell Lineage Underlies Postnatal Neurogenesis in the Enteric Nervous System

Elucidation of the cellular identity of neuronal precursors provides mechanistic insights into the development and pathophysiology of the nervous system. In the enteric nervous system (ENS), neurogenesis persists from midgestation to the postnatal period. Cellular mechanism underlying the long-term neurogenesis in the ENS has remained unclear. Using genetic fate mapping in mice, we show here that a subset of Schwann cell precursors (SCPs), which invades the gut alongside the extrinsic nerves, adopts a neuronal fate in the postnatal period and contributes to the ENS. We found SCP-derived neurogenesis in the submucosal region of the small intestine in the absence of vagal neural crest-derived ENS precursors. Under physiological conditions, SCPs comprised up to 20% of enteric neurons in the large intestine and gave rise mainly to restricted neuronal subtypes, calretinin-expressing neurons. Genetic ablation of Ret, the signaling receptor for glial cell line-derived neurotrophic factor, in SCPs caused colonic oligoganglionosis, indicating that SCP-derived neurogenesis is essential to ENS integrity. Identification of Schwann cells as a physiological neurogenic source provides novel insight into the development and disorders of neural crest-derived tissues.

SIGNIFICANCE STATEMENT

Elucidating the cellular identity of neuronal precursors provides novel insights into development and function of the nervous system. The enteric nervous system (ENS) is innervated richly by extrinsic nerve fibers, but little is known about the significance of extrinsic innervation to the structural integrity of the ENS. This report reveals that a subset of Schwann cell precursors (SCPs), which invades the gut alongside the extrinsic nerves, adopts a neuronal fate and differentiates into specific neuronal subtypes. SCP-specific ablation of the Ret gene leads to colonic oligoganglionosis, demonstrating a crucial role of SCP-derived neurogenesis in ENS development. Cross-lineage differentiation capacity in SCPs suggests their potential involvement in the development and pathology of a wide variety of neural crest-derived cell types.

Serotonin and colonic motility

The role of serotonin (5-hydroxytryptamine [5-HT]) in gastrointestinal motility has been studied for over 50 years. Most of the 5-HT in the body resides in the gut wall, where it is located in subsets of mucosal cells (enterochromaffin cells) and neurons (descending interneurons). Many studies suggest that 5-HT is important to normal and dysfunctional gut motility and drugs affecting 5-HT receptors, especially 5-HT3 and 5-HT4 receptors, have been used clinically to treat motility disorders; however, cardiovascular side effects have limited the use of these drugs. Recently studies have questioned the importance and necessity of 5-HT in general and mucosal 5-HT in particular for colonic motility. Recent evidence suggests the importance of 5-HT3 and 5-HT4 receptors for initiation and generation of one of the key colonic motility patterns, the colonic migrating motor complex (CMMC), in rat. The findings suggest that 5-HT3 and 5-HT4 receptors are differentially involved in two different types of rat CMMCs: the long distance contraction (LDC) and the rhythmic propulsive motor complex (RPMC). The understanding of the role of serotonin in colonic motility has been influenced by the specific motility pattern(s) studied, the stimulus used to initiate the motility (spontaneous vs induced), and the route of administration of drugs. All of these considerations contribute to the understanding and the controversy that continues to surround the role of serotonin in the gut.

The purinergic neurotransmitter revisited: a single substance or multiple players?

The past half century has witnessed tremendous advances in our understanding of extracellular purinergic signaling pathways. Purinergic neurotransmission, in particular, has emerged as a key contributor in the efficient control mechanisms in the nervous system. The identity of the purine neurotransmitter, however, remains controversial. Identifying it is difficult because purines are present in all cell types, have a large variety of cell sources, and are released via numerous pathways. Moreover, studies on purinergic neurotransmission have relied heavily on indirect measurements of integrated postjunctional responses that do not provide direct information for neurotransmitter identity. This paper discusses experimental support for adenosine 5'-triphosphate (ATP) as a neurotransmitter and recent evidence for possible contribution of other purines, in addition to or instead of ATP, in chemical neurotransmission in the peripheral, enteric and central nervous systems. Sites of release and action of purines in model systems such as vas deferens, blood vessels, urinary bladder and chromaffin cells are discussed. This is preceded by a brief discussion of studies demonstrating storage of purines in synaptic vesicles. We examine recent evidence for cell type targets (e.g., smooth muscle cells, interstitial cells, neurons and glia) for purine neurotransmitters in different systems. This is followed by brief discussion of mechanisms of terminating the action of purine neurotransmitters, including extracellular nucleotide hydrolysis and possible salvage and reuptake in the cell. The significance of direct neurotransmitter release measurements is highlighted. Possibilities for involvement of multiple purines (e.g., ATP, ADP, NAD(+), ADP-ribose, adenosine, and diadenosine polyphosphates) in neurotransmission are considered throughout.

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.

Toward a better understanding of gastrointestinal nitrergic neuromuscular transmission

BACKGROUND

Nitric oxide (NO) is an important inhibitory neurotransmitter in the gastrointestinal (GI) tract. The majority of nitrergic effects are transduced by NO-sensitive guanylyl cyclase (NO-GC) as the receptor for NO, and, thus, mediated by cGMP-dependent mechanisms. Work carried out during the past years has demonstrated NO to be largely involved in GI smooth muscle relaxation and motility. However, detailed investigation of nitrergic signaling has turned out to be complicated as NO-GC was identified in several different GI cell types such as smooth muscle cells, interstitial cells of Cajal and fibroblast-like cells. With regards to nitrergic neurotransmission, special focus has been placed on the role of interstitial cells of Cajal using mutant mice with reduced populations of ICC. Recently, global and cell-specific knockout mice for enzymes participating in nitrergic signaling have been generated providing a suitable approach to further examine the role of NO-mediated signaling in GI smooth muscle.

PURPOSE

This review discusses the current knowledge on nitrergic mechanisms in gastrointestinal neuromuscular transmission with a focus on genetic models and outlines possible further investigations to gain better understanding on NO-mediated effects in the GI tract.

Expression and function of NIK- and IKK2-binding protein (NIBP) in mouse enteric nervous system

BACKGROUND

NIK- and IKK2-binding protein (NIBP)/TRAPPC9 is expressed in brain neurons, and human NIBP mutations are associated with neurodevelopmental disorders. The cellular distribution and function of NIBP in the enteric nervous system (ENS) remain unknown.

METHODS

Western blot and reverse transcription-polymerase chain reaction analysis were used respectively to identify the protein and mRNA expression of NIBP and other neuronal markers. Multi-labeled immunofluorescent microscopy and confocal image analysis were used to examine the cellular distribution of NIBP-like immunoreactivity (IR) in whole mount intestine. Enteric neuronal cell line (ENC) was infected with lentivirus carrying NIBP or its shRNA expression vectors and treated with vehicle or tumor necrosis factor (TNF)α.

KEY RESULTS

NIBP is expressed at both mRNA and protein levels in different regions and layers of the mouse intestine. NIBP-like-IR was co-localized with various neuronal markers, but not with glial, smooth muscular, or interstitial cells of Cajal markers. A small population of NIBP-expressing cells and fibers in extra-ganglionic and intra-ganglionic area were negative for pan-neuronal markers HuD or Peripherin. Relatively high NIBP-like-IR was found in 35-44% of myenteric neurons and 9-10% of submucosal neurons. Approximately 98%, 87%, and 43% of these relatively high NIBP-expressing neurons were positive for choline acetyltransferase, neuronal nitric oxide synthase and Calretinin, respectively. NIBP shRNA knockdown in ENC inhibited TNFα-induced NFκB activation and neuronal differentiation, whereas NIBP overexpression promoted it.

CONCLUSIONS & INFERENCES

NIBP is extensively expressed in the ENS with relatively high level in a subpopulation of enteric neurons. Various NIBP expression levels in different neurons may represent dynamic trafficking or posttranslational modification of NIBP in some functionally active neurons and ultimately regulate ENS plasticity.

Neurogenic and myogenic motor activity in the colon of the guinea pig, mouse, rabbit, and rat

Gastrointestinal motility involves interactions between myogenic and neurogenic processes intrinsic to the gut wall. We have compared the presence of propagating myogenic contractions of the isolated colon in four experimental animals (guinea pig, mouse, rabbit, and rat), following blockade of enteric neural activity. Isolated colonic preparations were distended with fluid, with the anal end either closed or open. Spatiotemporal maps of changes in diameter were constructed from video recordings. Distension-induced peristaltic contractions were abolished by tetrodotoxin (TTX; 0.6 μM) in all animal species. Subsequent addition of carbachol (0.1-1 μM) did not evoke myogenic motor patterns in the mouse or guinea pig, although some activity was observed in rabbit and rat colon. These myogenic contractions propagated both orally and anally and differed from neurogenic propagating contractions in their frequency, extent of propagation, and polarity. Niflumic acid (300 μM), used to block myogenic activity, also blocked neural peristalsis and thus cannot be used to discriminate between these mechanisms. In all species, except the mouse colon, small myogenic "ripple" contractions were revealed in TTX, but in both rat and rabbit an additional, higher-frequency ripple-type contraction was superimposed. Following blockade of enteric nerve function, a muscarinic agonist can evoke propulsive myogenic peristaltic contractions in isolated rabbit and rat colon, but not in guinea pig or mouse colon. Marked differences between species exist in the ability of myogenic mechanisms to propel luminal content, but in all species there is normally a complex interplay between neurogenic and myogenic processes.

Characteristics of colonic migrating motor complexes in neuronal NOS (nNOS) knockout mice

It is well established that the intrinsic pacemaker mechanism that generates cyclical colonic migrating motor complexes (CMMCs) does not require endogenous nitric oxide (NO). However, pharmacological blockade of endogenous NO production potently increases the frequency of CMMCs, suggesting that endogenous NO acts normally to inhibit the CMMC pacemaker mechanism. In this study, we investigated whether mice with a life long genetic deletion of the neuronal nitric oxide synthase (nNOS) gene would show similar CMMC characteristics as wild type mice that have endogenous NO production acutely inhibited. Intracellular electrophysiological and mechanical recordings were made from circular muscle cells of isolated whole mouse colon in wild type and nNOS knockout (KO) mice at 35°C. In wild type mice, the NOS inhibitor, L-NA (100 μM) caused a significant increase in CMMC frequency and a significant depolarization of the CM layer. However, unexpectedly, the frequency of CMMCs in nNOS KO mice was not significantly different from control mice. Also, the resting membrane potential of CM cells in nNOS KO mice was not depolarized compared to controls; and the amplitude of the slow depolarization phase underlying MCs was of similar amplitude between KO and wild type offspring. These findings show that in nNOS KO mice, the major characteristics of CMMCs and their electrical correlates are, at least in adult mice, indistinguishable from wild type control offspring. One possibility why the major characteristics of CMMCs were no different between both types of mice is that nNOS KO mice may compensate for their life long deletion of the nNOS gene, and their permanent loss of neuronal NO production. In this regard, we suggest caution should be exercised when assuming that data obtained from adult nNOS KO mice can be directly extrapolated to wild type mice, that have been acutely exposed to an inhibitor of NOS.

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.

The Nav1.9 channel regulates colonic motility in mice

The colonic migrating motor complex (CMMC) is a major pattern of motility that is entirely generated and organized by the enteric nervous system. We have previously demonstrated that the Na_v1.9 channel underlies a tetrodotoxin-resistant sodium current which modulates the excitability of enteric neurons. The aim of this study was to observe the effect of loss of the Na_v1.9 channel in enteric neurons on mouse colonic motility in vitro. The mechanical activity of the circular muscle was simultaneously recorded from three sites, namely, proximal, mid- and distal, along the whole colon of male, age-matched wild-type and Na_v1.9 null mice. Spontaneous CMMCs were observed in all preparations. The mean frequency of CMMCs was significantly higher in the Na_v1.9 null mice (one every 2.87 ± 0.1 min compared to one every 3.96 ± 0.23 min in the wild type). The mean duration of CMMCs was shorter and the mean area-under-contraction was larger in the Na_v1.9 null mice compared to the wild type. In addition, CMMCs propagated preferentially in an aboral direction in the Na_v1.9 null mice. Our study demonstrates that CMMCs do occur in mice lacking the Na_v1.9 channel, but their characteristics are significantly different from controls. Up to now, the Na_v1.9 channel was mainly associated with nociceptive neurons and involved in their hyperexcitability after inflammation. Our result shows for the first time a role for the Na_v1.9 channel in a complex colonic motor pattern.

Differential release of β-NAD(+) and ATP upon activation of enteric motor neurons in primate and murine colons

BACKGROUND

The purinergic component of enteric inhibitory neurotransmission is important for normal motility in the gastrointestinal (GI) tract. Controversies exist about the purine(s) responsible for inhibitory responses in GI muscles: ATP has been assumed to be the purinergic neurotransmitter released from enteric inhibitory motor neurons; however, recent studies demonstrate that β-nicotinamide adenine dinucleotide (β-NAD(+)) and ADP-ribose mimic the inhibitory neurotransmitter better than ATP in primate and murine colons. The study was designed to clarify the sources of purines in colons of Cynomolgus monkeys and C57BL/6 mice.

METHODS

High-performance liquid chromatography with fluorescence detection was used to analyze purines released by stimulation of nicotinic acetylcholine receptors (nAChR) and serotonergic 5-HT(3) receptors (5-HT(3)R), known to be present on cell bodies and dendrites of neurons within the myenteric plexus.

KEY RESULTS

Nicotinic acetylcholine receptor or 5-HT(3)R agonists increased overflow of ATP and β-NAD(+) from tunica muscularis of monkey and murine colon. The agonists did not release purines from circular muscles of monkey colon lacking myenteric ganglia. Agonist-evoked overflow of β-NAD(+), but not ATP, was inhibited by tetrodotoxin (0.5 μmol L(-1)) or ω-conotoxin GVIA (50 nmol L(-1)), suggesting that β-NAD(+) release requires nerve action potentials and junctional mechanisms known to be critical for neurotransmission. ATP was likely released from nerve cell bodies in myenteric ganglia and not from nerve terminals of motor neurons.

CONCLUSIONS & INFERENCES

These results support the conclusion that ATP is not a motor neurotransmitter in the colon and are consistent with the hypothesis that β-NAD(+), or its metabolites, serve as the purinergic inhibitory neurotransmitter.

Ionic conductances regulating the excitability of colonic smooth muscles

The tunica muscularis of the gastrointestinal (GI) tract contains two layers of smooth muscle cells (SMC) oriented perpendicular to each other. SMC express a variety of voltage-dependent and voltage-independent ionic conductance(s) that develop membrane potential and control excitability. Resting membrane potentials (RMP) vary through the GI tract but generally are within the range of -80 to -40 mV. RMP sets the 'gain' of smooth muscle and regulates openings of voltage-dependent Ca(2+) channels. A variety of K(+) channels contribute to setting RMP of SMC. In most regions, RMP is considerably less negative than the K(+) equilibrium potential, due to a finely tuned balance between background K(+) channels and non-selective cation channels (NSCC). Variations in expression patterns and openings of K(+) channels and NSCC account for differences of the RMP in different regions of the GI tract. Smooth muscle excitability is also regulated by interstitial cells (interstitial cells of Cajal (ICC) and PDGFRα(+) cells) that express additional conductances and are electrically coupled to SMC. Thus, 'myogenic' activity results from the integrated behavior of the SMC/ICC/PDGFRα(+) cell (SIP) syncytium. Inputs from excitatory and inhibitory motor neurons are required to produce the complex motor patterns of the gut. Motor neurons innervate three cell types in the SIP, and receptors, second messenger pathways, and ion channels in these cells mediate postjunctional responses. Studies of isolated SIP cells have begun to unravel the mechanisms responsible for neural responses. This review discusses ion channels that set and regulate RMP of SIP cells and how neurotransmitters regulate membrane potential.

Muscarinic receptor subtypes involved in regulation of colonic motility in mice: functional studies using muscarinic receptor-deficient mice

Although muscarinic M(2) and M(3) receptors are known to be important for regulation of gastric and small intestinal motility, muscarinic receptor subtypes regulating colonic function remain to be investigated. The aim of this study was to characterize muscarinic receptors involved in regulation of colonic contractility. M(2) and/or M(3) receptor knockout (KO) and wild-type mice were used in in vivo (defecation, colonic propulsion) and in vitro (contraction) experiments. Amount of feces was significantly decreased in M(3)R-KO and M(2)/M(3)R-KO mice but not in M(2)R-KO mice. Ranking of colonic propulsion was wild-type=M(2)R-KO>M(3)R-KO>M(2)/M(3)R-KO. In vitro, the amplitude of migrating motor complexes in M(2)R-KO, M(3)R-KO and M(2)/M(3)R-KO mice was significantly lower than that in wild-type mice. Carbachol caused concentration-dependent contraction of the proximal colon and distal colon from wild-type mice. In M(2)R-KO mice, the concentration-contraction curves shifted to the right and downward. In contrast, carbachol caused non-sustained contraction and relaxation in M(3)R-KO mice depending on its concentration. Carbachol did not cause contraction but instead caused relaxation of colonic strips from M(2)/M(3)R-KO mice. 4-[[[(3-chlorophenyl)amino]carbonyl]oxy]-N,N,N-trimethyl-2-butyn-1-aminium chloride (McN-A-343) caused a non-sustained contraction of colonic strips from wild-type mice, and this contraction was changed to a sustained contraction by tetrodotoxin, pirenzepine and L-nitroarginine methylester (L-NAME). In the colon of M(2)/M(3)R-KO mice, McN-A-343 caused only relaxation, which was decreased by tetrodotoxin, pirenzepine and L-NAME. In conclusion, M(1), M(2) and M(3) receptors regulate colonic motility of the mouse. M(2) and M(3) receptors mediate cholinergic contraction, but M(1) receptors on inhibitory nitrergic nerves counteract muscarinic contraction.

Two independent networks of interstitial cells of cajal work cooperatively with the enteric nervous system to create colonic motor patterns

Normal motility of the colon is critical for quality of life and efforts to normalize abnormal colon function have had limited success. A better understanding of control systems of colonic motility is therefore essential. We report here a hypothesis with supporting experimental data to explain the origin of rhythmic propulsive colonic motor activity induced by general distention. The theory holds that both networks of interstitial cells of Cajal (ICC), those associated with the submuscular plexus (ICC-SMP) and those associated with the myenteric plexus (ICC-MP), orchestrate propagating contractions as pacemaker cells in concert with the enteric nervous system (ENS). ICC-SMP generate an omnipresent slow wave activity that causes propagating but non-propulsive contractions ("rhythmic propagating ripples") enhancing absorption. The ICC-MP generate stimulus-dependent cyclic depolarizations propagating anally and directing propulsive activity ("rhythmic propulsive motor complexes"). The ENS is not essential for both rhythmic motor patterns since distention and pharmacological means can produce the motor patterns after blocking neural activity, but it supplies the primary stimulus in vivo. Supporting data come from studies on segments of the rat colon, simultaneously measuring motility through spatiotemporal mapping of video recordings, intraluminal pressure, and outflow measurements.

β-nicotinamide adenine dinucleotide is an enteric inhibitory neurotransmitter in human and nonhuman primate colons

BACKGROUND & AIMS

An important component of enteric inhibitory neurotransmission is mediated by a purine neurotransmitter, such as adenosine 5'-triphosphate (ATP), binding to P2Y1 receptors and activating small conductance K(+) channels. In murine colon β-nicotinamide adenine dinucleotide (β-NAD) is released with ATP and mimics the pharmacology of inhibitory neurotransmission better than ATP. Here β-NAD and ATP were compared as possible inhibitory neurotransmitters in human and monkey colons.

METHODS

A small-volume superfusion assay and high-pressure liquid chromatography with fluorescence detection were used to evaluate spontaneous and nerve-evoked overflow of β-NAD, ATP, and metabolites. Postjunctional responses to nerve stimulation, β-NAD and ATP were compared using intracellular membrane potential and force measurements. Effects of β-NAD on smooth muscle cells (SMCs) were recorded by patch clamp. P2Y receptor transcripts were assayed by reverse transcription polymerase chain reaction.

RESULTS

In contrast to ATP, overflow of β-NAD evoked by electrical field stimulation correlated with stimulation frequency and was diminished by the neurotoxins, tetrodotoxin, and ω-conotoxin GVIA. Inhibitory junction potentials and responses to exogenous β-NAD, but not ATP, were blocked by P2Y receptor antagonists suramin, pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate (PPADS), 2'-deoxy-N6-methyladenosine 3',5'-bisphosphate (MRS 2179), and (1R,2S,4S,5S)-4-[2-Iodo-6-(methylamino)-9H-purin-9-yl]-2-(phosphonooxy)bicyclo[3.1.0]hexane-1-methanol dihydrogen phosphate ester tetraammonium salt (MRS 2500). β-NAD activated nonselective cation currents in SMCs, but failed to activate outward currents.

CONCLUSIONS

β-NAD meets the criteria for a neurotransmitter better than ATP in human and monkey colons and therefore may contribute to neural regulation of colonic motility. SMCs are unlikely targets for inhibitory purine neurotransmitters because dominant responses of SMCs were activation of net inward, rather than outward, current.

Purinergic and nitrergic neuromuscular transmission mediates spontaneous neuronal activity in the rat colon

Nitric oxide (NO) and ATP mediate smooth muscle relaxation in the gastrointestinal tract. However, the involvement of these neurotransmitters in spontaneous neuronal activity is unknown. The aim of the present work was to study spontaneous neuromuscular transmission in the rat midcolon. Microelectrode experiments were performed under constant stretch both in circular and longitudinal directions. Spontaneous inhibitory junction potentials (sIJP) were recorded. Tetrodotoxin (1 microM) and apamin (1 microM) depolarized smooth muscle cells and inhibited sIJP. N(omega)-nitro-l-arginine (l-NNA, 1 mM) depolarized smooth muscle cells but did not modify sIJP. In contrast, the P2Y(1) antagonist MRS-2500 (1 microM) did not modify the resting membrane potential (RMP) but reduced sIJP (IC(50) = 3.1 nM). Hexamethonium (200 microM), NF-023 (10 microM), and ondansetron (1 microM) did not modify RMP and sIJP. These results correlate with in vitro (muscle bath) and in vivo (strain gauges) data where l-NNA but not MRS-2500 induced a sustained increase of spontaneous motility. We concluded that, in the rat colon, inhibitory neurons regulate smooth muscle RMP and cause sIJP. In vitro, the release of inhibitory neurotransmitters is independent of nicotinic, P2X, and 5-hydroxytryptamine type 3 receptors. Neuronal NO causes a sustained smooth muscle hyperpolarization that is responsible for a constant inhibition of spontaneous motility. In contrast, ATP acting on P2Y(1) receptors is responsible for sIJP but does not mediate inhibitory neural tone. ATP and NO have complementary physiological functions in the regulation of gastrointestinal motility.

Critical role of 5-HT1A, 5-HT3, and 5-HT7 receptor subtypes in the initiation, generation, and propagation of the murine colonic migrating motor complex

The colonic migrating motor complex (CMMC) is necessary for fecal pellet propulsion in the murine colon. We have previously shown that 5-hydroxytryptamine (5-HT) released from enterochromaffin cells activates 5-HT(3) receptors on the mucosal processes of myenteric Dogiel type II neurons to initiate the events underlying the CMMC. Our aims were to further investigate the roles of 5-HT(1A), 5-HT(3), and 5-HT(7) receptor subtypes in generating and propagating the CMMC using intracellular microelectrodes or tension recordings from the circular muscle (CM) in preparations with and without the mucosa. Spontaneous CMMCs were recorded from the CM in isolated murine colons but not in preparations without the mucosa. In mucosaless preparations, ondansetron (3 microM; 5-HT(3) antagonist) plus hexamethonium (100 microM) completely blocked spontaneous inhibitory junction potentials, depolarized the CM. Ondansetron blocked the preceding hyperpolarization associated with a CMMC. Spontaneous CMMCs and CMMCs evoked by spritzing 5-HT (10 and 100 microM) or nerve stimulation in preparations without the mucosa were blocked by SB 258719 or SB 269970 (1-5 microM; 5-HT(7) antagonists). Both NAN-190 and (S)-WAY100135 (1-5 microM; 5-HT(1A) antagonists) blocked spontaneous CMMCs and neurally evoked CMMCs in preparations without the mucosa. Both NAN-190 and (S)-WAY100135 caused an atropine-sensitive depolarization of the CM. The precursor of 5-HT, 5-hydroxytryptophan (5-HTP) (10 microM), and 5-carboxamidotryptamine (5-CT) (5 microM; 5-HT(1/5/7) agonist) increased the frequency of spontaneous CMMCs. 5-HTP and 5-CT also induced CMMCs in preparations with and without the mucosa, which were blocked by SB 258719. 5-HT(1A), 5-HT(3), and 5-HT(7) receptors, most likely on Dogiel Type II/AH neurons, are important in initiating, generating, and propagating the CMMC. Tonic inhibition of the CM appears to be driven by ongoing activity in descending serotonergic interneurons; by activating 5-HT(7) receptors on AH neurons these interneurons also contribute to the generation of the CMMC.

The transwall gradient across the mouse colonic circular muscle layer is carbon monoxide dependent

Gastric and small intestinal circular smooth muscle layers have a transwall resting membrane potential (RMP) gradient that is dependent on release of carbon monoxide (CO) from interstitial cells of Cajal (ICCs). Our aim was to determine whether a RMP gradient exists in the mouse colon and whether the gradient is CO dependent. Microelectrodes were used to record RMPs from muscle cells at different depths of the circular muscle layer from wild-type and heme oxygenase-2-knockout (HO-2-KO) mice. A transwall RMP gradient was present in wild-type mice. The CO scavenger oxyhemoglobin (20 μM) and the heme oxygenase inhibitor chromium mesoporphyrin IX (CrMP, 5 μM) abolished the transwall gradient. The gradient was absent in HO-2-KO mice. Tetrodotoxin (1 μM) caused a significant depolarization in circular smooth muscle cells throughout the circular muscle layer and abolished the transwall gradient. Removal of the submucosal neurons abolished the gradient. The majority of submucosal neurons contained HO-2 immunoreactivity (HO-2-IR), while ICCs did not. These data show for the first time that a transwall gradient exists across the circular smooth muscle layer of the mouse colon, that the gradient is due to CO, and that the source of CO is the submucosal neurons.

Nitric oxide enhances inhibitory synaptic transmission and neuronal excitability in Guinea-pig submucous plexus

Varicosities immunoreactive for nitric oxide synthase (NOS) make synaptic connections with submucosal neurons in the guinea-pig small intestine, but the effects of nitric oxide (NO) on these neurons are unknown. We used intracellular recording to characterize effects of sodium nitroprusside (SNP, NO donor) and nitro-l-arginine (NOLA, NOS inhibitor), on inhibitory synaptic potentials (IPSPs), slow excitatory synaptic potentials (EPSPs) and action potential firing in submucosal neurons of guinea-pig ileum in vitro. Recordings were made from neurons with the characteristic IPSPs of non-cholinergic secretomotor neurons. SNP (100 μM) markedly enhanced IPSPs evoked by single stimuli applied to intermodal strands and IPSPs evoked by trains of 2–10 pulses (30 Hz). Both noradrenergic (idazoxan-sensitive) and non-adrenergic (idazoxan-insensitive) IPSPs were affected. SNP enhanced hyperpolarizations evoked by locally applied noradrenaline or somatostatin. SNP did not affect slow EPSPs evoked by single stimuli, but depressed slow EPSPs evoked by stimulus trains. NOLA (100 μM) depressed IPSPs evoked by one to three stimulus pulses and enhanced slow EPSPs evoked by trains of two to three stimuli (30 Hz). SNP also increased the number of action potentials and the duration of firing evoked by prolonged (500 or 1000 ms) depolarizing current pulses, but NOLA had no consistent effect on action potential firing. We conclude that neurally released NO acts post-synaptically to enhance IPSPs and depress slow EPSPs, but may enhance the intrinsic excitability of these neurons. Thus, NOS neurons may locally regulate several secretomotor pathways ending on common neurons.

Neural precursor death is central to the pathogenesis of intestinal aganglionosis in Ret hypomorphic mice

The RET tyrosine kinase is required for the migration, proliferation, and survival of the enteric neural crest-derived cells (ENCCs) that form the enteric nervous system (ENS). Hypomorphic RET alleles cause intestinal aganglionosis [Hirschsprung disease (HSCR)], in which delayed migration and successive nonapoptotic ENCC death are considered to be major contributory factors. The significance of ENCC death in intestinal aganglionosis, however, has remained unclear. We show that elevated expression of Bcl-xL inhibits ENCC death in both Ret-null and hypomorphic states. However, the rescued Ret-null mice showed ENS malfunction with reduced nitric oxide synthase expression in colonic neurons, revealing the requirement of RET for neuronal differentiation. In contrast, the inhibition of cell death allows morphologically and functionally normal ENS formation in Ret hypomorphic mice. These results indicate that ENCC death is a principal cause of intestinal aganglionosis in a Ret hypomorphic state, and suggest that the inhibition of cell death is a route to the prevention of HSCR.

The mechanisms underlying the generation of the colonic migrating motor complex in both wild-type and nNOS knockout mice

Colonic migrating motor complexes (CMMCs) propel fecal contents and are altered in diseased states, including slow-transit constipation. However, the mechanisms underlying the CMMCs are controversial because it has been proposed that disinhibition (turning off of inhibitory neurotransmission) or excitatory nerve activity generate the CMMC. Therefore, our aims were to reexamine the mechanisms underlying the CMMC in the colon of wild-type and neuronal nitric oxide synthase (nNOS)(-/-) mice. CMMCs were recorded from the isolated murine large bowel using intracellular recordings of electrical activity from circular muscle (CM) combined with tension recording. Spontaneous CMMCs occurred in both wild-type (frequency: 0.3 cycles/min) and nNOS(-/-) mice (frequency: 0.4 cycles/min). CMMCs consisted of a hyperpolarization, followed by fast oscillations (slow waves) with action potentials superimposed on a slow depolarization (wild-type: 14.0 +/- 0.6 mV; nNOS(-/-): 11.2 +/- 1.5 mV). Both atropine (1 microM) and MEN 10,376 [neurokinin 2 (NK2) antagonist; 0.5 microM] added successively reduced the slow depolarization and the number of action potentials but did not abolish the fast oscillations. The further addition of RP 67580 (NK1 antagonist; 0.5 microM) blocked the fast oscillations and the CMMC. Importantly, none of the antagonists affected the resting membrane potential, suggesting that ongoing tonic inhibition of the CM was maintained. Fecal pellet propulsion, which was blocked by the NK2 or the NK1 antagonist, was slower down the longer, more constricted nNOS(-/-) mouse colon (wild-type: 47.9 +/- 2.4 mm; nNOS(-/-): 57.8 +/- 1.4 mm). These observations suggest that excitatory neurotransmission enhances pacemaker activity during the CMMC. Therefore, the CMMC is likely generated by a synergistic interaction between neural and interstitial cells of Cajal networks.

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.

Immunohistochemical analysis of neuron types in the mouse small intestine

The definition of the nerve cell types of the myenteric plexus of the mouse small intestine has become important, as more researchers turn to the use of mice with genetic mutations to analyze roles of specific genes and their products in enteric nervous system function and to investigate animal models of disease. We have used a suite of antibodies to define neurons by their shapes, sizes, and neurochemistry in the myenteric plexus. Anti-Hu antibodies were used to reveal all nerve cells, and the major subpopulations were defined in relation to the Hu-positive neurons. Morphological Type II neurons, revealed by anti-neurofilament and anti-calcitonin gene-related peptide antibodies, represented 26% of neurons. The axons of the Type II neurons projected through the circular muscle and submucosa to the mucosa. The cell bodies were immunoreactive for choline acetyltransferase (ChAT), and their terminals were immunoreactive for vesicular acetylcholine transporter (VAChT). Nitric oxide synthase (NOS) occurred in 29% of nerve cells. Most were also immunoreactive for vasoactive intestinal peptide, but they were not tachykinin (TK)-immunoreactive, and only 10% were ChAT-immunoreactive. Numerous NOS terminals occurred in the circular muscle. We deduced that 90% of NOS neurons were inhibitory motor neurons to the muscle (26% of all neurons) and 10% (3% of all neurons) were interneurons. Calretinin immunoreactivity was found in a high proportion of neurons (52%). Many of these had TK immunoreactivity. Small calretinin neurons were identified as excitatory neurons to the longitudinal muscle (about 20% of neurons, with ChAT/calretinin/+/- TK chemical coding). Excitatory neurons to the circular muscle (about 10% of neurons) had the same coding. Calretinin immunoreactivity also occurred in a proportion of Type II neurons. Thus, over 90% of neurons in the myenteric plexus of the mouse small intestine can be currently identified by their neurochemistry and shape.

Disturbances of colonic motility in mouse models of Hirschsprung's disease

Mutations in genes encoding members of the GDNF and endothelin-3 (Et-3) signaling pathways can cause Hirschsprung's disease, a congenital condition associated with an absence of enteric neurons in the distal gut. GDNF signals through Ret, a receptor tyrosine kinase, and Et-3 signals through endothelin receptor B (Ednrb). The effects of Gdnf, Ret, and ET-3 haploinsufficiency and a null mutation in ET-3 on spontaneous motility patterns in adult and developing mice were investigated. Video recordings were used to construct spatiotemporal maps of spontaneous contractile patterns in colon from postnatal and adult mice in vitro. In Ret(+/-) and ET-3(+/-) mice, which have normal numbers of enteric neurons, colonic migrating motor complexes (CMMCs) displayed similar properties under control conditions and following inhibition of nitric oxide synthase (NOS) activity to wild-type mice. In the colon of Gdnf(+/-) mice and in the ganglionic region of ET-3(-/-) mice, there was a 50-60% reduction in myenteric neuron number. In Gdnf(+/-) mice, CMMCs were present, but abnormal, and the proportion of myenteric neurons containing NOS was not different from that of wild-type mice. In the ganglionic region of postnatal ET-3(-/-) mice, CMMCs were absent, and the proportion of myenteric neurons containing NOS was over 100% higher than in wild-type mice. Thus impairments in spontaneous motility patterns in the colon of Gdnf(+/-) mice and in the ganglionic region of ET-3(-/-) mice are correlated with a reduction in myenteric neuron density.

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.

Development of colonic motility in the neonatal mouse-studies using spatiotemporal maps

Colonic migrating motor complexes (CMMCs) are spontaneous, anally propagating constrictions, repeating every 3-5 min in mouse colon in vitro. They are regulated by the enteric nervous system and may be equivalent to mass movement contractions. We examined postnatal development of CMMCs and circular muscle innervation to gain insight into mechanisms regulating transit in the maturing colon. Video recordings of mouse colon in vitro were used to construct spatiotemporal maps of spontaneous contractile patterns. Development of nitric oxide synthase (NOS) and cholinergic nerve terminals in the circular muscle was examined immunohistochemically. In adults, CMMCs appeared regularly at 4.6 +/- 0.9-min intervals (n = 5). These intervals were reduced by inhibition of NOS (2.7 +/- 0.2 min; n = 5; P < 0.05). CMMCs were abolished by tetrodotoxin (n = 4). CMMCs at postnatal day (P)10 were indistinguishable from adult. At birth and P4, CMMCs were absent. Instead, small constrictions that propagated both orally and anally, "ripples," were seen. Ripples were unaffected by tetrodotoxin or inhibition of NOS and were present in Ret(-/-) mice (which lack enteric neurons) at embryonic day 18.5. In P6 mice, only ripples were seen in control, but NOS inhibition induced CMMCs (n = 8). NOS terminals were abundant in the circular muscle at birth; cholinergic terminals were sparse but were common by P10. In mouse, myogenic ripples are the only mechanism available to produce colonic transit at birth. At P6, neural circuits that generate CMMCs are present but are inhibited by tonic activity of nitric oxide. Adult patterns appear by P10.

Caveolin-1 gene knockout impairs nitrergic function in mouse small intestine

Caveolin-1 is a plasma membrane-associated protein that is responsible for caveolae formation. It plays an important role in the regulation of the function of different signaling molecules, among which are the different isoforms of nitric oxide synthase (NOS). Nitric oxide (NO) is known to be an important inhibitory mediator in the mouse gut. Caveolin-1 knockout mice (Cav1(-/-)) were used to examine the effect of caveolin-1 absence on the NO function in the mouse small intestine (ileum and jejunum) compared to their genetic controls and BALB/c controls. Immunohistochemical staining showed loss of caveolin-1 and NOS in the jejunal smooth muscles and myenteric plexus interstitial cells of Cajal (ICC) of Cav1(-/-) mice; however, nNOS immunoreactive nerves were still present in myenteric ganglia. Under nonadrenergic noncholinergic (NANC) conditions, small intestinal tissues from Cav1(-/-) mice relaxed to electrical field stimulation (EFS), as did tissues from control mice. Relaxation of tissues from control mice was markedly reduced by N-omega-nitro-L-arginine (10(-4) M), but relaxation of Cav1(-/-) animals was affected much less. Also, Cav1(-/-) mice tissues showed reduced relaxation responses to sodium nitroprusside (100 microM) compared to controls; yet there were no significant differences in the relaxation responses to 8-bromoguanosine-3': 5'-cyclic monophosphate (100 microM). Apamin (10(-6) M) significantly reduced relaxations to EFS in NANC conditions in Cav1(-/-) mice, but not in controls. The data from this study suggest that caveolin-1 gene knockout causes alterations in the smooth muscles and the ICC, leading to an impaired NO function in the mouse small intestine that could possibly be compensated by apamin-sensitive inhibitory mediators.

Synchronization of enteric neuronal firing during the murine colonic MMC

DiI (1,1'didodecyl-3,3,3',3'-tetramethylindocarbecyanine perchlorate) retrograde labelling and intracellular electrophysiological techniques were used to investigate the mechanisms underlying the generation of spontaneously occurring colonic migrating myoelectric complexes (colonic MMCs) in mice. In isolated, intact, whole colonic preparations, simultaneous intracellular electrical recordings were made from pairs of circular muscle (CM) cells during colonic MMC activity in the presence of nifedipine (1-2 microm). During the intervals between colonic MMCs, spontaneous inhibitory junction potentials (IJPs) were always present. The amplitudes of spontaneous IJPs were highly variable (range 1-20 mV) and occurred asynchronously in the two CM cells, when separated by 1 mm in the longitudinal axis. Colonic MMCs occurred every 151 +/- 7 s in the CM and consisted of a repetitive discharge of cholinergic rapid oscillations in membrane potential (range: 1-20 mV) that were superimposed on a slow membrane depolarization (mean amplitude: 9.6 +/- 0.5 mV; half-duration: 25.9 +/- 0.7 s). During the rising (depolarizing) phase of each colonic MMC, cholinergic rapid oscillations occurred simultaneously in both CM cells, even when the two electrodes were separated by up to 15 mm along the longitudinal axis of the colon. Smaller amplitude oscillations (< 5 mV) showed poor temporal correlation between two CM cells, even at short electrode separation distances (i.e. < 1 mm in the longitudinal axis). When the two electrodes were separated by 20 mm, all cholinergic rapid oscillations and IJPs in the CM (regardless of amplitude) were rarely, if ever, coordinated in time during the colonic MMC. Cholinergic rapid oscillations were blocked by atropine (1 microm) or tetrodotoxin (1 microm). Slow waves were never recorded from any CM cells. DiI labelling showed that the maximum projection length of CM motor neurones and interneurones along the bowel was 2.8 mm and 13 mm, respectively. When recordings were made adjacent to either oral or anal cut ends of the colon, the inhibitory or excitatory phases of the colonic MMC were absent, respectively. In summary, during the colonic MMC, cholinergic rapid oscillations of similar amplitudes occur simultaneously in two CM cells separated by large distances (up to 15 mm). As this distance was found to be far greater than the projection length of any single CM motor neurone, we suggest that the generation of each discrete cholinergic rapid oscillation represents a discreet cholinergic excitatory junction potential (EJP) that involves the synaptic activation of many cholinergic motor neurones simultaneously, by synchronous firing in many myenteric interneurones. Our data also suggest that ascending excitatory and descending inhibitory nerve pathways interact and reinforce each other.

Motility patterns and distribution of interstitial cells of Cajal and nitrergic neurons in the proximal, mid- and distal-colon of the rat

The aim of this work was to study the patterns of spontaneous motility in the circular and longitudinal muscle strips and to characterize the distribution of c-kit positive interstitial cells of Cajal (ICCs) and nitrergic neurons (nNOS) in the proximal, mid- and distal-colon of Sprague-Dawley rats. We described two types of spontaneous contractions: high frequency (HF) and low frequency (LF) contractions, which were recorded in the presence of tetrodotoxin, suggesting a non-neurogenic origin. Regional differences were found in the motility patterns depending on the muscle layer and on the part of the colon studied. Muscle strips without submuscular plexus (SMP) showed only LF contractions. The density of ICCs was of the same magnitude along the extent of the colon: about 90-120 cells mm(-2) at Auerbach's plexus (AP) and 50-60 cells mm(-2) at the SMP. nNOS positive cells were found at the level of the AP and the major density was found in the mid-colon. Electrical field stimulation abolished LF but did not affect HF contractions. Our results indicate that HF contractions are due to the ICC network found associated with the submuscular plexus (ICC-SMP). The origin of LF contractions is still unknown.

Structural differences in the enteric neural network in murine colon: impact on electrophysiology

The enteric neural network in the proximal murine colon shows a regularly occurring hypoganglionic region, which is here characterized by using anatomical and electrophysiological techniques. Staining with NADPH diaphorase, methylene blue, and cuprolinic blue in standard whole mounts and three-dimensional gut preparations of the murine proximal colon consistently revealed two hypoganglionic areas surrounded by a dense clustering of enteric neurons. This irregularity in the ganglionic plexus was found to be present in mice of three different genetic backgrounds, as well as in rats. The lack of myenteric ganglia in these regions was associated with an absence of the longitudinal muscle layer, as shown in cross sections. Histochemical identification of interstitial cells of Cajal in Kit(W-lacZ/+) transgenic mice showed Kit-positive cells oriented parallel to both muscle layers of the colon. Kit-positive cells oriented parallel to the longitudinal muscle layers were absent in the hypoganglionic area described. Electrical field stimulation elicited TTX-sensitive inhibitory junction potentials (IJPs), which showed region-specific characteristics. The initial partly apamin-sensitive hyperpolarization was present in all parts of the murine colon, whereas a second sustained NG-nitro-L-arginine-sensitive hyperpolarization was absent in the cecum and decreased from the proximal to the distal colon. Dissecting the hypoganglionic area from the surrounding tissue abolished the otherwise normal inhibitory neurotransmission to the circular muscle (1.6 +/- 1.4 and 2.6 +/- 1.7 mV for the fast and slow component of IJP amplitude in the hypoganglionic area vs. 16.5 +/- 1.9 and 23.7 +/- 2.7 mV for the fast and slow component of IJP amplitude in the neuron-rich area, respectively, P < 0.01, n = 6), whereas dissection of an area of identical size with an intact myenteric network showed normal inhibitory neurotransmission, indicating that the hypoganglionic area receives essential functional neural input from the neuron-rich surrounding tissue. In summary, in the murine and rat proximal colon, a constant and distinct hypoganglionic region is described with important concomitant changes in local electrophysiology.