Initiation of peristalsis by circumferential stretch of flat sheets of guinea-pig ileum

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.

The underlying mechanism, efficiency, and safety of manual therapy for functional gastrointestinal disorders: A narrative review

INTRODUCTION

Functional gastrointestinal disorders encompass a range of conditions resulting from complicated gut-brain interactions, which can negatively impact sufferers' lives. They are prevalent in clinical practice and the community, with a lifetime prevalence of almost 40 % worldwide. The challenge in diagnosing these disorders lies in the non-specificity of symptoms and the absence of reliable biomarkers. The existing literature suggests a multidisciplinary approach, including cognitive-behavioral therapy, dietary changes, psychotropic drug therapy, and improving gastrointestinal motility. Manual therapy applied to the abdomen and adjacent areas can potentially enhance gastrointestinal motility.

OBJECTIVES

This review aims to examine the types of manual interventions, their mechanisms, efficiency, and safety in managing functional disorders of the digestive system.

METHODS

We searched PubMed and Google Scholar in English from May 2022 to February 2023 with no date restriction. We prioritized systematic reviews, meta-analyses, and clinical trials and did not exclude any data sources.

RESULTS AND CONCLUSION

s: Initial evidence suggests that manual interventions on the abdomen and adjacent areas are effective in managing functional gastrointestinal disorders, with no reported adverse events and relatively low costs. However, further studies with rigorous scientific methodology are needed to understand better the unknown dimensions influencing the outcomes observed with abdominal massage and its positive impact on patients. Manual abdominal techniques are a promising therapy option for functional gastrointestinal disorders, and their efficacy, safety, and cost-effectiveness should be further explored.

The role of enteric inhibitory neurons in intestinal motility

The enteric nervous system controls much of the mixing and propulsion of nutrients along the digestive tract. Enteric neural circuits involve intrinsic sensory neurons, interneurons and motor neurons. While the role of the excitatory motor neurons is well established, the role of the enteric inhibitory motor neurons (IMNs) is less clear. The discovery of inhibitory transmission in the intestine in the 1960's in the laboratory of Geoff Burnstock triggered the search for the unknown neurotransmitter. It has since emerged that most neurons including the IMNs contain and may utilise more than one transmitter substances; for IMNs these include ATP, the neuropeptide VIP/PACAP and nitric oxide. This review distinguishes the enteric neural pathways underlying the 'standing reflexes' from the pathways operating physiologically during propulsive and non-propulsive movements. Morphological evidence in small laboratory animals indicates that the IMNs are located in the myenteric plexus and project aborally to the circular muscle, where they act by relaxing the muscle. There is ongoing 'tonic' activity of these IMNs to keep the intestinal muscle relaxed. Accommodatory responses to content further activate enteric pathways that involve the IMNs as the final neural element. IMNs are activated by mechanical and chemical stimulation induced by luminal contents, which activate intrinsic sensory enteric neurons and the polarised interneuronal ascending excitatory and descending inhibitory reflex pathways. The latter relaxes the muscle ahead of the advancing bolus, thus facilitating propulsion.

The virtual physiological human gets nerves! How to account for the action of the nervous system in multiphysics simulations of human organs

This article shows how to couple multiphysics and artificial neural networks to design computer models of human organs that autonomously adapt their behaviour to environmental stimuli. The model simulates motility in the intestine and adjusts its contraction patterns to the physical properties of the luminal content. Multiphysics reproduces the solid mechanics of the intestinal membrane and the fluid mechanics of the luminal content; the artificial neural network replicates the activity of the enteric nervous system. Previous studies recommended training the network with reinforcement learning. Here, we show that reinforcement learning alone is not enough; the input–output structure of the network should also mimic the basic circuit of the enteric nervous system. Simulations are validated against in vivo measurements of high-amplitude propagating contractions in the human intestine. When the network has the same input–output structure of the nervous system, the model performs well even when faced with conditions outside its training range. The model is trained to optimize transport, but it also keeps stress in the membrane low, which is exactly what occurs in the real intestine. Moreover, the model responds to atypical variations of its functioning with ‘symptoms’ that reflect those arising in diseases. If the healthy intestine model is made artificially ill by adding digital inflammation, motility patterns are disrupted in a way consistent with inflammatory pathologies such as inflammatory bowel disease.

Neurogenic Bowel Dysfunction in Children and Adolescents

Neurogenic/neuropathic bowel dysfunction (NBD) is common in children who are affected by congenital and acquired neurological disease, and negatively impacts quality of life. In the past, NBD received less attention than neurogenic bladder, generally being considered only in spina bifida (the most common cause of pediatric NBD). Many methods of conservative and medical management of NBD are reported, including relatively recently Transanal Irrigation (TAI). Based on the literature and personal experience, an expert group (pediatric urologists/surgeons/gastroenterologists with specific experience in NBD) focused on NBD in children and adolescents. A statement document was created using a modified Delphi method. The range of causes of pediatric NBD are discussed in this paper. The various therapeutic approaches are presented to improve clinical management. The population of children and adolescents with NBD is increasing, due both to the higher survival rate and better diagnosis. While NBD is relatively predictable in producing either constipation or fecal incontinence, or both, its various effects on each patient will depend on a wide range of underlying causes and accompanying comorbidities. For this reason, management of NBD should be tailored individually with a combined multidisciplinary therapy appropriate for the status of the affected child and caregivers.

Comparison of Effectiveness between Abdominal Vibration Stimulation and Walking Exercise for Bowel Cleansing before Therapeutic Colonoscopy

Background/Aims

Adequate bowel preparation is important for successful colonoscopy. We aimed to evaluate the clinical feasibility and effectiveness of abdominal vibration stimulation in bowel preparation before therapeutic colonoscopy.

Methods

A single center, prospective, randomized, investigator-blinded study was performed between January 2016 and December 2016. Patients for therapeutic colonoscopy were prospectively enrolled and assigned to either the vibrator group or walking group. Patients who refused to participate in this study as part of the experimental group consented to register in the control group instead. During the preparation period, patients assigned to the walking group walked ≥3,000 steps, whereas those assigned to the vibrator group received abdominal vibrator stimulation and restricted walking. All patients received the same colon cleansing regimen: 4-L split-dose polyethylene glycol (PEG) solution.

Results

Three hundred patients who received PEG solution for therapeutic colonoscopy were finally enrolled in this study (n=100 per group). Bowel cleansing with abdominal vibration stimulation showed almost similar results to that with walking exercise (Boston Bowel Preparation Scale score for the entire colon: vibrator vs walking vs control, 7.38±1.55 vs 7.39±1.55 vs 6.17±1.15, p<0.001). There were no significant differences between the vibrator group and walking group regarding instances of diarrhea after taking PEG, time to first diarrhea after taking PEG, total procedure time, and patient satisfaction.

Conclusions

This study indicates that, compared with conventional walking exercise, abdominal vibration stimulation achieved similar rates of bowel cleansing adequacy and colonoscopy success without compromising safety or patient satisfaction.

Comparison of Effectiveness between Abdominal Vibration Stimulation and Walking Exercise for Bowel Cleansing before Therapeutic Colonoscopy

Background/Aims

Adequate bowel preparation is important for successful colonoscopy. We aimed to evaluate the clinical feasibility and effectiveness of abdominal vibration stimulation in bowel preparation before therapeutic colonoscopy.

Methods

A single center, prospective, randomized, investigator-blinded study was performed between January 2016 and December 2016. Patients for therapeutic colonoscopy were prospectively enrolled and assigned to either the vibrator group or walking group. Patients who refused to participate in this study as part of the experimental group consented to register in the control group instead. During the preparation period, patients assigned to the walking group walked ≥3,000 steps, whereas those assigned to the vibrator group received abdominal vibrator stimulation and restricted walking. All patients received the same colon cleansing regimen: 4-L split-dose polyethylene glycol (PEG) solution.

Results

Three hundred patients who received PEG solution for therapeutic colonoscopy were finally enrolled in this study (n=100 per group). Bowel cleansing with abdominal vibration stimulation showed almost similar results to that with walking exercise (Boston Bowel Preparation Scale score for the entire colon: vibrator vs walking vs control, 7.38±1.55 vs 7.39±1.55 vs 6.17±1.15, p<0.001). There were no significant differences between the vibrator group and walking group regarding instances of diarrhea after taking PEG, time to first diarrhea after taking PEG, total procedure time, and patient satisfaction.

Conclusions

This study indicates that, compared with conventional walking exercise, abdominal vibration stimulation achieved similar rates of bowel cleansing adequacy and colonoscopy success without compromising safety or patient satisfaction.

TRPA1 Channel Activation Inhibits Motor Activity in the Mouse Colon

There is a growing awareness of the role that TRP channels play in regulating sensory and motor functions in the gastrointestinal tract. In this study we used an in-vitro murine model of colonic peristaltic-like complexes (CPMCs) to evaluate the role of exogenous and endogenous TRPA1 signaling processes in regulating colonic motility. Using in-vitro recordings of intraluminal pressure to monitor the presence of CPMCs in colonic segments we performed a series of experiments on male CD1 mice (2 months of age) and found that CPMC activity was attenuated by TRPA1 agonists. Bath application of the TRPA1 antagonist HC-030031 had no effect upon basal CPMC activity whereas application of the synthetic TRPA1 agonist ASP7663 caused a reversible dose dependent decrease in CPMC frequency that was blocked by HC-030031. Cinnamaldehyde and 4-hydroxy-2-nonenal elicited long lasting decreases in CPMC frequency that were blocked by HC-030031 whereas the decreased CPMC activity invoked by AITC could not be blocked by HC-030031. Our results show that any potential mechanosensory function of TRPA1 doesn’t involve contributing to distension induced colonic motor activity and that a role for TRPA1 in the colon is through regulating motility through exogenous and endogenous agonist induced inhibitory effects.

Physiotherapist management of a patient with spastic perineal syndrome and subsequent constipation: a case report

Background and Purpose: The purpose of this case report is to describe the benefits of manual therapy techniques, including mobilization and stretching, in the management of a patient with chronic constipation. Case Description: A 17-year-old male with an 8-month history of constipation and complaint of incomplete evacuation after defecation was referred for therapy. The patient was diagnosed with a spastic perineal syndrome. Isolated puborectalis relaxation exercise was not successful in alleviating constipation. Physical examination showed tightness of left side piriformis, thoracic kyphosis, apparent limb shortening on the left side, and a right-on-right sacrum forward torsion. The patient was treated with stretching of left piriformis and mobilization of thoracic and lumbar vertebrae and sacroiliac joints along with puborectalis relaxation exercises. Outcomes: After 4 weeks of treatment, there was a reduction in Bowel Function Index (BFI) from 74.6 to 27.2. The patient also reported spending less time (<3 minutes) during defecation. The patient maintained his improvement at 7 months. Discussion: A detailed neuromuscular examination assisted in identifying the pathophysiology related to obstructive defecation for this patient. Controlled studies regarding the effectiveness of various physiotherapy interventions in the treatment of obstructive constipation are warranted.

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.

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.

Damage from dissection is associated with reduced neuro-musclar transmission and gap junction coupling between circular muscle cells of guinea pig ileum, in vitro

Excitatory and inhibitory junction potentials of circular smooth muscle cells in guinea pig ileum and colon are suppressed 30–90 min after setting up in vitro preparations. We have previously shown this “unresponsive” period is associated with a transient loss of dye coupling between smooth muscle cells, which subsequently recovers over the ensuing 30–90 min; junction potentials recover in parallel with dye coupling (Carbone et al., 2012). Here, we investigated which components of dissection trigger the initial loss of coupling. Intracellular recordings were made from circular muscle cells of guinea pig ileum with micropipettes containing 5% carboxyfluorescein. After allowing 90–120 min for junction potentials to reach full amplitude, we re-cut all 4 edges of the preparation more than 1 mm from the recording sites. This caused a reduction in the amplitude of IJPs from 17.2 ± 0.7 mV to 9.5 ± 1.5 mV (P < 0.001, n = 12) and a significant reduction in dye coupling. Both recovered within 60 min. We repeated this experiment (n = 4), recording both 1 and 4 mm from the cut edge: both sites were equally affected by re-cutting the sides of the preparation. Equilibrated preparations were stretched to 150% of their original length, this had no significant effect on junction potentials or dye coupling. Setting up preparations in low calcium solution did not prevent the initial suppression of IJPs and dye coupling. Application of 3 μM indomethacin (n = 3), 10 μM ketotifen (n = 4) or 10 μM forskolin during dissection did not prevent the suppression of IJPs and dye coupling. If dissection damage was reduced, by leaving the mucosa and submucosa attached to the circular muscle, IJPs showed less initial suppression than in preparations where the layers were dissected off. We conclude that physical damage to the gut wall triggers loss of gap junction coupling and neuromuscular transmission, this is not due to stretch, influx of calcium ions, release of prostaglandins or mast cell degranulation. The mechanisms underlying this potent effect remain to be determined.

Electromechanical abdominal massage and colonic function in individuals with a spinal cord injury and chronic bowel problems

STUDY DESIGN

A prospective intervention of noninvasive abdominal massage using an electromechanical apparatus on bowel function in individuals with spinal cord injury (SCI).

OBJECTIVES

To evaluate the effects of noninvasive abdominal massage using an electromechanical apparatus on bowel function in individuals with SCI and chronic bowel problems. This easy-to-use apparatus can be applied by the patients at home without the help of a therapist.

SETTING

Homes of community-living individuals.

METHODS

Twenty-one subjects with SCI were instructed to use the massage apparatus daily for 20 min during a 10-week period. Compliance, effects, side effects and user satisfaction were assessed using questionnaires.

RESULTS

Fifteen subjects completed the 10-week period. Although some characteristics of defecation changed positively for some of the subjects (time to result, amount, consistency), none felt better or more confident after using the massage device. In addition, some individuals experienced negative side effects (predominantly pain or discomfort). The overall satisfaction with the device is ambiguous, with half of the group judging the device as insufficient and the other half as at least adequate.

CONCLUSION

The use of an electromechanical massage device does not improve bowel function in most individuals with SCI who have chronic bowel problems. Why some subjects benefit and others do not should be investigated in future studies.

Important role of mucosal serotonin in colonic propulsion and peristaltic reflexes: in vitro analyses in mice lacking tryptophan hydroxylase 1

Although there is general agreement that mucosal 5-hydroxytryptamine (5-HT) can initiate peristaltic reflexes in the colon, recent studies have differed as to whether or not the role of mucosal 5-HT is critical. We therefore tested the hypothesis that the secretion of 5-HT from mucosal enterochromaffin (EC) cells is essential for the manifestation of murine colonic peristaltic reflexes. To do so, we analysed the mechanisms underlying faecal pellet propulsion in isolated colons of mice lacking tryptophan hydroxylase 1 (Tph1(-/-) mice), which is the rate-limiting enzyme in the biosynthesis of mucosal but not neuronal 5-HT. We used video analysis of faecal pellet propulsion, tension transducers to record colonic migrating motor complexes (CMMCs) and intracellular microelectrodes to record circular muscle activity occurring spontaneously or following intraluminal distension. When compared with control (Tph1(+/+)) mice, Tph1(-/-) animals exhibited: (1) an elongated colon; (2) larger faecal pellets; (3) orthograde propulsion followed by retropulsion (not observed in Tph1(+/+) colon); (4) slower in vitro propulsion of larger faecal pellets (28% of Tph1(+/+)); (5) CMMCs that infrequently propagated in an oral to anal direction because of impaired descending inhibition; (6) reduced CMMCs and inhibitory responses to intraluminal balloon distension; (7) an absence of reflex activity in response to mucosal stimulation. In addition, (8) thin pellets that propagated along the control colon failed to do so in Tph1(-/-) colon; and (9) the 5-HT3 receptor antagonist ondansetron, which reduced CMMCs and blocked their propagation in Tph1(+/+) mice, failed to alter CMMCs in Tph1(-/-) animals. Our observations suggest that mucosal 5-HT is essential for reflexes driven by mucosal stimulation and is also important for normal propagation of CMMCs and propulsion of pellets in the isolated colon.

Automated algorithm for GI spike burst detection and demonstration of efficacy in ischemic small intestine

We present a novel, fully-automated gastrointestinal spike burst detection algorithm. Following pre-processing with SALPA (Wagenaar and Potter, J. Neurosci. Methods 120:113-120, 2002) and a Savitzky-Golay filter to remove unwanted low and high frequency components, candidate spike waveforms are detected utilizing the non-linear energy operator. Candidate waveforms are classified as spikes or artifact by a support vector machine. The new method achieves highly satisfactory performance with >90% sensitivity and positive prediction value. We also demonstrate an application of the new method to detect changes in spike rate and spatial propagation patterns upon induction of mesenteric ischemia in the small intestine. Spike rates were observed to transiently increase 10-20 fold for a duration of ≈600 s, relative to baseline conditions. In ischemic conditions, spike activity propagation patterns included retrograde-longitudinal wavefronts with occasional spontaneous conduction blocks, as well as self-terminating concentric-circumferential wavefronts. Longitudinal and circumferential velocities were 6.8-8.0 cm/s and 18.7 cm/s, respectively.

Identification and mechanosensitivity of viscerofugal neurons

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

Self-sustained peristaltic waves: explicit asymptotic solutions

A simple nonlinear model for the coupled problem of fluid flow and contractile wall deformation is proposed to describe peristalsis. In the context of the model the ability of a transporting system to perform autonomous peristaltic pumping is interpreted as the ability to propagate sustained waves of wall deformation. Piecewise-linear approximations of nonlinear functions are used to analytically demonstrate the existence of traveling-wave solutions. Explicit formulas are derived which relate the speed of self-sustained peristaltic waves to the rheological properties of the transporting vessel and the transported fluid. The results may contribute to the development of diagnostic and therapeutic procedures for cases of peristaltic motility disorders.

Loss of visceral pain following colorectal distension in an endothelin-3 deficient mouse model of Hirschsprung's disease

Endothelin peptides and their endogenous receptors play a major role in nociception in a variety of different organs. They also play an essential role in the development of the enteric nervous system. Mice with deletions of the endothelin-3 gene (lethal spotted mice, ls/ls) develop congenital aganglionosis. However, little is known about how nociception might be affected in the aganglionic rectum of mice deficient in endothelin-3. In this study we investigated changes in spinal afferent innervation and visceral pain transmission from the aganglionic rectum in ls/ls mice. Electromyogram recordings from anaesthetized ls/ls mice revealed a deficit in visceromotor responses arising from the aganglionic colorectum in response to noxious colorectal distension. Loss of visceromotor responses (VMRs) in ls/ls mice was selective, as no reduction in VMRs was detected after stimulation of the bladder or somatic organs. Calcitonin gene related peptide (CGRP) immunoreactivity, retrograde neuronal tracing and extracellular afferent recordings from the aganglionic rectum revealed decreased colorectal spinal innervation, combined with a reduction in mechanosensitivity of rectal afferents. The sensory defect in ls/ls mice is primarily associated with changes in low threshold wide dynamic range rectal afferents. In conclusion, disruption of endothelin 3 gene expression not only affects development and function of the enteric nervous system, but also specific classes of spinal rectal mechanoreceptors, which are required for visceral nociception from the colorectum.

Afferent mechanism in the urinary tract

Much of the current research on lower urinary tract dysfunction is focused on afferent mechanisms. The main goals are to define and modulate the signaling pathways by which afferent information is generated and conveyed to the central nervous system. Alterations in bladder afferent mechanisms are a potential source of voiding dysfunction and an emerging source of drug targets. Even some established drug therapies such as muscarinic receptor antagonists, as well as emerging therapies such as botulinum toxin type-A, may act partly through afferent mechanisms. This review presents up-to-date findings on the localization of afferent fiber types within the bladder wall, afferent receptors and transmitters, and how these may communicate with the urothelium, interstitial cells, and detrusor smooth muscle to regulate micturition in normal and pathological bladders. Peripheral and central mechanisms of afferent sensitization and myogenic mechanisms that lead to detrusor overactivity, overactive bladder symptoms, and urgency sensations are also covered as well as new therapeutic approaches and new and established methods of measuring afferent activity.

The use of abdominal massage to treat chronic constipation

Constipation is a disorder of gastrointestinal motility characterized by difficult or decreased bowel movements, and is a common condition in Western countries. Laxatives are the most common strategy for managing constipation. However, long-term use of some laxatives may be associated with harmful side-effects including increased constipation and fecal impaction. Abdominal massage, once an accepted method of treating constipation, is no longer standard of care, but may be a desirable therapy for this condition because it is inexpensive, non-invasive, free of harmful side-effects, and can be performed by patients themselves. However, until recently, evidence for its effectiveness was not strong enough to make a recommendation for its use in constipated patients. In 1999, Ernst reviewed all available controlled clinical trials, and found that there was no sound evidence for the effectiveness of abdominal massage in the treatment of chronic constipation. This article reviews scientific evidence from 1999 to the present, regarding abdominal massage as an intervention for chronic constipation. Since that time, studies have demonstrated that abdominal massage can stimulate peristalsis, decrease colonic transit time, increase the frequency of bowel movements in constipated patients, and decrease the feelings of discomfort and pain that accompany it. There is also good evidence that massage can stimulate peristalsis in patients with post-surgical ileus. Individual case reports show that massage has been effective for patients with constipation due to a variety of diagnosed physiologic abnormalities, as well as in patients with long-term functional constipation.

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.

Lactobacillus reuteri ingestion and IK(Ca) channel blockade have similar effects on rat colon motility and myenteric neurones

BACKGROUND

We have previously shown that ingestion of Lactobacillus reuteri may modulate colonic enteric neuron activity but with unknown effects on colon motility. The aim of the present report was to elucidate the neuronal mechanisms of action of the probiotic by comparing the effects on motility of L. reuteri ingestion with blockade of a specific ionic current in enteric neurons.

METHODS

We have used intraluminal pressure recordings from ex vivo rat colon segments and whole cell patch clamp recordings from neurons of rat longitudinal muscle myenteric plexus preparations to investigate the effects of L. reuteri and TRAM-34 on colon motility and neurophysiology. The effects of daily feeding of 10(9) L. reuteri bacteria or acute application of TRAM-34 on threshold fluid filling pressure or pulse pressure was measured.

KEY RESULTS

Lactobacillus reuteri increased intraluminal fluid filling pressure thresholds for evoking pressure pulses by 51% from 0.47 +/- 0.17 hPa; the probiotic also decreased the pulse pressure amplitudes, but not frequency, by 18% from 3.91 +/- 0.52 hPa. The intermediate conductance calcium-dependent potassium (IK(Ca)) channel blocker TRAM-34 (3 micromol L(-1)) increased filling threshold pressure by 43% from 0.52 +/- 0.22 hPa and reduced pulse pressure amplitude by 40% from 2.63 +/- 1.11 hPa; contraction frequency was unaltered. TRAM-34 (3 micromol L(-1)) reduced membrane polarization, leak conductance and the slow afterhyperpolarization current in 16/16 myenteric rat colon AH cells but 19/19 S cells were unaffected.

CONCLUSIONS & INFERENCES

The present results are consistent with L. reuteri enhancing tonic inhibition of colon contractile activity by acting via the IK(Ca) channel current in AH cells.

Mechanotransduction and chemosensitivity of two major classes of bladder afferents with endings in the vicinity to the urothelium

The guinea pig bladder is innervated by at least five distinct major classes of extrinsic sensory neurons. In this study, we have examined the mechanisms of mechanotransduction and chemosensitivity of two classes of bladder afferents that have their endings in the vicinity of the urothelium: stretch-sensitive muscle-mucosal mechanoreceptors and stretch-insensitive, mucosal high-responding afferents. The non-selective P2 purinoreceptor antagonist pyridoxal phosphate-6-azophenyl-2',4'-disulphonic acid did not affect stretch- or stroking-induced firing of these afferents but significantly reduced the excitatory action of alpha,beta-methylene ATP. Blocking synaptic transmission in Ca(2+)-free solution did not affect stretch-evoked firing but slightly reduced stretch-induced tension responses. Stroking-induced firing of both classes of afferents was also not affected in Ca(2+)-free solution. Of blockers of mechano-gated channels, benzamil (100 microM), but not amiloride (100 microM), Gd(3+) (100 microM) or SKF 96365 (50 microM), inhibited stretch- and stroking-induced firing. Serotonin (100 microM) applied directly onto receptive fields predominantly activated muscle-mucosal afferents. Muscarine (100 microM) and substance P (100 microM) in 24% and 36% cases activated only mucosal high-responding units. Bradykinin (10 microM), but not prostaglandin E2 (10 microM), excites predominantly mucosal units. High (80 mM) K(+) solution activated both afferent classes, but responses of mucosal units were 4 times greater. In contrast to muscle-mucosal units, most mucosal high-responding units were activated by hot Krebs solution (45-46 degrees C), low pH (pH 4) and capsaicin (3 microm). TRPV1 antagonist, capsazepine (10 microM) was without effect on mechanotransduction by mucosal high-responding afferents. The results show that mechanotransduction of these two types of afferents are not dependant upon Ca(2+)-dependent exocytotic release of mediators, or ATP, and it is likely that benzamil-sensitive stretch-activated ion channels on their endings are involved in direct mechanotransduction. The chemosensitivity to agonists and noxious stimuli differs significantly between these two major classes of bladder afferents that reflects their different physiological and pathophysiological roles in the bladder.

Identification of functional intramuscular rectal mechanoreceptors in aganglionic rectal smooth muscle from piebald lethal mice

The mechanosensitive endings of low-threshold, slowly adapting pelvic afferents that innervate the rectum have been previously identified as rectal intraganglionic laminar endings (rIGLEs) that lie within myenteric ganglia. We tested whether the aganglionic rectum of piebald-lethal (s(l)/s(l)) mice lacks rIGLEs and whether this could explain impaired distension-evoked reflexes from this region. Extracellular recordings were made from fine rectal nerves in C57BL/6 wild-type and s(l)/s(l) mice, combined with anterograde labeling. In C57BL/6 mice, graded circumferential stretch applied to the rectum activated graded increases in firing of slowly adapting rectal mechanoreceptors. In s(l)/s(l) mice, graded stretch of the aganglionic rectum activated similar graded increases in rectal afferent firing. Stretch-sensitive afferents responded at low mechanical thresholds and fired more intensely at noxious levels of stretch. They could also be activated by probing their receptive fields with von Frey hairs and by muscle contraction. Anterograde labeling from recorded rectal nerves identified the mechanoreceptors of muscular afferents in the aganglionic rectal smooth muscle. A population of afferents were also recorded in both C57BL/6 and s(l)/s(l) mice that were activated by von Frey hair probing, but not stretch. In summary, the aganglionic rectum is innervated by a population of stretch-sensitive rectal afferent mechanoreceptor which develops and functions in the absence of any enteric ganglia. These results suggest that in patients with Hirschsprung's disease the inability to activate extrinsic distension reflexes from the aganglionic rectum is unlikely to be due to the absence of stretch-sensitive extrinsic mechanoreceptors.

Identification of capsaicin-sensitive rectal mechanoreceptors activated by rectal distension in mice

Rodents detect visceral pain in response to noxious levels of rectal distension. However, the mechanoreceptors that innervate the rectum and respond to noxious levels of rectal distension have not been identified. Here, we have identified the mechanoreceptors of capsaicin-sensitive rectal afferents and characterized their properties in response to circumferential stretch of the rectal wall. We have also used the lethal spotted (ls/ls) mouse to determine whether rectal mechanoreceptors that respond to capsaicin and stretch may also develop in an aganglionic rectum that is congenitally devoid of enteric ganglia. In wild type (C57BL/6) mice, graded increases in circumferential stretch applied to isolated rectal segments activated a graded increase in firing of slowly-adapting rectal mechanoreceptors. Identical stimuli applied to the aganglionic rectum of ls/ls mice also activated similar graded increases in firing of stretch-sensitive rectal afferents. In both wild type and aganglionic rectal preparations, focal compression of the serosal surface using von Frey hairs identified mechanosensitive "hot spots," that were associated with brief bursts of action potentials. Spritzing capsaicin (10 microM) selectively onto each identified mechanosensitive hot spot activated an all or none discharge of action potentials in 32 of 56 identified hot spots in wild type mice and 24 of 62 mechanosensitive hot spots in the aganglionic rectum of ls/ls mice. Each single unit activated by both capsaicin and circumferential stretch responded to low mechanical thresholds (1-2 g stretch). No high threshold rectal afferents were ever recorded in response to circumferential stretch. Anterograde labeling from recorded rectal afferents revealed two populations of capsaicin-sensitive mechanoreceptor that responded to stretch: one population terminated within myenteric ganglia, the other within the circular and longitudinal smooth muscle layers. In the aganglionic rectum of ls/ls mice, only the i.m. mechanoreceptors were identified. Both myenteric and i.m. mechanoreceptors could be identified by their immunoreactivity to the anti-TRPV1 antibody and the vesicular glutamate transporter, Vglut2. Myenteric mechanoreceptors had a unique morphology, consisting of smooth bulbous nodules that ramified within myenteric ganglia. In summary, the rectum of wild type mice is innervated by at least two populations of capsaicin-sensitive rectal mechanoreceptor, both of which respond to low mechanical thresholds within the innocuous range. These findings suggest that the visceral pain pathway activated by rectal distension is likely to involve low threshold rectal mechanoreceptors that are activated within the normal physiological range.

Properties of the major classes of mechanoreceptors in the guinea pig bladder

Sensory neurons represent an attractive target for pharmacological treatment of various bladder disorders. However the properties of major classes of mechano-sensory neurons projecting to the bladder have not been systematically established. An in vitro bladder preparation was used to examine the effects of a range of mechanical stimuli (stretch, von Frey hair stroking and focal compression of receptive fields) and chemical stimuli (1 mm alpha,beta-methylene ATP, hypertonic solutions (500 mm NaCl) and 3 microm capsaicin) during electrophysiological recordings from guinea pig bladder afferents. Four functionally distinct populations of bladder sensory neurons were distinguished by these stimuli. The first class, muscle mechanoreceptors, were activated by stretch but not by mucosal stroking with light (0.05-0.1 mN) von Frey hairs or by hypertonic saline, alpha,beta-methylene ATP or capsaicin. Removal of the urothelium did not affect their stretch-induced firing. The second class, muscle-mucosal mechanoreceptors, were activated by both stretch and mucosal stroking with light von Frey hairs or by hypertonic saline and by alpha,beta-methylene ATP, but not by capsaicin. Removal of the urothelium reduced their stretch- and stroking-induced firing. The third class, mucosal high-responding mechanoreceptors, were stretch-insensitive but could be activated by mucosal stroking with light von Frey hairs or by hypertonic saline, alpha,beta-methylene ATP and capsaicin. Stroking-induced firing was significantly reduced by removal of the urothelium. The fourth class, mucosal low-responding mechanoreceptors, were stretch insensitive but could be weakly activated by mucosal stroking with light von Frey hairs but not by hypertonic saline, alpha,beta-methylene ATP or capsaicin. Removal of the urothelium reduced mucosal stroking-induced firing. All four populations of afferents conducted in the C-fibre range and showed class-dependent differences in spike amplitude and duration. At least four functional classes of bladder mechanoreceptors can be readily distinguished by different mechanisms of activation and are likely to transmit different types of information to the central nervous system.

An enteric occult reflex underlies accommodation and slow transit in the distal large bowel

BACKGROUND & AIMS

Transit of fecal material through the human colon takes > or =30 hours, whereas transit through the small intestine takes 24 hours. The mechanisms underlying colonic storage and slow transit have yet to be elucidated. Our aim was to determine whether an intrinsic neural mechanism underlies these phenomena.

METHODS

Recordings were made from circular muscle (CM) cells and myenteric neurons in the isolated guinea pig distal colon using intracellular recordings and Ca(2+) imaging techniques. Video imaging was used to determine the effects of colonic filling and pellet transit.

RESULTS

Circumferential stretch generated ongoing oral excitatory and anal inhibitory junction potentials in the CM. The application of longitudinal stretch inhibited all junction potentials. N-omega-nitro-L-arginine (100 micromol/L) completely reversed the inhibitory effects of longitudinal stretch suggesting that nitric oxide (NO) inhibited interneurons controlling peristaltic circuits. Ca(2+) imaging in preparations that were stretched in both axes revealed ongoing firing in nNOS +ve descending neurons, even when synaptic transmission was blocked. Inhibitory postsynaptic potentials were evoked in mechanosensitive interneurons that were blocked by N-omega-nitro-L-arginine (100 micromol/L). Pellet transit was inhibited by longitudinal stretch. Filling the colon with fluid led to colonic elongation and an inhibition of motility.

CONCLUSIONS

Our data support the novel hypothesis that slow transit and accommodation are generated by release of NO from descending (nNOS +ve) interneurons triggered by colonic elongation. We refer to this powerful inhibitory reflex as the intrinsic occult reflex (hidden from observation) because it withdraws motor activity from the muscle.

Activation of neural circuitry and Ca2+ waves in longitudinal and circular muscle during CMMCs and the consequences of rectal aganglionosis in mice

In mammals that develop rectal aganglionosis, the aganglionic segment still exhibits spontaneous phasic contractions that contribute to dysmotility and pseudoobstruction in this region. However, almost nothing is known about the mechanisms that generate these myogenic contractions or the effects of aganglionosis on the generation of Ca(2+) waves that underlie contractions of the longitudinal muscle (LM) and circular muscle (CM). In a mouse model of Hirschsprung's disease [endothelin type B receptor-deficient (Ednrb(s-l)/Ednrb(s-l)) mice], the Ca(2+) indicator fluo-4 was used to simultaneously monitor the temporal activation and spread of intercellular Ca(2+) waves in the LM and CM during spontaneous colonic motor activities. During the intervals between colonic migrating motor complexes (CMMCs) in control mice, Ca(2+) waves discharged asynchronously between the LM and CM. However, in these same mice, during CMMCs, a burst of discreet Ca(2+) waves fired simultaneously in both muscle layers, where the propagation velocity of Ca(2+) waves significantly increased, as did the rate of initiation and number of collisions between Ca(2+) waves. Hexamethonium (300 microM) or atropine (1 microM) prevented synchronized firing of Ca(2+) waves. In the aganglionic distal colon of Ednrb(s-l)/Ednrb(s-l) mice, not only were CMMCs absent, but Ca(2+) waves between the two muscle layers fired asynchronously, despite increased propagation velocity. The generation of CMMCs in control mice involves synchronized firing of enteric motor nerves to both the LM and CM, explaining the synchronized firing of discreet Ca(2+) waves between the two muscle layers. Aganglionosis results in a sporadic and sustained asynchrony in Ca(2+) wave firing between the LM and CM and an absence of CMMCs.

Anatomic modifications in the enteric nervous system of piebald mice and physiological consequences to colonic motor activity

It has been assumed that in piebald lethal mice that develop megacolon, impaired colonic motor activity is restricted to the aganglionic distal colon. Peristaltic mechanical recordings, immunohistochemistry, and quantitative PCR were used to investigate whether regions of the colon, other than the aganglionic segment, may also show anatomical modifications and dysfunctional colonic motor activity. Contrary to expectations, colonic migrating motor complexes (MMCs) were absent along the whole colon of piebald lethal homozygote mice and severely impaired in heterozygote siblings. Aganglionosis was detected not only in the distal colon of piebald homozygote lethal mice (mean length: 20.4 +/- 2.1 mm) but also surprisingly in their heterozygote siblings (mean length: 12.4 +/- 1.1 mm). Unlike homozygote lethal mice, piebald heterozygotes showed no signs of megacolon. Interestingly, mRNA expression for PGP 9.5 was also dramatically reduced (by 71-99%) throughout the entire small and large bowel in both homozygote lethal and heterozygous littermates (by 67-87%). Histochemical staining confirmed a significant reduction in myenteric ganglia along the whole colon. In summary, the piebald mutation in homozygote lethal and heterozygote siblings is associated with dramatic reductions in myenteric ganglia throughout the entire colon and not limited to the distal colon as originally thought. Functionally, this results in an absence or severe impairment of colonic MMC activity in both piebald homozygote lethal and heterozygote siblings, respectively. The observation that piebald heterozygotes have an aganglionic distal colon (mean length: 12 mm) but live a normal murine life span without megacolon suggests that aganglionosis >12 mm and the complete absence of colonic MMCs may be required before any symptoms of megacolon arise.

Ava[L-Pro9,N-MeLeu10] substance P(7-11) (GR 73632) and Sar9, Met(O2)11 increase distention-induced peristalsis through activation of neurokinin-1 receptors on smooth muscle and interstitial cells of cajal

Substance P is generally considered an excitatory neurotransmitter related to gut motor activity, although an inhibitory influence of neurokinin-1 (NK1) receptor activation on peristalsis has also been reported. With an optimized in vitro method to assess distention-induced peristalsis, our aim was to clarify the effect of NK1 receptor activation on peristaltic activity and to reveal the mechanisms by which NK1 activation alters peristalsis. Distention of the small intestine of the mouse and guinea pig induced periodic occurrence of rhythmic waves of propagating rings of circular muscle contraction, associated with slow waves and superimposed action potentials, that propelled intestinal contents aborally. Activation of NK1 receptors by Ava[l-Pro(9),N-MeLeu10] substance P(7-11) (GR 73632) and Sar(9), Met(O(2))(11) on smooth muscle cells resulted in prolongation of the activity periods and increased action potential generation occurring superimposed on the intestinal slow wave activity. Activation of NK1 receptors on interstitial cells of Cajal resulted in an increase in slow wave frequency. Slow wave amplitude increased, likely by increased cell-to-cell coupling. The NK1 antagonist (S)-1-(2-[3-(3,4-dichlorophenyl)-1-(3-isopropoxyphenylacetyl)piperidin-3-yl]ethyl)-4-phenyl-1-azoniabicyclo[2.2.2]octane chloride (SR 140333) induced a decrease in the slow wave frequency and duration of the activity periods evoked by distention, which makes it likely that NK1 receptor activation plays a role in the normal physiological distention-induced generation of peristaltic motor patterns. In summary, NK1 receptors play a role in normal development of peristalsis and NK1 receptor activation markedly increases propulsive peristaltic contractile activity.

Lack of pyloric interstitial cells of Cajal explains distinct peristaltic motor patterns in stomach and small intestine

The frequency and propagation velocity of distension-induced peristaltic contractions in the antrum and duodenum are distinctly different and depend on activation of intrinsic excitatory motoneurons as well as pacemaker cells, the interstitial cells of Cajal associated with Auerbach's plexus (ICC-AP). Because ICC are critical for coordination of motor activities along the long axis of many regions in the gut, the role of ICC in antroduodenal coordination was investigated. We used immunohistochemistry, electron microscopy, simultaneous multiple electrical recordings in vitro, and videofluoroscopy in vivo in mice and rats. A strongly reduced number of ICC-AP with loss of network characteristics was observed in a 4-mm area in the rat and a 1-mm area in the mouse pyloric region. The pyloric region showed a slow wave-free gap of 4.1 mm in rats and 1.3 mm in mice. Between antrum and duodenum, there was no interaction of electrical activities and in the absence of gastric emptying, there was no coordination of motor activities. When the pyloric sphincter opened, 2.4 s before the front of the antral wave reached the pylorus, the duodenum distended after receiving gastric content and aboral duodenal peristalsis was initiated, often disrupting other motor patterns. The absence of ICC-AP and slow wave activity in the pyloric region allows the antrum and duodenum to have distinct uncoordinated motor activities. Coordination of aborally propagating peristaltic antral and duodenal activity is initiated by opening of the pylorus, which is followed by distention-induced duodenal peristalsis. Throughout this coordinated motor activity, the pacemaker systems in antrum and duodenum remain independent.

Mechanisms of mechanotransduction by specialized low-threshold mechanoreceptors in the guinea pig rectum

The guinea pig rectum, but not the colon, is innervated by a specialized class of distension-sensitive mechanoreceptors that have transduction sites corresponding to rectal intraganglionic laminar endings (rIGLEs). Rectal mechanoreceptors recorded in vitro had low threshold to circumferential stretch, adapted slowly, and could respond within 2 ms to mechanical stimulation by a piezo-electric probe. Antagonists to ionotropic N-methyl-D-aspartate (NMDA; CGS 19755, memantine) and non-NMDA (6,7-dinitroquinoxaline-2,3-dione) glutamate receptors did not affect mechanotransduction. In normal Krebs solution, the P2X purinoreceptor agonist alpha,beta-methylene ATP reduced mechanoreceptor firing evoked by distension but simultaneously relaxed circular smooth muscle and inhibited stretch-induced contractions. Neither ATP nor alpha,beta-methylene ATP affected mechanotransduction when transduction sites were directly compressed with von Frey hairs. The P2 purinoreceptor antagonist pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid did not affect stretch-induced firing but reduced the inhibitory effect of alpha,beta-methylene ATP on stretch-induced firing. Under isometric conditions, blocking synaptic transmission in Ca2+-free solution reduced stretch-evoked firing but not when basal tension was restored to control levels. Under isotonic condition, Ca2+-free solution did not significantly affect load-evoked firing. The blockers of mechanogated and/or transient receptor potential channels, benzamil, Gd3+, SKF 96365, and ruthenium red inhibited stretch-induced firing but, in parallel, significantly reduced stretch-induced contractions. Benzamil and SKF 96365 were able to inhibit mechanotransduction when transduction sites were compressed with von Frey hairs. The results show that mechanotransduction is rapid but does not depend on fast exocytotic release of mediators. It is likely that stretch-activated ion channels on rIGLEs are involved in direct, physical mechanotransduction by rectal low-threshold mechanoreceptors.

Mechanical activation of rectal intraganglionic laminar endings in the guinea pig distal gut

The rectum receives specialized extrinsic afferent innervation by stretch-sensitive, low threshold, slowly adapting mechanoreceptors, with transduction sites shown to correspond to rectal intraganglionic laminar endings (IGLEs). Rectal IGLEs are located in myenteric ganglia, surrounded by the longitudinal and circular smooth muscle layers; in this study we investigated the mechanical stimuli to which they respond. Mechanoreceptors had graded responses to highly focal transmural compression with von Frey hairs. They were activated when stretched circumferentially by imposed increases of both length and load. Under both conditions, firing typically occurred in bursts associated with phasic muscle contractions. However, many contractions did not evoke firing. Longitudinal stretch also evoked firing, again associated with contractile activity. Thus, mechanoreceptors did not show directional sensitivity. Two agonists that excited smooth muscle directly (0.1 microm [beta-Ala(8)]-neurokinin A (4-10) and 1 microm carbachol) activated rectal mechanoreceptors, but not in the presence of Ca(2)(+)-free solution or when preparations were kept entirely slack. We measured the dimensions, in both longitudinal and circumferential axes, of receptive fields during smooth muscle contractile activity, using video micrography. Contractile activity within the receptive field often differed significantly from the behaviour of the preparation as a whole, providing an explanation for many of the discrepancies between gross contractility and firing. Simultaneous contraction of both muscle layers within the receptive field was the strongest predictor of mechanoreceptor activation. Our results suggest that rectal mechanoreceptors do not act simply as tension receptors: rather they appear to detect mechanical deformation of myenteric ganglia - especially flattening - associated with stretch of the receptive field, or contractions of smooth muscle within the receptive field.

Calcium imaging of gut activity

The major cell types regulating gut motility include enteric neurones, interstitial cells of Cajal (ICC) and their effector smooth muscle cells. These cells are arranged conveniently in nested layers through the gut wall. Our knowledge of how many of these cells in each layer are integrated to produce the various patterns of motility is largely unknown. So far, much of our knowledge of gut motility has usually been obtained by examining point sources of activity (e.g. intracellular recordings from enteric neurones, ICC and smooth muscle cells), rather than the spread of activity through these spatially distributed nerve and ICC networks, or smooth muscle syncitia. Our understanding of how these cells are integrated to produce gut movements would be greatly enhanced if we could image the activity in many of these cells in each layer, or many cells in several layers, simultaneously. Calcium (Ca2+) is a major signalling and regulatory molecule in most cells. In fact, electrical excitability in enteric neurones, ICC and smooth muscle is associated with robust rises in intracellular Ca2+ that long outlast the electrical events (e.g. action potentials in neurones and smooth muscle) that gave rise to them. These prolonged Ca2+ responses, together with the development of several high quality Ca2+ indicators, has provided a unique opportunity to image many cells in intact tissues simultaneously using ICCD video-rate cameras along with conventional microscopy. However, confocal microscopy has also been used, and has several advantages over the above systems. These include reduced photo-toxicity and bleaching and the elimination of out of focus light from different layers within the tissue. So far, despite some limitations with the calcium imaging techniques, the spread of activity through the two layers of smooth muscle, ICC networks and myenteric neurones in intact preparations, or cultured myenteric neuronal networks, is beginning to yield exciting new data about how these different cells interact and process information.

Segmentation induced by intraluminal fatty acid in isolated guinea-pig duodenum and jejunum

Small intestinal movements depend on the composition of the chyme with mixing predominating at high nutrient levels and propulsion being prevalent at low nutrient levels. The mechanisms coupling nutrients to motility are unknown. We used computer analysis of video recordings of isolated guinea-pig duodenum, jejunum and ileum to examine movements induced by a fatty acid, decanoic acid. Increasing intraluminal pressure past a threshold using control saline consistently evoked propulsive reflexes: lumen-occluding constrictions appeared at the oral end propagating at 20.4 +/- 2.4 mm s(-1) (mean +/-s.d., jejunum) to the anal end before being repeated until the intraluminal pressure was returned to control. Subthreshold pressure increases sometimes evoked a transient series of constrictions appearing at the oral end and propagating anally at 18.4 +/- 4.7 mm s(-1) (jejunum). At basal pressures, decanoic acid dose-dependently induced motor activity consisting of 40-60 s episodes of constrictions separated by 40-200 s periods of quiescence and lasting up to 2 h. Five contraction patterns were identified within episodes including localized stationary constrictions; constrictions that propagated slowly (5-8 mm s(-1)) for short distances orally or anally; and constrictions that propagated orally or anally for the length of the preparation at 14-20 mm s(-1). Decanoic acid induced motor activity was reversibly abolished by tetrodotoxin (3 microm), hyoscine (1 microm) and hexamethonium (100 microm), but was insensitive to blockade of P2 purinoceptors by PPADS (60 microm). Thus, decanoic acid induces motor activity equivalent to segmentation in guinea-pig small intestine in vitro and this depends on intrinsic neural pathways.

Inhibitory cotransmission or after-hyperpolarizing potentials can regulate firing in recurrent networks with excitatory metabotropic transmission

Recurrent networks of neurons communicating via excitatory connections are common in the nervous system. In the absence of mechanisms to control firing (collectively termed negative feedback), these networks are likely to be bistable and unable to meaningfully encode input signals. In most recurrent circuits, negative feedback is provided by a specialized subpopulation of interneurons, but such neurons are absent from some systems, which therefore require other forms of negative feedback. One such circuit is found within the enteric nervous system of the intestine, where AH/Dogiel type II neurons are interconnected via excitatory synapses acting through metabotropic receptors to produce slow excitatory postsynaptic potentials (slow EPSPs). Negative feedback in this recurrent network may come from either inhibitory postsynaptic potentials arising from the terminals that produce slow EPSPs or from the after hyperpolarizing potentials (AHPs) characteristic of these neurons. We have examined these possibilities using mathematical analysis, based on the Wilson-Cowan model, and computer simulations. Analysis of steady states showed that, under appropriate conditions, both types of negative feedback can provide robust regulation of firing allowing the networks to encode input signals. Numerical simulations were performed using large, anatomically realistic networks with realistic models for metabotropic transmission and suppression of the AHP. In the presence of constant exogenous input, parameters controlling aspects of synaptic events were varied, confirming the analytical results for static stimuli. The simulated networks also responded to time varying inputs in a manner consistent with known physiology. In addition, simulation revealed that neurons in networks with inhibitory contransmission fired in erratic bursts, a phenomenon observed in neurons in unparalysed tissue. Thus, either inhibitory contransmission or AHPs, or both, can allow recurrent networks of AH/Dogiel type II neurons to encode ongoing inputs in a biologically useful way. These neurons appear to be intrinsic primary afferent neurons (IPANs), which implies that the IPANs in a region act in a coordinated fashion.

Mechanotransduction by intraganglionic laminar endings of vagal tension receptors in the guinea-pig oesophagus

Vagal mechanoreceptors to the guinea-pig oesophagus, recorded extracellularly, in vitro, fired spontaneously at 3.3 +/- 0.2 Hz, (n = 75, from 57 animals), and had low thresholds to circumferential stretch. In this study, we have investigated whether mechanotransduction by intraganglionic laminar endings (IGLEs) directly relies on mechano-gated ion channels, or whether it is due to chemical activation by neurotransmitters (glutamate or ATP) released from other cells during mechanical distortion. Rapid distortion of focal transduction sites (IGLEs) evoked action potentials with a latency of < 10 ms. Antagonists to ionotropic (AP5, memantine and 6,7-dinitroquinoxaline-2,3-dione (DNQX)) and metabotropic glutamate receptors (N-phenyl-7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxamide (PHCCC) and (RS)-a-methyl-4-phosphono-phenylglycine (MPPG)) did not affect mechano-transduction. Glutamate, NMDA and the selective mGluR group II and III agonists, (2R, 4R)-APDC and L-AP4, had no effect on spontaneous or stretch-induced firing. The P2X purinoreceptor agonist, alpha,beta-methylene ATP, caused concentration-dependent excitation of vagal mechanoreceptors (EC50 = 22.2 microM) which was blocked by the non-selective P2 antagonist PPADS (30 microM). On its own, PPADS affected neither stretch-induced firing nor spontaneous firing. Neither Ca(2+)-free solution (1 mM EDTA, 3.6 mM Mg(2+)) solution nor Cd(2+) (100 microM) blocked stretch-induced firing. Thus chemical transmission is not involved in activation of vagal mechanoreceptors. The blocker of stretch-activated channels, Gd(3+) (300 microM), did not inhibit stretch-induced firing. However, benzamil (100 microM) significantly inhibited spontaneous and distension-evoked firing in a stretch-dependent manner; proportionally greater inhibition was seen with larger stretches. The results suggest that IGLEs of vagal tension receptors directly transduce mechanical stimuli probably via benzamil-sensitive, Gd3+-insensitive, stretch-activated ion channels, and that chemical transmission is not involved in transduction.

A smooth muscle tone-dependent stretch-activated migrating motor pattern in isolated guinea-pig distal colon

We have investigated the tone dependence of the intrinsic nervous activity generated by localized wall distension in isolated segments of guinea-pig distal colon using mechanical recordings and video imaging of wall movements. A segment of colon was threaded through two partitions, which divided the colon for pharmacological purposes into oral, stimulation and anal regions. An intraluminal balloon was located in the stimulation region between the two partitions (12 mm apart). Maintained colonic distension by an intraluminal balloon or an artificial faecal pellet held at a fixed location generated rhythmic (frequency 0.3 contractions min(-1); duration approximately 60 s) peristaltic waves of contraction. Video imaging of colonic wall movements or the selective application of pharmacological agents suggested that peristaltic waves originated just oral (< or = 4 mm) to the pellet and propagated both orally (approximately 11 mm s(-1)) and anally (approximately 1 mm s(-1)). Also, during a peristaltic wave the colon appears to passively shorten in front of a pellet, as a result of an active contraction of the longitudinal muscle oral to the pellet. Faecal pellet movement only occurred when a rhythmic peristaltic wave was generated. Rhythmic peristaltic waves were abolished in all regions by the smooth muscle relaxants isoproterenol (1 microM), nicardipine (1 microM) or papavarine (10 microM), and by the neural antagonists tetrodotoxin (TTX; 0.6 microM), hexamethonium (100 microM) or atropine (1 microM), when added selectively to the stimulation region. Nicardipine, atropine, TTX, or hexamethonium (100 microM) also blocked the evoked peristaltic waves when selectively added to the oral region. Nomega-nitro-L-arginine (L-NA; 100 microM) added to the anal region reduced the anal relaxation but increased the anal contraction, leading to an increase in the apparent conduction velocity of each peristaltic wave. In conclusion, maintained distension by a fixed artificial pellet generates propulsive, rhythmic peristaltic waves, whose enteric neural activity is critically dependent upon smooth muscle tone. These peristaltic waves usually originate just oral to the pellet, and their apparent conduction velocity is generated by activation of descending inhibitory nerve pathways.

Motility in the isolated mouse colon: migrating motor complexes, myoelectric complexes and pressure waves

This study has used mechanical, together with pressure/volume recordings or electrophysiological recordings, to investigate the spontaneous activity in isolated preparations of mouse colon. In the former preparations, when not distended with fluid, spontaneous colonic migrating motor complexes (CMMCs) were observed using isotonic transducers. When the colons were distended with fluid, CMMCs continued at an increased frequency and in addition were associated temporally, with rises in intraluminal pressure and pulses of distally ejected fluid. 5-Hydroxytryptamine (1 micro mol L-1) or NG-nitro-l-arginine (100 micro mol L-1) increased the frequency of propulsive activity and this activity was abolished by hexamethonium (500 micro mol L-1). In a second preparation, myoelectric complexes recorded from circular muscle cells in colons using intracellular microelectrodes, were found to correlate in frequency and phase with CMMCs. The experiments indicate that CMMCs are intimately related to pressure waves in the fluid-filled viscus and the muscle membrane potential changes that have been recorded during myoelectric complexes are likely to be analogous to those occurring during fluid-filled propulsive activity.

Stretch-activated neuronal pathways to longitudinal and circular muscle in guinea pig distal colon

The role of the longitudinal muscle (LM) layer during the peristaltic reflex in the small and large intestine is unclear. In this study, we have made double and quadruple simultaneous intracellular recordings from LM and circular muscle (CM) cells of guinea pig distal colon to correlate the electrical activities in the two different muscle layers during circumferential stretch. Simultaneous recordings from LM and CM cells (<200 microm apart) at the oral region of the colon showed that excitatory junction potentials (EJPs) discharged synchronously in both muscle layers for periods of up to 6 h. Similarly, at the anal region of the colon, inhibitory junction potentials (IJPs) discharged synchronously in the two muscle layers. Quadruple recordings from LM and CM orally at the same time as from the LM and CM anally revealed that IJPs occurred synchronously in the LM and CM anally at the same time as EJPs in LM and CM located 20 mm orally. Oral EJPs and anal IJPs were linearly related in amplitude between the two muscle layers. Spatiotemporal maps generated from simultaneous video imaging of the movements of the colon, combined with intracellular recordings, revealed that some LM contractions orally could be correlated in time with IJPs in CM cells anally. N(omega)-nitro-L-arginine (L-NA; 100 microM) abolished the IJP in LM, whereas a prominent L-NA-resistant "fast" IJP was always observed in CM. In summary, in stretched preparations, synchronized EJPs in both LM and CM orally are generated by synchronized firing of many ascending interneurons, which simultaneously activate excitatory motor neurons to both muscle layers. Similarly, synchronized IJPs in both LM and CM anally are generated by synchronized firing of many descending interneurons, which simultaneously activate inhibitory motor neurons to both muscle layers. This synchronized motor activity ensures that both muscles around the entire circumference are excited orally at the same time as inhibited anally, thus producing net aboral propulsion.

A rhythmic motor pattern activated by circumferential stretch in guinea-pig distal colon

Simultaneous intracellular recordings were made from pairs of circular muscle (CM) cells, at the oral and anal ends of a segment of guinea-pig distal colon, to investigate the neuronal mechanisms underlying faecal pellet propulsion. When a minimum degree of circumferential stretch was applied to sheet preparations of colon, recordings from CM cells revealed either no ongoing junction potentials, or alternatively, small potentials usually < 5 mV in amplitude. Maintained circumferential stretch applied to these preparations evoked an ongoing discharge of excitatory junction potentials (EJPs) at the oral recording site (range: 1-25 mV), which lasted for up to 6 h. The onset of each large oral EJP was time-locked with the onset of an inhibitory junction potential (IJP) at an anal recording electrode, located 2 cm from the oral recording. Similar results were obtained in isolated intact tube preparations of colon, when recordings were made immediately oral and anal of an artificial faecal pellet. The amplitudes of many large (> 5 mV) oral EJPs were linearly related to the amplitudes of anal IJPs occurring 20 mm apart. In the absence of an L-type Ca(2+) channel blocker, action potentials occurred on each large oral EJP. Synchronized discharges of stretch-activated EJPs and IJPs were preserved following pretreatment with capsaicin (10 microM), were unaffected by nifedipine (1 microM) and did not require the mucosa or submucous plexus. EJPs and IJPs were abolished by hexamethonium (300 microM) or tetrodotoxin (1 microM), but persisted in the presence of pyridoxal phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS; 10 microM) or an NK(3) tachykinin receptor antagonist (Neurokinin A 4-10; 100 nM to 5 microM). In summary, maintained circumferential stretch of the distal colon activates a population of intrinsic mechanosensory neurons that generate repetitive firing of ascending excitatory and descending inhibitory pathways to CM. These mechanosensory neurons, which may be interneurons, are stretch sensitive, rather than muscle tension sensitive, since they are resistant to muscular paralysis. We suggest the synchrony in onset of oral EJPs and anal IJPs over large regions of colon is due to synchronous synaptic activation of ascending and descending interneurons.

4-aminopyridine- and dendrotoxin-sensitive potassium channels influence excitability of vagal mechano-sensitive endings in guinea-pig oesophagus

1. Distension-sensitive vagal afferent fibres from the guinea-pig oesophagus were recorded extracellularly in vitro. Most recorded units were spontaneously active firing at 3.2+/-0.3 Hz (n=41, N=41) and had low thresholds (less than 1 mm) to circumferential stretch. Dynamic and adapted phases of stretch-evoked firing, as well as a silent period were linearly dependent on the amplitude of stretch. 2. High K+ (7-12 mM) Krebs solution dose-dependently increased both spontaneous and stretch-evoked firing and reduced the duration of the silent period. 3. Charybdotoxin (ChTX, 100 nM) slightly increased spontaneous and stretch-evoked firing and decreased the silent period, while neither iberiotoxin (100 nM) nor apamin (0.5 microM) had significant effects. omega-Conotoxin GVIA (0.5 microM) did not significantly affect firing of vagal mechanoreceptors. 4. In the majority of single units, 4-aminopyridine (4-AP) concentration-dependently (EC(50) approximately 28 microM) increased spontaneous firing, strongly reduced the silent period but did not affect stretch (3 mm)-induced firing. Firing evoked by 1-2 mm was increased by 4-AP. 5. Alpha-dendrotoxin (DnTX, 300 nM) and DnTX K (30 nM) slightly increased spontaneous and stretch-evoked firing. There was no additive effect on spontaneous firing when ChTX and DnTX K were applied simultaneously. 6. Barium (100 microM) increased stretch-induced firing, probably due to an increase in intramural tension. Glibenclamide (10 microM) had no effect on spontaneous or stretch-induced firing. 7. The results indicate that voltage-gated 4-AP- and dendrotoxin-sensitive K+ channels are the main type of K+ channels that influence excitability of vagal mechano-sensitive endings of the guinea-pig oesophagus. They were involved in control of spontaneous firing and in stretch-induced firing evoked by moderate stretch, but none of the K+ channels appeared to be involved in adaptation to maintained stretch by their slowly adapting vagal mechanoreceptors.

Similarities and differences in the propagation of slow waves and peristaltic waves

The relationship between slow waves and peristaltic reflexes has not been well analyzed. In this study, we have recorded the electrical activity of slow waves together with that generated by spontaneous peristaltic contractions at 240 extracellular sites simultaneously. Recordings were made from five isolated tubular and six sheet segments of feline duodenum superfused in vitro. In all preparations, slow waves propagated as broad wave fronts along the longitudinal axis of the preparation in either the aborad or the orad direction. Electrical potentials recorded during peristalsis (peristaltic waves) also propagated as broad wave fronts in either directions. Peristaltic waves often spontaneously stopped conducting (46%), in contrast to slow waves that never did. Peristaltic waves propagated at a lower velocity than the slow waves (0.98 +/- 0.25 and 1.29 +/- 0.28 cm/s, respectively; P < 0.001; n = 24) and in a direction independent of the preceding slow wave direction (64% in the same direction, 46% in the opposite direction). In conclusion, slow waves and peristaltic waves in the isolated feline duodenum seem to constitute two separate electrical events that may drive two different mechanisms of contraction in the small intestine.

ATP and 5-HT are the principal neurotransmitters in the descending excitatory reflex pathway of the guinea-pig ileum

Neurotransmission underlying descending excitatory reflexes evoked by distension was studied in opened segments of guinea-pig ileum and compared with peristalsis in intact segments. The opened segments were distended by inflating a balloon against the serosa at the oral end and changes in muscle length recorded from the anal end. Distension elicited contractions in both circular (CM) and longitudinal (LM) muscle layers. Granisetron, a 5-HT(3) receptor antagonist (10 nmol L-1 to 1 micromol L-1) reduced CM contractions (24% control), without affecting the LM. The P2 receptor antagonist, pyridoxal phosphate-6-azopheyl-2',4'-disulphonic acid (PPADS; 10 micromol L-1), reduced CM contractions to 31% and LM contractions to 39%. Hexamethonium (500 micromol L-1) enhanced LM contractions, but had no effect on CM contractions. Granisetron (1 micromol L-1) had no significant effect on the threshold for peristaltic contractions in a modified Trendelenburg preparation, but decreased the decay time of these contractions by 37%. PPADS (10 micromol L-1) had no significant effect in this preparation. Thus, the descending excitatory pathways to CM and LM can be distinguished pharmacologically; the former depend on 5-HT(3) and P2 ATP receptors, the latter are independent of 5-HT(3) receptors. Nicotinic receptors may have little part in either pathway. These properties differ from conventional peristaltic reflexes, which are effectively abolished by nicotinic blockade.

Somatostatin sst(2) receptors inhibit peristalsis in the rat and mouse jejunum

Somatostatin [somatotropin release-inhibitory factor (SRIF)] has widespread actions throughout the gastrointestinal tract, but the receptor mechanisms involved are not fully characterized. We have examined the effect of selective SRIF-receptor ligands on intestinal peristalsis by studying migrating motor complexes (MMCs) in isolated segments of jejunum from rats, mice, and sst(2)-receptor knockout mice. MMCs were recorded in 4- to 5-cm segments of jejunum mounted horizontally in vitro. MMCs occurred in rat and mouse jejunum with intervals of 104.4 +/- 10 and 131.2 +/- 8 s, respectively. SRIF, octreotide, and BIM-23027 increased the interval between MMCs, an effect fully or partially antagonized by the sst(2)-receptor antagonist Cyanamid154806. A non-sst(2) receptor-mediated component was evident in mouse as confirmed by the observation of an inhibitory action of SRIF in sst(2) knockout tissue. Blocking nitric oxide generation abolished the response to SRIF in rat but not mouse jejunum. sst(2) Receptors mediate inhibition of peristalsis in both rat and mouse jejunum, but a non-sst(2) component also exists in the mouse. Nitrergic mechanisms are differentially involved in rat and mouse jejunum.

Smooth muscle myosin heavy chain isoform distribution in the swine stomach

To evaluate the distribution of smooth muscle myosin heavy chain isoforms (SMB, with head insert), we examined frozen sections from the various regions of swine stomachs using isoform-specific antibodies. We previously reported variable SMB myosin heavy chain (MHC) expression in stomach cells that correlates with unloaded shortening velocities. This is consistent with the generalization of tonic fundic muscle having low expression and phasic antral muscle having high expression of the SMB MHC isoform. Using immunohistochemistry (IHC), we show a progression of the SMB MHC from very low immunoreactivity in the fundus to very intense immunoreactivity in the antrum. In the body, the average level of SMB MHC immunoreactivity lies between that of the antrum and fundus. Intercellular heterogeneity was observed in all stomach regions to a similar extent. However, the intercellular range in SMB MHC immunoreactivity decreases from fundus to antrum. All stomach regions show isolated pockets or clusters of cells with similar SMB MHC immunoreactivity. There is a non-uniform intracellular immunoreactivity in SMB MHC, with many cells showing greater-intensity staining of SMB MHC in their cell peripheries. This information may prove useful in helping to elucidate possible unique physiological roles of SMB MHC.