Orally projecting interneurones in the guinea-pig small intestine

Electrophysiological and morphological features of myenteric neurons of human colon revealed by intracellular recording and dye fills

BACKGROUND

Ex vivo intracellular recordings and dye fills, combined with immunohistochemistry, are a powerful way to analyze the enteric nervous system of laboratory animals.

METHODS

Myenteric neurons were recorded in isolated specimens of human colon. A key determinant of successful recording was near-complete removal of circular muscle from the surface of ganglia.

KEY RESULTS

Treatment with a collagenase/neutral protease mix before dissection significantly improved recording success and reduced damage to the plexus. Carboxyfluorescein in microelectrodes allowed recorded neurons to be routinely labeled, analyzed, and subjected to multi-layer immunohistochemistry. Carboxyfluorescein revealed morphological details that were not detected by immunohistochemical methods. Of 54 dye-filled myenteric neurons (n = 22), 45 were uni-axonal and eight were multi-axonal. There was a significant bias toward recordings from large neural somata. The close association between morphology and electrophysiology (long after-hyperpolarizations and fast EPSPs) seen in mice and guinea pigs did not hold for human myenteric neuron recordings. No slow EPSPs were recorded; however, disruption to the myenteric plexus during dissection may have led the proportion of cells receiving synaptic potentials to be underestimated. Neurons immunoreactive for nitric oxide synthase were more excitable than non-immunoreactive neurons. Distinctive grooves were observed on the serosal and/or mucosal faces of myenteric neurons in 3D reconstructions. These had varicose axons running through them and may represent a preferential site of synaptic inputs.

CONCLUSIONS

Human enteric neurons share many features with laboratory animals, but the combinations of features in individual cells appear more variable.

Identifying Types of Neurons in the Human Colonic Enteric Nervous System

Distinguishing and characterising the different classes of neurons that make up a neural circuit has been a long-term goal for many neuroscientists. The enteric nervous system is a large but moderately simple part of the nervous system. Enteric neurons in laboratory animals have been extensively characterised morphologically, electrophysiologically, by projections and immunohistochemically. However, studies of human enteric nervous system are less advanced despite the potential availability of tissue from elective surgery (with appropriate ethics permits). Recent studies using single cell sequencing have confirmed and extended the classification of enteric neurons in mice and human, but it is not clear whether an encompassing classification has been achieved. We present preliminary data on a means to distinguish classes of myenteric neurons in specimens of human colon combining immunohistochemical, morphological, projection and size data on single cells. A method to apply multiple layers of antisera to specimens was developed, allowing up to 12 markers to be characterised in individual neurons. Applied to multi-axonal Dogiel type II neurons, this approach demonstrated that they constitute fewer than 5% of myenteric neurons, are nearly all immunoreactive for choline acetyltransferase and tachykinins. Many express the calcium-binding proteins calbindin and calretinin and they are larger than average myenteric cells. This methodology provides a complementary approach to single-cell mRNA profiling to provide a comprehensive account of the types of myenteric neurons in the human colon.

Characterization of putative interneurons in the myenteric plexus of human colon

BACKGROUND

The enteric nervous system contains multiple classes of neurons, distinguishable by morphology, immunohistochemical markers, and projections; however, specific combinations differ between species. Here, types of enteric neurons in human colon were characterized immunohistochemically, using retrograde tracing combined with multiple labeling immunohistochemistry, focussing on non-motor neurons.

METHODS

The fluorescent carbocyanine tracer, DiI, was applied to the myenteric plexus in ex vivo preparations, filling neurons projecting within the plexus. Limits of projection lengths of motor neurons were established, allowing them to be excluded from the analysis. Long ascending and descending interneurons were then distinguished by labeling for discriminating immunohistochemical markers: calbindin, calretinin, enkephalin, 5-hydroxytryptamine, nitric oxide synthase, and substance P. These results were combined with a previous published study in which nitric oxide synthase and choline acetyltransferase immunoreactivities were established.

KEY RESULTS

Long ascending neurons (with projections longer than 8 mm, which excludes more than 95% motor neurons) formed four types, in descending order of abundance, defined by immunoreactivity for: (a) ChAT+/ENK+, (b) ChAT+/ENK+/SP+, (c) ChAT+/Calb+, and (d) ChAT+/ENK+/Calb+. Long descending neurons, up to 70 mm long also formed at least four types, distinguished by immunoreactivity for (a) NOS + cells (without ChAT), (b) ChAT+/NOS+, (c) ChAT+/Calret+, and (d) ChAT+/5HT + cells (with or without NOS).

CONCLUSIONS AND INFERENCES

Long interneurons, which do not innervate muscularis externa, are likely to coordinate neural activity over distances of many centimeters along the colon. Characterizing their neurochemical coding provides a basis for understanding their roles, investigating their connectivity, and building a comprehensive account of human colonic enteric neurons.

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

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

Distribution, projections, and association with calbindin baskets of motor neurons, interneurons, and sensory neurons in guinea-pig distal colon

Normal gut function relies on the activity of the enteric nervous system (ENS) found within the wall of the gastrointestinal tract. The structural and functional organization of the ENS has been extensively studied in the guinea pig small intestine, but less is known about colonic circuitry. Given that there are significant differences between these regions in function, observed motor patterns and pathology, it would be valuable to have a better understanding of the colonic ENS. Furthermore, disorders of colonic motor function, such as irritable bowel syndrome, are much more common. We have recently reported specialized basket-like structures, immunoreactive for calbindin, that likely underlie synaptic inputs to specific types of calretinin-immunoreactive neurons in the guinea-pig colon. Based on detailed immunohistochemical analysis, we postulated the recipient neurons may be excitatory motor neurons and ascending interneurons. In the present study, we combined retrograde tracing and immunohistochemistry to examine the projections of circular muscle motor neurons, myenteric interneurons, and putative sensory neurons. We focused on neurons with immunoreactivity for calbindin, calretinin and nitric oxide synthase and their relationship with calbindin baskets. Retrograde tracing using indocarbocyanine dye (DiI) revealed that many of the nerve cell bodies surrounded by calbindin baskets belong to motor neurons and ascending interneurons. Unique functional classes of myenteric neurons were identified based on morphology, neuronal markers and polarity of projection. We provide evidence for three groups of ascending motor neurons based on immunoreactivity and association with calbindin baskets, a finding that may have significant functional implications.

Electrical stimulation of gut motility guided by an in silico model

OBJECTIVE

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

APPROACH

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

MAIN RESULTS

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

SIGNIFICANCE

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

Mathematical modelling of enteric neural motor patterns

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

Ascending excitatory neural pathways modulate slow phasic myogenic contractions in the isolated human colon

BACKGROUND

In animal models, enteric reflex pathways have potent effects on motor activity; their roles have been much less extensively studied in human gut. The aim of this study was to determine if ascending excitatory interneuronal pathways can modulate spontaneous phasic contractions in isolated preparations of human colonic circular muscle.

METHODS

Human colonic preparations were cut into T shapes, with vertical bar of the 'T' pharmacologically isolated. Electrical stimulation and the nicotinic agonist, 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP), were applied to the isolated region and circular muscle contractile activity was measured from the cross-bar of the T, more than 10 mm orally from the region of stimulation.

KEY RESULTS

The predominant form of spontaneous muscle activity consisted of tetrodotoxin-resistant, large amplitude, slow phasic contractions (SPCs), occurring at average intervals of 124 ± 68 s. Addition of a high concentration of hexamethonium (1 mmol L(-1)) to the superfusing solution significantly increased the interval between SPCs to 278.1 ± 138.3 s (P < 0.005). Focal electrical stimulation more than 10 mm aboral to the muscle recording site advanced the onset of the next SPC, and this effect persisted in hexamethonium. However, the effect of electrical stimulation was blocked by tetrodotoxin (TTX, 1 μmol L(-1)). Application of the nicotinic agonist DMPP (1 mmol L(-1)) to the aboral chamber often stimulated a premature SPC (n = 4).

CONCLUSIONS & INFERENCES

The major form of spontaneous contractility in preparations of human colonic circular muscle is SPCs, which are myogenic in origin. Activation of ascending excitatory neural pathways, which involve nicotinic receptors, can modulate the timing of SPCs and thus influence human colonic motility.

Viscerofugal neurons recorded from guinea-pig colonic nerves after organ culture

BACKGROUND

Enteric viscerofugal neurons provide cholinergic synaptic inputs to prevertebral sympathetic neurons, forming reflex circuits that control motility and secretion. Extracellular recordings of identified viscerofugal neurons have not been reported.

METHODS

Preparations of guinea pig distal colon were maintained in organotypic culture for 4-6 days (n = 12), before biotinamide tracing, immunohistochemistry, or extracellular electrophysiological recordings from colonic nerves.

KEY RESULTS

After 4-6 days in organ culture, calcitonin gene-related peptide and tyrosine hydroxylase immunoreactivity in enteric ganglia was depleted, and capsaicin-induced firing (0.4 μmol L(-1) ) was not detected, indicating that extrinsic sympathetic and sensory axons degenerate in organ culture. Neuroanatomical tracing of colonic nerves revealed that viscerofugal neurons persist and increase as a proportion of surviving axons. Extracellular recordings of colonic nerves revealed ongoing action potentials. Interestingly, synchronous bursts of action potentials were seen in 10 of 12 preparations; bursts were abolished by hexamethonium, which also reduced firing rate (400 μmol L(-1) , P < 0.01, n = 7). DMPP (1,1-dimethyl-4-phenylpiperazinium; 10(-4) mol L(-1) ) evoked prolonged action potential discharge. Increased firing preceded both spontaneous and stretch-evoked contractions (χ(2) = 11.8, df = 1, P < 0.001). Firing was also modestly increased during distensions that did not evoke reflex contractions. All single units (11/11) responded to von Frey hairs (100-300 mg) in hexamethonium or Ca(2+) -free solution.

CONCLUSIONS & INFERENCES

Action potentials recorded from colonic nerves in organ cultured preparations originated from viscerofugal neurons. They receive nicotinic input, which coordinates ongoing burst firing. Large bursts preceded spontaneous and reflex-evoked contractions, suggesting their synaptic inputs may arise from enteric circuitry that also drives motility. Viscerofugal neurons were directly mechanosensitive to focal compression by von Frey hairs.

Activity in varicosities within the myenteric plexus between and during the colonic migrating motor complex in the isolated murine large intestine

BACKGROUND

  Neuronal communication within the myenteric plexus occurs when action potentials along nerve fibers produce Ca(2+) transients in varicosities leading to exocytosis of vesicles and neurotransmitters release. We used Ca(2+) transients in varicosities to monitor action potential activity in myenteric nerve pathways both between and during the colonic migrating motor complex (CMMC) in the isolated murine colon.

METHODS

  Strips of longitudinal muscle were removed to reveal the myenteric ganglia, which were then loaded with Fluo-4.

KEY RESULTS

  Many varicosities, including synaptotagmin 1 labeled varicosities, exhibited ongoing Ca(2+) transients (duration of unitary Ca(2+) transient 3.9 s). Between CMMCs, varicosities fired at a frequency of 0.6 Hz, which correlated with spontaneous inhibitory junction potentials in the circular muscle, suggesting they were mainly in inhibitory nerve pathways. During a CMMC other previously quiescent varicosities fired at 1.3 Hz (max. 2.0 Hz) for the duration (24 s) of the CMMC, suggesting they were on excitatory nerve pathways. Activity in varicosities was correlated with Ca(2+) transient responses in a number of neurons. Some varicosities appeared to release an inhibitory neurotransmitter that reduced activity in nNOS-positive neurons. Varicosities along the same nerve fiber exhibited identical patterns of activity that allowed nerve fibers to be traced throughout the myenteric plexus and internodal strands. Activity in varicosities was reduced by hexamethonium (100 μmol L(-1) ), and blocked by ω-conotoxin GVIA (200 nM) and tetrodotoxin (1 μmol L(-1) ; TTX).

CONCLUSIONS & INFERENCES

  Ca(2+) imaging of varicosities allows for a determination of activity in neural pathways within the enteric nervous system.

Partial, selective survival of nitrergic neurons in chagasic megacolon

One frequent chronic syndrome of Chagas’ disease is megacolon, an irreversible dilation of a colonic segment. Extensive enteric neuron loss in the affected segment is regarded as key factor for deficient motility. Here, we assessed the quantitative balance between cholinergic and nitrergic neurons representing the main limbs of excitatory and inhibitory colonic motor innervation, respectively. From surgically removed megacolonic segments of four patients, each three myenteric wholemounts (from non-dilated oral, megacolonic and non-dilated anal parts) was immunohistochemically triple-stained for choline acetyltransferase, neuronal nitric oxide synthase (NOS) and the panneuronal human neuronal protein Hu C/D. Degenerative changes were most pronounced in the megacolonic and anal regions, e.g. bulked, honeycomb-like ganglia with few neurons which were partly enlarged or atrophic or vacuolated. Neuron counts from each 15 ganglia of 12 megacolonic wholemounts were compared with those of 12 age- and region-matched controls. Extensive neuron loss, mainly in megacolonic and anal wholemounts, was obvious. In all three regions derived from megacolonic samples, the proportion of NOS-positive neurons (control: 55%) was significantly increased: in non-dilated oral parts to 61% (p = 0.003), in megacolonic regions to 72% (p < 0.001) and in non-dilated anal regions to 78% (p < 0.001). We suggest the chronic dilation of megacolonic specimens to be due to the preponderance of the nitrergic, inhibitory input to the intestinal muscle. However, the observed neuronal imbalance was not restricted to the dilated regions: the non-dilated anal parts may be innervated by ascending, cholinergic axons emerging from less affected, more anally located regions.

Non-peristaltic patterns of motor activity in the guinea-pig proximal colon

BACKGROUND

The guinea-pig proximal colon contains semi-solid feces which are propelled by intermittent neural peristaltic waves to the distal colon, where solid pellets are formed. Between propulsive periods, complex motor patterns underlie fluid re-absorption and mixing of contents.

METHODS

Spatio-temporal analysis of video recordings were used to investigate neural and myogenic patterns of non-peristaltic motor activity.

KEY RESULTS

At low distension (6 cmH(2)O), two major motor patterns were seen. Narrow rings of constriction (abrupt contractions) occurred at 19 cpm. These previously undescribed contractions occurred, almost simultaneously, at many points along the preparation, with a calculated propagation velocity of 110 mm s(-1). They were abolished by hexamethonium and by tetrodotoxin, indicating they were neurally mediated. Inhibition of nitric oxide synthase resulted in increased frequency of 'abrupt contractions' suggesting ongoing inhibitory modulation by endogenous nitric oxide. After tetrodotoxin, another distinct motor pattern was revealed; 'ripples'(1) consisted of shallow rings of contraction, occurring at 18 cpm and propagating at 2.7-2.9 mm s(-1) orally or aborally from multiple initiation sites. The frequency of 'ripples' increased as intraluminal pressure was raised, becoming very irregular at high distensions. L-type calcium channel blockers and openers affected the amplitude of 'ripples'. No frequency gradient of 'ripples' along the proximal colon was detected. This absence explains the multiple initiation sites which often shifted over time, and the oral and aboral propagation of 'ripples'.

CONCLUSIONS & INFERENCES

The interaction of myogenic 'ripples' with neurogenic 'abrupt contractions' generates localized alternating rings of contractions and dilatation, well suited to effective mixing of contents.

Innervation of tracheal parasympathetic ganglia by esophageal cholinergic neurons: evidence from anatomic and functional studies in guinea pigs

In the present study, we describe a subset of nerve fibers, characterized by their immunoreactivity for the calcium-binding protein calretinin, that are densely and selectively associated with cholinergic postganglionic neurons in the guinea pig tracheal ganglia. Retrograde neuronal tracing with cholera toxin B, combined with immunohistochemical analyses, showed that these nerve fibers do not originate from sensory neurons in the nodose, jugular, or dorsal root ganglia or from motor neurons in the nucleus ambiguus, dorsal motor nucleus of the vagus nerve, spinal cord, stellate ganglia, or superior cervical ganglia. Calretinin-immunoreactive nerve fibers disappeared from tracheal segments after 48 h in organotypic culture, indicating that the fibers were of extrinsic origin. However, calretinin-positive nerve fibers persisted in tracheal ganglia when tracheae were cocultured with the adjacent esophagus intact. Immunohistochemical analysis of the esophagus revealed a population of cholinergic neurons in the esophageal myenteric plexus that coexpressed calretinin. In functional studies, electrical stimulation of the esophagus in vitro evoked measurable contractions of the trachea. These contractions were not altered by prior organotypic culture of the trachea and esophagus to remove the extrinsic innervation to the airways but were significantly (P < 0.05) inhibited by the ganglionic blocker hexamethonium or by physical disruption of the tissue connecting the trachea and esophagus. These data suggest that a subset of esophageal neurons, characterized by the expression of calretinin and acetylcholine, provide a previously unrecognized excitatory input to tracheal cholinergic ganglia in guinea pigs.

Neuroanatomy and physiology of colorectal function and defaecation: from basic science to human clinical studies

Colorectal physiology is complex and involves programmed, coordinated interaction between muscular and neuronal elements. Whilst a detailed understanding remains elusive, novel information has emerged from recent basic science and human clinical studies concerning normal sensorimotor mechanisms and the organization and function of the key elements involved in the control of motility. This chapter summarizes these observations to provide a contemporary review of the neuroanatomy and physiology of colorectal function and defaecation.

Enteric nervous system development: Recent progress and future challenges

The enteric nervous system is the largest subdivision of the peripheral nervous system that plays a critical role in digestive functions. Despite considerable progress over the last 15 years in understanding the molecular and cellular mechanisms that control the development of the enteric nervous system, several questions remain unanswered. The present review will focus on recent progress on understanding the development of the mammalian enteric nervous system and highlight interesting directions of future research.

Plasticity and ambiguity of the electrophysiological phenotypes of enteric neurons

Advances in knowledge of enteric neurons electrophysiological characteristics have led to the realisation that the properties of the neurons are dependent on the state of the intestine, the region, the method of recording and the species. Thus, under different experimental conditions, electrophysiological studies cannot provide a reliable signature that identifies the functional type of neuron. In the normal guinea-pig small intestine, taken as a model tissue, neurons can be separated into two electrophysiological groups, S and AH neurons. Combined morphological and physiological studies place several classes of motor and interneurons in the S group, and intrinsic primary afferent neurons in the AH group. There is some evidence for subgroups of S neurons, in which electrophysiological differences are correlated with functional subtypes, but these subgroups have been incompletely investigated. Morphologically characterized Dogiel type II (DII) neurons are recognisable in many species, from mouse to human, but their electrophysiological characteristics are only partly conserved across species or cannot be satisfactorily defined due to technical difficulties. There is a strong need for a comprehensive analysis of channels and currents of S/Dogiel type I neuron subtypes, similar to the comprehensive analysis of AH/DII neurons in the guinea-pig, and similar studies need to be conducted in human and other species. The purpose of this review is to highlight that criteria used for electrophysiological definition of enteric neurons might not be sufficient to distinguish between functional classes of neurons, due to intrinsic properties of neuronal subpopulations, plasticity in pathological conditions and differences in recording techniques.

Targets of myenteric interneurons in the guinea-pig small intestine

Polarized outputs of myenteric interneurons in guinea-pig small intestine have been well studied. However, the variety of motility patterns exhibited suggests that some interneuron targets remain unknown. We used antisera selected to distinguish interneuron varicosities and known myenteric neuron types to investigate outputs of three interneuron classes in guinea-pig jejunum; two classes of descending interneurons immunoreactive (IR) for somatostatin (SOM) or nitric oxide synthase (NOS)/vasoactive intestinal peptide (VIP), and one class of ascending interneurons [calretinin/enkephalin (ENK)-IR]. Varicosities apposed to immunohistochemically identified cell bodies were quantified by confocal microscopy. Intrinsic sensory neurons (calbindin-IR) were apposed by few varicosities. Cholinergic secretomotor neurons (neuropeptide Y-IR) were apposed by many SOM-IR varicosities. Longitudinal muscle excitatory motor neurons (calretinin-IR) were apposed by some VIP- and ENK-IR varicosities, but few SOM-IR varicosities. Ascending interneurons (calretinin-IR) were apposed by many varicosities of all types. NOS-IR interneurons and inhibitory motor neurons were apposed by numerous VIP-IR and SOM-IR varicosities. NOS-IR short inhibitory motor neurons were apposed by significantly fewer ENK-IR varicosities than other NOS-IR neurons. Based on the specific chemical coding of ascending (ENK) and descending (SOM) interneurons, we conclude that cholinergic secretomotor neurons and short inhibitory neurons are located in descending reflex pathways, while ascending interneurons and NOS-IR descending interneurons are focal points at which ascending and descending pathways converge.

A new electrophysiological tool to investigate the spatial neuronal projections within the myenteric ascending reflex of the mouse colon

1. The intestinal peristaltic reflex is regulated by local microcircuits that, upon activation, result in an oral contraction and anal relaxation of the circular muscle. This contractile response is associated with typical electrophysiological changes in membrane potential resulting from excitatory and inhibitory myenteric pathways. 2. The aim of the present study was to investigate the influence of local electrical stimulation (ES; single pulses, 15 V, 0.3 msec duration) on the ascending gastrointestinal electrophysiological potentials of the mouse colon using a novel 12-channel stimulation electrode in a newly designed model of the ascending myenteric pathways with simultaneous intracellular recording. 3. Local myenteric reflex responses in the proximal colon were initiated by ES (12 bipolar stimulation electrodes (SE) 0.7 mm apart from each other) and excitatory and inhibitory junction potentials (EJP and IJP, respectively) were recorded from circular smooth muscle cells with intracellular recording techniques. In vivo colonic propulsion was determined by measuring the time to expulsion of a 3 mm glass bead inserted 2.5 cm into the distal colon of mouse. 4. Under basal conditions, circular smooth muscle cells displayed a stable membrane potential (-56.7 +/- 6.9 mV; n = 13). Electrical stimulation elicited a tetrodotoxin (3 micromol/L)-sensitive, neuronal-induced EJP (cholinergic; atropine (1 micromol/L) sensitive) and a biphasic IJP. Both the EJP and IJP showed characteristic responses dependent on the distance between stimulation and recording sites. The EJP could be recorded over long distances, resulting in a maximal EJP amplitude at a distance of 10 mm distance (represented by stimulation electrodes (SE) number 6/7) and a maximal projection distance of 18-20 mm. Both components of IJP were maximal during direct stimulation (at SE1; stimulation at the recording site) and gradually decreased to SE6/7 (10 mm). At distances greater than 10 mm apart, ES did not produce IJP. The ganglionic blocker hexamethonium (10-100 micromol/L) concentration dependently abolished all inhibitory junction potentials at distances greater than 10 mm and significantly reduced the amplitude of EJP for the first 10 mm. Colonic propulsion was decreased by hexamethonium (40 mg/kg) and atropine (0.7 mg/kg). 5. Neuronal circuits of the ascending myenteric reflex functionally project distances ranging up to 18-20 mm. Our newly designed setup allows simultaneous electrophysiological investigations of neuronal microcircuitry within the myenteric plexus over short and long distances and enables conclusions to be drawn regarding neuroneuronal and neuromuscular transmission.

ORL-1 receptor mediates the action of nociceptin on ascending myenteric reflex pathways in rats

BACKGROUND AND AIMS

Nociceptin is the endogenous agonist of the "orphan" opioid receptor-1 (ORL-1). We investigated whether activation of the ORL-1 receptor influences smooth muscle contractility and enteric neurotransmission within ascending myenteric reflex pathways of rats.

METHODS

Reverse transcriptase polymerase chain reaction was performed to evaluate the presence of ORL-1 receptors. The ascending part of the ascending myenteric reflex in rats was studied in ileal segments using a 3-chambered organ bath. Intracellular recordings were performed to evaluate pharmacologic effects on excitatory and inhibitory junction potentials (EJP; IJP). Single- and double-labeling immunohistochemistry was used to examine the distribution of ORL-1 within the intestinal wall.

RESULTS

ORL-1 expression and immunoreactivity was found in the large majority of myenteric neurons. In addition to the cholinergic myenteric neurons, all nitrergic myenteric neurons expressed the ORL-1 receptor. Nociceptin significantly reduced cholinergic twitch contractions, an effect that was reversed by the ORL-1 receptor antagonist [Nphe(1)]nociceptin(1-13)NH(2). Neither nociceptin nor [Nphe(1)]nociceptin(1-13)NH(2) had a direct influence on smooth muscle contractility. Nociceptin significantly reduced ascending myenteric reflex contractions and prolonged the latency from stimulation to contraction. Both effects were antagonized by [Nphe(1)]nociceptin(1-13)NH(2). Intracellular recordings demonstrated that nociceptin reduces the cholinergically mediated EJP and the nitrergic phase of IJP in a concentration-dependent manner, effects that were reversible in presence of [Nphe(1)]nociceptin(1-13)NH(2).

CONCLUSIONS

We conclude that activation of ORL-1 receptors on myenteric neurons reduce excitatory and inhibitory neurotransmission within the gastrointestinal tract. This is accompanied by a reduction of the small intestinal peristaltic reflex response. These effects might be used pharmacologically.

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.

Mapping 5-HT inputs to enteric neurons of the guinea-pig small intestine

5-HT released by gastrointestinal mucosa and enteric interneurons has powerful effects on gut behavior. However, the targets of 5-HT-containing neurons within enteric circuits are not well characterized. We used antisera against 5-HT and selected markers of known enteric neuron types to investigate the connections made by 5-HT-containing neurons in the guinea-pig jejunum. Confocal microscopy was used to quantify the number of 5-HT-immunoreactive varicosities apposed to immunohistochemically identified cell bodies. Large numbers of varicosities were identified apposing cholinergic secretomotor neurons, immunoreactive for neuropeptide Y, in both myenteric and submucous plexuses. Subgroups of neurons identified by calretinin (ascending interneurons) and nitric oxide synthase (descending interneurons and inhibitory motor neurons) immunoreactivity were also apposed by many varicosities. Longitudinal muscle motor neurons (calretinin immunoreactive) and AH/Dogiel type II (sensory) neurons (calbindin immunoreactive) were apposed by small numbers of varicosities. Combined retrograde tracing and immunohistochemistry were used to identify excitatory circular muscle motor neurons; these were encircled by 5-HT-immunoreactive varicosities, but the appositions could not be quantified. We suggest that 5-HT-containing interneurons are involved in secretomotor pathways and pathways to subgroups of other interneurons, but not longitudinal muscle motor neurons. There also appear to be connections between 5-HT-containing interneurons and excitatory circular muscle motor neurons. Physiological evidence demonstrates a functional connection between 5-HT-containing interneurons and AH/Dogiel type II neurons, but few 5-HT-immunoreactive varicosities were observed apposing calbindin-immunoreactive cell bodies. Taken together these results suggest that neural 5-HT may have significant roles in excitatory pathways regulating both motility and secretion.

Expression of sodium channel Nav1.6 in cholinergic myenteric neurons of guinea pig proximal colon

We wished to establish the functional identity of Na(v)1.6-expressing myenteric neurons of the guinea pig proximal colon by determining the extent of colocalization of Na(v)1.6 and selected neurochemical markers. Na(v)1.6-like immunoreactivity (-li) was primarily localized to the hillock and initial segments of myenteric neurons located near junctions with internodal fiber tracts. Immunoreactivity for Na(v)1.6 was co-localized with choline-acetyltransferase-li, representing 96% of Na(v)1.6-immunoreactive neurons; about 5% of these neurons showed co-localization with calretinin-li, but none with substance-P-li. Cholinergic neurons expressing Na(v)1.6 were amongst the smallest (somal area <300 mum(2)) of all cholinergic myenteric neurons observed. Only three of 234 Na(v)1.6-immunoreactive neurons exhibited nNOS-li, and none co-localized with calbindin-li. These data suggest that Na(v)1.6 is expressed in a small uniform population of cholinergic myenteric neurons that lie within the guinea pig proximal colon and that are likely to function as excitatory motor neurons.

Optical studies of nicotinic acetylcholine receptor subtypes in the guinea-pig enteric nervous system

Nicotinic transmission in the enteric nervous system (ENS) is extensive, but the role of individual nicotinic acetylcholine receptor (nAChR) subtypes in the functional connectivity of its plexuses has been elusive. Using monoclonal antibodies (mAbs) against neuronal alpha3-, alpha4-, alpha3/alpha5-, beta2-, beta4- and alpha7-subunits, combined with radioimmunoassays and immunocytochemistry, we demonstrate that guinea-pig enteric ganglia contain all of these nAChR-subunits with the exception of alpha4, and so, differ from mammalian brain. This information alone, however, is insufficient to establish the functional role of the identified nAChR-subtypes within the enteric networks and, ultimately, their specific contributions to gastrointestinal physiology. We have used voltage-sensitive dyes and a high-speed CCD camera, in conjunction with specific antagonists to various nAChRs, to elucidate some of the distinct contributions of the individual subtypes to the behaviour of enteric networks. In the guinea-pig, the submucous plexus has the extraordinary advantage that it is virtually two-dimensional, permitting optical recording, with single cell resolution, of the electrical activity of all of its neurones. In this plexus, the block of alpha3beta2-, alpha3beta4- and/or alpha7-nAChRs always results in a decrease in the magnitude of the synaptic response. However, the magnitude of the fast excitatory post-synaptic potentials (epsps) evoked by electrical stimulation of a neighbouring ganglion varies from cell to cell, reflecting the differential expression of subunits already observed using mAbs, as well as the strengths of the activated synaptic inputs. At the same time, we observe that submucous neurones have a substantial mecamylamine (Mec)-insensitive (non-nicotinic) component to their fast epsps, which may point to the presence of purinergic or serotonergic fast epsps in this system. In the myenteric plexus, on the other hand, the antagonist-induced changes in the evoked synaptic response vary depending upon the location of the stimulating electrode with respect to the ganglion under study. The range of activity patterns that follows sequential pharmacological elimination of individual subtypes suggests that nAChRs may be capable of regulating the activity of both excitatory and inhibitory pathways, in a manner similar to that described in the central nervous system.

Expression of the sodium channel Nav1.2 in chemically identified myenteric neurons in the guinea pig

Our purpose was to identify Na(v)1.2-expressing myenteric neurons of the small and large intestine of the guinea pig by using antibodies directed against Na(v)1.2 and selected neurochemical markers. Na(v)1.2-like immunoreactivity (-li) co-localized with immunoreactivity for choline acetyltransferase in all regions, representing 45%-67% of Na(v)1.2-positive neurons. Na(v)1.2-li co-localized with immunoreactivity for the neural form of nitric oxide synthase more frequently in the colon (20% of neurons exhibiting Na(v)1.2-li) than in the ileum (8%). Co-localization of Na(v)1.2-li with immunoreactivity for a form of neurofilament (NF145) was infrequently observed in the ileum and colon. Enkephalin-immunoreactive cell bodies co-localized with Na(v)1.2-li in all regions. Few myenteric cell bodies immunoreactive for neuropeptide Y were observed in the ileum, but all co-localized with Na(v)1.2-li. This and our previous data suggest that Na(v)1.2 is widely expressed within the guinea pig enteric nervous system, including the three main classes of myenteric neurons (sensory, motor, and interneurons), and is involved in both excitatory and inhibitory pathways. Notable exceptions include the excitatory motor neurons to the longitudinal smooth muscle, the ascending interneurons of the ileum, and the myenteric neurons immunoreactive for NF145, few of which are immunoreactive for Na(v)1.2.

Morphology of enkephalin-immunoreactive myenteric neurons in the human gut

The aim of this study was the morphological and further chemical characterisation of neurons immunoreactive for leu-enkephalin (leuENK). Ten wholemounts of small and large intestinal segments from nine patients were immunohistochemically triple-stained for leuENK/neurofilament 200 (NF)/substance P (SP). Based on their simultaneous NF-reactivity and 3D reconstruction of single NF-reactive cells, 97.5% of leuENK-positive neurons displayed the appearance of stubby neurons: small somata; short, stubby dendrites and one axon. Of these leuENK-reactive stubby neurons, 91.3% did not display co-reactivity for SP whereas 8.7% were SP-co-reactive. As to their axonal projection pattern, 50.4% of the recorded leuENK stubby neurons had axons running orally whereas in 29.4% they ran anally; the directions of the remaining 20.2% could not be determined. No axons were seen to enter into secondary strands of the myenteric plexus. Somal area measurements revealed clearly smaller somata of leuENK-reactive stubby neurons (between 259+/-47 microm(2) and 487+/-113 microm(2)) than those of putative sensory type II neurons (between 700+/-217 microm(2) and 1,164+/-396 microm(2)). The ratio dendritic field area per somal area of leuENK-reactive stubby neurons was between 2.0 and 2.8 reflecting their short dendrites. Additionally, we estimated the proportion of leuENK-positive neurons in comparison to the putative whole myenteric neuron population in four leuENK/anti-Hu doublestained wholemounts. This proportion ranged between 5.9% and 8.3%. We suggest leuENK-reactive stubby neurons to be muscle motor neurons and/or ascending interneurons. Furthermore, we explain why we do not use the term "Dogiel type I neurons" for this population.

Ileal myenteric plexus in aged guinea-pigs: loss of structure and calretinin-immunoreactive neurones

Myenteric plexus controls gastrointestinal motility by means of well organized circuits which are comprised of sensory neurones, interneurones and motor neurones to the muscular layers. Calretinin (CR) is a calcium-binding protein that, in guinea-pig ileum, has only been found in ascending interneurones, which also express neurofilament triplet proteins (NFT), and excitatory longitudinal muscle motor neurones, which do not. In spite of some evidence that age affects both function and structure of the myenteric plexus, little is known about the possible selectivity of the process regarding specific myenteric neuronal phenotypes. The influence of age on both the structure of the myenteric plexus and the presence of CR-immunoreactive (CR-IR) neurones was studied using conventional immunohistochemical procedures applied to ileal whole-mount preparations from guinea-pigs. Both a reduction in ganglionic size and changes in the distribution of neurones inside and outside the ganglia, together with a general neuronal loss were found in preparations from aged guinea-pigs. More interestingly, a relatively more pronounced age-related loss of CR-IR neurones, especially those lacking of NFT expression, was found. Specific myenteric neuronal phenotypes may show differential sensitivity to ageing, and this could, under certain circumstances, alter the functional balance of gastrointestinal motility in aged individuals.

Presynaptic modulation of cholinergic and non-cholinergic fast synaptic transmission in the myenteric plexus of guinea pig ileum

Abstract These studies investigated receptors modulating release of mediators of fast excitatory postsynaptic potentials (fEPSPs) in guinea pig ileum myenteric plexus using electrophysiological methods. Fast EPSPs inhibited by >95% by hexamethonium (100 micromol L(-1)) were cholinergic; mixed fEPSPs were inhibited <95% by hexamethonium. Non-cholinergic fEPSPs were studied in the presence of hexamethonium. The alpha2-adrenergic receptor agonist UK 14304 inhibited cholinergic (maximum inhibition = 76%, EC(50) = 18 nmol L(-1)), mixed (81%, 21 nmol L(-1)) and non-cholinergic (76%, 44 nmol L(-1)) fEPSPs equally. The 5-HT(1) receptor agonist 5-carboxamidotryptamine inhibited cholinergic, mixed and non-cholinergic fEPSPs equally. Renzapride, increased non-cholinergic (33%) less than mixed (97%, 13 micromol L(-1)) fEPSPs. Renzapride inhibited the purely cholinergic fEPSPs (-29%) but potentiated the cholinergic component of mixed fEPSPs (39%). Prucalopride potentiated all fEPSPs equally (30-33%). 5-HT (0.1 micromol L(-1)) induced potentiation of cholinergic (75%), mixed (97%) and non-cholinergic (84%) fEPSPs was not statistically different. The potentiating effects of renzapride and 5-HT on fEPSPs were inhibited by the 5-HT(4) receptor antagonist, SB 204070 (10 nmol L(-1)). Renzapride (0.3 micromol L(-1)) blocked 5-HT-induced increases in cholinergic fEPSPs. alpha2-Adrenergic and 5-HT(1) receptors mediate inhibition of transmitter release from cholinergic and mixed terminals. 5-HT and prucalopride, acting at 5-HT(4) receptors, facilitate all fEPSPs; renzapride facilitates the cholinergic and non-cholinergic components of mixed fEPSPs but not purely cholinergic fEPSPs. Cholinergic synapses may express few 5-HT(4) receptors or a renzapride-insensitive 5-HT(4) receptor isoform.

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

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

Enteric motor and interneuronal circuits controlling motility

The enteric nervous system regulates intestinal motility. It contains intrinsic sensory neurones, several types of interneurones and excitatory and inhibitory motor neurones. This review summarizes our knowledge of motor neurones and interneurones in simple motility reflex pathways (ascending and descending excitation, descending inhibition) and it focuses on guinea-pig ileum. Excitatory circular muscle motor neurones contain choline acetyltransferase (ChAT) and tachykinins and project orally 0.5-10 mm. They transmit via muscarinic acetylcholine receptors and tachykinins acting at NK1 and NK2 receptors. Inhibitory circular muscle motor neurones contain nitric oxide synthase (NOS), vasoactive intestinal peptide (VIP) and pituitary adenylyl cyclase activating peptide (PACAP), project anally up to 25 mm and transmit via ATP, nitric oxide and/or VIP. Ascending interneurones project up to 10 mm orally and contain ChAT and tachykinins. They transmit to each other via ACh at nicotinic receptors (nAChR), but to excitatory motor neurones via both nAChR and NK3 receptors. There are at least three types of descending interneurones and one transmits to inhibitory motor neurones via ATP acting at P2X receptors. NOS-containing descending interneurones receive input via P2Y receptors from other interneurones. Transmission to and from the other descending interneurones (ChAT/5-HT, ChAT/somatostatin) is yet to be characterized.

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.

Aging of the myenteric plexus: neuronal loss is specific to cholinergic neurons

Neuron loss occurs in the myenteric plexus of the aged rat. The myenteric plexus is composed of two mutually exclusive neuronal subpopulations expressing, respectively, nitrergic and cholinergic phenotypes. The goal of the present study, therefore, was to determine if neuron loss is specific to one phenotype, or occurs in both. Ad libitum fed virgin male Fischer 344 rats of 3 and 24 months of age were used in each of two neuronal staining protocols (n=10/age/neuron stain). The stomach, duodenum, jejunum, ileum, colon, and rectum were prepared as whole mounts and processed with either NADPHd or Cuprolinic Blue to stain, respectively, the nitrergic subpopulation or the entire population of myenteric neurons. Neuron numbers and sizes were determined for each preparation. Neuron counts from 24-month-old rats were corrected for changes in tissue area resulting from growth. There was no age-related loss of NADPHd-positive neurons for any of the regions sampled, whereas significant losses of Cuprolinic Blue-labeled neurons occurred in the small and large intestines of 24-month-old rats. At the two ages, the average neuron sizes were similar in the stomach and small intestine for both stains, but neurons in the large intestine were significantly larger at 24 months. In addition, numerous swollen NADPHd-positive axons were found in the large intestine at 24 months. These findings support the hypothesis that age-related cell loss in the small and large intestines occurs exclusively in the cholinergic subpopulation. It appears, however, from the somatic hypertrophy and the presence of swollen axons that the nitrergic neurons are not completely spared from the effects of age.

Electrical stimulation reveals complex neuronal input and activation patterns in single myenteric guinea pig ganglia

The myenteric plexus plays a key role in the control of gastrointestinal motility. We used confocal calcium imaging to study responses to electrical train stimulation (ETS) of interganglionic fiber tracts in entire myenteric ganglia of the guinea pig small intestine. ETS induced calcium transients in a subset of neurons: 52.2% responded to oral ETS, 65.4% to aboral ETS, and 71.7% to simultaneous oral and aboral ETS. A total of 41.3% of the neurons displayed convergence of oral and aboral ETS-induced responses. Responses could be reversibly blocked with TTX (10(-)6 M), demonstrating involvement of neuronal conduction, and by removal of extracellular calcium. omega-Conotoxin (5 x 10(-7) M) blocked the majority of responses and reduced the amplitude of residual responses by 45%, indicating the involvement of N-type calcium channels. Staining for calbindin and calretinin did not reveal different response patterns in these immunohistochemically identified neurons. We conclude that, at least for ETS close to a ganglion, confocal calcium imaging reveals complex oral and aboral input to individual myenteric neurons rather than a polarization in spread of activity.

Evidence for the expression of transient receptor potential proteins in guinea pig airway smooth muscle cells

OBJECTIVE

The present study investigates the expression of transient receptor potential (TRPC) proteins in airway smooth muscle (ASM) cells in order to determine whether these proteins may be candidate molecular counterparts of plasma membrane Ca2+-permeable channels involved in the contraction of ASM.

METHODS

Expression of TRPC mRNA was detected using specific primers and RT-PCR. Expression of the TRPC1, TRPC3 and TRPC6 proteins was detected using antibodies in immunoprecipitation and Western blot.

RESULTS

Guinea pig ASM cells exhibited thapsigargin- and acetylcholine-initiated Ca2+ inflow but none by 1-oleoyl-2-acetyl-sn-glycerol. mRNA encoding each of the TRPC1 to TRPC6 proteins was detected in ASM cells. mRNA encoding TRPC1, TRPC3, TRPC4 and TRPC6 was detected in ASM cells at a concentration approximately equivalent to that in guinea pig brain. mRNA encoding TRPC2 and TRPC5 was more abundant in ASM cells than in brain. The TRPC1 protein, but not the TRPC3 or TRPC6 proteins, was detected in extracts of ASM cells, while all three proteins were detected in brain.

CONCLUSION

The results provide evidence for a low level of expression of the TRPC1 to TRPC6 proteins in ASM cells. These proteins may function as store-operated Ca2+ and/or second messenger-activated non-selective cation channels in ASM cells.

Correlation of electrophysiological and morphological characteristics of enteric neurons in the mouse colon

We report on the first correlative study of the electrophysiological properties, shapes, and projections of enteric neurons in the mouse. Neurons in the myenteric plexus of the mouse colon were impaled with microelectrodes containing biocytin, their passive and active electrophysiological properties determined, and their responses to activation of synaptic inputs investigated. Biocytin, injected into the neurons from which recordings were made, was converted to an optically dense product and used to determine the shapes of neurons. By electrophysiological properties, almost all neurons belonged to one of two classes, AH neurons or S neurons. AH neurons had a biphasic repolarization of the action potential, and slow afterhyperpolarizing potentials usually followed the action potentials. S neurons had monophasic repolarizations, no slow afterhyperpolarization, and fast excitatory postsynaptic potentials in response to fibre tract stimulation. By shape, neurons were divided into Dogiel type II (28/136 neurons) and uniaxonal neurons. Dogiel type II neurons had large, smooth-surfaced cell bodies and several long processes that supplied branches within myenteric ganglia. All Dogiel type II neurons had AH electrophysiology; conversely, most AH neurons had Dogiel type II morphology. The majority of uniaxonal neurons had lamellar dendrites, i.e., Dogiel type I morphology. They projected to the circular muscle (circular muscle motor neurons), to the longitudinal muscle (longitudinal muscle motor neurons), and to other myenteric ganglia (interneurons) and in some cases could not be traced to target cells. All S neurons were uniaxonal. A small proportion of uniaxonal neurons (3/70) had AH electrophysiology. Fast excitatory synaptic potentials were only recorded from uniaxonal neurons and were in most cases blocked by nicotinic receptor antagonists. A small component of fast excitatory transmission in some neurons was antagonized by the purine receptor antagonist PPADS. Slow excitatory postsynaptic potentials were observed in both AH and S neurons. Slow inhibitory postsynaptic potentials were recorded from S neurons. We conclude that the major classes of neurons are Dogiel type II neurons with AH electrophysiological properties and Dogiel type I neurons with S electrophysiological properties. The S/Dogiel type I neurons include circular muscle motor neurons, longitudinal muscle motor neurons, and interneurons.

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.

Control of non-adrenergic non-cholinergic reflex motor responses in circular muscle of guinea-pig small intestine by Met-enkephalin

1 A triple organ bath method allowing the synchronous recording of the motor activity of the circular muscle layer belonging to the oral and anal segments of guinea-pig small intestine adjacent to an electrically stimulated middle segment was developed to study the ascending and descending reflex motor responses. 2 Electrical field stimulation (0.8 ms, 40 V, 5 Hz, 10 s) applied to the middle part of the segments elicited tetrodotoxin (1 microm)-sensitive ascending and descending contractile responses of the nonstimulated parts, oral and anal, respectively. The ascending contraction was more pronounced as compared with the descending contraction. 3 In the presence of phentolamine (5 microm), propranolol (5 microm) and atropine (3 microm) a significant decrease in the amplitude of the ascending contraction was seen and a descending relaxation, instead of a contraction was observed. 4 Met-enkephalin applied at a single concentration (0.1 microm) or cumulatively (0.001-1 microm) inhibited both non-adrenergic non-cholinergic (NANC) descending relaxation and ascending contraction with similar efficacy but different potency, IC50 being 5.9 +/- 0.3 and 39.0 +/- 4 nm, respectively. Naloxone (0.5 microm) prevented the effects of Met-enkephalin. 5 L-NNA (0.5 mm), an inhibitor of nitric oxide synthesis, increased the ascending contraction and strongly reduced but not abolished the descending relaxation. l-Arginine (0.5 mm) restored the motor responses to the initial level in l-NNA-pretreated preparations, d-Arginine (0.5 nm) had no effects. 6 Met-enkephalin (0.1 microm) depressed the l-NNA-dependent increase of the ascending contraction and failed to change the l-NNA-resistant part of the descending relaxation. 7 Met-enkephalin did not alter spontaneous NANC mechanical activity. SNP (1 or 10 microm), an exogenous donor of nitric oxide, caused a concentration-dependent relaxation. The effects of SNP persisted in Met-enkephalin (0.1 microm)-pretreated preparations. 8 NANC reflex ascending contraction and descending relaxation were synchronously induced by a local nerve stimulation indicating a functional coactivation of NANC orally projected excitatory and anally directed inhibitory pathways. Acting prejunctionally, Met-enkephalin provided a negative controlling mechanism inhibiting both ascending and descending, mainly nitric oxide mediated, reflex responses. A higher sensitivity of the descending relaxation to Met-enkephalin was observed suggesting an essential role of opioid(s) in reducing the efficacy of descending motor activity.

Responses of myenteric S neurones to low frequency stimulation of their synaptic inputs

Previous experiments have shown that prolonged low frequency stimulation of presynaptic inputs causes excitation of AH neurones that considerably outlasts the period of stimulation in the guinea-pig small intestine. The present experiments compare the responses of S neurones (which are motor neurones and interneurones) with responses of AH neurones (intrinsic primary afferent neurones) to low frequency stimulation of synaptic inputs. Neurones in the myenteric plexus of isolated segments of guinea-pig small intestine were recorded from with intracellular microelectrodes. During their impalement, the neurones were filled with a marker dye and they were later processed to reveal their shapes and immunohistochemical properties. One group of neurones, inhibitory motor neurones to the circular muscle, was depolarised by stimulation of synaptic inputs at 1 Hz for 100 s to 4 min. With 4-min trains of stimuli, peak depolarisation was 21+/-2 mV (mean+/-S.E.M.), which was reached at about 110 s. Depolarisation was accompanied by increased excitability; before stimulation, a test intracellular pulse (500 ms) triggered 3 action potentials, at the peak of excitability this reached 16 action potentials. Depolarisation began to decline immediately at the end of stimulation. This contrasts with responses of AH neurones, in which depolarisation persisted after the end of the stimulus (peak depolarisation at 300 s). The excitation and depolarisation of inhibitory motor neurones was blocked by the neurokinin 1 tachykinin receptor antagonist, SR140333 (100 nM), but excitation of AH neurones was not affected. Small or no responses to 1 Hz stimulation were recorded from descending filamentous interneurones, longitudinal muscle motor neurones and excitatory circular muscle motor neurones. In conclusion, this study indicates that sustained slow postsynaptic excitation only occurs in AH neurones, and that one type of S neurones, inhibitory motor neurones to the circular muscle, responds substantially, but not beyond the period of stimulation, to activation of synaptic inputs at 1 Hz. This slow excitatory postsynaptic potential evoked by low frequency stimulation is mediated by tachykinins.

Slow excitatory synaptic potentials evoked by distension in myenteric descending interneurones of guinea-pig ileum

The functional significance of the slow excitatory synaptic potentials (EPSPs) in myenteric neurones is unknown. We investigated this using intracellular recording from myenteric neurones in guinea-pig ileum, in vitro. In all, 121 neurones responded with fast EPSPs to distension of the intestine oral to the recording site. In 28 of these neurones, distension also evoked depolarizations similar to the slow EPSPs evoked by electrical stimulation in the same neurones. Intracellular injection of biocytin and immunohistochemistry revealed that neurones responding to distension with slow EPSPs were descending interneurones, which were immunoreactive for nitric oxide synthase (NOS). Other neurones, including inhibitory motor neurones and interneurones lacking NOS, did not respond to distension with slow EPSPs, but many had slow EPSPs evoked electrically. Slow EPSPs evoked electrically or by distension in NOS-immunoreactive descending interneurones were resistant to blockade of NK(1) or NK(3) tachykinin receptors (SR 140333, 100 nM; SR 142801, 100 nM, respectively) and group I metabotropic glutamate receptors (PHCCC, 10-30 microM), when the antagonists were applied in the recording chamber of a two-chambered organ bath. However, slow EPSPs evoked electrically in inhibitory motor neurones were substantially depressed by SR 140333 (100 nM). Blockade of synaptic transmission in the stimulation chamber of the organ bath abolished slow EPSPs evoked by distension, indicating that they arose from activity in interneurones, and not from anally directed, intrinsic sensory neurones. Thus, distension evokes slow EPSPs in a subset of myenteric neurones, which may be important for intestinal motility.

Role of alpha(2)-adrenoceptors in the sympathetic inhibition of motility reflexes of guinea-pig ileum

1. Sympathetic regulation of the motility of guinea-pig ileum was investigated using mesenteric nerve (MN) stimulation to inhibit motility reflexes, in vitro. 2. Transmural electrical stimulation (5 Hz, 1 s) in intact intestinal segments, or inflation of a balloon against the mucosa in opened segments, evoked contractions of the circular and longitudinal muscles oral to the stimulus. 3. MN stimulation (10 Hz, 5 s) usually abolished contractions of the longitudinal and circular muscles evoked by either electrical or mechanical stimuli. 4. The inhibition was mimicked by UK14,304 (70-100 nM) and abolished by idazoxan (100 nM), revealing an enhancement of circular muscle contractions. There was no evidence for alpha(2)-receptors on the muscle, suggesting sympathetic inhibition was via the myenteric plexus. 5. Possible sites of action of noradrenaline released from sympathetic nerves were investigated using intracellular recordings from the circular muscle in a multichambered organ bath. 6. When in the stimulation chamber, UK14,304 depressed (by 50 %) excitatory junction potentials (EJPs) recorded oral to a distension stimulus, but did not affect inhibitory junction potentials (IJPs) recorded anal to the stimulus. When added to a chamber between the stimulus and recording chambers, UK14,304 depressed EJPs by 40 %, but did not alter IJPs. When in the recording chamber, UK14,304 depressed EJPs by 20 %, but had no effect on IJPs. IJPs were inhibited, however, when UK14,304 was applied to the whole bath. 7. It is concluded that sympathetic activity inhibits intestinal motility mainly via alpha(2)-adrenoceptors on ascending interneurons and intrinsic sensory neurons of the orally directed reflex pathway.

Retrograde tracing of enteric neuronal pathways

Neuroanatomical tracing techniques, and retrograde labelling in particular, are widely used tools for the analysis of neuronal pathways in the central and peripheral nervous system. Over the last 10 years, these techniques have been used extensively to identify enteric neuronal pathways. In combination with multiple-labelling immunohistochemistry, quantitative data about the projections and neurochemical profile of many functional classes of cells have been acquired. These data have revealed a high degree of organization of the neuronal plexuses, even though the different classes of nerve cell bodies appear to be randomly assorted in ganglia. Each class of neurone has a predictable target, length and polarity of axonal projection, a particular combination of neurochemicals in its cell body and distinctive morphological characteristics. The combination of retrograde labelling with targeted intracellular recording has made it possible to target small populations of cells that would rarely be sampled during random impalements. These neuroanatomical techniques have also been applied successfully to human tissue and are gradually unravelling the complexity of the human enteric nervous system.

Classes of enteric nerve cells in the guinea-pig small intestine

The guinea-pig small intestine has been very widely used to study the physiology, pharmacology and morphology of the enteric nervous system. It also provides an ideal, simple mammalian preparation for studying how nerve cells are organised into functional circuits underlying simple behaviours. Many different types of nerve cells are present in the enteric nervous system and they show characteristic combinations of morphological features, projections, biophysical properties, neurochemicals, and receptors. To identify the different functional classes is an important prerequisite for systematic analysis of how the enteric nervous system controls normal gut behaviour. Based on combinations of multiple-labelling immunohistochemistry and retrograde tracing, it has been possible to account quantitatively for all of the neurones in the guinea-pig small intestine. This article summarises that account and updates it in the light of recent data. A total of 18 classes of neurones are currently distinguishable, including primary afferent neurones, motor neurones, interneurones, secretomotor and vasomotor neurones. It is now possible to take an individual nerve cell and use a few carefully chosen criteria to assign it to a functional class. This provides a firm anatomical foundation for the systematic analysis of how the enteric nervous system normally functions and how it goes wrong in various clinically important disorders.

A-type potassium current in myenteric neurons from guinea-pig small intestine

Biophysical properties of A-type K(+) currents (I(A)) in myenteric neurons from guinea-pig small intestine were studied. I(A) was present in both AH- and S-type myenteric neurons. Reduction of external Ca(2+) did not affect the current. Current density was 13.5+/-10.2 pA/pF in 68 AH-type neurons and 23.4+/-8.2 pA/pF in 31 S-type neurons. S-type neurons appeared to be a homogeneous group based on density of I(A). AH-type neurons were subdivided into two groups with current densities of 9.4+/-4.3 and 25.4+/-4.3 pA/pF. All other biophysical properties of the current were not statistically different for AH- and S-type neurons. Steady-state activation and inactivation curves showed half-activation potentials at -7 mV (k=15. 0 mV) and -86 mV (k=11.5 mV). The curves overlapped at potentials near the resting potential of approximately -55 mV. Time constants for activation ranged from 3.6 to 0.52 ms at test potentials between -20 and 50 mV. Inactivation time constants fell between 41.5 and 11 ms at test potentials between -20 and 50 mV. Time constants for recovery from inactivation fit a double-exponential curve with fast and slow recovery times of 11 and 550 ms. 4-Aminopyridine suppressed I(A) when it was activated at -20 mV following a pre-pulse to -110 mV. Addition of Zn(2+) in the external solution resulted in a concentration-dependent shift of the activation and inactivation curves in the depolarized direction. Zn(2+) slowed the activation and inactivation kinetics of I(A) by factors of 3.3- and 1.2-fold over a wide range of potentials. Elevation of external H(+) suppressed the effect of Zn(2+) with a pK of 7.3-7.4. The effects of Zn(2+) were interpreted as not being due to surface charge screening, because the affinity of Zn(2+) for its binding site on the A-channel was estimated to be between 170 and 312 microM, while the background concentration of Mg(2+) was 10 mM. The enteric nervous system is perceived as an independent integrative nervous system (brain-in-the-gut) that is responsible for local organizational control of motility and secretory patterns of gut behavior. AH- and S-type neurons are synaptically interconnected to form the microcircuits of the enteric nervous system. The results suggest that I(A) is a significant determinant of neuronal excitability for both the firing of nerve impulses and the various synaptic events in the two types of neurons.

Myenteric neurons activate submucosal vasodilator neurons in guinea pig ileum

This study examined whether myenteric neurons activate submucosal vasodilator pathways in in vitro combined submucosal-myenteric plexus preparations from guinea pig ileum. Exposed myenteric ganglia were electrically stimulated, and changes in the outside diameter of submucosal arterioles were monitored in adjoining tissue by videomicroscopy. Stimulation up to 18 mm from the recording site evoked large TTX-sensitive vasodilations in both orad and aborad directions. In double-chamber baths, which isolated the stimulating myenteric chamber from the recording submucosal chamber, hexamethonium or the muscarinic antagonist 4-diphenylacetoxy-N-(2-chloroethyl)-piperdine hydrochloride (4-DAMP) almost completely blocked dilations when superfused in the submucosal chamber. When hexamethonium was placed in the myenteric chamber approximately 50% of responses were hexamethonium sensitive in both orad and aboard orientations. The addition of 4-DAMP or substitution of Ca(2+)-free, 12 mM Mg(2+) solution did not cause further inhibition. These results demonstrate that polysynaptic pathways in the myenteric plexus projecting orad and aborad can activate submucosal vasodilator neurons. These pathways could coordinate intestinal blood flow and motility.

A parallel algorithm for simulation of large neural networks

The simulation of biologically realistic neural networks requires the numerical solution of very large systems of differential equations. Variables within the system can be changing at rates that vary by orders of magnitude, not only at different times of the solution, but at the same time in different parts of the network. Therefore, an efficient implementation must be able to vary the solution step size, and do so independently in different subsystems. A single processor algorithm is presented in which each neuron can be solved with its own step size by using a priority queue to integrate them in the correct order. But this leaves the problem of how communication and synchronisation between neurons should be managed when executing in parallel. The proposed solution uses an algorithm based on waveform relaxation, which allows groups of neurons on different processors to be solved independently and hence in parallel, for substantial parts of the computation. Realistic test problems were run on a distributed memory parallel computer and results show that speedups of 10 using 16 processors are achievable, and indicate that further speedups may be possible.

Recording ionic events from cultured, DiI-labelled myenteric neurons in the guinea-pig proximal colon

To date investigations of enteric neurons by patch clamping/calcium imaging have been limited by studying unidentified heterogeneous populations of neurons. In DiI-labelled colonic myenteric neurons, the feasibility of recording ionic events was determined by applying DiI either to the mucosa or the circular muscle, dispersing neurons after 48 h organotypic culture, and patch-clamping/calcium imaging labeled neurons after 3-7 days in culture. Myenteric neurons with diffuse DiI fluorescence were typically smooth and agranular. Neurons labeled after DiI was applied to circular muscle, fired in either a phasic or a tonic manner, and exhibited fast afterhyperpolarizations (100-300 ms duration) at the end of a depolarizing pulse. They expressed a fast inward current and at least three different outward currents. Action potentials elicited in DiI-labeled sensory neurons were followed by a prolonged afterhyperpolarization (AH, 4-6 s). The offset of a suprathreshold depolarizing step elicited a prolonged outward tail current that approximated the timecourse of the prolonged AH. In addition, in response to membrane depolarization in DiI-labeled neurons loaded with fura-2, robust Ca(2+) transients were recorded using the perforated patch technique. These results demonstrate that DiI labeling of cultured myenteric neurons is feasible, and patch clamp/Ca(2+) fluorescence recordings can be made from specific populations of cultured DiI-labeled colonic myenteric neurons.