An electrophysiological study of the projections of motor neurones that mediate non-cholinergic excitation in the circular muscle of the guinea-pig small intestine

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.

Propulsive activity induced by sequential electrical stimulation in the descending colon of the pig

This work was performed to study electrically induced contractions in the descending colon of pigs. Contractions were monitored using impedance planimetry and manometry. The luminal pressure, cross-sectional area (CSA), latency and velocity of CSA decrease were compared when using 3 ms, 9, 12, 15 or 30 mA pulses at 10 Hz for 10 s, and 15 mA, 0.03, 0.3 or 3 ms pulses at 10 Hz for 10 s. Stimulation was performed prior and after the application of N(G)-nitro-L-arginine methyl ester (L-NAME) and atropine. In the untreated colon, contraction was always of an 'off' type. A current increase from 9 to 30 mA increased the pressure. An increase of pulse duration from 0.03 to 3 ms shortened the latency, accelerated contraction and increased pressure. By sequential stimulation, contractions were coordinated to propel semi-fluid and solid luminal contents. L-NAME increased the magnitude of CSA decrease. Atropine induced inhibitory effects on contractions elicited by 3 ms pulses and abolished contractions induced by 0.03 and 0.3 ms pulses.

IN CONCLUSION

(i) electrical stimulation evokes'off' colon contractions, which can be coordinated to result in propulsion; (ii) the best combination for current and pulse duration to induce propulsive contractions is 15 mA and 3 ms; (iii) nitrergic and cholinergic pathways mediate responses to electrical stimulation.

Visceral hypersensitivity

Visceral hypersensitivity is considered one of the causes of functional gastrointestinal disorders. The objectives of this review are to provide a practical description of neuroanatomy and physiology of gut sensation, to describe the diverse tests of visceral sensation and the potential role of brain imaging to further our understanding of visceral sensitivity in health and disease. Changes in motor function in the gut may influence sensory levels, eg, during contractions or as a result of changes in viscus compliance. New insights on sensory end organs, such as intraganglionic laminar endings, and basic neurophysiologic studies showing afferent firing during changes in stretch rather than tension illustrate the importance of different types of stimuli, not just tension, to stimulate afferent sensation. These insights provide the basis for understanding visceral sensation in health and disease, which will be extensively discussed in subsequent articles.

Synchronization of enteric neuronal firing during the murine colonic MMC

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

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.

Spatial and temporal coordination of junction potentials in circular muscle of guinea-pig distal colon

1. In isolated, stretched, flat-sheet preparations of guinea-pig distal colon, simultaneous intracellular recordings were made from pairs of circular muscle (CM) cells to map the region of smooth muscle at which spontaneous junction potentials (sJPs) were coordinated in both space and time. 2. Spontaneous inhibitory junction potentials (sIJPs) and excitatory junction potentials (sEJPs) were recorded from all animals with varying frequencies and amplitudes (up to 25 mV). 3. Large amplitude (> or = 9 mV) sIJPs or sEJPs with near-identical amplitudes and time courses were recorded synchronously from two CM cells, even when the two electrodes were separated by up to 11 mm in the circumferential axis and < or = 4 mm in the longitudinal axis. However, smaller (< 9 mV) sIJPs or sEJPs were less coordinated and exhibited greater variability in their times to peak. 4. The standard deviation (S.D.) for the time difference between the peaks of sJPs was related to the amplitude of the events and the distance between the electrodes. The S.D. for large amplitude JPs (approximately 30 ms), which was less than that for small JPs (approximately 150 ms), remained constant across the circumferential axis (at least up to 6 mm), but declined rapidly for distances > or = 2 mm in the longitudinal axis. 5. Current injection (up to 8 nA) into a single CM cell elicited electrotonic potentials in neighbouring CM cells, only when the two electrodes were separated by less than 100 microm circumferentially. Beyond 50 microm electronic potentials were rarely detected. 6. Tetrodotoxin (TTX; 1 microM) abolished all sJPs, whereas hexamethonium (300 microM) either abolished, or substantially reduced all sJPs. 7. Nitro-L-arginine (L-NA; 100 microM) abolished the slow repolarisation phase of sIJPs without any apparent effect on the amplitude of sIJPs. Apamin abolished the fast, initial component of sIJPs, suggesting synchronous release of two inhibitory neurotransmitters during the sIJP. Atropine (1 microM) abolished sEJPs. 8. No sJPs were recorded from the CM layer when it was separated from the myenteric plexus. 9. In conclusion, sIJPs and sEJPs in colonic CM occur synchronously over large regions of the smooth muscle syncitium. The results are discussed in relation to the idea that spontaneous junction potentials in colonic CM are not monoquantal events, but are generated by the simultaneous release of transmitter from many release sites, due to the synchronous activation of many enteric motor neurons.

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.

Patch-clamp study of neurons and glial cells in isolated myenteric ganglia

Most of the physiological information on the enteric nervous system has been obtained from studies on preparations of the myenteric ganglia attached to the longitudinal muscle layer. This preparation has a number of disadvantages, e.g., the inability to make patch-clamp recordings and the occurrence of muscle movements. To overcome these limitations we used isolated myenteric ganglia from the guinea pig small intestine. In this preparation movement was eliminated because muscle was completely absent, gigaseals were obtained, and whole cell recordings were made from neurons and glial cells. The morphological identity of cells was verified by injecting a fluorescent dye by micropipette. Neurons displayed voltage-gated inactivating inward Na(+) and Ca(2+) currents as well as delayed-rectifier K(+) currents. Immunohistochemical staining confirmed that most neurons have Na(+) channels. Neurons responded to GABA, indicating that membrane receptors were retained. Glial cells displayed hyperpolarization-induced K(+) inward currents and depolarization-induced K(+) outward currents. Glia showed large "passive" currents that were suppressed by octanol, consistent with coupling by gap junctions among these cells. These results demonstrate the advantages of isolated ganglia for studying myenteric neurons and glial cells.

Purinergic and cholinergic neuro-neuronal transmission underlying reflexes activated by mucosal stimulation in the isolated guinea-pig ileum

1. We present evidence that adenosine triphosphate (ATP) plays a major role in excitatory neuro-neuronal transmission in ascending and descending reflex pathways to the longitudinal (LM) and circular muscle (CM). 2. A partitioned bath was used for the pharmacological isolation of a segment of guinea-pig ileum ( approximately 6 cm in length), allowing drugs to be selectively applied to an intermediate region between the region where mucosal stimulation was applied and that where mechanical recordings were made. 3. Brush stroking the mucosa (3 strokes) elicited a synchronous contraction of the LM and CM both above (ascending excitation) and below (descending excitation) the site of stimulation. All reflexes were abolished when tetrodotoxin (1 microM) was applied to the intermediate chamber. 4. Hexamethonium (300 microM) added to the intermediate chamber abolished the ascending contraction in 15 % of oral preparations (from 26 preparations, 18 animals) and the descending contraction in 13% of anal preparations studied (from 53 preparations, 48 animals). In the remaining 85% of oral preparations, hexamethonium usually attenuated the oral contraction of the LM and CM. However, in the remaining 87% of anal preparations, hexamethonium had no effect on the anal contraction of the LM and CM. 5. Oral and anal reflexes that were hexamethonium resistant were either abolished or attenuated by the further addition of the P2 purinergic receptor antagonist pyridoxal phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 10 microM) or alpha,beta-methylene ATP (50-100 microM) to the intermediate chamber. 6. 1,1-Dimethyl-4-phenyl-piperazinium iodide (DMPP, 20 microM) or alpha,beta-methylene ATP (50-100 microM) stimulated both ascending and descending excitatory pathways, when applied to the intermediate chamber. 7. In conclusion, ascending and descending neuro-neuronal transmission in excitatory nervous pathways to the LM and CM is complex and clearly involves neurotransmitter(s) other than acetylcholine (ACh). We suggest mucosal stimulation releases ACh and ATP in both ascending and descending excitatory reflex pathways that synapse with excitatory motoneurons to the LM and CM.

Nitric oxide modulates cholinergic reflex pathways to the longitudinal and circular muscle in the isolated guinea-pig distal colon

1. The involvement of nitric oxide (NO) in enteric neural pathways underlying reflex responses of the longitudinal muscle (LM) and circular muscle (CM) layers activated by mucosal stimulation was examined in the isolated guinea-pig distal colon. 2. A segment of colon spanned two partitions (10 mm apart), which divided the organ bath into three chambers: a recording chamber where LM and CM tension was measured; a stimulation chamber where mucosal stimulation was applied; and a middle chamber separating them. 3. Brushing the mucosa anal and oral to the recording site evoked simultaneous oral contraction and anal relaxation of both the LM and CM. 4. N omega-nitro-L-argininel-NA; 100 microM) or N omega-nitro-L-arginine methyl ester (L-NAME; 100 microM) applied to the middle chamber or stimulation chamber decreased the oral contractile response of the LM and CM (by about 30-40 %), but increased the anal relaxation (> 600 %) and exposed an anal contraction (> 1000 % increase) of both muscles. The addition of L-NA to the recording chamber reduced the anal relaxation of the LM and CM and the anal contraction of the LM, but slightly increased the anal contraction of the CM. 5. S-Nitroso-N-acetylpenicillamine (SNAP; 10 microM), an NO donor, reversed the effects of L-NA in the middle or stimulation chambers. 6. 1H-[1,2,4]oxadiazolo[4, 3-a]quinoxalin-1-one (ODQ; 10 microM), a soluble guanylate cyclase inhibitor, mimicked the effects of L-NAin the middle chamber or stimulation chamber, but these effects were not reversed by SNAP. 7. The oral contractile responses, and the anal relaxation and contractile responses of the LM and CM produced by L-NA in the stimulation or middle chambers, were blocked by hexamethonium (300 microM) in any chamber. Atropine (1 microM) in the recording chamber reduced the contractile responses of the LM and CM. 8. In conclusion, endogenous NO facilitates and depresses release of acetylcholine from interneurons in ascending and descending nervous pathways, respectively. These NO effects are mediated through soluble guanylate cyclase in cholinergic interneurons

Neuronal pathways and transmission to the lower esophageal sphincter of the guinea Pig

BACKGROUND & AIMS

The lower esophageal sphincter (LES) normally controls the opening and closing of the gastroesophageal junction to resist gastric reflux but allow swallowing. Neuronal pathways controlling the guinea pig LES were investigated anatomically and physiologically in isolated preparations.

METHODS

Intracellular recording from the LES with focal electrical stimulation and retrograde and anterograde neuronal tracing were used.

RESULTS

Electrical stimulation on the LES evoked inhibitory junction potentials (IJPs), which were reduced by 60% by 100 micromol/L N-nitro-L-arginine and subsequently blocked by 0.5 micromol/L apamin, unmasking excitatory junction potentials, which were abolished by 1 micromol/L hyoscine. Esophageal or vagal stimulation evoked IJPs, which were blocked by 100 micromol/L hexamethonium. Focal stimulation of the upper stomach evoked IJPs at 5-8 of 20 stimulation sites, which were abolished by cutting between the stimulation site and sphincter. Application of 1,1'-didodecyl-3,3,3', 3'-tetramethyl indocarbocyanine perchlorate (DiI) to the gastric sling muscle anterogradely labeled many motor axons in the sling muscle but few in the LES, confirming that the two muscles are separately innervated. DiI on the esophagus labeled nerve fibers, but not cell bodies, in the upper stomach.

CONCLUSIONS

The inhibitory motor neurons of the LES receive inputs from the vagus nerve, esophagus, and upper stomach.

Distension-evoked ascending and descending reflexes in the isolated guinea-pig stomach

Distension-evoked gastric reflexes were studied by intracellular recording from circular muscle cells in the gastric fundus, corpus and antrum in the isolated guinea-pig stomach. Localised electrical stimulation, 2 mm circumferential to the recording electrode, evoked inhibitory junctions potentials in all three gastric regions, sometimes followed by depolarisations in the antrum. In the mid corpus, the inhibitory responses were substantially reduced by Nw-nitro-L-arginine (100 microM), unmasking excitatory junction potentials. Residual hyperpolarisations were blocked by apamin (0.5 microM) which also enhanced the amplitude of excitatory junction potentials. These excitatory junction potentials were abolished by hyoscine (1 microM). Thus transmission from inhibitory motor neurons is mediated by both nitric oxide and an apamin-sensitive mechanism. Transmission from excitatory motor neurons to the circular muscle is mediated by acetylcholine via muscarinic receptors. Balloon distension of 10 s duration of the fundus or antrum elicited inhibitory junction potentials in circular muscle cells of the mid corpus. These inhibitory junction potentials were blocked by tetrodotoxin (0.6 microM) and were greatly reduced by Nw-nitro-L-arginine (100 microM). The residual hyperpolarisations were blocked by apamin (0.5 microM). This indicates the presence of ascending and descending inhibitory reflex pathways in the stomach. In 3 out of 7 experiments, following blockade of inhibitory transmission, small nerve-mediated excitatory junction potentials were evoked by antral distension indicating the presence of an additional ascending excitatory reflex pathway. Distension of the corpus elicited prominent inhibitory junction potentials, sometimes followed by large depolarisations, in circular muscle cells in the fundus, but not in the antrum. This suggests that there is also an ascending inhibitory reflex pathway from the corpus to the fundus but no distension-sensitive descending reflex pathway from the corpus to the antrum. These results demonstrate that within the stomach there are reflex pathways which can be activated by localised distension and project at some distance orally and aborally within the gastric wall. It is likely that the inhibitory reflex pathways are involved in gastric adaptive relaxation which occurs when the intact, isolated stomach is distended. The excitatory reflex pathways from the antrum to the corpus are likely to be involved in the intrinsic excitatory reflex responses observed in the isolated intact stomach to distension and thus be involved in the mixing and emptying of gastric contents.

Experimental basis for realistic large-scale computer simulation of the enteric nervous system

1. The enteric nervous system is perhaps the most accessible part of the mammalian nervous system in which it is feasible to attempt large scale computer simulation that is based closely on experimentally determined data. Here we summarize the data obtained for simulation of motility reflexes in the guinea-pig small intestine. 2. The chemistry, morphology and connectivity of each type of neuron involved in intrinsic reflexes have been investigated and most classes of neurons are physiologically well characterized. This includes primary sensory neurons, ascending and descending interneurons and motor neurons to circular and longitudinal muscle. 3. The responses of primary sensory neurons and the physiology of synaptic transmission from sensory neurons to interneurons and motor neurons, from interneurons to interneurons and from interneurons to motor neurons have been recorded during reflexes and in some cases the pharmacology of transmission has also been investigated. 4. Computer simulation, in which the activities of up to 30,000 neurons are modelled, produces patterns of activity that closely mimic those recorded in physiological experiments.

The role of ascending excitatory and descending inhibitory pathways in peristalsis in the isolated guinea-pig small intestine

1. The effects of experimental manipulations to alter the activation of ascending excitatory and descending inhibitory reflexes on peristalsis were studied in isolated segments of guinea-pig small intestine. 2. The normal site of initiation of the peristaltic contraction, the oral end, was always shifted to a point just anal to a crush which interrupted enteric neuronal pathways. 3. Shortening the functional length of the intestine by a series of crushes, thus effectively reducing the length of enteric neuronal reflex pathways, led to a progressive increase in the threshold distension for triggering the peristaltic contraction. 4. A sleeve placed around the intestine to prevent it from being distended by fluid led to a shift in the site of initiation of the peristaltic contraction to a point just anal to the sleeve. Furthermore, sleeves placed in the anal half of the intestine were able to stop propagation of the contraction. 5. The effect of these manipulations on peristalsis suggests that ascending excitatory and descending inhibitory enteric pathways, activated by distension, determine the pattern of peristaltic activity. The peristaltic contraction is initiated at the oral end as a result of the summation of ascending excitatory inputs and the relative absence of inputs from descending inhibitory pathways at this point. The magnitude of the distension for triggering this contraction is determined by a balance between ascending excitatory and descending inhibitory inputs to the circular muscle. Propagation of the circular muscle contraction requires the activation of ascending excitatory pathways at each point along the intestine and the sequential inactivation of the descending inhibitory reflex pathways anal to the contraction. The propagation of the circular muscle contraction stops when there is no longer a sufficient distension stimulus ahead.

Local neural control of intestinal motility: nerve circuits deduced for the guinea-pig small intestine

1. Propulsion of digesta along the intestine appears to occur by the action of a series of local reflexes which cause contraction oral to the digesta and relaxation of circular muscle on the anal side. 2. There is now substantial evidence available about the identities of the enteric neurons that mediate these reflexes. 3. The motor neurons and interneurons of the reflex pathways lie within the myenteric plexus. These neurons can be classified electrophysiologically as S-neurons and have distinctive projections and neurochemistries. 4. The sensory neurons may lie in the myenteric plexus, but there is some evidence for sensory neurons in the submucous plexus. A contribution from extrinsic sensory neurons to local motility reflexes cannot be ruled out. Intrinsic sensory neurons are probably AH-neurons and are large multi-axonal cells.

Neuronal nitric oxide in the gut

Motility of the gastrointestinal tract is directly controlled by enteric inhibitory and excitatory motor neurons that innervate the layers of smooth muscle. Inhibitory motor neurons mediate receptive and accommodative relaxations and control the opening of sphincters, thus playing an important role in normal gut motility. Recent studies have demonstrated that nitric oxide (NO) is an important neurotransmitter released by inhibitory motor neurons in animal and human gut. Antagonists of nitric oxide synthase (NOS), the synthetic enzyme for NO, reduce the effectiveness of transmission from inhibitory motor neurons. Exogenous NO mimics inhibitory nerve activation, and a variety of compounds that affect the availability of endogenously produced NO modulate relaxations of gastrointestinal smooth muscle. It is clear, however, that NO is unlikely to be the only transmitter released by enteric inhibitory motor neurons: several other substances such as vasoactive intestinal polypeptide (VIP), or related peptides, and adenosine triphosphate (ATP) are also likely to contribute to nerve-mediated inhibition. The identification of NO as a major inhibitory neurotransmitter to gastrointestinal smooth muscle fills an important gap in our understanding of the physiological control of motility and opens up a wide range of new experimental possibilities. It may eventually lead to the development of new drugs for motility disorders. It should be noted, however, that NO is important in the brain, in cardiovascular control, in blood cell function and in many other organ systems, suggesting that it may be difficult to achieve specific pharmacological intervention targeted on inhibitory neurotransmission in the gut, without undesirable side effects.

Post-stimulus depression of reflex changes in circular muscle activity in the guinea pig small intestine

The extent and time course of depression of successive reflex responses recorded with intracellular microelectrodes from the circular smooth muscle of the guinea pig small intestine were determined. Two stimuli were used, distension and distortion of the mucosa by compression; these were applied either at the same or at different sites. Excitatory responses oral and inhibitory responses anal to the stimuli were recorded. Post-stimulus depression of both ascending excitatory and descending inhibitory reflexes occurred, but the extent of depression was slightly less for the descending inhibition. A conditioning distension lasting 9 s depressed the excitatory response to a test distension applied 2 s later at the same site by 90%. After 30 s the depression was 50% and test responses were normal if inter-stimulus intervals were increased to 2 min. Increasing the duration of the conditioning stimulus increased the depression. Post-stimulus depression was less for compression stimuli than for distension stimuli and prior mucosal compression had almost no effect on responses to subsequent distension. The post-stimulus depression was greater if conditioning and test stimuli were at the same rather than different sites. For different sites, conditioning stimuli at 15 mm from the recording site (near) depressed responses to stimuli at 30 mm (far) to a greater extent than far stimuli depressed responses to near stimuli. If the conditioning stimulus at 15 mm was maintained until after the far test stimulus was applied, depression of the test response did not occur. It is concluded that the major sites of post-stimulus depression are at the synapses between primary sensory neurons and the first interneurons of reflex pathways, and that post-stimulus depression also occurs at other places in the pathway, presumably at synapses between interneurons or between interneurons and motor neurons.

Both ATP and the peptide VIP are inhibitory neurotransmitters in guinea-pig ileum circular muscle

1. Intracellular membrane potential recordings were made from circular smooth muscle cells of the guinea-pig ileum in the presence of atropine (1 microM) and nifedipine (0.1 microM) at 30 degrees C. 2. Perfusion with adenosine triphospate (ATP, 100 microM) and vasoactive intestinal peptide (VIP, 2 microM) resulted in membrane hyperpolarizations of 6.4 +/- 0.3 and 6.8 +/- 0.3 mV, respectively. Picospritzes of ATP (10 mM in pipette) and VIP (100 microM in pipette) resulted in membrane hyperpolarizations of 6.9 +/- 0.4 and 6.3 +/- 0.4 mV, respectively. 3. The ATP-induced hyperpolarizations were antagonized by alpha, beta-methylene ATP desensitization (100 microM for 30 min) and the ATP antagonist Reactive Blue 2 (200 microM), but were unaffected by the VIP antagonist VIP 10-28 (1 microM). 4. The VIP-induced hyperpolarizations were antagonized by VIP 10-28, but unaffected by alpha, beta-methylene ATP desensitization and Reactive Blue 2. 5. A single pulse of transmural nerve stimulation (2 ms, 15 mA) resulted in an inhibitory junction potential (IJP) that reached a maximal amplitude of 12.9 +/- 0.5 mV at 378 +/- 20 ms from the stimulus. This fast IJP was abolished by apamin (2 microM) or tetrodotoxin (1 microM), antagonized by alpha, beta-methylene ATP desensitization or Reactive Blue 2, but unaffected by VIP 10-28. 6. In the presence of apamin (1 microM), four pulses of transmural stimulation (2 ms, 20 Hz, 15 mA) resulted in an IJP that reached a maximal amplitude of 4.8 +/- 0.2 mV at 1.4 +/- 0.1 s from the stimulus. This slow IJP was antagonized by tetrodotoxin (1 microM) or VIP 10-28 (1 microM), augmented by Reactive Blue 2 (200 microM), and unaffected by alpha, beta-methylene ATP desensitization. 7. These findings provide evidence that both ATP and VIP are inhibitory neurotransmitters in the circular muscle layer of the ileum and that ATP may be the neurotransmitter responsible for the fast IJP and VIP the neurotransmitter responsible for the slow IJP.

The nature of noncholinergic membrane potential responses to transmural stimulation in guinea pig ileum

The effect of substance P antagonism on membrane potential responses to transmural nerve stimulation in the presence of atropine was examined in circular smooth muscle of the guinea pig ileum. Intracellular recordings of membrane potential responses recorded 3-5 mm oral to the transmural stimulus consisted of an inhibitory junction potential followed by two distinct depolarizations referred to as early and late excitatory junction potentials. Substance P antagonism was achieved by desensitization with high doses of substance P or use of the antagonist Spantide (Sigma Chemical Co., St. Louis, MO). Substance P antagonism had no effect on the amplitude of the inhibitory junction potential, caused an increase in the latter portion of the early excitatory junction potential, and abolished the late excitatory junction potential. The excitatory junction potential potentiated by substance P receptor antagonism was associated with a decrease in membrane resistance, increased in amplitude with conditioning hyperpolarizations to the estimated equilibrium potential for K+, and was blocked by the Cl-/HCO3- exchange inhibitor DIDS or prolonged perfusion with low-chloride solution. These studies suggest that a noncholinergic, non-substance P neurotransmitter is released from enteric motoneurons that produces excitation through an increase in smooth muscle chloride conductance.

Immunohistochemical identification of cholinergic neurons in the myenteric plexus of guinea-pig small intestine

It is well established that acetylcholine is a neurotransmitter at several distinct sites in the mammalian enteric nervous system. However, identification of the cholinergic neurons has not been possible due to an inability to selectively label enteric cholinergic neurons. In the present study an immunohistochemical method has been developed to localize choline acetyltransferase, the synthetic enzyme for acetylcholine, in order that cholinergic neurons can be visualized. The morphology, neurochemical coding and projections of cholinergic neurons in the guinea-pig small intestine were determined using double-labelling immunohistochemistry. These experiments have revealed that many myenteric neurons are cholinergic and that they can be distinguished by their specific combinations of immunoreactivity for neurochemicals such as calretinin, neurofilament protein triplet, substance P, enkephalin, somatostatin, 5-hydroxytryptamine, vasoactive intestinal peptide and calbindin. On the basis of their previously described projections, functional roles could be attributed to each of these populations. The identified cholinergic neurons are: motorneurons to the longitudinal muscle (choline acetyltransferase/calretinin); motorneurons to the circular muscle (choline acetyltransferase/neurofilament triplet protein/substance P, choline acetyltransferase/substance P and choline acetyltransferase alone); orally directed interneurons in the myenteric plexus (choline acetyltransferase/calretinin/enkephalin); anally directed interneurons in the myenteric plexus (choline acetyltransferase/somatostatin, choline acetyltransferase/5-hydroxytryptamine, choline acetyltransferase/vasoactive intestinal peptide); secretomotor neurons to the mucosa (choline acetyltransferase/somatostatin); and sensory neurons mediating myenteric reflexes (choline acetyltransferase/calbindin). This information provides a unique opportunity to identify functionally distinct populations of cholinergic neurons and will be of value in the interpretation of physiological and pharmacological studies of enteric neuronal circuitry.

Identification and immunohistochemistry of cholinergic and non-cholinergic circular muscle motor neurons in the guinea-pig small intestine

Motor neurons which innervate the circular muscle layer of the guinea-pig small intestine were retrogradely labelled, in vitro, with the carbocyanine dye, DiI, applied to the deep muscular plexus. By combining retrograde tracing and immunohistochemistry, the chemical coding of motor neurons was investigated. Five classes of neuron could be distinguished on the basis of the co-localization of immunoreactivity for the different antigens; the five classes were also characterized by different lengths and polarities of their axonal projections and by their cell body shapes. Two classes with local or orally directed axons were immunoreactive for choline acetyltransferase and substance P and are likely to be cholinergic excitatory motor neurons. Two other classes had anally directed axons; they were immunoreactive for vasoactive intestinal polypeptide and are likely to be inhibitory motor neurons. A small proportion of neurons with short projections to the circular muscle were immunoreactive for neither substance P nor for vasoactive intestinal polypeptide, but are likely to be cholinergic. The morphological and histochemical identification of excitatory and inhibitory motor neurons provides a neuroanatomical basis for the final motor pathways involved in the polarized reflex motor activity of the gut.

Identification of enteric motor neurones which innervate the circular muscle of the guinea pig small intestine

Retrograde transport of the carbocyanine dye DiI from the deep muscular plexus of guinea pig small intestine was carried out in organotypic culture to identify circular muscle motor neurones. Seventy-four % of DiI labelled neurones were located within 4 rows of myenteric ganglia oral or anal to the application site. Some motor neurones had axons extending up to 37 rows of myenteric ganglia in the anal direction, or long axons travelling up to 18 rows of ganglia in the oral direction. Ninety four percent of the filled cells had Dogiel type I soma morphology; of these, neurones oral to the application site consistently had lamellar dendrites and were readily distinguishable from those located anally which had short filamentous dendrites. The methods described in this study make it possible to distinguish circular muscle motor neurones from other cell types in the myenteric plexus for the first time.

Distribution of substance p-like immunoreactivity in nerves of the gastrointestinal tract of the frog Rana esculenta L

Substance P-like immunoreactivity in the alimentary canal of the frog Rana esculenta L. was studied by means of the indirect immunoperoxidase method. In all segments of the gastrointestinal tract, immunoreactivity was revealed in both the myenteric and the submucosa plexus. Stained nerve cells were observed in the myenteric plexus but not in the submucous plexus. The proximal part of the oesophagus and hindgut were free of immunoreactive perkarya. The stained nerve cells were of the Dogiel type I category in the foregut, and type II in the midgut. Both the musculature and gastrointestinal glands were supplied with immune-positive fibres. These results indicate that substance P may play similar roles in the frog gut, as described previously in mammals and fish.

An in vitro study of the projections of enteric vasoactive intestinal polypeptide-immunoreactive neurons in the human colon

The anatomical basis of the peptidergic neural control of the human colon is largely unknown. In this study, in vitro retrograde tracing methods have been used on fresh human colon to determine the projection pathways of the enteric nerves and, in particular, those containing vasoactive intestinal polypeptide, one of the most abundant and potent of the gut neuropeptides. Two components of the submucous plexus were identified, the inner one projecting to the lamina propria, and the outer to the circular muscle. The lengths of projections within the submucous plexus were up to 5-14 mm in all directions. Myenteric ganglion cells projected to both longitudinal and circular muscles, for distances of up to only 5 mm. The subpopulation of nerves containing vasoactive intestinal polypeptide arose mainly from the submucous plexus and projected up to 6.5 mm anally, 5 mm orally, and 14 mm within the submucous layer to the mucosa or circular muscle. These findings provide entirely new data on the neuroanatomy of the human colon and may help in the understanding of the neural control of colonic secretion and motility.

Distension-evoked ascending and descending reflexes in the circular muscle of guinea-pig ileum: an intracellular study

Reflex responses evoked by distension of the guinea-pig small intestine were recorded from the circular muscle with intracellular microelectrodes. For this purpose a mechanically stable preparation that allowed the intestinal wall to be distended within 9 mm of the recording site was developed. A segment of intestine was opened along the mesenteric border and pinned mucosa uppermost over a balloon set in the base of an organ bath, so that inflation of the balloon could distend the intestinal wall without simultaneously pushing against the mucosa. Compound excitatory junction potentials (EJPs) and compound inhibitory junction potentials (IJPs) were recorded at sites up to 40 mm oral and anal to the distending stimulus, respectively. The compound EJPs recorded orally had amplitudes of up to 24 mV and declined to baseline during distensions that exceeded 10-15 s. Distensions at intervals of less than 20 s evoked successively smaller oral compound EJPs; after four distensions in 30 the amplitude of the compound EJP had fallen to less than 10%. The amplitude of the oral compound EJP was reduced by hyoscine (1 microM), but the extent of the reduction depended on the degree of distension; responses to mild stimuli were blocked, whereas those to strong stimuli were only slightly reduced. The amplitude of the hyoscine-resistant component of the compound EJP was markedly reduced by antagonists of substance P receptors in the muscle. In the presence of muscarinic and substance P receptor antagonists, a transient compound IJP could be detected on the oral side of the stimulus. The compound IJPs recorded anal to the distension had amplitudes up to 22 mV but the potential returned to baseline during prolonged distension. In the presence of hyoscine (1 microM) some inhibitory activity continued throughout prolonged stimuli. Compound IJP amplitudes were not significantly reduced by repeated distensions separated by more than 6 s. At anal sites a transient depolarization (off-response) was recorded immediately following the termination of a distension in some preparations. The off-response was unaffected by hyoscine and was more readily observed after the further addition of substance P antagonists. The compound IJPs were almost completely blocked by apamin (0.2 microM). The compound EJPs and IJPs recorded orally were blocked by hexamethonium (100 microM), but the amplitudes of compound IJPs recorded anally were significantly reduced by hexamethonium (100-200 microM) only at recording sites greater than 15 mm from the centre of the balloon. The off-response was reduced by hexamethonium at all sites.(ABSTRACT TRUNCATED AT 400 WORDS)

A pharmacological analysis of the neuronal circuitry involved in distension-evoked enteric excitatory reflex

Isolated segments of guinea-pig small intestine were set up in a partitioned bath to study the enteric excitatory reflex evoked by distension. The gut was distended by a rubber balloon inserted at the aboral end and contractions of the circular muscle were recorded at the oral end. The oral and aboral ends of the gut were separated by an intermediate compartment of the bath. Inflation of the intraluminal balloon with 0.075-0.35 ml water elicited reproducible and distension-dependent contraction. This enteric orally directed (ascending) excitatory reflex was abolished by tetrodotoxin irrespective of the compartment in which it was applied. Hyoscine (0.3 microM) almost abolished the enteric excitatory reflex when it was applied to the oral compartment. This indicates that the transmission from the final motor neurons to the circular muscle is mainly cholinergic, acting via muscarinic receptors. Hyoscine had no effect on the enteric excitatory reflex when added to the intermediate compartment. When hyoscine was added to the aboral compartment, it decreased the enteric excitatory reflex elicited by low distension stimuli to 70% of control and decreased the enteric excitatory reflex elicited by higher distension stimuli to 95% of control. This indicates that ganglionic transmission involving muscarinic receptors at the site of distension in the aboral bath contributes to the enteric excitatory reflex. Hexamethonium (100 microm) greatly depressed the enteric excitatory reflex when applied to any compartment indicating that nicotinic transmission is most important in the afferent, intermediate and efferent components of the reflex and that the reflex pathway involves a polysynaptic chain of cholinergic interneurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Spontaneous junction potentials and slow waves in the circular muscle of isolated segments of guinea-pig ileum

Spontaneous electrical activity was recorded with intracellular microelectrodes from cells within the circular muscle of isolated, 2 cm long, intact segments of guinea-pig ileum that were unstretched, and in segments that had been slit open along the entire length of either their mesenteric or antimesenteric border and pinned flat under a minimum of tension. Intact segments usually exhibited fast spontaneous irregular oscillations in membrane potential (mean 1.6 Hz) which were unaffected by hyoscine (0.5 microM), the substance P antagonist D-Arg1, D-Pro2, D-Trp7.9, Leu11-substance P (10 microM), hexamethonium (100 microM), propranolol (1 microM) or phentolamine (1 microM) but were blocked by tetrodotoxin (0.4 microM) or apamin (0.4 microM). This irregular spontaneous activity is deduced to be due to ongoing firing of inhibitory motor neurons. After blockade with apamin or tetrodotoxin, a slow wave-like activity with a mean frequency of 16.4 cycles/min and maximum amplitude 2-14 mV was observed in 47% of intact segments. The amplitude of slow waves waxed and waned with a mean frequency of 0.9 cycles/min. Spontaneous cholinergic (hyoscine-sensitive) excitatory junction potentials were observed in some preparations. In contrast, in the majority of opened segments the resting membrane potential was quite stable, although slow waves that were similar to those in intact segments were observed in 14% of preparations. These studies indicate that spontaneous inhibitory junction potentials and slow waves can be recorded in intact segments of guinea-pig ileum. Their relative absence in opened segments suggests their normal expression is facilitated by the circumferential integrity of the intestine.

Involvement of substance P neurons in contractions of canine small intestine produced by mesenteric nerve stimulation

Pathways for contractions of in vivo canine small intestine produced by mesenteric nerve stimulation (MNS) were studied. In intact and chronically sympathectomized dogs, contractions of jejunal and ileal segments were largely reduced by intra-arterial infusion of capsaicin (10-100 microM, 0.07 ml/min), substance P (SP) antagonist, (D-Pro4, D-Trp7.9) SP (4-11) (100 microM, 0.14 ml/min), hexamethonium (100-1000 microM, 0.07 ml/min) or atropine (100 microM, 0.07 ml/min). In chronically vagotomized dogs, capsaicin, SP-antagonist or atropine significantly reduced MNS-induced contractions, but hexamethonium did not. In dogs in which the coeliac and superior mesenteric ganglia had previously been removed, MNS caused no response although intra-arterial injection of 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP, 0.1 mumol) caused marked contractions. It may therefore be suggested that extrinsic SP neurons probably originating in spinal ganglia and intrinsic SP neurons receiving input from vagal preganglionic cholinergic neurons are involved in the excitatory pathways to MNS-induced contractions and that activation of these neurons excites myenteric cholinergic neurons, thereby causing contractions of the small intestine.

Reflex changes in circular muscle activity elicited by stroking the mucosa: an electrophysiological analysis in the isolated guinea-pig ileum

A preparation of isolated small intestine from the guinea-pig was studied in which reflex responses of the circular muscle were recorded intracellularly when sensory receptors in the mucosa were stimulated mechanically. This preparation was used to examine the properties of mucosa to muscle reflexes that involve non-cholinergic motor neurons innervating the circular muscle. Reproducible stimulation of the mucosa was achieved by stroking with a motor-driven brush. Gentle brush-strokes applied to the mucosa typically evoked inhibitory junction potentials anal to the stimulus and excitatory junction potentials at recording sites oral to the stimulus. Both events were rapid in onset and up to 25 mV in amplitude. The reflexes were blocked by tetrodotoxin (0.5 microM). Junction potentials declined in amplitude with distance from the stimulus, the amplitude of the excitation 15 mm oral to the stimulus was half that at 5 mm from the stimulus, whereas the amplitude of the inhibitory potential at 40-45 mm was about 60% of that at 5-10 mm anal to the stimulus. Hexamethonium (100-200 microM) blocked the ascending excitation but only slightly reduced the descending inhibition. Ascending excitation was blocked by antagonists for substance P receptors in the muscle, and inhibition was substantially reduced by apamin (0.2 microM), both before and after hexamethonium. Both responses were abolished by removal of the mucosa from the stimulus site and when lesions were made through the myenteric plexus between the stimulation and recording sites, but persisted when similar lesions were made through the submucous plexus. It is concluded that there are neurons with mechanoreceptive nerve endings in the mucosa. Stimulation of such sensory neurons leads to activation of pathways in the myenteric plexus that excite motor neurons to the muscle both oral and anal to the stimulation site. The demonstration that mucosa to muscle reflexes can be consistently evoked in the small intestine in vitro provides an opportunity for close analysis of the reflex pathways. Such analysis is not so readily achieved when reflexes are initiated by distension that, by moving the intestine, can dislodge the recording electrode.