Intrinsic primary afferent neurons of the intestine

After a long period of inconclusive observations, the intrinsic primary afferent neurons of the intestine have been identified. The intestine is thus equipped with two groups of afferent neurons, those with cell bodies in cranial and dorsal root ganglia, and these recently identified afferent neurons with cell bodies in the wall of the intestine. The first, tentative, identification of intrinsic primary afferent neurons was by their morphology, which is type II in the terminology of Dogiel. These are multipolar neurons, with some axons that project to other nerve cells in the intestine and other axons that project to the mucosa. Definitive identification came only recently when action potentials were recorded intracellularly from Dogiel type II neurons in response to chemicals applied to the lumenal surface of the intestine and in response to tension in the muscle. These action potentials persisted after all synaptic transmission was blocked, proving the Dogiel type II neurons to be primary afferent neurons. Less direct evidence indicates that intrinsic primary afferent neurons that respond to mechanical stimulation of the mucosal lining are also Dogiel type II neurons. Electrophysiologically, the Dogiel type II neurons are referred to as AH neurons. They exhibit broad action potentials that are followed by early and late afterhyperpolarizing potentials. The intrinsic primary afferent neurons connect with each other at synapses where they transmit via slow excitatory postsynaptic potentials, that last for tens of seconds. Thus the intrinsic primary afferent neurons form self-reinforcing networks. The slow excitatory postsynaptic potentials counteract the late afterhyperpolarizing potentials, thereby increasing the period during which the cells can fire action potentials at high rates. Intrinsic primary afferent neurons transmit to second order neurons (interneurons and motor neurons) via both slow and fast excitatory postsynaptic potentials. Excitation of the intrinsic primary afferent neurons by lumenal chemicals or mechanical stimulation of the mucosa appears to be indirect, via the release of active compounds from endocrine cells in the epithelium. Stretch-induced activation of the intrinsic primary afferent neurons is at least partly dependent on tension generation in smooth muscle, that is itself sensitive to stretch. The intrinsic primary afferent neurons of the intestine are the only vertebrate primary afferent neurons so far identified with cell bodies in a peripheral organ. They are multipolar and receive synapses on their cell bodies, unlike cranial and spinal primary afferent neurons. They communicate with each other via slow excitatory synaptic potentials in self reinforcing networks and with interneurons and motor neurons via both fast and slow EPSPs.

Projections and chemical coding of neurons with immunoreactivity for nitric oxide synthase in the guinea-pig small intestine

The distribution of nitric oxide synthase (NOS) immunoreactivity was investigated in the guinea-pig small intestine. There were many immunoreactive nerve cell bodies in the myenteric plexus but very few in submucous ganglia. NOS immunoreactivity was not found in non-neuronal cells except for rare mucosal endocrine cells. Abundant immunoreactive nerve fibres in both myenteric and submucous ganglia, and in the circular muscle, arose from myenteric nerve cells whose axons projected anally along the intestine. NOS immunoreactivity coexisted with VIP-immunoreactivity, but not with substance P immunoreactivity. We conclude that nitric oxide synthase is located in a sub-population of enteric neurons, amongst which are inhibitory motor neurons that supply the circular muscle layer.

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.

The terminals of myenteric intrinsic primary afferent neurons of the guinea-pig ileum are excited by 5-hydroxytryptamine acting at 5-hydroxytryptamine-3 receptors

The aim of this study was to identify the receptor type(s) by which 5-hydroxytryptamine applied to the intestinal mucosa excites the terminals of myenteric AH neurons. The AH neurons have been identified as the intrinsic primary afferent (sensory) neurons in guinea-pig small intestine and 5-hydroxytryptamine has been identified as a possible intermediate in the sensory transduction process. Intracellular recordings were taken from AH neurons located within 1mm of intact mucosa to which 5-hydroxytryptamine was applied. Trains of action potentials and/or slow depolarizing responses were recorded in AH neurons in response to mucosal application of 5-hydroxytryptamine (10 or 20microM) or the 5-hydroxytryptamine-3 receptor agonist, 2-methyl-5-hydroxytryptamine (1 or 3mM), and to electrical stimulation of the mucosa. The 5-hydroxytryptamine-2 receptor agonist, alpha-methyl-5-hydroxytryptamine (100microM), and the 5-hydroxytryptamine-1,2,4 receptor agonist, 5-methoxytryptamine (10microM), did not elicit such responses. The 5-hydroxytryptamine-3 receptor-selective antagonist, granisetron (typically 1microM), and the 5-hydroxytryptamine-3,4 receptor antagonist, tropisetron (typically 1microM), each reduced or abolished the responses to 5-hydroxytryptamine, while the selective 5-hydroxytryptamine-4 receptor antagonist, SB 204070 (1microM), did not. It is concluded that application of 5-hydroxytryptamine to the mucosa activates a 5-hydroxytryptamine-3 receptor that triggers action potential generation in the mucosal nerve terminals of myenteric AH neurons.

Intracellular recording from myenteric neurons of the guinea-pig ileum that respond to stretch

1. Isolated longitudinal muscle-myenteric plexus preparations from guinea-pig ileum were used to investigate the activity of myenteric neurons when the tissue was stretched in the circumferential direction. Membrane potentials were recorded via flexibly mounted intracellular recording electrodes containing Neurobiotin in 1 M KCl. The preparations were stretched to constant widths (+20% and +40% beyond slack width). 2. Multipolar neurons (Dogiel type II morphology) discharged spontaneous action potentials and proximal process potentials during maintained stretching, three of twenty-one at +20% stretch and seven of nine at +40% stretch. At the maximum extent of stretch tried, +40% beyond slack tissue width, action potentials in Dogiel type II neurons occurred at 10-33 Hz. Neurons with other morphologies were all uniaxonal. Some displayed spontaneous fast EPSPs or action potentials, three of forty one at +20% stretch and seven of nineteen at +40% stretch. 3. In seven of eight Dogiel type II neurons, action potentials or proximal process potentials persisted when membrane hyperpolarization was imposed via the recording electrode. Action potential discharge was abolished by hyperpolarization in seven of nine uniaxonal neurons; the exceptions were two orally projecting neurons. 4. Dogiel type II and uniaxonal neurons were classified as rapidly accommodating if they discharged action potentials only at the beginning of a 500 ms intracellular depolarizing pulse and slowly accommodating if they discharged for more than 250 ms. For Dogiel type II neurons, three of thirteen were slowly accommodating at +20% stretch and two of four at 40% stretch. For uniaxonal neurons the corresponding data were twelve of twenty-six and fifteen of nineteen neurons. The slowly accommodating state was associated with increased cell input resistance in uniaxonal neurons. 5. The spontaneous action potential discharge in Dogiel type II and uniaxonal neurons ceased when the muscle was relaxed pharmacologically by nicardipine (3 microM) or isoprenaline (1 microM), although the applied stretch was maintained. At the same time, evoked spike discharge became rapidly accommodating. 6. We conclude that many Dogiel type II neurons, and possibly some orally projecting uniaxonal neurons, are intrinsic, stretch-sensitive, primary afferent neurons that respond to muscle tension with sustained action potential discharge.

Correlated electrophysiological and histochemical studies of submucous neurons and their contribution to understanding enteric neural circuits

Neither submucous ganglia, nor intestinal secretomotor reflexes are mentioned in the majority of the textbooks of physiology; because it has been realized only very recently that the submucous neurons may have important influences on whole body water and electrolyte balance. In the present review, we trace the rapid progress that has been made in determining the physiological properties of submucous neurons with known chemistry and projections in the guinea-pig small intestine, and we analyze how the work relates to studies in vivo of the neuronal control of intestinal trans-epithelial fluid transport. Four types of submucous neurons, which appear to be the full complement in the guinea-pig small intestine, have been identified through electrophysiological and histochemical analysis. (1) Cholinergic secretomotor neurons contain immunoreactivity for choline-acetyltransferase (ChAT), calcitonin gene-related peptide (CGRP), cholecystokinin (CCK), neuropeptide Y (NPY), somatostatin (SOM), and in the majority of cases galanin (GAL); these neurons project to the mucosal epithelium. (2) Non-cholinergic secretomotor neurons contain dynorphin (DYN), GAL and vasoactive intestinal peptide (VIP); these neurons project to the mucosa and provide collaterals to submucous arterioles. (3) Cholinergic interneurons contain ChAT alone; these neurons connect with the secretomotor neurons. (4) Presumed sensory neurons contain ChAT and substance P (SP) and have nerve endings in the mucosa. The two groups of secretomotor neurons receive cholinergic synaptic inputs from both myenteric and submucous ganglia. In addition, the DYN/GAL/VIP neurons receive sympathetic inhibitory inputs as well as inhibitory and non-cholinergic excitatory inputs from myenteric ganglia. The ChAT/SP nerve cells in submucous ganglia receive no or very ineffective inputs. From these data, from experiments on transmission from the neurons to the intestinal epithelium, and from studies of secretomotor reflexes in vivo, a correlated functional and structural circuitry of the submucous ganglia and their connections has been deduced. It is concluded that secretomotor reflexes are stimulated by the contents of the lumen during the digestion and absorption of food and that these reflexes cause a proportion of water and electrolytes that are absorbed with nutrients such as glucose to be returned to the lumen. The balance of absorption and secretion of water and electrolytes is controlled by sympathetic inhibitory inputs to secretomotor neurons, the activity in sympathetic pathways being varied to contribute to whole body water and electrolyte balance.

Chemotherapy-Induced Constipation and Diarrhea: Pathophysiology, Current and Emerging Treatments

Gastrointestinal (GI) side-effects of chemotherapy are a debilitating and often overlooked clinical hurdle in cancer management. Chemotherapy-induced constipation (CIC) and Diarrhea (CID) present a constant challenge in the efficient and tolerable treatment of cancer and are amongst the primary contributors to dose reductions, delays and cessation of treatment. Although prevalence of CIC is hard to estimate, it is believed to affect approximately 16% of cancer patients, whilst incidence of CID has been estimated to be as high as 80%. Despite this, the underlying mechanisms of both CID and CIC remain unclear, but are believed to result from a combination of intersecting mechanisms including inflammation, secretory dysfunctions, GI dysmotility and alterations in GI innervation. Current treatments for CIC and CID aim to reduce the severity of symptoms rather than combating the pathophysiological mechanisms of dysfunction, and often result in worsening of already chronic GI symptoms or trigger the onset of a plethora of other side-effects including respiratory depression, uneven heartbeat, seizures, and neurotoxicity. Emerging treatments including those targeting the enteric nervous system present promising avenues to alleviate CID and CIC. Identification of potential targets for novel therapies to alleviate chemotherapy-induced toxicity is essential to improve clinical outcomes and quality of life amongst cancer sufferers.

Identification of sensory nerve cells in a peripheral organ (the intestine) of a mammal

It is commonly believed that the cell bodies of mammalian sensory neurons are contained within spinal and cranial sensory ganglia associated with the central nervous system or within the central nervous system itself. However, strong circumstantial evidence implies that some sensory neurons are contained entirely within the gastrointestinal tract. We have investigated this possibility by using intracellular methods to record the responses of myenteric neurons in the guinea-pig small intestine to physiological stimuli applied to the neighbouring mucosa. The results show that the myenteric plexus contains a population of chemosensitive sensory neurons and that these neurons correspond to neurons with AH electrophysiological properties and Dogiel type II morphology. This is the first direct evidence that some sensory neurons are contained entirely within the peripheral nervous system.

Plurichemical transmission and chemical coding of neurons in the digestive tract

The enteric nervous system contains neurons with well-defined functions. However, when neurons of the same function are examined in different regions or species, they are found to show subtle differences in their pharmacologies of transmission and different chemical coding. Individual enteric neurons use more than one transmitter, i.e., transmission is plurichemical. For example, enteric inhibitory neurons have three or more primary transmitters, including nitric oxide, vasoactive intestinal peptide, and possibly adenosine triphosphate and pituitary adenylyl cyclase activating peptide. Primary transmitters are highly conserved, although their relative roles vary considerably between gut regions. Multiple substances, including transmitters and their synthesizing enzymes and nontransmitters (such as neurofilament proteins), provide neurons with a chemical coding through which their functions and projections can be identified. Although equivalent neurons in different regions have the same primary transmitters, other chemical markers differ substantially. Caution must be taken in extrapolating pharmacological and neurochemical observations between species or even between regions in the one species. On the other hand, careful interregion and interspecies comparisons lead to an understanding of the features of enteric neurons that are highly conserved and can be used in valid extrapolation.

The Role of the Gastrointestinal Mucus System in Intestinal Homeostasis: Implications for Neurological Disorders

Mucus is integral to gut health and its properties may be affected in neurological disease. Mucus comprises a hydrated network of polymers including glycosylated mucin proteins. We propose that factors that influence the nervous system may also affect the volume, viscosity, porosity of mucus composition and subsequently, gastrointestinal (GI) microbial populations. The gut has its own intrinsic neuronal network, the enteric nervous system, which extends the length of the GI tract and innervates the mucosal epithelium. The ENS regulates gut function including mucus secretion and renewal. Both dysbiosis and gut dysfunction are commonly reported in several neurological disorders such as Parkinson's and Alzheimer's disease as well in patients with neurodevelopmental disorders including autism. Since some microbes use mucus as a prominent energy source, changes in mucus properties could alter, and even exacerbate, dysbiosis-related gut symptoms in neurological disorders. This review summarizes existing knowledge of the structure and function of the mucus of the GI tract and highlights areas to be addressed in future research to better understand how intestinal homeostasis is impacted in neurological disorders.

Roles of peptides in transmission in the enteric nervous system

Studies of the enteric nervous system have proved to be important in the development of new concepts of the chemical nature of transmission from neurons. In particular, they have revealed the multiplicity of influences that peptides can have on transmission, such as their action as primary transmitters, and the fact that they often act as co-transmitters in enteric neurons. However, in other cases no roles can be attributed to neuropeptides in enteric neurons, and their involvement in short-term changes in excitability seems minor.

Electrophysiology of guinea-pig myenteric neurons correlated with immunoreactivity for calcium binding proteins

Experiments were undertaken to define the electrophysiological characteristics and shapes of neurons in the myenteric plexus of the guinea-pig ileum that are immunoreactive for calcium binding proteins. Recordings were made from the neurons with intracellular microelectrodes containing a mixture of the fluorescent dye Lucifer yellow and KCl solution. The neurons studied were filled with Lucifer yellow so that they could be re-identified after processing the tissue to reveal immunoreactivity for either the calcium binding protein (CaBP), spot 35 protein, or vitamin D-dependent CaBP. Neurons were characterized as being AH-neurons, in which each action potential is followed by a prolonged after-hyperpolarization (greater than 4 s), or S-neurons, in which the prolonged after-hyperpolarizations were not observed and focal stimulation of internodal strands evoked fast excitatory synaptic potentials. S-neurons were never immunoreactive for the CaBPs (108 cells), but most AH-neurons (62 of 74) were immunoreactive. Immunoreactive and non-immunoreactive AH-neurons were indistinguishable on the basis of their electrophysiological properties or their shapes (all the AH-neurons were Dogiel type II in shape, i.e. smooth soma and many long processes). The S-neurons had a variety of shapes, but none could be classified as Dogiel type II. It is concluded that most AH-neurons are immunoreactive for calcium binding proteins, and that these proteins are restricted to AH-neurons.

Calbindin neurons of the guinea-pig small intestine: quantitative analysis of their numbers and projections

The distribution of nerve cells with immunoreactivity for the calcium-binding protein, calbindin, has been studied in the small intestine of the guinea-pig, and the projections of these neurons have been analysed by tracing their processes and by examining the consequences of nerve lesions. The immunoreactive neurons were numerous in the myenteric ganglia; there were 3500 +/- 100 reactive nerve cells per cm2 of undistended intestine, which is 30% of all nerve cells. In contrast, reactive nerve cells were extremely rare in submucous ganglia. The myenteric nerve cells were oval in outline and gave rise to several long processes; this morphology corresponds to Dogiel's type-II classification. Processes from the cell bodies were traced through the circular muscle in perforating nerve fibre bundles. Other processes ran circumferentially in the myenteric plexus. Removal of the myenteric plexus, allowing time for subsequent fibre degeneration, showed that reactive nerve fibres in the submucous ganglia and mucosa came from the myenteric cell bodies. Operations to sever longitudinal or circumferential pathways in the myenteric plexus indicated that most reactive nerve terminals in myenteric ganglia arise from myenteric cell bodies whose processes run circumferentially for 1.5 mm, on average. It is deduced that the calbindin-reactive neurons are multipolar sensory neurons, with the sensitive processes in the mucosa and with other processes innervating neurons of the myenteric plexus.

Selective effects of (-)-baclofen on spinal synaptic transmission in the cat

When ejected microelectrophoretically near spinal interneurones of cats anaesthetised with pentobarbitone and under conditions where postsynaptic excitability was maintained artificially at a constant level, (-), but not (+), -baclofen selectively reduced monosynaptic excitation by impulses in low threshold muscle (Ia and Ib) and cutaneous (Aalpha) afferents. Polysynaptic excitation of interneurones and Renshaw cells by impulses in higher threshold afferents was less affected, and baclofen had little or no effect on the cholinergic monosynaptic excitation of Renshaw cells. Glycinergic and gabergic inhibitions of spinal neurones were relatively insensitive to baclofen. These stereospecific actions of baclofen, produced by either a reduction in the release of excitatory transmitter or postsynaptic antagonism, suggest that Ia, Ib, and Aalpha afferents may release the same excitatory transmitter which differs from that of spinal excitatory interneurones. Microelectrophoretic (-), but not (+), -baclofen also reduced primary afferent depolarization of ventral horn Ia extensor afferent terminations produced by impulses in low threshold flexor afferents, without altering either the electrical excitability of the terminations or their depolarization by electrophoretic GABA or L-glutamate. This stereospecific action of baclofen is interpreted as a reduction in the release of GABA at depolarizing axo-axonic synapses on Ia terminals.

Strain-specific genetics, anatomy and function of enteric neural serotonergic pathways in inbred mice

Serotonin (5-HT) powerfully affects small intestinal motility and 5-HT-immunoreactive (IR) neurones are highly conserved between species. 5-HT synthesis in central neurones and gastrointestinal mucosa depends on tissue-specific isoforms of the enzyme tryptophan hydroxylase (TPH). RT-PCR identified strain-specific expression of a polymorphism (1473C/G) of the tph2 gene in longitudinal muscle-myenteric plexus preparations of C57Bl/6 and Balb/c mice. The former expressed the high-activity C allele, the latter the low-activity G allele. Confocal microscopy was used to examine close contacts between 5-HT-IR varicosities and myenteric neurones immunoreactive for neuronal nitric oxide synthase (NOS) or calretinin in these two strains. Significantly more close contacts were identified to NOS- (P < 0.05) and calretinin-IR (P < 0.01) neurones in C57Bl/6 jejunum (NOS 1.6 +/- 0.3, n = 52; calretinin 5.2 +/- 0.4, n = 54), than Balb/c jejunum (NOS 0.9 +/- 0.2, n = 78; calretinin 3.5 +/- 0.3, n = 98). Propagating contractile complexes (PCCs) were identified in the isolated jejunum by constructing spatiotemporal maps from video recordings of cannulated segments in vitro. These clusters of contractions usually arose towards the anal end and propagated orally. Regular PCCs were initiated at intraluminal pressures of 6 cmH(2)O, and abolished by tetrodotoxin (1 microm). Jejunal PCCs from C57Bl/6 mice were suppressed by a combination of granisetron (1 microm, 5-HT(3) antagonist) and SB207266 (10 nm, 5-HT(4) antagonist), but PCCs from Balb/c mice were unaffected. There were, however, no strain-specific differences in sensitivity of longitudinal muscle contractions to exogenous 5-HT or blockade of 5-HT(3) and 5-HT(4) receptors. These data associate a genetic difference with significant structural and functional consequences for enteric neural serotonergic pathways in the jejunum.

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

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

White paper on guidelines concerning enteric nervous system stem cell therapy for enteric neuropathies

Over the last 20 years, there has been increasing focus on the development of novel stem cell based therapies for the treatment of disorders and diseases affecting the enteric nervous system (ENS) of the gastrointestinal tract (so-called enteric neuropathies). Here, the idea is that ENS progenitor/stem cells could be transplanted into the gut wall to replace the damaged or absent neurons and glia of the ENS. This White Paper sets out experts' views on the commonly used methods and approaches to identify, isolate, purify, expand and optimize ENS stem cells, transplant them into the bowel, and assess transplant success, including restoration of gut function. We also highlight obstacles that must be overcome in order to progress from successful preclinical studies in animal models to ENS stem cell therapies in the clinic.

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

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

Simultaneous intracellular recordings from enteric neurons reveal that myenteric AH neurons transmit via slow excitatory postsynaptic potentials

Simultaneous intracellular electrical recordings were made from pairs of neurons separated circumferentially by 100-200 microns of the myenteric plexus of the guinea-pig ileum in vitro. The recording electrodes were filled with the dye neurobiotin which was injected into impaled nerve cells, and later revealed histochemically. Intracellular current pulses were used to evoke action potentials via the recording electrode in one type of myenteric neuron, in most cases an AH neuron, while a second electrode was used to record from a simultaneously impaled S neuron or AH neuron. AH neurons are thought to be primary sensory neurons, whereas S neurons are interneurons and motor neurons. Ninety pairs of neurons were adequately tested for interaction. From these, 17 S neurons and three AH neurons that responded to AH neuron stimulation were detected. In each case, the response was a slow depolarization that was seen only in response to a train of stimuli at 10 Hz. The slow depolarizations were enhanced by passing depolarizing current and diminished by hyperpolarization. Responses were also diminished by lowering external Ca.2+ and elevating Mg2+. In all cases in which intracellular recording indicated communication between neurons, morphological evidence of connection was seen. In no case was there communication without connection, but in four instances, morphological connections appeared to exist, although no physiological evidence of communication was obtained.

Electrophysiology and enkephalin immunoreactivity of identified myenteric plexus neurones of guinea-pig small intestine

Intracellular injection of the fluorescent dye, Lucifer Yellow CH, revealed the shapes of neurones in the myenteric plexus of the guinea-pig ileum, and these shapes were correlated with the electrophysiological properties and enkephalin immunoreactivity of the neurones. A total of eighty-three neurones were filled using electrodes containing a 5% solution of the dye. Forty-six cells had many short processes and a single long process (Dogiel type 1) and twenty-four cells had essentially smooth somas and one to eight long processes (Dogiel type II). Thirteen cells could not be put into either group. Enkephalin-like immunoreactivity was detected in twenty-two of the forty-six Dogiel type I cells. Eighteen of these had club-like short processes. No other cells of the eighty-three showed enkephalin-like immunoreactivity. Electrodes filled with a 0.5% solution of Lucifer Yellow in 0.5 M-KCl were used to record from and simultaneously to inject dye into 240 neurones. Eighty-six nerve cells had a slow after-hyperpolarization following the action potential (AH cells) and forty-six nerve cells had no after-hyperpolarization but exhibited a fast excitatory synaptic potential (S cells). The other cells could not be unequivocally identified by their observed electrophysiological characteristics. Almost all S cells (forty-two of forty-six) were Dogiel type I, while eighty-two of the eighty-six AH cells were Dogiel type II. Fifty S cells (eight located geometrically, forty-two by dye injection) and ninety-one AH cells (twenty-six located geometrically, sixty-five by dye injection) were examined for enkephalin immunoreactivity. Fifteen of the S cells were reactive, whereas all of the AH cells were unreactive. It appears that prolonged impalements reduce immunoreactivity so that the proportion of reactive neurones in this series is an underestimate of the true proportion of S cells with enkephalin-like immunoreactivity. The results suggest that a substantial proportion of the S cells in myenteric ganglia contain enkephalin immunoreactivity while none of the AH cells do. The enkephalin neurones have a distinctive shape and are all Dogiel type I cells. AH cells are nearly always Dogiel type II.

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)

Shapes of nerve cells in the myenteric plexus of the guinea-pig small intestine revealed by the intracellular injection of dye

The shapes of myenteric neurons in the guinea-pig small intestine were determined after injecting living neurons with the dye Lucifer yellow via a microelectrode. The cells were fixed and the distribution of Lucifer yellow rendered permanent by an immunohistochemical method. Each of 204 nerve cells was examined in whole-mount preparations of the myenteric plexus and drawn using a camera lucida at 1250 x magnification. Four cell shapes were distinguished: (1) neurons with several long processes corresponding to type II of Dogiel; (2) neurons with a single long process and lamellar dendrites corresponding to type I of Dogiel; (3) neurons with numerous filamentous dendrites; and (4) small neurons with few processes. About 15% of the neurons could not be placed into these classes or into any single class. The type II neurons (39% of the sample) had generally smooth somata and up to 7 (average 3.3) long processes, most of which ran circumferentially. Dogiel type I neurons (34% of sampled neurons) had characteristic lamellar dendrites, i.e., broad dendrites that were flattened in the plane of the plexus. The filamentous neurons (7% of the sample), had, on average, 14 fine processes up to about 50 microns in length. Small neurons with smooth outlines and a few fine processes made up 5% of the neurons encountered. We conclude that myenteric neurons that have been injected with dye can be separated into morphologically distinct classes and that the different morphological classes probably correspond to different functional groupings of neurons.