Intrinsic primary afferent neurons and nerve circuits within the intestine

Effects of modulators of Ca(2+)-activated, intermediate-conductance potassium channels on motility of the rat small intestine, in vivo

The movements of the intestine shift between different motor patterns, including between propulsion and mixing, but there is little information concerning mechanisms that may lead to changes in the patterns of motility. We have investigated the influence on intestinal motility of drugs that affect the after-hyperpolarization potential (AHP) of intrinsic primary afferent neurons (IPANs). The current of the AHP is carried by the intermediate conductance, calcium-activated, potassium (IK) channel. In anaesthetized rats, the IK channel blocker, 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (0.05-1 mg kg(-1), i.v.) disrupted the regular propulsive pressure waves that occur in the small intestine and reduced propulsion of the contents (after 1 mg kg(-1), the fluid propelled was <25% of control). If the propulsion in the intestine was regular, the IK channel opener, 5,6-dichloro-1-ethyl-2-benzimidazolinone (DC-EBIO, 0.1 mg kg(-1) h(-1)) had no effect. DC-EBIO (0.1 mg kg(-1) h(-1)) restored propulsive activity after the nitric oxide synthase inhibitor, Nomega-nitro-l-arginine had changed motility to a mixing pattern. We suggest that the AHP determines the synchrony of action potential firing in synaptically coupled IPANs, and that this synchrony influences the patterns of firing of muscle motor neurons, and hence the pattern of contraction of the muscle and whether the pattern is predominantly propulsive or predominantly mixing.

Effects of Compounds That Influence IK (KCNN4) Channels on Afterhyperpolarizing Potentials, and Determination of IK Channel Sequence, in Guinea Pig Enteric Neurons

The late afterhyperpolarizing potential (AHP) that follows the action potential in intrinsic primary afferent neurons of the gastrointestinal tract has a profound influence on their firing patterns. There has been uncertainty about the identity of the channels that carry the late AHP current, especially in guinea pigs, where the majority of the physiological studies have been made. In the present work, the late AHP was recorded with intracellular microelectrodes from myenteric neurons in the guinea pig small intestine. mRNA was extracted from the ganglia to determine the identity of the guinea pig intermediate conductance potassium ( IK) channel gene transcript. The late AHP was inhibited by two blockers of IK channels, TRAM34 (0.1–1 μM) and clotrimazole (10 μM), and was enhanced by the potentiator of the opening of these channels, DC-EBIO (100 nM). Action potential characteristics were unchanged by TRAM34 or DC-EBIO. The full sequence of the gene transcript and the deduced amino acid sequence were determined from extracts including myenteric ganglia and from bladder urothelium, which is a rich source of IK channel mRNA. This showed that the guinea pig sequence has a high degree of homology with other mammalian sequences but that the guinea pig channel lacks a phosphorylation site that was thought to be critical for channel regulation. It is concluded that the channels that carry the current of the late afterhyperpolarizing potential in guinea pig enteric neurons are IK channels.

Persistent alterations to enteric neural signaling in the guinea pig colon following the resolution of colitis

Functional changes induced by inflammation persist following recovery from the inflammatory response, but the mechanisms underlying these changes are not well understood. Our aim was to investigate whether the excitability and synaptic properties of submucosal neurons remained altered 8 wk post-trinitrobenzene sulfonic acid (TNBS) treatment and to determine whether these changes were accompanied by alterations in secretory function in submucosal preparations voltage clamped in Ussing chambers. Mucosal serotonin (5-HT) release measurements and 5-HT reuptake transporter (SERT) immunohistochemistry were also performed. Eight weeks after TNBS treatment, colonic inflammation resolved, as assessed macroscopically and by myeloperoxidase assay. However, fast excitatory postsynaptic potential (fEPSP) amplitude was significantly increased in submucosal S neurons from previously inflamed colons relative to those in control tissue. In addition, fEPSPs from previously inflamed colons had a hexamethonium-insensitive component that was not evident in age-matched controls. AH neurons were hyperexcitable, had shorter action potential durations, and decreased afterhyperpolarization 8 wk following TNBS adminstration. Neuronally mediated colonic secretory function was significantly reduced after TNBS treatment, although epithelial cell signaling, as measured by responsiveness to both forskolin and bethanecol in the presence of tetrodotoxin, was comparable with control tissue. 5-HT levels and SERT immunoreactivity were comparable to controls 8 wk after the induction of inflammation, but there was an increase in glucagon-like peptide 2-immunoreactive L cells. In conclusion, sustained alterations in enteric neural signaling occur following the resolution of colitis, which are accompanied by functional changes in the absence of active inflammation.

Is there an experimental basis for the development of ischaemic colitis as a result of 5-HT3 antagonist treatment?

5-HT(3) antagonists are effective treatments for chemotherapy-induced emesis and diarrhoea and urgency and pain associated with irritable bowel syndrome. Reports of ischaemic colitis led to restricted use of the approved drug, alosetron. This article briefly reviews the controversial information from epidemiology and adverse reaction reports and addresses the experimental basis for the development of ischaemic colitis as a result of 5-HT(3) antagonist treatment. The author reviews the potential factors based involved in the ischaemic colitis and ways in which this class of compound may influence those factors based on experimental evidence, including the literature on any vascular effects of these agents. Finally, the article addresses the theoretical basis for the constipation as a predisposing factor for the development of ischaemic colitis. The evidence reviewed suggests that further studies are needed to explore the principles to prove or disprove the association.

Taste receptors in the gastrointestinal tract. IV. Functional implications of bitter taste receptors in gastrointestinal chemosensing

Changes in the luminal contents of the gastrointestinal tract modulate gastrointestinal functions, including absorption of nutrients, food intake, and protection against harmful substances. The current notion is that mucosal enteroendocrine cells act as primary chemoreceptors by releasing signaling molecules in response to changes in the luminal environment, which in turn activate nerve terminals. The recent discovery that taste receptors and G protein subunits alpha-gustducin and alpha-transducin, involved in gustatory signal transduction, are expressed in the gastrointestinal mucosa supports the concept of a chemosensory machinery in the gastrointestinal tract. An understanding of luminal sensing processes responsible for the generation of the appropriate functional response to specific nutrients and nonnutrients is of clinical importance since aberrant or unsteady responses to changes in luminal contents might result in disease states ranging from intoxication to feeding disorders and inflammation. The purpose of this theme article is to discuss the functional implications of bitter taste signaling molecules in the gastrointestinal tract deduced by their localization in selected populations of epithelial cells and their relationship with neural pathways responsible for the generation of specific responses to luminal contents.

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.

Bone morphogenetic protein-mediated modulation of lineage diversification during neural differentiation of embryonic stem cells

Embryonic stem cells (ES cells) can give rise to a broad spectrum of neural cell types. The biomedical application of ES cells will require detailed knowledge on the role of individual factors modulating fate specification during in vitro differentiation. Bone morphogenetic proteins (BMPs) are known to exert a multitude of diverse differentiation effects during embryonic development. Here, we show that exposure to BMP2 at distinct stages of neural ES cell differentiation can be used to promote specific cell lineages. During early ES cell differentiation, BMP2-mediated inhibition of neuroectodermal differentiation is associated with an increase in mesoderm and smooth muscle differentiation. In fibroblast growth factor 2-expanded ES cell-derived neural precursors, BMP2 supports the generation of neural crest phenotypes, and, within the neuronal lineage, promotes distinct subtypes of peripheral neurons, including cholinergic and autonomic phenotypes. BMP2 also exerts a density-dependent promotion of astrocyte differentiation at the expense of oligodendrocyte formation. Experiments involving inhibition of the serine threonine kinase FRAP support the notion that these effects are mediated via the JAK/STAT pathway. The preservation of diverse developmental BMP2 effects in differentiating ES cell cultures provides interesting prospects for the enrichment of distinct neural phenotypes in vitro.

Subepithelial fibroblasts in intestinal villi: roles in intercellular communication

Ingestion of food and water induces chemical and mechanical signals that trigger peristaltic reflexes in the gut. Intestinal villi are motile, equipped with chemosensors and mechanosensors, and transduce signaling to sensory neurons, but the exact mechanisms have not yet been elucidated. Subepithelial fibroblasts located under the villous epithelium form contractile cellular networks via gap junctions. The networks ensheathe lamina propria and are in close contact with epithelium, neural and capillary networks, smooth muscles, and immune cells. Unique characteristics of subepithelial fibroblasts have been revealed by primary cultures isolated from rat duodenal villi. They include rapid reversal changes in cell shape by cAMP reagents and endothelins, cell shape-dependent mechanosensitivity that induces ATP release as a paracrine mediator, contractile ability, and expression of various receptors for vasoactive and neuroactive substances. Herein, we review these characteristics that play a key role in the villi. They serve as a barrier/sieve, flexible mechanical frame, mechanosensor, and signal transduction machinery in the intestinal villi, which are regulated locally and dynamically by rapid cell shape conversion.

Calcitonin Gene-Related Peptide: A Marker for Putative Primary Afferent Neurons in the Pig Small Intestinal Myenteric Plexus?

For years, calcitonin gene-related peptide (CGRP) has been used as a marker peptide for Dogiel type II neurons, putative intrinsic primary afferent neurons, in the pig enteric nervous system. Recently, some studies showed CGRP-positive neurons displaying distinctly different shapes. The aims of this study were to evaluate (1) the proportion of myenteric type II neurons that contain CGRP and (2) the proportion of myenteric CGRP-positive neurons that display type II vs. non-type II morphologies and to conclude if this peptide could be suited as a marker for type II neurons. For this purpose, nine myenteric whole-mounts (each one from duodenum, jejunum, and ileum, respectively, derived from three pigs) were triple-immunostained for CGRP, neurofilaments (NF), and choline acetyl transferase (ChAT). Each whole-mount was evaluated twice. First, 50 NF-stained type II neurons were selected randomly and their coreactivities for CGRP and ChAT were observed. Second, 50 CGRP-positive neurons were located randomly and their NF morphology and ChAT coreactivity were observed. Altogether, 92% of all type II neurons investigated displayed CGRP immunoreactivity, whereas 94.9% of all CGRP-reactive neurons recorded displayed type II morphology. We observed three further shapes of CGRP-positive neurons: 7 type V neurons (all were ChAT-positive; mainly in the ileal whole-mounts), 6 type I-like neurons (all were ChAT-positive), and 14 type III-like neurons (mostly ChAT-negative; mainly in duodenal and jejunal specimens). We conclude that CGRP-antibodies can be used as markers for type II neurons in the pig small intestinal myenteric plexus in quantitative studies but it should be kept in mind that up to one-tenth of CGRP-reactive neurons may be non-type II neurons. In case of single cell evaluation, CGRP-immunoreactivity alone is not suited as a marker. In such cases additional, morphological analysis is necessary.

Sensory transmission in the gastrointestinal tract

The gastrointestinal (GI) tract must balance ostensibly opposite functions. On the one hand, it must undertake the process of digestion and absorption of nutrients. At the same time, the GI tract must protect itself from potential harmful antigenic and pathogenic material. Central to these processes is the ability to 'sense' the mechanical and chemical environment in the gut wall and lumen in order to orchestrate the appropriate response that facilitates nutrient assimilation or the rapid expulsion through diarrhoea and/or vomiting. In this respect, the GI tract is richly endowed with sensory elements that monitor the gut environment. Enteric neurones provide one source of such sensory innervation and are responsible for the ability of the decentralized gut to perform complex reflex functions. Extrinsic afferents not only contribute to this reflex control, but also contribute to homeostatic mechanisms and can give rise to sensations, under certain circumstances. The enteric and extrinsic sensory mechanisms share a number of common features but also some remarkably different properties. The purpose of this review is to summarize current views on sensory processing within both the enteric and extrinsic innervation and to specifically address the pharmacology of nociceptive extrinsic sensory pathways.

Turkey breeder hen age affects growth and systemic and intestinal inflammatory responses in female poults examined at different ages posthatch

This trial examined the effect of 2 turkey breeder hen ages (33 or 55 wk of age) on performance, intestinal histology, and inflammatory immune response of female turkey poults grown to market weight. Using a randomized design, female poults were separated by breeder flock age (n = 8 floor pens/breeder flock age; n = 26 poults/pen; 0.195 m(2)/bird), fed identical commercial diets (9 phases), and grown to market weight (approximately 11.4 kg/ bird). At young ages, poults from the older breeder flock tended to have higher BW (P < 0.01 for d 7, P < 0.09 for d 63), although feed consumed was not significantly different due to breeder flock age (P > 0.20 for all ages). After approximately 63 d posthatch, no difference in BW was observed, suggesting that poults from the younger breeder flock were eventually able to compensate for initial reductions in performance. In addition to growth measurements on d 10, 24, and 65 posthatch, poults were vaccinated with lipopolysaccharide (LPS, from Salmonella Typhimurium; 0.5 mg/kg of BW intraabdominally) or not vaccinated (control), and intestinal histology and plasma haptoglobin were assessed at 24 h postadministration. In control birds, intestinal villus length was greater for poults from the older breeder flock (P < 0.05), as was crypt depth (P < 0.05 for d 11 and 25). Plasma haptoglobin levels did not change in 11-d-old poults after LPS administration, but they increased with LPS at d 25 and 66 posthatch (P < 0.05 for each). At d 66 posthatch, poults from the younger flock had increased haptoglobin levels post-LPS compared with those from the older breeder flock (P < 0.05). In general, LPS administration increased villus width in the jejunum and ileum (P < 0.05 for each), increased lamina propria width in the duodenum and ileum (P < 0.05 for each), and decreased ileum crypt depth (P < 0.05). Overall, poults from the older breeder flock had reduced inflammatory responses, even at 9 to 10 wk posthatch, even though performance was similar in poults from the 2 flocks by this age.

Cholinergic regulation of epithelial ion transport in the mammalian intestine

Acetylcholine (ACh) is critical in controlling epithelial ion transport and hence water movements for gut hydration. Here we review the mechanism of cholinergic control of epithelial ion transport across the mammalian intestine. The cholinergic nervous system affects basal ion flux and can evoke increased active ion transport events. Most studies rely on measuring increases in short-circuit current (ISC = active ion transport) evoked by adding ACh or cholinomimetics to intestinal tissue mounted in Ussing chambers. Despite subtle species and gut regional differences, most data indicate that, under normal circumstances, the effect of ACh on intestinal ion transport is mainly an increase in Cl- secretion due to interaction with epithelial M3 muscarinic ACh receptors (mAChRs) and, to a lesser extent, neuronal M1 mAChRs; however, AChR pharmacology has been plagued by a lack of good receptor subtype-selective compounds. Mice lacking M3 mAChRs display intact cholinergically-mediated intestinal ion transport, suggesting a possible compensatory mechanism. Inflamed tissues often display perturbations in the enteric cholinergic system and reduced intestinal ion transport responses to cholinomimetics. The mechanism(s) underlying this hyporesponsiveness are not fully defined. Inflammation-evoked loss of mAChR-mediated control of epithelial ion transport in the mouse reveals a role for neuronal nicotinic AChRs, representing a hitherto unappreciated braking system to limit ACh-evoked Cl- secretion. We suggest that: i) pharmacological analyses should be supported by the use of more selective compounds and supplemented with molecular biology techniques targeting specific ACh receptors and signalling molecules, and ii) assessment of ion transport in normal tissue must be complemented with investigations of tissues from patients or animals with intestinal disease to reveal control mechanisms that may go undetected by focusing on healthy tissue only.

Characterization of Myenteric Sensory Neurons in the Mouse Small Intestine

We recorded from myenteric AH/Dogiel type II cells, demonstrated mechanosensitive responses, and characterized their basic properties. Recordings were obtained using the mouse longitudinal muscle myenteric plexus preparation with patch-clamp and sharp intracellular electrodes. The neurons had an action potential hump and a slow afterhyperpolarization (AHP) current. The slow AHP was carried by intermediate conductance Ca2+-dependent K+-channel currents sensitive to charybdotoxin and clotrimazole. All possessed a hyperpolarization-activated current that was blocked by extracellular cesium. They also expressed a TTX-resistant Na+ current with an onset near the resting potential. Pressing on the ganglion containing the patched neuron evoked depolarizing potentials in 17/18 cells. The potentials persisted after synaptic transmission was blocked. Volleys of presynaptic electrical stimuli evoked slow excitatory postsynaptic potentials (EPSPs) in 9/11 sensory neurons, but 0/29 cells received fast EPSP input. The slow EPSP was generated by removal of a voltage-insensitive K+ current. Patch-clamp recording with a KMeSO4-containing, but not a conventional KCl-rich, intracellular solution reproduced the single-spike slow AHPs and low input resistances seen with sharp intracellular recording. Cell-attached recording of intermediate conductance potassium channels supported the conclusion that the single-spike slow AHP is an intrinsic property of intestinal AH/sensory neurons. Unitary current recordings also suggested that the slow AHP current probably does not contribute significantly to the high resting background conductance seen in these cells. The characterization of mouse myenteric sensory neurons opens the way for the study of their roles in normal and pathological physiology.

The distribution of intermediate-conductance, calcium-activated, potassium (IK) channels in epithelial cells

Intermediate-conductance, calcium-activated, potassium (IK) channels were first identified by their roles in cell volume regulation, and were later shown to be involved in control of proliferation of lymphocytes and to provide a K+ current for epithelial secretory activity. Until now, there has been no systematic investigation of IK channel localization within different epithelia. IK channel immunoreactivity was present in most epithelia, where it occurred in surface membranes of epithelial cells. It was found in all stratified epithelia, including skin, cornea, oral mucosa, vaginal mucosa, urothelium and the oesophageal lining. It occurred in the ducts of fluid-secreting glands, the salivary glands, lacrimal glands and pancreas, and in the respiratory epithelium. A low level of expression was seen in serous acinar cells. It was also found in other epithelia with fluid-exchange properties, the choroid plexus epithelium, the ependyma, visceral pleura and peritoneum, bile ducts and intestinal lining epithelium. However, there was little or no expression in vascular endothelial cells, kidney tubules or collecting ducts, lung alveoli, or in sebaceous glands. It is concluded that the channel is present in surface epithelia (e.g. skin) where it has a cell-protective role against osmotic challenge, and in epithelia where there is anion secretion that is facilitated by a K+ current-dependent hyperpolarization. It was also in some epithelial cells where its roles are as yet unknown.

Binding of isolectin IB4 to neurons of the mouse enteric nervous system

The plant lectin, IB4, binds to primary afferent neurons of dorsal root and trigeminal ganglia, where it is selective for nociceptive neurons. In the enteric nervous system of the guinea-pig IB4 labels intrinsic primary afferent neurons, which are believed to have roles as nociceptors. Here we investigate whether IB4 binding is also a marker of intrinsic primary afferent neurons in the mouse. Neurons that bound IB4 were common in the enteric plexuses of the small intestine and colon. Labeled neurons were rare in the stomach, and absent from the esophagus and gallbladder. Binding was to the cell surface, initial parts of axons and to clumps in the cytoplasm. Similar binding occurred on small and medium sized neurons of dorsal root, nodose and trigeminal ganglia. In the enteric nervous system, IB4 revealed large round or oval (type II) neurons, type I neurons with prominent laminar dendrites and small neurons of myenteric ganglia. The type II neurons were immunoreactive for calretinin, and some type I neurons were immunoreactive for nitric oxide synthase. Most neurons in the submucosal ganglia bound IB4, and some of these were vasoactive intestinal peptide immunoreactive. Thus IB4 binds to specific subgroups of enteric neurons in the mouse. These include intrinsic primary afferent neurons, but other neurons, including secretomotor neurons, are labeled. The results suggest that IB4 is not a specific label for enteric nociceptive neurons.

Regulation of the slow afterhyperpolarization in enteric neurons by protein kinase A

The slow after-hyperpolarization (sAHP) following the action potential is an important determinant of the firing patterns of enteric neurons. The channel responsible for the sAHP thus serves as a critical control point at which neurotransmitters and inflammatory mediators modulate gut motility. Many of these receptor-evoked pathways are known to inhibit the sAHP and, thus, excite enteric neurons. They act through protein kinase A (PKA) which is a strong inhibitor of the sAHP current while protein phosphatases enhance the current. Increasing evidence suggests that the sAHP is mediated by the opening of intermediate-conductance Ca-activated potassium (IK) channels. This neuronal IK channel, previously known to be expressed in a variety of non-excitable cells, is strongly influenced by protein kinases. Investigation of the molecular basis for the modulation of IK channels by protein phosphorylation indicates that there are multiple mechanisms of channel control. Inhibition of channel activity by PKA involves phosphorylation sites located within the calmodulin-binding domain of the channel. The localization of these sites within the region involved in Ca2+ activation suggests that PKA-mediated phosphorylation of the channel opposes the conformational changes caused by binding of Ca/calmodulin, which would otherwise lead to opening of the channel. We suggest that the channel exists as a macromolecular complex involving calmodulin, protein kinases, protein phosphatase and possibly other proteins. The regulation of the channel through kinases and phophatases results in exquisite control of neuronal firing and subsequent modulation of enteric reflexes.

Tachykinin Receptor Expression and Function in Human Esophageal Smooth Muscle

Tachykinins are present in enteric nerves of the gastrointestinal tract and cause contraction of esophageal smooth muscle; however, the mechanisms involved are not understood. Our aim was to characterize tachykinin signaling in human esophageal smooth muscle. We investigated functional effects of tachykinins on human esophageal smooth muscle using tension recordings and isolated cells, receptor expression with reverse transcription (RT)-polymerase chain reaction (PCR) and immunoblotting, intracellular Ca2+ responses using fluorescent indicator dyes, and membrane currents with patch-clamp electrophysiology. The mammalian tachykinins [substance P and neurokinin (NK) A and NKB] elicited concentration-dependent contractions of human esophageal smooth muscle. These responses were not affected by muscarinic receptor or neuronal blockade indicating a direct effect on smooth muscle cells (SMCs). Immunofluorescence and RT-PCR identified tachykinin receptors (NK1, NK2, and NK3) on SMCs. Contraction was mediated through a combination of Ca2+ release from intracellular stores and influx through L-type Ca2+ channels. NK2 receptor blockade inhibited the largest proportion of tachykinin-evoked responses. NKA evoked a nonselective cation current (I(NSC)) with properties similar to that elicited by muscarinic stimulation. The following paradigm is suggested: tachykinin receptor binding to SMCs releases Ca2+ from stores along with activation of I(NSC), which in turn results in membrane depolarization, L-type Ca2+ channel opening, rise of Ca2+ concentration, and contraction. These studies reveal new aspects of tachykinin signaling in human esophageal SMCs. Excitatory tachykinin pathways may represent targets for pharmacological intervention in disorders of esophageal dysmotility.

Origin and mediation of secretion induced by oral phytohaemagglutinin (PHA) in rats

After oral administration several gut-binding lectins induce accumulation of liquor and amylase in the proximal small intestine. Orally administered Phaseolus vulgaris phytohaemagglutinin (PHA) was used to study the mediation of these effects in rats. The regulation of amylase secretion clearly differed from that of the liquor. The amylase activity was of pancreatic origin, in agreement with the known cholecystokinin-releasing effect of PHA. It appears that CCK exerts its effect both directly and by facilitating neural stimulatory pathways. Intestinal secretion was identified as the source of the liquor, without a contribution by other secretions. It was mediated by a local cholinergic reflex with the involvement of both muscarinic and nicotinic acetylcholine receptors. It is speculated that the observed enteric reflex may enable the gut to transport secreted antibacterial peptides or secretory antibodies from the crypts to adherent bacteria on adjacent villi.

Novel gut afferents: Intrinsic afferent neurons and intestinofugal neurons

Information about the conditions of all tissues in the body is conveyed to the central nervous system through afferent neurons. Uniquely amongst peripheral organs, the intestine has numerous additional afferent neurons, intrinsic primary afferent neurons that have their cell bodies and processes in the enteric plexuses and do not project to the central nervous system. They detect conditions within the gut and convey that information to intrinsic reflex pathways that are also entirely contained inside the gut wall. Intrinsic primary afferent neurons respond both to the presence of material in the gut lumen and to distension of the gut wall and initiate reflex changes in contractile activity, fluid transport across the mucosa and local blood flow. They also function as nociceptors that initiate tissue-protective propulsive and secretory reflexes to rid the gut of pathogens. The regulation of excitability of intrinsic primary afferent neurons is through multiple ion channels and ion channel regulators, and their excitability is critical to setting the strength of enteric reflexes. The intestine also provides afferent signals to sympathetic pre-vertebral ganglia. The signals are conveyed from the gut by intestinofugal neurons that have their cell bodies within enteric ganglia and form synapses in the sympathetic ganglia. Intestinofugal neurons form parts of the afferent limbs of entero-enteric inhibitory reflexes. Because the unusual afferent neurons of the small intestine and colon make their synaptic connections outside the central nervous system, the neurons and the reflex centres that they affect are potential targets for non-central penetrant therapeutic compounds.

Identification of neurons that express 5-hydroxytryptamine4 receptors in intestine

5-Hydroxytryptamine (5-HT) is an endogenous stimulant of intestinal propulsive reflexes. It exerts its effects partly through 5-HT4 receptors; 5-HT4 receptor agonists that are stimulants of intestinal transit are in clinical use. Both pharmacological and recent immunohistochemical studies indicate that 5-HT4 receptors are present on enteric neurons but the specific neurons that express the receptors have not been determined. In the present work, we describe the characterization of an anti-5-HT4 receptor antiserum that reveals immunoreactivity for enteric neurons and other cell types in the gastrointestinal tract. With this antiserum, 5-HT4 receptor immunoreactivity has been found in the muscularis mucosae of the rat oesophagus, a standard assay tissue for 5-HT4 receptors. It is also present in the muscularis mucosae of the guinea-pig and mouse oesophagus. In guinea-pig small intestine and rat and mouse colon, 5-HT4 receptor immunoreactivity occurs in subpopulations of enteric neurons, including prominent large neurons. Double-staining has shown that these large neurons in the guinea-pig small intestine are also immunoreactive for two markers of intrinsic primary afferent neurons, cytoplasmic NeuN and calbindin. Some muscle motor neurons in the myenteric ganglia are immunoreactive for this receptor, whereas it is rarely expressed by secretomotor neurons. Immunoreactivity also occurs in the interstitial cells of Cajal but is faint in the external muscle. Expression of the protein and mRNA has been confirmed in extracts containing enteric neurons. The observations suggest that one site of action of 5-HT4 receptor agonists is the intrinsic primary afferent neurons.

Effects of gastrointestinal inflammation on enteroendocrine cells and enteric neural reflex circuits

Inflammation of the gastrointestinal (GI) tract has pronounced effects on GI function. Many of the functions of the GI tract are subject to neural regulation by the enteric nervous system (ENS) and its extrinsic connections. Therefore, it is possible that inflammatory effects on the ENS contribute to altered function during GI inflammation. The reflex circuitry of the ENS is comprised of sensory transducers in the mucosa (enteroendocrine cells), afferent neurons, interneurons and motor neurons. This review focuses on recent data that describe inflammation-induced changes to the ENS and mucosal enteroendocrine cells. Studies of tissues from patients with inflammatory bowel disease (IBD) and from animal models of IBD have demonstrated marked changes in mucosal enteroendocrine cell signaling. These changes, which have been studied most intensely in 5-HT-containing enterochromaffin cells, involve changes in the number of cells, their signaling molecule content or their means of signal termination. Morphological evidence of enteric neuropathy during inflammation has been obtained from human samples and animal models of IBD. The neuropathy can reduce the number of enteric neurons in the inflamed region and is often accompanied by a change in the neurochemical coding of enteric neurons, both in the inflamed region and at distant sites. Electrophysiological recordings have been made from enteric neurons in inflamed regions of the colon of animal models of IBD. These studies have consistently found that inflammation increases excitability of intrinsic primary afferent neurons and alters synaptic transmission to interneurons and motor neurons. These data set the stage for a comprehensive examination of the role of altered neuronal and enteroendocrine cell signaling in symptom generation during GI inflammation.

Histopathological and immunohistochemical study of the enteric innervations among various types of aganglionoses including isolated and syndromic Hirschsprung disease

We investigated enteric innervations in 15 isolated and five syndromic cases of Hirschsprung disease (HSCR) with immunohistochemistry for the S100 protein (S100), class III a-tubulin (TUJ1), peripherin, neuronal nitric oxide synthase (nNOS) and CD34. The number of neurites per smooth muscle unit of the circular muscle layer (CML) was counted in the longitudinal sections. TUJ1 was the best marker to detect whole neuritic networks of the enteric nervous system. There were differences in the innervation patterns between isolated rectosigmoid aganglionosis (RS) and long segment aganglionosis (LS) including total colonic aganglionosis and extensive aganglionosis. In the aganglionic bowel (AGB) of LS, no nerve fibers innervated smooth muscle units in the CML in the area from the small bowel to the terminal descending colon. In the rectosigmoid region of every type of isolated HSCR, we observed transmural nerve fibers forming meshworks in the CML with TUJ1 and S100 antibodies. In RS, the neurites running parallel with smooth muscle cells gradually decreased in number in the distal portion. However, in the rectosigmoid AGB in LS, those neurites were absent and most neurites perpendicularly crossed the CML. Hypertrophic nerve trunks (HNT) in the submucous and myenteric plexuses were observed more frequently in the rectosigmoid region than in the rostral portion. Based on these data, it is suggested that the neuritic meshworks in the CML of the rectosigmoid AGB might derive from not only the sacral plexus, via HNT, but also intrinsic neurons in the oligoganglionic bowel. All of the syndromic HSCR were RS. In the AGB of RS with Down syndrome, the distribution of neurite meshworks in the CML is markedly reduced. In the AGB of RS with mental retardation suspected of having Mowat-Wilson syndrome, the density of intramuscular innervation was comparatively higher. In the rostral portion to the AGB of syndromic HSCR, myenteric ganglia were clearly small in size, and more numerous per smooth muscle unit with scarce internodal strands. These dysplastic features fall under neither hyperganglionosis nor hypoganglionosis classifications. We considered that syndromic HSCR might occur on the basis of a dysplastic enteric nervous system caused by genetic alteration.

Sensory collaterals, intramural ganglia and motor nerves in the guinea-pig bladder: evidence for intramural neural circuits

The afferent output from the bladder is important for triggering micturition. This study identifies different types of afferent nerve and explores the connections of their collateral fibres on intramural ganglia and potential ganglionic targets. The experiments were performed on tissues from male guinea-pigs (n=16). Fibres positive for choline acetyl transferase (ChAT(+)) were found to originate close to the urothelium, to transit the sub-urothelial interstitial cell layer and to pass into the lamina propria. A different population of fibres, immunopositive for calcitonin gene-related peptide (CGRP), capsaicin receptors or neurofilament protein (NF), were seen to intertwine with the ChAT(+) fibres in the lamina propria. The ChAT(+) fibres did not express NF. Ganglia with ChAT(+) and NF(+) neurones were found in the lamina propria and muscle. ChAT(+) fibres, with pronounced terminal varicosities, were present on the nerve cell bodies. Two types were noted: NF(+) terminals and those with little or no NF (NF(-)) suggesting that their origins were the ChAT(+) afferent collaterals and the adjacent ganglia. Fibres containing CGRP or substance P were seen on the ganglionic cells. alpha1B adrenergic receptors were also found on the neurones indicative of adrenergic synapses. Thus, the ganglia had multiple inputs. Different types of ChAT(+) nerves were seen in the muscle: NF(+) and NF(-). The ChAT(+)/NF(+) nerves may represent a ganglionic output to the muscle. This complex neuronal network may therefore represent the elements generating and modulating bladder sensations. The role of such a scheme in bladder pathology and the therapeutic sites of action of anticholinergic and sympathomimetic drugs are discussed.

The regulation of veratridine-stimulated electrogenic ion transport in mouse colon by neuropeptide Y (NPY), Y1 and Y2 receptors

1 Neuropeptide Y (NPY) is a prominent enteric neuropeptide with prolonged antisecretory effects in mammalian intestine. Veratridine depolarises neurons consequently causing epithelial anion secretion across mouse colon mucosa. Our aim was to characterise functionally, veratridine-stimulated mucosal responses and to determine the roles for NPY, Y(1), and Y(2) receptors in modulating these neurogenic effects. 2 Colon mucosae (with intact submucous innervation) from wild-type mice (+/+) and knockouts lacking either NPY (NPY-/-), Y(1)-/- or Y(2)-/- were placed in Ussing chambers and voltage clamped at 0 mV. Veratridine-stimulated short-circuit current (I(sc)) responses in +/+, Y(1) or Y(2) antagonist pretreated +/+ colon, Y(1)-/- and NPY-/- colon were insensitive to cholinergic blockade by atropine (At; 1 microM) and hexamethonium (Hex; 10 microM). Tetrodotoxin (TTX, 100 nM) abolished veratridine responses, but had no effect upon carbachol (CCh) or vasoactive intestinal polypeptide (VIP)-induced secretory responses. 3 To establish the functional roles for Y(1) and Y(2) receptors, +/+ tissues were pretreated with either the Y(1) or Y(2) receptor antagonist (BIBO3304 (300 nM) or BIIE0246 (1 microM), respectively) and veratridine responses were compared with those from Y(1)-/- or Y(2)-/- colon. Neither BIBO3304 nor Y(1)-/- altered veratridine-induced secretion, but Y(1) agonist responses were abolished in both preparations. In contrast, the Y(2) antagonist BIIE0246 significantly amplified veratridine responses in +/+ mucosa. Unexpectedly, NPY-/- colon exhibited significantly attenuated veratridine responses (between 1 and 5 min). 4 We demonstrate that electrogenic veratridine responses in mouse colon are noncholinergic and that NPY can act directly upon epithelia, a Y(1) receptor effect. The enhanced veratridine response observed in +/+ tissue following BIIE0246, indicates that Y(2) receptors are located on submucosal neurons and that their activation by NPY will inhibit enteric noncholinergic secretory neurotransmission. 5 We also demonstrate Y(1) and Y(2) receptor-mediated antisecretory tone in +/+ colon and show selective loss of each in Y(1) and Y(2) null colon respectively. In NPY-/- tissue, only Y(1)-mediated tone was present, this presumably being mediated by endogenous endocrine peptide YY. Y(2) tone was absent from NPY-/- (and Y(2)-/-) colon and we conclude that NPY activation of neuronal Y(2) receptors attenuates secretory neurotransmission thereby providing an absorptive electrolyte tone in isolated colon.

Inhibition of protein kinase A in murine enteric neurons causes lethal intestinal pseudo-obstruction

A number of in vitro studies suggest that many important developmental and functional events in the enteric nervous system are regulated by the intracellular signaling enzyme cAMP protein kinase A (PKA). To evaluate the in vivo significance of these observations, a Cre-inducible, dominant-negative, mutant regulatory subunit (RIalphaB) of PKA was activated in enteric neurons by either a Proteolipid protein-Cre transgene or a Hox11L1-Cre "knock-in" allele. In both models, RIalphaB activation resulted consistently in profound distension of the proximal small intestine within 2 weeks after birth. Intestinal transit of radio-opaque tracers was severely retarded in the double-transgenic animals, which died shortly after weaning. In the enteric nervous system, recombination was restricted to neurons as demonstrated by histochemical analysis and confocal microscopic colocalization of a Cre recombinase-dependent reporter gene with the neuronal marker Hu(C/D), in contrast with the glial marker S100. Histochemical analysis of beta-galactosidase expression and acetylcholinesterase activity, as well as neuronal counts, demonstrated that intestinal dysmotility was not associated with obvious malformation of the myenteric plexus. However, inhibition of PKA activity in enteric neurons disrupted the major motor complexes of isolated intestinal segments in vitro. These results provide strong evidence that PKA activity plays a critical role in enteric neurotransmission in vivo, and highlight neuronal PKA or related signaling molecules as potential therapeutic targets in gastrointestinal motility disorders.

Propionate modulates spontaneous contractions via enteric nerves and prostaglandin release in the rat distal colon

Short-chain fatty acids, such as propionate and acetate, are produced by a bacterial fermentation of carbohydrates in the colonic lumen. We examined the effects of propionate on the frequency and mean amplitude of spontaneous giant contractions (GCs) in circular muscle strips of the rat distal colon with the mucosa attached. An addition of propionate increased the frequency of GCs for about 20 min (> or =1 mm), but the mean amplitude was decreased (> or =0.1 mm). The propionate-induced increase in the frequency of GCs was blocked by the muscarinic acetylcholine receptor antagonist, atropine. In contrast, the nicotinic receptor antagonist, hexamethonium, augmented the response. The propionate-induced decrease in the mean amplitude of GCs was prevented by the cyclooxygenase inhibitor, piroxicam. A pretreatment of the tissues with acetate prevented the propionate-induced modulations of the frequency and amplitude of GCs. These results suggest that propionate increases the frequency of GCs by an activation of cholinergic motor neurons and decreases the mean amplitude by a prostaglandin release. Propionate as well as acetate may be involved in the regulation of spontaneous circular muscle activity in the rat distal colon.

Cytoplasmic, but not nuclear, expression of the neuronal nuclei (NeuN) antibody is an exclusive feature of Dogiel type II neurons in the guinea-pig gastrointestinal tract

This study aimed to reveal if NeuN, a neuronal nuclei (NeuN) antibody, is a selective marker of intrinsic primary afferent neurons (IPANs) in the guinea-pig gastrointestinal tract as previously hypothesised. The NeuN immunoreactivity was found in the enteric nervous system with exception of the esophagus. Two groups of NeuN-expressing neurons were observed: neurons with immunostained nuclei and cytoplasm (NeuN(NC)) and neurons only expressing immunoreactivity in their nuclei (NeuN(N)). The NeuN(N)-immunoreactive neurons were found in the myenteric plexus of the stomach and the colon. In the stomach, none of the NeuN(N)-expressing neurons, of which 55+/-3% co-expressed calbindin, had a Dogiel type I or II morphology. The NeuN(N)-positive neurons of the colon, which did not express calbindin, did not resemble a Dogiel type II morphology either, but were small-sized neurons. The NeuN(NC)-immunoreactive neurons were observed in both the small and large intestine. These neurons were smooth-contoured and bigger-sized, resembling a Dogiel type II morphology. Some of these neurons co-expressed calbindin. The present data reveal the existence of two populations of Dogiel type II neurons, exhibiting NeuN(NC)+/calbindin+ or NeuN(NC)+/calbindin- immunoreactivity, in the intestine. Assuming that all IPANs exhibit a Dogiel type II morphology, we conclude that the cytoplasmic expression of NeuN is an exclusive feature of IPANs.

Sensory peptide neurotransmitters mediating mucosal and distension evoked neural vasodilator reflexes in guinea pig ileum

The aim was to determine the role CGRP and/or tachykinins released from sensory neural mechanisms in enteric neural vasodilator pathways. These pathways project through the myenteric plexus to submucosal vasodilator neurons. Submucosal arterioles were exposed in the distal portion of an in vitro combined submucosal-myenteric guinea pig ileal preparation, and dilation was monitored with videomicroscopy. Vasodilator neural reflexes were activated by gently stroking the mucosa with a fine brush or by distending a balloon placed beneath the flat-sheet preparation in the proximal portion. Dilations evoked by mucosal stroking were inhibited 64% by the CGRP 8-37 and 37% by NK3 (SR 142801) antagonists. When the two antagonists were combined with hexamethonium, only a small vasodilation persisted. Balloon distension-evoked vasodilations were inhibited by NK3 antagonists (66%) but were not altered by CGRP 8-37. In preparations in which myenteric descending interneurons were directly activated by electrical stimulation, combined application of CGRP 8-37 and the NK antagonists had no effect. Stimulation of capsaicin sensitive nerves in the myenteric plexus did not activate these vasodilator reflexes. These findings suggest that mucosal-activated reflexes result from the release of CGRP and tachykinins from enteric sensory neurons. Distension-evoked responses were significantly blocked by NK3 antagonists, suggesting that stretch activation of myenteric sensory neurons release tachykinins that activate NK3 receptors on myenteric vasodilator pathways.

Review article: endocannabinoids and their receptors in the enteric nervous system

The therapeutic actions of cannabinoids have been known for centuries. In the last 25 years this area of research has grown exponentially with the discovery of specific cannabinoid receptors and endogenous ligands. In the enteric nervous system of gastrointestinal tract, cannabinoid receptors are located on enteric nerve terminals where they exert inhibitory actions on neurotransmission to reduce motility and secretion. Endogenous cannabinoids are present in the enteric nervous system, as are the degradative enzymes necessary to inhibit their action. The cellular mechanism of action of endocannabinoids has not been established in the enteric nervous system. Endocannabinoids not only act at cannabinoid receptors, but potentially also at vanilloid and 5-HT3 receptors, both of which are expressed in the gastrointestinal tract. The interactions between endocannabinoids and these other important receptor systems have not been extensively investigated. A greater understanding of the endocannabinoid system in the enteric nervous system could lead to advances with important therapeutic potential in the treatment of gastrointestinal disorders such as irritable bowel syndrome, inflammatory bowel disease, secretory diarrhoea and gastro-oesophageal reflux disease.

Extrinsic and intrinsic sources of calcitonin gene-related peptide immunoreactivity in the lamb ileum: a morphometric and neurochemical investigation

To investigate extrinsic origins of calcitonin gene-related peptide immunoreactive (CGRP-IR) nerve fibres in the sheep ileum, the retrograde fluorescent tracer Fast Blue (FB) was injected into the ileum wall. Sections of thoraco-lumbar dorsal root ganglia (DRG) and distal (nodose) vagal ganglia showing FB-labelled neurons were processed for CGRP immunohistochemistry. The distribution of CGRP-IR in fibres and nerve cell bodies in the ileum was also studied. CGRP-IR enteric neurons were morphometrically analysed in myenteric (MP) and submucosal plexuses (SMP) of lambs (2-4 months). Sensory neurons retrogradely labelled with FB were scattered in T5-L4 DRG but most were located at the upper lumbar levels (L1-L3); only a minor component of the extrinsic afferent innervation of the ileum was derived from nodose ganglia. In the DRG, 57% of retrogradely labelled neurons were also CGRP-IR. In cryostat sections, a dense network of CGRP-IR fibres was observed in the lamina propria beneath the epithelium, around the lacteals and lymphatic follicles (Peyer's platches), and along and around enteric blood vessels. Rare CGRP-IR fibres were also present in both muscle layers. Dense pericellular baskets of CGRP-IR fibres were observed around CGRP-negative somata. The only CGRP-IR nerve cells were well-defined Dogiel type II neurons localised in the MP and in the external and internal components of the SMP. CGRP-IR neurons in the myenteric ganglia were significantly larger than those in the submucosal ganglia (mean profile areas: about 1,400 mum(2) for myenteric neurons, 750 mum(2) for submucosal neurons). About 6% of myenteric neurons and 25% of submucosal neurons were CGRP-IR Dogiel type II neurons. The percentages of CGRP-IR neurons that were also tachykinin-IR were about 9% (MP) and 42% (SMP), whereas no CGRP-IR neurons exhibited immunoreactivity for vasoactive intestinal peptide, nitric oxide synthase or tyrosine hydroxylase in either plexus. Thus, CGRP immunoreactivity occurs in the enteric nervous system of the sheep ileum (as in human small intestine and MP of pig ileum) in only one morphologically defined type of neuron, Dogiel type II cells. These are probably intrinsic primary afferent neurons.

Motilin inhibits ganglionic transmission in the myenteric plexus of the guinea-pig ileum

Motilin is a key factor in triggering interdigestive migrating contractions. Our preceding study demonstrated that motilin caused membrane depolarizations in a minority of S and AH neurons in the myenteric plexus of the guinea-pig ileum after 18 h-fasting period; motilin depolarizations were small and seldom triggered action potentials. Then, the present study was undertaken to examine possible electrophysiological actions of motilin on the ganglionic transmission in the myenteric plexus. Intracellular recordings with sharp glass microelectrodes were made from myenteric S neurons having fast excitatory postsynaptic potentials (EPSPs), evoked by focal electrical stimulation. Motilin inhibited the fast EPSPs in amplitude, associated either with or without membrane depolarizations. Results obtained with the paired stimulus method suggested that the site for motilin-induced inhibition of fast EPSPs might be presynaptic. Furthermore, motilin did not decrease postsynaptic sensitivity to ACh, a main neurotransmitter mediating the fast EPSPs. Therefore, it is concluded that motilin might inhibit presynaptically ganglionic transmission in the myenteric plexus of the guinea-pig ileum.

Actions of orexins on individual myenteric neurons of the guinea-pig ileum: orexin A or B?

The actions of orexins (orexin A and B, 10-300 nM) on individual myenteric neurons of the guinea-pig ileum in vitro were compared using intracellular recording methods. Both orexins caused membrane depolarizations associated with an increase in input neuronal resistance in S and AH neurons via a direct action. Orexin depolarizations reversed at about -90 mV, indicating they were due to an inactivation of K+ channels. Orexins facilitated fast excitatory postsynaptic potentials without affecting postsynaptic sensitivity to acetylcholine and adenosine 5'-triphosphate, indicating that the peptides may facilitate ganglionic transmission by increasing presynaptic release of neurotransmitters. Orexin B was sometimes more effective than orexin A and vice versa. It is concluded that orexin B increased neuronal activity via mechanisms similar to orexin A in the guinea-pig myenteric plexus.

Neural and non-neural mediation of propionate-induced contractile responses in the rat distal colon

Short-chain fatty acids (SCFAs), including propionate, butyrate and acetate, are fermentation products of carbohydrates in the colon. We investigated the contractile effects of SCFAs on the rat distal colon. Mechanical activity of the circular muscle in strip preparations was recorded in vitro. Propionate and butyrate concentration-dependently (10 micromol L(-1)-10 mmol L(-1)) induced rapid, large amplitude phasic contractions (the first phase) followed by tonic contractions (the second phase). Acetate itself had no effect on muscle activity, although preincubation with acetate attenuated both phases of the propionate-induced response. The propionate-induced phasic contraction was attenuated by atropine, tetrodotoxin and the 5-HT4 receptor antagonist SB-204070. The propionate-induced tonic contraction was attenuated by the cyclo-oxygenase inhibitor piroxicam. Antagonists of 5-HT1A, 5-HT2A and 5-HT3 receptors had no effect on the responses. Propionate-induced responses were not observed in mucosa-free preparations. These results suggest that propionate acts on receptors in the mucosa causing the release of 5-HT from enterochromaffin cells. 5-HT acts through 5-HT4 receptors on the endings of intrinsic primary afferent neurones that in turn activate cholinergic motor neurones that contract the circular muscle. Propionate also causes tonic contraction, via prostaglandin release, in the rat distal colon.

Transcriptional regulation of inflammatory mediators secreted by human colonic circular smooth muscle cells

We investigated the transcriptional regulation of secretion of pro- and anti-inflammatory mediators by human colonic circular smooth muscle cells (HCCSMC) in response to tumor necrosis factor (TNF)-alpha. Gene chip array analysis indicated that HCCSMC express a specific panel of 11 cytokines, chemokines, and cell adhesion molecules in a time-dependent manner in response to TNF-alpha. The chip array data were supported by quantitative analysis of mRNA and protein expressions of interleukin (IL)-6, IL-8, intercellular adhesion molecule (ICAM)-1 and IL-11. The proinflammatory mediators were expressed early, whereas the anti-inflammatory cytokine IL-11 was expressed late after TNF-alpha treatment. The expression of ICAM-1 on HCCSMC increased lymphocyte adhesion to these cells, which was blocked by pretreatment with antibody to ICAM-1. TNF-alpha acted on both R(1) and R(2) receptors to induce the expression of ICAM-1. Pretreatment of HCCSMC with antisense oligonucleotides to p65 nuclear factor-kappaB (NF-kappaB) blocked the expression of ICAM-1, whereas pretreatment with antisense oligonucleotides to p50 NF-kappaB had little effect. The overexpression of p65 NF-kappaB enhanced the constitutive expression of ICAM-1, and TNF-alpha treatment had no further effect. The delayed expression of endogenous IL-11 limited the expression of ICAM-1, and pretreatment of HCCSMC with antisense oligonucleotides to IL-11 enhanced it. We conclude that TNF-alpha induces gene expression in HCCSMC for programmed synthesis and release of pro- and anti-inflammatory mediators.

Characteristics of subepithelial fibroblasts as a mechano-sensor in the intestine: cell-shape-dependent ATP release and P2Y1 signaling

Subepithelial fibroblasts form a cellular network just under the epithelium of the gastrointestinal tract. Using primary cultured cells isolated from rat duodenal villi, we previously found that subepithelial fibroblasts reversibly changed cell morphology between flat and stellate-shape depending on intracellular cAMP levels. In this paper, we examined cell-cell communication via released ATP and Ca2+ signaling in the cellular network. Subepithelial fibroblasts were sensitive to mechanical stress such as ;touching' a cell with a fine glass rod and ;stretching' cells cultured on elastic silicone chamber. Mechanical stimulations evoked Ca2+-increase in the cells and ATP-release from the cells. The released ATP activated P2Y receptors on the surrounding cells and propagated Ca2+-waves through the network. Concomitant with Ca2+-waves, a transient contraction of the network was observed. Histochemical, RT-PCR, western blotting and Ca2+ response analyses indicated P2Y1 is a dominant functional subtype. ATP-release and Ca2+ signaling were cell-shape dependent, i.e. they were abolished in stellate-shaped cells treated with dBcAMP, and recovered or further enhanced in re-flattened cells treated with endothelin. The response to ATP also decreased in stellate-shaped cells. These findings indicate cAMP-mediated intracellular signaling causes cell-shape change, which accompanies the changes in mechano- and ATP sensitivities. Using a co-culture system of neuronal cells (NG108-15) with subepithelial fibroblasts, we confirmed that mechanically induced Ca2+-waves propagated to neurons. From these findings we propose that subepithelial fibroblasts work as a mechanosensor in the intestine. Uptake of food, water and nutrients may cause mechanical stress on subepithelial fibroblasts in the villi. The ATP released by mechanical stimulation elicits Ca2+-wave propagation through the network via P2Y1 activation and also activates P2X on terminals of mucosal sensory neurons to regulate peristaltic motility.

N-methyl-D-aspartate receptors mediate endogenous opioid release in enteric neurons after abdominal surgery

BACKGROUND & AIMS

We tested the hypothesis that N-methyl-D-aspartate (NMDA) receptors mediate surgery-induced opioid release in enteric neurons.

METHODS

We used mu opioid receptor (muOR) internalization as a measure of opioid release with immunohistochemistry and confocal microscopy. MuOR internalization was quantified in enteric neurons from nondenervated and denervated ileal segments of guinea pig after abdominal laparotomy with and without pretreatment with NMDA-receptor antagonists acting at different recognition sites (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,b] cyclohepten-5,10-imine (MK-801) or (D) 2-amino-5-phosphopenoic acid (AP-5) at .5, 1 mg/kg; 8-chloro-4-hydroxy-1-oxo-1,2-dihydropyridazinol [4,5-]quinoline-5-oxide choline (MRZ 2/576) or 8-chloro-1,4-dioxo-1,2,3,4-tetrahydropyridazinol [4,5-]quinoline choline salt (MRZ 2/596) at .3, 1 mg/kg, or with an antagonist for the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, 6-cyano-7-nitroquinoxaline-2,3-dione (1, 3 mg/kg). To determine whether NMDA stimulation induces opioid release, (1) ilea were exposed to NMDA (100 micromol/L) and D-serine (10 micromol/L) with or without the antagonist MK-801 or AP-5 (50 micromol/L); and (2) neuromuscular preparations of the ileum were stimulated electrically (20 Hz, 20 min) with or without MK-801 or AP-5 (50 micromol/L).

RESULTS

MuOR endocytosis induced by abdominal laparotomy was inhibited significantly by NMDA-receptor antagonists in nondenervated and denervated ileal segments, but not by the AMPA-receptor antagonist. MuOR endocytosis in neurons exposed to NMDA or electrical stimulation was prevented by NMDA-R antagonists.

CONCLUSIONS

Abdominal laparotomy evokes local release of glutamate that results in endogenous opioid release through the activation of peripheral NMDA receptors. This suggests an interaction between the glutamatergic and opioid systems in response to the noxious and perhaps mechanosensory stimulation of surgery.

Primary afferent neurons intrinsic to the guinea-pig intestine, like primary afferent neurons of spinal and cranial sensory ganglia, bind the lectin, IB4

The plant lectin, IB4, binds to the surfaces of primary afferent neurons of the dorsal root and trigeminal ganglia and is documented to be selective for nociceptive neurons. Physiological data suggest that the intrinsic primary afferent neurons within the intestine are also nociceptors. In this study, we have compared IB4 binding to each of these neuron types in the guinea-pig. The only neurons in the intestine to be readily revealed by IB4 binding have Dogiel-type-II morphology; these neurons have been previously identified as intrinsic primary afferent neurons. Most of the neurons that are IB4-positive in the myenteric plexus are calbindin-immunoreactive, whereas those in the submucosal ganglia are immunoreactive for NeuN. The neurons that bind IB4 strongly have a similar appearance in enteric, dorsal root and trigeminal ganglia. Binding is to the cell surface, to the first part of axons and to cytoplasmic organelles. A low level of binding was found in the extracellular matrix. A few other neurons in all ganglia exhibit faint staining with IB4. Strongly reactive neurons are absent from the gastric corpus. Thus, IB4 binding reveals primary afferent neurons with similar morphologies, patterns of binding and physiological roles in enteric, dorsal root and trigeminal ganglia.

A quantitative approach to recording peristaltic activity from segments of rat small intestine in vivo

We have developed methods that allow correlation of propulsive reflexes of the intestine with measurements of intraluminal pressure, fluid movement and spatio-temporal maps of intestinal wall movements for the first time in vivo. A segment of jejunum was cannulated and set up in a Trendelenburg recording system while remaining connected to the vascular and nerve supply of the anaesthetized rat. The resting intraluminal pressure in intact intestine was 2-4 mmHg. Hydrostatic pressures of 2, 4, 8 and 16 mmHg were imposed. At a baseline pressure of 4 mmHg, propulsive waves generated pressures of 9 +/- 1 mmHg, that progressed oral to anal at 2-5 mm s(-1). Individual propulsive waves propelled 0.8 +/- 0.4 mL of fluid. The frequency of propulsive waves increased with pressure, but peristaltic efficiency (mL per contraction) decreased with pressure increase between 4 and 16 mmHg. Atropine, as a bolus, transiently blocked peristalsis, but caused maintained block when infused. Hexamethonium blocked propulsive contractions. Inhibition of nitrergic transmission converted regular peristalsis to non-propulsive contractions. These studies demonstrate the utility of an adapted Trendelenburg method for quantitative investigation of motility and pharmacology of enteric reflexes in vivo.

Dynamics of fast synaptic excitation during trains of stimulation in myenteric neurons of guinea-pig ileum

Fast excitatory postsynaptic potentials (fEPSPs) occur in bursts in the myenteric plexus during evoked motor reflexes in the guinea-pig ileum in vitro. This study used electrophysiological methods to study fEPSPs during stimulus trains to mimic bursts of synaptic activity in vitro. The amplitude of fEPSPs or fast excitatory postsynaptic currents (EPSCs) declined (rundown) during stimulus trains at frequencies of 0.5, 5, 10 and 20 Hz. At 0.5 Hz, fEPSP or fEPSC amplitude declined by 50% after the first stimulus but remained constant for the remainder of the train. At 5, 10 and 20 Hz, synaptic responses ran down completely with time constants of 0.35, 0.21 and 0.11 s, respectively. Recovery from rundown occurred with a time constant of 7 s. Mecamylamine, a nicotinic cholinergic receptor antagonist, or PPADS, a P2X receptor antagonist, reduced fEPSP amplitude, but they had no effect on rundown. Responses caused by trains of ionophoretically applied ATP or ACh (to mimic fEPSPs) did not rundown. Blockade of presynaptic inhibitory muscarinic, adenosine A1, opioid, alpha2-adrenergic and 5-HT1A receptors or pertussis toxin (PTX) treatment did not alter rundown. Antidromic action potentials followed a 10-Hz stimulus train. Iberiotoxin (100 nM), a blocker of large conductance calcium activated K+ (BK) channels, did not alter rundown. These data suggest that synaptic rundown is not due to: (a) action potential failure; (b) nicotinic or P2X receptor desensitization; (c) presynaptic inhibition mediated by pertussis-toxin sensitive G-proteins, or (d) BK channel activation. Synaptic rundown is likely due to depletion of a readily releasable pool (RRP) of neurotransmitter.

Synaptic facilitation and enhanced neuronal excitability in the submucosal plexus during experimental colitis in guinea-pig

Intestinal secretion is regulated by submucosal neurones of the enteric nervous system. Inflammation of the intestines leads to aberrant secretory activity; therefore we hypothesized that the synaptic and electrical behaviours of submucosal neurones are altered during colitis. To test this hypothesis, we used intracellular microelectrode recording to compare the excitability and synaptic properties of submucosal neurones from normal and trinitrobenzene sulphonic acid (TNBS)-inflamed guinea-pig colons. Inflammation differentially affected the electrophysiological characteristics of the two functional classes of submucosal neurones. AH neurones from inflamed colons were more excitable, had shorter action potential durations and reduced afterhyperpolarizations. Stimulus-evoked fast and slow excitatory postsynaptic potentials (EPSPs) in S neurones were larger during colitis, and the incidence of spontaneous fast EPSPs was increased. In control preparations, fast EPSPs were almost completely blocked by the nicotinic receptor antagonist hexamethonium, whereas fast EPSPs in inflamed S neurones were only partially inhibited by hexamethonium. In inflamed tissues, components of the fast EPSP in S neurones were sensitive to blockade of P2(X) and 5-HT(3) receptors while these antagonists had little effect in control preparations. Control and inflamed S neurones were equally sensitive to brief application of acetylcholine, ATP and 5-HT, suggesting that synaptic facilitation was due to a presynaptic mechanism. Immunoreactivity for 5-HT in the submucosal plexus was unchanged by inflammation; this indicates that altered synaptic transmission was not due to anatomical remodelling of submucosal nerve terminals. This is the first demonstration of alterations in synaptic pharmacology in the enteric nervous system during inflammation.

Investigation of PKC isoform-specific translocation and targeting of the current of the late afterhyperpolarizing potential of myenteric AH neurons

AH neurons in the enteric nervous system play an essential role in initiating intestinal reflexes and factors that control AH neuron excitability therefore influence the state of the digestive system. Prominent afterhyperpolarizations that follow action potentials in these neurons strongly affect their excitability. In the present work, we have investigated the regulation of the afterhyperpolarizing current (I(AHP)) by protein kinase C (PKC). Electrophysiological responses and protein translocation were investigated in AH neurons of freshly dissected preparations of myenteric ganglia from the guinea-pig ileum. The activator of conventional and novel PKCs, phorbol dibutyrate, but not the activator of novel PKCs, ingenol, blocked the I(AHP). Phorbol dibutyrate had no effect on the hyperpolarization-activated current (I(h)) or on the A current (I(A)). Stimulation of synaptic inputs to the neurons also reduced the I(AHP), and had no effect on I(h) or I(A). Phorbol dibutyrate also reduced a background outward current that was present after the I(AHP) current had been blocked by clotrimazole. Both phorbol dibutyrate and ingenol caused translocation of the novel PKC, PKCepsilon, in these neurons. Only phorbol dibutyrate caused translocation of PKCgamma, a conventional PKC. The studies thus indicate that the activation of PKC by phorbol esters and by nerve stimulation affects AH neurons in a similar way, and that PKC activation targets both the I(AHP) and another background K(+) current. The I(AHP) is targeted by a conventional PKC, suggested to be PKCgamma, as this is the only conventional PKC that is prominent in AH neurons.

Excitatory actions of motilin on myenteric neurons of the guinea-pig small intestine

Motilin is considered as a key factor in controlling interdigestive migrating contractions. The present electrophysiological experiments were performed in vitro to examine actions of motilin on myenteric neurons of guinea-pigs after 18-h fasting period. Superfusion of motilin depolarized both S and AH neurons; the lowest effective concentration was 10 nM, and motilin depolarization was observed in 9 of 23 S neurons and in 5 of 25 AH neurons. The motilin depolarizations were associated with an increase in neuronal input resistance. The motilin responses were preserved in Ca2+ free/high Mg2+ solution in which no Ca2+ dependent synaptic transmission occurred. The reversal potential of the motilin responses was estimated about -95 mV, close to the equilibrium potential for K+. Furthermore, muscarinic depolarizations were blocked during the motilin responses. All of these indicated that motilin directly excited myenteric neurons mainly by inactivating K+ channels. It is concluded that motilin might modulate neuronal excitability of the myenteric plexus, leading to the control of interdigestive migrating contractions.

Plasticity of the enteric nervous system during intestinal inflammation

Inflammation of the bowel causes structural and functional changes to the enteric nervous system (ENS). While morphological alterations to the ENS are evident in some inflammatory conditions, it appears that relatively subtle modifications to the neurophysiology of enteric microcircuits may play a role in gastrointestinal (GI) dysfunction. These include changes to the excitability and synaptic properties of enteric neurones. The response of the ENS to inflammation varies according to the site and type of inflammation, with the functional consequences depending on the nature of the inflammatory stimulus. It has become clear that inflammation at one site can produce changes that occur at remotes sites in the GI tract. Immunohistochemical data from patients with inflammatory bowel disease (IBD) and animal models indicate that inflammation alters the neurochemical content of some functional classes of enteric neurones. A growing body of evidence supports an active role for enteric glia in neuronal and neuroimmune communication in the GI tract, particularly during inflammation. In conclusion, plasticity of the ENS is a feature of intestinal inflammation. Elucidation of the mechanisms whereby inflammation alters enteric neural control of GI functions may lead to novel treatments for IBD.

Evidence for protein kinase involvement in long-term postsynaptic excitation of intrinsic primary afferent neurons in the intestine

We have investigated the effects of protein kinase inhibitors on the sustained slow postsynaptic excitation (SSPE) that is evoked by prolonged stimulation of synaptic inputs to intrinsic primary afferent neurons (IPANs) in the small intestines of guinea pigs. Stimulation of synaptic inputs to the IPANs caused depolarisation, increased input resistance, and increased excitation that continued after the cessation of stimulation. The excitation was substantially reduced by the broad-spectrum kinase inhibitor staurosporine (1 microM), PKC inhibitors Ro 31-8220 (3.3 microM) and calphostin C (1 microM), but not by the PKA inhibitor H89 (1 microM). At a higher concentration, 10 microM Ro 31-8220 reduced the excitability of axons to electrical stimulation. Phorbol dibutyrate (1 microM) caused excitability increases, membrane depolarisation, and increased input resistance that mimicked the SSPE. We conclude that the generation of the SSPE requires a phosphorylation step that is mediated by protein kinase C.

MULTIPLE LEVELS OF SENSORY INTEGRATION IN THE INTRINSIC SENSORY NEURONS OF THE ENTERIC NERVOUS SYSTEM

1. The enteric nervous system (ENS) is present in the wall of the gastrointestinal tract and contains all the functional classes of neuron required for complete reflex arcs. One of the most important and intriguing classes of neuron is that responsive to sensory stimuli: sensory neurons with cell bodies intrinsic to the ENS. 2. These neurons have three outstanding and interrelated features: (i) reciprocal connections with each other; (ii) a slow excitatory post-synaptic potential (EPSP) resulting from high-speed firing in other sensory neurons; and (iii) a large after-hyperpolarizing potential (AHP) at the soma. Slow EPSP depolarize the cell body, generate action potentials (APs) and reduce the AHP. Conversely, the AHP limits the firing rate and, hence, reduces transmission of slow EPSP. 3. Processing of sensory information starts at the input terminals as different patterns of APs depending on the sensory modality and recent sensory history. At the soma, the ability to fire APs and, hence, drive outputs is also strongly determined by the recent firing history of the neuron (through the AHP) and network activity (through the slow EPSP). Positive feedback within the population of intrinsic sensory neurons means that the network is able to drive outputs well beyond the duration of the stimuli that triggered them. 4. Thus, sensory input and subsequent reflex generation are integrated over several hierarchical levels within the network on intrinsic sensory neurons.