Localisation of NK1 receptor immunoreactivity to neurons and interstitial cells of the guinea-pig gastrointestinal tract

The Enteric Nervous System and Its Emerging Role as a Therapeutic Target

The gastrointestinal (GI) tract is innervated by the enteric nervous system (ENS), an extensive neuronal network that traverses along its walls. Due to local reflex circuits, the ENS is capable of functioning with and without input from the central nervous system. The functions of the ENS range from the propulsion of food to nutrient handling, blood flow regulation, and immunological defense. Records of it first being studied emerged in the early 19th century when the submucosal and myenteric plexuses were discovered. This was followed by extensive research and further delineation of its development, anatomy, and function during the next two centuries. The morbidity and mortality associated with the underdevelopment, infection, or inflammation of the ENS highlight its importance and the need for us to completely understand its normal function. This review will provide a general overview of the ENS to date and connect specific GI diseases including short bowel syndrome with neuronal pathophysiology and current therapies. Exciting opportunities in which the ENS could be used as a therapeutic target for common GI diseases will also be highlighted, as the further unlocking of such mechanisms could open the door to more therapy-related advances and ultimately change our treatment approach.

The Potential for Gut Organoid Derived Interstitial Cells of Cajal in Replacement Therapy

Effective digestion requires propagation of food along the entire length of the gastrointestinal tract. This process involves coordinated waves of peristalsis produced by enteric neural cell types, including different categories of interstitial cells of Cajal (ICC). Impaired food transport along the gastrointestinal tract, either too fast or too slow, causes a range of gut motility disorders that affect millions of people worldwide. Notably, loss of ICC has been shown to affect gut motility. Patients that suffer from gut motility disorders regularly experience diarrhoea and/or constipation, insomnia, anxiety, attention lapses, irritability, dizziness, and headaches that greatly affect both physical and mental health. Limited treatment options are available for these patients, due to the scarcity of human gut tissue for research and transplantation. Recent advances in stem cell technology suggest that large amounts of rudimentary, yet functional, human gut tissue can be generated in vitro for research applications. Intriguingly, these stem cell-derived gut organoids appear to contain functional ICC, although their frequency and functional properties are yet to be fully characterised. By reviewing methods of gut organoid generation, together with what is known of the molecular and functional characteristics of ICC, this article highlights short- and long-term goals that need to be overcome in order to develop ICC-based therapies for gut motility disorders.

Substance P and the neurokinin-1 receptor expression in dog ileum with and without inflammation

In the gastrointestinal tract, the tachykinin Substance P (SP) is involved in motility, fluid and electrolyte secretion, and blood flow and regulation of immunoinflammatory response. SP exerts its biological activity on target cells by interacting mainly with the neurokinin-1 receptor (NK₁R). The present study aims to quantify the percentage of SP-immunoreactive (SP-IR) enteric neurons and the density of SP-IR nerve fibers in the ileum of control dogs (CTRL-dogs; n=7) vs dogs with spontaneous ileal inflammation (INF-dogs; n=8). In addition, the percentage of enteric neurons bearing NK₁R, and nitrergic neurons (nNOS-IR) expressing NK₁R immunoreactivity were evaluated in both groups. The percentages of SP-IR neurons were similar in CTRL- and INF-dogs, in either the myenteric (MP) (15±8% vs. 16±7%, respectively) and submucosal plexus (SMP) (26±7% vs. 24±14%, respectively). In INF-dogs, the density of SP-IR mucosal nerve fibers showed a trend to decrease (P=0.07). Myenteric neurons of CTRL- and INF-dogs expressed similar percentages of NK₁R-immunoreactivity (39±5% vs. 38±20%, respectively). Submucosal NK₁R-IR neurons were occasionally observed in a CTRL-dog. MP nitrergic neurons bearing NK₁R showed a trend to decrease in INF-dogs vs. CTRL- dogs (41±22% vs. 65±10%, respectively; P=0.11). In INF-dogs, muscle cells and immune cells overexpressed NK₁R immunoreactivity. These findings should be taken as a warning for possible intestinal motility disorders, which might occur during administration of NK₁R-antagonist drugs. Conversely, the strong expression of NK₁R immunoreactivity observed in muscle and mucosal immune cells of inflamed tissues may provide a rationale for the use of NK₁R antagonist drugs in the treatment of intestinal inflammation.

Cholera Toxin Induces Sustained Hyperexcitability in Myenteric, but Not Submucosal, AH Neurons in Guinea Pig Jejunum

Background and Aims: Cholera toxin (CT)-induced hypersecretion requires activation of secretomotor pathways in the enteric nervous system (ENS). AH neurons, which have been identified as a population of intrinsic sensory neurons (ISNs), are a source of excitatory input to the secretomotor pathways. We therefore examined effects of CT in the intestinal lumen on myenteric and submucosal AH neurons.

Methods: Isolated segments of guinea pig jejunum were incubated for 90 min with saline plus CT (12.5 μg/ml) or CT + neurotransmitter antagonist, or CT + tetrodotoxin (TTX) in their lumen. After washing CT away, submucosal or myenteric plexus preparations were dissected keeping circumferentially adjacent mucosa intact. Submucosal AH neurons were impaled adjacent to intact mucosa and myenteric AH neurons were impaled adjacent to, more than 5 mm from, and in the absence of intact mucosa. Neuronal excitability was monitored by injecting 500 ms current pulses through the recording electrode.

Results: After CT pre-treatment, excitability of myenteric AH neurons adjacent to intact mucosa (n = 29) was greater than that of control neurons (n = 24), but submucosal AH neurons (n = 33, control n = 27) were unaffected. CT also induced excitability increases in myenteric AH neurons impaled distant from the mucosa (n = 6) or in its absence (n = 5). Coincubation with tetrodotoxin or SR142801 (NK3 receptor antagonist), but not SR140333 (NK1 antagonist) or granisetron (5-HT₃ receptor antagonist) prevented the increased excitability induced by CT. Increased excitability was associated with a reduction in the characteristic AHP and an increase in the ADP of these neurons, but not a change in the hyperpolarization-activated inward current, I_h.

Conclusions: CT increases excitability of myenteric, but not submucosal, AH neurons. This is neurally mediated and depends on NK3, but not 5-HT₃ receptors. Therefore, CT may act to amplify the secretomotor response to CT via an increase in the activity of the afferent limb of the enteric reflex circuitry.

Stimulus-induced pacemaker activity in interstitial cells of Cajal associated with the deep muscular plexus of the small intestine

BACKGROUND

The ICC-DMP have been proposed to generate stimulus-dependent pacemaker activity, rhythmic transient depolarizations, that take part in orchestrating segmentation and clustered propulsive motor patterns in the small intestine. However, little is known about the fundamental properties of ICC-DMP.

METHODS

This study was undertaken to increase our understanding of intrinsic properties of the ICC-DMP through calcium imaging and intracellular electrical recordings.

KEY RESULTS

Without stimulation, most ICC-DMP were quiescent. In some preparations ICC-DMP generated rhythmic low-frequency calcium oscillations (<10 cpm) with or without high frequency activity superimposed (>35 cpm). Immunohistochemistry proved the existence of NK1R on the ICC-DMP and close contacts between ICC-DMP and substance P-positive nerves. Substance P (25 nM) induced low-frequency calcium oscillations that were synchronized across the ICC-DMP network. Substance P also induced low frequency rhythmic transient depolarizations (<10cpm) in circular muscle cells close to the ICC-DMP. An intracellular recording from a positively identified ICC-DMP showed rhythmic transient depolarizations with superimposed high frequency activity. To investigate if quiescent ICC-DMP were chronically inhibited by nitrergic activity, nNOS was inhibited, but without effect.

CONCLUSIONS & INFERENCES

Substance P changes non-synchronized high frequency flickering or quiescence in ICC-DMP into strong rhythmic calcium transients that are synchronized within the network; they are associated with rhythmic transient depolarizations within the same frequency range. We hypothesize that Substance P, released from nerves, can evoke rhythmicity in ICC-DMP, thereby providing it with potential pacemaker activity.

The significance of interstitial cells in neurogastroenterology

Smooth muscle layers of the gastrointestinal tract consist of a heterogeneous population of cells that include enteric neurons, several classes of interstitial cells of mesenchymal origin, a variety of immune cells and smooth muscle cells (SMCs). Over the last number of years the complexity of the interactions between these cell types has begun to emerge. For example, interstitial cells, consisting of both interstitial cells of Cajal (ICC) and platelet-derived growth factor receptor alpha-positive (PDGFRα(+)) cells generate pacemaker activity throughout the gastrointestinal (GI) tract and also transduce enteric motor nerve signals and mechanosensitivity to adjacent SMCs. ICC and PDGFRα(+) cells are electrically coupled to SMCs possibly via gap junctions forming a multicellular functional syncytium termed the SIP syncytium. Cells that make up the SIP syncytium are highly specialized containing unique receptors, ion channels and intracellular signaling pathways that regulate the excitability of GI muscles. The unique role of these cells in coordinating GI motility is evident by the altered motility patterns in animal models where interstitial cell networks are disrupted. Although considerable advances have been made in recent years on our understanding of the roles of these cells within the SIP syncytium, the full physiological functions of these cells and the consequences of their disruption in GI muscles have not been clearly defined. This review gives a synopsis of the history of interstitial cell discovery and highlights recent advances in structural, molecular expression and functional roles of these cells in the GI tract.

The significance of interstitial cells in neurogastroenterology

Smooth muscle layers of the gastrointestinal tract consist of a heterogeneous population of cells that include enteric neurons, several classes of interstitial cells of mesenchymal origin, a variety of immune cells and smooth muscle cells (SMCs). Over the last number of years the complexity of the interactions between these cell types has begun to emerge. For example, interstitial cells, consisting of both interstitial cells of Cajal (ICC) and platelet-derived growth factor receptor alpha-positive (PDGFRα(+)) cells generate pacemaker activity throughout the gastrointestinal (GI) tract and also transduce enteric motor nerve signals and mechanosensitivity to adjacent SMCs. ICC and PDGFRα(+) cells are electrically coupled to SMCs possibly via gap junctions forming a multicellular functional syncytium termed the SIP syncytium. Cells that make up the SIP syncytium are highly specialized containing unique receptors, ion channels and intracellular signaling pathways that regulate the excitability of GI muscles. The unique role of these cells in coordinating GI motility is evident by the altered motility patterns in animal models where interstitial cell networks are disrupted. Although considerable advances have been made in recent years on our understanding of the roles of these cells within the SIP syncytium, the full physiological functions of these cells and the consequences of their disruption in GI muscles have not been clearly defined. This review gives a synopsis of the history of interstitial cell discovery and highlights recent advances in structural, molecular expression and functional roles of these cells in the GI tract.

Localisation and activation of the neurokinin 1 receptor in the enteric nervous system of the mouse distal colon

The substance P neurokinin 1 receptor (NK1R) regulates motility, secretion, inflammation and pain in the intestine. The distribution of the NK1R is a key determinant of the functional effects of substance P in the gut. Information regarding the distribution of NK1R in subtypes of mouse enteric neurons is lacking and is the focus of the present study. NK1R immunoreactivity (NK1R-IR) is examined in whole-mount preparations of the mouse distal colon by indirect immunofluorescence and confocal microscopy. The distribution of NK1R-IR within key functional neuronal subclasses was determined by using established neurochemical markers. NK1R-IR was expressed by a subpopulation of myenteric and submucosal neurons; it was mainly detected in large multipolar myenteric neurons and was colocalized with calcitonin gene-related peptide, neurofilament M, choline acetyltransferase and calretinin. The remaining NK1R-immunoreactive neurons were positive for nitric oxide synthase. NK1R was expressed by most of the submucosal neurons and was exclusively co-expressed with vasoactive intestinal peptide, with no overlap with choline acetyltransferase. Treatment with substance P resulted in the concentration-dependent internalisation of NK1R from the cell surface into endosome-like structures. Myenteric NK1R was mainly expressed by intrinsic primary afferent neurons, with minor expression by descending interneurons and inhibitory motor neurons. Submucosal NK1R was restricted to non-cholinergic secretomotor neurons. These findings highlight key differences in the neuronal distribution of NK1R-IR between the mouse, rat and guinea-pig, with important implications for the functional role of NK1R in regulating intestinal motility and secretion.

Chronic treatment with otilonium bromide induces changes in L-type Ca²⁺ channel, tachykinins, and nitric oxide synthase expression in rat colon muscle coat

BACKGROUND

Otilonium bromide (OB) is a quaternary ammonium derivative used for the treatment of intestinal hypermotility and is endowed with neurokinin2 receptor (NK2r) antagonist and Ca²⁺ channel blocker properties. Therefore, the possibility that OB might play a role in the neurokinin receptor/Substance-P/nitric oxide (NKr/SP/NO) circuit was investigated after chronic exposition to the drug.

METHODS

Rats were treated with OB 2-20 mg kg⁻¹ for 10 and 30 days. In the proximal colon, the expression and distribution of muscle NOsynthase 1 (NOS1), NK1r, NK2r, SP and Cav 1.2 subunit (for L-type Ca²⁺ channel) and the spontaneous activity and stimulated responses to NK1r and NK2r agonists were investigated.

KEY RESULTS

Immunohistochemistry showed a redistribution of NK1r and L-type Ca²⁺ channel in muscle cells with no change of NK2r at 30 days, a significant increase in muscle NOS1 expression at 10 days and a significant decrease in the SP content early in the ganglia and later in the intramuscular nerve fibers. Functional studies showed no change in spontaneous activity but a significant increase in maximal contraction induced by NK1r agonist.

CONCLUSIONS & INFERENCES

Chronic exposition to OB significantly affects the NKr/SP/NO circuit. The progressive decrease in SP-expression might be the consequence of the persistent presence of OB, the increase of NOS1 expression in muscle cells at 10 days in an attempt to guarantee an adequate NO production, and, at 30 days, the redistribution of the L-type Ca²⁺ channel and NK1r as a sign to compensate the drug channel block by re-cycling both of them. The physiological data suggest NK1r hypersensitivity.

Neurochemical mechanisms and pharmacologic strategies in managing nausea and vomiting related to cyclic vomiting syndrome and other gastrointestinal disorders

Nausea and vomiting are common gastrointestinal complaints which could be triggered by stimuli in both the peripheral and central nervous systems. They may be considered as defense mechanisms when threatening toxins/agents enter the gastrointestinal tract or there is excessive retention of gastrointestinal contents due to obstruction. The pathophysiology of nausea and vomiting is complex and much still remains unknown. Therefore, treatments are restricted or ineffective in many cases. Nausea and vomiting with functional etiologies including cyclic vomiting syndrome are challenging in gastroenterology. In this article, we review potential pathways, neurochemical transmitters, and their receptors which are possibly involved in the pathophysiology of nausea and vomiting including the entity cyclic vomiting syndrome.

Expression of β2 adrenoceptors within enteric neurons of the horse ileum

The activity of the gastrointestinal tract is regulated through the activation of adrenergic receptors (ARs). Since data concerning the distribution of ARs in the horse intestine is virtually absent, we investigated the distribution of β2-AR in the horse ileum using double-immunofluorescence. The β2-AR-immunoreactivity (IR) was observed in most (95%) neurons located in submucosal plexus (SMP) and in few (8%) neurons of the myenteric plexus (MP). Tyrosine hydroxylase (TH)-IR fibers were observed close to neurons expressing β2-AR-IR. Since β2-AR is virtually expressed in most neurons located in the horse SMP and in a lower percentage of neurons in the MP, it is reasonable to retain that this adrenergic receptor could regulate the activity of both secretomotor neurons and motor neurons innervating muscle layers and blood vessels. The high density of TH-IR fibers near β2-AR-IR enteric neurons indicates that the excitability of these cells could be directly modulated by the sympathetic system.

Roles of substance P and ATP in the subepithelial fibroblasts of rat intestinal villi

The ingestion of food and water induces chemical and mechanical signals that trigger peristaltic reflexes and also villous movement in the gut. In the intestinal villi, subepithelial fibroblasts under the epithelium form contractile cellular networks and closely contact to the varicosities of substance P and nonsubstance P afferent neurons. Subepithelial fibroblasts of the duodenal villi possess purinergic receptor P2Y1 and tachykinin receptor NK1. ATP and substance P induce increase in intracellular Ca(2+) and cell contraction in subepithelial fibroblasts. They are highly mechanosensitive and release ATP by mechanical stimuli. Released ATP spreads to form an ATP "cloud" with nearly 1μM concentration and activates the surroundings via P2Y1 and afferent neurons via P2X receptors. These findings suggest that villous subepithelial fibroblasts and afferent neurons interact via ATP and substance P. This mutual interaction may play important roles in the signal transduction of mechano reflex pathways including a coordinate villous movement and also in the maturation of the structure and function of the intestinal villi.

Regulation of gastrointestinal motility--insights from smooth muscle biology

Gastrointestinal motility results from coordinated contractions of the tunica muscularis, the muscular layers of the alimentary canal. Throughout most of the gastrointestinal tract, smooth muscles are organized into two layers of circularly or longitudinally oriented muscle bundles. Smooth muscle cells form electrical and mechanical junctions between cells that facilitate coordination of contractions. Excitation-contraction coupling occurs by Ca(2+) entry via ion channels in the plasma membrane, leading to a rise in intracellular Ca(2+). Ca(2+) binding to calmodulin activates myosin light chain kinase; subsequent phosphorylation of myosin initiates cross-bridge cycling. Myosin phosphatase dephosphorylates myosin to relax muscles, and a process known as Ca(2+) sensitization regulates the activity of the phosphatase. Gastrointestinal smooth muscles are 'autonomous' and generate spontaneous electrical activity (slow waves) that does not depend upon input from nerves. Intrinsic pacemaker activity comes from interstitial cells of Cajal, which are electrically coupled to smooth muscle cells. Patterns of contractile activity in gastrointestinal muscles are determined by inputs from enteric motor neurons that innervate smooth muscle cells and interstitial cells. Here we provide an overview of the cells and mechanisms that generate smooth muscle contractile behaviour and gastrointestinal motility.

CaMKII is essential for the function of the enteric nervous system

Background

Ca²⁺/calmodulin-dependent protein kinases (CaMKs) are major downstream mediators of neuronal calcium signaling that regulate multiple neuronal functions. CaMKII, one of the key CaMKs, plays a significant role in mediating cellular responses to external signaling molecules. Although calcium signaling plays an essential role in the enteric nervous system (ENS), the role of CaMKII in neurogenic intestinal function has not been determined. In this study, we investigated the function and expression pattern of CaMKII in the ENS across several mammalian species.

Methodology/Principal Findings

CaMKII expression was characterized by immunofluorescence analyses and Western Blot. CaMKII function was examined by intracellular recordings and by assays of colonic contractile activity. Immunoreactivity for CaMKII was detected in the ENS of guinea pig, mouse, rat and human preparations. In guinea pig ENS, CaMKII immunoreactivity was enriched in both nitric oxide synthase (NOS)- and calretinin-containing myenteric plexus neurons and non-cholinergic secretomotor/vasodilator neurons in the submucosal plexus. CaMKII immunoreactivity was also expressed in both cholinergic and non-cholinergic neurons in the ENS of mouse, rat and human. The selective CaMKII inhibitor, KN-62, suppressed stimulus-evoked purinergic slow EPSPs and ATP-induced slow EPSP-like response in guinea pig submucosal plexus, suggesting that CaMKII activity is required for some metabotropic synaptic transmissions in the ENS. More importantly, KN-62 significantly suppressed tetrodotoxin-induced contractile response in mouse colon, which suggests that CaMKII activity is a major determinant of the tonic neurogenic inhibition of this tissue.

Conclusion

ENS neurons across multiple mammalian species express CaMKII. CaMKII signaling constitutes an important molecular mechanism for controlling intestinal motility and secretion by regulating the excitability of musculomotor and secretomotor neurons. These findings revealed a fundamental role of CaMKII in the ENS and provide clues for the treatment of intestinal dysfunctions.

Enhanced nerve-stimulated muscarinic and neurokinin contractions of ileum from streptozotocin guinea-pigs

Diabetes mellitus can lead to neuropathy of enteric neurons, resulting in abnormal gut motility. These studies investigated voltage-dependent contributions of muscarinic M₃ receptor activation by acetylcholine and neurokinin NK₁ receptor activation by neurokinins to nerve-stimulated contractions of longitudinal ileal strips from STZ guinea-pigs, a type 1 diabetic model with insulin deficiency, but mild hyperglycaemia. Contractions to bethanechol, substance P methyl ester, and nerve stimulation were greater in diabetic as compared to control ileum. The muscarinic M₃ receptor antagonist 4-DAMP at lower voltages and the neurokinin NK₁ receptor antagonist SR140333 at higher voltages, but not the neurokinin NK₁ receptor antagonist CP-96,345, were more effective at inhibiting nerve-stimulated immediate peak contractions and total areas of contraction of ileum from diabetic as compared to control animals. For diabetic ileum, voltage-dependent increases in the areas of nerve-stimulated contraction were observed in the presence of 4-DAMP and CP-96,345 but not SR140333. At low voltages only, nerve-stimulated release of acetylcholine was greater from diabetic as compared to control ileum. Fluorescence intensity of tachykinin-like immunoreactivity was increased in ileal myenteric ganglia from diabetic as compared to control animals. In diabetic guinea-pigs, stronger ileal nerve-stimulated contractions reflected increased release of acetylcholine at lower voltages and tachykinins at higher voltages, as well as increased sensitivity of smooth muscle M₃ and NK₁ receptors to acetylcholine and tachykinins. Hypoinsulinaemia may be a primary contributor to intestinal motility dysfunction in type 1 diabetes mellitus.

Localization of NK1 receptors and roles of substance-P in subepithelial fibroblasts of rat intestinal villi

Subepithelial fibroblasts of the intestinal villi, which form a contractile cellular network beneath the epithelium, are in close contact with epithelial cells, nerve varicosities, capillaries, smooth muscles and immune cells, and secrete extracellular matrix molecules, growth factors and cytokines, etc. Cultured subepithelial fibroblasts of the rat duodenal villi display various receptors such as endothelins, ATP, substance-P and bradykinin, and release ATP in response to mechanical stimulation. In this study, the presence of functional NK1 receptors (NK1R) was pharmacologically confirmed in primary culture by Ca(2+) measurement, and the effects of substance-P were measured in an acute preparation of epithelium-free duodenal villi from 2- to 3-week-old rats using a two-photon laser microscope. Substance-P elicited an increase in the intracellular Ca(2+) concentration and contraction of the subepithelial fibroblasts in culture and the isolated villi. The localization of NK1R and substance-P in the villi was examined by light and electron microscopic immunohistochemistry. NK1R-like immunoreactivity was intensely localized on the plasma membrane of villous subepithelial fibroblasts in 10-day- to 4-week-old rats and mice and was decreased or absent in adulthood. The pericryptal fibroblasts of the small and large intestine were NK1R immuno-negative. These villous subepithelial fibroblasts form synapse-like structures with both substance-P-immunopositive and -immunonegative nerve varicosities. Here, we propose that the mutual interaction between villous subepithelial fibroblasts and afferent neurons via substance-P and ATP plays important roles in the maturation of the structure and function of the small intestine.

Neuroeffector apparatus in gastrointestinal smooth muscle organs

Control of gastrointestinal (GI) movements by enteric motoneurons is critical for orderly processing of food, absorption of nutrients and elimination of wastes. Work over the past several years has suggested that motor neurotransmission is more complicated than simple release of transmitter from nerve terminals and binding of receptors on smooth muscle cells. In fact the 'neuro-effector' junction in the tunica muscularis might consist of synaptic-like connectivity with specialized cells, and contributions from multiple cell types in integrated post-junctional responses. Interstitial cells of Cajal (ICC) were proposed as potential mediators in motor neurotransmission based on reduced post-junctional responses observed in W mutants that have reduced populations of ICC. More recent studies on W mutants have contradicted the original findings, and suggested that ICC may not be significant players in motor neurotransmission. This review examines the evidence for and against the role of ICC in motor neurotransmission and outlines areas for additional investigation that would help further resolve this controversy.

Synaptic transmission at functionally identified synapses in the enteric nervous system: roles for both ionotropic and metabotropic receptors

Digestion and absorption of nutrients and the secretion and reabsorption of fluid in the gastrointestinal tract are regulated by neurons of the enteric nervous system (ENS), the extensive peripheral nerve network contained within the intestinal wall. The ENS is an important physiological model for the study of neural networks since it is both complex and accessible. At least 20 different neurochemically and functionally distinct classes of enteric neurons have been identified in the guinea pig ileum. These neurons express a wide range of ionotropic and metabotropic receptors. Synaptic potentials mediated by ionotropic receptors such as the nicotinic acetylcholine receptor, P2X purinoceptors and 5-HT(3) receptors are seen in many enteric neurons. However, prominent synaptic potentials mediated by metabotropic receptors, like the P2Y(1) receptor and the NK(1) receptor, are also seen in these neurons. Studies of synaptic transmission between the different neuron classes within the enteric neural pathways have shown that both ionotropic and metabotropic synaptic potentials play major roles at distinct synapses within simple reflex pathways. However, there are still functional synapses at which no known transmitter or receptor has been identified. This review describes the identified roles for both ionotropic and metabotropic neurotransmission at functionally defined synapses within the guinea pig ileum ENS. It is concluded that metabotropic synaptic potentials act as primary transmitters at some synapses. It is suggested identification of the interactions between different synaptic potentials in the production of complex behaviours will require the use of well validated computer models of the enteric neural circuitry.

Distribution and morphological characteristics of the interstitial cells of Cajal in the ileocaecal junction of the guinea-pig

The guinea-pig ileocaecal junction including the valve was studied by immunohistochemistry to clarify the organization of the muscle bundles, the enteric nerves and the interstitial cells of Cajal (ICC). This region clearly exhibited characteristic features in the distribution patterns of ICC in a proximal to distal direction: (1) the thickened portion of the terminal ileum immediately adjacent to the ileocecal junction contained many ICC throughout the circular (ICC-CM) and longitudinal (ICC-LM) muscle layers, but ICC were few or absent in the rest of the ileum; (2) the ileal side of the valve contained ICC associated with the deep muscular plexus (ICC-DMP) as in the small intestine, whereas ICC-DMP were absent in the caecal side as in the caecum; (3) the valve contained many ICC-CM and ICC-LM in both the ileal and caecal sides; (4) many ICC associated with the myenteric plexus were observed in both the ileal and caecal sides of the valve, whereas they were only sparsely found in the caecum; (5) ICC were also observed around the submucosal plexus in a confined area of the terminal ileum and the ileocaecal valve. These observations provide morphological evidence that the terminal ileum and ileocaecal valve are specially equipped for their active involvement in the movement of the junctional area.

Updating old ideas and recent advances regarding the Interstitial Cells of Cajal

Since their discovery by Cajal in 1889, the Interstitial Cells of Cajal (ICC) have generated much controversy in the scientific community. Indeed, the nervous, muscle or fibroblastic nature of the ICC has remained under debate for more than a century, as has their possible physiological function. Cajal and his colleagues considered them to be neurons, while contemporary histologists like Kölliker and Dogiel categorized these cells as fibroblasts. More recently, the role of ICC in the origin of slow-wave peristaltism has been elucidated, and several studies have shown that they participate in neurotransmission (intercalation theory). The fact that ICC assemble in the circular muscular layer and that they originate from cells which emerge from the ventral neural tube (VENT cells), a source of neurons, glia and ICC precursors other than the neural crest, suggests a neural origin for this particular subset of ICC. The discovery that ICC express the Kit protein, a type III tyrosine kinase receptor encoded by the proto-oncogene c-kit, has helped better understand their physiological role and implication in pathological conditions. Gleevec, a novel molecule designed to inhibit the mutant activated version of c-Kit receptors, is the drug of choice to treat the so-called gastrointestinal stromal tumours (GIST), the most common non-epithelial neoplasm of the gastrointestinal tract. Here we review Cajal's original contributions with the aid of unique images taken from Cajal's histological slides (preserved at the Cajal Museum, Cajal Institute, CSIC). In addition, we present a historical review of the concepts associated with this particular cell type, emphasizing current data that has advanced our understanding of the role these intriguing cells fulfil.

In vitro characterization of the effects of rat/mouse hemokinin-1 on mouse colonic contractile activity: a comparison with substance P

Rat/mouse hemokinin-1 (r/m HK-1) has been identified as a member of the tachykinin family and its effect in colonic contractile activity remains unknown. We investigated the effects and mechanisms of actions of r/m HK-1 on the mouse colonic contractile activity in vitro by comparing it with that of substance P (SP). R/m HK-1 induced substantial contractions on the circular muscle of mouse colon. The maximal contractile responses to r/m HK-1 varied significantly among proximal-, mid- and distal-colon, suggesting that the action of r/m HK-1 was region-specific in mouse colon. The contractile response induced by r/m HK-1 is primarily via activation of tachykinin NK(1) receptors leading to activation of cholinergic excitatory pathways and with a minor contribution of NK(2) receptors, which may be on the smooth muscle itself. A direct action on colonic smooth muscles may be also involved. In contrast, SP induced biphasic colonic responses (contractile and relaxant responses) on the circular muscle, in which the contractile action of SP was equieffective with r/m HK-1. SP exerted its contractile effect predominantly through neural and muscular tachykinin NK(1) receptors, but unlike r/m HK-1 did not appear to act via NK(2) receptors. The relaxation induced by SP was largely due to release of nitric oxide (NO) produced via an action on neural NK(1) receptors. These results indicate that the receptors and the activation properties involved in r/m HK-1-induced mouse colonic contractile activity are different from those of SP.

Interstitial cells of Cajal in health and disease

The gastrointestinal tract serves the physiological function of digesting and absorbing nutrients from food and physically mixing and propelling these contents in an oral to anal direction. These functions require the coordinated interaction of several cell types, including enteric nerves, immune cells and smooth muscle. Interstitial cells of Cajal (ICC) are now recognized as another cell type that are required for the normal functioning of the gastrointestinal tract. Abnormalities in ICC numbers and networks are associated with several gastrointestinal motility disorders. This review will describe what is known about the function and role of ICC both in health and in a variety of motility disorders with a focus on unresolved issues pertaining to their role in the control of gastrointestinal motility.

A randomized, placebo-controlled, crossover, double-blind trial of the NK1 receptor antagonist aprepitant on gastrointestinal motor function in healthy humans

BACKGROUND

Little is known about the role of tachykinins on human gastrointestinal motility and no data exist on the possible effect of an NK1 receptor antagonist.

AIM

To examine the effect of an antiemetic dose of the selective NK1 receptor antagonist aprepitant on gastrointestinal propulsion in healthy humans.

METHODS

Twelve healthy volunteers participated in a crossover, double-blind study. In random order, each volunteer had a 125-mg capsule of aprepitant or placebo on day 1 followed by an 80-mg capsule of aprepitant or placebo on days 2-5. Gamma camera imaging was used to measure gastric emptying, small intestinal transit and colonic transit of a radiolabelled, 1600-kJ mixed liquid and solid meal ingested on day 2.

RESULTS

Aprepitant did not change gastric retention at 15 min, gastric half emptying time, gastric mean transit time, time to small intestinal transit of 10%, small intestinal mean transit time or colonic geometric centre after 24, 48 and 72 h.

CONCLUSION

A 125-mg capsule of aprepitant followed by an 80-mg capsule of aprepitant each of the next 2-5 days did not induce major changes in the propulsive function of the gastrointestinal tract in the small number of healthy volunteers investigated.

Involvement of thromboxane a(2) in the modulation of pacemaker activity of interstitial cells of cajal of mouse intestine

Although many studies show that thromboxane A(2) (TXA(2)) has the action of gastrointestinal (GI) motility using GI muscle cells and tissue, there are no reports on the effects of TXA(2) on interstitial cells of Cajal (ICC) that function as pacemaker cells in GI tract. So, we studied the modulation of pacemaker activities by TXA(2) in ICC with whole cell patch-clamp technique. Externally applied TXA(2) (5microM) produced membrane depolarization in current-clamp mode and increased tonic inward pacemaker currents in voltage-clamp mode. The tonic inward currents by TXA(2) were inhibited by intracellular application of GDP-beta-S. The pretreatment of ICC with Ca(2+) free solution and thapsigargin, a Ca(2+)-ATPase inhibitor in endoplasmic reticulum, abolished the generation of pacemaker currents and suppressed the TXA(2)-induced tonic inward currents. However, chelerythrine or calphostin C, protein kinase C inhibitors, did not block the TXA(2)-induced effects on pacemaker currents. These results suggest that TXA(2) can regulate intestinal motility through the modulation of ICC pacemaker activities. This modulation of pacemaker activities by TXA(2) may occur by the activation of G protein and PKC independent pathway via extra and intracellular Ca(2+) modulation.

Immunohistochemical localisation of cholinergic muscarinic receptor subtype 1 (M1r) in the guinea pig and human enteric nervous system

Little is known regarding the location of cholinergic muscarinic receptor 1 (M1r) in the ENS, even though physiological data suggest that M1rs are central to cholinergic neurotransmission. This study localised M1rs in the ENS of the guinea pig ileum and human colon using fluorescence immunohistochemistry and RT-PCR in human colon. Double labelling using antibodies against neurochemical markers was used to identify neuron subytpes bearing M1r. M1r immunoreactivity (IR) was present on neurons in the myenteric and submucosal ganglia. The two antibodies gave similar M1r-IR patterns and M1r-IR was abolished upon antibody preabsorption. M1r-IR was present on cholinergic and nNOS-IR nerve cell bodies in both guinea pig and human myenteric neurons. Presynaptic M1r-IR was present on NOS-IR and VAChT-IR nerve fibres in the circular muscle in the human colon. In the submucosal ganglia, M1r-IR was present on a population of neurons that contained cChAT-IR, but did not contain NPY-IR or calretinin-IR. M1r-IR was present on endothelial cells of blood vessels in the submucosal plexus. The localisation of M1r-IR in the guinea pig and human ENS shown in this study agrees with physiological studies. M1r-IR in cholinergic and nitrergic neurons and nerve fibres indicate that M1rs have a role in both cholinergic and nitrergic transmission. M1r-IR present in submucosal neurons suggests a role in mediating acetylcholine's effect on submucosal sensory and secretomotor/vasodilator neurons. M1r-IR present on blood vessel endothelial cells suggests that M1rs may also mediate acetylcholine's direct effect on vasoactivation.

Role for NK(1) and NK(2) receptors in the motor activity in mouse colon

The present study examined the effects induced by endogenous and exogenous activation of NK(1) and NK(2) receptors on the mechanical activity of mouse proximal colon. Experiments were performed in vitro recording the changes in intraluminal pressure from isolated colonic segments. Electrical field stimulation in the presence of atropine and guanethidine produced a small relaxation, followed by nonadrenergic noncholinergic (NANC) contraction. SR140333, NK(1) receptor antagonist, or SR48968, NK(2) receptor antagonist, significantly reduced the contraction, although SR48968 appeared more efficacious. The co-administration of SR140333 and SR48968 virtually abolished the NANC contraction. [Sar(9), Met(O(2))(11)]-substance P, selective NK(1) receptor agonist, induced a concentration-dependent biphasic effect, contraction followed by reduction of the mechanical spontaneous activity. Both effects were antagonized by SR140333, but not by SR48968. [beta-Ala(8)]-neurokinin A (4-10), selective NK(2) receptor agonist, evoked concentration-dependent contraction, which was antagonized by SR48968, but not by SR140333. The contraction induced by [Sar(9), Met(O(2))(11)]-substance P, but not by [beta-Ala(8)]-neurokinin A (4-10), was reduced by tetrodotoxin or atropine, and increased by N(omega)-nitro-L-arginine methyl ester (L-NAME), inhibitor of nitric oxide synthase. The inhibitory effects induced by [Sar(9), Met(O(2))(11)]-substance P were abolished by tetrodotoxin or L-NAME. The results of the present study suggest that in mouse colon both NK(1) and NK(2) receptors are junctionally activated by endogenous tachykinins to cause an additive response. NK(1) receptors appear to be located on cholinergic and on nitrergic neurons as well as on smooth muscle cells, whereas NK(2) receptors seem to be present exclusively on smooth muscle cells.

Roles of M2 and M3 muscarinic receptors in cholinergic nerve-induced contractions in mouse ileum studied with receptor knockout mice

BACKGROUND AND PURPOSE

The functional roles of M(2) and M(3) muscarinic receptors in neurogenic cholinergic contractions in gastrointestinal tracts remain to be elucidated. To address this issue, we studied cholinergic nerve-induced contractions in the ileum using mutant mice lacking M(2) or M(3) receptor subtypes.

EXPERIMENTAL APPROACH

Contractile responses to transmural electrical (TE) stimulation were isometrically recorded in ileal segments from M(2)-knockout (KO), M(3)-KO, M(2)/M(3)-double KO, and wild-type mice.

KEY RESULTS

TE stimulation at 2-50 Hz frequency-dependently evoked a fast, brief contraction followed by a slower, longer one in wild-type, M(2)-KO or M(3)-KO mouse preparations. Tetrodotoxin blocked both the initial and later contractions, while atropine only inhibited the initial contractions. The initial cholinergic contractions were significantly greater in wild-type than M(2)-KO or M(3)-KO mice; the respective mean amplitudes at 50 Hz were 91, 74 and 68 % of 70mM K(+)-induced contraction. Pretreatment with pertussis toxin blocked the cholinergic contractions in M(3)-KO but not in M(2)-KO mice. Cholinergic contractions also remained in wild-type preparations, but their sizes were reduced by 20-30 % at 10-50 Hz. In M(2)/M(3)-double KO mice, TE stimulation evoked only slow, noncholinergic contractions, which were significantly greater in sizes than in any of the other three mouse strains.

CONCLUSION AND IMPLICATIONS

These results demonstrate that M(2) and M(3) receptors participate in mediating cholinergic contractions in mouse ileum with the latter receptors assuming a greater role. Our data also suggest that the lack of both M(2) and M(3) receptors causes upregulation of noncholinergic excitatory innervation of the gut smooth muscle.

Involvement of intramuscular interstitial cells of Cajal in neuroeffector transmission in the gastrointestinal tract

Specialized cells known as interstitial cells of Cajal (ICC) are distributed in specific locations within the tunica muscularis of the gastrointestinal (GI) tract. ICC serve as electrical pacemakers, provide pathways for the active propagation of slow waves, are mediators of enteric motor neurotransmission and play a role in afferent neural signalling. Morphological studies have provided evidence that motor neurotransmission in the GI tract does not occur through poorly defined structures between nerves and smooth muscle, but rather via specialized synapses that exist between enteric nerve terminals and intramuscular ICC or ICC-IM. ICC-IM are coupled to smooth muscle cells via gap junctions and post-junctional responses elicited in ICC-IM are conducted to neighbouring smooth muscle cells. Electrophysiological studies from the stomachs and sphincters of wild-type and mutant animals that lack ICC-IM have provided functional evidence for the importance of ICC in cholinergic excitatory and nitrergic inhibitory motor neurotransmission. Intraperitoneal injection of animals with Kit neutralizing antibody or organ culture of gastrointestinal tissues in the presence of neutralizing antibody, which blocks the development and maintenance of ICC, has provided further evidence for the role of ICC in enteric motor transmission. ICC-IM also generate an ongoing discharge of unitary potentials in the gastric fundus and antrum that contributes to the overall excitability of the stomach.

Selective labeling and isolation of functional classes of interstitial cells of Cajal of human and murine small intestine

Specific functions of interstitial cells of Cajal (ICC) have been linked to distinct classes that differ by morphology and distribution. In the small intestine, slow wave-generating ICC are located in the myenteric region (ICC-MY), whereas ICC that mediate neuromuscular neurotransmission occur either throughout the circular muscle layer (intramuscular ICC, ICC-IM) or in association with the deep muscular plexus (ICC-DMP). Selective isolation of ICC to characterize specific properties has been difficult. Recently, neurokinin-1 receptors have been detected in murine ICC-DMP and neurons but not in ICC-MY. Here we identified and isolated ICC-DMP/IM by receptor-mediated internalization of fluorescent substance P and Kit immunofluorescence. Specificity of labeling was verified by confocal microscopy. Mouse and human ICC-DMP/IM were detected in suspension by fluorescent microscopy and harvested for RT-PCR with micropipettes. The isolated cells expressed Kit but not markers for neurons, smooth muscle, or antigen-presenting cells. ICC-DMP expressed neurokinin-1 receptor, M(2) and M(3) muscarinic receptors, P2Y(1) and P2Y(4) purinergic receptors, VIP receptor 2, soluble guanylate cyclase-1 subunits, and protein kinase G. L- or T-type Ca(2+) channels were not detected in these cells. ICC-MY and ICC-DMP were simultaneously detected and enumerated by flow cytometry and sorted to purity by fluorescence-activated cell sorting. In summary, functional classes of ICC have distinct molecular identities that can be used to selectively identify and harvest these cells with, for example, receptor-mediated uptake of substance P and Kit immunofluorescence. ICC-DMP express neurotransmitter receptors and signaling intermediate molecules that are consistent with their role in neuromuscular neurotransmission.

Substance P and Neurokinin 1 receptor - expression is affected in the ileum of mice with mutation in the W locus

The tachykinin substance P (SP) acts on the gut muscle coat via its preferred receptor, neurokinin 1 (NK1r). In the mouse ileum, NK1r-immunoreactivity (NK1r-IR) was detected in neurons, in the interstitial cells of Cajal at the deep muscular plexus (ICC-DMP) and the myoid cells of the villi. SP-IR was detected in neurons and varicose nerve fibers, which were especially numerous at the DMP and closely associated with the ICC-DMP. In mice with a mutation in the W locus (ckit mutant animals), innervation is suggested to be normal although few studies have actually tested this hypothesis. Indeed, studies demonstrating ICC-DMP integrity are lacking and whether SP- and NK1r-IR are normal in these animals has not been investigated. Our aim was to perform an immunohistochemical study on the ileum of a strain of heterozygous mice with a mutation in the W locus, the W(e/+) mice, to test this hypothesis. SP-IR nerve fibers were significantly more numerous than in wild type mice; NK1r-IR was clustered on the plasma membrane and also intracytoplasmatic in the neurons, but absent in the ICC-DMP. The richness in SP-IR nerve fibers and the NK1r-IR distribution in the neurons, similar to that of activated cells, might be attempts to compensate for the SP preferred receptor absence at the ICC-DMP. In conclusion, SP content and NK1r expression are noticeably different in c-kit mutants with respect to wild type mice, and probably causing an anomalous tachykininergic control of intestinal motility. Physiological studies on Wmutant mice have to take into account that innervation in this animal model is affected by the c-kit mutation.

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.

Role of NK1 and NK2 receptors in mouse gastric mechanical activity

1. The aim of the present study was to examine the role of NK1 and NK2 receptors in the control of mechanical activity of mouse stomach. In this view, the motor effects induced by NK1 and NK2 receptor agonists and antagonists were analyzed, measuring motility as intraluminal pressure changes in mouse-isolated stomach preparations. In parallel, immunohistochemical studies were performed to identify the location of NK1 and NK2 receptors on myenteric neurons and smooth muscle cells. 2. Substance P (SP) induced biphasic effects: a contraction followed by relaxation; neurokinin A (NKA) and [beta-Ala8]-NKA(4-10), selective agonist of NK2 receptors, evoked concentration-dependent contractions, whereas [Sar9, Met(O2)11]-SP, selective agonist of NK1 receptors, induced concentration-dependent relaxation. 3. SR48968, NK2 receptor antagonist, did not modify the spontaneous activity and reduced the contractile effects induced by tachykinins without affecting the relaxation. SR140333, NK1 receptor antagonist, did not modify the spontaneous activity and antagonized the relaxant response to tachykinins, failing to affect the contractile effects. 4. The relaxation to SP or to [Sar9, Met(O2)11]-SP was abolished by tetrodotoxin (TTX) and significantly reduced by N(omega)-nitro-L-arginine methyl ester (L-NAME). 5. NK2-immunoreactivity (NK2-IR) was seen at the level of the smooth muscle cells of both circular and longitudinal muscle layers. NK1-immunoreactive (NK1-IR) neurons were seen in the myenteric ganglia and NK1/nNOS double labeling revealed that some neurons were both NK1-IR and nNOS-IR. 6. These results suggest that, in mouse stomach, NK1 receptors, causing relaxant responses, are present on nitrergic inhibitory myenteric neurons, whereas NK2 receptors, mediating contractile responses, are present at muscular level.

Relationships between neurokinin receptor-expressing interstitial cells of Cajal and tachykininergic nerves in the gut

The so-called interstitial cells of Cajal (ICC) are distributed throughout the muscle coat of the alimentary tract with characteristic intramural location and species-variations in structure and staining. Several ICC sub-types have been identified: ICC-DMP, ICC-MP, ICC-IM, ICC-SM. Gut motility is regulated by ICC and each sub-type is responsible for the electrical activities typical of each gut region and/or muscle layer. The interstitial position of the ICC between nerve endings and smooth muscle cells has been extensively considered. Some of these nerve endings contain tachykinins. Three distinct tachykinin receptors (NK1r, NK2r and NK3r) have been demonstrated by molecular biology. Each of them binds with different affinities to a series of tachykinins (SP, NKA and NKB). In the ileum, SP-immunoreactive (SP-IR) nerve fibers form a rich plexus at the deep muscular plexus (DMP), distributed around SP-negative cells, and ICC-DMP intensely express the SP-preferred receptor NK1r; conversely a faint NK1r-IR is detected on the ICC-MP and mainly after receptor internalization was induced by agonists. ICC-IM are never stained in laboratory mammals, while those of the human antrum are NK1r- IR. RT-PCR conducted on isolated ileal ICC-MP and gastric ICC-IM showed that these cells express NK1r and NK3r. Colonic ICC, except those in humans, do not express NK1r-IR, at least in resting conditions. Outside the gut, NK1r-IR cells were seen in the arterial wall and exocrine pancreas. In the mouse gut only, NK1r-IR is present in non-neuronal cells located within the intestinal villi, so-called myoid cells, which are c-kit-negative and alpha-smooth muscle actin-positive. Immunohistochemistry and functional studies confirmed that ICC receive input from SP-IR terminals, with differences between ICC sub-types. In the rat, very early after birth, NK1r is expressed by the ICC-DMP and SP by the related nerve varicosities. Studies on pathological conditions are few and those on mutant strains practically absent. It has only been reported that in the inflamed ileum of rats the NK1r-IR ICC-DMP disappear and that at the peak of inflammatory conditions ICC-MP are NK1r-IR. In the ileum of mice with a mutation in the W locus, ICC-DMP were seen to express c-kit-IR but not NK1-IR, and SP-IR innervation seems unchanged. In summary, there are distinct ICC populations, each of them under a different tachykininergic control and, likely, having different functions. Further studies are recommended at the aim of understanding ICC involvement in modulating/transmitting tachykininergic inputs.

Expression of the neurokinin type 1 receptor in the human colon

The distribution of the neurokinin type 1 receptor (NK1r) in human intestine, mapped in a few immunohistochemical investigations in the antrum and the duodenum, is comparable to that widely studied in rodents. Importantly, despite pharmacological evidence of their presence in mammalian intestinal muscle, their immunohistochemical visualization in smooth muscle cells remains to be determined in human digestive tract. In the present work, we studied the distribution of NK1r in the human colon, with a particular view to visualize their expression in muscle cells. With this aim, part of colonic segments were incubated with nicardipine and TTX in order to induce accumulation of the NK1r on cell membrane. NK1r were visualized by using immunohistochemistry combined with fluorescence and confocal microscopy. Without incubation, NK1r-IR was clearly observed on the membrane and the cytoplasm of myenteric and submucous neurons and interstitial cells of Cajal, but could not be clearly determined in the longitudinal and circular muscle. NK1r-IR-expressing neurons and interstitial cells were closely surrounded by substance P (SP) immunoreactive nerves. Incubation of colonic segments with nicardipine and TTX at 4 degrees C for 1 h with SP allowed to reveal a strong NK1r-IR at the surface of muscle cells. Incubation with SP (10(-6) M) at 37 degrees C for 1 min induced a relocation of NK1r-IR into the cytoplasm of muscle. This is interpreted as an internalization of NK1r induced by the binding of SP on muscular NK1r. The present data contribute to emphasize the role of NK1r in tachykinin-mediated neuronal processes regulating intestinal motility.

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

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

Functional characterisation of tachykinin receptors in the circular muscle layer of the mouse ileum

OBJECTIVES

Tachykinins are important mediators in neuromuscular signalling but have not been thoroughly characterised in the mouse gut. We investigated the participation of tachykinin receptors in contractility of circular muscle strips of the mouse ileum.

RESULTS

Electrical field stimulation (EFS) of excitatory nonadrenergic noncholinergic (NANC) nerves induced frequency-dependent contractions which were mimicked by substance P (SP). Desensitisation of SP and NK(1), NK(2) or NK(3) receptors significantly reduced contractions to EFS. The NK(1) receptor blocker RP67580 significantly inhibited NANC contractions to EFS. The NK(2) and NK(3) receptor blockers nepadutant and SR142801 did not affect NANC contractions per se but increased the RP67580-induced inhibition of NANC contractions to EFS. Contractions to SP were significantly reduced by RP67580 but not affected by nepadutant or SR142801. The NK(1) and NK(2) receptor agonists, septide and [beta-ala(8)]-NKA 4-10 (beta-A-NKA), respectively, but not the NK(3) receptor agonist senktide-induced dose-dependent contractions. Atropine inhibited and l-NNA augmented contractions to septide. Contractions to beta-A-NKA were insensitive to atropine but augmented by l-NNA.

CONCLUSIONS

Tachykinins mediate NANC contractions to EFS in the mouse small intestine. Endogenously released tachykinins activate mainly NK(1) receptors, located on cholinergic nerves and smooth muscle cells and, to a lesser degree, NK(2) and NK(3) receptors, most likely located presynaptically.

Immunohistochemical localization of substance P NK1 receptor in guinea pig distal colon

BACKGROUND

Neurokinin receptors facilitate tachykinin mediated intestinal motility and secretion. Distribution of Substance P (SP) neurokinin 1 receptor (NK1r) immunoreactivity (IR) has been previously characterized in guinea pig ileum, but not colon. This study localizes NK1rs in guinea pig distal colon.

METHODS

Neurons were double labelled for NK1r and either acetylcholine transferase (ChAT), calbindin (calb), neuropeptide Y (NPY), nitric oxide synthase (NOS) or SP. The NK1r endocytosis was induced by 10(-5) mol L(-1) SP, septide, [SarMet] SP or neurokinin A.

RESULTS

In guinea pig distal colon, NK1r-IR was present on 70% of submucosal neurons. Sixty-threepercent of the NK1r-IR submucosal neurons were ChAT-IR, 16% calb/SP-IR, 19% NPY-IR and 14% NOS-IR neurons. The NK1r-IR was present on 5% of myenteric neurons. Of these 63% were ChAT-IR, 16% calb-IR neurons and 25% NOS-IR. The NK1rs were also on myenteric plexus interstitial cells of Cajal and on circular muscle.

CONCLUSION

In guinea pig distal colon, NK1rs were on 70% of submucosal neurons including all three secretomotor neuron subtypes and sensory neurons, suggesting NK1rs have a major role in neuronal control of mucosal reflexes. The NK1rs were on few myenteric neurons but were dense on muscle cells, suggesting NK1rs affect motility through neuro-muscular rather than neuro-neuronal transmission.

Subset of cells immunopositive for neurokinin-1 receptor identified as arterial interstitial cells of Cajal in human large arteries

In the adventitia of large arteries, dendritic cells are located between nerve fibers, some of which contain substance P. The aim of the present study was to examine whether neurokinin 1 receptor (NK-1R) was expressed by dendritic cells in the arterial wall. Parallel sections of aortic and carotid artery segments were immunostained with anti-NK-1R and cell-type-specific antibodies. Dendritic cells in the arterial wall expressed NK-1R, albeit at a low level. Other cells, which intensely expressed NK-1R, were located along the border between the media and adventitia. They did not co-express any dendritic cell markers, including fascin, CD1a, S100, or Lag-antigen, and were negative for CD68, CD3, and mast cell tryptase. These NK-1R(+) cells were laser-capture microdissected and studied by means of electron-microscopic analysis. The microdissected cells were in direct contact with nerve endings, and their ultrastructure was typical of the interstitial cells of Cajal present in the gastrointestinal tract. Further systematic electron-microscopic analysis revealed that the cells displaying the features typical of interstitial cells of Cajal were a basic element of the human arterial wall architectonics. Arterial interstitial cells of Cajal were negative for c-kit but they expressed vasoactive intestinal peptide receptor 1 (VIPR1). Destructive alterations of contacts between arterial interstitial cells of Cajal and nerve endings were observed in arterial segments with atherosclerotic lesions. The functional significance of the arterial interstitial cells of Cajal and their possible involvement in atherosclerosis and other vascular diseases need clarification.

Visceral hypersensitivity

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

Chronic constipation in children: organic disorders are a major cause

Diagnostic tools for paediatric chronic constipation have been limited, leading to over 90% of patients with treatment-resistant constipation being diagnosed with chronic idiopathic constipation, with no discernible organic cause. Work in our institution suggests that a number of children with intractable symptoms actually have slow colonic transit leading to slow transit constipation. This paper reviews recent data suggesting that a significant number of the children with chronic treatment-resistant constipation may have organic causes (slow colonic transit and outlet obstruction) and suggests new approaches to the management of children with chronic treatment-resistant constipation.

Intraganglionic laminar endings and their relationships with neuronal and glial structures of myenteric ganglia in the esophagus of rat and mouse

Intraganglionic laminar endings (IGLEs) represent the only vagal mechanosensory terminals in the tunica muscularis of the esophagus and may be involved in local reflex control. We recently detected extensive though not complete colocalization of the vesicular glutamate transporter 2 (VGLUT2) with markers for IGLEs. To elucidate this colocalization mismatch, this study aimed at identifying markers for nitrergic, cholinergic, peptidergic, and adrenergic neurons and glial cells, which may colocalize with VGLUT2 outside of IGLEs. Confocal imaging revealed, besides substantial colocalization of VGLUT2 and substance P (SP), no other significant colocalizations of VGLUT2 and immunoreactivity for any of these markers within the same varicosities. However, we found close contacts of VGLUT2-positive structures to vesicular acetylcholine transporter, choline acetyltransferase, neuronal nitric oxide synthase, galanin, neuropeptide Y, and vasoactive intestinal peptide immunoreactive cell bodies and varicosities, as well as to glial cells. Neuronal perikarya were never positive for VGLUT2. Thus, VGLUT2 was almost exclusively found in IGLEs and may serve as a specific marker for them. In addition, many IGLEs also contained SP. The close contacts established by IGLEs to myenteric cell bodies, dendrites, and varicose fibers suggest that IGLEs modulate various types of enteric neurons and vice versa.

Interstitial cells of Cajal are functionally innervated by excitatory motor neurones in the murine intestine

Recent studies have demonstrated that intramuscular interstitial cells of Cajal (ICC) are preferential targets for neurotransmission in the stomach. Terminals of enteric motor neurones also form tight, synaptic-like contacts with ICC in the small intestine and colon, but little is known about the role of these cells in neurotransmission. ICC at the deep muscular plexus (ICC-DMP) of the small intestine express neurokinin 1 receptors (NK1R) and internalize these receptors in response to exogenous substance P. We used NK1R internalization as an assay of functional innervation of ICC-DMP in the murine small intestine. Under basal conditions NK1R-like immunoreactivity (NK1R-LI) was mainly observed in ICC-DMP (519 cells counted, 100% were positive) and myenteric neurones. ICC-DMP were closely apposed to substance P-containing nerve fibres. Of 338 ICC-DMP examined, 65% were closely associated with at least one substance P-positive nerve fibre, 32% were associated with at least two, 2% were associated with more than two nerve fibres and 1% with none. After electrical field stimulation (EFS, 10 Hz; 1 min) NK1R-LI was internalized in more than 80% of ICC-DMP, as compared to 10% of cells before EFS. Internalization of NK1R was not observed in myenteric ICC or smooth muscle cells in response to nerve stimulation. Internalization of NK1R-LI was blocked by the specific NK1 receptor antagonist WIN 62577 (1 microm) and by tetrodotoxin (0.3 microm), suggesting that internalization resulted from stimulation of receptors with neurally released neurokinins. These data suggest that ICC-DMP are primary targets for neurokinins released from enteric motor neurones in the intestine.

Localization of neurokinin 1 receptor (NK1R) immunoreactivity in rat esophagus

The aim of the present immunohistochemical study was to investigate the localization of neurokinin 1 receptor (NK1R) in rat esophagus and examine the relationship between NK1Rs and intrinsic cholinergic, nitrergic, or substance P (SP) neurons. NK1R immunoreactivity (IR) was observed on the nerve cell bodies in the myenteric ganglia throughout the esophagus, but not on striated muscles and smooth muscle cells of the muscularis mucosae. The frequency of occurrence of NK1R neurons was highest in the cervical esophagus and lowest in the lower thoracic esophagus. Considerable immunoreactivity was seen on the nerve cell surfaces and was also present in the cytoplasm of cell somas and in the initial part of the axons, but not in any other nerve fibers or terminals. Dogiel type I-like morphology was observed in some of the NK1R neurons; however, the majority exhibited polymorphic morphology. Double immunolabeling indicated that a majority (77%) of the NK1R neurons were immunoreactive for choline acetyltransferase (ChAT), while a minority (23%) were immunoreactive for nitric oxide synthase (NOS)-IR. Most of the NK1R neurons (92%) were innervated by the SP nerve fibers. Triple immunolabeling indicated that 70% of the NK1R neurons were associated with intrinsic SP nerve fibers (without CGRP-IR), 59% were associated with extrinsic SP nerve fibers (with CGRP-IR), and 35% were associated with both intrinsic and extrinsic SP nerve fibers. These results suggest that SP/tachykinin released from the SP nerve fibers of intrinsic and/or extrinsic origin activates the predominantly intrinsic cholinergic neurons via NK1Rs to influence neuronal transmission or motility in rat esophagus.

Neurokinin-1 and -3 receptor blockade inhibits slow excitatory synaptic transmission in myenteric neurons and reveals slow inhibitory input

Recent studies have shown that tachykinins mediate slow synaptic transmission to myenteric AH (afterhyperpolarising) neurons via neurokinin-3 receptors (NK(3)R). This study investigated a similar role for neurokinin-1 receptors (NK(1)R) and compared the effect of selective receptor antagonists on non-cholinergic slow excitatory post-synaptic potentials (EPSPs) recorded in myenteric AH neurons of the guinea-pig ileum. Slow EPSPs evoked by electrical stimulation of circumferentially oriented presynaptic nerves were mimicked by application of senktide, an NK(3)R agonist. [Sar(9),Met(O(2))(11)]-substance P, an NK(1)R agonist, depolarised a smaller number of neurons. SR142801, a selective NK(3)R antagonist (100 nM), inhibited slow EPSPs and responses to senktide, but had no effect on depolarisations evoked by forskolin, an activator of adenylate cyclase. SR140333, a selective NK(1)R antagonist, inhibited slow EPSPs in a subset of neurons and blocked responses to [Sar(9),Met(O(2))(11)]-substance P, but not to senktide or forskolin. Slow EPSPs that were predominantly mediated by NK(1)R had significantly shorter latencies than those due to activation of NK(3)R. After blockade of slow EPSPs, slow hyperpolarizing responses to presynaptic nerve stimulation were revealed in one-third of neurons. These events, which were associated with a decrease in input resistance and blocked by tetrodotoxin, were equated with slow inhibitory postsynaptic potentials. They were abolished by the 5-hydroxytryptamine(1A) receptor antagonist 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]-piperazine (NAN-190), but unaffected by phentolamine, an alpha-adrenoceptor antagonist. In conclusion, these results provide the first direct evidence that NK(1)R mediate some slow excitatory synaptic input to myenteric AH neurons, and suggest that NK(1)R and NK(3)R activate distinct signal transduction pathways. These results also demonstrate that slow inhibitory synaptic transmission, which may be mediated by 5-hydroxytryptamine, is more prevalent in the myenteric plexus than previously indicated.

Protein kinases expressed by interstitial cells of Cajal

Interstitial cells of Cajal (ICC) are involved in the generation of electrical rhythmicity of intestinal muscle and in the transduction of neural inputs in the gut. Although the expression of receptors for neurotransmitters and hormones and some second messengers have been investigated in ICC, the protein kinases present in these cells have not been well documented. This study has demonstrated the immunohistochemical localisation of PKA, PKC gamma and PKC theta in ICC that were identified by the known ICC marker, c-Kit, in the guinea-pig gut. Other PKCs, PKC alpha, beta, delta, epsilon, eta, iota and lambda, and Ca(2+)-calmodulin-dependent protein kinase II were not localised in ICC. Double labelling studies were conducted on longitudinal muscle-myenteric plexus and external muscle-myenteric plexus preparations of the oesophagus, stomach (fundus, corpus and antrum), duodenum, distal ileum, caecum, proximal and distal colon, and rectum. The three protein kinases were detected in c-Kit-immunoreactive ICC at the level of the myenteric plexus (IC-MY), in the muscle (IC-IM) and at the level of the deep muscular plexus (IC-DMP) in the small intestine. PKA was found in over 90% of IC-IM in all regions examined, and in over 90% of IC-MY in the gastric body and antrum and throughout the small and large intestines. PKC gamma was in the majority of ICC in the gastric body and antrum and in the small intestine, but was largely absent from ICC in the oesophagus, proximal stomach and large intestine. PKC theta occurred in the majority of ICC in all regions except the rectum. The intensity of staining was greatest for PKA, with PKC gamma giving comparatively weak labelling of ICC. PKA was also detected in myenteric neurons, smooth muscle, macrophages and fibroblast-like cells. PKC gamma labelling occurred in large, multipolar neurons throughout the small and large intestine, as well as in lymph vessels and in capillaries. It is concluded that PKA, PKC gamma and PKC theta are all present in ICC, with the differences in their localisations suggesting specific roles for each in ICC function.

Cholinergic transmission to colonic circular muscle of children with slow-transit constipation is unimpaired, but transmission via NK2 receptors is lacking

Tachykinins (TKs) colocalize with acetylcholine in excitatory motor neurones supplying human colonic circular muscle (CCM). Some children with slow-transit constipation (STC) have reduced TK-immunoreactivity in nerve terminals in CCM suggesting a deficit in neuromuscular transmission. This study aimed to test this possibility. Seromuscular biopsies of transverse colon were obtained laparoscopically from STC children (37, 17 with low density of TK-immunoreactivity). Specimens of transverse (17) and sigmoid colon (20) were obtained from adults undergoing colonic resection for cancer. CCM contractions were measured isotonically and responses to carbachol, neurokinin A (NKA) and electrical field stimulation (EFS) recorded. Carbachol and NKA-evoked contractions in adult and STC colon. Hyoscine (2 micromol L-1) significantly depressed responses to EFS in all preparations. Blockade of NK2 receptors (SR 48968, 2 micromol L-1) significantly depressed EFS-evoked contractions of adult transverse CCM, but had no effect on STC preparations. Thus, neuromuscular transmission in both adults and STC children is predominantly cholinergic and this component is unimpaired in the latter, indicating that reduced TK-immunoreactivity is not a marker for depressed cholinergic responses. Although pharmacologically responsive TK receptors are present in STC colon, we did not detect neuromuscular transmission mediated by release of TKs in these preparations.

PKC-epsilon translocation in enteric neurons and interstitial cells of Cajal in response to muscarinic stimulation

Interstitial cells of Cajal in the deep muscular plexus (ICC-DMP) of the small intestine express excitatory neurotransmitter receptors. We tested whether ICC-DMP are functionally innervated by cholinergic neurons in the murine intestine. Muscles were stimulated by intrinsic nerves and ACh and processed for immunohistochemistry to determine these effects on PKC-epsilon activation. Under control conditions, PKC-epsilon-like immunoreactivy (PKC-epsilon-LI) was only observed in myenteric neurons within the tunica muscularis. Electrical field stimulation or ACh caused translocation of neural PKC-epsilon-LI from the cytosol to a peripheral compartment. After stimulation, PKC-epsilon-LI was found in spindle-shaped cells in the DMP. These cells were identified as ICC-DMP by Kit-LI and vimentin-LI. PKC-epsilon-LI in ICC-DMP and translocation of PKC epsilon-LI in neurons were blocked by tetrodotoxin or atropine, suggesting that these responses were due to activation of muscarinic receptors. Western blots also confirmed translocation of PKC-epsilon-LI. In conclusion, PKC-epsilon translocation is linked to muscarinic receptor activation in ICC-DMP and a subpopulation of myenteric neurons. These studies demonstrate that ICC-DMP are functionally innervated by excitatory motoneurons.