Projections of neurons with neuromedin U-like immunoreactivity in the small intestine of the guinea-pig

The enteric nervous system

Of all the organ systems in the body, the gastrointestinal tract is the most complicated in terms of the numbers of structures involved, each with different functions, and the numbers and types of signaling molecules utilized. The digestion of food and absorption of nutrients, electrolytes, and water occurs in a hostile luminal environment that contains a large and diverse microbiota. At the core of regulatory control of the digestive and defensive functions of the gastrointestinal tract is the enteric nervous system (ENS), a complex system of neurons and glia in the gut wall. In this review, we discuss 1) the intrinsic neural control of gut functions involved in digestion and 2) how the ENS interacts with the immune system, gut microbiota, and epithelium to maintain mucosal defense and barrier function. We highlight developments that have revolutionized our understanding of the physiology and pathophysiology of enteric neural control. These include a new understanding of the molecular architecture of the ENS, the organization and function of enteric motor circuits, and the roles of enteric glia. We explore the transduction of luminal stimuli by enteroendocrine cells, the regulation of intestinal barrier function by enteric neurons and glia, local immune control by the ENS, and the role of the gut microbiota in regulating the structure and function of the ENS. Multifunctional enteric neurons work together with enteric glial cells, macrophages, interstitial cells, and enteroendocrine cells integrating an array of signals to initiate outputs that are precisely regulated in space and time to control digestion and intestinal homeostasis.

Enteric neuroimmune interactions coordinate intestinal responses in health and disease

The enteric nervous system (ENS) of the gastrointestinal (GI) tract interacts with the local immune system bidirectionally. Recent publications have demonstrated that such interactions can maintain normal GI functions during homeostasis and contribute to pathological symptoms during infection and inflammation. Infection can also induce long-term changes of the ENS resulting in the development of post-infectious GI disturbances. In this review, we discuss how the ENS can regulate and be regulated by immune responses and how such interactions control whole tissue physiology. We also address the requirements for the proper regeneration of the ENS and restoration of GI function following the resolution of infection.

Neuronal regulation of the gut immune system and neuromodulation for treating inflammatory bowel disease

The gut immune system in the healthy intestine is anti-inflammatory, but can move to a pro-inflammatory state when the gut is challenged by pathogens or in disease. The nervous system influences the level of inflammation through enteric neurons and extrinsic neural connections, particularly vagal and sympathetic innervation of the gastrointestinal tract, each of which exerts anti-inflammatory effects. Within the enteric nervous system (ENS), three neuron types that influence gut immune cells have been identified, intrinsic primary afferent neurons (IPANs), vasoactive intestinal peptide (VIP) neurons that project to the mucosa, and cholinergic neurons that influence macrophages in the external muscle layers. The enteric neuropeptides, calcitonin gene-related peptide (CGRP), tachykinins, and neuromedin U (NMU), which are contained in IPANs, and VIP produced by the mucosa innervating neurons, all influence immune cells, notably innate lymphoid cells (ILCs). ILC2 are stimulated by VIP to release IL-22, which promotes microbial defense and tissue repair. Enteric neurons are innervated by the vagus, and, in the large intestine, by the pelvic nerves. Vagal nerve stimulation reduces gut inflammation, which may be both by stimulation of efferent (motor) pathways to the ENS, and stimulation of afferent pathways that connect to integrating centers in the CNS. Efferent pathways from the CNS have their anti-inflammatory effects through either or both vagal efferent neurons and sympathetic pathways. The final neurons in sympathetic pathways reduce gut inflammation by the action of noradrenaline on β2 adrenergic receptors expressed by immune cells. Activation of neural anti-inflammatory pathways is an attractive option to treat inflammatory bowel disease that is refractory to other treatments. Further investigation of the ways in which enteric reflexes, vagal pathways and sympathetic pathways integrate their effects to modulate the gut immune system and gut inflammation is needed to optimize neuromodulation therapy.

Neuromedin U, a Key Molecule in Metabolic Disorders

Obesity is now a public health concern. The leading cause of obesity is an energy imbalance between ingested and expended calories. The mechanisms of feeding behavior and energy metabolism are regulated by a complex of various kinds of molecules, including anorexigenic and orexigenic neuropeptides. One of these neuropeptides, neuromedin U (NMU), was isolated in the 1980s, and its specific receptors, NMUR1 and NMUR2, were defined in 2000. A series of subsequent studies has revealed many of the physiological roles of the NMU system, including in feeding behavior, energy expenditure, stress responses, circadian rhythmicity, and inflammation. Particularly over the past decades, many reports have indicated that the NMU system plays an essential and direct role in regulating body weight, feeding behavior, energy metabolism, and insulin secretion, which are tightly linked to obesity pathophysiology. Furthermore, another ligand of NMU receptors, NMS (neuromedin S), was identified in 2005. NMS has physiological functions similar to those of NMU. This review summarizes recent observations of the NMU system in relation to the pathophysiology of obesity in both the central nervous systems and the peripheral tissues.

Neuromedin U: potential roles in immunity and inflammation

Since the discovery of neuromedin U (NmU) from porcine spinal cord in 1985, this neuropeptide has been subsequently identified in many other species with multiple physiological and pathophysiological roles detected, ranging from smooth muscle contraction, feeding, energy balance to tumorigenesis. Intriguingly, NmU is also emerging to play pro-inflammatory roles involving immune cell activation and cytokine release in a neuron-dependent or neuron-independent manner. The NmU-mediated inflammatory responses have already been observed in worm infection, sepsis, autoimmune arthritis and allergic animal models. In this review, we focus on the roles of NmU in immunity and inflammation by highlighting the interactions between NmU and immune cells, summarizing the signalling mechanism involved in their reactions and discussing its potential contributions to inflammatory diseases.

Neuromedin U: a multifunctional neuropeptide with pleiotropic roles

BACKGROUND

Neuromedin U (NmU) belongs to the neuromedin family, comprising a series of neuropeptides involved in the gut-brain axis and including neuromedins B and C (bombesin-like), K (neurokinin B), L (neurokinin A or neurotensin), N, S, and U.

CONTENT

Although initially isolated from porcine spinal cord on the basis of their ability to induce uterine smooth muscle contraction, these peptides have now been found to be expressed in several different tissues and have been ascribed numerous functions, from appetite regulation and energy balance control to muscle contraction and tumor progression. NmU has been detected in several species to date, particularly in mammals (pig, rat, rabbit, dog, guinea pig, human), but also in amphibian, avian, and fish species. The NmU sequence is highly conserved across different species, indicating that this peptide is ancient and plays an important biological role. Here, we summarize the main structural and functional characteristics of NmU and describe its many roles, highlighting the jack-of-all-trades nature of this neuropeptide.

SUMMARY

NmU involvement in key processes has outlined the possibility that this neuropeptide could be a novel target for the treatment of obesity and cancer, among other disorders. Although the potential for NmU as a therapeutic target is obvious, the multiple functions of this molecule should be taken into account when designing an approach to targeting NmU and/or its receptors.

Expression of neuropeptides and anoctamin 1 in the embryonic and adult zebrafish intestine, revealing neuronal subpopulations and ICC-like cells

This immunohistochemical study in zebrafish aims to extend the neurochemical characterization of enteric neuronal subpopulations and to validate a marker for identification of interstitial cells of Cajal (ICC). The expression of neuropeptides and anoctamin 1 (Ano1), a selective ICC marker in mammals, was analyzed in both embryonic and adult intestine. Neuropeptides were present from 3 days postfertilization (dpf). At 3 dpf, galanin-positive nerve fibers were found in the proximal intestine, while calcitonin gene-related peptide (CGRP)- and substance P-expressing fibers appeared in the distal intestine. At 5 dpf, immunoreactive fibers were present along the entire intestinal length, indicating a well-developed peptidergic innervation at the onset of feeding. In the adult intestine, vasoactive intestinal peptide (VIP), pituitary adenylate cyclase-activating peptide (PACAP), galanin, CGRP and substance P were detected in nerve fibers. Colchicine pretreatment enhanced only VIP and PACAP immunoreactivity. VIP and PACAP were coexpressed in enteric neurons. Colocalization stainings revealed three neuronal subpopulations expressing VIP and PACAP: a nitrergic noncholinergic subpopulation, a serotonergic subpopulation and a subpopulation expressing no other markers. Ano1-immunostaining revealed a 3-dimensional network in the adult intestine containing multipolar cells at the myenteric plexus and bipolar cells interspersed between circular smooth muscle cells. Ano1 immunoreactivity first appeared at 3 dpf, indicative of the onset of proliferation of ICC-like cells. It is shown that the Ano1 antiserum is a selective marker of ICC-like cells in the zebrafish intestine. Finally, it is hypothesized that ICC-like cells mediate the spontaneous regular activity of the embryonic intestine.

The antiobesity effects of centrally administered neuromedin U and neuromedin S are mediated predominantly by the neuromedin U receptor 2 (NMUR2)

Neuromedin U (NMU) and neuromedin S (NMS) are structurally related neuropeptides that have been reported to modulate energy homeostasis. Pharmacological data have shown that NMU and NMS inhibit food intake when administered centrally and that NMU increases energy expenditure. Additionally, NMU-deficient mice develop obesity, whereas transgenic mice overexpressing NMU are lean and hypophagic. Two high-affinity NMU/NMS receptors, NMUR1 and NMUR2, have been identified. NMUR1 is predominantly expressed in the periphery, whereas NMUR2 is predominantly expressed in the brain, suggesting that the effects of centrally administered NMU and NMS are mediated by NMUR2. To evaluate the role of NMUR2 in the regulation of energy homeostasis, we characterized NMUR2-deficient (Nmur2(-/-)) mice. Nmur2(-/-) mice exhibited a modest resistance to diet-induced obesity that was at least in part due to reduced food intake. Acute central administration of NMU and NMS reduced food intake in wild-type but not in Nmur2(-/-) mice. The effects on activity and core temperature induced by centrally administered NMU were also absent in Nmur2(-/-) mice. Moreover, chronic central administration of NMU and NMS evoked significant reductions in body weight and sustained reductions in food intake in mice. In contrast, Nmur2(-/-) mice were largely resistant to these effects. Collectively, these data demonstrate that the anorectic and weight-reducing actions of centrally administered NMU and NMS are mediated predominantly by NMUR2, suggesting that NMUR2-selective agonists may be useful for the treatment of obesity.

Emerging pharmacology and physiology of neuromedin U and the structurally related peptide neuromedin S

Neuromedin U (NMU) has been paired with the G-protein-coupled receptors (GPRs) NMU(1) (formerly designated as the orphan GPR66 or FM-3) and NMU(2) (FM-4 or hTGR-1). Recently, a structurally related peptide, neuromedin S (NMS), which shares an amidated C-terminal heptapeptide motif, has been identified in both rat and human, and has been proposed as a second ligand for these receptors. Messenger RNA encoding NMU receptor subtypes shows differential expression: NMU(1) is predominantly expressed in peripheral tissues, particularly the gastrointestinal tract, whereas NMU(2) is abundant within the brain and spinal cord. NMU peptide parallels receptor distribution with highest expression in the gastrointestinal tract and specific structures within the brain, reflecting its major role in the regulation of energy balance. The NMU knockout mouse has an obese phenotype and, in agreement, the Arg165Trp amino acid variant of NMU-25 in humans, which is functionally inactive, co-segregated with childhood-onset obesity. Emerging physiological roles for NMU include vasoconstriction mediated predominantly via NMU(1) with nociception and bone remodelling via NMU(2). The NMU system has also been implicated in the pathogenesis of septic shock and cancers including bladder carcinoma and acute myeloid leukaemia. Intriguingly, NMS is more potent at NMU(2) receptors in vivo where it has similar central actions in suppression of feeding and regulation of circadian rhythms to NMU. Taken together with its vascular actions, NMU may be a functional link between energy balance and the cardiovascular system and may provide a future target for therapies directed against the disorders that comprise metabolic syndrome.

Neuromedin U can exert colon-specific, enteric nerve-mediated prokinetic activity, via a pathway involving NMU1 receptor activation

BACKGROUND AND PURPOSE

The neuromedin U (NMU) receptors, NMU1 and NMU2, are expressed in the gut but their functions are unclear. This study explores the role of NMU in gastrointestinal motility.

EXPERIMENTAL APPROACH

The effects of NMU were examined in the forestomach and colon isolated from NMU2R wild-type and NMU2R-/- (knockout) mice, looking for changes in muscle tension and in nerve-mediated responses evoked by electrical field stimulation (EFS), and in models of peristalsis in mouse colon and faecal pellet transit in guinea-pig colon.

KEY RESULTS

In the mouse forestomach, NMU (1 nM-10 microM) concentration-dependently induced muscle contraction, in the presence of tetrodotoxin and atropine, in preparations from both wild-type and NMU2R-/- mice (pEC50: 7.9, 7.6, Emax: 0.26, 0.20g tension, respectively, n=8 each concentration). The same concentrations of NMU had no consistent effects on the responses to EFS (n=8). In the mouse colon, NMU (0.1 nM-1 microM) had no significant effect on baseline muscle tension (n=8), but concentration-dependently potentiated EFS-evoked contractions in preparations from both wild-type and NMU2R-/- mice, pEC50: 8.1, 7.8, Emax: 24%, 21%, respectively, n=6-11. NMU (0.01 nM-0.1 microM, n=5-7) concentration-dependently decreased the interval between waves of peristalsis in the mouse colon (pEC50: 8.8) and increased the rate at which a faecal pellet moved along the guinea-pig colon.

CONCLUSIONS AND IMPLICATIONS

These results demonstrate that NMU exerts colon-specific, nerve-mediated, prokinetic activity, via a pathway involving activation of NMU1 receptors. This suggests that this receptor may represent a molecular target for the treatment of intestinal motility disorders.

Species-dependent smooth muscle contraction to Neuromedin U and determination of the receptor subtypes mediating contraction using NMU1 receptor knockout mice

The peptide ligand neuromedin U (NMU) has been implicated in an array of biological activities, including contraction of uterine, intestinal and urinary bladder smooth muscle. However, many of these responses appear to be species-specific. This study was undertaken to fully elucidate the range of smooth muscle-stimulating effects of NMU in rats, mice and guinea-pigs, and to examine the extent of the species differences. In addition, the NMU1 receptor knockout mouse was used to determine which receptor subtype mediates the contractile responses generated by NMU in the mouse. A range of isolated organ in vitro bioassays were carried out, which were chosen to re-confirm previous literature reports (uterine and stomach fundus contraction) and also to explore potentially novel smooth muscle responses to NMU. This investigation uncovered a number of previously unidentified NMU-mediated responses: contraction of rat lower esophageal sphinster (LES), rat ileum, mouse gallbladder, enhancement of electrically evoked contractions in rat and mouse vas deferens, and a considerable degree of cross-species differences. Studies using the NMU1 receptor knockout mice revealed that in the mouse fundus and gallbladder assays the NMU contractile response was mediated entirely through the NMU1 receptor subtype, whereas, in assays of mouse uterus and vas deferens, the response to NMU was unchanged in the NMU1 receptor knockout mouse, suggesting that the NMU response may be mediated through the NMU2 receptor subtype. NMU receptor subtype-selective antagonists are required to further elucidate the role of the individual receptor subtypes.

Neuromedin U stimulates contraction of human long saphenous vein and gastrointestinal smooth muscle in vitro

The neuropeptide Neuromedin U (NMU) stimulates smooth muscle contraction, and modulates local blood flow and adrenocortical function via two endogenous receptors, NMU1 and NMU2. Although its amino-acid sequence is highly conserved across species, the physiological effects of NMU are variable between species and little is known of its effects on human tissues. We have examined the contractile effects of NMU-25 on human smooth muscles of the gastrointestinal (GI) tract (ascending colon, gallbladder) and long saphenous vein (LSV) using in vitro organ bath bioassays. From LSV, ileum, gallbladder, caecum and colon, NMU receptor transcripts were amplified by RT-PCR and expression levels were determined by semi-quantitative scanning densitometry. NMU-25 produced a concentration-dependent, sustained contraction of isolated smooth muscle (p[A](50)+/-s.e.m., ascending colon, 8.93+/-0.18; gallbladder, 7.01+/-0.15; LSV, 8.67+/-0.09). NMU1 and NMU2 receptor transcription was detected in all tissues; transcription of both receptors was similar in gallbladder, but NMU1 receptor transcription was predominant in the sigmoid colon and LSV. In summary, these studies indicate that NMU may control tone in the human GI tract and LSV through an action on smooth muscle. Development of NMU receptor subtype-selective ligands will aid the further elucidation of the physiological roles of NMU and its two receptors.

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.

Neuromedin U and its receptors: structure, function, and physiological roles

Neuromedin U (NmU) is a structurally highly conserved neuropeptide. It is ubiquitously distributed, with highest levels found in the gastrointestinal tract and pituitary. Originally isolated from porcine spinal cord, it has since been isolated and sequenced from several species. Amino acid alignment of NmU from different species reveals a high level of conservation, and particular features within its structure are important for bioactivity. Specifically, the C terminus, including a terminal asparagine-linked amidation, is essential for activity. The conservation of NmU across a wide range of species indicates a strong evolutionary pressure to conserve this peptide and points to its physiological significance. Despite this, the precise physiological and indeed pathophysiological roles of NmU have remained elusive. NmU was first isolated based on its ability to contract rat uterine smooth-muscle (hence the suffix "U") and has since been implicated in the regulation of smooth-muscle contraction, blood pressure and local blood flow, ion transport in the gut, stress responses, cancer, gastric acid secretion, pronociception, and feeding behavior. Two G-protein-coupled receptors for NmU have recently been cloned. These receptors are widespread throughout the body but have differential distributions suggesting diverse but specific roles for the receptor subtypes. Here we detail the isolation and characterization of NmU, describe the discovery, cloning, distribution, and structure of its two receptors, and outline its possible roles in both physiology and pathophysiology. Ultimately the development of receptor-specific ligands and the generation of animals in which the receptors have been selectively knocked out will hopefully reveal the true extent of the biological roles of NmU and suggest novel therapeutic indications for selective activation or blockade of either of its receptors.

The distribution of P2X3 purine receptor subunits in the guinea pig enteric nervous system

Adenosine 5'-triphosphate (ATP) excites 70-90% of enteric neurons through P2X type purine receptors, and is likely to be an enteric neurotransmitter. Recent studies indicate that the P2X2 subunit is expressed by specific subgroups of enteric neurons, and that there are enteric neurons that are responsive to ATP but lack this subunit. In the present work, we have investigated whether the P2X3 subunit is similarly localised to specific subgroups of neurons, and whether these are different from the P2X2 subunit-expressing neurons. The P2X3 subunit was localised by immunohistochemistry to nerve cells of the myenteric ganglia of the stomach, small and large intestines, and nerve cells of the submucosal ganglia in the small and large intestines of the guinea pig. All immunoreactivity was absorbed with the P2X3 receptor peptide against which the antiserum was raised. In myenteric ganglia of the ileum, P2X3 receptor immunoreactivity was in calretinin, enkephalin and nitric oxide synthase (NOS)-immunoreactive neurons. In submucosal ganglia, all calretinin-immunoreactive nerve cells were P2X3 receptor immunoreactive. In the submucosal ganglia of the ileum, 13 +/- 3% of neuropeptide Y (NPY)-immunoreactive neurons were also P2X3 receptor immunoreactive, whereas in the distal colon, almost all NPY-expressing nerve cells were P2X3 receptor immunoreactive. The localisation of the P2X3 subunit was largely distinct from that of the P2X2 subunit, although both subunits occur in some NOS neurons, where P2X2 and P2X3 subunits may form heteromeric receptors. Unlike the P2X2 subunit, the P2X3 subunit is not expressed in intrinsic sensory neurons in the ileum. It is concluded that the P2X3 receptor subunit is expressed in specific functional groups of neurons; the major types are excitatory and inhibitory muscle motor neurons, ascending interneurons and cholinergic secretomotor neurons.

Characterization of neuromedin U effects in canine smooth muscle

Two endogenous receptors for the potent smooth muscle-stimulating peptide neuromedin U (NmU) have recently been identified and cloned. Pharmacological, binding, and expression studies were conducted in an attempt to determine the receptor(s) involved in the smooth muscle-stimulating effects of NmU. The NmU peptides caused a concentration-dependent contraction of canine isolated urinary bladder. NmU did not have this same effect in the urinary bladder from rat, guinea pig, rabbit, mouse, or ferret. Although NmU had no effect on canine uterus it did cause contraction of canine stomach, ileum, and colon. As well as causing contraction of canine bladder in vitro, NmU administered systemically resulted in a significant increase in urinary bladder pressure in vivo. High-affinity binding sites for NmU were identified in canine bladder. The four NmU peptides porcine NmU-8, rat NmU-23, human NmU-25, and porcine NmU-25 displaced (125)I-NmU-25 binding with similar K(i) values (0.08-0.24 nM). A different binding profile was revealed in human embryonic kidney-293 cells transiently expressed with the canine NmU-2 receptor where porcine NmU-8 (K(i) = 147.06 nM) was much less potent than the other NmU peptides. Using TaqMan, expression of NmU-1 was detected in human urinary bladder, small intestine, colon, and uterus. Expression of NmU-2 was much lower or absent in these human tissues and undetectable in canine bladder and stomach. The results of this study reveal significant species differences in the activity of NmU. The contractile activity in human and canine smooth muscle seems to be mediated by the recently cloned NmU-1 receptor.

Retrograde tracing of enteric neuronal pathways

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

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

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

Identification and functional characterization of a novel subtype of neuromedin U receptor

Neuromedin U is a bioactive peptide isolated originally from the porcine spinal cord. We recently identified neuromedin U as the cognate ligand for the orphan G protein-coupled receptor FM-3. In this study, we isolated cDNA coding for a novel G protein-coupled receptor, TGR-1, which was highly homologous with FM-3. We found that neuromedin U specifically and clearly elevated the extracellular acidification rates, arachidonic acid metabolite release, and intracellular Ca(2+) mobilization in Chinese hamster ovary cells expressing TGR-1. Radiolabeled neuromedin U specifically bound with high affinity to membrane fractions prepared from these cells. These results show that TGR-1, like FM-3, is a specific and functional receptor for neuromedin U. We analyzed TGR-1 mRNA tissue distribution in rats using quantitative reverse transcription-polymerase chain reaction and found it to considerably differ from that of FM-3 mRNA. TGR-1 mRNA was primarily expressed in the uterus, suggesting that TGR-1 mediates the contractile activity of neuromedin U in this tissue. The identification of specific and functional receptor subtypes for neuromedin U will facilitate the study of their physiological roles and the search for their specific agonists and antagonists.

Identification of neuromedin U as the cognate ligand of the orphan G protein-coupled receptor FM-3

Neuromedin U is a bioactive peptide first isolated from porcine spinal cord. In this paper, we demonstrate that neuromedin U is the cognate ligand for the orphan G protein-coupled receptor, FM-3, isolated originally as a homologue of neurotensin and growth hormone secretogogue receptors. Neuromedin U induced specific and evident elevation of extracellular acidification rates, arachidonic acid metabolite release, and intracellular Ca(2+) mobilization in Chinese hamster ovary cells expressing human FM-3. In addition, radiolabeled neuromedin U specifically bound to membrane fractions prepared from these cells with high affinity. We subsequently analyzed the tissue distribution of neuromedin U and FM-3 mRNAs in rats using quantitative reverse transcription-polymerase chain reaction. Neuromedin U mRNA was highly expressed in the gastrointestinal tract, and the highest expression was detected in the pituitary gland. On the other hand, FM-3 mRNA was highly expressed in the small intestine and lung, suggesting that neuromedin U plays important roles in these tissues. The identification of a specific and functional receptor for neuromedin U will facilitate studies on their physiological roles and the search for receptor agonists and antagonists.

Neurochemical classification of myenteric neurons in the guinea-pig ileum

A strategy has been developed to identify and quantify the different neurochemical populations of myenteric neurons in the guinea-pig ileum using double-labelling fluorescence immunohistochemistry of whole-mount preparations. First, six histochemical markers were used to identify exclusive, non-overlapping populations of nerve cell bodies. They included immunoreactivity for the calcium binding proteins calbindin and calretinin, the neuropeptides vasoactive intestinal polypeptide, substance P and somatostatin, and the amine, 5-hydroxytryptamine. The sizes of these populations of neurons were established directly or indirectly in double-labelling experiments using a marker for all nerve cell bodies. Each of these exclusive populations was further subdivided into classes by other markers, including immunoreactivity for enkephalins and neurofilament protein triplet. The size of each class was then established directly or by calculation. These distinct, neurochemically-identified classes were related to other published work on the histochemistry, electrophysiology and retrograde labelling of enteric neurons and to the simple Dogiel morphological classification. A classification scheme, consistent with previous studies, is proposed. It includes 14 distinct classes of myenteric neurons and accounts for nearly all neurons in the myenteric plexus of the guinea-pig ileum.

Chemical coding of neurons that project from different regions of intestine to the coeliac ganglion of the guinea pig

The chemical codings of neurons that project from the small intestine, caecum, proximal colon, distal colon and rectum to the coeliac ganglion of the guinea pig were investigated. The coeliac ganglion was injected with the retrogradely transported dye Fast Blue, and each of the regions was examined 6 days later in wholemounts that had been prepared for immunohistochemical localisation of pairs of antigens. In both the small and large intestines, all intestinofugal neurons were immunoreactive (IR) for choline acetyltransferase (ChAT). In each region of the large intestine, the largest population, representing 50-60% of retrogradely labelled neurons in each region, was immunoreactive for ChAT, bombesin (BN), calbindin (Calb) and nitric oxide synthase (NOS). Most intestinofugal neurons in the small intestine contain bombesin and VIP-IR along with ChAT-IR but none contain either Calb or NOS. Thus, nerve endings of enteric origin in the coeliac ganglion that contain NOS-IR or Calb-IR come from the large intestine and those with bombesin-IR but not NOS-IR are from the small intestine. The gastric wall was injected with Fast Blue in order to label noradrenergic (NA) neurons in the coeliac ganglion and to determine, by localisation of NOS and bombesin-IR, whether they receive inputs from the small and large intestine. Some NA neurons received inputs from the large intestine (and perhaps also from the small intestine) and some received inputs exclusively from the small intestine. Most NA neurons that received intestinofugal inputs had the chemical code NA/-; some were immunoreactive for somatostatin (NA/SOM neurons), but those with IR for neuropeptide Y (NA/NPY) rarely received intestinofugal inputs.

Sources of inputs to longitudinal muscle motor neurons and ascending interneurons in the guinea-pig small intestine

Light- and electron-microscopic studies were used to investigate connections between specific subgroups of neurons in the myenteric plexus of the guinea-pig small intestine. Inputs to two classes of calretinin-immunoreactive (IR) nerve cells, longitudinal muscle motor neurons and ascending interneurons, were examined. Inputs from calbindin-IR primary sensory neurons and from three classes of descending interneurons were studied. Electron-microscopic analysis showed that calbindin-IR axons formed two types of inputs, synapses and close contacts, on calretinin-IR neurons. About 40% of inputs to the longitudinal muscle motor neurons and 70% to ascending interneurons were calbindin-IR. Approximately 50% of longitudinal muscle motor neurons were surrounded by bombesin-IR dense pericellular baskets and 40% by closely apposed varicosities. At the electron-microscope level, the bombesin-IR varicosities were found to form synapses and close contacts with the motor neurons. Dense pericellular baskets with bombesin-IR surrounded 36% of all ascending interneurons, and a further 17% had closely apposed varicosities. Somatostatin- and 5-HT-IR descending interneurons provided no dense pericellular baskets to calretinin-IR nerve cells. Thus, calretinin-IR, longitudinal muscle motor neurons and ascending interneurons receive direct synaptic inputs from intrinsic primary sensory neurons and from non-cholinergic, bombesin-IR, descending interneurons.

Somal size and location within the ganglia for electrophysiologically identified myenteric neurons of the guinea pig ileum

The main goal of the present study was to examine the possibility of electrophysiologically identifying the excitable enteric S and AH neurons by use of one single criterion. Intracellular recordings were made from 189 cells of 64 ganglia in isolated preparations of the myenteric plexus of the guinea pig distal ileum. The recordings were made under visual control of the cells by using Hoffman Modulation Contrast optics at high magnification (600x). From photomicrographs, the soma size and the location within the ganglion of the individual (unstained) cells were determined. The cells were classified into three types according to their electrical excitability and the shape of the action potential. Excitable cells were classified as AH cells (n = 84) if the action potential showed a shoulder on the falling phase, otherwise as S cells (n = 56). Cells in which no action potential could be evoked by current injection were classified as nonspiking (NS) cells (n = 49). The three classes of cells showed significant differences with respect to membrane potential, input resistance and fast synaptic input. The AH cells had significantly larger somata (P < 0.01) than the S cells. The NS cells were significantly smaller than the AH and S cells (P < 0.01). AH and S cells were found to be randomly located in the ganglia, whereas the NS cells clustered (P < 0.008) in close proximity to the onsets of internodal strands. We conclude that the shoulder of the action potential can be used as a single criterion to distinguish "on line" S and AH neurons unequivocally.

Plurichemical transmission and chemical coding of neurons in the digestive tract

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

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

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

Morphological features of the myenteric plexus of the stomach of the axolotl, Ambystoma mexicanum, revealed by immunocytochemistry

The general morphology of the intramural innervation of the myenteric plexus of the axolotl stomach has been investigated using antisera raised against neuron-specific enolase and a microtubule-associated protein. Additionally, the occurrence of serotonin and several peptidergic neurotransmitter/neuromodulator substances was studied. Immunoreactivity for galanin, vasoactive intestinal polypeptide, substance P and neuromedin U was found in both fibres and intrinsic perikarya, whereas the serotonin and calcitonin gene-related peptide-like-substance-containing nerve fibres seemed to be of extrinsic origin. The axolotl stomach myenteric plexus appeared to be devoid of enkephalin-, neuropeptide Y-, somatostatin- and bombesin-like immunoreactive nerve fibres and nerve cell bodies. Double labelling experiments revealed the presence of a subpopulation of substance P/calcitonin gene-related peptide-like immunoreactive nerve fibres. Contrary to mammals, no coexistence of neuromedin U and substance P was found. Our findings illustrate that besides a number of similarities, considerable species differences exist between urodeles and anurans with regard to the organization of the enteric nervous system.

Primary structure and pharmacological activity of a nonapeptide related to neuromedin U isolated from chicken intestine

An extract of chicken intestine contained neuromedin U-like immunoreactivity (36 pmol/g wet tissue weight). The primary structure of the predominant molecular form (NMU-9), comprising 94% of the total immunoreactivity, was established as: Gly-Tyr-Phe-Phe-Phe-Arg-Pro-Arg- Asn-NH2. This sequence differs from that of pig neuromedin U-8 (NMU-8) by the substitution of Leu3 by Phe and, like the corresponding peptide from the guinea pig, is extended from the NH2-terminus by a Gly residue. A minor component of neuromedin U comprised 25 amino acid residues. An extract of chicken whole brain contained much less NMU-like immunoreactivity (1.5 pmol/g) and the nonpeptide was the only molecular form detected. Synthetic chicken NMU-9 produced a concentration-dependent contraction of smooth muscle from the rat uterus and its effect was unchanged in the presence of tetrodotoxin, atropine and indomethacin. The potency of chicken NMU-9 (EC50 360 +/- 60 nM; mean +/- S.E., n = 6) was approximately 8-fold less than that of pig NMU-8 (EC50 46 +/- 8 nM) but the maximum contraction produced by both agonists was not significantly different.

Comparison of the presence and actions of substance P and neurokinin A in guinea-pig taenia coli

The presence and sites of action of two closely related tachykinins, substance P (SP) and neurokinin A (NKA), were examined in the taenia coli of the guinea-pig. SP- and NKA-like immunoreactivity (LI) were demonstrated histochemically in nerve fibres supplying the taenia. Chromatographic characterization of aqueous acetic acid extracts of taenia showed only one peak of SP-LI, corresponding in retention time to authentic SP, whereas there were multiple peaks of NKA-LI, the major one of which corresponded to authentic NKA. SP-LI and NKA-LI, determined by radioimmunoassay, were in a molar ratio of SP equivalents to NKA equivalents of 8.5:1 in taenia extracts. Extrinsic denervation of the caecum had no significant effect on the concentration of either SP-LI or NKA-LI or on their immunohistochemical distributions. Both SP and NKA (10(-10) to 10(-5) M) caused contractions of the taenia that were unaffected by hyoscine (10(-6) M), mepyramine (10(-6) M) or tetrodotoxin (5 x 10(-7) M), indicating that both peptides act directly on the smooth muscle of the taenia. Contractions to SP occurred after a short, but concentration-dependent, delay, reached a peak quickly, and then decayed. In contrast, NKA caused contractions after longer latencies, the peak was reached more slowly, and the response was maintained for up to 10 min. (D-Pro2, D-Trp7,9)-SP (10(-5) M) antagonised responses to SP and NKA to a similar degree. It is concluded that both NKA and SP should be considered as transmitter candidates for non-cholinergic nerve-mediated excitation in the taenia.

Rabbit neuromedin U-25: lack of conservation of a posttranslational processing site

The rabbit small intestine contains neuromedin U-like immunoreactivity (22 pmol/g wet tissue weight) that was resolved into a single major molecular form by reversed-phase HPLC. The primary structure of the peptide was established as: Phe-Pro-Val-Asp-Glu-Glu-Phe-Gln-Ser-Pro10-Phe-Gly-Ser-Arg-Ser-Arg- Gly-Tyr-Phe- Leu20-Phe-Arg-Pro-Arg-Asn.NH2. In rabbit neuromedin U, the Arg16-Arg17 dibasic residue processing site that is found in pig and dog neuromedin U-25 is replaced by Arg-Gly, but this potential monobasic processing site is not utilized by cleavage enzyme(s) in the intestine.

The innervation of the gastrointestinal tract of a chelonian reptile, Pseudemys scripta elegans. II. Distribution of neuropeptides in the myenteric plexus

The myenteric plexus of the stomach, midgut and hindgut of the red-eared turtle, Pseudemys scripta elegans, has been investigated for the occurrence of immunoreactivity to nine neuropeptides. Neuropeptide Y (NPY)-, calcitonin gene-related peptide (CGRP)-, bombesin (BOM)- as well as substance P (SP)-like immunoreactivity (LI) were found in nerve fibres of all investigated gut regions. From all peptides investigated immunoreactivity for NPY was more pronounced. In the stomach NPY-LI was mainly found in the perikarya, while in the midgut region both NPY-immunoreactive (IR) somata and nerve fibres were revealed. The hindgut harboured few NPY-IR nerve cells and nerve fibres. A few SP-IR nerve cell bodies were observed in the stomach and midgut region. In the hindgut BOM-IR neuronal cell bodies were found. Neuromedin U (NMU)-LI was mainly observed in the stomach region, revealing both immunoreactive perikarya and nerve fibres. Immunoreactivity for vasoactive intestinal polypeptide, somatostatin, galanin and enkephalin could not be detected so far. Double labelling experiments revealed the coexistence of CGRP and SP in some nerve fibres in all three gut regions examined. Some SP-IR fibres in the midgut were immunoreactive for NMU.

The neuroendocrine system of the gut--an update

During the last few years the endocrine stomach has come into focus much due to the side-effects produced by powerful acid blockers. A sustained and marked inhibition of acid secretion in the rat results in hypergastrinemia, with gastrin cell hyperplasia, and a consequent hyperplasia of the ECL cells. This response of the ECL cells was predictable in view of previous observations that sustained hypergastrinemia causes ECL cell hyperplasia. While the gastrin cell hyperplasia levels off at about twice the normal cell density a few weeks after start of treatment, the ECL cells continue to proliferate for months to reach a five-fold higher density than normally. Evidence is accumulating that ECL cells proliferate through self replication. After life-long inhibition of acid production (high doses of ranitidine or omeprazole) or after extirpation of 75% of the acid-producing part of the stomach, ECL cell carcinoids develop. Endocrine cells in the gut often contain more than one putative messenger. Thus, gastrin cells in many species store GABA and peptide YY; in e.g. cat and man they store in addition a xenopsin-like peptide. Neuromedin U and pituitary adenylate cyclase activating peptide (PACAP) have recently been demonstrated in gut nerves. Their role in gut physiology remains to be identified.

Morphological and chemical identification of neurons that project from the colon to the inferior mesenteric ganglia in the guinea-pig

Labelled nerve cells were located in the distal colon of the guinea-pig 4-5 days after the retrograde tracing agent, Fast blue, was injected into the inferior mesenteric ganglia. Labelled neurons were only found in the myenteric plexus. Their frequency increased from oral to anal and was greater towards the mesenteric border, compared with the anti-mesenteric aspect, of the colon. Many retrogradely labelled neurons were immunoreactive for vasoactive intestinal peptide or calbindin. In the inferior mesenteric ganglia, vasoactive intestinal peptide and calbindin immunoreactive nerve fibres surrounded the same clumps of nerve cell bodies. Almost all calbindin and vasoactive intestinal peptide immunoreactive terminals degenerated after the nerves running from the large intestine to the inferior mesenteric ganglia were cut. It is concluded that the great majority of calbindin and vasoactive intestinal peptide immunoreactive terminals in the inferior mesenteric ganglia arise from nerve cell bodies in the myenteric plexus of the large intestine.

Isolation and microsequence analysis of a novel form of neuromedin U from guinea pig small intestine

A multidimensional chromatographic regimen has been used to isolate and purify a peptide showing immunoreactivity for neuromedin U from guinea pig small intestine. Microsequence Edman N-terminal analysis and C-terminal analysis by enzymatic digestion showed this peptide to be a nonapeptide with the following sequence: H-Gly-Tyr-Phe-Leu-Phe-Arg-Pro-Arg-Asn-NH2. The C-terminal octapeptide of this sequence is the same as porcine NMU-8, and the C-terminal heptapeptide is identical to rat NMU(17-23).