Nitric oxide synthase immunoreactivity in the enteric nervous system of the developing human digestive tract

Nitrergic and Substance P Immunoreactive Neurons in the Enteric Nervous System of the Bottlenose Dolphin (Tursiops truncatus) Intestine

Simple Summary

The gastrointestinal tract of the bottlenose dolphin (Tursiops truncatus) differs structurally and functionally from that of terrestrial mammals. In particular, the intestine does not show any macroscopic subdivision and lacks a caecum. In addition, the histological aspect of the intestine is relatively constant, without marked differences between the anterior and posterior parts. Although the intestine of these cetaceans presents differences in comparison with terrestrial mammals, little information is currently available on their enteric nervous system. The aim of the present study was to investigate the morphological and quantitative aspects of neurons immunoreactive (IR) for the neuronal nitric oxide synthase (nNOS) and Substance P (SP) in the intestine of bottlenose dolphins (Tursiops truncatus). In these dolphin specimens, a smaller number of nNOS-IR neurons in the submucosal plexus and a larger number of SP-IR neurons in the myenteric plexus were observed compared to other mammals. Interestingly, no co-localization between nNOS- and SP-IR neurons was detected in either of the plexuses, suggesting the existence of two completely distinct functional classes of neurons in the intestine of the bottlenose dolphin.

Abstract

Compared with other mammals, the digestive system of cetaceans presents some remarkable anatomical and physiological differences. However, the neurochemical features of the enteric nervous system (ENS) in these animals have only been described in part. The present study gives a description of the nitrergic and selected peptidergic systems in the myenteric plexus (MP) and submucosal plexus (SMP) of the intestine of the bottlenose dolphin (Tursiops truncatus). The distribution and morphology of neurons immunoreactive (IR) for the neuronal nitric oxide synthase (nNOS) and Substance P (SP) were immunohistochemically studied in formalin-fixed specimens from the healthy intestine of three animals, and the data were compared with those described in the literature on other mammals (human and non-human). In bottlenose dolphins, the percentages of nitrergic neurons (expressed as median and interquartile range—IQR) were 28% (IQR = 19–29) in the MP and 1% (IQR = 0–2) in the SMP, while the percentages of SP-IR neurons were 31% (IQR = 22–37) in the MP and 41% (IQR = 24–63) in the SMP. Although morphological features of nNOS- and SP-IR neurons were similar to those reported in other mammals, we found some noticeable differences in the percentages of enteric neurons. In fact, we detected a lower proportion of nNOS-IR neurons in the SMP and a higher proportion of SP-IR neurons in the MP compared to other mammals. To the best of the authors’ knowledge, this study represents the first description and quantification of nNOS-IR neurons and the first quantification of SP-IR neurons in the intestine of a cetacean species. As nNOS and SP are important mediators of intestinal functions and the nitrergic population is an important target for many neuroenteropathies, data obtained from a healthy intestine provide a necessary basis to further investigate and understand possible functional differences and motor intestinal dysfunctions/alterations in these special mammals.

Quantitative analysis of enteric neurons containing choline acetyltransferase and nitric oxide synthase immunoreactivities in the submucosal and myenteric plexuses of the porcine colon

The enteric nervous system (ENS) controls gastrointestinal functions. In large mammals' intestine, it comprises an inner (ISP) and outer (OSP) submucous plexus and a myenteric plexus (MP). This study quantifies enteric neurons in the ISP, OSP, and MP of the pig ascending (AC) and descending colon (DC) using the HuC/D, choline acetyltransferase (ChAT), and neuronal nitric oxide synthase (nNOS) neuronal markers in whole mount preparations with multiple labeling immunofluorescence. We established that the ISP contains the highest number of HuC/D neurons/mm², which were more abundant in AC vs. DC, followed by OSP and MP with similar density in AC and DC. In the ISP, the density of ChAT immunoreactive (IR) neurons was very similar in AC and DC (31% and 35%), nNOS-IR neurons were less abundant in AC than DC (15% vs. 42%, P < 0.001), and ChAT/nNOS-IR neurons were 5% and 10%, respectively. In the OSP, 39-44% of neurons were ChAT-IR in AC and DC, while 45% and 38% were nNOS-IR and 10-12% were ChAT/nNOS-IR (AC vs. DC P < 0.05). In the MP, ChAT-IR neurons were 44% in AC and 54% in DC (P < 0.05), nNOS-IR neurons were 50% in both, and ChAT/nNOS-IR neurons were 12 and 18%, respectively. The ENS architecture with multilayered submucosal plexuses and the distribution of functionally distinct groups of neurons in the pig colon are similar to humans, supporting the suitability of the pig as a model and providing the platform for investigating the mechanisms underlying human colonic diseases.

Neural Crest Cell Implantation Restores Enteric Nervous System Function and Alters the Gastrointestinal Transcriptome in Human Tissue-Engineered Small Intestine

Acquired or congenital disruption in enteric nervous system (ENS) development or function can lead to significant mechanical dysmotility. ENS restoration through cellular transplantation may provide a cure for enteric neuropathies. We have previously generated human pluripotent stem cell (hPSC)-derived tissue-engineered small intestine (TESI) from human intestinal organoids (HIOs). However, HIO-TESI fails to develop an ENS. The purpose of our study is to restore ENS components derived exclusively from hPSCs in HIO-TESI. hPSC-derived enteric neural crest cell (ENCC) supplementation of HIO-TESI establishes submucosal and myenteric ganglia, repopulates various subclasses of neurons, and restores neuroepithelial connections and neuron-dependent contractility and relaxation in ENCC-HIO-TESI. RNA sequencing identified differentially expressed genes involved in neurogenesis, gliogenesis, gastrointestinal tract development, and differentiated epithelial cell types when ENS elements are restored during in vivo development of HIO-TESI. Our findings validate an effective approach to restoring hPSC-derived ENS components in HIO-TESI and may implicate their potential for the treatment of enteric neuropathies.

Regenerative capacity of the enteric nervous system: is immaturity defining the point of no return?

BACKGROUND

Intestinal obstruction in newborns is associated with intestinal motility disorders after surgery. Alterations in the enteric nervous system (ENS) might cause abnormal peristalsis, which may then result in intestinal motility disorders. We aimed to quantify alterations in the myenteric plexus after a ligation and to test if these alterations were reversible.

METHODS

Small intestines of chicken embryos were ligated in ovo at embryonic day (ED) 11 for either 4 d (ED 11-15) or 8 d (ED 11-19). Both treated groups and control group were sacrificed and intestinal segments examined by means of both light and electron microscopy.

RESULTS

The number of proximal myenteric ganglia increased (ED 19, 30.7 ± 3.16 versus 23.1 ± 2.03; P < 0.001) in the 8-d ligature group but had values similar to the control group in the 4-d ligature group. The size distribution was skewed toward small ganglia in the 8-d ligature group (ED 19, 83.71 ± 11.60% versus 3.88 ± 4.74% in the control group; P < 0.001) but comparable with the control group in the 4-d ligature group. Subcellular alterations in the 4-d ligature group were reversible.

CONCLUSIONS

The pathologic alterations in the ENS were fully reversible in the 4-d ligature group. This reversibility might be linked to the degree of immaturity of the ENS.

Chagasic megacolon: enteric neurons and related structures

Megacolon, the irreversible dilation of a colonic segment, is a structural sign associated with various gastrointestinal disorders. In its hereditary, secondary form (e.g. in Hirschsprung's disease), dilation occurs in an originally healthy colonic segment due to an anally located, aganglionic zone. In contrast, in chronic Chagas' disease, the dilated segment itself displays pathohistological changes, and the earliest and most prominent being found was massive loss of myenteric neurons. This neuron loss was partial and selective, i.e. some neurons containing neuronal nitric oxide synthase and/or vasoactive intestinal peptide (VIP) were spared from neuron death. This disproportionate survival of inhibitory neurons, however, did not completely correlate with the calibre change along the surgically removed, megacolonic segments. A better correlation was observed as to potentially contractile muscle tissue elements and the interstitial cells of Cajal. Therefore, the decreased densities of α-smooth muscle actin- and c-kit-immunoreactive profiles were estimated along resected megacolonic segments. Their lowest values were observed in the megacolonic zones itself, whereas less pronounced decreases were found in the non-dilated, transitional zones (oral and anal to dilation). In contrast to the myenteric plexus, the submucosal plexus displayed only a moderate neuron loss. Neurons co-immunoreactive for VIP and calretinin survived disproportionately. As a consequence, these neurons may have contributed to maintain the epithelial barrier and allowed the chagasic patients to survive for decades, despite their severe disturbance of colonic motility. Due to its neuroprotective and neuroeffectory functions, VIP may play a key role in the development and duration of chagasic megacolon.

What neurons hide behind calretinin immunoreactivity in the human gut?

Calretinin (CALR) is often used as an immunohistochemical marker for the histopathological diagnosis of human intestinal neuropathies. However, little is known about its distribution pattern with respect to specific human enteric neuron types. Prior studies revealed CALR in both myenteric and submucosal neurons, most of which colabel with choline acetyl transferase (ChAT). Here, we specified the chemical code of CALR-positive neurons in small and large intestinal wholemounts in a series of 28 patients. Besides other markers, we evaluated the labeling pattern of CALR in combination with vasoactive intestinal peptide (VIP). In colonic submucosa, CALR and VIP were almost completely colocalized in about three-quarters of all submucosal neurons. In the small intestinal submucosa, both the colocalization rate of CALR and VIP as well as the proportion of these neurons were lower (about one-third). In the myenteric plexus of both small intestine and colon, CALR amounted to 11 and 10 %, respectively, whereas VIP to 5 and 4 % of the whole neuron population, respectively. Colocalization of both markers was found in only 2 and 3 % of myenteric neurons, respectively. In section specimens, nerve fibers coreactive for CALR and VIP were found in the mucosa but not in the muscle coat. Summarizing the present and earlier results, CALR was found in at least one submucosal and two myenteric neuron populations. Submucosal CALR+/VIP+/ChAT± neurons innervate mucosal structures. Furthermore, CALR immunoreactivity in the myenteric plexus was observed in morphological type II (supposed primary afferent) and spiny type I (supposed inter- or motor-) neurons.

Diabetes-related alterations in the enteric nervous system and its microenvironment

Gastric intestinal symptoms common among diabetic patients are often caused by intestinal motility abnormalities related to enteric neuropathy. It has recently been demonstrated that the nitrergic subpopulation of myenteric neurons are especially susceptible to the development of diabetic neuropathy. Additionally, different susceptibility of nitrergic neurons located in different intestinal segments to diabetic damage and their different levels of responsiveness to insulin treatment have been revealed. These findings indicate the importance of the neuronal microenvironment in the pathogenesis of diabetic nitrergic neuropathy. The main focus of this review therefore was to summarize recent advances related to the diabetes-related selective nitrergic neuropathy and associated motility disturbances. Special attention was given to the findings on capillary endothelium and enteric glial cells. Growing evidence indicates that capillary endothelium adjacent to the myenteric ganglia and enteric glial cells surrounding them are determinative in establishing the ganglionic microenvironment. Additionally, recent advances in the development of new strategies to improve glycemic control in type 1 and type 2 diabetes mellitus are also considered in this review. Finally, looking to the future, the recent and promising results of metagenomics for the characterization of the gut microbiome in health and disease such as diabetes are highlighted.

The enteric nervous system

The enteric nervous system (ENS), the intrinsic innervation of the gastrointestinal tract, consists of numerous types of neurons, and glial cells, that are distributed in two intramuscular plexuses that extend along the entire length of the gut and control co-ordinated smooth muscle contractile activity and other gut functions. All enteric neurons and glia are derived from neural crest cells (NCC). Vagal (hindbrain) level NCC provide the majority of enteric precursors along the entire length of the gut, while a lesser contribution, that is restricted to the hindgut, arises from the sacral region of the neuraxis. After leaving the dorsal neural tube NCC undergo extensive migration, proliferation, survival and differentiation in order to form a functional ENS. This article reviews the molecular mechanisms underlying these key developmental processes and highlights the major groups of molecules that affect enteric NCC proliferation and survival (Ret/Gdnf and EdnrB/Et-3 pathways, Sox10 and Phox2b transcription factors), cell migration (Ret and EdnrB signalling, semaphorin 3A, cell adhesion molecules, Rho GTPases), and the development of enteric neuronal subtypes and morphologies (Mash1, Gdnf/neurturin, BMPs, Hand2, retinoic acid). Finally, looking to the future, we discuss the need to translate the wealth of data gleaned from animal studies to the clinical area and thus better understand, and develop treatments for, congenital human diseases affecting the ENS.

Quantitation of cellular components of the enteric nervous system in the normal human gastrointestinal tract--report on behalf of the Gastro 2009 International Working Group

BACKGROUND

Patients with gastrointestinal neuromuscular diseases may undergo operative procedures that yield tissue appropriate to diagnosis of underlying neuromuscular pathology. Critical to accurate diagnosis is the determination of limits of normality based on the study of control human tissues. Although robust diagnostic criteria exist for many qualitative alterations in the neuromuscular apparatus, these do not include quantitative values due to lack of adequate control data.

PURPOSE

The aim of this report was to summarize all relevant available published quantitative data for elements of the human enteric nervous system (neuronal cell bodies, glial cells, and nerve fibers) from the perspective of the practicing pathologist. Forty studies meeting inclusion criteria were systematically reviewed with data tabulated in detail and discussed in the context of methodological variations and limitations. The results reveal a lack of concordance between observations of different investigators resulting in data insufficient to produce robust normal ranges. This diversity highlights the need to standardize the way pathologists collect, process, and quantitate neuronal and glial elements in enteric neuropathologic samples, as suggested by recent international guidelines on gastrointestinal neuromuscular pathology.

Neurochemical coding of enteric neurons in adult and embryonic zebrafish (Danio rerio)

Although the morphology and development of the zebrafish enteric nervous system have been extensively studied, the precise neurochemical coding of enteric neurons and their proportional enteric distribution are currently not known. By using immunohistochemistry, we determined the proportional expression and coexpression of neurochemical markers in the embryonic and adult zebrafish intestine. Tyrosine hydroxylase (TH), vasoactive intestinal peptide (VIP), and pituitary adenylate cyclase-activating peptide (PACAP) were observed only in nerve fibers, whereas other markers were also detected in neuronal cell bodies. Calretinin and calbindin had similar distributions. In embryos, all markers, except for choline acetyltransferase (ChAT) and TH, were present from 72 hours postfertilization. Nitrergic neurons, evenly distributed and remaining constant in time, constituted the major neuronal subpopulation. The neuronal proportions of the other markers increased during development and were characterized by regional differences. In the adult, all markers examined were expressed in the enteric nervous system. A large percentage of enteric neurons displayed calbindin and calretinin, and serotonin was the only marker showing significant distribution differences in the three intestinal regions. Colocalization studies showed that serotonin was not coexpressed with any of the other markers. At least five neuronal subpopulations were determined: a serotonergic, a nitrergic noncholinergic, two cholinergic nonnitrergic subpopulations along with one subpopulation expressing both ChAT and neuronal nitric oxide synthase. Analysis of nerve fibers revealed that nitrergic neurons coexpress VIP and PACAP, and that nitrergic neurons innervate the tunica muscularis, whereas serotonergic and cholinergic nonnitrergic neurons innervate the lamina propria and the tunica muscularis.

Tonic inhibition of human small intestinal motility by nitric oxide in children but not in adults

BACKGROUND

Gastrointestinal motility is dependent on neural influences that largely involve the enteric nervous system (ENS). The main motor patterns that occur in the fasted and fed state are noticeably different in children compared with adults. Although the development of the ENS continues after birth, there is no data on the contractile activity of segments of small intestine from young children. This study was designed to provide data on the development of muscle control by the human ENS with particular attention to acetylcholine (ACh) and nitric oxide (NO) as the primary neurotransmitters of enteric motor neurons, respectively.

METHODS

Small intestinal specimens were obtained from 11 children and six adults undergoing surgery for various diseases. The mechanical activity of the circular muscle was recorded in vitro. The effects of N(ω)-nitro-L-arginine methyl ester hydrochloride, an inhibitor of NO synthesis, and of atropine, an antagonist of muscarinic receptors, were tested on the spontaneous motility and responses to nerve stimulation.

KEY RESULTS

Spontaneous motility was observed in all preparations. Responses to nerve stimulation were identical in child and adult. No tonic cholinergic excitation of small intestinal motility was observed either in child or in adult. Inhibition of NO synthesis induced a major disinhibition of motility in child but not in adult.

CONCLUSIONS & INFERENCES

Spontaneous intestinal motility and cholinergic and nitrergic neurotransmission are present from birth. NO provides a tonic inhibition of intestinal motility only in child. Our study indicates that NO may be a major player in shaping the ontogenic development of intestinal motility in human.

Spatial pattern analysis of nitrergic neurons in the myenteric plexus of the duodenum of different mammalian species

Nitrergic myenteric neurons are especially susceptible to the development of neuropathy in functional gastrointestinal disorders. Investigations of the similarities and dissimilarities in the organization of nitrergic neurons in the various mammalian species are therefore important in an effort to determine the extent to which the results obtained in different animal models can be generalized. In the present work, the density and the spatial organization of the nitrergic neurons in the myenteric plexus of the duodenum were investigated in 7 mammalian species. After nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry, the Plexus Pattern Analysis software (PPAs) was applied to count the nuclei of nitrergic neurons, calculate the proportions of the areas covered by the plexus and perform randomization analysis. All 7 species exhibited a large population of nitrergic myenteric neurons, with densities in the range 12-56 cells/mm 2 . The distribution patterns of these neurons differed markedly in the different species, however, the rat was the only species in which the nitrergic neurons appeared to be randomly distributed. The PPAs in conjunction with NADPH-d histochemistry proved to be a simple and fast tool with which to reveal similarities and dissimilarities in the spatial arrangement of the nitrergic neurons in the different species.

Development of enteric neuron diversity

The mature enteric nervous system (ENS) is composed of many different neuron subtypes and enteric glia, which all arise from the neural crest. How this diversity is generated from neural crest-derived cells is a central question in neurogastroenterology, as defects in these processes are likely to underlie some paediatric motility disorders. Here we review the developmental appearance (the earliest age at which expression of specific markers can be localized) and birthdates (the age at which precursors exit the cell cycle) of different enteric neuron subtypes, and their projections to some targets. We then focus on what is known about the mechanisms underlying the generation of enteric neuron diversity and axon pathfinding. Finally, we review the development of the ENS in humans and the etiologies of a number of paediatric motility disorders.

The human neonatal small intestine has the potential for arginine synthesis; developmental changes in the expression of arginine-synthesizing and -catabolizing enzymes

Background

Milk contains too little arginine for normal growth, but its precursors proline and glutamine are abundant; the small intestine of rodents and piglets produces arginine from proline during the suckling period; and parenterally fed premature human neonates frequently suffer from hypoargininemia. These findings raise the question whether the neonatal human small intestine also expresses the enzymes that enable the synthesis of arginine from proline and/or glutamine. Carbamoylphosphate synthetase (CPS), ornithine aminotransferase (OAT), argininosuccinate synthetase (ASS), arginase-1 (ARG1), arginase-2 (ARG2), and nitric-oxide synthase (NOS) were visualized by semiquantitative immunohistochemistry in 89 small-intestinal specimens.

Results

Between 23 weeks of gestation and 3 years after birth, CPS- and ASS-protein content in enterocytes was high and then declined to reach adult levels at 5 years. OAT levels declined more gradually, whereas ARG-1 was not expressed. ARG-2 expression increased neonatally to adult levels. Neurons in the enteric plexus strongly expressed ASS, OAT, NOS1 and ARG2, while varicose nerve fibers in the circular layer of the muscularis propria stained for ASS and NOS1 only. The endothelium of small arterioles expressed ASS and NOS3, while their smooth-muscle layer expressed OAT and ARG2.

Conclusion

The human small intestine acquires the potential to produce arginine well before fetuses become viable outside the uterus. The perinatal human intestine therefore resembles that of rodents and pigs. Enteral ASS behaves as a typical suckling enzyme because its expression all but disappears in the putative weaning period of human infants.

Purinergic and nitrergic junction potential in the human colon

The aim of the present work is to investigate a putative junction transmission [nitric oxide (NO) and ATP] in the human colon and to characterize the electrophysiological and mechanical responses that might explain different functions from both neurotransmitters. Muscle bath and microelectrode techniques were performed on human colonic circular muscle strips. The NO donor sodium nitroprusside (10 microM), but not the P2Y receptor agonist adenosine 5'-O-2-thiodiphosphate (10 microM), was able to cause a sustained relaxation. NG-nitro-L-arginine (L-NNA) (1 mM), a NO synthase inhibitor, but not 2'-deoxy-N6-methyl adenosine 3',5'-diphosphate tetraammonium salt (MRS 2179) (10 microM), a P2Y antagonist, increased spontaneous motility. Electrical field stimulation (EFS) at 1 Hz caused fast inhibitory junction potentials (fIJPs) and a relaxation sensitive to MRS 2179 (10 microM). EFS at higher frequencies (5 Hz) showed biphasic IJP with fast hyperpolarization sensitive to MRS 2179 followed by sustained hyperpolarization sensitive to L-NNA; both drugs were needed to fully block the EFS relaxation at 2 and 5 Hz. Two consecutive single pulses induced MRS 2179-sensitive fIJPs that showed a rundown. The rundown mechanism was not dependent on the degree of hyperpolarization and was present after incubation with L-NNA (1 mM), hexamethonium (100 microM), MRS 2179 (1 microM), and NF023 (10 microM). We concluded that single pulses elicit ATP release from enteric motor neurons that cause a fIJP and a transient relaxation that is difficult to maintain over time; also, NO is released at higher frequencies causing a sustained hyperpolarization and relaxation. These differences might be responsible for complementary mechanisms of relaxation being phasic (ATP) and tonic (NO).

Immediate insulin treatment prevents gut motility alterations and loss of nitrergic neurons in the ileum and colon of rats with streptozotocin-induced diabetes

The streptozotocin-induced diabetic rat model was used to investigate the relation between the deranged gut motility and the segment-specific quantitative changes in the nitrergic myenteric neurons. Additionally, we studied the effectiveness of early insulin replacement to prevent the diabetes-induced changes. Rats were divided into three groups: controls, diabetics and insulin-treated diabetics. Ten weeks after the onset of diabetes, animals were chosen from each group for intestinal transit measurements. The remainder were killed and gut segments were processed for NADPH-diaphorase histochemistry and HuC/HuD immunohistochemistry. The diabetic rats displayed faster transit than that for the controls. In the insulin-treated group, the transit time was the same as that in the controls. In the duodenum of the diabetic rats, the number of nitrergic neurons was decreased, while the total neuronal number was not altered. In the jejunum, ileum and colon, both the total and the nitrergic neuronal cell number decreased significantly. Insulin treatment did not prevent the nitrergic cell loss significantly in the duodenum and jejunum, but it did prevent it significantly in the ileum and colon. These findings comprise the first evidence that the nitrergic neurons located in different intestinal segments exhibit different susceptibilities to a diabetic state and to insulin treatment.

The developmental stage and cell type dependent phosphorylation of eNOS in murine enteric mucosa and myenteric plexus

In order to clarify the developmental regulation of the eNOS activity in intestine by phosphorylation, we examined the immunohistochemical localizations of the eNOS phosphorylation sites at Ser(1177), Ser(116) and at Thr(495) in cells of the mouse enteric mucosa and myenteric plexus at E13.5, E14.5, E16.5, E18.5, E20.5 and P3. In addition, in cells of the E16.5 stage the protein levels of eNOS and the phosphorylation sites of eNOS at Ser(1177), Ser(116) and at Thr(495) were investigated by immunoblot. From E14.5 to P3, phosphorylation residues of eNOS at Ser(1177) and at Ser(116) were detected with different staining intensities in the enteric mucosa epithelium. In ganglion cells of the myenteric plexus Ser(116) was identified at E18.5 to P3. The absence of phosphorylated Thr(495) in cells of intestine during all developmental stages, was confirmed by immunoblot at E16.5. The immunoblot levels of eNOS and eNOS phosphorylated at Ser(1177) and at Ser(116) were comparable with the immunohistochemical results of E16.5 mouse intestine. It was concluded that development of epithelial cells of the enteric mucosa may be modulated by phosphorylation of eNOS at Ser(1177) and at Ser(116). The phosphorylation of eNOS in cells of the myenteric plexus is modulated at Ser(116). These data suggest that there is a developmental stage and cell type dependent phosphorylation of eNOS.

Alteration of nitrergic neuromuscular transmission as a result of acute experimental colitis in rat

Nitric oxide (NO) is a non-adrenergic, non-cholinergic neurotransmitter found in the enteric nervous system that plays a role in a variety of enteropathies, including inflammatory bowel disease. Alteration of nitrergic neurons has been reported to be dependent on the manner by which inflammation is caused. However, this observed alteration has not been reported with acetic acid-induced colitis. Therefore, the purpose of the current study was to investigate changes in nitrergic neuromuscular transmission in experimental colitis in a rat model. Distal colitis was induced by intracolonic administration of 4 % acetic acid in the rat. Animals were sacrificed at 4 h and 48 h postacetic acid treatment. Myeloperoxidase activity was significantly increased in the acetic acid-treated groups. However, the response to 60 mM KCl was not significantly different in the three groups studied. The amplitude of phasic contractions was increased by Nomega-nitro-L-arginine methyl ester (L-NAME) in the normal control group, but not in the acetic acid-treated groups. Spontaneous contractions disappeared during electrical field stimulation (EFS) in normal group. However, for the colitis groups, these contractions initially disappeared, and then reappeared during EFS. Moreover, the observed disappearance was diminished by L-NAME; this suggests that these responses were NO-mediated. In addition, the number of NADPH-diaphorase positive nerve cell bodies, in the myenteric plexus, was not altered in the distal colon; whereas the area of NADPH-diaphorase positive fibers, in the circular muscle layer, was decreased in the acetic acidtreated groups. These results suggest that NO-mediated inhibitory neural input, to the circular muscle, was decreased in the acetic acid-treated groups.

The effects of endogenous and exogenous nitric oxide on gut motility in zebrafish Danio rerio embryos and larvae

Using motion analysis, the ontogeny of the nitrergic control system in the gut was studied in vivo in zebrafish Danio rerio embryos and larvae. For the first time we show the presence of a nitrergic tonus,modulating both anterograde and retrograde contraction waves in the intestine of developing zebrafish. At 4 d.p.f. (days post fertilisation), the nitric oxide synthase (NOS) inhibitor l-NAME (three boluses of 50–100 nl, 10–3 mol l–1) increased the anterograde contraction wave frequency by 0.50±0.10 cycles min–1. Subsequent application of the NO donor sodium nitroprusside (SNP; three boluses of 50–100 nl, 10–4mol l–1) reduced the frequency of propagating anterograde waves (–0.71±0.20 cycles min–1). This coincided with the first appearance of an excitatory cholinergic tonus, observed in an earlier study. One day later, at 5 d.p.f., in addition to the effect on anterograde contraction waves, application of l-NAME increased(0.39±0.15 cycles min–1) and following SNP application reduced (–1.61±0.36 cycles min–1) the retrograde contraction wave frequency. In contrast, at 3 d.p.f., when no spontaneous motility is observed, application of l-NAME did not induce contraction waves in either part of the gut, indicating the lack of a functional inhibitory tonus at this early stage. Gut neurons expressing NOS-like immunoreactivity were present in the distal and middle intestine as early as 2 d.p.f., and at 1 day later in the proximal intestine. In conclusion, the present study suggests that a nitrergic inhibitory tonus develops shortly before or at the time for onset of exogenous feeding.

Gastrointestinal Function Regulation by Nitrergic Efferent Nerves

Gastrointestinal (GI) smooth muscle responses to stimulation of the nonadrenergic noncholinergic inhibitory nerves have been suggested to be mediated by polypeptides, ATP, or another unidentified neurotransmitter. The discovery of nitric-oxide (NO) synthase inhibitors greatly contributed to our understanding of mechanisms involved in these responses, leading to the novel hypothesis that NO, an inorganic, gaseous molecule, acts as an inhibitory neurotransmitter. The nerves whose transmitter function depends on the NO release are called "nitrergic", and such nerves are recognized to play major roles in the control of smooth muscle tone and motility and of fluid secretion in the GI tract. Endothelium-derived relaxing factor, discovered by Furchgott and Zawadzki, has been identified to be NO that is biosynthesized from l-arginine by the constitutive NO synthase in endothelial cells and neurons. NO as a mediator or transmitter activates soluble guanylyl cyclase and produces cyclic GMP in smooth muscle cells, resulting in relaxation of the vasculature. On the other hand, NO-induced GI smooth muscle relaxation is mediated, not only by cyclic GMP directly or indirectly via hyperpolarization, but also by cyclic GMP-independent mechanisms. Numerous cotransmitters and cross talk of autonomic efferent nerves make the neural control of GI functions complicated. However, the findingsrelated to the nitrergic innervation may provide us a new way of understanding GI tract physiology and pathophysiology and might result in the development of new therapies of GI diseases. This review article covers the discovery of nitrergic nerves, their functional roles, and pathological implications in the GI tract.

Fetal intestinal obstruction induces alteration of enteric nervous system development in human intestinal atresia

Intestinal motility disorders are a major cause of morbidity after surgical repair of intestinal atresia of unknown mechanism. We hypothesized that interruption of antenatal peristalsis may disturb the normal development of the enteric nervous system. Using a series of neuronal (synaptophysin, neuronal nitric oxide synthase, neurofilaments) and nonneuronal markers (glial acidic fibrillary protein and c-Kit) and immunohistochemistry, we have defined developmental steps of the enteric nervous system in normal intestine (12 fetuses, 15 children, and 4 adults) and their alterations above and below the obstacle in 22 human intestinal atresia compared with age-matched controls. Antisynaptophysin antibody revealed the progressive conversion of the myenteric plexus from a continuous belt into regularly spaced ganglions during normal fetal gut development and, by contrast, the significantly delayed appearance of individual neuronal ganglions in the distal segments of atresia (p < 0.05). Staging using three other markers for neuronal (neurofilaments and neuronal nitric oxide synthase) and nonneuronal cells (glial acidic fibrillary protein) confirmed that maturation of the myenteric plexus was significantly delayed below atresia (p < 0.01). These results indicate that intestinal atresia impairs the development of the enteric nervous system and provide an anatomical substrate for the motility disorders observed after surgical repair. They point to the role of peristalsis in normal gut development and suggest that stimulation of peristalsis might be used to accelerate recovery.

Spatial pattern analysis of nitrergic neurons in the developing myenteric plexus of the human fetal intestine

BACKGROUND

Enteric nervous system precursors derived from the neural crest migrate along defined pathways to colonize the bowel. The individual cells in different environments experience different growth, differentiation, and survival conditions. Hence, the spatial distribution of the neurons is determinant with regard to functional maturation. The question arises as to whether the distribution is random or nonrandom.

METHODS

Nitrergic cells were visualized by means of nicotinamide adenine dinucleotide phosphate diaphorase histochemistry. Stained specimens were photographed, and the borders of the myenteric plexus and the nuclei of the nitrergic neurons were digitalized. Plexus Pattern Analysis software was used to count the nuclei of nitrergic neurons, calculate the proportions of the areas covered by the plexus and the gut wall, and perform randomization analyses.

RESULTS

The distribution pattern of the nitrergic neurons changed markedly between weeks 14 and 22 of gestation. The nitrergic neurons were randomly distributed at week 14 but were aggregated in the plexus and within the individual ganglia at week 19. The dynamics of these changes exhibited regional differences.

CONCLUSIONS

The results suggest that, in addition to the gut wall and the plexus, other intraganglionic constituents may contribute to the aggregation of nitrergic cells and such examinations should be extended to other cell types in the future.

Coexistence of non-adrenergic non-cholinergic inhibitory and excitatory neurotransmitters in a large neuronal subpopulation in the vaginal segment of the chicken oviduct

The presence, distribution and smooth muscle motor effects of galanin and pituitary adenylate cyclase activating peptide (PACAP) were studied in the nerves of the vaginal part of the oviduct of egg-laying hens. Galanin and PACAP immunoreactivity were found both in neuronal perikarya and nerve fibres within the wall of the vaginal segment. Both populations showed a similar distribution pattern. Particularly the circular muscle and the intramural vascular net were richly innervated. A few galanin- and PACAP-IR nerve fibres extended up to the mucosal folds. Multiple labelling showed galanin to be colocalised with PACAP as well as with vasoactive intestinal polypeptide (VIP) and nitric oxide synthase (NOS) in a large, partly intrinsic neuronal subpopulation innervating the smooth muscle wall. Pharmacological in vitro experiments showed that isolated vaginal muscle strips had a spontaneous basal activity that was not affected by the neuronal conductance blocker tetrodotoxin (TTX). Galanin induced concentration-dependent contractions that were TTX-insensitive. PACAP, VIP, nitric oxide (NO) and the NO donor nitroglycerin caused concentration-dependent relaxations that were TTX-insensitive. Electrical field stimulation of isolated muscle strips induced frequency-dependent relaxations that were blocked by TTX and reduced by the NOS blocker L-nitroarginine. These data provide evidence that the vaginal part of the oviduct contains a largely intrinsic, neuronal subpopulation, capable of releasing multiple non-adrenergic, non-cholinergic (NANC) motor agents for the control of local muscular activities. In addition, we provided pharmacological evidence that VIP, NO and PACAP exert an inhibitory and galanin an excitatory action on isolated muscle strips of the vaginal part of the chicken oviduct. Our results suggest that these NANC neurotransmitters play an important role in the regulation of neuromuscular activity in this region.

Structural differences in the enteric neural network in murine colon: impact on electrophysiology

The enteric neural network in the proximal murine colon shows a regularly occurring hypoganglionic region, which is here characterized by using anatomical and electrophysiological techniques. Staining with NADPH diaphorase, methylene blue, and cuprolinic blue in standard whole mounts and three-dimensional gut preparations of the murine proximal colon consistently revealed two hypoganglionic areas surrounded by a dense clustering of enteric neurons. This irregularity in the ganglionic plexus was found to be present in mice of three different genetic backgrounds, as well as in rats. The lack of myenteric ganglia in these regions was associated with an absence of the longitudinal muscle layer, as shown in cross sections. Histochemical identification of interstitial cells of Cajal in Kit(W-lacZ/+) transgenic mice showed Kit-positive cells oriented parallel to both muscle layers of the colon. Kit-positive cells oriented parallel to the longitudinal muscle layers were absent in the hypoganglionic area described. Electrical field stimulation elicited TTX-sensitive inhibitory junction potentials (IJPs), which showed region-specific characteristics. The initial partly apamin-sensitive hyperpolarization was present in all parts of the murine colon, whereas a second sustained NG-nitro-L-arginine-sensitive hyperpolarization was absent in the cecum and decreased from the proximal to the distal colon. Dissecting the hypoganglionic area from the surrounding tissue abolished the otherwise normal inhibitory neurotransmission to the circular muscle (1.6 +/- 1.4 and 2.6 +/- 1.7 mV for the fast and slow component of IJP amplitude in the hypoganglionic area vs. 16.5 +/- 1.9 and 23.7 +/- 2.7 mV for the fast and slow component of IJP amplitude in the neuron-rich area, respectively, P < 0.01, n = 6), whereas dissection of an area of identical size with an intact myenteric network showed normal inhibitory neurotransmission, indicating that the hypoganglionic area receives essential functional neural input from the neuron-rich surrounding tissue. In summary, in the murine and rat proximal colon, a constant and distinct hypoganglionic region is described with important concomitant changes in local electrophysiology.

Myenteric nitrergic neurons along the rat esophagus: evidence for regional and strain differences in age-related changes

Several studies have suggested an age-related reduction in the number of myenteric neurons in the lower gastrointestinal (GI) tract linked to changes in GI neuromuscular functions. The present study, combining protein gene product 9.5 immunostaining and NADPH-diaphorase histochemistry, aimed at quantifying the proportion of nitrergic neurons compared to the overall number of enteric neurons in the esophagus of young (3-4.5 months) and aged (18-20 months) Sprague-Dawley and Wistar rats. In both strains, the neuron numbers per ganglion in the cervical region were almost twice as high as in the other esophageal regions. Irrespective of age or strain, the esophagus harbored a very high proportion of intrinsic nitrergic neurons (greater than approximately 65%). Both strains showed with aging an overall neuronal loss of approximately 27%. While a significant increase (young: 64-71%; aged: 82-89%) was observed in all esophageal regions in the Wistar strain, the proportion of nitrergic neurons remained stable with aging in the Sprague-Dawley strain (range: 72-82%). In conclusion, the age-related reduction in the overall number of myenteric, nitrergic, and non-nitrergic neurons observed in the rat esophagus, appears to be highly region- and strain-dependent. Therefore, a protective mechanism against neuronal cell loss, selectively present in specific (nitrergic) enteric subpopulations, as suggested in earlier reports, cannot be put forward as a general phenomenon throughout the entire GI tract.

Role of enteric glial cells in inflammatory bowel disease

Enteric glial cells (EGCs) represent an extensive but relatively poorly described cell population within the gastrointestinal tract. Accumulating data suggest that EGCs represent the morphological and functional equivalent of CNS astrocytes within the enteric nervous system (ENS). The EGC network has trophic and protective functions toward enteric neurons and is fully implicated in the integration and the modulation of neuronal activities. Moreover, EGCs seem to be active elements of the ENS during intestinal inflammatory and immune responses, sharing with astrocytes the ability to act as antigen-presenting cells and interacting with the mucosal immune system via the expression of cytokines and cytokine receptors. Transgenic mouse systems have demonstrated that specific ablation of EGC by chemical ablation or autoimmune T-cell targeting induces an intestinal pathology that shows similarities to the early intestinal immunopathology of Crohn's disease. EGCs may also share with astrocytes the ability to regulate tissue integrity, thereby postulating that similar interactions to those observed for the blood-brain barrier may also be partly responsible for regulating mucosal and vascular permeability in the gastrointestinal tract. Disruption of the EGC network in Crohn's disease patients may represent one possible cause for the enhanced mucosal permeability state and vascular dysfunction that are thought to favor mucosal inflammation.

A critical assessment of the neurodestructive and neuroprotective effects of nitric oxide

Whether nitric oxide is cytodestructive or cytoprotective is of obvious clinical importance. The debate on this subject in the past decade has generated much "heat and light". This paper focuses on the actions of NO on the nervous system and reexamines the controversial issue and the contribution of the authors and their colleagues in the light of recent findings. We also report new findings, critically assesses previous experimental data, and share perspectives on this important subject.

Distribution and effects of PACAP, VIP, nitric oxide and GABA in the gut of the African clawed frog Xenopus laevis

The distribution and possible effects on gastrointestinal motility of pituitary adenylate cyclase-activating polypeptide (PACAP), vasoactive intestinal polypeptide (VIP), nitric oxide and γ-amino-butyric acid(GABA) were investigated in the African clawed frog (Xenopus laevis)using immunohistochemistry and in vitro strip preparations. PACAP-and VIP-immunoreactive nerve fibres were common in the myenteric plexus as well as in the longitudinal and circular muscle layers all along the gastrointestinal tract. Double labelling demonstrated a close correlation between PACAP and VIP immunoreactivities, indicating that the two neurotransmitters are colocalised within the enteric nervous system. Occasionally, PACAP- and VIP-positive nerve cell bodies were seen in the myenteric or submucous plexa. In addition, VIP immunoreactivity coexisted with helospectin immunoreactivity. Nitric oxide synthase (NOS)-immunoreactive nerve cells were found in the myenteric plexus at an average density for the whole gastrointestinal tract of 4584±540 cells cm-2. The NOS-immunoreactive nerve cells were usually multipolar with an average size of 11.3±3.7 × 23.2±6.6 μm. Some NOS-immunoreactive nerve fibres were VIP-immunoreactive but not all VIP-positive fibres showed NOS immunoreactivity. GABA immunoreactivity was found in nerve fibres and nerve cells in the myenteric plexus of all regions of the gut. Few GABA-immunoreactive nerve fibres were VIP-immunoreactive. PACAP 27, VIP,sodium nitroprusside (a nitric oxide donor; NaNP) and GABA caused similar responses on spontaneously contracting circular preparations of the cardiac stomach of X. laevis. The mean force developed was decreased, mainly by a reduction in resting tension, while the amplitude of contractions was not necessarily affected. The NOS inhibitor NG-nitro-L-arginine methyl ester (L-NAME) increased the mean force developed, indicating a nitrergic tone in the preparations. In contrast, PACAP 27, VIP, NaNP, GABA and L-NAME had no significant effect on longitudinal strip preparations from the duodenum. These results indicate that PACAP, VIP, nitric oxide and GABA, which are known to be important inhibitory neurotransmitters in other vertebrates, are widely spread in the enteric nervous system of Xenopus laevis and may be involved in the inhibitory control of gastric motility. Although no effect of PACAP,VIP, nitric oxide or GABA on the longitudinal strips of the duodenum was seen in this study, this does not rule out the possibility that they might play an important role in controlling intestinal motility as well.

Arginine supplementation prevents necrotizing enterocolitis in the premature infant

OBJECTIVES

To determine whether supplementation with L -arginine reduces the incidence of all stages of necrotizing enterocolitis (NEC) in premature infants with birth weight < or =1250 g and gestational age < or =32 weeks.

STUDY DESIGN

In a randomized, double-blind, placebo-controlled study, 152 premature infants were prospectively, randomly assigned to receive either supplemental L -arginine (1.5 mmol/kg per day; n =75 [group A]) or placebo (control group; n = 77 [group B]) with oral feeds/parenteral nutrition during the first 28 days of life. Nutrient intake, plasma ammonia, arginine, and amino acid concentrations were measured in all infants at days 3, 14, and 28 and at the time of diagnosis of NEC.

RESULTS

NEC developed in 5 infants in group A compared with 21 infants in group B (P <.001). Arginine intake and plasma arginine concentrations were similar in both groups at study entry and (as expected) increased in group A at days 14 and 28. Plasma arginine concentrations were lower in both groups at time of diagnosis of NEC. No significant differences in maternal and neonatal demographics, nutrient intake, plasma ammonia and total and essential amino acid concentrations were present between the two groups.

CONCLUSIONS

Arginine supplementation (1.5 mmol/kg per day) in premature infants reduces the incidence of all stages of NEC.

The innervation of the human antro-pyloric region: Organization and composition

Although the composition of the gastric innervation has been determined in animal models, relatively little known about the innervation of the human antro-pyloric region. We used immunocytochemical techniques to establish the localization and co-expression of neuropeptides and nitric oxide in the human antrum and upper duodenum. Our results demonstrate the existence of a clearly defined submucosal plexus in the antral region that is absent in rats and guinea pigs. The abundant innervation of the lamina propria contains 3 major nerve populations: VIP- and NOS-, SP- and CGRP-, and GRP-immunoreactive. For the first time, NOS-containing nerve fibers were observed throughout the length of the antral glands. Within the antrum somatostatin was confined to endocrine cells, however, at the pyloric sphincter both enteric plexi contained immunoreactive neurons and nerve fibres. Within the pyloric sphincter CGRP- and SP-immunoreactive fibres were significantly increased, correlating with the presence of large ganglia in the submucosal plexus. In conclusion, the organization and composition of the innervation of human antro-pylorus differed substantially from that reported in other mammals. The presence of an abundant mucosal innervation paralled by a well-defined submucosal plexus indicates that the functional regulation of the gastric–pyloric region will be distinct from that of smaller animal models.Key words: gastric innervation, pyloric sphincter, neuropeptides, nitric oxide, somatostatin.

Mechanism mediating nitric oxide-induced inhibition in human jejunal longitudinal smooth muscle

BACKGROUND

Enteric neurotransmission is a complex process involving multiple neurotransmitters, including nitric oxide (NO). Our aim was to evaluate the role and mechanism(s) of action of NO in normal human jejunal longitudinal smooth muscle.

METHODS

Transmural strips of normal human jejunum obtained from subjects undergoing gastric bypass were studied in organ chambers. Effects of exogenous NO (7 x 10(-6) mol/L to 7 x 10(-5) mol/L) and electrical field stimulation (nonspecific release of endogenous neurotransmitters) on spontaneous contractile activity and on precontracted muscle strips (substance P, 10(-5) mol/L) were evaluated in the presence and absence of the competitive NO synthase inhibitor N(G)-amino-L-arginine (L-NNA, 10(-3) mol/L) and the specific soluble guanylyl cyclase inhibitor 1H-[1,2,4]-oxadiazaolo-[4,3-a]-quinoxalin-1-one (ODQ, 10(-5) mol/L and 10(-4) mol/L).

RESULTS

Exogenous NO dose-dependently inhibited spontaneous contractility and relaxed precontracted smooth muscle strips. The effects of NO were markedly attenuated or completely inhibited in the presence of ODQ. Electric field stimulation under nonadrenergic, noncholinergic conditions also inhibited spontaneous contractility and relaxed precontracted smooth muscle strips; both of these effects were attenuated, but not completely inhibited, in the presence of both ODQ and L-NNA.

CONCLUSIONS

NO is an endogenous inhibitory neurotransmitter in human jejunal longitudinal smooth muscle, acting at least in part via a mechanism mediated by guanylyl cyclase. Other (non-nitrergic) nonadrenergic, noncholinergic inhibitory neurotransmitters are likely active in this portion of the human gut.

Developmental changes in heme-oxygenase-2 and bNOS expression in enteric neurons in the pig duodenum

There exists much parallelism between carbon monoxide- and nitric oxide-generating systems. Therefore, we wondered whether developmental and functional differences along the duodenum similarly affect, part of them, namely, heme oxygenase-2-(HO-2) and neural isoform of nitric oxide synthase- (nNOS) expressing neurons. By applying NADPH diaphorase histochemistry and HO-2 immunohistochemistry on whole-mount preparations and by using stereologic methods, a qualitative and quantitative description of HO-2 and nNOS expression was obtained. Examinations were carried out on the duodenum of fetal, neonatal and weaned pigs. At all ages, three enteric plexuses were readily distinguished. The presence of both enzymes fits in with other morphological and physiological reports. However, the expression of both enzymes significantly changed during development. The number of HO-2-IR neurons increased approximately 20-fold in the inner submucous and almost doubled in the myenteric plexus. In addition, the number of nNOS-expressing neurons displayed a significant decrease in the outer submucous plexus after weaning. High levels of glucocorticoids may cause the perinatally increased HO-2 expression, whereas an influence on nNOS expression is doubtful. Therefore, it seems that notwithstanding the high similarity between both systems, their expression is regulated differently in the pig duodenum.

Three-dimensional morphology of c-Kit-positive cellular network and nitrergic innervation in the human gut

CONTEXT

-c-Kit-positive interstitial cells of Cajal (ICC) appear to play a key role in the normal motility function and development of intestine. Nitric oxide is considered to be the most important messenger of inhibitory nonadrenergic, noncholinergic nerves in the enteric nervous system.

OBJECTIVES

The aims of this study were to examine the distribution of nitrergic innervation and ICCs in normal human bowel and to demonstrate interconnections between ICCs and nitrergic nerves and smooth muscle fibers using histochemical and immunohistochemical double-staining methods with a whole-mount preparation technique and confocal laser scanning microscopy.

METHODS

Full-thickness small and large bowel specimens were obtained at autopsy from 18 children who died of nongastrointestinal diseases. A whole-mount preparation was performed for all specimens, and double staining was carried out with nicotinamide adenine dinucleotide phosphate (reduced form, NADPH)-diaphorase and c-Kit immunohistochemistry. Double immunofluorohistochemistry with neuronal nitric oxide synthase and c-Kit using confocal laser scanning microscopy was also performed in all specimens.

RESULTS

The whole-mount preparation facilitated 3-dimensional visualization of the meshlike network of NADPH-diaphorase-positive nerve fibers in the myenteric plexus surrounded by a reticular network of c-Kit-positive ICCs. The dense c-Kit-positive cellular network located between longitudinal and circular muscle layers and at the innermost part of circular muscle layer intermingled with the myenteric plexus. Short, fine processes of ICCs made connections with the muscle fibers and c-Kit-positive cells.

CONCLUSIONS

The development of double-NADPH-diaphorase histochemistry and c-Kit immunohistochemistry staining technique in a whole-mount preparation provides an easy and useful method for investigating the association between c-Kit-positive cellular network and nitrergic neuronal network in the human bowel wall. The characteristic profiles of the c-Kit-positive cellular network and nitrergic neuronal network and their relationship with the smooth muscle fibers provide a morphologic basis for investigating intestinal motility disorders.

Quantitative distribution of NADPH-diaphorase-positive myenteric neurons in different segments of the developing chicken small intestine and colon

NADPH-diaphorase (NADPH-d) was used as a marker for neuronal nitric oxide synthase in order to investigate the nitrergic neurons of the developing myenteric ganglia on whole-mount preparations in the proximal and distal segments of the small intestine and in the colon of the chicken embryo, between incubation days 12 and 19. Neurons that were positive for NADPH-d were counted in randomly selected myenteric ganglia. The data obtained from each area and each age group were subjected to two-way analysis of variance (ANOVA) and the Student-Newman-Keuls test. Between incubation days 12 and 19, the originally narrow-meshed myenteric plexus with its high ganglionic density progressively became wide-meshed and the ganglionic density decreased significantly. Quantitative analysis further revealed a significant decrease in the NADPH-d-positive nerve cell density with age. At the same time, the constant or even increasing number of nitrergic cells per ganglion may indicate that the decreasing cell density may be a result of the growth of the bowel with decreasing ganglion density rather than a decrease in the total number of myenteric nitrergic cells. Regional differences in the dynamics of the quantitative changes were revealed. A significant decrease in the nitrergic cell number appeared earlier in the proximal than in the distal segments of the small intestine or in the colon. In contrast, the significant decline of the ganglionic density was first noticed in the colon at the same time.

Altered nitric oxide production in the premature gut may increase susceptibility to intestinal damage in necrotizing enterocolitis

BACKGROUND/PURPOSE

Nitric oxide (NO) mediates enteric smooth muscle relaxation and mucosal protection. The authors have identified an ontogenically determined pattern of enteric NO neural maturation that may render the distal gut of premature piglets susceptible to injury.

METHODS

NO synthase (cNOS and iNOS) activities were measured in the developing piglet gut wall and compared with gut from an intraluminal model of necrotizing enterocolitis (NEC) at different times.

RESULTS

In premature animals, iNOS activity was significantly higher 3 hours after NEC induction compared with similarly treated 3-day-old piglets. INOS levels continued to rise 6 hours after NEC induction in prematures. Premature animals (labor induced by prostaglandins) failed to show such a rise in iNOS. In 3 day olds, iNOS levels increased significantly 16 hours after injury compared with the 3-hour group.

CONCLUSIONS

iNOS production increases in premature piglets with NEC compared with full-term NEC animals and continues to rise in the presence of intestinal damage regardless of developmental status. Maternal administration of prostaglandins attenuates this rise in iNOS activity. Elevated NO production in premature gut may contribute to increased susceptibility to damage in NEC.

The control of gut motility

Gut motility in non-mammalian vertebrates as in mammals is controlled by the presence of food, by autonomic nerves and by hormones. Feeding and the presence of food initiates contractions of the stomach wall and subsequently gastric emptying, peristalsis, migrating motor complexes and other patterns of motility follow. This overview will give examples of similarities and differences in control systems between species. Gastric receptive relaxation occurs in fish and is an enteric reflex. Cholecystokinin reduces the rate of gastric emptying in fish as in mammals. Inhibitory control of peristalsis is exerted, e.g. by VIP, PACAP, NO in fish and amphibians, while excitatory stimuli arise from nerves releasing tachykinins, acetylcholine or serotonin (5-HT). In crocodiles, we have found the presence of the same nerve types, although the effects on peristalsis have not been studied. Recent studies on signal transduction in the gut smooth muscle of fish and amphibians suggest that external Ca2+ is of great importance, but not the only source of Ca2+ recruitment in tachykinin-, acetylcholine- or serotonin-induced contractions of rainbow trout and Xenopus gastrointestinal smooth muscle. The effect of acetylcholine involves reduction of cAMP-levels in the smooth muscle cells. It is concluded that, in general, the control systems in non-mammalian vertebrates are amazingly similar between species and animal groups and in comparison with mammals.

Outer submucous plexus: an intrinsic nerve network involved in both secretory and motility processes in the intestine of large mammals and humans

The architecture of the enteric nerve networks in the gastrointestinal tract appears to be more complex in large mammals, including humans, than in small laboratory animals. At least two distinct ganglionic nerve plexuses could be identified in the submucous layer in the digestive tract of large mammals. While functionally and morphologically similar neuron populations are found in the intestinal wall of both small and large mammals, significant differences in their topographical organization and neurochemical features may be present. This short review clearly illustrates that the close and exclusive association, which has been assumed so far between the efferent pathways of the submucous plexus and regulation of intestinal secretion/absorption on the one hand and between the myenteric plexus and regulation of intestinal motility on the other hand, cannot be interpreted that strictly. An attempt has been made to give a briefoverview of the current status of the identification of distinct functional enteric neuronal classes in the gastrointestinal tract of large mammals using the pig and human intestine as references, and to compare these data with the more extensive information gathered from the guinea-pig intestine.

Quantitative distribution of NADPH-diaphorase-positive myenteric neurons in different segments of the developing chicken small intestine and colon

NADPH-diaphorase (NADPH-d) was used as a marker for neuronal nitric oxide synthase in order to investigate the nitrergic neurons of the developing myenteric ganglia on whole-mount preparations in the proximal and distal segments of the small intestine and in the colon of the chicken embryo, between incubation days 12 and 19. Neurons that were positive for NADPH-d were counted in randomly selected myenteric ganglia. The data obtained from each area and each age group were subjected to two-way analysis of variance (ANOVA) and the Student-Newman-Keuls test. Between incubation days 12 and 19, the originally narrow-meshed myenteric plexus with its high ganglionic density progressively became wide-meshed and the ganglionic density decreased significantly. Quantitative analysis further revealed a significant decrease in the NADPH-d-positive nerve cell density with age. At the same time, the constant or even increasing number of nitrergic cells per ganglion may indicate that the decreasing cell density may be a result of the growth of the bowel with decreasing ganglion density rather than a decrease in the total number of myenteric nitrergic cells. Regional differences in the dynamics of the quantitative changes were revealed. A significant decrease in the nitrergic cell number appeared earlier in the proximal than in the distal segments of the small intestine or in the colon. In contrast, the significant decline of the ganglionic density was first noticed in the colon at the same time.

Nitric oxide-containing neurons in the bovine gut, with special reference to their relationship with VIP and galanin

The presence and distribution of nicotinamide dinucleotide phosphate diaphorase (NADPH-d)-containing neurons have been studied by means of NADPH-d histochemistry in different regions of the adult cow gut, from the esophagus to the rectum. NADPH-d and nitric oxide synthase (NOS) were constantly recognized to be colocalized in the same neuron. The colocalization of vasoactive intestinal polypeptide (VIP) and galanin in such nitrergic neurons was also studied by means of combined histochemical and immunofluorescence techniques. NADPH-d-positive neurons were present along the myenteric plexus of the entire gut, and in the submucous plexus from the abomasum to the rectum. Notably, they formed two types of nerve networks in the submucous connective tissue of the jejunum-ileum. NADPH-d-positive innervation of the muscle layers occurred throughout the tract, and sometimes a clear correspondence was noted between the number of reactive fibres and the thickness of the muscle. Nitrergic fibres also occurred in the mucosa and often were in relation to glands and blood vessels. The nitrergic neurons varied in size, shape, and intensity of staining, and often their terminals were seen to surround unstained perikarya. Various types of neurons were recognized on the basis of their chemical content; one of them contained galanin, VIP and NOS simultaneously. The present results suggest that the nitrergic neurons of the bovine gastrointestinal tract play roles presumably for controlling the motility of the gut and the conduction of interneuronal impulses.

Marked morphological differences in the myenteric plexus between the mesenteric and antimesenteric sides of small bowel in premature infants

BACKGROUND

The gastrointestinal tract appears morphologically prepared for oral feeding by the end of the second trimester, but many of the physiological processes required for efficient enteral nutrition are not developed fully until 33 to 34 weeks' gestation. Myenteric plexus is well recognized as an important regulator of peristaltic activity. Whole-mount preparation technique produces a 3-dimensional picture to better show the neuronal networks branching and interconnections. The aim of this study was to investigate neurone density and morphology of the myenteric plexus in premature infants using whole-mount technique.

METHODS

Full-thickness small and large bowel specimens were collected at autopsy from 6 premature babies (gestational age, 26 to 32 weeks) who died without evidence of gastrointestinal disease. Whole-mount preparation of the myenteric plexus was made and stained with NADPH-diaphorase and Acetylcholinesterase (AChE) histochemistry. The stained myenteric network was measured with a computer image analysis system. Controls included 4 full-term babies who died of nongastrointestinal disease.

RESULTS

In premature infants there were striking differences in neuronal density of myenteric plexus in the mesenteric and antimesenteric border of small bowel. The differences in neuronal density in mesenteric and antimesenteric border of small bowel gradually became less striking as the gestation progressed with no differences evident at gestational age 32 weeks.

CONCLUSIONS

This study shows for the first time that the neurone density of myenteric plexus is significantly higher in the mesenteric border of the small bowel compared with antimesenteric border in premature infants. The marked morphological differences observed in neurone density in the small bowel of premature infants may contribute to immature small bowel activity.

Effect of exogenous ATP on canine jejunal smooth muscle

Intracellular recordings were made from the circular smooth muscle cells of the canine jejunum to study the effect of exogenous ATP and to compare the ATP response to the nonadrenergic, noncholinergic (NANC) inhibitory junction potential (IJP) evoked by electrical field stimulation (EFS). Under NANC conditions, exogenous ATP evoked a transient hyperpolarization (6.5 +/- 0.6 mV) and EFS evoked a NANC IJP (17 +/- 0.4 mV). Omega-conotoxin GVIA (100 nM) and a low-Ca(2+), high-Mg(2+) solution abolished the NANC IJP but had no effect on the ATP-evoked hyperpolarization. The ATP-evoked hyperpolarization and the NANC IJP were abolished by apamin (1 microM) and N(G)-nitro-L-arginine (100 microM). Oxyhemoglobin (5 microM) partially (38.8 +/- 5.5%) reduced the amplitude of the NANC IJP but had no effect on the ATP-evoked hyperpolarization. Neither the NANC IJP nor the ATP-evoked hyperpolarization was affected by P2 receptor antagonists or agonists, including suramin, reactive blue 2, 1-(N, O-bis-[5-isoquinolinesulfonyl]-N-methyl-L-tyrosyl)-4-phenylpiperazine , pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid, alpha, beta-methylene ATP, 2-methylthioadenosine 5'-triphosphate tetrasodium salt, and adenosine 5'-O-2-thiodiphosphate. The data suggest that ATP evoked an apamin-sensitive hyperpolarization in circular smooth muscle cells of the canine jejunum via local production of NO in a postsynaptic target cell.

Role of nitric oxide- and vasoactive intestinal polypeptide-containing neurones in human gastric fundus strip relaxations

The morphological pattern and motor correlates of nitric oxide (NO) and vasoactive intestinal polypeptide (VIP) innervation in the human isolated gastric fundus was explored. By using the nicotinamide adenine dinucleotide phosphate hydrogen (NADPH)-diaphorase and specific rabbit polyclonal NO-synthase (NOS) and VIP antisera, NOS- and VIP-containing varicose nerve fibres were identified throughout the muscle layer or wrapping ganglion cell bodies of the myenteric plexus. NOS-immunoreactive (IR) neural cell bodies were more abundant than those positive for VIP-IR. The majority of myenteric neurones containing VIP coexpressed NADPH-diaphorase. Electrical stimulation of fundus strips caused frequency-dependent NANC relaxations. N(G)-nitro-L-arginine (L-NOARG: 300 microM) enhanced the basal tone, abolished relaxations to 0.3 - 3 Hz (5 s) and those to 1 Hz (5 min), markedly reduced ( approximately 50%) those elicited by 10 - 50 Hz, and unmasked or potentiated excitatory cholinergic responses at frequencies > or =1 Hz. L-NOARG-resistant relaxations were virtually abolished by VIP (100 nM) desensitization at all frequencies. Relaxations to graded low mechanical distension (< or =1 g) were insensitive to tetrodotoxin (TTX: 1 microM) and L-NOARG (300 microM), while those to higher distensions (2 g) were slightly inhibited by both agents to the same extent ( approximately 25%). In the human gastric fundus, NOS- and VIP immunoreactivities are colocalized in the majority of myenteric neurones. NO and VIP mediate electrically evoked relaxations: low frequency stimulation, irrespective of the duration, caused NO release only, whereas shortlasting stimulation at high frequencies induced NO and VIP release. Relaxations to graded mechanical distension were mostly due to passive viscoelastic properties, with a slight NO-mediated neurogenic component at 2 g distension. The difference between NO and VIP release suggests that in human fundus accommodation is initiated by NO. British Journal of Pharmacology (2000) 129, 12 - 20

Projections of nitric oxide synthase and vasoactive intestinal polypeptide-reactive submucosal neurons in the human colon

BACKGROUND

The submucosal plexus is important in the control of secretomotor and motor function of the intestine. Our aim was to describe the projections of submucosal neurons to the mucosa within the submucosal plexus and to the circular muscle of human colon and to determine whether submucosal neurons that projected to different layers were located at different levels of the submucosa.

METHODS

A retrogradely transported fluorescent dye was applied to the mucosa, submucosa or circular muscle layer of human colon which was then maintained in organotypic culture for 5 days. The submucosa was then dissected into two preparations, one containing the inner layer of the submucosal plexus and the other containing both the intermediate and outer layers. The dissected preparations were labelled with antibodies to nitric oxide synthase (NOS) or vasoactive intestinal peptide (VIP).

RESULTS

Submucosal neurons projected to the mucosa, submucosa and circular muscle layers for mean distances of 3.7, 3.0 and 4.3 mm, respectively. Ninety-seven per cent of submucosal neurons labelled from the circular muscle were located in the outer or the intermediate layers, while 51% of those projecting to the mucosa were in inner layer and 49% in the intermediate/outer layers of the submucosal plexus. Eleven per cent of submucosal neurons projecting to the circular muscle were immunoreactive for NOS and 12% were immunoreactive for VIP. Forty-five per cent of those projecting within the submucosa were immunoreactive for VIP and 38% of those projecting to the mucosa were immunoreactive for VIP.

CONCLUSIONS

Submucosal neurons in the human colon innervate the mucosa, circular muscle and submucosa and different functional classes of neurons are located in different layers of the submucosal plexus.

Morphological changes of the myenteric plexus during early postnatal development of the rat

The enteric nervous system needs to adapt itself constantly to the postnatal changes of the developing gut. The aim of this study was to examine the morphological changes between the distal and proximal segments of the gastrointestinal (GI) tract during the first two postnatal weeks. Myenteric plexus from the duodenum, proximal and distal colon of 1-, 7- and 14-day-old rat pups was dissected and examined under the scanning electron microscope. Wholemounts from the same regions and postnatal stages were stained with cuprolinic blue. Neuronal numbers per ganglionic area were counted and neuronal sizes were measured. Furthermore, segments of the above-mentioned areas were embedded in resin and semithin sections were cut. The thickness of the circular and longitudinal muscle layers was measured. The morphology of the myenteric plexus depends on localization as well as on the age of the animal. While in younger animals the myenteric plexus is usually densely packed, the network expands with increasing age. Similarly, the thickness of the circular and the longitudinal muscle layers increases. Nerve cell numbers per ganglionic area increase from duodenum to distal colon and decrease from the 1-day (P1) to the 14-day-old (P14) animal. The longest diameters and the area of the nerve cells decrease from duodenum to distal colon and increase with age of the animal. The intensity of the cuprolinic blue staining varies also according to age and segment of the gut. During the first two postnatal weeks the three-dimensional architecture of the myenteric plexus as well as the size and densities of the enteric neurons change according to the increasing gut length and the thickness of the muscle layer. The differences between duodenum and colon might reflect the different physiological properties of the proximal and distal gut as well as a varying grade of maturity, which is also supported by a variation in the cuprolinic blue staining intensity.

A comparative study of esophageal and anorectal motility in myotonic dystrophy

Myotonic dystrophy may be associated with visceral abnormalities involving smooth muscle, the pathogenesis of which is not clear. Our aim was to evaluate the involvement of smooth and striated muscles at both ends of the gastrointestinal tract. Esophageal and anorectal manometric studies were performed in 13 patients and healthy controls. There was a correlation between: (1) the resting pressure in the upper esophageal sphincter and in the lower anal canal, (2) the amplitude and the coordination of contraction primary waves in the proximal and in the distal esophagus, and (3) the resting pressure in the higher anal canal and in the lower one. These results suggest that both ends of the gastrointestinal tract are disturbed in a similar fashion, both quantitatively and qualitatively and that there is a relationship between smooth and striated visceral muscle involvement in myotonic dystrophy.

Development of the human gastrointestinal tract: twenty years of progress

A combination of approaches has begun to elucidate the mechanisms of gastrointestinal development. This review describes progress over the last 20 years in understanding human gastrointestinal development, including data from both human and experimental animal studies that address molecular mechanisms. Rapid progress is being made in the identification of genes regulating gastrointestinal development. Genes directing initial formation of the endoderm as well as organ-specific patterning are beginning to be identified. Signaling pathways regulating the overall right-left asymmetry of the gastrointestinal tract and epithelial-mesenchymal interactions are being clarified. In searching for extrinsic developmental regulators, numerous candidate trophic factors have been proposed, but compelling evidence remains elusive. A critical gene that initiates pancreas development has been identified, as well as a number of genes regulating liver, stomach, and intestinal development. Mutations in genes affecting neural crest cell migration have been shown to give rise to Hirschsprung's disease. Considerable progress has been achieved in understanding specific phenomena, such as the transcription factors regulating expression of sucrase-isomaltase and fatty acid-binding protein. The challenge for the future is to integrate these data into a more complete understanding of the physiology of gastrointestinal development.

Distribution of NADPH diaphorase-positive neurons in the enteric nervous system of the rabbit intestine

Nitric oxide (NO) has been proposed as an inhibitory transmitter in gastrointestinal muscle relaxation. We analyzed the distribution of nitric-oxide producing neurons in the rabbit intestine through nicotinamide-adenine-dinucleotide-phosphate-diaphorase histochemistry. By this reliable and convenient method, we visualized neuronal nitric-oxide-synthase, the enzyme responsible for nitric oxide generation, in the rabbit intestine. In the ileum and rectum, nitric-oxide-synthase-related diaphorase activity was present in the myenteric plexus ganglion cells, and in the nerve fibers in the internodal strand, secondary, and tertiary plexuses. These fibers were particularly abundant in the deep circular rather than in the outer longitudinal muscle layer. In the inner submucosal plexus, we found scarce labeled neurons. Labeled neural somata showed a range of sizes and shapes suggesting different functional roles. The present basic information is required to use the rabbit as an experimental animal in neurochemical NO enteric research.