Neurotransmitter coding of enteric neurones in the submucous plexus is changed in non-inflamed rectum of patients with Crohn's disease

Immunoreactivity for high-affinity choline transporter colocalises with VAChT in human enteric nervous system

Cholinergic nerves are identified by labelling molecules in the ACh synthesis, release and destruction pathway. Recently, antibodies against another molecule in this pathway have been developed. Choline reuptake at the synapse occurs via the high-affinity choline transporter (CHT1). CHT1 immunoreactivity is present in cholinergic nerve fibres containing vesicular acetylcholine transporter (VAChT) in the human and rat central nervous system and rat enteric nervous system. We have examined whether CHT1 immunoreactivity is present in nerve fibres in human intestine and whether it is colocalised with markers of cholinergic, tachykinergic or nitrergic circuitry. Human ileum and colon were fixed, sectioned and processed for fluorescence immunohistochemistry with antibodies against CHT1, class III beta-tubulin (TUJ1), synaptophysin, common choline acetyl-transferase (cChAT), VAChT, nitric oxide synthase (NOS), substance P (SP) and vasoactive intestinal peptide (VIP). CHT1 immunoreactivity was present in many nerve fibres in the circular and longitudinal muscle, myenteric and submucosal ganglia, submucosa and mucosa in human colon and ileum and colocalised with immunoreactivity for TUJ1 and synaptophysin confirming its presence in nerve fibres. In nerve fibres in myenteric ganglia and muscle, CHT1 immunoreactivity colocalised with immunoreactivity for VAChT and cChAT. Some colocalisation occurred with SP immunoreactivity, but little with immunoreactivity for VIP or NOS. In the mucosa, CHT1 immunoreactivity colocalised with that for VIP and SP in nerve fibres and was also present in vascular nerve fibres in the submucosa and on epithelial cells on the luminal border of crypts. The colocalisation of CHT1 immunoreactivity with VAChT immunoreactivity in cholinergic enteric nerves in the human bowel thus suggests that CHT1 represents another marker of cholinergic nerves.

Contribution of bone marrow-derived cells to the pro-inflammatory effects of protease-activated receptor-2 in colitis

Objective

Our aim was to determine the contribution of proteinase-activated receptor-2 (PAR₂)-expressing bone marrow-derived cells on the development of colonic inflammation.

Materials

Chimeric mice were generated by injecting bone marrow cells from wildtype (PAR₂^+/+) or PAR₂ knockout mice (PAR₂^−/−) into irradiated PAR₂^+/+ or PAR₂^−/− mice. Treatments: Colitis was induced by giving 2.5% dextran sodium sulfate (DSS) solution for 7 days or by a single intracolonic administration of trinitrobenzene sulphonic acid (TNBS, 2 mg dissolved in 40% ethanol).

Methods

Seven days after the induction of colitis, bowel thickness, inflammatory parameters [myeloperoxidase (MPO) activity, macroscopic/microscopic damage scores], and leukocyte trafficking (visualized via intravital microscopy) were assessed.

Results

Total deficiency of PAR₂ resulted in a marked reduction in severity of both TNBS and DSS induced colitis as assessed by MPO activity, macroscopic damage, bowel thickness, and leukocyte adherence. Colitis was attenuated in all chimeric lines in which there was loss of PAR₂ in the host, non-bone marrow-derived tissue, independent of the status of PAR expression by bone marrow-derived cells. Interestingly, TNBS colitis was attenuated in PAR₂^+/+ chimeric mice with PAR₂^−/− derived bone marrow but these animals were not protected from DSS colitis.

Conclusions

Expression of PAR₂ by host-derived tissues plays a dominant role in regulating colonic inflammation. PAR₂ expression by bone marrow-derived cells appears to play a role in TNBS colitis but not in DSS induced injury.

Electronic supplementary material

The online version of this article (doi:10.1007/s00011-010-0181-9) contains supplementary material, which is available to authorized users.

Routine colonic biopsies as a new tool to study the enteric nervous system in living patients

Better characterization of enteric neuropathies during the course of gastrointestinal diseases could be of great diagnostic and/or therapeutic interest. However, studies using whole mounts of the enteric nervous system (ENS) are restricted to specific diseases requiring surgery and are also limited by the small number of specimens available. Therefore, we here describe a novel method to obtain whole mounts of submucosal plexus in routine colonic biopsies. We show that a single biopsy displays a substantial number of submucosal ganglia and neurons and that it can be reliably used to perform morphometric and neurochemical analysis and Western Blots quantification of neuronal or glial markers. This method of analysis of the human ENS will enable us to gain better insight into the characterization of enteric neuropathies in living patients.

Neuropeptides and inflammatory bowel disease

PURPOSE OF REVIEW

Inflammatory bowel disease (IBD) is a chronic intestinal inflammatory condition, the pathophysiology of which is not well understood. It has, however, become increasingly evident that interactions between the enteric nervous system and the immune system play an important role in the cause of IBD. Both the enteric nervous system and the central nervous system can amplify or modulate the aspects of intestinal inflammation through secretion of neuropeptides or small molecules. The purpose of this study is to present recent data on the role that neuropeptides play in the pathophysiology of IBD.

RECENT FINDINGS

The best studied of the neuropeptides thought to play a role in the pathogenesis of IBD include substance P, corticotropin-releasing hormone, neurotensin, and vasoactive intestinal peptide; small molecules include acetylcholine and serotonin. Recently discovered functions of each of these neuropeptides with a discussion of implications of the data for therapy are reviewed.

SUMMARY

Although the available data suggest an important role for neuropeptides in the pathophysiology of intestinal inflammation, there does yet not appear to be a function that can be taken as established for any of these molecules. The complexity of neuroimmune-endocrine systems, conflicting study results and dual mechanisms of action, warrant further research in this field. Clarification of the molecular mechanisms of action of neuropeptides and on immune and inflammatory reactions will likely yield new treatment options in the future.

Homeostatic and therapeutic roles of VIP in smooth muscle function: myo-neuroimmune interactions

We tested the hypothesis that spontaneous release of vasoactive intestinal peptide (VIP) from enteric neurons maintains homeostasis in smooth muscle function in mild inflammatory insults and that infusion of exogenous VIP has therapeutic effects on colonic smooth muscle dysfunction in inflammation. In vitro experiments were performed on human colonic circular smooth muscle tissues and in vivo on rats. The incubation of human colonic circular smooth muscle strips with TNF-alpha suppressed their contractile response to ACh and the expression of the pore-forming alpha(1C) subunit of Ca(v)1.2 channels. VIP reversed both effects by blocking the translocation of NF-kappaB to the nucleus and its binding to the kappaB recognition sites on halpha(1C)1b promoter. The translocation of NF-kappaB was inhibited by blocking the degradation of IkappaBbeta. Induction of inflammation by a subthreshold dose of 17 mg/kg trinitrobenzene sulfonic acid (TNBS) in rats moderately decreased muscularis externa concentration of VIP, and it had little effect on the contractile response of circular smooth muscle strips to ACh. The blockade of VIP and pituitary adenylate cyclase-activating peptide receptors 1/2 during mild inflammatory insult significantly worsened the suppression of contractility and the inflammatory response. The induction of more severe inflammation by 68 mg/kg TNBS induced marked suppression of colonic circular muscle contractility and decrease in serum VIP. Exogenous infusion of VIP by an osmotic pump reversed these effects. We conclude that the spontaneous release of VIP from the enteric motor neurons maintains homeostasis in smooth muscle function in mild inflammation by blocking the activation of NF-kappaB. The infusion of exogenous VIP mitigates colonic inflammatory response and smooth muscle dysfunction.

Post inflammatory damage to the enteric nervous system in diverticular disease and its relationship to symptoms

Some patients with colonic diverticula suffer recurrent abdominal pain and exhibit visceral hypersensitivity, though the mechanism is unclear. Prior diverticulitis increases the risk of being symptomatic while experimental colitis in animals increases expression of neuropeptides within the enteric nervous system (ENS) which may mediate visceral hypersensitivity. Our aim was to determine the expression of neuropeptides within the ENS in diverticulitis (study 1) and in patients with symptomatic disease (study 2). Study 1 - Nerves in colonic resection specimens with either acute diverticulitis (AD, n = 16) or chronic diverticulitis (CD, n = 16) were assessed for neuropeptide expression recording % area staining with protein gene product (PGP9.5), substance P (SP), neuropeptide K (NPK), pituitary adenylate cyclase activating polypeptide (PACAP), vasoactive intestinal polypeptide (VIP) and galanin. Study 2 - Seventeen symptomatic and 15 asymptomatic patients with colonic diverticula underwent flexible sigmoidoscopy and multiple peridiverticular mucosal biopsies. Study 1- Neural tissue, as assessed by PGP staining was increased to a similar degree in circular muscle in both AD and CD. The CD specimens showed significant increases in the immunoreactivity of SP, NPK and galanin in both mucosal and circular muscle layer compared with controls. Study 2 - Mucosal histology was normal and PGP9.5 staining was similar between groups however patients with symptomatic diverticular disease demonstrated significantly higher levels of SP, NPK, VIP, PACAP and galanin within the mucosal plexus. Patients with symptomatic diverticular disease exhibit increased neuropeptides in mucosal biopsies which may reflect resolved prior inflammation, as it parallels the changes seen in acute and chronic diverticulitis.

Plasticity of enteric nerve functions in the inflamed and postinflamed gut

Inflammation of the gut alters the properties of the intrinsic and extrinsic neurons that innervate it. While the mechanisms of neuroplasticity differ amongst the inflammatory models that have been used, amongst various regions of the gut, and between intrinsic vs extrinsic neurons, a number of consistent features have been observed. For example, intrinsic and extrinsic primary afferent neurons become hyperexcitable in response to inflammation, and interneuronal synaptic transmission is facilitated in the enteric circuitry. These changes contribute to alterations in gut function and sensation in the inflamed bowel as well as functional disorders, and these changes persist for weeks beyond the point at which detectable inflammation has subsided. Thus, gaining a more thorough understanding of the mechanisms responsible for inflammation-induced neuroplasticity, and strategies to reverse these changes are clinically relevant goals. The purpose of this review is to summarize our current knowledge regarding neurophysiological changes that occur during and following intestinal inflammation, and to identify and address gaps in our knowledge regarding the role of enteric neuroplasticity in inflammatory and functional gastrointestinal disorders.

Infiltration of mast cells in rat colon is a consequence of ischemia/reperfusion

Intestinal ischemia as well as mastocytosis occur in patients with inflammatory bowel disease and irritable bowel syndrome. Our aim was to clarify how ischemia with reperfusion (I/R) affects the structure, enteric neurons, and immune cells in the colon. Rats were subjected to colon ischemia for 1 h and reperfused for 1 day up to 20 weeks; sham-operated rats were used as controls. No structural remodeling of the intestinal segment was detected after I/R. The number and distribution of eosinophils were not affected by I/R. Local areas containing numerous mast cells were detected in the muscle layers, the serosa, and in and around the myenteric ganglia 4-20 weeks post ischemia. It was notable that myenteric ganglionic formations within mast-cell-rich areas virtually lacked neurons. Mast cells were rarely found in controls. In conclusion, I/R of the colon attracts mast cells, and death of myenteric neurons occurs in such locations.

Regional variation in the neurochemical coding of the myenteric plexus of the human colon and changes in patients with slow transit constipation

There are differences in the structure and function between regions of the colon. In patients with slow transit constipation the activity of all regions is markedly slowed. Counts of colonic neurones in slow transit constipation have been semiquantitative and led to varying results. We have applied new methods of quantification of markers in whole mounts of the colonic myenteric plexus to compare density of innervation between regions and between normal patients and those undergoing resection for severe constipation. Whole mounts of colonic myenteric plexus were made from specimens removed for cancer treatment (controls) and cases of severe constipation. All neurones were labelled by anti-human neuronal protein antibodies. Neurones synthesizing acetyl choline were labelled for choline acetyltransferase (ChAT) and those for nitric oxide by antisera to nitric oxide synthase (NOS). Four populations of neurones were distinguished and quantified according to the two selective markers, ChAT and NOS. In the normal major populations were NOS alone (51% of ascending colon neurones and 44% of descending colon neurones) and ChAT alone (41% ascending colon, 48% descending colon). Nitric oxide synthase/ChAT and NOS-/ChAT-comprised only small populations. In all regions in severe constipation, the percentage of NOS-only colonic myenteric neurones was raised (54% ascending colon, 49% descending colon) and ChAT only was reduced (36% ascending colon, 42% descending colon). The other populations were not changed. Accurate quantification of neuronal populations in whole mounts of human colon reveals inter-regional differences in innervation and marked changes in innervation in cases of very severe constipation.

Evaluation of differences between breeds for substance P, vasoactive intestinal polypeptide, and neurofilament 200 in the abomasal wall of cattle

OBJECTIVE

To compare the content of substance P, vasoactive intestinal polypeptide, and neurofilament 200 in biopsy specimens taken from the abomasal wall of healthy cows of 2 breeds.

SAMPLE POPULATION

Biopsy specimens taken from different sites of the abomasal wall from 20 German Holstein cows and 20 German Fleckvieh cows.

PROCEDURES

Biopsy specimens were examined immunohistochemically, and the content of substance P, vasoactive intestinal polypeptide, and neurofilament 200 was determined by measuring the immunoreactive areas.

RESULTS

Significant differences between the breeds were detected. Substance P-immuno-reactive area in the corpus abomasi was significantly smaller in the German Holsteins (geometric mean +/- geometric SD, 679 +/- 1.83 microm2) than in the German Fleckvieh cows (1,020 +/- 1.65 microm2). Concerning vasoactive intestinal polypeptide, differences between breeds were not significant. Overall nerve density in the antral abomasal wall was significantly greater in German Holsteins than in German Fleckvieh cows (immunoreactive areas for neurofilament 200 in German Holsteins was 4,842 +/- 1.29 microm2 and in German Fleckvieh cows was 3,333 +/- 1.63 microm2). Conclusions and Clinical Relevance-The significantly lower content of substance P in the corpus abomasi could explain why German Holstein cows are predisposed to abomasal displacement, compared with German Fleckvieh cows, in which this disease is a rare finding.

Functional gastrointestinal disorders and visceral hypersensitivity in children and adolescents suffering from Crohn's disease

BACKGROUND

Symptoms of abdominal pain are reported by children with active Crohn's disease (CD). During remissions abdominal pain improves in most children but some of them continue to experience pain. We hypothesized that these patients may suffer from protracted abdominal pain related to functional gastrointestinal disorders (FGID) and visceral hypersensitivity. The objective was to characterize the symptoms and to measure the rectal sensory threshold for pain (RSTP) by barostat in CD children and adolescents suffering from abdominal pain despite remission.

METHODS

Eight patients (median age 14.5 years; range 9.8-17) with quiescent CD but suffering from chronic abdominal pain were studied by rectal barostat. At the same time they completed validated questionnaires to assess FGID, anxiety, and depression. They were compared to 10 control children and 8 children with FGID also investigated in our laboratory.

RESULTS

All patients fulfilled Rome II criteria for irritable bowel syndrome (n = 5), functional abdominal pain (n = 2), and functional dyspepsia (n = 1). RSTP was significantly lower in CD patients compared to the normal controls: median (range) 25 mmHg (15-29) versus 40 mmHg (30-48) (P < 0.01). RSTP was similar in patients and children with FGID. Rectal compliance was similar in patients, children with FGID, and controls. Seven of the 8 patients had scores indicating an anxiety problem.

CONCLUSIONS

Protracted abdominal pain that affects children and adolescents with quiescent CD is related to FGID associated with visceral hypersensitivity and anxiety. The incidence of FGID in children suffering from CD requires further investigation.

Regulation of enteric functions by adenosine: pathophysiological and pharmacological implications

The wide distribution of ATP and adenosine receptors as well as enzymes for purine metabolism in different gut regions suggests a complex role for these mediators in the regulation of gastrointestinal functions. Studies in rodents have shown a significant involvement of adenosine in the control of intestinal secretion, motility and sensation, via activation of A1, A2A, A2B or A3 purinergic receptors, as well as the participation of ATP in the regulation of enteric functions, through the recruitment of P2X and P2Y receptors. Increasing interest is being focused on the involvement of ATP and adenosine in the pathophysiology of intestinal disorders, with particular regard for inflammatory bowel diseases (IBDs), intestinal ischemia, post-operative ileus and related dysfunctions, such as gut dysmotility, diarrhoea and abdominal discomfort/pain. Current knowledge suggests that adenosine contributes to the modulation of enteric immune and inflammatory responses, leading to anti-inflammatory actions. There is evidence supporting a role of adenosine in the alterations of enteric motor and secretory activity associated with bowel inflammation. In particular, several studies have highlighted the importance of adenosine in diarrhoea, since this nucleoside participates actively in the cross-talk between immune and epithelial cells in the presence of diarrhoeogenic stimuli. In addition, adenosine exerts complex regulatory actions on pain transmission at peripheral and spinal sites. The present review illustrates current information on the role played by adenosine in the regulation of enteric functions, under normal or pathological conditions, and discusses pharmacological interventions on adenosine pathways as novel therapeutic options for the management of gut disorders and related abdominal symptoms.

Effect of intestinal inflammation on neuronal survival and function in the dorsal motor nucleus of the vagus

BACKGROUND

The effects of intestinal inflammation on the central nervous system are unknown. The dorsal motor nucleus of the vagus (DMNV) integrates peripheral and central signals and sends efferent signals to the gastrointestinal system. The purpose of this study was to determine the effects of intestinal inflammation on the DMNV in an animal model and in vitro.

METHODS

Carbocyanine dye (DiI) was injected into the stomach wall of rats to label retrogradely the neurons of the DMNV. Colitis was induced with trinitrobenzene sulfonic acid (TNBS). Tissue was examined under fluorescent microscopy. In vitro studies were performed using primary culture of DMNV neurons. Cell proliferation was measured by BrdU incorporation. Apoptosis was measured by an enzyme sandwich-linked immunosorbent assay. Single-cell cytoplasmic calcium transients were determined using the fluorescence dye fura-2-AM. Reverse transcriptase-polymerase chain reaction of glutamate receptor was performed.

RESULTS

Animals treated with TNBS ate less and lost weight compared with controls. Microscopic analysis demonstrated a 77% decrease in DiI labeling in the DMNV of TNBS animals compared with controls. Cell proliferation in DMNV neurons after 24-hour exposure to the cytokines interleukin- (IL)-1 beta, IL-6, or tumor necrosis factor- (TNF)-alpha was significantly decreased. Similarly, apoptosis of DMNV neurons after 24 hours of incubation with IL-1 beta or TNF-alpha was significantly increased, but no changes resulted with IL-6. Exposure to each cytokine resulted in decreased glutamate-induced intracellular calcium transients. Transcription of glutamate receptor was decreased after 24-hour exposure to TNF-alpha.

CONCLUSIONS

DMNV neurons projecting to the stomach are reduced in number after induction of colitis in rats. In vitro, proinflammatory cytokines diminish DMNV cellular proliferation, increase apoptosis, and alter calcium responses to glutamate. These results indicate that intestinal inflammation affects adversely neuronal survival and function in the DMNV.

Quantitative assessment of glial cells in the human and guinea pig enteric nervous system with an anti-Sox8/9/10 antibody

Quantitative changes of enteric glia (EGC) have been implicated in gastrointestinal disorders. To facilitate future studies of EGC in human pathology, we aimed to characterize thoroughly glial markers in the human enteric nervous system (ENS) and to compare EGC in man and guinea pig. Whole-mount preparations of the enteric nerve plexuses from human and guinea pig ileum and colon were labeled with antibodies against S100b, glial fibrillary acidic protein (GFAP), and p75NGFR and the transcription factors Sox8/9/10 and neuronally counterstained. Abundant immunoreactivity (IR) for S100b, GFAP, p75NGFR, and Sox8/9/10 was detected in EGC of all studied regions. Although the cytoplasmatic staining pattern of most markers did not permit glial quantification, the nuclear localization of Sox8/9/10-IR allowed to identify and count all EGC individually. In both man and guinea pig, myenteric ganglia were larger and contained more EGC and neurons than submucous ganglia. Furthermore, there were more EGC in the human than in the guinea pig myenteric plexus (MP), glial density was consistently higher in the human ENS, and the glia index (glia:neuron ratio) ranged from 1.3 to 1.9 and from 5.9 to 7.0 in the human submucous plexus (SMP) and MP, respectively, whereas, in guinea pig, the glia index was 0.8-1.0 in the SMP and 1.7 in the MP. The glia index was the most robust quantitative descriptor within one species. This is a comprehensive set of quantitative EGC measures in man and guinea pig that provides a basis for pathological assessment of glial proliferation and/or degeneration in the diseased gut.

Neuro-glial crosstalk in inflammatory bowel disease

Inflammatory bowel disease (IBD) is a multifactorial disease in which environmental, immune and genetic factors are involved in the pathogenesis. Although biological therapies (antibodies anti-tumour necrosis factor-alpha or anti-integrin) have considerably improved the symptoms and quality of life of IBD patients, some drawbacks have emerged limiting their long-term use. In addition, prevention of relapses and treatment of resistant ulcers remains a clinical challenge. In this context, a better understanding of the pathophysiology of IBD and the development of novel therapeutic intervention would benefit from further basic and preclinical research into the role of the cellular microenvironment and the interaction between its cellular constituents. In this context, the role of the enteric nervous system (ENS) in the regulation of the intestinal epithelial barrier (IEB) and the gut immune response has fuelled an increased interest in the last few years. Recent advances, summarized in this review, have highlighted the ENS as playing a key role in the control of IEB functions and gut immune homeostasis, and that alterations of the ENS could be directly associated in the development of IBD and its associated symptoms.

Spatial organization of neurons in the dorsal motor nucleus of the vagus synapsing with intragastric cholinergic and nitric oxide/VIP neurons in the rat

The dorsal motor nucleus of the vagus (DMV) contains preganglionic neurons that control gastric motility and secretion. Stimulation of different parts of the DMV results in a decrease or an increase in gastric motor activities, suggesting a spatial organization of vagal preganglionic neurons in the DMV. Little is known about how these preganglionic neurons in the DMV synapse with different groups of intragastric motor neurons to mediate contraction or relaxation of the stomach. We used pharmacological and immunohistochemical methods to characterize intragastric neural pathways involved in mediating gastric contraction and relaxation in rats. Microinjections of L-glutamate (L-Glu) into the rostral or caudal DMV produced gastric contraction and relaxation, respectively, in a dose-related manner. Intravenous infusion of hexamethonium blocked these actions, suggesting mediation via preganglionic cholinergic pathways. Atropine inhibited gastric contraction by 85.5 +/- 4.5%. Gastric relaxation was reduced by intravenous administration of N(G)-nitro-L-arginine methyl ester (L-NAME; 52.5 +/- 11.9%) or VIP antagonist (56.3 +/- 14.9%). Combined administration of L-NAME and VIP antagonist inhibited gastric relaxation evoked by L-Glu (87.8 +/- 4.3%). Immunohistochemical studies demonstrated choline acetyltransferase immunoreactivity in response to L-Glu microinjection into the rostral DMV in 88% of c-Fos-positive intragastric myenteric neurons. Microinjection of L-Glu into the caudal DMV evoked expression of nitric oxide (NO) synthase and VIP immunoreactivity in 81 and 39%, respectively, of all c-Fos-positive intragastric myenteric neurons. These data indicate spatial organization of the DMV. Depending on the location, microinjection of L-Glu into the DMV may stimulate intragastric myenteric cholinergic neurons or NO/VIP neurons to mediate gastric contraction and relaxation.

Prolonged elevation of galanin and tachykinin expression in mucosal and myenteric enteric nerves in trinitrobenzene sulphonic acid colitis

Diverticulitis causes recurrent abdominal pain associated with increased mucosal expression of mucosal galanin and substance P (SP). We studied changes in mucosal and myenteric plexus neuropeptides in adult rats using a model of colonic inflammation, trinitrobenzenesulphonic acid colitis. We assessed the effects on the pan-neuronal markers protein gene product 9.5 (PGP9.5) and neurofilament protein, as well as specific neuropeptides at 1, 2, 3, 4, 6, 8, 10 and 14 weeks. Following the acute injury there was macroscopic resolution of inflammation but minor microscopic abnormalities persisted. Percent area stained of mucosal PGP9.5 fell initially but average levels on days 21 and 28 levels were significantly elevated (P < 0.001), returning to normal by day 42. Percent area staining of PGP9.5 in the muscle rose immediately and remained significantly elevated at 70 days (P < 0.001). SP, neuropeptide K and galanin followed a similar overall pattern. SP to PGP9.5 ratio was significantly increased in the muscle both acutely (days 1-28) and in the long term (days 70 and 98), whereas the galanin to PGP9.5 ratio was significantly increased in the mucosa throughout the study. Low-grade chronic inflammation after an acute initial insult causes a persistent increase in the expression of galanin in the mucosa and SP in muscle layer.

Review article: gastrointestinal sensory and motor disturbances in inflammatory bowel disease - clinical relevance and pathophysiological mechanisms

BACKGROUND

It is well known that inflammation has a profound impact on the neuromuscular apparatus of the gastrointestinal tract during the inflammatory insult and in periods of remission, at the site of inflammation and at distance from this site. The importance of this interaction is illustrated by the higher prevalence of functional gut disorders in patients with inflammatory bowel disease.

AIMS

To document the epidemiological and clinical significance of functional alterations of gut motility and sensitivity in patients with inflammatory bowel disease and to formulate potential pathophysiological mechanisms.

RESULTS AND CONCLUSIONS

Functional gut disorders occur frequently in patients with inflammatory bowel disease, both during inflammatory episodes and in periods of remission, and have a major impact on their quality of life. The clinical manifestations of these motility and sensitivity disorders vary and are often difficult to treat, mainly because therapeutic guidelines and specific diagnostic tests to distinguish inflammatory bowel disease from functional gut disorders are lacking. Chronic bowel inflammation results in a complicated interaction between neuroendocrine serotonin-predominant cells of the mucosa, inflammatory cells (particularly mast cells) in the submucosa, the intrinsic and extrinsic innervation and the muscular apparatus including the interstitial cells of Cajal. The outcome of this interaction is a perturbation of gastrointestinal motor function, both locally and at distance from the site of inflammation and during both acute inflammation and remission.

Neuro-immune interactions in inflammatory bowel disease and irritable bowel syndrome: future therapeutic targets

The gastro-intestinal tract is well known for its largest neural network outside the central nervous system and for the most extensive immune system in the body. Research in neurogastroenterology implicates the involvement of both enteric nervous system and immune system in symptoms of inflammatory bowel disease and irritable bowel syndrome. Since both disorders are associated with increased immune cell numbers, nerve growth and activation of both immune cells and nerves, we focus in this review on the involvement of immune cell-nerve interactions in inflammatory bowel disease and irritable bowel syndrome. Firstly, the possible effects of enteric nerves, especially of the nonadrenergic and noncholinergic nerves, on the intestinal immune system and their possible role in the pathogenesis of chronic intestinal inflammatory diseases are described. Secondly, the possible effects of immunological factors, from the innate (chemokines and Toll-like receptors) as well as the adaptive (cytokines and immunoglobulins) immune system, on gastro-intestinal nerves and its potential role in the development of inflammatory bowel disease and irritable bowel syndrome are reviewed. Investigations of receptor-mediated and intracellular signal pathways in neuro-immune interactions might help to develop more effective therapeutic approaches for chronic inflammatory intestinal diseases.

Activity-dependent regulation of tyrosine hydroxylase expression in the enteric nervous system

The regulation of neuromediator expression by neuronal activity in the enteric nervous system (ENS) is currently unknown. Using primary cultures of ENS derived from rat embryonic intestine, we have characterized the regulation of tyrosine hydroxylase (TH), a key enzyme involved in the synthesis of dopamine. Depolarization induced either by 40 mm KCl, veratridine or by electrical field stimulation produced a robust and significant increase in the proportion of TH immunoreactive (TH-IR) neurons (total neuronal population was identified with PGP9.5 or Hu) compared to control. This increase in the proportion of TH-IR neurons was significantly reduced by the sodium channel blocker tetrodotoxin (0.5 microm), demonstrating that neuronal activity was critically involved in the effects of these depolarizing stimuli. KCl also increased the proportion of VIP-IR but not nNOS-IR enteric neurons. The KCl-induced increase in TH expression was partly reduced in the presence of the nicotinic receptor antagonist hexamethonium (100 microm), of noradrenaline (1 microm) and of the alpha(2)-adrenoreceptor agonist clonidine (1 microm). Combining pharmacological and calcium imaging studies, we have further shown that L-type calcium channels were involved in the increase of TH expression induced by KCl. Finally, using specific inhibitors, we have shown that both protein kinases A and C as well as the extracellular signal-regulated kinases were required for the increase in the proportion of TH-IR neurons induced by KCl. These results are the first demonstration that TH phenotype of enteric neurons can be regulated by neuronal activity. They could also set the basis for the study of the pathways and mechanisms involved in the neurochemical plasticity observed both during ENS development and in inflammatory enteric neuropathies.

Mycobacterium avium subspecies paratuberculosis infection in cases of irritable bowel syndrome and comparison with Crohn's disease and Johne's disease: common neural and immune pathogenicities

Mycobacterium avium subsp. paratuberculosis causes Johne's disease, a systemic infection and chronic inflammation of the intestine that affects many species, including primates. Infection is widespread in livestock, and human populations are exposed. Johne's disease is associated with immune dysregulation, with involvement of the enteric nervous system overlapping with features of irritable bowel syndrome in humans. The present study was designed to look for an association between Mycobacterium avium subsp. paratuberculosis infection and irritable bowel syndrome. Mucosal biopsy specimens from the ileum and the ascending and descending colon were obtained from patients with irritable bowel syndrome attending the University of Sassari, Sassari, Sardinia, Italy. Crohn's disease and healthy control groups were also included. Mycobacterium avium subsp. paratuberculosis was detected by IS900 PCR with amplicon sequencing. Data on the potential risk factors for human exposure to these pathogens and on isolates from Sardinian dairy sheep were also obtained. Mycobacterium avium subsp. paratuberculosis was detected in 15 of 20 (75%) patients with irritable bowel syndrome, 3 of 20 (15%) healthy controls, and 20 of 23 (87%) people with Crohn's disease (P = 0.0003 for irritable bowel syndrome patients versus healthy controls and P = 0.0000 for Crohn's disease patients versus healthy controls). One subject in each group had a conserved single-nucleotide polymorphism at position 247 of IS900 that was also found in isolates from seven of eight dairy sheep. There was a significant association (P = 0.0018) between Mycobacterium avium subsp. paratuberculosis infection and the consumption of hand-made cheese. Mycobacterium avium subsp. paratuberculosis is a candidate pathogen in the causation of a proportion of cases of irritable bowel syndrome as well as in Crohn's disease.

Presence of a marked nonneuronal cholinergic system in human colon: study of normal colon and colon in ulcerative colitis

BACKGROUND

The body has not only a neuronal but also a nonneuronal cholinergic system. Both systems are likely to be very important, particularly in inflammatory conditions. The patterns and importance of the nonneuronal cholinergic system in patients with ulcerative colitis (UC) are largely unknown.

METHODS

The colons of UC and non-UC patients were examined for expression patterns of choline acetyltransferase (ChAT), vesicular acetylcholine transporter (VAChT), and the muscarinic receptor of the M(2) subtype.

RESULTS

ChAT and VAChT immunoreactions and mRNA reactions for ChAT were detected in epithelial and endocrine cells, in cells in the lamina propria, and in blood vessel walls. Furthermore, a marked M(2) immunoreaction was noted for epithelium, blood vessel walls, and smooth musculature. ChAT and VAChT immunoreactions were significantly higher in endocrine and epithelial cells, respectively, in non-UC mucosa than in UC mucosa. On the other hand, there was a tendency toward higher M(2) levels in epithelium of UC patients.

CONCLUSIONS

There is a pronounced nonneuronal cholinergic system in the colon, which has previously been ignored when discussing cholinergic influences in UC. Furthermore, it is evident that certain changes in the nonneuronal cholinergic system occur in response to inflammation/derangement in UC. Cholinergic effects in the colon can be considered to be related not only to nerve-related effects but also to effects of acetylcholine from nonneuronal local cells. Thus, the recently discussed phenomenon of a "cholinergic antiinflammatory pathway" in the intestine may have a pronounced nonneuronal component.

Mast cells and nerves tickle in the tummy: implications for inflammatory bowel disease and irritable bowel syndrome

Mast cells are well known as versatile cells capable of releasing and producing a variety of inflammatory mediators upon activation and are often found in close proximity of neurons. In addition, inflammation leads to local activation of neurons resulting in the release neuropeptides, which also play an important immune modulatory role by stimulation of immune cells. In intestinal disorders like inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS), the number of mast cells is known to be much higher than in the normal intestine. Moreover, both these disorders are also reported to be associated with alterations in neuropeptide content and in neural innervation. Mutual association between mast cells and enteric nerves has been demonstrated to be increased in pathophysiological conditions and contribute to spreading and amplification of the response in IBD and IBS. In this review the focus lies on studies appointed to the direct interaction between mast cells and nerves in IBD, IBS, and animal models for these disorders so far.

Histamine excites neurones in the human submucous plexus through activation of H1, H2, H3 and H4 receptors

Histamine is a major mast cell mediator of immunoneural signalling in the gut and mast cells play a role in the pathophysiology of functional and inflammatory bowel diseases. Histamine receptors are therefore promising drug targets to treat gut disorders. We aimed to study the so far unknown effect of histamine on neural activity in the human enteric nervous system (ENS) and to identify the pharmacology of histamine response. We used fast imaging techniques in combination with the potentiometric dye di-8-ANEPPS to monitor directly membrane potential changes and thereby neuronal excitability in the human submucous plexus from surgical specimens of 110 patients (2137 neurones, 273 ganglia). Local microejection of histamine resulted in action potential discharge in 37% of neurones. This excitatory effect was mimicked by the H(1) agonist HTMT-dimaleat, H(2) agonist dimaprit, H(3) agonist (R)-(-)-alpha-methylhistamine and H(4) agonist 4-methylhistamine. The excitatory actions of the agonists were specifically and selectively blocked by the H(1), H(2), H(3) or H(4) receptor antagonists pyrilamine, ranitidine, clobenpropit or J1-[(5-chloro-1H-indol-2-yl)carbonyl]-4-methylpiperazine (JNJ 7777120), respectively. Clobenproprit reduced the excitatory response to histamine. Unlike in the guinea-pig ENS (R)-(-)-alpha-methylhistamine had no presynaptic actions in human submucous plexus. Application of agonists revealed receptor clustering which was as follows: 29% H(1)/H(3), 27% H(2), 20% H(1)/H(2)/H(3), 10% H(3), 7% H(1)/H(2) and 7% H(2)/H(3). Histamine excites human enteric neurones and this effect involves all four histamine receptors; most striking was the identification of an excitatory H(3) mediated component and the discovery of H(4) mediated neuronal excitation. These data may form the basis of identification of new targets to treat inflammatory and functional gut disorders.

Role of neuropeptides in inflammatory bowel disease

Inflammatory bowel disease (IBD) is a chronic, relapsing condition involving complex interactions between genes and the environment. The mechanisms triggering the initial attack and relapses, however, are not well understood. In the past several years the enteric nervous system (ENS) has been implicated in the pathophysiology of IBD. Both the ENS and the central nervous system (CNS) can amplify or modulate aspects of intestinal inflammation through secretion of neuropeptides that serve as a link between the ENS and CNS. Neuropeptides are defined as any peptide released from the nervous system that serves as an intercellular signaling molecule. Neuropeptides thought to play a potentially key role in IBD include substance P, corticotropin-releasing hormone, neurotensin, vasoactive intestinal peptide, mu-opioid receptor agonists, and galanin. This review focuses on the role of these neuropeptides in the pathophysiology of IBD and discusses the cell types and mechanisms involved in this process. The available evidence that neuropeptide blockade may be considered a therapeutic approach in both Crohn's disease and ulcerative colitis will also be discussed.

Up-regulation of nerve growth factor and interleukin-10 in inflamed and non-inflamed intestinal segments in rats with experimental colitis

Inflammatory bowel diseases are characterized by dysregulated immune response to the normal microflora and structural and functional changes of the enteric nervous system which occur in inflamed as well as non-inflamed areas of the bowel. This study describes the changes in the expression of nerve growth factor (NGF) and interleukin-10 (IL-10) in the colon and in various segments of the small intestine in two rat models of experimental colitis induced by iodoacetamide or 2,4,6-trinitrobenzene sulfonic acid (TNBS). Levels of NGF and IL-10 were measured by ELISA in tissue homogenate sampled from duodenum, jejunum, ileum and colon at different time intervals. NGF and IL-10 increased significantly in homogenates of strips isolated from all small intestinal segments, 3-6h after iodoacetamide or TNBS administration and remained elevated until the colonic inflammation subsided. Similar but more pronounced increase occurred in areas of the colon adjacent to the ulcer. Histologic examinations revealed inflammatory changes in the colon; however, examination of sections from the small intestines did not reveal significant differences between controls and rats with colitis. The marked up-regulation of nerve growth factor and interleukin-10 in colitis suggests that they play a role in limiting or resolving inflammation and in preventing it from becoming uncontrolled. It also suggests that experimental colitis may be associated with latent inflammation in the small bowel.

Survival and neurotransmitter plasticity in cultured rat colonic myenteric neurons

The enteric nervous system is of great importance for maintenance and proper function of the gastrointestinal tract. The aim of this study was to quantify myenteric neuronal subpopulations expressing calcitonin gene-related peptide (CGRP), galanin, neuropeptide Y (NPY), somatostatin, vasoactive intestinal peptide (VIP) and nitric oxide synthase (NOS) in rat colon in vivo and after culturing. Further we investigated if culturing in the presence of CGRP, galanin, VIP, S-nitroso-N-acetyl-D,L-penicillamine (SNAP, a NO donor) or N-nitro-L-arginine methyl ester (L-NAME, a NOS inhibitor) affect neuronal survival. After 4 days of culturing the proportions of neurons expressing CGRP, NPY, somatostatin or VIP increased as compared to in vivo, while the proportions of neurons expressing galanin or NOS did not change. Neuronal survival was unaffected after culturing in media enriched with CGRP, galanin, VIP, SNAP or L-NAME. Neither did addition of CGRP, galanin nor VIP to the cultures affect the relative numbers of neurons expressing CGRP, galanin or VIP respectively. Addition of SNAP or L-NAME did not change the percentage of neurons expressing NOS. In conclusion, cultured rat colonic myenteric neurons increase their expression of CGRP, NPY, somatostatin and VIP, suggesting that these neuropeptides are of importance for neuronal survival.

The effect of age on colocalization of acetylcholinesterase and nicotinamide adenine dinucleotide phosphate diaphorase staining in enteric neurons in an experimental model

PURPOSE

Cholinergic and nitrergic neurons form 2 main subpopulations of the myenteric neurons, and they have been the targets of detailed morphological investigations in bowel motility disorders. However, little is known regarding the colocalization of neurotransmitters within the same enteric neurons. The aim of this study was to determine the histochemical colocalization of cholinergic and nitrergic neurons in the porcine distal large bowel myenteric plexus from fetal to adulthood.

METHODS

Distal large bowel specimens were taken from 6 randomly selected age groups (3 animals in each group) from midway of gestation to adulthood. The myenteric plexus was exposed using whole-mount technique. After nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) staining, cells per ganglion were counted. Then the specimens were stained with acetylcholinesterase (AChE), and the cells that were stained with individual enzymes and with both enzymes were counted.

RESULTS

Colocalization of AChE and NADPH-d was seen in all age groups, and it was highest during the mid part of gestation (30%) and decreased steadily thereafter into adulthood (8%). The individual number of NADPH-d- and AChE-positive neurons per ganglion remained constant till 4 weeks of age and significantly increased thereafter into adulthood.

CONCLUSION

The use of double-labeling histochemical technique shows for the first time the colocalization of cholinergic and nitrergic activity in a large population of enteric neurons in the late fetal and newborn period. Age-related loss of cholinergic and nitrergic colocalization in the myenteric plexus is most likely a maturational process.

The autonomic nervous system and inflammatory bowel disease

Crohn's disease and ulcerative colitis, collectively known as inflammatory bowel disease (IBD), are chronic, recurring, inflammatory conditions of the intestine. The precise mechanisms underlying the pathogenesis of IBD are not yet clear but they are believed to involve a number of precipitating factors, most notably genetic susceptibility and environmental influences. The autonomic nervous system (ANS) has long been known as a critical regulator of intestinal function and much evidence now exists to suggest that it also plays an important role in the development of IBD. Dramatic changes in the ANS in IBD are apparent from the cellular to the molecular level ultimately leading to altered communication between the ANS and effector cells of the intestine. This review aims to synthesize the current understanding of the pathogenesis of IBD with a particular emphasis on the role that the ANS plays in the progression of these diseases.

Cannabinoid 1 (CB1) receptors coupled to cholinergic motorneurones inhibit neurogenic circular muscle contractility in the human colon

The effects of cannabinoid subtype 1 (CB(1)) receptor activation were determined on smooth muscle, inhibitory and excitatory motorneuronal function in strips of human colonic longitudinal muscle (LM) and circular muscle (CM) in vitro. Electrical field stimulation (EFS; 0.5-20 Hz, 50 V) evoked a relaxation in LM and CM precontracted with a neurokinin-2 (NK-2) selective receptor agonist (beta-ala(8)-neurokinin A; 10(-6) M) in the presence of atropine (10(-6) M); this was unaltered following pretreatment with the CB(1)-receptor selective agonist arachidonyl-2-chloroethylamide (ACEA; 10(-6) M). In the presence of nitric oxide synthase blockade with N-nitro-L-arginine (10(-4) M), EFS evoked a frequency-dependent 'on-contraction' during stimulation and an 'off-contraction' following stimulus cessation. On-contractions were significantly inhibited in CM strips by pretreatment with ACEA (10(-6) M). These inhibitory effects were reversed in the presence of the CB(1) receptor-selective antagonist N-(piperidine-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (10(-7) M). ACEA did not alter LM or CM contractile responses to acetylcholine or NK-2 receptor-evoked contraction. Immunohistochemical studies revealed a colocalisation of CB(1) receptors to cholinergic neurones in the human colon based on colabelling with choline acetyltransferase, in addition to CB(1) receptor labelling in unidentified structures in the CM. In conclusion, activation of CB(1) receptors coupled to cholinergic motorneurones selectively and reversibly inhibits excitatory nerve transmission in colonic human colonic CM. These results provide evidence of a direct role for cannabinoids in the modulation of motor activity in the human colon by coupling to cholinergic motorneurones.

Cholinergic regulation of epithelial ion transport in the mammalian intestine

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

How inflammation changes neuromuscular function and its relevance to symptoms in diverticular disease

Diverticulosis is largely asymptomatic but recent evidence suggests that episodes of acute diverticulitis double the risk of subsequently suffering from recurrent noninflammatory pain. Numerous animal models demonstrate how inflammation is followed by circular muscle hypertrophy, abnormalities of innervation, and increased sensitivity to cholinergic agents. There is also an impairment of norepinephrine and acetylcholine release and damage to nitrergic neurons. These changes are also associated with visceral hypersensitivity. Many of the features, including visceral hypersensitivity are also seen in symptomatic patients with diverticulosis. The trinitrobenzene sulfonic acid colitis model demonstrates that inflammation is followed by long lasting increases in tachykinin and other neuropeptide immunoreactivity. These changes occur both in the mucosa and myenteric plexus and parallel changes seen in resections and mucosal biopsies in diverticular patients. These neural abnormalities may be responsible for the visceral hypersensitivity, which explains why symptoms correlate poorly with objective abnormalities such as intraluminal pressure or motor patterns. Treatment of visceral hypersensitivity might be more effective than current therapies that often leave pain unaltered.

Enteric neuroplasticity evoked by inflammation

Neuroplastic changes in the enteric nervous system (ENS) may be observed in physiological states, such as development and aging, or occur as a consequence of different pathological conditions, ranging from enteric neuropathies (e.g., Hirschsprung's disease) to intestinal (e.g., inflammatory bowel disease) or extra-intestinal diseases (e.g., Parkinson's disease). Studying ENS plasticity may help to elucidate the pathophysiology of several diseases and have a bearing on the development of new pharmacological interventions. In the present review, we would like to focus on neuronal plasticity evoked by gastrointestinal inflammation occurring in inflammatory bowel disease and in a subset of patients with severe derangement of gut motility due to an enteric neuropathy characterized by an inflammatory infiltrate of the enteric plexuses. Major features of neuroplasticity within the enteric microenvironment encompass structural abnormalities ranging from nerve re-arrangement (e.g., hypertrophy and hyperplasia) to degeneration and loss of enteric ganglion cells; altered synthesis, content and release of neurotransmitters as well as up- or down-regulation of receptor systems; gastrointestinal dysfunction characterized by sensory-motor and secretory impairment of the gut. Interestingly, neuronal changes may also occur in segments of the gastrointestinal tract remote from the site of the original inflammation, e.g. the ileum may show neuroplastic changes during colitis. Sometimes, the inflamed site may even be outside the gut. Among potential mechanisms underlying ENS plasticity, neurotrophins and enteric glia deserve special attention. A better comprehension of ENS plasticity during inflammation could be instrumental to develop new therapeutic options for patients with IBD and inflammatory enteric neuropathies.

Tachykinins and tachykinin receptors in the gut, with special reference to NK2 receptors in human

Tachykinins (TKs), substance P (SP), neurokinin A (NKA) and B (NKB) are important peptide modulators of intestinal motility in animal species studied so far, including humans. Modulation of motility by TKs can occur at various levels, since these peptides are expressed in cholinergic excitatory motor neurons projecting to both circular and longitudinal muscle, interneurons, and intramural and extramural sensory neurons. The effects of SP, NKA and NKB are preferentially mediated through the stimulation of NK1, NK2 and NK3 receptors, respectively; however, the selectivity of natural TKs for their preferred receptors is relative. In addition, SP and NKA are expressed in similar quantities in the human intestine and adequate stimuli can release similar amount of these TKs from enteric nerves. Furthermore, a single anatomical substrate can express more than one TK receptor type, so that the blockade of a single receptor type may not reveal functional effects in integrated models of motility. In isolated human small intestine and colon circular muscle strips, both NK1 and NK2 receptors mediate contractile effects. Indeed, in the human small intestine, smooth muscle electrical and motor events induced by electrical field stimulation (EFS) can involve either or both NK1 and NK2 receptors or these latter receptors predominantly, depending on the experimental conditions. In contrast, in the human colonic smooth muscle, only the NK2 receptor-mediated component of the response to EFS is prominent and some evidence would suggest that this component is the main excitatory motor mechanism at this level. Furthermore, a NK2 receptor-mediated secretory component in the human colonic mucosa has been recently demonstrated. Thus, it could be speculated that the blockade of both NK1 and NK2 receptors will be necessary to antagonise motor effects induced by exogenous administration or endogenous release of TKs in the small intestine, whereas the blockade of the NK2 receptors would be sufficient to disrupt physiological motor and, possibly, secretory activity at the colonic level. Available evidence indicates that, in healthy volunteers, the infusion of NKA (25 pmol/kg/min i.v.) stimulated small intestine motility and precipitated a series of intestinal and non-intestinal adverse events. Nepadutant (8 mg i.v.), a selective NK2 receptor antagonist, antagonised small intestine motility induced by NKA and prevented associated intestinal adverse events. In another study, the same dose of nepadutant increased colo-rectal compliance during isobaric balloon distension in healthy volunteers pretreated with a glycerol enema, disclosing a NK2 receptor-mediated component in the regulation of colonic smooth muscle tone. However, the prolonged blockade of NK2 receptors by nepadutant (16 mg i.v. b.i.d. for 8 days) did not affect bowel habits, neither in term of movements nor of stool consistency. Altogether, these results indicate that, even when there is a significant redundance in the effects of TKs and in the role of their receptors, the selective blockade of tachykinin NK2 receptors can have functional consequences on human intestinal motility and perception, but this can occur without the disruption of the physiological functions.

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

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

Indiscriminate loss of myenteric neurones in the TNBS-inflamed guinea-pig distal colon

This investigation was conducted to establish whether guinea-pig trinitrobenzene sulfonic acid (TNBS)-colitis was associated with a change in the number of neurones of the myenteric plexus, and, if so, whether select subpopulations of neurones were affected. Total neurones were quantified with human (Hu) antiserum, and subpopulations were evaluated with antisera directed against choline acetyltransferase, nitric oxide synthase, calretinin, neuronal nuclear protein or vasoactive intestinal peptide (VIP). Colitis was associated with a loss of 20% of the myenteric neurones, most of which occurred during the first 12 h past-TNBS administration. During this period, myenteric ganglia were infiltrated with neutrophils while lymphocytes appeared at a later time-point. The neuronal loss persisted at a 56-day time-point, when inflammation had resolved. The decrease in myenteric neurones was not associated with a decrease in any given subpopulation of neurones, but the proportion of VIP-immunoreactive neurones increased 6 days following TNBS administration and returned to the control range at the 56 days. These findings indicate that there is an indiscriminant loss of myenteric neurones that occurs during the onset of TNBS-colitis, and the loss of neurones may be associated with the appearance of neutrophils in the region.

Plasticity of the enteric nervous system during intestinal inflammation

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

Increased expression of nitric oxide synthase in cultured neurons from adult rat colonic submucous ganglia

Neuronal plasticity in the enteric nervous system (ENS) is probably a key step in intestinal adaptation during growth, maturation and ageing as well as in several pathophysiological situations. Studies on cultured myenteric neurons have revealed an increased vasoactive intestinal peptide (VIP) expression in neuronal nitric oxide synthase (NOS)-expressing neurons. In addition, both VIP and nitric oxide (NO) promote survival of cultured myenteric neurons. The aim of the present study was to investigate possible changes in the expression of VIP and NOS in cultured submucous neurons from adult rat large intestine. Submucous neurons were cultured as explants or as dissociated neurons for 3 and 8 days. Immunocytochemistry was used to determine the proportions of neurons containing VIP or NOS in preparations of uncultured controls (reflects the conditions in vivo) and in cultured explants of submucosa and dissociated submucous neurons. In situ hybridization was used to determine changes in the expressions of NOS and VIP mRNA. The relative number of NOS-expressing neurons increased significantly during culturing. The percentage of all neurons expressing NOS was 22% in controls, while approximately 50% of the cultured submucous neurons expressed NOS. VIP-expressing neurons constituted approximately 80% of all submucous neurons in controls as well as in cultured explants or dissociated neurons. Studies on coexistence revealed that the VIP-containing neurons were the ones that started to express NOS during culture. The induced expression of NOS in cultured adult submucous neurons indicates that nitric oxide, possibly in cooperation with VIP, is important for neuronal adaptation, maintenance and survival.

Immunohistochemical characterization of putative primary afferent (sensory) myenteric neurons in human small intestine

Pseudouni- or multiaxonal Dogiel type II neurons are the intrinsic primary afferent (sensory) neurons (IPANs) in the guinea pig small intestine. Our aim was to decipher the chemical code of human myenteric type II neurons and to establish their putative vertical projections, i.e., from the myenteric plexus to the submucosa/mucosa. Additionally, we tried to distinguish them chemically from uniaxonal, dendritic type V neurons displaying, at first glance, similar shapes, i.e., smoothly contoured cell bodies with several long processes. Wholemount preparations of the myenteric plexus were immunohistochemically double or triple stained for neurofilaments (NF) and one or two of the following peptides: calbindin, calretinin (CR), calcitonin gene-related peptide (CGRP), somatostatin (SOM) and substance P (SP). In each triple stained wholemount three counts were conducted: (1) NF-positive pseudouni- or multiaxonal (type II) neurons including their reactivities for the above peptides, (2) uniaxonal or NF-negative neurons displaying coreactivities for the above peptides and (3) NF-reactive type V neurons taking into account their reactivities for the above markers. Additionally, type II neurons, which had an axon leading into (disrupted) interconnecting strands towards the submucosa were counted and somal areas of types II and V neurons were measured. The majority of myenteric type II neurons displayed coreactivities for SOM/CR (89.6%), SOM/SP (86.6%) and SP/CR (81.6%), respectively. A minority of type II neurons was positive for CGRP or calbindin. A small population with type III morphology (uniaxonal, long and slender dendrites) displayed the same coreactivities as type II neurons. In contrast, not one single type V neuron was coreactive for SOM/CR, SOM/SP or SP/CR. Out of 627 type II neurons counted in six wholemounts, 84 type II neurons displayed an axon which could be followed into disrupted interconnecting strands indicating a vertical projection pattern. Somal areas of type II neurons were twice as big as those of type V neurons (904+/-210 versus 449+/-110 microm(2)). In conclusion, most human myenteric type II neurons contain SOM, SP and CR. We suggest they are the human IPANs. Type V neurons are both morphologically and chemically distinctly different from type II neurons and may represent descending interneurons. Further studies have to decipher the type-specific chemical code of type II neurons distinguishing them also from type III neurons.

The human enteric nervous system

Decades of work in animal models have demonstrated that the enteric nervous system (ENS) plays a key role in controlling gut functions. Recent advances made it possible to extend such studies to the ENS of man in health and even in disease. Such studies have already provided new insights into the pathophysiology of inflammatory and possibly functional bowel diseases. Studies on human ENS revealed both important similarities and differences between the ENS of man and of experimental animals. Here we summarize the current state of knowledge of the electrophysiology and neurochemistry of the human ENS, including relevant reflex mediated functions in the human gut. Additionally, we review disease associated changes in human ENS properties. Finally, we highlight some research areas that hold special promise in advancing our understanding of the human ENS.

Enteric glia

The enteric nervous system is composed of both enteric neurones and enteric glia. Enteric glial cells were first described by Dogiel and are now known to outnumber neurones approximately 4 : 1. In the past, these cells were assumed to subserve a largely supportive role; however, recent evidence indicates that enteric glial cells may play a more active role in the control of gut function. In transgenic mouse models, where enteric glial cells are selectively ablated, the loss of glia results in intestinal inflammation and disruption of the epithelial barrier. Enteric glia are activated specifically by inflammatory insults and may contribute actively to inflammatory pathology via antigen presentation and cytokine synthesis. Enteric glia also express receptors for neurotransmitters and so may serve as intermediaries in enteric neurotransmission. Thus, enteric glia may serve as a link between the nervous and immune systems of the gut and may also have an important role in maintaining the integrity of the mucosal barrier and in other aspects of intestinal homeostasis.

Substance P and other neuropeptides do not induce mediator release in isolated human intestinal mast cells

Neuropeptides such as substance P (SP) and related peptides are supposed to act as mast cell agonists, and thus as mediators of neuroimmune interactions. The data supporting this hypothesis were obtained mostly from rodent experiments. Here, we studied for the first time the effect of SP and other peptides on mediator release in human intestinal mast cells, either unpurified or enriched to 85-99% purity. We found that SP at 0.1-100 micromol L(-1), or other peptides including neurokinin A and B, calcitonin gene-related peptide, vasoactive intestinal peptide and serotonin at 1 micromol L(-1) do not induce release of mediators such as histamine, sulphidoleukotrienes, and tumour necrosis factor alpha. The peptides also failed to cause mediator release in mast cells isolated from inflamed tissue derived from Crohn's disease. Using reverse transcriptase-polymerase chain reaction, flow cytometry and immunohistochemistry, we could show that human intestinal mast cells do not express the tachykinin receptors NK-1, NK-2, or NK-3 under basal conditions. However, upon stimulation by immunoglobulin E (IgE) receptor-crosslinking, which induces an extensive mediator release reaction, a subpopulation of mast cells clearly expressed NK-1, the SP receptor. In conclusion, our data show that SP and other neuropeptides do not act as secretagogues in human intestinal mast cells that have not been pre-activated by IgE receptor-crosslinking.

Effects of the probiotic yeast Saccharomyces boulardii on the neurochemistry of myenteric neurones in pig jejunum

We studied the effects of food supplementation with Saccharomyces boulardii (S. boulardii; synonym S. cerevisiae HANSEN CBS 5926; 1 g per day for 9 days) on the presence and co-localization patterns of neuronal markers in myenteric neurones of the pig jejunum. The pan neuronal marker Hu revealed no change in the number of neuronal cell bodies per ganglion (37 +/- 7 in control vs 34 +/- 9 in the S. boulardii group). Ranked by size the following cell populations were identified: choline acetyltransferase (ChAT), calbindin-28k (CALB), substance P (SP), neurofilament 160 kD (NF-160), vasoactive intestinal polypeptide (VIP), nitric oxide synthase (NOS), calcitonin gene-related peptide (CGRP), calretinin (CALRET). We found a significant decrease in the number of CALB myenteric neurones in animals which received S. boulardii supplemented diet. None of the other neuronal markers revealed any difference between controls and S. boulardii treated animals. The study reports transmitter-localization patterns in the myenteric plexus of the pig jejunum and provides evidence that changes in the neurochemistry of enteric neurones occur with S. boulardii supplemented diet. Although only CALB expression was altered and the functional significance of this finding remains unknown, our study identified a possible new effector level of probiotics in the gut.

Neurochemical coding of myenteric neurones in the human gastric fundus

The major functions of the stomach are under the control of the enteric nervous system (ENS), but the neuronal circuits involved in this control are largely unknown in humans. Enteric neurones can be characterized by their neuromediator or marker content, i.e. by neurochemical coding. The purpose of this study was to characterize the presence and co-localization of neurotransmitters in myenteric neurones of the human gastric fundus. Choline acetyltransferase (ChAT), neurone-specific enolase (NSE), vasoactive intestinal polypeptide (VIP), nitric oxide synthase (NOS), substance P (SP) were detected by immunohistochemical methods in whole mounts of gastric fundus myenteric plexus (seven patients). Antibodies against ChAT and NOS labelled the majority of myenteric neurones identified by NSE (57.2 +/- 5.6% and 40.8 +/- 4.5%, respectively; mean +/- SD). The proportions of VIP- and SP-immunoreactive neurones were significantly smaller, constituting 19.6 +/- 6.9% and 16.0 +/- 3.7%, respectively. Co-localization studies revealed five major populations representing over 75% of the myenteric neurones: ChAT/-, 30.1 +/- 6.1%; NOS/-, 24.2 +/- 4.4%; ChAT/SP/-, 8.3 +/- 3.1%; NOS/VIP/-, 7.2 +/- 6.0%; ChAT/VIP/-, 4.9 +/- 2.6. Some similarities are apparent in the neurochemical coding of myenteric neurones in the stomach and intestine of humans, and between the stomach of humans and animals, but striking differences exist. The precise functional role of the neurochemically identified classes of neurones remains to be determined.

Human ENS regulates the intestinal epithelial barrier permeability and a tight junction-associated protein ZO-1 via VIPergic pathways

Although the enteric nervous system (ENS) has been shown to regulate various mucosal functions, its role in the physiological control of the human intestinal epithelial barrier is unknown. The aim of this study was to investigate whether the ENS is able to modulate epithelial barrier permeability and a key tight junction-associated protein, zonula occludens-1 (ZO-1). Therefore, we developed a co-culture model, consisting of human submucosa containing the submucosal neuronal network and human polarized colonic epithelial monolayers (HT29-Cl.16E or Caco-2). Submucosal neurons were activated by electrical field stimulation (EFS). Permeability was assessed by measuring the flux of paracellular permeability markers (FITC-dextran or FITC-inulin) across epithelial monolayers. Expression of ZO-1 was determined by immunofluorescence, quantitative immunoblot analysis, and real time RT-PCR. Using the coculture model, we showed that EFS of submucosal neurons resulted in a reduction in FITC-dextran or FITC-inulin fluxes, which was blocked by TTX. In HT29-Cl.16E, the effect of submucosal neuron activation was blocked by a VIP receptor antagonist (VIPra) and reproduced by VIP. Furthermore, ZO-1 expression (mRNA, protein) assessed in HT29-Cl.16E, was significantly increased after submucosal neuron activation by EFS. These effects on ZO-1 expression were blocked by TTX and VIPra and reproduced by VIP. In conclusion, our results strongly suggest a modulatory role of VIPergic submucosal neuronal pathways on intestinal epithelial barrier permeability and ZO-1 expression.