Nitric oxide is a neurotransmitter in the chloride secretory response to serotonin in rat colon

Reduction of epithelial secretion in male rat distal colonic mucosa by bile acid receptor TGR5 agonist, INT-777: role of submucosal neurons

BACKGROUND

Recent evidence from rat neuron-free mucosa study suggests that the membrane bile acid receptor TGR5 decreases colonic secretion under basal and stimulated conditions. As submucosal neurons are key players in secretory processes and highly express TGR5, we investigated their role in TGR5 agonist-induced inhibition of secretion and the pathways recruited.

METHODS

TGR5 expression and localization were assessed in rat proximal (pC) and distal (dC) colon by qPCR and immunohistochemistry with double labeling for cholinergic neurons in whole-mount preparations. The influence of a selective (INT-777) or weak (ursodeoxycholic acid, UDCA) TGR5 agonist on colonic secretion was assessed in Ussing chambers, in dC preparation removing seromuscular ± submucosal tissues, in the presence of different inhibitors of secretion pathways.

KEY RESULTS

TGR5 mRNA is expressed in full thickness dC and pC and immunoreactivity is located in colonocytes and pChAT-positive neurons. Addition of INT-777, and less potently UDCA, decreased colonic secretion in seromuscular stripped dC by -58.17± 2.6%. INT-777 effect on basal secretion was reduced in neuron-free and TTX-treated mucosal-submucosal preparations. Atropine, hexamethonium, indomethacin, and L-NAME all reduced significantly INT-777's inhibitory effect while the 5-HT₄ antagonist, RS-39604, and lidocaine abolished it. INT-777 inhibited stimulated colonic secretion induced by nicotine, but not cisapride, carbachol or PGE2.

CONCLUSIONS & INFERENCES

TGR5 activation inhibits basal and stimulated distal colonic secretion in rats by acting directly on epithelial cells and also inhibiting submucosal neurons. This could represent a counter-regulatory mechanism, at the submucosal level, of the known prosecretory effect of bile acids in the colon.

Germ-Free Mice Model for Studying Host-Microbial Interactions

Germ-free (GF) mice are a relevant model system to study host-microbial interactions in health and disease. In this chapter, we underscore the importance of using GF mice model to study host-microbial interactions in obesity, immune development and gastrointestinal physiology by reviewing current literature. Furthermore, we also provide a brief protocol on how to setup a gnotobiotic facility in order to properly maintain and assess GF status in mice colonies.

Luminal 5-HT stimulates colonic bicarbonate secretion in rats

BACKGROUND AND PURPOSE

The bioactive monoamine 5-HT, implicated in the pathogenesis of functional gastrointestinal disorders, is abundantly synthesized and stored in rat proximal colonic mucosa and released to the gut lumen and subepithelial space. Despite much data regarding its expression and function, the effects of luminal 5-HT on colonic anion secretion have not been fully investigated.

EXPERIMENTAL APPROACH

We measured short-circuit current (Isc ) as an indicator of ion transport in mucosa-submucosa or mucosa-only preparations of rat proximal colon. Total CO2 output was measured in vitro and in vivo. Immunohistochemistry was performed to investigate the localization of 5-HT4 , NOS1 and NOS2.

KEY RESULTS

Luminal 5-HT gradually increased the amplitude and sustained the elevation of Isc . Luminal 5-HT-evoked ΔIsc was acetazolamide sensitive and HCO3 (-) dependent, consistent with cytosolic carbonic anhydrase-dependent electrogenic HCO3 (-) secretion, while not affected by tetrodotoxin (TTX), atropine or indomethacin. Pretreatment with the selective 5-HT4 antagonist GR113808, but not antagonists for 5-HT3 , 5-HT6 or 5-HT7 , inhibited luminal 5-HT-evoked ΔIsc . Furthermore, luminal cisapride and tegaserod increased Isc to the same extent as did 5-HT in the presence of indomethacin and TTX. Removal of the submucosa or pretreatment with NOS inhibitors enhanced luminal 5-HT-evoked ΔIsc , suggesting that NO synthesized in the submucosa suppresses mucosal anion secretion. NOS1 and NOS2 were immunostained in the submucosal neurons and glial cells respectively. Luminal 5-HT-evoked HCO3 (-) secretion was confirmed in vivo, inhibited by co-perfusion of GR113808, but not by ondansetron.

CONCLUSIONS AND IMPLICATIONS

A novel apical 5-HT4 -mediated HCO3 (-) secretory pathway and an NO-dependent inhibitory mechanism are present in the proximal colon. Luminal 5-HT-evoked HCO3 (-) secretion may be important for the maintenance of mucosal integrity by regulating luminal pH.

Clinical presentation, diagnosis, pathogenesis and treatment options for lymphocytic colitis (Review)

Lymphocytic colitis (LC) is characterized by chronic or relapsing non-bloody watery diarrhea and a macroscopically normal colon. However, histopathological examination of colonic biopsy samples reveals an increased intraepithelial infiltration of lymphocytes (≥20/100 enterocytes), and increased inflammatory cells within the lamina propria, but with a normal mucosal architecture. The reported prevalence of LC varies from 14.2 to 45 per 100,000 individuals, while its reported incidence is between 0.6 and 16 per 100,000 individuals. LC has a high rate of spontaneous symptomatic remission and is not associated with an increased risk of colon cancer or inflammatory bowel disease. The diagnosis is based on the histopathological findings. The density of colonic chromogranin A-positive cells provides an effective diagnostic tool with high sensitivity and specificity in both the right and left colon. Gastrointestinal infections, drugs, and/or autoimmunity may trigger chronic colonic low-grade inflammation. Colonic nitric oxide, serotonin and peptide YY (PYY) cell densities are markedly increased in patients with LC. It has been hypothesized that the low-grade inflammation in LC through the endocrine-immune axis causes this increase. It has been postulated further that these abnormalities in the neuroendocrine system of the colon are responsible for the diarrhea observed in patients with LC. The benign course and rate of spontaneous remission of LC denotes that drugs with severe side-effects should be avoided if possible. The drug cost and drug coverage may also be limiting factors for some patients. These aspects should be taken into account when making decisions regarding treatment options.

Prostaglandin I(2) sensory input into the enteric nervous system during distension-induced colonic chloride secretion in rat colon

AIM

Intestinal pressure differences or experimental distension induce ion secretion via the enteric nervous system, the sensorial origin of which is only poorly understood. This study aimed to investigate sensorial inputs and the role of afferent and interneurones in mechanically activated submucosal secretory reflex circuits.

METHODS

Distension-induced rheogenic chloride secretion was measured as increase in short-circuit current 10 min after distension (DeltaI(SC)(10); distension parameters +/- 100 microL, 2 Hz, 20 s) in partially stripped rat distal colon in the Ussing-chamber in vitro. PGE(2) and PGI(2) were measured by radioimmunoassay.

RESULTS

DeltaI(SC)(10) was 2.0 +/- 0.2 micromol h(-1) cm(-2) and could be attenuated by lobeline, mecamylamine and dimethylphenylpiperazine, indicating an influence of nicotinergic interneurones. Additionally, a contribution of afferent neurones was indicated from the short-term potentiation of DeltaI(SC)(10) by capsaicin (1 microm). As evidence for its initial event, indomethacin (1 microm) inhibited distension-induced secretion and the release of PGI(2) was directly detected after distension. Furthermore, serotoninergic mediation was confirmed by granisetron (100 microm) which was functionally localized distally to PGI(2) in this reflex circuit, as granisetron inhibited an iloprost-induced I(SC), while indomethacin did not affect serotonin-activated ion secretion.

CONCLUSIONS

Distension-induced active electrogenic chloride secretion in rat colon is mediated by a neuronal reflex circuit which includes afferent neurones and nicotinergic interneurones. It is initiated by distension-induced PGI(2) release from subepithelial cells triggering this reflex via serotoninergic 5-HT(3) receptor transmission. Functionally, this mechanism may help to protect against intestinal stasis but could also contribute to luminal fluid loss, e.g. during intestinal obstruction.

Serotonin binds to purified neuronal nitric oxide synthase: a possible explanation for ROS production induced by 5HT in the presence of nNOS

Serotonin (5HT) was shown to induce in vitro the production of ROS in the presence of neuronal nitric oxide synthase (nNOS) in addition to the basal NO(+) formation. With the aim of understanding this mechanism, this study investigated the potential binding of 5HT to nNOS. By using [(3)H]5HT, it is reported here that 5HT binds to nNOS, but only when the enzyme is active and in a superoxide-dependent manner. This binding is prevented by DPI but not by L-NAME. The formation of 5HT-nNOS complex was shown to be very well correlated with the production of ROS by 5HT in the presence of nNOS. A mechanism involving nNOS only in its initial step is proposed to explain both the formation of 5HT-nNOS complex and the production of ROS observed in the presence of nNOS and 5HT.

The role of serotonin in intestinal luminal sensing and secretion

This mini-review addresses the role of the neuroendocrine substance serotonin (5-hydroxytryptamine, 5-HT) in intestinal luminal sensing and secretion. Intestinal sensory neurones are activated by several mechanisms, in particular following stimulation of 'specialized' cells in the mucosa. These specialized cells are the enteroendocrine cells, which contain a wide variety of neuroendocrine transmitters. One of these enteroendocrine cells is the enterochromaffin (EC) cell, which is present throughout the intestines and contains large amounts of serotonin, predominantly in the duodenum in humans. The EC cells act as mucosal sensory transducers and secrete serotonin in response to various physiological and pathological luminal stimuli. Following release, serotonin participates in several mucosal protecting processes, one being secretion. Serotonin stimulates active ion, mucus and fluid secretion. Epithelial 5-HT(2) receptors and neuronal 5-HT(1P), 5-HT(3) and 5-HT(4) receptors mediate the secretory effect of serotonin. A transmembrane serotonin transporter terminates epithelial serotonergic signalling. The transient receptor potential ion channel family is important for processing intestinal luminal sensory signalling. Accumulating evidence suggests a significant interaction between serotonin and one of the transient receptor potential ion channels, the capsaicin-sensitive transient receptor potential vanilloid type 1. Accordingly, EC cells, serotonergic receptors and transporter(s), and transient receptor potential vanilloid type-1 ion channels are all explored as pharmacological targets for treatment of some intestinal functional disorders.

Distinct morphology of serotonin-containing enterochromaffin (EC) cells in the rat distal colon

The present study was performed to examine the distribution and distinct morphology of the serotonin-containing enterochromaffin (EC) cells in the rat distal colon by immunohistochemical and electron microscopic methods. Serotonin-immunohistochemistry revealed that most of the serotonin-immunoreactive EC cells possessed extended cytoplasmic processes. In particular, the immunoreactive EC cells with long processes located along the body of the crypt were characterized by their bipolar processes comprising one with the terminal swellings extending vertically down to the basal crypt and the other running up along the luminal side - in many cases, with the apical ends reaching the glandular lumen. Moreover, a few EC cells had long processes which resembled neuronal processes with varicosities. Electron microscopic observations revealed rod-like, tortuous, oval, or round small pleomorphic granules in the long processbearing EC cells. The cell bodies and processes directly faced the crypt epithelial cells - including the enterocytes and goblet cells on one side and the basement membrane on the opposite side. The accumulation of the granules sometimes appeared within the cytoplasm on the side of the epithelial cells. These findings suggest that serotonin is released from the long processes of the EC cells and directly acts in a paracrine fashion on the crypt epithelial cells to secrete electrolytes and fluids into the colonic lumen. The long cytoplasmic processes of the EC cells may be a major contributor to the serotonininduced secretory events in the rat distal colon.

The endogenous neurotransmitter, serotonin, modifies neuronal nitric oxide synthase activities

Serotonin, an important neurotransmitter, is colocalized with neuronal nitric oxide synthase (nNOS), a homodimeric enzyme which catalyzes the production of nitric oxide (NO(.-)) and/or oxygen species. As many interactions have been reported between the nitrergic and serotoninergic systems, we studied the effect of serotonin on nNOS activities. Our results reveal that nNOS is activated by serotonin as both NADPH consumption and oxyhemoglobin (OxyHb) oxidation were enhanced. The generation of L-citrulline from L-arginine (L-Arg) was not affected by serotonin in the range of 0-200 microM, suggesting an additional production of oxygen-derived species. But 5-hydroxytryptamine (5HT) induced the formation of both O and H(2)O(2) by nNOS, as evidenced by electron paramagnetic resonance (EPR) and by using specific spin traps. Overall, these results demonstrate that serotonin is able to activate nNOS, leading to the generation of reactive oxygen species (ROS) in addition to the NO(.-) production. Such a property must be considered in vivo as various nNOS-derived products mediate different signaling pathways.

Luminal oxidants selectively modulate electrogenic ion transport in rat colon

AIM

To investigate the effects of luminal exposure to H2O(2) and two related thiol oxidizing agents on basal and stimulated chloride secretion in native colon using electrophysiological and pharmacological approaches.

METHODS

Unstripped rat distal colon segments were mounted in Ussing chambers. Potential difference, calculated resistance and short-circuit current across unstripped colon segments were monitored with a dual voltage/current clamp. Paracellular permeability was assessed by measuring the mucosa-to-serosa flux of a fluorescent probe (FITC).

RESULTS

Luminal exposure to hydrogen peroxide transitorily stimulated chloride secretion without altering barrier function. This stimulatory effect could be blocked by basolateral atropine but not indomethacin. The cysteine and methionine oxidizing compounds, phenylarsine oxide and chloramine T respectively, mimicked the effect of H2O(2), except for a drop in transcolonic resistance after 30 min. In contrast to the observed stimulatory effect on basal secretion, cAMP-stimulated electrogenic ion transport was blunted by luminal H2O(2). However, the Ca(2+)-activated response remained unchanged.

CONCLUSION

H2O(2) may be an important selective modulator of intestinal ion and water secretion in certain pathologic conditions such as inflammation or ischemia-reperfusion by multiple mechanisms.

Effects of nitric oxide in 5-hydroxytryptamine-, cholera toxin-, enterotoxigenic Escherichia coli- and Salmonella Typhimurium-induced secretion in the porcine small intestine

The effects of nitric oxide (NO) in the secretory response to the endogenous secretagogue 5-hydroxytryptamine (5-HT), the enterotoxins heat-labile enterotoxigenic Escherichia coli (ETEC) toxin (LT) and cholera toxin (CT), and various cultures of ETEC and Salmonella serotype Typhimurium in the porcine small intestine (Sus scrofa) were investigated. In anaesthetized pigs, jejunal tied-off loops were instilled with 5-HT, LT, CT, various cultures of ETEC or S. Typhimurium. Pigs were given intravenously isotonic saline or isotonic saline containing the NO synthase inhibitor, Nomega-nitro-L-arginine methyl ester (L-NAME). L-NAME significantly induced an increased fluid accumulation in loops induced by 5-HT, ETEC and stn-mutated S. Typhimurium. Fluid accumulation in loops instilled with wild-type S. Typhimurium was increased by L-NAME, although not significantly, while there was no effect on fluid accumulation induced by an invH-mutated isogenic strain. No significant effect of L-NAME was observed on the fluid accumulation induced by the purified enterotoxins LT and CT. The results also demonstrated a relatively large difference in the ability to induce fluid accumulation between the bacteria strains. Diastolic, systolic and mean blood pressures were significantly increased and the body temperature was significantly decreased in groups of pigs treated with L-NAME. In conclusion, the results suggest that NO has a proabsorptive effect in the intact porcine jejunum and is involved in the systemic vascular tone.

Effects of luminal ATPase inhibitors on electrogenic ion transport in rat distal colon

BACKGROUND

The involvement of transport proteins, other than chloride channels, expressed in the luminal membrane of epithelial cells in regulated chloride secretion in native colon remains poorly understood. There are at least two distinct ATPases expressed in the apical membrane of rat colonocytes. They can be distinguished by their different sensitivity to the vanadium-derived compound orthovanadate. The objective was to study the effects of luminal ATPase inhibitors on regulated chloride secretion using elecrophysiological and pharmacological approaches.

MATERIALS AND METHODS

Unstripped rat distal colon segments were mounted in Ussing chambers. Potential difference, transepithelial resistance, and short-circuit current across unstripped colon segments were monitored with a dual voltage/current clamp.

RESULTS

Luminal application of VO4(3-) did not alter baseline electrical values in rat distal colon but dose-dependently inhibited forskolin-stimulated Isc. Luminal ouabain (1 mm) did not blunt the response to the cAMP agonist. The inhibitory effect of luminal VO4(3-) occurred at a site distal to cAMP generation and was rather specific for the cyclic nucleotide-dependent signaling pathway, because the response to the Ca2+ agonist carbachol was largely preserved.

CONCLUSION

VO4(3-) inhibits cAMP-stimulated Cl- secretion in rat distal colon at a site distal to cAMP generation without altering intestinal permeability. Ouabain-sensitive luminal K+-ATPases do not seem to contribute to forskolin-stimulated electrogenic ion transport. These findings may suggest new therapeutic targets for secretory diarrhea.

The urinary 5-hydroxyindole acetic acid and plasma nitric oxide levels in irritable bowel syndrome: a preliminary study

BACKGROUND AND AIMS

Postprandial increase of 5-hydroxytryptamine (5-HT) has been implicated in irritable bowel syndrome (IBS). There is evidence that nitric oxide (NO) may act as a mediator of 5-HT-evoked secretions in the colon. Our aim is to investigate the role of urinary 5-hydroxyindole acetic acid (5-HIAA) and plasma NO levels (with diarrhoea) in IBS patients.

METHODS

Nineteen (with constipation) IBS patients (group 1), 22 IBS patients (group 2) and 18 healthy controls (group 3) were included in the study. The diagnosis of IBS was made according to the Rome I Criteria. The urine was collected for determination of 5-HIAA and venous blood was collected from each subject for the measurement of plasma NO levels.

RESULTS

The levels of urinary 5-HIAA mmol/day and plasma NO mmol/l of group 1 (22.4 +/- 2.2 and 29.4 +/- 2 respectively) were significantly higher than group 3 (14.2 +/- 2.3 and 21.3 +/- 2.1 respectively) (p = 0.036 and p = 0.019 respectively). The NO level of group 1 was also significantly higher than group 2 (21.8 +/- 1.9) (p = 0.021). The 5-HIAA level of group 1 was higher than group 2 (15.2 +/- 2.1) and the difference was marginally significant (p = 0.055). There was no difference between group 2 and group 3 with respect to 5-HIAA and NO levels.

CONCLUSIONS

The results of this preliminary study lend support to the involvement of 5-HT in some symptomatology of diarrhoea predominant IBS. Furthermore, NO may be one of the effector mediators of the 5-HT-induced symptoms in these patients.

Serotonin excites neurons in the human submucous plexus via 5-HT3 receptors

BACKGROUND & AIMS

Serotonin (5-hydroxytryptamine [5-HT]) is a key signaling molecule in the gut. Recently, the neural 5-HT3 receptor received a lot of attention as a possible target in functional bowel diseases. Yet, the 5-HT3 receptor-mediated changes in properties of human enteric neurons is unknown.

METHODS

We used a fast imaging technique in combination with the potentiometric dye 1-(3-sulfonatopropyl)-4-[beta[2-(di-n-octylamino)-6-naphthyl]vinyl]pyridinium betaine to monitor directly the membrane potential changes in neurons of human submucous plexus from surgical specimens of 21 patients. An Ussing chamber technique was used to study 5-HT3 receptor involvement in chloride secretion.

RESULTS

Local microejection of 5-HT directly onto ganglion cells resulted in a transient excitation of enteric neurons characterized by increased spike discharge. This response was mimicked by the 5-HT3 receptor agonist, 2-methyl-5-HT, and blocked by the 5-HT3 receptor antagonist, tropisetron. The proportions of 5-HT-responsive nerve cells per ganglion ranged from 25.5% +/- 18.4% in the duodenum to 54.2% +/- 46.9% in the colon. Interestingly, 2-methyl-5-HT did not evoke chloride secretion in the human intestine but it did in the guinea-pig intestine. Specific 5-HT3A and 5-HT3B receptor subunit immunoreactivity as well as 5-HT3A and 5-HT3B receptor-specific messenger RNA were detected in the tissue samples. Based on co-labeling with the pan-neuronal marker HuC/D we conclude that submucous nerve cells potentially express heteromeric 5-HT3A/B receptors.

CONCLUSIONS

We show that 5-HT excited human enteric neurons via 5-HT3 receptors, which may comprise both 5-HT3A and 5-HT3B receptor subunits.

Involvement of c-Src and protein kinase C delta in the inhibition of Cl(-)/OH- exchange activity in Caco-2 cells by serotonin

Serotonin (5-hydroxytryptamine (5-HT)) is an important neurotransmitter and intercellular messenger regulating various gastrointestinal functions, including electrolyte transport. To date, however, no information is available with respect to its effects on the human intestinal apical anion exchanger Cl(-)/OH- (HCO3-). The present studies were therefore undertaken to examine the direct effects of serotonin on OH- gradient-driven 4,4'-diisothiocyanato-stilbene-2, 2'-disulfonic acid-sensitive 36Cl- uptake utilizing the post-confluent transformed human intestinal epithelial cell line Caco-2. Our results demonstrate that serotonin inhibits Cl(-)/OH- exchange activity in Caco-2 cells via both tyrosine kinase and Ca(2+)-independent protein kinase C delta-mediated pathways involving either 5-HT3 or 5-HT4 receptor subtype. The data consistent with our inference are as follows. (i) The short term treatment of cells with 5-HT (0.1 microM) for 15-60 min significantly decreased Cl(-)/OH- exchange (50-70%, p < 0.05). (ii) The specific agonists for 5-HT3, m-chlorophenylbiguanide, and 5-HT4, 3-(4-allylpiperazin-1-yl)-2-quinoxaline chloronitrile, mimicked the effects of serotonin. (iii) Tropisetron dual inhibitor for both the 5-HT3/4 receptor subtypes significantly blocked the inhibition, whereas specific 5-HT3 (Y-25130) or 5-HT4 receptor (RS39604) antagonist failed to block the inhibitory effects of 5-HT. (iv) The Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl ester) had no effect on the serotonin-induced inhibition. (v) The specific protein kinase C (PKC) inhibitors chelerythrine chloride or calphostin C completely blocked the inhibition by 5-HT. (vi) The specific inhibitor for PKC delta, rottlerin, significantly blocked the inhibition by 5-HT. (vii) The specific tyrosine kinase inhibitor, herbimycin, or Src family kinase inhibitor, PP1, abolished the 5-HT-mediated inhibition of Cl(-)/OH- exchange activity. (viii) 5-HT stimulated tyrosine phosphorylation of c-Src kinase and PKC delta.

Role of serotonin in the pathophysiology of the irritable bowel syndrome

The irritable bowel syndrome (IBS) is a complex disorder that is associated with altered gastrointestinal motility, secretion, and sensation. Serotonin (5-HT) is an important neurotransmitter and paracrine signalling molecule in the gastrointestinal tract. 5-HT release from enterochromaffin (EC) cells initiates peristaltic, secretory, vasodilatory, vagal and nociceptive reflexes. The enteric nervous system (ENS) comprises a semiautonomous effector system that is connected to the central autonomic network. Parasympathetic and sympathetic nerves modulate the ENS via afferent and efferent communications. Ongoing, bidirectional brain-gut interactions involving 5-HT pathways occur that significantly influence the effector systems. Altered 5-HT signalling may lead to both intestinal and extraintestinal symptoms in IBS. 5-HT directly and indirectly affects intestinal motor and secretory function and abnormalities may lead to either constipation or diarrhea. 5-HT modulates sensation and perception of visceral stimulation at peripheral and central sites. Therapeutic agents targeting altered 5-HT signalling may provide new, effective treatments for patients with IBS.

Vitreal pathogenic role in optic pit foveolar retinoschisis and central serous retinopathy

PURPOSE

To expand on current theories concerning the vitreal-induced mechanism underlying the development of foveolar retinoschisis and macular sensory detachments associated with optic nerve head pits. To propose the notion that vitreal traction may contribute to the pathogenesis of serous detachments in central serous chorioretinopathy (CSC).

REPORTS

We describe two patients, one with macular retinoschisis and the other with central serous detachment. The first patient, a 45-year-old Hispanic female, presented with a temporally located optic nerve head pit, foveolar retinoschisis and schisis retinal spaces extending to the surrounding macula and to the disc. The second patient, a 43-year-old Haitian male, developed a central serous retinal detachment OS with decreased visual acuity one day following in-office administration of Apraclonidine (0.5 per cent Iopidine, Alcon) and Dorzolamide-Timolol Maleate (Cosopt, Merck) to lower elevated intraocular pressure (IOP). Macular retinal pigment mottling and epiretinal membrane sheen OU had been observed on his initial visit. Visual acuity improved within a three-day period with resolution of the serous detachment.

CONCLUSION

We suggest that the persistence of Cloquet's canal may permit fluid leakage into the proximal vitreous in cases of congenital optic nerve head pits. Tangential vitreal traction may promote the opening of a fistula at the optic pit and additionally thrust vitreal fluid into the pit and retinal space inducing the formation of schisis spaces, foveolar-schisis and underlying sensory serous detachment. We question whether a reduction in vitreous volume, induced by initial administration of anti-glaucoma medications, may contribute to the development and/or recurrence of central serous choroidopathy in predisposed individuals.

Effect of 5-hydroxytryptamine on duodenal mucosal bicarbonate secretion in mice

BACKGROUND & AIMS

5-hydroxytryptamine (5-HT) is an important neurotransmitter and intercellular messenger that modulates many gastrointestinal functions. Because little is known about the role of 5-HT in the regulation of duodenal bicarbonate secretion, we examined the role of 5-HT on duodenal bicarbonate secretion and define neural pathways involved in the actions of 5-HT.

METHODS

Duodenal mucosa from National Institutes of Health Swiss mice was stripped of seromuscular layers and mounted in Ussing chambers. The effect of 5-HT on duodenal bicarbonate secretion was determined by the pH stat technique. Acetylcholine (ACh) release from duodenal mucosa was assessed by preincubating the tissue with [(3)H] choline and measuring 5-HT-evoked release of tritium.

RESULTS

5-HT added to the serosal bath markedly stimulated duodenal bicarbonate secretion and short circuit current (Isc) in a dose-dependent manner (10(-7) mol/L to 10(-3) mol/L; P < 0.0001), whereas mucosally added 5-HT was without effect. 5-HT-stimulated bicarbonate secretion was independent of luminal Cl(-). Pretreatment with tetrodotoxin (TTX) (10(-6) mol/L) or atropine (10(-5) mol/L) markedly reduced 5-HT-stimulated duodenal bicarbonate secretion (by 60% and 65%, respectively; P < 0.001) and Isc (by 45% and 27%, respectively; P < 0.001 and P < 0.05). Pretreatment with N(omega)-nitro-l-arginine methyl ester (l-NAME) (10(-3) mol/L), propranolol (10(-5) mol/L), or phentolamine (10(-5) mol/L) did not significantly alter 5-HT-stimulated duodenal mucosal bicarbonate secretion or Isc. 5-HT concentration-dependently evoked ACh release from duodenal mucosal preparations (P < 0.0001). TTX markedly inhibited 5-HT-evoked ACh release (P < 0.001).

CONCLUSIONS

5-HT is a potent activator of duodenal mucosal bicarbonate secretion in mice. Duodenal bicarbonate secretion induced by 5-HT in vitro occurs principally via a cholinergic neural pathway.

Inhibition of neural nitric oxide synthase attenuates the chloride secretory response to stroking in human jejunum

BACKGROUND

Stroking of human jejunal mucosa induces serotonin release and a rise in short-circuit current (DeltaI(sc)). Nitric oxide is known to function as a nonadrenergic, noncholinergic neurotransmitter in response to neural serotonin receptor activation in the rat. We hypothesize that neural nitrergic mechanisms mediate the chloride secretory response to mucosal stroking in human jejunum.

METHODS

Segments of normal proximal jejunum were obtained from patients having gastric bypass surgery for obesity. Muscle-stripped segments of jejunum were mounted in Ussing chambers under short-circuit conditions. The neural nitric oxide synthase inhibitor, l-thiocitrulline, was added to experimental tissues. Mucosal stroking of control and experimental segments was then performed.

RESULTS

Pretreatment with l-thiocitrulline attenuated the short circuit rise seen after stroking in the experimental group when compared with the control (5.4 +/- 1.5 microA/cm(2) vs 8.0 +/- 1.6 microA/cm(2); P <.05, Student t test, paired data, n = 11), but did not affect baseline I(sc) before stroking. Serotonin released by stroking was not different in experimental versus control tissue.

CONCLUSIONS

The significantly reduced DeltaI(sc) in the group pretreated with the neural nitric oxide synthase inhibitor suggests that nitric oxide liberated from enteric neurons participates in the chloride secretory response to stroking in human jejunum in vitro.

Nitric oxide-induced Cl- secretion in isolated rat colon is mediated by the release of thromboxane A2

We have shown previously that thromboxane A2 (TXA2), which may be released by the anti-tumour drug irinotecan and by platelet-activating factor (PAF), causes Cl- secretion in the isolated rat colon. In the present study, the involvement of TXA2 in nitric oxide-induced Cl- secretion in isolated rat colon was investigated. In colonic mucosa set between Ussing chambers, the NO donor sodium nitroprusside (SNP; 100 microM) caused Cl- secretion, an effect that was almost completely inhibited by the NO scavenger carboxy-PTIO at 200 microM. The SNP-induced Cl- secretion was inhibited in a concentration-dependent manner by the TXA2 receptor antagonist ONO-3708 (IC50 = 2 microM) and the TX synthase inhibitor Y-20811 (IC50 = 0.4 microM). SNP significantly increased the release of TXA2 (measured as TXB2 release) from the mucosa. The SNP-induced increases in Cl- secretion and TXA2 release were blocked by a NO-sensitive guanylate cyclase inhibitor (ODQ). Dibutyryl cGMP (500 microM) also induced Cl- secretion, which was sensitive to ONO-3708 (10 microM) and Y-20811 (1 microM), and increased the release of TXA2 from the mucosa. PAF-induced (10 microM) Cl- secretion was inhibited by carboxy-PTIO (200 microM) and ODQ (10 microM), whereas irinotecan-induced (500 microM) Cl- secretion was not significantly inhibited by these drugs. A stable TXA2 analogue (STA2) but not SNP (100 microM) changed the membrane potential of epithelial cells in isolated colonic crypts under the whole-cell current-clamp condition. These results indicate that PAF elicits the NO-cGMP pathway and then stimulates the release of TXA2, which is a stimulant of colonic Cl- secretion. In contrast, the NO-cGMP pathway is not involved in the TXA2-mediated Cl- secretion induced by irinotecan.

Serotonergic modulating drugs for functional gastrointestinal diseases

After many years of basic research we have now begun to learn how to manipulate the serotonergic mechanisms within the gut. This has lead to a number of significant advances including 5HT3 antagonists for the treatment of functional diarrhoea, 5HT4 agonists for the treatment of constipation and 5HT1 agonists for the treatment of impaired fundal relaxation. Initial enthusiasm has been somewhat dented by the withdrawal of alosetron because of ischaemic colitis, but it remains to be seen whether this adverse event will be seen with other 5HT3 antagonists. Finally it should be recognized that, in a substantial proportion of patients attending clinics complaining of functional symptoms, anxiety is a major component. The drugs so far described are by and large devoid of CNS effects. It remains possible therefore that a drug which combines both peripheral and central effects would likely to be beneficial.

Mast cells and nitric oxide: control of production, mechanisms of response

Mast cells are involved in numerous activities ranging from control of the vasculature, to tissue injury and repair, allergic inflammation and host defences. They synthesize and secrete a variety of mediators, activating and modulating the functions of nearby cells and initiating complex physiological changes. Interestingly, NO produced by mast cells and/or other cells in the microenvironment appears to regulate these diverse roles. This review outlines some of the pathways central to the production of NO by mast cells and identifies many of the tightly controlled regulatory mechanisms involved. Several cofactors and regulatory elements are involved in NO production, and these act at transcriptional and post-translational sites. Their involvement in NO production will be outlined and the possibility that these pathways are critically important in mast cell functions will be discussed. The effects of NO on mast cell functions such as adhesion, activation and mediator secretion will be examined with a focus on molecular mechanisms by which NO modifies intracellular signalling pathways dependent or independent of cGMP and soluble guanylate cyclase. The possibility that NO regulates mast cell function through effects on selected ion channels will be discussed. Metabolic products of NO including peroxynitrite and other reactive species may be the critical elements that affect the actions of NO on mast cell functions. Further understanding of the actions of NO on mast cell activities may uncover novel strategies to modulate inflammatory conditions.

Both serotonin and a nitric-oxide donor cause chloride secretion in rat colonocytes by stimulating cGMP

BACKGROUND

Previous studies have demonstrated that an antagonist of nitric oxide synthase inhibits neurally mediated chloride secretion in response to serotonin (5-HT). The purpose of this study was to demonstrate that chloride secretion in rat colonocytes that were caused by stimulation of neural 5-HT receptors is mediated by way of a nitrergic pathway that involves the activation of guanylate cyclase.

METHODS

The nitric oxide (NO) donor, diethylenetriamine/NO (DNO), was added to an enriched suspension of rat colonocytes that were preloaded with (36)Cl(-). In parallel experiments, DNO (1 micromol/L) was added to cells that were pretreated with the specific inhibitor of soluble guanylate cyclase, NS2028 (2 micromol/L). In additional studies, the neural 5-HT(3) receptor agonist, 2-methyl-5-HT (10 micromol/L), was added to the serosal surface of muscle-stripped sheets of rat colonic mucosa that were mounted in Ussing chambers under voltage clamp conditions, both in the absence and presence of NS2028 (20 mircro).

RESULTS

DNO induced 18.0% +/- 8.0% greater (36)Cl(-) efflux than controls (P <.05; n = 14 animals). This efflux was abolished by previous treatment with NS2028. In the chamber experiments, 2-methyl-5-HT induced electrogenic chloride secretion that was significantly inhibited by previous treatment with NS2028 (2.2 +/- 0.5 microA/cm(2) vs 13.1 +/- 2.1 microA/cm(2); P <.001; n = 9 animals).

CONCLUSIONS

The predominant secretomotor neurotransmitter that mediates the chloride secretory effects of 5-HT in vitro is nitric oxide. Both the secretory effect initiated at the 5-HT(3) receptor on enteric neurons and at the NO(-) receptor on the rat colonocytes are mediated through the activation of intracellular guanylate cyclase and the production of cyclic guanosine monophosphate.