5-HT metabolism in the intestinal wall of the rat-I. The mucosa

Emerging Roles for Serotonin in Regulating Metabolism: New Implications for an Ancient Molecule

Serotonin is a phylogenetically ancient biogenic amine that has played an integral role in maintaining energy homeostasis for billions of years. In mammals, serotonin produced within the central nervous system regulates behavior, suppresses appetite, and promotes energy expenditure by increasing sympathetic drive to brown adipose tissue. In addition to these central circuits, emerging evidence also suggests an important role for peripheral serotonin as a factor that enhances nutrient absorption and storage. Specifically, glucose and fatty acids stimulate the release of serotonin from the duodenum, promoting gut peristalsis and nutrient absorption. Serotonin also enters the bloodstream and interacts with multiple organs, priming the body for energy storage by promoting insulin secretion and de novo lipogenesis in the liver and white adipose tissue, while reducing lipolysis and the metabolic activity of brown and beige adipose tissue. Collectively, peripheral serotonin acts as an endocrine factor to promote the efficient storage of energy by upregulating lipid anabolism. Pharmacological inhibition of serotonin synthesis or signaling in key metabolic tissues are potential drug targets for obesity, type 2 diabetes, and nonalcoholic fatty liver disease (NAFLD).

Colonic organoids derived from human induced pluripotent stem cells for modeling colorectal cancer and drug testing

With the goal of modeling human disease of the large intestine, we sought to develop an effective protocol for deriving colonic organoids (COs) from differentiated human embryonic stem cells (hESCs) or induced pluripotent stem cells (iPSCs). Extensive gene and immunohistochemical profiling confirmed that the derived COs represent colon rather than small intestine, containing stem cells, transit-amplifying cells, and the expected spectrum of differentiated cells, including goblet and endocrine cells. We applied this strategy to iPSCs derived from patients with familial adenomatous polyposis (FAP-iPSCs) harboring germline mutations in the WNT-signaling-pathway-regulator gene encoding APC, and we generated COs that exhibit enhanced WNT activity and increased epithelial cell proliferation, which we used as a platform for drug testing. Two potential compounds, XAV939 and rapamycin, decreased proliferation in FAP-COs, but also affected cell proliferation in wild-type COs, which thus limits their therapeutic application. By contrast, we found that geneticin, a ribosome-binding antibiotic with translational 'read-through' activity, efficiently targeted abnormal WNT activity and restored normal proliferation specifically in APC-mutant FAP-COs. These studies provide an efficient strategy for deriving human COs, which can be used in disease modeling and drug discovery for colorectal disease.

Rotavirus and Serotonin Cross-Talk in Diarrhoea

Rotavirus (RV) has been shown to infect and stimulate secretion of serotonin from human enterochromaffin (EC) cells and to infect EC cells in the small intestine of mice. It remains to identify which intracellularly expressed viral protein(s) is responsible for this novel property and to further establish the clinical role of serotonin in RV infection. First, we found that siRNA specifically silencing NSP4 (siRNA^NSP4) significantly attenuated secretion of serotonin from Rhesus rotavirus (RRV) infected EC tumor cells compared to siRNA^VP4, siRNA^VP6 and siRNA^VP7. Second, intracellular calcium mobilization and diarrhoeal capacity from virulent and avirulent porcine viruses correlated with the capacity to release serotonin from EC tumor cells. Third, following administration of serotonin, all (10/10) infants, but no (0/8) adult mice, responded with diarrhoea. Finally, blocking of serotonin receptors using Ondansetron significantly attenuated murine RV (strain EDIM) diarrhoea in infant mice (2.9 vs 4.5 days). Ondansetron-treated mice (n = 11) had significantly (p < 0.05) less diarrhoea, lower diarrhoea severity score and lower total diarrhoea output as compared to mock-treated mice (n = 9). Similarly, Ondansetron-treated mice had better weight gain than mock-treated animals (p < 0.05). A most surprising finding was that the serotonin receptor antagonist significantly (p < 0.05) also attenuated total viral shedding. In summary, we show that intracellularly expressed NSP4 stimulates release of serotonin from human EC tumor cells and that serotonin participates in RV diarrhoea, which can be attenuated by Ondansetron.

A review of the serotonin transporter and prenatal cortisol in the development of autism spectrum disorders

The diagnosis of autism spectrum disorder (ASD) during early childhood has a profound effect not only on young children but on their families. Aside from the physical and behavioural issues that need to be dealt with, there are significant emotional and financial costs associated with living with someone diagnosed with ASD. Understanding how autism occurs will assist in preparing families to deal with ASD, if not preventing or lessening its occurrence.

Serotonin plays a vital role in the development of the brain during the prenatal and postnatal periods, yet very little is known about the serotonergic systems that affect children with ASD. This review seeks to provide an understanding of the biochemistry and physiological actions of serotonin and its termination of action through the serotonin reuptake transporter (SERT). Epidemiological studies investigating prenatal conditions that can increase the risk of ASD describe a number of factors which elevate plasma cortisol levels causing such symptoms during pregnancy such as hypertension, gestational diabetes and depression. Because cortisol plays an important role in driving dysregulation of serotonergic signalling through elevating SERT production in the developing brain, it is also necessary to investigate the physiological functions of cortisol, its action during gestation and metabolic syndromes.

Life without peripheral serotonin: insights from tryptophan hydroxylase 1 knockout mice reveal the existence of paracrine/autocrine serotonergic networks

Since its identification, 75 years ago, the monoamine serotonin (5-HT) has attracted considerable attention toward its role as a neurotransmitter in the central nervous system. Yet, increasing evidence, from a growing number of research groups, substantiates the fact that 5-HT regulates important nonneuronal functions. Peripheral 5-HT, synthesized by the enzyme tryptophan hydroxyase (Tph) in intestinal cells, was assumed to be distributed throughout the entire body by blood platelets and to behave as a pleiotropic hormone. A decade ago, generation of a mouse model devoid of peripheral 5-HT lead to the discovery of a second isoform of the enzyme Tph and also suggested that 5-HT might act as a local regulator in various organs. The objective of this review is to highlight the newly discovered functions played by the monoamine using the Tph1 KO murine model and to outline current findings that led to the discovery of complete serotonergic systems in unexpected organs. Within an organ, both the presence of local Tph enzymatic activity and serotonergic components are of particular importance as they support the view that 5-HT meets the criteria to be qualified as a monoamine with a paracrine/autocrine function.

Signals from embryonic fibroblasts induce adult intestinal epithelial cells to form nestin-positive cells with proliferation and multilineage differentiation capacity in vitro

The intestinal epithelium has one of the greatest regenerative capacities in the body; however, neither stem nor progenitor cells have been successfully cultivated from the intestine. In this study, we applied an "artificial niche" of mouse embryonic fibroblasts to derive multipotent cells from the intestinal epithelium. Cocultivation of adult mouse and human intestinal epithelium with fibroblast feeder cells led to the generation of a novel type of nestin-positive cells (intestinal epithelium-derived nestin-positive cells [INPs]). Transcriptome analyses demonstrated that mouse embryonic fibroblasts expressed relatively high levels of Wnt/bone morphogenetic protein (BMP) transcripts, and the formation of INPs was specifically associated with an increase in Lef1, Wnt4, Wnt5a, and Wnt/BMP-responsive factors, but a decrease of BMP4 transcript abundance. In vitro, INPs showed a high but finite proliferative capacity and readily differentiated into cells expressing neural, pancreatic, and hepatic transcripts and proteins; however, these derivatives did not show functional properties. In vivo, INPs failed to form chimeras following injection into mouse blastocysts but integrated into hippocampal brain slice cultures in situ. We conclude that the use of embryonic fibroblasts seems to reprogram adult intestinal epithelial cells by modulation of Wnt/BMP signaling to a cell type with a more primitive embryonic-like stage of development that has a high degree of flexibility and plasticity.

5-hydroxytryptophol as a marker for recent alcohol intake

AIMS

To review the mechanism behind the alcohol-induced shift in serotonin metabolism, and the use of urinary 5-hydroxytryptophol (5-HTOL) as a biochemical marker of acute alcohol consumption.

BACKGROUND

The serotonin metabolite 5-HTOL is a normal, minor constituent of urine and is excreted mainly in conjugated form with glucuronic acid. The formation of 5-HTOL increases dramatically after alcohol intake, due to a metabolic interaction, and the elevated urinary excretion remains for some time (>5-15 hours depending on dose) after ethanol has been eliminated. This biochemical effect can be used for detection of recent alcohol intake.

RESULTS

5-HTOL is determined by the gas chromatography-mass spectrometry (GC-MS) or liquid chromatography and mass spectrometry (LC-MS) techniques. A new ELISA method for 5-HTOL glucuronide provides a promising clinical assay. The most robust way to use the marker is by measuring the ratio of 5-HTOL to 5-hydroxyindoleacetic acid, because this compensates for urine dilution and dietary intake of serotonin. 5-HTOL is a very sensitive and specific indicator of recent alcohol consumption and, as such, a valuable complement to self-report. In clinical use, 5-HTOL is effective for monitoring lapses into drinking during out-patient treatment and for objective evaluation of treatment efforts. Other applications include detection of high-risk patients in elective surgery, monitoring of disulfiram treatment and a method to rule out artefactual ethanol formation in forensic toxicology. 5-HTOL can also be used as a sensitive reference method for validation of self-report data in clinical alcohol research.

CONCLUSIONS

An elevated urinary 5-HTOL level can serve as a sensitive and reliable marker for recent alcohol intake with a number of clinical and forensic applications.

Disruption of the nonneuronal tph1 gene demonstrates the importance of peripheral serotonin in cardiac function

Serotonin (5-HT) controls a wide range of biological functions. In the brain, its implication as a neurotransmitter and in the control of behavioral traits has been largely documented. At the periphery, its modulatory role in physiological processes, such as the cardiovascular function, is still poorly understood. The rate-limiting enzyme of 5-HT synthesis, tryptophan hydroxylase (TPH), is encoded by two genes, the well characterized tph1 gene and a recently identified tph2 gene. In this article, based on the study of a mutant mouse in which the tph1 gene has been inactivated by replacement with the beta-galactosidase gene, we establish that the neuronal tph2 is expressed in neurons of the raphe nuclei and of the myenteric plexus, whereas the nonneuronal tph1, as detected by beta-galactosidase expression, is in the pineal gland and the enterochromaffin cells. Anatomic examination of the mutant mice revealed larger heart sizes than in wild-type mice. Histological investigation indicates that the primary structure of the heart muscle is not affected. Hemodynamic analyses demonstrate abnormal cardiac activity, which ultimately leads to heart failure of the mutant animals. This report links loss of tph1 gene expression, and thus of peripheral 5-HT, to a cardiac dysfunction phenotype. The tph1-/- mutant may be valuable for investigating cardiovascular dysfunction observed in heart failure in humans.

Urinary excretion patterns of 5-hydroxyindole-3-acetic acid and 5-hydroxytryptophol in various animal species: implications for studies on serotonin metabolism and turnover rate

The concentrations of the serotonin metabolites 5-hydroxyindole-3-acetic acid (5HIAA) and 5-hydroxytryptophol (5HTOL) were determined in spot urine samples of 12 mammalian and one fish species (cat, cow, dog, ferret, golden hamster, guinea pig, horse, monkey, mouse, rabbit, rainbow trout, rat, sheep) and compared with human data. The highest urinary concentrations of 5HTOL were found in the Sprague-Dawley rat (mean 9.5 micromol/L) and NMRI mouse (8.2 micromol/L), and the lowest in rainbow trout, cynomolgus macaque, and human urine (approximately 0.1 micromol/L). The highest 5HIAA concentrations were found in hamster (89.3 micromol/L) and mouse (85.2 micromol/L), and the lowest in rainbow trout, horse and sheep (range 2.0-3.7 micromol/L). Several species showed 5HIAA concentrations similar to that normally observed in human urine (approximately 5-40 micromol/L). This study demonstrated wide inter- and intra-species variations in the urinary concentrations of 5HIAA and 5HTOL, both separately and in the sum of concentrations. The 5HTOL/5HIAA ratio, which is used as an easily accessible index of the relative importance of the reductive and oxidative pathways for serotonin metabolism, also varied considerably between different species. This observation confirms that the much higher urinary 5HTOL/5HIAA ratio in rats (mean 0.35) compared with humans (< 0.01) is due to a higher baseline formation of 5HTOL in the rat. The monkey, ferret, hamster, and rabbit most closely resembled humans in this respect, and at least the two latter species appear to be more suitable than rats as animal models for studying serotonin metabolism and turnover rate, and the metabolic interaction with ethanol.

Involvement of NF-Y and Sp1 in basal and cAMP-stimulated transcriptional activation of the tryptophan hydroxylase (TPH ) gene in the pineal gland

The expression of the tryptophan hydroxylase (TPH) gene, encoding the rate-limiting enzyme of serotonin biosynthesis, is tightly regulated both at the transcriptional and at the post-transcriptional levels. In the pineal gland, transcription of the gene is activated in response to an intracellular circadian increase of the cAMP concentration. We have previously shown that transcription of a 2.1-kb fragment of the human TPH promoter is induced by cAMP, although it lacks the canonical cAMP responsive element, CRE. The minimal promoter (-73/+29) has only weak transcriptional activity but is responsive to cAMP. It contains an inverted CCAAT box, which was demonstrated to be involved in this response. Here, we have extended our investigation to the functional features of the inverted CCAAT box in the -252/+29 TPH promoter, which has a higher basal activity. We show that an additional cis -acting sequence, the adjacent GC-rich region, cooperates with the inverted CCAAT box for the full activation of basal transcription, and that both elements are essential for the full cAMP response. We also show that in pinealocytes, NF-Y and Sp1 transactivators bind the inverted CCAAT box and GC-rich-region, respectively. These factors participate in a novel pathway for the cAMP-mediated response of the TPH promoter, which is independent of the canonical CRE-mediated response.

CCAAT displacement protein (CDP/cut) binds a negative regulatory element in the human tryptophan hydroxylase gene

Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in the biosynthesis of serotonin, a neurotransmitter that has been implicated in many psychiatric illnesses. The mechanism of transcriptional regulation of the human TPH gene is largely unknown. We have identified a negative regulatory element located between nucleotides -310 and -220 in the human TPH (hTPH) gene. Electromobility shift analyses performed with the -310/-220 hTPH probe and nuclear extract from P815-HTR (a TPH-expressing cell line) revealed two slow migrating protein-DNA complexes, designated I and II. CCAAT displacement protein (CDP/Cut) is involved in complex I formation as shown in electromobility shift analysis, using consensus oligonucleotide competitor and antibody. Mutations in the CDP/Cut binding site not only disrupted the CDP-DNA complex but also disrupted the second complex, suggesting that the core binding sequences of the two proteins are overlapping. The functional importance of these protein-DNA interactions was assessed by transiently transfecting wild-type and mutant pTPH/luciferase reporter constructs into P815-HTR cells. Mutations in the core CDP/Cut site resulted in an approximately fourfold increase in relative luciferase activities. Because CDP/Cut has been shown to repress transcription of many target genes, we speculate that disruption of the CDP/Cut binding was responsible, at least in part, for the activation of hTPH gene.

Plasma levels of 5-HT and 5-HIAA increased after intestinal ischemia/reperfusion in rats

Intestinal ischemia/reperfusion (I/R) causes serious systemic injury, mainly from a variety of bioactive substances released from the injured intestine. To assess the possible roles of serotonin (5-hydroxytryptamine, 5-HT), a bioactive amine mainly stored in the intestine, in I/R injury, we assayed the levels of tryptophan, 5-HT, and 5-hydroxyindole acetic acid (5-HIAA) in the blood and intestine in a rat I/R model. Plasma 5-HT increased significantly over time after reperfusion; the plateau level was obtained 4 h after reperfusion and was associated with an increase in 5-HIAA. Plasma tryptophan levels declined gradually after reperfusion. The ratio of 5-HIAA/5-HT was significantly higher in I/R rats than in control rats, suggesting that elevated 5-HT was quickly metabolized in the systemic circulation. In the intestine, 5-HT decreased dramatically, whereas tryptophan increased. This phenomenon was prominent in the severely damaged intestine. These findings suggest that the injured intestine released large amounts of 5-HT, whereas its synthesis in the injured intestine was suppressed. An increase in 5-HT in the circulation may be related to various circulatory disturbances observed in humans after intestinal ischemia.

Polarized expression of the antidepressant-sensitive serotonin transporter in epinephrine-synthesizing chromaffin cells of the rat adrenal gland

Antidepressant-sensitive serotonin (5-hydroxytryptamine, 5HT) transporters (SERTs) clear the amine from extracellular spaces in the CNS and periphery as a mechanism for transmitter inactivation and recycling. Although it is known that SERTs are preferentially expressed on basolateral domains in transfected epithelial cells, details of the transporter's membrane localization in vivo are lacking. 5HT and 5HT receptors have been identified in the rodent adrenal gland. Using SERT antagonist autoradiography, we establish the presence of antidepressant-sensitive transport sites in the rat adrenal medulla. Immunofluorescence experiments using antibodies specific for the SERT COOH and NH2 termini, for 5HT, or for catecholamine biosynthetic enzymes suggest that SERT mediates intra-cellular 5HT accumulation by epinephrine-secreting chromaffin cells. Using confocal microscopy, we establish that SERT expression is nonuniformly distributed along the plasma membrane of chromaffin cells. Notably, SERT immunoreactivity is largely absent from plasma membranes bordering smooth muscle that surrounds vascular sinusoids. Rather, SERT is highly expressed in membranes adjoining other chromaffin cells, consistent with a role for 5HT and SERT in autocrine or paracrine control of chromaffin cell physiology. SNAP-25, a t-SNARE protein implicated in neurotransmitter release, was found to colocalize with SERT. In contrast, Na,K ATPase and NCAM are uniformly distributed along the entire perimeter of chromaffin cell membranes. These findings underscore a role for 5HT and SERT in adrenal physiology, reveal unrecognized polarity of chromaffin cell plasma membranes, and warrant a consideration of common targeting mechanisms localizing amine transporters near release sites.

5-Hydroxytryptamine and functional bowel disorders

The possibility that 5-hydroxytryptamine (5-HT) acts as a key sensitising agent in the aetiology of irritable bowel syndrome (IBS) is reviewed. The strategic locations of 5-HT and its receptors are described, the most dominant being the 5-HT3 and 5-HT4 type. 5-HT, acting mostly at 5-HT3 or 5-HT3-like receptors, enhances the sensitivity of visceral neurones projecting between the gut and the central nervous systems. 5-HT, acting at 5-HT4 receptors promotes the sensitivity of enteric neurones that react to luminal stimuli. 5-HT4 and 5-HT3 receptors also mediate, respectively, sensitising and physiological actions of 5-HT on gastro-intestinal motor and secretory functions. This distribution implies that some 5-HT3 receptor antagonists might reduce certain symptoms of IBS, such as pain, by reducing the reactivity of the visceral afferent neurones linking the gut with the brain and spinal cord. However, such antagonists are not likely to find widespread clinical acceptance because they can also affect normal lower bowel function and promote constipation. 5-HT4 receptor antagonists, by contrast, reduce 5-HT-induced enteric nerve hypersensitivity without notably affecting the function of the normal bowel. Accordingly, these agents may reduce the symptoms of IBS directly, by reducing the incidence of defecation and diarrhoea and indirectly, by reducing both 'rebound' constipation and the post-prandial discomfort and pain associated with gastrointestinal hyper-reactivity.

The 5-HT3 antagonist ondansetron reduces gastrointestinal side effects induced by a specific serotonin re-uptake inhibitor in man

The selective serotonin re-uptake inhibitors (SSRIs) are increasingly being used to treat depression and anxiety disorders. Gastrointestinal (GI) symptoms are the main side effects, probably resulting from the stimulation of central or peripheral 5-HT receptors. The present double-blind, placebo-controlled study was undertaken to see if the GI side effects of fluvoxamine could be attenuated by the co-administration of the 5-HT(3) antagonist ondansetron. The results demonstrate that, in volunteers, a single 100 mg oral dose of fluvoxamine can produce GI symptoms. Co-administration of ondansetron significantly reduced peak nausea and GI side effects, compared with placebo.

Effect of streptozotocin-induced diabetes mellitus on serotonin measures of peripheral tissues in rats

The present study was conducted to examine whether experimental diabetes (streptozotocin-induced) promotes changes in serotonin (5HT) measures of peripheral tissue. Platelet-free plasma 5HT, tryptophan and 5-hydroxyindolacetic acid (5HIAA), whole blood 5HT and renal, liver, intestinal and lung 5HT and 5HIAA levels were measured in rats of four experimental groups: control, diabetic, diabetic+insulin and non-diabetic+insulin. Several serotonin measures were unaltered in all four experimental groups, i.e. plasma, liver and lung 5HT and 5HIAA levels. Whole blood 5HT levels descended about 50% in diabetic rats, then recovered their proper levels after 1 week of insulin therapy. Diabetic animals had a significantly greater intestinal 5HT concentration (+50% versus control), while intestinal 5HIAA levels did not achieve statistical significance despite a -26% decrement in their value. Both renal 5HT and 5HIAA levels were decreased in diabetic animals and recovered with insulin therapy. Peripheral tissue 5HT measures were not varied by insulin administration to non-diabetic animals. The results are consistent with a 5HT release, which is diminished in enterochromaffin cells and enhanced in platelet concomitantly to a minor platelet 5HT uptake, for explaining alterations of plasma/blood 5HT measures in experimental diabetes, and with a diminished synthesis of 5HT for explaining renal changes.

Increase in serotonin levels in the dog ileum and blood by cisplatin as measured by microdialysis

Involvement of ileal and circulating serotonin (5-HT) levels in cisplatin-induced emesis was examined using a microdialysis technique and an extraction method in dogs. The 5-HT levels in the ileal dialysate were increased to 232-294% of the basal level from 100 to 180 min after cisplatin administration (3 mg/kg, i.v.) and had returned to the basal level 280 min after dosing. The 5-HT levels in the blood dialysate were increased to 424-2165% from 140 to 180 min after dosing. The concentrations of 5-HT determined by HPLC following extraction were increased to 271% in the ileal mucosa and to 478% in plasma 3 hr after dosing. In immunohistochemistry, the number of 5-HT-immunoreactive cells was increased to 166% in the ileal mucosa following cisplatin treatment. These results strongly suggest that increases in the release and synthesis of 5-HT in the gut, probably in the enterochromaffin cells, are intimately involved in cisplatin-induced emesis.

Serotonin synthesis in adrenochromaffin cells

The inter-renal (adrenal) gland of amphibians is composed of chromaffin and steroidogenic cells which can interact through a paracrine mode of communication. We have previously shown that serotonin is present in secretory granules of frog adrenochromaffin cells; concurrently, we have demonstrated that serotonin is a potent stimulator of corticosterone and aldosterone secretion by adrenocortical cells. The aim of the present study was to determine the origin of the amine contained in frog chromaffin cells. Using 3H-labelled tryptophan as a precursor, we observed the formation of substantial amounts of serotonin and its metabolite 5-hydroxyindoleacetic acid by frog inter-renal slices. Newly synthesized serotonin was secreted into the incubation medium and the release process was enhanced by depolarizing concentrations of KCl. Fluoxetine, and inhibitor of serotonin uptake, caused an increase of 3H-labelled serotonin in the incubation medium, suggesting that the indoleamine was taken up again by adrenal chromaffin cells. The capacity of the frog inter-renal gland to synthesize serotonin was also demonstrated by incubating inter-renal slices with non-labelled tryptophan or 5-hydroxytryptophan. In these conditions, we observed that the rate of synthesis was higher when 5-hydroxytryptophan was used as a a precursor, rather than tryptophan. Taken together, these results indicate that chromaffin cells, which have the capacity for synthesizing and releasing serotonin, behave like authentic serotonergic paraneurons. As far as is known, these data provide the first evidence for the occurrence of tryptophan-5-hydroxylase activity within the adrenal gland.

Regulation of serotonin release from the intestinal mucosa

In the mammalian intestine serotonin (5-hydroxytryptamine, 5-HT) is present in high concentrations in the enterochromaffin cells. The release of 5-HT from the intestinal mucosa is regulated by a complex pattern of neuronal and humoral inputs to the enterochromaffin cells. The enterochromaffin cells appear to be endowed with different inhibitory (alpha 2-adrenoceptors, GABAA- and GABAB-receptors, histamine H3-receptors, receptors for vasoactive intestinal polypeptide and somatostatin) as well as stimulatory receptors (beta-adrenoceptors, muscarine and nicotine receptors). The physiological significance of this complex system of receptors is suggested by experiments which demonstrate that the respective intrinsic neurotransmitters (catecholamines, acetylcholine, GABA and vasoactive intestinal polypeptide) released within the gut are involved in the regulation of the release of 5-HT from the enterochromaffin cells.

Spontaneous release of endogenous 5-hydroxytryptamine and 5-hydroxyindoleacetic acid from the isolated vascularly perfused ileum of the guinea-pig

The spontaneous release of 5-hydroxytryptamine and its metabolite 5-hydroxyindoleacetic acid from the enterochromaffin cells of the small intestine into the portal circulation was investigated in vitro using the vascularly perfused ileum of the guinea-pig. The release of 5-hydroxytryptamine decreased by 70% in a calcium-free medium and by 35% in the presence of tetrodotoxin. Inhibition of monoamine oxidase activity by pargyline (100 microM) had no effect on the spontaneous release of 5-hydroxytryptamine although it caused a 75% reduction in the outflow of 5-hydroxyindoleacetic acid. Imipramine (1 microM), an inhibitor of neuronal uptake of 5-hydroxytryptamine, reduced the 5-hydroxyindoleacetic acid outflow by 57% and increased the release of 5-hydroxytryptamine by 66%. The combination of both drugs showed no additional effect. The tissue content of 5-hydroxytryptamine and its metabolite was not changed after perfusion with the precursor L-tryptophan or monofluoromethyldopa, an inhibitor of the L-aromatic amino acid decarboxylase. The results show that the spontaneous release of 5-hydroxytryptamine and its metabolite reflects largely calcium-dependent exocytotic release of the amine. "Neuronal uptake" (into aminergic and/or enterochromaffin cells) followed by deamination appears to be the main pathway of 5-hydroxytryptamine catabolism in the guinea-pig ileum.

Pathogenesis and pharmacology of diarrhea

The etiological factors involved in diarrhea are multiple. Also the mechanisms and mediators involved are multiple: intracellular mediators (Ca, cAMP, cGMP, calmodulin, phospholipids), extracellular mediators (hormones, neurotransmitters, prostaglandins, enterotoxins...), intramural blood flow and oxygen, intestinal motility (local- and peristaltic motility). Till now, antidiarrheals are not so versatile that they provide a solution to all types of diarrhea. The mechanisms of action of fluid replacement therapy, loperamide, alpha 2 agonist and some nonsteroidal anti-inflammatory substances are reviewed.

Local release of monoamines in the gastrointestinal tract: an in vivo study in rabbits

Dialysis fibers chronically implanted into the gastric submucosa of rabbits allowed us to collect an interstitial fluid (I.S.F.) dialysate in which biogenic amine concentrations were measured, and compared with those obtained from plasma and tissue samples. The results suggest that I.S.F. concentrations represent a good assessment of the local release of the amines by enteric nerves and/or paracrine cells, under basal conditions. The fact that acetylcholine and neostigmine, when perfused through the dialysis system, increased I.S.F. serotonin (5-HT) concentrations, supports a cholinergic modulation of the release of 5-HT within the gastrointestinal wall, and validates the dialysis method as a powerful tool to monitor, in vivo, dynamic changes in I.S.F. monoamine concentrations.

Measurement of monoamines and their metabolites in the interstitial fluid of the gut

Bundles of hollow dialysis fibers were surgically implanted into the submucosa of the stomach and intestine allowing collection of an interstitial fluid (ISF) dialysate. Measurement of monoamines and their metabolites in ISF was performed using high-performance liquid chromatography with electrochemical detection. The local concentration of L-5-hydroxy-tryptophan (5-HTP) in ISF from the antrum was determined in conscious rats after intraperitoneal loading with 5-HTP. The metabolite was similarly evaluated after the administration of levodopa (L-DOPA). Validation of ISF dialysis as a method for determining changes of biogenic amine concentration in the extracellular fluid was determined for the antrum, duodenum, and colon in dogs after the oral or subcutaneous administration of 5-HTP as a precursor of serotonin and 5-hydroxyindole acetyl acetic acid. The proposed procedure provides a novel approach in pharmacologic studies, where a direct correlation between the distribution of a drug and its metabolites (pharmacokinetics) and their effects (pharmacodynamics) has to be measured.