Signal transduction pathways for serotonin as an intestinal secretagogue

The Role of Serotonin Neurotransmission in Gastrointestinal Tract and Pharmacotherapy

5-Hydroxytryptamine (5-HT, serotonin) is a neurotransmitter in both the central nervous system and peripheral structures, acting also as a hormone in platelets. Although its concentration in the gut covers >90% of all organism resources, serotonin is mainly known as a neurotransmitter that takes part in the pathology of mental diseases. Serotonin modulates not only CNS neurons, but also pain transmission and platelet aggregation. In the periphery, 5-HT influences muscle motility in the gut, bronchi, uterus, and vessels directly and through neurons. Serotonin synthesis starts from hydroxylation of orally delivered tryptophan, followed by decarboxylation. Serotonin acts via numerous types of receptors and clinically plays a role in several neural, mental, and other chronic disorders, such as migraine, carcinoid syndrome, and some dysfunctions of the alimentary system. 5-HT acts as a paracrine hormone and growth factor. 5-HT receptors in both the brain and gut are targets for drugs modifying serotonin neurotransmission. The aim of the present article is to review the 5-HT receptors in the gastrointestinal (GI) tract to determine the role of serotonin in GI physiology and pathology, including known GI diseases and the role of serotonin in GI pharmacotherapy.

The Ketogenic Diet Improves Gut-Brain Axis in a Rat Model of Irritable Bowel Syndrome: Impact on 5-HT and BDNF Systems

Altered gut-brain communication can contribute to intestinal dysfunctions in the intestinal bowel syndrome. The neuroprotective high-fat, adequate-protein, low-carbohydrate ketogenic diet (KD) modulates the levels of different neurotransmitters and neurotrophins. The aim was to evaluate the effects of KD on levels of 5-HT, the receptors 5-HT_3B and 5-HT₄, the 5-HT transporter SERT, the neurotrophin BDNF, and its receptor TrkB in the colon and brain of a rat model of irritable bowel syndrome (IBS). Samples from Wistar rats exposed to maternal deprivation as newborns and then fed with a standard diet (IBS-Std) or KD (IBS-KD) for ten weeks were analyzed. As controls, unexposed rats (Ctrl-Std and Ctrl-KD) were studied. IBS-Std rats had a disordered enteric serotoninergic signaling shown by increased mucosal 5-HT content and reduced SERT, 5-HT_3B, and 5-HT₄ levels compared to controls. In the brain, these animals showed up-regulation of the BDNF receptor TrkB as a counteracting response to the stress-induced reduction of the neurotrophin. KD showed a dual effect in improving the altered 5-HT and BDNF systems. It down-regulated the increased mucosal 5-HT without affecting transporter and receptor levels. KD improved brain BDNF levels and established negative feedback, leading to a compensatory downregulation of TrkB to maintain a physiological steady state.

Nano-analysis reveals high fraction of serotonin release during exocytosis from a gut epithelium model cell

Electrochemical methods were used to explore the exocytotic nature of serotonin (5‐HT) release in human carcinoid BON cells, an in vitro human enterochromaffin cell model, to understand the mechanisms operating the release of gut‐derived 5‐HT in the intestinal mucosal epithelium. We show that the fractional vesicular 5‐HT release in BON cells is 80 % compared to previous work in pancreatic beta cells (34 %). The fractional release increased from 80 % in control BON cells to 87 % with 5‐HT preincubation and nearly 100 % with the combination of 5‐HT and the 5‐HT₄ autoreceptor agonist, cisapride. Thus, partial release is the primary mechanism of exocytosis in BON cells, resulting in a variable amount of the vesicular content being released. Factors that control secretion of 5‐HT from enterochromaffin cells or BON cells are important as partial release provides a mechanism for development of effective therapeutic strategies to treat gastrointestinal diseases.

Serotonin Transporter Deficiency is Associated with Dysbiosis and Changes in Metabolic Function of the Mouse Intestinal Microbiome

Serotonin transporter (SERT) plays a critical role in regulating extracellular availability of serotonin (5-HT) in the gut and brain. Mice with deletion of SERT develop metabolic syndrome as they age. Changes in the gut microbiota are being increasingly implicated in Metabolic Syndrome and Diabetes. To investigate the relationship between the gut microbiome and SERT, this study assessed the fecal and cecal microbiome profile of 11 to 12 week-old SERT+/+ and SERT-/- mice. Microbial DNA was isolated, processed for metagenomics shotgun sequencing, and taxonomic and functional profiles were assessed. 34 differentially abundant bacterial species were identified between SERT+/+ and SERT-/-. SERT-/- mice displayed higher abundances of Bacilli species including genera Lactobacillus, Streptococcus, Enterococcus, and Listeria. Furthermore, SERT-/- mice exhibited significantly lower abundances of Bifidobacterium species and Akkermansia muciniphilia. Bacterial community structure was altered in SERT-/- mice. Differential abundance of bacteria was correlated with changes in host gene expression. Bifidobacterium and Bacilli species exhibited significant associations with host genes involved in lipid metabolism pathways. Our results show that SERT deletion is associated with dysbiosis similar to that observed in obesity. This study contributes to the understanding as to how changes in gut microbiota are associated with metabolic phenotype seen in SERT deficiency.

Human-derived gut microbiota modulates colonic secretion in mice by regulating 5-HT3 receptor expression via acetate production

Serotonin [5-hydroxytryptamine (5-HT)], an important neurotransmitter and a paracrine messenger in the gastrointestinal tract, regulates intestinal secretion by its action primarily on 5-HT₃ and 5-HT₄ receptors. Recent studies highlight the role of gut microbiota in 5-HT biosynthesis. In this study, we determine whether human-derived gut microbiota affects host secretory response to 5-HT and 5-HT receptor expression. We used proximal colonic mucosa-submucosa preparation from age-matched Swiss Webster germ-free (GF) and humanized (HM; ex-GF colonized with human gut microbiota) mice. 5-HT evoked a significantly greater increase in short-circuit current (ΔI_sc) in GF compared with HM mice. Additionally, 5-HT₃ receptor mRNA and protein expression was significantly higher in GF compared with HM mice. Ondansetron, a 5-HT₃ receptor antagonist, inhibited 5-HT-evoked ΔI_sc in GF mice but not in HM mice. Furthermore, a 5-HT₃ receptor-selective agonist, 2-methyl-5-hydroxytryptamine hydrochloride, evoked a significantly higher ΔI_sc in GF compared with HM mice. Immunohistochemistry in 5-HT_3A-green fluorescent protein mice localized 5-HT₃ receptor expression to enterochromaffin cells in addition to nerve fibers. The significant difference in 5-HT-evoked ΔI_sc between GF and HM mice persisted in the presence of tetrodotoxin (TTX) but was lost after ondansetron application in the presence of TTX. Application of acetate (10 mM) significantly lowered 5-HT₃ receptor mRNA in GF mouse colonoids. We conclude that host secretory response to 5-HT may be modulated by gut microbiota regulation of 5-HT₃ receptor expression via acetate production. Epithelial 5-HT₃ receptor may function as a mediator of gut microbiota-driven change in intestinal secretion.NEW & NOTEWORTHY We found that gut microbiota alters serotonin (5-HT)-evoked intestinal secretion in a 5-HT₃ receptor-dependent mechanism and gut microbiota metabolite acetate alters 5-HT₃ receptor expression in colonoids.View this article's corresponding video summary at https://www.youtube.com/watch?v=aOMYJMuLTcw&feature=youtu.be.

Effects of San-Huang-Xie-Xin-tang, a traditional Chinese prescription for clearing away heat and toxin, on the pacemaker activities of interstitial cells of Cajal from the murine small intestine

ETHNOPHARMACOLOGICAL RELEVANCE

San-Huang-Xie-Xin-Tang (SHXXT) is a traditional Chinese medicinal formula composed of Coptidis rhizoma (Coptis chinesis Franch), Scutellariae radix (Scutellaria baicalensis Georgi), and Rhei rhizoma (Rheum officinale Baill) and is widely used in Eastern Asia, especially to ameliorate the symptoms of gastrointestinal (GI) disorders related to gastritis, gastric bleeding, peptic ulcers, and abnormal GI motility

AIM OF THE STUDY

Interstitial cells of Cajal (ICCs) are pacemaker cells in the GI tract that generate rhythmic oscillations in membrane potentials known as slow waves. Because GI disorders, especially abnormal GI motility, are major lifelong problems, the authors investigated the effects of SHXXT on mouse small intestine ICCs, and sought to identify the receptors and the action mechanisms involved.

MATERIALS AND METHODS

Enzymatic digestions were used to dissociate ICCs from small intestines, and the whole-cell patch-clamp configuration was used to record potentials generated by cultured ICCs.

RESULTS

SHXXT produced membrane depolarization in current-clamp mode, and Y25130 (a 5-HT3 receptor antagonist) and RS39604 (a 5-HT4 receptor antagonist) blocked SHXXT-induced membrane depolarizations, whereas SB269970 (a 5-HT7 receptor antagonist) did not. However, during external Ca2+ free conditions or in the presence of thapsigargin, SHXXT did not exhibit membrane depolarization. Furthermore, the application of flufenamic acid (a nonselective cation channel (NSCC) blocker) or DIDS (a chloride channel blocker) abolished pacemaker potential generation and blocked SHXXT-induced membrane depolarizations. In addition, SHXXT-induced membrane depolarizations, which are dependent on G-protein, in ICCs were blocked by PD 98059 (a p42/44 mitogen-activated protein kinase (MAPK) inhibitor), SB203580 (a p38 MAPK inhibitor), and by a c-jun NH2-terminal kinase (JNK) II inhibitor. Regarding the components of SHXXT, Coptidis rhizome and Rhei rhizoma modulated ICC pacemaking activity, whereas Scutellariae radix did not.

CONCLUSION

SHXXT modulates pacemaker potentials via 5-HT3 and 5-HT4 receptor-mediated pathways, external Ca2+ influx, and Ca2+ release from internal stores. Furthermore, NSCCs and Cl- channels play important roles in the regulation of pacemaking activity in a MAPK dependent manner in ICCs. The regulation of pacemaking activity by SHXXT may be due to the activity of Coptidis rhizome and Rhei rhizome. The study shows SHXXT can modulate the pacemaking activity of ICCs in the GI tract, and thus, suggests SHXXT has potential pharmacological relevance for the treatment of GI motility disorders.

Pharmacological profile of DA-6886, a novel 5-HT4 receptor agonist to accelerate colonic motor activity in mice

DA-6886, the gastrointestinal prokinetic benzamide derivative is a novel 5-HT4 receptor agonist being developed for the treatment of constipation-predominant irritable bowel syndrome (IBS-C). The purpose of this study was to characterize in vitro and in vivo pharmacological profile of DA-6886. We used various receptor binding assay, cAMP accumulation assay, organ bath experiment and colonic transit assay in normal and chemically constipated mice. DA-6886 exhibited high affinity and selectivity to human 5-HT4 receptor splice variants, with mean pKi of 7.1, 7.5, 7.9 for the human 5-HT4a, 5-HT4b and 5-HT4d, respectively. By contrast, DA-6886 did not show significant affinity for several receptors including dopamine D2 receptor, other 5-HT receptors except for 5-HT2B receptor (pKi value of 6.2). The affinity for 5-HT4 receptor was translated into functional agonist activity in Cos-7 cells expressing 5-HT4 receptor splice variants. Furthermore, DA-6886 induced relaxation of the rat oesophagus preparation (pEC50 value of 7.4) in a 5-HT4 receptor antagonist-sensitive manner. The evaluation of DA-6886 in CHO cells expressing hERG channels revealed that it inhibited hERG channel current with an pIC50 value of 4.3, indicating that the compound was 1000-fold more selective for the 5-HT4 receptor over hERG channels. In the normal ICR mice, oral administration of DA-6886 (0.4 and 2mg/kg) resulted in marked stimulation of colonic transit. Furthermore, in the loperamide-induced constipation mouse model, 2mg/kg of DA-6886 significantly improved the delay of colonic transit, similar to 10mg/kg of tegaserod. Taken together, DA-6886 is a highly potent and selective 5-HT4 receptor agonist to accelerate colonic transit in mice, which might be therapeutic agent having a favorable safety profile in the treatment of gastrointestinal motor disorders such as IBS-C and chronic constipation.

Involvement of MAPKs and PLC pathways in modulation of pacemaking activity by So-Cheong-Ryong-Tang in interstitial cells of Cajal from murine small intestine

Purpose. Interstitial cells of Cajal (ICCs) are the pacemaker cells that generate slow waves in the gastrointestinal (GI) tract. We have aimed to investigate the effects of Socheongryong-Tang (SCRT) in ICCs from mouse's small intestine. Methods. The whole-cell patch-clamp configuration was used to record membrane potentials from cultured ICCs. Intracellular Ca²⁺ ([Ca²⁺]_i) increase was studied in cultured ICCs using fura-2 AM. Results. ICCs generated pacemaker potentials in mouse's small intestine. SCRT produced membrane depolarization in current clamp mode. Y25130 (5-HT₃ receptor antagonist) and RS39604 (5-HT₄ receptor antagonist) blocked SCRT-induced membrane depolarizations, whereas SB269970 (5-HT₇ receptor antagonist) did not. When GDP-β-S (1 mM) was in the pipette solution, SCRT did not induce the membrane depolarizations. [Ca²⁺]_i analysis showed that SCRT increased [Ca²⁺]_i. In the presence of PD98059 (p42/44 MAPK inhibitor), SCRT did not produce membrane depolarizations. In addition, SB203580 (p38 MAPK inhibitor) and JNK inhibitors blocked the depolarizations by SCRT in pacemaker potentials. Furthermore, the membrane depolarizations by SCRT were not inhibited by U-73122, an active phospholipase C (PLC) inhibitor, but by U-73343, an inactive PLC inhibitor. Conclusion. These results suggest that SCRT might affect GI motility by the modulation of pacemaker activity through MAPKs and PLC pathways in the ICCs.

Poncirus trifoliate fruit modulates pacemaker activity in interstitial cells of Cajal from the murine small intestine

ETHNOPHARMACOLOGICAL RELEVANCE

Poncirus fructus (PF) has been widely used as a traditional medicine in Eastern Asia, especially to ameliorate the symptoms of gastrointestinal (GI) disorders related to abnormal GI motility.

AIM OF THE STUDY

Poncirus fructus (PF), also known as Poncirus trifoliata (L.) Raf. (Rutaceae), is widely used as a traditional medicine in Eastern Asia mainly to ameliorate the symptoms of gastrointestinal (GI) disorders related to abnormal GI motility. In a previous study, a methanol extract of PF was found to have particularly potent gastroprokinetic effects. Interstitial cells of Cajal (ICCs) are pacemaker cells in the gastrointestinal tract, but the action mechanisms of PF extract in mouse small intestinal ICCs have not been investigated. Therefore, in the present study, we investigated the effects of a methanol extract of PF (MPF) in mouse small intestinal ICCs. In addition, we sought to identify the receptors involved.

MATERIALS AND METHODS

Enzymatic digestions were used to dissociate ICCs from small intestines. The whole-cell patch-clamp configuration was used to record potentials (current clamp) from cultured ICCs. In addition, we analyzed intracellular Ca(2+) concentrations ([Ca(2+)]i).

RESULTS

MPF decreased the amplitudes of pacemaker potentials in ICCs, and depolarized resting membrane potentials in a concentration dependent manner. Y25130 (a 5-HT3 receptor antagonist) and RS39604 (a 5-HT4 receptor antagonist) blocked MPF-induced membrane depolarizations, whereas SB269970 (a 5-HT7 receptor antagonist) did not. Pretreatment with Na(+) or Ca(2+)-free solution or thapsigargin (a Ca(2+)-ATPase inhibitor in endoplasmic reticulum) abolished the generation of pacemaker potentials and suppressed MPF-induced activity. [Ca(2+)]i analysis showed that MPF increased [Ca(2+)]i. Furthermore, treatments with PD 98059, SB203580, or JNK II inhibitor blocked MPF-induced membrane depolarizations in ICCs.

CONCLUSION

These results suggest that MPF modulates pacemaker potentials through 5-HT3 and 5-HT4 receptor-mediated pathways via external Na(+) and Ca(2+) influx, and via Ca(2+) release from internal stores in a mitogen-activated protein kinase dependent manner. The study shows MPF is a good candidate for the development of a gastroprokinetic agent. In view of the effects of MPF on ICCs, further research is required, particularly to identify the active compound(s) involved and to determine their action mechanisms.

Serotonin: from top to bottom

Serotonin is a monoamine neurotransmitter, which is phylogenetically conserved in a wide range of species from nematodes to humans. In mammals, age-related changes in serotonin systems are known risk factors of age-related diseases, such as diabetes, faecal incontinence and cardiovascular diseases. A decline in serotonin function with aging would be consistent with observations of age-related changes in behaviours, such as sleep, sexual behaviour and mood all of which are linked to serotonergic function. Despite this little is known about serotonin in relation to aging. This review aims to give a comprehensive analysis of the distribution, function and interactions of serotonin in the brain; gastrointestinal tract; skeletal; vascular and immune systems. It also aims to demonstrate how the function of serotonin is linked to aging and disease pathology in these systems. The regulation of serotonin via microRNAs is also discussed, as are possible applications of serotonergic drugs in aging research and age-related diseases. Furthermore, this review demonstrates that serotonin is potentially involved in whole organism aging through its links with multiple organs, the immune system and microRNA regulation. Methods to investigate these links are discussed.

5-hydroxytryptamine generates tonic inward currents on pacemaker activity of interstitial cells of cajal from mouse small intestine

In this study we determined whether or not 5-hydroxytryptamine (5-HT) has an effect on the pacemaker activities of interstitial cells of Cajal (ICC) from the mouse small intestine. The actions of 5-HT on pacemaker activities were investigated using a whole-cell patch-clamp technique, intracellular Ca(2+) ([Ca(2+)](i)) analysis, and RT-PCR in ICC. Exogenously-treated 5-HT showed tonic inward currents on pacemaker currents in ICC under the voltage-clamp mode in a dose-dependent manner. Based on RT-PCR results, we found the existence of 5-HT(2B, 3, 4, and 7) receptors in ICC. However, SDZ 205557 (a 5-HT(4) receptor antagonist), SB 269970 (a 5-HT7 receptor antagonist), 3-tropanylindole - 3 - carboxylate methiodide (3-TCM; a 5-HT(3) antagonist) blocked the 5-HT-induced action on pacemaker activity, but not SB 204741 (a 5-HT(2B) receptor antagonist). Based on [Ca(2+)](i) analysis, we found that 5-HT increased the intensity of [Ca(2+)](i). The treatment of PD 98059 or JNK II inhibitor blocked the 5-HT-induced action on pacemaker activity of ICC, but not SB 203580. In summary, these results suggest that 5-HT can modulate pacemaker activity through 5-HT(3, 4, and 7) receptors via [Ca(2+)](i) mobilization and regulation of mitogen-activated protein kinases.

The Pharmacology of TD-8954, a Potent and Selective 5-HT(4) Receptor Agonist with Gastrointestinal Prokinetic Properties

This study evaluated the in vitro and in vivo pharmacological properties of TD-8954, a potent and selective 5-HT(4) receptor agonist. TD-8954 had high affinity (pK(i) = 9.4) for human recombinant 5-HT(4(c)) (h5-HT(4(c))) receptors, and selectivity (>2,000-fold) over all other 5-hydroxytryptamine (5-HT) receptors and non-5-HT receptors, ion channels, enzymes and transporters tested (n = 78). TD-8954 produced an elevation of cAMP in HEK-293 cells expressing the h5-HT(4(c)) receptor (pEC(50) = 9.3), and contracted the guinea pig colonic longitudinal muscle/myenteric plexus preparation (pEC(50) = 8.6). TD-8954 had moderate intrinsic activity in the in vitro assays. In conscious guinea pigs, subcutaneous administration of TD-8954 (0.03-3 mg/kg) increased the colonic transit of carmine red dye, reducing the time taken for its excretion. Following intraduodenal dosing to anesthetized rats, TD-8954 (0.03-10 mg/kg) evoked a dose-dependent relaxation of the esophagus. Following oral administration to conscious dogs, TD-8954 (10 and 30 μg/kg) produced an increase in contractility of the antrum, duodenum, and jejunum. In a single ascending oral dose study in healthy human subjects, TD-8954 (0.1-20 mg) increased bowel movement frequency and reduced the time to first stool. It is concluded that TD-8954 is a potent and selective 5-HT(4) receptor agonist in vitro, with robust in vivo stimulatory activity in the gastrointestinal (GI) tract of guinea pigs, rats, dogs, and humans. TD-8954 may have clinical utility in patients with disorders of reduced GI motility.

Activation of submucosal 5-HT(3) receptors elicits a somatostatin-dependent inhibition of ion secretion in rat colon

BACKGROUND AND PURPOSE

5-Hydroxytryptamine (5-HT) is a key regulator of the gastrointestinal system and we have shown that submucosal neuronal 5-HT(3) receptors exerted a novel inhibitory effect on colonic ion transport. The aim of the present study was to investigate the precise mechanism(s) underlying this inhibitory effect.

EXPERIMENTAL APPROACH

Mucosa/submucosa or mucosa-only preparations from rat distal colon were mounted in Ussing chambers for measurement of short-circuit current (I(sc)) as an indicator of ion secretion. Somatostatin release was determined with radioimmunoassay. Intracellular cAMP content was measured with enzyme-linked immunoadsorbent assay (elisa). Immunohistochemical techniques were used to study the expression of 5-HT(3) receptors, somatostatin and somatostatin receptors in colonic tissue.

KEY RESULTS

In rat distal colonic mucosa/submucosa preparations, pretreatment with 5-HT(3) receptor antagonists enhanced 5-HT-induced increases in I(sc). However, in mucosa-only preparations without retained neural elements, pretreatment with 5-HT(3) receptor antagonists inhibited 5-HT-induced DeltaI(sc). Pretreatment with a somatostatin-2 (sst(2)) receptor antagonist in mucosa/submucosa preparations augmented 5-HT-induced DeltaI(sc). Combination of sst(2) and 5-HT(3) receptor antagonists did not cause further enhancement of 5-HT-induced DeltaI(sc). Moreover, both sst(2) and 5-HT(3) receptor antagonists enhanced 5-HT-induced increase in intracellular cAMP concentration in the mucosa/submucosa preparations. 5-HT released somatostatin from rat colonic mucosa/submucosa preparations, an effect prevented by pretreatment with 5-HT(3) receptor antagonists. Immunohistochemical staining demonstrated the presence of 5-HT(3) receptors on submucosal somatostatin neurons and of sst(2) receptors on colonic mucosa.

CONCLUSION AND IMPLICATIONS

Activation of neuronal 5-HT(3) receptors in the submucosal plexus of rat colon suppressed 5-HT-induced ion secretion by releasing somatostatin from submucosal neurons.

Gut sensing mechanisms

The mechanisms by which the gut senses and responds to nutrients involve the interplay of multiple complex pathways. In addition to regulating digestion and absorption, the pathways stimulated by molecules in the gut lumen mediate gastric motility, food intake, and satiety. Furthermore, protective mechanisms are activated as necessary to prevent injury, promote healing, and limit intake and absorption of potentially toxic substances. This review provides an update on the current knowledge and recent findings related to gastric sensing of nutrients, highlighting recent research and future endeavors in the field.

Persistent gut motor dysfunction in a murine model of T-cell-induced enteropathy

BACKGROUND

Inflammatory bowel disease (IBD) patients in remission often experience irritable bowel syndrome (IBS)-like symptoms. We investigated the mechanism for intestinal muscle hypercontractility seen in T-cell-induced enteropathy in recovery phase.

METHODS

BALB/c mice were treated with an anti-CD3 antibody (100 microg per mouse) and euthanized at varying days post-treatment to investigate the histological changes, longitudinal smooth muscle cell contraction, cytokines (Th1, Th2 cytokines, TNF-alpha) and serotonin (5-HT)-expressing enterochromaffin cell numbers in the small intestine. The role of 5-HT in anti-CD3 antibody-induced intestinal muscle function in recovery phase was assessed by inhibiting 5-HT synthesis using 4-chloro-DL-phenylalanine (PCPA).

KEY RESULTS

Small intestinal tissue damage was observed from 24 h after the anti-CD3 antibody injection, but had resolved by day 5. Carbachol-induced smooth muscle cell contractility was significantly increased from 4 h after injection, and this muscle hypercontractility was evident in recovery phase (at day 7). Th2 cytokines (IL-4, IL-13) were significantly increased from 4 h to day 7. 5-HT-expressing cells in the intestine were increased from day 1 to day 7. The 5-HT synthesis inhibitor PCPA decreased the anti-CD3 antibody-induced muscle hypercontractility in recovery phase.

CONCLUSIONS & INFERENCES

Intestinal muscle hypercontractility in remission is maintained at the smooth muscle cell level. Th2 cytokines and 5-HT in the small intestine contribute to the maintenance of the altered muscle function in recovery phase.

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.

The in vivo gastrointestinal activity of TD-5108, a selective 5-HT(4) receptor agonist with high intrinsic activity

The in vivo preclinical pharmacodynamic profile of TD-5108, a selective 5-HT(4) receptor agonist with high intrinsic activity, was compared to that of the clinically studied gastrointestinal pro-kinetic agents, tegaserod, cisapride and mosapride. The activity of TD-5108 was evaluated in guinea pig colonic transit, rat oesophageal relaxation and dog gastrointestinal smooth muscle contractility models. Subcutaneous administration of TD-5108, tegaserod, cisapride and mosapride increased guinea pig colonic transit (rank order of potencies: TD-5108 > tegaserod > cisapride > mosapride). Following intravenous and intraduodenal dosing, TD-5108, tegaserod, cisapride and mosapride produced dose-dependent relaxation of the rat oesophagus. On a molar basis, TD-5108 was approximately twofold less potent than tegaserod following intravenous dosing but 6- or 86-fold more potent than cisapride or mosapride, respectively, and 9- or 18-fold more potent than tegaserod or cisapride, respectively, after intraduodenal administration. Orally dosed TD-5108 increased the contractility of the canine antrum, duodenum and jejunum with higher potency than tegaserod. The selective 5-HT(4) receptor agonist, TD-5108, demonstrates robust in vivo activity in the guinea pig, rat and dog gastrointestinal tracts.

The in vitro pharmacological profile of TD-5108, a selective 5-HT(4) receptor agonist with high intrinsic activity

The in vitro pharmacological profile of TD-5108, a novel, selective 5-HT(4) receptor agonist, was compared to that of clinically efficacious gastroprokinetic 5-HT(4) receptor agonists. TD-5108 produced an elevation of cyclic adenosine monophosphate in human embryonic kidney 293 cells expressing the human recombinant 5-HT(4(c)) (h5-HT(4(c))) receptor (pEC(50) = 8.3) and 5-HT(4) receptor-mediated relaxation of the rat esophagus (pEC(50) = 7.9) and contraction of the guinea pig colon (pEC(50) = 7.9). In all in vitro assays, TD-5108 was a high intrinsic activity agonist, unlike tegaserod, mosapride, and cisapride which, in the majority of test systems, had lower intrinsic activity. TD-5108 had high affinity (pK (i) = 7.7) and selectivity (> or =25-fold) for h5-HT(4(c)) receptors over other biogenic amine receptors. TD-5108 was >500-fold selective over other 5-HT receptors (including h5-HT(2B) and h5-HT(3A)) and, at 3 microM, had no effect on human ether-à-go-go-related gene K+ channels. In conclusion, TD-5108 is a selective 5-HT(4) receptor agonist in vitro. The high intrinsic activity and preferential binding of TD-5108 to 5-HT4 over other 5-HT receptors may result in an improved clinical profile for the treatment of gastrointestinal disorders of reduced motility.

A dual role of 5-hydroxytryptamine receptor 3 in serotonin induced ion transport in rat distal colon

5-hydroxytryptamine (5-HT)-evoked intestinal secretion can be divided into neural and non-neural pathway. Recently, 5-HT(3) receptor in neural pathway received much attention as a possible target in bowel diseases. The present study aims to investigate the effects of 5-HT(3) receptor in different enteric neural plexus (myenteric plexus and submucosal plexus) on rat colonic ion transport by using rat intact colon and mucosa/submucosa preparations. Ussing chamber and real-time PCR techniques were performed in our present study. Surprisingly, we found that in mucosa/submucosa preparations, 5-HT-induced DeltaI(SC) (change in short-circuit current) was not inhibited, but further increased by pretreatment with 5-HT(3) receptor antagonists, MDL72222 and Tropanyl-3, 5-dimethylbenzoate. And this response was significantly attenuated in the presence of tetrodotoxin (TTX). Conversely, in rat intact colon, 5-HT(3) receptor antagonists significantly inhibited 5-HT-induced DeltaI(SC). The results from real-time PCR proved the existence of 5-HT(3) receptor in muscularis externa and submucosa. Taken together, 5-HT(3) receptors possess a role of dual regulation on electrolyte secretion in rat distal colon, the neural stimulatory effect of 5-HT(3) receptor in myenteric plexus and the inhibitory effect of 5-HT(3) receptor in submucosal plexus.

Serotonin pharmacology in the gastrointestinal tract: a review

Serotonin (5-hydroxytryptamine or 5-HT) plays a critical physiological role in the regulation of gastrointestinal (GI) function. 5-HT dysfunction may also be involved in the pathophysiology of a number of functional GI disorders, such as chronic constipation, irritable bowel syndrome and functional dyspepsia. This article describes the role of 5-HT in the enteric nervous system (ENS) of the mammalian GI tract and the receptors with which it interacts. Existing serotonergic therapies that have proven effective in the treatment of GI functional disorders and the potential of drugs currently in development are also highlighted. Advances in our understanding of the physiological and pathophysiological roles of 5-HT in the ENS and the identification of selective receptor ligands bodes well for the future development of more efficacious therapies for patients with functional GI disorders.

Recent advances in understanding the role of serotonin in gastrointestinal motility in functional bowel disorders: alterations in 5-HT signalling and metabolism in human disease

Serotonin (5-hydroxytryptamine, 5-HT) is present in abundance within the gut, most stored in enterochromaffin cell granules. It is released by a range of stimuli, most potently by mucosal stroking. Released 5-HT stimulates local enteric nervous reflexes to initiate secretion and propulsive motility. It also acts on vagal afferents altering motility and in large amounts induces nausea. Rapid reuptake by a specific transporter (serotonin transporter, SERT) limits its diffusion and actions. Abnormally increased 5-HT is found in a range of gastrointestinal disorders including chemotherapy-induced nausea and vomiting, carcinoid syndrome, coeliac disease, inflammatory bowel disease and irritable bowel syndrome (IBS) with diarrhoea (IBS-D), especially that developing following enteric infection. Impaired SERT has been described in IBS-D and might account for some of the increase in mucosal 5-HT availability. 5-HT(3) receptor antagonists inhibit chemotherapy-induced nausea and diarrhoea associated with both carcinoid syndrome and IBS. While IBS-D is associated with increased 5-HT postprandially, IBS with constipation (IBS-C) is associated with impaired 5-HT response and responds to 5-HT(4) agonists such as Prucalopride and 5-HT(4) partial agonists such as Tegaserod.

Peripheral serotonin dynamics in the rainbow trout (Oncorhynchus mykiss)

Serotonin (5-hydroxytryptamine, 5-HT) occurs in a wide range of tissues throughout the body of the rainbow trout. Results reported here indicate that the main peripheral sources of serotonin are the intestinal tract and the gill epithelium (levels above 1500 ng/g). The high intestinal serotonin concentration is mostly due to serotoninergic nerve fibres, which are present at high density in the intestinal wall. Only about 2% of serotonin is associated with mucosal enterochromaffin cells. In the remaining tissues studied serotonin concentration was below 160 ng/g: the highest concentrations were seen in the anterior and posterior kidneys, followed by the liver, heart, and spleen. 5-Hydroxyindolacetic acid (5-HIAA) levels, except in plasma, were generally lower than serotonin levels, and were below our detection limits in heart, spleen and posterior kidney. Acute d-fenfluramine treatment (5 or 15 mg/kg i.p.) significantly increased 5-HIAA/5-HT ratio in the anterior intestine, pyloric caeca and plasma. Serotonin released from intestinal serotoninergic fibres in response to d-fenfluramine treatment is metabolized locally, and only a small part reaches the blood, from where it can be taken up and metabolized by other peripheral tissues, such as the liver and gill epithelium. The non-metabolized serotonin pool in the blood appears to be located extracellularly, not intracellularly as in mammals. In view of these findings, we present an overview of peripheral serotonin dynamics in rainbow trout.

Serotonin increases protective duodenal bicarbonate secretion via enteric ganglia and a 5-HT4-dependent pathway

OBJECTIVE

Serotonin (5-HT) is present in much larger amounts in the gut than in the central nervous system and is predominantly synthesized and stored in mucosal enterochromaffin cells. Bicarbonate secretion by the duodenal mucosa is the major mechanism in maintaining mucosal integrity, neutralizing invading protons within the surface mucus gel. In this study the role of local 5-HT in the control of the protective secretion was investigated.

MATERIAL AND METHODS

A segment of proximal duodenum was perfused in situ in anaesthetized rats and the alkaline secretion was continuously recorded by pH-stat. Intracellular calcium signalling was measured in clusters of human and rat duodenal enterocytes devoid of neural tissue. After loading with the fluorescent probe, fura-2, the clusters were attached to the bottom of a temperature-controlled perfusion chamber.

RESULTS

Close intra-arterial infusion to the duodenal segment of 5-HT (20-200 nmol kg(-1) h(-1)) dose-dependently increased duodenal mucosal HCO3 secretion. A higher dose (2000 nmol kg(-1) h(-1)) did not further increase secretion. Responses were inhibited by the ganglionic blocker and nicotinic receptor antagonist hexamethonium, and were abolished by the 5-HT4 receptor antagonist SB 204070. The 5-HT3 antagonist tropisetron, in contrast, caused only slight inhibition. Viable human and rat duodenal enterocytes responded to 5-HT (100-500 nM) with an increase in intracellular calcium concentration. Pretreatment with SB 204070 or removal of external calcium abolished the response.

CONCLUSIONS

Stimulation of the duodenal protective secretion by 5-HT thus involves receptors of the 5-HT4 subtype as well as nicotinic transmission, the myenteric plexus being a likely location. In addition, serotonin acts on enterocyte membrane receptors, inducing intracellular calcium signalling.

Functional characterization of serotonin receptor subtypes in human duodenal secretion

Serotonin (5-HT) stimulates ion secretion in the gastrointestinal tract and the sensitivity for 5-HT might be altered in dyspeptic patients infected with Helicobacter pylori. The purpose of the present study was to characterize the 5-HT-induced electrogenic ion transport in the duodenum of dyspeptic patients with or without Helicobacter pylori infection, and to determine the 5-HT receptor subtypes functionally involved. Biopsies from the second part of duodenum were obtained from 43 dyspeptic patients during routine endoscopy. Biopsies were mounted in modified Ussing chambers with air suction for measurements of short-circuit current by a previously validated technique. Short-circuit current was measured before and after application of graded cumulative doses of 5-HT and a single dose of bumetanide (an inhibitor of chloride/bicarbonate transport), or one of the selective 5-HT receptor antagonists: ketanserin, ondansetron, or SB-204070 (1-butyl-4 piperidinmethyl-8-amino-7-chloro-2,3-dihydro-1,4-benzodioxin-5-carboxylate HCl). Histological examination was performed on duodenal biopsies. Helicobacter urease testing and histological examination determined Helicobacter pylori infection. 5-HT induced a dose-dependent and bumetanide-sensitive short-circuit current, which was independent of the presence of Helicobacter pylori infection. All the three 5-HT receptor antagonists failed to significantly effect basal and 5-HT-induced short-circuit current. Our results indicate that in human duodenum 1) 5-HT is a potent stimulator of bumetanide-sensitive secretion, 2) the serotonergic receptor subtype, which acts as the main mediator of 5-HT-induced secretion is different from the 5-HT(2), 5-HT(3), and the 5-HT(4) subtype and, 3) the sensitivity to 5-HT is not altered by Helicobacter pylori infection.

Serotonin-Containing Cells in the Gastrointestinal Tract of Newborn Foals and Adult Horses

Serotonin (5-hydroxytryptamine, 5-HT), a regulatory amine of mucosal enterochromaffin cells plays an important role in the control of gastrointestinal smooth muscle contraction and epithelial secretion. Serotonin has also been associated with gastric ulcers, diarrhoea and abdominal pain. In spite of the high incidence of these gastrointestinal disorders in newborn foals and adult horses, no data are available regarding 5-HT immunoreactive cells (i.c.) in the gastrointestinal tract (GIT) of foals, and for adult horses, data are incomplete and contradictory. In this study, the distribution and relative frequency of 5-HT i.c. in the GIT of newborn foals and adult horses were determined immunohistochemically. In foals as in adults, a relatively large number of 5-HT i.c. were detected in all portions of the GIT. In foals, a significantly higher amount of cells was found in the pyloric region and margo plicatus of the stomach, as well as in the caecum and colon ascendens compared with adults. Our results provided rationale for further research concerning the role of 5-HT i.c. during the milk diet or in the regulation of gastrointestinal growth/cell proliferation, and in the pathogenesis of gastric ulcers, especially in newborn foals.

Mediation of neurogenic ion transport by acetylcholine, prostanoids and 5-hydroxytryptamine in porcine ileum

Enteric neural activity modulates active transepithelial ion transport in the intestine. We investigated the neural circuits mediating neurogenic secretion in mucosal explants from porcine ileum. Transmural electrical stimulation increased short-circuit current, a measure of active ion transport, by 35+/-2 microA/cm2. The neuronal Na+ channel blocker saxitoxin, the muscarinic cholinergic receptor antagonist atropine, the 5-hydroxytryptamine3 receptor antagonist tropisetron, and the cyclooxygenase inhibitor indomethacin inhibited this response. In addition, tropisetron inhibited the atropine-resistant portion of the response, and both atropine and indomethacin attenuated the saxitoxin-resistant component. Neurogenic secretion in porcine ileum appears to be mediated by tryptaminergic and prostanoid-sensitive cholinergic pathways.

Serotonin inhibits Na+/H+ exchange activity via 5-HT4 receptors and activation of PKC alpha in human intestinal epithelial cells

BACKGROUND & AIMS

Increased serotonin levels have been implicated in the pathophysiology of diarrhea associated with celiac and inflammatory diseases. However, the effects of serotonin on Na+ /H+ exchange (NHE) activity in the human intestine have not been investigated fully. The present studies examined the acute effects of 5-hydroxytryptamine (5-HT) on NHE activity using Caco-2 cells as an in vitro model.

METHODS

Caco-2 cells were treated with 5-HT (.1 micromol/L, 1 h) and NHE activity was measured as ethyl-isopropyl-amiloride (EIPA)-sensitive 22Na uptake. The effect of 5-HT receptor-specific agonists and antagonists was examined. The role of signaling intermediates in 5-HT-mediated effects on NHE activity was elucidated using pharmacologic inhibitors and immunoblotting.

RESULTS

NHE activity was inhibited significantly (approximately 50%-75%, P < .05) by .1 micromol/L 5-HT via inhibition of maximal velocity (Vmax) without any changes in apparent affinity (Km) for the substrate Na+ . NHE inhibition involved a decrease of both NHE2 and NHE3 activities. Studies using specific inhibitors and agonists showed that the effects of 5-HT were mediated by 5-HT4 receptors. 5-HT-mediated inhibition of NHE activity was dependent on phosphorylation of phospholipase C gamma 1 (PLC gamma 1) via activation of src-kinases. Signaling pathways downstream of PLC gamma 1 involved increase of intracellular Ca 2+ levels and subsequent activation of protein kinase C alpha (PKC alpha). The effects of 5-HT on NHE activity were not cell-line specific because T84 cells also showed NHE inhibition.

CONCLUSIONS

A better understanding of the regulation of Na+ absorption by 5-HT offers the potential for providing insights into molecular and cellular mechanisms involved in various diarrheal and inflammatory disorders.

Regulation of ion transport by 5-hydroxytryptamine in rat colon

1. 5-Hydroxytryptamine (5-HT) modulates the motility and secretion of the gastrointestinal tract. To examine the direct effect of 5-HT on the secretions of colonic epithelial cells, a short-circuit current was used to measure electrolyte transport in the rat stripped distal colon. A neuronal Na+ channel blocker and a cyclo-oxygenase inhibitor were routinely added in experiments to abolish the effects of the enteric nervous system and endogenous prostaglandin, respectively. 2. Basolateral application of 5-HT (10 micromol/L) induced an increase in the short circuit current (ISC). Removal of extracellular Cl-, HCO3- or both resulted in a 59.6, 76.4 and 90% reduction of 5-HT-elicited responses, respectively. The Ca(2+)-dependent Cl- channel blocker 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) had no effect on the 5-HT-induced increase in ISC, but the selective cystic fibrosis transmembrane conductance regulator (CFTR) channel blocker glibenclamide (1 mmol/L) inhibited 5-HT-induced increases in ISC by approximately 92.9%. Removal of apical Na+ reduced the 5-HT-induced increase in ISC by 33.3%. 3. Basolateral pretreatment with 100 micromol/L bumetanide (an inhibitor of the Na(+)-K(+)-2Cl- cotransporter), 200 micromol/L DIDS (an inhibitor of the Na(+)-HCO3- transporter or the Cl-/HCO3- exchanger) or both decreased the DeltaISC induced by 5-HT by approximately 75.5, 59.0 and 86.3%, respectively. Removal of basolateral Na+ also reduced the current evoked by 5-HT. 4. The selective 5-HT4 antagonist GR113808 (1 micromol/L) totally abolished the 5-HT-induced increase in ISC, whereas 2-methyl-5-HT (100 micromol/L) induced a weak ISC response. 5. In conclusion, the present study has demonstrated that 5-HT can elicit Cl(-)- and HCO3- anion secretion and Na+ absorption by acting directly on colonic epithelial cells via 5-HT4 receptors.

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.

Distribution of mRNA that codes for 5-hydroxytryptamine receptor subtypes in the gastrointestinal tract of dairy cows

OBJECTIVE

To describe the distribution of mRNA that codes for 8 subtypes of 5-hydroxytryptamine receptors (5-HTRs) in the digestive tract of dairy cows.

SAMPLE POPULATION

Fresh full-thickness wall specimens from the abomasum (fundus, corpus, and antrum), ileum, cecum, proximal loop of ascending colon, and 4 locations of the spiral colon collected from 10 healthy cows at slaughter.

PROCEDURE

Concentrations of mRNA that code for 5-HTR subtypes (5-HTR1A. 5-HTR1B, 5-HTR1D, 5-HTR1F, 5-HTR2A, 5-HTR2B, 5-HTR2C, and 5-HTR4) in the bovine digestive tract were measured by use of a quantitative real-time reverse transcription-polymerase chain reaction assay. Results were reported in relation to mRNA expression of the housekeeping gene glyceraldehyde phosphate dehydrogenase (GAPDH).

RESULTS

Mean relative mRNA concentrations for 5-HTR were low (range, 0% to 1.32% of GAPDH), and mRNA that codes for 5-HTR1A was not detected. In the abomasum, mRNA expression was highest for 5-HTR1B and 5-HTR2B, followed by subtypes 1F 2A, 1D, and 4, whereas 5-HTR2C was not detected. In intestinal samples, concentrations of subtypes 1B, 2B, and 4 were highest, followed by 1D, 1F, 2A, and 2C. Relative concentrations of mRNA that code for 5-HTR2A were significantly higher in the abomasum than the intestines, but lower for 5-HTR2B, 5-HTR2C, and 5-HTR4.

CONCLUSIONS AND CLINICAL RELEVANCE

Relative concentrations of mRNA that code for 5-HTRs differ among locations in the gastrointestinal tract of cattle. Understanding differences in the distribution of 5-HTRs in healthy cattle and cattle with gastrointestinal tract disease may lead to improved therapeutic approaches for abomasal and cecal motility disorders.

The 5-HT4 receptor agonist, tegaserod, is a potent 5-HT2B receptor antagonist in vitro and in vivo

1 Tegaserod (Zelnorm) is a potent 5-hydroxytryptamine4 (5-HT4) receptor agonist with clinical efficacy in disorders associated with reduced gastrointestinal motility and transit. The present study investigated the interaction of tegaserod with 5-HT2 receptors, and compared its potency in this respect to its 5-HT4 receptor agonist activity. 2 Tegaserod had significant binding affinity for human recombinant 5-HT2A, 5-HT2B and 5-HT2C receptors (pKi=7.5, 8.4 and 7.0, respectively). The 5-HT2B receptor-binding affinity of tegaserod was identical to that at human recombinant 5-HT4(c) receptors (mean pKi=8.4) in human embryonic kidney-293 (HEK-293) cells stably transfected with the human 5-HT4(c) receptor. 3 Tegaserod (0.1-3 microm) inhibited 5-HT-mediated contraction of the rat isolated stomach fundus potently (pA2=8.3), consistent with 5-HT(2B) receptor antagonist activity. Tegaserod produced, with similar potency, an elevation of adenosine 3',5' cyclic monophosphate in HEK-293 cells stably transfected with the human 5-HT4(c) receptor (mean pEC50=8.6), as well as 5-HT4) receptor-mediated relaxation of the rat isolated oesophagus (mean pEC50=8.2) and contraction of the guinea-pig isolated colon (mean pEC50=8.3). 4 Following subcutaneous administration, tegaserod (0.3 or 1 mg kg(-1)) inhibited contractions of the stomach fundus in anaesthetized rats in response to intravenous dosing of alpha-methyl 5-HT (0.03 mg kg(-1)) and BW 723C86 (0.3 mg kg(-1)), selective 5-HT2B receptor agonists. At similar doses, tegaserod (1 and 3 mg kg(-1) subcutaneously) evoked a 5-HT4 receptor-mediated increase in colonic transit in conscious guinea-pigs. 5 The data from this study indicate that tegaserod antagonizes 5-HT2B receptors at concentrations similar to those that activate 5-HT4 receptors. It remains to be determined whether this 5-HT2B receptor antagonist activity of tegaserod contributes to its clinical profile.

Influence of weaning and effect of post weaning dietary zinc and copper on electrophysiological response to glucose, theophylline and 5-HT in piglet small intestinal mucosa

This study aimed to examine how weaning and how dietary zinc and/or copper fed post weaning may affect the electrophysiological response to glucose and to chloride secretagogues in piglet small intestine in vitro. Study 1 included 54 piglets (six litters of nine piglets). One piglet from every litter was killed 1 day before weaning. The remaining 48 piglets were allocated at weaning (28 d) to four dietary zinc treatments and subsequently killed 1-2, 5-6 or 14-15 days after weaning. Study 2 included 48 piglets (six litters of eight piglets) allocated to four dietary treatments, consisting of high or low dietary zinc with or without high dietary copper. All piglets in study 2 were killed 5-7 days after weaning. The in vitro studies in Ussing chambers showed that weaning resulted in increased ileal glucose absorption as well as increased neuroendocrine-regulated (activated by 5-HT) and cAMP-dependent (activated by theophylline) chloride secretion. High zinc supplementation reduced the responses to 5-HT and theophylline. The study did not reveal any influence of copper on these parameters. It is concluded that the positive effect of zinc supplementation on diarrhoea in weaned piglets may be due to zinc reducing the intestinal mucosal susceptibility to secretagogues that activate chloride secretion.

5-HT induces duodenal mucosal bicarbonate secretion via cAMP- and Ca2+-dependent signaling pathways and 5-HT4 receptors in mice

In previous studies, we have found that 5-hydroxytryptamine (5-HT) is a potent stimulant of duodenal mucosal bicarbonate secretion (DMBS) in mice. The aim of the present study was to determine the intracellular signaling pathways and 5-HT receptor subtypes involved in 5-HT-induced DMBS. Bicarbonate secretion by murine duodenal mucosa was examined in vitro in Ussing chambers. 5-HT receptor involvement in DMBS was inferred from pharmacological studies by using selective 5-HT receptor antagonists and agonists. The expression of 5-HT(4) receptor mRNA in duodenal mucosa and epithelial cells was analyzed by RT-PCR. cAMP-dependent signaling pathway inhibitors MDL-12330A, Rp-cAMP, and H-89 and Ca(2+)-dependent signaling pathway inhibitors verapamil and W-13 markedly reduced 5-HT-stimulated duodenal bicarbonate secretion and short-circuit current (I(sc)), whereas cGMP-dependent signaling pathway inhibitors NS-2028 and KT-5823 failed to alter these responses. Both SB-204070 and high-dose ICS-205930 (selective 5-HT(4) receptor antagonists) markedly inhibited 5-HT-stimulated bicarbonate secretion and I(sc), whereas methiothepine (5-HT(1) receptor antagonist), ketanserin (5-HT(2) receptor antagonist), and a low concentration of ICS-205930 (5-HT(3) receptor antagonist) had no effect. RS-67506 (partial 5-HT(4) receptor agonist) concentration-dependently increased bicarbonate secretion and I(sc), whereas 5-carboxamidotryptamine (5-HT(1) receptor agonist), alpha-methyl-5-HT (5-HT(2) receptor agonist), and phenylbiguanide (5-HT(3) receptor agonist) did not significantly increase bicarbonate secretion or I(sc). RT-PCR analysis confirmed the expression of 5-HT(4) receptor mRNA in murine duodenal mucosa and epithelial cells. These results demonstrate that 5-HT regulates DMBS via both cAMP- and Ca(2+)-dependent signaling pathways and 5-HT(4) receptors located in the duodenal mucosa and/or epithelial cells.

Pharmacological aspects of anticancer drug-induced emesis with emphasis on serotonin release and vagal nerve activity

Cytotoxic drug-induced nausea and vomiting are the side effects most feared by cancer patients. Emesis is an instinctive defense reaction caused by the somatoautonomic nerve reflex, which is integrated in the medulla oblongata. Emesis caused by cytotoxic drugs such as cisplatin is associated with an increase in the concentration of 5-hydroxytryptamine (5-HT) in the intestine and the brainstem. It is proposed that cytotoxic drugs evoke 5-HT release from the enterochromaffin (EC) cells in the intestinal mucosa and that the released 5-HT stimulates the 5-HT receptors on the adjacent vagal afferent nerves. The depolarization of the vagal afferent nerves stimulates the vomiting center in the brainstem and eventually induces a vomiting reflex. 5-HT released from EC cells seems to mediate the cisplatin-induced emesis sensitive to 5-HT(3) receptor antagonists. The release of 5-HT from the EC cells, however, is regulated by polymodal mechanisms on autoreceptors or heteroreceptors. The precise role of 5-HT on the occurrence of vomiting has not been fully elucidated. The present review aims to describe the role of 5-HT in anticancer drug-induced emesis from the viewpoint of 5-HT release and afferent vagus nerve activity. Various methods for predicting emesis are also evaluated.

The enteric nervous system III: a target for pharmacological treatment

The past decade has seen major advances in the pharmacological understanding of the nervous system of the gastrointestinal tract, the enteric nervous system, and its importance for gut functions in several states of disease. Indeed, the enteric nervous system has become a promising target in the treatment of many gastrointestinal symptoms and disorders. Some of these new therapeutic concepts, such as botulinum toxin for achalasia and serotonergic drugs for functional bowel diseases, are already in clinical use. This paper is part 3 of three Minireviews in Pharmacology & Toxicology, and presents the neurogastrointestinal pharmacological therapeutic options in gastrointestinal pain, functional gastrointestinal disorders, inflammatory bowel diseases, cancer and related conditions with focus on future drug targets. The diagnosis of gastrointestinal neuropathy, the role of serotonin and related neuroendocrine transmitters, serotonergic drugs, and neurotrophic factors in neurogastrointestinal pharmacology will be addressed in this context.

The enteric nervous system II: gastrointestinal functions

The enteric nervous system is involved in most of the physiological and pathophysiological processes in the gastrointestinal tract. This Minireview is part two of three and describes the role of the enteric nervous system in gastrointestinal functions (motility, exocrine and endocrine secretions, blood flow, and immune processes) in health and some disease states. In this context, the functional importance of the enteric nervous system for food intake, the gall bladder, and pancreas will be addressed. In specific, dysmotility, diarrhoea, constipation, non-occlusive intestinal ischaemia (intestinal angina), inflammation, cholelithiasis, cholecystitis, postcholecystectomy syndrome, and pancreatitis can be treated with neuroactive pharmacological agents. For example, serotonin receptor type four agonists can be used for the treatment of constipation, while nitric oxide synthesis inhibitors can be employed for the treatment of intestinal angina.

Chronic effects of copper exposure versus endocrine toxicity: two sides of the same toxicological process?

Chronic sub-lethal exposure to copper (Cu) causes a series of cellular and physiological changes in fish that enable the animal to survive. Copper is also an endocrine disrupting metal in the aquatic environment, and has a number of normal neuro-endocrine roles in vertebrates. This paper explores whether the chronic effects of Cu exposure can be explained by the effects of Cu on neuro-endocrine functions in fish. Chronic Cu exposure involves complex physiological adjustments in many body systems, including increased oxygen consumption, reduced mean swimming speed, up-regulation of ionic regulation, decreasing lymphocyte levels and increasing neutrophils, altered immunity, modulation of Cu-dependent and independent enzyme activities, and proliferation of epithelial cells in gills or intestine. These responses can occur with exposure via the food or the water and can be rationalised into three major categories: (1) up-regulation of enzymes/metabolism (2) altered haematopoietic responses and (3) altered cellularity (cell type, turnover or size) in tissues. Some of these responses can be explained by stimulation of general stress responses, including the adrenergic response and stimulated cortisol release via the hypothalamic-pituitary-interrenal axis. This can occur despite evidence of vacuolation and foci of necrosis in the brain, and increased macrophage activity, in the kidney of fish exposed to dietary Cu. In addition to generic stress responses, Cu regulates specific neuro-endocrine functions, including the loss of circadian rhythm during dietary Cu exposure that involves the failure to respond to circulating melatonin and a loss of circulating serotonin. We conclude that the chronic physiological effects of Cu and apparent endocrine disrupting effects of Cu are two sides of the same toxicological process.

Active bicarbonate-dependent secretion evoked by 5-hydroxytryptamine in porcine ileal mucosa is mediated by opioid-sensitive enteric neurons

5-Hydroxytryptamine (5-HT) mediates intestinal hypersecretion associated with infection and inflammation. We tested the hypothesis that 5-HT-induced anion secretion is mediated by an opioid-sensitive enteric neural circuit. 5-HT, at a contraluminal concentration of 10 microM, increased short-circuit current by 58 +/- 7 microA/cm(2) in sheets of porcine ileal mucosa with attached inner submucosal plexus. Responses to 5-HT were inhibited by saxitoxin or indomethacin, and reduced in tissues bathed in Cl(-)- or HCO(3)(-)-deficient media. 5-HT action was attenuated by saxitoxin in tissues bathed in Cl(-)-free media, but not HCO(3)-free media. The delta-opioid receptor agonist [D-Pen(2,5)]enkephalin (0.1 microM) blunted the 5-HT change in short-circuit current by a mechanism sensitive to the delta-opioid receptor antagonist naltrindole. The inhibitory actions of [D-Pen(2,5)]enkephalin and saxitoxin were not additive. These results suggest that 5-HT stimulates HCO(3)(-)-dependent ion transport through a mechanism involving prostanoids and an enteric neural pathway modulated by opioids.

5-HT(2A) receptors: location and functional analysis in intestines of wild-type and 5-HT(2A) knockout mice

The distribution and function of the 5-hydroxytryptamine (5-HT(2A)) receptor were investigated in the intestines of wild-type (5-HT(2A) +/+) and knockout (5-HT(2A) -/-) mice. In 5-HT(2A) +/+ mice, rats, and guinea pigs, 5-HT(2A) receptor immunoreactivity was found on circular and longitudinal smooth muscle cells, neurons, enterocytes, and Paneth cells. Muscular 5-HT(2A) receptors were concentrated in caveolae; neuronal 5-HT(2A) receptors were found intracellularly and on the plasma membranes of nerve cell bodies and axons. Neuronal 5-HT(2A) immunoreactivity was detected as early as E14 in ganglia, intravillus nerves, and the deep muscle plexus. The 5-HT(2A) -/- colon did not express 5-HT(2A) receptors and did not contract in response to exogenous 5-HT. 5-HT(2A) -/- enterocytes were smaller, Paneth cells fewer, and muscle layers thinner (and showed degeneration) compared with those of 5-HT(2A) +/+ littermates. The 5-HT(2A) receptor may thus be required for the maintenance and/or development of enteric neuroeffectors and other enteric functions, although gastrointestinal and colonic transit times in 5-HT(2A) -/- and +/+ mice did not differ significantly.