Histamine inhibits 5-hydroxytryptamine release from the porcine small intestine: involvement of H3 receptors

Potential Roles of Enterochromaffin Cells in Early Life Stress-Induced Irritable Bowel Syndrome

Irritable bowel syndrome (IBS) is one of the most common functional gastrointestinal disorders, also known as disorders of the gut–brain interaction; however, the pathophysiology of IBS remains unclear. Early life stress (ELS) is one of the most common risk factors for IBS development. However, the molecular mechanisms by which ELS induces IBS remain unclear. Enterochromaffin cells (ECs), as a prime source of peripheral serotonin (5-HT), play a pivotal role in intestinal motility, secretion, proinflammatory and anti-inflammatory effects, and visceral sensation. ECs can sense various stimuli and microbiota metabolites such as short-chain fatty acids (SCFAs) and secondary bile acids. ECs can sense the luminal environment and transmit signals to the brain via exogenous vagal and spinal nerve afferents. Increasing evidence suggests that an ECs-5-HT signaling imbalance plays a crucial role in the pathogenesis of ELS-induced IBS. A recent study using a maternal separation (MS) animal model mimicking ELS showed that MS induced expansion of intestinal stem cells and their differentiation toward secretory lineages, including ECs, leading to ECs hyperplasia, increased 5-HT production, and visceral hyperalgesia. This suggests that ELS-induced IBS may be associated with increased ECs-5-HT signaling. Furthermore, ECs are closely related to corticotropin-releasing hormone, mast cells, neuron growth factor, bile acids, and SCFAs, all of which contribute to the pathogenesis of IBS. Collectively, ECs may play a role in the pathogenesis of ELS-induced IBS. Therefore, this review summarizes the physiological function of ECs and focuses on their potential role in the pathogenesis of IBS based on clinical and pre-clinical evidence.

5-HT containing enteroendocrine cells characterised by morphologies, patterns of hormone co-expression, and relationships with nerve fibres in the mouse gastrointestinal tract

5-HT containing enteroendocrine cells (EEC), the most abundant type of EEC in the gut, regulate many functions including motility, secretion and inflammatory responses. We examined the morphologies of 5-HT cells from stomach to rectum, patterns of hormone co-expression in the stomach and colon, and the relationship of 5-HT cells with nerve fibres. We also reviewed some of the relevant literature. The morphologies of 5-HT cells were distinct, depending on their location in the gut. A noticeable feature of some 5-HT cells in the antrum and colon was their long basal processes, which resembled processes of neurons, whereas 5-HT cells in the small intestinal mucosa lacked basal processes. In the stomach, numerous 5-HT cells, including cells with basal processes, were identified as enterochromaffin-like cells by their expression of histidine decarboxylase. In the colon, we observed a small number of 5-HT cells that were in close contact with, but distinct from, oxyntomodulin (OXM) and PYY immunoreactive EEC. We did not find specific relationships between nerve fibres and the processes of colonic 5-HT cells. We conclude that five major features, i.e., gut region, morphology, hormone content, receptor repertoire and cell lineage, can be used to define 5-HT cells.

Mechanosensitive Piezo Channels in the Gastrointestinal Tract

Sensation of mechanical forces is critical for normal function of the gastrointestinal (GI) tract and abnormalities in mechanosensation are linked to GI pathologies. In the GI tract there are several mechanosensitive cell types-epithelial enterochromaffin cells, intrinsic and extrinsic enteric neurons, smooth muscle cells and interstitial cells of Cajal. These cells use mechanosensitive ion channels that respond to mechanical forces by altering transmembrane ionic currents in a process called mechanoelectrical coupling. Several mechanosensitive ionic conductances have been identified in the mechanosensory GI cells, ranging from mechanosensitive voltage-gated sodium and calcium channels to the mechanogated ion channels, such as the two-pore domain potassium channels K2P (TREK-1) and nonselective cation channels from the transient receptor potential family. The recently discovered Piezo channels are increasingly recognized as significant contributors to cellular mechanosensitivity. Piezo1 and Piezo2 are nonselective cationic ion channels that are directly activated by mechanical forces and have well-defined biophysical and pharmacologic properties. The role of Piezo channels in the GI epithelium is currently under investigation and their role in the smooth muscle syncytium and enteric neurons is still not known. In this review, we outline the current state of knowledge on mechanosensitive ion channels in the GI tract, with a focus on the known and potential functions of the Piezo channels.

Endothelium-dependent relaxation of rat aorta to a histamine H(3) agonist is reduced by inhibitors of nitric oxide synthase, guanylate cyclase and Na,K-ATPase

The possible involvement of different effector systems (nitric oxide synthase, guanylate cyclase, β-adrenergic and muscarinic cholinergic receptors, cyclooxygenase and lipoxygenase, and Na⁺,K⁺-ATPase) was evaluated in a histamine H₃ receptor agonist-induced ((R)α-methylhistamine, (R)α-MeHA) endothelium-dependent rat aorta relaxation assay. (R)α-MeHA (0.1 nM – 0.01 mM) relaxed endothelium-dependent rat aorta, with a pD₂ value of 8.22 ± 0.06, compared with a pD₂ value of 7.98 ± 0.02 caused by histamine (50% and 70% relaxation, respectively). The effect of (R)α-MeHA (0.1 nM – 0.01 mM) was competitively antagonized by thioperamide (1, 10 and 30 nM) (pA₂ = 9.21 ± 0.40; slope = 1.03 ± 0.35) but it was unaffected by pyrilamine (100 nM), cimetidine (1 μM), atropine (10 μM), propranolol (1 μM), indomethacin (10 μM) or nordthydroguaiaretic acid (0.1 mM). Inhibitors of nitric oxide synthase, L-N^G-monomethylarginine (L-NMMA, 10 μM) and N^G-nitro-L-arginine methylester (L-NOARG, 10 μM) inhibited the relaxation effect of (R)α-MeHA, by approximately 52% and 70%, respectively). This inhibitory effect of L-NMMA was partially reversed by L-arginine (10 μM). Methylene blue (10 μM) and ouabain (10 μM) inhibited relaxation (R)α-MeHA-induced by approximately 50% and 90%, respectively. The products of cyclooxygenase and lipoxygenase are not involved in (R)α-MeHA-induced endothelium-dependent rat aorta relaxation nor are the muscarinic cholinergic and β-adrenergic receptors. The results also suggest the involvement of NO synthase, guanylate cyclase and Na⁺,K⁺-ATPase in (R)α-MeHA-induced endothelium-dependent rat aorta relaxation.

The evidence for histamine H(3) receptor-mediated endothelium-dependent relaxation in isolated rat aorta

The presence of histamine H₃ receptors was evaluated on the rat aorta endothelium. In the presence of pyrilamine (1 nM, 7 nM, 10 nM) or thioperamide (1 nM, 10 nM, 30 nM) the concentration–response curve for histamine-induced (0.1 nM − 0.01 mM) endothelium-dependent rat aorta relaxation was shifted to the right without significant change of the E_max indicating competitive antagonism by pyrilamine (pA₂ = 9.33 ± 0.34, slope = 1.09 ± 0.36) or thioperamide (pA₂ =9.31 ± 0.16, slope=0.94 ± 0.10). Cimetidine (1 μM) did not influence histamine-induced endothelium-dependent rat aorta relaxation. In the presence of thioperamide (1 nM, 10 nM, 30 nM) the concentration–response curve for (R)α-MeHA-induced (0.1 nM − 0.01 mM) endothelium-dependent relaxation was shifted to the right without significant change of E_max indicated competitive antagonism by thioperamide (pA₂ = 9.21 ± 0.4, slope = 1.03 ± 0.35). Pyrilamine (100 nM) or cimetidine (1 μM) did not influence (R)α-MeHA-induced endothelium-dependent rat aorta relaxation. These results suggest the presence of a heterogenous population of histamine receptors, H₁ and H₃, on rat aorta endothelium.

Serotonin and Anticancer Drug-induced Emesis

Cytotoxic drug-induced nausea and vomiting are the side effects most feared by cancer patients. Emesis is an instinctive defense reaction caused by the somato-autonomic nerve reflex, which is integrated in the medulla oblongata. Emesis caused by anticancer drugs is associated with an increase in the concentration of serotonin (5-HT) (5-HT) in the intestinal mucosa and brainstem. 5-HT released from the enterochromaffin (EC) cells, which synthesize and secrete 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 appears to mediate the cisplatin-induced emesis sensitive to 5-HT(3) receptor antagonists. The precise role of 5-HT in the occurrence of vomiting has not been fully elucidated. The present review describes the role of 5-HT in anticancer drug-induced emesis from the viewpoint of 5-HT release and afferent vagal nerve activity. Various models and methods for predicting emesis are also evaluated.

Calcitonin gene-related peptide facilitates serotonin release from guinea-pig colonic mucosa via myenteric neurons and tachykinin NK2/NK3 receptors

The ability of calcitonin gene-related peptide (CGRP), to alter the outflow of 5-hydroxytryptamine (5-HT) from the guinea-pig proximal colon, was evaluated using three different isolated preparations: whole colon, mucosa-free muscle layer and submucosa/mucosa preparations. In the presence of the monoamine oxidase A inhibitor, clorgyline, CGRP elicited a concentration-dependent increase in 5-HT outflow from the whole colon, but not from mucosa-free muscle layer preparations. The CGRP-evoked 5-HT outflow was sensitive to tetrodotoxin (TTX) or hexamethonium, but was not detectable in submucosa/mucosa preparations. HCGRP8-37 (3 microM) inhibited the submaximal effect of CGRP on the 5-HT outflow. [Cys(ACM)2,7]hCGRP had a slight stimulant influence on the 5-HT outflow. The selective NK2 and NK3 receptor antagonists, SR48968 or SR142801, respectively, prevented the enhancing effect of CGRP. By contrast, a selective NK1 receptor antagonist L703606, failed to block the effect of CGRP. The enhancing effect of CGRP was mimicked by the NK2 receptor agonist [beta-Ala8]-neurokinin A (NKA)4-10 and the NK3 receptor agonist senktide. The effect of [beta-Ala8]-NKA4-10 on the 5-HT outflow was unaffected by TTX, while the effect of senktide was prevented by TTX, hexamethonium or SR48968. The present data also demonstrated a synergistic action of the NK2 and NK3 receptor agonists on the CGRP-evoked 5-HT outflow. We concluded that CGRP facilitates 5-HT release from the guinea-pig colonic mucosa through an action on myenteric neurons and that this effect is mediated by endogenously released tachykinins, acting via tachykinin NK2/NK3 receptors in cascade. British Journal of Pharmacology (2004) 141, 385-390. doi:10.1038/sj.bjp.0705624

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.

Study of serotonin interactions with brush border membrane of rabbit jejunum enterocytes

Recent studies have demonstrated that serotonin (5-hydroxytryptamine, 5-HT) may interact with either specific receptors or with a specific transporter that takes up 5-HT in the gastrointestinal tract. The purpose of the present work was to study the 5-HT interactions with brush border membrane from rabbit jejunum enterocytes. The results obtained showed that 5-HT did not seem to be transported by any specific system of transport in brush border membrane vesicles. Nevertheless, [3H]5-HT seemed to bind specifically to this membrane. The kinetic analysis indicated a saturable and dissociable specific binding with a dissociation constant K(D)=14x10(-9) M. The saturation studies with [3H]5-HT indicated the presence of one specific site in the brush border membrane. The results of displacement of [3H]5-HT specific binding from the brush border membrane showed that both unlabelled 5-HT and unlabelled GR113080 ([1-[(2-methyl sulphonyl) amino] ethyl-4-piperidinyl] methyl-1-methyl-1H-indole-3-carboxylate), a specific competitive antagonist of 5-HT(4) receptors, inhibited the specific binding of [3H]5-HT to this membrane.

Effect of ondansetron on Salmonella typhimurium-induced net fluid accumulation in the pig jejunum in vivo

Two major pathophysiological mechanisms explaining the diarrhoea induced by Salmonella typhimurium have been suggested to be: (a) invasion of the intestine by the bacteria, and (b) an enterotoxin resembling Vibrio cholerae toxin. Cholera toxin is a potent secretagogue in pig small intestine and induces secretion partly by activating 5-hydroxytryptamine receptors, following release of 5-hydroxytryptamine. Ondansetron is a selective 5-hydroxytryptamine-3 receptor antagonist, which reduces the cholera toxin-evoked fluid accumulation in pig jejunum. The aim of this study was to investigate the effect of ondansetron on Salmonella typhimurium-induced fluid accumulation in ligated loops of pig jejunum in vivo. 10(10) colony-forming units of the bacteria was injected into loops and incubated for 8 hr. 200 mg x kg-1 ondansetron given subcutaneously reduced the Salmonella typhimurium-induced fluid accumulation by about 40%. This results suggests the involvement of 5-hydroxytryptamine and 5-hydroxytryptamine-3 receptors in Salmonella typhimurium-induced diarrhoea.

Effect of 5-HT2 and 5-HT3 receptor antagonists on cholera toxin-induced fluid hypersecretion in the pig jejunum

5-Hydroxytryptamine is a mediator in cholera toxin-induced hypersecretion in the small intestine. The aim of this study was to determine the effect of the 5-hydroxytryptamine receptor antagonists ketanserin, granisetron, ondansetron and tropisetron on cholera toxin-induced hypersecretion in the pig jejunum. Hypersecretion was induced by cholera toxin in ligated jejunal loops. The antagonists were administered subcutaneously at a dose of 100 micrograms/kg. Furthermore, the effect of intraluminally instilled ondansetron was studied. None of the antagonists altered basal absorption or caused fluid hypersecretion. Cholera toxin caused a dose-dependent electrolyte and fluid hypersecretion. The apparent maximal effect, 6.8 +/- 0.4 mg fluid x mg dry loop-1, was reduced by ondansetron, granisetron and tropisetron by about 40%, 30%, and 20%, respectively, whereas ketanserin had no effect. Intraluminal ondansetron reduced the effect of cholera toxin by about 50%. These results demonstrate that 5-hydroxytryptamine3 antagonists administered subcutaneously reduce the cholera toxin-induced hypersecretion in the pig jejunum. Finally, the results support species differences with respect to the antagonistic effect of the tested drugs in cholera toxin-induced hypersecretion.

The effect of alpha-trinositol on cholera toxin-induced hypersecretion and morphological changes in pig jejunum

alpha-Trinositol (D-myo-inositol 1,2,6-trisphosphate, PP56) is a novel antiinflammatory drug. This study elucidates the effect of intravenous alpha-trinositol on basal and acute fluid transport and morphological changes following cholera toxin administration in pig jejunum in vivo. Using isolated jejunal tied-off loops, the fluid hypersecretory (accumulation) effect of different doses of cholera toxin was studied in pigs treated intravenously with saline added different doses (0, 4, 8, 16 and 32 mg x kg-1 x hr-1) of alpha-trinositol. Levels of alpha-trinositol, as well as stereomicroscopical, light microscopical and scanning electron microscopical morphological studies were performed. Cholera toxin evoked a dose-dependent fluid hypersecretion. Treatment with alpha-trinositol caused a dose-dependent inhibition of the cholera toxin-induced fluid hypersecretion and did not affect basal fluid absorption. The 16 mg x kg-1 x hr-1 alpha-trinositol dose gave a maximal inhibition of 36%. Morphological studies showed only minor changes following 6 hr of exposure to 20 micrograms x loop-1 cholera toxin. These changes consisted of dilation of the villus capillaries, an increase of apical membrane blebbing and a reduction of the intercellular space. Treatment with 16 mg x kg-1 x hr-1 alpha-trinositol alone did not induce any morphological changes, and did not alter the morphological changes induced by cholera toxin, which caused fluid hypersecretion and only minor acute morphological changes. In conclusion, alpha-trinositol treatment reduced cholera toxin-induced fluid hypersecretion without altering basal fluid absorption, basal morphology, or cholera toxin-induced morphological changes in pig jejunum in vivo.

Regulation of 5-HT release from enterochromaffin cells

Large amounts of 5-HT are present in the mammalian intestine where the amine is concentrated in the enterochromaffin cells (ECs) of the mucosa. ECs have the enzymes to synthesize 5-HT, are endowed with a specific, imipramine-sensitive 5-HT uptake mechanism and can store 5-HT in specific secretory vesicles. ECs can secrete 5-HT in a calcium-dependent manner. In particular, calcium influx through voltage-regulated channels and receptor-mediated liberation of intracellular calcium can evoke 5-HT release. 5-HT secretion from ECs occurs predominantly at the interstitial side and is controlled by a complex pattern of receptor-mediated mechanisms. Stimulatory receptors (beta-adrenoceptors, muscarine, nicotine and 5-HT3 receptors) and inhibitory receptors (alpha 2-adrenoceptors, histamine H3, GABAA- and GABAB-, A2 and P2y alpha purine and 5-HT4 receptors as well as receptors for vasoactive intestinal polypeptide (VIP), pituitary adenylate cyclase stimulating peptide (PACAP) and somatostatin) have been shown to be involved in the control of 5-HT release from the ECs.

An update on histamine H3 receptors and gastrointestinal functions

The distribution and functions of histamine H3 receptors in the gastrointestinal tract is reviewed with particular reference to the effects on gastric acid secretion, mucosal protection, and intestinal motility. Histamine H3 receptor activation has negative effects on acid secretion induced by indirect secretagogues in cats, dogs, and rabbits; less clear effects were found in rats. An inhibitory effect on histamine release induced by different stimuli was observed in rats, rabbits, and dogs after H3 receptor agonists, thus supporting the idea that H3 receptors occur in ECL cells. (R)-alpha-methylhistamine has a marked protective effect against gastric lesions induced by ethanol in rats, being slightly less effective against aspirin and stress. H3 receptor activation decreases the intestinal motility induced by electrical stimulation in a variety of gut preparations, reducing both cholinergic and NANC neurotransmitter release. In this tissue the inhibitory effects mediated by histamine H3 receptors seem to be coupled, via a G protein, to a restriction of Ca2+ access into the nerve terminal; other mechanisms, however, have been suggested in the gastric mucosa. Histamine H3 receptors have already been subdivided into two receptor subtypes, H3A and H3B, the former being the subtype predominant in the gastrointestinal tissue. The increasing availability of selective agonists and antagonists of H3 receptors will unravel possible novel actions and physiological roles of histamine.

R-alpha-methylhistamine-induced inhibition of gastric acid secretion in pylorus-ligated rats via central histamine H3 receptors

1. The effect of central H3 histamine receptor activation on gastric acid and pepsin production has been investigated in pylorus-ligated rats. 2. Intracerebroventricular injections (i.c.v.) of the selective H3 agonist, R-alpha-methylhistamine (0.5-50 nmol per rat) caused a dose-dependent inhibition of gastric acid secretion while intravenous administration (5-500 nmol per rat) was completely ineffective. 3. I.c.v. microinjections of mepyramine, tiotidine and thioperamide (51 nmol per rat), selective antagonists at H1-, H2- and H3-sites respectively, failed to modify the acid secretory response to pylorus ligation. 4. The antisecretory effect of R-alpha-methylhistamine (5 nmol per rat, i.c.v.) was selectively prevented by the H3-blocker, thioperamide (51 nmol per rat, i.c.v.), mepyramine and tiotidine pretreatment being completely inactive. 5. Unlike acid secretion, pepsin production was not significantly affected by all the tested compounds. 6. These findings provide the first pharmacological evidence that the activation of central H3 histamine receptors exerts a negative control in the regulation of gastric acid secretion in conscious pylorus-ligated rats.

H3 receptors: modulation of histamine-stimulated neural pathways influencing electrogenic ion transport in the guinea pig colon

The role of H3 receptors in neurally-evoked recurrent increases in chloride ion secretion evaluated from changes in short-circuit current (Isc) was examined during application of histamine or histamine analogs. Muscle-stripped or whole thickness segments of distal colon were set up in flux chambers. Histamine and dimaprit, an H2 receptor agonist, caused recurrent increases in Isc. Dimaprit-evoked recurrent responses were reduced by the H3 receptor agonists, N alpha-methylhistamine and R alpha-methylhistamine, and the inhibition was reversed by the H3 receptor antagonist, burimamide. Histamine-evoked recurrent increases in Isc were enhanced by the H3 receptor antagonists burimamide and thioperamide. The results indicate that H3 receptors play an inhibitory role in histamine-evoked, neurally-mediated recurrent increases in Isc in guinea pig colon.

Ketanserin and granisetron reduce cholera toxin-induced hypersecretion in pig jejunum

BACKGROUND

Serotonin antagonists have been proven antisecretory in cholera toxin (CT)-induced hypersecretion in the small intestine of rodents. The pig small intestine is a good model for the human small intestine with regard to physiologic and pharmacologic processes.

METHODS

The antisecretory effect of intraluminally administered methysergide, renzapride, ketanserin, granisetron, and tropisetron on CT-induced hypersecretion was tested in isolated pig jejunal loops in vivo.

RESULTS

Methysergide, ketanserin, and granisetron reduced the hypersecretory effect of CT maximally by 25%, 80%, and 50%, respectively. Tropisetron enhanced whereas renzapride did not alter the CT response. Combination of ketanserin and granisetron gave a maximal inhibitory effect of about 85%. Surprisingly, renzapride, granisetron, and tropisetron each induced hypersecretion. Taking into account the hypersecretory effect of the antagonists, they all reduced this CT-elicited hypersecretion.

CONCLUSIONS

Results suggest involvement of the 5-hydroxytryptamine-2 and 5-hydroxytryptamine-3 receptor subtypes as mediators in CT-induced hypersecretion in pig jejunum, and antidiarrheal therapeutic potentials of ketanserin and granisetron.

5-Hydroxytryptamine2 and 5-hydroxytryptamine3 receptors mediate serotonin-induced short-circuit current in pig jejunum

The purpose of this study was to determine the effect of methysergide, ketanserin, granisetron, cisapride, and renzapride on serotonin (5-hydroxytryptamine-evoked short-circuit current in muscle and myenteric plexus-stripped pig jejunum using the Ussing chamber technique. Ketanserin, granisetron, cisapride, and renzapride all reduced the 5-hydroxytryptamine-induced increase in short-circuit current by about 50%. Combination of ketanserin and granisetron only reduced the 5-hydroxytryptamine-induced peak increase in short-circuit current by 25%. Cisapride caused a small concentration-dependent increase in short-circuit current. Atropine and hexamethonium both almost completely suppressed the cisapride-induced peak increase in short-circuit current. Atropine and hexamethonium both almost completely suppressed the cisapride-induced peak increase in short-circuit current. Ketanserin, granisetron, methysergide, and renzapride did not alter the basal short-circuit current. These results suggest that 5-hydroxytryptamine elicits an increase in short-circuit current by activating epithelial and submucosal 5-hydroxytryptamine2 and 5-hydroxytryptamine3 receptor subtypes. Furthermore, the short-circuit current-increasing effect of cisapride, is due to activation of at least muscarinic and nicotinic receptors.

The importance of 5-hydroxytryptamine receptors in the gut

Most of the 5-hydroxytryptamine (5-HT) present in the adult human body is located in the gastrointestinal tract. The vast majority is contained in enteroendocrine cells, the rest exists mainly in myenteric interneurons separated from the mucosa by an intraenteric barrier. Physiological studies suggest that 5-HT plays a vital role in mediating both sensory and reflex responses to gastrointestinal stimuli and, thus, this transmitter is closely implicated in gut reactions. This review outlines some of the evidence for different 5-HT receptors, summarizes the role of 5-HT in mediating gut sensitivity and motor activity, secretion and more complex activities, such as emesis and diarrhoea and identifies the clinical role of drugs acting on 5-HT receptors in the treatment of emesis, diarrhoea, the control of abdominal pain and discomfort and the rectification of gastrointestinal motility.

Serotonin-induced short-circuit current in pig jejunum

This paper presents the effects of serotonin (5-HT) on short-circuit current (SCC), sodium and chloride fluxes, and prostaglandin E2 release in pig jejunum, using the Ussing-chamber technique. 5-HT elicited a dose-dependent increase in SCC, yielding an EC50 of 6 +/- 4 microM and EMAX of 77 +/- 8 microA.cm-2 using about 100 microM. Inhibited sodium absorption and stimulated chloride secretion carried part of the 5-HT-induced SCC. 5-HT caused a dose-independent PGE2 release, and indomethacin reduced the SCC-inducing effect of 5-HT by 40%. Octreotide, a long-lasting somatostatin analogue, also reduced 5-HT-induced SCC by about 40%, while tetrodotoxin (TTX) did not alter the effect of 5-HT. In conclusion, 5-HT causes a dose-dependent indomethacin and octreotide-sensitive, and TTX-insensitive increase in SCC, and a chloride secretion and inhibited sodium absorption and an increased release of PGE2 in pig jejunum in vitro.

Pituitary adenylate cyclase-activating peptide is a potent modulator of human colonic motility

Pituitary adenylate cyclase-activating peptide (PACAP) represents a novel brain-gut peptide with high sequence homology to vasoactive intestinal polypeptide (VIP). Since PACAP has been identified in the human gut, the effect of the two molecular forms PACAP-(1-38) and PACAP-(1-27), the hybrid PACAP-(1-23)VIP-(24-28), and VIP on the contractility of the longitudinal muscle of human sigmoid colon was tested in vitro. All peptides inhibited the spontaneous phasic contractions and relaxed concentration-dependently carbachol-precontracted preparations. The effects of the peptides remained unaffected by tetrodotoxin, by inhibition of phosphodiesterase activity, and by inhibition of nitric oxide synthesis. Apamin reduced only the effects of the PACAP peptides, whereas tetraethylammonium blocked only the effect of VIP. In conclusion, PACAP peptides and VIP mediate their relaxant effects via activation of specific PACAP and VIP receptors coupled to different potassium channels.

Nicotinic and muscarinic modulation of 5-hydroxytryptamine (5-HT) release from porcine and canine small intestine

Strips of porcine and canine small intestine were incubated in vitro and the release of 5-hydroxytryptamine (5-HT) was determined by HPLC with electrochemical detection. The spontaneous outflow of 5-HT from the porcine and canine small intestine largely reflects calcium-dependent 5-HT secretion from enterochromaffin cells which are under a spontaneous neuronal, excitatory input as indicated by the inhibitory effect (30-40%) of tetrodotoxin. In both species, nicotine enhanced the release of 5-HT in a concentration-dependent manner by a maximum of about 50% at 100 microM. This effect was blocked by the nicotine receptor antagonist hexamethonium, but not by the subtype-selective nicotine receptor antagonist alpha-bungarotoxin. The effect of nicotine was rapidly desensitized. The presence of tetrodotoxin abolished the effect of nicotine on 5-HT release in canine tissue but not in porcine tissue. The presence of the muscarine receptor antagonist scopolamine prevented the effect of nicotine on 5-HT release from canine tissue, but significantly enhanced 5-HT release from porcine tissue. The muscarine receptor agonist oxotremorine inhibited 5-HT release from porcine tissue, but increased 5-HT release from canine tissue. However, in the presence of tetrodotoxin, oxotremorine enhanced 5-HT release in tissue from both species. In conclusion, activation of nicotine receptors induce the release of 5-HT from porcine and canine small intestine. In the dog, the effect of nicotine is mediated via the release of acetylcholine which then stimulates 5-HT release via muscarine receptors on the enterochromaffin cells. In the pig, the stimulatory effect of nicotine appears to be located directly on the enterochromaffin cells. In addition, activation of neuronal muscarine receptors in the porcine small intestine induced the release of a previously unidentified neurotransmitter which inhibited 5-HT release. Nicotine, via cholinergic interneurons, also appears to induce the release of this inhibitory neurotransmitter which opposes the direct stimulatory action of nicotine on 5-HT release.