Serotonergic regulation of canine enteric motility (measured as electrical activity) and absorption: physiologic and morphologic evidence

Localization of the Serotonin Transporter in the Dog Intestine and Comparison to the Rat and Human Intestines

Serotonin is crucial in gastrointestinal functions, including motility, sensitivity, secretion, and the inflammatory response. The serotonin transporter (SERT), responsible for serotonin reuptake and signaling termination, plays a prominent role in gastrointestinal physiology, representing a promising therapeutic target in digestive disorders. Serotonin transporter expression has been poorly investigated in veterinary medicine, under both healthy and pathological conditions, including canine chronic enteropathy, in which the serotonin metabolism seems to be altered. The aim of the present study was to determine the distribution of SERT immunoreactivity (SERT-IR) in the dog intestine and to compare the findings with those obtained in the rat and human intestines. Serotonin transporter-IR was observed in canine enterocytes, enteric neurons, lamina propria cells and the tunica muscularis. Data obtained in dogs were consistent with those obtained in rats and humans. Since the majority of the serotonin produced by the body is synthesized in the gastrointestinal tract, SERT-expressing cells may exert a role in the mechanism of serotonin reuptake.

Serotonin modifies cytoskeleton and brush-border membrane architecture in human intestinal epithelial cells

Serotonin or 5-hydroxytryptamine (5-HT) influences numerous functions in the gastrointestinal tract. We previously demonstrated that 5-HT treatment of Caco-2 cells inhibited Na(+)/H(+) exchangers (NHE) and Cl(-)/OH(-) exchange activities via distinct signaling mechanisms. Since regulation of several ion transporters such as NHE3 is influenced by intact cytoskeleton, we hypothesized that 5-HT modifies actin cytoskeleton and/or brush-border membrane architecture via involvement of signaling pathways. Ultrastructural analysis showed that 5-HT (0.1 muM, 1 h) treatment of Caco-2 cells caused the apical membrane to assume a convex dome shape that was associated with shortening of microvilli. To examine whether these cellular architecture changes are cytoskeleton driven, we analyzed actin cytoskeleton by fluorescence microscopy. 5-HT induced basal stress fibers with prominent cortical actin filaments via 5-HT3 and 5-HT4 receptor subtypes. This induction was partially attenuated by chelation of intracellular Ca(2+) and PKCalpha inhibition (Go6976). In vitro assays revealed that PKCalpha interacted with actin and this association was increased by 5-HT. Our data provide novel evidence that 5-HT-induced signaling via 5-HT3/4 receptor subtypes to cause Ca(2+) and PKCalpha-dependent regulation of actin cytoskeleton may play an important role in modulation of ion transporters that contribute to pathophysiology of diarrheal conditions associated with elevated levels of 5-HT.

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.

5-HT receptor subtypes involved in the serotonin-induced inhibition of L-leucine absorption in rabbit jejunum

The aim of the present study was to determine the 5-HT receptor subtypes involved in the serotonin-induced inhibition of L-leucine absorption across rabbit jejunum in vitro. A number of agonists and antagonists were used to characterize the receptors through which serotonin inhibits this absorption. The results show that 2.5x10(-6) M 5-HT inhibits the amino acid absorption by about 20%. The 5-HT receptor agonists, alpha-methyl-5-HT (5-HT2), 2-methyl-5-HT (5-HT3) and zacopride (5-HT4) at concentrations 2.5x10(-6) and 2.5x10(-5) M produced 10-30% inhibition on L-leucine intestinal absorption. 5-carboxyamidotryptamine (5-HT1) did not produce any inhibition. The 5-HT antagonists, GR 113808A (5-HT4) at 2.5x10(-6) M and ritanserin (5-HT2) and ondansetron (5-HT3) at 2.5x10(-5) M completely blocked the effect of 5-HT. However, methiothepin (5-HT1) did not produce any effect on serotonin action in the intestinal absorption of amino acid. It can be concluded that 5-HT2, 5-HT3 and 5-HT4 receptors could mediate inhibition of L-leucine absorption across rabbit jejunum.

Effects of 5-hydroxytryptamine agonists on myoelectric activity of the forestomach and antroduodenal area in sheep

To increase knowledge of the role of 5-hydroxytryptamine (5-HT) receptors in the regulation of reticuloruminal, omasal and antroduodenal myoelectric activity in sheep, the effects of 5-HT agonists on forestomach and antroduodenal myoelectric activity have been investigated in conscious sheep. 5-Carboxamidotryptamine, methysergide, alpha-methyl-5-HT, 2-methyl-5-HT, cisapride, zacopride or metoclopramide were infused intravenously for 5 min and myoelectric recordings were obtained from electrodes chronically implanted in the reticulum, rumen (dorsal sac), omasal body, abomasal antrum and duodenal bulb. The integrated activity of the reticular and ruminal spike bursts was modified only by the highest doses of alpha-methyl-5-HT, 2-methyl-5-HT, metoclopramide and cisapride. A phase III-like activity pattern was recorded in the antroduodenal area with all 5-HT-ergic agents and a dose-dependent inhibition of myoelectric activity was recorded in both reticulorumen and omasum at the same time as the antroduodenal effects. In the forestomach, methysergide alone induced inhibition of ruminal secondary contractions; 5-HT, alpha-methyl-5-HT, cisapride and metoclopramide, moreover, evoked an initial dose-dependent increase in antral activity. These results suggest that 5-HT1, 5-HT2, 5-HT3 and 5-HT4 receptors are involved in the regulation of the migrating myoelectric complex in sheep and in the genesis of forestomach hypomotility that is occasionally recorded concomitantly with the spontaneous duodenal phase III in sheep. 5-HT4 receptors also have a prokinetic action in the antral area.

Inhibitory action of 5-hydroxytryptamine on propulsive activity of the guinea-pig isolated colon

The effects of 5-hydroxytryptamine (5-HT) and its receptor subtype agonists on spontaneous propulsive activities of the segment, and on changes in tension of the muscle strips were examined in the guinea-pig isolated distal colon. 5-HT, 5-carboxamidotryptamine (5-CT), alpha-methyl-5-HT and t-methoxytryptamine (5-MeOH), applied to the serosal side of the segment, inhibited spontaneous propulsive activities, but 2-methyl-5-HT did not. On the indomethacin-treated segment, 5-HT and its receptor subtype agonists all stimulated propulsive activity. 5-HT, 5-CT, alpha-methyl-5-HT and 5-MeOH relaxed circular muscle strips, which were inhibited in the presence of tetrodotoxin. However, these agonists showed contractile effects on the indomethacin-treated circular muscle strips. These results suggest that 5-HT may inhibit spontaneous propulsive activities of the colonic segment via release of endogenous PGs (e.g., E and I types) in the circular muscle cells mediated by an inhibitory neurotransmitter, which release was stimulated by 5-HT1-like 5-HT2 and 5-HT4 receptors on the myenteric neurones in the circular muscle layer.

Are muscarinic receptors involved in the effect of serotonin on gastrointestinal electrical activity in the conscious piglet?

In conscious piglets provided chronically with electrodes in the wall of the antrum pylori, duodenum, jejunum and ileum, the effect of intravenous infusion of 5-HT, 4 micrograms/kg/min for 2 h, with and without pre-treatment with atropine, 0.5 mg/kg, on gastrointestinal myoelectrical activity was studied. In the antrum, fast oscillations were partially inhibited by 5-HT and nearly completely blocked by the atropine/5-HT combination and by atropine alone. In the small intestine 5-HT induced a decrease in MMC interval, an increase in phase III activity in duodenum and jejunum and an acceleration of propagation velocity as measured for the jejunum. These effects were not influenced by atropine. Following atropine, phase II activity in the jejunum was significantly inhibited by 5-HT. The ileum was rather insensitive to 5-HT. It is concluded that the inhibitory effect of 5-HT on antral electrical activity is enhanced by atropine, and that 5-HT has a stimulatory effect on small intestinal activity which is not dependent on a muscarinic action.

Effect of serotonin on gastrointestinal electrical activity in the conscious piglet

In conscious piglets with electrodes implanted in the wall of the antrum pylori, duodenum, jejunum and ileum, the influence of intravenous infusion of serotonin (5-HT), 4 micrograms/kg/min for 2 hours, on gastrointestinal electrical activity and arterial pressure was examined. 5-HT was without significant influence on antral electrical activity and arterial pressure. In the small intestinal parts 5-HT provoked a shortening of the interval of the migrating myoelectrical complex (MMC) by reducing the duration of phase I and phase II activities, and an increase in propagation velocity. 5-HT also induced an increase in phase III activity. A possible involvement of 5-HT in the initiation and propagation of the MMC is suggested.

Effects of serotonin on rat ileocolonic transit and fluid transfer in vivo: possible mechanisms of action

The aim was to investigate the action of serotonin (5HT) on function of the ileocolonic junction (ICJ) in vivo. In anaesthetised rats, models were developed to study the effects of intra-aortic (ia) serotonin on ileocolonic and colonic transit, and the effects on transit of a number of 5HT receptor antagonists. In the first series of experiments, a bolus of saline labelled with 99mTc DTPA was instilled 20 cm proximal to the ICJ and transit was assessed three hours later by the geometric centre of the spread of isotope. In the second series, similar techniques were used on the postcaecal colon and transit assessed two hours later. In the third series of experiments, the effects of ia 5HT on ileal net fluid flux was evaluated by standard perfusion experiments with 14C polyethylene glycol (PEG) 4000 as a non-absorbable marker in rat plasma-like electrolyte solution. Compared with ia saline, 5HT accelerated ICJ transit significantly (p < 0.05). This acceleration was comparable with the effect of ia bethanechol. The effects of 5HT on ICJ transit were inhibited by the intraperitoneal (ip) infusion of atropine, the 5HT receptor antagonists, methysergide, ketanserin, zacopride, and the 5HT4 agonist, SC53116. Methysergide, zacopride, and SC53116 given with ia 5HT slowed ICJ transit to rates below those of ia saline alone. When these same agents were given together with ia saline, the ICJ transit was not significantly altered. Serotonin, at the dose that accelerated ICJ transit, did not significantly alter colonic transit or ileal fluid transport. In conclusion, 5HT is a potent pharmacological stimulant of transit across the rat ICJ in vivo; the action of 5HT is mediated partly through muscarinic neurones and several 5HT receptor subtypes.

Effect of putative carcinoid mediators on gastric and small bowel transit in rats and the role of 5-HT receptors

Whereas serotonin and substance P stimulate in-vivo and in-vitro myoelectric activity in the small intestine, their effects on transit are unclear. We used a validated in-vivo transit model in the chloral hydrate-anaesthetized rat to study the effects of serotonin, substance P and motilin, three putative mediators of carcinoid diarrhoea, on transit through the upper digestive tract. Intra-arterial serotonin accelerated gastric emptying of a radiolabelled liquid, while motilin accelerated overall upper gastrointestinal transit. Substance P slowed overall upper gastrointestinal transit without altering gastric emptying. The antagonists to serotonin receptor subtypes, R-zacopride (5-HT3) and ketanserin (5-HT2), also accelerated rat gastric emptying of liquids; in contrast, a 5-HT4 agonist, SC53116, resulted in a less pronounced effect on gastric emptying at the dose tested. We conclude that circulating substance P is unlikely to be an important accelerator of transit through the upper digestive tract; in contrast, hyperserotoninaemia significantly accelerates transit through the stomach, and 5-HT2 and 5-HT3 receptor subtypes may play a role in the motor effects of serotonin in the stomach.

Receptors implicated in the actions of serotonin on chicken ileum longitudinal smooth muscle

The presence of serotonin (5-HT) in the chicken gastrointestinal tract has been previously reported, but its motor effects have been poorly described. The aims of this work were: A) to define the effects of 5-HT on chicken longitudinal ileum; B) to explore the mechanisms by which such effects occur and C) to identify the subtype(s) of 5-HT-ergic receptors implicated. The motor responses to 5-HT were assayed in vitro using ileal strips taken from male White Leghorn chickens 7-9 week old. 5-HT elicited ileal contraction (EC50 9.6 x 10(-8) M), which was markedly decreased in the presence of tetrodotoxin (TTX). Repeated exposure of the tissue to supramaximal concentrations of 5-HT did not however lead to desensitization. Atropine (10(-6) M), ketanserin (10(-5) M), methysergide (10(-5) M) and methiothepine (10(-6) M) attenuated the response to 5-HT. Ketanserin was an effective inhibitor of the residual response to 5-HT obtained even in the presence of TTX. Several serotonergic agonists were assayed to further analyse the type of receptors involved in the response to 5-HT. 5-methoxytryptamine (5-MOT), a mixed 5-HT1, 5-HT2 and 5-HT4 agonist, reproduced all the effects of 5-HT. 8-OH-DPAT, a selective 5-HT1A agonist, trifluoromethylphenylpiperazine, a mixed 5-HT1B/C agonist, and m-chlorophenylbiguanide, a 5-HT3 agonist, did not induce any consistent contractile effects. Sumatriptan, a 5-HT1D agonist, exerted a slight agonistic effect which was blocked by methiothepine and decreased by TTX but not by atropine. Cisapride, a 5-HT4 partial agonist in mammals, decreased the effects of both 5-HT and 5-MOT. These results indicate that chicken ileum contains 5-HT1 receptors similar to the 5-HT1D mammalian subtype but not the 5-HT1A, 5-HT1B, 5-HT1C or 5-HT3 subtypes. 5-HT2 receptors are also present and would appear to be located on smooth muscle.

Review article: 5-hydroxytryptamine agonists and antagonists in the modulation of gastrointestinal motility and sensation: clinical implications

Serotonin (5-hydroxytryptamine; 5-HT) is found in the enteric nervous system where it has been implicated in controlling gastrointestinal motor function. A number of receptor or recognition sites have been identified in the gut, but recently most attention has focused on the 5-HT3 and 5-HT4 receptors. The functional role of the 5-HT3 receptor remains incompletely understood, but it is probably involved in the modulation of colonic motility and visceral pain in the gut. A number of selective 5-HT3 antagonists have been developed including ondansetron, granisetron, tropisetron renzapride and zacopride. While the substituted benzamide prokinetics (for example, metoclopramide, cisapride) also block 5-HT3 receptors in high concentrations, their prokinetic action is believed to be on the basis of their agonist effects on the putative 5-HT4 receptor. Some 5-HT3 antagonists have 5-HT4 agonist activity (for example, renzapride, zacopride) and others do not (for example, ondansetron, granisetron), while tropisetron in high concentrations is a 5-HT4 antagonist. Based on the pharmacological data, it has been suggested that specific 5-HT antagonists and agonists may prove to be beneficial in a number of gastrointestinal disorders including the irritable bowel syndrome, functional dyspepsia, non-cardiac chest pain, gastrooesophageal reflux and refractory nausea. In this review, the rationale for the use of these compounds is discussed, and the available experimental evidence is summarized.

The gastrointestinal motor effect of benzamide derivatives is unrelated to 5-HT3 receptor blockade

The prokinetic properties of a number of 5-HT3 antagonists containing the benzamide moiety (metoclopramide, cisapride, BRL 24924, zacopride) were compared with those of the chemically unrelated antagonist, ICS 205-930. Their 5-HT3 antagonistic potency was evaluated using the Bezold-Jarisch test. All compounds accelerated gastric emptying of beads in the rat, with potencies comparable to those found for inhibiting the Bezold-Jarisch reflex. Metoclopramide, cisapride and zacopride potentiated the twitch contraction of guinea pig ileum and contracted the isolated guinea pig colon in a concentration-dependent manner. Furthermore, they increased the contractility of the gastric Heidenhain pouch in the conscious dog. In contrast, ICS 205-930 was devoid of agonist or antagonist activities in all models except gastric emptying in the rat. Two findings, (1) that benzamide derivatives showed high efficacy in all models of gastrointestinal motility in contrast to ICS 205-930 that was active only to increase gastric emptying, and (2) the different potency order of benzamides in the Bezold-Jarisch test as compared to the in vitro motility tests, indicate that 5-HT3 receptors are involved in gastric emptying, whereas a different receptor operates in the other models used.