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

Intravenous infusion of 5-hydroxytryptophan to mid-lactation Holstein cows transiently affects milk production and circulating amino acid concentrations

In dairy cows, the lactating mammary glands synthesize serotonin, which acts in an autocrine-paracrine manner in the glands and is secreted into the periphery. Serotonin signaling during lactation modulates nutrient metabolism in peripheral tissues such as adipose and liver. We hypothesized that the elevation of circulating serotonin during lactation would increase nutrient partitioning to the mammary glands, thereby promoting milk production. Our objective was to elevate circulating serotonin via intravenous infusion of the serotonin precursor 5-hydroxytryptophan (5-HTP) to determine its effects on mammary supply and extraction efficiency of AA, and milk components production. Twenty-two multiparous mid-lactation Holstein cows were intravenously infused with 5-HTP (1 mg/kg body weight) or saline, in a crossover design with two 21-d periods. Treatments were infused via jugular catheters for 1 h/d, on d 1 to 3, 8 to 10, and 15 to 17 of each period, to maintain consistent elevation of peripheral serotonin throughout the period. Milk and blood samples were collected in the last 96 h of each period. Whole-blood serotonin concentration was elevated above saline control for 96 h after the last 5-HTP infusion. Dry matter intake was decreased for cows receiving 5-HTP, and on average they lost body weight over the 21-d period, in contrast to saline cows who gained body weight. Milk production and milk protein yield were lower in cows receiving 5-HTP during the 3 infusion days, but both recovered to saline cow yields in the days after. Although milk fat yield exhibited a day-by-treatment interaction, no significant difference occurred on any given day. Milk urea nitrogen concentration was lower in 5-HTP cows on the days following the end of infusions, but not different from saline cows on infusion days. Meanwhile, plasma urea nitrogen was not affected by 5-HTP infusion. Circulating concentrations of AA were overall transiently decreased by 5-HTP, with concentrations mostly returning to baseline within 7 h after the end of 5-HTP infusion. Mammary extraction efficiency of AA was unaffected by 5-HTP infusion. Overall, both lactation performance and circulating AA were transiently reduced in cows infused with 5-HTP, despite sustained elevation of circulating serotonin concentration.

Crosstalk Between Intestinal Serotonergic System and Pattern Recognition Receptors on the Microbiota-Gut-Brain Axis

Disruption of the microbiota-gut-brain axis results in a wide range of pathologies that are affected, from the brain to the intestine. Gut hormones released by enteroendocrine cells to the gastrointestinal (GI) tract are important signaling molecules within this axis. In the search for the language that allows microbiota to communicate with the gut and the brain, serotonin seems to be the most important mediator. In recent years, serotonin has emerged as a key neurotransmitter in the gut-brain axis because it largely contributes to both GI and brain physiology. In addition, intestinal microbiota are crucial in serotonin signaling, which gives more relevance to the role of the serotonin as an important mediator in microbiota-host interactions. Despite the numerous investigations focused on the gut-brain axis and the pathologies associated, little is known regarding how serotonin can mediate in the microbiota-gut-brain axis. In this review, we will mainly discuss serotonergic system modulation by microbiota as a pathway of communication between intestinal microbes and the body on the microbiota-gut-brain axis, and we explore novel therapeutic approaches for GI diseases and mental disorders.

The Apelinergic System Immuno-Detection in the Abomasum and Duodenum of Sheep Grazing on Semi-Natural Pasture

Simple Summary

The semi-natural pastures in the Apennines represent the feed source for ovine, whose grazing activity helps to preserve the grassland’s biodiversity. Summer drought stress decreases the grassland pastoral value and affects the morpho-functional features of sheep’s digestive systems. A better knowledge of the gastrointestinal system of sheep may contribute to guaranteeing their welfare, a prerequisite for the sustainability of livestock production. This study aimed to immune-localize the apelinergic system in the abomasum and duodenum of sheep grazing on semi-natural pasture during the spring–summer season and to compare its behavior among animal groups fed with or without supplementation. The apelinergic system, composed of apelin and its receptor, is involved in foodintake and the secretion and absorption activities of the digestive apparatus. Apelinergic system molecules were localized at the abomasum lining epithelium and fundic glands level and at the duodenum lining and crypt epithelium, in addition to the neuroendocrine cells. Variations in reactivity were observed in the different feed groups; feed supplementation seemed to maintain the functionality of the apelinergic system in the organs near the status related to the better pasture phase, suggesting that it may be a suitable solution able to counteract the harmful effects of summer drought stress.

Abstract

Apelin (APLN) is an adipokine mainly produced by adipose tissue and related to an individual’s nutritional status as well as digestive apparatus functions. In this work, APLN and its receptor (APLNR) were investigated, by immunohistochemistry, in the abomasum and duodenum of 15 Comisana × Appenninica adult sheep reared in a semi-natural pasture. Organ samples were collected after maximum pasture flowering (M × F group) and after maximum pasture dryness (M × D group); the experimental group (E × p group) received a feed supplementation of 600 grams/day/head of barley and corn in addition to M × D group feeding. APLN and APLNR were identified in the lining epithelium and the fundic gland chief cells of the abomasum. APLNR was observed in the lining epithelium, in the crypts and the serotonin secreting cells of the duodenum. Similar reactivity was observed between the M × F and E × p groups, while the M × D group showed a lower intensity of immunostaining for both APLN and APLNR in all positive structures but the duodenal serotonin neuroendocrine cells. Hence, our findings show that the E × p group presents a picture quite overlapped with M × F and suggest that food supplementation has a maintaining effect on the apelinergic system expression in the investigated digestive tracts of the sheep.

Melatonin inhibits serotonin transporter activity in intestinal epithelial cells

Gastrointestinal serotonin (5-HT) and melatonin are two closely related neuromodulators which are synthesised in the enterochromaffin cells of the intestinal epithelium and which have been shown to be involved in the physiopathology of the gastrointestinal tract. The effects of 5-HT depend on 5-HT availability which is, in part, modulated by the serotonin transporter (SERT). This transporter provides an efficient 5-HT uptake after release and is expressed in the membrane of the enterocytes. Although the origin and effects of 5-HT and melatonin are similar, the interrelationship between them in the gastrointestinal tract is unknown. The main aim of this study was to determine whether melatonin affects SERT activity and expression, and, if so, to elucidate the mechanisms involved. Caco-2 cell line was used to carry out the study as these cells have been shown to endogenously express SERT. The results showed that melatonin inhibits SERT activity by affecting both V(max) and kt kinetic constants although SERT synthesis or intracellular trafficking did not appear to be affected. The melatonin effect seemed to be independent of melatonin receptors MT(1) and MT(2) and protein kinase C and cAMP intracellular pathways. Our results suggest that the inhibition of SERT might be due to a catalytic effect of melatonin on the allosteric citalopram-sensitive site in SERT. This study shows, for the first time, that melatonin modulates SERT activity, thus demonstrating the feedback system between melatonin and the serotoninergic system in the gastrointestinal tract.

Lipopolysaccharide induces alteration of serotonin transporter in human intestinal epithelial cells

Intestinal serotoninergic activity and serotonin transporter (SERT) function have been shown to be altered in intestinal inflammatory diseases. Serotonin (5-HT) plays a critical role in the regulation of gastrointestinal physiology. Activity of 5-HT depends on its extracellular availability, partly modulated by SERT that transports 5-HT into the cell. Lipopolysaccharide (LPS) is a component of Gram-negative bacteria outer membrane, which acts as a potent activator of the inflammatory system in the intestine. The aim of this work was to determine, in the enterocyte-like cell line Caco-2, whether LPS treatment affects serotoninergic activity by acting on SERT. The results demonstrate that LPS treatment diminishes SERT activity in a dose- and period-dependent way. The kinetic study shows that V(max) was significantly reduced after treatment with LPS. The LPS effect on 5-HT uptake was, in part, mediated by protein kinase C (PKC) activation. The molecular expression of SERT revealed that LPS treatment did not affect the mRNA level or the SERT protein content in cell homogenate. The level of SERT protein, however, was reduced on brush border membrane. The LPS effect might be due to an alteration of the intracellular traffic of SERT which may, in part, be mediated by PKC activity.

Regulation of serotonin transporter activity by adenosine in intestinal epithelial cells

Serotonin plays a critical role in the regulation of intestinal physiology. The serotonin transporter (SERT) expressed in the intestinal epithelium determines 5-HT availability and activity. The serotoninergic system and SERT activity have been described as being altered in chronic intestinal pathologies such as inflammatory diseases. Adenosine has also been shown to be involved in a variety of intestinal functions and to play a central role in the regulation of inflammatory responses of injured tissue. Since the modulation of SERT by adenosine in the intestine remains unknown, the aim of the present work was to study the effect of adenosine on SERT activity and expression and to determine the molecular mechanism involved. The study has been carried out using human enterocyte-like Caco-2 cells which endogenously express SERT. The results show that adenosine diminishes SERT activity in both the apical and basal membranes by acting in the intrinsic molecule with no alteration of either SERT mRNA or protein levels. The effect of adenosine appears to be mediated by A(2) receptors and activation of the cAMP/PKA signalling pathway. Moreover, the adenosine effect did not seem to involve the activation of AMP activated protein kinase. Adenosine effects are reached at high concentrations, which suggests that adenosine modulation of SERT may be significant under conditions of inflammation and tissue injury.

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.

Regulation of the human serotonin transporter mediated by long-term action of serotonin in Caco-2 cells

AIM

The aim of this study was to determine the effect of long-term serotonin (5-hydroxytryptamine, 5-HT) treatment on the human serotonin transporter (hSERT) function and its expression.

METHODS

This study was carried out in the enterocyte-like cell line Caco-2. These cells constitutively express the hSERT and have been shown to be an excellent model for the study of this protein. We measured serotonin transport, levels of mRNA expression and of the SERT protein after treating the cells with serotonin.

RESULTS

Serotonin treatment diminished hSERT activity in a concentration and period-dependent way by increasing the K(t) value and reducing V(max). This inhibition was reversible and was not mediated by either the action of 5-HT(2), 5-HT(3) or 5-HT(4) receptors, or by the intracellular second messengers, protein kinase C and cAMP. 5-HT did not seem to affect either the mRNA level of the SERT or the protein transporter measured in either the membrane or the cell lysate. The 5-HT treatment effect was additive to the inhibitory effect of treatment with a low concentration of citalopram and fluoxetine. Nevertheless, 5-HT did not increase the inhibition yielded by treatment with high concentration citalopram.

CONCLUSION

The chronic increase in serotonin in the extracellular medium diminishes the function of the SERT. This effect seems to be due to an effect on the transporter molecule itself in the membrane, without altering protein synthesis, intracellular traffic, or its availability.

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.

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.

Up-regulation of P-glycoprotein expression in small intestine under chronic serotonin-depleted conditions in rats

To investigate the role of serotonin (5-HT), an important neurotransmitter and hormone/paracrine agent in the small intestine, in the transport activity of P-glycoprotein (P-gp), the intestinal transport of quinidine, a P-gp substrate, was examined in 5-HT-depleted rats prepared by intraperitoneal administration of p-chlorophenylalanine, a specific inhibitor of tryptophan hydroxylase in 5-HT biosynthesis. In the in vitro transport study, quinidine transport across rat jejunum was significantly enhanced in both the secretory and absorptive directions under 5-HT-depleted conditions, although the secretory transport was still predominant. The electrophysiological study suggested that the quinidine transport via passive diffusion was enhanced presumably through a paracellular route. This might be due to looser tight junctions under 5-HT-depleted conditions. The voltage-clamp technique clearly indicated that the secretory transport of quinidine through the transcellular pathway was also enhanced by the depletion of 5-HT. Furthermore, 5-HT depletion increased verapamil-sensitive secretory transport of quinidine in rat jejunum. These results indicate that the secretory transport of quinidine via P-gp was significantly enhanced under 5-HT-depleted conditions. The level of ATP, an energy source for functioning P-gp, wet weight of jejunum, and total protein level in rat jejunal mucosa were not changed by 5-HT depletion, but the expression of P-gp in the brush-border membrane of rat jejunum was significantly induced, which is partly responsible for the enhancement of P-gp activity under the 5-HT-depleted condition.

In vitro and in vivo effect of fluoxetine on the permeability of3H-serotonin across rat intestine

The aim of this work was to characterize the mucosal-to-serosal (apical to basolateral; AP-BL) and serosal-to-mucosal (basolateral to apical; BL-AP) transport of serotonin (5-HT) across rat jejunum, ileum, and colon, and to determine the influence of serotonin neuronal transporter inhibitors on this transport. The AP-BL apparent permeability (Papp) of3H-5-HT increased in the order colon = jejunum < ileum, and the BL-AP Pappof3H-5-HT increased in the order colon < jejunum = ileum. In vitro, neither fluoxetine (0.02 or 0.2 µmol/L) nor desipramine (0.4 or 4 µmol/L) had a significant effect upon the AP-BL or BL-AP Pappof3H-5-HT in any of the intestinal regions. However, fluoxetine (0.2 µmol/L) decreased the accumulation of3H-5-HT in the ileum (to 65% of control) in the BL-AP experiments. In vivo, chronic fluoxetine (10 mg/kg daily administered orally for 15 days), as assessed in the ileum, significantly increased (to ±180% of control levels) the BL-AP Pappof3H-5-HT and tended to increase the AP-BL Pappof3H-5-HT. In conclusion, the increase in the Pappof3H-5-HT after chronic administration of fluoxetine suggests that this treatment is able to increase the extracellular concentration of3H-5-HT at the intestinal level.Key words: fluoxetine, serotonin, rat intestine, permeability.

Uptake of serotonin at the apical and basolateral membranes of human intestinal epithelial (Caco-2) cells occurs through the neuronal serotonin transporter (SERT)

Serotonin plays important physiological functions at the intestinal level. However, nothing is known concerning its inactivation mechanisms in the human intestine. So, the aim of this work was to characterize the uptake of serotonin at the apical and basolateral membranes of human intestinal epithelial (Caco-2) cells. Uptake of [3H]serotonin at the apical membrane of Caco-2 cells was specific and Na+-, Cl--, and potential-dependent. It was concentration dependently inhibited by several monoamines (with the following rank order of potency: serotonin >> dopamine > or = noradrenaline) and tricyclic and nontricyclic antidepressants (with the following rank order of potency: fluoxetine > desipramine > cocaine > GBR 12909). In contrast, it was not affected by corticosterone (0.01-100 micro M) and was only partially inhibited by decynium-22 (0.001-10 micro M). Transepithelial apparent permeability (Papp) to [3H]serotonin in the apical-to-basolateral direction was reduced by desipramine (0.4 micro M) and fluoxetine (0.02 micro M), and it was not Na+-dependent nor affected by corticosterone (100 micro M). Uptake of [3H]serotonin at the basolateral membrane of Caco-2 cells was Na+-dependent and reduced by desipramine (0.4 micro M) and fluoxetine (0.02 micro M), and it was not affected by corticosterone (100 micro M). The Papp to [3H]serotonin in the basolateral-to-apical direction was reduced by desipramine (0.4 micro M) and fluoxetine (0.02 micro M), and it was not affected by Na+ omission or by corticosterone (100 micro M). Reverse transcriptase-polymerase chain reaction indicates that mRNA of the neuronal serotonin transporter (SERT) is present in Caco-2 cells and in human small intestine. In conclusion, these results suggest that human intestinal epithelial Caco-2 cells functionally express SERT, both at their apical and basolateral cell membranes.

Serotonin in the rabbit ileum: localization, uptake, and effect on motility

Repeated experiments to localise serotonin in the myenteric plexus of rabbit ileum failed. After preincubation in serotonin (10(-5) M), an extensive varicose fibre system was detected by immunocytochemical methods. Stained fibres left the myenteric plexus and ran to the muscle layers. Labelled cell bodies could not be found, even after pretreatment with colchicine or pargyline. Application of reserpine (10(-5) M) and fluoxetine (10(-5) M) prevented serotonin uptake. Antisera against tryptophan hydroxylase revealed a rich fibre system, including those processes that entered the tertiary plexus. These fibres were able to accumulate serotonin, but again the cell bodies could not be detected. Serotonin caused concentration-dependent contraction in the longitudinal muscle layer of the rabbit ileum. Pretreatment with tetrodotoxin strongly reduced the effect of serotonin. Preapplication of atropine caused a slight decrease of response evoked by serotonin. Combined administration of tetrodotoxin and atropine significantly reduced the responses to serotonin, but did not abolish them. At the same time, agonists of 5-HT(2) and 5-HT(4) receptors caused concentration-dependent contractions. Our studies show that: 1). Without pretreatment, serotonin cannot be detected in the myenteric plexus of rabbit ileum. 2). An extensive uptake system works in this plexus. If released from myenteric nerve fibres, serotonin may evoke contractions in indirect and direct ways. 3). There may be an extrinsic serotoninergic innervation from the mesenteric ganglia. 4). Serotonin exerts its effect through 5-HT(2) and 5-HT(4) receptors on smooth muscle cells and nerve elements.

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.

Effects of serotonin on the physiology of the rabbit small intestine

Serotonin has been shown to alter the intestinal transport of ions and intestinal motility. These effects may interfere with each other, modulating the whole physiology of the intestine. We have previously shown that serotonin also alters the transport of nutrients. Thus, the aims of the present work were to determine the possible interference between the secretagogue effect of serotonin and the mechanism by which serotonin inhibits the absorption of nutrients, and to study the effect of serotonin on the digestive activity of nutrients of the brush border membrane jejunum enterocyte in the rabbit. The results show that the secretagogue effect of serotonin neither affects the inhibitory effect of serotonin on the intestinal absorption of the nutrients, nor affects the activity of Na+/K+-ATPase. The activity of sucrase and aminopeptidase N was also not affected by serotonin in the rabbit jejunum. Finally, we also studied different parameters of the motility in the rabbit small intestine. Serotonin seemed to stimulate the motility of the rabbit small intestine by increasing integrated mechanical activity and tone of muscle fibers in duodenum, jejunum, and ileum. In conclusion, serotonin might alter or modulate the whole intestinal physiology.

Key words: serotonin, small intestine, motility, absorption, rabbit.