Uptake of Serotonin at the Apical and Basolateral Membranes of Human Intestinal Epithelial (Caco-2) Cells Occurs through the Neuronal Serotonin Transporter (SERT)

Abstract

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 [ 3 H]serotonin at the apical membrane of Caco-2 cells was specific and Na ϩ -, Cl Ϫ -, and potentialdependent. It was concentration dependently inhibited by several monoamines (with the following rank order of potency: serotonin Ͼ Ͼ dopamine Ն noradrenaline) and tricyclic and nontricyclic antidepressants (with the following rank order of potency: fluoxetine Ͼ desipramine Ͼ cocaine Ͼ GBR 12909). In contrast, it was not affected by corticosterone (0.01-100 M) and was only partially inhibited by decynium-22 (0.001-10 M). Transepithelial apparent permeability (P app ) to [ 3 H]serotonin in the apical-to-basolateral direction was reduced by desipramine (0.4 M) and fluoxetine (0.02 M), and it was not Na ϩ -dependent nor affected by corticosterone (100 M). Uptake of [3 H]serotonin at the basolateral membrane of Caco-2 cells was Na ϩ -dependent and reduced by desipramine (0.4 M) and fluoxetine (0.02 M), and it was not affected by corticosterone (100 M).The P app to [3 H]serotonin in the basolateral-to-apical direction was reduced by desipramine (0.4 M) and fluoxetine (0.02 M), and it was not affected by Na ϩ omission or by corticosterone (100 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.The largest store of serotonin in the body is found in the gastrointestinal tract, corresponding to over 95% of the body's serotonin (Erspamer, 1966). Most of the gastrointestinal serotonin is contained in enterochromaffin (EC) cells of the mucosal epithelium, within which serotonin is synthesized from L-tryptophan and stored in secretory granules. Serotonin is also present in serotonergic neurons of the enteric nervous system. Several functions of enteric serotonin have been identified in the past years. First, serotonin acts as the neurotransmitter of a subset of myenteric interneurons (Wade et al., 1994). Second, serotonin contained in EC cells, being released in response to chemical or mechanical stimuli, affects gastrointestinal motility (it initiates peristaltic reflexes by acting on intrinsic sensory neurons and musculature) and intestinal electrolyte transport (it initiates secretory reflexes by acting on intrinsic sensory neurons and mucosal cells) (Engel et al., 1984;Imada-Shirakata et al., 1997). Moreover, extrinsic sensory neurons activated by serotonin initiate sensations from the bowel, which may include nausea, bloating, and pain (for review, see Gershon, 1999). Additionally, serotonin present in EC cells...

IFN-γ and TNF-α decrease serotonin transporter function and expression in Caco2 cells

“…To elucidate whether naturally produced inflammatory mediators alter SERT function in Caco2 cells, human lymphocytes were isolated and activated with either the bacterial endotoxin (LPS) or E. coli. Wild-type Caco2 cells take up radiolabeled 5-HT in a fluoxetine-sensitive manner (24). We have found this uptake to be between 90 and 95% specific and temperature sensitive (data not shown).…”

“…The goal of this study was to identify potential proinflammatory mediators that can decrease SERT function, as seen in animal models of intestinal inflammation as well as in human patients with IBS. For these studies we have used an immortalized human intestinal epithelial cell line (Caco2 cells), which natively express SERT (24). Originally derived from a human carcinoma of the colon, these cells form a polarized monolayer when grown in vitro (12,13,17).…”

Regulation of serotonin transporter activity by adenosine in intestinal epithelial cells

“…5-HT effects on intestinal physiology depend on 5-HT availability, which is, in part, regulated by SERT expressed in enterocytes [6,7]. Recent reports have confirmed the relevance of the SERT function for the control of intestinal activity [30,31].…”

“…This transporter is responsible for 5-HT uptake to enable its clearance from the extracellular milieu, thus determining 5-HT availability. SERT expression and activity have been described in the epithelial cells of the GI mucosa [5][6][7][8]. Changes in SERT activity can lead to alteration in the GI function, as evidenced by the GI side-effects associated with SERT selective inhibitors [9] that are widely used in disturbances in which serotoninergic activity is affected.…”

“…The aim of this work was to assess whether adenosine affects SERT activity and expression in human enterocyte-like Caco-2 cells and thus to elucidate possible mediators of SERT alteration in intestinal inflammation. The Caco-2 cell line has been used to carry out this study as it has been described as constitutively expressing SERT [6,7] and results obtained in our laboratory have shown that these cells are an excellent model for the study of intestinal SERT regulation [22,23].…”