Facilitation by endogenous acetylcholine and nitric oxide of luminal serotonin release from the guinea-pig colon

Morphologies and distributions of 5-HT containing enteroendocrine cells in the mouse large intestine

Serotonin (5-HT)-containing gastrointestinal endocrine cells contribute to regulation of numerous bodily functions, but whether these functions are related to differences in cell shape is not known. The current study identified morphologies and localization of subtypes of 5-HT-containing enteroendocrine cells in the mouse large intestine. 5-HT cells were most frequent in the proximal colon compared with cecum and distal colon. The large intestine harbored both open (O) cells, with apical processes that reached the lumen, and closed (C) cells, not contacting the lumen, classified into O1, O2, and O3 and C1, C2, and C3 cells, by the lengths of their basal processes. O1 and C1 cells, with basal processes sometimes longer that 100 µm, were most common in the distal colon. Their long basal processes ran against the inner surfaces of the mucosal epithelial cells and were strongly immunoreactive for 5-HT; these processes are ideally placed to communicate with the epithelium and to react to mechanical forces. O2 and C2 cells that had similar but shorter basal processes were also most common in the distal colon. O3 and C3 cells had no or very short basal processes. The O3 open type 5-HT cells were abundant in the proximal colon, particularly at the luminal surface, where they could release 5-HT into the lumen to act on luminal 5-HT receptors. Numerous O3 type 5-HT cells occurred in the lower (submucosal) region of the crypts in all segments and might release 5-HT to influence cell renewal in the crypt proliferative zones.

Certain Diet and Lifestyle May Contribute to Islet β-cells Protection in Type-2 Diabetes via the Modulation of Cellular PI3K/AKT Pathway

PI3K/AKT pathway has been shown to play a pivotal role on islet β-cell protection, enhancing β-cell survival by stimulating cell proliferation and inhibiting cell apoptosis. Accordingly, this pathway appears to be crucial in type-2 diabetes. Understanding the regulations of this pathway may provide a better efficacy of new therapeutic approaches. In this review, we summarize advances on the involvement of the PI3K/AKT pathway in hypothetical intra-cellular signaling of islet β-cells. As recent findings may show the nutritional regulation of the survival pathway in the islet β-cells through activation of the PI3K/AKT pathway, we also review studies on the features of several diets, correlated lifestyle, and its signaling pathway involved in type-2 diabetes. The molecular mechanisms contributing to the disease are the subject of considerable investigation, as a better understanding of the pathogenesis will lead to novel therapies against a condition of the disease.

Luminal amino acid sensing in the rat gastric mucosa

Recent advancements in molecular biology in the field of taste perception in the oral cavity have raised the possibility for ingested nutrients to be "tasted" in the upper gastrointestinal tract. The purpose of this study was to identify the existence of a nutrient-sensing system by the vagus in the rat stomach. Afferent fibers of the gastric branch increased their firing rate solely with the intragastric application of the amino acid glutamate. Other amino acids failed to have the same effect. This response to glutamate was blocked by the depletion of serotonin (5-HT) and inhibition of serotonin receptor(3) (5-HT(3)) or nitric oxide (NO) synthase enzyme. Luminal perfusion with the local anesthesia lidocaine abolished the glutamate-evoked afferent activation. The afferent response was also mimicked by luminal perfusion with a NO donor, sodium nitroprusside. In addition, the NO donor-induced afferent activation was abolished by 5-HT(3) blockade as well. Altogether, these results strongly suggest the existence of a sensing system for glutamate in the rat gastric mucosa. Thus luminal glutamate would enhance the electrophysiological firing rate of afferent fibers from the vagus nerve of the stomach through the production of mucosal bioactive substances such as NO and 5-HT. Assuming there is a universal coexistence of free glutamate with dietary protein, a glutamate-sensing system in the stomach could contribute to the gastric phase of protein digestion.

The suppressant effect of GEA3162 on spontaneous serotonin release from human colonic mucosa in vitro

The effect of a lipophilic nitric oxide (NO)-releasing compound 5-amino-3-(3,4-dichlorophenyl) 1,2,3,4-oxatriazolium (GEA3162) on the spontaneous release of 5-hydroxytryptamine (5-HT) from human colonic mucosa was investigated in vitro. In the presence of tetrodotoxin, spontaneous outflow of 5-HT from the human colonic mucosa was measured by high-performance liquid chromatography with electrochemical detection. GEA3162 concentration-dependently suppressed the 5-HT outflow, but neither the NO-activated soluble guanylate cyclase inhibitor 1H-[1,2,4]-oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) nor peroxynitrite scavenger ebselen affected the suppressant effect of GEA3162. Moreover, neither the L-type calcium channel blocker nicardipine, NO synthase inhibitor l-N(G)-nitroarginine methyl ester nor guanylate cyclase activator guanylin affected the spontaneous 5-HT outflow. These results indicate that human colonic mucosa is capable of eliciting tetrodotoxin-resistant and nicardipine-insensitive 5-HT release, and that GEA3162 can suppress the 5-HT release via an action on colonic mucosa through mechanism independent of ODQ-sensitive cyclic GMP system or peroxynitrite generation.

Loperamide inhibits tachykinin NK3-receptor-triggered serotonin release without affecting NK2-receptor-triggered serotonin release from guinea pig colonic mucosa

The effect of loperamide on tachykinin NK(2)- and NK(3)-receptor-mediated 5-HT outflow from guinea pig colonic mucosa was investigated in vitro. The selective tachykinin NK(2)-receptor agonist [beta-Ala(8)]-neurokinin A(4-10) (betaAla-NKA) or the selective NK(3)-receptor agonist senktide elicited an increase in 5-HT outflow from whole colonic strips, but not from mucosa-free muscle layer preparations. The enhancing effect of betaAla-NKA and senktide was prevented by the selective NK(2)-receptor antagonist GR94800 or the selective NK(3)-receptor antagonist SB222200. Loperamide concentration-dependently suppressed the senktide-evoked 5-HT outflow, but failed to affect the betaAla-NKA-evoked 5-HT outflow. The kappa-opioid receptor antagonist nor-binaltorphimine or the delta-opioid receptor antagonist naltrindole displaced the concentration-response curve for the suppressant action of loperamide to the right without significant depression of the maximum. However, the mu-opioid receptor antagonist CTOP did not affect the suppressant effect of loperamide. We concluded that the NK(3) receptor-triggered 5-HT release from colonic mucosa is suppressed by loperamide-sensitive mechanisms, whereas the NK(2)-receptor-triggered 5-HT release is loperamide-insensitive. Our data also suggest that the suppressant effect of loperamide is probably mediated by the activation of kappa- and delta-opioid receptors located on intrinsic neurons.

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

Modification of 5-hydroxytryptophan-evoked 5-hydroxytryptamine formation of guinea pig colonic mucosa by reactive oxygen species

We studied whether reactive oxygen species (ROS) generated by normal colonic mucosa affect 5-hydroxytryptophan (5-HTP)-evoked 5-HT formation (measured as the sum of 5-HT plus 5-hydroxyindole acetic acid (5-HIAA) accumulation) of guinea pig's isolated colonic mucosa. Catalase (3000-6000 U/ml), a hydrogen peroxide (H2O2) scavenger or diphenylene iodonium (DPI, 10-100 microM), an NADPH oxidase inhibitor, concentration-dependently caused an increase of the sum of 5-HT plus 5-HIAA accumulation in the presence of 5-HTP (10 microM), but these drugs did not significantly affect the 5-HT-metabolite in the colonic mucosa measured as the ratio of 5-HIAA/5-HT. Exogenously applied H2O2 (10-100 microM) concentration-dependently inhibited the sum of 5-HT plus 5-HIAA accumulation. In contrast, neither superoxide dismutase (SOD, 100-300 U/ml), superoxide anion scavenger, nor dimetyl sulfoxide (1-5%, DMSO), a hydroxyl radical scavenger affected the sum of 5-HT plus 5-HIAA accumulation. Moreover, mucosa ROS generation was estimated using the chemiluminescence technique. SOD (100-300 U/ml), catalase (3000-6000 U/ml) or DPI (10-100 microM), concentration-dependently reduced luminol-enhanced chemiluminescence signal from the colonic mucosa, while allopurinol (10-100 microM), a xanthine oxidase inhibitor, did not affect the chemiluminescence signal. These results suggest that ROS is formed through an NADPH oxidase system in the guinea pig colonic mucosa, where it exerts a modulatory effect on mucosal 5-HT formation upon addition of 5-HTP. Thus, ROS formation from normal colonic mucosa could be considered to contribute to the control of 5-HT production in mucosa enterochromaffin cells.

Investigation into the 5-hydroxytryptophan-evoked luminal 5-hydroxytryptamine release from the guinea pig colon

The effect of an endogenous 5-hydroxytryptamine (5-HT) precursor, 5-hydroxytryptophan (5-HTP), on the luminal outflow of 5-HT was examined using the luminally perfused isolated colon of the guinea pig, a model that would facilitate the pharmacological analysis of luminal 5-HT release from enterochromaffin cells (EC cells). 5-HTP (1-10 microM) concentration-dependently caused an increase of the luminal outflow of 5-HT. Either tetrodotoxin (0.3 microM) or atropine (0.2 microM) did not affect the 5-HTP-evoked increase in luminal 5-HT outflow, while the L-type calcium channel blocker, nicardipine (1 microM) or diltiazem (1 microM) reduced the 5-HTP-evoked 5-HT outflow by 47% and 61%, respectively. SB203186 (1 microM), a 5-HT4-receptor antagonist, enhanced the 5-HTP-evoked 5-HT outflow, while ramosetron (1 microM), a 5-HT3-receptor antagonist reduced the stimulating effect of 5-HTP by 66%. Ketanserin (0.1 microM), a 5-HT2A-receptor antagonist did not modify the stimulatory effect of 5-HTP. It is concluded that in the guinea pig colon, 5-HTP facilitates the luminal 5-HT release from EC cells, with no involvement of neuronal mechanisms and a non-neuronal cholinergic system. Furthermore, non-neuronal 5-HT3 and 5-HT4 receptors appear to contribute to the regulation of the luminal 5-HT release evoked by 5-HTP. This new bioassay of the guinea pig colon allows the pharmacological characterization of uncomplicated luminal 5-HT release from EC cells.

KW-5092, a novel gastrokinetic agent, facilitates luminal serotonin release from the guinea-pig colon

The present study was designed to determine the influence of KW-5092 ((1-[2-[[[5-(piperidinomethyl)-2-furanyl]methyl]amino]ethyl]-2- imidazolidinylidene) propanedinitrile fumarate), a novel gastroprokinetic agent on intraluminal serotonin (5-hydroxytryptamine, 5-HT) release which reflects the release of 5-HT from enterochromaffin cells, using the luminally perfused isolated guinea-pig proximal colon in vitro. 5-HT was determined by high-performance liquid chromatography with electrochemical detection. KW-5092 (1-10 microM) concentration-dependently caused an increase in the luminal 5-HT outflow. In the presence of atropine (0.2 microM) or tetrodotoxin (0.3 microM), the stimulatory action of KW-5092 (10 microM) was inhibited by 94% and 74%, respectively. These results suggest that KW-5092 stimulates intraluminal 5-HT release from luminally perfused proximal colon of the guinea-pig via the stimulation of cholinergic neurons. Because 5-HT is recognized as an important messenger substance in the control of intestinal motility, this stimulatory effect could be considered as an indirect action of KW-5092 that may contribute to its prokinetic effects.