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

Oxidative Stress is Closely Associated with Increased Arterial Stiffness, Especially in Aged Male Smokers without Previous Cardiovascular Events: A Cross-Sectional Study

Aim: Cigarette smoking is one of the major risk factors for cardiovascular diseases and induces deleterious vascular damage. Oxidative stress is involved in vascular inflammation, the process of atherosclerosis. The purpose of the present study was to investigate whether the effects of oxidative stress on the arterial wall differ between smokers and non-smokers.

Methods: Male smokers and non-smokers without physical deconditioning who visited Enshu hospital for an annual physical check-up were enrolled in the study. To assess oxidative stress, serum levels of derivative reactive oxygen metabolites (d-ROM) were measured. The radial augmentation index (RAI) was measured using an automated device and was used as an index for arterial stiffness.

Results: Univariate and multivariate linear regression analysis showed that RAI was independently associated with d-ROM levels only in smokers. Moreover, RAI was significantly higher in smokers than in non-smokers. Logistic regression analysis with the endpoint of a higher RAI than the mean revealed that older age (> 65 years), hypertension, and smoking were independently associated with higher RAI. Similarly, logistic regression analysis with the endpoint of higher d-ROM levels than the mean showed that older age and smoking were independently associated with higher d-ROM levels.

Conclusions: Increased RAI is significantly associated with smoking and, in smokers, with increased d-ROM levels. These results suggest that the effects of oxidative stress on arterial properties differ between smokers and non-smokers and that oxidative stress is closely associated with arterial stiffness, especially in smokers.

Luminal regulation of normal and neoplastic human EC cell serotonin release is mediated by bile salts, amines, tastants, and olfactants

Mechanisms by which gut luminal content regulates secretion and motility are ill understood. We evaluated whether neuroendocrine enterochromaffin (EC) cells act as luminal sensors for a wide variety of nutrients and defined the secretory mechanisms of this process. Pure (98-99%) FACS-sorted human EC cells and neoplastic EC cells (KRJ-I) were studied. RT-PCR identified transcripts for T2R1 (bitter), OR1G1 (class II olfactory) and trace amine (TAR1) G protein-coupled receptors (GPCRs) and transporters for glutamine (SNAT1/2), glucose (GLUT1/3/SGLT1), and bile salts (ABST). Glutamine and sodium deoxycholate stimulated 5-HT release (EC(50) = 0.002-0.2 microM; 2-fold release) but were 10-100 times more potent in neoplastic EC cells, which also secreted 6-13 times more 5-HT. Tastants (caffeine, tyramine, octopamine) and olfactants (thymol and eugenol) also stimulated normal and neoplastic EC cell 5-HT secretion (EC(50) = 1.2 nM to 2.1 microM and 0.05 nM to 0.1 microM release, respectively); 2-deoxyglucose and the artificial sweetener sucralose also stimulated (EC(50) = 9.2 and 0.38 nM). 5-HT release was associated with ERK phosphorylation (1.5-fold, P < 0.02) and could be inhibited by a somatostatin analog (IC(50) = 1 pM). Eleven secretory associated genes including the vesicle docking inhibitor STXBP3 were upregulated in response to glutamine and bile salt stimulation in neoplastic EC cells. Targeting STXBP3 expression by use of antisense knockdown significantly (P < 0.05) reduced 5-HT secretion. In conclusion, EC cells express GPCRs and transporters for luminal tastants, olfactants, glutamine, glucose, and bile salts. Activation includes a panel of secretory genes, ERK phosphorylation, and 5-HT secretion. Luminal EC cell regulation is likely to be as important as G cell regulation in gastric acid secretion; development of agents to target EC cell function is therefore a critical therapeutic goal.

High mucosal serotonin availability in neonatal guinea pig ileum is associated with low serotonin transporter expression

BACKGROUND & AIMS

5-Hydroxytryptamine (5-HT) is a neurotransmitter and paracrine signaling molecule in the gut. Paracrine signaling by enterochromaffin cells (EC), which release 5-HT, has not been studied in neonates. Our aim was to compare 5-HT disposition in the intestinal mucosa of neonatal and adult guinea pigs.

METHODS

5-HT was locally measured in vitro from intestinal segments using a diamond microelectrode and continuous amperometry. The serotonin transporter (SERT) was measured using immunohistochemical and Western blot techniques. 5-HT intestinal content was measured using immunohistochemistry and high-performance liquid chromatography with electrochemical detection.

RESULTS

An oxidation current, reflective of local 5-HT release, was recorded with the microelectrode near the mucosal surface, and this current was larger in neonatal than in adult tissues. Mechanically stimulating the mucosa with a fine glass probe evoked an additional current in adult but not neonatal tissues. Oxidation currents were reduced by tetrodotoxin and were blocked in calcium-free solutions. Fluoxetine (1 microM) potentiated oxidation currents in adult but not neonatal tissues. SERT levels were lower in neonatal vs adult tissues. There was no difference in 5-HT content between neonates and adults but 5-hydroxyindole acetic acid/5-HT ratios were higher in adults. EC cell counts showed no difference in cell number, but EC cells were found in the crypts in neonatal and along the villi in adult tissues.

CONCLUSIONS

SERT expression is low in neonates, and this is associated with high levels of free mucosal 5-HT and reduced metabolism. Postnatal maturation of 5-HT signaling may be important for development of neurohumoral control of intestinal motor reflexes.

The serotonin signaling system: from basic understanding to drug development for functional GI disorders

Serotonin is an important gastrointestinal signaling molecule. It is a paracrine messenger utilized by enterochromaffin (EC) cells, which function as sensory transducers. Serotonin activates intrinsic and extrinsic primary afferent neurons to, respectively, initiate peristaltic and secretory reflexes and to transmit information to the central nervous system. Serotonin is also a neurotransmitter utilized by a system of long descending myenteric interneurons. Serotonin is synthesized through the actions of 2 different tryptophan hydroxylases, TpH1 and TpH2, which are found, respectively, in EC cells and neurons. Serotonin is inactivated by the serotonin reuptake transporter (SERT)-mediated uptake into enterocytes or neurons. The presence of many serotonin receptor subtypes enables selective drugs to be designed to therapeutically modulate gastrointestinal motility, secretion, and sensation. Current examples include tegaserod, a 5-HT(4) partial agonist, which has been approved for treatment of irritable bowel syndrome (IBS) with constipation in women and for chronic constipation in men and women. The 5-HT(3) antagonists, granisetron and ondansetron, are useful in combating the nausea associated with cancer chemotherapy, and alosetron is employed in the treatment of IBS with diarrhea. Serotonergic signaling abnormalities have also been putatively implicated in the pathogenesis of functional bowel diseases. Other compounds, for which efficacy has not been rigorously established, but which may have value, include tricyclic antidepressants and serotonin selective reuptake inhibitors to combat IBS, and 5-HT(1) agonists, which enhance gastric accommodation, to treat functional dyspepsia. The initial success encountered with serotonergic agents holds promise for newer and more potent insights and therapies of brain-gut disorders.

Real-time measurement of serotonin release and motility in guinea pig ileum

Enterochromaffin (EC) cells are sensors that detect chemical or mechanical stimuli and respond with release of serotonin (5-HT). 5-HT activates local motor reflexes, but whether local motor reflexes also evoke 5-HT release is unknown. The aim of the present study was to establish the relationship between the release of 5-HT and the enteric neural circuits controlling the movements of the intestine. Recordings were made from full-thickness preparations of guinea pig ileum using electrochemical techniques with carbon fibre electrodes to measure local concentrations of 5-HT. The tension in the circular muscle (CM) and longitudinal muscle (LM) was recorded with force transducers. The release of 5-HT from the EC cells was detected selectively and the timing of the events quantified. Pressure-evoked peristalsis caused detectable 5-HT release only when the recording site was invaded by a ring of CM contraction. Spontaneous and stretch-evoked reflex contraction of the CM and LM occurred simultaneously with 5-HT release. Paralysis of the smooth muscle significantly reduced the stretch-evoked release. Muscarinic agonists evoked reflexes that were associated with increases in tension in CM and LM simultaneous with 5-HT release. Tetrodotoxin abolished the coordination between the CM contraction and 5-HT release but not the direct activation of the CM and EC cells by the agonists. In conclusion, the correlation between local motor reflexes and 5-HT release observed in the present study is caused primarily by the contraction of the smooth muscle and subsequent deformation of the mucosa. The EC cell is, thus, a site of convergence for mechanical forces that contribute to the release of 5-HT during motor reflexes.

Statistical distribution of blood serotonin as a predictor of early autistic brain abnormalities

BACKGROUND

A wide range of abnormalities has been reported in autistic brains, but these abnormalities may be the result of an earlier underlying developmental alteration that may no longer be evident by the time autism is diagnosed. The most consistent biological finding in autistic individuals has been their statistically elevated levels of 5-hydroxytryptamine (5-HT, serotonin) in blood platelets (platelet hyperserotonemia). The early developmental alteration of the autistic brain and the autistic platelet hyperserotonemia may be caused by the same biological factor expressed in the brain and outside the brain, respectively. Unlike the brain, blood platelets are short-lived and continue to be produced throughout the life span, suggesting that this factor may continue to operate outside the brain years after the brain is formed. The statistical distributions of the platelet 5-HT levels in normal and autistic groups have characteristic features and may contain information about the nature of this yet unidentified factor.

RESULTS

The identity of this factor was studied by using a novel, quantitative approach that was applied to published distributions of the platelet 5-HT levels in normal and autistic groups. It was shown that the published data are consistent with the hypothesis that a factor that interferes with brain development in autism may also regulate the release of 5-HT from gut enterochromaffin cells. Numerical analysis revealed that this factor may be non-functional in autistic individuals.

CONCLUSION

At least some biological factors, the abnormal function of which leads to the development of the autistic brain, may regulate the release of 5-HT from the gut years after birth. If the present model is correct, it will allow future efforts to be focused on a limited number of gene candidates, some of which have not been suspected to be involved in autism (such as the 5-HT4 receptor gene) based on currently available clinical and experimental studies.

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.

The inhibition of cholera toxin-induced 5-HT release by the 5-HT(3) receptor antagonist, granisetron, in the rat

1. The secretagogue 5-hydroxytryptamine (5-HT) is implicated in the pathophysiology of cholera. 5-HT released from enterochromaffin cells after cholera toxin exposure is thought to activate non-neuronally (5-HT(2) dependent) and neuronally (5-HT(3) dependent) mediated water and electrolyte secretion. CT-secretion can be reduced by preventing the release of 5-HT. Enterochromaffin cells possess numerous receptors that, under basal conditions, modulate 5-HT release. 2. These include basolateral 5-HT(3) receptors, the activation of which is known to enhance 5-HT release. 3. Until now, 5-HT(3) receptor antagonists (e.g. granisetron) have been thought to inhibit cholera toxin-induced fluid secretion by blockading 5-HT(3) receptors on secretory enteric neurones. Instead we postulated that they act by inhibiting cholera toxin-induced enterochromaffin cell degranulation. 4. Isolated intestinal segments in anaesthetized male Wistar rats, pre-treated with granisetron 75 microg kg(-1), lidoocaine 6 mg kg(-1) or saline, were instilled with a supramaximal dose of cholera toxin or saline. Net fluid movement was determined by small intestinal perfusion or gravimetry and small intestinal and luminal fluid 5-HT levels were determined by HPLC with fluorimetric detection. 5. Intraluminal 5-HT release was proportional to the reduction in tissue 5-HT levels and to the onset of water and electrolyte secretion, suggesting that luminal 5-HT levels reflect enterochromaffin cell activity. 6. Both lidocaine and granisetron inhibited fluid secretion. However, granisetron alone, and proportionately, reduced 5-HT release. 7. The simultaneous inhibition of 5-HT release and fluid secretion by granisetron suggests that 5-HT release from enterochromaffin cells is potentiated by endogenous 5-HT(3) receptors. The accentuated 5-HT release promotes cholera toxin-induced fluid secretion.

Nicotine stereoisomers and cotinine stimulate prostaglandin E2 but inhibit thromboxane B2 and leukotriene E4 synthesis in whole blood

The effects of (-)-nicotine (0.0005-500 microM), (+)-nicotine (0.0005-50 microM) and (-)-cotinine (0.0005-500 microM) on arachidonic acid metabolism were investigated in Ca2+ ionophore A23187 (calcimycin)-stimulated human whole blood in vitro. (-)-Nicotine and (-)-cotinine stimulated prostaglandin E2 but inhibited thromboxane B2 synthesis, as has been observed previously in A23187-stimulated polymorphonuclear leukocytes and platelet-rich plasma [Saareks, V., Riutta, A., Mucha, I., Alanko, J., Vapaatalo, H., 1993. Nicotine and cotinine modulate eicosanoid production in human leukocytes and platelet rich plasma. Eur. J. Pharmacol., 248, 345-349.]. (+)-Nicotine also stimulated prostaglandin E2 but inhibited thromboxane B2 synthesis. High concentrations of (-)-nicotine and (-)-cotinine and even nanomolar concentrations of (+)-nicotine inhibited leukotriene E4 synthesis. These results indicate that (-)-nicotine and (-)-cotinine stimulate cyclooxygenase but inhibit thromboxane synthase and 5-lipoxygenase in whole blood in vitro. (+)-Nicotine is capable of affecting in the same direction as well.

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.