Recent advances in understanding the role of serotonin in gastrointestinal motility in functional bowel disorders: alterations in 5-HT signalling and metabolism in human disease

Abstract  Serotonin (5-hydroxytryptamine, 5-HT) is present in abundance within the gut, most stored in enterochromaffin cell granules. It is released by a range of stimuli, most potently by mucosal stroking. Released 5-HT stimulates local enteric nervous reflexes to initiate secretion and propulsive motility. It also acts on vagal afferents altering motility and in large amounts induces nausea. Rapid reuptake by a specific transporter (serotonin transporter, SERT) limits its diffusion and actions. Abnormally increased 5-HT is found in a range of gastrointestinal disorders including chemotherapy-induced nausea and vomiting, carcinoid syndrome, coeliac disease, inflammatory bowel disease and irritable bowel syndrome (IBS) with diarrhoea (IBS-D), especially that developing following enteric infection. Impaired SERT has been described in IBS-D and might account for some of the increase in mucosal 5-HT availability. 5-HT₃ receptor antagonists inhibit chemotherapy-induced nausea and diarrhoea associated with both carcinoid syndrome and IBS. While IBS-D is associated with increased 5-HT postprandially, IBS with constipation (IBS-C) is associated with impaired 5-HT response and responds to 5-HT₄ agonists such as Prucalopride and 5-HT₄ partial agonists such as Tegaserod.     

Effect of cannabidiol on sepsis-induced motility disturbances in mice: involvement of CB1 receptors and fatty acid amide hydrolase

Abstract  Sepsis is an inflammatory condition that is associated with reduced propulsive gastrointestinal motility (ileus). A therapeutic option to treat sepsis is to promote intestinal propulsion preventing bacterial stasis, overgrowth and translocation. Recent evidence suggests that anti-oxidants improve sepsis-induced ileus. Cannabidiol, a non-psychotropic component of Cannabis sativa , exerts strong anti-oxidant and anti-inflammatory effects without binding to cannabinoid CB₁ or CB₂ receptors. Cannabidiol also regulates the activity of fatty acid amide hydrolase (FAAH) which is the main enzyme involved in endocannabinoid breakdown and which modulates gastrointestinal motility. Because of the therapeutic potential of cannabidiol in several pathologies, we investigated its effect on sepsis-induced ileus and on cannabinoid receptor and FAAH expression in the mouse intestine. Sepsis was induced by treating mice with lipopolysaccharides for 18 h. Sepsis led to a decrease in gastric emptying and intestinal transit. Cannabidiol further reduced gastrointestinal motility in septic mice but did not affect gastrointestinal motility in control mice. A low concentration of the CB₁ antagonist AM251 did not affect gastrointestinal motility in control mice but reversed the effect of cannabidiol in septic mice. Sepsis was associated with a selective upregulation of intestinal CB₁ receptors without affecting CB₂ receptor expression and with increased FAAH expression. The increase in FAAH expression was completely reversed by cannabidiol but not affected by AM251. Our results show that sepsis leads to an imbalance of the endocannabinoid system in the mouse intestine. Despite its proven anti-oxidant and anti-inflammatory properties, cannabidiol may be of limited use for the treatment of sepsis-induced ileus.     

Central regulatory mechanisms of visceral sensation in response to colonic distension with special reference to brain orexin

Visceral hypersensitivity is a major pathophysiology in irritable bowel syndrome (IBS). Although brain-gut interaction is considered to be involved in the regulation of visceral sensation, little had been known how brain controls visceral sensation. To improve therapeutic strategy in IBS, we should develop a novel approach to control visceral hypersensitivity. Here, we summarized recent data on central control of visceral sensation by neuropeptides in rats. Orexin, ghrelin or oxytocin in the brain is capable of inducing visceral antinociception. Dopamine, cannabinoid, adenosine, serotonin or opioid in the central nervous system (CNS) plays a role in the visceral hyposensitivity. Central ghrelin, levodopa or morphine could induce visceral antinociception via the orexinergic signaling. Orexin induces visceral antinociception through dopamine, cannabinoid, adenosine or oxytocin. Orexin nerve fibers are identified widely throughout the CNS and orexins are implicated in a number of functions. With regard to gastrointestinal functions, in addition to its visceral antinociception, orexin acts centrally to stimulate gastrointestinal motility and improve intestinal barrier function. Brain orexin is also involved in regulation of sleep/awake cycle and anti-depressive action. From these evidence, we would like to make a hypothesis that decreased orexin signaling in the brain may play a role in the pathophysiology in a part of patients with IBS who are frequently accompanied with sleep disturbance, depressive state and disturbed gut functions such as gut motility disturbance, leaky gut and visceral hypersensitivity.     

Importance of 5-hydroxytryptamine receptors on intestinal afferents in the regulation of visceral sensitivity

Abstract  Serotonin (5-HT) plays an important role as a signalling molecule in the gastrointestinal (GI) tract. The regulation of GI sensitivity via 5-HT is mediated by specific 5-HT receptor subytypes on intrinsic and extrinsic afferents. This review discusses visceral afferent hypersensitivity in irritable bowel syndrome (IBS) and the importance of 5HT₃, 5HT₄, and 5HT_2B receptor-mediated mechanisms in the regulation of visceral sensitivity.     

Do interactions between stress and immune responses lead to symptom exacerbations in irritable bowel syndrome?

Irritable bowel syndrome (IBS) is a common, debilitating gastrointestinal (GI) disorder, with a worldwide prevalence of between 10% and 20%. This functional gut disorder is characterized by episodic exacerbations of a cluster of symptoms including abdominal pain, bloating and altered bowel habit, including diarrhea and/or constipation. Risk factors for the development of IBS include a family history of the disorder, childhood trauma and prior gastrointestinal infection. It is generally accepted that brain–gut axis dysfunction is fundamental to the development of IBS; however the underlying pathophysiological mechanisms remain elusive. Additional considerations in comprehending the chronic relapsing pattern that typifies IBS symptoms are the effects of both psychosocial and infection-related stresses. Indeed, co-morbidity with mood disorders such as depression and anxiety is common in IBS. Accumulating evidence points to a role for a maladaptive stress response in the initiation, persistence and severity of IBS-associated symptom flare-ups. Moreover, mechanistically, the stress-induced secretion of corticotropin-releasing factor (CRF) is known to mediate changes in GI function. Activation of the immune system also appears to be important in the generation of IBS symptoms and increasing evidence now implicates low-grade inflammation or immune activation in IBS pathophysiology. There is a growing body of research focused on understanding at a molecular, cellular and in vivo level, the relationship between the dysregulated stress response and immune system alterations (either individually or in combination) in the etiology of IBS and to the occurrence of symptoms.     

The Role of the Endocannabinoid System in Atherosclerosis

Our current understanding of the pathophysiology of atherosclerosis suggests a prominent role for immune responses from its initiation through its complications. Given the increasing prevalence of cardiovascular risk factors worldwide, there is an urgent need to better understand the underlying mechanisms to improve current treatment protocols. A growing body of evidence suggests that endocannabinoid signalling plays a critical role in the pathogenesis of atherogenesis and its clinical manifestations. Blocking CB₁ receptors has been shown to mediate not only weight reduction, but also several cardiometabolic effects in rodents and humans, indicating a potential relevance for the process of atherosclerosis. Activation of CB₂ receptors with Δ⁹-tetrahydrocannabinol (THC) has been shown to inhibit atherosclerotic plaque progression in mice, mainly by inhibiting macrophage recruitment. Endocannabinoids released from endothelial cells, macrophages or platelets, reduce hypertension in rodents, a major risk factor for atherosclerosis. In addition, anandamide inhibits inflammatory gene expression in endothelial cells, and consequently monocyte adhesion. Conversely, endocannabinoids might also mediate pro-atherosclerotic effects by inducing platelet activation. In conclusion, the precise role of the endocannabinoid system during atherosclerosis is not yet understood. Whether increased endocannabinoid signalling is associated with disease progression and increased risk of acute thrombotic events remains to be determined.     

Serotonin and neuroprotection in functional bowel disorders

Abstract  The 5-HT₄ partial agonist tegaserod is effective in the treatment of chronic constipation and constipation predominant irritable bowel syndrome. 5-HT₄ receptors are located on presynaptic terminals in the enteric nervous system. Stimulation of 5-HT₄ receptors enhances the release of acetylcholine and calcitonin gene related peptide from stimulated nerve terminals. This action strengthens neurotransmission in prokinetic pathways, enhancing gastrointestinal motility. The knockout of 5-HT₄ receptors in mice not only slows gastrointestinal activity but also, after 1 month of age, increases the age-related loss of enteric neurons and decreases the size of neurons that survive. 5-HT₄ receptor agonists, tegaserod and RS67506, increase numbers of enteric neurons developing from precursor cells and/or surviving in culture; they also increase neurite outgrowth and decrease apoptosis. The 5-HT₄ receptor antagonist, GR113808, blocks all of these effects, which are thus specific and 5-HT₄-mediated. 5-HT₄ receptor agonists, therefore, are neuroprotective and neurotrophic for enteric neurons. Because the age-related decline in numbers of enteric neurons may contribute to the dysmotilities of the elderly, the possibility that the neuroprotective actions of 5-HT agonists can be utilized to prevent the occurrence or worsening of these conditions should be investigated.     

The incretin hormones GIP and GLP-1 in diabetic rats: effects on insulin secretion and small bowel motility

Abstract  Incretin hormones often display inhibitory actions on gut motility. The aim of this study was to investigate if altered responsiveness to glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide-1 (GLP-1) as regards insulin release and small bowel motility could bring further clarity to the pathophysiology of diabetes in the Goto-Kakizaki (GK) rat. The isolated perfused pancreas was studied in male GK and Wistar rats (controls) under euglycemic and hyperglycemic conditions. Glucose-dependent insulinotropic peptide (10 nmol L⁻¹) or GLP-1 (10 nmol L⁻¹) were added to the medium and perfusate was collected and analysed for insulin. Moreover, GK and Wistar rats were supplied with bipolar electrodes in the small bowel and myoelectric activity was recorded during intravenous administration of GIP (1–400 pmol kg⁻¹ min⁻¹) or GLP-1 (0.1–20 pmol kg⁻¹ min⁻¹). Finally, tissue was collected from GK and Wistar rats for RNA extraction. Under euglycemia, GIP and GLP-1 stimulated the initial insulin response by 10-fold in GK rats ( P  < 0.05). At later hyperglycemia, the insulin response to GIP and GLP-1 was blunted to about one-third compared with controls ( P  < 0.05). In the bowel GLP-1 was about 2.6–16.7 times more potent than GIP in abolishing the migrating myoelectric complex in the GK and control rats. Polymerase chain reaction (PCR) showed GIP and GLP-1 receptor gene expression in pancreatic islets and in small bowel. The initially high, but later low insulin responsiveness to stimulation with GIP and GLP-1 along with inhibition of small bowel motility in the GK rat indicates a preserved incretin response on motility in diabetes type 2.     

Options for patients with irritable bowel syndrome: contrasting traditional and novel serotonergic therapies

This article reviews the efficacy and tolerability of traditional therapies for irritable bowel syndrome (IBS) and concludes that they are limited by both poor efficacy and adverse effects. Serotonin, a neurotransmitter found mainly in the gut, appears to represent a link in IBS pathophysiological processes -- altered gut motility, abnormal intestinal secretion and visceral hypersensitivity. Recently, available treatments for IBS have targeted serotonin receptors that are involved in motor, sensory and secretory functions of the gut. Serotonergic agents, such as alosetron (a 5-HT3 receptor antagonist) and tegaserod (a selective 5-HT4 receptor partial agonist), provide global relief of the multiple symptoms of IBS with diarrhoea and IBS with constipation, respectively, and represent important additions to the IBS treatment armamentarium.     

Cannabinoid Type 1 Receptor: Another Arrow in the Adipocytes' Bow

The endocannabinoid system has recently emerged as an important modulator of several functions of adipose tissue, including cell proliferation, differentiation and secretion. Here, we will review the effects of cannabinoid type 1 (CB₁) receptor activation/blockade in adipocytes by summarising the data in the literature since the discovery of the presence of this receptor in adipose tissue. We will also discuss our original data obtained in mouse 3T3-L1 adipocyte cells using WIN55 212, a CB₁/CB₂ receptor agonist and SR141716 (rimonabant), a specific CB₁ receptor antagonist, respectively, in different experimental settings.     

The identification of peroxisome proliferator-activated receptor alpha-independent effects of oleoylethanolamide on intestinal transit in mice

Abstract  Oleoylethanolamide (OEA) is an endogenous lipid produced in the intestine that mediates satiety by activation of peroxisome proliferator-activated receptor alpha (PPARα). OEA inhibits gastric emptying and intestinal motility, but the mechanism of action remains to be determined. We investigated whether OEA inhibits intestinal motility by activation of PPARα. PPARα immunoreactivity was examined in whole mount preparations of mouse gastrointestinal (GI) tract. The effect of OEA on motility was assessed in wildtype, PPARα, cannabinoid CB₁ receptor and CB₂ receptor gene-deficient mice and in a model of accelerated GI transit. In addition, the effect of OEA on motility was assessed in mice injected with the PPARα antagonist GW6471, transient receptor potential vanilloid 1 antagonist SB366791 or the glucagon-like peptide 1 antagonist exendin-3(9-39) amide. PPARα immunoreactivity was present in neurons in the myenteric and submucosal plexuses throughout the GI tract. OEA inhibited upper GI transit in a dose-dependent manner, but was devoid of an effect on whole gut transit or colonic propulsion. OEA-induced inhibition of motility was still present in PPARα, CB₁ and CB₂ receptor gene-deficient mice and in the presence of GW6471, SB366791 and exendin-3(9-39) amide, suggesting neither PPARα nor the cannabinoids and other likely receptors are involved in mediating the effects of OEA. OEA blocked stress-induced accelerated upper GI transit at a dose that had no effect on physiological transit. We show that PPARα is found in the enteric nervous system, but our results suggest that PPARα is not involved in the suppression of motility by OEA.     

Irritable bowel syndrome: is the colonic mucosa to blame?

Irritable bowel syndrome (IBS) is characterized by chronic abdominal pain or discomfort and altered gastrointestinal function of unknown etiology. Studies of colonic mucosal biopsies from patients with IBS have suggested altered immune system function as a potential mechanism in the pathophysiology of IBS, but efforts to identify the mucosal mediators responsible for the manifestation of symptoms that define the disorder have been limited. In this issue of Neurogastroenterology and Motility, Buhner et al. and Balestra et al. report findings from studies linking increased excitability of the enteric nervous system with mucosal mediators released from biopsies of patients with IBS. These studies provide evidence for the concept that mediators present in the colonic mucosa may contribute to the manifestation of clinical symptoms present in IBS.     

The quest for biomarkers in IBS—where should it lead us?

Irritable bowel syndrome (IBS) is a prevalent functional gastrointestinal disorder which represents a major cost to health‐care services. The diagnosis of IBS is currently performed by means of symptom‐based diagnostic criteria, but there has been an ongoing interest in developing biomarkers which could simplify the diagnosis and/or evaluating the effect of treatments. This article reviews the current literature concerning the proposed biomarkers including those of altered gut motility, of visceral hypersensitivity, of abnormal brain mechanisms, of serum, fecal and mucosal inflammation and of increased intestinal permeability.     

The multi-herbal drug STW 5 (Iberogast®) has prosecretory action in the human intestine

Abstract  There is growing evidence that STW 5 (Iberogast^®, fixed combination of hydroethanolic herbal extracts), besides being effective in functional dyspepsia, also improves symptoms in irritable bowel syndrome (IBS). Clinical data indicate that modulation of mucosal secretion is a promising approach to treat intestinal disorders associated with IBS. We therefore explored the effect of STW 5 on secretion in the human intestine and the mechanisms by which it acts. The Ussing chamber technique was used to measure mucosal secretion in human intestinal mucosa/submucosa preparations and in human epithelial cell line T84. In addition, we recorded STW 5 effects on human enteric neurons with voltage sensitive dye imaging. In human tissue and T84 cells STW 5 induced a dose-dependent increase in ion secretion that was significantly reduced by the Na–K–Cl cotransporter blocker bumetanide, the adenylate cyclase inhibitor MDL-12 330, the non-specific and selective cystic fibrosis transmembrane conductance regulator (CFTR) inhibitors glibenclamide and CFTR_inh-172, respectively, and the blocker of calcium dependent Cl⁻ channels (ClCa) SITS (4-acetamido-4-isothiocyanatostilbene-2,2-disulphonic acid). It was unaffected by amiloride, a blocker of epithelial Na⁺ channels. In human tissue, the nerve blocker tetrodotoxin significantly suppressed the STW 5 response. STW 5 evoked an increased spike discharge in 51% of human submucous neurons. Results suggest that STW 5 is a secretogogue in the human intestine by direct epithelial actions and through activation of enteric neurons. The prosecretory effect is due to increased epithelial Cl⁻ fluxes via CFTR and Ca-dependent ClCa channels. STW 5 may be a novel option to treat secretory disorders associated with IBS and constipation.     

Endocannabinoids: Some Like it Fat (and Sweet Too)

There is growing interest in the commercialisation of the CB₁ receptor antagonist Rimonabant in Europe for the treatment of obesity and the metabolic syndrome. Clinical trials have shown that CB₁ receptor blockers are able to reduce not only food intake but also abdominal adiposity and its metabolic sequelae. Accordingly, CB₁ receptors, and tissue concentrations of endocannabinoids sufficient to activate them, are present in all brain and peripheral organs involved in the control of energy balance, including the hypothalamus, nucleus accumbens, pancreas, adipose tissue, skeletal muscle and liver. At the central level, the endocannabinoid system seems to play a dual role in the regulation of food intake by hedonic and homeostatic energy regulation. At the peripheral level, the endocannabinoid system seems to behave as a system that reduces energy expenditure and directs energy balance towards energy storage into fat. The emerging role of the endocannabinoid system in energy balance at both central and peripheral levels will be discussed in this review.     

Endocannabinoids and the Regulation of Bone Metabolism

In mammals, including humans, bone metabolism is manifested as an ongoing modelling/remodelling process whereby the bone mineralised matrix is being continuously renewed. Recently, the main components of the endocannabinoid system have been reported in the skeleton. Osteoblasts, the bone forming cells, and other cells of the osteoblastic lineage, as well as osteoclasts, the bone resorbing cells, and their precursors, synthesise the endocannabinoids anandamide and 2-arachidonoylglycerol (2-AG). CB₁ cannabinoid receptors are present in sympathetic nerve terminals in close proximity to osteoblasts. Activation of these CB₁ receptors by elevated bone 2-AG levels communicates brain-to-bone signals as exemplified by traumatic brain injury-induced stimulation of bone formation. In this process, the retrograde CB₁ signalling inhibits noradernaline release and alleviates the tonic sympathetic restrain of bone formation. CB₂ receptors are expressed by osteoblasts and osteoclasts. Their activation stimulates bone formation and suppresses bone resorption. CB₂-deficient mice display a markedly accelerated age-related bone loss. Ovariectomy-induced bone loss can be both prevented and rescued by a CB₂ specific agonist. Hence, synthetic CB₂ ligands, which are stable and orally available, provide a basis for developing novel anti-osteoporotic therapies, free of psychotropic effects. The CNR2 gene (encoding CB₂) in women is associated with low bone mineral density, offering an assay for identifying females at risk of developing osteoporosis.     

Effect of various doses of erythromycin on colonic myoelectrical activity in IBS patients

Abstract Erythromycin mimics the effects of motilin but its action on colonic motility is still controversial. The aim of this study was to evaluate the effect of intravenous infusions of various doses of erythromycin on colonic motility in patients with irritable bowel syndrome (IBS).
At 09.00 hours, day 1 (D1), an intraluminal probe was introduced up to the right colon in 24 IBS patients with myoelectrical activity recorded for 48 h. Patients were randomised in three groups of eight, receiving either erythromycin 1.5, 3 or 7 mg kg⁻¹. All were investigated as follows: at 09.00 hours, D2, fasted patients were infused with erythromycin in 30 min; at 09.00 hours, D3, glycerol 10 ml was injected intraluminally. Colonic activity was measured 30 min before the infusion started, during and 30 min after it, separately in the left and right colons, and expressed in time occupied during the recording period.
Basal activity was similar in both parts of the colon. During and after erythromycin 1.5 and 3 mg kg⁻¹, colonic activity remained unchanged. Erythromycin 7 mg kg⁻¹ induced a moderate increase in spiking activity (long spike bursts or LSBs + 33%; migrating LSBs or MLSBs + 41%) in the right colon, no change being statistically significant; activities in the left colon were similar to basal level.
During the first 30 min following glycerol injection, mean short spike burst or SSB activity did not change, mean LSB activity increased by seven fold (P = 0.014) and MLSB activity by ten fold (P = 0.023).
Erythromycin at doses from 1.5 to 7 mg kg⁻¹ failed to stimulate colonic motility in IBS patients. By contrast, response to glycerol was strong, suggesting normal colonic responsiveness. Erythromycin stimulatory effects could be limited to the upper gut at least in IBS patients.     

Recto-colonic reflex is impaired in patients with irritable bowel syndrome

Motor and sensory dysfunction of the gut are present in a subset of patients with irritable bowel syndrome (IBS). Recent studies have demonstrated the presence of a recto-colonic inhibitory reflex in healthy humans. It is not known whether this reflex exists in IBS. We studied rectal compliance, perception and the recto-colonic reflex by measuring volume responses of the descending colon to rectal distentions by barostat in 26 IBS patients and 13 healthy controls under both fasting and postprandial conditions. In the fasting state, rectal distention inhibited colonic tone and phasic motility to a similar extent in health and IBS. After a meal, rectal distention inhibited colonic tone and phasic motility to a lesser degree (P < 0.05) in IBS than health. Under postprandial but not fasting conditions, rectal distentions of increasing intensity were associated with higher pain scores in IBS than in health. Rectal distention inhibits tonic and phasic motility of the descending colon in healthy controls and in IBS patients. Postprandially this recto-colonic inhibitory reflex is impaired and attenuated in IBS patients compared with controls. These findings point to an altered reflex function in IBS and have implications for pathophysiology and therapy.     

Effects of serotonin transporter inhibition on gastrointestinal motility and colonic sensitivity in the mouse

Serotonin-selective reuptake transporter (SERT) expression is decreased in animal models of intestinal inflammation and in individuals with inflammatory bowel disease (IBD) or irritable bowel syndrome (IBS), and it is possible that resultant changes in intestinal serotonin signalling contribute to the manifestation of clinical features associated with these disorders. The objective of this investigation was to determine whether inhibition of SERT function leads to changes in gut motility and sensitivity. Mice underwent a 14-day treatment with the SERT inhibitor, paroxetine (20 mg kg(-1)), or vehicle (saline/propylene glycol). Gastrointestinal (GI) transit following charcoal gavage, colonic motility, stool frequency and visceromotor responses to colorectal distension were evaluated. In mice treated with paroxetine, stool output was decreased, upper GI transit was delayed, and colonic sensitivity to a nociceptive stimulus was attenuated. These results demonstrate that reduced SERT function (via pharmacological blockade) significantly alters GI motility and sensitivity in mice, and support the concept that altered SERT expression and function could contribute to symptoms associated with IBS and IBD.