Review article: serotonin receptors and transporters -- roles in normal and abnormal gastrointestinal motility

A survey of commonalities relevant to function and dysfunction in pelvic and sexual organs

Micturition, defecation and sexual function are all programmed through spinal reflexes that are under descending control from higher centres. Interaction between these reflexes can clearly be perceived, and evidence is accumulating the dysfunction in one reflex is often associated with dysfunction in another. In this article, we describe some of the basic properties and neural control of the smooth muscles mediating the reflexes, reviewing the common features that underlie these reflex functions, and what changes may be responsible for dysfunction. We propose that autonomic control within the pelvis predisposes pelvic and sexual organs to crosstalk, with the consequence that diseases and conditions of the pelvis are subject to convergence on a functional level. It should be expected that disturbance of the function of one system will inevitably impact adjacent systems.

5-HT receptors on interstitial cells of Cajal, smooth muscle and enteric nerves

The majority of the body's serotonin (5-HT) is produced by the gastrointestinal tract. 5-HT has several functions in the gastrointestinal tract. 5-HT is a paracrine signalling molecule released from enterochromaffin cells, a survival and proliferating factor and a neurotransmitter. The actions of 5-HT are transduced by a large family of 5-HT receptors, several of which are expressed on different gastrointestinal cell types including enteric nerves, smooth muscle and interstitial cells of Cajal (ICC). This review will summarize recent advances in understanding the role of 5-HT in regulating function of ICC, and the expression and function of 5-HT receptors on muscle and enteric nerves in human tissue. Rodent ICC express several 5-HT receptors including 5-HT(2B) receptors which regulate ICC survival and proliferation. Human smooth muscle and enteric neurons also express several 5-HT receptor subtypes. Expression and function of these receptors is significantly different from small laboratory animals. 5-HT(7) receptor activation causes relaxation of muscle, whereas 5-HT(2B) receptors increase muscle activity. The 5-HT(4) receptor appears to mediate both inhibition and activation of smooth muscle involving myogenic as well as neural actions. Despite the abundant expression of 5-HT(3) receptors in the human enteric nervous system no functional correlate has been as yet demonstrated.

Pharmacogenomics and serotonergic agents: research observations and potential clinical practice implications

Pharmacogenomics of serotonin are potentially relevant in research and clinical practice. There are few proven examples of the importance of pharmacogenetics of serotonin-modifying agents used in functional gastrointestinal or motility disorders. Drug metabolism is dependent on function of the cytochrome P450 enzymes, such as 2D6 and 3A4. Genetic variations in transporters and translation mechanisms have been associated with responses to treatment in irritable bowel syndrome and in obesity. Research on the impact of polymorphisms of key proteins on the pharmacokinetics and pharmacodynamics of drugs that alter serotonin-mediated signalling will assist in explaining diverse responses to those drugs and ultimately improve clinical practice, individualizing medicine.

Impact of 5-HT3 receptor blockade on colonic haemodynamic responses to ischaemia and reperfusion in the rat

5-HT(3) receptor antagonists are clinically available for treating patients with irritable bowel syndrome (IBS) but their use is restricted because of a link with some episodes of ischaemic colitis. However, the role of 5-HT3 receptors in regulating colonic blood flow has not been systematically investigated. Thus, we examined acute and chronic treatment with alosetron, a potent and selective antagonist of the 5-HT3 receptor, on baseline colonic blood flow and haemodynamic responses during occlusion and reactive hyperaemia in the pentobarbitone-anaesthetized rat. Colonic haemodynamics were assessed using ultrasonic recordings of superior mesenteric blood flow (MBF) and laser Doppler recordings of colonic vascular perfusion (VP). Blood pressure (BP) was also monitored and in some experiments tissue oxygen was detected polarographically. Alosetron (10, 30, 100 microg kg(-1), i.v.) had no effect on baseline haemodynamics nor responses to nitric oxide synthase inhibition with N(omega)-nitro-l-arginine methyl ester (l-NAME) (16 mg kg(-1)). Arterial occlusion (5 min) reduced MBF (-98.6 +/- 0.6%) and VP (-70.7 +/- 5.4%) followed by a post-occlusion reactive hyperaemia (MBF = +94.5 +/- 19.1%; VP = +60.0 +/- 22.3%) the magnitude of which was unchanged following acute (30 microg kg(-1)) or chronic alosetron administration (0.5 mg kg(-1) twice daily, 5 days). Alosetron did not significantly alter baseline colonic blood flow in the anaesthetized rat; nor did it interfere with vascular control mechanisms activated during occlusion and reactive hyperaemia.

Regional differences in expression of TPH-1, SERT, 5-HT(3) and 5-HT(4) receptors in the human stomach and duodenum

The aim of this study was to increase the understanding of the role of serotonergic signalling in normal gastroduodenal function at a molecular level. Mucosal biopsy specimens were collected from the fundus, antrum and duodenum of 11 healthy subjects. Serotonin (5-HT)-positive cells were counted and the mRNA levels of tryptophan hydroxylase (TPH), serotonin transporter (SERT), 5-HT(4) receptor and 5-HT(3) receptor subunits were quantified by real-time reverse transcription polymerase chain reaction. The number of 5-HT-positive cells was larger in the duodenum compared with the stomach (P < 0.001). Serotonin transport protein expression was 19-fold higher in the duodenum compared with the antrum and 457-fold higher compared with the fundus (P < 0.001). Tryptophan hydroxylase-1 expression was lower in the duodenum compared with the antrum and fundus (regional differences -2.3 and -3.6, respectively). The 5-HT(4) receptor and the 5-HT(3C) and 5-HT(3E) receptor subunits were more abundantly expressed in duodenum compared with the stomach (P < 0.001). The larger number of 5-HT-positive cells, the higher expression of 5-HT(3) and 5-HT(4) receptors, and in particularly the higher uptake capacity of 5-HT in the duodenum, point out to a more prominent role of serotonergic signalling at the mucosal level in the duodenum compared with the stomach.

Enterochromaffin cells of the human gut: sensors for spices and odorants

BACKGROUND & AIMS

Release of serotonin from mucosal enterochromaffin cells triggered by luminal substances is the key event in the regulation of gut motility and secretion. We were interested to know whether nasal olfactory receptors are also expressed in the human gut mucosa by enterochromaffin cells and whether their ligands and odorants present in spices, fragrances, detergents, and cosmetics cause serotonin release.

METHODS

Receptor expression was studied by the reverse-transcription polymerase chain reaction method in human mucosal enterochromaffin cells isolated by laser microdissection and in a cell line derived from human enterochromaffin cells. Activation of the cells by odorants was investigated by digital fluorescence imaging using the fluorescent Ca(2+) indicator Fluo-4. Serotonin release was measured in culture supernatants by a serotonin enzyme immunoassay and amperometry using carbon fiber microelectrodes placed on single cells.

RESULTS

We found expression of 4 olfactory receptors in microdissected human mucosal enterochromaffin cells and in a cell line derived from human enterochromaffin cells. Ca(2+) imaging studies revealed that odorant ligands of the identified olfactory receptors cause Ca(2+) influx, elevation of intracellular free Ca(2+) levels, and, consequently, serotonin release.

CONCLUSIONS

Our results show that odorants present in the luminal environment of the gut may stimulate serotonin release via olfactory receptors present in human enterochromaffin cells. Serotonin controls both gut motility and secretion and is implicated in pathologic conditions such as vomiting, diarrhea, and irritable bowel syndrome. Thus, olfactory receptors are potential novel targets for the treatment of gastrointestinal diseases and motility disorders.

Significant enhancement of esophageal pre-epithelial defense by tegaserod: implications for an esophagoprotective effect

BACKGROUND & AIMS

Tegaserod, a serotonin 5-hydroxytryptamine (5-HT)4 receptor agonist, is thought to stimulate intestinal secretions. The aim of the current study was to assess the effect of tegaserod vs placebo on salivary and esophageal protective factors in patients with gastroesophageal reflux disease (GERD).

METHODS

This study was a randomized, double-blind, placebo-controlled, cross-over trial in 38 GERD patients treated with tegaserod 6 mg twice a day vs placebo. Salivary samples were collected basally and during mastication. In addition, in 32 GERD patients, salivary and esophageal secretions also were collected during infusion of NaCl, HCl/pepsin, and NaCl in a consecutive fashion using a specially designed esophageal catheter. Saliva and esophageal perfusates were assessed for the pH, volume, content of buffers, protein, mucin, epidermal growth factor (EGF), transforming growth factor alpha (TGFalpha), and prostaglandin E (PGE)2 and analyzed statistically.

RESULTS

Salivary flow rates during administration of tegaserod increased over corresponding values during both basal conditions (P < .01) and mastication (P < .001). The rate of secretion of salivary bicarbonate and nonbicarbonate buffers also increased in basal conditions (P < .001 and P < .01, respectively) and during mastication (P < .05 and P = .05). Salivary EGF increased during mastication (P < .05), whereas PGE2 and TGF alpha increased in basal conditions (P < .05 and P < .01). Esophageal perfusate volumes increased during administration of tegaserod in basal conditions (P < .05), whereas esophageal EGF secretion increased after mucosal exposure to HCl/pepsin and subsequent final perfusion with NaCl (P < .05).

CONCLUSIONS

Significant stimulatory impact of 5-HT4 agonist on several salivary protective factors as well as esophageal EGF secretion may have esophagoprotective implications in patients with GERD and may help to address new therapies in the future.

Autonomic nervous system and secretion across the intestinal mucosal surface

Chloride secretion is important because it is the driving force for fluid movement into the intestinal lumen. The flow of accumulated fluid flushes out invading micro-organisms in defense of the host. Chloride secretion is regulated by neurons in the submucosal plexus of the enteric nervous system. Mechanosensitive enterochromaffin cells that release 5-hydroxytryptamine (5-HT) and activate intrinsic afferent neurons in the submucosal plexus and initiate chloride secretion. Mechanical stimulation by distention may also trigger reflexes by a direct action on intrinsic afferent neurons. Dysregulation of 5-HT release or altered activity of intrinsic afferents is likely to occur in states of inflammation and other disorders.

Serotonin and its role in colonic function and in gastrointestinal disorders

Serotonin (5-HT) is most commonly thought of as a neurotransmitter in the central nervous system. However, the predominant site of serotonin synthesis, storage, and release is the enterochromaffin cells of the intestinal mucosa. Within the intestinal mucosa, serotonin released from EC cells activates neural reflexes associated with intestinal secretion, motility, and sensation. Two important receptors for serotonin that are located in the neural circuitry of the intestines are the 5-HT(3) and 5-HT(4) receptors; these are the targets of drugs designed to treat gastrointestinal disorders. 5-HT(3) receptor antagonists are used to treat nausea and emesis associated with chemotherapy and for functional disorders associated with diarrhea. 5-HT(4) receptor agonists are used as promotility agents to promote gastric emptying and to alleviate constipation. Because of the importance of serotonin in normal gut function and sensation, a number of studies have investigated potential changes in mucosal serotonin signaling in pathologic conditions. Despite the inconsistencies in the current literature, changes in serotonin signaling have now been demonstrated in inflammatory bowel disease, irritable bowel syndrome, postinfectious irritable bowel syndrome, and idiopathic constipation. Emerging evidence has led to many contradictory theories regarding serotonin signaling and its roles in the pathology of gut disorders. This review summarizes the current medications affecting serotonin signaling and provides an overview of our current knowledge of the changes in serotonin that occur in pathologic conditions.

Tegaserod in the treatment of irritable bowel syndrome (IBS) with constipation as the prime symptom

Irritable bowel syndrome with constipation (IBS-C) as the predominant bowel symptom is a prevalent disorder, characterized by recurring abdominal pain/discomfort, bloating, and constipation, and imposes a significant socio-economic burden. Traditional treatments generally address just one of the multiple IBS symptoms. The efficacy and safety profile of tegaserod, a serotonin 5-HT(4) receptor agonist, has been demonstrated in several randomized, placebo-controlled, and open-label trials. This review discusses the major clinical trials of tegaserod, which have involved 8948 IBS patients. Overall, data reveal that tegaserod is an effective treatment for IBS-C, providing statistically significant relief of overall and multiple individual IBS-C symptoms (abdominal pain/discomfort, bloating, and constipation) in both placebo-controlled and "real-life" open-label settings. Repeat treatments with tegaserod were also shown to be effective, which is noteworthy given the chronic and episodic nature of IBS. Moreover, tegaserod was associated with improvements in patients' quality of life and work productivity. Data also indicate that tegaserod is well tolerated over the short-term (4 weeks), long-term (12 months), and repeated treatments. Diarrhea is the only adverse event consistently associated with tegaserod and was generally mild and transient. Overall, tegaserod has been demonstrated to offer effective and well-tolerated treatment of IBS-C, providing patients with meaningful symptom relief.

Enhancement of the intrinsic defecation reflex by mosapride, a 5-HT4 agonist, in chronically lumbosacral denervated guinea pigs

The defecation reflex is composed of rectal distension-evoked rectal (R-R) reflex contractions and synchronous internal anal sphincter (R-IAS) reflex relaxations in guinea pigs. These R-R and R-IAS reflexes are controlled via extrinsic sacral excitatory nerve pathway (pelvic nerves), lumbar inhibitory nerve pathways (colonic nerves) and by intrinsic cholinergic excitatory and nitrergic inhibitory nerve pathways. The effect of mosapride (a prokinetic benzamide) on the intrinsic reflexes, mediated via enteric 5-HT(4) receptors, was evaluated by measuring the mechanical activity of the rectum and IAS in anesthetized guinea pigs using an intrinsic R-R and R-IAS reflex model resulting from chronic (two to nine days) lumbosacral denervation (PITH). In this model, the myenteric plexus remains undamaged and the distribution of myenteric and intramuscular interstitial cells of Cajal is unchanged. Although R-R and R-IAS reflex patterns markedly changed, the reflex indices (reflex pressure or force curve-time integral) of both the R-R contractions and the synchronous R-IAS relaxations were unchanged. The frequency of the spontaneous R and IAS motility was also unchanged. Mosapride (0.1-1.0 mg/kg) dose-dependently increased both intrinsic R-R (maximum: 1.82) and R-IAS reflex indices (maximum: 2.76) from that of the control (1.0) 6-9 days following chronic PITH. The dose-response curve was similar to that in the intact guinea pig, and had shifted to the left from that in the guinea pig after acute PITH. A specific 5-HT(4) receptor antagonist, GR 113808 (1.0 mg/kg), decreased both reflex indices by approximately 50% and antagonized the effect of mosapride 1.0 mg/kg. This was quite different from the result in the intact guinea pig where GR 113808 (1.0 mg/kg) did not affect either of the reflex indices. The present results indicate that mosapride enhanced the intrinsic R-R and R-IAS reflexes and functionally compensated for the deprivation of extrinsic innervation. The actions of mosapride were mediated through endogenously active, intrinsic 5-HT(4) receptors which may be post-synaptically located in the myenteric plexus of the anorectum.

Mapping 5-HT inputs to enteric neurons of the guinea-pig small intestine

5-HT released by gastrointestinal mucosa and enteric interneurons has powerful effects on gut behavior. However, the targets of 5-HT-containing neurons within enteric circuits are not well characterized. We used antisera against 5-HT and selected markers of known enteric neuron types to investigate the connections made by 5-HT-containing neurons in the guinea-pig jejunum. Confocal microscopy was used to quantify the number of 5-HT-immunoreactive varicosities apposed to immunohistochemically identified cell bodies. Large numbers of varicosities were identified apposing cholinergic secretomotor neurons, immunoreactive for neuropeptide Y, in both myenteric and submucous plexuses. Subgroups of neurons identified by calretinin (ascending interneurons) and nitric oxide synthase (descending interneurons and inhibitory motor neurons) immunoreactivity were also apposed by many varicosities. Longitudinal muscle motor neurons (calretinin immunoreactive) and AH/Dogiel type II (sensory) neurons (calbindin immunoreactive) were apposed by small numbers of varicosities. Combined retrograde tracing and immunohistochemistry were used to identify excitatory circular muscle motor neurons; these were encircled by 5-HT-immunoreactive varicosities, but the appositions could not be quantified. We suggest that 5-HT-containing interneurons are involved in secretomotor pathways and pathways to subgroups of other interneurons, but not longitudinal muscle motor neurons. There also appear to be connections between 5-HT-containing interneurons and excitatory circular muscle motor neurons. Physiological evidence demonstrates a functional connection between 5-HT-containing interneurons and AH/Dogiel type II neurons, but few 5-HT-immunoreactive varicosities were observed apposing calbindin-immunoreactive cell bodies. Taken together these results suggest that neural 5-HT may have significant roles in excitatory pathways regulating both motility and secretion.

Comparison of 5-HT4 and 5-HT7 receptor expression and function in the circular muscle of the human colon

Serotonin receptors are potential targets for treating functional bowel disorders. This study investigated the functional roles and expression of the 5-HT4 and the 5-HT7 receptor, which coexist in human colon circular smooth muscle. 5-HT3 receptor expression was also investigated. Part of the relaxant response to 5-HT was due to activation of 5-HT4 receptors as the apparent pKB value of the selective 5-HT4 antagonist, GR 113808, was 9.36. 5-HT4 mRNA levels were low in five tissues and undetectable in four others, but all responded to 5-HT with an EC50 value of 102.54+/-19.32 nM. The contribution of 5-HT7 receptors to the response was not readily demonstrated using the selective 5-HT7 antagonist, SB-269970, as its apparent pKB value of 7.19 (5-HT4 block with 1 microM GR 113808) was lower than the value obtained using the 5-HT7 guinea pig ileum assay (8.62). Nevertheless, the 5-HT7 receptor was expressed more consistently than the 5-HT4, but at similar levels. The 5-HT(3Ashort) and 5-HT(3B) subunits were co-expressed at similar levels, but the 5-HT(3Along) subunit was detected in only five of the nine samples tested. The findings show that 5-HT4-induced relaxation occurs at low to undetectable levels of tissue mRNA, as measured by qPCR. Although 5-HT7 receptor mRNA is detected at low, but consistent levels, the functional activity of this receptor is not readily identified given the currently available drugs.

Embryonic differentiation of serotonin-containing neurons in the enteric nervous system of the locust (Locusta migratoria)

The enteric nervous system (ENS) of the locust consists of four ganglia (frontal and hypocerebral ganglion, and the paired ingluvial ganglia) located on the foregut, and nerve plexus innervating fore- and midgut. One of the major neurotransmitters of the ENS, serotonin, is known to play a vital role in gut motility and feeding. We followed the anatomy of the serotonergic system throughout embryonic development. Serotonergic neurons are generated in the anterior neurogenic zones of the foregut and migrate rostrally along the developing recurrent nerve to contribute to the frontal ganglion. They grow descending neurites, which arborize in all enteric ganglia and both nerve plexus. On the midgut, the neurites closely follow the leading migrating midgut neurons. The onset of serotonin synthesis occurs around halfway through development-the time of the beginning of midgut closure. Cells developing to serotonergic phenotype express the serotonin uptake transporter (SERT) significantly earlier, beginning at 40% of development. The neurons begin SERT expression during migration along the recurrent nerve, indicating that they are committed to a serotonergic phenotype before reaching their final destination. After completion of the layout of the enteric ganglia (at 60%) a maturational phase follows, during which serotonin-immunoreactive cell bodies increase in size and the fine arborizations in the nerve plexus develop varicosities, putative sites of serotonin release (at 80%). This study provides the initial step for future investigation of potential morphoregulatory functions of serotonin during ENS development.

Serotonergic and non-serotonergic targets in the pharmacotherapy of visceral hypersensitivity

Visceral hypersensitivity is considered a key mechanism in the pathogenesis of functional gastrointestinal (GI) disorders. Targeting visceral hypersensitivity seems an attractive approach to the development of drugs for functional GI disorders. This review summarizes current knowledge on targets for the treatment of visceral hypersensitivity, and the status of current and future drug and probiotic treatment development, and the role of pharmacogenomic factors.

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.

Irritable bowel syndrome in the 21st century: perspectives from Asia or South-east Asia

Asian irritable bowel syndrome (IBS) studies not only confirm the truth of this functional disorder but also describe the current disease situation of this continent, with its variable socioeconomic backgrounds. Most Asian community IBS prevalence is within 5-10%, regardless of gender or ethnic character. As well as meeting the main Rome II criteria, Asian IBS subjects also have many minor symptoms. Thus this recommendation remains useful to diagnose Asian IBS. Also, female patients commonly express constipation-predominant (C-) symptoms. Extra-colonic symptoms are common in Asia, for example dyspepsia, insomnia and irritable urinary bladder. Asian IBS subjects do experience psychological disturbances including anxiety, depression, agoraphobia and neuroticism. Accordingly, their quality of life is poor and there is absenteeism leading to excessive physician visits. Abnormal gut motor and sensory functions have been indicated among the Asian IBS subjects. Now, there is evidence of altered colonic neuroimmune function leading to gut hypersensitivity and dysmotility. An Asia-Pacific trial also confirmed tegaserod efficacy on female C-IBS subjects. More than 90% of nurses have very limited IBS knowledge, and are unable even to explain it clearly. In conclusion, Western recommended criteria clearly diagnose Asian IBS and many factors are mutual leading to IBS. Current IBS treatments remain useful but additional reeducation for medical professionals appears to be needed.

The bidirectional communication between neurons and mast cells within the gastrointestinal tract

Normal or disordered behaviour of the gastrointestinal tract is determined by a complex interplay between the epithelial barrier, immune cells, blood vessels, smooth muscle and intramurally located nerve elements. Mucosal mast cells (MMCs), which are able to detect noxious and antigenic threats and to generate or amplify signals to the other cells, are assigned a rather central position in this complex network. Signal input from MMCs to intrinsic enteric neurons is particularly crucial, because the enteric nervous system fulfils a pivotal role in the control of gastrointestinal functions. Activated enteric neurons are able to generate an alarm program involving alterations in motility and secretion. MMC signalling to extrinsic nerve fibres takes part in pathways generating visceral pain or extrinsic reflexes contributing to the disturbed motor and secretory function. Morphological and functional studies, especially studies concerning physiological stress, have provided evidence that, apart from the interaction between the enteric nervous system and MMCs, there is also a functional communication between the central nervous system and these mast cells. Psychological factors trigger neuronal pathways, which directly or indirectly affect MMCs. Further basic and clinical research will be needed to clarify in more detail whether basic patterns of this type of interactions are conserved between species including humans.

Histopathological alterations in irritable bowel syndrome

Irritable bowel syndrome is a common disorder defined by a symptom complex including abdominal pain and altered bowel habit. The etiopathogenesis appears to be multifactorial and to involve altered gastrointestinal motor function, enhanced perception of visceral stimuli and psychosocial factors. More recently a role for mucosal immune activation has been suggested. Routine histologic examination reveals no mucosal abnormality in the majority of cases but quantitative histological, immunohistochemical and ultrastructural analyses reveal subtle morphologic changes involving lymphocytes, mast cells, enterochromaffin cells and enteric nerves. The recent appreciation of these changes has led to new hypotheses linking central and enteric nervous systems to immune processes. This review highlights the spectrum of morphologic changes that occur in irritable bowel syndrome, examines their relationship to the pathophysiology of irritable bowel syndrome and considers their relevance to daily pathology practice.

Functional CRF receptors in BON cells stimulate serotonin release

BON cells are human, pancreatic carcinoid-derived, endocrine-like cells that share functional similarities with intestinal enterochromaffin (EC) cells. We investigated the presence of corticotropin-releasing factor (CRF) receptors, their signalling pathways and the functional effects of their stimulation in BON cells (clone #7). Expression analysis showed that BON cells contain mRNA for the CRF receptor types 1 and 2 (CRF1/2), although CRF2 mRNA levels were 23-fold higher than those of CRF1 mRNA. The CRF1/2 ligand, rat/human (r/h)CRF (EC50 = 233 nM), and the selective CRF2 ligand, human urocortin 3 (Ucn 3) (EC50 = 48 nM), induced a dose-dependent increase in cAMP formation. Effects of r/hCRF were blocked by 44% with the selective CRF1 antagonist DMP-696, while the selective CRF2 antagonist antisauvagine-30 had only marginal effects. Both ligands (100 nM) stimulated the release of serotonin with similar efficacy (3-fold increase over basal). Effects of r/hCRF, but not Ucn 3, were blocked by pre-incubation with antisauvagine-30. These observations demonstrate that the EC cell-related BON cells express functional CRF2 receptors linked to the release of serotonin. This suggests that EC cells may be a target for CRF and/or Ucn 3 in the intestine during stress-related responses. Actions of CRF/Ucn 3 and EC cell-derived mediators, such as serotonin, might underlie several motor, secretory and/or sensory disorders of the gastrointestinal (GI) tract which may play a role in the pathophysiology of functional GI disorders, such as irritable bowel syndrome.

In vitro continuous amperometric monitoring of 5-hydroxytryptamine release from enterochromaffin cells of the guinea pig ileum

A diamond microelectrode was used to sensitively, reproducibly and stably record overflow of 5-hydroxytryptamine (5-HT, serotonin) from enterochromaffin cells (EC) of the intenstinal mucosal layer. 5-HT is an important neurotransmitter and paracrine signalling molecule in the gastrointestinal tract. The diamond microelectrode was formed by overcoating a sharpened 76 microm diameter Pt wire with a thin layer of conducting diamond. After insulation with polypropylene, the conically-shaped microelectrode had a diameter of about 10 microm at the tip and 80 microm at the cylindrical portion. The exposed length was 100-200 microm. Continuous amperometry with the microelectrode poised at a detection potential of 700 mV vs. Ag|AgCl was used to measure 5-HT overflow as an oxidation current. 5-HT overflow was elicited by both mechanical and electrical stimulation. Some minor electrode fouling, a common problem with the oxidative detection of 5-HT, was seen for diamond but the response stabilized enabling recording in vitro. Both 5-HT and the paracrine hormone, melatonin, were detected in the extracellular solution. The 5-HT oxidation current increased in the presence of the serotonin transporter (SERT) inhibitor, fluoxetine (1 microM), providing evidence that the oxidation current was associated with 5-HT.

Irritable bowel syndrome: recent and novel therapeutic approaches

Irritable bowel syndrome (IBS) is a highly prevalent functional gastrointestinal disorder affecting up to 3-15% of the general population in Western countries. It is characterised by unexplained abdominal pain, discomfort and bloating in association with altered bowel habits. The pathophysiology of IBS is considered to be multifactorial, involving disturbances of the brain-gut-axis: IBS has been associated with abnormal gastrointestinal motor functions, visceral hypersensitivity, psychosocial factors, autonomic dysfunction and mucosal inflammation. Traditional IBS therapy is mainly symptom oriented and often unsatisfactory. Hence, there is a need for new treatment strategies. Increasing knowledge of brain-gut physiology, mechanisms, and neurotransmitters and receptors involved in gastrointestinal motor and sensory function have led to the development of several new therapeutic approaches. This article provides a systematic overview of recently approved or novel medications that show promise for the treatment of IBS; classification is based on the physiological systems targeted by the medication. The article includes agents acting on the serotonin receptor or serotonin transporter system, novel selective anticholinergics, alpha-adrenergic agonists, opioid agents, cholecystokinin antagonists, neurokinin antagonists, somatostatin receptor agonists, neurotrophin-3, corticotropin releasing factor antagonists, chloride channel activators, guanylate cyclase-c agonists, melatonin and atypical benzodiazepines. Finally, the role of probiotics and antibacterials in the treatment of IBS is summarised.

Clinical application of pharmacogenetics in gastrointestinal diseases

As knowledge of the human genome grows, there will be a direct impact on the management of specific diseases. Within gastroenterology and hepatology, there has been a change in the understanding of how variations or mutations in genes involved in drug metabolism or disease pathophysiology affect response to therapy. This review discusses the application of clinical pharmacogenetics to the following diseases and disorders: inflammatory bowel disease, Helicobacter pylori infections, gastroesophageal reflux disease, irritable bowel syndrome, functional dyspepsia, liver transplantation and colon cancer. Although only a few genotyping tests are regularly used in clinical practice, it is anticipated that studies will propel the routine use of many of the tests described in this review, in the future.

Ontogeny of brain and blood serotonin levels in 5-HT receptor knockout mice: potential relevance to the neurobiology of autism

The most consistent neurochemical finding in autism has been elevated group mean levels of blood platelet 5-hydroxytryptamine (5-HT, serotonin). The origin and significance of this platelet hyperserotonemia remain poorly understood. The 5-HT(1A) receptor plays important roles in the developing brain and is also expressed in the gut, the main source of platelet 5-HT. Post-natal tissue levels of 5-HT, 5-hydroxyindoleacetic acid (5-HIAA) and tryptophan were examined in the brain, duodenum and blood of 5-HT(1A) receptor-knockout and wild-type mice. At 3 days after birth, the knockout mice had lower mean brain 5-HT levels and normal mean platelet 5-HT levels. Also, at 3 days after birth, the mean tryptophan levels in the brain, duodenum and blood of the knockout mice were around 30% lower than those of the wild-type mice. By 2 weeks after birth, the mean brain 5-HT levels of the knockout mice normalized, but their mean platelet 5-HT levels became 24% higher than normal. The possible causes of these dynamic shifts were explored by examining correlations between central and peripheral levels of 5-HT, 5-HIAA and tryptophan. The results are discussed in relation to the possible role of 5-HT in the ontogeny of autism.

Review article: modulation of the brain-gut axis as a therapeutic approach in gastrointestinal disease

BACKGROUND

The importance of bi-directional brain-gut interactions in gastrointestinal illness is increasingly being recognized, most prominently in the area of functional gastrointestinal disorders. Numerous current and emerging therapies aimed at normalizing brain-gut interactions are a focus of interest, particularly for irritable bowel syndrome and functional dyspepsia.

METHODS

A literature search was completed for preclinical and clinical studies related to central modulation of gastrointestinal functions and published in English between 1980 and 2006.

RESULTS

Existing data, while sparse, support the use of different classes of antidepressant drugs, including tricyclics, and selective and non-selective serotonin reuptake inhibitors in irritable bowel syndrome. Serotonin receptor agonists and antagonists with peripheral and possibly central effects are effective in treating specific subtypes of irritable bowel syndrome. Based largely on theoretical and preclinical evidence, several novel compounds that selectively target receptors at multiple levels within the brain-gut axis such as neurokinin, somatostatin and corticotropin-releasing factor receptor antagonists are promising.

CONCLUSIONS

This review discusses the rationale for modulation of the brain-gut axis in the treatment of functional gastrointestinal disorders and highlights the most promising current and future therapeutic strategies.

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.

The 5-HT4 antagonist R216073 does not affect gastric motor and sensory function in patients with functional dyspepsia

BACKGROUND

Serotonin and the 5-HT4 receptor play an important role in gastrointestinal motor and sensory functions. While 5-HT4 agonists are known for their prokinetics properties, the effect of 5-HT4 antagonists on upper gastrointestinal functions is largely unknown.

AIM

To assess the effect of a 5-HT4 receptor antagonist (R216073) on gastric relaxation and visceral sensitivity in patients with functional dyspepsia. Secondly, the influence of a functional polymorphism in the gene encoding the serotonin transport protein on drug response was determined.

METHODS

A double-blind, randomized, placebo-controlled, two-period crossover study was performed in 20 functional dyspepsia patients. The effect of a single dose of 2,000 mg R216073 on gastric relaxation and sensitivity was tested using three-dimensional ultrasonography and a nutrient drinktest.

RESULTS

R216073 did not affect partial gastric volumes or upper abdominal sensations scored during three-dimensional ultrasonography (P > 0.05). The maximum tolerated volume or upper abdominal sensations induced by the drinktest were not affected by R216073 (P > 0.05). The serotonin transport protein promoter polymorphism was not associated with any of the end-points of the study.

CONCLUSIONS

A single dose of R216073 had no effect on fundic relaxation, drinking capacity, or upper abdominal symptoms in patients with functional dyspepsia.

Acidity induces c-Fos expression in a subpopulation of human colonic submucosal neurons

Enteric neurons responding to chemical challenge of the mucosa have been characterized in animal models mainly in the myenteric plexus. However, in humans, the existence of enteric neurons responding to chemical stimulation of the mucosa remains currently unknown. Therefore, the aim of our study was to identify and characterize human submucosal neurons activated by mucosal challenge with butyrate or hydrochloric acid. Segments of human colon were placed in a modified Ussing chamber and incubated on the mucosal side with butyric acid (20 mM, pH 6.5), sodium butyrate (20 mM, pH 7.5), hydrochloric acid (10 mM, pH 6.5) or culture medium (pH 7.5). After 90 min of culture, tissues were fixed and microdissected to obtain whole mount preparation of submucosa containing the Meissner's plexus. Neuron specific enolase (NSE), c-Fos, vasoactive intestinal peptide (VIP) and substance P (SP) were detected using immunohistochemical methods. Tetrodotoxin (TTX, 1 microM) was used to inhibit neuronal activity. After 90 min of culture, butyric acid induced a significant 5.6-fold increase in the proportion of c-Fos-immunoreactive neurons compared to control (19 +/- 4% versus 4 +/- 1%, respectively, p < 0.001). 41 +/- 5% of c-Fos-immunoreactive neurons were VIP-immunoreactive and 3 +/- 2% were SP-immunoreactive. Butyric acid did not modify the proportion of VIP-immunoreactive neurons. The increase in c-Fos-immunoreactive neurons induced by butyric acid was reproduced with hydrochloric acid at the same pH but not with sodium butyrate. Finally, preincubation of the tissue with TTX prevented the effect of butyric acid. In conclusion, our results demonstrate that acidic mucosal challenge induced the activation of a population of human submucosal neurons with a specific neurochemical coding.

Emerging drugs for irritable bowel syndrome

Irritable bowel syndrome (IBS) is one of the most common chronic gastrointestinal disorders, yet its pathophysiology is incompletely understood and pharmacological treatments remain unsatisfactory. Current therapeutic choices include a range of drugs aimed at normalising bowel habits, reducing pain or treating comorbid psychological symptoms. However, this individual symptom-targeted approach remains unsatisfactory in terms of global symptom relief and patient satisfaction. In the last decade, further characterisation of IBS pathophysiology has provided new and exciting targets at different levels of the brain-gut axis for the development of several candidate drugs. Advances in clinical trial design will help to evaluate these compounds in different IBS patient populations.

Contribution of 5-HT2A receptor in nematode infection-induced murine intestinal smooth muscle hypercontractility

BACKGROUND & AIMS

Enteric nematode infection induces a smooth muscle hypercontractility that depends on interleukin (IL)-4 and IL-13 activation of the signal transducer and activator of transcription (STAT) 6. Serotonin (5-HT) is involved in the physiologic regulation of gut function. The present study investigated the contribution of 5-HT and its receptors in nematode-induced intestinal smooth muscle hypercontractility.

METHODS

Mice were infected with Nippostrongylus brasiliensis (N brasiliensis) or Heligmosomoides polygyrus (H polygyrus) or injected intravenously with IL-13. Segments of jejunum were suspended in organ baths, and smooth muscle responses to 5-HT were determined in the presence or absence of specific 5-HT antagonists. IL-4, IL-13, and 5-HT receptor messenger RNA expressions were determined by real-time quantitative polymerase chain reaction.

RESULTS

5-HT evoked a modest contraction of smooth muscle in wild-type (WT) mice that was unaltered by the 5-HT2A antagonist ketanserin. N brasiliensis infection induced a smooth muscle hypercontractility to 5-HT that was abolished by 5-HT(2A) antagonists but not by other 5-HT antagonists. Infection-induced up-regulation of 5-HT2A expression was correlated with the smooth muscle hypercontractility to 5-HT. The infection-induced up-regulation of 5-HT2A in WT mice was observed also in IL-4(-/-) mice but was not seen in IL-13(-/-) or STAT6(-/-) mice. In addition, smooth muscle responses to 5-HT and 5-HT2A expression in WT mice were also enhanced by IL-13 or H polygyrus infection.

CONCLUSIONS

These data show that 5-HT2A is one of the molecules downstream from STAT6 activation that mediates changes in smooth muscle function. 5-HT2A represents a novel therapeutic target for modulating immune-mediated effects on intestinal motility.

Peripheral mechanisms of sibutramine involving proximal gastric motility in dogs

OBJECTIVE

Sibutramine, a serotonin-norepinephrine uptake inhibitor, has been used for treating obesity. However, its possible mechanisms involving gastric motility have not been reported. The aim of this study was to evaluate the effects of sibutramine on gastric accommodation and antral motility.

RESEARCH METHODS AND PROCEDURES

The study was performed in seven dogs with a stomach cannula and composed of two separate experiments: antral contractions and gastric tone. Each experiment included two sessions on 2 separate days in a randomized order: a control session and a treatment session with sibutramine (5 mg/kg per os) administrated 2 hours before the study.

RESULTS

Sibutramine significantly increased fasting gastric tone; the gastric volume in the fasting state at baseline was 103.8 +/- 12.3 mL and significantly decreased to 35.3 +/- 16.0 mL with sibutramine (p = 0.0075). Sibutramine also impaired gastric accommodation. The average postprandial gastric volume was 472.1 +/- 16.7 mL in the control session and reduced to 302.2 +/- 53.6 mL with sibutramine (p = 0.013). The average postprandial increase in gastric volume during the 60-minute postprandial period with sibutramine was significantly lower than the corresponding values in the control session: 266.8 +/- 46.1 vs. 393.9 +/- 15.3 mL (p = 0.03). Sibutramine had no effects on postprandial antral contractions.

DISCUSSION

Sibutramine increases gastric tone and impairs gastric accommodation to an orally ingested meal. The inhibitory effect of sibutramine on gastric accommodation may partially explain the reduced food intake with sibutramine in patients with obesity.

Adaptive changes of the enterochromaffin and gastrin cells in the rat gastrointestinal tract following subtotal colectomy

OBJECTIVE

Colectomized patients often have diarrhoea and increased gastric acid secretion. Although serotonin influences gastrointestinal (GI) motility and secretion, GI serotonin-producing enterochromaffin (EC) cells have not been investigated after colectomy, nor have the antral gastrin cells. The aim of this experimental study was to investigate the GI tract in rats 8 weeks after subtotal colectomy, with particular emphasis on the frequency and distribution of EC and gastrin cells.

MATERIAL AND METHODS

Immunohistochemical techniques were used to identify the two endocrine cell types.

RESULTS

The colectomized animals had diarrhoea. Body-weight was lower and the small intestine shorter in the colectomized animals compared with sham-operated and untreated controls. In the two surgically treated groups, the antral mucosa was thinner and the small intestinal mucosa was thicker compared with that of the untreated rats, whereas the thickness of the rectum of the colectomized rats was increased compared with that of the control groups. In the colectomized animals, the number of EC cells was increased in the small intestine and rectum, whereas the numbers of both EC and gastrin cells were decreased in the antrum.

CONCLUSIONS

The results indicate that colectomy exerts a significant influence on the GI mucosa and on the endocrine cell systems studied. An increased number of EC cells can result in alterations in motility and secretion, which may be important in the pathogenesis of the diarrhoea that often occurs after colectomy.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Identification of neurons that express 5-hydroxytryptamine4 receptors in intestine

5-Hydroxytryptamine (5-HT) is an endogenous stimulant of intestinal propulsive reflexes. It exerts its effects partly through 5-HT4 receptors; 5-HT4 receptor agonists that are stimulants of intestinal transit are in clinical use. Both pharmacological and recent immunohistochemical studies indicate that 5-HT4 receptors are present on enteric neurons but the specific neurons that express the receptors have not been determined. In the present work, we describe the characterization of an anti-5-HT4 receptor antiserum that reveals immunoreactivity for enteric neurons and other cell types in the gastrointestinal tract. With this antiserum, 5-HT4 receptor immunoreactivity has been found in the muscularis mucosae of the rat oesophagus, a standard assay tissue for 5-HT4 receptors. It is also present in the muscularis mucosae of the guinea-pig and mouse oesophagus. In guinea-pig small intestine and rat and mouse colon, 5-HT4 receptor immunoreactivity occurs in subpopulations of enteric neurons, including prominent large neurons. Double-staining has shown that these large neurons in the guinea-pig small intestine are also immunoreactive for two markers of intrinsic primary afferent neurons, cytoplasmic NeuN and calbindin. Some muscle motor neurons in the myenteric ganglia are immunoreactive for this receptor, whereas it is rarely expressed by secretomotor neurons. Immunoreactivity also occurs in the interstitial cells of Cajal but is faint in the external muscle. Expression of the protein and mRNA has been confirmed in extracts containing enteric neurons. The observations suggest that one site of action of 5-HT4 receptor agonists is the intrinsic primary afferent neurons.

Review article: intestinal serotonin signalling in irritable bowel syndrome

Alterations in motility, secretion and visceral sensation are hallmarks of irritable bowel syndrome. As all of these aspects of gastrointestinal function involve serotonin signalling between enterochromaffin cells and sensory nerve fibres in the mucosal layer of the gut, potential alterations in mucosal serotonin signalling have been explored as a possible mechanism of altered function and sensation in irritable bowel syndrome. Literature related to intestinal serotonin signalling in normal and pathophysiological conditions has been searched and summarized. Elements of serotonin signalling that are altered in irritable bowel syndrome include: enterochromaffin cell numbers, serotonin content, tryptophan hydroxylase message levels, 5-hydroxyindoleacedic acid levels, serum serotonin levels and expression of the serotonin-selective reuptake transporter. Both genetic and epigenetic factors could contribute to decreased serotonin-selective reuptake transporter in irritable bowel syndrome. A serotonin-selective reuptake transporter gene promoter polymorphism may cause a genetic predisposition, and inflammatory mediators can induce serotonin-selective reuptake transporter downregulation. While a psychiatric co-morbidity exists with IBS, changes in mucosal serotonin handling support the concept that there is a gastrointestinal component to the aetiology of irritable bowel syndrome. Additional studies will be required to gain a more complete understanding of changes in serotonin signalling that are occurring, their cause and effect relationship, and which of these changes have pathophysiological consequences.

Differential tissue distribution of tryptophan hydroxylase isoforms 1 and 2 as revealed with monospecific antibodies

Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in the synthesis of the neurotransmitter serotonin. Once thought to be a single-gene product, TPH is now known to exist in two isoforms-TPH1 is found in the pineal and gut, and TPH2 is selectively expressed in brain. Heretofore, probes used for localization of TPH protein or mRNA could not distinguish between the TPH isoforms because of extensive homology shared by them at the nucleotide and amino acid level. We have produced monospecific polyclonal antibodies against TPH1 and TPH2 using peptide antigens from nonoverlapping sequences in the respective proteins. These antibodies allow the differentiation of TPH1 and TPH2 upon immunoblotting, immunoprecipitation, and immunocytochemical staining of tissue sections from brain and gut. TPH1 and TPH2 antibodies do not cross-react with either tyrosine hydroxylase or phenylalanine hydroxylase. Analysis of mouse tissues confirms that TPH1 is the predominant form expressed in pineal gland and in P815 mastocytoma cells with a molecular weight of 51 kDa. TPH2 is the predominant enzyme form expressed in brain extracts from mesencephalic tegmentum, striatum, and hippocampus with a molecular weight of 56 kDa. Antibody specificity against TPH1 and TPH2 is retained across mouse, rat, rabbit, primate, and human tissues. Antibodies that distinguish between the isoforms of TPH will allow studies of the differential regulation of their expression in brain and periphery.

Effect of 5-HT1 agonist (sumatriptan) on anorectal function in irritable bowel syndrome patients

AIM: To evaluate the effect of sumatriptan, a selective 5-HT₁ agonist, on anorectal function in irritable bowel syndrome (IBS) patients.

METHODS: Twenty-two IBS patients selected according to the Rome II criteria (F 15, M 7; mean age 29.3±6.8, range 22-44 years) were examined. The study was blind, randomized and placebo-controlled with a crossover design. Anorectal manometry and rectal balloon distension test were performed before and after the administration of placebo and sumatriptan.

RESULTS: The administration of sumatriptan caused a significant increase in the resting anal canal pressure from 9.2±2.0 kPa to 13.1±3.3 kPa (P<0.0001) connected with the increase in the anal sphincter length and high pressure zone. After sumatriptan injection a remarkable increase in the threshold for the first sensation from 27±9 mL to 34±12 mL (P<0.05) and urge sensation from 61±19 mL to 68±18 mL (P<0.01) was observed. Sumatriptan did not affect either the volume evoking the rectoanal inhibitory reflex or the results of the straining test.

CONCLUSION: 5-HT₁ receptors participate in the regulation of anorectal function. Elucidation of the role of 5-HT₁ receptors in the pathophysiological mechanisms of IBS may have some therapeutic implications.

Serotonin transporter polymorphisms and side effects in antidepressant therapy – a pilot study

Objectives: To assess the influence of the serotonin transporter variable number of tandem repeat (HTT-VNTR) polymorphism and the serotonin transporter-gene-linked polymorphic region (HTTLPR) polymorphism on development of side effects under antidepressant therapy. Methods: A total of 109 depressive in-patients treated with various antidepressants according to local clinical practice were included in the investigation. Four weeks after admission to hospital, side effects were assessed by using a modified version of the dosage record and treatment emergent symptoms scale (DOTES). Differences in side effects between the genotype groups of both polymorphisms were analyzed using the Fisher’s exact test. Results: A total of 65 patients received mirtazapine (25 of them in combination with other antidepressants), and 44 patients were predominantly treated with antidepressants acting via HTT, such as selective serotonin re-uptake inhibitors (SSRIs) and tricyclic antidepressants (TCAs). When patients were treated with HTT-blocking antidepressants, a significantly higher occurrence of side effects in patients with the HTTVNTR 2.10/2.10 genotype (52.6%) than in patients with the 2.10/2.12 (12.5%) and 2.12/2.12 (0%) genotypes (p = 0.004) was found. With regard to the HTTLPR polymorphism, patients predominantly on HTT-blocking antidepressants with the s/s genotype suffered more frequently from side effects (50.0%) than heterozygotes (40.0%) and homozygotes for the l-allele (0%) (p = 0.002). In contrast, no association of the HTTVNTR polymorphism was found in patients treated with mirtazapine. The risk groups defined by a combined genotype from both polymorphisms demonstrated a major effect on the incidence of adverse drug events in patients treated with predominantly HTT-blocking antidepressants (p = 0.00018; low risk: 0%, 0/13, medium risk: 13.3%, 2/15, high risk: 62.5%, 10/16). Conclusion: These results support the hypothesis that both polymorphisms influence tolerability to drugs primarily acting via HTT inhibition, such as SSRIs, TCAs and venlafaxine. Tolerability to mirtazapine was not influenced, probably owing to a different mode of action. As there are limitations due to the heterogeneity of treatment and concomitant therapy, further studies are required to confirm the obtained results.

Mouse tryptophan hydroxylase isoform 2 and the role of proline 447 in enzyme function

Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in the synthesis of the neurotransmitter serotonin (5-HT). Once thought to be a single gene product, TPH is now known to exist in two isoforms. Isoform 1 (TPH1) is found in the pineal gland and gut, and isoform 2 (TPH2) is selectively expressed in brain. A single-nucleotide polymorphism in TPH2 results in a proline-to-arginine mutation at residue 447 and substantially lowers catalytic activity. In view of the importance of TPH in determining brain 5-HT function, we cloned TPH2 and produced the P447R mutant to assess the importance of this proline in enzyme function. Catalytically active TPH2 and the P447R mutant were expressed at the predicted subunit molecular mass of 56 kDa. The P447R mutant expressed less than 50% of the activity of TPH2. Mutation of this conserved proline in TPH1 (P403R) also resulted in an enzyme with significantly lower activity than the wild-type enzyme. The P447R mutant had a V(max) 50% lower than that of TPH2. The P447R mutation did not alter the oligomeric assembly of the protein, nor change its responsiveness to cysteine modification. The P447R mutation did not alter enzyme substrate specificity or stability, but conferred slightly enhanced sensitivity to inhibition by dopamine and diminished sensitivity to iron in catalysis. The conserved proline in TPH (residue 447 in TPH2 and 403 in TPH1) plays an important role in enzyme function by regulating V(max) of the catalytic reaction.

5-Hydroxytryptamine2C receptors on pudendal motoneurons innervating the external anal sphincter

The aim of this study was to determine the localization of 5-hydroxytryptamine2C (5-HT2C) receptors on the motoneurons innervating the external anal sphincter (EAS) of male rats. Motoneurons were retrogradely labeled after percutaneous intramuscular injection of Fluorogold (FG) into the EAS. Using fluorescent immunohistochemistry, FG-positive EAS motoneurons that were immunoreactive for the 5-HT2C receptor (5-HT2C-IR) were targeted for specific examination with widefield microscopy or confocal laser scanning microscopy with spectral separation. Widefield microscopy revealed distributions of FG-positive EAS motoneurons in the L5-S1 gray matter corresponding to the dorsomedial cell group. 5-HT2C-IR positive cells were distributed in the intermediolateral cell column and the ventral horn. Ventral horn 5-HT2C-IR labeling included the dorsomedial cell group as well as the dorsolateral, ventromedial and ventrolateral areas. Confocal analysis of FG-positive EAS motoneurons and 5-HT2C-IR positive motoneuron profiles adjacent to EAS motoneurons that were not labeled with FG but presumably innervate the bulbospongiosus muscle confirmed that EAS motoneurons were immunopositive for the 5-HT2C receptor. These data suggest that previously identified descending serotonergic immunopositive fibers observed terminating on EAS motoneurons might mediate their input through the activation of 5-HT2C receptors.

Pediatric gastrointestinal motility--future directions and challenges

The study of gastrointestinal motility has evolved to a sophisticated diagnostic technique that is widely used clinically to further guide management of children with complex gastrointestinal problems. Thorough comprehension requires a multidisciplinary approach with the integration of molecular and cell biology, organ physiology, and clinical observations. During the past decade there has been a dramatic increase in our knowledge of the enteric neuromuscular system. Continued exploration of targeted gene mutations in animal models has the potential of enhancing our understanding of congenital disorders of gastrointestinal motility. Experiments studying polymorphisms in serotonin transporter gene (SERT) and different therapeutic responses to serotonergic agents in adults with irritable bowel syndrome need to be carried out in children with functional bowel disorders. Additional considerations that need to be addressed if advances are to continue include increasing the number of specialists interest in motility disorders and identifying funding sources to support the establishment of research consortiums among pediatric centers.

Effects of 5-hydroxytryptamine on electrical responses of circular smooth muscle isolated from the guinea-pig gastric antrum

The effects of 5-hydroxytryptamine (5-HT) on electrical responses of the membrane were investigated in circular smooth muscle isolated from the guinea-pig stomach antrum. Small segment of circular muscle tissue produced a periodical generation of slow potentials at frequency of 0.1-2 cycles min(-1), during random generation of unitary potentials. Application of 5-HT (10(-7)-10(-5) M) hyperpolarized the membrane and either increased or decreased the frequency of slow potentials, both with associated increase in amplitude of slow potential. These effects of 5-HT were abolished by methysergide. N(omega)-nitro-L-arginine (L-NA) increased the frequency of spontaneously generated slow potentials and also increased the frequency of slow potentials generated during stimulation with 5-HT, suggesting an involvement of the increased production of nitric oxide (NO) by 5-HT. Atropine did not alter spontaneous and 5-HT-induced electrical responses. The hyperpolarization produced by 5-HT was associated with a decrease in input resistance and time constant of the membrane. The amplitude of the 5-HT-induced hyperpolarization was increased in low [K(+)](o) solution and decreased in high [K(+)](o) solution or in the presence of glybenclamide, suggesting that the hyperpolarization was produced by activation of ATP-sensitive K-channels. The increase in amplitude of slow potentials by 5-HT may be secondary due to hyperpolarization of the membrane. The inhibition by 5-HT of the frequency of slow potentials may be partly due to the increased release of NO, however the mechanism by which dual effects of 5-HT on the frequency of slow potentials remains unsolved.

The role of gender and biological sex in irritable bowel syndrome

Irritable bowel syndrome (IBS), which is characterized by abdominal pain or discomfort that is associated with altered bowel function (diarrhea, constipation, or alteration between the two), is one of several gastrointestinal motility disorders. IBS affects up to one in five North Americans, mostly women. The reason(s) this disorder is reported more often by women than men, and the role of gender and biological sex in the prevalence, pathophysiology, symptom presentation, impact on quality of life, diagnosis, and response to treatment, are poorly understood. The purpose of this article is to review the evidence surrounding the roles of gender and biological sex in IBS.

Symptom overlap and comorbidity of irritable bowel syndrome with other conditions

Irritable bowel syndrome (IBS) is one of several highly prevalent, multi-symptom gastrointestinal motility disorders that have a wide clinical spectrum and are associated with symptoms of gastrointestinal dysmotility and visceral hypersensitivity. Symptom overlap and comorbidity between IBS and other gastrointestinal motility disorders (eg, chronic constipation, functional dyspepsia, gastroesophageal reflux disease), with gastrointestinal disorders that are not related to motility (eg, celiac disease, lactose intolerance), and with somatic conditions (eg, fibromyalgia, chronic fatigue syndrome), are frequent. The clinical associations and pathophysiologic links between IBS and these disorders continue to be explored. This review discusses overlapping symptoms and comorbidity of IBS with select gastrointestinal and non-gastrointestinal disorders and attempts to identify commonalities among these conditions.