Serotonin excites neurons in the human submucous plexus via 5-HT3 receptors

Functional and Structural Investigation of Myenteric Neurons in the Human Colon

Background and Aims

The enteric nervous system independently controls gastrointestinal function including motility, which is primarily mediated by the myenteric plexus, therefore also playing a crucial role in functional intestinal disorders. Live recordings from human myenteric neurons proved to be challenging due to technical difficulties. Using the neuroimaging technique, we are able to record human colonic myenteric neuronal activity and investigate their functional properties in a large cohort of patients.

Methods

Activity from myenteric neurons in wholemount preparations of different sampling sites of fresh, human colonic tissue was recorded using neuroimaging with the voltage sensitive dye 1-(3-sulfanatopropyl)-4-[beta[2-(di-n-octylamino)-6-naphthyl]vinyl]pyridinium betaine. Neuronal responses were analyzed following stimulation with nicotine and serotonin (5-HT) for differences based on the donor's age, the disorder indicative for surgery and the colonic region. Immunohistochemistry was performed to calculate the total neuronal numbers.

Results

Stimulation with nicotine and 5-HT elicited reproducible action potential discharge in a proportion of human myenteric neurons. The responses to 5-HT were significantly greater in tissues from older patients and from those with inflammatory disorders, while neuronal activity to nicotinergic stimulation was comparable in all patients. Neuronal numbers declined with rising patient's age and was highest in the sigmoid colon.

Conclusion

Neuroimaging with 1-(3-sulfanatopropyl)-4-[beta[2-(di-n-octylamino)-6-naphthyl]vinyl]pyridinium betaine was successfully adapted to record reproducible responses from human colonic myenteric neurons upon pharmacological stimulation. Evidence exists for an impact of age and inflammation on the serotonergic neuronal signaling and for differences in neuronal numbers in the distinct colonic regions as well as a neuronal decrease with age.

Biopsy samples from patients with irritable bowel syndrome, but not from those with mastocytosis or unspecific gastrointestinal complaints reveal unique nerve activation in all gut regions independent of mast cell density, histamine content or specific gastrointestinal symptoms

Introduction

We previously showed enteric nerve activation after application of colonic mucosal biopsy supernatants from patients with irritable bowel syndrome (IBS). The question remains whether this is a region-specific or a generalized sensitization. We tested the nerve-activating properties of supernatants from large and small intestinal regions of IBS patients with diarrhea (IBS-D) in comparison to those from mastocytosis patients with diarrhea (MC-D) or non-IBS/non-MC patients with GI-complaints. MC-D patients were included to test samples from patients with an established, severe mast cell disorder, because mast cells are suggested to play a role in IBS.

Methods

Voltage-sensitive dye imaging was used to record the effects of mucosal biopsy supernatants from IBS-D, MC-D, and non-IBS/non-MC on guinea pig submucous neurons. Mast cell density and histamine concentrations were measured in all samples.

Results

The median neuroindex (spike frequency × % responding neurons in Hz × %) was significantly (all p < 0.001) increased for IBS-D (duodenum and colon, proximal and distal each, 49.3; 50.5; 63.7; 71.9, respectively) compared to non-IBS/non-MC (duodenum and colon, proximal and distal each, 8.7; 4.9; 6.9; 5.4, respectively) or MC-D supernatants (duodenum and colon, proximal and distal each, 9.4; 11.9; 0.0; 7.9, respectively). Nerve activation by MC-D and non-IBS/non-MC supernatants was comparable (p>0.05). Mast cell density or histamine concentrations were not different between IBS-D, MC-D, and non-IBS/non-MC samples.

Discussion

Nerve activation by biopsy supernatants is an IBS hallmark that occurs throughout the gut, unrelated to mast cell density or histamine concentration. At least as important is our finding that GI complaints per se were not associated with biopsy supernatant-induced nerve activation, which further stresses the relevance of altered nerve behavior in IBS.

Metabotropic 5-HT receptor-mediated effects in the human submucous plexus

BACKGROUND

Serotonin (5-HT) is an important mediator in the gastrointestinal tract, acting on different neuronal 5-HT receptors. The ionotropic 5-HT₃ receptor mediates immediate but transient spike discharge in human enteric neurons. We studied the role of the metabotropic 5-HT_1P , 5-HT₄ , and 5-HT₇ receptors to activate human submucous neurons.

METHODS

Neuroimaging using the voltage sensitive dye Di-8-ANEPPS was performed in submucous plexus preparations from human surgical specimens of the small and large intestine. We synthesized a new, stable 5-HT_1P agonist, 5-benzyloxyhydrazonoindalpine (5-BOHIP).

KEY RESULTS

5-HT evoked a fast and late-onset spike discharge in enteric neurons. The fast component was blocked by the 5-HT₃ receptor antagonist cilansetron, while the remaining sustained response was significantly reduced by the 5-HT_1P receptor antagonist 5-hydroxytryptophanyl-5-hydroxytryptophan amide (5-HTP-DP). The newly synthesized 5-HT_1P agonist 5-BOHIP induced a slowly developing, long-lasting activation of submucous neurons, which was blocked by 5-HTP-DP. We could not demonstrate any 5-HT₇ receptor-induced spike discharge based on the lack of response to 5-carboxamidotryptamine. Similarly, the 5-HT₄ agonists 5-methoxytryptamine and prucalopride evoked no immediate or late-onset spike discharge.

CONCLUSIONS & INFERENCES

Our work demonstrated for the first time the presence of functional 5-HT_1P receptors on human submucous neurons. Furthermore, we found no evidence for a role of 5-HT₄ or 5-HT₇ receptors in the postsynaptic activation of human submucous neurons by 5-HT.

Role of Ion Channels in the Chemotransduction and Mechanotransduction in Digestive Function and Feeding Behavior

The gastrointestinal tract constantly communicates with the environment, receiving and processing a wide range of information. The contents of the gastrointestinal tract and the gastrointestinal tract generate mechanical and chemical signals, which are essential for regulating digestive function and feeding behavior. There are many receptors here that sense intestinal contents, including nutrients, microbes, hormones, and small molecule compounds. In signal transduction, ion channels are indispensable as an essential component that can generate intracellular ionic changes or electrical signals. Ion channels generate electrical activity in numerous neurons and, more importantly, alter the action of non-neurons simply and effectively, and also affect satiety, molecular secretion, intestinal secretion, and motility through mechanisms of peripheral sensation, signaling, and altered cellular function. In this review, we focus on the identity of ion channels in chemosensing and mechanosensing in the gastrointestinal tract.

Impact of Metabolic Surgery on Gut Microbiota and Sera Metabolomic Patterns among Patients with Diabetes

Metabolic surgery is a promising treatment for obese individuals with type 2 diabetes mellitus (T2DM), but the mechanism is not completely understood. Current understanding of the underlying ameliorative mechanisms relies on alterations in parameters related to the gastrointestinal hormones, biochemistry, energy absorption, the relative composition of the gut microbiota, and sera metabolites. A total of 13 patients with obesity and T2DM undergoing metabolic surgery treatments were recruited. Systematic changes of critical parameters and the effects and markers after metabolic surgery, in a longitudinal manner (before surgery and three, twelve, and twenty-four months after surgery) were measured. The metabolomics pattern, gut microbiota composition, together with the hormonal and biochemical characterizations, were analyzed. Body weight, body mass index, total cholesterol, triglyceride, fasting glucose level, C-peptide, HbA1c, HOMA-IR, gamma-glutamyltransferase, and des-acyl ghrelin were significantly reduced two years after metabolic surgery. These were closely associated with the changes of sera metabolomics and gut microbiota. Significant negative associations were found between the Eubacterium eligens group and lacosamide glucuronide, UDP-L-arabinose, lanceotoxin A, pipercyclobutanamide B, and hordatine B. Negative associations were identified between Ruminococcaceae UCG-003 and orotidine, and glucose. A positive correlation was found between Enterococcus and glutamic acid, and vindoline. Metabolic surgery showed positive effects on the amelioration of diabetes and metabolic syndromes, which were closely associated with the change of sera metabolomics, the gut microbiota, and other disease-related parameters.

Tapping into 5-HT3 Receptors to Modify Metabolic and Immune Responses

5-hydroxytryptamine type 3 (5-HT₃) receptors are ligand gated ion channels, which clearly distinguish their mode of action from the other G-protein coupled 5-HT or serotonin receptors. 5-HT₃ receptors are well established targets for emesis and gastrointestinal mobility and are used as adjunct targets in treating schizophrenia. However, the distribution of these receptors is wider than the nervous system and there is potential that these additional sites can be targeted to modulate inflammatory and/or metabolic conditions. Recent progress in structural biology and pharmacology of 5-HT₃ receptors have provided profound insights into mechanisms of their action. These advances, combined with insights into clinical relevance of mutations in genes encoding 5-HT₃ subunits and increasing understanding of their implications in patient's predisposition to diseases and response to the treatment, open new avenues for personalized precision medicine. In this review, we recap on the current status of 5-HT₃ receptor-based therapies using a biochemical and physiological perspective. We assess the potential for targeting 5-HT₃ receptors in conditions involving metabolic or inflammatory disorders based on recent findings, underscoring the challenges and limitations of this approach.

Fast synaptic excitatory neurotransmission in the human submucosal plexus

BACKGROUND

Acetylcholine is the main excitatory neurotransmitter in the enteric nervous system (ENS) in all animal models examined so far. However, data for the human ENS is scarce.

METHODS

We used neuroimaging using voltage and calcium dyes, Ussing chamber, and immunohistochemistry to study fast synaptic neurotransmission in submucosal plexus neurons of the human gut.

KEY RESULTS

Electrical stimulation of intraganglionic fiber tracts led to fast excitatory postsynaptic potentials (fEPSPs) in 29 submucosal neurons which were all blocked by the nicotinic antagonist hexamethonium. The nicotinic agonist DMPP mimicked the effects of electrical stimulation and had excitatory effects on 56 of 73 neurons. The unselective NMDA antagonist MK-801 blocked fEPSPs in 14 out of 22 neurons as well as nicotine evoked spike discharge. In contrast, the application of NMDA showed only weak effects on excitability or calcium transients. This agreed with the finding that the specific NMDA antagonist D-APV reduced fEPSPs in only 1 out of 40 neurons. Application of AMPA or kainite had no effect in 41 neurons or evoked spike discharge in only one out of 41 neurons, respectively. Immunohistochemistry showed that 98.7 ± 2.4% of all submucosal neurons (n = 6 preparations, 1003 neurons) stained positive for the nicotinic receptor (α₁ , α₂ or α₃ -subunit). Hexamethonium (200 µM) reduced nerve-evoked chloride secretion by 34.3 ± 18.6% (n = 14 patients), whereas D-APV had no effect.

CONCLUSION & INFERENCE

Acetylcholine is the most important mediator of fast excitatory postsynaptic transmission in human submucous plexus neurons whereas glutamatergic fEPSPs were rarely encountered.

The role of serotonin in the control of esophageal sensitivity assessed by multimodal stimulation in health

BACKGROUND

Esophageal hypersensitivity is considered an important pathophysiological mechanism in refractory gastroesophageal reflux disease (GERD) patients. Serotonin (5-HT) plays an important role in the regulation of GI (gastrointestinal) secretion, motility and sensitivity. Previous studies found that altered 5-HT availability has no clear effects on esophageal/GI sensations. Our aim was therefore to investigate the role of 5-HT in esophageal sensitivity in healthy volunteers (HV).

METHODS

Esophageal sensitivity to thermal, mechanical, electrical, and chemical stimuli was assessed in 3 different placebo-controlled studies. In the first study, the effect of citalopram (40 mg; 5-HT reuptake inhibitor; intravenous) was investigated (n = 14). In the second study, the effect of buspirone (20 mg; 5HT1A agonist; oral) was investigated (n = 10). In the third study, acute tryptophan depletion (ATD) was used to decrease 5-HT levels to investigate the effect of reduced 5-HT availability on esophageal sensitivity (n = 15).

KEY RESULTS

No difference was observed in esophageal sensitivity after the administration of citalopram or buspirone (all p > 0.06). In contrast, pain perception threshold to chemical stimulation was increased after ATD (p = 0.017, Cohen's d+ = 0.67). No effect was found on the first perception or pain tolerance threshold. ATD had no influence on esophageal sensitivity to thermal, mechanical, and electrical stimulation compared with placebo.

CONCLUSIONS AND INFERENCES

ATD, which induces 5-HT depletion, significantly decreased pain perception threshold during chemical stimulation, without affecting sensitivity to mechanical, thermal, or electrical stimulation. These findings confirm the involvement of 5-HT in the control of esophageal acid sensitivity, but identifying the receptors involved requires more ligands and studies.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Submucosal enteric neurons of the cavine distal colon are sensitive to hypoosmolar stimuli

KEY POINTS

Neurons of the enteric submucous plexus are challenged by osmolar fluctuations during digestion and absorption of nutrients. Central neurons are very sensitive to changes in osmolality but knowledge on that issue related to enteric neurons is sparse. The present study focuses on investigation of osmosensitivity of submucosal neurons including potential molecular mediating mechanisms. Results show that submucosal neurons respond to hypoosmolar stimuli with increased activity which is partially mediated by the transient receptor potential vanilloid 4 channel. We provided important information on osmosensitive properties of enteric neurons. These data are fundamental to better explain the nerve-mediated control of the gastrointestinal functions during physiological and pathophysiological (diarrhoea) conditions.

ABSTRACT

Enteric neurons are located inside the gut wall, where they are confronted with changes in osmolality during (inter-) digestive periods. In particular, neurons of the submucous plexus (SMP), located between epithelial cells and blood vessels may sense and respond to osmotic shifts. The present study was conducted to investigate osmosensitivity of enteric submucosal neurons and the potential role of the transient receptor potential vanilloid 4 channel (TRPV4) as a mediator of enteric neuronal osmosensitivity. Therefore, freshly dissected submucosal preparations from guinea pig colon were investigated for osmosensitivity using voltage-sensitive dye and Ca²⁺ imaging. Acute hypoosmolar stimuli (final osmolality reached at ganglia of 94, 144 and 194 mOsm kg^-1 ) were applied to single ganglia using a local perfusion system. Expression of TRPV4 in the SMP was quantified using qRT-PCR, and GSK1016790A and HC-067047 were used to activate or block the receptor, respectively, revealing its relevance in enteric osmosensitivity. On average, 11.0 [7.0/17.0] % of submucosal neurons per ganglion responded to the hypoosmolar stimulus. The Ca²⁺ imaging experiments showed that glia responded to the hypoosmolar stimulus, but with a delay in comparison with neurons. mRNA expression of TRPV4 could be shown in the SMP and blockade of the receptor by HC-067047 significantly decreased the number of responding neurons (0.0 [0.0/6.3] %) while the TRPV4 agonist GSK1016790A caused action potential discharge in a subpopulation of osmosensitive enteric neurons. The results of the present study provide insight into the osmosensitivity of submucosal enteric neurons and strongly indicate the involvement of TRPV4 as an osmotransducer.

Compression and stretch sensitive submucosal neurons of the porcine and human colon

The pig is commonly believed to be a relevant model for human gut functions-however, there are only a few comparative studies and none on neural control mechanisms. To address this lack we identified as one central aspect mechanosensitive enteric neurons (MEN) in porcine and human colon. We used neuroimaging techniques to record responses to tensile or compressive forces in submucous neurons. Compression and stretch caused Ca-transients and immediate spike discharge in 5-11% of porcine and 15-24% of human enteric neurons. The majority of these MEN exclusively responded to either stimulus quality but about 9% responded to both. Most of the MEN expressed choline acetyltransferase and substance P; nitric oxide synthase-positive MEN primarily occurred in distal colon. The findings reveal common features of MEN in human and pig colon which we interpret as a result of species-independent evolutionary conservation rather than a specific functional proximity between the two species.

CSTI-300 (SMP-100); a Novel 5-HT3 Receptor Partial Agonist with Potential to Treat Patients with Irritable Bowel Syndrome or Carcinoid Syndrome

The 5-hydroxytryptamine (5-HT) (serotonin) 5-HT₃ receptor represents a clinical target for antagonists to deliver symptomatic relief to patients with diarrhea-predominant irritable bowel syndrome (IBS-d) or carcinoid syndrome. Unfortunately, this pharmacological strategy can present side effects (e.g., severe constipation). The present study investigates the potential of a novel 5-HT₃ receptor partial agonist, CSTI-300, to treat patients with IBS-d and other conditions associated with discomfort from colonic distension, with a predicted reduced side-effect profile. The in vitro and in vivo preclinical pharmacology of the drug CSTI-300 was investigated to explore the potential to treat patients with IBS-d. CSTI-300 displayed selective high affinity for the human and rat 5-HT₃ receptor (K_i approximately 2.0 nM) and acted as a partial agonist (approximately 30%-50% intrinsic efficacy) in vitro. In an in vivo model of IBS-d, the rat colon distension model, CSTI-300 displayed dose-dependent efficacy. In addition, oral administration of CSTI-300 to dogs that achieved plasma levels of the drug exceeding the K_i value for the 5-HT₃ receptor failed to either evoke emesis or alter the state of feces. Pharmacokinetics for CSTI-300 in rat and dog identified high levels of oral availability with t _1/2 range of 1.6-4.4 hours. The preclinical pharmacology of the lead candidate drug, CSTI-300, supports the potential of this novel drug to offer symptomatic relief to patients with irritable bowel syndrome and carcinoid syndrome with a rationale for a reduced "on-target" side-effect profile relative to 5-HT₃ receptor antagonists, such as alosetron. SIGNIFICANCE STATEMENT: There is a lack of effective current treatment for diarrhea-predominant irritable bowel syndrome and carcinoid syndrome, and in both conditions, overactivity of the 5-hydroxytryptamine (5-HT) 5-HT₃ receptor is thought to be implicated in the pathophysiology. Because 5-HT₃ receptor blockade with antagonists results in significant side effects, we present evidence that treatment with a suitable 5-HT₃ receptor partial agonist will alleviate some symptoms associated with these conditions yet, without fully inhibiting the receptor, predict a less pronounced side-effect profile associated with this therapeutic strategy.

Development of the intrinsic innervation of the small bowel mucosa and villi

Detection of nutritional and noxious food components in the gut is a crucial component of gastrointestinal function. Contents in the gut lumen interact with enteroendocrine cells dispersed throughout the gut epithelium. Enteroendocrine cells release many different hormones, neuropeptides, and neurotransmitters that communicate either directly or indirectly with the central nervous system and the enteric nervous system, a network of neurons and glia located within the gut wall. Several populations of enteric neurons extend processes that innervate the gastrointestinal lamina propria; however, how these processes develop and begin to transmit information from the mucosa is not fully understood. In this study, we found that Tuj1-immunoreactive neurites begin to project out of the myenteric plexus at embryonic day (E)13.5 in the mouse small intestine, even before the formation of villi. Using live calcium imaging, we discovered that neurites were capable of transmitting electrical information from stimulated villi to the plexus by E15.5. In unpeeled gut preparations where all layers were left intact, we also mimicked the basolateral release of 5-HT from enteroendocrine cells, which triggered responses in myenteric cell bodies at postnatal day (P)0. Altogether, our results show that enteric neurons extend neurites out of the myenteric plexus early during mouse enteric nervous system development, innervating the gastrointestinal mucosa, even before villus formation in mice of either sex. Neurites are already able to conduct electrical information at E15.5, and responses to 5-HT develop postnatally.NEW & NOTEWORTHY How enteric neurons project into the gut mucosa and begin to communicate with the epithelium during development is not known. Our study shows that enteric neurites project into the lamina propria as early as E13.5 in the mouse, before development of the submucous plexus and before formation of intestinal villi. These neurites are capable of transmitting electrical signals back to their cell bodies by E15.5 and respond to serotonin applied to neurite terminals by birth.

The gut microbiota to the brain axis in the metabolic control

The regulation of glycemia is under a tight neuronal detection of glucose levels performed by the gut-brain axis and an efficient efferent neuronal message sent to the peripheral organs, as the pancreas to induce insulin and inhibit glucagon secretions. The neuronal detection of glucose levels is performed by the autonomic nervous system including the enteric nervous system and the vagus nerve innervating the gastro-intestinal tractus, from the mouth to the anus. A dysregulation of this detection leads to the one of the most important current health issue around the world i.e. diabetes mellitus. Furthemore, the consequences of diabetes mellitus on neuronal homeostasis and activities participate to the aggravation of the disease establishing a viscious circle. Prokaryotic cells as bacteria, reside in our gut. The strong relationship between prokaryotic cells and our eukaryotic cells has been established long ago, and prokaryotic and eukaryotic cells in our body have evolved synbiotically. For the last decades, studies demonstrated the critical role of the gut microbiota on the metabolic control and how its shift can induce diseases such as diabetes. Despite an important increase of knowledge, few is known about 1) how the gut microbiota influences the neuronal detection of glucose and 2) how the diabetes mellitus-induced gut microbiota shift observed participates to the alterations of autonomic nervous system and the gut-brain axis activity.

Effect of Metformin on Antipsychotic-Induced Metabolic Dysfunction: The Potential Role of Gut-Brain Axis

Antipsychotics are the first-line medications prescribed for patients with schizophrenia or other mental disorders. Cumulative evidence has revealed that metabolic dysfunctions frequently occur in patients receiving antipsychotics, especially second-generation antipsychotics, and these effects may decrease patient compliance and increase health costs. Metformin is an effective pharmaceutical adjuvant for ameliorating antipsychotic-induced metabolic dysfunction (AIMD) in clinical practice. However, the mechanism of the effects of metformin on AIMD remains unclear. The gut-brain axis is a bidirectional communication system between the gastrointestinal tract and the central nervous system and has been associated with many pathological and physiological conditions, such as those related to metabolism. Antipsychotics interact with and have affinity for dopamine receptors and other receptors in the brain, and treatment with these antipsychotics has been shown to influence gut microbiota metabolism and composition, as observed in both animal and human studies. Metformin exerts an antidiabetic effect that is correlated with activation of AMP-kinase in the hypothalamus, and metformin also influences gut flora. Therefore, the gut-brain axis may play a role in the effect of metformin on AIMD. Since no direct evidence is available, this perspective may provide a direction for further research.

Imaging of mast cells

Mast cells are a part of the innate immune system implicated in allergic reactions and the regulation of host-pathogen interactions. The distribution, morphology and biochemical composition of mast cells has been studied in detail in vitro and on tissue sections both at the light microscopic and ultrastructural level. More recently, the development of fluorescent reporter strains and intravital imaging modalities has enabled first glimpses of the real-time behavior of mast cells in situ. In this review, we describe commonly used imaging approaches to study mast cells in cell culture as well as within normal and diseased tissues. We further describe the interrogation of mast cell function via imaging by providing a detailed description of mast cell-nerve plexus interactions in the intestinal tract. Together, visualizing mast cells has expanded our view of these cells in health and disease.

The C and E subunits of the serotonin 5-HT3 receptor subtly modulate electrical properties of the receptor

Serotonin type 3 (5-hydroxytrptamine-3, 5-HT₃) receptors are ligand-gated cation channels present in both central and peripheral nervous systems. In humans there are five different subunits (A, B, C, D and E) of 5-HT₃ receptors which can form homomeric or heteromeric receptors that may account for discrepancies in patient responses to treatments. The present study commences characterisation of the profiles of human 5-HT₃ receptors containing C and/or E subunits. Recombinant 5-HT₃ receptors were expressed transiently in HEK293T cells and expression was checked via immunocytochemistry staining against each epitope-tagged subunits. Functional characterisation of different combinations of 5-HT₃ receptor complexes was studied via patch clamp whole cell recordings. In this study, increased current was seen in cells containing A and C subunits but only subtle changes were seen in the electrical properties of cells expressing A, AE, or ACE subunits in response to the ligand, 5-HT. Both di- and tri-heteromeric 5-HT₃ receptors were significantly inhibited by the antagonists, ondansetron and palonosetron. Notably, palonosetron exerted stronger and more rapid inhibition on the 5-HT₃ receptor ACE tri-heteromer compared to homomeric and di-heteromeric counterparts. This study demonstrated that the C and E subunits when assembled as simple or complex heteromeric 5-HT₃ receptors may alter efficacies of 5-HT and clinically used antagonists such as ondansetron and palonosetron, and this in turn may have implications for patient responses to therapies.

Sensitivity to Strain and Shear Stress of Isolated Mechanosensitive Enteric Neurons

Within the enteric nervous system, the neurons in charge to control motility of the gastrointestinal tract reside in a particular location nestled between two perpendicular muscle layers which contract and relax. We used primary cultured myenteric neurons of male guinea pigs to study mechanosensitivity of enteric neurons in isolation. Ultrafast Neuroimaging with a voltage-sensitive dye technique was used to record neuronal activity in response to shear stress and strain. Strain was induced by locally deforming the elastic cell culture substrate next to a neuron. Measurements showed that substrate strain was mostly elongating cells. Shear stress was exerted by hydrodynamic forces in a microchannel. Both stimuli induced excitatory responses. Strain activated 14% of the stimulated myenteric neurons that responded with a spike frequency of 1.9 (0.7/3.2) Hz, whereas shear stress excited only a few neurons (5.6%) with a very low spike frequency of 0 (0/0.6) Hz. Thus, shear stress does not seem to be an adequate stimulus for mechanosensitive enteric neurons (MEN) while strain activates enteric neurons in a relevant manner. Analyzing the adaptation behavior of MEN showed that shear stress activated rapidly/slowly/ultraslowly adapting MEN (2/62/36%) whereas strain only slowly (46%) and ultraslowly (54%) MEN. Paired experiments with strain and normal stress revealed three mechanosensitive enteric neuronal populations: one strain-sensitive (37%), one normal stress-sensitive (17%) and one strain- and stress-sensitive (46%). These results indicate that shear stress does not play a role in the neuronal control of motility but normal stress and strain.

Tryptase potentiates enteric nerve activation by histamine and serotonin: Relevance for the effects of mucosal biopsy supernatants from irritable bowel syndrome patients

BACKGROUND

We previously showed that mucosal biopsy supernatants from irritable bowel syndrome patients activated neurons despite low concentrations of tryptase, histamine, and serotonin which individually would not cause spike discharge. We studied the potentiating responses between these mediators on excitability of enteric neurons.

METHODS

Calcium-imaging was performed using the calcium-sensitive dye Fluo-4 AM in human submucous plexus preparations from 45 individuals. Histamine, serotonin, and tryptase were applied alone and in combinations to evaluate nerve activation which was assessed by analyzing increase in intracellular Ca²⁺ ([Ca²⁺ ]_i ), the proportion of responding neurons and the product of both defined as Ca-neuroindex (NI). Protease activated receptor (PAR) 2 activating peptide, PAR2 antagonist and the serine protease-inhibitor FUT-175 were used to particularly investigate the role of proteases.

KEY RESULTS

Histamine or serotonin (1 μmol/L each) evoked only few small responses (median NI [25%/75%]: 0 [0/148]; 85 [0/705] respectively). Their combined application evoked statistically similar responses (216 [21/651]). Addition of the PAR2 activator tryptase induced a significantly higher Ca-NI (1401 [867/4075]) compared to individual application of tryptase or to coapplied histamine and serotonin. This synergistic potentiation was neither mimicked by PAR2 activating peptide nor reversed by the PAR2 antagonist GB83, but abolished by FUT-175.

CONCLUSIONS & INFERENCES

We observed synergistic potentiation between histamine, serotonin, and tryptase in enteric neurons, which is mediated by proteolytic activity rather than PAR2 activation. This explained neuronal activation by a cocktail of these mediators despite their low concentrations and despite a relatively small PAR2-mediated response in human submucous neurons.

Lamotrigine, an antiepileptic drug, inhibits 5-HT3 receptor currents in NCB-20 neuroblastoma cells

Lamotrigine is an antiepileptic drug widely used to treat epileptic seizures. Using whole-cell voltage clamp recordings in combination with a fast drug application approach, we investigated the effects of lamotrigine on 5-hydroxytryptamine (5-HT)₃ receptors in NCB-20 neuroblastoma cells. Co-application of lamotrigine (1~300 µM) resulted in a concentration-dependent reduction in peak amplitude of currents induced by 3 µM of 5-HT for an IC₅₀ value of 28.2±3.6 µM with a Hill coefficient of 1.2±0.1. These peak amplitude decreases were accompanied by the rise slope reduction. In addition, 5-HT₃-mediated currents evoked by 1 mM dopamine, a partial 5-HT₃ receptor agonist, were inhibited by lamotrigine co-application. The EC₅₀ of 5-HT for 5-HT₃ receptor currents were shifted to the right by co-application of lamotrigine without a significant change of maximal effect. Currents activated by 5-HT and lamotrigine co-application in the presence of 1 min pretreatment of lamotrigine were similar to those activated by 5-HT and lamotrigine co-application alone. Moreover, subsequent application of lamotrigine in the presence of 5-HT and 5-hydroxyindole, known to attenuate 5-HT₃ receptor desensitization, inhibited 5-HT₃ receptor currents in a concentration-dependent manner. The deactivation of 5-HT₃ receptor was delayed by washing with an external solution containing lamotrigine. Lamotrigine accelerated the desensitization process of 5-HT₃ receptors. There was no voltage-dependency in the inhibitory effects of lamotrigine on the 5-HT₃ receptor currents. These results indicate that lamotrigine inhibits 5-HT₃-activated currents in a competitive manner by binding to the open state of the channels and blocking channel activation or accelerating receptor desensitization.

Anti-Hu antibodies activate enteric and sensory neurons

IgG of type 1 anti-neuronal nuclear antibody (ANNA-1, anti-Hu) specificity is a serological marker of paraneoplastic neurological autoimmunity (including enteric/autonomic) usually related to small-cell lung carcinoma. We show here that IgG isolated from such sera and also affinity-purified anti-HuD label enteric neurons and cause an immediate spike discharge in enteric and visceral sensory neurons. Both labelling and activation of enteric neurons was prevented by preincubation with the HuD antigen. Activation of enteric neurons was inhibited by the nicotinic receptor antagonists hexamethonium and dihydro-β-erythroidine and reduced by the P2X antagonist pyridoxal phosphate-6-azo (benzene-2,4-disulfonic acid (PPADS) but not by the 5-HT₃ antagonist tropisetron or the N-type Ca-channel blocker ω-Conotoxin GVIA. Ca⁺⁺ imaging experiments confirmed activation of enteric neurons but not enteric glia. These findings demonstrate a direct excitatory action of ANNA-1, in particular anti-HuD, on visceral sensory and enteric neurons, which involves nicotinic and P2X receptors. The results provide evidence for a novel link between nerve activation and symptom generation in patients with antibody-mediated gut dysfunction.

Synthesis and Pharmacological Evaluation of [11C]Granisetron and [18F]Fluoropalonosetron as PET Probes for 5-HT3 Receptor Imaging

Serotonin-gated ionotropic 5-HT₃ receptors are the major pharmacological targets for antiemetic compounds. Furthermore, they have become a focus for the treatment of irritable bowel syndrome (IBS) and there is some evidence that pharmacological modulation of 5-HT₃ receptors might alleviate symptoms of other neurological disorders. Highly selective, high-affinity antagonists, such as granisetron (Kytril) and palonosetron (Aloxi), belong to a family of drugs (the "setrons") that are well established for clinical use. To enable us to better understand the actions of these drugs in vivo, we report the synthesis of 8-fluoropalonosetron (15) that has a binding affinity (K_i = 0.26 ± 0.05 nM) similar to the parent drug (K_i = 0.21 ± 0.03 nM). We radiolabeled 15 by nucleophilic ¹⁸F-fluorination of an unsymmetrical diaryliodonium palonosetron precursor and achieved the radiosynthesis of 1-(methyl-¹¹C)-N-granisetron ([¹¹C]2) through N-alkylation with [¹¹C]CH₃I, respectively. Both compounds [¹⁸F]15 (chemical and radiochemical purity >95%, specific activity 41 GBq/μmol) and [¹¹C]2 (chemical and radiochemical purity ≥99%, specific activity 170 GBq/μmol) were evaluated for their utility as positron emission tomography (PET) probes. Using mouse and rat brain slices, in vitro autoradiography with both [¹⁸F]15 and [¹¹C]2 revealed a heterogeneous and displaceable binding in cortical and hippocampal regions that are known to express 5-HT₃ receptors at significant levels. Subsequent PET experiments suggested that [¹⁸F]15 and [¹¹C]2 are of limited utility for the PET imaging of brain 5-HT₃ receptors in vivo.

The Human Serotonin Type 3 Receptor Gene (HTR3A-E) Allelic Variant Database

Serotonin type 3 (5-HT₃ ) receptors are ligand-gated ion channels formed by five subunits (5-HT3A-E), which are encoded by the HTR3A, HTR3B, HTR3C, HTR3D, and HTR3E genes. Functional receptors are pentameric complexes of diverse composition. Different receptor subtypes confer a predisposition to nausea and vomiting during chemotherapy, pregnancy, and following surgery. In addition, different subtypes contribute to neurogastroenterologic disorders such irritable bowel syndrome (IBS) and eating disorders as well as comorbid psychiatric conditions. 5-HT₃ receptor antagonists are established treatments for emesis and IBS and are beneficial in the treatment of psychiatric diseases. Several case-control and pharmacogenetic studies have demonstrated an association between HTR3 variants and psychiatric and neurogastroenterologic phenotypes. Recently, their potential as predictors of nausea and vomiting and treatment of psychiatric disorders became evident. This information is now available in the serotonin receptor 3 HTR3 gene allelic variant database (www.htr3.uni-hd.de), which contains five sub-databases, one for each of the five different serotonin receptor genes HTR3A-E. Information on HTR3 variants, their functional relevance, associated phenotypes, and pharmacogenetic data such as drug response and side effects are available. This central information pool should help clinicians as well as scientists to evaluate their findings and to use the relevant information for subsequent genotype-phenotype correlation studies and pharmacogenetic approaches.

The antidepressant bupropion is a negative allosteric modulator of serotonin type 3A receptors

The FDA-approved antidepressant and smoking cessation drug bupropion is known to inhibit dopamine and norepinephrine reuptake transporters, as well as nicotinic acetylcholine receptors (nAChRs) which are cation-conducting members of the Cys-loop superfamily of ion channels, and more broadly pentameric ligand-gated ion channels (pLGICs). In the present study, we examined the ability of bupropion and its primary metabolite hydroxybupropion to block the function of cation-selective serotonin type 3A receptors (5-HT_3ARs), and further characterized bupropion's pharmacological effects at these receptors. Mouse 5-HT_3ARs were heterologously expressed in HEK-293 cells or Xenopus laevis oocytes for equilibrium binding studies. In addition, the latter expression system was utilized for functional studies by employing two-electrode voltage-clamp recordings. Both bupropion and hydroxybupropion inhibited serotonin-gated currents from 5-HT_3ARs reversibly and dose-dependently with inhibitory potencies of 87 μM and 112 μM, respectively. Notably, the measured IC₅₀ value for hydroxybupropion is within its therapeutically-relevant concentrations. The blockade by bupropion was largely non-competitive and non-use-dependent. Unlike its modulation at cation-selective pLGICs, bupropion displayed no significant inhibition of the function of anion-selective pLGICs. In summary, our results demonstrate allosteric blockade by bupropion of the 5-HT_3AR. Importantly, given the possibility that bupropion's major active metabolite may achieve clinically relevant concentrations in the brain, our novel findings delineate a not yet identified pharmacological principle underlying its antidepressant effect.

Mechanical stress activates neurites and somata of myenteric neurons

The particular location of myenteric neurons, sandwiched between the 2 muscle layers of the gut, implies that their somata and neurites undergo mechanical stress during gastrointestinal motility. Existence of mechanosensitive enteric neurons (MEN) is undoubted but many of their basic features remain to be studied. In this study, we used ultra-fast neuroimaging to record activity of primary cultured myenteric neurons of guinea pig and human intestine after von Frey hair evoked deformation of neurites and somata. Independent component analysis was applied to reconstruct neuronal morphology and follow neuronal signals. Of the cultured neurons 45% (114 out of 256, 30 guinea pigs) responded to neurite probing with a burst spike frequency of 13.4 Hz. Action potentials generated at the stimulation site invaded the soma and other neurites. Mechanosensitive sites were expressed across large areas of neurites. Many mechanosensitive neurites appeared to have afferent and efferent functions as those that responded to deformation also conducted spikes coming from the soma. Mechanosensitive neurites were also activated by nicotine application. This supported the concept of multifunctional MEN. 14% of the neurons (13 out of 96, 18 guinea pigs) responded to soma deformation with burst spike discharge of 17.9 Hz. Firing of MEN adapted rapidly (RAMEN), slowly (SAMEN), or ultra-slowly (USAMEN). The majority of MEN showed SAMEN behavior although significantly more RAMEN occurred after neurite probing. Cultured myenteric neurons from human intestine had similar properties. Compared to MEN, dorsal root ganglion neurons were activated by neurite but not by soma deformation with slow adaptation of firing. We demonstrated that MEN exhibit specific features very likely reflecting adaptation to their specialized functions in the gut.

Ion channels gated by acetylcholine and serotonin: structures, biology, and drug discovery

The nicotinic acetylcholine receptors (nAChRs) and the 5-HT3 receptors (5-HT3Rs) are cation-selective members of the pentameric ligand-gated ion channels (pLGICs), which are oligomeric protein assemblies that convert a chemical signal into an ion flux through postsynaptic membrane. They are critical components for synaptic transmission in the nervous system, and their dysfunction contributes to many neurological disorders. The diverse subunit compositions of pLGICs give rise to complex mechanisms of ligand recognition, channel gating, and ion-selective permeability, which have been demonstrated in numerous electrophysiological and molecular biological studies, and unraveled by progress in studying the structural biology of this protein family. In this review, we discuss recent insights into the structural and functional basis of two cation-selective pLGICs, the nAChR and the 5-HT3R, including their subunit compositions, ligand binding, and channel gating mechanisms. We also discuss their relevant pharmacology and drug discovery for treating various neurological disorders. Finally, we review a model of two alternative ion conducting pathways based on the latest 5-HT3A crystal structure.

Distribution of 5-HT3, 5-HT4, and 5-HT7 Receptors Along the Human Colon

Background/Aims

Several disorders of the gastrointestinal tract are associated with abnormal serotonin (5-HT) signaling or metabolism where the 5-HT₃ and 5-HT₄ receptors are clinically relevant. The aim was to examine the distribution of 5-HT₃, 5-HT₄, and 5-HT₇ receptors in the normal human colon and how this is associated with receptor interacting chaperone 3, G protein coupled receptor kinases, and protein LIN-7 homologs to extend previous observations limited to the sigmoid colon or the upper intestine.

Methods

Samples from ascending, transverse, descending, and sigmoid human colon were dissected into 3 separate layers (mucosa, longitudinal, and circular muscles) and ileum samples were dissected into mucosa and muscle layers (n = 20). Complementary DNA was synthesized by reverse transcription from extracted RNA and expression was determined by quantitative or end point polymerase chain reaction.

Results

The 5-HT₃ receptor subunits were found in all tissues throughout the colon and ileum. The A subunit was detected in all samples and the C subunit was expressed at similar levels while the B subunit was expressed at lower levels and less frequently. The 5-HT₃ receptor E subunit was mainly found in the mucosa layers. All splice variants of the 5-HT₄ and 5-HT₇ receptors were expressed throughout the colon although the 5-HT₄ receptor d, g, and i variants were expressed less often.

Conclusions

The major differences in 5-HT receptor distribution within the human colon are in relation to the mucosa and muscular tissue layers where the 5-HT₃ receptor E subunit is predominantly found in the mucosal layer which may be of therapeutic relevance.

Neuropharmacology of purinergic receptors in human submucous plexus: Involvement of P2X₁, P2X₂, P2X₃ channels, P2Y and A₃ metabotropic receptors in neurotransmission

RATIONALE

The role of purinergic signaling in human ENS is not well understood. We sought to further characterize the neuropharmacology of purinergic receptors in human ENS and test the hypothesis that endogenous purines are critical regulators of neurotransmission.

EXPERIMENTAL APPROACH

LSCM-Fluo-4/(Ca(2+))-imaging of postsynaptic Ca(2+) transients (PSCaTs) was used as a reporter of synaptic transmission evoked by fiber tract electrical stimulation in human SMP surgical preparations. Pharmacological analysis of purinergic signaling was done in 1,556 neurons (identified by HuC/D-immunoreactivity) in 235 ganglia from 107 patients; P2XR-immunoreactivity was evaluated in 19 patients. Real-time MSORT (Di-8-ANEPPS) imaging tested effects of adenosine on fast excitatory synaptic potentials (fEPSPs).

RESULTS

Synaptic transmission is sensitive to pharmacological manipulations that alter accumulation of extracellular purines: Apyrase blocks PSCaTs in a majority of neurons. An ecto-NTPDase-inhibitor 6-N,N-diethyl-D-β,γ-dibromomethyleneATP or adenosine deaminase augments PSCaTs. Blockade of reuptake/deamination of eADO inhibits PSCaTs. Adenosine inhibits fEPSPs and PSCaTs (IC50 = 25 µM), sensitive to MRS1220-antagonism (A3AR). A P2Y agonist ADPβS inhibits PSCaTs (IC50 = 111 nM) in neurons without stimulatory ADPbS responses (EC50 = 960 nM). ATP or a P2X1,2,2/3 (α,β-MeATP) agonist evokes fast, slow, biphasic Ca(2+) transients or Ca(2+) oscillations (ATP,EC50 = 400 mM). PSCaTs are sensitive to P2X1 antagonist NF279. Low (20 nM) or high (5 µM) concentrations of P2X antagonist TNP-ATP block PSCaTs in different neurons; proportions of neurons with P2XR-immunoreactivity follow the order P2X2 > P2X1 >> P2X3; P2X1 + P2X2 and P2X3 + P2X2 are co-localized. RT-PCR identified mRNA-transcripts for P2X1-7, P2Y1,2,12-14R.

CONCLUSIONS

Purines are critical regulators of neurotransmission in human ENS. Purinergic signaling involves P2X1, P2X2, P2X3 channels, P2X1 + P2X2 co-localization and inhibitory P2Y or A3 receptors. These are potential novel therapeutic targets for neurogastroenterology.

Reduced Responses of Submucous Neurons from Irritable Bowel Syndrome Patients to a Cocktail Containing Histamine, Serotonin, TNFα, and Tryptase (IBS-Cocktail)

BACKGROUND AND AIMS

Malfunctions of enteric neurons are believed to play an important role in the pathophysiology of irritable bowel syndrome (IBS). Our aim was to investigate whether neuronal activity in biopsies from IBS patients is altered in comparison to healthy controls (HC).

METHODS

Activity of human submucous neurons in response to electrical nerve stimulation and local application of nicotine or a mixture of histamine, serotonin, tryptase, and TNF-α (IBS-cocktail) was recorded in biopsies from 17 HC and 35 IBS patients with the calcium-sensitive-dye Fluo-4 AM. The concentrations of the mediators resembeled those found in biopsy supernatants or blood. Neuronal activity in guinea-pig submucous neurons was studied with the voltage-sensitive-dye di-8-ANEPPS.

RESULTS

Activity in submucous ganglia in response to nicotine or electrical nerve stimulation was not different between HC and IBS patients (P = 0.097 or P = 0.448). However, the neuronal response after application of the IBS-cocktail was significantly decreased (P = 0.039) independent of whether diarrhea (n = 12), constipation (n = 5) or bloating (n = 5) was the predominant symptom. In agreement with this we found that responses of submucous ganglia conditioned by overnight incubation with IBS mucosal biopsy supernatant to spritz application of this supernatant was significantly reduced (P = 0.019) when compared to incubation with HC supernatant.

CONCLUSION

We demonstrated for the first time reduced neuronal responses in mucosal IBS biopsies to an IBS mediator cocktail. While excitability to classical stimuli of enteric neurons was comparable to HC, the activation by the IBS-cocktail was decreased. This was very likely due to desensitization to mediators constantly released by mucosal and immune cells in the gut wall of IBS patients.

Inhibition of native 5-HT3 receptor-evoked contractions in guinea pig and mouse ileum by antimalarial drugs

Quinine, chloroquine and mefloquine are commonly used to treat malaria, however, with associated gastrointestinal (GI) side-effects. These drugs act as antagonists at recombinant 5-HT3 receptors and modulate gut peristalsis. These gastrointestinal side effects may be the result of antagonism at intestinal 5-HT3 receptors. Ileum from male C57BL/6 mice and guinea pigs was mounted longitudinally in organ baths. The concentration-response curves for 5-HT and the selective 5-HT3 agonist 2-Me-5-HT were obtained with 5-HT (pEC50 = 7.57 ± 0.33, 12) more potent (P = 0.004) than 2-Me-5-HT (pEC50 = 5.45 ± 0.58, n = 5) in mouse ileum. There was no difference in potency of 5-HT (pEC50 = 5.42 ± 0.15, n = 8) and 2-Me-5-HT (pIC50 = 5.01 ± 0.55, n = 11) in guinea pig ileum (P > 0.05). Quinine, chloroquine or mefloquine was applied for 10 min and inhibitions prior to submaximal agonist application. In mouse ileum, quinine, chloroquine and mefloquine antagonised 5-HT-induced contractions (pIC50 = 4.9 ± 0.17, n = 7; 4.76 ± 0.14, n = 5; 6.21 ± 0.2, n = 4, correspondingly) with mefloquine most potent (P < 0.05). Quinine, chloroquine and mefloquine antagonised 2-me-5-HT-induced contractions (pIC50 = 6.35 ± 0.11, n = 8; 4.64 ± 0.2, n = 7; 5.11 ± 0.22, n = 6, correspondingly) with quinine most potent (P < 0.05). In guinea-pig ileum, quinine, chloroquine and mefloquine antagonised 5-HT-induced contractions (pIC50 = 5.02 ± 0.15, n = 6; 4.54 ± 0.1, n = 7; 5.32 ± 0.13, n = 5) and 2-me-5-HT-induced contractions (pIC50 = 4.62 ± 0.25, n = 5; 4.56 ± 0.14, n = 6; 5.67 ± 0.12, n = 4) with chloroquine least potent against 5-HT and mefloquine most potent against 2-me-5-HT (P < 0.05). These results support previous studies identifying anti-malarial drugs as antagonists at recombinant 5-HT3 receptors and may also demonstrate the ability of these drugs to influence native 5-HT3 receptor-evoked contractile responses which may account for their associated GI side-effects.

Natural compounds boldine and menthol are antagonists of human 5-HT3 receptors: implications for treating gastrointestinal disorders

BACKGROUND

Impaired 5-HT3 receptor function is likely involved in the pathogenesis of functional gastrointestinal disorders (FGID) and 5-HT3 receptor antagonists are effective treatments for chemotherapy-induced nausea and vomiting (CINV) and irritable bowel syndrome (IBS). The monoterpene alcohol menthol and the aporphine alkaloid boldine combat symptoms of gastrointestinal diseases; both interact with other members of the Cys-loop ligand-gated ion channel family and may therefore also act on 5-HT3 receptors.

METHODS

The impact of boldine and menthol on human recombinant homomeric 5-HT3 A- and heteromeric 5-HT3 AB receptors in HEK293 cells was determined by radioligand binding, a luminescence-based Ca(2+) assay, and a membrane potential assay. 5-HT3 protein and mRNA expression was assessed in human colon tissue.

KEY RESULTS

Boldine and menthol inhibited the 5-HT-induced activation of 5-HT3 receptors in the low and middle micromolar range, respectively. Boldine was a competitive antagonist of both receptors being 6.5- to 10-fold more potent at 5-HT3 A- vs 5-HT3 AB receptors. Menthol non-competitively and stereoselectively inhibited both receptors: In contrast to (+)-menthol, (-)-menthol was significantly more potent toward 5-HT3 A- vs 5-HT3 AB receptors. We show co-expression of 5-HT3A and 5-HT3B subunits in the human gut epithelium, the lamina propria, the myenteric plexus, and the muscular cell layer.

CONCLUSIONS & INFERENCES

The demonstrated 5-HT3 inhibitory effects may be relevant for boldine's and menthol's alleviating properties on FGID and may encourage clinical studies with the compounds or the plant extracts for CINV and IBS treatment. The found receptor-discriminative properties make boldine and (-)-menthol to potentially useful tools for analyzing structural differences between these receptor subtypes.

Luminal trypsin induces enteric nerve-mediated anion secretion in the mouse cecum

Proteases play a diverse role in health and disease. An excessive concentration of proteases has been found in the feces of patients with inflammatory bowel disease or irritable bowel syndrome and been implicated in the pathogenesis of such disorders. This study examined the effect of the serine protease, trypsin, on intestinal epithelial anion secretion when added to the luminal side. A mucosal-submucosal sheet of the mouse cecum was mounted in Ussing chambers, and the short-circuit current (I sc) was measured. Trypsin added to the mucosal (luminal) side increased I sc with an ED50 value of approximately 10 μM. This I sc increase was suppressed by removing Cl(-) from the bathing solution. The I sc increase induced by 10-100 μM trypsin was substantially suppressed by tetrodotoxin, and partially inhibited by a neurokinin-1 receptor antagonist, but not by a muscarinic or nicotinic ACh-receptor antagonist. The trypsin-induced I sc increase was also significantly inhibited by a 5-hydroxytryptamine-3 receptor (5-HT3) antagonist and substantially suppressed by the simultaneous addition of both 5-HT3 and 5-HT4 receptor antagonists. We conclude that luminal trypsin activates the enteric reflex to induce anion secretion, 5-HT and substance P playing important mediating roles in this secreto-motor reflex. Luminal proteases may contribute to the cause of diarrhea occurring with some intestinal disorders.

Translational neuropharmacology: the use of human isolated gastrointestinal tissues

Translational sciences increasingly emphasize the measurement of functions in native human tissues. However, such studies must confront variations in patient age, gender, genetic background and disease. Here, these are discussed with reference to neuromuscular and neurosecretory functions of the human gastrointestinal (GI) tract. Tissues are obtained after informed consent, in collaboration with surgeons (surgical techniques help minimize variables) and pathologists. Given the difficulties of directly recording from human myenteric neurones (embedded between muscle layers), enteric motor nerve functions are studied by measuring muscle contractions/relaxations evoked by electrical stimulation of intrinsic nerves; responses are regionally dependent, often involving cholinergic and nitrergic phenotypes. Enteric sensory functions can be studied by evoking the peristaltic reflex, involving enteric sensory and motor nerves, but this has rarely been achieved. As submucosal neurones are more accessible (after removing the mucosa), direct neuronal recordings are possible. Neurosecretory functions are studied by measuring changes in short-circuit current across the mucosa. For all experiments, basic questions must be addressed. Because tissues are from patients, what are the controls and the influence of disease? How long does it take before function fully recovers? What is the impact of age- and gender-related differences? What is the optimal sample size? Addressing these and other questions minimizes variability and raises the scientific credibility of human tissue research. Such studies also reduce animal use. Further, the many differences between animal and human GI functions also means that human tissue research must question the ethical validity of using strains of animals with unproved translational significance.

Effect of hyoscine butylbromide (Buscopan®) on cholinergic pathways in the human intestine

BACKGROUND

Hyoscine butylbromide (HBB, Buscopan(®) ) is clinically used to treat intestinal cramps and visceral pain. Various studies, mainly on animal tissues, suggested that its antimuscarinic action is responsible for its spasmolytic effect. However, functional in vitro studies with human tissue have not been performed so far.

METHODS

We wanted to provide a comprehensive study on the mode of action of HBB in human intestinal samples and investigated HBB (1 nmol L(-1) -10 μmol L(-1)) effects on muscle activity with isometric force transducers and calcium imaging, on epithelial secretion with Ussing chamber technique and on enteric neurons using fast neuroimaging.

KEY RESULTS

Hyoscine butylbromide concentration dependently reduced muscle contractions, calcium mobilization, and epithelial secretion induced by the muscarinic agonist bethanechol with IC50 values of 429, 121, and 224 nmol L(-1), respectively. Forskolin-induced secretion was not altered by HBB. Cholinergic muscarinic muscle and epithelial responses evoked by electrical nerve stimulation were inhibited by 1-10 μmol L(-1) HBB. Moreover, HBB significantly reduced the bethanechol-induced action potential discharge in enteric neurons. Interestingly, we observed that high concentrations of HBB (10 μmol L(-1)) moderately decreased nicotinic receptor-mediated secretion, motility, and nerve activity.

CONCLUSIONS & INFERENCES

The results demonstrated the strong antimuscarinic action of HBB whereas the nicotinic antagonism at higher concentrations plays at most a moderate modulatory role. The muscle relaxing effect of HBB and its inhibition of muscarinic nerve activation likely explain its clinical use as an antispasmodic drug. Our results further highlight a so far unknown antisecretory action of HBB which warrants further clinical studies on its use in secretory disorders.

Ginger and its pungent constituents non-competitively inhibit activation of human recombinant and native 5-HT3 receptors of enteric neurons

BACKGROUND

Beneficial effects of ginger in the treatment of gastrointestinal (GI) problems and chemotherapy-induced nausea and vomiting are well accepted. In rodents, the action of ginger seems to be mediated by the inhibition of 5-HT3 receptors, which are established targets to combat emesis and irritable bowel syndrome.

METHODS

Heterologously expressed human 5-HT3 A or 5-HT3 AB receptors were characterized by means of Ca(2+) influx studies using HEK293 cells. Complementing Ca(2+) measurements in Fluo-4-AM-stained whole-mount preparations of the human submucous plexus were carried out. Furthermore, [3H]GR65630 binding assays were performed to reveal the mode of action of ginger and its pungent compounds.

KEY RESULTS

We show for the first time that ginger extracts and its pungent arylalkane constituents concentration-dependently inhibit activation of human 5-HT3 receptors. Ginger extracts inhibited both receptors with increasing content of pungent compounds, confirming that these are part of ginger's active principle. Inhibition potencies of the arylalkanes 6-gingerol and 6-shogaol on both receptors were in the low micromolar range. A lipophilic ginger extract and 6-gingerol had no influence on 5-HT potency, but reduced the 5-HT maximum effect, indicating non-competitive inhibition. The non-competitive action was confirmed by [(3) H]GR65630 binding, showing that the ginger extract did not displace the radioligand from 5-HT3 A and 5-HT3 AB receptors. The potential relevance of the inhibitory action of ginger on native 5-HT3 receptors in the gut was confirmed in whole-mount preparations of the human submucous plexus. While a general neurotoxic effect of 6-gingerol was ruled out, it inhibited the 2-methyl-5-HT-mediated activation of 5-HT3 receptors residing on enteric neurons.

CONCLUSIONS & INFERENCES

Our findings may encourage the use of ginger extracts to alleviate nausea in cancer patients receiving chemotherapy and to treat functional GI disorders.

Submucous rather than myenteric neurons are activated by mucosal biopsy supernatants from irritable bowel syndrome patients

BACKGROUND

We previously showed that colonic mucosal biopsy supernatants from patients with irritable bowel syndrome (IBS) activate neurons of the human submucous plexus, an area with densely packed immune cells. Based on the concept that mucosa-nerve signaling is altered in IBS, we tested in this study whether the nerve sensitizing effect of IBS mucosal biopsy supernatants is more prominent in the submucous than myenteric plexus.

METHODS

Fast neuroimaging with the voltage-sensitive dye Di-8-ANEPPS was used to record activity of guinea-pig submucous and myenteric neurons after application of constipation (C)- and diarrhea (D)-IBS supernatants (three each) and four supernatants from healthy control subjects. Results are based on recordings from 4731 neurons.

KEY RESULTS

Control supernatants did not evoke significant responses in submucous or myenteric neurons. In contrast, all IBS supernatants evoked a significant spike discharge (median 3.6 Hz) in 46% of submucous neurons. This activation was significantly stronger than in the myenteric plexus where even twice the amount of supernatants evoked a lower spike frequency (median 2.1Hz) in only 8.5% of neurons. Pharmacological studies revealed serotonin, histamine, and proteases as components mediating neuronal activation. Individual application of these components revealed that only serotonin evoked a significantly stronger activation of submucous compared with myenteric neurons.

CONCLUSIONS & INFERENCES

Direct neuronal activation by IBS mucosal biopsy supernatants is primarily a feature of submucous rather than myenteric neurons. This is associated with a stronger excitation of submucous neurons by serotonin. The plexus-specific effects support the concept that altered mucosa-nerve signaling underlies disturbances in IBS.

Activity of protease-activated receptors in primary cultured human myenteric neurons

Activity of the four known protease-activated receptors (PARs) has been well studied in rodent enteric nervous system and results in animal models established an important role for neuronal PAR2. We recently demonstrated that, unlike in rodents, PAR1 is the dominant neuronal protease receptor in the human submucous plexus. With this study we investigated whether this also applies to the human myenteric plexus. We used voltage sensitive dye recordings to detect action potential discharge in primary cultures of human myenteric neurons in response to PAR activating peptides (APs). Application of the PAR1-AP (TFLLR) or PAR4-AP (GYPGQV) evoked spike discharge in 79 or 23% of myenteric neurons, respectively. The PAR1-AP response was mimicked by the endogenous PAR1 activator thrombin and blocked by the PAR1 antagonists SCH79797. Human myenteric neurons did not respond to PAR2-AP. This was not due to culture conditions because all three PAR-APs evoked action potentials in cultured guinea pig myenteric neurons. Consecutive application of PAR-APs revealed coexpression (relative to the population responding to PAR-APs) of PAR1/PAR2 in 51%, PAR1/PAR4 in 43%, and of PAR2/PAR4 in 29% of guinea pig myenteric neurons. Our study provided further evidence for the prominent role of neuronal PAR1 in the human enteric nervous system.

Enteroendocrine and neuronal mechanisms in pathophysiology of acute infectious diarrhea

BACKGROUND

While enterocyte secretion is the predominant mechanism considered responsible for secretory diarrhea in response to acute enteric infections, there are several lines of evidence that support alternative mechanisms controlling fluid and electrolyte secretion in diarrhea.

AIM

To review enteroendocrine and neuronal mechanisms that participate in the development of acute infectious diarrhea.

RECENT ADVANCES

Acute infectious diarrheas due to bacterial toxins (e.g., cholera, E. coli heat-stable enterotoxin, C. difficile) and rotavirus are all associated with secretion of transmitters from enteroendocrine cells (e.g., 5-HT) and activation of afferent neurons that stimulate submucosal secretomotor neurons. The latter secrete acetylcholine (which binds to muscarinic receptors on epithelial cells) and VIP. Involvement of nerves was demonstrated by inhibition of bacterial toxin-induced secretion by hexamethonium (nicotinic), tetrodotoxin (Na(+) channel blocker), and lidocaine (visceral/mucosal afferents). Nicotinic receptors are present on secretomotoneurons and these are activated by release of acetylcholine from enteric interneurons or extrinsic efferent fibers. Specific organisms also modify other mechanisms that may contribute to development of acute diarrhea. Thus, mucin secretion, activation of motor mechanisms, increased mucosal permeability and inhibition of bile acid absorption have been reported in specific types of acute infectious diarrhea.

CONCLUSION

New therapies targeting neural and transmitter mediation including 5-HT, VIP, NPY, as well as toxin receptors and channels activated during acute infectious diarrhea could usher in a novel approach to enhancing glucose-electrolyte solutions used in the treatment of acute diarrhea.

Activity of protease-activated receptors in the human submucous plexus

BACKGROUND & AIMS

Protease-activated receptors (PARs) are expressed in the enteric nervous system. Excessive release of proteases has been reported in functional and inflammatory bowel diseases. Studies in several animal models indicate the involvement of neural PARs. We studied the actions of different PAR-activating peptides (AP) in the human submucous plexus and performed comparative studies in guinea pig submucous neurons.

METHODS

We used voltage- and calcium-sensitive dye recordings to study the effects of PAR1-AP, PAR2-AP, PAR4-AP, the PAR1 activator thrombin, and the PAR2 activator tryptase on neurons and glia in human and guinea pig submucous plexus. Human preparations were derived from surgical resections. Levels of mucosal secretion evoked by PAR-APs were measured in Ussing chambers.

RESULTS

PAR1-AP and thrombin evoked a prominent spike discharge and intracellular Ca(2+) concentration ([Ca](i)) transients in most human submucous neurons and glia. PAR2-AP, tryptase, and PAR4-AP caused significantly weaker responses in a minor population. In contrast, PAR2-AP evoked much stronger responses in enteric neurons and glia of guinea pigs than did PAR1-AP or PAR4-AP. PAR1-AP, but not PAR2-AP or PAR4-AP, evoked a nerve-mediated secretion in human epithelium. The PAR1 antagonist SCH79797 inhibited the PAR1-AP, and thrombin evoked responses on neurons, glia, and epithelial secretion. In the submucous layer of human intestine, but not guinea pig intestine, PAR2-AP evoked [Ca](i) signals in CD68(+) macrophages.

CONCLUSIONS

In the human submucous plexus, PAR1, rather than PAR2 or PAR4, activates nerves and glia. These findings indicate that PAR1 should be the focus of future studies on neural PAR-mediated actions in the human intestine; PAR1 might be developed as a therapeutic target for gastrointestinal disorders associated with increased levels of proteases.

Effect of 5-HT and electrical stimulating on gastric electric activity in rabbits with gastric dysrhythmia

AIM: To evaluate the effect of 5-hydroxytryptamine (5-HT) and electrical stimulation on gastric electric activity (GEA) in rabbits with gastric dysrhythmia (GD).

METHODS: GD was induced in rabbits by peripheral injection of glucagon. Peripheral injection of 5-HT, gastric pacemaking, and electrical stimulation of the paraventricular nucleus (PVN) and raphe magnus (RM, P7) were performed in GD rabbits. Four pairs of bipolar Ag-AgCl electrodes implanted on the serosa along the gastric greater curve were used for recording GEA. The stimulation of PVN and RM was carried out using brain stereotaxis apparatus (SN-2). The analyzed parameters included frequency (F), phase difference (PD), ratio of negative PD (RNPD) and corresponding rate of waves (CRW).

RESULTS: The F of GEA in the gastric corpus 2 increased (from 4.29 ± 0.60 to 4.56 ± 0.59, P = 0.05) and that in the gastric antrum decreased (from 4.54 ± 0.51 to 4.27 ± 0.44, P = 0.013) in GD rabbits. Injection of 500 μg 5-HT accelerated the F of GEA in the gastric corpus 1 (from 4.06 ± 0.45 to 4.25 ± 0.37, P = 0.031), corpus 3 (from 4.32 ± 0.51 to 4.58 ± 0.36, P = 0.041) and gastric antrum (from 4.54 ± 0.47 to 4.73 ± 0.44, P = 0.017) and improved the CRW in the gastric corpus 3 (from 0.78 ± 0.13 to 0.83 ± 0.10, P = 0.030) in GD rabbits. The F of GEA in the gastric corpus 3 was (from 4.27 ± 0.53 to 4.52 ± 0.47, P = 0.022) increased by gastric pacing. Electrical stimulation of PVN decreased the F of GEA in the gastric corpus 3 (from 4.47 ± 0.44 to 4.14 ± 0.46, P = 0.046) and gastric antrum (from 4.05 ± 0.54 to 3.69 ± 0.55, n = 12, P = 0.039), increased the RNPD in the gastric antrum (from 0.32 ± 0.19 to 0.40 ± 0.19, P = 0.046), and greatly inhibited GEA in GD rabbits. Electrical stimulation of P7 slowed the F of GEA in the gastric antrum (from 4.31 ± 0.44 to 3.86 ± 0.47, P = 0.012) in GD rabbits.

CONCLUSION: A rabbit model of GD has been successfully developed by injection of glucagon. Peripheral injection of 500 μg 5-HT and gastric pacemaking prominently improved the waves of GEA. Electrical stimulation of the PVN and RM decreased the F of GEA in GD rabbits, which demonstrate that there exists a "descending inhibition system" in the centre nervous system.

Liver regeneration is promoted by increasing serotonin content in rat liver with secondary biliary cirrhosis

AIM

  Liver cirrhosis clinically shows thrombocytopenia and hypersplenism. Although splenectomy is performed to achieve higher platelet count and better hemostasis, the effect of splenectomy for liver cirrhosis remains unclear. The aim of the present study that was focused on serotonin was to investigate the relationship between splenectomy and liver regeneration in rats with secondary biliary cirrhosis.

METHODS

  Liver cirrhosis was induced in Sprague-Dawley rats by bile duct ligation (BDL). In addition, splenectomy and administration of ketanserin, which selectively antagonizes 5-HT2A and 2B serotonin receptors, were performed. Three weeks after the interventions, whole blood, plasma, serum, and liver specimens were obtained for the following studies: peripheral platelet counts, hemodynamics of serotonin, histopathological examination, immunostaining, and quantification of mRNA expression.

RESULTS

  Splenectomy induced thrombocytosis, and increased serotonin content in cirrhotic liver. Stimulation of liver regeneration was indicated by the following parameters: hepatocyte ratio to the entire liver area, Ki67-positive hepatocyte count, and expression of phosphorylated extracellular signal-regulated kinases. This enhancement of liver regeneration was negated by ketanserin.

CONCLUSION

  Our results showed that splenectomy promoted liver regeneration by increasing serotonin content in liver even under cirrhotic conditions.

Rotavirus stimulates release of serotonin (5-HT) from human enterochromaffin cells and activates brain structures involved in nausea and vomiting

Rotavirus (RV) is the major cause of severe gastroenteritis in young children. A virus-encoded enterotoxin, NSP4 is proposed to play a major role in causing RV diarrhoea but how RV can induce emesis, a hallmark of the illness, remains unresolved. In this study we have addressed the hypothesis that RV-induced secretion of serotonin (5-hydroxytryptamine, 5-HT) by enterochromaffin (EC) cells plays a key role in the emetic reflex during RV infection resulting in activation of vagal afferent nerves connected to nucleus of the solitary tract (NTS) and area postrema in the brain stem, structures associated with nausea and vomiting. Our experiments revealed that RV can infect and replicate in human EC tumor cells ex vivo and in vitro and are localized to both EC cells and infected enterocytes in the close vicinity of EC cells in the jejunum of infected mice. Purified NSP4, but not purified virus particles, evoked release of 5-HT within 60 minutes and increased the intracellular Ca²⁺ concentration in a human midgut carcinoid EC cell line (GOT1) and ex vivo in human primary carcinoid EC cells concomitant with the release of 5-HT. Furthermore, NSP4 stimulated a modest production of inositol 1,4,5-triphosphate (IP₃), but not of cAMP. RV infection in mice induced Fos expression in the NTS, as seen in animals which vomit after administration of chemotherapeutic drugs. The demonstration that RV can stimulate EC cells leads us to propose that RV disease includes participation of 5-HT, EC cells, the enteric nervous system and activation of vagal afferent nerves to brain structures associated with nausea and vomiting. This hypothesis is supported by treating vomiting in children with acute gastroenteritis with 5-HT₃ receptor antagonists.

Rotavirus (RV) can cause severe dehydration and is a leading cause of childhood deaths worldwide. While most deaths occur due to excessive loss of fluids and electrolytes through vomiting and diarrhoea, the pathophysiological mechanisms that underlie this life-threatening disease remain to be clarified. Our previous studies revealed that drugs that inhibit the function of the enteric nervous system can reduce symptoms of RV disease in mice. In this study we have addressed the hypothesis that RV infection triggers the release of serotonin (5-hydroxytryptamine, 5-HT) from enterochromaffin (EC) cells in the intestine leading to activation of vagal afferent nerves connected to brain stem structures associated with vomiting. RV activated Fos expression in the nucleus of the solitary tract of CNS, the main target for incoming fibers from the vagal nerve. Both secreted and recombinant forms of the viral enterotoxin (NSP4), increased intracellular Ca²⁺ concentration and released 5-HT from EC cells. 5-HT induced diarrhoea in mice within 60 min, thereby supporting the role of 5-HT in RV disease. Our study provides novel insight into the complex interaction between RV, EC cells, 5-HT and nerves.

Serotonin receptor diversity in the human colon: Expression of serotonin type 3 receptor subunits 5-HT3C, 5-HT3D, and 5-HT3E

Since the first description of 5-HT₃ receptors more than 50 years ago, there has been speculation about the molecular basis of their receptor heterogeneity. We have cloned the genes encoding novel 5-HT3 subunits 5-HT3C, 5-HT3D, and 5-HT3E and have shown that these subunits are able to form functional heteromeric receptors when coexpressed with the 5-HT3A subunit. However, whether these subunits are actually expressed in human tissue remained to be confirmed. In the current study, we performed immunocytochemistry to locate the 5-HT3A as well as the 5-HT3C, 5-HT3D, and 5-HT3E subunits within the human colon. Western blot analysis was used to confirm subunit expression, and RT-PCR was employed to detect transcripts encoding 5-HT₃ receptor subunits in microdissected tissue samples. This investigation revealed, for the first time, that 5-HT3C, 5-HT3D, and 5-HT3E subunits are coexpressed with 5-HT3A in cell bodies of myenteric neurons. Furthermore, 5-HT3A and 5-HT3D were found to be expressed in submucosal plexus of the human large intestine. These data provide a strong basis for future studies of the roles that specific 5-HT₃ receptor subtypes play in the function of the enteric and central nervous systems and the contribution that specific 5-HT₃ receptors make to the pathophysiology of gastrointestinal disorders such as irritable bowel syndrome and dyspepsia.

Calretinin and somatostatin immunoreactivities label different human submucosal neuron populations

In human myenteric plexus, calretinin (CALR) and somatostatin (SOM) coexist in Dogiel Type II neurons, which were considered as intrinsic primary afferent neurons in the guinea pig. The aims of this study were to test if also human submucosal neurons costain immunohistochemically for CALR and SOM and whether these or other neurons display Type II morphology. Two sets of submucosal wholemounts of small and large intestine from 29 patients (median age 65 years) were triple stained for CALR, SOM, and human neuronal protein Hu C/D (HU, a pan-neuronal marker) as well as for CALR, SOM, and peripherin (PER), respectively. Only exceptionally, neurons coreactive for both CALR and SOM were found. The three major groups of neurons were CALR-/HU-coreactive (CALR-neurons), SOM-/HU-coreactive (SOM-neurons), and HU-alone-positive neurons. We observed significantly more CALR-neurons in the external submucosal plexus (ESP) of all regions and more SOM-neurons in the internal submucosal plexus (ISP), although with substantial interindividual variations. Comparisons of small vs. large intestine revealed more SOM-neurons (ESP: 29% vs. 4%, ISP: 40% vs. 13%) but fewer CALR-neurons (ESP: 37% vs. 77%, ISP: 21% vs. 67%) in small intestine. Morphologically, CALR-neurons had multiple processes; in some cases, we identified multidendritic/uniaxonal neurons. In contrast, SOM-neurons had mostly only one process. The functions of both populations as possible primary afferent neurons, interneurons, secretomotor neurons, or vasomotor neurons are discussed. Future morphochemical distinction of these groups may reveal different subgroups.

Influence of acute tryptophan depletion on gastric sensorimotor function in humans

Peripheral serotonin (5-hydrodytryptamine; 5-HT) is involved in the regulation of gastrointestinal motility and sensation, whereas centrally it plays a role in mood regulation. A dysfunctional serotonergic system may provide a plausible link between functional dyspepsia symptoms and its high psychosocial comorbidity such as anxiety and depression. The aim of this study was to evaluate the effect of decreased 5-HT synthesis by acute tryptophan depletion (ATD) on gastric sensorimotor function and nutrient tolerance, anxiety scores, and gastrointestinal mucosal 5-HT concentrations in healthy volunteers. All subjects were studied under a control condition and during ATD. Gastric sensorimotor function and nutrient tolerance were assessed using a barostat (n = 16, mean age 28.8 ± 1.4 yr) and a satiety drinking test (n = 13, mean age 27.3 ± 1.4 yr). Anxiety during the barostat was evaluated using State-Trait Anxiety Inventory (STAI) questionnaire. 5-HT concentrations were measured in fundic and duodenal mucosal biopsies by means of ELISA and immunohistochemistry. ATD significantly decreased plasma tryptophan levels compared with control in every experiment. ATD did not affect gastric sensitivity and compliance but decreased the sensation of nausea during balloon distension (AUC: 17.4 ± 4.3 vs. 11.4 ± 3.4 mm·mmHg, P = 0.030). ATD enhanced the postprandial volume increase (ANOVA, P < 0.05), but this was not accompanied by augmented nutrient tolerance (848 ± 110 vs. 837 ± 99 ml, nonsignificant). ATD had no effect on STAI state anxiety scores. No evidence was found for an effect on the number of enterochromaffin cells, but ATD reduced 5-HT levels in the duodenal mucosa. ATD alters gastric postprandial motor function and distension-induced nausea. These findings confirm involvement of 5-HT in the control of gastric accommodation and sensitivity.

Effects of a 5-HT(3) antagonist, ondansetron, on fasting and postprandial small bowel water content assessed by magnetic resonance imaging

BACKGROUND

5-HT(3) antagonists have been shown to be effective in relieving the symptoms of irritable bowel syndrome with diarrhoea (IBS-D). Using a recently validated magnetic resonance imaging (MRI) method, we have demonstrated reduced fasting small bowel water content (SBWC) in IBS-D associated with accelerated small bowel transit. We hypothesized that slowing of transit with ondansetron would lead to an increase in SBWC by inhibiting fasting motility.

AIM

To assess the effects of ondansetron compared with placebo in healthy volunteers on SBWC and motility in two different groups of subjects, one studied using MRI and another using manometry.

METHODS

Healthy volunteers were given either a placebo or ondansetron on the day prior to and on the study day. Sixteen volunteers underwent baseline fasting and postprandial MRI scans for 270 min. In a second study, a separate group of n = 18 volunteers were intubated and overnight migrating motor complex (MMC) recorded. Baseline MRI scans were carried out after the tube was removed.

RESULTS

Fasting SBWC was markedly increased by ondansetron (P < 0.0007). Ondansetron reduced the overall antroduodenal Motility Index (P < 0.04). The subjects who were intubated had significantly lower fasting SBWC (P < 0.0002) compared with the group of subjects who were not intubated.

CONCLUSIONS

The 5-HT(3) receptor antagonism increased fasting small bowel water. This was associated with reduced fasting antroduodenal Motility Index which may explain the clinical benefit of such drugs.

Recordings from human myenteric neurons using voltage-sensitive dyes

Voltage-sensitive dye (VSD) imaging became a powerful tool to detect neural activity in the enteric nervous system, including its routine use in submucous neurons in freshly dissected human tissue. However, VSD imaging of human myenteric neurons remained a challenge because of limited visibility of the ganglia and dye accessibility. We describe a protocol to apply VSD for recordings of human myenteric neurons in freshly dissected tissue and myenteric neurons in primary cultures. VSD imaging of guinea-pig myenteric neurons was used for reference. Electrical stimulation of interganglionic fiber tracts and exogenous application of nicotine or elevated KCl solution was used to evoke action potentials. Bath application of the VSDs Annine-6Plus, Di-4-ANEPPS, Di-8-ANEPPQ, Di-4-ANEPPDHQ or Di-8-ANEPPS revealed no neural signals in human tissue although most of these VSD worked in guinea-pig tissue. Unlike methylene blue and FM1-43, 4-Di-2-ASP did not influence spike discharge and was used in human tissue to visualize myenteric ganglia as a prerequisite for targeted intraganglionic VSD application. Of all VSDs, only intraganglionic injection of Di-8-ANEPPS by a volume controlled injector revealed neuronal signals in human tissue. Signal-to-noise ratio increased by addition of dipicrylamine to Di-8-ANEPPS (0.98±0.16 vs. 2.4±0.62). Establishing VSD imaging in primary cultures of human myenteric neurons led to a further improvement of signal-to-noise ratio. This allowed us to routinely record spike discharge after nicotine application. The described protocol enabled reliable VSD recordings from human myenteric neurons but may also be relevant for the use of other fluorescent dyes in human tissues.

5-HT(3) receptors: role in disease and target of drugs

Serotonin type 3 (5-HT(3)) receptors are pentameric ion channels belonging to the superfamily of Cys-loop receptors. Receptor activation either leads to fast excitatory responses or modulation of neurotransmitter release depending on their neuronal localisation. 5-HT(3) receptors are known to be expressed in the central nervous system in regions involved in the vomiting reflex, processing of pain, the reward system, cognition and anxiety control. In the periphery they are present on a variety of neurons and immune cells. 5-HT(3) receptors are known to be involved in emesis, pain disorders, drug addiction, psychiatric and GI disorders. Progress in molecular genetics gives direction to personalised medical strategies for treating complex diseases such as psychiatric and functional GI disorders and unravelling individual drug responses in pharmacogenetic approaches. Here we discuss the molecular basis of 5-HT(3) receptor diversity at the DNA and protein level, of which our knowledge has greatly extended in the last decade. We also evaluate their role in health and disease and describe specific case-control studies addressing the involvement of polymorphisms of 5-HT3 subunit genes in complex disorders and responses to drugs. Furthermore, we focus on the actual state of the pharmacological knowledge concerning not only classical 5-HT(3) antagonists--the setrons--but also compounds of various substance classes targeting 5-HT(3) receptors such as anaesthetics, opioids, cannabinoids, steroids, antidepressants and antipsychotics as well as natural compounds derived from plants. This shall point to alternative treatment options modulating the 5-HT(3) receptor system and open new possibilities for drug development in the future.

Polymorphism in HTR3D shows different risks for acute chemotherapy-induced vomiting after anthracycline chemotherapy

Aims: Serotonin (5-hydroxytryptamine 3; 5-HT3) receptors are involved in chemotherapy-induced nausea and vomiting (CINV), and 5-HT3 antagonists are part of the ‘gold standard’ antiemetic treatment during chemotherapy. We investigated the correlation of common variants in 5-HT3 receptor subunit genes with the occurrence of CINV. Materials & methods: A total of 110 previously characterized chemotherapy-naive women with primary breast cancer treated with anthracycline-containing chemotherapy served as a study group for mutational analysis by direct sequencing. Eight common SNPs in the 5-HT3 receptor genes, HTR3A, HTR3B, HTR3D and HTR3E, were selected for association analysis. Results: A nonsynonymous variant in HTR3D, p.G36A (rs6443930), was found to be over-represented in nonresponders, assuming a log-additive inheritance model (p = 0.048). Cox proportional regression analysis resulted in a hazards ratio of 0.36 for homozygous carriers of the C allele to vomit within 24 h after first chemotherapy administration (p = 0.049). Conclusion: Our data supports the hypothesis that 5-HT3 receptors play an important role in the pathogenesis of CINV. Along with previously identified HTR3 polymorphisms, the HTR3D p.G36A variant could also contribute to facilitating individual risk predictions.