Effects of 5-hydroxytryptamine (5-HT) on the discharge of vagal mechanoreceptors and motility in the upper gastrointestinal tract of the ferret

Structure, Function and Physiology of 5-Hydroxytryptamine Receptors Subtype 3

5-hydroxytryptamine receptor subtype 3 (5-HT₃R) is a pentameric ligand-gated ion channel (pLGIC) involved in neuronal signaling. It is best known for its prominent role in gut-CNS signaling though there is growing interest in its other functions, particularly in modulating non-serotonergic synaptic activity. Recent advances in structural biology have provided mechanistic understanding of 5-HT₃R function and present new opportunities for the field. This chapter gives a broad overview of 5-HT₃R from a physiological and structural perspective and then discusses the specific details of ion permeation, ligand binding and allosteric coupling between these two events. Biochemical evidence is summarized and placed within a physiological context. This perspective underscores the progress that has been made as well as outstanding challenges and opportunities for future 5-HT₃R research.

Implication of neurohormonal-coupled mechanisms of gastric emptying and pancreatic secretory function in diabetic gastroparesis

Recently, diabetic gastroparesis (DGP) has received much attention as its prevalence is increasing in a dramatic fashion and management of patients with DGP represents a challenge in the clinical practice due to the limited therapeutic options. DGP highlights an interrelationship between the gastric emptying and pancreatic secretory function that regulate a wide range of digestive and metabolic functions, respectively. It well documented that both gastric emptying and pancreatic secretion are under delicate control by multiple neurohormonal mechanisms including extrinsic parasympathetic pathways and gastrointestinal (GI) hormones. Interestingly, the latter released in response to various determinants that related to the rate and quality of gastric emptying. Others and we have provided strong evidence that the central autonomic nuclei send a dual output (excitatory and inhibitory) to the stomach and the pancreas in response to a variety of hormonal signals from the abdominal viscera. Most of these hormones released upon gastric emptying to provide feedback, and control this process and simultaneously regulate pancreatic secretion and postprandial glycemia. These findings emphasize an important link between gastric emptying and pancreatic secretion and its role in maintaining homeostatic processes within the GI tract. The present review deals with the neurohormonal-coupled mechanisms of gastric emptying and pancreatic secretory function that implicated in DGP and this provides new insights in our understanding of the pathophysiology of DGP. This also enhances the process of identifying potential therapeutic targets to treat DGP and limit the complications of current management practices.

The Vagus Nerve at the Interface of the Microbiota-Gut-Brain Axis

The microbiota, the gut, and the brain communicate through the microbiota-gut-brain axis in a bidirectional way that involves the autonomic nervous system. The vagus nerve (VN), the principal component of the parasympathetic nervous system, is a mixed nerve composed of 80% afferent and 20% efferent fibers. The VN, because of its role in interoceptive awareness, is able to sense the microbiota metabolites through its afferents, to transfer this gut information to the central nervous system where it is integrated in the central autonomic network, and then to generate an adapted or inappropriate response. A cholinergic anti-inflammatory pathway has been described through VN's fibers, which is able to dampen peripheral inflammation and to decrease intestinal permeability, thus very probably modulating microbiota composition. Stress inhibits the VN and has deleterious effects on the gastrointestinal tract and on the microbiota, and is involved in the pathophysiology of gastrointestinal disorders such as irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD) which are both characterized by a dysbiosis. A low vagal tone has been described in IBD and IBS patients thus favoring peripheral inflammation. Targeting the VN, for example through VN stimulation which has anti-inflammatory properties, would be of interest to restore homeostasis in the microbiota-gut-brain axis.

Distinct Expression of Phenotypic Markers in Placodes- and Neural Crest-Derived Afferent Neurons Innervating the Rat Stomach

BACKGROUND

Visceral pain is initiated by activation of primary afferent neurons among which the capsaicin-sensitive (TRPV1-positive) neurons play an important role. The stomach is a common source of visceral pain. Similar to other organs, the stomach receives dual spinal and vagal afferent innervation. Developmentally, spinal dorsal root ganglia (DRG) and vagal jugular neurons originate from embryonic neural crest and vagal nodose neurons originate from placodes. In thoracic organs the neural crest- and placodes-derived TRPV1-positive neurons have distinct phenotypes differing in activation profile, neurotrophic regulation and reflex responses. It is unknown to whether such distinction exists in the stomach.

AIMS

We hypothesized that gastric neural crest- and placodes-derived TRPV1-positive neurons express phenotypic markers indicative of placodes and neural crest phenotypes.

METHODS

Gastric DRG and vagal neurons were retrogradely traced by DiI injected into the rat stomach wall. Single-cell RT-PCR was performed on traced gastric neurons.

RESULTS

Retrograde tracing demonstrated that vagal gastric neurons locate exclusively into the nodose portion of the rat jugular/petrosal/nodose complex. Gastric DRG TRPV1-positive neurons preferentially expressed markers PPT-A, TrkA and GFRα₃ typical for neural crest-derived TRPV1-positive visceral neurons. In contrast, gastric nodose TRPV1-positive neurons preferentially expressed markers P2X₂ and TrkB typical for placodes-derived TRPV1-positive visceral neurons. Differential expression of neural crest and placodes markers was less pronounced in TRPV1-negative DRG and nodose populations.

CONCLUSIONS

There are phenotypic distinctions between the neural crest-derived DRG and placodes-derived vagal nodose TRPV1-positive neurons innervating the rat stomach that are similar to those described in thoracic organs.

The possible role of gastrointestinal endocrine cells in the pathophysiology of irritable bowel syndrome

The etiology of irritable bowel syndrome (IBS) is unknown, but several factors appear to play a role in its pathophysiology, including abnormalities of the gastrointestinal endocrine cells. The present review illuminates the possible role of gastrointestinal hormones in the pathophysiology of IBS and the possibility of utilizing the current knowledge in treating the disease. Areas covered: Research into the intestinal endocrine cells and their possible role in the pathophysiology of IBS is discussed. Furthermore, the mechanisms underlying the abnormalities in the gastrointestinal endocrine cells in IBS patients are revealed. Expert commentary: The abnormalities observed in the gastrointestinal endocrine cells in IBS patients explains their visceral hypersensitivity, gastrointestinal dysmotility, and abnormal intestinal secretion, as well as the interchangeability of symptoms over time. Clarifying the role of the intestinal stem cells in the pathophysiology of IBS may lead to new treatment methods for IBS.

Role of central vagal 5-HT3 receptors in gastrointestinal physiology and pathophysiology

Vagal neurocircuits are vitally important in the co-ordination and modulation of GI reflexes and homeostatic functions. 5-hydroxytryptamine (5-HT; serotonin) is critically important in the regulation of several of these autonomic gastrointestinal (GI) functions including motility, secretion and visceral sensitivity. While several 5-HT receptors are involved in these physiological responses, the ligand-gated 5-HT3 receptor appears intimately involved in gut-brain signaling, particularly via the afferent (sensory) vagus nerve. 5-HT is released from enterochromaffin cells in response to mechanical or chemical stimulation of the GI tract which leads to activation of 5-HT3 receptors on the terminals of vagal afferents. 5-HT3 receptors are also present on the soma of vagal afferent neurons, including GI vagal afferent neurons, where they can be activated by circulating 5-HT. The central terminals of vagal afferents also exhibit 5-HT3 receptors that function to increase glutamatergic synaptic transmission to second order neurons of the nucleus tractus solitarius within the brainstem. While activation of central brainstem 5-HT3 receptors modulates visceral functions, it is still unclear whether central vagal neurons, i.e., nucleus of the tractus solitarius (NTS) and dorsal motor nucleus of the vagus (DMV) neurons themselves also display functional 5-HT3 receptors. Thus, activation of 5-HT3 receptors may modulate the excitability and activity of gastrointestinal vagal afferents at multiple sites and may be involved in several physiological and pathophysiological conditions, including distention- and chemical-evoked vagal reflexes, nausea, and vomiting, as well as visceral hypersensitivity.

Vagal pathways for microbiome-brain-gut axis communication

There is now strong evidence from animal studies that gut microorganism can activate the vagus nerve and that such activation plays a critical role in mediating effects on the brain and behaviour. The vagus appears to differentiate between non-pathogenic and potentially pathogenic bacteria even in the absence of overt inflammation and vagal pathways mediate signals that can induce both anxiogenic and anxiolytic effects, depending on the nature of the stimulus. Certain vagal signals from the gut can instigate an anti-inflammatory reflex with afferent signals to the brain activating an efferent response, releasing mediators including acetylcholine that, through an interaction with immune cells, attenuates inflammation. This immunomodulatory role of the vagus nerve may also have consequences for modulation of brain function and mood.What is currently lacking are relevant data on the electrophysiology of the system. Certainly, important advances in our understanding of the gut-brain and microbiome- gut-brain axis will come from studies of how distinct microbial and nutritional stimuli activate the vagus and the nature of the signals transmitted to the brain that lead to differential changes in the neurochemistry of the brain and behaviour.Understanding the induction and transmission of signals in the vagus nerve may have important implications for the development of microbial-or nutrition based therapeutic strategies for mood disorders.

Serotonin activates catecholamine neurons in the solitary tract nucleus by increasing spontaneous glutamate inputs

Serotonin (5-HT) is a critical neurotransmitter in the control of autonomic functions. 5-HT(3) receptors participate in vagal afferent feedback to decrease food intake and regulate cardiovascular reflexes; however, the phenotype of the solitary tract nucleus (NTS) neurons involved is not known. A(2)/C(2) catecholamine (CA) neurons in the NTS are directly activated by visceral afferents and are important for the control of food intake and cardiovascular function, making them good candidates to respond to and mediate the effects of serotonin at the level of the NTS. This study examines serotonin's effects on NTS-CA neurons using patch-clamp techniques and transgenic mice expressing an enhanced green fluorescent protein driven by the tyrosine hydroxylase (TH) promoter (TH-EGFP) to identify catecholamine neurons. Serotonin increased the frequency of spontaneous glutamate excitatory postsynaptic currents (sEPSCs) in >90% of NTS-TH-EGFP neurons, an effect blocked by the 5-HT(3) receptor antagonist ondansetron and mimicked by the 5-HT(3) receptor agonists SR5227 and mCPBG. In contrast, 5-HT(3) receptor agonists increased sEPSCs on a minority (<30%) of non-TH neurons. 5-HT(3) receptor agonists increased the frequency, but not the amplitude, of mini-EPSCs, suggesting that their actions are presynaptic. 5-HT(3) receptor agonists increased the firing rate of TH-EGFP neurons, an effect dependent on the increased spontaneous glutamate inputs as it was blocked by the ionotropic glutamate antagonist NBQX, but independent of visceral afferent activation. These results demonstrate a cellular mechanism by which serotonin activates NTS-TH neurons and suggest a pathway by which it can increase catecholamine release in target regions to modulate food intake, motivation, stress, and cardiovascular function.

Viscerofugal neurons recorded from guinea-pig colonic nerves after organ culture

BACKGROUND

Enteric viscerofugal neurons provide cholinergic synaptic inputs to prevertebral sympathetic neurons, forming reflex circuits that control motility and secretion. Extracellular recordings of identified viscerofugal neurons have not been reported.

METHODS

Preparations of guinea pig distal colon were maintained in organotypic culture for 4-6 days (n = 12), before biotinamide tracing, immunohistochemistry, or extracellular electrophysiological recordings from colonic nerves.

KEY RESULTS

After 4-6 days in organ culture, calcitonin gene-related peptide and tyrosine hydroxylase immunoreactivity in enteric ganglia was depleted, and capsaicin-induced firing (0.4 μmol L(-1) ) was not detected, indicating that extrinsic sympathetic and sensory axons degenerate in organ culture. Neuroanatomical tracing of colonic nerves revealed that viscerofugal neurons persist and increase as a proportion of surviving axons. Extracellular recordings of colonic nerves revealed ongoing action potentials. Interestingly, synchronous bursts of action potentials were seen in 10 of 12 preparations; bursts were abolished by hexamethonium, which also reduced firing rate (400 μmol L(-1) , P < 0.01, n = 7). DMPP (1,1-dimethyl-4-phenylpiperazinium; 10(-4) mol L(-1) ) evoked prolonged action potential discharge. Increased firing preceded both spontaneous and stretch-evoked contractions (χ(2) = 11.8, df = 1, P < 0.001). Firing was also modestly increased during distensions that did not evoke reflex contractions. All single units (11/11) responded to von Frey hairs (100-300 mg) in hexamethonium or Ca(2+) -free solution.

CONCLUSIONS & INFERENCES

Action potentials recorded from colonic nerves in organ cultured preparations originated from viscerofugal neurons. They receive nicotinic input, which coordinates ongoing burst firing. Large bursts preceded spontaneous and reflex-evoked contractions, suggesting their synaptic inputs may arise from enteric circuitry that also drives motility. Viscerofugal neurons were directly mechanosensitive to focal compression by von Frey hairs.

Detection and signaling of glucose in the intestinal mucosa--vagal pathway

Intestinal luminal exposure to glucose initiates changes in food intake and gastrointestinal (GI) motor and secretory function. It does this by stimulating the release of GI hormones and 5-hydroxytryptamine (5-HT) from enteroendocrine and enterochromaffin cells (EC), respectively, which in turn activate intrinsic and extrinsic neuronal pathways. An article in this issue of the journal provides new insight into the mechanisms involved in luminal glucose sensing. Vincent et al. have used a novel in vivo technique to determine activation of gut epithelial cells and vagal afferent pathways in rats by staining for activated calcium-calmodulin kinase II (pCaMKII) along the pathway. In the mucosa, they found that intraluminal glucose activated EC cells and brush cells. At the next stage, pCaMKII was seen in neurons of the myenteric plexus and vagal afferent neurons in the nodose ganglia. In the central nervous system (CNS), activation was seen in second- and higher-order neurons in the dorsal vagal complex and hypothalamus. They found that 5-HT(3) receptors were involved in initiating neural signaling as activation of neurons, but not EC cells, was reduced by 5-HT(3) receptor antagonism. Selectively stimulating the sodium-glucose cotransporter (SGLT-3) had similar effects to glucose. This suggests that SGLT-3 behaves as a glucose sensor, mainly on EC cells, inducing the release of 5-HT, which activates 5-HT(3) receptors on vagal afferent endings nearby and in turn, their connections in the CNS. There is evidence elsewhere that other sensors and transmitter mechanisms are involved in this pathway, so the possibility exists of multiple redundant systems.

Roles of gastro-oesophageal afferents in the mechanisms and symptoms of reflux disease

Oesophageal pain is one of the most common reasons for physician consultation and/or seeking medication. It is most often caused by acid reflux from the stomach, but can also result from contractions of the oesophageal muscle. Different forms of pain are evoked by oesophageal acid, including heartburn and non-cardiac chest pain, but the basic mechanisms and pathways by which these are generated remain to be elucidated. Both vagal and spinal afferent pathways are implicated by basic research. The sensitivity of afferent fibres within these pathways may become altered after acid-induced inflammation and damage, but the severity of symptoms in humans does not necessarily correlate with the degree of inflammation. Gastro-oesophageal reflux disease (GORD) is caused by transient relaxations of the lower oesophageal sphincter, which are triggered by activation of gastric vagal mechanoreceptors. Vagal afferents are therefore an emerging therapeutic target for GORD. Pain in the absence of excess acid reflux remains a major challenge for treatment.

Nitric oxide as an endogenous peripheral modulator of visceral sensory neuronal function

Nitric oxide (NO) plays important roles in CNS and smooth muscle function. Here we reveal an additional function in peripheral sensory transmission. We hypothesized that endogenous NO modulates the function of gastrointestinal vagal afferent endings. The nonselective NO synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester hydrochloride increased responses to tactile mechanical stimuli of mucosal afferent endings in two species, in some cases severalfold. This was mimicked by a neuronal NOS inhibitor but not an endothelial NOS inhibitor. NOS inhibitors did not affect the responsiveness of smooth muscle afferent endings, suggesting that the endogenous source of NO is exclusively accessible to mucosal receptors. The role of the NO-soluble guanylyl cyclase (sGC)-cGMP pathway was confirmed using the sGC inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxaline-1-one and the cGMP phosphodiesterase 5' inhibitor sildenafil. The first enhanced and the second inhibited mechanosensory function. Exogenous NO, from the donor S-nitroso-N-acetylpenicillamine, significantly reduced mechanosensitivity of both types of ending. Up to one-third of stomach-projecting afferent neurons in the nodose ganglia expressed neuronal NOS (nNOS). However, anterograde-traced vagal endings were nNOS negative, indicating NOS is not transported peripherally and there are alternative sources of NO for afferent modulation. A subpopulation of enteroendocrine cells in the gut mucosa were nNOS positive, which were found anatomically in close apposition with mucosal vagal afferent endings. These results indicate an inhibitory neuromodulatory role of epithelial NO, which targets a select population of vagal afferents. This interaction is likely to play a role in generation of symptoms and behaviors from the upper gastrointestinal system.

Effects of ghrelin on interdigestive contractions of the rat gastrointestinal tract

Ghrelin causes interdigestive contractions of the stomach in rats. However, it remains unknown whether ghrelin causes interdigestive contractions in the small intestine. Four strain gauge transducers were implanted on the antrum, duodenum, proximal and distal jejunum. After an overnight fast, gastrointestinal (GI) contractions were recorded in freely moving conscious rats. Spontaneous phase III-like contractions were observed at every 13-16 min in rat GI tract. The fasted motor patterns were replaced by the fed motor pattern immediately after food intake. Two minutes after finishing the spontaneous phase III-like contractions in the antrum, acyl ghrelin (0.8, 2.4 and 8.0 μg/kg per min) was continuously infused for 30 min. Three-five minutes after the starting ghrelin infusion, augmented phase III-like contractions were observed at the antrum, duodenum, and jejunum. Ghrelin infusion (0.8, 2.4 and 8.0 μg/kg per min) significantly increased motility index of phase III-like contractions at the antrum and jejunum in a dose dependent manner, compared to that of saline injection. Thus, it is likely that exogenously administered ghrelin causes phase III-like contraction at the antrum, which migrates to the duodenum and jejunum. The possible role of 5-HT, in addition to ghrelin, in mediating intestinal migrating motor complex (MMC), is discussed.

5-Hydroxytryptamine selectively activates the vagal nodose C-fibre subtype in the guinea-pig oesophagus

The afferent neurons innervating the oesophagus originate from two embryonic sources: neurons located in vagal nodose ganglia originate from embryonic placodes and neurons located in vagal jugular and spinal dorsal root ganglia (DRG) originate from the neural crest. Here, we address the hypothesis that 5-hydroxytryptamine (5-HT) differentially stimulates afferent nerve subtypes in the oesophagus. Extracellular recordings of single unit activity originating from nerve terminals were made in the isolated innervated guinea-pig oesophagus. Whole cell patch clamp recordings (35 degrees C) were made from the primary afferent neurons retrogradely labelled from the oesophagus. 5-Hydroxytryptamine (10 micromol L(-1)) activated vagal nodose C-fibres (70%) in the oesophagus but failed to activate overtly vagal jugular nerve fibres and oesophagus-specific spinal DRG neurons. The response to 5-HT in nodose C-fibre nerve terminals was mimicked by the selective 5-HT(3) receptor agonist 2-methyl-5-HT (10 micromol L(-1)) and nearly abolished by the 5-HT(3) receptor antagonists ondansetron (10 micromol L(-1)) and Y-25130 (10 micromol L(-1)). In patch clamp studies, 2-methyl-5-HT (10 micromol L(-1)) activated a proportion of isolated oesophagus-specific nodose capsaicin-sensitive neurons (putative cell bodies of nodose C-fibres). We conclude that the responsiveness to 5-HT discriminates placode-derived (vagal nodose) C-fibres from the neural crest-derived (vagal jugular and spinal DRG) afferent nerves in the oesophagus. The response to 5-HT in nodose C-fibres is mediated by the 5-HT(3) receptor in their neuronal membrane.

Endogenous ghrelin and 5-HT regulate interdigestive gastrointestinal contractions in conscious rats

Endogenous ghrelin causes interdigestive contractions of the stomach in rats. In contrast, previous studies showed that 5-HT(3) and 5-HT(4) receptors were involved in regulating intestinal interdigestive contractions. We studied the possible role of endogenous ghrelin and 5-HT regulating interdigestive gastrointestinal (GI) contractions in rats. Four strain gauge transducers were implanted on the antrum, duodenum, and proximal and distal jejunum. After an overnight fast, GI contractions were recorded in freely moving conscious rats and ghrelin receptor antagonists [(d-lys3)GHRP6; 1 micromol/kg], 5-HT(3) antagonists (Ondansetron; 0.5 mg/kg) and 5-HT(4) antagonists (GR 125,487; 1 mg/kg) were administered (bolus iv). To evaluate the relationship between the luminal concentrations of 5-HT and phase III-like contractions of the duodenum, duodenal juice was collected via the intraduodenal catheter. 5-HT content of the duodenal juice was measured by HPLC. (d-lys3)GHRP6 significantly attenuated the occurrence and amplitude of phase III-like contractions of the antrum, but not the duodenum and jejunum. 5-HT(4) antagonists significantly reduced spontaneous phase III-like contractions of the jejunum, without affecting those of the antrum and duodenum. In contrast, 5-HT(3) antagonists did not affect phase III-like contractions in GI tract. Luminal concentration of 5-HT at the phase III-like contraction (36.0 +/- 13.3 ng/ml, n = 9) was significantly higher than that at the phase I-like contractions of the duodenum (4.9 +/- 1.6 ng/ml, n = 9, P < 0.05). It is suggested that released ghrelin from the gastric mucosa mediates gastric phase III-like contractions, whereas 5-HT released from enterochromaffin cells of the duodenal mucosa mediates intestinal phase III-like contractions via 5-HT(4) receptors.

Chemical coding and central projections of gastric vagal afferent neurons

Vagal afferents that innervate gastric muscle or mucosa transmit distinct sensory information from their endings to the nucleus of the tractus solitarius (NTS). While these afferent subtypes are functionally distinct, no neurochemical correlate has been described and it is unknown whether they terminate in different central locations. This study aimed to identify gastric vagal afferent subtypes in the nodose ganglion (NG) of ferrets, their terminal areas in NTS and neurochemistry for isolectin-B4 (IB4) and calcitonin gene-related peptide (CGRP). Vagal afferents were traced from gastric muscle or mucosa and IB4 and CGRP labelling assessed in NG and NTS. 7 +/- 1% and 6 +/- 1% of NG neurons were traced from gastric muscle or mucosa respectively; these were more likely to label for CGRP or for both CGRP and IB4 than other NG neurons (P < 0.01). Muscular afferents were also less likely than others to label with IB4 (P < 0.001). Less than 1% of NG neurons were traced from both muscle and mucosa. Central terminals of both afferent subtypes occurred in the subnucleus gelatinosus of the NTS, but did not overlap completely. This region also labelled for CGRP and IB4. We conclude that while vagal afferents from gastric muscle and mucosa differ little in their chemical coding for CGRP and IB4, they can be traced selectively from their peripheral endings to NG and to overlapping and distinct regions of NTS. Thus, there is an anatomical substrate for convergent NTS integration for both types of afferent input.

Mechanisms of cancer-related fatigue

Cancer-related fatigue (CRF) is one of the most prevalent symptoms patients with cancer experience, both during and after treatment. CRF is pervasive and affects patients' quality of life considerably. It is important, therefore, to understand the underlying pathophysiology of CRF in order to develop useful strategies for prevention and treatment. At present, the etiology of CRF is poorly understood and the relative contributions of the neoplastic disease, various forms of cancer therapy, and comorbid conditions (e.g., anemia, cachexia, sleep disorders, depression) remain unclear. In any individual, the etiology of CRF probably involves the dysregulation of several physiological and biochemical systems. Mechanisms proposed as underlying CRF include 5-HT neurotransmitter dysregulation, vagal afferent activation, alterations in muscle and ATP metabolism, hypothalamic-pituitary-adrenal axis dysfunction, circadian rhythm disruption, and cytokine dysregulation. Currently, these hypotheses are largely based on evidence from other conditions in which fatigue is a characteristic, in particular chronic fatigue syndrome and exercise-induced fatigue. The mechanisms that lead to fatigue in these conditions provide a theoretical basis for future research into the complex etiology of this distressing and debilitating symptom. An understanding of relevant mechanisms may offer potential routes for its prevention and treatment in patients with cancer.Disclosure of potential conflicts of interest is found at the end of this article.

Sensitization and Activation of Intracranial Meningeal Nociceptors by Mast Cell Mediators

Intracranial headaches such as migraine are thought to result from activation of sensory trigeminal pain neurons that supply intracranial blood vessels and the meninges, also known as meningeal nociceptors. Although the mechanism underlying the triggering of such activation is not completely understood, our previous work indicates that the local activation of the inflammatory dural mast cells can provoke a persistent sensitization of meningeal nociceptors. Given the potential importance of mast cells to the pain of migraine it is important to understand which mast cell-derived mediators interact with meningeal nociceptors to promote their activation and sensitization. In the present study, we have used in vivo electrophysiological single-unit recording of meningeal nociceptors in the trigeminal ganglion of anesthetized rats to examine the effect of a number of mast cell mediators on the activity level and mechanosensitivity of meningeal nociceptors. We have found that that serotonin (5-HT), prostaglandin I(2) (PGI(2)), and to a lesser extent histamine can promote a robust sensitization and activation of meningeal nociceptors, whereas the inflammatory eicosanoids PGD(2) and leukotriene C(4) are largely ineffective. We propose that dural mast cells could promote headache by releasing 5-HT, PGI(2), and histamine.

Sensing of glucose in the gastrointestinal tract

In general, nutrient sensing mechanisms in the intestine are not well understood. Potential sensors include the terminals of extrinsic afferent nerves, enteric nerves, endocrine cells and other epithelial cells including enterocytes and immune cells. This short review will concentrate on the neural pathways that are activated by the presence of glucose in the intestinal lumen and the role of a specialized endocrine cell, the enterochromaffin cell in glucose-sensing and the subsequent activation of extrinsic neural pathways.

Ileal brake: neuropeptidergic control of intestinal transit

Digestion and absorption of a meal are time-intensive processes. To optimize digestion and absorption, transit of the meal through the gastrointestinal tract is regulated by a complex integration of neuropeptidergic signals generated as the jejunal brake and ileal brake response to nutrients. Mediators involved in the slowing of transit responses include peptide YY (PYY), chemosensitive afferent neurons, intestinofugal nerves, noradrenergic nerves, myenteric serotonergic neurons, and opioid neurons. The activation of this circuitry modifies the peristaltic reflex to convert the intestinal motility pattern from propagative to segmenting. Fat is the most potent trigger of these transit control mechanisms. The integrated circuitry of gut peptides and neurons involved in transit control in response to nutrients is described in this review.

Luminal glucose sensing in the rat intestine has characteristics of a sodium-glucose cotransporter

The presence of glucose in the intestinal lumen elicits a number of changes in gastrointestinal function, including inhibition of gastric emptying and food intake and stimulation of pancreatic and intestinal secretion. The present study tested the hypothesis that Na(+)-glucose cotransporter (SGLT)-3, a member of the SGLT family of transport proteins, is involved in detection of luminal glucose in the intestine. Gastric emptying, measured in awake rats, was significantly inhibited by perfusion of the intestine with glucose (60 and 90 mg); this effect was mimicked by alpha-methyl glucose (nonmetabolizable substrate of SGLT-1 and -3) but not 2-deoxy-d-glucose (substrate for GLUT-2) or isoosmotic mannitol. Gastric motility and intestinal fluid secretion, measured in anesthetised rats, were significantly inhibited and stimulated, respectively, by duodenal glucose but not galactose, which has a much lower affinity for SGLT-3 than glucose. Duodenal glucose but not galactose stimulated the release of 5-HT into mesenteric lymph and stimulated the discharge of duodenal vagal afferent fibers. mRNA for SGLT-3 was identified in the duodenal mucosa. Together these data suggest that detection of glucose in the intestine may involve SGLT-3, possibly expressed by enterochromaffin cells in the intestinal mucosa, and release of 5-HT.

Evidence for a vagal pathophysiology for bulimia nervosa and the accompanying depressive symptoms

BACKGROUND

The bilateral vagus nerves (Cranial X) provide both afferent and efferent connections between the viscera and the caudal medulla. The afferent branches increasingly are being recognized as providing significant input to the central nervous system for modulation of complex behaviors. In this paper, we review evidence from our laboratory that increases in vagal afferent activity are involved in perpetuating binge-eating and vomiting in bulimia nervosa. Preliminary findings are also presented which suggest that a subgroup of depressions may have a similar pathophysiology.

METHODS

Two main approaches were used to study the role of vagal afferents. Ondansetron (ONDAN), a 5-HT3 antagonist, was used as a pharmacological tool for inhibiting or reducing vagal afferent neurotransmission. Second, somatic pain detection thresholds were assessed for monitoring a physiological process known to be modulated by vagal afferents, including the gastric branches involved in meal termination and satiety. High levels of vagal activity result in an increase in pain detection thresholds. Depressive symptoms were assessed using the Beck Depression Inventory (BDI). Positron Emission Tomography (PET) was used to identify higher cortical brain areas activated by vagal stimulation produced by proximal gastric distention in normal eating subjects.

RESULTS

Double-blind treatment of severe bulimia nervosa subjects with ONDAN resulted in a rapid and significant decrease in binge-eating and vomiting compared to placebo controls. The decrease in abnormal eating episodes was accompanied by a return of normal satiety. Pain detection thresholds measured weekly over the course of the treatment protocol were found to dynamically fluctuate in association with bulimic episodes. Thresholds were the most elevated during periods of short-term abstinence from the behaviors, suggesting that not engaging in a binge/vomit episode is accompanied by an increase in vagal activity. ONDAN also resulted in abolition of the fluctuations in pain thresholds. Depressive symptoms in these subjects also were reduced by ONDAN. Like pain thresholds, depressive symptoms varied dynamically with the bulimic behaviors, with BDI scores increasing (more depressed) as more time elapsed since the last bulimic episode. PET studies indicated that mechanical distention of the stomach with a balloon (a non-nutritive stimulus) was associated with the activation of several brain loci, including those associated with vagal activation (parabrachial nucleus), emotive aspects of eating (lateral inferior frontal and orbitofrontal), and depressive symptoms (anterior cingulate).

CONCLUSIONS

The results of the ONDAN study in bulimia nervosa subjects suggest that cyclic increases in vagal activity drive the urge to binge-eat and vomit. The alterations in vagal firing patterns are possibly a physiological adaptation to the high levels of vagal stimulation initially provided by voluntarily binge-eating and vomiting for weight control. The depressive symptoms that occur in association with the urge to binge-eat are also likely due to the cyclic increase in vagal activity. This suggestion is supported by the reduction of depressive symptoms during ONDAN treatment in bulimia subjects and PET imaging studies in normal eating subjects showing that brain loci classically involved in depression are activated by vagal stimulation administered by mechanical gastric distention. In normal eating individuals, depressions accompanying visceral diseases may also be vagally mediated. Ondansetron and other drugs known to modulate vagal activity may be helpful in treating depressions of this origin.

The herbal medicine, Dai-Kenchu-to, accelerates delayed gastrointestinal transit after the operation in rats

BACKGROUND

Post-operative ileus (POI) is a transient bowel dysmotility after operation. We have previously shown that laparotomy alone significantly delayed gastrointestinal (GI) transit, compared to anesthesia alone. The GI transit was further delayed after laparotomy plus intestinal manipulation. Dai-Kenchu-to (DKT), an herbal medicine, has been used for treating adhesive bowel obstruction in Japan. We studied whether DKT improves delayed GI transit after the operation, with or without morphine administration in rats.

MATERIALS AND METHODS

Under isoflurane anesthesia, POI was induced by laparotomy with intestinal manipulation. Immediately after the operation, the rats received 51Cr by gavage. Three hours after the operation, the rats were sacrificed and GI transit was estimated by calculating the geometric center (GC). DKT (120, 360, and 1,200 mg/kg) were administered by gavage after the operation, with or without morphine administration (1 mg/kg s.c.). A muscarinic receptor antagonist (atropine; 50 mug/kg), a 5HT3 receptor antagonist (ondansetron; 1 mg/kg) and a 5HT4 receptor antagonist (GR113,808; 3 mg/kg) were administered before the operation. Truncal vagotomy was performed preceding the operation.

RESULTS

Laparotomy with intestinal manipulation produced a significant delay in GI transit (GC = 2.93 +/- 0.16), compared to that of anesthesia alone (9.51 +/- 0.45). DKT at the dose of 360 mg/kg (GC = 3.77 +/- 0.10, P < 0.01) and 1,200 mg/kg (GC = 3.77 +/- 0.20, P < 0.01) significantly accelerated delayed GI transit induced by operation. Ondansetron, GR113,808, atropine, and truncal vagotomy abolished the stimulatory effect of DKT (360 mg/kg). When morphine was administered, GI transit was further reduced (GC = 1.97 +/- 0.10). DKT at the dose of 360 mg/kg (GC = 2.81 +/- 0.22, P < 0.05) and 1,200 mg/kg (GC = 2.87 +/- 0.23, P < 0.05) significantly improved delayed GI transit in morphine treated rats.

CONCLUSIONS

DKT accelerates delayed GI transit induced by intestinal manipulation with and without concomitant morphine administration. DKT treatment may be useful for the patients with POI.

Short-chain fatty acids stimulate colonic transit via intraluminal 5-HT release in rats

We studied whether physiological concentration of short-chain fatty acids (SCFAs) affects colonic transit and colonic motility in conscious rats. Intraluminal administration of SCFAs (100-200 mM) into the proximal colon significantly accelerated colonic transit. The stimulatory effect of SCFAs on colonic transit was abolished by perivagal capsaicin treatment, atropine, hexamethonium, and vagotomy, but not by guanethidine. The stimulatory effect of SCFAs on colonic transit was also abolished by intraluminal pretreatment with lidocaine and a 5-hydroxytryptamine (HT)(3) receptor antagonist. Intraluminal administration of SCFAs provoked contractions at the proximal colon, which migrated to the mid- and distal colon. SCFAs caused a significant increase in the luminal concentration of 5-HT of the vascularly isolated and luminally perfused rat colon ex vivo. It is suggested that the release of 5-HT from enterochromaffin cells in response to SCFAs stimulates 5-HT(3) receptors located on the vagal sensory fibers. The sensory information is transferred to the vagal efferent and stimulates the release of acetylcholine from the colonic myenteric plexus, resulting in muscle contraction.

Excitation of rat colonic afferent fibres by 5-HT(3) receptors

The gastrointestinal tract contains most of the body's 5-hydroxytryptamine (5-HT) and releases large amounts after meals or exposure to toxins. Increased 5-HT release occurs in patients with irritable bowel syndrome (IBS) and their peak plasma 5-HT levels correlate with pain episodes. 5-HT(3) receptor antagonists reduce symptoms of IBS clinically, but their site of action is unclear and the potential for other therapeutic targets is unexplored. Here we investigated effects of 5-HT on sensory afferents from the colon and the expression of 5-HT(3) receptors on their cell bodies in the dorsal root ganglia (DRG). Distal colon, inferior mesenteric ganglion and the lumbar splanchnic nerve bundle (LSN) were placed in a specialized organ bath. Eighty-six single fibres were recorded from the LSN. Three classes of primary afferents were found: 70 high-threshold serosal afferents, four low-threshold muscular afferents and 12 mucosal afferents. Afferent cell bodies were retrogradely labelled from the distal colon to the lumbar DRG, where they were processed for 5-HT(3) receptor-like immunoreactivity. Fifty-six percent of colonic afferents responded to 5-HT (between 10(-6) and 10(-3) M) and 30 % responded to the selective 5-HT(3) agonist, 2-methyl-5-HT (between 10(-6) and 10(-2) M). Responses to 2-methyl-5-HT were blocked by the 5-HT(3) receptor antagonist alosetron (2 x 10(-7) M), whereas responses to 5-HT were only partly inhibited. Twenty-six percent of L1 DRG cell bodies retrogradely labelled from the colon displayed 5-HT(3) receptor-like immunoreactivity. We conclude that colonic sensory neurones expressing 5-HT(3) receptors also functionally express the receptors at their peripheral endings. Our data reveal actions of 5-HT on colonic afferent endings via both 5-HT(3) and non-5-HT(3) receptors.

Cannabidiol: an overview of some pharmacological aspects

Over the past few years, considerable attention has focused on cannabidiol (CBD), a major nonpsychotropic constituent of cannabis. The authors present a review on the chemistry of CBD and discuss the anticonvulsive, antianxiety, antipsychotic, antinausea, and antirheumatoid arthritic properties of CBD. CBD does not bind to the known cannabinoid receptors, and its mechanism of action is yet unknown. It is possible that, in part at least, its effects are due to its recently discovered inhibition of anandamide uptake and hydrolysis and to its antioxidative effect.

Serotonergic modulating drugs for functional gastrointestinal diseases

After many years of basic research we have now begun to learn how to manipulate the serotonergic mechanisms within the gut. This has lead to a number of significant advances including 5HT3 antagonists for the treatment of functional diarrhoea, 5HT4 agonists for the treatment of constipation and 5HT1 agonists for the treatment of impaired fundal relaxation. Initial enthusiasm has been somewhat dented by the withdrawal of alosetron because of ischaemic colitis, but it remains to be seen whether this adverse event will be seen with other 5HT3 antagonists. Finally it should be recognized that, in a substantial proportion of patients attending clinics complaining of functional symptoms, anxiety is a major component. The drugs so far described are by and large devoid of CNS effects. It remains possible therefore that a drug which combines both peripheral and central effects would likely to be beneficial.

Vagal mechanoreceptors and chemoreceptors in mouse stomach and esophagus

We used a novel in vitro mouse vagus-gastro-esophageal preparation to study the properties of peripheral vagal afferent endings. We found two types of mechanoreceptive fiber, mucosal receptors and tension receptors. These were distinguished by their sensitivity to mucosal stroking with von Frey hairs and circular tension applied via a claw-cantilever system. A comparison was made with gastro-esophageal afferents found in a similar preparation of ferret tissue. Responses of mouse tension receptors to circular tension were significantly greater than ferret tension and tension/mucosal receptors. Similarly the responses of mouse mucosal receptors to mucosal stroking were significantly greater than ferret mucosal and tension/mucosal receptors. Forty-seven percent of mouse mucosal receptors and 50% of tension receptors responded to one or more drugs or chemical stimuli applied to the receptive field. These included alpha,beta-methylene ATP (10(-6) to 10(-3) M), 5-hydroxytryptamine (10(-6) to 10(-3) M), and hydrochloric acid (10(-2) to 10(-1) M). Drug responses were concentration dependent. One hundred percent of mucosal receptors and 61% of tension receptors tested responded to bile (1:8 to 1:1 dilution). A third type of fiber was recruited by bile. These fibers were mechanically insensitive and silent prior to bile exposure. In conclusion, we have shown three types of gastro-esophageal vagal afferent fibers in the mouse: mucosal mechanoreceptors, tension receptors, and specific chemoreceptors activated by bile.

Acute effects of capsaicin on gastrointestinal vagal afferents

Capsaicin is an important tool for investigation of thin afferent fibres, but its acute effects on subtypes of vagal afferent endings are unknown. In the gastrointestinal tract, these subtypes are: muscle endings (thought to be purely tension sensitive), mucosal endings (sensitive to stroking and chemical stimuli) and endings in the oesophagus with both properties. Acute capsaicin sensitivity was investigated in ferrets using in vivo and in vitro methods. Single-fibre activity was recorded from 63 vagal afferents: 12 Adelta-fibres, 15 C-fibres and 36 unclassified fibres with endings in the oesophagus (n=42), stomach (n=19) and duodenum (n=2). Responses to capsaicin occurred independently of motility changes and were therefore due to direct activation of the receptor ending. In the oesophagus in vivo, two of 10 tension receptors and one of one mucosal receptor responded to intraluminal application of 3.25 mM capsaicin. In the stomach and duodenum, five of 14 tension receptors and two of four mucosal receptors responded to close-systemic (32-164 nmol) capsaicin. In an in vitro gastro-oesophageal preparation, three of five tension, four of 21 mucosal and two of eight tension/mucosal receptors responded to topical application of 1mM capsaicin. Occurrence of responses was therefore unrelated to location of endings and isolation of tissue. Responsiveness was also unrelated to conduction velocity. Capsaicin caused desensitization of responses to further capsaicin application in 37% of afferents. It additionally caused cross-desensitization to mechanical stimuli, which was also seen in afferents that did not respond directly to capsaicin. In conclusion, capsaicin acutely activates all subtypes of gut vagal afferents in vivo and in vitro, although responsiveness is restricted to 30% of fibres and follows no specific pattern. Acute desensitization may be induced with or without a response.

An in vitro study of the properties of vagal afferent fibres innervating the ferret oesophagus and stomach

1. A novel preparation of the oesophagus with attached vagus nerve from the ferret maintained in vitro was used to study the properties of single vagal afferent nerve fibres with identified receptive fields. 2. Recordings were made from three types of gastro-oesophageal vagal afferent fibres that were classified on the basis of their sensitivity to mechanical stimulation. There were those responding to mucosal stroking (mucosal receptors), to circular tension (tension receptors) and those responding to mucosal stroking and circular tension, which we have termed tension/mucosal (TM) receptors. 3. The conduction velocities for mucosal, TM and tension receptor fibres were 6.38 +/- 1.22 m s-1 (n = 22), 6.20 +/- 1.49 m s-1 (n = 13) and 5.33 +/- 0.86 m s-1 (n = 22), respectively. 4. Receptive fields of afferents showed random topographical distribution by fibre type and conduction velocity. They were found mainly distal but also occasionally proximal to the point of vagal dissection. 5. Twenty-eight per cent of mucosal, 63% of TM and 43% of tension receptors responded to one or more drugs or chemical stimuli applied to the receptive field. 6. In conclusion, this experimental preparation provides evidence for the existence of three types of oesophageal vagal afferent fibre, namely mucosal, tension and the newly identified tension/mucosal receptors.

Vagal efferent fibre responses to gastric and oesophageal mechanical and chemical stimuli in the ferret

Gastric and oesophageal afferent inputs to vagal efferent fibres were investigated in Urethane anaesthetized ferrets. Mechanical, chemical, and pharmacological stimuli were tested and efferent activity recorded from single cervical vagal fibres. Fibres showed either no basal discharge or low frequency, irregular patterns of resting discharge; only those which showed > 50% excitation or inhibition of basal activity with both gastric distension and oesophageal balloon distension were studied further. These responses were rapid and maintained only for the duration of the stimuli. 18/32 efferent fibres tested also showed changes in discharge in response to acid infused slowly into the distal oesophagus. These responses were larger after repeated acid infusions. Subsequent intra-oesophageal capsaicin elicited a similar response in 7/8 fibres. These responses were reproducible with repeated capsaicin infusions in 2/4 fibres and desensitized in 2/4 fibres. 2 capsaicin-responsive fibres were unresponsive to oesophageal acidification. 4/12 fibres tested responded to close intraarterial injections of capsaicin and 9/12 to close intraarterial bradykinin. These responses were brief and of short latency. Vagal efferent responses to mechanical and chemical stimuli above were unchanged after the NK-1 receptor antagonist CP96,345 (4 mg/kg i.v.). Subsequently, bilateral vagotomy caudal to the recording site abolished the basal activity in 4/7 fibres. In the 3 fibres where spontaneous activity remained, none of these responded to oesophageal distension or intra-oesophageal acid (2/2 fibres tested) after vagotomy, whereas 2/2 fibres tested still responded to gastric distension. The response of 1 fibre to intraarterial bradykinin and capsaicin was unchanged by vagotomy. We conclude that vagal efferent neurones respond to gastro-oesophageal mechanical inputs and also receive convergent input from oesophageal acid-sensitive and gastrointestinal bradykinin- and capsaicin-sensitive afferents. These afferent inputs are not mediated via NK-1 receptors. There also exists a nonvagal afferent input onto vagal efferent neurones which is probably spinal and likewise non NK-1 receptor mediated.

Gastro-oesophageal afferent and serotonergic inputs to vagal efferent neurones

Peripheral 5-HT3 receptor mechanisms are involved in activation of gastrointestinal (GI) mucosal vagal afferent fibres. 5-HT3 receptor mechanisms in the central nervous system (CNS) may be involved in behavioural and reflex motility responses. This study investigates the processing of different sensory inputs in the CNS and the involvement of 5-HT3 receptors at these different levels. In Urethane (1.5 g/kg, i.p.) anaesthetized, splanchnectomized ferrets, the jugular vein was cannulated for intravenous (i.v.) drug injection, and the coeliac axis for intraarterial (i.a.) injection close to the upper GI tract. The carotid artery was intubated with a T-cannula for CNS-directed intracarotid (i.c.) injections. An intragastric cannula was used for fluid distension (40-50 ml), and an oesophageal catheter for balloon distension (2 ml). Efferent fibres were dissected from the right cervical vagus for single-unit recording. Nineteen single vagal efferent fibres were selected, with low frequency resting discharge (2.5 +/- 0.3 impulses/s), but no respiratory or cardiovascular phasic input. All responded rapidly (< 2.5 s) to gastric distension (532 +/- 230% change in firing rate) and oesophageal distension (300 +/- 170%). Gastric distension caused excitation in 14 fibres, inhibition in 4 fibres, and a biphasic response in 1. Oesophageal distension excited 16 and inhibited 3. Discharge was also influenced by i.a. injection of 5-HT or the 5-HT3 receptor agonist 2-methyl 5-HT (10-100 micrograms) in all fibres tested. These responses consisted of rapid (< 2.5 s) and powerful changes in firing rate, with excitation, inhibition or biphasic responses. 65% of responses to i.c. or i.v. injection were opposite in direction to those after close i.a. injection, indicating the activation of a different population of receptors. No differences were seen between effects of i.c. and i.v. injections. The 5-HT3 receptor antagonist granisetron (100 micrograms/kg, i.v.) blocked or reduced efferent responses to 5-HT receptor agonists, whereas responses to gastric and oesophageal distension were unchanged. Thus there is extensive convergence of inputs from gastric and oesophageal mechanoreceptors onto vagal motorneurones. These central effects of mechanical stimuli do not involve 5-HT3 receptor mechanisms. Other 5-HT3 receptor inputs are evident, probably peripherally from GI mucosal afferent fibres and from within the CNS.

Effects of 5-hydroxytryptamine on discharge of vagal mucosal afferent fibres from the upper gastrointestinal tract of the ferret

This study was undertaken to determine the effects of 5-hydroxytryptamine (5-HT) on vagal mucosal afferent endings and how this may relate to their sensitivity to other stimuli. Single afferent fibres with receptive fields in the mucosa of the upper gastrointestinal tract were recorded from the cervical vagus of Urethane anaesthetized ferrets. The selection criteria included failure to respond to luminal distension (i.e., vagal tension receptors were excluded). All fibres tested responded to mucosal stroking. The majority of these (28/32) also responded to close-intrarterially applied 5-HT (10 micrograms) with a brief burst (usually < 15 s) of action potentials, which in 6/6 cases was reduced or abolished by the 5-HT3 receptor antagonist granisetron (0.1-0.5 mg/kg i.v.), as were four responses to cholecystokinin-octapeptide (100-400 pmol close I.A.). The response to 5-HT was shown to be dose-dependent over the dose range 2-75 micrograms on six occasions. Responses to luminal stimuli, which included 150 mM HCl, 1 M NaCl, and mucosal stroking, were not blocked by granisetron although in three fibres, the resting discharge was reduced by the antagonist, suggesting that resting discharge in vagal mucosal afferents may be influenced by endogenously released 5-HT.