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

Superficial oesophageal mucosal innervation may contribute to severity of symptoms in oesophageal motility disorders

BACKGROUND

Mechanisms underlying perception of dysphagia and chest pain have not been completely elucidated, although oesophageal mucosal afferent nerves might play an important role.

AIMS

To evaluate the relationship between oesophageal mucosal afferent nerves and the severity of dysphagia and chest pain in oesophageal motility disorders.

METHODS

We prospectively recruited patients with oesophageal motility disorders having dysphagia and/or chest pain from whom oesophageal biopsies were obtained. High-resolution manometry classified patients into disorders of oesophagogastric junction (OGJ) outflow and disorders of peristalsis. Symptom severity was assessed using validated questionnaires including Brief Oesophageal Dysphagia Questionnaire (BEDQ). Immunohistochemistry was performed on oesophageal biopsies to evaluate the location of calcitonin gene-related peptide (CGRP)-immunoreactive mucosal afferent nerves. Findings were compared to existing data from 10 asymptomatic healthy volunteers.

RESULTS

Of 79 patients, 61 patients had disorders of OGJ outflow and 18 had disorders of peristalsis. CGRP-immunoreactive mucosal nerves were more superficially located in the mucosa of patients with oesophageal motility disorders compared to healthy volunteers. Within disorders of OGJ outflow, the location of CGRP-immunoreactive nerves negatively correlated with BEDQ score both in the proximal (ρ = -0.567, p < 0.001) and distal oesophagus (ρ = -0.396, p = 0.003). In the proximal oesophagus, strong chest pain was associated with more superficially located mucosal nerves than weak chest pain (p = 0.04). Multivariate analysis showed superficial nerves in the proximal oesophagus was independently associated with severe dysphagia in disorders of OGJ outflow (p = 0.008).

CONCLUSIONS

Superficial location of mucosal nerves in the proximal oesophagus might contribute to symptoms, especially severe dysphagia, in disorders of OGJ outflow.

Plasticity of gastrointestinal vagal afferents in terms of feeding-related physiology and pathophysiology

Gastrointestinal vagal afferents play an important role in communicating food related information from the gut to the brain. This information initiates vago-vagal reflexes essential for gut functions, including gut motility and secretions. These afferents also play a role in energy homeostasis, signalling the arrival, amount and nutrient composition of a meal to the central nervous system where it is processed ultimately leading to termination of a meal. Vagal afferent responses to food related stimuli demonstrate a high degree of plasticity, responding to short term changes in nutritional demand, such as the fluctuations that occur across a 24-hr or in response to a fast, as well as long term changes in energy demand, such as occurs during pregnancy. This plasticity is disrupted in disease states, such as obesity or chronic stress where there is hypo- and hypersensitivity of these afferents, respectively. Improved understanding of the plasticity of these afferents will enable identification of new treatment options for diseases associated with vagal afferent function.

Prox2 and Runx3 vagal sensory neurons regulate esophageal motility

Vagal sensory neurons monitor mechanical and chemical stimuli in the gastrointestinal tract. Major efforts are underway to assign physiological functions to the many distinct subtypes of vagal sensory neurons. Here, we use genetically guided anatomical tracing, optogenetics, and electrophysiology to identify and characterize vagal sensory neuron subtypes expressing Prox2 and Runx3 in mice. We show that three of these neuronal subtypes innervate the esophagus and stomach in regionalized patterns, where they form intraganglionic laminar endings. Electrophysiological analysis revealed that they are low-threshold mechanoreceptors but possess different adaptation properties. Lastly, genetic ablation of Prox2 and Runx3 neurons demonstrated their essential roles for esophageal peristalsis in freely behaving mice. Our work defines the identity and function of the vagal neurons that provide mechanosensory feedback from the esophagus to the brain and could lead to better understanding and treatment of esophageal motility disorders.

Ginger Constituent 6-Shogaol Attenuates Vincristine-Induced Activation of Mouse Gastroesophageal Vagal Afferent C-Fibers

Chemotherapeutic agent-induced nausea and vomiting are the severe adverse effects that are induced by their stimulations on the peripheral and/or central emetic nerve pathways. Even though ginger has been widely used as an herbal medicine to treat emesis, mechanisms underlying its neuronal actions are still less clear. The present study aimed to determine the chemotherapeutic agent vincristine-induced effect on gastroesophageal vagal afferent nerve endings and the potential inhibitory role of ginger constituent 6-shogaol on such response. Two-photon neuron imaging studies were performed in ex vivo gastroesophageal-vagal preparations from Pirt-GCaMP6 transgenic mice. Vincristine was applied to the gastroesophageal vagal afferent nerve endings, and the evoked calcium influxes in their intact nodose ganglion neuron somas were recorded. The responsive nodose neuron population was first characterized, and the inhibitory effects of 5-HT3 antagonist palonosetron, TRPA1 antagonist HC-030031, and ginger constituent 6-shogaol were then determined. Vincristine application at gastroesophageal vagal afferent nerve endings elicited intensive calcium influxes in a sub-population of vagal ganglion neurons. These neurons were characterized by their positive responses to P2X2/3 receptor agonist α,β-methylene ATP and TRPA1 agonist cinnamaldehyde, suggesting their nociceptive placodal nodose C-fiber neuron lineages. Pretreatment with TRPA1 selective blocker HC-030031 inhibited vincristine-induced calcium influxes in gastroesophageal nodose C-fiber neurons, indicating that TRPA1 played a functional role in mediating vincristine-induced activation response. Such inhibitory effect was comparable to that from 5-HT3 receptor antagonist palonosetron. Alternatively, pretreatment with ginger constituent 6-shogaol significantly attenuated vincristine-induced activation response. The present study provides new evidence that chemotherapeutic agent vincristine directly activates vagal nodose nociceptive C-fiber neurons at their peripheral nerve endings in the upper gastrointestinal tract. This activation response requires both TRPA1 and 5-HT3 receptors and can be attenuated by ginger constituent 6-shogaol.

Gut feelings: mechanosensing in the gastrointestinal tract

The primary function of the gut is to procure nutrients. Synchronized mechanical activities underlie nearly all its endeavours. Coordination of mechanical activities depends on sensing of the mechanical forces, in a process called mechanosensation. The gut has a range of mechanosensory cells. They function either as specialized mechanoreceptors, which convert mechanical stimuli into coordinated physiological responses at the organ level, or as non-specialized mechanosensory cells that adjust their function based on the mechanical state of their environment. All major cell types in the gastrointestinal tract contain subpopulations that act as specialized mechanoreceptors: epithelia, smooth muscle, neurons, immune cells, and others. These cells are tuned to the physical properties of the surrounding tissue, so they can discriminate mechanical stimuli from the baseline mechanical state. The importance of gastrointestinal mechanosensation has long been recognized, but the latest discoveries of molecular identities of mechanosensors and technical advances that resolve the relevant circuitry have poised the field to make important intellectual leaps. This Review describes the mechanical factors relevant for normal function, as well as the molecules, cells and circuits involved in gastrointestinal mechanosensing. It concludes by outlining important unanswered questions in gastrointestinal mechanosensing.

Esophageal mucosal sensory nerves and potential mechanoreceptors in patients with ineffective esophageal motility

BACKGROUND

Ineffective esophageal motility (IEM) is the most common motility disorder. However, little is known about its pathophysiology. Vagal afferent nerves convey esophageal intraluminal bolus information to solitary nucleus, which is likely to be involved with esophageal primary and secondary peristalsis (SP). We hypothesized that altered mucosal sensory afferents underlie the pathogenesis of IEM.

METHODS

We prospectively collected esophageal biopsies from 38 patients with proton pump inhibitor-refractory reflux symptoms from January to December 2019. All patients underwent high-resolution manometry for the evaluation of primary and secondary peristalsis, and off-PPI 24-h impedance-pH studies. Biopsies were analyzed using immunohistochemistry for identification of calcitonin gene-related peptide-immunoreactive (CGRP-IR) nerves and qPCR for mRNA expression of potential mechanoreceptors.

KEY RESULTS

Overall 32 patients were finally analyzed which consisted of 11 patients with normal motility and 21 patients with IEM. The position of mucosal CGRP-IR nerves from the esophageal lumen did not differ between the two groups (the proximal esophagus (p = 0.52), the mid-esophagus (p = 0.92), the distal esophagus (p = 0.29)) with the similar reflux profile. No difference was seen in the position of CGRP-IR nerves between patients with successful triggering of SP and those unable to trigger SP. There was also no difference in mRNA expression of each potential mechanoreceptors (TRPA1, TRPV1, TRPV4, ASIC1, ASIC3) between the two groups.

CONCLUSIONS AND INFERENCES

Our study showed that mucosal sensory afferents nerve position and mRNA expression of potential mechanoreceptors did not correlate to weak esophageal contraction.

Neural signalling of gut mechanosensation in ingestive and digestive processes

Eating and drinking generate sequential mechanosensory signals along the digestive tract. These signals are communicated to the brain for the timely initiation and regulation of diverse ingestive and digestive processes - ranging from appetite control and tactile perception to gut motility, digestive fluid secretion and defecation - that are vital for the proper intake, breakdown and absorption of nutrients and water. Gut mechanosensation has been investigated for over a century as a common pillar of energy, fluid and gastrointestinal homeostasis, and recent discoveries of specific mechanoreceptors, contributing ion channels and the well-defined circuits underlying gut mechanosensation signalling and function have further expanded our understanding of ingestive and digestive processes at the molecular and cellular levels. In this Review, we discuss our current understanding of the generation of mechanosensory signals from the digestive periphery, the neural afferent pathways that relay these signals to the brain and the neural circuit mechanisms that control ingestive and digestive processes, focusing on the four major digestive tract parts: the oral and pharyngeal cavities, oesophagus, stomach and intestines. We also discuss the clinical implications of gut mechanosensation in ingestive and digestive disorders.

Contribution of tetrodotoxin-sensitive, voltage-gated sodium channels (NaV1) to action potential discharge from mouse esophageal tension mechanoreceptors

Action potentials depend on voltage-gated sodium channels (NaV1s), which have nine α subtypes. NaV1 inhibition is a target for pathologies involving excitable cells such as pain. However, because NaV1 subtypes are widely expressed, inhibitors may inhibit regulatory sensory systems. Here, we investigated specific NaV1s and their inhibition in mouse esophageal mechanoreceptors-non-nociceptive vagal sensory afferents that are stimulated by low threshold mechanical distension, which regulate esophageal motility. Using single fiber electrophysiology, we found mechanoreceptor responses to esophageal distension were abolished by tetrodotoxin. Single-cell RT-PCR revealed that esophageal-labeled TRPV1-negative vagal neurons expressed multiple tetrodotoxin-sensitive NaV1s: NaV1.7 (almost all neurons) and NaV1.1, NaV1.2, and NaV1.6 (in ∼50% of neurons). Inhibition of NaV1.7, using PF-05089771, had a small inhibitory effect on mechanoreceptor responses to distension. Inhibition of NaV1.1 and NaV1.6, using ICA-121341, had a similar small inhibitory effect. The combination of PF-05089771 and ICA-121341 inhibited but did not eliminate mechanoreceptor responses. Inhibition of NaV1.2, NaV1.6, and NaV1.7 using LSN-3049227 inhibited but did not eliminate mechanoreceptor responses. Thus, all four tetrodotoxin-sensitive NaV1s contribute to action potential initiation from esophageal mechanoreceptors terminals. This is different to those NaV1s necessary for vagal action potential conduction, as demonstrated using GCaMP6s imaging of esophageal vagal neurons during electrical stimulation. Tetrodotoxin-sensitive conduction was abolished in many esophageal neurons by PF-05089771 alone, indicating a critical role of NaV1.7. In summary, multiple NaV1 subtypes contribute to electrical signaling in esophageal mechanoreceptors. Thus, inhibition of individual NaV1s would likely have minimal effect on afferent regulation of esophageal motility.

Selective stimulation of the ferret abdominal vagus nerve with multi-contact nerve cuff electrodes

Dysfunction and diseases of the gastrointestinal (GI) tract are a major driver of medical care. The vagus nerve innervates and controls multiple organs of the GI tract and vagus nerve stimulation (VNS) could provide a means for affecting GI function and treating disease. However, the vagus nerve also innervates many other organs throughout the body, and off-target effects of VNS could cause major side effects such as changes in blood pressure. In this study, we aimed to achieve selective stimulation of populations of vagal afferents using a multi-contact cuff electrode wrapped around the abdominal trunks of the vagus nerve. Four-contact nerve cuff electrodes were implanted around the dorsal (N = 3) or ventral (N = 3) abdominal vagus nerve in six ferrets, and the response to stimulation was measured via a 32-channel microelectrode array (MEA) inserted into the left or right nodose ganglion. Selectivity was characterized by the ability to evoke responses in MEA channels through one bipolar pair of cuff contacts but not through the other bipolar pair. We demonstrated that it was possible to selectively activate subpopulations of vagal neurons using abdominal VNS. Additionally, we quantified the conduction velocity of evoked responses to determine what types of nerve fibers (i.e., Aδ vs. C) responded to stimulation. We also quantified the spatial organization of evoked responses in the nodose MEA to determine if there is somatotopic organization of the neurons in that ganglion. Finally, we demonstrated in a separate set of three ferrets that stimulation of the abdominal vagus via a four-contact cuff could selectively alter gastric myoelectric activity, suggesting that abdominal VNS can potentially be used to control GI function.

Endocannabinoids and the Gut-Brain Control of Food Intake and Obesity

Gut-brain signaling controls food intake and energy homeostasis, and its activity is thought to be dysregulated in obesity. We will explore new studies that suggest the endocannabinoid (eCB) system in the upper gastrointestinal tract plays an important role in controlling gut-brain neurotransmission carried by the vagus nerve and the intake of palatable food and other reinforcers. A focus will be on studies that reveal both indirect and direct interactions between eCB signaling and vagal afferent neurons. These investigations identify (i) an indirect mechanism that controls nutrient-induced release of peptides from the gut epithelium that directly interact with corresponding receptors on vagal afferent neurons, and (ii) a direct mechanism via interactions between eCBs and cannabinoid receptors expressed on vagal afferent neurons. Moreover, the impact of diet-induced obesity on these pathways will be considered.

Rapid activation of esophageal mechanoreceptors alters the pharyngeal phase of swallow: Evidence for inspiratory activity during swallow

Swallow is a complex behavior that consists of three coordinated phases: oral, pharyngeal, and esophageal. Esophageal distension (EDist) has been shown to elicit pharyngeal swallow, but the physiologic characteristics of EDist-induced pharyngeal swallow have not been specifically described. We examined the effect of rapid EDist on oropharyngeal swallow, with and without an oral water stimulus, in spontaneously breathing, sodium pentobarbital anesthetized cats (n = 5). Electromyograms (EMGs) of activity of 8 muscles were used to evaluate swallow: mylohyoid (MyHy), geniohyoid (GeHy), thyrohyoid (ThHy), thyropharyngeus (ThPh), thyroarytenoid (ThAr), cricopharyngeus (upper esophageal sphincter: UES), parasternal (PS), and costal diaphragm (Dia). Swallow was defined as quiescence of the UES with overlapping upper airway activity, and it was analyzed across three stimulus conditions: 1) oropharyngeal water infusion only, 2) rapid esophageal distension (EDist) only, and 3) combined stimuli. Results show a significant effect of stimulus condition on swallow EMG amplitude of the mylohyoid, geniohyoid, thyroarytenoid, diaphragm, and UES muscles. Collectively, we found that, compared to rapid cervical esophageal distension alone, the stimulus condition of rapid distension combined with water infusion is correlated with increased laryngeal adductor and diaphragm swallow-related EMG activity (schluckatmung), and post-swallow UES recruitment. We hypothesize that these effects of upper esophageal distension activate the brainstem swallow network, and function to protect the airway through initiation and/or modulation of a pharyngeal swallow response.

Gastrointestinal Vagal Afferents and Food Intake: Relevance of Circadian Rhythms

Gastrointestinal vagal afferents (VAs) play an important role in food intake regulation, providing the brain with information on the amount and nutrient composition of a meal. This is processed, eventually leading to meal termination. The response of gastric VAs, to food-related stimuli, is under circadian control and fluctuates depending on the time of day. These rhythms are highly correlated with meal size, with a nadir in VA sensitivity and increase in meal size during the dark phase and a peak in sensitivity and decrease in meal size during the light phase in mice. These rhythms are disrupted in diet-induced obesity and simulated shift work conditions and associated with disrupted food intake patterns. In diet-induced obesity the dampened responses during the light phase are not simply reversed by reverting back to a normal diet. However, time restricted feeding prevents loss of diurnal rhythms in VA signalling in high fat diet-fed mice and, therefore, provides a potential strategy to reset diurnal rhythms in VA signalling to a pre-obese phenotype. This review discusses the role of the circadian system in the regulation of gastrointestinal VA signals and the impact of factors, such as diet-induced obesity and shift work, on these rhythms.

Intact vagal gut-brain signalling prevents hyperphagia and excessive weight gain in response to high-fat high-sugar diet

Aim

The tools that have been used to assess the function of the vagus nerve lack specificity. This could explain discrepancies about the role of vagal gut‐brain signalling in long‐term control of energy balance. Here we use a validated approach to selectively ablate sensory vagal neurones that innervate the gut to determine the role of vagal gut‐brain signalling in the control of food intake, energy expenditure and glucose homoeostasis in response to different diets.

Methods

Rat nodose ganglia were injected bilaterally with either the neurotoxin saporin conjugated to the gastrointestinal hormone cholecystokinin (CCK), or unconjugated saporin as a control. Food intake, body weight, glucose tolerance and energy expenditure were measured in both groups in response to chow or high‐fat high‐sugar (HFHS) diet. Willingness to work for fat or sugar was assessed by progressive ratio for orally administered solutions, while post‐ingestive feedback was tested by measuring food intake after an isocaloric lipid or sucrose pre‐load.

Results

Vagal deafferentation of the gut increases meal number in lean chow‐fed rats. Switching to a HFHS diet exacerbates overeating and body weight gain. The breakpoint for sugar or fat solution did not differ between groups, suggesting that increased palatability may not drive HFHS‐induced hyperphagia. Instead, decreased satiation in response to intra‐gastric infusion of fat, but not sugar, promotes hyperphagia in CCK‐Saporin‐treated rats fed with HFHS diet.

Conclusions

We conclude that intact sensory vagal neurones prevent hyperphagia and exacerbation of weight gain in response to a HFHS diet by promoting lipid‐mediated satiation.

Pregnancy-related plasticity of gastric vagal afferent signals in mice

Gastric vagal afferents (GVAs) sense food-related mechanical stimuli and signal to the central nervous system, to integrate control of meal termination. Pregnancy is characterized by increased maternal food intake, which is essential for normal fetal growth and to maximize progeny survival and health. However, it is unknown whether GVA function is altered during pregnancy to promote food intake. This study aimed to determine the mechanosensitivity of GVAs and food intake during early, mid-, and late stages of pregnancy in mice. Pregnant mice consumed more food compared with nonpregnant mice, notably in the light phase during mid- and late pregnancy. The increased food intake was predominantly due to light-phase increases in meal size across all stages of pregnancy. The sensitivity of GVA tension receptors to gastric distension was significantly attenuated in mid- and late pregnancy, whereas the sensitivity of GVA mucosal receptors to mucosal stroking was unchanged during pregnancy. To determine whether pregnancy-associated hormonal changes drive these adaptations, the effects of estradiol, progesterone, prolactin, and growth hormone on GVA tension receptor mechanosensitivity were determined in nonpregnant female mice. The sensitivity of GVA tension receptors to gastric distension was augmented by estradiol, attenuated by growth hormone, and unaffected by progesterone or prolactin. Together, the data indicate that the sensitivity of GVA tension receptors to tension is reduced during pregnancy, which may attenuate the perception of gastric fullness and explain increased food intake. Further, these adaptations may be driven by increases in maternal circulating growth hormone levels during pregnancy.NEW & NOTEWORTHY This study provides first evidence that gastric vagal afferent signaling is attenuated during pregnancy and inversely associated with meal size. Growth hormone attenuated mechanosensitivity of gastric vagal afferents, adding support that increases in maternal growth hormone may mediate adaptations in gastric vagal afferent signaling during pregnancy. These findings have important implications for the peripheral control of food intake during pregnancy.

An Atlas of Vagal Sensory Neurons and Their Molecular Specialization

Sensory functions of the vagus nerve are critical for conscious perceptions and for monitoring visceral functions in the cardio-pulmonary and gastrointestinal systems. Here, we present a comprehensive identification, classification, and validation of the neuron types in the neural crest (jugular) and placode (nodose) derived vagal ganglia by single-cell RNA sequencing (scRNA-seq) transcriptomic analysis. Our results reveal major differences between neurons derived from different embryonic origins. Jugular neurons exhibit fundamental similarities to the somatosensory spinal neurons, including major types, such as C-low threshold mechanoreceptors (C-LTMRs), A-LTMRs, Aδ-nociceptors, and cold-, and mechano-heat C-nociceptors. In contrast, the nodose ganglion contains 18 distinct types dedicated to surveying the physiological state of the internal body. Our results reveal a vast diversity of vagal neuron types, including many previously unanticipated types, as well as proposed types that are consistent with chemoreceptors, nutrient detectors, baroreceptors, and stretch and volume mechanoreceptors of the respiratory, gastrointestinal, and cardiovascular systems.

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A comprehensive molecular identification of neuronal types in vagal ganglion complex

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Prdm12⁺ jugular ganglion neurons share features with spinal somatosensory neurons

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Phox2b⁺ viscerosensory nodose neurons are molecularly versatile and highly specialized

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Nodose neuron types are consistent with chemo-, baro-, stretch-, tension-, and volume-sensors

Dissecting the Role of Subtypes of Gastrointestinal Vagal Afferents

Gastrointestinal (GI) vagal afferents convey sensory signals from the GI tract to the brain. Numerous subtypes of GI vagal afferent have been identified but their individual roles in gut function and feeding regulation are unclear. In the past decade, technical approaches to selectively target vagal afferent subtypes and to assess their function has significantly progressed. This review examines the classification of GI vagal afferent subtypes and discusses the current available techniques to study vagal afferents. Investigating the distribution of GI vagal afferent subtypes and understanding how to access and modulate individual populations are essential to dissect their fundamental roles in the gut-brain axis.

Vagus Nerves Provide a Robust Afferent Innervation of the Mucosa Throughout the Body of the Esophagus in the Mouse

The vagal afferent nerves regulate swallowing and esophageal motor reflexes. However, there are still gaps in the understanding of vagal afferent innervation of the esophageal mucosa. Anatomical studies found that the vagal afferent mucosal innervation is dense in the upper esophageal sphincter area but rare in more distal segments of the esophagus. In contrast, electrophysiological studies concluded that the vagal afferent nerve fibers also densely innervate mucosa in more distal esophagus. We hypothesized that the transfection of vagal afferent neurons with adeno-associated virus vector encoding green fluorescent protein (AAV-GFP) allows to visualize vagal afferent nerve fibers in the esophageal mucosa in the mouse. AAV-GFP was injected into the vagal jugular/nodose ganglia in vivo to sparsely label vagal afferent nerve fibers. The esophageal tissue was harvested 4-6 weeks later, the GFP signal was amplified by immunostaining, and confocal optical sections of the entire esophagi were obtained. We found numerous GFP-labeled fibers in the mucosa throughout the whole body of the esophagus. The GFP-labeled mucosal fibers were located just beneath the epithelium, branched repeatedly, had mostly longitudinal orientation, and terminated abruptly without forming terminal structures. The GFP-labeled mucosal fibers were concentrated in random areas of various sizes in which many fibers could be traced to a single parental axon. We conclude that the vagus nerves provide a robust afferent innervation of the mucosa throughout the whole body of the esophagus in the mouse. Vagal mucosal fibers may contribute to the sensing of intraluminal content and regulation of swallowing and other reflexes.

Modulatory effect of methanandamide on gastric vagal afferent satiety signals depends on nutritional status

SIGNIFICANCE STATEMENT

Gastric vagal afferent responses to tension are dampened in high fat diet-induced obesity. Endocannabinoids are known to dose-dependently inhibit and excite gastric vagal afferents but their effect on gastric vagal afferents in diet-induced obesity are unknown. In individual gastric vagal afferent neurons of diet-induced obese mice the co-expression of components of the endocannabinoid system, including CB1, GHSR, TRPV1 and FAAH, was increased compared with lean mice. In high fat diet-induced obese mice, methanandamide only inhibited gastric vagal afferent responses to tension, possibly due to the observed change in the balance of receptors, hormones and breakdown enzymes in this system. Collectively, these data suggest that endocannabinoid signalling, by gastric vagal afferents, is altered in diet-induced obesity which may impact satiety and gastrointestinal function.

ABSTRACT

Gastric vagal afferents (GVAs) play a role in appetite regulation. The endocannabinoid anandamide (AEA) dose-dependently inhibits and excites tension-sensitive GVAs. However, it is also known that high fat diet (HFD) feeding alters GVA responses to stretch. The aim of this study was to determine the role of AEA in GVA signalling in lean and HFD-induced obese mice. Male C57BL/6 mice were fed (12 weeks) a standard laboratory diet (SLD) or HFD. Protein and mRNA expression of components of the cannabinoid system was determined in individual GVA cell bodies and the gastric mucosa. An in vitro GVA preparation was used to assess the effect of methanandamide (mAEA) on tension-sensitive GVAs and the second messenger pathways involved. In individual GVA cell bodies, cannabinoid 1 (CB1) and ghrelin (GHSR) receptor mRNA was higher in HFD mice than SLD mice. Conversely, gastric mucosal AEA and ghrelin protein levels were lower in HFD mice than SLD mice. In SLD mice, mAEA exerted dose-dependent inhibitory and excitatory effects on tension-sensitive GVAs. Only an inhibitory effect of mAEA was observed in HFD mice. The excitatory effect of mAEA was dependent on CB1, transient receptor potential vanilloid 1 (TRPV1) and the protein kinase C. Conversely, the inhibitory effect was dependent on CB1, growth hormone secretagogue receptor, TRPV1 and the protein kinase A. Endocannabinoids, acting through CB1 and TRPV1, have a pivotal role in modulating GVA satiety signals depending on the second messenger pathway utilised. In HFD mice only an inhibitory effect was observed. These changes may contribute to the development and/or maintenance of obesity.

Circadian regulation of appetite and time restricted feeding

The circadian system plays an important role in the temporal regulation of metabolic processes as well as food intake to ensure energy efficiency. The 'master' clock is located within the superchiasmatic nucleus and receives input from the retina so that it can be entrained by the light:dark cycle. In turn, the master clock entrains other clocks in the central nervous system, including areas involved in energy homeostasis such as the arcuate nucleus, and the periphery (e.g. adipose tissue and the gastrointestinal tract). This master clock is reinforced by other zeitgebers such as the timing of food intake and activity. If these zeitgebers desynchronise, such as occurs in high fat diet-induced obesity or shift work conditions, it can lead to a misalignment of circadian clocks, disruption of metabolic processes and the development of metabolic disorders. The timing of food intake is a strong zeitgeber, particularly in the gastrointestinal tract, and therefore time restricted feeding offers potential for the treatment of diet and shift work induced metabolic disorders. This review will focus on the role of the circadian system in food intake regulation and the effect of environment factors, such as high fat diet feeding or shift work, on the temporal regulation of food intake along with the benefits of time restricted feeding.

Chronic stress induces hypersensitivity of murine gastric vagal afferents

BACKGROUND

Stress exposure is known to trigger and exacerbate functional dyspepsia (FD) symptoms. Increased gastric sensitivity to food-related stimuli is widely observed in FD patients and is associated with stress and psychological disorders. The mechanisms underlying the hypersensitivity are not clear. Gastric vagal afferents (GVAs) play an important role in sensing meal-related mechanical stimulation to modulate gastrointestinal function and food intake. This study aimed to determine whether GVAs display hypersensitivity after chronic stress, and whether its interaction with leptin was altered by stress.

METHODS

Eight-week-old male C57BL/6 mice were exposed to unpredictable chronic mild stress or no stress (control) for 8 weeks. The metabolic rate, gastric emptying rate, and anxiety- and depression-like behaviors were determined. GVA mechanosensitivity, and its modulation by leptin, was determined using an in vitro single fiber recording technique. QRT-PCR was used to establish the levels of leptin and leptin receptor mRNA in the stomach and nodose ganglion, respectively.

KEY RESULTS

The stressed mice had lower body weight and food intake, and increased anxiety-like behavior compared to the control mice. The mechanosensitivity of mucosal and tension-sensitive GVAs was higher in the stressed mice. Leptin potentiated mucosal GVA mechanosensitivity in control but not stressed mice. The expression of leptin mRNA in the gastric mucosa was lower in the stressed mice.

CONCLUSIONS AND INFERENCES

In conclusion, chronic stress enhances GVA mechanosensitivity, which may contribute to the gastric hypersensitivity in FD. In addition, the modulatory effect of leptin on GVA signaling is lost after chronic stress exposure.

Biphasic effects of methanandamide on murine gastric vagal afferent mechanosensitivity

KEY POINTS

The fine control of food intake is important for the maintenance of a healthy metabolic state. Gastric vagal afferents (GVAs) are involved in the peripheral regulation of food intake via signalling the degree of distension of the stomach which ultimately leads to feelings of fullness and satiety. This study provides evidence that endocannabinoids such as anandamide are capable of regulating GVA sensitivity in a concentration-dependent biphasic manner. This biphasic effect is dependent upon interactions between the CB1, TRPV1 and GHSR receptors. These data have important implications for the peripheral control of food intake.

ABSTRACT

Gastric vagal afferents (GVAs) signal to the hindbrain resulting in satiety. Endocannabinoids are endogenous ligands of cannabinoid 1 receptor (CB1) and transient receptor potential vanilloid-1 (TRPV1) channels. The endocannabinoid anandamide (AEA) is expressed in the stomach, and its receptor CB1 is expressed in ghrelin-positive gastric mucosal cells. Further, TRPV1, CB1 and growth hormone secretagogue receptor (ghrelin receptor, GHSR) are expressed in subpopulations of GVA neurons. This study aimed to determine the interaction between TRPV1, CB1, GHSR and endocannabinoids in the modulation of GVA signalling. An in vitro electrophysiology preparation was used to assess GVA mechanosensitivity in male C57BL/6 mice. Effects of methanandamide (mAEA; 1-100 nm), on GVA responses to stretch were determined in the absence and presence of antagonists of CB1, TRPV1, GHSR, protein kinase-A (PKA), protein kinase-C (PKC) and G-protein subunits Gα_i/o , or Gα_q . Low doses (1-10 nm) of mAEA reduced GVA responses to 3 g stretch, whereas high doses (30-100 nm) increased the response. The inhibitory and excitatory effects of mAEA (1-100 nm) were reduced/lost in the presence of a CB1 and TRPV1 antagonist. PKA, Gα_i/o or GHSR antagonists prevented the inhibitory effect of mAEA on GVA mechanosensitivity. Conversely, in the presence of a PKC or Gα_q antagonist the excitatory effect of mAEA was reduced or lost, respectively. Activation of CB1, by mAEA, can activate or inhibit TRPV1 to increase or decrease GVA responses to stretch, depending on the pathway activated. These interactions could play an important role in the fine control of food intake.

Effects of esophageal acid infusion vs mosapride on distension-induced secondary peristalsis in humans

Secondary peristalsis contributes to the clearance of the refluxate from the esophagus. Acute administration of 5-hydroxytryptamine 4 (5-HT₄ ) receptors agonist, mosapride or esophageal infusion of hydrochloric acid (HCl) facilitates secondary peristalsis. The aim of this study was to determine whether esophageal acid infusion and administration of mosapride had different effects on secondary peristalsis. Secondary peristalsis was performed with esophageal distension with rapid and slow air injections in 16 healthy subjects. We performed two separate sessions with HCl (0.1 N) and 40 mg oral mosapride to compare their influence on secondary peristaltic parameters. The threshold volume of secondary peristalsis was significantly lower with HCl infusion than mosapride (P = 0.01) by slow air injections. The threshold volume to generate secondary peristalsis was significantly lower with HCl infusion than mosapride (P = 0.002) by rapid air injections. More secondary peristalsis was trigged by rapid air injections after HCl infusion than mosapride (P = 0.003). Infusion of HCl or mosapride administration has similar effects on peristaltic wave amplitude and duration of primary and secondary peristalsis. Acute esophageal acid infusion can induce greater mechanosensitivity to distension-induced secondary peristalsis than 5-HT₄ receptors agonist mosapride. The data suggest that acid-sensitive afferents are more likely to contribute to sensory modulation of esophageal secondary peristalsis; however, the motility aspects of secondary peristalsis are comparable between acute esophageal acidification and 5-HT₄ receptors activation via mosapride.

Characterization of peristaltic motility in the striated muscle portion of the esophagus using a novel in vivo method in rats

BACKGROUND

Esophageal peristalsis is controlled by the brainstem via vago-vagal reflex. However, the precise regulatory mechanisms in the striated muscle portion are largely unknown. The aim of this study was to characterize peristaltic motility in the portion of the esophagus using a novel in vivo method in rats.

METHODS

A balloon-tipped catheter was placed in the esophagus of a rat anesthetized with urethane. To induce esophageal peristalsis, the balloon was inflated by water injection.

KEY RESULTS

When the balloon was inflated near the bronchial bifurcation, the balloon was transported in the aboral direction. Vagotomy abolished the peristaltic response. The threshold volume for inducing esophageal peristalsis varied according to the velocity of balloon distention; the volume being effective to induce peristalsis at a low inflation speed was smaller than the threshold volume at a rapid inflation speed. Even in the absence of inflation, keeping the balloon inside the esophagus during an interval period prevented subsequent induction of peristaltic motility. In addition, a nitric oxide synthase inhibitor abolished the induction of esophageal peristalsis.

CONCLUSIONS AND INFERENCES

Our findings suggest that (a) in addition to the intensity, the velocity of distention is important for activating the mechanosensory mechanism to induce esophageal peristalsis, (b) tonic inputs from afferent fibers located at the mucosa may reduce the excitability of mechanosensors which is necessary for inducing peristalsis, and (c) nitric oxide plays essential roles in the induction of esophageal peristalsis. These results provide novel insights into the regulatory mechanisms of esophageal motility.

QX-314 inhibits acid-induced activation of esophageal nociceptive C fiber neurons

INTRODUCTION

Acid reflux in the esophagus can induce painful sensations such as heartburn and non-cardiac chest pain. These nociceptive symptoms are initiated by activation of TRPV1-positive afferent C fibers in the esophagus. The present study aimed to explore a novel C fiber inhibition approach. We hypothesized that activation of TRPV1 by acid enabled QX-314, a membrane impermeable sodium channel blocker, to inhibit acid-induced activation of esophageal nociceptive C fiber neurons.

METHOD

We determined the inhibitory effect of QX-314 in the presence of acid in guinea pig esophageal nociceptive vagal jugular C fiber neurons by both patch clamp recording in neuron soma and by extra-cellular recording at nerve terminals.

KEY RESULTS

Our data demonstrated QX-314 alone did not inhibit sodium currents. However, when applied along with capsaicin to activate TRPV1, QX-314 was able to block sodium currents in esophageal-specific jugular C fiber neurons. We then showed that in the presence of acid, QX-314 significantly blocked acid-evoked activation of jugular C fiber neurons. This effect was attenuated by TRPV1 antagonist AMG9810, suggesting acid-mediated inhibitory effect of QX-314 was TRPV1-dependent. Finally, we provided evidence at nerve endings that acid-evoked action potential discharges in esophageal jugular C fibers were inhibited by QX-314 when applied in the presence of acid.

CONCLUSION AND INFERENCES

Our data demonstrated that activation of TRPV1 by acid enabled membrane impermeable sodium channel blocker QX-314 to inhibit acid-induced activation in esophageal nociceptive C fibers. This supports a localized application of QX-314 in the esophagus to block esophageal nociception in acid reflux disorders.

Effects of esophageal acidification on esophageal reflexes controlling the upper esophageal sphincter

Esophageal acid exposure can alter upper esophageal sphincter (UES) function, but the mechanism is unknown. The aim of this study was to determine the effects of esophageal acid exposure on esophago-UES relaxation (EURR) and contractile (EUCR) reflexes. Cats, decrebrate ( n = 27) or chronic ( n = 4), were implanted with electromyographic electrodes on pharynx, larynx, and esophagus. The esophagus was infused with either NaCl (0.9%) or HCl (0.1 N). The EUCR was activated by balloon distension in acute cats and slow air injection in chronic cats, and the EURR was activated by rapid air injection in both sets of cats. We found that NaCl infused for 15 or 30 min had no effect on EUCR or EURR in acute cats. HCl infused for 15, 30, or 45 min significantly ( P < 0.05) decreased the sensitivity to activate EUCR. HCl infused for 15 min significantly ( P < 0.05) increased and for 45 min significantly ( P < 0.05) decreased sensitivity to activate EURR. In chronic cats, HCl infused for 15 min/day increased sensitivity to activate EURR and decreased ( P < 0.05) sensitivity to activate EUCR after 4 days of infusion. EURR occurred spontaneously during HCl infusions on the 3rd and 4th ( P < 0.05) days of HCl infusion. We conclude that esophageal acid exposure initially sensitizes the esophagus to activation of EURR and desensitizes to activation of EUCR, but with longer exposure desensitizes to both. The alteration in sensitivity to activate EURR and EUCR caused by gastroesophageal reflux may play a role in the generation of supraesophageal reflux. NEW & NOTEWORTHY In acute studies, short-term esophageal acid exposure sensitizes esophagus to activation of esophago-upper esophageal sphincter relaxation response (EURR), whereas longer-term exposure inhibits EURR. Short- or long-term esophageal acid exposure decreases sensitivity to activation of esophago-upper esophageal sphincter contractile response (EUCR). In chronic studies, short-term esophageal acid exposure has the same effects on EURR and EUCR as occur acutely, but these effects take days to develop. Alteration in EURR and EUCR caused by gastroesophageal reflux may play a role in reflux disease.

The metabolic role of vagal afferent innervation

The regulation of energy and glucose balance contributes to whole-body metabolic homeostasis, and such metabolic regulation is disrupted in obesity and diabetes. Metabolic homeostasis is orchestrated partly in response to nutrient and vagal-dependent gut-initiated functions. Specifically, the sensory and motor fibres of the vagus nerve transmit intestinal signals to the central nervous system and exert biological and physiological responses. In the past decade, the understanding of the regulation of vagal afferent signals and of the associated metabolic effect on whole-body energy and glucose balance has progressed. This Review highlights the contributions made to the understanding of the vagal afferent system and examines the integrative role of the vagal afferent in gastrointestinal regulation of appetite and glucose homeostasis. Investigating the integrative and metabolic role of vagal afferent signalling represents a potential strategy to discover novel therapeutic targets to restore energy and glucose balance in diabetes and obesity.

Meal-Sensing Signaling Pathways in Functional Dyspepsia

The upper gastrointestinal tract plays an important role in sensing the arrival, amount and chemical composition of a meal. Ingestion of a meal triggers a number of sensory signals in the gastrointestinal tract. These include the response to mechanical stimulation (e.g., gastric distension), from the presence of food in the gut, and the interaction of various dietary nutrients with specific "taste" receptors on specialized enteroendocrine cells in the small intestine culminating in the release of gut hormones. These signals are then transmitted to the brain where they contribute to food intake regulation by modulating appetite as well as feedback control of gastrointestinal functions (e.g., gut motility). There is evidence that the sensitivity to these food related stimuli is abnormally enhanced in functional dyspepsia leading to symptoms such nausea and bloating. In addition, these gut-brain signals can modulate the signaling pathways involved in visceral pain. This review will discuss the role of gut-brain signals in appetite regulation and the role dysregulation of this system play in functional dyspepsia.

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.

Involvement of TRPV1 Channels in Energy Homeostasis

The ion channel TRPV1 is involved in a wide range of processes including nociception, thermosensation and, more recently discovered, energy homeostasis. Tightly controlling energy homeostasis is important to maintain a healthy body weight, or to aid in weight loss by expending more energy than energy intake. TRPV1 may be involved in energy homeostasis, both in the control of food intake and energy expenditure. In the periphery, it is possible that TRPV1 can impact on appetite through control of appetite hormone levels or via modulation of gastrointestinal vagal afferent signaling. Further, TRPV1 may increase energy expenditure via heat production. Dietary supplementation with TRPV1 agonists, such as capsaicin, has yielded conflicting results with some studies indicating a reduction in food intake and increase in energy expenditure, and other studies indicating the converse. Nonetheless, it is increasingly apparent that TRPV1 may be dysregulated in obesity and contributing to the development of this disease. The mechanisms behind this dysregulation are currently unknown but interactions with other systems, such as the endocannabinoid systems, could be altered and therefore play a role in this dysregulation. Further, TRPV1 channels appear to be involved in pancreatic insulin secretion. Therefore, given its plausible involvement in regulation of energy and glucose homeostasis and its dysregulation in obesity, TRPV1 may be a target for weight loss therapy and diabetes. However, further research is required too fully elucidate TRPV1s role in these processes. The review provides an overview of current knowledge in this field and potential areas for development.

Development of an oesophageal stimulation method to elicit swallowing reflex in humans

Swallowing reflex is known to be evoked by gastroesophageal regurgitation or oesophageal stimulation in animal studies. However, details regarding the stimulating material, bolus size and stimulation area remain unclear for the stimulation-induced type of swallowing reflex in humans. Here, we evaluated the effects of different kinds of stimulation via water and air injection of the oesophagus on the initiation of the swallowing reflex. Nine healthy individuals participated in this study. A fibre-optic endoscope was passed transnasally, and a thin catheter for injection was passed through the other side. The tip of the catheter was placed at the upper, upper middle, lower middle or lower region of the oesophagus, and the rate of injection was controlled at 0.2 mL/s. Swallowing reflex latency was calculated as the time from injection via air or thin/thick fluid until the onset of white-out in endoscopic images. Reflex latency was significantly shorter when injection occurred at the upper region of the oesophagus than at the lower region, for both thin and thick fluids (P < .01). At the upper region of the oesophagus, the latency was significantly shorter after injection of thin fluid than with thick fluid (P < .05). Injection of air did not induce the swallowing reflex at all sites. These findings suggest that while the swallowing reflex is evoked by stimulation via fluid injection of the oesophagus in humans, sensitivity is greatest in the upper region of the oesophagus compared with the lower region and can vary depending on the injecting material.

Characterization and mechanisms of the supragastric belch in the cat

A response in which a belch occurs without gastric involvement, i.e., the supragastric belch (SGB), has been characterized in humans. The aims of this study were to determine whether animals have an SGB and, if so, to determine its mechanisms. Studies were conducted in decerebrate cats (n = 30) with electromyographic electrodes on hyoid, pharyngeal, esophageal, and diaphragm muscles. The effects of distending different regions of the esophagus in different manners using a balloon were quantified to determine the most appropriate stimulus for activating the cat SGB. The effects of esophageal perfusion of lidocaine (n = 3), vagus nerve transection (n = 3), or esophageal acidification (n = 5) on activation of the SGB were determined. Rapid large distensions of the thoracic esophagus best activated responses similar to the human SGB, i.e., rapid inhalation followed by a belch. The rapid inhalation was associated with activation of hiatal fibers and the belch with activation of dome fibers of the diaphragm. The rapid inhalation response was independent of the belch response. Lidocaine perfusion of the esophagus blocked the belch response without blocking the rapid inhalation, HCl perfusion sensitized the esophagus to activation of both the rapid inhalation and the belch response, and vagotomy blocked both responses. We conclude that the cat has an SGB that is composed of two independent reflex responses, i.e., rapid inhalation and belch, that are mediated by the vagus nerves and tension/mucosal receptors of the esophagus and sensitized by esophageal acid exposure. We hypothesize that the SGB is a learned voluntarily activated reflex response.NEW & NOTEWORTHY Rapid strong distension of the thoracic esophagus activates rapid inhalation followed by a belch, which is the sequence of responses that compose the human supragastric belch (SGB). The rapid inhalation and belch phases of the cat SGB are activated by hiatal and dome fibers of the diaphragm, respectively, and are mediated by the vagus nerves and tension/mucosal receptors of the esophagus and sensitized by esophageal acid exposure. There are many similarities between the cat and human SGB.

Plasticity of vagal afferent signaling in the gut

Vagal sensory neurons mediate the vago-vagal reflex which, in turn, regulates a wide array of gastrointestinal functions including esophageal motility, gastric accommodation and pancreatic enzyme secretion. These neurons also transmit sensory information from the gut to the central nervous system, which then mediates the sensations of nausea, fullness and satiety. Recent research indicates that vagal afferent neurons process non-uniform properties and a significant degree of plasticity. These properties are important to ensure that vagally regulated gastrointestinal functions respond rapidly and appropriately to various intrinsic and extrinsic factors. Similar plastic changes in the vagus also occur in pathophysiological conditions, such as obesity and diabetes, resulting in abnormal gastrointestinal functions. A clear understanding of the mechanisms which mediate these events may provide novel therapeutic targets for the treatment of gastrointestinal disorders due to vago-vagal pathway malfunctions.

A novel single compartment in vitro model for electrophysiological research using the perfluorocarbon FC-770

Electrophysiological studies of whole organ systems in vitro often require measurement of nerve activity and/or stimulation of the organ via the associated nerves. Currently two-compartment setups are used for such studies. These setups are complicated and require two fluids in two separate compartments and stretching the nerve across one chamber to the other, which may damage the nerves. We aimed at developing a simple single compartment setup by testing the electrophysiological properties of FC-770 (a perfluorocarbon) for in vitro recording of bladder afferent nerve activity and electrical stimulation of the bladder. Perflurocarbons are especially suitable for such a setup because of their high oxygen carrying capacity and insulating properties. In male Wistar rats, afferent nerve activity was recorded from postganglionic branches of the pelvic nerve in vitro, in situ and in vivo. The bladder was stimulated electrically via the efferent nerves. Organ viability was monitored by recording spontaneous contractions of the bladder. Additionally, histological examinations were done to test the effect of FC-770 on the bladder tissue. Afferent nerve activity was successfully recorded in a total of 11 rats. The bladders were stimulated electrically and high amplitude contractions were evoked. Histological examinations and monitoring of spontaneous contractions showed that FC-770 maintained organ viability and did not cause damage to the tissue. We have shown that FC-770 enables a simple, one compartment in vitro alternative for the generally used two compartment setups for whole organ electrophysiological studies.

Effects of prucalopride on esophageal secondary peristalsis in humans

Objectives:

Prucalopride, a high-affinity 5-hydroxytrypatamine 4 (5-HT₄) receptors agonist, has been shown to improve colon motility in adults. Secondary peristalsis helps the clearance of retained food bolus and refluxate from the esophagus, but the effects of prucalopride on esophageal secondary peristalsis are unknown. We aimed to assess the effects of prucalopride on distension-induced secondary peristalsis in healthy adults.

Methods:

Two separate sessions with prucalopride and placebo were performed in 11 healthy adults to test the effects on secondary peristalsis. Secondary peristalsis was performed with slow and rapid mid-esophageal injections of air after a baseline recording of esophageal motility.

Results:

Prucalopride significantly decreased the threshold volume to generate secondary peristalsis during slow air injection (9.8±1.4 vs. 14.4±0.9 ml, P=0.005) and rapid air injection (3.9±0.3 vs. 5.2±0.4 ml, P=0.008). Secondary peristalsis was generated more frequently after application of prucalopride (80% (70–100%) vs. 70% (60–73%), P=0.01). Prucalopride increased the wave amplitude of distal esophagus during slow air injection (147.9±28.5 vs. 104.2±16.8 mm Hg, P=0.048) and rapid air injection (128.0±13.3 vs. 105.7±12.3 mm Hg, P=0.016). Primary peristaltic amplitudes were also significantly increased by the application of prucalopride.

Conclusions:

Acute administration of prucalopride enhances mechanosensitivity of distension-induced secondary peristalsis and promotes esophageal contractility in healthy adults. Whether prucalopride could be a therapeutic option for the treatment of subjects with esophageal hypomotility needs further study.

Characterization and mechanisms of the pharyngeal swallow activated by stimulation of the esophagus

Stimulation of the esophagus activates the pharyngeal swallow response (EPSR) in human infants and animals. The aims of this study were to characterize the stimulus and response of the EPSR and to determine the function and mechanisms generating the EPSR. Studies were conducted in 46 decerebrate cats in which pharyngeal, laryngeal, and esophageal motility was monitored using EMG, strain gauges, or manometry. The esophagus was stimulated by balloon distension or luminal fluid infusion. We found that esophageal distension increased the chance of occurrence of the EPSR, but the delay was variable. The chance of occurrence of the EPSR was related to the position, magnitude, and length of the stimulus in the esophagus. The most effective stimulus was long, strong, and situated in the cervical esophagus. Acidification of the esophagus activated pharyngeal swallows and sensitized the receptors that activate the EPSR. The EPSR was blocked by local anesthesia applied to the esophageal lumen, and electrical stimulation of the recurrent laryngeal nerve caudal to the cricoid cartilage (RLNc) activated the pharyngeal swallow response. We conclude that the EPSR is activated in a probabilistic manner. The receptors mediating the EPSR are probably mucosal slowly adapting tension receptors. The sensory neural pathway includes the RLNc and superior laryngeal nerve. We hypothesize that, because the EPSR is observed in human infants and animals, but not human adults, activation of EPSR is related to the elevated position of the larynx. In this situation, the EPSR occurs rather than secondary peristalsis to prevent supraesophageal reflux when the esophageal bolus is in the proximal esophagus.

Differences in the Control of Secondary Peristalsis in the Human Esophagus: Influence of the 5-HT4 Receptor versus the TRPV1 Receptor

Objective

Acute administration of 5-hydroxytryptamine4 (5-HT₄) receptor agonist, mosapride or esophageal infusion of the transient receptor potential vanilloid receptor-1 (TRPV₁) agonist capsaicin promotes secondary peristalsis. We aimed to investigate whether acute esophageal instillation of capsaicin-containing red pepper sauce or administration of mosapride has different effects on the physiological characteristics of secondary peristalsis.

Methods

Secondary peristalsis was induced with mid-esophageal air injections in 14 healthy subjects. We compared the effects on secondary peristalsis subsequent to capsaicin-containing red pepper sauce (pure capsaicin, 0.84 mg) or 40 mg oral mosapride.

Results

The threshold volume for generating secondary peristalsis during slow air distensions was significantly decreased with capsaicin infusion compared to mosapride (11.6 ± 1.0 vs. 14.1 ± 0.8 mL, P = 0.02). The threshold volume required to produce secondary peristalsis during rapid air distension was also significantly decreased with capsaicin infusion (4.6 ± 0.5 vs. 5.2 ± 0.6 mL, P = 0.02). Secondary peristalsis was noted more frequently in response to rapid air distension after capsaicin infusion than mosapride (80% [60–100%] vs. 65% [5–100%], P = 0.04). Infusion of capsaicin or mosapride administration didn’t change any parameters of primary or secondary peristalsis.

Conclusions

Esophageal infusion with capsaicin-containing red pepper sauce suspension does create greater mechanosensitivity as measured by secondary peristalsis than 5-HT₄ receptor agonist mosapride. Capsaicin-sensitive afferents appear to be more involved in the sensory modulation of distension-induced secondary peristalsis.

The Physiology of Eructation

Eructation is composed of three independent phases: gas escape, upper barrier elimination, and gas transport phases. The gas escape phase is the gastro-LES inhibitory reflex that causes transient relaxation of the lower esophageal sphincter, which is activated by distension of stretch receptors of the proximal stomach. The upper barrier elimination phase is the transient relaxation of the upper esophageal sphincter along with airway protection. This phase is activated by stimulation of rapidly adapting mechanoreceptors of the esophageal mucosa. The gas transport phase is esophageal reverse peristalsis mediated by elementary reflexes, and it is theorized that this phase is activated by serosal rapidly adapting tension receptors. Alteration of the receptors which activate the upper barrier elimination phase of eructation by gastro-esophageal reflux of acid may in part contribute to the development of supra-esophageal reflux disease.

TRP channel functions in the gastrointestinal tract

Transient receptor potential (TRP) channels are predominantly distributed in both somatic and visceral sensory nervous systems and play a crucial role in sensory transduction. As the largest visceral organ system, the gastrointestinal (GI) tract frequently accommodates external inputs, which stimulate sensory nerves to initiate and coordinate sensory and motor functions in order to digest and absorb nutrients. Meanwhile, the sensory nerves in the GI tract are also able to detect potential tissue damage by responding to noxious irritants. This nocifensive function is mediated through specific ion channels and receptors expressed in a subpopulation of spinal and vagal afferent nerve called nociceptor. In the last 18 years, our understanding of TRP channel expression and function in GI sensory nervous system has been continuously improved. In this review, we focus on the expressions and functions of TRPV1, TRPA1, and TRPM8 in primary extrinsic afferent nerves innervated in the esophagus, stomach, intestine, and colon and briefly discuss their potential roles in relevant GI disorders.

Neuropeptide W modulation of gastric vagal afferent mechanosensitivity: Impact of age and sex

AIM

Gastric vagal afferents are activated in response to mechanical stimulation, an effect attenuated by neuropeptide W (NPW) in 20-week-old female mice. In this study we aimed to determine whether there were age and sex dependent effects of NPW on gastric vagal afferent mechanosensitivity.

METHODS

An in vitro gastro-oesophageal preparation was used to determine the effect of NPW on gastric vagal afferent mechanosensitivity from 8 and 20-week-old male and female C57BL/6 mice. Retrograde tracing and laser capture microdissection were used to selectively collect gastric vagal afferent cell bodies. Expression of NPW in the gastric mucosa and its receptor, GPR7, in gastric vagal afferent cell bodies was determined using quantitative RT-PCR.

RESULTS

NPW inhibited gastric tension sensitive vagal afferents from 20-week-old male and female mice, but not 8-week-old mice. In contrast, NPW inhibited the mechanosensitivity of gastric mucosal vagal afferents in 8-week-old male and female mice, but not 20-week-old mice. NPW mRNA expression in the gastric mucosa was higher in 20-week-old male mice compared to 8-week-old male mice. GPR7 mRNA expression in vagal afferent neurons innervating the gastric muscular layers was higher in 20-week-old mice compared to 8-week-old mice in both sexes.

CONCLUSION

The inhibitory effect of NPW on gastric tension sensitive and mucosal vagal afferents is age but not sex-dependent. These findings suggest that the physiological role of NPW varies depending on the age of the mice.

Physiology and pathogenesis of gastroesophageal reflux disease

Gastroesophageal reflux disease (GERD) is one of the most common problems treated by primary care physicians. Almost 20% of the population in the United States experiences occasional regurgitation, heartburn, or retrosternal pain because of GERD. Reflux disease is complex, and the physiology and pathogenesis are still incompletely understood. However, abnormalities of any one or a combination of the three physiologic processes, namely, esophageal motility, lower esophageal sphincter function, and gastric motility or emptying, can lead to GERD. There are many diagnostic and therapeutic approaches to GERD today, but more studies are needed to better understand this complex disease process.

Characterization of synapses in the rat subnucleus centralis of the nucleus tractus solitarius

The nucleus tractus solitarius (NTS) receives subdiaphragmatic visceral sensory information via vagal A- or C-fibers. We have recently shown that, in contrast to cardiovascular NTS medialis neurons, which respond to either purinergic or vanilloid agonists, the majority of esophageal NTS centralis (cNTS) neurons respond to vanilloid agonists, whereas a smaller subset responds to both vanilloid and purinerigic agonists. The present study aimed to further investigate the neurochemical and synaptic characteristics of cNTS neurons using whole cell patch-clamp, single cell RT-PCR and immunohistochemistry. Excitatory postsynaptic currents (EPSCs) were evoked in cNTS by tractus solitarius stimulation, and in 19 of 64 neurons perfusion with the purinergic agonist αβ-methylene ATP (αβMeATP) increased the evoked EPSC amplitude significantly. Furthermore, neurons with αβMeATP-responsive synaptic inputs had different probabilities of release compared with nonresponsive neurons. Single cell RT-PCR revealed that 8 of 13 αβMeATP-responsive neurons expressed metabotropic glutamate receptor 8 (mGluR8) mRNA, which our previous studies have suggested is a marker of glutamatergic neurons, whereas only 3 of 13 expressed glutamic acid dehydroxylase, a marker of GABAergic neurons. A significantly lower proportion of αβMeATP-nonresponsive neurons expressed mGluR8 (2 of 30 neurons), whereas a greater proportion expressed glutamic acid dehydroxylase (12 of 30 neurons). Esophageal distension significantly increased the number of colocalized mGluR8- and c-Fos-immunoreactive neurons in the cNTS from 8.0 ± 4% to 20 ± 2.5%. These data indicate that cNTS comprises distinct neuronal subpopulations that can be distinguished based on their responses to purinergic agonists and that these subpopulations have distinct neurochemical and synaptic characteristics, suggesting that integration of sensory inputs from the esophagus relies on a discrete organization of synapses between vagal afferent fibers and cNTS neurons.

Mechanism of UES relaxation initiated by gastric air distension

The aim of this study was to determine the mechanism of initiation of transient upper esophageal sphincter relaxation (TUESR) caused by gastric air distension. Cats (n = 31) were decerebrated, EMG electrodes were placed on the cricopharyngeus, a gastric fistula was formed, and a strain gauge was sewn on the lower esophageal sphincter (n = 8). Injection of air (114 ± 13 ml) in the stomach caused TUESR (n = 18) and transient lower esophageal sphincter relaxation (TLESR, n = 6), and this effect was not significantly (P > 0.05) affected by thoracotomy. Free air or bagged air (n = 6) activated TLESR, but only free air activated TUESR. Closure of the gastroesophageal junction blocked TUESR (9/9), but not TLESR (4/4), caused by air inflation of the stomach. Venting air from distal esophagus during air inflation of the stomach prevented TUESR (n = 12) but did not prevent air escape from the stomach to the esophagus (n = 4). Rapid injection of air on the esophageal mucosa always caused TUESR (9/9) but did not always (7/9) cause an increase in esophageal pressure. The time delay between the TUESR and the rapid air pulse was significantly more variable (P < 0.05) than the time delay between the rapid air pulse and the rise in esophageal pressure. We concluded that the TUESR caused by gastric air distension is dependent on air escape from the stomach, which stimulates receptors in the esophagus, but is not dependent on distension of the stomach or esophagus, or the TLESR. Therefore, the TUESR caused by gastric air distension is initiated by stimulation of receptors in the esophageal mucosa.

Diet-dependent modulation of gastro-oesphageal vagal afferent mechanosensitivity by endogenous nitric oxide

Neuronal nitric oxide (NO) plays an important role in gastric motor activity and modulates the mechanosensitivity of gastro-oesophageal vagal afferents. Effects of NO on food intake are dependent on feeding status. We sought to determine the effect of NO on gastro-oesophageal vagal afferent activity in the normally fed and food-restricted states and the second messenger pathways mediating these effects. Eight week old female C56BL/6 mice were fed ad libitum or food restricted for 14 h. An in vitro preparation was used to determine the functional effects of NO and the second messenger pathways involved. Expression of NO signal transduction molecules in vagal afferents was determined by reverse-transcription polymerase chain reaction (RT-PCR). Endogenous NO and the NO donor S-nitroso-N-acetylpenicillamine (SNAP) inhibited vagal mucosal afferent responses to tactile stimuli in mice fed ad libitum. After a 14 h fast endogenous NO and SNAP potentiated tension and mucosal afferent responses to mechanical stimulation. The excitatory effect of NO was blocked by the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitor apocynin. After a 14 h fast expression of NADPH oxidase 2 (NOX2) mRNA in whole nodose ganglia was significantly reduced and the excitatory effect of NO on gastro-oesophageal vagal afferents was lost. Under fasting conditions the inhibitory effect of NO was blocked with the hyperpolarisation-activated cyclic nucleotide-gated (HCN) channel blocker ivabradine and mRNA expression of HCN3 in the nodose ganglia was elevated. In conclusion, the role of NO in the peripheral modulation of gastro-oesophageal vagal afferents is dynamic and dependent on feeding status.

Plasticity of gastro-intestinal vagal afferent endings

Vagal afferents are a vital link between the peripheral tissue and central nervous system (CNS). There is an abundance of vagal afferents present within the proximal gastrointestinal tract which are responsible for monitoring and controlling gastrointestinal function. Whilst essential for maintaining homeostasis there is a vast amount of literature emerging which describes remarkable plasticity of vagal afferents in response to endogenous as well as exogenous stimuli. This plasticity for the most part is vital in maintaining healthy processes; however, there are increased reports of vagal plasticity being disrupted in pathological states, such as obesity. Many of the disruptions, observed in obesity, have the potential to reduce vagal afferent satiety signalling which could ultimately perpetuate the obese state. Understanding how plasticity occurs within vagal afferents will open a whole new understanding of gut function as well as identify new treatment options for obesity.

Pathophysiology of gastroesophageal reflux disease

Gastroesophageal reflux disease (GERD) is one of the most common digestive diseases in the Western world, with typical symptoms, such as heartburn, regurgitation, or retrosternal pain, reported by 15% to 20% of the general population. The pathophysiology of GERD is multifactorial. Our understanding of these factors has significantly improved in recent years, with increased understanding of the acid pocket and hiatal hernia and how these factors interact. Although our insight has significantly increased over the past years, more studies are required to better understand symptom generation in GERD, especially in patients with therapy-resistant symptoms.

Intraluminal acid activates esophageal nodose C fibers after mast cell activation

Acid reflux in the esophagus can induce esophageal painful sensations such as heartburn and noncardiac chest pain. The mechanisms underlying acid-induced esophageal nociception are not clearly understood. In our previous studies, we characterized esophageal vagal nociceptive afferents and defined their responses to noxious mechanical and chemical stimulation. In the present study, we aim to determine their responses to intraluminal acid infusion. Extracellular single-unit recordings were performed in nodose ganglion neurons with intact nerve endings in the esophagus using ex vivo esophageal-vagal preparations. Action potentials evoked by esophageal intraluminal acid perfusion were compared in naive and ovalbumin (OVA)-challenged animals, followed by measurements of transepithelial electrical resistance (TEER) and the expression of tight junction proteins (zona occludens-1 and occludin). In naive guinea pigs, intraluminal infusion with either acid (pH = 2-3) or capsaicin did not evoke an action potential discharge in esophageal nodose C fibers. In OVA-sensitized animals, following esophageal mast cell activation by in vivo OVA inhalation, intraluminal acid infusion for about 20 min started to evoke action potential discharges. This effect is further confirmed by selective mast cell activation using in vitro tissue OVA challenge in esophageal-vagal preparations. OVA inhalation leads to decreased TEER and zona occludens-1 expression, suggesting an impaired esophageal epithelial barrier function after mast cell activation. These data for the first time provide direct evidence of intraluminal acid-induced activation of esophageal nociceptive C fibers and suggest that mast cell activation may make esophageal epithelium more permeable to acid, which subsequently may increase esophageal vagal nociceptive C fiber activation.

Comparable effects of capsaicin-containing red pepper sauce and hydrochloric acid on secondary peristalsis in humans

BACKGROUND AND AIM

We aimed to evaluate whether acute esophageal instillation of capsaicin and hydrochloric acid had different effects on distension-induced secondary peristalsis.

METHODS

Secondary peristalsis was induced by slow and rapid air injections into the mid-esophagus after the evaluation of baseline motility in 16 healthy subjects. The effects on secondary peristalsis were determined by esophageal instillation with capsaicin-containing red pepper sauce (pure capsaicin, 0.84 mg) and hydrochloric acid (0.1 N).

RESULTS

The administration of capsaicin induced a significant increase in the visual analogue scale score for heartburn as compared with hydrochloric acid (P = 0.002). The threshold volume for generating secondary peristalsis during slow and rapid air distensions did not differ between capsaicin and hydrochloric acid infusions. Hydrochloric acid significantly increased the frequency of secondary peristalsis in response to rapid air distension compared with capsaicin infusion (P = 0.03). Pressure wave amplitude during slow air distension was greater with the infusion of hydrochloric acid than capsaicin infusion (P = 0.001). The pressure wave duration during rapid air distension was longer after capsaicin infusion than hydrochloric acid infusion (P = 0.01). The pressure wave amplitude during rapid air distension was similar between capsaicin and hydrochloric acid infusions.

CONCLUSIONS

Despite subtle differences in physiological characteristics of secondary peristalsis, acute esophageal instillation of capsaicin and hydrochloric acid produced comparable effects on distension-induced secondary peristalsis. Our data suggest the coexistence of both acid- and capsaicin-sensitive afferents in human esophagus which produce similar physiological alterations in secondary peristalsis.

Modulation of murine gastric vagal afferent mechanosensitivity by neuropeptide W

AIM

Neuropeptide W (NPW) is an endogenous ligand for the receptors GPR7 and GPR8 and is involved in central regulation of energy homeostasis. NPW in the periphery is found in gastric gastrin (G) cells. In the stomach, energy intake is influenced by vagal afferent signals, so we aimed to determine the effect of NPW on mechanosensitive gastric vagal afferents under different feeding conditions.

METHODS

Female C57BL/6 mice (N > 10 per group) were fed a standard laboratory diet (SLD), high-fat diet (HFD) or were food restricted. The relationship between NPW immunopositive cells and gastric vagal afferent endings was determined by anterograde tracing and NPW immunohistochemistry. An in vitro gastro-oesophageal preparation was used to determine the functional effects of NPW on gastric vagal afferents. Expression of NPW in the gastric mucosa and GPR7 in whole nodose ganglia was determined by quantitative RT-PCR (QRT-PCR). The expression of GPR7 in gastric vagal afferent neurones was determined by retrograde tracing and QRT-PCR.

RESULTS

Neuropeptide W immunoreactive cells were found in close proximity to traced vagal afferents. NPW selectively inhibited responses of gastric vagal tension receptors to stretch in SLD but not HFD or fasted mice. In the nodose ganglia, GPR7 mRNA was specifically expressed in gastric vagal afferent neurones. In fasted mice gastric mucosal NPW and nodose GPR7, mRNA was reduced compared with SLD. A HFD had no effect on gastric NPW mRNA, but down-regulated nodose GPR7 expression.

CONCLUSION

Neuropeptide W modulates gastric vagal afferent activity, but the effect is dynamic and related to feeding status.

Influence of intraluminal acidification on esophageal secondary peristalsis in humans

BACKGROUND

Secondary peristalsis contributes to clearance of retained refluxate or material from the esophagus.

AIM

The goal of our study was to investigate the effects of hydrochloric acid (HCl) on physiological characteristics of esophageal secondary peristalsis in healthy adults.

METHODS

After recording esophageal motility baseline for primary peristalsis, secondary peristalsis was stimulated with slow and rapid air injections in the mid-esophageal in 16 healthy subjects. Normal saline and HCl (0.1 N) were separately infused into the esophagus to test whether they had effects on secondary peristalsis.

RESULTS

After infusion of HCl, the threshold volume to generate secondary peristalsis was significantly decreased during rapid and slow air infusions (both P < 0.05). The frequency of secondary peristalsis was increased after HCl infusion (90 % [72.5-100 %] versus 85 % [72.5-90 %], P = 0.002). Infusion of HCl significantly increased pressure wave amplitude during rapid and slow air infusions (both P < 0.05). Infusion of saline did not affect any parameters of secondary peristalsis. The occurrence of heartburn was generated in 7 of 16 subjects after infusion of HCl with an increase in visual analogue scale score (12.5).

CONCLUSIONS

Our data show that acute esophageal acid infusion enhances sensitivity of distension-induced secondary peristalsis and enhances secondary peristaltic activity. The study supports the evidence of the presence of acid-sensitive afferents in the modulation of distension-induced secondary peristalsis in humans.

Extrinsic primary afferent signalling in the gut

Visceral sensory neurons activate reflex pathways that control gut function and also give rise to important sensations, such as fullness, bloating, nausea, discomfort, urgency and pain. Sensory neurons are organised into three distinct anatomical pathways to the central nervous system (vagal, thoracolumbar and lumbosacral). Although remarkable progress has been made in characterizing the roles of many ion channels, receptors and second messengers in visceral sensory neurons, the basic aim of understanding how many classes there are, and how they differ, has proven difficult to achieve. We suggest that just five structurally distinct types of sensory endings are present in the gut wall that account for essentially all of the primary afferent neurons in the three pathways. Each of these five major structural types of endings seems to show distinctive combinations of physiological responses. These types are: 'intraganglionic laminar' endings in myenteric ganglia; 'mucosal' endings located in the subepithelial layer; 'muscular-mucosal' afferents, with mechanosensitive endings close to the muscularis mucosae; 'intramuscular' endings, with endings within the smooth muscle layers; and 'vascular' afferents, with sensitive endings primarily on blood vessels. 'Silent' afferents might be a subset of inexcitable 'vascular' afferents, which can be switched on by inflammatory mediators. Extrinsic sensory neurons comprise an attractive focus for targeted therapeutic intervention in a range of gastrointestinal disorders.