Local differences in vagal afferent innervation of the rat esophagus are reflected by neurochemical differences at the level of the sensory ganglia and by different brainstem projections

Mucosal neuroimmune mechanisms in gastro-oesophageal reflux disease (GORD) pathogenesis

Gastro-oesophageal reflux disease (GORD) is a chronic condition characterised by visceral pain in the distal oesophagus. The current first-line treatment for GORD is proton pump inhibitors (PPIs), however, PPIs are ineffective in a large cohort of patients and long-term use may have adverse effects. Emerging evidence suggests that nerve fibre number and location are likely to play interrelated roles in nociception in the oesophagus of GORD patients. Simultaneously, alterations in cells of the oesophageal mucosa, namely epithelial cells, mast cells, dendritic cells, and T lymphocytes, have been a focus of GORD research for several years. The oesophagus of GORD patients exhibits both macro- and micro-inflammation as a response to chronic acidic reflux at the epithelium. In other conditions of the GI tract, such as IBS and IBD, well-characterised bidirectional processes between immune cells and mucosal nerve fibres contribute to pathogenesis and symptom generation. Sensory alterations in these conditions such as nerve fibre outgrowth and hypersensitivity can be driven by inflammatory processes, which promote visceral pain signalling. This review will examine what is currently known of the molecular pathways linking inflammation and sensory perception leading to the development of GORD symptoms and explore potentially relevant mechanisms in other GI regions which may indicate new areas in GORD research.

Gabapentin alleviated the cough hypersensitivity and neurogenic inflammation in a guinea pig model with repeated intra-esophageal acid perfusion

OBJECTIVE

The anti-tussive effect of gabapentin and its underlying neuromodulatory mechanism were investigated via a modified guinea pig model of gastroesophageal reflux-related cough (GERC).

METHODS

Intra-esophageal perfusion with hydrochloric acid (HCl) was performed every other day 12 times to establish the GERC model. High-dose gabapentin (48 mg/kg), low-dose gabapentin (8 mg/kg), or saline was orally administered for 2 weeks after modeling. Cough sensitivity, airway inflammation, lung and esophagus histology, levels of substance P (SP), and neurokinin-1 (NK1)-receptors were monitored.

RESULTS

Repeated intra-esophageal acid perfusion aggravated the cough sensitivity in guinea pigs in a time-dependent manner. The number of cough events was significantly increased after 12 times HCl perfusion, and the hypersensitivity period was maintained for 2 weeks. The SP levels in BALF, trachea, lung, distal esophagus, and vagal ganglia were increased in guinea pigs receiving HCl perfusion. The intensity of cough hypersensitivity in the GERC model was significantly correlated with increased SP expression in the airways. Both high and low doses of gabapentin administration could reduce cough hypersensitivity exposed to HCl perfusion, attenuate airway inflammatory damage, and inhibit neurogenic inflammation by reducing SP expression from the airway and vagal ganglia.

CONCLUSIONS

Gabapentin can desensitize the cough sensitivity in the GERC model of guinea pig. The anti-tussive effect is associated with the alleviated peripheral neurogenic inflammation as reflected in the decreased level of SP.

Pharmacological activation of transient receptor potential vanilloid 4 promotes triggering of the swallowing reflex in rats

The swallowing reflex is an essential physiological reflex that allows food or liquid to pass into the esophagus from the oral cavity. Delayed triggering of this reflex is a significant health problem in patients with oropharyngeal dysphagia for which no pharmacological treatments exist. Transient receptor potential channels have recently been discovered as potential targets to facilitate triggering of the swallowing reflex. However, the ability of transient receptor potential vanilloid 4 (TRPV4) to trigger the swallowing reflex has not been studied. Here, we demonstrate the involvement of TRPV4 in triggering the swallowing reflex in rats. TRPV4 immunoreactive nerve fibers were observed in the superior laryngeal nerve (SLN)-innervated swallowing-related regions. Retrograde tracing with fluorogold revealed localization of TRPV4 on approximately 25% of SLN-afferent neurons in the nodose-petrosal-jugular ganglionic complex. Among them, approximately 49% were large, 35% medium, and 15% small-sized SLN-afferent neurons. Topical application of a TRPV4 agonist (GSK1016790A) to the SLN-innervated regions dose-dependently facilitated triggering of the swallowing reflex, with the highest number of reflexes triggered at a concentration of 250 μM. The number of agonist-induced swallowing reflexes was significantly reduced by prior topical application of a TRPV4 antagonist. These findings indicate that TRPV4 is expressed on sensory nerves innervating the swallowing-related regions, and that its activation by an agonist can facilitate swallowing. TRPV4 is a potential pharmacological target for the management of oropharyngeal dysphagia.

Physiological and Pathological Significance of Esophageal TRP Channels: Special Focus on TRPV4 in Esophageal Epithelial Cells

Transient receptor potential vanilloid 4 (TRPV4) is a non-selective cation channel that is broadly expressed in different human tissues, including the digestive system, where it acts as a molecular sensor and a transducer that regulates a variety of functional activities. Despite the extensive research to determine the role of this channel in the physiology and pathophysiology of different organs, the unique morphological and functional features of TRPV4 in the esophagus remain largely unknown. Ten years ago, TRPV4 was shown to be highly expressed in esophageal epithelial cells where its activation induces Ca²⁺-dependent ATP release, which, in turn, mediates several functions, ranging from mechanosensation to wound healing. This review summarizes the research progress on TRPV4, and focuses on the functional expression of TRPV4 in esophageal epithelium and its possible role in different esophageal diseases that would support TRPV4 as a candidate target for future therapeutic approaches to treat patients with these conditions.

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.

Functional anatomy of the vagus system - Emphasis on the somato-visceral interface

Due to its pivotal role in autonomic networks, the vagus attracts continuous interest from both basic scientists and clinicians. In particular, recent advances in vagus nerve stimulation strategies and their application to pathological conditions beyond epilepsy provide a good opportunity to recall basic features of vagal peripheral and central anatomy. In addition to the "classical" vagal brainstem nuclei, i.e., dorsal motor nucleus, nucleus ambiguus and nucleus tractus solitarii, the spinal trigeminal and paratrigeminal nuclei come into play as targets of vagal afferents. On the other hand, the nucleus of the solitary tract receives and integrates not only visceral but also somatic afferents. Thus, the vagus system participates significantly in what may be defined as "somato-visceral interface".

Esophageal afferent innervation and its role in gastro-esophageal reflux disease symptoms

PURPOSE OF REVIEW

Despite the wide prevalence of gastro-esophageal reflux disease (GERD), the neurophysiological mechanisms underlying heartburn perception in the esophagus of patients with GERD remains incompletely understood. Recent studies have highlighted the potential influence sensory afferent nerves innervating the oesophageal epithelium may have on heartburn pathogenesis. The purpose of this review is to consider the current understanding of esophageal afferent neuronal innervation, including the nociceptive role of acid-sensing receptors expressed on these sensory nerves, in relation to pain perception in the esophagus of GERD patients.

RECENT FINDINGS

Central and peripheral pathways of sensitization following noxious stimulation of nociceptive receptors expressed on afferent nerves can regulate the strength of sensory nerve activation in the esophagus, which can result in the amplification or suppression of afferent signal transmission. The localization and characterization of mucosal sensory afferent nerves vary between GERD phenotypes and may explain the heterogeneity of symptom perception in patients with apparently similar levels of reflux.

SUMMARY

In this review, we discuss the relevance of afferent esophageal innervation in heartburn perception, with a particular focus on the pathways of reflux-induced activation of nociceptive nerves.

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.

Targeting Chemosensory Ion Channels in Peripheral Swallowing-Related Regions for the Management of Oropharyngeal Dysphagia

Oropharyngeal dysphagia, or difficulty in swallowing, is a major health problem that can lead to serious complications, such as pulmonary aspiration, malnutrition, dehydration, and pneumonia. The current clinical management of oropharyngeal dysphagia mainly focuses on compensatory strategies and swallowing exercises/maneuvers; however, studies have suggested their limited effectiveness for recovering swallowing physiology and for promoting neuroplasticity in swallowing-related neuronal networks. Several new and innovative strategies based on neurostimulation in peripheral and cortical swallowing-related regions have been investigated, and appear promising for the management of oropharyngeal dysphagia. The peripheral chemical neurostimulation strategy is one of the innovative strategies, and targets chemosensory ion channels expressed in peripheral swallowing-related regions. A considerable number of animal and human studies, including randomized clinical trials in patients with oropharyngeal dysphagia, have reported improvements in the efficacy, safety, and physiology of swallowing using this strategy. There is also evidence that neuroplasticity is promoted in swallowing-related neuronal networks with this strategy. The targeting of chemosensory ion channels in peripheral swallowing-related regions may therefore be a promising pharmacological treatment strategy for the management of oropharyngeal dysphagia. In this review, we focus on this strategy, including its possible neurophysiological and molecular mechanisms.

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.

Functional involvement of acid-sensing ion channel 3 in the swallowing reflex in rats

BACKGROUND

Difficulty swallowing represents a major health problem. Swallowing function is improved by incorporating weak acids in suspensions/food boluses, implicating acid-sensing ion channels (ASICs) in the swallowing reflex. However, the functional involvement of ASICs in the swallowing reflex has not been fully elucidated.

METHODS

We localized ASIC3s in swallowing-related regions innervated by the superior laryngeal nerves (SLNs) and those in the nodose-petrosal-jugular ganglionic complex (NPJc) and examined their functional involvement in evoking the swallowing reflex in rats.

KEY RESULTS

We localized ASIC3s on epithelial cells and nerve fibers in swallowing-related regions innervated by the SLNs. In the NPJc, around half of the SLN-afferent neurons expressed ASIC3. Two-thirds of ASIC3s were localized on unmyelinated neurons in the nodose and petrosal ganglia. In the jugular ganglia, they were equally distributed on unmyelinated and myelinated neurons. Topical application of a synthetic non-proton ASIC3 activator, 2-guanidine-4-methylquinazoline (GMQ), and its natural endogenous ligand agmatine (a metabolite of the amino acid arginine) in swallowing-related regions evoked a considerable number of swallowing reflexes. Increasing the concentration of GMQ and agmatine up to a certain concentration increased the number of evoked reflexes and shortened the interval between the evoked reflexes. Agmatine was less potent than GMQ in its ability to evoke swallowing reflexes. Prior topical application of an ASIC3 antagonist significantly attenuated the number of GMQ- and agmatine-evoked swallowing reflexes.

CONCLUSIONS & INFERENCES

Acid-sensing ion channel 3s localized on nerves and epithelial cells in swallowing-related regions are functional in evoking the swallowing reflex and activation of these channels via a pharmacological agonist appears to improve swallowing behavior.

Phenotypic distinctions between the nodose and jugular TRPV1-positive vagal sensory neurons in the cynomolgus monkey

Vagal capsaicin-sensitive afferent C-fibers play an important role in the maintenance of visceral homeostasis and contribute to symptoms in visceral diseases. Based on their developmental origin two functionally distinct types of vagal C-fibers are recognized: those with neurons in the vagal nodose ganglia (derived from epibranchial placodes) and in the vagal jugular ganglia (from neural crest). Studies in nonprimate species demonstrated that the vagal nodose and jugular C-fibers differ in activation profile, neurotrophic regulation, and expression of neurotransmitters. We hypothesized that the expression of selected markers related to key phenotypic properties of vagal C-fibers in the cynomolgus monkey is similar to that reported in nonprimate species. We performed single-cell RT-PCR on nodose and jugular putative C-fiber (TRPV1-positive) neurons isolated from the cynomolgus monkey. We found that the expression of purinergic P2X receptors that underlie selective responsiveness of nodose C-fiber terminals to ATP was conserved in that P2X2 and P2X3 subunits were expressed in nodose, but only P2X3 subunit was expressed in jugular TRPV1-positive neurons. Also conserved was the preferential expression of neurotrophic receptor TrkB in the nodose and preprotachykinin-A in the jugular TRPV1-positive neurons. Several key distinctions in gene expression between nodose and jugular TRPV1-positive (C-fiber) neurons that have been noted in mice, rats, and guinea pigs, are conserved in the cynomolgus monkey. Our results support the translatability of distinct vagal C-fiber phenotypes to primates.

Activation of TRPV1 and TRPM8 Channels in the Larynx and Associated Laryngopharyngeal Regions Facilitates the Swallowing Reflex

The larynx and associated laryngopharyngeal regions are innervated by the superior laryngeal nerve (SLN) and are highly reflexogenic. Transient receptor potential (TRP) channels have recently been detected in SLN innervated regions; however, their involvement in the swallowing reflex has not been fully elucidated. Here, we explore the contribution of two TRP channels, TRPV1 and TRPM8, located in SLN-innervated regions to the swallowing reflex. Immunohistochemistry identified TRPV1 and TRPM8 on cell bodies of SLN afferents located in the nodose-petrosal-jugular ganglionic complex. The majority of TRPV1 and TRPM8 immunoreactivity was located on unmyelinated neurons. Topical application of different concentrations of TRPV1 and TRPM8 agonists modulated SLN activity. Application of the agonists evoked a significantly greater number of swallowing reflexes compared with the number evoked by distilled water. The interval between the reflexes evoked by the agonists was shorter than that produced by distilled water. Prior topical application of respective TRPV1 or TRPM8 antagonists significantly reduced the number of agonist-evoked reflexes. The findings suggest that the activation of TRPV1 and TRPM8 channels present in the swallowing-related regions can facilitate the evoking of swallowing reflex. Targeting the TRP channels could be a potential therapeutic strategy for the management of dysphagia.

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.

Superficial Esophageal Mucosal Afferent Nerves May Contribute to Reflux Hypersensitivity in Nonerosive Reflux Disease

BACKGROUND & AIMS

Little is known about the causes of heartburn in patients with gastro-esophageal reflux disease. Visible epithelial damage is seldom associated with symptom severity, evidenced by the significant symptom burden in patients with nonerosive reflux disease (NERD) compared with patients with erosive reflux disease (ERD) or Barrett's esophagus (BE). We studied the distribution of mucosal nerve fibers in patients with NERD, ERD, and BE, and compared the results with those of healthy subjects.

METHODS

We performed a prospective study of 13 patients with NERD, 11 patients with ERD, and 16 patients with BE undergoing endoscopic evaluation in the United Kingdom or Greece. Biopsies were obtained from the proximal and distal esophageal mucosa of patients with NERD, from the distal esophageal mucosa of patients with ERD, and the distal-most squamous epithelium of patients with BE. These were examined for the presence and location of nerve fibers that reacted with a labeled antibody against calcitonin gene-related peptide (CGRP), a marker of nociceptive sensory nerves. The results were compared with those from 10 healthy volunteers (controls).

RESULTS

The distribution of CGRP-positive nerves did not differ significantly between the distal esophageal mucosa of controls (median, 25.5 cell layers to surface; interquartile range [IQR], 21.4-28.8) vs patients with ERD (median, 23 cell layers to surface; IQR, 16-27.5), or patients with BE (median, 21.5 cell layers to surface; IQR, 16.1-27.5). However, CGRP-positive nerves were significantly more superficial in mucosa from patients with NERD-both distal (median, 9.5 cell layers to surface; IQR, 1.5-13.3; P < .0001 vs ERD, BE, and controls) and proximal (median, 5.0 cell layers to surface; IQR, 2.5-9.3 vs median 10.4 cell layers to surface; IQR, 8.0-16.9; P = .0098 vs controls).

CONCLUSIONS

Proximal and distal esophageal mucosa of patients with NERD have more superficial afferent nerves compared with controls or patients with ERD or BE. Acid hypersensitivity in patients with NERD might be partially explained by the increased proximity of their afferent nerves to the esophageal lumen, and therefore greater exposure to noxious substances in refluxate.

Distinct and common expression of receptors for inflammatory mediators in vagal nodose versus jugular capsaicin-sensitive/TRPV1-positive neurons detected by low input RNA sequencing

Capsaicin-sensitive sensory C-fibers derived from vagal ganglia innervate the visceral organs, and respond to inflammatory mediators and noxious stimuli. These neurons play an important role in maintenance of visceral homeostasis, and contribute to the symptoms of visceral inflammatory diseases. Vagal sensory neurons are located in two ganglia, the jugular ganglia (derived from the neural crest), and the nodose ganglia (from the epibranchial placodes). The functional difference, especially in response to immune mediators, between jugular and nodose neurons is not fully understood. In this study, we microscopically isolated murine nodose and jugular capsaicin-sensitive / Trpv1-expressing C-fiber neurons and performed transcriptome profiling using ultra-low input RNA sequencing. RNAseq detected genes with significantly differential expression in jugular and nodose neurons, which were mostly involved in neural functions. Transcriptional regulators, including Cited1, Hoxb5 and Prdm12 showed distinct expression patterns in the two C-fiber neuronal populations. Common and specific expression of immune receptor proteins was characterized in each neuronal type. The expression of immune receptors that have received little or no attention from vagal sensory biologists is highlighted including receptors for certain chemokines (CXCLs), interleukins (IL-4) and interferons (IFNα, IFNγ). Stimulation of immune receptors with their cognate ligands led to activation of the C-fibers in isolated functional assays.

Immunocytochemical organization and sour taste activation in the rostral nucleus of the solitary tract of mice

Sensory inputs from the oropharynx terminate in both the trigeminal brainstem complex and the rostral part of the nucleus of the solitary tract (nTS). Taste information is conveyed via the facial and glossopharyngeal nerves, while general mucosal innervation is carried by the trigeminal and glossopharyngeal nerves. In contrast, the caudal nTS receives general visceral information largely from the vagus nerve. Although the caudal nTS shows clear morphological and molecularly delimited subdivisions, the rostral part does not. Thus, linking taste-induced patterns of activity to morphological subdivisions in the nTS is challenging. To test whether molecularly defined features of the rostral nTS correlate with patterns of taste-induced activity, we combined immunohistochemistry for markers of various visceral afferent and efferent systems with c-Fos-based activity maps generated by stimulation with a sour tastant, 30 mM citric acid. We further dissociated taste-related activity from activity arising from acid-sensitive general mucosal innervation by comparing acid-evoked c-Fos in wild-type and "taste blind" P2X₂ /P2X₃ double knockout (P2X-dbl KO) mice. In wild-type mice, citric acid stimulation evoked significant c-Fos activation in the central part of the rostral nTS-activity that was largely absent in the P2X-dbl KO mice. P2X-dbl KO mice, like wild-type mice, did exhibit acid-induced c-Fos activity in the dorsomedial trigeminal brainstem nucleus situated laterally adjacent to the rostral nTS. This dorsomedial nucleus also showed substantial innervation by trigeminal nerve fibers immunoreactive for calcitonin gene-related peptide (CGRP), a marker for polymodal nociceptors, suggesting that trigeminal general mucosal innervation carries information about acids in the oral cavity. J. Comp. Neurol. 525:271-290, 2017. © 2016 Wiley Periodicals, Inc.

Central nervous system integration of sensorimotor signals in oral and pharyngeal structures: oropharyngeal kinematics response to recurrent laryngeal nerve lesion

Safe, efficient liquid feeding in infant mammals requires the central coordination of oropharyngeal structures innervated by multiple cranial and spinal nerves. The importance of laryngeal sensation and central sensorimotor integration in this system is poorly understood. Recurrent laryngeal nerve lesion (RLN) results in increased aspiration, though the mechanism for this is unclear. This study aimed to determine the effect of unilateral RLN lesion on the motor coordination of infant liquid feeding. We hypothesized that 1) RLN lesion results in modified swallow kinematics, 2) postlesion oropharyngeal kinematics of unsafe swallows differ from those of safe swallows, and 3) nonswallowing phases of the feeding cycle show changed kinematics postlesion. We implanted radio opaque markers in infant pigs and filmed them pre- and postlesion with high-speed videofluoroscopy. Markers locations were digitized, and swallows were assessed for airway protection. RLN lesion resulted in modified kinematics of the tongue relative to the epiglottis in safe swallows. In lesioned animals, safe swallow kinematics differed from unsafe swallows. Unsafe swallow postlesion kinematics resembled prelesion safe swallows. The movement of the tongue was reduced in oral transport postlesion. Between different regions of the tongue, response to lesion was similar, and relative timing within the tongue was unchanged. RLN lesion has a pervasive effect on infant feeding kinematics, related to the efficiency of airway protection. The timing of tongue and hyolaryngeal kinematics in swallows is a crucial locus for swallow disruption. Laryngeal sensation is essential for the central coordination in feeding of oropharyngeal structures receiving motor inputs from different cranial nerves.

The effect of spinal cord injury on the neurochemical properties of vagal sensory neurons

The vagus nerve is composed primarily of nonmyelinated sensory neurons whose cell bodies are located in the nodose ganglion (NG). The vagus has widespread projections that supply most visceral organs, including the bladder. Because of its nonspinal route, the vagus nerve itself is not directly damaged from spinal cord injury (SCI). Because most viscera, including bladder, are dually innervated by spinal and vagal sensory neurons, an impact of SCI on the sensory component of vagal circuitry may contribute to post-SCI visceral pathologies. To determine whether SCI, in male Wistar rats, might impact neurochemical characteristics of NG neurons, immunohistochemical assessments were performed for P2X3 receptor expression, isolectin B4 (IB4) binding, and substance P expression, three known injury-responsive markers in sensory neuronal subpopulations. In addition to examining the overall population of NG neurons, those innervating the urinary bladder also were assessed separately. All three of the molecular markers were represented in the NG from noninjured animals, with the majority of the neurons binding IB4. In the chronically injured rats, there was a significant increase in the number of NG neurons expressing P2X3 and a significant decrease in the number binding IB4 compared with noninjured animals, a finding that held true also for the bladder-innervating population. Overall, these results indicate that vagal afferents, including those innervating the bladder, display neurochemical plasticity post-SCI that may have implications for visceral homeostatic mechanisms and nociceptive signaling.

Distinct afferent innervation patterns within the human proximal and distal esophageal mucosa

Little is known about the mucosal phenotype of the proximal human esophagus. There is evidence to suggest that the proximal esophagus is more sensitive to chemical and mechanical stimulation compared with the distal. This may have physiological relevance (e.g., in prevention of aspiration of gastroesophageal refluxate), but also pathological relevance (e.g., in reflux perception or dysphagia). Reasons for this increased sensitivity are unclear but may include impairment in mucosal barrier integrity or changes in sensory innervation. We assessed mucosal barrier integrity and afferent nerve distribution in the proximal and distal esophagus of healthy human volunteers. In 10 healthy volunteers baseline proximal and distal esophageal impedance was measured in vivo. Esophageal mucosal biopsies from the distal and proximal esophagus were taken, and baseline transepithelial electrical resistance (TER) was measured in Ussing chambers. Biopsies were examined immunohistochemically for presence and location of calcitonin gene-related peptide (CGRP)-immunoreactive nerve fibers. In a further four healthy volunteers we investigated for colocalization of CGRP and protein gene product (PGP) 9.5 immunoreactivity in nerve fibers. Baseline impedance was higher in the proximal than in the distal esophagus [2,936 Ω (SD578) vs. 2,229 Ω (SD821); P = 0.03], however, baseline TER was not significantly different between them. Mucosal CGRP-immunoreactive nerves were found in the epithelium of both proximal and distal esophagus, but were located more superficially in the proximal mucosa compared with the distal [11.5 (SD7) vs. 21.7 (SD5) cell layers from lumen, P = 0.002] 19% of proximal, and 10% of distal mucosal PGP-immunoreactive fibers colocalized with CGRP. PGP-immunoreactive fibers were also significantly closer to the luminal surface in the proximal compared with the distal esophagus (P < 0.001). We conclude that mucosal barrier integrity is similar in proximal and distal esophagus, but proximal mucosal afferent nerves are in a more superficial location. The enhanced sensitivity to reflux-evoked symptoms of the proximal esophagus most likely has an anatomical basis.

The expression profile of acid-sensing ion channel (ASIC) subunits ASIC1a, ASIC1b, ASIC2a, ASIC2b, and ASIC3 in the esophageal vagal afferent nerve subtypes

Acid-sensing ion channels (ASICs) have been implicated in esophageal acid sensing and mechanotransduction. However, insufficient knowledge of ASIC subunit expression profile in esophageal afferent nerves hampers the understanding of their role. This knowledge is essential because ASIC subunits form heteromultimeric channels with distinct functional properties. We hypothesized that the esophageal putative nociceptive C-fiber nerves (transient receptor potential vanilloid 1, TRPV1-positive) express multiple ASIC subunits and that the ASIC expression profile differs between the nodose TRPV1-positive subtype developmentally derived from placodes and the jugular TRPV1-positive subtype derived from neural crest. We performed single cell RT-PCR on the vagal afferent neurons retrogradely labeled from the esophagus. In the guinea pig, nearly all (90%-95%) nodose and jugular esophageal TRPV1-positive neurons expressed ASICs, most often in a combination (65-75%). ASIC1, ASIC2, and ASIC3 were expressed in 65-75%, 55-70%, and 70%, respectively, of both nodose and jugular TRPV1-positive neurons. The ASIC1 splice variants ASIC1a and ASIC1b and the ASIC2 splice variant ASIC2b were similarly expressed in both nodose and jugular TRPV1-positive neurons. However, ASIC2a was found exclusively in the nodose neurons. In contrast to guinea pig, ASIC3 was almost absent from the mouse vagal esophageal TRPV1-positive neurons. However, ASIC3 was similarly expressed in the nonnociceptive TRPV1-negative (tension mechanoreceptors) neurons in both species. We conclude that the majority of esophageal vagal nociceptive neurons express multiple ASIC subunits. The placode-derived nodose neurons selectively express ASIC2a, known to substantially reduce acid sensitivity of ASIC heteromultimers. ASIC3 is expressed in the guinea pig but not in the mouse vagal esophageal TRPV1-positive neurons, indicating species differences in ASIC expression.

Brain-derived neurotrophic factor immunoreactive vagal sensory neurons innervating the gastrointestinal tract of the rat

We have determined whether brain-derived neurotrophic factor immunoreactive (BDNF-ir) neurons in the vagal ganglia innervate the gastrointestinal tract. Many BDNF-ir neurons were medium in size and located throughout the jugular and nodose ganglia. When Fluorogold was injected into the wall of the cervical esophagus, many retrogradely Fluorogold-labeled neurons were found in both the jugular ganglion and the nodose ganglion. When Fluorogold was injected into the body of the stomach or applied to the cut end of the subdiaphragmatic vagus nerve, numerous Fluorogold-labeled neurons were found mostly in the nodose ganglion. Double-labeling combining immunohistochemistry for BDNF and retrograde tracing with Fluorogold showed that more than 90% of the neurons in the jugular ganglion and the nodose ganglion projecting to the cervical esophagus contained BDNF-like immunoreactivity. In the cases of both Fluorogold injection into the stomach and Fluorogold application to the subdiaphragmatic vagus nerve, almost all Fluorogold-labeled neurons in the nodose ganglion contained BDNF-like immunoreactivity. These results indicated that almost all vagal sensory neurons located in either the jugular ganglion or the nodose ganglion that innervate the gastrointestinal tract are BDNF-ir neurons.

Role of peripheral reflexes in the initiation of the esophageal phase of swallowing

The aim of this study was to determine the role of peripheral reflexes in initiation of the esophageal phase of swallowing. In 10 decerebrate cats, we recorded electromyographic responses from the pharynx, larynx, and esophagus and manometric data from the esophagus. Water (1-5 ml) was injected into the nasopharynx to stimulate swallowing, and the timing of the pharyngeal and esophageal phases of swallowing was quantified. The effects of transection or stimulation of nerves innervating the esophagus on swallowing and esophageal motility were tested. We found that the percent occurrence of the esophageal phase was significantly related to the bolus size. While the time delays between the pharyngeal and esophageal phases of swallowing were not related to the bolus size, they were significantly more variable than the time delays between activation of muscles within the pharyngeal phase. Transection of the sensory innervation of the proximal cervical esophagus blocked or significantly inhibited activation of the esophageal phase in the proximal cervical esophagus. Peripheral electrical stimulation of the pharyngoesophageal nerve activated the proximal cervical esophagus, peripheral electrical stimulation of the vagus nerve activated the distal cervical esophagus, and peripheral electrical stimulation the superior laryngeal nerve (SLN) had no effect on the esophagus. Centripetal electrical stimulation of the SLN activated the cervical component of the esophageal phase of swallowing before initiation of the pharyngeal phase. Therefore, we concluded that initiation of the esophageal phase of swallowing depends on feedback from peripheral reflexes acting through the SLN, rather than a central program.

Localization of receptors for calcitonin-gene-related peptide to intraganglionic laminar endings of the mouse esophagus: peripheral interaction between vagal and spinal afferents?

The calcitonin-gene-related peptide (CGRP) receptor is a heterodimer of calcitonin-receptor-like receptor (CLR) and receptor-activity-modifying protein 1 (RAMP1). Despite the importance of CGRP in regulating gastrointestinal functions, nothing is known about the distribution and function of CLR/RAMP1 in the esophagus, where up to 90 % of spinal afferent neurons contain CGRP. We detected CLR/RAMP1 in the mouse esophagus using immunofluorescence and confocal laser scanning microscopy and examined their relationship with neuronal elements of the myenteric plexus. Immunoreactivity for CLR and RAMP1 colocalized with VGLUT2-positive intraganglionic laminar endings (IGLEs), which were contacted by CGRP-positive varicose axons presumably of spinal afferent origin, typically at sites of CRL/RAMP1 immunoreactivity. This provides an anatomical basis for interaction between spinal afferent fibers and IGLEs. Immunoreactive CLR and RAMP1 also colocalized in myenteric neurons. Thus, CGRP-containing spinal afferents may interact with both vagal IGLEs and myenteric neurons in the mouse esophagus, possibly modulating motility reflexes and inflammatory hypersensitivity.

Generation of Gastroesophageal Reflux Disease Symptoms During Esophageal Acid Infusion With Concomitant Esophageal pH Monitoring in Healthy Adults

Background/Aims

The sensitivity of the upper and lower esophageal mucosa to acid is considered to differ. We investigated the relationship between pH changes in different sites of the esophagus and generation of gastroesophageal reflux symptoms during an acid infusion test.

Methods

An acid infusion catheter was placed at 5 or 15 cm above the lower esophageal sphincter (LES) in 18 healthy volunteers, while a 2-channel pH sensor catheter was also placed in each with the sensors set at 5 and 15 cm above the LES. Solutions containing water and hydrochloric acid at different concentrations were infused through the infusion catheter.

Results

Acid infusion in the upper esophagus caused a pH drop in both upper and lower esophageal sites, whereas that in the lower esophagus resulted in a significant pH drop only in the lower without a corresponding pH decline in the upper esophagus. Stronger heartburn, chest pain, and chest oppression symptoms were noted when acid was infused in the upper as compared to the lower esophagus, while increased intra-esophageal acidity strengthened each symptom. Regurgitations caused by upper and lower esophageal acid infusions were similar, and not worsened by a larger drop in intra-esophageal pH. Chest pain was caused only by lowered intra-esophageal pH, while heartburn, chest oppression, and regurgitation were induced by a less acidic solution.

Conclusions

Higher intra-esophageal acidity caused stronger heartburn, chest pain, and chest oppression symptoms. However, regurgitation was not significantly influenced by intra-esophageal acidity. The upper esophagus showed higher acid sensitivity than the lower esophagus.

Changes in calcitonin gene-related peptide (CGRP) receptor component and nitric oxide receptor (sGC) immunoreactivity in rat trigeminal ganglion following glyceroltrinitrate pretreatment

BACKGROUND

Nitric oxide (NO) is thought to play an important role in the pathophysiology of migraine. Infusion of the nitrovasodilator glyceroltrinitrate (nitroglycerin, GTN), which mobilizes NO in the organism, is an approved migraine model in humans. Calcitonin gene-related peptide (CGRP) is regarded as another key mediator in migraine. Increased plasma levels of CGRP have been found during spontaneous as well as nitrovasodilator-induced migraine attacks. The nociceptive processes and interactions underlying the NO and CGRP mediated headache are poorly known but can be examined in animal experiments. In the present study we examined changes in immunofluorescence of CGRP receptor components (CLR and RAMP1) and soluble guanylyl cyclase (sGC), the intracellular receptor for NO, in rat trigeminal ganglia after pretreatment with GTN.

METHODS

Isoflurane anaesthetised rats were intravenously infused with GTN (1 mg/kg) or saline for four hours and two hours later the trigeminal ganglia were processed for immunohistochemistry. Different primary antibodies recognizing CLR, RAMP1, CGRP and sGC coupled to fluorescent secondary antibodies were used to examine immunoreactive cells in serial sections of trigeminal ganglia with epifluorescence and confocal laser scanning microscopy. Several staining protocols were examined to yield optimized immunolabeling.

RESULTS

In vehicle-treated animals, 42% of the trigeminal ganglion neurons were immunopositive for RAMP1 and 41% for CLR. After GTN pretreatment CLR-immunopositivity was unchanged, while there was an increase in RAMP1-immunopositive neurons to 46%. RAMP1 and CLR immunoreactivity was also detected in satellite cells. Neurons immunoreactive for sGC were on average smaller than sGC-immunonegative neurons. The percentage of sGC-immunopositive neurons (51% after vehicle) was decreased after GTN infusion (48%).

CONCLUSIONS

Prolonged infusion of GTN caused increased fractions of RAMP1- and decreased fractions of sGC-immunopositive neurons in the trigeminal ganglion. The observed alterations are likely immunophenotypic correlates of the pathophysiological processes underlying nitrovasodilator-induced migraine attacks and indicate that signalling via CGRP receptors but not sGC-mediated mechanisms may be enhanced through endogenous NO production.

The neural crest- and placodes-derived afferent innervation of the mouse esophagus

BACKGROUND

The mouse is an invaluable model for mechanistic studies of esophageal nerves, but the afferent innervation of the mouse esophagus is incompletely understood. Vagal afferent neurons are derived from two embryonic sources: neural crest and epibranchial placodes. We hypothesized that both neural crest and placodes contribute to the TRPV1-positive (potentially nociceptive) vagal innervation of the mouse esophagus.

METHODS

Vagal jugular/nodose ganglion (JNG) and spinal dorsal root ganglia (DRG) neurons were retrogradely labeled from the cervical esophagus. Single cell RT-PCR was performed on the labeled neurons.

KEY RESULTS

In the Wnt1Cre/R26R mice expressing a reporter in the neural crest-derived cells we found that both the neural crest- and the placodes-derived vagal JNG neurons innervate the mouse esophagus. In the wild-type mouse the esophageal vagal JNG TRPV1-positive neurons segregated into two subsets: putative neural crest-derived purinergic receptor P2X(2) -negative/preprotachykinin-A (PPT-A)-positive subset and putative placodes-derived P2X(2) -positive/PPTA-negative subset. These subsets also segregated by the expression of TrkA and GFRα(3) in the putative neural crest-derived subset, and TrkB in the putative placodes-derived subset. The TRPV1-positive esophageal DRG neurons had the phenotype similar to the vagal putative neural crest-derived subset.

CONCLUSIONS & INFERENCES

The TRPV1-positive (potentially nociceptive) vagal afferent neurons innervating the mouse esophagus originate from both neural crest and placodes. The expression profile of the receptors for neurotrophic factors is similar between the neural crest-derived vagal and spinal nociceptors, but distinct from the vagal placodes-derived nociceptors.

The role of the superior laryngeal nerve in esophageal reflexes

The aim of this study was to determine the role of the superior laryngeal nerve (SLN) in the following esophageal reflexes: esophago-upper esophageal sphincter (UES) contractile reflex (EUCR), esophago-lower esophageal sphincter (LES) relaxation reflex (ELIR), secondary peristalsis, pharyngeal swallowing, and belch. Cats (N = 43) were decerebrated and instrumented to record EMG of the cricopharyngeus, thyrohyoideus, geniohyoideus, and cricothyroideus; esophageal pressure; and motility of LES. Reflexes were activated by stimulation of the esophagus via slow balloon or rapid air distension at 1 to 16 cm distal to the UES. Slow balloon distension consistently activated EUCR and ELIR from all areas of the esophagus, but the distal esophagus was more sensitive than the proximal esophagus. Transection of SLN or proximal recurrent laryngeal nerves (RLN) blocked EUCR and ELIR generated from the cervical esophagus. Distal RLN transection blocked EUCR from the distal cervical esophagus. Slow distension of all areas of the esophagus except the most proximal few centimeters activated secondary peristalsis, and SLN transection had no effect on secondary peristalsis. Slow distension of all areas of the esophagus inconsistently activated pharyngeal swallows, and SLN transection blocked generation of pharyngeal swallows from all levels of the esophagus. Slow distension of the esophagus inconsistently activated belching, but rapid air distension consistently activated belching from all areas of the esophagus. SLN transection did not block initiation of belch but blocked one aspect of belch, i.e., inhibition of cricopharyngeus EMG. Vagotomy blocked all aspects of belch generated from all areas of esophagus and blocked all responses of all reflexes not blocked by SLN or RLN transection. In conclusion, the SLN mediates all aspects of the pharyngeal swallow, no portion of the secondary peristalsis, and the EUCR and ELIR generated from the proximal esophagus. Considering that SLN is not a motor nerve for any of these reflexes, the role of the SLN in control of these reflexes is sensory in nature only.

In gastroesophageal reflux disease, differential gene expression in the duodenum points towards enhanced chylomicron production and secretion

BACKGROUND

Duodenal signaling affects esophageal motility and perception, both pathophysiological factors in gastroesophageal reflux disease (GERD). Duodenal gene expression abnormalities, contributing to altered esophageal sensorimotor function, have not been reported to date.

AIM

To identify differentially expressed genes in GERD patients' duodenum.

METHODS

Twenty GERD patients (total 24-h acid exposure 6-12%, SAP ≥95%) and ten healthy controls (HC) were included. Two weeks prior to duodenal biopsy collection, ten patients discontinued proton pump inhibitor (PPI) treatment and ten took maximum dose PPI. RNA was profiled on an Affymetrix Human Genome U133 Plus 2.0 array (Affymetrix, Santa Clara, CA, USA). Genes exhibiting a fold change ≥ 1.4 (t test p value <1E-4) were considered differentially expressed. A subset of 21 differentially expressed genes was selected for confirmatory TaqMan low-density array RT-PCR. Mucosal apolipoprotein A-IV (apoA-IV) and cholecystokinin (CCK) concentrations were determined by ELISA and RIA, respectively.

RESULTS

In GERD patients off PPI, 23 up- and 23 down-regulated genes relative to HC were found. In GERD patients on PPI, 33 and five genes were higher, respectively, lower expressed. The majority of up-regulated genes were associated with lipid absorption, particularly triglyceride resynthesis and intracellular vesicular transport, rate-limiting processes for chylomicron production and secretion. Differential expression of 11 genes was confirmed by RT-PCR. Mucosal apoA-IV and CCK concentrations (signaling proteins released upon chylomicron secretion) were similar in GERD patients and HC.

CONCLUSIONS

The identified mRNA expression differences suggest that in GERD patients' duodenum, the chylomicron production and secretion potential is elevated, and may underlie a mechanism by which postprandial duodenal signaling contributes to GERD symptom generation.

ATP: a mediator for HCl-induced TRPV1 activation in esophageal mucosa

In esophageal mucosa, HCl causes TRPV1-mediated release of calcitonin gene-related peptide (CGRP) and substance P (SP) from submucosal neurons and of platelet-activating factor (PAF) from epithelial cells. CGRP and SP release was unaffected by PAF antagonists but reduced by the purinergic antagonist suramin. ATP caused CGRP and SP release from esophageal mucosa, confirming a role of ATP in the release. The human esophageal epithelial cell line HET-1A was used to identify epithelial cells as the site of ATP release. HCl caused ATP release from HET-1A, which was reduced by the TRPV1 antagonist 5-iodoresiniferatoxin. Real-time PCR demonstrated the presence of mRNA for several P2X and P2Y purinergic receptors in epithelial cells. HCl also increased activity of lyso-PAF acetyl-CoA transferase (lyso-PAF AT), the enzyme responsible for production of PAF. The increase was blocked by suramin. ATP caused a similar increase, confirming ATP as a mediator for the TRPV1-induced increase in enzyme activity. Repeated exposure of HET-1A cells to HCl over 2 days caused upregulation of mRNA and protein expression for lyso-PAF AT. Suramin blocked this response. Repeated exposure to ATP caused a similar mRNA increase, confirming ATP as a mediator for upregulation of the enzyme. Thus, HCl-induced activation of TRPV1 causes ATP release from esophageal epithelial cells that causes release of CGRP and SP from esophageal submucosal neurons and activation of lyso-PAF AT, the enzyme responsible for the production of PAF in epithelial cells. Repeated application of HCl or of ATP causes upregulation of lyso-PAF AT in epithelial cells.

The central projections of the laryngeal nerves in the rat

The larynx serves respiratory, protective, and phonatory functions. The motor and sensory innervation to the larynx controlling these functions is provided by the superior laryngeal nerve (SLN) and the recurrent laryngeal nerve (RLN). Classical studies state that the SLN innervates the cricothyroid muscle and provides sensory innervation to the supraglottic cavity, whereas the RLN supplies motor innervation to the remaining intrinsic laryngeal muscles and sensory innervation to the infraglottic cavity, but recent data suggest a more complex anatomical and functional organisation. The current neuroanatomical tracing study was undertaken to provide a comprehensive description of the central brainstem connections of the axons within the SLN and the RLN, including those neurons that innervate the larynx. The study has been carried out in 41 adult male Sprague-Dawley rats. The central projections of the laryngeal nerves were labelled following application of biotinylated dextran amines onto the SLN, the RLN or both. The most remarkable result of the study is that in the rat the RLN does not contain any afferent axons from the larynx, in contrast to the pattern observed in many other species including man. The RLN supplied only special visceromotor innervation to the intrinsic muscles of the larynx from motoneurons in the nucleus ambiguus (Amb). All the afferent axons innervating the larynx are contained within the SLN, and reach the nucleus of the solitary tract. The SLN also contained secretomotor efferents originating from motoneurons in the dorsal motor nucleus of the vagus, and special visceral efferent fibres from the Amb. In conclusion, the present study shows that in the rat the innervation of the larynx differs in significant ways from that described in other species.

Transient receptor potential vanilloid 4 (TRPV4)-dependent calcium influx and ATP release in mouse oesophageal keratinocytes

Gastro-oesophageal reflux disease (GERD) is a multi-factorial disease that may involve oesophageal hypersensitivity to mechanical or heat stimulus as well as acids. Intraganglionic laminar endings (IGLEs) are the most prominent terminal structures of oesophageal vagal mechanosensitive afferents and may modulate mechanotransduction via purinergic receptors. Transient receptor potential channel vanilloid 4 (TRPV4) can detect various stimuli such as warm temperature, stretch and some chemicals, including 4α-phorbol 12,13-didecanoate (4α-PDD) and GSK1016790A. TRPV4 is expressed in many tissues, including renal epithelium, skin keratinocytes and urinary bladder epithelium, but its expression and function in the oesophagus is poorly understood. Here, we show anatomical and functional TRPV4 expression in mouse oesophagus and its involvement in ATP release. TRPV4 mRNA and protein were detected in oesophageal keratinocytes. Several known TRPV4 activators (chemicals, heat and stretch stimulus) increased cytosolic Ca2+ concentrations in cultured WT keratinocytes but not in TRPV4 knockout (KO) cells. Moreover, the TRPV4 agonist GSK1016790A and heat stimulus evoked TRPV4-like current responses in isolated WT keratinocytes, but not in TRPV4KO cells. GSK1016790A and heat stimulus also significantly increased ATP release from WT oesophageal keratinocytes compared to TRPV4KO cells. The vesicle-trafficking inhibitor brefeldin A (BFA) inhibited the ATP release. This ATP release could be mediated by the newly identified vesicle ATP transporter, VNUT, which is expressed by oesophageal keratinocytes at the mRNA and protein levels. In conclusion, in response to heat, chemical and possibly mechanical stimuli, TRPV4 contributes to ATP release in the oesophagus. Thus, TRPV4 could be involved in oesophageal mechano- and heat hypersensitivity.

Transgene expression and effective gene silencing in vagal afferent neurons in vivo using recombinant adeno-associated virus vectors

Vagal afferent fibres innervating thoracic structures such as the respiratory tract and oesophagus are diverse, comprising several subtypes of functionally distinct C-fibres and A-fibres. Both morphological and functional studies of these nerve subtypes would be advanced by selective, effective and long-term transduction of vagal afferent neurons with viral vectors. Here we addressed the hypothesis that vagal sensory neurons can be transduced with adeno-associated virus (AAV) vectors in vivo, in a manner that would be useful for morphological assessment of nerve terminals, using enhanced green fluorescent protein (eGFP), as well as for the selective knock-down of specific genes of interest in a tissue-selective manner. We found that a direct microinjection of AAV vectors into the vagal nodose ganglia in vivo leads to selective, effective and long-lasting transduction of the vast majority of primary sensory vagal neurons without transduction of parasympathetic efferent neurons. The transduction of vagal neurons by pseudoserotype AAV2/8 vectors in vivo is sufficiently efficient such that it can be used to functionally silence TRPV1 gene expression using short hairpin RNA (shRNA). The eGFP encoded by AAV vectors is robustly transported to both the central and peripheral terminals of transduced vagal afferent neurons allowing for bright imaging of the nerve endings in living tissues and suitable for structure-function studies of vagal afferent nerve endings. Finally, the AAV2/8 vectors are efficiently taken up by the vagal nerve terminals in the visceral tissue and retrogradely transported to the cell body, allowing for tissue-specific transduction.

Differential activation of pontomedullary nuclei by acid perfusion of different regions of the esophagus

The objective of this study was to determine the brain stem nuclei and physiological responses activated by esophageal acidification. The effects of perfusion of the cervical (ESOc), or thoracic (ESOt) esophagus with PBS or HCl on c-fos immunoreactivity of the brain stem or on physiological variables, and the effects of vagotomy were examined in anesthetized cats. We found that acidification of the ESOc increased the number of c-fos positive neurons in the area postrema (AP), vestibular nucleus (VN), parabrachial nucleus (PBN), nucleus ambiguus (NA), dorsal motor nucleus (DMN), and all subnuclei of the nucleus tractus solitarius (NTS), but one. Acidification of the ESOt activated neurons in the central (CE), caudal (CD), dorsomedial (DM), dorsolateral (DL), ventromedial (VM) subnuclei of NTS, and the DMN. Vagotomy blocked all c-fos responses to acid perfusion of the whole esophagus (ESOw). Perfusion of the ESOc or ESOt with PBS activated secondary peristalsis (2P), but had no effect on blood pressure, heart rate, or respiratory rate. Perfusion of the ESOc, but not ESOt, with HCl activated pharyngeal swallowing (PS), profuse salivation, or physiological correlates of emesis. Vagotomy blocked all physiological effects of ESOw perfusion. We conclude that acidification of the ESOc and ESOt activate different sets of pontomedullary nuclei and different physiological responses. The NTSce, NTScom, NTSdm, and DMN are associated with activation of 2P, the NTSim and NTSis, are associated with activation of PS, and the AP, VN, and PBN are associated with activation of emesis and perhaps nausea. All responses to esophageal fluid perfusion or acidification are mediated by the vagus nerves.

Increase in CGRP- and nNOS-immunoreactive neurons in the rat trigeminal ganglion after infusion of an NO donor

Background: Nitrovasodilators, such as glyceroltrinitrate (GTN), which produce nitric oxide (NO) in the organism, are known to cause delayed headaches in migraineurs, accompanied by increased plasma levels of calcitonin gene-related peptide (CGRP) in the cranial venous outflow. Increases in plasma CGRP and NO metabolites have also been found in spontaneous migraine attacks. In a rat model of meningeal nociception, infusion of NO donors induced activity of neurons in the spinal trigeminal nucleus.

Methods: Isoflurane-anaesthetised rats were intravenously infused with GTN (250 µg/kg) or saline for two hours and fixed by perfusion after a further four hours. Cryosections of dissected trigeminal ganglia were immunostained for detection of CGRP and neuronal NO synthase (nNOS). The ganglion neurons showing immunofluorescence for either of these proteins were counted.

Results: The proportions of CGRP- and nNOS- as well as double-immunopositive neurons were increased after GTN infusion compared to saline treatment in all parts of the trigeminal ganglion (CGRP) or restricted to the ophthalmic region (nNOS). The size of immunopositive neurons was not significantly different compared to controls.

Conclusion: High levels of NO may induce the expression or availability of CGRP and nNOS. Similar changes may be involved in nitrovasodilator-induced and spontaneous headache attacks in migraineurs.

An essential component to brainstem cough gating identified in anesthetized guinea pigs

Coughing protects and clears the airways and lungs of inhaled irritants, particulates, pathogens, and accumulated secretions. An initial urge to cough, and an almost binary output suggests gating mechanisms that encode and modulate this defensive reflex. Whether this "gate" has a physical location for the physiological barrier it poses to cough is unknown. Here we describe a critical component to cough gating, the central terminations of the cough receptors. A novel microinjection strategy defined coordinates for microinjection of glutamate receptor antagonists that nearly abolished cough evoked from the trachea and larynx in anesthetized guinea pigs while having no effect on basal respiratory rate and little or no effect on reflexes attributed to activating other afferent nerve subtypes. Comparable microinjections in adjacent brainstem locations (0.5-2 mm distal) were without effect on coughing. Subsequent transganglionic and dual tracing studies confirmed that the central terminations of the cough receptors and their primary relay neurons are found bilaterally within nucleus tractus solitarius (nTS), lateral to the commissural subnucleus and perhaps in the medial subnuclei. These synapses possess the physiological characteristics of a cough gate. Their localization should facilitate more mechanistic studies of the encoding and gating of cough.

Innervation of tracheal parasympathetic ganglia by esophageal cholinergic neurons: evidence from anatomic and functional studies in guinea pigs

In the present study, we describe a subset of nerve fibers, characterized by their immunoreactivity for the calcium-binding protein calretinin, that are densely and selectively associated with cholinergic postganglionic neurons in the guinea pig tracheal ganglia. Retrograde neuronal tracing with cholera toxin B, combined with immunohistochemical analyses, showed that these nerve fibers do not originate from sensory neurons in the nodose, jugular, or dorsal root ganglia or from motor neurons in the nucleus ambiguus, dorsal motor nucleus of the vagus nerve, spinal cord, stellate ganglia, or superior cervical ganglia. Calretinin-immunoreactive nerve fibers disappeared from tracheal segments after 48 h in organotypic culture, indicating that the fibers were of extrinsic origin. However, calretinin-positive nerve fibers persisted in tracheal ganglia when tracheae were cocultured with the adjacent esophagus intact. Immunohistochemical analysis of the esophagus revealed a population of cholinergic neurons in the esophageal myenteric plexus that coexpressed calretinin. In functional studies, electrical stimulation of the esophagus in vitro evoked measurable contractions of the trachea. These contractions were not altered by prior organotypic culture of the trachea and esophagus to remove the extrinsic innervation to the airways but were significantly (P < 0.05) inhibited by the ganglionic blocker hexamethonium or by physical disruption of the tissue connecting the trachea and esophagus. These data suggest that a subset of esophageal neurons, characterized by the expression of calretinin and acetylcholine, provide a previously unrecognized excitatory input to tracheal cholinergic ganglia in guinea pigs.

Calcitonin gene-related peptide immunoreactive neurons innervating the soft palate, the root of tongue, and the pharynx in the superior glossopharyngeal ganglion of the rat

We have examined whether calcitonin gene-related peptide immunoreactive (CGRP-ir) neurons in the glossopharyngeal ganglia innervate the soft palate, the root of tongue, and the pharynx of the rat. Immunohistochemical observations revealed that numerous CGRP-ir neurons are located in the superior glossopharyngeal ganglion located ventrolateral to the medulla oblongata in the cranial cavity, and that CGRP-ir neurons are also located in the inferior glossopharyngeal ganglion at the jugular foramen. When Fluorogold was injected into the soft palate, the root of tongue, or the pharyngeal constrictor muscles, many retrogradely Fluorogold-labeled neurons were found in the superior glossopharyngeal ganglion and the nodose ganglion, and several Fluorogold-labeled neurons were found in the inferior glossopharyngeal ganglion. Double labeling with immunohistochemistry for CGRP and Fluorogold showed that in every case of injections of Fluorogold into the soft palate, the root of tongue, or the pharynx, about 30% of the Fluorogold-labeled neurons in the superior glossopharyngeal ganglion expressed CGRP-like immunoreactivity, while no double-labeled neurons were found in the inferior glossopharyngeal ganglion or the nodose ganglion. These results indicate that nociceptive sensory information from the soft palate, the root of tongue, and the pharynx might be conveyed by the neurons in the superior glossopharyngeal ganglion to the nucleus tractus solitarii.

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.

Vagal afferent nerves with the properties of nociceptors

Vagal afferent nerves are essential for optimal neural regulation of visceral organs, but are not often considered important for their defense. However, there are well-defined subsets of vagal afferent nerves that have activation properties indicative of specialization to detect potentially harmful stimuli (nociceptors). This is clearly exemplified by the vagal bronchopulmonary C-fibers that are quiescent in healthy lungs but are readily activated by noxious chemicals and inflammatory molecules. Vagal afferent nerves with similar activation properties have been also identified in the esophagus and probably exist in other visceral tissues. In addition, these putative vagal nociceptors often initiate defensive reflexes, can be sensitized, and have the capacity to induce central sensitization. This set of properties is a characteristic of nociceptors in somatic tissues.

Brain stem control of the phases of swallowing

The phases of swallowing are controlled by central pattern-generating circuitry of the brain stem and peripheral reflexes. The oral, pharyngeal, and esophageal phases of swallowing are independent of each other. Although central pattern generators of the brain stem control the timing of these phases, the peripheral manifestation of these phases depends on sensory feedback through reflexes of the pharynx and esophagus. The dependence of the esophageal phase of swallowing on peripheral feedback explains its absence during failed swallows. Reflexes that initiate the pharyngeal phase of swallowing also inhibit the esophageal phase which ensures the appropriate timing of its occurrence to provide efficient bolus transport and which prevents the occurrence of multiple esophageal peristaltic events. These inhibitory reflexes are probably partly responsible for deglutitive inhibition. Three separate sets of brain stem nuclei mediate the oral, pharyngeal, and esophageal phases of swallowing. The trigeminal nucleus and reticular formation probably contain the oral phase pattern-generating neural circuitry. The nucleus tractus solitarius (NTS) probably contains the second-order sensory neurons as well as the pattern-generating circuitry of both the pharyngeal and esophageal phases of swallowing, whereas the nucleus ambiguus and dorsal motor nucleus contain the motor neurons of the pharyngeal and esophageal phases of swallowing. The ventromedial nucleus of the NTS may govern the coupling of the pharyngeal phase to the esophageal phase of swallowing.

Cough and gastroesophageal reflux: insights from animal models

Chronic cough in gastroesophageal reflux disease (GERD) has been attributed to irritation of the esophagus and/or upper airways by reflux of gastric content. Animal models have provided insight into both of these putative mechanisms. In patients with chronic cough and GERD, stimuli associated with reflex in the esophagus sensitize the cough reflex. This sensitization can be reproduced in the guinea pig and is most likely mediated by the esophageal afferent nerve fibers carried by the vagus nerves. Studies in animals have identified several subtypes of vagal esophageal C-fibers that may subserve this function. The putative nociceptive vagal C-fibers in the guinea pig esophagus are stimulated by acid and express the TRPV1 and TRPA1 receptors that confer responsiveness to disparate noxious stimuli. Acute and/or chronic irritation of the upper airways by reflux may contribute to cough by stimulation and/or sensitization of the airway afferent nerves. Studies in animals have identified airway nerves that likely initiate cough due to aspirated reflux; have characterized their pharmacology; and have provided insight into changes of their sensitivity. Studies in animal models have also described the neurophysiology of reflexes that protect the airways from reflux. In conclusion, animal models provide mechanistic insight into the modulation of cough from the esophagus and the pharmacology of neural pathways mediating cough in GERD.

Release of calcitonin gene-related peptide from the jugular-nodose ganglion complex in rats--a new model to examine the role of cardiac peptidergic and nitrergic innervation

OBJECTIVE

Afferent information from the heart and the lung is conveyed to the brainstem by primary afferent fibers originating from vagal sensory neurons (jugular-nodose ganglion complex, JNC). The present study was made to evaluate if release of the sensory neuropeptide calcitonin gene-related peptide (CGRP) from the JNC can be used as a model for future studies on changes in neuropeptide release under pathological conditions of the heart.

METHODS

Freshly isolated rat JNC's were passed through a series of solutions based on oxygenated synthetic interstitial fluid (SIF). Substances such as the TRPV1 receptor agonist capsaicin and the nitric oxide (NO) donor sodium nitroprusside (SNP) were added as excitatory test stimuli. The eluates were processed using an enzyme immuno-assay (EIA) for measurement of CGRP concentrations. Immunohistochemistry was used to visualize CGRP containing and NO producing neurons in the JNC.

RESULTS

Both SNP and capsaicin caused significant increases in CGRP release. CGRP-immunoreactive neurons (somata) were preferentially found in the jugular ganglion, whereas neurons immunoreactive for neuronal NO synthase were mostly localized in the nodose ganglion.

CONCLUSION

The present study demonstrates an easily reproducible model for measuring stimulated CGRP release from vagal afferents arising from the JNC. Nitric oxide produced by vagal afferents may stimulate CGRP release upon afferent activation.