Mast cell-mediated long-lasting increases in excitability of vagal C fibers in guinea pig esophagus

Animal models of eosinophilic esophagitis

Eosinophilic esophagitis is a chronic inflammatory disorder of the esophagus. Over the past 25 yr, great strides have been made toward understanding its pathogenesis, in part due to studies in several types of animal models. The vast majority of these models have been characterized in mice. In this review, we summarize the histopathological features of eosinophilic esophagitis recapitulated by these animal models, as well as discuss their strengths and weaknesses.

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.

The diversity of neuroimmune circuits controlling lung inflammation

It is becoming increasingly appreciated that the nervous and immune systems communicate bidirectionally to regulate immunological outcomes in a variety of organs including the lung. Activation of neuronal signaling can be induced by inflammation, tissue damage, or pathogens to evoke or reduce immune cell activation in what has been termed a neuroimmune reflex. In the periphery, these reflexes include the cholinergic anti-inflammatory pathway, sympathetic reflex, and sensory nociceptor-immune cell pathways. Continual advances in neuroimmunology in peripheral organ systems have fueled small-scale clinical trials that have yielded encouraging results for a range of immunopathologies such as rheumatoid arthritis. Despite these successes, several limitations should give clinical investigators pause in the application of neural stimulation as a therapeutic for lung inflammation, especially if inflammation arises from a novel pathogen. In this review, the general mechanisms of each reflex, the evidence for these circuits in the control of lung inflammation, and the key knowledge gaps in our understanding of these neuroimmune circuits will be discussed. These limitations can be overcome not only through a better understanding of neuroanatomy but also through a systematic evaluation of stimulation parameters using immune activation in lung tissues as primary readouts. Our rapidly evolving understanding of the nervous and immune systems highlights the importance of communication between these cells in health and disease. This integrative approach has tremendous potential in the development of targeted therapeutics if specific challenges can be overcome.

Capsaicin-Sensitive Vagal Afferent Nerve-Mediated Interoceptive Signals in the Esophagus

Heartburn and non-cardiac chest pain are the predominant symptoms in many esophageal disorders, such as gastroesophageal reflux disease (GERD), non-erosive reflux disease (NERD), functional heartburn and chest pain, and eosinophilic esophagitis (EoE). At present, neuronal mechanisms underlying the process of interoceptive signals in the esophagus are still less clear. Noxious stimuli can activate a subpopulation of primary afferent neurons at their nerve terminals in the esophagus. The evoked action potentials are transmitted through both the spinal and vagal pathways to their central terminals, which synapse with the neurons in the central nervous system to induce esophageal nociception. Over the last few decades, progress has been made in our understanding on the peripheral and central neuronal mechanisms of esophageal nociception. In this review, we focus on the roles of capsaicin-sensitive vagal primary afferent nodose and jugular C-fiber neurons in processing nociceptive signals in the esophagus. We briefly compare their distinctive phenotypic features and functional responses to mechanical and chemical stimulations in the esophagus. Then, we summarize activation and/or sensitization effects of acid, inflammatory cells (eosinophils and mast cells), and mediators (ATP, 5-HT, bradykinin, adenosine, S1P) on these two nociceptive C-fiber subtypes. Lastly, we discuss the potential roles of capsaicin-sensitive esophageal afferent nerves in processing esophageal sensation and nociception. A better knowledge of the mechanism of nociceptive signal processes in primary afferent nerves in the esophagus will help to develop novel treatment approaches to relieve esophageal nociceptive symptoms, especially those that are refractory to proton pump inhibitors.

Cow's Milk Allergy: Immunomodulation by Dietary Intervention

Cow’s milk proteins cause allergic symptoms in 2% to 3% of all infants. In these individuals, the physiological mechanism of tolerance is broken with subsequent possible sensitization to antigens, which can lead eventually to allergic responses. The present review aims to provide an overview of different aspects of immune modulation by dietary intervention in cow’s milk allergy (CMA). It focuses on pathogenetic mechanisms of different CMA related disorders, e.g., gastroesophageal reflux and eosinophilic esophagitis, highlighting the role of dietary management on innate and adaptive immune systems. The traditional dietary management of CMA has greatly changed in the last years, moving from a passive approach, consisting of an elimination diet to relieve symptoms, to a “proactive” one, meaning the possibility to actively modulate the immune system. Thus, new insights into the role of hydrolysates and baked milk in immunomodulation are addressed here. Additionally, nutritional components, such as pre- and probiotics, may target the immune system via microbiota, offering a possible road map for new CMA prevention and treatment strategies.

Mechanisms of organophosphorus pesticide toxicity in the context of airway hyperreactivity and asthma

Numerous epidemiologic studies have identified an association between occupational exposures to organophosphorus pesticides (OPs) and asthma or asthmatic symptoms in adults. Emerging epidemiologic data suggest that environmentally relevant levels of OPs may also be linked to respiratory dysfunction in the general population and that in utero and/or early life exposures to environmental OPs may increase risk for childhood asthma. In support of a causal link between OPs and asthma, experimental evidence demonstrates that occupationally and environmentally relevant OP exposures induce bronchospasm and airway hyperreactivity in preclinical models. Mechanistic studies have identified blockade of autoinhibitory M2 muscarinic receptors on parasympathetic nerves that innervate airway smooth muscle as one mechanism by which OPs induce airway hyperreactivity, but significant questions remain regarding the mechanism(s) by which OPs cause neuronal M2 receptor dysfunction and, more generally, how OPs cause persistent asthma, especially after developmental exposures. The goals of this review are to 1) summarize current understanding of OPs in asthma; 2) discuss mechanisms of OP neurotoxicity and immunotoxicity that warrant consideration in the context of OP-induced airway hyperreactivity and asthma, specifically, inflammatory responses, oxidative stress, neural plasticity, and neurogenic inflammation; and 3) identify critical data gaps that need to be addressed in order to better protect adults and children against the harmful respiratory effects of low-level OP exposures.

Mast cell deficiency attenuates acupuncture analgesia for mechanical pain using c-kit gene mutant rats

Background

Acupuncture therapy plays a pivotal role in pain relief, and increasing evidence demonstrates that mast cells (MCs) may mediate acupuncture analgesia. The present study aims to investigate the role of MCs in acupuncture analgesia using c-kit gene mutant-induced MC-deficient rats.

Materials and methods

WsRC-Ws/Ws rats and their wild-type (WT) littermates (WsRC-+/+) were used. The number of MCs in skin of ST36 area was compared in two rats after immunofluorescence labeling. Mechanical withdrawal latency (MWL), mechanical withdrawal threshold (MWT), and thermal withdrawal latency (TWL) were measured on bilateral plantar for pain threshold evaluation before and after each stimulus. Acupuncture- and moxibustion-like stimuli (43°C, 46°C heat, 1 mA electroacupuncture [EA], 3 mA EA, and manual acupuncture [MA]) were applied randomly on different days.

Results

Fewer MCs were observed in the skin of ST36 in mutant rats compared to WT rats (P<0.001). For pain thresholds, MWL and MWT were higher in WsRC-Ws/Ws compared to WsRC-+/+ on bilateral paws (P<0.05), but TWL was not different between the two rats (P>0.05). Bilateral MWL and MWT in WsRC-+/+ rats increased significantly after each stimulus compared to baseline (P<0.01, P<0.001). In WsRC-Ws/Ws rats, only noxious stimuli could produce anti-nociceptive effects for mechanical pain (46°C, 3 mA EA, MA) (P<0.01, P<0.001). Additionally, the net increases in MWL and MWT induced by most stimuli were greater in WT than in mutant rats (P<0.05). For thermal nociception, either high- or low-intensity stimuli could significantly augment TWL in two rats (P<0.001), and the net increases of TWL evoked by most stimuli were to the same extent in two genetic variants.

Conclusion

MCs influence the basic mechanical but not thermal pain threshold. MCs participate in acupuncture analgesia in mechanical but not in thermal nociception, in that MC deficiency may attenuate the mechanical analgesia evoked by high-intensity stimuli and eliminate analgesia provoked by low-intensity stimuli.

Mechanisms of the adenosine A2A receptor-induced sensitization of esophageal C fibers

Clinical studies indicate that adenosine contributes to esophageal mechanical hypersensitivity in some patients with pain originating in the esophagus. We have previously reported that the esophageal vagal nodose C fibers express the adenosine A2A receptor. Here we addressed the hypothesis that stimulation of the adenosine A2A receptor induces mechanical sensitization of esophageal C fibers by a mechanism involving transient receptor potential A1 (TRPA1). Extracellular single fiber recordings of activity originating in C-fiber terminals were made in the ex vivo vagally innervated guinea pig esophagus. The adenosine A2A receptor-selective agonist CGS21680 induced robust, reversible sensitization of the response to esophageal distention (10-60 mmHg) in a concentration-dependent fashion (1-100 nM). At the half-maximally effective concentration (EC50: ≈3 nM), CGS21680 induced an approximately twofold increase in the mechanical response without causing an overt activation. This sensitization was abolished by the selective A2A antagonist SCH58261. The adenylyl cyclase activator forskolin mimicked while the nonselective protein kinase inhibitor H89 inhibited mechanical sensitization by CGS21680. CGS21680 did not enhance the response to the purinergic P2X receptor agonist α,β-methylene-ATP, indicating that CGS21680 does not nonspecifically sensitize to all stimuli. Mechanical sensitization by CGS21680 was abolished by pretreatment with two structurally different TRPA1 antagonists AP18 and HC030031. Single cell RT-PCR and whole cell patch-clamp studies in isolated esophagus-specific nodose neurons revealed the expression of TRPA1 in A2A-positive C-fiber neurons and demonstrated that CGS21682 potentiated TRPA1 currents evoked by allylisothiocyanate. We conclude that stimulation of the adenosine A2A receptor induces mechanical sensitization of nodose C fibers by a mechanism sensitive to TRPA1 antagonists indicating the involvement of TRPA1.

Cough reflex sensitization from esophagus and nose

The diseases of the esophagus and nose are among the major factors contributing to chronic cough although their role in different patient populations is debated. Studies in animal models and in humans show that afferent C-fiber activators applied on esophageal or nasal mucosa do not initiate cough, but enhance cough induced by inhaled irritants. These results are consistent with the hypothesis that activation of esophageal and nasal C-fibers contribute to cough reflex hypersensitivity observed in chronic cough patients with gastroesophageal reflux disease (GERD) and chronic rhinitis, respectively. The afferent nerves mediating cough sensitization from the esophagus are probably the neural crest-derived vagal jugular C-fibers. In addition to their responsiveness to high concentration of acid typical for gastroesophageal reflux (pH < 5), esophageal C-fibers also express receptors for activation by weakly acidic reflux such as receptors highly sensitive to acid and receptors for bile acids. The nature of sensory pathways from the nose and their activators relevant for cough sensitization are less understood. Increased cough reflex sensitivity was also reported in many patients with GERD or rhinitis who do not complain of cough indicating that additional endogenous or exogenous factors may be required to develop chronic coughing in these diseases.

Sex- and afferent-specific differences in histamine receptor expression in vagal afferents of rats: A potential mechanism for sexual dimorphism in prevalence and severity of asthma

The incidence of asthma is more common in boys than in girls during the childhood, and more common in premenopausal female than age-matched males. Our previous study demonstrated a gender difference in histamine-mediated neuroexcitability in nodose ganglia neurons (NGNs), highlighting a possibility of histamine-mediated gender difference in asthma via visceral afferent function. In the present study, we aimed to explore the gender difference in expression profiles of histamine receptors (HRs) in nodose ganglia (NG) and individual identified NGNs to provide deeper insights into the mechanisms involved in sexual dimorphism of asthma. Western-blot and SYBR green RT-PCR showed that H2R and H3R were highly expressed in NG of females compared with males and downregulated in ovariectomized females. H1R was equally expressed in NG of both sexes and not altered by ovariectomy. Furthermore, this highly expressive H2R and H3R were distributed in both myelinated and unmyelinated NGNs isolated from adult female rats by immunofluorescence and single-cell RT-PCR. H3R widely distributed in all tested neuron subtypes and its expression did not show significant difference among neuron subtypes. H2R was widely and highly expressed in low-threshold and sex-specific subpopulation of myelinated Ah-types compared with myelinated A- and unmyelinated C-type NGNs. Unexpectedly, weak expression of H1R was detected in both myelinated and unmyelinated NGNs by immunofluorescence, which was further confirmed by single-cell RT-PCR. Our results suggest that the sexual dimorphism in the expression of H2R and H3R in vagal afferents very likely contributes, at least partially, to the gender difference in prevalence and severity of asthma.

Mast cell activation is involved in stress-induced epithelial barrier dysfunction in the esophagus

OBJECTIVE

We aimed to investigate the role of mast cell in stress-induced barrier dysfunction in the esophagus and its possible pathway involved using mast cell-deficient (Ws/Ws) rats.

METHODS

Ws/Ws rats and normal (+/+) rats were submitted to chronic restraint stress (CRS) 2 h/day for 7 days. Tissues were obtained from distal esophagus. Mast cells were counted under Alcian blue-safranin O stain. Activation of mast cells was assessed using transmission electron microscope. Esophageal epithelial barrier dysfunction was evaluated by measuring intercellular spaces (ICS) and by quantifying tight junction (TJ) proteins. The localization and expression of mast cell-derived tryptase and proteinase activated receptor 2 (PAR-2) were assessed.

RESULTS

A higher number of mast cells and higher proportion of activated mast cells were observed in CRS +/+ rats compared with non-stress controls. Increased ICS and decreased expression of some TJ proteins were observed in the CRS +/+ rats but not in the CRS Ws/Ws rats. Tryptase and its receptor PAR-2 were found elevated concomitantly by nearly 100% in CRS +/+ rats, but not in CRS Ws/Ws rats.

CONCLUSIONS

Mast cells play an important role in stress-induced epithelial barrier dysfunction in esophagus. The mechanism may involve the activation of PAR-2 by mast cell-derived tryptase, causing proinflammatory responses and the subsequent disruption of the epithelial TJ proteins and a disturbed cytoskeleton function, resulting in dilated intercellular spaces.

Allergen challenge sensitizes TRPA1 in vagal sensory neurons and afferent C-fiber subtypes in guinea pig esophagus

Transient receptor potential A1 (TRPA1) is a newly defined cationic ion channel, which selectively expresses in primary sensory afferent nerve, and is essential in mediating inflammatory nociception. Our previous study demonstrated that TRPA1 plays an important role in tissue mast cell activation-induced increase in the excitability of esophageal vagal nodose C fibers. The present study aims to determine whether prolonged antigen exposure in vivo sensitizes TRPA1 in a guinea pig model of eosinophilic esophagitis (EoE). Antigen challenge-induced responses in esophageal mucosa were first assessed by histological stains and Ussing chamber studies. TRPA1 function in vagal sensory neurons was then studied by calcium imaging and by whole cell patch-clamp recordings in 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI)-labeled esophageal vagal nodose and jugular neurons. Extracellular single-unit recordings were performed in vagal nodose and jugular C-fiber neuron subtypes using ex vivo esophageal-vagal preparations with intact nerve endings in the esophagus. Antigen challenge significantly increased infiltrations of eosinophils and mast cells in the esophagus. TRPA1 agonist allyl isothiocyanate (AITC)-induced calcium influx in nodose and jugular neurons was significantly increased, and current densities in esophageal DiI-labeled nodose and jugular neurons were also significantly increased in antigen-challenged animals. Prolonged antigen challenge decreased esophageal epithelial barrier resistance, which allowed intraesophageal-infused AITC-activating nodose and jugular C fibers at their nerve endings. Collectively, these results demonstrated that prolonged antigen challenge sensitized TRPA1 in esophageal sensory neurons and afferent C fibers. This novel finding will help us to better understand the molecular mechanism underlying esophageal sensory and motor dysfunctions in EoE.

Prostaglandin D2 receptor D-type prostanoid receptor 2 mediates eosinophil trafficking into the esophagus

Eosinophilic esophagitis is characterized by eosinophil-predominant inflammation in the esophagus. How eosinophils migrate and infiltrate into the esophagus, however, is less clear. Our previous study demonstrated that mast cell activation led to eosinophil infiltration in the esophagus. Prostaglandin D2 (PGD2) is an important mediator released from activated mast cells. The present study aims to determine whether PGD2 induces eosinophil infiltration into the esophagus via a d-type prostanoid receptor 2 (DP2) receptor-dependent mechanism. Using an in vivo guinea pig model, PGD2, d-type prostanoid receptor 1 (DP1) agonist, or DP2 agonist were injected into the esophagus. Esophageal tissues were removed 2 hours after injections and proceeded to either hematoxylin-eosin (HE) staining or immunofluorescent staining of eosinophil major basic protein (MBP) to compare each treatment-induced eosinophil infiltration in the esophagus. In a separate study, ovalbumin (OVA)-sensitized guinea pigs were pretreated with either DP2 or DP1 antagonists, followed by inhalation of OVA to induce mast cell activation. Esophageal tissues were then processed for immunofluorescent staining of MBP. PGD2 injection in the esophagus led to an increase of eosinophil infiltration in esophageal epithelium at the injection site as revealed by HE staining. Increased infiltration of eosinophils was further confirmed by the increased presence of MBP-labeled immunopositive (MBP-LI) cells in esophageal epithelium. Injection with DP2 agonist 15(R)-PGD2, but not DP1 agonist BW 245C, mimicked the PGD2-induced response. In OVA-sensitized animals, antigen inhalation increased MBP-LI cells in esophageal epithelium. Pretreatment with DP2 antagonist BAY-u3405, but not DP1 antagonist BW 868C, inhibited the antigen inhalation-induced increase of MBP-LI cells in esophageal epithelium. These data support the hypothesis that PGD2 induces eosinophil trafficking into the esophageal epithelium via a DP2-mediated pathway, suggesting a role of DP2 antagonist in the prevention of eosinophilic esophagitis.

Increased acid responsiveness in vagal sensory neurons in a guinea pig model of eosinophilic esophagitis

Eosinophilic esophagitis (EoE) is characterized with eosinophils and mast cells predominated allergic inflammation in the esophagus and present with esophageal dysfunctions such as dysphagia, food impaction, and heartburn. However, the underlying mechanism of esophageal dysfunctions is unclear. This study aims to determine whether neurons in the vagal sensory ganglia are modulated in a guinea pig model of EoE. Animals were actively sensitized by ovalbumin (OVA) and then challenged with aerosol OVA inhalation for 2 wk. This results in a mild esophagitis with increases in mast cells and eosinophils in the esophageal wall. Vagal nodose and jugular neurons were disassociated, and their responses to acid, capsaicin, and transient receptor potential vanilloid type 1 (TRPV1) antagonist AMG-9810 were studied by calcium imaging and whole cell patch-clamp recording. Compared with naïve animals, antigen challenge significantly increased acid responsiveness in both nodose and jugular neurons. Their responses to capsaicin were also increased after antigen challenge. AMG-9810, at a concentration that blocked capsaicin-evoked calcium influx, abolished the increase in acid-induced activation in both nodose and jugular neurons. Vagotomy strongly attenuated those increased responses of nodose and jugular neurons to both acid and capsaicin induced by antigen challenge. These data for the first time demonstrated that prolonged antigen challenge significantly increases acid responsiveness in vagal nodose and jugular ganglia neurons. This sensitization effect is mediated largely through TRPV1 and initiated at sensory nerve endings in the peripheral tissues. Allergen-induced enhancement of responsiveness to noxious stimulation by acid in sensory nerve may contribute to the development of esophageal dysfunctions such as heartburn in EoE.

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.

Mechanisms underlying the neuronal-based symptoms of allergy

Persons with allergies present with symptoms that often are the result of alterations in the nervous system. Neuronally based symptoms depend on the organ in which the allergic reaction occurs but can include red itchy eyes, sneezing, nasal congestion, rhinorrhea, coughing, bronchoconstriction, airway mucus secretion, dysphagia, altered gastrointestinal motility, and itchy swollen skin. These symptoms occur because mediators released during an allergic reaction can interact with sensory nerves, change processing in the central nervous system, and alter transmission in sympathetic, parasympathetic, and enteric autonomic nerves. In addition, evidence supports the idea that in some subjects this neuromodulation is, for reasons poorly understood, upregulated such that the same degree of nerve stimulus causes a larger effect than seen in healthy subjects. There are distinctions in the mechanisms and nerve types involved in allergen-induced neuromodulation among different organ systems, but general principles have emerged. The products of activated mast cells, other inflammatory cells, and resident cells can overtly stimulate nerve endings, cause long-lasting changes in neuronal excitability, increase synaptic efficacy, and also change gene expression in nerves, resulting in phenotypically altered neurons. A better understanding of these processes might lead to novel therapeutic strategies aimed at limiting the suffering of those with allergies.

Gender differences in histamine-induced depolarization and inward currents in vagal ganglion neurons in rats

Evidence has shown gender differences regarding the critical roles of histamine in the prevalence of asthma, anaphylaxis, and angina pectoris. Histamine depolarizes unmyelinated C-type neurons without any effects on myelinated A-type vagal ganglion neurons (VGNs) in male rats. However, little is known if VGNs from females react to histamine in a similar manner. Membrane depolarization and inward currents were tested in VGNs isolated from adult rats using a whole-cell patch technique. Results from males were consistent with the literature. Surprisingly, histamine-induced depolarization and inward currents were observed in both unmyelinated C-type and myelinated A- and Ah-type VGNs from female rats. In Ah-type neurons, responses to 1.0 μM histamine were stronger in intact females than in males and significantly reduced in ovariectomized (OVX) females. In C-type neurons, histamine-induced events were significantly smaller (pA/pF) in intact females compared with males and this histamine-induced activity was dramatically increased by OVX. Female A-types responded to histamine, which was further increased following ovariectomy. Histamine at 300 nM depolarized Ah-types in females, but not Ah-types in OVX females. In contrast, the sensitivity of A- and C-types to histamine was upregulated by OVX. These data demonstrate gender differences in VGN chemosensitivity to histamine for the first time. Myelinated Ah-types showed the highest sensitivity to histamine across female populations, which was changed by OVX. These novel findings improve the understanding of gender differences in the prevalence of asthma, anaphylaxis, and pain. Changes in sensitivity to histamine by OVX may explain alterations in the prevalence of certain pathophysiological conditions when women reach a postmenopausal age.

Identifying spinal sensory pathways activated by noxious esophageal acid

BACKGROUND

The transient receptor potential vanilloid 1 (TRPV1) channel is critical for spinal afferent signaling of burning pain throughout the body. Such pain frequently originates from the esophagus, following acid reflux. The contribution of TRPV1 to spinal nociceptor signaling from the esophagus remains unclear. We aimed to identify the spinal afferent pathways that convey nociceptive signaling from the esophagus, specifically those sensitive to acid, and the extent to which TRPV1 contributes.

METHODS

Acid/pepsin (150 mM HCl/1 mg mL(-1) pepsin) or saline/pepsin was perfused into the esophageal lumen of anesthetized wild-type and TRPV1 null mice over 20 min, followed by atraumatic perfuse fixation and removal of the cervical and thoracic spinal cord and dorsal root ganglia (DRG). To identify neurons responsive to esophageal perfusate, immunolabeling for neuronal activation marker phosphorylated extracellular receptor-regulated kinase (pERK) was used. Labeling for calcitonin gene-related peptide (CGRP) and isolectin B4 (IB4) was then used to characterize responsive neurons.

KEY RESULTS

Esophageal acid/pepsin perfusion significantly increased the number of pERK-immunoreactive (IR) neurons in the DRG and the cervical and thoracic spinal cord dorsal horn (DH) relative to saline/pepsin (DRG P < 0.01; cervical DH P < 0.05 and thoracic DH P < 0.005). The number of pERK-IR neurons following acid perfusion was significantly attenuated in TRPV1 -/- mice (DH P < 0.05 and DRG P < 0.05).

CONCLUSIONS & INFERENCES

This study has identified populations of spinal afferent DRG neurons and DH neurons involved in signaling of noxious acid from the esophagus. There is a major contribution of TRPV1 to signaling within these pathways.

A focus on mast cells and pain

Mast cells (MCs) are immunocytes with secretory functions that act locally in peripheral tissues to modulate local hemodynamics, nociceptor activation and pain. They are also able to infiltrate the central nervous system (CNS), especially the spinal cord and the thalamus, but their cerebral function remains an enigma. A role in regulating the opening of the blood-brain barrier has been proposed. Paracrine-like action of MCs on synaptic transmission might also signal a modulation of the nervous system by the immune system. In this review, we examine the link between MCs and nociceptive process, at the periphery as well as in the CNS.

Role of prostaglandin D2 in mast cell activation-induced sensitization of esophageal vagal afferents

Sensitization of esophageal afferents plays an important role in esophageal nociception, but the mechanism is less clear. Our previous studies demonstrated that mast cell (MC) activation releases the preformed mediators histamine and tryptase, which play important roles in sensitization of esophageal vagal nociceptive C fibers. PGD2 is a lipid mediator released by activated MCs. Whether PGD2 plays a role in this sensitization process has yet to be determined. Expression of the PGD2 DP1 and DP2 receptors in nodose ganglion neurons was determined by immunofluorescence staining, Western blotting, and RT-PCR. Extracellular recordings were performed in ex vivo esophageal-vagal preparations. Action potentials evoked by esophageal distension were compared before and after perfusion of PGD2, DP1 and DP2 receptor agonists, and MC activation, with or without pretreatment with antagonists. The effect of PGD2 on 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI)-labeled esophageal nodose neurons was determined by patch-clamp recording. Our results demonstrate that DP1 and DP2 receptor mRNA and protein were expressed mainly in small- and medium-diameter neurons in nodose ganglia. PGD2 significantly increased esophageal distension-evoked action potential discharges in esophageal nodose C fibers. The DP1 receptor agonist BW 245C mimicked this effect. PGD2 directly sensitized DiI-labeled esophageal nodose neurons by decreasing the action potential threshold. Pretreatment with the DP1 receptor antagonist BW A868C significantly inhibited PGD2 perfusion- or MC activation-induced increases in esophageal distension-evoked action potential discharges in esophageal nodose C fibers. In conclusion, PGD2 plays an important role in MC activation-induced sensitization of esophageal nodose C fibers. This adds a novel mechanism of visceral afferent sensitization.

Evaluation of the effect of GABA(B) agonists on the vagal nodose C-fibers in the esophagus

Clinical studies showed that GABA(B) receptor agonists improve symptoms in patients with gastroesophageal reflux disease. One proposed mechanism of this effect is direct inhibition of the gastroesophageal vagal tension mechanosensors by GABA(B) agonists leading to reduction of reflux. In addition to tension mechanosensors, the vagal nodose ganglion supplies the esophagus with nociceptive C-fibers that likely contribute to impairment of esophageal reflex regulation in diseases. We hypothesized that GABA(B) agonists inhibit mechanically-induced activation of vagal esophageal nodose C-fibers in baseline and/or in sensitized state induced by inflammatory mediators. Ex vivo extracellular recordings were made from the esophageal nodose C-fibers in the isolated vagally-innervated guinea pig esophagus. We found that the selective GABA(B) agonist baclofen (100-300 microM) did not inhibit activation of esophageal nodose C-fibers evoked by esophageal distention (10-60 mmHg). The mechanical response of esophageal nodose C-fibers can be sensitized by different pathways including the stimulation of the histamine H(1) receptor and the stimulation the adenosine A(2A) receptor. Baclofen failed to inhibit mechanical sensitization of esophageal nodose C-fibers induced by histamine (100 microM) or the selective adenosine A(2A) receptor agonist CGS21680 (3 nM). Our data suggest that the direct mechanical inhibition of nodose C-fibers in the esophagus is unlikely to contribute to beneficial effects of GABA(B) agonists in patients with esophageal diseases.

Mast cells: an expanding pathophysiological role from allergy to other disorders

The mast cells are multi-effector cells with wide distribution in the different body parts and traditionally their role has been well-defined in the development of IgE-mediated hypersensitivity reactions including bronchial asthma. Due to the availability of genetically modified mast cell-deficient mice, the broadened pathophysiological role of mast cells in diverse diseases has been revealed. Mast cells exert different physiological and pathophysiological roles by secreting their granular contents, including vasoactive amines, cytokines and chemokines, and various proteases, including tryptase and chymase. Furthermore, mast cells also synthesize plasma membrane-derived lipid mediators, including prostaglandins and leukotrienes, to produce diverse biological actions. The present review discusses the pathophysiological role of mast cells in different diseases, including atherosclerosis, pulmonary hypertension, ischemia-reperfusion injury, male infertility, autoimmune disorders such as rheumatoid arthritis and multiple sclerosis, bladder pain syndrome (interstitial cystitis), anxiety, Alzheimer's disease, nociception, obesity and diabetes mellitus.

ERK1/2 signaling pathway in mast cell activation-induced sensitization of esophageal nodose C-fiber neurons

Sensitization of esophageal nociceptive afferents by inflammatory mediators plays an important role in esophageal inflammatory nociception. Our previous studies demonstrated that esophageal mast cell activation increases the excitability of esophageal nodose C-fibers. But the intracellular mechanism of this sensitization process is still less clear. We hypothesize that extracellular signal-regulated kinases 1 and 2 (ERK1/2) signaling pathway plays an important role in mast cell activation-induced sensitization of esophageal nodose C-fiber neurons. Mast cell activation and in vivo esophageal distension-induced phosphorylations of ERK1/2 were studied by immuno-staining and Western blot in esophageal nodose neurons. Extracellular recordings were performed from nodose neurons using ex vivo esophageal-vagal preparations with intact nerve endings in the esophagus. Nerve excitabilities were compared by action potentials evoked by esophageal distensions before and after mast cell activations with/without pretreatment of mitogen-activated protein kinases (MAPK)/ERK kinase inhibitor U0126. The expressions of phospho-ERK1/2 (p-ERK1/2) in the same nodose ganglia were then studied by Western blot. Mast cell activation enhances in vivo esophageal distension-induced phosphorylation of ERK1/2 in nodose neurons. This can be prevented by pretreatment with mast cell stabilizer cromolyn. In ex vivo esophageal-vagal preparations, both mast cell activation and proteinase-activated receptor 2 (PAR2)-activating peptide perfusion increases esophageal distension-induced mechano-excitability of esophageal nodose C-fibers and phosphorylation of ERK1/2 in nodose neurons. Pretreatment with MAPK/ERK kinase inhibitor U0126 prevents these potentiation effects. Collectively, our data demonstrated that mast cell activation enhances esophageal distension-induced mechano-excitability and phosphorylation of ERK1/2 in esophageal nodose C-fiber neurons. This reveals a new intracellular pathway of esophageal peripheral sensitization and inflammatory nociception.

Neuroplasticity in vagal afferent neurons involved in cough

Neurotrophic factors are produced in the airways during allergic and viral inflammation. Their selective interaction with cognate receptors on sensory nerves likely accounts for some of the neuroplasticity that can accompany inflammatory diseases. We have previously described a nodose Aδ fiber in the guinea pig trachea that evokes cough upon stimulation. These nerves do not express TRPV1 and accordingly are capsaicin-insensitive. We evaluated the neurotrophic factor expression in nodose tracheal Aδ fiber neurons using single identified neuron RT-PCR. We found these neuron expressed mainly TRKB; the receptor for brain-derived neurotrophic factor, (BDNF) and NT4. They also expressed GFRα1; the receptor for glial-derived neurotrophic factor (GDNF). Treating the trachea with BDNF, to activate the TRKB receptors, caused a phenotypic change in the vast majority of nodose Aδ neurons such that they expressed TRPV1. These results strengthen the conclusion that the phenotypic characteristics of afferent nerves involved in cough may vary, depending on the context in which they are studied.

Adenosine-induced activation of esophageal nociceptors

Clinical studies implicate adenosine acting on esophageal nociceptive pathways in the pathogenesis of noncardiac chest pain originating from the esophagus. However, the effect of adenosine on esophageal afferent nerve subtypes is incompletely understood. We addressed the hypothesis that adenosine selectively activates esophageal nociceptors. Whole cell perforated patch-clamp recordings and single-cell RT-PCR analysis were performed on the primary afferent neurons retrogradely labeled from the esophagus in the guinea pig. Extracellular recordings were made from the isolated innervated esophagus. In patch-clamp studies, adenosine evoked activation (inward current) in a majority of putative nociceptive (capsaicin-sensitive) vagal nodose, vagal jugular, and spinal dorsal root ganglia (DRG) neurons innervating the esophagus. Single-cell RT-PCR analysis indicated that the majority of the putative nociceptive (transient receptor potential V1-positive) neurons innervating the esophagus express the adenosine receptors. The neural crest-derived (spinal DRG and vagal jugular) esophageal nociceptors expressed predominantly the adenosine A(1) receptor while the placodes-derived vagal nodose nociceptors expressed the adenosine A(1) and/or A(2A) receptors. Consistent with the studies in the cell bodies, adenosine evoked activation (overt action potential discharge) in esophageal nociceptive nerve terminals. Furthermore, the neural crest-derived jugular nociceptors were activated by the selective A(1) receptor agonist CCPA, and the placodes-derived nodose nociceptors were activated by CCPA and/or the selective adenosine A(2A) receptor CGS-21680. In contrast to esophageal nociceptors, adenosine failed to stimulate the vagal esophageal low-threshold (tension) mechanosensors. We conclude that adenosine selectively activates esophageal nociceptors. Our data indicate that the esophageal neural crest-derived nociceptors can be activated via the adenosine A(1) receptor while the placodes-derived esophageal nociceptors can be activated via A(1) and/or A(2A) receptors. Direct activation of esophageal nociceptors via adenosine receptors may contribute to the symptoms in esophageal diseases.

Understanding eosinophilic esophagitis: the cellular and molecular mechanisms of an emerging disease

Eosinophilic esophagitis (EoE) has been increasingly recognized as a unique clinicopathological entity over the past two decades. In this short time, the mechanisms of a complex disease have begun to emerge. Patient studies suggest that EoE is an immunologic disease related to atopy. At the cellular level, eosinophils, mast cells, and B and T lymphocytes are increased in the esophageal mucosa in a patchy distribution throughout the length of the esophagus. Laboratory investigations have implicated aeroallergens, food allergens, and a unique T helper type 2 cytokine profile. EoE appears to be an antigen-driven hypersensitivity reaction characterized by a mixed IgE-dependent/delayed-type reaction and a distinct cascade of cytokines and growth factors. The causative events that lead to EoE in humans remain unknown.

Pharmacologic attenuation of pelvic pain in a murine model of interstitial cystitis

BACKGROUND

Interstitial cystitis/painful bladder syndrome (IC/PBS) is a bladder disease that causes debilitating pelvic pain of unknown origin, and IC/PBS symptoms correlate with elevated bladder lamina propria mast cell counts. Similar to IC/PBS patients, pseudorabies virus (PRV) infection in mice induces a neurogenic cystitis associated with bladder lamina propria mast cell accumulation and pelvic pain. We evaluated several drugs to determine the effectiveness of reducing PRV-induced pelvic pain.

METHODS

Neurogenic cystitis was induced by the injection of Bartha's strain of PRV into the abductor caudalis dorsalis tail base muscle of female C57BL/6 mice. Therapeutic modulation of pelvic pain was assessed daily for five days using von Frey filament stimulation to the pelvic region to quantify tactile allodynia.

RESULTS

Significant reduction of PRV-induced pelvic pain was observed for animals treated with antagonists of neurokinin receptor 1 (NK1R) and histamine receptors. In contrast, the H1R antagonist hydroxyzine, proton pump inhibitors, a histamine receptor 3 agonist, and gabapentin had little or no effect on PRV-induced pelvic pain.

CONCLUSION

These data demonstrate that bladder-associated pelvic pain is attenuated by antagonists of NK1R and H2R. Therefore, NK1R and H2R represent direct therapeutic targets for pain in IC/PBS and potentially other chronic pain conditions.

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.

TRPA1 in mast cell activation-induced long-lasting mechanical hypersensitivity of vagal afferent C-fibers in guinea pig esophagus

Sensitization of esophageal sensory afferents by inflammatory mediators plays an important role in esophageal nociception. We have shown esophageal mast cell activation induces long-lasting mechanical hypersensitivity in vagal nodose C-fibers. However, the roles of mast cell mediators and downstream ion channels in this process are unclear. Mast cell tryptase via protease-activated receptor 2 (PAR2)-mediated pathways sensitizes sensory nerves and induces hyperalgesia. Transient receptor potential A1 (TRPA1) plays an important role in mechanosensory transduction and nociception. Here we tested the hypothesis that mast cell activation via a PAR2-dependent mechanism sensitizes TRPA1 to induce mechanical hypersensitivity in esophageal vagal C-fibers. The expression profiles of PAR2 and TRPA1 in vagal nodose ganglia were determined by immunostaining, Western blot, and RT-PCR. Extracellular recordings from esophageal nodose neurons were performed in ex vivo guinea pig esophageal-vagal preparations. Action potentials evoked by esophageal distention and chemical perfusion were compared. Both PAR2 and TRPA1 expressions were identified in vagal nodose neurons by immunostaining, Western blot, and RT-PCR. Ninety-one percent of TRPA1-positive neurons were of small and medium diameters, and 80% coexpressed PAR2. Esophageal mast cell activation significantly enhanced the response of nodose C-fibers to esophageal distension (mechanical hypersensitivity). This was mimicked by PAR2-activating peptide, which sustained for 90 min after wash, but not by PAR2 reverse peptide. TRPA1 inhibitor HC-030031 pretreatment significantly inhibited mechanical hypersensitivity induced by either mast cell activation or PAR2 agonist. Collectively, our data provide new evidence that sensitizing TRPA1 via a PAR2-dependent mechanism plays an important role in mast cell activation-induced mechanical hypersensitivity of vagal nodose C-fibers in guinea pig esophagus.

Cooperation between mast cells and neurons is essential for antigen-mediated bronchoconstriction

Mast cells are important sentinels guarding the interface between the environment and the body: a breach in the integrity of this interface can lead to the release of a plethora of mediators that engage the foreign agent, recruit leukocytes, and initiate adaptive physiological changes in the organism. While these capabilities make mast cells critical players in immune defense, it also makes them important contributors to the pathogenesis of diseases such as asthma. Mast cell mediators induce dramatic changes in smooth muscle physiology, and the expression of receptors for these factors by smooth muscle suggests that they act directly to initiate constriction. Contrary to this view, we show herein that mast cell-mediated bronchoconstriction is observed only in animals with intact innervation of the lung and that serotonin release alone is required for this action. While ablation of sensory neurons does not limit bronchoconstriction, constriction after Ag challenge is absent in mice in which the cholinergic pathways are compromised. Linking mast cell function to the cholinergic system likely provides an important means of modulating the function of these resident immune cells to physiology of the lung, but may also provide a safeguard against life-threatening anaphylaxis during mast cell degranulation.

Characterization of mast cell subtypes, distribution, and antigen-induced activation in the guinea pig esophagus

The number of esophageal mucosa mast cells (MCs) increases in allergic and inflammation conditions in the esophagus, but their role in these conditions is less clear. MCs are derived from bone marrow, migrate and mature in the peripheral tissues. Two subsets of MCs have been characterized as mucosal MC (MMC) and connective tissue MC (CTMC) defined by anatomic location, granule contents, and functions. Whether esophageal MCs share typical features with either MMC or CTMC has yet to be determined. This study characterized esophageal MCs subtypes, distribution, antigen-induced sensitization, and degranulation as measured by MC staining and histamine release assay. Immunofluorescent double staining of MC tryptase and chymase were performed in the esophagus, intestine, and skin from normal and ovalbumin (OVA) actively sensitized guinea pigs. Histamine release was measured in the esophagus from OVA-sensitized guinea pigs following in vitro antigen challenge. Similar to the MCs in the intestine and skin, esophageal MCs contained three subtypes, which included 62% MCtc (tryptase+/chymase+), 17% MCc (chymase+/tryptase-), and 21% MCt (tryptase+/chymase-). In contrast to the ileal MCs, which were distributed all over the mucosa, submucosa, and serosa, MCs in the esophagus almost all (more than 98%) lined along the lamina propria. OVA active sensitization significantly increased the esophageal MC subtype MCtc. OVA in vitro challenge of the esophagus from sensitized guinea pig significantly decreased tryptase-positive MC subtypes MCtc and MCt, and released a significant amount of tissue histamine content. In conclusion, MCs in the guinea pig esophagus have unique features in immunophenotypes, distribution, and degranulation response to OVA challenge with the release of significant amounts of proteases and histamine into the tissue. These characteristics may indicate that OVA in vitro challenge in OVA-sensitized guinea pig esophagus could be a good model to study the role of esophageal MCs in allergic and inflammation conditions.

TRPA1 in bradykinin-induced mechanical hypersensitivity of vagal C fibers in guinea pig esophagus

Bradykinin (BK) activates sensory nerves and causes hyperalgesia. Transient receptor potential A1 (TRPA1) is expressed in sensory nerves and mediates cold, mechanical, and chemical nociception. TRPA1 can be activated by BK. TRPA1 knockout mice show impaired responses to BK and mechanical nociception. However, direct evidence from sensory nerve terminals is lacking. This study aims to determine the role of TRPA1 in BK-induced visceral mechanical hypersensitivity. Extracellular recordings of action potentials from vagal nodose and jugular neurons are performed in an ex vivo guinea pig esophageal-vagal preparation. Peak frequencies of action potentials of afferent nerves evoked by esophageal distension and chemical perfusion are recorded and compared. BK activates most nodose and all jugular C fibers. This activation is repeatable and associated with a significant increase in response to esophageal distension, which can be prevented by the B2 receptor antagonist WIN64338. TRPA1 agonist allyl isothiocyanate (AITC) activates most BK-positive nodose and jugular C fibers. This is associated with a transient loss of response to mechanical distensions and desensitization to a second AITC perfusion. Desensitization with AITC and pretreatment with TRPA1 inhibitor HC-030031 both inhibit BK-induced mechanical hypersensitivity but do not affect BK-evoked activation in nodose and jugular C fibers. In contrast, esophageal vagal afferent Adelta fibers do not respond to BK or AITC and fail to show mechanical hypersensitivity after BK perfusion. This provides the first evidence directly from visceral sensory afferent nerve terminals that TRPA1 mediates BK-induced mechanical hypersensitivity. This reveals a novel mechanism of visceral peripheral sensitization.

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.

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

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

Mast cell-derived histamine mediates cystitis pain

Background

Mast cells trigger inflammation that is associated with local pain, but the mechanisms mediating pain are unclear. Interstitial cystitis (IC) is a bladder disease that causes debilitating pelvic pain of unknown origin and without consistent inflammation, but IC symptoms correlate with elevated bladder lamina propria mast cell counts. We hypothesized that mast cells mediate pelvic pain directly and examined pain behavior using a murine model that recapitulates key aspects of IC.

Methods and Findings

Infection of mice with pseudorabies virus (PRV) induces a neurogenic cystitis associated with lamina propria mast cell accumulation dependent upon tumor necrosis factor alpha (TNF), TNF-mediated bladder barrier dysfunction, and pelvic pain behavior, but the molecular basis for pelvic pain is unknown. In this study, both PRV-induced pelvic pain and bladder pathophysiology were abrogated in mast cell-deficient mice but were restored by reconstitution with wild type bone marrow. Pelvic pain developed normally in TNF- and TNF receptor-deficient mice, while bladder pathophysiology was abrogated. Conversely, genetic or pharmacologic disruption of histamine receptor H1R or H2R attenuated pelvic pain without altering pathophysiology.

Conclusions

These data demonstrate that mast cells promote cystitis pain and bladder pathophysiology through the separable actions of histamine and TNF, respectively. Therefore, pain is independent of pathology and inflammation, and histamine receptors represent direct therapeutic targets for pain in IC and other chronic pain conditions.