Effect of reflux-induced inflammation on transient receptor potential vanilloid one (TRPV1) expression in primary sensory neurons innervating the oesophagus of rats

Revisiting the Role of Esophageal Mucosal Dilated Intercellular Spaces in the Diagnosis and Pathophysiology of Heartburn

Background/Aims

Dilated intercellular spaces (DISs) facilitate the diffusion of noxious agents into the deep layers of the esophageal epithelium. The role of DIS in heartburn pathogenesis is still controversial. Therefore, we aim to reinvestigate DIS in an extensively evaluated group of patients and healthy controls (HCs).

Methods

We classified 149 subjects into the following groups: 15 HC, 58 mild erosive reflux disease (ERD), 17 severe ERD, 25 nonerosive reflux disease (NERD), 15 reflux hypersensitivity (RH), and 19 functional heartburn (FH). A total of 100 length measurements were performed for each patient's biopsy.

Results

The overall intercellular spaces (ISs) value of gastroesophageal reflux disease (GERD) patients was higher than that of HC (P = 0.020). In phenotypes, mild ERD (vs HC [P = 0.036], NERD [P = 0.004], RH [P = 0.014]) and severe ERD (vs HC [P = 0.002], NERD [P < 0.001], RH [P = 0.001], FH [P = 0.004]) showed significantly higher IS. There was no significant difference between the HC, NERD, RH, and FH groups. The 1.12 μm DIS cutoff value had 63.5% sensitivity and 66.7% specificity in the diagnosis of GERD. There was a weak correlation (r = 0.302) between the IS value and acid exposure time, and a weak correlation (r = -0.359) between the IS value and baseline impedance. A strong correlation was shown between acid exposure time and baseline impedance (r = -0.783).

Conclusions

Since the IS length measurement had better discrimination power only in erosive groups, it is not feasible to use in daily routine to discriminate other nonerosive phenotypes and FH. The role of DIS in heartburn in nonerosive patients should be reconsidered.

TRPV1 is a risk factor for sleep disturbance in patients with gastro-oesophageal reflux disease: a case control study

BACKGROUND/AIMS

Gastro-oesophageal reflux disease (GORD) is a chronic high-morbidity disease with a bidirectional relationship with sleep disturbance (SD) that may occur via the transient receptor potential vanilloid type 1 receptor (TRPV1) in the oesophageal mucosa. Yet the related mechanism was still unclear, the aim of this study is to investigate whether TRPV1 is associated with the presence of SD in GORD patients.

METHODS

A case-control study was performed. After the screening, A total of 88 subjects were assigned to GORD without sleep disturbance (GORD + NOSD, n = 28), GORD comorbid sleep disturbance (GORD + SD, n = 30) and matched healthy controls (n = 30). Mucosal tissue was obtained from the participants by digestive endoscopy, the levels of TRPV1 expressed in the oesophageal mucosa were detected via RT-qPCR and western blot in different groups, and the correlation between GORD and SD were also analysed.

RESULTS

In this study, we found that the Gastroesophageal Reflux Disease Diagnostic Questionnaire (GerdQ) scores was positively correlated with Pittsburgh Sleep Quality Index (PSQI) scores but negatively correlated with total sleep time (TST). We also found that the level of TRPV1 expressed in the oesophageal mucosa of GORD + SD was significantly higher than GORD + NOSD patients, and they were all higher than healthy controls.

CONCLUSION

The current study suggested a closer link exists between GORD and sleep disturbance, and TRPV1 in oesophageal mucosa may be a crucial factor affecting sleep in GORD patients.

Localization of substance P (SP)-immunoreactivity in the myenteric plexus of the rat esophagus

The present immunohistochemical study was performed to examine the number, distribution, and chemical coding of intrinsic substance P (SP) neurons and nerve fibers within the esophagus and discuss their functional roles. Many SP neurons and nerve fibers were found in the myenteric plexus, and the SP neurons gradually decreased from the oral side toward the aboral side of the esophagus. Double-immunolabeling showed that most SP neurons were cholinergic (positive for choline acetyltransferase), and few were nitrergic (positive for nitric oxide synthase). Some cholinergic SP nerve terminals surrounded cell bodies of several myenteric neurons. In the muscularis mucosa and lower esophageal sphincter, and around blood vessels, numerous SP nerve endings were present, and many of them were cholinergic. Also, SP nerve endings were found on only a few motor endplates of the striated muscles, and most of them were calcitonin gene-related peptide (CGRP)-positive. Retrograde tracing using Fast Blue (FB) showed that numerous sensory neurons in the dorsal root ganglia (DRGs) and nodose ganglion (NG) projected to the esophagus, and most FB-labeled SP neurons were CGRP-positive. These results suggest that the intrinsic SP neurons in the rat esophagus may play roles as, at least, motor neurons, interneurons, and vasomotor neurons, which are involved in local regulation of smooth muscle motility, neuronal transmission, and blood circulation, respectively. Moreover, SP nerve endings on only a minority of motor endplates may be extrinsic, derived from DRGs or NG, and possibly detect chemical circumstances within motor endplates to modulate esophageal motility.

Gastric vagal afferent neuropathy following experimental spinal cord injury

Dramatic impairment of gastrointestinal (GI) function accompanies high-thoracic spinal cord injury (T3-SCI). The vagus nerve contains mechano- and chemosensory fibers as well as the motor fibers necessary for the central nervous system (CNS) control of GI reflexes. Cell bodies for the vagal afferent fibers are located within the nodose gangla (NG) and the majority of vagal afferent axons are unmyelinated C fibers that are sensitive to capsaicin through activation of transient receptor potential vanilloid-1 (TRPV1) channels. Vagal afferent fibers also express receptors for GI hormones, including cholecystokinin (CCK). Previously, T3-SCI provokes a transient GI inflammatory response as well as a reduction of both gastric emptying and centrally-mediated vagal responses to GI peptides, including CCK. TRPV1 channels and CCK-A receptors (CCKar) expressed in vagal afferents are upregulated in models of visceral inflammation. The present study investigated whether T3-SCI attenuates peripheral vagal afferent sensitivity through plasticity of TRPV1 and CCK receptors. Vagal afferent response to graded mechanical stimulation of the stomach was significantly attenuated by T3-SCI at 3-day and 3-week recovery. Immunocytochemical labeling for CCKar and TRPV1 demonstrated expression on dissociated gastric-projecting NG neurons. Quantitative assessment of mRNA expression by qRT-PCR revealed significant elevation of CCKar and TRPV1 in the whole NG following T3-SCI in 3-day recovery, but levels returned to normal after 3-weeks. Three days after injury, systemic administration of CCK-8 s showed a significantly diminished gastric vagal afferent response in T3-SCI rats compared to control rats while systemic capsaicin infusion revealed a significant elevation of vagal response in T3-SCI vs control rats. These findings demonstrate that T3-SCI provokes peripheral remodeling and prolonged alterations in the response of vagal afferent fibers to the physiological signals associated with digestion.

The Esophageal Epithelial Barrier in Health and Disease

Dysfunction in the esophageal epithelial barrier function is a major source for morbidity. To better understand the pathophysiologic pathways of the diseases associated with barrier dysfunction, including gastroesophageal reflux disease, eosinophilic esophagitis, Barrett's esophagus, and obesity, it is important to understand the esophageal epithelial embryologic development, microscopic anatomy with a special focus on the barrier structure and function, extraepithelial defense mechanisms, and how these change in the diseased state. In recent years, significant progress has been made in elucidating the esophageal barrier structure and function both in vitro and in vivo. This has enhanced the understanding of mechanisms of disease, and may also allow identification of therapeutic targets that can help in the management of these diseases. This review provides a detailed discussion regarding the esophageal epithelial barrier structure and function, the current and historical techniques used to study the barrier, and how it is affected by common esophageal diseases.

Role of TRPV1 receptor in inflammation and impairment of esophageal mucosal integrity in a murine model of nonerosive reflux disease

BACKGROUND

Microscopic inflammation and impairment of the esophageal epithelial barrier are considered relevant for perception of symptoms in patients with nonerosive reflux disease (NERD). In these patients, the receptor transient receptor potential vanilloid 1 (TRPV1) is overexpressed in the esophageal mucosa, but its role is not yet fully understood. We evaluated the role of TRPV1 in esophageal inflammation and mucosal barrier impairment in a murine model of NERD.

METHODS

Nonerosive reflux disease was surgically induced in Swiss mice by pyloric substenosis and ligature of the gastric fundus, and the mice were killed 7 days post surgery. The experimental groups were: I, sham surgery (negative control); II, NERD untreated; III and IV, NERD + SB366791 or capsazepine (TRPV1 antagonists); and V, NERD + resiniferatoxin (for long-term desensitization of TRPV1). The esophagus was collected for western blotting and histopathology and for evaluation of wet weight, myeloperoxidase (MPO), keratinocyte-derived chemokine (KC), transepithelial electrical resistance (TEER), and basal permeability to fluorescein.

KEY RESULTS

Compared to sham, NERD mice had increased esophageal wet weight and MPO and KC levels. The mucosa had no ulcers but exhibited inflammation. NERD mice showed mucosal TRPV1 overexpression, a more pronounced decrease in TEER at pH 0.5 (containing pepsin and taurodeoxycholic acid), and increased basal permeability. Pharmacological modulation of TRPV1 prevented esophageal inflammation development, TEER changes by acidic exposure, and increase in esophageal permeability.

CONCLUSIONS & INFERENCES

The TRPV1 receptor has a critical role in esophageal inflammation and mucosal barrier impairment in NERD mice, suggesting that TRPV1 might be a pharmacological target in patients with NERD.

Expression of CGRP neurotransmitter and vascular genesis marker mRNA is age-dependent in superior cervical ganglia of senescence-accelerated prone mice

Calcitonin gene-related peptide (CGRP) is a neurotransmitter that is released from the superior cervical ganglion (SCG) and causes head and neck pain. The morphological properties of human SCG neurons, including neurotransmitter content, are altered during aging. However, morphological changes in CGRP in the SCG during aging are not known. Therefore, we investigated CGRP and other markers in the SCG during aging in an aging model of senescence-accelerated prone mouse (SAMP8) and senescence-accelerated resistant mice (SAMR1) using real-time RT-PCR mRNA analyses and in situ hybridization. The abundance of neurotransmitter (CGRP, NPY, TRPV1), vascular genesis marker (CD31, LYVE-1), and cytochrome C mRNA differed between 12-week-old and 24-week-old SAMP8 and SAMR1. Abundance of TRPV1, CD31 and cytochrome C mRNAs of SAMP8 decreased between 12- and 24-week-old. The ratio of CGRP mRNA positive cells and CGRP mRNA abundance levels of the SCG of aging mouse such as SAMP8 have already been also higher than that of SAMR1 at 12-week-old. The CGRP positive shrunken ganglion cells was increased from 12- to 24-weeks-old mouse in SAMR1 and SAMP8. The SCG primarily affected the internal and external carotid arteries, larynx thyroid gland, and pharyngeal muscle during aging.

A novel murine model of esophageal nonerosive reflux disease: from inflammation to impairment in mucosal integrity

Nonerosive reflux disease (NERD) is a highly prevalent phenotype of the gastroesophageal reflux disease. In this study, we developed a novel murine model of NERD in mice with microscopic inflammation and impairment in the epithelial esophageal barrier. Female Swiss mice were subjected to the following surgical procedure: the transitional region between the forestomach and the glandular portion of the stomach was ligated, and a nontoxic ring was placed around the duodenum near the pylorus. The control group underwent sham surgery. The animals were euthanized at 1, 3, 7, and 14 days after surgery. Survival and body weight were monitored daily. Esophageal wet weight, macroscopic lesion, histopathological alterations, myeloperoxidase (MPO) activity, cytokine levels, transepithelial electrical resistance (TEER), and mucosal permeability were evaluated. The survival rate was 78% at 14 days, with mild loss in body weight. Surgery did not induce erosive esophagitis but instead induced microscopic inflammation and increased esophageal wet weight, IL-6, keratinocyte-derived cytokine (KC) levels, and MPO activity with maximal peak between 3 and 7 days and resolution at 14 days postsurgery. Epithelial esophageal barrier was evaluated in operated mice at 7 and 14 days postsurgery; a decrease in TEER and increase in the esophageal epithelial permeability were observed compared with the sham-operated group. In addition, the inhibition of acid secretion with omeprazole significantly prevented the esophageal inflammation and impairment of barrier function at 7 days postsurgery. Thus we established a novel experimental model of NERD in mice, which can contribute to understanding the pathophysiological events associated with NERD.NEW & NOTEWORTHY In this study, we standardized an experimental model of nonerosive reflux disease (NERD) in mice. This model involves an acute inflammatory response followed by impaired esophageal mucosal integrity, even in the absence of inflammation. Thus this model can serve for evaluation of pathophysiological aspects of NERD and open new perspectives for therapeutic strategies for patients with this disorder.

Capsaicin, Nociception and Pain

Capsaicin, the pungent ingredient of the hot chili pepper, is known to act on the transient receptor potential cation channel vanilloid subfamily member 1 (TRPV1). TRPV1 is involved in somatic and visceral peripheral inflammation, in the modulation of nociceptive inputs to spinal cord and brain stem centers, as well as the integration of diverse painful stimuli. In this review, we first describe the chemical and pharmacological properties of capsaicin and its derivatives in relation to their analgesic properties. We then consider the biochemical and functional characteristics of TRPV1, focusing on its distribution and biological effects within the somatosensory and viscerosensory nociceptive systems. Finally, we discuss the use of capsaicin as an agonist of TRPV1 to model acute inflammation in slices and other ex vivo preparations.

Inhibition of p38 MAPK activation attenuates esophageal mucosal damage in a chronic model of reflux esophagitis

BACKGROUND

Reflux esophagitis (RE) is one of the common gastrointestinal diseases that are increasingly recognized as a significant health problem. This study was designed to investigate the role of p38 mitogen-activated protein kinase (MAPK) in experimental chronic RE model of rats.

METHODS

Chronic acid RE rats were induced by fundus ligation and partial obstruction of the pylorus and treated with SB203580 (a p38 MAPK inhibitor, i.p., 1 mg/kg/day) for 14 days.

KEY RESULTS

Immunohistochemical staining and Western blotting results revealed the activation of p38 MAPK signaling in the esophagus mucosa 14 days post injury. Through gross and histological assessment, we found that inhibition of p38 MAPK activation by SB203580 attenuated esophageal mucosal damage in RE rats. Inhibition of p38 MAPK activation in RE rats attenuated esophageal barrier dysfunction, through enhancing the expression of tight junction proteins and reducing the expression of matrix matalloproteinases-3 and -9. Inhibition of p38 MAPK activation in RE rats reduced CD68-positive cells in esophagus mucosa and mRNA levels of tumor necrosis factor (TNF)-α, interleukin (IL)-6, and IL-1β in esophagus and protein levels of TNF-α, IL-6, and IL-1β in serum. In addition, we found that inhibition of p38 MAPK activation in RE rats suppressed protein expression of inducible nitric oxide synthase and reduced formation of nitric oxide (NO), 3-nitrotyrosin, and malondialdehyde in esophagus.

CONCLUSIONS & INFERENCES

Inhibition of p38 MAPK activation attenuated esophageal mucosal damage in acid RE rats, possibly by modulating esophageal barrier function and regulating inflammatory cell recruitment, and the subsequent formation of cytokines, NO, and reactive oxygen species.

Role of satellite glial cells in gastrointestinal pain

Gastrointestinal (GI) pain is a common clinical problem, for which effective therapy is quite limited. Sensations from the GI tract, including pain, are mediated largely by neurons in the dorsal root ganglia (DRG), and to a smaller extent by vagal afferents emerging from neurons in the nodose/jugular ganglia. Neurons in rodent DRG become hyperexcitable in models of GI pain (e.g., gastric or colonic inflammation), and can serve as a source for chronic pain. Glial cells are another element in the pain signaling pathways, and there is evidence that spinal glial cells (microglia and astrocytes) undergo activation (gliosis) in various pain models and contribute to pain. Recently it was found that satellite glial cells (SGCs), the main type of glial cells in sensory ganglia, might also contribute to chronic pain in rodent models. Most of that work focused on somatic pain, but in several studies GI pain was also investigated, and these are discussed in the present review. We have shown that colonic inflammation induced by dinitrobenzene sulfonic acid (DNBS) in mice leads to the activation of SGCs in DRG and increases gap junction-mediated coupling among these cells. This coupling appears to contribute to the hyperexcitability of DRG neurons that innervate the colon. Blocking gap junctions (GJ) in vitro reduced neuronal hyperexcitability induced by inflammation, suggesting that glial GJ participate in SGC-neuron interactions. Moreover, blocking GJ by carbenoxolone and other agents reduces pain behavior. Similar changes in SGCs were also found in the mouse nodose ganglia (NG), which provide sensory innervation to most of the GI tract. Following systemic inflammation, SGCs in these ganglia were activated, and displayed augmented coupling and greater sensitivity to the pain mediator ATP. The contribution of these changes to visceral pain remains to be determined. These results indicate that although visceral pain is unique, it shares basic mechanisms with somatic pain, suggesting that therapeutic approaches to both pain types may be similar. Future research in this field should include additional types of GI injury and also other types of visceral pain.

TRP channel functions in the gastrointestinal tract

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

Changes in Gene Expression Patterns of Circadian-Clock, Transient Receptor Potential Vanilloid-1 and Nerve Growth Factor in Inflamed Human Esophagus

Circadian rhythm is driven by the molecular circadian-clock system and regulates many physiological functions. Diurnal rhythms in the gastrointestinal tract are known to be related to feeding pattern, but whether these rhythms are also related to the gastrointestinal damage or injuries; for example, gastroesophageal reflux disease (GERD), is unclear. This study was conducted to determine whether expression of circadian-clock genes or factors involved in vagal stimulation or sensitization were altered in the esophagus of GERD patients. Diurnal patterns of PER1, PER2, BMAL1, CRY2, TRPV1, and NGF mRNA expression were found in patient controls, and these patterns were altered and significantly correlated to the GERD severity in GERD patients. Although levels of CRY1, TIM, CB1, NHE3, GDNF, and TAC1 mRNA expression did not show diurnal patterns, they were elevated and also correlated with GERD severity in GERD patients. Finally, strong correlations among PER1, TRPV1, NGF and CRY2 mRNA expression, and among PER2, TRPV1 and CRY2 expression were found. Expression levels of CRY1 mRNA highly correlated with levels of TIM, CB1, NHE3, GDNF and TAC1. This study suggests that the circadian rhythm in the esophagus may be important for the mediation of and/or the response to erosive damage in GERD patients.

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.

Distribution of transient receptor potential cation channel subfamily V member 1-expressing nerve fibers in mouse esophagus

Transient receptor potential cation channel subfamily V member 1 (TRPV1) plays a role in esophageal function. However, the distribution of TRPV1 nerve fibers in the esophagus is currently not well understood. In the present study, we investigated the distribution of TRPV1 and neurotransmitters released from TRPV1 nerve fibers in the mouse lower esophagus. Furthermore, we investigated changes in the presence of TRPV1 in the mouse model of esophagitis. Numerous TRPV1-immunoreactive nerve fibers were seen in both the submucosal layer and myenteric plexus of the lower esophagus and colocalized with calcitonin gene-related peptide (CGRP). TRPV1 colocalized with substance P in axons in the submucosal layer and myenteric plexus. TRPV1 colocalized with neuronal nitric oxide synthase in the myenteric plexus. We observed some colocalization of CGRP with the vesicular acetylcholine (ACh) transporter, packaging of ACh into synaptic vesicles after its synthesis in terminal cytoplasm, in the submucosal layer and myenteric plexus. In the esophagitis model, the number of the TRPV1 nerve fibers did not change, but their immunoreactive intensity increased compared with sham-operated mice. Inhibitory effect of exogenous capsaicin on electrically stimulated twitch contraction significantly increased in esophagitis model compared with the effect in sham-operated mice. Overall, these results suggest that TRPV1 nerve fibers projecting to both the submucosal and muscle layer of the esophagus are extrinsic spinal and vagal afferent neurons. Furthermore, TRPV1 nerve fibers contain CGRP, substance P, nitric oxide, and ACh. Therefore, acid influx-mediated TRPV1 activation may play a role in regulating esophageal relaxation.

Effect of rikkunshito on the expression of substance P and CGRP in dorsal root ganglion neurons and voluntary movement in rats with experimental reflux esophagitis

BACKGROUND

While there are reports that the herbal medicine rikkunshito (RKT) relieves upper gastrointestinal disease symptoms, the effect of RKT on primary afferent neurons is unknown.

METHODS

A model of reflux esophagitis (RE) was implemented using male Wistar rats aged 6-7 weeks. Ten days after surgery, the total area of esophageal mucosal erosion sites was determined. Th8-10 dorsal root ganglia (DRG) were dissected out and the expression of substance P (SP), calcitonin gene-related peptide (CGRP), and phosphorylated extracellular signal-regulated kinase 1/2 (p-ERK1/2) was determined in DRG using immunohistochemistry. RKT (0.6%/WV) or omeprazole (OME) (10 mg/kg) was administered for 10 days beginning on the day after surgery. Voluntary movement was measured with an infrared sensor for 22 h each day.

KEY RESULTS

RE rats showed esophageal mucosal erosion and significantly increased number of SP/CGRP- and p-ERK1/2-immunoreactive neurons in DRG. Treatment with OME improved the size of erosive lesions in the esophageal mucosa of RE rats, while RKT did not. Treatment with RKT or OME significantly reduced the expression of SP/CGRP and p-ERK1/2 in DRG, and significantly increased voluntary movement in RE rats.

CONCLUSIONS & INFERENCES

RKT inhibited the activation of ERK1/2 and decreased the expression of SP and CGRP in DRG of RE rats, which may be associated with the observed amelioration of voluntary movement.

Mechanisms of visceral pain in health and functional gastrointestinal disorders

Background and aims Chronic visceral pain is common both in patients with identifiable organic disease and also in those without any structural, biochemical or immunological abnormality such as in the functional gastrointestinal disorders (FGIDs). We aim to provide a contemporaneous summary of pathways involved in visceral nociception and how a variety of mechanisms may influence an individual's experience of visceral pain. Methods In this narrative review, we have brought together evidence through a detailed search of Medline in addition to using our experience and exposure to recent research developments from ourselves and other research groups. Results FGIDs are a heterogeneous group of disorders whose aetiology largely remains an enigma. The germane hypothesis for the genesis and maintenance of chronic visceral pain in FGIDs is the concept of visceral hypersensitivity. A number of peripheral and central mechanisms have been proposed to account for this epiphenomenon. In the periphery, inflammatory mediators activate and sensitize nociceptive afferent nerves by reducing their transduction thresholds and by inducing the expression and recruitment of hitherto silent nociceptors culminating in an increase in pain sensitivity at the site of injury known as primary hyperalgesia. Centrally, secondary hyperalgesia, defined as an increase in pain sensitivity in anatomically distinct sites, occurs at the level of the spinal dorsal horn. Moreover, the stress responsive physiological systems, genetic and psychological factors may modulate the experience of visceral pain. We also address some novel aetiological concepts in FGIDs, namely the gastrointestinal microbiota, connective tissue abnormalities and the gastrointestinal neuromuscular disorders. Firstly, the gastrointestinal microbiota is a diverse and dynamic ecosystem, that safeguards the host from external pathogens, aids in the metabolism of polysaccharides and lipids, modulates intestinal motility, in addition to modulating visceral perception. Secondly, connective tissue disorders, which traditionally have been considered to be confined largely to the musculoskeletal system, have an increasing evidence base demonstrating the presence of visceral manifestations. Since the sensorimotor apparatus of the GI tract is embedded within connective tissue it should not be surprising that such disorder may result in visceral pain and abnormal gut motility. Thirdly, gastrointestinal neuromuscular diseases refer to a heterogeneous group of disorders in which symptoms arise from impaired GI motor activity often manifesting as abnormal transit with or without radiological evidence of transient or persistent dilation of the viscera. Although a number of these are readily recognizable, such as achalasia or Hirschsprung's disease, the cause in a number of patients is not. An international working group has recently addressed this "gap", providing a comprehensive morphologically based diagnostic criteria. Conclusions/implications Although marked advances have been made in understanding the mechanisms that contribute to the development and maintenance of visceral pain, many interventions have failed to produce tangible improvement in patient outcomes. In the last part of this review we highlight an emerging approach that has allowed the definition and delineation of temporally stable visceral pain clusters, which may improve participant homogeneity in future studies, potentially facilitate stratification of treatment in FGID and lead to improvements in diagnostic criteria and outcomes.

Medical treatments of GERD: the old and new

The mainstay of pharmacological therapy for GERD is gastric acid suppression with proton pump inhibitors (PPIs), which are superior to histamine-2 receptor antagonists for healing erosive esophagitis and achieving symptomatic relief. However, up to one-third of patients may not respond to PPI therapy, creating the need for alternative treatments. Potential approaches include transient lower esophageal sphincter relaxation inhibitors, augmentation esophageal defense mechanisms by improving esophageal clearance or enhancing epithelial repair, and modulation of sensory pathways responsible for GERD symptoms. This review discusses the effectiveness of acid suppression and the data on alternative pharmacological approaches for the treatment of GERD.

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.

Role of nociceptors/neuropeptides in the pathogenesis of visceral hypersensitivity of nonerosive reflux disease

BACKGROUND AND AIMS

Esophageal visceral hypersensitivity has been proposed to be a pathogenesis of heartburn in nonerosive reflux disease (NERD), but its further mechanisms are unclear. Recently, it has been suggested that nociceptors and neuropeptides control sensory and pain mechanisms. Therefore, the objective of the present study was to estimate expression of acid-sensitive nociceptors such as transient receptor potential vanilloid 1 (TRPV1) and acid-sensing ion channel 3, protease-activated receptor 2 (PAR2), neuropeptides such as substance P and calcitonin-gene-related peptide, and their receptors such as neurokinin 1 receptor (NK1R) and receptor activity-modifying protein 1 in the esophageal mucosa of NERD patients.

METHODS

Biopsy samples were taken from NERD patients and healthy control subjects without heartburn. The expression level of nociceptors, neuropeptides, and their receptors were assessed by real-time RT-PCR and enzyme immunoassay. Localization of substance P and CGRP in the esophageal mucosa was determined by immunohistochemical staining.

RESULTS

Expression of mRNA for TRPV1 and PAR2 was significantly elevated in the esophageal mucosa of NERD patients. Substance P protein level and its receptor NK1R mRNA also increased in NERD patients. A positive correlation between the substance P protein level and reflux symptoms was observed. Immunohistochemical study revealed the presence of substance P-positive nerves in the lamina propria of the esophagus.

CONCLUSIONS

These findings suggest that visceral hypersensitivity in NERD patients is involved in neurogenic inflammation showing the increase in both substance P release and NK1R expression, which may be associated with the activation of TRPV1 and PAR2.

MicroRNA-mediated GABA Aα-1 receptor subunit down-regulation in adult spinal cord following neonatal cystitis-induced chronic visceral pain in rats

The nociceptive transmission under pathological chronic pain conditions involves transcriptional and/or translational alteration in spinal neurotransmitters, receptor expressions, and modification of neuronal functions. Studies indicate the involvement of microRNA (miRNA) - mediated transcriptional deregulation in the pathophysiology of acute and chronic pain. In the present study, we tested the hypothesis that long-term cross-organ colonic hypersensitivity in neonatal zymosan-induced cystitis is due to miRNA-mediated posttranscriptional suppression of the developing spinal GABAergic system. Cystitis was produced by intravesicular injection of zymosan (1% in saline) into the bladder during postnatal (P) days P14 through P16 and spinal dorsal horns (L6-S1) were collected either on P60 (unchallenged groups) or on P30 after a zymosan re-challenge on P29 (re-challenged groups). miRNA arrays and real-time reverse transcription-polymerase chain reaction (RT-PCR) revealed significant, but differential, up-regulation of mature miR-181a in the L6-S1 spinal dorsal horns from zymosan-treated rats compared with saline-treated controls in both the unchallenged and re-challenged groups. The target gene analysis demonstrated multiple complementary binding sites in miR-181a for GABA(A) receptor subunit GABA(Aα-1) gene with a miRSVR score of -1.83. An increase in miR-181a concomitantly resulted in significant down-regulation of GABA(Aα-1) receptor subunit gene and protein expression in adult spinal cords from rats with neonatal cystitis. Intrathecal administration of the GABA(A) receptor agonist muscimol failed to attenuate the viscero-motor response (VMR) to colon distension in rats with neonatal cystitis, whereas in adult zymosan-treated rats the drug produced significant decrease in VMR. These results support an integral role for miRNA-mediated transcriptional deregulation of the GABAergic system in neonatal cystitis-induced chronic pelvic pain.

Neural plasticity in the gastrointestinal tract: chronic inflammation, neurotrophic signals, and hypersensitivity

Neural plasticity is not only the adaptive response of the central nervous system to learning, structural damage or sensory deprivation, but also an increasingly recognized common feature of the gastrointestinal (GI) nervous system during pathological states. Indeed, nearly all chronic GI disorders exhibit a disease-stage-dependent, structural and functional neuroplasticity. At structural level, GI neuroplasticity usually comprises local tissue hyperinnervation (neural sprouting, neural, and ganglionic hypertrophy) next to hypoinnervated areas, a switch in the neurochemical (neurotransmitter/neuropeptide) code toward preferential expression of neuropeptides which are frequently present in nociceptive neurons (e.g., substance P/SP, calcitonin-gene-related-peptide/CGRP) and of ion channels (TRPV1, TRPA1, PAR2), and concomitant activation of peripheral neural glia. The functional counterpart of these structural alterations is altered neuronal electric activity, leading to organ dysfunction (e.g., impaired motility and secretion), together with reduced sensory thresholds, resulting in hypersensitivity and pain. The present review underlines that neural plasticity in all GI organs, starting from esophagus, stomach, small and large intestine to liver, gallbladder, and pancreas, actually exhibits common phenotypes and mechanisms. Careful appraisal of these GI neuroplastic alterations reveals that--no matter which etiology, i.e., inflammatory, infectious, neoplastic/malignant, or degenerative--neural plasticity in the GI tract primarily occurs in the presence of chronic tissue- and neuro-inflammation. It seems that studying the abundant trophic and activating signals which are generated during this neuro-immune-crosstalk represents the key to understand the remarkable neuroplasticity of the GI tract.

Mechanism of capsaicin receptor TRPV1-mediated toxicity in pain-sensing neurons focusing on the effects of Na(+)/Ca(2+) fluxes and the Ca(2+)-binding protein calretinin

Transient receptor potential vanilloid subtype 1 (TRPV1) receptor is a pain-sensing, ligand-gated, non-selective cation channel expressed in peripheral sensory neurons. Prolonged activation of TRPV1 by capsaicin leads to cell swelling and formation of membrane blebs in rat dorsal root ganglion (DRG) neurons. Similar results were obtained in NIH3T3 fibroblast cells stably expressing TRPV1. Here, we assessed the contribution of Ca(2+) and Na(+) ions to TRPV1-mediated changes. Cell swelling was caused by a substantial influx of extracellular Na(+) via TRPV1 channels, causing concomitant transport of water. In the absence of extracellular Na(+), the membrane blebbing was completely inhibited, but Ca(2+) influx did not change under these conditions. Na(+) influx was modulated by the intracellular Ca(2+) concentration ([Ca(2+)]i). Elevation of [Ca(2+)]i by ionomycin sensitized/activated TRPV1 channels causing cell swelling in TRPV1-positive cells. In the absence of extracellular Ca(2+), capsaicin caused only little increase in [Ca(2+)]i indicating that the increase in [Ca(2+)]i observed after capsaicin application is derived essentially from extracellular Ca(2+) and not from internal Ca(2+) stores. In the absence of extracellular Ca(2+) also the process of cell swelling was considerably slower. Calretinin is a Ca(2+) buffer protein, which is expressed in a subset of TRPV1-positive neurons. Calretinin decreased the amplitude, but slowed down the decay of Ca(2+) signals evoked by ionomycin. Cells co-expressing TRPV1 and calretinin were less sensitive to TRPV1-mediated, capsaicin-induced volume increases. In TRPV1-expressing NIH3T3 cells, calretinin decreased the capsaicin-induced Ca(2+) and Na(+) influx. Swelling and formation of membrane blebs resulted in impaired plasma membrane integrity finally leading to cell death. Our results hint towards a mechanistic explanation for the apoptosis-independent capsaicin-evoked neuronal loss and additionally reveal a protective effect of calretinin; we propose that the Ca(2+)-buffering capacity of calretinin reduces the susceptibility of calretinin-expressing DRG neurons against cell swelling/death caused by overstimulation of TRPV1 channels. This article is part of a Special Issue entitled:12th European Symposium on Calcium.

Oxidative stress-induced posttranslational modification of TRPV1 expressed in esophageal epithelial cells

Human esophageal epithelium is continuously exposed to physical stimuli or to gastric acid that sometimes causes inflammation of the mucosa. Transient receptor potential vanilloid 1 (TRPV1) is a nociceptive, Ca(2+)-selective ion channel activated by capsaicin, heat, and protons. It has been reported that activation of TRPV1 expressed in esophageal mucosa is involved in gastroesophageal reflux disease (GERD) or in nonerosive GERD symptoms. In this study, we examined the expression and function of TRPV1 in the human esophageal epithelial cell line Het1A, focusing in particular on the role of oxidative stress. Interleukin-8 (IL-8) secreted by Het1A cells upon stimulation by capsaicin or acid with/without 4-hydroxy-2-nonenal (HNE) was measured by ELISA. Following capsaicin stimulation, the intracellular production of reactive oxygen species (ROS) was determined using a redox-sensitive fluorogenic probe, and ROS- and HNE-modified proteins were determined by Western blotting using biotinylated cysteine and anti-HNE antibody, respectively. HNE modification of TRPV1 proteins was further investigated by immunoprecipitation after treatment with synthetic HNE. Capsaicin and acid induced IL-8 production in Het1A cells, and this production was diminished by antagonists of TRPV1. Capsaicin also significantly increased the production of intracellular ROS and ROS- or HNE-modified proteins in Het1A cells. Moreover, IL-8 production in capsaicin-stimulated Het1A cells was enhanced by synthetic HNE treatment. Immunoprecipitation studies revealed that TRPV1 was modified by HNE in synthetic HNE-stimulated Het1A cells. We concluded that TRPV1 functions in chemokine production in esophageal epithelial cells, and this function may be regulated by ROS via posttranslational modification of TRPV1.

Transient receptor potential (TRP) channels as drug targets for diseases of the digestive system

Approximately 20 of the 30 mammalian transient receptor potential (TRP) channel subunits are expressed by specific neurons and cells within the alimentary canal. They subserve important roles in taste, chemesthesis, mechanosensation, pain and hyperalgesia and contribute to the regulation of gastrointestinal motility, absorptive and secretory processes, blood flow, and mucosal homeostasis. In a cellular perspective, TRP channels operate either as primary detectors of chemical and physical stimuli, as secondary transducers of ionotropic or metabotropic receptors, or as ion transport channels. The polymodal sensory function of TRPA1, TRPM5, TRPM8, TRPP2, TRPV1, TRPV3 and TRPV4 enables the digestive system to survey its physical and chemical environment, which is relevant to all processes of digestion. TRPV5 and TRPV6 as well as TRPM6 and TRPM7 contribute to the absorption of Ca²⁺ and Mg²⁺, respectively. TRPM7 participates in intestinal pacemaker activity, and TRPC4 transduces muscarinic acetylcholine receptor activation to smooth muscle contraction. Changes in TRP channel expression or function are associated with a variety of diseases/disorders of the digestive system, notably gastro-esophageal reflux disease, inflammatory bowel disease, pain and hyperalgesia in heartburn, functional dyspepsia and irritable bowel syndrome, cholera, hypomagnesemia with secondary hypocalcemia, infantile hypertrophic pyloric stenosis, esophageal, gastrointestinal and pancreatic cancer, and polycystic liver disease. These implications identify TRP channels as promising drug targets for the management of a number of gastrointestinal pathologies. As a result, major efforts are put into the development of selective TRP channel agonists and antagonists and the assessment of their therapeutic potential.

Acid sensing by visceral afferent neurones

Acidosis in the gastrointestinal tract can be both a physiological and pathological condition. While gastric acid serves digestion and protection from pathogens, pathological acidosis is associated with defective acid containment, inflammation and ischaemia. The pH in the oesophagus, stomach and intestine is surveyed by an elaborate network of acid-sensing mechanisms to maintain homeostasis. Deviations from physiological values of extracellular pH (7.4) are monitored by multiple acid sensors expressed by epithelial cells and sensory neurones. Protons evoke multiple currents in primary afferent neurones, which are carried by several acid-sensitive ion channels. Among these, acid-sensing ion channels (ASICs) and transient receptor potential (TRP) vanilloid-1 (TRPV1) ion channels have been most thoroughly studied. ASICs survey moderate decreases in extracellular pH whereas TRPV1 is activated only by severe acidosis resulting in pH values below 6. Other molecular acid sensors comprise TRPV4, TRPC4, TRPC5, TRPP2 (PKD2L1), epithelial Na(+) channels, two-pore domain K(+) (K₂(P)) channels, ionotropic purinoceptors (P2X), inward rectifier K(+) channels, voltage-activated K(+) channels, L-type Ca²(+) channels and acid-sensitive G-protein-coupled receptors. Most of these acid sensors are expressed by primary sensory neurones, although to different degrees and in various combinations. As upregulation and overactivity of acid sensors appear to contribute to various forms of chronic inflammation and pain, acid-sensitive ion channels and receptors are also considered as targets for novel therapeutics.

Esophageal sensation and esophageal hypersensitivity - overview from bench to bedside

Noxious stimuli in the esophagus activate nociceptive receptors on esophageal mucosa, such as transient receptor potential, acid-sensing ion channel and the P2X family, a family of ligand-gated ion channels responsive to ATP, and this generates signals that are transmitted to the central nervous system via either spinal nerves or vagal nerves, resulting in esophageal sensation. Among the noxious stimuli, gastric acid and other gastric contents are clinically most important, causing typical reflux symptoms such as heartburn and regurgitation. A conventional acid penetration theory has been used to explain the mechanism of heartburn, but much recent evidence does not support this theory. Therefore, it may be necessary to approach the causes of heartburn symptoms from a new conceptual framework. Hypersensitivity of the esophagus, like that of other visceral organs, includes peripheral, central and probably psychosocial factor-mediated hypersensitivity, and is known to play crucial roles in the pathoegenesis of nonerosive reflux disease, functional heartburn and non-cardiac chest pain. There also are esophagitis patients who do not perceive typical symptoms. This condition is known as silent gastroesophageal reflux disease. Although the pathogenesis of silent gastroesophageal reflux disease is still not known, hyposensitivity to reflux of acid may possibly explain the condition.

Evidence for neurotrophic factors associating with TRPV1 gene expression in the inflamed human esophagus

BACKGROUND

Transient receptor potential vanilloid-1 (TRPV1) receptor has been implicated in the mechanism of acid induced inflammation in gastro-esophageal reflux disease (GERD). It has been demonstrated that the increase in nerve growth factor (NGF) and glial derived neurotrophic factor (GDNF) was associated with the increased expression of TRPV1. We aimed to determine whether expression of TRPV1 was increased in severe inflamed human esophagus, and to test the hypothesis whether the expression of TRPV1 was mediated by neurotrophic factors such as NGF and GDNF.

METHODS

We compared biopsies taken from the distal esophagus of 15 patients with erosive GERD, 16 asymptomatic patients (AP), and 10 healthy controls. We assessed the biopsies with reverse transcription polymerase chain reaction (RT-PCR) and real-time quantitative polymerase chain reaction (qPCR) for TRPV1, NGF, and GDNF. Immunohistochemical analysis of TRPV1 protein expression was also determined.

KEY RESULTS

Transient receptor potential vanilloid-1 mRNA level and its protein expression were significantly greater in patients with erosive esophagitis than AP (P < 0.001) and healthy controls (P < 0.001). Nerve growth factor and GDNF gene levels in the esophageal mucosa were also significantly increased in patients with erosive esophagitis compared with AP and healthy controls (all P < 0.001). Transient receptor potential vanilloid-1 mRNA correlated well with NGF (r = 0.61, P < 0.001) and GDNF (r = 0.58, P < 0.001).

CONCLUSIONS & INFERENCES

These results support the association of NGF and GDNF in the up-regulation of TRPV1 receptors in patients with erosive esophagitis.

Increased TRPV1 gene expression in esophageal mucosa of patients with non-erosive and erosive reflux disease

BACKGROUND

Transient receptor potential channel vanilloid subfamily member-1 (TRPV1) may play a role in esophageal perception. TRPV1 mRNA and protein expression were examined in the esophageal mucosa of non-erosive reflux disease (NERD) and erosive esophagitis (EE) patients and correlated to esophageal acid exposure.

METHODS

Seventeen NERD patients, eight EE patients and 10 healthy subjects underwent endoscopy after a 3-week washout from proton pump inhibitors or H2 antagonists. Biopsies, obtained from the distal esophagus, were used for conventional histology, for Western blot analysis and/or quantitative real-time polymerase chain reaction (qPCR). Overall 13 NERD patients, four EE patients and five controls underwent ambulatory pH-testing.

KEY RESULTS

TRPV1 expression was increased in all NERD and EE patients, as measured by Western blot analysis (0.65 +/- 0.07 and 0.8 +/- 0.05 VS 0.34 +/- 0.04 in controls; P < 0.01) and by qPCR (1.98 +/- 0.21 and 2.52 +/- 0.46 VS 1.00 +/- 0.06; P < 0.01). Neutrophilic infiltration, in the mucosa, was detected only in EE patients.

CONCLUSIONS & INFERENCES

Non-erosive reflux disease and EE patients presented increased TRPV1 receptors mRNA and protein, although no correlation with acid exposure was demonstrated. Increased TRPV1 in the esophageal mucosa may contribute to symptoms both in NERD and EE patients and possibly account for peripheral mechanisms responsible for esophageal hypersensitivity in NERD patients.

Cross-organ sensitization of thoracic spinal neurons receiving noxious cardiac input in rats with gastroesophageal reflux

Gastroesophageal reflux (GER) frequently triggers or worsens cardiac pain or symptoms in patients with coronary heart disease. This study aimed to determine whether GER enhances the activity of upper thoracic spinal neurons receiving noxious cardiac input. Gastric fundus and pyloric ligations as well as a longitudinal myelotomy at the gastroesophageal junction induced acute GER in pentobarbital-anesthetized, paralyzed, and ventilated male Sprague-Dawley rats. Manual manipulations of the stomach and lower esophagus were used as surgical controls in another group. At 4-9 h after GER surgery, extracellular potentials of single neurons were recorded from the T3 spinal segment. Intrapericardial bradykinin (IB) (10 microg/ml, 0.2 ml, 1 min) injections were used to activate cardiac nociceptors, and esophageal distensions were used to activate esophageal afferent fibers. Significantly more spinal neurons in the GER group responded to IB compared with the control group (69.1 vs. 38%, P < 0.01). The proportion of IB-responsive neurons in the superficial laminae of GER animals was significantly different from those in deeper layers (1/8 vs. 46/60, P < 0.01); no difference was found in control animals (7/25 vs. 20/46, P > 0.05). Excitatory responses of spinal neurons to IB in the GER group were greater than in the control group [32.4 +/- 3.5 impulses (imp)/s vs. 13.3 +/- 2.3 imp/s, P < 0.01]. Forty-five of 47 (95.7%) neurons responded to cardiac input and ED, which was higher than the control group (61.5%, P < 0.01). These results indicate that acute GER enhanced the excitatory responses of thoracic spinal neurons in deeper laminae of the dorsal horn to noxious cardiac stimulus.

Rikkunshito, a traditional Japanese medicine, may relieve abdominal symptoms in rats with experimental esophagitis by improving the barrier function of epithelial cells in esophageal mucosa

BACKGROUND

A traditional Japanese medicine, rikkunshito, has been reported to relieve dyspepsia symptoms. We investigated the effect of rikkunshito on RE-induced abdominal dyspepsia, and performed experiments to elucidate the mechanism of that effect.

METHODS

RE model rats were prepared using 8-week-old male Wistar rats, and rikkunshito was administered in drinking water. Voluntary movement was used as an index of RE-induced abdominal dyspepsia, which was monitored by an infrared sensor. On the tenth day after surgery, the total area of esophageal erosion was measured, and samples of nonerosive mucosa were collected. Using those samples, intercellular spaces of epithelial mucosa were examined by transmission electron microscopy, and the NP-40-soluble and -insoluble levels of the tight junction proteins claudin-1, -3 and -4 and their mRNAs were determined.

RESULTS

Rikkunshito did not reduce the average total area of erosive lesions in the esophageal mucosa of RE model rats. On day 10, voluntary movement was significantly decreased in the RE model rats and rikkunshito significantly increased it. Nonerosive esophageal mucosa from RE rats showed dilation of intercellular spaces in epithelium, and significantly decreased claudin-3 mRNA and protein levels. Rikkunshito significantly suppressed intercellular space dilation and significantly increased the level of NP-40-insoluble claudin-3, but it did not affect the mRNA level, suggesting that it promoted tight junction formation by facilitating the translocation of proteins.

CONCLUSION

Rikkunshito increased voluntary movement in RE model rats. This may have been because rikkunshito ameliorated the symptoms of RE by improving the barrier function of esophageal mucosa.

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.

Visceral pain: the neurophysiological mechanism

The mechanism of visceral pain is still less understood compared with that of somatic pain. This is primarily due to the diverse nature of visceral pain compounded by multiple factors such as sexual dimorphism, psychological stress, genetic trait, and the nature of predisposed disease. Due to multiple contributing factors there is an enormous challenge to develop animal models that ideally mimic the exact disease condition. In spite of that, it is well recognized that visceral hypersensitivity can occur due to (1) sensitization of primary sensory afferents innervating the viscera, (2) hyperexcitability of spinal ascending neurons (central sensitization) receiving synaptic input from the viscera, and (3) dysregulation of descending pathways that modulate spinal nociceptive transmission. Depending on the type of stimulus condition, different neural pathways are involved in chronic pain. In early-life psychological stress such as maternal separation, chronic pain occurs later in life due to dysregulation of the hypothalamic-pituitary-adrenal axis and significant increase in corticotrophin releasing factor (CRF) secretion. In contrast, in early-life inflammatory conditions such as colitis and cystitis, there is dysregulation of the descending opioidergic system that results excessive pain perception (i.e., visceral hyperalgesia). Functional bowel disorders and chronic pelvic pain represent unexplained pain that is not associated with identifiable organic diseases. Often pain overlaps between two organs and approximately 35% of patients with chronic pelvic pain showed significant improvement when treated for functional bowel disorders. Animal studies have documented that two main components such as (1) dichotomy of primary afferent fibers innervating two pelvic organs and (2) common convergence of two afferent fibers onto a spinal dorsal horn are contributing factors for organ-to-organ pain overlap. With reports emerging about the varieties of peptide molecules involved in the pathological conditions of visceral pain, it is expected that better therapy will be achieved relatively soon to manage chronic visceral pain.

Altered expression of P2X3 in vagal and spinal afferents following esophagitis in rats

Purinergic P2X(3) receptors are predominantly expressed in small diameter primary afferent neurons and activation of these receptors by adenosine triphosphate is reported to play an important role in nociceptive signaling. The objective of this study was to investigate the expression of P2X(3) receptors in spinal and vagal sensory neurons and esophageal tissues following esophagitis in rats. Two groups of rats were used including 7 days fundus-ligated (7D-ligated) esophagitis and sham-operated controls. Esophagitis was produced by ligating the fundus and partial obstruction of pylorus that initiated reflux of gastric contents. The sham-operated rats underwent midline incision without surgical manipulation of the stomach. Expressions of P2X(3) receptors in thoracic dorsal root ganglia (DRGs), nodose ganglia (NGs), and esophageal tissues were evaluated by RT-PCR, western blot and immunohistochemistry. Esophageal neurons were identified by retrograde transport of Fast Blue from the esophagus. There were no significant differences in P2X(3) mRNA expressions in DRGs (T1-T3) and NGs between 7D-ligated and sham-operated rats. However, there was an upregulation of P2X(3) mRNA in DRGs (T6-T12) and in the esophageal muscle. At protein level, P2X(3) exhibited significant upregulation both in DRGs and in NGs of rats having chronic esophagitis. Immunohistochemical analysis exhibited a significant increase in P2X(3) and TRPV1 co-expression in DRGs and NGs in 7D-ligated rats compared to sham-operated rats. The present findings suggest that chronic esophagitis results in upregulation of P2X(3) and its co-localization with TRPV1 receptor in vagal and spinal afferents. Changes in P2X(3) expression in vagal and spinal sensory neurons may contribute to esophageal hypersensitivity following acid reflux-induced esophagitis.

Airway irritability--a burning issue?

Systems biology is being increasingly used to probe the underlying pathophysiology of asthma, although serious challenges remain to decipher the physiologic significance of the information revealed in these studies relating to gene expression and regulatory gene networks often used to understand gene-gene interactions. One phenotypic change characteristic of asthma is increased airway irritability, or bronchial hyperresponsiveness (BHR) which is still poorly understood. While the precise mechanism(s) remain(s) to be identified, a number of hypotheses have been posited to account for this phenomenon, including airways inflammation, alteration in airway smooth muscle function, and airway remodeling. However, the role of sensory nerves in this phenomenon has received scant attention yet offers a potentially new target for the development of novel drugs.

The neural regulation of the mammalian esophageal motility and its implication for esophageal diseases

In contrast to the tunica muscularis of the stomach, small intestine and large intestine, the external muscle layer of the mammalian esophagus contains not only smooth muscle but also striated muscle fibers. Although the swallowing pattern generator initiates the peristaltic movement via vagal preganglionic neurons that project to the myenteric ganglia in the smooth muscle esophagus, the progressing front of contraction is organized by a local reflex circuit composed by intrinsic neurons similarly to other gastrointestinal tracts. On the other hand, the peristalsis of the striated muscle esophagus is both initiated and organized by the swallowing pattern generator via vagal motor neurons that directly innervate the muscle fibers. The presence of a distinct ganglionated myenteric plexus in the striated muscle portion of the esophagus had been enigmatic and neglected in terms of peristaltic control for a long time. Recently, the regulatory roles of intrinsic neurons in the esophageal striated muscle have been clarified. It was reported that esophageal striated muscle receives dual innervation from both vagal motor fibers originating in the brainstem and varicose intrinsic nerve fibers originating in the myenteric plexus, which is called 'enteric co-innervation' of esophageal motor endplates. Moreover, a putative local neural reflex pathway that can control the motility of the striated muscle was identified in the rodent esophagus. This reflex circuit consists of primary afferent neurons and myenteric neurons, which can modulate the release of neurotransmitters from vagal motor neurons in the striated muscle esophagus. The pathogenesis of some esophageal disorders such as achalasia and gastroesophageal reflux disease might be involved in dysfunction of the neural networks including alterations of the myenteric neurons. These evidences indicate the physiological and pathological significance of intrinsic nervous system in the regulation of the esophageal motility. In addition, it is assumed that the components of intrinsic neurons might be therapeutic targets for several esophageal diseases.

Differential effects of transient receptor vanilloid one (TRPV1) antagonists in acid-induced excitation of esophageal vagal afferent fibers of rats

Gastro-esophageal acid reflux can stimulate esophageal vagal sensory afferents by activating proton-sensitive ion channel transient receptor vanilloid one (TRPV1). The objective of this study was to investigate the response characteristics of vagal afferent fibers of rats to acid (0.1 N HCl) and capsaicin (CAP) following esophagitis and differential effects of two classes of TRPV1 antagonists on responses of vagal afferent fibers. The chronic reflux was induced by ligating the fundus of the stomach and partial constriction of pylorus. Extracellular single fiber recordings were made from the cervical vagal afferent fibers from naive control and fundus-ligated (FL) esophagitis rats. Innervations of fibers were identified to esophageal distension (ED) and subsequently tested to CAP and acid before and after injection of TRPV1 antagonist JYL1421 or AMG9810 (10 micromol/kg i.v.). Seventy-five vagal afferent fibers from 70 rats were identified to ED. Intra-esophageal CAP (0.1 ml of 1 mg/ml) excited 39.5% (17/43, 5/22 from naive and 12/21 from FL rats) fibers. In contrast, i.v. injection of CAP (0.03-0.3 micromol/kg) dose-dependently excited 72% (42/58) fibers. Responses to CAP were significantly greater for fibers from FL rats (n=32) than naive rats (n=25). TRPV1 antagonists JYL1421 and AMG9810 (10 micromol/kg) significantly blocked response to CAP. Intra-esophageal acid infusion stimulated 5/17 (29.4%) fibers from naive rats and 12/28 (42%) from FL rats. Effect of acid was significantly blocked by AMG9810, but not by JYL1421. Results indicate that following esophagitis the number of fibers responsive to CAP and acid is greater than noninflamed esophagus, which may contribute to esophageal hypersensitivity. Acid-induced excitation of vagal sensory afferents can be differentially attenuated by different classes of TRPV1 antagonists. Therefore, TRPV1 antagonists play a key role in attenuation of hypersensitivity following reflux-induced esophagitis. The use of TRPV1 antagonists could be an alternative to the traditional symptoms-based treatment of chronic acid reflux and esophageal hypersensitivity.

Basic and clinical aspects of gastrointestinal pain

The gastrointestinal (GI) tract is a system of organs within multicellular animals which facilitates the ingestion, digestion, and absorption of food with subsequent defecation of waste. A complex arrangement of nerves and ancillary cells contributes to the sensorimotor apparatus required to subserve such essential functions that are with the exception of the extreme upper and lower ends of the GI tract normally subconscious. However, it also has the potential to provide conscious awareness of injury. Although this function can be protective, when dysregulated, particularly on a chronic basis, the same system can lead to considerable morbidity. The anatomical and molecular basis of gastrointestinal nociception, conditions associated with chronic unexplained visceral pain, and developments in treatment are presented in this review.

Acid-sensitive ion channels and receptors

Acidosis is a noxious condition associated with inflammation, ischaemia or defective acid containment. As a consequence, acid sensing has evolved as an important property of afferent neurons with unmyelinated and thinly myelinated nerve fibres. Protons evoke multiple currents in primary afferent neurons, which are carried by several acid-sensitive ion channels. Among these, acid-sensing ion channels (ASICs) and transient receptor potential (TRP) vanilloid-1 (TRPV1) ion channels have been most thoroughly studied. ASICs survey moderate decreases in extracellular pH, whereas TRPV1 is activated only by severe acidosis resulting in pH values below 6. Two-pore-domain K(+) (K(2P)) channels are differentially regulated by small deviations of extra- or intracellular pH from physiological levels. Other acid-sensitive channels include TRPV4, TRPC4, TRPC5, TRPP2 (PKD2L1), ionotropic purinoceptors (P2X), inward rectifier K(+) channels, voltage-activated K(+) channels, L-type Ca(2+) channels, hyperpolarization-activated cyclic nucleotide gated channels, gap junction channels, and Cl(-) channels. In addition, acid-sensitive G protein coupled receptors have also been identified. Most of these molecular acid sensors are expressed by primary sensory neurons, although to different degrees and in various combinations. Emerging evidence indicates that many of the acid-sensitive ion channels and receptors play a role in acid sensing, acid-induced pain and acid-evoked feedback regulation of homeostatic reactions. The existence and apparent redundancy of multiple pH surveillance systems attests to the concept that acid-base regulation is a vital issue for cell and tissue homeostasis. Since upregulation and overactivity of acid sensors appear to contribute to various forms of chronic pain, acid-sensitive ion channels and receptors are considered as targets for novel analgesic drugs. This approach will only be successful if the pathological implications of acid sensors can be differentiated pharmacologically from their physiological function.

Alterations in N-methyl-D-aspartate receptor subunits in primary sensory neurons following acid-induced esophagitis in cats

The excitatory amino acid glutamate plays an important role in the development of neuronal sensitization and the ionotropic N-methyl-d-aspartate receptor (NMDAR) is one of the major receptors involved. The objective of this study was to use a cat model of gastroesophageal reflux disease (GERD) to investigate the expression of the NR1 and NR2A subunits of NMDAR in the vagal and spinal afferent fibers innervating the esophagus. Two groups of cats (Acid-7D and PBS-7D) received 0.1 N HCl (pH 1.2) or 0.1 M PBS (pH 7.4) infusion in the esophagus (1 ml/min for 30 min/day for 7 days), respectively. NR1 splice variants (both NH(2) and COOH terminals) and NR2A in the thoracic dorsal root ganglia (DRGs), nodose ganglia (NGs), and esophagus were evaluated by RT-PCR, Western blot, and immunohistochemistry. Acid produced marked inflammation and a significant increase in eosinophil peroxidase and myeloperoxidase contents compared with PBS-infused esophagus. The NR1-4 splice variant gene exhibited a significant upregulation in DRGs and esophagus after acid infusion. In DRGs, NGs, and esophagus, acid infusion resulted in significant upregulation of NR1 and downregulation of NR2A subunit gene expression. A significant increase in NR1 polypeptide expression was observed in DRGs and NGs from Acid-7D compared with control. In conclusion, long-term acid infusion in the cat esophagus resulted in ulcerative esophagitis and differential expressions of NR1 and NR2A subunits. It is possible that these changes may in part contribute to esophageal hypersensitivity observed in reflux esophagitis.

The pharmacological challenge to tame the transient receptor potential vanilloid-1 (TRPV1) nocisensor

The transient receptor potential vanilloid-1 (TRPV1) cation channel is a receptor that is activated by heat (>42 degrees C), acidosis (pH<6) and a variety of chemicals among which capsaicin is the best known. With these properties, TRPV1 has emerged as a polymodal nocisensor of nociceptive afferent neurones, although some non-neuronal cells and neurones in the brain also express TRPV1. The activity of TRPV1 is controlled by a multitude of regulatory mechanisms that either cause sensitization or desensitization of the channel. As many proalgesic pathways converge on TRPV1 and this nocisensor is upregulated and sensitized by inflammation and injury, TRPV1 is thought to be a central transducer of hyperalgesia and a prime target for the pharmacological control of pain. As a consequence, TRPV1 agonists causing defunctionalization of sensory neurones and a large number of TRPV1 blockers have been developed, some of which are in clinical trials. A major drawback of many TRPV1 antagonists is their potential to cause hyperthermia, and their long-term use may carry further risks because TRPV1 has important physiological functions in the peripheral and central nervous system. The challenge, therefore, is to pharmacologically differentiate between the physiological and pathological implications of TRPV1. There are several possibilities to focus therapy specifically on those TRPV1 channels that contribute to disease processes. These approaches include (i) site-specific TRPV1 antagonists, (ii) modality-specific TRPV1 antagonists, (iii) uncompetitive TRPV1 (open channel) blockers, (iv) drugs interfering with TRPV1 sensitization, (v) drugs interfering with intracellular trafficking of TRPV1 and (vi) TRPV1 agonists for local administration.

TRPV1 receptors on unmyelinated C-fibres mediate colitis-induced sensitization of pelvic afferent nerve fibres in rats

Patients with inflammatory bowel disease often suffer from gastrointestinal motility and sensitivity disorders. The aim of the current study was to investigate the role of transient receptor potential of the vanilloid type 1 (TRPV1) receptors in the pathophysiology of colitis-induced pelvic afferent nerve sensitization. Trinitrobenzene sulphate (TNBS) colitis (7.5 mg, 30% ethanol) was induced in Wistar rats 72 h prior to the experiment. Single-fibre recordings were made from pelvic nerve afferents in the decentralized S1 dorsal root. Fibres responding to colorectal distension (CRD) were identified in controls and rats with TNBS colitis. The effect of the TRPV1 antagonist N-(4-tertiarybutylphenyl)-4-(3-chlorophyridin-2-yl)tetrahydropyrazine-1(2H)carboxamide (BCTC; 0.25-5 mg kg(-1)) or its vehicle (hydroxypropyl-beta-cyclodextrin) was tested on the afferent response to repetitive distensions (60 mmHg). Immunocytochemical staining of TRPV1 and NF200, a marker for A-fibre neurons, was performed in the dorsal root ganglia L6-S1. TNBS colitis significantly increased the response to colorectal distension of pelvic afferent C-fibres. BCTC did not significantly affect the C-fibre response in controls, but normalized the sensitized response in rats with colitis. TNBS colitis increased the spontaneous activity of C-fibres, an effect which was insensitive to administration of BCTC. TNBS colitis had no effect on Adelta-fibres, nor was their activity modulated by BCTC. TNBS colitis caused an immunocytochemical up-regulation of TRPV1 receptors in the cell bodies of pelvic afferent NF200 negative neurons. TRPV1 signalling mediates the colitis-induced sensitization of pelvic afferent C-fibres to CRD, while Adelta-fibres are neither sensitized by colitis nor affected by TRPV1 inhibition.