Modulation of visceral hypersensitivity by glial cell line-derived neurotrophic factor family receptor α-3 in colorectal afferents

An anesthesia protocol for robust and repeatable measurement of behavioral visceromotor responses to colorectal distension in mice

Introduction

Visceral motor responses (VMR) to graded colorectal distension (CRD) have been extensively implemented to assess the level of visceral pain in awake rodents, which are inevitably confounded by movement artifacts and cannot be conveniently implemented to assess invasive neuromodulation protocols for treating visceral pain. In this report, we present an optimized protocol with prolonged urethane infusion that enables robust and repeatable recordings of VMR to CRD in mice under deep anesthesia, providing a two-hour window to objectively assess the efficacy of visceral pain management strategies.

Methods

During all surgical procedures, C57BL/6 mice of both sexes (8-12 weeks, 25-35 g) were anesthetized with 2% isoflurane inhalation. An abdominal incision was made to allow Teflon-coated stainless steel wire electrodes to be sutured to the oblique abdominal musculature. A thin polyethylene catheter (Φ 0.2 mm) was placed intraperitoneally and externalized from the abdominal incision for delivering the prolonged urethane infusion. A cylindric plastic-film balloon (Φ 8 mm x 15 mm when distended) was inserted intra-anally, and its depth into the colorectum was precisely controlled by measuring the distance between the end of the balloon and the anus. Subsequently, the mouse was switched from isoflurane anesthesia to the new urethane anesthesia protocol, which consisted of a bout of infusion (0.6 g urethane per kg weight, g/kg) administered intraperitoneally via the catheter and continuous low-dose infusion throughout the experiment at 0.15-0.23 g per kg weight per hour (g/kg/h).

Results

Using this new anesthesia protocol, we systematically investigated the significant impact of balloon depth into the colorectum on evoked VMR, which showed a progressive reduction with increased balloon insertion depth from the rectal region into the distal colonic region. Intracolonic TNBS treatment induced enhanced VMR to CRD of the colonic region (>10 mm from the anus) only in male mice, whereas colonic VMR was not significantly altered by TNBS in female mice.

Discussion

Conducting VMR to CRD in anesthetized mice using the current protocol will enable future objective assessments of various invasive neuromodulatory strategies for alleviating visceral pain.

Optical clearing reveals TNBS-induced morphological changes of VGLUT2-positive nerve fibers in mouse colorectum

Colorectal hypersensitivity and sensitization of both mechanosensitive and mechanically insensitive afferents develop after intracolonic instillation of 2,4,6-trinitrobenzenesulfonic acid (TNBS) in the mouse, a model of postinfectious irritable bowel syndrome. In mice in which ∼80% of extrinsic colorectal afferents were labeled genetically using the promotor for vesicular glutamate transporter type 2 (VGLUT2), we systematically quantified the morphology of VGLUT2-positive axons in mouse colorectum 7-28 days following intracolonic TNBS treatment. After removal, the colorectum was distended (20 mmHg), fixed with paraformaldehyde, and optically cleared to image VGLUT2-positive axons throughout the colorectal wall thickness. We conducted vector path tracing of individual axons to allow systematic quantification of nerve fiber density and shape. Abundant VGLUT2-positive nerve fibers were present in most layers of the colorectum, except the serosal and longitudinal muscular layers. A small percentage of VGLUT2-positive myenteric plexus neurons was also detected. Intracolonic TNBS treatment significantly reduced the number of VGLUT2-positive nerve fibers in submucosal, myenteric plexus, and mucosal layers at day 7 post-TNBS, which mostly recovered by day 28. We also found that almost all fibers in the submucosa were meandering and curvy, with ∼10% showing pronounced curviness (quantified by the linearity index). TNBS treatment resulted in a significant reduction of the proportions of pronounced curvy fibers in the rectal region at 28 days post-TNBS. Altogether, the present morphological study reveals profound changes in the distribution of VGLUT2-positive fibers in mouse colorectum undergoing TNBS-induced colitis and draws attention to curvy fibers in the submucosa with potential roles in visceral nociception.NEW & NOTEWORTHY We conducted genetic labeling and optical clearing to visualize extrinsic sensory nerve fibers in whole-mount colorectum, which revealed widespread presence of axons in the submucosal layer. Remarkably, axons in the submucosa were meandering and curvy, in contrast to axons in other layers generally aligned with the basal tissues. Intracolonic TNBS treatment led to pronounced changes of nerve fiber density and curviness, suggesting nerve fiber morphologies as potentially contributing factors to sensory sensitization.

High-Throughput Functional Characterization of Visceral Afferents by Optical Recordings From Thoracolumbar and Lumbosacral Dorsal Root Ganglia

Functional understanding of visceral afferents is important for developing the new treatment to visceral hypersensitivity and pain. The sparse distribution of visceral afferents in dorsal root ganglia (DRGs) has challenged conventional electrophysiological recordings. Alternatively, Ca²⁺ indicators like GCaMP6f allow functional characterization by optical recordings. Here we report a turnkey microscopy system that enables simultaneous Ca²⁺ imaging at two parallel focal planes from intact DRG. By using consumer-grade optical components, the microscopy system is cost-effective and can be made broadly available without loss of capacity. It records low-intensity fluorescent signals at a wide field of view (1.9 × 1.3 mm) to cover a whole mouse DRG, with a high pixel resolution of 0.7 micron/pixel, a fast frame rate of 50 frames/sec, and the capability of remote focusing without perturbing the sample. The wide scanning range (100 mm) of the motorized sample stage allows convenient recordings of multiple DRGs in thoracic, lumbar, and sacral vertebrae. As a demonstration, we characterized mechanical neural encoding of visceral afferents innervating distal colon and rectum (colorectum) in GCaMP6f mice driven by VGLUT2 promotor. A post-processing routine is developed for conducting unsupervised detection of visceral afferent responses from GCaMP6f recordings, which also compensates the motion artifacts caused by mechanical stimulation of the colorectum. The reported system offers a cost-effective solution for high-throughput recordings of visceral afferent activities from a large volume of DRG tissues. We anticipate a wide application of this microscopy system to expedite our functional understanding of visceral innervations.

Visceral pain from colon and rectum: the mechanotransduction and biomechanics

Visceral pain is the cardinal symptom of functional gastrointestinal (GI) disorders such as the irritable bowel syndrome (IBS) and the leading cause of patients' visit to gastroenterologists. IBS-related visceral pain usually arises from the distal colon and rectum (colorectum), an intraluminal environment that differs greatly from environment outside the body in chemical, biological, thermal, and mechanical conditions. Accordingly, visceral pain is different from cutaneous pain in several key psychophysical characteristics, which likely underlies the unsatisfactory management of visceral pain by drugs developed for other types of pain. Colorectal visceral pain is usually elicited from mechanical distension/stretch, rather than from heating, cutting, pinching, or piercing that usually evoke pain from the skin. Thus, mechanotransduction, i.e., the encoding of colorectal mechanical stimuli by sensory afferents, is crucial to the underlying mechanisms of GI-related visceral pain. This review will focus on colorectal mechanotransduction, the process of converting colorectal mechanical stimuli into trains of action potentials by the sensory afferents to inform the central nervous system (CNS). We will summarize neurophysiological studies on afferent encoding of colorectal mechanical stimuli, highlight recent advances in our understanding of colorectal biomechanics that plays critical roles in mechanotransduction, and review studies on mechano-sensitive ion channels in colorectal afferents. This review calls for focused attention on targeting colorectal mechanotransduction as a new strategy for managing visceral pain, which can also have an added benefit of limited CNS side effects, because mechanotransduction arises from peripheral organs.

Effect of moxibustion on the expression of GDNF and its receptor GFRα3 in the colon and spinal cord of rats with irritable bowel syndrome

BACKGROUND

Moxibustion treatment has been found to ameliorate clinical symptoms including abdominal pain, diarrhoea and constipation in patients with irritable bowel syndrome (IBS). Herein we investigated the mechanisms underlying the use of moxibustion in a rat model of IBS.

METHODS

In our study, an IBS model was established in rats by colorectal distension (CRD) stimulus and mustard oil enema. The rats were randomly divided into a normal group, model group, mild moxibustion group, electroacupuncture group, probiotic group and dicetel group. Abdominal withdrawal reflex (AWR) scores were determined within 90 min of the last treatment. The expression of GDNF/GFRα3 protein and mRNA in the colon and spinal cord were detected by immunohistochemistry and quantitative real-time-PCR, respectively.

RESULTS

The IBS model rats had significantly higher AWR scores than the normal group (P<0.01). After mild moxibustion treatment, the AWR score was significantly reduced (20 mm Hg, P<0.05; 40 mm Hg, 60 mm Hg and 80 mm Hg, P<0.01). The model group showed significantly more colonic glial cell line-derived neurotrophic factor (GDNF/GFRα3 (GDNF family receptor α3) protein and mRNA expression in the colon and spinal cord than the normal group (P<0.01). Compared with the model group, the expression of GDNF/GFRα3 protein and mRNA in the colon and spinal cord of the rats were significantly decreased in the mild moxibustion group (colon: GDNF and GFRα3 protein, P<0.01; GDNF and GFRα3 mRNA, P<0.01; spinal cord: GDNF and GFRα3 protein, P<0.01; GDNF mRNA, P<0.05, GFRα3 mRNA, P<0.01).

CONCLUSIONS

Our data suggest that moxibustion therapy may mitigate CRD-induced increases in the expression of GDNF and its receptor GFRα3 in the colon and spinal cord in a rat model of IBS.

Optical recording reveals topological distribution of functionally classified colorectal afferent neurons in intact lumbosacral DRG

Neuromodulation as a non-drug alternative for managing visceral pain in irritable bowel syndrome (IBS) may target sensitized afferents of distal colon and rectum (colorectum), especially their somata in the dorsal root ganglion (DRG). Developing selective DRG stimulation to manage visceral pain requires knowledge of the topological distribution of colorectal afferent somata which are sparsely distributed in the DRG. Here, we implemented GCaMP6f to conduct high-throughput optical recordings of colorectal afferent activities in lumbosacral DRG, that is, optical electrophysiology. Using a mouse ex vivo preparation with distal colorectum and L5-S1 DRG in continuity, we recorded 791 colorectal afferents' responses to graded colorectal distension (15, 30, 40, and 60 mmHg) and/or luminal shear flow (20-30 mL/min), then functionally classified them into four mechanosensitive classes, and determined the topological distribution of their somata in the DRG. Of the 791 colorectal afferents, 90.8% were in the L6 DRG, 8.3% in the S1 DRG, and only 0.9% in the L5 DRG. L6 afferents had all four classes: 29% mucosal, 18.4% muscular-mucosal, 34% low-threshold (LT) muscular, and 18.2% high-threshold (HT) muscular afferents. S1 afferents only had three classes: 19.7% mucosal, 34.8% LT muscular, and 45.5% HT muscular afferents. All seven L5 afferents were HT muscular. In L6 DRG, somata of HT muscular afferents were clustered in the caudal region whereas somata of the other classes did not cluster in specific regions. Outcomes of this study can directly inform the design and improvement of next-generation neuromodulation devices that target the DRG to alleviate visceral pain in IBS patients.

Measurement of novel intestinal secretory and barrier pathways and effects of proteases

The epithelial lining of the gastrointestinal (GI) tract in conjunction with the enteric nervous system (ENS) plays an important role in mediating solute absorption and secretion. A dysregulated ionic movement across the epithelium can result in GI diseases that manifest as either watery diarrhea or constipation. Hirschsprung disease is an example of an ENS disorder characterized by absence of enteric ganglia in distal gut resulting in obstructive phenotype. Receptor rearranged during transfection (RET) gene variants are the most commonly recognized genetic associations with Hirschsprung disease. In this issue of Neurogastroenterology and Motility, Russell et al demonstrate that RET mediates colonic ion transport through modulation of cholinergic nerves. They go on to show inhibition of RET can attenuate accelerated transit in a rat model. Normalizing secretory and absorptive defects has been an attractive therapeutic strategy. In addition to the intrinsic regulation of secretory processes, luminal mediators like bile acids, short-chain fatty acids, and proteases can affect both secretion and barrier function of the intestinal epithelium. Elevated levels of proteases have been identified in a wide range of GI diseases including irritable bowel syndrome. Proteases are known to cause visceral hypersensitivity and barrier disruption in vitro and in animal models. The goals of this review are to describe fundamental concepts related to intestinal epithelial secretion, the utility of Ussing chambers to measure ionic mechanisms and to discuss examples of novel signaling pathways; namely the RET signaling cascade in secretomotor neurons and effects of luminal proteases on barrier and ionic secretion.

Differential biomechanical properties of mouse distal colon and rectum innervated by the splanchnic and pelvic afferents

Visceral pain is one of the principal complaints of patients with irritable bowel syndrome, and this pain is reliably evoked by mechanical distension and stretch of distal colon and rectum (colorectum). This study focuses on the biomechanics of the colorectum that could play critical roles in mechanical neural encoding. We harvested the distal 30 mm of the colorectum from mice, divided evenly into three 10-mm-long segments (colonic, intermediate and rectal), and conducted biaxial mechanical stretch tests and opening-angle measurements for each tissue segment. In addition, we determined the collagen fiber orientations and contents across the thickness of the colorectal wall by nonlinear imaging via second harmonic generation (SHG). Our results reveal a progressive increase in tissue compliance and prestress from colonic to rectal segments, which supports prior electrophysiological findings of distinct mechanical neural encodings by afferents in the lumbar splanchnic nerves (LSN) and pelvic nerves (PN) that dominate colonic and rectal innervations, respectively. The colorectum is significantly more viscoelastic in the circumferential direction than in the axial direction. In addition, our SHG results reveal a rich collagen network in the submucosa and orients approximately ±30° to the axial direction, consistent with the biaxial test results presenting almost twice the stiffness in axial direction versus the circumferential direction. Results from current biomechanical study strongly indicate the prominent roles of local tissue biomechanics in determining the differential mechanical neural encoding functions in different regions of the colorectum. NEW & NOTEWORTHY Mechanical distension and stretch-not heat, cutting, or pinching-reliably evoke pain from distal colon and rectum. We report different local mechanics along the longitudinal length of the colorectum, which is consistent with the existing literature on distinct mechanotransduction of afferents innervating proximal and distal regions of the colorectum. This study draws attention to local mechanics as a potential determinant factor for mechanical neural encoding of the colorectum, which is crucial in visceral nociception.

Exploring the Potential of RET Kinase Inhibition for Irritable Bowel Syndrome: A Preclinical Investigation in Rodent Models of Colonic Hypersensitivity

Abdominal pain represents a significant complaint in patients with irritable bowel syndrome (IBS). While the etiology of IBS is incompletely understood, prior exposure to gastrointestinal inflammation or psychologic stress is frequently associated with the development of symptoms. Inflammation or stress-induced expression of growth factors or cytokines may contribute to the pathophysiology of IBS. Here, we aimed to investigate the therapeutic potential of inhibiting the receptor of glial cell line-derived neurotrophic factor, rearranged during transfection (RET), in experimental models of inflammation and stress-induced visceral hypersensitivity resembling IBS sequelae. In RET-cyan fluorescent protein [(CFP) Ret^CFP/+] mice, thoracic and lumbosacral dorsal root ganglia were shown to express RET, which colocalized with calcitonin gene-related peptide. To understand the role of RET in visceral nociception, we employed GSK3179106 as a potent, selective, and gut-restricted RET kinase inhibitor. Colonic hyperalgesia, quantified as exaggerated visceromotor response to graded pressures (0-60 mm Hg) of isobaric colorectal distension (CRD), was produced in multiple rat models induced 1) by colonic irritation, 2) following acute colonic inflammation, 3) by adulthood stress, and 4) by early life stress. In all the rat models, RET inhibition with GSK3179106 attenuated the number of abdominal contractions induced by CRD. Our findings identify a role for RET in visceral nociception. Inhibition of RET kinase with a potent, selective, and gut-restricted small molecule may represent a novel therapeutic strategy for the treatment of IBS through the attenuation of post-inflammatory and stress-induced visceral hypersensitivity.

Discovery of a First-in-Class Gut-Restricted RET Kinase Inhibitor as a Clinical Candidate for the Treatment of IBS

Abdominal pain and abnormal bowel habits represent major symptoms for irritable bowel syndrome (IBS) patients that are not adequately managed. Although the etiology of IBS is not completely understood, many of the functions of the gastrointestinal (GI) tract are regulated by the enteric nervous system (ENS). Inflammation or stress-induced expression of growth factors or cytokines may lead to hyperinnervation of visceral afferent neurons in GI tract and contribute to the pathophysiology of IBS. Rearranged during transfection (RET) is a neuronal growth factor receptor tyrosine kinase critical for the development of the ENS as exemplified by Hirschsprung patients who carry RET loss-of-function mutations and lack normal colonic innervation leading to colonic obstruction. Similarly, RET signaling in the adult ENS maintains neuronal function by contributing to synaptic formation, signal transmission, and neuronal plasticity. Inhibition of RET in the ENS represents a novel therapeutic strategy for the normalization of neuronal function and the symptoms of IBS patients. Herein, we describe our screening effort and subsequent structure-activity relationships (SARs) in optimizing potency, selectivity, and mutagenicity of the series, which led to the discovery of a first-in-class, gut-restricted RET kinase inhibitor, 2-(4-(4-ethoxy-6-oxo-1,6-dihydropyridin-3-yl)-2-fluorophenyl)-N-(5-(1,1,1-trifluoro-2-methylpropan-2-yl)isoxazol-3-yl)acetamide (15, GSK3179106), as a clinical candidate for the treatment of IBS. GSK3179106 is a potent, selective, and gut-restricted pyridone hinge binder small molecule RET kinase inhibitor with a RET IC₅₀ of 0.3 nM and is efficacious in vivo.

Critical Evaluation of Animal Models of Gastrointestinal Disorders

Preclinical research remains an important tool for discovery and validation of novel therapeutics for gastrointestinal disorders. While in vitro assays can be used to verify receptor-ligand interactions and test for structural activity of new compounds, only whole-animal studies can demonstrate drug efficacy within the gastrointestinal system. Most major gastrointestinal disorders have been modeled in animals; however the translational relevance of each model is not equal. The purpose of this chapter is to provide a critical evaluation of common animal models that are being used to develop pharmaceuticals for gastrointestinal disorders. For brevity, the models are presented for upper gastrointestinal disorders involving the esophagus, stomach, and small intestine and lower gastrointestinal disorders that focus on the colon. Particular emphasis is used to explain the face and construct validity of each model, and the limitations of each model, including data interpretation, are highlighted. This chapter does not evaluate models that rely on surgical or other non-pharmacological interventions for treatment.

Zeta Inhibitory Peptide as a Novel Therapy to Control Chronic Visceral Hypersensitivity in a Rat Model

Background

The pathogenesis of multiple chronic visceral pain syndromes, such as irritable bowel syndrome (IBS), is not well known, and as a result current therapies are ineffective. The objective of this study was to investigate the effect of spinal protein kinase M zeta (PKMζ) on visceral pain sensitivity in rats with IBS to better understand the pathogenesis and investigate the effect of zeta inhibitory peptide (ZIP) as a therapy for chronic visceral pain.

Methods

Visceral hypersensitivity rats were produced by neonatal maternal separation (NMS). Visceral pain sensitivity was assessed by electromyographic (EMG) responses of abdominal muscles to colorectal distention (CRD). Spinal PKMζ and phosphorylated PKMζ (p-PKMζ) were detected by western blot. Varying doses of ZIP were intrathecally administered to investigate the role of spinal PKMζ in chronic visceral hypersensitivity. The open field test was used to determine if ZIP therapy causes spontaneous motor activity side effects.

Results

Graded CRD pressure significantly increased EMG responses in NMS rats compared to control rats (p < 0.05). p-PKMζ expression increased in the thoracolumbar and lumbosacral spinal cord in the IBS-like rats with notable concomitant chronic visceral pain compared to control rats (p < 0.05). EMG data revealed that intrathecal ZIP injection (1, 5, and 10 μg) dose-dependently attenuated visceral pain hypersensitivity in IBS-like rats.

Conclusions

Phosphorylated PKMζ may be involved in the spinal central sensitization of chronic visceral hypersensitivity in IBS, and administration of ZIP could effectively treat chronic visceral pain with good outcomes in rat models.

Persistent alterations in colonic afferent innervation in a rat model of postinfectious gut dysfunction: Role for changes in peripheral neurotrophic factors

BACKGROUND

Visceral hypersensitivity in the inflamed gut is related partly to the effects of peripheral neurotrophic factors (NTFs) on local afferent neurons. However, alterations in sensory afferents of distant areas remain unexplored. Using the Trichinella spiralis infection model, which causes a jejunitis, we investigated the remodeling of colonic afferents and the potential role of NTFs.

METHODS

Rats were infected with T. spiralis. Inflammatory-like changes, mucosal mast cells (MMCs) dynamics, and expression of nerve growth factor and glial cell line-derived NTFs (glial cell-derived neurotrophic factor, artemin, and neurturin) were determined in the colon up to day 30 postinfection. Functional responses of colonic afferents were determined assessing changes in the expression of sensory-related markers in thoracolumbar (TL)/lumbosacral (LS) dorsal root ganglias (DRGs) following intracolonic capsaicin.

KEY RESULTS

Trichinella spiralis induced an inflammatory-like response within the colon, partly resolved at day 30 postinfection, except for a persistent MMC infiltrate. While the jejunum of infected animals showed an up-regulation in the expression of NTFs, a transitory down-regulation was observed in the colon. Overall, T. spiralis effects on DRGs gene expression were restricted to a transient down-regulation of TPRV1. Stimulation with intracolonic capsaicin induced a down-regulation of TRPV1 levels in TL and LS DRGs, an effect enhanced in LS DRGs of infected animals, regardless the postinfection time considered.

CONCLUSIONS & INFERENCES

During intestinal inflammation, spread morphological and functional alterations, including remodeling of visceral afferents, are observed outside the primary region affected by the insult. Similar mechanisms might be operating in states of widespread alterations of visceral sensitivity.

Animal models of gastrointestinal and liver diseases. Animal models of visceral pain: pathophysiology, translational relevance, and challenges

Visceral pain describes pain emanating from the thoracic, pelvic, or abdominal organs. In contrast to somatic pain, visceral pain is generally vague, poorly localized, and characterized by hypersensitivity to a stimulus such as organ distension. Animal models have played a pivotal role in our understanding of the mechanisms underlying the pathophysiology of visceral pain. This review focuses on animal models of visceral pain and their translational relevance. In addition, the challenges of using animal models to develop novel therapeutic approaches to treat visceral pain will be discussed.

In vitro functional characterization of mouse colorectal afferent endings

This video demonstrates in detail an in vitro single-fiber electrophysiological recording protocol using a mouse colorectum-nerve preparation. The approach allows unbiased identification and functional characterization of individual colorectal afferents. Extracellular recordings of propagated action potentials (APs) that originate from one or a few afferent (i.e., single-fiber) receptive fields (RFs) in the colorectum are made from teased nerve fiber fascicles. The colorectum is removed with either the pelvic (PN) or lumbar splanchnic (LSN) nerve attached and opened longitudinally. The tissue is placed in a recording chamber, pinned flat and perfused with oxygenated Krebs solution. Focal electrical stimulation is used to locate the colorectal afferent endings, which are further tested by three distinct mechanical stimuli (blunt probing, mucosal stroking and circumferential stretch) to functionally categorize the afferents into five mechanosensitive classes. Endings responding to none of these mechanical stimuli are categorized as mechanically-insensitive afferents (MIAs). Both mechanosensitive and MIAs can be assessed for sensitization (i.e., enhanced response, reduced threshold, and/or acquisition of mechanosensitivity) by localized exposure of RFs to chemicals (e.g., inflammatory soup (IS), capsaicin, adenosine triphosphate (ATP)). We describe the equipment and colorectum-nerve recording preparation, harvest of colorectum with attached PN or LSN, identification of RFs in the colorectum, single-fiber recording from nerve fascicles, and localized application of chemicals to the RF. In addition, challenges of the preparation and application of standardized mechanical stimulation are also discussed.

Condition-specific role of colonic inflammatory molecules in persistent functional colorectal hypersensitivity in the mouse

BACKGROUND

A low-level inflammation has been hypothesized to mediate visceral hypersensitivity in functional bowel disorders that persist after or even in the absence of gut inflammation. We aimed to test the efficacy of a steroidal anti-inflammatory treatment, and identify local inflammatory molecules mediating post- and non-inflammatory colorectal hypersensitivity using two mouse models.

METHODS

Visceromotor responses to colorectal distension were quantified as a measure of colorectal sensitivity. On day 1, mice received intracolonic saline (control), trinitrobenzenesulfonic acid (postinflammatory on day 15), or acidified hypertonic saline (non-inflammatory). Colorectal sensitivity before (day 10) and after (day 15) 4-day dexamethasone (Dex) treatment was compared, and colonic gene expression of inflammatory molecules was quantified.

KEY RESULTS

Dexamethasone effectively inhibited gene expression of inflammatory molecules such as interleukin (IL)-1β and mast cell protease-1 in the colon, but did not attenuate colorectal hypersensitivity in either model. Gene expression of inflammatory molecules in the colon did not differ between control and the non-inflammatory model, but the postinflammatory model showed increased IL-10 and tight junction protein 2, and decreased IL-6, transforming growth factor (TGF)-β, a precursor of β-endorphin, occludin, and mucin 2. While no common molecule explained colorectal hypersensitivity in these models, hypersensitivity was positively correlated with TGF-β2 mRNA in control, and with IL-1β, inhibin βA, and prostaglandin E2 synthase in the Dex-treated postinflammatory model. In the non-inflammatory model, cyclooxygenase-2 mRNA was negatively correlated with colorectal sensitivity.

CONCLUSIONS & INFERENCES

These results suggest that persistent functional colorectal hypersensitivity is mediated by condition-specific mediators whose gene expression in the colon is not inevitably sensitive to steroidal anti-inflammatory treatment.

Characterization of axons expressing the artemin receptor in the female rat urinary bladder: a comparison with other major neuronal populations

Artemin is a member of the glial cell line-derived neurotrophic factor (GDNF) family that has been strongly implicated in development and regeneration of autonomic nerves and modulation of nociception. Whereas other members of this family (GDNF and neurturin) primarily target parasympathetic and nonpeptidergic sensory neurons, the artemin receptor (GFRα3) is expressed by sympathetic and peptidergic sensory neurons that are also the primary sites of action of nerve growth factor, a powerful modulator of bladder nerves. Many bladder sensory neurons express GFRα3 but it is not known if they represent a specific functional subclass. Therefore, our initial aim was to map the distribution of GFRα3-immunoreactive (-IR) axons in the female rat bladder, using cryostat sections and whole wall thickness preparations. We found that GFRα3-IR axons innervated the detrusor, vasculature, and urothelium, but only part of this innervation was sensory. Many noradrenergic sympathetic axons innervating the vasculature were GFRα3-IR, but the noradrenergic innervation of the detrusor was GFRα3-negative. We also identified a prominent source of nonneuronal GFRα3-IR that is likely to be glial. Further characterization of bladder nerves revealed specific structural features of chemically distinct classes of axon terminals, and a major autonomic source of axons labeled with neurofilament-200, which is commonly used to identify myelinated sensory axons within organs. Intramural neurons were also characterized and quantified. Together, these studies reveal a diverse range of potential targets by which artemin could influence bladder function, nerve regeneration, and pain, and provide a strong microanatomical framework for understanding bladder physiology and pathophysiology.

Characterization of bladder sensory neurons in the context of myelination, receptors for pain modulators, and acute responses to bladder inflammation

Bladder sensation is mediated by lumbosacral dorsal root ganglion neurons and is essential for normal voiding and nociception. Numerous electrophysiological, structural, and molecular changes occur in these neurons following inflammation. Defining which neurons undergo these changes is critical for understanding the mechanism underlying bladder pain and dysfunction. Our first aim was to define the chemical classes of bladder sensory neurons that express receptors for the endogenous modulators of nociceptor sensitivity, glial cell line-derived neurotrophic factor (GDNF), the related neurotrophic factor, artemin, and estrogens. Bladder sensory neurons of adult female Sprague-Dawley rats were identified with retrograde tracer. Diverse groups of neurons express these receptors, and some neurons express receptors for both neurotrophic factors and estrogens. Lumbar and sacral sensory neurons showed some distinct differences in their expression profile. We also distinguished the chemical profile of myelinated and unmyelinated bladder sensory neurons. Our second aim was to identify bladder sensory neurons likely to be undergoing structural remodeling during inflammation. Following systemic administration of cyclophosphamide (CYP), its renal metabolite acrolein causes transient urothelial loss, exposing local afferent terminals to a toxic environment. CYP induced expression of the injury-related immediate-early gene product, activating transcription factor-3 (ATF-3), in a small population of sacral nitrergic bladder sensory neurons. In conclusion, we have defined the bladder sensory neurons that express receptors for GDNF, artemin and estrogens. Our study has also identified a sub-population of sacral sensory neurons that are likely to be undergoing structural remodeling during acute inflammation of the bladder. Together these results contribute to increased understanding of the neurons that are known to be involved in pain modulation and hyperreflexia during inflammation.

Combined genetic and pharmacological inhibition of TRPV1 and P2X3 attenuates colorectal hypersensitivity and afferent sensitization

The ligand-gated channels transient receptor potential vanilloid 1 (TRPV1) and P2X3 have been reported to facilitate colorectal afferent neuron sensitization, thus contributing to organ hypersensitivity and pain. In the present study, we hypothesized that TRPV1 and P2X3 cooperate to modulate colorectal nociception and afferent sensitivity. To test this hypothesis, we employed TRPV1-P2X3 double knockout (TPDKO) mice and channel-selective pharmacological antagonists and evaluated combined channel contributions to behavioral responses to colorectal distension (CRD) and afferent fiber responses to colorectal stretch. Baseline responses to CRD were unexpectedly greater in TPDKO compared with control mice, but zymosan-produced CRD hypersensitivity was absent in TPDKO mice. Relative to control mice, proportions of mechanosensitive and -insensitive pelvic nerve afferent classes were not different in TPDKO mice. Responses of mucosal and serosal class afferents to mechanical probing were unaffected, whereas responses of muscular (but not muscular/mucosal) afferents to stretch were significantly attenuated in TPDKO mice; sensitization of both muscular and muscular/mucosal afferents by inflammatory soup was also significantly attenuated. In pharmacological studies, the TRPV1 antagonist A889425 and P2X3 antagonist TNP-ATP, alone and in combination, applied onto stretch-sensitive afferent endings attenuated responses to stretch; combined antagonism produced greater attenuation. In the aggregate, these observations suggest that 1) genetic manipulation of TRPV1 and P2X3 leads to reduction in colorectal mechanosensation peripherally and compensatory changes and/or disinhibition of other channels centrally, 2) combined pharmacological antagonism produces more robust attenuation of mechanosensation peripherally than does antagonism of either channel alone, and 3) the relative importance of these channels appears to be enhanced in colorectal hypersensitivity.

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.

Colitis decreases mechanosensitive K2P channel expression and function in mouse colon sensory neurons

TREK-1, TREK-2 and TRAAK are mechanosensitive two-pore domain K(+) (K(2P)) channels thought to be involved in the attenuation of mechanotransduction. Because colon inflammation is associated with colon mechanohypersensitivity, we hypothesized that the role of these channels in colon sensory (dorsal root ganglion, DRG) neurons would be reduced by colon inflammation. Accordingly, we studied the functional expression of mechanosensitive K(2P) channels in colon sensory neurons in both thoracolumbar (TL) and lumbosacral (LS) DRG that represent the splanchnic and pelvic nerve innervations of the colon, respectively. In colon DRG neurons identified by retrograde tracer previously injected into the colon wall, 62% of TL neurons and 83% of LS neurons expressed at least one of three K(2P) channel mRNAs; the proportion of neurons expressing the TREK-1 gene was greater in LS than in TL DRG. In electrophysiological studies, single-channel activities of TREK-1a, TREK-1b, TREK-2, and TRAAK-like channels were detected in cultured colon DRG neuronal membranes. After trinitrobenzene sulfonic acid-induced colon inflammation, we observed significant decreases in the amount of TREK-1 mRNA, in the response of TREK-2-like channels to membrane stretch, and in the whole cell outward current during osmotic stretch in LS colon DRG neurons. These findings document that the majority of DRG neurons innervating the mouse colon express mechanosensitive K(2P) channels and suggest that a decrease in their expression and activities contributes to the increased colon mechanosensitivity that develops in inflammatory bowel conditions.

Loss of visceral pain following colorectal distension in an endothelin-3 deficient mouse model of Hirschsprung's disease

Endothelin peptides and their endogenous receptors play a major role in nociception in a variety of different organs. They also play an essential role in the development of the enteric nervous system. Mice with deletions of the endothelin-3 gene (lethal spotted mice, ls/ls) develop congenital aganglionosis. However, little is known about how nociception might be affected in the aganglionic rectum of mice deficient in endothelin-3. In this study we investigated changes in spinal afferent innervation and visceral pain transmission from the aganglionic rectum in ls/ls mice. Electromyogram recordings from anaesthetized ls/ls mice revealed a deficit in visceromotor responses arising from the aganglionic colorectum in response to noxious colorectal distension. Loss of visceromotor responses (VMRs) in ls/ls mice was selective, as no reduction in VMRs was detected after stimulation of the bladder or somatic organs. Calcitonin gene related peptide (CGRP) immunoreactivity, retrograde neuronal tracing and extracellular afferent recordings from the aganglionic rectum revealed decreased colorectal spinal innervation, combined with a reduction in mechanosensitivity of rectal afferents. The sensory defect in ls/ls mice is primarily associated with changes in low threshold wide dynamic range rectal afferents. In conclusion, disruption of endothelin 3 gene expression not only affects development and function of the enteric nervous system, but also specific classes of spinal rectal mechanoreceptors, which are required for visceral nociception from the colorectum.