Peripheral and central P2X receptor contributions to colon mechanosensitivity and hypersensitivity in the mouse

P2X7 and P2Y1 receptors in DRG mediate electroacupuncture to inhibit peripheral sensitization in rats with IBS visceral pain

Although multiple purinergic receptors mediate the analgesic effects of acupuncture, it remains unclear whether there is mutual interaction between purinergic receptors to jointly mediate the electroacupuncture inhibition of peripheral sensitization in visceral pain. Visceral hypersensitivity was induced by intracolonic 2,4,6-trinitrobenzene sulfonic acid (TNBS) in rat. The antinociception effect of electroacupuncture on visceral pain was evaluated by morphology, behaviors, neuroelectrophysiology and molecular biology techniques. After labeling the colon-related primary sensory neurons with neural retrograde tracer and employing neuropharmacology, neuroelectrophysiology, and molecular biotechnology, the mechanisms of P2X7R, P2Y1R, and P2X3R in colon-related dorsal root ganglion (DRG) neurons alleviating visceral hypersensitivity of irritable bowel syndrome (IBS) by electroacupuncture at Zusanli and Sanyinjiao acupoints.were elucidated from the perspective of peripheral sensitization. Electroacupuncture significantly inhibited TNBS-induced colonic hypersensitivity in rats with IBS, and Satellite Glial Cells (SGCs) in DRG were found to be involved in electroacupuncture-mediated regulation of the electrophysiological properties of neurons. P2X7R was found to play a pain-inducing role in IBS visceral hypersensitivity by affecting P2X3R, and electroacupuncture exerted an analgesic effect by inhibiting P2X7R activation. P2Y1R was found to play an analgesic role in the process of visceral pain, mediating electroacupuncture to relieve visceral hypersensitivity. P2Y1R relieved visceral pain by inhibiting P2X3R in neurons associated with nociception, with P2X7R identified as upstream of P2Y1R up-regulation by electroacupuncture. Our study suggests that the P2X7R → P2Y1R → P2X3R inhibitory pathway in DRG mediates the inhibition of peripheral sensitization by electroacupuncture in rats with IBS visceral hypersensitivity.

Sex differences in zymosan-induced behavioral visceral hypersensitivity and colorectal afferent sensitization

Sex differences in visceral nociception have been reported in clinical and preclinical studies, but the potential differences in sensory neural encoding of the colorectum between males and females are not well understood. In this study, we systematically assessed sex differences in colorectal neural encoding by conducting high-throughput optical recordings in intact dorsal root ganglia (DRGs) from control and visceral hypersensitive mice. We found an apparent sex difference in zymosan-induced behavioral visceral hypersensitivity: enhanced visceromotor responses to colorectal distension were observed only in male mice, not in female mice. In addition, a higher number of mechanosensitive colorectal afferents were identified per mouse in the zymosan-treated male group than in the saline-treated male group, whereas the mechanosensitive afferents identified per mouse were comparable between the zymosan- and saline-treated female groups. The increased number of identified afferents in zymosan-treated male mice was predominantly from thoracolumbar (TL) innervation, which agrees with the significant increase in the TL afferent proportion in the zymosan group as compared with the control group in male mice. In contrast, female mice showed no difference in the proportion of colorectal neurons between saline- and zymosan-treated groups. Our results revealed a significant sex difference in colorectal afferent innervation and sensitization in the context of behavioral visceral hypersensitivity, which could drive differential clinical symptoms in male and female patients.NEW & NOTEWORTHY We used high-throughput GCaMP6f recordings to study 2,275 mechanosensitive colorectal afferents in mice. Our results revealed significant sex differences in the zymosan-induced behavioral visceral hypersensitivity, which were present in male but not female mice. Male mice also showed sensitization of colorectal afferents in the thoracolumbar pathway, whereas female mice did not. These findings highlight sex differences in sensory neural anatomy and function of the colorectum, with implications for sex-specific therapies for treating visceral pain.

Role and therapeutic target of P2X2/3 receptors in visceral pain

Visceral pain (VP) is caused by internal organ disease. VP is involved in nerve conduction and related signaling molecules, but its specific pathogenesis has not yet been fully elucidated. Currently, there are no effective methods for treating VP. The role of P2X2/3 in VP has progressed. After visceral organs are subjected to noxious stimulation, cells release ATP, activate P2X2/3, enhance the sensitivity of peripheral receptors and the plasticity of neurons, enhance sensory information transmission, sensitize the central nervous system, and play an important role in the development of VP. However, antagonists possess the pharmacological effect of relieving pain. Therefore, in this review, we summarize the biological functions of P2X2/3 and discuss the intrinsic link between P2X2/3 and VP. Moreover, we focus on the pharmacological effects of P2X2/3 antagonists on VP therapy and provide a theoretical basis for its targeted therapy.

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.

Electrophysiology as a Tool to Decipher the Network Mechanism of Visceral Pain in Functional Gastrointestinal Disorders

Abdominal pain, including visceral pain, is prevalent in functional gastrointestinal (GI) disorders (FGIDs), affecting the overall quality of a patient's life. Neural circuits in the brain encode, store, and transfer pain information across brain regions. Ascending pain signals actively shape brain dynamics; in turn, the descending system responds to the pain through neuronal inhibition. Pain processing mechanisms in patients are currently mainly studied with neuroimaging techniques; however, these techniques have a relatively poor temporal resolution. A high temporal resolution method is warranted to decode the dynamics of the pain processing mechanisms. Here, we reviewed crucial brain regions that exhibited pain-modulatory effects in an ascending and descending manner. Moreover, we discussed a uniquely well-suited method, namely extracellular electrophysiology, that captures natural language from the brain with high spatiotemporal resolution. This approach allows parallel recording of large populations of neurons in interconnected brain areas and permits the monitoring of neuronal firing patterns and comparative characterization of the brain oscillations. In addition, we discussed the contribution of these oscillations to pain states. In summary, using innovative, state-of-the-art methods, the large-scale recordings of multiple neurons will guide us to better understanding of pain mechanisms in FGIDs.

Upregulation of P2X3 receptors in primary afferent pathways involves in colon-to-bladder cross-sensitization in rats

Background: Clinical investigation indicates a high level of co-morbidity between bladder overactivity and irritable bowel syndrome. The cross-sensitization of afferent pathways has been demonstrated to be the main reason for the cross-organ sensitization, but the underlying mechanism is unclear.

Methods: A single dose of 2, 4, 6-trinitrobenzene sulfonic acid (TNBS) was applied to induce the colitis rat models by intracolonic administration. All rats were randomly divided into three groups: control, TNBS-3-day, and TNBS-7-day groups. Western blot and immunofluorescent staining were performed to detect the expression of the P2X3 receptor. The spontaneous contractions of the detrusor strip were measured to evaluate the detrusor contractility function. The micturition function was measured by a cystometry experiment. The intercontractile interval (ICI) and maximum bladder pressure (BP) were recorded.

Results: The distal colon from colitis showed serious tissue damage or chronic inflammation after TNBS instillation (p < 0.01). However, there were no detectable histological changes in bladder among groups (p > 0.05). TNBS-induced colitis significantly increased P2X3 receptor expression on the myenteric and submucosal plexus of the distal colon and urothelium of the bladder, especially at day 3 post-TNBS (p < 0.05). Meanwhile, the expression of the P2X3 receptor on DRG neurons was increased in TNBS-induced colitis (p < 0.01). The detrusor strip of rats exhibited detrusor overactivity after days 3 and 7 of TNBS administration (p < 0.01), but inhibition of the P2X3 receptor had no effect (p > 0.05). Moreover, the rats with colitis exhibited the micturition pattern of bladder overactivity, manifested by decreased ICI and increased maximum BP (p < 0.05). Interestingly, inhibition of the P2X3 receptor by intrathecal injection of A-317491 alleviated bladder overactivity evoked by TNBS-induced colitis (p < 0.05).

Conclusion: The upregulation of the P2X3 receptor in an afferent pathway involved in bladder overactivity evoked by TNBS-induced colonic inflammation, suggesting that the P2X3 receptor antagonist may be an available and novel strategy for the control of bladder overactivity.

Oral cancer induced TRPV1 sensitization is mediated by PAR2 signaling in primary afferent neurons innervating the cancer microenvironment

Oral cancer patients report sensitivity to spicy foods and liquids. The mechanism responsible for chemosensitivity induced by oral cancer is not known. We simulate oral cancer-induced chemosensitivity in a xenograft oral cancer mouse model using two-bottle choice drinking and conditioned place aversion assays. An anatomic basis of chemosensitivity is shown in increased expression of TRPV1 in anatomically relevant trigeminal ganglion (TG) neurons in both the xenograft and a carcinogen (4-nitroquinoline 1-oxide)-induced oral cancer mouse models. The percent of retrograde labeled TG neurons that respond to TRPV1 agonist, capsaicin, is increased along with the magnitude of response as measured by calcium influx, in neurons from the cancer models. To address the possible mechanism of TRPV1 sensitivity in tongue afferents, we study the role of PAR₂, which can sensitize the TRPV1 channel. We show co-expression of TRPV1 and PAR₂ on tongue afferents and using a conditioned place aversion assay, demonstrate that PAR₂ mediates oral cancer-induced, TRPV1-evoked sensitivity in an oral cancer mouse model. The findings provide insight into oral cancer-mediated chemosensitivity.

Pannexin 1 role in the trigeminal ganglion in infraorbital nerve injury-induced mechanical allodynia

OBJECTIVES

The detailed pathological mechanism of orofacial neuropathic pain remains unknown. We aimed to examine the pannexin 1 (Panx1) signaling in the trigeminal ganglion (TG) involvement in infraorbital nerve injury (IONI)-induced orofacial neuropathic pain.

MATERIALS AND METHODS

Mechanical head-withdrawal threshold (MHWT) was measured in IONI-treated rats receiving intra-TG Panx1 inhibitor or metabotropic glutamate receptor 5 (mGluR5) antagonist administration and MHWTs in naive rats receiving intra-TG mGluR5 agonist administration post-IONI. Glutamate and Panx1 in the TG were measured post-IONI. Panx1, mGluR5, and glutamine synthetase expression in TG were immunohistochemically identified, and changes in the number of mGluR5-P2X₃ -expressed TG neurons were examined.

RESULTS

MHWT was significantly decreased post-IONI, and this decrease was reversed by Panx1 inhibition or mGluR5 antagonism. mGluR5 agonism induced a decrease in the MHWT. IONI increased extracellular glutamate in TG. Panx1 was expressed in satellite glial cells and TG neurons, and intra-TG mGluR5 antagonism decreased the number of mGluR5 and P2X₃ positive TG neurons post-IONI.

CONCLUSIONS

IONI facilitates glutamate release via Panx1 that activates mGluR5 which was expressed in the nociceptive TG neurons innervating the orofacial region. In turn, P2X₃ receptor-expressed TG neurons is enhanced via mGluR5 signaling, resulting in orofacial neuropathic pain.

Pathophysiological Role of Purinergic P2X Receptors in Digestive System Diseases

P2X receptors (P2XRs) are trimeric, non-selective cation channels activated by extracellular ATP and widely distributed in the digestive system. P2XRs have an important role in the physiological function of the digestive system, such as neurotransmission, ion transports, proliferation and apoptosis, muscle contraction, and relaxation. P2XRs can be involved in pain mechanisms both centrally and in the periphery and confirmed the association of P2XRs with visceral pain. In the periphery, ATP can be released as a result of tissue injury, visceral distension, or sympathetic activation and can excite nociceptive primary afferents by acting at homomeric P2X(3)R or heteromeric P2X(2/3)R. Thus, peripheral P2XRs, and homomeric P2X(3) and/or heteromeric P2X(2/3)R in particular, constitute attractive targets for analgesic drugs. Recently studies have shown that P2XRs have made significant advances in inflammation and cancer. P2X7R mediates NLRP3 inflammasome activation, cytokine and chemokine release, T lymphocyte survival and differentiation, transcription factor activation, and cell death. The P2X7R is a potent stimulant of inflammation and immunity and a promoter of cancer cell growth. This makes P2X7R an appealing target for anti-inflammatory and anti-cancer therapy. It is believed that with the further study of P2XRs and its subtypes, P2XRs and its specific antagonists will be expected to be widely used in the treatment of human digestive diseases in the future.

Purinergic receptor ligands: the cytokine storm attenuators, potential therapeutic agents for the treatment of COVID-19

The coronavirus disease-19 (COVID-19), at first, was reported in Wuhan, China, and then rapidly became pandemic throughout the world. Cytokine storm syndrome (CSS) in COVID-19 patients is associated with high levels of cytokines and chemokines that cause multiple organ failure, systemic inflammation, and hemodynamic instabilities. Acute respiratory distress syndrome (ARDS), a common complication of COVID-19, is a consequence of cytokine storm. In this regard, several drugs have been being investigated to suppress this inflammatory condition. Purinergic signaling receptors comprising of P1 adenosine and P2 purinoceptors play a critical role in inflammation. Therefore, activation or inhibition of some subtypes of these kinds of receptors is most likely to be beneficial to attenuate cytokine storm. This article summarizes suggested therapeutic drugs with potential anti-inflammatory effects through purinergic receptors.

Gene expression analysis in NSAID-induced rat small intestinal disease model with the intervention of berberine by the liquid chip technology

Objective

Investigate the effect and mechanism of berberine on the small intestinal mucosa of non-steroidal anti-inflammatory drugs (NSAIDs) related small intestinal injury.

Materials and methods

Twenty-four SD rats were randomly divided into control group, model group and intervention group. The model group and intervention group were treated with diclofenac (7.5 mg/kg·d, 2/d), a total of 4 days tube feeding, and the intervention group was treated with 50 mg/kg·d intragastric administration of berberine after 2 days. The control group was treated with 7.5 mg/kg·d, 2/d 0.9% saline tube feeding. Then we screened differential expression of colonic mucosal gene by the liquid chip technology.

Results

Compared with the control group, macroscopic and histology score of the model group increased significantly (P < 0.05), HTR4, HTR1a, F2RL3, CALCA, NPY, CRHR2, IL1b, P2RX3, TPH1, HMOX1, TRPV1, VIP, F2RL1, SLC6A4, TFF2, AQP8 content were significantly increased (P < 0.05), NOS1 content decreased significantly (P < 0.05); Compared with the model group, macroscopic and histology score of the intervention group improved significantly (P < 0.05), and HTR4, F2RL3, NPY, CRHR2, IL1b, VIP, AQP8 content were significantly lower (P < 0.05), NOS1 content increased significantly (P < 0.05).

Conclusion

Berberine has a protective effect on NSAID-associated small intestinal injury, the mechanism may be that berberine decreases the expression of intestinal mucosa HTR4, F2RL3, NPY, CRHR2, IL1b, VIP, AQP8, and increases the expression of NOS1, that to reduce intestinal permeability and protect intestinal mucosal barrier.

Purinergic P2 Receptors: Novel Mediators of Mechanotransduction

Mechanosensing and mechanotransduction are vital processes in mechanobiology and play critical roles in regulating cellular behavior and fate. There is increasing evidence that purinergic P2 receptors, members of the purinergic family, play a crucial role in cellular mechanotransduction. Thus, information on the specific mechanism of P2 receptor-mediated mechanotransduction would be valuable. In this review, we focus on purinergic P2 receptor signaling pathways and describe in detail the interaction of P2 receptors with other mechanosensitive molecules, including transient receptor potential channels, integrins, caveolae-associated proteins and hemichannels. In addition, we review the activation of purinergic P2 receptors and the role of various P2 receptors in the regulation of various pathophysiological processes induced by mechanical stimuli.

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.

Neuromechanism of acupuncture regulating gastrointestinal motility

Acupuncture has been used in China for thousands of years and has become more widely accepted by doctors and patients around the world. A large number of clinical studies and animal experiments have confirmed that acupuncture has a benign adjustment effect on gastrointestinal (GI) movement; however, the mechanism of this effect is unclear, especially in terms of neural mechanisms, and there are still many areas that require further exploration. This article reviews the recent data on the neural mechanism of acupuncture on GI movements. We summarize the neural mechanism of acupuncture on GI movement from four aspects: acupuncture signal transmission, the sympathetic and parasympathetic nervous system, the enteric nervous system, and the central nervous system.

Epithelial-Neuronal Communication in the Colon: Implications for Visceral Pain

Visceral hypersensitivity and pain result, at least in part, from increased excitability of primary afferents that innervate the colon. In addition to intrinsic changes in these neurons, emerging evidence indicates that changes in lining epithelial cells may also contribute to increased excitability. Here we review recent studies on how colon epithelial cells communicate directly with colon afferents. Specifically, anatomical studies revealed specialized synaptic connections between epithelial cells and nerve fibers and studies using optogenetic activation of the epithelium showed initiation of pain-like responses. We review the possible mechanisms of epithelial-neuronal communication and provide an overview of the possible neurotransmitters and receptors involved. Understanding the biology of this interface and how it changes in pathological conditions may provide new treatments for visceral pain conditions.

Update on novel purinergic P2X3 and P2X2/3 receptor antagonists and their potential therapeutic applications

Introduction: Purinergic P2X3-P2X2/3 receptors are placed in nociceptive neurons' strategic location and show unique desensitization properties; hence, they represent an attractive target for many pain-related diseases. Therefore, a broad interest from academic and pharmaceutical scientists has focused on the search for P2X3 and P2X2/3 receptor ligands and has led to the discovery of numerous new selective antagonists. Some of them have been studied in clinical trials for the treatment of pathological conditions such as bladder disorders, gastrointestinal and chronic obstructive pulmonary diseases.Areas covered: This review provides a summary of the patents concerning the discovery of P2X3 and/or P2X2/3 receptor antagonists published between 2015 and 2019 and their potential clinical use. Thus, the structures and biological data of the most representative molecules are reported.Expert opinion: The 2016 publication of the crystallographic structure of the human P2X3 receptor subtype gave an improvement of published patents in 2017. Hence, a great number of small molecules with dual antagonist activity on P2X3-P2X2/3 receptors, a favorable pharmacokinetic profile, and reasonable oral bioavailability was discovered. The most promising compounds are the phenoxy-diaminopyrimidines including gefapixant (AF-219), and the imidazo-pyridines like BLU-5937, which are in phase III and phase II clinical trials, respectively, for refractory chronic cough.

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.

Inhibitory effect of estrogen receptor beta on P2X3 receptors during inflammation in rats

Estrogen receptor beta (ERβ) has been shown to play a therapeutic role in inflammatory bowel disease (IBD). However, the mechanism underlying how ERβ exerts therapeutic effects and its relationship with P2X3 receptors (P2X3R) in rats with inflammation is not known. In our study, animal behavior tests, visceromotor reflex recording, and Western blotting were used to determine whether the therapeutic effect of ERβ in rats with inflammation was related with P2X3R. In complete Freund adjuvant (CFA)-induced chronic inflammation in rats, paw withdrawal threshold was significantly decreased which were then reversed by systemic injection of ERβ agonists, DPN or ERB-041. In 2,4,6-trinitrobenzene sulfonic acid (TNBS)-induced colitis in rats, weight loss, higher DAI scores, increased visceromotor responses, and inflammatory responses were reversed by application of DPN or ERB-041. The higher expressions of P2X3R in dorsal root ganglia (DRG) of CFA-treated rats and those in rectocolon and DRG of TNBS-treated rats were all decreased by injection of DPN or ERB-041. DPN application also inhibited P2X3R-evoked inward currents in DRG neurons from TNBS rats. Mechanical hyperalgesia and increased P2X3 expression in ovariectomized (OVX) CFA-treated rats were reversed by estrogen replacements. Furthermore, the expressions of extracellular signal-regulated kinase (ERK) in DRG and spinal cord dorsal horn (SCDH) and c-fos in SCDH were significantly decreased after estrogen replacement compared with those of OVX rats. The ERK antagonist U0126 significantly reversed mechanical hyperalgesia in the OVX rats. These results suggest that estrogen may play an important therapeutic role in inflammation through down-regulation of P2X3R in peripheral tissues and the nervous system, probably via ERβ, suggesting a novel therapeutic strategy for clinical treatment of inflammation.

Co-localization of Pirt protein and P2X2 receptors in the mouse enteric nervous system

P2X2 receptors, with other P2X receptor subtypes, have an important role mediating synaptic transmission in regulating the functions of the gastrointestinal tract. Our recent work has found a new regulator of P2X receptor function, called phosphoinositide-interacting regulator of transient receptor potential channels (Pirt). In the present work, we have shown that Pirt immunoreactivity was localized in nerve cell bodies and nerve fibers in the myenteric plexus of the stomach, ileum, proximal, and distal colon and in the submucosal plexus of the jejunum, ileum, proximal, and distal colon. Almost all the Pirt-immunoreactive (ir) neurons were also P2X2-ir, and co-immunoprecipitation experiments have shown that Pirt co-precipitated with the anti-P2X2 antibody. This work provides detailed information about the expression of Pirt in the gut and its co-localization with P2X2, indicating its potential role in influencing P2X2 receptor function.

Neuroimmunomodulation in the Gut: Focus on Inflammatory Bowel Disease

Intestinal immunity is finely regulated by several concomitant and overlapping mechanisms, in order to efficiently sense external stimuli and mount an adequate response of either tolerance or defense. In this context, a complex interplay between immune and nonimmune cells is responsible for the maintenance of normal homeostasis. However, in certain conditions, the disruption of such an intricate network may result in intestinal inflammation, including inflammatory bowel disease (IBD). IBD is believed to result from a combination of genetic and environmental factors acting in concert with an inappropriate immune response, which in turn interacts with nonimmune cells, including nervous system components. Currently, evidence shows that the interaction between the immune and the nervous system is bidirectional and plays a critical role in the regulation of intestinal inflammation. Recently, the maintenance of intestinal homeostasis has been shown to be under the reciprocal control of the microbiota by immune mechanisms, whereas intestinal microorganisms can modulate mucosal immunity. Therefore, in addition to presenting the mechanisms underlying the interaction between immune and nervous systems in the gut, here we discuss the role of the microbiota also in the regulation of neuroimmune crosstalk involved in intestinal homeostasis and inflammation, with potential implications to IBD pathogenesis.

P2Y Receptors Sensitize Mouse and Human Colonic Nociceptors

Activation of visceral nociceptors by inflammatory mediators contributes to visceral hypersensitivity and abdominal pain associated with many gastrointestinal disorders. Purine and pyrimidine nucleotides (e.g., ATP and UTP) are strongly implicated in this process following their release from epithelial cells during mechanical stimulation of the gut, and from immune cells during inflammation. Actions of ATP are mediated through both ionotropic P2X receptors and metabotropic P2Y receptors. P2X receptor activation causes excitation of visceral afferents; however, the impact of P2Y receptor activation on visceral afferents innervating the gut is unclear. Here we investigate the effects of stimulating P2Y receptors in isolated mouse colonic sensory neurons, and visceral nociceptor fibers in mouse and human nerve-gut preparations. Additionally, we investigate the role of Nav1.9 in mediating murine responses. The application of UTP (P2Y2 and P2Y4 agonist) sensitized colonic sensory neurons by increasing action potential firing to current injection and depolarizing the membrane potential. The application of ADP (P2Y1, P2Y12, and P2Y13 agonist) also increased action potential firing, an effect blocked by the selective P2Y1 receptor antagonist MRS2500. UTP or ADP stimulated afferents, including mouse and human visceral nociceptors, in nerve-gut preparations. P2Y1 and P2Y2 transcripts were detected in 80% and 56% of retrogradely labeled colonic neurons, respectively. Nav1.9 transcripts colocalized in 86% of P2Y1-positive and 100% of P2Y2-positive colonic neurons, consistent with reduced afferent fiber responses to UTP and ADP in Na(v)1.9(-/-) mice. These data demonstrate that P2Y receptor activation stimulates mouse and human visceral nociceptors, highlighting P2Y-dependent mechanisms in the generation of visceral pain during gastrointestinal disease.

Optogenetic activation of mechanically insensitive afferents in mouse colorectum reveals chemosensitivity

The sensory innervation of the distal colorectum includes mechanically insensitive afferents (MIAs; ∼25%), which acquire mechanosensitivity in persistent visceral hypersensitivity and thus generate de novo input to the central nervous system. We utilized an optogenetic approach to bypass the process of transduction (generator potential) and focus on transformation (spike initiation) at colorectal MIA sensory terminals, which is otherwise not possible in typical functional studies. From channelrhodopsin2-expressing mice (driven by Advillin-Cre), the distal colorectum with attached pelvic nerve was harvested for ex vivo single-fiber recordings. Afferent receptive fields (RFs) were identified by electrical stimulation and tested for response to mechanical stimuli (probing, stroking, and stretch), and afferents were classified as either MIAs or mechanosensitive afferents (MSAs). All MIA and MSA RFs were subsequently stimulated optically and MIAs were also tested for activation/sensitization with inflammatory soup (IS), acidic hypertonic solution (AHS), and/or bile salts (BS). Responses to pulsed optical stimuli (1-10 Hz) were comparable between MSAs and MIAs whereas 43% of MIAs compared with 86% of MSAs responded tonically to stepped optical stimuli. Tonic-spiking MIAs responded preferentially to AHS (an osmotic stimulus) whereas non-tonic-spiking MIAs responded to IS (an inflammatory stimulus). A significant proportion of MIAs were also sensitized by BS. These results reveal transformation as a critical factor underlying the differences between MIAs (osmosensors vs. inflammatory sensors), revealing a previously unappreciated heterogeneity of MIA endings. The current study draws attention to the sensory encoding of MIA nerve endings that likely contribute to afferent sensitization and thus have important roles in visceral pain.

Purinergic Mechanisms and Pain

There is a brief introductory summary of purinergic signaling involving ATP storage, release, and ectoenzymatic breakdown, and the current classification of receptor subtypes for purines and pyrimidines. The review then describes purinergic mechanosensory transduction involved in visceral, cutaneous, and musculoskeletal nociception and on the roles played by receptor subtypes in neuropathic and inflammatory pain. Multiple purinoceptor subtypes are involved in pain pathways both as an initiator and modulator. Activation of homomeric P2X3 receptors contributes to acute nociception and activation of heteromeric P2X2/3 receptors appears to modulate longer-lasting nociceptive sensitivity associated with nerve injury or chronic inflammation. In neuropathic pain activation of P2X4, P2X7, and P2Y12 receptors on microglia may serve to maintain nociceptive sensitivity through complex neural-glial cell interactions and antagonists to these receptors reduce neuropathic pain. Potential therapeutic approaches involving purinergic mechanisms will be discussed.

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.

Acupoint Specificity on Colorectal Hypersensitivity Alleviated by Acupuncture and the Correlation with the Brain-Gut Axis

This project was focused on the study of the effect of the different acupoints on visceral hypersensitivity and the correlation with the brain-gut axis. By using a mouse model of zymosan-induced colorectal hypersensitivity, and observing the response of hypersensitivity model to colorectal distension stimulation in acupuncture at different acupoints, we selected the specific acupoints. With immunohistochemical staining method, we observed c-fos expression, distribution and changes after acupuncture on sensory pathway, including colorectum, spinal dorsal horn and different regions of brain center in the model with colorectal distension stimulation, and evaluated the acupuncture effect on brain-gut axis. The results revealed that the effectiveness of acupuncture for alleviating visceral hypersensitivity was different at individual acupoint, meaning Tianshu (ST25), Zusanli (ST36) and Shangjuxu (ST37) > Quchi (LI11) and Dachangshu (BL25) > Ciliao (BL32). C-fos expression was concentrated in anterior cingulate cortex, hypothalamus, spinal dorsal horn and colorectum in model of zymosan-induced colorectal hypersensitivity and it was down-regulated after acupuncture. The results demonstrates that the acupoint specificity presents in acupuncture for relieving visceral hypersensitivity and the effects are more predominated at the acupoints on stomach meridian innervated by the same or adjacent spinal ganglion segments. The model of zymosan-induced colorectal hypersensitivity can be the animal model simulating brain-gut interaction.

P2X3 receptors mediate visceral hypersensitivity during acute chemically-induced colitis and in the post-inflammatory phase via different mechanisms of sensitization

Objectives

Experiments using P2X₃ knock-out mice or more general P2X receptor antagonists suggest that P2X₃ receptors contribute to visceral hypersensitivity. We aimed to investigate the effect of the selective P2X₃ antagonist A-317491 on visceral sensitivity under physiological conditions, during acute colitis and in the post-inflammatory phase of colitis.

Methods

Trinitrobenzene sulphonic-acid colitis was monitored by colonoscopy: on day 3 to confirm the presence of colitis and then every 4 days, starting from day 10, to monitor convalescence and determine the exact timepoint of endoscopic healing in each rat. Visceral sensitivity was assessed by quantifying visceromotor responses to colorectal distension in controls, rats with acute colitis and post-colitis rats. A-317491 was administered 30 min prior to visceral sensitivity testing. Expression of P2X₃ receptors (RT-PCR and immunohistochemistry) and the intracellular signalling molecules cdk5, csk and CASK (RT-PCR) were quantified in colonic tissue and dorsal root ganglia. ATP release in response to colorectal distension was measured by luminiscence.

Results

Rats with acute TNBS-colitis displayed significant visceral hypersensitivity that was dose-dependently, but not fully, reversed by A-317491. Hypersenstivity was accompanied by an increased colonic release of ATP. Post-colitis rats also displayed visceral hypersensitivity that was dose-dependently reduced and fully normalized by A-317491 without increased release of ATP. A-317491 did not modify visceral sensitivity in controls. P2X₃ mRNA and protein expression in the colon and dorsal root ganglia were similar in control, acute colitis and post-colitis groups, while colonic mRNA expression of cdk5, csk and CASK was increased in the post-colitis group only.

Conclusions

These findings indicate that P2X₃ receptors are not involved in sensory signaling under physiological conditions whereas they modulate visceral hypersensitivity during acute TNBS-colitis and even more so in the post-inflammatory phase, albeit via different mechanisms of sensitization, validating P2X₃ receptors as potential new targets in the treatment of abdominal pain syndromes.

Effect of electroacupuncture on P2X3 receptor regulation in the peripheral and central nervous systems of rats with visceral pain caused by irritable bowel syndrome

The aim of this study is to investigate the role of the purinergic receptor P2X3 in the peripheral and central nervous systems during acupuncture treatment for the visceral pain of irritable bowel syndrome (IBS). A total of 24 8-day-old Sprague-Dawley (SD) neonatal male rats (SPF grade) were stimulated using colorectal distention (CRD) when the rats were awake. The modeling lasted for 2 weeks with one stimulation per day. After 6 weeks, the rats were randomly divided into three groups of eight each: (1) the normal group (NG, n = 8); (2) the model group (MG, n = 8); and (3) the model + electroacupuncture group (EA, n = 8) that received electroacupuncture at a needling depth of 5 mm at the Shangjuxu (ST37, bilateral) and Tianshu (ST25, bilateral) acupoints. The parameters of the Han's acupoint nerve stimulator (HANS) were as follows: sparse-dense wave with a frequency of 2/100 Hz, current of 2 mA, 20 min/stimulation, and one stimulation per day; the treatment was provided for seven consecutive days. At the sixth week after the treatment, the abdominal withdrawal reflex (AWR) score was determined; immunofluorescence and immunohistochemistry were used to measure the expression of the P2X3 receptor in myenteric plexus neurons, prefrontal cortex, and anterior cingulate cortex; and, a real-time PCR assay was performed to measure the expression of P2X3 messenger RNA (mRNA) in the dorsal root ganglion (DRG) and spinal cord. After stimulation with CRD, the expression levels of the P2X3 receptor in the inter-colonic myenteric plexus, DRG, spinal cord, prefrontal cortex, and anterior cingulate cortex were upregulated, and the sensitivity of the rats to IBS visceral pain was increased. Electroacupuncture (EA) could downregulate the expression of the P2X3 receptor and ease the sensitivity to visceral pain. The P2X3 receptor plays an important role in IBS visceral pain. The different levels of P2X3 in the peripheral enteric nervous system and central nervous system mediate the effects of the EA treatment of the visceral hyperalgesia of IBS.

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.

Acupuncture for visceral pain: neural substrates and potential mechanisms

Visceral pain is the most common form of pain caused by varied diseases and a major reason for patients to seek medical consultation. Despite much advances, the pathophysiological mechanism is still poorly understood comparing with its somatic counterpart and, as a result, the therapeutic efficacy is usually unsatisfactory. Acupuncture has long been used for the management of numerous disorders in particular pain and visceral pain, characterized by the high therapeutic benefits and low adverse effects. Previous findings suggest that acupuncture depresses pain via activation of a number of neurotransmitters or modulators including opioid peptides, serotonin, norepinephrine, and adenosine centrally and peripherally. It endows us, by advancing the understanding of the role of ion channels and gut microbiota in pain process, with novel perspectives to probe the mechanisms underlying acupuncture analgesia. In this review, after describing the visceral innervation and the relevant afferent pathways, in particular the ion channels in visceral nociception, we propose three principal mechanisms responsible for acupuncture induced benefits on visceral pain. Finally, potential topics are highlighted regarding the future studies in this field.

Potential for developing purinergic drugs for gastrointestinal diseases

Treatments for inflammatory bowel disease (IBD), irritable bowel syndrome (IBS), functional dyspepsia, or motility disorders are not adequate, and purinergic drugs offer exciting new possibilities. Gastrointestinal symptoms that could be targeted for therapy include visceral pain, inflammatory pain, dysmotility, constipation, and diarrhea. The focus of this review is on the potential for developing purinergic drugs for clinical trials to treat gastrointestinal symptoms. Purinergic receptors are divided into adenosine P1 (A(1), A(2A), A(2B), A(3)), ionotropic ATP-gated P2X ion channel (P2X(1-7)), or metabotropic P2Y(1,2,4,6,11-14) receptors. There is good experimental evidence for targeting A(2A), A(2B), A(3), P2X(7), and P2X(3) receptors or increasing endogenous adenosine levels to treat IBD, inflammatory pain, IBS/visceral pain, inflammatory diarrhea, and motility disorders. Purine genes are also potential biomarkers of disease. Advances in medicinal chemistry have an accelerated pace toward clinical trials: Methotrexate and sulfasalazine, used to treat IBD, act by stimulating CD73-dependent adenosine production. ATP protects against NSAID-induced enteropathy and has pain-relieving properties in humans. A P2X(7)R antagonist AZD9056 is in clinical trials for Crohn's disease. A(3) adenosine receptor drugs target inflammatory diseases (e.g., CF101, CF102). Dipyridamole, a nucleoside uptake inhibitor, is in trials for endotoxemia. Drugs for pain in clinical trials include P2X(3)/P2X(2/3) (AF-219) and P2X(7) (GSK1482160) antagonists and A(1) (GW493838) or A(2A) (BVT.115959) agonists. Iberogast is a phytopharmacon targeting purine mechanisms with efficacy in IBS and functional dyspepsia. Purinergic drugs have excellent safety/efficacy profile for prospective clinical trials in IBD, IBS, functional dyspepsia, and inflammatory diarrhea. Genetic polymorphisms and caffeine consumption may affect susceptibility to treatment. Further studies in animals can clarify mechanisms and test new generation drugs. Finally, there is still a huge gap in our knowledge of human pathophysiology of purinergic signaling.

Effects of NB001 and gabapentin on irritable bowel syndrome-induced behavioral anxiety and spontaneous pain

Irritable bowel syndrome (IBS) is characterized by recurrent abdominal discomfort, spontaneous pain, colorectal hypersensitivity and bowel dysfunction. Patients with IBS also suffer from emotional anxiety and depression. However, few animal studies have investigated IBS-induced spontaneous pain and behavioral anxiety. In this study, we assessed spontaneous pain and anxiety behaviors in an adult mouse model of IBS induced by zymosan administration. By using Fos protein as a marker, we found that sensory and emotion related brain regions were activated at day 7 after the treatment with zymosan; these regions include the prefrontal cortex, anterior cingulate cortex, insular cortex and amygdala. Behaviorally, zymosan administration triggered spontaneous pain (decreased spontaneous activities in the open field test) and increased anxiety-like behaviors in three different tests (the open field, elevated plus maze and light/dark box tests). Intraperitoneal injection of NB001, an adenylyl cyclase 1 (AC1) inhibitor, reduced spontaneous pain but had no significant effect on behavioral anxiety. In contrast, gabapentin reduced both spontaneous pain and behavioral anxiety. These results indicate that NB001 and gabapentin may inhibit spontaneous pain and anxiety-like behaviors through different mechanisms.

Main ion channels and receptors associated with visceral hypersensitivity in irritable bowel syndrome

Irritable bowel syndrome (IBS) is a very frequent functional gastrointestinal disorder characterized by recurrent abdominal pain or discomfort and alteration of bowel habits. The IBS physiopathology is extremely complex. Visceral hypersensitivity plays an important role in the pathogenesis of abdominal pain in both in vitro and in vivo models of this functional disorder. In order to obtain a general view of the participation of the main ion channels and receptors regarding the visceral hypersensitivity in the IBS and to describe their chemical structure, a literature review was carried out. A bibliographical research in the following electronic databases: Pubmed and Virtual Library in Health (BVS) was fulfilled by using the search terms "ion channels" "or" "receptors" "and" "visceral hypersensitivity" "or" "visceral nociception" "and" "irritable bowel syndrome". Original and review articles were considered for data acquisition. The activation of the ATP ion-gated channels, voltage-gated sodium (Na_v) and calcium (Ca_v) channels, as well as the activation of protease-activated receptors (PAR2), transient receptor potential vanilloide-1, serotonin, cannabinoids and cholecystokinin are involved in the genesis of visceral hypersensitivity in IBS. The involvement of ion channels and receptors concerning visceral hypersensitivity is noteworthy in IBS models.

Introduction and perspective, historical note

P2 nucleotide receptors were proposed to consist of two subfamilies based on pharmacology in 1985, named P2X and P2Y receptors. Later, this was confirmed following cloning of the receptors for nucleotides and studies of transduction mechanisms in the early 1990s. P2X receptors are ion channels and seven subtypes are recognized that form trimeric homomultimers or heteromultimers. P2X receptors are involved in neuromuscular and synaptic neurotransmission and neuromodulation. They are also expressed on many types of non-neuronal cells to mediate smooth muscle contraction, secretion, and immune modulation. The emphasis in this review will be on the pathophysiology of P2X receptors and therapeutic potential of P2X receptor agonists and antagonists for neurodegenerative and inflammatory disorders, visceral and neuropathic pain, irritable bowel syndrome, diabetes, kidney failure, bladder incontinence and cancer, as well as disorders if the special senses, airways, skin, cardiovascular, and musculoskeletal systems.

VGLUTs in Peripheral Neurons and the Spinal Cord: Time for a Review

Vesicular glutamate transporters (VGLUTs) are key molecules for the incorporation of glutamate in synaptic vesicles across the nervous system, and since their discovery in the early 1990s, research on these transporters has been intense and productive. This review will focus on several aspects of VGLUTs research on neurons in the periphery and the spinal cord. Firstly, it will begin with a historical account on the evolution of the morphological analysis of glutamatergic systems and the pivotal role played by the discovery of VGLUTs. Secondly, and in order to provide an appropriate framework, there will be a synthetic description of the neuroanatomy and neurochemistry of peripheral neurons and the spinal cord. This will be followed by a succinct description of the current knowledge on the expression of VGLUTs in peripheral sensory and autonomic neurons and neurons in the spinal cord. Finally, this review will address the modulation of VGLUTs expression after nerve and tissue insult, their physiological relevance in relation to sensation, pain, and neuroprotection, and their potential pharmacological usefulness.

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.

Afferent Arteriolar Responses to β,γ-methylene ATP and 20-HETE are not Blocked by ENaC Inhibition

Afferent arteriolar myogenic and tubuloglomerular feedback responses are critical for the proper maintenance of renal hemodynamics and water and electrolyte homeostasis. Adenosine triphosphate (ATP) P2X receptor activation and 20-hydroxyeicosatetraenoic acids (20-HETE) have been implicated in afferent arteriolar autoregulatory responses. Besides these two participants, members of the degenerin/epithelial Na⁺ channel (DEG/ENaC) family have been demonstrated to play a pivotal role in the afferent arteriolar myogenic response. The aim of this study was to determine if ENaC contributes to P2X receptor- or 20-HETE-mediated afferent arteriolar vasoconstriction. As previously demonstrated, afferent arteriolar diameter responses to increasing perfusion pressure from 100 to 160 mmHg were abolished by ENaC inhibitors amiloride or benzamil. Afferent arteriolar diameter decreased by 29% under control conditions and by 1% and 5% in the presence of amiloride or benzamil, respectively. The P2X receptor agonist β,γ-methylene ATP decreased afferent arteriolar diameter by 3 ± 1%, 7 ± 1%, 12 ± 2%, and 17 ± 3% in response to 0.1, 1, 10, and 100 μmol/L, respectively. ENaC inhibition did not alter the afferent arteriolar vasoconstrictor response to the P2X receptor agonist β,γ-methylene ATP. Like P2X receptor activation, 20-HETE dose-dependently decreased afferent arteriolar diameter and this vasoconstrictor response was not altered by the presence of ENaC inhibitors amiloride or benzamil. These results suggest that DEG/ENaC channels are required for afferent arteriolar autoregulatory responses; however, DEG/ENaC channels do not contribute to P2X receptor- or 20-HETE-mediated afferent arteriolar vasoconstriction.

Purinergic mechanisms and pain--an update

There is a brief summary of the background literature about purinergic signalling. The review then considers purinergic mechanosensory transduction involved in visceral, cutaneous and musculoskeletal nociception and on the roles played by P2X3, P2X2/3, P2X4, P2X7 and P2Y₁₂ receptors in neuropathic and inflammatory pain. Current developments of compounds for the therapeutic treatment of both visceral and neuropathic pain are discussed.

Acupuncture Alleviates Colorectal Hypersensitivity and Correlates with the Regulatory Mechanism of TrpV1 and p-ERK

Here we used a mouse model of zymosan-induced colorectal hypersensitivity, a similar model of IBS in our previous work, to evaluate the effectiveness of the different number of times of acupuncture and elucidate its potential mechanism of EA treatment. Colorectal distension (CRD) tests show that intracolonic zymosan injection does, while saline injection does not, induce a typical colorectal hypersensitivity. EA treatment at classical acupoints Zusanli (ST36) and Shangjuxu (ST37) in both hind limbs for 15 min slightly attenuated and significantly blunted the hypersensitive responses after first and fifth acupunctures, respectively, to colorectal distention in zymosan treatment mice, but not in saline treatment mice. Western blot results indicated that ion channel and TrpV1 expression in colorectum as well as ERK1/2 MAPK pathway activation in peripheral and central nerve system might be involved in this process. Hence, we conclude that EA is a potential therapeutic tool in the treatment and alleviation of chronic abdominal pain, and the effectiveness of acupuncture analgesia is accumulative with increased number of times of acupuncture when compared to that of a single time of acupuncture.

Neuromodulation by extracellular ATP and P2X receptors in the CNS

Extracellular adenosine 5' triphosphate (ATP) is a widespread cell-to-cell signaling molecule in the brain, where it activates cell surface P2X and P2Y receptors. P2X receptors define a protein family unlike other neurotransmitter-gated ion channels in terms of sequence, subunit topology, assembly, and architecture. Within milliseconds of binding ATP, they catalyze the opening of a cation-selective pore. However, recent data show that P2X receptors often underlie neuromodulatory responses on slower time scales of seconds or longer. Herein, we review these findings at molecular, cellular and systems levels. We propose that, while P2X receptors are fast ligand-gated cation channels, they are most adept at mediating slow neuromodulatory functions that are more widespread and more physiologically utilized than fast ATP synaptic transmission in the CNS.

Drug management of visceral pain: concepts from basic research

Visceral pain is experienced by 40% of the population, and 28% of cancer patients suffer from pain arising from intra- abdominal metastasis or from treatment. Neuroanatomy of visceral nociception and neurotransmitters, receptors, and ion channels that modulate visceral pain are qualitatively or quantitatively different from those that modulate somatic and neuropathic pain. Visceral pain should be recognized as distinct pain phenotype. TRPV1, Na 1.8, and ASIC3 ion channels and peripheral kappa opioid receptors are important mediators of visceral pain. Mu agonists, gabapentinoids, and GABAB agonists reduce pain by binding to central receptors and channels. Combinations of analgesics and adjuvants in animal models have supra-additive antinociception and should be considered in clinical trials. This paper will discuss the neuroanatomy, receptors, ion channels, and neurotransmitters important to visceral pain and provide a basic science rationale for analgesic trials and management.

Long-term sensitization of mechanosensitive and -insensitive afferents in mice with persistent colorectal hypersensitivity

Afferent input contributes significantly to the pain and colorectal hypersensitivity that characterize irritable bowel syndrome. In the present study, we investigated the contributions of mechanically sensitive and mechanically insensitive afferents (MIAs; or silent afferents) to colorectal hypersensitivity. The visceromotor response to colorectal distension (CRD; 15-60 mmHg) was recorded in mice before and for weeks after intracolonic treatment with zymosan or saline. After CRD tests, the distal colorectum with the pelvic nerve attached was removed for single-fiber electrophysiological recordings. Colorectal afferent endings were located by electrical stimulation and characterized as mechanosensitive or not by blunt probing, mucosal stroking, and circumferential stretch. Intracolonic zymosan produced persistent colorectal hypersensitivity (>24 days) associated with brief colorectal inflammation. Pelvic nerve muscular-mucosal but not muscular mechanosensitive afferents recorded from mice with colorectal hypersensitivity exhibited persistent sensitization. In addition, the proportion of MIAs (relative to control) was significantly reduced from 27% to 13%, whereas the proportion of serosal afferents was significantly increased from 34% to 53%, suggesting that MIAs acquired mechanosensitivity. PGP9.5 immunostaining revealed no significant loss of colorectal nerve fiber density, suggesting that the reduction in MIAs is not due to peripheral fiber loss after intracolonic zymosan. These results indicate that colorectal MIAs and sensitized muscular-mucosal afferents that respond to stretch contribute significantly to the afferent input that sustains hypersensitivity to CRD, suggesting that targeted management of colorectal afferent input could significantly reduce patients' complaints of pain and hypersensitivity.

Repeated vulvovaginal fungal infections cause persistent pain in a mouse model of vulvodynia

Provoked vestibulodynia, the most common form of vulvodynia (unexplained pain of the vulva), is a prevalent, idiopathic pain disorder associated with a history of recurrent candidiasis (yeast infections). It is characterized by vulvar allodynia (painful hypersensitivity to touch) and hyperinnervation. We tested whether repeated, localized exposure of the vulva to a common fungal pathogen can lead to the development of chronic pain. A subset of female mice subjected to recurrent Candida albicans infection developed mechanical allodynia localized to the vulva. The mice with allodynia also exhibited hyperinnervation with peptidergic nociceptor and sympathetic fibers (as indicated by increased protein gene product 9.5, calcitonin gene-related peptide, and vesicular monoamine transporter 2 immunoreactivity in the vaginal epithelium). Long-lasting behavioral allodynia in a subset of mice was also observed after a single, extended Candida infection, as well as after repeated vulvar (but not hind paw) inflammation induced with zymosan, a mixture of fungal antigens. The hypersensitivity and hyperinnervation were both present at least 3 weeks after the resolution of infection and inflammation. Our data show that infection can cause persistent pain long after its resolution and that recurrent yeast infection replicates important features of human provoked vulvodynia in the mouse.

Expression of vesicular glutamate transporters type 1 and 2 in sensory and autonomic neurons innervating the mouse colorectum

Vesicular glutamate transporters (VGLUTs) have been extensively studied in various neuronal systems, but their expression in visceral sensory and autonomic neurons remains to be analyzed in detail. Here we studied VGLUTs type 1 and 2 (VGLUT(1) and VGLUT(2) , respectively) in neurons innervating the mouse colorectum. Lumbosacral and thoracolumbar dorsal root ganglion (DRG), lumbar sympathetic chain (LSC), and major pelvic ganglion (MPG) neurons innervating the colorectum of BALB/C mice were retrogradely traced with Fast Blue, dissected, and processed for immunohistochemistry. Tissue from additional naïve mice was included. Previously characterized antibodies against VGLUT(1) , VGLUT(2) , and calcitonin gene-related peptide (CGRP) were used. Riboprobe in situ hybridization, using probes against VGLUT(1) and VGLUT(2) , was also performed. Most colorectal DRG neurons expressed VGLUT(2) and often colocalized with CGRP. A smaller percentage of neurons expressed VGLUT(1) . VGLUT(2) -immunoreactive (IR) neurons in the MPG were rare. Abundant VGLUT(2) -IR nerves were detected in all layers of the colorectum; VGLUT(1) -IR nerves were sparse. A subpopulation of myenteric plexus neurons expressed VGLUT2 protein and mRNA, but VGLUT1 mRNA was undetectable. In conclusion, we show 1) that most colorectal DRG neurons express VGLUT(2) , and to a lesser extent, VGLUT(1) ; 2) abundance of VGLUT2-IR fibers innervating colorectum; and 3) a subpopulation of myenteric plexus neurons expressing VGLUT(2). Altogether, our data suggests a role for VGLUT(2) in colorectal glutamatergic neurotransmission, potentially influencing colorectal sensitivity and motility.

Targeting the visceral purinergic system for pain control

Experimental evidence is presented to support the hypothesis that purinergic mechanosensory transduction can initiate visceral pain in urinary bladder, ureter, gut and uterus. In general, physiological reflexes are mediated via P2X3 and P2X2/3 receptors on low threshold sensory fibres, while these receptors on high threshold sensory fibres mediate pain. Potential therapeutic strategies are considered for the treatment of visceral pain in such conditions as renal colic, interstitial cystitis and inflammatory bowel disease by purinergic agents, including P2X3 and P2X2/3 receptor antagonists that are orally bioavailable and stable in vivo and agents that modulate ATP release and breakdown.

Involvement of peripheral ionotropic glutamate receptors in orofacial thermal hyperalgesia in rats

Background

The purpose of the present study was to elucidate the mechanisms that may underlie the sensitization of trigeminal spinal subnucleus caudalis (Vc) and upper cervical spinal cord (C1-C2) neurons to heat or cold stimulation of the orofacial region following glutamate (Glu) injection.

Results

Glu application to the tongue or whisker pad skin caused an enhancement of head-withdrawal reflex and extracellular signal-regulated kinase (ERK) phosphorylation in Vc-C2 neurons. Head-withdrawal reflex and ERK phosphorylation were also enhanced following cold stimulation of the tongue but not whisker pad skin in Glu-injected rats, and the head-withdrawal reflex and ERK phosphorylation were enhanced following heat stimulation of the tongue or whisker pad skin. The enhanced head-withdrawal reflex and ERK phosphorylation after heat stimulation of the tongue or whisker pad skin, and those following cold stimulation of the tongue but not whisker pad skin were suppressed following ionotropic glutamate receptor antagonists administration into the tongue or whisker pad skin. Furthermore, intrathecal administration of MEK1/2 inhibitor PD98059 caused significant suppression of enhanced head-withdrawal reflex in Glu-injected rats, heat head-withdrawal reflex in the rats with Glu injection into the tongue or whisker pad skin and cold head-withdrawal reflex in the rats with Glu injection into the tongue.

Conclusions

The present findings suggest that peripheral Glu receptor mechanisms may contribute to cold hyperalgesia in the tongue but not in the facial skin, and also contribute to heat hyperalgesia in the tongue and facial skin, and that the mitogen-activated protein kinase cascade in Vc-C2 neurons may be involved in these Glu-evoked hyperalgesic effects.

Laminar shear stress modulates the activity of heterologously expressed P2X(4) receptors

P2X(4) receptors are involved in mechanotransduction processes, but it is unknown whether or not P2X(4) receptors form mechanosensitive ion channels. This study questioned, whether laminar shear stress (LSS) can modulate P2X(4) receptor activity. Mouse P2X(4) receptor was cloned and heterologously expressed in Xenopus laevis oocytes. In two-electrode-voltage-clamp experiments the application of ATP (100μM) produced a transient inward current that was decreased by about 50% upon a second ATP application, corresponding to the desensitization behavior of P2X(4) receptors. In P2X(4) expressing oocytes LSS (shear forces of ~5.1dynes/cm(2)) did not produce any effect. However, LSS modulated the response of P2X(4) to ATP. With LSS (~5.1dynes/cm(2)) the desensitization of the current due to the second ATP application was diminished. Ivermectin (IVM), a compound which stabilizes the open state of P2X(4) receptors, mimicked the effect of LSS (~5.1dynes/cm(2)), since there was no additional effect of LSS after pre-incubation with IVM detected. This indicates that LSS like IVM stabilizes the open state of the receptor, although the particular mechanism remains unknown. These data demonstrate that LSS modulates the activity of P2X(4) receptors by eliminating the desensitization of the receptors in response to ATP probably by stabilizing the open state of the channel.

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

Irritable bowel syndrome is characterized by colorectal hypersensitivity and contributed to by sensitized mechanosensitive primary afferents and recruitment of mechanoinsensitive (silent) afferents. Neurotrophic factors are well known to orchestrate dynamic changes in the properties of sensory neurons. Although pain modulation by proteins in the glial cell line-derived neurotrophic factor (GDNF) family has been documented in various pathophysiological states, their role in colorectal hypersensitivity remains unexplored. Therefore, we investigated the involvement of the GDNF family receptor α-3 (GFRα3) signaling in visceral hypersensitivity by quantifying visceromotor responses (VMR) to colorectal distension before and after intracolonic treatment with 2,4,6-trinitrobenzene sulfonic acid (TNBS). Baseline responses to colorectal distension did not differ between C57BL/6 and GFRα3 knockout (KO) mice. Relative to intracolonic saline treatment, TNBS significantly enhanced the VMR to colorectal distension in C57BL/6 mice 2, 7, 10, and 14 days posttreatment, whereas TNBS-induced visceral hypersensitivity was significantly suppressed in GFRα3 KO mice. The proportion of GFRα3 immunopositive thoracolumbar and lumbosacral colorectal dorsal root ganglion neurons was significantly elevated 2 days after TNBS treatment. In single fiber recordings, responses to circumferential stretch of colorectal afferent endings in C57BL/6 mice were significantly increased (sensitized) after exposure to an inflammatory soup, whereas responses to stretch did not sensitize in GFRα3 KO mice. These findings suggest that enhanced GFRα3 signaling in visceral afferents may contribute to development of colorectal hypersensitivity.

Traditional thoughts on the pathophysiology of irritable bowel syndrome

The pathogenesis of symptoms in irritable bowel syndrome (IBS) is multifactorial and varies from patient to patient. Disturbances of motor function in the small intestine and colon and smooth-muscle dysfunction in other gut and extraintestinal regions are prominent. Abnormalities of sensory function in visceral and somatic structures are detected in most patients with IBS, which may relate to peripheral sensitization or altered central nervous system processing of afferent information. Contributions from psychosocial disturbances are observed in patients from tertiary centers and primary practice. Proof of causation of symptom genesis for most of these factors is limited.