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

Catechol-O-Methyltransferase Loss Drives Cell-Specific Nociceptive Signaling via the Enteric Catechol-O-Methyltransferase/microRNA-155/Tumor Necrosis Factor α Axis

BACKGROUND & AIMS

The etiology of abdominal pain in postinfectious, diarrhea-predominant irritable bowel syndrome (PI-IBS-D) is unknown, and few treatment options exist. Catechol-O-methyltransferase (COMT), an enzyme that inactivates and degrades biologically active catecholamines, plays an important role in numerous physiologic processes, including modulation of pain perception. Our objective was to determine the mechanism(s) of how decreased colonic COMT in PI-IBS-D patients contributes to the chronic abdominal pain phenotype after enteric infections.

METHODS

Colon neurons, epithelial cells, and macrophages were procured with laser capture microdissection from PI-IBS-D patients to evaluate cell-specific colonic COMT, microRNA-155 (miR-155), and tumor necrosis factor (TNF) α expression levels compared to recovered patients (infection cleared: did not develop PI-IBS-D) and control individuals. COMT-/-, colon-specific COMT-/-, and miR-155-/- mice and human colonoids were used to model phenotypic expression of COMT in PI-IBS-D patients and to investigate signaling pathways linking abdominal pain. Citrobacter rodentium and trinitrobenzene sulfonic acid animal models were used to model postinflammatory changes seen in PI-IBS-D patients.

RESULTS

Colonic COMT levels were significantly decreased and correlated with increased visual analog scale abdominal pain ratings in PI-IBS-D patients compared to recovered patients and control individuals. Colonic miR-155 and TNF-α were increased in PI-IBS-D patients with diminished colonic COMT. COMT-/- mice had significantly increased expression of miR-155 and TNF-α in both colon tissues and dorsal root ganglia. Introduction of cV1q antibody (anti-TNF-α) into mice reversed visceral hypersensitivity after C rodentium and trinitrobenzene sulfonic acid.

CONCLUSIONS

Decreased colonic COMT in PI-IBS-D patients drives abdominal pain phenotypes via the COMT/miR-155/TNF-α axis. These important findings will allow new treatment paradigms and more targeted and personalized medicine approaches for gastrointestinal disorders after enteric infections.

The neurobiology of irritable bowel syndrome

Irritable bowel syndrome (IBS) is the most prevalent disorder of brain-gut interactions that affects between 5 and 10% of the general population worldwide. The current symptom criteria restrict the diagnosis to recurrent abdominal pain associated with altered bowel habits, but the majority of patients also report non-painful abdominal discomfort, associated psychiatric conditions (anxiety and depression), as well as other visceral and somatic pain-related symptoms. For decades, IBS was considered an intestinal motility disorder, and more recently a gut disorder. However, based on an extensive body of reported information about central, peripheral mechanisms and genetic factors involved in the pathophysiology of IBS symptoms, a comprehensive disease model of brain-gut-microbiome interactions has emerged, which can explain altered bowel habits, chronic abdominal pain, and psychiatric comorbidities. In this review, we will first describe novel insights into several key components of brain-gut microbiome interactions, starting with reported alterations in the gut connectome and enteric nervous system, and a list of distinct functional and structural brain signatures, and comparing them to the proposed brain alterations in anxiety disorders. We will then point out the emerging correlations between the brain networks with the genomic, gastrointestinal, immune, and gut microbiome-related parameters. We will incorporate this new information into a systems-based disease model of IBS. Finally, we will discuss the implications of such a model for the improved understanding of the disorder and the development of more effective treatment approaches in the future.

Two Cases of Perihepatitis With the Liver Capsule Irritation Sign: A New Physical Examination Technique

Perihepatitis, including Fitz-Hugh-Curtis syndrome, is an uncommon, chronic manifestation of pelvic inflammatory disease usually affecting premenopausal women. It causes right upper quadrant pain due to inflammation of the liver capsule and adhesion of the peritoneum. Since delayed diagnosis of Fitz-Hugh-Curtis syndrome can lead to infertility and other complications, physical examination findings need to be investigated to predict perihepatitis in the early stages of the disease. Here, we hypothesized that perihepatitis is characterized by increased tenderness and spontaneous pain in the right upper abdomen when the patient is placed in the left lateral recumbent position (we termed this indication the "liver capsule irritation sign"). We examined the patients physically for the presence of this liver capsule irritation sign for an early diagnosis of perihepatitis. We report the first two cases of perihepatitis due to Fitz-Hugh-Curtis syndrome in which the liver capsule irritation sign observed during the physical examination was used for diagnosis. The liver capsule irritation sign is caused by two mechanisms: first, the liver falls gravitationally into the left lateral recumbent position, which makes the liver easier to palpate; and second, the peritoneum is stretched and thus stimulated. The second mechanism is that the transverse colon running around the right upper abdomen slumps gravitationally when the patient is in the left lateral recumbent position, allowing for direct palpation of the liver. The liver capsule irritation sign can be a useful physical finding, suggestive of perihepatitis due to Fitz-Hugh-Curtis syndrome. It may also be suitable in cases of perihepatitis caused by factors other than Fitz-Hugh-Curtis syndrome.

Acupuncture and related therapies for the anxiety and depression in irritable bowel syndrome with diarrhea (IBS-D): A network meta-analysis of randomized controlled trials

Objective

A growing number of clinical studies have suggested the value of acupuncture-related therapies for patients with irritable bowel syndrome with diarrhea (IBS-D), and the patient's mental state plays an important role, but there are many types of acupuncture-related therapies involved. This study aimed to evaluate the mental status, efficacy and safety of the different acupuncture-related therapies for IBS-D patients.

Methods

We searched seven databases to collect randomized controlled trials of acupuncture-related therapies for IBS-D. After independent literature screening and data extraction, the quality of the final included literature was evaluated. Hamilton anxiety rating scale (HAMA), hamilton depression rating scale (HAMD), self-rating anxiety scale (SAS), and self-rating depression scale (SDS) was used as the primary outcome indicator. And the network meta-analysis (NMA) was performed by using Revman 5.4, Stata 15.0 and WinBUGS 1.4.3 software, and the surface under the cumulative ranking curve was conducted to rank the included interventions.

Results

We analyzed 24 eligible studies with 1,885 patients, involving eight types of acupuncture and related therapies along with comprehensive therapies. The NMA result shows that: for SAS scores, combined therapies were more efficacious than anti-diarrheal or antispasmodic (western medicine, WM) (SMD: -8.92; 95% CI: -15.30, -2.47); for SDS scores, combined therapies were more efficacious than WM (SMD: -8.45; 95% CI: -15.50, -1.41). For HAMA scores, moxibustion (MOX) was more efficacious than placebo (SMD: -8.66; 95% CI: -16.64, -0.38). For HAMD scores, MOX was more efficacious than all other included interventions. For response rate, MOX was more efficacious than the following interventions: acupuncture (ACU) (SMD:0.29; 95% CI:0.08,0.93), Chinese herb medicine (CH) (SMD:0.09; 95% CI:0.02,0.36), combined therapies (SMD:0.23; 95% CI:0.06, 0.85), electroacupuncture (EA) (SMD:0.06; 95% CI:0.01,0.33), warm acupuncture (WA) (SMD:22.16; 95% CI:3.53,148.10), WM (SMD:15.59; 95% CI:4.68,61.21), and placebo (SMD:9.80; 95% CI:2.90,45.51). Combined therapies were more efficacious than the following interventions: CH (SMD:0.39; 95% CI:0.19,0.80), WA (SMD:4.96; 95% CI:1.30,21.62), and WM (SMD:3.62; 95% CI:2.35,5.66). The comprehensive ranking results show that MOX, ACU, combined therapies, and EA had high SUCRA rankings involving different outcome indicators.

Conclusion

MOX, ACU, combined therapies, and EA better alleviate anxiety and depression among IBS-D patients, and with a higher safety level, may be the optimal therapies. In addition, combining acupuncture-related treatments and other therapies also delivers a higher global benefit level.

Systematic review registration

[https://www.crd.york.ac.uk/], identifier [CRD42022364560].

The Effect of Serine Protease Inhibitors on Visceral Pain in Different Rodent Models With an Intestinal Insult

Background: Serine proteases are believed to play a key role in the origin of abdominal pain in IBD and IBS. We previously demonstrated a reduction of visceral pain in a post-inflammatory IBS rat model after a single intraperitoneal or intracolonic administration of a serine protease inhibitor. The aim of this study was to investigate the efficacy of serine protease inhibition on visceral pain in two different animal models involving a colonic insult based either on acute inflammation or on neonatal irritation. Moreover, protease profiling was explored in the acute colitis model. Methods: An acute 2,4,6-trinitrobenzenesulphonic acid (TNBS) colitis rat model and a chronic neonatal acetic acid mouse model were used in this study. Visceral sensitivity was quantified by visceromotor responses (VMRs) to colorectal distension, 30 min after intraperitoneal administration of the serine protease inhibitors nafamostat, UAMC-00050 or their vehicles. Colonic samples from acute colitis rats were used to quantify the mRNA expression of a panel of serine proteases and mast cell tryptase by immunohistochemistry. Finally, proteolytic activities in colonic and fecal samples were characterized using fluorogenic substrates. Key Results: We showed a significant and pressure-dependent increase in visceral hypersensitivity in acute colitis and neonatal acetic acid models. UAMC-00050 and nafamostat significantly reduced VMRs in both animal models. In acute colitis rats, the administration of a serine protease inhibitor did not affect the inflammatory parameters. Protease profiling of these acute colitis animals revealed an increased tryptase immunoreactivity and a downregulation of matriptase at the mRNA level after inflammation. The administration of UAMC-00050 resulted in a decreased elastase-like activity in the colon associated with a significantly increased elastase-like activity in fecal samples of acute colitis animals. Conclusion: In conclusion, our results suggest that serine proteases play an important role in visceral hypersensitivity in an acute TNBS colitis model in rats and a neonatal acetic acid model in mice. Moreover, we hypothesize a potential mechanism of action of UAMC-00050 via the alteration of elastase-like proteolytic activity in acute inflammation. Taken together, we provided fundamental evidence for serine protease inhibitors as a promising new therapeutic strategy for abdominal pain in gastrointestinal diseases.

Visceral sensitivity modulation by faecal microbiota transplantation: the active role of gut bacteria in pain persistence

ABSTRACT

Recent findings linked gastrointestinal disorders characterized by abdominal pain to gut microbiota composition. The present work aimed to evaluate the power of gut microbiota as a visceral pain modulator and, consequently, the relevance of its manipulation as a therapeutic option in reversing postinflammatory visceral pain persistence. Colitis was induced in mice by intrarectally injecting 2,4-dinitrobenzenesulfonic acid (DNBS). The effect of faecal microbiota transplantation from viscerally hypersensitive DNBS-treated and naive donors was evaluated in control rats after an antibiotic-mediated microbiota depletion. Faecal microbiota transplantation from DNBS donors induced a long-lasting visceral hypersensitivity in control rats. Pain threshold trend correlated with major modifications in the composition of gut microbiota and short chain fatty acids. By contrast, no significant alterations of colon histology, permeability, and monoamines levels were detected. Finally, by manipulating the gut microbiota of DNBS-treated animals, a counteraction of persistent visceral pain was achieved. The present results provide novel insights into the relationship between intestinal microbiota and visceral hypersensitivity, highlighting the therapeutic potential of microbiota-targeted interventions.

Comparison of five diarrhea-predominant irritable bowel syndrome (IBS-D) rat models in the brain-gut-microbiota axis

The brain-gut-microbiota (BGM) axis is known to be essential for diarrhea-predominant irritable bowel syndrome (IBS-D). In order to evaluate the effects of IBS-D rat models (the central sensitization model, the peripheral sensitization model and the compound model) on the BGM axis, five models were induced in Wistar rats with 4% acetic acid (AD, dissolved 0.4 ml of AD in 9.6 ml of ultrapure water) + wrap restrain stress (WRS), 4% AD, colorectal distention (CRD), WRS, and neonatal maternal separation (NMS). Abdominal withdrawal reflex (AWR) scale scores and the moisture content of feces (MCF) were evaluated on the day of completing modeling. Body weight was measured every 7 days during modeling. Brain gut peptides, cytokine levels, the activity of spinal cord neurons, intestinal mucosal barrier function, and gut microbiota were determined after induction of IBS-D. We found intervention with 4% AD + WRS, 4% AD, CRD, WRS, and NMS induced a similar course of effects on the BGM axis. Among the five models, AWR scores (60 mmHg, 80 mmHg) were all increased. The levels of 5-hydroxytryptamine, corticotropin-releasing factor, substance P, and calcitonin gene-related protein in serum rapidly increased, whereas that of neuropeptide Y decreased. C-fos in the spinal cord showed increased neuronal activity. The intestinal permeability was increased and the composition and structure of gut microbiota were changed. In conclusion, the five models could cause changes in BGM axis, but the 4% AD + WRS model was closer to the changes BGM axis of post-inflammatory models of IBS-D.

TRPA1 Role in Inflammatory Disorders: What Is Known So Far?

The transient receptor potential ankyrin 1 (TRPA1), a member of the TRP superfamily of channels, is primarily localized in a subpopulation of primary sensory neurons of the trigeminal, vagal, and dorsal root ganglia, where its activation mediates neurogenic inflammatory responses. TRPA1 expression in resident tissue cells, inflammatory, and immune cells, through the indirect modulation of a large series of intracellular pathways, orchestrates a range of cellular processes, such as cytokine production, cell differentiation, and cytotoxicity. Therefore, the TRPA1 pathway has been proposed as a protective mechanism to detect and respond to harmful agents in various pathological conditions, including several inflammatory diseases. Specific attention has been paid to TRPA1 contribution to the transition of inflammation and immune responses from an early defensive response to a chronic pathological condition. In this view, TRPA1 antagonists may be regarded as beneficial tools for the treatment of inflammatory conditions.

The Isolated Mouse Jejunal Afferent Nerve Assay as a Tool to Assess the Effect of Botulinum Neurotoxins in Visceral Nociception

For the past two decades, botulinum neurotoxin A (BoNT/A) has been described as a strong candidate in the treatment of pain. With the production of modified toxins and the potential new applications at the visceral level, there is a real need for tools allowing the assessment of these compounds. In this study, we evaluated the jejunal mesenteric afferent nerve assay to investigate BoNT/A effects on visceral nociception. This ex vivo model allowed the continuous recording of neuronal activity in response to various stimuli. BoNT/A was applied intraluminally during three successive distensions, and the jejunum was distended every 15 min for 3 h. Finally, samples were exposed to external capsaicin. BoNT/A intoxication was validated at the molecular level with the presence of cleaved synaptosomal-associated protein of 25 (SNAP25) in nerve terminals in the mucosa and musculosa layers 3 h after treatment. BoNT/A had a progressive inhibitory effect on multiunit discharge frequency induced by jejunal distension, with a significant decrease from 1 h after application without change in jejunal compliance. The capsaicin-induced discharge was also affected by the toxin. This assay allowed the description of an inhibitory effect of BoNT/A on afferent nerve activity in response to distension and capsaicin, suggesting BoNT/A could alleviate visceral nociception.

Animal Model of Video-Assisted Cecum and Ileum Instrumentation for Equine Visceral Pain Study

This study aimed to describe an animal model for studying equine visceral pain using minimally invasive and video-assisted cecum and ileum instrumentation. The access to the cecum and ileum was affected because of a previous typhlostomy. For video-assisted distention of the cecum and ileum, a distention device, which we developed using an endotracheal Rusch probe, was used, adapted, and coupled to a cuffometer to inflate and measure the pressure of the cuff attached to its distal portion. In a video-assisted manner, the distal portion of the device was introduced into the cecum and ileum, which contained the cuff in its distal portion, properly positioning it in the lumen. The cuff of the distension device was insufflated after the measurement of baseline physiological parameters of the animals and video-assisted confirmation of its right placement in the cecum and ileum lumen (M0). Was performed in one moment through two simultaneous cuff compressions and 1 minute of animal observation to evaluate the degree of abdominal discomfort manifestations (M1). To cease these stimuli, the cuff was deflated by disconnecting the extensor of the distension device attached to its proximal portion (M2). The procedure was easily performed in most cases. Slow and progressive insufflation allowed subjective adjustment of the intensity of the pain stimulus based on behavioral manifestations. Even with a low rate of complications, the model is feasible and reproducible for studies on visceral pain and the evaluation of analgesic effects.

Systematic Review of Systemic and Neuraxial Effects of Acetaminophen in Preclinical Models of Nociceptive Processing

Acetaminophen (APAP) in humans has robust effects with a high therapeutic index in altering postoperative and inflammatory pain states in clinical and experimental pain paradigms with no known abuse potential. This review considers the literature reflecting the preclinical actions of acetaminophen in a variety of pain models. Significant observations arising from this review are as follows: 1) acetaminophen has little effect upon acute nociceptive thresholds; 2) acetaminophen robustly reduces facilitated states as generated by mechanical and thermal hyperalgesic end points in mouse and rat models of carrageenan and complete Freund’s adjuvant evoked inflammation; 3) an antihyperalgesic effect is observed in models of facilitated processing with minimal inflammation (eg, phase II intraplantar formalin); and 4) potent anti-hyperpathic effects on the thermal hyperalgesia, mechanical and cold allodynia, allodynic thresholds in rat and mouse models of polyneuropathy and mononeuropathies and bone cancer pain. These results reflect a surprisingly robust drug effect upon a variety of facilitated states that clearly translate into a wide range of efficacy in preclinical models and to important end points in human therapy. The specific systems upon which acetaminophen may act based on targeted delivery suggest both a spinal and a supraspinal action. Review of current targets for this molecule excludes a role of cyclooxygenase inhibitor but includes effects that may be mediated through metabolites acting on the TRPV1 channel, or by effect upon cannabinoid and serotonin signaling. These findings suggest that the mode of action of acetaminophen, a drug with a long therapeutic history of utilization, has surprisingly robust effects on a variety of pain states in clinical patients and in preclinical models with a good therapeutic index, but in spite of its extensive use, its mechanisms of action are yet poorly understood.

Acute visceral pain relief mediated by A3AR agonists in rats: involvement of N-type voltage-gated calcium channels

ABSTRACT

Pharmacological tools for chronic visceral pain management are still limited and inadequate. A3 adenosine receptor (A3AR) agonists are effective in different models of persistent pain. Recently, their activity has been related to the block of N-type voltage-gated Ca2+ channels (Cav2.2) in dorsal root ganglia (DRG) neurons. The present work aimed to evaluate the efficacy of A3AR agonists in reducing postinflammatory visceral hypersensitivity in both male and female rats. Colitis was induced by the intracolonic instillation of 2,4-dinitrobenzenesulfonic acid (DNBS; 30 mg in 0.25 mL 50% EtOH). Visceral hypersensitivity was assessed by measuring the visceromotor response and the abdominal withdrawal reflex to colorectal distension. The effects of A3AR agonists (MRS5980 and Cl-IB-MECA) were evaluated over time after DNBS injection and compared to that of the selective Cav2.2 blocker PD173212, and the clinically used drug linaclotide. A3AR agonists significantly reduced DNBS-evoked visceral pain both in the postinflammatory (14 and 21 days after DNBS injection) and persistence (28 and 35 days after DNBS) phases. Efficacy was comparable to effects induced by linaclotide. PD173212 fully reduced abdominal hypersensitivity to control values, highlighting the role of Cav2.2. The effects of MRS5980 and Cl-IB-MECA were completely abolished by the selective A3AR antagonist MRS1523. Furthermore, patch-clamp recordings showed that A3AR agonists inhibited Cav2.2 in dorsal root ganglia neurons isolated from either control or DNBS-treated rats. The effect on Ca2+ current was PD173212-sensitive and prevented by MRS1523. A3AR agonists are effective in relieving visceral hypersensitivity induced by DNBS, suggesting a potential therapeutic role against abdominal pain.

Peripheral Guanylate Cyclase-C modulation of corticolimbic activation and corticotropin-releasing factor signaling in a rat model of stress-induced colonic hypersensitivity

BACKGROUND

Psychological stress is a risk factor for irritable bowel syndrome, a functional gastrointestinal pain disorder featuring abnormal brain-gut connectivity. The guanylate cyclase-C (GC-C) agonist linaclotide has been shown to relieve abdominal pain in IBS-C and exhibits antinociceptive effects in rodent models of post-inflammatory visceral hypersensitivity. However, the role GC-C signaling plays in psychological stress-induced visceral hypersensitivity is unknown. Here, we test the hypothesis that GC-C agonism reverses stress-induced colonic hypersensitivity via inhibition of nociceptive afferent signaling resulting in normalization of stress-altered corticotropin-releasing factor (CRF) expression in brain regions involved in pain perception and modulation.

METHODS

Adult female rats were exposed to water avoidance stress or sham stress for 10 days, and the effects of linaclotide on stress-induced changes in colonic sensitivity, corticolimbic phospho-extracellular signal-regulated kinase (pERK), and CRF expression were measured using a combination of behavioral assessments, immunohistochemistry, and qRT-PCR.

KEY RESULTS

Stressed rats exhibited colonic hypersensitivity and elevated corticolimbic pERK on day 11, which was inhibited by linaclotide. qRT-PCR analysis revealed dysregulated CRF expression in the medial prefrontal cortex, paraventricular nucleus of the hypothalamus, and central nucleus of the amygdala on day 28. Dysregulated CRF expression was not affected by linaclotide treatment.

CONCLUSIONS AND INFERENCES

Our results demonstrate that exposure to repeated stress induces chronic colonic hypersensitivity in conjunction with altered corticolimbic activation and CRF expression. GC-C agonism attenuated stress-induced colonic hypersensitivity and ERK phosphorylation, but had no effect on CRF expression, suggesting the analgesic effects of linaclotide occur independent of stress-driven CRF gene expression in corticolimbic circuitry.

The Brain-Gut-Microbiotal Axis: A framework for understanding functional GI illness and their therapeutic interventions

Functional gastrointestinal disorders (FGIDs), characterized by chronic abdominal complaints without a structural or biochemical cause, are common diseases that are frequently encountered by specialists in internal medicine. Collectively, irritable bowel syndrome (IBS) and functional dyspepsia are estimated to affect up to 22% of the population, and are often associated with additional somatic and pain complaints, all without an obvious structural source [1,2]. An appreciation of the current understanding of the mechanistic basis for these disorders is key to developing treatment goals and optimization of patient management strategies. In recent years, the brain-gut axis increasingly has been recognized as a central factor in the experience of functional abdominal pain disorders, including the most recent Rome IV guidelines which identify FGIDs as disorders of gut-brain interaction [3]. The brain-gut axis (BGA), simply defined, is a complex network of bidirectional communication between the central and enteric nervous systems. This axis broadly includes all the systems involved with communication between the GI tract and central nervous system (CNS), with principle inputs into this network occurring between the CNS, enteric nervous system (ENS), and autonomic nervous systems (ANS), but also includes interfaces with numerous other factors, including endocrine hormones and immune effector cells as well as interactions with the gut microbiota. Perturbances to this system have been found to play a critical role in the development of visceral hypersensitivity, bowel dysregulation, and mood. This review will summarize the principle processes involved in the neurologic and biologic function of the brain-gut axis, our current understanding of its role in functional GI disorders, and potential targets for therapeutic intervention.

Dependency on sex and stimulus quality of nociceptive behavior in a conscious visceral pain rat model

Visceral pain may be influenced by many factors. The aim of this study was to analyze the impact of sex and quality of intracolonic mechanical stimulus on the behavioral manifestations of visceral pain in a preclinical model. Male and female young adult Wistar rats were sedated, and a 5 cm long latex balloon was inserted into the colon. Sedation was reverted and behavior was recorded. The pressure of the intracolonic balloon was gradually increased using a sphygmomanometer. Visceral sensitivity was measured as abdominal contractions in response to mechanical intracolonic stimulation. Two different types of stimulation were used: tonic and phasic. Phasic stimulation consisted of repeating several times (3x) the same short stimulus (20 s) within a 5 min interval allowing a 1 min break between individual stimuli. For tonic stimulation the stimulus was maintained throughout the whole 5 min interval. Both phasic and tonic stimulation produced a pressure-dependent increase of abdominal contractions. The abdominal response was more intense under phasic than under tonic stimulation, but with differences depending on the sex of the animals: females exhibited more contractions than males and of similar duration at all pressures, whereas duration of contractions pressure-dependently increased in males. The duration of tonically stimulated contractions was lower and not sex- or pressure-dependent. In the rat, responses to colonic distension depend on the quality of the stimulus, which also produces sex-dependent differences that must be taken into account in the development of models of pathology and visceral pain treatments.

Clinical, Physiological, and Psychological Correlates of the Improvement of Defecation during Menses in Women with Functional Gastrointestinal Disorders

BACKGROUND/AIMS

Little is known about the improvement in defecation frequently reported by women around menses. We aimed to describe clinical, physiological, and psychological correlates of this improvement in those with functional bowel disorders.

PATIENTS AND METHODS

We recruited 478 consecutive premenopausal adult females with no indication of gynecologic or psychiatric disease, who were attending an outpatient functional bowel disorders clinic. Patients completed a Rome III questionnaire, psychological evaluation stool form, and a 10-point Likert scale for constipation, diarrhea, bloating, and abdominal pain. These patients underwent physiological tests, anorectal manometry, and colonic transit time and were classified according to the presence or the absence of improvement in defecation during menses. The reverse selection procedure was used for model selection during multivariate logistic regression where statistically significant variables (p < 0.01) remained in the adjusted model.

RESULTS

Ninety-seven patients (20%) reported easier defecation during menstruation. These patients were younger (p < 0.001) but had similar body mass indices and psychological profiles as the other patients. Clinically, they only reported more frequent irritable bowel syndrome (IBS) with constipation (p = 0.007), with harder stools (p = 0.005) and delayed left colon transit time (p = 0.002). No anorectal manometric parameter was different between the 2 groups.

CONCLUSION

Improvement of constipation during menses is mainly associated with younger age and constipation-IBS phenotype and not with functional constipation.

P2X3 Receptor in Primary Afferent Neurons Mediates the Relief of Visceral Hypersensitivity by Electroacupuncture in an Irritable Bowel Syndrome Rat Model

Background

Electroacupuncture (EA) has been confirmed effectiveness in the treatment of irritable bowel syndrome (IBS), and P2X₃ receptors in the peripheral and central neurons participate in the acupuncture-mediated relief of the visceral pain in IBS.

Objective

To reveal the neurobiological mechanism that P2X₃ receptor of colonic primary sensory neurons in the dorsal root ganglia of the lumbosacral segment is involved in the alleviation of visceral hypersensitivity by EA in an IBS rat model.

Methods

The IBS chronic visceral pain rat model was established according to the method of Al-Chaer et al. EA at the bilateral He-Mu points, including ST25 and ST37, was conducted for intervention. The behavioral studies, histopathology of colon, electrophysiology, immunofluorescence histochemistry, and real-time polymerase chain reaction assays were used to observe the role of P2X₃ receptor in the colon and related DRG in relieving visceral hypersensitivity by EA.

Results

EA significantly reduced the behavior scores of the IBS rats under different levels (20, 40, 60, 80 mmHg) of colorectal distention stimulation and downregulated the expression levels of P2X₃ receptor protein and mRNA in colon and related DRG of the IBS rats. EA also regulated the electrical properties of the membranes, including the resting membrane potential, rheobase, and action potential of colon-associated DRG neurons in the IBS rats.

Conclusion

EA can regulate the P2X₃ receptor protein and mRNA expression levels in the colon and related DRG of IBS rats with visceral pain and then regulate the excitatory properties of DRG neurons.

Possible implications of animal models for the assessment of visceral pain

Acute pain, provoked generally after the activation of peripheral nociceptors, is an adaptive sensory function that alerts the individual to avoid noxious stimuli. However, uncontrolled acute pain has a maladaptive role in sensory activity leading to development of a chronic pain state which persists even after the damage is resolved, or in some cases, in the absence of an initial local acute injury. Huge numbers of people suffer from visceral pain at least once during their life span, leading to substantial health care costs. Although studies reporting on the mechanism of visceral pain are accumulating, it is still not precisely understood. Therefore, this review aims to elucidate the mechanism of visceral pain through an evaluation of different animal models and their application to develop novel therapeutic approaches for treating visceral pain. To assess the nociceptive responses in viscera, several visceral pain models such as inflammatory, traction, stress and genetic models utilizing different methods of measurement have been devised. Among them, the inflammatory and traction models are widely used for studying the visceral pain mechanism of different disease conditions and post-operative surgery in humans and animals. A hapten, 2,4,6-trinitrobenzene sulfonic acid (TNBS), has been extensively used as an inflammatory agent to induce visceral pain. The traction model seems to cause a strong pain stimulation and autonomic reaction and could thus be the most appropriate model for studying the underlying visceral pain mechanism and for probing the therapeutic efficacies of various anesthetic and analgesics for the treatment of visceral pain and hyperalgesia.

Attenuation of Visceral and Somatic Nociception by Ghrelin Mimetics

Purpose

The anti-nociceptive properties of ghrelin have been demonstrated in alleviating inflammatory and neuropathic pain. Whether a ghrelin receptor-mediated mechanism attenuates visceral and somatic pain in the absence of active inflammation remains to be explored. Here, we investigate the efficacy of peripherally restricted (ipamorelin) and a globally active (HM01) selective ghrelin receptor agonist in an experimental model of non-inflammatory visceral hypersensitivity and somatic mechanical allodynia.

Materials and Methods

Visceral hypersensitivity was induced by dilute acetic acid (0.6%) infusion in the colon of rats in the absence of colonic epithelial inflammation. Ghrelin mimetics HM01 and ipamorelin were administered orally or intravenously, respectively. The ghrelin receptor antagonist H0900 was administered orally. Colonic sensitivity was assessed via a visceromotor behavioral response (VMR) quantified as the number of abdominal contractions in response to graded isobaric pressures (0-60 mmHg) of colorectal distension (CRD). Somatic mechanical allodynia was quantified by the number of ipsilateral paw withdrawals in response to a calibrated von Frey filament.

Results

Compared to vehicle controls, ghrelin mimetics HM01 and ipamorelin significantly attenuated colonic hypersensitivity and somatic allodynia. The anti-nociceptive effects of the ghrelin mimetics were blocked after administration of the ghrelin receptor antagonist H0900.

Conclusion

We have shown that ghrelin receptor-mediated mechanisms are involved in visceral and somatic hypersensitivity in the absence of active colonic inflammation. Furthermore, visceral and somatic hypersensitivity could be attenuated by a peripherally restricted ghrelin mimetic. These results highlight a potential novel approach for treating acute visceral and somatic pain by ghrelin mimetics.

Critical evaluation of animal models of visceral pain for therapeutics development: A focus on irritable bowel syndrome

The classification of chronic visceral pain is complex, resulting from persistent inflammation, vascular (ischemic) mechanisms, cancer, obstruction or distension, traction or compression, and combined mechanisms, as well as unexplained functional mechanisms. Despite the prevalence, treatment options for chronic visceral pain are limited. Given this unmet clinical need, the development of novel analgesic agents, with defined targets derived from preclinical studies, is urgently needed. While various animal models have played an important role in our understanding of visceral pain, our knowledge is far from complete. Due to the complexity of visceral pain, this document will focus on chronic abdominal pain, which is the major complaint in patients with disorders of the gut-brain interaction, also referred to as functional gastrointestinal disorders, such as irritable bowel syndrome (IBS). Models for IBS are faced with challenges including a complex clinical phenotype, which is comorbid with other conditions including anxiety, depression, painful bladder syndrome, and chronic pelvic pain. Based upon the multifactorial nature of IBS with complicated interactions between biological, psychological, and sociological variables, no single experimental model recapitulates all the symptoms of IBS. This position paper will contextualize chronic visceral pain using the example of IBS and focus on its pathophysiology while providing a critical review of current animal models that are most relevant, robust, and reliable in which to screen promising therapeutics to alleviate visceral pain and delineate the gaps and challenges with these models. We will also highlight, prioritize, and come to a consensus on the models with the highest face/construct validity.

Gastrointestinal pain

Gastrointestinal (GI) pain - a form of visceral pain - is common in some disorders, such as irritable bowel syndrome, Crohn's disease and pancreatitis. However, identifying the cause of GI pain frequently represents a diagnostic challenge as the clinical presentation is often blurred by concomitant autonomic and somatic symptoms. In addition, GI pain can be nociceptive, neuropathic and associated with cancer, but in many cases multiple aetiologies coexist in an individual patient. Mechanisms of GI pain are complex and include both peripheral and central sensitization and the involvement of the autonomic nervous system, which has a role in generating the symptoms that frequently accompany pain. Treatment of GI pain depends on the precise type of pain and the primary disorder in the patient but can include, for example, pharmacological therapy, cognitive behavioural therapies, invasive surgical procedures, endoscopic procedures and lifestyle alterations. Owing to the major differences between organ involvement, disease mechanisms and individual factors, treatment always needs to be personalized and some data suggest that phenotyping and subsequent individual management of GI pain might be options in the future.

Stress and the brain-gut axis in functional and chronic-inflammatory gastrointestinal diseases: A transdisciplinary challenge

The broad role of stress in the brain-gut axis is widely acknowledged, with implications for multiple prevalent health conditions that are characterized by chronic gastrointestinal symptoms. These include the functional gastrointestinal disorders (FGID), such as irritable bowel syndrome and functional dyspepsia, as well as inflammatory bowel diseases (IBD) like ulcerative colitis and Crohn's disease. Although the afferent and efferent pathways linking the gut and the brain are modulated by stress, the fields of neurogastroenterology and psychoneuroendocrinology (PNE)/ psychoneuroimmunology (PNI) remain only loosely connected. We aim to contribute to bringing these fields closer together by drawing attention to a fascinating, evolving research area, targeting an audience with a strong interest in the role of stress in health and disease. To this end, this review introduces the concept of the brain-gut axis and its major pathways, and provides a brief introduction to epidemiological and clinical aspects of FGIDs and IBD. From an interdisciplinary PNE/PNI perspective, we then detail current knowledge regarding the role of chronic and acute stress in the pathophysiology of FGID and IBD. We provide an overview of evidence regarding non-pharmacological treatment approaches that target central or peripheral stress mechanisms, and conclude with future directions, particularly those arising from recent advances in the neurosciences and discoveries surrounding the gut microbiota.

Risk and Protective Factors Related to Early Adverse Life Events in Irritable Bowel Syndrome

BACKGROUND

Irritable bowel syndrome (IBS) is a stress-sensitive disorder of brain-gut interactions associated with a higher prevalence of early adverse life events (EALs). However, it is incompletely understood how trauma severity or disclosure influence the risk of developing IBS or symptom severity.

AIMS

To determine whether (1) IBS patients report a greater number of EALs compared with healthy controls; (2) trauma severity and first age of EAL increase the odds of IBS; (3) confiding in others reduces the odds of IBS; (4) the number, trauma severity, and first age of EAL are associated with symptom severity; (5) sex differences exist.

METHODS

In total, 197 IBS patients (72% women, mean age=30.28 y) and 165 healthy controls (59% women, mean age=30.77 y) completed the Childhood Traumatic Events Scale, measuring severity of EALs and degree of confiding in others. Regression analyses were used to predict IBS status from EALs and association between gastrointestinal symptoms and EALs.

RESULTS

A greater number of EALs [odds ratio (OR)=1.36, 95% confidence interval (CI), 1.14-1.62; P<0.001] and higher perceived trauma severity (OR=1.13, 95% CI, 1.08-1.19; P<0.001) were associated with increased odds of IBS. Confiding in others decreased the odds of having IBS (OR=0.83, 95% CI, 0.72-0.96; P=0.012). The first age of EAL was not predictive of IBS. No sex differences were found.

CONCLUSIONS

Assessing the traumatic severity of EALs and amount of confiding in others is important as they can affect the risk of having IBS. Our findings emphasize early intervention to improve health outcomes in individuals with EALs.

Gastrointestinal hypoalgesia in inflammatory bowel disease

Background:

Pain perception is critical for detection of noxious bodily insults. Gastrointestinal hypoalgesia in inflammatory bowel disease (IBD) is a poorly understood phenomenon previously linked to poor patient outcomes. We aimed to evaluate the risk factors associated with this condition and to discern characteristics that might differentiate these patients from pain-free quiescent counterparts.

Methods:

We performed a retrospective analysis using an IBD natural history registry based in a single tertiary care referral center. We compared demographic and clinical features in 3 patient cohorts defined using data from simultaneous pain surveys and ileocolonoscopy: a) active IBD without pain (hypoalgesic IBD); b) active IBD with pain; and c) inactive IBD without pain.

Results:

One hundred fifty-three IBD patients had active disease and 43 (28.1%) exhibited hypoalgesia. Hypoalgesic IBD patients were more likely to develop non-perianal fistulae (P=0.03). On logistic regression analysis, hypoalgesic IBD was independently associated with male sex, advancing age and mesalamine use, and inversely associated with anxious/depressed state and opiate use. Hypoalgesic IBD patients were demographically and clinically similar to the pain-free quiescent IBD cohort (n=59). Platelet count and C-reactive protein were more likely to be pathologically elevated in hypoalgesic IBD (P=0.03), though >25% did not exhibit elevated inflammatory markers.

Conclusions:

Hypoalgesia is common in IBD, particularly in male and older individuals, and is associated with an increased incidence of fistulae and corticosteroid use. Novel noninvasive diagnostic tools are needed to screen for this population, as inflammatory markers are not always elevated.

Evidence for Somatic Hypersensitivity in Veterans With Gulf War Illness and Gastrointestinal Symptoms

INTRODUCTION

Over 25% of Persian Gulf War (PGW) veterans with Gulf War Illness (GWI) (chronic health symptoms of undetermined etiology) developed gastrointestinal (GI) (diarrhea and abdominal pain) and other somatic symptoms.

OBJECTIVES

Our study objective was to determine if veterans with GWI and GI symptoms exhibit heightened patterns of somatic pain perception (hypersensitivity) across nociceptive stimuli modalities.

METHODS

Participants were previously deployed GW Veterans with GWI and GI symptoms (n=53); veterans with GWI without GI symptom (n=47); and veteran controls (n=38). We determined pain thresholds for contact thermal, cold pressor, and ischemic stimuli.

RESULTS

Veterans with GWI and GI symptoms showed lower pain thresholds (P<0.001) for each stimulus. There was also overlap of somatic hypersensitivities among veterans with GI symptoms with 20% having hypersensitivity to all 3 somatic stimuli. Veterans with GWI and GI symptoms also showed a significant correlation between mechanical visual analog scale abdominal pain ratings and heat pain threshold, cold pressor threshold, and ischemic pain threshold/tolerance.

DISCUSSION

Our findings show that there is widespread somatic hypersensitivity in veterans with GWI/GI symptoms that is positively correlated with abdominal pain ratings. In addition, veterans with somatic hypersensitivity that overlap have the greatest number of extraintestinal symptoms. These findings may have a translational benefit: strategies for developing more effective therapeutic agents that can reduce and/or prevent somatic and GI symptoms in veterans deployed to future military conflicts.

Role of brain imaging in disorders of brain-gut interaction: a Rome Working Team Report

Imaging of the living human brain is a powerful tool to probe the interactions between brain, gut and microbiome in health and in disorders of brain-gut interactions, in particular IBS. While altered signals from the viscera contribute to clinical symptoms, the brain integrates these interoceptive signals with emotional, cognitive and memory related inputs in a non-linear fashion to produce symptoms. Tremendous progress has occurred in the development of new imaging techniques that look at structural, functional and metabolic properties of brain regions and networks. Standardisation in image acquisition and advances in computational approaches has made it possible to study large data sets of imaging studies, identify network properties and integrate them with non-imaging data. These approaches are beginning to generate brain signatures in IBS that share some features with those obtained in other often overlapping chronic pain disorders such as urological pelvic pain syndromes and vulvodynia, suggesting shared mechanisms. Despite this progress, the identification of preclinical vulnerability factors and outcome predictors has been slow. To overcome current obstacles, the creation of consortia and the generation of standardised multisite repositories for brain imaging and metadata from multisite studies are required.

Intestinal Hyperpermeability in Gulf War Veterans With Chronic Gastrointestinal Symptoms

BACKGROUND

Well over 700,000 United States military personnel participated in the Persian Gulf War in which they developed chronic health disorders of undetermined etiology. Up to 25% of Veterans had persistent and chronic gastrointestinal (GI) symptoms, which they suspected were related to their military service in the Gulf.

AIM

The overall aim of the current study was to evaluate intestinal permeability in previously deployed Gulf War Veterans who developed chronic GI symptoms during their tour in the Persian Gulf.

METHODS

To accomplish this, we evaluated intestinal permeability (IP) using the urinary lactulose/mannitol test. Measurements of intestinal permeability were then correlated with mean ratings of daily abdominal pain, frequency of bowel movements, and consistency of bowel movements on the Bristol Stool Scale in all Veterans.

RESULTS

A total of 73 veterans had documented chronic GI symptoms (diarrhea, abdominal pain) and were included in the study. A total of 29/73 (39%) of veterans has increased IP and had a higher average daily stool frequency (P<0.05); increased liquid stools as indicated by a higher Bristol Stool Scale (P<0.01); and a higher mean M-VAS abdominal pain rating (P<0.01). Pearson correlation coefficients revealed that there was a positive correlation between increased IP and stool frequency, Bristol Stool Scale, and M-VAS abdominal pain rating.

CONCLUSIONS

Our study demonstrates that deployed Gulf War Veterans with persistent GI symptoms commonly have increased intestinal permeability that potentiates the severity of abdominal pain, diarrhea, and stool consistency. These new findings in our study are important as they may lead to novel diagnostic biomarkers for returning Gulf War Veterans who suffer from chronic functional gastrointestinal disorders. These advances are also important for an increasing number of veterans who are now serving in the Persian Gulf and are at a high risk of developing these chronic pain disorders.

Brain and Gut CRF Signaling: Biological Actions and Role in the Gastrointestinal Tract

BACKGROUND

Corticotropin-releasing factor (CRF) pathways coordinate behavioral, endocrine, autonomic and visceral responses to stress. Convergent anatomical, molecular, pharmacological and functional experimental evidence supports a key role of brain CRF receptor (CRF-R) signaling in stress-related alterations of gastrointestinal functions. These include the inhibition of gastric acid secretion and gastric-small intestinal transit, stimulation of colonic enteric nervous system and secretorymotor function, increase intestinal permeability, and visceral hypersensitivity. Brain sites of CRF actions to alter gut motility encompass the paraventricular nucleus of the hypothalamus, locus coeruleus complex and the dorsal motor nucleus while those modulating visceral pain are localized in the hippocampus and central amygdala. Brain CRF actions are mediated through the autonomic nervous system (decreased gastric vagal and increased sacral parasympathetic and sympathetic activities). The activation of brain CRF-R2 subtype inhibits gastric motor function while CRF-R1 stimulates colonic secretomotor function and induces visceral hypersensitivity. CRF signaling is also located within the gut where CRF-R1 activates colonic myenteric neurons, mucosal cells secreting serotonin, mucus, prostaglandin E2, induces mast cell degranulation, enhances mucosal permeability and propulsive motor functions and induces visceral hyperalgesia in animals and humans. CRF-R1 antagonists prevent CRF- and stressrelated gut alterations in rodents while not influencing basal state.

DISCUSSION

These preclinical studies contrast with the limited clinical positive outcome of CRF-R1 antagonists to alleviate stress-sensitive functional bowel diseases such as irritable bowel syndrome.

CONCLUSION

The translational potential of CRF-R1 antagonists in gut diseases will require additional studies directed to novel anti-CRF therapies and the neurobiology of brain-gut interactions under chronic stress.

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.

Do MicroRNAs Modulate Visceral Pain?

Visceral pain, a common characteristic of multiple diseases relative to viscera, impacts millions of people worldwide. Although hundreds of studies have explored mechanisms underlying visceral pain, it is still poorly managed. Over the past decade, strong evidence emerged suggesting that microRNAs (miRNAs) play a significant role in visceral nociception through altering neurotransmitters, receptors and other genes at the posttranscriptional level. Under pathological conditions, one kind of miRNA may have several target mRNAs and several kinds of miRNAs may act on one target, suggesting complex interactions and mechanisms between miRNAs and target genes lead to pathological states. In this review we report on recent progress in examining miRNAs responsible for visceral sensitization and provide miRNA-based therapeutic targets for the management of visceral pain.

Aluminum Ingestion Promotes Colorectal Hypersensitivity in Rodents

Background & Aims

Irritable bowel syndrome (IBS) is a multifactorial disease arising from a complex interplay between genetic predisposition and environmental influences. To date, environmental triggers are not well known. Aluminum is commonly present in food, notably by its use as food additive. We investigated the effects of aluminum ingestion in rodent models of visceral hypersensitivity, and the mechanisms involved.

Methods

Visceral hypersensitivity was recorded by colorectal distension in rats administered with oral low doses of aluminum. Inflammation was analyzed in the colon of aluminum-treated rats by quantitative PCR for cytokine expression and by immunohistochemistry for immune cells quantification. Involvement of mast cells in the aluminum-induced hypersensitivity was determined by cromoglycate administration of rats and in mast cell-deficient mice (KitW-sh/W-sh). Proteinase-activated receptor-2 (PAR2) activation in response to aluminum was evaluated and its implication in aluminum-induced hypersensitivity was assessed in PAR2 knockout mice.

Results

Orally administered low-dose aluminum induced visceral hypersensitivity in rats and mice. Visceral pain induced by aluminum persisted over time even after cessation of treatment, reappeared and was amplified when treatment resumed. As observed in humans, female animals were more sensitive than males. Major mediators of nociception were up-regulated in the colon by aluminum. Activation of mast cells and PAR2 were required for aluminum-induced hypersensitivity.

Conclusions

These findings indicate that oral exposure to aluminum at human dietary level reproduces clinical and molecular features of IBS, highlighting a new pathway of prevention and treatment of visceral pain in some susceptible patients.

Newly developed serine protease inhibitors decrease visceral hypersensitivity in a post-inflammatory rat model for irritable bowel syndrome

BACKGROUND AND PURPOSE

Serine proteases have been re suggested as important mediators of visceral pain. We investigated their effect by using newly developed serine protease inhibitors with a well-characterized inhibitory profile in a rat model of post-inflammatory irritable bowel syndrome (IBS).

EXPERIMENTAL APPROACH

Colitis was induced in rats receiving intrarectal trinitrobenzenesulphonic acid; controls received 0.9% NaCl. Colonoscopies were performed on day 3, to confirm colitis, and later until mucosal healing. Visceral hypersensitivity was quantified by visceromotor responses (VMRs) to colorectal distension, 30 min after i.p. injection of the serine protease inhibitors nafamostat, UAMC-00050 or UAMC-01162. Serine proteases, protease-activated receptors (PARs) and TRP channels were quantified by qPCR and immunohistochemistry. Proteolytic activity was characterized using fluorogenic substrates.

KEY RESULTS

VMR was significantly elevated in post-colitis rats. Nafamostat normalized VMRs at the lowest dose tested. UAMC-00050 and UAMC-01162 significantly decreased VMR dose-dependently. Expression of mRNA for tryptase-αβ-1and PAR4, and tryptase immunoreactivity was significantly increased in the colon of post-colitis animals. Trypsin-like activity was also significantly increased in the colon but not in the faeces. PAR2 and TRPA1 immunoreactivity co-localized with CGRP-positive nerve fibres in control and post-colitis animals.

CONCLUSIONS AND IMPLICATIONS

Increased expression of serine proteases and activity together with increased expression of downstream molecules at the colonic and DRG level and in CGRP-positive sensory nerve fibres imply a role for serine proteases in post-inflammatory visceral hypersensitivity. Our results support further investigation of serine protease inhibitors as an interesting treatment strategy for IBS-related visceral pain.

Novel Insights Into the Mechanisms of Abdominal Pain in Obstructive Bowel Disorders

Obstructive bowel disorders (OBD) are characterized by lumen distention due to mechanical or functional obstruction in the gut. Abdominal pain is one of the main symptoms in OBD. In this article, we aim to critically review the potential mechanisms for acute and chronic pain in bowel obstruction (BO). While clustered contractions and associated increase of intraluminal pressure may account for colicky pain in simple obstruction, ischemia may be involved in acute pain in severe conditions such as closed loop obstruction. Recent preclinical studies discovered that visceral sensitivity is increased in BO, and visceral hypersensitivity may underlie the mechanisms of chronic abdominal pain in BO. Mounting evidence suggests that lumen distension, as a circumferential mechanical stretch, alters gene expression (mechano-transcription) in the distended bowel, and mechano-transcription of nociceptive and inflammatory mediators plays a critical role in the development of visceral hypersensitivity in BO. Mechano-transcription of nerve growth factor (NGF) in gut smooth muscle cells is found to increase voltage-gated Na⁺ channel (Na_v) activity of the primary sensory neurons by up-regulating expression of TTX-resistant Na_v1.8, whereas mechanical stretch-induced brain-derived neurotrophic factor (BDNF) reduces K_v currents especially A-type (IA) currents by down-regulating expression of specific IA subtypes such as K_v1.4. The NGF and BDNF mediated changes in gene expression and channel functions in the primary sensory neurons may constitute the main mechanisms of visceral hypersensitivity in OBD. In addition, mechanical stretch-induced COX-2 and other inflammatory mediators in the gut may also contribute to abdominal pain by activating and sensitizing nociceptors.

Reversal of visceral hypersensitivity in rat by Menthacarin® , a proprietary combination of essential oils from peppermint and caraway, coincides with mycobiome modulation

BACKGROUND

Irritable bowel syndrome (IBS) is a common gastrointestinal disorder associated with altered gastrointestinal microflora and increased nociception to colonic distension. This visceral hypersensitivity can be reversed in our rat maternal separation model by fungicides. Menthacarin^® is a proprietary combination of essential oils from Mentha x piperita L. and Carum carvi. Because these oils exhibit antifungal and antibacterial properties, we investigated whether Menthacarin^® can reverse existing visceral hypersensitivity in maternally separated rats.

METHODS

In non-handled and maternally separated rats, we used the visceromotor responses to colorectal distension as measure for visceral sensitivity. We evaluated this response before and 24 hours after water-avoidance stress and after 7 days treatment with Menthacarin^® or control. The pre- and post-treatment mycobiome and microbiome were characterized by sequencing of fungal internal transcribed spacer 1 (ITS-1) and bacterial 16s rDNA regions. In vitro antifungal and antimicrobial properties of Menthacarin^® were studied with radial diffusion assay.

KEY RESULTS

Menthacarin^® inhibited in vitro growth of yeast and bacteria. Water-avoidance caused visceral hypersensitivity in maternally separated rats, and this was reversed by treatment. Multivariate analyses of ITS-1 and 16S high throughput data showed that maternal separation, induced changes in the myco- and microbiome. Menthacarin^® treatment of non-handled and maternally separated rats shifted the mycobiomes to more similar compositions.

CONCLUSIONS & INFERENCES

The development of visceral hypersensitivity in maternally separated rats and the Menthacarin^® -mediated reversal of hypersensitivity is associated with changes in the mycobiome. Therefore, Menthacarin^® may be a safe and effective treatment option that should be tested for IBS.

Functional Bowel Disorders: A Roadmap to Guide the Next Generation of Research

In June 2016, the National Institutes of Health hosted a workshop on functional bowel disorders (FBDs), particularly irritable bowel syndrome, with the objective of elucidating gaps in current knowledge and recommending strategies to address these gaps. The workshop aimed to provide a roadmap to help strategically guide research efforts during the next decade. Attendees were a diverse group of internationally recognized leaders in basic and clinical FBD research. This document summarizes the results of their deliberations, including the following general conclusions and recommendations. First, the high prevalence, economic burden, and impact on quality of life associated with FBDs necessitate an urgent need for improved understanding of FBDs. Second, preclinical discoveries are at a point that they can be realistically translated into novel diagnostic tests and treatments. Third, FBDs are broadly accepted as bidirectional disorders of the brain-gut axis, differentially affecting individuals throughout life. Research must integrate each component of the brain-gut axis and the influence of biological sex, early-life stressors, and genetic and epigenetic factors in individual patients. Fourth, research priorities to improve diagnostic and management paradigms include enhancement of the provider-patient relationship, longitudinal studies to identify risk and protective factors of FBDs, identification of biomarkers and endophenotypes in symptom severity and treatment response, and incorporation of emerging "-omics" discoveries. These paradigms can be applied by well-trained clinicians who are familiar with multimodal treatments. Fifth, essential components of a successful program will include the generation of a large, validated, broadly accessible database that is rigorously phenotyped; a parallel, linkable biorepository; dedicated resources to support peer-reviewed, hypothesis-driven research; access to dedicated bioinformatics expertise; and oversight by funding agencies to review priorities, progress, and potential synergies with relevant stakeholders.

Mechanisms of Stress-induced Visceral Pain

Evidence suggests that long-term stress facilitates visceral pain through sensitization of pain pathways and promotes chronic visceral pain disorders such as the irritable bowel syndrome (IBS). This review will describe the importance of stress in exacerbating IBS-induced abdominal pain. Additionally, we will briefly review our understanding of the activation of the hypothalamic-pituitary-adrenal axis by both chronic adult stress and following early life stress in the pathogenesis of IBS. The review will focus on the glucocorticoid receptor and corticotropin-releasing hormone-mediated mechanisms in the amygdala involved in stress-induced visceral hypersensitivity. One potential mechanism underlying persistent effects of stress on visceral sensitivity could be epigenetic modulation of gene expression. While there are relatively few studies examining epigenetically mediated mechanisms involved in stress-induced visceral nociception, alterations in DNA methylation and histone acetylation patterns within the brain, have been linked to alterations in nociceptive signaling via increased expression of pro-nociceptive neurotransmitters. This review will discuss the latest studies investigating the long-term effects of stress on visceral sensitivity. Additionally, we will critically review the importance of experimental models of adult stress and early life stress in enhancing our understanding of the basic molecular mechanisms of nociceptive processing.

Optogenetic and chemogenetic techniques for neurogastroenterology

Optogenetics and chemogenetics comprise a wide variety of applications in which genetically encoded actuators and indicators are used to modulate and monitor activity with high cellular specificity. Over the past 10 years, development of these genetically encoded tools has contributed tremendously to our understanding of integrated physiology. In concert with the continued refinement of probes, strategies to target transgene expression to specific cell types have also made much progress in the past 20 years. In addition, the successful implementation of optogenetic and chemogenetic techniques thrives thanks to ongoing advances in live imaging microscopy and optical technology. Although innovation of optogenetic and chemogenetic methods has been primarily driven by researchers studying the central nervous system, these techniques also hold great promise to boost research in neurogastroenterology. In this Review, we describe the different classes of tools that are currently available and give an overview of the strategies to target them to specific cell types in the gut wall. We discuss the possibilities and limitations of optogenetic and chemogenetic technology in the gut and provide an overview of their current use, with a focus on the enteric nervous system. Furthermore, we suggest some experiments that can advance our understanding of how the intrinsic and extrinsic neural networks of the gut control gastrointestinal function.

Stereotaxic Exposure of the Central Nucleus of the Amygdala to Corticosterone Increases Colonic Permeability and Reduces Nerve-Mediated Active Ion Transport in Rats

Background: Irritable bowel syndrome (IBS) is characterized by visceral pain and abnormal bowel habits that are worsened during stress. Evidence also suggests altered intestinal barrier function in IBS. Previously, we demonstrated that stereotaxic application of the stress hormone corticosterone (CORT) onto the central nucleus of the amygdala (CeA) induces colonic hyperalgesia and anxiety-like behavior in a rat model, however the effect on intestinal permeability and mucosal function remain to be evaluated. Methods: Male Fischer 344 rats underwent bilateral stereotaxic implantation of CORT or inert cholesterol (CHOL)-containing micropellets (30 μg) onto the dorsal margin of the CeA. Seven days later, colonic tissue was isolated to assess tissue permeability in modified Ussing chambers via transepithelial electrical resistance (TEER) and macromolecular flux of horseradish peroxidase (HRP). Secretory responses to electrical field stimulation (EFS) of submucosal enteric nerves as well as activation with forskolin were used to assess movements of ions across the isolated colonic tissues. In a separate cohort, colonic histology, and mast cell infiltration was assessed. Key Results: Compared to CHOL-implanted controls, we determined that exposing the CeA to elevated levels of CORT significantly increased macromolecular flux across the colonic epithelial layer without changing TEER. Nerve-mediated but not cAMP-mediated active transport was inhibited in response to elevated amygdala CORT. There were no histological changes or increases in mast cell infiltration within colonic tissue from CORT treated animals. Conclusion and Inferences: These observations support a novel role for the CeA as a modulator of nerve-mediated colonic epithelial function.

Stress-Induced Chronic Visceral Pain of Gastrointestinal Origin

Visceral pain is generally poorly localized and characterized by hypersensitivity to a stimulus such as organ distension. In concert with chronic visceral pain, there is a high comorbidity with stress-related psychiatric disorders including anxiety and depression. The mechanisms linking visceral pain with these overlapping comorbidities remain to be elucidated. Evidence suggests that long term stress facilitates pain perception and sensitizes pain pathways, leading to a feed-forward cycle promoting chronic visceral pain disorders such as irritable bowel syndrome (IBS). Early life stress (ELS) is a risk-factor for the development of IBS, however the mechanisms responsible for the persistent effects of ELS on visceral perception in adulthood remain incompletely understood. In rodent models, stress in adult animals induced by restraint and water avoidance has been employed to investigate the mechanisms of stress-induce pain. ELS models such as maternal separation, limited nesting, or odor-shock conditioning, which attempt to model early childhood experiences such as neglect, poverty, or an abusive caregiver, can produce chronic, sexually dimorphic increases in visceral sensitivity in adulthood. Chronic visceral pain is a classic example of gene × environment interaction which results from maladaptive changes in neuronal circuitry leading to neuroplasticity and aberrant neuronal activity-induced signaling. One potential mechanism underlying the persistent effects of stress on visceral sensitivity could be epigenetic modulation of gene expression. While there are relatively few studies examining epigenetically mediated mechanisms involved in visceral nociception, stress-induced visceral pain has been linked to alterations in DNA methylation and histone acetylation patterns within the brain, leading to increased expression of pro-nociceptive neurotransmitters. This review will discuss the potential neuronal pathways and mechanisms responsible for stress-induced exacerbation of chronic visceral pain. Additionally, we will review the importance of specific experimental models of adult stress and ELS in enhancing our understanding of the basic molecular mechanisms of pain processing.

Amygdala-mediated mechanisms regulate visceral hypersensitivity in adult females following early life stress: importance of the glucocorticoid receptor and corticotropin-releasing factor

Alterations in amygdala activity are apparent in women who report a history of early life stress (ELS) and those diagnosed with chronic pain disorders. Chronic stress in adulthood induces visceral hypersensitivity by alterations in glucocorticoid receptor (GR) and corticotropin-releasing factor (CRF) expression within the central amygdala (CeA). Here, we hypothesized that unpredictable ELS, previously shown to induce visceral hypersensitivity in adult female rats, alters GR and CRF expression in the CeA. After neonatal ELS, visceral sensitivity and GR and CRF gene expression were quantified in adult female rats. After unpredictable ELS, adult female rats exhibited visceral hypersensitivity and increased expression of GR and CRF in the CeA. After predictable ELS, adult female rats demonstrated normosensitive behavioral pain responses and upregulation of GR but not CRF in the CeA. After the ELS paradigms, visceral sensitivity and gene expression within the CeA were unaffected in adult male rats. The role of GR and CRF in modulating visceral sensitivity in adult female rats after ELS was investigated using oligodeoxynucleotide sequences targeted to the CeA for knockdown of GR or CRF. Knockdown of GR increased visceral sensitivity in all rats but revealed an exaggerated visceral hypersensitivity in females with a history of predictable or unpredictable ELS compared with that of controls. Knockdown of CRF expression or antagonism of CRF1R in the CeA attenuated visceral hypersensitivity after unpredictable ELS. This study highlights a shift in GR and CRF regulation within the CeA after ELS that underlies the development of visceral hypersensitivity in adulthood.

Animal models of pain and migraine in drug discovery

Preclinical research activities in relation to pain typically involve the 'holy trinity' of nociceptive, inflammatory and neuropathic pain for purposes of target validation and defining target product profiles of novel analgesic compounds. For some reason it seems that headache or migraine are rarely considered as additional entities to explore. Frontline medications used in the treatment of, for example, inflammatory pain, neuropathic pain and migraine (NSAIDs versus pregabalin/duloxetine versus triptans) reveal distinct differences in pathophysiology that partially explain this approach. Nevertheless, for many patients enduring chronic pain, regardless of aetiology, high unmet needs remain. By focusing more on commonalities shared between neuropathic pain and headache disorders such as migraine, drug discovery efforts could be spread more efficiently across a larger indication area. Here, some of the most commonly used models and methods employed within 'pain and migraine' drug development will be presented. Recent advances within these disciplines suggest that, with the addition of a few extra carefully chosen ancillary models and/or endpoints, the relative value in terms of resources used, reciprocal flow of information and net worth of a 'typical' package could be increased substantially for the pain and migraine fields.

The Role of the Gastrointestinal Microbiota in Visceral Pain

A growing body of preclinical and clinical evidence supports a relationship between the complexity and diversity of the microorganisms that inhabit our gut (human gastrointestinal microbiota) and health status. Under normal homeostatic conditions this microbial population helps maintain intestinal peristalsis, mucosal integrity, pH balance, immune priming and protection against invading pathogens. Furthermore, these microbes can influence centrally regulated emotional behaviour through mechanisms including microbially derived bioactive molecules (amino acid metabolites, short-chain fatty acids, neuropeptides and neurotransmitters), mucosal immune and enteroendocrine cell activation, as well as vagal nerve stimulation.The microbiota-gut-brain axis comprises a dynamic matrix of tissues and organs including the brain, autonomic nervous system, glands, gut, immune cells and gastrointestinal microbiota that communicate in a complex multidirectional manner to maintain homeostasis and resist perturbation to the system. Changes to the microbial environment, as a consequence of illness, stress or injury, can lead to a broad spectrum of physiological and behavioural effects locally including a decrease in gut barrier integrity, altered gut motility, inflammatory mediator release as well as nociceptive and distension receptor sensitisation. Centrally mediated events including hypothalamic-pituitary-adrenal (HPA) axis, neuroinflammatory events and neurotransmitter systems are concomitantly altered. Thus, both central and peripheral pathways associated with pain manifestation and perception are altered as a consequence of the microbiota-gut-brain axis imbalance.In this chapter the involvement of the gastrointestinal microbiota in visceral pain is reviewed. We focus on the anatomical and physiological nodes whereby microbiota may be mediating pain response, and address the potential for manipulating gastrointestinal microbiota as a therapeutic target for visceral pain.

Visceral pain triggered by traction on the ileocecal ligament with ileitis

Background

Visceral pain is a common symptom of several gastrointestinal disorders. Despite tremendous progress in understanding its basic mechanisms, it remains a significant health challenge for clinicians. The present study quantified the intensity of visceral pain using ileocecal ligament traction on an inflamed ileum in goats.

Materials and methods

A total of 36 male goats weighing 20.05±2.1 kg were randomly allocated equally into a 2,4,6-trinitrobenzenesulfonic acid (TNBS) group (n=18) and a saline group (n=18). Ileitis was induced via the injection of 30 mg TNBS dissolved in 30% ethanol into the ileal wall through a laparotomy. An equal volume of normal saline was injected into the ileal wall of the saline goats. Behavioral responses to traction (2, 4, and 6 N) on the ileocecal ligament were observed on days 3, 7, and 14. Six goats from each group received a laparotomy and partial intestinal resection for ileal sample collection immediately after behavioral testing on days 3, 7, and 14. Ileal histopathological changes were assessed and concentrations of myeloperoxidase, IL-1β, IL-6, and TNFα investigated using enzyme-linked immunosorbent assay.

Results

The TNBS-treated goats exhibited remarkably increased macroscopic scores, mast-cell counts, myeloperoxidase, and TNFα concentrations on days 3 and 7 compared to the saline group, and increased microscopic scores and IL-1β and IL-6 concentrations on days 3–14. The TNBS-treated goats exhibited behavioral changes in response to traction in the same pattern as their microscopic changes and cytokine levels. The traction force correlated positively with pain-behavior responses.

Conclusion

Traction on the ileocecal ligament of goats with ileitis provoked an apparent, stable, and reproducible ileum-derived pain. The current model may be helpful in evaluating the efficacy of new drugs for the management of visceral pain and in investigating its underlying mechanisms.

Opioidergic effects on enteric and sensory nerves in the lower GI tract: basic mechanisms and clinical implications

Opioids are one of the most prescribed drug classes for treating acute pain. However, chronic use is often associated with tolerance as well as debilitating side effects, including nausea and dependence, which are mediated by the central nervous system, as well as constipation emerging from effects on the enteric nervous system. These gastrointestinal (GI) side effects limit the usefulness of opioids in treating pain in many patients. Understanding the mechanism(s) of action of opioids on the nervous system that shows clinical benefit as well as those that have unwanted effects is critical for the improvement of opioid drugs. The opioidergic system comprises three classical receptors (μ, δ, κ) and a nonclassical receptor (nociceptin), and each of these receptors is expressed to varying extents by the enteric and intestinal extrinsic sensory afferent nerves. The purpose of this review is to discuss the role that the opioidergic system has on enteric and extrinsic afferent nerves in the lower GI tract in health and diseases of the lower GI tract, particularly inflammatory bowel disease and irritable bowel syndrome, and the implications of opioid treatment on clinical outcomes. Consideration is also given to emerging developments in our understanding of the immune system as a novel source of endogenous opioids and the mechanisms underlying opioid tolerance, including the potential influence of opioid receptor splice variants and heteromeric complexes.

Neurobiology of early life stress and visceral pain: translational relevance from animal models to patient care

BACKGROUND

Epidemiological studies show that females are twice as likely to receive a diagnosis of irritable bowel syndrome (IBS) than their male counterparts. Despite evidence pointing to a role for sex hormones in the onset or exacerbation of IBS symptoms, the mechanism by which ovarian hormones may predispose women to develop IBS remains largely undefined. On the other hand, there is a growing body of research showing a correlation between reports of early life stress (ELS) and the diagnosis of IBS. Current treatments available for IBS patients target symptom relief including abdominal pain and alterations in bowel habits, but are not directed to the etiology of the disease.

PURPOSE

To better understand the mechanisms by which sex hormones and ELS contribute to IBS, animal models have been developed to mirror complex human experiences allowing for longitudinal studies that investigate the lifelong consequences of ELS. These preclinical models have been successful in recapitulating ELS-induced visceral pain. Moreover, in female rats the influence of cycling hormones on visceral hypersensitivity resembles that seen in women with IBS. Such studies suggest that rodent models of ELS may serve as pivotal tools in determining (i) the etiology of IBS, (ii) novel future treatments for IBS, and (iii) improving individualized patient care. The current review aims to shed light on the progress and the challenges observed by clinicians within the field of gastroenterology and the preclinical science aimed at addressing those challenges in an effort to understand and more efficiently treat functional gastrointestinal disorders (FGIDs) in both children and adults.