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

Endocannabinoids regulate enteric neuron-glia networks and visceral hypersensitivity following inflammation through a glial-dependent mechanism

Acute gastrointestinal (GI) inflammation induces neuroplasticity that produces long-lasting changes in gut motor function and pain. The endocannabinoid system is an attractive target to correct pain and dysmotility, but how inflammation changes endocannabinoid control over cellular communication in enteric neurocircuits is not understood. Enteric glia modulate gut neurons that control motility and pain and express monoacylglycerol lipase (MAGL) which controls endocannabinoid availability. We used a combination of in situ calcium imaging, chemogenetics, and selective drugs to study how endocannabinoid mechanisms affect glial responses and subsequent enteric neuron activity in health and following colitis in Wnt1Cre;GCaMP5g-tdT;GFAP::hM3Dq mice. Trpv1Cre;GCaMP5gtdT mice were used to study nociceptor sensitivity and Sox10CreERT2;Mgllf/f mice were used to test the role of glial MAGL in visceral pain. The data show that endocannabinoid signaling regulates neuro-glial signaling in gut neurocircuits in a sexually dimorphic manner. Inhibiting MAGL in healthy samples decreased glial responsiveness but this effect was lost in females following colitis and converted to an excitatory effect in males. Manipulating CB1 and CB2 receptors revealed further sex differences amongst neuro-glia signaling that were impacted following inflammation. Inflammation increased gut nociceptor sensitivity in both sexes but only females exhibited visceral hypersensitivity in vivo. Blocking MAGL normalized nociceptor responses in vitro and deleting glial Mgll in vivo rescued visceral hypersensitivity in females. These results show that sex and inflammation impact endocannabinoid mechanisms that regulate intercellular enteric glia-neuron communication. Further, targeting glial MAGL could provide therapeutic benefits for visceral nociception in a sex-dependent manner.

A Synbiotic Combining Chitin-Glucan and Lactobacillus acidophilus NCFM Induces a Colonic Molecular Signature Soothing Intestinal Pain and Inflammation in an Animal Model of IBS

Chitin-glucan (CG) is a new generation of prebiotic. Lactobacillus acidophilus NCFM® (NCFM) is a probiotic with the ability to decrease abdominal pain. We evaluate the functional and molecular gastrointestinal responses to a synbiotic administration combining CG and NCFM in a rat model of long-lasting colon hypersensitivity. The intracolonic pressure was assessed during the 9-week experiment in animals receiving CG in association or not with NCFM and compared to that in Lacticaseibacillus paracasei Lpc-37®-treated animals and control rats receiving tap water. The effects of the synbiotic were evaluated using the Wallace score, the quantification of colon myeloperoxidase (MPO) and the master genes driving analgesia and inflammation. CG 1.5 alone and NCFM 109 colony forming units (CFU) alone similarly decreased the visceral pain sensitivity. Lpc-37 had no significant effect. The best profile of pain perception inhibition was obtained with the combination of CG 1.5 g and NCFM 109 CFU, confirming a synbiotic property. This synbiotic treatment significantly reduced macroscopic colonic lesions and MPO concentrations, and induced master genes involved in analgesia (CB1, CB2, MOR, PPARα), with a downregulation of inflammatory cytokines (IL-1β, TNFα) and an induction of IL-10 and PPARγ. In conclusion, CG 1.5 g + NCFM 109 CFU significantly decreased visceral pain perception and intestinal inflammation through the regulation of master genes.

Targeting nerve growth factor for pain relief: pros and cons

Nerve growth factor (NGF) is a neurotrophic protein that has crucial roles in survival, growth and differentiation. It is expressed in neuronal and non-neuronal tissues. NGF exerts its effects via two types of receptors including the high affinity receptor, tropomyosin receptor kinase A and the low affinity receptor p75 neurotrophin receptor highlighting the complex signaling pathways that underlie the roles of NGF. In pain perception and transmission, multiple studies shed light on the effects of NGF on different types of pain including inflammatory, neuropathic, cancer and visceral pain. Also, the binding of NGF to its receptors increases the availability of many nociceptive receptors such as transient receptor potential vanilloid 1, transient receptor potential ankyrin 1, N-methyl-D-aspartic acid, and P2X purinoceptor 3 as well as nociceptive transmitters such as substance P and calcitonin gene-related peptide. The role of NGF in pain has been documented in pre-clinical and clinical studies. This review aims to shed light on the role of NGF and its signaling in different types of pain.

Effects of Stress Exposure to Pain Perception in Pre-Clinical Studies: Focus on the Nociceptin/Orphanin FQ-NOP Receptor System

Exposure to physical and psychological stress modulates pain transmission in a dual manner. Stress-induced analgesia (SIA) refers to the reduction in pain sensitivity that can occur in response to acute stress. On the contrary, chronic stress exposure may lead to a phenomenon named stress-induced hyperalgesia (SIH). SIH is a clinically relevant phenomenon since it has been well documented that physical and psychological stress exacerbates pain in patients with several chronic pain syndromes, including migraine. The availability of animal models of SIA and SIH is of high importance for understanding the biological mechanisms leading to these phenomena and for the identification of pharmacological targets useful to alleviate the burden of stress-exacerbated chronic pain. Among these targets, the nociceptin/orphanin FQ (N/OFQ)-N/OFQ peptide (NOP) receptor system has been identified as a key modulator of both pain transmission and stress susceptibility. This review describes first the experimental approaches to induce SIA and SIH in rodents. The second part of the manuscript summarizes the scientific evidence that suggests the N/OFQ-NOP receptor system as a player in the stress-pain interaction and candidates NOP antagonists as useful drugs to mitigate the detrimental effects of stress exposure on pain perception.

Photobiomodulation improves acute restraint stress-induced visceral hyperalgesia in rats

The purpose of this study is to explore the potential application of photobiomodulation to irritable bowel syndrome. We established the following experimental groups: the Non-Stress + Sham group, which consisted of rats that were not restrained and were only subjected to sham irradiation; the Stress + Sham group, which underwent 1 hour of restraint stress followed by sham irradiation; and the Stress + Laser group, which was subjected to restraint stress and percutaneous laser irradiation bilaterally on the L6 dorsal root ganglia for 5 minutes each. The experiment was conducted twice, with three and two laser conditions examined. Following laser irradiation, a barostat catheter was inserted into the rat's colon. After a 30-minute acclimatization period, the catheter was inflated to a pressure of 60 mmHg, and the number of abdominal muscle contractions was measured over a 5-minute period. The results showed that photobiomodulation significantly suppressed the number of abdominal muscle contractions at average powers of 460, 70, and 18 mW. However, no significant suppression was observed at average powers of 1 W and 3.5 mW. This study suggests that photobiomodulation can alleviate visceral hyperalgesia induced by restraint stress, indicating its potential applicability to irritable bowel syndrome.

Chitin-glucan improves important pathophysiological features of irritable bowel syndrome

BACKGROUND

Irritable bowel syndrome (IBS) is one of the most frequent and debilitating conditions leading to gastroenterological referrals. However, recommended treatments remain limited, yielding only limited therapeutic gains. Chitin-glucan (CG) is a novel dietary prebiotic classically used in humans at a dosage of 1.5-3.0 g/d and is considered a safe food ingredient by the European Food Safety Authority. To provide an alternative approach to managing patients with IBS, we performed preclinical molecular, cellular, and animal studies to evaluate the role of chitin-glucan in the main pathophysiological mechanisms involved in IBS.

AIM

To evaluate the roles of CG in visceral analgesia, intestinal inflammation, barrier function, and to develop computational molecular models.

METHODS

Visceral pain was recorded through colorectal distension (CRD) in a model of long-lasting colon hypersensitivity induced by an intra-rectal administration of TNBS [15 milligrams (mg)/kilogram (kg)] in 33 Sprague-Dawley rats. Intracolonic pressure was regularly assessed during the 9 wk-experiment (weeks 0, 3, 5, and 7) in animals receiving CG (n = 14) at a human equivalent dose (HED) of 1.5 g/d or 3.0 g/d and compared to negative control (tap water, n = 11) and positive control (phloroglucinol at 1.5 g/d HED, n = 8) groups. The anti-inflammatory effect of CG was evaluated using clinical and histological scores in 30 C57bl6 male mice with colitis induced by dextran sodium sulfate (DSS) administered in their drinking water during 14 d. HT-29 cells under basal conditions and after stimulation with lipopolysaccharide (LPS) were treated with CG to evaluate changes in pathways related to analgesia (µ-opioid receptor (MOR), cannabinoid receptor 2 (CB2), peroxisome proliferator-activated receptor alpha, inflammation [interleukin (IL)-10, IL-1b, and IL-8] and barrier function [mucin 2-5AC, claudin-2, zonula occludens (ZO)-1, ZO-2] using the real-time PCR method. Molecular modelling of CG, LPS, lipoteichoic acid (LTA), and phospholipomannan (PLM) was developed, and the ability of CG to chelate microbial pathogenic lipids was evaluated by docking and molecular dynamics simulations. Data were expressed as the mean ± SEM.

RESULTS

Daily CG orally-administered to rats or mice was well tolerated without including diarrhea, visceral hypersensitivity, or inflammation, as evaluated at histological and molecular levels. In a model of CRD, CG at a dosage of 3 g/d HED significantly decreased visceral pain perception by 14% after 2 wk of administration (P < 0.01) and reduced inflammation intensity by 50%, resulting in complete regeneration of the colonic mucosa in mice with DSS-induced colitis. To better reproduce the characteristics of visceral pain in patients with IBS, we then measured the therapeutic impact of CG in rats with TNBS-induced inflammation to long-lasting visceral hypersensitivity. CG at a dosage of 1.5 g/d HED decreased visceral pain perception by 20% five weeks after colitis induction (P < 0.01). When the CG dosage was increased to 3.0 g/d HED, this analgesic effect surpassed that of the spasmolytic agent phloroglucinol, manifesting more rapidly within 3 wk and leading to a 50% inhibition of pain perception (P < 0.0001). The underlying molecular mechanisms contributing to these analgesic and anti-inflammatory effects of CG involved, at least in part, a significant induction of MOR, CB2 receptor, and IL-10, as well as a significant decrease in pro-inflammatory cytokines IL-1b and IL-8. CG also significantly upregulated barrier-related genes including muc5AC, claudin-2, and ZO-2. Molecular modelling of CG revealed a new property of the molecule as a chelator of microbial pathogenic lipids, sequestering gram-negative LPS and gram-positive LTA bacterial toxins, as well as PLM in fungi at the lowesr energy conformations.

CONCLUSION

CG decreased visceral perception and intestinal inflammation through master gene regulation and direct binding of microbial products, suggesting that CG may constitute a new therapeutic strategy for patients with IBS or IBS-like symptoms.

Gut–Brain Axis, Microbiota and Probiotics—Current Knowledge on Their Role in Irritable Bowel Syndrome: A Review

Irritable bowel syndrome (IBS) is a common digestive disorder with a significant impact on both individuals and society in terms of quality of life and healthcare costs. A growing body of research has identified various communication pathways between the microbiota and the brain in relation to motility disorders, with the gut–brain axis being key to the pathogenesis of IBS. Multiple factors contribute to the pathogenetic pathways in IBS, including immune mechanisms, psychosocial factors, increased oxidative stress and pro-inflammatory cytokine release, as well as genetic and hormonal factors. Increased permeability of the normal intestinal barrier allows bacterial products to access the lamina propria, providing a mechanism for perpetuating chronic inflammation and characteristic symptoms. The microbiota influences inflammatory processes in IBS by altering the balance between pro-inflammatory factors and host defence. Probiotics modulate the pathophysiological mechanisms involved in IBS by influencing the composition of the microbiota and improving intestinal motility disorders, visceral hypersensitivity, immune function of the intestinal epithelium, metabolic processes in the intestinal lumen, dysfunction of the microbiota-GBA, and are recognised as effective and safe in IBS therapy. Our study aimed to provide a comprehensive overview of the relationship between the gut–brain axis, microbiota, and IBS, based on current information.

Enteric glia promote visceral hypersensitivity during inflammation through intercellular signaling with gut nociceptors

Inflammation in the intestines causes abdominal pain that is challenging to manage. The terminals of sensory neurons innervating the gut are surrounded by glia. Here, using a mouse model of acute colitis, we found that enteric glia contribute to visceral pain by secreting factors that sensitized sensory nerves innervating the gut in response to inflammation. Acute colitis induced a transient increase in the production of proinflammatory cytokines in the intestines of male and female mice. Of these, IL-1β was produced in part by glia and augmented the opening of the intercellular communication hemichannel connexin-43 in glia, which made normally innocuous stimuli painful in female mice. Chemogenetic glial activation paired with calcium imaging in nerve terminals demonstrated that glia sensitized gut-innervating nociceptors only under inflammatory conditions. This inflammatory, glial-driven visceral hypersensitivity involved an increased abundance of the enzyme COX-2 in glia, resulting in greater production and release of prostaglandin E2 that activated EP4 receptors on sensory nerve terminals. Blocking EP4 receptors reduced nociceptor sensitivity in response to glial stimulation in tissue samples from colitis-model mice, and impairing glial connexin-43 reduced visceral hypersensitivity induced by IL-1β in female mice. The findings suggest that therapies targeting enteric glial-neuron signaling might alleviate visceral pain caused by inflammatory disorders.

The Gut Microbial Bile Acid Modulation and Its Relevance to Digestive Health and Diseases

The human gut microbiome has been linked to numerous digestive disorders, but its metabolic products have been much less well characterized, in part due to the expense of untargeted metabolomics and lack of ability to process the data. In this review, we focused on the rapidly expanding information about the bile acid repertoire produced by the gut microbiome, including the impacts of bile acids on a wide range of host physiological processes and diseases, and discussed the role of short-chain fatty acids and other important gut microbiome-derived metabolites. Of particular note is the action of gut microbiome-derived metabolites throughout the body, which impact processes ranging from obesity to aging to disorders traditionally thought of as diseases of the nervous system, but that are now recognized as being strongly influenced by the gut microbiome and the metabolites it produces. We also highlighted the emerging role for modifying the gut microbiome to improve health or to treat disease, including the "engineered native bacteria'' approach that takes bacterial strains from a patient, modifies them to alter metabolism, and reintroduces them. Taken together, study of the metabolites derived from the gut microbiome provided insights into a wide range of physiological and pathophysiological processes, and has substantial potential for new approaches to diagnostics and therapeutics of disease of, or involving, the gastrointestinal tract.

Spinal VGLUT3 lineage neurons drive visceral mechanical allodynia but not sensitized visceromotor reflexes

Visceral pain is among the most prevalent and bothersome forms of chronic pain, but their transmission in the spinal cord is still poorly understood. Here, we conducted focal colorectal distention (fCRD) to drive both visceromotor responses (VMRs) and aversion. We first found that spinal CCK neurons were necessary for noxious fCRD to drive both VMRs and aversion under naive conditions. We next showed that spinal VGLUT3 neurons mediate visceral allodynia, whose ablation caused loss of aversion evoked by low-intensity fCRD in mice with gastrointestinal (GI) inflammation or spinal circuit disinhibition. Importantly, these neurons were dispensable for driving sensitized VMRs under both inflammatory and central disinhibition conditions. Anatomically, a subset of VGLUT3 neurons projected to parabrachial nuclei, whose photoactivation sufficiently generated aversion in mice with GI inflammation, without influencing VMRs. Our studies suggest the presence of different spinal substrates that transmit nociceptive versus affective dimensions of visceral sensory information.

TRIM27 maintains gut homeostasis by promoting intestinal stem cell self-renewal

Dysregulation of gut homeostasis is associated with irritable bowel syndrome (IBS), a chronic functional gastrointestinal disorder affecting approximately 11.2% of the global population. The poorly understood pathogenesis of IBS has impeded its treatment. Here, we report that the E3 ubiquitin ligase tripartite motif-containing 27 (TRIM27) is weakly expressed in IBS but highly expressed in inflammatory bowel disease (IBD), a frequent chronic organic gastrointestinal disorder. Accordingly, knockout of Trim27 in mice causes spontaneously occurring IBS-like symptoms, including increased visceral hyperalgesia and abnormal stool features, as observed in IBS patients. Mechanistically, TRIM27 stabilizes β-catenin and thus activates Wnt/β-catenin signaling to promote intestinal stem cell (ISC) self-renewal. Consistent with these findings, Trim27 deficiency disrupts organoid formation, which is rescued by reintroducing TRIM27 or β-catenin. Furthermore, Wnt/β-catenin signaling activator treatment ameliorates IBS symptoms by promoting ISC self-renewal. Taken together, these data indicate that TRIM27 is critical for maintaining gut homeostasis, suggesting that targeting the TRIM27/Wnt/β-catenin axis could be a potential treatment strategy for IBS. Our study also indicates that TRIM27 might serve as a potential biomarker for differentiating IBS from IBD.

Multi-omics for biomarker approaches in the diagnostic evaluation and management of abdominal pain and irritable bowel syndrome: what lies ahead

Reliable biomarkers for common disorders of gut-brain interaction characterized by abdominal pain, including irritable bowel syndrome (IBS), are critically needed to enhance care and develop individualized therapies. The dynamic and heterogeneous nature of the pathophysiological mechanisms that underlie visceral hypersensitivity have challenged successful biomarker development. Consequently, effective therapies for pain in IBS are lacking. However, recent advances in modern omics technologies offer new opportunities to acquire deep biological insights into mechanisms of pain and nociception. Newer methods for large-scale data integration of complementary omics approaches have further expanded our ability to build a holistic understanding of complex biological networks and their co-contributions to abdominal pain. Here, we review the mechanisms of visceral hypersensitivity, focusing on IBS. We discuss candidate biomarkers for pain in IBS identified through single omics studies and summarize emerging multi-omics approaches for developing novel biomarkers that may transform clinical care for patients with IBS and abdominal pain.

Long-lasting visceral hypersensitivity in a model of DSS-induced colitis in rats

BACKGROUND

Persistent visceral hypersensitivity is a key component of functional and inflammatory gastrointestinal diseases. Current animal models fail to fully reproduce the characteristics of visceral pain in humans, particularly as it relates to persistent hypersensitivity. This work explores the validity of DSS-induced colitis in rats as a model to mimic chronic intestinal hypersensitivity.

METHODS

Exposure to DSS (5% for 7 days) was used to induce colitis in rats. Thereafter, changes in viscerosensitivity (visceromotor responses to colorectal distension-CRD), the presence of somatic referred pain (mechanosensitivity of the hind paws, von Frey test) and the expression (qRT-PCR) of sensory-related markers (colon, lumbosacral DRGs, and lumbosacral spinal cord) were assessed at different times during the 35 days period after colitis induction.

RESULTS

Following colitis, a sustained increase in visceromotor responses to CRD were observed, indicative of the presence of visceral hypersensitivity. Responses in animals without colitis remained stable over time. In colitic animals, somatic referred hypersensitivity was also detected. DSS-induced colitis was associated to a differential expression of sensory-related markers (with both pro- and anti-nociceptive action) in the colon, lumbosacral DRGs and lumbosacral spinal cord; indicating the presence of peripheral and central sensitization.

CONCLUSIONS AND INFERENCES

DSS-induced colitis in rats is associated to the generation of a long-lasting state of visceral (colonic) hypersensitivity, despite clinical colitis resolution. This model reproduces the changes in intestinal sensitivity characteristics of inflammatory and functional gastrointestinal disorders in humans and can be used in the characterization of new pharmacological treatments against visceral pain.

Regional brain activation during rectal distention and attenuation with alosetron in a nonhuman primate model of irritable bowel syndrome

Greater understanding of the mechanism that mediates visceral pain and hypersensitivity associated with irritable bowel syndrome (IBS) would facilitate the development of effective therapeutics to manage these symptoms. An objective marker associated with the underlying mechanisms of visceral pain and hypersensitivity could be used to guide therapeutic development. The current study examined brain activation evoked by rectal distention with functional magnetic resonance imaging (fMRI) in a cynomolgus macaque model of visceral hypersensitivity. Male, cynomolgus macaques underwent five four-week treatments of dextran sodium sulfate (DSS)-distilled water (DW), which induced mild-moderate colitis with remission during each treatment cycle. Balloon rectal distention (RD) was performed under anesthesia 14 weeks after the final DSS-DW treatment. Colonoscopy confirmed the absence of colitis prior to the start of RD. In naïve, untreated macaques, 10, 20 and 30 ml RD did not evoke brain activation. However, insular cortex/somatosensory II cortex and cerebellum were significantly activated in DSS-treated macaques at 20 and 30 ml rectal distention. Intra-rectal pressure after DSS treatment was not significantly different from that of naïve, untreated macaques, indicating lack of alteration of rectal functioning following DSS-treatment. Treatment with 5-HT3 receptor antagonist alosetron (p.o.) reduced distension-evoked brain activation and decreased intra-rectal pressure. The current findings demonstrated activation of brain regions to RD following DSS treatments which was not present in naïve macaques, suggesting visceral hypersensitivity. Brain activation in turn was reduced by alosetron, which could underlie the analgesic effect alosetron in IBS patients.

Editorial: natural history of irritable bowel syndrome in inflammatory bowel disease

LINKED CONTENT

This article is linked to Fairbrass et al papers. To view these articles, visit https://doi.org/10.1111/apt.17193 and https://doi.org/10.1111/apt.17215

The involvement of Pellino-1 downregulation in the modulation of visceral hypersensitivity via the TLR4/NF-κB pathway in the rat fastigial nucleus

Irritable bowel syndrome (IBS) is a common functional bowel disorder whose key characteristics include chronic visceral hypersensitivity (CVH) and abnormal brain-gut interactions. Pellino-1 is an E3 ubiquitin ligase, mediating the degradation or modification of targeted proteins. Some brain regions, such as the fastigial nucleus (FN), may play important roles in CVH; however, the molecular mechanism underlying this phenomenon is not clear. In this study, we assessed the roles of Pellino-1 within the FN in modulating VH by generating a colorectal distention (CRD) model in male Sprague-Dawley rats. Our results showed that the downregulation of Pellino-1 in the fastigial nucleus (FN) was involved in the modulation of visceral hypersensitivity. The expression of Pellino-1 was downregulated in the FN of adult CRD rats compared with control rats, whereas TLR4 and NF-κB were upregulated in the CRD model. To overexpress Pellino-1, a lentivirus specifically expressing Pellino-1 and green fluorescent protein was administered into the FN. The overexpression of Pellino-1 increased the visceral sensitivity of CRD rats, and the expression of TLR4 and NF-κB increased further. After administration of TAK-242 (a specific TLR4 inhibitor), the visceral response to overexpression of Pellino-1 was reversed. Overall, the findings indicated the involvement of the FN in the development of CVH; the downregulation of Pellino-1 in the FN acted through the TLR4/NF-κB pathway to protect against CVH in a CRD rat model.

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.

Chronic psychological stress alters gene expression in rat colon epithelial cells promoting chromatin remodeling, barrier dysfunction and inflammation

Chronic stress is commonly associated with enhanced abdominal pain (visceral hypersensitivity), but the cellular mechanisms underlying how chronic stress induces visceral hypersensitivity are poorly understood. In this study, we examined changes in gene expression in colon epithelial cells from a rat model using RNA-sequencing to examine stress-induced changes to the transcriptome. Following chronic stress, the most significantly up-regulated genes included Atg16l1, Coq10b, Dcaf13, Nat2, Ptbp2, Rras2, Spink4 and down-regulated genes including Abat, Cited2, Cnnm2, Dab2ip, Plekhm1, Scd2, and Tab2. The primary altered biological processes revealed by network enrichment analysis were inflammation/immune response, tissue morphogenesis and development, and nucleosome/chromatin assembly. The most significantly down-regulated process was the digestive system development/function, whereas the most significantly up-regulated processes were inflammatory response, organismal injury, and chromatin remodeling mediated by H3K9 methylation. Furthermore, a subpopulation of stressed rats demonstrated very significantly altered gene expression and transcript isoforms, enriched for the differential expression of genes involved in the inflammatory response, including upregulation of cytokine and chemokine receptor gene expression coupled with downregulation of epithelial adherens and tight junction mRNAs. In summary, these findings support that chronic stress is associated with increased levels of cytokines and chemokines, their downstream signaling pathways coupled to dysregulation of intestinal cell development and function. Epigenetic regulation of chromatin remodeling likely plays a prominent role in this process. Results also suggest that super enhancers play a primary role in chronic stress-associated intestinal barrier dysfunction.

Spinal Microglia and Astrocytes: Two Key Players in Chronic Visceral Pain Pathogenesis

Chronic visceral pain (CVP) is one of the common symptoms of many diseases triggered by underlying diseases of the internal organs of the human body. Its causes include vascular mechanisms, mechanical factors, persistent inflammation, and unexplained functional mechanisms. Although the pathogenesis is unclear, more and more research has begun to shift from the neuronal aspect to the glial cells in recent years. Some data highlight that the spinal glial cells, particularly the microglia and astrocytes, play an essential role in CVP. Based on this, we highlight the mechanisms of microglia and astrocytes in CVP concerning the release of cytokines, chemokines, and neuroactive substances and alterations in intracellular signaling pathways during the process. Finally, because CVP is widespread in various diseases, we present future perspectives targeting microglia and astrocytes for treatment.

Guanylate cyclase-C agonists as peripherally acting treatments of chronic visceral pain

Irritable bowel syndrome (IBS) is a chronic gastrointestinal disorder characterized by abdominal pain and altered bowel habit that affects ~11% of the global population. Over the past decade, preclinical and clinical studies have revealed a variety of novel mechanisms relating to the visceral analgesic effects of guanylate cyclase-C (GC-C) agonists. Here we discuss the mechanisms by which GC-C agonists target the GC-C/cyclic guanosine-3',5'-monophosphate (cGMP) pathway, resulting in visceral analgesia as well as clinically relevant relief of abdominal pain and other sensations in IBS patients. Due to the preponderance of evidence we focus on linaclotide, a 14-amino acid GC-C agonist with very low oral bioavailability that acts within the gut. Collectively, the weight of experimental and clinical evidence supports the concept that GC-C agonists act as peripherally acting visceral analgesics.

Review article: current and future treatment approaches for pain in IBS

BACKGROUND

Abdominal pain is a core symptom of IBS and a primary driver of care seeking. Visceral hypersensitivity is a key pathophysiological mechanism and therapeutic target for pain in IBS, with components of peripheral and central sensitisation and psychological factors.

AIM

To review current and future treatment approaches specifically for the pain component of IBS.

METHODS

Pubmed search terms included combinations of irritable bowel, pain, visceral hypersensitivity, novel, new, emerging, future and advances.

RESULTS

Established non-pharmacological treatments for IBS pain include the low FODMAP diet, probiotics and psychological interventions, especially hypnotherapy. Tricyclics remain the best evidenced pharmacological approach with GCC agonists, tenapanor, lubiprostone, eluxadoline and 5HT3 antagonists second line according to patient characteristics and availability. Less well-evidenced current options include anti-spasmodics, peppermint oil, SSRIs, SNRIs, alpha 2 delta ligands, melatonin and histamine antagonists. Patients are vulnerable to iatrogenesis and harmful approaches to be avoided include opioids and unwarranted surgical interventions. For severe pain, the concept of augmentation with combined gut-brain neuromodulators and psychotherapy in a multi-disciplinary setting is considered. A plethora of molecular targets and ligands are emerging from pre-clinical studies, together with early clinical evidence for a range of pharmacological, dietary, neurostimulation and novel psychological treatment delivery methods which are reviewed. The history of such emerging approaches, however, merits both caution and optimism in equal measure.

CONCLUSIONS

Despite good in-roads and emerging options, the management of abdominal pain remains one of the biggest challenges and research priorities for patients with IBS.

Animal models of visceral pain and the role of the microbiome

Visceral pain refers to pain arising from the internal organs and is distinctly different from the expression and mechanisms of somatic pain. Diseases and disorders with increased visceral pain are associated with significantly reduced quality of life and incur large financial costs due to medical visits and lost work productivity. In spite of the notable burden of illness associated with those disorders involving increased visceral pain, and some knowledge regarding etiology, few successful therapeutics have emerged, and thus increased attention to animal models of visceral hypersensitivity is warranted in order to elucidate new treatment opportunities. Altered microbiota-gut-brain (MGB) axis communication is central to the comorbid gastrointestinal/psychiatric diseases of which increased visceral (intestinal) sensitivity is a hallmark. This has led to a particular focus on intestinal microbiome disruption and its potential role in the etiology of heightened visceral pain. Here we provide a review of studies examining models of heightened visceral pain due to altered bidirectional communication of the MGB axis, many of which are conducted on a background of stress exposure. We discuss work in which the intestinal microbiota has either been directly manipulated (as with germ-free, antibiotic, and fecal microbial transplantation studies) or indirectly affected through early life or adult stress, inflammation, and infection. Animal models of visceral pain alterations with accompanying changes to the intestinal microbiome have the highest face and construct validity to the human condition and are the focus of the current review.

Depression and Pain: use of antidepressant

Background:

Emotional disorders are common comorbid affectations that exacerbate the severity and persistence of chronic pain. Specifically, depressive symptoms can lead to an excessive duration and intensity of pain. Clinical and preclinical studies have been focused on the underlying mechanisms of chronic pain and depression comorbidity and the use of antidepressants to reduce pain.

Aim:

This review provides an overview of the comorbid relationship of chronic pain and depression, the clinical and pre-clinical studies performed on the neurobiological aspects of pain and depression, and the use of antidepressants as analgesics.

Methods:

A systematic search of literature databases was conducted according to pre-defined criteria. The authors independently conducted a focused analysis of the full-text articles.

Results:

Studies suggest that pain and depression are highly intertwined and may co-exacerbate physical and psychological symptoms. One important biochemical basis for pain and depression focuses on the serotonergic and norepinephrine system, which have been shown to play an important role in this comorbidity. Brain structures that codify pain are also involved in mood. It is evident that using serotonergic and norepinephrine antidepressants are strategies commonly employed to mitigate pain

Conclusion:

Literature indicates that pain and depression impact each other and play a prominent role in the development and maintenance of other chronic symptoms. Antidepressants continue to be a major therapeutic tool for managing chronic pain. Tricyclic antidepressants (TCAs) are more effective in reducing pain than Selective Serotonin Reuptake Inhibitors (SSRIs) and Serotonin-Noradrenaline Reuptake Inhibitors (SNRIs).

Changes in fecal short-chain fatty acids following fecal microbiota transplantation in patients with irritable bowel syndrome

BACKGROUND

Short-chain fatty acids (SCFAs) may play a role in the pathophysiology of irritable bowel syndrome (IBS). This study analyzed fecal SCFAs after performing fecal microbiota transplantation (FMT) in the IBS patients who were included in our previous study of the efficacy of FMT.

METHODS

This study included 142 of the 164 IBS patients who participated in our previous study. They were belonging to three groups: placebo (own feces), 30-g (superdonor feces), and 60-g (superdonor feces) FMT. The patients completed the IBS Severity Scoring System (IBS-SSS) Birmingham IBS Symptom, Fatigue Assessment Scale (FAS), the IBS Quality of Life (IBS-QoL) and Short-Form Nepean Dyspepsia Index (SF-NDI) questionnaires and delivered fecal samples at the baseline and 1 month after FMT. The SCFA levels were determined by vacuum distillation followed by gas chromatography.

KEY RESULTS

The fecal butyric acid level was significantly increased after FMT in both the 30-g and 60-g groups (both P ≤ 0.001). In the 60-g group, the levels of total SCFAs and isobutyric, isovaleric, and valeric acids increased after FMT. Butyric acid levels in the responders in both the 30-g and 60-g FMT groups were significantly inversely correlated with IBS-SSS and FAS scores (P = 0.001, r = -0.3 and P = 0.0001. r=- 0.3, respectively). There were no differences in the SCFA levels in the placebo group after FMT.

CONCLUSION AND INFERENCES

FMT increases the fecal SCFA levels in IBS patients. The increase in the butyric acid level is inversely correlated with symptoms in IBS patients following FMT, suggesting that SCFAs might play a role in the pathophysiology of IBS. www.clini​caltr​ials.gov (NCT03822299).