Importance of epigenetic mechanisms in visceral pain induced by chronic water avoidance stress

Identification of Arginine-Vasopressin Receptor 1a (Avpr1a/Avpr1a) as a Novel Candidate Gene for Chronic Visceral Pain Sheds Light on the Potential Role of Enteric Neurons in the Development of Visceral Hypersensitivity

Chronic abdominal pain in the absence of ongoing disease is the hallmark of disorders of gut-brain interaction (DGBIs), including irritable bowel syndrome (IBS). While the etiology of DGBIs remains poorly understood, there is evidence that both genetic and environmental factors play a role. In this study, we report the identification and validation of arginine-vasopressin receptor 1A (Avpr1a) as a novel candidate gene for visceral hypersensitivity (VH), a primary peripheral mechanism underlying abdominal pain in DGBI/IBS. Comparing 2 C57BL/6 (BL/6) substrains (C57BL/6NTac and C57BL/6J) revealed differential susceptibility to the development of chronic VH following intrarectal zymosan instillation, a validated preclinical model for postinflammatory IBS. Using whole-genome sequencing, we identified a single-nucleotide polymorphism differentiating the 2 strains in the 5' intergenic region upstream of Avpr1a, encoding the protein Avpr1a. We used behavioral, histological, and molecular approaches to identify distal colon-specific gene expression and neuronal hyperresponsiveness covarying with Avpr1a genotype and VH susceptibility. While the 2 BL/6 substrains did not differ across other gastrointestinal phenotypes (eg, fecal water retention), VH-susceptible BL/6NTac mice had higher colonic Avpr1a mRNA and protein expression. These results parallel findings that patients' colonic Avpr1a mRNA expression corresponded to higher pain ratings. Moreover, neurons of the enteric nervous system were hyperresponsive to the Avpr1a agonist arginine-vasopressin, suggesting a role for enteric neurons in the pathology underlying VH. Taken together, these findings implicate differential regulation of Avpr1a as a novel mechanism of VH susceptibility as well as a potential therapeutic target specific to VH. PERSPECTIVE: This article presents evidence of Avpr1a as a novel candidate gene for VH in a mouse model of IBS. Avpr1a genotype and/or tissue-specific expression represents a potential biomarker for chronic abdominal pain susceptibility.

Pain-sensorimotor interactions: New perspectives and a new model

How pain and sensorimotor behavior interact has been the subject of research and debate for many decades. This article reviews theories bearing on pain-sensorimotor interactions and considers their strengths and limitations in the light of findings from experimental and clinical studies of pain-sensorimotor interactions in the spinal and craniofacial sensorimotor systems. A strength of recent theories is that they have incorporated concepts and features missing from earlier theories to account for the role of the sensory-discriminative, motivational-affective, and cognitive-evaluative dimensions of pain in pain-sensorimotor interactions. Findings acquired since the formulation of these recent theories indicate that additional features need to be considered to provide a more comprehensive conceptualization of pain-sensorimotor interactions. These features include biopsychosocial influences that range from biological factors such as genetics and epigenetics to psychological factors and social factors encompassing environmental and cultural influences. Also needing consideration is a mechanistic framework that includes other biological factors reflecting nociceptive processes and glioplastic and neuroplastic changes in sensorimotor and related brain and spinal cord circuits in acute or chronic pain conditions. The literature reviewed and the limitations of previous theories bearing on pain-sensorimotor interactions have led us to provide new perspectives on these interactions, and this has prompted our development of a new concept, the Theory of Pain-Sensorimotor Interactions (TOPSMI) that we suggest gives a more comprehensive framework to consider the interactions and their complexity. This theory states that pain is associated with plastic changes in the central nervous system (CNS) that lead to an activation pattern of motor units that contributes to the individual's adaptive sensorimotor behavior. This activation pattern takes account of the biological, psychological, and social influences on the musculoskeletal tissues involved in sensorimotor behavior and on the plastic changes and the experience of pain in that individual. The pattern is normally optimized in terms of biomechanical advantage and metabolic cost related to the features of the individual's musculoskeletal tissues and aims to minimize pain and any associated sensorimotor changes, and thereby maintain homeostasis. However, adverse biopsychosocial factors and their interactions may result in plastic CNS changes leading to less optimal, even maladaptive, sensorimotor changes producing motor unit activation patterns associated with the development of further pain. This more comprehensive theory points towards customized treatment strategies, in line with the management approaches to pain proposed in the biopsychosocial model of pain.

Identification of arginine-vasopressin receptor 1a (Avpr1a/AVPR1A) as a novel candidate gene for chronic visceral pain

Chronic abdominal pain in the absence of ongoing disease is the hallmark of disorders of gut-brain interaction (DGBIs), including irritable bowel syndrome (IBS). While the etiology of DGBIs remains poorly understood, there is evidence that both genetic and environmental factors play a role. In this study, we report the identification and validation of Avpr1a as a novel candidate gene for visceral hypersensitivity (VH), a primary peripheral mechanism underlying abdominal pain in DGBI/IBS. Comparing two C57BL/6 (BL/6) substrains (C57BL/6NTac and C57BL/6J) revealed differential susceptibility to the development of chronic VH following intrarectal zymosan (ZYM) instillation, a validated preclinical model for post-inflammatory IBS. Using whole genome sequencing, we identified a SNP differentiating the two strains in the 5' intergenic region upstream of Avpr1a, encoding the protein arginine-vasopressin receptor 1A (AVPR1A). We used behavioral, histological, and molecular approaches to identify distal colon-specific gene expression differences and neuronal hyperresponsiveness covarying with Avpr1a genotype and VH susceptibility. While the two BL/6 substrains did not differ across other gastrointestinal (GI) phenotypes (e.g., GI motility), VH-susceptible BL/6NTac mice had higher colonic Avpr1a mRNA and protein expression. Moreover, neurons of the enteric nervous system were hyperresponsive to the AVPR1A agonist AVP, suggesting a role for enteric neurons in the pathology underlying VH. These results parallel our findings that patients' colonic Avpr1a mRNA expression was higher in patients with higher pain ratings. Taken together, these findings implicate differential regulation of Avpr1a as a novel mechanism of VH-susceptibility as well as a potential therapeutic target specific to VH.

The impact of acute and chronic stress on gastrointestinal physiology and function: a microbiota-gut-brain axis perspective

The physiological consequences of stress often manifest in the gastrointestinal tract. Traumatic or chronic stress is associated with widespread maladaptive changes throughout the gut, although comparatively little is known about the effects of acute stress. Furthermore, these stress-induced changes in the gut may increase susceptibility to gastrointestinal disorders and infection, and impact critical features of the neural and behavioural consequences of the stress response by impairing gut-brain axis communication. Understanding the mechanisms behind changes in enteric nervous system circuitry, visceral sensitivity, gut barrier function, permeability, and the gut microbiota following stress is an important research objective with pathophysiological implications in both neurogastroenterology and psychiatry. Moreover, the gut microbiota has emerged as a key aspect of physiology sensitive to the effects of stress. In this review, we focus on different aspects of the gastrointestinal tract including gut barrier function as well as the immune, humoral and neuronal elements involved in gut-brain communication. Furthermore, we discuss the evidence for a role of stress in gastrointestinal disorders. Existing gaps in the current literature are highlighted, and possible avenues for future research with an integrated physiological perspective have been suggested. A more complete understanding of the spatial and temporal dynamics of the integrated host and microbial response to different kinds of stressors in the gastrointestinal tract will enable full exploitation of the diagnostic and therapeutic potential in the fast-evolving field of host-microbiome interactions.

Narrative Review of the Complex Interaction between Pain and Trauma in Children: A Focus on Biological Memory, Preclinical Data, and Epigenetic Processes

The incidence and collective impact of early adverse experiences, trauma, and pain continue to increase. This underscores the urgent need for translational efforts between clinical and preclinical research to better understand the underlying mechanisms and develop effective therapeutic approaches. As our understanding of these issues improves from studies in children and adolescents, we can create more precise preclinical models and ultimately translate our findings back to clinical practice. A multidisciplinary approach is essential for addressing the complex and wide-ranging effects of these experiences on individuals and society. This narrative review aims to (1) define pain and trauma experiences in childhood and adolescents, (2) discuss the relationship between pain and trauma, (3) consider the role of biological memory, (4) decipher the relationship between pain and trauma using preclinical data, and (5) examine the role of the environment by introducing the importance of epigenetic processes. The ultimate scope is to better understand the wide-ranging effects of trauma, abuse, and chronic pain on children and adolescents, how they occur, and how to prevent or mitigate their effects and develop effective treatment strategies that address both the underlying causes and the associated physiological and psychological effects.

New insights into irritable bowel syndrome pathophysiological mechanisms: contribution of epigenetics

Irritable bowel syndrome (IBS) is a complex multifactorial condition including alterations of the gut-brain axis, intestinal permeability, mucosal neuro-immune interactions, and microbiota imbalance. Recent advances proposed epigenetic factors as possible regulators of several mechanisms involved in IBS pathophysiology. These epigenetic factors include biomolecular mechanisms inducing chromosome-related and heritable changes in gene expression regardless of DNA coding sequence. Accordingly, altered gut microbiota may increase the production of metabolites such as sodium butyrate, a prominent inhibitor of histone deacetylases. Patients with IBS showed an increased amount of butyrate-producing microbial phila as well as an altered profile of methylated genes and micro-RNAs (miRNAs). Importantly, gene acetylation as well as specific miRNA profiles are involved in different IBS mechanisms and may be applied for future diagnostic purposes, especially to detect increased gut permeability and visceromotor dysfunctions. In this review, we summarize current knowledge of the role of epigenetics in IBS pathophysiology.

Epigenetic mechanisms underlying stress-induced visceral pain: Resilience versus vulnerability in a two-hit model of early life stress and chronic adult stress

BACKGROUND

Women with a history of early life stress (ELS) have a higher risk of developing irritable bowel syndrome (IBS). In addition, chronic stress in adulthood can exacerbate IBS symptoms such as abdominal pain due to visceral hypersensitivity. We previously showed that sex and the predictability of ELS determine whether rats develop visceral hypersensitivity in adulthood. In female rats, unpredictable ELS confers vulnerability and results in visceral hypersensitivity, whereas predictable ELS induces resilience and does not induce visceral hypersensitivity in adulthood. However, this resilience is lost after exposure to chronic stress in adulthood leading to an exacerbation of visceral hypersensitivity. Evidence suggests that changes in histone acetylation at the promoter regions of glucocorticoid receptor (GR) and corticotrophin-releasing factor (CRF) in the central nucleus of the amygdala (CeA) underlie stress-induced visceral hypersensitivity. Here, we aimed to investigate the role of histone acetylation in the CeA on visceral hypersensitivity in a two-hit model of ELS followed by chronic stress in adulthood.

METHODS

Male and female neonatal rats were exposed to unpredictable, predictable ELS, or odor only (no stress control) from postnatal days 8 to 12. In adulthood, rats underwent stereotaxic implantation of indwelling cannulas. Rats were exposed to chronic water avoidance stress (WAS, 1 h/day for 7 days) or SHAM stress and received infusions of vehicle, the histone deacetylase inhibitor trichostatin A (TSA) or the histone acetyltransferase inhibitor garcinol (GAR) after each WAS session. 24 h after the final infusion, visceral sensitivity was assessed and the CeA was removed for molecular experiments.

RESULTS

In the two-hit model (ELS + WAS), female rats previously exposed to predictable ELS, showed a significant reduction in histone 3 lysine 9 (H3K9) acetylation at the GR promoter and a significant increase in H3K9 acetylation at the CRF promoter. These epigenetic changes were associated with changes in GR and CRF mRNA expression in the CeA and an exacerbation of stress-induced visceral hypersensitivity in female animals. TSA infusions in the CeA attenuated the exacerbated stress-induced visceral hypersensitivity, whereas GAR infusions only partially ameliorated ELS+WAS induced visceral hypersensitivity.

CONCLUSION

The two-hit model of ELS followed by WAS in adulthood revealed that epigenetic dysregulation occurs after exposure to stress in two important periods of life and contributes to the development of visceral hypersensitivity. These aberrant underlying epigenetic changes may explain the exacerbation of stress-induced abdominal pain in IBS patients.

The Unmet Needs for Studying Chronic Pelvic/Visceral Pain Using Animal Models

The different definitions of chronic pelvic/visceral pain used by international societies have changed over the years. These differences have a great impact on the way researchers study chronic pelvic/visceral pain. Recently, the role of systemic changes, including the role of the central nervous system, in the perpetuation and chronification of pelvic/visceral pain has gained weight. Consequently, researchers are using animal models that resemble those systemic changes rather than using models that are organ- or tissue-specific. In this review, we discuss the advantages and disadvantages of using bladder-centric and systemic models, enumerating some of the central nervous system changes and pain-related behaviors occurring in each model. We also present some drawbacks when using animal models and pain-related behavior tests and raise questions about possible, yet to be demonstrated, investigator-related bias. We also suggest new approaches to study chronic pelvic/visceral pain by refining existing animal models or using new ones.

Targeting the endocannabinoid system for the treatment of abdominal pain in irritable bowel syndrome

The management of visceral pain in patients with disorders of gut-brain interaction, notably irritable bowel syndrome, presents a considerable clinical challenge, with few available treatment options. Patients are increasingly using cannabis and cannabinoids to control abdominal pain. Cannabis acts on receptors of the endocannabinoid system, an endogenous system of lipid mediators that regulates gastrointestinal function and pain processing pathways in health and disease. The endocannabinoid system represents a logical molecular therapeutic target for the treatment of pain in irritable bowel syndrome. Here, we review the physiological and pathophysiological functions of the endocannabinoid system with a focus on the peripheral and central regulation of gastrointestinal function and visceral nociception. We address the use of cannabinoids in pain management, comparing them to other treatment modalities, including opioids and neuromodulators. Finally, we discuss emerging therapeutic candidates targeting the endocannabinoid system for the treatment of pain in irritable bowel syndrome.

Chronic stress increases DNA methylation of the GR promoter in the central nucleus of the amygdala of female rats

The central pathophysiological mechanisms underlying irritable bowel syndrome (IBS), a female-predominant gastrointestinal disorder characterized by abdominal pain and abnormal bowel habits, remain poorly understood. IBS patients often report that chronic stress exacerbates their symptoms. Brain imaging studies have revealed that the amygdala, a stress-responsive brain region, of IBS patients is overactive when compared to healthy controls. Previously, we demonstrated that downregulation of the glucocorticoid receptor (GR) in the central nucleus of the amygdala (CeA) underlies stress-induced visceral hypersensitivity in female rats. In the current study, we aimed to evaluate in the CeA of female rats whether chronic water avoidance stress (WAS) alters DNA methylation of the GR exon 1₇ promoter region, a region homologous to the human GR promoter. As histone deacetylase (HDAC) inhibitors are able to change DNA methylation, we also evaluated whether administration of the HDAC inhibitor trichostatin A (TSA) directly into the CeA prevented WAS-induced increases in DNA methylation of the GR exon 1₇ promoter. We found that WAS increased overall and specific CpG methylation of the GR promoter in the CeA of female rats, which persisted for up to 28 days. Administration of the TSA directly into the CeA prevented these stress-induced changes of DNA methylation at the GR promoter. Our results suggest that, in females, changes in DNA methylation are involved in the regulation of GR expression in the CeA. These changes in DNA methylation may contribute to the central mechanisms responsible for stress-induced visceral hypersensitivity.

Epigenetic modulation of visceral nociception

Epigenetics is a process that alters gene activity or phenotype without any changes in the underlying DNA sequence or genotype. These biological changes may have deleterious effects and can lead to various human diseases. Ongoing research is continuing to illuminate the role of epigenetics in a variety of pathophysiologic processes. Several categories of epigenetic mechanisms have been studied including chromatin remodeling, DNA methylation, histone modification, and non-coding RNA mechanisms. These epigenetic changes can have a long-term effect on gene expression without any underlying changes in the DNA sequences. The underlying pathophysiology of disorders of brain-gut interaction and stress-induced visceral pain are not fully understood and the role of epigenetic mechanisms in these disorders are starting to be better understood. Current work is underway to determine how epigenetics plays a role in the neurobiology of patients with chronic visceral pain and heightened visceral nociception. More recently, both animal models and human studies have shown how epigenetic regulation modulates stress-induced visceral pain. While much more work is needed to fully delineate the mechanistic role of epigenetics in the neurobiology of chronic visceral nociception, the current study by Louwies et al., in Neurogastroenterology and Motility provides additional evidence supporting the involvement of epigenetic alterations in the central nucleus of the amygdala in stress-induced visceral hypersensitivity in rodents.

Epigenetics of Autism Spectrum Disorder: Histone Deacetylases

The etiology of autism spectrum disorder (ASD) remains unknown, but gene-environment interactions, mediated through epigenetic mechanisms, are thought to be a key contributing factor. Prenatal environmental factors have been shown to be associated with both increased risk of ASD and altered histone deacetylases (HDACs) or acetylation levels. The relationship between epigenetic changes and gene expression in ASD suggests that alterations in histone acetylation, which lead to changes in gene transcription, may play a key role in ASD. Alterations in the acetylome have been demonstrated for several genes in ASD, including genes involved in synaptic function, neuronal excitability, and immune responses, which are mechanisms previously implicated in ASD. We review preclinical and clinical studies that investigated HDACs and autism-associated behaviors and discuss risk genes for ASD that code for proteins associated with HDACs. HDACs are also implicated in neurodevelopmental disorders with a known genetic etiology, such as 15q11-q13 duplication and Phelan-McDermid syndrome, which share clinical features and diagnostic comorbidities (e.g., epilepsy, anxiety, and intellectual disability) with ASD. Furthermore, we highlight factors that affect the behavioral phenotype of acetylome changes, including sensitive developmental periods and brain region specificity in the context of epigenetic programming.

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.

Persistent inflammatory pain is linked with anxiety-like behaviors, increased blood corticosterone, and reduced global DNA methylation in the rat amygdala

Chronic pain increases the risk of developing anxiety, with limbic areas being likely neurological substrates. Despite high clinical relevance, little is known about the precise behavioral, hormonal, and brain neuroplastic correlates of anxiety in the context of persistent pain. Previous studies have shown that decreased nociceptive thresholds in chronic pain models are paralleled by anxiety-like behavior in rats, but there are conflicting ideas regarding its effects on the stress response and circulating corticosterone levels. Even less is known about the molecular mechanisms through which the brain encodes pain-related anxiety. This study examines how persistent inflammatory pain in a rat model would impact anxiety-like behaviors and corticosterone release, and whether these changes would be reflected in levels of global DNA methylation in brain areas involved in stress regulation. Complete Freund's adjuvant (CFA) or saline was administered in the right hindpaw of adult male Wistar rats. Behavioral testing included the measurement of nociceptive thresholds (digital anesthesiometer), motor function (open field test), and anxiety-like behaviors (elevated plus maze and the dark-light box test). Corticosterone was measured via radioimmunoassay. Global DNA methylation (enzyme immunoassay) as well as DNMT3a levels (western blotting) were quantified in the amygdala, prefrontal cortex, and ventral hippocampus. CFA administration resulted in persistent reduction in nociceptive threshold in the absence of locomotor abnormalities. Increased anxiety-like behaviors were observed in the elevated plus maze and were accompanied by increased blood corticosterone levels 10 days after pain induction. Global DNA methylation was decreased in the amygdala, with no changes in DNMT3a abundance in any of the regions examined. Persistent inflammatory pain promotes anxiety -like behaviors, HPA axis activation, and epigenetic regulation through DNA methylation in the amygdala. These findings describe a molecular mechanism that links pain and stress in a well-characterized rodent model.

Microbiota and Pain: Save Your Gut Feeling

Recently, a growing body of evidence has emerged regarding the interplay between microbiota and the nervous system. This relationship has been associated with several pathological conditions and also with the onset and regulation of pain. Dysregulation of the axis leads to a huge variety of diseases such as visceral hypersensitivity, stress-induced hyperalgesia, allodynia, inflammatory pain and functional disorders. In pain management, probiotics have shown promising results. This narrative review describes the peripheral and central mechanisms underlying pain processing and regulation, highlighting the role of the gut-brain axis in the modulation of pain. We summarized the main findings in regard to the stress impact on microbiota’s composition and its influence on pain perception. We also focused on the relationship between gut microbiota and both visceral and inflammatory pain and we provided a summary of the main evidence regarding the mechanistic effects and probiotics use.

An enriched environment reduces chronic stress-induced visceral pain through modulating microglial activity in the central nucleus of the amygdala

Cognitive behavioral therapy (CBT) improves the quality of life for patients with brain-gut disorders; however, the underlying mechanisms of CBT remain to be explored. Previously, we showed that environmental enrichment (EE), an experimental paradigm that mirrors positive behavioral intervention, ameliorates chronic stress-induced visceral hypersensitivity in a rodent model via mechanisms involving altered activity in the central nucleus of amygdala (CeA). In the present study, we investigated whether microglia-mediated synaptic plasticity in the CeA is a potential mechanism underlying the protective effects of EE against stress-induced visceral hypersensitivity. We stereotaxically implanted corticosterone (CORT) micropellets onto the dorsal margin of the CeA shown previously to induce colonic hypersensitivity. Animals were housed in EE cages or standard cages for 14 days after CORT implantation. Visceral sensitivity was assessed via visceromotor behavioral response to colorectal distension. Microglial morphology, microglia-mediated synaptic engulfment, and the expression of synaptic pruning-related signals complement component 1q (C1q), complement component 3 (C3), and C3 receptor (C3R) were measured using immunofluorescence and RNAscope assay. We found that housing CORT implanted rats in EE cages for 14 days attenuated visceral hypersensitivity in both male and female rats as compared with control rats maintained in standard housing. EE reduced CORT-induced microglial remodeling and microglia-mediated synaptic pruning with reduced C1q and CR3, but not C3, expression. Our data suggest that exposure to EE is sufficient to ameliorate stress-induced visceral pain via reducing amygdala microglia-modulated neuronal plasticity.NEW & NOTEWORTHY Clinical studies show that cognitive behavioral therapy (CBT) is effective in ameliorating visceral pain in patient with irritable bowel syndrome (IBS), yet the underlying mechanisms remain unexplored. By using environmental enrichment (EE), an experimental paradigm that mirrors positive behavioral intervention, we demonstrated that microglia-mediated synaptic plasticity in the CeA explains, plays a role, at least in part, in the positive effects of EE to reduce visceral hypersensitivity.

Reciprocal interaction between depression and pain: results from a comprehensive bidirectional Mendelian randomization study and functional annotation analysis

ABSTRACT

To understand a putative causal link for depression and pain, we retrieved summary statistics from genome-wide association studies conducted for pain at 7 different body sites (N = 151,922-226,683) and major depression disorder (MDD, Ncase/control = 246,363/561,190). We conducted a bidirectional Mendelian randomization analysis using distinct genome-wide association studies-identified single nucleotide polymorphisms for each trait as instrumental variables and performed several sensitivity analyses to verify Mendelian randomization assumptions. We also conducted functional annotation analysis using 396 tissue-specific annotations from the roadmap project. Across 7 different body sites, genetic predisposition to depression was associated with pain at the neck/shoulder (odds ratio [OR] = 1.08 per one log-unit increase in depression risk, 95% confidence interval [CI]: 1.06-1.10), back (OR = 1.05, 95% CI: 1.04-1.07), abdominal/stomach (OR = 1.03, 95% CI: 1.02-1.04), as well as headache (OR = 1.10, 95% CI: 1.07-1.12), but not with pain on the face, hip, and knee. In the reverse direction, genetically instrumented multisite chronic pain (OR = 1.78 per one increment in the number of pain site, 95% CI: 1.51-2.11) and headache (OR = 1.55 per one log-unit increase in headache risk, 95% CI = 1.13-2.10) were associated with MDD. Functional annotation analysis showed differential clustering patterns where depression clustered closely with headache and neck/shoulder pain, exhibiting substantial brain tissue enrichment. Our study indicates that depression is a causal risk factor for headache and pain localized at neck/shoulder, back, and abdominal/stomach, rather than pain at face, hip, and knee, and suggests common neurological pathologies underlying the development of depression, headache, and neck/shoulder pain.

Stress-induced visceral pain in female rats is associated with epigenetic remodeling in the central nucleus of the amygdala

Stress and anxiety contribute to the pathophysiology of irritable bowel syndrome (IBS), a female-predominant disorder of the gut-brain axis, characterized by abdominal pain due to heightened visceral sensitivity. In the current study, we aimed to evaluate in female rats whether epigenetic remodeling in the limbic brain, specifically in the central nucleus of the amygdala (CeA), is a contributing factor in stress-induced visceral hypersensitivity. Our results showed that 1 h exposure to water avoidance stress (WAS) for 7 consecutive days decreased histone acetylation at the GR promoter and increased histone acetylation at the CRH promoter in the CeA. Changes in histone acetylation were mediated by the histone deacetylase (HDAC) SIRT-6 and the histone acetyltransferase CBP, respectively. Administration of the HDAC inhibitor trichostatin A (TSA) into the CeA prevented stress-induced visceral hypersensitivity through blockade of SIRT-6 mediated histone acetylation at the GR promoter. In addition, HDAC inhibition within the CeA prevented stress-induced histone acetylation of the CRH promoter. Our results suggest that, in females, epigenetic modifications in the limbic brain regulating GR and CRH expression contribute to stress-induced visceral hypersensitivity and offer a potential explanation of how stress can trigger symptoms in IBS patients.

A Monoclonal Anti-Calcitonin Gene-Related Peptide Antibody Decreases Stress-Induced Colonic Hypersensitivity

Irritable bowel syndrome (IBS) is a brain-gut disorder characterized by abdominal pain and altered bowel habits. Although the etiology of IBS remains unclear, stress in adulthood or in early life has been shown to be a significant factor in the development of IBS symptomatology. Evidence suggests that aberrant calcitonin gene-related peptide (CGRP) signaling may be involved in afferent sensitization and visceral organ hypersensitivity. Here, we used a monoclonal anti-CGRP divalent antigen-binding fragment [F(ab')₂] antibody to test the hypothesis that inhibition of peripheral CGRP signaling reverses colonic hypersensitivity induced by either chronic adult stress or early life stress. A cohort of adult male rats was exposed to repeated water avoidance stress. Additionally, a second cohort consisting of female rats was exposed to a female-specific neonatal odor-attachment learning paradigm of unpredictable early life stress. Colonic sensitivity was then assessed in adult animals via behavioral responses to colorectal distension (CRD). To analyze spinal nociceptive signaling in response to CRD, dorsal horn extracellular signal-regulated kinase (ERK) 1/2 phosphorylation was measured via immunohistochemistry. Repeated psychologic stress in adulthood or unpredictable stress in early life induced colonic hypersensitivity and enhanced evoked ERK1/2 phosphorylation in the spinal cord after CRD in rats. These phenotypes were reversed by administration of a monoclonal anti-CGRP F(ab')₂ fragment antibody. Stress-induced changes in visceral sensitivity and spinal nociceptive signaling were reversed by inhibition of peripheral CGRP signaling, which suggests a prominent role for CGRP in central sensitization and the development of stress-induced visceral hypersensitivity. SIGNIFICANCE STATEMENT: Targeting peripheral calcitonin gene-related peptide (CGRP) with a monoclonal anti-CGRP divalent antigen-binding fragment antibody reduced central sensitization and attenuated colonic hypersensitivity induced by either chronic adult stress or early life stress. CGRP-targeting antibodies are approved for migraine prevention, and the results of this study suggest that targeting CGRP may provide a novel treatment strategy for irritable bowel syndrome-related, stress-induced visceral pain.

Estrogen and serotonin enhance stress-induced visceral hypersensitivity in female rats by up-regulating brain-derived neurotrophic factor in spinal cord

BACKGROUND

We previously reported that female offspring of dams subjected to chronic prenatal stress (CPS) develop enhanced visceral hypersensitivity (VHS) following exposure to chronic stress in adult life that is mediated by up-regulation of spinal cord BDNF. The aims of this study were to examine the roles of estrogen receptor alpha (ERα) and an increase in spinal serotonin signaling in promoting this enhanced VHS in female rats and up-regulation of spinal cord BDNF transcription.

METHODS

Pregnant dams were exposed to chronic stress from E11 until delivery. At 8 weeks, a chronic adult stress (CAS) protocol was applied for nine days.

KEY RESULTS

Ovariectomy before CAS or treatment with letrozole before and during CAS significantly prevented the development of enhanced VHS in female CPS+CAS rats. Intrathecal application of ERα siRNA significantly reduced VHS, decreased lumbar-sacral spinal cord expression of both ERα and BDNF, and reversed pro-transcriptional epigenetic modifications at BDNF promoter lX. Cerebrospinal fluid serotonin levels and 5HT3A receptor expression in the LS spinal cord were both significantly increased in female CPS+CAS rats. During CAS, intrathecal infusion of alosetron significantly decreased VHS, reduced BDNF and ERα expression in the LS spinal cord, and attenuated RNA pol II and ERα binding to the BNDF core promoter IX.

CONCLUSIONS & INFERENCES

Serotonin-mediated activation of 5HT3A receptors in the spinal cord drives the development of enhanced female-specific VHS in our two hit CPS+CAS through up-regulation of spinal cord ERα.

Histone post-translational modifications as potential therapeutic targets for pain management

Effective pharmacological management of pain associated with tissue pathology is an unmet medical need. Transcriptional modifications in nociceptive pathways are pivotal for the development and the maintenance of pain associated with tissue damage. Accumulating evidence has shown the importance of the epigenetic control of transcription in nociceptive pathways via histone post-translational modifications (PTMs). Hence, histone PTMs could be targets for novel effective analgesics. Here, we discuss the current understanding of histone PTMs in the modulation of gene expression affecting nociception and pain phenotypes following tissue injury. We also provide a critical view of the translational implications of preclinical models and discuss opportunities and challenges of targeting histone PTMs to relieve pain in clinically relevant tissue injuries.

Environmental enrichment prevents stress-induced epigenetic changes in the expression of glucocorticoid receptor and corticotrophin releasing hormone in the central nucleus of the amygdala to inhibit visceral hypersensitivity

INTRODUCTION

Stress is a known trigger for the symptoms of irritable bowel syndrome (IBS), a gastrointestinal (GI) disorder that presents with abnormal bowel habits and abdominal pain due to visceral hypersensitivity. While behavioral therapies have been used to attenuate IBS symptoms, the underlying mechanisms by which these therapies interact with stress-induced pathology remains to be delineated. Here we use a rat model to test the hypothesis that exposure to environmental enrichment (EE) inhibits stress-induced changes within the brain-gut axis to prevent visceral and somatic hypersensitivity and colonic hyperpermeability.

METHODS

Female rats (n = 8/group) were housed in EE one week before and one week during exposure to water avoidance stress (WAS) while controls were housed in standard cages (SH). One day after the final WAS exposure, colonic and somatic sensitivity were assessed by the visceromotor response (VMR) to colorectal distension (CRD) and withdrawal threshold elicited by an electronic von Frey on the hind paw of the rats respectively. All rats were returned to SH for 3 weeks before colonic and somatic sensitivity were reassessed on day 28. The rats were then immediately euthanized and the spinal cord was collected to assess changes in neuronal activation (assessed via ERK phosphorylation) in response to noxious CRD. A separate cohort of animals (n = 8/group) that did not undergo behavioral assessments was euthanized the day after the final WAS exposure and the central nucleus of the amygdala (CeA) was collected to investigate WAS and EE induced epigenetic changes at the glucocorticoid receptor (GR) and corticotrophin releasing hormone (CRH) promoter. The colon from these rats was also collected to assess colonic permeability via changes in transepithelial electrical resistance (TEER) in vitro.

RESULTS

Exposure to stress persistently increased VMR to CRD (P < 0.01) and decreased the hind paw withdrawal threshold (P < 0.001) in female rats. WAS also decreased TEER in the colon tissue of female rats (p = 0.05). In the CeA, WAS induced a decrease in histone acetylation at the GR promoter but increased histone acetylation at the CRH promoter and reduced GR-CRH interactions in the CeA. Analysis of the spinal cord showed that WAS increased CRD-evoked ERK phosphorylation in the dorsal horn. Exposure to EE prevented WAS-induced changes in the CeA, dorsal horn and colon respectively to prevent visceral and somatic hypersensitivity.

CONCLUSION

Our data reveals that behavioral therapies can produce long lasting molecular and epigenetic changes that can prevent stress-induced pathologies even after completion of the therapy. These results highlight the potential mechanisms by which behavioral therapies may ameliorate visceral pain associated stress-related pathologies such as the irritable bowel syndrome.

Kaiso Regulates DNA Methylation Homeostasis

Gain and loss of DNA methylation in cells is a dynamic process that tends to achieve an equilibrium. Many factors are involved in maintaining the balance between DNA methylation and demethylation. Previously, it was shown that methyl-DNA protein Kaiso may attract NCoR, SMRT repressive complexes affecting histone modifications. On the other hand, the deficiency of Kaiso resulted in reduced methylation of ICR in H19/Igf2 locus and Oct4 promoter in mouse embryonic fibroblasts. However, nothing is known about how Kaiso influences DNA methylation at the genome level. Here we show that deficiency of Kaiso led to whole-genome hypermethylation, using Kaiso deficient human renal cancer cell line obtained via CRISPR/CAS9 genome editing. However, Kaiso serves to protect genic regions, enhancers, and regions with a low level of histone modifications from demethylation. We detected hypomethylation of binding sites for Oct4 and Nanog in Kaiso deficient cells. Kaiso immunoprecipitated with de novo DNA methyltransferases DNMT3a/3b, but not with maintenance methyltransferase DNMT1. Thus, Kaiso may attract methyltransferases to surrounding regions and modulate genome methylation in renal cancer cells apart from being methyl DNA binding protein.

Amygdala microglia modify neuronal plasticity via complement C1q/C3-CR3 signaling and contribute to visceral pain in a rat model

Stress can trigger symptoms in patients with irritable bowel syndrome (IBS). Previously we demonstrated that chronic psychological stress induced microglial remodeling in the central nucleus of amygdala (CeA) and contributed to the development of visceral hypersensitivity via synaptic engulfment. However, the specific signaling mechanisms that microglia depend upon to recognize target neurons to facilitate visceral pain remain unknown. Here, we test the hypothesis that the microglia in the CeA contribute to chronic stress-induced visceral hypersensitivity via complement C1q/C3-CR3 signaling-mediated synaptic remodeling. In male and female Fischer-344 rats, micropellets of corticosterone (CORT) or cholesterol (control) were stereotaxically implanted bilaterally onto the CeA. After 7 days, microglial C1q, complement receptor 3 (CR3) expression, and microglia-mediated synaptic engulfment were assessed via RNAscope, quantitative PCR, and immunofluorescence. The microglial inhibitor minocycline, CR3 antagonist neutrophil inhibitory factor (NIF), or vehicle were daily infused into the CeA following CORT implantations. Visceral sensitivity was assessed via a visceromotor response (VMR) to graded pressures of isobaric colorectal distension (CRD). Our results suggest that chronic exposure to elevated CORT in the CeA induced visceral hypersensitivity and amygdala microglial morphological remodeling. CORT increased microglial C1q and CR3 expression and increased microglia-mediated synaptic engulfment. Both groups of animals with minocycline or NIF infusions reversed microglia-mediated synaptic remodeling and attenuated CORT-induced visceral hypersensitivity. Our findings demonstrate that C1q/C3-CR3 signaling is critical for microglia-mediated synaptic remodeling in the CeA and contributes to CORT-induced visceral hypersensitivity.NEW & NOTEWORTHY Patients with irritable bowel syndrome (IBS) show altered amygdala activity. We showed previously that stress induces visceral hypersensitivity partially through microglia-modulated synaptic plasticity in the central nucleus of the amygdala (CeA). Our current data suggest that the C1q/C3-CR3 cascade initiates microglia-mediated synaptic remodeling in the CeA. Blocking C3-CR3 interaction attenuates stress-induced visceral hypersensitivity. These findings uncover a role of microglia-synapse signaling in the brain-gut regulation and support a future therapeutic target to treat visceral pain.

The Role of Epigenomic Regulatory Pathways in the Gut-Brain Axis and Visceral Hyperalgesia

The gut-brain axis (GBA) is broadly accepted to describe the bidirectional circuit that links the gastrointestinal tract with the central nervous system (CNS). Interest in the GBA has grown dramatically over past two decades along with advances in our understanding of the importance of the axis in the pathophysiology of numerous common clinical disorders including mood disorders, neurodegenerative disease, diabetes mellitus, non-alcohol fatty liver disease (NAFLD) and enhanced abdominal pain (visceral hyperalgesia). Paralleling the growing interest in the GBA, there have been seminal developments in our understanding of how environmental factors such as psychological stress and other extrinsic factors alter gene expression, primarily via epigenomic regulatory mechanisms. This process has been driven by advances in next-generation multi-omics methods and bioinformatics. Recent reviews address various components of GBA, but the role of epigenomic regulatory pathways in chronic stress-associated visceral hyperalgesia in relevant regions of the GBA including the amygdala, spinal cord, primary afferent (nociceptive) neurons, and the intestinal barrier has not been addressed. Rapidly developing evidence suggests that intestinal epithelial barrier dysfunction and microbial dysbiosis play a potentially significant role in chronic stress-associated visceral hyperalgesia in nociceptive neurons innervating the lower intestine via downregulation in intestinal epithelial cell tight junction protein expression and increase in paracellular permeability. These observations support an important role for the regulatory epigenome in the development of future diagnostics and therapeutic interventions in clinical disorders affecting the GBA.

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.

Epigenetics of the developing and aging brain: Mechanisms that regulate onset and outcomes of brain reorganization

Brain development is a life-long process that encompasses several critical periods of transition, during which significant cognitive changes occur. Embryonic development, puberty, and reproductive senescence are all periods of transition that are hypersensitive to environmental factors. Rather than isolated episodes, each transition builds upon the last and is influenced by consequential changes that occur in the transition before it. Epigenetic marks, such as DNA methylation and histone modifications, provide mechanisms by which early events can influence development, cognition, and health outcomes. For example, parental environment influences imprinting patterns in gamete cells, which ultimately impacts gene expression in the embryo which may result in hypersensitivity to poor maternal nutrition during pregnancy, raising the risks for cognitive impairment later in life. This review explores how epigenetics induce and regulate critical periods, and also discusses how early environmental interactions prime a system towards a particular health outcome and influence susceptibility to disease or cognitive impairment throughout life.

Sex-Bias in Irritable Bowel Syndrome: Linking Steroids to the Gut-Brain Axis

Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder that is more common in females. Despite its high global incidence, the disease mechanism is still unclear and therapeutic options remain limited. The sexual dimorphism in IBS incidence suggests that sex steroids play a role in disease onset and symptoms severity. This review considers sex steroids and their involvement in IBS symptoms and the underlying disease mechanisms. Estrogens and androgens play important regulatory roles in IBS symptomology, including visceral sensitivity, gut motility and psychological conditions, possibly through modulating the gut-brain axis. Steroids are regulators of hypothalamic-pituitary-adrenal activity and autonomic nervous system function. They also modulate gut microbiota and enteric nervous systems, impacting serotonin and mast cell signaling. Sex steroids also facilitate bidirectional cross-talk between the microbiota and host following bacterial transformation and recycling of steroids by the intestine. The sex-specific interplay between sex steroids and the host provides neuroendocrinology insight into the pathophysiology, epigenetics and treatment of IBS patients.

Histone H3K9 methylation regulates chronic stress and IL-6-induced colon epithelial permeability and visceral pain

BACKGROUND

Chronic stress is associated with activation of the HPA axis, elevation in pro-inflammatory cytokines, decrease in intestinal epithelial cell tight junction (TJ) proteins, and enhanced visceral pain. It is unknown whether epigenetic regulatory pathways play a role in chronic stress-induced intestinal barrier dysfunction and visceral hyperalgesia.

METHODS

Young adult male rats were subjected to water avoidance stress ± H3K9 methylation inhibitors or siRNAs. Visceral pain response was assessed. Differentiated Caco-2/BBE cells and human colonoids were treated with cortisol or IL-6 ± antagonists. Expression of TJ, IL-6, and H3K9 methylation status at gene promoters was measured. Transepithelial electrical resistance and FITC-dextran permeability were evaluated.

KEY RESULTS

Chronic stress induced IL-6 up-regulation prior to a decrease in TJ proteins in the rat colon. The IL-6 level inversely correlated with occludin expression. Treatment with IL-6 decreased occludin and induced visceral hyperalgesia. Chronic stress and IL-6 increased H3K9 methylation and decreased transcriptional GR binding to the occludin gene promoter, leading to down-regulation of protein expression and increase in paracellular permeability. Intrarectal administration of a H3K9 methylation antagonist prevented chronic stress-induced visceral hyperalgesia in the rat. In a human colonoid model, cortisol decreased occludin expression, which was prevented by the GR antagonist RU486, and IL-6 increased H3K9 methylation and decreased TJ protein levels, which were prevented by inhibitors of H3K9 methylation.

CONCLUSIONS & INFERENCES

Our findings support a novel role for methylation of the repressive histone H3K9 to regulate chronic stress, pro-inflammatory cytokine-mediated reduction in colon TJ protein levels, and increase in paracellular permeability and visceral hyperalgesia.

Comparative expression profiling in the intestine of patients with Giardia-induced postinfectious functional gastrointestinal disorders

BACKGROUND

A Giardia outbreak in Bergen, Norway, caused postinfectious functional gastrointestinal disorders (PI-FGIDs). Despite the devastating effects of this outbreak, it presented a unique chance to investigate the implication on the dysregulation of genetic pathways in PI-FGID.

METHODS

We performed the first comparative expression profiling of miRNAs and their potential target genes in microdissected rectal biopsies from 20 Giardia-induced PI-FGID patients vs 18 healthy controls by nCounter analysis. Subsequently, candidates were validated on protein level by immunostaining.

KEY RESULTS

miRNA profiling on rectal biopsy samples from 5 diarrhea-predominant PI-IBS cases compared to 10 healthy controls revealed differential expression in the epithelial layer. The top five regulated miRNAs were implicated in GI disease, inflammatory response, and immunological disease. Subsequently, these miRNAs and 100 potential mRNA targets were examined in 20 PI-FGID cases and 18 healthy controls in both the mucosal epithelium and the lamina propria. Although deregulation of the selected miRNAs could not be verified in the larger sample set, mRNAs involved in barrier function were downregulated in the epithelium. Pro-inflammatory genes and genes implicated in epigenetic modifications were upregulated in the lamina propria. Immunostaining for selected candidates on 17 PI-FGID cases and 16 healthy controls revealed increased tryptase levels as well as a decreased and aberrant subcellular expression of occludin.

CONCLUSIONS AND INFERENCES

Genes relevant to immune and barrier function as well as stress response and epigenetic modulation are differentially expressed in PI-FGIDs and may contribute to disease manifestation.

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.

Environmental enrichment prevents chronic stress-induced brain-gut axis dysfunction through a GR-mediated mechanism in the central nucleus of the amygdala

BACKGROUND

Cognitive behavioral therapy (CBT) improves quality of life of patients with irritable bowel syndrome (IBS), a disorder characterized by chronic visceral pain and abnormal bowel habits. Whether CBT can actually improve visceral pain in IBS patients is still unknown. The aim of this study is to evaluate whether environment enrichment (EE), the animal analog of CBT, can prevent stress-induced viscero-somatic hypersensitivity through changes in glucocorticoid receptor (GR) signaling within the central nucleus of the amygdala (CeA).

METHODS

Rats were housed in either standard housing (SH) or EE for 7 days before and during daily water avoidance stress (WAS) exposure (1-h/d for 7 days). In the first cohort, visceral and somatic sensitivity were assessed via visceromotor response to colorectal distention and von Frey Anesthesiometer 24 hous and 21 days after WAS. In another cohort, the CeA was isolated for GR mRNA quantification.

KEY RESULTS

Environment enrichment for 7 days before and during the 7 days of WAS persistently attenuated visceral and somatic hypersensitivity when compared to rats placed in SH. Environment enrichment exposure also prevented the WAS-induced decrease in GR expression in the CeA.

CONCLUSION & INFERENCES

Pre-exposure to short-term EE prevents the stress-induced downregulation of GR, and inhibits visceral and somatic hypersensitivity induced by chronic stress. These results suggest that a positive environment can ameliorate stress-induced pathology and provide a non-pharmacological therapeutic option for disorders such as IBS.

Inhibition of Microglial Activation in the Amygdala Reverses Stress-Induced Abdominal Pain in the Male Rat

BACKGROUND & AIMS

Psychological stress is a trigger for the development of irritable bowel syndrome and associated symptoms including abdominal pain. Although irritable bowel syndrome patients show increased activation in the limbic brain, including the amygdala, the underlying molecular and cellular mechanisms regulating visceral nociception in the central nervous system are incompletely understood. In a rodent model of chronic stress, we explored the role of microglia in the central nucleus of the amygdala (CeA) in controlling visceral sensitivity. Microglia are activated by environmental challenges such as stress, and are able to modify neuronal activity via synaptic remodeling and inflammatory cytokine release. Inflammatory gene expression and microglial activity are regulated negatively by nuclear glucocorticoid receptors (GR), which are suppressed by the stress-activated pain mediator p38 mitogen-activated protein kinases (MAPK).

METHODS

Fisher-344 male rats were exposed to water avoidance stress (WAS) for 1 hour per day for 7 days. Microglia morphology and the expression of phospho-p38 MAPK and GR were analyzed via immunofluorescence. Microglia-mediated synaptic remodeling was investigated by quantifying the number of postsynaptic density protein 95-positive puncta. Cytokine expression levels in the CeA were assessed via quantitative polymerase chain reaction and a Luminex assay (Bio-Rad, Hercules, CA). Stereotaxic infusion into the CeA of minocycline to inhibit, or fractalkine to activate, microglia was followed by colonic sensitivity measurement via a visceromotor behavioral response to isobaric graded pressures of tonic colorectal distension.

RESULTS

WAS induced microglial deramification in the CeA. Moreover, WAS induced a 3-fold increase in the expression of phospho-p38 and decreased the ratio of nuclear GR in the microglia. The number of microglia-engulfed postsynaptic density protein 95-positive puncta in the CeA was increased 3-fold by WAS, while cytokine levels were unchanged. WAS-induced changes in microglial morphology, microglia-mediated synaptic engulfment in the CeA, and visceral hypersensitivity were reversed by minocycline whereas in stress-naïve rats, fractalkine induced microglial deramification and visceral hypersensitivity.

CONCLUSIONS

Our data show that chronic stress induces visceral hypersensitivity in male rats and is associated with microglial p38 MAPK activation, GR dysfunction, and neuronal remodeling in the CeA.

Epigenetic Mechanisms in Irritable Bowel Syndrome

Irritable bowel syndrome (IBS) is a brain-gut axis disorder characterized by abdominal pain and altered bowel habits. IBS is a multifactorial, stress-sensitive disorder with evidence for familial clustering attributed to genetic or shared environmental factors. However, there are weak genetic associations reported with IBS and a lack of evidence to suggest that major genetic factor(s) contribute to IBS pathophysiology. Studies on animal models of stress, including early life stress, suggest a role for environmental factors, specifically, stress associated with dysregulation of corticotropin releasing factor and hypothalamus-pituitary-adrenal (HPA) axis pathways in the pathophysiology of IBS. Recent evidence suggests that epigenetic mechanisms, which constitute molecular changes not driven by a change in gene sequence, can mediate environmental effects on central and peripheral function. Epigenetic alterations including DNA methylation changes, histone modifications, and differential expression of non-coding RNAs (microRNA [miRNA] and long non-coding RNA) have been associated with several diseases. The objective of this review is to elucidate the molecular factors in the pathophysiology of IBS with an emphasis on epigenetic mechanisms. Emerging evidence for epigenetic changes in IBS includes changes in DNA methylation in animal models of IBS and patients with IBS, and various miRNAs that have been associated with IBS and endophenotypes, such as increased visceral sensitivity and intestinal permeability. DNA methylation, in particular, is an emerging field in the realm of complex diseases and a promising mechanism which can provide important insights into IBS pathogenesis and identify potential targets for treatment.

Role of estrogen and stress on the brain-gut axis

Symptoms of functional gastrointestinal disorders (FGIDs), including fullness, bloating, abdominal pain, and altered gastrointestinal (GI) motility, present a significant clinical problem, with a reported prevalence of 25%-40% within the general population. More than 60% of those affected seek and require healthcare, and affected individuals report a significantly decreased quality of life. FGIDs are highly correlated with episodes of acute and chronic stress and are increased in prevalence and reported severity in women compared with men. Although there is evidence that sex and stress interact to exacerbate FGID symptoms, the physiological mechanisms that mediate these sex-dependent disparities are incompletely understood, although hormonal-related differences in GI motility and visceral sensitivity have been purported to play a significant role in the etiology. In this mini review, we will discuss brain-gut axis control of GI motility and sensitivity, the influence of estrogen on GI motility and sensitivity, and stress modulation of the brain-gut axis.

Mucosal RNA and protein expression as the next frontier in IBS: abnormal function despite morphologically intact small intestinal mucosa

Irritable bowel syndrome (IBS) is one of the commonest gastrointestinal disorders. Although long-time considered a pure functional disorder, intense research in past years has rendered a very complex and varied array of observations indicating the presence of structural and molecular abnormalities underlying characteristic motor and sensitive changes and clinical manifestations. Analysis of gene and protein expression in the intestinal mucosa has shed light on the molecular mechanisms implicated in IBS physiopathology. This analysis uncovers constitutive and inductive genetic and epigenetic marks in the small and large intestine that highlight the role of epithelial barrier, immune activation, and mucosal processing of foods and toxins and several new molecular pathways in the origin of IBS. The incorporation of innovative high-throughput techniques into IBS research is beginning to provide new insights into highly structured and interconnected molecular mechanisms modulating gene and protein expression at tissue level. Integration and correlation of these molecular mechanisms with clinical and environmental data applying systems biology/medicine and data mining tools emerge as crucial steps that will allow us to get meaningful and more definitive comprehension of IBS-detailed development and show the real mechanisms and causality of the disease and the way to identify more specific diagnostic biomarkers and effective treatments.

Targeting epigenetic mechanisms for chronic visceral pain: A valid approach for the development of novel therapeutics

BACKGROUND

Chronic visceral pain is persistent pain emanating from thoracic, pelvic, or abdominal origin that is poorly localized with regard to the specific organ affected. The prevalence can range up to 25% in the adult population as chronic visceral pain is a common feature of many visceral disorders, which may or may not be accompanied by distinct structural or histological abnormalities within the visceral organs. Mounting evidence suggests that changes in epigenetic mechanisms are involved in the top-down or bottom-up sensitization of pain pathways and the development of chronic pain. Epigenetic changes can lead to long-term alterations in gene expression profiles of neurons and consequently alter functionality of peripheral neurons, dorsal root ganglia, spinal cord, and brain neurons. However, epigenetic modifications are dynamic, and thus, detrimental changes may be reversible. Hence, external factors/therapeutic interventions may be capable of modulating the epigenome and restore normal gene expression for extended periods of time.

PURPOSE

The goal of this review is to highlight the latest discoveries made toward understanding the epigenetic mechanisms that are involved in the development or maintenance of chronic visceral pain. Furthermore, this review will provide evidence supporting that targeting these epigenetic mechanisms may represent a novel approach to treat chronic visceral pain.

The molecular neurobiology of chronic pain-induced depression

The increasing number of individuals with comorbidities poses an urgent need to improve the management of patients with multiple co-existing diseases. Among these comorbidities, chronic pain and mood disorders, two long-lasting disabling conditions that significantly reduce the quality of life, could be cited first. The recent development of animal models accelerated the studies focusing on the underlying mechanisms of the chronic pain and depression/anxiety comorbidity. This review provides an overview of clinical and pre-clinical studies performed over the past two decades addressing the molecular aspects of the comorbid relationship of chronic pain and depression. We thus focused on the studies that investigated the molecular characteristics of the comorbid relationship between chronic pain and mood disorders, especially major depressive disorders, from the genetic and epigenetic point of view to key neuromodulators which have been shown to play an important role in this comorbidity.

Comparison of Different Histone Deacetylase Inhibitors in Attenuating Inflammatory Pain in Rats

Histone deacetylase inhibitors (HDACIs), which interfere with the epigenetic process of histone acetylation, have shown analgesic effects in animal models of persistent pain. The HDAC family comprises 18 genes; however, the different effects of distinct classes of HDACIs on pain relief remain unclear. The aim of this study was to determine the efficacy of these HDACIs on attenuating thermal hyperalgesia in persistent inflammatory pain. Persistent inflammatory pain was induced by injecting Complete Freund's Adjuvant (CFA) into the left hind paw of rats. Then, HDACIs targeting class I (entinostat (MS-275)) and class IIa (sodium butyrate, valproic acid (VPA), and 4-phenylbutyric acid (4-PBA)), or class II (suberoylanilide hydoxamic acid (SAHA), trichostatin A (TSA), and dacinostat (LAQ824)) were administered intraperitoneally once daily for 3 or 4 days. We found that the injection of SAHA once a day for 3 days significantly attenuated CFA-induced thermal hyperalgesia from day 4 and lasted 7 days. In comparison with SAHA, suppression of hyperalgesia by 4-PBA peaked on day 2, whereas that by MS-275 occurred on days 5 and 6. Fatigue was a serious side effect seen with MS-275. These findings will be beneficial for optimizing the selection of specific HDACIs in medical fields such as pain medicine and neuropsychiatry.

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.

Chronic stress-associated visceral hyperalgesia correlates with severity of intestinal barrier dysfunction

In humans, chronic psychological stress is associated with increased intestinal paracellular permeability and visceral hyperalgesia, which is recapitulated in the chronic intermittent water avoidance stress (WAS) rat model. However, it is unknown whether enhanced visceral pain and permeability are intrinsically linked and correlate. Treatment of rats with lubiprostone during WAS significantly reduced WAS-induced changes in intestinal epithelial paracellular permeability and visceral hyperalgesia in a subpopulation of rats. Lubiprostone also prevented WAS-induced decreases in the epithelial tight junction protein, occludin (Ocln). To address the question of whether the magnitude of visceral pain correlates with the extent of altered intestinal permeability, we measured both end points in the same animal because of well-described individual differences in pain response. Our studies demonstrate that visceral pain and increased colon permeability positively correlate (0.6008, P = 0.0084). Finally, exposure of the distal colon in control animals to Ocln siRNA in vivo revealed that knockdown of Ocln protein inversely correlated with increased paracellular permeability and enhanced visceral pain similar to the levels observed in WAS-responsive rats. These data support that Ocln plays a potentially significant role in the development of stress-induced increased colon permeability. We believe this is the first demonstration that the level of chronic stress-associated visceral hyperalgesia directly correlates with the magnitude of altered colon epithelial paracellular permeability.

Transcriptome and methylome profiling in a rat model of irritable bowel syndrome induced by stress

Irritable bowel syndrome (IBS) is a common gastrointestinal disorder that is associated with psychological stress. However, the full landscape of IBS-related epigenetic factors remains unveiled and needs to be elucidated. The water-avoidance stress (WAS) method was used to induce a rat IBS model. Each rat was monitored, and its defecation and behavior were recorded. Total colon RNA was isolated and subjected to Affymetrix GeneChip analysis. Reduced Representation Bisulfate Sequencing (RRBS) was applied to determine the genome-wide methylation pattern in both IBS and control rats. Rats with IBS egested a significantly increased amount of dry and loose stools compared with the control animals, without significant changes in body weight. Compared with the control group, 309 genes were upregulated and 224 genes were downregulated in the colon of the IBS rats. Notch signaling and focal adhesion were increased in the differentially expressed genes (DEGs). A total of 541 genes had significant lower methylation level and 626 genes had significantly higher methylation level in their promoter regions. Adherens junction and leukocyte transendothelial migration were enriched in the differentially methylated genes (DMGs). Few genes were identified in common in both DEGs and DMGs, suggesting that gene expression was not altered by promoter methylation. Reverse transcription-quantitative polymerase chain reaction validation revealed that the mRNA levels of SSX2IP, PARD3 and VCL were significantly downregulated in the IBS group, in accordance with hypermethylation of their promoters. In summary, the present study used a WAS-induced IBS rat model to provide transcriptome and methylome profiling. Most DEGs were associated with Notch signaling and focal adhesion, and only a few were altered by promoter methylation. The present results demonstrated that psychological stress could influence the integrity of the intestinal mucosa barrier and regulate inflammatory response.

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.

Reappraising Preclinical Models of Separation Anxiety Disorder, Panic Disorder, and CO2 Sensitivity: Implications for Methodology and Translation into New Treatments

Separation anxiety applies to multiple forms of distress responses seen in mammals during postnatal development, including separation from a caregiver. Childhood separation anxiety disorder is an important risk factor for developing panic disorder in early adulthood, and both conditions display an increased sensitivity to elevated CO₂ concentrations inhaled from the air. By interfacing epidemiological, genetic, and physiological knowledge with preclinical animal research models, it is possible to decipher the mechanisms that are central to separation anxiety and panic disorders while also suggesting possible therapies. Preclinical research models allow for environmentally controlled studies of early interferences with parental care. These models have shown that different forms of early maternal separation in mice and rats induce elevated CO₂ respiratory sensitivity, an important biomarker of separation anxiety and panic disorders. In mice, this is likely due to gene-environment interactions that affect multiple behavioural and physical phenotypes after exposure to this early adversity. Although several questions regarding the causal mechanism of separation anxiety and panic disorder remain unanswered, the identification and improved understanding of biomarkers that link these mental health conditions under the guise of preclinical research models in conjunction with human longitudinal cohort studies can help resolve these issues.

Transplantation of fecal microbiota from patients with irritable bowel syndrome alters gut function and behavior in recipient mice

Irritable bowel syndrome (IBS) is a common disorder characterized by altered gut function and often is accompanied by comorbid anxiety. Although changes in the gut microbiota have been documented, their relevance to the clinical expression of IBS is unknown. To evaluate a functional role for commensal gut bacteria in IBS, we colonized germ-free mice with the fecal microbiota from healthy control individuals or IBS patients with diarrhea (IBS-D), with or without anxiety, and monitored gut function and behavior in the transplanted mice. Microbiota profiles in recipient mice clustered according to the microbiota profiles of the human donors. Mice receiving the IBS-D fecal microbiota showed a taxonomically similar microbial composition to that of mice receiving the healthy control fecal microbiota. However, IBS-D mice showed different serum metabolomic profiles. Mice receiving the IBS-D fecal microbiota, but not the healthy control fecal microbiota, exhibited faster gastrointestinal transit, intestinal barrier dysfunction, innate immune activation, and anxiety-like behavior. These results indicate the potential of the gut microbiota to contribute to both intestinal and behavioral manifestations of IBS-D and suggest the potential value of microbiota-directed therapies in IBS patients.

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.