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

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Summary

Epigenetic molecular mechanisms, which include DNA methylation and histone deacetylation, are implicated in the dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis. Previously, we demonstrated that repeated water avoidance stress (WAS), a validated model of chronic psychological stress, induces heightened visceral pain behaviors in rodents that resemble irritable bowel syndrome (IBS) sequelae. However, the involvement of epigenetic molecular mechanisms in the pathophysiology of stress-induced visceral pain has not been explored. Our hypothesis is that epigenetic mechanisms within the central nervous system (CNS) are important to chronic stress-induced visceral hypersensitivity. Adult male F-344 rats with intracerebroventricular (i.c.v.) cannulae were exposed to 7 days of repeated WAS. Controls received a SHAM stress. Following the daily 1 h stressor, trichostatin A (TSA; 100 ng/ml), a potent histone deacetylase inhibitor, or vehicle (VEH; 0.1% DMSO/saline,) as control was administered via the i.c.v. cannula. Visceral sensitivity was assessed 24 h after the final WAS and quantified the visceromotor response (VMR) by recording the number of abdominal contractions in response to graded pressures (20–60 mmHg) of colorectal distensions (CRD). From a separate group of rats that were exposed to repeated WAS or SHAM stress, the amygdala was isolated to assess the methylation status of glucocorticoid receptor (GR) and corticotropin releasing-factor (CRF) genes via bisulfite sequencing and verified by pyrosequencing. GR and CRF gene expression was quantified via qRT-PCR. Stressed rats exhibited visceral hypersensitivity that was significantly attenuated by TSA. Compared to SHAM controls, methylation of the GR gene was increased following WAS while expression of the GR gene was decreased. Methylation of the CRF promoter was decreased with WAS with a concomitant increase in CRF expression. This study demonstrates the involvement of central epigenetic mechanisms in regulating stress-induced visceral hypersensitivity and provides a foundation for exploring the epigenetic mechanisms that may contribute to IBS-like symptomatology.

Introduction

Stress and episodes of anxiety are strongly implicated in the pathophysiology of irritable bowel syndrome (IBS), a chronic functional gastrointestinal (GI) disorder characterized by abdominal pain and abnormal bowel habits (Longstreth et al., 2006, Lydiard, 2001, Mayer et al., 2009, Posserud et al., 2004). This comorbidity of stress with the exacerbation of IBS symptomatology is supported by an abnormal hypothalamic-pituitary-adrenal (HPA) axis in IBS patients (Chang et al., 2009, Dinan et al., 2006). Further supporting the link between stress and IBS, the symptoms of IBS persist long after a stressful event and are exacerbated by subsequent stress (Blanchard et al., 2008). Several preclinical experiments have revealed key factors involved in the stress-related pathology of IBS (Bradesi et al., 2005, Greenwood-Van Meerveld et al., 2001, Myers and Greenwood-Van Meerveld, 2010, Myers and Greenwood-Van Meerveld, 2012, Venkova et al., 2010). For example, in a rodent model, chronic psychological stress can induce visceral hyperalgesia. Specifically, rats exposed to chronic water avoidance stress (WAS), a well-characterized experimental model of psychological stress, exhibit many of the classical sequelae reported by IBS patients including visceral hyperalgesia and colonic dysmotility (Bradesi et al., 2005, Myers and Greenwood-Van Meerveld, 2012). Much like the persistent symptoms experienced by IBS patients, visceral hyperalgesia induced by chronic WAS persisted following cessation of WAS (Bradesi et al., 2005).

Our previous studies provided further insight into the mechanisms involved in the stress-induced pathophysiology of IBS. Specifically, we demonstrated that stereotaxic implantation of the stress hormone corticosterone (CORT) onto the dorsal margin of the central amygdala (CeA), a primary brain region involved in physiological responses to stress, induces chronic visceral hypersensitivity and colonic dysmotility in rats (Greenwood-Van Meerveld et al., 2001, Myers et al., 2007, Venkova et al., 2010). Molecular and pharmacological experiments revealed that glucocorticoid receptors (GR) and mineralocorticoid receptors (MR) in the central nucleus of the amygdala (CeA) (Myers and Greenwood-Van Meerveld, 2007) and corticotropin-releasing factor (CRF) (Myers et al., 2005, Shepard et al., 2000) are critical for CORT-induced visceral hypersensitivity. Although these studies identify key central receptor-mediated mechanisms that may be involved in the pathophysiology of IBS, the complex mechanisms by which stress leads to persistent abnormalities in GI function remain unexplored.

Recent studies have shown that remodeling of the epigenome by the environment, or during chronic stress, may result in long-term changes in gene expression (Weaver, 2009). Gene expression patterns are controlled by the epigenome, which can include changes in the specific chromosome structure through differential histone acetylation or methylation patterns of the DNA. Here we test the hypothesis that central epigenetic changes contribute to stress-induced visceral hypersensitivity. To address this hypothesis we first examined the effect of a histone deacetylase inhibitor administered directly into the brain on stress-induced visceral hypersensitivity. We then focused our study on the amygdala to determine whether stress alters the methylation patterns of GR and CRF, two key genes previously identified to be involved in the stress-induced pathology.

Section snippets

Animals

Experiments were performed on male Fischer-344 rats, weighing 175–200 g upon arrival (Charles Rivers Laboratory, Wilmington, MA). All animals were single-housed to prevent post-surgery complications and maintained on a 12 h light/dark cycle (lights on at 5:30 AM) at 21 °C and 70% humidity with ad libitum access to food and water. Rats were acclimated to the animal facility for one week and to the experimenter and the laboratory for an additional week before experimentation. The experiments were

General effects of repeated WAS

No changes in general behavior were observed following WAS and drug treatment, which is consistent with previous studies using TSA treatment (Minetti et al., 2006, Moreira et al., 2003, Vigushin et al., 2001, Weaver, 2009). There were no significant differences in percent body weight in any groups following 7 days of WAS alone (F(1,22) = 0.10, p > 0.05) or 7 days of WAS with drug infusions (F(1,22) = 0.54, p > 0.05). However, WAS significantly increased FPO compared to SHAM treated animals (F(1,22) = 

Discussion

The overall goal of our study was to investigate the involvement of epigenetic mechanisms in the pathophysiology of stress-induced visceral pain. To this end, we confirmed our hypothesis that epigenetic mechanisms within the CNS are important in stress-induced visceral hypersensitivity. Consistent with previous work, the present study demonstrated that daily exposure to a repeated acute stressor produces visceral hypersensitivity in a rodent model. Furthermore, to our knowledge we are the first

Role of the funding source

Funding was provided by the US Department of Veterans Affairs in support of Veteran healthcare research. The funding source had no role in the execution of the experiments or the writing of the manuscript.

Conflict of interest statement

The authors declare no conflict of interests.

Acknowledgements

Beverley Greenwood-Van Meerveld graciously acknowledges the generous support from the Department of Veterans Affairs for her Research Career Scientist and Merit Review Awards.

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