The Gut-Brain Axis in Inflammatory Bowel Disease: Assessment of the Central Inflammatory Response to Dextran Sulfate Sodium-Induced Colitis in Rats
Citation:DEMPSEY, ELAINE, The Gut-Brain Axis in Inflammatory Bowel Disease: Assessment of the Central Inflammatory Response to Dextran Sulfate Sodium-Induced Colitis in Rats, Trinity College Dublin.School of Pharmacy & Pharma. Sciences.PHARMACY AND PHARMACEUTICAL SCIENCES, 2018
Inflammatory bowel disease (IBD) is a chronic relapsing-remitting inflammatory condition of the gastrointestinal (GI) tract. Evidence indicates a comorbidity between IBD and psychological symptoms in patients. It is thought that GI inflammation is communicated to the brain causing alterations which contribute to these psychological symptoms. Using dextran sulfate sodium (DSS) -induced colitis to model active and recovered IBD, this project aims to identify behavioural changes associated with colitis, and link them to changes in central inflammation, magnetic resonance imaging (MRI) measures of brain structure and composition, and Fos mapping of neuronal activity. DSS (5%; dissolved in home cage drinking water) induced an acute colitis in male Wistar rats, measured by symptoms of weight loss, diarrhoea and rectal bleeding, which peaked after 6 days of exposure. Animals recovered from these symptoms following return to tap water. Damage to crypts and epithelial cells were observed in the colon along with immune cell infiltration. The colon exhibits a pro-inflammatory cytokine profile with increased IL-1β, TNF-α, IL-6, iNOS, GFAP and MMP mRNA expression and MMP activity. At 7 days following cessation of DSS exposure, increased IL-1β, IL-6, iNOS, and IFNγ mRNA were measured in the colon. Similarly, a pro-inflammatory profile was observed in the cortex with increased IL-6 and iNOS mRNA expression in acute colitis and recovery. In acute colitis, sickness behaviour was observed in the open field test, and this recovers by recovery day 4. However, rats in recovery showed symptoms of anhedonia in the saccharin preference test, behavioural despair in the forced swim test, and anxiety in the elevated plus maze, light/dark box and marble burying test. As the inflammatory marker iNOS showed the greatest increases relative to control, iNOS mRNA expression was investigated further and was seen to be increased in colon, liver, spleen, cortex, hippocampus, and hypothalamus in acute colitis and recovery. iNOS expression in these regions correlated with iNOS measures in the colon and cortex. Central inflammation was examined further using immunohistochemistry and revealed that DSS-induced colitis leads to neuroinflammation surrounding two regions of the brain in particular: the subfornical organ (SFO) and the median eminence (ME), both of which are circumventricular organs (CVO). CVOs are brain regions with a more permeable blood brain barrier (BBB). Exposure to DSS-induced colitis was associated with increases in iNOS immunoreactivity in the parenchyma surrounding the SFO and the ME in acute DSS-induced colitis and in recovery. iNOS is an enzyme induced in response to inflammation, which is capable of generating neurotoxic levels of nitric oxide and of altering protein structure by nitration. The nitration marker 3-NT confirms an increase in iNOS activity at CVOs. Being the resident immune cells of the brain, microglia were assessed, using the marker IBA1, and found to be increased around the SFO and ME following DSS-induced colitis. Morphological analysis confirms that they are in an amoeboid, immunologically activated state. Dual immunohistochemical staining demonstrated co-localisation of iNOS and IBA1 indicating that iNOS is produced by microglia in response to DSS-induced colitis. To address a potential mechanism by which peripheral inflammation may be communicated to the brain, BBB integrity was investigated following DSS-induced colitis. An increase in immunoreactivity for endogenous circulating immunoglobulin IgG, was observed at the SFO in acute DSS-induced colitis indicating an impaired barrier. Anatomical MRI analysis revealed an increase in ventricular volumes in both acute and recovered colitis. Changes to T2 but not T1 relaxometry times in cortical regions were observed in the recovery period indicating a change in tissue composition. Mapping of neuronal activation using the immediate early gene deltaFosB revealed an increase in neuronal activity at the NAc and DRN in acute and recovered colitis, potentially indicating activation of compensatory mechanisms in the reward and serotonergic centres of the brain to counteract the anhedonia and despair observed in this model. These experiments provide further evidence supporting the link between GI and central inflammation, and anxiety/depression-related behaviours. This link may be key to understanding the increased risk for psychological symptoms in IBD patients.
Author: Harkin, Andrew
Publisher:Trinity College Dublin. School of Pharmacy & Pharma. Sciences. Discipline of Pharmacy
Type of material:Thesis
Availability:Full text available