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

Abstract

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.

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

“…Long‐Evans rats were also used to investigate alterations in corticolimbic CRF signaling following stress exposure and/or linaclotide treatment based on previous studies showing adult stress induced significantly elevated HPA axis activity within this strain 31–33 . Importantly, the homotypic stressor employed during this study (water avoidance stress) induces comparable behavioral changes in Fischer 344 and Long‐Evans rats, with both strains exhibiting an increased fecal‐pellet output during the stress exposure and a subsequent increase in the visceromotor response to colorectal distension following the exposure 34–37 . Rats were double‐housed on Sani‐Chip bedding in standard individually ventilated cages at 23°C with a relative humidity of 50% on a 12‐hr light/dark cycle (7:00 a.m. to 7:00 p.m.).…”

“…[31][32][33] Importantly, the homotypic stressor employed during this study (water avoidance stress) induces comparable behavioral changes in Fischer 344 and Long-Evans rats, with both strains exhibiting an increased fecal-pellet output during the stress exposure and a subsequent increase in the visceromotor response to colorectal distension following the exposure. [34][35][36][37] Rats were double-housed on Sani-Chip bedding in standard individually ventilated cages at 23°C with a relative humidity of 50% on a 12-hr light/dark cycle (7:00 a.m. to 7:00 p.m.). Food (5053 Irradiated PicoLab Rodent Diet; LabDiet) and water were available ad libitum.…”

Differential Activation of Colonic Afferents and Dorsal Horn Neurons Underlie Stress-Induced and Comorbid Visceral Hypersensitivity in Female Rats

“…Previous studies in animals have shown that stress increases visceral sensitivity 7,16,22,31,32,46,57,70,71 . The visceromotor response (VMR) is a contraction of core muscles in response to hollow organ distension recorded as a change in magnitude of the abdominal muscle EMG.…”

SIRT1 Activation Attenuates Microglia Mediated Synaptic Engulfment in Postoperative Cognitive Dysfunction

“…Work in animal models of other neurodegenerative or neurological disease have shown alteration in microglia morphology during activation [32,33]. Rami ed microglia convert to amoeboid subtype, which is characterized by expanded cell bodies and retracted branches [34]. Although no notable changes were observed in surface area and microglia size, we found decreased occupied area and increased soma size of microglia in hippocampus after anesthesia and surgery.…”