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

Abstract Background and Aims Psychological stress is a trigger for the development of irritable bowel syndrome (IBS) and associated symptoms including abdominal pain. Although IBS patients exhibit increased activation in the limbic brain, including the amygdala, the underlying molecular and cellular mechanisms regulating visceral nociception in the central nervous system (CNS) are incompletely understood. In a rodent model of chronic stress, we explored the role of microglia in the central nucleus of 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 negatively regulated by nuclear glucocorticoid receptors (GR), which are suppressed by the stress-activated pain mediator P38 MAPK. Methods Fisher-344 male rats were exposed to water avoidance stress (WAS) for 1 hr. /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 post-synaptic density protein 95 (PSD95)-positive puncta. Cytokine expression levels in the CeA were assessed via quantitative PCR and a Luminex assay. 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 (VMR) to isobaric graded pressures of tonic colorectal distension (CRD). 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 PSD95-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 whilst in stress-naive rats, fractalkine induced microglial deramification and visceral hypersensitivity. Conclusion Our data demonstrate 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.