The Effect of Social Stress on Cognitive Bias and Neural Inflammation in Rats

Abstract

One of the most prevalent mood disorders is Major Depressive Disorder (MDD). A major symptom of MDD is a negative cognitive bias, which is the perception of an ambiguous context as negative. Females are twice as likely to experience severe cognitive symptoms of depression as males. The major factors contributing to the development of MDD are stress and the female sex. In particular, social stress targets adolescents more than adults. However, the link between social stress and MDD is unclear. MDD is further correlated with increased neural inflammation. Neural inflammation involves the expression of microglial activation and pro-inflammatory cytokines. This study observes the sex-specific role of social stress in cognitive bias and neural inflammation using rats as a model. The rats were exposed to social instability stress during adolescence and underwent a fear-based cognitive bias task from adolescence to adulthood to determine their cognitive biases. This study further examined pro-inflammatory cytokine levels (IL-1β, TNF-α, and IL-6), anti-inflammatory cytokine levels (IL-4, IL-10, and IL-6), and a microglia marker (IBA-1) in the ventral hippocampus, dorsal hippocampus, and basolateral amygdala. In the dorsal hippocampus, socially stressed rats exhibited elevated cytokine levels compared to the control rats. Additionally, the microglial morphology in this region appeared more activated in socially stressed rats with reduced complexity and shorter branch lengths. This study aims to build on neural explanations of MDD so that the treatments for this mood disorder can be improved.