Chronic Mild Stress Induces Memory Deficit With Asymmetrical Damages in The Hippocampus in Rats

Background: Major depressive disorder (MDD) is associated with deficits in memory which is often caused by hippocampal damages. Left-right asymmetry of human brain has been understood for a century. Chronic mild stress (CMS) protocol is widely used to evoke depressive-like behaviors in laboratory rats. Few studies have been done to examine the relation between depression and left-right asymmetry hippocampal damages. This paper aimed to investigate the impacts of CMS on both sides of the hippocampus, learning and memory. Methods: Body weight and sucrose preference and a modified open-field test was performed to check the depressive-like behaviors of CMS model. Learning and memory, apoptosis rates and chemiluminescence density of the hippocampus were determined. Results: Compared with normal control, CMS group showed less weight gain with age, decreased sucrose intake and reduced open-field exploration indicative of depression; CMS had a negative effect on performance in the step-through test, reflecting a deficit in learning memory; CMS group had a greater increase in the apoptosis rates and chemiluminescence density in the left hippocampus, but with no significant change in the right one. Conclusions: These findings suggest that CMS may cause the asymmetrical damages in the hippocampus which induce a deficit in learning and memory in rats.