Low dose of corticosterone treatment with exercise increases hippocampal cell proliferation, and improves cognition

Intermediate level of stress is beneficial for brain functions, whereas extreme low level or high level of stress is deleterious. We have previously shown that chronic exposure to high doses of corticosterone (CORT) suppressed hippocampal plasticity and physical exercise in terms of running counteracted the detrimental effects of CORT treatment. We aimed to study whether a mild stress, that mimicked by a treatment with low CORT dose, improved hippocampal plasticity in terms of hippocampal cell proliferation and dendritic remodeling, and to examine whether running with CORT treatment showed an additive effect on improving hippocampal plasticity. The rats were treated with 20 mg/kg CORT for 14 days with or without running, followed by Morris water maze test or forced swim test. The hippocampal proliferating cells was labeled by intraperitoneal injection of 5-bromo-2'-deoxyuridine. The dendritic morphology was analyzed using Golgi staining method. Treatment with 20 mg/kg CORT alone yielded a higher number of hippocampal cell proliferation and significantly increased dendritic branching compared to vehicle-treated non-runners, but had no behavioral effects. In contrast, CORT treatment with running showed an additive increase in hippocampal cell proliferation and dendritic remodeling that was associated with improved spatial learning and decreased depression-like behavior; however, there was no additive improvement in behavior compared to vehicle-treated runners. These findings suggest that mild stress does not always cause detrimental effect on the brain, and combining mild stress with running could promote hippocampal plasticity via inducing cell proliferation and dendritic remodeling.