Effect of chronic psychosocial stress and long-term cortisol treatment on hippocampus-mediated memory and hippocampal volume: a pilot-study in tree shrews.

We investigated the impact of chronic psychosocial stress and long-term cortisol treatment on hippocampus-mediated memory processes and hippocampal volume in male tree shrews. By combining cognitive tests on a hole board, magnetic resonance imaging (MRI), and saliva cortisol analysis, we were able to follow in individual animals the stress- and cortisol-induced temporal effects on HPA axis activity and the hippocampus and its executive functions during 15 weeks. Four weeks of either cortisol treatment or psychosocial stress affected hippocampus-mediated memory. Cortisol-induced impairments were observed only at the end of the treatment phase while in stressed animals a negative effect on hippocampus-mediated memory was monitored after 7 weeks of recovery. A trend towards a reduction of the hippocampal volume was found in both experimental groups. The present preclinical study demonstrated that major life events, such as psychosocial stress, or chronic cortisol treatment leave traces in hippocampus-dependent memory. This finding requires systematic analysis to understand how brain areas critical for information processing such as the hippocampal formation are affected by stress and stress hormones.