The effect of predictable chronic stress and enriched environment in adolescence in rats on spatial memory and hippocampal asymmetry

Many stimuli exposed during adolescence are structurally and functionally affecting hippocampus, critical brain region for learning-memory. Negative effects of chronic stress on hippocampus are known. However, with few conflicting results, positive effects of short-term predictable stress on memory have been reported. In addition, positive effects of the enriched environment in acquisition of cognitive abilities have been shown in numerous studies. In our study, after investigating effects of chronic immobilization stress applied to adolescent rats for 15 minutes a day for 4 weeks and effects of enriched environment for an hour a day for 4 weeks in a separate group on memory, critical mediators of LTP which is neurobiological basis of leaning-memory process (BDNF, CREB, NOS), neuronal (NeuN), and oxidative stress (MDA, PCO, SOD) markers were evaluated in left and right hippocampus. In both experimental groups, spatial memory was significantly improved compared to control group. We found left>right asymmetry in all markers in control group. In experimental groups; hippocampal BDNF significantly increased in both hemispheres, showing different asymmetry according to group. Right>left asymmetry was detected for other markers. Interestingly, we found an increase in MDA in the right hippocampus and decrease of NeuN positive cell number in DG in both side. Furthermore, while nNOS showed different asymmetry in hippocampus subreigons, eNOS in DG showed asymmetry parallel to BDNF. Our original and first findings reveal the asymmetric expression of neurobiological mediators in hippocampus, which is effective in formation of spatial memory, and will contribute to development of new hypotheses in this area.