Effects of environmental enrichment on cognitive disorders caused by chronic stress in rats

It has been previously shown in many studies that chronic unpredictable mild stress (CUMS) negatively affects learning and memory performances. On the other hand, environmental enrichment (EE) applications have been proven to improve learning and memory functions. In our study, we aimed to determine the effects of environmental enrichment on memory impairment and neuroinflammation in rats subjected to chronic unpredictable mild stress (CUMS) by behavioural experiments, ELISA measurements and biochemical parameters. In our study, 32 3-month-old adult male Wistar Albino rats were used. The rats were divided into 4 different groups with 8 rats in each group by paying attention to their body weights. These groups 1) Control, 2) CUMS, 3) CUMS+EE, 4) EE. The chronic unpredictable mild stress procedure was created by randomly applying simultaneous and equivalent stressors 7 days a week for 3 weeks. The chronic unpredictable mild stress procedure was applied to rats in group 2 and group 3 in standard cages with appropriate conditions, and the environmental enrichment procedure was applied to rats in group 3 and group 4 in cages built in accordance with the literature for 21 days. Animals were subjected to novel object recognition test and Morris water maze test to evaluate learning and memory performances without sacrificing them. At the end of the experiment, hippocampus and prefrontal cortex (PFC) tissues were isolated from the brain tissues of the rats and ELISA and biochemical studies were performed. BDNF, TrkB, IL-1β, IL-6, NF-κB, TNF-α levels in hippocampus and PFC tissues, corticosterone levels in plasma, MDA and GSH levels in brain tissue were analysed. When the results of novel object recognition test in behavioural tests were examined, it was observed that discrimination index (DI) values, which indicate good memory performance, decreased with the use of CUMS and increased with the application of EE, but in the Morris water maze test, it was found that the application of EE decreased the time to find the platform, which is accepted as an indicator of learning, only in the group in which it was applied with stress. In addition, there was no effect of the treatments on the time spent in the target quadrant in the probe test in which the long-term memory of the animals was evaluated. It was observed that BDNF and TrkB levels decreased, IL-1β and IL-6 levels increased, but TNF-α and NFκB levels did not change significantly in hippocampus and PFC tissues. In addition, it was observed that it increased MDA levels and decreased GSH levels in brain tissues. It was observed that environmental enrichment applications prevented the negative effects on learning and memory induced by CUMS with an increase in BDNF and TrkB levels, neuroinflammation with a decrease in IL-1β and IL-6 levels, oxidative stress with a decrease in MDA levels and an increase in GSH levels, but did not cause a significant change in TNF-α and NFκB levels. Plasma corticosterone levels were found to be significantly increased with the application of CUMS and the application of EE was found to be successful in decreasing the levels. The findings of our study showed that the learning and memory impairments and oxidative alterations induced by the administration of CUMS were ameliorated by environmental enrichment including many different types of stimuli, especially through BDNF/TrkB signalling pathways.