Protective role of vitamin-D on chronic stress induced-learning and memory deficits in rats

Introduction: Long-term stress disturbs HPA axis and increases glucocorticoids release, which disturbs neural plasticity in the hippocampus and may lead to learning and memory deficits. Noticeably, vitamin D plays a protective role in the nervous system and has receptors in the hippocampus. This behavioral study investigates the possible protective role of vitamin-D against the negative effects of chronic stress on learning and memory. Materials and Methods: All the Rats were randomly divided to two groups; stress and control groups (each had 3 subgroups). Stress-group animals were exposed to restraint stress for 28 days, 3 hours/day. During 28 days, vitamin D (5 or 10 mg/kg), or vehicle was injected (IP) twice weekly to both groups. At day 29, blood sample collected for serum corticosterone assay. Morris water maze (MWM) test was performed in the order of 4 days training, one-day probe testing and finally working memory test. Passive avoidance test was performed after MWM. Results: Stress group revealed higher levels of serum corticosterone (P <0.0001) relative to controls. In MWM test, all groups learned the location of the platform during training. Latency in reaching the platform was reduced by the training days (P = 0.0001) for all the groups. Probe test indicated that stress groups reached the platform sooner than the control groups. Higher dose of vitamin D reduced this latency (P = 0.0002). Conversely, in passive avoidance test, stress groups spent less time in the dark than the control group (P = 0.0997).  Vitamin D (10 μg / kg) did not change the results. Conclusion: In this study, chronic restraint stress increased blood corticosterone levels and changed the behavioral effects of long and short term spatial and conditional memory. Treatment with vitamin D improved spatial learning.