The effect of photoperiodic stress on anxiety-like behaviors, learning, memory, locomotor activity and memory consolidation in rats

Introduction: Light-dark cycles regulate the body's physiological activity; hence, marked changes in these cycles could lead to conditions with impaired brain functions and disrupted moods (e.g., stress). Therefore, this study compared the impact of stress due to various photoperiodic durations on anxiety-like behavior, learning, memory, locomotor activity and memory consolidation in rats. Methods: Thirty-five male rats were divided into five groups with different light(L)-dark(D) cycles: L20/D4, L16/D8, L12/D12 (control), L8/D16 and L4/D20 groups. After14 days, the elevated plus-maze (EPM) and passive avoidance (PA) tests were performed to assess the anxiety-like behaviors and brain functions. Results: The percentage of spent time, number of entries to the open arm of the EPM test and the entrance latency to the dark room of the PA test decreased significantly in the L20/D4 and L4/D20 groups; however, the reduction of latency to enter the dark room was particularly significant in the L20/D4 group. In addition, there were significant differences between the initial latency and latency after one day (as learning) in all experimental groups. The total dark stay time increased significantly in different photoperiods. Conclusion: An abnormal light-dark length could disrupt certain brain functions, such as learning, memory, locomotor activity, memory consolidation and anxiety-like behavioral responses at different levels in a time-independent manner. The light-dark length (both minimum and especially the maximum day length) led to increased learning impairment and memory deficits, as well as worsened anxiety-like behaviors. The memory consolidation was also disrupted with various photoperiods.