High Intensity Stress Enhances Contextual Fear Memory Recall

Abstract

Animal studies have reported an impairing effect of stress on the recall of contextual fear memory as indicated by a reduction in freezing behavior, a frequently use index of fear. However, fear behavior in rodents is not limited to freezing but reflected in other fear behavior such as hiding, head-out vigilance-like behavior, and locomotion. Therefore, this Honors thesis re-examined the role of acute stress on contextual fear memory recall in rats by measuring not only freezing but also other fear-related responses in a runway apparatus with a hide box. Rats were trained to associate a hide box (conditioned stimulus) with a fear-eliciting cat-odor cloth (unconditioned stimulus) in the runway test box. To investigate the effects of stress on contextual fear memory retrieval, rats were placed in a shock box the day after conditioning and exposed to no stress or the stress of 10 footshocks (0.8 or 1.2 mA, 1-s duration) with each shock delivered at 2 min intervals. Immediately after exposure to footshock stress, rats were returned to the runway apparatus and fear behavior was measured over a 22 min period. Rats were returned to the runway apparatus over the next 4 days to measure extinction over time. Results appear consistent with previous studies showing no significant effects of stress on freezing. However, high intensity footshocks decreased head out behavior and reduced locomotor exploratory behavior. These novel behavioral results suggest that exposure to high intensity stress immediately prior to memory reactivation enhances the retrieval of contextual fear memory.