Moderate Levels of Stress Inhibit Auditory Fear Memory Extinction in Rats

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Weber, Dylan
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The effects of stress on memory formation are well documented, but very few studies have investigated the effects of stress on memory retrieval and extinction. The few previous studies reported mixed results, which may be due to varying methods, memory tests, and behavioral measures. This study specifically investigated the effects of acute stress on auditory fear memory extinction using a rat model of Posttraumatic Stress Disorder (PTSD). Rats were classically conditioned to associate auditory clicks with predator odor, then tested for fear memory extinction in a hide box runway apparatus by re‐exposing rats to the auditory clicks once daily over the next five days. However, immediately before the first extinction day, each rat was exposed to acute foot shock stress of 0.8 mA, 1.2 mA, or 0.0 mA as a no-shock control. Behavioral responses scored during extinction testing included freezing, head out vigilance, hiding, and locomotion. Results showed that although the high stress 1.2 mA group did not significantly differ from the control in freezing, the 0.8 mA shock group displayed significantly elevated levels of freezing across all extinction trials. These results suggest that exposure to moderate acute stress immediately prior to the first extinction trial appears to enhance fear memory recall or inhibit extinction memory formation. The effects of different intensities of stress on fear memory retrieval may reflect a Yerkes-Dodson arousal curve where low (0.0 mA) and very high (1.2 mA) levels of stress induce similar behavioral effects in comparison to effects elicited by moderate levels of stress (0.8 mA). Understanding these issues may facilitate our understanding of PTSD and other anxiety and panic-based disorders, and hopefully will highlight new avenues of treatment and prevention.

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