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A history of stress exposure predisposes people to developing post-traumatic stress disorder (PTSD) after a traumatic experience. To investigate why, investigators exposed mice to two 1-hour sessions a day apart consisting of either immobilization stress or a control condition (handling). A day later, the mice underwent pairing of tones and footshocks. Freezing in response to the tone was assessed as a measure of retention of fear conditioning in both the short term and long term (2 and 24 hours after conditioning, respectively).
Initial stress exposure did not affect fear acquisition or short-term memory of fear conditioning (freezing to the tone), but animals with earlier stress had more long-term memory of fear conditioning than nonstress…