Indirectly acquired fear memories have distinct, sex-specific molecular signatures from directly acquired fear memories.

Post-traumatic stress disorder (PTSD) is a severe anxiety disorder that affects women more than men. About 30% of patients suffering from PTSD develop the disorder by witnessing a traumatic event happen to someone else. However, as the focus has remained on those directly experiencing the traumatic event, whether indirectly acquired fear memories that underlie PTSD have the same molecular signature as those that are directly acquired remains unknown. Here, using a rodent indirect fear learning paradigm where one rat (observer) watches another rat (demonstrator) associate an auditory cue with foot shock, we found that fear can be indirectly acquired by both males and females regardless of the sex or novelty (familiarity) of the demonstrator animal. However, behaviorally, indirectly acquired fear responses resemble those of pseudoconditioning, a behavioral response that is thought to not represent learning. Despite this, using unbiased proteomics, we found that indirectly acquired fear memories have distinct protein degradation profiles in the amygdala and anterior cingulate cortex (ACC) relative to directly acquired fear memories and pseudoconditioning, which further differed significantly by sex. Additionally, Egr2 and c-fos expression in the retrosplenial cortex of observer animals resembled that of demonstrator rats but was significantly different than that of pseudoconditioned rats. Together, these findings reveal that indirectly acquired fear memories have sex-specific molecular signatures that differ from those of directly acquired fear memories or pseudoconditioning. These data have important implications for understanding the neurobiology of indirectly acquired fear memories that may underlie bystander PTSD.