Generalization of contextual fear depends on associative rather than non-associative memory components

Posttraumatic stress disorder (PTSD) is characterized by the presence of three major symptom clusters: persistent fear memories, hyperarousal, and avoidance. With a passage of time after the trauma, PTSD patients show an increase in unspecific fear and avoidance, a phenomenon termed "fear generalization". It is not clear whether fear generalization arises from the time-dependent growth of hyperarousal or changes in associative fear. The present study investigated behavioral and neuroanatomical correlates of non-associative and associative fear memory one week vs. one month after a trauma in a mouse model of PTSD with immediate vs. delayed foot shock application. The immediate shock procedure led to a lower contextual fear, but did not influence the hyperarousal (i.e. increased acoustic startle responses) assessed within the first week after the trauma. Only delayed shocked mice demonstrated generalization of contextual fear and an increase in generalized avoidance behavior, with no changes in hyperarousal one month after trauma. We observed the same increase in c-Fos expression following delayed and immediate shock presentation within the lateral, basolateral, central amygdala and CA1, CA3 and dentate gyrus of hippocampus, suggesting that all of these structures contribute to the development of hyperarousal. Only basolateral amygdala and dentate gyrus appeared to be additionally involved in encoding of contextual information. In summary, our results demonstrate the independence of associative and non-associative trauma-related fear. They support the hypothesis that generalized fear emerges in consequence of forgetting specific stimulus attributes associated with the shock context.