Figure 3.

Matching of fear levels to analyze the mechanisms underlying the within- and between-session extinction deficit of PTSD-like rats. (A1,A2) Higher levels of conditioned fear do not explain within-session extinction deficit seen in PTSD-like rats. Freezing levels at the beginning of the extinction training session (arrow) were matched by dropping subjects at opposite poles of the distributions in the two rat groups. Two samples of six rats remained. (A1) Time spent freezing (y-axis) during the various phases of the fear-conditioning protocol (x-axis). (A2) Bar graph plots freezing levels in these subsets of PTSD-like (red) and resilient (black) rats during the last six CSs of the extinction training session (left) and first four CSs of the extinction testing session (right). (B1,B2) Higher freezing levels of PTSD-like rats during extinction recall are not due to a within-session extinction deficit. Freezing levels at the end of extinction training (last two CSs, arrow) were matched between the resilient and PTSD-like rats. To achieve this, we dropped subjects at the opposite ends of the distributions in the PTSD-like and resilient rats. We remained with samples of six PTSD-like and five resilient rats. (B1) Graph plotting percentage of time spent freezing (y-axis) during the various phases of the fear-conditioning protocol (x-axis) in these two subsets of rats. (B2) Bar graph plots freezing levels in these subsets of PTSD-like (red) and resilient (black) rats during the last two CSs of the extinction training session (left) and first two CSs of the extinction testing session (right). Even after matching freezing levels at the end of extinction training, we still observed significantly higher freezing at the beginning of the extinction test, suggesting that PTSD-like rats are deficient in the overnight consolidation of extinction.