Enhanced sensitivity to learning fearful associations during adolescence

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Abstract

The majority of anxiety disorders emerge during adolescence, yet there is a paucity of research examining factors that contribute to the “storm and stress” of this period. Understanding how juvenile (P23), adolescent (P35), and adult (P90) rats differ on basic fear conditioning tasks may shed light on this issue. In Experiment 1, P23, P35, and P90 rats were given 6 CS–US presentations. There were four training conditions: Delay (i.e., CS co-terminating with the US), Trace 20 and Trace 40 (i.e., an interval of 20 s and 40 s between the CS and US, respectively), and Unpaired (i.e., explicitly Unpaired presentations of the CS and US). Twenty-four hours after conditioning, freezing was measured to assess fear of the CS in a novel context. At test, there were no age differences in CS-elicited freezing in group Delay, and this condition exhibited significantly higher levels of freezing compared to group Unpaired. However, the adolescent rats were the only age group to exhibit higher levels of freezing following training with the 20 s and 40 s trace intervals, compared to Unpaired controls. Experiment 2 replicated the finding that adolescent but not adult rats exhibit fear following conditioning with a 20 s trace interval, while also demonstrating that both age groups display learning with a shorter trace interval of 5 s. Experiment 3 showed that exposure to corticosterone (200 μg/ml) in the drinking water for 1 week prior to conditioning selectively disrupts Trace 20 but not Delay conditioning during adolescence. Lastly, in Experiment 4 the test procedures were changed such that freezing was measured both during the CS and during a stimulus free trace interval. Once again, P35 but not P90 rats exhibited fear following training with a 20 s trace interval. Taken together, these findings demonstrate that adolescent rats show a heightened propensity to learn fearful associations, and that this is disrupted following exposure to corticosterone.

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