Abstract
Stress is known to exert its detrimental effects not only by enhancing fear, but also by impairing its extinction. However, in earlier studies stress exposure preceded both processes. Thus, compared to unstressed animals, stressed animals had to extinguish fear memories that were strengthened by prior exposure to stress. Here, we dissociate the two processes to examine if stress specifically impairs the acquisition and recall of fear extinction. Strikingly, when fear memories were formed before stress exposure, thereby allowing animals to initiate extinction from comparable levels of fear, recall of fear extinction was unaffected. Despite this, we observed a persistent increase in theta activity in the BLA. Theta activity in the mPFC, by contrast, was normal. Stress also disrupted mPFC-BLA theta-frequency synchrony and directional coupling. Thus, in the absence of the fear-enhancing effects of stress, the expression of fear during and after extinction reflects normal regulation of theta activity in the mPFC, not theta hyperactivity in the amygdala.
Highlights
Accumulating evidence from animal models shows that stress elicits divergent patterns of plasticity across brain regions (Chattarji et al, 2015)
These animals were divided into two groups – one was subjected to 10 days of chronic immobilization stress (Days 1–10, Figure 1a) while the other served as unstressed control. 24 hr after the end of chronic stress there was no difference in conditioned stimulus (CS)-induced freezing behavior between the two groups (Day 11, Block1, Figure 1b)
This result differs from past reports of stress-induced deficits in fear extinction
Summary
Accumulating evidence from animal models shows that stress elicits divergent patterns of plasticity across brain regions (Chattarji et al, 2015). Animal studies confirm that chronic stress shrinks brain cells in the prefrontal cortex, but grows them bigger in the amygdala These and other studies have led scientists to believe that stress causes people to both form stronger fear memories and have difficulties getting rid of such memories. The behavioral data suggest that prior exposure to stress may have led to stronger fear memories manifested as higher levels of freezing in stressed animals at the beginning of extinction training (Chauveau et al, 2012; Hoffman et al, 2014; Miracle et al, 2006) This too could have contributed to the subsequent deficit in fear extinction recall.
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