The Mechanisms of Fear Sensitization Caused by Acute Traumatic Stress: from Induction to Expression to Long-Lasting Reversal
Fear is an adaptive response that is normally proportional to the level of imposed threat, which allows for a balance between defensive behavior and other behaviors necessary for survival. However, fear becomes maladaptive when the level is inappropriate to the level of imposed threat. Exposure to a severe stressor can alter future fear learning to become disproportionate to the actual level of threat, potentially leading to generalized fear to less threatening circumstances (Rau, DeCola, and Fanselow, 2005). Inappropriate fear regulation after severe stress is a hallmark of post-traumatic stress disorder (PTSD). The primary goal of the experiments in this dissertation is to investigate the biological mechanisms that underlie both induction and expression of stress-enhanced fear learning (SEFL), a model developed and tested in rats to demonstrate that an acute footshock stressor nonassociatively and permanently enhances conditional fear learning.
In the SEFL procedure, rats are given 15 unsignaled footshocks in a certain context, and some time later, are given a single footshock in a novel context. When rats are tested for changes in freezing levels in the novel context in absence of a shock, they show exaggerated levels of freezing behavior, which is called SEFL. Many features of SEFL are similar to the symptoms of PTSD. Experiments in Chapter 1 of this dissertation show that corticosterone (CORT) is necessary to induce SEFL. This effect is demonstrated using intraperitoneal injections of metyrapone, a CORT synthesis blocker. Metyrapone before, but not after the 15 shocks dose-dependently attenuated SEFL and plasma CORT levels during the 15-shock stressor. Moreover, it is shown that the basolateral amygdala (BLA) must be functional during, but not after the 15-shock stressor. A glucocorticoid receptor (GR) antagonist infused into the BLA also attenuated SEFL; so, CORT acting on GRs in the BLA is necessary to induce SEFL.
Further work in Chapter 2 explored the mechanisms underlying expression of SEFL. CORT drove long-term alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR) subunit glutamate receptor 1 (GluA1), expression in the BLA, but not GluA2, or the glutamate N-methyl-D aspartate receptor (NMDAR) subunit GluN1. Infusing an AMPAR antagonist into the BLA after the severe stressor temporarily prevented sensitized fear expression. Experiments in Chapter 3 targeted GluA1 synthesis in the BLA using antisense oligonucleotide (ASO) treatments post-stress, which surprisingly reversed SEFL long-term. The most interesting finding in this set of experiments was that reversal of SEFL by ASO treatment did not prevent fear learning or amygdala function, nor did it affect associative fear to the stressor context. Moreover, in Chapter 3 we examined the functional importance of increased GluA1 in the BLA after SEFL. This was accomplished with post-stressor intra-BLA infusions of a GluA2-lacking AMPAR blocker, IEM-1460, which reduced SEFL.
In conclusion, these results elucidate novel neurobiological mechanisms underlying sensitized behavioral responses observed using the SEFL model in rats, with potential relevance to PTSD treatments in humans. Specifically, the collective findings show that that CORT acts on GRs in the BLA during the stressor to upregulate the GluA1 subunit of the AMPAR long-term, which elucidates novel mechanisms for the induction and the expression of SEFL. Furthermore, the finding that a single ASO treatment directed at the AMPARs within the BLA restored normal fear responding is especially relevant for developing novel and potentially more effective treatments for PTSD. Clinical implications are discussed throughout the present work.