Fifty to 90% of Americans are exposed to trauma that can lead to Post-traumatic Stress Disorder (PTSD). Only a few develop PTSD. Identifying the vulnerable and modifying the processes that translate risk into illness could reduce the public health burden of this serious disease. Risk factors and neurobiological mechanism are being identified, but much remains unknown. Emerging models identify relevant genes, environments, brain circuits and behavior, expanding our focus beyond simple fear learning to incorporate more complex neural circuits that modulate responses to threat. Studying these circuits and their functions has generated a novel model of PTSD pathophysiology that focuses on deficits in context processing (CP) and on the hippocampal- prefrontal circuitry that subserves CP functions. This model is supported by growing evidence, explains much of PTSD's phenomenology, and integrates much of its neurobiology.
The aim of this project is to further develop and test this model, and explore implications for treatment and, potentially, for prevention. PTSD patients respond fearfully to ambiguous cues (e.g., loud noise) even when in safe contexts (e.g., home backyard). Difficulty linking cues to contexts may be a core problem for them, undermining access to contextual information that should modulate adaptive responses. The hippocampus (Hpc) plays a key role in this process, mediating core CP functions like pattern separation (PS) and pattern completion (PC). PS/PC deficits may underlie CP difficulties in PTSD, contributing to an inability to remember that something once threatening is now safe (extinction recall) or to recognize potential danger when danger signals are contextual (fear renewal). Glucocorticoid (GC) signaling in Hpc can impair CP functions, so evidence of increased GC receptor sensitivity in PTSD is consistent with the CP model. Genetic and developmental factors known to shape GC sensitivity may contribute to PTSD risk through impact on CP functions like PS/PC, perhaps mediated by activity/connectivity within Hpc-prefrontal (PFC) circuits. This project will test the CP model, examining links between CP functions like PS/PC and the Hpc-PFC neural pathways subserving these functions, the role of glucocorticoid signaling in moderating these pathways and functions, and the ability of GCs to improve or undermine CP functions. It will do so by studying 120 healthy subjects performing PS/PC and fear learning tasks in fMRI, under low cortisol, physiological cortisol, and elevated (moderate and high) cortisol levels. ?Baseline? levels of GC signaling will be assessed via integrated cortisol secretion (hair cortisol) and GC receptor sensitivity (in vitro lysozyme inhibition). The project will also study 150 PTSD patients in the same paradigm, to determine whether PS/PC processes are core deficits in PTSD, linked to symptom severity and extinction recall/fear renewal deficits via Hpc-PFC dysfunction, and to test the ability of GCs to ?rescue? CP deficits in PTSD, via impact on Hpc-PFC pathways.
PTSD is a major public health problem, but it may ultimately be preventable. It involves deficits in the use of contextual information to process threat information and is associated with abnormalities in both the neural circuits that subserve contextual processing and in a stress hormone system that influences activity in this circuit. This study will deepen our understanding of how contextual information about threats is processed in these brain circuits and how stress hormones can impact these processes, both in healthy people and in people with PTSD, with the goal of understanding mechanisms that may help us reduce the detrimental effects of stress and trauma on human health.
