Mild Traumatic Brain Injury Impairs Fear Extinction and Network Excitability in the Infralimbic Cortex.

Traumatic brain injury (TBI) is a leading cause of morbidity and disability, with mild TBI (concussions) representing over 80% of cases. Although often considered benign, mild TBI is associated with persistent neuropsychiatric conditions, including post-traumatic stress disorder, anxiety, and depression. A hallmark of these conditions is impaired fear extinction (FE), the process by which learned fear responses are inhibited in safe contexts. This dysfunction contributes to maladaptive fear expression and is linked to altered neurocircuitry, particularly in the infralimbic cortex (IL), a key region in FE. Despite extensive evidence of impaired FE in patients with mild TBI and animal models, the specific mechanisms underlying this deficit remain poorly understood. This study aimed to address this gap by combining cued-FE behavior, local field potential recordings, and whole-cell patch-clamp techniques to investigate how mild TBI affects IL network activity and excitability in a mouse model of TBI. Our results demonstrate that mild lateral fluid percussion injury significantly impairs FE memory, as evidenced by an elevated cued-fear response during extinction testing 10 days post-injury. Field potential recordings revealed decreased activation of the IL network in both layers II/III and V, which was consistent with the observed behavioral deficits. Further analysis of synaptic physiology revealed an imbalance in excitatory and inhibitory neurotransmission (E/I imbalance) in the IL, characterized by reduced excitatory input and enhanced inhibitory input to neurons in both layers. Moreover, intrinsic excitability was altered in IL neurons after mild TBI. This study provides novel insights into how mild TBI disrupts the neurocircuitry underlying FE, specifically by suppressing IL excitability. These results highlight the importance of understanding the mechanistic disruptions in IL activity for developing therapeutic strategies to address fear-based disorders in patients with mild TBI.