Effects of repetitive low-level blast exposure on fear learning and extinction in a rat model of mild traumatic brain injury

Traumatic brain injury (TBI) has been linked to a variety of chronic mental health problems in veterans. A striking feature in the most recent veterans returning from Iraq and Afghanistan has been the overlap between a history of blast-related mild TBI (mTBI) and post-traumatic stress disorder (PTSD). In many symptoms that

follow mTBI persist and evolve into a chronic postconcussion syndrome. In addition, in some new symptoms develop or old symptoms progress and mounting evidence suggests TBI is a risk factor for later development of neurodegenerative diseases. In collaboration with a Department of Defense investigator, Dr. Stephen

Ahlers, we have been studying a rat model of blast overpressure injury that was developed to mimic a blast- related human mild TBI or subclinical blast exposure. Blast-exposed rats exhibit chronic cognitive and PTSD- related behavioral traits including altered fear learning. These traits are present for over one year after blast

exposure and develop in a delayed and progressive manner. Fear learning has been extensively studied in rodents for its relevance to PTSD. Blast-exposed rats thus provide a model for how a physical injury alters fear responses. An mGluR2/3 antagonist improved blast-related behavioral traits including reverting altered fear

learning. We later found mGluR2 elevated in a time dependent manner that correlated with the blast-related behavioral phenotype's appearance. Here building on insights that we have developed from over a decade of study of this model, we will explore how blast affects the expression and pathobiology of fear leaning. In

Specific Aim 1 we will explore how in a time dependent and region-specific way blast exposure alters mGluR2 related signaling and synaptic protein formation during Pavlovian fear learning. These studies will in particular test the hypothesis that development of PTSD-related behavioral traits following blast exposure involves a

degenerative process that preferentially affects hippocampus and involves altered mGluR2 related signaling and synaptic protein formation. We will study fear learning in rats at 6 weeks (before altered fear responses develop) and 8 months (after altered fear responses appear) following blast exposure. Studies will include

male and female rats treated with the mGluR2/3 antagonist LY341495. Specific Aim 2 will examine how in a time dependent and region-specific way blast exposure alters the anatomic activation pattern of immediate early genes during Pavlovian fear learning as well as whether dendritic spines and their dynamic remodeling

are affected. This aim will provide the anatomic correlates to test the hypothesis that blast injury preferentially affects hippocampus. The mGluR2 dependence of blast effects on fear learning and expression will be explored in Specific Aim #3 by knocking down mGluR2 on a regional basis using adeno viral associated

vectors (AAV). In specific aim #4 using single nuclei RNA transcriptomics we will explore why blast exposure alters fear learning including why an mGluR2/3 antagonist rescues blast effects on fear learning and whether tumor necrosis factor α (TNFα)-related signaling may be involved. Collectively, these studies will examine the

anatomic and molecular basis for why blast exposure affects fear responses using a rat model, which mimics human mTBI or subclinical blast exposure.