Effects of combined psychosocial stress and mild traumatic brain injury on anxiety-like behavior and fear extinction
Approximately 15 percent of American soldiers received a mild traumatic brain injury (mTBI) during their deployment to Iraq or Afghanistan, of which 44 percent were also diagnosed with posttraumatic stress disorder (PTSD), suggesting a relationship between mTBI and PTSD. Within the general population, 23 percent of individuals who receive an mTBI also suffer from generalized anxiety disorder (GAD) or PTSD. Heightened contextual fear conditioning and impaired fear extinction are associated with PTSD. Animal models of mTBI have shown heightened contextual fear conditioning and anxiety-like behavior, but not altered fear extinction. To better mimic the combat environment and to further test whether mTBI directly results in symptoms associated with PTSD, rats underwent psychosocial stress (social defeat) immediately prior to the experimentally-induced mTBI. Rats undergoing mTBI with no stress, sham surgery with stress, and the combination of mTBI and stress, all showed significantly heightened anxiety-like behavior and contextual fear conditioning compared to controls, during elevated plus maze and contextual fear conditioning testing. The mTBI and stress group also displayed an impairment in fear extinction and greater anxiety-like behavior when compared to all other groups. Overall, results suggest that the combination of mTBI and psychosocial stress in rats mimics symptoms of PTSD and GAD.