Assessment of Sex Differences Following Repeated Mild Head Injuries

Background: Traumatic brain injury (TBI) is a major cause of disability, morbidity, and mortality in the U.S. Although there is a growing understanding of the effects of moderate and severe TBI, less is understood about the effects of repetitive mild TBI (rmTBI). Nevertheless, some studies show that long term participants in contact sports have an increased risk for neurodegenerative disease. In addition, there is limited information about sex differences in TBI, despite some studies suggesting females participating in contact sports experience more head injuries than males. With an increasing number of females participating in contact sports, it's important to explore the effects of rmTBI in females. Purpose: This study will test the hypothesis that rmTBI will lead to more severe neurological deficits in female mice than in male mice. Methods: C57BL/6 female mice were assigned to sham and rmTBI groups (n=30/group). Lightly anesthetized mice received 25 mild head injuries, once a day (M-F) over 5 weeks using a weight drop model that included a free fall with rotational injury. Acute effects of injury were assessed by righting reflex and balance beam tests weekly. Chronic effects were tested with rotarod, Morris water maze (MWM), elevated plus maze (EPM), and T-maze beginning 5 or 25 weeks after the last injury. Effects in female mice will be compared to previously collected data in male mice. Inflammation and white matter injury will be assessed with western blotting and immunohistochemistry, respectively. Results: Acutely, rmTBI female mice performed worse than sham injured mice on the balance beam (F (1,28) =4.309, P=0.0472) whereas there was no difference in males. Five weeks after injury, both male and female mice in the rmTBI group performed significantly (T-test P< 0.01) worse on the Rotarod. Neither males nor females displayed deficits in cognition on the T-Maze or learning phase of the MWM, although males had a significant impairment on MWM memory (Probe T-test P< 0.05). Neither sex showed deficits in the EPM. Fifteen weeks after injury, male mice displayed significant deficits in learning in the MWM (T-test P< 0.05) and EPM (T-test P< 0.05). Male mice in the rmTBI group also showed increased astrogliosis and Tau phosphorylation in the cerebral cortex compared to sham injured mice. Additional assessments of white matter injury are planned, but assessment of female mice 15 weeks after injury is incomplete at this time. Conclusion: Acutely, female mice showed balance deficits that were not apparent in males. Five weeks after injury, both sexes continued to show motor deficits on the rotarod, but only males had mild deficits in cognition. Ongoing studies will assess whether these differences persist or new differences between males and females appear chronically. AUP: 2021-0035