Repetitive mild traumatic brain injury alters diurnal locomotor activity and response to the light change in mice.

Mild traumatic brain injury (mTBI) is a common cause of brain damage with a high incidence of multiple mTBIs found among athletes and soldiers. The purpose of this study is to examine the diurnal behavioral changes after multiple mTBIs. Adult mice were anesthetized; mTBI was conducted by dropping a 30-g weight to the right temporal skull once (mTBI1) or three times (mTBI3) over 3-week. Open-field motor behavior was recorded for 3 days after the last mTBI. In the first 4-hour exploratory phase, mTBI1 or mTBI3 equally reduced locomotor activity. A significant reduction of locomotor activity was found in the dark cycle between 4-72 hour in mTBI1 or mTBI3 mice; higher motor activity was seen after mTBI3 compared to mTBI1. In the light cycle, mTBI3 mice demonstrated an earlier immobilization followed by hyperactivity. The response to light change significantly correlated with the number of impacts. The IBA1 and BAX protein levels were equally increased in the lesioned cortex after mTBI1 and mTBI3. mTBI3 selectively upregulated the expression of circadian clock gene Per1 in hypothalamus and hippocampus as well as iNOS expression in the lesioned side cortex. Our data suggest multiple mTBIs alter diurnal locomotor activity and response to the change of light, which may involve Per1 expression in the lesioned brain.