Repeated Mild Concussive Events Heighten the Vulnerability of Brain to Blast Exposure.

Mild concussive events without loss of consciousness are typically left untreated and can result in neurological abnormalities at later stages of life. No systematic studies have been carried out to determine the effect of concussion or repeated mild concussive episodes on brain vulnerability towards blast exposure. We have evaluated the effect of repeated mild concussive events on the vulnerability of brain to blast exposure using neurobehavioral functional assessments. Rats were subjected to either repeated mild concussive impacts (two impacts 1 week apart using a modified Marmarou weight drop model), a single blast exposure (19 psi using an advanced blast simulator), or a single blast exposure one day after the second mild concussive impact. Neurobehavioral changes were monitored using rotating pole test, open field exploration test, and novel object recognition test. Rotating pole test results indicated that vestibulomotor function was unaffected by blast or repeated mild concussive impacts, but significant impairment was observed in the blast exposed animals who had prior repeated mild concussive impacts. Novel object recognition test revealed short-term memory loss at 1 month post-blast only in rats subjected to both repeated mild concussive impacts and blast. Horizontal activity count, ambulatory activity count, center time and margin time legacies in the open field exploratory activity test indicated that only those rats exposed to both repeated mild concussive impacts and blast develop anxiety-like behaviors at both acute and sub-acute time-points. The results indicate that a history of repeated mild concussive episodes heightens brain vulnerability to blast exposure.