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Abstract

Introduction: Repetitive traumatic brain injury (TBI) occurs in a significant portion of trauma patients, especially in specific populations, such as abusive head trauma (AHT) victims or athletes suffering sports concussion. While accumulating evidence suggests that repeated mild TBI (rmTBI) may cause long-term cognitive dysfunction in adults, it is unknown whether rmTBI causes similar deficits in the immature brain. Methods: 18 day old rats were divided into two groups. The 1st (n=3/grp) received either two closed head impacts (CHI, 9.5mm impactor, 4m/s velocity, 1mm depth), one CHI paired with sham (S) or two sham (S/S) insults 24h apart. Histological analyses at 7d included silver staining to detect axonal injury, Iba-1 immunohistochemistry to assess microglial activation, H&E to evaluate cell survival, and phospho-Tau, a footprint of traumatic encephalopathy. The 2nd group received three CHI (n=18) or three sham insults (n=13) 24h apart and evaluated for motor (beam balance and inclined plane tests, d1-5) and cognitive (Morris water maze (MWM, d11-16) and (novel object recognition (NOR), d17-18) dysfunction. Results: H&E staining showed no overt neuronal cell loss. Silver staining revealed argyrophilia and axonal staining in the ipsilateral cortex and external capsule and bilateral corpus callosum in CHI/S and CHI/CHI groups. Increased Iba1 positivity with morphological appearance of activated microglia was observed in ipsilateral cortex and bilateral amygdala after CHI/CHI. CHI/CHI group showed greater Iba1 positivity vs CHI/S or S/S groups. Tau phosphorylation was not detected in sham or injured rats. There were no differences in beam balance, inclined plane, and MWM performance between S/S/S and CHI/CHI/CHI groups. However, rmTBI rats were significantly impaired in the NOR vs. sham controls (P < 0.05, 65 ± 2.43 vs 75 ± 3.75%). Conclusions: RmTBI amplifies the mTBI response producing diffuse axonal injury, microglial activation and memory deficits. Defining the mechanisms underlying damage from second impact could be vital to therapy development for AHT or sports concussion in children. Support: NS061817, U19AIO68021, NS076511