Abstracts

Rationale: Approximately 10% of soldiers in Iraq and Afghanistan experienced one or multiple mild or moderate blast exposures, leading to traumatic brain injury (TBI). Blasts give rise to substantial barotraumatic shearing forces that damage tissues.  Although blast trauma is correlated with epilepsy development, it is unknown if blast injury causes early seizures that contribute to epilepsy development. Indeed, there has as yet been no experimental evidence of blast-induced seizures in humans or animal models. As a first step towards developing therapeutics that prevent the development of seizures and epilepsy following blast-induced brain injury, we developed a mouse model of post blast seizures. Methods: We used a blast tube apparatus located at the US Army Institute for Surgical Research at Fort Houston that produces pressure waves having the characteristic “Friedlander” waveform exhibiting peak pressures and total energy characteristic of a free-field blast wave. 10 week old C57BL/6J mice were subjected to repeated (3X in 3 days) blasts (14 kPa) directed head-on.  During blast exposure, mice were anesthetized using ketamine (25-75 mg/kg) + dexmedetomidine (0.25 mg/kg). The day following the last blast exposure, mice were implanted with screw-type EEG electrodes and allowed 24 hours to recover. They were then recorded continuously over the following 48 hours. A subset of identically treated mice was sacrificed and brains harvested for brain slice electrophysiology, protein immunochemistry or histology. Results: We used immunoblot detection to semi-quantitatively determine levels of glial fibrillary acid protein (GFAP), an established biomarker of brain trauma. We found exposure to 3 repeated blasts induced a significant increase in GFAP levels, despite no effect on mortality or obvious external trauma severity.  We also found 5/10 repetitive blast-subjected mice exhibited generalized seizures ranging in frequency from 1/day to 9/day, with durations of 5-36 sec per seizure (average 22 sec). The seizures correlated with behavioral arrest in some, but not all animals. Patch clamp recording from hippocampus dentate gyrus granule neurons in brain slices indicated that blast injury increased intrinsic neuronal excitability. This was evident as a significant depolarization of resting membrane potentials and hyperpolarization of action potential threshold. There was no effect on synaptic currents.  Whereas biochemical studies also indicated no change in levels of total Tau protein, we did identify a significant increase in phospho-tau (S202) levels, suggesting a link to neural degeneration. Conclusions: The findings here indicate that repetitive moderate blast injuries cause early seizures and neuronal hyper-excitability. Future studies will determine if this paradigm also models post-blast epilepsy development. The animal studies here also suggest that soldiers that receive moderate blast injury may experience early, undiagnosed seizures given that seizures are often non-convulsive. These seizures may perhaps underlie some of the symptoms of blast concussions that overlap with non-convulsive seizures such as memory loss, confusion, mood disturbances and others. Funding: Funded by DoD-CDMRP grants PR141246P3 (J.D.L.) and W81XWH-15-1-0284 (M.S.S.).