Abstracts

Rationale: Traumatic brain injury (TBI) is a major risk factor for the development of refractory epilepsy. Although hyperexcitable cortical and hippocampal networks are associated with TBI, the precise mechanisms by which alterations in neurocircuitry lead to epilepsy are unknown. In this study, we investigated how controlled cortical impact (CCI), a model of TBI, leads to glutamate network hyperactivity in the cortex.Methods: 10-week old, male C57BL/6 mice underwent sham or CCI injury, and acute cortical brain slices were obtained 2-4 weeks post-injury; a time after injury but prior to the onset of spontaneous seizures. Using FRET-based glutamate biosensors, we optically mapped the spatial and temporal parameters of cortical glutamate release in real-time. Slices from CCI injured cortex were loaded with glutamate biosensor immediately prior to imaging. Extracellular and intracellular electrophysiological recordings were used to assess both network-wide and single-cell level changes in cortical connectivity. Western blot analysis was used to measure changes in protein expression of the astrocytic glutamate transporters GLT-1 and GLAST. GABAergic interneurons were quantified by immunohistochemical staining for parvalbumin and somatostatin. Results were compared to cortical slices from sham injured mice.Results: Electrical stimulation evoked polyphasic, epileptiform field potentials and disrupted input-output relationships in slices from CCI injured cortex. High-speed glutamate biosensor imaging showed that glutamate signaling evoked by electrical stimulation was significantly increased in CCI injured slices. Elevated glutamate responses were consistent with epileptiform activity, were highest in the area directly adjacent to the injury, spread via deep cortical layers, and were NMDA receptor dependent. Cortical glutamate release in slices from CCI injured cortex was sequentially activated with the same temporal parameters as disinhibited slices from sham controls. Sham and CCI injured slices showed no differences in astrocytic GLT-1 and GLAST protein expression. Areas of cortical glutamate signaling were consistent with loss of GABAergic interneurons and showed a decrease in spontaneous IPSCs compared to sham injured slices.Conclusions: Our results suggest that loss of GABAergic inhibition in specific cortical sub-networks during the latent period following TBI may facilitate the spread of epileptiform activity. This study provides a possible mechanism for epileptogenesis following TBI.