Abstracts

Rationale: Post-traumatic epilepsy (PTE) is characterized by spontaneous recurrent seizures (SRS) following traumatic brain injury (TBI), a leading cause of acquired epilepsy. PTE is poorly controlled by current medications and the molecular pathophysiology of PTE is unclear.  A critical step for preventing PTE is to find a key mechanism that drives disease progression during the latent period after TBI. In this study, we sought to demonstrate a causal pathological role of neuroinflammation and/or neurodegeneration in the onset and development of PTE in mice. Methods: TBI was induced in mice by a CCI paradigm and they were monitored by video-EEG recording for 120 days with an electrode in the contralateral hippocampus and cortex (n=5-6/time-point). At days 1, 3, 7, 30, 60, 120 post-surgery, brains were fixed for immunohistochemistry and analyzed for principal neurons (NeuN+), interneurons (PV+), astrogliosis (GFAP), microgliosis (IBA1), and neurogenesis (DCX). To identify a critical correlative pathogenic mechanism of PTE, we utilized linear repeated mixed-effect regression models to account for within-animal correlations. Results: SRS developed progressively in TBI mice with an average latency of 39 days. Hippocampal inflammatory markers of microgliosis and astrogliosis in the contralateral hemisphere reached peak levels within the first week and steadily declined throughout the 120-day observation period. In contrast, neurodegeneration of NeuN(+) cells and PV(+) interneurons was not evident in these early stages, but cell loss of these populations became unmistakable starting 30 days post-TBI. Nearly 30% of PV(+) interneurons were lost shortly before onset of SRS at 30 days (p=0.000324), with the dentate gyrus serving as the greatest region of cell loss (~40% loss of interneurons, p=0.0000885). These results suggest that neuroinflammation that begins early after TBI may catapult neurodegeneration, which appears critical for the onset of epileptogenesis. Specifically, progressive loss of PV(+) GABAergic interneurons within the dentate gyrus correlated with the progression of SRS from 30 to 60 days, coinciding with the onset of SRS at 39 days post-TBI. Conclusions: These data demonstrate that degeneration of hippocampal inhibitory interneurons during the latent period after TBI is a key epileptogenic mechanism that could be causally involved in the onset and progression of PTE in mice model. These findings are reminiscent of a preferential loss of inhibitory interneurons in epilepsy patients and in animal epilepsy models. Funding: Funded by DOD Award #W81XWH-16-1-0660