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Abstract
Continuous seizures endanger neuronal populations in the brain. Seizures lasting in excess of 30 min are injurious, triggering neuronal death in endangered populations, whereas vulnerable neurons survive shorter seizure durations. Furthermore, pre-exposure to non-injurious seizures protects endangered cells from seizure-evoked neurodegeneration. We hypothesized that DNA damage may mediate the transition from cell endangerment to neuronal death in the brain. Thus, we predicted that following seizures, neuronal fate will be influenced by the (a) extent of DNA damage, (b) appropriate detection of DNA damage, and (c) induction of compensatory DNA repair. To begin addressing these hypotheses, we investigated the regulation of DNA repair factors in the brain following both injurious and non-injurious durations of continuous seizures. We found that the expression of DNA repair genes from multiple repair pathways was induced by both injurious and non-injurious seizures. Furthermore, we found that non-injurious durations of continuous seizures were sufficient for inducing the double-strand break (DSB) repair protein, Ku 70. Notably, pre-exposure to neuroprotective electroconvulsive seizures (ECS) prevented injury-evoked Ku 70 expression. We next focused on DSB induction and detection in mature neurons by investigating the phosphorylation of histone H2A.X (termed gamma-H2AX), which occurs rapidly in response to DSBs and facilitates the DSB repair process. To further test our original hypotheses, we asked the following questions: (1) Do DNA-damaging agents (i.e., ionizing radiation) evoke gamma-H2AX in the mature brain? (2) Do seizures induce gamma-H2AX in endangered neural populations in the brain? (3) Does neuroprotective seizure preconditioning prevent subsequent seizure-evoked gamma-H2AX? Both ionizing radiation and ionotropic glutamate receptor activation induced gamma-H2AX in cortical neurons in vitro. Furthermore, both ionizing radiation and continuous seizures evoked gamma-H2AX in neurons in vivo. Intriguingly, pre-exposure to ECS attenuated seizure-evoked gamma-H2AX formation in endangered neurons. Our findings indicate that seizures induce DNA damage and compensatory repair responses in the mature brain. Furthermore, pre-exposure to non-injurious seizures attenuates subsequent seizure-evoked DNA damage, suggesting that the neuroprotection effects of ECS are mediated, at least in part, by a decrease in the cellular damage elicited by subsequent insults.
