Abstracts

Rationale: Acquired epilepsies are characterized by spontaneous, recurrent seizures that emerge following injury. Brain injuries account for 20-60% of all epilepsy and one third of patients with post-traumatic epilepsy are refractory to current treatment options. In order to guide the development of new treatment options, we need to know more about the process of seizure initiation, or “ictogenesis”, in chronically epileptic networks. Here, we investigated patterns of preictal activity in the hippocampal organotypic slice culture model of post-traumatic epilepsy. Methods: In this study, chronic calcium imaging in mouse organotypic hippocampal slice cultures was employed to record the network dynamics of hundreds of seizure onsets. Organotypic slice culture become spontaneously epileptic following the widespread axotomy of slicing, and offer a highly accessible model for investigating ictogenesis in post-traumatic epilepsy. This reduced preparation allows us to survey the entire network, and ensures we have captured the seizure focus (as no external input exist). Network dynamics were imaged using GCaMP7-based calcium indicators in combination with tdtomato-labeled interneurons, to parse the contributions of principal cells vs interneurons to ictogenesis. Chronic imaging was achieved using the newly developed “Incuscope”, allowing for the longitudinal study of the epileptic networks. Results: Interestingly, we found that both low voltage fast activity-like (LAF- low amplitude fast) and hypersynchronous-like (HYP) seizures were generated from individual organotypic slice cultures. LAF onset was consistently observed early during epileptogenesis, with HYP onset and high amplitude activity onset emerging later. K-means clustering was used to identify the earliest and most highly active cells during ictogenesis (i.e. the likely drivers of seizure onset). We found a reliable cluster of highly active cells in LAF onset seizures, which contained a high number of interneurons. Conversely, the highly active cluster in hypersynchronous onset was more variable seizure to seizure and featured similar numbers of both principal cells and interneurons. Conclusions: We found that an organotypic slice culture (i.e., a single seizure focus) can spontaneously generate both low amplitude fast onset seizures and hypersynchronous onset seizures, with LAF being the more immature seizure onset type observed. Our results further suggest that hyper-activation of interneurons may be an important driver of LAF seizure onset in particular. Future work is aimed at testing the causality of these activity patterns in driving different seizure onset types. Funding: NIH NINDS