Abstracts

Rationale: The basic assumption of epileptiform activity arising from recurrent and hypersynchronous runaway excitation has fueled the development of anti-epileptic drugs (AEDs) that broadly restrict excitability. Converging evidence demonstrates that neural activity during inter-ictal and ictal discharges are highly heterogeneous, suggesting complex interactions among different neural groups. While no study to date has shown this phenomenon beyond small brain regions, recent advances in imaging technology enable the acquisition of whole brain neural activity at single cell resolution in larval zebrafish. We therefore examined the interplay of macro and microscale network activity to find evidence of a macro-micro disconnect at the level of whole brain. Collectively, elucidating this gap will provide a critical window to develop more strategic therapies. Methods: We used the chemoconvulsant pentylenetetrazol (PTZ) model of acute seizures to concurrently investigate macro and micro-scale seizure networks in larval zebrafish with pan-neuronal expression of the intracellular calcium indicator GCaMP6. Whole brain neural activity was recorded at single cell resolution using light-sheet microscopy. Macro and micro networks were constructed using dynamic functional connectivity and synchronization was captured using correlation matrix analysis. Results: Imaging was conducted on two fish with 7,172 and 5,916 neurons after preprocessing. The macroscale brain displayed elevated synchronization after PTZ application relative to baseline (p < 0.001) in both fish, as observed in prior literature. The microscale brain displayed transient elevation in synchronization during the initial PTZ application and during ictal discharge. In contrast to macroscale synchronization, microscale synchronization was not continuously elevated during the pre-seizure state. Collectively, synchronization during both pre-seizure and seizure states was significantly elevated in the macroscale network relative to the microscale network (p < 0.001). Analysis of the functional microscale connectome showed that the number of edges a given neurons makes prior to PTZ application follows a traditional inverse relationship on the log-log scale. However, during both the pre-seizure and seizure state, this distribution shifts towards increasingly higher number of edges. Conclusions: Using a combination of advanced imaging and analytic methodologies, these data suggest that there is a disconnect between macroscale and microscale neural activity at the level of whole brain. Furthermore, we provided preliminary evidence of dynamic reorganization of the functional connectome prior to the ictal discharge. These results indicate that addressing whole brain seizure networks at single cell resolution will provide unique insights into the nature of seizure generation and propagation at both the macro- and micro-scales. Funding: Stanford Interdisciplinary Graduate Fellowship in collaboration with the Wu Tsai Neurosciences Institute (DH); US National Institute of Health Grant NS094668 (IS).