Abstracts

Rationale: Ictal events develop through unknown mechanisms, though differences in inhibitory and excitatory neuron activity have been implicated. In mammals, recent studies have reported that inhibitory interneurons often lead seizure activity, while excitatory neurons are recruited later. The larval zebrafish model presents an opportunity to examine ictal development over large regions of the brain, while also allowing for imaging of different neuronal subtypes at cellular resolution. Here we use larval zebrafish and a pharmacological seizure model preparation in conjunction with fast in vivo calcium imaging to examine how ictal events develop across brain regions at different depths and between excitatory and inhibitory cell types. Methods: Larval zebrafish (Tg(HuC:h2b-GCaMP6f) x Tg(Vglut2a:dsRed)) were treated with the convulsant Pentylenetetrazole (PTZ, 15 mM) by bath application. 2-photon calcium imaging with a custom microscope was used to examine activity profiles of individual neurons across brain regions during ictal events. Slow (2-4 Hz) and rapid (10-100 Hz) imaging was applied to examine ictal propagation patterns and differences in excitatory vs inhibitory neuron responses during ictal events, respectively. Results: 2-photon imaging revealed that, within individual animals, ictal events follow similar propagation patterns through brain regions. However, across animals, we observed ictal events emanating from different brain structures. Further, we observed weak involvement by the telencephalon relative to other brain regions. Fast (10-100 Hz) imaging revealed that within subnetworks neuronal recruitment times differed by hundreds of milliseconds; however, examination of inhibitory vs excitatory neurons revealed no strong differences in cell-type-specific recruitment by ictal events. Conclusions: PTZ-induced ictal events in the larval zebrafish model do not follow stereotyped patterns across animals, though they do follow patterns within animals. The telencephalon, in particular, is not typically involved in ictal events. Further, PTZ-induced ictal events in larval zebrafish do not appear to recruit inhibitory interneurons prior to excitatory neurons. Funding: Weill Cornell Medicine Collaborative Research Seed Grant