Project Summary/ Abstract: Temporal Lobe Epilepsy (TLE) is the most frequent type of post-traumatic epilepsy, causing significant morbidity in the veteran population. Approximately 60% of adult epilepsy cases are due to TLE, which is often medication-resistant requiring surgery. Even after surgery, ~30% of patients continue to have ictal events. TLE causes cognitive deficits, including significant executive, memory, and neuropsychiatric dysfunction. After initiation by a precipitating event, a seizure-free “epileptogenic” period typically follows before TLE sets in. The network mechanisms that lead to the development of epilepsy during this “epileptogenic” period are poorly understood. A deep and precise understanding of these mechanisms is critical for developing new, more effective, methods of intervention to treat temporal epilepsy without the side- effects of medications and the potential disability of large surgical resections. TLE seizures are thought to be initiated at a restricted temporal focus and then entrain cortical networks. Recent evidence suggests that a large network of areas, including neocortex, play an active role in TLE. Sheybani et al. [7] reports that a self-sustained epileptic network developed during epileptogenesis, becoming gradually able to generate pathological electrical activity independent of the initial hippocampal focus. Together with other experimental and clinical observations, this strongly suggests that extra-hippocampal cortical areas are involved in epileptogenesis. However, how cortical circuits get modified during epileptogenesis remains unknown. We combine chronic, in-vivo, large-field (Mesoscopic) two-photon microscopy with optogenetic modulation of specific cortical interneuron classes to study at single-cell resolution: i) how aberrant activity emerges in neocortical circuits over the course of epileptogenesis in the pilocarpine model of TLE, and ii) whether it is possible to interrupt the hippocampo-cortical cycle of epileptic activity by modulating optogenetically specific types of cortical interneurons. We hypothesize that hypersynchronous firing of parvalbumin positive (PV+) and progressively decreased engagement of SST+ interneurons emerges in cortical circuits during the epileptogenic period. Pathological circuit dynamics will be particularly observed during the 200-400 Hz high frequency oscillations (HFOs) shown to be a marker for circuit hyper-excitability. In aim 1, we measure how the profile of recruitment of different types of cortical neurons during high frequency oscillations (HFOs) changes as a function of time during epileptogenesis in the pilocarpine model of TLE. We expect that over time cortical excitability will increase and autonomous hyper-synchronous activity patterns that may be hippocampally independent will emerge. In aim 2, we will u...