PROJECT SUMMARY & ABSTRACT The SLC6A1 gene has been recently implicated in a spectrum of neurodevelopmental disorders including epilepsy, autism, and intellectual and motor disability, collectively named SLC6A1 syndrome. SLC6A1 encodes the GABA transporter GAT1, but the cellular and circuit mechanisms by which SLC6A1 mutations cause SLC6A1 syndrome remain unknown, thus hampering the development of effective treatments. To fill this gap, I propose to dissect the mechanisms that cause epilepsy in a new mouse model carrying a human SLC6A1 mutation. My preliminary data show that Slc6a1 S295L/+ mutation leads to state-dependent seizures in thalamocortical circuits, disrupting sleep. Motivated by these findings, I will test the central hypothesis that thalamic circuits are key regulators of epileptic seizures associated with SLC6A1 syndrome. To test this hypothesis, I will determine the impact of S295L mutation on synaptic and intrinsic properties of thalamocortical neurons and thalamic circuit excitability in brain slices, and on thalamocortical function in vivo. I will also investigate whether thalamic targeting with optogenetic tools can treat epileptic seizures. The proposed work will harness in vitro and in vivo electrophysiological and optogenetic circuit dissection techniques. These results will elucidate our basic understanding of GAT1 dysfunction and assess how the subsequent increased tonic GABA current might impact the thalamocortical circuit in SLC6A1 syndrome. Ultimately, this work will help identify potential therapeutic targets to treat SLC6A1-related epilepsy.