PROJECT SUMMARY Over 3.4 million patients in the U.S. have epilepsy, with no preventative treatments and approximately one- third of patients being refractory to drug treatment. This makes elucidating the basic mechanisms of epilepsy development a pressing priority so that more effective therapeutics can be developed. Abnormal excitatory neurons are a common pathological feature of childhood epilepsies, including tuberous sclerosis complex and focal cortical dysplasia, but it is unclear whether, and which, other cells are responsible for regulating these neurons. This proposal will address this deficit by evaluating the role of a potentially critical subpopulation of interneurons. The Danzer lab has developed a mouse model of epilepsy in which Pten, an mTOR pathway inhibitor, is deleted from a subset of hippocampal granule cells (DGCs) to introduce abnormal (DGC Pten knockout (KO)) DGCs into an otherwise normal brain. Epilepsy development in these mice provides evidence of granule cell involvement in epileptogenesis. Although not directly targeted in this model, animals develop impaired inhibition and interneuron loss. Findings suggest that epilepsy may develop via accumulation of aberrant DGCs followed by disruption of inhibitory restraint of those abnormal cells. This proposal will test the hypothesis that somatostatin-expressing (SST) interneurons provide critical inhibitory control over Pten KO DGCs. A key prediction of this hypothesis is that SST interneuron disruption will dramatically exacerbate epilepsy severity in these animals. The objective of this research is to evaluate the significance of hippocampal SST interneurons in the Pten KO model and determine whether ablation of these sprouted SST neurons induces an increase in disease severity. The hypothesis underlying this proposal is that SST interneurons restrain hyperexcitable granule cells in the Pten KO model and that loss of these sprouted SST interneurons will amplify epilepsy severity. To reveal SST interneurons, DGC Pten KO mice that express a fluorescent reporter in those interneurons have been generated. A viral strategy will be used to induce expression of a silencing archaerhodopsin in hippocampal SST neurons. Aim 1 will establish whether SST input to Pten KO cells is enhanced using morphological and patch clamp electrophysiology approaches. Aim 2 will functionally assess the importance of SST neuron-mediated inhibition by determining whether SST interneuron ablation in Pten KO mice exacerbates their epilepsy phenotype. To evaluate the impact of the loss of SST interneurons, SST interneurons will be removed either alone or in combination with Pten KO in DGCs to determine whether interneuron ablation and Pten KO in DGCs will result in more severe epilepsy than either insult alone. Virally-induced expression of diphtheria toxin receptor in SST interneurons will enable their ablation after treatment with diphtheria toxin. These studies will reveal how somatostatin interneurons ...