PROJECT SUMMARY / ABSTRACT Deep brain stimulation of the anterior thalamic nuclei can reduce the number of seizures in patients with intractable temporal lobe epilepsy, but rarely leads to complete remission. The mechanisms of action underlying these therapeutic benefits remain unknown. We have identified a specific pathway by which anterior thalamic stimulation may induce an inhibitory firewall in a brain region called the retrosplenial cortex, potentially preventing the spread of seizures. The hippocampal formation sends dense outputs to the superficial layers of the retrosplenial granular cortex (RSG) via the subiculum. Anterior thalamic axons converge onto exactly the same RSG circuit. The retrosplenial cortex then projects to dozens of neocortical regions, including the secondary motor cortex, and is thus a critical gateway via which seizures can spread from the hippocampus to the neocortex, leading to secondarily generalized motor seizures. Despite this important connectivity, the cells and circuits of the retrosplenial cortex are massively understudied in epilepsy. We have discovered a new cell type in the RSG and found that this brain region is dominated by local inhibition. Our recordings show that anterior thalamic inputs to the retrosplenial cortex strongly recruit this inhibitory circuitry and identify a unique pathway for the thalamic recruitment of fast-spiking inhibitory neurons in RSG, via both layer 1 and layer 3. This pronounced inhibition can silence the excitatory neurons of the RSG, potentially preventing the propagation of seizure activity to the rest of the neocortex. Our retrosplenial gate hypothesis posits that the inhibition-dominated retrosplenial circuitry recruited by anterior thalamic stimulation can prevent the spread of temporal lobe seizures to the neocortex and partially explain the therapeutic mechanisms underlying anterior thalamic DBS. In Aim 1 we will characterize simultaneous neuronal dynamics in the hippocampus, retrosplenial cortex, secondary motor cortex and anterior thalamus in a rodent model of chronic temporal lobe epilepsy. In Aim 2, we will causally test the retrosplenial gate hypothesis by attempting to prevent the spread of temporal lobe seizures to the neocortex using the closed- loop optogenetic stimulation of anterior thalamic projections specifically to the retosplenial cortex. Therapeutic benefits will be compared to optogenetic stimulation of anterior thalamic projections to alternative cortical regions. The successful completion of these aims has the potential to identify a novel, precise and rational therapeutic pathway for the treatment of temporal lobe epilepsy.