PROJECT SUMMARY Lewy body dementias (LBD), including Parkinson’s disease dementia (PDD) and dementia with Lewy bodies (DLB), are the second most common type of degenerative dementia in the world. Unfortunately, there are currently no therapies to slow or halt the progression of LBD. Seizures associated with LBD deserve more attention because, despite the harmful impact on the patients, seizure activity can go unrecognized and untreated. Seizures or myoclonus occur in over half of DLB patients, and these symptoms hasten cognitive decline. Our preliminary studies indicate that abnormal α-synuclein (αS), a key component of Lewy bodies, causes cognitive deficits preceded by epileptic activity. We also show that αS-dependent synaptic and cognitive deficits and epileptic activity require endogenous tau expression. This is reminiscent of data showing that the tau-dependence of cognitive deficits and epileptic activity in the mouse models of Alzheimer’s disease (AD). Preventing epileptic activity with antiseizure drugs improves memory in models of AD, and antiseizure drugs are currently in early phase clinical trials for AD. However, antiseizure drugs have not been well investigated for α- synucleinopathy. To better define the role of seizure activity in LBD, the following aims are proposed: Aim 1, Determine the causal relationship between the epileptiform activity and αS-dependent cognitive deficits; Aim 2, Determine if αS fibril inoculation model of PDD/DLB causes tau-dependent cognitive deficits mediated by epileptiform activity; and Aim 3, Define the circuitry and cellular signaling mechanisms contributing to epileptiform activity in mutant αS models. The results of this investigation could lead to new strategies, such as antiseizure drugs and reducing tau levels, as therapies for LBD.