Project Summary/Abstract Accumulation of Aβ peptide in the brain appears to initiate Alzheimer’s disease (AD), including tau aggregation which then plays a major role in disease progression. In humans and animal models of AD, brain regions with the highest levels of synaptic activity show the greatest amount of Aβ plaques, suggesting Aβ production is closely related to synaptic transmission. Studies from our lab have demonstrated that direct modulation of synaptic activity dynamically regulates brain Aβ and tau levels in awake animals, with increased synaptic activity rapidly increasing brain interstitial fluid (ISF) Aβ and tau levels and vice versa for suppressed activity. These findings strongly suggest a close temporal relationship between synaptic activity and Aβ and tau levels in the brain ISF. The brain extracellular space plays a particularly important role in AD biology. Aβ plaques are extracellular structures with the majority of Aβ that builds onto an existing plaque coming from the brain ISF. Hyperphosphorylated tau tangles are intracellular structures and were long thought to be independent of the ISF; however, recent studies demonstrate that tau can be secreted from one neuron into the ISF, then internalized into a naïve neuron to corrupt intracellular tau in that neuron and cause aggregation. Understanding mechanisms that regulate Aβ and tau kinetics within the brain ISF should provide valuable insight into disease pathogenesis. We hypothesize that pathological forms of Aβ and tau are spread between brain regions in a synaptic- dependent manner. We propose that neurons function within a network to regulate not only the amount of Aβ and tau released, but also the conformation (monomer, oligomers, etc.) of Aβ and tau that is released. Aim 1 will determine the relationship between synaptic frequency and Aβ/tau levels and species. Aim 2 will determine how glutamatergic and GABAergic neurons independently impact Aβ and tau in the ISF. And Aim 3 will determine how local synaptic activity within target brain region affects tau propagation. While several studies suggest that synaptic activity within the initiating brain region drives Aβ and tau secretion in the target region, how local synaptic activity within the dendritic target region affects tau uptake, propagation and seeding is unknown. We have developed a novel micro-immunoelectrode electrode (MIE) technology that detects Aβ and tau with very high temporal resolution in the brains of living mice (measures Aβ in vivo every 60 seconds over several hours). MIEs are highly selective for either Aβ40, Aβ42, tau, or their aggregates, enabling us to determine how synaptic activity regulates the rapid dynamics of these peptides and conformations in vivo. In this proposal we will utilize MIEs to measure rapid changes in Aβ and tau levels within intact neuronal networks and from specific cell types. Studies will include young APP/PS1 mice and young P301S mice prior to Aβ or tau patholo...