instnjctions): The Synaptic Ap Microdialysis Core will serve projects within this PPG to assess dynamic changes in extracellular, or interstitial fluid (ISF), Ap levels in living mice. Microdialysis enables us to measure Ap levels in the context of a living brain with intact neural networks, a functioning blood-brain barrier, and a normal extracellular milieau. This means that studies are performed in a physiologic setting. Having a core facility organize and conduct these studies, particularly for an intricate technique such as microdialysis, ensures experiments and data across all projects can be combined and compared. RELEVANCE (See instructions): Alzheimer's disease is characterized and likely caused by accumulation of the Ap peptide within the bran extracellular space. Understanding how extracellular Ap changes over time will be critical for determining what factors contribute to disease risk as well as for developing therapeutic interventions. Microdialysis enables us to measure how genes, proteins, or behaviors rapidly change the levels of extracellular Ap.