Project Summary Alzheimer's disease (AD) is the most common age-related dementia, accounting for the progressive cognitive impairment and compromised life quality of approximately 5.8 million people in the United States. AD is characterized by neuroinflammation and microglia have a complex role in both the early and late stages of the disease. AD is also characterized by dysregulation of norepinephrine (NE) signaling, degeneration of noradrenergic neurons in the locus coeruleus (LC) and sleep disturbances that further alter normal patterns of NE release. Signaling through β2 adrenergic receptors (β2 ARs) has been shown to be anti-inflammatory and to alter microglial behavior in the context of AD, although its effects on AD pathology have been controversial and the details of NE signaling to microglia have not been defined. In this supplement proposal, we will build on the aims of the parent RO1 in which we have discovered that endogenous NE signaling to microglia through microglial β2 ARs reduces the size of the microglial arbor and inhibits microglial surveillance of the brain, limiting microglial interactions with neurons and impairing experience-dependent plasticity. Based on this work, we believe that the normal patterns of NE release that fluctuate as a function of arousal state allow microglia to cycle through “active” and “passive” states. When NE release is blunted due to LC neuron degeneration or sleep disruption in AD, microglia no longer cycle through these different states. This alters their behavior and impacts their ability to interact with amyloid deposits, protect synapses from elimination, and respond appropriately to cues in their environment. The work presented in this supplement will generate preliminary data to show how β2 AR signals are altered in microglia over time in a mouse model of AD, and how this correlates with AD pathology. We will also determine how long-term periodic pharmacological stimulation of β2 AR signaling alters microglial dynamics and AD pathology to set the stage for future experiments where we use microglial specific knock out of β2 ARs to test the contribution of microglia in this context. Lastly, we will use these microglial specific β2 AR KO mice, in the absence of AD pathology, to determine whether long-term loss of β2 AR signaling in microglia alters inflammatory responses. These aims are highly synergistic with those of the parent R01 and have clear relevance to AD. Together they will generate supporting data for a future grant application built on our broad hypothesis that cyclical stimulation of microglial β2 ARs is critical for the maintenance of microglial responses that are beneficial in the context of AD pathology.