Project Summary/Abstract Circadian clocks are ubiquitous in tissues, regulate many biological functions, and their misalignment or disruption contributes to deleterious health consequences. Although molecular underpinning of the circadian clock is well-studied, the role of the clock in tissue-specific biological functions, such as that of the blood-brain barrier (BBB), is poorly understood. The BBB is an interface between the vasculature and the brain that both protects the brain from peripheral insults and allows transports of endogenous molecules. Our recent work has found that the circadian clock regulates ATPase binding cassette transporter-mediated xenobiotic efflux from the brain. This proposal examines the mechanisms of BBB circadian clock regulation of endogenous ligands, its relevance to behaviors such as sleep and feeding, and its perturbation under conditions of inflammation. I propose that in addition to xenobiotic efflux, the circadian clock in the BBB regulates endocytosis of particles from brain (Aim 1). Based on published and preliminary data, I hypothesize that the BBB clock influences behavior through transporter-regulated endogenous ligands (Aim 2 and 3). Further, I will interrogate the effect of inflammation induced by either endotoxin or sleep deprivation on the rhythms of the BBB (Aim 4). To pursue these aims, I will use a combination of molecular assays (qPCR, intracellular ion measurements, metabolomics), functional bioassays (endocytosis, xenobiotic permeability) and behavioral assays (sleep, feeding). Successful completion of this project will offer important advances in understanding both the BBB circadian clock and behavior. First, understanding molecular mechanism of the BBB clock will further define the role of the clock in gating blood to brain communication. Second, it will provide new insights into the regulation of behavior. Third, it will identify novel endogenous sleep-promoting compounds. Understanding the temporal gating of the BBB and its effects on behavior under pathophysiologic conditions is important for developing interventions to improve human health.