PROJECT SUMMARY Making correct choices, and learning from them, requires a detailed understanding of biological needs. Such needs are governed by fluctuating drives (hunger, thirst) and by larger, underlying rhythms such as the Circadian Cycle. Forebrain circuits, and axonal inputs to them from the Ventral Tegmental Area (VTA), are crucial to computations underlying choice and learning. A key source of body state information for the Forebrain are circulating signals in the vasculature. However, the blood-brain barrier (BBB) is commonly reported to block almost all such signaling molecules to protect the brain from contamination. Our lab has been testing a potential resolution to this tension: Specifically, we have found that BBB permeability is dynamic, only ‘opening’ at behaviorally-relevant moments when the risk of contamination is worth the reward of higher quality state information. Our extensive Preliminary Data show that BBB permeability events occur in response to local VTA axon activity both endogenous and optogenetically-driven. I build on these new findings and my graduate studies, where I discovered substantial vascular dynamics between sleep and waking (Turner et al., 2020)1 and how these relate to indicators of arousal (Turner et al., 2023)2. In Aim I, I will test the hypothesis that VTA axon activity and local dopaminergic (DA) levels peak early in the active (dark) phase, and then decrease progressively. I will test this hypothesis with 1P and 2P imaging of VTA axon activity and DA signaling in Neocortex and Striatum, leveraging behavioral paradigms shown to drive distinct patterns of VTA axon activity (Hamid et al., 2021)3. In Aim II, I will test the hypothesis that VTA-driven BBB permeability events show the same relationship to Circadian Cycle. Specifically, that single axon spikes and optogenetic input drive larger amplitude permeability events early in the active (dark) phase followed by a decreased relative impact across this phase. In these studies, I will test endogenous and optogenetic-driven VTA axon activation while imaging local BBB permeability events and systematically testing that VTA-driven BBB permeability events allow the transmission of multiple active signaling molecules including sex hormones and metabolic cues. In Aim III, I will test the hypothesis that integration of prior reward history will have the strongest impact early in the active (dark) phase. In these studies, I will employ a two- armed bandit task in conjunction with fiber photometry recordings of DA levels in SI, mFC, DS, and NAc. I will fit this data into behavioral computation frameworks developed in my co-sponsor Dr. Michael Frank’s laboratory. These studies will generate unique data that tests how a major determinant of behavioral state, the Circadian Cycle, relates to VTA axon activity, rapid changes in BBB permeability, and behavior itself. Further, these studies are ideal for my training, directly supporting my central career goal of unders...