SUMMARY: Anxiety disorders are the most common psychiatric conditions, yet no new anxiolytic drug has been approved for treatment in the last decade. Therefore, there is a significant unmet need to develop new effective pharmacotherapies. In rodent and human studies, the bed nucleus of the stria terminalis (BNST) has emerged as a key brain region translating prolonged exposure to uncertain threats into sustained fear. The BNST is hyperactive in patients suffering from anxiety disorders and post-traumatic stress disorder. Human imaging studies demonstrate an enhanced functional connectivity between the BNST and the central amygdala (CeA) in anxious individuals. In rats, the BNST sends inhibitory projection to the CeA (BNSTàCeA) but there is a significant knowledge gap on how the BNST interacts with the CeA to modulate fear processing. As the CeA and BNST drive fear responses to predictable and unpredictable threats, respectively, increased activity of the BNSTàCeA neurons will favor responses to the unpredictable threats and can precipitate hypervigilance, an important hallmark of anxiety disorders in humans. Therefore, the factors that inhibit this BNSTàCeA output have untapped potential as novel therapeutic targets. Recent findings from the lab show that one such target involves oxytocin receptors (OTR) in the BNST, as OTR activation inhibits BNSTàCeA output and facilitates fear to predictable threats (cued fear). In contrast, our compelling preliminary data indicate that activation of vasopressin 1a receptor (V1aR) in the BNST directly excites Type III/Corticotropin- releasing factor (CRF) neurons, which promote sustained fear responses. Based on this scientific premise, the central hypothesis of this proposal is that by inhibiting and exciting the BNSTàCeA neurons, OTR and V1aR facilitate fear responses to predictable vs. unpredictable threats, respectively. This innovative concept will be investigated using cutting-edge experimental approaches, including chemogenetic and optogenetic manipulations of BNSTàCeA neurons in recently developed transgenic rat models (OTR-Cre, AVP-Cre, CRF- Cre). Rigorously designed behavioral and electrophysiological experiments will test the hypothesis with the following specific aims: 1) Determine the integrated role of V1aR and OTR in modulating activity of BNST neurons, 2) Determine whether predictable vs. unpredictable threats differentially modulate activity of BNSTàCeA neurons via OTR and V1aR, 3) Determine the role of BNSTàCeA neurons in mediating fear responses to predictable vs. unpredictable threats, and the contribution of OTR vs. V1aR. By refining mechanisms underlying the activity of BNSTàCeA neurons, this proposal will have a positive impact on the understanding how an imbalance in processing of predictable vs. unpredictable threats can lead to a hypervigilance and precipitate the onset of anxiety disorders.