Project Summary/Abstract Sleep disorders constitute a major public health problem in the United States, and abnormalities in arousal often occur in concert with other neurological or neuropsychiatric diseases. The locus coeruleus (NE), the major source of norepinephrine (NE) in the brain, promotes arousal, attention, and wakefulness, but the neural substrates that transduce these actions of the LC have not been fully identified. One intriguing candidate is an understudied population of wake-promoting dopamine (DA) neurons in the ventral periaqueductal gray (vPAG). The LC projects to the vPAG, and our preliminary data indicate that NE provides excitatory drive onto DA neurons in this brain region via α1-adrenergic receptors (α1ARs), suggesting that NE-DA interactions in the vPAG may promote arousal. The goal of this proposal is to delineate this LC-vPAG circuit. In Aim 1, we will use tract tracing and immunohistochemistry at the electron microscopic level to map the connections between the LC and vPAG as well as specific localization of α1ARs. In Aim 2, we will use ex vivo slice electrophysiology to determine the cellular mechanisms underlying the ability of NE to enhance excitatory drive onto vPAG DA neurons. In Aim 3, we will use DREADD chemogenetics to assess the behavioral consequences of inhibiting or stimulating various nodes of the LC-vPAG arousal circuit. These studies will define a novel arousal circuit and identify candidate neuroanatomical and molecular targets for the treatment of both primary and disease- associated deficits.