Depression is among the most common, disabling, and expensive disorders afflicting our nation's veterans. Cellular and molecular studies of circadian (ca. 24 hr) biological clocks have recently provided tantalizing clues that clock defects may be involved not just in sleep disorders but in a wide range of clinical maladies, including mood disorders. Objectives: Our long-term goal is to define the precise relationship between genetically determined cellular circadian clock function and human mood dysregulation. Our overall hypothesis is that normal mood regulation requires proper circadian timing within brain circuits, and that mood disorders are therefore associated with dysfunction of the circadian clock at a cellular level. We hypothesize that vulnerability to mood disorders arises from clock defects not in the hypothalamic suprachiasmatic nucleus (SCN), the brain's master circadian pacemaker, but rather in the nucleus accumbens (NAc), a brain region implicated in mood regulation that contains a separate circadian clock. Our specific hypotheses are that: (1) circadian clocks are weak in human cells from depressed patients, (2) circadian clocks are also weak in NAc neurons in a mouse model of depression, and (3) weak circadian clocks in the NAc can increase vulnerability to depression in mice by removing normal daily cyclic repression of the CREB signaling pathway by the clock protein CRY. Specifically, we aim to test for circadian dysfunction in skin cells from depressed patients and in neurons from mice exhibiting depression-like behavior. We will then test whether manipulating the circadian clock or CREB signaling in NAc cells of mice alters their vulnerability to depression-like behavior. Research Design & Methodology: We will use viral vectors to introduce a firefly luciferase circadian reporter gene into human skin cells obtained by our collaborators. We will then monitor circadian rhythms of cell bioluminescence by luminometry and single-cell imaging, and analyze comprehensively the functioning of the clock, as well as expression levels of core clock genes. Next, we will use behaviorally induced learned helplessness as a mouse model of depression. Mice will harbor a bioluminescent circadian reporter, allowing us to test circadian clock function in NAc neuronal cells as we did in human cells. We will also measure levels of core clock gene expression and CREB activity in NAc brain slices, expecting to find low Cry expression and high P-CREB. Finally, we will manipulate clock or CREB function in mouse NAc by injection of various inhibitory RNAs or CREB constructs, respectively, followed by testing vulnerability to induced learned helplessness. We expect that weakening NAc rhythms by Cry knockdown, but not by knockdown of a different clock gene, will increase vulnerability to learned helplessness, and that selectively reducing CREB function at dawn, by properly phased rhythmic expression of a dominant-negative CREB construct, will be optimal for reducing vulnerability. Clinical Relationships: Our goal is an improved understanding of the relationship between neuronal circadian clocks and depression. This project may lead to improved diagnostic and therapeutic approaches to mood disorders, which are major afflictions and sources of disability for veterans.