Summary Major depressive disorder (MDD) is one of the leading causes of disability and lost productivity. Nearly half of all clinically depressed patients fail to respond to the first prescribed antidepressant, and about a third fail to respond to all medications. Development of new approaches will require better understand of the mechanisms underlying the disorder. This project has identified and is examining a signaling pathway not previously implicated in anxiety and depression-like behavior, bone morphogenetic protein (BMP) signaling. MDD is associated with reductions in volume of the hippocampus (HC) in humans and in neurogenesis in the HC in animal models of the disorder. Reduction of BMP signaling in the HC in mice is sufficient to produce antidepressant-like changes in behavior and to increase neurogenesis. Treatment with several different classes of antidepressant drugs reduces BMP signaling in the HC, and prevention of this reduction in BMP signaling blocks the effects of the drugs on both behavior and neurogenesis. Inhibition of BMP signaling in the HC also blocks the effects of unpredictable chronic mild stress on both depression- like behavior and neurogenesis. Thus BMP signaling in the hippocampus regulates both depression-like behavior. However, a causal link between the changes in neurogenesis and behavior has not been established. The proposed studies will determine whether there is a causal relationship between changes in neurogenesis, electrophysiological activity of newly generated neurons, and behavior after inhibition of BMP signaling in HC stem/progenitor cells. They also will define the role of BMP signaling in cellular and behavioral responses to stress, and test the hypothesis that that gene expression changes due to elevated BMP signaling contribute to the decrease in neurogenesis, increased proportion of quiescent neural stem cells, and behavioral changes associated with stress/depression.