Project Summary Metabotropic glutamate (mGlu) receptor 2 is a presynaptic receptor enriched in active zones of glutamatergic nerve terminals within the striatum and is pivotal for brain functions and some mental illnesses. Recently, we found that Ca2+/calmodulin-dependent protein kinase II (CaMKII) directly binds to mGlu2 receptors and phosphorylates mGlu2 receptors at a threonine residue in the intracellular C-terminal tail. These findings for the first time reveal the mGlu2 receptor as a direct substrate of CaMKII. Encouraged by this novel discovery, we propose this renewal application to systematically study this previously unrecognized CaMKII-mGlu2 coupling and to explore roles of CaMKII and mGlu2 receptors in the pathogenesis and symptomatology of a common mental illness. Our hypothesis is that CaMKII regulates mGlu2 receptors and links mGlu2 plasticity to depression-like behavior. Using multidisciplinary approaches, this hypothesis will be tested both in vitro and in vivo in the following four inter-supportive Aims. Aim I will characterize fundamental protein biochemistry of the CaMKII-mGlu2 interplay in vitro. Aim II will define the regulation of CaMKII-mGlu2 interactions and mGlu2 receptor phosphorylation by changing Ca2+ signals in striatal glutamatergic nerve terminals in vivo. Aim III will explore functional roles of CaMKII in modulating trafficking and subcellular expression of mGlu2 receptors and in controlling the efficacy of various mGlu2 signaling events. Aim IV will first monitor long- lasting neuroadaptations of the striatal CaMKII-mGlu2 system in response to prolonged social isolation in adult rats, a chronic stress paradigm modeling anhedonic depression in adulthood animals. Aim IV will then clarify functional roles of CaMKII-mGlu2 interactions in the isolation- induced depression-like behavior. Results achieved here will conceptually advance our current understanding of a phosphorylation-dependent mechanism in regulating glutamate receptor signaling at presynaptic sites. They will also ultimately contribute to the development of novel pharmacotherapies, by targeting CaMKII and mGlu2 receptors, for the treatment of core symptoms of depression.