Synaptic plasticity is essential for brain function and development. The overall objective of this proposal is to understand the molecular mechanisms by which activation of NMDA receptors (NMDARs) can lead to bidirectional modification of synaptic strength (LTP and LTD) in the hippocampus. Our Preliminary Studies suggest that NR2A- and NR2B-containing NMDARs are selectively coupled to LTP or LTD of hippocampal CA1 synapses, respectively. Our overall hypothesis, supported by preliminary experiments, is that NR2A and NR2B subunits are differentially associated with postsynaptic signaling molecules that mediate plasticity changes. These findings offer exciting new inroads into understanding NMDAR signaling, which is a central question in neuroscience and relevant to brain maturation, learning and memory, and many neurological and neuropsychiatric diseases. The Specific Aims of this grant will extend these observations and investigate the underlying mechanisms of NMDAR signaling in systematic fashion. Aim 1 will characterize the differential roles of NR2A- versus NR2B-NMDARs in Ras-MAP kinase signaling, AMPAR trafficking and hippocampal synaptic plasticity, using a combination of pharmacological and molecular genetic approaches. Aim 2 will test the idea that different members of the PSD-95 family of scaffold proteins (which bind to NR2A/2B) have different functions in AMPAR trafficking, synaptic transmission and LTP/LTD. Aim 3 will use immunoprecipitation of NMDAR complexes from brain to identify and subsequently characterize the specific signaling proteins that are preferentially associated with NR2A- versus NR2B-NMDARs. Finally, Aim 4 will identify and characterize the effector proteins of the small GTPase Rap, which is emerging as an important player in synaptic depression and possibly synapse elimination. Together these aims will allow us to gain novel and fundamental insight into the molecular organization of NMDARs and their postsynaptic signaling pathways in central excitatory neurons.