Project Summary In the previous funding period we investigated the mechanism of rapid antidepressant activity of ketamine, an ionotropic glutamatergic n-methyl-d-aspartate (NMDA) receptor antagonist. We demonstrated that Brain- derived neurotrophic factor (BDNF), and its high affinity receptor TrkB, are required for the rapid antidepressant effects of ketamine as these effects are lost in forebrain specific BDNF knockout mice and conditional forebrain specific TrkB knockout mice. We found the antidepressant effects of ketamine require protein translation, but not transcription, resulting in increases in BDNF protein levels in the hippocampus that are critical for the behavioral effect. Ketamine's blockade of spontaneous NMDA receptor mediated neurotransmission inactivates eukaryotic elongation factor 2 kinase (eEF2K) resulting in dephosphorylation of its only known substrate, eukaryotic elongation factor 2, thereby increasing protein translation of target transcripts, including BDNF. In turn, BDNF is postulated to act via eliciting insertion of 3-hydroxy-5-methyl-4- isoxazolepropionic acid (AMPA) receptors at the postsynaptic membrane, which results in potentiation of AMPA receptor-mediated CA3-CA1 field excitatory postsynaptic potentials (fEPSPs) by ketamine. While this potentiation is suggested to be a cellular correlate of rapid antidepressant effects, its properties deviate from classical Hebbian forms of plasticity and rather coincide with homeostatic synaptic scaling seen after activity suppression. These data provide the basis for the novel hypothesis that BDNF-TrkB signaling regulates synaptic scaling in vivo that is critical for the rapid antidepressant effects of ketamine. The objective of this renewal is to specifically test the causal and instructive role of BDNF-TrkB signaling in the hippocampus in ketamine-mediated synaptic scaling and rapid antidepressant effects. Collectively, this information will provide novel information on the synaptic locus, as well as key molecules, necessary for ketamine's rapid antidepressant effects.