Project Summary/Abstract Depression is the leading cause of disability worldwide and new treatments are needed. This therapeutic challenge stems from the fact that depression involves deficits distributed among neuronal subtypes and multiple brain regions. A growing body of clinical research demonstrates that a single dose of N-methyl-D-aspartate receptor , induce rapid and durable improvements in depressive symptoms persisting for days to weeks in patients refractory to conventional antidepressant therapies. Despite nitrous oxide being the oldest and one of the safest anesthetics in current (NMDA-R) antagonists, inhaled nitrous oxide or intravenous ketamine clinical practice, our understanding of how nitrous oxide modulates neuronal activity and circuit function to produce its clinical effects is extraordinarily limited. While it has been hypothesized that the therapeutic effect of NMDA-R antagonists is related to their ability to induce plasticity, the mechanisms that initiate and sustain this plasticity remain unclear. By combining in vivo imaging of synaptic structure, functional calcium imaging, electrophysiology, and behavioral recordings, this proposal will test an innovative hypothesis that changes in neuronal activity imposed by nitrous oxide acutely, automatically give rise to sustained plasticity through activity-dependent mechanisms. This hypothesis is based on extensive work on activity- dependent plasticity in the neocortex and on our own novel preliminary results showing that nitrous oxide specifically and directly activates layer 5 pyramidal neurons, which mediate cortico-cortical and cortico- subcortical connectivity, through a novel mechanism. All animal models and techniques have been established in the studies that generated the preliminary results, making the proposal highly achievable. In line with the mission of the NIGMS, the immediate goal of this proposal is to lay the foundation for advances in the treatment of depression by understanding the mechanisms of action of nitrous oxide in cortical circuits. Such an understanding may explain how acute pharmacologic interventions can lead to sustained therapeutic benefits in the setting of depression and potentially other neuropsychiatric diseases.