This project describes the aims that will be addressed during the R00 phase in the Department of Pharmacology and Physiology at the University of Rochester. Currently available antidepressants have serious limitations for treating major depressive disorder (MDD), including low response rates, a significant number of treatment resistant patients, and a time-lag before there is a therapeutic response. Notably, ketamine, an NMDA receptor blocker, has demonstrated promise in clinical trials because of its rapid and sustained antidepressant effects. Although its mechanisms of action are still to be elucidated, our previous studies suggest that ketamine first inhibits cortical GABA interneurons, leading to disinhibition of excitatory pyramidal neurons, and subsequently, a glutamate burst, which results in synaptic plasticity and fast antidepressant responses. However, the effects of ketamine seem to be more complex than a simple enhancement of glutamatergic function, since MDD subjects and stressed animals show robust GABAergic deficits in cortical brain areas, which can be reversed by ketamine treatment. In addition, drugs that target the GABA system via α5-containing GABAA receptors (α5-GABAAR) have also been shown to produce fast and sustained behavioral effects in rodents. Therefore, the goal of this project is to extend our previous work and investigate how excitatory and inhibitory neuronal mechanisms interact to promote GABA-mediated plasticity that culminates in ketamine-induced behavioral responses, and explore additional GABAergic compounds relevant to MDD treatment, including α5-GABAAR modulators. The candidate will test the novel hypothesis that, in addition to glutamate-induced plasticity, increased GABA function in the medial prefrontal cortex (mPFC), specifically through α5-GABAAR, is critical for the synaptic and behavioral effects of fast-acting antidepressants. This hypothesis will be investigated by integrating multiple levels of analysis, including pharmacological, molecular, genetic, behavioral and circuit-level approaches. The lab will address the following aims: 1) To investigate cellular and synaptic mechanisms involved in the behavioral actions of α5-GABAAR NAMs and PAMs and, 2) To investigate the role of α5GABAAR in mediating cell type-specific neuronal activity in the mPFC and associated behavioral outcomes. In addition to significant scientific advances in understanding the pathophysiology of depression, this project will guide efforts to develop a new generation of agents to treat MDD.