Project Summary/Abstract Major depression is one of the most common mental health disorders in the United States, affecting approximately 20% of the population at least once in their lifetime. Pharmacologic treatment has been anchored in monoamine reuptake inhibitors; however up to a third of patients find no benefit from these agents. Recently, the development of novel rapid-acting antidepressant treatments has provided improvement of depression symptoms for some patients refractory to traditional pharmacotherapy. Brexanolone, a formulation of the endogenous neurosteroid allopregnanolone (AlloP), is a rapid-acting agent for postpartum depression; however, undesirable side effects limit the feasibility of its widespread use. It is crucial to identify neurophysiologic actions underlying the antidepressant effects of rapid acting agents to inform continued development of safe and effective pharmacotherapies for depression. One proposed mechanism for rapid acting antidepressants is the disinhibition of neural circuits. Here, we focus on hippocampus as a nexus of debilitative cognitive symptoms in depression and other neuropsychiatric illness. We will characterize AlloP- induced changes to neural activity at cellular, circuit, and network levels to identify specific changes to neurophysiology induced by this rapidly acting antidepressant. We hypothesize that low-dose AlloP cultivates paradoxical disinhibition that generates distinct in vivo electrophysiological characteristics of rapid antidepressants. We will test this hypothesis in a series of experimental aims. Aim 1 will test the hypothesis that AlloP disinhibits CA1 through cellular and population measures of activity in vitro. We will test whether sub-sedative concentrations of AlloP disinhibits CA1 pyramidal cells and query the involvement of interneurons in this phenomenon. Aim 2 will investigate the hypothesis that clinically relevant concentrations of AlloP have preferential actions onto hippocampal interneurons compared to CA1 pyramidal neurons. We compare direct actions of AlloP on inhibition of interneurons and pyramidal cells, and probe contributions by GABAAR subpopulations. Aim 3 will identify features of neural oscillations induced by AlloP in vivo at doses relevant for antidepressant effects. Mice will be monitored with video EEG and by depth LFP recordings after administration of drug to probe physiologic markers that differentiate AlloP from comparators and thus may mark antidepressant potential. The results will provide a better understanding of how neurosteroid drugs affect different cell and receptor types to ultimately regulate circuit and network activity. The combination of diverse experimental approaches provides excellent training potential for my scientific training. Coupled with the outstanding clinical training and mentorship provided by Washington University School of Medicine, this proposal will help me achieve my career goal of becoming an independent physician-scien...