Project Summary This project is a continuation of the work proposed in the original K99 application. Having secured a tenure- track assistant professorship at the University of Connecticut School of Medicine, the remaining work will be carried out at that institution. The aims proposed here are substantively unchanged from those originally proposed for the independent phase of the award. The ability to flexibly adapt to changing circumstance is critical for navigating through the world. In order to effectively use cues from the environment to inform choices and guide decisions, irrelevant cues must be effectively ignored, and often an appropriate response in one situation becomes inappropriate in another. This type of behavior, referred to as set-shifting, represents a form of cognitive flexibility. Chronic stress can impair the ability to set-shift and may be related to the impairments in set-shifting that accompany psychiatric disorders such as schizophrenia and depression, as well as neurodegenerative disorders such as Alzheimer’s disease. An extensive body of research in humans and in translational animal models has established a critical role for the prefrontal cortex (PFC) in maintaining cognitive flexibility. However, the precise anatomical and information processing characteristics of the neural circuits within the PFC that enable this behavior remain unknown. Here I propose to leverage powerful imaging techniques to survey the activity of specific populations of prefrontal neurons in a mouse performing a set-shifting task. First, lab members will follow up on preliminary findings that chronic stress results in a deficit in cognitive flexibility by examining the task-related coding properties of projection-specific neuronal populations in mice undergoing chronic stress. The goal of this project will be to determine the effect of stress on behaviorally relevant information coding among prefrontal neurons. Second, lab members will examine the effects of prefrontal glutamate release on stress-induced deficits in cognitive flexibility by testing whether this deficit can be rescued by pharmacogenetic activation of glutamatergic signaling in prefrontal projection neurons. Third, lab members will extend the investigation of the effects of stress on prefrontal activity by using high-speed imaging to examine the effects of stress on rapid, network-level state transitions. Together, these experiments will advance our understanding of the role of the prefrontal cortex in supporting behavior related to cognitive flexibility and of the circuit-level mechanisms by which stress may impair cognitive flexibility in psychiatric illness-related cognitive deficits.