PROJECT SUMMARY The regulation of oxygen in brain tissue is one of the most important fundamental questions in neuroscience and medicine. The brain has high metabolic demands and its healthy function depends on maintaining tissue oxygen within a relatively narrow range that is sufficiently high to prevent hypoxia and low enough to minimize generation of toxic oxygen species. The regulation of brain tissue oxygen in the cerebral cortex is influenced by excitatory activity, consisting of thalamocortical input and pyramidal cells, as well as the activity of interneurons, yet how they shape this vital process remains poorly understood. Based on published and preliminary data we hypothesize that interneurons are primarily responsible for regulating brain tissue PO2 spontaneous fluctuations and stimulus-evoked responses according to their degree of neuronal firing. In Aim 1, we will show that interneurons are required for PO2 regulation and blocking interneuron output will result in decreased PO2 response in parallel with increased neuronal activity. In Aim 2, we will determine whether interneurons are able to generate PO2 response without excitatory activity. In Aim 3, we will establish that the amplitude and frequency of PO2 fluctuations depend on inhibitory and excitatory spontaneous neuronal activity by demonstrating that interneurons are primarily responsible for high-frequency (8-15 cpm) PO2 fluctuations. Simultaneous oxygen, MRI, and electrophysiological measurements combined with localized pharmacological manipulations and sensory and optogenetic stimulation will provide information about brain tissue oxygen regulation. These studies will provide a deeper understanding of the physiology of interneurons and how they shape the oxygen level in brain tissue.