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The cerebral cortex is organized in cortical layers that differ in their cellular density, composition, and wiring. Cortical laminar architecture is also readily revealed by staining for cytochrome oxidase – the last enzyme in the respiratory electron transport chain located in the inner mitochondrial membrane. I has been hypothesized that a high-density band of cytochrome oxidase in cortical layer IV reflects higher oxygen consumption under baseline (unstimulated) conditions. Here, we test the hypothesis using direct measurements of the partial pressure of O2 (pO2) in cortical tissue obtained with 2-photon phosphorescence lifetime microscopy (2PLM). We revisit our previously developed method for extraction of cerebral metabolic rate of O2 (CMRO2) based on 2-photon pO2 measurements around diving arterioles and apply this method to estimate baseline CMRO2 in awake mice across cortical layers. Our results show that, in contrary to the common notion, baseline CMRO2 in layer IV is not higher than that in upper layers. We speculate that the cytochrome oxidase band in layer IV may reflect higher metabolic demands during transient surges in neuronal activity. These findings are important for interpretation of cortical-layer-resolved Blood Oxygen Level Dependent (BOLD) functional Magnetic Resonance Imaging (fMRI) studies.
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