Baseline oxygen consumption decreases with cortical depth: Artery06_010919_VG729

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. It 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 tested the above hypothesis using direct measurements of the partial pressure of O2 (pO2) in cortical tissue by means of 2-photon phosphorescence lifetime microscopy (2PLM). We revisited our previously developed method for extraction of the cerebral metabolic rate of O2 (CMRO2) based on 2-photon pO2 measurements around diving arterioles and applied this method to estimate baseline CMRO2 in awake mice across cortical layers. To our surprise, our results revealed a decrease in baseline CMRO2 from layer I to layer IV. This decrease of CMRO2 with cortical depth was paralleled by an increase in tissue oxygenation. Higher baseline oxygenation and cytochrome density in layer IV may serve as an O2 reserve during surges of neuronal activity or certain metabolically active brain states rather than baseline energy needs. Our study provides the first quantification of microscopically resolved CMRO2 across cortical layers as a step towards better understanding of the brain energy metabolism.

大脑皮层（cerebral cortex）按照皮层层次（cortical layers）组织，各层次的细胞密度、组成及连接模式均存在差异。皮层的层状结构可通过细胞色素氧化酶（cytochrome oxidase）染色便捷显现——该酶是定位于线粒体内膜的呼吸电子传递链的最后一个酶。已有假说提出，皮层第四层中高浓度的细胞色素氧化酶条带，反映了基线（未受刺激）状态下更高的氧消耗量。本研究通过双光子磷光寿命显微镜（2-photon phosphorescence lifetime microscopy, 2PLM）直接测量皮层组织的氧分压（partial pressure of O2, pO2），对上述假说开展验证。我们重新优化了此前开发的基于下行小动脉（diving arterioles）周围双光子pO2测量的脑氧代谢率（cerebral metabolic rate of O2, CMRO2）提取方法，并将该方法用于估算清醒小鼠各皮层层次的基线CMRO2。令人意外的是，我们的结果显示，基线CMRO2从皮层第一层到第四层呈递减趋势。这一随皮层深度增加而降低的CMRO2变化，与组织氧合水平的升高同步出现。第四层中更高的基线氧合水平与细胞色素密度，或许是为了在神经元活动激增或特定代谢活跃的脑状态下储备氧气，而非满足基线状态下的能量需求。本研究首次实现了皮层层次分辨率下的微观CMRO2定量分析，为更深入理解大脑能量代谢迈出了重要一步。