Brain tissue oxygen consumption and supply induced by neural activation: Determined under suppressed hemodynamic response conditions in the anesthetized rat cerebral cortex

The dynamic changes in cerebral metabolic rate of oxygen (CMRO₂) and oxygen supply during brain functions have not been well-characterized. To examine this issue, experiments with electrophysiology, oxygen microelectrode and laser-Doppler flowmetry were performed in the anesthetized rat somatosensory cortex. During neural activation, brain tissue partial pressure of oxygen (Po2) and local cerebral blood flow (CBF) were similarly increased. To separate the Po2 changes originating from the increase in CMRO₂ and the increase in oxygen supply, the same experiments were repeated under a vasodilator-induced hypotension condition in which evoked CBF change was minimal. In this condition, evoked Po2 monotonically decreased, indicating an increase in CMRO₂. Then, CMRO₂ was determined at resting as well as activation periods using a dynamic oxygen exchange model. Our results indicated that the changes in CMRO₂ were linearly related with the summation of evoked field potentials and further showed that the oxygen supply in the normal condition was about 2.5 times larger than the demand. However, this oxygen oversupply was not explainable by the change in CBF alone, but at least partly by the increase in oxygenation levels at pre-capillary arterioles (e.g., 82% to 90% O₂ saturation level) when local neural activity was evoked.