"Overshoot" of O2 Is Required to Maintain Baseline Tissue Oxygenation at Locations Distal to Blood Vessels

In vivo imaging of cerebral tissue oxygenation is important in defining healthy physiology and pathological departures associated with cerebral disease. We used a recently developed two-photon microscopy method, based on a novel phosphorescent nanoprobe, to image tissue oxygenation in the rat primary sensory cortex in response to sensory stimulation. Our measurements showed that a stimulus-evoked increase in tissue pO(2) depended on the baseline pO(2) level. In particular, during sustained stimulation, the steady-state pO(2) at low-baseline locations remained at the baseline, despite large pO(2) increases elsewhere. In contrast to the steady state, where pO(2) never decreased below the baseline, transient decreases occurred during the "initial dip" and "poststimulus undershoot." These results suggest that the increase in blood oxygenation during the hemodynamic response, which has been perceived as a paradox, may serve to prevent a sustained oxygenation drop at tissue locations that are remote from the vascular feeding sources.