A new technique for the mapping of oxygen tension on the brain surface

Most measurements of oxygen tension (PO2) in the brain have been performed using oxygen microelectrodes. However, the insertion of microelectrodes into the brain per se causes cortical injury and hence could lead to erroneous PO2 measurements. The recently developed “quenching lifetime method” requires the injection of fluorescent chemicals into the blood circulation. To address this issue, we tested the feasibility of our O2-sensitive fluorescent membrane technique in the rat brain, and visualized the spatial distribution of PO2 on the brain surface as epifluorescent microscopic patterns. An O2-quenching fluorescence dye, tris (1,10-phenanthroline) Ru2+, was immobilized in a highly gas-permeable, thin silicone-rubber film formed on a microscope coverslip. Unlike the original method, which was intended for transparent rat mesenteric tissue, any change in the redox state in the brain tissue will influence the optical measurement of PO2. Thus, in the present study, the O2-sensing membrane was further coated with a thin opaque silicone-rubber to minimize this type of influence. This new method enabled us to visualize the PO2 gradient on the rat brain without causing cortical injuries. In an ischemia/reperfusion model using Pulsinelli's four-vessel occlusion rats, the changes in the PO2 were highly heterogeneous during the ischemic period and this heterogeneity, both temporal and spatial, was higher in the off-arteriolar area than in the peri-arteriolar area.