No evidence for early decrease in blood oxygenation in rat whisker cortex in response to functional activation

Using optical methods through a closed cranial window over the rat primary sensory cortex in chloralose/urethane-anesthetized rats we evaluated the ti me course of oxygen delivery and consumption in response to a physiological stimulus (whisker deflection). Independent methodological approaches (opti cal imaging spectroscopy, single fiber spectroscopy, oxygen-dependent phosp horescence quenching) were applied to different modes of whisker deflection (single whisker, full whisker pad). Spectroscopic data were evaluated usin g different algorithms (constant pathlength, differential pathlength correc tion). We found that whisker deflection is accompanied by a significant inc rease of oxygenated hemoglobin (oxy-Hb), followed by an undershoot. An earl y increase in deoxygenated hemoglobin (deoxy-Hb) proceeded hyperoxygenation when spectroscopic data were analyzed by constant pathlength analysis. How ever, correcting for the wavelength dependence of photon pathlength in brai n tissue (differential pathlength correction) completely eliminated the inc rease in deoxy-sb. Oxygen-dependent phosphorescence quenching did not repro ducibly detect early deoxygenation. Together with recent fMRI data, our res ults argue against significant early deoxygenation as a universal phenomeno n in functionally activated mammalian brain. Interpreted with a diffusion-l imited model of oxygen delivery to brain tissue our results are compatible with coupling between neuronal activity and cerebral blood flow throughout stimulation, as postulated 110 years ago by C, Roy and C, Sherrington (1890 , J. Physiol. 11:85-108), (C) 2001 Academic Press.