Neuronal activation induced changes in microcirculatory haemoglobin oxygenation: to dip or not to dip

Abstract

A tight temporal and spatial relationship exists between neuronal activity, metabolism, and blood flow in the brain, and different temporal and spatial kinetics of oxygen consumption and blood flow lead to complex changes in regional cerebral haemoglobin oxygenation. Until now, it remains unclear whether neuronal activation leads to an early deoxygenation termed ‘initial dip’ preceding the rCBF response accompanied by hyperoxygenation. Although several studies have reported the ‘initial dip’, other studies did not confirm these findings, regardless of the species used. In an extensive series of experiments in anesthetized rats, we did not robustly find the dip using various modes of cortical O 2-measurements. Our findings for the first time demonstrate that the appearance of an initial dip and its amplitude may be correlated to the kinetics of the rCBF response. In addition, we show that the mode of analysis of spectroscopic data critically affects results. In our experiments, the finding of an initial dip was sensitive to the mode of spectroscopic analysis. Advanced models of spectroscopic analysis, including differential pathlength correction, should become standard in optical imaging spectroscopy studies. We conclude that in the rodent the initial dip is not a robust phenomenon, the occurrence of which may depend on the level of baseline CBF and thus mean transit time.