The use of neutral red as an intracellular pH indicator in rat brain cortex in vivo

Intracellular pH in the intact, normally perfused rat brain cortex was determined by rapid scanning reflectance spectrophotometry of Neutral Red. Neutral Red, a pH indicator dye, was administered intraperitoneally to rats. Reflectance spectra recorded from the exposed dural surface of 11 anesthetized rats were used to calculate an intracellular pH of 7.04 +/- 0.01. Detailed studies on the interactions of the dye with brain tissue were carried out in vitro to define the in vivo calibration curves. In addition, the physiological effect of dye administration on systemic blood pressure was determined, as well as uptake curves for Neutral Red into plasma and brain. It is concluded that Neutral Red can be used as an in vivo brain intracellular pH indicator and compares favorably with other methods of brain intracellular pH measurement with respect to accuracy, sensitivity, noninvasiveness, and stability and has the potential to exceed any existing method in time resolution.