Umbelliferone, a pH sensitive fluorescent indicator, can be used to determine intracellular pH measurements and analyze the pathway of a fat soluble substance across the blood-brain barrier (BBB). The 'equivalent intracellular pH' determined by this technique corresponds closely with the calculated intracellular pH derived from mathematical abstractions. The pH of the indicator's environment along its pathway into and out of brain tissue can be determined from a ratio analysis of various points along the calibrated fluorescent tissue clearance curves from 340 and 370 nm excitation. This analysis indicates that immediately upon leaving the intravascular space the indicator enters an environment that is too acid to represent the extracellular space. This suggests that fat soluble substances follow an intracellular pathway across the BBB (capillary endothelium to glial cell to neuron). © 1980.
Sundt, T. M., & Anderson, R. E. (1980). Intracellular brain pH and the pathway of a fat soluble pH indicator across the blood-brain barrier. Brain Research, 186(2), 355–364. https://doi.org/10.1016/0006-8993(80)90981-6