Intrinsic optical signals in vitro: A tool to measure alterations in extracellular space with two-dimensional resolution

In excitable tissues, extensive neuronal activity or pathophysiological con ditions, such as spreading depression, ischemic infarct, or epileptic seizu re, are accompanied by changes in extracellular space volume. Extracellular space volume, in turn, influences neuronal excitability and extracellular ion concentrations and is, therefore, an important parameter of brain activ ity. Unfortunately, determination of changes in extracellular space by ion- selective microelectrodes is tedious, restricted to one spot in space at a time and limited in time resolution. In this study we present intrinsic opt ical signals in vitro as a tool to measure relative changes in extracellula r space volume in brain slice preparations with two-dimensional spatial and sufficient time resolution. Evidence is given that the intensity of intrin sic optical signals is linearly correlated to the amplitude of extracellula r space volume changes. In contrast, the optical signal is poorly correlate d to the concomitant increase in extracellular potassium concentration. We conclude that intrinsic optical signals in vitro are a useful tool to measu re the spread of changes in extracellular space volume with high resolution in time and space. In combination with the measurement of the extracellula r space at one location using ion-selective microelectrodes, it is possible to calibrate the optical signal to percentile alterations of extracellular space volume. (C) 1999 Elsevier Science Inc.