INTRINSIC OPTICAL SIGNALS IN RAT NEOCORTICAL SLICES MEASURED WITH NEAR-INFRARED DARK-FIELD MICROSCOPY REVEAL CHANGES IN EXTRACELLULAR-SPACE

In the last decade, the measurement of activity-dependent intrinsic op tical signals (IOSs) in excitable tissues has become a useful tool for collecting data about spatial patterns of information processing in m ammalian brain and spread of excitation. Although the extent of the IO S correlates well with the extent of electrical excitation, its time c ourse is much slower, suggesting that it does not directly monitor the electrical activity. The aim of this study was to investigate the mec hanisms responsible for generation of IOSs. Coronal neocortical brain slices of juvenile rats were electrically stimulated at the border of layer VI and the white matter. The induced columnar-shaped IOSs were r ecorded using dark-field video microscopy. At corresponding locations, alterations in extracellular K+ concentration and extracellular space (ECS) volume were registered using ion-selective microelectrodes. Aft er stimulation, a transient increase of extracellular K+ concentration up to 10 mM and a transient decrease of ECS volume by similar to 4% c ould be observed. The comparison of the time courses of these paramete rs yielded considerable differences between extracellular K+ concentra tion increase and IOS, but obvious similarities between alterations in ECS volume and IOS. To test the hypothesis that changes in IOS reflec t changes in ECS, but not extracellular K+ concentration, we recorded under conditions that are known to prevent activity-induced changes in ECS, i.e., in low Cl- solutions and in the presence of furosemide. Bo th treatments similarly decreased stimulation-induced IOSs and alterat ions of ECS. However, the effect of these treatments on changes of ext racellular K+ was different and did not correspond to the changes of I OS. We conclude that activity-dependent IOSs in rat neocortical slices measured by near-infrared video microscopy reveal changes in ECS. Fur thermore, the pharmacological and ion substitutional experiments make it likely that activity-induced IOSs are attributable to cell swelling via a net KCl uptake and a concomitant water influx.