Changes in intrinsic optical signals could be related to cell swelling; however, the evidence is not compelling. We measured light transmittance, ECS volume fraction (alpha), and extracellular K+ in rat spinal cord slices during electrical stimulation and the application of elevated potassium, NMDA, or anisoosmotic solutions. Dorsal root stimulation (10 Hz/1 min) induced an elevation in extracellular K+ to 6-8 mM, a light transmittance increase of 6-8%, and a relative ECS volume decrease of less than 5%; all of these changes had different time courses. The application of 6 or 10 mM K+ or NMDA (10(-5) M) had no measurable effect on alpha, but light transmittance increased by 20-25%. The application of 50 or 80 mM K+ evoked a 72% decrease in alpha while the light transmittance increase remained as large as that in 6 or 10 mM K+. While the change in alpha persisted throughout the 45-min application, light transmittance, after peaking in 6-8 min, quickly returned to control levels and decreased below them. Astrocytic hypertrophy was observed in 6, 10, and 50 mM K+. The same results followed the application of 10(-4) M NMDA or hypotonic solution (160 mmol/kg). The elevation of extracellular K+ after NMDA application, corresponding to increased neuronal activity, had a similar time course as the light transmittance changes. Furosemide, Cl(-)-free, or Ca(2+)-free solution blocked or slowed down the decreases in alpha, while the light transmittance increases were unaffected. In hypertonic solution (400 mmol/kg), alpha increased by 30-40%, while light transmittance decreased by 15-20%. Thus, light transmittance changes do not correlate with changes in ECS volume but are associated with neuronal activity and morphological changes in astrocytes.

Department of Neuroscience 2nd Medical Faculty Charles University Institute of Experimental Medicine Academy of Sciences of the Czech Republic Prague Czech Republic

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Diffusion in brain extracellular space