The relationship between changes in intrinsic optical signals and cell swelling in rat spinal cord slices.

Changes in intrinsic optical signals could be related to cell swelling; however, the evidence is not compelling. We measured light transmittance, ECS volume fraction (α), 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 α, but light transmittance increased by 20–25%. The application of 50 or 80 mM K+ evoked a 72% decrease in α while the light transmittance increase remained as large as that in 6 or 10 mM K+. While the change in α 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 Ca2+-free solution blocked or slowed down the decreases in α, while the light transmittance increases were unaffected. In hypertonic solution (400 mmol/kg), α 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.