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Hippocampal slice (K+)e: depth profiles and changes induced by stimulation or anoxia.

In an attempt to develop a technique which would allow early assessment of the functional state of explanted brain tissue, (K+)e was measured in the CA1 region of rat hippocampal slices using K+-selective microelectrodes. In slices (450 micron) maintained at the boundary between the incubation medium and 95% O2/5% CO2 atmosphere, (K+)e was highest (up to 25-30 mmol/l) immediately below the exposed surface and gradually decreased with depth to (K+) of the bathing fluid (5 mmol/l). (K+)e below the exposed surface remained high throughout the 2 h of incubation. In submersed slices, (K+)e was the highest in the center of the slice (200 micron, 10 mmol/l) and decreased towards both surfaces. During 2 h incubation, (K+)e decreased in the center of the slice to 6 mmol/l in viable preparations remaining high in the deteriorating ones. Electrical stimulation of Schaffer's collaterals (15 V; 0.2 ms; 10 Hz) increased (K+)e of viable slices 200 micron below the surface by 2-3 mmol/l. Similar but slower (K+)e changes were elicited by brief (3 min) anoxic episodes (perfusion with incubation medium equilibrated with 95% N2/5% CO2). It is concluded that submersed slices have a more uniform (K+)e profile as compared to the exposed ones and that low (K+)e in the early phase of incubation is a good predictor of slice viability.