Effects of extracellular potassium concentration on the excitability of the parallel fibres of the rat cerebellum.

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Abstract
Field potentials and extracellular K+ [K+]0, were recorded from the rat cerebellar cortex using ion-selective micro-electrodes, following microstimulation of the cerebellar surface. The compound action potential of the parallel fibers (pf) showed changes indicative of a supernormal period (snp) when conditioned by a previous pf volley and was studied in relation to [K+]0. Repetitive stimulation of the pf (> 10 Hz) elicited an alternation in pf excitability from supernormality to subnormality simultaneous to a steady increase in [K+]0. Superfusion with various levels of K+ led to changes in the pf conduction properties. Small increases in [K+]0 above the resting 3.0 mM level led to an increase in pf conduction velocity while greater increases led to conduction slowing and eventually block. Repetitive activation of a row of pf elicited increases in [K+]0 in the vicinity of neighboring non-activated fibers. These fibers displayed an increase in excitability that was quantitatively related to [K+]0. After introduction of 4-aminopyridine (4-AP; 100 .mu.M) into the superfusate, a single stimulus would elicit relatively large (up to 15 mM) increases in [K+]0 around neighboring non-activated pf. The excitability of the adjacent non-activated fibers was either increased or decreased, and was quantitatively related to [K+]0. Strophanthidin application (15 .mu.M) led to a slow and continuous increase in [K+]0. The excitability of the pf initially increased as [K+]0 increased, but subsequently decreased, eventually resulting in conduction block. Small increases in [K+]0 may elicit an increase in pf excitability while greater increases lead to a decrease in pf excitability.