Undershoots following stimulus-induced rises of extracellular potassium concentration in cerebral cortex of cat

Abstract

Extracellular potassium activity (a K) was recorded with potassium-sensitive electrodes in the sensorimotor cortex of cats. Resting activity was 2.8–3.4 mEquiv./l. Electrical stimulation of the cortical surface and the nucleus ventroposterolateralis of the thalamus brough about an increase in a K followed by an undershoot and return to normal value. The lowest observed value of a K was 2.1 mEquiv./l. Size and duration (range 0.5–4 min) of the undershoots of a K increased with increasing peak amplitudes of the preceding rise in a K. Following the rise in a K, a period reduced neuronal activity was observed which was usually shorter lasting than the decrease in extracellular a K. An undershoot of a K and a concomitant reduction of neuronal discharge frequency can also occur in immediate response to antidromic stimulation of the pyramidal tract. To compare the K + redistribution at normal and reduced levels of a K electrophoretic K + signals were produced with constant current pulses from a proximate KCl-filled capillary. Both amplitudes and half times of decay of these K + signals were found to decrease during the phase of poststimulatory undershoot in a K (19 and 23% respectively). It is suggested that an activated reuptake of potassium contributes to the decrease in extracellular a K in addition to inhibitory processes.