Potassium translocation and spreading depression induced by electrical stimulation of the brain

Cathodal current pulses with durations from 20 ms to 80 s were applied to the surface of rat parietal neocortex, and the strength-duration properties of the threshold stimulus for initiation of spreading depression was determined. In one series, extradural stimulation was used. In a second series, a liquid electrode was used, the dura was opened, and K+-sensitive microelectrodes were used to determine the time course of extracellular K+ concentration during and after each stimulus pulse. With the dura open, the strength-duration curve was of the form It0.55 = constant. With extradural stimulation, the slope of the log-log plot relating current intensity to pulse duration changed gradually as the pulse duration increased, averaging 0.63. Theoretical analysis suggests that diffusion of K+ away from a zone of current-induced accumulation can account for these slope data. Applicability of this mechanism of K+ accumulation to observed changes in sensitivity of neurons to repeated stimulation at subcortical sites is considered.