Initiation of spreading depression can be blocked by transcortical polarization of rat cerebral cortex

Spreading depression (SD) was elicited in urethane anesthetized rats by pricking the cortical gray matter with a needle. The SDs were monitored by recording changes of direct current (DC) potentials and changes of extracellular potassium concentrations ([K+]e). Simultaneous recordings were made at cortical depths of 400 microns and 1200 microns by an array of two double-barrelled electrodes, one served to measure DC the other contained an ion-sensitive resin. An additional DC microelectrode was inserted in the gray matter near the point of SD elicitation at a depth of about 400 microns. An epicortical Ag-AgCl wire electrode surrounding the recording site and a remote Ag-AgCl electrode penetrating the cortex in the contralateral hemisphere were used for polarizing DC currents. These currents were applied 5 min before elicitation of SD by a needle prick and were sustained for a period ending 3 min after SD. Cathodic polarization of cortical surface with intensities of 30 microA and higher blocked the SD completely. Lower intensities of polarizing currents (10 or 20 microA) had no effect. After ending polarizations normal SDs could be elicited. The polarizing and restitution effects were replicable in the same animal. The results suggest that longer lasting DC polarization of the cortex blocks initiation of SD but not propagation.