Spreading depression can be restricted to distinct depths of the rat cerebral cortex

In cerebral cortex of rats recurrent and single spreading depressions (SDs) were elicited by KCl application and by needle prick, respectively. SDs were monitored by recording changes of DC (direct current) potentials and of K + concentration ([K +] o) in the extracellular space using K +-selective microelectrodes. Profiles of DC potential and of [K +] o were obtained by stepwise lowering a microelectrode array consisting of up to four electrodes into the brain cortex. Recurrent SDs propagating from the site of KCl application had lower frequencies and longer duration in superficial compared to deeper cortical structures. Single SD elicited by needle prick 3 mm away from the recording sites usually invaded the whole grey matter and showed DC potential shifts that differed in shape from the recurrent SDs. At a depth restricted to 1000 μm SD-related DC potential shifts and rises in [K +] o were drastically diminished. In 2 of 8 experiments prick-elicited SD was absent either above or below cortical depths of 800–1000 μm. The results suggest a barrier for vertical SD propagation in cortical depth between 800 and 1200 μm. The observations are relevant for application of noninvasive techniques (DC electroencephalography, magnetoencephalography) to detect SD in the brain.