Identification of the extent of cortical spreading depression propagation by Npas4 mRNA expression

Cortical spreading depression (CSD) is a phenomenon associated with a propagating large shift in direct current (DC) potential followed by suppression of electrophysiological activity. For temporal analysis of CSD propagation, electrophysiological recording is the most reliable tool. However, it is difficult to completely identify the spatial area of the brain influenced by CSD, because recording sites are technically limited. Histological post hoc identification of activated neurons by labeling the induction of an immediate early gene (IEG) could determine areas of CSD propagation. We found that cortical application of potassium chloride induced expression of Npas4 IEG mRNA in the ipsilateral dorsal cortex. Interestingly, induction of Npas4 was never observed in the ipsilateral hippocampus and there was a clear boundary to the area of Npas4 expression. To determine whether the boundary of the area of Npas4 mRNA expression was the limit of CSD propagation, we recorded local field potentials from multiple sites that crossed the boundary of Npas4 expression. We found that the area of Npas4 mRNA expression coincided with the area of DC-potential shift propagation. We propose that induction of Npas4 identifies the area influenced by CSD propagation.