RBC (oxygen-carrier) behaviour in the cerebrocortical microvasculature during K^+^-induced cortical spreading depression (CSD) was examined in urethane-anesthetized male Wistar rats (n=10). The movements of FITC-labeled RBCs in single capillaries in the cortical region were traced with a high-speed camera laser scanning confocal fluorescence microscope and analyzed with Matlab domain software, KEIO-IS2, to obtain the velocities of all labeled RBCs appearing in local capillaries during CSD wave propagation. We found that CSD induced periodic decreases in both RBC number and velocity until RBCs halted or disappeared for 3.3 +/- 2.3 s, and then RBC flow was restored. The RBC flow stall was statistically significant (P < 0.05). During capillary flow failure in association with CSD spread, systemic arterial blood pressure remained unchanged. We conclude that RBCs are transiently sieved and stalled in capillaries during neuronal depolarization, and we suggest that this neuro-capillary coupling involves a hemorheological (viscosity-related) mechanism
