Time-varying spreading depression waves in rat cortex revealed by optical intrinsic signal imaging

This study aimed to investigate the variation of propagation patterns of successive spreading depression (SD) waves induced by K+ in rat cortex. SD was elicited by 1 M KCl solution in the frontal cortex of 18 Sprague–Dawley rats under α-chloralose/urethane anesthesia. We applied optical intrinsic signal imaging (OISI) at an isosbestic point of hemoglobin (550 nm) to examine regional cerebral blood volume (CBV) changes in the parieto-occipital cortex. In 6 of the 18 rats, OISI was performed in conjunction with DC potential recording of the cortex. CBV changes appeared as repetitive propagation of wave-like hyperemia at a speed of 3.7 ± 0.4 mm/min, which was characterized by a significant negative peak (−14.3 ± 3.2%) in the reflectance signal. Among the observed 186 SDs, the first wave always propagated through the entire imaged cortex in every rat, whereas following waves were likely to bypass the medial area of the imaged cortex (partially propagated waves, n = 65, 35%). Correspondingly, DC potential shifts showed non-uniform in the medial area, and they seemed closely related to the changes in reflectance. For partially propagated SD waves, the mean time interval to the previous SD wave (217.0 ± 24.3 s) was significantly shorter than for fully propagated SD waves (251.2 ± 29.0 s). The results suggest that the propagation patterns of a series of SD waves are time-varying in different regions of rat cortex, and the variation is related to the interval between SD waves.