Intrinsic Optical Imaging Reveals Regionally Different Manifestation of Spreading Depression in Hippocampal and Entorhinal Structures in Vitro

The spatiotemporal features of spreading depression (SD) were analyzed in vitro by using combined hippocampal-entorhinal cortex slices. SDs were induced by microinjection of 1 M KCl in the stratum radiatum of the CA1 region of the hippocampus. Measurements of extracellular field potentials, extracellular space (ECS) volume changes and intrinsic optical signal changes were combined to study SD features in different regions of the slice. Each SD was associated with a pronounced shrinkage of the extracellular space (ECS) volume and a decrease in light transmittance. The beginning of the optical signal change occurred simultaneously with the electrographic onset as measured with extracellular microelectrodes but outlasted the dc shift for tens of seconds. The amplitude of the intrinsic optical signal change displayed marked regional variations with greatest changes of 12% in cortical regions. The signal amplitudes were considerably lower in hippocampal regions. The analysis of spread patterns revealed two types of waves: fully propagated waves spreading from CA1 all the way to the temporal neocortex and abortive waves that ceased earlier. The spread velocities displayed pronounced regional differences with highest velocities of 5.4 +/- 0.3 mm/min in the area CA3 of the hippocampal formation and lowest velocities of 2.7 +/- 0.1 mm/min in cortical regions.