Optical Imaging Reveals Characteristic Seizure Onsets, Spread Patterns, and Propagation Velocities in Hippocampal–Entorhinal Cortex Slices of Juvenile Rats

We have combined recordings with extracellular microelectrodes or ion-sensitive electrodes and imaging of intrinsic optical signal changes to study the spatiotemporal pattern of seizure onset and spread during development. We have employed the entorhinal cortex-hippocampus brain slice preparation of juvenile rats at different stages of postnatal maturation. Three age groups were analyzed: 4-6 days (age group I), 10-14 days (age group II), and 20-23 days (age group III). Seizure-like events were induced by perfusion of slices with Mg(2+)-free artificial cerebrospinal fluid thereby removing the Mg(2+) block of the N-methyl-d-aspartate receptor. Seizure susceptibility was highest in age groups II and III. In age group I seizure-like events originated mainly in the hippocampus proper. Seizure-like events in age group II originated mainly in the entorhinal cortex and this tendency was even more pronounced in age group III. Invasion of the hippocampal formation via the perforant path-dentate gyrus and via the subiculum was seen in age groups I and II. In contrast, in age group III the hippocampus was invaded exclusively via the subiculum pathway. The velocity of spread at which seizure-like events propagated within different regions of the slice increased with postnatal age. The characteristics of onset, spread patterns, and propagation velocities as revealed by this study allow insight into the evolving properties of the developing brain.