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-sensi tive electrodes and imaging of intrinsic optical signal changes to study th e 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 group s were analyzed: 4-6 days (age group 1), 10-14 days (age group II), and 20- 23 days (age group Ill). Seizure-like events were induced by perfusion of s lices with Mg2+-free artificial cerebrospinal fluid thereby removing the Mg 2+ block of the N-methyl-D-aspartate receptor. Seizure susceptibility was h ighest in age groups II and III. In age group I seizure-like events origina ted mainly in the hippocampus proper. Seizure-like events in age group II o riginated mainly in the entorhinal cortex and this tendency was even more p ronounced in age group III. Invasion of the hippocampal formation via the p erforant 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 exclusive ly via the subiculum pathway. The velocity of spread at which seizure-like events propagated within different regions of the slice increased with post natal age. The characteristics of onset, spread patterns, and propagation v elocities as revealed by this study allow insight into the evolving propert ies of the developing brain. (C) 2000 Academic Press.