Intracortical and intrahippocampal injections of kainic acid in developing rats: an electrographic study

Electrographic patterns induced by neocortical and hippocampal microinjections of kainic acid (KA) have been studied in curarized 4-30-day-old rats. In younger (4-6-day-old) animals, both hippocampal and neocortical KA application induced, with a long delay, the appearance of sequences of slow spikes, simultaneously occurring in the cortex and hippocampus. The same pattern was observed in about 60% of animals 7-9 days old. In the remaining 40% of the rats of this age, epileptic abnormalities, initially localized in the neocortical or hippocampal injection site were obtained. The latter pattern always appeared in 10-14-day-old rats. In some of these status epilepticus was also reached. In older (15-30 days) animals, the hippocampal injection caused the appearance of hippocampal seizures, always evolving into status epilepticus. In neocortically injected animals, cortical bursts of polyspikes appeared, with or without hippocampal involvement. After 40-60 min, typical hippocampal seizures occurred, later leading to status epilepticus. The simultaneous hippocampal and neocortical response observed in younger rats is attributed to a massive activation of the immature brain structures. The focal response seems to be correlated with a maturational process of glutamate and/or kainate receptors at both hippocampal and neocortical levels. This process is completed during the third week, when a typical selective activation of the limbic structures is obtained.