Intrahippocampal kainic acid, seizures and local neuronal degeneration: relationships assessed in unanesthetized rats.

Intrahippocampal infusion of nanogram amounts of the neurotoxin kainic acid were used to investigate possible relationships between the convulsive and the local neurodegenerative properties of the amino acid. Bilateral hippocampal depth electrodes and cortical leads were employed to provide simultaneous and continuous electroencephalographic records following kainate injection in unanesthetized freely-behaving rats. In every animal, morphological analysis was performed 3-5 days after administration of kainic acid and attempts were made to correlate neuronal destruction with electroencephalographic patterns. Doses as low as 500 pg kainate led to behavioral sequelae consisting of grooming, scratching and enhanced locomotor activity. In a roughly dose-dependent fashion (range 500 gp-250 ng), these behaviors increased in frequency and at the highest doses the rats also displayed wet-dog shakes, stereotype mouth movements and occasional facial myoclonus. Apart from these automatisms, generalized motor seizures were never seen. Following kainic acid, a spectrum of electroencephalographic changes could occur consisting of one or more of the following: high voltage fast activity, slow and fast high voltage spiking, paroxysmal bursts, spindle bursts or postictal depression periods. The combination of any two of these changes were defined as an ictal episode if they occurred in all four leads simultaneously. Upon morphological examination, only the highest dose used (250 ng) resulted reliably in the degeneration of CA3, CA4 and, partly, CA1 pyramidal cells on the injected side. While the duration of electroencephalographic changes at this dose was significantly higher than at any of the lower doses, the number of seizures or the total time spent in seizures was not different at 250 ng from that at 50 ng. At the latter dose, however, only marginal cell damage could be found. Our data indicate that very low doses of kainic acid directly applied to hippocampal CA3 neurons, can elicit bilateral changes in the electroencephalogram indicative of repetitive limbic seizures which are not necessarily accompanied by neuronal degeneration. At higher doses (250 ng), kainic acid treatment results in both seizure activity and nerve cell death but the two effects appear mechanistically unrelated. While there is no clear-cut dose-response relationship between neuronal damage and seizures, extended electroencephalographic changes of a 15-30 Hz fast activity or simple spiking phenomena may be instrumental for the degenerative process. This dissociation between convulsive and neurodegenerative properties of kainic acid, however, does not argue against a role of an endogenous substance related to kainic acid in the etiology of temporal lobe seizure disorders.