In acute experiments on cats the effect of kainic acid microinjected into the raphe dorsal nucleus on the EEG activity recorded from various brain structures was investigated. Kainic acid in doses of 0.2 to 32.0 nmol was injected by glass micropipets and an air-pressure system. The analysis of the EEG and the EEG power analysis in the usual frequency bands (delta, theta, alpha, and beta) revealed an activating to hyperexcitatory effect of kainic acid. This hyperexcitatory effect was manifested by the appearance of paroxysmal EEG patterns of different types which developed in some cases into seizure episodes or an epileptic state. Abnormal EEG changes occurred first in the thalamus and mesencephalic reticular formation. A dose-response relationship existed: the duration of the EEG with pathological patterns increased whereas the latency of their appearance decreased with increase of the dose of kainic acid. Five to seven hours after the injection there were no severe histopathologic changes in the region of the raphe dorsal nucleus. The hyperexcitatory effect of kainic acid on brain excitability might be a result of the excitatory action of kainic acid and of triggering of circuits of neuronal hyperactivity in structures connected with the raphe dorsal nucleus.
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