Effect of cortisol on the excitability of limbic structures of the brain in freely moving rats.

The effects of cortisol on the excitability of the dorsal hippocampus, septum, hypothalamus and the pontine reticular formation in unrestrained, unanesthetized rats with permanently-implanted electrodes were investigated. The hormone produced a slowing in the spontaneous activity in these regions. The stimulation of the septum, hypothalamus and reticular formation had no appreciable influence on the local or propagated electrical activity of the brain after cortisol injection; however in 14 out of 29 experiments hippocampal stimulation with the same voltages as before cortisol administration, induced generalized convulsive activity. The attacks consisted of high-voltage spikes and slow-wave activity and were followed by a post-seizure exhaustion in the hippocampus. In half of the rats behavioral convulsions also appeared. The convulsive effects of cortisol on the brain are briefly reviewed and the specificity of hippocampal involvement in the present experiments is emphasized. The possible significance of the present findings in relation to the feedback of glucocorticoids on the brain, in the regulation of ACTH secretion, is discussed. The experiments described may also contribute to our understanding of the mechanisms of the convulsive effects of cortisol on the brain.