Spontaneous Hyperpolarizations in Pyramidal Cells of Chronically Stimulated Rabbit Hippocampus

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Through daily electrical stimulation of one of the hippocampi in the rabbit, EEG spikes were “kindled” in both hippocampi. Such hippocampi (termed kindled hippocampi) were then studied with the microelectrode in acute experiments. The electrical activities of the kindled hippocampus were characterized by spontaneous occurrence of hyperpolarizing potentials of various magnitudes (3-27 mV) and various durations (50-600 msec) in the pyramidal cell. These hyperpolarizations were never preceded by a prolonged spike burst. Furthermore, they could occur without any preceding depolarizations. They consisted of two components, i.e., Cl-dependent (somatic inhibitory postsynaptic potential) and Cl-non-dependent (dendritic hyperpolarization) components. They were the main abnormal events occurring within the pyramidal cell of the kindled hippocampus. It was therefore considered that they were the intracellular correlates of the EEG spikes.