Two types of depolarizing after-potentials in hippocampal pyramidal cells of rabbits.

(1) Depolarizing after-potentials (DAPs) of hippocampal pyramidal cells were studied with intracellular microelectrodes in rabbits anesthetized with Nembutal and, in addition, curaized. (2) DAPs were classifiedl into two types. One was linearly related in magnitude to the spike and the membrane potential of the cell body while the other was not. Therefore, the former was tentatively called the true DAP and the latter the pseudo-DAP. (3) In accordance with reduction in the membrane potential, the magnitude of the true DAP diminished and at a membrane of--60 to--50 mV, the true DAP disappeared. On further reduction in the membrane potential, a hyperpolarization-going dip appeared in place of the true DAP. (4) The pseudo-DAP usually appeared when the membrane potential of the cell body was reduced beyond--60 to--50 mV. But the pseudo-DAP was not quantitatively related to the membrane potential. It was shown that the pseudo-DAP was a nonsynaptic process, and its possible relation to the dendritic spike was discussed. (5) It was demonstrated that it was the pseudo-DAP that led to the genesis of the so-called inactivation response, i.e. a large and prolonged depolarization of the cell membrane with a strong tendency to spike inactivation.