1) The rabbit hippocampus was kindled through daily electrical stimulation. The rabbit was then anesthetized with Nembutal, and interictal and brain stem-induced potentials, together with fornix-evoked potentials, were studied by observing the depth profiles of field potentials. 2) Interictal field potentials were characterized by the existence of a large slow positivity when recorded in CA1-CA2 apical dendrite layer. On the other hand, brain stem induced field potential having the shortest

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... ncy was recorded invariably as a positive deflection in the apical dendrite layer. These positivities were thought to be the extracellular correlates of the Cl-nondependent hyperpolarization of the pyramidal cell, because they were the largest in magnitude in the apical dendrite layer. 3) The largest interictal field potential and supramaximal fornix-evoked field potential were identical with each other in waveform. Both potentials had one or two population spikes at the onset (CA3-CA4) or at the early part (CA1-CA2). This indicated that the synchronous discharges of pyramidal cells were instrumental in generating the large interictal hyperpolarization through synchronous activation of recurrent inhibitory circuits.