Activation of hippocampal neurons by mossy fiber stimulation in thin brain sections in vitro.

Abstract

1. Transmission through the synapse between the mossy fiber and the CA3 neurons was studied in vitro in thin transverse sections from the hippocampus of the guinea-pig. 2. Stimulation to the granular layer elicited in the pyramidal cell layer of the regio CA3 a mainly negative focal potential consisting of the early and late waves. Conditioning stimulation potentiated the late wave but not the early wave. The late wave was strongly suppressed by an excess of Mg++ or a reduction in Ca++ in the medium, but the early wave was affected only a little. 3. The action potentials, EPSP, and IPSP recorded intracellularly differed little from those observed in vivo except that most of the action potentials lacked the depolarizing afterpotential. The EPSP was markedly potentiated during low frequency stimulation. At nearly threshold strength, it showed a wide fluctuation in amplitude. 4. The latency and properties of the action potential triggered by the EPSP agreed with those of the late wave, which was therefore concluded to represent activation of the CA3 neurons by chemical transmission. 5. Corresponding to the early wave in the field potential, some cells generated action potentials before the onset of the EPSP. Probable explanations were offered for the activation of the CA3 neurons without preceding EPSPs.