Facilitation of hippocampal CA3 pyramidal cell firing by electrical fields generated antidromically

In experiments on rats under urethane anaesthesia--in which the fimbria and hippocampal commissure had been cut previously to eliminate orthodromic inputs--the negative antidromic population spike evoked in CA3 by fimbrial stimulation was measured inside and outside 73 neurons in the stratum pyramidale. Subtraction of the extracellular from the intracellular records showed that on the average 39.2% (S.E. 1.93) of the extracellular population spikes appeared as a positive, depolarizing transmembrane potential. Similar measurements in the dendritic zone of CA3, where the extracellular antidromic population spike is positive, revealed a smaller and hyperpolarizing transmembrane potential, whereas presumed neuroglia showed no consistent transmembrane potential in either direction. Further tests demonstrated clear facilitation of individual pyramidal cell firing, synchronous with the antidromic population spike. These observations are consistent with the possibility that, owing to the unusually close packing and regular alignment of the pyramidal neurons, electrical field interactions in CA3 tend to promote synchronized mass discharges.