Hippocampal output to the subicular cortex: an electrophysiological study.

The hippocampal output to the subicular cortex was studied in the guinea pig in experiments of field potential analysis. Perforant path volleys, synaptically elicited by stimulation of the dorsal hippocampal commissure, were used to obtain the activation of pyramidal neurons in the lamellar circuits of the dorsal hippocampus and the consequent activation of pyramidal neurons in the ventral hippocampus. Discharge of the pyramidal neurons was followed by excitatory synaptic effects consisting of neuron depolarization and discharge throughout the ipsilateral subiculum. The laminar site of generation of these effects shifted from the deep to the superficial layers going from the dorsal to the ventral subiculum. All dorsoventral levels of the subiculum were activated by impulses coming from the corresponding hippocampal segments. Field CA3 appeared to be solely responsible for the subiculum activation. The data provide physiologic demonstration of powerful segmentally organized hippocampus-subiculum connections and suggest that the lamellar circuit should be extended to include the subiculum as a further element funneling the hippocampal output.