Electrophysiological analysis of the hippocampal output to the presubiculum

The hippocampal output to the dorsal presubiculum was analysed in the guinea-pig by field potential analysis. Perforant path volleys, synaptically elicited by stimulation of the dorsal psalterium of one side, were used to activate the lamellar circuit of the same side and, through interhippocampal impulses, the hippocampal pyramidal neurons of the opposite side. The discharge of the pyramidal neurons of the hippocampus was followed by a negative fast wave with associated unit firing in lamina principalis interna and in the innermost part of lamina principalis externa of the presubicular cortex. These findings suggested the generation of excitatory synaptic effects in lamina principalis interna and innermost lamina principalis externa. The presubicular response was present in preparations with the fornix bilaterally sectioned. It was eliminated by interruption of the caudally-directed hippocampal projections, which suggested that it was mediated by these hippocampal efferents. The generation site of the presubicular response in deep cell laminae was not consistent with the terminal field of the hippocampus-presubiculum projections but was with that of the subiculum-presubiculum projections. The hippocampal discharge evoked in the dorsal subiculum a response with shorter latency and lower threshold than the presubicular response. These data suggest that the presubicular response might be evoked through hippocampus-subiculum-presubiculum connections. The results show that the hippocampal output evokes excitatory synaptic effects in the presubiculum and that these effects are segregated in deep cell laminae. The fact that these laminae give origin to an important projection to the anterior thalamic nuclei suggests that hippocampal impulses may be transferred to subcortical structures through the presubiculum.