Hippocampal-entorhinal relationships: electrophysiological analysis of the ventral hippocampal projections to the ventral entorhinal cortex.

The hippocampal output to the ventral entorhinal area was studied in the guinea-pig by field potential analysis. Perforant path volleys, synaptically elicited by stimulation of dorsal psalterium fibers, were used to obtain activation of the lamellar circuit of the dorsal hippocampal formation and the subsequent activation, through intrahippocampal longitudinal connections, of pyramidal neurons in the ventral hippocampus. The latter activation was obtained by low-frequency (0.1-2.0/s) repetitive dorsal psalterium stimulation. A response occurred in the ventral entorhinal area only following low-frequency (0.1-2.0/s) repetitive stimulation. The ventral entorhinal response occurred both in the medial and lateral divisions of the ventral entorhinal area. It consisted of a negative wave with associated unit firing in all cellular layers of the medial and lateral ventral entorhinal area. The latency of the entorhinal response increased moving from the deep to the superficial layers. These findings suggest the generation of excitatory synaptic effects in temporal sequence in the deep and then in the superficial layers of the entorhinal cortex. The ventral entorhinal response showed longer latency and a higher threshold than the ventral hippocampal response, and was eliminated by interruption of the caudally directed ventral hippocampal projections. These data suggest that the ventral entorhinal response was mediated by projections from the ventral hippocampus. The results show that the ventral hippocampal output evokes excitatory synaptic effects in all cellular layers of the medial and the lateral ventral entorhinal area. The massive involvement of the entorhinal area, together with the widespread distribution of the entorhinal projections, support the idea that the entorhinal cortex represents a crucial link between the hippocampus and the other brain regions.