Experiments were carried out to study the spatiotemporal organization of medial entorhinal inputs to the hippocampal system. They were performed in the isolated guinea pig brain in vitro preparation as it provides easy access to the medial entorhinal cortex (mEC) which is difficult to reach in vivo. Multiple simultaneous field potential recordings along the septotemporal extent of the dentate granular layer revealed that the mEC projection to the dentate gyrus (DG) is organized topographically. Thus, stimulation of the caudal regions of the mEC elicited population spikes (PSs) in the septa1 pole of the D G while successively more rostral stimulation sites activated progressively more temporal sectors of the DG. However, threshold mEC stimuli never elicited PSs over more than one-third of the DG.
In the CAI pyramidal layer, only trisynaptic PSs were evoked by the mEC stimulation (latency >20 ms at 30°C). However, PSs were widely distributed in the transverse and longitudinal axes of the hippocampus and, irrespective of the mEC stimulation site, the latency of CA1 PSs gradually increased from the CA3/CA1 border toward the subiculum. By contrast, in the longitudinal axis, each segment of the CA1 region responded at a shorter latency to stimulation of a given rostrocaudal level of the mEC. Septa1 CA1 levels responded at shorter latencies to caudal mEC stimulation sites while more temporal CA1 levels responded at shorter latencies to rostral mEC stimulation sites. When stimulated at threshold stimulation intensity, the initial CA1 response propagated to the rest of the CA1 field with a conduction velocity of 0.5-0.9 d s .
These results suggest that the associative pathways linking CA3 and CA1 regions can transmit mEC-elicited DG activities to large segments of the hippocampal formation while preserving the topographical relationship between the mEC and the hippocampus in the time domain. These findings are in contrast to the so-called lamellar hypothesis. 01994 Wiley-Liss, Inc.