Entorhinal cortex of the rat: topographic organization of the cells of origin of the perforant path projection to the dentate gyrus.

By using three-dimensional computer reconstruction techniques and the production of two-dimensional unfolded maps, we analyzed the topographic organization of projections from the entorhinal cortex of the rat to the dentate gyrus. The retrograde tracers, Fast blue and Diamidino yellow, were injected at all septotemporal levels of the dentate gyrus, and the distribution of retrogradely labeled layer II cells in the entorhinal cortex was plotted by using computer-aided microscopy systems. Discrete injections of fluorescent dyes into the dentate gyrus labeled bands of layer II neurons in the entorhinal cortex that covered approximately 45% of its surface area. Injections confined to the septal half of the dentate gyrus resulted in a band that occupied the most lateral and caudomedial portions of the entorhinal cortex. Although there were subtle changes in the density of labeled cells in this region, essentially the same region of cells was labeled after any injection into the septal half of the dentate gyrus. Injections into mid-septotemporal levels of the dentate gyrus (50-75% of the distance from the septal pole) led to a distinctly different pattern of retrograde labeling. A more medial portion of the lateral entorhinal cortex and a more rostral portion of the medial entorhinal area were labeled in these cases. Another change in entorhinal labeling occurred when the injection involved the most temporal quarter of the dentate gyrus. Injections into this area led to a constrained region of entorhinal labeling that included the most medial portion of the lateral entorhinal area and the most rostral portion of the medial entorhinal area. Although the domains of cells projecting to septal, mid-septotemporal, and temporal levels of the dentate gyrus were not entirely segregated, there was relatively little overlap of the three populations of neurons. These data raise the possibility that different portions of the entorhinal-hippocampal circuit are capable of semiautonomous information processing, at least at the stage of input to the dentate gyrus.