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

Abstract

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.

Cortical efferents of the perirhinal, postrhinal, and entorhinal cortices of the rat

“…Labeled fibers arising from EC injection sites waser also analyzed with respect to the band of origin. The location of the bands were estimated based on the analyses and descriptions reported in the earlier study by Dolorfo and Amaral (1998b; 1998a). Labeled fibers were quantified for subregions within the piriform, frontal, insular, temporal, cingulate, parietal, and occipital cortices (see Nomenclature section and Figure 2).…”

“…The EC in rats and monkeys is organized into multiple bands that project to different septotemporal levels of the dentate gyrus (Witter, 1989; Witter et al, 1989b; Dolorfo and Amaral, 1998b; Dolorfo and Amaral, 1998a). Dolorfo and Amaral (1998b; 1998a) showed that the EC is organized into three bands that project to different septotemporal levels of the dentate gyrus (DG, Figure 1B).…”

“…Dolorfo and Amaral (1998b; 1998a) showed that the EC is organized into three bands that project to different septotemporal levels of the dentate gyrus (DG, Figure 1B). Each of the three bands spans both the LEA and MEA.…”

“…The location of each injection site in the EC was identified with respect to subregion (LEA or MEA) based on the local cytoarchitectonic features and with respect to the finer parcellation of Insausti et al (1997). Sites were further characterized as belonging to one of the dentate-projecting bands by referencing material to coronal sections shown by Dolorfo and Amaral (1998b; 1998a).…”

Distribution of calbindin D‐28k in the entorhinal, perirhinal, and parahippocampal cortices of the macaque monkey

“…The perirhinal and parahippocampal cortices have strong and largely reciprocal interconnections with the frontal, temporal, and parietal lobes as well as the cingulate, retrosplenial, and insular cortices (Van Hoesen et al, 1972;Martin-Elkins and Horel, 1992;Suzuki and Amaral, 1994a;Lavenex et al, 2001). The perirhinal and parahippocampal cortices also have strong and reciprocal projections with the entorhinal cortex Suzuki and Amaral, 1994b), the major cortical input of the hip-pocampus (Dolorfo and Amaral, 1998). Convergent findings from human neuropsychological studies (Scoville and Milner, 1957;Stefanacci et al, 2000) together with findings from lesion (Mishkin, 1978;Zola-Morgan and Squire, 1985) and neurophysiological (Miller et al, 1993;Suzuki et al, 1997) studies have shown that that entorhinal, perirhinal, and parahippocampal cortices, together with the hippocampus, are essential for normal declarative memory.…”