Entorhinal cortex of the rat: organization of intrinsic connections.

Two sets of experiments were carried out to examine the organization of associational connections within the rat entorhinal cortex. First, a comprehensive analysis of the areal and laminar distribution of intrinsic projections was performed by using the anterograde tracers Phaseolus vulgaris-leuocoagglutinin (PHA-L) and biotinylated dextran amine (BDA). Second, retrograde tracers were injected into the dentate gyrus and PHA-L and BDA were injected into the entorhinal cortex to determine the extent to which entorhinal neurons that project to different septotemporal levels of the dentate gyrus are linked by intrinsic connections. The regional distribution of intrinsic projections within the entorhinal cortex was related to the location of the cells of origin along the mediolateral axis of the entorhinal cortex. Cells located in the lateral regions of the entorhinal cortex gave rise to intrinsic connections that largely remained within the lateral reaches of the entorhinal cortex, i.e., within the rostrocaudally situated entorhinal band of cells that projected to septal levels of the dentate gyrus. Cells located in the medial regions of the entorhinal cortex gave rise to intrinsic projections confined to the medial portion of the entorhinal cortex. Injections made into mid-mediolateral regions of the entorhinal cortex mainly gave rise to projections to mid-mediolateral levels, although some fibers did enter either lateral or medial portions of the entorhinal cortex. These patterns were the same regardless of whether the projections originated from the superficial (II-III) or deep (V-VI) layers of the entorhinal cortex. This organizational scheme indicates, and our combined retrograde/anterograde labeling studies confirmed, that laterally situated entorhinal neurons that project to septal levels of the dentate gyrus are not in direct communication with neurons projecting to the temporal portions of the dentate gyrus. These results suggest that entorhinal intrinsic connections allow for both integration (within a band) and segregation (across bands) of entorhinal cortical information processing.