Topography between the entorhinal cortex and the dentate septotemporal axis in rats: I. Medial and intermediate entorhinal projecting cells.

Retrograde tracing experiments were performed to clarify the topographic projection from medial (area 28m) and intermediate (area 28i) divisions of the entorhinal cortex to the dentate gyrus. Pipets filled with horseradish peroxidase (HRP) were positioned by electrophysiologic guidance at one of several septotemporal (S-T) levels in the dentate molecular layer of anesthetized rats; the tracer was expelled iontrophoretically to minimize its spread. Retrograde labeling of neurons within areas 28m and 28i was analyzed in relation to cytoarchitectonic as well as spatial features of the region (obtained by histologic reconstruction). Regardless of the S-T level, ejections of HRP which were confined to the dentate gyrus labeled only layer II neurons of each area. Following septal pole ejections, labeled neurons were located in the posterolateral, extreme posterior, and posteromedial parts of both areas 28m and 28i. Mid S-T ejections produced not only a ventral, but also an anteromedial, shift in the location of entorhinal projection cells; no cells were labeled posterolaterally. After temporal dentate ejections labeled neurons occupied the most anteromedial part of these entorhinal areas. For both areas, but especially for area 28i, convergence of entorhinal efferents upon a single S-T level in the dentate gyrus occurred from neurons which lay in a dorsoventral (i.e., frontal), and to a lesser extent a rostrocaudal, plane. The efferent axes of both areas 28m and 28i thus appear to be curved and are therefore best described in three dimensions. The entorhinal axes begin in a posterodorsolateral location, wrap around the posterior cortical convexity, and end in an anteroventromedial position. The results provide a useful map for in situ exploration of entorhinodentate connections in the rat, emphasize the parallel innervation of the dentate gyrus by distinct entorhinal fiber systems, and reflect the importance of the S-T axis as a framework for interpreting hippocampal organization.