Topographical relationship between the entorhinal cortex and the septotemporal axis of the dentate gyrus in rats: II. Cells projecting from lateral entorhinal subdivision

Abstract

Projections from the rat lateral entorhinal cortex (area 28‐l) to the dentate gyrus were traced and then interpreted according to a parcellation scheme that recognized four cytoarchitectonic subdivisions of area 28‐1: areas dorsolateral (dl), ventrolateral (vl), ventromedial (vm), and TR. Following lesions of area 28‐l, anterograde degeneration was traced with the Fink‐Heimer method. In parallel experiments iontophoretic injections of horseradish peroxidase (HRP) were made in the lateral perforant path terminal zone of the dentate molecular layer. Retrograde neuronal labeling patterns within area 28‐l were charted following dorsal, midseptotemporal (mid ST), and ventral dentate injections. In two additional cases HRP was deposited in the ventral subiculum. Lesions of area dl (which lies entirely on the posterolateral cortex) produced terminal degeneration that was confined to the dorsal one‐half of the dentate gyrus. Lesions involving primarily areas vl and vm (which lie on the posteroinferior cortex) caused a complementary pattern of degeneration; silver grains predominated in the ventral dentate gyrus. Injections of HRP into the outer dentate molecular layer labeled layer II neurons within area 28‐l. Deposits of HRP in the dorsal one‐third of the dentate gyrus labeled a rostrocaudal strip of neurons within the dorsal one‐third of area dl; no other subdivisions of area 28vl contained labeling. After mid‐ST deposits of HRP, a rostrocaudally oriented strip of labeled cells appeared in the ventral one‐third of area dl. Mid‐ST injections also labeled neurons in the caudolateral quadrant of area vl. Injection of HRP into the ventral dentate gyrus labeled neurons in the caudomedial quadrant of area vl as well as a few neurons in caudal area vm. No labeled cells were ever found in area dl following ventral dentate HRP deposits. Neurons within area TR were never retrogradely labeled from injections of HRP into the dentate perforant path zone. However, ventral subicular injections of HRP labeled a few cells in the posterior part of area TR, as well as hundreds of neurons throughout the rostrocaudal extent of area vl. The results indicate a highly organized innervation of the dentate gyrus by several subdivisions of area 28‐l. In area dl, rostrocaudal strips of layer II neurons innervate distinct segments of the dorsal ST axis. The posterior half of areas vl and vm innervates the ventral half of the ST axis; a lateromedial gradient there corresponds to increasingly ventral terminations along the dentate ST axis. Finally, in contrast to the other lateral entorhinal subdivisions area TR does not appear to innervate the distal dendrites of dentate granule cells. The implications of this organization for the study of functional diversity along the ST axis are considered.