Septal innervation of mossy cells in the hilus of the rat dentate gyrus: an anterograde tracing and intracellular labeling study.

Mossy cells in the hilus of the rat dentate gyrus are the main cells of origin of the dentate commissural and associational projections. They project along the septotemporal axis of the dentate gyrus and may thus influence the hippocampal signal flow in a longitudinal direction. To analyze the septal innervation of these hilar neurons, anterograde tracing with Phaseolus vulgaris leucoagglutinin (PHAL) was used in combination with intracellular labeling of mossy cells (Lucifer yellow). Anterogradely labeled septal fibers impinge on proximal and distal dendrites of hilar mossy cells but spare the cell body. In contrast, numerous aspiny hilar neurons, presumably GABAergic interneurons, receive a septal innervation on their somata and proximal primary dendrites. These data demonstrate that septal fibers show a specificity for the dendritic segments of hilar mossy cells. Since mossy cells project predominantly to adjacent hippocampal lamellae, the activity of adjacent portions of the dentate gyrus may be influenced by the septal input onto these neurons.