Topographical projections from the medial septum-diagonal band complex to the hippocampus: a retrograde tracing study with multiple fluorescent dyes in rats.

Direct projections from the medial septum-diagonal band complex (MS-DB) to the hippocampus were studied in rats using the retrograde labeling method with fluorescent dyes. In the first series of experiments, the distribution patterns of retrogradely labeled cells in the MS-DB were examined in sagittal sections, following simultaneous injection of two different fluorescent dyes separately into the ventral hippocampus (VHPC) and the dorsal hippocampus (DHPC). It was found that the DHPC received fiber terminals mainly from the rostral half of the MS-vertical limb of the diagonal band (VDB) and the core part of the horizontal limb of the diagonal band (HDB), whereas the VHPC received fiber terminals largely from the caudal half part of the MS-VDB and the marginal part of the HDB. The marginal part appears to form a 'C' and not a complete circle around the core part. In the second series of experiments, the distribution patterns of retrogradely labeled cells in the MS-DB were investigated in sagittal sections, following simultaneous injection of three or two different fluorescent dyes separately into the subfields of the DHPC. The CA1 received fiber terminals mainly from the HDB and VDB, while the hilus received fiber terminals mainly from the MS. The CA2-CA3 received fiber terminals almost equally from the MS, VDB and HDB. The present results reveal distinctive topographical organizations of the septohippocampal connections.