Patterns of direct projections from the hippocampus to the medial septum-diagonal band complex: anterograde tracing with Phaseolus vulgaris leucoagglutinin combined with immunohistochemistry of choline acetyltransferase.

The projections from the Ammon's horn to the cholinergic cell groups in the medial septal and diagonal band nuclei were investigated with anterograde tracing of Phaseolus vulgaris leucoagglutinin combined with immunocytochemical detection of choline acetyltransferase, in the rat. Tracer injections were placed into various fields of the septal and temporal parts of the Ammon's horn (CA1-3). These injections revealed differential distributions of Phaseolus vulgaris leucoagglutinin-labeled projections in both the lateral septal area and the medial septum-diagonal band complex. In addition to the labeling of dense axonal networks in the lateral septal area, significant numbers of arborizing fibers were labeled in the medial septal and diagonal band nuclei, in particular after tracer injections into the fields CA2-3. The distributions of the projections to the medial septum-diagonal band complex arising from the septal portion of fields CA1 and CA2-3 are similar. In contrast, the septal part and temporal portion of field CA3 project in a topographically differentiated manner to the medial septum and nuclei of the diagonal band. The septal pole of the Ammon's horn innervates the dorsal and medial parts of the medial septal nucleus and the anterior and dorsal parts of the vertical limb of the diagonal band. Axons of the temporal pole of the hippocampus reach the lateral and ventral parts of the medial septum and the intermediate, caudal and ventral parts of the vertical limb of the diagonal band. These results demonstrate direct feedback projections of the Ammon's horn to the medial septum-diagonal band complex, which show a topographic organization mainly as a function of the septotemporal level of the hippocampus. Within the medial septal and diagonal band nuclei, the labeled varicosities are formed in close proximity to the cell bodies and dendrites of the cholinergic neurons.