Cortical projection patterns of the medial septum-diagonal band complex

A detailed analysis of the cortical projections of the medial septum-diagonal band (MS/DB) complex was carried out by means of anterograde transport of Phaseolus vulgaris leucoagglutinin (PHA-L). The tracer was injected iontophoretically into cell groups of the medial septum (MS) and the vertical and horizontal limbs of the diagonal band of Broca (VDB and HDB), and sections were processed immunohistochemically for the intra-axonally transported PHA-L. The labeled efferents showed remarkable differences in regional distribution in the cortical mantle dependent on the position of the injection site in the MS/DB complex, revealing a topographic organization of the MS/DB-cortical projection. In brief, the lateral and intermediate aspects of the HDB, also referred to as the magnocellular preoptic area, predominantly project to the olfactory nuclei and the lateral entorhinal cortex. The medial part of the HDB and adjacent caudal (angular) part of the VDB are characterized by widespread, abundant projections to medial mesolimbic, occipital, and lateral entorhinal cortices, olfactory bulb, and dorsal aspects of the subicular and hippocampal areas. Projections from the rostromedial part of the VDB and from the MS are preponderantly aimed at the entire hippocampal and retrohippocampal regions and to a lesser degree at the medial mesolimbic cortex. Furthermore, the MS projections are subject to a clear mediolateral topographic arrangement, such that the lateral MS predominantly projects to the ventral/temporal aspects of the subicular complex and hippocampus and to the medial portion of the entorhinal cortex, whereas more medially located cells in the MS innervate more septal/dorsal parts of the hippocampal and subicular areas and more lateral parts of the entorhinal cortex. PHA-L filled axons have been observed to course through a number of pathways, i.e., the fimbria-fornix system, supracallosal stria, olfactory peduncle, and lateral piriform route (the latter two mainly by the HDB and caudal VDB). Generally, labeled projections were distributed throughout all cortical layers, although clear patterns of lamination were present in several target areas. The richly branching fibers were abundantly provided with both "boutons en passant" and terminal boutons. Both distribution and morphology of the labeled basal forebrain efferents in the prefrontal, cingulate, and occipital cortices closely resemble the distribution and morphology of the cholinergic innervation as revealed by immunohistochemical demonstration of choline acetyltransferase. In contrast, the labeled projections to the olfactory, hippocampal, subicular, and entorhinal areas showed a heterogeneous morphology. Here, the distribution of only the thin varicose projections resembled the distribution of cholinergic fibers.