Cortical projection patterns of magnocellular basal nucleus subdivisions as revealed by anterogradely transported Phaseolus vulgaris leucoagglutinin.

The present paper deals with a detailed analysis of cortical projections from the magnocellular basal nucleus (MBN) and horizontal limb of the diagonal band of Broca (HDB) in the rat. The MBN and HDB were injected iontophoretically with the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L). After immunocytochemical visualization of labeled efferents, the distribution of projections over the cortical mantle, olfactory regions and amygdala were studied by light microscopy. Based on differences in cortical projection patterns, the MBN was subdivided in anterior, intermediate and posterior portions (MBNa, MBNi and MBNp). All subdivisions maintain neocortical projections and are subject to an anterior to posterior topographic arrangement. In the overall pattern, however, the frontal cortex is the chief target. Furthermore, all MBN parts project to various regions of meso- and allocortex, which are progressively more dense when the tracer injection is more anteriorly placed. The most conspicuous finding, however, was a ventrolateral to dorsomedial cortical projection pattern as the PHA-L injection site moved from posterior to anterior. Thus, the posterior MBN projects predominantly to lateral neo- and mesocortex while the anterior MBN sends more fibers to the medial cortical regions. Furthermore, the MBNa is a source of considerable afferent input to the olfactory nuclei and as such should be regarded as a transition to the HDB. The HDB, apart from projecting densely to olfactory bulb and related nuclei, maintains a substantial output to the medial prefrontal cortical regions and entorhinal cortex, as well. Comparison of young vs aged cases indicate that aging does not appear to have a profound influence on cortical innervation patterns, at least as studied with the PHA-L method.