Development of basal forebrain projections to visual cortex: DiI studies in rat.

We performed experiments using retrograde and anterograde labeling with DiI to examine the development of basal forebrain (BFB) projections to the visual cortex in postnatal rats. DiI placed in occipital cortex led to retrograde labeling of BFB neurons as early as postnatal day 0 (P0); labeled cells were found mainly in the diagonal band complex but also in the medial septum, globus pallidus, and substantia innominata. The retrogradely labeled BFB cells displayed remarkably well-developed dendritic arbors, even in younger animals, and showed increases in soma size, dendritic arbors, and dendritic spines over the first 2 postnatal weeks. DiI placements in the diagonal band led to anterogradely labeled axons in cortex. At early ages (P0-P1), labeled axons were largely confined to white matter. With increasing age, greater numbers of labeled axons were seen in the white matter and in deep cortical layers, and labeled axons extended into superficial layers. The leading edge of labeled fibers reached layer V of visual cortex by P2 and layer IV by P4 and were found throughout the cortical layers by P6. Numbers and densities of labeled axons in visual cortex were greater in older animals, at least through P14. The time of ingrowth of labeled BFB axons into visual cortex indicates that these afferents grow into particular cortical layers after those layers have differentiated from the cortical plate. These data indicate that basal forebrain projections arrive in occipital cortex after cortical lamination is well underway and after the entry of primary thalamocortical projections.