Topographic projections to the visual cortex from the basal forebrain in the rat.

Recent studies have shown that cholinergic neurons within the basal forebrain give rise to a projection that terminates throughout the neocortex. The purpose of the present study is to determine the topographic organization of the basal forebrain projection to a single cortical region: the visual cortex. Injections of wheat germ agglutinin-horseradish peroxidase were placed within the infragranular layers of areas 17, 18a or 18b in Long-Evans hooded rats. Following injections placed within area 18a, retrogradely labeled neurons were located primarily within the caudal components of the basal forebrain including the basal nucleus of Meynert. Injections placed within area 18b, on the other hand, resulted in retrograde labeling of numerous neurons within rostral basal forebrain nuclei, including the horizontal limb of the diagonal band of Broca, whereas only a few labeled cells were located within the basal nucleus. Two patterns of labeled cells were evident following injections placed within area 17 and they resembled the results from injections within the adjacent extrastriate area; i.e. either area 18a or area 18b. Thus, injections restricted to the lateral portions of area 17 resulted in retrograde labeling of neurons located within the caudal levels of the basal forebrain, whereas injections within the medial portions of area 17 were followed by retrograde labeling of neurons within the rostral levels of the basal forebrain. Based on these results we suggest that the cholinergic corticopetal projection terminates within the visual cortex in a medial to lateral pattern.