Basal Forebrain Projections to the Retrosplenial and Cingulate Cortex in Rats

The basal forebrain (BF) plays a crucial role in modulating cortical activation through its widespread projections across the cortical mantle. Previous anatomical studies have demonstrated that each cortical region receives a specific projection from the BF. In this study, we examined BF cholinergic and non-cholinergic projections to the retrosplenial cortex (RSC) and anterior cingulate cortex (ACC) using two retrograde tracers, Fast Blue (FB) and Fluoro-Gold (FG), in combination with choline acetyltransferase (ChAT) immunostaining in rats. The RSC and ACC receive cholinergic and non-cholinergic projections mainly from the medial part of the horizontal limb of the diagonal band (HDB) and the vertical limb of the diagonal band (VDB). The main difference of BF projections to the RSC, ACC, and prelimbic cortex (PL) is that the ACC and PL receive projections from the rostral half of the medial globus pallidus (GP), whereas the RSC receives stronger non-cholinergic projections from the VDB and medial septum (MS). As the injection site shifts from rostral (PL) to caudal (RSC) through the ACC, the strong GP and weak MS/VDB projections of non-cholinergic neurons are reversed. Cholinergic projection neurons make up a similar proportion of the total projection neurons in both ACC (37%) and RSC (33%) injections. Double retrograde tracer injections in the RSC and ACC revealed a small number of double-labeled projection neurons in the MS/VDB and HDB. These findings indicate that the ACC and RSC receive both spatially overlapping and differential projections from the BF, with the cholinergic and non-cholinergic projections varying between BF subregions and different rostrocaudal cortical regions.