The ventral striatopallidal complex: an immunocytochemical analysis of medium-sized striatal neurons and striatopallidal fibers in the basal forebrain of the rat

The borders of the ventral striatum and ventral pallidum of the rat brain have been studied with immunocytochemistry for two protein markers that are present in these regions. One of these, DARPP-32 (dopamine and cyclic AMP-regulated phosphoprotein, Mr 32,000), is specifically enriched in medium-sized spiny neurons of the neostriatum and in the projection of these neurons upon pallidal regions. The second protein, synaptophysin, a marker for nerve terminals, effectively labels pallidal synapses. In the ventral striatum, neurons strongly immunoreactive for DARPP-32 were densely packed throughout its three components, i.e., the fundus striati and the nucleus accumbens septi, the olfactory tubercle, and the cell bridges that connect the tubercle with the overlying caudatoputamen and the nucleus accumbens. In the ventral pallidum, axons and axon terminals immunoreactive for DARPP-32 and axon terminals immunoreactive for synaptophysin were clearly delineated. As defined by these markers, the ventral pallidum was traced rostroventrally from the globus pallidus to the superficial layers of the olfactory tubercle, medially to the insula Calleja magna and the lateral septum, laterally to the pyriform cortex, and caudally to the anterior amygdaloid area. The ventral striatum and pallidum were densely intermingled in parts of the olfactory tubercle and medial forebrain bundle regions, and clearly separated in more caudal regions. The insulae Callejae did not contain typical striatal or pallidal staining patterns. Our results indicate that the ventral striatopallidal complex in the rat extends both rostrocaudally and dorsoventrally, in a highly complex, intermingled fashion, throughout most of the basal forebrain.