Distribution of striatal afferents from the mesencephalon in the cat.

In a series of cats, various quantities of horseradish peroxidase were injected in different parts of the striatum to map out the extent and the topography of the afferent projections from the mesencephalon. Following large injections in the head of the caudate nucleus and putamen, the majority of cells were retrogradely labeled in the substantia nigra pars compacta (SNc) and in its dorsal extension the retrorubral nucleus or pars dorsalis (SNd), confirming previous reports. Numerous HRP positive neurons were also present in the ventral tegmental area (VTA) and the rostral raphe nuclei, including the rostral linear (LR) and intrafascicular (IF) nuclei, forming a mediolateral continuum with SNc. More caudally, labeled cells were found in central linear (LC), superior central (CS) and dorsal raphe nuclei (DR), in the ventrolateral tegmentum and between the decussating fibers of the brachium conjunctivum. Small injections in different mediolateral, dorsoventral and anteroposterior sites of the neostriatum resulted in retrograde transport of HRP to a variety of cell groups in this striatal projecting system indicating a topographical organization. Efferents from SNc and SNd distribute in the striatum in a mediolateral, anteroposterior and to some extent in an inverse dorsoventral topographical pattern. The terminal fields in the striatum overlap considerably. The topographical relations are more evident mediolaterally than in the other principal planes. The projections of the raphe nuclei and VTA terminate in the nucleus accumbens and the ventral half of the striatum. Within this territory, they display a mediolateral topography. In accord with earlier reports, fibers from the dorsal raphe nuclei have a wider distribution. An inverse topographical relationship is also established between the VTA cells and the ventral parts of the striatum.