The visual cortico-striato-nigral pathway in the rat.

The organization of the visual cortico-striato-nigral pathway in the rat has been investigated in two sets of experiments using anterograde autoradiographic tracing techniques. First, in one group of animals, injections of tritiated amino acids were placed throughout the visual cortex to demonstrate the visual corticostriatal pathway. The results indicate that visual corticostriatal fibres terminate in a distinctive clustered pattern throughout the entire length of the ipsilateral dorsomedial striatum. The projection shows a longitudinal topographic organization with cortical loci projecting onto narrow longitudinal regions of the dorsomedial striatum. In addition to the ipsilateral projection, a substantially smaller contralateral visual corticostriatal projection was also demonstrated. In the second set of experiments, the visual corticorecipient region of the striatum demonstrated in the first set of experiments was injected with tritiated amino acids to demonstrate the "visual" striatonigral projection. The results indicate that striatonigral fibres from the dorsomedial striatum project throughout the rostrocaudal extent of the ventral region of the pars reticulata. As with the visual corticostriatal pathway, the projection shows a longitudinal topographic organization with striatal loci projecting onto longitudinal regions of the ventral pars reticulata; the most rostral regions of the dorsal striatum project to the most medial regions of the ventral pars reticulata and, likewise, successively more caudal regions of the dorsal striatum project to successively more lateral regions of the ventral pars reticulata. In addition to the main projection to the ventral pars reticulata, a second smaller component of the striatonigral pathway was traced to adjacent regions of the dorsal pars reticulata and ventral pars compacta. These findings provide evidence of a visual cortico-striato-nigral pathway to both the pars reticulata and pars compacta in the rat. It is suggested that the major projection onto the ventral pars reticulata may provide an input onto nigrotectal neurons and thereby complete an indirect visual corticotectal connection mediated via links in the basal ganglia.