Histochemical mapping of dopamine neurons and fiber pathways in dog mesencephalon

A topographic mapping of dopamine (DA)-containing neurons and fibers was done mainly in the mesencephalon of the dog using the fluorescent histochemical technique of Falck and Hillarp. The extensive DA neuron system was found to be located in the ventral and medial regions of the mesencephalon; the pars compacta of the substantia nigra, the area almost corresponding to the ventral tegmental area of Tsai (which consists of three groups, a caudal, the nucleus parabrachialis pigmentosus, a ventral, the nucleus paranigralis and a rostral, the caudal part of the nucleus tegmentalis ventralis of Tsai), the nucleus linearis of the raphe, and the mesencephalic reticular formation. The nigro-neostriatal projection can be traced in the non-treated or nialamide plus L-dopa treated puppies without the lesion-degeneration technique. Most fibers arising from these DA cell groups assemble at the prerubral area and ascend just dorsal to the medial forebrain bundle. Most fibers turn laterally at the lateral hypothalamus and enter the neostriatum via the dorsal part of the subthalamic nucleus, the zona incerta and the capsula interna. These findings show that the distribution of DA neurons and the nigro-neostriatal pathway are fundamentally similar to those in other mammals. In this study, the processes of the nigral and paranigral DA neurons have been demonstrated to project into the pars reticulata in the dog.