Distribution of nigrothalamic projections in the dog.

The distribution of nigrothalamic projections was studied in the dog by using the autoradiographic tracing method. On the basis of a systematic series of tritiated amino acid injections into different portions of the substantia nigra, we found that the rostral three-fourths of the substantia nigra pars reticulata (SNr) gives rise to widespread thalamic projections. The nigrothalamic label occupied a longitudinal band extending from rostral ventral anterior nucleus (VA) through to caudal mediodorsal nucleus (MD). In the dog, VA is histochemically identified as an acetylthiocholinesterase (AChE)-negative region and is distinct from the adjacent ventral lateral nuclei which stain positively for AChE. In rostral thalamus, the dense autoradiographic label was observed in rostral VA within the AchE-negative region lying alongside the mammillothalamic tract (MT). The adjacent AChE-positive ventral lateral nuclei did not contain autoradiographic label. In addition, homogeneously distributed silver grains were observed throughout the ventromedial nucleus (VM) bilaterally. More caudally, as the ventral lateral thalamic compartment emerges, label was also observed within the internal medullary lamina region, including the paralaminar portion of VA, the central lateral nucleus (CL), and the paralaminar portion of MD. Finally, in caudal thalamus, the central portion of MD, as well as the parafascicular nucleus (Pf), contained autoradiographic label. The overall nigrothalamic distribution observed in the dog was similar to the distribution of nigrothalamic projections in monkeys with one exception. Unlike that of primates, the distribution of nigral efferents is to the whole extent of VM in the dog as in other non-primate species. Overall, we found that nigral efferents primarily project to three main thalamic targets: the VA/MD region, VM, and the internal medullary lamina region, which includes dorsal CL and paralaminar VA and MD. We propose that these three nigrothalamic territories may constitute critical links subserving different functional channels.