Convergent Synaptic Input From the Neostriatum and the Subthalamus Onto Identified Nigrothalamic Neurons in the Rat

The two major afferents of the substantia nigra pars reticulata are the subthalamic nucleus and the striatum. Stimulation of these afferents has opposing physiological effects on the output neurons of the substantia nigra pars reticulata. In order to better understand the role of these afferents in the flow of information through the basal ganglia and to better understand the ways in which they might interact, experiments have been performed to test the possibility that single‐output neurons of the substantia nigra pars reticulata receive convergent synaptic input from the subthalamic nucleus and the neostriatum. To address this, rats received iontophoretic deposits of the anterograde tracer Phaseolus vulgaris leucoagglutinin in the subthalamic nucleus, injections of the anterograde tracer biocytin in the neostriatum and injections of the retrograde tracer horseradish peroxidase conjugated to wheat‐germ agglutinin in the ventral medial nucleus of the thalamus. Following appropriate survival times the animals were perfusion‐fixed and sections of the substantia nigra were processed to reveal the transported tracers and prepared for electron microscopy. Light microscopic examination revealed that the substantia nigra contained rich plexuses of anterogradely labelled subthalamic and striatal terminals, as well as many retrogradely labelled nigrothalamic neurons. The anterogradely labelled terminals were often seen apposed to the retrogradely labelled neurons. In the electron microscope the subthalamic terminals were seen to form asymmetrical synaptic contacts (subthalamic type 1) with the identified nigrothalamic neurons as well as unlabelled perikarya and both proximal and distal dendrites. In confirmation of previous findings, the striatal terminals made symmetrical synaptic contact with the nigrothalamic neurons as well as unlabelled neurons. In areas of overlap between the two classes of terminals, identified nigrothalamic neurons and unlabelled nigral neurons were found to receive convergent synaptic input from the subthalamic nucleus and the neostriatum. In addition to the anterogradely labelled subthalamic terminals that formed asymmetrical synaptic specializations, a second, much rarer class was also observed (subthalamic type 2). These terminals were much larger and formed symmetrical synapses; several lines of evidence suggest that they originated not in the subthalamic nucleus but in the globus pallidus. These terminals were found to make synaptic contacts with identified nigrothalamic neurons and non‐labelled neurons and to form convergent synaptic contacts with subthalamic type 1 terminals and striatal terminals. It is concluded that the topographical and synaptic organization of the so‐called direct (striatum to substantia nigra pars reticulata) and indirect pathways (i.e. pathways involving the subthalamic nucleus andlor the globus pallidus) of information flow through the basal ganglia underlies the inhibition and excitation of the output neurons of the substantia nigra pars reticulata that occur following stimulation of the striatum.