Evidence for a dopaminergic innervation of the subthalamic nucleus in the rat

The dopaminergic connection from the substantia nigra pars compacta (SNc) and the ventral tegmental area (VTA) to the subthalamic nucleus in the rat was investigated using anterograde and retrograde tracers. Iontophoretic injection of the retrograde tracer fluoro-gold (FG) into the subthalamic nucleus resulted in a substantial number of labeled neurons in the SNc. Immunohistochemistry of tyrosine hydroxylase (TH) confirmed the dopaminergic nature of these labeled neurons. Retrogradely labeled neurons were also found in the VTA. Injection of the anterograde tracer biocytin into the SNc produced biocytin-labeled terminals in the subthalamic nucleus hence providing clear evidence for a dopaminergic innervation of this nucleus. Quantitative analysis of labeled axons revealed that there were 15-38 terminal branches per axon, each branch being 50-150 microm long. The overall dimensions of one terminal arborization were 400 x 250 x 150 microm. There was no clear-cut topographical organization of the projection, but a slight mediolateral difference in the density of terminals. This direct dopaminergic projection is thought to interact with cortical and pallidal inputs in the subthalamic nucleus, which implies that the functions of the subthalamic nucleus are more complex than previously assumed.