Deep brain stimulation (DBS) of the subthalamic nucleus (STN) and globus pallidus internus (GPi) is an established neuromodulation intervention for the treatment of Parkinson’s Disease (PD). Although highly successful, DBS of these nuclei can produce side effects due to their involvement as part of the complex hyperdirect pathway. The primary motor cortex (M1) projects to the STN and plays a role in PD. Improving characterization of M1 projections through the hyperdirect pathway could present potential new targets for improved selectivity and symptom relief in DBS for PD. In this study, we injected red fluorescent retrobeads into the STN of a healthy rat to retrogradely label its afferent projections. Tissue was sliced in 40 um sections, processed, and analyzed using an immunohistochemistry approach with DAPI staining, imaged at 20x magnification and quantified via fluorescent density. Retrobeads were found in a total of nineteen brain structures. The structures which appeared most often containing retrobeads across all twenty-four images were the ventral posteromedial thalamic nucleus (VPM), ventral posterolateral thalamic nucleus (VPL), zona incerta-ventral (ZIV), subthalamic nucleus (STN), internal capsule (IC), reticular nucleus of the thalamus (Rt), and the primary somatosensory cortex (S1). All other structures containing retrobeads had very limited fluorescence intensity. Contrary to expectations, no retrobead labeling was found in the M1. Additionally, the internal capsule showed the highest retrobead signals, suggesting a critical role in this pathway. The structure with the lowest retrobead signal was the basal nucleus of Meynert. While limited by the sample size, these preliminary findings support the complexity of the hyperdirect pathway and the important role of the internal capsule in the basal ganglia and hyperdirect pathway. Above all, these results highlight the potential of retrobead-based circuit mapping to re ne understanding of the STN’s afferents, ultimately improve DBS targeting strategies for PD.
