Selective Activation of Subthalamic Nucleus Output Quantitatively Scales Movements.

The subthalamic nucleus (STN) is a common target for deep brain stimulation (DBS) treatments of Parkinsonian motor symptoms. According to the dominant model, the STN output can suppress movement by enhancing inhibitory basal ganglia (BG) output via the indirect pathway, and disrupting STN output using DBS can restore movement in Parkinson's patients. But the mechanisms underlying STN DBS remain poorly understood, as previous studies usually relied on electrical stimulation, which cannot selectively target STN output neurons. Here, we selectively stimulated STN projection neurons using optogenetics and quantified behavior in male and female mice using 3D motion capture. STN stimulation resulted in movements with short latencies (10-15 ms). A single pulse of light was sufficient to generate movement, and there was a highly linear relationship between stimulation frequency and kinematic measures. Unilateral stimulation caused movement in the ipsiversive direction (toward the side of stimulation) and quantitatively determined head yaw and head roll, while stimulation of either STN raises the head (pitch). Bilateral stimulation does not cause turning but raised the head twice as high as unilateral stimulation of either STN. Optogenetic stimulation increased the firing rate of STN neurons in a frequency-dependent manner, and the increased firing is responsible for stimulation-induced movements. Finally, stimulation of the STN's projection to the brainstem mesencephalic locomotor region was sufficient to reproduce the behavioral effects of STN stimulation. These results question the common assumption that the STN suppresses movement, and instead suggest that STN output can precisely specify action parameters via direct projections to the brainstem.