Reversal of hyperactive subthalamic circuits differentially mitigates pain hypersensitivity phenotypes in parkinsonian mice.

Although pain is a prevalent nonmotor symptom in Parkinson's disease (PD), it is undertreated, in part because of our limited understanding of the underlying mechanisms. Considering that the basal ganglia are implicated in pain sensation, and that their synaptic outputs are controlled by the subthalamic nucleus (STN), we hypothesized that the STN might play a critical role in parkinsonian pain hypersensitivity. To test this hypothesis, we established a unilateral parkinsonian mouse model with moderate lesions of dopaminergic neurons in the substantia nigra. The mice displayed pain hypersensitivity and neuronal hyperactivity in the ipsilesional STN and in central pain-processing nuclei. Optogenetic inhibition of STN neurons reversed pain hypersensitivity phenotypes in parkinsonian mice, while hyperactivity in the STN was sufficient to induce pain hypersensitivity in control mice. We further demonstrated that the STN differentially regulates thermal and mechanical pain thresholds through its projections to the substantia nigra pars reticulata (SNr) and the internal segment of the globus pallidus (GPi)/ventral pallidum (VP), respectively. Interestingly, optogenetic inhibition of STN-GPi/STN-VP and STN-SNr projections differentially elevated mechanical and thermal pain thresholds in parkinsonian mice. In summary, our results support the hypothesis that the STN and its divergent projections play critical roles in modulating pain processing under both physiological and parkinsonian conditions, and suggest that inhibition of individual STN projections may be a therapeutic strategy to relieve distinct pain phenotypes in PD.