The primary output structures of the basal ganglia are the globus pallidus interna (GPi) and substantia nigra reticulata (SNr), though the SNr is thought to play a more prominent role in rodents.  The GABAergic neurons of these nuclei integrate the activity of the direct and indirect striatal pathways to regulate important motor regions including the thalamus.  Combining neurophysiology with optogenetic and chemogenetic approaches, we are measuring how the activity of SNr changes in Parkinson’s disease and levodopa-induced dyskinesia models, how the direct modulation of SNr neurons alters the associated behavioral abnormalities, and how modulation of SNr neurons changes the activity of SNr target brain regions.