Circuits that connect the cerebral cortex to the basal ganglia, and the connections back through thalamus to the cortex are a major feature of the mammalian forebrain. Each area of allo-, meso-or neocortex projects to a striatal or striatal-like subregion, which then communicates to downstream basal ganglia regions that ultimately connect to thalamo-cortical neurons (Gerfen 1992, Haber and Calzavara 2009). Within the basal ganglia, regions including the globus pallidus internal and external segments (GPi and GPe respectively), and the substantia nigra reticulata (SNr) process striatal output to provide feedback that ultimately influences cortical output. Monoaminergic transmission from midbrain and brainstem regions modulates circuit function mainly through actions in the striatal regions, but also at other subregions within the circuitry (Molliver 1987, Berridge, Stratford et al. 1997, Delfs, Zhu et al. 1998, Beier, Steinberg et al. 2015, Lerner, Shilyansky et al. 2015) . These networks interface with the large thalamic network via thalamo-striatal projections. The overall loop of the cortico-basal ganglia-cortical (C-BG-C) system can be separated into numerous subcircuits based on anatomical and functional divisions (Alexander and Crutcher 1990). The best
