expresses expresses medium spiny neuron direct pathway medium spiny neuron A GABAergic medium spiny neuron located in the striatum that gives rise to the direct basal ganglia pathway. It projects directly to the GPi and SNr, where it inhibits tonically active GABAergic output neurons to disinhibit thalamic and brainstem motor targets (Gerfen. 2023). In mice and humans, this cell selectively expresses the D1 dopamine receptor (DRD1), which couples to stimulatory G-proteins (Gs/Gαolf) to increase cAMP and activate PKA-dependent signaling cascades (Gerfen et al., 1990; Kebabian & Calne, 1979). Functionally, activation of this cell promotes selected motor actions by disinhibiting thalamic and midbrain motor centers (Mink, 1996; Cui et al., 2013). Despite the direct and indirect circuits are often described not only as functionally opposing but also as anatomically segregated there are several anatomical and functional studies that have challenged this notion of pure segregation based on extensive collateral projections between pathways, dual-projection neurons, simultaneous co-activation during behavior. dopamine 1 medium spiny neuron D(1A) dopamine receptor