(A) Representative images of axons in the GP, striatum, and STN originated from the primary motor cortex (M1), labeled with biotinylated dextran amine (BDA; visualized in black). Cortical axons in the GP are predominantly distributed in the calbindin (CB)-negative regions. CB is visualized in brown. Irrespective of differences in axon density among the GP, striatum, and STN, the morphology of the axons and varicosities is similar (A2). (B) Drawings and an image of cortical axons in the GP. The collaterals (thin black lines in B1) were issued from thick axon trunks (thick gray lines in B1; arrowhead in B2). (C) Magnified views of cortical axon varicosities (arrowheads) in the GP; left, M1 axons; right, M2 axons. The images are composites from multiple focal planes. The sections were counter-stained with an anti-parvalbumin (PV) antibody, visualized in brown. (D), Cortical axons in the GP were found exclusively in the ipsilateral hemisphere. M2 axons were visualized with AAV. Similar to axons in the STN, fluorescent signals were not detected in the contralateral GP even by over-exposure.