(A1–D1) Injection sites for fluorescent tracing experiments injecting FG into the DLS (A1; bregma +0.86 mm), DMS (B1; bregma +0.74 mm), lNAcc (C1; bregma +0.86 mm), mNacc (D1; bregma +1.54 mm). (Left images) merged TH-(green) and FG-(blue) signals. (Right images) FG mono. (A2–D2) locations of retrogradely traced DA neurons (accumulated from n = 3 animals) at a rostral (−3.16 mm) and a caudal (−3.64 mm) midbrain section, shown as colored heat maps. DLS-projecting DA neurons (A2) are shown in green, DMS-projecting DA neurons (B2) in cyan, lNAcc-projecting DA neurons (C2) in magenta, mNAcc-projecting DA neurons (D2) in orange. The border between lSN and mSN is indicated with a yellow dotted line. Note that DLS-projecting DA neurons are located throughout the mediolateral extent of the SN (A2) and that the mSN is a substantial midbrain source area for three mayor projection-defined systems (A2–C2). (E1) Local cell population matrix (%, average of three animals). Note that the lSN, PBP, PN and IF are populated mainly by one projection-defined population each (lSN - DLS, PBP - lNAcc, PN and IF - mNAcc). However, the mSN is a region of overlap of DLS-, DMS- and lNAcc- projecting DA neurons. (F1) Projection population matrix (%, average of three animals). Note that the DLS receives dopaminergic input from both SN source areas (lSN and mSN). On the other side, DMS mainly receives input from the mSN, lNAcc from PBP, mNAcc from PN. (E2) Shared sources and distinct targets. The mSN is a region of overlap of three different projection target-defined systems (DLS, DMS and lNAcc). The VTA harbors both the mNAcc and lNAcc projection system. (F2) Distinct sources and shared target. The DLS receives dopaminergic input from two distinct midbrain source areas (lSN and mSN).