Mice were anesthetized with isoflurane (4 mg/ml) supplemented in the air, while placed in a separate cage, and afterward, they were mounted in a stereotaxic frame (David Kopf Instruments, Tujunga, CA) equipped with a mouse adapter. Mice were kept anesthetized throughout the procedure by inhaling isoflurane at a concentration of 2 mg/ml. Anterograde tracing of the mDA neurons was achieved by injecting a Cre-dependent AAV expressing EGFP (AAV2/2.pCAG.FLEX.EGFP.WPRE.bGH) (26 (link)) in the midbrain of 8-month-old mice. Using bregma as a reference point, 1 μl of the AAV virus was injected unilaterally reaching the SN [anteroposterior (AP): −2.9, mediolateral (ML): −1.25, and dorsoventral (DV): −4.5] and 1 μl reaching the VTA (AP: −3.1, ML: −0.5, and DV: −4.2). All coordinates are in millimeters relative to bregma according to The Mouse Brain in Stereotaxic Coordinates (Academic Press, San Diego, CA, 2012). Virus injection was executed at a dispense rate of 0.2 μl/min. Animals were euthanized 3 weeks after surgery by CO2.