CAMK2A and infection: To verify whether rAAV9-Retro and rAAV2-Retro can retrogradely infect the same brain regions that project to the injection site and avoid the difference caused by injection site deviation or individual variation of animals, rAAV2-Retro-CaMKIIa-mCherry and rAAV9-Retro-CaMKIIa-EGFP viruses were mixed at the particle ratio of 1:1 and injected into the VTA at 300 nl (Fig. 3A), and local infection and projection regions were imaged using an Olympus VS120 Slide Scanner microscope at 21 days post-injection (DPI).