Evidence supports negative feedback from the 5-HT1A receptor in the synthesis and release of serotonin, which may be one of the reasons why Slit2-Tg mice exhibit anxiety- and depression-like behaviors (Frey et al., 2008) This hypothesis seems to be consistent with the result of a study reporting that transgenic mice expressing human mutant LRRK2 G2019S exhibited anxiety-/depression-like behavior, and a significant increase in 5-HT1A receptor level was detected in the hippocampus of transgenic mice (Lim et al., 2018). This evidence concerns the gene SLIT2 and major depressive disorder.