Until now, human PET studies using TSPO ligands have suggested over microglial activation in a variety of psychiatric patients such as schizophrenia, at-risk stage of psychosis, autism and depression (van Berckel et al., 2008; Doorduin et al., 2009; Takano et al., 2010; Suzuki et al., 2013; Setiawan et al., 2015, 2018; Bloomfield et al., 2016). Here, TSPO is linked to major depressive disorder.