TSPO and major depressive disorder: The hypothesis of a potential direct action of peripheral cytokines trespassing the blood brain barrier (BBB) to activate brain microglia (D'Mello et al., 2009) has spurred a number of imaging studies of neuroinflammation in the central nervous system (CNS) in MDD that have been conducted using positron emission tomography (PET) and radiotracers targeting the 18 kDa mitochondrial translocator protein (TSPO), a protein that is consistently upregulated in activated microglia (see review in (Mondelli et al., 2017)).