In mammals, TKs play roles as neuromodulators/cotransmitters in central brain circuits, as well as in pain, stress, anxiety, depressive disorder, aggression, memory formation, inflammation, cancer, immune function, gut function, hematopoiesis, sensory processing, reproduction and cytokine and hormone regulation [see (Otsuka and Yoshioka, 1993; Felipe et al., 1998; Hökfelt et al., 2001; Holsboer, 2009; Onaga, 2014; Steinhoff et al., 2014; Lénárd et al., 2018; Zieglgänsberger, 2019)]. Here, CDC42 is linked to depressive disorder.