Notably, lower PKA expression and ADCY activity (Perez et al., 2001; Hines et al., 2005) in platelets, which express only Adcy3 but not other Adcy isoforms (Katsel et al., 2003), are detected in major depressive disorder subjects and attenuated by the use of various drugs including antidepressants, analgesics, and addictive drugs (Hines et al., 2005). This evidence concerns the gene ADCY3 and major depressive disorder.