SLC6A4 and major depressive disorder: For example, SERT knockout (SERT–/–) rats, which exhibit depression- and anxiety-like behaviors (Kalueff et al., 2010), show decreased mRNA expression of key glutamatergic substrates within the habenula, including GLT-1, NMDA subunits (GluN1, GluN2A, and GluN2B), and AMPA subunits (GluA1 and GluA2) (Caffino et al., 2019).