SLC6A4 and major depressive disorder: In those studies, we demonstrated decreased brain pH and increased lactate levels in various animal models of neuropsychiatric disorders, including those with face and construct validity for depressive disorder, such as SERT KO mice [19, 20], mice exposed to social defeat stress [21, 22], and mouse models of comorbid depression in diabetes mellitus and colitis [23, 24] (Fig. S4) [18].