VDR and major depressive disorder: Moreover, VDR was found in neurons and glia in many regions of the brain (prefrontal cortex, substantia nigra, cingulate cortex, hippocampus, and hypothalamus) that may play a role in the pathophysiology of depression [8], and vitamin D is biologically suggested to not only be involved in the synthesis of neurotransmitters such as serotonin, dopamine, adrenalin, and noradrenalin through VDR but also to moderate the hypothalamic–pituitary–adrenal axis (HPA) and γ-aminobutyric acid A (GABA-A) receptor activity [56].