VDR and major depressive disorder: Vitamin D receptors are widely present in brain regions associated with depression, such as the amygdala, substantia nigra, and hippocampus (40, 41) and vitamin D crosses the blood–brain barrier, activates vitamin D receptors, and plays a role in the control of behavior in humans (42), and its deficiency has been associated with a reduction in brain tissue and hippocampal volume in animals or humans (43).