Additionally, experimental models of depression have found that vitamin D contributes to improved serotonergic metabolism in the brain, as it not only increases serotonin synthesis by induction of the TPH2 gene expression but influences the expression of serotonin reuptake transporter (SERT) and the levels of monoamine oxidase-A (MAO-A), responsible to serotonin catabolism. This evidence concerns the gene MAOA and major depressive disorder.