VDR and major depressive disorder: The active metabolite of vitamin D - 1,25 dihydroxyvitamin D (1,25(OH)2D) - is synthesized in the brain by the enzyme 1α-hydroxylase [17], enabling local activation of vitamin D. Moreover, the vitamin D receptor (VDR) is present in several brain areas important for depression and emotional behaviour, including the hippocampus and hypothalamus [18].