BDNF and depressive symptom measurement: The same study highlighted the implication of the hippocampal BDNF in depression symptoms since the administration of a BDNF-binding protein (TrkB-IgG) reversed the antidepressant role of vitamin D. Although the exact mechanism underlying vitamin D levels with BDNF upregulation is unrevealed yet, however, evidence indicates an involvement of vitamin D receptor in the synthesis of neurotrophins including BDNF (Xu and Liang 2021).