BDNF and depressive disorder: Similarly, Xu and Liang (2021) showed that Vitamin D3/VDR signaling mitigates post-stroke depression by upregulating hippocampal BDNF [29], and Numakawa and Kajihara (2025) emphasized that impaired BDNF–TrkB signaling is a hallmark of depression, schizophrenia, and neurodegeneration [6].