BDNF and major depressive disorder: In addition to the enhanced protein expression and gene transcription of BDNF, PF can also activate the expression of tropomyosin receptor kinase B (high-affinity receptor for BDNF) (Chen et al., 2019) and importantly can promote hippocampal neurogenesis and up-regulate BDNF levels and thus help to relieve the symptoms of depression (see Table 2).