BDNF and major depressive disorder: Moreover, L. rhamnosus zz-1 alleviated depression-like behaviors in mice induced by chronic unpredictable mild stress (CUMS) improved hypothalamic–pituitary–adrenal (HPA) axis hyperactivity, and increased monoamine neurotransmitters, brain-derived neurotrophic factor (BDNF), and tyrosine kinase receptor B (TrkB) through regulation of gut microbiota, such as recovering the relative abundances of Lachnospiraceae NK4A136, Bacteroides as well as Muribaculum [79].