BDNF and major depressive disorder: Consequently, BDNF crucially regulates learning and memory processes in young and adult mammals (see, e.g., Boschen and Klintsova 2017; Gomez-Pinilla and Vaynman 2005) such that imbalances in BDNF levels and downstream signaling via its cognate TrkB tyrosine kinase receptor are associated with neurodegenerative and psychiatric diseases, like Alzheimer’s disease, major depressive disorder (Castren and Hen 2013), or schizophrenia (Mohammadi et al. 2018).