NTRK2 and major depressive disorder: Furthermore, expression levels of BDNF, proBDNF, and its intrinsic receptor, TrkB, are reduced in the cerebella of patients with neuropsychiatric disorders, including schizophrenia, bipolar disorder (BPD), and major depressive disorder (MDD) as well as in a rodent model for ASD (Soontornniyomkij et al., 2011; Yang et al., 2017; Alò et al., 2021).