BDNF and major depressive disorder: Brain-derived neurotrophic factor (BDNF), a key neurotrophin involved in neuronal survival, synaptic plasticity, and neurogenesis, has been implicated in learning, memory, and the regulation of mood and emotional functioning, with alterations linked to depression and other psychiatric conditions (Castrén and Rantamäki, 2010; Autry and Monteggia, 2012).