BDNF and depressive symptom measurement: One of the most relevant is the neurotrophic/neuroplastic hypothesis of depression, which postulates that a reduction in neurotrophic factors (such as the brain-derived neurotrophic factor, BDNF), in brain areas such as the hippocampus and the prefrontal cortex, underlies the neuronal atrophy associated with this disease in preclinical [2,3,4] and clinical studies [5,6].