Lower serum levels of neurotrophic factors, such as brain-derived neurotrophic factor (BDNF) are often observed in patients with depression (Pisoni et al., 2018) and increased expression of neurotrophic factors, such as BDNF or glial cell-derived neurotrophic factor (GDNF), have been reported in multiple studies as a response to antidepressant treatment. Here, BDNF is linked to depressive disorder.