BDNF and major depressive disorder: Some proposed biomarkers of MDD include abnormal functional connectivity and activity in networks important to affective and cognitive processing, such as the anterior cingulate cortex, amygdala, and dorsolateral prefrontal cortex [4–6]; inflammatory cytokines and neuroinflammation, which are consistently elevated in depressed individuals [7, 8]; decreased levels of brain-derived neurotrophic factor (BDNF) [9–11], and dysregulation in the expression of a wide array of non-coding RNAs [12, 13].