BDNF and depressive disorder: In recent years, brain-derived neurotrophic factor (BDNF), a neurotrophic growth factor, has been shown to be associated with several neuropsychiatric disorders and some CVDs, such as acute coronary syndrome (ACS) and stroke [22,23,24]; therefore, serum BDNF levels have been proposed as a reliable biomarker for depression [25,26] and a consistent parameter for predicting cardiovascular risk [27,28].