BDNF and major depressive disorder: Currently, depression is understood to be fueled by a complex interplay of various molecular mechanisms, primarily including a neurotransmitter reduction, abnormally stressed hypothalamus–pituitary–adrenal (HPA) axis, a decline in brain-derived neurotrophic factor (BDNF) levels, increased pro-inflammatory intestinal responses, and interaction between the intestinal microbiota and the brain via the vagus nerve [11,12,13,14,15,16].