BDNF and depressive disorder: The inflammatory hypothesis proposes that depression is caused by the interaction of inflammatory cytokine with the hypothalamic–pituitary–adrenal (HPA) axis, consequently affecting the synthesis and reuptake of neurotransmitters [10,11], which subsequently triggers glucocorticoid resistance, glutamate excitotoxicity, and the reduction of brain-derived neurotrophic factor (BDNF) expression [12].