BDNF and depressive symptom measurement: Neurobiologically, depression involves monoaminergic dysregulation, hypothalamic-pituitary-adrenal axis (HPA axis) hyperactivity, neuroinflammation, reduced neurotrophic signaling (e.g., serum brain-derived neurotrophic factor), and altered cortical–subcortical circuit function (Li et al., 2021; Plante, 2021).