BDNF and post-traumatic stress disorder: Numerous biomarkers are listed and organized in the meta-data in the PTSD Biomarker Database [11], associated with the neurotransmitters dopamine, noradrenaline, and serotonin [14]; neurotrophic factors such as brain derived neurotrophic factor (BDNF) [15]; the hypothalamic-pituitary-adrenal (HPA) axis; and the immune system [16,17].