BDNF and depressive disorder: Biological mechanisms may include [64,82,148–155] down-regulation of the hypothalamic-pituitary-adrenal axis and sympathetic nervous system, increase in vagal tone with activation of the parasympathetic nervous system, increase in central nervous system neurotransmitters such as serotonin and gamma-aminobutyric acid (GABA), increase in brain-derived neurotropic factor (BDNF), promotion of cerebral alpha wave activity, and increase in telomerase activity with mitigation of depression-related cellular decline.