117 Oxidative stress markers are also correlated with decreased telomerase activity.118 Further, diminished levels of antioxidants reportedly lower BDNF activity.119 Interestingly, antidepressants decrease oxidative stress.120 Since cellular oxidative damage may be an important component of the aging process, prolonged or repeated Inhibitors,research,lifescience,medical exposure to oxidative stress might accelerate aspects of biological aging and promote the development of aging-related diseases in depressed individuals.114 It is unknown whether antioxidant treatment would retard stress- or depressionrelated aging; this is discussed below under “novel treatment implications.” Brain-derived
neurotrophic factor The “neurotrophic model” of depression74 emphasizes the centrality of neurogenesis
and neuronal plasticity in the pathophysiology of depression. It posits that diminished hippocampal Inhibitors,research,lifescience,medical BDNF activity, caused by stress or excessive GCs, impairs the ability of stem cells in the subgranular zone of the dentate gyrus (as well Inhibitors,research,lifescience,medical as cells in the subventricular zone, projecting to the prefrontal cortex) to remain viable and to proliferate into mature cells. It is not known whether such effects can cause depression, but they may be relevant to the mechanism of action of antidepressant treatments.121 Unmedicated patients with depression have decreased hippocampal (at autopsy) and serum mTOR cancer concentrations of BDNF121,122 Over 20 studies have documented decreased serum concentrations of BDNF in unmedicated depressed individuals; this is now one of the most consistently replicated biochemical Inhibitors,research,lifescience,medical findings in major depression.121,123 Further, serum BDNF concentrations increase with antidepressant treatment.121,123 The relationship
of peripheral BDNF concentrations to central ones is Inhibitors,research,lifescience,medical not known, but even peripherally administered BDNF abrogates depressive and anxiety-like behaviors and increases hippocampal neurogenesis in mice, suggesting that serum BDNF concentrations are functionally significant for brain function and are more than merely a biomarker.124 A role of BDNF in antidepressant mechanisms of action is supported by findings that hippocampal Idoxuridine neurogenesis (in animals) and serum BDNF concentrations (in depressed humans) increase with antidepressant treatment,121,123 and that hippocampal neurogenesis and intact BDNF expression are required for behavioral effects of antidepressants in animals.125,126 Apart from its direct neurotrophic actions, BDNF also has anti-inflammatory and antioxidant effects that may contribute to its neuroprotective efficacy,127 and BDNF, in concert with telomerase (discussed below) promotes the growth of developing neurons.