Alterations of neuroplasticity in depression: the hippocampus and beyond

Early hypotheses on the pathophysiology of major depression were based on aberrant intrasynaptic concentrations of mainly the neurotransmitters serotonin and norepinephrine. However, recent neuroimaging studies have demonstrated selective structural changes across various limbic and nonlimbic circuits in the brains of depressed patients. In addition, postmortem morphometric studies revealed decreased glial and neuron densities in selected brain structures supporting the idea that major depression may be related to impairments of structural plasticity. Stressful life events are among the major predisposing risk factors for developing depression. Using the chronic psychosocial stress paradigm in male tree shrews, an animal model with a high validity for the pathophysiology of depressive disorders, we found that 1 month of stress reduced the in vivo concentrations of the brain metabolites N-acetyl-aspartate, choline-containing compounds, and (phospho)-creatine, as well as the proliferation rate in the dentate gyrus and the hippocampal volume. Even though long-lasting social conflict does not lead to a loss of principal cells, the hippocampal changes were accompanied by modifications in the incidence of apoptosis. Notably, these suppressive effects of social conflict on hippocampal structure could be counteracted by treatment with the antidepressant tianeptine. These findings support current theories proposing that major depressive disorders may be associated with impairment of structural plasticity and neural cellular resilience, and that antidepressants may act by correcting this dysfunction.