Jennifer Warner‐Schmidt

The hippocampus is one of several limbic brain structures implicated in the pathophysiology and treatment of mood disorders. Preclinical and clinical studies demonstrate that stress and depression lead to reductions of the total volume of this structure and atrophy and loss of neurons in the adult hippocampus. One of the cellular mechanisms that could account for alterations of hippocampal structure as well as function is the regulation of adult neurogenesis. Stress exerts a profound effect on neurogenesis, leading to a rapid and prolonged decrease in the rate of cell proliferation in the adult hippocampus. In contrast, chronic antidepressant treatment up-regulates hippocampal neurogenesis, and could thereby block or reverse the atrophy and damage caused by stress. Recent studies also demonstrate that neurogenesis is required for the actions of antidepressants in behavioral models of depression. This review discusses the literature that has lead to a neurogenic hypothesis of depression and antidepressant action, as well as the molecular and cellular mechanisms that underlie the regulation of adult neurogenesis by stress and antidepressant treatment.

The neural mechanisms underlying the cellular and behavioral responses to antidepressants are not yet known. Up-regulation of growth factors and adult neurogenesis suggest a role for one or more of these factors in the action of antidepressants. One candidate of interest is vascular endothelial growth factor (VEGF), which was initially characterized for its role in angiogenesis, but also exerts direct mitogenic effects on neural progenitors in vitro. Results of this study demonstrate that VEGF is induced by multiple classes of antidepressants at time points consistent with the induction of cell proliferation and therapeutic action of these treatments. We find that VEGF signaling through the Flk-1 receptor is required for antidepressant-induced cell proliferation. We also show that VEGF-Flk-1 signaling is required and sufficient for behavioral responses in two chronic and two subchronic antidepressant models. Taken together, these studies identify VEGF and VEGF-Flk-1 signaling as mediators of antidepressant actions and potential targets for therapeutic intervention.

Antiinflammatory drugs achieve their therapeutic actions at least in part by regulation of cytokine formation. A "cytokine hypothesis" of depression is supported by the observation that depressed individuals have elevated plasma levels of certain cytokines compared with healthy controls. Here we investigated a possible interaction between antidepressant agents and antiinflammatory agents on antidepressant-induced behaviors and on p11, a biochemical marker of depressive-like states and antidepressant responses. We found that widely used antiinflammatory drugs antagonize both biochemical and behavioral responses to selective serotonin reuptake inhibitors (SSRIs). In contrast to the levels detected in serum, we found that frontal cortical levels of certain cytokines (e.g., TNFα and IFNγ) were increased by serotonergic antidepressants and that these effects were inhibited by antiinflammatory agents. The antagonistic effect of antiinflammatory agents on antidepressant-induced behaviors was confirmed by analysis of a dataset from a large-scale real-world human study, "sequenced treatment alternatives to relieve depression" (STAR*D), underscoring the clinical significance of our findings. Our data indicate that clinicians should carefully balance the therapeutic benefits of antiinflammatory agents versus the potentially negative consequences of antagonizing the therapeutic efficacy of antidepressant agents in patients suffering from depression.

All classes of antidepressants increase hippocampal cell proliferation and neurogenesis, which contributes, in part, to the behavioral actions of these treatments. Among antidepressant treatments, electroconvulsive seizure (ECS) is the most robust stimulator of hippocampal cell proliferation and the most efficacious treatment for depression, but the cellular mechanisms underlying the actions of ECS are unknown. To address this question, we investigated the effect of ECS on proliferation of neural stem-like and/or progenitor cells in the subgranular zone of rat dentate gyrus. We define the neural differentiation cascade from stem-like cells to early neural progenitors (also referred to as quiescent and amplifying neural progenitors, respectively) by coexpression of selective cellular and mitotic activity markers. We find that at an early mitotic phase ECS increases the proliferation of quiescent progenitors and then at a later phase increases the proliferation of amplifying progenitors. We further demonstrate that vascular endothelial growth factor (VEGF) signaling is necessary for ECS induction of quiescent neural progenitor cell proliferation and is sufficient to produce this effect. These findings demonstrate that ECS and subsequent induction of VEGF stimulates the proliferation of neural stem-like cells and neural progenitor cells, thereby accounting for the superior neurogenic actions of ECS compared with chemical antidepressants.