Chronic stress in the adult dentate gyrus reduces cell proliferation near the vasculature and VEGF and Flk-1 protein expression

Intracellular signaling pathways pave roads to recovery for mood disorders

Mood disorders, including major depression and bipolar disorder, remain a major unmet medical need as current antidepressant and mood stabilizing therapies require chronic treatment for efficacy and are not effective in all patients. Multiple deficits, including cell atrophy and loss, have been observed in limbic and cortical brain regions of patients with mood disorders and in stressed animals. It is thought that antidepressant and mood stabilizing medications restore these deficits by reestablishing proper patterns of gene expression and function. In support of this hypothesis, numerous changes in gene expression and activity have been observed in limbic and cortical brain regions of mood disorder patients, and thymoleptic therapies have been shown to reciprocally regulate many of these changes. These findings have implicated four main signaling pathways in the pathophysiology and/or treatment of mood disorders, namely the cyclic-AMP, phosphoinositol, mitogen-activated protein kinase, and glycogen synthase kinase signaling cascades. Below we review this literature, and discuss potential targets for novel antidepressant and mood stabilizing drug design that are highlighted by these findings.

Role of corticosteroid hormones in the dentate gyrus

Dentate granule cells are enriched with receptors for the stress hormone corticosterone, i.e., the high-affinity mineralocorticoid receptor (MR), which is already extensively occupied with low levels of the hormone, and the glucocorticoid receptor (GR), which is particularly activated after stress. More than any other cell type in the brain studied so far, dentate granule cells require hormone levels to be within the physiological range. In the absence of corticosteroids, proliferation and apoptotic cell death are dramatically enhanced. Dendritic morphology and synaptic transmission are compromised. Conversely, prolonged exposure of animals to a high level of corticosterone suppresses neurogenesis and presumably makes dentate granule cells more vulnerable to delayed cell death. These corticosteroid effects on dentate cell and network function are translated into behavioral consequences, in health and disease.

A model for the involvement of neural cell adhesion molecules in stress-related mood disorders

Critical interactions between genetic and environmental factors -- among which stress is one of the most potent non-genomic factors -- are involved in the development of mood disorders. Intensive work during the past decade has led to the proposal of the network hypothesis of depression [Castren E: Nat Rev Neurosci 2005;6:241-246]. In contrast to the earlier chemical hypothesis of depression that emphasized neurochemical imbalance as the cause of depression, the network hypothesis proposes that problems in information processing within relevant neural networks might underlie mood disorders. Clinical and preclinical evidence supporting this hypothesis are mainly based on observations from depressed patients and animal stress models indicating atrophy (with basic research pointing at structural remodeling and decreased neurogenesis as underlying mechanisms) and malfunctioning of the hippocampus and prefrontal cortex, as well as the ability of antidepressant treatments to have the opposite effects. A great research effort is devoted to identify the molecular mechanisms that are responsible for the network effects of depression and antidepressant actions, with a great deal of evidence pointing at a key role of neurotrophins (notably the brain-derived neurotrophic factor) and other growth factors. In this review, we present evidence that implicates alterations in the levels of the neural cell adhesion molecules of the immunoglobulin superfamily, NCAM and L1, among the mechanisms contributing to stress-related mood disorders and, potentially, in antidepressant action.

Aging does not alter the number or phenotype of putative stem/progenitor cells in the neurogenic region of the hippocampus

To investigate whether dramatically waned dentate neurogenesis during aging is linked to diminution in neural stem/progenitor cell (NSC) number, we counted cells immunopositive for Sox-2 (a putative marker of NSCs) in the subgranular zone (SGZ) of young, middle-aged and aged F344 rats. The young SGZ comprised approximately 50,000 Sox-2+ cells and this amount did not diminish with aging. Quantity of GFAP+ cells and vimentin+ radial glia also remained stable during aging in this region. Besides, in all age groups, analogous fractions of Sox-2+ cells expressed GFAP (astrocytes/NSCs), NG-2 (oligodendrocyte-progenitors/NSCs), vimentin (radial glia), S-100beta (astrocytes) and doublecortin (new neurons). Nevertheless, analyses of Sox-2+ cells with proliferative markers insinuated an increased quiescence of NSCs with aging. Moreover, the volume of rat-endothelial-cell-antigen-1+ capillaries (vascular-niches) within the SGZ exhibited an age-related decline, resulting in an increased expanse between NSCs and capillaries. Thus, decreased dentate neurogenesis during aging is not attributable to altered number or phenotype of NSCs. Instead, it appears to be an outcome of increased quiescence of NSCs due to changes in NSC milieu.

A neurotrophic model for stress-related mood disorders

There is a growing body of evidence demonstrating that stress decreases the expression of brain-derived neurotrophic factor (BDNF) in limbic structures that control mood and that antidepressant treatment reverses or blocks the effects of stress. Decreased levels of BDNF, as well as other neurotrophic factors, could contribute to the atrophy of certain limbic structures, including the hippocampus and prefrontal cortex that has been observed in depressed subjects. Conversely, the neurotrophic actions of antidepressants could reverse neuronal atrophy and cell loss and thereby contribute to the therapeutic actions of these treatments. This review provides a critical examination of the neurotrophic hypothesis of depression that has evolved from this work, including analysis of preclinical cellular (adult neurogenesis) and behavioral models of depression and antidepressant actions, as well as clinical neuroimaging and postmortem studies. Although there are some limitations, the results of these studies are consistent with the hypothesis that decreased expression of BDNF and possibly other growth factors contributes to depression and that upregulation of BDNF plays a role in the actions of antidepressant treatment.

The effect of chronic exposure to highly aggressive mice on hippocampal gene expression of non-aggressive subordinates

Exposure to a chronic psychosocial stressor changes the behavioral and neuroendocrine response pattern and causes structural changes in the rodent hippocampus. However, the underlying molecular mechanism of these changes induced by chronic stress is largely unknown. Recently, it was shown that exposure to a dominant highly aggressive mouse in the sensory contact model induced long-lasting stress symptoms in subordinate mice genetically selected for long attack latency (LAL mice). The aim of the present study was to study the effect of chronic stress on hippocampal gene expression in these subordinate LAL mice. GeneChips (Affymetrix) were used to compare gene expression profiles of LAL mice exposed to a sensory contact stressor for 25 days and their controls (one array per mouse, n=5 per line). After this stress paradigm, 131 genes were found differentially expressed (P<0.01). Strikingly, all of these genes showed a subtle downregulation in response to a chronic stressor. Interestingly, a significant overrepresentation of genes encoding structural components of ribosomes were found, suggesting diminished protein biosynthesis in the hippocampus of chronically stressed LAL mice. In addition, several genes of the NFkappaB signaling cascade, a pathway crucially involved in neuronal viability and neurite growth, were found to be downregulated. Together, we hypothesize that reduced NFkappaB signaling and diminished protein biosynthesis form part of the molecular mechanisms by which a chronic psychosocial stressor induces structural alterations in hippocampus of LAL mice.

Hippocampal neurogenesis: Opposing effects of stress and antidepressant treatment

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.

Neurotrophic factors and regulation of mood: role of exercise, diet and metabolism

Results from basic and clinical studies demonstrate that stress and depression decrease neurotrophic factor expression and neurogenesis in brain, and that antidepressant treatment blocks or reverses these effects, leading to a neurotrophic hypothesis of depression. Neurotrophic factor expression and neurogenesis are also decreased during aging and could be risk factors for depression. In contrast, exercise and enriched environment increase neurotrophic support and neurogenesis, which could contribute to the blockade of the effects of stress and aging and produce antidepressant effects. A brief overview of this work and the specific neurotrophic factors involved are discussed in this review.