Hippocampal angiogenesis and progenitor cell proliferation are increased with antidepressant use in major depression

Chronically restricted or disrupted sleep as a causal factor in the development of depression

Sleep problems are a common complaint in the majority of people suffering from depression. While sleep complaints were traditionally seen as a symptom of mood disorders, accumulating evidence suggests that in many cases the relationship may be reverse as well. A long list of longitudinal studies shows that sleep complaints often precede the onset of depression and constitute an independent risk factor for the development of the disorder. Additionally, experimental studies in animals show that chronically restricted or disrupted sleep may gradually induce neurobiological changes that are very similar to what has been reported for depressed patients. The mechanisms through which insufficient sleep increases the risk for depression are poorly understood but may include effects of sleep disturbance on neuroendocrine stress systems, serotonergic neurotransmission, and various interacting signaling pathways involved in the regulation of neuronal plasticity and neurogenesis. Because sleep is considered to play a crucial role in regulating neuronal plasticity and synaptic strength, chronically insufficient sleep may contribute to depression through an impairment of these plasticity processes leading to altered connectivity and communication within and between brain regions involved in the regulation of mood.

Role of adult hippocampal neurogenesis in cognition in physiology and disease: pharmacological targets and biomarkers

Adult hippocampal neurogenesis is a remarkable form of brain structural plasticity by which new functional neurons are generated from adult neural stem cells/precursors. Although the precise role of this process remains elusive, adult hippocampal neurogenesis is important for learning and memory and it is affected in disease conditions associated with cognitive impairment, depression, and anxiety. Immature neurons in the adult brain exhibit an enhanced structural and synaptic plasticity during their maturation representing a unique population of neurons to mediate specific hippocampal function. Compelling preclinical evidence suggests that hippocampal neurogenesis is modulated by a broad range of physiological stimuli which are relevant in cognitive and emotional states. Moreover, multiple pharmacological interventions targeting cognition modulate adult hippocampal neurogenesis. In addition, recent genetic approaches have shown that promoting neurogenesis can positively modulate cognition associated with both physiology and disease. Thus the discovery of signaling pathways that enhance adult neurogenesis may lead to therapeutic strategies for improving memory loss due to aging or disease. This chapter endeavors to review the literature in the field, with particular focus on (1) the role of hippocampal neurogenesis in cognition in physiology and disease; (2) extrinsic and intrinsic signals that modulate hippocampal neurogenesis with a focus on pharmacological targets; and (3) efforts toward novel strategies pharmacologically targeting neurogenesis and identification of biomarkers of human neurogenesis.

Benzodiazepines and the potential trophic effect of antidepressants on dentate gyrus cells in mood disorders

Modest antidepressant response rates of mood disorders (MD) encourage benzodiazepine (BZD) co-medication with debatable benefit. Adult hippocampal neurogenesis may underlie antidepressant responses, but diazepam co-administration impairs murine neuron maturation and survival in response to fluoxetine. We counted neural progenitor cells (NPCs), mitotic cells, and mature granule neurons post-mortem in dentate gyrus (DG) from subjects with: untreated Diagnostic and Statistical Manual of Mental Disorders (DSM) IV MD (n = 17); antidepressant-treated MD (MD*ADT, n = 10); benzodiazepine-antidepressant-treated MD (MD*ADT*BZD, n = 7); no psychopathology or treatment (controls, n = 18). MD*ADT*BZD had fewer granule neurons vs. MD*ADT in anterior DG and vs. controls in mid DG, and did not differ from untreated-MD in any DG subregion. MD*ADT had more granule neurons than untreated-MD in anterior and mid DG and comparable granule neuron number to controls in all dentate subregions. Untreated-MD had fewer granule neurons than controls in anterior and mid DG, and did not differ from any other group in posterior DG. MD*ADT*BZD had fewer NPCs vs. MD*ADT in mid DG. MD*ADT had more NPCs vs. untreated-MD and controls in anterior and mid DG. MD*ADT*BZD and MD*ADT had more mitotic cells in anterior DG vs. controls and untreated-MD. There were no between-group differences in mid DG in mitotic cells or in posterior DG for any cell type. Our results in mid-dentate, and to some degree anterior dentate, gyrus are consistent with murine findings that benzodiazepines counteract antidepressant-induced increases in neurogenesis by interfering with progenitor proliferation. We also confirmed, in this expanded sample, our previous finding of granule neuron deficit in untreated MD.

Rodent models of treatment-resistant depression

Major depression is a prevalent and debilitating disorder and a substantial proportion of patients fail to reach remission following standard antidepressant pharmacological treatment. Limited efficacy with currently available antidepressant drugs highlights the need to develop more effective medications for treatment- resistant patients and emphasizes the importance of developing better preclinical models that focus on treatment- resistant populations. This review discusses methods to adapt and refine rodent behavioral models that are predictive of antidepressant efficacy to identify populations that show reduced responsiveness or are resistant to traditional antidepressants. Methods include separating antidepressant responders from non-responders, administering treatments that render animals resistant to traditional pharmacological treatments, and identifying genetic models that show antidepressant resistance. This review also examines pharmacological and non-pharmacological treatments regimes that have been effective in refractory patients and how some of these approaches have been used to validate animal models of treatment-resistant depression. The goals in developing rodent models of treatment-resistant depression are to understand the neurobiological mechanisms involved in antidepressant resistance and to develop valid models to test novel therapies that would be effective in patients that do not respond to traditional monoaminergic antidepressants.

A ventral view on antidepressant action: roles for adult hippocampal neurogenesis along the dorsoventral axis

Adult hippocampal neurogenesis is implicated in antidepressant action, stress responses, and cognitive functioning. The hippocampus is functionally segregated along its longitudinal axis into dorsal (dHi) and ventral (vHi) regions in rodents, and analogous posterior and anterior regions in primates, whereby the vHi preferentially regulates stress and anxiety, while the dHi preferentially regulates spatial learning and memory. Given the role of neurogenesis in functions preferentially regulated by the dHi or vHi, it is plausible that neurogenesis is preferentially regulated in either the dHi or vHi depending upon the stimulus. We appraise here the literature on the effects of stress and antidepressants on neurogenesis along the hippocampal longitudinal axis and explore whether preferential regulation of neurogenesis in the vHi/anterior hippocampus contributes to stress resilience and antidepressant action.

Basolateral amygdala volume and cell numbers in major depressive disorder: a postmortem stereological study

Functional imaging studies consistently report abnormal amygdala activity in major depressive disorder (MDD). Neuroanatomical correlates are less clear: imaging studies have produced mixed results on amygdala volume, and postmortem neuroanatomic studies have only examined cell densities in portions of the amygdala or its subregions in MDD. Here, we present a stereological analysis of the volume of, and the total number of, neurons, glia, and neurovascular (pericyte and endothelial) cells in the basolateral amygdala in MDD. Postmortem tissues from 13 subjects with MDD and 10 controls were examined. Sections (~15/subject) taken throughout the rostral-caudal extent of the basolateral amygdala (BLA) were stained for Nissl substance and utilized for stereological estimation of volume and cell numbers. Results indicate that depressed subjects had a larger lateral nucleus than controls and a greater number of total BLA neurovascular cells than controls. There were no differences in the number or density of neurons or glia between depressed and control subjects. These findings present a more detailed picture of BLA cellular anatomy in depression than has previously been available. Further studies are needed to determine whether the greater number of neurovascular cells in depressed subjects may be related to increased amygdala activity in depression.

Wnt signaling in neuropsychiatric disorders: ties with adult hippocampal neurogenesis and behavior

In an effort to better understand and treat mental disorders, the Wnt pathway and adult hippocampal neurogenesis have received increased attention in recent years. One is a signaling pathway regulating key aspects of embryonic patterning, cell specification and adult tissue homeostasis. The other is the generation of newborn neurons in adulthood that integrate into the neural circuit and function in learning and memory, and mood behavior. In this review, we discuss the growing relationship between Wnt signaling-mediated regulation of adult hippocampal neurogenesis as it applies to neuropsychiatric disorders. Evidence suggests dysfunctional Wnt signaling may aberrantly regulate new neuron development and cognitive function. Indeed, altered expression of key Wnt pathway components are observed in the hippocampus of patients suffering from neuropsychiatric disorders. Clinically-utilized mood stabilizers also proceed through modulation of Wnt signaling in the hippocampus, while Wnt pathway antagonists can regulate the antidepressant response. Here, we review the role of Wnt signaling in disease etiology and pathogenesis, regulation of adult neurogenesis and behavior, and the therapeutic targeting of disease symptoms.

Hippocampal GSK3β as a Molecular Link Between Obesity and Depression

Obesity is considered as a risk factor for mood disorders including depression. Nevertheless, the mechanisms underlying this association are not clearly understood. To address this issue, we investigated the impact of high-fat (HF)-diet-induced obesity on depressive-like behavior and on serotonin (5-HT)-dependent Akt/glycogen synthase kinase 3β (GSK3β) signaling in the dentate gyrus (DG) of the hippocampus, which has been associated with mood regulation. We first showed that a HF diet induced significant overweight and hyperglycemia as well as a depressive-like behavior in adult Wistar rats. By using an ex vivo approach on brain slices, we demonstrated that 5-HT activates the Akt/GSK3β cascade in the DG of control chow (C) diet-fed animals and that a 16-week HF diet feeding abolishes this activation, concurrently with a desensitization of leptin and insulin signaling in the same region. Furthermore, depressive-like behavior inversely correlated with 5-HT-induced phosphorylation of GSK3β in the subgranular neurons of the DG. Interestingly, a substitution of HF with C diet for 6 weeks induced a total loss of depressive symptoms, whereas body weight and glycemia remained significantly higher compared to control rats. In addition, food restoration led to a recovery of the Akt/GSK3β signaling pathway activation in the DG. In parallel, we observed a negative correlation between body weight and cell proliferation in the subgranular zone of the DG. To conclude, we provide evidence for a desensitization of 5-HT-induced Akt/GSK3β signaling and an impaired cell proliferation in the DG by HF diet, suggesting novel molecular mechanisms linking obesity to depression.

The role of serotonin in adult hippocampal neurogenesis

Serotonin is probably best known for its role in conveying a sense of contentedness and happiness. It is one of the most unique and pharmacologically complex monoamines in both the peripheral and central nervous system (CNS). Serotonin has become in focus of interest for the treatment of depression with multiple serotonin-mimetic and modulators of adult neurogenesis used clinically. Here we will take a broad view of serotonin from development to its physiological role as a neurotransmitter and its contribution to homeostasis of the adult rodent hippocampus. This chapter reflects the most significant findings on cellular and molecular mechanisms from neuroscientists in the field over the last two decades. We illustrate the action of serotonin by highlighting basic receptor targeting studies, and how receptors impact brain function. We give an overview of recent genetically modified mouse models that differ in serotonin availability and focus on the role of the monoamine in antidepressant response. We conclude with a synthesis of the most recent data surrounding the role of serotonin in activity and hippocampal neurogenesis. This synopsis sheds light on the mechanisms and potential therapeutic model by which serotonin plays a critical role in the maintenance of mood.

Identifying novel interventional strategies for psychiatric disorders: integrating genomics, 'enviromics' and gene-environment interactions in valid preclinical models

Psychiatric disorders affect a substantial proportion of the population worldwide. This high prevalence, combined with the chronicity of the disorders and the major social and economic impacts, creates a significant burden. As a result, an important priority is the development of novel and effective interventional strategies for reducing incidence rates and improving outcomes. This review explores the progress that has been made to date in establishing valid animal models of psychiatric disorders, while beginning to unravel the complex factors that may be contributing to the limitations of current methodological approaches. We propose some approaches for optimizing the validity of animal models and developing effective interventions. We use schizophrenia and autism spectrum disorders as examples of disorders for which development of valid preclinical models, and fully effective therapeutics, have proven particularly challenging. However, the conclusions have relevance to various other psychiatric conditions, including depression, anxiety and bipolar disorders. We address the key aspects of construct, face and predictive validity in animal models, incorporating genetic and environmental factors. Our understanding of psychiatric disorders is accelerating exponentially, revealing extraordinary levels of genetic complexity, heterogeneity and pleiotropy. The environmental factors contributing to individual, and multiple, disorders also exhibit breathtaking complexity, requiring systematic analysis to experimentally explore the environmental mediators and modulators which constitute the 'envirome' of each psychiatric disorder. Ultimately, genetic and environmental factors need to be integrated via animal models incorporating the spatiotemporal complexity of gene-environment interactions and experience-dependent plasticity, thus better recapitulating the dynamic nature of brain development, function and dysfunction.

Impact of social status and antidepressant treatment on neurogenesis in the baboon hippocampus

Adult hippocampal neurogenesis is critically implicated in rodent models of stress and anxiety as well as behavioral effects of antidepressants. Whereas similar factors such as psychiatric disorder and antidepressant administration are correlated with hippocampal volume in humans, the relationship between these factors and adult neurogenesis is less well understood. To better bridge the gap between rodent and human physiology, we examined the numbers of proliferating neural precursors and immature cells in the hippocampal dentate gyrus (DG) as well as in vivo magnetic resonance imaging (MRI)-estimated whole hippocampal volume in eight socially dominant- or subordinate-like (SL) baboons administered the antidepressant fluoxetine or vehicle. SL baboons had lower numbers of proliferating cells and immature neurons than socially dominant-like baboons. Fluoxetine treatment was associated with a larger whole hippocampal volume but surprisingly resulted in lower numbers of immature neurons. These findings are the first to indicate that adult neurogenesis in the baboon hippocampal DG may be functionally relevant in the context of social stress and mechanisms of antidepressant action.

Antidepressant-induced vascular dynamics in the hippocampus of adult mouse brain

New neurons are continuously added to hippocampal circuitry involved with spatial learning and memory throughout life. These new neurons originate from neural stem/progenitor cells (NSPCs) in the subgranular zone (SGZ) of the dentate gyrus (DG). Recent studies indicate that vascular reconstruction is closely connected with neurogenesis, but little is known about its mechanism. We have examined vascular reconstruction in the hippocampus of adult mouse brain after the administration of the antidepressant fluoxetine, a potent inducer of hippocampal neurogenesis. The immunohistochemistry of laminin and CD31 showed that filopodia of endothelial cells sprouted from existing thick microvessels and often formed a bridge between two thick microvessels. These filopodia were frequently seen at the molecular layer and dentate hilus of the DG, the stratum lacunosum-moleculare of the CA1, and the stratum oriens of the CA3. The filopodia were exclusively localized along cellular processes of astrocytes, but such intimate association was not seen with cell bodies and processes of NSPCs. The administration of fluoxetine significantly increased vascular density by enlarging the luminal size of microvessels and eliminating the filopodia of endothelial cells in the molecular layer and dentate hilus. Treatment with fluoxetine increased the number of proliferating NSPCs in the granule cell layer and dentate hilus, and that of endothelial cells in the granule cell layer. Thus, antidepressant-induced vascular dynamics in the DG are possibly attributable to the alteration of the luminal size of microvessels rather than to proliferation of endothelial cells.

Desvenlafaxine may accelerate neuronal maturation in the dentate gyri of adult male rats

Adult hippocampal neurogenesis has been linked to the effects of anti-depressant drugs on behavior in rodent models of depression. To explore this link further, we tested whether the serotonin-norepinephrine reuptake inhibitor (SNRI) venlafaxine impacted adult hippocampal neurogenesis differently than its primary active SNRI metabolite desvenlafaxine. Adult male Long Evans rats (n = 5-6 per group) were fed vehicle, venlafaxine (0.5 or 5 mg) or desvenlafaxine (0.5 or 5 mg) twice daily for 16 days. Beginning the third day of drug treatment, the rats were given a daily bromodeoxyuridine (BrdU; 50 mg/kg) injection for 5 days to label dividing cells and then perfused 2 weeks after the first BrdU injection to confirm total new hippocampal cell numbers and their phenotypes. The high desvenlafaxine dose increased total new BrdU+ cell number and appeared to accelerate neuronal maturation because fewer BrdU+ cells expressed maturing neuronal phenotypes and more expressed mature neuronal phenotypes in the dentate gyri of these versus vehicle-treated rats. While net neurogenesis was not increased in the dentate gyri of rats treated with the high desvenlafaxine dose, significantly more mature neurons were detected. Our data expand the body of literature showing that antidepressants impact adult neurogenesis by stimulating NPC proliferation and perhaps the survival of neuronal progeny and by showing that a high dose of the SNRI antidepressant desvenlafaxine, but neither a high nor low venlafaxine dose, may also accelerate neuronal maturation in the adult rat hippocampus. These data support the hypothesis that hippocampal neurogenesis may indeed serve as a biomarker of depression and the effects of antidepressant treatment, and may be informative for developing novel fast-acting antidepressant strategies.

Induction of depressive-like effects by subchronic exposure to cocaine or heroin in laboratory rats

The effect of psychoactive drugs on depression has usually been studied in cases of prolonged drug addiction and/or withdrawal, without much emphasis on the effects of subchronic or recreational drug use. To address this issue, we exposed laboratory rats to subchronic regimens of heroin or cocaine and tested long-term effects on (i) depressive-like behaviors, (ii) brain-derived neurotrophic factor (BDNF) levels in reward-related brain regions, and (iii) depressive-like behavior following an additional chronic mild stress procedure. The long-term effect of subchronic cocaine exposure was a general reduction in locomotor activity whereas heroin exposure induced a more specific increase in immobility during the forced swim test. Both cocaine and heroin exposure induced alterations in BDNF levels that are similar to those observed in several animal models of depression. Finally, both cocaine and heroin exposure significantly enhanced the anhedonic effect of chronic mild stress. These results suggest that subchronic drug exposure induces depressive-like behavior which is accompanied by modifications in BDNF expression and increases the vulnerability to develop depressive-like behavior following chronic stress. Implications for recreational and small-scale drug users are discussed. In the present study, we examined the long-term effects of limited subchronic drug exposure on depressive-like symptoms. Our results demonstrate that short-term, subchronic administration of either cocaine or heroin promotes some depressive-like behaviors, while inducing alterations in BDNF protein levels similar to alterations observed in several animal models of depression. In addition, subchronic cocaine or heroin enhanced the anhedonic effect of chronic stress.

Hippocampal neurogenesis and antidepressive therapy: shocking relations

Speculations on the involvement of hippocampal neurogenesis, a form of neuronal plasticity, in the aetiology of depression and the mode of action of antidepressive therapies, started to arise more than a decade ago. But still, conclusive evidence that adult neurogenesis contributes to antidepressive effects of pharmacological and physical therapies has not been generated yet. This review revisits recent findings on the close relation between the mode(s) of action of electroconvulsive therapy (ECT), a powerful intervention used as second-line treatment of major depression disorders, and the neurogenic response to ECT. Following application of electroconvulsive shocks, intricate interactions between neurogenesis, angiogenesis, and microglia activation, the hypothalamic-pituitary-adrenal axis and the secretion of neurotrophic factors have been documented. Furthermore, considering the fact that neurogenesis strongly diminishes along aging, we investigated the response to electroconvulsive shocks in young as well as in aged cohorts of mice.

Effects of chronic fluoxetine treatment on neurogenesis and tryptophan hydroxylase expression in adolescent and adult rats

The antidepressant drug fluoxetine (Prozac) has been increasingly prescribed to children and adolescents with depressive disorders despite a lack of thorough understanding of its therapeutic effects in the paediatric population and of its putative neurodevelopmental effects. Within the framework of PRIOMEDCHILD ERA-NET, we investigated; a) effects of chronic fluoxetine treatment on adult hippocampal neurogenesis, a structural readout relevant for antidepressant action and hippocampal development; b) effects on tryptophan hydroxylase (TPH) expression, a measure of serotonin synthesis; c) whether treatment effects during adolescence differed from treatment at an adult age, and d) whether they were subregion-specific. Stereological quantification of the number of proliferating (Ki-67+) cells and of the number of young migratory neurons (doublecortin+), revealed a significant age-by-treatment interaction effect, indicating that fluoxetine affects both proliferation and neurogenesis in adolescent-treated rats differently than it does in adult-treated rats. In terms of subregional differences, fluoxetine enhanced proliferation mainly in the dorsal parts of the hippocampus, and neurogenesis in both the suprapyramidal and infrapyramidal blades of the dentate gyrus in adolescent-treated rats, while no such differences were seen in adult-treated rats. Fluoxetine exerted similar age-by-treatment interaction effects on TPH cells mainly in the ventral portion of the dorsal raphe nucleus. We conclude that fluoxetine exerts divergent effects on structural plasticity and serotonin synthesis in adolescent versus adult-treated rats. These preliminary data indicate a differential sensitivity of the adolescent brain to this drug and thus warrant further research into their behavioural and translational aspects. Together with recent related findings, they further call for caution in prescribing these drugs to the adolescent population.

Adult neuroplasticity: more than 40 years of research

Within the last four decades, our view of the mature vertebrate brain has changed significantly. Today it is generally accepted that the adult brain is far from being fixed. A number of factors such as stress, adrenal and gonadal hormones, neurotransmitters, growth factors, certain drugs, environmental stimulation, learning, and aging change neuronal structures and functions. The processes that these factors may induce are morphological alterations in brain areas, changes in neuron morphology, network alterations including changes in neuronal connectivity, the generation of new neurons (neurogenesis), and neurobiochemical changes. Here we review several aspects of neuroplasticity and discuss the functional implications of the neuroplastic capacities of the adult and differentiated brain with reference to the history of their discovery.

Angiogenesis inhibition and depression in older men

BACKGROUND

Cardiovascular diseases have been associated with depression in later life, and a potential mechanism is inhibition of angiogenesis. We designed this study to determine if depression is associated with higher serum concentration of endostatin, an endogenous angiogenesis inhibitor.

METHODS

We performed a cross-sectional examination of a random sample of men aged 69-86 years. Those who scored 7 or higher on the 15-item Geriatric Depression Scale were deemed depressed. We determined the concentration of serum endostatin using a reproducible assay. Other measures included age, education, body mass index, smoking, history of depression, use of antidepressants, hypertension, diabetes, coronary heart disease and stroke, high sensitivity C-reactive protein, plasma homocysteine, triglycerides and cholesterol. We used logistic regression to investigate the association between endostatin and depression, and adjusted the analyses for confounding factors.

RESULTS

Our sample included 1109 men. Sixty-three (5.7%) men were depressed. Their serum endostatin was higher than that of nondepressed participants (p = 0.021). Men in the highest decile of endostatin had greater adjusted odds of depression (odds ratio [OR] 1.78, 95% confidence interval [CI] 1.03-3.06). A doubling of endostatin doubled the odds of depression (OR 1.93, 95% CI 1.31-2.84). The probability of depression increased with the concentration of endostatin in a log-linear fashion up to a maximum of about 20%-25%.

LIMITATIONS

The cross-sectional design limits the study's ability to ascribe causality to the association between high endostatin and depression.

CONCLUSION

Serum endostatin is associated with depression in older men. It remains to be established whether correction of this imbalance is feasible and could decrease the prevalence of depression in later life.

Fresh approaches to antidepressant drug discovery

INTRODUCTION

The success of antidepressant research has long been challenged by a limited mechanistic understanding of depression pathogenesis and antidepressant treatment response. Progress in this field has thereby consistently been hindered by a lack of novel conceptual approaches and sophisticated experimental techniques to dissect the highly intricate neurobiology of depression. Using fresh approaches to investigate the cellular and molecular mechanisms underlying depression will thus be vital for discovery of novel antidepressant targets.

AREAS COVERED

This article provides an overview of some fundamental problems that depression research is currently facing and critically evaluates how these issues could be addressed by future research. It also discusses novel conceptual and technological advances in the field of neuroscience, particularly in regard to how they may help in providing unprecedented insight into the molecular mechanisms of depression pathogenesis.

EXPERT OPINION

Although progress in antidepressant drug discovery has been limited over recent years, modern innovations in neuroscience, molecular biology, genetics and bioinformatics are just beginning to revolutionize depression research and to reveal novel and promising treatment targets. Integrating findings from a range of relevant experimental models and using the most advanced technology will be vital for the future success of antidepressant drug discovery.

Can pharmacological and psychological treatment change brain structure and function in PTSD? A systematic review

While there is evidence of clinical improvement of posttraumatic stress disorder (PTSD) with treatment, its neural underpinnings are insufficiently clear. Moreover, it is unknown whether similar neurophysiological changes occur in PTSD specifically after child abuse, given its enduring nature and the developmental vulnerability of the brain during childhood. We systematically reviewed PTSD treatment effect studies on structural and functional brain changes from PubMed, EMBASE, PsycINFO, PILOTS and the Cochrane Library. We included studies on adults with (partial) PTSD in Randomized Controlled Trials (RCT) or pre-post designs (excluding case studies) on pharmacotherapy and psychotherapy. Risk of bias was evaluated independently by two raters. Brain coordinates and effect sizes were standardized for comparability. We included 15 studies (6 RCTs, 9 pre-post), four of which were on child abuse. Results showed that pharmacotherapy improved structural abnormalities (i.e., increased hippocampus volume) in both adult-trauma and child abuse related PTSD (3 pre-post studies). Functional changes were found to distinguish between groups. Adult-trauma PTSD patients showed decreased amygdala and increased dorsolateral prefrontal activations post-treatment (4 RCTs, 5 pre-post studies). In one RCT, child abuse patients showed no changes in the amygdala, but decreased dorsolateral prefrontal, dorsal anterior cingulate and insula activation post-treatment. In conclusion, pharmacotherapy may reduce structural abnormalities in PTSD, while psychotherapy may decrease amygdala activity and increase prefrontal, dorsal anterior cingulate and hippocampus activations, that may relate to extinction learning and re-appraisal. There is some evidence for a distinct activation pattern in child abuse patients, which clearly awaits further empirical testing.

Functional dissociation of adult-born neurons along the dorsoventral axis of the dentate gyrus

Adult-born granule cells in the mammalian dentate gyrus have long been implicated in hippocampal dependent spatial learning and behavioral effects of chronic antidepressant treatment. Although recent anatomical and functional evidence indicates a dissociation of the dorsal and ventral regions of the hippocampus, it is not known if adult neurogenesis within each region specifically contributes to distinct functions or whether adult-born cells along the entire dorsoventral axis are required for these behaviors. We examined the role of distinct subpopulations of adult-born hippocampal granule cells in learning- and anxiety-related behaviors using low-dose focal x-irradiation directed specifically to the dorsal or ventral dentate gyrus. Our findings indicate a functional dissociation between adult-born neurons along the longitudinal axis of the dentate gyrus wherein new neurons in the dorsal dentate gyrus are required for timely acquisition of contextual discrimination while immature neurons in the ventral dentate gyrus are necessary for anxiolytic/antidepressant-related effects of fluoxetine. Interestingly, when contexts are presented with altered temporal cues, or fluoxetine is administered alongside chronic glucocorticoid treatment, this dissociation is abrogated such that adult-born neurons across the entire dorsoventral extent of the dentate gyrus appear to contribute to these behaviors. Our results suggest that individual subpopulations of adult-born hippocampal neurons may be sufficient to mediate distinct behaviors in certain conditions, but are required to act in concert in more challenging situations.

Consequences of cancer treatments on adult hippocampal neurogenesis: implications for cognitive function and depressive symptoms

The human brain is capable of generating new functional neurons throughout life, a phenomenon known as adult neurogenesis. The generation of new neurons is sustained throughout adulthood due to the proliferation and differentiation of adult neural stem cells. This process in humans is uniquely located in the subgranular zone of the dentate gyrus in the hippocampus. Adult hippocampal neurogenesis (AHN) is thought to play a major role in hippocampus-dependent functions, such as spatial awareness, long-term memory, emotionality, and mood. The overall aim of current treatments for cancer (such as radiotherapy and chemotherapy) is to prevent aberrant cell division of cell populations associated with malignancy. However, the treatments in question are absolutist in nature and hence inhibit all cell division. An unintended consequence of this cessation of cell division is the impairment of adult neural stem cell proliferation and AHN. Patients undergoing treatment for cancerous malignancies often display specific forms of memory deficits, as well as depressive symptoms. This review aims to discuss the effects of cancer treatments on AHN and propose a link between the inhibition of the neurogenetic process in the hippocampus and the advent of the cognitive and mood-based deficits observed in patients and animal models undergoing cancer therapies. Possible evidence for coadjuvant interventions aiming to protect neural cells, and subsequently the mood and cognitive functions they regulate, from the ablative effects of cancer treatment are discussed as potential clinical tools to improve mental health among cancer patients.

Adult hippocampal neurogenesis in depression: behavioral implications and regulation by the stress system

Adult hippocampal neurogenesis, the birth of new neurons in the dentate gyrus of the adult brain, can be regulated by stress and antidepressant treatment, and has consistently been implicated in the behavioral neurobiology of stress-related disorders, especially depression and anxiety. A reciprocal relationship between hippocampal neurogenesis and the hypothalamus-pituitary-adrenal (HPA) axis has recently been suggested, which may play a crucial role in the development and in the resolution of depressive symptoms. This chapter will review some of the existing evidence for stress- and antidepressant-induced changes in adult hippocampal neurogenesis, and critically evaluate the behavioral effects of these changes for depression and anxiety. The potential role of neurogenesis as a neurobiological mechanism for sustained remission from depressive symptoms will be discussed, integrating existing data from clinical studies, animal work, and cellular models. The effect of glucocorticoid hormones and the glucocorticoid receptor (GR) will thereby be evaluated as a central mechanism by which stress and antidepressant may exert their opposing effects on neurogenesis, and ultimately, on mood and behavior.

Age- and sex-dependent effects of early life stress on hippocampal neurogenesis

Early life stress is a well-documented risk factor for the development of psychopathology in genetically predisposed individuals. As it is hard to study how early life stress impacts human brain structure and function, various animal models have been developed to address this issue. The models discussed here reveal that perinatal stress in rodents exerts lasting effects on the stress system as well as on the structure and function of the brain. One of the structural parameters strongly affected by perinatal stress is adult hippocampal neurogenesis. Based on compiled literature data, we report that postnatal stress slightly enhances neurogenesis until the onset of puberty in male rats; when animals reach adulthood, neurogenesis is reduced as a consequence of perinatal stress. By contrast, female rats show a prominent reduction in neurogenesis prior to the onset of puberty, but this effect subsides when animals reach young adulthood. We further present preliminary data that transient treatment with a glucocorticoid receptor antagonist can normalize cell proliferation in maternally deprived female rats, while the compound had no effect in non-deprived rats. Taken together, the data show that neurogenesis is affected by early life stress in an age- and sex-dependent manner and that normalization may be possible during critical stages of brain development.

Neuropathology of stress

Environmental challenges are part of daily life for any individual. In fact, stress appears to be increasingly present in our modern, and demanding, industrialized society. Virtually every aspect of our body and brain can be influenced by stress and although its effects are partly mediated by powerful corticosteroid hormones that target the nervous system, relatively little is known about when, and how, the effects of stress shift from being beneficial and protective to becoming deleterious. Decades of stress research have provided valuable insights into whether stress can directly induce dysfunction and/or pathological alterations, which elements of stress exposure are responsible, and which structural substrates are involved. Using a broad definition of pathology, we here review the "neuropathology of stress" and focus on structural consequences of stress exposure for different regions of the rodent, primate and human brain. We discuss cytoarchitectural, neuropathological and structural plasticity measures as well as more recent neuroimaging techniques that allow direct monitoring of the spatiotemporal effects of stress and the role of different CNS structures in the regulation of the hypothalamic-pituitary-adrenal axis in human brain. We focus on the hypothalamus, hippocampus, amygdala, nucleus accumbens, prefrontal and orbitofrontal cortex, key brain regions that not only modulate emotions and cognition but also the response to stress itself, and discuss disorders like depression, post-traumatic stress disorder, Cushing syndrome and dementia.

Neurogenesis in neurological and psychiatric diseases and brain injury: from bench to bedside

Researchers who have uncovered the presence of stem cells in an adult's central nervous system have not only challenged the dogma that new neurons cannot be generated during adulthood, but also shed light on the etiology and disease mechanisms underlying many neurological and psychiatric disorders. Brain trauma, neurodegenerative diseases, and psychiatric disorders pose enormous burdens at both personal and societal levels. Although medications for these disorders are widely used, the treatment mechanisms underlying the illnesses remain largely elusive. In the past decade, an increasing amount of evidence indicate that adult neurogenesis (i.e. generating new CNS neurons during adulthood) may be involved in the pathology of different CNS disorders, and thus neurogenesis may be a potential target area for treatments. Although new neurons were shown to be a major player in mediating treatment efficacy of neurological and psychotropic drugs on cognitive functions, it is still debatable if the altered production of new neurons can cause the disorders. This review hence seeks to discuss pre and current clinical studies that demonstrate the functional impact adult neurogenesis have on neurological and psychiatric illnesses while examining the related underlying disease mechanisms.

Examination of age-dependent effects of fetal ethanol exposure on behavior, hippocampal cell counts, and doublecortin immunoreactivity in rats

Ethanol is known as a potent teratogen having adverse effects on brain and behavior. However, some of the behavioral deficits caused by fetal alcohol exposure and well expressed in juveniles ameliorate with maturation may suggest some kind of functional recovery occurring during postnatal development. The aim of this study was to reexamine age-dependent behavioral impairments in fetal-alcohol rats and to investigate the changes in neurogenesis and gross morphology of the hippocampus during a protracted postnatal period searching for developmental deficits and/or delays that would correlate with behavioral impairments in juveniles and for potential compensatory processes responsible for their amelioration in adults. Ethanol was delivered to the pregnant dams by intragastric intubation throughout 7-21 gestation days at daily dose of 6 g/kg. Isocaloric intubation and intact control groups were included. Locomotor activity, anxiety, and spatial learning tasks were applied to juvenile and young-adult rats from all groups. Unbiased stereological estimates of hippocampal volumes, the total number of pyramidal and granular cells, and double cortin expressing neurons were carried out for postnatal days (PDs) PD1, PD10, PD30, and PD60. Alcohol insult during second trimester equivalent caused significant deficits in the spatial learning in juvenile rats; however, its effect on hippocampal morphology was limited to a marginally lower number of granular cells in dentate gyrus (DG) on PD30. Thus, initial behavioral deficits and the following functional recovery in fetal-alcohol subjects may be due to more subtle plastic changes within the hippocampal formation but also in other structures of the extended hippocampal circuit. Further investigation is required.

Stress, serotonin, and hippocampal neurogenesis in relation to depression and antidepressant effects

Chronic stressful life events are risk factors for developing major depression, the pathophysiology of which is strongly linked to impairments in serotonin (5-HT) neurotransmission. Exposure to chronic unpredictable stress (CUS) has been found to induce depressive-like behaviours, including passive behavioural coping and anhedonia in animal models, along with many other affective, cognitive, and behavioural symptoms. The heterogeneity of these symptoms represents the plurality of corticolimbic structures involved in mood regulation that are adversely affected in the disorder. Chronic stress has also been shown to negatively regulate adult hippocampal neurogenesis, a phenomenon that is involved in antidepressant effects and regulates subsequent stress responses. Although there exists an enormous body of data on stress-induced alterations of 5-HT activity, there has not been extensive exploration of 5-HT adaptations occurring presynaptically or at the level of the raphe nuclei after exposure to CUS. Similarly, although hippocampal neurogenesis is known to be negatively regulated by stress and positively regulated by antidepressant treatment, the role of neurogenesis in mediating affective behaviour in the context of stress remains an active area of investigation. The goal of this review is to link the serotonergic and neurogenic hypotheses of depression and antidepressant effects in the context of stress. Specifically, chronic stress significantly attenuates 5-HT neurotransmission and 5-HT1A autoreceptor sensitivity, and this effect could represent an endophenotypic hallmark for mood disorders. In addition, by decreasing neurogenesis, CUS decreases hippocampal inhibition of the hypothalamic-pituitary-adrenal (HPA) axis, exacerbating stress axis overactivity. Similarly, we discuss the possibility that adult hippocampal neurogenesis mediates antidepressant effects via the ventral (in rodents; anterior in humans) hippocampus' influence on the HPA axis, and mechanisms by which antidepressants may reverse chronic stress-induced 5-HT and neurogenic changes. Although data are as yet equivocal, antidepressant modulation of 5-HT neurotransmission may well serve as one of the factors that could drive neurogenesis-dependent antidepressant effects through these stress regulation-related mechanisms.

Mechanisms of antidepressant resistance

Depression is one of the most frequent and severe mental disorder. Since the discovery of antidepressant (AD) properties of the imipramine and then after of other tricyclic compounds, several classes of psychotropic drugs have shown be effective in treating major depressive disorder (MDD). However, there is a wide range of variability in response to ADs that might lead to non response or partial response or in increased rate of relapse or recurrence. The mechanisms of response to AD therapy are poorly understood, and few biomarkers are available than can predict response to pharmacotherapy. Here, we will first review markers that can be used to predict response to pharmacotherapy, such as markers of drug metabolism or blood-brain barrier (BBB) function, the activity of specific brain areas or neurotransmitter systems, hormonal dysregulations or plasticity, and related molecular targets. We will describe both clinical and preclinical studies and describe factors that might affect the expression of these markers, including environmental or genetic factors and comorbidities. This information will permit us to suggest practical recommendations and innovative treatment strategies to improve therapeutic outcomes.

Adult neurogenesis in the mammalian hippocampus: why the dentate gyrus?

In the adult mammalian brain, newly generated neurons are continuously incorporated into two networks: interneurons born in the subventricular zone migrate to the olfactory bulb, whereas the dentate gyrus (DG) of the hippocampus integrates locally born principal neurons. That the rest of the mammalian brain loses significant neurogenic capacity after the perinatal period suggests that unique aspects of the structure and function of DG and olfactory bulb circuits allow them to benefit from the adult generation of neurons. In this review, we consider the distinctive features of the DG that may account for it being able to profit from this singular form of neural plasticity. Approaches to the problem of neurogenesis are grouped as "bottom-up," where the phenotype of adult-born granule cells is contrasted to that of mature developmentally born granule cells, and "top-down," where the impact of altering the amount of neurogenesis on behavior is examined. We end by considering the primary implications of these two approaches and future directions.

Sex, hormones and neurogenesis in the hippocampus: hormonal modulation of neurogenesis and potential functional implications

The hippocampus is an area of the brain that undergoes dramatic plasticity in response to experience and hormone exposure. The hippocampus retains the ability to produce new neurones in most mammalian species and is a structure that is targeted in a number of neurodegenerative and neuropsychiatric diseases, many of which are influenced by both sex and sex hormone exposure. Intriguingly, gonadal and adrenal hormones affect the structure and function of the hippocampus differently in males and females. Adult neurogenesis in the hippocampus is regulated by both gonadal and adrenal hormones in a sex- and experience-dependent way. Sex differences in the effects of steroid hormones to modulate hippocampal plasticity should not be completely unexpected because the physiology of males and females is different, with the most notable difference being that females gestate and nurse the offspring. Furthermore, reproductive experience (i.e. pregnancy and mothering) results in permanent changes to the maternal brain, including the hippocampus. This review outlines the ability of gonadal and stress hormones to modulate multiple aspects of neurogenesis (cell proliferation and cell survival) in both male and female rodents. The function of adult neurogenesis in the hippocampus is linked to spatial memory and depression, and the present review provides early evidence of the functional links between the hormonal modulation of neurogenesis that may contribute to the regulation of cognition and stress.

Increased hippocampal neurogenesis and p21 expression in depression: dependent on antidepressants, sex, age, and antipsychotic exposure

The mammalian hippocampus continues to generate new neurons throughout life. The function of adult-generated neurons remains controversial, but adult neurogenesis in the hippocampus is related to depression. Studies show that neurogenesis in the hippocampus is regulated by antidepressants in both humans and rodents, but no studies have examined the effects of age, sex, or antipsychotic exposure on the relationship between depression, antidepressant exposure, and hippocampal neurogenesis in humans. Hippocampal sections were obtained from the Stanley Medical Research Institute and were immunohistochemically labeled for the immature neuron marker doublecortin and the cell cycle arrest marker p21. We compared the number of cells in the granule cell layer and subgranular zone that expressed these proteins in brains from control subjects (n=12), patients with major depressive disorder (MDD) without psychotic symptoms (n=12), and patients with MDD and psychotic symptoms (n=12). We show here that the density of doublecortin/NeuN expression was increased in MDD patients compared with controls and MDD patients with psychosis, with the effect greater in women. Further, we show that older depressed patients without psychosis had higher levels of p21/NeuN expression and that depressed individuals prescribed antidepressants had higher levels of p21/NeuN expression, but only in older women. We show for the first time that changes in neurogenesis due to prescribed antidepressants or depression are dependent on age, sex, and the presence of antipsychotics or psychotic symptoms.

Huntingtin mediates anxiety/depression-related behaviors and hippocampal neurogenesis

Huntington disease (HD) is associated with early psychiatric symptoms including anxiety and depression. Here, we demonstrate that wild-type huntingtin, the protein mutated in HD, modulates anxiety/depression-related behaviors according to its phosphorylation at serines 1181 and 1201. Genetic phospho-ablation at serines 1181 and 1201 in mouse reduces basal levels of anxiety/depression-like behaviors. We observe that the reduction in anxiety/depression-like phenotypes is associated with increased adult hippocampal neurogenesis. By improving the attachment of molecular motors to microtubules, huntingtin dephosphorylation increases axonal transport of BDNF, a crucial factor for hippocampal adult neurogenesis. Consequently, the huntingtin-mediated increased BDNF dynamics lead to an increased delivery and signaling of hippocampal BDNF. These results support the notion that huntingtin participates in anxiety and depression-like behavior and is thus relevant to the etiology of mood disorders and anxiety/depression in HD.

VEGF and depression: a comprehensive assessment of clinical data

Vascular Endothelial Growth Factor (VEGF), initially identified as an angiogenic mitogen, is believed to play a role in hippocampal neurogenesis and response to stress. It exerts neuroprotective effects and influences synaptic transmission. The possible role of VEGF in depression has been hypothesized in the context of the neurotrophic model of depression, which postulates that stress can lead to decreased level of neurotrophins. Since VEGF has emerged as a potential component in the pathophysiology of stress and stress-related disorders, animal and clinical studies have attempted to delineate its precise role. In this review article we provide a synopsis of basic studies that are of direct relevance to the clinical findings in depression and antidepressant drug action. We have classified the studies on the basis of higher, lower or no different levels of VEGF as compared to control subjects. It became evident that there is conflicting data regarding VEGF levels in depressed patients. The fact that no definitive trend is apparent in the published data is likely attributable to differences in study designs. However, promising leads have emerged in our effort to understand and clarify this wide variation in results. Further study could establish the potential use of VEGF as a biomarker to aid in making a correct diagnosis and a successful treatment plan. Delineating the relationship of VEGF and depression ultimately has the potential to shed light on the still elusive neural mechanisms underlying the pathophysiology of depression and the mechanisms by which antidepressants exert their effects.

Structural changes in hippocampal subfields in major depressive disorder: a high-field magnetic resonance imaging study

BACKGROUND

Magnetic resonance imaging (MRI) has shown lower hippocampal volume in major depressive disorder (MDD). Preclinical and postmortem studies show that chronic stress and MDD may affect hippocampal subfields differently, but MRI spatial resolution has previously been insufficient to measure subfield volumes.

METHODS

Twenty MDD participants (9 unmedicated and 11 medicated, both > 6 months) and 27 healthy control subjects were studied. We used T2-weighted two-dimensional fast spin echo and T1-weighted three-dimensional magnetization prepared rapid acquisition gradient-echo sequences at 4.7 T to compare hippocampal subfield volumes at .09 μL voxel volume.

RESULTS

Unmedicated MDD participants had a lower dentate gyrus volume than control subjects or medicated MDD participants and a lower cornu ammonis (CA1-3) volume in the hippocampal body subregion than control subjects.

CONCLUSIONS

Hippocampal volumes in unmedicated MDD showed evidence of localization to specific subfields and subregions, findings that appear, on the surface, consistent with preclinical evidence for localized mechanisms of hippocampal neuroplasticity. Strengths include in vivo measurement of entire hippocampal subfields and separation between unmedicated and medicated MDD. Limitations include power to control for multiple comparisons and that MRI landmarks approximate the subfields defined by cellular microstructure.

Wnt signaling in the regulation of adult hippocampal neurogenesis

In the adult brain new neurons are continuously generated mainly in two regions, the subventricular zone (SVZ) of the lateral ventricles and the subgranular zone (SGZ) in the hippocampal dentate gyrus. In the SGZ, radial neural stem cells (NSCs) give rise to granule cells that integrate into the hippocampal circuitry and are relevant for the plasticity of the hippocampus. Loss of neurogenesis impairs learning and memory, suggesting that this process is important for adult hippocampal function. Adult neurogenesis is tightly regulated by multiple signaling pathways, including the canonical Wnt/β-catenin pathway. This pathway plays important roles during the development of neuronal circuits and in the adult brain it modulates synaptic transmission and plasticity. Here, we review current knowledge on the regulation of adult hippocampal neurogenesis by the Wnt/β-catenin signaling cascade and the potential mechanisms involved in this regulation. Also we discuss the evidence supporting that the canonical Wnt pathway is part of the signaling mechanisms involved in the regulation of neurogenesis in different physiological conditions. Finally, some unsolved questions regarding the Wnt-mediated regulation of neurogenesis are discussed.

Interplay between pro-inflammatory cytokines and growth factors in depressive illnesses

The development of depressive disorders had long been attributed to monoamine variations, and pharmacological treatment strategies likewise focused on methods of altering monoamine availability. However, the limited success achieved by treatments that altered these processes spurred the search for alternative mechanisms and treatments. Here we provide a brief overview concerning a possible role for pro-inflammatory cytokines and growth factors in major depression, as well as the possibility of targeting these factors in treating this disorder. The data suggest that focusing on one or another cytokine or growth factor might be counterproductive, especially as these factors may act sequentially or in parallel in affecting depressive disorders. It is also suggested that cytokines and growth factors might be useful biomarkers for individualized treatments of depressive illnesses.

Hippocampal granule neuron number and dentate gyrus volume in antidepressant-treated and untreated major depression

Smaller hippocampal volume is reported in major depressive disorder (MDD). We hypothesize that it may be related to fewer granule neurons (GN) in the dentate gyrus (DG), a defect possibly reversible with antidepressants. We studied age-, sex-, and postmortem interval-matched groups: no major psychopathology (controls); unmedicated-MDD; and MDD treated with serotonin reuptake inhibitors (MDD*SSRI) or tricyclics (MDD*TCA). Frozen right hippocampi were fixed, sectioned (50 μm), immunostained with neuronal nuclear marker (NeuN), and counterstained with hematoxylin. GN and glial number, and DG and granule cell layer (GCL) volumes were stereologically estimated. Fewer GNs in the anterior DG were present in unmedicated-MDDs compared with controls (p=0.013). Younger age of MDD onset correlated with fewer GNs (p=0.021). Unmedicated-MDDs had fewer mid-DG GNs than MDD*SSRIs (p=0.028) and controls (p=0.032). Anterior GCL glial number did not differ between groups. Anterior/mid GCL volume was smaller in unmedicated-MDDs vs controls (p=0.008) and larger in MDD*SSRIs vs unmedicated-MDDs (p<0.001), MDD*TCAs (p<0.001), and controls (p<0.001). Anterior GCL volume and GN number (r=0.594, p=0.001), and mid DG volume and GN number (r=0.398, p=0.044) were correlated. Anterior DG capillary density correlated with GN number (p=0.027), and with GCL (p=0.024) and DG (r=0.400, p=0.047) volumes. Posterior DG volume and GN number did not differ between groups. Fewer GNs in unmedicated-MDD without fewer neuronal progenitor cells, as previously reported, suggests a cell maturation or survival defect, perhaps related to MDD duration. This may contribute to a smaller hippocampus and is potentially reversed by SSRIs. Postmortem studies are correlative and animal studies are needed to test implied causal relationships.

Hippocampal neurogenesis: a biomarker for depression or antidepressant effects? Methodological considerations and perspectives for future research

Whereas animal models of depression are associated with decreased adult hippocampal neurogenesis, antidepressant treatments, including pharmacotherapy but also electroconvulsive therapy, have the opposite action, as they stimulate cell proliferation and the survival and maturation of newborn dentate gyrus neurons. Although the lack of these new cells is not causally involved in depression, as their absence does not trigger a depressive-episode per se, their loss has been shown to be causally involved in the ability of chronic monoaminergic antidepressants to achieve remission. However, the process by which the stimulation of hippocampal neurogenesis can elicit recovery after a depressive-like episode is poorly understood. The accepted view is that hippocampal newborn neurons integrate into the hippocampal network and thus participate in hippocampal cognitive functions crucial for remission. The hippocampus is associated with a wide range of such functions, including spatial navigation, pattern separation, encoding of new contextual information, emotional behavior and control over the hypothalamic-pituitary-adrenal axis. The present review aims at discussing each of these functions and tries to identify the process by which newborn cells participate in remission after successful therapy. Finally, future directions are proposed for a better understanding of these mechanisms.

The neural plasticity theory of depression: assessing the roles of adult neurogenesis and PSA-NCAM within the hippocampus

Depression is a devastating and prevalent disease, with profound effects on neural structure and function; however the etiology and neuropathology of depression remain poorly understood. Though antidepressant drugs exist, they are not ideal, as only a segment of patients are effectively treated, therapeutic onset is delayed, and the exact mechanism of these drugs remains to be elucidated. Several theories of depression do exist, including modulation of monoaminergic neurotransmission, alterations in neurotrophic factors, and the upregulation of adult hippocampal neurogenesis, and are briefly mentioned in the review. However none of these theories sufficiently explains the pathology and treatment of depression unto itself. Recently, neural plasticity theories of depression have postulated that multiple aspects of brain plasticity, beyond neurogenesis, may bridge the prevailing theories. The term “neural plasticity” encompasses an array of mechanisms, from the birth, survival, migration, and integration of new neurons to neurite outgrowth, synaptogenesis, and the modulation of mature synapses. This review critically assesses the role of adult hippocampal neurogenesis and the cell adhesion molecule, PSA-NCAM (which is known to be involved in many facets of neural plasticity), in depression and antidepressant treatment.

Cognitive impairments and cancer chemotherapy: translational research at a crossroads

Cancer chemotherapy is often associated with cognitive deficits which may remain after the treatment has ended. As more people survive cancer, concern is increasing about the impact of these problems with memory and executive function when they return to everyday life. When chemotherapeutic drugs are administered to healthy animals in dosing regimens modeling those used in humans, cognitive deficits also occur, and these preclinical studies can provide information about the biological mechanisms by which the cancer fighting drugs affect the brain. Evidence from animal studies points to damage to hippocampus, particularly a disruption of neurogenesis, whereas human studies emphasize cognitive deficits associated with impairments in frontal cortical function. This discrepancy may be due more to the tasks selected by researchers, and the choice of biochemical endpoints than inherently different effects of chemotherapy in humans and rodents. These differences in approach must be reconciled if common underlying mechanisms are to be identified, with the hope of leading to novel drug or non-pharmacological treatments. This may be achieved by broadening the scope of human and animal studies, and by looking outside the topic of chemotherapy-induced cancer deficits to learn from the advances being made by studying the effects of stress and somatic disease on brain function, and the cognitive impairments now recognized to result from a wide range of mental and physical illnesses.

Adult hippocampal neurogenesis: an actor in the antidepressant-like action

Depression and anxiety are psychiatric illnesses that are major burdens in society and affect as much as 7% of the world's population. The heterogeneous nature of depression suggests an involvement of multiple distinct brain regions including amygdala, prefrontal cortex and the hippocampus, which may be responsible for the diversity of the symptoms. Besides its critical role in learning and memory, the hippocampus is one of only two areas in mammalian brain where adult neurogenesis occurs. Of the current leading hypotheses of the pathophysiology and treatment of depression, the neurogenesis hypothesis of depression deserves particular attention because changes in neurogenesis are only seen after chronic, but not acute, antidepressant treatment. This review revisits the role of adult hippocampal neurogenesis in the pathophysiology of mood disorders, especially anxiety/depression, and also in the antidepressant-like responses, especially in stressed rodents.

In vivo imaging of adult human hippocampal neurogenesis: progress, pitfalls and promise

New neurons are produced within the hippocampus of the mammalian brain throughout life. Evidence from animal studies has suggested that the function of these adult-born neurons is linked to cognition and emotion. Until we are able to detect and measure levels of adult neurogenesis in living human brains-a formidable challenge for now-we cannot establish its functional importance in human health, disease and new treatment development. Current non-invasive neuroimaging modalities can provide live snapshots of the brain's structure, chemistry, activity and connectivity. This review explores whether existing macroscopic imaging methods can be used to understand the microscopic dynamics of adult hippocampal neurogenesis in living individuals. We discuss recent studies that have found correlations between neuroimaging measures of human hippocampal biology and levels of pro- or anti-neurogenic stimuli, weigh whether these correlations reflect changes in adult neurogenesis, detail the conceptual and technical limitations of these studies and elaborate on what will be needed to validate in vivo neuroimaging measures of adult neurogenesis for future investigations.

Amygdalar expression of proteins associated with neuroplasticity in major depression and suicide

INTRODUCTION

Doublecortin (DCX) and polysialilated neural cell adhesion molecule (PSA-NCAM), two proteins associated with immature neuronal phenotypes and elevated neuroplasticity in the adult brain, have recently been identified in the mammalian amygdala, and evidence from rodent studies suggests that stress may modify their expression in this brain region. The purpose of the present study was to investigate whether the expression of proteins involved in neuroplasticity is altered in the amygdala of individuals with depression.

METHODS

Basolateral amygdala (BLA) and central amygdala (CeA) postmortem human brain samples were collected from individuals with a history of depression (n = 22 and 25, respectively) and psychiatrically healthy controls (CTRL; n = 14). Proteins associated with neuroplasticity were quantified using Western blotting.

RESULTS

Immunoblots revealed that depressed subjects displayed increased expression of DCX (p = 0.033) and PSA-NCAM (p = 0.027) in the BLA as compared to CTRLs. Subsequent analyses revealed that this effect was due primarily to a subset of depressed subjects who had not died by suicide (DNS). DNS subjects displayed higher DCX than CTRLs (p < 0.001) and depressed suicides (p = 0.001), and higher PSA-NCAM than CTRLs (p = 0.007). Conversely, within the CeA, DNS subjects displayed a tendency toward lower DCX expression than CTRLs (p = 0.062), and higher BDNF levels than DS subjects (p = 0.045).

CONCLUSION

These results suggest that the BLA and CeA display contrasting patterns of neuroplasticity in depression, and that greater impairment of amygdalar neuroplasticity may be associated with increased risk of suicide.

Understanding resilience

Resilience is the ability to adapt successfully in the face of stress and adversity. Stressful life events, trauma, and chronic adversity can have a substantial impact on brain function and structure, and can result in the development of posttraumatic stress disorder (PTSD), depression and other psychiatric disorders. However, most individuals do not develop such illnesses after experiencing stressful life events, and are thus thought to be resilient. Resilience as successful adaptation relies on effective responses to environmental challenges and ultimate resistance to the deleterious effects of stress, therefore a greater understanding of the factors that promote such effects is of great relevance. This review focuses on recent findings regarding genetic, epigenetic, developmental, psychosocial, and neurochemical factors that are considered essential contributors to the development of resilience. Neural circuits and pathways involved in mediating resilience are also discussed. The growing understanding of resilience factors will hopefully lead to the development of new pharmacological and psychological interventions for enhancing resilience and mitigating the untoward consequences.