Age-dependent decline in hippocampal neurogenesis is not altered by chronic treatment with fluoxetine

Fluoxetine mitigates depressive-like behaviors in mice via anti-inflammation and enhancing neuroplasticity

Neuroplasticity and inflammation represent a common final pathway for effective antidepressant treatment. SSRIs are the most commonly prescribed medications for depression and have demonstrated efficacy in reducing depressive symptoms. However, the precise impact of SSRIs on neuroplasticity and inflammation remains unclear. In this study, we aimed to investigate the influence of long-term treatment with SSRIs on hippocampal neuron, inflammation, synaptic function and morphology. Our findings revealed that fluoxetine treatment significantly alleviated behavioral despair, anhedonia, and anxiety in reserpine-treated mice. Moreover, fluoxetine mitigated hippocampal neuron impairment, inhibited inflammatory release, and increased the expression of synaptic proteins markers (SYP and PSD95) in mice. Notably, fluoxetine also suppressed reserpine-induced synapse loss in the hippocampus. Based on these results, fluoxetine has been demonstrated effectively to ameliorate depressive mood and cognitive dysfunction, possibly through the enhancement of synaptic plasticity. Overall, our study contributes to a further understanding of the mechanisms underlying the therapeutic effects of fluoxetine and its potential role in improving depressive symptoms and cognitive impairments.

The Effect of a Diet Enriched with Jerusalem artichoke, Inulin, and Fluoxetine on Cognitive Functions, Neurogenesis, and the Composition of the Intestinal Microbiota in Mice

The aim of the study was to assess the effect of long-term administration of natural prebiotics: Jerusalem artichoke (topinambur, TPB) and inulin (INU) as well as one of the most popular antidepressants, fluoxetine (FLU), on the proliferation of neural stem cells, learning and memory functions, and the composition of the intestinal microbiota in mice. Cognitive functions were assessed using the Morris Water Maze (MWM)Test. Cells were counted using a confocal microscope and ImageJ software. We performed 16S rRNA sequencing to assess changes in the gut microbiome of the mice. The obtained results showed that the 10-week supplementation with TPB (250 mg/kg) and INU (66 mg/kg) stimulates the growth of probiotic bacteria, does not affect the learning and memory process, and does not disturb the proliferation of neural stem cells in the tested animals. Based on this data, we can assume that both TPB and INU seem to be safe for the proper course of neurogenesis. However, 2-week administration of FLU confirmed an inhibitory impact on Lactobacillus growth and negatively affected behavioral function and neurogenesis in healthy animals. The above studies suggest that the natural prebiotics TPB and INU, as natural supplements, may have the potential to enrich the diversity of intestinal microbiota, which may be beneficial for the BGM axis, cognitive functions, and neurogenesis.

Boosting Neurogenesis in the Adult Hippocampus Using Antidepressants and Mesenchymal Stem Cells

The hippocampus is one of the few privileged regions (neural stem cell niche) of the brain, where neural stem cells differentiate into new neurons throughout adulthood. However, dysregulation of hippocampal neurogenesis with aging, injury, depression and neurodegenerative disease leads to debilitating cognitive impacts. These debilitating symptoms deteriorate the quality of life in the afflicted individuals. Impaired hippocampal neurogenesis is especially difficult to rescue with increasing age and neurodegeneration. However, the potential to boost endogenous Wnt signaling by influencing pathway modulators such as receptors, agonists, and antagonists through drug and cell therapy-based interventions offers hope. Restoration and augmentation of hampered Wnt signaling to facilitate increased hippocampal neurogenesis would serve as an endogenous repair mechanism and contribute to hippocampal structural and functional plasticity. This review focuses on the possible interaction between neurogenesis and Wnt signaling under the control of antidepressants and mesenchymal stem cells (MSCs) to overcome debilitating symptoms caused by age, diseases, or environmental factors such as stress. It will also address some current limitations hindering the direct extrapolation of research from animal models to human application, and the technical challenges associated with the MSCs and their cellular products as potential therapeutic solutions.

Effects of chronic fluoxetine treatment on anxiety- and depressive-like behaviors in adolescent rodents - systematic review and meta-analysis

Background

Drugs prescribed for psychiatric disorders in adolescence should be studied very extensively since they can affect developing and thus highly plastic brain differently than they affect the adult brain. Therefore, we aimed to summarize animal studies reporting the behavioral consequences of chronic exposure to the most widely prescribed antidepressant drug among adolescents i.e., fluoxetine.

Methods

Electronic databases (Medline via Pubmed, Web of Science Core Collection, ScienceDirect) were systematically searched until April 12, 2022, for published, peer-reviewed, controlled trials concerning the effects of chronic fluoxetine administration vs. vehicle on anxiety and depression measures in naïve and stress-exposed adolescent rodents. All of the relevant studies were selected and critically appraised, and a meta-analysis of eligible studies was performed.

Results

A total of 18 studies were included in the meta-analysis. In naïve animals, chronic adolescent fluoxetine administration showed dose-related anxiogenic-like effects, measured as a reduction in time spent in the open arms of the elevated plus maze. No significant effects of chronic adolescent fluoxetine on depression-like behavior were reported in naïve animals, while in stress-exposed rodents chronic adolescent fluoxetine significantly decreased immobility time in the forced swim test compared to vehicle.

Conclusions

These results suggest that although chronic fluoxetine treatment proves positive effects in animal models of depression, it may simultaneously increase anxiety in adolescent animals in a dose-related manner. Although the clinical implications of the data should be interpreted with extreme caution, adolescent patients under fluoxetine treatment should be closely monitored.

Supplementary Information

The online version contains supplementary material available at 10.1007/s43440-022-00420-w.

Adolescent cadmium exposure impairs cognition and hippocampal neurogenesis in C57BL/6 mice

Cadmium (Cd) is a toxic heavy metal and a significant public health concern. Epidemiological studies suggest that Cd is a potential neurotoxicant, and its exposure is associated with cognitive deficits in children, adults, and seniors. Our previous study has found that adulthood-only Cd exposure can impair cognition in mice. However, few studies have addressed the effects of Cd exposure during adolescence on cognitive behavior in animals later in life. In the present study, we exposed 4-week-old male C57BL/6 mice to 3 mg/L Cd via drinking water for 28 weeks and assessed their hippocampus-dependent learning and memory. Cd did not affect anxiety or locomotor activity in the open field test. However, Cd exposure impaired short-term spatial memory and contextual fear memory in mice. A separate cohort of 4-week-old mice was similarly exposed to Cd for 13 weeks to investigate the potential mechanism of Cd neurotoxicity on cognition. We observed that Cd-treated mice had fewer adult-born cells, adult-born neurons, and a reduced proportion of adult-born cells that differentiated into mature neurons in the subgranular zone of the dentate gyrus. These results suggest that Cd exposure from adolescence to adulthood is sufficient to cause cognitive deficits and impair key processes of hippocampal neurogenesis in mice.

The knowns and unknowns of SSRI treatment in young people with depression and anxiety: efficacy, predictors, and mechanisms of action

The use of SSRIs for the treatment of depression and anxiety in young people is increasing. However, the effects of SSRIs in adolescence, a time when there are substantial changes in neural, cognitive, and social functioning, are not well understood. Here, we review evidence from clinical trials about the benefits and risks of SSRIs in young people and consider their mechanisms of action, as shown through human experimental work and animal models. We emphasise key outstanding questions about the effects of SSRIs in youth, identified through gaps in the literature and in consultation with young people with lived experience. It is crucial to characterise the mechanisms underpinning risks and benefits of SSRIs in this age group to progress the field, and to narrow the chasm between the widespread use of SSRIs in youth and the science on which this use is based.

Novel pharmacotherapy: NNI-362, an allosteric p70S6 kinase stimulator, reverses cognitive and neural regenerative deficits in models of aging and disease

Aging is known to slow the neurogenic capacity of the hippocampus, one of only two mammalian adult neurogenic niches. The reduction of adult-born neurons with age may initiate cognitive decline progression which is exacerbated in chronic neurodegenerative disorders, e.g., Alzheimer's disease (AD). With physiologic neurogenesis diminished, but still viable in aging, non-invasive therapeutic modulation of this neuron regeneration process remains possible. The discovery of truly novel neuron regenerative therapies could be identified through phenotypic screening of small molecules that promote adult-born neurons from human neural progenitor cells (hNPCs). By identifying neuron-generating therapeutics and potentially novel mechanism of actions, therapeutic benefit could be confirmed through in vivo proof-of-concept studies. The key aging and longevity mTOR/p70S6 kinase axis, a commonly targeted pathway, is substrate for potential selective kinase modulators to promote new hippocampal neurons from NPCs. The highly regulated downstream substrate of mTOR, p70S6 kinase, directly controls pleiotropic cellular activities, including translation and cell growth. Stimulating this kinase, selectively in an adult neurogenic niche, should promote NPC proliferation, and cell growth and survival in the hippocampus. Studies of kinase profiling and immunocytochemistry of human progenitor neurogenesis suggest that the novel small molecule NNI-362 stimulates p70S6 kinase phosphorylation, which, in turn, promotes proliferation and differentiation of NPCs to neurons. NNI-362 promoted the associative reversal of age- and disease-related cognitive deficits in aged mice and Down syndrome-modeled mice. This oral, allosteric modulator may ultimately be beneficial for age-related neurodegenerative disorders involving hippocampal-dependent cognitive impairment, specifically AD, by promoting endogenous hippocampal regeneration.

Rotated nursing environment with underfeeding: A form of early-life adversity with sex- and age-dependent effects on coping behavior and hippocampal neurogenesis

We investigated how a unique form of early-life adversity (ELA), caused by rotated nursing environment to induce underfeeding, alters anxiety-like and stress-coping behaviors in male and female Sprague Dawley rats in adolescence and adulthood. Adult female rats underwent either thelectomy (thel; surgical removal of teats), sham surgery, or no surgery (control) before mating. Following parturition, litters were rotated between sham and thel rats every 12 h to generate a group of rats that experienced ELA (rotated housing, rotated mother, and 50% food restriction) from postnatal day 0 to 26. Control litters remained with their natal, nursing dams. Regardless of age and sex, ELA reduced activity in the periphery of the open field. ELA increased immobility in the forced swim test, particularly in adults. We used doublecortin immunohistochemistry to identify immature neurons in the hippocampus. ELA increased the number and density of immature neurons in the dentate gyrus of adolescent males (but not females) and reduced the density of immature neurons in adult males (but not females). This research indicates that a unique form of ELA alters stress-related passive coping and hippocampal neurogenesis in an age- and sex-dependent manner.

Hormonal Regulation of Mammalian Adult Neurogenesis: A Multifaceted Mechanism

Adult neurogenesis—resulting in adult-generated functioning, integrated neurons—is still one of the most captivating research areas of neuroplasticity. The addition of new neurons in adulthood follows a seemingly consistent multi-step process. These neurogenic stages include proliferation, differentiation, migration, maturation/survival, and integration of new neurons into the existing neuronal network. Most studies assessing the impact of exogenous (e.g., restraint stress) or endogenous (e.g., neurotrophins) factors on adult neurogenesis have focused on proliferation, survival, and neuronal differentiation. This review will discuss the multifaceted impact of hormones on these various stages of adult neurogenesis. Specifically, we will review the evidence for hormonal facilitation (via gonadal hormones), inhibition (via glucocorticoids), and neuroprotection (via recruitment of other neurochemicals such as neurotrophin and neuromodulators) on newly adult-generated neurons in the mammalian brain.

Sex Differences and Similarities in Hippocampal Cellular Proliferation and the Number of Immature Neurons during Adolescence in Rats

Adolescence is associated with significant reductions in hippocampal cellular proliferation and neurogenesis, the physiological and behavioral implications of which are unclear. Though sex differences exist in these proliferative processes in adulthood, relatively little is known about the role sex plays in these adolescent-related changes. To address this gap, we examined cross-sectional area of the dentate gyrus and cellular proliferation, as measured by Ki-67 immunohistochemistry, in pre- (30 days), mid- (45 days), and post-adolescent (70 days) male and female rats. We also investigated the number of immature neurons using doublecortin (DCX) immunohistochemistry in pre- and post-adolescent males and females. Despite increases in the size of the dentate gyrus during adolescence, we found significant adolescent-related decreases in hippocampal proliferation in both males and females, with a more dramatic decrease in males, indicating both age- and sex-dependent changes in the dentate gyrus. We also found an adolescent-related decline in the number of immature neurons in the dentate gyrus of male rats and a female-biased sex difference in the number of immature neurons in adults. Given these significant changes in the dentate gyrus, these data suggest that this period in development might be particularly sensitive to internal and external factors known to modulate neurogenesis, with potential sex-specific neurobehavioral ramifications.

Exposure of Rat Neural Stem Cells to Ethanol Affects Cell Numbers and Alters Expression of 28 Proteins

Developing brain cells express many proteins but little is known of how their protein composition responds to chronic exposure to alcohol and/or how such changes might relate to alcohol toxicity. We used cultures derived from embryonic rat brain (previously shown to contain mostly neural stem cells; rat NSC, rNSC), exposed them to ethanol (25-100 mM) for up to 96 h and studied how they reacted. Ethanol (50 and 100 mM) reduced cell numbers indicating either compromised cell proliferation, cytotoxicity or both. Increased lipid peroxidation was consistent with the presence of oxidative stress accompanying alcohol-induced cytotoxicity. Proteomics revealed 28 proteins as altered by ethanol (50 mM for 96 h). Some were constituents of cytoskeleton, others were involved in transcription/translation, signal transduction and oxidative stress. Nucleophosmin (NPM1) and dead-end protein homolog 1 (DND1) were further studied by immunological techniques in cultured neurons and astrocytes (derived from brain tissue at embryonic ages E15 and E20, respectively). In the case of DND1 (but not NPM1) ethanol induced similar pattern of changes in both types of cells. Given the critical role of the protein NPM1 in cell proliferation and differentiation, its reduced expression in the ethanol-exposed rNSC could, in part, explain the lower cells numbers. We conclude that chronic ethanol profoundly alters protein composition of rNSC to the extent that their functioning-including proliferation and survival-would be seriously compromised. Translated to humans, such changes could point the way towards mechanisms underlying the fetal alcohol spectrum disorder and/or alcoholism later in life.

Pattern Separation: A Potential Marker of Impaired Hippocampal Adult Neurogenesis in Major Depressive Disorder

Adult neurogenesis involves the generation of new neurons, particularly in the dentate gyrus of the hippocampus. Decreased hippocampal neurogenesis has been implicated in both animal models of depression and in patients with major depressive disorder (MDD), despite some inconsistency in the literature. Here, we build upon current models to generate a new testable hypothesis, linking impaired neurogenesis to downstream psychological outcomes commonly observed in MDD. We contend that disruption in adult neurogenesis impairs pattern separation, a hippocampus-dependent function requiring the careful discrimination and storage of highly similar, but not identical, sensory inputs. This, in turn, can affect downstream processing and response selection, of relevance to emotional wellbeing. Specifically, disrupted pattern separation leads to misperceived stimuli (i.e., stimulus confusion), triggering the selection and deployment of established responses inappropriate for the actual stimuli. We speculate that this may be akin to activation of automatic thoughts, described in the Cognitive Behavior Theory of MDD. Similarly, this impaired ability to discriminate information at a fundamental sensory processing level (e.g., impaired pattern separation) could underlie impaired psychological flexibility, a core component of Acceptance and Commitment Therapy of MDD. We propose that research is needed to test this model by examining the relationship between cognitive functioning (e.g., pattern separation ability), psychological processes (e.g., perseveration and psychological inflexibility), and neurogenesis, taking advantage of emerging magnetic resonance spectroscopy-based imaging that measures neurogenesis in-vivo.

Hilar granule cells of the mouse dentate gyrus: effects of age, septotemporal location, strain, and selective deletion of the proapoptotic gene BAX

The dentate gyrus (DG) principal cells are glutamatergic granule cells (GCs), and they are located in a compact cell layer. However, GCs are also present in the adjacent hilar region, but have been described in only a few studies. Therefore, we used the transcription factor prospero homeobox 1 (Prox1) to quantify GCs at postnatal day (PND) 16, 30, and 60 in a common mouse strain, C57BL/6J mice. At PND16, there was a large population of Prox1-immunoreactive (ir) hilar cells, with more in the septal than temporal hippocampus. At PND30 and 60, the size of the hilar Prox1-ir cell population was reduced. Similar numbers of hilar Prox1-expressing cells were observed in PND30 and 60 Swiss Webster mice. Prox1 is usually considered to be a marker of postmitotic GCs. However, many Prox1-ir hilar cells, especially at PND16, were not double-labeled with NeuN, a marker typically found in mature neurons. Most hilar Prox1-positive cells at PND16 co-expressed doublecortin (DCX) and calretinin, markers of immature GCs. Double-labeling with a marker of actively dividing cells, Ki67, was not detected. These results suggest that, surprisingly, a large population of cells in the hilus at PND16 are immature GCs (Type 2b and Type 3 cells). We also asked whether hilar Prox1-ir cell numbers are modifiable. To examine this issue, we conditionally deleted the proapoptotic gene BAX in Nestin-expressing cells at a time when there are numerous immature GCs in the hilus, PND2-8. When these mice were examined at PND60, the numbers of Prox1-ir hilar cells were significantly increased compared to control mice. However, deletion of BAX did not appear to change the proportion that co-expressed NeuN, suggesting that the size of the hilar Prox1-expressing population is modifiable. However, deleting BAX, a major developmental disruption, does not appear to change the proportion that ultimately becomes neurons.

Stress and adolescent hippocampal neurogenesis: diet and exercise as cognitive modulators

Adolescence is a critical period for brain maturation. Deciphering how disturbances to the central nervous system at this time affect structure, function and behavioural outputs is important to better understand any long-lasting effects. Hippocampal neurogenesis occurs during development and continues throughout life. In adulthood, integration of these new cells into the hippocampus is important for emotional behaviour, cognitive function and neural plasticity. During the adolescent period, maturation of the hippocampus and heightened levels of hippocampal neurogenesis are observed, making alterations to neurogenesis at this time particularly consequential. As stress negatively affects hippocampal neurogenesis, and adolescence is a particularly stressful time of life, it is important to investigate the impact of stressor exposure at this time on hippocampal neurogenesis and cognitive function. Adolescence may represent not only a time for which stress can have long-lasting effects, but is also a critical period during which interventions, such as exercise and diet, could ameliorate stress-induced changes to hippocampal function. In addition, intervention at this time may also promote life-long behavioural changes that would aid in fostering increased hippocampal neurogenesis and cognitive function. This review addresses both the acute and long-term stress-induced alterations to hippocampal neurogenesis and cognition during the adolescent period, as well as changes to the stress response and pubertal hormones at this time which may result in differential effects than are observed in adulthood. We hypothesise that adolescence may represent an optimal time for healthy lifestyle changes to have a positive and long-lasting impact on hippocampal neurogenesis, and to protect against stress-induced deficits. We conclude that future research into the mechanisms underlying the susceptibility of the adolescent hippocampus to stress, exercise and diet and the consequent effect on cognition may provide insight into why adolescence may be a vital period for correct conditioning of future hippocampal function.

Fluoxetine effects on molecular, cellular and behavioral endophenotypes of depression are driven by the living environment

Selective serotonin reuptake inhibitors (SSRIs) represent the most common treatment for major depression. However, their efficacy is variable and incomplete. In order to elucidate the cause of such incomplete efficacy, we explored the hypothesis positing that SSRIs may not affect mood per se but, by enhancing neural plasticity, render the individual more susceptible to the influence of the environment. Consequently, SSRI administration in a favorable environment promotes a reduction of symptoms, whereas in a stressful environment leads to a worse prognosis. To test such hypothesis, we exposed C57BL/6 mice to chronic stress in order to induce a depression-like phenotype and, subsequently, to fluoxetine treatment (21 days), while being exposed to either an enriched or a stressful condition. We measured the most commonly investigated molecular, cellular and behavioral endophenotypes of depression and SSRI outcome, including depression-like behavior, neurogenesis, brain-derived neurotrophic factor levels, hypothalamic-pituitary-adrenal axis activity and long-term potentiation. Results showed that, in line with our hypothesis, the endophenotypes investigated were affected by the treatment according to the quality of the living environment. In particular, mice treated with fluoxetine in an enriched condition overall improved their depression-like phenotype compared with controls, whereas those treated in a stressful condition showed a distinct worsening. Our findings suggest that the effects of SSRI on the depression- like phenotype is not determined by the drug per se but is induced by the drug and driven by the environment. These findings may be helpful to explain variable effects of SSRI found in clinical practice and to device strategies aimed at enhancing their efficacy by means of controlling environmental conditions.

Antidepressant-like effects and basal immobility depend on age and serotonin transporter genotype

Monoamine uptake inhibitors are common treatments for depression; however, the therapeutic efficacy of these drugs varies widely. Two factors that are commonly linked to clinical outcome are age and serotonin transporter (SERT) genotype. Mouse models provide powerful tools to study consequences of age and genotype on antidepressant-like efficacy; however, to date, systematic studies of this nature are lacking. Here, we used the tail suspension test (TST), a preclinical assay for antidepressant efficacy, to gain insight into age and SERT genotype dependency of immobility time in the TST under control conditions (saline injection) and in response to the tricyclic antidepressant, desipramine (DMI). Immobility after saline injection in juvenile, adolescent, adult, mature adult and middle-aged mice (postnatal days 21, 28, 90, 210 and 300, respectively) significantly increased with age; however, the rate of increase was slower for SERT null (-/-) mice than for wild-type (+/+) or heterozygote (+/-) mice. Desipramine reduced immobility across ages and SERT genotypes. Middle-aged, but not adult, SERT(-/-) mice were significantly more sensitive to DMI than age-matched SERT(+/+) or SERT(+/-) mice. Desipramine was less potent in middle-aged SERT(+/+) and SERT(+/-) mice than in adult SERT(+/+) or SERT(+/-) mice. Regardless of age, DMI's maximal effects were greater in SERT(-/-) mice than in SERT(+/+) or SERT(+/-) mice. These results show that immobility time in the TST varies as a function of age and SERT genotype, underscoring the utility of the TST as a potential model to examine age- and SERT genotype-dependent influences on antidepressant response.

Fluoxetine Enhances Neurogenesis in Aged Rats with Cortical Infarcts, but This is not Reflected in a Behavioral Recovery

Age is associated with poor outcome and impaired functional recovery after stroke. Fluoxetine, which is widely used in clinical practice, can regulate hippocampal neurogenesis in young rodents. As the rate of neurogenesis is dramatically reduced during aging, we studied the effect of post-stroke fluoxetine treatment on neurogenesis in the subventricular zone (SVZ) and subgranular zone (SGZ) of dentate gyrus (DG) and whether this would be associated with any behavioral recovery after the cortical infarct in aged rats. Aged rats were randomly assigned to four groups: sham-operated rats, sham-operated rats treated with fluoxetine, rats subjected to cerebral ischemia, and rats with ischemia treated with fluoxetine. Focal cortical ischemia was induced by intracranial injection of vasoconstrictive peptide, endothelin-1 (ET-1). Fluoxetine was administered in the drinking water for 3 weeks starting 1 week after ischemia at a dose of 18 mg/kg/day. Behavioral recovery was evaluated on post-stroke days 29 to 31 after which the survival rate and fate of proliferating cells in the SVZ and DG were assessed by immunohistochemistry. Apoptosis was measured with the TUNEL assay. The results indicated that chronic fluoxetine treatment after stroke enhanced the proliferation of newborn neurons in the SVZ, but not in SGZ, and it suppressed perilesional apoptosis. Fluoxetine treatment did not affect the survival or differentiation of newly generated cells in the SVZ i.e., the enhanced neurogenesis was not translated into a behavioral outcome.

Fluoxetine induces input-specific hippocampal dendritic spine remodeling along the septotemporal axis in adulthood and middle age

Fluoxetine, a selective serotonin-reuptake inhibitor (SSRI), is known to induce structural rearrangements and changes in synaptic transmission in hippocampal circuitry. In the adult hippocampus, structural changes include neurogenesis, dendritic, and axonal plasticity of pyramidal and dentate granule neurons, and dedifferentiation of dentate granule neurons. However, much less is known about how chronic fluoxetine affects these processes along the septotemporal axis and during the aging process. Importantly, studies documenting the effects of fluoxetine on density and distribution of spines along different dendritic segments of dentate granule neurons and CA1 pyramidal neurons along the septotemporal axis of hippocampus in adulthood and during aging are conspicuously absent. Here, we use a transgenic mouse line in which mature dentate granule neurons and CA1 pyramidal neurons are genetically labeled with green fluorescent protein (GFP) to investigate the effects of chronic fluoxetine treatment (18 mg/kg/day) on input-specific spine remodeling and mossy fiber structural plasticity in the dorsal and ventral hippocampus in adulthood and middle age. In addition, we examine levels of adult hippocampal neurogenesis, maturation state of dentate granule neurons, neuronal activity, and glutamic acid decarboxylase-67 expression in response to chronic fluoxetine in adulthood and middle age. Our studies reveal that while chronic fluoxetine fails to augment adult hippocampal neurogenesis in middle age, the middle-aged hippocampus retains high sensitivity to changes in the dentate gyrus (DG) such as dematuration, hypoactivation, and increased glutamic acid decarboxylase 67 (GAD67) expression. Interestingly, the middle-aged hippocampus shows greater sensitivity to fluoxetine-induced input-specific synaptic remodeling than the hippocampus in adulthood with the stratum-oriens of CA1 exhibiting heightened structural plasticity. The input-specific changes and circuit-level modifications in middle-age were associated with modest enhancement in contextual fear memory precision, anxiety-like behavior and antidepressant-like behavioral responses.

The current state of the neurogenic theory of depression and anxiety

Newborn neurons are continuously added to the adult hippocampus. Early studies found that adult neurogenesis is impaired in models of depression and anxiety and accelerated by antidepressant treatment. This led to the theory that depression results from impaired adult neurogenesis and restoration of adult neurogenesis leads to recovery. Follow up studies yielded a complex body of often inconsistent results, and the veracity of this theory is uncertain. We propose five criteria for acceptance of this theory, we review the recent evidence for each criterion, and we draw the following conclusions: Diverse animal models of depression and anxiety have impaired neurogenesis. Neurogenesis is consistently boosted by antidepressants in animal models only when animals are stressed. Ablation of neurogenesis in animal models impairs cognitive functions relevant to depression, but only a minority of studies find that ablation causes depression or anxiety. Recent human neuroimaging and postmortem studies are consistent with the neurogenic theory, but they are indirect. Finally, a novel drug developed based on the neurogenic theory is promising in animal models.

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.

Neural plasticity and proliferation in the generation of antidepressant effects: hippocampal implication

It is widely accepted that changes underlying depression and antidepressant-like effects involve not only alterations in the levels of neurotransmitters as monoamines and their receptors in the brain, but also structural and functional changes far beyond. During the last two decades, emerging theories are providing new explanations about the neurobiology of depression and the mechanism of action of antidepressant strategies based on cellular changes at the CNS level. The neurotrophic/plasticity hypothesis of depression, proposed more than a decade ago, is now supported by multiple basic and clinical studies focused on the role of intracellular-signalling cascades that govern neural proliferation and plasticity. Herein, we review the state-of-the-art of the changes in these signalling pathways which appear to underlie both depressive disorders and antidepressant actions. We will especially focus on the hippocampal cellularity and plasticity modulation by serotonin, trophic factors as brain-derived neurotrophic factor (BDNF), and vascular endothelial growth factor (VEGF) through intracellular signalling pathways—cAMP, Wnt/β-catenin, and mTOR. Connecting the classic monoaminergic hypothesis with proliferation/neuroplasticity-related evidence is an appealing and comprehensive attempt for improving our knowledge about the neurobiological events leading to depression and associated to antidepressant therapies.

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.

The pubertal-related decline in cellular proliferation and neurogenesis in the dentate gyrus of male rats is independent of the pubertal rise in gonadal hormones

Pubertal development is marked by significant decreases in cellular proliferation and neurogenesis in the dentate gyrus of the hippocampal formation. Although it is unclear what mediates these developmental changes in the dentate gyrus, gonadal hormones have been implicated in modulating many neurobiological processes during puberty and various parameters of neurogenesis in adulthood. Thus, it is possible that the gradual and sustained increase in gonadal hormones experienced during puberty plays a role in these changes in neurogenesis. In this experiments, we first quantified cellular proliferation and neurogenesis using 5-bromo-2'-deoxyuridine (BrdU) and doublecortin (DCX) immunohistochemistry, respectively, in the dentate gyrus of prepubertal (30 d), midpubertal (45 d), and adult (90 d) male rats. We found the decline in BrdU and DCX cell numbers throughout these ages was coincident with increases in their plasma testosterone levels. We next tested whether exposure to the pubertal rise in gonadal hormones was necessary for this decrease in hippocampal neurogenesis to occur. Thus, we examined cellular proliferation and neurogenesis in intact 30 day (prepubertal) and 60-day-old (late-pubertal) rats, as well as 60-day-old rats that had previously been gonadectomized or sham-gonadectomized at 30 days of age. Although we again found the expected decline in BrdU and DCX cell numbers between 30 and 60 days of age in the intact groups, there were no differences among the 60-day-old animals, regardless of gonadal status. These data indicate that the pubertal-related decline in hippocampal cellular proliferation and neurogenesis is independent of the pubertal change in gonadal hormones.

Novel insights into depression and antidepressants: a synergy between synaptogenesis and neurogenesis?

Major depressive disorder has been associated with manifold pathophysiological changes. These include metabolic abnormalities in discreet brain areas; modifications in the level of stress hormones, neurotransmitters, and neurotrophic factors; impaired spinogenesis and synaptogenesis in crucial brain areas, such as the prefrontal cortex and the hippocampus; and impaired neurogenesis in the hippocampus. Antidepressant therapy facilitates remission by reversing most of these disturbances, indicating that these dysfunctions may participate causally in depressive symptomatology. However, few attempts have been made to integrate these different pathophysiologies into one model. The present chapter endeavors (1) to review the extant literature in the field, with particular focus on the role of neurogenesis and synaptogenesis in depression; (2) and to suggest a possible interplay between these two processes, as well as, describe the ways by which improving both neurogenesis and synaptogenesis may enable effective recovery by acting on a larger neuronal network.

Hyperforin modulates dendritic spine morphology in hippocampal pyramidal neurons by activating Ca(2+) -permeable TRPC6 channels

The standardized extract of the St. John's wort plant (Hypericum perforatum) is commonly used to treat mild to moderate depression. Its active constituent is hyperforin, a phloroglucinol derivative that reduces the reuptake of serotonin and norepinephrine by increasing intracellular Na(+) concentration through the activation of nonselective cationic TRPC6 channels. TRPC6 channels are also Ca(2+) -permeable, resulting in intracellular Ca(2+) elevations. Indeed, hyperforin activates TRPC6-mediated currents and Ca(2+) transients in rat PC12 cells, which induce their differentiation, mimicking the neurotrophic effect of nerve growth factor. Here, we show that hyperforin modulates dendritic spine morphology in CA1 and CA3 pyramidal neurons of hippocampal slice cultures through the activation of TRPC6 channels. Hyperforin also evoked intracellular Ca(2+) transients and depolarizing inward currents sensitive to the TRPC channel blocker La(3+) , thus resembling the actions of the neurotrophin brain-derived neurotrophic factor (BDNF) in hippocampal pyramidal neurons. These results suggest that the antidepressant actions of St. John's wort are mediated by a mechanism similar to that engaged by BDNF.

From the stressed adolescent to the anxious and depressed adult: investigations in rodent models

Anxiety and depression are the most prevalent of the psychiatric disorders. The average age of onset of these disorders is in adolescence, and stressful experiences are recognized as an important pathway to such dysfunction. Until recently, however, most animal models of these disorders involved adult males. We provide a brief overview of anxiety and depression and the extent to which adolescent rodents are a valid model for their investigation, and briefly review the main measures of anxiety-like and depressive behaviour in rodents. The focus of the review is investigations in which adolescent rodents were exposed to chronic stressors, describing our research using social instability stress and that of other researchers using various social and non-social stressors. The evidence to date suggests stress in adolescence alters the trajectory of brain development, and particularly that of the hippocampus, increasing anxiety and depressive behaviour in adulthood.

Is neurogenesis relevant in depression and in the mechanism of antidepressant drug action? A critical review

OBJECTIVES

Major depression is a complex disorder that involves genetic, epigenetic and environmental factors in its aetiology. Recent research has suggested that hippocampal neurogenesis may play a role in antidepressant action. However, careful examination of the literature suggests that the complex biological and psychological changes associated with depression cannot be attributed to disturbance in hippocampal neurogenesis alone. While antidepressants may induce hippocampal neurogenesis in non-human primates, there is a paucity of evidence that such effects are sufficient for full therapeutic action in humans.

METHODS

This review examines the literature on neurogenesis and discusses the stress-induced cortisol neurotoxicity and antidepressant-induced neurogenesis rescue model of depression. The disparity between a simple antidepressant-induced neurogenesis rescue model in the hippocampus and the complexity of clinical depression is analyzed through critical evaluation of recent research data.

RESULTS AND CONCLUSIONS

Major depression is a complex brain disorder with multiple symptoms and disturbances reflecting dysfunction in more than one single brain area. Initial research suggesting a model of hippocampal degeneration as basis of depression, and reversal by antidepressants through neurogenesis seems to be over-simplified given the emergence of new data. Synaptogenesis and re-organization or re-integration of new neurons rather than simple addition of new neurons may underlie the role of antidepressant drugs in the reversal of some but not all symptoms in depression. The importance of the neurogenesis hypothesis of depression and antidepressant action lies in stimulating further research into the possible roles played by the new neurons and synapses generated.

Effects of risperidone treatment in adolescence on hippocampal neurogenesis, parvalbumin expression, and vascularization following prenatal immune activation in rats

Maternal infection in pregnancy is an environmental risk factor for the development of schizophrenia and related disorders in the offspring, and this association is recapitulated in animal models using gestational infection or immune stimulation. We have recently shown that behavioral abnormalities and altered hippocampal morphology emerging in adult offspring of dams treated with the viral mimic polyriboinosinic-polyribocytidilic acid (poly I:C) are prevented by treatment with the atypical antipsychotic drug risperidone (RIS) in adolescence. Here we used a battery of cellular markers and Nissl stain to morphometrically analyze different hippocampal cell populations in the offspring of poly I:C and saline-treated mothers that received saline or RIS in adolescence, at different time points of postnatal development. We report that impaired neurogenesis, disturbed micro-vascularization and loss of parvalbumin-expressing hippocampal interneurons, are found in the offspring of poly I:C-treated dams. Most, but not all, of these neuropathological changes are not present in poly I:C offspring that had been treated with RIS. These effects may be part of the complex processes underlying the capacity of RIS treatment in adolescence to prevent structural and behavioral abnormalities deficits in the poly I:C offspring.

Chronic fluoxetine treatment in middle-aged rats induces changes in the expression of plasticity-related molecules and in neurogenesis

Background

Antidepressants promote neuronal structural plasticity in young-adult rodents, but little is known of their effects on older animals. The polysialylated form of the neural cell adhesion molecule (PSA-NCAM) may mediate these structural changes through its anti-adhesive properties. PSA-NCAM is expressed in immature neurons and in a subpopulation of mature interneurons and its expression is modulated by antidepressants in the telencephalon of young-adult rodents.

Results

We have analyzed the effects of 14 days of fluoxetine treatment on the density of puncta expressing PSA-NCAM and different presynaptic markers in the medial prefrontal cortex, hippocampus and amygdala of middle-aged (8 months old) rats. The density of puncta expressing PSA-NCAM increased in the dorsal cingulate cortex, as well as in different hippocampal and amygdaloid regions. In these later regions there were also increases in the density of puncta expressing glutamic acid decarboxylase 65/67 (GAD6), synaptophysin (SYN), PSA-NCAM/SYN and PSA-NCAM/GAD6, but a decrease of those expressing vesicular glutamate transporter 1 (VGluT1). Since there is controversy on the effects of antidepressants on neurogenesis during aging, we analyzed the number of proliferating cells expressing Ki67 and that of immature neurons expressing doublecortin or PSA-NCAM. No significant changes were found in the subgranular zone, but the number of proliferating cells decreased in the subventricular zone.

Conclusions

These results indicate that the effects of fluoxetine in middle-aged rats are different to those previously described in young-adult animals, being more restricted in the mPFC and even following an opposite direction in the amygdala or the subventricular zone.

Role of gonadal hormones in anxiety and fear memory formation and inhibition in male mice

Recent research investigating Pavlovian fear conditioning and fear extinction has elucidated the neurocircuitry involved in acquisition and inhibition of fear responses. Modulatory factors that may underlie individual differences in fear acquisition and inhibition, however, are not well understood. Testosterone is known to affect anxiety-like behavior and cognitive processing. In this study, we hypothesized that castration would increase anxiety and reduce memory for contextual fear conditioning in an age-dependent manner. In addition, castration would reduce the rate of extinction to context, as high levels of testosterone correlate with reduced PTSD-like symptoms. We compared behaviors in male mice that were castrated at one of two different time points, either before puberty (at 4 weeks) or after puberty (at 10 weeks) to sham-operated control mice. The behaviors investigated included: anxiety, cued and contextual fear conditioning, and extinction of the fear memory. An interaction of hormone status and age and a significant effect of age were measured in the elevated plus maze, a measure of anxiety. Castration caused a significant reduction of contextual fear memory, but no effect on cued fear memory. There was no significant effect of castration on extinction. Interestingly, a significant effect of age of the mouse at the time of testing was observed on extinction. These results suggest that endogenous androgens during puberty are important for anxiety and fear memory formation. In addition, these results define a late post-adolescent developmental time point for changes in anxiety and fear extinction.

The appropriateness of unbiased optical fractionators to assess cell proliferation in the adult hippocampus

Optical fractionators have dominated the field of neural cell counting for two decades. These unbiased stereological techniques are often used for the quantification of hippocampal cell proliferation in neurogenesis experiments. However, the heterogeneous distribution of labeled cells, especially in the form of clusters, confounds the application of these techniques. A critical evaluation of the applicability of the optical fractionator suggests that absolute counting achieves higher efficiency in the quantification of cell proliferation than unbiased estimations.

Depression, antidepressants, and neurogenesis: a critical reappraisal

The neurogenesis hypothesis of depression posits (1) that neurogenesis in the subgranular zone of the dentate gyrus is regulated negatively by stressful experiences and positively by treatment with antidepressant drugs and (2) that alterations in the rate of neurogenesis play a fundamental role in the pathology and treatment of major depression. This hypothesis is supported by important experimental observations, but is challenged by equally compelling contradictory reports. This review summarizes the phenomenon of adult hippocampal neurogenesis, the initial and continued evidence leading to the development of the neurogenesis hypothesis of depression, and the recent studies that have disputed and/or qualified those findings, to conclude that it can be affected by stress and antidepressants under certain conditions, but that these effects do not appear in all cases of psychological stress, depression, and antidepressant treatment.

Antidepressants and adolescent brain development

Despite being a first-line treatment for adolescent depression and anxiety, antidepressant drugs appear to have questionable efficacy and carry an increased risk of adverse effects in this population. The neural mechanisms underlying this phenomenon are currently unknown. Recent research into the neural effects of alcohol and recreational drugs suggests that the developmental trajectory of the adolescent brain may be particularly vulnerable to pharmacological disturbance. It is therefore important to consider whether prescription psychotropic drugs may have analogous effects. This article reviews the contribution of recent preclinical, clinical and pharmacogenetic literature to current knowledge on the short-term and enduring neural effects of antidepressants on the adolescent brain, with a particular focus on the major neurotransmitter systems and neuroplasticity.

The age-dependent effects of selective serotonin reuptake inhibitors in humans and rodents: A review

The selective serotonin reuptake inhibitor (SSRI) Prozac® (fluoxetine) is widely prescribed for the treatment of depression and anxiety-related disorders. While extensive research has established that fluoxetine is safe for adults, safety is not guaranteed for (unborn) children and adolescents. Some clinical studies have reported adverse outcomes, such as premature birth, neonatal cardiovascular abnormalities, and pulmonary hypertension in children whose mothers used SSRIs during pregnancy. In addition, several reports show that adolescent fluoxetine treatment increases risk for suicidal behavior. Despite these studies, fluoxetine is not contraindicated in the treatment of depressed pregnant women and adolescents. Longitudinal research in humans is limited because of ethical reasons and time constraints, and to overcome these limitations, rodents are used to increase insight in the age-dependent effects of fluoxetine exposure. It has been established that neonatal and adolescent fluoxetine exposure leads to paradoxical anxiety- and depression-like features in later life of rats and mice, although in some studies adolescent fluoxetine exposure was without effects. These age-dependent outcomes of fluoxetine may be explained by serotonin's neurotrophic effects, which may vary according to the developmental stage of the brain due to epigenetic modifications. Here we review the existing evidence for the age-dependent effects of fluoxetine in humans and rodents, address the gaps in our current knowledge and propose directions for future research. Given the overlap between human and rodent findings, rodents provide heuristic value in further research on the age-dependent effects of SSRIs.

Lithium, but not fluoxetine or the corticotropin-releasing factor receptor 1 receptor antagonist R121919, increases cell proliferation in the adult dentate gyrus

Several antidepressant drugs have previously been reported to increase neurogenesis in the dentate gyrus of the hippocampus in laboratory animals. We found no effect of the selective serotonin reuptake inhibitor fluoxetine or the corticotropin-releasing factor receptor 1 antagonist R121919 [3-[6-(dimethylamino)-4-methylpyridin-3-yl]-2,5-dimethyl-N,N-dipropyl-1H-pyrazolo[1,5-a]pyrimidin-8-ium-7-amine] on the rate of cell proliferation or hippocampal brain-derived neurotrophic factor (BDNF) mRNA expression in either adult or adolescent rats after long-term administration. In adults, the mood stabilizer lithium was found to significantly increase cell proliferation; the atypical antipsychotic paliperidone did not affect proliferation, either alone or when combined with lithium. Fourteen-day survival of neuronally fated cells showed a significant interaction effect of lithium and paliperidone but no effect of either drug alone. BDNF mRNA expression was significantly decreased by lithium in the CA1/2 cell fields and increased by paliperidone in the CA1/2, CA3, and dentate gyrus. These results raise questions concerning the hypothesis that all antidepressants increase neurogenesis under nonstressed conditions. They also confirm and extend previous reports of lithium-induced increases in cell proliferation but not survival.

Insights into neurogenesis and aging: potential therapy for degenerative disease?

Neurogenesis is the process by which new neural cells are generated from a small population of multipotent stem cells in the adult CNS. This natural generation of new cells is limited in its regenerative capabilities and also declines with age. The use of stem cells in the treatment of neurodegenerative disease may hold great potential; however, the age-related incidence of many CNS diseases coincides with reduced neurogenesis. This review concisely summarizes current knowledge related to adult neurogenesis and its alteration with aging and examines the feasibility of using stem cell and gene therapies to combat diseases of the CNS with advancing age.

Comparison of neurogenic effects of fluoxetine, duloxetine and running in mice

Hippocampal neurogenesis can be regulated by extrinsic factors, such as exercise and antidepressants. While there is evidence that the selective serotonin reuptake inhibitor (SSRI) fluoxetine enhances neurogenesis, the new dual serotonergic-noradrenergic reuptake inhibitor (SNRI) duloxetine has not been evaluated in this context. In addition, it is unclear whether effects of antidepressants and running on cell genesis and behavior are of similar magnitude in mice. Here, we assessed neurogenesis and open-field behavior in 2-month-old female C57Bl/6 mice after 28days of treatment with either fluoxetine (18mg/kg), duloxetine (2, 6 or 18mg/kg) or exercise. New cell survival, as measured by 5-bromo-2'-deoxyuridine (BrdU)-labeled cells, was enhanced by 200% in the running group only. Both running and fluoxetine, but not duloxetine, increased the percentage of new cells that became neurons. In the open-field test, animals treated with either drug spent less time in the center than controls and runners. In addition, fluoxetine treatment resulted in reduced locomotor activity. Together, these data show that the neurogenic response to exercise is much stronger than to antidepressants and imply a low likelihood that anxiolytic effects of these drugs are mediated by adult neurogenesis in C57Bl/6 mice.

Hippocampal adult neurogenesis is enhanced by chronic eszopiclone treatment in rats

The adult hippocampal dentate gyrus (DG) exhibits cell proliferation and neurogenesis throughout life. We examined the effects of daily administration of eszopiclone (Esz), a commonly used hypnotic drug and gamma-aminobutyric acid (GABA) agonist, compared with vehicle, on DG cell proliferation and neurogenesis, and on sleep-wake patterns. Esz was administered during the usual sleep period of rats, to mimic typical use in humans. Esz treatment for 7 days did not affect the rate of cell proliferation, as measured by 5-bromo-2'-deoxyuridine (BrdU) immunostaining. However, twice-daily Esz administration for 2 weeks increased survival of newborn cells by 46%. Most surviving cells exhibited a neuronal phenotype, identified as BrdU-neuronal nuclei (NeuN) double-labeling. NeuN is a marker of neurons. Non-rapid eye movement sleep was increased on day 1, but not on days 7 or 14 of Esz administration. Delta electroencephalogram activity was increased on days 1 and 7 of treatment, but not on day 14. There is evidence that enhancement of DG neurogenesis is a critical component of the effects of antidepressant treatments of major depressive disorder (MDD). Adult-born DG cells are responsive to GABAergic stimulation, which promotes cell maturation. The present study suggests that Esz, presumably acting as a GABA agonist, has pro-neurogenic effects in the adult DG. This result is consistent with evidence that Esz enhances the antidepressant treatment response of patients with MDD with insomnia.

Rosiglitazone, an agonist of peroxisome proliferator-activated receptor gamma, decreases immunoreactivity of markers for cell proliferation and neuronal differentiation in the mouse hippocampus

In the present study, we investigated the regulating effects of rosiglitazone (RSG), a synthetic agonist of peroxisome proliferator-activated receptor gamma, treatment for 28days on the cell proliferation and neuronal differentiation in the mouse hippocampal dentate gyrus by 5-bromo-2'-deoxyuridine (BrdU), Ki67 and doublecortin (DCX) immunohistochemistry. These markers were detected in the subgranular zone (SGZ) of the dentate gyrus in vehicle- and RSG-treated groups. In the RSG-treated group, the number of BrdU-, Ki67- and DCX-immunoreactive cells was significantly decreased compared to those in the vehicle-treated group. In addition, brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor levels were significantly decreased in the dentate gyrus of the RSG-treated groups compared to the vehicle-treated group. These results indicate that RSG treatment decreases immunoreactivities of markers for cell proliferation and neuronal differentiation and levels of neurotrophic factors in the SGZ of the hippocampal dentate gyrus.

Roles of neural stem cells and adult neurogenesis in adolescent alcohol use disorders

This review discusses the contributions of a newly considered form of plasticity, the ongoing production of new neurons from neural stem cells, or adult neurogenesis, within the context of neuropathologies that occur with excessive alcohol intake in the adolescents. Neural stem cells and adult neurogenesis are now thought to contribute to the structural integrity of the hippocampus, a limbic system region involved in learning, memory, behavioral control, and mood. In adolescents with alcohol use disorders (AUDs), the hippocampus appears to be particularly vulnerable to the neurodegenerative effects of alcohol, but the role of neural stem cells and adult neurogenesis in alcoholic neuropathology has only recently been considered. This review encompasses a brief overview of neural stem cells and the processes involved in adult neurogenesis, how neural stem cells are affected by alcohol, and possible differences in the neurogenic niche between adults and adolescents. Specifically, what is known about developmental differences in adult neurogenesis between the adult and adolescent is gleaned from the literature, as well as how alcohol affects this process differently among the age groups. Finally, this review suggests differences that may exist in the neurogenic niche between adults and adolescents and how these differences may contribute to the susceptibility of the adolescent hippocampus to damage. However, many more studies are needed to discern whether these developmental differences contribute to the vulnerability of the adolescent to developing an AUD.

Role of the amygdala in antidepressant effects on hippocampal cell proliferation and survival and on depression-like behavior in the rat

The stimulation of adult hippocampal neurogenesis by antidepressants has been associated with multiple molecular pathways, but the potential influence exerted by other brain areas has received much less attention. The basolateral complex of the amygdala (BLA), a region involved in anxiety and a site of action of antidepressants, has been implicated in both basal and stress-induced changes in neural plasticity in the dentate gyrus. We investigated here whether the BLA modulates the effects of the SSRI antidepressant fluoxetine on hippocampal cell proliferation and survival in relation to a behavioral index of depression-like behavior (forced swim test). We used a lesion approach targeting the BLA along with a chronic treatment with fluoxetine, and monitored basal anxiety levels given the important role of this behavioral trait in the progress of depression. Chronic fluoxetine treatment had a positive effect on hippocampal cell survival only when the BLA was lesioned. Anxiety was related to hippocampal cell survival in opposite ways in sham- and BLA-lesioned animals (i.e., negatively in sham- and positively in BLA-lesioned animals). Both BLA lesions and low anxiety were critical factors to enable a negative relationship between cell proliferation and depression-like behavior. Therefore, our study highlights a role for the amygdala on fluoxetine-stimulated cell survival and on the establishment of a link between cell proliferation and depression-like behavior. It also reveals an important modulatory role for anxiety on cell proliferation involving both BLA-dependent and -independent mechanisms. Our findings underscore the amygdala as a potential target to modulate antidepressants' action in hippocampal neurogenesis and in their link to depression-like behaviors.