Adult hippocampal neuroplasticity triggers susceptibility to recurrent depression

Potential role of gut microbiota in major depressive disorder: A review

Interactions between the gut microbiota and host immunity are sophisticated, dynamic, and host-dependent. Scientists have recently conducted research showing that disturbances in the gut bacterial community can lead to a decrease in some metabolites and, consequently, to behaviors such as depression. Exposure to stressors dropped the relative abundance of bacteria in the genus Bacteroides while soaring the relative abundance of bacteria in the genus Clostridium, Coprococcus, Dialister, and Oscillibacter, which were also reduced in people with depression. Microbiota and innate immunity are in a bilateral relationship. The gut microbiota has been shown to induce the synthesis of antimicrobial proteins such as catalysidins, type C lectins, and defensins. Probiotic bacteria can modulate depressive behavior through GABA signaling. The gut microbiome produces essential metabolites such as neurotransmitters, tryptophan metabolites, and short-chain fatty acids (SCFAs) that can act on the CNS. In the case of dysbiosis, due to mucin changes, the ratio of intestinal-derived molecules may change and contribute to depression. Psychotropics, including Bifidobacterium longum NCC3001, Clostridium butyricum CBM588, and Lactobacillus acidophilus, have mental health benefits, and can have a positive effect on the host-brain relationship, and have antidepressant effects. This article reviews current studies on the association between gut microbiota dysbiosis and depression. Comprehensively, these findings could potentially lead to novel approaches to improving depressive symptoms via gut microbiota alterations, including probiotics, prebiotics, and fecal microbiota transplantation.

Comprehensive analysis of genetic risk loci uncovers novel candidate genes and pathways in the comorbidity between depression and Alzheimer's disease

There is growing evidence of a shared pathogenesis between Alzheimer's disease and depression. Therefore, we aimed to further investigate their shared disease mechanisms. We made use of publicly available brain-specific eQTL data and gene co-expression networks of previously reported genetic loci associated with these highly comorbid disorders. No direct genetic overlap was observed between Alzheimer's disease and depression in our dataset, but we did detect six shared brain-specific eQTL genes: SRA1, MICA, PCDHA7, PCDHA8, PCDHA10 and PCDHA13. Several pathways were identified as shared between Alzheimer's disease and depression by conducting clustering pathway analysis on hippocampal co-expressed genes; synaptic signaling and organization, myelination, development, and the immune system. This study highlights trans-synaptic signaling and synaptoimmunology in the hippocampus as main shared pathomechanisms of Alzheimer's disease and depression.

Glial-restricted precursors stimulate endogenous cytogenesis and effectively recover emotional deficits in a model of cytogenesis ablation

Adult cytogenesis, the continuous generation of newly-born neurons (neurogenesis) and glial cells (gliogenesis) throughout life, is highly impaired in several neuropsychiatric disorders, such as Major Depressive Disorder (MDD), impacting negatively on cognitive and emotional domains. Despite playing a critical role in brain homeostasis, the importance of gliogenesis has been overlooked, both in healthy and diseased states. To examine the role of newly formed glia, we transplanted Glial Restricted Precursors (GRPs) into the adult hippocampal dentate gyrus (DG), or injected their secreted factors (secretome), into a previously validated transgenic GFAP-tk rat line, in which cytogenesis is transiently compromised. We explored the long-term effects of both treatments on physiological and behavioral outcomes. Grafted GRPs reversed anxiety-like deficits and demonstrated an antidepressant-like effect, while the secretome promoted recovery of only anxiety-like behavior. Furthermore, GRPs elicited a recovery of neurogenic and gliogenic levels in the ventral DG, highlighting the unique involvement of these cells in the regulation of brain cytogenesis. Both GRPs and their secretome induced significant alterations in the DG proteome, directly influencing proteins and pathways related to cytogenesis, regulation of neural plasticity and neuronal development. With this work, we demonstrate a valuable and specific contribution of glial progenitors to normalizing gliogenic levels, rescuing neurogenesis and, importantly, promoting recovery of emotional deficits characteristic of disorders such as MDD.

SYK-623, a δ Opioid Receptor Inverse Agonist, Mitigates Chronic Stress-Induced Behavioral Abnormalities and Disrupted Neurogenesis

The δ opioid receptor (DOR) inverse agonist has been demonstrated to improve learning and memory impairment in mice subjected to restraint stress. Here, we investigated the effects of SYK-623, a new DOR inverse agonist, on behavioral, immunohistochemical, and biochemical abnormalities in a mouse model of imipramine treatment-resistant depression. Male ddY mice received daily treatment of adrenocorticotropic hormone (ACTH) combined with chronic mild stress exposure (ACMS). SYK-623, imipramine, or the vehicle was administered once daily before ACMS. After three weeks, ACMS mice showed impaired learning and memory in the Y-maze test and increased immobility time in the forced swim test. SYK-623, but not imipramine, significantly suppressed behavioral abnormalities caused by ACMS. Based on the fluorescent immunohistochemical analysis of the hippocampus, ACMS induced a reduction in astrocytes and newborn neurons, similar to the reported findings observed in the postmortem brains of depressed patients. In addition, the number of parvalbumin-positive GABA neurons, which play a crucial role in neurogenesis, was reduced in the hippocampus, and western blot analysis showed decreased glutamic acid decarboxylase protein levels. These changes, except for the decrease in astrocytes, were suppressed by SYK-623. Thus, SYK-623 mitigates behavioral abnormalities and disturbed neurogenesis caused by chronic stress.

Antidepressant-like effects of the Punica granatum and citalopram combination are associated with structural changes in dendritic spines of granule cells in the dentate gyrus of rats

Introduction: Natural products such as phytoestrogens-enriched foods or supplements have been considered as an alternative therapy to reduce depressive symptoms associated with menopause. It is known that the aqueous extract of Punica granatum (AE-PG) exerts antidepressant-like effects by activating β-estrogen receptors and facilitates the antidepressant response of the clinical drug citalopram (CIT). However, the effects on neuroplasticity are unknown. Objectvie investigated the antidepressant-like response of combining AE-PG and CIT at sub-optimal doses, analyzing their effects on the formation and maturation of dendrite spines in granule cells as well as on the dendrite complexity. Methods: Ovariectomized Wistar rats (3-month-old) were randomly assigned to one of the following groups: A) control (saline solution as vehicle of CIT and AE-PG, B) AE-PG at a sub-threshold dose (vehicle of CIT plus AE-PG at 0.125 mg/kg), C) CIT at a sub-threshold dose (0.77 mg/kg plus vehicle of AE-PG), and D) a combination of CIT plus AE-PG (0.125 mg/kg and 0.77 mg/kg, respectively). All rats were treated intraperitoneally for 14 days. Antidepressant-like effects were evaluated using the force swimming test test (FST). The complexity of dendrites and the number and morphology of dendrite spines of neurons were assessed in the dentate gyrus after Golgi-Cox impregnation. The expressions of the mature brain-derived neurotrophic factor (mBDNF) in plasma and of mBDNF and synaptophysin in the hippocampus, as markers of synaptogenesis, were also determined. Results: Administration of CIT combined with AE-PG, but not alone, induced a significant antidepressant-like effect in the FST with an increase in the dendritic complexity and the number of dendritic spines in the dentate gyrus (DG) of the hippocampus, revealed by the thin and stubby categories of neurons at the granular cell layer. At the same time, an increase of mBDNF and synaptophysin expression was observed in the hippocampus of rats that received the combination of AE-PG and CIT.

Research on Xiaoyao Powder in the treatment of depression based on epigenetics and quality markers

Depression has become one of the most common public health issues around the world, and the incidence has been increasing in recent years. A large amount of clinical investigations have proven that the treatment of depression is difficult. The prognosis is poor, and the fatality rate is high. At present, western medicine is the preferred treatment for depression, but it often causes adverse clinical reactions such as dry mouth, blurred vision, and memory loss, etc. The herbal compound Xiaoyao Powder is a traditional medicine for soothing the liver and relieving depression, strengthening the spleen, and nourishing the blood. It can reduce adverse reactions. It is effective in treating depression. In this study, we elucidate the function of Xiaoyao Powder in anti-depression from the perspective of clinical application and pharmacological mechanisms such as regulating epigenetic and chemical quality markers to provide empirical and experimental theoretical results that contribute to developing future depression therapy with Xiaoyao Powder.

Home-cage behavior is impacted by stress exposure in rats

Being social animals, rats exhibit a range of social behaviors that help them build social bonds and maintain group cohesion. Behavior is influenced by multiple factors, including stress exposure, and the expression of the impact of stress on both social and non-social behaviors may also be affected by the living conditions of rats. In this study, we explored the physiological and behavioral effects of chronic unpredictable stress on group-housed rats in the PhenoWorld (PhW), a socially and physically enriched environment closer to real-life conditions. Two independent experiments were performed: one in the control condition (PhW control, n = 8) and one in the stress condition (PhW stress, n = 8). Control animals remained undisturbed except for cage cleaning and daily handling procedures. Stress group animals were all exposed to chronic unpredictable stress. Data confirm that stress exposure triggers anxiety-like behavior in the PhW. In terms of home-cage behaviors, we found that stress affects social behaviors (by decreased playing and increased huddling behaviors) and non-social behaviors (as shown by the decrease in rearing and walking behaviors). These results are of relevance to expand our knowledge on the influence of stress on social and non-social behaviors, which are of importance to understand better species-typical behaviors.

Fluoxetine Can Cause Epileptogenesis and Aberrant Neurogenesis in Male Wild-Type Mice

Antidepressants in general, and fluoxetine in particular, increase adult hippocampal neurogenesis (AHN) in mice. Here we asked how the antidepressant fluoxetine affects behavior and AHN in a corticosterone model of depression. In three groups of adult male C57BL/6j mice, we administered either vehicle (VEH), corticosterone (CORT) treatment to induce a depression-like state, or corticosterone plus a standard dose of fluoxetine (CORT+FLX). Following treatment, mice performed the open field test, the novelty suppressed feeding (NSF) test, and the splash test. Neurogenesis was assessed by means of immunohistochemistry using BrdU and neuronal maturation markers. Unexpectedly, 42% of the CORT+FLX-treated mice exhibited severe weight loss, seizures, and sudden death. As expected, the CORT-treated group had altered behaviors compared to the VEH group, but the CORT+FLX mice that survived did not show any behavioral improvement compared to the CORT group. Antidepressants generally increase neurogenesis and here we also found that compared to CORT mice, CORT+FLX mice that survived had a significantly greater density of BrdU+, BrdU+DCX+, and BrdU+NeuN+ cells, suggesting increased neurogenesis. Moreover, the density of BrdU+NeuN+ cells was increased in an aberrant location, the hilus, of CORT+FLX mice, similar to previous studies describing aberrant neurogenesis following seizures. In conclusion, fluoxetine could induce considerable adverse effects in wild-type mice, including seizure-like activity. Fluoxetine-induced neurogenesis increases could be related to this activity; therefore, proneurogenic effects of fluoxetine and other antidepressants, especially in the absence of any behavioral therapeutic effects, should be interpreted with caution.

Glial-Restricted Precursors stimulate endogenous cytogenesis and effectively recover emotional deficits in a model of cytogenesis ablation

Adult cytogenesis, the continuous generation of newly-born neurons (neurogenesis) and glial cells (gliogenesis) throughout life, is highly impaired in several neuropsychiatric disorders, such as Major Depressive Disorder (MDD), impacting negatively on cognitive and emotional domains. Despite playing a critical role in brain homeostasis, the importance of gliogenesis has been overlooked, both in healthy and diseased states. To examine the role of newly formed glia, we transplanted Glial Restricted Precursors (GRPs) into the adult hippocampal dentate gyrus (DG), or injected their secreted factors (secretome), into a previously validated transgenic GFAP-tk rat line, in which cytogenesis is transiently compromised. We explored the long-term effects of both treatments on physiological and behavioral outcomes. Grafted GRPs reversed anxiety-like and depressive-like deficits, while the secretome promoted recovery of only anxiety-like behavior. Furthermore, GRPs elicited a recovery of neurogenic and gliogenic levels in the ventral DG, highlighting the unique involvement of these cells in the regulation of brain cytogenesis. Both GRPs and their secretome induced significant alterations in the DG proteome, directly influencing proteins and pathways related to cytogenesis, regulation of neural plasticity and neuronal development. With this work, we demonstrate a valuable and specific contribution of glial progenitors to normalizing gliogenic levels, rescueing neurogenesis and, importantly, promoting recovery of emotional deficits characteristic of disorders such as MDD.

Early-life stress biases responding to negative feedback and increases amygdala volume and vulnerability to later-life stress

Early-life stress (ELS) or adversity, particularly in the form of childhood neglect and abuse, is associated with poor mental and physical health outcomes in adulthood. However, whether these relationships are mediated by the consequences of ELS itself or by other exposures that frequently co-occur with ELS is unclear. To address this question, we carried out a longitudinal study in rats to isolate the effects of ELS on regional brain volumes and behavioral phenotypes relevant to anxiety and depression. We used the repeated maternal separation (RMS) model of chronic ELS, and conducted behavioral measurements throughout adulthood, including of probabilistic reversal learning (PRL), responding on a progressive ratio task, sucrose preference, novelty preference, novelty reactivity, and putative anxiety-like behavior on the elevated plus maze. Our behavioral assessment was combined with magnetic resonance imaging (MRI) for quantitation of regional brain volumes at three time points: immediately following RMS, young adulthood without further stress, and late adulthood with further stress. We found that RMS caused long-lasting, sexually dimorphic biased responding to negative feedback on the PRL task. RMS also slowed response time on the PRL task, but without this directly impacting task performance. RMS animals were also uniquely sensitive to a second stressor, which disproportionately impaired their performance and slowed their responding on the PRL task. MRI at the time of the adult stress revealed a larger amygdala volume in RMS animals compared with controls. These behavioral and neurobiological effects persisted well into adulthood despite a lack of effects on conventional tests of 'depression-like' and 'anxiety-like' behavior, and a lack of any evidence of anhedonia. Our findings indicate that ELS has long-lasting cognitive and neurobehavioral effects that interact with stress in adulthood and may have relevance for understanding the etiology of anxiety and depression in humans.

A thalamic-primary auditory cortex circuit mediates resilience to stress

Resilience enables mental elasticity in individuals when rebounding from adversity. In this study, we identified a microcircuit and relevant molecular adaptations that play a role in natural resilience. We found that activation of parvalbumin (PV) interneurons in the primary auditory cortex (A1) by thalamic inputs from the ipsilateral medial geniculate body (MG) is essential for resilience in mice exposed to chronic social defeat stress. Early attacks during chronic social defeat stress induced short-term hyperpolarizations of MG neurons projecting to the A1 (MG^A1 neurons) in resilient mice. In addition, this temporal neural plasticity of MG^A1 neurons initiated synaptogenesis onto thalamic PV neurons via presynaptic BDNF-TrkB signaling in subsequent stress responses. Moreover, optogenetic mimicking of the short-term hyperpolarization of MG^A1 neurons, rather than merely activating MG^A1 neurons, elicited innate resilience mechanisms in response to stress and achieved sustained antidepressant-like effects in multiple animal models, representing a new strategy for targeted neuromodulation.

Early life adversity across different cell- types in the brain

Early life adversity (ELA)- which includes physical, psychological, emotional, and sexual abuse is one of the most common predictors to diverse psychopathologies later in adulthood. As ELA has a lasting impact on the brain at a developmental stage, recent findings from the field highlighted the specific contributions of different cell types to ELA and their association with long lasting consequences. In this review we will gather recent findings describing morphological, transcriptional and epigenetic alterations within neurons, glia and perineuronal nets and their associated cellular subpopulation. The findings reviewed and summarized here highlight important mechanisms underlying ELA and point to therapeutic approaches for ELA and related psychopathologies later in life.

Stress-induced reduction of Na+/H+ exchanger isoform 1 promotes maladaptation of neuroplasticity and exacerbates depressive behaviors

Major depression disorder (MDD) is a neuropsychiatric disorder characterized by abnormal neuronal activity in specific brain regions. A factor that is crucial in maintaining normal neuronal functioning is intracellular pH (pHi) homeostasis. In this study, we show that chronic stress, which induces depression-like behaviors in animal models, down-regulates the expression of the hippocampal Na⁺/H⁺ exchanger isoform 1, NHE1, a major determinant of pHi in neurons. Knockdown of NHE1 in CA1 hippocampal pyramidal neurons leads to intracellular acidification, promotes dendritic spine loss, lowers excitatory synaptic transmission, and enhances the susceptibility to stress exposure in rats. Moreover, E3 ubiquitin ligase cullin4A may promote ubiquitination and degradation of NHE1 to induce these effects of an unbalanced pHi on synaptic processes. Electrophysiological data further suggest that the abnormal excitability of hippocampal neurons caused by maladaptation of neuroplasticity may be involved in the pathogenesis of this disease. These findings elucidate a mechanism for pHi homeostasis alteration as related to MDD.

Resilience to chronic mild stress-induced anhedonia preserves the ability of the ventral hippocampus to respond to an acute challenge

Stress is a major precipitating factor for psychiatric disorders and its effects may depend on its duration and intensity. Of note, there are differences in individual susceptibility to stress, with some subjects displaying vulnerability and others showing resistance. Furthermore, the ability to react to stressful-life events can alter the response to a subsequent new stressor. Hence, we investigated whether the vulnerability and resilience to the chronic mild stress (CMS) paradigm, in terms of the hedonic phenotype, are paralleled by a different response when facing a novel acute challenge. Specifically, rats submitted to CMS were stratified based on their sucrose intake into vulnerable (anhedonic rats showing reduce intake of sucrose) and resilient (rats not showing the anhedonic-like behavior) subgroups and then further exposed to an acute restraint stress (ARS). Then, neuronal activation was investigated by measuring the gene expression of early immediate (IEG) genes such as Arc and Cfos and early response (ERG) genes, such as Gadd45β, Sgk1, Dusp1, and Nr4a1, in brain regions that play a crucial role in the stress response. We found that resilient rats preserve the ability to increase ERG expression following the ARS selectively in the ventral hippocampus. Conversely, such ability is lost in vulnerable rats. Interestingly, the recovery from the anhedonic phenotype observed in vulnerable rats after 3 weeks of rest from the CMS procedure also parallels the restoration of the ability to adequately respond to the challenge. In conclusion, these findings support the role of the ventral subregion of the hippocampus in the management of both chronic and acute stress response and point to this brain subregion as a critical target for a potential therapeutic strategy aimed at promoting stress resilience.

Dissecting the role of adult hippocampal neurogenesis towards resilience versus susceptibility to stress-related mood disorders

Adult hippocampal neurogenesis in the developmental process of generating and integrating new neurons in the hippocampus during adulthood and is a unique form of structural plasticity with enormous potential to modulate neural circuit function and behaviour. Dysregulation of this process is strongly linked to stress-related neuropsychiatric conditions such as anxiety and depression, and efforts have focused on unravelling the contribution of adult-born neurons in regulating stress response and recovery. Chronic stress has been shown to impair this process, whereas treatment with clinical antidepressants was found to enhance the production of new neurons in the hippocampus. However, the precise role of adult hippocampal neurogenesis in mediating the behavioural response to chronic stress is not clear and whether these adult-born neurons buffer or increase susceptibility to stress-induced mood-related maladaptation remains one of the controversial issues. In this review, we appraise evidence probing the causal role of adult hippocampal neurogenesis in the regulation of emotional behaviour in rodents. We find that the relationship between adult-born hippocampal neurons and stress-related mood disorders is not linear, and that simple subtraction or addition of these neurons alone is not sufficient to lead to anxiety/depression or have antidepressant-like effects. We propose that future studies examining how stress affects unique properties of adult-born neurons, such as the excitability and the pattern of connectivity during their critical period of maturation will provide a deeper understanding of the mechanisms by which these neurons contribute to functional outcomes in stress-related mood disorders.

Inhibition of Hippocampal Neurogenesis Starting in Adolescence Increases Anxiodepressive Behaviors Amid Stress

Stressors during the adolescent period can affect development of the brain and have long-lasting impacts on behavior. Specifically, adolescent stress impairs hippocampal neurogenesis and can increase risk for anxiety, depression, and a dysregulated stress response in adulthood. In order to model the functional effects of reduced hippocampal neurogenesis during adolescence, a transgenic neurogenesis ablation rat model was used to suppress neurogenesis during the adolescent period and test anxiodepressive behaviors and stress physiology during adulthood. Wildtype and transgenic (TK) rats were given valganciclovir during the first two weeks of adolescence (4-6 weeks old) to knock down neurogenesis in TK rats. Starting in young adulthood (13 weeks old), blood was sampled for corticosterone at several time points following acute restraint stress to measure negative feedback of the stress response, and rats were tested on a battery of anxiodepressive tests at baseline and following acute restraint stress. Although TK rats had large reductions in both cell proliferation during adolescence, as measured by bromodeoxyuridine (BrdU), and ongoing neurogenesis in adulthood (by doublecortin), resulting in decreased volume of the dentate gyrus, negative feedback of the stress response following acute restraint was similar across all rats. Despite similar stress responses, TK rats showed higher anxiety-like behavior at baseline. In addition, only TK rats had increased depressive-like behavior when tested after acute stress. Together, these results suggest that long-term neurogenesis ablation starting in adolescence produces hippocampal atrophy and increases behavioral caution and despair amid stressful environments.

The Duration of Stress Determines Sex Specificities in the Vulnerability to Depression and in the Morphologic Remodeling of Neurons and Microglia

Stress exposure has been shown to induce a variety of molecular and functional alterations associated with anxiety and depression. Some studies suggest that microglia, the immune cells of the brain, play a significant role in determining neuronal and behavioral responses to chronic stress and also contribute to the development of stress-related psychopathologies. However, little is known about the impact of the duration of stress exposure upon microglia and neurons morphology, particularly considering sex differences. This issue deserves particular investigation, considering that the process of morphologic remodeling of neurons and microglia is usually accompanied by functional changes with behavioral expression. Here, we examine the effects of short and long unpredictable chronic mild stress (uCMS) protocols on behavior, evaluating in parallel microglia and neurons morphology in the dorsal hippocampus (dHIP) and in the nucleus accumbens (NAc), two brain regions involved in the etiology of depression. We report that long-term uCMS induced more behavioral alterations in males, which present anxiety and depression-like phenotypes (anhedonia and helplessness behavior), while females only display anxiety-like behavior. After short-term uCMS, both sexes presented anxiety-like behavior. Microglia cells undergo a process of morphologic adaptation to short-term uCMS, dependent on sex, in the NAc: we observed a hypertrophy in males and an atrophy in females, transient effects that do not persist after long-term uCMS. In the dHIP, the morphologic adaptation of microglia is only observed in females (hypertrophy) and after the protocol of long uCMS. Interestingly, males are more vulnerable to neuronal morphological alterations in a region-specific manner: dendritic atrophy in granule neurons of the dHIP and hypertrophy in the medium spiny neurons of the NAc, both after short- or long-term uCMS. The morphology of neurons in these brain regions were not affected in females. These findings raise the possibility that, by differentially affecting neurons and microglia in dHIP and NAc, chronic stress may contribute for differences in the clinical presentation of stress-related disorders under the control of sex-specific mechanisms.

Rodent models of stress and dendritic plasticity – Implications for psychopathology

Stress, as commonplace as it is, is a major environmental risk factor for psychopathology. While this association intuitively, anecdotally, and empirically makes sense, we are still very early in the process of understanding what the neurobiological manifestations of this risk truly are. Seminal work from the past few decades has established structural plasticity in the brain as a potential key mechanism. In this review we discuss evidence linking particularly chronic stress exposure in rodent models to plasticity at the dendrites, like remodeling of dendritic branches and spines, in a range of brain regions. A number of candidate mechanisms that seek to explain how stress influences neuroanatomy at this level have been proposed, utilizing in vivo, ex vivo and in vitro methods. However, a large gap still remains in our knowledge of how such dynamic structural changes ultimately relate to downstream effects such as altered affective and cognitive states relevant for psychopathology. We propose that future work expand our understanding of plasticity of specific stress-related brain circuits and cell-types. We also note that the vast majority of the work has been conducted solely on male rodents. The next big strides in our understanding of the neurobiology of psychopathology will require the inclusion of female subjects, as several studies have suggested both sex divergent and convergent features. By understanding plasticity, we can harness it. The growth of this body of knowledge will inform our efforts to improve the therapeutic options for stress-related psychopathology.

Beyond New Neurons in the Adult Hippocampus: Imipramine Acts as a Pro-Astrogliogenic Factor and Rescues Cognitive Impairments Induced by Stress Exposure

Depression is a prevalent, socially burdensome disease. Different studies have demonstrated the important role of astrocytes in the pathophysiology of depression as modulators of neurotransmission and neurovascular coupling. This is evidenced by astrocyte impairments observed in brains of depressed patients and the appearance of depressive-like behaviors upon astrocytic dysfunctions in animal models. However, little is known about the importance of de novo generated astrocytes in the mammalian brain and in particular its possible involvement in the precipitation of depression and in the therapeutic actions of current antidepressants (ADs). Therefore, we studied the modulation of astrocytes and adult astrogliogenesis in the hippocampal dentate gyrus (DG) of rats exposed to an unpredictable chronic mild stress (uCMS) protocol, untreated and treated for two weeks with antidepressants—fluoxetine and imipramine. Our results show that adult astrogliogenesis in the DG is modulated by stress and imipramine. This study reveals that distinct classes of ADs impact differently in the astrogliogenic process, showing different cellular mechanisms relevant to the recovery from behavioral deficits induced by chronic stress exposure. As such, in addition to those resident, the newborn astrocytes in the hippocampal DG might also be promising therapeutic targets for future therapies in the neuropsychiatric field.

Inhibition of adult neurogenesis reduces avoidance behavior in male, but not female, mice subjected to early life adversity

Early life adversity (ELA) increases the risk of developing neuropsychiatric illnesses such as anxiety disorders. However, the mechanisms connecting these negative early life experiences to illness later in life remain unclear. In rodents, plasticity mechanisms, specifically adult neurogenesis in the ventral hippocampus, have been shown to be altered by ELA and important for buffering against detrimental stress-induced outcomes. The current study sought to explore whether adult neurogenesis contributes to ELA-induced changes in avoidance behavior. Using the GFAP-TK transgenic model, which allows for the inhibition of adult neurogenesis, and CD1 littermate controls, we subjected mice to an ELA paradigm of maternal separation and early weaning (MSEW) or control rearing. We found that mice with intact adult neurogenesis showed no behavioral changes in response to MSEW. After reducing adult neurogenesis, however, male mice previously subjected to MSEW had an unexpected decrease in avoidance behavior. This finding was not observed in female mice, suggesting that a sex difference exists in the role of adult-born neurons in buffering against ELA-induced changes in behavior. Taken together with the existing literature on ELA and avoidance behavior, this work suggests that strain differences exist in susceptibility to ELA and that adult-born neurons may play a role in regulating adaptive behavior.

Mindful nonreactivity moderates the relationship between posttraumatic stress disorder and depression

PTSD and depression represent major individual and societal burdens. Depression is commonly comorbid with PTSD among veterans, although buffers of this relationship are unclear. We evaluated whether facets of mindfulness moderated the relationship between PTSD and depression in veterans with PTSD (N = 70). Three facets - nonjudging, acting with awareness, and nonreactivity - were assessed as moderators. Results indicated nonreactivity significantly attenuated the relationship between PTSD and depression (p=.013), such that veterans with high nonreactivity (+1 SD) showed a nonsignificant relationship between PTSD and depression, whereas veterans with average (Mean; p<.001) and low (-1 SD; p<.001) nonreactivity exhibited a significant relationship.

Altered responsiveness of the antioxidant system in chronically stressed animals: modulation by chronic lurasidone treatment

RATIONALE

Although the occurrence of stressful events is very common during life, their impact may be different depending on the experience severity and duration. Specifically, acute challenges may trigger adaptive responses and even improve the individual's performance. However, such a physiological positive coping can only take place if the underlying molecular mechanisms are properly functioning. Indeed, if these systems are compromised by genetic factors or previous adverse conditions, the response set in motion by an acute challenge may be maladaptive and even cause the insurgence or the relapse of stress-related psychiatric disorders.

OBJECTIVES

On these bases, we evaluated in the rat brain the role of the antioxidant component of the redox machinery on the acute stress responsiveness and its modulation by potential detrimental or beneficial events.

METHODS

The expression of several antioxidant enzymes was assessed in different brain areas of adult male rats exposed to acute stress 3 weeks after a chronic immobilization paradigm with or without a concomitant treatment with the antipsychotic lurasidone.

RESULTS

The acute challenge was able to trigger a marked antioxidant response that, despite the washout period, was impaired by the previous adverse experience and restored by lurasidone in an anatomical-specific manner.

CONCLUSIONS

We found that a working antioxidant machinery takes part in acute stress response and may be differentially affected by other experiences. Given the essential role of stress responsiveness in almost every life process, the identification of the underlying mechanisms and their potential pharmacological modulation add further translational value to our data.

Constitutive deficiency of the neurogenic hippocampal modulator AP2γ promotes anxiety-like behavior and cumulative memory deficits in mice from juvenile to adult periods

The transcription factor activating protein two gamma (AP2γ) is an important regulator of neurogenesis both during embryonic development as well as in the postnatal brain, but its role for neurophysiology and behavior at distinct postnatal periods is still unclear. In this work, we explored the neurogenic, behavioral, and functional impact of a constitutive and heterozygous AP2γ deletion in mice from early postnatal development until adulthood. AP2γ deficiency promotes downregulation of hippocampal glutamatergic neurogenesis, altering the ontogeny of emotional and memory behaviors associated with hippocampus formation. The impairments induced by AP2γ constitutive deletion since early development leads to an anxious-like phenotype and memory impairments as early as the juvenile phase. These behavioral impairments either persist from the juvenile phase to adulthood or emerge in adult mice with deficits in behavioral flexibility and object location recognition. Collectively, we observed a progressive and cumulative impact of constitutive AP2γ deficiency on the hippocampal glutamatergic neurogenic process, as well as alterations on limbic-cortical connectivity, together with functional behavioral impairments. The results herein presented demonstrate the modulatory role exerted by the AP2γ transcription factor and the relevance of hippocampal neurogenesis in the development of emotional states and memory processes.

Hippocampal cytogenesis abrogation impairs inter-regional communication between the hippocampus and prefrontal cortex and promotes the time-dependent manifestation of emotional and cognitive deficits

Impaired ability to generate new cells in the adult brain has been linked to deficits in multiple emotional and cognitive behavioral domains. However, the mechanisms by which abrogation of adult neural stem cells (NSCs) impacts on brain function remains controversial. We used a transgenic rat line, the GFAP-Tk, to selectively eliminate NSCs and assess repercussions on different behavioral domains. To assess the functional importance of newborn cells in specific developmental stages, two parallel experimental timeframes were adopted: a short- and a long-term timeline, 1 and 4 weeks after the abrogation protocol, respectively. We conducted in vivo electrophysiology to assess the effects of cytogenesis abrogation on the functional properties of the hippocampus and prefrontal cortex, and on their intercommunication. Adult brain cytogenesis abrogation promoted a time-specific installation of behavioral deficits. While the lack of newborn immature hippocampal neuronal and glial cells elicited a behavioral phenotype restricted to hyperanxiety and cognitive rigidity, specific abrogation of mature new neuronal and glial cells promoted the long-term manifestation of a more complex behavioral profile encompassing alterations in anxiety and hedonic behaviors, along with deficits in multiple cognitive modalities. More so, abrogation of 4 to 7-week-old cells resulted in impaired electrophysiological synchrony of neural theta oscillations between the dorsal hippocampus and the medial prefrontal cortex, which are likely to contribute to the described long-term cognitive alterations. Hence, this work provides insight on how newborn neurons and astrocytes display different functional roles throughout different maturation stages, and establishes common ground to reconcile contrasting results that have marked this field.

Cell Cycle Regulation of Hippocampal Progenitor Cells in Experimental Models of Depression and after Treatment with Fluoxetine

Changes in adult hippocampal cell proliferation and genesis have been largely implicated in depression and antidepressant action, though surprisingly, the underlying cell cycle mechanisms are largely undisclosed. Using both an in vivo unpredictable chronic mild stress (uCMS) rat model of depression and in vitro rat hippocampal-derived neurosphere culture approaches, we aimed to unravel the cell cycle mechanisms regulating hippocampal cell proliferation and genesis in depression and after antidepressant treatment. We show that the hippocampal dentate gyrus (hDG) of uCMS animals have less proliferating cells and a decreased proportion of cells in the G2/M phase, suggesting a G1 phase arrest; this is accompanied by decreased levels of cyclin D1, E, and A expression. Chronic fluoxetine treatment reversed the G1 phase arrest and promoted an up-regulation of cyclin E. In vitro, dexamethasone (DEX) decreased cell proliferation, whereas the administration of serotonin (5-HT) reversed it. DEX also induced a G1-phase arrest and decreased cyclin D1 and D2 expression levels while increasing p27. Additionally, 5-HT treatment could partly reverse the G1-phase arrest and restored cyclin D1 expression. We suggest that the anti-proliferative actions of chronic stress in the hDG result from a glucocorticoid-mediated G1-phase arrest in the progenitor cells that is partly mediated by decreased cyclin D1 expression which may be overcome by antidepressant treatment.

Adult brain cytogenesis in the context of mood disorders: From neurogenesis to the emergent role of gliogenesis

Psychiatric disorders severely impact patients' lives. Motivational, cognitive and emotional deficits are the most common symptoms observed in these patients and no effective treatment is still available, either due to the adverse side effects or the low rate of efficacy of currently available drugs. Neurogenesis recovery has been one important focus in the treatment of psychiatric disorders, which undeniably contributes to the therapeutic action of antidepressants. However, glial plasticity is emerging as a new strategy to explore the deficits observed in mood disorders and the efficacy of therapeutic interventions. Thus, it is crucial to understand the mechanisms behind glio- and neurogenesis to better define treatments and preventive therapies, once adult cytogenesis is of pivotal importance to cognitive and emotional components of behavior, both in healthy and pathological contexts, including in psychiatric disorders. Here, we review the concepts and history of neuro- and gliogenesis, providing as well a reflection on the functional importance of cytogenesis in the context of disease.

Maternal chewing improves prenatal stress-induced cognitive deficit and anxiety-like behavior associated with alterations of the apoptotic response and serotonin pathway in mouse offspring

OBJECTIVE

To examine whether maternal chewing affects prenatal stress-induced behavioral alternations associated with the changes in apoptosis-related proteins and serotonin pathway of the mouse offspring.

DESIGN

Pregnant mice were assigned to control, stress, and stress/chewing groups. Stress mice were placed in restraint tubes, from gestational day 12 until parturition. Stress/chewing mice were given a wooden stick for chewing during stress period. Morris water maze and hole-board tests were applied for behavioral alterations in one-month-old male pups. Hippocampal mRNA expression of B-cell lymphoma 2 (Bcl-2) and Bcl-2 associated X protein (Bax) was analyzed by quantitative real-time PCR. Serotonin and tryptophan hydroxylase expression level in the dorsal raphe nucleus was investigated immunohistochemically.

RESULTS

Prenatal stress impaired the spatial learning, induced anxiety-like behavior, increased the ratio of hippocampal Bax/Bcl-2 expression, and decreased the expression of serotonin and tryptophan hydroxylase in dorsal raphe nucleus of the offspring. Maternal chewing ameliorated prenatal stress-induced cognitive impairment, anxiety-like behavior, and attenuated the increased ratio of hippocampal Bax/Bcl-2 expression, and the downregulated serotonin signaling in dorsal raphe nucleus of the offspring.

CONCLUSIONS

Our results indicate that maternal chewing could improve prenatal stress-related anxiety-like behavior and cognitive impairment in mouse offspring, at least in part by affecting hippocampal apoptotic response and central serotonin pathway.

Adult Neurogenesis and Antidepressant Treatment: The Surprise Finding by Ron Duman and the Field 20 Years Later

Of Duman's many influential findings, the finding that long-term treatment with antidepressant drugs produces an increase in neurogenesis in the subgranular zone of the adult hippocampus may be one of the most enduring and far-reaching. This novel discovery and his decades of continued research in the field led to a new hypothesis about the mechanism of action of antidepressants, providing a critical step in our understanding of the neurotrophic hypothesis of depression and synaptic plasticity. It is now accepted that antidepressant treatments can oppose and even reverse the effects of stress on the brain and on newly born hippocampal cells, possibly via neurotrophic factors, which Duman had continued to explore. Furthermore, ablation studies have shown preclinically that hippocampal neurogenesis may be necessary for some of the clinical effects of antidepressant drugs. Duman's laboratory continued to interrogate neurotrophins and synaptic plasticity, demonstrating that newer clinically approved antidepressant compounds also affect neurogenesis and synaptic plasticity. In this review, we summarize Duman's original findings and discuss the current state of the field of neurogenesis with respect to animal models and human studies and the implications of those findings on the field of drug discovery.

Comparative Proteomic Characterization of Ventral Hippocampus in Susceptible and Resilient Rats Subjected to Chronic Unpredictable Stress

Chronic stress is an essential factor leading to depression. However, there exist individual differences in people exposed to the same stressful stimuli. Some people display negative psychology and behavior, while others are normal. Given the importance of individual difference, finding differentially expressed proteins in stress-resistant and stress-susceptible groups has great significance for the study of pathogenesis and treatment of depression. In this study, stress-susceptible rats and stress-resilient rats were first distinguished by sucrose preference test. These stress-susceptible rats also displayed depression-like behaviors in forced swimming test and open field test. Then, we employed label-free quantitative proteomics to analyze proteins in the ventral hippocampus. There were 4,848 proteins totally identified. Based on statistical analysis, we found 276 differentially expressed proteins. Bioinformatics analysis revealed that the biological processes of these differential proteins were related to mitochondrion organization, protein localization, coenzyme metabolic process, cerebral cortex tangential migration, vesicle-mediated transport, and so on. The KEGG pathways were mainly involved in metabolic pathways, axon guidance, autophagy, and tight junction. Furthermore, we ultimately found 20 stress-susceptible proteins and two stress-resilient proteins. These stress-related proteins could not only be potential biomarkers for depression diagnosis but also contribute to finding new therapeutic targets and providing personalized medicine.

Brain circuitry underlying the ABC model of anxiety

Anxiety Disorders are prevalent and often chronic, recurrent conditions that reduce quality of life. The first-line treatments, such as serotonin reuptake inhibitors and cognitive behavioral therapy, leave a significant proportion of patients symptomatic. As psychiatry moves toward targeted circuit-based treatments, there is a need for a theory that unites the phenomenology of anxiety with its underlying neural circuits. The Alarm, Belief, Coping (ABC) theory of anxiety describes how the neural circuits associated with anxiety interact with each other and domains of the anxiety symptoms, both temporally and spatially. The latest advancements in neuroimaging techniques offer the ability to assess these circuits in vivo. Using Neurosynth, a large open-access meta-analytic imaging database, the association between terms related to specific neural circuits was explored within the ABC theory framework. Alarm-related terms were associated with the amygdala, anterior cingulum, insula, and bed nucleus of stria terminalis. Belief-related terms were associated with medial prefrontal cortex, precuneus, bilateral temporal poles, and hippocampus. Coping-related terms were associated with the ventrolateral and dorsolateral prefrontal cortices, basal ganglia, and anterior cingulate. Neural connections underlying the functional neuroanatomy of the ABC model were observed. Additionally, there was considerable interaction and overlap between circuits associated with the symptom domains. Further neuroimaging research is needed to explore the dynamic interaction between the functional domains of the ABC theory. This will pave the way for probing the neuroanatomical underpinnings of anxiety disorders and provide an evidence-based foundation for the development of targeted treatments, such as neuromodulation.

Chronic pain susceptibility is associated with anhedonic behavior and alterations in the accumbal ubiquitin-proteasome system

ABSTRACT

It remains unknown why on similar acute/subacute painful conditions, pain persists in some individuals while in others it resolves. Genetic factors, mood, and functional alterations, particularly involving the mesolimbic network, seem to be key. To explore potential susceptibility or resistance factors, we screened a large population of rats with a peripheral neuropathy and we isolated a small subset (<15%) that presented high thresholds (HTs) to mechanical allodynia (reduced pain manifestation). The phenotype was sustained over 12 weeks and was associated with higher hedonic behavior when compared with low-threshold (LT) subjects. The nucleus accumbens of HT and LT animals were isolated for proteomic analysis by Sequential Window Acquisition of All Theoretical Mass Spectra. Two hundred seventy-nine proteins displayed different expression between LT and HT animals or subjects. Among several protein families, the proteasome pathway repeatedly emerged in gene ontology enrichment and KEGG analyses. Several alpha and beta 20S proteasome subunits were increased in LT animals when compared with HT animals (eg, PSMα1, PSMα2, and PSMβ5). On the contrary, UBA6, an upstream ubiquitin-activating enzyme, was decreased in LT animals. Altogether these observations are consistent with an overactivation of the accumbal proteasome pathway in animals that manifest pain and depressive-like behaviors after a neuropathic injury. All the proteomic data are available through ProteomeXchange with identifier PXD022478.

Neurogenic hypothesis of positive psychology in stress-induced depression: Adult hippocampal neurogenesis, neuroinflammation, and stress resilience

Stress is an important risk factor for depression. Emerging evidence supports the hypothesis that stress-mediated neuroinflammation destroys brain function and leads to anxiety-like and depression-like behaviors. Previous studies of stress-induced depression have mainly focused on pathological damage; however, the rise of positive psychology has attracted the interest of many researchers in environmental enrichment to promote stress resilience. The hippocampus is one of the most severely damaged brain regions in stress-induced depression. In addition, the hippocampus is one of the most unique regions in the brain, as new neurons are produced in the adult hippocampus, a process known as adult hippocampal neurogenesis (AHN). AHN is an important core component of the neurogenic hypothesis and has also become a major innovative breakthrough in positive psychology, in which environmental enrichment mediates stress resilience. Neuroinflammation, by activating microglia and releasing some proinflammatory cytokines, is increasingly shown to be one of the key determinant pathophysiological factors that negatively affects AHNand cognitive reserve. AHN is mainly related to remodeling stress response mechanisms, such as memory clearing, emotional control, and pattern separation, suggesting that a correlation may exist between neuroinflammation and AHN in stress resilience. Therefore, we summarized the previous research results to systematically expound on the relationship between AHN, stress resilience, and neuroinflammation. We hope this neurogenic hypothesis of positive psychology in stress-induced depression will provide a new perspective for the study of depression and antidepressant therapy.

Collapsin response mediator protein 5 (CRMP5) modulates susceptibility to chronic social defeat stress in mice

Collapsin response mediator protein 5 (CRMP5), a member of the CRMP family, is expressed in the brain, particularly in the hippocampus, an area of the brain that can modulate stress responses. Social stress has a well-known detrimental effect on health and can lead to depression, but not all individuals are equally sensitive to stress. To date, researchers have not conclusively determined how social stress increases the susceptibility of the brain to depression. Here, we used the chronic social defeat stress (CSDS) model and observed higher hippocampal CRMP5 expression in stress-susceptible (SS) mice than in control and stress-resilient (RES) mice. A negative correlation was observed between the expression levels of CRMP5 and the social interaction (SI) ratio. Reduced hippocampal CRMP5 expression increased the SI ratio in SS mice, whereas CRMP5 overexpression was sufficient to induce social avoidance behaviors in control mice following exposure to subthreshold social stress induced by lentivirus-based overexpression and inducible tetracycline-on strategies to upregulate CRMP5. Interestingly, increased CRMP5 expression in SS and lenti-CRMP5-treated mice also caused serum corticosterone concentrations to increase. These findings improve our understanding of the potential mechanism by which CRMP5 triggers susceptibility to social stress, and they support the further development of therapeutic agents for the treatment of stress disorders in humans.

Resilience to stress and sex-specific remodeling of microglia and neuronal morphology in a rat model of anxiety and anhedonia

Prenatal exposure to stress or glucocorticoids (GC) is associated with the appearance of psychiatric diseases later in life. Microglia, the immune cells of the brain, are altered in stress-related disorders.

Synthetic GC such as dexamethasone (DEX) are commonly prescribed in case of preterm risk labour in order to promote fetal lung maturation. Recently, we reported long-lasting differences in microglia morphology in a model of in utero exposure to DEX (iuDEX), that presents an anxious phenotype. However, it is still unclear if stress differentially affects iuDEX males and females.

In this work, we evaluated how iuDEX animals of both sexes cope with chronic mild stress for 2 weeks. We evaluated emotional behavior and microglia and neuronal morphology in the dorsal hippocampus (dHIP) and nucleus accumbens (NAc), two brain regions involved in emotion-related disorders. We report that males and females prenatally exposed to DEX have better performance in anxiety- and depression-related behavioral tests after chronic stress exposure in adulthood than non-exposed animals. Interestingly, iuDEX animals present sex-dependent changes in microglia morphology in the dHIP (hypertrophy in females) and in the NAc (atrophy in females and hypertrophy in males). After chronic stress, these cells undergo sex-specific morphological remodeling. Paralleled to these alterations in cytoarchitecture of microglia, we report inter-regional differences in dendritic morphology in a sex-specific manner. iuDEX females present fewer complex neurons in the NAc, whereas iuDEX males presented less complex neuronal morphology in the dHIP. Interestingly, these alterations were modified by stress exposure.

Our work shows that stressful events during pregnancy can exert a preserved sex-specific effect in adulthood. Although the role of the observed cellular remodeling is still unknown, sex-specific differences in microglia plasticity induced by long-term stress exposure may anticipate differences in drug efficacy in the context of stress-induced anxiety- or depression-related behaviors.

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iuDEX induces anxiety- and depression-related behavioral in both sexes.

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iuDEX induces sex dependent fine structural alterations in neurons and microglia morphology in the dHIP and in the NAc.

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uCMS in combination to iuDEX normalize the behavior as well the morphology of neurons in the NAc.

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Stressful events during pregnancy can exert a preserved sex-specific effect in adulthood.

Identification of the antidepressive properties of C1, a specific inhibitor of Skp2, in mice

We have reported that SMIP004, an inhibitor of S-phase kinase-associated protein 2 (Skp2), displays antidepressant-like activities in stress-naïve and chronically stressed mice. Here, we investigated the antidepressant-like effect of C1, another inhibitor of Skp2, in mouse models following acute or chronic drug administration at different doses and treatment times by using the tail suspension test (TST), forced swimming test (FST), and social interaction test (SIT). The time- and dose-dependent results showed that the antidepressant-like effect of C1 occurred 8 days after the drug treatment, and C1 produced antidepressant-like activities at the dose of 5 and 10 but not 1 mg/kg in male or female mice. C1 administration (5 mg/kg) also induced antidepressant-like effects in stress-naïve mice in a three-times administration mode within 24 h (24, 5, and 1 h before the test) but not in an acute administration mode (1 h before the test). The C1 and fluoxetine co-administration produced additive effect on depression-like behaviors in stress-naïve mice. The antidepressant-like effect of C1 was not associated with the change in locomotor activity, as no increased locomotor activity was observed in different treatment modes. Furthermore, the long-term C1 treatment (5 mg/kg) was found to ameliorate the depression-like behaviors in chronic social defeat stress-exposed mice, suggesting that C1 can produce antidepressant-like actions in stress conditions. Since C1 is a specific inhibitor of Skp2, our results demonstrate that inhibition of Skp2 might be a potential strategy for the treatment of depression, and Skp2 may be potential target for the development of novel antidepressants.

The Epac2 coding gene (RAPGEF4) rs3769219 polymorphism is associated with protection against major depressive disorder in the Chinese Han population

BACKGROUND

Adult hippocampal neurogenesis has been demonstrated to be associated with the occurrence of major depressive disorder (MDD). A recent study indicated that deletion of the Epac2 gene (RAPGEF4) caused downregulation of hippocampal neurogenesis. This study aimed to analyze the association between genetic variants of the RAPGEF4 gene and the risk of MDD.

METHODS

We recruited 502 patients with MDD and 504 healthy controls who matched for age and gender. Genomic DNA was extracted from whole blood samples and genotyping was performed by next-generation sequencing. In addition, we conducted subgroup analysis according to the gender and recurrence, respectively.

RESULTS

We found no significant association between RAPGEF4 gene rs3769219 variant and MDD in all subjects. However, the A-allele and GA + AA genotypes at rs3769219 were significantly associated with a reduced risk of MDD in the male population but not in the female population. Similarly, our study identified the A-allele and GA + AA genotypes at rs3769219 as protective factors for recurrent MDD (rMDD).

CONCLUSION

Our findings suggest that RAPGEF4 gene rs3769219 mutation is associated with a reduced risk of MDD in male population and rMDD in total population.

Chronic Restraint Stress Inhibits the Response to a Second Hit in Adult Male Rats: A Role for BDNF Signaling

Depression is a recurrent disorder, with about 50% of patients experiencing relapse. Exposure to stressful events may have an adverse impact on the long-term course of the disorder and may alter the response to a subsequent stressor. Indeed, not all the systems impaired by stress may normalize during symptoms remission, facilitating the relapse to the pathology. Hence, we investigated the long-lasting effects of chronic restraint stress (CRS) and its influence on the modifications induced by the exposure to a second hit on brain-derived neurotrophic factor (BDNF) signaling in the prefrontal cortex (PFC). We exposed adult male Sprague Dawley rats to 4 weeks of CRS, we left them undisturbed for the subsequent 3 weeks, and then we exposed animals to one hour of acute restraint stress (ARS). We found that CRS influenced the release of corticosterone induced by ARS and inhibited the ability of ARS to activate mature BDNF, its receptor Tropomyosin receptor kinase B (TRKB), and their associated intracellular cascades: the TRKB-PI3K-AKT), the MEK-MAPK/ERK, and the Phospholipase C γ (PLCγ) pathways, positively modulated by ARS in non-stressed animals. These results suggest that CRS induces protracted and detrimental consequences that interfere with the ability of PFC to cope with a challenging situation.

Nucleus Reuniens Lesion and Antidepressant Treatment Prevent Hippocampal Neurostructural Alterations Induced by Chronic Mild Stress in Male Rats

The hippocampus-prefrontal cortex circuit plays a major role in stress and in the neurobiology of depression and its treatment. Disruption of this circuit by lesioning the thalamic nucleus reuniens (RE) has been shown to prevent the detrimental effects of chronic mild stress on prefrontal cortex neuroplasticity indices in male rats. However, it remains unknown whether hippocampal neurostructural response to stress is modified by RE lesion. In the present study, adult male rats were subjected to the chronic mild stress model of depression and were treated with either vehicle or an antidepressant (i.e. sertraline). Moreover, a group of animals was subjected to RE lesion before stress exposure with or without sertraline treatment. We demonstrated that chronic mild stress induced hippocampal CA1 dendritic atrophy and this was prevented by pre-stress RE lesion to the same extent that antidepressant treatment reversed it. The present findings highlight the importance of hippocampal-prefrontal cortex communication in chronic stress effects on hippocampal neuroplasticity and contribute to the elucidation of the role of RE in neurostructural changes underlying stress-driven depression and its treatment.

Alleviation of Depression by Glucagon-Like Peptide 1 Through the Regulation of Neuroinflammation, Neurotransmitters, Neurogenesis, and Synaptic Function

Depression has emerged as a major cause of mortality globally. Many studies have reported risk factors and mechanisms associated with depression, but it is as yet unclear how these findings can be applied to the treatment and prevention of this disorder. The onset and recurrence of depression have been linked to diverse metabolic factors, including hyperglycemia, dyslipidemia, and insulin resistance. Recent studies have suggested that depression is accompanied by memory loss as well as depressive mood. Thus, many researchers have highlighted the relationship between depressive behavior and metabolic alterations from various perspectives. Glucagon-like peptide-1 (GLP-1), which is secreted from gut cells and hindbrain areas, has been studied in metabolic diseases such as obesity and diabetes, and was shown to control glucose metabolism and insulin resistance. Recently, GLP-1 was highlighted as a regulator of diverse pathways, but its potential as the therapeutic target of depressive disorder was not described comprehensively. Therefore, in this review, we focused on the potential of GLP-1 modulation in depression.

Adult Neurogenesis, Glia, and the Extracellular Matrix

In the adult mammalian hippocampus, new neurons arise from stem and progenitor cell division, in a process known as adult neurogenesis. Adult-generated neurons are sensitive to experience and may participate in hippocampal functions, including learning and memory, anxiety and stress regulation, and social behavior. Increasing evidence emphasizes the importance of new neuron connectivity within hippocampal circuitry for understanding the impact of adult neurogenesis on brain function. In this Review, we discuss how the functional consequences of new neurons arise from the collective interactions of presynaptic and postsynaptic neurons, glial cells, and the extracellular matrix, which together form the "tetrapartite synapse."

Depressive-like state sensitizes 5-HT1A and 5-HT1B auto-receptors in the dorsal raphe nucleus sub-system

Dorsal raphe (DR) and median raphe (MR) 5-HT neurons are two distinct sub-systems known to be regulated by 5-HT_1A and 5-HT_1B auto-receptors. Whether the auto-receptors in each sub-system are functionally altered in depressive-like state remains unknown. The present study is aimed to study a specific circuit (DR-ventral hippocampus and MR-dorsal hippocampus) within each sub-system to investigate changes in receptor sensitivity in the pathogenesis of depression. A mouse model of depression was developed through the social defeat paradigm, and was then treated with fluoxetine (FLX). 5-HT_1A auto-receptor in the neuronal cell body (DR or MR) and 5-HT_1B auto-receptor in the axonal terminal (ventral or dorsal hippocampus) were directly targeted by local perfusion of antagonists (5-HT_1A: WAY100635; 5-HT_1B: GR127935) through reverse microdialysis. Time courses of dialysate 5-HT measured at the axonal terminal were subsequently determined for each circuit. At baseline, 5-HT_1A and 5-HT_1B antagonists dose-dependently increased dialysate 5-HT, with sub-circuit specificity. In the depressive-like state, greater increases in dialysate 5-HT were observed only in the DR-ventral hippocampus circuit following local delivery of both antagonists, which were then fully restored following the FLX treatment. In contrast, no changes were observed in the MR-dorsal hippocampus circuit. Our results demonstrate differential changes in sensitivities of 5-HT_1A and 5-HT_1B auto-receptors in the DR-ventral hippocampus and MR-dorsal hippocampus circuits. 5-HT_1A and 5-HT_1B auto-receptors in the DR-ventral hippocampus circuit are sensitized in the depressive-like state. Taken together, these results suggest that the DR sub-system maybe the neural substrate mediating depressive phenotypes.

Stress-Induced Morphological, Cellular and Molecular Changes in the Brain-Lessons Learned from the Chronic Mild Stress Model of Depression

Major depressive disorder (MDD) is a severe illness imposing an increasing social and economic burden worldwide. Numerous rodent models have been developed to investigate the pathophysiology of MDD. One of the best characterized and most widely used models is the chronic mild stress (CMS) model which was developed more than 30 years ago by Paul Willner. More than 2000 published studies used this model, mainly to assess novel compounds with potential antidepressant efficacy. Most of these studies examined the behavioral consequences of stress and concomitant drug intervention. Much fewer studies focused on the CMS-induced neurobiological changes. However, the stress-induced cellular and molecular changes are important as they may serve as potential translational biomarkers and increase our understanding of the pathophysiology of MDD. Here, we summarize current knowledge on the structural and molecular alterations in the brain that have been described using the CMS model. We discuss the latest neuroimaging and postmortem histopathological data as well as molecular changes including recent findings on microRNA levels. Different chronic stress paradigms occasionally deliver dissimilar findings, but the available experimental data provide convincing evidence that the CMS model has a high translational value. Future studies examining the neurobiological changes in the CMS model in combination with clinically effective antidepressant drug intervention will likely deliver further valuable information on the pathophysiology of MDD.

Fluoxetine exposure for more than 2 days decreases the neuronal plasticity mediated by CRMP2 in differentiated PC12 cells

BACKGROUND

Recent studies indicate that antidepressants treatment restores neuronal plasticity. In contrast, some researchers claim that serotonergic antidepressants, including fluoxetine (FLU), may exacerbate neuronal plasticity, which is contradictory and rarely studied. Since almost those studies exposed cells with drugs for 1-2 days as treatment models of antidepressants, it is possible that FLU exposure for longer periods would have opposite effects on neuronal plasticity.

RESULTS

In the present study, we examined the effects of FLU exposure (up to 3 days) on the neuronal plasticity in differentiated PC12 cells. The cell viability shown a slight decrease at day 2 (93.5 ± 3.5 %), followed by a highly significant decrease at day 3(71.4 ± 4.4 %). As previously reported, neuronal plasticity was significantly upregulated by FLU exposure at day 1. However, the neurite length, activity-regulated cytoskeleton-associated protein (Arc) and c-Fos mRNA were inhibited with FLU exposure at day 3. Similarly, the expression of tubulin, which play important roles in the neuronal plasticity, was the same result. Furthermore, we found α-tubulin interacted with collapsing response mediator protein 2(CRMP2), which is related to neuronal plasticity, and the regulation of CRMP2 activity influenced the neurite length, Arc, c-Fos and tubulin expression.

CONCLUSIONS

The results demonstrated that neuronal plasticity was increased by FLU exposure at day 1, but exposure with FLU for more than 2 days had opposite effect on it. The reduction in neuronal plasticity with FLU exposure for more than 2 days might be involved in some aspects of the therapeutic effect of antidepressant on depression.

Astrocytic plasticity at the dorsal dentate gyrus on an animal model of recurrent depression

Astrocytes are now known to play crucial roles in the central nervous system, supporting and closely interacting with neurons and therefore able to modulate brain function. Both human postmortem studies in brain samples from patients diagnosed with Major Depressive Disorder and from animal models of depression reported numerical and morphological astrocytic changes specifically in the hippocampus. In particular, these studies revealed significant reductions in glial cell density denoted by a decreased number of S100B-positive cells and a decrease in GFAP expression in several brain regions including the hippocampus. To reveal plastic astrocytic changes in the context of recurrent depression, we longitudinally assessed dynamic astrocytic alterations (gene expression, cell densities and morphologic variations) in the hippocampal dentate gyrus under repeated exposure to unpredictable chronic mild stress (uCMS) and upon treatment with two antidepressants, fluoxetine and imipramine. Both antidepressants decreased astrocytic complexity immediately after stress exposure. Moreover, we show that astrocytic alterations, particularly an increased number of S100B-positive cells, are observed after recurrent stress exposure. Interestingly, these alterations were prevented at the long-term by either fluoxetine or imipramine treatment.

Animal models of major depression: drawbacks and challenges

Major depression is a leading contributor to the global burden of disease. This situation is mainly related to the chronicity and/or recurrence of the disorder, and to poor response to antidepressant therapy. Progress in this area requires valid animal models. Current models are based either on manipulating the environment to which rodents are exposed (during the developmental period or adulthood) or biological underpinnings (i.e. gene deletion or overexpression of candidate genes, targeted lesions of brain areas, optogenetic control of specific neuronal populations, etc.). These manipulations can alter specific behavioural and biological outcomes that can be related to different symptomatic and pathophysiological dimensions of major depression. However, animal models of major depression display substantial shortcomings that contribute to the lack of innovative pharmacological approaches in recent decades and which hamper our capabilities to investigate treatment-resistant depression. Here, we discuss the validity of these models, review putative models of treatment-resistant depression, major depression subtypes and recurrent depression. Furthermore, we identify future challenges regarding new paradigms such as those proposing dimensional rather than categorical approaches to depression.

Adult-generated neurons born during chronic social stress are uniquely adapted to respond to subsequent chronic social stress

Chronic stress is a recognized risk factor for psychiatric and psychological disorders and a potent modulator of adult neurogenesis. Numerous studies have shown that during stress, neurogenesis decreases; however, during the recovery from the stress, neurogenesis increases. Despite the increased number of neurons born after stress, it is unknown if the function and morphology of those neurons are altered. Here we asked whether neurons in adult mice, born during the final 5 days of chronic social stress and matured during recovery from chronic social stress, are similar to neurons born with no stress conditions from a quantitative, functional and morphological perspective, and whether those neurons are uniquely adapted to respond to a subsequent stressful challenge. We observed an increased number of newborn neurons incorporated in the dentate gyrus of the hippocampus during the 10-week post-stress recovery phase. Interestingly, those new neurons were more responsive to subsequent chronic stress, as they showed more of a stress-induced decrease in spine density and branching nodes than in neurons born during a non-stress period. Our results replicate findings that the neuronal survival and incorporation of neurons in the adult dentate gyrus increases after chronic stress and suggest that such neurons are uniquely adapted in the response to future social stressors. This finding provides a potential mechanism for some of the long-term hippocampal effects of stress.

Chronic stress triggers divergent dendritic alterations in immature neurons of the adult hippocampus, depending on their ultimate terminal fields

Chronic stress, a suggested precipitant of brain pathologies, such as depression and Alzheimer's disease, is known to impact on brain plasticity by causing neuronal remodeling as well as neurogenesis suppression in the adult hippocampus. Although many studies show that stressful conditions reduce the number of newborn neurons in the adult dentate gyrus (DG), little is known about whether and how stress impacts on dendritic development and structural maturation of these newborn neurons. We, herein, demonstrate that chronic stress impacts differentially on doublecortin (DCX)-positive immature neurons in distinct phases of maturation. Specifically, the density of the DCX-positive immature neurons whose dendritic tree reaches the inner molecular layer (IML) of DG is reduced in stressed animals, whereas their dendritic complexity is increased. On the contrary, no change on the density of DCX-positive neurons whose dendritic tree extends to the medial/outer molecular layer (M/OML) of the DG is found under stress conditions, whereas the dendritic complexity of these cells is diminished. In addition, DCX+ cells displayed a more complex and longer arbor in the dendritic compartments located in the granular cell layer of the DG under stress conditions; on the contrary, their dendritic segments localized into the M/OML were shorter and less complex. These findings suggest that the neuroplastic effects of chronic stress on dendritic maturation and complexity of DCX+ immature neurons vary based on the different maturation stage of DCX-positive cells and the different DG sublayer, highlighting the complex and dynamic stress-driven neuroplasticity of immature neurons in the adult hippocampus.

Longitudinal in vivo Diffusion Tensor Imaging Detects Differential Microstructural Alterations in the Hippocampus of Chronic Social Defeat Stress-Susceptible and Resilient Mice

Background: Microstructural alterations in the hippocampus may underlie stress-related disorders and stress susceptibility. However, whether these alterations are pre-existing stress vulnerability biomarkers or accumulative results of chronic stress remain unclear. Moreover, examining the whole hippocampus as one unit and ignoring the possibility of a lateralized effect of stress may mask some stress effects and contribute to the heterogeneity of previous findings. Methods: After C57BL/6 mice were exposed to a 10-day chronic social defeat stress (CSDS) paradigm, different stress phenotypes, i.e., susceptible (n = 10) and resilient (n = 7) mice, were discriminated by the behavior of the mice in a social interaction test. With in vivo diffusion tensor imaging (DTI) scans that were conducted both before and after the stress paradigm, we evaluated diffusion properties in the left and right, dorsal (dHi) and ventral hippocampus (vHi) of experimental mice. Results: A significantly lower fractional anisotropy (FA) was found in the right vHi of the susceptible mice prior to the CSDS paradigm than that found in the resilient mice, suggesting that pre-existing microstructural abnormalities may result in stress susceptibility. However, no significant group differences were found in the post-stress FA values of any of the hippocampal regions of interest (ROIs). In addition, mean diffusivity (MD) and radial diffusivity (RD) values were found to be significantly greater only in the right dHi of the resilient group compared to those of the susceptible mice. Furthermore, a significant longitudinal decrease was only observed in the right dHi RD value of the susceptible mice. Moreover, the social interaction (SI) ratio was positively related to post-stress left MD, right dHi MD, and right dHi RD values and the longitudinal right dHi MD percent change. Meanwhile, a negative relationship was detected between the SI ratio and bilateral mean of the post-stress left relative to right vHi FA value, highlighting the important role of right hippocampus in stress-resilience phenotype. Conclusion: Our findings demonstrated different longitudinal microstructural alterations in the bilateral dHi and vHi between stress-susceptible and resilient subgroups and indicated a right-sided lateralized stress effect, which may be useful in the diagnosis and prevention of stress-related disorders as well as their intervention.