Chronic stress effects on hippocampal structure and synaptic function: relevance for depression and normalization by anti-glucocorticoid treatment

The association between childhood adversity and hippocampal volumes is moderated by romantic relationship experiences

Reduced hippocampal volumes are a feature of many mental disorders. Childhood maltreatment is a known risk factor for the development of psychopathology and has consistently been linked to hippocampal volume reductions in adults, but not in children and adolescents. We propose that maltreatment-related difficulties in coping with developmental tasks in adolescence and young adulthood might underlie the delayed emergence of hippocampal volume reductions in maltreated individuals. In a study with 196 healthy young adults (mean age [years]: 24.0 ± 3.2, 50% female, 20.6% living with a partner (missings: n = 2)), we investigated the interaction between childhood maltreatment (Childhood Trauma Screener) and the breakup of a steady romantic relationship (List of Threatening Experiences Questionnaire) on hippocampal magnetic resonance imaging grey matter volumes. The experience of a romantic relationship breakup moderated the association between childhood maltreatment and bilateral hippocampal volumes, revealing more negative associations with hippocampal volumes in participants with at least one breakup compared to those with no breakup experience (right hippocampus: β = - 0.05 ± 0.02, p = 0.031, p (FDR) = 0.031; left hippocampus: β = -0.06 ± 0.02, p = 0.005, p (FDR) = 0.009). Moreover, our findings provide some evidence that childhood maltreatment is related to smaller bilateral hippocampal volumes only in those adults who suffered from a relationship breakup (right hippocampus: β = -0.23 ± 0.10, p = 0.018, p (FDR) = 0.018; left hippocampus: β = -0.24 ± 0.10, p = 0.016, p (FDR) = 0.018;). Our study highlights the interaction of adult social bonds with early adversity on vulnerability to psychopathology.

Effects of chronic empathic stress on synaptic efficacy, as well as short-term and long-term plasticity at the Schaffer collateral/commissural- CA1 synapses in the dorsal hippocampus of rats

Empathy, the ability to comprehend and share others' emotional states, impacts brain functions. This in vivo electrophysiological study explored the influence of chronic empathic stress on synaptic efficacy, as well as short-term and long-term plasticity at the Schaffer collateral/Commissural - CA1 synapses in the dorsal hippocampus of rats, in situations of social equality and inequality. Forty-eight male rats were randomized into six groups: control, pseudo-observer, pseudo-demonstrator, observer, demonstrator, and co-demonstrator (Co, Pse-Ob, Pse-De, Ob, De, Co-De) groups. Stress induction (2h/day, 21 days) was performed in situations of equality and inequality. Serum corticosterone levels, slope, amplitude, and area under the curve (AUC) of field excitatory postsynaptic potentials (fEPSPs) were assessed in the hippocampal CA1 area using input-output (I/O) functions, paired-pulse (PP) responses with different interpulse intervals (IPIs), and long-term potentiation (LTP) after high-frequency stimulation (HFS). The fEPSP slope, amplitude, and AUC significantly decreased in all stress groups, especially in the De and Pse-De groups. These parameters were significantly increased in the Co-De and Ob groups compared to the De group. Notably, the corticosterone levels strongly confirmed the electrophysiological findings. Chronic empathic stress could disrupt synaptic efficacy and plasticity in the CA1 area. Empathic stress, involving the presence of cagemates in situations of social equality and inequality, can modify long-term plasticity and serum corticosterone levels in demonstrators and co-demonstrators. Under empathic stress related to situations of inequality, freely moving observers may influence the demonstrators' stress experience. Therefore, the presence of a conspecific in the social inequality conditions had significant suppressive effects on long-term plasticity, while conversely, under equality conditions, long-term plasticity was favorably improved through social buffering.

Hypothalamic-Pituitary-Adrenal Axis Dysfunction Elevates SUDEP Risk in a Sex-Specific Manner

Epilepsy is often comorbid with psychiatric illnesses, including anxiety and depression. Despite the high incidence of psychiatric comorbidities in people with epilepsy, few studies address the underlying mechanisms. Stress can trigger epilepsy and depression. Evidence from human and animal studies supports that hypothalamic-pituitary-adrenal (HPA) axis dysfunction may contribute to both disorders and their comorbidity ( Kanner, 2003). Here, we investigate if HPA axis dysfunction may influence epilepsy outcomes and psychiatric comorbidities. We generated a novel mouse model (Kcc2/Crh KO mice) lacking the K+/Cl- cotransporter, KCC2, in corticotropin-releasing hormone (CRH) neurons, which exhibit stress- and seizure-induced HPA axis hyperactivation ( Melon et al., 2018). We used the Kcc2/Crh KO mice to examine the impact on epilepsy outcomes, including seizure frequency/burden, comorbid behavioral deficits, and sudden unexpected death in epilepsy (SUDEP) risk. We found sex differences in HPA axis dysfunction's effect on chronically epileptic KCC2/Crh KO mice seizure burden, vulnerability to comorbid behavioral deficits, and SUDEP. Suppressing HPA axis hyperexcitability in this model using pharmacological or chemogenetic approaches decreased SUDEP incidence, suggesting that HPA axis dysfunction may contribute to SUDEP. Altered neuroendocrine markers were present in SUDEP cases compared with people with epilepsy or individuals without epilepsy. Together, these findings implicate HPA axis dysfunction in the pathophysiological mechanisms contributing to psychiatric comorbidities in epilepsy and SUDEP.

Investigating the relationship between hippocampus/dentate gyrus volume and hypothalamus metabolism in participants with major depressive disorder

Reduced hippocampal volume occurs in major depressive disorder (MDD), potentially due to elevated glucocorticoids from an overactivated hypothalamus-pituitary-adrenal (HPA) axis. To examine this in humans, hippocampal volume and hypothalamus (HPA axis) metabolism was quantified in participants with MDD before and after antidepressant treatment. 65 participants (n = 24 males, n = 41 females) with MDD were treated in a double-blind, randomized clinical trial of escitalopram. Participants received simultaneous positron emission tomography (PET)/magnetic resonance imaging (MRI) before and after treatment. Linear mixed models examined the relationship between hippocampus/dentate gyrus volume and hypothalamus metabolism. Chi-squared tests and multivariable logistic regression examined the association between hippocampus/dentate gyrus volume change direction and hypothalamus activity change direction with treatment. Multiple linear regression compared these changes between remitter and non-remitter groups. Covariates included age, sex, and treatment type. No significant linear association was found between hippocampus/dentate gyrus volume and hypothalamus metabolism. 62% (38 of 61) of participants experienced a decrease in hypothalamus metabolism, 43% (27 of 63) of participants demonstrated an increase in hippocampus size (51% [32 of 63] for the dentate gyrus) following treatment. No significant association was found between change in hypothalamus activity and change in hippocampus/dentate gyrus volume, and this association did not vary by sex, medication, or remission status. As this multimodal study, in a cohort of participants on standardized treatment, did not find an association between hypothalamus metabolism and hippocampal volume, it supports a more complex pathway between hippocampus neurogenesis and hypothalamus metabolism changes in response to treatment.

Septohippocampal cholinergic system at the intersection of stress and cognition: Current trends and translational implications

Deficits in hippocampus-dependent memory processes are common across psychiatric and neurodegenerative disorders such as depression, anxiety and Alzheimer's disease. Moreover, stress is a major environmental risk factor for these pathologies and it exerts detrimental effects on hippocampal functioning via the activation of hypothalamic-pituitary-adrenal (HPA) axis. The medial septum cholinergic neurons extensively innervate the hippocampus. Although, the cholinergic septohippocampal pathway (SHP) has long been implicated in learning and memory, its involvement in mediating the adaptive and maladaptive impact of stress on mnemonic processes remains less clear. Here, we discuss current research highlighting the contributions of cholinergic SHP in modulating memory encoding, consolidation and retrieval. Then, we present evidence supporting the view that neurobiological interactions between HPA axis stress response and cholinergic signalling impact hippocampal computations. Finally, we critically discuss potential challenges and opportunities to target cholinergic SHP as a therapeutic strategy to improve cognitive impairments in stress-related disorders. We argue that such efforts should consider recent conceptualisations on the dynamic nature of cholinergic signalling in modulating distinct subcomponents of memory and its interactions with cellular substrates that regulate the adaptive stress response.

Neural circuits for the adaptive regulation of fear and extinction memory

The regulation of fear memories is critical for adaptive behaviors and dysregulation of these processes is implicated in trauma- and stress-related disorders. Treatments for these disorders include pharmacological interventions as well as exposure-based therapies, which rely upon extinction learning. Considerable attention has been directed toward elucidating the neural mechanisms underlying fear and extinction learning. In this review, we will discuss historic discoveries and emerging evidence on the neural mechanisms of the adaptive regulation of fear and extinction memories. We will focus on neural circuits regulating the acquisition and extinction of Pavlovian fear conditioning in rodent models, particularly the role of the medial prefrontal cortex and hippocampus in the contextual control of extinguished fear memories. We will also consider new work revealing an important role for the thalamic nucleus reuniens in the modulation of prefrontal-hippocampal interactions in extinction learning and memory. Finally, we will explore the effects of stress on this circuit and the clinical implications of these findings.

Investigating The Relationship Between Hippocampus:Dentate Gyrus Volume and Hypothalamus Metabolism in Participants with Major Depressive Disorder

Reduced hippocampal volume occurs in major depressive disorder (MDD), theoretically due to elevated glucocorticoids from an overactivated hypothalamus-pituitary-adrenal (HPA) axis. To examine this in humans, hippocampal volume, and hypothalamus (HPA axis) metabolism was quantified in participants with MDD before and after antidepressant treatment. 65 participants (n = 24 males, n = 41 females) with MDD were treated in a double-blind, randomized clinical trial of escitalopram. Participants received simultaneous positron emission tomography (PET) / magnetic resonance imaging (MRI) before and after treatment. Linear mixed models examined the relationship between hippocampus/dentate gyrus volume and hypothalamus metabolism. Chi-squared tests and multivariable logistic regression examined the association between hippocampus/dentate gyrus volume change direction and hypothalamus activity change direction with treatment. Multiple linear regression compared these changes between remitter and non-remitter groups. Covariates included age, sex, and treatment type. No significant linear association was found between hippocampus/dentate gyrus volume and hypothalamus metabolism. 62% (38 of 61) of participants experienced a decrease in hypothalamus metabolism, 43% (27 of 63) of participants demonstrated an increase in hippocampus size (51% [32 of 63] for the dentate gyrus) following treatment. No significant association was found between change in hypothalamus activity and change in hippocampus/dentate gyrus volume, and this association did not vary by sex, medication, or remission status. As this multimodal study, in a cohort of participants on standardized treatment, did not find an association between hypothalamus metabolism and hippocampal volume, it supports a more complex pathway between hippocampus neurogenesis and treatment response.

Hyperoxygenation Ameliorates Stress-induced Neuronal and Behavioral Deficits

Hyperoxygenation therapy remediates neuronal injury and improves cognitive function in various animal models. In the present study, the optimal conditions for hyperoxygenation treatment of stress-induced maladaptive changes were investigated. Mice exposed to chronic restraint stress (CRST) produce persistent adaptive changes in genomic responses and exhibit depressive-like behaviors. Hyperoxygenation treatment with 100% O₂ (HO₂) at 2.0 atmospheres absolute (ATA) for 1 h daily for 14 days in CRST mice produces an antidepressive effect similar to that of the antidepressant imipramine. In contrast, HO₂ treatment at 2.0 ATA for 1 h daily for shorter duration (3, 5, or 7 days), HO₂ treatment at 1.5 ATA for 1 h daily for 14 days, or hyperbaric air treatment at 2.0 ATA (42% O₂) for 1 h daily for 14 days is ineffective or less effective, indicating that repeated sufficient hyperoxygenation conditions are required to reverse stress-induced maladaptive changes. HO₂ treatment at 2.0 ATA for 14 days restores stress-induced reductions in levels of mitochondrial copy number, stress-induced attenuation of synaptophysin-stained density of axon terminals and MAP-2-staining dendritic processes of pyramidal neurons in the hippocampus, and stress-induced reduced hippocampal neurogenesis. These results suggest that HO₂ treatment at 2.0 ATA for 14 days is effective to ameliorate stress-induced neuronal and behavioral deficits.

The Dual Dose-Dependent Effects of Corticosterone on Hippocampal Cell Apoptosis After Traumatic Brain Injury Depend on the Activation Ratio of Mineralocorticoid Receptors to Glucocorticoid Receptors

In our recent studies, we reported that mineralocorticoid receptor (MR) had the opposite effects of glucocorticoid receptor (GR) on neural cell survival after traumatic brain injury (TBI). However, whether short-term use of high-dose natural glucocorticoids, which are mixed agonists of both MR and GR, leads to neurotoxic effects by inducing excessive GR activation is unclear, as is the threshold GR activation level and the possible signaling pathways remain unclear. In this study, we examined the dual dose-dependent effects of corticosterone (CORT) on spatial memory, hippocampal cell survival and receptor-mediated downstream signaling pathways after TBI. We found that different doses of CORT exhibited dual effects on hippocampal cell survival and rat spatial memory. Low doses of CORT (0.3 and 3 mg/kg) significantly increased MR activation, upregulated Akt/CREB/Bad phosphorylation and Bcl-2 concentration, reduced the number of apoptotic neural cells, and subsequently improved rat spatial memory. In contrast, a high dose of CORT (30 mg/kg) exerted the opposite effects by overactivating GR, upregulating P53/Bax levels, and inhibiting Erk/CREB activity. The results suggest that the neuroprotective and neurotoxic effects of endogenous GC depend on a threshold level and that a higher dose of GC, even for short-term use, should be avoided after TBI.

Corticotropin-releasing factor induces functional and structural synaptic remodelling in acute stress

Biological responses to stress are complex and highly conserved. Corticotropin-releasing factor (CRF) plays a central role in regulating these lifesaving physiological responses to stress. We show that, in mice, CRF rapidly changes Schaffer Collateral (SC) input into hippocampal CA1 pyramidal cells (PC) by modulating both functional and structural aspects of these synapses. Host exposure to acute stress, in vivo CRF injection, and ex vivo CRF application all result in fast de novo formation and remodeling of existing dendritic spines. Functionally, CRF leads to a rapid increase in synaptic strength of SC input into CA1 neurons, e.g., increase in spontaneous neurotransmitter release, paired-pulse facilitation, and repetitive excitability and improves synaptic plasticity: long-term potentiation (LTP) and long-term depression (LTD). In line with the changes in synaptic function, CRF increases the number of presynaptic vesicles, induces redistribution of vesicles towards the active zone, increases active zone size, and improves the alignment of the pre- and postsynaptic compartments. Therefore, CRF rapidly enhances synaptic communication in the hippocampus, potentially playing a crucial role in the enhanced memory consolidation in acute stress.

Type 1 interferon mediates chronic stress-induced neuroinflammation and behavioral deficits via complement component 3-dependent pathway

Chronic stress is a major risk factor in the pathophysiology of many neuropsychiatric disorders. Further, chronic stress conditions can promote neuroinflammation and inflammatory responses in both humans and animal models. Type I interferons (IFN-I) are critical mediators of the inflammatory response in the periphery and responsible for the altered mood and behavior. However, the underlying mechanisms are not well understood. In the present study, we investigated the role of IFN-I signaling in chronic stress-induced changes in neuroinflammation and behavior. Using the chronic restraint stress model, we found that chronic stress induces a significant increase in serum IFNβ levels in mice, and systemic blockade of IFN-I signaling attenuated chronic stress-induced infiltration of macrophages into prefrontal cortex and behavioral abnormalities. Furthermore, complement component 3 (C3) mediates systemic IFNβ-induced changes in neuroinflammation and behavior. Also, we found significant increases in the mRNA expression levels of IFN-I stimulated genes in the prefrontal cortex of depressed suicide subjects and significant correlation with C3 and inflammatory markers. Together, these findings from animal and human postmortem brain studies identify a crucial role of C3 in IFN-I-mediated changes in neuroinflammation and behavior under chronic stress conditions.

Early Life Stress Exacerbates Outcome after Traumatic Brain Injury

The neurocognitive impairments associated with mild traumatic brain injury (TBI) often resolve within 1-2 weeks; however, a subset of people exhibit persistent cognitive dysfunction for weeks to months after injury. The factors that contribute to these persistent deficits are unknown. One potential risk factor for worsened outcome after TBI is a history of stress experienced by a person early in life. Early life stress (ELS) includes maltreatment such as neglect, and interferes with the normal construction of cortical and hippocampal circuits. We hypothesized that a history of ELS contributes to persistent learning and memory dysfunction following a TBI. To explore this interaction, we modeled ELS by separating Sprague Dawley pups from their nursing mothers from post-natal days 2-14 for 3 h daily. At 2 months of age, male rats received sham surgery or mild to moderate parasagittal fluid-percussion brain injury. We found that the combination of ELS with TBI in adulthood impaired hippocampal-dependent learning, as assessed with contextual fear conditioning, the water maze task, and spatial working memory. Cortical atrophy was significantly exacerbated in TBI animals exposed to ELS compared with normal-reared TBI animals. Changes in corticosterone in response to restraint stress were prolonged in TBI animals that received ELS compared with TBI animals that were normally reared or sham animals that received ELS. Our findings indicate that ELS is a risk factor for worsened outcome after TBI, and results in persistent learning and memory deficits, worsened cortical pathology, and an exacerbation of the hormonal stress response.

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.

Sex and chronic stress alter delta opioid receptor distribution within rat hippocampal CA1 pyramidal cells following behavioral challenges

Following oxycodone (Oxy) conditioned place preference (CPP), delta opioid receptors (DORs) differentially redistribute in hippocampal CA3 pyramidal cells in female and male rats in a manner that would promote plasticity and opioid-associative learning processes. However, following chronic immobilization stress (CIS), males do not acquire Oxy-CPP and the trafficking of DORs in CA3 pyramidal neurons is attenuated. Here, we examined the subcellular distribution of DORs in CA1 pyramidal cells using electron microscopy in these same cohorts.

CPP

Saline (Sal)-females compared to Sal-males have more cytoplasmic and total DORs in dendrites and more DOR-labeled spines. Following Oxy-CPP, DORs redistribute from near-plasmalemma pools in dendrites to spines in males.

CIS

Control females compared to control males have more near-plasmalemmal dendritic DORs. Following CIS, dendritic DORs are elevated in the cytoplasm in females and near-plasmalemma in males.

CIS plus CPP

CIS Sal-females compared to CIS Sal-males have more DORs on the plasmalemma of dendrites and in spines. After Oxy, the distribution of DORs does not change in either females or males.

Conclusion

Following Oxy-CPP, DORs within CA1 pyramidal cells remain positioned in naïve female rats to enhance sensitivity to DOR agonists and traffic to dendritic spines in naïve males where they can promote plasticity processes. Following CIS plus behavioral enrichment, DORs are redistributed within CA1 pyramidal cells in females in a manner that could enhance sensitivity to DOR agonists. Conversely, CIS plus behavioral enrichment does not alter DORs in CA1 pyramidal cells in males, which may contribute to their diminished capacity to acquire Oxy-CPP.

Long-term stability and characteristics of behavioral, biochemical, and molecular markers of three different rodent models for depression

OBJECTIVE

The present study was designed to explore the long-term differences between three mouse models for depression.

METHOD

In the present study, the unpredictable chronic mild stress (UCMS) model, the glucocorticoid/corticosterone model, and the olfactory bulbectomy model were compared at two, three, and five weeks after model induction. Behavioral testing performed included forced-swimming, tail suspension, open-field and elevated plus-maze tests. In addition, 5-hydroxytryptamine (5-HT) and dopamine levels, and mRNA and protein expressions related to 5-HT synthesis, transport, and signaling were analyzed in the hippocampus of tested animals.

RESULTS

Our results revealed that each model demonstrated a specific profile of markers, whereas the stability of them differed over testing time.

CONCLUSIONS

Each model provided a unique set of advantages that can be considered depending on the context and aims of each study. Among the three models, the UCMS model was mostly stable and appeared to the best model for testing long-term depression-like state.

Effects of Acupuncture on Chronic Stress-Induced Depression-Like Behavior and Its Central Neural Mechanism

Depression is a serious psychiatric disorder with an enormous socioeconomic burden, and it is commonly comorbid with pain, chronic fatigue, or other inflammatory diseases. Recent studies have shown that acupuncture is an effective therapeutic method for reducing depressive symptoms; however, the underlying mechanism remains unknown. In this study, we investigated the effects of acupuncture on chronic stress-induced depression-like behavior and its central neural mechanisms in the brain. We induced chronic restraint stress (CRS) in male C57BL/6 mice for 14 or 28 consecutive days. Acupuncture treatment was performed at KI10·LR8·LU8·LR4 or control points for 7 or 14 days. Depression-like behavior was assessed with the open field test. Then, brain neural activity involving c-Fos and serotonin-related mechanisms via the 5-HT1A and 5-HT1B receptors were investigated. Acupuncture treatment at KI10·LR8·LU8·LR4 points rescued the depressive-like behavior, while control points (LU8·LR4·HT8·LR2) and non-acupoints on the hips did not. Brain neural activity was changed in the hippocampus, cingulate cortex, motor cortex, insular cortex, thalamus, and the hypothalamus after acupuncture treatment. Acupuncture treatment increased expression of 5-HT1A receptor in the cortex, hippocampus, thalamus, and the hypothalamus, and of 5-HT1B in the cortex and thalamus. In conclusion, acupuncture treatment at KI10·LR8·LU8·LR4 was effective in alleviating the depressive-like behavior in mice, and this therapeutic effect was produced through central brain neural activity and serotonin receptor modulation.

Neuroendocrine Regulation of Brain Cytokines After Psychological Stress

There is growing evidence that stress-induced brain cytokines are important in the etiology of depression and anxiety. Here, we review how the neuroendocrine responses to psychological stressors affect the immediate and long-term regulation of inflammatory cytokines within the brain and highlight how the regulation changes across time with repeated stress exposure. In doing so, we report on the percentage of studies in the literature that observed increases in either IL-1β, TNF-α, or IL-6 within the hypothalamus, hippocampus, or prefrontal cortex after either acute or chronic stress exposure. The key takeaway is that catecholamines and glucocorticoids play critical roles in the regulation of brain cytokines after psychological stress exposure. Central catecholamines stimulate the release of IL-1β from microglia, which is a key factor in the further activation of microglia and recruitment of monocytes into the brain. Meanwhile, the acute elevation of glucocorticoids inhibits the production of brain cytokines via two mechanisms: the suppression of noradrenergic locus coeruleus neurons and inhibition of the NFκB signaling pathway. However, glucocorticoids and peripheral catecholamines facilitate inflammatory responses to future stimuli by stimulating monocytes to leave the bone marrow, downregulating inhibitory receptors on microglia, and priming inflammatory responses mediated by peripheral monocytes or macrophages. The activation of microglia and the elevation of peripheral glucocorticoid and catecholamine levels are both necessary during times of stress exposure for the development of psychopathologies.

Protective and therapeutic effects of exercise on stress-induced memory impairment

The objective of this paper was to systematically evaluate the potential preventive and therapeutic effects of exercise in attenuating stress-induced memory impairment. A systematic review was employed, searching PubMed, PsychInfo, Sports Discus and Google Scholar databases. For eligibility, studies had to be published in English, employ an experimental design, have the acute or chronic bout of exercise occur prior to, during or after the stressor, implement a psychophysiological stressor, and have an assessment of memory function occurring after the stressor. In total, 23 studies were evaluated, all of which were conducted among animal models. All 23 studies employed a chronic exercise protocol and a chronic stress protocol. Eight studies evaluated a preventive model, three employed a concurrent model, ten studies employed a therapeutic model, and two studies evaluated both a preventive and therapeutic model within the same study. Among the eight studies employing a preventive model, all eight demonstrated that the stress regimen impaired memory function. In all eight of these studies, when exercise occurred prior to the stressor, exercise attenuated the stress-induced memory impairment effect. Among the ten studies employing a therapeutic model, one study showed that the stress protocol enhanced memory function, one showed that the stress protocol did not influence memory, and eight demonstrated that the stress regimen impaired memory function. Among the eight studies showing that the stress protocol impaired memory function, all eight studies demonstrated that exercise, after the stressor, attenuated stress-induced memory impairment. Within animal models, chronic stress is associated with memory impairment and chronic exercise has both a preventive and therapeutic effect in attenuating stress-induced memory impairment. Additional experimental work in human studies is needed. Such work should also examine acute exercise and stress protocols.

Absence of Wdr13 Gene Predisposes Mice to Mild Social Isolation - Chronic Stress, Leading to Depression-Like Phenotype Associated With Differential Expression of Synaptic Proteins

We earlier reported that the male mice lacking the Wdr13 gene (Wdr13^-/0) showed mild anxiety, better memory retention, and up-regulation of synaptic proteins in the hippocampus. With increasing evidences from parallel studies in our laboratory about the possible role of Wdr13 in stress response, we investigated its role in brain. We observed that Wdr13 transcript gets up-regulated in the hippocampus of the wild-type mice exposed to stress. To further dissect its function, we analyzed the behavioral and molecular phenotypes of Wdr13^-/0 mice when subjected to mild chronic psychological stress, namely; mild (attenuated) social isolation. We employed iTRAQ based quantitative proteomics, real time PCR and western blotting to investigate molecular changes. Three weeks of social isolation predisposed Wdr13^-/0 mice to anhedonia, heightened anxiety-measured by Open field test (OFT), increased behavior despair- measured by Forced swim test (FST) and reduced dendritic branching along with decreased spine density of hippocampal CA1 neurons as compared to wild-type counterparts. This depression-like-phenotype was however ameliorated when treated with anti-depressant imipramine. Molecular analysis revealed that out of 1002 quantified proteins [1% False discovery rate (FDR), at-least two unique peptides], strikingly, a significant proportion of synaptic proteins including, SYN1, CAMK2A, and RAB3A were down-regulated in the socially isolated Wdr13^-/0 mice as compared to its wild-type counterparts. This was in contrast to the elevated levels of these proteins in non-stressed mutants as compared to the controls. We hypothesized that a de-regulated transcription factor upstream of the synaptic genes might be responsible for the observed phenotype. Indeed, in the socially isolated Wdr13^-/0 mice, there was an up-regulation of GATA1 – a transcription factor that negatively regulates synaptic genes and has been associated with Major Depression (MD) in humans. The present study demonstrates significant genotype × enviornment interaction for Wdr13 gene as shown by the reversal in the expression levels of several synaptic proteins in the mutant vis-à-vis wild-type mouse when exposed to social isolation stress.

Sex differences after chronic stress in the expression of opioid-, stress- and neuroplasticity-related genes in the rat hippocampus

Opioid peptides and their receptors re-organize within hippocampal neurons of female, but not male, rats following chronic immobilization stress (CIS) in a manner that promotes drug-related learning. This study was conducted to determine if there are also sex differences in gene expression in the hippocampus following CIS. Adult female and male rats were subjected to CIS (30 min/day) for 10 days. Twenty-four hours after the last stressor, the rats were euthanized, the brains were harvested and the medial (dentate gyrus/CA1) and lateral (CA2/CA3) dorsal hippocampus were isolated. Following total RNA isolation, cDNA was prepared for gene expression analysis using a RT² Profiler PCR expression array. This custom designed qPCR expression array contained genes for opioid peptides and receptors, as well as genes involved in stress-responses and candidate genes involved in synaptic plasticity, including those upregulated following oxycodone self-administration in mice. Few sex differences are seen in hippocampal gene expression in control (unstressed) rats. In response to CIS, gene expression in the hippocampus was altered in males but not females. In males, opioid, stress, plasticity and kinase/signaling genes were all down-regulated following CIS, except for the gene that codes for corticotropin releasing hormone, which was upregulated. Changes in opioid gene expression following chronic stress were limited to the CA2 and CA3 regions (lateral sample). In conclusion, modest sex- and regional-differences are seen in expression of the opioid receptor genes, as well as genes involved in stress and plasticity responses in the hippocampus following CIS.

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Unstressed female rats have less Arc expression in hippocampus than males.

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Chronic immobilization stress (CIS) down-regulates opioid gene expression in males.

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CIS up-regulates Crh but down-regulates other stress genes in male hippocampi.

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CIS down-regulates Arc and other plasticity genes in male hippocampi.

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CIS down-regulates select kinases and signaling molecules in male hippocampi.

Stress and Corticosteroids Modulate Muscarinic Long Term Potentiation (mLTP) in the Hippocampus

Stress influences synaptic plasticity, learning and memory in a steroid hormone receptor dependent manner. Based on these findings it has been proposed that stress could be a major risk factor for the development of cognitive decline and dementia. Interestingly, evidence has been provided that stress also affects muscarinic, i.e., acetylcholine (ACh)-mediated neurotransmission. To learn more about the impact of stress and steroids on synaptic plasticity, in this study, we investigated the effects of stress on muscarinic long term potentiation (mLTP). We report that multiple, unpredictable exposure to stress depresses carbachol (0.5 μM)-induced mLTP, while this effect of stress is not observed in hippocampal slices prepared from mice exposed only to a single stressful procedure. Furthermore, we demonstrate that activation of distinct steroid hormone receptors is involved in stress-mediated alterations of mLTP. Activation of mineralocorticoid receptors (MR) promotes mLTP, while glucocorticoid receptor (GR) activity impairs mLTP. These effects of multiple unpredictable stress on mLTP are long-lasting since they are detected even two weeks after the last stressful experience. Thus, multiple unpredictable events rather than a single stressful experience affect mLTP in a steroid hormone receptor dependent manner, suggesting that chronic unpredictable stress can lead to lasting alterations in hippocampal cholinergic plasticity.

Electron Microscopic Analysis of Hippocampal Axo-Somatic Synapses in a Chronic Stress Model for Depression

Stress can alter the number and morphology of excitatory synapses in the hippocampus, but nothing is known about the effect of stress on inhibitory synapses. Here, we used an animal model for depression, the chronic mild stress model, and quantified the number of perisomatic inhibitory neurons and their synapses. We found reduced density of parvalbumin‐positive (PV+) neurons in response to stress, while the density of cholecystokinin‐immunoreactive (CCK+) neurons was unaffected. We did a detailed electron microscopic analysis to quantify the frequency and morphology of perisomatic inhibitory synapses in the hippocampal CA1 area. We analyzed 1100 CA1 pyramidal neurons and 4800 perisomatic terminals in five control and four chronically stressed rats. In the control animals we observed the following parameters: Number of terminals/soma = 57; Number of terminals/100 µm cell perimeter = 10; Synapse/terminal ratio = 32%; Synapse number/100 terminal = 120; Average terminal length = 920nm. None of these parameters were affected by the stress exposure. Overall, these data indicate that despite the depressive‐like behavior and the decrease in the number of perisomatic PV+ neurons in the light microscopic preparations, the number of perisomatic inhibitory synapses on CA1 pyramidal cells was not affected by stress. In the electron microscope, PV+ neurons and the axon terminals appeared to be normal and we did not find any apoptotic or necrotic cells. This data is in sharp contrast to the remarkable remodeling of the excitatory synapses on spines that has been reported in response to stress and depressive‐like behavior. © 2016 The Authors Hippocampus Published by Wiley Periodicals, Inc.

Fetal brain 11β-hydroxysteroid dehydrogenase type 2 selectively determines programming of adult depressive-like behaviors and cognitive function, but not anxiety behaviors in male mice

Stress or elevated glucocorticoids during sensitive windows of fetal development increase the risk of neuropsychiatric disorders in adult rodents and humans, a phenomenon known as glucocorticoid programming. 11β-Hydroxysteroid dehydrogenase type 2 (11β-HSD2), which catalyses rapid inactivation of glucocorticoids in the placenta, controls access of maternal glucocorticoids to the fetal compartment, placing it in a key position to modulate glucocorticoid programming of behavior. However, the importance of the high expression of 11β-HSD2 within the midgestational fetal brain is unknown. To examine this, a brain-specific knockout of 11β-HSD2 (HSD2BKO) was generated and compared to wild-type littermates. HSD2BKO have markedly diminished fetal brain 11β-HSD2, but intact fetal body and placental 11β-HSD2 and normal fetal and placental growth. Despite normal fetal plasma corticosterone, HSD2BKO exhibit elevated fetal brain corticosterone levels at midgestation. As adults, HSD2BKO show depressive-like behavior and have cognitive impairments. However, unlike complete feto-placental deficiency, HSD2BKO show no anxiety-like behavioral deficits. The clear mechanistic separation of the programmed components of depression and cognition from anxiety implies distinct mechanisms of pathogenesis, affording potential opportunities for stratified interventions.

Preferential loss of dorsal-hippocampus synapses underlies memory impairments provoked by short, multimodal stress

The cognitive effects of stress are profound, yet it is unknown if the consequences of concurrent multiple stresses on learning and memory differ from those of a single stress of equal intensity and duration. We compared the effects on hippocampus-dependent memory of concurrent, hours-long light, loud noise, jostling and restraint (multimodal stress) with those of restraint or of loud noise alone. We then examined if differences in memory impairment following these two stress types might derive from their differential impact on hippocampal synapses, distinguishing dorsal and ventral hippocampus. Mice exposed to hours-long restraint or loud noise were modestly or minimally impaired in novel object recognition, whereas similar-duration multimodal stress provoked severe deficits. Differences in memory were not explained by differences in plasma corticosterone levels or numbers of Fos-labeled neurons in stress-sensitive hypothalamic neurons. However, although synapses in hippocampal CA3 were impacted by both restraint and multimodal stress, multimodal stress alone reduced synapse numbers severely in dorsal CA1, a region crucial for hippocampus-dependent memory. Ventral CA1 synapses were not significantly affected by either stress modality. Probing the basis of the preferential loss of dorsal synapses after multimodal stress, we found differential patterns of neuronal activation by the two stress types. Cross-correlation matrices, reflecting functional connectivity among activated regions, demonstrated that multimodal stress reduced hippocampal correlations with septum and thalamus and increased correlations with amygdala and BST. Thus, despite similar effects on plasma corticosterone and on hypothalamic stress-sensitive cells, multimodal and restraint stress differ in their activation of brain networks and in their impact on hippocampal synapses. Both of these processes might contribute to amplified memory impairments following short, multimodal stress.

Regulation of 11β-hydroxysteroid dehydrogenase type 1 and 7α-hydroxylase CYP7B1 during social stress

11β-hydroxysteroid dehydrogenase type 1 (11HSD1) is an enzyme that amplifies intracellular glucocorticoid concentration by the conversion of inert glucocorticoids to active forms and is involved in the interconversion of 7-oxo- and 7-hydroxy-steroids, which can interfere with the activation of glucocorticoids. The presence of 11HSD1 in the structures of the hypothalamic-pituitary-adrenal (HPA) axis suggests that this enzyme might play a role in the regulation of HPA output. Here we show that the exposure of Fisher 344 rats to mild social stress based on the resident-intruder paradigm increased the expression of 11HSD1 and CYP7B1, an enzyme that catalyzes 7-hydroxylation of steroids. We found that social behavioral profile of intruders was significantly decreased whereas their plasma levels of corticosterone were increased more than in residents. The stress did not modulate 11HSD1 in the HPA axis (paraventricular nucleus, pituitary, adrenal cortex) but selectively upregulated 11HSD1 in some regions of the hippocampus, amygdala and prelimbic cortex. In contrast, CYP7B1 was upregulated not only in the hippocampus and amygdala but also in paraventricular nucleus and pituitary. Furthermore, the stress downregulated 11HSD1 in the thymus and upregulated it in the spleen and mesenteric lymphatic nodes whereas CYP7B1 was upregulated in all of these lymphoid organs. The response of 11HSD1 to stress was more obvious in intruders than in residents and the response of CYP7B1 to stress predominated in residents. We conclude that social stress induces changes in enzymes of local metabolism of glucocorticoids in lymphoid organs and in brain structures associated with the regulation of the HPA axis. In addition, the presented data clearly suggest a role of 11HSD1 in modulation of glucocorticoid feedback of the HPA axis during stress.

Vitamin A status regulates glucocorticoid availability in Wistar rats: consequences on cognitive functions and hippocampal neurogenesis?

A disruption of the vitamin A signaling pathway has been involved in age-related memory decline and hippocampal plasticity alterations. Using vitamin A deficiency (VAD), a nutritional model leading to a hyposignaling of the retinoid pathway, we have recently demonstrated that retinoic acid (RA), the active metabolite of vitamin A, is efficient to reverse VAD-induced spatial memory deficits and adult hippocampal neurogenesis alterations. Besides, excess of glucocorticoids (GCs) occurring with aging is known to strongly inhibit hippocampal plasticity and functions and few studies report on the counteracting effects of RA signaling pathway on GCs action. Here, we have addressed whether the modulation of brain GCs availability could be one of the biological mechanisms involved in the effects of vitamin A status on hippocampal plasticity and functions. Thus, we have studied the effects of a vitamin A-free diet for 14 weeks and a 4-week vitamin A supplementation on plasma and hippocampal corticosterone (CORT) levels in Wistar rats. We have also investigated corticosteroid binding globulin (CBG) binding capacity and 11beta-Hydrosteroid Dehydrogenase type 1 (11β-HSD1) activity, both important modulators of CORT availability at the peripheral and hippocampal levels respectively. Interestingly, we show that the vitamin A status regulates levels of free plasma CORT and hippocampal CORT levels, by acting through a regulation of CBG binding capacity and 11β-HSD1 activity. Moreover, our results suggest that increased CORT levels in VAD rats could have some deleterious consequences on spatial memory, anxiety-like behavior and adult hippocampal neurogenesis whereas these effects could be corrected by a vitamin A supplementation. Thus, the modulation of GCs availability by vitamin A status is an important biological mechanism that should be taken into account in order to prevent age-related cognitive decline and hippocampal plasticity alterations.

Stress during first pregnancy increases seizure threshold in adult male offspring

OBJECTIVE(S)

Stress induces many homeostatic aberrations which are followed by lifelong allostatic responses. Epilepsy is developed or influenced by different environmental factors, i.e. prenatal stress which makes many contradictory developmental changes in seizure threshold and intensity. We investigated the potential seizure response of the rat offspring to prenatal stress; the stress which was applied to their mothers.

MATERIALS AND METHODS

Nine day heterogeneous sequential stress (HSS) model was used before and during the first and before the second pregnancy. The kindling was induced using 13 IP injections of pentylenetetrazol (PTZ) every 48 hr to adult male Wistar rat's offspring.

RESULTS

The results of the present study demonstrated that, before pregnancy stress decreased the rate of kindling (P<0.05) in the offspring, while stress which was applied during pregnancy completely prevented kindling (P <0.001). Further, their convulsive latency was increased and tonic clonic seizure duration was decreased. In contrast, previous pregnancy and between pregnancies stress could not change kindling process. Although maternal separation stress did not change kindling development, it could increase convulsive intensities by elongating the duration of seizures (P<0.05) and reducing convulsion latency (P <0.05).

CONCLUSION

It is concluded that stress detrimental effects could be prevented by stress which was applied around first pregnancy; however this beneficial effect is weakened by before second pregnancy stress.

Local potentiation of excitatory synapses by serotonin and its alteration in rodent models of depression

The causes of major depression remain unknown. Antidepressants elevate concentrations of monoamines, particularly serotonin, but it remains uncertain which downstream events are critical to their therapeutic effects. We found that endogenous serotonin selectively potentiated excitatory synapses formed by the temporoammonic pathway with CA1 pyramidal cells via activation of serotonin receptors (5-HT(1B)Rs), without affecting nearby Schaffer collateral synapses. This potentiation was expressed postsynaptically by AMPA-type glutamate receptors and required calmodulin-dependent protein kinase-mediated phosphorylation of GluA1 subunits. Because they share common expression mechanisms, long-term potentiation and serotonin-induced potentiation occluded each other. Long-term consolidation of spatial learning, a function of temporoammonic-CA1 synapses, was enhanced by 5-HT(1B)R antagonists. Serotonin-induced potentiation was quantitatively and qualitatively altered in a rat model of depression, restored by chronic antidepressants, and required for the ability of chronic antidepressants to reverse stress-induced anhedonia. Changes in serotonin-mediated potentiation, and its recovery by antidepressants, implicate excitatory synapses as a locus of plasticity in depression.

Strain differences in the effects of chronic corticosterone exposure in the hippocampus

Stress hormones are thought to be involved in the etiology of depression, in part, because animal models show they cause morphological damage to the brain, an effect that can be reversed by chronic antidepressant treatment. The current study examined two mouse strains selected for naturalistic variation of tissue regeneration after injury for resistance to the effects of chronic corticosterone (CORT) exposure on cell proliferation and neurotrophin mobilization. The wound healer MRL/MpJ and control C57BL/6J mice were implanted subcutaneously with pellets that released CORT for 7 days. MRL/MpJ mice were resistant to reductions of hippocampal cell proliferation by chronic exposure to CORT when compared to vulnerable C57BL/6J mice. Chronic CORT exposure also reduced protein levels of brain-derived neurotrophic factor (BDNF) in the hippocampus of C57BL/6J but not MRL/MpJ mice. CORT pellet exposure increased circulating levels of CORT in the plasma of both strains in a dose-dependent manner although MRL/MpJ mice may have larger changes from baseline. The strains did not differ in circulating levels of corticosterone binding globulin (CBG). There were also no strain differences in CORT levels in the hippocampus, nor did CORT exposure alter glucocorticoid receptor or mineralocorticoid receptor expression in a strain-dependent manner. Strain differences were found in the N-methyl-D-aspartate (NMDA) receptor, and BDNF I and IV promoters. Strain and CORT exposure interacted to alter tropomyosine-receptor-kinase B (TrkB) expression and this may be a potential mechanism protecting MRL/MpJ mice. In addition, differences in the inflammatory response of matrix metalloproteinases (MMPs) may also contribute to these strain differences in resistance to the deleterious effects of CORT to the brain.

Sculpting the hippocampus from within: stress, spines, and CRH

Learning and memory processes carried out within the hippocampus are influenced by stress in a complex manner, and the mechanisms by which stress modulates the physiology of the hippocampus are not fully understood. This review addresses how the production and release of the neuropeptide corticotropin-releasing hormone (CRH) within the hippocampus during stress influences neuronal structure and hippocampal function. CRH functions in the contexts of acute and chronic stresses taking place during development, adulthood and aging. Current challenges are to uncover how the dynamic actions of CRH integrate with the well-established roles of adrenal-derived steroid stress hormones to shape the cognitive functions of the hippocampus in response to stress.