Mechanisms for acute stress-induced enhancement of glutamatergic transmission and working memory

Elevated tumor necrosis factor-alpha receptor subtype 1 and the association with abnormal brain function in treatment-resistant depression

BACKGROUND

Major depressive disorder (MDD) patients have shown elevated plasma levels of pro-inflammatory biomarkers compared to healthy controls. We hypothesized increased serum tumor necrosis factor-alpha receptor subtype 1 (TNF-α R1) is more associated with impaired brain function in patients with treatment-resistant depression (TRD) than those without TRD.

METHODS

34 MDD patients and 34 healthy subjects were recruited and we separated MDD patients to TRD group (n = 20) and non-TRD (n = 14) group. Pro-inflammatory cytokines were assessed by enzyme-linked immunosorbent assays. A standardized uptake values (SUV) of glucose metabolism measured by ¹⁸F-FDG positron-emission-tomography (PET) was applied to all subjects for subsequent region-of- interest analyses and whole-brain voxel-wise analyses. ¹⁸F-FDG-PET measures glucose uptake into astrocytes in response to glutamate release from neuronal cells, and was thus used as a proxy measure to quantify glutamatergic neurotransmission in the human brain.

RESULTS

Post-hoc analysis revealed that TRD group had higher serum concentrations of TNF-α R1 compared to healthy control or non-TRD group. In the MDD group, higher serum concentrations of TNF-α R1 significantly correlated with decreased SUV in anterior cingulate cortex (ACC) and bilateral caudate nucleus. The ROI analysis further supported the negative correlations of plasma TNF-α R1 and SUV in the ACC and caudate nucleus. Such correlation is more consistent in TRD group than in non-TRD and HC groups.

LIMITATION

Glutamate neurotransmission and the effect of chronic stress on glutamate release in the brain were not measured directly.

CONCLUSIONS

Increased TNF-α R1 was associated with impaired glutamatergic neurotransmission of caudate nucleus and ACC in MDD patients, particularly in the TRD.

Salivary kynurenic acid response to psychological stress: inverse relationship to cortical glutamate in schizophrenia

Frontal glutamatergic synapses are thought to be critical for adaptive, long-term stress responses. Prefrontal cortices, including the anterior cingulate cortex (ACC) contribute to stress perception and regulation, and are involved in top-down regulation of peripheral glucocorticoid and inflammatory responses to stress. Levels of kynurenic acid (KYNA) in saliva increase in response to psychological stress, and this stress-induced effect may be abnormal in people with schizophrenia. Here we test the hypothesis that ACC glutamatergic functioning may contribute to the stress-induced salivary KYNA response in schizophrenia. In 56 patients with schizophrenia and 58 healthy controls, our results confirm that levels of KYNA in saliva increase following psychological stress. The magnitude of the effect correlated negatively with proton magnetic resonance spectroscopy (MRS) glutamate + glutamine (r = -.31, p = .017) and glutamate (r = -0.27, p = .047) levels in the ACC in patients but not in the controls (all p ≥ .45). Although, a causal relationship cannot be ascertained in this cross-sectional study, these findings suggest a potentially meaningful link between central glutamate levels and kynurenine pathway response to stress in individuals with schizophrenia.

Chemicogenetic Restoration of the Prefrontal Cortex to Amygdala Pathway Ameliorates Stress-Induced Deficits

Corticosteroid stress hormones exert a profound impact on cognitive and emotional processes. Understanding the neuronal circuits that are altered by chronic stress is important for counteracting the detrimental effects of stress in a brain region- and cell type-specific manner. Using the chemogenetic tool, Designer Receptors Exclusively Activated by Designer Drugs (DREADDs), which enables the remote, noninvasive and long-lasting modulation of cellular activity and signal transduction in discrete neuronal populations in vivo, we sought to identify the specific pathways that play an essential role in stress responses. We found that prolonged severe stress induced the diminished glutamatergic projection from pyramidal neurons in prefrontal cortex (PFC) to GABAergic interneurons in basolateral amygdala (BLA), leading to the loss of feedforward inhibition and ensuing hyperexcitability of BLA principal neurons, which caused a variety of behavioral abnormalities. Activating PFC pyramidal neurons with hM3D(Gq) DREADD restored the functional connection between PFC and BLA in stressed animals, resulting in the rescue of recognition memory, normalization of locomotor activity and reduction of aggressive behaviors. Inhibiting BLA principal neurons directly with hM4D(Gi) DREADD also blocked BLA hyperactivity and aggressive behaviors in stressed animals. These results have offered an effective avenue to counteract the stress-induced disruption of circuitry homeostasis.

Potential implication of SGK1-dependent activity change in BV-2 microglial cells

It has recently been established that microglial activation is involved in the pathophysiology of various neurological and psychiatric disorders such as amyotrophic lateral sclerosis and schizophrenia. The pathological molecular machineries underlying microglial activation and its accelerating molecules have been precisely described in the diseased central nervous system (CNS). However, to date, the details of physiological mechanism, which represses microglial activation, are still to be elucidated. Our latest report demonstrated that serum- and glucocorticoid-inducible kinases (SGK1 and SGK3) were expressed in multiple microglial cell lines, and their inhibitor enhanced the toxic effect of lipopolysaccharide on microglial production of inflammatory substances such as TNFα and iNOS. In the present report, we prepared SGK1-lacked microglial cell line (BV-2) and demonstrated that deficiency of SGK1 in microglia induced its toxic conversion, in which it took amoeboid morphology characteristic of reactive microglia, increased CD68 expression, quickened its proliferation, and showed higher susceptibility to ATP and subsequent cell death. Our data indicate that SGK1 plays pivotal roles in inhibiting its pathological activation, and suggest its potential function as a therapeutic target for the treatment of various disorders related to the inflammation in the CNS.

Importance of the brain corticosteroid receptor balance in metaplasticity, cognitive performance and neuro-inflammation

Bruce McEwen's discovery of receptors for corticosterone in the rat hippocampus introduced higher brain circuits in the neuroendocrinology of stress. Subsequently, these receptors were identified as mineralocorticoid receptors (MRs) that are involved in appraisal processes, choice of coping style, encoding and retrieval. The MR-mediated actions on cognition are complemented by slower actions via glucocorticoid receptors (GRs) on contextualization, rationalization and memory storage of the experience. These sequential phases in cognitive performance depend on synaptic metaplasticity that is regulated by coordinate MR- and GR activation. The receptor activation includes recruitment of coregulators and transcription factors as determinants of context-dependent specificity in steroid action; they can be modulated by genetic variation and (early) experience. Interestingly, inflammatory responses to damage seem to be governed by a similarly balanced MR:GR-mediated action as the initiating, terminating and priming mechanisms involved in stress-adaptation. We conclude with five questions challenging the MR:GR balance hypothesis.

Acute stress enhances learning and memory by activating acid-sensing ion channels in rats

Acute stress has been shown to enhance learning and memory ability, predominantly through the action of corticosteroid stress hormones. However, the valuable targets for promoting learning and memory induced by acute stress and the underlying molecular mechanisms remain unclear. Acid-sensing ion channels (ASICs) play an important role in central neuronal systems and involves in depression, synaptic plasticity and learning and memory. In the current study, we used a combination of electrophysiological and behavioral approaches in an effort to explore the effects of acute stress on ASICs. We found that corticosterone (CORT) induced by acute stress caused a potentiation of ASICs current via glucocorticoid receptors (GRs) not mineralocorticoid receptors (MRs). Meanwhile, CORT did not produce an increase of ASICs current by pretreated with GF109203X, an antagonist of protein kinase C (PKC), whereas CORT did result in a markedly enhancement of ASICs current by bryostatin 1, an agonist of PKC, suggesting that potentiation of ASICs function may be depended on PKC activating. More importantly, an antagonist of ASICs, amiloride (10 μM) reduced the performance of learning and memory induced by acute stress, which is further suggesting that ASICs as the key components involves in cognitive processes induced by acute stress. These results indicate that acute stress causes the enhancement of ASICs function by activating PKC signaling pathway, which leads to potentiated learning and memory.

Dehydroepiandrosterone and Dehydroepiandrosterone-Sulfate and Emotional Processing

Steroid hormones are important regulators of brain development, physiological function, and behavior. Among them, dehydroepiandrosterone (DHEA) and dehydroepiandrosterone-sulfate (DHEAS) also do modulate emotional processing and may have mood enhancement effects. This chapter reviews the studies that bear relation to DHEA and DHEAS [DHEA(S)] and brain emotional processing and behavior. A brief introduction to the mechanisms of action and variations of DHEA(S) levels throughout life has also been forward in this chapter. Higher DHEA(S) levels may reduce activity in brain regions involved in the generation of negative emotions and modulate activity in regions involved in regulatory processes. At the electrophysiological level, higher DHEA-to-cortisol and DHEAS-to-DHEA ratios were related to shorter P300 latencies and shorter P300 amplitudes during the processing of negative stimuli, suggesting less interference of negative stimuli with the task and less processing of the negative information, which in turn may suggest a protective mechanism against negative information overload. Present knowledge indicates that DHEA(S) may play a role in cortical development and plasticity, protecting against negative affect and depression, and at the same time enhancing attention and overall working memory, possibly at the cost of a reduction in emotional processing, emotional memory, and social understanding.

Glucocorticoids, genes and brain function

The identification of key genes in transcriptomic data constitutes a huge challenge. Our review of microarray reports revealed 88 genes whose transcription is consistently regulated by glucocorticoids (GCs), such as cortisol, corticosterone and dexamethasone, in the brain. Replicable transcriptomic data were combined with biochemical and physiological data to create an integrated view of the effects induced by GCs. The most frequently reported genes were Errfi1 and Ddit4. Their up-regulation was associated with the altered transcription of genes regulating growth factor and mTORC1 signaling (Gab1, Tsc22d3, Dusp1, Ndrg2, Ppp5c and Sesn1) and progression of the cell cycle (Ccnd1, Cdkn1a and Cables1). The GC-induced reprogramming of cell function involves changes in the mRNA level of genes responsible for the regulation of transcription (Klf9, Bcl6, Klf15, Tle3, Cxxc5, Litaf, Tle4, Jun, Sox4, Sox2, Sox9, Irf1, Sall2, Nfkbia and Id1) and the selective degradation of mRNA (Tob2). Other genes are involved in the regulation of metabolism (Gpd1, Aldoc and Pdk4), actin cytoskeleton (Myh2, Nedd9, Mical2, Rhou, Arl4d, Osbpl3, Arhgef3, Sdc4, Rdx, Wipf3, Chst1 and Hepacam), autophagy (Eva1a and Plekhf1), vesicular transport (Rhob, Ehd3, Vps37b and Scamp2), gap junctions (Gjb6), immune response (Tiparp, Mertk, Lyve1 and Il6r), signaling mediated by thyroid hormones (Thra and Sult1a1), calcium (Calm2), adrenaline/noradrenaline (Adcy9 and Adra1d), neuropeptide Y (Npy1r) and histamine (Hdc). GCs also affected genes involved in the synthesis of polyamines (Azin1) and taurine (Cdo1). The actions of GCs are restrained by feedback mechanisms depending on the transcription of Sgk1, Fkbp5 and Nr3c1. A side effect induced by GCs is increased production of reactive oxygen species. Available data show that the brain's response to GCs is part of an emergency mode characterized by inactivation of non-core activities, restrained inflammation, restriction of investments (growth), improved efficiency of energy production and the removal of unnecessary or malfunctioning cellular components to conserve energy and maintain nutrient supply during the stress response.

Interactions between stress and physical activity on Alzheimer's disease pathology

Physical activity and stress are both environmental modifiers of Alzheimer's disease (AD) risk. Animal studies of physical activity in AD models have largely reported positive results, however benefits are not always observed in either cognitive or pathological outcomes and inconsistencies among findings remain. Studies using forced exercise may increase stress and mitigate some of the benefit of physical activity in AD models, while voluntary exercise regimens may not achieve optimal intensity to provide robust benefit. We evaluated the findings of studies of voluntary and forced exercise regimens in AD mouse models to determine the influence of stress, or the intensity of exercise needed to outweigh the negative effects of stress on AD measures. In addition, we show that chronic physical activity in a mouse model of AD can prevent the effects of acute restraint stress on Aβ levels in the hippocampus. Stress and physical activity have many overlapping and divergent effects on the body and some of the possible mechanisms through which physical activity may protect against stress-induced risk factors for AD are discussed. While the physiological effects of acute stress and acute exercise overlap, chronic effects of physical activity appear to directly oppose the effects of chronic stress on risk factors for AD. Further study is needed to identify optimal parameters for intensity, duration and frequency of physical activity to counterbalance effects of stress on the development and progression of AD.

Recovery of Chronic Stress-Triggered Changes of Hippocampal Glutamatergic Transmission

Chronic stress results in neurochemical, physiological, immune, molecular, cellular, and structural changes in the brain and often dampens the cognition. The hippocampus has been one major focus in studying the stress responsivity and neural mechanisms underlying depression. Both acute and chronic stress stimuli lead to dynamic changes in excitatory transmission in the hippocampus. The present study examined the potential effects of spontaneous recovery after chronic stress on spatial memory function and glutamatergic transmission in the hippocampus. The results showed that chronic unpredicted mild stress transiently increased AMPA receptor GluA2/3 subunit expression, together with elevated PICK-1 protein expression. Spontaneous recovery restored the behavioral deficits in Barnes maze test, as well as the glutamate receptor expression changes. In conclusion, spontaneous recovery acts as an important mechanism in system homeostasis.

Partial Amelioration of Synaptic and Cognitive Deficits by Inhibiting Cofilin Dephosphorylation in an Animal Model of Alzheimer's Disease

The loss of synaptic structure and function has been linked to the cognitive impairment of Alzheimer's disease (AD). Dysregulation of the actin cytoskeleton, which plays a key role in regulating the integrity of synapses and the transport of synaptic proteins, has been suggested to contribute to the pathology of AD. In this study, we found that glutamate receptor surface expression and synaptic function in frontal cortical neurons were significant diminished in a familial AD (FAD) model, which was correlated with the reduction of phosphorylated cofilin, a key protein regulating the dynamics of actin filaments. Injecting a cofilin dephosphorylation inhibitory peptide to FAD mice led to the partial rescue of the surface expression of AMPA and NMDA receptor subunits, as well as the partial restoration of AMPAR- and NMDAR-mediated synaptic currents. Moreover, the impaired working memory and novel object recognition memory in FAD mice were partially ameliorated by injections of the cofilin dephosphorylation inhibitory peptide. These results suggest that targeting the cofilin-actin signaling holds promise to mitigate the physiological and behavioral abnormality in AD.

The Interleukin (IL)-23/T helper (Th)17 Axis in Experimental Autoimmune Encephalomyelitis and Multiple Sclerosis

T helper (Th)17 cells are responsible for host defense against fungi and certain extracellular bacteria but have also been reported to play a role in a variety of autoimmune diseases. Th17 cells respond to environmental cues, are very plastic, and might also be involved in tissue homeostasis and regeneration. The imprinting of pathogenic properties in Th17 cells in autoimmunity seems highly dependent on interleukin (IL)-23. Since Th17 cells were first described in experimental autoimmune encephalomyelitis, they have been suggested to also promote tissue damage in multiple sclerosis (MS). Indeed, some studies linked Th17 cells to disease severity in MS, and the efficacy of anti-IL-17A therapy in MS supported this idea. In this review, we will summarize molecular features of Th17 cells and discuss the evidence for their function in experimental models of autoimmune diseases and MS.

FoxO1, A2M, and TGF-β1: three novel genes predicting depression in gene X environment interactions are identified using cross-species and cross-tissues transcriptomic and miRNomic analyses

To date, gene-environment (GxE) interaction studies in depression have been limited to hypothesis-based candidate genes, since genome-wide (GWAS)-based GxE interaction studies would require enormous datasets with genetics, environmental, and clinical variables. We used a novel, cross-species and cross-tissues "omics" approach to identify genes predicting depression in response to stress in GxE interactions. We integrated the transcriptome and miRNome profiles from the hippocampus of adult rats exposed to prenatal stress (PNS) with transcriptome data obtained from blood mRNA of adult humans exposed to early life trauma, using a stringent statistical analyses pathway. Network analysis of the integrated gene lists identified the Forkhead box protein O1 (FoxO1), Alpha-2-Macroglobulin (A2M), and Transforming Growth Factor Beta 1 (TGF-β1) as candidates to be tested for GxE interactions, in two GWAS samples of adults either with a range of childhood traumatic experiences (Grady Study Project, Atlanta, USA) or with separation from parents in childhood only (Helsinki Birth Cohort Study, Finland). After correction for multiple testing, a meta-analysis across both samples confirmed six FoxO1 SNPs showing significant GxE interactions with early life emotional stress in predicting depressive symptoms. Moreover, in vitro experiments in a human hippocampal progenitor cell line confirmed a functional role of FoxO1 in stress responsivity. In secondary analyses, A2M and TGF-β1 showed significant GxE interactions with emotional, physical, and sexual abuse in the Grady Study. We therefore provide a successful 'hypothesis-free' approach for the identification and prioritization of candidate genes for GxE interaction studies that can be investigated in GWAS datasets.

Arc/Arg3.1 protein expression in dorsal hippocampal CA1, a candidate event as a biomarker for the effects of exercise on chronic stress-evoked behavioral abnormalities

[Purpose]

Chronic stress is a risk factor for behavioral deficits, including impaired memory processing and depression. Exercise is well known to have beneficial impacts on brain health.

[Methods]

Mice were forced to treadmill running (4-week) during chronic restraint stress (6h/21d), and then behavioral tests were conducted by Novel object recognition, forced swimming test: FST, sociality test: SI. Dissected brain was stained with anti-calbindin-d28k and anti-Arc antibodies. Also, mice were treated with CX546 intraperitoneally during chronic restraint stress, and behavioral tests were assessed using Morris water maze, FST, and SI. Dissected brain was stained with anti-Arc antibody.

[Results]

The current study demonstrated that chronic stress-induced impairment of memory consolidation and depression-like behaviors, along with the changes in calbindin-d28k and Arc protein levels in the hippocampal CA1 area, were attenuated by regular treadmill running. Further, prolonged ampakine treatment prevented chronic stress-evoked behavioral abnormalities and nuclear Arc levels in hippocampal CA1 neurons. Nuclear localization of Arc protein in hippocampal CA1 neurons, but not total levels, was correlated with behavioral outcome in chronically stressed mice in response to a regular exercise regimen.

[Conclusion]

These results suggest that nuclear levels of Arc are strongly associated with behavioral changes, and highlight the role of exercise acting through an α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor (AMPAR)-mediated mechanisms in a chronic stress-induced maladaptive condition.

Molecular and Epigenetic Mechanisms for the Complex Effects of Stress on Synaptic Physiology and Cognitive Functions

Evidence over the past decades has found that stress, particularly through the corticosterone stress hormones, produces complex changes in glutamatergic signaling in prefrontal cortex, which leads to the alteration of cognitive processes medicated by this brain region. Interestingly, the effects of stress on glutamatergic transmission appear to be "U-shaped," depending upon the duration and severity of the stressor. These biphasic effects of acute vs chronic stress represent the adaptive vs maladaptive responses to stressful stimuli. Animal studies suggest that the stress-induced modulation of excitatory synaptic transmission involves changes in presynaptic glutamate release, postsynaptic glutamate receptor membrane trafficking and degradation, spine structure and cytoskeleton network, and epigenetic control of gene expression. This review will discuss current findings on the key molecules involved in the stress-induced regulation of prefrontal cortex synaptic physiology and prefrontal cortex-mediated functions. Understanding the molecular and epigenetic mechanisms that underlie the complex effects of stress will help to develop novel strategies to cope with stress-related mental disorders.

Gynostemma pentaphyllum is neuroprotective in a rat model of cardiopulmonary resuscitation

Asphyxial cardiac arrest (ACA)-induced ischemia results in acute and delayed neuronal cell death. The early reperfusion phase is critical for the outcome. Intervention strategies directed to this period are promising to reduce ACA/resuscitation-dependent impairments. This study focused on the evaluation of the protective potential of an extract from Gynostemma pentaphyllum (GP), a plant used in traditional medicine with antioxidative, glucose lowering and neuroprotective activities, in an ACA rat model. We tested the following parameters: i) Basic systemic parameters such as pCO₂ and blood glucose value within the first 30 min post-ACA; ii) mitochondrial response by determining activities of citrate synthase, respiratory chain complexes I + III and II + III, and the composition of cardiolipin 6 and 24 h post-ACA; iii) neuronal vitality of the CA1 hippocampal region by immunohistochemistry 24 h and 7 days post-ACA; and iv) cognitive function by a novel object recognition test 7 days post-ACA. GP, administered after reaching spontaneous circulation, counteracted the following: i) ACA-mediated increases in arterial CO₂ tension and blood glucose values; ii) transient increase in the activity of the respiratory chain complexes II + III; iii) elevation in cardiolipin content; iv) hippocampal CA1 neurodegeneration, and v) loss of normal novelty-object seeking. The protective effects of GP were accompanied by side effects of the vehicle DMSO, such as the stimulation of citrate synthase activity in control animals, inhibition of cardiolipin synthesis in ACA animals and complex II + III activity in both control and ACA animals. The results emphasize the importance of the early post-resuscitation phase for the neurological outcome after ACA/resuscitation, and demonstrated the power of GP substitution as neuroprotective intervention. Moreover, the results underline the need of a careful handling of the popular vehicle DMSO.

Whole-transcriptome microarray analysis reveals regulation of Rab4 by RBM5 in neurons

RNA binding motif 5 (RBM5) is a nuclear protein that modulates gene transcription and mRNA splicing in cancer cells. The brain is among the highest RBM5-expressing organ in the body but its mRNA target(s) or functions in the CNS have not been elucidated. Here we knocked down (KO) RBM5 in primary rat cortical neurons and analyzed total RNA extracts by gene microarray vs. neurons transduced with lentivirus to deliver control (non-targeting) shRNA. The mRNA levels of Sec23A (involved in ER-Golgi transport) and the small GTPase Rab4a (involved in endocytosis/protein trafficking) were increased in RBM5 KO neurons relative to controls. At the protein level, only Rab4a was significantly increased in RBM5 KO extracts. Also, elevated Rab4a levels in KO neurons were associated with decreased membrane levels of oligomeric serotonin transporters (SERT). Finally, RBM5 KO was associated with increased uptake of membrane-derived monomeric SERT.

SIGNIFICANCE

Rab4a is involved in the regulation of endocytosis and protein trafficking in cells. In the CNS it regulates diverse neurobiological functions including (but not limited to) trafficking of transmembrane proteins involved in neurotransmission (e.g. SERT), maintaining dendritic spine size, promoting axonal growth, and modulating cognition. Our findings suggest that RBM5 regulates Rab4a in rat neurons.

A sexually dimorphic pre-stressed translational signature in CA3 pyramidal neurons of BDNF Val66Met mice

Males and females use distinct brain circuits to cope with similar challenges. Using RNA sequencing of ribosome-bound mRNA from hippocampal CA3 neurons, we found remarkable sex differences and discovered that female mice displayed greater gene expression activation after acute stress than males. Stress-sensitive BDNF Val66Met mice of both sexes show a pre-stressed translational phenotype in which the same genes that are activated without applied stress are also induced in wild-type mice by an acute stressor. Behaviourally, only heterozygous BDNF Val66Met females exhibit spatial memory impairment, regardless of acute stress. Interestingly, this effect is not observed in ovariectomized heterozygous BDNF Val66Met females, suggesting that circulating ovarian hormones induce cognitive impairment in Met carriers. Cognitive deficits are not observed in males of either genotype. Thus, in a brain region not normally associated with sex differences, this work sheds light on ways that genes, environment and sex interact to affect the transcriptome’s response to a stressor.

Animals’ response to acute stress is known to be influenced by sex and genetics. Here the authors performed RNA-seq on actively translated mRNAs in hippocampal CA3 neurons in mice, and document the effects of sex and genotype (i.e., BDNF Val66Met) on acute stress-induced gene expression.

Stress hormone rapidly tunes synaptic NMDA receptor through membrane dynamics and mineralocorticoid signalling

Stress hormones, such as corticosteroids, modulate the transmission of hippocampal glutamatergic synapses and NMDA receptor (NMDAR)-dependent synaptic plasticity, favouring salient behavioural responses to the environment. The corticosterone-induced synaptic adaptations partly rely on changes in NMDAR signalling, although the cellular pathway underlying this effect remains elusive. Here, we demonstrate, using single molecule imaging and electrophysiological approaches in hippocampal neurons, that corticosterone specifically controls GluN2B-NMDAR surface dynamics and synaptic content through mineralocorticoid signalling. Strikingly, extracellular corticosterone was sufficient to increase the trapping of GluN2B-NMDAR within synapses. Functionally, corticosterone-induced potentiation of AMPA receptor content in synapses required the changes in NMDAR surface dynamics. These high-resolution imaging data unveiled that, in hippocampal networks, corticosterone is a natural, potent, fast and specific regulator of GluN2B-NMDAR membrane trafficking, tuning NMDAR-dependent synaptic adaptations.

Genome-wide association study of treatment response to venlafaxine XR in generalized anxiety disorder

We conducted the first genome-wide association study (GWAS) in Generalized Anxiety Disorder (GAD) to identify potential predictors of venlafaxine XR treatment outcome. Ninety-eight European American patients participated in a venlafaxine XR clinical trial for GAD, with Hamilton Anxiety Scale (HAM-A) response/remission at 24 weeks as the primary outcome measure. All participants were genotyped with the Illumina PsychChip, and 266,820 common single nucleotide polymorphisms (SNPs) were analyzed. Although no SNPs reached genome-wide significance, 8 SNPs were marginally associated with treatment response/remission and HAM-A reduction at week 12 and 24 (p<0.00001). Several identified genes may indicate markers crossing neuropsychiatric diagnostic categories.

Coordination of AMPA receptor trafficking by Rab GTPases

Synaptic connections in the brain are continuously weakened or strengthened in response to changes in neuronal activity. This process, known as synaptic plasticity, is the cellular basis for learning and memory, and is thought to be altered in several neuronal disorders. An important aspect of synaptic plasticity is the tightly controlled trafficking and synaptic targeting of the AMPA-type glutamate receptors, which are the major mediators of fast excitatory transmission in the brain. This review addresses the role of Rab GTPases in AMPA receptor trafficking in neurons under basal conditions and during activity-induced synaptic plasticity, especially during long-term potentiation (LTP) and long-term depression (LTD). We highlight the importance of the tight spatio-temporal control of Rab activity and suggest that this is critical for proper neuronal functions. We also discuss how abnormal AMPA receptor trafficking and malfunctioning of Rabs can lead to neurologic disorders or memory problems.

Ketamine Treatment and Global Brain Connectivity in Major Depression

Capitalizing on recent advances in resting-state functional connectivity magnetic resonance imaging (rs-fcMRI) and the distinctive paradigm of rapid mood normalization following ketamine treatment, the current study investigated intrinsic brain networks in major depressive disorder (MDD) during a depressive episode and following treatment with ketamine. Medication-free patients with MDD and healthy control subjects (HC) completed baseline rs-fcMRI. MDD patients received a single infusion of ketamine and underwent repeated rs-fcMRI at 24 h posttreatment. Global brain connectivity with global signal regression (GBCr) values were computed as the average of correlations of each voxel with all other gray matter voxels in the brain. MDD group showed reduced GBCr in the prefrontal cortex (PFC) but increased GBCr in the posterior cingulate, precuneus, lingual gyrus, and cerebellum. Ketamine significantly increased GBCr in the PFC and reduced GBCr in the cerebellum. At baseline, 2174 voxels of altered GBCr were identified, but only 310 voxels significantly differed relative to controls following treatment (corrected α<0.05). Responders to ketamine showed increased GBCr in the lateral PFC, caudate, and insula. Follow-up seed-based analyses illustrated a pattern of dysconnectivity between the PFC/subcortex and the rest of the brain in MDD, which appeared to normalize postketamine. The extent of the functional dysconnectivity identified in MDD and the swift and robust normalization following treatment suggest that GBCr may serve as a treatment response biomarker for the development of rapid acting antidepressants. The data also identified unique prefrontal and striatal circuitry as a putative marker of successful treatment and a target for antidepressants' development.

Stress Degrades Prefrontal Cortex Neuronal Coding of Goal-Directed Behavior

Stress, pervasive in modern society, impairs prefrontal cortex (PFC)-dependent cognitive processes, an action implicated in multiple psychopathologies and estimated to contribute to nearly half of all work place accidents. However, the neurophysiological bases for stress-related impairment of PFC-dependent function remain poorly understood. The current studies examined the effects of stress on PFC neural coding during a working memory task in rats. Stress suppressed responses of medial PFC (mPFC) neurons strongly tuned to a diversity of task events, including delay and outcome (reward, error). Stress-related impairment of task-related neuronal activity included multidimensional coding by PFC neurons, an action that significantly predicted cognitive impairment. Importantly, the effects of stress on PFC neuronal signaling were highly conditional on tuning strength: stress increased task-related activity in the larger population of PFC neurons weakly tuned to task events. Combined, stress elicits a profound collapse of task representations across the broader population of PFC neurons.

The Effects of Acute Exercise on Mood, Cognition, Neurophysiology, and Neurochemical Pathways: A Review

A significant body of work has investigated the effects of acute exercise, defined as a single bout of physical activity, on mood and cognitive functions in humans. Several excellent recent reviews have summarized these findings; however, the neurobiological basis of these results has received less attention. In this review, we will first briefly summarize the cognitive and behavioral changes that occur with acute exercise in humans. We will then review the results from both human and animal model studies documenting the wide range of neurophysiological and neurochemical alterations that occur after a single bout of exercise. Finally, we will discuss the strengths, weaknesses, and missing elements in the current literature, as well as offer an acute exercise standardization protocol and provide possible goals for future research.

Deficits in cognitive flexibility induced by chronic unpredictable stress are associated with impaired glutamate neurotransmission in the rat medial prefrontal cortex

Deficits in cognitive flexibility, the ability to modify behavior in response to changes in the environment, contribute to the onset and maintenance of stress-related neuropsychiatric illnesses, such as depression. Cognitive flexibility depends on medial prefrontal cortex (mPFC) function, and in depressed patients, cognitive inflexibility is associated with hypoactivity and decreased glutamate receptor expression in the mPFC. Rats exposed to chronic unpredictable stress (CUS) exhibit compromised mPFC function on the extradimensional (ED) set-shifting task of the attentional set-shifting test. Moreover, CUS-induced ED deficits are associated with dendritic atrophy and decreased glutamate receptor expression in the mPFC. This evidence suggests that impaired glutamate signaling may underlie stress-induced deficits in cognitive flexibility. To test this hypothesis, we first demonstrated that blocking NMDA or AMPA receptors in the mPFC during ED replicated CUS-induced deficits in naïve rats. Secondly, we found that expression of activity-regulated cytoskeleton-associated protein (Arc) mRNA, a marker of behaviorally induced glutamate-mediated plasticity, was increased in the mPFC following ED. We then showed that CUS compromised excitatory afferent activation of the mPFC following pharmacological stimulation of the mediodorsal thalamus (MDT), indicated by a reduced induction of c-fos expression. Subsequently, in vivo recordings of evoked potentials in the mPFC indicated that CUS impaired afferent activation of the mPFC evoked by MDT stimulation, but not the ventral hippocampus. Lastly, glutamate microdialysis showed that CUS attenuated the acute stress-evoked increase in extracellular glutamate in the mPFC. Together, these results demonstrate that CUS-induced ED deficits are associated with compromised glutamate neurotransmission in the mPFC.

Hepatoprotective effect of blocking N-methyl-d-aspartate receptors in male albino rats exposed to acute and repeated restraint stress

Stress affects many organs in addition to the brain, including the liver. We assessed the effects on the liver of blocking N-methyl-d-aspartate (NMDA) glutamate receptors with memantine in acute and repeated restraint stress. Forty-two male albino rats were divided into 7 groups; control, acute restraint stress (ARS), ARS + memantine, repeated restraint stress, repeated restraint + memantine, and positive control groups. We measured serum iron, zinc, alanine transferase and aspartame transferase, hepatic malondialdehyde, tumor necrosis factor-α (TNF-α), glutathione peroxidase, superoxide dismutase, metallothionein content, zinc transporter ZRT/IRT-like protein 14 mRNA expression, and hepcidin expression. We conducted a histopathological evaluation via histological staining and immunostaining for glial fibrillary acidic protein and synaptophysin expression, both of which are markers of hepatic stellate cell (HSC) activation. Both ARS and repeated stress increased markers of hepatic cell injury, oxidative stress, and HSC activation. Blocking NMDA with memantine provided a hepatoprotective effect in acute and repeated restraint stress and decreased hepatic cell injury, oxidative stress, and HSC activation.

Acute stress enhances the glutamatergic transmission onto basoamygdala neurons embedded in distinct microcircuits

Amygdala activation is known to be critical for the processing of stressful events in brain. Recent studies have shown that the projection neurons (PNs) in amygdala, although architecturally intermingled, are integrated into distinct microcircuits and thus play divergent roles in amygdala-related behaviors. It remains unknown how stress regulates the individual amygdala PNs embedded in distinct microcircuits. Here, by using retrograde tracing and electrophysiological recording in in vitro slices, we explored the modulation of acute immobilization stress (AIS) on the basoamygdala (BA) PNs projecting either to medial prefrontal cortex (mPFC) or elsewhere, which we designated as BA-mPFC and non-BA-mPFC PNs respectively. The results showed that in the control mice, both the excitatory and inhibitory postsynaptic currents (sEPSCs/sIPSCs) were comparable between these two subsets of BA PNs. The influences of AIS on sEPSCs and sIPSCs were overall similar between the two neuronal populations. It markedly increased the sEPSCs amplitude but left unaltered their frequency as well as the sIPSCs amplitude and frequency. Despite this, several differences emerged between the effects of AIS on the distribution of sEPSCs/sIPSCs frequency in these two groups of BA PNs. Similar changes were also observed in the sEPSCs/sIPSCs of the two PN populations from mice experiencing forced swimming stress. Their intrinsic excitability, on the other hand, was nearly unaltered following AIS. Our results thus suggest that acute stress recruit both BA-mPFC and non-BA-mPFC PNs mainly through enhancing the glutamatergic transmission they receive.

Acute stress effects on GABA and glutamate levels in the prefrontal cortex: A 7T 1H magnetic resonance spectroscopy study

There is ample evidence that the inhibitory GABA and the excitatory glutamate system are essential for an adequate response to stress. Both GABAergic and glutamatergic brain circuits modulate hypothalamus-pituitary-adrenal (HPA)-axis activity, and stress in turn affects glutamate and GABA levels in the rodent brain. However, studies examining stress-induced GABA and glutamate levels in the human brain are scarce. Therefore, we investigated the influence of acute psychosocial stress (using the Trier Social Stress Test) on glutamate and GABA levels in the medial prefrontal cortex of 29 healthy male individuals using 7 Tesla proton magnetic resonance spectroscopy. In vivo GABA and glutamate levels were measured before and 30 min after exposure to either the stress or the control condition. We found no associations between psychosocial stress or cortisol stress reactivity and changes over time in medial prefrontal glutamate and GABA levels. GABA and glutamate levels over time were significantly correlated in the control condition but not in the stress condition, suggesting that very subtle differential effects of stress on GABA and glutamate across individuals may occur. However, overall, acute psychosocial stress does not appear to affect in vivo medial prefrontal GABA and glutamate levels, at least this is not detectable with current practice ¹H-MRS.

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Psychosocial stress did not alter glutamate and GABA levels in the medial prefrontal cortex in healthy male individuals.

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Moreover, cortisol stress reactivity was not associated with medial prefrontal glutamate and GABA level change over time.

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Together, acute stress does not seem to affect in vivo medial prefrontal 7T MRI GABA and glutamate levels in humans.

Genome to Phenome: A Systems Biology Approach to PTSD Using an Animal Model

Post-traumatic stress disorder (PTSD) is a debilitating illness that imposes significant emotional and financial burdens on military families. The understanding of PTSD etiology remains elusive; nonetheless, it is clear that PTSD is manifested by a cluster of symptoms including hyperarousal, reexperiencing of traumatic events, and avoidance of trauma reminders. With these characteristics in mind, several rodent models have been developed eliciting PTSD-like features. Animal models with social dimensions are of particular interest, since the social context plays a major role in the development and manifestation of PTSD.For civilians, a core trauma that elicits PTSD might be characterized by a singular life-threatening event such as a car accident. In contrast, among war veterans, PTSD might be triggered by repeated threats and a cumulative psychological burden that coalesced in the combat zone. In capturing this fundamental difference, the aggressor-exposed social stress (Agg-E SS) model imposes highly threatening conspecific trauma on naïve mice repeatedly and randomly.There is abundant evidence that suggests the potential role of genetic contributions to risk factors for PTSD. Specific observations include putatively heritable attributes of the disorder, the cited cases of atypical brain morphology, and the observed neuroendocrine shifts away from normative. Taken together, these features underscore the importance of multi-omics investigations to develop a comprehensive picture. More daunting will be the task of downstream analysis with integration of these heterogeneous genotypic and phenotypic data types to deliver putative clinical biomarkers. Researchers are advocating for a systems biology approach, which has demonstrated an increasingly robust potential for integrating multidisciplinary data. By applying a systems biology approach here, we have connected the tissue-specific molecular perturbations to the behaviors displayed by mice subjected to Agg-E SS. A molecular pattern that links the atypical fear plasticity to energy deficiency was thereby identified to be causally associated with many behavioral shifts and transformations.PTSD is a multifactorial illness sensitive to environmental influence. Accordingly, it is essential to employ the optimal animal model approximating the environmental condition that elicits PTSD-like symptoms. Integration of an optimal animal model with a systems biology approach can contribute to a more knowledge-driven and efficient next-generation care management system and, potentially, prevention of PTSD.

Context-dependent memory following recurrent hypoglycaemia in non-diabetic rats is mediated via glucocorticoid signalling in the dorsal hippocampus

AIMS/HYPOTHESIS

Recurrent hypoglycaemia is primarily caused by repeated over-administration of insulin to patients with diabetes. Although cognition is impaired during hypoglycaemia, restoration of euglycaemia after recurrent hypoglycaemia is associated with improved hippocampally mediated memory. Recurrent hypoglycaemia alters glucocorticoid secretion in response to hypoglycaemia; glucocorticoids are well established to regulate hippocampal processes, suggesting a possible mechanism for recurrent hypoglycaemia modulation of subsequent cognition. We tested the hypothesis that glucocorticoids within the dorsal hippocampus might mediate the impact of recurrent hypoglycaemia on hippocampal cognitive processes.

METHODS

We characterised changes in the dorsal hippocampus at several time points to identify specific mechanisms affected by recurrent hypoglycaemia, using a well-validated 3 day model of recurrent hypoglycaemia either alone or with intrahippocampal delivery of glucocorticoid (mifepristone) and mineralocorticoid (spironolactone) receptor antagonists prior to each hypoglycaemic episode.

RESULTS

Recurrent hypoglycaemia enhanced learning and also increased hippocampal expression of glucocorticoid receptors, serum/glucocorticoid-regulated kinase 1, cyclic AMP response element binding (CREB) phosphorylation, and plasma membrane levels of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartic acid (NMDA) receptors. Both hippocampus-dependent memory enhancement and the molecular changes were reversed by glucocorticoid receptor antagonist treatment.

CONCLUSIONS/INTERPRETATION

These results indicate that increased glucocorticoid signalling during recurrent hypoglycaemia produces several changes in the dorsal hippocampus that are conducive to enhanced hippocampus-dependent contextual learning. These changes appear to be adaptive, and in addition to supporting cognition may reduce damage otherwise caused by repeated exposure to severe hypoglycaemia.

Sex-Specific Effects of Prenatal Stress on Bdnf Expression in Response to an Acute Challenge in Rats: a Role for Gadd45β

Exposure to early adversities represents a major risk factor for psychiatric disorders. We have previously shown that exposure to prenatal stress (PNS) in rats alters the developmental expression of brain-derived neurotrophic factor (Bdnf) with a specific temporal profile. However, exposure to early-life stress is known to alter the ability to cope with challenging events later in life, which may contribute to the enhanced vulnerability to stress-related disorders. Since Bdnf is also an important player for activity-dependent plasticity, we investigated whether the exposure to PNS in rats could alter Bdnf responsiveness to an acute challenge at adulthood. We found that exposure to PNS produces significant changes in Bdnf responsiveness with brain region- and gender-specific selectivity. Indeed, exposure to an acute stress upregulates Bdnf expression in the prefrontal cortex, but not in the hippocampus, of control animals. Moreover, such modulatory activity is selectively impaired in PNS female rats, an effect that was associated with changes in the modulation of the DNA demethylase Gadd45β. Our results suggest that exposure to PNS may reprogram gene transcription through epigenetic mechanisms reducing the ability to cope under adverse conditions, a trait that is disrupted in psychiatric diseases.

Acute stress diminishes M-current contributing to elevated activity of hypothalamic-pituitary-adrenal axis

Acute stress stimulates corticotrophin-releasing hormone (CRH)-expressing neurons in the hypothalamic paraventricular nucleus (PVN), which is an essential component of hypothalamic-pituitary-adrenal (HPA) axis. However, the cellular and molecular mechanisms remain unclear. The M-channel is a voltage-dependent K⁺ channel involved in stabilizing the neuronal membrane potential and regulating neuronal excitability. In this study, we tested our hypothesis that acute stress suppresses expression of Kv7 channels to stimulate PVN-CRH neurons and the HPA axis. Rat PVN-CRH neurons were identified by expressing enhanced green fluorescent protein driven by Crh promoter. Acute restraint stress attenuated the excitatory effect of Kv7 blocker XE-991 on the firing activity of PVN-CRH neurons and blunted the increase in plasma corticosterone (CORT) levels induced by microinjection of XE-991 into the PVN. Furthermore, acute stress significantly decreased the M-currents in PVN-CRH neurons and reduced PVN expression of Kv7.3 subunit in the membrane. In addition, acute stress significantly increased phosphorylated AMP-activated protein kinase (AMPK) levels in the PVN tissue. Intracerebroventricular injection of the AMPK inhibitor dorsomorphin restored acute stress-induced elevation of CORT levels and reduction of membrane Kv7.3 protein level in the PVN. Dorsomorphin treatment increased the M-currents and reduced the firing activity of PVN-CRH neurons in acutely stressed rats. Collectively, these data suggest that acute stress diminishes Kv7 channels to stimulate PVN-CRH neurons and the HPA axis potentially via increased AMPK activity.

Chronic Stress Increases Prefrontal Inhibition: A Mechanism for Stress-Induced Prefrontal Dysfunction

BACKGROUND

Multiple neuropsychiatric disorders, e.g., depression, are linked to imbalances in excitatory and inhibitory neurotransmission and prefrontal cortical dysfunction, and are concomitant with chronic stress.

METHODS

We used electrophysiologic (n = 5-6 animals, 21-25 cells/group), neuroanatomic (n = 6-8/group), and behavioral (n = 12/group) techniques to test the hypothesis that chronic stress increases inhibition of medial prefrontal cortex (mPFC) glutamatergic output neurons.

RESULTS

Using patch clamp recordings from infralimbic mPFC pyramidal neurons, we found that chronic stress selectively increases the frequency of miniature inhibitory postsynaptic currents with no effect on amplitude, which suggests that chronic stress increases presynaptic gamma-aminobutyric acid release. Elevated gamma-aminobutyric acid release under chronic stress is accompanied by increased inhibitory appositions and terminals onto glutamatergic cells, as assessed by both immunohistochemistry and electron microscopy. Furthermore, chronic stress decreases glucocorticoid receptor immunoreactivity specifically in a subset of inhibitory neurons, which suggests that increased inhibitory tone in the mPFC after chronic stress may be caused by loss of a glucocorticoid receptor-mediated brake on interneuron activity. These neuroanatomic and functional changes are associated with impairment of a prefrontal-mediated behavior. During chronic stress, rats initially make significantly more errors in the delayed spatial win-shift task, an mPFC-mediated behavior, which suggests a diminished impact of the mPFC on decision making.

CONCLUSIONS

Taken together, the data suggest that chronic stress increases synaptic inhibition onto prefrontal glutamatergic output neurons, limiting the influence of the prefrontal cortex in control of stress reactivity and behavior. Thus, these data provide a mechanistic link among chronic stress, prefrontal cortical hypofunction, and behavioral dysfunction.

Neonatal isolation augments social dominance by altering actin dynamics in the medial prefrontal cortex

Social separation early in life can lead to the development of impaired interpersonal relationships and profound social disorders. However, the underlying cellular and molecular mechanisms involved are largely unknown. Here, we found that isolation of neonatal rats induced glucocorticoid-dependent social dominance over nonisolated control rats in juveniles from the same litter. Furthermore, neonatal isolation inactivated the actin-depolymerizing factor (ADF)/cofilin in the juvenile medial prefrontal cortex (mPFC). Isolation-induced inactivation of ADF/cofilin increased stable actin fractions at dendritic spines in the juvenile mPFC, decreasing glutamate synaptic AMPA receptors. Expression of constitutively active ADF/cofilin in the mPFC rescued the effect of isolation on social dominance. Thus, neonatal isolation affects spines in the mPFC by reducing actin dynamics, leading to altered social behavior later in life.

The ADHD-linked human dopamine D4 receptor variant D4.7 induces over-suppression of NMDA receptor function in prefrontal cortex

The human dopamine D4 receptor (hD4R) variants with long tandem repeats in the third intracellular loop have been strongly associated with attention deficit hyperactivity disorder (ADHD) and risk taking behaviors. To understand the potential molecular mechanism underlying the connection, we have investigated the synaptic function of human D4R polymorphism by virally expressing the ADHD-linked 7-repeat allele, hD4.7, or its normal counterpart, hD4.4, in the prefrontal cortex (PFC) of D4R knockout mice. We found that hD4R bound to the SH3 domain of PSD-95 in a state-dependent manner. Activation of hD4.7 caused more reduction of NR1/PSD-95 binding and NR1 surface expression than hD4.4 in PFC slices. Moreover, the NMDAR-mediated excitatory postsynaptic currents (NMDAR-EPSC) in PFC pyramidal neurons were suppressed to a larger extent by hD4.7 than hD4.4 activation. Direct stimulation of NMDARs with the partial agonist d-cycloserine prevented the NMDAR hypofunction induced by hD4.7 activation. Moreover, hD4.7-expressing mice exhibited the increased exploratory and novelty seeking behaviors, mimicking the phenotypic hallmark of human ADHD. d-cycloserine administration ameliorated the ADHD-like behaviors in hD4.7-expressing mice. Our results suggest that over-suppression of NMDAR function may underlie the role of hD4.7 in ADHD, and enhancing NMDAR signaling may be a viable therapeutic strategy to ADHD humans carrying the D4.7 allele.

Dynamic Regulation of AMPAR Phosphorylation In Vivo Following Acute Behavioral Stress

The tuning of glutamatergic transmission is an essential mechanism for neuronal communication. α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) are ionotropic glutamate receptors that mediate fast synaptic transmission. The phosphorylation states of specific serine residues on the GluA1 and GluA2 AMPAR subunits are considered critical post-translational modifications that regulate AMPAR activity and subcellular trafficking. While behavioral stress, via stress hormones, exerts specific alterations on such glutamatergic processes, there have been conflicting data concerning the influence of stress on AMPAR phosphorylation in different brain regions, and the post-stress signaling mechanisms mediating these processes are not well delineated. Here, we examined the dynamics of phosphorylation at three AMPAR serine residues (ser831-GluA1, ser845-GluA1, and ser880-GluA2) in four brain regions [amygdala, medial prefrontal cortex (mPFC), dorsal hippocampus, and ventral hippocampus] of the rat during the hour following behavioral stress. We also tested the impact of post-stress corticosteroid receptor blockade on AMPAR phosphorylation. Both GluA1 subunit residues exhibited elevated phosphorylation after stress, yet post-stress administration of corticosteroid receptor antagonists curtailed these effects only at ser831-GluA1. In contrast, ser880-GluA2 displayed a time-dependent tendency for early decreased phosphorylation (that was selectively augmented by mifepristone treatment in the amygdala and mPFC of stressed animals) followed by increased phosphorylation later on. These findings show that the in vivo regulation of AMPAR phosphorylation after stress is a dynamic and subunit-specific process, and they provide support for the hypothesis that corticosteroid receptors have an ongoing role in the regulation of ser831-GluA1 phosphorylation during the post-stress interval.

HPA Axis Interactions with Behavioral Systems

Perhaps the most salient behaviors that individuals engage in involve the avoidance of aversive experiences and the pursuit of pleasurable experiences. Engagement in these behaviors is regulated to a significant extent by an individual's hormonal milieu. For example, glucocorticoid hormones are produced by the hypothalamic-pituitary-adrenocortical (HPA) axis, and influence most aspects of behavior. In turn, many behaviors can influence HPA axis activity. These bidirectional interactions not only coordinate an individual's physiological and behavioral states to each other, but can also tune them to environmental conditions thereby optimizing survival. The present review details the influence of the HPA axis on many types of behavior, including appetitively-motivated behaviors (e.g., food intake and drug use), aversively-motivated behaviors (e.g., anxiety-related and depressive-like) and cognitive behaviors (e.g., learning and memory). Conversely, the manuscript also describes how engaging in various behaviors influences HPA axis activity. Our current understanding of the neuronal and/or hormonal mechanisms that underlie these interactions is also summarized. © 2016 American Physiological Society. Compr Physiol 6:1897-1934, 2016.

Prednisolone increases neural reactivity to negative socio-emotional stimuli in healthy young men

Exogenous glucocorticoids are known to trigger affective changes, but these are highly variable across individuals. A better understanding of how synthetic glucocorticoids impact the processing of negative emotions in the human brain might help to predict such changes. In the present functional magnetic resonance imaging (fMRI) study, we sought to uncover the slow effects of a synthetic glucocorticoid infusion on the neural response to socio-emotional scenes using a within-participant, double-blind, placebo-controlled design. In two separate sessions, 20 young males were given either an intravenous prednisolone dose (250mg) or placebo in a cross-over, randomized order. Four hours later, they were scanned while viewing drawings of persons in a neutral or negative emotional situation. On the next morning participants provided a blood sample for serum cortisol measurement, which served as a manipulation check. Prednisolone strongly suppressed morning cortisol, and heightened brain reactivity to emotional stimuli in left amygdala, left caudate head, right inferior frontal gyrus, bilateral supplementary motor area, and right somatosensory cortex. Amygdala reactivity was related to lower self-reported fatigue and higher irritability in the prednisolone condition. Moreover, prednisolone blunted inferior frontal and amygdala connectivity with other regions of the emotion-processing neural circuitry. Our results suggest specific brain pathways through which exogenous glucocorticoids may labilize affect.

Histone Modification of Nedd4 Ubiquitin Ligase Controls the Loss of AMPA Receptors and Cognitive Impairment Induced by Repeated Stress

Stress and the major stress hormone corticosterone induce profound influences in the brain. Altered histone modification and transcriptional dysfunction have been implicated in stress-related mental disorders. We previously found that repeated stress caused an impairment of prefrontal cortex (PFC)-mediated cognitive functions by increasing the ubiquitination and degradation of AMPA-type glutamate receptors via a mechanism depending on the E3 ubiquitin ligase Nedd4. Here, we demonstrated that in PFC of repeatedly stressed rats, active glucocorticoid receptor had the increased binding to the glucocorticoid response element of histone deacetylase 2 (HDAC2) promoter, resulting in the upregulation of HDAC2. Inhibition or knock-down of HDAC2 blocked the stress-induced impairment of synaptic transmission, AMPAR expression, and recognition memory. Furthermore, we found that, in stressed animals, the HDAC2-dependent downregulation of histone methyltransferase Ehmt2 (G9a) led to the loss of repressive histone methylation at the Nedd4-1 promoter and the transcriptional activation of Nedd4. These results have provided an epigenetic mechanism and a potential treatment strategy for the detrimental effects of chronic stress.

SIGNIFICANCE STATEMENT

Prolonged stress exposure can induce altered histone modification and transcriptional dysfunction, which may underlie the profound influence of stress in regulating brain functions. We report an important finding about the epigenetic mechanism controlling the detrimental effects of repeated stress on synaptic transmission and cognitive function. First, it has revealed the stress-induced alteration of key epigenetic regulators HDAC2 and Ehmt2, which determines the synaptic and behavioral effects of repeated stress. Second, it has uncovered the stress-induced histone modification of the target gene Nedd4, an E3 ligase that is critically involved in the ubiquitination and degradation of AMPA receptors and cognition. Third, it has provided the epigenetic approach, HDAC2 inhibition or knock-down, to rescue synaptic and cognitive functions in stressed animals.

Stress-induced mechanisms in mental illness: A role for glucocorticoid signalling

Stress represents the main environmental risk factor for mental illness. Exposure to stressful events, particularly early in life, has been associated with increased incidence and susceptibility of major depressive disorders as well as of other psychiatric illnesses. Among the key players in these events are glucocorticoid receptors. Dysfunctional glucocorticoid signalling may indeed contribute to psychopathology through a number of mechanisms that regulate the response to acute or chronic stress and that affect the function of genes and systems known to be relevant for mood disorders. Indeed, exposure to chronic stress early in life as well as in adulthood has been shown to reduce the expression of glucocorticoid receptors (GR), also through epigenetic mechanisms, and to up-regulate the expression of the co-chaperone gene FKBP5, which restrains GR activity by limiting the translocation of the receptor complex to the nucleus. Another mechanism that contributes to changes in GR responsiveness is the state of receptor phosphorylation that controls activation, subcellular localization as well as its transcriptional activity. Moreover, GR phosphorylation may represent an important mechanism for the cross talk between neurotrophic signalling and GR-dependent transcription, bridging two important players for mood disorders. One gene that lies downstream from GR and may contribute to stress-related changes is serum glucocorticoid kinase-1 (SGK1). We have demonstrated that the expression of SGK1 is significantly increased after exposure to chronic stress in rodents as well as in the blood of drug-free depressed patients. We have also shown that SGK1 up-regulation may ultimately reduce hippocampal neurogenesis and contribute to the structural abnormalities that have been reported to occur in depressed patients. In summary, GR signalling may represent a point of convergence as well as of divergence for defects associated with pathologic conditions characterized by heightened vulnerability to stress. The characterization of these abnormalities is crucial to identify novel targets for therapeutic intervention that may counteract more effectively stress-induced neurobiological abnormalities.

Estrogen in prefrontal cortex blocks stress-induced cognitive impairments in female rats

Animal and human studies have found that males and females show distinct stress responses. Recent studies suggest the contribution of estrogen in the brain to this sexual dimorphism. Repeated stress has been found to impair cognitive behaviors via suppressing glutamatergic transmission and glutamate receptor surface expression in pyramidal neurons of prefrontal cortex (PFC) in male rats. On the contrary, female rats exposed to the same stress paradigms show normal synaptic function and PFC-mediated cognition. The level of aromatase, the enzyme for the biosynthesis of estrogen, is significantly higher in the PFC of females than males. The stress-induced glutamatergic deficits and memory impairment are unmasked by blocking estrogen receptors or aromatase in females, suggesting a protective role of estrogen against the detrimental effects of repeated stress.

Neonatal pain and reduced maternal care: Early-life stressors interacting to impact brain and behavioral development

Advances in neonatal intensive care units (NICUs) have drastically increased the survival chances of preterm infants. However, preterm infants are still exposed to a wide range of stressors during their stay in the NICU, which include painful procedures and reduced maternal contact. The activation of the hypothalamic-pituitary-adrenal (HPA) axis, in response to these stressors during this critical period of brain development, has been associated with many acute and long-term adverse biobehavioral outcomes. Recent research has shown that Kangaroo care, a non-pharmacological analgesic based on increased skin-to-skin contact between the neonate and the mother, negates the adverse outcomes associated with neonatal pain and reduced maternal care, however the biological mechanism remains widely unknown. This review summarizes findings from both human and rodent literature investigating neonatal pain and reduced maternal care independently, primarily focusing on the role of the HPA axis and biobehavioral outcomes. The physiological and positive outcomes of Kangaroo care will also be discussed in terms of how dampening of the HPA axis response to neonatal pain and increased maternal care may account for positive outcomes associated with Kangaroo care.

Effects of stress on behavioral flexibility in rodents

Cognitive flexibility is the ability to switch between different rules or concepts and behavioral flexibility is the overt physical manifestation of these shifts. Behavioral flexibility is essential for adaptive responses and commonly measured by reversal learning and set-shifting performance in rodents. Both tasks have demonstrated vulnerability to stress with effects dependent upon stressor type and number of repetitions. This review compares the effects of stress on reversal learning and set-shifting to provide insight into the differential effect of stress on cognition. Acute and short-term repetition of stress appears to facilitate reversal learning whereas the longer term repetition of stress impairs reversal learning. Stress facilitated intradimensional set-shifting within a single, short-term stress protocol but otherwise generally impaired set-shifting performance in acute and repeated stress paradigms. Chronic unpredictable stress impairs reversal learning and set-shifting whereas repeated cold intermittent stress selectively impairs reversal learning and has no effect on set-shifting. In considering the mechanisms underlying the effects of stress on behavioral flexibility, pharmacological manipulations performed in conjunction with stress are also reviewed. Blocking corticosterone receptors does not affect the facilitation of reversal learning following acute stress but the prevention of corticosterone synthesis rescues repeated stress-induced set-shifting impairment. Enhancing post-synaptic norepinephrine function, serotonin availability, and dopamine receptor activation rescues and/or prevents behavioral flexibility performance following stress. While this review highlights a lack of a standardization of stress paradigms, some consistent effects are apparent. Future studies are necessary to specify the mechanisms underlying the stress-induced impairments of behavioral flexibility, which will aid in alleviating these symptoms in patients with some psychiatric disorders.

Stress Response and Perinatal Reprogramming: Unraveling (Mal)adaptive Strategies

Environmental stressors induce coping strategies in the majority of individuals. The stress response, involving the activation of the hypothalamic-pituitary-adrenocortical axis and the consequent release of corticosteroid hormones, is indeed aimed at promoting metabolic, functional, and behavioral adaptations. However, behavioral stress is also associated with fast and long-lasting neurochemical, structural, and behavioral changes, leading to long-term remodeling of glutamate transmission, and increased susceptibility to neuropsychiatric disorders. Of note, early-life events, both in utero and during the early postnatal life, trigger reprogramming of the stress response, which is often associated with loss of stress resilience and ensuing neurobehavioral (mal)adaptations. Indeed, adverse experiences in early life are known to induce long-term stress-related neuropsychiatric disorders in vulnerable individuals. Here, we discuss recent findings about stress remodeling of excitatory neurotransmission and brain morphology in animal models of behavioral stress. These changes are likely driven by epigenetic factors that lie at the core of the stress-response reprogramming in individuals with a history of perinatal stress. We propose that reprogramming mechanisms may underlie the reorganization of excitatory neurotransmission in the short- and long-term response to stressful stimuli.

Interactions between N-Ethylmaleimide-sensitive factor and GluA2 contribute to effects of glucocorticoid hormones on AMPA receptor function in the rodent hippocampus

Glucocorticoid hormones, via activation of their receptors, promote memory consolidation, but the exact underlying mechanisms remain elusive. We examined how corticosterone regulates AMPA receptor (AMPAR) availability in the synapse, which is important for synaptic plasticity and memory formation. Peptides which specifically block the interaction between N-Ethylmaleimide-Sensitive Factor (NSF) and the AMPAR-subunit GluA2 prevented the increase in synaptic transmission and surface expression of AMPARs known to occur after corticosterone application to hippocampal neurons. Combining a live imaging Fluorescence Recovery After Photobleaching (FRAP) approach with the use of the pH-sensitive GFP-AMPAR tagging revealed that this NSF/GluA2 interaction was also essential for the increase of the mobile fraction and reduction of the diffusion of AMPARs after treating hippocampal neurons with corticosterone. We conclude that the interaction between NSF and GluA2 contributes to the effects of corticosterone on AMPAR function. © 2016 Wiley Periodicals, Inc.

The dynamics of the stress neuromatrix

Stressful stimuli in healthy subjects trigger activation of a consistent and reproducible set of brain regions; yet, the notion that there is a single and constant stress neuromatrix is not sustainable. Indeed, after chronic stress exposure there is activation of many brain regions outside that network. This suggests that there is a distinction between the acute and the chronic stress neuromatrix. Herein, a new working model is proposed to understand the shift between these networks. The understanding of the factors that modulate these networks and their interplay will allow for a more comprehensive and holistic perspective of how the brain shifts 'back and forth' from a healthy to a stressed pattern and, ultimately, how the latter can be a trigger for several neurological and psychiatric conditions.