Neural correlates of learning and memory are altered by early-life stress.

The ability to learning and remember, which is fundamental for behavioral adaptation, is susceptible to stressful experiences during the early postnatal period, such as abnormal levels of maternal care. The exact mechanisms underlying these effects still remain elusive. This study examined in male mice whether early life stress (ELS) alters memory and brain activation patterns, by studying the expression of the immediate early genes (IEGs) c-Fos and Arc in the dentate gyrus (DG) and basolateral amygdala (BLA) after training and memory retrieval in a fear conditioning task. Furthermore, we examined the potential of RU38486 (RU486), a glucocorticoid receptor antagonist, to mitigate ELS-induced memory deficits by blocking stress signalling during adolescence. Arc::dVenus reporter mice, which allow investigating experience-dependent expression of the immediate early gene Arc also at more remote time points, were exposed to ELS by housing dams and offspring with limited bedding and nesting material (LBN) between postnatal days (PND) 2-9 and trained in a fear conditioning task at adult age. We found that ELS reduced both fear acquisition and contextual memory retrieval. RU486 did not prevent these effects. ELS reduced the number of Arc::dVenus+ cells in DG and BLA after training, while the number of c-Fos+ cells were left unaffected. After memory retrieval, ELS decreased c-Fos+ cells in the ventral DG and BLA. ELS also disrupted the colocalization of c-Fos+ cells with (training activated) Arc::dVenus+ cells in the ventral DG, possibly indicating impaired engram allocation in the ventral DG after memory retrieval. Altered correlated activity during training and changes in IEG expression over time were also found in ELS animals. In conclusion, this study shows that ELS alters neuronal activation patterns after fear acquisition and retrieval, which may provide mechanistic insights into enduring impact of early-life stress on the processing of fear memories, possibly via changes in cell (co-) activation and engram cell allocation.

Early-life stress and amyloidosis in mice share pathogenic pathways involving synaptic mitochondria and lipid metabolism.

INTRODUCTION: Early-life stress (ES) increases the risk for Alzheimer's disease (AD). We and others have shown that ES aggravates amyloid-beta (Aß) pathology and promotes cognitive dysfunction in APP/PS1 mice, but underlying mechanisms remain unclear. METHODS: We studied how ES affects the hippocampal synaptic proteome in wild-type (WT) and APP/PS1 mice at early and late pathological stages, and validated hits using electron microscopy and immunofluorescence. RESULTS: The hippocampal synaptosomes of both ES-exposed WT and early-stage APP/PS1 mice showed a relative decrease in actin dynamics-related proteins and a relative increase in mitochondrial proteins. ES had minimal effects on older WT mice, while strongly affecting the synaptic proteome of advanced stage APP/PS1 mice, particularly the expression of astrocytic and mitochondrial proteins. DISCUSSION: Our data show that ES and amyloidosis share pathogenic pathways involving synaptic mitochondrial dysfunction and lipid metabolism, which may underlie the observed impact of ES on the trajectory of AD.

Elevated corticosterone after fear learning impairs remote auditory memory retrieval and alters brain network connectivity.

Glucocorticoids are potent memory modulators that can modify behavior in an adaptive or maladaptive manner. Elevated glucocorticoid levels after learning promote memory consolidation at recent time points, but their effects on remote time points are not well established. Here we set out to assess whether corticosterone (CORT) given after learning modifies remote fear memory. To that end, mice were exposed to a mild auditory fear conditioning paradigm followed by a single 2 mg/kg CORT injection, and after 28 d, auditory memory was assessed. Neuronal activation was investigated using immunohistochemistry for the immediate early gene c-Fos, and coactivation of brain regions was determined using a correlation matrix analysis. CORT-treated mice displayed significantly less remote auditory memory retrieval. While the net activity of studied brain regions was similar compared with the control condition, CORT-induced remote memory impairment was associated with altered correlated activity between brain regions. Specifically, connectivity of the lateral amygdala with the basal amygdala and the dorsal dentate gyrus was significantly reduced in CORT-treated mice, suggesting disrupted network connectivity that may underlie diminished remote memory retrieval. Elucidating the pathways underlying these effects could help provide mechanistic insight into the effects of stress on memory and possibly provide therapeutic targets for psychopathology.

Assessing the degree of urbanisation using a single-item self-report measure: a validation study.

The differential impact of rural versus urban residence on mental health remains a controversial topic that requires more in-depth investigations. This calls for a valid and easy measure to assess the degree of urbanisation. The purpose of the present study was to determine the utility of a single-item self-report measure (SIDU) as a tool to classify areas along the rural-urban continuum. The validity of the SIDU was assessed by comparing its scores (1-7) to a commonly used objective surrogate measure of the degree of urbanisation (i.e. surrounding address density, SAD) in two independent older adult samples (A: N = 36, 65+; B: N = 121, 55+). SIDU scores approximated SAD scores, with r = .77 to 0.82, (A), and r = .79 to 0.83 (B). A SIDU threshold score of 6 most accurately distinguished extremely urbanised areas from other areas. Altogether, our findings suggest that SIDU scores could be used as proxy of SAD. Since self-report leaves room for the consideration of additional aspects that confer an urban settlement, this single-item scale may be even more comprehensive, and circumvents the collection and handling of highly sensitive location data when the primary goal is solely to distinguish urbanisation subgroups.

An emerging role for microglia in stress-effects on memory.

Stressful experiences evoke, among others, a rapid increase in brain (nor)epinephrine (NE) levels and a slower increase in glucocorticoid hormones (GCs) in the brain. Microglia are key regulators of neuronal function and contain receptors for NE and GCs. These brain cells may therefore potentially be involved in modulating stress effects on neuronal function and learning and memory. In this review, we discuss that stress induces (1) an increase in microglial numbers as well as (2) a shift toward a pro-inflammatory profile. These microglia have (3) impaired crosstalk with neurons and (4) disrupted glutamate signaling. Moreover, microglial immune responses after stress (5) alter the kynurenine pathway through metabolites that impair glutamatergic transmission. All these effects could be involved in the impairments in memory and in synaptic plasticity caused by (prolonged) stress, implicating microglia as a potential novel target in stress-related memory impairments.

Effects of corticosterone on mild auditory fear conditioning and extinction; role of sex and training paradigm.

Multiple lines of evidence suggest that glucocorticoid hormones enhance memory consolidation of fearful events. However, most of these studies involve male individuals. Since anxiety, fear, and fear-associated disorders present differently in male and female subjects we investigated in mice whether male and female mice perform differently in a mild, auditory fear conditioning task and tested the modulatory role of glucocorticoid hormones. Using an auditory fear conditioning paradigm with different footshock intensities (0.1, 0.2, and 0.4 mA) and frequencies (1× or 3×), we find that intraperitoneal injections with corticosterone (2 mg/kg) immediately after training, altered freezing behavior when repeated footshocks were applied, and that the direction of the effects were opposite in male and female mice. Effects were independent of footshock intensity. In male mice, corticosterone consistently increased freezing behavior in response to the tone, whereas in female mice, corticosterone reduced freezing behavior 24 h after training. These effects were not related to the phase of the oestrous cycle. In addition, corticosterone enhanced extinction learning for all tones, in both male and female mice. These results emphasize that glucocorticoid hormones influence memory consolidation and retrieval, and underscore sex-specific effects of glucocorticoid hormones in modulating conditioned fear responses.

Early postnatal handling reduces hippocampal amyloid plaque formation and enhances cognitive performance in APPswe/PS1dE9 mice at middle age.

In rodents, fragmented and low levels of maternal care have been implicated in age-related cognitive decline and the incidence and progression of Alzheimer's pathology. In contrast, enhancing early postnatal maternal care has been associated with improved cognitive function later in life. Here we examined whether early postnatal handling of mouse pups from postnatal days 2-9 enhanced maternal care and whether this affected cognition and Alzheimer pathology at 5 and 11months of age in the APPswe/PS1dE9 mouse model for Alzheimer's disease. Brief, 15min daily episodes of separating offspring from their dams from postnatal days 2-9 (early handling, EH) increased maternal care of the dam towards her pups upon reunion. At 11 (but not 5) months of age, EH APPswe/PS1dE1 mice displayed significantly reduced amyloid plaque pathology in the hippocampus. At this age, EH also prevented short-term working memory deficits while restoring impairments in contextual fear memory formation in APPswe/PS1dE9 mice. EH did not modulate amyloid pathology in the amygdala, nor did it affect auditory fear conditioning deficits in APPswe/PS1dE9 mice. We conclude that increased levels of maternal care during the early life period delays amyloid accumulation and cognitive decline in an Alzheimer's mouse model, involving the hippocampus, but not to the amygdala. These studies highlight the importance of the early postnatal period in modulating resilience to develop Alzheimer's pathology later in life.

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.

Blocking glucocorticoid receptors at adolescent age prevents enhanced freezing between repeated cue-exposures after conditioned fear in adult mice raised under chronic early life stress.

Early life adversity can have long-lasting impact on learning and memory processes and increase the risk to develop stress-related psychopathologies later in life. In this study we investigated (i) how chronic early life stress (ELS) - elicited by limited nesting and bedding material from postnatal day 2 to 9 - affects conditioned fear in adult mice and (ii) whether these effects can be prevented by blocking glucocorticoid receptors (GRs) at adolescent age. In adult male and female mice, ELS did not affect freezing behavior to the first tone 24h after training in an auditory fear-conditioning paradigm. Exposure to repeated tones 24h after training also resulted in comparable freezing behavior in ELS and control mice, both in males and females. However, male (but not female) ELS compared to control mice showed significantly more freezing behavior between the tone-exposures, i.e. during the cue-off periods. Intraperitoneal administration of the GR antagonist RU38486 during adolescence (on postnatal days 28-30) fully prevented enhanced freezing behavior during the cue-off period in adult ELS males. Western blot analysis revealed no effects of ELS on hippocampal expression of glucocorticoid receptors, neither at postnatal day 28 nor at adult age, when mice were behaviorally tested. We conclude that ELS enhances freezing behavior in adult mice in a potentially safe context after cue-exposure, which can be normalized by brief blockade of glucocorticoid receptors during the critical developmental window of adolescence.

mTOR is essential for corticosteroid effects on hippocampal AMPA receptor function and fear memory.

Glucocorticoid hormones, via activation of their receptors, promote memory consolidation, but the exact underlying mechanisms remain elusive. We examined how corticosterone regulates AMPA receptors (AMPARs), which are crucial for synaptic plasticity and memory formation. Combining a live imaging fluorescent recovery after photobleaching approach with the use of the pH-sensitive GFP-AMPAR tagging revealed that corticosterone enhances the AMPAR mobile fraction and increases synaptic trapping of AMPARs in hippocampal cells. In parallel, corticosterone-enhanced AMPAR-mediated synaptic transmission. Blocking the mammalian target of rapamycin (mTOR) pathway prevented the effects of corticosterone on both AMPAR trapping-but not on the mobile fraction-and synaptic transmission. Blocking the mTOR pathway also prevented the memory enhancing effects of corticosterone in a contextual fear-conditioning paradigm. We conclude that activation of the mTOR pathway is essential for the effects of corticosterone on synaptic trapping of AMPARs and, possibly as a consequence, fearful memory formation.

Dysregulation of Rho GTPases in the αPix/Arhgef6 mouse model of X-linked intellectual disability is paralleled by impaired structural and synaptic plasticity and cognitive deficits.

Mutations in the ARHGEF6 gene, encoding the guanine nucleotide exchange factor αPIX/Cool-2 for the Rho GTPases Rac1 and Cdc42, cause X-linked intellectual disability (ID) in humans. We show here that αPix/Arhgef6 is primarily expressed in neuropil regions of the hippocampus. To study the role of αPix/Arhgef6 in neuronal development and plasticity and gain insight into the pathogenic mechanisms underlying ID, we generated αPix/Arhgef6-deficient mice. Gross brain structure in these mice appeared to be normal; however, analysis of Golgi-Cox-stained pyramidal neurons revealed an increase in both dendritic length and spine density in the hippocampus, accompanied by an overall loss in spine synapses. Early-phase long-term potentiation was reduced and long-term depression was increased in the CA1 hippocampal area of αPix/Arhgef6-deficient animals. Knockout animals exhibited impaired spatial and complex learning and less behavioral control in mildly stressful situations, suggesting that this model mimics the human ID phenotype. The structural and electrophysiological alterations in the hippocampus were accompanied by a significant reduction in active Rac1 and Cdc42, but not RhoA. In conclusion, we suggest that imbalance in activity of different Rho GTPases may underlie altered neuronal connectivity and impaired synaptic function and cognition in αPix/Arhgef6 knockout mice.

Pharmacotherapy in the aftermath of trauma; opportunities in the 'golden hours'.

Several lines of research have demonstrated that memories for fearful events become transiently labile upon re-exposure. Activation of molecular mechanisms is required in order to maintain retrieved information. This process is called reconsolidation. Targeting reconsolidation - as in exposure-based psychotherapy - offers therefore a potentially interesting tool to manipulate fear memories, and subsequently to treat disorders such as post-traumatic stress disorder (PTSD). In this paper we discuss the evidence for reconsolidation in rodents and humans and highlight recent studies in which clinical research on normal and abnormal fear extinction reduction of the expression of fear was obtained by targeting the process of reconsolidation. We conclude that reconsolidation presents an interesting opportunity to modify or alter fear and fear-related memories. More clinical research on normal and abnormal fear extinction is required.

Early-life stress impairs acquisition and retrieval of fear memories: sex-effects, corticosterone modulation, and partial prevention by targeting glucocorticoid receptors at adolescent age.

The early postnatal period is a sensitive time window that is characterized by several neurodevelopmental processes that define neuronal architecture and function later in life. Here, we examined in young adult mice, using an auditory fear conditioning paradigm, whether stress during the early postnatal period 1) impacts fear acquisition and memory consolidation in male and female mice; 2) alters the fear responsiveness to corticosterone and 3) whether effects of early-life stress (ELS) can be prevented by treating mice with a glucocorticoid (GR) antagonist at adolescence. Male and female mice were exposed to a limited nesting and bedding model of ELS from postnatal day (PND) 2-9 and injected i.p with RU38486 (RU486) at adolescent age (PND 28-30). At two months of age, mice were trained in the fear conditioning (FC) paradigm (with and without post training administration of corticosterone - CORT) and freezing behavior during fear acquisition and contextual and auditory memory retrieval was scored. We observed that ELS impaired fear acquisition specifically in male mice and reduced both contextual and auditory memory retrieval in male and female mice. Acute post-training administration of CORT increased freezing levels during auditory memory retrieval in female mice but reduced freezing levels during the tone presentation in particular in control males. Treatment with RU486 prevented ELS-effects in acquisition in male mice and in females during auditory memory retrieval. In conclusion, this study highlights the long-lasting consequences of early-life stress on fear memory processing and further illustrates 1) the potential of a glucocorticoid antagonist intervention during adolescence to mitigate these effects and 2) the partial modulation of the auditory retrieval upon post training administration of CORT, with all these effects being sex-dependent.

Shaping Memories via Stress: A Synaptic Engram Perspective.

Stress modulates the activity of various memory systems and can thereby guide behavioral interaction with the environment in an adaptive or maladaptive manner. At the cellular level, a large body of evidence indicates that (nor)adrenaline and glucocorticoid release induced by acute stress exposure affects synapse function and synaptic plasticity, which are critical substrates for learning and memory. Recent evidence suggests that memories are supported in the brain by sparsely distributed neurons within networks, termed engram cell ensembles. While the physiological and molecular effects of stress on the synapse are increasingly well characterized, how these synaptic modifications shape the multiscale dynamics of engram cell ensembles is still poorly understood. In this review, we discuss and integrate recent information on how acute stress affects synapse function and how this may alter engram cell ensembles and their synaptic connectivity to shape memory strength and memory precision. We provide a mechanistic framework of a synaptic engram under stress and put forward outstanding questions that address knowledge gaps in our understanding of the mechanisms that underlie stress-induced memory modulation.

Targeted mutation of Cyln2 in the Williams syndrome critical region links CLIP-115 haploinsufficiency to neurodevelopmental abnormalities in mice.

Williams syndrome is a neurodevelopmental disorder caused by the hemizygous deletion of 1.6 Mb on human chromosome 7q11.23. This region comprises the gene CYLN2, encoding CLIP-115, a microtubule-binding protein of 115 kD. Using a gene-targeting approach, we provide evidence that mice with haploinsufficiency for Cyln2 have features reminiscent of Williams syndrome, including mild growth deficiency, brain abnormalities, hippocampal dysfunction and particular deficits in motor coordination. Absence of CLIP-115 also leads to increased levels of CLIP-170 (a closely related cytoplasmic linker protein) and dynactin at the tips of growing microtubules. This protein redistribution may affect dynein motor regulation and, together with the loss of CLIP-115-specific functions, underlie neurological alterations in Williams syndrome.

Improving goal striving and resilience in older adults through a personalized metacognitive self-help intervention: a protocol paper.

BACKGROUND: Successful aging is often linked to individual's ability to demonstrate resilience: the maintenance or quick recovery of functional ability, well-being, and quality of life despite losses or adversity. A crucial element of resilience is behavioral adaptability, which refers to the adaptive changes in behavior in accordance with internal or external demands. Age-related degradation of executive functions can, however, lead to volition problems that compromise flexible adjustment of behavior. In contrast, the reliance on habitual control has been shown to remain relatively intact in later life and may therefore provide an expedient route to goal attainment among older adults. In the current study, we examine whether a metacognitive self-help intervention (MCSI), aimed at facilitating goal striving through the gradual automatization of efficient routines, could effectively support behavioral adaptability in favor of resilience among older adults with and without (sub-clinical) mental health problems. METHODS: This metacognitive strategy draws on principles from health and social psychology, as well as clinical psychology, and incorporates elements of established behavioral change and activation techniques from both fields. Additionally, the intervention will be tailored to personal needs and challenges, recognizing the significant diversity that exist among aging individuals. DISCUSSION: Despite some challenges that may limit the generalizability of the results, our MCSI program offers a promising means to empower older adults with tools and strategies to take control of their goals and challenges. This can promote autonomy and independent functioning, and thereby contribute to adaptability and resilience in later life. TRIAL REGISTRATION: Pre-registered, partly retrospectively. This study was pre-registered before the major part of the data was collected, created, and realized. Only a small part of the data of some participants (comprising the baseline and other pre-intervention measures), and the full dataset of the first few participants, was collected prior to registration, but it was not accessed yet. See: https://osf.io/5b9xz.

Early life stress lastingly alters the function and AMPA-receptor composition of glutamatergic synapses in the hippocampus of male mice.

Early postnatal life is a sensitive period of development that shapes brain structure and function later in life. Exposure to stress during this critical time window can alter brain development and may enhance the susceptibility to psychopathology and neurodegenerative disorders later in life. The developmental effects of early life stress (ELS) on synaptic function are not fully understood, but could provide mechanistic insights into how ELS modifies later brain function and disease risk. We here assessed the effects of ELS on synaptic function and composition in the hippocampus of male mice. Mice were subjected to ELS by housing dams and pups with limited bedding and nesting material from postnatal days (P) 2-9. Synaptic strength was measured in terms of miniature excitatory postsynaptic currents (mEPSCs) in the hippocampal dentate gyrus at three different developmental stages: the early postnatal phase (P9), preadolescence (P21, at weaning) and adulthood at 3 months of age (3MO). Hippocampal synaptosome fractions were isolated from P9 and 3MO tissue and analyzed for protein content to assess postsynaptic composition. Finally, dendritic spine density was assessed in the DG at 3MO. At P9, ELS increased mEPSC frequency and amplitude. In parallel, synaptic composition was altered as PSD-95, GluA3 and GluN2B content were significantly decreased. The increased mEPSC frequency was sustained up to 3MO, at which age, GluA3 content was significantly increased. No differences were found in dendritic spine density. These findings highlight how ELS affects the development of hippocampal synapses, which could provide valuable insight into mechanisms how ELS alters brain function later in life.

Corrigendum: The interplay between quality of life and resilience factors in later life: a network analysis.

[This corrects the article DOI: 10.3389/fpsyg.2021.752564.].

Early Life Stress Enhances Cognitive Decline and Alters Synapse Function and Interneuron Numbers in Young Male APP/PS1 Mice.

BACKGROUND: Exposure to stress early in life increases the susceptibility to Alzheimer's disease (AD) pathology in aged AD mouse models. So far, the underlying mechanisms have remained elusive. OBJECTIVE: To investigate 1) effects of early life stress (ELS) on early functional signs that precede the advanced neuropathological changes, and 2) correlate synaptosomal protein content with cognition to identify neural correlates of AD. METHODS: APPswe/PS1dE9 mice and littermates were subjected to ELS by housing dams and pups with limited bedding and nesting material from postnatal days 2-9. At 3 months of age, an age where no cognitive loss or amyloid-ß (Aß) pathology is typically reported in this model, we assessed hippocampal Aß pathology, synaptic strength and synapse composition and interneuron populations. Moreover, cognitive flexibility was assessed and correlated with synaptosomal protein content. RESULTS: While ELS did not affect Aß pathology, it increased synaptic strength and decreased the number of calretinin+ interneurons in the hippocampal dentate gyrus. Both genotype and condition further affected the level of postsynaptic glutamatergic protein content. Finally, APP/PS1 mice were significantly impaired in cognitive flexibility at 3 months of age, and ELS exacerbated this impairment, but only at relatively high learning criteria. CONCLUSIONS: ELS reduced cognitive flexibility in young APP/PS1 mice and altered markers for synapse and network function. These findings at an early disease stage provide novel insights in AD etiology and in how ELS could increase AD susceptibility.

Adult hippocampal glucocorticoid receptor expression and dentate synaptic plasticity correlate with maternal care received by individuals early in life.

Maternal care in mammals is the prevailing environmental influence during perinatal development. The adult rat offspring of mothers exhibiting increased levels of pup licking/grooming (LG; High LG mothers), compared to those reared by Low LG dams, show increased hippocampal glucocorticoid receptor expression, complex dendritic tree structure, and an enhanced capacity for synaptic potentiation. However, these data were derived from studies using the total amount of maternal care directed toward the entire litter, thus ignoring possible within-litter variation. We show that the amount of LG received by individual pups within a litter varies considerably. Therefore, we questioned if the amount of LG received by individual pups correlates with and thus putatively predicts later hippocampal structure and function. To this end, LG-scores were determined during the first postnatal week for all pups in 32 litters and correlated with neuroendocrine and hippocampal parameters in young-adulthood. Pup LG-score positively correlated with the glucocorticoid receptor mRNA expression in the adult hippocampus. Moreover, the ability to induce synaptic potentiation in the dentate gyrus in vitro was enhanced in animals with high LG-scores. Structural plasticity correlated less reliably with LG-scores early in life and differed between sexes. Male offspring with high LG-scores displayed fewer newborn neurons, higher brain derived neurotrophic factor expression and tended to have more complex granule cell dendritic trees. We conclude that even moderate variations in early life environment have a major impact on adult hippocampal function. This principle could provide a mechanistic basis for individual differences in susceptibility to psychopathology.