Stress or no stress: Mineralocorticoid receptors in the forebrain regulate behavioral adaptation

Prenatal exposure to the mineralocorticoid receptor antagonist spironolactone disrupts hippocampal area CA2 connectivity and alters behavior in mice

In the brain, the hippocampus is enriched with mineralocorticoid receptors (MR; Nr3c2), a ligand-dependent transcription factor stimulated by the stress hormone corticosterone in rodents. Recently, we discovered that MR is required for the acquisition and maintenance of many features of mouse area CA2 neurons. Notably, we observed that immunofluorescence for the vesicular glutamate transporter 2 (vGluT2), likely representing afferents from the supramammillary nucleus (SuM), was disrupted in the embryonic, but not postnatal, MR knockout mouse CA2. To test whether pharmacological perturbation of MR activity in utero similarly disrupts CA2 connectivity, we implanted slow-release pellets containing the MR antagonist spironolactone in mouse dams during mid-gestation. After confirming that at least one likely active metabolite crossed from the dams' serum into the embryonic brains, we found that spironolactone treatment caused a significant reduction of CA2 axon fluorescence intensity in the CA1 stratum oriens, where CA2 axons preferentially project, and that vGluT2 staining was significantly decreased in both CA2 and dentate gyrus in spironolactone-treated animals. We also found that spironolactone-treated animals exhibited increased reactivity to novel objects, an effect similar to what is seen with embryonic or postnatal CA2-targeted MR knockout. However, we found no difference in preference for social novelty between the treatment groups. We infer these results to suggest that persistent or more severe disruptions in MR function may be required to interfere with this type of social behavior. These findings do indicate, though, that developmental disruption in MR signaling can have persistent effects on hippocampal circuitry and behavior.

Imbalanced glucocorticoid and mineralocorticoid stress hormone receptor function has sex-dependent and independent regulatory effects in the mouse hippocampus

Many stress-related neuropsychiatric disorders display pronounced sex differences in their frequency and clinical symptoms. Glucocorticoids are primary stress hormones that have been implicated in the development of these disorders but whether they contribute to the observed sex bias is poorly understood. Glucocorticoids signal through two closely related nuclear receptors, the glucocorticoid (GR) and mineralocorticoid receptor (MR). To elucidate the sex-specific and independent actions of glucocorticoids in the hippocampus, we developed knockout mice lacking hippocampal GR, MR, or both GR and MR. Mice deficient in hippocampal MR or both GR and MR showed an altered molecular phenotype of CA2 neurons and reduced anxiety-like behavior in both sexes, but altered stress adaptation behavior only in females and enhanced fear-motivated cue learning only in males. All three knockout mouse models displayed reduced sociability but only in male mice. Male and female mice deficient in both hippocampal GR and MR exhibited extensive neurodegeneration in the dentate gyrus. Global transcriptomic analysis revealed a marked expansion in the number of dysregulated genes in the hippocampus of female knockout mice compared to their male counterparts; however, the overall patterns of gene dysregulation were remarkably similar in both sexes. Within and across sex comparisons identified key GR and MR target genes and associated signaling pathways underlying the knockout phenotypes. These findings define major sex-dependent and independent effects of GR/MR imbalances on gene expression and functional profiles in the hippocampus and inform new strategies for treating men and women with stress-related neuropsychiatric disorders.

From Ligands to Behavioral Outcomes: Understanding the Role of Mineralocorticoid Receptors in Brain Function

Stress is a normal response to situational pressures or demands. Exposure to stress activates the hypothalamic-pituitary-adrenal (HPA) axis and leads to the release of corticosteroids, which act in the brain via two distinct receptors: mineralocorticoid receptors (MR) and glucocorticoid receptors (GR). Persistent HPA axis overactivation or dysregulation can disrupt an individual's homeostasis, thereby contributing to an increased risk for mental illness. On the other hand, successful coping with stressful events involves adaptive and cognitive processes in the brain that render individuals more resilient to similar stressors in the future. Here we review the role of the MR in these processes, starting with an overview of the physiological structure, ligand binding, and expression of MR, and further summarizing its role in the brain, its relevance to psychiatric disorders, and related rodent studies. Given the central role of MR in cognitive and emotional functioning, and its importance as a target for promoting resilience, future research should investigate how MR modulation can be used to alleviate disturbances in emotion and behavior, as well as cognitive impairment, in patients with stress-related psychiatric disorders.

Choosing memory retrieval strategies: A critical role for inhibition in the dentate gyrus

Remembering the location of food is essential for survival. Rodents and humans employ mainly hippocampus-dependent spatial strategies, but when being stressed they shift to striatum-mediated stimulus-based strategies. To investigate underlying brain circuits, we tested mice with a heightened stress susceptibility due to a lack of the GABA-synthetizing enzyme GAD65 (GAD65−/− mice) in a dual solution task. Here, GAD65−/− mice preferred to locate a food reward in an open field via a proximal cue, while their wildtype littermates preferred a spatial strategy. The analysis of cFos co-activation across brain regions and of stress-induced mRNA expression changes of GAD65 pointed towards the hippocampal dorsal dentate gyrus (dDG) as a central structure for mediating stress effects on strategy choices via GAD65. Reducing the GAD65 expression locally in the dDG by a shRNA mediated knock down was sufficient to replicate the phenotype of the global GAD65 knock out and to increase dDG excitability. Using DREADD vectors to specifically interfere with dDG circuit activity during dual solution retrieval but not learning confirmed that the dDG modulates strategy choices and that a balanced excitability of this structure is necessary to establish spatial strategy preference. These data highlight the dDG as a critical hub for choosing between spatial and non-spatial foraging strategies.

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Stress reduces spatial memory preferences for locating rewards in an open field.

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GAD65 deficient mice show reduced preferences for spatial memory strategy.

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Dorsal dentate gyrus knock down of GAD65 is sufficient to reduce spatial strategies.

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Excitability in the dorsal dentate gyrus modulates retrieval strategy choices.

Nuclear receptors modulate inflammasomes in the pathophysiology and treatment of major depressive disorder

Major depressive disorder (MDD) is highly prevalent and is a significant cause of mortality and morbidity worldwide. Currently, conventional pharmacological treatments for MDD produce temporary remission in < 50% of patients; therefore, there is an urgent need for a wider spectrum of novel antidepressants to target newly discovered underlying disease mechanisms. Accumulated evidence has shown that immune inflammation, particularly inflammasome activity, plays an important role in the pathophysiology of MDD. In this review, we summarize the evidence on nuclear receptors (NRs), such as glucocorticoid receptor, mineralocorticoid receptor, estrogen receptor, aryl hydrocarbon receptor, and peroxisome proliferator-activated receptor, in modulating the inflammasome activity and depression-associated behaviors. This review provides evidence from an endocrine perspective to understand the role of activated NRs in the pathophysiology of MDD, and to provide insight for the discovery of antidepressants with novel mechanisms for this devastating disorder.

Combinatorial actions of glucocorticoid and mineralocorticoid stress hormone receptors are required for preventing neurodegeneration of the mouse hippocampus

Chronic stress contributes to numerous human pathologies including cognition impairments and psychiatric disorders. Glucocorticoids are primary stress hormones that activate two closely related nuclear receptors, the glucocorticoid (GR) and mineralocorticoid receptor (MR), that are both highly expressed in the hippocampus. To investigate potential combinatorial actions of hippocampal GR and MR, we developed mice with conditional knockout of both GR and MR in the hippocampus and compared them to their single knockout counterparts. Mice lacking MR alone or both GR and MR in the hippocampus exhibited altered expression of multiple CA2-specific neuronal markers and enhanced cue-dependent learning in a conditioned fear test. Provocatively, in contrast to the single knockouts, mice depleted of both GR and MR showed profound neurodegeneration of the hippocampus. Neuronal death was increased and neurogenesis was reduced in the dentate gyrus of the double knockout mice. Global gene expression assays of the knockout mice revealed a synergistic increase in the number of dysregulated genes in the hippocampus lacking both GR and MR. This large cohort of genes reliant on both GR and MR for expression was strongly associated with cell death and cell proliferation pathways. GR/MR complexes were detected in CA1 and dentate gyrus neurons suggesting receptor heterodimers contribute to the joint actions of GR and MR. These findings reveal an obligate role for MR signaling in regulating the molecular phenotype of CA2 neurons and demonstrate that combinatorial actions of GR and MR are essential for preserving dentate gyrus neurons and maintaining hippocampal health.

Mineralocorticoid receptors dampen glucocorticoid receptor sensitivity to stress via regulation of FKBP5

Responding to different dynamic levels of stress is critical for mammalian survival. Disruption of mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) signaling is proposed to underlie hypothalamic-pituitary-adrenal (HPA) axis dysregulation observed in stress-related psychiatric disorders. In this study, we show that FK506-binding protein 51 (FKBP5) plays a critical role in fine-tuning MR:GR balance in the hippocampus. Biotinylated-oligonucleotide immunoprecipitation in primary hippocampal neurons reveals that MR binding, rather than GR binding, to the Fkbp5 gene regulates FKBP5 expression during baseline activity of glucocorticoids. Notably, FKBP5 and MR exhibit similar hippocampal expression patterns in mice and humans, which are distinct from that of the GR. Pharmacological inhibition and region- and cell type-specific receptor deletion in mice further demonstrate that lack of MR decreases hippocampal Fkbp5 levels and dampens the stress-induced increase in glucocorticoid levels. Overall, our findings demonstrate that MR-dependent changes in baseline Fkbp5 expression modify GR sensitivity to glucocorticoids, providing insight into mechanisms of stress homeostasis.

Glucocorticoid Therapy is Associated with a Lower Risk of Dementia

BACKGROUND

Recent evidence indicates an important role for neuroinflammation in the pathological cascade of Alzheimer's disease (AD), and neuroinflammation is increasingly being recognized as a potential therapeutic target.

OBJECTIVE

To assess the impact of glucocorticoids on the risk of developing dementia.

METHODS

We used health insurance data of the largest German health insurer from 2004-2013 with a baseline sample of 176,485 persons aged 50 years and older to study the association of glucocorticoid treatment and incidence of dementia. Cox proportional-hazard models were calculated adjusting for sex, age, and comorbidities known to be major risk factors for dementia and were given as hazard ratios (HR) with 95% confidence intervals (CI). We further stratified glucocorticoid treatment by route of application and treatment duration.

RESULTS

Of the 176,485 dementia-free persons, 19,938 were diagnosed with dementia by the end of 2013. The risk of suffering from dementia was significantly lower for glucocorticoid users compared to non-users (HR = 0.81, CI = 0.78-0.84). The lowest risk was found among users of inhaled glucocorticoid (HR = 0.65, CI = 0.57-0.75), followed by nasal (HR = 0.76, CI = 0.66-0.87), other (HR = 0.84, CI = 0.80-0.88), and oral users (HR = 0.83, CI = 0.78-0.88). We found no difference in risk reduction between long- and short-term-users.

CONCLUSION

Longitudinal German health insurance data indicate that the use of glucocorticoids is associated with a lower risk of dementia. Prospective clinical trials will be necessary to determine whether glucocorticoids can have a positive impact on neuroinflammation and thus protect persons against dementia.

Age-dependent shift in spontaneous excitation-inhibition balance of infralimbic prefrontal layer II/III neurons is accelerated by early life stress, independent of forebrain mineralocorticoid receptor expression

In this study we tested the hypothesis i) that age-dependent shifts in the excitation-inhibition balance of prefrontal neurons are accelerated by early life stress, a risk factor for the etiology of many psychiatric disorders; and if so, ii) that this process is exacerbated by genetic forebrain-specific downregulation of the mineralocorticoid receptor, a receptor that was earlier found to be a protective factor for negative effects of early life stress in both rodents and humans. In agreement with the literature, an age-dependent downregulation of the excitation-inhibition balance was found both with regard to spontaneous and evoked synaptic currents. The age-dependent shift in spontaneous excitatory relative to inhibitory currents was significantly accelerated by early life stress, but this was not exacerbated by reduction in mineralocorticoid receptor expression. The age-dependent changes in the excitation-inhibition balance were mirrored by similar changes in receptor subunit expression and morphological alterations, particularly in spine density, which could thus potentially contribute to the functional changes. However, none of these parameters displayed acceleration by early life stress, nor depended on mineralocorticoid receptor expression. We conclude that, in agreement with the hypothesis, early life stress accelerates the developmental shift of the excitation-inhibition balance but, contrary to expectation, there is no evidence for a putative protective role of the mineralocorticoid receptor in this system. In view of the modest effect of early life stress on the excitation-inhibition balance, alternative mechanisms potentially underlying the development of psychiatric disorders should be further explored.

Three Brothers With Autism Carry a Stop-Gain Mutation in the HPA-Axis Gene NR3C2

Whole exome sequencing and copy-number variant analysis was performed on a family with three brothers diagnosed with autism. Each of the siblings shares an alteration in the nuclear receptor subfamily 3 group C member 2 (NR3C2) gene that is predicted to result in a stop-gain mutation (p.Q919X) in the mineralocorticoid receptor (MR) protein. This variant was maternally inherited and provides further evidence for a connection between the NR3C2 and autism. Interestingly, the NR3C2 gene encodes the MR protein, a steroid hormone-regulated transcription factor that acts in the hypothalamic-pituitary-adrenal axis and has been connected to stress and anxiety, both of which are features often seen in individuals with autism. Autism Res 2020, 13: 523-531. © 2020 International Society for Autism Research, Wiley Periodicals, Inc. LAY SUMMARY: Given the complexity of the genetics underlying autism, each gene contributes to risk in a relatively small number of individuals, typically less than 1% of all autism cases. Whole exome sequencing of three brothers with autism identified a rare variant in the nuclear receptor subfamily 3 group C member 2 gene that is predicted to strongly interfere with its normal function. This gene encodes the mineralocorticoid receptor protein, which plays a role in how the body responds to stress and anxiety, features that are often elevated in people diagnosed with autism. This study adds further support to the relevance of this gene as a risk factor for autism.

Pex3 is involved in the genetic regulation of Nr3c2 expression in the amygdala of mice

The mineralocorticoid receptor (Nr3c2) has received increased attention as an important stress-related gene. Here, we sought to uncover candidate genes regulating the expression of Nr3c2. Using a genetical genomics approach, we identified a significant trans-regulated expression quantitative trait locus (eQTL) at Chromosome 10 for Nr3c2 expression in the amygdala of BXD RI strains. We then examined genes upstream of the eQTL to identify likely regulatory candidates of Nr3c2 expression. Pex3 (peroxisomal) expression was highly correlated with that of Nr3c2, had a significant cis-regulated eQTL that mapped to the Nr3c2 eQTL region and thus emerged as the most likely regulatory candidate of Nr3c2 expression. In vitro studies showed that silencing of Pex3 by siRNA decreased Nr3c2 expression in HEK293T and SHSY5 cell lines while overexpression increased Nr3c2 expression. A relationship between the expression of these two genes was further supported by our observations that expression levels of Pex3 and Nr3c2 decreased in the amygdala of mice exposed to chronic unpredictable stress. Our findings provide insight into the genetic regulation of Nr3c2 expression and suggest a new role for Pex3 in stress responses. Future characterization of Pex3's role in the regulation of Nr3c2 expression and the pathways involved may lead to a better understanding of stress responses and risk for stress-related pathology.

Sex-Dependent Modulation of Acute Stress Reactivity After Early Life Stress in Mice: Relevance of Mineralocorticoid Receptor Expression

Early life stress (ELS) is considered a major risk factor for developing psychopathology. Increasing evidence points towards sex-dependent dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis as a contributing mechanism. Additionally, clinical studies suggest that the mineralocorticoid receptor (MR) may further confer genetic vulnerability/resilience on a background of ELS. The link between ELS, sex and the HPA axis and how this interacts with MR genotype is understudied, yet important to understand vulnerability/resilience to stress. We used the early life-limited nesting and bedding model to test the effect of ELS on HPA properties in adult female and male mice carrying a forebrain-specific heterozygous knockout for MR. Basal HPA axis activity was measured by circadian peak and nadir corticosterone levels, in addition to body weight and weight of stress-sensitive tissues. HPA axis reactivity was assessed by mapping corticosterone levels after 10 min immobilization. Additionally, we measured the effects of ELS on steroid receptor [MR and glucocorticoid receptor (GR)] levels in the dorsal hippocampus and medial prefrontal cortex (mPFC) with western blot. Finally, behavioral reactivity towards a novel environment was measured as a proxy for anxiety-like behavior. Results show that HPA axis activity under rest conditions was not affected by ELS. HPA axis reactivity after immobilization was decreased by ELS in females and increased, at trend-level in males. This effect in females was further exacerbated by low expression of the MR. We also observed a sex*ELS interaction regarding MR and GR expression in the dorsal hippocampus, with a significant upregulation of MR in males only. The sex-dependent interaction with ELS was not reflected in the behavioral response to novel environment and time spent in a sheltered compartment. We did find increased locomotor activity in all groups after a history of ELS, which attenuated after 4 h in males but not females regardless of condition. Our findings support that ELS alters HPA axis functioning sex-dependently. Genetic predisposition to low MR function may render females more susceptible to the harmful effect of ELS whereas in males low MR function promotes resilience. We propose that this model may be a useful tool to investigate the underlying mechanisms of sex-dependent and genetic vulnerability/resilience to stress-related psychopathology.

Hyperthermia-induced seizures followed by repetitive stress are associated with age-dependent changes in specific aspects of the mouse stress system

Stress is among the most frequently self-reported factors provoking epileptic seizures in children and adults. It is still unclear, however, why some people display stress-sensitive seizures and others do not. Recently, we showed that young epilepsy patients with stress-sensitive seizures exhibit a dysregulated hypothalamic-pituitary-adrenal (HPA)-axis. Most likely, this dysregulation gradually develops, and is triggered by stressors occurring early in life (early-life stress [ELS]). ELS may be particularly impactful when overlapping with the period of epileptogenesis. To examine this in a controlled and prospective manner, the present study investigated the effect of repetitive variable stressors or control treatment between postnatal day (PND) 12 and 24 in male mice exposed on PND10 to hyperthermia (HT)-induced prolonged seizures (control: normothermia). A number of peripheral and central indices of HPA-axis activity were evaluated at pre-adolescent and young adult age (ie, at PND25 and 90, respectively). At PND25 but not at PND90, body weight gain and absolute as well as relative (to body weight) thymus weight were reduced by ELS (vs control), whereas relative adrenal weight was enhanced, confirming the effectiveness of the stress treatment. Basal and stress-induced corticosterone levels were unaffected, though, by ELS at both ages. HT by itself did not affect any of these peripheral markers of HPA-axis activity, nor did it interact with ELS. However, centrally we did observe age-specific interaction effects of HT and ELS with regard to hippocampal glucocorticoid receptor mRNA expression, neurogenesis with the immature neurone marker doublecortin and the number of hilar (ectopic) granule cells using Prox1 staining. This lends some support to the notion that exposure to repetitive stress after HT-induced seizures may dysregulate central components of the stress system in an age-dependent manner. Such dysregulation could be one of the mechanisms conferring higher vulnerability of individuals with epilepsy to develop seizures in the face of stress.

Influence of glucocorticoid and mineralocorticoid receptor stimulation on task switching

The influence of stress on executive functions has been demonstrated in numerous studies and is potentially mediated by the stress-induced cortisol release. Yet, the impact of cortisol on cognitive flexibility and task switching in particular remains equivocal. In this study, we investigated the influence of pharmacological glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) stimulation, two corticosteroid receptor types known to be responsible for cortisol effects on the brain. We conducted two experiments, each with 80 healthy participants (40 women and 40 men), and tested the effect of the unspecific MR/GR agonist hydrocortisone (Experiment I) and the more specific MR agonist fludrocortisone (Experiment II) on switch costs and task rule congruency in a bivalent, cued task switching paradigm. The results did not confirm our hypotheses; we found no significant effects of our manipulations on task switching capacity, although general switching and congruency effects were observed. We discuss the absence of MR/GR-mediated effects and propose alternative mechanisms that could explain stress induced effects on task switching.

Stress and the Etiopathogenesis of Alzheimer's Disease and Depression

Alzheimer's disease (AD) is a multifactorial neurodegenerative disorder with a complex physiopathology whose initiators are poorly defined. Accumulating clinical and experimental evidence suggests a causal role of lifetime stress in AD. This chapter summarizes current knowledge about how chronic stress and its accompanying high levels of glucocorticoid (GC) secretion, trigger the two main pathomechanisms of AD: (i) misprocessing of amyloid precursor protein (APP) and the generation of amyloid beta (Aβ) and (ii) Tau hyperphosphorylation and aggregation. Given that depression is a well-known stress-related illness, and the evidence that depression may precede AD, this chapter also explores neurobiological mechanisms that may be common to depressive and AD pathologies. This review also discusses emerging insights into the role of Tau and its malfunction in disrupting neuronal cascades and neuroplasticity and, thus triggering brain pathology.

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.

On the Developmental Timing of Stress: Delineating Sex-Specific Effects of Stress across Development on Adult Behavior

Stress, and the chronic overactivation of major stress hormones, is associated with several neuropsychiatric disorders. However, clinical literature on the exact role of stress either as a causative, triggering, or modulatory factor to mental illness remains unclear. We suggest that the impact of stress on the brain and behavior is heavily dependent on the developmental timing at which the stress has occurred, and as such, this may contribute to the overall variability reported on the association of stress and mental illness. Here, animal models provide a way to comprehensively assess the temporal impact of stress on behavior in a controlled manner. This review particularly focuses on the long-term impact of stress on behavior in various rodent stress models at three major developmental time points: early life, adolescence, and adulthood. We characterize the various stressor paradigms into physical, social, and pharmacological, and discuss commonalities and differences observed across these various stress-inducing methods. In addition, we discuss here how sex can influence the impact of stress at various developmental time points. We conclude here that early postnatal life and adolescence represent particular periods of vulnerability, but that stress exposure during early life can sometimes lead to resilience, particularly to fear-potentiated memories. In the adult brain, while shorter periods of stress tended to enhance spatial memory, longer periods caused impairments. Overall, males tended to be more vulnerable to the long-term effects of early life and adolescent stress, albeit very few studies incorporate both sexes, and further well-powered sex comparisons are needed.

Coping with Stress During Aging: The Importance of a Resilient Brain

BACKGROUND

Resilience is the ability to achieve a positive outcome when we are in the face of adversity. It supposes an active resistance to adversity by coping mechanisms in which genetic, molecular, neural and environmental factors are involved. Resilience has been usually studied in early ages and few is known about it during aging.

METHODS

In this review, we will address the age-related changes in the brain mechanisms involved in regulating the stress response. Furthermore, using the EE paradigm, we analyse the resilient potential of this intervention and its neurobiological basis. In this case, we will focus on identifying the characteristics of a resilient brain (modifications in HPA structure and function, neurogenesis, specific neuron types, glia, neurotrophic factors, nitric oxide synthase or microRNAs, among others).

RESULTS

The evidence suggests that a healthy lifestyle has a crucial role to promote a resilient brain during aging. Along with the behavioral changes described, a better regulation of HPA axis, enhanced levels of postmitotic type-3 cells or changes in GABAergic neurotransmission are some of the brain mechanisms involved in resilience.

CONCLUSION

Future research should identify different biomarkers that increase the resistance to develop mood disorders and based on this knowledge, develop new potential therapeutic targets.

A Haplotype Associated with Enhanced Mineralocorticoid Receptor Expression Facilitates the Stress-Induced Shift from "Cognitive" to "Habit" Learning

Stress induces a shift from hippocampus-dependent "cognitive" toward dorsal striatum-dependent "habit" memory. However, not all individuals are susceptible to this shift under stress. Based on pharmacological studies indicating a critical role of the mineralocorticoid receptor (MR) in the stress-induced bias toward dorsal striatal learning, we hypothesized that MR gene variants contribute to these individual differences. In two experiments, healthy participants were genotyped, exposed to a stressor or control manipulation and performed a learning task that can be solved using hippocampal or dorsal striatal systems, while electroencephalography (EEG; Experiment I) or functional magnetic resonance imaging (fMRI; Experiment II) measurements were taken. Stress led to a shift from hippocampal to dorsal striatal learning which was more pronounced in homo- and heterozygous carriers of a six single nucleotide polymorphisms (SNPs)-comprising haplotype containing the alleles of two MR SNPs associated with increased MR expression and transactivational activity (MR-2G/C C [rs2070951], MR-I180V A [rs5522]). This stress-induced shift toward habit memory was paralleled by an increased feedback-related negativity (FRN), which may reflect striatal processing, and increased caudate activation. Carriers of the MR haplotype showed a reduced P3a, an event-related potential thought to indicate cognitive processing, and reduced hippocampal activity after stress. Moreover, stress resulted in reduced amygdala-hippocampus connectivity and the decrease in amygdala connectivity to the parahippocampal cortex was particularly pronounced in MR haplotype carriers. Our findings indicate that genetic variants associated with enhanced MR expression facilitate a stress-induced shift from hippocampal toward dorsal striatal learning, most likely via impaired hippocampal processing and reduced amygdala-hippocampus cross talk, allowing the dorsal striatum to guide behavior under stress.

Differential effects of imipramine and CORT118335 (Glucocorticoid receptor modulator/mineralocorticoid receptor antagonist) on brain-endocrine stress responses and depression-like behavior in female rats

Depression is commonly associated with hypothalamic-pituitary adrenal (HPA) axis dysfunction that primarily manifests as aberrant glucocorticoid secretion. Glucocorticoids act on Type I mineralocorticoid (MR) and Type II glucocorticoid receptors (GR) to modulate mood and endocrine responses. Successful antidepressant treatment normalizes HPA axis function, in part due to modulatory effects on MR and GR in cortico-limbic structures. Although women are twice as likely to suffer from depression, little is known about how antidepressants modulate brain, endocrine, and behavioral stress responses in females. Here, we assessed the impact of CORT118335 (GR modulator/MR antagonist) and imipramine (tricyclic antidepressant) on neuroendocrine and behavioral responses to restraint or forced swim stress (FST) in female rats (n=10-12/group). Increased immobility CORT118335 in the FST is purported to reflect passive coping or depression-like behavior. CORT118335 dampened adrenocorticotropic hormone (ACTH) and corticosterone responses to the FST, but did not affect immobility. Imipramine suppressed ACTH, but had minimal effects on corticosterone responses to FST. Despite these marginal effects, imipramine decreased immobility, suggesting antidepressant efficacy. In an effort to link brain-endocrine responses with behavior, c-Fos was assessed in HPA axis and mood modulatory regions in response to the FST. CORT118335 upregulated c-Fos expression in the paraventricular nucleus of the hypothalamus. Imipramine decreased c-Fos in the basolateral amygdala and hippocampus (CA1 and CA3), but increased c-Fos in the central amygdala. These data suggest the antidepressant-like (e.g., active coping) properties of imipramine may be due to widespread effects on cortico-limbic circuits that regulate emotional and cognitive processes.

Effects of mineralocorticoid-receptor stimulation on risk taking behavior in young healthy men and women

Risk taking is influenced by stress, with riskier decisions after exposure to an acute stressor and consecutively elevated cortisol levels. In the brain, cortisol acts on two receptors with different functional profiles: the glucocorticoid receptor (GR) and the mineralocorticoid receptor (MR). In the current study we investigated the effect of MR stimulation on risk taking behavior in 80 young healthy participants (40 women, mean age=23.9). We administered 0.4mg fludrocortisone, a MR agonist, in a between-subjects, placebo controlled design. Subsequently, participants conducted an established risk taking paradigm, the Balloon-Analogue-Risk-Task (BART). We also used two questionnaires to assess risk taking and decision behavior as trait measures. We found a treatment effect with riskier decisions in the fludrocortisone group. Furthermore, we found a sex effect with more risk taking in men. There was no statistically significant interaction between both factors. Our results indicate that acute MR stimulation leads to riskier decisions in women and men. Our findings argue for an important role of the MR in decision making processes.

Mineralocorticoid receptor stimulation effects on spatial memory in healthy young adults: A study using the virtual Morris Water Maze task

OBJECTIVES

Stress hormones such as cortisol are known to influence a wide range of cognitive functions, including hippocampal based spatial memory. In the brain, cortisol acts via two different receptors: the glucocorticoid (GR) and the mineralocorticoid receptor (MR). As the MR has a high density in the hippocampus, we examined the effects of pharmacological MR stimulation on spatial memory.

METHODS

Eighty healthy participants (40 women, 40 men, mean age=23.9years±SD=3.3) completed the virtual Morris Water Maze (vMWM) task to test spatial encoding and spatial memory retrieval after receiving 0.4mg fludrocortisone, a MR agonist, or placebo.

RESULTS

There was no effect of MR stimulation on spatial encoding during the vMWM task. However, participants who received fludrocortisone exhibited improved spatial memory retrieval performance. There was neither a main effect of sex nor a sex-by-treatment interaction.

CONCLUSION

In young healthy participants, MR stimulation improved hippocampal based spatial memory retrieval in a virtual Morris Water Maze task. Our study not only confirms the importance of MR function in spatial memory, but suggests beneficial effects of acute MR stimulation on spatial memory retrieval in humans.

Learning about stress: neural, endocrine and behavioral adaptations

In this review, nonassociative learning is advanced as an organizing principle to draw together findings from both sympathetic-adrenal medullary and hypothalamic-pituitary-adrenocortical (HPA) axis responses to chronic intermittent exposure to a variety of stressors. Studies of habituation, facilitation and sensitization of stress effector systems are reviewed and linked to an animal's prior experience with a given stressor, the intensity of the stressor and the appraisal by the animal of its ability to mobilize physiological systems to adapt to the stressor. Brain pathways that regulate physiological and behavioral responses to stress are discussed, especially in light of their regulation of nonassociative processes in chronic intermittent stress. These findings may have special relevance to various psychiatric diseases, including depression and post-traumatic stress disorder (PTSD).

Effects of Mineralocorticoid Receptor Overexpression on Anxiety and Memory after Early Life Stress in Female Mice

Early-life stress (ELS) is a risk factor for the development of psychopathology, particularly in women. Human studies have shown that certain haplotypes of NR3C2, encoding the mineralocorticoid receptor (MR), that result in gain of function, may protect against the consequences of stress exposure, including childhood trauma. Here, we tested the hypothesis that forebrain-specific overexpression of MR in female mice would ameliorate the effects of ELS on anxiety and memory in adulthood. We found that ELS increased anxiety, did not alter spatial discrimination and reduced contextual fear memory in adult female mice. Transgenic overexpression of MR did not alter anxiety but affected spatial memory performance and enhanced contextual fear memory formation. The effects of ELS on anxiety and contextual fear were not affected by transgenic overexpression of MR. Thus, MR overexpression in the forebrain does not represent a major resilience factor to early life adversity in female mice.

Overexpression of mineralocorticoid receptors does not affect memory and anxiety-like behavior in female mice

Mineralocorticoid receptors (MRs) have been implicated in behavioral adaptation and learning and memory. Since-at least in humans-MR function seems to be sex-dependent, we examined the behavioral relevance of MR in female mice exhibiting transgenic MR overexpression in the forebrain. Transgenic MR overexpression did not affect contextual fear memory or cued fear learning and memory. Moreover, MR overexpressing and control mice discriminated equally well between fear responses in a combined cue and context fear conditioning paradigm. Also context-memory in an object recognition task was unaffected in MR overexpressing mice. We conclude that MR overexpression in female animals does not affect fear conditioned responses and object recognition memory.

The effects of environmental enrichment and age-related differences on inhibitory avoidance in zebrafish (Danio rerio Hamilton)

The inhibitory avoidance paradigm allows the study of mechanisms underlying learning and memory formation in zebrafish (Danio rerio Hamilton). For zebrafish, the physiology and behavior associated with this paradigm are as yet poorly understood. We therefore assessed the effects of environmental enrichment and fish age on inhibitory avoidance learning. Fish raised in an environmentally enriched tank showed decreased anxiety-like behavior and increased exploration. Enrichment greatly reduced inhibitory avoidance in 6-month (6M)- and 12-month (12 M)-old fish. Following inhibitory avoidance, telencephalic mRNA levels of proliferating cell nuclear antigen (pcna), neurogenic differentiation (neurod), cocaine- and amphetamine-regulated transcript 4 (cart4), and cannabinoid receptor 1 (cnr1) were lower in enriched-housed fish, while the ratios of mineralocorticoid receptor (nr3c2)/glucocorticoid receptor α [nr3c1(α)] and glucocorticoid receptor β [nr3c1(β)]/glucocorticoid receptor α [nr3c1(α)] were higher. This was observed for 6M-old fish only, not for 24-month (24 M) old fish. Instead, 24 M-old fish showed delayed inhibitory avoidance, no effects of enrichment, and reduced expression of neuroplasticity genes. Overall, our data show strong differences in inhibitory avoidance behavior between zebrafish of different ages and a clear reduction in avoidance behavior following housing under environmental enrichment.

The brain mineralocorticoid receptor and stress resilience

Stress exposure activates the HPA-axis and results in the release of corticosteroids which bind to two receptor types in the brain: the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR). While the role of the GR in stress reactivity has been extensively studied, the MR has received less attention. Nevertheless, pioneering in-depth studies over the past two decades have shown the importance of the brain MR in the processing of stressful information. Moreover, a membrane-bound MR mediating the rapid effects of cortisol was recently discovered. This review summarizes how the MR may play a role in stress resilience. Both preclinical and clinical studies suggest that the MR is an important stress modulator and influences basal as well as stress-induced HPA-axis activity, stress appraisal, and fear-related memories. These MR effects are mediated by both genomic and non-genomic MRs and appear to be at least partially sex-dependent. Moreover, the majority of studies indicate that high MR functionality or expression may confer resilience to traumatic stress. This has direct clinical implications. First, increasing activity or expression of brain MRs may prevent or reverse symptoms of stress-related depression. Second, individuals with a relatively low MR functionality may possess an increased stress susceptibility for depression. Nevertheless, the number of clinical MR studies is currently limited. In conclusion, the recent emergence of the MR as a putative stress resilience factor is important and may open up new avenues for the prevention and treatment of psychiatric disorders.

Mineralocorticoid receptor stimulation improves cognitive function and decreases cortisol secretion in depressed patients and healthy individuals

Memory and executive function are often impaired in patients with major depression, while cortisol secretion is increased. Mineralocorticoid receptors (MR) are abundantly expressed in the hippocampus and in the prefrontal cortex, brain areas critical for memory, executive function, and cortisol inhibition. Here, we investigated whether MR stimulation with fludrocortisone (1) improves memory and executive function and (2) decreases cortisol secretion in depressed patients and healthy individuals. Twenty-four depressed patients without medication and 24 age-, sex-, and education-matched healthy participants received fludrocortisone (0.4 mg) or placebo in a randomized, double-blind, within-subject cross-over design. We measured verbal memory, visuospatial memory, executive function, psychomotor speed, and salivary cortisol secretion during cognitive testing between 1400 and 1700 hours. For verbal memory and executive function, we found better performance after fludrocortisone compared with placebo across groups. No treatment effect on other cognitive domains emerged. Depressed patients performed worse than healthy individuals in psychomotor speed and executive function. No group effect or group × treatment interaction emerged on other cognitive domains. Fludrocortisone decreased cortisol secretion across groups and there was a significant correlation between cortisol inhibition and verbal memory performance. Our data suggest a crucial role of MR in verbal memory and executive function and demonstrate the possibility to improve cognition in depressed patients and healthy individuals through MR stimulation.

Unpredictable chronic stress decreases inhibitory avoidance learning in Tuebingen long-fin zebrafish: stronger effects in the resting phase than in the active phase

Zebrafish (Danio rerio Hamilton) are increasingly used as a model to study the effects of chronic stress on brain and behaviour. In rodents, unpredictable chronic stress (UCS) has a stronger effect on physiology and behaviour during the active phase than during the resting phase. Here, we applied UCS during the daytime (active phase) for 7 and 14 days or during the night-time (resting phase) for 7 nights in an in-house-reared Tuebingen long-fin (TLF) zebrafish strain. Following UCS, inhibitory avoidance learning was assessed using a 3 day protocol where fish learn to avoid swimming from a white to a black compartment where they will receive a 3 V shock. Latencies of entering the black compartment were recorded before training (day 1; first shock) and after training on day 2 (second shock) and day 3 (no shock, tissue sampling). Fish whole-body cortisol content and expression levels of genes related to stress, fear and anxiety in the telencephalon were quantified. Following 14 days of UCS during the day, inhibitory avoidance learning decreased (lower latencies on days 2 and 3); minor effects were found following 7 days of UCS. Following 7 nights of UCS, inhibitory avoidance learning decreased (lower latency on day 3). Whole-body cortisol levels showed a steady increase compared with controls (100%) from 7 days of UCS (139%), to 14 days of UCS (174%) to 7 nights of UCS (231%), suggestive of an increasing stress load. Only in the 7 nights of UCS group did expression levels of corticoid receptor genes (mr, grα, grβ) and of bdnf increase. These changes are discussed as adaptive mechanisms to maintain neuronal integrity and prevent overload, and as being indicative of a state of high stress load. Overall, our data suggest that stressors during the resting phase have a stronger impact than during the active phase. Our data warrant further studies on the effect of UCS on stress axis-related genes, especially grβ; in mammals this receptor has been implicated in glucocorticoid resistance and depression.

Inhibitory avoidance learning in zebrafish (Danio rerio): effects of shock intensity and unraveling differences in task performance

The zebrafish (Danio rerio) is increasingly used as a model in neurobehavioral and neuroendocrine studies. The inhibitory avoidance paradigm has been proposed as tool to study mechanisms underlying learning and memory in zebrafish. In this paradigm subjects receive a shock after entering the black compartment of a black-white box. On the next day, latency to enter the black compartment is assessed; higher latencies are indicative of increased avoidance learning. Here, we aimed to understand the effects of different shock intensities (0, 1, 3, and 9 V) and to unravel variation in inhibitory avoidance learning in an in-house reared Tuebingen Long-Fin zebrafish (D. rerio) strain. While median latencies had increased in the 1, 3, and 9 V groups, no increase in median latency was found in the 0 V group. In addition, higher shock intensities resulted in a higher number of avoiders (latency ≥180 s) over nonavoiders (latency <60 s). Both changes are indicative of increased avoidance learning. We assessed whole-body cortisol content and the expression levels of genes relevant to stress, anxiety, fear, and learning 2 h after testing. Shock intensity was associated with whole-body cortisol content and the expression of glucocorticoid receptor alpha [nr3c1(alpha)], cocaine- and amphetamine-regulated transcript (cart4), and mineralocorticoid receptor (nr3c2), while avoidance behavior was associated with whole-body cortisol content only. The inhibitory avoidance paradigm in combination with measuring whole-body cortisol content and gene expression is suitable to unravel (genetic) mechanisms of fear avoidance learning. Our data further show differences in brain-behavior relationships underlying fear avoidance learning and memory in zebrafish. These findings serve as starting point for further unraveling differences in brain-behavior relationships underlying (fear avoidance) learning and memory in zebrafish.

Adrenal steroids in the brain: role of the intrinsic expression of corticosteroid-binding globulin (CBG) in the stress response

The complex interaction between hypothalamus, pituitary and adrenal glands is a key component of the neuroendocrine stress response. The major stress hormones--glucocorticoids--have both central and peripheral effects. Among the factors regulating their availability to target tissues are levels of corticosteroid-binding globulin, as the major transport protein for glucocorticoids in systemic circulation. Our recent findings demonstrated expression of corticosteroid-binding globulin in various brain regions and in different cell populations (neurons and glial cells). We showed at the cellular level the presence of corticosteroid-binding globulin in the human hypothalamus, where it was co-localized with the classical neurohypophyseal neurohormones--vasopressin and oxytocin. For the first time we demonstrated in mouse that the same gene encodes brain and liver corticosteroid-binding globulin. The full-length sequencing of hypothalamic corticosteroid-binding globulin revealed a full homology with liver corticosteroid-binding globulin cDNA. Thus, we confirmed that corticosteroid-binding globulin mRNA is produced locally within various cerebral regions and thus not transported from blood. However, the amounts of mRNA encoding corticosteroid-binding globulin are in liver about 200 times higher than in brain. The wide distribution of corticosteroid-binding globulin, distinct from the localization of glucocorticoid receptors, observed in our comparative study in rodents, led us to propose two possibilities: (1) corticosteroid-binding globulin is made in certain neurons to deliver glucocorticoids into the cell and within the cell in the absence of cytoplasmic glucocorticoid receptors or (2) is internalized into neurons specifically to deliver glucocorticoids to classical glucocorticoid receptors. Brain corticosteroid-binding globulin may be involved in the response to changing systemic glucocorticoid levels either additionally to known nuclear and membrane corticosteroid receptors or in glucocorticoid responsive brain regions devoid of these receptors. Clearly the multiple locations of corticosteroid-binding globulin within the central nervous system of humans and rodents imply multiple functional properties in normal and/or pathological conditions, which are yet to be determined. Most likely, the importance of brain corticosteroid-binding globulin exceeds the function of a mere steroid transporter.

Mineralocorticoid receptor and heat shock protein expression levels in peripheral lymphocytes from war trauma-exposed men with and without PTSD

Alterations in the number and functional status of mineralocorticoid (MR) and glucocorticoid receptors (GR) may contribute to vulnerability to posttraumatic stress disorder (PTSD). Corticosteroid receptors are chaperoned by heat shock proteins Hsp90 and Hsp70. We examined relations between corticosteroid receptor and heat shock protein expression levels, and related them with war trauma exposure, PTSD and resilience to PTSD. Relative levels of MR, Hsp90 and Hsp70 were determined by immunoblotting in lymphocytes from war trauma-exposed men with current PTSD (current PTSD group, n=113), with life-time PTSD (life-time PTSD group, n=61) and without PTSD (trauma control group, n=88), and from non-traumatized healthy controls (healthy control group, n=85). Between-group differences in MR, Hsp90 and Hsp70 levels and in MR/GR ratio were not observed. The level of MR was correlated with both Hsp90 and Hsp70 levels in trauma control and healthy control groups. On the other hand, GR level was correlated only with Hsp90 level, and this correlation was evident in current PTSD and trauma control groups. In conclusion, PTSD and exposure to trauma are not related to changes in lymphocyte MR, Hsp90 or Hsp70 levels, but may be associated with disturbances in corticosteroid receptors interaction with heat shock proteins.

Deletion of the forebrain mineralocorticoid receptor impairs social discrimination and decision-making in male, but not in female mice

Social interaction with unknown individuals requires fast processing of information to decide whether it is friend or foe. This process of discrimination and decision-making is stressful and triggers secretion of corticosterone activating mineralocorticoid receptor (MR) and glucocorticoid receptor (GR). The MR is involved in appraisal of novel experiences and risk assessment. Recently, we have demonstrated in a dual-solution memory task that MR plays a role in the early stage of information processing and decision-making. Here we examined social approach and social discrimination in male and female mice lacking MR from hippocampal-amygdala-prefrontal circuitry and controls. The social approach task allows the assessment of time spent with an unfamiliar mouse and the ability to discriminate between familiar and unfamiliar conspecifics. The male and female test mice were both more interested in the social than the non-social experience and deletion of their limbic MR increased the time spent with an unfamiliar mouse. Unlike controls, the male MR^CaMKCre mice were not able to discriminate between an unfamiliar and the familiar mouse. However, the female MR mutant had retained the discriminative ability between unfamiliar and familiar mice. Administration of the MR antagonist RU28318 to male mice supported the role of the MR in the discrimination between an unfamiliar mouse and a non-social stimulus. No effect was found with a GR antagonist. Our findings suggest that MR is involved in sociability and social discrimination in a sex-specific manner through inhibitory control exerted putatively via limbic-hippocampal efferents. The ability to discriminate between familiar and unfamiliar conspecifics is of uttermost importance for territorial defense and depends on a role of MR in decision-making.

Mineralocorticoid receptors guide spatial and stimulus-response learning in mice

Adrenal corticosteroid hormones act via mineralocorticoid (MR) and glucocorticoid receptors (GR) in the brain, influencing learning and memory. MRs have been implicated in the initial behavioral response in novel situations, which includes behavioral strategies in learning tasks. Different strategies can be used to solve navigational tasks, for example hippocampus-dependent spatial or striatum-dependent stimulus-response strategies. Previous studies suggested that MRs are involved in spatial learning and induce a shift between learning strategies when animals are allowed a choice between both strategies. In the present study, we further explored the role of MRs in spatial and stimulus-response learning in two separate circular holeboard tasks using female mice with forebrain-specific MR deficiency and MR overexpression and their wildtype control littermates. In addition, we studied sex-specific effects using male and female MR-deficient mice. First, we found that MR-deficient compared to control littermates and MR-overexpressing mice display altered exploratory and searching behavior indicative of impaired acquisition of novel information. Second, female (but not male) MR-deficient mice were impaired in the spatial task, while MR-overexpressing female mice showed improved performance in the spatial task. Third, MR-deficient mice were also impaired in the stimulus-response task compared to controls and (in the case of females) MR-overexpressing mice. We conclude that MRs are important for coordinating the processing of information relevant for spatial as well as stimulus-response learning.

Mineralocorticoid receptor blockade prevents stress-induced modulation of multiple memory systems in the human brain

BACKGROUND

Accumulating evidence suggests that stress may orchestrate the engagement of multiple memory systems in the brain. In particular, stress is thought to favor dorsal striatum-dependent procedural over hippocampus-dependent declarative memory. However, the neuroendocrine mechanisms underlying these modulatory effects of stress remain elusive, especially in humans. Here, we targeted the role of the mineralocorticoid receptor (MR) in the stress-induced modulation of dorsal striatal and hippocampal memory systems in the human brain using a combination of event-related functional magnetic resonance imaging and pharmacologic blockade of the MR.

METHODS

Eighty healthy participants received the MR antagonist spironolactone (300 mg) or a placebo and underwent a stressor or control manipulation before they performed, in the scanner, a classification task that can be supported by the hippocampus and the dorsal striatum.

RESULTS

Stress after placebo did not affect learning performance but reduced explicit task knowledge and led to a relative increase in the use of more procedural learning strategies. At the neural level, stress promoted striatum-based learning at the expense of hippocampus-based learning. Functional connectivity analyses showed that this shift was associated with altered coupling of the amygdala with the hippocampus and dorsal striatum. Mineralocorticoid receptor blockade before stress prevented the stress-induced shift toward dorsal striatal procedural learning, same as the stress-induced alterations of amygdala connectivity with hippocampus and dorsal striatum, but resulted in significantly impaired performance.

CONCLUSIONS

Our findings indicate that the stress-induced shift from hippocampal to dorsal striatal memory systems is mediated by the amygdala, required to preserve performance after stress, and dependent on the MR.

Lifetime achievement from a brain-adrenal perspective: on the CRF-urocortin-glucocorticoid balance

This contribution dedicated to Wylie Vale is focused on the action of the glucocorticoid hormone aimed to counterbalance the stress response orchestrated by the corticotrophin releasing factor (CRF) and urocortin (Ucn) family of peptides. It appears that the release and action of these stress hormones themselves are subjected to intrinsic self-regulatory feedback loops that operate as checks and balances in stress adaptation. One of these feedback loops is operated by the mineralocorticoid (MR) and glucocorticoid receptors (GR) that mediate in complementary fashion the action of endogenous cortisol/corticosterone in brain circuits underlying the onset and termination of the stress response. By affecting appraisal processes MR has an important role in coordinating emotional expression and cognitive flexibility with the onset of the stress response, while GR's role is prominent in the management of behavioral and physiological adaptations during the recovery phase. Genetic variation in interaction with environmental input and experience-related factors can modulate this balance between susceptibility and recovery governed by a balanced MR:GR signaling. Thanks to the Wylie Vale School of scientists a parallel balanced regulation between the CRF/CRF-1 and Ucn/CRF-2 receptor systems is being uncovered, leading inexorably to the question: how do the CRF/Ucn and glucocorticoid systems interact in multiple brain sites to maintain homeostasis and health?

Stress and glucocorticoid receptor-dependent mechanisms in long-term memory: from adaptive responses to psychopathologies

A proper response against stressors is critical for survival. In mammals, the stress response is primarily mediated by secretion of glucocorticoids via the hypothalamic-pituitary-adrenocortical (HPA) axis and release of catecholamines through adrenergic neurotransmission. Activation of these pathways results in a quick physical response to the stress and, in adaptive conditions, mediates long-term changes in the brain that lead to the formation of long-term memories of the experience. These long-term memories are an essential adaptive mechanism that allows an animal to effectively face similar demands again. Indeed, a moderate stress level has a strong positive effect on memory and cognition, as a single arousing or moderately stressful event can be remembered for up to a lifetime. Conversely, exposure to extreme, traumatic, or chronic stress can have the opposite effect and cause memory loss, cognitive impairments, and stress-related psychopathologies such as anxiety disorders, depression and post-traumatic stress disorder (PTSD). While more effort has been devoted to the understanding of the negative effects of chronic stress, much less has been done thus far on the identification of the mechanisms engaged in the brain when stress promotes long-term memory formation. Understanding these mechanisms will provide critical information for use in ameliorating memory processes in both normal and pathological conditions. Here, we will review the role of glucocorticoids and glucocorticoid receptors (GRs) in memory formation and modulation. Furthermore, we will discuss recent findings on the molecular cascade of events underlying the effect of GR activation in adaptive levels of stress that leads to strong, long-lasting memories. Our recent data indicate that the positive effects of GR activation on memory consolidation critically engage the brain-derived neurotrophic factor (BDNF) pathway. We propose and will discuss the hypothesis that stress promotes the formation of strong long-term memories because the activation of hippocampal GRs after learning is coupled to the recruitment of the growth and pro-survival BDNF/cAMP response element-binding protein (CREB) pathway, which is well-know to be a general mechanism required for long-term memory formation. We will then speculate about how these results may explain the negative effects of traumatic or chronic stress on memory and cognitive functions.

Functional profile of the binary brain corticosteroid receptor system: mediating, multitasking, coordinating, integrating

This contribution is focused on the action of the naturally occurring corticosteroids, cortisol and corticosterone, which are secreted from the adrenals in hourly pulses and after stress with the goal to maintain resilience and health. To achieve this goal the action of the corticosteroids displays an impressive diversity, because it is cell-specific and context-dependent in coordinating the individual's response to changing environments. These diverse actions of corticosterone are mediated by mineralocorticoid- and glucocorticoid-receptors that operate as a binary system in concert with neurotransmitter and neuropeptide signals to activate and inhibit stress reactions, respectively. Classically MR and GR are gene transcription factors, but recently these receptors appear to mediate also rapid non-genomic actions on excitatory neurotransmission suggesting that they integrate functions over time. Hence the balance of receptor-mediated actions is crucial for homeostasis. This balanced function of mineralo- and glucocorticoid-receptors can be altered epigenetically by a history of traumatic (early) life events and the experience of repeated stressors as well as by predisposing genetic variants in signaling pathways of these receptors. One of these variants, mineralocorticoid receptor haplotype 2, is associated with dispositional optimism in appraisal of environmental challenges. Imbalance in receptor-mediated corticosterone actions was found to leave a genomic signature highlighting the role of master switches such as cAMP response element-binding protein and mammalian target of rapamycin to compromise health, and to promote vulnerability to disease. Diabetic encephalopathy is a pathology of imbalanced corticosterone action, which can be corrected in its pre-stage by a brief treatment with the antiglucocorticoid mifepristone.

Mineralocorticoid receptor antagonist spironolactone prevents chronic corticosterone induced depression-like behavior

High level of serum corticosteroid is frequently associated with depression, in which a notable HPA (hypothalamus-pituitary-adrenal) axis hyperactivity is often observed. There are two types of corticosteroid receptors expressed in the hippocampus that provide potent negative feedback regulation on the HPA axis but dysfunction during depression, i.e. the glucocorticoid receptor (GR) and the mineralocorticoid receptor (MR). The balance between hippocampal MR and GR during chronic stress plays an important role in the occurrence of depression. The aim of this study is to explore if chronic corticosterone administration would induce depression-like behavior and affect the expression and function of hippocampal MR and GR, in addition to assess whether manipulation of corticosteroid receptors would modulate depressive behaviors. Hence, mice were treated with corticosterone (40 mg/kg) for 21 days followed by assessment in a battery of depression-like behaviors. The results show that chronic corticosterone-treated animals displayed an increased immobility time in a forced-swimming test, decreased preference to sucrose solution and novel object recognition performance, and enhanced hippocampal serotonin but decreased MR expression in both hippocampus and hypothalamus. On the other hand, co-administration of MR antagonist, spironolactone (25mg/kg, i.p. × 7 days) in corticosteroid-treated animals reduced immobility time in a forced-swimming test and improved performance in a novel object recognition test. In conclusion, we demonstrate that chronic corticosterone treatment triggers several depression-like behaviors, and in parallel, down-regulates MR expression in the hippocampus and hypothalamus. Administration of an MR antagonist confers an anti-depressant effect in chronic corticosterone-treated animals.

Spatial learning of female mice: a role of the mineralocorticoid receptor during stress and the estrous cycle

Corticosterone facilitates behavioral adaptation to a novel experience in a coordinate manner via mineralocorticoid (MR) and glucocorticoid receptors (GR). Initially, MR mediates corticosterone action on appraisal processes, risk assessment and behavioral flexibility and then, GR activation promotes consolidation of the new information into memory. Here, we studied on the circular holeboard (CHB) the spatial performance of female mice with genetic deletion of MR from the forebrain (MR^CaMKCre) and their wild type littermates (MR^flox/flox mice) over the estrous cycle and in response to an acute stressor. The estrous cycle had no effect on the spatial performance of MR^flox/flox mice and neither did the acute stressor. However, the MR^CaMKCre mutants needed significantly more time to find the exit and made more hole visit errors than the MR^flox/flox mice, especially when in proestrus and estrus. In addition, stressed MR^CaMKCre mice in estrus had a shorter exit latency than the control estrus MR^CaMKCre mice. About 70% of the female MR^CaMKCre and MR^flox/flox mice used a hippocampal (spatial, extra maze cues) rather than the caudate nucleus (stimulate-response, S-R, intra-maze cue) strategy and this preference did neither change over the estrous cycle nor after stress. However, stressed MR^CaMKCre mice using the S-R strategy needed significantly more time to find the exit hole as compared to the spatial strategy using mice suggesting that the MR could be needed for the stress-induced strategy switch toward a spatial strategy. In conclusion, the results suggest that loss of MR interferes with performance of a spatial task especially when estrogen levels are high suggesting a strong interaction between stress and sex hormones.

The role of the glucocorticoids in developing resilience to stress and addiction

There is emerging evidence that individuals have the capacity to learn to be resilient by developing protective mechanisms that prevent them from the maladaptive effects of stress that can contribute to addiction. The emerging field of the neuroscience of resilience is beginning to uncover the circuits and molecules that protect against stress-related neuropsychiatric diseases, such as addiction. Glucocorticoids (GCs) are important regulators of basal and stress-related homeostasis in all higher organisms and influence a wide array of genes in almost every organ and tissue. GCs, therefore, are ideally situated to either promote or prevent adaptation to stress. In this review, we will focus on the role of GCs in the hypothalamic-pituitary adrenocortical axis and extra-hypothalamic regions in regulating basal and chronic stress responses. GCs interact with a large number of neurotransmitter and neuropeptide systems that are associated with the development of addiction. Additionally, the review will focus on the orexinergic and cholinergic pathways and highlight their role in stress and addiction. GCs play a key role in promoting the development of resilience or susceptibility and represent important pharmacotherapeutic targets that can reduce the impact of a maladapted stress system for the treatment of stress-induced addiction.

Differences in colocalization of corticosteroid-binding globulin and glucocorticoid receptor immunoreactivity in the rat brain

Endocrine regulation of central and systemic stress response as well as learning and memory are in part controlled by systemic glucocorticoid levels. So far steroids have been thought to act on the brain predominantly through nuclear receptors. However, some brain systems known to respond to glucocorticoids seem to be devoid of the respective receptor proteins (GR). It is likely that known central actions of adrenal steroids may also be mediated by non-genomic actions involving intrinsic binding globulins. In recent studies we described the intrinsic expression of corticosteroid-binding globulin (CBG) in rat, mouse and human brains. Here we report an immunohistochemical mapping study on the colocalization of CBG and of GR in the rat brain. In the nucleus accumbens, septum, hippocampus, globus pallidus, medial and basolateral amygdale nuclei, magnocellular preoptic nuclei, diagonal band of Broca high intensity of CBG immunoreactivity was accompanied by weak or moderate GR staining, and vice versa. In the caudate putamen, bed nucleus of stria terminalis, septohypothalamic nucleus and parvocellular subdivision of the paraventricular nucleus strong GR immunoreactivity was observed, but CBG was almost undetectable. In contrast, throughout the supraoptic nucleus and magnocellular subdivision of the paraventricular nucleus numerous strongly CBG-positive cells were observed, devoid of specific GR immunoreactivity. It is most likely that CBG in the brain may be involved in the response to changing systemic glucocorticoid levels in addition to known nuclear and membrane corticosteroid receptors, or in glucocorticoid responsive regions devoid of these receptors.

Stress and estrous cycle affect strategy but not performance of female C57BL/6J mice

Stress induces a switch in learning strategies of male C57BL/6J mice from predominantly spatial to more stimulus-response learning. To study generalization of these findings over sex, we investigated female C57BL/6J mice at three phases of the estrous cycle under non stress and acute (10 min) restraint stress conditions. On a circular hole board (CHB) task, about half of the naive female mice used spatial and stimulus-response strategies to solve the task. Under stress, female mice favored spatial over stimulus-response strategies, with 100% of female mice in the estrus phase. Performance expressed as latency to solve the task is only improved in stressed female mice in the estrus phase. We conclude that the use of learning strategies is influenced by sex and this difference between sexes is aggravated by acute stress.

Sex differences in fear memory and extinction of mice with forebrain-specific disruption of the mineralocorticoid receptor

Previous studies showed that the mineralocorticoid receptor (MR) is needed for behavioral flexibility in a fear conditioning paradigm. Female mice with forebrain-specific deletion of the MR gene (MR(CaMKCre) ) were unable to show extinction of contextual fear, and could not discriminate between cue and context fear unlike control mice. In the present study, male and female (MR(CaMKCre) ) mice and control littermates were used to study sex-specific fear conditioning, memory performance and extinction. The fear conditioning paradigm assessed both context- and cue-related fear within one experimental procedure. We observed that at the end of the conditioning all mice acquired the fear-motivated response. During the first minutes of the memory test, both male and female MR(CaMKCre) mice remembered and feared the context more than the control mice. Furthermore, female MR(CaMKCre) mice were not able to extinguish this memory even on the second day of memory testing. The female mutants also could not discriminate between cue (more freezing) and context periods (less freezing). In contrast, male MR(CaMKCre) mice and the controls showed extinction and were capable to discriminate, although the MR(CaMKCre) mice needed more time before they started extinction. These findings further support the relevance of MR for behavioral flexibility and extinction of fear-motivated behavior. In conclusion, the loss of MR in the forebrain results in large differences in emotional and cognitive behaviors between female and male mice, which suggests a role of this receptor in the female prevalence of stress- and anxiety-regulated disorders.