Bidirectional rescue of extreme genetic predispositions to anxiety: impact of CRH receptor 1 as epigenetic plasticity gene in the amygdala

Chronic mild stress induces differential depression-like symptoms and c-Fos and 5HT1A protein levels in high-anxiety female Long Evans rats

Depression and anxiety disorders overlap in clinical populations, suggesting common mechanisms that may be further investigated in reliable animal models. We used filial 8 female Long-Evans rats bred for high (HAn; n = 19) and low anxiety (LAn)-like behavior (n = 21) to assess forced swim test mobility strategies and chronic mild stress (CMS)-induced depression-like symptoms. We measured (1) weight, (2) fur piloerection, (3) sweet food consumption, (4) grooming behavior, and (5) circulating estradiol (E2). One month after CMS terminated and following a terminal forced swim test, brains were processed for immunohistochemistry targeting c-Fos and serotonin 1 A receptor (5-HT1AR) protein in the paraventricular nucleus (PVN) of the hypothalamus. HAn female rats showed increased anxiety-like behavior (i.e., lower open to closed arm ratios, increased closed arm entries), more swimming (i.e., mobility), and less floating (i.e., immobility) behavior in the forced swim test. Overall, HAn females weighed less than their LAn counterparts. After chronic mild stress, HAn lines displayed even greater mobility and consumed fewer Froot Loops™. Fur and grooming analyses indicated no significant differences in mean counts across experimental groups. One month after CMS, cycling E2 concentrations (pg/ml) did not differ between HAn and LAn animals. Elevated c-Fos and 5-HT1AR expression were observed in the PVN, where HAn CMS rats expressed the most c-Fos and 5-HT1AR immunoreactivity. In summary, outbred HAn rats show robust anxiety-like behavior, exhibit more mobility in the forced swim test, and are more sensitive to chronic mild stress-induced grooming and decline in palatable food ingestion.

Enriched Environment Attenuates Enhanced Trait Anxiety in Association with Normalization of Aberrant Neuro-Inflammatory Events

Neuroinflammation is discussed to play a role in specific subgroups of different psychiatric disorders, including anxiety disorders. We have previously shown that a mouse model of trait anxiety (HAB) displays enhanced microglial density and phagocytic activity in key regions of anxiety circuits compared to normal-anxiety controls (NAB). Using minocycline, we provided causal evidence that reducing microglial activation within the dentate gyrus (DG) attenuated enhanced anxiety in HABs. Besides pharmacological intervention, "positive environmental stimuli", which have the advantage of exerting no side-effects, have been shown to modulate inflammation-related markers in human beings. Therefore, we now investigated whether environmental enrichment (EE) would be sufficient to modulate upregulated neuroinflammation in high-anxiety HABs. We show for the first time that EE can indeed attenuate enhanced trait anxiety, even when presented as late as adulthood. We further found that EE-induced anxiolysis was associated with the attenuation of enhanced microglial density (using Iba-1 as the marker) in the DG and medial prefrontal cortex. Additionally, EE reduced Iba1 + CD68+ microglia density within the anterior DG. Hence, the successful attenuation of trait anxiety by EE was associated in part with the normalization of neuro-inflammatory imbalances. These results suggest that pharmacological and/or positive behavioral therapies triggering microglia-targeted anti-inflammatory effects could be promising as novel alternatives or complimentary anxiolytic therapeutic approaches in specific subgroups of individuals predisposed to trait anxiety.

Mitochondrial brain proteome acetylation levels and behavioural responsiveness to amphetamine are altered in mice lacking Sirt3

Post-translational modification of mitochondrial proteins represents one mechanism by which the functional activity of mitochondria can be regulated. In the brain, these modifications can influence the functional properties of different neural circuitries. Given that the sirtuin family member Sirt3 represents the primary protein deacetylase enzyme in mitochondria, we tested whether brain mitochondrial proteome acetylation would increase in male or female mice lacking Sirt3. Our results confirm that whole brain mitochondrial proteome acetylation levels are indeed elevated in both sexes of Sirt3-KO mice relative to controls. Consistently, we found the mitochondria of mouse embryonic fibroblast (MEF) cells derived from Sirt3-KO mice were smaller in size, and fewer in number than in wild-type MEFs, and that mitochondrial free calcium levels were elevated within the mitochondria of these cells. As protein acetylation can influence mitochondrial function, and changes in mitochondrial function have been linked to alterations in neural circuit function regulating motor activity and anxiety-like behavior, we tested whether Sirt3-deficient mice would display sensitized responsiveness to the stimulant amphetamine. Both male and female Sirt3-KO mice displayed hyper-locomotion and attenuated anxiety-like behavior in response to a dose of amphetamine that was insufficient to promote any behavioural responses in wild-type mice. Collectively, these results confirm that Sirt3 regulates mitochondrial proteome acetylation levels in brain tissue, and that the absence of Sirt3 increases the sensitivity of neural systems to amphetamine-induced behavioural responses.

Transgenerational inheritance of fetal alcohol effects on proopiomelanocortin gene expression and methylation, cortisol response to stress, and anxiety-like behaviors in offspring for three generations in rats: Evidence for male germline transmission

Previously it has been shown that fetal alcohol exposure increases the stress response partly due to lowering stress regulatory proopiomelanocortin (Pomc) gene expression in the hypothalamus via epigenetic mechanisms for multiple generations in mixed-breed rats. In this study we assess the induction of heritable epigenetic changes of Pomc-related variants by fetal alcohol exposure in isogenic Fischer 344 rats. Using transgenerational breeding models and fetal alcohol exposure procedures, we determined changes in hypothalamic Pomc gene expression and its methylation levels, plasma corticosterone hormone response to restraint stress, and anxiety-like behaviors using elevated plus maze tests in fetal alcohol-exposed offspring for multiple generations in isogenic Fischer rats. Fetal alcohol-exposed male and female rat offspring showed significant deficits in POMC neuronal functions with increased Pomc gene methylation and reduced expression. These changes in POMC neuronal functions were associated with increased plasma corticosterone response to restraint stress and increased anxiety-like behavior. These effects of fetal alcohol exposure persisted in the F1, F2, and F3 progeny of the male germline but not of the female germline. These data suggest that fetal alcohol exposure induces heritable changes in Pomc-related variants involving stress hyperresponsiveness and anxiety-like behaviors which perpetuate into subsequent generations through the male germline via epigenetic modifications.

CRF-receptor1 modulation of the dopamine projection to prelimbic cortex facilitates cognitive flexibility after acute and chronic stress

Stress reduces cognitive flexibility and dopamine D1 receptor-related activity in the prelimbic cortex (PL), effects hypothesized to depend on reduced corticotropic releasing factor receptor type 1 (CRFr1) regulation of dopamine neurons in the ventral tegmental area (VTA). We assessed this hypothesis in rats by examining the effect of chronic unpredictable restraint stress (CUS), mild acute stress, or their combination on cognitive flexibility, CRFr1 expression in the VTA and D1-related activity in PL. In Experiment 1, rats received either CUS or equivalent handling for 14 days before being trained to press two levers to earn distinct food outcomes. Initial learning was assessed using an outcome devaluation test after which cognitive flexibility was assessed by reversing the outcomes earned by the actions. Prior to each reversal training session, half the CUS and controls receiving acute stress with action-outcome updating assessed using a second devaluation test and CRFr1 expression in the VTA assessed using in-situ hybridisation. Although CUS did not itself affect action-outcome learning, its combination with acute stress blocked reversal learning and decreased VTA CRFr1 expression after acute shock. The relationship between these latter two effects was assessed in Experiment 2 by pharmacologically disconnecting the VTA and PL, unilaterally blocking neurons expressing CRFr1 in the VTA and D1 receptors in the contralateral PL during reversal learning after acute stress. Acute stress again blocked reversal learning but only in the group with VTA-PL disconnection, demonstrating that VTA CRFr1-induced facilitation of dopaminergic activity in the PL is necessary for maintaining cognitive flexibility after acute stress. [250].

•

Acute stress increased CRF receptor1 expression in the VTA.

•

Chronic stress attenuated the effect of acute stress on CRFr1 expression.

•

Chronic stress plus acute stress produced a loss of cognitive flexibility.

•

Blocking VTA CFRr1 and dopamine D1r in PL reduced cognitive flexibility following stress.

Long term effects of early life stress on HPA circuit in rodent models

Adaptation to environmental challenges represents a critical process for survival, requiring the complex integration of information derived from both external cues and internal signals regarding current conditions and previous experiences. The Hypothalamic-pituitary-adrenal axis plays a central role in this process inducing the activation of a neuroendocrine signaling cascade that affects the delicate balance of activity and cross-talk between areas that are involved in sensorial, emotional, and cognitive processing such as the hippocampus, amygdala, Prefrontal Cortex, Ventral Tegmental Area, and dorsal raphe. Early life stress, especially early critical experiences with caregivers, influences the functional and structural organization of these areas, affects these processes in a long-lasting manner and may result in long-term maladaptive and psychopathological outcomes, depending on the complex interaction between genetic and environmental factors. This review summarizes the results of studies that have modeled this early postnatal stress in rodents during the first 2 postnatal weeks, focusing on the long-term effects on molecular and structural alteration in brain areas involved in Hypothalamic-pituitary-adrenal axis function. Moreover, a brief investigation of epigenetic mechanisms and specific genetic targets mediating the long-term effects of these early environmental manipulations and at the basis of differential neurobiological and behavioral effects during adulthood is provided.

Sex-dependent effects of chronic variable stress on discrete corticotropin-releasing factor receptor 1 cell populations

Anxiety and depression are strikingly more prevalent in women compared with men. Dysregulation of corticotropin-releasing factor (CRF) binding to its cognate receptor (CRFR1) is thought to play a critical role in the etiology of these disorders. In the present study, we investigated whether there were sex differences in the effects of chronic variable stress (CVS) on CRFR1 cells using CRFR1-GFP reporter mice experiencing a 9-day CVS paradigm. Brains were collected from CVS and stress naïve female and male mice following exposure to the open field test. This CVS paradigm effectively increased anxiety-like behavior in female and male mice. In addition, we assessed changes in activation of CRFR1 cells (co-localization with c-Fos and phosphorylated CREB (pCREB)) in stress associated brain structures, including two sexually dimorphic CRFR1 cell groups in the anteroventral periventricular nucleus (AVPV/PeN; F>M) and paraventricular hypothalamus (PVN; M>F). CVS increased CRFR1-GFP cell number as well as the number of CRFR1/pCREB co-expressing cells in the female but not male AVPV/PeN. In the PVN, the number of CRFR1/pCREB co-expressing cells was overall greater in males regardless of treatment and CVS resulted in a male-specific reduction of CRFR1/c-Fos cells. In addition, CVS induced a female-specific reduction in CRFR1/c-Fos cells within the anteroventral bed nucleus of the stria terminalis and both sexes exhibited a reduction in CRFR1/c-Fos co-expressing cells following CVS within the ventral basolateral amygdala. Overall, these sex-specific effects of CVS on CRFR1 populations may have implications for sex differences in stress-induction of mood disorders.

Towards precision medicine in generalized anxiety disorder: Review of genetics and pharmaco(epi)genetics

Generalized anxiety disorder (GAD) is a prevalent and chronic mental disorder that elicits widespread functional impairment. Given the high degree of non-response/partial response among patients with GAD to available pharmacological treatments, there is a strong need for novel approaches that can optimize outcomes, and lead to medications that are safer and more effective. Although investigations have identified interesting targets predicting treatment response through pharmacogenetics (PGx), pharmaco-epigenetics, and neuroimaging methods, these studies are often solitary, not replicated, and carry several limitations. This review provides an overview of the current status of GAD genetics and PGx and presents potential strategies to improve treatment response by combining better phenotyping with PGx and improved analytical methods. These strategies carry the dual benefit of delivering data on biomarkers of treatment response as well as pointing to disease mechanisms through the biology of the markers associated with response. Overall, these efforts can serve to identify clinical, genetic, and epigenetic factors that can be incorporated into a pharmaco(epi)genetic test that may ultimately improve treatment response and reduce the socioeconomic burden of GAD.

Epigenetic Mechanisms Within the Cingulate Cortex Regulate Innate Anxiety-Like Behavior

BACKGROUND

Pathological anxiety originates from a complex interplay of genetic predisposition and environmental factors, acting via epigenetic mechanisms. Epigenetic processes that can counteract detrimental genetic risk towards innate high anxiety are not well characterized.

METHODS

We used female mouse lines of selectively bred high (HAB)- vs low (LAB)-innate anxiety-related behavior and performed select environmental and pharmacological manipulations to alter anxiety levels as well as brain-specific manipulations and immunohistochemistry to investigate neuronal mechanisms associated with alterations in anxiety-related behavior.

RESULTS

Inborn hyperanxiety of high anxiety-like phenotypes was effectively reduced by environmental enrichment exposure. c-Fos mapping revealed that hyperanxiety in high anxiety-like phenotypes was associated with blunted challenge-induced neuronal activation in the cingulate-cortex, which was normalized by environmental enrichment. Relating this finding with epigenetic modifications, we found that high anxiety-like phenotypes (compared with low-innate anxiety phenotypes) showed reduced acetylation in the hypoactivated cingulate-cortex neurons following a mild emotional challenge, which again was normalized by environmental enrichment. Paralleling the findings using environmental enrichment, systemic administration of histone-deacetylase-inhibitor MS-275 elicited an anxiolytic-like effect, which was correlated with increased acetylated-histone-3 levels within cingulate-cortex. Finally, as a proof-of-principle, local MS-275 injection into cingulate-cortex rescued enhanced innate anxiety and increased acetylated-histone-3 within the cingulate-cortex, suggesting this epigenetic mark as a biomarker for treatment success.

CONCLUSIONS

Taken together, the present findings provide the first causal evidence that the attenuation of high innate anxiety-like behavior via environmental/pharmacological manipulations is epigenetically mediated via acetylation changes within the cingulate-cortex. Finally, histone-3 specific histone-deacetylase-inhibitor could be of therapeutic importance in anxiety disorders.

Postnatal impoverished housing impairs adolescent risk-assessment and increases risk-taking: A sex-specific effect associated with histone epigenetic regulation of Crfr1 in the medial prefrontal cortex

While increasing evidence posits poor decision-making as a central feature of mental disorders, very few studies investigated the effects of early-life stress (ELS) on specific components of reward-related choice behaviors. Risk-taking (RT) involves the exposure to some danger, or negative consequences, in order to achieve a goal-directed behavior. Such behaviors are likely to be preceded by risk-assessment (RA), which is a dynamic cognitive process involving the acquisition of information in potentially dangerous situations. Here, we investigated the effects of being raised in impoverished housing conditions during early life (P2-P9) on RT, RA and dopaminergic and corticotrophinergic gene expression of adolescent male and female mice. Phenotypes were assessed by two protocols: the elevated plus-maze (EPM) and the predator-odor risk-taking (PORT). We found decreased RA in mice exposed to impoverished housing in the absence of a reward (EPM), with a more pronounced effect among females. Moreover, when exposed to a predatory olfactory cue, increased RT was observed in these females in a reward-related task (PORT), as well as decreased HPA axis responsivity. This sex-specific behavioral effect was associated with increased Crfr1 mRNA expression in the medial prefrontal cortex (mPFC) and higher levels of the histone mark H3R2^me2s, a histone modification known to be involved in transcriptional activation, within the promoter of the Crfr1 gene. These findings revealed that ELS exposure can impair the acquisition of environmental information in dangerous situations and increase RT in reward-related scenarios among females, with an important role regarding epigenetic regulation of the Crfr1 gene.

Epigenetic Inheritance: Concepts, Mechanisms and Perspectives

Parents’ stressful experiences can influence an offspring’s vulnerability to many pathological conditions, including psychopathologies, and their effects may even endure for several generations. Nevertheless, the cause of this phenomenon has not been determined, and only recently have scientists turned to epigenetics to answer this question. There is extensive literature on epigenetics, but no consensus exists with regard to how and what can (and must) be considered to study and define epigenetics processes and their inheritance. In this work, we aimed to clarify and systematize these concepts. To this end, we analyzed the dynamics of epigenetic changes over time in detail and defined three types of epigenetics: a direct form of epigenetics (DE) and two indirect epigenetic processes—within (WIE) and across (AIE). DE refers to changes that occur in the lifespan of an individual, due to direct experiences with his environment. WIE concerns changes that occur inside of the womb, due to events during gestation. Finally, AIE defines changes that affect the individual’s predecessors (parents, grandparents, etc.), due to events that occur even long before conception and that are somehow (e.g., through gametes, the intrauterine environment setting) transmitted across generations. This distinction allows us to organize the main body of epigenetic evidence according to these categories and then focus on the latter (AIE), referring to it as a faster route of informational transmission across generations—compared with genetic inheritance—that guides human evolution in a Lamarckian (i.e., experience-dependent) manner. Of the molecular processes that are implicated in this phenomenon, well-known (methylation) and novel (non-coding RNA, ncRNA) regulatory mechanisms are converging. Our discussion of the chief methods that are used to study epigenetic inheritance highlights the most compelling technical and theoretical problems of this discipline. Experimental suggestions to expand this field are provided, and their practical and ethical implications are discussed extensively.

In Vivo and In Vitro Neuronal Plasticity Modulation by Epigenetic Regulators

Prenatal stress (PS) induces molecular changes that alter neural connectivity, increasing the risk for neuropsychiatric disorders. Here we analyzed -in the hippocampus of adult rats exposed to PS- the epigenetic signature mediating the PS-induced neuroplasticity changes. Furthermore, using cultured hippocampal neurons, we investigated the effects on neuroplasticity of an epigenetic modulator. PS induced significant modifications in the mRNA levels of stress-related transcription factor MEF2A, SUV39H1 histone methyltransferase, and TET1 hydroxylase, indicating that PS modifies gene expression through chromatin remodeling. In in vitro analysis, histone acetylation inhibition with apicidin increased filopodium density, suggesting that the external regulation of acetylation levels might modulate neuronal morphology. These results offer a way to enhance neural connectivity that could be considered to revert PS effects.

Regulation of anxiety-like behavior and Crhr1 expression in the basolateral amygdala by LMO3

The LIM domain only protein LMO3 is a transcriptional regulator that has been shown to regulate several behavioral responses to alcohol. Specifically, Lmo3 null (Lmo3^Z) mice consume more ethanol in a binge-drinking test and show enhanced ethanol-induced sedation. Due to the high comorbidity of alcohol use and anxiety, we investigated anxiety-like behavior in Lmo3^Z mice. Lmo3^Z mice spent more time in the open arms of the elevated plus maze compared with their wild-type littermates, but the effect was confounded by reduced locomotor activity. To verify the anxiety phenotype in the Lmo3^Z mice, we tested them for novelty-induced hypophagia and found that they also showed reduced anxiety in this test. We next explored the mechanism by which LMO3 might regulate anxiety by measuring mRNA and protein levels of corticotropin releasing factor (encoded by the Crh gene) and its receptor type 1 (Crhr1) in Lmo3^Z mice. Reduced Crhr1 mRNA and protein was evident in the basolateral amygdala (BLA) of Lmo3^Z mice. To examine whether Lmo3 in the amygdala is important for anxiety-like behavior, we locally reduced Lmo3 expression in the BLA of wild type mice using a lentiviral vector expressing a short hairpin RNA targeting the Lmo3 transcript. Mice with Lmo3 knockdown in the BLA exhibited decreased anxiety-like behavior relative to control mice. These results suggest that Lmo3 promotes anxiety-like behavior specifically in the BLA, possibly by altering Crhr1 expression. This study is the first to support a role for Lmo3 in anxiety-like behavior.

Epigenetics applied to psychiatry: Clinical opportunities and future challenges

Psychiatric disorders are clinically heterogeneous and debilitating chronic diseases resulting from a complex interplay between gene variants and environmental factors. Epigenetic processes, such as DNA methylation and histone posttranslational modifications, instruct the cell/tissue to correctly interpret external signals and adjust its functions accordingly. Given that epigenetic modifications are sensitive to environment, stable, and reversible, epigenetic studies in psychiatry could represent a promising approach to better understanding and treating disease. In the present review, we aim to discuss the clinical opportunities and challenges arising from the epigenetic research in psychiatry. Using selected examples, we first recapitulate key findings supporting the role of adverse life events, alone or in combination with genetic risk, in epigenetic programming of neuropsychiatric systems. Epigenetic studies further report encouraging findings about the use of methylation changes as diagnostic markers of disease phenotype and predictive tools of progression and response to treatment. Then we discuss the potential of using targeted epigenetic pharmacotherapy, combined with psychosocial interventions, for future personalized medicine for patients. Finally, we review the methodological limitations that could hinder interpretation of epigenetic data in psychiatry. They mainly arise from heterogeneity at the individual and tissue level and require future strategies in order to reinforce the biological relevance of epigenetic data and its translational use in psychiatry. Overall, we suggest that epigenetics could provide new insights into a more comprehensive interpretation of mental illness and might eventually improve the nosology, treatment, and prevention of psychiatric disorders.

Polymorphism in Tmem132d regulates expression and anxiety-related behavior through binding of RNA polymerase II complex

TMEM132D is a candidate gene, where risk genotypes have been associated with anxiety severity along with higher mRNA expression in the frontal cortex of panic disorder patients. Concurrently, in a high (HAB) and low (LAB) trait anxiety mouse model, Tmem132d was found to show increased expression in the anterior cingulate cortex (aCC) of HAB as compared to LAB mice. To understand the molecular underpinnings underlying the differential expression, we sequenced the gene and found two single-nucleotide polymorphisms (SNPs) in the promoter differing between both lines which could explain the observed mRNA expression profiles using gene reporter assays. In addition, there was no difference in basal DNA methylation in the CpG Island that encompasses the HAB vs. LAB Tmem132d promoter region. Furthermore, we found significantly higher binding of RNA polymerase II (POLR2A) to the proximal HAB-specific SNP (rs233264624) than the corresponding LAB locus in an oligonucleotide pull-down assay, suggesting increased transcription. Virus mediated overexpression of Tmem132d in the aCC of C57BL/6 J mice could confirm its role in mediating an anxiogenic phenotype. To model gene-environmental interactions, HAB mice exposed to enriched environment (HAB-EE) responded with decreased anxiety levels but, had enhanced Tmem132d mRNA expression as compared to standard-housed HAB (HAB-SH) mice. While LAB mice subjected to unpredictable chronic mild stress (LAB-UCMS) exhibited higher anxiety levels and had lower mRNA expression compared to standard-housed LAB (LAB-SH) mice. Chromatin immunoprecipitation revealed significantly higher binding of POLR2A to rs233264624 in HAB-EE, while LAB-UCMS had lower POLR2A binding at this locus, thus explaining the enhanced or attenuated expression of Tmem132d compared to their respective SH controls. To further investigate gene-environment interactions, DNA methylation was assessed using Illumina 450 K BeadChip in 74 panic disorder patients. Significant methylation differences were observed in two CpGs (cg26322591 and cg03283235) located in TMEM132D depending on the number of positive life events supporting the results of an influence of positive environmental cues on regulation of Tmem132d expression in mice.

Evaluation of Alcohol Preference and Drinking in msP Rats Bearing a Crhr1 Promoter Polymorphism

Alcoholism is a pervasive societal problem, yet available pharmacotherapies fail to treat most sufferers. The type 1 corticotropin-releasing factor (CRF₁) receptor has received much attention for its putative role in the progression to alcohol dependence, although at present its success in clinical trials has been limited. Two single-nucleotide polymorphisms in the rat Crhr1 promoter have been identified in the Marchigian substrain of Sardinian alcohol-preferring (msP) rats. Unlike other Wistar-derived alcohol-preferring lines, nondependent msP rats reduce their alcohol self-administration in response to CRF₁ antagonists and show increased brain CRF₁ expression. The current study tested the hypotheses that the A alleles in the Crhr1 promoter polymorphisms are: (1) unique to msP (vs. CRF₁ antagonist-insensitive) alcohol-preferring lines and (2) associate with greater alcohol preference or intake. Two related polymorphisms were observed in which both loci on a given chromosome were either mutant variant (A) or wild-type (G) alleles within the distal Crhr1 promoter of 17/25 msP rats (68%), as compared to 0/23 Indiana P rats, 0/20 Sardinian alcohol-preferring rats bred at Scripps (Scr:sP) and 0/21 outbred Wistar rats. Alcohol consumption in msP rats did not differ according to the presence of Crhr1 A alleles, but greater alcohol preference (98%) was observed in A allele homozygous msP rats (AA) compared to msP rats with wild-type (GG, 91%) or heterozygous (GA, 91%) genotypes. The greater alcohol preference reflected decreased water intake, accompanied by reduced total calories consumed by AA rats. The data show that msP rats differentially possess mutant A variant alleles in the polymorphic promoter region of the Crhr1 gene that may differentially regulate consumption.

Dopamine and Stress System Modulation of Sex Differences in Decision Making

Maladaptive decision making is associated with several neuropsychiatric disorders, including problem gambling and suicidal behavior. The prevalence of these disorders is higher in men vs women, suggesting gender-dependent regulation of their pathophysiology underpinnings. We assessed sex differences in decision making using the rat version of the Iowa gambling task. Female rats identified the most optimal choice from session 1, whereas male rats from session 5. Male, but not female rats, progressively improved their advantageous option responding and surpassed females. Estrus cycle phase did not affect decision making. To test whether pharmacological manipulations targeting the dopaminergic and stress systems affect decision making in a sex-dependent manner, male and female rats received injections of a dopamine D₂ receptor (D₂R) antagonist (eticlopride), D₂R agonist (quinpirole), corticotropin-releasing factor 1 (CRF₁) antagonist (antalarmin), and α₂-adrenergic receptor antagonist (yohimbine; used as a pharmacological stressor). Alterations in mRNA levels of D₂R and CRF₁ were also assessed. Eticlopride decreased advantageous responding in male, but not female rats, whereas quinpirole decreased advantageous responding specifically in females. Yohimbine dose-dependently decreased advantageous responding in female rats, whereas decreased advantageous responding was only observed at higher doses in males. Antalarmin increased optimal choice responding only in female rats. Higher Drd2 and Crhr1 expression in the amygdala were observed in female vs male rats. Higher amygdalar Crhr1 expression was negatively correlated with advantageous responding specifically in females. This study demonstrates the relevance of dopaminergic- and stress-dependent sex differences to maladaptive decision making.

The integration of multiple signaling pathways provides for bidirectional control of CRHR1 gene transcription in rat pituitary cell during hypoxia

Hypoxia upregulates hypothalamic corticotrophin releasing hormone (CRH) and its receptor type-1 (CRHR1) expression and activates the HPA axis and induces hypoxic sickness and behavioral change. The transcriptional mechanism by which hypoxia differently regulates CRHR1 expression remains unclear. Here we report hypoxia time-dependently induced biphasic expression of CRHR1mRNA in rat pituitary during different physiological status. Short exposure of gestational dams to hypoxia reduced CRHR1mRNA in the pituitary of P1-P14 male rat offspring. A short- and prolonged-hypoxia evoked biphasic response of CRHR1mRNA characterized initially by decreases and subsequently by persistent increases, mediated by a rapid negative feedback via CRHR1 signaling and positive transcriptional control via NF-κB, respectively. Further analysis of CRHR1 promoter in cultured primary anterior pituitary and AtT20 cells showed that c-Jun/AP-1 delivered negative while HIF-1α and NF-κB delivered positive control of transcription at CRHR1 promoter. The negative and positive inputs are integrated by hypoxic initiation and duration in CRHR1 transcription.

Epigenetic programming of the neuroendocrine stress response by adult life stress

The hypothalamic-pituitary-adrenal (HPA) axis is critically involved in the neuroendocrine regulation of stress adaptation, and the restoration of homeostasis following stress exposure. Dysregulation of this axis is associated with stress-related pathologies like major depressive disorder, post-traumatic stress disorder, panic disorder and chronic anxiety. It has long been understood that stress during early life can have a significant lasting influence on the development of the neuroendocrine system and its neural regulators, partially by modifying epigenetic regulation of gene expression, with implications for health and well-being in later life. Evidence is accumulating that epigenetic plasticity also extends to adulthood, proposing it as a mechanism by which psychological trauma later in life can long-lastingly affect HPA axis function, brain plasticity, neuronal function and behavioural adaptation to neuropsychological stress. Further corroborating this claim is the phenomenon that these epigenetic changes correlate with the behavioural consequences of trauma exposure. Thereby, epigenetic modifications provide a putative molecular mechanism by which the behavioural phenotype and transcriptional/translational potential of genes involved in HPA axis regulation can change drastically in response to environmental challenges, and appear an important target for treatment of stress-related disorders. However, improved insight is required to increase their therapeutic (drug) potential. Here, we provide an overview of the growing body of literature describing the epigenetic modulation of the (primarily neuroendocrine) stress response as a consequence of adult life stress and interpret the implications for, and the challenges involved in applying this knowledge to, the identification and treatment of stress-related psychiatric disorders.

DNA methylation and expression of stress related genes in PBMC of MDD patients with and without serious suicidal ideation

Stress plays an important role in major depressive disorder (MDD) and is one of the state dependent factors in suicidal behavior. A dysfunctional hypothalamic-pituitary-adrenal axis is a common feature in this disorder. The involvement of environmental factors has added additional complexity to understanding depression or suicidal behavior. In this regard, epigenetic regulation has been considered a mechanistic interface between environmental stress stimuli and altered functioning of underlying gene network that may increase susceptibility to depression or suicidal behavior. The present study examined whether epigenetic modifications of stress related genes are associated with MDD and whether there are differences in these epigenetic marks between depressed individuals with and without serious suicidal ideation. Using MeDIP analysis in genomic DNA isolated from peripheral blood mononuclear cells (PBMC) of healthy controls (n = 20), MDD patients with (n = 14) or without serious suicidal ideation (n = 10), we studied methylation of the stress-associated genes, Brain Derived Neurotrophic Factor (BDNF), Nuclear Receptor Subfamily 3 Group C Member 1 (NR3C1), FK506 Binding Protein 5 (FKBP5), Corticotropin Releasing Hormone Binding Protein (CRHBP), and Corticotropin Releasing Hormone Receptor 1 (CRHR1). In addition, we determined their transcript levels in RNAs isolated from the same PBMC. We found that BDNF, FKBP5, CRHBP, and NR3C1 gene promoters were significantly hypermethylated in MDD patients with and without suicidal ideation. We also found concomitant reductions in expression of BDNF, FKBP5 transcript variants (1, 2 and 3), and NR3C1 genes in these patients, suggesting that promoter hypermethylation in these genes may functionally be associated with their observed downregulation in MDD patients. In a secondary analysis, methylation of these genes was compared between MDD patients with or without serious suicidal ideation and controls. The MDD with serious suicidal ideation were significantly different from controls while the MDD without were not, although MDD with or without suicidal ideation were not different from each other, likely owning to a relatively small sample size. Thus, our findings underline the importance of epigenetic modifications of stress-associated genes in depression and, possibly, suicidal behavior, which, in future, needs to be confirmed in a larger patient population.

CRHR1 promoter hypomethylation: An epigenetic readout of panic disorder?

The corticotropin releasing hormone receptor 1 (CRHR1) is crucially involved in the hypothalamic-pituitary-adrenal axis and thus a major regulator of the stress response. CRHR1 gene variation is associated with several mental disorders including anxiety disorders. Studies in rodents have demonstrated epigenetic regulation of CRHR1 gene expression to moderate response to stressful environment. In the present study, we investigated CRHR1 promoter methylation for the first time regarding its role in panic disorder applying a case-control approach (N=131 patients, N=131 controls). In an independent sample of healthy volunteers (N=255), CRHR1 methylation was additionally analyzed for association with the Beck Anxiety Inventory (BAI) score as a dimensional panic-related intermediate phenotype. The functional relevance of altered CRHR1 promoter methylation was investigated by means of luciferase-based reporter gene assays. In panic disorder patients, a significantly decreased CRHR1 methylation was discerned (p<0.001). Accordingly, healthy controls with high BAI scores showed significantly decreased CRHR1 methylation. Functional analyses revealed an increased gene expression in presence of unmethylated as compared to methylated pCpGl_CRHR1 reporter gene vectors. The present study identified a potential role of CRHR1 hypomethylation - conferring increased CRHR1 expression - in panic disorder and a related dimensional intermediate phenotype. This up-regulation of CRHR1 gene expression driven by de-methylation might constitute a link between the stress response and panic disorder risk.

Modeling Trait Anxiety: From Computational Processes to Personality

Computational methods are increasingly being applied to the study of psychiatric disorders. Often, this involves fitting models to the behavior of individuals with subclinical character traits that are known vulnerability factors for the development of psychiatric conditions. Anxiety disorders can be examined with reference to the behavior of individuals high in "trait" anxiety, which is a known vulnerability factor for the development of anxiety and mood disorders. However, it is not clear how this self-report measure relates to neural and behavioral processes captured by computational models. This paper reviews emerging computational approaches to the study of trait anxiety, specifying how interacting processes susceptible to analysis using computational models could drive a tendency to experience frequent anxious states and promote vulnerability to the development of clinical disorders. Existing computational studies are described in the light of this perspective and appropriate targets for future studies are discussed.

Preconception Alcohol Increases Offspring Vulnerability to Stress

The effect of preconception drinking by the mother on the life-long health outcomes of her children is not known, and therefore, in this study using an animal model, we determined the impact of preconception alcohol drinking of the mother on offspring stress response during adulthood. In our preconception alcohol exposure model, adult female rats were fed with 6.7% alcohol in their diet for 4 weeks, went without alcohol for 3 weeks and were bred to generate male and female offspring. Preconception alcohol-exposed offsprings' birth weight, body growth, stress response, anxiety-like behaviors, and changes in stress regulatory gene and protein hormone levels were evaluated. In addition, roles of epigenetic mechanisms in preconception alcohol effects were determined. Alcohol feeding three weeks prior to conception significantly affected pregnancy outcomes of female rats, with respect to delivery period and birth weight of offspring, without affecting maternal care behaviors. Preconception alcohol negatively affected offspring adult health, producing an increased stress hormone response to an immune challenge. In addition, preconception alcohol was associated with changes in expression and methylation profiles of stress regulatory genes in various brain areas. These changes in stress regulatory genes were normalized following treatment with a DNA methylation blocker during the postnatal period. These data highlight the novel possibility that preconception alcohol affects the inheritance of stress-related diseases possibly by epigenetic mechanisms.

Maternal stress and diet may influence affective behavior and stress-response in offspring via epigenetic regulation of central peptidergic function

It has been shown that maternal stress and malnutrition, or experience of other adverse events, during the perinatal period may alter susceptibility in the adult offspring in a time-of-exposure dependent manner. The mechanism underlying this may be epigenetic in nature. Here, we summarize some recent findings on the effects on gene-regulation following maternal malnutrition, focusing on epigenetic regulation of peptidergic activity. Numerous neuropeptides within the central nervous system are crucial components in regulation of homeostatic energy-balance, as well as affective health (i.e. health events related to affective disorders, psychiatric disorders also referred to as mood disorders). It is becoming evident that expression, and function, of these neuropeptides can be regulated via epigenetic mechanisms during fetal development, thereby contributing to the development of the adult phenotype and, possibly, modulating disease susceptibility. Here, we focus on two such neuropeptides, neuropeptide Y (NPY) and corticotropin-releasing hormone (CRH), both involved in regulation of endocrine function, energy homeostasis, as well as affective health. While a number of published studies indicate the involvement of epigenetic mechanisms in CRH-dependent regulation of the offspring adult phenotype, NPY has been much less studied in this context and needs further work.

Reduced Anxiety-Like Behavior and Altered Hippocampal Morphology in Female p75NTR(exon IV-/-) Mice

The presence of the p75 neurotrophin receptor (p75NTR) in adult basal forebrain cholinergic neurons, precursor cells in the subventricular cell layer and the subgranular cell layer of the hippocampus has been linked to alterations in learning as well as anxiety- and depression- related behaviors. In contrast to previous studies performed in a p75NTR^{exon III−/−} model still expressing the short isoform of the p75NTR, we focused on locomotor and anxiety–associated behavior in p75NTR^{exon IV−/−} mice lacking both p75NTR isoforms. Comparing p75NTR^{exon IV−/−} and wildtype mice for both male and female animals showed an anxiolytic-like behavior as evidenced by increased central activities in the open field paradigm and flex field activity system as well as higher numbers of open arm entries in the elevated plus maze test in female p75NTR knockout mice. Morphometrical analyses of dorsal and ventral hippocampus revealed a reduction of width of the dentate gyrus and the granular cell layer in the dorsal but not ventral hippocampus in male and female p75NTR^{exon IV−/−} mice. We conclude that germ-line deletion of p75NTR seems to differentially affect morphometry of dorsal and ventral dentate gyrus and that p75NTR may play a role in anxiety-like behavior, specifically in female mice.

Don't worry; be informed about the epigenetics of anxiety

Epigenetic processes regulate gene expression independent of the DNA sequence and are increasingly being investigated as contributors to the development of behavioral disorders. Environmental insults, such as stress, diet, or toxin exposure, can affect epigenetic mechanisms, including chromatin remodeling, DNA methylation, and non-coding RNAs that, in turn, alter the organism's phenotype. In this review, we examine the literature, derived at both the preclinical (animal) and clinical (human) levels, on epigenetic alterations associated with anxiety disorders. Using animal models of anxiety, researchers have identified epigenetic changes in several limbic and cortical brain regions known to be involved in stress and emotion responses. Environmental manipulations have been imposed prior to conception, during prenatal or early postnatal periods, and at juvenile and adult ages. Time of perturbation differentially affects the epigenome and many changes are brain region-specific. Although some sex-dependent effects are reported in animal studies, more research employing both sexes is needed particularly given that females exhibit a disproportionate number of anxiety disorders. The human literature is in its infancy but does reveal some epigenetic associations with anxiety behaviors and disorders. In particular, effects in monoaminergic systems are seen in line with evidence from etiological and treatment research. Further, there is evidence that epigenetic changes may be inherited to affect subsequent generations. We speculate on how epigenetic processes may interact with genetic contributions to inform prevention and treatment strategies for those who are at risk for or have anxiety disorders.

Cross-generational influences on childhood anxiety disorders: pathways and mechanisms

Anxiety disorders are common across the lifespan, cause severe distress and impairment, and usually have their onset in childhood. Substantial clinical and epidemiological research has demonstrated the existence of links between anxiety and its disorders in children and parents. Research on the pathways and mechanisms underlying these links has pointed to both behavioral and biological systems. This review synthesizes and summarizes several major aspects of this research. Behavioral systems include vicarious learning, social referencing, and modeling of parental anxiety; overly protective or critical parenting styles; and aspects of parental responses to child anxiety including family accommodation of the child's symptoms. Biological systems include aspects of the prenatal environment affected by maternal anxiety, development and functioning of the oxytocinergic system, and genetic and epigenetic transmission. Implications for the prevention and treatment of child anxiety disorders are discussed, including the potential to enhance child anxiety treatment outcomes through biologically informed parent-based interventions.

Epigenetic regulation of G protein coupled receptor signaling and its implications in psychiatric disorders

G protein-coupled receptors (GPCRs) act as a relay center through which extracellular signals, in the form of neurotransmitters or therapeutics, are converted into an intracellular response, which ultimately shapes the overall response at the tissue and behavioral level. Remarkably in similar ways, epigenetic mechanisms also modulate the expression pattern of a large number of genes in response to the dynamic environment inside and outside of the body, and consequently overall response. Emerging evidences from the pharmacogenomics and preclinical studies clearly suggest that these two distinct mechanisms criss-cross each other in several neurological disorders. At one hand such cross-talks between two distinct mechanisms make disease etiology more challenging to understand, while on the other hand if dealt appropriately, such situations might provide an opportunity to find novel druggable target and strategy for the treatment of complex diseases. In this review article, we have summarized and highlighted the main findings that tie epigenetic mechanisms to GPCR mediated signaling in the pathophysiology of central nervous system (CNS) disorders, including depression, addiction and pain.

Activation of corticotropin releasing factor-containing neurons in the rat central amygdala and bed nucleus of the stria terminalis following exposure to two different anxiogenic stressors

Rats exposed to the odor of a predator or to the elevated plus maze (EPM) express unique unconditioned fear behaviors. The extended amygdala has previously been demonstrated to mediate the response to both predator odor and the EPM. We seek to determine if divergent amygdalar microcircuits are associated with the different behavioral responses. The current experiments compared activation of corticotropin-releasing factor (CRF)-containing neuronal populations in the central amygdala and bed nucleus of the stria terminalis (BNST) of rats exposed to either the EPM (5 min) versus home cage controls, or predator (ferret) odor versus butyric acid, or no odor (30 min). Sections of the brains were prepared for dual-labeled immunohistochemistry and counts of c-Fos co-localized with CRF were made in the centrolateral and centromedial amygdala (CLA and CMA) as well as the dorsolateral (dl), dorsomedial (dm), and ventral (v) BNST. Ferret odor-exposed rats displayed an increase in duration and a decrease in latency of defensive burying versus control rats. Exposure to both predator stress and EPM induced neuronal activation in the BNST, but not the central amygdala, and similar levels of neuronal activation were seen in both the high and low anxiety groups in the BNST after EPM exposure. Dual-labeled immunohistochemistry showed a significant increase in the percentage of CRF/c-Fos co-localization in the vBNST of ferret odor-exposed rats compared to control and butyric acid-exposed groups as well as EPM-exposed rats compared to home cage controls. In addition, an increase in the percentage of CRF-containing neurons co-localized with c-Fos was observed in the dmBNST after EPM exposure. No changes in co-localization of CRF with c-Fos was observed with these treatments in either the CLA or CMA. These results suggest that predator odor and EPM exposure activates CRF neurons in the BNST to a much greater extent than CRF neurons of the central amygdala, and indicates unconditioned anxiogenic stimuli may activate unique anatomical circuits in the extended amygdala.

Plasticity as a developing trait: exploring the implications

Individual differences in plasticity have been classically framed as genotype-by-environment interactions, with different genotypes showing different reaction norms in response to environmental conditions. However, research has shown that early experience can be a critical factor in shaping an individual's plasticity to later environmental factors. In other words, plasticity itself can be investigated as a developing trait that reflects the combined action of an individual's genes and previous interactions with the environment. In this paper I explore some implications of the idea that the early environment modulates long-term plasticity, with an emphasis on plasticity in behavioral traits. I begin by focusing on the mechanisms that mediate plasticity at the proximate level, and discussing the possibility that some traits may work as generalized mediators of plasticity by affecting the sensitivity of multiple phenol types across developmental contexts. I then tackle the complex problem of the evolution of reaction norms for plasticity. Next, I consider a number of potential implications for research on parental effects and phenotypic matching, and conclude by discussing how plasticity may become a target of evolutionary conflict between parents and offspring. In total, I aim to show how the idea of plasticity as a developing trait offers a rich source of questions and insights that may inform future research in this area.

Genetics of anxiety disorders: Genetic epidemiological and molecular studies in humans

This review provides a broad overview of the state of research in the genetics of anxiety disorders (AD). Genetic epidemiological studies report a moderate level of familial aggregation (odds ratio: 4-6) and heritability estimates are about 30-50%. Twin studies suggest that the genetic architecture of AD is not isomorphic with their classifications, sharing risk factors with each other. So far, linkage and association studies of AD have produced inconclusive results. Genome-wide association studies of AD can provide an unbiased survey of common genetic variations across the entire genome. Given the shared causes of AD that transcend our current diagnostic classifications, clustering anxiety phenotypes into broader groups may be a powerful approach to identifying susceptibility locus for AD. Using such a shared genetic risk factor, meta-analyses of genome-wide association studies of AD conducted by large consortia are needed. Environmental factors also make a substantial contribution to the cause of AD. Although candidate gene studies of gene by environmental (G × E) interaction have appeared recently, no genome-wide search for G × E interactions have been performed. Epigenetic modification of DNA appears to have important effects on gene expression mediating environmental influences on disease risk. Given that G × E can be linked to an epigenetic modification, a combination analysis of genome-wide G × E interaction and methylation could be an alternative method to find risk variants for AD. This genetic research will enable us to utilize more effective strategies for the prevention and treatment of AD in the near future.

Chronic stress and brain plasticity: Mechanisms underlying adaptive and maladaptive changes and implications for stress-related CNS disorders

Stress responses entail neuroendocrine, autonomic, and behavioral changes to promote effective coping with real or perceived threats to one's safety. While these responses are critical for the survival of the individual, adverse effects of repeated exposure to stress are widely known to have deleterious effects on health. Thus, a considerable effort in the search for treatments to stress-related CNS disorders necessitates unraveling the brain mechanisms responsible for adaptation under acute conditions and their perturbations following chronic stress exposure. This paper is based upon a symposium from the 2014 International Behavioral Neuroscience Meeting, summarizing some recent advances in understanding the effects of stress on adaptive and maladaptive responses subserved by limbic forebrain networks. An important theme highlighted in this review is that the same networks mediating neuroendocrine, autonomic, and behavioral processes during adaptive coping also comprise targets of the effects of repeated stress exposure in the development of maladaptive states. Where possible, reference is made to the similarity of neurobiological substrates and effects observed following repeated exposure to stress in laboratory animals and the clinical features of stress-related disorders in humans.

Blunted HPA axis reactivity reveals glucocorticoid system dysbalance in a mouse model of high anxiety-related behavior

Depression and anxiety disorders are often characterized by altered hypothalamic-pituitary-adrenal (HPA) axis re-/activity. However, the presence of a molecular link between dysbalanced neuroendocrine regulation and psychopathologies is not yet fully established. Earlier, we reported that high (HAB), normal (NAB) and low (LAB) anxiety-related behavior mice express divergent anxiety-related and passive/active coping phenotypes. Here, we studied mechanisms that might contribute to the different HPA axis reactivity observed in HAB, NAB and LAB mice and their involvement in the regulation of anxiety-related behavior and passive/active coping style. We found that HAB mice respond with significantly reduced corticosterone (CORT) secretion to an acute stressful stimulus and a blunted response in the Dex/CRH test compared to NAB and LAB mice. At the molecular level, higher expression of the glucocorticoid receptor (GR/Nr3c1) and decreased corticotropin-releasing hormone receptor 1 (CRHR1) expression were observed in the pituitary of HAB mice. We further analyzed whether these stress mediators differed between the HAB, NAB and LAB lines in limbic system-associated brain regions and whether their interplay contributes to the phenotype. Interestingly, not only in the pituitary but also in almost all brain regions investigated, GR expression was significantly higher in HAB mice. In contrast, the amount of CORT in the brain structures analyzed was significantly lower in these animals. The expression of CRHR1 varied in the prefrontal cortex only. Since glucocorticoids regulate both GR and CRHR1, we treated HAB and NAB mice chronically with CORT. After 6 weeks of administration, reduced anxiety- and depression-like behaviors were observed in HAB mice, whereas increased anxiety was found in NABs. In both groups, GR, but not CRHR1, were significantly reduced. Taken together, our study proposes HAB mice as an animal model of simultaneous features of increased anxiety-related and depression-like behaviors with blunted HPA axis reactivity suggesting a dysregulated GR/CORT system as one key mechanism behind their phenotype.

Early-life stress interactions with the epigenome: potential mechanisms driving vulnerability toward psychiatric illness

Throughout the 20th century a body of literature concerning the long-lasting effects of the early environment was produced. Adverse experiences in early life, or early-life stress (ELS), is associated with a higher risk of developing various psychiatric illnesses. The mechanisms driving the complex interplay between ELS and adult phenotype has baffled many investigators for decades. Over the last decade, the new field of neuroepigenetics has emerged as one possible mechanism by which ELS can have far-reaching effects on adult phenotype, behavior, and risk for psychiatric illness. Here we review two commonly investigated epigenetic mechanisms, histone modifications and DNA methylation, and the emerging field of neuroepigenetics as they relate to ELS. We discuss the current animal literature demonstrating ELS-induced epigenetic modulation of gene expression that results in altered adult phenotypes. We also briefly discuss other areas in which neuroepigenetics has emerged as a potential mechanism underlying environmental and genetic interactions.

Environmental manipulations generate bidirectional shifts in both behavior and gene regulation in a crossbred mouse model of extremes in trait anxiety

Although gene-environment interactions are known to significantly influence psychopathology-related disease states, only few animal models cover both the genetic background and environmental manipulations. Therefore, we have taken advantage of the bidirectionally inbred high (HAB) and low (LAB) anxiety-related behavior mouse lines to generate HAB × LAB F1 hybrids that intrinsically carry both lines’ genetic characteristics, and subsequently raised them in three different environments—standard, enriched (EE) and chronic mild stress (CMS). Assessing genetic correlates of trait anxiety, we focused on two genes already known to play a role in HAB vs. LAB mice, corticotropin releasing hormone receptor type 1 (Crhr1) and high mobility group nucleosomal binding domain 3 (Hmgn3). While EE F1 mice showed decreased anxiety-related and increased explorative behaviors compared to controls, CMS sparked effects in the opposite direction. However, environmental treatments affected the expression of the two genes in distinct ways. Thus, while expression ratios of Hmgn3 between the HAB- and LAB-specific alleles remained equal, total expression resembled the one observed in HAB vs. LAB mice, i.e., decreased after EE and increased after CMS treatment. On the other hand, while total expression of Crhr1 remained unchanged between the groups, the relative expression of HAB- and LAB-specific alleles showed a clear effect following the environmental modifications. Thus, the environmentally driven bidirectional shift of trait anxiety in this F1 model strongly correlated with Hmgn3 expression, irrespective of allele-specific expression patterns that retained the proportions of basic differential HAB vs. LAB expression, making this gene a match for environment-induced modifications. An involvement of Crhr1 in the bidirectional behavioral shift could, however, rather be due to different effects of the HAB- and LAB-specific alleles described here. Both candidate genes therefore deserve attention in the complex regulation of anxiety-related phenotypes including environment-mediated effects.