Effects of a simulated wolf encounter on brain and blood biomarkers of stress-related psychological disorders in beef cows with or without previous exposure to wolves

This experiment compared mRNA expression of brain-blood biomarkers associated with stress-related psychological disorders, including post-traumatic stress disorder (PTSD), in beef cows from wolf-naïve and wolf-experienced origins that were subjected to a simulated wolf encounter. Multiparous, non-pregnant, non-lactating Angus-crossbred cows from the Eastern Oregon Agricultural Research Center (Burns, OR; CON; = 10) and from a commercial operation near Council, ID (WLF; = 10) were used. To date, gray wolves are not present around Burns, OR, and thus CON were naïve to wolves. Conversely, wolves are present around Council, ID, and WLF cows were selected from a herd that had experienced multiple wolf-predation episodes from 2008 to 2015. After a 60-d commingling and adaptation period, CON and WLF cows were allocated to groups A or B (d -1; 5 CON and 5 WLF cows in each group). On d 0, cows from group A were sampled for blood and immediately slaughtered, and samples were analyzed to evaluate inherent differences between CON and WLF cows. On d 1, cows from group B were exposed in pairs (1 CON and 1 WLF cow) to experimental procedures. Cows were sampled for blood, moved to 2 adjacent drylot pens (1 WLF and 1 CON cow/pen) and subjected to a simulated wolf encounter event for 20 min. The encounter consisted of (1) cotton plugs saturated with wolf urine attached to the drylot fence, (2) reproduction of wolf howls, and (3) three leashed dogs that were walked along the fence perimeter. Thereafter, another blood sample was collected and cows were slaughtered. Upon slaughter, the brain was removed and dissected for collection of the hypothalamus, and one longitudinal slice of the medial pre-frontal cortex, amygdala, and Cornu Ammonis (1 region of the hippocampus from both hemispheres). Within cows from group A, expression of in hippocampus and amygdala were greater ( < 0.01) in WLF vs. CON cows. Within cows from group B, expression of hippocampal mRNA and expression of c mRNA in hippocampus and amygdala were less ( ≤ 0.04) in WLF vs. CON cows. These are key biological markers known to be downregulated during stress-related psychological disorders elicited by fear, particularly PTSD. Hence, cows originated from a wolf-experienced herd presented biological evidence suggesting a psychological disorder, such as PTSD, after the simulated wolf encounter when compared with cows originated from a wolf-naïve herd.

Noradrenergic Stimulation Impairs Memory Generalization in Women

Memory generalization is essential for adaptive decision-making and action. Our ability to generalize across past experiences relies on medial-temporal lobe structures, known to be highly sensitive to stress. Recent evidence suggests that stressful events may indeed interfere with memory generalization. Yet, the mechanisms involved in this generalization impairment are unknown. We tested here whether a pharmacological elevation of major stress mediators-noradrenaline and glucocorticoids-is sufficient to disrupt memory generalization. In a double-blind, placebo-controlled design, healthy men and women received orally a placebo, hydrocortisone, the α2-adrenoceptor antagonist yohimbine that leads to increased noradrenergic stimulation, or both drugs, before they completed an associative learning task probing memory generalization. Drugs left learning performance intact. Yohimbine, however, led to a striking generalization impairment in women, but not in men. Hydrocortisone, in turn, had no effect on memory generalization, neither in men nor in women. The present findings indicate that increased noradrenergic activity, but not cortisol, is sufficient to disrupt memory generalization in a sex-specific manner, with relevant implications for stress-related mental disorders characterized by generalization deficits.

Selective dentate gyrus disruption causes memory impairment at the early stage of experimental multiple sclerosis

Memory impairment is an early and disabling manifestation of multiple sclerosis whose anatomical and biological substrates are still poorly understood. We thus investigated whether memory impairment encountered at the early stage of the disease could be explained by a differential vulnerability of particular hippocampal subfields. By using experimental autoimmune encephalomyelitis (EAE), a mouse model of multiple sclerosis, we identified that early memory impairment was associated with selective alteration of the dentate gyrus as pinpointed in vivo with diffusion-tensor-imaging (DTI). Neuromorphometric analyses and electrophysiological recordings confirmed dendritic degeneration, alteration in glutamatergic synaptic transmission and impaired long-term synaptic potentiation selectively in the dentate gyrus, but not in CA1, together with a more severe pattern of microglial activation in this subfield. Systemic injections of the microglial inhibitor minocycline prevented DTI, morphological, electrophysiological and behavioral impairments in EAE-mice. Furthermore, daily infusions of minocycline specifically within the dentate gyrus were sufficient to prevent memory impairment in EAE-mice while infusions of minocycline within CA1 were inefficient. We conclude that early memory impairment in EAE is due to a selective disruption of the dentate gyrus associated with microglia activation. These results open new pathophysiological, imaging, and therapeutic perspectives for memory impairment in multiple sclerosis.

Changes in C57BL6 Mouse Hippocampal Transcriptome Induced by Hypergravity Mimic Acute Corticosterone-Induced Stress

Centrifugation is a widely used procedure to study the impact of altered gravity on Earth, as observed during spaceflights, allowing us to understand how a long-term physical constraint can condition the mammalian physiology. It is known that mice, placed in classical cages and maintained during 21 days in a centrifuge at 3G gravity level, undergo physiological adaptations due to hypergravity, and/or stress. Indeed, an increase of corticosterone levels has been previously measured in the plasma of 3G-exposed mice. Corticosterone is known to modify neuronal activity during memory processes. Although learning and memory performances cannot be assessed during the centrifugation, literature largely described a large panel of proteins (channels, second messengers, transcription factors, structural proteins) which expressions are modified during memory processing. Thus, we used the Illumina technology to compare the whole hippocampal transcriptome of three groups of C57Bl6/J mice, in order to gain insights into the effects of hypergravity on cerebral functions. Namely, a group of 21 days 3G-centrifuged mice was compared to (1) a group subjected to an acute corticosterone injection, (2) a group receiving a transdermal chronic administration of corticosterone during 21 days, and (3) aged mice because aging could be characterized by a decrease of hippocampus functions and memory impairment. Our results suggest that hypergravity stress induced by corticosterone administration and aging modulate the expression of genes in the hippocampus. However, the modulations of the transcriptome observed in these conditions are not identical. Hypergravity affects per-se the hippocampus transcriptome and probably modifies its activity. Hypergravity induced changes in hippocampal transcriptome were more similar to acute injection than chronic diffusion of corticosterone or aging.

Anxiety

Anxiety disorders (separation anxiety disorder, selective mutism, specific phobias, social anxiety disorder, panic disorder, agoraphobia, and generalised anxiety disorder) are common and disabling conditions that mostly begin during childhood, adolescence, and early adulthood. They differ from developmentally normative or stress-induced transient anxiety by being marked (ie, out of proportion to the actual threat present) and persistent, and by impairing daily functioning. Most anxiety disorders affect almost twice as many women as men. They often co-occur with major depression, alcohol and other substance-use disorders, and personality disorders. Differential diagnosis from physical conditions-including thyroid, cardiac, and respiratory disorders, and substance intoxication and withdrawal-is imperative. If untreated, anxiety disorders tend to recur chronically. Psychological treatments, particularly cognitive behavioural therapy, and pharmacological treatments, particularly selective serotonin-reuptake inhibitors and serotonin-noradrenaline-reuptake inhibitors, are effective, and their combination could be more effective than is treatment with either individually. More research is needed to increase access to and to develop personalised treatments.

Amygdala Circuits for Fear Memory: A Key Role for Dopamine Regulation

In addition to modulating a number of cognitive functions including reward, punishment, motivation, and salience, dopamine (DA) plays a pivotal role in regulating threat-related emotional memory. Changes in neural circuits of the amygdala nuclei are also critically involved in the acquisition and expression of emotional memory. In this review, we summarize the regulation of amygdala circuits by DA. Specifically, we describe DA signaling in the amygdala, and DA regulation of synaptic transmission and synaptic plasticity of the amygdala neurons. Finally, we discuss a potential contribution of DA-related mechanisms to the pathogenesis of posttraumatic stress disorder.

Cortisol, heart rate, and blood pressure as early markers of PTSD risk: A systematic review and meta-analysis

Individuals with posttraumatic stress disorder (PTSD) typically exhibit altered hypothalamic-pituitary-adrenal (HPA) function and sympathetic nervous system (SNS) activity. The goals of this study were to determine whether HPA and SNS alterations in the immediate aftermath of trauma predict subsequent PTSD symptom development and whether inconsistencies observed between studies can be explained by key demographic and methodological factors. This work informs secondary prevention of PTSD by identifying subgroups of trauma survivors at risk for PTSD. This meta-analysis (26 studies, N=5186 individuals) revealed that higher heart rate measured soon after trauma exposure was associated with higher PTSD symptoms subsequently (r=0.13). Neither cortisol (r=-0.07) nor blood pressure (diastolic: r=-0.01; systolic: r=0.02) were associated with PTSD symptoms which may be influenced by methodological limitations. Associations between risk markers (heart rate, cortisol, systolic blood pressure) and PTSD symptoms were in the positive direction for younger samples and negative direction for older samples. These findings extend developmental traumatology models of PTSD by revealing an age-related shift in the presentation of early risk markers. More work will be needed to identify risk markers and pathways to PTSD while addressing methodological limitations in order to shape and target preventive interventions.

Plasticity-augmented psychotherapy for refractory depressive and anxiety disorders

Psychotherapy and pharmacotherapy have been the mainstays of treatment for depression and anxiety disorders during the last century. However, treatment response has not improved in the last few decades, with only half of all patients responding satisfactorily to typical antidepressants. To fulfill the needs of the remaining patients, new treatments with better efficacy are in demand. The addition of psychotherapy to antidepressant treatment has been shown to be superior to pharmacotherapy alone. However, the time costs of psychotherapy limit its use for clinicians and patients. Advancements in neuroscience have contributed to an improved understanding of the pathogenesis of depressive and anxiety disorders. In particular, recent advances in the field of fear conditioning have provided valuable insight into the treatment of refractory depressive and anxiety disorders. In this review, we studied the reconsolidation-updating paradigm and the concept of epigenetic modification, which has been shown to permanently attenuate remote fear memory. This has implications for drug-augmented, e.g. antidepressant and valproic acid, psychotherapy. Future research on more sophisticated psychotherapy techniques will increase the desirability of this treatment modality for both clinicians and patients.

Posttraumatic stress disorder: A metabolic disorder in disguise?

Posttraumatic stress disorder (PTSD) is a heterogeneous psychiatric disorder that affects individuals exposed to trauma and is highly co-morbid with other adverse health outcomes, including cardiovascular disease and obesity. The unique pathophysiological feature of PTSD is the inability to inhibit fear responses, such that individuals suffering from PTSD re-experience traumatic memories and are unable to control psychophysiological responses to trauma-associated stimuli. However, underlying alterations in sympathetic nervous system activity, neuroendocrine systems, and metabolism associated with PTSD are similar to those present in traditional metabolic disorders, such as obesity and diabetes. The current review highlights existing clinical, translational, and preclinical data that support the notion that underneath the primary indication of impaired fear inhibition, PTSD is itself also a metabolic disorder and proposes altered function of inflammatory responses as a common underlying mechanism. The therapeutic implications of treating PTSD as a whole-body condition are significant, as targeting any underlying biological system whose activity is altered in both PTSD and metabolic disorders, (i.e. HPA axis, sympathetic nervous systems, inflammation) may elicit symptomatic relief in individuals suffering from these whole-body adverse outcomes.

Anxiolytic effects of GLYX-13 in animal models of posttraumatic stress disorder-like behavior

In the present study, we investigated the effectiveness of GLYX-13, an NMDA receptor glycine site functional partial agonist, to alleviate the enhanced anxiety and fear response in both a mouse and rat model of stress-induced behavioral changes that might be relevant to posttraumatic stress disorder (PTSD). Studies over the last decades have suggested that the hyperactivity of hypothalamic-pituitary-adrenal (HPA) axis is one of the most consistent findings in stress-related disease. Herein, we used these animal models to further investigate the effect of GLYX-13 on the stress hormone levels and glucocorticoid receptor (GR) expression. We found that exposure to foot shock induced long-lasting behavioral deficiencies in mice, including freezing and anxiety-like behaviors, that were significantly ameliorated by the long-term administration of GLYX-13 (5 or 10 mg/kg). Our enzyme-linked immunosorbent assay results showed that long-term administration of GLYX-13 at behaviorally effective doses (5 or 10 mg/kg) significantly decreased the elevated serum levels of both corticosterone and its upstream stress hormone adrenocorticotropic hormone in rats subjected to the TDS procedure. These results suggest that GLYX-13 exerts a therapeutic effect on PTSD-like stress responding that is accompanied by (or associated with) modulation of the HPA axis, including inhibition of stress hormone levels and upregulation of hippocampal GR expression.

Understanding posttraumatic stress disorder through fear conditioning, extinction and reconsolidation

Careaga MBL, Girardi CEN, Suchecki D. Understanding posttraumatic stress disorder through fear conditioning, extinction and reconsolidation. NEUROSCI BIOBEHAV REV -Posttraumatic stress disorder (PTSD) is a psychopathology characterized by exacerbation of fear response. A dysregulated fear response may be explained by dysfunctional learning and memory, a hypothesis that was proposed decades ago. A key component of PTSD is fear conditioning and the study of this phenomenon in laboratory has expanded the understanding of the underlying neurobiological changes in PTSD. Furthermore, traumatic memories are strongly present even years after the trauma and maintenance of this memory is usually related to behavioral and physiological maladaptive responses. Persistence of traumatic memory may be explained by a dysregulation of two memory processes: extinction and reconsolidation. The former may explain the over-expression of fear responses as an imbalance between traumatic and extinction memory. The latter, in turn, explains the maintenance of fear responses as a result of enhancing trauma-related memories. Thus, this review will discuss the importance of fear conditioning for the establishment of PTSD and how failure in extinction or abnormal reconsolidation may contribute to the maintenance of fear response overtime.

Estradiol enhances retention but not organization of hippocampus-dependent memory in intact male mice

Because estrogens have mostly been studied in gonadectomized females, effects of chronic exposure to environmental estrogens in the general population are underestimated. Estrogens can enhance hippocampus-dependent memory through the modulation of information storage. However, declarative memory, the hippocampus-dependent memory of facts and events, demands more than abilities to retain information. Specifically, memory of repetitive events of everyday life such as "where I parked" requires abilities to organize/update memories to prevent proactive interference from similar memories of previous "parking events". Whether such organizational processes are estrogen-sensitive is unknown. We here studied, in intact young and aged adult mice, drinking-water (1μM) estradiol effects on both retention and organizational components of hippocampus-dependent memory, using a radial-maze task of everyday-like memory. Demand on retention vs organization was manipulated by varying the time-interval separating repetitions of similar events. Estradiol increased performance in young and aged mice under minimized organizational demand, but failed to improve the age-associated memory impairment and diminished performance in young mice under high organizational demand. In fact, estradiol prolonged mnemonic retention of successive events without improving organization abilities, hence resulted in more proactive interference from irrelevant memories. c-Fos imaging of testing-induced brain activations showed that the deterioration of young memory was associated with dentate gyrus dysconnectivity, reminiscent of that seen in aged mice. Our findings support the view that estradiol is promnesic but also reveal that such property can paradoxically impair memory. These findings have important outcomes regarding health issues relative to the impact of environmental estrogens in the general population.

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

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

Temporal Memory and Its Enhancement by Estradiol Requires Surface Dynamics of Hippocampal CA1 N-Methyl-D-Aspartate Receptors

BACKGROUND

Identifying the underlying cellular mechanisms of episodic memory is an important challenge, since this memory, based on temporal and contextual associations among events, undergoes preferential degradation in aging and various neuropsychiatric disorders. Memory storage of temporal and contextual associations is known to rely on hippocampal N-methyl-D-aspartate receptor (NMDAR)-dependent synaptic plasticity, which depends ex vivo on dynamic organization of surface NMDARs. Whether NMDAR surface trafficking sustains the formation of associative memory, however, remains unknown.

METHODS

We tested this hypothesis, using single nanoparticle imaging, electrophysiology, and behavioral approaches, in hippocampal networks challenged with a potent modulator of NMDAR-dependent synaptic plasticity and memory, 17β-estradiol (E2).

RESULTS

We demonstrate that E2 modulates NMDAR surface trafficking, a necessary condition for E2-induced potentiation at hippocampal cornu ammonis 1 synapses. Strikingly, cornu ammonis 1 NMDAR surface trafficking controls basal and E2-enhanced mnemonic retention of temporal, but not contextual, associations.

CONCLUSIONS

NMDAR surface trafficking and its modulation by the sex hormone E2 is a cellular mechanism critical for a major component of episodic memory, opening a new and noncanonical research avenue in the physiopathology of cognition.

Fear conditioning in mouse lines genetically selected for binge-like ethanol drinking

The comorbidity of substance- and alcohol-use disorders (AUD) with other psychiatric conditions, especially those related to stress such as post-traumatic stress disorder (PTSD), is well-established. Binge-like intoxication is thought to be a crucial stage in the development of the chronic relapsing nature of the addictions, and self-medication through binge-like drinking is commonly seen in PTSD patients. We have selectively bred two separate High Drinking in the Dark (HDID-1 and HDID-2) mouse lines to reach high blood ethanol concentrations (BECs) after a 4-h period of access to 20% ethanol starting shortly after the onset of circadian dark. As an initial step toward the eventual goal of employing binge-prone HDID mice to study PTSD-like behavior including alcohol binge drinking, we sought first to determine their ability to acquire conditioned fear. We asked whether these mice acquired, generalized, or extinguished conditioned freezing to a greater or lesser extent than unselected control HS/Npt mice. In two experiments, we trained groups of 16 adult male mice in a standard conditioned fear protocol. Mice were tested for context-elicited freezing, and then, in a novel context, for cue-induced freezing. After extinction tests, renewal of conditioned fear was tested in the original context. Mice of all three genotypes showed typical fear responding. Context paired with shock elicited freezing behavior in a control experiment, but cue unpaired with shock did not. These studies indicate that fear learning per se does not appear to be influenced by genes causing predisposition to binge drinking, suggesting distinct neural mechanisms. However, HDID mice are shown to be a suitable model for studying the role of conditioned fear specifically in binge-like drinking.

From Memory Impairment to Posttraumatic Stress Disorder-Like Phenotypes: The Critical Role of an Unpredictable Second Traumatic Experience

Arousal and stress critically regulate memory formation and retention. Increasing levels of stress produce an inverted U-shaped effect on cognitive performance, including the retention of explicit memories, and experiencing a severe stress during a traumatic event may lead to posttraumatic stress disorder (PTSD). The molecular mechanisms underlying the impairing effect of a severe stress on memory and the key contribution of traumatic experiences toward the development of PTSD are still unknown. Here, using increasing footshock intensities in an inhibitory avoidance paradigm, we reproduced the inverted U-shaped curve of memory performance in rats. We then show that the inverted U profile of memory performance correlates with an inverted U profile of corticosterone level in the circulation and of brain-derived neurotrophic factor, phosphorylated tropomyosin-receptor kinase B, and methyl CpG binding protein in the dorsal hippocampus. Furthermore, training with the highest footshock intensity (traumatic experience) led to a significant elevation of hippocampal glucocorticoid receptors. Exposure to an unpredictable, but not to a predictable, highly stressful reminder shock after a first traumatic experience resulted in PTSD-like phenotypes, including increased memory of the trauma, high anxiety, threat generalization, and resistance to extinction. Systemic corticosterone injection immediately after the traumatic experience, but not 3 d later, was sufficient to produce PTSD-like phenotypes. We suggest that, although after a first traumatic experience a suppression of the corticosterone-dependent response protects against the development of an anxiety disorder, experiencing more than one trauma (multiple hits) is a critical contributor to the etiology of PTSD.

Behavioral and Neural Mechanisms of Overgeneralization in Anxiety

Overgeneralization of dangerous stimuli is a possible etiological account for anxiety disorders, yet the underlying behavioral and neural origins remain vague. Specifically, it is unclear whether this is a choice behavior in an unsafe environment ("better safe than sorry") or also a fundamental change in how the stimulus is perceived. We show that anxiety patients have wider generalization for loss-conditioned tone when compared to controls and do so even in a safe context that requires a different behavioral policy. Moreover, patients overgeneralized for gain-conditioned tone as well. Imaging (fMRI) revealed that in anxiety only, activations during conditioning in the dACC and the putamen were correlated with later overgeneralization of loss and gain, respectively, whereas valence distinction in the amygdala and hippocampus during conditioning mediated the difference between loss and gain generalization. During generalization itself, neural discrimination based on multivoxel patterns in auditory cortex and amygdala revealed specific stimulus-related plasticity. Our results suggest that overgeneralization in anxiety has perceptual origins and involves affective modulation of stimulus representations in primary cortices and amygdala.

Bi-directional effects of pituitary adenylate cyclase-activating polypeptide (PACAP) on fear-related behavior and c-Fos expression after fear conditioning in rats

Pituitary adenylate cyclase-activating polypeptide (PACAP) is implicated in stress regulation and learning and memory. PACAP has neuromodulatory actions on brain structures within the limbic system that could contribute to its acute and persistent effects in animal models of stress and anxiety-like behavior. Here, male Sprague-Dawley rats were implanted with intracerebroventricular (ICV) cannula for infusion of PACAP-38 (0.5, 1, or 1.5 μg) or vehicle followed 30 min later by fear conditioning. Freezing was measured early (1, 4, and 7 days) or following a delay (7, 10, and 13 days) after conditioning. PACAP (1.5 μg) produced a bi-phasic response in freezing behavior across test days: relative to controls, PACAP-treated rats showed a reduction in freezing when tested 1 or 7 days after fear conditioning that evolved into a significant elevation in freezing by the third test session in the early, but not delayed, group. Corticosterone (CORT) levels were significantly elevated in PACAP-treated rats following fear conditioning, but not at the time of testing (Day 1). Brain c-Fos expression revealed PACAP-dependent alterations within, as well as outside of, areas typically implicated in fear conditioning. Our findings raise the possibility that PACAP disrupts fear memory consolidation by altering synaptic plasticity within neurocircuits normally responsible for encoding fear-related cues, producing a type of dissociation or peritraumatic amnesia often seen in people early after exposure to a traumatic event. However, fear memories are retained such that repeated testing and memory reactivation (e.g., re-experiencing) causes the freezing response to emerge and persist at elevated levels. PACAP systems may represent an axis on which stress and exposure to trauma converge to promote maladaptive behavioral responses characteristic of psychiatric illnesses such as post-traumatic stress disorder (PTSD).

Adult Hippocampal Neurogenesis, Fear Generalization, and Stress

The generalization of fear is an adaptive, behavioral, and physiological response to the likelihood of threat in the environment. In contrast, the overgeneralization of fear, a cardinal feature of posttraumatic stress disorder (PTSD), manifests as inappropriate, uncontrollable expression of fear in neutral and safe environments. Overgeneralization of fear stems from impaired discrimination of safe from aversive environments or discernment of unlikely threats from those that are highly probable. In addition, the time-dependent erosion of episodic details of traumatic memories might contribute to their generalization. Understanding the neural mechanisms underlying the overgeneralization of fear will guide development of novel therapeutic strategies to combat PTSD. Here, we conceptualize generalization of fear in terms of resolution of interference between similar memories. We propose a role for a fundamental encoding mechanism, pattern separation, in the dentate gyrus (DG)-CA3 circuit in resolving interference between ambiguous or uncertain threats and in preserving episodic content of remote aversive memories in hippocampal-cortical networks. We invoke cellular-, circuit-, and systems-based mechanisms by which adult-born dentate granule cells (DGCs) modulate pattern separation to influence resolution of interference and maintain precision of remote aversive memories. We discuss evidence for how these mechanisms are affected by stress, a risk factor for PTSD, to increase memory interference and decrease precision. Using this scaffold we ideate strategies to curb overgeneralization of fear in PTSD.

A Stress-Induced Shift From Trace to Delay Conditioning Depends on the Mineralocorticoid Receptor

BACKGROUND

Fear learning in stressful situations is highly adaptive for survival by steering behavior in subsequent situations, but fear learning can become disproportionate in vulnerable individuals. Despite the potential clinical significance, the mechanism by which stress modulates fear learning is poorly understood. Memory theories state that stress can cause a shift away from more controlled processing depending on the hippocampus toward more reflexive processing supported by the amygdala and striatum. This shift may be mediated by activation of the mineralocorticoid receptor (MR) for cortisol. We investigated how stress shifts processes underlying cognitively demanding learning versus less demanding fear learning using a combined trace and delay fear conditioning paradigm.

METHODS

In a pharmacological functional magnetic resonance imaging study, we tested 101 healthy men probing the effects of stress (socially evaluated cold pressor vs. control procedure) and MR-availability (400 mg spironolactone vs. placebo) in a randomized, placebo-controlled, full-factorial, between-subjects design.

RESULTS

Effective stress induction and successful conditioning were confirmed by subjective, physiologic, and somatic data. In line with a stress-induced shift, stress enhanced later recall of delay compared with trace conditioning in the MR-available groups as indexed by skin conductance responses. During learning, this was accompanied by a stress-induced reduction of learning-related hippocampal activity for trace conditioning. The stress-induced shift in fear and neural processing was absent in the MR-blocked groups.

CONCLUSIONS

Our results are in line with a stress-induced shift in fear learning, mediated by the MR, resulting in a dominance of cognitively less demanding amygdala-based learning, which might be particularly prominent in individuals with high MR sensitivity.

ApoE2 Exaggerates PTSD-Related Behavioral, Cognitive, and Neuroendocrine Alterations

Apolipoprotein E (apoE) is an essential component of lipoprotein particles in both the brain and periphery, and exists in three isoforms in the human population: E2, E3, and E4. ApoE has numerous, well-established roles in neurobiology. Most notably, E4 is associated with earlier onset and increased risk of Alzheimer's disease (AD). Although possession of E2 is protective in the context of AD, E2 appears to confer an increased incidence and severity of posttraumatic stress disorder (PTSD). However, the biological processes underlying this link remain unclear. In this study, we began to elucidate these associations by examining the effects of apoE on PTSD severity in combat veterans, and on PTSD-like behavior in mice with human apoE. In a group of 92 veterans with PTSD, we observed significantly higher Clinician-Administered PTSD Scale and PTSD Checklist scores in E2+ individuals, as well as alterations in salivary cortisol levels. Furthermore, we measured behavioral and biological outcomes in mice expressing human apoE after a single stressful event as well as following a period of chronic variable stress, a model of combat-related trauma. Mice with E2 showed impairments in fear extinction, and behavioral, cognitive, and neuroendocrine alterations following trauma. To the best of our knowledge, these data constitute the first translational demonstration of PTSD severity in men and PTSD-like symptoms in mice with E2, and point to apoE as a novel biomarker of susceptibility, and potential therapeutic target, for PTSD.

Abnormal Fear Memory as a Model for Posttraumatic Stress Disorder

For over a century, clinicians have consistently described the paradoxical co-existence in posttraumatic stress disorder (PTSD) of sensory intrusive hypermnesia and declarative amnesia for the same traumatic event. Although this amnesia is considered as a critical etiological factor of the development and/or persistence of PTSD, most current animal models in basic neuroscience have focused exclusively on the hypermnesia, i.e., the persistence of a strong fear memory, neglecting the qualitative alteration of fear memory. The latest is characterized by an underrepresentation of the trauma in the context-based declarative memory system in favor of its overrepresentation in a cue-based sensory/emotional memory system. Combining psychological and neurobiological data as well as theoretical hypotheses, this review supports the idea that contextual amnesia is at the core of PTSD and its persistence and that altered hippocampal-amygdalar interaction may contribute to such pathologic memory. In a first attempt to unveil the neurobiological alterations underlying PTSD-related hypermnesia/amnesia, we describe a recent animal model mimicking in mice some critical aspects of such abnormal fear memory. Finally, this line of argument emphasizes the pressing need for a systematic comparison between normal/adaptive versus abnormal/maladaptive fear memory to identify biomarkers of PTSD while distinguishing them from general stress-related, potentially adaptive, neurobiological alterations.

Stress effects on the hippocampus: a critical review

Uncontrollable stress has been recognized to influence the hippocampus at various levels of analysis. Behaviorally, human and animal studies have found that stress generally impairs various hippocampal-dependent memory tasks. Neurally, animal studies have revealed that stress alters ensuing synaptic plasticity and firing properties of hippocampal neurons. Structurally, human and animal studies have shown that stress changes neuronal morphology, suppresses neuronal proliferation, and reduces hippocampal volume. Since the inception of stress research nearly 80 years ago, much focus has been on the varying levels of hypothalamic-pituitary-adrenal (HPA) axis neuroendocrine hormones, namely glucocorticoids, as mediators of the myriad stress effects on the hippocampus and as contributing factors to stress-associated psychopathologies such as post-traumatic stress disorder (PTSD). However, reports of glucocorticoid-produced alterations in hippocampal functioning vary widely across studies. This review provides a brief history of stress research, examines how the glucocorticoid hypothesis emerged and guides contemporary stress research, and considers alternative approaches to understanding the mechanisms underlying stress effects on hippocampal functioning.

Long-lasting memory abnormalities following exposure to the mouse defense test battery: An animal model of PTSD

Memory dysfunctions are thought to play a crucial role both in the development and the maintenance of posttraumatic stress disorder (PTSD). Patients suffering from this condition persistently re-experience the traumatic event particularly when exposed to trauma-related cues and they display memory alterations. The objective of the present study was to investigate the long-term effects of a traumatic stress exposure on defensive behaviors and memory performance in mice confronted with a natural threat (i.e. a rat) in the defense test battery (MDTB), a procedure developed by the Blanchard group in the early nineties. The object recognition task,which addresses certain aspects of episodic memory, was used to assess the long-term consequences of stress on memory function. Mice were exposed to the MDTB followed two weeks later by a re-exposure to the test apparatus, but in the absence of the threat stimulus. Two hours after the second exposure to the MDTB apparatus, mice were exposed to the object recognition task (ORT). Another set of animals was used which were either exposed to the first or to the second MDTB session, before being tested in the ORT. Results showed that MDTB exposure produced long-lasting alterations in some defensive behaviors, such as escape attempts from the apparatus, which were increased during the re-exposure session at day 14 compared to non-exposed control mice.While exposure to the MDTB context only did not affect memory performance in the ORT, confrontation with the threat stimulus in the MDTB on day 1 impaired episodic memory two weeks after the stressful event. Finally, mice confronted both with the rat on day 1 and the MDTB context on day 14 displayed intact episodic memory performance in the ORT. We hypothesize that re-exposure to the context following a stressful event resulted in an increase of arousal, which subsequently led to an improvement in cognitive performance, a phenomenon also described in PTSD patients. The MDTB is a typical example of the tremendous efforts of Blanchard's lab to increase the translatability potential of the behavioral models of central nervous system disorders.

Brain organic cation transporter 2 controls response and vulnerability to stress and GSK3β signaling

Interactions between genetic and environmental factors, like exposure to stress, have an important role in the pathogenesis of mood-related psychiatric disorders, such as major depressive disorder. The polyspecific organic cation transporters (OCTs) were shown previously to be sensitive to the stress hormone corticosterone in vitro, suggesting that these transporters might have a physiologic role in the response to stress. Here, we report that OCT2 is expressed in several stress-related circuits in the brain and along the hypothalamic-pituitary-adrenocortical (HPA) axis. Genetic deletion of OCT2 in mice enhanced hormonal response to acute stress and impaired HPA function without altering adrenal sensitivity to adrenocorticotropic hormone (ACTH). As a consequence, OCT2(-/-) mice were potently more sensitive to the action of unpredictable chronic mild stress (UCMS) on depression-related behaviors involving self-care, spatial memory, social interaction and stress-sensitive spontaneous behavior. The functional state of the glycogen synthase kinase-3β (GSK3β) signaling pathway, highly responsive to acute stress, was altered in the hippocampus of OCT2(-/-) mice. In vivo pharmacology and western blot experiments argue for increased serotonin tonus as a main mechanism for impaired GSK3β signaling in OCT2(-/-) mice brain during acute response to stress. Our findings identify OCT2 as an important determinant of the response to stress in the brain, suggesting that in humans OCT2 mutations or blockade by certain therapeutic drugs could interfere with HPA axis function and enhance vulnerability to repeated adverse events leading to stress-related disorders.

Stimulation of α1-adrenoceptors facilitates GABAergic transmission onto pyramidal neurons in the medial prefrontal cortex

Whereas activation of α1-adrenoceptors (α1-ARs) modulates glutamatergic transmission, the roles of α1-ARs in GABAergic transmission in the medial prefrontal cortex (mPFC) are elusive. Here, we examined the effects of the α1-AR agonist phenylephrine (Phe) on GABAergic transmission onto pyramidal neurons in the deep layers of the mPFC. We found that bath application of Phe dose-dependently increased the amplitude of evoked IPSCs (eIPSCs). Phe increased the frequency but not the amplitude of miniature IPSCs (mIPSCs). Ca(2+) influx through T-type voltage-gated calcium channels is required for Phe-induced increases in GABA release. Phe increases GABA release probability and the number of releasable vesicles. Phe depolarizes the fast-spiking (FS) interneurons without effects on the firing rate of action potentials (APs) of interneurons. Phe-induced depolarization is independent of extracellular Na(+), Ca(2+) and T-type calcium channels, but requires inward rectifier K(+) channels (Kirs). The present study demonstrates that Phe enhances GABAergic transmission onto mPFC pyramidal neurons through inhibiting interneurons Kirs, which further depolarizes interneurons leading to increase in Ca(2+) influx via T-type calcium channels. Our results may provide a cellular and molecular mechanism that helps explain α1-AR-induced PFC dysfunction.

Juvenile obesity enhances emotional memory and amygdala plasticity through glucocorticoids

In addition to metabolic and cardiovascular disorders, obesity is associated with adverse cognitive and emotional outcomes. Its growing prevalence during adolescence is particularly alarming since recent evidence indicates that obesity can affect hippocampal function during this developmental period. Adolescence is a decisive period for maturation of the amygdala and the hypothalamic-pituitary-adrenal (HPA) stress axis, both required for lifelong cognitive and emotional processing. However, little data are available on the impact of obesity during adolescence on amygdala function. Herein, we therefore evaluate in rats whether juvenile high-fat diet (HFD)-induced obesity alters amygdala-dependent emotional memory and whether it depends on HPA axis deregulation. Exposure to HFD from weaning to adulthood, i.e., covering adolescence, enhances long-term emotional memories as assessed by odor-malaise and tone-shock associations. Juvenile HFD also enhances emotion-induced neuronal activation of the basolateral complex of the amygdala (BLA), which correlates with protracted plasma corticosterone release. HFD exposure restricted to adulthood does not modify all these parameters, indicating adolescence is a vulnerable period to the effects of HFD-induced obesity. Finally, exaggerated emotional memory and BLA synaptic plasticity after juvenile HFD are alleviated by a glucocorticoid receptor antagonist. Altogether, our results demonstrate that juvenile HFD alters HPA axis reactivity leading to an enhancement of amygdala-dependent synaptic and memory processes. Adolescence represents a period of increased susceptibility to the effects of diet-induced obesity on amygdala function.

Models and mechanisms for hippocampal dysfunction in obesity and diabetes

Clinical studies suggest that obesity and Type 2 (insulin-resistant) diabetes impair the structural integrity of medial temporal lobe regions involved in memory and confer greater vulnerability to neurological insults. While eliminating obesity and its endocrine comorbidities would be the most straightforward way to minimize cognitive risk, structural barriers to physical activity and the widespread availability of calorically dense, highly palatable foods will likely necessitate additional strategies to maintain brain health over the lifespan. Research in rodents has identified numerous correlates of hippocampal functional impairment in obesity and diabetes, with several studies demonstrating causality in subsequent mechanistic studies. This review highlights recent work on pathways and cell-cell interactions underlying the synaptic consequences of obesity, diabetes, or in models with both pathological conditions. Although the mechanisms vary across different animal models, immune activation has emerged as a shared feature of obesity and diabetes, with synergistic exacerbation of neuroinflammation in model systems with both conditions. This review discusses these findings with reference to the benefits of incorporating existing models from the fields of obesity and metabolic disease. Many transgenic lines with basal metabolic alterations or differential susceptibility to diet-induced obesity have yet to be characterized with respect to their cognitive and synaptic phenotype. Adopting these models, and building on the extensive knowledge base used to generate them, is a promising avenue for understanding interactions between peripheral disease states and neurodegenerative disorders.

Orexin neuropeptides contribute to the development and persistence of generalized avoidance behavior in the rat

RATIONALE

Avoidance of contexts directly associated with fearful experiences represents an adaptive behavioral survival strategy. Over-interpretation of contextual cues leading to generalized avoidance of situations that are only remotely similar to the original fear context represents a pathologic process that contributes to anxiety disorders. Orexin neuropeptides modulate anxiety-like behavioral and physiological responses.

OBJECTIVE

The objective of this paper was to investigate the impact of pharmacological orexin receptor blockade on generalized avoidance behavior.

METHODS

Rats received a single electric foot-shock in the dark side of a two-compartment shuttle box followed by situational context reminders. After shock, rats were treated chronically (3 weeks) with the orexin receptor antagonist almorexant or with the selective serotonin reuptake inhibitor sertraline, used as positive anxiolytic control. In week 3, avoidance behavior was measured under conditions of high (dark-light (DL)-box) and low (elevated plus maze (EPM)) similarity to the original shock context. Avoidance behavior was re-assessed 5 and 17 weeks after treatment termination.

RESULTS

Avoidance in the DL box (contextual fear memory) remained unaffected by any treatment and lasted 20 weeks post-shock exposure. Avoidance in the EPM (neophobic fear generalization) was partially attenuated during treatment with almorexant and sertraline at week 3. Following 5 and 17 weeks of drug washout, avoidance in the EPM was significantly reduced in almorexant- but not in sertraline-treated rats. Almorexant also reduced persistent avoidance in the EPM upon treatment initiation 3 weeks after shock exposure.

CONCLUSION

Chronic orexin receptor blockade in rats reduces both the development and persistence of generalized avoidance in situations with low similarity to the initial shock context.

The role of brain interleukin-1 in stress-enhanced fear learning

Posttraumatic stress disorder (PTSD) has been shown to be associated with pro-inflammatory markers, including elevated plasma levels of interleukin-1β (IL-1β). However, the precise role of neuroinflammation and central immune signaling on the development of this debilitating psychological disorder is not known. Here, we used stress-enhanced fear learning (SEFL), an animal model of the disorder, to examine the role of central IL-1β in PTSD. The results show that the severe stressor in SEFL induces a time-dependent increase in IL-1β immunoreactivity and mRNA expression within the dentate gyrus of the dorsal hippocampus (DH). There was no increase in IL-1β in the basolateral amygdala or the perirhinal cortex. Moreover, blocking the action of IL-1β following the severe stressor with IL-1 receptor antagonist (10 μg, intracerebroventricular (i.c.v.), 24 and 48 h after the stressor) prevented the development of SEFL. To provide further support for the role of IL-1β in the development of SEFL, we show that systemic morphine, a treatment which is known to reduce both PTSD and SEFL, also reduces IL-1β expression in the DH induced by the severe stressor. These studies provide the first evidence that IL-1 is involved SEFL and suggest that IL-1 signaling in the brain may have a critical role in the development of PTSD.

Adrenal fasciculata cells express T-type and rapidly and slowly activating L-type Ca2+ channels that regulate cortisol secretion

In whole cell patch-clamp recordings, we characterized the L-type Ca(2+) currents in bovine adrenal zona fasciculata (AZF) cells and explored their role, along with the role of T-type channels, in ACTH- and angiotensin II (ANG II)-stimulated cortisol secretion. Two distinct dihydropyridine-sensitive L-type currents were identified, both of which were activated at relatively hyperpolarized potentials. One activated with rapid kinetics and, in conjunction with Northern blotting and PCR, was determined to be Cav1.3. The other, expressed in approximately one-half of AZF cells, activated with extremely slow voltage-dependent kinetics and combined properties not previously reported for an L-type Ca(2+) channel. The T-type Ca(2+) channel antagonist 3,5-dichloro-N-[1-(2,2-dimethyl-tetrahydro-pyran-4-ylmethyl)-4-fluoro-piperidin-4-ylmethyl]-benzamide (TTA-P2) inhibited Cav3.2 current in these cells, as well as ACTH- and ANG II-stimulated cortisol secretion, at concentrations that did not affect L-type currents. In contrast, nifedipine specifically inhibited L-type currents and cortisol secretion, but less effectively than TTA-P2. Diphenylbutylpiperidine Ca(2+) antagonists, including pimozide, penfluridol, and fluspirilene, and the dihydropyridine niguldipine blocked Cav3.2 and L-type currents and inhibited ACTH-stimulated cortisol secretion with similar potency. This study shows that bovine AZF cells express three Ca(2+) channels, the voltage-dependent gating and kinetics of which could orchestrate complex mechanisms linking peptide hormone receptors to cortisol secretion through action potentials or sustained depolarization. The function of the novel, slowly activating L-type channel is of particular interest in this respect. Regardless, the well-correlated selective inhibition of T- and L-type currents and ACTH- and ANG II-stimulated cortisol secretion by TTA-P2 and nifedipine establish the critical importance of these channels in AZF cell physiology.

β-Alanine supplemented diets enhance behavioral resilience to stress exposure in an animal model of PTSD

This study investigated the effects of β-alanine (BA) ingestion on the behavioral and neuroendocrine response of post-traumatic stress disorder (PTSD) in a murine model. Animals were fed a normal diet with or without (PL) BA supplementation (100 mg kg(-1)) for 30 days. Animals were then exposed to a predator-scent stress (PSS) or a sham (UNEX). Behaviors were evaluated using an elevated plus maze (EPM) and acoustic startle response (ASR) 7 days following exposure to the PSS. Corticosterone concentrations (CS), expression of brain-derived neurotrophic factor (BDNF), and brain carnosine concentrations were analyzed a day later. Animals in PSS+PL spent significantly less time in the open arms and in the number of entries in the EPM than PSS+BA, UNEX+BA, or UNEX+PL. Animals in PSS+BA had comparable scores to UNEX+BA. Anxiety index was higher (p < 0.05) in PSS+PL compared to PSS+BA or animals that were unexposed. ASR and freezing were greater (p < 0.05) in animals exposed to PSS compared to animals unexposed. CS expression was higher (p < 0.05) in animals exposed to PSS compared to unexposed animals. Brain carnosine concentrations in the hippocampus and other brain sections were significantly greater in animals supplemented with BA compared to PL. BDNF expression in the CA1 and DG subregions of the hippocampus was lower (p < 0.05) in animals exposed and fed a normal diet compared to animals exposed and supplemented with BA, or animals unexposed. In conclusion, BA supplementation in rats increased brain carnosine concentrations and resulted in a reduction in PTSD-like behavior, which may be mediated in part by maintaining BDNF expression in the hippocampus.

Adaptive emotional memory: the key hippocampal-amygdalar interaction

For centuries philosophical and clinical studies have emphasized a fundamental dichotomy between emotion and cognition, as, for instance, between behavioral/emotional memory and explicit/representative memory. However, the last few decades cognitive neuroscience have highlighted data indicating that emotion and cognition, as well as their underlying neural networks, are in fact in close interaction. First, it turns out that emotion can serve cognition, as exemplified by its critical contribution to decision-making or to the enhancement of episodic memory. Second, it is also observed that reciprocally cognitive processes as reasoning, conscious appraisal or explicit representation of events can modulate emotional responses, like promoting or reducing fear. Third, neurobiological data indicate that reciprocal amygdalar-hippocampal influences underlie such mutual regulation of emotion and cognition. While supporting this view, the present review discusses experimental data, obtained in rodents, indicating that the hippocampal and amygdalar systems not only regulate each other and their functional outcomes, but also qualify specific emotional memory representations through specific activations and interactions. Specifically, we review consistent behavioral, electrophysiological, pharmacological, biochemical and imaging data unveiling a direct contribution of both the amygdala and hippocampal-septal system to the identification of the predictor of a threat in different situations of fear conditioning. Our suggestion is that these two brain systems and their interplay determine the selection of relevant emotional stimuli, thereby contributing to the adaptive value of emotional memory. Hence, beyond the mutual quantitative regulation of these two brain systems described so far, we develop the idea that different activations of the hippocampus and amygdala, leading to specific configurations of neural activity, qualitatively impact the formation of emotional memory representations, thereby producing either adaptive or maladaptive fear memories.

Corticosterone mitigates the stress response in an animal model of PTSD

Activation of glucocorticoid receptor signaling in the stress response to traumatic events has been implicated in the pathogenesis of stress-associated psychiatric disorders such as post-traumatic stress disorder (PTSD). Elevated startle response and hyperarousal are hallmarks of PTSD, and are generally considered to evince fear (DSM V). To further examine the efficacy of corticosterone in treating hyperarousal and elevated fear, the present study utilized a learned helplessness stress model in which rats are restrained and subjected to tail shock for three days. These stressed rats develop a delayed long-lasting exaggeration of the acoustic startle response (ASR) and retarded body weight growth, similar to symptoms of PTSD patients (Myers et al., 2005; Speed et al., 1989). We demonstrate that both pre-stress and post-stress administration of corticosterone (3 mg/kg/day) mitigates a subsequent exaggeration of the ASR measured 14 days after cessation of the stress protocol. Furthermore, the mitigating efficacy of pre-stress administration of corticosterone (3 mg/kg/day for three days) appeared to last significantly longer, up to 21 days after the cessation of the stress protocol, in comparison to that of post-stress administration of corticosterone. However, pre-stress administration of corticosterone at 0.3 mg/kg/day for three days did not mitigate stress-induced exaggeration of the ASR measured at both 14 and 21 days after the cessation of the stress protocol. In addition, pre-stress administration of corticosterone (3 mg/kg/day for three days) mitigates the retardation of body weight growth otherwise resulting from the stress protocol. Congruently, co-administration of the corticosterone antagonist RU486 (40 mg/kg/day for three days) with corticosterone (3 mg/kg/day) prior to stress diminished the mitigating efficacy of the exogenous corticosterone on exaggerated ASR and stress-retarded body weight. The relative efficacy of pre versus post administration of corticosterone and high versus low dose of corticosterone on stress-induced exaggeration of innate fear response and stress-retarded body weight growth indicate that exogenous corticosterone administration within an appropriate time window and dosage are efficacious in diminishing traumatic stress induced pathophysiological processes. Clinical implications associated with the efficacy of prophylactic and therapeutic corticosterone therapy for mitigating symptoms of PTSD are discussed, particularly in relation to diminishing hyperarousal and exaggerated innate fear response.

Fear generalization in the primate amygdala

Broad generalization of negative memories is a potential etiology for anxiety disorders, yet the underlying mechanisms remain unknown. We developed a non-human primate model that replicates behavioral observations in humans and identifies specific changes in tuning properties of amygdala neurons: the width of auditory tuning increases with the distance of its center from the conditioned stimulus. This center-width relationship can account for better detection and at the same time explain the wide stimulus generalization.

Tracking explicit and implicit long-lasting traces of fearful memories in humans

Recent accounts of Posttraumatic Stress Disorder (PTSD) suggest that the encoding of an episode within a fearful context generates different implicit and explicit memory representations. Whilst implicit memory traces include the associated emotional states, explicit traces include a recoding into an abstract or gist-based structural context of the episode. Theoretically, the long-term preservation of implicit memory traces may facilitate the often untreatable memory intrusions in PTSD. Here, we tracked in two experiments how implicit and explicit memory traces for fearful episodes dissociate and evolve over time. Subjects (N=86) were presented with semantically-related word-lists in a contextual fear paradigm and tested for explicit memories either immediately (i.e., 30 min) or after a delay (i.e., 1 or 2 weeks) with a verbal recognition task. Skin Conductance Response (SCR) was used to assess implicit memory responses. Subjects showed high memory accuracy for words when tested immediately after encoding. At test, SCR was higher during the presentation of verbatim but not gist-based words encoded in a fearful context, and remained unchanged after 2 weeks, despite subjects being unaware of words' encoding context. We found no clear evidence of accurate explicit memory traces for the fearful or neutral contexts of words presented during encoding, either 30 min or 2 weeks afterwards. These findings indicate that the implicit, but not the explicit, memory trace of a fearful context of an episode can be detected at long-term through SCR and is dissociated from the gist-based memory. They may have implicationstowards the understanding of how the processing of fearful memoriescould lead to PTSD.

BDNF-TrkB signaling through Erk1/2 MAPK phosphorylation mediates the enhancement of fear memory induced by glucocorticoids

Activation of glucocorticoid receptors (GR) by glucocorticoid hormones (GC) enhances contextual fear memories through the activation of the Erk1/2(MAPK) signaling pathway. However, the molecular mechanism mediating this effect of GC remains unknown. Here we used complementary molecular and behavioral approaches in mice and rats and in genetically modified mice in which the GR was conditionally deleted (GR(NesCre)). We identified the tPA-BDNF-TrkB signaling pathway as the upstream molecular effectors of GR-mediated phosphorylation of Erk1/2(MAPK) responsible for the enhancement of contextual fear memory. These findings complete our knowledge of the molecular cascade through which GC enhance contextual fear memory and highlight the role of tPA-BDNF-TrkB-Erk1/2(MAPK) signaling pathways as one of the core effectors of stress-related effects of GC.

A conceptual framework for the neurobiological study of resilience

The well-replicated observation that many people maintain mental health despite exposure to severe psychological or physical adversity has ignited interest in the mechanisms that protect against stress-related mental illness. Focusing on resilience rather than pathophysiology in many ways represents a paradigm shift in clinical-psychological and psychiatric research that has great potential for the development of new prevention and treatment strategies. More recently, research into resilience also arrived in the neurobiological community, posing nontrivial questions about ecological validity and translatability. Drawing on concepts and findings from transdiagnostic psychiatry, emotion research, and behavioral and cognitive neuroscience, we propose a unified theoretical framework for the neuroscientific study of general resilience mechanisms. The framework is applicable to both animal and human research and supports the design and interpretation of translational studies. The theory emphasizes the causal role of stimulus appraisal (evaluation) processes in the generation of emotional responses, including responses to potential stressors. On this basis, it posits that a positive (non-negative) appraisal style is the key mechanism that protects against the detrimental effects of stress and mediates the effects of other known resilience factors. Appraisal style is shaped by three classes of cognitive processes--positive situation classification, reappraisal, and interference inhibition--that can be investigated at the neural level. Prospects for the future development of resilience research are discussed.

From receptor balance to rational glucocorticoid therapy

Corticosteroids secreted as end product of the hypothalamic-pituitary-adrenal axis act like a double-edged sword in the brain. The hormones coordinate appraisal processes and decision making during the initial phase of a stressful experience and promote subsequently cognitive performance underlying the management of stress adaptation. This action exerted by the steroids on the initiation and termination of the stress response is mediated by 2 related receptor systems: mineralocorticoid receptors (MRs) and glucocorticoid receptors (GRs). The receptor types are unevenly distributed but colocalized in abundance in neurons of the limbic brain to enable these complementary hormone actions. This contribution starts from a historical perspective with the observation that phasic occupancy of GR during ultradian rhythmicity is needed to maintain responsiveness to corticosteroids. Then, during stress, initially MR activation enhances excitability of limbic networks that are engaged in appraisal and emotion regulation. Next, the rising hormone concentration occupies GR, resulting in reallocation of energy to limbic-cortical circuits with a role in behavioral adaptation and memory storage. Upon MR:GR imbalance, dysregulation of the hypothalamic-pituitary-adrenal axis occurs, which can enhance an individual's vulnerability. Imbalance is characteristic for chronic stress experience and depression but also occurs during exposure to synthetic glucocorticoids. Hence, glucocorticoid psychopathology may develop in susceptible individuals because of suppression of ultradian/circadian rhythmicity and depletion of endogenous corticosterone from brain MR. This knowledge generated from testing the balance hypothesis can be translated to a rational glucocorticoid therapy.

Alterations in DNA methylation of Fkbp5 as a determinant of blood-brain correlation of glucocorticoid exposure

BACKGROUND

Epigenetic studies that utilize peripheral tissues to identify molecular substrates of neuropsychiatric disorders rely on the assumption that disease-relevant, cellular alterations that occur in the brain are mirrored and detectable in peripheral tissues such as blood. We sought to test this assumption by using a mouse model of Cushing's disease and asking whether epigenetic changes induced by glucocorticoids can be correlated between these tissue types.

METHODS

Mice were treated with different doses of glucocorticoids in their drinking water for four weeks to assess gene expression and DNA methylation (DNAm) changes in the stress response gene Fkbp5.

RESULTS

Significant linear relationships were observed between DNAm and four-week mean plasma corticosterone levels for both blood (R(2)=0.68, P=7.1×10(-10)) and brain (R(2)=0.33, P=0.001). Further, degree of methylation change in blood correlated significantly with both methylation (R(2)=0.49, P=2.7×10(-5)) and expression (R(2)=0.43, P=3.5×10(-5)) changes in hippocampus, with the notable observation that methylation changes occurred at different intronic regions between blood and brain tissues.

CONCLUSION

Although our findings are limited to several intronic CpGs in a single gene, our results demonstrate that DNA from blood can be used to assess dynamic, glucocorticoid-induced changes occurring in the brain. However, for such correlation analyses to be effective, tissue-specific locations of these epigenetic changes may need to be considered when investigating brain-relevant changes in peripheral tissues.

The effect of glucose on hippocampal-dependent contextual fear conditioning

BACKGROUND

The metabolic challenge of trauma disrupts hippocampal functioning, which is necessary for processing the complex co-occurring elements comprising the traumatic context. Poor contextual memory of trauma may subsequently contribute to intrusive memories and overgeneralization of fear. Glucose consumption following trauma may be a means to protect hippocampal functioning and contextual fear learning. This study experimentally examined the effect of glucose on hippocampal-dependent contextual learning versus cued fear learning in humans.

METHODS

Forty-two male participants underwent cued conditioning with an unconditional stimulus (US) (shock) paired with a discrete conditional stimulus (geometric shape) and context conditioning (requiring hippocampal processing) with a US unpredictably paired with a background context (picture of room). Participants were then blindly randomized to consume either a 25 g glucose or sweet-tasting placebo drink and returned for a test phase 24 hours later. Measures included acoustic startle response, US expectancy, blood glucose levels, and arousal ratings.

RESULTS

The glucose group showed superior retention of hippocampal-dependent contextual learning at test relative to the placebo group, as demonstrated by acoustic startle response and US expectancy ratings. Glucose and placebo groups did not differ on any measure of cued fear learning at test.

CONCLUSIONS

This study provides experimental evidence that in mildly stressed humans postconditioning glucose consumption improves retention of hippocampal-dependent contextual learning but not cued learning. Ultimately, glucose consumption following trauma may be a means of improving learning about the traumatic context, thereby preventing subsequent development of symptoms of posttraumatic stress.

Is meditation associated with altered brain structure? A systematic review and meta-analysis of morphometric neuroimaging in meditation practitioners

Numerous studies have begun to address how the brain's gray and white matter may be shaped by meditation. This research is yet to be integrated, however, and two fundamental questions remain: Is meditation associated with altered brain structure? If so, what is the magnitude of these differences? To address these questions, we reviewed and meta-analyzed 123 brain morphology differences from 21 neuroimaging studies examining ∼300 meditation practitioners. Anatomical likelihood estimation (ALE) meta-analysis found eight brain regions consistently altered in meditators, including areas key to meta-awareness (frontopolar cortex/BA 10), exteroceptive and interoceptive body awareness (sensory cortices and insula), memory consolidation and reconsolidation (hippocampus), self and emotion regulation (anterior and mid cingulate; orbitofrontal cortex), and intra- and interhemispheric communication (superior longitudinal fasciculus; corpus callosum). Effect size meta-analysis (calculating 132 effect sizes from 16 studies) suggests a global 'medium' effect size (Cohen's d¯=0.46; r¯=.19). Publication bias and methodological limitations are strong concerns, however. Further research using rigorous methods is required to definitively link meditation practice to altered brain morphology.