Sumatriptan attenuates fear-learning despair induced by social isolation stress in mice: Mediating role of hypothalamic-pituitary-adrenal axis

OBJECTIVES

Research has demonstrated that chronic stress experienced early in life can lead to impairments in memory and learning. These deficits are attributed to an imbalance in the interaction between glucocorticoids, the end product of the hypothalamic-pituitary-adrenal (HPA) axis, and glucocorticoid receptors in brain regions responsible for mediating memory, such as the hippocampus. This imbalance can result in detrimental conditions like neuroinflammation. The aim of this study was to assess the impact of sumatriptan, a selective agonist for 5-HT 1B/1D receptors, on fear learning capabilities in a chronic social isolation stress model in mice, with a particular focus on the role of the HPA axis.

METHODS

Mice were assigned to two opposing conditions, including social condition (SC) and isolated condition (IC) for a duration of five weeks. All mice underwent passive avoidance test, with their subsequent freezing behavior serving as an indicator of fear retrieval. Mice in the IC group were administered either a vehicle, sumatriptan, GR-127935 (a selective antagonist for 5-HT 1B/1D receptors), or a combination of sumatriptan and GR-127935 during the testing sessions. At the end, all mice were sacrificed and samples of their serum and hippocampus were collected for further analysis.

RESULTS

Isolation was found to significantly reduce freezing behavior (p<0.001). An increase in the freezing response among IC mice was observed following the administration of varying doses of sumatriptan, as indicated by a one-way ANOVA analysis (p<0.001). However, the mitigating effects of sumatriptan were reversed upon the administration of GR-127935. An ELISA assay conducted before and after the passive avoidance test revealed no significant change in serum corticosterone levels among SC mice. In contrast, a significant increase was observed among IC mice, suggesting hyper-responsiveness of the HPA axis in isolated animals. This hyper-responsiveness was ameliorated following the administration of sumatriptan. Furthermore, both the sumatriptan and SC groups exhibited a similar trend, showing a significant increase in the expression of hippocampal glucocorticoid receptors following the stress of the passive avoidance test. Lastly, the elevated production of inflammatory cytokines (TNF-α, IL-1β) observed following social isolation was attenuated in the sumatriptan group.

CONCLUSION

Sumatriptan improved fear learning probably through modulation of HPA axis and hippocampus neuroinflammation.

Fear generalization predicts post-traumatic stress symptoms: A two-year follow-up study in Dutch fire fighters

INTRODUCTION

Excessive fear generalization has been associated with pathological anxiety, including posttraumatic stress disorder (PTSD). However, studies investigating the longitudinal relationship between generalization and the development of anxiety symptomatology are scarce. This study aims to test the predictive value of fear generalization for PTSD symptoms in a high-risk profession sample and to explore the relationship between generalization and neuroticism, which are both linked to PTSD.

METHOD

Longitudinal data from a multi-wave study in 529 Dutch fire-fighters were used. Fear generalization, PTSD symptoms and neuroticism were assessed at baseline. PTSD symptoms were reevaluated at six, 12, 18, and 24 months. Generalization was assessed in a differential conditioning paradigm by measuring expectancies of an aversive outcome when presented with stimuli similar to previously conditioned stimuli.

RESULTS

Higher expectancy ratings towards stimuli most similar to safety signals predicted PTSD symptoms at follow-up after controlling for baseline PTSD symptoms, whereas higher expectancy ratings towards stimuli most similar to danger signals was associated with neuroticism. Neuroticism weakened the predictive power of fear generalization when considered simultaneously.

DISCUSSION

These findings suggest that heightened fear generalization is associated with the development of anxiety and trauma-related symptoms. Targeting problematic fear generalization may be a promising intervention approach.

A test of pre-exposure spacing and multiple context pre-exposure on the mechanisms of latent inhibition of dental fear: A study protocol

BACKGROUND

Latent inhibition occurs when exposure to a stimulus prior its direct associative conditioning impairs learning. Results from naturalistic studies suggest that latent inhibition disrupts the learning of dental fear from aversive associative conditioning and thereby reduces the development of dental phobia. Although theory suggests latent inhibition occurs because pre-exposure changes the expected relevance and attention directed to the pre-exposed stimulus, evidence supporting these mechanisms in humans is limited. The aim of this study is to determine if two variables, pre-exposure session spacing and multiple context pre-exposure, potentiate the hypothesized mechanisms of expected relevance and attention and, in turn, increase latent inhibition of dental fear.

METHODS

In a virtual reality simulation, child and adult community members (ages 6 to 35) will take part in pre-exposure and conditioning trials, followed by short- and long-term tests of learning. A 100ms puff of 60 psi air to a maxillary anterior tooth will serve as the unconditioned stimulus. Pre-exposure session spacing (no spacing vs. sessions spaced) and multiple context pre-exposure (single context vs. multiple contexts) will be between-subject factors. Stimulus type (pre-exposed to-be conditioned stimulus, a non-pre-exposed conditioned stimulus, and an unpaired control stimulus) and trial will serve as within-subject factors. Baseline pain sensitivity will also be measured as a potential moderator.

DISCUSSION

It is hypothesized that spaced pre-exposure and pre-exposure in multiple contexts will increase the engagement of the mechanisms of expected relevance and attention and increase the latent inhibition of dental fear. It is expected that the findings will add to theory on fear learning and provide information to aid the design of future interventions that leverage latent inhibition to reduce dental phobia.

From cellular to fear memory: An epigenetic toolbox to remember

Throughout development, the neuronal epigenome is highly sensitive to external stimuli, yet capable of safeguarding cellular memory for a lifetime. In the adult brain, memories of fearful experiences are rapidly instantiated, yet can last for decades, but the mechanisms underlying such longevity remain unknown. Here, we showcase how fear memory formation and storage - traditionally thought to exclusively affect synapse-based events - elicit profound and enduring changes to the chromatin, proposing epigenetic regulation as a plausible molecular template for mnemonic processes. By comparing these to mechanisms occurring in development and differentiation, we notice that an epigenetic machinery similar to that preserving cellular memories might be employed by brain cells so as to form, store, and retrieve behavioral memories.

The guanine nucleotide exchange factor RapGEF2 is required for ERK-dependent immediate-early gene (Egr1) activation during fear memory formation

The MAP kinase ERK is important for neuronal plasticity underlying associative learning, yet specific molecular pathways for neuronal ERK activation are undetermined. RapGEF2 is a neuron-specific cAMP sensor that mediates ERK activation. We investigated whether it is required for cAMP-dependent ERK activation leading to other downstream neuronal signaling events occurring during associative learning, and if RapGEF2-dependent signaling impairments affect learned behavior. Camk2α-cre+/-::RapGEF2fl/fl mice with depletion of RapGEF2 in hippocampus and amygdala exhibit impairments in context- and cue-dependent fear conditioning linked to corresponding impairment in Egr1 induction in these two brain regions. Camk2α-cre+/-::RapGEF2fl/fl mice show decreased RapGEF2 expression in CA1 and dentate gyrus associated with abolition of pERK and Egr1, but not of c-Fos induction, following fear conditioning, impaired freezing to context after fear conditioning, and impaired cAMP-dependent long-term potentiation at perforant pathway and Schaffer collateral synapses in hippocampal slices ex vivo. RapGEF2 expression is largely eliminated in basolateral amygdala, also involved in fear memory, in Camk2α-cre+/-::RapGEF2fl/fl mice. Neither Egr1 nor c-fos induction in BLA after fear conditioning, nor cue-dependent fear learning, are affected by ablation of RapGEF2 in BLA. However, Egr1 induction (but not that of c-fos) in BLA is reduced after restraint stress-augmented fear conditioning, as is freezing to cue after restraint stress-augmented fear conditioning, in Camk2α-cre+/-::RapGEF2fl/fl mice. Cyclic AMP-dependent GEFs have been genetically associated as risk factors for schizophrenia, a disorder associated with cognitive deficits. Here we show a functional link between one of them, RapGEF2, and cognitive processes involved in associative learning in amygdala and hippocampus.

A study protocol testing pre-exposure dose and compound pre-exposure on the mechanisms of latent inhibition of dental fear

BACKGROUND

Dental stimuli can evoke fear after being paired - or conditioned - with aversive outcomes (e.g., pain). Pre-exposing the stimuli before conditioning can impair dental fear learning via a phenomenon known as latent inhibition. Theory suggests changes in expected relevance and attention are two mechanisms responsible for latent inhibition. In the proposed research, we test whether pre-exposure dose and degree of pre-exposure novelty potentiate changes in expected relevance and attention to a pre-exposed stimulus. We also assess if the manipulations alter latent inhibition and explore the possible moderating role of individual differences in pain sensitivity.

METHODS

Participants will be healthy individuals across a wide range of ages (6 to 35 years), from two study sites. Participants will undergo pre-exposure and conditioning followed by both a short-term and long-term test of learning, all in a novel virtual reality environment. The unconditioned stimulus will be a brief pressurized puff of air to a maxillary anterior tooth. Pre-exposure dose (low vs. high) and pre-exposure novelty (element stimulus vs. compound stimuli) will be between-subject factors, with stimulus type (pre-exposed to-be conditioned stimulus, a non-pre-exposed conditioned stimulus, and an unpaired control stimulus) and trial as within-subject factors. Pain sensitivity will be measured through self-report and a cold pressor test. It is hypothesized that a larger dose of pre-exposure and compound pre-exposure will potentiate the engagement of the target mechanisms and thereby result in greater latent inhibition in the form of reduced fear learning. Further, it is hypothesized that larger effects will be observed in participants with greater baseline pain sensitivity.

DISCUSSION

The proposed study will test whether pre-exposure dose and compound stimulus presentation change expected relevance and attention to the pre-exposed stimulus, and thereby enhance latent inhibition of dental fear. If found, the results will add to our theoretical understanding of the latent inhibition of dental fear and inform future interventions for dental phobia prevention.

Hippocampal Involvement in Safety Signal Learning Varies With Anxiety Among Healthy Adults

Background

Safety signal learning (SSL), based on conditioned inhibition of fear in the presence of learned safety, can effectively attenuate threat responses in animal models and humans. Difficulty regulating threat responses is a core feature of anxiety disorders, suggesting that SSL may provide a novel mechanism for fear reduction. Cross-species evidence suggests that SSL involves functional connectivity between the anterior hippocampus and the dorsal anterior cingulate cortex. However, the neural mechanisms supporting SSL have not been examined in relation to trait anxiety or while controlling for the effect of novelty.

Methods

Here, we investigated the neural mechanisms involved in SSL and associations with trait anxiety in a sample of 64 healthy (non-clinically anxious) adults (ages 18-30 years; 43 female, 21 male) using physiological, behavioral, and neuroimaging (functional magnetic resonance imaging) data collected during an SSL task.

Results

During SSL, compared with individuals with lower trait anxiety, individuals with higher trait anxiety showed less fear reduction as well as altered hippocampal activation and hippocampal-dorsal anterior cingulate cortex functional connectivity, and lower inferior frontal gyrus and ventrolateral prefrontal cortex activation. Importantly, the findings show that SSL reduces threat responding, across learning and over and above the effect of novelty, and involves hippocampal activation.

Conclusions

These findings provide new insights into the nature of SSL and suggest that there may be meaningful variation in SSL and related neural correlates as a function of trait anxiety, with implications for better understanding fear reduction and optimizing interventions for individuals with anxiety disorders.

Pharmacological modulation of conditioned fear in the fear-potentiated startle test: a systematic review and meta-analysis of animal studies

RATIONALE AND OBJECTIVES

Fear conditioning is an important aspect in the pathophysiology of anxiety disorders. The fear-potentiated startle test is based on classical fear conditioning and over the years, a broad range of drugs have been tested in this test. Synthesis of the available data may further our understanding of the neurotransmitter systems that are involved in the expression of conditioned fear.

METHODS

Following a comprehensive search in Medline and Embase, we included 68 research articles that reported on 103 drugs, covering 56 different drug classes. The systematic review was limited to studies using acute, systemic drug administration in naive animals.

RESULTS

Qualitative data synthesis showed that most clinically active anxiolytics, but not serotonin-reuptake inhibitors, reduced cued fear. Anxiogenic drugs increased fear potentiation in 35% of the experiments, reduced fear potentiation in 29% of the experiments, and were without effect in 29% of the experiments. Meta-analyses could be performed for five drug classes and showed that benzodiazepines, buspirone, 5-HT1A agonists, 5-HT1A antagonists, and mGluR2,3 agonists reduced cued conditioned fear. The non-cued baseline startle response, which may reflect contextual anxiety, was only significantly reduced by benzodiazepines and 5-HT1A antagonists. No associations were found between drug effects and methodological characteristics, except for strain.

CONCLUSIONS

The fear-potentiated startle test appears to have moderate to high predictive validity and may serve as a valuable tool for the development of novel anxiolytics. Given the limited available data, the generally low study quality and high heterogeneity additional studies are warranted to corroborate the findings of this review.

The effect of SSRIs on fear learning: a systematic review and meta-analysis

RATIONALE

Selective serotonin reuptake inhibitors (SSRIs) are considered first-line medication for anxiety-like disorders such as panic disorder, generalized anxiety disorder, and post-traumatic stress disorder. Fear learning plays an important role in the development and treatment of these disorders. Yet, the effect of SSRIs on fear learning are not well known.

OBJECTIVE

We aimed to systematically review the effect of six clinically effective SSRIs on acquisition, expression, and extinction of cued and contextual conditioned fear.

METHODS

We searched the Medline and Embase databases, which yielded 128 articles that met the inclusion criteria and reported on 9 human and 275 animal experiments.

RESULTS

Meta-analysis showed that SSRIs significantly reduced contextual fear expression and facilitated extinction learning to cue. Bayesian-regularized meta-regression further suggested that chronic treatment exerts a stronger anxiolytic effect on cued fear expression than acute treatment. Type of SSRI, species, disease-induction model, and type of anxiety test used did not seem to moderate the effect of SSRIs. The number of studies was relatively small, the level of heterogeneity was high, and publication bias has likely occurred which may have resulted in an overestimation of the overall effect sizes.

CONCLUSIONS

This review suggests that the efficacy of SSRIs may be related to their effects on contextual fear expression and extinction to cue, rather than fear acquisition. However, these effects of SSRIs may be due to a more general inhibition of fear-related emotions. Therefore, additional meta-analyses on the effects of SSRIs on unconditioned fear responses may provide further insight into the actions of SSRIs.

Threat learning in space: How stimulus-outcome spatial compatibility modulates conditioned skin conductance response

A central question in Pavlovian conditioning concerns the critical conditions that drive the acquisition and maintenance of the stimulus-outcome association. The spatial relationship between the conditioned (CS) and unconditioned (US) stimuli is considered to exert strong effects on learning. However, how spatial information modulates Pavlovian learning remains mostly unexplored in humans. Here, we test how the compatibility between the CS and the US location influences the acquisition, extinction, and recovery (following reinstatement) of Pavlovian conditioned threat. Participants (N = 20) completed a differential threat conditioning task in which visual CSs appeared on the same (compatible) or opposite (incompatible) hemispace as the US delivery (aversive shock to one hand), while their skin conductance response served as an index of learning. Results show that initial threat expectations were biased in favor of compatible CSs before conditioning. Nevertheless, this bias was revised during acquisition to reflect current stimulus-outcome contingencies. Computational modeling suggested that this effect occurred through a higher reliance on positive aversive prediction errors for incompatible CSs, thereby facilitating learning of their association with the US. Additionally, the conditioned response to incompatible CSs was associated with initially slower extinction and a greater recovery after threat reinstatement. These findings suggest that spatial information conveyed by stimuli and outcomes can be flexibly used to enact defensive responses to the current source of danger, highlighting the adaptive nature of Pavlovian learning.

Role of nitric oxide on defensive behavior and long-term aversive learning induced by chemical stimulation of the dorsolateral periaqueductal gray matter

The midbrain periaqueductal gray matter, especially the dorsolateral portion (dlPAG), coordinates immediate defensive responses (DR) to threats, but also ascends forebrain information for aversive learning. The synaptic dynamics in the dlPAG regulate the intensity and type of behavioral expression, as well as long-term processes such as memory acquisition, consolidation, and retrieval. Among several neurotransmitters and neural modulators, nitric oxide seems to play an important regulatory role in the immediate expression of DR, but it remains unclear if this gaseous on-demand neuromodulator contributes to aversive learning. Therefore, the role of nitric oxide in the dlPAG was investigated, during conditioning in an olfactory aversive task. The behavioral analysis consisted of freezing and crouch-sniffing in the conditioning day after glutamatergic NMDA agonist injection into the dlPAG. Two days later, rats were re-exposed to the odor cue and avoidance was measured. 7NI, a selective neuronal nitric oxide synthase inhibitor (40 and 100 nmol), injected before NMDA (50 pmol) impaired immediate DR and consequent aversive learning. The scavenging of extrasynaptic nitric oxide by C-PTIO (1 and 2 nmol) induced similar results. Moreover, spermine NONOate, a nitric oxide donor (5, 10, 20, 40, and 80 nmol), produced DR by itself, but only the low dose also promoted learning. The following experiments utilized a fluorescent probe, DAF-FM diacetate (5 µM), directly into the dlPAG, to quantify nitric oxide in the three previous experimental situations. Nitric oxide levels were increased after NMDA stimulation, decreased after 7NI, and increased after spermine NONOate, in line with alterations in defensive expression. Altogether, the results indicate that nitric oxide plays a modulatory and decisive role in the dlPAG regarding immediate DR and aversive learning.

Study protocol of an investigation of attention and prediction error as mechanisms of action for latent inhibition of dental fear in humans

BACKGROUND

Evidence suggests that dental anxiety and phobia are frequently the result of direct associative fear conditioning but that pre-exposure to dental stimuli prior to conditioning results in latent inhibition of fear learning. The mechanisms underlying the pre-exposure effect in humans, however, are poorly understood. Moreover, pain sensitivity has been linked to dental fear conditioning in correlational investigations and theory suggests it may moderate the latent inhibition effect, but this hypothesis has not been directly tested. These gaps in our understanding are a barrier to the development of evidence-based dental phobia prevention efforts.

METHODS

Healthy volunteers between the ages of 6 and 35 years will be enrolled across two sites. Participants will complete a conditioning task in a novel virtual reality environment, allowing for control over pre-exposure and the examination of behaviour. A dental startle (a brief, pressurized puff of air to a tooth) will serve as the unconditioned stimulus. Using a within-subjects experimental design, participants will experience a pre-exposed to-be conditioned stimulus, a non-pre-exposed to-be conditioned stimulus, and a neutral control stimulus. Two hypothesized mechanisms, changes in prediction errors and attention, are expected to mediate the association between stimulus condition and fear acquisition, recall, and retention. To ascertain the involvement of pain sensitivity, this construct will be measured through self-report and the cold pressor task.

DISCUSSION

Dental phobia negatively affects the dental health and overall health of individuals. This study aims to determine the mechanisms through which pre-exposure retards conditioned dental fear acquisition, recall, and retention. A randomized control trial will be used to identify these mechanisms so that they can be precisely targeted and maximally engaged in preventative efforts.

Getting Better with Age? A Review of Psychophysiological Studies of Fear Extinction Learning Across Development

A critical developmental task is learning what constitutes reliable threat and safety signals in the environment. In humans, atypical fear learning processes are implicated in many mental health conditions, particularly fear and anxiety disorders, pointing to the potential for laboratory measures of fear learning to facilitate early identification of at-risk individuals. This chapter reviews studies of fear learning and extinction learning that incorporate peripheral measures of psychophysiological response and include a developmental sample. Broadly, these studies indicate substantial consistency in differential learning and extinction across development, as assessed with multiple paradigms, across physiological indices. Importantly, though, response coherence across measures (e.g., physiological, neural, and behavioral) was inconsistent across studies. There was also less consistency in results from studies that probed associations between anxiety and fear learning processes. These mixed findings highlight the need for additional examination of when and why there is variability, both across development and in relation to individual differences factors, including mental health, childhood adversity, and sex. In addition, there remains a need for studies that test for developmental change in extinction recall learning and whether stimulus type impacts learning across development. Longitudinal studies designed to address these questions could provide novel insight into the developmental trajectory of fear learning and extinction.

Fear modulates parental orienting during childhood and adolescence

Adults quickly orient toward sources of danger and deploy fight-or-flight tactics to manage threatening situations. In contrast, infants who cannot implement the safety strategies available to adults and depend heavily on caregivers for survival are more likely to turn toward familiar adults, such as their parents, to help them navigate threatening circumstances. However, work has yet to investigate how readily children and adolescents orient toward their parents in threatening or fearful contexts. The current work addressed this question using a visual search paradigm that included arrays of parents' and strangers' faces as target and distractor stimuli, preceded by a fear or neutral emotional priming procedure. Linear mixed-effects models showed that children and adolescents (N = 88, age range = 4-17 years; 42M/46F) were faster to search for the face of their parent than of a stranger. However, fear priming attenuated this effect of the parent on search times, such that children and adolescents were significantly slower to orient toward their parent in an array of strangers' faces if they were first primed with fear as opposed to a neutral video. This work indicates that fear priming may phasically interfere with parental orienting during childhood and adolescence, possibly because fear reallocates attention away from parents and toward (potentially threatening) unfamiliar people in the environment to facilitate the development of independent threat learning and coping systems.

The association between mnemonic discrimination ability and differential fear learning

BACKGROUND AND OBJECTIVES

It is important to be able to learn which stimuli in our surroundings predict aversive outcomes. To maintain emotional well-being, it is similarly important to be able to learn which stimuli predict safety. The ability to discriminate between stimuli that predict danger and safety has been suggested to not only have an emotional component, but also a cognitive one. One such candidate mechanism is mnemonic discrimination (MD), the ability to differentiate between two memories that are similar but not identical. In the present study, we wanted to examine if MD performance helps to explain inter-individual differences in the ability to acquire a differentiated fear response during fear conditioning.

METHODS

Participants performed a task assessing MD ability, and then underwent a fear conditioning procedure. Fear responses were measured using skin conductance responses (SCRs).

RESULTS

Results revealed no support for MD ability being associated with to which degree a differentiated fear response was acquired, or with the time needed to acquire such a response.

LIMITATIONS

Our only outcome measurement was SCRs. Future studies need to include fear ratings, expectancy ratings and neural responses. Future studies also need to examine this using a stimulus material where the conditioned stimulus and the safety stimulus are more difficult to distinguish from each other.

CONCLUSIONS

If MD ability has a role in inhibiting overgeneralization of fear learning, this does not seem to be driven by MD already during the initial learning.

Differential effects of chronic stress on anxiety-like behavior and contextual fear conditioning in the TgF344-AD rat model of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative brain disorder that leads to severe cognitive and functional impairments. Many AD patients also exhibit neuropsychiatric symptoms, such as anxiety and depression, prior to the clinical diagnosis of dementia. Chronic stress is associated with numerous adverse health consequences and disease states, and AD patients exhibit altered stress systems. Thus, stress may represent a causal link between neuropsychiatric symptoms and AD. To address this possibility, we examined the effects of chronic stress in the TgF344-AD rat model that co-expresses the mutant human amyloid precursor protein (APPsw) and presenilin 1 (PS1ΔE9) genes. Adult male transgenic (Tg+) and wild-type (WT) rats (6-7.5 months of age), with and without a history of chronic restraint stress, were tested for footshock-induced conditioned fear and for anxiety-like behavior in the elevated plus-maze. We found that non-stressed Tg+ rats showed increased anxiety-like behavior compared to non-stressed WT rats. In contrast, Tg+ and WT rats did not differ in levels of freezing immediately following footshock or during contextual re-exposure. Additionally, stressed Tg+ rats were not significantly different from stressed WT rats on any measures of anxiety or fear. Thus, while stress has been linked as a risk factor for AD-related pathology, it appears from the present findings that two weeks of daily restraint stress did not further enhance anxiety- or fear-like behaviors in TgF344-AD rats.

A Multivoxel Pattern Analysis of Anhedonia During Fear Extinction: Implications for Safety Learning

BACKGROUND

Pavlovian learning processes are central to the etiology and treatment of anxiety disorders. Anhedonia and related perturbations in reward processes have been implicated in Pavlovian learning. Associations between anhedonia symptoms and neural indices of Pavlovian learning can inform transdiagnostic associations among depressive and anxiety disorders.

METHODS

Participants ages 18 to 19 years (67% female) completed a fear extinction (n = 254) and recall (n = 249) paradigm during functional magnetic resonance imaging. Symptom dimensions of general distress (common to anxiety and depression), fears (more specific to anxiety), and anhedonia-apprehension (more specific to depression) were evaluated. We trained whole-brain multivoxel pattern decoders for anhedonia-apprehension during extinction and extinction recall and tested the decoders' ability to predict anhedonia-apprehension in an external validation sample. Specificity analyses examined effects covarying for general distress and fears. Decoding was repeated within canonical brain networks to highlight candidate neurocircuitry underlying whole-brain effects.

RESULTS

Whole-brain decoder training succeeded during both tasks. Prediction of anhedonia-apprehension in the external validation sample was successful for extinction (R² = 0.047; r = 0.276, p = .002) but not extinction recall (R² < 0.001, r = -0.063, p = .492). The extinction decoder remained significantly associated with anhedonia-apprehension covarying for fears and general distress (t₁₂₁ = 3.209, p = .002). Exploratory results highlighted activity in the cognitive control, default mode, limbic, salience, and visual networks related to these effects.

CONCLUSIONS

Results suggest that patterns of brain activity during extinction, particularly in the cognitive control, default mode, limbic, salience, and visual networks, can be predictive of anhedonia symptoms. Future research should examine associations between anhedonia and extinction, including studies of exposure therapy or positive affect treatments among anhedonic individuals.

Fear learning, avoidance, and generalization are more context-dependent for adults than adolescents

This study examined developmental differences in contextual and perceptual generalization of fear and avoidance learning. Adults (N = 39) and adolescents (N = 44) completed differential fear acquisition wherein each conditional stimulus (CS) appeared in a background context. In the dangerous context, one stimulus (CS+) predicted an aversive sound, and the other stimulus (CS-) did not. In the safe context, the aversive sound was never administered with either CS. During fear generalization, participants were presented with three generalization stimuli (GSs), ranging on a perceptual continuum from threat to safety stimuli, in both contexts. Participants then completed avoidance conditioning and avoidance generalization phases, allowing them to actively avoid the upcoming aversive sound by pressing an avoidance button. Developmental differences emerged in threat perception, physiological arousal, avoidance behavior, and eye movements during contextual fear learning and generalization. Adolescents showed less discrimination between stimuli and contexts than adults, resulting primarily from their elevated fear responses to safety and generalized stimuli. Developmental differences in fear learning should be further explored in future research, as they could explain why adolescence is a sensitive developmental period for anxiety.

Glucocorticoids Promote Fear Generalization by Increasing the Size of a Dentate Gyrus Engram Cell Population

BACKGROUND

Traumatic experiences, such as conditioned threat, are coded as enduring memories that are frequently subject to generalization, which is characterized by (re-) expression of fear in safe environments. However, the neurobiological mechanisms underlying threat generalization after a traumatic experience and the role of stress hormones in this process remain poorly understood.

METHODS

We examined the influence of glucocorticoid hormones on the strength and specificity of conditioned fear memory at the level of sparsely distributed dentate gyrus (DG) engram cells in male mice.

RESULTS

We found that elevating glucocorticoid hormones after fear conditioning induces a generalized contextual fear response. This was accompanied by a selective and persistent increase in the excitability and number of activated DG granule cells. Selective chemogenetic suppression of these sparse cells in the DG prevented glucocorticoid-induced fear generalization and restored contextual memory specificity, while leaving expression of auditory fear memory unaffected.

CONCLUSIONS

These results implicate the sparse ensemble of DG engram cells as a critical cellular substrate underlying fear generalization induced by glucocorticoid stress hormones.

Effects of increased attention allocation to threat and safety stimuli on fear extinction and its recall

BACKGROUND AND OBJECTIVES

Attention plays an important role in the treatment of anxiety. Increased attention to threat has been shown to yield improved treatment outcomes in anxious patients following exposure-based therapy. This study examined whether increasing attention to learned stimuli during fear extinction, an experimental analogue for exposure-based treatments, could improve extinction learning and its maintenance.

METHODS

Sixty-five healthy adults were randomized into experimental or control conditions. All completed a differential fear conditioning task. During extinction, a subtle attentional manipulation was implemented in the experimental group, designed to increase participants' attention to both threat and safety cues. Three days later, an extinction recall test was conducted using the original cues and two perceptually similar morphs.

RESULTS

Fear conditioning was achieved in both behavioral and psychophysiological measures. In addition, between-group differences emerged during extinction. The experimental group exhibited increased attention to stimuli and lower fear responses in physiological measure than the control group. Similarly, during extinction recall, the experimental group exhibited lower startle responses than the control group. Last, across groups, attending to the safety cue during extinction was associated with lower self-reported risk of the two generalization morphs displayed during extinction recall.

LIMITATIONS

Skin conductance response (SCR) was not measured during extinction recall. Future research should include both SCR and additional generalization morphs so as to allow for the examination of more subtle individual differences.

CONCLUSIONS

Results indicate that the attentional manipulation increased attention allocation to stimuli during extinction; this, in turn, affected fear-related physiological response.

Appetitive and aversive sensory preconditioning in rats is impaired by disruption of the postrhinal cortex

Episodic memory involves binding stimuli and/or events together in time and place. Furthermore, memories become more complex when new experiences influence the meaning of stimuli within the original memory. Thus collectively, complex episodic memory formation and maintenance involves processes such as encoding, storage, retrieval, updating and reconsolidation, which can be studied using animal models of higher-order conditioning. In the present study aversive and appetitive sensory preconditioning paradigms were used to test the hypothesis that the postrhinal cortex (POR), which is a component of the hippocampal memory system, is involved in higher-order conditioning. Drawing on the known role of the POR in contextual learning, Experiment 1 employed a four-phase sensory preconditioning task that involved fear learning and context discrimination in rats with or without permanent lesions of the POR. In parallel, to examine POR function during higher-order conditioning in the absence of a particular spatial arrangement, Experiments 2 and 3 used a three-phase sensory preconditioning paradigm involving phasic stimuli. In Experiment 2, bilateral lesions of the POR were made and in Experiment 3, a chemogenetic approach was used to temporarily inactivate POR neurons during each phase of the paradigm. Evidence of successful sensory preconditioning was observed in sham rats which, during the critical context discrimination test, demonstrated higher levels of freezing behavior when re-exposed to the paired versus the unpaired context, whereas POR-lesioned rats did not. Data from the appetitive sensory preconditioning paradigm also confirmed the hypothesis in that during the critical auditory discrimination test, sham rats showed greater food cup responding following presentations of the paired compared to the unpaired auditory stimulus, whereas POR-lesioned rats did not. Lastly, in Experiment 3, when the POR was inactivated only during preconditioning or only during conditioning, discrimination during the critical auditory test was impaired. Thus, regardless of whether stimulus-stimulus associations were formed between static or phasic stimuli or whether revaluation of the paired stimulus occurred through association with an aversive or an appetitive unconditioned stimulus, the effects were the same; POR lesions disrupted the ability to use higher-order conditioned stimuli to guide prospective behavior.

Emerging Domain-Based Treatments for Pediatric Anxiety Disorders

Domain-specific cognitive training treatments for pediatric anxiety disorders rely on accurate and reliable identification of specific underlying deficits and biases in neurocognitive functions. Once identified, such biases can serve as specific targets for therapeutic intervention. Clinical translations typically reflect mechanized training protocols designed to rectify the identified biases. Here, we review and synthesize research on key neurocognitive processes that emerge as potential targets for specialized cognitive training interventions in pediatric anxiety disorders in the domains of attention, interpretation, error monitoring, working memory, and fear learning. For each domain, we describe the current status of target establishment (i.e., an association between pediatric anxiety and a specific neurocognitive process), and then review extant translational efforts regarding these targets and the evidence supporting their clinical utility in youths. We then localize each of the domains within the path leading to efficacious, evidence-supported treatments for pediatric anxiety, providing a roadmap for future research. The review indicates that specific cognitive targets in pediatric anxiety have been established in all the reviewed domains except for fear learning, where a clear target is yet to be elucidated. In contrast, evidence for clinical efficacy emerged only in the threat-related attention domain, with some preliminary findings in the domains of interpretation and working memory. The path to clinical translation in the domain of error monitoring is yet unclear. Implications and potential avenues for future research and translation are discussed.

Pavlovian Learning Processes in Pediatric Anxiety Disorders: A Critical Review

Deficits in associative and Pavlovian learning are thought to lie at the center of anxiety-related disorders. However, the majority of studies have been carried out in adult populations. The aim of this review was to critically examine the behavioral and neuroimaging literature on Pavlovian learning in pediatric anxiety disorders. We conclude that although there is evidence for deficits in Pavlovian processes (e.g., heightened reactivity to safety cues in anxious samples), the extant literature suffers from key methodological and theoretical issues. We conclude with theoretical and methodological recommendations for future research in order to further elucidate the role of Pavlovian learning in the etiology, maintenance, and treatment of pediatric anxiety disorders.

Impaired fear learning and extinction, but not generalization, in anxious and non-anxious depression

Fear conditioning and generalization are well-known mechanisms in the pathogenesis of anxiety disorders. Extinction of conditioned fear responses is crucial for the psychotherapeutic treatment of these diseases. Anxious depression as a subtype of major depression shares characteristics with anxiety disorders. We therefore aimed to compare fear learning mechanisms in patients with anxious versus non-anxious depression. Fear learning mechanisms in patients with major depression (n = 79; for subgroup analyses n = 41 patients with anxious depression and n = 38 patients with non-anxious depression) were compared to 48 healthy participants. We used a well-established differential fear conditioning paradigm investigating acquisition, generalization, and extinction. Ratings of valence, arousal and probability of expected threat were assessed as well as skin conductance response as an objective psychophysiological measure. Patients with major depression showed impaired acquisition of conditioned fear. In addition, depressed patients showed impaired extinction of conditioned fear responses after successful fear conditioning. Generalization was not affected. However, there was no difference between patients with anxious and non-anxious depression. Results differed between objective and subjective measures. Our findings show altered fear acquisition and extinction in major depression as compared to healthy controls, but they do not favor differential fear learning and extinction mechanisms in the pathogenesis of anxious versus non-anxious depression. The results of impaired extinction warrant future studies addressing extinction learning elements in the treatment of depression.

Sleep to remember, sleep to forget: Rapid eye movement sleep can have inverse effects on recall and generalization of fear memories

Rapid Eye Movement (REM) sleep has been shown to modulate the consolidation of fear memories, a process that may contribute to the development of Post-Traumatic Stress Disorder (PTSD). However, contradictory findings have been reported regarding the direction of this modulation and its differential effects on recall versus generalization. In two complementary experiments, we addressed this by employing sleep deprivation protocols together with a novel fear-conditioning paradigm that required the discrimination between coexisting threat and safety signals. Using skin conductance responses and functional imaging (fMRI), we found two opposing effects of REM sleep: While REM impaired recall of the original threat memories, it improved the ability to generalize these memories to novel situations that emphasized the discrimination between threat and safety signals. These results, as well as previous findings in healthy participants and patients diagnosed with PTSD, could be explained by the degree to which the balance between threat and safety signals for a given stimulus was predictive of threat. We suggest that this account can be integrated with contemporary theories of sleep and fear learning, such as the REM recalibration hypothesis.

Animal Models for OCD Research

OCD has lagged behind other psychiatric illnesses in the identification of molecular treatment targets, due in part to a lack of significant findings in genome-wide association studies. However, while progress in this area is being made, OCD's symptoms of obsessions, compulsions, and anxiety can be deconstructed into distinct neural functions that can be dissected in animal models. Studies in rodents and non-human primates have highlighted the importance of cortico-basal ganglia-thalamic circuits in OCD pathophysiology, and emerging studies in human post-mortem brain tissue point to glutamatergic synapse abnormalities as a potential cellular substrate for observed dysfunctional behaviors. In addition, accumulated evidence points to a potential role for neuromodulators including serotonin and dopamine in both OCD pathology and treatment. Here, we review current efforts to use animal models for the identification of molecules, cell types, and circuits relevant to OCD pathophysiology. We start by describing features of OCD that can be modeled in animals, including circuit abnormalities and genetic findings. We then review different strategies that have been used to study OCD using animal model systems, including transgenic models, circuit manipulations, and dissection of OCD-relevant neural constructs. Finally, we discuss how these findings may ultimately help to develop new treatment strategies for OCD and other related disorders.

Learning to predict pain: differences in people with persistent neck pain and pain-free controls

Background

Learning to predict threatening events enables an organism to engage in protective behavior and prevent harm. Failure to differentiate between cues that truly predict danger and those that do not, however, may lead to indiscriminate fear and avoidance behaviors, which in turn may contribute to disability in people with persistent pain. We aimed to test whether people with persistent neck pain exhibit contingency learning deficits in predicting pain relative to pain-free, gender-and age-matched controls.

Method

We developed a differential predictive learning task with a neck pain-relevant scenario. During the acquisition phase, images displaying two distinct neck positions were presented and participants were asked to predict whether these neck positions would lead to pain in a fictive patient with persistent neck pain (see fictive patient scenario details in Appendix A). After participants gave their pain-expectancy judgment in the hypothetical scenario, the verbal outcome (PAIN or NO PAIN) was shown on the screen. One image (CS+) was followed by the outcome “PAIN”, while another image (CS−) was followed by the outcome “NO PAIN”. During the generalization phase, novel but related images depicting neck positions along a continuum between the CS+ and CS− images (generalization stimuli; GSs) were introduced to assess the generalization of acquired predictive learning to the novel images; the GSs were always followed by the verbal outcome “NOTES UNREADABLE” to prevent extinction learning. Finally, an extinction phase was included in which all images were followed by “NO PAIN” assessing the persistence of pain-expectancy judgments following disconfirming information.

Results

Differential pain-expectancy learning was reduced in people with neck pain relative to controls, resulting from patients giving significantly lower pain-expectancy judgments for the CS+, and significantly higher pain-expectancy judgments for the CS−. People with neck pain also demonstrated flatter generalization gradients relative to controls. No differences in extinction were noted.

Discussion

The results support the hypothesis that people with persistent neck pain exhibit reduced differential pain-expectancy learning and flatter generalization gradients, reflecting deficits in predictive learning. Contrary to our hypothesis, no differences in extinction were found. These findings may be relevant to understanding behavioral aspects of chronic pain.

No persistent attenuation of fear memories in humans: A registered replication of the reactivation-extinction effect

It has been proposed that memory retrieval can destabilize consolidated memories, after which they need to be reconsolidated in order to be retained. The presentation of relevant information during memory reconsolidation could then result in the modification of a destabilized memory trace, by allowing the memory trace to be updated before being reconsolidated. In line with this idea, Schiller et al. (2010) have demonstrated that memory retrieval shortly before extinction training can prevent the later recovery of conditioned fear responding that is observed after regular extinction training. Those findings have been the subject of considerable controversy, due in part to theoretical reasons but also due to a number of failures to obtain similar results in conceptual replication attempts. Here, we report the results of a highly powered, direct, independent replication of the critical conditions of Schiller et al. (2010, Experiment 1). Due to misrepresentation of the exclusion criteria in the original Schiller et al. (2010) report, data collection was considerably delayed. When we eventually managed to attain our pre-registered sample size, we found that we could not observe any benefit of reactivation-extinction over regular extinction training in preventing recovery of conditioned fear. The results of the present study, along with the mixed findings in the literature and the misreporting in Schiller et al. (2010), give cause to question whether there is robust evidence that reactivation-extinction prevents the return of fear in humans.

Conserved features of anterior cingulate networks support observational learning across species

The ability to observe, interpret, and learn behaviors and emotions from conspecifics is crucial for survival, as it bypasses direct experience to avoid potential dangers and maximize rewards and benefits. The anterior cingulate cortex (ACC) and its extended neural connections are emerging as important networks for the detection, encoding, and interpretation of social signals during observational learning. Evidence from rodents and primates (including humans) suggests that the social interactions that occur while individuals are exposed to important information in their environment lead to transfer of information across individuals that promotes adaptive behaviors in the form of either social affiliation, alertness, or avoidance. In this review, we first showcase anatomical and functional connections of the ACC in primates and rodents that contribute to the perception of social signals. We then discuss species-specific cognitive and social functions of the ACC and differentiate between neural activity related to 'self' and 'other', extending into the difference between social signals received and processed by the self, versus observing social interactions among others. We next describe behavioral and neural events that contribute to social learning via observation. Finally, we discuss some of the neural mechanisms underlying observational learning within the ACC and its extended network.

Deficiency of the immunoproteasome subunit β5i/LMP7 supports the anxiogenic effects of mild stress and facilitates cued fear memory in mice

Proteolysis as mediated by one of the major cellular protein degradation pathways, the ubiquitin-proteasome system (UPS), plays an essential role in learning and memory formation. However, the functional relevance of immunoproteasomes in the healthy brain and especially their impact on normal brain function including processes of learning and memory has not been investigated so far. In the present study, we analyzed the phenotypic effects of an impaired immunoproteasome formation using a β5i/LMP7-deficient mouse model in different behavioral paradigms focusing on locomotor activity, exploratory behavior, innate anxiety, startle response, prepulse inhibition, as well as fear and safety conditioning. Overall, our results demonstrate no strong effects of constitutive β5i/LMP7-deficiency on gross locomotor abilities and anxiety-related behavior in general. However, β5i/LMP7-deficient mice expressed more anxiety after mild stress and increased cued fear after fear conditioning. These findings indicate that the basal proper formation of immunoproteasomes and/or at least the expression of β5i/LMP7 in healthy mice seem to be involved in the regulation of anxiety and cued fear levels.

Animal Models of PTSD: A Critical Review

The goals of animal research in post-traumatic stress disorder (PTSD) include better understanding the neurophysiological etiology of PTSD, identifying potential targets for novel pharmacotherapies, and screening drugs for their potential use as PTSD treatment in humans. Diagnosis of PTSD relies on a patient interview and, as evidenced by changes to the diagnostic criteria in the DSM-5, an adequate description of this disorder in humans is a moving target. Therefore, it may seem insurmountable to model the construct of PTSD in animals such as rodents. Fortunately, the neural circuitry involved in fear and anxiety, thought to be essential to the etiology of PTSD in humans, is highly conserved throughout evolution. Furthermore, many symptoms can be modeled using behavioral tests that have face, construct, and predictive validity. Because PTSD is precipitated by a definite traumatic experience, animal models can simulate the induction of PTSD, and test causal factors with longitudinal designs. Accordingly, several animal models of physical and psychological trauma have been established. This review discusses the widely used animal models of PTSD in rodents, and overviews their strengths and weaknesses in terms of face, construct, and predictive validity.

Sleep Disruption, Safety Learning, and Fear Extinction in Humans: Implications for Posttraumatic Stress Disorder

Fear learning is critical in the development and maintenance of posttraumatic stress disorder (PTSD) symptoms, and safety learning and extinction are necessary for recovery. Studies in animal models suggest that sleep disruption, and REM sleep fragmentation in particular, interfere with safety learning and extinction processes, and recently, studies are extending these findings to humans. A discussion of the human literature is presented here, which largely consists of experimental studies in healthy human control subjects. A theoretical model for the relationship between fear learning, sleep disruption, and impaired safety learning and extinction is proposed, which provides an explanatory framework for sleep disruption and its relationship to PTSD. Overall, findings suggest that sleep disruption plays a role in the development and maintenance of PTSD symptoms, and thus presents an important modifiable target in PTSD treatment.

Functional neuroimaging of associative learning and generalization in specific phobia

BACKGROUND

Theoretical models have implicated classical fear conditioning, fear generalization, and extinction learning in the development of anxiety disorders. To date, it is largely unknown to what extent these mechanisms and the underlying neurobiology may be altered in specific phobia, a disorder characterized by focal fears. The current study systematically examined fear conditioning, fear generalization, extinction learning, and extinction recall in a sample of individuals with a specific phobia.

METHODS

Participants with a specific phobia (SP) of spiders (n = 46) and healthy controls (HC) (n = 48) underwent a 3-day fMRI cue-conditioning protocol, including a fear acquisition and a fear generalization phase (day 1), an extinction learning phase (day 2), and an extinction recall phase (day 3). Stimuli were phobia-irrelevant, as geometrical shapes served as conditioned threat (CS+) and safety stimuli (CS-), and an electrical shock as the unconditioned stimulus (US). Self-reported fear, US expectancy, and blood-oxygen-level dependent responses were measured.

RESULTS

Behavioral results only revealed enhanced CS+/CS-differentiation in fear scores during acquisition retention in SP. Some neural differences were observed during other task phases. During early fear acquisition, SP showed enhanced differential activation in the angular gyrus and lateral occipital cortex, and during extinction recall, more precuneus deactivation was found in SP compared to HC. There were no clear indications of altered neural fear generalization or extinction learning mechanisms in the SP group.

CONCLUSIONS

Results indicate that spider phobia may be characterized by enhanced differential fear retention and altered brain activation patterns during fear acquisition and extinction recall. The findings provide insight into the nature of fear learning alterations in specific phobia, and how these may differ from those found in disorders characterized by broad anxious distress.

Problematic attention processing and fear learning in adolescent anxiety: Testing a combined cognitive and learning processes model

BACKGROUND AND OBJECTIVES

Anxiety in adolescence is characterised by disturbances in attentional processes and the overgeneralisation of fear, however, little is known about the combined and reciprocal effects of and between these factors on youth anxiety. The present study investigated whether attention (attention allocation and control) and fear generalisation processes together predict more variance on adolescent anxiety symptoms than each factor in isolation, and explored their interrelations.

METHODS

197 adolescents completed a novel conditioning task, which paired balloon cues with mildly aversive or neutral outcomes. A spatial cueing task, and self-report measures of emotional attentional control and anxiety, were also completed.

RESULTS

Threat-avoidant attention allocation biases, impaired attention control, and exaggerated fear generalisation together predicted greater variance in anxiety symptoms (55.3%), than each set of fear and attention processes in isolation. Results also provided evidence of an interplay between these factors. Individual differences in threat-avoidant attention allocation biases predicted variability in the generalisation of fear, whilst the association between heightened anxiety and the overgeneralization of fear was moderated by poor attention control.

CONCLUSIONS

This study provides unique evidence of the combined effects of attention and fear generalisation mechanisms in explaining youth anxiety, and interrelations between these factors. Importantly, results suggested that deficiencies in attention control may bring out anxiety-associated impairments in fear generalisation.

LIMITATIONS

We relied on self-reported ratings of fear during generalization and also of attention control. Thus demand effects cannot be discounted. Reaction-time measures of attention focus are also indirect assessments of attention that may lack precision.

Exploring the involvement of Tac2 in the mouse hippocampal stress response through gene networking

Tachykinin 2 (Tac2) is expressed in a number of areas throughout the brain, including the hippocampus. However, knowledge about its function has been only well explored in the hypothalamus in the context of reproductive health. In this study, we identified and validated increased hippocampal Tac2 mRNA expression in response to chronic mild stress in mice. Expression quantitative trait locus (eQTL) analysis showed Tac2 is cis-regulated in the hippocampus. Using a systems genetics approach, we constructed a Tac2 co-expression network to better understand the relationship between Tac2 and the hippocampal stress response. Our network identified 69 total genes associated with Tac2, several of which encode major neuropeptides involved in hippocampal stress signaling as well as critical genes for producing neural plasticity, indicating that Tac2 is involved in these processes. Pathway analysis for the member of Tac2 gene network revealed a strong connection between Tac2 and neuroactive ligand-receptor interaction, calcium signaling pathway, as well as cardiac muscle contraction. In addition, we also identified 46 stress-related phenotypes, specifically fear conditioning response, that were significantly correlated with Tac2 expression. Our results provide evidence for Tac2 as a strong candidate gene who likely plays a role in hippocampal stress processing and neural plasticity.

Increased anxiety-like behavior following circuit-specific catecholamine denervation in mice

Parkinson's disease (PD) presents with a constellation of non-motor symptoms, notably increased anxiety, which are currently poorly treated and underrepresented in animal models of the disease. Human post-mortem studies report loss of catecholaminergic neurons in the pre-symptomatic phases of PD when anxiety symptoms emerge, and a large literature from rodent and human studies indicate that catecholamines are important mediators of anxiety via their modulatory effects on limbic regions such as the amygdala. On the basis of these observations, we hypothesized that anxiety in PD could result from an early loss of catecholaminergic inputs to the amygdala and/or other limbic structures. To interrogate this hypothesis, we bilaterally injected the neurotoxin 6-OHDA in the mouse basolateral amygdala (BL). This produced a restricted pattern of catecholaminergic (tyrosine-hydroxylase-labeled) denervation in the BL, intercalated cell masses and ventral hippocampus, but not the central amygdala or prefrontal cortex. We found that this circuit-specific lesion did not compromise performance on multiple measures of motor function (home cage, accelerating rotarod, beam balance, pole climbing), but did increase anxiety-like behavior in the elevated plus-maze and light-dark exploration tests. Fear behavior in the pavlovian cued conditioning and passive avoidance assays was, by contrast, unaffected; possibly due to preservation of catecholamine innervation of the central amygdala from the periaqueductal gray. These data provide some of the first evidence implicating loss of catecholaminergic neurotransmission in midbrain-amygdala circuits to increased anxiety-like behavior. Our findings offer an initial step towards identifying the neural substrates for pre-motor anxiety symptoms in PD.

Activation of somatodendritic 5-HT1A autoreceptors reduces the acquisition and expression of cued fear in the rat fear-potentiated startle test

RATIONALE

Fear conditioning is an important factor in the etiology of anxiety disorders. Previous studies have demonstrated a role for serotonin (5-HT)_1A receptors in fear conditioning. However, the relative contribution of somatodendritic 5-HT_1A autoreceptors and post-synaptic 5-HT_1A heteroreceptors in fear conditioning is still unclear.

OBJECTIVE

To determine the role of pre- and post-synaptic 5-HT_1A receptors in the acquisition and expression of cued and contextual conditioned fear.

METHODS

We studied the acute effects of four 5-HT_1A receptor ligands in the fear-potentiated startle test. Male Wistar rats were injected with the 5-HT_1A receptors biased agonists F13714 (0-0.16 mg/kg, IP), which preferentially activates somatodendritic 5-HT_1A autoreceptors, or F15599 (0-0.16 mg/kg, IP), which preferentially activates cortical post-synaptic 5-HT_1A heteroreceptors, with the prototypical 5-HT_1A receptor agonist R(+)8-OH-DPAT (0-0.3 mg/kg, SC) or the 5-HT_1A receptor antagonist WAY100,635 (0-1.0 mg/kg, SC).

RESULTS

F13714 (0.16 mg/kg) and R(+)-8-OH-DPAT (0.03 mg/kg) injected before training reduced cued fear acquisition. Pre-treatment with F15599 or WAY100,635 had no effect on fear learning. In the fear-potentiated startle test, F13714 (0.04-0.16 mg/kg) and R(+)-8-OH-DPAT (0.1-0.3 mg/kg) reduced the expression of cued and contextual fear, whereas F15599 had no effect. WAY100,635 (0.03-1.0 mg/kg) reduced the overall startle response.

CONCLUSIONS

The current findings indicate that activation of somatodendritic 5-HT_1A autoreceptors reduces cued fear learning, whereas 5-HT_1A receptors seem not involved in contextual fear learning. Moreover, activation of somatodendritic 5-HT_1A autoreceptors may reduce cued and contextual fear expression, whereas we found no evidence for the involvement of cortical 5-HT_1A heteroreceptors in the expression of conditioned fear.

Poor between-session recall of extinction learning and hippocampal activation and connectivity in children

BACKGROUND

In healthy adults, successful between-session recall of extinction learning depends on the hippocampus and ventromedial prefrontal cortex (vmPFC), especially when tested in the extinction context. Poor extinction recall and dysfunction within hippocampal-vmPFC circuitry are associated with fear-based disorders (e.g., anxiety, posttraumatic stress disorder). Despite the early age of onset of virtually all fear-based disorders and the protracted development of the hippocampus and vmPFC across the first two decades of life, little is known about extinction recall and the underlying neural correlates in children.

METHODS

Here, we tested extinction recall in 43 pre-adolescent children (ages 6-11 yrs) by coupling functional magnetic resonance imaging and virtual reality with a novel interpersonal threat-related two-day (ABBA) fear-extinction paradigm. Conditioned fear responding was assessed at behavioral, subjective, physiological, and neural levels.

RESULTS

Although children demonstrated intact within-session extinction, there was poor between-session recall of extinction learning (retention index: 13.56%), evidenced by elevations in skin conductance, avoidant behavioral responses, and subjective ratings. Elevations in conditioning fear responding were accompanied by activation in the hippocampus and insula, and increased connectivity of the hippocampus with the insula and dorsal anterior cingulate cortex - regions implicated in the return of fear in adult studies. Children who kept more distance from the extinguished cue during extinction subsequently demonstrated heightened hippocampal-cingulate coupling during recall, suggesting that avoidant behavior interferes with extinction retention.

CONCLUSIONS

Poor extinction recall in children may have implications for developmental vulnerability to fear-based disorders, and for the application of therapeutic strategies that rely on principles of extinction (e.g., exposure therapy) to pediatric samples.

Enhancement of fear learning in PPARα knockout mice

Peroxisome proliferator-activated receptor alpha (PPARα) is a member of the nuclear receptor superfamily and regulates fatty acid oxidation. Although PPARα is expressed not only in the peripheral tissues but also in the brain, its role in higher brain function is unclear. In this study, we investigated the role of PPARα in the control of behavior, including memory/learning and mood change, using PPARα knockout (KO) mice. A significant difference between wild-type (WT) and KO mice was seen in the passive avoidance test, demonstrating that KO mice showed enhanced fear leaning. In the amygdala of KO mice, the levels of dopamine and its metabolites were increased, and the mRNA expression of dopamine degrading enzyme was decreased. When dopamine D1 receptor antagonist was administered, the enhanced fear learning observed in KO mice was attenuated. These results suggest that PPARα is involved in the regulation of emotional memory via the dopamine pathway in the amygdala.

Assessing Classical Olfactory Fear Conditioning by Behavioral Freezing in Mice

Classical fear conditioning typically involves pairing a discrete cue with a foot shock. Quantifying behavioral freezing to the learned cue is a crucial assay for neuroscience studies focused on learning and memory. Many paradigms utilize discrete stimuli such as tones; however, given mice are odor-driven animals and the wide variety of odorants commercially available, using odors as conditioned stimuli presents advantages for studies involving learning. Here, we describe detailed procedures for assembling systems for presenting discrete odor cues during single-day fear conditioning and subsequent analysis of freezing behavior to assess learning.

Distinct Activity Patterns of the Human Bed Nucleus of the Stria Terminalis and Amygdala during Fear Learning

The amygdala and, more recently, also the bed nucleus of the stria terminalis, have been widely implicated in fear and anxiety. Much of our current knowledge is derived from animal studies and suggests an intricate convergence and divergence in functions related to defensive responding. In a recent paper, Klumpers and colleagues set out to examine these functions in a human fear learning procedure using functional magnetic resonance imaging. Their main findings were a role for the bed nucleus of the stria terminalis in threat anticipation, and for the amygdala in threat confrontation. Here, we provide a critical summary of this interesting study and point out some important issues that were not addressed by its authors. In particular, we first take a closer look at the striking differences between both samples that were combined for the study, and, secondly, we provide an in-depth discussion of their findings in relation to existing neurobehavioral models.

Who is studied in de novo fear conditioning paradigms? An examination of demographic and stimulus characteristics predicting fear learning

A common challenge in fear conditioning studies is that a relatively large proportion of individuals fail to acquire a differential conditioned skin conductance response (SCR). Researchers have identified demographic factors associated with poorer fear learning and explored the use of different fear conditioning paradigms across various populations. However, few studies have strategically aimed to enhance acquisition by manipulating the unconditioned stimulus (UCS). In the current manuscript, we examined whether demographic factors predicted failure to condition (n = 274) and explored whether modifications to the UCS enhanced fear learning (n = 143). Results indicated that race, but not age, education, or gender, predicted failure to condition. Stepwise logistic regression demonstrated that race was the most influential of these predictors; African Americans were less likely to acquire a conditioned SCR, compared to non-African Americans. Also, use of a compound UCS (i.e., electric shock combined with a scream noise) led to nearly double the rate of acquisition of a conditioned SCR. Hence, use of a compound UCS may provide a way to reduce the number of excluded individuals in studies of fear-conditioned SCR and thereby improve the representativeness of research samples.

Amygdala Inhibitory Circuits Regulate Associative Fear Conditioning

Associative memory formation is essential for an animal's survival by ensuring adaptive behavioral responses in an ever-changing environment. This is particularly important under conditions of immediate threats such as in fear learning. One of the key brain regions involved in associative fear learning is the amygdala. The basolateral amygdala is the main entry site for sensory information to the amygdala complex, and local plasticity in excitatory basolateral amygdala principal neurons is considered to be crucial for learning of conditioned fear responses. However, activity and plasticity of excitatory circuits are tightly controlled by local inhibitory interneurons in a spatially and temporally defined manner. In this review, we provide an updated view on how distinct interneuron subtypes in the basolateral amygdala contribute to the acquisition and extinction of conditioned fear memories.

Neurogenetic Approaches to Stress and Fear in Humans as Pathophysiological Mechanisms for Posttraumatic Stress Disorder

In this review article, genetic variation associated with brain responses related to acute and chronic stress reactivity and fear learning in humans is presented as an important mechanism underlying posttraumatic stress disorder. We report that genes related to the regulation of the hypothalamic-pituitary-adrenal axis, as well as genes that modulate serotonergic, dopaminergic, and neuropeptidergic functions or plasticity, play a role in this context. The strong overlap of the genetic targets involved in stress and fear learning suggests that a dimensional and mechanistic model of the development of posttraumatic stress disorder based on these constructs is promising. Genome-wide genetic analyses on fear and stress mechanisms are scarce. So far, reliable replication is still lacking for most of the molecular genetic findings, and the proportion of explained variance is rather small. Further analysis of neurogenetic stress and fear learning needs to integrate data from animal and human studies.

Impaired contextual fear-conditioning in MAM rodent model of schizophrenia

The methylazoxymethanol acetate (MAM) rodent neurodevelopmental model of schizophrenia exhibits aberrant dopamine system activation attributed to hippocampal dysfunction. Context discrimination is a component of numerous behavioral and cognitive functions and relies on intact hippocampal processing. The present study explored context processing behaviors, along with dopamine system activation, during fear learning in the MAM model. Male offspring of dams treated with MAM (20mg/kg, i.p.) or saline on gestational day 17 were used for electrophysiological and behavioral experiments. Animals were tested on the immediate shock fear conditioning paradigm, with either different pre-conditioning contexts or varying amounts of context pre-exposure (0-10 sessions). Amphetamine-induced locomotor activity and dopamine neural activity was measured 1-week after fear conditioning. Saline, but not MAM animals, demonstrated enhanced fear responses following a single context pre-exposure in the conditioning context. One week following fear learning, saline rats with 2 or 7min of context pre-exposure prior to fear conditioning also demonstrated enhanced amphetamine-induced locomotor response relative to MAM animals. Dopamine neuron recordings showed fear learning-induced reductions in spontaneous dopamine neural activity in MAM rats that was further reduced by amphetamine. Apomorphine administration confirmed that reductions in dopamine neuron activity in MAM animals resulted from over excitation, or depolarization block. These data show a behavioral insensitivity to contextual stimuli in MAM rats that coincide with a less dynamic dopamine response after fear learning.

Chronic nicotine differentially alters spontaneous recovery of contextual fear in male and female mice

Post-traumatic stress disorder (PTSD) is a devastating disorder with symptoms such as flashbacks, hyperarousal, and avoidance of reminders of the traumatic event. Exposure therapy, which attempts to extinguish fear responses, is a commonly used treatment for PTSD but relapse following successful exposure therapy is a frequent problem. In rodents, spontaneous recovery (SR), where extinguished fear responses resurface following extinction treatment, is used as a model of fear relapse. Previous studies from our lab showed that chronic nicotine impaired fear extinction and acute nicotine enhanced SR of contextual fear in adult male mice. In addition, we showed that acute nicotine's effects were specific to SR as acute nicotine did not affect recall of contextual fear conditioning in the absence of extinction. However, effects of chronic nicotine administration on SR are not known. Therefore, in the present study, we investigated if chronic nicotine administration altered SR or recall of contextual fear in adult male and female C57BL/6J mice. Our results showed that chronic nicotine significantly enhanced SR in female mice and significantly decreased SR in males. Chronic nicotine had no effect on recall of contextual fear in males or females. Female sham mice also had significantly less baseline SR than male sham mice. Overall, these results demonstrate sex differences in SR of fear memories and that chronic nicotine modulates these effects on SR but nicotine does not alter recall of contextual fear.

A novel perceptual discrimination training task: Reducing fear overgeneralization in the context of fear learning

Generalization is an adaptive learning mechanism, but it can be maladaptive when it occurs in excess. A novel perceptual discrimination training task was therefore designed to moderate fear overgeneralization. We hypothesized that improvement in basic perceptual discrimination would translate into lower fear overgeneralization in affective cues. Seventy adults completed a fear-conditioning task prior to being allocated into training or placebo groups. Predesignated geometric shape pairs were constructed for the training task. A target shape from each pair was presented. Thereafter, participants in the training group were shown both shapes and asked to identify the image that differed from the target. Placebo task participants only indicated the location of each shape on the screen. All participants then viewed new geometric pairs and indicated whether they were identical or different. Finally, participants completed a fear generalization test consisting of perceptual morphs ranging from the CS + to the CS-. Fear-conditioning was observed through physiological and behavioural measures. Furthermore, the training group performed better than the placebo group on the assessment task and exhibited decreased fear generalization in response to threat/safety cues. The findings offer evidence for the effectiveness of the novel discrimination training task, setting the stage for future research with clinical populations.

A developmental analysis of threat/safety learning and extinction recall during middle childhood

The current study examined developmental changes in fear learning and generalization in 54 healthy 5-10-year old children using a novel fear conditioning paradigm. In this task, the conditioned stimuli (CS+/CS-) were two blue and yellow colored cartoon bells, and the unconditioned stimulus was an unpleasant loud alarm sound presented with a red cartoon bell. Physiological and subjective data were acquired. Three weeks after conditioning, 48 of these participants viewed the CS-, CS+, and morphed images resembling the CS+. Participants made threat-safety discriminations while appraising threat and remembering the CS+. Although no age-related differences in fear learning emerged, patterns of generalization were qualified by child age. Older children demonstrated better discrimination between the CS+ and CS morphs than younger age groups and also reported more fear to stimuli resembling the CS+ than younger children. Clinical implications and future directions are discussed.

Common Polymorphisms in the Age of Research Domain Criteria (RDoC): Integration and Translation

The value of common polymorphisms in guiding clinical psychiatry is limited by the complex polygenic architecture of psychiatric disorders. Common polymorphisms have too small an effect on risk for psychiatric disorders as defined by clinical phenomenology to guide clinical practice. To identify polymorphic effects that are large and reliable enough to serve as biomarkers requires detailed analysis of a polymorphism's biology across levels of complexity from molecule to cell to circuit and behavior. Emphasis on behavioral domains rather than clinical diagnosis, as proposed in the Research Domain Criteria framework, facilitates the use of mouse models that recapitulate human polymorphisms because effects on equivalent phenotypes can be translated across species and integrated across levels of analysis. A knockin mouse model of a common polymorphism in the brain-derived neurotrophic factor gene (BDNF) provides examples of how such a vertically integrated translational approach can identify robust genotype-phenotype relationships that have relevance to psychiatric practice.

NOX2 Mediated-Parvalbumin Interneuron Loss Might Contribute to Anxiety-Like and Enhanced Fear Learning Behavior in a Rat Model of Post-Traumatic Stress Disorder

Post-traumatic stress disorder (PTSD) is a common psychiatric disease following exposure to a severe traumatic event or physiological stress, yet the precise mechanisms underlying PTSD remains largely to be determined. Using an animal model of PTSD induced by a single prolonged stress (SPS), we assessed the role of hippocampal nicotinamide adenosine dinucleotide phosphate (NADPH) oxidase 2 (NOX2) and parvalbumin (PV) interneurons in the development of PTSD symptoms. In the present study, behavioral tests were performed by the open field (day 13 after SPS) and fear conditioning tests (days 13 and 14 after SPS). For the interventional study, rats were chronically treated with a NADPH oxidase inhibitor apocynin either by early or delayed administration. The levels of tumor necrosis factor-alpha, interleukin (IL)-1β, IL-6, IL-10, malondialdehyde, superoxide dismutase, NOX2, 4-hydroxynonenal, and PV in the hippocampus were measured at the indicated time points. In the present study, we showed that SPS rats displayed anxiety-like and enhanced fear learning behavior, which was accompanied by the increased expressions of malondialdehyde, IL-6, NOX2, 4-hydroxynonenal, and decreased PV expression. Notably, early but not delayed treatment with apocynin reversed all these abnormalities after SPS. In conclusion, our results provided evidence that NOX2 activation in the hippocampus, at least in part, contributes to oxidative stress and neuroinflammation, which further results in PV interneuron loss and consequent PTSD symptoms in a rat model of PTSD induced by SPS.