Predictability and heritability of individual differences in fear learning

Agmatine diminishes behavioral and endocrine alterations in a rat model of post-traumatic stress disorder

Post-traumatic stress disorder (PTSD), is a severe anxiety disorder characterized by associative fear conditioning. Single prolonged stress (SPS) is a widely accepted reliable animal model to stimulate PTSD. Agmatine is an endogenous neuromodulator of stress; however, its effect on PTSD remains to be investigated. This study explored the role of agmatine in conditioned fear response (CFR) in PTSD and highlighted the role of imidazoline receptors in the effect of agmatine. Intra-cerebroventricular (icv) surgery was done in order to facilitate drug administration. Animals were subjected to SPS. Agmatine and the involvement of imidazoline receptors (I1 and I2) were assessed for their effect in fear conditioning apparatus. During weeks 1, 2, and 3, in CFR, agmatine (40 µg/rat, icv) showed significantly decreased freezing time whereas other doses of agmatine (10 and 20 µg/rat, icv). Imidazoline (I1 and I2) receptor agonists Moxonidine (25 µg/rat, icv) and 2-BFI, (10 µg/rat, icv) respectively, at their sub-effective doses, with a submaximal dose of agmatine (20 µg/rat, icv) significantly decreased the altered freezing time during weeks 1, 2 and 3 compared to SPS animals. Moreover, the effective dose of agmatine (40 µg/rat, icv) with imidazoline (I1 and I2) receptor antagonists Efaroxan (10 µg/rat, icv) and Idazoxan (4 µg/rat, icv) respectively does not reversed the effect of agmatine on freezing. Agmatine and its combination with I1 and I2 agonists, normalized the altered freezing behavior, corticosterone level, organ coefficient of adrenal gland, neuroinflammatory and neurotrophic factor due to SPS during CFR projecting its strong therapeutic effect in SPS induced PTSD.

A fear conditioned cue orchestrates a suite of behaviors in rats

Pavlovian fear conditioning has been extensively used to study the behavioral and neural basis of defensive systems. In a typical procedure, a cue is paired with foot shock, and subsequent cue presentation elicits freezing, a behavior theoretically linked to predator detection. Studies have since shown a fear conditioned cue can elicit locomotion, a behavior that - in addition to jumping, and rearing - is theoretically linked to imminent or occurring predation. A criticism of studies observing fear conditioned cue-elicited locomotion is that responding is non-associative. We gave rats Pavlovian fear discrimination over a baseline of reward seeking. TTL-triggered cameras captured 5 behavior frames/s around cue presentation. Experiment 1 examined the emergence of danger-specific behaviors over fear acquisition. Experiment 2 examined the expression of danger-specific behaviors in fear extinction. In total, we scored 112,000 frames for nine discrete behavior categories. Temporal ethograms show that during acquisition, a fear conditioned cue suppresses reward seeking and elicits freezing, but also elicits locomotion, jumping, and rearing - all of which are maximal when foot shock is imminent. During extinction, a fear conditioned cue most prominently suppresses reward seeking, and elicits locomotion that is timed to shock delivery. The independent expression of these behaviors in both experiments reveals a fear conditioned cue to orchestrate a temporally organized suite of behaviors.

Carbon Dioxide Reactivity Differentially Predicts Fear Expression After Extinction and Retrieval-Extinction in Rats

Background

Cues present during a traumatic event may result in persistent fear responses. These responses can be attenuated through extinction learning, a core component of exposure therapy. Exposure/extinction is effective for some people, but not all. We recently demonstrated that carbon dioxide (CO2) reactivity predicts fear extinction memory and orexin activation and that orexin activation predicts fear extinction memory, which suggests that a CO2 challenge may enable identification of whether an individual is a good candidate for an extinction-based approach. Another method to attenuate conditioned responses, retrieval-extinction, renders the original associative memory labile via distinct neural mechanisms. The purpose of the current study was to examine whether we could replicate previous findings that retrieval-extinction is more effective than extinction at preventing the return of fear and that CO2 reactivity predicts fear memory after extinction. We also examined whether CO2 reactivity predicts fear memory after retrieval-extinction.

Methods

Male rats first underwent a CO2 challenge and fear conditioning and were assigned to receive either standard extinction (n = 28) or retrieval-extinction (n = 28). Then, they underwent a long-term memory (LTM) test and a reinstatement test.

Results

We found that retrieval-extinction resulted in lower freezing during extinction, LTM, and reinstatement than standard extinction. Using the best subset approach to linear regression, we found that CO2 reactivity predicted LTM after extinction and also predicted LTM after retrieval-extinction, although to a lesser degree.

Conclusions

CO2 reactivity could be used as a screening tool to determine whether an individual may be a good candidate for an extinction-based therapeutic approach.

CO2 reactivity is associated with individual differences in appetitive extinction memory

Pavlovian conditioning can underly the maladaptive behaviors seen in psychiatric disorders such as anxiety and addiction. In both the lab and the clinic, these responses can be attenuated through extinction learning, but often return with the passage of time, stress, or a change in context. Extinction to fear and reward cues are both subject to these return of behavior phenomena and have overlap in neurocircuitry, yet it is unknown whether they share any common predictors. The orexin system has been implicated in both fear and appetitive extinction and can be activated through a CO2 challenge. We previously found that behavioral CO2 reactivity predicts fear extinction and orexin activation. Here, we sought to extend our previous findings to determine whether CO2 reactivity might also predict extinction memory for appetitive light-food conditioning. We find that the same subcomponent of behavioral CO2 reactivity that predicted fear extinction also predicts appetitive extinction, but in the opposite direction. We show evidence that this subcomponent remains stable across two CO2 challenges, suggesting it may be a stable trait of both behavioral CO2 reactivity and appetitive extinction phenotype. Our findings further the possibility for CO2 reactivity to be used as a transdiagnostic screening tool to determine whether an individual would be a good candidate for exposure therapy.

Examining the long-term effects of traumatic brain injury on fear extinction in male rats

There is a strong association between traumatic brain injuries (TBIs) and the development of psychiatric disorders, including post-traumatic stress disorder (PTSD). Exposure-based therapy is a first-line intervention for individuals who suffer from PTSD and other anxiety-related disorders; however, up to 50% of individuals with PTSD do not respond well to this approach. Fear extinction, a core mechanism underlying exposure-based therapy, is a procedure in which a repeated presentation of a conditioned stimulus in the absence of an unconditioned stimulus leads to a decrease in fear expression, and is a useful tool to better understand exposure-based therapy. Identifying predictors of extinction would be useful in developing alternative treatments for the non-responders. We recently found that CO₂ reactivity predicts extinction phenotypes in rats, likely through the activation of orexin receptors in the lateral hypothalamus. While studies have reported mixed results in extinction of fear after TBI, none have examined the long-term durability of this phenotype in the more chronically injured brain. Here we tested the hypothesis that TBI results in a long-term deficit in fear extinction, and that CO₂ reactivity would be predictive of this extinction phenotype. Isoflurane-anesthetized adult male rats received TBI (n = 59) (produced by a controlled cortical impactor) or sham surgery (n = 29). One month post-injury or sham surgery, rats underwent a CO₂ or air challenge, followed by fear conditioning, extinction, and fear expression testing. TBI rats exposed to CO₂ (TBI-CO₂) showed no difference during extinction or fear expression relative to shams exposed to CO₂ (sham-CO₂). However, TBI-CO₂ rats, showed significantly better fear expression than TBI rats exposed to air (TBI-air). In contrast to previous findings, we observed no relationship between CO₂ reactivity and post-extinction fear expression in either the sham or TBI rats. However, compared to the previously observed naïve sample, we observed more variability in post-extinction fear expression but a very similar distribution of CO₂ reactivity in the current sample. Isoflurane anesthesia may lead to interoceptive threat habituation, possibly via action on orexin receptors in the lateral hypothalamus, and may interact with CO₂ exposure, resulting in enhanced extinction. Future work will directly test this possibility.

ChAT::Cre transgenic rats show sex-dependent altered fear behaviors, ultrasonic vocalizations and cholinergic marker expression

The cholinergic system is a critical regulator of Pavlovian fear learning and extinction. As such, we have begun investigating the cholinergic system's involvement in individual differences in cued fear extinction using a transgenic ChAT::Cre rat model. The current study extends behavioral phenotyping of a transgenic ChAT::Cre rat line by examining both freezing behavior and ultrasonic vocalizations (USVs) during a Pavlovian cued fear learning and extinction paradigm. Freezing, 22 kHz USVs, and 50 kHz USVs were compared between male and female transgenic ChAT::Cre+ rats and their wildtype (Cre-) littermates during fear learning, contextual and cue-conditioned fear recall, cued fear extinction, and generalization to a novel tone. During contextual and cued fear recall ChAT::Cre+ rats froze slightly more than their Cre- littermates, and displayed significant sex differences in contextual and cue-conditioned freezing, 22 kHz USVs, and 50 kHz USVs. Females showed more freezing than males in fear recall trials, but fewer 22 kHz distress calls during fear learning and recall. Females also produced more 50 kHz USVs during exposure to the testing chambers prior to tone (or shock) presentation compared with males, but this effect was blunted in ChAT::Cre+ females. Corroborating previous studies, ChAT::Cre+ transgenic rats overexpressed vesicular acetylcholine transporter immunolabeling in basal forebrain, striatum, basolateral amygdala, and hippocampus, but had similar levels of acetylcholinesterase and numbers of ChAT+ neurons as Cre- rats. This study suggests that variance in behavior between ChAT::Cre+ and wildtype rats is sex dependent and advances theories that distinct neural circuits and processes regulate sexually divergent fear responses.

Reactivity to conditioned threat cues is distinct from exploratory drive in the elevated plus maze

Fear and anxiety are adaptive states that allow humans and animals alike to respond appropriately to threatening cues in their environment. Commonly used tasks for studying behaviour akin to fear and anxiety in rodent models are Pavlovian threat conditioning and the elevated plus maze (EPM), respectively. In threat conditioning the rodents learn to associate an aversive event with a specific stimulus or context. The learnt association between the two stimuli (the 'memory') can then be recalled by re-exposing the subject to the conditioned stimulus. The elevated plus maze is argued to measure the agoraphobic avoidance of the brightly lit open maze arms in crepuscular rodents. These two tasks have been used extensively, yet research into whether they interact is scarce. We investigated whether recall of an aversive memory, across contextual, odour or auditory modalities, would potentiate anxiety-like behaviour in the elevated plus maze. The data did not support that memory recall, even over a series of time points, could influence EPM behaviour. Furthermore, there was no correlation between EPM behaviour and conditioned freezing in independent cohorts tested in the EPM before or after auditory threat conditioning. Further analysis found the production of 22 kHz ultrasonic vocalisations revealed the strongest responders to a conditioned threat cue. These results are of particular importance for consideration when using the EPM and threat conditioning to identify individual differences and the possibility to use the tasks in batteries of tests without cross-task interference.

Sex differences in fear responses: Neural circuits

Women have increased vulnerability to PTSD and anxiety disorders compared to men. Understanding the neurobiological underpinnings of these disorders is critical for identifying risk factors and developing appropriate sex-specific interventions. Despite the clear clinical relevance of an examination of sex differences in fear responses, the vast majority of pre-clinical research on fear learning and memory formation has exclusively used male animals. This review highlights sex differences in context and cued fear conditioning, fear extinction and fear generalization with a focus on the neural circuits underlying these behaviors in rodents. There are mixed reports of behavioral sex differences in context and cued fear conditioning paradigms, which can depend upon the behavioral indices of fear. However, there is greater evidence of differential activation of the hippocampus, amygdalar nuclei and the prefrontal cortical regions in male and female rodents during context and cued fear conditioning. The bed nucleus of the stria terminalis (BNST), a sexually dimorphic structure, is of particular interest as it differentially contributes to fear responses in males and females. In addition, while the influence of the estrous cycle on different phases of fear conditioning is delineated, the clearest modulatory effect of estrogen is on fear extinction processes. Examining the variability in neural responses and behavior in both sexes should increase our understanding of how that variability contributes to the neurobiology of affective disorders. This article is part of the Special Issue on 'Fear, anxiety and PTSD'.

Robust group- but limited individual-level (longitudinal) reliability and insights into cross-phases response prediction of conditioned fear

Here, we follow the call to target measurement reliability as a key prerequisite for individual-level predictions in translational neuroscience by investigating (1) longitudinal reliability at the individual and (2) group level, (3) internal consistency and (4) response predictability across experimental phases. One hundred and twenty individuals performed a fear conditioning paradigm twice 6 months apart. Analyses of skin conductance responses, fear ratings and blood oxygen level dependent functional magnetic resonance imaging (BOLD fMRI) with different data transformations and included numbers of trials were conducted. While longitudinal reliability was rather limited at the individual level, it was comparatively higher for acquisition but not extinction at the group level. Internal consistency was satisfactory. Higher responding in preceding phases predicted higher responding in subsequent experimental phases at a weak to moderate level depending on data specifications. In sum, the results suggest that while individual-level predictions are meaningful for (very) short time frames, they also call for more attention to measurement properties in the field.

Individual Differences in Conditioned Fear and Extinction in Female Rats

The inability to extinguish a traumatic memory is a key aspect of post-traumatic stress disorder (PTSD). While PTSD affects 10–20% of individuals who experience a trauma, women are particularly susceptible to developing the disorder. Despite this notable female vulnerability, few studies have investigated this particular resistance to fear extinction observed in females. Similar to humans, rodent models of Pavlovian fear learning and extinction show a wide range of individual differences in fear learning and extinction, although female rodents are considerably understudied. Therefore, the present study examined individual differences in fear responses, including freezing behavior and ultrasonic vocalizations (USVs), of female Long–Evans rats during acquisition of fear conditioning and cued fear extinction. Similar to prior studies in males, female rats displayed individual variation in freezing during cued fear extinction and were divided into extinction competent (EC) and extinction resistant (ER) phenotypes. Differences in freezing between ER and EC females were accompanied by shifts in rearing during extinction, but no darting was seen in any trial. Freezing behavior during fear learning did not differ between the EC and ER females. Vocalizations emitted in the 22 and 50 kHz ranges during fear learning and extinction were also examined. Unlike vocalizations seen in previous studies in males, very few 22 kHz distress vocalizations were emitted by female rats during fear acquisition and extinction, with no difference between ER and EC groups. Interestingly, all female rats produced significant levels of 50 kHz USVs, and EC females emitted significantly more 50 kHz USVs than ER rats. This difference in 50 kHz USVs was most apparent during initial exposure to the testing environment. These results suggest that like males, female rodents show individual differences in both freezing and USVs during fear extinction, although females appear to vocalize more in the 50 kHz range, especially during initial periods of exposure to the testing environment, and emit very few of the 22 kHz distress calls that are typically observed in males during fear learning or extinction paradigms. Overall, these findings show that female rodents display fear behavior repertoires divergent from males.

Cholinergic neurotransmission in the basolateral amygdala during cued fear extinction

Cholinergic neuromodulation plays an important role in numerous cognitive functions including regulating arousal and attention, as well as associative learning and extinction processes. Further, studies demonstrate that cholinergic inputs from the basal forebrain cholinergic system influence physiological responses in the basolateral amygdala (BLA) as well as fear extinction processes. Since rodent models display individual differences in conditioned fear and extinction responses, this study investigated if cholinergic transmission in the BLA during fear extinction could contribute to differences between extinction resistant and extinction competent phenotypes in outbred Long-Evans male rats. Experiment 1 used in vivo microdialysis to test the hypothesis that acetylcholine (ACH) efflux in the BLA would increase with presentation of an auditory conditioned stimulus (CS+) during extinction learning. Acetylcholine efflux was compared in rats exposed to the CS+, a CS- (the tone never paired with a footshock), or to a context shift alone (without CS+ tone presentation). Consistent with acetylcholine's role in attention and arousal, ACH efflux in the BLA was increased in all three groups (CS+, CS-, Shift Alone) by the initial context shift into the extinction learning chamber, but returned more rapidly to baseline levels in the Shift Alone group (no CS+). In contrast, in the group exposed to the CS+, ACH efflux in the BLA remained elevated during continued presentation of conditioned cues and returned to baseline more slowly, leading to an overall increase in ACH efflux compared with the Shift Alone group. Based on the very dense staining in the BLA for acetylcholinesterase (ACHE), Experiment 2 examined if individual differences in fear extinction were associated with differences in cholinesterase enzyme activity (CHE) in the BLA and/or plasma with a separate cohort of animals. Cholinesterase activity (post-testing) in both the BLA and plasma was higher in extinction competent rats versus rats resistant to extinction learning. There was also a significant negative correlation between BLA CHE activity and freezing during extinction learning. Taken together, our results support a role for ACH efflux in the BLA during cued fear extinction that may be modulated by individual differences in ACHE activity, and are associated with behavioral responses during fear extinction. These findings implicate individual differences in cholinergic regulation in the susceptibility to disorders with dysregulation of extinction learning, such post-traumatic stress disorder (PTSD) in humans.

The relationship between monoaminergic gene expression, learning, and optimism in red junglefowl chicks

Intra-species cognitive variation is commonly observed, but explanations for why individuals within a species differ in cognition are still understudied and not yet clear. Cognitive processes are likely influenced by genetic differences, with genes in the monoaminergic systems predicted to be important. To explore the potential role of these genes in association with individual variation in cognition, we exposed red junglefowl (Gallus gallus) chicks to behavioural assays measuring variation in learning (discriminative learning, reversal learning, and cognitive flexibility) and optimism (measured in a cognitive judgement bias test). Following this, we analysed prefrontal cortex gene expression of several dopaminergic and serotonergic genes in these chicks. Of our explored genes, serotonin receptor genes 5HT2A and 5HT2B, and dopaminergic receptor gene DRD1 were associated with measured behaviour. Chicks that had higher 5HT2A were less flexible in the reversal learning task, and chicks with higher 5HT2B also tended to be less cognitively flexible. Additionally, chicks with higher DRD1 were more optimistic, whilst chicks with higher 5HT2A tended to be less optimistic. These results suggest that the serotonergic and dopaminergic systems are linked to observed cognitive variation, and, thus, individual differences in cognition can be partially explained by variation in brain gene expression.

Making translation work: Harmonizing cross-species methodology in the behavioural neuroscience of Pavlovian fear conditioning

Translational neuroscience bridges insights from specific mechanisms in rodents to complex functions in humans and is key to advance our general understanding of central nervous function. A prime example of translational research is the study of cross-species mechanisms that underlie responding to learned threats, by employing Pavlovian fear conditioning protocols in rodents and humans. Hitherto, evidence for (and critique of) these cross-species comparisons in fear conditioning research was based on theoretical viewpoints. Here, we provide a perspective to substantiate these theoretical concepts with empirical considerations of cross-species methodology. This meta-research perspective is expected to foster cross-species comparability and reproducibility to ultimately facilitate successful transfer of results from basic science into clinical applications.

Multiple genetic loci affect place learning and memory performance in Drosophila melanogaster

Learning and memory are critical functions for all animals, giving individuals the ability to respond to changes in their environment. Within populations, individuals vary, however the mechanisms underlying this variation in performance are largely unknown. Thus, it remains to be determined what genetic factors cause an individual to have high learning ability and what factors determine how well an individual will remember what they have learned. To genetically dissect learning and memory performance, we used the Drosophila synthetic population resource (DSPR), a multiparent mapping resource in the model system Drosophila melanogaster, consisting of a large set of recombinant inbred lines (RILs) that naturally vary in these and other traits. Fruit flies can be trained in a "heat box" to learn to remain on one side of a chamber (place learning) and can remember this (place memory) over short timescales. Using this paradigm, we measured place learning and memory for ~49 000 individual flies from over 700 DSPR RILs. We identified 16 different loci across the genome that significantly affect place learning and/or memory performance, with 5 of these loci affecting both traits. To identify transcriptomic differences associated with performance, we performed RNA-Seq on pooled samples of seven high performing and seven low performing RILs for both learning and memory and identified hundreds of genes with differences in expression in the two sets. Integrating our transcriptomic results with the mapping results allowed us to identify nine promising candidate genes, advancing our understanding of the genetic basis underlying natural variation in learning and memory performance.

Chronic, Intermittent Microdoses of the Psychedelic N,N-Dimethyltryptamine (DMT) Produce Positive Effects on Mood and Anxiety in Rodents

Drugs capable of ameliorating symptoms of depression and anxiety while also improving cognitive function and sociability are highly desirable. Anecdotal reports have suggested that serotonergic psychedelics administered in low doses on a chronic, intermittent schedule, so-called "microdosing", might produce beneficial effects on mood, anxiety, cognition, and social interaction. Here, we test this hypothesis by subjecting male and female Sprague Dawley rats to behavioral testing following the chronic, intermittent administration of low doses of the psychedelic N,N-dimethyltryptamine (DMT). The behavioral and cellular effects of this dosing regimen were distinct from those induced following a single high dose of the drug. We found that chronic, intermittent, low doses of DMT produced an antidepressant-like phenotype and enhanced fear extinction learning without impacting working memory or social interaction. Additionally, male rats treated with DMT on this schedule gained a significant amount of body weight during the course of the study. Taken together, our results suggest that psychedelic microdosing may alleviate symptoms of mood and anxiety disorders, though the potential hazards of this practice warrant further investigation.

Sign-tracking behavior is difficult to extinguish and resistant to multiple cognitive enhancers

The attribution of incentive-motivational value to drug-related cues underlies relapse and craving in drug addiction. One method of addiction treatment, cue-exposure therapy, utilizes repeated presentations of drug-related cues in the absence of drug (i.e., extinction learning); however, its efficacy has been limited due to an incomplete understanding of extinction and relapse processes after cues have been imbued with incentive-motivational value. To investigate this, we used a Pavlovian conditioned approach procedure to screen for rats that attribute incentive-motivational value to reward-related cues (sign-trackers; STs) or those that do not (goal-trackers; GTs). In Experiment 1, rats underwent Pavlovian extinction followed by reinstatement and spontaneous recovery tests. For comparison, a separate group of rats underwent PCA training followed by operant conditioning, extinction, and tests of reinstatement and spontaneous recovery. In Experiment 2, three cognitive enhancers (sodium butyrate, D-cycloserine, and fibroblast growth factor 2) were administered following extinction training to facilitate extinction learning. STs but not GTs displayed enduring resistance to Pavlovian, but not operant, extinction and were more susceptible to spontaneous recovery. In addition, none of the cognitive enhancers tested affected extinction learning. These results expand our understanding of extinction learning by demonstrating that there is individual variation in extinction and relapse processes and highlight potential difficulties in applying extinction-based therapies to drug addiction treatment in the clinic.

Individual variability in the recall of fear extinction is associated with phosphorylation of mitogen-activated protein kinase in the infralimbic cortex

RATIONALE

Although most individuals will be exposed to trauma at some point, only a small portion of individuals develops posttraumatic stress disorder (PTSD), suggesting there are factors which render some individuals particularly susceptible to the development of this disorder. One cardinal feature of PTSD is the failure to extinguish fear responses to cues that once signaled danger. Rodent studies of fear learning and extinction have provided insight into the neural mechanisms underlying extinction; however, most of these studies have focused on mechanisms involved in typical responses and fewer have identified mechanisms that distinguish animals that extinguish well versus those that do not extinguish their fear responses. Investigation of individual differences in fear extinction might help us better understand the susceptibility to and development of PTSD.

OBJECTIVES

In order to understand the neural mechanisms underlying such variation, we assessed phosphorylated mitogen-activated protein kinase (P-MAPK) levels in infralimbic cortex (IL), basolateral amygdala (BLA), and dorsal hippocampus in subsets of rats which exhibited good or poor recall of extinction.

RESULTS

We found a relationship between extinction recall and P-MAPK in the IL such that rats which had good extinction recall had higher levels of P-MAPK than those which had poor extinction recall. We also found that rats which had good extinction recall had higher levels of P-MAPK in the dorsal hippocampus than control rats.

CONCLUSIONS

Our findings suggest that individual differences in the recall of extinction learning can be explained by altered cell signaling in the IL.

Fear Extinction Retention: Is It What We Think It Is?

There has been an explosion of research on fear extinction in humans in the past 2 decades. This has not only generated major insights, but also brought a new goal into focus: how to maintain extinction memory over time (i.e., extinction retention). We argue that there are still important conceptual and procedural challenges in human fear extinction research that hamper advancement in the field. We use extinction retention and the extinction retention index to exemplarily illustrate these challenges. Our systematic literature search identified 16 different operationalizations of the extinction retention index. Correlation coefficients among these different operationalizations as well as among measures of fear/anxiety show a wide range of variability in four independent datasets, with similar findings across datasets. Our results suggest that there is an urgent need for standardization in the field. We discuss the conceptual and empirical implications of these results and provide specific recommendations for future work.

Data-driven criteria to assess fear remission and phenotypic variability of extinction in rats

Fear conditioning is widely employed to examine the mechanisms that underlie dysregulations of the fear system. Various manipulations are often used following fear acquisition to attenuate fear memories. In rodent studies, freezing is often the main output measure to quantify 'fear'. Here, we developed data-driven criteria for defining a standard benchmark that indicates remission from conditioned fear and for identifying subgroups with differential treatment responses. These analyses will enable a better understanding of individual differences in treatment responding.This article is part of a discussion meeting issue 'Of mice and mental health: facilitating dialogue between basic and clinical neuroscientists'.

Neurochemical and molecular mechanisms underlying the retrieval-extinction effect

Extinction within the reconsolidation window, or 'retrieval-extinction', has received much research interest as a possible technique for targeting the reconsolidation of maladaptive memories with a behavioural intervention. However, it remains to be determined whether the retrieval-extinction effect-a long-term reduction in fear behaviour, which appears resistant to spontaneous recovery, renewal and reinstatement-depends specifically on destabilisation of the original memory (the 'reconsolidation-update' account) or represents facilitation of an extinction memory (the 'extinction-facilitation' account). We propose that comparing the neurotransmitter systems, receptors and intracellular signalling pathways recruited by reconsolidation, extinction and retrieval-extinction will provide a way of distinguishing between these accounts.

Predicting extinction phenotype to optimize fear reduction

Fear conditioning is widely employed to study dysregulations of the fear system. The repeated presentation of a conditioned stimulus in the absence of a reinforcer leads to a decrease in fear responding-a phenomenon known as extinction. From a translational perspective, identifying whether an individual might respond well to extinction prior to intervention could prove important to treatment outcomes. Here, we test the hypothesis that CO₂ reactivity predicts extinction phenotype in rats, and that variability in CO₂ reactivity as well as extinction long-term memory (LTM) significantly predicts orexin activity in the lateral hypothalamus (LH). Our results validate a rat model of CO₂ reactivity and show that subcomponents of behavioral reactivity following acute CO₂ exposure explain a significant portion of the variance in extinction LTM. Furthermore, we show evidence that variability in CO₂ reactivity is also significantly predictive of orexin activity in the LH, and that orexin activity, in turn, significantly accounts for LTM variance. Our findings open the possibility that we may be able to use CO₂ reactivity as a screening tool to determine if individuals are good candidates for an extinction/exposure-based approach.

Disrupting reconsolidation: memory erasure or blunting of emotional/motivational value?

When memories are retrieved they become labile, and subject to alteration by a process known as reconsolidation. Disruption of memory reconsolidation decreases the performance of learned responses, which is often attributed to erasure of the memory; in the case of Pavlovian learning, to a loss of the association between a conditioned stimulus (CS) and unconditioned stimulus (US). However, an alternative interpretation is that disrupting reconsolidation does not erase memories, but blunts their emotional/motivational impact. It is difficult to parse the predictive vs. emotional/motivational value of CSs in non-human animals, but studies on variation in the form of conditioned responses (CRs) in a Pavlovian conditioned approach task suggest a way to do this. In this task a lever-CS paired with a food reward (US) acquires predictive value in all rats, but is attributed with emotional/motivational value to a greater extent in some rats (sign-trackers) than others (goal-trackers). We report that the post-retrieval administration of propranolol selectively attenuates a sign-tracking CR, and the associated neural activation of brain "motive circuits", while having no effect on conditioned orienting behavior in sign-trackers, or on goal-tracking CRs evoked by either a lever-CS or a tone-CS. We conclude that the disruption of reconsolidation by post-retrieval propranolol degrades the emotional/motivational impact of the CS, required for sign-tracking, but leaves the CS-US association intact. The possibility that post-retrieval interventions can reduce the emotional/motivational aspects of memories, without actually erasing them, has important implications for treating maladaptive memories that contribute to some psychiatric disorders.

Effects of N, N-Dimethyltryptamine on Rat Behaviors Relevant to Anxiety and Depression

Depression and anxiety disorders are debilitating diseases resulting in substantial economic costs to society. Traditional antidepressants often take weeks to months to positively affect mood and are ineffective for about 30% of the population. Alternatives, such as ketamine, a dissociative anesthetic capable of producing hallucinations, and the psychoactive tisane ayahuasca, have shown great promise due to their fast-acting nature and effectiveness in treatment-resistant populations. Here, we investigate the effects of N, N-dimethyltryptamine (DMT), the principle hallucinogenic component of ayahuasca, in rodent behavioral assays relevant to anxiety and depression using adult, male, Sprague-Dawley rats. We find that while DMT elicits initial anxiogenic responses in several of these paradigms, its long-lasting effects tend to reduce anxiety by facilitating the extinction of cued fear memory. Furthermore, DMT reduces immobility in the forced swim test, which is a characteristic behavioral response induced by many antidepressants. Our results demonstrate that DMT produces antidepressant and anxiolytic behavioral effects in rodents, warranting further investigation of ayahuasca and classical psychedelics as treatments for depression and post-traumatic stress disorder.

Brain region-specific expression of genes mapped within quantitative trait loci for behavioral responsiveness to acute stress in Fisher 344 and Wistar Kyoto male rats

Acute stress responsiveness is a quantitative trait that varies in severity from one individual to another; however, the genetic component underlying the individual variation is largely unknown. Fischer 344 (F344) and Wistar Kyoto (WKY) rat strains show large differences in behavioral responsiveness to acute stress, such as freezing behavior in response to footshock during the conditioning phase of contextual fear conditioning (CFC). Quantitative trait loci (QTL) have been identified for behavioral responsiveness to acute stress in the defensive burying (DB) and open field test (OFT) from a reciprocal F2 cross of F344 and WKY rat strains. These included a significant QTL on chromosome 6 (Stresp10). Here, we hypothesized that the Stresp10 region harbors genes with sequence variation(s) that contribute to differences in multiple behavioral response phenotypes between the F344 and WKY rat strains. To test this hypothesis, first we identified differentially expressed genes within the Stresp10 QTL in the hippocampus, amygdala, and frontal cortex of F344 and WKY male rats using genome-wide microarray analyses. Genes with both expression differences and non-synonymous sequence variations in their coding regions were considered candidate quantitative trait genes (QTGs). As a proof-of-concept, the F344.WKY-Stresp10 congenic strain was generated with the Stresp10 WKY donor region into the F344 recipient strain. This congenic strain showed behavioral phenotypes similar to those of WKYs. Expression patterns of Gpatch11 (G-patch domain containing 11), Cdkl4 (Cyclin dependent kinase like 4), and Drc1 (Dynein regulatory complex subunit 1) paralleled that of WKY in the F344.WKY-Stresp10 strain matching the behavioral profiles of WKY as opposed to F344 parental strains. We propose that these genes are candidate QTGs for behavioral responsiveness to acute stress.

Fear extinction in the human brain: A meta-analysis of fMRI studies in healthy participants

The study of fear extinction represents an important example of translational neuroscience in psychiatry and promises to improve the understanding and treatment of anxiety and fear-related disorders. We present the results of a set of meta-analyses of human fear extinction studies in healthy participants, conducted with functional magnetic resonance imaging (fMRI) and reporting whole-brain results. Meta-analyses of fear extinction learning primarily implicate consistent activation of brain regions linked to threat appraisal and experience, including the dorsal anterior cingulate and anterior insular cortices. An overlapping anatomical result was obtained from the meta-analysis of extinction recall studies, except when studies directly compared an extinguished threat stimulus to an unextinguished threat stimulus (instead of a safety stimulus). In this latter instance, more consistent activation was observed in dorsolateral and ventromedial prefrontal cortex regions, together with other areas including the hippocampus. While our results partially support the notion of a shared neuroanatomy between human and rodent models of extinction processes, they also encourage an expanded account of the neural basis of human fear extinction.

Activation of orexin/hypocretin neurons is associated with individual differences in cued fear extinction

Identifying the neurobiological mechanisms that underlie differential sensitivity to stress is critical for understanding the development and expression of stress-induced disorders, such as post-traumatic stress disorder (PTSD). Preclinical studies have suggested that rodents display different phenotypes associated with extinction of Pavlovian conditioned fear responses, with some rodent populations being resistant to extinction. An emerging literature also suggests a role for orexins in the consolidation processes associated with fear learning and extinction. To examine the possibility that the orexin system might be involved in individual differences in fear extinction, we used a Pavlovian conditioning paradigm in outbred Long-Evans rats. Rats showed significant variability in the extinction of cue-conditioned freezing and extinction recall, and animals were divided into groups based on their extinction profiles based on a median split of percent freezing behavior during repeated exposure to the conditioned cue. Animals resistant to extinction (high freezers) showed more freezing during repeated cue presentations during the within trial and between trial extinction sessions compared with the group showing significant extinction (low freezers), although there were no differences between these groups in freezing upon return to the conditioned context or during the conditioning session. Following the extinction recall session, activation of orexin neurons was determined using dual label immunohistochemistry for cFos in orexin positive neurons in the hypothalamus. Individual differences in the extinction of cue conditioned fear were associated with differential activation of hypothalamic orexin neurons. Animals showing poor extinction of cue-induced freezing (high freezers) had significantly greater percentage of orexin neurons with Fos in the medial hypothalamus than animals displaying significant extinction and good extinction recall (low freezers). Further, the freezing during extinction learning was positively correlated with the percentage of activated orexin neurons in both the lateral and medial hypothalamic regions. No differences in the overall density of orexin neurons or Fos activation were seen between extinction phenotypes. Although correlative, our results support other studies implicating a role of the orexinergic system in regulating extinction of conditioned responses to threat.

Preventing the return of fear using reconsolidation updating and methylene blue is differentially dependent on extinction learning

Many factors account for how well individuals extinguish conditioned fears, such as genetic variability, learning capacity and conditions under which extinction training is administered. We predicted that memory-based interventions would be more effective to reduce the reinstatement of fear in subjects genetically predisposed to display more extinction learning. We tested this hypothesis in rats genetically selected for differences in fear extinction using two strategies: (1) attenuation of fear memory using post-retrieval extinction training, and (2) pharmacological enhancement of the extinction memory after extinction training by low-dose USP methylene blue (MB). Subjects selectively bred for divergent extinction phenotypes were fear conditioned to a tone stimulus and administered either standard extinction training or retrieval + extinction. Following extinction, subjects received injections of saline or MB. Both reconsolidation updating and MB administration showed beneficial effects in preventing fear reinstatement, but differed in the groups they targeted. Reconsolidation updating showed an overall effect in reducing fear reinstatement, whereas pharmacological memory enhancement using MB was an effective strategy, but only for individuals who were responsive to extinction.

Don't fear 'fear conditioning': Methodological considerations for the design and analysis of studies on human fear acquisition, extinction, and return of fear

The so-called 'replicability crisis' has sparked methodological discussions in many areas of science in general, and in psychology in particular. This has led to recent endeavours to promote the transparency, rigour, and ultimately, replicability of research. Originating from this zeitgeist, the challenge to discuss critical issues on terminology, design, methods, and analysis considerations in fear conditioning research is taken up by this work, which involved representatives from fourteen of the major human fear conditioning laboratories in Europe. This compendium is intended to provide a basis for the development of a common procedural and terminology framework for the field of human fear conditioning. Whenever possible, we give general recommendations. When this is not feasible, we provide evidence-based guidance for methodological decisions on study design, outcome measures, and analyses. Importantly, this work is also intended to raise awareness and initiate discussions on crucial questions with respect to data collection, processing, statistical analyses, the impact of subtle procedural changes, and data reporting specifically tailored to the research on fear conditioning.

Individual Differences in Male Rats in a Behavioral Test Battery: A Multivariate Statistical Approach

Animal models for anxiety, depressive-like and cognitive diseases or aging often involve testing of subjects in behavioral test batteries. The large number of test variables with different mean variations and within and between test correlations often constitute a significant problem in determining essential variables to assess behavioral patterns and their variation in individual animals as well as appropriate statistical treatment. Therefore, we applied a multivariate approach (principal component analysis) to analyse the behavioral data of 162 male adult Sprague-Dawley rats that underwent a behavioral test battery including commonly used tests for spatial learning and memory (holeboard) and different behavioral patterns (open field, elevated plus maze, forced swim test) as well as for motor abilities (Rota rod). The high dimensional behavioral results were reduced to fewer components associated with spatial cognition, general activity, anxiety-, and depression-like behavior and motor ability. The loading scores of individual rats on these different components allow an assessment and the distribution of individual features in a population of animals. The reduced number of components can be used also for statistical calculations like appropriate sample sizes for valid discriminations between experimental groups, which otherwise have to be done on each variable. Because the animals were intact, untreated and experimentally naïve the results reflect trait patterns of behavior and thus individuality. The distribution of animals with high or low levels of anxiety, depressive-like behavior, general activity and cognitive features in a local population provides information of the probability of their appeareance in experimental samples and thus may help to avoid biases. However, such an analysis initially requires a large cohort of animals in order to gain a valid assessment.

Emergence in extinction of enhanced and persistent responding to ambiguous aversive cues is associated with high MAOA activity in the prelimbic cortex

There is a great deal of individual variability in the emotional outcomes of potentially traumatic events, and the underlying mechanisms are only beginning to be understood. In order to further our understanding of individual trajectories to trauma, its vulnerability and resilience, we adapted a model of fear expression to ambiguous vs perfect cues in adult male rats, and examined long-term fear extinction, 2, 3, and 50 days from acquisition. After the final conditioned fear test, mitochondrial enzyme monoamine oxidase A (MAOA) function was examined. In order to identify associations between this function and behavioral expression, an a posteri median segregation approach was adopted, and animals were classified as high or low responding according to level of freezing to the ambiguous cue at remote testing, long after the initial extinction. Those individuals characterized by their higher response showed a freezing pattern that persisted from their previous extinction sessions, in spite of their acquisition levels being equivalent to the low-freezing group. Furthermore, unlike more adaptive individuals, freezing levels of high-freezing animals even increased at initial extinction, to almost double their acquisition session levels. Controlling for perfect cue response at remote extinction, greater ambiguous threat cue response was associated with enhanced prelimbic cortex MAOA functional activity. These findings underscore MAOA as a potential target for the development of interventions to mitigate the impact of traumatic experiences.

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Potentially traumatic event outcomes vary and mechanisms are poorly understood.

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We examined fear extinction of perfect or ambiguous cues in adult male rats.

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Higher freezing to ambiguous cue in extinction yet followed equivalent acquisition.

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Ambiguous cue response was associated with higher prelimbic cortex MAOA function.

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These findings support targeting MAOA to mitigate impact of traumatic experiences.

Stress and Fear Extinction

Stress has a critical role in the development and expression of many psychiatric disorders, and is a defining feature of posttraumatic stress disorder (PTSD). Stress also limits the efficacy of behavioral therapies aimed at limiting pathological fear, such as exposure therapy. Here we examine emerging evidence that stress impairs recovery from trauma by impairing fear extinction, a form of learning thought to underlie the suppression of trauma-related fear memories. We describe the major structural and functional abnormalities in brain regions that are particularly vulnerable to stress, including the amygdala, prefrontal cortex, and hippocampus, which may underlie stress-induced impairments in extinction. We also discuss some of the stress-induced neurochemical and molecular alterations in these brain regions that are associated with extinction deficits, and the potential for targeting these changes to prevent or reverse impaired extinction. A better understanding of the neurobiological basis of stress effects on extinction promises to yield novel approaches to improving therapeutic outcomes for PTSD and other anxiety and trauma-related disorders.

A dendritic organization of lateral amygdala neurons in fear susceptible and resistant mice

Subtle differences in neuronal microanatomy may be coded in individuals with genetic susceptibility for neuropsychiatric disorders. Genetic susceptibility is a significant risk factor in the development of anxiety disorders, including post-traumatic stress disorder (PTSD). Pavlovian fear conditioning has been proposed to model key aspects of PTSD. According to this theory, PTSD begins with the formation of a traumatic memory which connects relevant environmental stimuli to significant threats to life. The lateral amygdala (LA) is considered to be a key network hub for the establishment of Pavlovian fear conditioning. Substantial research has also linked the LA to PTSD. Here we used a genetic mouse model of fear susceptibility (F-S) and resistance (F-R) to investigate the dendritic and spine structure of principal neurons located in the LA. F-S and F-R lines were bi-directionally selected based on divergent levels of contextual and cued conditioned freezing in response to fear-evoking footshocks. We examined LA principal neuron dendritic and spine morphology in the offspring of experimentally naive F-S and F-R mice. We found differences in the spatial distribution of dendritic branch points across the length of the dendrite tree, with a significant increase in branch points at more distal locations in the F-S compared with F-R line. These results suggest a genetic predisposition toward differences in fear memory strength associated with a dendritic branch point organization of principal neurons in the LA. These micro-anatomical differences in neuron structure in a genetic mouse model of fear susceptibility and resistance provide important insights into the cellular mechanisms of pathophysiology underlying genetic predispositions to anxiety and PTSD.

Rats with differential self-grooming expression in the elevated plus-maze do not differ in anxiety-related behaviors

Individual differences are important biological predictors for reactivity to stressful stimulation. The extent to which trait differences underlie animal's reactions to conditioned and unconditioned fear stimuli, for example, is still to be clarified. Although grooming behavior has been associated with some aspects of the obsessive-compulsive disorder in humans, its relation with other anxiety disorders is still unknown. Given that grooming behavior could be a component of the whole spectrum of these disorders, in the present study we allocated male Wistar rats in low, intermediate and high self-grooming groups according to the duration of such behavior in the elevated plus-maze (EPM). These groups were then evaluated in unconditioned fear tests, such as the EPM and the open-field, and in conditioned fear tests, such as fear-potentiated startle and fear extinction retention. Additionally, we studied the expression of unconditioned behaviors in marble burying test and the sensorimotor gate function with prepulse inhibition test. Neurochemicals and neuroendocrine parameters were also evaluated, with the quantification of basal corticosterone in the plasma, and dopamine, serotonin and their metabolites in brain structures involved with fear processing. In general, rats classified according to grooming expression showed similar performance in all behavioral tests. Accordingly, corticosterone and monoamine concentrations were similar among groups. Thus, despite grooming expression elicited by different approaches--especially pharmacological ones--has been related with some aspects of anxiety disorders, rats with different expression of spontaneous self-grooming in the EPM do not differ in anxiety-like behaviors nor in neurochemical and neuroendocrine parameters generally associated with anxiety disorders.