Glucocorticoids are required for extinction of predator stress-induced hyperarousal

Background

The role of glucocorticoids in extinction of traumatic memories has not been fully characterized despite its potential as a therapeutic target for acquired posttraumatic stress disorder (PTSD). The predator stress paradigm allows us to determine whether glucocorticoids mediate the extinction of both context-dependent and context-independent fear memories.

Methods

Male C57BL/6J mice were exposed to a predator (cat) then repeatedly exposed to the predator stress context in the absence of the cat. Context-dependent (associative) fear memory was assessed as suppression of activity during re-exposure to the predator stress context without the cat (extinction trials). Context-independent fear (non-associative) was assessed seven days after extinction trials using measures of hyperarousal and anxiety-like behaviours in environments unlike the predator stress context. To assess the role of glucocorticoids, mice were injected with metyrapone (50 mg/kg) 90 min prior to extinction trials in predator stressed mice and context-dependent and context-independent fear memories were assessed. Finally, metyrapone-treated predator stressed mice were injected with corticosterone (5 or 10 mg/kg) immediately following extinction trials and context-dependent and context-independent fear memories were assessed.

Results

Repeated re-exposure to the predator stress context without the cat present extinguished context-dependent fear memory, and also reduced hyperarousal, a generalized, chronic PTSD-like symptom. We show that extinction of context-independent predator stress-induced hyperarousal is dependent on endogenous glucocorticoids during the extinction trials. Furthermore, the inhibition of extinction by metyrapone on startle amplitude was reduced by exogenous administration of corticosterone following extinction trials. Overall, these data implicate glucocorticoids in the extinction of hyperarousal, a core symptom of PTSD.     

Extinction treatment in multiple contexts attenuates ABC renewal in humans

Renewal has been implicated as one of the underlying mechanisms in return of fear following exposure therapy. ABC renewal is clinically more relevant than ABA renewal and yet it is a weaker form of renewal, suggesting that conducting extinction treatment in multiple contexts may be sufficient to attenuate ABC renewal. Using self-reported expectancy of shock and startle blink responses the current study examined the effects of conducting extinction treatment in multiple contexts on ABC fear renewal. Participants (N = 68) received conditional stimulus (CS) and unconditional stimulus (US) pairings in one context (A) followed by extinction treatment (CS presentations alone) in either one other context (B) or three other contexts (BCD). Non-reinforced test trials in a novel context (E) resulted in renewal of extinguished conditioned behaviour for those who received extinction in only one context. However, renewal was attenuated for those who received extinction treatment in three contexts. No renewal was found for the control group that received the test trial in the same context as during extinction. Suggestions are provided for clinicians seeking to prevent or attenuate return of fear following exposure therapy.     

Effects of multiple contexts and context similarity on the renewal of extinguished conditioned behaviour in an ABA design with humans

The ABA renewal procedure involves pairing a conditional stimulus (CS) and an unconditional stimulus (US) in one context (A), presenting extinction trials of the CS alone in a second context (B), and nonreinforced test trials of the CS in the acquisition context (A). The renewal of extinguished conditioned behaviour is observed during test. The current study tested the effects of multiple extinction contexts and context similarity in attenuating renewal. Participants (N = 99) took part in a fear conditioning ABA renewal procedure. Using a measure of self-reported expectancy of the US, ABA renewal was observed when a single extinction context that was dissimilar to the test context was used. Renewal was attenuated, though still present, when extinction occurred in multiple dissimilar extinction contexts or in a single extinction context that was similar to the test context. Renewal was completely abolished when multiple extinction contexts that were similar to the test context were combined. Multiple extinction contexts and context similarity act additively in their effect on attenuating renewal. The results are discussed in relation to the design of exposure therapy programs that seek to reduce relapse that can occur via renewal.     

Different mechanisms of fear extinction dependent on length of time since fear acquisition

Fear extinction is defined as a decline in conditioned fear responses (CRs) following nonreinforced exposure to a feared conditioned stimulus (CS). Behavioral evidence indicates that extinction is a form of inhibitory learning: Extinguished fear responses reappear with the passage of time (spontaneous recovery), a shift of context (renewal), and unsignaled presentations of the unconditioned stimulus (reinstatement). However, there also is evidence to suggest that extinction is an "unlearning" process corresponding to depotentiation of potentiated synapses within the amygdala. Because depotentiation is induced more readily at short intervals following LTP induction and is not inducible at all at a sufficient delay, it may be that extinction initiated shortly following fear acquisition preferentially engages depotentiation/"unlearning," whereas extinction initiated at longer delays recruits a different mechanism. We investigated this possibility through a series of behavioral experiments examining the recoverability of conditioned fear following extinction. Consistent with an inhibitory learning mechanism of extinction, rats extinguished 24-72 h following acquisition exhibited moderate to strong reinstatement, renewal, and spontaneous recovery. In contrast, and consistent with an erasure mechanism, rats extinguished 10 min to 1 h after acquisition exhibited little or no reinstatement, renewal, or spontaneous recovery. These data support a model in which different neural mechanisms are recruited depending on the temporal delay of fear extinction.     

Extinction in multiple contexts does not necessarily make extinction less vulnerable to relapse

Three fear-conditioning experiments with rat subjects examined the effects of extinction in multiple contexts on a final relapse (renewal) effect that occurred when the extinguished fear cue was tested in a new context (Experiments 1 and 3) or in the context in which fear conditioning had first occurred (Experiment 2). Rats that received extinction in three contexts demonstrated more fear during extinction than rats that received the same number and temporal distribution of extinction trials in one context; extinction was partially lost with each context switch. Although extinction in multiple contexts thus had an impact on extinction behavior, it did not reduce the size of the final renewal effect. Fear during extinction was occasionally positively correlated with fear during final testing, but the two were never negatively correlated. The results suggest that extinction in multiple contexts does not necessarily weaken fear renewal, and that extinction procedures that generate high levels of responding in extinction do not necessarily make extinction learning less context-specific.     

Inhibitory Learning for Anxiety-Related Disorders

This article reviews the articles in this issue that describe the strategies derived from the inhibitory learning model as applied to exposure therapy for anxiety disorders. The major principles of inhibitory learning are to create and strengthen nonthreat associations in memory (largely by engaging prefrontal cortical regions), and to effectively retrieve those nonthreat associations in the long term. Several case vignettes are provided that demonstrate how the principles of inhibitory learning (which include maximizing expectancy violations, limiting distraction, fear antagonistic actions, deepened extinction, elimination of safety behaviors, occasional reinforced extinction, increasing variability of exposures and offsetting reinstatement and context renewal effects) can be applied in clinical practice.     

Single prolonged stress disrupts retention of extinguished fear in rats

Clinical research has linked post-traumatic stress disorder (PTSD) with deficits in fear extinction. However, it is not clear whether these deficits result from stress-related changes in the acquisition or retention of extinction or in the regulation of extinction memories by context, for example. In this study, we used the single prolonged stress (SPS) animal model of PTSD and fear conditioning procedures to examine the effects of prior traumatic stress on the acquisition, retention, and context-specificity of extinction. SPS administered one week prior to fear conditioning had no effect on the acquisition of fear conditioning or extinction but disrupted the retention of extinction memories for both contextual and cued fear. This SPS effect required a post-stress incubation period to manifest. The results demonstrate that SPS disrupts extinction retention, leading to enhanced fear renewal; further research is needed to identify the neurobiological processes through which SPS induces these effects.     

Expanding the Intertrial Interval During Extinction: Response Cessation and Recovery

We examined trial spacing during extinction following a human contingency learning task. Specifically, we assessed if an expanding retrieval practice schedule ( [Bjork and Bjork, 1992] and [Bjork and Bjork, 2006]), in which the spacing between extinction trials was progressively increased, would result in faster immediate extinction and less recovery from extinction than uniformly spaced extinction trials. We used an ABB vs. ABA renewal design and observed that, whereas the expanding group extinguished faster during extinction treatment, the expanding and constant groups showed the same level of extinction with an immediate test in the extinction context (ABB) and the two groups showed equivalent ABA renewal at test in the training context. We conclude that the faster extinction observed in the expanding groups could be misleading in clinical treatment, if the therapist used the absence of fear during extinction as the basis for terminating treatment.     

Block of glucocorticoid synthesis during re-activation inhibits extinction of an established fear memory

BackgroundThe pharmacology of traumatic memory extinction has not been fully characterized despite its potential as a therapeutic target for established, acquired anxiety disorders, including post-traumatic stress disorder (PTSD). Here we examine the role of endogenous glucocorticoids in traumatic memory extinction.MethodsMale C57BL/6J mice were injected with corticosterone (10 mg/kg, i.p.) or metyrapone (50 mg/kg, s.c.) during re-activation of a contextual fear memory, and compared to vehicle groups (N = 10–12 per group). To ensure that metyrapone was blocking corticosterone synthesis, we measured corticosterone levels following re-activation of a fear memory in metyrapone- and vehicle-treated animals.ResultsCorticosterone administration following extinction trials caused a long-lasting inhibition of the original fear memory trace. In contrast, blockade of corticosteroid synthesis with metyrapone prior to extinction trials enhanced retrieval and prevented extinction of context-dependent fear responses in mice. Further behavioral analysis suggested that the metyrapone enhancement of retrieval and prevention of extinction were not due to non-specific alterations in locomotor or anxiety-like behavior. In addition, the inhibition of extinction by metyrapone was rescued by exogenous administration of corticosterone following extinction trials. Finally, we confirmed that the rise in corticosterone during re-activation of a contextual fear memory was blocked by metyrapone.ConclusionsWe demonstrate that extinction of a classical contextual fear memory is dependent on endogenous glucocorticoid synthesis during re-activation of a fear memory. Our data suggest that decreased glucocorticoids during fear memory re-activation may contribute to the inability to extinguish a fear memory, thus contributing to one of the core symptoms of PTSD.     

Contextual control of human fear associations in a renewal paradigm

The original model of behavior change suggests that extinction is context dependent whereas fear acquisition is context independent [Bouton, M. E. & Ricker, S. T. (1994). Renewal of extinguished responding in a second context. Animal Learning and Behavior, 22, 317-324]. Supportive evidence stems mainly from animal studies, showing that after acquisition (conditioned stimulus-unconditioned stimulus (CS-US)) in Context A and extinction in Context B, fear is renewed by presenting the CS in acquisition Context A (ABA renewal) or in a novel Context C (ABC renewal). By implication, the model predicts equal ABA and ABC renewal. However, there is also evidence to suggest that the context dependency of extinction and the context independency of acquisition may be less stringent than originally proposed. The present study investigated renewal in humans using a differential fear conditioning paradigm with a shock US and online shock expectancy ratings and electrodermal responses as dependent variables. Experiment 1 compared an ABA condition with an AAA condition. Experiment 2 compared three conditions: ABA, ABC, and AAA. Both experiments demonstrated ABA renewal. Most importantly, Experiment 2 showed larger ABA than ABC renewal. Overall, results for expectancy ratings were more convincing than for electrodermal responses. In line with the extinction model, the present findings support the context dependency of extinction in humans. In contrast to the model, the findings suggest that in humans not only extinction learning, but also fear acquisition is controlled by its current context.     

D-cycloserine facilitates context-specific fear extinction learning

D-cycloserine (DCS) may facilitate fear extinction learning, but the behavioral consequences and mechanisms behind this effect are not well understood at present. In this paper, we re-analyze data from previously reported null result experiments and find that rats showing above-median extinction learning during DCS treatment benefited from the drug, whereas rats showing below-median (and in this case little) extinction learning did not. Two additional experiments found that DCS facilitated extinction learning when specifically combined with a moderate, but not a small, number of extinction trials. DCS thus facilitates extinction learning only if the behavioral procedure first engages the extinction learning process. The benefits of the drug, however, were specific to the context in which extinction was learned--i.e., DCS did not prevent or influence the renewal of fear observed when the extinguished cue was tested in the original conditioning context.     

Reciprocal patterns of c-Fos expression in the medial prefrontal cortex and amygdala after extinction and renewal of conditioned fear

After extinction of conditioned fear, memory for the conditioning and extinction experiences becomes context dependent. Fear is suppressed in the extinction context, but renews in other contexts. This study characterizes the neural circuitry underlying the context-dependent retrieval of extinguished fear memories using c-Fos immunohistochemistry. After fear conditioning and extinction to an auditory conditioned stimulus (CS), rats were presented with the extinguished CS in either the extinction context or a second context, and then sacrificed. Presentation of the CS in the extinction context yielded low levels of conditioned freezing and induced c-Fos expression in the infralimbic division of the medial prefrontal cortex, the intercalated nuclei of the amygdala, and the dentate gyrus (DG). In contrast, presentation of the CS outside of the extinction context yielded high levels of conditioned freezing and induced c-Fos expression in the prelimbic division of the medial prefrontal cortex, the lateral and basolateral nuclei of the amygdala, and the medial division of the central nucleus of the amygdala. Hippocampal areas CA1 and CA3 exhibited c-Fos expression when the CS was presented in either context. These data suggest that the context specificity of extinction is mediated by prefrontal modulation of amygdala activity, and that the hippocampus has a fundamental role in contextual memory retrieval.Considerable interest has emerged in recent years in the neural mechanisms underlying the associative extinction of learned fear (Maren and Quirk 2004; Myers et al. 2006; Quirk and Mueller 2008). Notably, extinction is a useful model for important aspects of exposure-based therapies for the treatment of human anxiety disorders such as panic disorder and post-traumatic stress disorder (PTSD) (Bouton et al. 2001, 2006). During extinction, a conditioned stimulus (CS) is repeatedly presented in the absence of the unconditioned stimulus (US), a procedure that greatly reduces the magnitude and probability of the conditioned response (CR). After the extinction of fear, there is substantial evidence that extinction does not erase the original fear memory, but results in a transient inhibition of fear. For example, extinguished fear responses return after the mere passage of time (i.e., spontaneous recovery) or after a change in context (i.e., renewal) (Bouton et al. 2006; Ji and Maren 2007). In other words, extinguished fear is context specific. The return of fear after extinction is a considerable challenge for maintaining long-lasting fear suppression after exposure-based therapies (Rodriguez et al. 1999; Hermans et al. 2006; Effting and Kindt 2007; Quirk and Mueller 2008).In the last several years, considerable progress has been made in understanding the neural mechanisms underlying the context specificity of fear extinction. For example, lesions or inactivation of the hippocampus prevent the renewal of fear when an extinguished CS is presented outside of the extinction context (Corcoran and Maren 2001, 2004; Corcoran et al. 2005; Ji and Maren 2005, 2008; Hobin et al. 2006). In addition, neurons in the basolateral complex of the amygdala exhibit context-specific spike firing to extinguished CSs (Hobin et al. 2003; Herry et al. 2008), and this requires hippocampal input (Maren and Hobin 2007). Indeed, amygdala neurons that fire more to extinguished CSs outside of the extinction context are monosynaptically excited by hippocampal stimulation (Herry et al. 2008). In contrast, neurons that responded preferentially to extinguished CSs in the extinction context receive synaptic input from the medial prefrontal cortex (Herry et al. 2008).The prevalent theory of the interactions between the prefrontal cortex, hippocampus, and amygdala that lead to regulation of fear by context assumes that when animals experience an extinguished CS in the extinction context, the hippocampus drives prefrontal cortex inhibition of the amygdala to suppress fear (Hobin et al. 2003; Maren and Quirk 2004; Maren 2005). When animals encounter an extinguished CS outside of the extinction context, the hippocampus is posited to inhibit the prefrontal cortex and thereby promote amygdala activity required to renew fear. The hippocampus may also drive fear renewal through its direct projections to the basolateral amygdala (Herry et al. 2008). Although this model accounts for much of the extant literature on the context specificity of extinction, it is not known whether the nodes of this hypothesized neural network are coactive during the retrieval of fear and extinction memories. As a first step in addressing this issue, we used ex vivo c-Fos immunohistochemistry (e.g., Knapska et al. 2007) to generate a functional map of the neural circuits involved in the contextual retrieval of fear memory after extinction. Our results reveal reciprocal activity in prefrontal-amygdala circuits involved in extinction and renewal and implicate the hippocampus in hierarchical control of contextual memory retrieval within these circuits.     

Recent exposure to a dangerous context impairs extinction and reinstates lost fear reactions

Rats were shocked in a context and then exposed to that context in the absence of shock. Shorter intervals between these extinction trials produced more long-term freezing than did longer ones, and shorter intervals between the final extinction trial and test produced more freezing than did longer ones. A short interval between a context extinction trial and test with an extinguished conditioned stimulus (CS) produced more freezing than did a longer one, and a short interval between a nonreinforced context exposure and an extinguished CS reinstated freezing when the CS was tested 24 hr later. The results suggest that recent fear acts to favor subsequent retrieval of the memory formed at conditioning rather than extinction and to render the retrieved memory more salient.     

Contextual-specificity of short-delay extinction in humans: renewal of fear-potentiated startle in a virtual environment

A recent fear-potentiated startle study in rodents suggested that extinction was not context dependent when extinction was conducted after a short delay following acquisition, suggesting that extinction can lead to erasure of fear learning in some circumstances. The main objective of this study was to attempt to replicate these findings in humans by examining the context specificity of short-delay extinction in an ABA renewal procedure using virtual reality environments. A second objective was to examine whether renewal, if any, would be influenced by context conditioning. Subjects underwent differential aversive conditioning in virtual context A, which was immediately followed by extinction in virtual context B. Extinction was followed by tests of renewal in context A and B, with the order counterbalanced across subjects. Results showed that extinction was context dependent. Evidence for renewal was established using fear-potentiated startle as well as skin conductance and fear ratings. In addition, although contextual anxiety was greater in the acquisition context than in the extinction context during renewal, as assessed with startle, context conditioning did not influence the renewal effect. These data do not support the view that extinction conducted shortly after acquisition is context independent. Hence, they do not provide evidence that extinction can lead to erasure of a fear memory established via Pavlovian conditioning.     

Kappa opioid receptors mediate where fear is expressed following extinction training

Six experiments used a within-subjects renewal design to examine the involvement of kappa opioid receptors (KORs) in regulating the expression and recovery of extinguished fear. Rats were trained to fear a tone conditioned stimulus (CS) via pairings with foot shock in a distinctive context (A). This was followed by extinction training of the CS in a second context (B). Finally, all rats were tested for fear to the tone in the extinction context (ABB) and the training (ABA) or a novel (ABC) context. Intracerebroventricular (ICV) infusion of the KOR antagonist nor-binaltorphimine dihydrochloride (nor-BNI) dose-dependently prevented ABA renewal of fear, but had no effect on the expression of ABC renewal, the expression of extinction, or the expression of nonextinguished fear. Conversely, pretest infusion of the KOR agonist U50,488 hydrochloride (U50,488) selectively facilitated the expression of ABA renewal and had no effect on the expression of extinction. Pretest infusion of nor-BNI had no effect on the expression of context-specific latent inhibition. Collectively, these results suggest that KORs gate the expression of fear following extinction training and may comprise a critical neuropeptide component of the circuitry underlying context-dependent expression of fear.     

睡眠对恐惧学习的影响及其认知神经机制

睡眠问题可能会诱发恐惧相关情绪障碍(焦虑、创伤性应激障碍、恐怖症等),研究睡眠影响恐惧学习的认知神经机制,有助于增强对恐惧相关情绪障碍的预测、诊断和治疗。以往研究表明睡眠剥夺影响恐惧习得和消退主要是通过抑制vmPFC活动,阻碍其与杏仁核的功能连接,从而导致恐惧习得增强或是消退学习受损。进一步研究发现睡眠不同阶段对恐惧学习相关脑区有独特的影响:剥夺(缺乏)快速眼动睡眠会抑制vmPFC活动、增强杏仁核、海马激活,导致恐惧习得增强,消退学习受损,此外边缘皮层的功能连接减少破坏了记忆巩固(恐惧记忆和消退记忆);而慢波睡眠主要与海马变化有关,慢波睡眠期间进行目标记忆重激活可促进恐惧消退学习。未来研究需要增加睡眠影响恐惧泛化的神经机制研究、及昼夜节律中断对恐惧消退的影响,以及关注动物睡眠研究向人类睡眠研究转化中存在的问题。     

The effects of physical context changes and multiple extinction contexts on two forms of renewal in a conditioned suppression task with humans

Three experiments examined the effects of physical context changes and multiple extinction contexts on the renewal of conditioned suppression in humans. A conditioned suppression task used an undesirable event as the unconditional stimulus (US). One conditional stimulus (CS+) predicted the occurrence of the US and another (CS−) predicted US absence. In Experiment 1 (N = 32), conditioned suppression was acquired to the CS+ in one context and extinguished in a different context. An increase in suppression was found for the CS+ and not for the CS− when subsequent test trials were conducted in the acquisition context (ABA renewal). Experiment 2 (N = 32) tested for ABC Renewal and showed increased suppression to both the CS+ and CS− when test was conducted in a novel context. Experiment 3 (N = 80) showed that these two effects were abolished when extinction was conducted in multiple contexts. The experiments extend the ABA renewal of conditioned suppression found with non-human animal subjects and the reduction of renewal by extinction in multiple contexts. Context changes may also facilitate cue competition effects after training with elementary stimuli, as shown by the effects of US omission in the ABA and ABC renewal groups.     

Methylene blue facilitates the extinction of fear in an animal model of susceptibility to learned helplessness

The objectives were to (1) extend previous findings on fear extinction deficits in male congenitally helpless rats (a model for susceptibility to learned helplessness) to female congenitally helpless rats, and (2) attempt a therapeutic intervention with methylene blue, a metabolic enhancer that improves memory retention, to alleviate the predicted extinction deficits. In the first experiment, fear acquisition (four tone-shock pairings in operant chamber) was followed by extinction training (60 tones in open field). Congenitally helpless rats showed fear acquisition similar to controls but had dramatic extinction deficits, and did not display the gradual extinction curves observed in controls. Congenitally helpless rats demonstrated greater tone-evoked freezing as compared to controls in both the acquisition and extinction contexts one week after extinction training, and also in the extinction probe conducted one month later. In the second experiment (which began one month after the first experiment) congenitally helpless subjects were further exposed to tones for 5 days, each followed by 4 mg/kg methylene blue or saline IP, and had a fear renewal test in the acquisition context. Methylene blue administration improved retention of the extinction memory as demonstrated by significant decreases in fear renewal as compared to saline-administered congenitally helpless subjects. The impaired ability to extinguish fear to a traumatic memory in congenitally helpless rats supports the validity of this strain as an animal model for vulnerability to post-traumatic stress disorder, and this study further suggests that methylene blue may facilitate fear extinction as an adjunct to exposure therapy.     

Learning and memory in conditioned fear extinction: effects of D-cycloserine

This review addresses the effects of the cognitive enhancer D-cycloserine (DCS) on the memory processes that occur in conditioned fear extinction, which is the experimental model for exposure techniques to reduce clinical anxiety. All reported rat studies show an enhanced fear extinction effect when DCS is administered acutely before or shortly after extinction training. DCS also promotes the generalization of this fear extinction effect. In addition, DCS reduces some forms of relapse (reduced reinstatement, reduced spontaneous recovery), but not others (contextual renewal, rapid reacquisition). It is argued that this pattern of results is best explained by assuming that DCS promotes extinction learning to the background context, resulting in enhanced contextual inhibition. Four human studies have produced mixed results, but some methodological issues complicate the reported failures. It is concluded that DCS is a promising tool as an adjunct to extinction techniques in exposure treatment, but that more pre-clinical and clinical research is needed to fully characterize its behavioral consequences.     

Contextual specificity of extinction of delay but not trace eyeblink conditioning in humans

Renewal of an extinguished conditioned response has been demonstrated in humans and in animals using various types of procedures, except renewal of motor learning such as eyeblink conditioning. We tested renewal of delay and trace eyeblink conditioning in a virtual environment in an ABA design. Following acquisition in one context (A, e.g., an airport) and extinction in a different context (B, e.g., a city), tests for renewal took place in the acquisition (A) and extinction context (B), in a counterbalanced order. Results showed renewal of the extinguished conditioned response in the delay but not trace condition.