The exclusive induction of extinction is gated by BDNF

We have previously reported that the reconsolidation and extinction of hippocampal-dependent contextual fear memory can be initiated by a single context conditioned stimulus (CS) presentation of either short or long duration, and that both processes require protein synthesis in this brain region. Furthermore, reconsolidation depends on Zif268 activity in this region. Here we show that by infusing a recombinant brain-derived neurotrophic factor (rBDNF) directly into the brain of rats, that high levels of mature BDNF in the hippocampus at retrieval constrain the extinction of the fear memory after prolonged memory recall. We also show after a short CS exposure that reconsolidation was impaired using antisense oligonucleotides targeting Zif268, and that, similarly, reductions in conditioned behavior were observed after prolonged CS presentation when extinction is constrained by high levels of BDNF. This is direct evidence that in the mammalian brain extinction proceeds exclusively after prolonged CS exposure. In addition, that BDNF activity in the hippocampus contributes to a molecular switch for the extinction of hippocampal-dependent memory.     

Extinction memory in the crab Chasmagnathus: recovery protocols and effects of multi-trial extinction training

A decline in the frequency or intensity of a conditioned behavior following the withdrawal of the reinforcement is called experimental extinction. However, the experimental manipulation necessary to trigger memory reconsolidation or extinction is to expose the animal to the conditioned stimulus in the absence of reinforcement. Recovery protocols were used to reveal which of these two processes was developed. By using the crab contextual memory model (a visual danger stimulus associated with the training context), we investigated the dynamics of extinction memory in Chasmagnathus. Here, we reveal the presence of three recovery protocols that restore the original memory: the old memory comes back 4 days after the extinction training, or when a weak training is administered later, or once the VDS is presented in a novel context 24 h after the extinction session. Another objective was to evaluate whether the administration of multi-trial extinction training could trigger an extinction memory in Chasmagnathus. The results evince that the extinction memory appears only when the total re-exposure time is around 90 min independently of the number of trials employed to accumulate it. Thus, it is feasible that the mechanisms described for the case of the extinction memory acquired through a single training trial are valid for multi-trial extinction protocols. Finally, these results are in agreement with those reports obtained with models phylogenetically far apart from the crab. Behind this attempt is the idea that in the domain of studies on memory, some principles of behavior organization and basic mechanisms have universal validity.     

Extinction learning, reconsolidation and the internal reinforcement hypothesis

Retrieving a consolidated memory--by exposing an animal to the learned stimulus but not to the associated reinforcement--leads to two opposing processes: one that weakens the old memory as a result of extinction learning, and another that strengthens the old, already-consolidated memory as a result of some less well-understood form of learning. This latter process of memory strengthening is often referred to as "reconsolidation", since protein synthesis can inhibit this form of memory formation. Although the behavioral phenomena of the two antagonizing forms of learning are well documented, the mechanisms behind the corresponding processes of memory formation are still quite controversial. Referring to results of extinction/reconsolidation experiments in honeybees, we argue that two opposing learning processes--with their respective consolidation phases and memories--are initiated by retrieval trials: extinction learning and reminder learning, the latter leading to the phenomenon of spontaneous recovery from extinction, a process that can be blocked with protein synthesis inhibition.     

Contextual Pavlovian conditioning in the crab Chasmagnathus

In contextual conditioning, a complex pattern of information is processed to associate the characteristics of a particular place with incentive or aversive reinforcements. This type of learning has been widely studied in mammals, but studies of other taxa are scarce. The context-signal memory (CSM) paradigm of the crab Chasmagnathus has been extensively used as a model of learning and memory. Although initially interpreted as habituation, some characteristics of contextual conditioning have been described. However, no anticipatory response has been detected for animals exposed to the training context. Thus, CSM could be interpreted either as an associative habituation or as contextual conditioning that occurs without a context-evoked anticipatory response. Here, we describe a training protocol developed for contextual Pavlovian conditioning (CPC). For each training trial, the context (conditioned stimulus, CS) was discretely presented and finished together with the unconditioned stimulus (US). In agreement with the CSM paradigm, a robust freezing response was acquired during the 15 training trials, and clear retention was found when tested with the US presentation after short (2 and 4 h) and long (1–4 days) delays. This CPC memory showed forward but not simultaneous presentation conditioning and was context specific and protein synthesis dependent. Additionally, a weak CPC memory was enhanced during consolidation. One day after training, CPC was extinguished by repeated CS presentation, while one presentation induced a memory labilisation–reconsolidation process. Finally, we found an anticipatory conditioned response (CR) during the CS presentation for both short-term (4 h) and long-term memory (24 h). These findings support the conditioning nature of the new paradigm.     

提取-消退范式中复合刺激对恐惧消退的影响

情绪障碍治疗的关键在于消退条件性恐惧记忆,研究证明基于记忆再巩固的提取-消退范式能有效消除或改写原有的恐惧记忆。本研究将提取-消退范式应用到更复杂的恐惧记忆中,采用多感官复合刺激(声音+图片)作为条件刺激,以皮电反应作为恐惧反应指标,考察采用单个线索(声音或图片)、复合线索(声音+图片)进行提取-消退对条件性恐惧记忆的消退效果有何差异。结果表明:声音线索提取-消退组出现了自发恢复和重建效应,图片提取-消退组只出现了重建效应,复合刺激提取-消退组未出现自发恢复和重建效应。说明由复合刺激线索引发的条件性恐惧,采用复合刺激中的单个较强线索或原有完整线索进行提取-消退,对恐惧记忆的消退效果最好。     

The resistance of renewal to instructions that devalue the role of contextual cues in a conditioned suppression task with humans

The renewal of extinguished conditioned behaviour appears to reflect context-dependent learning. The present research used a conditioned suppression task with humans to examine whether instructions concerning the context could influence renewal. Pairings of a conditional stimulus (CS) and unconditional stimulus (US) were made in one context, followed by extinction trials of CS alone in a second context, and test trials of CS alone upon return to the original learning context. Four experiments tested whether the renewal of conditioned suppression observed during test would be attenuated if participants were instructed that the context changes were irrelevant to the predictive relationship between the CS and US. Using a differential conditioning design, no attenuation was found when the instructions were given prior to conditioning (Experiment 1) or immediately prior to the test trials (Experiment 2). The latter result was replicated with a single-cue conditioning design and further controls for exposure to the extinction contexts (Experiment 3). The collection of on-line ratings about the relationship between the contextual changes and the predictive nature of the CS indicated that participants did attend to and believe the instructions (Experiment 4). The results point to the resistance of renewal to explicit instructions that attempt to devalue the role of the contextual cues.     

Differential roles for hippocampal areas CA1 and CA3 in the contextual encoding and retrieval of extinguished fear

Recent studies demonstrate that context-specific memory retrieval after extinction requires the hippocampus. However, the contribution of hippocampal subfields to the context-dependent expression of extinction is not known. In the present experiments, we examined the roles of areas CA1 and CA3 of the dorsal hippocampus in the context specificity of extinction. After pairing an auditory conditional stimulus (CS) with an aversive footshock (unconditional stimulus or US), rats received extinction sessions in which the CS was presented without the US. In Experiment 1, pretraining neurotoxic lesions in either CA1 or CA3 eliminated the context dependence of extinguished fear. In Experiment 2, lesions of CA1 or CA3 were made after extinction training. In this case, only CA1 lesions impaired the context dependence of extinction. Collectively, these results reveal that both hippocampal areas CA1 and CA3 contribute to the acquisition of context-dependent extinction, but that only area CA1 is required for contextual memory retrieval.     

Deficits in trace cued fear conditioning in galanin-treated rats and galanin-overexpressing transgenic mice

Galanin inhibits the release of several neurotransmitters and produces performance deficits in a variety of spatial and aversive learning and memory tasks. The experiments in this study investigated the role galanin has in emotional learning and memory using a standard delay cued and contextual fear conditioning task. Rats were administered galanin into the lateral ventricles before training, and scored for freezing behavior in the same context and in a novel context with and without an auditory cue (CS) that had been paired previously with an aversive stimulus (US). Galanin-overexpressing transgenic mice were tested in an identical behavioral protocol. The galanin-administered rats and the transgenic mice were not significantly different from their respective controls on this task. A more challenging trace cued and contextual fear conditioning procedure was administered to separate groups of galanin-treated rats and galanin-overexpressing transgenic mice. Subjects were trained with the same CS and US, however, a 2.5-sec delay was inserted between CS offset and US onset. Following the trace conditioning, rats administered galanin and mice overexpressing galanin both exhibited significantly less freezing to the CS in the novel context as compared with their control groups. These results indicate that the observed disruption of cued fear conditioning was specific to the more difficult trace conditioning task. These findings are the first demonstration that galanin impairs performance on an emotional memory task and support the hypothesis that galanin-induced deficits are specific to more difficult cognitive tasks.     

Role of conditioned contextual stimuli in reinstatement of extinguished fear

If the unconditioned stimulus (US) is presented independently of the conditioned stimulus (CS) following extinction, the conditioned response may be reinstated to the CS. Three experiments are reported that suggest that reinstatement is mediated by conditioning to contextual stimuli that are present during both US presentation and testing. Shocks presented to rats following the extinction of conditioned suppression reliably reinstated suppression to the CS, but only when they were presented in the context in which testing was later to occur. Reinstatement was also reversed by extinguishing fear to the context through nonreinforced exposure to the context between shock presentation and testing. Reinstatement was obtained in these experiments in spite of procedures that have been used in the past to minimize the influence of context conditioning. Moreover, fear of the context was never detected directly by depressed bar-press rates in the absence of the CS. The results do not support the hypothesis that reinstatement results from an increment in the strength of a memory of the US that has been weakened during extinction. Problems inherent in controlling and detecting levels of context conditioning that may influence behavior toward nominal CSs are discussed.     

The histone deacetylase inhibitor valproic acid enhances acquisition, extinction, and reconsolidation of conditioned fear

Histone modifications contribute to the epigenetic regulation of gene expression, a process now recognized to be important for the consolidation of long-term memory. Valproic acid (VPA), used for many years as an anticonvulsant and a mood stabilizer, has effects on learning and memory and enhances the extinction of conditioned fear through its function as a histone deacetylase inhibitor (HDAC). Here we report that VPA enhances long-term memory for both acquisition and extinction of cued-fear. Interestingly, VPA enhances extinction, but also enhances renewal of the original conditioned fear when tested in a within-subjects design. This effect appears to be related to a reconsolidation-like process since a single CS reminder in the presence of VPA can enhance long-term memory for the original fear in the context in which fear conditioning takes place. We also show that by modifying the intertrial interval during extinction training, VPA can strengthen reconsolidation of the original fear memory or enhance long-term memory for extinction such that it becomes independent of context. These findings have important implications for the use of HDAC inhibitors as adjuncts to behavior therapy in the treatment of phobia and related anxiety disorders.     

Factors regulating the effects of hippocampal inactivation on renewal of conditional fear after extinction

After extinction of fear to a Pavlovian conditional stimulus (CS), contextual stimuli come to regulate the expression of fear to that CS. There is growing evidence that the context dependence of memory retrieval after extinction involves the hippocampus. In the present experiment, we examine whether hippocampal involvement in memory retrieval after extinction is related to the history of CS presentations in the context used for retrieval testing. We used infusions of muscimol to inactivate the dorsal hippocampus (DH) during postextinction retrieval tests that were conducted in contexts that differed in their history of CS presentations in that context. We found that DH inactivation affected the context-dependent retrieval of extinction (i.e., renewal) when testing occurred in a context that had no history of CS exposure, but not in a context that reliably predicted the CS. These results are discussed in terms of theories regarding the role of the hippocampus in contextual memory retrieval.     

Post-retrieval beta-adrenergic receptor blockade: effects on extinction and reconsolidation of cocaine-cue memories

Contexts and discrete cues associated with drug-taking are often responsible for relapse among addicts. Animal models have shown that interference with the reconsolidation of drug-cue memories can reduce seeking of drugs or drug-paired stimuli. One such model is conditioned place preference (CPP) in which an animal is trained to associate a particular environment with the rewarding effects of a drug. Previous work from this laboratory has shown that intra-nucleus accumbens core infusions of a MEK inhibitor can interfere with reconsolidation of these drug-cue memories. A question that remains is whether post-retrieval drug effects on subsequent memories represent an interference with reconsolidation processes or rather a facilitation of extinction. In this experiment, we explore the effect of post-retrieval injections of propranolol, a beta-adrenergic receptor antagonist, on reconsolidation and extinction of cocaine CPP. After acquisition of cocaine CPP, animals were given post-retrieval propranolol injections once or each day during a protocol of unreinforced preference tests, until the animals showed no preference for the previously cocaine-paired environment. Following a cocaine priming injection, the animals that received daily post-test propranolol injections did not reinstate their preference for the drug-paired side. In contrast, a single post-retrieval propranolol injection followed by multiple days of unreinforced preference tests failed to blunt subsequent cocaine reinstatement of the memory. These data suggest that daily post-retrieval systemic injections of propranolol decrease the conditioned preference by interfering with reconsolidation of the memory for the association between the drug-paired side and the reinforcing effects of the drug, rather than facilitating new extinction learning.     

Temporal specificity of extinction in autoshaping

Three experiments investigated the effects of varying the conditioned stimulus (CS) duration between training and extinction. Ring doves (Streptopelia risoria) were autoshaped on a fixed CS-unconditioned stimulus (US) interval and extinguished with CS presentations that were longer, shorter, or the same as the training duration. During a subsequent test session, the training CS duration was reintroduced. Results suggest that the cessation of responding during an extinction session is controlled by generalization of excitation between the training and extinction CSs and by the number of nonreinforced CS presentations. Transfer of extinction to the training CS is controlled by the similarity between the extinction and training CSs. Extinction learning is temporally specific.     

Administration of corticosterone after memory reactivation disrupts subsequent retrieval of a contextual conditioned fear memory: dependence upon training intensity

Reactivation of stabilized memories returns them to a labile state and causes them to undergo extinction or reconsolidation processes. Although it is well established that administration of glucocorticoids after training enhance consolidation of contextual fear memories, but their effects on post-retrieval processes are not known. In this study, we first asked whether administration of corticosterone after memory reactivation would modulate subsequent expression of memory in rats. Additionally, we examined whether this modulatory action would depend upon the strength of the memory. We also tested the effect of propranolol after memory reactivation. Adult male Wistar rats were trained in a fear conditioning system using moderate (0.4 mA) or high shock (1.5 mA) intensities. For reactivation, rats were returned to the chamber for 90 s 24h later. Immediately after reactivation, rats were injected with corticosterone (1, 3 or 10mg/kg) or vehicle. One, 7 and 14 days after memory reactivation, rats were returned to the context for 5 min, and freezing behavior was scored. The findings indicated that corticosterone when injected after memory reactivation had no significant effect on recall of a moderate memory, but it impaired recall of a strong memory at a dose of 3mg/kg. Propranolol (5mg/kg) given after the reactivation treatment produced a modest impairment that persisted over three test sessions. Further, the results showed that corticosterone, but not propranolol deficit was reversed by a reminder shock. These findings provide evidence that administration of glucocorticoids following memory reactivation reduces subsequent retrieval of strong, but not moderate, contextual conditioned fear memory likely via acceleration of memory extinction. On the other hand, propranolol-induced amnesia may result from blockade of reconsolidation process. Further studies are needed to determine the underlying mechanisms.     

Is evaluative conditioning really resistant to extinction? Evidence for changes in evaluative judgements without changes in evaluative representations

Evaluative conditioning (EC) is defined as the change in the evaluation of a conditioned stimulus (CS) due to its pairing with a positive or negative unconditioned stimulus (US). Although several individual studies suggest that EC is unaffected by unreinforced presentations of the CS without the US, a recent meta-analysis indicates that EC effects are less pronounced for post-extinction measurements than post-acquisition measurements. The disparity in research findings suggests that extinction of EC may depend on yet unidentified conditions. In an attempt to uncover these conditions, three experiments (N = 784) investigated the influence of unreinforced post-acquisition CS presentations on EC effects resulting from simultaneous versus sequential pairings and pairings with single versus multiple USs. For all four types of CS–US pairings, EC effects on self-reported evaluations were reduced by unreinforced CS presentations, but only when the CSs had been rated after the initial presentation of CS–US pairings. EC effects on an evaluative priming measure remained unaffected by unreinforced CS presentations regardless of whether the CSs had been rated after acquisition. The results suggest that reduced EC effects resulting from unreinforced CS presentations are due to judgement-related processes during the verbal expression of CS evaluations rather than genuine changes in the underlying evaluative representations.     

Memory is not extinguished along with CS presentation but within a few seconds after CS-offset

Prior work with the crab's contextual memory model showed that CS-US conditioned animals undergoing an unreinforced CS presentation would either reconsolidate or extinguish the CS-US memory, depending on the length of the reexposure to the CS. Either memory process is only triggered once the CS is terminated. Based on these results, the following questions are raised. First, when is extinction memory acquired, if not along extinction training, and how long does it take? Second, can acquisition and consolidation of extinction memory be pharmacologically dissected? Here we address these questions performing three series of experiments: a first one aimed to study systematically the relationship between extinction and increasing periods of unreinforced CS presentations, a second one to determine the time boundaries of the extinction memory acquisition, and the third one to assay the requirement for protein synthesis and NMDA-like receptors of acquisition and consolidation of extinction memory. Our results confirm that it is CS-offset and not the mere retrieval (CS-onset) that triggers acquisition of extinction memory and that it is completed in less than 45 sec after CS-offset. In addition, protein synthesis is required for consolidation but not for acquisition of this memory and, conversely, NMDA-like receptor activity is required for its acquisition but not for its consolidation. Finally, we offer an interpretative scheme of our results and we discuss to what extent it could apply to multitrial extinction.     

The effects of context changes on the reinstatement of extinguished conditioned behavior in a conditioned suppression task with humans

Reinstatement refers to the return of previously extinguished conditioned responses to test trials of a conditional stimulus (CS) when presentations of the unconditional stimulus (US) alone are given following extinction. Four experiments were conducted to determine whether reinstatement could be found in a conditioned suppression task with humans and whether contextual changes can abolish it. Experiment 1 demonstrated the reinstatement of conditioned suppression when acquisition, extinction, US alone, and test trials were all given in the same context. Experiments 2 and 3 suggested that the reinstatement effect was still present when the US alone presentations were given in a different context to the subsequent test trials. Experiment 4 replicated this effect using additional controls over the amount of exposure to the various contexts. The results suggest that reinstatement can be robust across changing contexts. Aspects of the conditioned suppression task that promote the transfer of learning across contexts or the establishment of configural context-CS stimuli may underlie the apparent lack of contextual control over reinstatement.     

Electrolytic lesions of the dorsal hippocampus disrupt renewal of conditional fear after extinction

There is a growing body of evidence that the hippocampus is critical for context-dependent memory retrieval. In the present study, we used Pavlovian fear conditioning in rats to examine the role of the dorsal hippocampus (DH) in the context-specific expression of fear memory after extinction (i.e., renewal). Pre-training electrolytic lesions of the DH blunted the expression of conditional freezing to an auditory conditional stimulus (CS), but did not affect the acquisition of extinction to that CS. In contrast, DH lesions impaired the context-specific expression of extinction, eliminating the renewal of fear normally observed to a CS presented outside of the extinction context. Post-extinction DH lesions also eliminated the context dependence of fear extinction. These results are consistent with those using pharmacological inactivation of the DH and suggest that the DH is required for using contextual stimuli to regulate the expression of fear to a Pavlovian CS after extinction.     

Instructed fear learning,extinction, and recall: additive effects of cognitive information on emotional learning of fear

The effects of instruction on learning of fear and safety are rarely studied. We aimed to examine the effects of cognitive information and experience on fear learning. Fourty healthy participants, randomly assigned to three groups, went through fear conditioning, extinction learning, and extinction recall with two conditioned stimuli (CS+). Information was presented about the presence or absence of conditioned stimulus–unconditioned stimulus (CS–US) contingency at different stages of the experiment. Information about the CS–US contingency prior to fear conditioning enhanced fear response and reduced extinction recall. Information about the absence of CS–US contingency promoted extinction learning and recall, while omission of this information prior to recall resulted in fear renewal. These findings indicate that contingency information can facilitate fear expression during fear learning, and can facilitate extinction learning and recall. Information seems to function as an element of the larger context in which conditioning occurs.