Context extinction and associative learning in Lymnaea

Aerial respiratory behavior in the pond snail Lymnaea was operantly conditioned so that snails learned not to perform aerial respiration in a hypoxic environment. Snails were trained in either the standard context (no food odorant) or a carrot (food-odorant) context. An operant training procedure of two 45-min training sessions with a 1-h interval between the sessions followed by a third 45-min training session 18 h later was sufficient to produce associative learning and long-term memory (LTM) that persisted for at least 5 days. If, however, following the third operant training session snails received three 45-min extinction training sessions, with each extinction session separated by at least a 1-h interval, LTM was not observed when tested the following day. That is, the memory was extinguished. Extinction, however, did not occur if the context of the extinction training was different from the context of the associative training. That is, in the snails trained in the standard context, extinction did not occur if the extinction training sessions were performed in the food-odorant context and vice versa.     

Reconsolidation of declarative memory in humans

The reconsolidation hypothesis states that a consolidated memory could again become unstable and susceptible to facilitation or impairment for a discrete period of time after a reminder presentation. The phenomenon has been demonstrated in very diverse species and types of memory, including the human procedural memory of a motor skill task but not the human declarative one. Here we provide evidence for both consolidation and reconsolidation in a paired-associate learning (i.e., learning an association between a cue syllable and the respective response syllable). Subjects were given two training sessions with a 24-h interval on distinct verbal material, and afterward, they received at testing two successive retrievals corresponding to the first and second learning, respectively. Two main results are noted. First, the first acquired memory was impaired when a reminder was presented 5 min before the second training (reconsolidation), and also when the second training was given 5 min instead of 24 h after the first one (consolidation). Second, the first retrieval proved to influence negatively on the later one (the retrieval-induced forgetting [RIF] effect), and we used the absence of this RIF effect as a very indicator of the target memory impairment. We consider the demonstration of reconsolidation in human declarative memory as backing the universality of this phenomenon and having potential clinical relevance. On the other hand, we discuss the possibility of using the human declarative memory as a model to address several key topics of the reconsolidation hypothesis.     

Cycloheximide produces amnesia for extinction and reconsolidation in an appetitive odor discrimination task in rats

A considerable literature has shown deficits in memory resulting from the administration of protein synthesis inhibitors; however, most of the past literature in this field has focused on acquisition of new memory using aversively-motivated tasks. The effect of protein synthesis inhibition on appetitive learning and memory as well as extinction is less clear. The present study employed an appetitive odor discrimination paradigm to examine the effects of acute cycloheximide administration (1 mg/kg) on reconsolidation and extinction.Male, Long-Evans adult rats were trained to discriminate between two odors (i.e., cocoa and cinnamon) and then received extinction trials following an intraperitoneal injection of cycloheximide or vehicle. Twenty-four hours later, rats were tested via one non-reinforced test trial. Results showed amnesia for extinction as well as original training (i.e., correct odor choice) in cycloheximide-injected rats in this appetitive task, while vehicle-injected controls showed good memory for extinction. These data add to a growing literature showing the importance of protein synthesis inhibition for extinction and reconsolidation in appetitive learning and memory.     

Disrupting reconsolidation: pharmacological and behavioral manipulations

We previously demonstrated that disrupting reconsolidation by pharmacological manipulations "deleted" the emotional expression of a fear memory in humans. If we are to target reconsolidation in patients with anxiety disorders, the disruption of reconsolidation should produce content-limited modifications. At the same time, the fear-erasing effects should not be restricted to the feared cue itself considering that fear generalization is a main characteristic of anxiety disorders. In Experiment I and Experiment I(b), we addressed these issues using a within-subject differential startle fear conditioning paradigm and a test of fear generalization. In Experiment II, we tested whether a behavioral approach targeting the reconsolidation through extinction learning was also effective in weakening the original fear memory. A behavioral procedure is evidently preferred over drug manipulations provided that similar effects can be obtained. Here, the extinction procedure subsequent to retrieval did not "erase" the emotional expression of the fear memory as the retrieval techniques (i.e., reminder shocks and reacquisition) unveiled a return of the startle fear response to the fear-relevant stimuli. In contrast, β-adrenergic receptor blockade during reconsolidation selectively deleted the fear-arousing aspects of the memory (i.e., startle fear response) along with its category-related information. The pharmacological manipulation rendered the core memory trace too weak to observe fear generalization after successful reacquisition. Hence, relearning following the disruption of reconsolidation seems to be qualitatively different from initial learning. Our findings demonstrate that disrupting reconsolidation by pharmacological manipulations, although selective, undermines the generalization of fear, a key feature of anxiety disorders.     

Both differences in encoding processes and monitoring at retrieval reduce false alarms when distinctive information is studied

ABSTRACT

Recent research has provided evidence for memory modifications when a post-reactivation treatment (e.g., drugs, new learning) interferes with the memory re-stabilisation (reconsolidation) process. This finding contradicts the long-standing consolidation theory and has high practical and theoretical implications. With an object-learning paradigm, it was shown that episodic memory is highly susceptible to interfering material presented after its reactivation [Hupbach, A., Gomez, R., Hardt, O., &; Nadel, L. (2007). Reconsolidation of episodic memories: A subtle reminder triggers integration of new information. Learning &; Memory, 14, 47–53. doi:10.1101/lm.365707]. The reactivation of a learned list (List 1) before a second learned list (List 2) led to intrusion errors from List 2 when trying to recall List 1, but not vice-versa. Their work has been widely cited and their findings have been explained according to reconsolidation theory. For the first time, we systematically explored the role of retrieval context as an alternative explanation for Hupbach’s results. Our results showed that the intrusion effect occurs independently of the retrieval context (Experiment 1). Additionally, even when the intrusion rate probability is increased (i.e., List 1 memory test is performed in the List 2 learning context), the groups that did not reactivate the original list did not commit intrusion errors (Experiment 2). In sum, we found that the intrusion effect critically depends on the presence of reactivation, discarding alternative interpretations of the results.     

Epigenetic alterations in the lateral amygdala are required for reconsolidation of a Pavlovian fear memory

Epigenetic mechanisms have been widely implicated in synaptic plasticity and in memory consolidation, yet little is known about the role of epigenetic mechanisms in memory reconsolidation processes. In the present study, we systematically examine the role of histone acetylation and DNA methylation in the reconsolidation of an amygdala-dependent Pavlovian fear memory. We first show that the acetylation of histone 3 (H3), but not histone 4 (H4), is regulated following auditory fear memory retrieval in the lateral nucleus of the amygdala (LA). We next show that histone deacetylase (HDAC) inhibition in the LA enhances both retrieval-induced histone acetylation and reconsolidation of an auditory fear memory. Conversely, inhibition of DNA methytransferase (DNMT) activity in the LA significantly impairs both retrieval-related H3 acetylation and fear memory reconsolidation. The effects of HDAC and DNMT inhibitors on fear memory reconsolidation were observed to be time-limited and were not evident in the absence of memory reactivation. Further, memories lost following DNMT inhibition were not observed to be vulnerable to spontaneous recovery, reinstatement, or to a shift in testing context, suggesting that memory impairment was not the result of facilitated extinction. Finally, pretreatment with the HDAC inhibitor was observed to rescue the reconsolidation deficit induced by the DNMT inhibitor. These findings collectively suggest that histone acetylation and DNA methylation are critical for reconsolidation of fear memories in the LA.     

Interaction between the basolateral amygdala and dorsal hippocampus is critical for cocaine memory reconsolidation and subsequent drug context-induced cocaine-seeking behavior in rats

Contextual stimulus control over instrumental drug-seeking behavior relies on the reconsolidation of context-response-drug associative memories into long-term memory storage following retrieval-induced destabilization. According to previous studies, the basolateral amygdala (BLA) and dorsal hippocampus (DH) regulate cocaine-related memory reconsolidation; however, it is not known whether these brain regions interact or independently control this phenomenon. To investigate this question, rats were trained to lever press for cocaine reinforcement in a distinct environmental context followed by extinction training in a different context. Rats were then briefly re-exposed to the cocaine-paired context to destabilize cocaine-related memories, or they were exposed to an unpaired context. Immediately thereafter, the rats received unilateral microinfusions of anisomycin (ANI) into the BLA plus baclofen/muscimol (B/M) into the contralateral (BLA/DH disconnection) or ipsilateral DH, or they received contralateral or ipsilateral microinfusions of vehicle. They then remained in their home cages overnight or for 21 d, followed by additional extinction training and a test of cocaine-seeking behavior (nonreinforced active lever responding). BLA/DH disconnection following re-exposure to the cocaine-paired context, but not the unpaired context, impaired subsequent drug context-induced cocaine-seeking behavior relative to vehicle or ipsilateral ANI + B/M treatment. Prolonged home cage stay elicited a time-dependent increase, or incubation, of drug-context-induced cocaine-seeking behavior, and BLA/DH disconnection inhibited this incubation effect despite some recovery of cocaine-seeking behavior. Thus, the BLA and DH interact to regulate the reconsolidation of cocaine-related associative memories, thereby facilitating the ability of drug-paired contexts to trigger cocaine-seeking behavior and contributing to the incubation of cocaine-seeking behavior.     

Reconsolidation of episodic memories: a subtle reminder triggers integration of new information

Recent demonstrations of "reconsolidation" suggest that memories can be modified when they are reactivated. Reconsolidation has been observed in human procedural memory and in implicit memory in infants. This study asks whether episodic memory undergoes reconsolidation. College students learned a list of objects on Day 1. On Day 2, they received a reminder or not, and then learned a second list. Memory for List 1 was tested immediately on Day 2 (Experiment 2) or on Day 3 (Experiment 1). Although the reminder did not moderate the number of items recalled from List 1 on either day, subjects who received a reminder incorrectly intermixed items from the second list when recalling List 1 on Day 3. Experiment 2 showed that this effect does not occur immediately and thus is time-dependent. The reminder did not affect memory for List 2 on Day 3 (Experiment 3), demonstrating that modification occurred only for the original memory (List 1). The study demonstrates the crucial role of reminders for the modification of episodic memory, that reconsolidation of episodic memory is time-dependent, and, in contrast to previous reconsolidation findings, that reconsolidation is also a constructive process, one that supports the incorporation of new information in memory.     

Paradoxical effects of testing: retrieval enhances both accurate recall and suggestibility in eyewitnesses

Although retrieval practice typically enhances memory retention, it can also impair subsequent eyewitness memory accuracy (Chan, Thomas, & Bulevich, 2009). Specifically, participants who had taken an initial test about a witnessed event were more likely than nontested participants to recall subsequently encountered misinformation—an effect we called retrieval-enhanced suggestibility (RES). Here, we sought to test the generality of RES and to further elucidate its underlying mechanisms. To that end, we tested a dual mechanism account, which suggests that RES occurs because initial testing (a) enhances learning of the later misinformation by reducing proactive interference and (b) causes the reactivated memory trace to be more susceptible to later interference (i.e., a reconsolidation account). Three major findings emerged. First, RES was found after a 1-week delay, where a robust testing benefit occurred for event details that were not contradicted by later misinformation. Second, blockage of reconsolidation was unnecessary for RES to occur. Third, initial testing enhanced learning of the misinformation even when proactive interference played a minimal role.     

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.     

Anisomycin infused into the hippocampus fails to block "reconsolidation" but impairs extinction: the role of re-exposure duration

Recent studies have reported new evidence consistent with the hypothesis that reactivating a memory by re-exposure to a training context destabilizes the memory and induces "reconsolidation." In the present experiments, rats' memory for inhibitory avoidance (IA) training was tested 6 h (Test 1), 2 d (Test 2), and 6 d (Test 3) after training. On Test 1 the rats were either removed from the shock compartment immediately after entry or retained in the shock context for 200 sec, and intrahippocampal infusions of the protein synthesis inhibitor anisomycin (75 microg/side) were administered immediately after the test. Anisomycin infusions administered after Test 1 impaired IA performance on Test 2 in animals given the brief re-exposure, but impaired extinction in animals exposed to the context for 200 sec. Rats with anisomycin-induced retention impairment on Test 2 demonstrated spontaneous recovery of retention performance on Test 3, whereas rats showing extinction on Test 2 showed further extinction on Test 3. The findings indicate that post-retrieval administration of anisomycin impairs subsequent retention performance only in the absence of extinction and that this impairment is temporary.     

Systemic and intrahippocampal administrations of the glucocorticoid receptor antagonist RU38486 impairs fear memory reconsolidation in rats

Reconsolidation is the process by which previously consolidated memories are stabilized after retrieval. Several lines of evidence indicate that glucocorticoids modulate distinct phases of learning and memory. These effects are considered to be mediated by mineralocorticoid receptors and glucocorticoid receptors (GRs), which display a high concentration and distinct distribution in the hippocampus. The role of glucocorticoid system in fear memory reconsolidation is the subject of some controversy. Moreover, we found no studies that assessed the role of hippocampal GRs in fear memory reconsolidation. Here, we investigated the effect of GR blockade on fear memory reconsolidation in rats. Rats were trained and tested in an inhibitory avoidance task. Intrahippocampal or systemic administration of the GR antagonist RU38486 immediately following memory reactivation produced a deficit in post-retrieval long-term memory that persisted over test sessions, and memory did not re-emerge following a footshock reminder. These results indicate that hippocampal GRs are required for reconsolidation of fear-based memory.     

Remaking memories: reconsolidation updates positively motivated spatial memory in rats

There is strong evidence that reactivation of a memory returns it to a labile state, initiating a restabilization process termed reconsolidation, which allows for updating of the memory. In this study we investigated reactivation-dependent updating using a new positively motivated spatial task in rodents that was designed specifically to model a human list-learning paradigm. On Day 1, rats were trained to run to three feeders (List 1) for rewards. On Day 2, rats were trained to run to three different feeders (List 2) in either the same (Reminder condition) or a different (No Reminder condition) experimental context than on Day 1. On Day 3, rats were cued to recall List 1. Rats in the Reminder condition made significantly more visits to List 2 feeders (intrusions) during List 1 recall than rats in the No Reminder condition, indicating that the reminder triggered reactivation and allowed integration of List 2 items into List 1. This reminder effect was selective for the reactivated List 1 memory, as no intrusions occurred when List 2 was recalled on Day 3. No intrusions occurred when retrieval took place in a different context from the one used at encoding, indicating that the expression of the updated memory is dependent upon the retrieval context. Finally, the level of intrusions was highest when retrieval took place immediately after List 2 learning, and generally declined when retrieval occurred 1-4 h later, indicating that the List 2 memory competed with short-term retrieval of List 1. These results demonstrate the dynamic nature of memory over time and the impact of environmental context at different stages of memory processing.     

The effects of the dopamine D1 receptor antagonist SCH23390 on memory reconsolidation following reminder-activated retrieval in day-old chicks

This series of experiments examined the involvement of the dopamine D1 receptor antagonist, SCH23390, on memory reconsolidation following reminder-activated retrieval. Day-old male New HampshirexWhite Leghorn chicks were trained on a single trial passive avoidance task. A dose of 0.5 mg/kg of SCH23390 was administered subcutaneously 5 min before reminder trials, which were presented at 30, 60, and 90 min following training. Memory deficits were observed when reminder trials were presented at 30 and 60 min following training, but not when a reminder was presented at 90 min. No effect on memory retention was observed when reminder trials were not presented, suggesting that reconsolidation mechanisms were both contingent on the presentation of the reminder and independent of the consolidation process. Following a reminder presented at 60 min post-training, deficits in memory retention emerged between 45 and 60 min. The deficit was prolonged, lasting for up until 48 h after reminder presentation. The results indicate an important role for the D1 receptor in reconsolidation processes.     

Retrieval per se is not sufficient to trigger reconsolidation of human fear memory

Ample evidence suggests that consolidated memories, upon their retrieval, enter a labile state, in which they might be susceptible to change. It has been proposed that memory labilization allows for the integration of relevant information in the established memory trace (memory updating). Memory labilization and reconsolidation do not necessarily occur when a memory is being reactivated, but only when there is something to be learned during memory retrieval (prediction error). Thus, updating of a fear memory trace should not occur under retrieval conditions in which the outcome is fully predictable (no prediction error). Here, we addressed this issue, using a human differential fear conditioning procedure, by eliminating the very possibility of reinforcement of the reminder cue. A previously established fear memory (picture-shock pairings) was reactivated with shock-electrodes attached (Propranolol group, n=18) or unattached (Propranolol No-Shock Expectation group, n=19). We additionally tested a placebo-control group with the shock-electrodes attached (Placebo group, n=18). Reconsolidation was not triggered when nothing could be learned during the reminder trial, as noradrenergic blockade did not affect expression of the fear memory 24h later in the Propranolol No-Shock Expectation group. Only when the outcome of the retrieval cue was not fully predictable, propranolol, contrary to placebo, reduced the startle fear response and prevented the return of fear (reinstatement) the following day. In line with previous studies, skin conductance response and shock expectancies were not affected by propranolol. Remarkably, a double dissociation emerged between the emotional (startle response) and more cognitive expression (expectancies, SCR) of the fear memory. Our findings have important implications for reconsolidation blockade as treatment strategy for emotional disorders. First, fear reducing procedures that target the emotional component of fear memory do not necessarily affect the cognitive component and vice versa. Second, mere retrieval of the fear memory is not sufficient to induce its labilization and reconsolidation.     

Low environmental calcium blocks long-term memory formation in a freshwater pulmonate snail

The freshwater snail Lymnaea stagnalis (L.) is considered a calciphile and exhibits reduced growth and survival in environments containing less than 20 mg/l environmental calcium. Although it has no apparent effect on survival at 20 mg/l, reducing environmental calcium increases metabolic demand, and as such we consider that this level of calcium acts as a stressor on the snail. We exposed snails to acute periods of low environmental calcium and tested their ability to form intermediate-term memory (ITM) and long-term memory (LTM) following one trial operant conditioning (1TT) to reduce aerial respiratory activity in hypoxic conditions. We also assessed whether there were changes in the electrophysiological properties of a single neuron, right pedal dorsal 1 (RPeD1), which has been demonstrated to be necessary for LTM formation. Following training in high (80 mg/l) environmental calcium, L. stagnalis formed ITM and LTM lasting 24 h and demonstrated a significant reduction in all activity measured from RPeD1; however when snails were exposed to low (20 mg/l) environmental calcium they were able to form ITM but not LTM. Although no behavioral LTM was formed, a partial reduction in RPeD1 activtiy measured 24 h after training was observed, indicating a residual effect of training. The strong effect that environmental calcium concentration had on physiology and behavior in response to training to reduce aerial respiration in L. stagnalis suggests that it is an element of gastropod husbandry that needs to be carefully considered when studying other traits. This study also indicates that L. stagnalis found naturally in low calcium environments may be less able to adapt to novel stressors than populations found in harder waters.     

Retrieval does not induce reconsolidation of inhibitory avoidance memory

It has been suggested that retrieval during a nonreinforced test induces reconsolidation instead of extinction of the mnemonic trace. Reconsolidation would preserve the original memory from the labilization induced by its nonreinforced recall through a hitherto uncharacterized mechanism requiring protein synthesis. Given the importance that such a process would have in terms of maintaining, as part of the animal behavioral repertoire, a learned response that has been devalued by experience, we analyzed its existence for the memory associated with a one-trial, step-down inhibitory avoidance task (IA), a memory whose consolidation and extinction require protein synthesis in the CA1 region of the dorsal hippocampus (CA1) and involve the participation of the basolateral amygdala (BLA) and entorhinal cortex (ENT). Rats were trained in IA, and 24 h later they were submitted either to a pure reactivation session (retrieval without stepping down), which was unable by itself to initiate extinction of the avoidance response, or to a second training session. Fifteen minutes before or 3 h after either the reactivation or the retraining sessions, animals were infused with the protein synthesis inhibitor anisomycin (ANI) into CA1, BLA, or ENT. Contrary to the prediction of the reconsolidation hypothesis, none of these treatments affected subsequent memory retention. Because reconsolidation is regarded to be a direct consequence of retrieval, one would expect that, when given before a retention test or a pure reactivation session, enhancers of memory expression should permanently improve retention and, therefore, facilitate retrieval both in that and in subsequent sessions. Using two well-known retrieval enhancers, noradrenaline and adrenocorticotropin(1-24), we could not find any evidence suggestive of reconsolidation. Hence, our results indicate that there is no retrieval-induced, protein synthesis-dependent process that would cause reconsolidation of IA memory.     

Role of matrix metalloproteinases in the acquisition and reconsolidation of cocaine-induced conditioned place preference

Persistent drug seeking/taking behavior involves the consolidation of memory. With each drug use, the memory may be reactivated and reconsolidated to maintain the original memory. During reactivation, the memory may become labile and susceptible to disruption; thus, molecules involved in plasticity should influence acquisition and/or reconsolidation. Recently, matrix metalloproteinases (MMPs) have been shown to influence neuronal plasticity, presumably by their regulation of extracellular matrix (ECM) molecules involved in synaptic reorganization during learning. We hypothesized that inhibition of MMP activity would impair the acquisition and/or reconsolidation of cocaine-conditioned place preference (CPP) in rats. Intracerebral ventricular (i.c.v.) microinjection of a broad spectrum MMP inhibitor, FN-439, prior to cocaine training suppressed acquisition of CPP and attenuated cocaine-primed reinstatement in extinguished animals. In a separate experiment, the cocaine memory was reactivated on two consecutive days with a cocaine priming injection. On these two days, artificial cerebral spinal fluid (aCSF) or FN-439 was administered either 30 min prior to or 1 min after cocaine-primed reinstatement sessions. Infusion of FN-439 partially impaired retrieval of the cocaine-associated context when given 30 min prior to cocaine. In both groups, however, FN-439 suppressed reinstatement compared with controls on the third consecutive test for cocaine-primed reinstatement, when no FN-439 was given. Control experiments demonstrated that two injections of FN-439 + cocaine given in the home cage, or of FN-439 + saline priming injections in the CPP chambers did not disrupt subsequent cocaine-primed reinstatement. These results show for the first time that (1) MMPs play a critical role in acquisition and reconsolidation of cocaine-induced CPP, and (2) rats demonstrate apparent disruption of reconsolidation by an MMP inhibitor after extinction and while they are under the influence of cocaine during reinstatement.     

Reconsolidation Reconsidered

Some of the considerations that led to a consolidation interpretation of retrograde amnesia (RA), which states that RA results from the disruption of memory processing and storage when neural activity is interrupted by a brain insult, are reviewed here. The time-dependent gradient of memory loss (i.e., new memories are more vulnerable to amnesia than old memories) that characterizes RA seemed to fit nicely with the notion of a cascade of cellular events occurring during the immediate post-acquisition period that would transform a labile representation into a more stable form (i.e., consolidate the memory). However, a variety of observations came to challenge the storage-disruption model, and among these was the finding of amnesia for old but reactivated memories. A recent study by Nader, Schafe, and LeDoux (2000) provides an important analytic extension of the work on "reconsolidation" by showing that inhibition of protein synthesis in the lateral and basal nuclei of the amygdala immediately following the reactivation of old memory will induce retrograde amnesia. We offer a retrieval-oriented conceptualization to account for the temporal gradient and the "reconsolidation" phenomena.