D-cycloserine potentiates the reconsolidation of cocaine-associated memories

Exercise Influences the Brain's Metabolic Response to Chronic Cocaine Exposure in Male Rats

Cocaine use is associated with negative health outcomes: cocaine use disorders, speedballing, and overdose deaths. Currently, treatments for cocaine use disorders and overdose are non-existent when compared to opioid use disorders, and current standard cocaine use disorder treatments have high dropout and recidivism rates. Physical exercise has been shown to attenuate addiction behavior as well as modulate brain activity. This study examined the differential effects of chronic cocaine use between exercised and sedentary rats. The effects of exercise on brain glucose metabolism (BGluM) following chronic cocaine exposure were assessed using Positron Emission Tomography (PET) and [18F]-Fluorodeoxyglucose (FDG). Compared to sedentary animals, exercise decreased metabolism in the SIBF primary somatosensory cortex. Activation occurred in the amygdalopiriform and piriform cortex, trigeminothalamic tract, rhinal and perirhinal cortex, and visual cortex. BGluM changes may help ameliorate various aspects of cocaine abuse and reinstatement. Further investigation is needed into the underlying neuronal circuits involved in BGluM changes and their association with addiction behaviors.

Drug memory reconsolidation: from molecular mechanisms to the clinical context

Since its rediscovery at the beginning of the 21st Century, memory reconsolidation has been proposed to be a therapeutic target for reducing the impact of emotional memories that can go awry in mental health disorders such as drug addiction (substance use disorder, SUD). Addiction can be conceptualised as a disorder of learning and memory, in which both pavlovian and instrumental learning systems become hijacked into supporting drug-seeking and drug-taking behaviours. The past two decades of research have characterised the details of the molecular pathways supporting the reconsolidation of pavlovian cue-drug memories, with more recent work indicating that the reconsolidation of instrumental drug-seeking memories also relies upon similar mechanisms. This narrative review considers what is known about the mechanisms underlying the reconsolidation of pavlovian and instrumental memories associated with drug use, how these approaches have translated to experimental medicine studies, and the challenges and opportunities for the clinical use of reconsolidation-based therapies.

Rottlerin, BDNF, and the impairment of inhibitory avoidance memory

RATIONALE AND OBJECTIVE

As a eukaryotic elongation factor 2 kinase (eEF2K) inhibitor and a mitochondrial uncoupler, oncologists have extensively studied rottlerin. Neuroscientists, however, have accumulated scarce data on the role of rottlerin in affective and cognitive functions. Only two prior studies have, respectively, documented its antidepressant-like effect and how it impairs psychostimulant-supported memory. Whether or not rottlerin would affect aversive memory remains unknown. Hence, we sought to investigate the effects of rottlerin on aversive memory in the inhibitory avoidance (IA) task in mice.

MATERIALS AND METHODS

Male C57BL/6J mice were trained to acquire the IA task. Rottlerin (5 mg/kg, i.p. or 3 μg bilaterally in the hippocampus) or the vehicle was administered before footshock training (acquisition), after footshock training (consolidation), after the memory reactivation (reconsolidation), and before the test (retrieval) in the IA task.

RESULTS

Systemic and intrahippocampal rottlerin impaired the acquisition, consolidation, and retrieval of IA memory, without affecting the reconsolidation process. Rottlerin (5 mg/kg, i.p.) induced a fast-onset and long-lasting increase in the brain-derived neurotrophic factor (BDNF) protein levels in the mouse hippocampus. Systemic injection of 7,8-dihydroxyflavone (7,8-DHF, 30 mg/kg), a BDNF tropomyosin receptor kinase B (TrkB) agonist impaired IA memory consolidation, and treatment with K252a (5 μg/kg), a Trk receptor antagonist, reversed the suppressing effect of rottlerin on IA memory consolidation.

CONCLUSION

Rottlerin impairs IA memory consolidation through the enhancement of BDNF signaling in the mouse hippocampus. Excessive brain BDNF levels can be detrimental to cognitive function. Rottlerin is likely to affect the original memory-associated neuroplasticity. Thus, it can be combined with exposure therapy to facilitate the forgetting of maladaptive aversive memory, such as post-traumatic stress disorder (PTSD).

Molecular and circuit mechanisms regulating cocaine memory

Risk of relapse is a major challenge in the treatment of substance use disorders. Several types of learning and memory mechanisms are involved in substance use and have implications for relapse. Associative memories form between the effects of drugs and the surrounding environmental stimuli, and exposure to these stimuli during abstinence causes stress and triggers drug craving, which can lead to relapse. Understanding the neural underpinnings of how these associations are formed and maintained will inform future advances in treatment practices. A large body of research has expanded our knowledge of how associative memories are acquired and consolidated, how they are updated through reactivation and reconsolidation, and how competing extinction memories are formed. This review will focus on the vast literature examining the mechanisms of cocaine Pavlovian associative memories with an emphasis on the molecular memory mechanisms and circuits involved in the consolidation, reconsolidation, and extinction of these memories. Additional research elucidating the specific signaling pathways, mechanisms of synaptic plasticity, and epigenetic regulation of gene expression in the circuits involved in associative learning will reveal more distinctions between consolidation, reconsolidation, and extinction learning that can be applied to the treatment of substance use disorders.

Randomized controlled trial of d-cycloserine in cocaine dependence: Effects on contingency management and cue-induced cocaine craving in a naturalistic setting

Cocaine dependence constitutes a significant public health concern. This randomized, double-blind, placebo-controlled trial tested a novel approach to reducing cocaine use among cocaine-dependent individuals with d-cycloserine, a drug known to enhance learning and some learning-based therapies. Urine samples and cocaine craving were assessed across three phases: induction (Weeks 1-2), treatment (Weeks 3-5; urinalysis-based contingency management plus exposure therapy), and posttreatment (Weeks 6-7). During the treatment phase, either 50 mg of d-cycloserine or placebo was administered after delivery of urinalysis feedback with potential monetary reward and before exposure therapy sessions in naturalistic contexts individually associated with cocaine use. d-cycloserine significantly improved learning on an operant laboratory task. Contingency management significantly reduced cocaine use and craving. d-cycloserine did not significantly affect cocaine use or craving in the treatment phase. Craving significantly increased for the d-cycloserine group during the post treatment phase. Therefore, although the study showed that d-cycloserine was capable of improving learning, enhancement of learning-based therapy was not observed. Moreover, no differences in behavioral measures of cocaine demand (cocaine purchasing task) or monetary or sexual delay discounting were observed across phases or between groups in any phase. These results are somewhat consistent with previous findings suggesting that d-cycloserine administration increases cocaine craving, although they differ from other findings showing that d-cycloserine administration reduces alcohol or nicotine cravings. Methodological variables (e.g., guided vs. unguided exposure therapy sessions, length of extinction exposure) likely play a role in dissimilar findings observed across studies. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Calcineurin Promotes Neuroplastic Changes in the Amygdala Associated with Weakened Cocaine-Cue Memories

Interfering with memory reconsolidation or inducing memory extinction are two approaches for weakening maladaptive memories in disorders such as addiction and post-traumatic stress disorder. Both extinction and reconsolidation are regulated by intracellular protein kinases and phosphatases, and interfering with these signaling molecules can alter memory strength. The calcium-dependent protein phosphatase, calcineurin (CaN), has been implicated in both the consolidation and extinction of fear memories. However, the role of CaN in regulating drug-cue associative memories has not been investigated. Prior studies have demonstrated that plasticity at thalamo-lateral amygdala (T-LA) synapses is critically involved in the regulation of cocaine-cue memories. Therefore, in the present study, we tested the effects of LA administration of an activator of CaN, chlorogenic acid (CGA), on behavioral and electrophysiological indices of cocaine cue memory reconsolidation and extinction. Male, Sprague-Dawley rats were trained to self-administer cocaine paired with an audiovisual cue. The cue memory was then either briefly reactivated, extinguished, or not manipulated, followed immediately by LA infusion of CGA. Rats were tested 24 h later for cue-induced reinstatement, or LA slices were prepared for electrophysiological recordings. We found that intra-LA infusions of CGA following cue extinction or reconsolidation reduced cue-induced reinstatement, which was blocked by co-infusion of the CaN inhibitor, FK-506. Similarly, CGA infusions following cue re-exposure significantly attenuated EPSC amplitude at T-LA synapses, suggesting that CaN affects cocaine-cue memory reconsolidation and extinction by altering T-LA synaptic strength. Therefore, CaN signaling in the LA may represent a novel target for disrupting cocaine-associated memories to reduce relapse.SIGNIFICANCE STATEMENT Repetitive drug use induces synaptic plasticity that underlies the formation of long-lasting associative memories for environmental cues paired with the drug. We previously identified thalamo-amygdala synapses (T-LA) that project via the interal capsule, as an important locus for the regulation of cocaine-cue memories. These synapses are strengthened by repeated cocaine-cue pairings, but this is reversed by extinction training or by optogenetic induction of in vivo long-term depression (LTD). Here, we demonstrate that activating calcineurin, a calcium-dependent phosphatase, following the reactivation or extinction of a cocaine-cue memory, induces LTD-like changes at T-LA synapses, and a corresponding decrease in cue-induced reinstatement, suggesting that calcineurin may be a potential therapeutic target for relapse prevention.

Can cocaine-induced neuroinflammation explain maladaptive cocaine-associated memories?

Persistent and intrusive memories define a number of psychiatric disorders, including posttraumatic stress disorder and substance use disorder. In the latter, memory for drug-paired cues plays a critical role in sustaining compulsive drug use as these are potent triggers of relapse. As with many drugs, cocaine-cue associated memory is strengthened across presentations as cues become reliable predictors of drug availability. Recently, the targeting of cocaine-associated memory through disruption of the reconsolidation process has emerged as a potential therapeutic strategy; reconsolidation reflects the active process by which memory is re-stabilized after retrieval. In addition, a separate line of work reveals that neuroinflammatory markers, regulated by cocaine intake, play a role in memory processes. Our review brings these two literatures together by summarizing recent findings on cocaine-associated reconsolidation and cocaine-induced neuroinflammation. We discuss the interactions between reconsolidation processes and neuroinflammation following cocaine use, concluding with a new perspective on treatment to decrease risk of relapse to cocaine use.

Limited replicability of drug-induced amnesia after contextual fear memory retrieval in rats

With the ultimate goal of investigating boundary conditions for post-reactivation amnesia, we set out to replicate studies in which systemic, post-reactivation administration of midazolam, propranolol, or cycloheximide resulted in amnesia for contextual fear memories. Our experiments involved conceptual as well as exact replications of previously published studies. In most of our experiments, we adopted a procedure that conformed to the standard 3-day protocol typically used in the literature, with contextual fear conditioning on day 1, unreinforced re-exposure to the conditioning context followed by systemic injection of the amnestic drug on day 2, and a memory retention test on day 3. Given the plethora of successful studies with large effects sizes and the absence of any failed replications in the literature, we were surprised to find that we were generally unable to replicate those findings. Our results suggest that post-reactivation amnesia by systemic drug administration in rats is more difficult to obtain than what would be expected based on published empirical reports. At present, it remains unclear which conditions determine the success of this procedure.

Cocaine conditioned place preference: unexpected suppression of preference due to testing combined with strong conditioning

Conditioned place preference (CPP) is widely used for evaluating the rewarding effects of drugs. Like other memories, CPP is proposed to undergo reconsolidation during which it is unstable and sensitive to pharmacological inhibition. Previous studies have shown that cocaine CPP can be apparently erased by extracellular signal-regulated kinase (ERK) pathway inhibition during cocaine reconditioning (re-exposure to the drug-paired environment in the presence of the drug). Here, we show that blockade of D1 receptors during reconditioning prevented ERK activation and induced a loss of CPP. However, we also unexpectedly observed a CPP disappearance in mice that underwent testing and reconditioning with cocaine alone, specifically in strong conditioning conditions. The loss was due to the intermediate test. CPP was not recovered with reconditioning or priming in the short term, but it spontaneously reappeared after a month. When we challenged the D1 antagonist-mediated erasure, we observed that both a high dose of cocaine and a first CPP test were required for this effect. Our results also suggest a balance between D1-dependent ERK pathway activation and an A2a-dependent mechanism in D2 striatal neurons in controlling CPP expression. Our data reveal that, paradoxically, a simple CPP test can induce a complete (but transient) loss of place preference following strong but not weak cocaine conditioning. This study emphasizes the complex nature of CPP memory and the importance of multiple parameters that must be taken into consideration when investigating reconsolidation.

Molecular and synaptic mechanisms regulating drug-associated memories: Towards a bidirectional treatment strategy

The successful treatment of substance use disorders is dependent on the establishment of a long-term abstinent state. Relapse can be suppressed by interfering with memories of drug use that are evoked by re-exposure to drug-associated contexts and cues. Two strategies for accomplishing this goal are either to prevent drug-memory reconsolidation or to induce the formation of a competing, extinction memory. However, clinical attempts to prolong abstinence by behavioral modification of drug-related memories have had limited success. One approach to improve behavioral treatment strategies is to identify the molecular mechanisms that regulate these memory processes and then use pharmacological tools as supplements to improve efficacy. Still, due to the involvement of several overlapping signaling cascades in both reconsolidation and extinction, it is difficult to specifically modify one of the two processes. For example, attempting to elicit extinction may instead initiate reconsolidation, resulting in the unintentional strengthening of drug-related memories. A better approach is to identify diverging components of the two processes, whereby a single medication would simultaneously weaken reconsolidation and enhance extinction. This review will provide an overview of the neural substrates that are involved in the regulation of drug-associated memories, and will discuss emerging approaches to pharmacologically weaken these memories, including recent efforts to precisely and bidirectionally target reconsolidation and extinction. Ultimately, pharmacologically-enhanced memory-based approaches have the potential to produce more informed relapse-prevention therapies.

Phosphoproteomic Analysis Reveals a Novel Mechanism of CaMKIIα Regulation Inversely Induced by Cocaine Memory Extinction versus Reconsolidation

Successful addiction treatment depends on maintaining long-term abstinence, making relapse prevention an essential therapeutic goal. However, exposure to environmental cues associated with drug use often thwarts abstinence efforts by triggering drug using memories that drive craving and relapse. We sought to develop a dual approach for weakening cocaine memories through phosphoproteomic identification of targets regulated in opposite directions by memory extinction compared with reconsolidation in male Sprague-Dawley rats that had been trained to self-administer cocaine paired with an audiovisual cue. We discovered a novel, inversely regulated, memory-dependent phosphorylation event on calcium-calmodulin-dependent kinase II α (CaMKIIα) at serine (S)331. Correspondingly, extinction-associated S331 phosphorylation inhibited CaMKIIα activity. Intra-basolateral amygdala inhibition of CaMKII promoted memory extinction and disrupted reconsolidation, leading to a reduction in subsequent cue-induced reinstatement. CaMKII inhibition had no effect if the memory was neither retrieved nor extinguished. Therefore, inhibition of CaMKII represents a novel mechanism for memory-based addiction treatment that leverages both extinction enhancement and reconsolidation disruption to reduce relapse-like behavior.

SIGNIFICANCE STATEMENT

Preventing relapse to drug use is an important goal for the successful treatment of addictive disorders. Relapse-prevention therapies attempt to interfere with drug-associated memories, but are often hindered by unintentional memory strengthening. In this study, we identify phosphorylation events that are bidirectionally regulated by the reconsolidation versus extinction of a cocaine-associated memory, including a novel site on CaMKIIα. Additionally, using a rodent model of addiction, we show that CaMKII inhibition in the amygdala can reduce relapse-like behavior. Together, our data supports the existence of mechanisms that can be used to enhance current strategies for addiction treatment.

The computational nature of memory modification

Retrieving a memory can modify its influence on subsequent behavior. We develop a computational theory of memory modification, according to which modification of a memory trace occurs through classical associative learning, but which memory trace is eligible for modification depends on a structure learning mechanism that discovers the units of association by segmenting the stream of experience into statistically distinct clusters (latent causes). New memories are formed when the structure learning mechanism infers that a new latent cause underlies current sensory observations. By the same token, old memories are modified when old and new sensory observations are inferred to have been generated by the same latent cause. We derive this framework from probabilistic principles, and present a computational implementation. Simulations demonstrate that our model can reproduce the major experimental findings from studies of memory modification in the Pavlovian conditioning literature.

A Meta-Analysis of D-Cycloserine in Exposure-Based Treatment: Moderators of Treatment Efficacy, Response, and Diagnostic Remission

OBJECTIVE

This meta-analysis examined treatment efficacy, treatment response, and diagnostic remission effect sizes and moderators of D-cycloserine-augmented exposure treatment in randomized controlled trials (RCTs) of individuals with anxiety disorders, obsessive-compulsive disorder (OCD), and posttraumatic stress disorder (PTSD).

DATA SOURCES AND STUDY SELECTION

The terms D-cycloserine AND randomized controlled trial were used to search the PubMed (1965-May 2015), PsycINFO, and Scopus databases for randomized placebo-controlled trials of D-cycloserine-augmented exposure therapy for anxiety disorders, OCD, and PTSD.

DATA EXTRACTION

Clinical variables and effect sizes were extracted from 20 RCTs (957 participants). A random-effects model calculated the effect sizes for treatment efficacy, treatment response, and diagnostic remission using standardized rating scales. Subgroup analyses and meta-regression were used to examine potential moderators.

RESULTS

A small, nonsignificant benefit of D-cycloserine augmentation compared to placebo augmentation was identified across treatment efficacy (g = 0.15), response (risk ratio [RR] = 1.08), and remission (RR = 1.109), with a moderately significant effect (P = .03) for anxiety disorders specifically (g = 0.33). At initial follow-up assessments, a small, nonsignificant effect size of D-cycloserine augmentation compared to placebo was found for treatment efficacy (g = 0.21), response (RR = 1.06), and remission (RR = 1.12). Specific treatment moderators (eg, comorbidity, medication status, gender, publication year) were found across conditions for both acute treatment and initial follow-up assessments.

CONCLUSIONS

D-Cycloserine does not universally enhance treatment outcomes but demonstrates promise for anxiety disorders. Distinct treatment moderators may account for discrepant findings across RCTs and disorders. Future trials may be strengthened by accounting for identified moderators in their design, with ongoing research needed on the mechanisms of D-cycloserine to tailor treatment protocols and maximize its benefit.

Translational Approaches Targeting Reconsolidation

Maladaptive learned responses and memories contribute to psychiatric disorders that constitute a significant socio-economic burden. Primary treatment methods teach patients to inhibit maladaptive responses, but do not get rid of the memory itself, which explains why many patients experience a return of symptoms even after initially successful treatment. This highlights the need to discover more persistent and robust techniques to diminish maladaptive learned behaviours. One potentially promising approach is to alter the original memory, as opposed to inhibiting it, by targeting memory reconsolidation. Recent research shows that reactivating an old memory results in a period of memory flexibility and requires restorage, or reconsolidation, for the memory to persist. This reconsolidation period allows a window for modification of a specific old memory. Renewal of memory flexibility following reactivation holds great clinical potential as it enables targeting reconsolidation and changing of specific learned responses and memories that contribute to maladaptive mental states and behaviours. Here, we will review translational research on non-human animals, healthy human subjects, and clinical populations aimed at altering memories by targeting reconsolidation using biological treatments (electrical stimulation, noradrenergic antagonists) or behavioural interference (reactivation-extinction paradigm). Both approaches have been used successfully to modify aversive and appetitive memories, yet effectiveness in treating clinical populations has been limited. We will discuss that memory flexibility depends on the type of memory tested and the brain regions that underlie specific types of memory. Further, when and how we can most effectively reactivate a memory and induce flexibility is largely unclear. Finally, the development of drugs that can target reconsolidation and are safe for use in humans would optimize cross-species translations. Increasing the understanding of the mechanism and limitations of memory flexibility upon reactivation should help optimize efficacy of treatments for psychiatric patients.

Drug Addiction: Updating Actions to Habits to Compulsions Ten Years On

A decade ago, we hypothesized that drug addiction can be viewed as a transition from voluntary, recreational drug use to compulsive drug-seeking habits, neurally underpinned by a transition from prefrontal cortical to striatal control over drug seeking and taking as well as a progression from the ventral to the dorsal striatum. Here, in the light of burgeoning, supportive evidence, we reconsider and elaborate this hypothesis, in particular the refinements in our understanding of ventral and dorsal striatal mechanisms underlying goal-directed and habitual drug seeking, the influence of drug-associated Pavlovian-conditioned stimuli on drug seeking and relapse, and evidence for impairments in top-down prefrontal cortical inhibitory control over this behavior. We further review animal and human studies that have begun to define etiological factors and individual differences in the propensity to become addicted to drugs, leading to the description of addiction endophenotypes, especially for cocaine addiction. We consider the prospect of novel treatments for addiction that promote abstinence from and relapse to drug use.

Disrupting astrocyte-neuron lactate transfer persistently reduces conditioned responses to cocaine

A central problem in the treatment of drug addiction is the high risk of relapse often precipitated by drug-associated cues. The transfer of glycogen-derived lactate from astrocytes to neurons is required for long-term memory. Whereas blockade of drug memory reconsolidation represents a potential therapeutic strategy, the role of astrocyte-neuron lactate transport in long-term conditioning has received little attention. By infusing an inhibitor of glycogen phosphorylase into the basolateral amygdala of rats, we report that disruption of astrocyte-derived lactate not only transiently impaired the acquisition of a cocaine-induced conditioned place preference but also persistently disrupted an established conditioning. The drug memory was rescued by L-Lactate co-administration through a mechanism requiring the synaptic plasticity-related transcription factor Zif268 and extracellular signal-regulated kinase (ERK) signalling pathway but not the brain-derived neurotrophic factor (Bdnf). The long-term amnesia induced by glycogenolysis inhibition and the concomitant decreased expression of phospho-ERK were both restored with L-Lactate co-administration. These findings reveal a critical role for astrocyte-derived lactate in positive memory formation and highlight a novel amygdala-dependent reconsolidation process, whose disruption may offer a novel therapeutic target to reduce the long-lasting conditioned responses to cocaine.

Epigenetics and memory: causes, consequences and treatments for post-traumatic stress disorder and addiction

Understanding the interaction between fear and reward at the circuit and molecular levels has implications for basic scientific approaches to memory and for understanding the etiology of psychiatric disorders. Both stress and exposure to drugs of abuse induce epigenetic changes that result in persistent behavioral changes, some of which may contribute to the formation of a drug addiction or a stress-related psychiatric disorder. Converging evidence suggests that similar behavioral, neurobiological and molecular mechanisms control the extinction of learned fear and drug-seeking responses. This may, in part, account for the fact that individuals with post-traumatic stress disorder have a significantly elevated risk of developing a substance use disorder and have high rates of relapse to drugs of abuse, even after long periods of abstinence. At the behavioral level, a major challenge in treatments is that extinguished behavior is often not persistent, returning with changes in context, the passage of time or exposure to mild stressors. A common goal of treatments is therefore to weaken the ability of stressors to induce relapse. With the discovery of epigenetic mechanisms that create persistent molecular signals, recent work on extinction has focused on how modulating these epigenetic targets can create lasting extinction of fear or drug-seeking behavior. Here, we review recent evidence pointing to common behavioral, systems and epigenetic mechanisms in the regulation of fear and drug seeking. We suggest that targeting these mechanisms in combination with behavioral therapy may promote treatment and weaken stress-induced relapse.

Memory as a new therapeutic target

This review aims to demonstrate how an understanding of the brain mechanisms involved in memory provides a basis for; (i) reconceptualizing some mental disorders; (ii) refining existing therapeutic tools; and (iii) designing new ones for targeting processes that maintain these disorders. First, some of the stages which a memory undergoes are defined, and the clinical relevance of an understanding of memory processing by the brain is discussed. This is followed by a brief review of some of the clinical studies that have targeted memory processes. Finally, some new insights provided by the field of neuroscience with implications for conceptualizing mental disorders are presented.

Neural and psychological mechanisms underlying compulsive drug seeking habits and drug memories--indications for novel treatments of addiction

This review discusses the evidence for the hypothesis that the development of drug addiction can be understood in terms of interactions between Pavlovian and instrumental learning and memory mechanisms in the brain that underlie the seeking and taking of drugs. It is argued that these behaviours initially are goal-directed, but increasingly become elicited as stimulus-response habits by drug-associated conditioned stimuli that are established by Pavlovian conditioning. It is further argued that compulsive drug use emerges as the result of a loss of prefrontal cortical inhibitory control over drug seeking habits. Data are reviewed that indicate these transitions from use to abuse to addiction depend upon shifts from ventral to dorsal striatal control over behaviour, mediated in part by serial connectivity between the striatum and midbrain dopamine systems. Only some individuals lose control over their drug use, and the importance of behavioural impulsivity as a vulnerability trait predicting stimulant abuse and addiction in animals and humans, together with consideration of an emerging neuroendophenotype for addiction are discussed. Finally, the potential for developing treatments for addiction is considered in light of the neuropsychological advances that are reviewed, including the possibility of targeting drug memory reconsolidation and extinction to reduce Pavlovian influences on drug seeking as a means of promoting abstinence and preventing relapse.

The transcription factor Zif268/Egr1, brain plasticity, and memory

The capacity to remember our past experiences and organize our future draws on a number of cognitive processes that allow our brain to form and store neural representations that can be recalled and updated at will. In the brain, these processes require mechanisms of neural plasticity in the activated circuits, brought about by cellular and molecular changes within the neurons activated during learning. At the cellular level, a wealth of experimental data accumulated in recent years provides evidence that signaling from synapses to nucleus and the rapid regulation of the expression of immediate early genes encoding inducible, regulatory transcription factors is a key step in the mechanisms underlying synaptic plasticity and the modification of neural networks required for the laying down of memories. In the activated neurons, these transcriptional events are thought to mediate the activation of selective gene programs and subsequent synthesis of proteins, leading to stable functional and structural remodeling of the activated networks, so that the memory can later be reactivated upon recall. Over the past few decades, novel insights have been gained in identifying key transcriptional regulators that can control the genomic response of synaptically activated neurons. Here, as an example of this approach, we focus on one such activity-dependent transcription factor, Zif268, known to be implicated in neuronal plasticity and memory formation. We summarize current knowledge about the regulation and function of Zif268 in different types of brain plasticity and memory processes.

Rapamycin prevents drug seeking via disrupting reconsolidation of reward memory in rats

The maladaptive drug memory developed between the drug-rewarding effect and environmental cues contributes to difficulty in preventing drug relapse. Established reward memories can be disrupted by pharmacologic interventions following their reactivation. Rapamycin, an inhibitor of mammalian target of rapamycin (mTOR) kinase, has been proved to be involved in various memory consolidation. However, it is less well characterized in drug memory reconsolidation. Using a conditioned place preference (CPP) procedure, we examined the effects of systemically administered rapamycin on reconsolidation of drug memory in rats. We found that systemically administered rapamycin (0.1 or 10 mg/kg, i.p.) after re-exposure to drug-paired environment, dose dependently decreased the expression of CPP 1 d later, and the effect lasted for up to 14 d and could not be reversed by a priming injection of morphine. The effect of rapamycin on morphine-associated memory was specific to drug-paired context, and rapamycin had no effect on subsequent CPP expression when rats were exposed to saline-paired context or homecage. These results indicated that systemic administration of rapamycin after memory reactivation can persistently inhibit the drug seeking behaviour via disruption of morphine memory reconsolidation in rats. Additionally, the effect of rapamycin on memory reconsolidation was reproduced in cocaine CPP and alcohol CPP. Furthermore, rapamycin did not induce conditioned place aversion and had no effect on locomotor activity and anxiety behaviour. These findings suggest that rapamycin could erase the acquired drug CPP in rats, and that mTOR activity plays an important role in drug reconsolidation and is required for drug relapse.

Opposing effects of D-cycloserine on fear despite a common extinction duration: interactions between brain regions and behavior

A number of studies have reported that D-cycloserine (DCS), a partial agonist of the N-methyl-D-aspartate glutamate receptor, can facilitate the loss of conditioned fear if it is administered during an extinction trial. Here we examine the effects of DCS injected into the hippocampus or amygdala on extinction of context-evoked freezing after contextual fear conditioning in C57BL/6 mice. We find that DCS administered prior to an extinction session decreased freezing from the outset of the session regardless of which brain region was targeted. Retention tests revealed opposite effects on fear expression despite identical behavioral treatments: intra-hippocampal DCS inhibited fear expression while intra-amygdala DCS potentiated fear expression. Following post-extinction session injections of DCS, we found a similar though less pronounced effect. Closer inspection of the data revealed that the effects of DCS interacted with the behavior of the subjects during extinction. Intra-hippocampal injections of DCS enhanced extinction in those mice that showed the greatest amount of within-session extinction, but had less pronounced effects on mice that showed the least within-session extinction. Intra-amygdala injections of DCS impaired extinction in those mice that showed the least within-session extinction, but there was some evidence that the effect in the amygdala did not depend on behavior during extinction. These findings demonstrate that even with identical extinction trial durations, the effects of DCS administered into the hippocampus and amygdala can heavily depend on the organism's behavior during the extinction session. The broader implication of these findings is that the effects of pharmacological treatments designed to enhance extinction by targeting hippocampal or amygdalar processes may depend on the responsivity of the subject to the behavioral treatment.

Mechanisms governing the reactivation-dependent destabilization of memories and their role in extinction

The extinction of learned associations has traditionally been considered to involve new learning, which competes with the original memory for control over behavior. However, a recent resurgence of interest in reactivation-dependent amnesia has revealed that the retrieval of fear-related memory (with what is essentially a brief extinction session) can result in its destabilization. This review discusses some of the cellular and molecular mechanisms that are involved in the destabilization of a memory following its reactivation and/or extinction, and investigates the evidence that extinction may involve both new learning as well as a partial destabilization-induced erasure of the original memory trace.

Outcome specificity in deepened extinction may limit treatment feasibility: co-presentation of a food cue interferes with extinction of cue-elicited cocaine seeking

BACKGROUND

We previously showed that presenting two cocaine cues simultaneously during extinction deepens the extinction of cue-elicited cocaine seeking (Kearns et al., 2012). The present study investigated whether compounding a non-drug appetitive cue with a cocaine cue would similarly deepen extinction.

METHODS

In Experiment 1, tone and click were each first established as discriminative stimuli for cocaine-reinforced responding and light was a cue for food-reinforced responding. In an initial extinction phase, all stimuli were presented individually. Then, during an additional compound extinction session, rats received 8 presentations of one of the cocaine cues (counterbalanced over subjects) simultaneously with light and 8 presentations of the other cue alone. A spontaneous recovery test was used to evaluate the effectiveness of the extinction treatments. Experiment 2 was performed under conditions designed to match those of Experiment 1, except food was the reinforcer in tone and click instead of cocaine.

RESULTS

In Experiment 1, the cocaine cue compounded with the food cue during extinction controlled greater spontaneous recovery of cocaine seeking than the cocaine cue always presented alone. In contrast, Experiment 2 demonstrated deepened extinction of responding to a food cue when both compounded cues were food cues.

CONCLUSIONS

Results suggest that deepened extinction depends on the compound presentation of cues associated with the same reinforcer. Compound presentation of cues associated with different reinforcers could lead to an enhancement of responding. Care is urged in attempts to deepen the extinction of cue-elicited drug seeking by compounding drug cues with non-drug cues.

Targeting cocaine versus heroin memories: divergent roles within ventromedial prefrontal cortex

In the search for novel treatments for addiction, most research has been propelled by the hope for a 'magic bullet' that would cure all forms of addiction. More recently, the field has started to appreciate the differences between psychostimulants versus opiates. Recent data suggest that the ventromedial prefrontal cortex (vmPFC) may fundamentally serve different roles in cocaine versus heroin addiction: acting as a neural OFF switch for cocaine seeking, but an ON switch for heroin seeking. We discuss the relevance of this distinction in relationship to three main functions of the vmPFC: (i) extinction memory, (ii) the suppression of impulsive behaviors, and (iii) the transition from goal-directed behaviors to habits. We highlight the importance of dopamine in modulating corticostriatal circuits for each of these functions. Finally, we conclude by discussing the implications for treatment strategies.

Memory reconsolidation in aversive and appetitive settings

Memory reconsolidation has been observed across species and in a number of behavioral paradigms. The majority of memory reconsolidation studies have been carried out in Pavlovian fear conditioning and other aversive memory settings, with potential implications for the treatment of post-traumatic stress disorder. However, there is a growing literature on memory reconsolidation in appetitive reward-related memory paradigms, including translational models of drug addiction. While there appears to be substantial similarity in the basic phenomenon and underlying mechanisms of memory reconsolidation across unconditioned stimulus valence, there are also notable discrepancies. These arise both when comparing aversive to appetitive paradigms and also across different paradigms within the same valence of memory. We review the demonstration of memory reconsolidation across different aversive and appetitive memory paradigms, the commonalities and differences in underlying mechanisms and the conditions under which each memory undergoes reconsolidation. We focus particularly on whether principles derived from the aversive literature are applicable to appetitive settings, and also whether the expanding literature in appetitive paradigms is informative for fear memory reconsolidation.

Addiction: a drug-induced disorder of memory reconsolidation

Persistent maladaptive memories that maintain drug seeking and are resistant to extinction are a hallmark of addiction. As such, disruption of memory reconsolidation after retrieval has received attention for its therapeutic potential. Unrestrained reconsolidation may have the opposite effect, leading to reiterative and cumulative strengthening of memory over long periods of time. Here we review the molecular mechanisms underlying reconsolidation of appetitive and drug-rewarded memories, and discuss how these findings contribute to our understanding of the nature of this process. Finally, we suggest that drug-induced alterations to signal transduction might lead to dysregulation of reconsolidation, causing enhancements of drug-related memory after retrieval, and significantly contribute to the compulsive drug seeking that is a core component of addiction.

Drink, drugs and disruption: memory manipulation for the treatment of addiction

Addiction is a complex disorder, and one characterised by the acquisition of maladaptive instrumental (drug-seeking and drug-taking) and pavlovian (cue-drug associations) memories. These memories markedly contribute to the long-term risk of relapse, so reduction of the impact of these memories on behaviour could potentially be an important addition to current therapies for addiction. Memory reconsolidation may provide such a target for disrupting well-consolidated pavlovian cue-drug memories following an extensive drug history. Reconsolidation can be disrupted either by administering amnestic drugs in conjunction with a memory reactivation session, or by updating the memory adaptively through the induction of 'superextinction'. More work is needed before these therapies are ready for translation to the clinic, but if found clinically effective memory manipulation promises a radical new way of treating addiction.

The context dependency of extinction negates the effectiveness of cognitive enhancement to reduce cocaine-primed reinstatement

With respect to the treatment of addiction, the objective of extinction training is to decrease drug-seeking behavior by repeatedly exposing the patient to cues in the absence of unconditioned reinforcement. Such exposure therapy typically takes place in a novel (clinical) environment. This is potentially problematic, as the effects of extinction training include a context dependent component and therefore diminished efficacy is expected upon the patient's return to former drug-seeking/taking environments. We have reported that treatment with the NMDAR coagonist d-serine is effective in facilitating the effects of extinction to reduce cocaine-primed reinstatement. The present study assesses d-serine's effectiveness in reducing drug-primed reinstatement under conditions in which extinction training occurs in a novel environment. After 22 days of cocaine self-administration (0.5 mg/kg) in context "A", animals underwent 5 extinction training sessions in context "B". Immediately after each extinction session in "B", animals received either saline or d-serine (60 mg/kg) treatment. Our results indicate that d-serine treatment following extinction in "B" had no effect on either IV or IP cocaine-primed reinstatement conducted in "A". These results stand in contrast to our previous findings where extinction occurred in "A", indicating that d-serine's effectiveness in facilitating extinction training to reduce drug-primed reinstatement is not transferable to a novel extinction environment. This inability of d-serine treatment to reduce the context specificity of extinction training may explain the inconsistent effects observed in clinical studies published to date in which adjunctive cognitive enhancement treatment has been combined with behavioral therapy without significant benefit.

D-cycloserine enhancement of fear extinction is specific to successful exposure sessions: evidence from the treatment of height phobia

BACKGROUND

Whereas some studies have shown clear evidence for an augmentation effect of D-cycloserine (DCS) on exposure therapy for anxiety disorders, other studies have shown weak effects or no effect at all. Some preclinical data suggest that the DCS augmentation effect is moderated by the success of extinction learning. Therefore, we conducted a reanalysis of existing data to examine whether the effects of DCS on clinical outcome would vary as a function of response to the exposure session (i.e., exposure success).

METHODS

In a clinical trial, patients with height phobia received two sessions involving 30 minutes of virtual reality exposure therapy and were randomly assigned to a pill placebo (n = 14) or 50 mg of DCS (n = 15) immediately after each session.

RESULTS

Mixed-effects regression analysis showed that the effects of DCS administration on clinical improvement was moderated by the level of fear experienced just before concluding exposure sessions. Patients receiving DCS exhibited significantly greater improvement in symptoms relative to patients who received placebo when fear was low at the end of the exposure. In contrast, when end fear was still elevated, patients receiving DCS improved less compared with those receiving placebo.

CONCLUSIONS

D-cycloserine appears to enhance the benefits of exposure treatment when applied after a successful session, but it seems to have detrimental effects when administered after inadequate/unsuccessful exposure sessions.

Reconsolidation and extinction of an appetitive pavlovian memory

When memories are retrieved, they can enter a labile state during which the memory may be modified and subsequently restabilized through the process of reconsolidation. However, this does not occur in all situations, and certain "boundary conditions" determine whether a memory will undergo reconsolidation. Naïve male lister hooded rats were trained for 5 days to press a lever in order to retrieve a food reward associated with a pavlovian light stimulus. Three days post-training, animals were injected with either MK-801 (0.1 mgkg(-1); i.p.) or saline vehicle, 30 min before they were placed back into the training context for a retrieval session. Lever pressing was reinforced only by the light stimulus and was restricted to either 10, 30 or 50 presentations of the light conditioned stimulus. After 48 h, animals were again returned to the boxes and light-reinforced lever-pressing activity was recorded. MK-801-treated animals in the 10CS group significantly reduced lever pressing at test, compared to saline controls. In contrast, MK-801-treated rats in the 50CS group demonstrated a significant increase. There was no effect of MK-801 in the 30CS group. Additionally, there were no effects of MK-801 in an analogous, pure instrumental, setting when the cue lights were omitted. The opposing effects of MK-801 under different parametric conditions likely reflect impairments of appetitive pavlovian memory reconsolidation and extinction, respectively. These results demonstrate a competition between reconsolidation and extinction. However, there are also conditions under which MK-801 fails to impair either process.

Learning to forget: manipulating extinction and reconsolidation processes to treat addiction

Finding effective long-lasting treatments for drug addiction has been an elusive goal. Consequently, researchers are beginning to investigate novel treatment strategies including manipulations of drug-associated memories. When environmental stimuli (cues) become associated with drug use, they become powerful motivators of continued drug use and relapse after abstinence. Reducing the strength of these cue-drug memories could decrease the number of factors that induce craving and relapse to aid in the treatment of addiction. Enhancing the consolidation of extinction learning and/or disrupting cue-drug memory reconsolidation are two strategies that have been proposed to reduce the strength of cues in motivating drug-seeking and drug-taking behavior. Here, we review the latest basic and clinical research elucidating the mechanisms underlying consolidation of extinction and reconsolidation of cue-drug memories in the hopes of developing pharmacological tools that exploit these signaling systems to treat addiction.

A randomized, placebo-controlled laboratory study of the effects of D-cycloserine on craving in cocaine-dependent individuals

RATIONALE

D-Cycloserine (DCS), a partial glutamate N-methyl-D-aspartate (NMDA) receptor agonist, enhances extinction of conditioned fear responding; preliminary data suggest that it may facilitate extinction of drug cue reactivity.

OBJECTIVE

This study investigates DCS effects on cocaine cue craving and drug use in cocaine-dependent subjects.

METHODS

Thirty-two subjects were randomly assigned to receive (1) DCS only, (2) DCS before sessions 1 and 3, placebo (PBO) before session 2, or (3) PBO only 15-min before each of 3 1-h cocaine cue exposure sessions conducted 1 day apart. Craving ratings were obtained before, during, and after sessions. Drug use and cue-induced craving were assessed 1 week after the last cue session.

RESULTS

Repeated presentation of cocaine cues resulted in decreased craving both within and between sessions. DCS did not facilitate extinction learning and may have enhanced craving. The group that received three doses of DCS had significantly higher craving than the PBO group at the baseline ratings taken before sessions 2 and 3, as well as significantly higher cue-induced craving at follow-up. The group that received two doses of DCS did not differ from the PBO group. There were no group differences in postextinction cocaine use.

CONCLUSIONS

The reduction of cocaine cue reactivity in the PBO group suggests that the study procedures were sufficient to produce extinction. Under these conditions, DCS did not facilitate extinction and may have enhanced craving. Further studies of glutamatergic agents and extinction in cocaine dependence should include consideration of procedural variables that could have a major impact on study outcomes.

NMDA Receptor Modulators in the Treatment of Drug Addiction

Glutamate plays a pivotal role in drug addiction, and the N-methyl-d-aspartate (NMDA) glutamate receptor subtype serves as a molecular target for several drugs of abuse. In this review, we will provide an overview of NMDA receptor structure and function, followed by a review of the mechanism of action, clinical efficacy, and side effect profile of NMDA receptor ligands that are currently in use or being explored for the treatment of drug addiction. These ligands include the NMDA receptor modulators memantine and acamprosate, as well as the partial NMDA agonist d-Cycloserine. Data collected to date suggest that direct NMDA receptor modulators have relatively limited efficacy in the treatment of drug addiction, and that partial agonism of NMDA receptors may have some efficacy with regards to extinction learning during cue exposure therapy. However, the lack of consistency in results to date clearly indicates that additional studies are needed, as are studies examining novel ligands with indirect mechanisms for altering NMDA receptor function.

D-Serine facilitates the effectiveness of extinction to reduce drug-primed reinstatement of cocaine-induced conditioned place preference

Addiction is a disease that is characterized by compulsive drug-seeking and use despite negative health and social consequences. One obstacle in treating addiction is a high susceptibility for relapse which persists despite prolonged periods of abstinence. Relapse can be triggered by drug predictive stimuli such as environmental context and drug associated cues, as well as the addictive drug itself. The conditioned place preference (CPP) behavioral model is a useful paradigm for studying the ability of these drug predictive stimuli to reinstate drug-seeking behavior. The present study investigated the dose-dependent effects of D-serine (10 mg/kg, 30 mg/kg and 100 mg/kg) on extinction training and drug-primed reinstatement in cocaine-conditioned rats. In the first experiment, D-serine had no effect on the acquisition or development of cocaine-induced locomotor sensitization or CPP. In the second experiment, D-serine treatment resulted in significantly decreased time spent in the drug-paired compartment following completion of an extinction protocol. A cocaine-primed reinstatement test indicated that the combination of extinction training along with D-serine treatment resulted in a significant reduction of drug-seeking behavior. The third experiment assessed D-serine's long-term effects to diminish drug-primed reinstatement. D-serine treatment given during extinction was effective in reducing drug-seeking for more than four weeks of abstinence after the last cocaine exposure. These findings demonstrate that D-serine may be an effective adjunct therapeutic agent along with cognitive behavioral therapy for the treatment of cocaine addiction. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Facilitation of fear extinction by the 5-HT(1A) receptor agonist tandospirone: possible involvement of dopaminergic modulation

Fear extinction-based exposure treatment is an important component of psychotherapy for anxiety disorders such as posttraumatic stress disorder (PTSD). Recent studies have focused on pharmacological approaches combined with exposure therapy to augment extinction. In this study, we elucidated the therapeutic potential of the serotonin 1A (5-HT(1A) ) receptor agonist tandospirone compared with the effects of the N-methyl-D-aspartate partial agonist D-cycloserine (DCS), focusing on the possible involvement of dopaminergic mechanisms. We used a rat model of juvenile stress [aversive footshock (FS)] exposure during the third postnatal week (3wFS). The 3wFS group exhibited extinction deficit reflected in sustained fear-related behavior and synaptic dysfunction in the hippocampal CA1 field and medial prefrontal cortex (mPFC), which are responsible for extinction processes. Tandospirone administration (5 mg/kg, i.p.) before and after the extinction trials ameliorated both the behavioral deficit and synaptic dysfunction, i.e., synaptic efficacy in the CA1 field and mPFC associated with extinction training and retrieval, respectively, was potentiated in the tandospirone-treated 3wFS group. Extracellular dopamine release in the mPFC was increased by extinction retrieval in the non-FS control group. This facilitation was not observed in the 3wFS group; however, tandospirone treatment increased cortical dopamine levels after extinction retrieval. DCS (15 mg/kg, i.p.) also ameliorated the extinction deficit in the 3wFS group, but impaired extinction in the non-FS control group. These results suggest that tandospirone has therapeutic potential for enhancing synaptic efficacy associated with extinction processes by involving dopaminergic mechanisms. Pharmacological agents that target cortical dopaminergic systems may provide new insights into the development of therapeutic treatments of anxiety disorders, including PTSD.

Divergent cellular pathways of hippocampal memory consolidation and reconsolidation

The reconsolidation of memories after their retrieval involves cellular mechanisms that recapitulate much of the initial consolidation process. However, we have previously demonstrated that there are independent cellular mechanisms of consolidation and reconsolidation in the dorsal hippocampus for contextual fear memories. Expression of BDNF was required for consolidation, while Zif268 expression was necessary for reconsolidation. Given the dichotomy between the obvious mechanistic similarity and notable dissociations between consolidation and reconsolidation, we sought to determine whether the separation at the level of gene expression reflected either parallel and independent upstream signaling pathways, or common upstream mechanisms that diverge by the level of transcriptional activation. Here we show that while consolidation and reconsolidation are commonly dependent upon NMDA receptor activation in the dorsal hippocampus there is a double dissociation between the effects of the MEK inhibitor U0126 and the IKK inhibitor sulfasalazine. Moreover, rescue experiments and western blot analyses show that there are functional NMDA receptor-ERK1-BDNF and NMDA receptor-IKKα-Zif268 pathways for consolidation and reconsolidation, respectively. Therefore, there are divergent pathways of hippocampal memory consolidation and reconsolidation, involving commonality at the cell surface, but separable downstream kinase cascades and transcriptional regulation.

D-cycloserine in the basolateral amygdala prevents extinction and enhances reconsolidation of odor-reward associative learning in rats

It is well established that D-cycloserine (DCS), a partial agonist of the NMDA receptor glycine site, enhances learning and memory processes. Although the effects of DCS have been especially elucidated in the extinction and reconsolidation of aversive behavioral paradigms or drug-related behaviors, they have not been clearly determined in appetitive tasks using natural reinforcers. The current study examined the effects of pre-retrieval intra-basolateral amygdala (BLA) infusions of DCS on the extinction and reconsolidation of an appetitive odor discrimination task. Rats were trained to discriminate between three odors, one of which was associated with a palatable food reward, and, 20 min prior to extinction learning (experiment 1) or reactivation (experiment 2), they received bilateral intra-BLA infusions of DCS or vehicle. In experiment 1, DCS infusion reduced the rate of extinction learning, weakened extinction retention in a post-extinction test and enhanced reacquisition of the ODT task. In experiment 2, DCS improved subsequent memory expression in the reconsolidation test performed one day after the reactivation session. Such results indicate the involvement of BLA NMDA receptors in odor-food reward associative memory and suggest that DCS may potentiate the persistence or strength of the original memory trace.

Learning deficits in an odor reward-task induced by parafascicular thalamic lesions are ameliorated by pretraining D-cycloserine in the prelimbic cortex

We investigated whether the N-methyl-D-aspartate (NMDA) receptor partial agonist D-cycloserine (DCS) infused into the prelimbic cortex (PLC) would reverse the learning deficits caused by bilateral excitotoxic lesions of the parafascicular nucleus (PFn) in an odor discrimination task (ODT). Rats with PFn lesions received a bilateral infusion of DCS (10 μg/side) into the PLC 20 min before ODT acquisition. The task retention was evaluated in a drug-free test carried out 24 h later. DCS significantly attenuated the PFn lesion-induced deficits as measured by both latency to nose-poke the rewarded odor and number of errors committed during ODT acquisition and retention. Therefore, DCS may be an enhancing memory treatment in animal models of cognitive impairment, such as PFn-lesioned rats. The PFn contribution to learning and memory may possibly be linked to its role in the modulation of glutamatergic PLC activity.

Adult neurogenesis in the central and peripheral nervous systems

Neurogenesis occurs during embryonic development and continues throughout adulthood. Although spontaneous adult neurogenesis is restricted to selective germinal regions, including the subventricular zone and dentate gyrus of the hippocampus, neural stem cells (NSCs) are widely distributed in the central and peripheral nervous systems. Besides their ability to integrate into existing neural networks during physiological conditions, NSCs also proliferate and differentiate in response to injury, thus promising the potential use of endogenous NSCs for the treatment of neuropathological conditions. In this review, we discuss recent progress in the understanding of adult neurogenesis in the brain and peripheral nervous system.

D-cycloserine effects on extinction of conditioned responses to drug-related cues

D-cycloserine (DCS) is an N-methyl-D-aspartate (NMDA) receptor partial agonist that facilitates extinction of conditioned fear in animals and cue exposure therapy (CET) for fear and anxiety disorders in people. Recent preclinical and clinical studies have examined the effect of DCS on extinction of conditioned responses elicited by cues paired with administration of or withdrawal from drugs of abuse, including physiological responses, craving, withdrawal, and drug-seeking behavior. DCS facilitates extinction and blunts postextinction recovery of these responses in animal models, including place conditioning and drug self-administration, but DCS effects on CET in substance users/abusers are less robust. Some of the null effects in the clinical literature might be attributable to issues related to sample size, data characteristics, DCS administration, and participant characteristics, among others. In this review we describe the preclinical and clinical literatures on DCS modulation of extinction of addiction-related conditioned responses, consider possible limitations of the clinical studies that have been published to date, and propose ways of designing future clinical studies so as to maximize the probability of detecting a DCS effect. We also discuss concerns with regard to potential harmful effects of DCS-coupled CET in addicts and describe how these concerns might be mitigated. We conclude that it is as yet unclear whether DCS-coupled CET might be a useful approach in the treatment of addiction.

Reconsolidation of drug memories

Persistent, unwanted memories are believed to be key contributors to drug addiction and the chronic relapse problem over the lifetime of the addict. Contrary to the long-held idea that memories are static and fixed, new studies in the last decade have shown that memories are dynamic and changeable. However, they are changeable only under specific conditions. When a memory is retrieved (reactivated), it becomes labile for a period of minutes to hours and then is reconsolidated to maintain long-term memory. Recent findings indicate that even well-established long-term memories may be susceptible to disruption by interfering with reconsolidation through delivery of certain amnestic agents during memory retrieval. Here I review the growing literature on memory reconsolidation in animal models of addiction, including sensitization, conditioned place preference and self-administration. I also discuss (a) several issues that need to be considered in interpreting the findings from reconsolidation studies and (b) future challenges and directions for memory reconsolidation studies in the field of addiction. The findings indicate promise for using this approach as a therapy for disrupting the long-lasting memories that can trigger relapse.

Inhibiting glycine transporter-1 facilitates cocaine-cue extinction and attenuates reacquisition of cocaine-seeking behavior

BACKGROUND

Combining extinction training with cognitive-enhancing pharmacotherapy represents a novel strategy for improving the efficacy of exposure therapy for drug relapse prevention. We investigated if the selective glycine transporter-1 (GlyT-1) inhibitor RO4543338 could facilitate extinction of cocaine-conditioned responses and attenuate reacquisition of cocaine-seeking behavior.

METHODS

Rats were trained to self-administer cocaine (0.3mg/kg), which was associated with a 2-s light cue under a second-order schedule of i.v. drug injection. Rats received vehicle, 30 or 45mg/kg of RO4543338 prior to three 1-h extinction-training sessions spaced at weekly intervals. Responses were extinguished by substituting saline for cocaine while maintaining response-contingent cue presentations. Reacquisition of cocaine-seeking behavior during self-administration sessions began 1 week after the last extinction session. Control experiments were conducted under conditions that precluded explicit extinction of cocaine-conditioned responses.

RESULTS

Compared to vehicle, 30 and 45mg/kg RO4543338 significantly decreased responding early in extinction training and during subsequent reacquisition sessions. The latter effect persisted for at least five sessions. In control studies, reacquisition of cocaine-seeking behavior was not altered when RO4543338 was administered either prior to weekly self-administration control sessions or prior to weekly control sessions in which cocaine and cues were omitted and the levers retracted.

CONCLUSIONS

As the GlyT-1 inhibitor facilitated cocaine-cue extinction learning and attenuated subsequent reacquisition of cocaine-seeking behavior, this class of compounds may have utility as a pharmacological adjunct to cocaine-cue exposure therapy in addicts.

The role of metaplasticity mechanisms in regulating memory destabilization and reconsolidation

Memory allows organisms to predict future events based on prior experiences. This requires encoded information to persist once important predictors are extracted, while also being modifiable in response to changes within the environment. Memory reconsolidation may allow stored information to be modified in response to related experience. However, there are many boundary conditions beyond which reconsolidation may not occur. One interpretation of these findings is that the event triggering memory retrieval must contain new information about a familiar stimulus in order to induce reconsolidation. Presently, the mechanisms that affect the likelihood of reconsolidation occurring under these conditions are not well understood. Here we speculate on a number of systems that may play a role in protecting memory from being destabilized during retrieval. We conclude that few memories may enter a state in which they cannot be modified. Rather, metaplasticity mechanisms may serve to alter the specific reactivation cues necessary to destabilize a memory. This might imply that destabilization mechanisms can differ depending on learning conditions.