Long-term memory of cocaine-associated context: disruption and reinstatement

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.

The Role of Beta-Endorphin in Cocaine-Induced Conditioned Place Preference, Its Extinction, and Reinstatement in Male and Female Mice

Endogenous opioids have been implicated in cocaine reward. However, the role of each opioid peptide in this regard is unknown. Notably, the role of each peptide in extinction and reinstatement is not fully characterized. Thus, we assessed whether cocaine-induced conditioned place preference (CPP) and its extinction and reinstatement would be altered in the absence of beta-endorphin. We also examined if sex-related differences would exist in these processes. Male and female mice lacking beta-endorphin and their respective controls were tested for baseline place preference on day 1. On day 2, mice were treated with saline/cocaine (15 mg/kg) and confined to the vehicle- or drug-paired chamber for 30 min, respectively. In the afternoon, mice were treated with the alternate treatment and confined to the opposite chamber. Mice were then tested for CPP on day 3. Mice then received additional conditioning on this day as well as on day 4. Mice were then tested for CPP on day 5. Mice then received extinction training on day 9. On day 10, mice were tested for extinction and then reinstatement of CPP following a priming dose of cocaine (7.5 mg/kg). Male and female mice lacking beta-endorphin did not exhibit CPP following single conditioning with cocaine. On the other hand, only male mice lacking beta-endorphin failed to show CPP after repeated conditioning. Nonetheless, reinstatement of CPP was blunted in both male and female mice lacking beta-endorphin compared to controls. The present results suggest that beta-endorphin plays a functional role in cocaine-induced CPP and its reinstatement, and sex-related differences exist in the regulatory action of beta-endorphin on the acquisition but not reinstatement of cocaine CPP.

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.

Cocaine-induced Changes in the Expression of NMDA Receptor Subunits

Cocaine use disorder is manifested by repeated cycles of drug seeking and drug taking. Cocaine exposure causes synaptic transmission in the brain to exhibit persistent changes, which are poorly understood, while the pharmacotherapy of this disease has not been determined. Multiple potential mechanisms have been indicated to be involved in the etiology of co-caine use disorder. The glutamatergic system, especially N-methyl-D-aspartate (NMDA) receptors, may play a role in sever-al physiological processes (synaptic plasticity, learning and memory) and in the pathogenesis of cocaine use disorder. The composition of the NMDA receptor subunits changes after contingent and noncontingent cocaine administration and after drug abstinence in a region-specific and time-dependent manner, as well as depending on the different protocols used for co-caine administration. Changes in the expression of NMDA receptor subunits may underlie the transition from cocaine abuse to dependence, as well as the transition from cocaine dependence to cocaine withdrawal. In this paper, we summarize the cur-rent knowledge regarding neuroadaptations within NMDA receptor subunits and scaffolding proteins observed following voluntary and passive cocaine intake, as well as the effects of NMDA receptor antagonists on cocaine-induced behavioral changes during cocaine seeking and relapse.

Spermidine, a positive modulator of the NMDA receptor, facilitates extinction and prevents the reinstatement of morphine-induced conditioned place preference in mice

RATIONALE

Individuals with opioid use disorders often relapse into drug-seeking behavior after recalling memories linked to the drug use experience. Improving extinction efficacy has been used as a strategy to treat substance use disorders and suppress relapse. Although N-methyl-D-aspartate receptor (NMDAr) agonists facilitate acquisition, consolidation, and extinction, no study has addressed whether spermidine (SPD), a natural polyamine ligand of the NMDA receptor, facilitates the extinction and reinstatement of morphine-induced conditioned place preference (CPP).

OBJECTIVES AND METHODS

The aim of the present study was to investigate the effect of SPD, an NMDAr agonist, on the extinction and reinstatement of morphine-induced CPP in mice. Adult male albino Swiss mice received saline (0.9% NaCl) or morphine (5 mg/kg) intraperitoneally (i.p.) and were respectively confined to a black or a white compartment for 30 min for four consecutive days for CPP induction. SPD (10-30 mg/kg, i.p.) or ifenprodil (NMDAr antagonist, 0.1-1 mg/kg, i.p.) were injected 15 min before extinction training.

RESULTS

SPD and ifenprodil facilitated the extinction of morphine-induced CPP. SPD treatment during the extinction period impaired reinstatement induced by a priming dose of morphine (1.25 mg/kg). Ifenprodil (0.1 mg/kg) prevented the facilitatory effect of spermidine on the extinction of morphine-induced CPP but did not prevent reinstatement induced by morphine.

CONCLUSIONS

These results suggest that SPD facilitated the extinction of morphine-induced CPP by modulating the polyamine binding site of the NMDA receptor. Our findings reveal important effects of SPD and ifenprodil on the re-exposure-induced decrease in morphine-induced CPP, which may be promising for developing novel pharmacological strategies to treat opioid use disorder.

Activation of GSK3β induced by recall of cocaine reward memories is dependent on GluN2A/B NMDA receptor signaling

Glycogen synthase kinase-3β (GSK3β) is a critical regulator of the balance between long-term depression and long-term potentiation which is essential for learning and memory. Our previous study demonstrated that GSK3β activity is highly induced during cocaine memory reactivation, and that reconsolidation of cocaine reward memory is attenuated by inhibition of GSK3β. NMDA receptors and protein phosphatase 1 (PP1) are activators of GSK3β. Thus, this study investigated the roles of NMDA receptor subtypes and PP1in the reconsolidation of cocaine contextual reward memory. Cocaine contextual memories were established and evaluated using cocaine conditioned place preference methods. The regulation of GSK3β activity in specific brain areas was assessed by measuring its phosphorylation state using immunoblot assays. Mice underwent cocaine place conditioning for 8 days and were tested for place preference on day 9. Twenty-four hours later, mice were briefly confined to the compartment previous paired with cocaine to reactivate cocaine-associated memories. Administration of the GluN2A- and GluN2B-NMDA receptor antagonists, NVP-AAM077 and ifenprodil, respectively, immediately following recall abrogated an established cocaine place preference, while preventing the activation of GSK3β in the amygdala, nucleus accumbens, and hippocampus during cocaine memory reactivation. PP1 inhibition with okadaic acid also blocked the activation of GSK3β and attenuated a previously established cocaine place preference. These findings suggest that the dephosphorylation of GSK3β that occurred upon activation of cocaine-associated reward memories may be initiated by the activation of PP1 during the induction of NMDA receptor-dependent reconsolidation of cocaine mnemonic traces. Moreover, the importance of NMDA receptors and PP1 in reconsolidation of cocaine memory makes them potential therapeutic targets in treatment of cocaine use disorder and prevention of relapse.

NMDA receptor dependent changes in c-fos and p-CREB signaling following extinction and reinstatement of morphine place preference

Neural circuitry comprising the ventral tegmental area, nucleus accumbens (NAc), prefrontal cortex (PFC) and hippocampus (HIP) has a main role in reward phenomena. Previous behavioral studies indicated that intracerebroventricular administration of AP5 (NMDA glutamate receptor antagonist) and CNQX (AMPA/kainate glutamate receptor antagonist) during the extinction and before reinstatement of morphine-induced conditioned place preference (CPP) reduced the extinction period and reinstatement of morphine-CPP. Therefore, in the present study, we tried to evaluate the effect of antagonism of NMDA glutamate receptors on the p-CREB/CREB ratio and c-fos expression in the NAc, PFC and HIP during these two phases of morphine-CPP in male adult albino Wistar rats. The p-CREB/CREB ratio and c-fos levels were estimated by Western blot analysis. The results revealed that these two factors decreased by antagonism of NMDA glutamate receptors (different doses of AP5) compared to saline-control group in aforementioned regions. The reduction of molecular markers, especially the p-CREB/CREB ratio, after AP5 administration was more during the extinction period. Therefore, it can be assumed that consolidation and reconsolidation of morphine memory via intra-PFC, -NAc and -HIP NMDA glutamate receptors are in accordance with changes in p-CREB/CREB ratio and c-fos levels.

Infralimbic GluN2A-Containing NMDA Receptors Modulate Reconsolidation of Cocaine Self-Administration Memory

Addiction is characterized by high relapse susceptibility, and relapse can be triggered by drug-associated cues. Cue presentation induces retrieval of the drug-cue memory, which becomes labile and must be reconsolidated into long-term storage. Repeated unpaired cue presentation, however, promotes extinction. Cue-reactivity can be reduced by blocking reconsolidation or facilitating extinction, which are mediated by NMDA receptors (NMDArs). However, the role of NMDArs in either process following self-administration is unclear. Thus, to determine their role in extinction, rats learned to self-administer cocaine before receiving injections of the NMDAr antagonist CPP immediately after four 45-min extinction sessions. During a subsequent 90-min extinction retention test, CPP-treated rats lever pressed less than saline-treated rats indicating that NMDAr blockade facilitated extinction or disrupted drug-cue memory reconsolidation. In addition, infusing CPP into the infralimbic medial prefrontal cortex (IL-mPFC), a structure implicated in extinction, before four 45-min or immediately after four 30min extinction sessions, had similar results during the extinction retention tests. Next, the GluN2A-selective antagonist NVP or GluN2B-selective antagonist Ro25 was infused into IL-mPFC or nucleus accumbens (NAc) shell, another structure implicated in extinction, after four 45-min extinction sessions. Blocking GluN2A-, but not GluN2B-, containing NMDArs, in IL-mPFC or NAc shell reduced lever pressing during the extinction retention tests. Finally, to dissociate reconsolidation from extinction, NVP was infused into IL-mPFC after four 10-min reactivation sessions, which resulted in reduced lever pressing during the retention test. These results indicate that IL-mPFC GluN2A-containing NMDArs modulate reconsolidation, and suggest a novel treatment strategy, as reducing cue reactivity could limit relapse susceptibility.

NMDAR dependent intracellular responses associated with cocaine conditioned place preference behavior

The aim of this study was to investigate the intracellular responses associated with the acquisition and expression of cocaine-context associations. ERK (extracellular regulated kinase), CREB (cAMP responsive element binding protein), FosB and ΔFosB proteins were of particular interest due to their involvement in cocaine reward and in synaptic plasticity underlying learning and memory. We used the conditioned place preference (CPP) paradigm, which employs a Pavlovian conditioning procedure to establish an association between a drug-paired environment and the drug's rewarding effects, to study the role of these signaling pathways in cocaine-context associations. N-methyl-D-aspartate receptor (NMDAR) antagonism prior to cocaine administration during conditioning blocked the acquisition of cocaine CPP and reduced Nucleus Accumbens (NAc) phosphorylated-ERK (pERK) and phosphorylated CREB (pCREB) levels following the CPP test (drug-free). We also show that cocaine-induced increases in Caudate Putamen (CPu) FosB and ΔFosB levels are decreased after MK-801 pre-treatment during conditioning. In addition, our results provide evidence for the involvement of striatal SIRT (Silent Information Regulator of Transcription) proteins in cocaine-CPP. These results will aid in the advancement of general knowledge about the molecular formation and retrieval of cocaine-associated memories that can be used in the future when designing treatments for cocaine addiction that target both prevention and relapse.

NMDA antagonist MK 801 in nucleus accumbens core but not shell disrupts the restraint stress-induced reinstatement of extinguished cocaine-conditioned place preference in rats

Relapse is a common feature of cocaine addiction. In rodents, it can be elicited by cues, stress or the drug. Restraint stress-induced reinstatement of cocaine-conditioned place preference (CPP) is a useful model to study the mechanisms involved in stress-induced relapse of drug-seeking behavior. There is evidence that the glutamate NMDA receptors are critically involved in drug- and cue-induced reinstatement of seeking behavior and drug-CPP responses. The aim of this study was to investigate the contribution of NMDA receptors within core vs. shell nucleus accumbens (NAc) subregions to restraint stress-induced reinstatement of extinguished cocaine-CPP. After extinction of cocaine-conditioned preference, animals were administered MK 801 systemically or directly into intra-core or intra-shell, and restrained for 30min or left undisturbed in their home-cages. First, we demonstrated that restraint stress-induced reinstatement of extinguished cocaine-CPP depends on the duration of restraint as well as on the context in which it is applied. Second, this effect was blocked by systemic MK 801 administration either before or after restraint. Third, intra-core but not intra-shell administration abrogated the restraint stress-induced reinstatement. These findings show that NMDA receptors within NAc core, but not shell, play a critical role in restraint stress-induced reinstatement of cocaine-CPP.

Cocaine Self-Administration Elevates GluN2B within dmPFC Mediating Heightened Cue-Elicited Operant Responding

Cue-elicited drug-craving correlates with hyperactivity within prefrontal cortex (PFC), which is theorized to result from dysregulated excitatory neurotransmission. The NMDA glutamate receptor is highly implicated in addiction-related neuroplasticity. As NMDA receptor function is regulated critically by its GluN2 subunits, herein, we assayed the relation between incubated cue-elicited cocaine-seeking following extended access to intravenous cocaine (6 h/d; 0.25 mg/infusion for 10 d) and the expression of GluN2A/B receptor subunits within PFC sub regions during early versus late withdrawal (respectively, 3 vs. 30 days). Cocaine-seeking rats exhibited elevated GluN2B expression within the dorsomedial aspect of the PFC (dmPFC); this effect was apparent at both 3 and 30 days withdrawal and occurred in cocaine-experienced rats, regardless of experiencing an extinction test or not. Thus, elevated dmPFC GluN2B expression appears to reflect a pharmacodynamic response to excessive cocaine intake that is independent of the duration of drug withdrawal or re-exposure to drug-taking context. The functional relevance of elevated dmPFC GluN2B expression for drug-seeking was assessed by the local infusion of the prototypical GluN2B-selective antagonist ifenprodil (1.0 µg/side). Ifenprodil did not alter cue-elicited responding in animals with a history of saline self-administration. In contrast, ifenprodil lowered cue-elicited cocaine-seeking, while potentiating cue-elicited sucrose-seeking. Thus, the effects of an intra-dmPFC ifenprodil infusion upon cue reactivity are reinforcer-specific, arguing in favor of targeting GluN2B-containing NMDA receptors as a pharmacological strategy for reducing behavioral reactivity to drug-associated cues with the potential benefit of heightening the reinforcing properties of cues associated with non-drug primary rewards.

Variations in the stimulus salience of cocaine reward influences drug-associated contextual memory

Drugs of abuse act as reinforcers because they influence learning and memory processes resulting in long-term memory of drug reward. We have previously shown that mice conditioned by fixed daily dose of cocaine (Fix-C) or daily escalating doses of cocaine (Esc-C) resulted in short- and long-term persistence of drug memory, respectively, suggesting different mechanisms in acquisition of cocaine memory. The present study was undertaken to investigate the differential contribution of N-methyl-D-aspartate receptor (NMDAR) subunits in the formation of Fix-C and Esc-C memory in C57BL/6J mice. Training by Esc-C resulted in marked elevation in hippocampal expression of Grin2b mRNA and NR2B protein levels compared with training by Fix-C. The NR2B-containing NMDAR antagonist ifenprodil had similar attenuating effects on acquisition and reconsolidation of Fix-C and Esc-C memory. However, the NMDAR antagonist MK-801 had differential effects: (1) higher doses of MK-801 were required for post-retrieval disruption of reconsolidation of Esc-C memory than Fix-C memory; and (2) pre-retrieval MK-801 inhibited extinction of Fix-C memory but it had no effect on Esc-C memory. In addition, blockade of NMDAR downstream signaling pathways also showed differential regulation of Fix-C and Esc-C memory. Inhibition of neuronal nitric oxide synthase attenuated acquisition and disrupted reconsolidation of Fix-C but not Esc-C memory. In contrast, the mitogen-activating extracellular kinase inhibitor SL327 attenuated reconsolidation of Esc-C but not Fix-C memory. These results suggest that NMDAR downstream signaling molecules associated with consolidation and reconsolidation of cocaine-associated memory may vary upon changes in the salience of cocaine reward during conditioning.

Contribution of an SFK-Mediated Signaling Pathway in the Dorsal Hippocampus to Cocaine-Memory Reconsolidation in Rats

Environmentally induced relapse to cocaine seeking requires the retrieval of context-response-cocaine associative memories. These memories become labile when retrieved and must undergo reconsolidation into long-term memory storage to be maintained. Identification of the molecular underpinnings of cocaine-memory reconsolidation will likely facilitate the development of treatments that mitigate the impact of cocaine memories on relapse vulnerability. Here, we used the rat extinction-reinstatement procedure to test the hypothesis that the Src family of tyrosine kinases (SFK) in the dorsal hippocampus (DH) critically controls contextual cocaine-memory reconsolidation. To this end, we evaluated the effects of bilateral intra-DH microinfusions of the SFK inhibitor, PP2 (62.5 ng per 0.5 μl per hemisphere), following re-exposure to a cocaine-associated (cocaine-memory reactivation) or an unpaired context (no memory reactivation) on subsequent drug context-induced instrumental cocaine-seeking behavior. We also assessed alterations in the phosphorylation state of SFK targets, including GluN2A and GluN2B N-methyl-D-aspartate (NMDA) and GluA2 α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunits at the putative time of memory restabilization and following PP2 treatment. Finally, we evaluated the effects of intra-DH PEAQX (2.5 μg per 0.5 μl per hemisphere), a GluN2A-subunit-selective NMDAR antagonist, following, or in the absence of, cocaine-memory reactivation on subsequent drug context-induced cocaine-seeking behavior. GluN2A phosphorylation increased in the DH during putative memory restabilization, and intra-DH PP2 treatment inhibited this effect. Furthermore, PP2-as well as PEAQX-attenuated subsequent drug context-induced cocaine-seeking behavior, in a memory reactivation-dependent manner, relative to VEH. These findings suggest that hippocampal SFKs contribute to the long-term stability of cocaine-related memories that underlie contextual stimulus control over cocaine-seeking behavior.

NMDA and dopamine D1 receptors within NAc-shell regulate IEG proteins expression in reward circuit during cocaine memory reconsolidation

Reactivation of consolidated memory initiates a memory reconsolidation process, during which the reactivated memory is susceptible to strengthening, weakening or updating. Therefore, effective interference with the memory reconsolidation process is expected to be an important treatment for drug addiction. The nucleus accumbens (NAc) has been well recognized as a pathway component that can prevent drug relapse, although the mechanism underlying this function is poorly understood. We aimed to clarify the regulatory role of the NAc in the cocaine memory reconsolidation process, by examining the effect of applying different pharmacological interventions to the NAc on Zif 268 and Fos B expression in the entire reward circuit after cocaine memory reactivation. Through the cocaine-induced conditioned place preference (CPP) model, immunohistochemical and immunofluorescence staining for Zif 268 and Fos B were used to explore the functional activated brain nuclei after cocaine memory reactivation. Our results showed that the expression of Zif 268 and Fos B was commonly increased in the medial prefrontal cortex (mPFC), the infralimbic cortex (IL), the NAc-core, the NAc-shell, the hippocampus (CA1, CA2, and CA3 subregions), the amygdala, the ventral tegmental area (VTA), and the supramammillary nucleus (SuM) following memory reconsolidation, and Zif 268/Fos B co-expression was commonly observed (for Zif 268: 51-68%; for Fos B: 52-66%). Further, bilateral NAc-shell infusion of MK 801 and SCH 23390, but not raclopride or propranolol, prior to addictive memory reconsolidation, decreased Zif 268 and Fos B expression in the entire reward circuit, except for the amygdala, and effectively disturbed subsequent CPP-related behavior. In summary, N-methyl-d-aspartate (NMDA) and dopamine D1 receptors, but not dopamine D2 or β adrenergic receptors, within the NAc-shell, may regulate Zif 268 and Fos B expression in most brain nuclei of the reward circuit after cocaine memory reactivation. These findings indicated that the NAc played a key role in regulating addictive memory reconsolidation by influencing the function of the entire addictive memory network.

Nicotine-seeking reinstatement is reduced by inhibition of instrumental memory reconsolidation

The reinforcing properties of nicotine play a major role in instrumental conditioning to nicotine taking in smokers. Retrieval of nicotine-related memories may promote relapse to nicotine seeking after prolonged abstinence. Once consolidated, memories are stable, but they return to a labile phase, called reconsolidation, after their retrieval. The aim of our study was to investigate whether it was possible to interfere with the reconsolidation of instrumental nicotine-related memories by acting at glutamatergic receptors [N-methyl-D-aspartate receptors (NMDARs)] to prevent relapse to nicotine-seeking behaviour in the rat. We assessed whether the NMDAR antagonist MK-801, administered before or after nicotine-related instrumental memory retrieval, can reduce reinstatement of nicotine-seeking behaviour in rats previously trained to nicotine self-administration. Following a period of forced abstinence, MK-801 (0.1 mg/kg intraperitoneally) was administered 30 min before or 1 h after the re-exposure to 20 lever presses without any contingency in the training context to retrieve instrumental memory. MK-801 administered after, but not before, retrieval inhibited reinstatement compared with vehicle controls and groups without retrieval of instrumental memory. Interestingly, a retrieval factor effect was observed as an increase of reinstatement in vehicle-treated groups, suggesting a behavioural outcome of the occurrence of instrumental memory reconsolidation. Our findings suggest that, by acting on NMDARs, it is possible to reduce the reinstatement of nicotine-seeking behaviour through inhibition of instrumental nicotine-related memory reconsolidation.

Single dose propranolol does not affect physiologic or emotional reactivity to smoking cues

BACKGROUND

Smoking cue exposure reactivates salient smoking-related memories, triggering craving to smoke, a phenomenon associated with maintenance of smoking behavior and relapse after periods of abstinence. Acute β-adrenergic blockade with propranolol reduces physiologic reactivity during subsequent recollection of traumatic events by inhibiting reconsolidation of reactivated memories in a process called memory reconsolidation blockade.

OBJECTIVE

The objective of this study is to determine whether a single dose of propranolol prior to retrieval of smoking-related memories reduces subsequent physiologic reactivity to personally salient smoking imagery scripts in current smokers.

METHODS

Fifty-four overnight-abstinent, adult smokers received a single-dose propranolol or placebo prior to reactivation of smoking-related memories in a randomized, double-blind, placebo-controlled trial and resumed smoking afterward. One week later, skin conductance (SC), heart rate (HR), left corrugator electromyogram (EMG), self-reported emotional state, and craving were assessed following script-driven imagery with neutral and personalized smoking-related scripts.

RESULTS

Smoking scripts were associated with increased physiologic activation (SC, HR, EMG), craving, and negative emotional state compared with neutral scripts. Propranolol did not moderate the effect of script type on any outcome.

CONCLUSION

Personalized smoking script-driven imagery robustly increased physiologic activation, negative emotional state, and craving, and a single dose of propranolol prior to memory reactivation did not moderate this effect.

Glucocorticoid receptor antagonism disrupts the reconsolidation of social reward-related memories in rats

Reconsolidation is the process whereby consolidated memories are destabilized upon retrieval and restabilized to persist for later use. Although the neurobiology of the reconsolidation of both appetitive and aversive memories has been intensively investigated, reconsolidation of memories of physiologically relevant social rewards has received little attention. Social play, the most characteristic social behaviour displayed by young mammals, is highly rewarding, illustrated by the fact that it can induce conditioned place preference (CPP). Here, we investigated the role of signalling mechanisms implicated in memory processes, including reconsolidation, namely glucocorticoid, mineralocorticoid, NMDA glutamatergic and CB1 cannabinoid receptors, in the reconsolidation of social play-induced CPP in rats. Systemic treatment with the glucocorticoid receptor antagonist mifepristone before, but not immediately after, retrieval disrupted the reconsolidation of social play-induced CPP. Mifepristone did not affect social play-induced CPP in the absence of memory retrieval. Treatment with the NMDA receptor antagonist MK-801 modestly affected the reconsolidation of social play-induced CPP. However, the reconsolidation of social play-induced CPP was not affected by treatment with the mineralocorticoid and CB1 cannabinoid receptor antagonists spironolactone and rimonabant, respectively. We conclude that glucocorticoid neurotransmission mediates the reconsolidation of social reward-related memories in rats. These data indicate that the neural mechanisms of the reconsolidation of social reward-related memories only partially overlap with those underlying the reconsolidation of other reward-related memories.

Involvement of insular muscarinic cholinergic receptors in morphine-induced conditioned place preference in rats

RATIONALE

Drug addiction represents a pathological usurpation of neural processes involved in learning and memory. Retrieval of drug-related memories can result in drug craving and relapse. Recently, the insula was identified as part of the neuronal circuit responsible for the processing of drug memory; however, its precise role remains unclear.

OBJECTIVE

To investigate the involvement of insular muscarinic acetylcholine receptors (mAChRs) in the processing of drug memory.

METHOD

The morphine-induced conditioned place preference (CPP) was used to assess drug memory. All rats were first trained with morphine to establish the CPP. Sub-groups of these rats were used for contextual cue-induced CPP reinstatement. Other sub-groups of rats underwent extinction of the CPP, and 5 m/kg morphine was used for priming-induced CPP reinstatement. Microinjection of mAChR antagonists or agonists into the insula was performed prior to the CPP tests in order to evaluate their effect on CPP expression.

RESULTS

Insular microinjections of the nonselective mAChR antagonist, scopolamine, and the M₁ antagonist, pirenzepine, significantly inhibited CPP expression in both contextual cue- and priming-induced CPP reinstatement; the M₁ agonist, MCN-A-343, and the M₄ antagonist, tropicamide, enhanced CPP expression. The M₄ agonist, LY2033298, inhibited CPP expression. The M₂ antagonist, methoctramine, and M₃ antagonist, 4-DAMP, had no effect on CPP expression.

CONCLUSION

Our results demonstrate that insular mAChRs play a role in the processing of drug memory. M₁ and M₄ mAChRs work paradoxically; M₁ activation and M₄ inhibition attenuate the expression of drug memory, while M₁ inhibition and M₄ activation augment the expression of drug memory.

Activation of exchange protein activated by cAMP in the rat basolateral amygdala impairs reconsolidation of a memory associated with self-administered cocaine

The intracellular mechanisms underlying memory reconsolidation critically involve cAMP signaling. These events were originally attributed to PKA activation by cAMP, but the identification of Exchange Protein Activated by cAMP (Epac), as a distinct mediator of cAMP signaling, suggests that cAMP-regulated processes that subserve memory reconsolidation are more complex. Here we investigated how activation of Epac with 8-pCPT-cAMP (8-CPT) impacts reconsolidation of a memory that had been associated with cocaine self-administration. Rats were trained to lever press for cocaine on an FR-1 schedule, in which each cocaine delivery was paired with a tone+light cue. Lever pressing was then extinguished in the absence of cue presentations and cocaine delivery. Following the last day of extinction, rats were put in a novel context, in which the conditioned cue was presented to reactivate the cocaine-associated memory. Immediate bilateral infusions of 8-CPT into the basolateral amygdala (BLA) following reactivation disrupted subsequent cue-induced reinstatement in a dose-dependent manner, and modestly reduced responding for conditioned reinforcement. When 8-CPT infusions were delayed for 3 hours after the cue reactivation session or were given after a cue extinction session, no effect on cue-induced reinstatement was observed. Co-administration of 8-CPT and the PKA activator 6-Bnz-cAMP (10 nmol/side) rescued memory reconsolidation while 6-Bnz alone had no effect, suggesting an antagonizing interaction between the two cAMP signaling substrates. Taken together, these studies suggest that activation of Epac represents a parallel cAMP-dependent pathway that can inhibit reconsolidation of cocaine-cue memories and reduce the ability of the cue to produce reinstatement of cocaine-seeking behavior.

The strength of aversive and appetitive associations and maladaptive behaviors

Certain maladaptive behaviors are thought to be acquired through classical Pavlovian conditioning. Exaggerated fear response, which can develop through Pavlovian conditioning, is associated with acquired anxiety disorders such as post-traumatic stress disorders (PTSDs). Inflated reward-seeking behavior, which develops through Pavlovian conditioning, underlies some types of addictive behavior (e.g., addiction to drugs, food, and gambling). These maladaptive behaviors are dependent on associative learning and the development of long-term memory (LTM). In animal models, an aversive reinforcer (fear conditioning) encodes an aversive contextual and cued LTM. On the other hand, an appetitive reinforcer results in conditioned place preference (CPP) that encodes an appetitive contextual LTM. The literature on weak and strong associative learning pertaining to the development of aversive and appetitive LTM is relatively scarce; thus, this review is particularly focused on the strength of associative learning. The strength of associative learning is dependent on the valence of the reinforcer and the salience of the conditioned stimulus that ultimately sways the strength of the memory trace. Our studies suggest that labile (weak) aversive and appetitive LTM may share similar signaling pathways, whereas stable (strong) aversive and appetitive LTM is mediated through different pathways. In addition, we provide some evidence suggesting that extinction of aversive fear memory and appetitive drug memory is likely to be mediated through different signaling molecules. We put forward the importance of studies aimed to investigate the molecular mechanisms underlying the development of weak and strong memories (aversive and appetitive), which would ultimately help in the development of targeted pharmacotherapies for the management of maladaptive behaviors that arise from classical Pavlovian conditioning.

Differential effects of propranolol on conditioned hyperactivity and locomotor sensitization induced by morphine in rats

According to memory reconsolidation theory, when long-term memory is reactivated by relevant clues, the memory traces become labile, which can be altered by pharmacological manipulations. Accumulating evidence reveals that memory related to drug abuse can be erased by disrupting reconsolidation process. We used an animal model that could simultaneously measure conditioned hyperactivity and locomotor sensitization induced by morphine. β-adrenoceptor antagonist propranolol or saline were administered following conditioned stimuli (CS) or a small dose of morphine reactivation. The results showed that the conditioned hyperactivity could be disrupted by propranolol treatment following CS reactivation. However, the expression of locomotor sensitization could not be disrupted by propranolol administration following CS or morphine reactivation. Furthermore, morphine injection and propranolol intervention enhanced the locomotor sensitization effect. These data suggest that blocking the reconsolidation process can disrupt the conditioned hyperactivity induced by environmental cues associated with morphine treatment, but not morphine-induced locomotor sensitization.

A bout of voluntary running enhances context conditioned fear, its extinction, and its reconsolidation

Three experiments used rats to examine the effect of a single bout of voluntary activity (wheel running) on the acquisition, extinction, and reconsolidation of context conditioned fear. In Experiment 1, rats provided with access to a wheel for 3 h immediately before or after a shocked exposure to a context froze more when tested in that context than rats provided with access to the wheels 6 h after the shocked exposure or rats not provided with access to the wheels. In Experiment 2, rats provided with access to the wheels immediately before or after a nonshocked exposure to the conditioned context froze less when tested in that context than rats provided with access to the wheels 6 h after the nonshocked exposure or rats not provided with access to the wheels. In Experiment 3, rats provided with access to wheels immediately after an extended nonshocked exposure to the conditioned context again froze less, whereas rats provided with access to the wheels after a brief nonshocked exposure froze more on the subsequent test than sedentary controls. These results show that a single bout of running can enhance acquisition, extinction, and reconsolidation of context conditioned fear.

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.

On the persistence of cocaine-induced place preferences and aversions in rats

RATIONALE

Rats develop preferences for places associated with the immediate rewarding effects of cocaine and aversions for places paired with the drug's delayed negative effects. The motivation to seek cocaine should therefore depend upon the relative magnitude of these two opposing effects of the drug.

OBJECTIVE

The current study tested this notion by assessing the relative persistence of the positive and negative associations formed between environmental cues and the immediate or delayed effects of cocaine.

METHODS

Rats were administered 1.0 mg/kg intravenous cocaine and placed into a distinctive environment either immediately or 15-min after injection, alternating daily with pairings of a second environment with saline. After four drug-place and four saline-place pairings, rats were returned to their home cages for 1, 7, or 21 days after which a 15-min place preference test was conducted. In a second experiment, the effectiveness of a single reconditioning session (one drug-place and one saline-place pairing) to reactivate learned cocaine-place associations was assessed after 1 or 3 weeks of drug abstinence.

RESULTS

Places associated with the immediate effects of cocaine were preferred (CPP), while places associated with the delayed effects of cocaine were avoided (CPA). The persistence of these effects differed with CPP remaining viable at 3 weeks of withdrawal, while CPA was no longer present after 1 week. Reconditioning with an additional cocaine-place pairing failed to reinstate the CPA.

CONCLUSIONS

Cue-induced "relapse" of cocaine-seeking behavior may be fueled in part by an increased persistence of positive relative to negative associations with drug-paired stimuli.

Inhibition of glycine transporter-1 reduces cue-induced nicotine-seeking, but does not promote extinction of conditioned nicotine cue responding in the rat

Pharmacological stimulation of N-methyl-D-aspartate receptors (NMDAr) could enhance the outcome of cue-exposure therapy for smoking cessation. NMDAr stimulation can be achieved by increasing pharmacologically the synaptic levels of glycine, a necessary co-agonist. Here, we evaluate the effects of SSR504734, a selective inhibitor of glycine type I transporter (GlyT1) in an extinction-reinstatement procedure inducing robust and lasting nicotine-seeking behavior in rats. Male Wistar rats were trained to associate discriminative stimuli (S(D)s) with the availability of nicotine (0.03 mg/kg/65 μL/2 second/infusion) or sucrose (45-mg pellet) versus non-reward in two-lever operant cages. Reinforced response was followed by cue signaling 20-second time-out (CSs). Once the training criterion was met, rats underwent extinction of lever presses, in the absence of reinforcers, S(D) s and CSs. Re-exposure to nicotine or sucrose S(D+)/CS(+), but not non-reward S(D-)/CS(-), revived responding at the previously reinforced lever. Acute pre-treatment with SSR504734 (10 mg/kg i.p.) reduced nicotine-seeking but not sucrose-seeking behavior without influencing rats' locomotor activity. Sub-chronic treatment (10 mg/kg i.p. for 5 days) during daily exposure to S(D+)/CS(+) reduced nicotine-seeking; however, this effect was transient, with return to S(D+)/CS(+) responding at 72 hours. Full recovery to S(D+)/CS(+) responding was observed after 1 month suggesting that SSR504734 sub-acute treatment did not engage the long-term plasticity mechanisms probably involved in nicotine-seeking. In conclusion, GlyT1-inhibitors might offer a therapeutic opportunity for acute cue-controlled nicotine-seeking, but the lack of persistent effects of the sub-chronic treatment associated with nicotine cues exposure suggests that short-term administration of GlyT1-inhibitor SSR504734 is not sufficient to promote extinction of nicotine-cue conditioned responding.

D-cycloserine, sarcosine and D-serine diminish the expression of cocaine-induced conditioned place preference

Reactivation of cocaine-associated memories plays a critical role in reinstating the cocaine-seeking behavior and causing relapse. Cocaine-induced conditioned place preference (CPP) was used as a behavioral paradigm indicative of cocaine-associated memory and repeated cocaine-free preference tests served as a behavioral procedure to retrieve such a memory in this study. Since D-cycloserine was reported to eradicate drug-associated memories, two other N-methyl-D-aspartate (NMDA) receptor agonists were assessed for their efficacy on facilitating the extinction of cocaine-induced CPP. Although D-cycloserine (30 mg/kg) abolished cocaine (10 mg/kg)-induced CPP, sarcosine (300 and 600 mg/kg) and D-serine (600 mg/kg) diminished the expression of such a cocaine memory. Sarcosine (600 mg/kg) and D-serine (600 mg/kg) did not affect the storage of this cocaine memory. It was of interest to note that D-cycloserine facilitated the extinction of cocaine-induced CPP in a fast and early-onset manner, while sarcosine and D-serine decreased cocaine-induced CPP expression in a delay-onset manner. D-cycloserine (30 mg/kg), D-serine (600 mg/kg) and sarcosine (600 mg/kg) did not affect the consolidation of cocaine (5 mg/kg)-induced CPP. Finally, sarcosine (at 600 mg/kg/day for 3 consecutive days) and D-serine (at 600 mg/kg/day for 3 consecutive days) did not produce observable aversive effect associated with their administration in a conditioned place aversion paradigm. Likewise, a similar dosing regimen of sarcosine or D-serine did not cause evident activity-impairing effect. In addition to D-cycloserine treatment, our results indicate that long-term treatment with D-serine and sarcosine may afford a therapeutic advance in suppressing the expression of cocaine-associated memory.

Sodium butyrate-induced histone acetylation strengthens the expression of cocaine-associated contextual memory

The conditioned place preference (CPP) paradigm entails Pavlovian conditioning and allows evaluating the acquisition and extinction of drug-associated memory. Epigenetic regulation of chromatin structure by acetylation and deacetylation of histone proteins is critical for formation of long-term memory (LTM). We have recently shown that a single administration of the histone deacetylase (HDAC) inhibitor sodium butyrate (NaB) facilitated extinction of fear-associated memory in mice. Using the CPP paradigm, the present study investigated the effect of NaB on cocaine-associated memory. C57B/6 mice were conditioned by either fixed daily doses of cocaine (5mg/kg×4 and 15mg/kg×4days) or an escalating schedule (3, 6, 12 and 24mg/kg). Acute administration of NaB (1.2g/kg) prior to conditioning by fixed doses of cocaine increased the expression and impaired the extinction of place preference compared to control subjects. Subjects that were conditioned by 15mg/kg×4 cocaine and received a single injection of NaB following the first or the second CPP test showed impaired extinction compared to control mice that received saline instead of NaB. Subjects that were conditioned by escalating schedule of cocaine and subsequently received repeated injections of NaB during daily reexposure to nonreinforced context showed either enhancement or no effect on place preference. Acute administration of NaB (1.2g/kg) to naïve mice resulted in marked increase in acetylation of histone H3 lysine 14 (H3K14) and histone H4 lysine 8 (H4K8) in hippocampus but not amygdala. Results suggest that regardless of the scheduling of either cocaine or NaB administration, NaB-induced histone hyperacetylation in the hippocampus may strengthen cocaine-associated contextual memory.

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.

Common influences of non-competitive NMDA receptor antagonists on the consolidation and reconsolidation of cocaine-cue memory

RATIONALE

Environmental stimuli or contexts previously associated with rewarding drugs contribute importantly to relapse among addicts, and research has focused on neurobiological processes maintaining those memories. Much research shows contributions of cell surface receptors and intracellular signaling pathways in maintaining associations between rewarding drugs (e.g., cocaine) and concurrent cues/contexts; these memories can be degraded at the time of their retrieval through reconsolidation interference. Much less studied is the consolidation of drug-cue memories during their acquisition.

OBJECTIVE

The present experiments use the cocaine-conditioned place preference (CPP) paradigm in rats to directly compare, in a consistent setting, the effects of N-methyl-D-aspartate (NMDA) glutamate receptor antagonists MK-801 and memantine on the consolidation and reconsolidation of cocaine-cue memories.

METHODS

For the consolidation studies, animals were systemically administered MK-801 or memantine immediately following training sessions. To investigate the effects of these NMDA receptor antagonists on the retention of previously established cocaine-cue memories, animals were systemically administered MK-801 or memantine immediately after memory retrieval.

RESULTS

Animals given either NMDA receptor antagonist immediately following training sessions did not establish a preference for the cocaine-paired compartment. Post-retrieval administration of either NMDA receptor antagonist attenuated the animals' preference for the cocaine-paired compartment. Furthermore, animals given NMDA receptor antagonists post-retrieval showed a blunted response to cocaine-primed reinstatement.

CONCLUSIONS

Using two distinct NMDA receptor antagonists in a common setting, these findings demonstrate that NMDA receptor-dependent processes contribute both to the consolidation and reconsolidation of cocaine-cue memories, and they point to the potential utility of treatments that interfere with drug-cue memory reconsolidation.

Appetitive Pavlovian goal-tracking memories reconsolidate only under specific conditions

Despite extensive evidence that appetitive memories undergo reconsolidation, two notable failures to observe reconsolidation have been reported: instrumental responding and goal-tracking. However, these studies do not provide conclusive evidence for a lack of memory reconsolidation due to the numerous boundary conditions that dictate whether a memory will undergo reconsolidation. In this study we sought to reexamine reconsolidation in an appetitive, Pavlovian conditioned approach procedure and the behavioral boundary conditions within which memories are destabilized and reconsolidated. This study demonstrated that a Pavlovian goal-tracking memory, previously thought to be resistant to destabilization, will undergo memory reconsolidation under discrete conditions that favor reconsolidation as opposed to extinction, and that this is dependent on the amount of training rats received. With restricted training, systemic administration of MK-801 impaired memory extinction. In contrast, with more extended training, MK-801 administration impaired memory reconsolidation. We also demonstrate that behavioral boundary conditions that exist for appetitive memory reconsolidation are much more complex than simple parametric calculations. Moreover, extinction per se is not a boundary on reconsolidation, in that MK-801 also has no behavioral effect under some conditions.

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.

Pharmacologically-mediated reactivation and reconsolidation blockade of the psychostimulant-abuse circuit: a novel treatment strategy

Psychostimulant abuse continues to present legal, socioeconomic and medical challenges as a primary psychiatric disorder, and represents a significant comorbid factor in major psychiatric and medical illnesses. To date, monotherapeutic drug treatments have not proven effective in promoting long-term abstinence in psychostimulant abusers. In contrast to clinical trials utilizing monotherapies, combinations of dopamine (DA) agonists and selective 5-HT(3), 5HT(2A/2C), or NK(1) antagonists have shown robust efficacy in reversing behavioral and neurobiological alterations in animal models of psychostimulant abuse. One important temporal requirement for these treatments is that the 5-HT or NK(1) receptor antagonist be given at a critical time window after DA agonist administration. This requirement may reflect a necessary dosing regimen towards normalizing underlying dysfunctional neural circuits and "addiction memory" states. Indeed, chronic psychostimulant abuse can be conceptualized as a consolidated form of dysfunctional memory maintained by repeated drug- or cue-induced reactivation of neural circuit and subsequent reconsolidation. According to this concept, the DA agonist given first may reactivate this memory circuit, thereby rendering it transiently labile. The subsequent antagonist is hypothesized to disrupt reconsolidation necessary for restabilization, thus leading progressively to a therapeutically-mediated abolishment of dysfunctional synaptic plasticity. We propose that long-term abstinence in psychostimulant abusers may be achieved not only by targeting putative mechanistic pathways, but also by optimizing drug treatment regimens designed to disrupt the neural processes underlying the addicted state.

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.

Inhibition of β-adrenergic receptors induces a persistent deficit in retrieval of a cocaine-associated memory providing protection against reinstatement

Drug-seeking behavior is maintained by encounters with drug-associated cues. Preventing retrieval of drug-associated memories that these cues provoke would therefore limit relapse susceptibility; however, little is known regarding the mechanisms of retrieval. Here, we show that β-adrenergic receptor activation is necessary for the retrieval of a cocaine-associated memory. Using a conditioned place preference (CPP) procedure, rats were conditioned to associate one chamber, but not another, with cocaine. When administered before a CPP trial, propranolol, but not saline, prevented retrieval of a cocaine-associated CPP. In subsequent drug-free trials, rats previously treated with propranolol continued to show a retrieval deficit, as no CPP was evident. This retrieval deficit was long lasting and robust, as the CPP did not re-emerge during a test for spontaneous recovery 14 days later or reinstate following a priming injection of cocaine. Moreover, the peripheral β-adrenergic receptor antagonist sotalol did not affect retrieval. Thus, retrieval of cocaine-associated memories is mediated by norepinephrine acting at central β-adrenergic receptors. Our findings support the use of propranolol, a commonly prescribed β-blocker, as an adjunct to exposure therapy for the treatment of addiction by preventing retrieval of drug-associated memories during and long after treatment, and by providing protection against relapse.

Glutamatergic targets for enhancing extinction learning in drug addiction

The persistence of the motivational salience of drug-related environmental cues and contexts is one of the most problematic obstacles to successful treatment of drug addiction. Behavioral approaches to extinguishing the salience of drug-associated cues, such as cue exposure therapy, have generally produced disappointing results which have been attributed to, among other things, the context specificity of extinction and inadequate consolidation of extinction learning. Extinction of any behavior or conditioned response is a process of new and active learning, and increasing evidence suggests that glutamatergic neurotransmission, a key component of the neural plasticity that underlies normal learning and memory, is also involved in extinction learning. This review will summarize findings from both animal and human studies that suggest that pharmacological enhancement of glutamatergic neurotransmission facilitates extinction learning in the context of drug addiction. Pharmacological agents that have shown potential efficacy include NMDA partial agonists, mGluR5 receptor positive allosteric modulators, inhibitors of the GlyT1 glycine transporter, AMPA receptor potentiators, and activators of the cystine-glutamate exchanger. These classes of cognition-enhancing compounds could potentially serve as novel pharmacological adjuncts to cue exposure therapy to increase success rates in attenuating cue-induced drug craving and relapse.

The activation of NMDA receptor-ERK pathway in the central amygdala is required for the expression of morphine-conditioned place preference in the rat

Reinforcing effects of addictive drugs can be evaluated with the conditioned place preference (CPP) test which involves both the action of drugs and environmental cues. However, the encoded neural circuits and underlying signaling mechanism are not fully understood. In this study, we have used morphine-CPP model in the rat and characterized the role of N-methyl-D: -aspartate (NMDA) receptor and the phosphorylation of extracellular signal-regulated kinase (ERK) in the central nuclei of amygdala (CeA) in the expression of morphine-induced CPP. We have found that morphine repeated pairing treatment causes a significant preference for compartment paired with morphine after 1 day or 7 days post-training, which is associated with increased ERK1/2 phosphorylation (p-ERK1/2, a measure of ERK activity) in the CeA. More than 80% of the positive p-ERK1/2 neurons express NMDA receptor subunit NR1 by double immunofluorescence studies. The infusion of either MEK inhibitor U0126 or NMDA receptor antagonist MK-801 in the CeA not only suppresses the activation of ERK1/2 in the CeA but also abolishes the expression of CPP. These results suggest that the activation of the NMDA receptor-ERK signaling pathway in the CeA is required for the expression of morphine-induced place preference in the rat.

Aberrant learning and memory in addiction

Over the past several years, drug addiction has increasingly been accepted to be a disease of the brain as opposed to simply being due to a lack of willpower or personality flaw. Exposure to addictive substances has been shown to create enduring changes in brain structure and function that are thought to underlie the transition to addiction. Specific genetic and environmental vulnerability factors also influence the impact of drugs of abuse on the brain and can enhance the likelihood of becoming an addict. Long-lasting alterations in brain function have been found in neural circuits that are known to be responsible for normal appetitive learning and memory processes and it has been hypothesized that drugs of abuse enhance positive learning and memory about the drug while inhibiting learning about the negative consequences of drug use. Therefore, the addict's behavior becomes increasingly directed towards obtaining and using drugs of abuse, while at the same time developing a poorer ability to stop using, even when the drug is less rewarding or interferes with functioning in other facets of life. In this review we will discuss the clinical evidence that addicted individuals have altered learning and memory and describe the possible neural substrates of this dysfunction. In addition, we will explore the pre-clinical evidence that drugs of abuse cause a progressive disorder of learning and memory, review the molecular and neurobiological changes that may underlie this disorder, determine the genetic and environmental factors that may increase vulnerability to addiction, and suggest potential strategies for treating addiction through manipulations of learning and memory.

Cognitive enhancers for facilitating drug cue extinction: insights from animal models

Given the success of cue exposure (extinction) therapy combined with a cognitive enhancer for reducing anxiety, it is anticipated that this approach will prove more efficacious than exposure therapy alone in preventing relapse in individuals with substance use disorders. Several factors may undermine the efficacy of exposure therapy for substance use disorders, but we suspect that neurocognitive impairments associated with chronic drug use are an important contributing factor. Numerous insights on these issues are gained from research using animal models of addiction. In this review, the relationship between brain sites whose learning, memory and executive functions are impaired by chronic drug use and brain sites that are important for effective drug cue extinction learning is explored first. This is followed by an overview of animal research showing improved treatment outcome for drug addiction (e.g. alcohol, amphetamine, cocaine, heroin) when explicit extinction training is conducted in combination with acute dosing of a cognitive-enhancing drug. The mechanism by which cognitive enhancers are thought to exert their benefits is by facilitating consolidation of drug cue extinction memory after activation of glutamatergic receptors. Based on the encouraging work in animals, factors that may be important for the treatment of drug addiction are considered.

Pharmacological enhancement of drug cue extinction learning: translational challenges

Augmentation of cue exposure (extinction) therapy with cognitive-enhancing pharmacotherapy may constitute a rational strategy for the clinical management of drug relapse. While certain success has been reported for this form of therapy in anxiety disorders, in this paper we highlight several obstacles that may undermine the efficacy of exposure therapy for substance use disorders. We also review translational studies that have evaluated the facilitative effects of the cognitive enhancer D-cycloserine on extinction targeting drug-related cues. Finally, important considerations for the design and implementation of future studies evaluating exposure therapy combined with pharmacotherapy for substance use disorders are discussed.

Glutamate receptors in extinction and extinction-based therapies for psychiatric illness

Some psychiatric illnesses involve a learned component. For example, in posttraumatic stress disorder, memories triggered by trauma-associated cues trigger fear and anxiety, and in addiction, drug-associated cues elicit drug craving and withdrawal. Clinical interventions to reduce the impact of conditioned cues in eliciting these maladaptive conditioned responses are likely to be beneficial. Extinction is a method of lessening conditioned responses and involves repeated exposures to a cue in the absence of the event it once predicted. We believe that an improved understanding of the behavioral and neurobiological mechanisms of extinction will allow extinction-like procedures in the clinic to become more effective. Research on the role of glutamate-the major excitatory neurotransmitter in the mammalian brain-in extinction has led to the development of pharmacotherapeutics to enhance the efficacy of extinction-based protocols in clinical populations. In this review, we describe what has been learned about glutamate actions at its three major receptor types (N-methyl-D-aspartate (NMDA) receptors, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, and metabotropic glutamate receptors) in the extinction of conditioned fear, drug craving, and withdrawal. We then discuss how these findings have been applied in clinical research.

Disruption of Long-Term Alcohol-Related Memory Reconsolidation: Role of β-Adrenoceptors and NMDA Receptors

Disrupting reconsolidation of drug-related memories may be effective in reducing the incidence of relapse. In the current study we examine whether alcohol-related memories are prone to disruption by the β-adrenergic receptor antagonist propranolol (10 mg/kg) and the NMDA receptor antagonist MK801 (0.1 mg/kg) following their reactivation. In operant chambers, male Wistar rats were trained to self-administer a 12% alcohol solution. After 3 weeks of abstinence, the animals were placed in the self-administration cages and were re-exposed to the alcohol-associated cues for a 20-min retrieval period, immediately followed by a systemic injection of either propranolol, MK801 or saline. Rats were tested for cue-induced alcohol seeking on the following day. Retrieval session, injection and test were repeated on two further occasions at weekly intervals. Both propranolol and MK801 administration upon reactivation did not reduce alcohol seeking after the first reactivation test. However, a significant reduction of alcohol seeking was observed over three post-training tests in propranolol treated animals, and MK801 treated animals showed a strong tendency toward reduced alcohol seeking (p = 0.06). Our data indicate that reconsolidation of alcohol-related memories can be disrupted after a long post-training interval and that particularly β-adrenergic receptors may represent novel targets for pharmacotherapy of alcoholism, in combination with cue-exposure therapies.

The psychological and neurochemical mechanisms of drug memory reconsolidation: implications for the treatment of addiction

Memory reconsolidation is the process by which memories, destabilised at retrieval, require restabilisation to persist in the brain. It has been demonstrated that even old, well-established memories require reconsolidation following retrieval; therefore, memory reconsolidation could potentially be exploited to disrupt, or even erase, aberrant memories that underlie psychiatric disorders, thereby providing a novel therapeutic target. Drug addiction is one such disorder; it is both chronic and relapsing, and one prominent risk factor for a relapse episode is the presentation of environmental cues that have previously been associated with drugs of abuse. This 'cue-induced relapse' can be accounted for in psychological terms by reinforcing memories of the pavlovian association between the cue and the drug, which can thus influence behaviour through at least three psychologically and neurobiologically dissociable mechanisms: conditioned reinforcement, conditioned approach and conditioned motivation. As each of these psychological processes could contribute to the resumption of drug-seeking following abstinence, it is important to develop treatments that can reduce drug-seeking re-established via influences on each or all of these pavlovian processes, in order to minimise the risk of a subsequent relapse. Investigation of the memory reconsolidation mechanisms of the memories underlying conditioned reinforcement, conditioned approach and conditioned motivation indicate that they depend upon different neurochemical systems, including the glutamatergic and adrenergic systems within limbic corticostriatal circuitry. We also discuss here the subsequent translation to the clinic of this preclinical work.