D-cycloserine facilitates extinction the first time but not the second time: an examination of the role of NMDA across the course of repeated extinction sessions

Neuropharmacological Modulation of N-methyl-D-aspartate, Noradrenaline and Endocannabinoid Receptors in Fear Extinction Learning: Synaptic Transmission and Plasticity

Learning to recognize and respond to potential threats is crucial for survival. Pavlovian threat conditioning represents a key paradigm for investigating the neurobiological mechanisms of fear learning. In this review, we address the role of specific neuropharmacological adjuvants that act on neurochemical synaptic transmission, as well as on brain plasticity processes implicated in fear memory. We focus on novel neuropharmacological manipulations targeting glutamatergic, noradrenergic, and endocannabinoid systems, and address how the modulation of these neurobiological systems affects fear extinction learning in humans. We show that the administration of N-methyl-D-aspartate (NMDA) agonists and modulation of the endocannabinoid system by fatty acid amide hydrolase (FAAH) inhibition can boost extinction learning through the stabilization and regulation of the receptor concentration. On the other hand, elevated noradrenaline levels dynamically modulate fear learning, hindering long-term extinction processes. These pharmacological interventions could provide novel targeted treatments and prevention strategies for fear-based and anxiety-related disorders.

Presynaptic GABAB receptor inhibition sex dependently enhances fear extinction and attenuates fear renewal

Anxiety and trauma-related disorders are highly prevalent worldwide, and are associated with altered associative fear learning. Despite the effectiveness of exposure therapy, which aims to reduce associative fear responses, relapse rates remain high. This is due, in part, to the context specificity of exposure therapy, which is a form of extinction. Many studies show that fear relapses when mice are tested outside the extinction context, and this is known as fear renewal. Using Pavlovian fear conditioning and extinction, we can study the mechanisms underlying extinction and renewal. The aim of the current experiment was to identify the role of presynaptic GABAB receptors in these two processes. Previous work from our lab showed that genetic deletion or pharmacological inhibition of GABAB(1a) receptors that provide presynaptic inhibition on glutamatergic terminals reduces context specificity and leads to generalization. We therefore hypothesized that inactivation of these presynaptic GABAB receptors could be used to reduce the context specificity associated with fear extinction training and suppress renewal when mice are tested outside of the extinction context. Using CGP 36216, an antagonist specific for presynaptic GABAB receptors, we blocked presynaptic GABAB receptors using intracerebroventricular injections during various time points of extinction learning in male and female mice. Results showed that blocking these receptors pre- and post-extinction training led to enhanced extinction learning in male mice only. We also found that post-extinction infusions of CGP reduced renewal rates in male mice when they were tested outside of the extinction context. In an attempt to localize the function of presynaptic GABAB receptors within regions of the extinction circuit, we infused CGP locally within the basolateral amygdala or dorsal hippocampus. We failed to reduce renewal when CGP was infused directly within these regions, suggesting that presynaptic inhibition within these regions per se may not be necessary for driving context specificity during extinction learning. Together, these results show an important sex-dependent role of presynaptic GABAB receptors in extinction and renewal processes and identify a novel receptor target that may be used to design pharmacotherapies to enhance the effectiveness of exposure therapy.

D-cycloserine augmentation of cognitive behavioral therapy for delusions: A randomized clinical trial

OBJECTIVE

D-cycloserine (DCS) promotes consolidation of extinction learning. This study extends earlier work by examining whether DCS can enhance cognitive behavioral therapy (CBT) for delusions.

METHODS

Adults reporting moderate or greater delusions were randomly assigned to receive 50 mg of DCS or placebo prior to 10 weekly CBT sessions. The primary outcome was change in severity of delusions measured with the Psychotic Symptom Rating Scale delusion subscale (PSYRATS-D). Secondary outcomes included persistence of response at 3 and 6 month follow-up and the effects of DCS on memory consolidation and cognitive flexibility. Fifty-eight participants were randomized and 44 completed the trial.

RESULTS

The DCS and placebo groups did not differ in change from baseline to end of CBT on PSYRATS-D, nor did DCS improve memory consolidation or cognitive flexibility compared to placebo. However, at the 3 month follow-up visit (week 24), 47% of participants who completed treatment with DCS reported a 20% or greater decrease on PSYRATS-D compared to 15% in the placebo group (p = .04). Change in distress across CBT sessions interacted with treatment group to predict change from baseline to week 24 in PSYRATS-D total score (p = .03) such that response at week 24 was greatest in DCS-treated participants who experienced a decrease in distress during CBT sessions.

CONCLUSIONS

DCS augmentation of CBT did not improve delusions compared to placebo during treatment; however, DCS was associated with a higher response rate at 3-month follow-up. DCS may produce a delayed therapeutic effect, associated with successful CBT sessions, but this finding requires replication.

d-Cycloserine and estradiol enhance fear extinction in nulliparous but not primiparous female rats

Female reproductive experience has been shown to alter the hormonal, neurobiological and behavioural features of fear extinction, which is the laboratory basis of exposure therapy. This raises uncertainties as to whether pharmacological agents that enhance fear extinction in reproductively inexperienced females are equally effective in reproductively experienced females. The aim of the current study was therefore to compare the effects of two pharmacological enhancers of fear extinction, d-cycloserine (DCS) and estradiol, between nulliparous (virgin) and primiparous (reproductively experienced) female rats. In Experiment 1, nulliparous and primiparous females received systemic administration of either DCS or saline immediately after extinction training, and were tested for extinction recall the following day. DCS enhanced extinction recall in nulliparous females that showed low levels of freezing at the end of extinction training, but not among those that showed high levels of freezing at the end of extinction training. DCS did not enhance fear extinction in primiparous females, regardless of their level of freezing at the end of extinction training. In Experiment 2, nulliparous and primiparous female rats received systemic administration of either estradiol or vehicle prior to extinction training. Estradiol enhanced extinction recall among nulliparous females, but not primiparous females. Increasing the dose of estradiol administered prior to extinction training did not alter the outcomes in primiparous females (Experiment 3). Together, these findings suggest that reproductive status may be an important individual difference factor associated with the response to pharmacological modulators of extinction in rats. The implications of these findings for the pharmacological augmentation of exposure therapy in clinical populations are discussed.

Vagus nerve stimulation reverses the extinction impairments in a model of PTSD with prolonged and repeated trauma

We have shown that vagus nerve stimulation (VNS) enhances extinction of conditioned fear and reduces anxiety in rat models of PTSD using moderate stress. However, it is still unclear if VNS can be effective in enhancing extinction of severe fear after prolonged and repeated trauma. Severe fear was induced in adult male rats by combining single prolonged stress (SPS) and protracted aversive conditioning (PAC). After SPS and PAC procedures, rats were implanted with stimulating cuff electrodes, exposed to five days of extinction training with or without VNS, and then tested for extinction retention, return of fear in a new context and reinstatement. The elevated plus maze, open field and startle were used to test anxiety. Sham rats showed no reduction of fear during extensive extinction training. VNS-paired with extinction training reduced freezing at the last extinction session by 70% compared to sham rats. VNS rats exhibited half as much fear as shams, as well as less fear renewal. Sham rats exhibited significantly more anxiety than naive controls, whereas VNS rats did not. These results demonstrate that VNS enhances extinction and reduces anxiety in a severe model of PTSD that combined SPS and a conditioning procedure that is 30 times more intense than the conditioning procedures in previous VNS studies. The broad utility of VNS in enhancing extinction learning in rats and the strong clinical safety record of VNS suggest that VNS holds promise as an adjuvant to exposure-based therapy in people with PTSD and other complex forms of this condition.

On the transition from reconsolidation to extinction of contextual fear memories

Retrieval of an associative memory can lead to different phenomena. Brief reexposure sessions tend to trigger reconsolidation, whereas more extended ones trigger extinction. In appetitive and fear cued Pavlovian memories, an intermediate "null point" period has been observed where neither process seems to be engaged. Here we investigated whether this phenomenon extends to contextual fear memory. Adult rats were subjected to a contextual fear conditioning paradigm, reexposed to the context 2 d later for 3, 5, 10, 20, or 30 min, with immediate injections of MK-801 or saline following reexposure, and tested on the following day. We observed a significant effect of MK-801 with the 3- and 30-min sessions, impairing reconsolidation and extinction, respectively. However, it did not have significant effects with 5-, 10-, or 20-min sessions, even though freezing decreased from reexposure to test. Further analyses indicated that this is not likely to be due to a variable transition point at the population level. In conclusion, the results show that in contextual fear memories there is a genuine "null point" between the parameters that induce reconsolidation and extinction, as defined by the effects of MK-801, although NMDA receptor-independent decreases in freezing can still occur in these conditions.

Breaking boundaries: optimizing reconsolidation-based interventions for strong and old memories

Recent research has demonstrated that consolidated memories can enter a temporary labile state after reactivation, requiring restabilization in order to persist. This process, known as reconsolidation, potentially allows for the modification and disruption of memory. Much interest in reconsolidation stems from the possibility that maladaptive memory traces—a core feature of several psychiatric conditions—could be tackled by disrupting their reconsolidation. However, research has indicated a range of supposed boundary conditions on the induction of reconsolidation. Stronger memories, often resulting from exposure to stressful conditions, or older memories, appear to be relatively resistant to undergoing reconsolidation. This may be taken as a potential stumbling block for reconsolidation-based interventions: in clinical practice, old and strong maladaptive memories are the norm rather than the exception. Yet, boundary conditions have been derived from limited experimental evidence, are not unique to reconsolidation-based interventions, and do not seem to be absolute. In this paper, we review a range of experimental studies that have aimed to disrupt old memories, or memories that were strengthened by stress manipulations, through reconsolidation. Such research highlights several techniques that could be used to optimize reconsolidation-based approaches and overcome putative boundary conditions. We supplement this review of experimental literature with a case study of a reconsolidation-based treatment of a strong and decades-old phobia for mice, further suggesting that age and strength of memory may not be insurmountable barriers. Translating findings from basic science, to human experiments, to clinical applications and back again, can potentially unlock powerful new treatments for the many people who suffer daily from anxiety disorders.

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.

D-cycloserine in Schizophrenia: New Strategies for Improving Clinical Outcomes by Enhancing Plasticity

Background

Dysregulation of N-methyl D-aspartate (NMDA) receptor signaling is strongly implicated in schizophrenia. Based on the ketamine model of NMDA receptor hypoactivity, therapeutic approaches designed to maintain a sustained increase in agonist activity at the glycine site of the NMDA receptor have produced promising, although inconsistent, efficacy for negative symptoms.

Methods

A review of the published literature on D-cycloserine (DCS) pharmacology in animal models and in clinical studies was performed. Findings relevant to DCS effects on memory and plasticity and their potential clinical application to schizophrenia were summarized.

Results

Studies in animals and clinical trials in patients with anxiety disorders have demonstrated that single or intermittent dosing with DCS enhances memory consolidation. Preliminary trials in patients with schizophrenia suggest that intermittent dosing with DCS may produce persistent improvement of negative symptoms and enhance learning when combined with cognitive behavioral therapy for delusions or with cognitive remediation. The pharmacology of DCS is complex, since it acts as a “super agonist” at NMDA receptors containing GluN2C subunits and, under certain conditions, it may act as an antagonist at NMDA receptors containing GluN2B subunits.

Conclusions

There are preliminary findings that support a role for D-cycloserine in schizophrenia as a strategy to enhance neuroplasticity and memory. However, additional studies with DCS are needed to confirm these findings. In addition, clinical trials with positive and negative allosteric modulators with greater specificity for NMDA receptor subtypes are needed to identify the optimal strategy for enhancing neuroplasticity in schizophrenia.

The Therapeutic Role of d-Cycloserine in Schizophrenia

The ketamine model for schizophrenia has led to several therapeutic strategies for enhancing N-methyl d-aspartate (NMDA) receptor activity, including agonists directed at the glycine receptor site and inhibitors of glycine reuptake. Because ketamine may primarily block NMDA receptors on inhibitory interneurons, drugs that reduce glutamate release have also been investigated as a means of countering a deficit in inhibitory input. These approaches have met with some success for the treatment of negative and positive symptoms, but results have not been consistent. An emerging approach with the NMDA partial agonist, d-cycloserine (DCS), aims to enhance plasticity by intermittent treatment. Early trials have demonstrated benefit with intermittent DCS dosing for negative symptoms and memory. When combined with cognitive remediation, intermittent DCS treatment enhanced learning on a practiced auditory discrimination task and when added to cognitive behavioral therapy, DCS improved delusional severity in subjects who received DCS with the first CBT session. These studies require replication, but point toward a promising strategy for the treatment of schizophrenia and other disorders of plasticity.

D-Cycloserine in Neuropsychiatric Diseases: A Systematic Review

D-Cycloserine, known from tuberculosis therapy, has been widely introduced to neuropsychiatric studies, since its central active mechanism as a partial NMDA-agonist has been found. In this review, we evaluate its therapeutic potential in neuropsychological disorders and discuss its pitfalls in terms of dosing and application frequency as well as its safety in low-dose therapy. Therefore, we identified 91 clinical trials by performing a Medline search. We demonstrate in part preliminary but increasing evidence that D-cycloserine may be effective in various psychiatric diseases, including schizophrenia, anxiety disorders, addiction, eating disorders, major depression, and autism as well as in neurological diseases, including dementia, Alzheimer's disease, and spinocerebellar degeneration. D-Cycloserine in low-dose therapy is safe, but there is still a need for new drugs with higher specificity to the different N-methyl-D-aspartate-receptor subunits.

Drug development in schizophrenia: are glutamatergic targets still worth aiming at?

PURPOSE OF REVIEW

The ketamine model has dominated drug discovery in schizophrenia over the past decade, supported by genetic and postmortem evidence implicating glutamatergic transmission. This review assesses recent successes and disappointments of glutamatergic agents and identifies promising new directions.

RECENT FINDINGS

Strategies focused on enhancing activity of the N-methyl D-aspartate (NMDA) receptor via direct agonists at the glycine site or by inhibition of glycine reuptake have produced modest and often inconsistent evidence of efficacy, as have approaches to reduce excessive glutamate release by lamotrigine or by mGluR2/3 agonists. Strategies targeting α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors have also met with only limited success. Newer approaches include selective allosteric modulation of NMDA receptor subunits and of mGluR5 receptors. In addition, intracellular pathways downstream of NMDA receptors may also provide new treatment targets, as exemplified by phosphodiesterase (PDE) inhibitors.

SUMMARY

Targeting glutamatergic transmission remains one of the most promising strategies in schizophrenia, particularly early in the course of illness, but therapeutic approaches may require greater specificity for receptor subtype type, illness phase, and individual biology in order to enhance efficacy and overcome problems with reproducibility of clinical results.

Pharmacology of cognitive enhancers for exposure-based therapy of fear, anxiety and trauma-related disorders

Pathological fear and anxiety are highly debilitating and, despite considerable advances in psychotherapy and pharmacotherapy they remain insufficiently treated in many patients with PTSD, phobias, panic and other anxiety disorders. Increasing preclinical and clinical evidence indicates that pharmacological treatments including cognitive enhancers, when given as adjuncts to psychotherapeutic approaches [cognitive behavioral therapy including extinction-based exposure therapy] enhance treatment efficacy, while using anxiolytics such as benzodiazepines as adjuncts can undermine long-term treatment success. The purpose of this review is to outline the literature showing how pharmacological interventions targeting neurotransmitter systems including serotonin, dopamine, noradrenaline, histamine, glutamate, GABA, cannabinoids, neuropeptides (oxytocin, neuropeptides Y and S, opioids) and other targets (neurotrophins BDNF and FGF2, glucocorticoids, L-type-calcium channels, epigenetic modifications) as well as their downstream signaling pathways, can augment fear extinction and strengthen extinction memory persistently in preclinical models. Particularly promising approaches are discussed in regard to their effects on specific aspects of fear extinction namely, acquisition, consolidation and retrieval, including long-term protection from return of fear (relapse) phenomena like spontaneous recovery, reinstatement and renewal of fear. We also highlight the promising translational value of the preclinial research and the clinical potential of targeting certain neurochemical systems with, for example d-cycloserine, yohimbine, cortisol, and L-DOPA. The current body of research reveals important new insights into the neurobiology and neurochemistry of fear extinction and holds significant promise for pharmacologically-augmented psychotherapy as an improved approach to treat trauma and anxiety-related disorders in a more efficient and persistent way promoting enhanced symptom remission and recovery.

Bidirectional effects of inhibiting or potentiating NMDA receptors on extinction after cocaine self-administration in rats

RATIONALE

Extinction of drug seeking is facilitated by NMDA receptor (NMDAr) agonists, but it remains unclear whether extinction is dependent on NMDAr activity.

OBJECTIVES

We investigated the necessity of NMDArs for extinction of cocaine seeking and whether extinction altered NMDAr expression within extinction-related neuroanatomical loci.

METHODS

Rats were trained to lever press for i.v. infusions of cocaine or sucrose reinforcement prior to extinction training or withdrawal.

RESULTS

Administration of the NMDAr competitive antagonist CPP prior to four brief extinction sessions impaired subsequent extinction retention. In contrast, administration of the NMDAr coagonist D-serine after four brief extinction sessions attenuated lever pressing during subsequent extinction, indicative of facilitated consolidation of extinction. Furthermore, expression of the NMDAr subunits, GluN2A and GluN2B, was not altered in the ventromedial prefrontal cortex. However, both GluN2A and GluN2B subunit expression in the nucleus accumbens increased following cocaine self-administration, and this increased expression was relatively resistant to modulation by extinction.

CONCLUSIONS

Our findings demonstrate that extinction of cocaine seeking is bidirectionally mediated by NMDArs and suggest that selective modulation of NMDAr activity could facilitate extinction-based therapies for treatment of cocaine abuse.

D-serine enhances fear extinction by increasing GluA2-containing AMPA receptor endocytosis

Activation of the N-methyl-D-aspartate receptor (NMDAR) glycine site has been shown to enhance memory extinction in physiological and pathological conditions. In the current study, we examined the effects of D-serine, an endogenous NMDAR glycine site agonist, on fear extinction and α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) endocytosis in the hippocampus during the process of fear extinction. In inhibitory avoidance task, systemic administration of D-serine (800 mg/kg, i.p.) significantly accelerated memory extinction. The Western blot analyses showed that the acceleration of memory extinction was accompanied by an increase in postsynaptic AMPAR endocytosis in the hippocampus. Furthermore, the application of a synthetic peptide Tat-GluA23Y (3.0 μmol/kg, i.p.) that interferes with the endocytosis of AMPARs succeeded in preventing the enhancement of fear extinction and AMPAR endocytosis induced by D-serine. These results suggest that d-serine might enhance fear extinction through increasing GluA2-containing AMPA receptor endocytosis, and that d-serine may be a potential therapeutic agent against learning and memory disorders.

d-Cycloserine augmentation of cognitive remediation in schizophrenia

d-Cycloserine (DCS) has been shown to enhance memory and, in a previous trial, once-weekly DCS improved negative symptoms in schizophrenia subjects. We hypothesized that DCS combined with a cognitive remediation (CR) program would improve memory of a practiced auditory discrimination task and that gains would generalize to performance on unpracticed cognitive tasks. Stable, medicated adult schizophrenia outpatients participated in the Brain Fitness CR program 3-5 times per week for 8weeks. Subjects were randomly assigned to once-weekly adjunctive treatment with DCS (50mg) or placebo administered before the first session each week. Primary outcomes were performance on an auditory discrimination task, the MATRICS cognitive battery composite score and the Scale for the Assessment of Negative Symptoms (SANS) total score. 36 subjects received study drug and 32 completed the trial (average number of CR sessions=26.1). Performance on the practiced auditory discrimination task significantly improved in the DCS group compared to the placebo group. DCS was also associated with significantly greater negative symptom improvement for subjects symptomatic at baseline (SANS score ≥20). However, improvement on the MATRICS battery was observed only in the placebo group. Considered with previous results, these findings suggest that DCS augments CR and alleviates negative symptoms in schizophrenia patients. However, further work is needed to evaluate whether CR gains achieved with DCS can generalize to other unpracticed cognitive tasks.

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.

A window of vulnerability: impaired fear extinction in adolescence

There have been significant advances made towards understanding the processes mediating extinction of learned fear. However, despite being of clear theoretical and clinical significance, very few studies have examined fear extinction in adolescence, which is often described as a developmental window of vulnerability to psychological disorders. This paper reviews the relatively small body of research examining fear extinction in adolescence. A prominent finding of this work is that adolescents, both humans and rodents, exhibit a marked impairment in extinction relative to both younger (e.g., juvenile) and older (e.g., adult) groups. We then review some potential mechanisms that could produce the striking extinction deficit observed in adolescence. For example, one neurobiological candidate mechanism for impaired extinction in adolescence involves changes in the functional connectivity within the fear extinction circuit, particularly between prefrontal cortical regions and the amygdala. In addition, we review research on emotion regulation and attention processes that suggests that developmental changes in attention bias to threatening cues may be a cognitive mechanism that mediates age-related differences in extinction learning. We also examine how a differential reaction to chronic stress in adolescence impacts upon extinction retention during adolescence as well as in later life. Finally, we consider the findings of several studies illustrating promising approaches that overcome the typically-observed extinction impairments in adolescent rodents and that could be translated to human adolescents.

The effects of intra-hippocampal microinfusion of D-cycloserine on fear extinction, and the expression of NMDA receptor subunit NR2B and neurogenesis in the hippocampus in rats

Pharmacological and behavior interventions for inhibiting fear and anxiety are important in the treatment of different types of anxiety disorder. Fear extinction, as a novel form of associative learning, is the most extensively studied models to understand the neural mechanisms of fear-related and anxiety disorders. One of the possible mechanisms of neural plasticity in extinction learning may depend on activation of NMDA receptors in the amygdale; however, the role played by the hippocampus in extinction remains largely unclear. In the present study, using a fear conditioning paradigm, we repeatedly microinfused D-cycloserine, a partial agonist of NMDA receptor, into the hippocampus and investigated the effects of repeated infusions of DCS on extinction behavior and protein levels of NMDA receptor subunit NR2B. We also examined the effects of DCS on neurogenesis in adult rat hippocampus. Our results showed that the administration of DCS facilitated the acquisition and retrieval of extinction memory, and enhanced the expression of NR2B protein in the dentate gyrus, CA1 and CA3 of the hippocampus. We also found that repeated microinfusions of DCS increased proliferation of newly born cells in the hippocampus. These findings suggest that neural plasticity mediated by NMDA receptors in the hippocampus is involved in the enhancement of acquisition and retrieval of extinction memory.

How to erase memory traces of pain and fear

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Currently emerging concepts of maladaptive pain and fear suggest that they share basic neuronal circuits and cellular mechanisms of memory formation.

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Recent studies have revealed processes of erasing memory traces of pain and fear that may be promising targets for future therapies.

Pain and fear are both aversive experiences that strongly impact on behaviour and well being. They are considered protective when they lead to meaningful, adaptive behaviour such as the avoidance of situations that are potentially dangerous to the integrity of tissue (pain) or the individual (fear). Pain and fear may, however, become maladaptive if expressed under inappropriate conditions or at excessive intensities for extended durations. Currently emerging concepts of maladaptive pain and fear suggest that basic neuronal mechanisms of memory formation are relevant for the development of pathological forms of pain and fear. Thus, the processes of erasing memory traces of pain and fear may constitute promising targets for future therapies.

Effects of D-cycloserine on cue-induced craving and cigarette smoking among concurrent cocaine- and nicotine-dependent volunteers

Rates of cigarette smoking are 3- to 4-fold greater among those with cocaine-dependence, and compared to non-users, cocaine users are at greater risk of incurring smoking-related negative health effects and death. The current study examined D-cycloserine's (0 or 50mg once weekly) effects on 1) extinction of cue-induced craving for cigarettes, 2) cigarette smoking in conjunction with cognitive-behavioral therapy, and 3) safety and tolerability in cocaine-dependent smokers. This was a double-blind, placebo-controlled, between groups, outpatient study. Participants (N=29) were concurrent cocaine- and nicotine-dependent volunteers seeking treatment for their cigarette smoking. Study visits were 3 times per week for 4 consecutive weeks. At each visit, participants received cognitive-behavioral therapy for smoking, were exposed to smoking cues. A subset of participants (N=22) returned for 6-month follow-up visits. While craving decreased, no significant effects of D-cycloserine treatment were observed. Likewise, significant decreases in smoking were observed at study days 6 (p<0.002) and 12 (p<0.0001) relative to baseline, although no participants achieved complete abstinence. However, there was no effect of D-cycloserine on cigarette smoking during treatment or at 6-mos follow-up. The treatment was safe and tolerable, with nearly 90% of treatment sessions attended based on an intent-to-treat analysis. While no effects of D-cycloserine on craving or smoking were observed in the current study, the results do suggest that smoking treatment is well accepted and may be effective for cocaine-dependent individuals.

From resilience to vulnerability: mechanistic insights into the effects of stress on transitions in critical period plasticity

While early experiences are proposed to be important for the emergence of anxiety and other mental health problems, there is little empirical research examining the impact of such experiences on the development of emotional learning. Of the research that has been performed in this area, however, a complex picture has emerged in which the maturation of emotion circuits is influenced by the early experiences of the animal. For example, under typical laboratory rearing conditions infant rats rapidly forget learned fear associations (infantile amnesia) and express a form of extinction learning which is relapse-resistant (i.e., extinction in infant rats may be due to fear erasure). In contrast, adult rats exhibit very long-lasting memories of past learned fear associations, and express a form of extinction learning that is relapse-prone (i.e., the fear returns in a number of situations). However, when rats are reared under stressful conditions then they exhibit adult-like fear retention and extinction behaviors at an earlier stage of development (i.e., good retention of learned fear and relapse-prone extinction learning). In other words, under typical rearing conditions infant rats appear to be protected from exhibiting anxiety whereas after adverse rearing fear learning appears to make those infants more vulnerable to the later development of anxiety. While the effects of different experiences on infant rats' fear retention and extinction are becoming better documented, the mechanisms which mediate the early transition seen following stress remain unclear. Here we suggest that rearing stress may lead to an early maturation of the molecular and cellular signals shown to be involved in the closure of critical period plasticity in sensory modalities (e.g., maturation of GABAergic neurons, development of perineuronal nets), and speculate that these signals could be manipulated in adulthood to reopen infant forms of emotional learning (i.e., those that favor resilience).

Pharmacological treatment of anxiety disorders: current treatments and future directions

Modern pharmacological treatments for anxiety disorders are safer and more tolerable than they were 30 years ago. Unfortunately, treatment efficacy and duration have not improved in most cases despite a greater understanding of the pathophysiology of anxiety. Moreover, innovative treatments have not reached the market despite billions of research dollars invested in drug development. In reviewing the literature on current treatments, we argue that evidence-based practice would benefit from better research on the causes of incomplete treatment response as well as the comparative efficacy of drug combinations and sequencing. We also survey two broad approaches to the development of innovative anxiety treatments:the continued development of drugs based on specific neuroreceptors and the pharmacological manipulation of fear-related memory. We highlight directions for future research, as neither of these approaches is ready for routine clinical use.

D-cycloserine: an evolving role in learning and neuroplasticity in schizophrenia

As a partial agonist at the glycine site of the NMDA receptor, D-cycloserine (DCS) has been viewed as lacking potency to fully test the NMDA receptor hypofunction theory of schizophrenia. However, findings of full agonist activity at a subset of NMDA receptors that may have particular relevance to schizophrenia, plus a growing body of evidence demonstrating enhancement of learning and neuroplasticity in animal models, suggest novel therapeutic strategies with DCS in schizophrenia. Preliminary studies with once-weekly administration have supported this potential new role for DCS in schizophrenia by demonstrating benefit for negative symptoms, memory consolidation, and facilitation of cognitive behavioral therapy for delusions.

Neural and cellular mechanisms of fear and extinction memory formation

Over the course of natural history, countless animal species have evolved adaptive behavioral systems to cope with dangerous situations and promote survival. Emotional memories are central to these defense systems because they are rapidly acquired and prepare organisms for future threat. Unfortunately, the persistence and intrusion of memories of fearful experiences are quite common and can lead to pathogenic conditions, such as anxiety and phobias. Over the course of the last 30 years, neuroscientists and psychologists alike have attempted to understand the mechanisms by which the brain encodes and maintains these aversive memories. Of equal interest, though, is the neurobiology of extinction memory formation as this may shape current therapeutic techniques. Here we review the extant literature on the neurobiology of fear and extinction memory formation, with a strong focus on the cellular and molecular mechanisms underlying these processes.

Cue exposure and response prevention with heavy smokers: a laboratory-based randomised placebo-controlled trial examining the effects of D-cycloserine on cue reactivity and attentional bias

RATIONALE

Treatments based on exposure/response prevention (Exp/RP) produce only modest benefits in substance dependence disorders. However, a new strategy, which has shown promise in animal models of addiction involves combining Exp/RP with extinction-enhancing pharmacological treatments. A prototype of the latter is D-cycloserine (DCS), a partial agonist at the glycine site of the NMDA receptor.

METHODS

In a laboratory-based randomised, double-blind, placebo-controlled trial with non-treatment-seeking heavy smokers (n = 32), we examined the efficacy of Exp/RP combined with DCS (125 mg). Two sessions of Exp/RP were carried out during which cue reactivity was monitored. Effects on attentional bias and/or subjective craving and smoking behaviour were also evaluated after at least 48 h and 2 weeks following session 2 of Exp/RP.

RESULTS

Within- and between-session reductions in cue reactivity were observed in both treatment groups, although the DCS group did not show an enhanced reduction by the end of session 2. However, a subtle effect of DCS on the emotionality subscale of the Tobacco Craving Questionnaire was observed, with a trend towards a sustained reduction in this aspect of craving at 2-week follow-up.

CONCLUSION

Our findings suggest that two sessions of Exp/RP combined with DCS does not enhance the reduction in episodic cue reactivity in non-treatment seeking smokers. A trend towards a greater sustained reduction in the emotionality scale of the TCQ in the DCS group suggests that further detailed study of the effects of combined Exp/RP-DCS on different aspects of craving is warranted, especially in smokers with a current intention to quit.

Pharmacological enhancement of fear reduction: preclinical models

Anxiety disorders have a high prevalence, and despite the substantial advances in the psychological treatment of anxiety, relapse is still a common problem. One approach to improving existing psychological treatments for anxiety has been to develop pharmacological agents that can be used to enhance the processes underlying exposure therapy, which is the most commonly used and empirically validated psychological treatment for anxiety during which individuals are taught to appropriately inhibit fear. Animal models of exposure therapy, particularly fear extinction, have proved to be a very useful way of examining the neural and molecular correlates of fear inhibition, which has in turn led to the identification of numerous drugs that enhance these processes in rats. Several of these drugs have subsequently been tested as novel pharmacological adjuncts to exposure therapy in humans with a range of anxiety disorders. The purpose of this review is to outline the key animal models of exposure therapy and to describe how these have been used to develop potential pharmacological adjuncts for anxiety disorders. Drugs that are currently in clinical use, as well as those currently in the preclinical stages of investigation, are described.

NMDA receptors and fear extinction: implications for cognitive behavioral therapy

Based primarily on studies that employ Pavlovian fear conditioning, extinction of conditioned fear has been found to be mediated by N-methyl-D-aspartate (NMDA) receptors in the amygdala and medial prefrontal cortex. This led to the discovery that an NMDA partial agonist, D-cycloserine, could facilitate fear extinction when given systemically or locally into the amygdala. Because many forms of cognitive behavioral therapy depend on fear extinction, this led to the successful use of D-cycloserine as an adjunct to psychotherapy in patients with so-called simple phobias (fear of heights), social phobia, obsessive-compulsive behavior, and panic disorder. Data in support of these conclusions are reviewed, along with some of the possible limitations of D-cycloserine as an adjunct to psychotherapy.

Acute, but not chronic, exposure to d-cycloserine facilitates extinction and modulates spontaneous recovery of a conditioned taste aversion

D-cycloserine, the glutamate N-methyl-D-aspartate receptor partial agonist, has been reported to facilitate the extinction of learned fears acquired in both naturalistic and laboratory settings. The current study extended this literature by evaluating the ability of either chronic or acute administrations of DCS to modulate the extinction and spontaneous recovery of a conditioned taste aversion (CTA). Twenty-three hour fluid-deprived Sprague-Dawley rats acquired a strong CTA following 3 pairings of a conditioned stimulus (CS; 0.3% oral saccharin)+unconditioned stimulus [US; 81 mg/kg (i.p.) lithium chloride (LiCl)]. In separate groups of rats, we then employed 2 different extinction paradigms: (1) CS-only (CSO-EXT) in which saccharin was presented every-other day, or (2) Explicitly Unpaired (EU-EXT) in which both saccharin and LiCl were presented but on alternate days. Previous studies have indicated that the EU-EXT procedure speeds up the extinction process. Further, spontaneous recovery of a CTA emerges following CSO-EXT but the EU-EXT paradigm causes a suppression of spontaneous recovery. DCS (15 mg/kg, i.p.) was administered immediately after daily liquid presentations (saccharin or water, alternate days) during the extinction period. In an acute drug manipulation, DCS (15 mg/kg, i.p.) or saline control injections were administered for 4 days only. This was done during one of 3 different phases of extinction [i.e., static (2-5%), early dynamic (8-16%), or middle dynamic (20-40%) saccharin reacceptance]. Other animals assigned to the chronic DCS condition received daily DCS (15 mg/kg, i.p.) throughout extinction. Changes in saccharin drinking in these animals were compared to the data from rats that received no drug (saline controls). Once rats met our criterion for asymptotic extinction (90% reacceptance of the CS) they entered a 30-day latency period during which they received water for 1 h/day. The day after the completion of the latency period, a final opportunity to drink saccharin was provided (spontaneous recovery test). Saline-treated control rats that went through the EU-EXT procedure achieved asymptotic extinction more quickly than did the CSO-EXT rats and did not exhibit a spontaneous recovery of the CTA. Chronic DCS treatments did not significantly reduce the time to achieve asymptotic CTA extinction in rats exposed to either CSO or EU extinction methods. Further, animals treated with DCS throughout EU-EXT exhibited a spontaneous recovery of the CTA whereas the saline-treated, EU-EXT rats did not. Thus, chronic DCS treatment did not shorten the time to extinguish a CTA and this treatment eliminated the ability of EU-EXT to block spontaneous recovery of the CTA. Acute DCS treatments were more effective in reducing the time required to extinguish a CTA than were chronic drug treatments. Moreover, the timing of these acute DCS treatments affected spontaneous recovery of the CTA depending on the extinction method employed. Acute DCS administrations later in extinction were more effective in reducing spontaneous recovery than were early administrations if the rats went through the CSO-EXT procedure. However, late-in-extinction administrations of DCS facilitated spontaneous recovery of the CTA in rats that experienced the EU-EXT method. These data agree with other findings suggesting that DCS treatments are more effective when administered a limited number of times. Our data extend these findings to the CTA paradigm and further suggest that, depending on the extinction paradigm employed, acute exposure to DCS can speed up CTA extinction and reduce spontaneous recovery of the aversion. The timing of the acute DCS treatment during extinction is generally less important than its duration in predicting the rate of CTA extinction. However, the timing of acute DCS treatments during extinction and the method of extinction employed can interact to affect spontaneous recovery of a CTA.

D-cycloserine facilitation of cognitive behavioral therapy for delusions in schizophrenia

Glutamatergic N-methyl-D-aspartate (NMDA) receptor hypofunction has been proposed as a mechanism underlying psychosis. D-cycloserine, a partial agonist at the glycine site of the NMDA receptor, enhances learning in animal models, although tachyphylaxis develops with repeated dosing. Once-weekly dosing of D-cycloserine produces persistent improvement when combined with cognitive behavioral therapy (CBT) in anxiety disorders. Delusional beliefs can be conceptualized as a learning deficit, characterized by the failure to use contradictory evidence to modify the belief. CBT techniques have been developed with modest success to facilitate such reality-testing (or new learning) in delusional beliefs. The current study evaluated whether D-cycloserine could potentiate beneficial effects of CBT on delusional severity. Twenty-one outpatients with schizophrenia or schizoaffective disorder and moderately severe delusions were randomized in a double-blind cross-over design to receive a single-dose of either D-cycloserine 50mg or placebo in a counterbalanced order on two consecutive weeks 1h prior to a CBT intervention involving training in the generation of alternative beliefs. Assessments were completed at baseline, 7 days following the first study drug administration and 7 days following the second study drug administration. Contrary to prediction, there was no significant d-cycloserine treatment effect on delusional distress or severity as measured by the SAPS or PSYRATS. An unexpected finding was an order effect, whereby subjects who received D-cycloserine first had significantly reduced delusional severity, distress, and belief conviction on PSYRATS compared to subjects who received placebo first. However, this finding is consistent with animal models in which D-cycloserine enhances learning only when accompanying the first exposure to training.

Fear conditioning, synaptic plasticity and the amygdala: implications for posttraumatic stress disorder

Posttraumatic stress disorder (PTSD) is an anxiety disorder that can develop after a traumatic experience such as domestic violence, natural disasters or combat-related trauma. The cost of such disorders on society and the individual can be tremendous. In this article, we review how the neural circuitry implicated in PTSD in humans is related to the neural circuitry of fear. We then discuss how fear conditioning is a suitable model for studying the molecular mechanisms of the fear components that underlie PTSD, and the biology of fear conditioning with a particular focus on the brain-derived neurotrophic factor (BDNF)-tyrosine kinase B (TrkB), GABAergic and glutamatergic ligand-receptor systems. We then summarize how such approaches might help to inform our understanding of PTSD and other stress-related disorders and provide insight to new pharmacological avenues of treatment of PTSD.

Characterization of NMDAR-Independent Learning in the Hippocampus

It is currently thought that memory formation requires the activation of NMDA receptors (NMDARs) in the hippocampus. However, recent studies indicate that these receptors are not necessary for all forms of learning. The current experiments examine this issue using context fear conditioning in mice. First, we show that context fear can be acquired without NMDAR activation in previously trained animals. Mice trained in one environment (context A) are subsequently able to learn about a second environment (context B) in the presence of NMDAR antagonists. Second, we demonstrate that NMDAR-independent learning requires the hippocampus and is dependent on protein synthesis. However, unlike NMDAR-dependent learning, it is not contingent on the expression of activity-regulated cytoskeleton-associated protein (Arc). Lastly, we present data that suggests NMDAR-independent learning is only observed when recently stimulated neurons are reactivated during conditioning. These data suggest that context fear conditioning modifies synaptic plasticity mechanisms in the hippocampus and allows subsequent learning to occur in the absence of NMDAR activation.

Fibroblast growth factor-2 alters the nature of extinction

These experiments examined the effects of the NMDA-receptor (NMDAr) antagonist MK801 on reacquisition and re-extinction of a conditioned fear that had been previously extinguished before injection of fibroblast growth factor-2 (FGF2) or vehicle. Recent findings have shown that relearning and re-extinction, unlike initial learning and extinction, do not depend on NMDAr activation. Three experiments demonstrated that FGF2 prevents the switch from NMDAr-dependent to NMDAr-independent reacquisition and re-extinction, suggesting that FGF2 may lead to the partial erasure of the original fear memory. These findings add to a growing body of work suggesting that FGF2 may be a novel pharmacological enhancer of exposure therapy for humans with anxiety disorders.

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.

The effect of D-cycloserine on immediate vs. delayed extinction of learned fear

We compared the effect of D-cycloserine (DCS) on immediate (10 min after conditioning) and delayed (24 h after conditioning) extinction of learned fear in rats. DCS facilitated both immediate and delayed extinction when the drug was administered after extinction training. However, DCS did not facilitate immediate extinction when administered prior to extinction training (i.e., when the interval between drug administration and shock was reduced). In addition, administering five, but not two, shocks prior to extinction training disrupted the facilitating effects of DCS on delayed extinction. These results suggest that aversive experiences prior to DCS administration can prevent it from facilitating extinction.

Impaired extinction retention in adolescent rats: effects of D-cycloserine

The developmental trajectory of the prefrontal cortex (PFC) in both rats and humans is nonlinear, with a notable decline in synaptic density during adolescence, potentially creating a 'natural lesion' preparation at this age. Given that the PFC is critically involved in retention of extinction of learned fear in adult humans and rodents, the present study examined whether adolescent rats exhibit impaired extinction retention. The results of experiment 1 showed that adolescent rats were impaired in extinction retention, compared with both younger and older rats. The partial NMDA receptor agonist D-cycloserine (DCS) improved extinction retention in adolescent rats (experiment 2), but only if administered immediately after extinction training (experiment 3). In addition, providing extended extinction training improved extinction retention in adolescent rats in a manner similar to that of DCS (experiment 4). The results of this study show that adolescent rats exhibit impaired extinction retention, and that this can be reduced through either DCS or extended extinction training. These novel findings have potential implications for clinical treatments of fear and anxiety disorders in adolescent patients.

Extinction in preweanling rats does not involve NMDA receptors

Past research has shown that the N-methyl-d-aspartate receptor (NMDAr) is critically involved in the extinction of learned fear in adult rats with NMDAr antagonists impairing extinction retention and NMDAr agonists enhancing it. In the present study we examined the effects of the NMDAr antagonist MK-801 on extinction in the developing rat. In Experiment 1, rats were given pairings of a white-noise conditioned stimulus (CS) and a shock unconditioned stimulus (US) on postnatal day (P)16. An extinction session, where the CS was presented without the US, occurred on P17 or P24. Prior to extinction rats were injected with MK-801 or saline. All rats were tested for fear of the CS on P25, while drug free. Saline-treated rats exhibited good retention of extinction whether they were extinguished at P17 or P25. Rats treated with MK-801 exhibited impaired extinction retention but only if extinction occurred on P24. These findings show that extinction is NMDAr-dependent at P24 but NMDAr-independent at P17. Experiment 2 further examined the involvement of NMDA receptors in extinction at different stages of development by taking advantage of a finding from several recent studies on re-extinction. These studies have shown, in adult rats, a transition from NMDAr-dependent extinction to NMDAr-independent re-extinction. That is, if rats are trained, extinguished, re-trained to the same CS, and then extinguished again (i.e., re-extinction), then NMDA receptors are not required for extinction the second time. In Experiment 2, rats were trained to fear the CS at P16; this fear was extinguished at either P17 or P24. All rats were then re-trained to fear the CS at P25, re-extinguished at P26, and tested at P27. Prior to re-extinction, rats were injected with MK-801 or saline. Rats initially extinguished at P24, an age where NMDA receptors are involved in extinction, exhibited the transition to an NMDAr-independent re-extinction process. In contrast, rats initially extinguished at P17, an age where NMDA receptors are not involved in extinction, did not (i.e., these rats still exhibited an impairment in extinction retention if given MK-801 prior to re-extinction). Taken together, these findings demonstrate that a qualitatively different system mediates extinction early in life.

Alterations of excitatory transmission in the lateral amygdala during expression and extinction of fear memory

Understanding the neurophysiology of fear extinction has important implications for the treatment of post-traumatic stress disorders. Here we report that fear conditioning resulted in an increase in AMPA/NMDA ratio as well as depression of paired-pulse facilitation (PPF) in neurons of the lateral nucleus of amygdala. These conditioning-induced changes in synaptic transmission were not affected by extinction training. D-cycloserine (DCS), a partial agonist at the glycine-binding site of the NMDA receptor, facilitated extinction and reversed the increase in AMPA/NMDA ratio without altering the depression of PPF when administered before extinction training. Extinction training, however, significantly increased the frequency and amplitude of miniature inhibitory post-synaptic currents and these effects were unaffected by the DCS treatment. Disruption of AMPA receptor endocytosis with a synthetic peptide containing a short C-terminal sequence of GluR2 (869YKEGYNVYG877, GluR23Y) specifically blocked DCS-induced reversal of AMPA/NMDA ratio and the facilitation of extinction. These results suggest that extinction training mainly increases inhibitory transmission leaving conditioning-induced excitatory association unaltered. DCS does not affect inhibitory transmission but reverses the conditioning-induced post-synaptic memory trace when administered before extinction training.

D-cycloserine deters reacquisition of cocaine self-administration by augmenting extinction learning

Augmentation of cue exposure (extinction) therapy with cognitive-enhancing pharmacotherapy may offer an effective strategy to combat cocaine relapse. To investigate this possibility at the preclinical level, rats and squirrel monkeys were trained to self-administer cocaine paired with a brief visual cue. Lever pressing was subsequently extinguished by withholding cocaine injections while maintaining response-contingent presentations of the cue. The glycine partial agonist D-cycloserine (DCS; 15 and 30 mg/kg in rats, 3 and 10 mg/kg in monkeys) was evaluated for its effects on the rate of extinction and subsequent reacquisition of cocaine self-administration. Compared with vehicle, pretreatment with 30 mg/kg DCS 0.5 h before extinction training reduced the number of responses and latency to reach the extinction criterion in rats, but neither dose of DCS altered these measures in monkeys. In both species, pretreatment with the higher dose of DCS before extinction training significantly attenuated reacquisition of cocaine self-administration compared with either extinction training in the absence of DCS or DCS in the absence of explicit extinction. Furthermore, treatment with 30 mg/kg DCS accompanied by brief handling (a stress induction) immediately after but not 6 h after extinction training attenuated reacquisition of cocaine self-administration in rats. No adverse effects of 10 mg/kg DCS were evident in quantitative observational studies in monkeys. The results suggest that DCS augmented consolidation of extinction learning to deter reacquisition of cocaine self-administration in rats and monkeys. The results suggest that DCS combined with exposure therapy may constitute a rational strategy for the clinical management of cocaine relapse.

The temporal specificity of the switch from NMDAr-dependent extinction to NMDAr-independent re-extinction

Recent findings show that the switch from NMDAr-dependent extinction to NMDAr-independent re-extinction is both context and stimulus specific. In this study we examined whether this switch was temporally specific as well. Re-extinction was found to be NMDAr-independent when it occurred 2 days after initial extinction but NMDAr-dependent when it occurred 21 days following initial extinction, thereby illustrating the importance of time as a type of context that modulates the mechanisms involved in extinction.