D-cycloserine potentiates the reconsolidation of cocaine-associated memories

The persistence of maladaptive memory: addiction, drug memories and anti-relapse treatments

Addiction is a chronic, relapsing disorder, characterised by the long-term propensity of addicted individuals to relapse. A major factor that obstructs the attainment of abstinence is the persistence of maladaptive drug-associated memories, which can maintain drug-seeking and taking behaviour and promote unconscious relapse of these habits. Thus, addiction can be conceptualised as a disorder of aberrant learning of the formation of strong instrumental memories linking actions to drug-seeking and taking outcomes that ultimately are expressed as persistent stimulus-response habits; of previously neutral environmental stimuli that become associated with drug highs (and/or withdrawal states) through pavlovian conditioning, and of the subsequent interactions between pavlovian and instrumental memories to influence relapse behaviour. Understanding the psychological, neurobiological and molecular basis of these drug memories may produce new methods of pro-abstinence, anti-relapse treatments for addiction.

D-cycloserine facilitates socially reinforced learning in an animal model relevant to autism spectrum disorders

BACKGROUND

There are no drugs that specifically target the social deficits of autism spectrum disorders (ASD). This may be due to a lack of behavioral paradigms in animal models relevant to ASD. Partner preference formation in the prairie vole represents a social cognitive process involving socially reinforced learning. D-cycloserine (DCS) is a cognitive enhancer that acts at the N-methyl-D-aspartate receptor to promote learning. If DCS enhances socially reinforced learning in the partner preference paradigm, it may be useful in combination with behavioral therapies for enhancing social functioning in ASD.

METHODS

Female prairie and meadow voles were given DCS either peripherally or directly into one of three brain regions: nucleus accumbens, amygdala, or caudate putamen. Subjects were then cohabited with a male vole under conditions that do not typically yield a partner preference. The development of a preference for that stimulus male vole over a novel male vole was assessed using a partner preference test.

RESULTS

A low dose of DCS administered peripherally enhanced preference formation in prairie voles but not meadow voles under conditions in which it would not otherwise occur. These effects were replicated in prairie voles by microinfusions of DCS into the nucleus accumbens, which is involved in reinforcement learning, and the amygdala, which is involved in social information processing.

CONCLUSIONS

Partner preference in the prairie vole may provide a behavioral paradigm with face, construct, and predictive validity for identifying prosocial pharmacotherapeutics. D-cycloserine may be a viable treatment strategy for social deficits of ASD when paired with social behavioral therapy.

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.

Glutamatergic medications for the treatment of drug and behavioral addictions

Historically, most pharmacological approaches to the treatment of addictive disorders have utilized either substitution-based methods (i.e., nicotine replacement or opioid maintenance) or have targeted monoaminergic or endogenous opioidergic neurotransmitter systems. However, substantial evidence has accumulated indicating that ligands acting on glutamatergic transmission are also of potential utility in the treatment of drug addiction, as well as various behavioral addictions such as pathological gambling. The purpose of this review is to summarize the pharmacological mechanisms of action and general clinical efficacy of glutamatergic medications that are currently approved or are being investigated for approval for the treatment of addictive disorders. Medications with effects on glutamatergic transmission that will be discussed include acamprosate, N-acetylcysteine, d-cycloserine, gabapentin, lamotrigine, memantine, modafinil, and topiramate. We conclude that manipulation of glutamatergic neurotransmission is a relatively young but promising avenue for the development of improved therapeutic agents for the treatment of drug and behavioral addictions.

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.

D-cycloserine reduces the context specificity of pavlovian extinction of cocaine cues through actions in the nucleus accumbens

Extinction therapy has been proposed as a method to reduce the motivational impact of drug-associated cues to prevent relapse. Cue extinction therapy, however, takes place in a novel context (e.g., treatment facility), and is unlikely to be effective due to the context specificity of extinction. We tested the hypothesis that d-cycloserine (DCS), which enhances extinction in other procedures, would enhance extinction of cocaine-associated cues in a novel context to reduce cue-induced reinstatement. Male Sprague Dawley rats were trained to self-administer cocaine associated with a cue. The cue was later extinguished in the drug-taking context (context A) or a novel context (context B) using a Pavlovian cue extinction procedure designed to mimic human cue exposure therapy. DCS was administered systemically or into a specific brain region immediately following the cue extinction sessions to enhance the consolidation of extinction learning. We demonstrate that DCS given postextinction session in context B reduces reinstatement in context A, indicating a reduction in the context specificity of extinction learning. The effect of systemic DCS was recapitulated by administration of DCS into the nucleus accumbens core, but not in the basolateral amygdala, dorsal hippocampus, infralimbic or prelimbic prefrontal cortex. DCS treatment caused a reduction in cue-induced reinstatement only when it was given after cue extinction sessions, and not when given 1) in the absence of extinction or 2) after a brief memory reactivation session. A pharmacological method that can render extinction context independent may provide an innovative method to reduce cue-induced relapse in addicts and to study the neurobiology of addiction.

Increased voluntary ethanol consumption and c-Fos expression in selected brain areas induced by fear memory retrieval in ethanol withdrawn rats

Withdrawal from chronic ethanol administration facilitated the formation of contextual fear memory. The effect of fear memory retrieval on subsequent ethanol consumption, by employing a two-bottle free-choice procedure with either water or ethanol (2-8% v/v), was investigated in ethanol withdrawn rats. The effect of fear memory extinction with or without d-cycloserine (DCS, 5mg/kgi.p.) on subsequent ethanol consumption was also evaluated. In addition, we examined c-Fos expression in different brain areas following the fear memory recall. The retrieval of such fear memory induced a significant increase in ethanol consumption in ethanol withdrawn but not in control animals. Regardless of DCS treatment, this increase was attenuated by extinction training. In ethanol withdrawn rats, context-dependent memory retrieval was accompanied by an increased c-Fos expression in the basolateral amygdala, ventrolateral periaqueductal gray, dentate gyrus and dorsomedial periaqueductal gray. Among these brain areas suggested to be implicated in the modulation of motivation and of emotional states, the basolateral amygdala has a crucial role in the emergence of negative affective state during ethanol withdrawal. These data suggest that retrieval of fear memory in ethanol withdrawn rats affected ethanol consumption and that amygdala may be involved in this effect.

Reconsolidation of a cocaine-associated stimulus requires amygdalar protein kinase A

Drug addiction is a chronic disorder associated with recurrent craving and relapse often precipitated by the presence of drug-associated stimuli. Pharmacological and behavioral treatments that disrupt drug-associated stimulus memories could be beneficial in the treatment of addictive disorders. Memory restabilization (or reconsolidation) following retrieval of drug-paired stimuli depends upon the amygdala. Here we assessed whether amygdalar PKA is required for the reconsolidation of an appetitive, cocaine-paired stimulus. Rats were trained to lever press for intravenous cocaine infusions paired with a light/tone conditioned stimulus. After 12 d of acquisition, rats either underwent lever extinction (8-12 d) followed by light/tone reactivation and subsequent cue-induced and cocaine-induced (15 mg/kg, i.p.) reinstatement testing or were subsequently tested to assess the ability of the light/tone stimulus to serve as a conditioned reinforcer in the acquisition of a new instrumental response (nose poking). Bilateral intra-amygdalar infusions of the PKA inhibitor Rp-cAMPS (18 microg per side) given immediately following light/tone stimulus reactivation decreased subsequent cue-induced reinstatement and responding with a conditioned reinforcer, while having no effect on cocaine-induced reinstatement. Intra-amygdalar infusions of Rp-cAMPS made 3 h following reactivation or immediately following no stimulus reactivation had no effect on subsequent cue-induced reinstatement. These data show that memory reconsolidation for a cocaine-paired stimulus is retrieval dependent and time limited and critically depends upon amygdalar PKA.

Cocaine-induced neuroadaptations in glutamate transmission: potential therapeutic targets for craving and addiction

A growing body of evidence indicates that repeated exposure to cocaine leads to profound changes in glutamate transmission in limbic nuclei, particularly the nucleus accumbens. This review focuses on preclinical studies of cocaine-induced behavioral plasticity, including behavioral sensitization, self-administration, and the reinstatement of cocaine seeking. Behavioral, pharmacological, neurochemical, electrophysiological, biochemical, and molecular biological changes associated with cocaine-induced plasticity in glutamate systems are reviewed. The ultimate goal of these lines of research is to identify novel targets for the development of therapies for cocaine craving and addiction. Therefore, we also outline the progress and prospects of glutamate modulators for the treatment of cocaine addiction.

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.