An epigenetic mechanism for over-consolidation of fear memories.

Excessive fear is a hallmark of anxiety disorders, a major cause of disease burden worldwide. Substantial evidence supports a role of prefrontal cortex-amygdala circuits in the regulation of fear and anxiety, but the molecular mechanisms that regulate their activity remain poorly understood. Here, we show that downregulation of the histone methyltransferase PRDM2 in the dorsomedial prefrontal cortex enhances fear expression by modulating fear memory consolidation. We further show that Prdm2 knock-down (KD) in neurons that project from the dorsomedial prefrontal cortex to the basolateral amygdala (dmPFC-BLA) promotes increased fear expression. Prdm2 KD in the dmPFC-BLA circuit also resulted in increased expression of genes involved in synaptogenesis, suggesting that Prdm2 KD modulates consolidation of conditioned fear by modifying synaptic strength at dmPFC-BLA projection targets. Consistent with an enhanced synaptic efficacy, we found that dmPFC Prdm2 KD increased glutamatergic release probability in the BLA and increased the activity of BLA neurons in response to fear-associated cues. Together, our findings provide a new molecular mechanism for excessive fear responses, wherein PRDM2 modulates the dmPFC -BLA circuit through specific transcriptomic changes.

Cocaine falls into oblivion during volitional initiation of choice trials.

When facing a choice, most animals quit drugs in favour of a variety of nondrug alternatives. We recently found, rather unexpectedly, that choice of the nondrug alternative is in fact inflexible and habitual. One possible contributing factor to habitual choice is the intermittency and uncontrollability of choice trials in previous studies. Here, we asked whether and to what extent volitional control over the occurrence of choice trials could change animals' preference by preventing habitual choice. To do so, rats were trained to nosepoke in a hole to trigger the presentation of two operant levers: one associated with cocaine, the other with saccharin. Rats were then free to choose among the two levers to obtain the corresponding reward, after which both levers retracted until rats self-initiated the next choice trial. Overall, we found that volitional control over choice trials did not change preference. Most rats preferred saccharin over cocaine and selected this option almost exclusively. Intriguingly, after repeated choice and consumption of saccharin, rats transiently lost interest in this option (i.e., due to sensory-specific satiety), but they did not switch to cocaine, preferring instead to pause during long periods of time before reinitiating a choice trial for saccharin. This finding suggests that during volitional initiation of a choice trial, rats fail to consider cocaine as an option. We discuss a possible associative mechanism to explain this perplexing behaviour.

Neurobiology of alcohol seeking behavior.

Alcohol addiction is a chronic relapsing brain disease characterized by an impaired ability to stop or control alcohol use despite adverse consequences. A main challenge of addiction treatment is to prevent relapse, which occurs in more than >50% of newly abstinent patients with alcohol disorder within 3 months. In people suffering from alcohol addiction, stressful events, drug-associated cues and contexts, or re-exposure to a small amount of alcohol trigger a chain of behaviors that frequently culminates in relapse. In this review, we first present the preclinical models that were developed for the study of alcohol seeking behavior, namely the reinstatement model of alcohol relapse and compulsive alcohol seeking under a chained schedule of reinforcement. We then provide an overview of the neurobiological findings obtained using these animal models, focusing on the role of opioids systems, corticotropin-release hormone and neurokinins, followed by dopaminergic, glutamatergic, and GABAergic neurotransmissions in alcohol seeking behavior.

Recent Advances in the Potential of Positive Allosteric Modulators of the GABAB Receptor to Treat Alcohol Use Disorder.

AIMS: The effects of alcohol on gamma-aminobutyric acid (GABA) transmission are key for the development and maintenance of alcohol use disorder (AUD). Previous research consistently indicates that GABAB receptor agonists such as baclofen can attenuate addiction-related behaviors in preclinical models of AUD. More importantly, baclofen has also shown promise in clinical studies, particularly in severely alcohol-dependent patients. However, despite this promise, other clinical studies have not confirmed its efficacy and chiefly, larger clinical trials have not been conducted. Therefore, with the exception of France, baclofen is not approved for the treatment of AUD in any other country. Furthermore, it is also important to keep in mind that some patients treated with baclofen may experience important side-effects, including sedation, drowsiness and sleepiness. METHODS: This short review will first discuss the history of baclofen for AUD treatment. We will then summarize preclinical behavioral results that have investigated the efficacy of GABAB PAMs for addiction treatment, with a special focus on our recent work that investigated the effects of ADX71441, a novel GABAB PAM, on several alcohol-related behaviors in rats that model important aspects of human AUD. Finally, in light of the recent criticism about the translational value of animal models of addiction, the specific translational potential of our work and of other preclinical studies that have unanimously reported the efficacy of GABAB PAMs to attenuate multiple alcohol-related behaviors will be discussed. RESULTS: Positive allosteric modulators (PAMs) of the GABAB receptor offer an attractive alternative approach to baclofen and have the potential to achieve mechanistic and therapeutic effects similar to GABAB agonists, while avoiding the tolerance and toxicity issues associated with baclofen. To date, all preclinical behavioral results have invariably shown the efficacy of GABAB PAMs for addiction treatment. CONCLUSIONS: Preclinical studies indicate that GABAB PAMs have a higher therapeutic index than orthosteric agonists, at least in terms of mitigating the sedative effects of GABAB agonism. This predicts that GABAB PAMs have a high translational potential in humans and merit being tested clinically, in particular in patients with severe AUD.

Stress-induced escalation of alcohol self-administration, anxiety-like behavior, and elevated amygdala Avp expression in a susceptible subpopulation of rats.

Comorbidity between alcohol use and anxiety disorders is associated with more severe symptoms and poorer treatment outcomes than either of the conditions alone. There is a well-known link between stress and the development of these disorders, with post-traumatic stress disorder as a prototypic example. Post-traumatic stress disorder can arise as a consequence of experiencing traumatic events firsthand and also after witnessing them. Here, we used a model of social defeat and witness stress in rats, to study shared mechanisms of stress-induced anxiety-like behavior and escalated alcohol self-administration. Similar to what is observed clinically, we found considerable individual differences in susceptibility and resilience to the stress. Both among defeated and witness rats, we found a subpopulation in which exposure was followed by emergence of increased anxiety-like behavior and escalation of alcohol self-administration. We then profiled gene expression in tissue from the amygdala, a key brain region in the regulation of stress, alcohol use, and anxiety disorders. When comparing "comorbid" and resilient socially defeated rats, we identified a strong upregulation of vasopressin and oxytocin, and this correlated positively with the magnitude of the alcohol self-administration and anxiety-like behavior. A similar trend was observed in comorbid witness rats. Together, our findings provide novel insights into molecular mechanisms underpinning the comorbidity of escalated alcohol self-administration and anxiety-like behavior.

Loss of metabotropic glutamate receptor 2 escalates alcohol consumption.

Identification of genes influencing complex traits is hampered by genetic heterogeneity, the modest effect size of many alleles, and the likely involvement of rare and uncommon alleles. Etiologic complexity can be simplified in model organisms. By genomic sequencing, linkage analysis, and functional validation, we identified that genetic variation of Grm2, which encodes metabotropic glutamate receptor 2 (mGluR2), alters alcohol preference in animal models. Selectively bred alcohol-preferring (P) rats are homozygous for a Grm2 stop codon (Grm2 *407) that leads to largely uncompensated loss of mGluR2. mGluR2 receptor expression was absent, synaptic glutamate transmission was impaired, and expression of genes involved in synaptic function was altered. Grm2 *407 was linked to increased alcohol consumption and preference in F2 rats generated by intercrossing inbred P and nonpreferring rats. Pharmacologic blockade of mGluR2 escalated alcohol self-administration in Wistar rats, the parental strain of P and nonpreferring rats. The causal role of mGluR2 in altered alcohol preference was further supported by elevated alcohol consumption in Grm2 (-/-) mice. Together, these data point to mGluR2 as an origin of alcohol preference and a potential therapeutic target.

cAMP-induced mitochondrial compartment biogenesis: role of glutathione redox state.

Cell fate and proliferation are tightly linked to the regulation of the mitochondrial energy metabolism. Hence, mitochondrial biogenesis regulation, a complex process that requires a tight coordination in the expression of the nuclear and mitochondrial genomes, has a major impact on cell fate and is of high importance. Here, we studied the molecular mechanisms involved in the regulation of mitochondrial biogenesis through a nutrient-sensing pathway, the Ras-cAMP pathway. Activation of this pathway induces a decrease in the cellular phosphate potential that alleviates the redox pressure on the mitochondrial respiratory chain. One of the cellular consequences of this modulation of cellular phosphate potential is an increase in the cellular glutathione redox state. The redox state of the glutathione disulfide-glutathione couple is a well known important indicator of the cellular redox environment, which is itself tightly linked to mitochondrial activity, mitochondria being the main cellular producer of reactive oxygen species. The master regulator of mitochondrial biogenesis in yeast (i.e. the transcriptional co-activator Hap4p) is positively regulated by the cellular glutathione redox state. Using a strain that is unable to modulate its glutathione redox state (Δglr1), we pinpoint a positive feedback loop between this redox state and the control of mitochondrial biogenesis. This is the first time that control of mitochondrial biogenesis through glutathione redox state has been shown.

Wistar rats choose alcohol over social interaction in a discrete-choice model.

Animal models of substance use disorders have been criticized for their limited translation. One important factor behind seeking and taking that has so far been largely overlooked is the availability of alternative non-drug rewards. We recently reported that only about 15% of outbred Wistar rats will choose alcohol over a sweet solution of saccharin. It was also shown using a novel operant model of choice of drugs over social rewards that social interaction consistently attenuates self-administration and incubation of craving for stimulants and opioids. Whether this is also true for alcohol and choice of alcohol over a sweet reward translates to social rewards is currently unknown. We therefore evaluated choice between alcohol and a social reward in different experimental settings in both male and female Wistar rats. We found, in contrast to prior work that employed discrete choice of drugs vs. social reward, that rats almost exclusively prefer alcohol over social interaction, irrespective of the nature of the social partner (cagemate vs. novel rat), the length of interaction, housing conditions and sex. Alcohol choice was reduced when the response requirement for alcohol was increased. However, rats persisted in choosing alcohol, even when the effort required to obtain it was 10-16 times higher (for females and males respectively) than the one for the social reward. Altogether, these results indicate that the social choice model may not generalize to alcohol, pointing to the possibility that specific interactions between alcohol and social reward, not seen when a sweet solution is used as an alternative to the drug, may play a crucial role in alcohol vs. social choice experiments.

Activation of GABAB receptors in central amygdala attenuates activity of PKCδ + neurons and suppresses punishment-resistant alcohol self-administration in rats.

Alcohol use despite negative consequences is a core phenomenon of alcohol addiction. We recently used alcohol self-administration that is resistant to footshock punishment as a model of this behavior, and found that activity of PKCδ + GABAergic neurons in the central amygdala (CeA) is a determinant of individual susceptibility for punishment resistance. In the present study, we examined whether activation of GABAB receptors in CeA can attenuate the activity of PKCδ + neurons in this region, and whether this will result in suppression of punishment- resistant alcohol self-administration in the minority of rats that show this behavior. Systemic administration of the clinically approved GABAB agonist baclofen (1 and 3 mg/kg) dose- dependently reduced punishment-resistant alcohol self-administration. Bilateral microinjections of baclofen into CeA (64 ng in 0.3 µl/side) reduced the activity of PKCδ + neurons, as measured by Fos expression. This manipulation also selectively suppressed punished alcohol self-administration in punishment-resistant rats. Expression analysis indicated that virtually all CeA PKCδ + neurons express the GABAB receptor. Using in vitro electrophysiology, we found that baclofen induced hyperpolarization of CeA neurons, reducing their firing rate in response to depolarizing current injections. Together, our findings provide a potential mechanism that contributes to the clinical efficacy of baclofen in alcohol addiction. Therapeutic use of baclofen itself is limited by problems of tolerance and need for dose escalation. Our findings support a mechanistic rationale for developing novel, improved alcohol addiction medications that target GABAB receptors, and that lack these limitations, such as e.g., GABAB positive allosteric modulators (PAM:s).

Diazepam promotes choice of abstinence in cocaine self-administering rats.

When facing a choice between cocaine and a potent, albeit inessential, non-drug alternative (i.e. water sweetened with saccharin), most cocaine self-administering rats abstain from cocaine in favor of the non-drug pursuit, regardless of the dose available and even after extended drug use. Only a minority continues to take the drug despite the opportunity of making a different choice and increasing stakes. This pattern of individual variation could suggest that the majority of rats are resilient to addiction, taking cocaine by default of other options. Only a minority would be vulnerable to addiction. This study tested the hypothesis that rats choose to refrain from cocaine self-administration because cocaine would be conflictual, having both rewarding and anxiogenic properties. Contrary to this hypothesis, however, we report here that diazepam-a broad-spectrum benzodiazepine anxiolytic-did not decrease, but instead, further increased cocaine abstinence. Interestingly, although diazepam decreased locomotion, rats adapted to this effect by spending more time near the lever associated with the preferred reward, a behavior that minimized the need for locomotion at the moment of choice. When responding for cocaine or saccharin was analyzed separately, we found that diazepam decreased responding for cocaine without affecting responding for saccharin. Finally, the abstinence-promoting effects of diazepam were also induced in cocaine-preferring rats treated chronically with diazepam. Overall, this study demonstrates that abstinence from cocaine cannot be explained away by the anxiogenic effects of cocaine, thereby reinforcing the notion of resilience to addiction. It also supports the use of benzodiazepines in the treatment of cocaine addiction.

Downregulation of Synaptotagmin 1 in the Prelimbic Cortex Drives Alcohol-Associated Behaviors in Rats.

BACKGROUND: Alcohol addiction is characterized by persistent neuroadaptations in brain structures involved in motivation, emotion, and decision making, including the medial prefrontal cortex, the nucleus accumbens, and the amygdala. We previously reported that induction of alcohol dependence was associated with long-term changes in the expression of genes involved in neurotransmitter release. Specifically, Syt1, which plays a key role in neurotransmitter release and neuronal functions, was downregulated. Here, we therefore examined the role of Syt1 in alcohol-associated behaviors in rats. METHODS: We evaluated the effect of Syt1 downregulation using an adeno-associated virus (AAV) containing a short hairpin RNA against Syt1. Cre-dependent Syt1 was also used in combination with an rAAV2 retro-Cre virus to assess circuit-specific effects of Syt1 knockdown (KD). RESULTS: Alcohol-induced downregulation of Syt1 is specific to the prelimbic cortex (PL), and KD of Syt1 in the PL resulted in escalated alcohol consumption, increased motivation to consume alcohol, and increased alcohol drinking despite negative consequences ("compulsivity"). Syt1 KD in the PL altered the excitation/inhibition balance in the basolateral amygdala, while the nucleus accumbens core was unaffected. Accordingly, a projection-specific Syt1 KD in the PL-basolateral amygdala projection was sufficient to increase compulsive alcohol drinking, while a KD of Syt1 restricted to PL-nucleus accumbens core projecting neurons had no effect on tested alcohol-related behaviors. CONCLUSIONS: Together, these data suggest that dysregulation of Syt1 is an important mechanism in long-term neuroadaptations observed after a history of alcohol dependence, and that Syt1 regulates alcohol-related behaviors in part by affecting a PL-basolateral amygdala brain circuit.

A neural substrate of compulsive alcohol use.

Alcohol intake remains controlled in a majority of users but becomes "compulsive," i.e., continues despite adverse consequences, in a minority who develop alcohol addiction. Here, using a footshock-punished alcohol self-administration procedure, we screened a large population of outbred rats to identify those showing compulsivity operationalized as punishment-resistant self-administration. Using unsupervised clustering, we found that this behavior emerged as a stable trait in a subpopulation of rats and was associated with activity of a brain network that included central nucleus of the amygdala (CeA). Activity of PKCδ+ inhibitory neurons in the lateral subdivision of CeA (CeL) accounted for ~75% of variance in punishment-resistant alcohol taking. Activity-dependent tagging, followed by chemogenetic inhibition of neurons activated during punishment-resistant self-administration, suppressed alcohol taking, as did a virally mediated shRNA knockdown of PKCδ in CeA. These findings identify a previously unknown mechanism for a core element of alcohol addiction and point to a novel candidate therapeutic target.

Nicotine increases alcohol self-administration in male rats via a µ-opioid mechanism within the mesolimbic pathway.

BACKGROUND AND PURPOSE: Alcohol and nicotine use disorders are commonly comorbid. Both alcohol and nicotine can activate opioid systems in reward-related brain regions, leading to adaptive changes in opioid signalling upon chronic exposure. The potential role of these adaptations for comorbidity is presently unknown. Here, we examined the contribution of µ and κ-opioid receptors to nicotine-induced escalation of alcohol self-administration in rats. EXPERIMENTAL APPROACH: Chronic nicotine was tested on alcohol self-administration and motivation to obtain alcohol. We then tested the effect of the κ antagonist CERC-501 and the preferential µ receptor antagonist naltrexone on basal and nicotine-escalated alcohol self-administration. To probe µ or κ receptor adaptations, receptor binding and G-protein coupling assays were performed in reward-related brain regions. Finally, dopaminergic activity in response to alcohol was examined, using phosphorylation of DARPP-32 in nucleus accumbens as a biomarker. KEY RESULTS: Nicotine robustly induced escalation of alcohol self-administration and motivation to obtain alcohol. This was blocked by naltrexone but not by CERC-501. Escalation of alcohol self-administration was associated with decreased DAMGO-stimulated µ receptor signalling in the ventral tegmental area (VTA) and decreased pDARPP-32 in the nucleus accumbens shell in response to alcohol. CONCLUSIONS AND IMPLICATIONS: Collectively, these results suggest that nicotine contributes to escalate alcohol self-administration through a dysregulation of µ receptor activity in the VTA. These data imply that targeting µ rather than κ receptors may be the preferred pharmacotherapeutic approach for the treatment of alcohol use disorder when nicotine use contributes to alcohol consumption.

Developing neuroscience-based treatments for alcohol addiction: A matter of choice?

Excessive alcohol use is the cause of an ongoing public health crisis, and accounts for ~5% of global disease burden. A minority of people with recreational alcohol use develop alcohol addiction (hereafter equated with "alcohol dependence" or simply "alcoholism"), a condition characterized by a systematically biased choice preference for alcohol at the expense of healthy rewards, and continued use despite adverse consequences ("compulsivity"). Alcoholism is arguably the most pressing area of unmet medical needs in psychiatry, with only a small fraction of patients receiving effective, evidence-based treatments. Medications currently approved for the treatment of alcoholism have small effect sizes, and their clinical uptake is negligible. No mechanistically new medications have been approved since 2004, and promising preclinical results have failed to translate into novel treatments. This has contributed to a reemerging debate whether and to what extent alcohol addiction represents a medical condition, or reflects maladaptive choices without an underlying brain pathology. Here, we review this landscape, and discuss the challenges, lessons learned, and opportunities to retool drug development in this important therapeutic area.

A molecular mechanism for choosing alcohol over an alternative reward.

Alcohol addiction leads to increased choice of alcohol over healthy rewards. We established an exclusive choice procedure in which ~15% of outbred rats chose alcohol over a high-value reward. These animals displayed addiction-like traits, including high motivation to obtain alcohol and pursuit of this drug despite adverse consequences. Expression of the γ-aminobutyric acid (GABA) transporter GAT-3 was selectively decreased within the amygdala of alcohol-choosing rats, whereas a knockdown of this transcript reversed choice preference of rats that originally chose a sweet solution over alcohol. GAT-3 expression was selectively decreased in the central amygdala of alcohol-dependent people compared to those who died of unrelated causes. Impaired GABA clearance within the amygdala contributes to alcohol addiction, appears to translate between species, and may offer targets for new pharmacotherapies for treating this disorder.

A Method for Evaluating the Reinforcing Properties of Ethanol in Rats without Water Deprivation, Saccharin Fading or Extended Access Training.

Operant oral self-administration methods are commonly used to study the reinforcing properties of ethanol in animals. However, the standard methods require saccharin/sucrose fading, water deprivation and/or extended training to initiate operant responding in rats. This paper describes a novel and efficient method to quickly initiate operant responding for ethanol that is convenient for experimenters and does not require water deprivation or saccharin/sucrose fading, thus eliminating the potential confound of using sweeteners in ethanol operant self-administration studies. With this method, Wistar rats typically acquire and maintain self-administration of a 20% ethanol solution in less than two weeks of training. Furthermore, blood ethanol concentrations and rewards are positively correlated for a 30 min self-administration session. Moreover, naltrexone, an FDA-approved medication for alcohol dependence that has been shown to suppress ethanol self-administration in rodents, dose-dependently decreases alcohol intake and motivation to consume alcohol for rats self-administering 20% ethanol, thus validating the use of this new method to study the reinforcing properties of alcohol in rats.

The GABAB Positive Allosteric Modulator ADX71441 Attenuates Alcohol Self-Administration and Relapse to Alcohol Seeking in Rats.

GABAergic signaling is involved in modulating the reinforcing properties of alcohol, and GABAB receptors have been proposed as a potential target for clinical treatment of alcoholism. The orthosteric GABAB receptor agonist baclofen has been shown to suppress operant self-administration of alcohol in animals and alcohol use in alcohol-dependent patients, but its utility is limited by a narrow therapeutic index. We tested the effects of ADX71441, a novel GABAB receptor positive allosteric modulator, on alcohol-related behaviors in rats. We first assessed the effects of ADX71441 (1, 3, 10 and 30 mg/kg, I.P.) on both non-dependent and dependent male Wistar rats trained to self-administer 20% alcohol. We then determined the effects of ADX71441 on stress-induced as well as cue-induced relapse-like behavior. Finally, we sought to identify the brain regions through which ADX71441 may act to prevent relapse-like behavior by mapping the neuronal activation induced by stress-induced reinstatement of alcohol-seeking using c-Fos immunohistochemistry. ADX71441 dose-dependently decreased alcohol self-administration of both dependent and non-dependent animals, but its potency was higher in alcohol-dependent rats. Furthermore, both cue- and stress-induced alcohol seeking were blocked by the GABAB receptor positive allosteric modulator. Finally, pretreatment with 3 mg/kg of ADX71441 before stress-induced reinstatement significantly decreased c-Fos expression in a network of brain regions implicated in stress-induced relapse, comprising the nucleus accumbens shell, the dorsal raphe nucleus and the medial prefrontal cortex. Our findings support a causal role of GABAB receptors in alcohol reinforcement and relapse to alcohol seeking. These effects are observed in the absence of significant sedative side effects. Jointly, these observations indicate that GABAB receptor positive allosteric modulators merit being tested clinically for the treatment of alcoholism. Our data also point to a potential biomarker of target engagement for early clinical studies.

The mGluR2 Positive Allosteric Modulator, AZD8529, and Cue-Induced Relapse to Alcohol Seeking in Rats.

Group II metabotropic glutamate receptors (mGluR2 and mGluR3) may control relapse of alcohol seeking, but previously available Group II agonists were unable to discriminate between mGluR2 and mGluR3. Here we use AZD8529, a novel positive allosteric mGluR2 modulator, to determine the role of this receptor for alcohol-related behaviors in rats. We assessed the effects of AZD8529 (20 and 40 mg/kg s.c.) on male Wistar rats trained to self-administer 20% alcohol and determined the effects of AZD8529 on self-administration, as well as stress-induced and cue-induced reinstatement of alcohol seeking. The on-target nature of findings was evaluated in Indiana P-rats, a line recently shown to carry a mutation that disrupts the gene encoding mGluR2. The behavioral specificity of AZD8529 was assessed using self-administration of 0.2% saccharin and locomotor activity tests. AZD8529 marginally decreased alcohol self-administration at doses that neither affected 0.2% saccharin self-administration nor locomotor activity. More importantly, cue- but not stress-induced alcohol seeking was blocked by the mGluR2 positive allosteric modulator. This effect of AZD8529 was completely absent in P rats lacking functional mGluR2s, demonstrating the receptor specificity of this effect. Our findings provide evidence for a causal role of mGluR2 in cue-induced relapse to alcohol seeking. They contribute support for the notion that positive allosteric modulators of mGluR2 block relapse-like behavior across different drug categories.

A choice-based screening method for compulsive drug users in rats.

We describe a protocol for screening compulsive drug users among cocaine self-administering rats, the most frequently used animal model in addiction research. Rats are first trained on several alternating days to self-administer either cocaine (i.v.) or saccharin-sweetened water (by mouth)--a potent, albeit nonessential, nondrug reward. Then rats are allowed to choose between the two rewards over several days until the preference stabilizes. Most rats choose to stop using cocaine and pursue the alternative reward. Only a minority of Wistar strain rats (generally 15%) persist in taking the drug, regardless of the severity of past cocaine use and even when made hungry and offered the possibility to relieve their physiological need. Persistence of cocaine use in the face of a high-stakes choice is a core defining feature of compulsion. This choice-based screening method for compulsive drug users is easy to implement, has several important applications, and compares well with other methods in the field.