Animal models for assessing drugs of abuse

Proposing an integrative, dynamic and transdiagnostic model for addictions: dysregulation phenomena of the three main modes of the predostatic mind

Several theories have been proposed to explain the complex diagnostic aspects related to addiction disorders and their development. Recent frameworks tend to focus on dimensional perspectives of symptoms rather than categorical systems, since substance use disorders are frequently comorbid with other psychiatric and especially personality disorders. However, useful transdiagnostic models that could integrate clinical evaluation derived from neuroscientific theories are lacking. In the present manuscript, the authors propose a model based on a new paradigm, in an attempt to better explain this complex, multifaceted phenomenon. The new paradigm presupposes that emotions and behavior are a response to risk prediction. Individuals make choices and engage in actions to manage potential risks/rewards in order to seek or maintain homeostasis in their internal and external environments - a mechanism that the authors call predostatic (predictive mechanism with homeostatic purpose). The model considers three main modes of the predostatic mind: (1) Alarm Mode, activated by high and/or imminent risk prediction; (2) Seek Mode, activated by long-term risk or reward prediction; and (3) Balance Mode, a self-regulating state of mind related to low risk prediction, a soothing system and a calm state. Addiction is seen as a chronic dysregulation of organism systems leading to internalizing or externalizing phenomena mainly related to the Seek and Alarm Modes, which are persistently activated by reward and risk prediction, respectively, thus hindering Balance. Addiction neuroscience research has shown that chronic drug use or engagement in addictive behaviors can lead to neuroadaptations in the brain reward circuitry, disrupting normal balance and the regulation of reward processes. This dysregulation can contribute to persistent drug-seeking/addictive behaviors despite negative consequences. This newly proposed dynamic and integrative model, named dysregulation based on externalizing and internalizing phenomena of the three main modes of the predostatic mind (DREXI3), proposes six dysregulation dimensions with basic emotional and behavioral symptoms, such as neurophysiological alterations, impulsivity, compulsion, cognitive impairment/psychosis, mood, and anxiety/anger. In this paper, the authors explain the rationale behind DREXI3 and present some hypothetical clinical examples to better illustrate the use of the model in clinical practice. The development of this innovative model could possibly guide tailored treatment interventions in the addiction field.

Pharmacological and behavioral divergence of ketamine enantiomers: implications for abuse liability

Ketamine, a racemic mixture of (S)-ketamine and (R)-ketamine enantiomers, has been used as an anesthetic, analgesic and more recently, as an antidepressant. However, ketamine has known abuse liability (the tendency of a drug to be used in non-medical situations due to its psychoactive effects), which raises concerns for its therapeutic use. (S)-ketamine was recently approved by the United States' FDA for treatment-resistant depression. Recent studies showed that (R)-ketamine has greater efficacy than (S)-ketamine in preclinical models of depression, but its clinical antidepressant efficacy has not been established. The behavioral effects of racemic ketamine have been studied extensively in preclinical models predictive of abuse liability in humans (self-administration and conditioned place preference [CPP]). In contrast, the behavioral effects of each enantiomer in these models are unknown. We show here that in the intravenous drug self-administration model, the gold standard procedure to assess potential abuse liability of drugs in humans, rats self-administered (S)-ketamine but not (R)-ketamine. Subanesthetic, antidepressant-like doses of (S)-ketamine, but not of (R)-ketamine, induced locomotor activity (in an opioid receptor-dependent manner), induced psychomotor sensitization, induced CPP in mice, and selectively increased metabolic activity and dopamine tone in medial prefrontal cortex (mPFC) of rats. Pharmacological screening across thousands of human proteins and at biological targets known to interact with ketamine yielded divergent binding and functional enantiomer profiles, including selective mu and kappa opioid receptor activation by (S)-ketamine in mPFC. Our results demonstrate divergence in the pharmacological, functional, and behavioral effects of ketamine enantiomers, and suggest that racemic ketamine's abuse liability in humans is primarily due to the pharmacological effects of its (S)-enantiomer.

Translational value of non-human primates in opioid research

Preclinical opioid research using animal models not only provides mechanistic insights into the modulation of opioid analgesia and its associated side effects, but also validates drug candidates for improved treatment options for opioid use disorder. Non-human primates (NHPs) have served as a surrogate species for humans in opioid research for more than five decades. The translational value of NHP models is supported by the documented species differences between rodents and primates regarding their behavioral and physiological responses to opioid-related ligands and that NHP studies have provided more concordant results with human studies. This review highlights the utilization of NHP models in five aspects of opioid research, i.e., analgesia, abuse liability, respiratory depression, physical dependence, and pruritus. Recent NHP studies have found that (1) mixed mu opioid and nociceptin/orphanin FQ peptide receptor partial agonists appear to be safe, non-addictive analgesics and (2) mu opioid receptor- and mixed opioid receptor subtype-based medications remain the only two classes of drugs that are effective in alleviating opioid-induced adverse effects. Given the recent advances in pharmaceutical sciences and discoveries of novel targets, NHP studies are posed to identify the translational gap and validate therapeutic targets for the treatment of opioid use disorder. Pharmacological studies using NHPs along with multiple outcome measures (e.g., behavior, physiologic function, and neuroimaging) will continue to facilitate the research and development of improved medications to curb the opioid epidemic.

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.

Establishing performance characteristics for positive control article selection in drug self-administration studies

The selection of a controlled substance (CS) for use as the positive control article in a nonclinical drug abuse liability (DAL) assessment study should be contemplated carefully and with full understanding of the stated intent of the study design. Any CS that can maintain day-to-day stable baseline responding of voluntary intravenous intakes in animals may be selected under the current guidelines. Schedule I - IV CNS stimulants, depressants, and sedative/hypnotics can serve as maintenance drugs in these protocols, but not all of these compounds will provide comparatively efficient, robust, and stable daily intakes. Each Sponsor is directed to select a positive control article and training dose that will provide the most balanced, predictive, and scientifically-sound comparison consistent with the mechanism of action or therapeutic target of the test article. The SA study design is not a "one-size-fits-all" assay. This is a discussion of the critical design factors to be considered in selecting the most appropriate positive control article to use for a SA study.

Animal models of fatigue in major depressive disorder

Fatigue is common in a host of neurological and psychiatric disorders including depression and often continues unabated even after primary symptoms of disease are treated. Its high estimated prevalence combined with the lack of effective medicines has engaged the preclinical research community to search for fatigue models. The present review briefly summarizes the most common animal models that have been brought forward along with some of the associated pharmacological validation data. Like all preclinical models, these models have issues that need to be appreciated in the generation and interpretation of data for the purposes of translation to human disease; specifically, there are deficiencies in construct validity, a lack of medicines that effectively address residual fatigue symptoms, and difficulties in defining specificity with respect to drug effects on fatigue per se. Nonetheless, existing animal models of fatigue arguably serve the valuable purpose of encouraging research in this large area of unmet medical need. Data from these models are predicted to engender human experimentation and the further development of improved model systems.

Ghrelin Receptor Antagonism of Methamphetamine-Induced Conditioned Place Preference and Intravenous Self-Administration in Rats

Methamphetamine abuse imposes a significant burden on individuals and society worldwide, and an effective therapy of methamphetamine addiction would provide distinguished social benefits. Ghrelin significantly participates in reinforcing neurobiological mechanisms of stimulants, including amphetamines; thus, ghrelin antagonism is proposed as a promising addiction treatment. The aim of our study was to elucidate whether the pretreatment with growth hormone secretagogue receptor (GHS-R1A) antagonist, substance JMV2959, could reduce the methamphetamine intravenous self-administration (IVSA) and the tendency to relapse, and whether JMV2959 could reduce or prevent methamphetamine-induced conditioned place preference (CPP) in rats. Following an adequate maintenance period, JMV2959 3 mg/kg was administered intraperitoneally 20 min before three consequent daily 180 min sessions of methamphetamine IVSA under a fixed ratio FR1, which significantly reduced the number of active lever-pressings, the number of infusions, and the amount of the consumed methamphetamine dose. Pretreatment with JMV2959 also reduced or prevented relapse-like behavior tested in rats on the 12th day of the abstinence period. Pretreatment with JMV2959 significantly reduced the expression of methamphetamine-induced CPP. Simultaneous administration of JMV2959 with methamphetamine during the conditioning period significantly reduced the methamphetamine-CPP. Our results encourage further research of the ghrelin antagonism as a potential new pharmacological tool for methamphetamine addiction treatment.

Role of Central Amygdala Neuronal Ensembles in Incubation of Nicotine Craving

The craving response to smoking-associated cues in humans or to intravenous nicotine-associated cues in adult rats progressively increases or incubates after withdrawal. Here, we further characterized incubation of nicotine craving in the rat model by determining whether this incubation is observed after adolescent-onset nicotine self-administration. We also used the neuronal activity marker Fos and the Daun02 chemogenetic inactivation procedure to identify cue-activated neuronal ensembles that mediate incubation of nicotine craving. We trained adolescent and adult male rats to self-administer nicotine (2 h/d for 12 d) and assessed cue-induced nicotine seeking in extinction tests (1 h) after 1, 7, 14, or 28 withdrawal days. In both adult and adolescent rats, nicotine seeking in the relapse tests followed an inverted U-shaped curve, with maximal responding on withdrawal day 14. Independent of the withdrawal day, nicotine seeking in the relapse tests was higher in adult than in adolescent rats. Analysis of Fos expression in different brain areas of adolescent and adult rats on withdrawal days 1 and 14 showed time-dependent increases in the number of Fos-positive neurons in central and basolateral amygdala, orbitofrontal cortex, ventral and dorsal medial prefrontal cortex, and nucleus accumbens core and shell. In adult Fos-lacZ transgenic rats, selective inactivation of nicotine-cue-activated Fos neurons in central amygdala, but not orbitofrontal cortex, decreased "incubated" nicotine seeking on withdrawal day 14. Our results demonstrate that incubation of nicotine craving occurs after adolescent-onset nicotine self-administration and that neuronal ensembles in central amygdala play a critical role in this incubation.

SIGNIFICANCE STATEMENT

The craving response to smoking-associated cues in humans or to intravenous nicotine-associated cues in adult rats progressively increases or incubates after withdrawal. It is currently unknown whether incubation of craving also occurs after adolescent-onset nicotine self-administration. The brain areas that mediate such incubation are also unknown. Here, we used a rat model of incubation of drug craving, the neuronal activity marker Fos, and the Daun02 chemogenetic inactivation method to demonstrate that incubation of nicotine craving is also observed after adolescent-onset nicotine self-administration and that neuronal ensembles in the central nucleus of the amygdala play a critical role in this incubation in adult rats.

Time to connect: bringing social context into addiction neuroscience

Research on the neural substrates of drug reward, withdrawal and relapse has yet to be translated into significant advances in the treatment of addiction. One potential reason is that this research has not captured a common feature of human addiction: progressive social exclusion and marginalization. We propose that research aimed at understanding the neural mechanisms that link these processes to drug seeking and drug taking would help to make addiction neuroscience research more clinically relevant.

Glutamatergic transmission in drug reward: implications for drug addiction

Individuals addicted to drugs of abuse such as alcohol, nicotine, cocaine, and heroin are a significant burden on healthcare systems all over the world. The positive reinforcing (rewarding) effects of the above mentioned drugs play a major role in the initiation and maintenance of the drug-taking habit. Thus, understanding the neurochemical mechanisms underlying the reinforcing effects of drugs of abuse is critical to reducing the burden of drug addiction in society. Over the last two decades, there has been an increasing focus on the role of the excitatory neurotransmitter glutamate in drug addiction. In this review, pharmacological and genetic evidence supporting the role of glutamate in mediating the rewarding effects of the above described drugs of abuse will be discussed. Further, the review will discuss the role of glutamate transmission in two complex heterogeneous brain regions, namely the nucleus accumbens (NAcc) and the ventral tegmental area (VTA), which mediate the rewarding effects of drugs of abuse. In addition, several medications approved by the Food and Drug Administration that act by blocking glutamate transmission will be discussed in the context of drug reward. Finally, this review will discuss future studies needed to address currently unanswered gaps in knowledge, which will further elucidate the role of glutamate in the rewarding effects of drugs of abuse.

Preclinical Abuse Potential Assessment

Although laboratories have been conducting scientific evaluations of the abused drugs for many years, preclinical evaluations of the abuse potential of new drugs have been an integral component of new drug applications more recently. The development of a unified testing approach is crucial prior to initiating individual studies to address abuse potential. The core preclinical studies that will be required include a dependence/withdrawal study, an assessment of the discriminative cue produced by the new drug, and an assessment of whether the drug will be self-administered. This discussion is focused on the requirements for drug scheduling recommendations from the FDA and how to conduct the evaluations that will be used to make those recommendations and how to select parameter details such as preclinical species, test doses, test conditions, route of drug administration, comparator compounds, and behavioral test designs recommended.

Neuronal activity-regulated pentraxin expressed in medial prefrontal cortex neurons is not necessary for extinction of heroin self-administration

The medial prefrontal cortex (mPFC) plays a key role in extinction learning. Previously, we found that expression of a neuronal activity-regulated pentraxin (Narp) dominant-negative construct in the mPFC of mice blocked extinction of morphine-conditioned place preference. To further investigate the role of mPFC Narp in the extinction of drug seeking, we tested whether mPFC Narp is necessary for the extinction of heroin self-administration in rats. Specifically, we injected an adeno-associated viral vector expressing a dominant-negative form of Narp (NarpN) into the infralimbic region of the mPFC of rats and compared lever presses during extinction to those of rats injected with a control virus. In contrast to our previous study, we found that injection of NarpN did not affect extinction of heroin self-administration. Our findings suggest that mPFC Narp is necessary for extinction of opiate seeking in the Pavlovian-conditioned place preference paradigm but not in the operant paradigm of drug self-administration.

Reward, interrupted: Inhibitory control and its relevance to addictions

There are broad individual differences in the ability to voluntarily and effortfully suppress motivated, reward-seeking behaviors, and this review presents the hypothesis that these individual differences are relevant to addictive disorders. On one hand, cumulative experience with drug abuse appears to alter the molecular, cellular and circuit mechanisms that mediate inhibitory abilities, leading to increasingly uncontrolled patterns of drug-seeking and -taking. On the other, native inter-individual differences in inhibitory control are apparently a risk factor for aspects of drug-reinforced responding and substance use disorders. In both cases, the behavioral manifestation of poor inhibitory abilities is linked to relatively low striatal dopamine D2-like receptor availability, and evidence is accumulating for a more direct contribution of striatopallidal neurons to cognitive control processes. Mechanistic research is now identifying genes upstream of dopamine transmission that mediate these relationships, as well as the involvement of other neurotransmitter systems, acting alone and in concert with dopamine. The reviewed research stands poised to identify new mechanisms that can be targeted by pharmacotherapies and/or by behavioral interventions that are designed to prevent or treat addictive behaviors and associated behavioral pathology. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Extended heroin access increases heroin choices over a potent nondrug alternative

Epidemiological research shows that the proportion of drug users who become addicted to heroin is higher than to cocaine. Here we tested whether this difference could be due to a difference in the addiction liability between the two drugs. Addiction liability was assessed under a discrete-trials choice procedure by measuring the proportion of rats that prefer the drug over a potent alternative reward (ie, water sweetened with saccharin). Previous research on choice between self-administration of i.v. cocaine or sweet water showed that the proportion of cocaine-preferring rats remains relatively low and invariable (ie, 15%), even after extended drug access and regardless of past drug consumption (ie, total drug use before choice testing). By contrast, the present study shows that under similar choice conditions, the proportion of heroin-preferring rats considerably increases with extended heroin access (6-9 h per day for several weeks) and with past heroin consumption, from 11 to 51% at the highest past drug consumption level. At this level, the proportion of drug-preferring rats was about three times higher with heroin than with cocaine (51% vs 15%). This increase in the rate of heroin preference after extended heroin access persisted even after recovery from acute heroin withdrawal. Overall, these findings show that choice procedures are uniquely sensitive to different drugs and suggest that heroin is more addictive than cocaine. This higher addiction liability may contribute to explain why more drug users become addicted to heroin than to cocaine in epidemiological studies.

Place preference test in rodents

Conditioned place preference is a behavioral model currently used to measure the rewarding properties induced by the administration of a drug. In this paradigm, the rewarding properties of a compound are associated with the particular characteristics of a given environment. Advantages of this test are the absence of any instrumental learning and the possibility of evaluating reinforcing effects in the absence of the test substance. Due to the recent availability of multiple lines of genetically modified mice, this unit has been updated to include procedures for testing place preference in both rats and mice. Details are also provided for evaluating drug effects using this procedure.

Analysis of opioid efficacy, tolerance, addiction and dependence from cell culture to human

Opioid agonists are the most effective treatment for pain, but their use is limited by side effects, tolerance and fears of addiction and dependence. A major goal of opioid research is to develop agonists that have high analgesic efficacy and a low profile for side effects, tolerance, addiction and dependence. Unfortunately, there is a serious lack of experimental data comparing the degree to which different opioids produce these effects in humans. In contrast, a wide range of experimental techniques from heterologous expression systems to behaviour assessment in whole animals have been developed to study these problems. The objective of this review is to describe and evaluate these techniques as they are used to study opioid efficacy, tolerance, addiction and dependence.

Opiate versus psychostimulant addiction: the differences do matter

The publication of the psychomotor stimulant theory of addiction in 1987 and the finding that addictive drugs increase dopamine concentrations in the rat mesolimbic system in 1988 have led to a predominance of psychobiological theories that consider addiction to opiates and addiction to psychostimulants as essentially identical phenomena. Indeed, current theories of addiction - hedonic allostasis, incentive sensitization, aberrant learning and frontostriatal dysfunction - all argue for a unitary account of drug addiction. This view is challenged by behavioural, cognitive and neurobiological findings in laboratory animals and humans. Here, we argue that opiate addiction and psychostimulant addiction are behaviourally and neurobiologically distinct and that the differences have important implications for addiction treatment, addiction theories and future research.

A multivariate assessment of individual differences in sensation seeking and impulsivity as predictors of amphetamine self-administration and prefrontal dopamine function in rats

Drug abuse vulnerability has been linked to sensation seeking (behaviors likely to produce rewards) and impulsivity (behaviors occurring without foresight). Since previous preclinical work has been limited primarily to using single tasks as predictor variables, the present study determined if measuring multiple tasks of sensation seeking and impulsivity would be useful in predicting amphetamine self-administration in rats. Multiple tasks were also used as predictor variables of dopamine transporter function in the medial prefrontal and orbitofrontal cortexes, as these neural systems have been implicated in sensation seeking and impulsivity. Rats were tested on six behavioral tasks as predictor variables to evaluate sensation seeking (locomotor activity, novelty place preference, and sucrose reinforcement on a progressive ratio schedule) and impulsivity (delay discounting, cued go/no-go, and passive avoidance), followed by d-amphetamine self-administration (0.0056-0.1 mg/kg infusion) and kinetic analysis of dopamine transporter function as outcome variables. The combination of these predictor variables into a multivariate approach failed to yield any clear relationship among predictor and outcome measures. Using multivariate approaches to understand the relation between individual predictor and outcome variables in preclinical models may be hindered by alterations in behavior due to training and thus, the relation between various individual differences in behavior and drug self-administration may be better assessed using a univariate approach in which a only a single task is used as the predictor variable.

Methylphenidate as a reinforcer for rats: contingent delivery and intake escalation

Methylphenidate (MPH) is one of the most widely prescribed drugs for treating attention-deficit hyperactivity disorder. Previous research suggested that MPH is a reinforcer for rats, but not all of the manipulations to show that lever pressing is controlled by the contingency to obtain MPH have been examined. In Experiment 1, responding for MPH on a progressive ratio (PR) schedule was assessed. Rats self-administered varying doses of MPH (0.056-1.0 mg/kg/infusion) on a PR schedule of reinforcement, and self-administered more MPH than saline, with maximal responding occurring at a unit dose of 0.56 mg/kg/infusion. Experiment 2 examined if there were differences in responding between contingent and noncontingent MPH (0.56 mg/kg/infusion) on a fixed ratio schedule of reinforcement. Results showed that rats responded for contingent MPH, and that responding was not maintained when MPH was delivered noncontingently. Experiment 3 examined self-administration of MPH (0.1 or 0.3 mg/kg/infusion) during long access (6 hr) compared to short access sessions (1 hr). Results showed that rats given long access to MPH showed an escalation of intake across sessions, with this escalation being more pronounced at the lower unit dose (0.1 mg/kg/infusion); in contrast, rats given short access to MPH did not show an increase in MPH self-administration across sessions at either MPH dose tested. Taken together, these results indicate that MPH is an effective intravenous reinforcer for rats and that, similar to other stimulants such as cocaine, amphetamine and methamphetamine, MPH is subject to abuse as reflected by dysregulated intake across repeated long access sessions.

Effect of stress on opioid-seeking behavior: evidence from studies with rats

Studies concerned with the relation between exposure to stress and the behavioral effects of opioid agonists in animal models of drug use are reviewed. These studies, which primarily utilized male rats, indicate that under certain conditions short-term mild stressors increase self-administration of opioid drugs and reinstate herein-seeking behavior following a drug-free period. On the other hand, there is evidence that long-term chronic inescapable stressors and severe acute stressors reduce the reinforcing effects of morphine as measured by a conditioned place preference procedure and decrease the behavioral effects of other positive reinforcers. The results of the studies reviewed suggest that stressors are important modulators of opioid-taking behavior, especially during drug-free periods. The implications of these findings to the understanding of the neurobiology of relapse to opioid-seeking behavior and for strategies for medication development to prevent relapse to heroin are discussed.

Correlating human and animal studies of cocaine abuse and gene expression

Gene expression changes resulting from cocaine abuse in both humans and animal models have been studied for several decades. Although human studies have been very useful at illuminating cocaine-related expression changes, there are many factors complicating these studies, including the difficulty of obtaining high-quality postmortem brain tissue and patient comorbidities. Animal models of cocaine abuse have served as valuable additions to human data and allow examination of specific aspects of cocaine abuse, including immediate early gene expression and the molecular effects of abstinence and relapse. In total, human and animal studies of cocaine abuse have uncovered gene expression changes in the brain related to a number of molecular functions, including the extracellular matrix, synaptic communication and neuroplasticity, receptors, ion channels and transporters, oligodendrocytes and myelin, apoptosis and cell death, mitochondrial function, signal transduction, and transcription factors. In addition, the mitogen-activated protein kinase and synaptic long-term potentiation signal transduction pathways are highlighted as pathways in which multiple components are altered by cocaine. Pathways and processes affected by changes in gene expression that overlap among multiple species may be promising pharmacotherapeutic targets for reducing the behavioral effects of cocaine abuse and the relapse potential observed in humans.

Place preference test in rodents

Conditioned place preference is a behavioral model currently used to measure the rewarding properties induced by the administration of a drug. In this paradigm, the rewarding properties of a compound are associated with the particular characteristics of a given environment. Due to the recent availability of multiple lines of genetically modified mice, this unit includes procedures for testing place preference in both rats and mice.

Neuroplasticity in the mesolimbic dopamine system and cocaine addiction

The main characteristics of cocaine addiction are compulsive drug use despite adverse consequences and high rates of relapse during periods of abstinence. A current popular hypothesis is that compulsive cocaine use and cocaine relapse is due to drug-induced neuroadaptations in reward-related learning and memory processes, which cause hypersensitivity to cocaine-associated cues, impulsive decision making and abnormal habit-like learned behaviours that are insensitive to adverse consequences. Here, we review results from studies on the effect of cocaine exposure on selected signalling cascades, growth factors and physiological processes previously implicated in neuroplasticity underlying normal learning and memory. These include the extracellular signal-regulated kinase (ERK) signalling pathway, brain-derived neurotrophic factor (BDNF), glutamate transmission, and synaptic plasticity (primarily in the form of long-term potentiation and depression, LTP and LTD). We also discuss the degree to which these cocaine-induced neuroplasticity changes in the mesolimbic dopamine system mediate cocaine psychomotor sensitization and cocaine-seeking behaviours, as assessed in animal models of drug addiction. Finally, we speculate on how these factors may interact to initiate and sustain cocaine psychomotor sensitization and cocaine seeking.

Self-administration of cannabinoids by experimental animals and human marijuana smokers

Drug self-administration behavior has been one of the most direct and productive approaches for studying the reinforcing effects of psychoactive drugs, which are critical in determining their abuse potential. Cannabinoids, which are usually abused by humans in the form of marijuana, have become the most frequently abused illicit class of drugs in the United States. The early elucidation of the structure and stereochemistry of delta-9-tetrahydrocannabinol (THC) in 1964, which is now recognized as the principal psychoactive ingredient in marijuana, activated cannabinoid research worldwide. This review examines advances in research on cannabinoid self-administration behavior by humans and laboratory animals. There have been numerous laboratory demonstrations of the reinforcing effects of cannabinoids in human subjects, but reliable self-administration of cannabinoids by laboratory animals has only recently been demonstrated. It has now been shown that strong and persistent self-administration behavior can be maintained in experimentally and drug-naïve squirrel monkeys by doses of THC comparable to those in marijuana smoke inhaled by humans. Furthermore, reinforcing effects of some synthetic CB1 cannabinoid agonists have been recently reported using intravenous and intracerebroventricular self-administration procedures in rats and mice. These findings support previous conclusions that THC has a pronounced abuse liability comparable to other drugs of abuse under certain experimental conditions. Self-administration of THC by squirrel monkeys provides the most reliable animal model for human marijuana abuse available to date. This animal model now makes it possible to study the relative abuse liability of other natural and synthetic cannabinoids and to preclinically assess new therapeutic strategies for the treatment or prevention of marijuana abuse in humans.

Intravenous self-administration of abused solvents and anesthetics in mice

Volatile organic solvents, fuels and anesthetics are subject to abuse. The aim of the present study was to evaluate i.v. self-administration of several of these chemicals in drug- and experiment-naive mice using a commercially available vehicle, intralipid. Two strains of mice (DBA/2 and Swiss) were allowed to self-administer toluene (0.0017-0.17 micromol/infusion), 1,1,1-trichloroethane (0.006-0.19 micromol/infusion), ethanol (0.32-1.6 micromol/infusion), cyclohexane (0.0017-0.052 micromol/infusion), propofol (0.01-0.53 micromol/infusion) and flurothyl (0.00042-0.072 micromol/infusion) or their vehicles during 30-min tests. During the test, each nose-poke of the master mouse resulted in a 1.88-microl i.v. infusion to the master mouse and a yoked control mouse. When the delivery line was loaded with a reinforcing drug solution, the number of nose-pokes of the master mice significantly exceeded that for yoked control mice. In the present experiments, significant differences in rates of nose-poking were observed between mice receiving response-contingent and response-noncontingent deliveries of ethanol and toluene in both strains of mice and of 1,1,1-trichloroethane in Swiss mice. These data suggest that the reinforcing effects of abused inhalants can be studied using i.v. self-administration procedures.

Effect of environmental stressors on opiate and psychostimulant reinforcement, reinstatement and discrimination in rats: a review

Studies in humans suggest that exposure to life stressors is correlated with compulsive drug abuse and relapse to drugs during periods of abstinence. The behavioral and neurobiological mechanisms involved in the effect of stress on drug abuse, however, are not known. Here, we review data from studies using preclinical models in rats on the effect of environmental stressors on opiate and psychostimulant reinforcement, as measured by the intravenous drug self-administration and conditioned place preference procedures, on relapse to these drugs, as measured by the reinstatement procedure, and on the subjective effects of these drugs, as measured by the drug discrimination procedure. The results of the studies reviewed here suggest that while stressors are important modulators of the behavioral effects of opiate and psychostimulant drugs, the effect of stress on behavior in these animal models is stressor-specific, and to some degree, procedure- and drug-class-specific. The review of studies on the neurobiological mechanisms underlying stress-drug interactions in these animal models indicate that central noradrenaline and extrahypothalamic corticotropin-releasing factor mediate the effect of one form of stress (intermittent footshock) on reinstatement of opiate and psychostimulant seeking after prolonged drug-free periods. At present, however, little is known about the neuronal events that mediate the effect of environmental stressors on opiate and psychostimulant reinforcement or discrimination. The broader implications of the data reviewed here for future research and for the treatment of opiate and psychostimulant addiction are briefly discussed.

Guidelines and methodological reviews concerning drug abuse liability assessment

Regulatory control of drugs with abuse liability is an important component of drug control policy and is believed to help prevent nonmedical use. To be maximally effective, this requires a scientific assessment of abuse liability of drugs considered for regulatory control. These assessments have relied extensively on laboratory-based animal and human testing, but also utilize information from clinical trials, actual abuse and other sources. Here, we discuss recommendations and guidelines that have been proposed for abuse liability assessment and describe important review papers and conference proceedings that have addressed this matter, focusing primarily on drugs with medical usefulness. Historically, there is substantial consensus about how to approach abuse liability evaluation of drugs with actions similar to those of abused opiates, stimulants, depressants, and to a somewhat lesser extent, cannabinoids and hallucinogens, and much of what has been recommended for abuse potential assessment in the past remains valid and useful. On the other hand, novel CNS-active medications which cannot be readily classified with these traditional drugs of abuse are increasingly under development. In addition, advances in the science of abuse liability assessment need to be incorporated into future guidelines and recommendations on this subject. Developers of new medications need guidance on how to utilize scientific research to maximize therapeutic benefit while minimizing risk for abuse. Thus, another goal of this review has been to identify areas where critical thinking and new guideline development are needed.

Effects of training dose on amphetamine drug discrimination: dose-response functions and generalization to cocaine

Rats were trained to discriminate one of three doses of amphetamine (AM), 0.5, 1, or 2 mg/kg, from vehicle (VEH) in a two-lever, food-reinforced, drug-discrimination task. The purpose of the study was to investigate the nature of the shift of the dose-response curve and generalization to cocaine (COC) as a function of training dose. In order to preclude potential differences among the groups in stimulus control, the three training-dose groups were required to perform the discrimination at high and equivalent levels of accuracy. The shift of the dose-response functions to the right as a function of increasing training dose was not parallel. The slope decreased as training dose increased. There was a dose-dependent increase in AM lever responding to test doses of COC that tended to be affected by training dose. The results suggest that proper evaluation of training-dose effects requires that groups be trained to equivalent levels of stimulus control.

Gamma-hydroxybutyric acid in alcohol preference, dependence and withdrawal

Gamma-hydroxybutyric acid (GHB) is an endogenous constituent of the mammalian brain, where it likely functions as a neurotransmitter or a neuromodulator. Its exogenous administration exerts a number of pharmacological effects, including reduction of intensity of alcohol withdrawal syndrome and alcohol consumption in both laboratory animals and human alcoholics.The clinical studies conducted to date, although often testing samples of limited size, feature GHB as an effective, well-tolerated and safe drug for the treatment of alcohol dependence. Behavioural data in rats suggest that GHB may produce alcohol-like effects. This similarity may explain why GHB produces positively reinforcing properties, being subsequently self-administered by rodents and sometimes abused by humans (although episodes of self-directed intake of GHB appear to be a limited phenomenon in alcoholics); in addition it provides support to the hypothesis that GHB constitutes for alcoholism a replacement therapy similar to methadone in heroin addiction.

Conditioned changes in dopamine oxidation currents in the nucleus accumbens of rats by stimuli paired with self-administration or yoked-administration of d-amphetamine

In vivo chronoamperometry was used to monitor changes in dopamine oxidation currents corresponding to dopamine efflux in the nucleus accumbens of rats after presentation of a conditioned light stimulus repeatedly paired with either yoked- or self-administered intravenous injections of the psychostimulant d-amphetamine. Daily conditioning trials began with a non-contingent drug injection, paired with a conditioned stimulus consisting of a 5 s flashing light and 30 s lights out, after which a house light was illuminated during the 3 h session, signalling drug availability. Each subsequent injection of d-amphetamine was paired with the conditioned stimulus. Electrochemical measures were taken on conditioning trials 4-7, and on each trial, intravenous d-amphetamine (0.25 mg/kg per injection) self-administration produced a significant maximal increase in mean dopamine oxidation currents of approximately 8 nA above baseline. Dopamine oxidation currents in rats receiving yoked d-amphetamine were approximately 5 nA above baseline by the fourth day of drug administration and reached approximately 8 nA on the seventh and final day of drug administration. On day 9 the first presentation of the vehicle injection and conditioned stimulus, in combination with illumination of the house lights, induced an immediate increase in nucleus accumbens dopamine oxidation currents in all rats that had previously received d-amphetamine. Subsequent presentations of the conditioned stimulus at 30 min intervals induced further increases in extracellular dopamine oxidation currents in both drug-treated groups. By the end of the 3 h session, both groups had similar maximal conditioned increases in dopamine oxidation currents of approximately 6 nA. These data are discussed with relation to the neurochemistry of drug craving.

Effects of haloperidol in a response-reinstatement model of heroin relapse

The present study employed an animal model of drug relapse in which previously extinguished heroin self-administration behavior was reinstated following a single reinforced trial. Male albino rats were trained to traverse a straight-alley for a reinforcer consisting of a single IV injection of 0.06 mg/kg diacetylmorphine (heroin). Once the alley-running had been established, the heroin reinforcer was removed and the operant behavior permitted to extinguish over trials. On treatment day, animals were injected 45 min prior to testing with 0.0, 0.075, 0.10, 0.15 or 0.3 mg/kg of the dopamine receptor antagonist, haloperidol. A single trial was then conducted during which some animals continued to experience extinction conditions while others were injected with the heroin reinforcer upon entry into the goal box. The effects of these manipulations were determined during an additional single test trial conducted 24 h later when the subjects were no longer drugged. While heroin produced a reliable reinstatement in operant responding, this effect was dose-dependently prevented by pretreatment with haloperidol. These data suggest that dopamine receptor antagonism alters the reinforcing consequences of heroin administration as measured by heroin's ability to reinstate operant behavior following a prolonged period of nonreinforced responding.

Diazepam dependence prevented by glutamate antagonists

Long-term treatment leads to tolerance to and dependence on benzodiazepines. Abrupt termination of benzodiazepine administration triggers the expression of signs of dependence. Mice withdrawn from chronic treatment with diazepam showed a time-related evolution of anxiety, muscle rigidity, and seizures between days 4 and 21 after treatment discontinuation. A period between withdrawal days 1 and 3 was symptom-free. Surprisingly, during this "silent phase" the susceptibility of mice to alpha-amino-3-hydroxy-5-tert-butyl-4-isoxazolepropionate (ATPA) and kainate seizures and the magnitude of monosynaptic reflexes mediated by non-N-methyl-D-aspartate (NMDA) mechanisms were enhanced. In apparent contrast, the "active phase", between withdrawal days 4 and 21, was characterized by increased susceptibility to NMDA seizures and enhanced magnitude of polysynaptic reflexes, which are NMDA dependent. Treatment of mice with alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) antagonists 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466) or 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(f)quinoxaline but not with the NMDA antagonist 3-[(+/-)-2-carboxypiperazin-4-yl]-propyl-1-phosphonate (CPP) during the silent phase prevented signs of dependence. In contrast, treatment with CPP but not with GYKI 52466 during the active phase prevented the symptoms. The development of tolerance to and dependence on diazepam was prevented by concurrent treatment of mice with CPP but was not prevented by GYKI 52466. These data indicate that NMDA-dependent mechanisms contribute to the development of tolerance to diazepam and to the expression of signs of dependence in mice after termination of long-term treatment with diazepam. Nevertheless, the non-NMDA-mediated silent phase is essential for triggering the symptoms. Therefore, AMPA antagonists may offer a therapeutic approach for preventing dependence on benzodiazepines that is an alternative to NMDA antagonism.

Animal models of drug craving

Drug craving, the desire to experience the effect(s) of a previously experienced psychoactive substance, has been hypothesized to contribute significantly to continued drug use and relapse after a period of abstinence in humans. In more theoretical terms, drug craving can be conceptualized within the framework of incentive motivational theories of behavior and be defined as the incentive motivation to self-administer a psychoactive substance. The incentive-motivational value of drugs is hypothesized to be determined by a continuous interaction between the hedonic rewarding properties of drugs (incentive) and the motivational state of the organism (organismic state). In drug-dependent individuals, the incentive-motivational value of drugs (i.e., drug craving) is greater compared to non-drug-dependent individuals due to the motivational state (i.e., withdrawal) developed with repeated drug administration. In this conceptual framework, animal models of drug craving would reflect two aspects of the incentive motivation to self-administer a psychoactive substance. One aspect would be the unconditioned incentive (reinforcing) value of the drug itself. The other aspect would be relatively independent of the direct (unconditioned) incentive value of the drug itself and could be reflected in the ability of previously neutral stimuli to acquire conditioned incentive properties that could elicit drug-seeking and drug-taking behavior. Animal models of drug craving that permit the investigation of the behavioral and neurobiological components of these two aspects of drug craving are reviewed and evaluated. The models reviewed are the progressive ratio, choice, extinction, conditioned reinforcement and second-order schedule paradigms. These animal models are evaluated according to two criteria that are established herein as necessary and sufficient criteria for the evaluation of animal models of human psychopathology: reliability and predictive validity. The development of animal models of drug craving will have heuristic value and allow a systematic investigation of the neurobiological mechanisms of craving.