Withdrawal from chronic amphetamine elevates baseline intracranial self-stimulation thresholds

Effects of regular cannabis and nicotine use on acute stress responses: chronic nicotine, but not cannabis use, is associated with blunted adrenocortical and cardiovascular responses to stress

RATIONALE

Cannabis is one of the most prevalent substances used by tobacco smokers and, in light of the growing list of states and territories legalizing cannabis, it is expected that co-use of cannabis and nicotine will escalate significantly and will lead to continuing challenges with tobacco use.

OBJECTIVES

This study was conducted to examine the interactive effects of chronic cannabis and nicotine use on adrenocortical, cardiovascular, and psychological responses to stress and to explore sex differences in these effects.

METHODS

Participants (N = 231) included cannabis-only users, nicotine-only users, co-users of both substances, and a non/light-user comparison group. After attending a medical screening session, participants completed a laboratory stress session during which they completed measures of subjective states, cardiovascular responses, and salivary cortisol during baseline (rest) and after exposure to acute stress challenges.

RESULTS

Nicotine use, but not cannabis use, was associated with blunted cortisol and cardiovascular responses to stress across both men and women. Men exhibited larger cortisol responses to stress than women. Co-users had significantly larger stress-related increases in cannabis craving than cannabis-only users. Cannabis users reported smaller increases in anxiety during stress than cannabis non/light-users, and both male nicotine-only users and male cannabis-only users experienced significantly smaller increases in stress than their non/light-user control counterparts.

CONCLUSIONS

This study replicates and extends earlier research on the impacts of sex and nicotine use on stress responses, and it provides novel findings suggesting that when co-used with nicotine, cannabis use may not confer additional alterations to physiological nor subjective responses to stress. Co-use, however, was associated with enhanced stress-related craving for cannabis.

Anticipation: An Essential Feature of Anhedonia

The following essay addresses the evolution of the term "anhedonia" as a key construct in biological psychiatry, especially as it pertains to positive emotional and motivational states central to mental health and well-being. In its strictest definition, anhedonia was intended to convey an inability to experience "pleasure" derived from ingestion of sweet tastes or the experience of pleasant odors and tactile sensations, among a host of positive sensations. However, this definition has proved to be too restrictive to capture the complexity of key psychological factors linked to major depression, schizophrenia, and substance use disorders it was originally intended to address. Despite the appeal of the elegant simplicity of the term anhedonia, its limitations soon became apparent when used to explain psychological constructs including aspects of learning, memory, and incentive motivation that are major determinants of success in securing the necessities of life. Accordingly, the definition of anhedonia has morphed into a much broader term that includes key roles in the disturbance of motivation in the form of anergia, impaired incentive motivation, along with deficits in associative learning and key aspects of memory, on which the ability to predict the consequences of one's actions are based. Here we argue that it is this latter capacity, namely predicting the likely consequences of motivated behavior, which can be termed "anticipation," that is especially important in the key deficits implied by the general term anhedonia in the context of neuropsychiatric conditions.

Energy sensors in drug addiction: A potential therapeutic target

Addiction is defined as the repeated exposure and compulsive seek of psychotropic drugs that, despite the harmful effects, generate relapse after the abstinence period. The psychophysiological processes associated with drug addiction (acquisition/expression, withdrawal, and relapse) imply important alterations in neurotransmission and changes in presynaptic and postsynaptic plasticity and cellular structure (neuroadaptations) in neurons of the reward circuits (dopaminergic neuronal activity) and other corticolimbic regions. These neuroadaptation mechanisms imply important changes in neuronal energy balance and protein synthesis machinery. Scientific literature links drug-induced stimulation of dopaminergic and glutamatergic pathways along with presence of neurotrophic factors with alterations in synaptic plasticity and membrane excitability driven by metabolic sensors. Here, we provide current knowledge of the role of molecular targets that constitute true metabolic/energy sensors such as AMPK, mTOR, ERK, or K_ATP in the development of the different phases of addiction standing out the main brain regions (ventral tegmental area, nucleus accumbens, hippocampus, and amygdala) constituting the hubs in the development of addiction. Because the available treatments show very limited effectiveness, evaluating the drug efficacy of AMPK and mTOR specific modulators opens up the possibility of testing novel pharmacotherapies for an individualized approach in drug abuse.

Impacts of Different Anesthetic Agents on Left Ventricular Systolic Function in Mice Assessed by Echocardiography

The present experiments were performed to study the effects and time trends of different anesthetic agents on the left ventricular (LV) systolic function and heart rate by high-resolution echocardiography in mice. Ten male C57BL/6J mice were submitted to echocardiography imaging separated by 72-hour intervals under the following conditions: 1) conscious mice, 2) mice anesthetized with isoflurane (ISO, inhaled), 3) mice anesthetized with tribromoethanol (TBE, intraperitoneal), 4) mice anesthetized with chloral hydrate (CH, intraperitoneal), and 5) mice anesthetized with pentobarbital sodium (PS, intraperitoneal). The effect of ISO, TBE, CH, and PS on LV systolic function was measured at 0, 1, 2, 3, 4, 6, 8, and 10 min after anesthesia. The results showed that LV systolic function and heart rate (HR) of anesthetized mice were reduced significantly (P<0.05), compared with results in the same mice studied in the conscious state. In addition, the results indicated that the anesthetic with the least effect on LV function was CH, and followed by TBE, PS, ISO. We conclude that different anesthetic agents always depressed the HR and LV systolic function of mice, and, furthermore, the effects and time trends of different anesthetics on LV function are different. In echocardiographic experiments, we should choose proper anesthetic agents according to the experimental requirements.

Bipolar Disorder: Its Etiology and How to Model in Rodents

Characterized by the switch of manic and depressive phases, bipolar disorder was described as early as the fifth century BC. Nevertheless up to date, the underlying neurobiology is still largely unclear, assuming a multifactor genesis with both biological-genetic and psychosocial factors. Significant process has been achieved in recent years in researching the causes of bipolar disorder with modern molecular biological (e.g., genetic and epigenetic studies) and imaging techniques (e.g., positron emission tomography (PET) and functional magnetic resonance imaging (fMRI)). In this chapter we will first summarize our recent knowledge on the etiology of bipolar disorder. We then discuss how several factors observed to contribute to bipolar disorder in human patients can be manipulated to generate rodent models for bipolar disorder. Finally, we will give an overview on behavioral test that can be used to assess bipolar-disorder-like behavior in rodents.

Dopamine and addiction: what have we learned from 40 years of research

Among the neurotransmitters involved in addiction, dopamine (DA) is clearly the best known. The critical role of DA in addiction is supported by converging evidence that has been accumulated in the last 40 years. In the present review, first we describe the dopaminergic system in terms of connectivity, functioning and involvement in reward processes. Second, we describe the functional, structural, and molecular changes induced by drugs within the DA system in terms of neuronal activity, synaptic plasticity and transcriptional and molecular adaptations. Third, we describe how genetic mouse models have helped characterizing the role of DA in addiction. Fourth, we describe the involvement of the DA system in the vulnerability to addiction and the interesting case of addiction DA replacement therapy in Parkinson's disease. Finally, we describe how the DA system has been targeted to treat patients suffering from addiction and the result obtained in clinical settings and we discuss how these different lines of evidence have been instrumental in shaping our understanding of the physiopathology of drug addiction.

Animal models for bipolar disorder: from bedside to the cage

Bipolar disorder is characterized by recurrent manic and depressive episodes. Patients suffering from this disorder experience dramatic mood swings with a wide variety of typical behavioral facets, affecting overall activity, energy, sexual behavior, sense of self, self-esteem, circadian rhythm, cognition, and increased risk for suicide. Effective treatment options are limited and diagnosis can be complicated. To overcome these obstacles, a better understanding of the neurobiology underlying bipolar disorder is needed. Animal models can be useful tools in understanding brain mechanisms associated with certain behavior. The following review discusses several pathological aspects of humans suffering from bipolar disorder and compares these findings with insights obtained from several animal models mimicking diverse facets of its symptomatology. Various sections of the review concentrate on specific topics that are relevant in human patients, namely circadian rhythms, neurotransmitters, focusing on the dopaminergic system, stressful environment, and the immune system. We then explain how these areas have been manipulated to create animal models for the disorder. Even though several approaches have been conducted, there is still a lack of adequate animal models for bipolar disorder. Specifically, most animal models mimic only mania or depression and only a few include the cyclical nature of the human condition. Future studies could therefore focus on modeling both episodes in the same animal model to also have the possibility to investigate the switch from mania-like behavior to depressive-like behavior and vice versa. The use of viral tools and a focus on circadian rhythms and the immune system might make the creation of such animal models possible.

Cocaine-induced reward enhancement measured with intracranial self-stimulation in rats bred for low versus high saccharin intake

Rats selectively bred for high (HiS) or low (LoS) saccharin intake are a well-established model of drug-abuse vulnerability, with HiS rats being more likely to consume sweets and cocaine than LoS rats. Still, the nature of these differences is poorly understood. This study examined whether the motivational consequences of cocaine exposure are differentially expressed in HiS and LoS rats by measuring intracranial self-stimulation (ICSS) thresholds following acute injections of cocaine (10 mg/kg). Reductions in ICSS thresholds following cocaine injection were greater in HiS rats than in LoS rats, suggesting that the reward-enhancing effects of cocaine are greater in the drug-vulnerable HiS than LoS rats. Higher cocaine-induced reward, indicated by lower ICSS thresholds, may explain the higher rates of drug consumption in sweet-preferring animal models, providing a clue to the etiology of cocaine addiction in vulnerable populations.

Chronic treatment with the vasopressin 1b receptor antagonist SSR149415 prevents the dysphoria associated with nicotine withdrawal in rats

Nicotine addiction is a chronic brain disorder that is characterized by dysphoria upon smoking cessation and relapse after brief periods of abstinence. It has been hypothesized that the negative mood state associated with nicotine withdrawal is partly mediated by a heightened activity of brain stress systems. Animal studies suggest that blockade of vasopressin 1b (V1b) receptors diminishes high levels of drug intake in dependent animals and attenuates the emotional response to stressors. The goal of the present studies was to investigate the effect of acute and chronic treatment with the V1b receptor antagonist SSR149415 on the negative mood state associated with nicotine withdrawal in rats. An intracranial self-stimulation (ICSS) procedure was used to assess mood states and nicotine dependence was induced using minipumps. The nicotinic receptor antagonist mecamylamine was used to precipitate withdrawal. Mecamylamine elevated the brain reward thresholds of the nicotine dependent rats, which reflects a negative mood state. Mecamylamine did not affect the brain reward thresholds of the saline-treated control rats. Chronic treatment with SSR149415 completely prevented the elevations in brain reward thresholds associated with nicotine withdrawal while acute treatment only partly prevented nicotine withdrawal. These data suggest that chronic treatment with V1b receptor antagonists may prevent the dysphoria associated with smoking cessation and thereby improve relapse rates.

A critical role for the melanocortin 4 receptor in stress-induced relapse to nicotine seeking in rats

Tobacco addiction is characterized by a lack of control over smoking and relapse after periods of abstinence. Smoking cessation leads to a dysphoric state that contributes to relapse to smoking. After the acute withdrawal phase, exposure to stressors increases the risk for relapse. Blockade of melanocortin 4 (MC4 ) receptors has anxiolytic and antidepressant-like effects in animal models. The aim of these studies was to investigate the role of MC4 receptors in the dysphoria associated with nicotine withdrawal and stress-induced reinstatement of nicotine seeking. To study stress-induced reinstatement, rats self-administered nicotine for 16 days and then nicotine seeking was extinguished by substituting saline for nicotine. Nicotine seeking was reinstated by intermittent footshock stress. The intracranial self-stimulation (ICSS) procedure was used to assess the negative mood state associated with nicotine withdrawal. Elevations in the ICSS thresholds are indicative of a dysphoric state. The selective MC4 receptor antagonists HS014 and HS024 prevented stress-induced reinstatement of extinguished nicotine seeking. Drug doses that prevented stress-induced relapse did not affect responding for food pellets, which indicates that the drugs did not induce sedation or motor impairments. In the ICSS experiments, the nicotinic acetylcholine receptor antagonist mecamylamine elevated the ICSS thresholds of the nicotine-dependent rats. Pre-treatment with HS014 or HS024 did not prevent the elevations in ICSS thresholds. These studies indicate that MC4 receptors play a critical role in stress-induced reinstatement of nicotine seeking, but these receptors may not play a role in the dysphoria associated with acute nicotine withdrawal.

Anhedonia in substance use disorders: a systematic review of its nature, course and clinical correlates

OBJECTIVE

There is growing evidence that anhedonia is a commonly experienced symptom among substance-using populations. This systematic review synthesises findings across a range of substances to address questions regarding the time course of anhedonia, how anhedonia relates to other symptoms of substance dependence and whether it is similarly prevalent across all addictive drugs.

METHOD

A literature search was conducted on PubMed, PsycINFO and MEDLINE, yielding 32 studies that used self-report measures of anhedonia among participants with a history of a substance abuse, dependence or long-term daily use of addictive substances.

RESULTS

Findings from these studies indicate that anhedonia (1) is elevated in samples dependent on a range of substances; (2) typically appears as a consequence of substance abuse or dependence, and diminishes with abstinence; and (3) predicts increased drug cravings and the likelihood of relapse in those attempting abstinence.

CONCLUSIONS

The common experience of anhedonia in substance-dependent populations, and its relationship to relapse, emphasises the importance of developing therapeutic interventions that specifically target anhedonia in the treatment of all substance use disorders.

Use of intracranial self-stimulation to evaluate abuse-related and abuse-limiting effects of monoamine releasers in rats

BACKGROUND AND PURPOSE

Monoamine releasers constitute a class of drugs that promote the release of dopamine (DA), serotonin (5-HT) and/or norepinephrine. Although some drugs in this class are well-known drugs of abuse (amphetamine, methamphetamine), others are thought to have reduced (3,4-methylenedioxy-N-methylamphetamine [MDMA]) or no (fenfluramine) abuse potential. The purpose of this study was to further elucidate the role of dopamine versus serotonin selectivity on expression of abuse-related effects produced by monoamine releasers in an assay of intracranial self-stimulation (ICSS) in rats.

EXPERIMENTAL APPROACH

This study evaluated effects produced in a frequency-rate ICSS procedure by 11 monoamine releasers that vary in selectivity to release DA versus 5-HT.

KEY RESULTS

Efficacy of monoamine releasers to facilitate ICSS correlated with DA-selectivity, such that DA-selective releasers exclusively facilitated ICSS, a 5-HT-selective releaser exclusively depressed ICSS, and mixed-action releasers both facilitated low ICSS rates and depressed high ICSS rates. Fixed-proportion mixtures of a DA-selective releaser and a 5-HT-selective releaser recapitulated effects of mixed-action releasers. Efficacy of monoamine releasers to facilitate ICSS also correlated with previously published data on efficacy to maintain self-administration in rhesus monkeys responding under a progressive-ratio schedule of reinforcement.

CONCLUSIONS AND IMPLICATIONS

These data support the importance of selectivity for DA versus 5-HT in determining abuse potential of monoamine releasers and demonstrate a novel correlation between rat ICSS and nonhuman primate self-administration measures of abuse-related effects. Taken together, these results support the use of ICSS in rats as an experimental tool to study the expression and pharmacological determinants of abuse-related effects of monoamine releasers.

Stress response dysregulation and stress-induced analgesia in nicotine dependent men and women

Alterations in the stress response and endogenous pain regulation mechanisms may contribute directly and indirectly to maintenance of nicotine dependence and relapse. We examined the extent to which nicotine dependence alters endogenous pain regulatory systems, including the hypothalamic-pituitary-adrenocortical axis, cardiovascular activity, and stress-induced analgesia. Smokers and nonsmokers attended a laboratory session that included assessment of hormonal and cardiovascular responses to stress. Smokers smoked at their regular rate prior to the session. The hand cold pressor and heat thermal pain tests were completed twice, once after acute stress (public speaking and math tasks) and the other after rest. While smokers and nonsmokers exhibited significant hormonal and cardiovascular responses to stress, smokers exhibited blunted stress responses relative to nonsmokers. They also exhibited diminished stress-induced analgesia. Results demonstrate altered stress response and diminished stress-induced analgesia among chronic smokers, and suggest that these dysregulated physiological responding may contribute to altered endogenous pain regulation.

Blockade of CRF1 receptors in the central nucleus of the amygdala attenuates the dysphoria associated with nicotine withdrawal in rats

The majority of smokers relapse during the acute withdrawal phase when withdrawal symptoms are most severe. The goal of the present studies was to investigate the role of corticotropin-releasing factor (CRF) and noradrenergic transmission in the central nucleus of the amygdala (CeA) in the dysphoria associated with smoking cessation. It was investigated if blockade of CRF1 receptors, blockade of α1-adrenergic receptors, or stimulation of α2-adrenergic receptors in the CeA diminishes the deficit in brain reward function associated with nicotine withdrawal in rats. Nicotine dependence was induced by implanting minipumps that delivered a nicotine solution. Withdrawal was precipitated with the nicotinic acetylcholine receptor antagonist mecamylamine. A discrete-trial intracranial self-stimulation procedure was used to assess the negative affective aspects of nicotine withdrawal. Elevations in brain reward thresholds are indicative of a deficit in brain reward function. In all the experiments, mecamylamine elevated the brain reward thresholds of the rats chronically treated with nicotine and did not affect the brain reward thresholds of the saline-treated control rats. Intra-CeA administration of the CRF1 receptor antagonist R278995/CRA0450 completely prevented the mecamylamine-induced elevations in brain reward thresholds in the nicotine-treated rats and did not affect the brain reward thresholds of the saline-treated control rats. R278995/CRA0450 has also been shown to block sigma-1 receptors but there is no evidence that this could affect negative mood states. Intra-CeA administration of the α1-adrenergic receptor antagonist prazosin or the α2-adrenergic receptor agonist clonidine did not affect the brain reward thresholds of the nicotine or saline-treated rats. These studies suggest that CRF1 receptor antagonists may diminish the dysphoria associated with smoking cessation by blocking CRF1 receptors in the CeA.

Natural rewards, neuroplasticity, and non-drug addictions

There is a high degree of overlap between brain regions involved in processing natural rewards and drugs of abuse. "Non-drug" or "behavioral" addictions have become increasingly documented in the clinic, and pathologies include compulsive activities such as shopping, eating, exercising, sexual behavior, and gambling. Like drug addiction, non-drug addictions manifest in symptoms including craving, impaired control over the behavior, tolerance, withdrawal, and high rates of relapse. These alterations in behavior suggest that plasticity may be occurring in brain regions associated with drug addiction. In this review, I summarize data demonstrating that exposure to non-drug rewards can alter neural plasticity in regions of the brain that are affected by drugs of abuse. Research suggests that there are several similarities between neuroplasticity induced by natural and drug rewards and that, depending on the reward, repeated exposure to natural rewards might induce neuroplasticity that either promotes or counteracts addictive behavior.

Effects of prazosin, clonidine, and propranolol on the elevations in brain reward thresholds and somatic signs associated with nicotine withdrawal in rats

RATIONALE

Tobacco withdrawal is characterized by a negative mood state and relatively mild somatic symptoms. Increased noradrenergic transmission has been reported to play an important role in opioid withdrawal, but little is known about the role of noradrenergic transmission in nicotine withdrawal.

OBJECTIVES

The aim of these experiments was to investigate the effects of prazosin, clonidine, and propranolol on the negative mood state and somatic signs associated with nicotine withdrawal in rats.

METHODS

A discrete-trial intracranial self-stimulation procedure was used to assess the negative affective state of nicotine withdrawal. Elevations in brain reward thresholds are indicative of a deficit in brain reward function.

RESULTS

In all the experiments, the nicotinic acetylcholine receptor antagonist mecamylamine (3 mg/kg) elevated the brain reward thresholds of the nicotine-treated rats and did not affect those of the control rats. The α1-adrenergic receptor antagonist prazosin (0.0625 and 0.125 mg/kg) dose-dependently attenuated the elevations in brain reward thresholds associated with precipitated nicotine withdrawal. The α2-adrenergic receptor agonist clonidine (10-40 μg/kg) and the nonselective β-adrenergic receptor antagonist propranolol (2.5-10 mg/kg) did not attenuate the elevations in brain reward thresholds associated with nicotine withdrawal. Furthermore, mecamylamine (2 mg/kg) induced more somatic signs in the nicotine-treated rats than in the control rats. Clonidine and propranolol, but not prazosin, decreased the total number of somatic signs associated with nicotine withdrawal.

CONCLUSION

Blockade of α1-adrenergic receptors attenuates the deficit in brain reward function associated with nicotine withdrawal. Antagonism of β-adrenergic receptors or stimulation of α2-adrenergic receptors attenuates the somatic symptoms of nicotine withdrawal.

The pursuit of value: sensitization or tolerance?

Two issues are raised. (1) What is the nature of the drug effect Heyman thinks confers value to drugs? (2) What is the evidence that drug use decreases the value of drugs and of conventional incentives over the long-term? There is considerable evidence for the opposite; a persistent increase in the sensitivity of neural systems that mediate drug value.

Future directions for the melioration model of addiction

For use in applied settings, the melioration model will need to incorporate changes in the shapes of local value functions over time, treat current value as a continuous function of time since previous choices, and take into account discounting of the effects of current behavior on future value. The policy implications of the model for regulating drugs are limited.

Behavioral choice theory can enhance our understanding of drug dependence and other behavioral disorders

I support the major theme of Heyman's target article that behavioral choice theory can enhance our understanding of drug dependence, but I raise concerns about the critique of the operant model of drug dependence, the underscoring of melioration to the exclusion of other theories of choice, and assertions about the unique effects of drug reinforcement.

Stimulus factors in addiction

Heyman's analysis of addiction in terms of matching to local relative value can be supplemented by stimulus-control processes. Stimulus equivalence can broaden the set of situations that occasion addictive behavior, and the situation-reinforcer correlation can enhance its persistence. The joint effects of stimulus-control and reinforcement processes may complicate treatment.

Understanding addiction: Conventional rewards and lack of control

The conflict between drug and conventional rewards leads to a paradox: Sanctions against drug use decrease access to conventional rewards and push drug users toward drug abuse, whereas increased access to the rewards of family, friends, and work may help reduce drug abuse. Lack of control is not specific to drug addiction and is unlikely to yield to a shift in bookkeeping.

Who determines the value of drug-taking behavior? Cultural considerations for a theory of behavioral choice

Heyman's analysis of addiction suggests that drug taking is irrational. The irrationality of drug taking, however, may depend on the acceptance of mainstream society's view of what is valuable. Consideration of the addict's viewpoint and cultural aspects of drug taking may be useful in trying to fathom the “rationality” of drug taking.

An economic perspective on addiction and matching

Economic models of addiction are choice-based. These models push the choice paradigm too far by modeling addiction as rational, normative behavior. Heyman's target article provides a sensible alternative to this economic approach by emphasizing that addiction is characterized by ambivalence and a perceived loss of self control. However, matching may not be a satisfactory platform on which to build this alternative model. Matching experiments do not provide evidence of ambivalence or perceived loss of self control.

Resolving the contradictions of addiction

Research findings on addiction are contradictory. According to biographical records and widely used diagnostic manuals, addicts use drugs compulsively, meaning that drug use is out of control and independent of its aversive consequences. This account is supported by studies that show significant heritabilities for alcoholism and other addictions and by laboratory experiments in which repeated administration of addictive drugs caused changes in neural substrates associated with reward. Epidemiological and experimental data, however, show that the consequences of drug consumption can significantly modify drug intake in addicts. The disease model can account for the compulsive features of addiction, but not occasions in which price and punishment reduced drug consumption in addicts. Conversely, learning models of addiction can account for the influence of price and punishment, but not compulsive drug taking. The occasion for this target article is that recent developments in behavioral choice theory resolve the apparent contradictions in the addiction literature. The basic argument includes the following four statements: First, repeated consumption of an addictive drug decreases its future value and the future value of competing activities. Second, the frequency of an activity is a function of its relative (not absolute) value. This implies that an activity that reduces the values of competing behaviors can increase in frequency even if its own value also declines. Third, a recent experiment (Heyman & Tanz 1995) shows that the effective reinforcement contingencies are relative to a frame of reference, and this frame of reference can change so as to favor optimal or suboptimal choice. Fourth, if the frame of reference is local, reinforcement contingencies will favor excessive drug use, but if the frame of reference is global, the reinforcement contingencies will favor controlled drug use. The transition from a global to a local frame of reference explains relapse and other compulsive features of addiction.

In search of the relevant behavioral variables

Heyman s analysis of the relevant complex behavioral variables associated with addiction is evaluated in relation to identifying the appropriate variables in behavior analysis. The model of behavioral allocation and choice known as melioration, discussed by Heyman as a way to understand the complexities of addiction, is examined and contrasted with another model of matching called ratio invariance, which is offered in this commentary as another behavioral account with a significant potential for resolving the contradictions of addiction.

Positive reinforcement, the matching law and morality

Addictive behavior has never seemed rational because it persists in spite of drastic aversive consequences. This is a particular problem for models of addiction such as operant psychology which hold that behavior is controlled by its consequences. Inspite of claims to the contrary, Heymans target article illustrates how operant psychology resolves this contradiction. By using the matching law, Heyman suggests a mechanism that explains why delayed aversive events may not control behavior, and a conceptual framework in which we can understand successful therapies.

Melioration and addiction

I discuss various theoretical issues concerning maximizing, matching, and melioration. The model of addiction based on melioration has the key feature that a reduction in drag use increases both the value of drag-taking and the value of conventional activities. I found Heyman's target article stimulating. I don't feel competent to comment on drags but I do have some thoughts, both general and specific, on the theoretical issues.

Is melioration the addiction theory of choice?

Heyman makes a convincing case that a melioration choice strategy is sufficient to produce addictive behavior. But given a plethora of addiction theories, the question is whether melioration theory is superior to rivals more sophisticated than a simple disease model or operant conditioning account. Heyman offers little direct evidence that melioration actually causes the addictions we observe.

A scientific fix for the classical account of addiction

Heyman's two crucial theses are that people over-value immediate rewards, and that addictive substances “subvert the value of competing commodities.” These perennial ideas were discussed by Plato. Whereas Heyman provides scientific clarification and support for the first, the second remains problematic. I outline how this deficiency might be remedied via evolutionary considerations.

Control versus causation of addiction

Heyman explains useful ways to bring addictive drug use under environmental control. We doubt that relapse is explained by drug features such as immediate reinforcement, clouding of judgment, and so forth. Relapse may require explanation in terms of enduring sensitization of incentive neural substrates, but even if its causal assumptions are wrong, Heyman's model makes useful predictions for behavioral control.

Self-control and impulsiveness: Resolution of apparent contradictions in choice behavior

Choosing the smaller, less delayed reinforcers of drug abuse (including cessation of withdrawal) over the larger, more delayed reinforcers of a healthy life can be described as impulsiveness (the opposite of self-control). Examination of drug abuse using a self-control/impulsiveness framework can help explain many of the puzzles about this behavior, while tying drug abuse into a large research database.

Who is at risk for addiction?

Numerous individual differences determine the effectiveness of the multiple external reinforcement contingencies that control drug seeking behavior. Differences in the capacity to frame reinforcement and also the toxic effects of drugs modulate susceptibility to addiction. As a result, not all individuals are at the same degree of risk for addiction.

Tobacco smoke exposure induces nicotine dependence in rats

RATIONALE

Tobacco smoke contains nicotine and many other compounds that act in concert on the brain reward system. Therefore, animal models are needed that allow the investigation of chronic exposure to the full spectrum of tobacco smoke constituents.

OBJECTIVES

The aim of these studies was to investigate if exposure to tobacco smoke leads to nicotine dependence in rats.

METHODS

The intracranial self-stimulation procedure was used to assess the negative affective aspects of nicotine withdrawal. Somatic signs were recorded from a checklist of nicotine abstinence signs. Nicotine self-administration sessions were conducted to investigate if tobacco smoke exposure affects the motivation to self-administer nicotine. Nicotinic receptor autoradiography was used to investigate if exposure to tobacco smoke affects central alpha7 nicotinic acetylcholine receptor (nAChR) and non-alpha7 nAChR levels (primarily alpha4beta2 nAChRs).

RESULTS

The nAChR antagonist mecamylamine dose-dependently elevated the brain reward thresholds of the rats exposed to tobacco smoke and did not affect the brain reward thresholds of the untreated control rats. Furthermore, mecamylamine induced more somatic withdrawal signs in the smoke-exposed rats than in the control rats. Nicotine self-administration was decreased 1 day after the last tobacco smoke exposure sessions and was returned to control levels 5 days later. Tobacco smoke exposure increased the alpha7 nAChR density in the CA2/3 area and the stratum oriens and increased the non-alpha7 nAChR density in the dentate gyrus.

CONCLUSION

Tobacco smoke exposure leads to nicotine dependence as indicated by precipitated affective and somatic withdrawal signs and induces an upregulation of nAChRs in the hippocampus.

Neural substrates of psychostimulant withdrawal-induced anhedonia

Psychostimulant drugs have powerful reinforcing and hedonic properties and are frequently abused. Cessation of psychostimulant administration results in a withdrawal syndrome characterized by anhedonia (i.e., an inability to experience pleasure). In humans, psychostimulant withdrawal-induced anhedonia can be debilitating and has been hypothesized to play an important role in relapse to drug use. Hence, understanding the neural substrates involved in psychostimulant withdrawal-induced anhedonia is essential. In this review, we first summarize the theoretical perspectives of psychostimulant withdrawal-induced anhedonia. Experimental procedures and measures used to assess anhedonia in experimental animals are also discussed. The review then focuses on neural substrates hypothesized to play an important role in anhedonia experienced after termination of psychostimulant administration, such as with cocaine, amphetamine-like drugs, and nicotine. Both neural substrates that have been extensively investigated and some that need further evaluation with respect to psychostimulant withdrawal-induced anhedonia are reviewed. In the context of reviewing the various neurosubstrates of psychostimulant withdrawal, we also discuss pharmacological medications that have been used to treat psychostimulant withdrawal in humans. This literature review indicates that great progress has been made in understanding the neural substrates of anhedonia associated with psychostimulant withdrawal. These advances in our understanding of the neurobiology of anhedonia may also shed light on the neurobiology of nondrug-induced anhedonia, such as that seen as a core symptom of depression and a negative symptom of schizophrenia.

Corticotropin-releasing factor-1 receptor activation mediates nicotine withdrawal-induced deficit in brain reward function and stress-induced relapse

BACKGROUND

Tobacco addiction is a chronic brain disorder that is characterized by a negative affective state upon smoking cessation and relapse after periods of abstinence. Previous research has shown that blockade of corticotropin-releasing factor (CRF) receptors with a nonspecific CRF1/CRF2 receptor antagonist prevents the deficit in brain reward function associated with nicotine withdrawal and stress-induced reinstatement of extinguished nicotine-seeking in rats. The aim of these studies was to investigate the role of CRF1 and CRF2 receptors in the deficit in brain reward function associated with precipitated nicotine withdrawal and stress-induced reinstatement of nicotine-seeking.

METHODS

The intracranial self-stimulation (ICSS) procedure was used to assess the negative affective state of nicotine withdrawal. Elevations in brain reward thresholds are indicative of a deficit in brain reward function. Stress-induced reinstatement of nicotine-seeking was investigated in animals in which responding for intravenously infused nicotine was extinguished by substituting saline for nicotine.

RESULTS

In the ICSS experiments, the nicotinic receptor antagonist mecamylamine elevated the brain reward thresholds of the nicotine-dependent rats but not those of the control rats. The CRF1 receptor antagonist R278995/CRA0450 but not the CRF2 receptor antagonist astressin-2B prevented the elevations in brain reward thresholds associated with precipitated nicotine withdrawal. Furthermore, R278995/CRA0450 but not astressin-2B prevented stress-induced reinstatement of extinguished nicotine-seeking. Neither R278995/CRA0450 nor astressin-2B affected operant responding for chocolate-flavored food pellets.

CONCLUSIONS

These studies indicate that CRF(1) receptors but not CRF(2) receptors play an important role in the anhedonic-state associated with acute nicotine withdrawal and stress-induced reinstatement of nicotine-seeking.

Corticotropin-releasing factor within the central nucleus of the amygdala and the nucleus accumbens shell mediates the negative affective state of nicotine withdrawal in rats

Tobacco addiction is a chronic disorder that is characterized by a negative affective state upon smoking cessation and relapse after periods of abstinence. Previous research has shown that an increased central release of corticotropin-releasing factor (CRF) at least partly mediates the deficit in brain reward function associated with nicotine withdrawal in rats. The aim of these studies was to investigate the role of CRF in the central nucleus of the amygdala (CeA), the lateral bed nucleus of the stria terminalis (BNST), and the nucleus accumbens shell (Nacc shell) in the deficit in brain reward function associated with precipitated nicotine withdrawal. The intracranial self-stimulation procedure was used to assess the negative affective aspects of nicotine withdrawal. Elevations in brain reward thresholds are indicative of a deficit in brain reward function. In all experiments, the nicotinic receptor antagonist mecamylamine (3 mg/kg) elevated the brain reward thresholds of the nicotine-dependent rats (9 mg/kg per day of nicotine salt) and did not affect the brain reward thresholds of the saline-treated control rats. The administration of the nonspecific CRF1/2 receptor antagonist D-Phe CRF((12-41)) into the CeA and the Nacc shell prevented the mecamylamine-induced elevations in brain reward thresholds in the nicotine-dependent rats. Blockade of CRF1/2 receptors in the lateral BNST did not prevent the mecamylamine-induced elevations in brain reward thresholds in the nicotine-dependent rats. These studies indicate that the negative emotional state associated with precipitated nicotine withdrawal is at least partly mediated by an increased release of CRF in the CeA and the Nacc shell.

Deficit in brain reward function and acute and protracted anxiety-like behavior after discontinuation of a chronic alcohol liquid diet in rats

RATIONALE

Discontinuation of chronic and excessive alcohol consumption leads to a dysphoric state in humans. It is not known if there are changes in brain reward function after the discontinuation of an alcohol liquid in rats.

OBJECTIVES

The aim of these studies was to investigate the effect of withdrawal from an alcohol liquid diet on brain reward function and acute and protracted anxiety-like behavior.

MATERIALS AND METHODS

The intracranial self-stimulation procedure was used to assess brain reward function, and the elevated plus maze test was used to assess anxiety-like behavior.

RESULTS

Discontinuation of chronic, 12 weeks, exposure to a 6.2% v/v alcohol liquid diet lead to a minor deficit in brain reward function and did not increase anxiety-like behavior. Discontinuation of chronic, 12 weeks, exposure to a 10% v/v alcohol liquid diet lead to a pronounced deficit in brain reward function and increased anxiety-like behavior. Two weeks after discontinuation of the 10% v/v alcohol liquid diet, the rats with a history of alcohol dependence did not display increased anxiety-like behavior. Restraint stress increased anxiety-like behavior in the rats with a history of alcohol dependence, but not in the control rats. Brain reward thresholds were assessed during the chronic 10% v/v alcohol exposure period. During this period, there were no differences between the brain rewards thresholds of the alcohol and control rats.

CONCLUSION

These findings indicate that withdrawal from a 10% v/v alcohol liquid diet leads to a pronounced deficit in brain reward function and acute and protracted anxiety-like behavior in rats.

Effects of mood stabilizers on brain reward processes in rats: studies using the intracranial self-stimulation paradigm

Bipolar disorder is characterized by dysregulated motivation and increased hedonistic drive. d-Amphetamine induces manic symptoms in humans and exacerbates mania in bipolar disorder patients, effects that are counteracted by mood stabilizers. We utilized intracranial self-stimulation (ICSS) to examine how lithium (LiCl), valproate (VPA) or their combination that is commonly used in the clinic affect brain reward function in rats, and how these drugs affect d-amphetamine's reward-facilitating effects. Acute intraperitoneal (i.p.) administration of LiCl (100, 200 mg/kg), VPA (400 mg/kg) or combined administration of subthreshold doses of LiCl (50 mg/kg) and VPA (200 mg/kg) increased ICSS thresholds. LiCl (100 mg/kg) and combined administration of LiCl and VPA (50 and 200 mg/kg), but not VPA alone (200, 400 mg/kg), attenuated d-amphetamine's reward-facilitating effects. These results suggest that ICSS combined with d-amphetamine constitutes a useful model to explore the elation and increased hedonistic drive observed in bipolar patients and ultimately help to identify novel pharmacotherapies for bipolar disorder.

Effects of NPY and the specific Y1 receptor agonist [D-His(26)]-NPY on the deficit in brain reward function and somatic signs associated with nicotine withdrawal in rats

Tobacco addiction is a chronic disorder that is characterized by dysphoria upon smoking cessation and relapse after periods of abstinence. Previous research suggests that Neuropeptide Y (NPY) and Y1 receptor agonists attenuate negative affective states and somatic withdrawal signs. The aim of the present experiments was to investigate the effects of NPY and the specific Y1 receptor agonist [D-His(26)]-NPY on the deficit in brain reward function and somatic signs associated with nicotine withdrawal in rats. The intracranial self-stimulation procedure was used to assess the effects of nicotine withdrawal on brain reward function as this procedure can provide a quantitative measure of emotional states in rodents. Elevations in brain reward thresholds are indicative of a deficit in brain reward function. In the first experiment, NPY did not prevent the elevations in brain reward thresholds associated with precipitated nicotine withdrawal and elevated the brain reward thresholds of the saline-treated control rats. Similar to NPY, [D-His(26)]-NPY did not prevent the elevations in brain reward thresholds associated with precipitated nicotine withdrawal and elevated the brain reward thresholds of the saline-treated control rats. Neither NPY nor [D-His(26)]-NPY affected the response latencies. In a separate experiment, it was demonstrated that the specific Y1 receptor antagonist BIBP-3226 prevented the NPY-induced elevations in brain reward thresholds. NPY attenuated the overall somatic signs associated with precipitated nicotine withdrawal. [D-His(26)]-NPY did not affect the overall somatic signs associated with precipitated nicotine withdrawal, but decreased the number of abdominal constrictions. Both NPY and [D-His(26)]-NPY attenuated the overall somatic signs associated with spontaneous nicotine withdrawal. These findings indicate that NPY and [D-His(26)]-NPY attenuate somatic nicotine withdrawal signs, but do not prevent the deficit in brain reward function associated with precipitated nicotine withdrawal. In addition, NPY decreases the sensitivity to rewarding electrical stimuli via an Y1 dependent mechanism.

Exposure to nicotine and sensitization of nicotine-induced behaviors

Evidence for an important link between sensitization of midbrain dopamine (DA) neuron reactivity and enhanced self-administration of amphetamine and cocaine has been reported. To the extent that exposure to nicotine also sensitizes nucleus accumbens DA reactivity, it is likely that it will also impact subsequent drug taking. It is thus necessary to gain an understanding of the long-term effects of exposure to nicotine on nicotinic acetylcholine receptors (nAChRs), neuronal excitability and behavior. A review of the literature is presented in which different regimens of nicotine exposure are assessed for their effects on upregulation of nAChRs, induction of LTP in interconnected midbrain nuclei and development of long-lasting locomotor and DA sensitization. Exposure to nicotine upregulates nAChRs and nAChR currents and produces LTP of excitatory inputs to midbrain DA neurons. These effects appear in the hours to days following exposure. Exposure to nicotine also leads to long-lasting sensitization of nicotine's nucleus accumbens DA and locomotor activating effects. These effects appear days to weeks after drug exposure. A model is proposed in which nicotine exposure regimens that produce transient nAChR upregulation and LTP consequently produce long-lasting sensitization of midbrain DA neuron reactivity and nicotine-induced behaviors. These neuroadaptations are proposed to constitute critical components of the mechanisms underlying the initiation, maintenance and escalation of drug use.

The amphetamine-induced sensitized state as a model of schizophrenia

Schizophrenia is a serious psychiatric disorder which impacts a broad range of cognitive, behavioural and emotional domains. In animals, exposure to an intermittent, escalating dose regimen of amphetamine induces a sensitized state that appears to share a number of behavioural and neurochemical similarities with schizophrenia. In humans repeated exposure to amphetamine, or other psychomotor stimulants, can induce sensitization as well as psychosis. The following paper evaluates the evidence for the amphetamine-induced sensitized state as an animal model of schizophrenia, focussing separately on the positive, cognitive and negative symptoms associated with this disease. Current evidence supports the use of amphetamine sensitization as a model of the positive symptoms observed in schizophrenia. Additionally, there is increasing evidence for long-lasting cognitive deficits in sensitized animals, especially in the area of attention and/or cognitive flexibility. Other areas of cognition, such as long-term memory, appear to be unaltered in sensitized animals. Finally, little evidence currently exists to either support or refute the use of amphetamine sensitization as a model of negative symptoms. It is concluded that amphetamine sensitization likely impacts behaviour by altering the functioning of mesolimbic dopamine systems and prefrontal cortical function and can serve as a model of certain domains of schizophrenia.

Dynorphin and the pathophysiology of drug addiction

Drug addiction is a chronic relapsing disease in which drug administration becomes the primary stimulus that drives behavior regardless of the adverse consequence that may ensue. As drug use becomes more compulsive, motivation for natural rewards that normally drive behavior decreases. The discontinuation of drug use is associated with somatic signs of withdrawal, dysphoria, anxiety, and anhedonia. These consequences of drug use are thought to contribute to the maintenance of drug use and to the reinstatement of compulsive drug use that occurs during the early phase of abstinence. Even, however, after prolonged periods of abstinence, 80-90% of human addicts relapse to addiction, suggesting that repeated drug use produces enduring changes in brain circuits that subserve incentive motivation and stimulus-response (habit) learning. A major goal of addiction research is the identification of the neural mechanisms by which drugs of abuse produce these effects. This article will review data showing that the dynorphin/kappa-opioid receptor (KOPr) system serves an essential function in opposing alterations in behavior and brain neurochemistry that occur as a consequence of repeated drug use and that aberrant activity of this system may not only contribute to the dysregulation of behavior that characterizes addiction but to individual differences in vulnerability to the pharmacological actions of cocaine and alcohol. We will provide evidence that the repeated administration of cocaine and alcohol up-regulates the dynorphin/KOPr system and that pharmacological treatments that target this system may prove effective in the treatment of drug addiction.

Neurochemical consequences of dysphoric state during amphetamine withdrawal in animal models: a review

Chronic abuse of amphetamines, such as d-amphetamine (AMPH) and d-methamphetamine, results in psychological dependence, a condition in which the drug produces a feeling of satisfaction and a drive that requires periodic or continuous administration of the drug to produce overwhelming pleasure or to avoid discomfort such as dysphoria. The dysphoric state of AMPH withdrawal has been recognized as depressive syndromes, such as anhedonia, depression, anxiety, and social inhibition, in early drug abstinence. Medication for treatment of the dysphoric state is important for AMPH abusers to avoid impulsive self-injurious behavior or acts that are committed with unconscious or uncontrolled suicidal ideation. However, successful treatments for AMPH withdrawal remain elusive, since the exact molecular basis of the expression of dysphoria has not been fully elucidated. This review focuses on the molecular aspects of AMPH withdrawal as indexed by neurochemical parameters under a variety of injection regimens (for example, levels of brain monoamines and their metabolites, and gamma-aminobutyric acid, expression of genes and proteins involved in neuronal activity, and monoamine metabolism and availability) in rodent models which exhibit significant phenotypic features relevant to the syndromes of AMPH withdrawal in humans.

Morphine withdrawal decreases responding reinforced by sucrose self-administration in progressive ratio

Previous studies have shown that withdrawal from psychostimulant drugs such as d-amphetamine or methamphetamine decreases motivation to work for a natural reinforcement, which is thought to be associated with the withdrawal-induced depressive state and hypofunction of the mesolimbic dopamine system. However, to our knowledge, studies exploring the effect of morphine withdrawal on motivation for a natural reinforcement are lacking. The purpose of the present study was to examine whether motivation to work for a natural reinforcement changes during morphine withdrawal. Three groups of male Sprague-Dawley rats were trained to respond on a nose poke for a 4% sucrose solution under a progressive ratio schedule and were subsequently administered a 10-day regimen of injection of high or low dose of morphine or saline. Their duration of break point and withdrawal symptoms were assessed. The finding showed that break points were significantly reduced on day 1 and persisted to at least day 10 of withdrawal without change in locomotor activity. There were hardly any differences bear mentioning when comparing the magnitude of the decrease between the high- and the low-dose group, whereas the withdrawal scales were significant greater in the high-dose group than in the low-dose group. The results suggest that the morphine withdrawal resulted in decreased motivation to obtain the natural reinforcement. The progressive ratio procedure may be a useful technique for evaluation of changes in motivation for natural reinforcing stimuli following withdrawal from opiates.

Animal models and treatments for addiction and depression co-morbidity

The high rates of co-morbidity of drug addiction with depression may be attributable to shared neurobiology. Here, we discuss shared neurobiological substrates in drug withdrawal and depression, with an emphasis on changes in brain reward circuitry that may underlie anhedonia, a core symptom of depression and drug withdrawal. We explored experimentally whether clinical antidepressant medications or other treatments would reverse the anhedonia observed in rats undergoing spontaneous nicotine or amphetamine withdrawal, defined operationally as elevated brain reward thresholds. The co-administration of selective serotonin reuptake inhibitors with a serotonin-1A receptor antagonist, or the tricyclic antidepressant desipramine, or the atypical antidepressant bupropion ameliorated nicotine or amphetamine withdrawal in rats. Thus, increases in monoaminergic neurotransmission, or neuroadaptations induced by increased monoaminergic neurotransmission, ameliorated depression-like aspects of drug withdrawal. Further, chronic pretreatment with the atypical antipsychotic clozapine, that has some efficacy in the treatment of the depression-like symptoms of schizophrenia, attenuated nicotine and amphetamine withdrawal. Finally, a metabotropic glutamate 2/3 receptor antagonist reversed threshold elevations associated with nicotine withdrawal. The effects of these pharmacological manipulations are consistent with the altered neurobiology observed in drug withdrawal and depression. Thus, these data support the hypothesis of common substrates mediating the depressive symptoms of drug withdrawal and those seen in psychiatric patients. Accordingly, the anhedonic state associated with drug withdrawal can be used to study the neurobiology of anhedonia, and thus contribute to the identification of novel targets for the treatment of depression-like symptoms seen in various psychiatric and neurological disorders.

Antagonism of CRF receptors prevents the deficit in brain reward function associated with precipitated nicotine withdrawal in rats

Nicotine dependence is a chronic mental illness that is characterized by a negative affective state upon tobacco smoking cessation and relapse after periods of abstinence. It has been hypothesized that cessation of nicotine administration results in the activation of brain corticotropin-releasing factor (CRF) systems that leads to the negative affective state of withdrawal. The aim of our experiments was to investigate the role of brain CRF systems in the deficit in brain reward function associated with the cessation of nicotine administration in rats. The intracranial self-stimulation procedure was used to assess to negative affective aspects of nicotine withdrawal as this procedure can provide a quantitative measure of emotional distress in rats. In the first experiment, mecamylamine induced a dose-dependent elevation in brain reward thresholds in nicotine-treated rats. In the follow-up experiment, it was shown that pretreatment with the corticotropin-receptor antagonist D-Phe CRF((12-41)) prevents the elevations in brain reward thresholds associated with precipitated nicotine withdrawal. In the third experiment, the effect of D-Phe CRF((12-41)) on the elevations in brain reward thresholds associated with spontaneous nicotine withdrawal was investigated. Administration of D-Phe CRF((12-41)) 6 h after the explantation of the nicotine pumps, did not result in a lowering of the brain reward thresholds. These findings indicate that antagonism of CRF receptors prevents, but not reverses, the deficit in brain associated with nicotine withdrawal. These data provide support for the hypothesis that a hyperactivity of brain CRF systems may at least partly mediate the initiation of the negative affective aspects of nicotine withdrawal.

Effects of terguride, ropinirole, and acetyl-l-carnitine on methamphetamine withdrawal in the rat

Withdrawal from psychostimulants, including methamphetamine, induces a depressive state associated with lethargy, dysphoria, hyperphagia and psychomotor retardation. Previous work with repeated administration of amphetamine in rats has shown that amphetamine withdrawal produces decreased motivation to work for a non-drug reward, and this withdrawal is reversed by administration of a dopamine partial agonist. The purpose of the present study was to examine decreased motivation to work for a non-drug reward during methamphetamine withdrawal and explore the effects of a dopamine agonist, dopamine partial agonist, and indirect monoamine agonist on methamphetamine withdrawal. During withdrawal from repeated methamphetamine administration, rats showed reduced responding for a sweet solution in a progressive-ratio schedule of reinforcement, and this effect was significantly more pronounced than previously observed with amphetamine. Repeated systemic treatment with the dopamine partial agonist terguride (0.2 and 0.4 mg/kg, i.p., twice daily), the full dopamine agonist ropinirole (1 mg/kg, i.p., twice daily), and acetyl-L-carnitine (60 and 100 mg/kg, i.p.), a compound with a potential antidepressant effect, during methamphetamine withdrawal restored responding for the sweet solution, suggesting that these drugs may represent potential therapeutic strategies for the treatment of methamphetamine addiction during the withdrawal phase.

Imbalance between drug and non-drug reward availability: A major risk factor for addiction

Laboratory animals self-administer most, though not all, drugs of abuse. Recent evidence shows that with increased drug availability, most laboratory rats develop all the major behavioral signs of addiction, including: 1) drug intake escalation, 2) increased motivation for the drug, 3) difficulty to abstain, 4) decreased reward function, and 5) inflexible drug use. The large prevalence of addicted rats may suggest that they are particularly vulnerable to develop compulsive drug use. I review evidence showing that this apparent vulnerability results in large part from the lack of positive (i.e., alternative non-drug rewards) and negative (i.e., costs) incentives capable of turning animals away from the pursuit of drugs. In particular, most animals seem to take drugs and eventually become addicted, not because drugs are intrinsically addictive, but more likely because drugs are the only significant sources of reward available in the laboratory. Laboratory animals would therefore represent more of a model of high-risk human groups than of the general population. Consequently, they should be more suited for searching factors that protect from, rather than predispose to, drug addiction. Reconsidering the environmental background of drug self-administration experiments in laboratory animals raises intriguing implications for understanding the initial demand for drug consumption and the transition to drug addiction, and for extrapolation from laboratory animals to humans.