Dissociation of psychomotor sensitization from compulsive cocaine consumption

Heroin and its metabolites: relevance to heroin use disorder

Heroin is an opioid agonist commonly abused for its rewarding effects. Since its synthesis at the end of the nineteenth century, its popularity as a recreational drug has ebbed and flowed. In the last three decades, heroin use has increased again, and yet the pharmacology of heroin is still poorly understood. After entering the body, heroin is rapidly deacetylated to 6-monoacetylmorphine (6-MAM), which is then deacetylated to morphine. Thus, drug addiction literature has long settled on the notion that heroin is little more than a pro-drug. In contrast to these former views, we will argue for a more complex interplay among heroin and its active metabolites: 6-MAM, morphine, and morphine-6-glucuronide (M6G). In particular, we propose that the complex temporal pattern of heroin effects results from the sequential, only partially overlapping, actions not only of 6-MAM, morphine, and M6G, but also of heroin per se, which, therefore, should not be seen as a mere brain-delivery system for its active metabolites. We will first review the literature concerning the pharmacokinetics and pharmacodynamics of heroin and its metabolites, then examine their neural and behavioral effects, and finally discuss the possible implications of these data for a better understanding of opioid reward and heroin addiction. By so doing we hope to highlight research topics to be investigated by future clinical and pre-clinical studies.

Choosing between cocaine and sucrose under the influence: testing the effect of cocaine tolerance

RATIONALE

Cocaine use not only depends on the reinforcing properties of the drug, but also on its pharmacological effects on alternative nondrug activities. In animal models investigating choice between cocaine and alternative sweet rewards, the latter influence can have a dramatic impact on choice outcomes. When choosing under cocaine influence is prevented by imposing sufficiently long intervals between choice trials, animals typically prefer the sweet reward. However, when choosing under the drug influence is permitted, animals shift their preference in favor of cocaine.

OBJECTIVES

We previously hypothesized that this preference shift is mainly due to a direct suppression of responding for sweet reward by cocaine pharmacological effects. Here we tested this hypothesis by making rats tolerant to this drug-induced behavioral suppression.

RESULTS

Contrary to our expectation, tolerance did not prevent rats from shifting their preference to cocaine when choosing under the influence.

CONCLUSION

Thus, other mechanisms must be invoked to explain the influence of cocaine intoxication on choice outcomes.

Deep brain stimulation of the nucleus accumbens shell attenuates cocaine withdrawal but increases cocaine self-administration, cocaine-induced locomotor activity, and GluR1/GluA1 in the central nucleus of the amygdala in male cocaine-dependent rats

BACKGROUND

Cocaine addiction is a major public health problem. Despite decades of intense research, no effective treatments are available. Both preclinical and clinical studies strongly suggest that deep brain stimulation of the nucleus accumbens (NAcc) is a viable target for the treatment of cocaine use disorder (CUD).

OBJECTIVE

Although previous studies have shown that DBS of the NAcc decreases cocaine seeking and reinstatement, the effects of DBS on cocaine intake in cocaine-dependent animals have not yet been investigated.

METHODS

Rats were made cocaine dependent by allowing them to self-administer cocaine in extended access conditions (6 h/day, 0.5 mg/kg/infusion). The effects of monophasic bilateral high-frequency DBS (60 μs pulse width and 130 Hz frequency) stimulation with a constant current of 150 μA of the NAcc shell on cocaine intake was then evaluated. Furthermore, cocaine-induced locomotor activity, irritability-like behavior during cocaine abstinence, and the levels of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunits 1 and 2 (GluR1/GluA1 and GluR2/GluA2) after DBS were investigated.

RESULTS

Contrary to our expectations, DBS of the NAcc shell induced a slight increase in cocaine self-administration, and increased cocaine-induced locomotion after extended access of cocaine self-administration. In addition, DBS decreased irritability-like behavior 18 h into cocaine withdrawal. Finally, DBS increased both cytosolic and synaptosomal levels of GluR1, but not GluR2, in the central nucleus of the amygdala but not in other brain regions.

CONCLUSIONS

These preclinical results with cocaine-dependent animals support the use of high-frequency DBS of the NAcc shell as a therapeutic approach for the treatment of the negative emotional state that emerges during cocaine abstinence, but also demonstrate that DBS does not decrease cocaine intake in active, long-term cocaine users. These data, together with the existing evidence that DBS of the NAcc shell reduces the reinstatement of cocaine seeking in abstinent animals, suggest that NAcc shell DBS may be beneficial for the treatment of the negative emotional states and craving during abstinence, although it may worsen cocaine use if individuals continue drug use.

Animal Models of the Behavioral Symptoms of Substance Use Disorders

To more effectively manage substance use disorders, it is imperative to understand the neural, genetic, and psychological underpinnings of addictive behavior. To contribute to this understanding, considerable efforts have been made to develop translational animal models that capture key behavioral characteristics of addiction on the basis of DSM5 criteria of substance use disorders. In this review, we summarize empirical evidence for the occurrence of addiction-like behavior in animals. These symptoms include escalation of drug use, neurocognitive deficits, resistance to extinction, exaggerated motivation for drugs, increased reinstatement of drug seeking after extinction, preference for drugs over nondrug rewards, and resistance to punishment. The occurrence of addiction-like behavior in laboratory animals has opened the opportunity to investigate the neural, genetic, and psychological background of key aspects of addiction, which may ultimately contribute to the prevention and treatment of substance use disorders.

The Cocaine and Oxycodone Biobanks, Two Repositories from Genetically Diverse and Behaviorally Characterized Rats for the Study of Addiction

The rat oxycodone and cocaine biobanks contain samples that vary by genotypes (by using genetically diverse genotyped HS rats), phenotypes (by measuring addiction-like behaviors in an advanced SA model), timepoints (samples are collected longitudinally before, during, and after SA, and terminally at three different timepoints in the addiction cycle: intoxication, withdrawal, and abstinence or without exposure to drugs through age-matched naive rats), samples collected (organs, cells, biofluids, feces), preservation (paraformaldehyde-fixed, snap-frozen, or cryopreserved) and application (proteomics, transcriptomics, microbiomics, metabolomics, epigenetics, anatomy, circuitry analysis, biomarker discovery, etc.Substance use disorders (SUDs) are pervasive in our society and have substantial personal and socioeconomical costs. A critical hurdle in identifying biomarkers and novel targets for medication development is the lack of resources for obtaining biological samples with a detailed behavioral characterization of SUD. Moreover, it is nearly impossible to find longitudinal samples. As part of two ongoing large-scale behavioral genetic studies in heterogeneous stock (HS) rats, we have created two preclinical biobanks using well-validated long access (LgA) models of intravenous cocaine and oxycodone self-administration (SA) and comprehensive characterization of addiction-related behaviors. The genetic diversity in HS rats mimics diversity in the human population and includes individuals that are vulnerable or resilient to compulsive-like responding for cocaine or oxycodone. Longitudinal samples are collected throughout the experiment, before exposure to the drug, during intoxication, acute withdrawal, and protracted abstinence, and include naive, age-matched controls. Samples include, but are not limited to, blood plasma, feces and urine, whole brains, brain slices and punches, kidney, liver, spleen, ovary, testis, and adrenal glands. Three preservation methods (fixed in formaldehyde, snap-frozen, or cryopreserved) are used to facilitate diverse downstream applications such as proteomics, metabolomics, transcriptomics, epigenomics, microbiomics, neuroanatomy, biomarker discovery, and other cellular and molecular approaches. To date, >20,000 samples have been collected from over 1000 unique animals and made available free of charge to non-profit institutions through https://www.cocainebiobank.org/ and https://www.oxycodonebiobank.org/.

Dopamine 'ups and downs' in addiction revisited

Repeated drug use can change dopamine (DA) function in ways that promote the development and persistence of addiction, but in what direction? By one view, drug use blunts DA neurotransmission, producing a hypodopaminergic state that fosters further drug use to overcome a DA deficiency. Another view is that drug use enhances DA neurotransmission, producing a sensitized, hyperdopaminergic reaction to drugs and drug cues. According to this second view, continued drug use is motivated by sensitization of drug 'wanting'. Here we discuss recent evidence supporting the latter view, both from preclinical studies using intermittent cocaine self-administration procedures that mimic human patterns of use and from related human neuroimaging studies. These studies have implications for the modeling of addiction in the laboratory and for treatment.

Ethanol-induced locomotor sensitization: Neuronal activation in the nucleus accumbens and medial prefrontal cortex

Ethanol consumption may promote neuroplasticity and alterations in synapses, resulting in modifications in neuronal activity. Here, we treated male Swiss mice with ethanol (2.2 g/kg) or saline once per day for 21 consecutive days. Nine days after the last ethanol administration, they received a challenge injection of ethanol or saline, and we assessed locomotor activity. After the behavioral analysis, we evaluated neuronal activation in the medial Prefrontal Cortex (Cingulate, Prelimbic, and Infralimbic) and the Nucleus Accumbens (Shell and Core) using Fos/DAB immunohistochemistry. In another group of animals, we performed the quantitative analysis of the ARC and PSD-95 protein levels by Western blotting in the medial prefrontal cortex and nucleus accumbens brain areas. Repeated ethanol administration produced locomotor sensitization, accompanied by an increase in the nucleus accumbens shell's activation but not core. Furthermore, the ethanol pretreatment reduced ARC expression in the nucleus accumbens and medial prefrontal cortex. Our results suggest the participation of the nucleus accumbens shell in ethanol behavioral sensitization and add new pieces of evidence that neuroplasticity in synapses may contribute to the mechanism underlying this behavior.

Vulnerability to ethanol sensitization predicts higher intake and motivation to self-administer ethanol: Proof of the incentive salience sensitization theory?

Ethanol-induced behavioral sensitization (EIBS) is thought to play a key role in addiction. However, whether EIBS is linked to an increase in the motivation to self-administerethanol in an operant paradigm has never been demonstrated, and thus, the motivational sensitization theory (increase in drug wanting) has not been yet confirmed. We investigated using the operant ethanol self-administrationparadigm if the motivation to self-administerethanol (breakpoint) is increased in female mice prone to develop EIBS. Outbred female Swiss mice were treated once a day with 2.5-g ethanol per kilogram during 10 days and challenged with the same dose of ethanol 7 days later. EIBS-pronegroup was characterized by a significant increase in locomotion between the challenge day and day 1. When the difference was not significant, mice were considered as the "EIBS-resistant"group. Mice were then trained to nose poke for a 20% ethanol solution reinforcer under a FR1 and then a FR-2schedule of reinforcement. Motivation was assessed more directly with a progressive ratio schedule. Our results show that there is a positive correlation between EIBS and both the level of intake and motivation. Interestingly, acquisition of ethanol self-administrationwas faster in sensitized mice that also display a quick and long-lastingincrease in ethanol intake together with a lack of effect of alcohol challenge on c-Fosexpression restricted to the dorsolateral striatum. These results further support that EIBS vulnerability is crucial in the development of addictive behaviors and suggest a potential link with habit learning processes.

Extinction and drug-induced reinstatement of cocaine seeking following self-administration or conditioned place preference in adolescent and adult rats

Adolescence marks a particularly vulnerable period to developing substance use disorders, and people who start using drugs in adolescence are more likely to relapse. A limited number of studies have investigated age difference in relapse following re-exposure to the drug after a period of abstinence. Using a cocaine self-administration paradigm, we showed no age difference in acquisition or extinction of self-administration. Interestingly, adolescent rats displayed impaired cocaine-primed reinstatement of cocaine seeking. Using the same dose as that self-administered in the first experiment, we then investigated age differences in acquisition and extinction of conditioned place preference, as well as locomotor sensitization. While there were no differences in locomotor activity or acquisition of preference, adolescents failed to extinguish their preference, even when the number of extinction sessions was doubled from what adults received. Taken together, these results suggest that while cocaine has similar rewarding and reinforcing effects regardless of age, adolescents may attribute stronger salience to the drug-associated context. In addition, re-exposure to cocaine itself may not be a strong relapse trigger in adolescence. Overall, these findings suggest that we should focus more on alleviating drug-context salience compared to re-exposure to substance in order to reduce relapse of drug seeking in adolescents.

Intermittent access cocaine self-administration produces psychomotor sensitization: effects of withdrawal, sex and cross-sensitization

RATIONALE

With repeated administration, the psychomotor activating effects of drugs such as cocaine or amphetamine can change in very different ways-showing sensitization or tolerance-depending on whether they are administered more or less intermittently. This behavioral plasticity is thought to reflect, at least in part, changes in dopamine (DA) neurotransmission, and therefore, may provide insights into the development of substance use disorders. Indeed, the most widely used preclinical model of cocaine addiction, which involves Long Access (LgA) self-administration procedures, is reported to produce tolerance to cocaine's psychomotor activating effects and effects on DA activity. In contrast, Intermittent Access (IntA) cocaine self-administration is more effective than LgA in producing addiction-like behavior, but sensitizes DA neurotransmission. There is, however, very little information concerning the effects of IntA experience on the psychomotor activating effects of cocaine.

OBJECTIVE

The objective of this study was to determine whether IntA experience produces psychomotor sensitization with similar characteristics to that produced by the intermittent, noncontingent administration of cocaine.

RESULTS

IntA to cocaine did indeed produce psychomotor sensitization that (1) was greater after a long (30 days) vs. short (1 day) period of withdrawal, (2) was greater in females than males, and (3) resulted in cross-sensitization to another psychomotor stimulant drug, amphetamine.

CONCLUSION

The tolerance sometimes associated with LgA cocaine self-administration has been cited in support of the idea that, in addiction, drug-seeking and drug-taking is motivated to overcome a DA deficiency and associated anhedonia. In contrast, the neurobehavioral sensitization associated with IntA cocaine self-administration favors an incentive-sensitization view.

The role of 6-acetylmorphine in heroin-induced reward and locomotor sensitization in mice

We have previously demonstrated that heroin's first metabolite, 6-acetylmorphine (6-AM), is an important mediator of heroin's acute effects. However, the significance of 6-AM to the rewarding properties of heroin still remains unknown. The present study therefore aimed to examine the contribution of 6-AM to heroin-induced reward and locomotor sensitization. Mice were tested for conditioned place preference (CPP) induced by equimolar doses of heroin or 6-AM (1.25-5 μmol/kg). Psychomotor activity was recorded during the CPP conditioning sessions for assessment of drug-induced locomotor sensitization. The contribution of 6-AM to heroin reward and locomotor sensitization was further examined by pretreating mice with a 6-AM specific antibody (anti-6-AM mAb) 24 hours prior to the CPP procedure. Both heroin and 6-AM induced CPP in mice, but heroin generated twice as high CPP scores compared with 6-AM. Locomotor sensitization was expressed after repeated exposure to 2.5 and 5 μmol/kg heroin or 6-AM, but not after 1.25 μmol/kg, and we found no correlation between the expression of CPP and the magnitude of locomotor sensitization for either opioid. Pretreatment with anti-6-AM mAb suppressed both heroin-induced and 6-AM-induced CPP and locomotor sensitization. These findings provide evidence that 6-AM is essential for the rewarding and sensitizing properties of heroin; however, heroin caused stronger reward compared with 6-AM. This may be explained by the higher lipophilicity of heroin, providing more efficient drug transfer to the brain, ensuring rapid increase in the brain 6-AM concentration.

Dopamine D3 Receptor Antagonism Reverses the Escalation of Oxycodone Self-administration and Decreases Withdrawal-Induced Hyperalgesia and Irritability-Like Behavior in Oxycodone-Dependent Heterogeneous Stock Rats

Prescription opioids, such as oxycodone, are highly effective analgesics for clinical pain management, but approximately 25% of patients who are prescribed opioids misuse them, and 5%–10% develop an opioid use disorder (OUD). Effective therapies for the prevention and treatment of opioid abuse and addiction need to be developed. The present study evaluated the effects of the highly selective dopamine D₃ receptor antagonist VK4-116 ([R]-N-[4-(4-[3-chloro-5-ethyl-2-methoxyphenyl]piperazin-1-yl)-3-hydroxybutyl]-1H-indole-2-carboxamide) on oxycodone addictive-like behaviors. We used a model of extended access to oxycodone self-administration and tested the effects of VK4-116 on the escalation of oxycodone self-administration and withdrawal-induced hyperalgesia and irritability-like behavior in male and female rats. Pretreatment with VK4-116 (5–25 mg/kg, i.p.) dose-dependently decreased the escalation of oxycodone self-administration and reduced withdrawal-induced hyperalgesia and irritability-like behavior in opioid-dependent rats. These findings demonstrate a key role for D₃ receptors in both the motivation to take opioids and negative emotional states that are associated with opioid withdrawal and suggest that D₃ receptor antagonism may be a viable therapeutic approach for the treatment of OUD.

Incentive and dopamine sensitization produced by intermittent but not long access cocaine self-administration

The temporal pattern of drug use (pharmacokinetics) has a profound effect on the ability of self-administered cocaine to produce addiction-like behavior in rodents, and to change the brain. To further address this issue, we compared the effects of long access (LgA) cocaine self-administration, which is widely used to model the transition to addiction, with intermittent access (IntA), which is thought to better reflect the pattern of drug use in humans, on the ability of a single, self-administered injection of cocaine to increase dopamine (DA) overflow in the core of the nucleus accumbens (using in vivo microdialysis), and to produce addiction-like behavior. IntA experience was more effective than LgA in producing addiction-like behavior-a drug experience-dependent increase in motivation for cocaine assessed using behavioral economic procedures, and cue-induced reinstatement-despite much less total drug consumption. There were no group differences in basal levels of DA in dialysate [DA], but a single self-administered IV injection of cocaine increased [DA] in the core of the nucleus accumbens to a greater extent in rats with prior IntA experience than those with LgA or limited access experience, and the latter two groups did not differ. Furthermore, high motivation for cocaine was associated with a high [DA] response. Thus, IntA, but not LgA, produced both incentive and DA sensitization. This is consistent with the notion that a hyper-responsive dopaminergic system may contribute to the transition from casual patterns of drug use to the problematic patterns that define addiction.

Revisiting long-access versus short-access cocaine self-administration in rats: intermittent intake promotes addiction symptoms independent of session length

In rats, continuous cocaine access during long self-administration sessions (6 versus 1-2 hours) promotes the development of behavioral symptoms of addiction. This has led to the assumption that taking large amounts of drug during extended daily bouts is necessary to develop an addiction phenotype. Recent work shows that within-session intermittent access (IntA) to cocaine produces much less drug intake than continuous-access procedures (i.e. long-access sessions) but evokes addiction symptoms more effectively. IntA-sessions are also long, typically lasting 6 hours. It is not known whether IntA-sessions must be extended to promote addiction-relevant changes in drug use over time. Here, we determined the influence of IntA-session length on patterns of cocaine use relevant to addiction. Two groups of male Wistar rats self-administered cocaine (0.25 mg/kg/injection, injected over 5 seconds) during 18 daily IntA-sessions. One group had long 6-hour sessions (Long-IntA), the other group had shorter, 2-hour sessions (Short-IntA). Only Long-IntA rats escalated their cocaine intake over sessions, but both groups developed a burst-like pattern of drug use over time and similar levels of psychomotor sensitization. The two groups also showed robust and similar levels of both responding for cocaine under a progressive ratio schedule of reinforcement and cocaine-induced reinstatement of extinguished drug-seeking behavior. In summary, long IntA-sessions lead to greater cocaine intake than shorter IntA-sessions, but the two conditions are equally effective in evoking the patterns of drug-taking and drug-seeking that define addiction. This suggests that chronic intermittent cocaine use, even during short daily bouts, is sufficient to promote addiction symptoms.

An extended history of drug self-administration results in multiple sources of control over drug seeking behavior

It is widely recognized that across the development of drug addiction, cues associated with drug use come to exert increasing control over drug seeking and taking behaviors. However, there remain gaps in our knowledge regarding how the different types of drug related cues affect drug seeking and taking behaviors, and how the emergence of cue control over these behaviors relates to the onset of drug seeking compulsions. This paper reviews the literature on drug self-administration in animals to address these gaps. It first identifies the different types of cues that acquire control over reward seeking behavior generally, and examines whether the same types of cues acquire control over drug seeking behavior specifically. It then examines how the role of drug related cues in motivating and reinforcing drug seeking behavior changes across an extended drug-taking history, with a particular focus on the case of nicotine. The evidence reviewed shows that, after an extended history of drug taking, drug seeking behaviors are controlled by contextual cues associated with the development of drug seeking habits, response contingent cues that accompany delivery of the drug, as well as internal states that correlate with levels of drug intake. These multiple sources of control over drug seeking are discussed in relation to the generation of an addicted phenotype in animal models and the hypothesized progression from internal control over drug use to compulsive drug seeking.

The active heroin metabolite 6-acetylmorphine has robust reinforcing effects as assessed by self-administration in the rat

Previous studies have suggested that at least some of the behavioral effects of heroin might be mediated by its active metabolite 6-acetylmorphine (6-AM). The aim of the present study was to investigate the reinforcing effects of 6-AM and its role in mediating those of heroin. We used an intravenous self-administration procedure in male Sprague-Dawley rats including four phases: acquisition, extinction, reinstatement of drug-seeking, and re-acquisition. Independent groups of rats readily learned to self-administer equimolar doses (0.135 μmol/kg) of either 6-AM (44.3 μg/kg) or heroin (50 μg/kg). Under a fixed ratio 1 (FR1) schedule of reinforcement, the rate of responding was the same for 6-AM and heroin, but it was significantly higher for 6-AM than for heroin under a FR2 schedule. A non-contingent infusion ('priming') of 0.068 μmol/kg of either 6-AM or heroin reinstated non-reinforced drug-seeking (relapse). The rats readily re-acquired self-administration behaviour when given access to one of two doses (0.068 and 0.135 μmol/kg) of 6-AM or heroin. Pretreatment with a specific monoclonal antibody (mAb) against 6-AM blocked the priming effect of 6-AM, and modified the rate of lever-pressing on re-acquisition of 6-AM self-administration in a manner compatible with a shift to the right of the dose-effect curve. The mAb did not affect heroin responding. The present results show that 6-AM possesses reinforcing effects similar to those of heroin. The lack of effect of 6-AM mAb on heroin priming and heroin self-administration calls for further studies to clarify the role of heroin and its metabolites in heroin reward. This article is part of the Special Issue entitled 'Opioid Neuropharmacology: Advances in treating pain and opioid addiction'.

Adolescent cannabinoid exposure induces irritability-like behavior and cocaine cross-sensitization without affecting the escalation of cocaine self-administration in adulthood

Cannabis use is typically initiated during adolescence and is a significant risk factor for the development of cocaine use in adulthood. However, no preclinical studies have examined the effects of adolescent cannabinoid exposure on cocaine dependence in adulthood using the escalation model of cocaine self-administration and the assessment of negative emotional states. In the present study, we found that exposure to the cannabinoid receptor agonist WIN55,212-2 (WIN) in adolescence produced irritability-like behavior and psychomotor cross-sensitization to cocaine in adolescence. In adulthood, rats were allowed to self-administer cocaine. The acquisition of cocaine self-administration was lower in rats with adolescent WIN exposure compared with controls. However, both WIN-exposed and control rats escalated their cocaine intake at the same rate, had similar responding under a progressive-ratio schedule of reinforcement, and had similar psychomotor responses to cocaine. Interestingly, the increase in irritability-like behavior that was previously observed in adolescence after WIN exposure persisted into adulthood. Whether the persisting increase in irritability-like behavior after WIN exposure has translational relevance remains to be studied. In summary, these results suggest that psychoactive cannabinoid exposure during adolescence is unlikely to have a major effect on the escalation of cocaine intake or the development of compulsive-like responding per se in adulthood in a rat model of cocaine self-administration. However, whether the persisting irritability-like behavior may predispose an individual to mood-related impairments in adulthood or predict such impairments warrants further investigation.

Pre-trial cocaine biases choice toward cocaine through suppression of the nondrug option

Being under the influence during choice between drug and nondrug options can have a dramatic effect on choice outcomes. When rats face a choice between cocaine and sweet water and are not under the influence, they prefer sweet water. In contrast, when they are under the influence of cocaine, this causes them to shift their choice to cocaine nearly exclusively. Here we sought to characterize the behavioral mechanisms underlying the influence of cocaine on choice. In theory, rats under the influence of cocaine should be in a mixed motivational state, at least temporarily, with both their motivation for cocaine and their motivation for the nondrug option suppressed by the drug satiating and anorexic effects of cocaine, respectively. For this mixed state to shift choice to cocaine, the satiated motivation for cocaine should recover before the suppressed motivation for the preferred nondrug option. The goal of the present study was to test this prediction in rats that expressed a preference for sweet water after extended access to cocaine self-administration. We measured their choice and response latencies to each option after pre-trial, passive administration of cocaine to estimate the duration of its drug satiating and anorexic effects. As expected, pre-trial cocaine caused most rats to shift their choice to cocaine. Though this shift was not simply due to a longer latency to respond for sweet water than for cocaine after pre-trial cocaine, it nevertheless occurred while rats' motivation for the nondrug option was still partially suppressed. Thus, cocaine seems to bias choice toward more cocaine mainly via suppression of the nondrug option.

Contributions of prolonged contingent and non-contingent cocaine exposure to escalation of cocaine intake and glutamatergic gene expression

Similar to the pattern observed in people with substance abuse disorders, laboratory animals will exhibit escalation of cocaine intake when the drug is available over prolonged periods of time. Here, we investigated the contribution of behavioral contingency of cocaine administration on escalation of cocaine intake and gene expression in the dorsal medial prefrontal cortex (dmPFC) in adult male rats. Rats were allowed to self-administer intravenous cocaine (0.25 mg/infusion) under either limited cocaine-(1 h/day), prolonged cocaine-(6 h/day), or limited cocaine-(1 h/day) plus yoked cocaine-access (5 h/day); a control group received access to saline (1 h/day). One day after the final self-administration session, the rats were euthanized and the dmPFC was removed for quantification of mRNA expression of critical glutamatergic signaling genes, Homer2, Grin1, and Dlg4, as these genes and brain region have been previously implicated in addiction, learning, and memory. All groups with cocaine-access showed escalated cocaine intake during the first 10 min of each daily session, and within the first 1 h of cocaine administration. Additionally, the limited-access + yoked group exhibited more non-reinforced lever responses during self-administration sessions than the other groups tested. Lastly, Homer2, Grin1, and Dlg4 mRNA were impacted by both duration and mode of cocaine exposure. Only prolonged-access rats exhibited increases in mRNA expression for Homer2, Grin1, and Dlg4 mRNA. Taken together, these findings indicate that both contingent and non-contingent "excessive" cocaine exposure supports escalation behavior, but the behavioral contingency of cocaine-access has distinct effects on the patterning of operant responsiveness and changes in mRNA expression.

High and escalating levels of cocaine intake are dissociable from subsequent incentive motivation for the drug in rats

RATIONALE

Taking high and increasing amounts of cocaine is thought to be necessary for the development of addiction. Consequently, a widely used animal model of drug self-administration involves giving animals continuous drug access during long sessions (LgA), as this produces high and escalating levels of intake. However, human cocaine addicts likely use the drug with an intermittent rather than continuous pattern, producing spiking brain cocaine levels.

OBJECTIVES

Using an intermittent-access (IntA) cocaine self-administration procedure in rats, we studied the relationship between escalation of cocaine intake and later incentive motivation for the drug, as measured by responding under a progressive ratio schedule of cocaine reinforcement.

RESULTS

First, under IntA, rats escalated their cocaine use both within and between sessions. However, escalation did not predict later incentive motivation for the drug. Second, incentive motivation for cocaine was similar in IntA-rats limited to low- and non-escalating levels of drug intake (IntA-Lim) and in IntA-rats that took high and escalating levels of drug. Finally, IntA-Lim rats took much less cocaine than rats given continuous drug access during each self-administration session (LgA-rats). However, IntA-Lim rats later responded more for cocaine under a progressive ratio schedule of reinforcement.

CONCLUSIONS

Taking large and escalating quantities of cocaine does not appear necessary to increase incentive motivation for the drug. Taking cocaine in an intermittent pattern-even in small amounts-is more effective in producing this addiction-relevant change. Thus, beyond the amount of drug taken, the temporal kinetics of drug use predict change in drug use over time.

The effects of social contact on cocaine intake under extended-access conditions in male rats

Social learning theories of drug use propose that drug use is influenced by the behavior of peers. We previously reported that cocaine self-administration under limited-access conditions can be either facilitated or inhibited by social contact, depending on the behavior of a peer. The purpose of this study was to determine whether social contact influences cocaine self-administration under conditions that are more representative of problematic patterns of drug use. Male rats were assigned to either isolated or pair-housed conditions in which a social partner either had access to cocaine or did not have access to cocaine. Pair-housed rats were tested in custom-built operant conditioning chambers that allowed both rats to be tested simultaneously in the same chamber. In Experiment 1, rats were tested for 14 consecutive days during daily 6-hr test sessions. In Experiment 2, different doses of cocaine were tested in 23-hr test sessions conducted every 3 days. All groups of rats escalated their cocaine intake in Experiment 1; however, pair-housed rats with a partner without access to cocaine had lower levels of intake throughout the 14 days of testing. In Experiment 2, pair-housed rats with a partner without access to cocaine had lower levels of cocaine intake than did rats with a partner with access to cocaine, and this effect was observed at all doses of cocaine tested. These data indicate that the behavior of a social partner (i.e., whether or not that partner is also self-administering cocaine) influences cocaine self-administration under conditions that model problematic patterns of drug use. (PsycINFO Database Record

Choosing Under the Influence: A Drug-Specific Mechanism by Which the Setting Controls Drug Choices in Rats

Ample evidence shows that the setting can control drug choices in both humans and animals. Here we reveal in rats that a major mechanism of this control involves a regulation of the drug influence on other competing options at the time of choice. Briefly, rats were offered a choice between a drug dose (cocaine or heroin) and a brief access to water sweetened with saccharin in two different settings. In one setting, choosing under the influence was not possible and rats largely preferred saccharin over either cocaine or heroin. In contrast, when the same rats were shifted to a setting where choosing under the influence was possible, they chose the drug either nonexclusively or exclusively depending on whether the drug enhanced or suppressed sweet reward, respectively. Thus, when rats were under the orexigenic influence of heroin at the time of choice, they more frequently chose saccharin in alternation with heroin. In contrast, when rats were under the anorexic influence of cocaine, they stopped choosing saccharin and continued taking cocaine exclusively. These setting- and drug-specific changes in preference were rapid and reversible, and could be induced by passively administering cocaine or heroin before choice. Finally, rats behaved as if they were oblivious to the drug influence on their choices. This behavior could explain why rats are vulnerable to harm themselves, sometimes to the point of death, in settings where choices are made under the drug influence, notably if this influence excludes other important options or, conversely, enhances harmful ones.

Motivational Effects of Methylphenidate are Associated with GABRA2 Variants Conferring Addiction Risk

Background: Variations in the GABRA2 gene, encoding α2 subunits of GABA_A receptors, have been associated with risk for addiction to several drugs, but the mechanisms by which variations in non-coding regions of GABRA2 increase risk for addictions are not understood. Mice with deletion of GABRA2 show deficits in the ability of psychostimulants to facilitate responding for conditioned reinforcers, offering a potential explanation.

Methods: We report human and mouse studies investigating a potential endophenotype underlying this association. Healthy human volunteers carrying either cocaine-addiction “risk” or “protective” GABRA2 single nucleotide polymorphism (SNPs) were tested for their subjective responses to methylphenidate, and methylphenidate’s ability to facilitate conditioned reinforcement (CRf) for visual stimuli (CS+) associated with monetary reward. In parallel, methylphenidate’s ability to facilitate responding for a visual CRf was studied in wildtype and α2 knockout (α2^−/−) mice.

Results: Methylphenidate increased the number of CS+ presentations obtained by human subjects carrying protective, but not risk SNPs. In mice, methylphenidate increased responding for a CS+ in wildtype, but not α2^−/− mice. Human subjects carrying protective SNPs felt stimulated, aroused and restless following methylphenidate, while individuals carrying risk SNPs did not.

Conclusion: Human risk SNP carriers were insensitive to methylphenidate’s effects on mood or in facilitating CRf. That mice with the gene deletion were also insensitive to methylphenidate’s ability to increase responding for CRf, suggests a potential mechanism whereby low α2-subunit levels increase risk for addictions. Circuits employing GABA_A-α2 subunit-containing receptors may protect against risk for addictions.

Abstinence from cocaine and sucrose self-administration reveals altered mesocorticolimbic circuit connectivity by resting state MRI

Previous preclinical studies have emphasized that drugs of abuse, through actions within and between mesocorticolimbic (MCL) regions, usurp learning and memory processes normally involved in the pursuit of natural rewards. To distinguish MCL circuit pathobiological neuroadaptations that accompany addiction from general learning processes associated with natural reward, we trained two groups of rats to self-administer either cocaine (IV) or sucrose (orally) followed by an identically enforced 30 day abstinence period. These procedures are known to induce behavioral changes and neuroadaptations. A third group of sedentary animals served as a negative control group for general handling effects. We examined low-frequency spontaneous fluctuations in the functional magnetic resonance imaging (fMRI) signal, known as resting-state functional connectivity (rsFC), as a measure of intrinsic neurobiological interactions between brain regions. Decreased rsFC was seen in the cocaine-SA compared with both sucrose-SA and housing control groups between prelimbic (PrL) cortex and entopeduncular nucleus and between nucleus accumbens core (AcbC) and dorsomedial prefrontal cortex (dmPFC). Moreover, individual differences in cocaine SA escalation predicted connectivity strength only in the Acb-dmPFC circuit. These data provide evidence of fronto-striatal plasticity across the addiction trajectory, which are consistent with Acb-PFC hypoactivity seen in abstinent human drug addicts, indicating potential circuit level biomarkers that may inform therapeutic interventions. They further suggest that available data from cross-sectional human studies may reflect the consequence of rather a predispositional predecessor to their dependence.

Amphetamine sensitization and cross-sensitization with acute restraint stress: impact of prenatal alcohol exposure in male and female rats

RATIONALE

Individuals with fetal alcohol spectrum disorder (FASD) are at increased risk for substance use disorders (SUD). In typically developing individuals, susceptibility to SUD is associated with alterations in dopamine and hypothalamic-pituitary-adrenal (HPA) systems, and their interactions. Prenatal alcohol exposure (PAE) alters dopamine and HPA systems, yet effects of PAE on dopamine-HPA interactions are unknown. Amphetamine-stress cross-sensitization paradigms were utilized to investigate sensitivity of dopamine and stress (HPA) systems, and their interactions following PAE.

METHODS

Adult Sprague-Dawley offspring from PAE, pair-fed, and ad libitum-fed control groups were assigned to amphetamine-(1-2 mg/kg) or saline-treated conditions, with injections every other day for 15 days. Fourteen days later, all animals received an amphetamine challenge (1 mg/kg) and 5 days later, hormones were measured under basal or acute stress conditions. Amphetamine sensitization (augmented locomotion, days 1-29) and cross-sensitization with acute restraint stress (increased stress hormones, day 34) were assessed.

RESULTS

PAE rats exhibited a lower threshold for amphetamine sensitization compared to controls, suggesting enhanced sensitivity of dopaminergic systems to stimulant-induced changes. Cross-sensitization between amphetamine (dopamine) and stress (HPA hormone) systems was evident in PAE, but not in control rats. PAE males exhibited increased dopamine receptor expression (medial prefrontal cortex (mPFC)) compared to controls.

CONCLUSIONS

PAE alters induction and expression of sensitization/cross-sensitization, as reflected in locomotor, neural, and endocrine changes, in a manner consistent with increased sensitivity of dopamine and stress systems. These results provide insight into possible mechanisms that could underlie increased prevalence of SUD, as well as the impact of widely prescribed stimulant medications among adolescents with FASD.

Effects of rimonabant on the development of single dose-induced behavioral sensitization to ethanol, morphine and cocaine in mice

RATIONALE

The endocannabinoid system has been implicated in the neurobiological mechanism underlying drug addiction, especially the primary rewarding dopamine-dependent processes. Therefore, endocannabinoid receptor antagonists, such as the CB1 cannabinoid antagonist rimonabant, have been proposed as candidates for preventive addiction therapies.

OBJECTIVES

Investigate the possible involvement of CB1 receptors in the development of behavioral sensitization to ethanol, morphine and cocaine in mice.

METHODS

We compared the effects of different doses of rimonabant (0.3, 1, 3 and 10mg/kg) on spontaneous locomotor activity in the open-field, hyperlocomotion induced by acute administration of ethanol (1.8g/kg), morphine (20mg/kg) or cocaine (10mg/kg) and on subsequent drug-induced locomotor sensitization using a two-injection protocol in mice. We also investigated a possible depressive-like effect of an acute rimonabant challenge at the highest dose and its potential anxiogenic property.

RESULTS

At the highest dose, rimonabant abolished ethanol- and cocaine-induced hyperlocomotion and behavioral sensitization without modifying spontaneous and central locomotor activity or inducing depressive-like behavior on the forced swim test in mice. The other doses of rimonabant also selectively blocked acute ethanol-induced central hyperlocomotion. Although rimonabant at 0.3 and 1mg/kg potentiated the central hyperlocomotion induced by acute morphine injection, it was effective in attenuating morphine-induced behavioral sensitization at all doses.

CONCLUSIONS

Because the neural basis of behavioral sensitization has been proposed to correspond to some components of addiction, our findings indicate that the endocannabinoid system might be involved in ethanol, cocaine and morphine abuse.

Escalation of cocaine consumption in short and long access self-administration procedures

BACKGROUND

Escalation of consumption is a hallmark of cocaine addiction. Many animal models reveal escalation by increasing the duration of drug access (e.g., 6-24 h/day) after longer histories of self-administration. We recently developed a method that reveals escalation early post-acquisition under shorter access conditions. However, whether or not rats will escalate cocaine consumption both early post-acquisition under short access (2 h/day) conditions, and later under long access (6 h/day) conditions, has not been demonstrated.

METHODS

All rats acquired cocaine self-administration (0.8 mg/kg, i.v.) under 2 h conditions, and then continued 2h self-administration for an additional 13 sessions. Then, rats were assigned either to 2 or 6h conditions, and self-administered cocaine (0.8 mg/kg, i.v.) for an additional 19 sessions. In addition, four cocaine-induced locomotor activity measurements were taken for each rat: before cocaine exposure, after non-contingent cocaine administration, and after escalation in the short and long access experimental phases.

RESULTS

Following acquisition, rats displayed a robust escalation of intake during 2 h sessions. Rats that self-administered cocaine in continued 2h sessions exhibited stable intake, whereas rats that self-administered cocaine in 6h sessions further escalated intake. Despite the second escalation in 6h rats, cocaine-induced locomotor activity did not differ between 2 and 6h rats.

CONCLUSIONS

Escalation of cocaine self-administration can occur in the same rats both early post-acquisition, and later under long access conditions. Importantly, this early post-acquisition period provides a new opportunity to determine the mechanisms first involved in the escalation phenomenon.

Predisposing effects of neonatal visceral pain on abuse-related effects of morphine in adult male Sprague Dawley rats

RATIONALE

Adverse early life experiences are risk factors for drug abuse and addiction. Changes in brain opioid systems have been demonstrated in response to neonatal visceral pain (NVP), but the impact of these changes on abuse-related effects of morphine are unknown. The NVP procedure used models chronic visceral hyperalgesia persisting across development.

OBJECTIVES

Intravenous self-administration, drug discrimination, and locomotor activity were used to compare the abuse-related effects of morphine in NVP and control rats.

METHODS

Rats self-administered 0.3 mg/kg/inj morphine under an FR1 schedule, and dose-effect functions for morphine were then established. Separate rats were trained to discriminate 3.2 mg/kg morphine from saline under an FR20 schedule, and morphine dose-effect functions were then determined in the absence and presence of 0.1 mg/kg naltrexone. A third group of rats was tested with a range of morphine doses in an assay of locomotor activity, then injected daily with 10 mg/kg morphine to assess locomotor sensitization.

RESULTS

NVP rats self-administered more morphine than controls at reinforcing doses. Discriminative stimulus effects of morphine were similar between groups, but in the presence of naltrexone, the ED50 for morphine was more than 12× greater in control rats than in NVP animals. Morphine did not stimulate locomotor activity at any tested dose in NVP rats, although significant effects were observed in controls. Finally, significant locomotor sensitization was observed only in NVP rats.

CONCLUSIONS

NVP-induced changes in brain opioid systems have persistent pharmacological consequences into adulthood and may increase sensitivity to abuse-related effects of opioids across development.

Rat ultrasonic vocalizations demonstrate that the motivation to contextually reinstate cocaine-seeking behavior does not necessarily involve a hedonic response

Human self-reports often indicate that changes in mood are a major contributor to drug relapse. Still, arguments have been made that instances of drug-seeking following abstinence in animal models (i.e. relapse/reinstatement) may be outside of hedonic control. Therefore, the present study utilized ultrasonic vocalizations in the rat in order to evaluate affect during cocaine self-administration and contextual reinstatement of cocaine-seeking in a pre-clinical model of drug relapse (abstinence-reinstatement model). Results show that while subjects effectively reinstated drug-seeking (lever pressing) following 30 days of abstinence, and spontaneously recovered/reinstated drug-seeking following 60 days of abstinence, ultrasonic vocalizations did not increase over baseline levels during either reinstatement session. These results are consistent with previous results from our laboratory and current theories of addiction suggesting that cues that are weakly associated with drug consumption can motivate drug-seeking behavior that is outside of hedonic processing.

Tolerance to the locomotor-activating effects of 3,4-methylenedioxymethamphetamine (MDMA) predicts escalation of MDMA self-administration and cue-induced reinstatement of MDMA seeking in rats

Pre-clinical studies of individual differences in addiction vulnerability have been increasing over recent years, but the amphetamine derivative 3,4-methylenedioxymethamphetamine (MDMA; ecstasy) has received relatively little attention in this regard. Previously, we reported large individual differences both in rats' initial behavioral response to experimenter-administered MDMA and their degree of behavioral sensitization to repeated administration. To determine whether these differences could predict subsequent patterns of MDMA-taking or -seeking behaviors we used the self-administration-extinction-reinstatement model to examine addiction-like behavior (i.e., escalation of MDMA self-administration and cue-induced reinstatement of MDMA seeking) in rats a priori characterized for either locomotor sensitization or tolerance to MDMA. Rats that developed tolerance to the locomotor-activating effects of MDMA had a significantly larger locomotor response to the first MDMA injection relative to rats that developed sensitization. Importantly, rats that developed tolerance subsequently displayed an escalation of MDMA self-administration over days, as well as clear cue-induced reinstatement of MDMA seeking following extinction. Conversely, rats that developed locomotor sensitization to MDMA subsequently maintained relatively stable levels of MDMA self-administration over days and showed no cue-induced reinstatement of MDMA seeking. These results show that differences in the level of psychomotor activation following acute and repeated MDMA administration can reliably predict two important addiction-like behaviors in rats, which may have implications in the prediction of compulsive MDMA use in humans.

Time course of cocaine-induced behavioral and neurochemical plasticity

Factors that result in augmented reinstatement, including increased withdrawal period duration and high levels of cocaine consumption, may provide insight into relapse vulnerability. The neural basis of augmented reinstatement may arise from more pronounced changes in plasticity required for reinstatement and/or the emergence of plasticity expressed only during a specific withdrawal period or under specific intake conditions. In this study, we examined the impact of withdrawal period duration and cocaine intake on the magnitude of cocaine-primed reinstatement and extracellular glutamate in the nucleus accumbens, which has been shown to be required for cocaine-primed reinstatement. Rats were assigned to self-administer under conditions resulting in low (2 hours/day; 0.5 mg/kg/infusion, IV) or high (6 hours/day; 1.0 mg/kg/infusion, IV) levels of cocaine intake. After 1, 21 or 60 days of withdrawal, drug seeking and extracellular glutamate levels in the nucleus accumbens were measured before and after a cocaine injection. Cocaine-reinstated lever pressing and elevated extracellular glutamate at every withdrawal time point tested, which is consistent with the conclusion that increased glutamatergic signaling in the nucleus accumbens, is required for cocaine-induced reinstatement. Interestingly, high-intake rats exhibited augmented reinstatement at every time point tested, yet failed to exhibit higher levels of cocaine-induced increases in extracellular glutamate relative to low-intake rats. Our current data indicate that augmented reinstatement in high-intake rats is not due to relative differences in extracellular levels of glutamate in the nucleus accumbens, but rather may stem from intake-dependent plasticity.

Addiction as a stress surfeit disorder

Drug addiction has been conceptualized as a chronically relapsing disorder of compulsive drug seeking and taking that progresses through three stages: binge/intoxication, withdrawal/negative affect, and preoccupation/anticipation. Drug addiction impacts multiple motivational mechanisms and can be conceptualized as a disorder that progresses from positive reinforcement (binge/intoxication stage) to negative reinforcement (withdrawal/negative affect stage). The construct of negative reinforcement is defined as drug taking that alleviates a negative emotional state. Our hypothesis is that the negative emotional state that drives such negative reinforcement is derived from dysregulation of key neurochemical elements involved in the brain stress systems within the frontal cortex, ventral striatum, and extended amygdala. Specific neurochemical elements in these structures include not only recruitment of the classic stress axis mediated by corticotropin-releasing factor (CRF) in the extended amygdala as previously hypothesized but also recruitment of dynorphin-κ opioid aversive systems in the ventral striatum and extended amygdala. Additionally, we hypothesized that these brain stress systems may be engaged in the frontal cortex early in the addiction process. Excessive drug taking engages activation of CRF not only in the extended amygdala, accompanied by anxiety-like states, but also in the medial prefrontal cortex, accompanied by deficits in executive function that may facilitate the transition to compulsive-like responding. Excessive activation of the nucleus accumbens via the release of mesocorticolimbic dopamine or activation of opioid receptors has long been hypothesized to subsequently activate the dynorphin-κ opioid system, which in turn can decrease dopaminergic activity in the mesocorticolimbic dopamine system. Blockade of the κ opioid system can also block anxiety-like and reward deficits associated with withdrawal from drugs of abuse and block the development of compulsive-like responding during extended access to drugs of abuse, suggesting another powerful brain stress/anti-reward system that contributes to compulsive drug seeking. Thus, brain stress response systems are hypothesized to be activated by acute excessive drug intake, to be sensitized during repeated withdrawal, to persist into protracted abstinence, and to contribute to the development and persistence of addiction. The recruitment of anti-reward systems provides a powerful neurochemical basis for the negative emotional states that are responsible for the dark side of addiction. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Adaptations underlying the development of excessive alcohol intake in selectively bred mice

BACKGROUND

This commentary discusses the important contributions of the article in this issue by Matson and colleagues entitled "Selectively bred crossed high-alcohol-preferring mice drink to intoxication and develop functional tolerance, but not locomotor sensitization during free-choice ethanol access" as well as providing comparison to studies on other drugs of abuse.

METHODS

The findings of the target article are evaluated and compared to the larger literature of intake escalation and vulnerability to addiction observed with other drugs of abuse.

RESULTS

In their study, Matson and colleagues report that mice derived by crossing different lines selectively bred for high alcohol intake exhibit initial alcohol intakes associated with motor impairment followed by marked escalation of consumption and tolerance to the effects of alcohol on motor coordination. In contrast, no evidence of pharmacokinetic tolerance or sensitization of alcohol-induced locomotion was observed. These results demonstrate that the cHAP mice constitute an appropriate model for the study of excessive drinking, which is produced by escalated alcohol intake and functional changes, leading to excessive intoxication.

CONCLUSIONS

Future work should assess adaptations in motivational processes and subjective effects of alcohol as well as the potential genetic and epigenetic bases of escalated alcohol intake.

Netrin-1 receptor-deficient mice show age-specific impairment in drug-induced locomotor hyperactivity but still self-administer methamphetamine

RATIONALE

The mesocorticolimbic dopamine system undergoes significant reorganization of neuronal connectivity and functional refinement during adolescence. Deleted in colorectal cancer (DCC), a receptor for the guidance cue netrin-1, is involved in this reorganization. Previous studies have shown that adult mice with a heterozygous (het) loss-of-function mutation in DCC exhibit impairments in sensitization and conditioned place preference (CPP) to psychostimulants. However, the commonly abused psychostimulant methamphetamine (METH) has not been assessed, and the role of DCC in drug self-administration remains to be established.

OBJECTIVES

Using dcc het mice and wildtype (WT) littermates, we extended previous findings on dcc haplodeficiency by examining self-administration of METH in adult mice, including cue-induced drug seeking following abstinence. We also examined hyperactivity, sensitization, and CPP to a METH-paired context in adult and adolescent mice.

RESULTS

While adult dcc het mice expressed largely similar METH self-administration and cue-induced drug seeking as WT littermates, they failed to modulate responding according to dose of METH. Compared to WT, both adult and adolescent dcc het mice expressed impaired locomotor hyperactivity to acute METH but nevertheless showed comparable behavioral sensitization. Conditioned hyperactivity increased with age in WT but not in dcc het mice.

CONCLUSIONS

Impaired METH-induced hyperactivity and dose-related responding in adult dcc het mice suggest that reduced DCC alters METH-related behaviors. Adolescence is identified as a vulnerable period during which impairment in hyperactivity due to reduced DCC can be overcome with repeated METH injections. Nevertheless, DCC appears to have a somewhat limited role in METH-consumption and seeking following abstinence.

Addiction is Not a Natural Kind

I argue that addiction is not an appropriate category to support generalizations for the purposes of scientific prediction. That is, addiction is not a natural kind. I discuss the Homeostatic Property Cluster (HPC) theory of kinds, according to which members of a kind share a cluster of properties generated by a common mechanism or set of mechanisms. Leading accounts of addiction in literature fail to offer a mechanism that explains addiction across substances. I discuss popular variants of the disease conception and demonstrate that at least one class of substances that fails to confirm a major prediction of each account. When no mechanism can be found to explain the occurrence of the relevant properties in members of a category, the HPC view suggests that we revise our categories. I discuss options offered by the HPC view, including category revision and category replacement. I then conclude that talk of addiction as a prediction-supporting category should be replaced with categories such as "S-addiction" and "T-addiction," where S and T are substances or sets of substances of abuse, as these categories are genuine natural kinds.

Extending therapeutic use of psychostimulants: focus on serotonin-1A receptor

INTRODUCTION

Despite a number of medicinally important pharmacological effects, the therapeutic use of psychostimulants is limited because of abuse potential and psychosis following long term use. Development of pharmacological agents for improving and extending therapeutic use of psychostimulants in narcolepsy, attention deficit hyperactivity disorder, Parkinson's disease, obesity and as cognitive enhancer is an important research imperative. In this regard, one potential target system is the 5-hydroxytryptamine (5-HT; serotonin) neurotransmitter system. The focus of the present article is to evaluate a potential role of 5-HT-1A receptor in the alleviation of abuse potential and psychosis-induced by prescription psychostimulants amphetamines and apomorphine.

METHOD

Synaptic contacts between dopamine systems and 5-HT-1A receptors are traced. Studies on serotonin-1A influences on the modulation of dopamine neurotransmission and psychostimulant-induced behavioral sensitization are accumulated.

RESULTS

Inhibition of amphetamine and apomorphine-induced behavioral sensitization by co administration of 5-HT-1A agonists cannot be explained in terms of direct activation of 5-HT-1A receptors, because activation of pre- as well as postsynaptic 5-HT-1A receptors tends to increase dopamine neurotransmission.

CONCLUSION

Long term use of amphetamine and apomorphine produces adaptive changes in 5-HT-1A receptor mediated functions, which are prevented by the co-use of 5-HT-1A agonists. In view of extending medicinal use of psychostimulants, it is important to evaluate the effects of co-use of 5-HT-1A agonists on potential therapeutic profile of amphetamine and apomorphine in preclinical research. It is also important to evaluate the functional significance of 5-HT-1A receptors on psychostimulant-induced behaviors in other addiction models such as drug self-administration and reinstatement of drug seeking behavior.

Addiction is a Reward Deficit and Stress Surfeit Disorder

Drug addiction can be defined by a three-stage cycle - binge/intoxication, withdrawal/negative affect, and preoccupation/anticipation - that involves allostatic changes in the brain reward and stress systems. Two primary sources of reinforcement, positive and negative reinforcement, have been hypothesized to play a role in this allostatic process. The negative emotional state that drives negative reinforcement is hypothesized to derive from dysregulation of key neurochemical elements involved in the brain reward and stress systems. Specific neurochemical elements in these structures include not only decreases in reward system function (within-system opponent processes) but also recruitment of the brain stress systems mediated by corticotropin-releasing factor (CRF) and dynorphin-κ opioid systems in the ventral striatum, extended amygdala, and frontal cortex (both between-system opponent processes). CRF antagonists block anxiety-like responses associated with withdrawal, block increases in reward thresholds produced by withdrawal from drugs of abuse, and block compulsive-like drug taking during extended access. Excessive drug taking also engages the activation of CRF in the medial prefrontal cortex, paralleled by deficits in executive function that may facilitate the transition to compulsive-like responding. Neuropeptide Y, a powerful anti-stress neurotransmitter, has a profile of action on compulsive-like responding for ethanol similar to a CRF1 antagonist. Blockade of the κ opioid system can also block dysphoric-like effects associated with withdrawal from drugs of abuse and block the development of compulsive-like responding during extended access to drugs of abuse, suggesting another powerful brain stress system that contributes to compulsive drug seeking. The loss of reward function and recruitment of brain systems provide a powerful neurochemical basis that drives the compulsivity of addiction.

Compulsive drug use and its neural substrates

Drug addiction is a chronic relapsing brain disease, characterized by compulsive drug use. Despite the fact that drug addiction affects millions of people worldwide, treatments for this disorder are limited in number and efficacy. In our opinion, understanding the neural underpinnings of drug addiction would open new avenues for the development of innovative treatments for this disorder. Based on an awareness that drug use and drug reward do not equal drug addiction, there has been increasing interest in developing animal models of addiction that mimick the loss of control over drug use more closely than existing models aimed at studying drug reward. The present review provides an overview of animal studies of compulsive drug use and the neural mechanisms involved. First, the employed models are summarized, with a particular emphasis on models of escalation of drug use and resistance to punishment. Next, we discuss mechanisms within the (ventral and dorsal) striatum and (central) amygdala that have recently been implicated in the compulsive seeking and taking of alcohol and cocaine. The studies discussed here provide a promising line of research that will advance our knowledge of the neural circuits involved in the self-destructive behavior that characterizes drug addiction.

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.

Animal studies of addictive behavior

It is increasingly recognized that studying drug taking in laboratory animals does not equate to studying genuine addiction, characterized by loss of control over drug use. This has inspired recent work aimed at capturing genuine addiction-like behavior in animals. In this work, we summarize empirical evidence for the occurrence of several DSM-IV-like symptoms of addiction in animals after extended drug use. These symptoms include escalation of drug use, neurocognitive deficits, resistance to extinction, increased motivation for drugs, preference for drugs over nondrug rewards, and resistance to punishment. The fact that addiction-like behavior can occur and be studied in animals gives us the exciting opportunity to investigate the neural and genetic background of drug addiction, which we hope will ultimately lead to the development of more effective treatments for this devastating disorder.

Discriminative inhibitory control of cocaine seeking involves the prelimbic prefrontal cortex

BACKGROUND

Recent neuroimaging studies have shown that people with cocaine addiction retain some degree of control over drug craving that correlates with neural activity in the lateral prefrontal cortex (PFC). Here, we report similar findings in a rat model of inhibitory control of cocaine seeking.

METHODS

Rats actively responding for cocaine were trained to stop responding when presented with a discriminative stimulus that signaled lack of reinforcement. Rats were then tested for inhibitory control of cocaine seeking in novel behavioral contexts and in circumstances when cocaine seeking is particularly intense (e.g., following drug priming). The role of neuronal activity in different subregions of the PFC was assessed using local pharmacologic inactivation and c-Fos immunohistochemistry.

RESULTS

Rats progressively acquired the ability to stop cocaine seeking, even during drug intoxication and after a long history of cocaine self-administration. Inhibitory control of cocaine seeking was flexible, sufficiently strong to block cocaine-primed reinstatement, and selectively depended on increased neuronal activity within the prelimbic PFC, which is considered the rodent functional homolog of the human lateral PFC.

CONCLUSIONS

Parallel evidence in both animal models and humans indicate that recruitment of prefrontal inhibitory control of drug seeking is still functional after prolonged cocaine use. Preclinical investigation of the mechanisms underlying this capacity may contribute to designing new behavioral and/or pharmacologic strategies to promote its use for the prevention of relapse in addiction.

Drug specificity in extended access cocaine and heroin self-administration

Increased drug availability can precipitate a rapid escalation of drug consumption in both vulnerable humans and laboratory animals. Drug intake escalation is observed across a broad spectrum of drugs of abuse, including stimulants, opiates, ethanol and phencyclidine. Whether and to what extent the processes underlying escalated levels of drug intake vary across different substances is poorly understood. The present study sought to address this question in rats self-administering both cocaine and heroin-two addictive drugs with both common and different neurobiological effects. In experiment 1, we determined how cocaine intake is initially related to heroin intake in non-escalated rats with a limited access to both drugs. In experiment 2, two groups of rats were initially allowed to self-administer either cocaine or heroin for 1 hour per day and then after behavioral stabilization, for 6 hours per day to precipitate drug intake escalation. In each group, dose-injection functions for cocaine and heroin self-administration were generated. In experiment 1, regardless of the dose, rats with a high intake of one drug did not necessarily have a high intake of the alternate drug. In experiment 2, escalated levels of heroin or cocaine self-administration did not generalize to the other drug. This outcome was confirmed in a third drug substitution experiment following different access lengths to cocaine self-administration (i.e. 1, 4 and 8 hours). The processes underlying spontaneous and escalated drug overconsumption appear thus to vary across different drugs of abuse. More research should be devoted in the future to these differences.

fMRI response in the medial prefrontal cortex predicts cocaine but not sucrose self-administration history

Repeated cocaine exposure induces long-lasting neuroadaptations that alter subsequent responsiveness to the drug. However, systems-level investigation of these neuroplastic consequences is limited. We employed a rodent model of drug addiction to investigate neuroadaptations associated with prolonged forced abstinence after long-term cocaine self-administration (SA). Since natural rewards also activate the mesolimbic reward system in a partially overlapping fashion as cocaine, our design also included a sucrose SA group. Rats were trained to self-administer cocaine or sucrose using a fixed-ratio one, long-access schedule (6 h/day for 20 days). A third group of naïve, sedentary rats served as a negative control. After 30 days of abstinence, the reactivity of the reward system was assessed with functional magnetic resonance imaging (fMRI) following an intravenous cocaine injection challenge. A strong positive fMRI response, as measured by fractional cerebral blood volume changes relative to baseline (CBV%), was seen in the sedentary control group in such cortico-limbic regions as medial prefrontal cortex and anterior cingulate cortex. In contrast, both the cocaine and sucrose SA groups demonstrated a very similar initial negative fMRI response followed by an attenuated positive response. The magnitude of the mPFC response was significantly correlated with the total amount of reinforcer intake during the training sessions for the cocaine SA but not for the sucrose SA group. Given that the two SA groups had identical histories of operant training and handling, this region-specific group difference revealed by regression analysis may reflect the development of neuroadaptive mechanisms specifically related to the emergence of addiction-like behavior that occurs only in cocaine SA animals.