Environmental modulation of cocaine self-administration in the rat

Locus coeruleus activation during environmental novelty gates cocaine-induced long-term hyperactivity of dopamine neurons

A key feature of the brain is the ability to handle novelty. Anything that is new will stimulate curiosity and trigger exploration. Novelty preference has been proposed to predict increased sensitivity to cocaine. Different brain circuits are activated by novelty, but three specific brain regions are critical for exploring a novel environment: the noradrenergic neurons originating from the locus coeruleus (LC), the dopaminergic neurons from the ventral tegmental area (VTA), and the hippocampus. However, how exploring a novel environment can interfere with the reward system and control cocaine impact on VTA dopamine neuron plasticity is unclear. Here, we first investigated the effects of exposure to a novel environment on the tonic electrophysiological properties of VTA dopamine neurons. Then, we explored how exposure to a novel environment controls cocaine-evoked plasticity in dopamine neurons. Our findings indicate that LC controls VTA dopamine neurons under physiological conditions but also after cocaine.

Factors modulating the incubation of drug and non-drug craving and their clinical implications

It was suggested in 1986 that cue-induced cocaine craving increases progressively during early abstinence and remains high during extended periods of time. Clinical evidence now supports this hypothesis and that this increase is not specific to cocaine but rather generalize across several drugs of abuse. Investigators have identified an analogous incubation phenomenon in rodents, in which time-dependent increases in cue-induced drug seeking are observed after abstinence from intravenous drug or palatable food self-administration. Incubation of craving is susceptible to variation in magnitude as a function of biological and/or the environmental circumstances surrounding the individual. During the last decade, the neurobiological correlates of the modulatory role of biological (sex, age, genetic factors) and environmental factors (environmental enrichment and physical exercise, sleep architecture, acute and chronic stress, abstinence reinforcement procedures) on incubation of drug craving has been investigated. In this review, we summarized the behavioral procedures adopted, the key underlying neurobiological correlates and clinical implications of these studies.

Presence of a social peer enhances acquisition of remifentanil self-administration in male rats

BACKGROUND

Social peers influence human drug use at every stage of addiction. Using a dual-compartment apparatus that allows for limited social contact, recent work has shown that cocaine self-administration is enhanced when two rats are trained to self-administer at the same time compared to rats trained alone or trained in the presence of a saline self-administration control peer. The current study measured social influence on self-administration of the short-acting opioid remifentanil using a dual-compartment operant conditioning chamber.

METHODS

Adult male rats were placed in one of five groups: (1) REMI-REMI group, in which both rats self-administered remifentanil; (2) REMI-SAL group, in which rats self-administered remifentanil in the presence of a peer that self-administered saline; (3) SAL-REMI group, in which rats self-administered saline in the presence of a peer that self-administered remifentanil; and (4) REMI ALONE and (5) SAL ALONE groups, in which rats administered their respective drugs alone (no peer). Self-administration was measured using a 2-lever procedure during acquisition, maintenance, increasing fixed-ratio, and dose-response phases.

RESULTS

The presence of a social peer enhanced drug intake during acquisition, regardless of the drug exposure of their peer. Additionally, active lever position significantly affected remifentanil intake during acquisition and maintenance, with the greatest influence occurring when the active lever was close to the peer.

CONCLUSION

The presence of a social peer in the drug-taking context potentiates remifentanil self-administration, regardless of the peer's drug access. Future studies utilizing the dual-compartment apparatus will help elucidate the neural mechanisms underlying social influence on opioid abuse.

Non-pharmacological factors that determine drug use and addiction

Based on their pharmacological properties, psychoactive drugs are supposed to take control of the natural reward system to finally drive compulsory drug seeking and consumption. However, psychoactive drugs are not used in an arbitrary way as pure pharmacological reinforcement would suggest, but rather in a highly specific manner depending on non-pharmacological factors. While pharmacological effects of psychoactive drugs are well studied, neurobiological mechanisms of non-pharmacological factors are less well understood. Here we review the emerging neurobiological mechanisms beyond pharmacological reinforcement which determine drug effects and use frequency. Important progress was made on the understanding of how the character of an environment and social stress determine drug self-administration. This is expanded by new evidence on how behavioral alternatives and opportunities for drug instrumentalization generate different patterns of drug choice. Emerging evidence suggests that the neurobiology of non-pharmacological factors strongly determines pharmacological and behavioral drug action and may, thus, give rise for an expanded system's approach of psychoactive drug use and addiction.

Distinct Populations of Neurons Activated by Heroin and Cocaine in the Striatum as Assessed by catFISH

Despite the still prevailing notion of a shared substrate of action for all addictive drugs, there is evidence suggesting that opioid and psychostimulant drugs differ substantially in terms of their neurobiological and behavioral effects. These differences may reflect separate neural circuits engaged by the two drugs. Here we used the catFISH (cellular compartment analysis of temporal activity by fluorescence in situ hybridization) technique to investigate the degree of overlap between neurons engaged by heroin versus cocaine in adult male Sprague Dawley rats. The catFISH technique is a within-subject procedure that takes advantage of the different transcriptional time course of the immediate-early genes homer 1a and arc to determine to what extent two stimuli separated by an interval of 25 min engage the same neuronal population. We found that throughout the striatal complex the neuronal populations activated by noncontingent intravenous injections of cocaine (800 μg/kg) and heroin (100 and 200 μg/kg), administered at an interval of 25 min from each other, overlapped to a much lesser extent than in the case of two injections of cocaine (800 μg/kg), also 25 min apart. The greatest reduction in overlap between populations activated by cocaine and heroin was in the dorsomedial and dorsolateral striatum (∼30% and ∼22%, respectively, of the overlap observed for the sequence cocaine-cocaine). Our results point toward a significant separation between neuronal populations activated by heroin and cocaine in the striatal complex. We propose that our findings are a proof of concept that these two drugs are encoded differently in a brain area believed to be a common neurobiological substrate to drug abuse.

Opposite environmental gating of the experienced utility ('liking') and decision utility ('wanting') of heroin versus cocaine in animals and humans: implications for computational neuroscience

BACKGROUND

In this paper, we reviewed translational studies concerned with environmental influences on the rewarding effects of heroin versus cocaine in rats and humans with substance use disorder. These studies show that both experienced utility ('liking') and decision utility ('wanting') of heroin and cocaine shift in opposite directions as a function of the setting in which these drugs were used. Briefly, rats and humans prefer using heroin at home but cocaine outside the home. These findings appear to challenge prevailing theories of drug reward, which focus on the notion of shared substrate of action for drug of abuse, and in particular on their shared ability to facilitate dopaminergic transmission.

AIMS

Thus, in the second part of the paper, we verified whether our findings could be accounted for by available computational models of reward. To account for our findings, a model must include a component that could mediate the substance-specific influence of setting on drug reward RESULTS: It appears of the extant models that none is fully compatible with the results of our studies.

CONCLUSIONS

We hope that this paper will serve as stimulus to design computational models more attuned to the complex mechanisms responsible for the rewarding effects of drugs in real-world contexts.

Heroin versus cocaine: opposite choice as a function of context but not of drug history in the rat

RATIONALE

Previous studies have shown that rats trained to self-administer heroin and cocaine exhibit opposite preferences, as a function of setting, when tested in a choice paradigm. Rats tested at home prefer heroin to cocaine, whereas rats tested outside the home prefer cocaine to heroin. Here, we investigated whether drug history would influence subsequent drug preference in distinct settings. Based on a theoretical model of drug-setting interaction, we predicted that regardless of drug history rats would prefer heroin at home and cocaine outside the home.

METHODS

Rats with double-lumen catheters were first trained to self-administer either heroin (25 μg/kg) or cocaine (400 μg/kg) for 12 consecutive sessions. Twenty-six rats were housed in the self-administration chambers (thus, they were tested at home), whereas 30 rats lived in distinct home cages and were transferred to self-administration chambers only for the self-administration session (thus, they were tested outside the home). The rats were then allowed to choose repeatedly between heroin and cocaine within the same session for seven sessions.

RESULTS

Regardless of the training drug, the rats tested outside the home preferred cocaine to heroin, whereas the rats tested at home preferred heroin to cocaine. There was no correlation between drug preference and drug intake during the training phase.

CONCLUSION

Drug preferences were powerfully influenced by the setting but, quite surprisingly, not by drug history. This suggests that, under certain conditions, associative learning processes and drug-induced neuroplastic adaptations play a minor role in shaping individual preferences for one drug or the other.

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'.

The Affective and Neural Correlates of Heroin versus Cocaine Use in Addiction Are Influenced by Environmental Setting But in Opposite Directions

Previous studies have shown that individuals with heroin and cocaine addiction prefer to use these drugs in distinct settings: mostly at home in the case of heroin and mostly outside the home in the case of cocaine. Here we investigated whether the context would modulate the affective and neural responses to these drugs in a similar way. First, we used a novel emotional task to assess the affective state produced by heroin or cocaine in different settings, based on the recollections of male and female drug users. Then we used fMRI to monitor neural activity during drug imagery (re-creating the setting of drug use) in male drug users. Consistent with our working hypothesis, the majority of participants reported a shift in the affective valence of heroin from mostly pleasant at home to mostly unpleasant outside the home (p < 0.0001). The opposite shift was observed for cocaine; that is, most participants who found cocaine pleasant outside the home found it unpleasant when taken at home (p < 0.0014). Furthermore, we found a double dissociation, as a function of drug and setting imagery, in BOLD signal changes in the left PFC and caudate, and bilaterally in the cerebellum (all p values <0.01), suggesting that the fronto-striatal-cerebellar network is implicated in the contextualization of drug-induced affect. In summary, we report that the same setting can influence in opposite directions the affective and neural response to psychostimulants versus opiates in humans, adding to growing evidence of distinct substrates for the rewarding effects of these two drug classes.SIGNIFICANCE STATEMENT The rewarding effects of addictive drugs are often thought to depend on shared substrates. Yet, environmental influences can unmask striking differences between psychostimulants and opiates. Here we used emotional tasks and fMRI to explore the influence of setting on the response to heroin versus cocaine in individuals with addiction. Simply moving from one setting to another significantly decreased heroin pleasure but increased cocaine pleasure, and vice versa. Similar double dissociation was observed in the activity of the fronto-striatal-cerebellar network. These findings suggest that the effects of opiates and psychostimulants depend on dissociable psychological and neural substrates and that therapeutic approaches to addiction should take into account the peculiarities of different drug classes and the settings of drug use.

Chronic cocaine administration induces long-term impairment in the drive to obtain natural reinforcers in high- but not low-demanding tasks

Repeated drug exposure induces short- and long-term neuroadaptations in brain reward circuitries that are normally involved in the regulation of motivation. Hence, repeated drug exposure has been suggested to also affect the drive to acquire natural reinforcers. Here, we tested how chronic exposure of rats to cocaine, as well as a subsequent withdrawal period, affects acquisition of natural reinforcers in high- and low-demanding tasks (HD and LD tasks, respectively). We chronically administered cocaine (i.p., 15 mg/kg once daily, or saline in control) for 30 days, followed by a 30-day withdrawal period. We tested the effect of this treatment on the acquisition of two natural appetitive reinforcers, namely self-administering a 10% sucrose solution and mounting a receptive female, under LD and HD conditions. During the cocaine exposure period, behavioral testing took place 18 hours after cocaine injection, namely after the acute pharmacologic effect of the drug dissipated. We show that chronic i.p. cocaine exposure decreased procurement of both reinforcers in HD but not in LD tasks. The effect was observed throughout the administration period with partial recovery after withdrawal. Taken together, we present empirical evidence that chronic exposure to a constant dose of cocaine is sufficient to reduce natural reinforcement, and that this decrease can outlast drug exposure. Importantly, such effects are observed only when high demands are opposing the consumption of the natural reinforcer.

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.

Ultrasonic vocalization in rats self-administering heroin and cocaine in different settings: evidence of substance-specific interactions between drug and setting

RATIONALE

Clinical and preclinical evidence indicates that the setting of drug use affects drug reward in a substance-specific manner. Heroin and cocaine co-abusers, for example, indicated distinct settings for the two drugs: heroin being used preferentially at home and cocaine preferentially outside the home. Similar results were obtained in rats that were given the opportunity to self-administer intravenously both heroin and cocaine.

OBJECTIVES

The goal of the present study was to investigate the possibility that the positive affective state induced by cocaine is enhanced when the drug is taken at home relative to a non-home environment, and vice versa for heroin.

METHODS

To test this hypothesis, we trained male rats to self-administer both heroin and cocaine on alternate days and simultaneously recorded the emission of ultrasonic vocalizations (USVs), as it has been reported that rats emit 50-kHz USVs when exposed to rewarding stimuli, suggesting that these USVs reflect positive affective states.

RESULTS

We found that Non-Resident rats emitted more 50-kHz USVs when they self-administered cocaine than when self-administered heroin whereas Resident rats emitted more 50-kHz USVs when self-administering heroin than when self-administering cocaine. Differences in USVs in Non-Resident rats were more pronounced during the first self-administration (SA) session, when the SA chambers were completely novel to them. In contrast, the differences in USVs in Resident rats were more pronounced during the last SA sessions.

CONCLUSION

These findings indicate that the setting of drug taking exerts a substance-specific influence on the ability of drugs to induce positive affective states.

Inflammatory Pain Promotes Increased Opioid Self-Administration: Role of Dysregulated Ventral Tegmental Area μ Opioid Receptors

Pain management in opioid abusers engenders ethical and practical difficulties for clinicians, often resulting in pain mismanagement. Although chronic opioid administration may alter pain states, the presence of pain itself may alter the propensity to self-administer opioids, and previous history of drug abuse comorbid with chronic pain promotes higher rates of opioid misuse. Here, we tested the hypothesis that inflammatory pain leads to increased heroin self-administration resulting from altered mu opioid receptor (MOR) regulation of mesolimbic dopamine (DA) transmission. To this end, the complete Freund's adjuvant (CFA) model of inflammation was used to assess the neurochemical and functional changes induced by inflammatory pain on MOR-mediated mesolimbic DA transmission and on rat intravenous heroin self-administration under fixed ratio (FR) and progressive ratio (PR) schedules of reinforcement. In the presence of inflammatory pain, heroin intake under an FR schedule was increased for high, but attenuated for low, heroin doses with concomitant alterations in mesolimbic MOR function suggested by DA microdialysis. Consistent with the reduction in low dose FR heroin self-administration, inflammatory pain reduced motivation for a low dose of heroin, as measured by responding under a PR schedule of reinforcement, an effect dissociable from high heroin dose PR responding. Together, these results identify a connection between inflammatory pain and loss of MOR function in the mesolimbic dopaminergic pathway that increases intake of high doses of heroin. These findings suggest that pain-induced loss of MOR function in the mesolimbic pathway may promote opioid dose escalation and contribute to opioid abuse-associated phenotypes.

SIGNIFICANCE STATEMENT

This study provides critical new insights that show that inflammatory pain alters heroin intake through a desensitization of MORs located within the VTA. These findings expand our knowledge of the interactions between inflammatory pain and opioid abuse liability, and should help to facilitate the development of novel and safer opioid-based strategies for treating chronic pain.

Differential vulnerability to relapse into heroin versus cocaine-seeking as a function of setting

RATIONALE

Previous studies have shown that the effect of setting on drug-taking is substance specific in both humans and rats. In particular, we have shown that when the setting of drug self-administration (SA) coincides with the home environment of the rats (resident rats), the rats tend to prefer heroin to cocaine. The opposite was found in nonresident rats, for which the SA chambers represented a distinct environment.

OBJECTIVES

The aim of the present study was to investigate the influence of setting on the ability of different doses of cocaine and heroin to prime cocaine- versus heroin-seeking in rats that had been trained to self-administer both drugs and had then undergone an extinction procedure.

METHODS

Resident (N = 62) and nonresident (N = 63) rats with double-lumen intra-jugular catheters were trained to self-administer cocaine (400 μg/kg/infusion) and heroin (25 μg/kg/infusion) on alternate days for 10 consecutive daily sessions (3 h each). After the extinction phase, independent groups of rats were given a noncontingent intravenous infusion of heroin (25, 50, or 100 μg/kg) or cocaine (400, 800, or 1600 μg/kg), and drug-seeking was quantified by counting nonreinforced lever presses.

RESULTS

All resident and nonresident rats acquired heroin and cocaine SA. However, cocaine primings reinstated cocaine-seeking only in nonresident rats, whereas heroin primings reinstated heroin-seeking only in resident rats.

CONCLUSIONS

We report here that the susceptibility to relapse into drug-seeking behavior is drug-specific and setting-specific, confirming the crucial role played by drug, set, and setting interactions in drug addiction.

Basal ganglia circuit loops, dopamine and motivation: A review and enquiry

Dopamine neurons located in the midbrain play a role in motivation that regulates approach behavior (approach motivation). In addition, activation and inactivation of dopamine neurons regulate mood and induce reward and aversion, respectively. Accumulating evidence suggests that such motivational role of dopamine neurons is not limited to those located in the ventral tegmental area, but also in the substantia nigra. The present paper reviews previous rodent work concerning dopamine's role in approach motivation and the connectivity of dopamine neurons, and proposes two working models: One concerns the relationship between extracellular dopamine concentration and approach motivation. High, moderate and low concentrations of extracellular dopamine induce euphoric, seeking and aversive states, respectively. The other concerns circuit loops involving the cerebral cortex, basal ganglia, thalamus, epithalamus, and midbrain through which dopaminergic activity alters approach motivation. These models should help to generate hypothesis-driven research and provide insights for understanding altered states associated with drugs of abuse and affective disorders.

Neurocircuitry of drug reward

In recent years, neuroscientists have produced profound conceptual and mechanistic advances on the neurocircuitry of reward and substance use disorders. Here, we will provide a brief review of intracranial drug self-administration and optogenetic self-stimulation studies that identified brain regions and neurotransmitter systems involved in drug- and reward-related behaviors. Also discussed is a theoretical framework that helps to understand the functional properties of the circuitry involved in these behaviors. The circuitry appears to be homeostatically regulated and mediate anticipatory processes that regulate behavioral interaction with the environment in response to salient stimuli. That is, abused drugs or, at least, some may act on basic motivation and mood processes, regulating behavior-environment interaction. Optogenetics and related technologies have begun to uncover detailed circuit mechanisms linking key brain regions in which abused drugs act for rewarding effects. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Substance-specific environmental influences on drug use and drug preference in animals and humans

Epidemiological, clinical, and preclinical evidence indicate that the setting of drug use can exert a powerful modulatory influence on drug reward and that this influence is substance-specific. When heroin and cocaine co-abusers, for example, report on the circumstances of drug use, they indicate distinct settings for the two drugs: heroin being used preferentially at home and cocaine being used preferentially outside the home. Similar results were obtained in laboratory rats. These findings will be interpreted in the light of a novel model of drug reward, based on the emotional appraisal of central and peripheral drug effects as a function of environmental context. I argue here that drug addiction research has not paid sufficient attention to the substance-specific aspects of drug abuse and this may have contributed to the present dearth of effective treatments. Pharmacological and cognitive-behavioral therapy, for example, should be tailored so as to allow the addict to anticipate, and cope with, the risks associated, in a substance-specific manner, to the different settings of drug use.

Exploratory studies in sensory reinforcement in male rats: effects of methamphetamine

Understanding sensory reinforcement and the effects of stimulant drugs on sensory reinforcers is potentially important for understanding their influence on addiction processes. Experiment 1 explored the reinforcing properties of a visual stimulus and the effects of methamphetamine (METH) on responding maintained by a visual reinforcer (VRF) in male rats. Snout poke responses to the active alternative produced the VRF according to variable interval (VI) schedules of reinforcement, and responses to an inactive alternative had no programmed effect. Experiment 2 explored the effects of METH on choice between the VRF and a water reinforcer (H2ORF) using concurrent VI schedules in male rats. In Experiment 1, response-contingent onset of the VRF produced an increase in both the relative frequency and absolute rate of active responding. The rate of both active and inactive responding declined across the 40-min test sessions. METH did not differentially enhance active responding for the VRF. Instead, METH nondifferentially increased the rate of responding and attenuated the within-session decline of responding. In Experiment 2, METH differentially increased the rate of responding for the VRF relative to the H2ORF. The results of these exploratory experiments indicate that the reinforcing effects of the VRF were weak and transient. In addition, METH treatment increased responding, and the specificity of the enhancement of METH was dependent upon the testing conditions. Potential explanations of these differences, such as novelty and reinforcer type, are discussed.

Opiate versus psychostimulant addiction: the differences do matter

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

The role of setting in the oral self-administration of alcohol in the rat

RATIONALE

We have previously found that rats that were kept at all times in the self-administration (SA) chambers (resident group) self-administered more heroin than rats that were transferred to the SA chambers immediately before testing (Non-Resident group). Alcohol resembles heroin in its ability to produce, at recreational doses, mood elevation, euphoria, drowsiness, and sedation. Furthermore, alcohol presents some similarities with the mechanisms of action of heroin at the levels of the mesostriatal circuitry. Therefore, we predicted that, as for heroin, alcohol intake would be greater in the Resident than in the Non-Resident group.

MATERIALS AND METHODS

In Experiment 1, oral self-administration of ethanol and wine solutions (2.5%, 5%, and 10%, v/v) was assessed in Resident and Non-Resident rats using both one-bottle (three sessions) and two-bottle (seven sessions) tests. In addition, we also assessed the intake of water (Experiment 2) and of 0.04% saccharin-0.003% quinine solution (Experiment 3).

RESULTS

During the one-bottle sessions, alcohol intake of Resident rats was up to two times that of Non-Resident rats. During the two-bottle sessions, Resident rats drank two times more 5% alcohol than water, whereas Non-Resident rats took equal amount of the two fluids. The average daily intake of pure ethanol for Resident rats given access to 5% solutions was 0.71 ± 0.076 vs. 0.46 ± 0.078 g/kg for Non-Resident rats. No group differences in the intake of water and of saccharin-quinine solution were found.

CONCLUSION

The present report demonstrates at a preclinical level the importance of setting for alcohol self-administration.

A novel IV cocaine self-administration procedure in rats: differential effects of dopamine, serotonin, and GABA drug pre-treatments on cocaine consumption and maximal price paid

RATIONALE

Behavior occurring during cocaine self-administration can be classified as either consummatory or appetitive. These two concepts are usually addressed independently using separate reinforcement schedules. For example, appetitive behavior can be assessed with a progressive ratio schedule, whereas consummatory behavior is typically measured using a fixed ratio schedule.

OBJECTIVES

Depending on the schedule used, it is often difficult to determine whether a particular drug pretreatment is affecting self-administration through an effect on appetitive responding, consummatory responding, or perhaps both. In the present study, we tested the effect of pretreating rats with four different drugs on appetitive and consummatory behaviors.

MATERIALS AND METHODS

We recently developed a technique that provides an independent assessment of both behavioral concepts within the same experimental session. In this threshold procedure, rats are offered a descending series of 11 unit doses (422-1.3 μg/injection) during consecutive timed intervals under a fixed-ratio schedule. Consummatory behavior can be analyzed by assessing intake at high unit doses; an estimate of appetitive responding can be determined from responding occurring at the threshold dose. Applying behavioral economics to these data provides dependent measures of consumption when minimally constrained by price and the maximal price paid (P (max)) for cocaine.

RESULTS

Haloperidol increased cocaine consumption when minimally constrained by price but decreased P (max). In contrast, D: -amphetamine increased P (max). Fluoxetine decreased P (max) and consumption when minimally constrained by price. Baclofen selectively decreased P (max).

CONCLUSIONS

These data suggest that drug pretreatments can alter consummatory and appetitive behavior differently because each concept involves distinct neural mechanisms.

Ketamine self-administration in the rat: evidence for a critical role of setting

RATIONALE

The abuse of ketamine has been reported to be on the rise over the past 15 years, but its abuse appears to be limited almost exclusively to the context of music and dance settings, indicating a major role of context in modulating its reinforcing effects. We have previously reported that amphetamine, cocaine, and heroin self-administration (SA) in the rat are differentially influenced by the setting in which testing takes place. The aim of the present study is to extend this pre-clinical model to ketamine.

MATERIALS AND METHODS

Independent groups of rats with intravenous catheters were given the possibility to self-administer different doses of ketamine (125, 250, and 500 μg/kg per infusion) under two environmental conditions. Some animals were housed in the SA chambers (resident rats) whereas other rats were transported to the SA chambers only for the test sessions (non-resident rats). After training, within-subject dose effect curves (125, 250, 500, and 1,000 μg/kg per infusion) and break-point (during a progressive ratio session) were calculated.

RESULTS

Non-resident rats readily acquired ketamine self-administration. In contrast, resident rats self-administered only the highest dose of ketamine (500 μg/kg), but still four times less than non-resident rats (11.0 ± 6.0 vs 44.4 ± 5.2 infusions during the last training session). No significant differences in break-point were found during the progressive ratio session.

CONCLUSIONS

The present study confirms at a preclinical level the importance of setting for ketamine SA and further validates a previously described animal model of drug-environment interaction.

Drug context differently regulates cocaine versus heroin self-administration and cocaine- versus heroin-induced Fos mRNA expression in the rat

RATIONALE

We have previously reported that cocaine self-administration is facilitated in male rats not residing in the test chambers (Non Resident rats) relative to rats living in the test chambers at all times (Resident rats). Surprisingly, the opposite was found for heroin.

MATERIALS AND METHODS

We predicted that, when given access to both cocaine and heroin on alternate days, Non Resident rats would take more cocaine relative to heroin than Resident rats. Heroin (25.0 microg/kg) and cocaine (400 microg/kg), were made alternately available for 14 self-administration sessions, on a fixed ratio (FR) schedule that was progressively increased from FR1 to FR5. Next, some rats underwent a progressive-ratio procedure for heroin and cocaine. The other rats continued to alternate heroin and cocaine self-administration for 12 additional sessions, during which the FR schedule was progressively increased from FR10 to FR100. The second aim of the study was to investigate Fos mRNA expression in Resident and Non Resident rats treated with non-contingent intravenous infusion of "self-administration doses" of heroin (25.0 microg/kg) and cocaine (400 microg/kg).

RESULTS

We found that: (1) drug-taking context differentially modulates intravenous cocaine versus heroin self-administration; (2) very low doses of cocaine and heroin are sufficient to induce Fos mRNA expression in the posterior caudate; (3) drug-administration context differentially modulates cocaine- versus heroin-induced Fos mRNA expression.

CONCLUSIONS

Our study indicates that the context of drug taking can play a powerful role in modulating cocaine versus heroin intake in the laboratory rat.

Ambience and drug choice: cocaine- and heroin-taking as a function of environmental context in humans and rats

BACKGROUND

We have recently observed an unforeseen dissociation in the effect of environmental context on heroin versus cocaine self-administration in rats. Rats housed in the self-administration chambers (Residents) took more heroin than rats that were transferred to the self-administration chambers only for the test sessions (Nonresidents). The contrary was found for cocaine. The twofold aim of the present study was to investigate: 1) drug choice as a function of ambience in rats given access to both cocaine and heroin, and 2) ambience of choice for cocaine- versus heroin-taking in human addicts.

METHODS

Resident and Nonresident rats with double-lumen intrajugular catheters were trained to self-administer cocaine (400 microg/kg/infusion) and heroin (25 microg/kg/infusion) on alternate days and then given the opportunity to choose between the two drugs during seven daily sessions. In the human study, we asked heroin and cocaine abusers where they preferred to take these drugs.

RESULTS

Approximately 46.7% of Resident rats exhibited a preference for heroin over cocaine; 33.3% preferred cocaine, and 20% expressed no preference. In contrast, only 8.3% of Nonresident rats preferred heroin, whereas 66.7% preferred cocaine, and 25% expressed no preference. In the human study, 73% of co-abusers reported that they used heroin exclusively or mostly at home (22% used it outside the home), whereas only 25% reported using cocaine at home (67% took it outside their homes).

CONCLUSIONS

Environmental context plays an important role in drug choice in both humans and rats self-administering heroin and cocaine.

Opposite environmental regulation of heroin and amphetamine self-administration in the rat

RATIONALE

The circumstances of drug taking are thought to play a role in drug abuse but the evidence of it is anecdotal. Previous studies have shown that the intravenous self-administration of cocaine is facilitated in rats non-residing in the test chambers relative to rats that live in the test chambers at all times. We investigated here whether environmental context could exert its modulatory influence on heroin and amphetamine self-administration as well.

MATERIALS AND METHODS

Independent groups of rats were given the possibility to self-administer different doses of heroin or amphetamine (12.5, 25.0, or 50.0 microg/kg). Some animals were housed in the self-administration chambers (resident groups) whereas other rats were transported to the self-administration chambers only for the test sessions (non-resident groups).

RESULTS

Amphetamine-reinforcing effects were more pronounced in non-resident rats than in resident rats, as previously reported for cocaine. Quite unexpectedly, the opposite was found for heroin. Because of this surprising dissociation, some of the rats trained to self-administer amphetamine were later given the opportunity to self-administer heroin. Also in this case, resident rats took more heroin than non-resident rats.

CONCLUSIONS

These findings suggest an unforeseen dissociation between opioid and psychostimulant reward and demonstrate that even in the laboratory rat some contexts are associated with the propensity to self-administer more opioid than psychostimulant drugs and vice versa, thus indicating that drug taking is influenced not only by economical or cultural factors but also can be modulated at a much more basic level by the setting in which drugs are experienced.

Hippocampal regulation of contextual cue-induced reinstatement of cocaine-seeking behavior

Associations between cocaine and cues facilitate development and maintenance of addiction. We hypothesized that the ventral hippocampus is important for acquisition of these associations. Rats were trained to self-administer cocaine, with or without pre-exposure to distinct sets of cocaine- and saline-paired contextual cues. Next, rats were conditioned for 3 days with the distinct sets of contextual cues paired with cocaine and saline along with distinct discrete cues. Vehicle or lidocaine was infused into the ventral hippocampus prior to conditioning sessions. Following extinction, reinstatement of cocaine-seeking behavior was examined following exposure to contextual cues, discrete cues, or their combination. Inactivation of the ventral hippocampus during conditioning blocked acquisition of the association between cocaine and cocaine-paired contextual cues in that only lidocaine-treated rats with short-term cue exposure failed to reinstate responding in the presence of cocaine-paired contextual cues. Lidocaine also prevented rats in both cue exposure groups from discriminating between cocaine- and saline-paired contextual cues during reinstatement tests. Reinstatement induced by cocaine-paired discrete cues or by contextual and discrete cues together was not impaired for either cue exposure condition. The hippocampus is important for acquisition of the association between cocaine and context and in maintaining discrimination between cocaine-relevant and -irrelevant contextual cues.

Teratogenic effects of maternal antidepressant exposure on neural substrates of drug-seeking behavior in offspring

If neurotransmitter balance is upset in the developing nervous system by exposure to antidepressant drugs, structural and functional hedonic phenotypes of offspring may be affected. In order to test this hypothesis, two groups of pregnant Wistar dams were exposed to vehicle or fluoxetine by implantation on gestational day 14 of osmotic minipumps delivering 0 or 10 mg/kg/day fluoxetine for 14 days. The consequences of perinatal fluoxetine exposure on offspring conflict-exploratory behavior were quantified using the elevated plus-maze on postnatal day (PND) 30. Beginning on PND 60, the reinforcing properties of acutely administered cocaine were examined using a place conditioning procedure. Beginning on PND 90, a subset of rats were implanted with jugular catheters and allowed to acquire self-administration of cocaine in an operant environment. In support of the hedonic modulation hypothesis, perinatal fluoxetine produced a significant decline in both nucleus accumbens cell count (-9%) and serotonin transporter-like immunoreactivity in the raphe nucleus (-35%) on PND 120. In the elevated plus-maze, perinatal fluoxetine exposure decreased (-21%) overall activity. In the place conditioning trial, only the fluoxetine-treated group exhibited a significant place preference for the compartment paired previously with cocaine. In a cocaine self-administration extinction trial, there was a statistically significant increase (350%) in extinction response rate among fluoxetine-exposed offspring. These findings suggest that perinatal exposure to fluoxetine perturbs adult serotonergic neurotransmission and produces a positive hedonic shift for conditioned reinforcing effects of cocaine.

Modeling the role of environment in addiction

The aim of this review is to provide an overview of the main types of animal models used to investigate the modulatory role of environment on drug addiction. The environment can alter the responsiveness to addictive drugs in at least three major ways. First, adverse life experiences can make an individual more vulnerable to develop drug addiction or to relapse into drug seeking. Second, neutral environmental cues can acquire, through Pavlovian conditioning, the ability to trigger drug seeking even after long periods of abstinence. Third, the environment immediately surrounding drug taking can alter the behavioral, subjective, and rewarding effects of a given drug, thus influencing the propensity to use the same drug again. We have focused in particular on the results obtained using an animal model we have developed to study the latter type of drug-environment interaction.