Preclinical research on cocaine self-administration: environmental determinants and their interaction with pharmacological treatment

Nonhuman animal models of substance use disorders: Translational value and utility to basic science

BACKGROUND

The National Institute on Drug Abuse (NIDA) recently released a Request for Information (RFI) soliciting comments on nonhuman animal models of substance use disorders (SUD).

METHODS

A literature review was performed to address the four topics outlined in the RFI and one topic inspired by the RFI: (1) animal models that best recapitulate SUD, (2) animal models that best balance the trade-offs between resources and ecological validity, (3) animal models whose translational value are frequently misrepresented or overrepresented by the scientific community, (4) aspects of SUD that are not currently being modeled in animals, and (5) animal models that are optimal for examining the basic mechanisms by which drugs produce their abuse-related effects.

RESULTS

Models that employ response-contingent drug administration, use complex schedules of reinforcement, measure behaviors that mimic the distinguishing features of SUD, and use animals that are phylogenetically similar to humans have the greatest translational value. Models that produce stable and reproducible baselines of behavior, lessen the number of uncontrolled variables, and minimize the influence of extraneous factors are best at examining basic mechanisms contributing to drug reward and reinforcement.

CONCLUSIONS

Nonhuman animal models of SUD have undergone significant refinements to increase their utility for basic science and translational value for SUD. The existing literature describes numerous examples of how these models may best be utilized to answer mechanistic questions of drug reward and identify potential therapeutic interventions for SUD. Progress in the field could be accelerated by further collaborations between researchers using animals versus humans.

Abstinence environment contributes to age differences in reinstatement of cocaine seeking between adolescent and adult male rats

RATIONALE

Extinction responding and cue-induced reinstatement of cocaine seeking after 60-days of forced abstinence are attenuated in male rats that self-administered cocaine during adolescence, compared with adults. Given that environmental enrichment during abstinence decreases reinstatement among adults, a possible explanation for attenuated reinstatement among adolescents is that standard pair-housing in prior studies creates a more stimulating environment for younger rats.

OBJECTIVE

Therefore, we tested whether standard pair-housing is necessary for the attenuated reinstatement among adolescents by determining whether an impoverished environment during abstinence would increase reinstatement among adolescents, up to adult levels. Conversely, we also tested whether environmental enrichment could further decrease reinstatement among adolescents, and whether we could replicate effects of environmental enrichment to decrease reinstatement among adults down to adolescent levels (positive controls).

METHODS

Adolescent and adult male Wistar rats self-administered cocaine intravenously for 12days (fixed ratio 1; 0.36mg/kg per infusion; 2h sessions). Rats were then moved into enriched (grouped, large cages, novel toys), standard (pair-housed, shoebox cages), or impoverished (isolated, hanging cages) housing conditions. After 60days, extinction and cue-induced reinstatement of cocaine seeking were tested, followed by drug-primed reinstatement (0, 5, 10mg/kg cocaine, i.p.).

RESULTS

Consistent with previous results, extinction and cue-induced reinstatement were attenuated in adolescent-onset groups compared with adults; this age difference also extended to drug-primed reinstatement. In support of the present hypothesis, an impoverished environment during abstinence increased reinstatement among adolescents to levels that were not different from adult standard-housing levels.

CONCLUSIONS

These data suggest that abstinence environment influences the enduring effects of cocaine among adolescents as well as adults.

Serotonin 5-HT2 receptor interactions with dopamine function: implications for therapeutics in cocaine use disorder

Cocaine exhibits prominent abuse liability, and chronic abuse can result in cocaine use disorder with significant morbidity. Major advances have been made in delineating neurobiological mechanisms of cocaine abuse; however, effective medications to treat cocaine use disorder remain to be discovered. The present review will focus on the role of serotonin (5-HT; 5-hydroxytryptamine) neurotransmission in the neuropharmacology of cocaine and related abused stimulants. Extensive research suggests that the primary contribution of 5-HT to cocaine addiction is a consequence of interactions with dopamine (DA) neurotransmission. The literature on the neurobiological and behavioral effects of cocaine is well developed, so the focus of the review will be on cocaine with inferences made about other monoamine uptake inhibitors and releasers based on mechanistic considerations. 5-HT receptors are widely expressed throughout the brain, and several different 5-HT receptor subtypes have been implicated in mediating the effects of endogenous 5-HT on DA. However, the 5-HT2A and 5-HT2C receptors in particular have been implicated as likely candidates for mediating the influence of 5-HT in cocaine abuse as well as to traits (e.g., impulsivity) that contribute to the development of cocaine use disorder and relapse in humans. Lastly, new approaches are proposed to guide targeted development of serotonergic ligands for the treatment of cocaine use disorder.

Cocaine use in the past year is associated with altitude of residence

OBJECTIVES

Recently, increased rates of suicide in US counties at higher altitudes have been noted. Because of the documented association between cocaine use and suicide, we hypothesized that there would be a correlation between incidence of cocaine use and altitude of residence.

METHODS

Cocaine use data were obtained from the Substate Substance Abuse Estimates from the 1999-2001 National Surveys on Drug Use and Health. Data related to the percentages of people 12 years or older who used cocaine in the past year. Average elevation for US counties was calculated using the Shuttle Radar Topography Mission elevation data set, and subject region elevation was calculated by averaging the weighted elevations of each region's relevant counties. The correlation between elevation of a substate region and incidence of cocaine use in that region was calculated using Pearson correlation coefficients.

RESULTS

A significant correlation exists between mean altitude of a substate region and incidence of cocaine use in that region (r = 0.34; P < 0.0001). Regression analysis controlling for age, sex, race, education level, income, unemployment, and population density was performed. Altitude remained a significant factor (P = 0.007), whereas male sex (P = 0.008) and possessing less than a college education (P < 0.0001) were also significant predictors of self-reported cocaine use in the past year. It is important to note that cocaine use was assessed in isolation of other drugs of abuse, an additional confounding variable.

CONCLUSIONS

This study demonstrates a significant correlation between altitude of substate region of residence and incidence of cocaine use. It is possible that stress response due to hypoxia is responsible; however, this requires further investigation. However, because other substance use was not assessed, specificity of this association is unknown. In addition, this correlation may help explain the increased rate of suicide in areas of higher elevation.

Access to a running wheel decreases cocaine-primed and cue-induced reinstatement in male and female rats

BACKGROUND

Relapse to drug use after a period of abstinence is a persistent problem in the treatment of cocaine dependence. Physical activity decreases cocaine self-administration in laboratory animals and is associated with a positive prognosis in human substance-abusing populations. The purpose of this study was to examine the effects of long-term access to a running wheel on drug-primed and cue-induced reinstatement of cocaine-seeking behavior in male and female rats. methods: Long-Evans rats were obtained at weaning and assigned to sedentary (no wheel) and exercising (access to wheel) groups for the duration of the study. After 6 weeks, rats were implanted with intravenous catheters and trained to self-administer cocaine for 14 days. After training, saline was substituted for cocaine and responding was allowed to extinguish, after which cocaine-primed reinstatement was examined in both groups. Following this test, cocaine self-administration was re-established in both groups for a 5-day period. Next, a second period of abstinence occurred in which both cocaine and the cocaine-associated cues were withheld. After 5 days of abstinence, cue-induced reinstatement was examined in both groups.

RESULTS

Sedentary and exercising rats exhibited similar levels of cocaine self-administration, but exercising rats responded less than sedentary rats during extinction. In tests of cocaine-primed and cue-induced reinstatement, exercising rats responded less than sedentary rats, and this effect was apparent in both males and females.

CONCLUSIONS

These data indicate that long-term access to a running wheel decreases drug-primed and cue-induced reinstatement, and that physical activity may be effective at preventing relapse in substance-abusing populations.

The inhibition of histone deacetylases reduces the reinstatement of cocaine-seeking behavior in rats

Drug addiction is a chronic brain disease characterized by a persistent risk of relapse, even after a long period of abstinence. A current hypothesis states that relapse results from lasting neuroadaptations that are induced in response to repeated drug administration. The adaptations require gene expression, some of which being under the control of stable epigenetic regulations. We have previously demonstrated that pretreatment with histone deacetylase (HDAC) inhibitors reduces the cocaine reinforcing properties as well as the motivation of rats for cocaine. We show here that the same HDAC inhibitors, trichostatin A and phenylbutyrate, significantly reduced the cocaine-seeking behavior induced by the combination of a cocaine injection together with the exposure to a light cue previously associated with cocaine taking. Reinstatement of drug-seeking behavior was carried out after a 3-week withdrawal period, which came after ten daily sessions of cocaine intravenous self-administration. Our results suggest that pharmacological treatment aimed at modulating epigenetic regulation, and particularly treatment that would inhibit HDAC activity, could reduce the risk of relapse, a major drawback in the treatment of drug addiction.

Hypocretin 1/orexin A in the ventral tegmental area enhances dopamine responses to cocaine and promotes cocaine self-administration

RATIONALE

Recent evidence indicates that the hypocretin/orexin system participates in the regulation of reinforcement and addiction processes. For example, manipulations that decrease hypocretin neurotransmission result in disruptions of neurochemical and behavioral responses to cocaine.

OBJECTIVES

To further assess the relationship between the hypocretin system and cocaine reinforcement, the current studies used microdialysis and in vivo voltammetry to examine the effects of hypocretin 1 on cocaine-induced enhancement of dopamine signaling in the nucleus accumbens core. Fixed ratio, discrete trials, and progressive ratio self-administration procedures were also used to assess whether hypocretin 1 promotes cocaine self-administration behavior.

RESULTS

Infusions of hypocretin 1 into the ventral tegmental area increased the effects of cocaine on tonic and phasic dopamine signaling and increased the motivation to self-administer cocaine on the discrete trials and progressive ratio schedules.

CONCLUSIONS

Together with previous observations demonstrating that a hypocretin 1 receptor antagonist disrupts dopamine signaling and reduces self-administration of cocaine, the current observations further indicate that the hypocretin system participates in reinforcement processes likely through modulation of the mesolimbic dopamine system.

Cocaine self-administration alters the expression of chromatin-remodelling proteins; modulation by histone deacetylase inhibition

Injection of the histone deacetylases inhibitor trichostatin A to rats has been shown to decrease the reinforcing properties of cocaine. In the present study, we investigated alterations in gene expression patterns in the anterior cingulate cortex, caudate-putamen and nucleus accumbens of rats self-administering cocaine and treated with trichostatin A. As recent studies highlighted the importance of chromatin remodelling in the regulation of gene transcription in neurons, we studied the expression of Mecp2 and of several histone deacetylases. Cocaine self-administration was accompanied by an increased synthesis of Mecp2, HDAC2 and HDAC11 and by a decreased nuclear localization of HDAC5 and of the phospho-form of HDAC5, suggesting a nuclear export of this protein in response to the drug. The latter mechanism was further addressed by the demonstration of an enhanced expression of MEF2C transcription factor. Among the genes we examined, treatment with trichostatin A before each cocaine self-administration session was found to mostly affect Mecp2 and HDAC11 expression. A correlation was found between the modification of Mecp2 and MEF2C gene expression and the reinforcing property of cocaine. The two factors known to regulate gene transcription are likely to play a role in the neurobiological mechanism underlying a decrease in the reinforcing properties of cocaine.

Is slow-onset long-acting monoamine transport blockade to cocaine as methadone is to heroin? Implication for anti-addiction medications

The success of methadone in treating opiate addiction has suggested that long-acting agonist therapies may be similarly useful for treating cocaine addiction. Here, we examined this hypothesis, using the slow-onset long-acting monoamine reuptake inhibitor 31,345, a trans-aminotetralin analog, in a variety of addiction-related animal models, and compared it with methadone's effects on heroin's actions in the same animal models. Systemic administration of 31,345 produced long-lasting enhancement of electrical brain-stimulation reward (BSR) and extracellular nucleus accumbens (NAc) dopamine (DA). Pretreatment with 31,345 augmented cocaine-enhanced BSR, prolonged cocaine-enhanced NAc DA, and produced a long-term (24-48  h) reduction in cocaine self-administration rate without obvious extinction pattern, suggesting an additive effect of 31,345 with cocaine. In contrast, methadone pretreatment not only dose-dependently inhibited heroin self-administration with an extinction pattern but also dose-dependently inhibited heroin-enhanced BSR and NAc DA, suggesting functional antagonism by methadone of heroin's actions. In addition, 31,345 appears to possess significant abuse liability, as it produces dose-dependent enhancement of BSR and NAc DA, maintains a low rate of self-administration behavior, and dose-dependently reinstates drug-seeking behavior. In contrast, methadone only partially maintains self-administration with an extinction pattern, and fails to induce reinstatement of drug-seeking behavior. These findings suggest that 31,345 is a cocaine-like slow-onset long-acting monoamine transporter inhibitor that may act as an agonist therapy for cocaine addiction. However, its pattern of action appears to be significantly different from that of methadone. Ideal agonist substitutes for cocaine should fully emulate methadone's actions, that is, functionally antagonizing cocaine's action while blocking monoamine transporters to augment synaptic DA.

The hypocretin-orexin system regulates cocaine self-administration via actions on the mesolimbic dopamine system

Recent evidence suggests that the hypocretin-orexin system participates in the regulation of reinforcement processes. The current studies examined the extent to which hypocretin neurotransmission regulates behavioral and neurochemical responses to cocaine, and behavioral responses to food reinforcement. These studies used a combination of fixed ratio, discrete trials, progressive ratio and threshold self-administration procedures to assess whether the hypocretin 1 receptor antagonist, SB-334867, reduces cocaine self-administration in rats. Progressive ratio sucrose self-administration procedures were also used to assess the extent to which SB-334867 reduces responding to a natural reinforcer in food-restricted and food-sated rats. Additionally, these studies used microdialysis and in vivo voltammetry in rats to examine whether SB-334867 attenuates the effects of cocaine on dopamine signaling within the nucleus accumbens core. Furthermore, in vitro voltammetry was used to examine whether hypocretin knockout mice display attenuated dopamine responses to cocaine. Results indicate that when SB-334867 was administered peripherally or within the ventral tegmental area, it reduced the motivation to self-administer cocaine and attenuated cocaine-induced enhancement of dopamine signaling. SB-334867 also reduced the motivation to self-administer sucrose in food-sated but not food-restricted rats. Finally, hypocretin knockout mice displayed altered baseline dopamine signaling and reduced dopamine responses to cocaine. Combined, these studies suggest that hypocretin neurotransmission participates in reinforcement processes, likely through modulation of the mesolimbic dopamine system. Additionally, the current observations suggest that the hypocretin system may provide a target for pharmacotherapies to treat cocaine addiction.

Toward a nonhuman model of contingency management: effects of reinforcing abstinence from nicotine self-administration in rats with an alternative nondrug reinforcer

RATIONALE

Reinforcing abstinence from drug use with alternative nondrug reinforcers (e.g., contingency management) is one of the most effective interventions for drug abuse. While nonhuman studies have also shown that access to alternative nondrug reinforcers reduces drug self-administration, this effect has not been examined in nicotine self-administration models. Moreover, abstinence contingencies per se under free-operant conditions have not been examined.

OBJECTIVE

The objective of this experiment was to begin development of a model of contingency management interventions by employing a differential-reinforcement-of-alternative-behavior (DRA) schedule of alternative nondrug reinforcement in rats self-administering nicotine.

METHODS

Two groups of rats were trained to self-administer nicotine under a multiple schedule of nicotine and sucrose delivery. The DRA-group was then exposed to an interlocking FR3 nicotine DRA t-sec sucrose schedule. Under this schedule, nicotine continued to be available under the FR schedule while a sucrose pellet was made available contingent upon every pause in self-administration responding (DRA interval) of 40, 80, or 160 s. The FT-group was exposed to noncontingent delivery of sucrose under fixed time (FT) schedules at an average rate equal to that obtained under the DRA schedule in the DRA-group.

RESULTS

The DRA schedule significantly reduced NSA by 73, 69, and 59% at the DRA 40, 80, and 160 s intervals, respectively, compared to baseline, while noncontingent sucrose had no significant effect. The effect of the DRA schedule was apparent throughout the NSA sessions.

CONCLUSIONS

The present assay approximates the abstinence contingencies arranged in contingency management interventions for drug abuse and provides a preliminary nonhuman model of such interventions.

Histone deacetylase inhibitors decrease cocaine but not sucrose self-administration in rats

Regulation of gene expression is known to contribute to the long-term adaptations taking place in response to drugs of abuse. Recent studies highlighted the regulation of gene transcription in neurons by chromatin remodeling, a process in which posttranslational modifications of histones play a major role. To test the involvement of epigenetic regulation on drug-reinforcing properties, we submitted rats to the cocaine operant self-administration paradigm. Using the fixed ratio 1 schedule, we found that the histone deacetylase (HDAC) inhibitors trichostatin A and phenylbutyrate dose-dependently reduced cocaine self-administration. Under the progressive ratio schedule, both trichostatin A and depudecin significantly reduced the breaking point, indicating that HDAC inhibition attenuated the motivation of rats for cocaine. Conversely, HDAC inhibition did not decrease self-administration for the natural reinforcer sucrose. This observation was correlated with measurements of HDAC activity in the frontal cortex, which was inhibited in response to cocaine, but not to sucrose self-administration. Control experiments showed that the decrease in the motivation for the drug was not attributable to a general motivational dysfunction because trichostatin A had no adverse effect on locomotion during the habituation session or on cocaine-induced hyperlocomotion. It was not attributable to anhedonia because the inhibitor had no effect on the sucrose preference test. In contrast, trichostatin A completely blocked the cocaine-induced behavioral sensitization. Together, the data show that epigenetic regulation of gene transcription in adult brain is able to influence a motivated behavior and suggest that HDAC inhibition may counteract the neural sensitization leading to drug dependence.

Differential Rearing Conditions Alter Operant Responding for Ethanol in Outbred Rats

BACKGROUND

Differential rearing environments affect a number of behaviors displayed by rats in adulthood. For example, rats reared in an impoverished condition (IC; reared alone in hanging metal cages), social condition (SC; reared in standard shoebox cages, 2 per cage), or enriched condition (EC; reared in a large metal cage with bedding, 14 novel objects, and 10 cohorts) display clear differences in the amount of drug they consume and/or self-administer through operant responding. Animals reared in an EC consume greater amounts of ethanol compared with rats reared in an IC when provided free access, but it is not known how differential rearing conditions affect operant responding for ethanol.

METHODS

Twenty-eight male Long-Evans rats were reared in 1 of 3 environments (IC, SC, or EC) during postnatal days 21 to 111. At the conclusion of the rearing period, all rats underwent sucrose/ethanol fading and then were tested for lever press responding for 10% ethanol as well as ethanol preference.

RESULTS

Rats reared in an IC responded for 10% ethanol at significantly higher rates than SC and EC rats. A greater percentage of IC rats were able to switch lever responding when the ethanol availability was changed to a second lever. Lastly, the IC group was the only one to display a clear preference for 10% ethanol when both this fluid and water were available.

CONCLUSION

Rats reared in an IC show greater proclivity to respond operantly for 10% ethanol compared with rats raised in either SC or EC (which did not differ from each other). These findings agree with a number of studies that have shown isolate reared animals to consume greater amounts of ethanol compared with their socially reared counterparts yet contrast some studies showing EC animals consume greater amounts of ethanol than IC rats. The current findings illustrate that rearing environment also plays an important role in an animal's proclivity to respond for ethanol.

Modulation of the endocannabinoid system: therapeutic potential against cocaine dependence

Dependence on cocaine is still a main unresolved medical and social concern, and in spite of research efforts, no pharmacological therapy against cocaine dependence is yet available. Recent studies have shown that the endocannabinoid system participates in specific stages and aspects of drug dependence in general, and some of this evidence suggests an involvement of the cannabinoid system in cocaine effects. For example, cocaine administration has been shown to alter brain endocannabinoid levels, and the endocannabinoid system has been involved in long-term modifications of brain processes that might play a role in neuro/behavioral effects of psychostimulant drugs like cocaine. Human studies show that marijuana dependence is frequently associated with cocaine dependence, and that the cannabinoid receptor CNR1 gene polymorphism might be related to cocaine addiction. This article will review the main papers in the field showing how a modulation of different components of the cannabinoid system might interact with some of the neurobiological/behavioral effects of cocaine related to its reinforcing effects, evaluated in preclinical models or in clinical settings. The goal of this review will be to provide insights into the complex picture of cocaine abuse and addiction, and to extrapolate from such endocannabinoid-cocaine interactions useful information to test the therapeutic potential of cannabinoid ligands and endocannabinoid-level enhancers against cocaine dependence for future preclinical/clinical trials.

Blocking of conditioning to a cocaine-paired stimulus: testing the hypothesis that cocaine perpetually produces a signal of larger-than-expected reward

According to a recent account of addiction, dopaminergic effects of drugs like cocaine mimic the neuronal signal that occurs when a natural reward has a larger value than expected. Consequently, the drug's expected reward value increases with each administration, leading to an over-selection of drug-seeking behavior. One prediction of this hypothesis is that the blocking effect, a cornerstone of contemporary learning theory, should not occur with drug reinforcers. To test this prediction, two groups of rats were trained to self-administer cocaine with a nose-poking response. For 5 sessions, a tone was paired with each self-administered injection (blocking group), or no stimulus was paired with injection (non-blocking group). Then, in both groups, the tone and a light were both paired with each injection for 5 sessions. In subsequent testing, the light functioned as a conditioned reinforcer for a new response (lever-pressing) in the non-blocking group, but not the blocking group. Thus, contrary to prediction, pre-training with the tone blocked conditioning to the light. Although these results fail to support a potentially powerful explanation of addiction, they are consistent with the fact that most conditioning and learning phenomena that occur with non-drug reinforcers can also be demonstrated with drug reinforcers.

Effects of the selective dopamine D3 receptor antagonist SB-277011A on the reinforcing effects of nicotine as measured by a progressive-ratio schedule in rats

The dopamine D3 receptor is primarily localized within the mesocorticolimbic system, and may therefore have potential as a pharmacotherapeutic target for the treatment of drug dependence. Studies have shown that the selective dopamine D3 receptor antagonist SB-277011A reduces a variety of dependence-related behavioral effects of cocaine, alcohol and heroin. A previous study examining SB-277011A on nicotine self-administration using relatively low doses of the antagonist and a low response requirement for nicotine found no effect on drug-taking behavior per se, whereas reinstatement of nicotine-seeking was reduced. The purpose of the present study was to further examine the effects of higher doses of SB-277011A on nicotine self-administration in rats under a progressive-ratio (PR) schedule, which imposes relatively high response requirements for nicotine. Rats were trained to respond under a PR schedule of either nicotine or food reinforcement. Once responding was stable, SB-277011A (3-56 mg/kg) or vehicle was administered i.p. 1 h prior to the operant session. The highest dose tested significantly decreased the mean number of reinforcers and mean response rates in the nicotine self-administration group, but had no effect on either the mean number of reinforcers or response rate in the food group. In a separate set of experiments, the effects of SB-277011A on locomotor activity were measured. At the dose that significantly decreased nicotine self-administration, total distance traveled was also significantly decreased, suggesting that the effect on operant responding at the high dose of SB-277011A is at a threshold for motor effects and may not be directly mediated by an action at dopamine D3 receptors.

Serotonin and psychostimulant addiction: Focus on 5-HT1A-receptors

Serotonin(1A)-receptors (5-HT(1A)-Rs) are important components of the 5-HT system in the brain. As somatodendritic autoreceptors they control the activity of 5-HT neurons, and, as postsynaptic receptors, the activity in terminal areas. Cocaine (COC), amphetamine (AMPH), methamphetamine (METH) and 3,4-methylenedioxymethamphetamine ("Ecstasy", MDMA) are psychostimulant drugs that can lead to addiction-related behavior in humans and in animals. At the neurochemical level, these psychostimulant drugs interact with monoamine transporters and increase extracellular 5-HT, dopamine and noradrenalin activity in the brain. The increase in 5-HT, which, in addition to dopamine, is a core mechanism of action for drug addiction, hyperactivates 5-HT(1A)-Rs. Here, we first review the role of the various 5-HT(1A)-R populations in spontaneous behavior to provide a background to elucidate the contribution of the 5-HT(1A)-Rs to the organization of psychostimulant-induced addiction behavior. The progress achieved in this field shows the fundamental contribution of brain 5-HT(1A)-Rs to virtually all behaviors associated with psychostimulant addiction. Importantly, the contribution of pre- and postsynaptic 5-HT(1A)-Rs can be dissociated and frequently act in opposite directions. We conclude that 5-HT(1A)-autoreceptors mainly facilitate psychostimulant addiction-related behaviors by a limitation of the 5-HT response in terminal areas. Postsynaptic 5-HT(1A)-Rs, in contrast, predominantly inhibit the expression of various addiction-related behaviors directly. In addition, they may also influence the local 5-HT response by feedback mechanisms. The reviewed findings do not only show a crucial role of 5-HT(1A)-Rs in the control of brain 5-HT activity and spontaneous behavior, but also their complex role in the regulation of the psychostimulant-induced 5-HT response and subsequent addiction-related behaviors.

Reinforcing effects of methylenedioxy amphetamine congeners in rhesus monkeys: are intravenous self-administration experiments relevant to MDMA neurotoxicity?

RATIONALE

Many animal models relevant to the persistent effects of drugs of abuse necessitate the application of interspecies dose scaling procedures to approximate drug administration regimens in humans, but drug self-administration procedures differ in that they allow animal subjects to control their own drug intake.

OBJECTIVES

This report reviews the reinforcing effects of 3,4-methylenedioxymethamphetamine (MDMA), its enantiomers, and several structural analogs in rhesus monkeys, paying particular attention to the pharmacological mechanisms of such reinforcing effects, the development of structure activity relationships among these compounds, the stability of MDMA self-administration behavior over time, and the persistent effects of self-administered MDMA on monoamines.

RESULTS

The methylenedioxy amphetamine congeners MDMA, 3,4-methylenedioxyamphetamine, N-ethyl-3,4-methylenedioxyamphetamine, and N-methyl-1-(3,4-methylenedioxyphenyl)-2-butanamine function as reinforcers in rhesus monkeys, maintaining self-administration behavior greater than that engendered by contingent saline but less than that engendered by traditional psychostimulants. These findings are remarkable as structurally distinct serotonergic hallucinogen-like drugs do not maintain reliable self-administration in laboratory animals. During prolonged MDMA self-administration, MDMA-maintained responding progressively weakens, and MDMA eventually fails to maintain significant self-administration. The neurochemical correlates of this effect have not yet been identified.

CONCLUSIONS

Procedures in which MDMA and related compounds are self-administered can be established in rhesus monkeys. These techniques can be used to engender contingent MDMA exposure without resorting to controversial methods of interspecies dose scaling. As such, further application of self-administration methods may provide important new insights into the persistent effects of MDMA on brain and behavior in nonhuman primates.

Role of dopamine and glutamate receptors in cocaine-induced social effects in isolated and grouped male OF1 mice

Cocaine administration in paired male mice decreases social contacts as well as increases avoidance and flee elements. As dopamine (DA) and glutamate seem to be involved in some of cocaine's effects, an attempt was made to assess whether a range of associated receptors influenced the social impacts of this drug of abuse. The NMDA antagonist memantine (10 and 40 mg/kg); the AMPA antagonist CNQX (1 and 20 mg/kg); the DA release inhibitor CGS 10746b (2 and 8 mg/kg): the DA D1 antagonist SCH 23390 (0.05 and 0.5 mg/kg); and the DA D2/D3 antagonist raclopride (0.03 and 0.3 mg/kg) were administered prior to 25 mg/kg of cocaine and behaviour was evaluated during an encounter between an experimental and a standard opponent in a neutral cage for 10 min. Memantine reverts cocaine-induced social withdrawal and the increase in avoidance and flee, CNQX being effective only in these latter actions. On the other hand, SCH 23390 counteracts the social as well as the defensive action of cocaine, raclopride being effective only in blocking the cocaine-induced increase in avoidance and flee behaviours. In conclusion, although both neurotransmitter systems are involved in the effects of cocaine on social behaviour, NMDA and D1DA receptors seem to have an important role.

Neural substrates of cocaine-cue associations that trigger relapse

Learned associations that occur during the process of repeated drug use in addiction can later manifest as trigger factors in relapse to renewed drug-seeking and drug-taking behavior. The process of conditioned-cued relapse of drug-seeking behavior has been successfully modeled in animals using the reinstatement procedure, in which chronic drug self-administration can be extinguished or withheld, and then reinstated using conditioned stimuli previously paired with the drug. Our laboratory has extensively studied the neural circuitry underlying conditioned-cued drug-seeking during the expression of reinstatement. In order to study the learning process of drug-cue pairings, we further developed a procedure whereby discrete cocaine-cue pairings can be conducted in a single pavlovian training session in animals previously trained to self-administer cocaine. Presentation of these cues during later reinstatement trials produces robust responding over extinction levels at levels similar to those seen when animals experience the cues on a daily basis. In a series of experiments, we have shown that reversible pharmacological inactivation of the basolateral complex of the amygdala just prior to acquisition of cocaine-cue associations blocks the ability of cocaine-paired stimuli to elicit conditioned-cued reinstatement. This learning process is mediated in part by muscarinic acetylcholine and dopaminergic inputs to the basolateral complex of the amygdala, as intra-amygdala infusion of selective receptor antagonists at the time of acquisition significantly affects reinstatement. We have also recently found that disruption of neural activity within the basolateral complex of the amygdala at the time of consolidation (just after cocaine-cue pairings) will disrupt reinstatement. Taken together, these results reveal the importance of the amygdala in the acquisition, consolidation, and expression of drug-stimulus learning that drives relapse to drug-seeking behavior.

Effects of anorectic drugs on food intake under progressive-ratio and free-access conditions in rats

The effects of two anorectic drugs, dexfenfluramine and phentermine, on food intake under different food-access conditions were examined. Experiment 1 compared the effects of these drugs on food intake under a progressive-ratio (PR) schedule and free-access conditions. Dexfenfluramine decreased food intake under both conditions, but the doses required to decrease intake under free-access conditions were higher than those required to reduce intake under the PR condition. Intermediate doses of phentermine sometimes increased breaking points, and higher doses decreased them. Phentermine decreased food intake at the same doses under both access conditions. Thus the potency of dexfenfluramine, but not phentermine, to decrease food-maintained behavior depended upon the food-access condition. Experiment 2 used a novel mixed progressive-ratio schedule of food delivery to study the duration of drug effects. Sessions consisted of five components separated by 3-hr timeouts. The ratio requirement reset at the beginning of each component and a new breaking point was obtained. Both dexfenfluramine and phentermine dose-dependently decreased breaking points early in the session. In some rats, compensatory increases in breaking point were observed. That is, breaking points later in the session increased over control levels, resulting in no change in the total number of food pellets earned for the session compared to control. The present findings suggest that the effects of some anorectic drugs depend upon the access conditions for food; increasing the effort to obtain food may enhance their ability to decrease food-maintained behavior.

Binge self-administration and deprivation produces sensitization to the reinforcing effects of cocaine in rats

RATIONALE

Behavioral procedures that incorporate dynamic changes in drug-maintained behavior are needed to model the development of cocaine addiction in humans.

OBJECTIVES

Because sensitization may occur to some aspects of drug administration during the addiction process, the objective of the present study was to define the critical features of self-administration histories that result in subsequent increases in the reinforcing efficacy of cocaine (measured using the progressive ratio (PR) schedule).

METHODS

Animals were trained to self-administer cocaine on a fixed ratio (FR) schedule, baseline performance on a PR schedule was determined, and animals were given various histories of cocaine self-administration and drug deprivation. PR performance was reassessed following this experience.

RESULTS

Cocaine self-administration under a discrete-trials procedure (24 h/day) for 10 days, followed by a 7-day deprivation period resulted in sensitization to the reinforcing effects of cocaine as assessed by the PR schedule (increases in maximal breakpoints maintained by cocaine with no change in sensitivity at lower doses). Similar levels of daily cocaine intake on a FR schedule (typically completed within 6 h) coupled with a deprivation period failed to produce changes in breakpoint. Providing access to cocaine during the "deprivation period" by repeated testing on a PR schedule prevented the sensitization.

CONCLUSIONS

These data suggest that these self-administration-induced changes in breakpoint reflect sensitization, and show that a drug-free deprivation period is necessary, but not sufficient, to produce this increase.

Herrnstein??s hyperbolic matching equation and behavioral pharmacology: review and critique

Behavioral pharmacologists have enlisted Herrnstein's (1970) hyperbolic matching equation to understand the behavioral effects of drugs. Herrnstein's hyperbola describes the relation between absolute response rate and reinforcement rate. The equation has two fitted parameters. The parameter k represents the asymptotic response rate, and the parameter r(e) represents the reinforcement rate necessary to obtain half the asymptotic response rate. According to one interpretation of the equation, changes in k should reflect changes in response or motoric variables, and changes in r(e) should reflect changes in reinforcer or motivational variables, or changes in reinforcement from sources extraneous to the instrumental response. We review research that has applied Herrnstein's equation to distinguish the motoric from the motivational effects of drugs, and to identify additional independent variables responsible for drug effects, such as extraneous reinforcement. The validity of inferences about drug effects depends on the consistency of how k and r(e) respond to environmental manipulations: k should change only with response or motoric variables, and r(e) should change with reinforcer or motivational variables and with the rate of extraneous reinforcement. Empirical tests of these predictions, however, have produced inconsistent results. The review suggests that Herrnstein's theory has not fulfilled its promise of identifying the behavioral mechanisms of drug action. Modifications to the equation, known as bias and sensitivity, may explain some of these inconsistent results, and the modified equation may have utility in behavioral pharmacology.

Acute administration of the GABA reuptake inhibitor tiagabine does not alter the effects of oral cocaine in humans

Drugs affecting central gamma-aminobutyric acid (GABA) systems may have promise as treatments for cocaine addiction. In the present study, tiagabine (Gabatril), a GABA reuptake inhibitor, was investigated for its ability to modify the discriminative-stimulus, reinforcing, subject-rated, performance and cardiovascular effects of oral cocaine in non-treatment seeking cocaine users. Initially, acute doses of 4 mg tiagabine were tested alone and in combination with oral cocaine in four participants to establish the safety of cocaine-tiagabine combinations. A higher dose of tiagabine (8 mg) was then tested in a larger group (n = 6). Participants learned to discriminate 150 mg of oral cocaine. The effects of cocaine (0-150 mg, p.o.) administered alone and in combination with tiagabine were then determined. Subject-rated, performance and cardiovascular measures were obtained at regular intervals. The reinforcing effects of cocaine, tiagabine and cocaine-tiagabine combinations were assessed using the Multiple-Choice Procedure. Cocaine alone produced prototypical behavioral and physiological effects (i.e., functioned as a discriminative and reinforcing stimulus, produced stimulant-like subject-rated effects, improved performance and increased heart rate). In general, acute administration of tiagabine did not alter the effects of oral cocaine. These findings suggest that tiagabine would not be effective at preventing continued cocaine use by blocking its acute, abuse-related effects.

Characterization of dopamine D1 and D2 receptor function in socially housed cynomolgus monkeys self-administering cocaine

RATIONALE

Social rank has been shown to influence dopamine (DA) D(2) receptor function and vulnerability to cocaine self-administration in cynomolgus monkeys. The present studies were designed to extend these findings to maintenance of cocaine reinforcement and to DA D(1) receptors.

OBJECTIVE

Examine the effects of a high-efficacy D(1) agonist on an unconditioned behavior (eyeblinking) and a low-efficacy D(1) agonist on cocaine self-administration, as well as the effects of cocaine exposure on D(2) receptor function across social ranks, as determined by positron emission tomography (PET).

METHODS

Effects of the high-efficacy D(1) agonist SKF 81297 and cocaine (0.3-3.0 mg/kg) on spontaneous blinking were characterized in eight monkeys during 15-min observation periods. Next, the ability of the low-efficacy D(1) agonist SKF 38393 (0.1-17 mg/kg) to decrease cocaine self-administration (0.003-0.1 mg/kg per injection, IV) was assessed in 11 monkeys responding under a fixed-ratio 50 schedule. Finally, D(2) receptor levels in the caudate and putamen were assessed in nineteen monkeys using PET.

RESULTS

SKF 81297, but not cocaine, significantly increased blinking in all monkeys, with slightly greater potency in dominant monkeys. SKF 38393 dose-dependently decreased cocaine-maintained response rates with similar behavioral potency and efficacy across social rank. After an extensive cocaine self-administration history, D(2) receptor levels did not differ across social ranks.

CONCLUSIONS

These results suggest that D(1) receptor function is not substantially influenced by social rank in monkeys from well-established social groups. While an earlier study showed that dominant monkeys had higher D(2) receptor levels and were less sensitive to the reinforcing effects of cocaine during initial exposure, the present findings indicate that long-term cocaine use changed D(2) receptor levels such that D(2) receptor function and cocaine reinforcement were not different between social ranks. These findings suggest that cocaine exposure attenuated the impact of social housing on DA receptor function.

The effect of sertraline and environmental context on treating depression and illicit substance use among methadone maintained opiate dependent patients: a controlled clinical trial

Psychiatric comorbidity, particularly depressive disorders, is associated with continued substance use and poor social functioning among methadone maintained patients. Evidence suggests similar neurochemical and environmental pathways may link the two disorders and it is reasonable to hypothesize that pharmacological and environmental factors play important roles in the treating comorbid depression and substance use. The present study tested the efficacy of sertraline for treating syndromally defined depressive disorders among non-abstinent methadone maintained opiate dependent patients. The moderating effects of environmental context on treatment outcome were also examined. Ninety-five patients were randomized in a 12-week, double-blind, placebo-controlled trial of sertraline, a serotonin-selective re-uptake inhibitor. There was no main effect of sertraline on either depression or substance use outcomes. However, sertraline demonstrated significant ameliorative effects on depression among patients with a more positive environment or less negative environment. The odds of being abstinent from heroin and cocaine were greater for patients on sertraline in environments with relatively less adversity. The findings support the hypothesis that contextual factors moderate the efficacy of pharmacological treatment for depression among methadone patients. They also suggest future research should examine a pharmacological treatment that is combined with a behavioral intervention targeting the accessibility of reinforcement or reducing the impact of aversive environmental interactions.

Acquisition of lever pressing for cocaine in C57BL/6J mice: effects of prior Pavlovian conditioning

The purpose of this study was to determine (1) if C57BL/6J (C57) mice would lever-press for intravenous cocaine infusions in a limited-access paradigm without previously establishing the instrumental response with natural reinforcers and (2) if prior Pavlovian conditioning of cocaine to the response contingent stimulus complex used in the cocaine self-administration sessions would facilitate acquisition of lever responding for cocaine. After implanting jugular catheters, some mice received Pavlovian conditioning during which 12 passive cocaine infusions (0.1 or 1 mg/kg unit doses) were paired with the tone/light/pump sound stimulus complex used in the self-administration sessions. The remaining mice simply began the cocaine self-administration sessions for 0.1 or 1 mg/kg unit doses of cocaine. Twenty-seven of the 33 mice with patent catheters acquired stable lever responding within an average of 5 to 6 days without previously establishing the instrumental response with natural rewards. Prior Pavlovian pairing of cocaine with the response contingent stimulus complex used in the self-administration sessions did not influence the acquisition of cocaine self-administration at the highest cocaine dose (1 mg/kg). This conditioning procedure using the low cocaine dose (0.1 mg/kg/infusion) reduced the number of mice acquiring cocaine self-administration to 50%, and the number of mice developing stable response patterns was only 25%. The results establish that C57 mice can acquire cocaine self-administration over several unit doses in a limited-access paradigm without previously establishing the instrumental response with natural reinforcers. Furthermore, prior pairing of response contingent cues with cocaine via Pavlovian conditioning did not facilitate the acquisition of cocaine self-administration.

Targeting the dopamine D3 receptor cannot influence continuous reinforcement cocaine self-administration in rats

Recent studies point out the important role of dopamine D3 receptors in drug addiction. Therefore, D3 receptor ligands have been proposed as candidate medications for the treatment of cocaine dependence. The present study was designed to compare several dopamine D3 ligands of various selectivity in an animal model of drug-dependence, the cocaine self-administration paradigm. None of the doses of SB-277011 (5, 20 mg/kg), the most selective dopamine D3 antagonist to date, and the lower dose (12 mg/kg) of the moderately D3 selective antagonist U-99194A could influence the rate of self-administration. At the higher dose (24 mg/kg), U-99194A decreased the lever-pressing for cocaine. Both the dopamine D1 selective SCH-23390 (0.2, 0.1 mg/kg) and the dopamine D2 receptor preferring haloperidol (0.5, 0.2 mg/kg) increased the lever-pressing. Both the most dopamine D3 selective agonist PD-128907 (1.0 mg/kg) and the less selective 7-OH-DPAT (0.1, 0.5 mg/kg, s.c.) caused significant decrease in lever-pressing. At lower dose (0.2 mg/kg) PD-128907 was ineffective. The partial agonist BP-897 (1 mg/kg) evoked slight but significant increase in self-administration, while the lower dose (0.5 mg/kg) was ineffective. In all, in contrast to the dopamine D1 and D2 receptors acute inhibition or stimulation of the D3 receptor do not appear to exert considerable influence on the acute reinforcing effect of cocaine.

Increases in the reinforcing efficacy of cocaine after particular histories of reinforcement

In humans, the progression to cocaine addiction presumably involves increases in the effectiveness of cocaine to function as a reinforcer. Here we use breakpoints assessed using the progressive ratio (PR) schedule as an index of the efficacy of cocaine as a reinforcer. To date, no preclinical studies have demonstrated an increase in breakpoint as a consequence of self-administration history. In the current study, baseline performances on fixed ratio (FR) and PR schedules were determined. Rats were then exposed to different self-administration histories and deprivation periods, and responding under FR and PR schedules was reassessed. Exposure to a discrete-trials procedure (access to cocaine 4 times/hour, 24 hours/day; DT4) for 7 or 10 days, coupled with a deprivation period of 7 days, resulted in increases in breakpoint on a PR schedule, with no change in FR1 schedule responding. Exposure to an FR1 schedule for 72 consecutive hours followed by 7 days of deprivation, failed to change breakpoints, but increased rates of intake assessed with an FR1 schedule. Thus, the type of self-administration history and the length of deprivation experienced contribute to changes in the reinforcing efficacy of cocaine as measured by a PR schedule.

Effects of interleukin-1beta on food-maintained behavior in the mouse

The present studies compared the effect of parenteral administration of the proinflammatory cytokine interleukin-1beta (IL-1beta) on food-seeking behavior under various conditions. IL-1beta (100 ng/mouse) decreased home cage consumption of sweetened milk to a greater extent in ad libitum fed mice than in mice that were food-restricted to maintain 85-90% of their free-feeding body weight. When operant responding for milk was maintained under a fixed-ratio 10 response (FR10) schedule of milk delivery, IL-1beta (30-300 ng/mouse) significantly decreased milk-maintained responding in mice fed ad libitum, but not in food-restricted mice. When food-restricted mice were trained under either an FR4 or FR32 response schedule of milk delivery, IL-1beta (100-300 ng/mouse) produced significant decreases in FR32, but not in FR4 responding. When responding was maintained under a progressive-ratio 10 response (PR10) schedule of milk delivery, IL-1beta (30-300 ng/mouse) dose-dependently decreased breaking points. These results indicate that the effects of IL-1beta on food-maintained behavior depend on both the level of motivation (as assessed by food restriction) and on the response cost for the milk (as assessed by ratio requirement). These findings suggest that motivational factors may be capable of attenuating some of the behavioral effects of these agents.

Conditioning history and the reinforcing effects of drugs: comment on Alessi, Roll, Reilly, and Johanson (2002)

Drug reinforcement plays a central role in drug abuse. Drugs of abuse are unconditioned reinforcers whose functional effects are mediated through neuropharmacological mechanisms. The role of conditioning has received attention, but the focus has been on associations between environmental cues and unconditioned drug stimuli. Recent studies by C.-E. Johanson, A. Mattox, and C. R. Schuster (1995) and S. M. Alessi, J. M. Roll, M. P. Reilly, and C.-E. Johanson (2002) have demonstrated that drugs can also acquire functional control over human behavior through conditioning processes. Given the potent effects that conditioned reinforcers (e.g., money) can have on human behavior, studies by Johanson et al. and Alessi et al. should engender further examination of conditioning processes and drug reinforcement.

A comparison of patterns of methamphetamine and cocaine use

Typical use patterns of methamphetamine (MA) users were examined using self-report measures from 120 MA and 63 cocaine users. Twenty (14 MA and 6 cocaine) of the participants also took part in structured interviews designed to provide more specific descriptions of their drug use. The typical MA user uses more than 20 days a month. Use is evenly spaced throughout the day, and although the amount of drug used per day is not different, MA users use fewer times per day than do cocaine users. Fewer of the cocaine users are continuous users, and they use in the evening rather than the daytime. The cocaine pattern of fewer days of use, evening use, and more frequent doses per day fits a picture of recreational use, whereas the all-day-most-days methamphetamine pattern does not.

The pharmacokinetic determinants of the frequency and pattern of intravenous cocaine self-administration in rats by pharmacokinetic modeling

We investigated the pharmacokinetic determinants of the frequency of intravenous cocaine self-administration in 2.5-h sessions. Two groups of rats were implanted with dual catheters that permitted cocaine infusion and blood sampling via the femoral and jugular vein catheters, respectively. Half of the animals in each group self-administered one of the two cocaine unit doses (0.5 and 1 mg/kg/infusion) by pressing a lever under a continuous schedule of reinforcement. To monitor serum cocaine concentrations, the remaining animals received concurrent, response-independent infusions whenever the matched animals self-administered cocaine infusions. Multiple concentration-time data in two successive self-administrations were determined to monitor the extent of fluctuation in concentrations by pharmacokinetic modeling. Behavioral analyses revealed the higher unit dose (1 mg/kg) resulted in less frequent cocaine self-administration, and a longer interinfusion interval, whereas the total doses were similar for the two groups (24.5-27.0 mg/kg/2.5 h). Cocaine decayed biexponentially. Both the values of clearance and terminal elimination rate constant for the self-administration paradigm were significantly greater than those after the bolus cocaine dosing series (0.5 and 1 mg/kg, separated by 3 days). The regularity in cocaine self-administration produced relatively stable serum cocaine concentrations that oscillated between maximum (C(max)) and minimum (C(min)) values regardless of dose size and interinfusion interval. Although the C(max) for the 1-mg/kg unit dose (1.47 microg/ml) was significantly higher than that for the 0.5-mg/kg dose (0.82 microg/ml), the C(min) values between the groups approximated each other (0.28, and 0.34 microg/ml, respectively). Hence, the C(min) is the determinant of the initiation of the next drug-taking behavior.

Social dominance in monkeys: dopamine D2 receptors and cocaine self-administration

Disruption of the dopaminergic system has been implicated in the etiology of many pathological conditions, including drug addiction. Here we used positron emission tomography (PET) imaging to study brain dopaminergic function in individually housed and in socially housed cynomolgus macaques (n = 20). Whereas the monkeys did not differ during individual housing, social housing increased the amount or availability of dopamine D2 receptors in dominant monkeys and produced no change in subordinate monkeys. These neurobiological changes had an important behavioral influence as demonstrated by the finding that cocaine functioned as a reinforcer in subordinate but not dominant monkeys. These data demonstrate that alterations in an organism's environment can produce profound biological changes that have important behavioral associations, including vulnerability to cocaine addiction.

The effects of D-amphetamine on responding for candy and fruit drink using a fixed ratio and a progressive ratio schedule of reinforcer delivery

The first purpose of this study was to compare the effects of D-amphetamine (AMPH) on operant responding reinforced under fixed ratio (FR) or progressive ratio (PR) schedules of reinforcement, testing the hypothesis that responding reinforced under a PR operant schedule would be disrupted by lower doses of AMPH than responding reinforced under a FR operant schedule. The second purpose of this study was to test the generalizability of the first hypothesis by comparing the effects of AMPH on responding reinforced by two different reinforcers under both FR and PR operant schedules. Rhesus monkeys had five to six candy and five to six fruit drink sessions per day, and could receive two reinforcers per session. Responding was initially reinforced under a PR procedure, such that the ratio size increased with each subsequent session. The parameters of the PR schedule were individually selected so that monkeys consumed a similar number of candy and fruit-drink reinforcers each day. The effects of oral AMPH (0.5, 0.75, 1.0 mg/kg) on responding were assessed. Responding was then stabilized using a FR schedule with parameters individually selected so that monkeys consumed a similar number of candy and fruit-drink reinforcers each day, and the effects of oral AMPH were again assessed. The PR breakpoint was significantly greater for candy than fruit-drink. AMPH produced dose-related decreases in both candy and fruit-drink intake, but each AMPH dose decreased the number of fruit-drink deliveries to a greater extent than the number of candy deliveries. The results failed to support the hypothesis that responding under PR schedules of reinforcement would be disrupted by lower doses of AMPH.

Assessing the abuse potential of methylphenidate in nonhuman and human subjects: a review

Methylphenidate (MPH) is widely used for the treatment of Attention Deficit Hyperactivity Disorder (ADHD) in children, adolescents, and adults. Methylphenidate is clearly effective for the treatment of ADHD, but there is controversy as to whether it has significant abuse potential like other psychostimulants (e.g., D-amphetamine and cocaine). In general, the drug is believed to be abused at rates much lower than those for other stimulants. The present review examines studies that investigated the behavioral pharmacological profile of methylphenidate and discusses how results from these studies address its abuse liability. Using MEDLINE search terms methylphenidate, drug discrimination, reinforcement, self-administration, subjective effects, subject-rated effects, abuse potential, and abuse liability, along with a review of the references from identified articles, 60 studies were located in which the reinforcing, discriminative-stimulus, or subjective effects of methylphenidate were directly assessed in nonhumans or humans. Forty-eight (80.0%) of the studies reviewed indicate that methylphenidate either functions in a manner similar to D-amphetamine or cocaine (e.g., functions as a reinforcer, substitutes fully in drug discrimination experiments), or produces a pattern of subjective effects suggestive of abuse potential. The results are discussed as they pertain to factors that may account for the apparent discrepancy in abuse rates between methylphenidate and other stimulants, including characterization of actual abuse rates, defining abuse and misuse, pharmacokinetic factors, and validity of abuse liability assays.