PET Imaging of Dopamine D2 Receptor and Transporter Availability During Acquisition of Cocaine Self-administration in Rhesus Monkeys

PET imaging of dopamine transporters and D2/D3 receptors in female monkeys: effects of chronic cocaine self-administration

Brain imaging studies using positron emission tomography (PET) have shown that long-term cocaine use is associated with lower levels of dopamine (DA) D2/D3 receptors (D2/D3R); less consistent are the effects on DA transporter (DAT) availability. However, most studies have been conducted in male subjects (humans, monkeys, rodents). In this study, we used PET imaging in nine drug-naïve female cynomolgus monkeys to determine if baseline measures of DAT, with [18F]FECNT, and D2/D3R availability, with [11C]raclopride, in the caudate nucleus, putamen and ventral striatum were associated with rates of cocaine self-administration and if these measures changed during long-term (~13 months) cocaine self-administration and following time-off (3-9 months) from cocaine. Cocaine (0.2 mg/kg/injection) and 1.0 g food pellets were available under a multiple fixed-interval (FI) 3-min schedule of reinforcement. In contrast to what has been observed in male monkeys, baseline D2/D3R availability was positively correlated with rates of cocaine self-administration only during the first week of exposure; DAT availability did not correlate with cocaine self-administration. D2/D3R availability decreased ~20% following cumulative intakes of 100 and 1000 mg/kg cocaine; DAT availability did not significantly change. These reductions in D2/D3R availability did not recover over 9 months of time-off from cocaine. To determine if these reductions were reversible, three monkeys were implanted with osmotic pumps that delivered raclopride for 30 days. We found that chronic treatment with the D2/D3R antagonist raclopride increased D2/D3R availability in the ventral striatum but not in the other regions when compared to baseline levels. Over 13 months of self-administration, tolerance did not develop to the rate-decreasing effects of self-administered cocaine on food-reinforced responding, but number of injections and cocaine intake significantly increased over the 13 months. These data extend previous findings to female monkeys and suggest sex differences in the relationship between D2/D3R availability related to vulnerability and long-term cocaine use.

Association of dopamine D2-like and D3 receptor function with initial sensitivity to cocaine reinforcement in male rhesus monkeys

Identifying neurobiological characteristics that predict the development of cocaine use disorder would be of great value in prevention efforts. Because of their importance in mediating the abuse-related effects of cocaine, brain dopamine receptors are logical candidates for investigation. We analyzed data from two recently published studies that characterized availability of dopamine D2-like receptors (D2R) with [11C]raclopride PET imaging and dopamine D3 receptor (D3R) sensitivity with quinpirole-induced yawning in cocaine-naïve rhesus monkeys who subsequently acquired cocaine self-administration and completed a cocaine self-administration dose-effect curve. The present analysis compared D2R availability in several brain areas and characteristics of quinpirole-induced yawning, both acquired when monkeys were drug-naïve, with measures of initial sensitivity to cocaine. D2R availability in the caudate nucleus was negatively correlated with the ED50 of the cocaine self-administration curve, although the significance of this relationship was driven by an outlier and was not present after the outlier was removed. No other significant associations were observed between D2R availability in any examined brain region and measures of sensitivity to cocaine reinforcement. However, there was a significant negative correlation between D3R sensitivity, represented by the ED50 of the quinpirole-induced yawning curve, and the dose at which monkeys acquired cocaine self-administration. We also report no change from baseline D2R availability when a second PET scan was conducted after completion of the dose-effect curves. These data suggest the utility of D3R sensitivity, but not D2R availability, as a biomarker for vulnerability and resilience to cocaine. The well-established relationships between dopamine receptors and cocaine reinforcement in cocaine-experienced humans and animals may require extensive cocaine exposure.

Atlas of type 2 dopamine receptors in the human brain: Age and sex dependent variability in a large PET cohort

BACKGROUND

The dopamine system contributes to a multitude of functions ranging from reward and motivation to learning and movement control, making it a key component in goal-directed behavior. Altered dopaminergic function is observed in neurological and psychiatric conditions. Numerous factors have been proposed to influence dopamine function, but due to small sample sizes and heterogeneous data analysis methods in previous studies their specific and joint contributions remain unresolved.

METHODS

In this cross-sectional register-based study we investigated how age, sex, body mass index (BMI), as well as cerebral hemisphere and regional volume influence striatal type 2 dopamine receptor (D₂R) availability in the human brain. We analyzed a large historical dataset (n=156, 120 males and 36 females) of [11C]raclopride PET scans performed between 2004 and 2018.

RESULTS

Striatal D₂R availability decreased through age for both sexes (2-5 % in striatal ROIs per 10 years) and was higher in females versus males throughout age (7-8% in putamen). BMI and striatal D₂R availability were weakly associated. There was no consistent lateralization of striatal D₂R. The observed effects were independent of regional volumes. These results were validated using two different spatial normalization methods, and the age and sex effects also replicated in an independent sample (n=135).

CONCLUSIONS

D₂R availability is dependent on age and sex, which may contribute to the vulnerability of neurological and psychiatric conditions involving altering D₂R expression.

Early onset consumption of coca paste associated with executive-attention vulnerability markers linked to caudate-frontal structural and functional abnormalities

Coca paste is the most popular form of smoked cocaine (SC) in Latin America and also the most widespread among adolescents in vulnerable sectors of society, thus representing a significant public health concern. Despite evidence suggesting that abnormal executive-attention function is predictive of addiction to stimulant drugs, no study to date has compared clinically relevant neuropsychological (NPS) and physiological variables between individuals with histories of smoked cocaine dependence (SCD) and insufflated cocaine hydrochloride dependence (ICD). In this study we evaluated 25 SCD and 22 ICD subjects matched by poly-consumption profiles, and 25 healthy controls (CTR) matched by age, gender, education, and socioeconomic status. An exhaustive NPS battery was used to assess cognitive domains (attention, executive functions, fluid intelligence, memory, language and social cognition). We complemented this assessment with structural (MRI) and functional (fMRI) neuroimaging data. We found that executive function and attention impairments could be explained by the administration route of cocaine, with strongest impairments for the SCD group. SCD also presented reduced grey matter density relative to ICD and CTR in the bilateral caudate, a key area for executive and attentional function. Functional connectivity between left caudate and inferior frontal regions mediated the association between brain structure and behavioral performance. Our results highlight the relevance of assessing the route of administration of stimulants, both in clinical and research settings.

Sex differences and Tat expression affect dopaminergic receptor expression and response to antioxidant treatment in methamphetamine-sensitized HIV Tat transgenic mice

Methamphetamine (Meth) abuse is a common HIV comorbidity. Males and females differ in their patterns of Meth use, associated behaviors, and responses, but the underlying mechanisms and impact of HIV infection are unclear. Transgenic mice with inducible HIV-1 Tat protein in the brain (iTat) replicate many neurological aspects of HIV infection in humans. We previously showed that Tat induction enhances the Meth sensitization response associated with perturbation of the dopaminergic system, in male iTat mice. Here, we used the iTat mouse model to investigate sex differences in individual and interactive effects of Tat and Meth challenge on locomotor sensitization, brain expression of dopamine receptors (DRDs) and regulatory adenosine receptors (ADORAs). Because Meth administration increases the production of reactive oxygen species (ROS), we also determined whether the effects of Meth could be rescued by concomitant treatment with the ROS scavenger N-acetyl cysteine (NAC). After Meth sensitization and a 7-day abstinence period, groups of Tat+ and Tat-male and female mice were challenged with Meth in combination with NAC. We confirmed that Tat expression and Meth challenge suppressed DRD mRNA and protein in males and females' brains, and showed that females were particularly susceptible to the effects of Meth on D1-like and D2-like DRD subtypes and ADORAs. The expression of these markers differed strikingly between males and females, and between females in different phases of the estrous cycle, in a Tat -dependent manner. NAC attenuated Meth-induced locomotor sensitization and preserved DRD expression in all groups except for Tat + females. These data identify complex interactions between sex, Meth use, and HIV infection on addiction responses, with potential implications for the treatment of male and female Meth users in the context of HIV, especially those with cognitive disorders.

Experimental Traumatic Brain Injury during Adolescence Enhances Cocaine Rewarding Efficacy and Dysregulates Dopamine and Neuroimmune Systems in Brain Reward Substrates

Although clinical studies identify traumatic brain injury (TBI) as a risk factor for the development of substance use disorder, much remains unknown about the possible underlying pathogenesis and age-specific effects. Thus, the aim of this study is to test the hypothesis that at an age of ongoing maturation, adolescent TBI alters elements of the reward pathway, resulting in increased sensitivity to the rewarding effects of a subthreshold dose of cocaine that does not induce significant behavioral changes in naïve, non-injured mice. Specifically, these results were derived from the combination of the controlled cortical impact model of TBI, performed on either adolescent (6 weeks) or young adult (8 weeks) mice, followed by the cocaine-induced conditioned place preference assay 2 weeks later. Using three-dimensional isosurface rendering and volumetric image analysis, TBI was found to induce neuromorphological changes such as decreased dendritic complexity and reduced spine density in brain regions essential for reward perception and processing of drug-induced euphoria. Further, we demonstrated that these neuronal changes may affect the differential expression of dopamine-associated genes. Our analysis also provided evidence for age-related differences in immune response and the distinct involvement of augmented microglial phagocytic activity in the remodeling of neuronal structures in the adolescent TBI brain. Our studies suggest that TBI during adolescence, a period associated with ongoing maturation of dopaminergic systems, may subsequently enhance the abuse liability of cocaine in adulthood.

Stress as a Risk Factor for Substance Use Disorders: A Mini-Review of Molecular Mediators

The extant literature supports the role of stress in enhancing the susceptibility of drug abuse progressing to a substance use disorder diagnosis. However, the molecular mediators by which stress enhances the progression from cocaine abuse to cocaine use disorder via the mesolimbic pathway remain elusive. In this mini-review article, we highlight three mechanisms by which glucocorticoids (GCs) and the dopaminergic system interact. First, GCs upregulate tyrosine hydroxylase (TH), the rate-limiting enzyme in dopamine (DA) synthesis. Second, GCs downregulate monoamine-oxidase (MAO), an enzyme responsible for DA removal. Lastly, GCs are hypothesized to decrease DA reuptake, subsequently increasing synaptic DA. Based on these interactions, we review preclinical literature highlighting how stress modulates the mesolimbic pathway, including the ventral tegmental area (VTA) and nucleus accumbens (NAcs), to alter cocaine abuse-related effects. Taken together, stress enhances cocaine's abuse-related effects at multiple points along the VTA mesolimbic projection, and uniquely in the NAcs through a positive feedback type mechanism. Furthermore, we highlight future directions to elucidate the interaction between the prefrontal cortex (PFC) and key intermediaries including ΔFosB, cAMP response element binding protein (CREB) and cyclin-dependent kinase 5 (CDK5) to highlight possible mechanisms that underlie stress-induced acceleration of the progression to a cocaine use disorder diagnosis.

Neural Correlates of Exposure to Cocaine Cues in Rhesus Monkeys: Modulation by the Dopamine Transporter

BACKGROUND

A major goal of treatments for cocaine addiction is to reduce relapse-associated cravings, which are typically induced by environmental stimuli associated with cocaine use and related to changes in dopamine neurotransmission.

METHODS

The present study used an animal model of cocaine seeking to determine functional consequences of cue exposure using fluorodeoxyglucose positron emission tomography and to relate findings to juvenile levels of dopamine transporter and D₂-like receptor availabilities determined before any drug exposure. Adult male rhesus monkeys (N = 11) self-administered cocaine (0.2 mg/kg per injection) under a second-order schedule of reinforcement, in which responding was maintained by conditioned reinforcers. Positron emission tomography scans assessing glucose utilization, a marker of functional activation, were conducted during cocaine-cue responding and food-reinforced responding in a context where cocaine was never available.

RESULTS

Compared with the noncocaine condition, we found significant functional activation in the medial prefrontal cortex, anterior cingulate, precuneus region of the parietal cortex, and striatum-findings similar to those reported in humans who abuse cocaine. Furthermore, these functional activations in the prefrontal, cingulate, and parietal cortex measured during cocaine-cue responding were significantly correlated with juvenile measures of dopamine transporter availability, whereas no significant relationship with prior D₂-like receptor availability was observed in any brain region.

CONCLUSIONS

The similarity between the present findings and findings in humans who use cocaine supports the use of this model for examination of factors that affect the development and intensity of cue-induced drug seeking and provides evidence for potential biomarkers for the evaluation of potential treatments (behavioral and pharmacologic) for cocaine abuse.

Brain dopaminergic system changes in drug addiction: a review of positron emission tomography findings

Dopamine (DA) is considered crucial for the rewarding effects of drugs of abuse, but its role in addiction remains unclear. Positron emission tomography (PET) is the first technology used for in vivo measurement of components of the dopaminergic system in the human brain. In this article, we review the major findings from PET imaging studies on the involvement of DA in drug addiction, including presynaptic DA synthesis, vesicular monoamine transporter 2, the DA transporter, and postsynaptic DA receptors. These results have corroborated the role of DA in addiction and increased the understanding of its underlying mechanisms.

PET studies in nonhuman primate models of cocaine abuse: translational research related to vulnerability and neuroadaptations

The current review highlights the utility of positron emission tomography (PET) imaging to study the neurobiological substrates underlying vulnerability to cocaine addiction and subsequent adaptations following chronic cocaine self-administration in nonhuman primate models of cocaine abuse. Environmental (e.g., social rank) and sex-specific influences on dopaminergic function and sensitivity to the reinforcing effects of cocaine are discussed. Cocaine-related cognitive deficits have been hypothesized to contribute to high rates of relapse and are described in nonhuman primate models. Lastly, the long-term consequences of cocaine on neurobiology are discussed. PET imaging and longitudinal, within-subject behavioral studies in nonhuman primates have provided a strong framework for designing pharmacological and behavioral treatment strategies to aid drug-dependent treatment seekers. Non-invasive PET imaging will allow for individualized treatment strategies. Recent advances in radiochemistry of novel PET ligands and other imaging modalities can further advance our understanding of stimulant use on the brain. This article is part of the Special Issue Section entitled 'Neuroimaging in Neuropharmacology'.

Environmental modulation of drug taking: Nonhuman primate models of cocaine abuse and PET neuroimaging

The current review highlights the importance of environmental variables on cocaine self-administration in nonhuman primate models of drug abuse. In addition to describing the behavioral consequences, potential mechanisms of action are discussed, based on imaging results using the non-invasive and translational technique of positron emission tomography (PET). In this review, the role of three environmental variables - both positive and negative - are described: alternative non-drug reinforcers; social rank (as an independent variable) and punishment of cocaine self-administration. These environmental stimuli can profoundly influence brain function and drug self-administration. We focus on environmental manipulations involving non-drug alternatives (e.g., food reinforcement) using choice paradigms. Manipulations such as response cost and social variables (e.g., social rank, social stress) also influence the behavioral effects of drugs. Importantly, these manipulations are amenable to brain imaging studies. Taken together, these studies emphasize the profound impact environmental variables can have on drug taking, which should provide important information related to individual-subject variability in treatment responsiveness, and the imaging work may highlight pharmacological targets for medications related to treating drug abuse. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Chronic treatment with extended release methylphenidate does not alter dopamine systems or increase vulnerability for cocaine self-administration: a study in nonhuman primates

Despite the widespread use of stimulant medications for the treatment of attention deficit hyperactivity disorder, few studies have addressed their long-term effects on the developing brain or susceptibility to drug use in adolescence. Here, we determined the effects of chronic methylphenidate (MPH) treatment on brain dopamine (DA) systems, developmental milestones, and later vulnerability to substance abuse in juvenile nonhuman primates. Male rhesus monkeys (approximately 30 months old) were treated daily with either a sustained release formulation of MPH or placebo (N=8 per group). Doses were titrated to achieve initial drug blood serum levels within the therapeutic range in children and adjusted throughout the study to maintain target levels. Growth, including measures of crown-rump length and weight, was assessed before and after 1 year of treatment and after 3-5 months washout. In addition, positron emission tomography scans were performed to quantify binding availability of D2/D3 receptors and dopamine transporters (DATs). Distribution volume ratios were calculated to quantify binding of [¹⁸F]fluoroclebopride (DA D2/D3) and [¹⁸F]-(+)-N-(4-fluorobenzyl)-2β-propanoyl-3β-(4-chlorophenyl)tropane (DAT). Chronic MPH did not differentially alter the course of weight gain or other measures of growth, nor did it influence DAT or D2/D3 receptor availability after 1 year of treatment. However, after washout, the D2/D3 receptor availability of MPH-treated animals did not continue to decline at the same rate as control animals. Acquisition of intravenous cocaine self-administration was examined by first substituting saline for food reinforcement and then cocaine doses (0.001-0.1 mg/kg per injection) in ascending order. Each dose was available for at least five consecutive sessions. The lowest dose of cocaine that maintained response rates significantly higher than saline-contingent rates was operationally defined as acquisition of cocaine reinforcement. There were no differences in rates of acquisition, overall response rates, or cocaine intake as a function of cocaine dose between groups. In an animal model that closely mimics human development; chronic treatment with therapeutic doses of sustained release MPH did not have a significant influence on the regulation of DATs or D2/D3 receptors, or on standard measures of growth. Furthermore, this treatment regimen and subsequent drug washout did not have an impact on vulnerability to cocaine abuse.

Positron emission tomography molecular imaging of dopaminergic system in drug addiction

Dopamine (DA) is involved in drug reinforcement, but its role in drug addiction remains unclear. Positron emission tomography (PET) is the first technology used for the direct measurement of components of the dopaminergic system in the living human brain. In this article, we reviewed the major findings of PET imaging studies on the involvement of DA in drug addiction, especially in heroin addiction. Furthermore, we summarized PET radiotracers that have been used to study the role of DA in drug addiction. To investigate presynaptic function in drug addiction, PET tracers have been developed to measure DA synthesis and transport. For the investigation of postsynaptic function, several radioligands targeting dopamine one (D1) receptor and dopamine two (D2) receptor are extensively used in PET imaging studies. Moreover, we also summarized the PET imaging findings of heroin addiction studies, including heroin-induced DA increases and the reinforcement, role of DA in the long-term effects of heroin abuse, DA and vulnerability to heroin abuse and the treatment implications. PET imaging studies have corroborated the role of DA in drug addiction and increase our understanding the mechanism of drug addiction.

Nonhuman primate models of addiction and PET imaging: dopamine system dysregulation

This chapter highlights the use of nonhuman primate models of cocaine addiction and the use of positron emission tomography (PET) imaging to study the role of individual differences in vulnerability and how environmental and pharmacological variables can impact cocaine abuse. The chapter will describe studies related to the dopamine (DA) neurotransmitter system, and focus primarily on the D2-like DA receptor, the DA transporter and the use of fluorodeoxyglucose to better understand the neuropharmacology of cocaine abuse. The use of nonhuman primates allows for within-subject, longitudinal studies that have provided insight into the human condition and serve as an ideal model of translational research. The combination of nonhuman primate behavior, pharmacology and state-of-the-art brain imaging using PET will provide the foundation for future studies aimed at developing behavioral and pharmacological treatments for drug addiction in humans.

Influence of cocaine history on the behavioral effects of Dopamine D(3) receptor-selective compounds in monkeys

Although dopamine D(3) receptors have been associated with cocaine abuse, little is known about the consequences of chronic cocaine on functional activity of D(3) receptor-preferring compounds. This study examined the behavioral effects of D(3) receptor-selective 4-phenylpiperazines with differing in vitro functional profiles in adult male rhesus monkeys with a history of cocaine self-administration and controls. In vitro assays found that PG 619 (N-(3-hydroxy-4-(4-(2-methoxyphenyl)piperazin-1-yl)butyl)-4-(pyridin-2-yl)benzamide HCl) was a potent D(3) antagonist in the mitogenesis assay, but a fully efficacious agonist in the adenylyl cyclase assay, NGB 2904 (N-(4-(4-(2,3-dichlorophenyl)piperazin-1-yl)butyl)-9H-fluorene-2-carboxamide HCl) was a selective D(3) antagonist, whereas CJB 090 (N-(4-(4-(2,3-dichlorophenyl)piperazin-1-yl)butyl)-4-(pyridin-2-yl)benzamide HCl) exhibited a partial agonist profile in both in vitro assays. In behavioral studies, the D(3) preferential agonist quinpirole (0.03-1.0 mg/kg, i.v.) dose-dependently elicited yawns in both groups of monkeys. PG 619 and CJB 090 elicited yawns only in monkeys with an extensive history of cocaine, whereas NGB 2904 did not elicit yawns, but did antagonize quinpirole and PG 619-elicited yawning in cocaine-history monkeys. In another experiment, doses of PG 619 that elicited yawns did not alter response rates in monkeys self-administering cocaine (0.03-0.3 mg/kg per injection). Following saline extinction, cocaine (0.1 mg/kg) and quinpirole (0.1 mg/kg), but not PG 619 (0.1 mg/kg), reinstated cocaine-seeking behavior. When given before a cocaine prime, PG 619 decreased cocaine-elicited reinstatement. These findings suggest that (1) an incongruence between in vitro and in vivo assays, and (2) a history of cocaine self-administration can affect in vivo efficacy of D(3) receptor-preferring compounds PG 619 and CJB 090, which appear to be dependent on the behavioral assay.

Nonhuman primate neuroimaging and the neurobiology of psychostimulant addiction

Neuroimaging techniques have led to significant advances in our understanding of the neurobiology and treatment of drug addiction in humans. The capability to conduct parallel studies in nonhuman primates and human subjects provides a powerful translational approach to link findings in human and animal research. A significant advantage of nonhuman primate models is the ability to use drug-naïve subjects in longitudinal designs that document the neurobiological changes that are associated with chronic drug use. Moreover, experimental therapeutics can be evaluated in subjects with well-documented histories of drug exposure. The in vivo distribution and pharmacokinetics of drug binding in brain have been related to the time-course of behavioral effects associated with the addictive properties of stimulants. Importantly, the characterization of drug interactions with specific protein targets in brain has identified potential targets for medication development. Neuroimaging has proven especially useful in studying the dynamic changes in neuronal function that may be associated with environmental variables. Last, neuroimaging has been used effectively in nonhuman primates to characterize both transient and long-lasting changes in brain chemistry associated with chronic drug exposure. Although there is some evidence to suggest neurotoxicity in humans with long histories of stimulant use, parallel studies in nonhuman primates have not identified consistent long-term changes in such neurochemical markers. Collectively, the results of these studies of nonhuman primates have enhanced our understanding of the neurobiological basis of stimulant addiction and should have a significant impact on efforts to develop medications to treat stimulant abuse.

Review. Positron emission tomography imaging studies of dopamine receptors in primate models of addiction

Animal models have provided valuable information related to trait and state variables associated with vulnerability to drug addiction. Our brain imaging studies in monkeys have implicated D2 receptors in cocaine addiction. For example, an inverse relationship between D2 receptor availability and rates of cocaine self-administration has been documented. Moreover, environmental variables, such as those associated with formation of the social hierarchy, can impact receptor availability and sensitivity to the abuse-related effects of cocaine. Similarly, both D2 receptor availability and cocaine self-administration can be altered by chronic drug administration and fluctuations in hormone levels. In addition, cocaine self-administration can be altered in an orderly fashion by presentation of an acute stressor, such as acting as an intruder into an unfamiliar social group, which can shift the cocaine dose-response curve to the left in subordinate monkeys and to the right in dominant animals, suggesting an interaction between social variables and acute stressors. Conversely, irrespective of social rank, acute environmental enrichment, such as increasing the size of the living space, shifts the cocaine dose-response curve to the right. These findings highlight a pervasive influence of the environment in modifying the reinforcing effects of cocaine and strongly implicate brain D2 receptors.

The influence of reinforcing effects of cocaine on cocaine-induced increases in extinguished responding in cynomolgus monkeys

RATIONALE

Although reinstatement of extinguished cocaine self-administration is widely used as an animal model of relapse, it is unclear which behavioral effects of the drug stimulus (i.e., unconditioned, discriminative or reinforcing) mediate the increases in responding after extinction.

OBJECTIVE

To examine the influence of experience with cocaine as a reinforcer on the ability of response-independent cocaine injections to increase extinguished responding.

MATERIALS AND METHODS

Effects of noncontingent injections of cocaine (0.01-1.0 mg/kg, i.v.) were assessed in two groups of cynomolgus monkeys, those with extensive histories of cocaine self-administration when responding was maintained under a concurrent fixed ratio (FR) 50 schedule of saline and food presentation (n = 8) and cocaine-naive monkeys (n = 5) responding under an FR 50 schedule of food presentation. In the latter group, the effects of noncontingent cocaine and food (one or five pellets) were examined before and after a brief history of cocaine (0.03 mg/kg/inj) self-administration under an FR 50 schedule.

RESULTS

In the cocaine-experienced subjects responding under a concurrent schedule of saline and food availability, noncontingent cocaine dose-dependently increased injection-lever responding. In the initially cocaine-naive subjects, no dose of cocaine increased extinguished food-maintained responding before or after a brief exposure to cocaine self-administration. In contrast, noncontingent delivery of five food pellets significantly increased extinguished food-maintained responding after cocaine self-administration.

CONCLUSIONS

These results support the view that, under self-administration conditions, the discriminative stimulus effects of cocaine play a prominent role in the ability of cocaine to increase extinguished responding.