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

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.

Effects of chronic cocaine and ethanol self‐administration on brain dopamine receptors in a rhesus monkey model of polysubstance abuse

Most individuals with cocaine use disorder also use alcohol; however, little is known about the behavioural and pharmacological mechanisms that promote co‐abuse. For example, although studies in humans and animals have documented that chronic use of either alcohol or cocaine alone decreases D2‐like receptor (D2R) availability, effects of co‐abuse of these substances on dopamine receptor function have not been characterized. These studies examined the effects of long‐term cocaine self‐administration in 12 male rhesus monkeys who also consumed either ethanol or an ethanol‐free solution each day (n = 6 per group). Specifically, all monkeys self‐administered cocaine (0.1 mg/kg per injection) 5 days per week in the morning. In the afternoon, six monkeys consumed 2.0 g/kg ethanol over 1 h to model binge drinking and six monkeys drank an ethanol‐free solution. Assessment of D2R availability using positron emission tomography (PET) and [¹¹C]raclopride occurred when monkeys were drug‐naïve and again when monkeys had self‐administered approximately 400‐mg/kg cocaine. D₃R function was assessed at the same time points by determining the potency of the D₃R‐preferring agonist quinpirole to elicit yawns. Chronic cocaine self‐administration decreased D2R availability in subregions of the basal ganglia in control monkeys, but not those that also drank ethanol. In contrast, D₃R sensitivity increased significantly after chronic cocaine self‐administration in ethanol‐drinking monkeys but not controls. These results suggest that co‐use of ethanol substantially changes the effects of chronic cocaine self‐administration on dopamine receptors, specifically implicating D₃R as a target for medications in these individuals.

Can pharmacotherapy improve treatment outcomes in people with co-occurring major depressive and cocaine use disorders?

Introduction: Major depressive disorder (MDD) and cocaine use disorder (CUD) are prevalent and frequently co-occur. When co-occurring, the presence of one disorder typically negatively impacts the prognosis for the other. Given the clinical relevance, we sought to examine pharmacotherapies for co-occurring CUD and MDD. While multiple treatment options have been examined in the treatment of each condition individually, studies exploring pharmacological options for their comorbidity are fewer and not conclusive.Areas Covered: For this review, the authors searched the literature in PubMed using clinical query options for therapies and keywords relating to each condition. Then, they described potentially promising pharmacologic therapeutic options based on shared mechanisms between the two conditions and/or results from individual clinical trials conducted to date.Expert opinion: Medications like stimulants, dopamine (D3) receptors partial agonists or antagonists, antagonists of kappa opioid receptors, topiramate, and ketamine could be promising as there is significant overlap relating to reward deficiency models, antireward pathways, and altered glutamatergic systems. However, the available clinical literature on any one of these types of agents is mixed. Additionally, for some agents there is possible concern related to abuse potential (e.g. ketamine and stimulants).

2016 Philip S. Portoghese Medicinal Chemistry Lectureship: Designing Bivalent or Bitopic Molecules for G-Protein Coupled Receptors. The Whole Is Greater Than the Sum of Its Parts

The genesis of designing bivalent or bitopic molecules that engender unique pharmacological properties began with Portoghese's work directed toward opioid receptors, in the early 1980s. This strategy has evolved as an attractive way to engineer highly selective compounds for targeted G-protein coupled receptors (GPCRs) with optimized efficacies and/or signaling bias. The emergence of X-ray crystal structures of many GPCRs and the identification of both orthosteric and allosteric binding sites have provided further guidance to ligand drug design that includes a primary pharmacophore (PP), a secondary pharmacophore (SP), and a linker between them. It is critical to note the synergistic relationship among all three of these components as they contribute to the overall interaction of these molecules with their receptor proteins and that strategically designed combinations have and will continue to provide the GPCR molecular tools of the future.

Dopamine D3 Receptors Modulate the Ability of Win-Paired Cues to Increase Risky Choice in a Rat Gambling Task

Similar to other addiction disorders, the cues inherent in many gambling procedures are thought to play an important role in mediating their addictive nature. Animal models of gambling-related behavior, while capturing dimensions of economic decision making, have yet to address the impact that these salient cues may have in promoting maladaptive choice. Here, we determined whether adding win-associated audiovisual cues to a rat gambling task (rGT) would influence decision making. Thirty-two male Long-Evans rats were tested on either the cued or uncued rGT. In these tasks, animals chose between four options associated with different magnitudes and frequencies of reward and punishing time-out periods. As in the Iowa Gambling Task, favoring options associated with smaller per-trial rewards but smaller losses and avoiding the tempting "high-risk, high-reward" decks maximized profits. Although the reinforcement contingencies were identical in both task versions, rats' choice of the disadvantageous risky options was significantly greater on the cued task. Furthermore, a D3 receptor agonist increased choice of the disadvantageous options, whereas a D3 antagonist had the opposite effects, only on the cued task. These findings are consistent with the reported role of D3 receptors in mediating the facilitatory effects of cues in addiction. Collectively, these results indicate that the cued rGT is a valuable model with which to study the mechanism by which salient cues can invigorate maladaptive decision making, an important and understudied component of both gambling and substance use disorders. Significance statement: We used a rodent analog of the Iowa Gambling Task to determine whether the addition of audiovisual cues would affect choice preferences. Adding reward-concurrent cues significantly increased risky choice. This is the first clear demonstration that reward-paired cues can bias cost/benefit decision making against a subject's best interests in a manner concordant with elevated addiction susceptibility. Choice on the cued task was uniquely sensitive to modulation by D3 receptor ligands, yet these drugs did not alter decision making on the uncued task. The relatively unprecedented sensitivity of choice on the cued task to D3-receptor-mediated neurotransmission data suggest that similar neurobiological processes underlie the ability of cues to both bias animals toward risky options and facilitate drug addiction.

Neuronal circuitry underlying the impact of D3 receptor ligands in drug addiction

Since the cloning of the D3 receptor in the early 1990s, there has been a great deal of interest in this receptor as a possible therapeutic target for drug addiction. The development of a D3 ligand suitable for use in humans has remained elusive, so the study of the function of the D3 receptor and its possible therapeutic efficacy has largely been restricted to animals. Pre-clinical studies have established that systemic administration of D3 ligands, particularly antagonists and partial agonists, can alter drug-seeking in animals. Despite over a decade of research, few studies have investigated the effects of intra-cerebral infusion of D3 ligands on drug-seeking. In the present review, these studies are summarized, which have largely focused on stimulus-controlled behaviors. Converging evidence from studies of D3 receptor expression, Fos and pharmacological Magnetic Resonance Imaging (phMRI) is also provided to delineate some of the D3 brain systems involved in drug-seeking and taking. The data so far indicate that different brain systems may be involved in different types of stimulus control as well as drug taking.

Effects of buspirone and the dopamine D3 receptor compound PG619 on cocaine and methamphetamine self-administration in rhesus monkeys using a food-drug choice paradigm

RATIONALE

The dopamine (DA) D2 and D3 receptors have been associated with cocaine abuse. A recent study with the D3 receptor (D3R) partial agonist PG619 found that it attenuated cocaine-induced reinstatement and the D2-like receptor antagonist buspirone has shown positive outcomes in two studies of cocaine abuse in monkeys. However, a recent clinical trial indicated that buspirone did not improve abstinence in treatment-seeking cocaine abusers.

OBJECTIVE

The objective of the study was to examine PG619 and buspirone under a food-drug choice paradigm in order to better model the clinical findings. In addition, we extended the characterization of both compounds to include methamphetamine (MA) self-administration (SA).

METHODS

Six adult male rhesus monkeys were trained to respond under a concurrent food (1.0-g pellets) and drug (0.01-0.3 mg/kg/injection cocaine or MA) choice paradigm in which complete SA dose-response curves were determined each session (N = 3/group). Monkeys received 5 days of treatment with either PG619 (0.1-3.0 mg/kg, i.v.) or buspirone (0.01-1.0 mg/kg, i.m.). In a follow-up study, the SA doses were reduced (0.003-0.1 mg/kg/injection) to increase reinforcement frequency and buspirone was retested.

RESULTS

PG619 did not affect cocaine or MA choice, while buspirone increased low-dose cocaine choice. Changing the SA doses increased the number of reinforcers received each session, but buspirone did not decrease drug choice.

CONCLUSIONS

Consistent with clinical findings, these results do not support the use of buspirone for psychostimulant abuse and suggest that food-drug choice paradigms may have greater predictive validity than the use of other schedules of reinforcement.

Identifying Medication Targets for Psychostimulant Addiction: Unraveling the Dopamine D3 Receptor Hypothesis

The dopamine D3 receptor (D3R) is a target for developing medications to treat substance use disorders. D3R-selective compounds with high affinity and varying efficacies have been discovered, providing critical research tools for cell-based studies that have been translated to in vivo models of drug abuse. D3R antagonists and partial agonists have shown especially promising results in rodent models of relapse-like behavior, including stress-, drug-, and cue-induced reinstatement of drug seeking. However, to date, translation to human studies has been limited. Herein, we present an overview and illustrate some of the pitfalls and challenges of developing novel D3R-selective compounds toward clinical utility, especially for treatment of cocaine abuse. Future research and development of D3R-selective antagonists and partial agonists for substance abuse remains critically important but will also require further evaluation and development of translational animal models to determine the best time in the addiction cycle to target D3Rs for optimal therapeutic efficacy.

Eating high fat chow and the behavioral effects of direct-acting and indirect-acting dopamine receptor agonists in female rats

Eating high fat chow increases the sensitivity of male rats to some behavioral effects of the direct-acting dopamine receptor agonist quinpirole; it is not known whether sensitivity to quinpirole is similarly enhanced in female rats eating high fat chow. Female Sprague-Dawley rats had free access to standard chow (5.7% fat) or either free or restricted access (i.e. body weight matched to rats eating standard chow) to high fat (34.3% fat) chow. Quinpirole (0.0032-0.32 mg/kg) produced hypothermia and a low frequency of yawning. Eating high fat chow produced insulin resistance without affecting quinpirole-induced yawning or hypothermia. Pretreatment with the dopamine D2 receptor antagonist L-741,626 failed to increase quinpirole-induced yawning, indicating that the low frequency of yawning was not due to enhanced D2 receptor sensitivity. Compared with younger (postnatal day 75), drug-naive female rats in a previous study, rats in the present study (postnatal day 275) were more sensitive to cocaine-elicited (1-17.8 mg/kg) locomotion and the development of sensitization across 5 weeks; however, eating high fat chow did not further enhance these effects. These results suggest that drug history and age might modulate the effects of diet on sensitivity to drugs acting on dopamine systems.

Beyond small-molecule SAR: using the dopamine D3 receptor crystal structure to guide drug design

The dopamine D3 receptor is a target of pharmacotherapeutic interest in a variety of neurological disorders including schizophrenia, restless leg syndrome, and drug addiction. The high protein sequence homology between the D3 and D2 receptors has posed a challenge to developing D3 receptor-selective ligands whose behavioral actions can be attributed to D3 receptor engagement, in vivo. However, through primarily small-molecule structure-activity relationship (SAR) studies, a variety of chemical scaffolds have been discovered over the past two decades that have resulted in several D3 receptor-selective ligands with high affinity and in vivo activity. Nevertheless, viable clinical candidates remain limited. The recent determination of the high-resolution crystal structure of the D3 receptor has invigorated structure-based drug design, providing refinements to the molecular dynamic models and testable predictions about receptor-ligand interactions. This chapter will highlight recent preclinical and clinical studies demonstrating potential utility of D3 receptor-selective ligands in the treatment of addiction. In addition, new structure-based rational drug design strategies for D3 receptor-selective ligands that complement traditional small-molecule SAR to improve the selectivity and directed efficacy profiles are examined.

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'.

Further characterization of quinpirole-elicited yawning as a model of dopamine D3 receptor activation in male and female monkeys

The dopamine (DA) D3 receptor (D3R) has been associated with impulsivity, pathologic gambling, and drug addiction, making it a potential target for pharmacotherapy development. Positron emission tomography studies using the D3R-preferring radioligand [(11)C]PHNO ([(11)C](+)-propyl-hexahydro-naphtho-oxazin) have shown higher binding potentials in drug abusers compared with control subjects. Preclinical studies have examined D3R receptor activation using the DA agonist quinpirole and the unconditioned behavior of yawning. However, the relationship between quinpirole-elicited yawning and D3R receptor availability has not been determined. In Experiment 1, eight drug-naive male rhesus monkeys were scanned with [(11)C]PHNO, and the ability of quinpirole (0.01-0.3 mg/kg i.m.) to elicit yawning was examined. Significant positive (globus pallidus) and negative (caudate nucleus, putamen, ventral pallidum, and hippocampus) relationships between D3R receptor availability and quinpirole-induced yawns were noted. Experiment 2 replicated earlier findings that a history of cocaine self-administration (n = 11) did not affect quinpirole-induced yawning and extended this to examine monkeys (n = 3) with a history of methamphetamine (MA) self-administration and found that monkeys with experience self-administering MA showed greater potency and significantly higher quinpirole-elicited yawning compared with controls. Finally, quinpirole-elicited yawning was studied in drug-naive female monkeys (n = 6) and compared with drug-naive male monkeys (n = 8). Sex differences were noted, with quinpirole being more potent and eliciting significantly more yawns in males compared with females. Taken together these findings support the use of quinpirole-elicited yawning as a behavioral tool for examining D3R activation in monkeys and that both drug history and sex may influence individual sensitivity to the behavioral effects of D3R compounds.

Reduction of cocaine self-administration and D3 receptor-mediated behavior by two novel dopamine D3 receptor-selective partial agonists, OS-3-106 and WW-III-55

Dopamine D3 receptor (D3R)-selective compounds may be useful medications for cocaine dependence. In this study, we identified two novel arylamide phenylpiperazines, OS-3-106 and WW-III-55, as partial agonists at the D3R in the adenylyl cyclase inhibition assay. OS-3-106 and WW-III-55 have 115- and 862-fold D3R:D2 receptor (D2R) binding selectivity, respectively. We investigated their effects (0, 3, 5.6, or 10 mg/kg) on operant responding by using a multiple variable-interval (VI) 60-second schedule that alternated components with sucrose reinforcement and components with intravenous cocaine reinforcement (0.375 mg/kg). Additionally, we evaluated the effect of OS-3-106 (10 mg/kg) on the dose-response function of cocaine self-administration and the effect of WW-III-55 (0-5.6 mg/kg) on a progressive ratio schedule with either cocaine or sucrose reinforcement. Both compounds were also examined for effects on locomotion and yawning induced by a D3R agonist. OS-3-106 decreased cocaine and sucrose reinforcement rates, increased latency to first response for cocaine but not sucrose, and downshifted the cocaine self-administration dose-response function. WW-III-55 did not affect cocaine self-administration on the multiple-variable interval schedule, but it reduced cocaine and sucrose intake on the progressive ratio schedule. Both compounds reduced locomotion at doses that reduced responding, and both compounds attenuated yawning induced by low doses of 7-OH-DPAT (a D3R-mediated behavior), but neither affected yawning on the descending limb of the 7-OH-DPAT dose-response function (a D2R-mediated behavior). Therefore, both compounds blocked a D3R-mediated behavior. However, OS-3-106 was more effective in reducing cocaine self-administration. These findings support D3Rs, and possibly D2Rs, as targets for medications aimed at reducing the motivation to seek cocaine.

Dopamine D3 and 5-HT1B receptor dysregulation as a result of psychostimulant intake and forced abstinence: Implications for medications development

Addiction to psychostimulants, including cocaine and amphetamine, is associated with dysregulation of dopamine and serotonin (5-HT) neurotransmitter systems. Neuroadaptations in these systems vary depending on the stage of the drug taking-abstinence-relapse cycle. Consequently, the effects of potential treatments that target these systems may vary depending on whether they are given during abstinence or relapse. In this review, we discuss evidence that dopamine D3 receptors (D3Rs) and 5-HT1B receptors (5-HT1BRs) are dysregulated in response to both chronic psychostimulant use and subsequent abstinence. We then review findings from preclinical self-administration models which support targeting D3Rs and 5-HT1BRs as potential medications for psychostimulant dependence. Potential side effects of the treatments are discussed and attention is given to studies reporting positive treatment outcomes that depend on: 1) whether testing occurs during self-administration versus abstinence, 2) whether escalation of drug self-administration has occurred, 3) whether the treatments are given repeatedly, and 4) whether social factors influence treatment outcomes. We conclude that D3/D2 agonists may decrease psychostimulant intake; however, side effects of D3/D2R full agonists may limit their therapeutic potential, whereas D3/D2R partial agonists have fewer undesirable side effects. D3-selective antagonists may not reduce psychostimulant intake during relapse, but nonetheless, may decrease motivation for seeking psychostimulants with relatively few side-effects. 5-HT1BR agonists provide a striking example of treatment outcomes that are dependent on the stage of the addiction cycle. Specifically, these agonists initially increase cocaine's reinforcing effects during maintenance of self-administration, but after a period of abstinence they reduce psychostimulant seeking and the resumption of self-administration. In conclusion, we suggest that factors contributing to dysregulation of monoamine systems, including drug history, abstinence, and social context, should be considered when evaluating potential treatments to better model treatment effects in humans. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Effects of chronic cocaine self-administration on cognition and cerebral glucose utilization in Rhesus monkeys

BACKGROUND

Chronic cocaine use is associated with neurobiological and cognitive deficits that persist into abstinence, hindering success of behavioral treatment strategies and perhaps increasing likelihood of relapse. The effects of current cocaine use and abstinence on neurobiology and cognition are not well characterized.

METHODS

Adult male rhesus monkeys with an extensive cocaine self-administration history (∼ 5 years) and age-matched control animals (n = 4/group) performed cognitive tasks in morning sessions and self-administered cocaine or food in afternoon sessions. Positron emission tomography and [(18)F]-fluorodeoxyglucose were employed to assess cerebral metabolic rates of glucose utilization during cognitive testing.

RESULTS

Cocaine-experienced monkeys required significantly more trials and committed more errors on reversal learning and multidimensional discriminations, compared with control animals. Cocaine-naive, but not cocaine-experienced, monkeys showed greater metabolic rates of glucose utilization during a multidimensional discrimination task in the caudate nucleus, hippocampus, anterior and posterior cingulate, and regions associated with attention, error detection, memory, and reward. Using a delayed match-to-sample task, there were no differences in baseline working memory performance between groups. High-dose cocaine self-administration disrupted delayed match-to-sample performance but tolerance developed. Acute abstinence from cocaine did not affect performance, but by day 30 of abstinence, accuracy increased significantly, while performance of cocaine-naive monkeys was unchanged.

CONCLUSIONS

These data document direct effects of cocaine self-administration on cognition and neurobiological sequelae underlying cognitive deficits. Improvements in working memory can occur in abstinence, albeit across an extended period critical for treatment seekers, suggesting pharmacotherapies designed to enhance cognition may improve success of current behavioral modification strategies.

Phenylpiperazine derivatives with selectivity for dopamine D3 receptors modulate cocaine self-administration in rats

This study examined cocaine self-administration after pretreatments with three structurally related compounds that bind selectively to dopamine D3 receptors (D3Rs) relative to the D2 receptor subtype (D2Rs) and exhibit varying intrinsic activities in the forskolin-stimulated adenylyl cyclase assay. The compounds are: a) WC10, a D3R weak partial agonist/antagonist with 42-fold D3R:D2R selectivity, b) WC26, a 51-fold selective D3R partial agonist, c) WC44, a 23-fold selective D3R agonist. Rats were stabilized on a multiple variable-interval 60-s (VI60) schedule with alternating components of sucrose (45 mg pellets) or cocaine reinforcement (0.375 mg/kg, IV) and then tested for effects of the WC compounds (0.0, 1.0, 3.0, 5.6, or 10.0 mg/kg, IP). Another cohort was trained to self-administer cocaine (0.75 mg/kg, IV) on a VI60 schedule then tested with various doses of cocaine available (0.0-1.5 mg/kg, IV) following pretreatment with WC10 (5.6 or 10.0 mg/kg) or WC44 (10.0 mg/kg). WC10 and WC26 decreased both cocaine and sucrose reinforcement rates at the 10.0 mg/kg dose, whereas WC44 decreased only cocaine reinforcement rate at this dose. Furthermore, WC26 and WC44 increased response latency for cocaine but not sucrose. In the cocaine dose-response experiment, WC10 and WC44 flattened the dose-effect function of cocaine reinforcement rate. All compounds decreased spontaneous locomotion. WC10 and WC26 also reduced cocaine-induced locomotion. These results support the targeting of D3Rs for treatments for cocaine dependence. WC26 and WC44, in particular, show promise as they increased the latency to respond for cocaine but not sucrose, suggesting selective reduction of the motivation for cocaine.

Effects of nicotinic acetylcholine receptor agonists on cognition in rhesus monkeys with a chronic cocaine self-administration history

Cocaine use is associated with impaired cognitive function, which may negatively impact treatment outcomes. One pharmacological strategy to improve cognition involves nicotinic acetylcholine receptor (nAChR) stimulation. However, the effects of chronic cocaine exposure on nAChR distribution and function have not been characterized. Thus, one goal of this study was to examine nAChR availability in rhesus monkeys with an extensive cocaine self-administration history (n = 4; ~6 years, mean intake, 1463 mg/kg) compared to age-matched cocaine-naive control monkeys (n = 5). Using [¹¹C]-nicotine and positron emission tomography (PET) imaging, cocaine-experienced monkeys showed significantly higher receptor availability in the hippocampus compared to cocaine-naive monkeys. A second goal was to examine the effects of nAChR agonists on multiple domains of cognitive performance in these same monkeys. For these studies, working memory was assessed using a delayed match-to-sample (DMS) task, associative learning and behavioral flexibility using stimulus discrimination and reversal learning tasks. When administered acutely, the nonselective high-efficacy agonist nicotine, the low-efficacy α4β2* subtype-selective agonist varenicline and the high-efficacy α7 subtype-selective agonist, PNU-282987 significantly improved DMS performance in both cocaine-naive and cocaine-experienced monkeys. Individual doses of nicotine and varenicline that engendered maximum cognitive enhancing effects on working memory did not affect discrimination or reversal learning, while PNU-282987 disrupted reversal learning in the cocaine-naive monkeys. These findings indicate that a cocaine self-administration history influenced nAChR distribution and the effects of nAChR agonists on cognitive performance, including a reduced sensitivity to the disrupting effects on reversal learning. The cognitive enhancing effects of nAChR agonists may be beneficial in combination with behavioral treatments for cocaine addiction. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Medication discovery for addiction: translating the dopamine D3 receptor hypothesis

The dopamine D3 receptor (D3R) has been investigated as a potential target for medication development to treat substance use disorders (SUDs) with a particular focus on cocaine and methamphetamine. Currently, there are no approved medications to treat cocaine and methamphetamine addiction and thus developing pharmacotherapeutics to complement existing behavioral strategies is a fundamental goal. Novel compounds with high affinity and D3R selectivity have been evaluated in numerous animal models of drug abuse and favorable outcomes in nonhuman primate models of self-administration and relapse have provided compelling evidence to advance these agents into the clinic. One approach is to repurpose drugs that share the D3R mechanism and already have clinical utility, and to this end buspirone has been identified as a viable candidate for clinical trials. A second, but substantially more resource intensive and risky approach involves the development of compounds that exclusively target D3R, such as GSK598809 and PG 619. Clinical investigation of these drugs or other novel D3R-selective agents will provide a better understanding of the role D3R plays in addiction and whether or not antagonists or partial agonists that are D3R selective are effective in achieving abstinence in this patient population.