Ethanol effects on self-administration of alfentanil, cocaine, and nomifensine in rhesus monkeys

Organic cation transporters in psychiatric and substance use disorders

Psychiatric and substance use disorders inflict major public health burdens worldwide. Their widespread burden is compounded by a dearth of effective treatments, underscoring a dire need to uncover novel therapeutic targets. In this review, we summarize the literature implicating organic cation transporters (OCTs), including three subtypes of OCTs (OCT1, OCT2, and OCT3) and the plasma membrane monoamine transporter (PMAT), in the neurobiology of psychiatric and substance use disorders with an emphasis on mood and anxiety disorders, alcohol use disorder, and psychostimulant use disorder. OCTs transport monoamines with a low affinity but high capacity, situating them to play a central role in regulating monoamine homeostasis. Preclinical evidence discussed here suggests that OCTs may serve as promising targets for treatment of psychiatric and substance use disorders and encourage future research into their therapeutic potential.

Ethanol inhibits dopamine uptake via organic cation transporter 3: Implications for ethanol and cocaine co-abuse

Concurrent cocaine and alcohol use is among the most frequent drug combination, and among the most dangerous in terms of deleterious outcomes. Cocaine increases extracellular monoamines by blocking dopamine (DA), norepinephrine (NE) and serotonin (5-HT) transporters (DAT, NET and SERT, respectively). Likewise, ethanol also increases extracellular monoamines, however evidence suggests that ethanol does so independently of DAT, NET and SERT. Organic cation transporter 3 (OCT3) is an emergent key player in the regulation of monoamine signaling. Using a battery of in vitro, in vivo electrochemical, and behavioral approaches, as well as wild-type and constitutive OCT3 knockout mice, we show that ethanol's actions to inhibit monoamine uptake are dependent on OCT3. These findings provide a novel mechanistic basis whereby ethanol enhances the neurochemical and behavioral effects of cocaine and encourage further research into OCT3 as a target for therapeutic intervention in the treatment of ethanol and ethanol/cocaine use disorders.

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.

Effect of chronic binge-like ethanol consumption on subsequent cocaine reinforcement in rhesus monkeys

BACKGROUND

Although most individuals with cocaine use disorder also abuse alcohol, little is known about the behavioral and pharmacological mechanisms that promote co-abuse. For example, it is unclear whether prior experience with alcohol renders individuals more sensitive to cocaine when it is subsequently experienced.

METHODS

This study examined the effects of chronic ethanol consumption on subsequent cocaine reinforcement in rhesus monkeys. Six monkeys consumed 2.0 g/kg ethanol in a binge-drinking paradigm and 6 monkeys drank a non-alcoholic solution 5 days per week. After 9 months, each monkey's sensitivity to acquiring cocaine self-administration was determined. Monkeys performed an operant response resulting in food pellet delivery under a fixed-ratio 30 schedule of reinforcement. Saline, then ascending doses of cocaine, were substituted for food pellets until a cocaine dose was reached at which the number of cocaine injections delivered differed significantly from saline injections delivered. Following acquisition, a complete cocaine dose-effect curve was generated to determine whether ethanol consumption altered the reinforcing potency of cocaine determined by calculating the ED₅₀ of the ascending limb of each subject's curve.

RESULTS

Although individual variability was observed, the cocaine dose which initially served as a reinforcer did not differ between ethanol-drinking and control groups and, within the ethanol-drinking group, was not related to the amount of ethanol consumed. Moreover, the reinforcing potency of cocaine did not differ between groups.

CONCLUSION

Taken together, the data suggest that a history of binge-like alcohol consumption does not affect sensitivity to cocaine when it is subsequently first experienced.

The orexin (hypocretin) neuropeptide system is a target for novel therapeutics to treat cocaine use disorder with alcohol coabuse

An estimated 50-90% of individuals with cocaine use disorder (CUD) also report using alcohol. Cocaine users report coabusing alcohol to 'self-medicate' against the negative emotional side effects of the cocaine 'crash', including the onset of anxiety. Thus, pharmaceutical strategies to treat CUD would ideally reduce the motivational properties of cocaine, alcohol, and their combination, as well as reduce the onset of anxiety during drug withdrawal. The hypothalamic orexin (hypocretin) neuropeptide system offers a promising target, as orexin neurons are critically involved in activating behavioral and physiological states to respond to both positive and negative motivators. Here, we seek to describe studies demonstrating efficacy of orexin receptor antagonists in reducing cocaine, alcohol- and stress-related behaviors, but note that these studies have largely focused on each of these phenomena in isolation. For orexin-based compounds to be viable in the clinical setting, we argue that it is imperative that their efficacy be tested in animal models that account for polysubstance use patterns. To begin to examine this, we present new data showing that rats' preferred level of cocaine intake is significantly increased following chronic homecage access to alcohol. We also report that cocaine intake and motivation are reduced by a selective orexin-1 receptor antagonist when rats have a history of cocaine + alcohol, but not a limited history of cocaine alone. In light of these proof-of-principle data, we outline what we believe to be the key priorities going forward with respect to further examining the orexin system in models of polysubstance use. This article is part of the special issue on Neurocircuitry Modulating Drug and Alcohol Abuse.

One Is Not Enough: Understanding and Modeling Polysubstance Use

Substance use disorder (SUD) is a chronic, relapsing disease with a highly multifaceted pathology that includes (but is not limited to) sensitivity to drug-associated cues, negative affect, and motivation to maintain drug consumption. SUDs are highly prevalent, with 35 million people meeting criteria for SUD. While drug use and addiction are highly studied, most investigations of SUDs examine drug use in isolation, rather than in the more prevalent context of comorbid substance histories. Indeed, 11.3% of individuals diagnosed with a SUD have concurrent alcohol and illicit drug use disorders. Furthermore, having a SUD with one substance increases susceptibility to developing dependence on additional substances. For example, the increased risk of developing heroin dependence is twofold for alcohol misusers, threefold for cannabis users, 15-fold for cocaine users, and 40-fold for prescription misusers. Given the prevalence and risk associated with polysubstance use and current public health crises, examining these disorders through the lens of co-use is essential for translatability and improved treatment efficacy. The escalating economic and social costs and continued rise in drug use has spurred interest in developing preclinical models that effectively model this phenomenon. Here, we review the current state of the field in understanding the behavioral and neural circuitry in the context of co-use with common pairings of alcohol, nicotine, cannabis, and other addictive substances. Moreover, we outline key considerations when developing polysubstance models, including challenges to developing preclinical models to provide insights and improve treatment outcomes.

Establishing performance characteristics for positive control article selection in drug self-administration studies

The selection of a controlled substance (CS) for use as the positive control article in a nonclinical drug abuse liability (DAL) assessment study should be contemplated carefully and with full understanding of the stated intent of the study design. Any CS that can maintain day-to-day stable baseline responding of voluntary intravenous intakes in animals may be selected under the current guidelines. Schedule I - IV CNS stimulants, depressants, and sedative/hypnotics can serve as maintenance drugs in these protocols, but not all of these compounds will provide comparatively efficient, robust, and stable daily intakes. Each Sponsor is directed to select a positive control article and training dose that will provide the most balanced, predictive, and scientifically-sound comparison consistent with the mechanism of action or therapeutic target of the test article. The SA study design is not a "one-size-fits-all" assay. This is a discussion of the critical design factors to be considered in selecting the most appropriate positive control article to use for a SA study.

The importance of considering polysubstance use: lessons from cocaine research

BACKGROUND

Polysubstance use (PSU) is prevalent among individuals with substance use disorders, but the vast majority of preclinical substance use research has focused on individual substances in isolation. Cocaine has been prevalent in the repertoire of persons who use more than one illicit substance.

METHODS

We conducted a meta-analysis combining results from literature searches and secondary data analyses to estimate the prevalence of simultaneous and concurrent cocaine + alcohol and cocaine + cannabis use among cocaine users. We next summarized the small body of literature on behavioral, cognitive and neurobiological consequences of cocaine PSU across species, with a focus on alcohol and cannabis. Finally, we used systematic literature searches to assess the extent to which human and animal studies on the neurobiological consequences of cocaine include PSU subjects.

RESULTS

The estimated prevalence of simultaneous and concurrent alcohol use among human cocaine users was 74% and 77%, respectively. The estimated prevalence of simultaneous and concurrent cannabis use among cocaine users was 38% and 64%, respectively. Consumption of alcohol or cannabis with cocaine enhances subjective responses to cocaine, concomitant with changes in cocaine metabolism that increase blood cocaine levels, and, in the case of alcohol, produce the psychoactive metabolite cocaethylene. There is also consistent evidence for neurobiological effects of cocaine + alcohol combinations. However, animal PSU research with cocaine lags behind human research.

CONCLUSION

Based on the prevalence and known consequences of PSU, consideration of PSU in both human and animal research is vital for understanding patterns of substance use.

Effects of ethanol on cocaine self-administration in monkeys responding under a second-order schedule of reinforcement

BACKGROUND

Concurrent alcohol use among cocaine abusers is common but the behavioral variables that promote co-abuse are not well understood. The present study examined the effects of intragastric (i.g.) ethanol (EtOH) administration in monkeys responding under a schedule of cocaine reinforcement in which extensive drug seeking was maintained by conditioned stimuli.

METHODS

Four adult male cynomolgus monkeys (Macaca fascicularis) were trained to respond under a second-order fixed-interval (FI) 600s (fixed-ratio (FR) 30:S) schedule of cocaine (0.003-0.56mg/kg/injection) presentation. Sessions ended after 5 injections or 90min had elapsed. Different EtOH doses (0.5-2.0g/kg, i.g.) were administered 30min before the session, typically on Tuesdays and Fridays. Blood ethanol concentrations (BECs) were also assessed. Pattern of FI responding was assessed by determining quarter-life (QL) values.

RESULTS

Cocaine self-administration was characterized as an inverted U-shaped function of dose; QL values increased monotonically with dose. EtOH pretreatments dose-dependently decreased self-administration at several cocaine doses in 3 of 4 monkeys. In one animal, EtOH increased low-dose cocaine-maintained responding. For all monkeys, QL values were increased by EtOH when low- and high-cocaine doses were self-administered, suggesting additive effects of EtOH and cocaine. Furthermore, BECs were not altered following cocaine self-administration.

CONCLUSIONS

The reductions in cocaine self-administration and the increases in QL values following EtOH, suggest that EtOH was enhancing cocaine-related conditioned reinforcement. A better understanding of the behavioral mechanisms that mediate the co-abuse of alcohol and cocaine will lead to improved treatments for both drugs.

Lack of effect of ethanol on cocaine prime-induced reinstatement of extinguished cocaine self-administration in rhesus monkeys

Cocaine and alcohol are commonly co-abused for reasons that are incompletely understood. Laboratory animal studies have suggested that, although the reinforcing effects of low cocaine doses are increased following chronic ethanol (EtOH) consumption, acute EtOH administration does not consistently alter cocaine self-administration. The present study examined whether EtOH influences another abuse-related effect of cocaine: reinstatement of extinguished responding. Rhesus monkeys that had previously consumed EtOH for 8 weeks (2.0 g/kg over 1 h, 5 days/week) self-administered up to 10 injections per day of 0.1 mg/kg cocaine under a fixed-interval 300-s schedule. After responding had been extinguished by substituting saline for cocaine, a pre-session infusion of saline or EtOH (0.5 or 1.0 g/kg, intravenously over 10 min) was followed by a 'priming' injection of saline or cocaine (intravenously). Responding was increased significantly by priming injections of cocaine, but not saline. EtOH infusions neither reinstated behavior when administered before a saline prime nor altered the priming effect of cocaine. The inability of EtOH to alter the response-reinstating ability of cocaine provides further evidence for a lack of acute behavioral interactions between cocaine and EtOH.

Effects of chronic binge-like ethanol consumption on cocaine self-administration in rhesus monkeys

BACKGROUND

Most cocaine abusers also abuse alcohol, but little is known about interactions that promote co-abuse. These experiments in rhesus monkeys determined the effects of >8 weeks of ethanol (EtOH) consumption on cocaine self-administration (n=6), effects of dopamine (DA) receptor antagonists on cocaine reinforcement (n=3-4 per drug) and the ability of the D2-like DA receptor agonist quinpirole to elicit yawning (n=3).

METHODS

Monkeys self-administered cocaine (0.0-1.0mg/kg/injection, i.v.) under a 300-s fixed-interval schedule and the above-listed variables were measured before EtOH exposure. Next, monkeys consumed a sweetened, 4% EtOH solution in the home cage under binge-like conditions: 1h, 5 days/week with daily intake equaling 2.0g/kg EtOH. After approximately 8 weeks, measures were re-determined, then EtOH drinking was discontinued. Finally, acute effects of EtOH on cocaine self-administration were determined by infusing EtOH (0.0-1.0g/kg. i.v.) prior to cocaine self-administration sessions (n=4).

RESULTS

In five of six monkeys, EtOH drinking increased self-administration of low cocaine doses but did not alter reinforcing effects of higher doses. Self-administration returned to baseline after EtOH access was terminated (n=3). Effects of DA receptor antagonists on cocaine self-administration were not consistently altered after EtOH consumption, but the ability of quinpirole to induce yawning was enhanced in two of three monkeys. Acute EtOH infusions only decreased self-administration of lower cocaine doses.

CONCLUSIONS

Taken together, the data suggest that long-term EtOH exposure can increase sensitivity to cocaine, possibly by increasing D3 receptor sensitivity. Data do not support a role for acute pharmacological interactions in promoting cocaine/EtOH co-abuse.