Effects of self- and experimenter-administered cocaine on subsequent ethanol drinking in rhesus monkeys.

BACKGROUND: One possible reason for the lack of FDA-approved pharmacotherapies to treat cocaine use disorder (CUD) is that, although cocaine is typically used in combination with alcohol, it is studied in isolation in preclinical studies. A better understanding of the cocaine-alcohol interactions that promote polysubstance use (PSU) will improve animal models of CUD and hasten pharmacotherapy development. We used a rhesus monkey model of cocaine-alcohol PSU to investigate one possible mechanism: that alcohol is used to mitigate negative effects associated with termination of cocaine use. METHODS: In 6 adult male rhesus monkeys, the relationship between self-administered cocaine intake and oral ethanol intake 2hours later was examined during self-administration of cocaine (0.0003-0.3mg/kg per injection, i.v.) under a fixed-ratio 30 schedule (FR30) or a progressive-ratio (PR) schedule. Next, ethanol consumption was measured 0-120minutes after experimenter-administered cocaine (0.3-1.7mg/kg, i.v.). RESULTS: Self-administered cocaine intake under both FR30 and PR schedules was unrelated to oral ethanol intakes 2hours later. When cocaine was administered non-contingently, cocaine decreased ethanol intake as well as intake of a non-alcoholic solution in monkeys who never consumed ethanol (n=4) in a time- and dose-dependent manner. CONCLUSIONS: Taken together, the results do not provide evidence for cocaine-induced increases in ethanol consumption. By extension, the results do not support the hypothesis that cocaine users drink alcohol to counteract negative effects that occur after terminating use. This finding implies either that such effects do not exist or that such effects exist but are unaffected by ethanol.

Cognitive-Enhancing Effects of Acetylcholine Receptor Agonists in Group-Housed Cynomolgus Monkeys Who Drink Ethanol.

The cognitive impairments that are often observed in patients with alcohol use disorder (AUD) partially contribute to the extremely low rates of treatment initiation and adherence. Brain acetylcholine receptors (AChR) mediate and modulate cognitive and reward-related behavior, and their distribution can be altered by long-term heavy drinking. Therefore, AChRs are promising pharmacotherapeutic targets for treating the cognitive symptoms of AUD. In the present study, the procognitive efficacy of two AChR agonists, xanomeline and varenicline, were evaluated in group-housed monkeys who self-administered ethanol for more than 1 year. The muscarinic AChR antagonist scopolamine was used to disrupt performance of a serial stimulus discrimination and reversal (SDR) task designed to probe cognitive flexibility, defined as the ability to modify a previously learned behavior in response to a change in reinforcement contingencies. The ability of xanomeline and varenicline to remediate the disruptive effects of scopolamine was compared between socially dominant and subordinate monkeys, with lighter and heavier drinking histories, respectively. We hypothesized that subordinate monkeys would be more sensitive to all three drugs. Scopolamine dose-dependently impaired performance on the serial SDR task in all monkeys at doses lower than those that produced nonspecific impairments (e.g., sedation); its potency did not differ between dominant and subordinate monkeys. However, both AChR agonists were effective in remediating the scopolamine-induced deficit in subordinate monkeys but not in dominant monkeys. These findings suggest xanomeline and varenicline may be effective for enhancing cognitive flexibility in individuals with a history of heavy drinking. SIGNIFICANCE STATEMENT: Procognitive effects of two acetylcholine (ACh) receptor agonists were assessed in group-housed monkeys who had several years' experience drinking ethanol. The muscarinic ACh receptor agonist xanomeline and the nicotinic ACh receptor agonist varenicline reversed a cognitive deficit induced by the muscarinic ACh receptor antagonist scopolamine. However, this effect was observed only in lower-ranking (subordinate) monkeys and not higher-ranking (dominant monkeys). Results suggest that ACh agonists may effectively remediate alcohol-induced cognitive deficits in a subpopulation of those with alcohol use disorder.

Influence of social rank on the development of long-term ethanol drinking trajectories in cynomolgus monkeys.

BACKGROUND: Chronic stress contribute to the development of alcohol use disorder (AUD). However, characterizing the role of chronic social stressors in the development of problematic drinking trajectories in humans is complicated by practical and ethical constraints. Group-housed nonhuman primates develop social dominance hierarchies that represent a continuum of social experiences from enrichment in higher-ranked (dominant) monkeys to chronic social stress in lower-ranked (subordinate) individuals. This framework provides a translationally relevant model of chronic social stress that can be used to characterize its effects on vulnerability to AUD. METHODS: Twelve male cynomolgus monkeys living in three social groups with established social dominance hierarchies were provided access to ethanol and water for 22 h/day, 4-5 days/week, for 1 year. Ethanol-free periods (2- or 3-day "weekends" or longer periods up to 10 days) were spent in social groups to maintain the stability of the social hierarchies. Observational studies conducted 6 months into the year of drinking assessed signs of ethanol withdrawal. After 1 year, monkeys were individually housed 24 h/day, 7 days/week for four consecutive weeks to examine the effect of eliminating the "weekends" spent socially housed. RESULTS: Subordinate monkeys had significantly higher mean daily ethanol intakes than dominant monkeys across 1 year of open access. Subordinates also had higher intakes on the first day back drinking following ethanol-free periods of 9-10 days. Moreover, during the last 4 weeks of open access, intakes on the first drinking day after an ethanol-free weekend increased significantly in subordinate monkeys. This effect diminished when all monkeys were individually housed for 4 weeks, indicating that the increased intake in subordinates was driven by the social environment. CONCLUSIONS: These data demonstrate that social subordination, which is associated with chronic social stress, results in increased vulnerability to the development and maintenance of heavy drinking trajectories.

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 [11 C]raclopride occurred when monkeys were drug-naïve and again when monkeys had self-administered approximately 400-mg/kg cocaine. D3 R function was assessed at the same time points by determining the potency of the D3 R-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, D3 R 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 D3 R as a target for medications in these individuals.

Punishment of ethanol choice in rhesus monkeys.

A defining characteristic of individuals diagnosed with alcohol use disorder (AUD) is that negative outcomes related to drinking do not lead them to reduce their alcohol use. In rodent models of AUD, this characteristic has been studied by adding the bitter tastant quinine to an ethanol solution. In this study, we extended this approach to a nonhuman primate model in which the ability of quinine to decrease the choice of a 4% ethanol solution vs. water was measured. Five adult female rhesus monkeys with 7.3 years of experience drinking ethanol were given access to a 4% ethanol solution and water for 3 h per day. When ethanol choice was stable, a single quinine concentration (0.03-5.6 g /L) was added to the ethanol solution for 1 day until a quinine concentration-effect curve was generated. After determining the quinine concentration that reduced ethanol choice by half (the quinine EC 50 ), the relative reinforcing strength of ethanol was manipulated by adding quinine or sucrose to the water alternative depending on the monkey's baseline choice. Adding quinine to ethanol produced a concentration-dependent decrease in ethanol choice and intake. Importantly, water intake increased, indicating an effect on response allocation rather than simply a decrease in fluid consumption. Consistent with this conclusion, the addition of quinine or sucrose to the water alternative resulted in predictable increases and decreases, respectively, in ethanol choice. These studies establish a model of punishment of ethanol choice in nonhuman primates that can be used to understand the contextual, biologic and pharmacologic factors that influence sensitivity to the punishment of alcohol drinking.

Rich Club Characteristics of Alcohol-Naïve Functional Brain Networks Predict Future Drinking Phenotypes in Rhesus Macaques.

Purpose: A fundamental question for Alcohol use disorder (AUD) is how and when naïve brain networks are reorganized in response to alcohol consumption. The current study aimed to determine the progression of alcohol's effect on functional brain networks during transition from the naïve state to chronic consumption. Procedures: Resting-state brain networks of six female rhesus macaque (Macaca mulatta) monkeys were acquired using magnetoencephalography (MEG) prior to alcohol exposure and after free-access to alcohol using a well-established model of chronic heavy alcohol consumption. Functional brain network metrics were derived at each time point. Results: The average connection frequency (p < 0.024) and membership of the Rich Club (p < 0.022) changed significantly over time. Metrics describing network topology remained relatively stable from baseline to free-access drinking. The minimum degree of the Rich Club prior to alcohol exposure was significantly predictive of future free-access drinking (r = -0.88, p < 0.001). Conclusions: Results suggest naïve brain network characteristics may be used to predict future alcohol consumption, and that alcohol consumption alters functional brain networks, shifting hubs and Rich Club membership away from previous regions in a non-systematic manner. Further work to refine these relationships may lead to the identification of a high-risk drinking phenotype.

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.

PET Imaging of [11C]MPC-6827, a Microtubule-Based Radiotracer in Non-Human Primate Brains.

Dysregulation of microtubules is commonly associated with several psychiatric and neurological disorders, including addiction and Alzheimer's disease. Imaging of microtubules in vivo using positron emission tomography (PET) could provide valuable information on their role in the development of disease pathogenesis and aid in improving therapeutic regimens. We developed [11C]MPC-6827, the first brain-penetrating PET radiotracer to image microtubules in vivo in the mouse brain. The aim of the present study was to assess the reproducibility of [11C]MPC-6827 PET imaging in non-human primate brains. Two dynamic 0-120 min PET/CT imaging scans were performed in each of four healthy male cynomolgus monkeys approximately one week apart. Time activity curves (TACs) and standard uptake values (SUVs) were determined for whole brains and specific regions of the brains and compared between the "test" and "retest" data. [11C]MPC-6827 showed excellent brain uptake with good pharmacokinetics in non-human primate brains, with significant correlation between the test and retest scan data (r = 0.77, p = 0.023). These initial evaluations demonstrate the high translational potential of [11C]MPC-6827 to image microtubules in the brain in vivo in monkey models of neurological and psychiatric diseases.

Effects of stimulation of mu opioid and nociceptin/orphanin FQ peptide (NOP) receptors on alcohol drinking in rhesus monkeys.

Alcohol use disorder (AUD) persists as a devastating public health problem; widely effective pharmacological treatments are needed. Evidence from rodent models suggests that stimulating brain receptors for the neuropeptide nociceptin/orphanin FQ (NOP) can decrease ethanol drinking. We characterized the effects of the mu opioid peptide (MOP) receptor agonist buprenorphine and the buprenorphine analog (2S)-2-[(5R,6R,7R,14S)-N-cyclopropylmethyl-4,5-epoxy-6,14-ethano-3-hydroxy-6 methoxymorphinan-7-yl]-3,3-dimethylpentan-2-ol (BU08028), which stimulates MOP and NOP receptors, in a translational nonhuman primate model of AUD. Rhesus monkeys drank a 4% ethanol solution 6 h per day, 5 days per week via an operant behavioral panel in their home cages. To assess behavioral selectivity, monkeys responded via a photo-optic switch to earn food pellets. After characterizing the acute effects of BU08028 (0.001-0.01 mg/kg, i.m.) and buprenorphine (0.003-0.056 mg/kg, i.m.), the drugs were administered chronically using a model of pharmacotherapy assessment that incorporates clinical aspects of AUD and treatment. Acutely, both drugs decreased ethanol drinking at doses that did not affect food-maintained responding. During chronic treatment, effects of BU08028 and buprenorphine were maintained for several weeks without development of tolerance or emergence of adverse effects. BU08028 was ~0.5 and 1.0 log units more potent in acute and chronic studies, respectively. The selective NOP receptor agonist SCH 221510 also selectively decreased ethanol intakes when given acutely (0.03-1.0 mg/kg, i.m.), whereas the MOP antagonist naltrexone (1.7-5.6 mg/kg, i.m.) decreased both ethanol intake and food pellets delivered. These data demonstrate that bifunctional MOP/NOP agonists, which may have therapeutic advantages to MOP-selective drugs, can decrease alcohol drinking in nonhuman primates.