The impact of social variables in preclinical models of cocaine abuse

Ethanol consumption prior to cocaine self-administration reduced vulnerability to cocaine reinforcement in socially subordinate female and male monkeys

RATIONALE

Although ethanol consumption is ubiquitous among individuals who use cocaine, most preclinical work investigating factors that contribute to the development of problematic cocaine use do not incorporate ethanol into their experimental designs. Given that only a subset of individuals who try cocaine go on to develop a cocaine use disorder (CUD) research is needed to identify factors, such as ethanol consumption, that may influence vulnerability to cocaine reinforcement.

OBJECTIVES

Thus, this study aimed to determine how a history of ethanol self-administration and exposure immediately prior to the first experience with cocaine self-administration influenced the potency of cocaine to function as a reinforcer in socially housed male and female cynomolgus monkeys.

METHODS

For these experiments, one group of monkeys (n = 7) was trained to self-administer up to 1.5 g/kg of ethanol prior to cocaine self-administration while another group of monkeys (n = 13) remained ethanol-naïve through the study. Acquisition of cocaine self-administration was studied in both groups of monkeys under a fixed-ratio schedule of reinforcement where ascending doses of cocaine were substituted for food pellets. Cocaine ED50 values from the ascending limb were examined statistically.

RESULTS

The results showed that subordinate monkeys that self-administered ethanol prior to cocaine self-administration required higher cocaine doses to function as a reinforcer compared with subordinate monkeys not exposed to ethanol.

CONCLUSIONS

One possible explanation for this finding is that ethanol, due to its acute anxiolytic effects, mitigated the effect of chronic stress in subordinate monkeys and thereby blunted the reinforcing effects of initial cocaine exposure. Future research is needed to examine whether variables such as environmental enrichment or treatment with clinically effective anxiolytics in chronically stressed individuals can modify the initiation and continued use of cocaine.

Animal models of cocaine use: importance of social context and co-use

Cocaine-use disorders (CUDs) continue to be a major public health problem that requires effective treatments. Despite decades of preclinical research, there are no FDA-approved pharmacotherapies for cocaine use. While there are numerous potential reasons why no efficacious treatments have been identified or approved for cocaine use, we discuss two possible reasons in this review: the low number of studies incorporating social variables and the overlooking of the clinical reality of polysubstance use. These variables impact drug use across the substance-use cycle, including vulnerability, maintenance, and treatment. Recent preclinical and clinical data suggest that cocaine users who engage in polysubstance use should be viewed as a distinct and more prevalent population who require unique behavioral and pharmacological approaches to reduce cocaine use. Therefore, to understand the neurobiology and eventual treatments for CUDs, both variables should be included in animal models.

The Protective Effect of Social Reward on Opioid and Psychostimulant Reward and Relapse: Behavior, Pharmacology, and Brain Regions

Until recently, most modern neuroscience research on addiction using animal models did not incorporate manipulations of social factors. Social factors play a critical role in human addiction: social isolation and exclusion can promote drug use and relapse, while social connections and inclusion tend to be protective. Here, we discuss the state of the literature on social factors in animal models of opioid and psychostimulant preference, self-administration, and relapse. We first summarize results from rodent studies on behavioral, pharmacological, and circuit mechanisms of the protective effect of traditional experimenter-controlled social interaction procedures on opioid and psychostimulant conditioned place preference, self-administration, and relapse. Next, we summarize behavioral and brain-mechanism results from studies using newer operant social-interaction procedures that inhibit opioid and psychostimulant self-administration and relapse. We conclude by discussing how the reviewed studies point to future directions for the addiction field and other neuroscience and psychiatric fields, and their implications for mechanistic understanding of addiction and development of new treatments.SIGNIFICANCE STATEMENT In this review, we propose that incorporating social factors into modern neuroscience research on addiction could improve mechanistic accounts of addiction and help close gaps in translating discovery to treatment. We first summarize rodent studies on behavioral, pharmacological, and circuit mechanisms of the protective effect of both traditional experimenter-controlled and newer operant social-interaction procedures. We then discuss potential future directions and clinical implications.