The effects of social contact on cocaine intake under extended-access conditions in male rats

The positive reinforcing effects of cocaine and opposite-sex social contact: roles of biological sex and estrus

RATIONALE

Preclinical studies report that drug use and social contact mutually influence the reinforcing effects of one another. Most of these studies have used same-sex dyads exclusively, and the role of factors related to biological sex and hormonal fluctuations are not well understood.

OBJECTIVES

The purpose of this study was to examine the reinforcing effects of cocaine and social contact with an opposite-sex partner in male and female rats, and how these effects are modulated by ovarian hormones.

METHODS

Male and female rats were trained in a nonexclusive choice procedure in which cocaine and social contact with an opposite-sex partner were simultaneously available on concurrent progressive ratio schedules of reinforcement. To examine the effects of ovarian hormones related to estrous cycling, Experiment 1 used naturally cycling, gonadally intact females, whereas Experiment 2 used ovariectomized females, and estrus was artificially induced with exogenous hormones.

RESULTS

In both experiments, cocaine and social contact functioned as robust reinforcers, and there were no significant effects of biological sex or estrus status of the females. The positive reinforcing effects of both cocaine and social contact increased as a function of cocaine dose, indicating that contingent cocaine administration increases the reinforcing effects of social contact.

CONCLUSIONS

These data suggest that cocaine use among opposite-sex partners may enhance factors that contribute to social bonding.

Treatment with dextroamphetamine decreases the reacquisition of cocaine self-administration: Consistency across social contexts

RATIONALE

A return to cocaine use following abstinence frequently occurs in a social context, and the presence of other individuals using cocaine may contribute to the likelihood of use. Previous studies have reported that chronic d-amphetamine treatment decreases cocaine self-administration in laboratory animals and reduces a return to cocaine use following abstinence in humans.

OBJECTIVE

The purpose of this study was to examine the effects of chronic d-amphetamine treatment on the reacquisition of cocaine use in rats self-administering cocaine in different social contexts.

METHODS

Male and female rats were implanted with intravenous catheters and trained to self-administer cocaine during daily 6-hr sessions. After 14 days, cocaine self-administration was extinguished by substituting saline for the cocaine stimulus. At this time, rats were randomized to receive chronic treatment with either d-amphetamine or saline. After 9 days of extinction, cocaine was again made available during daily 6-hr sessions. At this time, rats were further randomized into three social conditions: (1) rats continued self-administering cocaine in isolation, (2) rats self-administered cocaine in the presence of a same-sex partner that also self-administered cocaine, or (3) rats self-administered cocaine in the presence of a same-sex partner that did not have access to cocaine. Daily treatment with d-amphetamine or saline continued for the duration of reacquisition testing.

RESULTS

Chronic treatment with d-amphetamine decreased cocaine intake during reacquisition, but these effects were not influenced by the social context. No sex differences were observed.

CONCLUSION

These data support previous studies reporting that d-amphetamine decreases cocaine intake and demonstrate its efficacy across social contexts.

Role of social context in addiction etiology and recovery

While social context has long been considered central to substance use disorder prevention and treatment and many drug-taking events occur in social settings, experimental research on social context has historically been limited. Recent years have seen an emergence of concerted preclinical and human laboratory research documenting the direct impact of social context on substance use, delineating behavioral and neurobiological mechanisms underlying social influence's role. We review this emerging preclinical and human laboratory literature from a theoretical lens that considers distinct stages of the addiction process including drug initiation/acquisition, escalation, and recovery. A key conclusion of existing research is that the impact of the social environment is critically moderated by the drug-taking behavior and drug use history of a social peer. Specifically, while drug-free social contexts can reduce the likelihood of drug use initiation and act as a competitive non-drug alternative preventing escalation, drug-using peers can equally facilitate initiation and escalation through peer modeling as a contingent reward of use. Likewise, social context may facilitate recovery or serve as a barrier that increases the chances of a return to regular use. We conclude by discussing evidence-based treatments and recovery support services that explicitly target social mechanisms or that have identified social context as a mechanism of change within treatment. Ultimately, new areas for research including the expansion of drug classes studied and novel human laboratory designs are needed to further translate emerging findings into clinical practice.

Demand and cross-price elasticity of cocaine and social contact in a free-operant procedure of nonexclusive choice

Research examining the social determinants of addiction has advanced significantly with the recent development of preclinical models of drug use and the social environment. These models reveal that drug use and social contact compete with one another for behavioral expression in discrete-trial choice procedures using concurrent schedules of reinforcement. The purpose of this study was to determine how concurrent access to cocaine and a social partner influences the demand for each alternative under free-operant conditions in which responding maintained by each reinforcer is independent and nonexclusive of the other. To this end, male rats were trained under a free-operant, concurrent schedule of reinforcement in which responding maintained by cocaine and access to a social partner operated independently of one another. Measures of economic demand (e.g., intensity, Omax, cross-price elasticity) were determined by manipulating the response requirement (i.e., fixed ratio value) across sessions. Tests were conducted in which the social partner was either treated or not treated with cocaine to determine whether the intoxication state of the partner influenced demand. The principal findings of this study are (1) demand for a cocaine-treated partner is greater than demand for a cocaine-free partner, (2) demand for cocaine is greater in the presence of a cocaine-treated partner than a cocaine-free partner, and (3) concurrent access to cocaine decreases demand for social contact. Notably, measures of cross-price elasticity indicated that social contact is a robust economic substitute for cocaine.

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.

Lack of evidence for positive reinforcing and prosocial effects of MDMA in pair-housed male and female rats

3,4-Methylenedioxymethamphetamine (MDMA) is classified as an entactogen, producing feelings of emotional openness and relatedness. One unique feature of MDMA is that people tend to selectively take this drug in social and/or intimate situations. Although MDMA is recognized as having abuse liability, preclinical studies report that it has weak reinforcing effects in animals. The objective of this study was to characterize the positive reinforcing and prosocial effects of MDMA in a translational model of the social environment in which two rats have simultaneous and contingent access to MDMA in close physical proximity. To this end, MDMA self-administration was examined on both fixed and progressive ratio schedules of reinforcement in six groups of rats: (1) isolated males, (2) isolated females, (3) male-male dyads, (4) female-female dyads, (5) male-female dyads, and (6) female-male dyads. For pair-housed rats, data from both rats were analyzed. Next, social preferences were examined in a partner preference test. MDMA failed to produce positive reinforcing effects under all conditions examined. Across a 30-fold dose range (0.01-1.0 mg/kg/infusion), MDMA did not maintain higher responding than saline on both schedules of reinforcement and in all groups tested. In partner preference tests, a history of shared exposure to MDMA did not establish a social preference, and acute administration of MDMA failed to establish a preference for another MDMA-treated rat. These data suggest that social contact does not increase the positive reinforcing effects of MDMA in rats, and that neither contingent nor noncontingent MDMA administration establishes a social preference in rats.

Social Contact Reinforces Cocaine Self-Administration in Young Adult Male Rats: The Role of Social Reinforcement in Vulnerability to Drug Use

Drug-using peers are recognized as a leading factor influencing drug use among adolescents and young adults. One mechanism by which peers influence drug use is by providing social reinforcement for using drugs. Social reinforcement may be provided in multiple ways, including by making social contact contingent on drug use (i.e., an individual must use drugs to gain/maintain access to a peer). The purpose of this study was to develop a preclinical model in which intravenous cocaine self-administration was positively reinforced by access to a social partner. Young adult male rats were trained to self-administer cocaine in operant conditioning chambers with a guillotine door that could be opened to an adjacent compartment housing either a social partner or a non-social stimulus. Once cocaine self-administration was established, the guillotine door was activated, and cocaine intake was reinforced by brief access to either a social (age- and sex-matched peer) or non-social (black-and-white athletic sock) stimulus. Contingent access to a social partner rapidly increased cocaine self-administration. Total cocaine intake was 2- to 3-fold greater in rats assigned to the social versus non-social condition across a 100-fold dose range. Cocaine intake rapidly increased when rats in the original non-social group were later provided with social partners, whereas cocaine intake resisted change and remained elevated when rats in the original social group had their partners removed. These data indicate that contingent access to a social partner increases drug intake and suggest that social reinforcement may represent a vulnerability factor that is particularly resistant to psychosocial interventions.

Environmental influence on the preclinical evaluation of substance use disorder therapeutics

Substance use disorders (SUD) develop as a result of complex interactions between the environment, the subject, and the drug of abuse. Preclinical basic research investigating each of these tripartite components of SUD individually has resulted in advancements in our fundamental knowledge regarding the progression from drug abuse to SUD and severe drug addiction and the underlying behavioral and neurobiological mechanisms. How these complex interactions between the environment, the subject, and the drug of abuse impact the effectiveness of candidate or clinically used medications for SUD has not been as extensively investigated. The focus of this chapter will address the current state of our knowledge how these environmental, subject, and pharmacological variables have been shown to impact candidate or clinical SUD medication evaluation in preclinical research using drug self-administration procedures as the primary dependent measure. The results discussed in this chapter highlight the importance of considering environmental variables such as the schedule of reinforcement, concurrent availability of alternative nondrug reinforcers, and experimental housing conditions in the context of SUD therapeutic evaluation. The thesis of this chapter is that improved understanding of environmental variables in the context of SUD research will facilitate the utility of preclinical drug self-administration studies in the evaluation and development of candidate SUD therapeutics.

Enhancement of Drug Seeking Following Drug Taking in a Sexual Context Requires Anterior Cingulate Cortex Activity in Male Rats

Individual variance in vulnerability to develop addictions is influenced by social factors. Specifically, drug-taking in a sexual context appears to enhance further drug-seeking behavior in human users, as these users identify the effects of drugs to enhance sexual pleasure as a primary reason for continued drug use. Methamphetamine (Meth) is commonly used in this context. Similarly, male rats that self-administered Meth immediately followed by sexual behavior display enhanced drug-seeking behavior, including attenuation of extinction and increased reinstatement to seeking of Meth-associated cues. Hence, drug-taking in a sexual context enhances vulnerability for addiction. However, the neural mechanisms by which this occurs are unknown. Here the hypothesis was tested that medial prefrontal cortex is essential for this effect of Meth and sex when experienced concurrently. First it was shown that CaMKII neurons in the anterior cingulate area (ACA) were co-activated by both Meth and sex. Next, chemogenetic inactivation of ACA CaMKII cells using AAV5-CaMKIIa-hM4Di-mCherry was shown not to affect Meth-induced locomotor activity or sexual behavior. Subsequently, chemogenetic inactivation of ACA CaMKII neurons during Meth self-administration followed by sexual behavior was shown to prevent the effects of Meth and sex on enhanced reinstatement of Meth-seeking but did not affect enhanced drug-seeking during extinction tests. These results indicate that ACA CaMKII cell activation during exposure to Meth in a sexual context plays an essential role in the subsequent enhancement of drug-seeking during reinstatement tests.

Presence of a social peer enhances acquisition of remifentanil self-administration in male rats

BACKGROUND

Social peers influence human drug use at every stage of addiction. Using a dual-compartment apparatus that allows for limited social contact, recent work has shown that cocaine self-administration is enhanced when two rats are trained to self-administer at the same time compared to rats trained alone or trained in the presence of a saline self-administration control peer. The current study measured social influence on self-administration of the short-acting opioid remifentanil using a dual-compartment operant conditioning chamber.

METHODS

Adult male rats were placed in one of five groups: (1) REMI-REMI group, in which both rats self-administered remifentanil; (2) REMI-SAL group, in which rats self-administered remifentanil in the presence of a peer that self-administered saline; (3) SAL-REMI group, in which rats self-administered saline in the presence of a peer that self-administered remifentanil; and (4) REMI ALONE and (5) SAL ALONE groups, in which rats administered their respective drugs alone (no peer). Self-administration was measured using a 2-lever procedure during acquisition, maintenance, increasing fixed-ratio, and dose-response phases.

RESULTS

The presence of a social peer enhanced drug intake during acquisition, regardless of the drug exposure of their peer. Additionally, active lever position significantly affected remifentanil intake during acquisition and maintenance, with the greatest influence occurring when the active lever was close to the peer.

CONCLUSION

The presence of a social peer in the drug-taking context potentiates remifentanil self-administration, regardless of the peer's drug access. Future studies utilizing the dual-compartment apparatus will help elucidate the neural mechanisms underlying social influence on opioid abuse.

Subthalamic nucleus mediates the modulation on cocaine self-administration induced by ultrasonic vocalization playback in rats

Drug intake is known to be under the influence of social context. We have recently shown that presence of a peer influences drug intake in both rats and humans. Whether or not social acoustic communications between the peers play a role during cocaine or sucrose self-administration (SA) was investigated here using playback of ultrasonic vocalizations (USVs) at 50 and 22 kHz, conveying, respectively, positive and negative internal affective states in adult rats. To assess the neurobiological substrate of a potential USV influence on drug and food intake, we tested the effects of subthalamic nucleus (STN) lesions, given its role in emotional and motivational processes. In sham-control rats, playback of USV associated with positive affective states induced long-term decreased cocaine consumption, while USV associated with negative affective states induced short-term increase. Interestingly, no effect of USV playback was observed on sucrose intake, whatever the frequency. STN lesions abolished the influence of USV on cocaine intake, highlighting the influence of STN in emotional processes induced by USV emitted by a peer. These results show how acoustic social communication is important to regulate drug intake in rats and how STN modulation could interfere with addiction processes.

Nonhuman animal models of substance use disorders: Translational value and utility to basic science

BACKGROUND

The National Institute on Drug Abuse (NIDA) recently released a Request for Information (RFI) soliciting comments on nonhuman animal models of substance use disorders (SUD).

METHODS

A literature review was performed to address the four topics outlined in the RFI and one topic inspired by the RFI: (1) animal models that best recapitulate SUD, (2) animal models that best balance the trade-offs between resources and ecological validity, (3) animal models whose translational value are frequently misrepresented or overrepresented by the scientific community, (4) aspects of SUD that are not currently being modeled in animals, and (5) animal models that are optimal for examining the basic mechanisms by which drugs produce their abuse-related effects.

RESULTS

Models that employ response-contingent drug administration, use complex schedules of reinforcement, measure behaviors that mimic the distinguishing features of SUD, and use animals that are phylogenetically similar to humans have the greatest translational value. Models that produce stable and reproducible baselines of behavior, lessen the number of uncontrolled variables, and minimize the influence of extraneous factors are best at examining basic mechanisms contributing to drug reward and reinforcement.

CONCLUSIONS

Nonhuman animal models of SUD have undergone significant refinements to increase their utility for basic science and translational value for SUD. The existing literature describes numerous examples of how these models may best be utilized to answer mechanistic questions of drug reward and identify potential therapeutic interventions for SUD. Progress in the field could be accelerated by further collaborations between researchers using animals versus humans.

Sexual cues influence cocaine-induced locomotion, anxiety and the immunoreactivity of oestrogen receptor alpha and tyrosine hydroxylase in both sexes

Dyadic physical social interaction influences cocaine-seeking behaviour, although whether limited sexual cues (LSC) from an opposite-sex partner influence the behavioural responses to cocaine is unclear. We investigated this issue using a cylindrical wire cage containing a stimulus mouse; the subject mouse (of the opposite sex) had access to this stimulus mouse during a "binge" injection pattern (injected with cocaine or saline vehicle twice a day at 6-hour intervals). Following the second injection, locomotion and anxiety-like behaviours were examined using the open-field and elevated plus maze test, at the same time as oestrogen receptor (ER)α and tyrosine hydroxylase (TH) immunoreactivities were also examined. The data indicate that LSC enhanced cocaine-stimulated locomotion in both sexes and inhibited the levels of anxiety caused by cocaine in males only. Accompanying these changes, the interaction between LSC and cocaine altered ERα immunoreactivity in the ventral medial nuclei of the hypothalamus (VMH) and medial amygdaloid nucleus (MeA) of males, whereas such interaction effects occurred in the VMH, MeA, arcuate nucleus (AR), bed nucleus of the stria terminalis (BNST) and lateral septum (LS) of females. LSC increased cocaine-induced ERα immunoreactivity in the VMH in males and reduced cocaine-induced ERα immunoreactivity in the AR and LS in females. LSC up-regulated cocaine-induced increases in ventral tegmental area (VTA) TH immunoreactivity in females only. Our present data suggest that interactions between LSC and cocaine led to changes in ERα and TH immunoreactivity in a brain region-specific manner, which showed subtle differences in both sexes. The effects of LSC-mediated cocaine-induced locomotion and anxiety may be associated with alterations in ERα and dopamine activation.

Social modulation of drug use and drug addiction

This review aims to demonstrate how social science and behavioral neurosciences have highlighted the influence of social interactions on drug use in animal models. In neurosciences, the effect of global social context that are distal from drug use has been widely studied. For human and other social animals such as monkeys and rodents, positive social interactions are rewarding, can overcome drug reward and, in all, protect from drug use. In contrast, as other types of stress, negative social experiences facilitate the development and maintenance of drug abuse. However, interest recently emerged in the effect of so-called "proximal" social factors, that is, social interactions during drug-taking. These recent studies have characterized the role of the drug considered, the sharing of drug experience and the familiarity of the peer which interaction are made with. We also examine the few studies regarding the sensorial mediator of social behaviors and critically review the neural mediation of social factors on drug use. However, despite considerable characterization of the factors modulating distal influences, the mechanisms for proximal influences on drug use remain largely unknown. This article is part of the Special Issue entitled 'The neuropharmacology of social behavior: from bench to bedside'.

The effects of social contact on cocaine intake in female rats

BACKGROUND

Studies conducted in male rats report that social contact can either facilitate or inhibit drug intake depending on the behavior of social partners. The purpose of the present study was to: (1) examine the effects of social contact on cocaine intake in female rats, (2) examine the behavioral mechanisms by which social contact influences cocaine intake, and (3) examine whether the estrous cycle moderates the effects of social contact on cocaine intake.

METHODS

Female rats were assigned to either isolated or pair-housed conditions in which a social partner either had access to cocaine (cocaine partner) or did not have access to cocaine (abstinent partner). Pair-housed rats were tested in custom-built operant conditioning chambers that allowed both rats to be tested simultaneously in the same chamber.

RESULTS

Rats housed with a cocaine partner self-administered more cocaine than isolated rats and rats housed with an abstinent partner. A behavioral economic analysis indicated that these differences were driven by a greater intensity of cocaine demand (i.e., greater intake at lower unit prices) in rats housed with a cocaine partner. Multivariate modeling revealed that the estrous cycle did not moderate the effects of social contact on cocaine intake.

CONCLUSIONS

These findings indicate that: (1) social contact influences cocaine self-administration in females in a manner similar to that reported in males, (2) these effects are due to differences in the effects of social contact on the intensity of cocaine demand, and (3) these effects are consistent across all phases of the estrous cycle.

The Effects of Excitatory and Inhibitory Social Cues on Cocaine-Seeking Behavior

Social partners influence the likelihood of using drugs, developing a substance use disorder and relapse to drug use after a period of abstinence. Preclinical studies report that social cues influence the acquisition of cocaine use, the escalation of cocaine use over time, and the compulsive patterns of cocaine use that emerge during an extended binge. The purpose of this study was to examine the effects of social cues on the reinstatement of cocaine-seeking behavior after a period of abstinence. Male rats were obtained at weaning, assigned to triads (three rats/cage), reared to adulthood and implanted with intravenous catheters. Rats from each triad were then assigned to one of three conditions: (1) test rats were trained to self-administer cocaine and were tested for reinstatement; (2) cocaine partners were trained to self-administer cocaine and were predictive of response-contingent cocaine delivery; and (3) abstinent partners were not given access to cocaine and were predictive of extinction. The test rats alternated social partners every 5 days for 20 days such that responding was reinforced with cocaine in the presence of the cocaine partner (S⁺) for 10 days and not reinforced with cocaine in the presence of the abstinent partner (S⁻) for 10 days. Responding of the test rats was then extinguished over 7 days under isolated conditions. Tests of reinstatement were then conducted in the presence of the cocaine partner and abstinent partner under extinction conditions. Neither social partner reinstated responding relative to that observed on the final day of extinction; however, responding was greater in the presence of the cocaine partner (S⁺) than the abstinent partner (S⁻) during the reinstatement test. These data fail to demonstrate that a social partner reinstates cocaine-seeking behavior after a period of abstinence, but they do indicate that social partners can serve as either excitatory or inhibitory discriminative stimuli to influence drug-seeking responses.