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

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.

Response-contingent cocaine increases the reinforcing effectiveness of social contact

Epidemiological studies report a high concordance rate of drug use within groups, suggesting an interplay between drug reinforcement and social cohesion. Preclinical studies reveal that (a) contingent access to a social partner increases cocaine intake and (b) experimenter-delivered cocaine increases the reinforcing effects of social contact. The purpose of this study was to determine if response-contingent cocaine increases the reinforcing effectiveness of social contact. Male rats were implanted with intravenous catheters and trained on a fixed ratio (FR1) schedule for 30-s access to a social partner. The reinforcing effectiveness of social contact was then determined using a progressive ratio (PR) schedule. After the PR test, rats were divided into two groups in which each response on an FR1 schedule produced social access and either response-contingent cocaine (0.5 mg/kg/infusion) or saline. After 9 days, the reinforcing effectiveness of social contact in the absence of infusions was determined again on the PR schedule. The cocaine and saline reinforcers were then switched between groups and the latter procedures were repeated. Recent exposure to response-contingent cocaine increased the reinforcing effectiveness of social contact on the PR schedule. This effect was transient, and the reinforcing effectiveness of social contact returned to baseline levels once response-contingent cocaine was replaced with saline. These data indicate that recent exposure to response-contingent cocaine transiently increases the reinforcing effectiveness of social contact and suggest that cocaine use may strengthen social cohesion by increasing the reinforcing effects of social contact with other individuals. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

Social influence and the likelihood of using cannabis: Role of source physical attractiveness

The presence of another individual may increase or decrease the likelihood a person will use drugs, depending on factors such as whether the source (i.e., the other individual) is also using drugs. The purpose of this study was to determine whether the physical attractiveness of the source influences the likelihood a person will use cannabis. Heterosexual men and women were recruited via a crowdsourcing platform and asked to rank order the physical attractiveness of 13 opposite-gender people. Participants were then presented with hypothetical scenarios in which they reported the likelihood of engaging in drug use (i.e., "use marijuana") and a nondrug control activity ("enjoy the view" from a private balcony) when they were alone versus in the presence of an opposite-gender person they rated low, moderate, or high in relative physical attractiveness. The likelihood of participating in both drug and nondrug activities increased as a function of the relative physical attractiveness of the other individual (i.e., the source); however, notable gender differences were observed in the likelihood of using cannabis. Women were less likely to use cannabis in the presence of less attractive men relative to using cannabis alone, whereas men were more likely to use cannabis in the presence of more attractive women than using cannabis alone. These data suggest the presence of an opposite-gender person can either inhibit or facilitate drug use depending on the physical attractiveness of the source and gender of the subject. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

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.

Rats choose alcohol over social reward in an operant choice procedure

The interaction between social factors and alcohol addiction is complex, with potential for both positive and negative contributions to drug use and abstinence. Positive social connections are an important component in successful abstinence, and yet the social context of alcohol use can also lead to relapse. Recently it was shown that rats overwhelmingly choose social reward over methamphetamine, cocaine, and heroin in a discrete choice procedure, and that prolonged choice for social reward attenuates incubation of drug craving. The extent to which this effect generalises to rats trained to self-administer alcohol is not known. In this study we aimed to test the effect of social reward on choice for alcohol in male and female rats. We first validated social reward self-administration in both male and female Long-Evans rats, and found that 60 s access to a social partner of the same sex can serve as an operant reinforcer. Next we trained rats to self-administer both social reward and alcohol (20% ethanol in water), and then used discrete choice trial based tests to determine whether there is a choice preference for alcohol or social reward. Our main finding is that both male and female rats showed persistent choice for alcohol over social reward, with only minor differences between the sexes. We also show that choice for alcohol could be reduced via increased response requirement for alcohol, pre-choice alcohol exposure, and also decreasing the alcohol percentage. This study shows that preference for social rewards over drugs may not generalise to rats self-administering alcohol, and we describe several conditions where choice for social reward can be developed. This study highlights the important contribution of social factors to alcohol abuse, and future studies can investigate the neurobiology underlying a shift in preference from alcohol to social rewards.

A network-based analysis detects cocaine-induced changes in social interactions in Drosophila melanogaster

Addiction is a multifactorial biological and behavioral disorder that is studied using animal models, based on simple behavioral responses in isolated individuals. A couple of decades ago it was shown that Drosophila melanogaster can serve as a model organism for behaviors related to alcohol, nicotine and cocaine (COC) addiction. Scoring of COC-induced behaviors in a large group of flies has been technologically challenging, so we have applied a local, middle and global level of network-based analyses to study social interaction networks (SINs) among a group of 30 untreated males compared to those that have been orally administered with 0.50 mg/mL of COC for 24 hours. In this study, we have confirmed the previously described increase in locomotion upon COC feeding. We have isolated new network-based measures associated with COC, and influenced by group on the individual behavior. COC fed flies showed a longer duration of interactions on the local level, and formed larger, more densely populated and compact, communities at the middle level. Untreated flies have a higher number of interactions with other flies in a group at the local level, and at the middle level, these interactions led to the formation of separated communities. Although the network density at the global level is higher in COC fed flies, at the middle level the modularity is higher in untreated flies. One COC specific behavior that we have isolated was an increase in the proportion of individuals that do not interact with the rest of the group, considered as the individual difference in COC induced behavior and/or consequence of group influence on individual behavior. Our approach can be expanded on different classes of drugs with the same acute response as COC to determine drug specific network-based measures and could serve as a tool to determinate genetic and environmental factors that influence both drug addiction and social interaction.

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.

Wistar rats choose alcohol over social interaction in a discrete-choice model

Animal models of substance use disorders have been criticized for their limited translation. One important factor behind seeking and taking that has so far been largely overlooked is the availability of alternative non-drug rewards. We recently reported that only about 15% of outbred Wistar rats will choose alcohol over a sweet solution of saccharin. It was also shown using a novel operant model of choice of drugs over social rewards that social interaction consistently attenuates self-administration and incubation of craving for stimulants and opioids. Whether this is also true for alcohol and choice of alcohol over a sweet reward translates to social rewards is currently unknown. We therefore evaluated choice between alcohol and a social reward in different experimental settings in both male and female Wistar rats. We found, in contrast to prior work that employed discrete choice of drugs vs. social reward, that rats almost exclusively prefer alcohol over social interaction, irrespective of the nature of the social partner (cagemate vs. novel rat), the length of interaction, housing conditions and sex. Alcohol choice was reduced when the response requirement for alcohol was increased. However, rats persisted in choosing alcohol, even when the effort required to obtain it was 10-16 times higher (for females and males respectively) than the one for the social reward. Altogether, these results indicate that the social choice model may not generalize to alcohol, pointing to the possibility that specific interactions between alcohol and social reward, not seen when a sweet solution is used as an alternative to the drug, may play a crucial role in alcohol vs. social choice experiments.

Peer-induced cocaine seeking in rats: Comparison to nonsocial stimuli and role of paraventricular hypothalamic oxytocin neurons

The purpose of this study was to determine if social vs nonsocial cues (peer vs light/tone) can serve as discriminative stimuli to reinstate cocaine seeking. In addition, to assess a potential mechanism, an oxytocin (OT) promoter-linked hM3Dq DREADD was infused into the paraventricular nucleus of the hypothalamus to determine whether peer-induced cocaine seeking is decreased by activation of OT neurons. Male rats underwent twice-daily self-administration sessions, once with cocaine in the presence of one peer (S+) and once with saline in the presence of a different peer (S-). Another experiment used similar procedures, except the discriminative stimuli were nonsocial (constant vs flashing light/tone), with one stimulus paired with cocaine (S+) and the other paired with saline (S-). A third experiment injected male and female rats with OTp-hM3Dq DREADD or control virus into PVN and tested them for peer-induced reinstatement of cocaine seeking following clozapine (0.1 mg/kg). Although acquisition of cocaine self-administration was similar in rats trained with either peer or light/tone discriminative stimuli, the latency to first response was reduced by the peer S+, but not by the light/tone S+. In addition, the effect of the conditioned stimulus was overshadowed by the peer S+ but not by the light/tone S+. Clozapine blocked the effect of the peer S+ in rats receiving the OTp-hM3Dq DREADD virus, but not in rats receiving the control virus. These results demonstrate that a social peer can serve as potent trigger for drug seeking and that OT in PVN modulates peer-induced reinstatement of cocaine seeking.