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

Subthalamic high-frequency deep brain stimulation reduces addiction-like alcohol use and the possible negative influence of a peer presence

RATIONALE

The immediate social context significantly influences alcohol consumption in humans. Recent studies have revealed that peer presence could modulate drugs use in rats. The most efficient condition to reduce cocaine intake is the presence of a stranger peer, naive to drugs. Deep brain stimulation (DBS) of the Subthalamic Nucleus (STN), which was shown to have beneficial effects on addiction to cocaine or alcohol, also modulates the protective influence of peer's presence on cocaine use.

OBJECTIVES

This study aimed to: 1) explore how the presence of an alcohol-naive stranger peer affects recreational and escalated alcohol intake, and 2) assess the involvement of STN on alcohol use and in the modulation induced by the presence of an alcohol-naïve stranger peer.

METHODS

Rats with STN DBS and control animals self-administered 10% (v/v) ethanol in presence, or absence, of an alcohol-naive stranger peer, before and after escalation of ethanol intake (observed after intermittent alcohol (20% (v/v) ethanol) access).

RESULTS

Neither STN DBS nor the presence of an alcohol-naive stranger peer modulated significantly recreational alcohol intake. After the escalation procedure, STN DBS reduced ethanol consumption. The presence of an alcohol-naive stranger peer increased consumption only in low drinkers, which effect was suppressed by STN DBS.

CONCLUSIONS

These results highlight the influence of a peer's presence on escalated alcohol intake, and confirm the role of STN in addiction-like alcohol intake and in the social influence on drug consumption.

Evaluating the impact of concurrent sucrose availability on operant ethanol self-administration in male and female Long Evans rats

Investigating how environmental factors, such as the availability of non-ethanol alternative reinforcers, influences ethanol self-administration is critical for understanding the pathology of alcohol use disorder (AUD). Here we established the first operant choice paradigm that leverages the strengths of the sipper tube self-administration model to investigate how concurrent access to sucrose altered ethanol self-administration in male and female Long Evans rats. Choice behavior was examined using two distinct paradigms, including a novel adaptation of the response requirement paradigm. Under both a fixed-ratio or response requirement paradigm, we observed that concurrent availability of an alternative reinforcer significantly reduced appetitive and consummatory ethanol drinking-related behaviors. Furthermore, we assessed the sensitivity of the response requirement choice paradigm by administering the pharmacological stressor yohimbine and by altering the taste of the ethanol solution. Yohimbine administration non-selectively increased ethanol and sucrose intake, but not seeking, while taste adulteration decreased ethanol seeking and intake. These experiments demonstrate the utility of two concurrent choice paradigms that can more accurately capture AUD-like phenotypes, such as ethanol-directed choice in the face of alternative reinforcers. Future studies should investigate how models of vulnerability and dependence alter ethanol choice behavior under these paradigms.

Social context and drug cues modulate inhibitory control in cocaine addiction: involvement of the STN evidenced through functional MRI

Addictions often develop in a social context, although the influence of social factors did not receive much attention in the neuroscience of addiction. Recent animal studies suggest that peer presence can reduce cocaine intake, an influence potentially mediated, among others, by the subthalamic nucleus (STN). However, there is to date no neurobiological study investigating this mediation in humans. This study investigated the impact of social context and drug cues on brain correlates of inhibitory control in individuals with and without cocaine use disorder (CUD) using functional Magnetic Resonance Imaging (fMRI). Seventeen CUD participants and 17 healthy controls (HC) performed a novel fMRI "Social" Stop-Signal Task (SSST) in the presence or absence of an observer while being exposed to cocaine-related (vs. neutral) cues eliciting craving in drug users. The results showed that CUD participants, while slower at stopping with neutral cues, recovered control level stopping abilities with cocaine cues, while HC did not show any difference. During inhibition (Stop Correct vs Stop Incorrect), activity in the right STN, right inferior frontal gyrus (IFG), and bilateral orbitofrontal cortex (OFC) varied according to the type of cue. Notably, the presence of an observer reversed this effect in most areas for CUD participants. These findings highlight the impact of social context and drug cues on inhibitory control in CUD and the mediation of these effects by the right STN and bilateral OFC, emphasizing the importance of considering the social context in addiction research. They also comfort the STN as a potential addiction treatment target.

An investigation of economic interactions between social reinforcement and heroin or cocaine in rats

The primary goal of the present study was to determine the economic relationship between heroin and social reinforcement in rats: are they substitutes, independents, or complements? In Experiment 1, one group of rats was given a budget of responses that they could allocate between heroin and social reinforcement offered at various combinations of prices. A second group chose between two levers that each resulted in social reinforcement at varying prices when pressed. There was no relationship between the relative allocation of responses between heroin and social reinforcement and changes in their relative prices, indicating that these reinforcers are best viewed as independents. In contrast, when choosing between two sources of social reinforcement, rats increased the allocation of behavior to the cheaper option, confirming that the method used here was sensitive to detecting substitution effects. In Experiment 2, the same method was used to compare one group that chose between heroin and social reinforcement with a second group that chose between cocaine and social reinforcement. The finding that heroin and social reinforcement were independents was replicated. Additionally, there was some evidence that cocaine and social reinforcement were substitutes, at least when the first few minutes of the session were excluded. These results add to our knowledge of how drug and nondrug reinforcers interact in choice situations in rats and may model factors that influence drug use in humans.

Genetic and functional adaptations and alcohol-biased signaling in the mediodorsal thalamus of alcohol dependent mice

Alcohol Use Disorder (AUD) is a significant health concern characterized by an individual's inability to control alcohol intake. With alcohol misuse increasing and abstinence rates declining, leading to severe social and health consequences, it is crucial to uncover effective treatment strategies for AUD by focusing on understanding neuroadaptations and cellular mechanisms. The mediodorsal thalamus (MD) is a brain region essential for cognitive functioning and reward-guided choices. However, the effects of alcohol (ethanol) dependence on MD neuroadaptations and how dependence alters MD activity during choice behaviors for alcohol and a natural reward (sucrose) are not well understood. Adult C57BL/6J mice treated with chronic intermittent ethanol (CIE) exposure were used to assess genetic and functional adaptations in the MD. Fiber photometry-based recordings of GCaMP6f expressed in the MD of C57BL/6J mice were acquired to investigate in vivo neural adaptations during choice drinking sessions for alcohol (15%) and either water or sucrose (3%). There were time-dependent changes in cFos and transcript expression during acute withdrawal and early abstinence. Differentially expressed genes were identified in control mice across different circadian time points and when comparing control and alcohol dependent mice. Gene Ontology enrichment analysis of the alcohol-sensitive genes revealed disruption of genes that control glial function, axonal myelination, and protein binding. CIE exposure also increased evoked firing in MD cells at 72 hours of withdrawal. In alcohol-dependent male and female mice that show increased alcohol drinking and preference for alcohol over water, we observed an increase in alcohol intake and preference for alcohol when mice were given a choice between alcohol and sucrose. Fiber photometry recordings demonstrated that MD activity is elevated during and after licking bouts for alcohol, water, and sucrose, and the signal for alcohol is significantly higher than that for water or sucrose during drinking. The elevated signal during alcohol bouts persisted in alcohol dependent mice. These findings demonstrate that CIE causes genetic and functional neuroadaptations in the MD and that alcohol dependence enhances alcohol-biased behaviors, with the MD uniquely responsive to alcohol, even in dependent mice.

Sex differences in the social motivation of rats: Insights from social operant conditioning, behavioural economics, and video tracking

BACKGROUND

Social behaviour plays a key role in mental health and wellbeing, and developing greater understanding of mechanisms underlying social interaction-particularly social motivation-holds substantial transdiagnostic impact. Common rodent behavioural assays used to assess social behaviour are limited in their assessment of social motivation, whereas the social operant conditioning model can provide unique and valuable insights into social motivation. Further characterisation of common experimental parameters that may influence social motivation within the social operant model, as well as complementary methodological and analytical approaches, are warranted.

METHODS

This study investigated the effects of biological sex, housing condition, and time-of-day, on social motivation using the social operant model. This involved training rats to lever press (FR1) for 60-s access to a social reward (same-sex conspecific stimulus). Subjects were male and female Wistar rats, housed under individual or paired conditions, and sessions were conducted either in the mid-late light phase (ZT6-10) or early-mid dark phase (ZT13-17). A behavioural economics approach was implemented to measure social demand and the influence of stimulus partner sex (same- vs. opposite-sex stimulus) on social operant responding. Additionally, video tracking analyses were conducted to assess the degree of convergence between social appetitive and consummatory behaviours.

RESULTS

Biological sex, housing conditions, the interaction between sex and housing, and stimulus partner sex potently influenced social motivation, whereas time-of-day did not. Behavioural economics demonstrated that sex, housing, and their interaction influence both the hedonic set-point and elasticity of social demand. Video analysis of social interaction during social operant sessions revealed that social appetitive and consummatory behaviours are not necessarily convergent, and indicate potential social satiety. Lastly, oestrus phase of female experimental and stimulus rats did not impact social motivation within the model.

CONCLUSIONS

Social isolation-dependent sex differences exist in social motivation for rats, as assessed by social operant conditioning. The social operant model represents an optimal preclinical assay that comprehensively evaluates social motivation and offers a platform for future investigations of neurobiological mechanisms underlying sex differences in social motivation. These findings highlight the importance of continued consideration and inclusion of sex as a biological variable in future social operant conditioning studies. Humans are social creatures-our everyday interactions with others and the support this provides play a key role in our wellbeing. For those experiencing mental health conditions, people's motivation to engage with others can wane, which can lead them to withdraw from those who support them. Therefore, to develop better treatment strategies for these conditions, we need to gain a deeper understanding of social motivation. Studying social behaviour in animals can facilitate this investigation of social motivation as it allows for a causal understanding of underlying neurobiology that is not possible in human experiments. An optimal way to study social motivation in animals is using the social operant conditioning model, where rats learn to press a lever that opens a door and allows them to interact with another rat for a short time. This study characterised the social operant model by testing whether sex, housing conditions, time-of-day, and the sex of the stimulus partner influence rats' motivation to seek interaction with another rat. We found that female rats were more socially motivated than males, and that rats living alone were more motivated than those living with another rat; interestingly, this effect of housing affected females more than males. Regardless of sex, rats were more motivated to interact with a rat of the opposite sex. These findings provide insights into sex differences in social motivation in rats and new insights into the social operant model which will help guide future research into social motivation and other mental health conditions.

The importance of choice and agency in animal models of addiction

The aim of this short commentary is twofold. First, it uncovers and outlines in broad strokes a historical, albeit oft-overlooked, trend toward a growing place for choice and agency in the design of animal models of addiction. Next, it tries to draw from this historical trend some perspectives and implications for future research.

Altered reward sensitivity to sucrose outcomes prior to drug exposure in alcohol preferring rats

Addiction involves key impairments in reward sensitivity (RS). The current study explored impaired RS to natural reward as a predisposing factor to addictive-like behavior. Alcohol preferring (P) rats are selectively bred based on significantly greater ethanol consumption and preference and offer the ability to inspect differences in subjects with a positive family history of addictive-like behavior. P rat's RS was compared to RS in the well-used Sprague-Dawley (SD) strain. To assess RS in a novel manner, instrumental incentive contrast, discrimination and consumption of sucrose solution were examined. Animals performed in a free operant situation for different sucrose concentration solutions using a block of 'mixed' trials with alternating outcome concentrations (e.g., 5 and 10 % sucrose) to change outcome value in a predictable manner. Animals also performed for reward in blocks of single outcome trials (5 or 10 or 20 or 40 % sucrose daily exposure) surrounding the mixed block. RS (e.g., reward discrimination and contrast effects between and within-sessions) was measured by changes in trials completed, instrumental response latency and consumption. P rats expressed an altered profile of RS with a greater tendency toward equivalent responding to different outcomes within the same session and an absence of incentive contrast from diverse reward comparisons. In contrast, SD animals expressed within-session reward discrimination and a subset of incentive contrast effects. These effects were moderated by food deprivation more consistently in SD compared to P rats. P rat alterations in processing natural rewards could predispose them to addictive-like behaviors including greater alcohol consumption and preference.

Different mechanisms underlie compulsive alcohol self-administration in male and female rats

BACKGROUND

Sex is an important factor in the progression and treatment of alcohol addiction, and therapeutic approaches may have to be tailored to potential sex differences. This highlights the importance of understanding sex differences in behaviors that reflect key elements of clinical alcohol addiction, such as continued use despite negative consequences ("compulsive use"). Studies in experimental animals can help provide an understanding of the role sex plays to influence these behaviors.

METHODS

Large populations of genetically heterogeneous male and female Wistar rats were tested in an established model of compulsive alcohol self-administration, operationalized as alcohol responding despite contingent foot shock punishment. We also tested baseline (fixed ratio, unpunished) operant alcohol self-administration, motivation to self-administer alcohol (progressive ratio), and temporal discounting for alcohol reward. In search of predictors of compulsivity, animals were screened for novelty-induced place preference, anxiety-like behavior, pain sensitivity and corticosterone levels. The estrous cycle was monitored throughout the study.

RESULTS

Unpunished self-administration of alcohol did not differ between males and females when alcohol intake was corrected for body weight. Overall, females showed higher levels of compulsive responding for alcohol. Compulsive response rates showed bimodal distributions in male but not in female rats when intermediate shock intensities were used (0.2 and 0.25 mA); at higher shock intensities, responding was uniformly suppressed in both males and females. We also found less steep discounting in females when alcohol was devalued by delaying its delivery. Males exhibited a stronger motivation to obtain alcohol under unpunished conditions, while females showed higher corticosterone levels at baseline. Factor analysis showed that an underlying dimension related to stress and pain predicted compulsivity in females, while compulsivity in males was predicted by a reward factor. We did not find differences in alcohol-related behaviors throughout the various stages of the estrous cycle.

CONCLUSIONS

Our results suggest that mechanisms promoting compulsivity, a key feature of alcohol addiction, likely differ between males and females. This underscores the importance of considering sex as a biological variable in both preclinical and clinical research, and has potential treatment implications in alcohol addiction.

Molecular fingerprints in the hippocampus of alcohol seeking during withdrawal

Alcohol use disorder (AUD) is characterized by excessive alcohol seeking and use. Here, we investigated the molecular correlates of impaired extinction of alcohol seeking using a multidimentional mouse model of AUD. We distinguished AUD-prone and AUD-resistant mice, based on the presence of ≥ 2 or < 2 criteria of AUD and utilized RNA sequencing to identify genes that were differentially expressed in the hippocampus and amygdala of mice meeting ≥ 2 or < 2 criteria, as these brain regions are implicated in alcohol motivation, seeking, consumption and the cognitive inflexibility characteristic of AUD. Our findings revealed dysregulation of the genes associated with the actin cytoskeleton, including actin binding molecule cofilin, and impaired synaptic transmission in the hippocampi of mice meeting ≥ 2 criteria. Overexpression of cofilin in the polymorphic layer of the dentate gyrus (PoDG) inhibited ML-DG synapses, increased motivation to seek alcohol and impaired extinction of alcohol seeking, resembling the phenotype observed in mice meeting ≥ 2 criteria. Overall, our study uncovers a novel mechanism linking increased hippocampal cofilin expression with the AUD phenotype.