Propensity for social interaction predicts nicotine-reinforced behaviors in outbred rats

Inbred rat heredity and sex affect oral oxycodone self-administration and augmented intake in long sessions: correlations with anxiety and novelty-seeking

Oxycodone abuse begins with prescription oral oxycodone, yet vulnerability factors determining abuse are largely undefined. We evaluated genetic vulnerability in a rat model of oral oxycodone self-administration (SA): increasing oxycodone concentration/session (0.025-0.1mg/ml; 1,4,16-h) followed by extinction and reinstatement. Active licks and oxycodone intake were greater in females than males during 4-h and 16-h sessions (p< 0.001). Each sex increased intake during 16-h vs 4-h sessions (p<2e-16), but a subset of strains dramatically augmented intake at 16-h (p=0.0005). Heritability (h 2) of active licks/4-h at increasing oxycodone dose ranged from 0.30-0.53. Under a progressive ratio schedule, breakpoints were strain-dependent (p<2e-16). Cued reinstatement was greater in females (p<0.001). Naive rats were assessed by elevated plus maze (EPM), open field (OF) and novel object interaction (NOI). We correlated these behaviors with 28 parameters of oxycodone SA. Anxiety-defining EPM traits were most associated with SA in both sexes, whereas more OF and NOI traits were SA-associated in males. Sex and heredity are major determinants of motivation to take and seek oxycodone; intake augments dramatically during extended access in specific strains; and pleiotropic genes affect anxiety and multiple SA parameters.

Sex Differences in Social Cognition

In this review we explore the sex differences underlying various types of social cognition. Particular focus will be placed on the behaviors of social recognition, social learning, and aggression. Known similarities and differences between sexes in the expressions of these behaviors and the known brain regions where these behaviors are mediated are discussed. The role that the sex hormones (estrogens and androgens) have as well as possible interactions with other neurochemicals, such as oxytocin, vasopressin, and serotonin is reviewed as well. Finally, implications about these findings on the mediation of social cognition are mediated and the sex differences related to humans are considered.

Genome-Wide Association Study on Three Behaviors Tested in an Open Field in Heterogeneous Stock Rats Identifies Multiple Loci Implicated in Psychiatric Disorders

Many personality traits are influenced by genetic factors. Rodents models provide an efficient system for analyzing genetic contribution to these traits. Using 1,246 adolescent heterogeneous stock (HS) male and female rats, we conducted a genome-wide association study (GWAS) of behaviors measured in an open field, including locomotion, novel object interaction, and social interaction. We identified 30 genome-wide significant quantitative trait loci (QTL). Using multiple criteria, including the presence of high impact genomic variants and co-localization of cis-eQTL, we identified 17 candidate genes (Adarb2, Ankrd26, Cacna1c, Cacng4, Clock, Ctu2, Cyp26b1, Dnah9, Gda, Grxcr1, Eva1a, Fam114a1, Kcnj9, Mlf2, Rab27b, Sec11a, and Ube2h) for these traits. Many of these genes have been implicated by human GWAS of various psychiatric or drug abuse related traits. In addition, there are other candidate genes that likely represent novel findings that can be the catalyst for future molecular and genetic insights into human psychiatric diseases. Together, these findings provide strong support for the use of the HS population to study psychiatric disorders.

Strain and sex matters: Differences in nicotine self-administration between outbred and recombinase-driver transgenic rat lines

Preclinical studies of nicotine self-administration provide important value for the field as they are highly rigorous, controlled, can be conducted quickly, and are generalizable to humans. Given the translational value of the nicotine self-administration model, and the relatively new guidelines of the National Institutes of Health to include sex as a biological variable, strain and sex differences in nicotine acquisition were examined here in two outbred rat strains. Sprague-Dawley (SD) and Long-Evans (LE; wildtype and cholinergic acetyltransferase cre-recombinase transgenic) rats of each sex were implanted with indwelling intravenous jugular catheters. Rats were trained to self-administer nicotine (0.02 mg/kg per infusion, paired with contingent light + tone stimuli). Acquisition criteria were set at a minimum active:inactive response ratio of 2:1 and a minimum of 10 infusions per session, both of which had to be met for a minimum of 10 sessions. Across 10 sessions, male SD rats self-administered significantly more nicotine than female SD rats (p < .05), indicating a sex difference in this strain. LE females self-administered more nicotine than SD females indicative of a strain difference between females (p < .05). SD males increased nicotine infusions across sessions compared to LE males and SD females (p < .05). No strain or sex differences were observed in the number of sessions to reach criteria. No differences between wildtype and transgenic LE rats were observed. These results demonstrate sex and strain differences in nicotine self-administration between SD and LE rats and may lend insight into development of other nicotine self-administration models, where sex and strain may impact acquisition. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Administration of N-acetylcysteine Plus Acetylsalicylic Acid Markedly Inhibits Nicotine Reinstatement Following Chronic Oral Nicotine Intake in Female Rats

Background

Nicotine is the major addictive component of cigarette smoke and the prime culprit of the failure to quit smoking. Common elements perpetuating the use of addictive drugs are (i) cues associated with the setting in which drug was used and (ii) relapse/reinstatement mediated by an increased glutamatergic tone (iii) associated with drug-induced neuroinflammation and oxidative stress.

Aims

The present study assessed the effect of the coadministration of the antioxidant N-acetylcysteine (NAC) plus the anti-inflammatory acetylsalicylic acid (ASA) on oral nicotine reinstatement intake following a post-deprivation re-access in female rats that had chronically and voluntarily consumed a nicotine solution orally. The nicotine-induced oxidative stress and neuroinflammation in the hippocampus and its effects on the glutamate transporters GLT-1 and XCT mRNA levels in prefrontal cortex were also analyzed.

Results

The oral coadministration of NAC (40 mg/kg/day) and ASA (15 mg/kg/day) inhibited by 85% of the oral nicotine reinstatement intake compared to control (vehicle), showing an additive effect of both drugs. Acetylsalicylic acid and N-acetylcysteine normalized hippocampal oxidative stress and blunted the hippocampal neuroinflammation observed upon oral nicotine reinstatement. Nicotine downregulated GLT-1 and xCT gene expression in the prefrontal cortex, an effect reversed by N-acetylcysteine, while acetylsalicylic acid reversed the nicotine-induced downregulation of GLT-1 gene expression. The inhibitory effect of N-acetylcysteine on chronic nicotine intake was blocked by the administration of sulfasalazine, an inhibitor of the xCT transporter.

Conclusion

Nicotine reinstatement, following post-deprivation of chronic oral nicotine intake, downregulates the mRNA levels of GLT-1 and xCT transporters, an effect reversed by the coadministration of N-acetylcysteine and acetylsalicylic acid, leading to a marked inhibition of nicotine intake. The combination of these drugs may constitute a valuable adjunct in the treatment of nicotine-dependent behaviors.

Housing conditions during self-administration determine motivation for cocaine in mice following chronic social defeat stress

RATIONALE

Stress exposure has a lasting impact on motivated behavior and can exacerbate existing vulnerabilities for developing a substance use disorder. Several models have been developed to examine how stressful experiences shape drug reward. These range from locomotor sensitization and conditioned place preference to the propensity for drug self-administration or responding to drug-predictive cues. While self-administration studies are considered to have more translational relevance, most of the studies to date have been conducted in rats. Further, many self-administration studies are conducted in single-housed animals, adding the additional stressor of social isolation.

OBJECTIVES

We sought to establish how chronic social defeat stress (CSDS) and social housing conditions impact cocaine self-administration and cocaine-seeking behaviors in C57BL/6 mice.

METHODS

We assessed self-administration behavior (cocaine or saline, 0.5 mg/kg/infusion) in C57BL/6 mice subjected to 10-day CSDS or in unstressed controls. Mice were housed either in pairs or in isolation during self-administration. We compared the effect of housing on acquisition of self-administration, seeking, extinction, drug-induced reinstatement, and after re-exposure to the social stressor.

RESULTS

Pair-housing during self-administration revealed increased social avoidance after CSDS is associated with decreased cocaine intake. In contrast, single-housing revealed stress-sensitive cocaine intake, with increased social avoidance after CSDS associated with increased early cocaine intake. Pair-, but not single-housed mice are susceptible to drug-induced reinstatement independent of CSDS history. Stress re-exposure sensitized cocaine-seeking in stressed single-housed mice.

CONCLUSIONS

The social context surrounding cocaine intake can bidirectionally influence cocaine-related behaviors after psychosocial stress and should be considered when studying stress and drug cross-sensitization.

Role of Brain Derived Extracellular Vesicles in Decoding Sex Differences Associated with Nicotine Self-Administration

Smoking remains a significant health and economic concern in the United States. Furthermore, the emerging pattern of nicotine intake between sexes further adds a layer of complexity. Nicotine is a potent psychostimulant with a high addiction liability that can significantly alter brain function. However, the neurobiological mechanisms underlying nicotine’s impact on brain function and behavior remain unclear. Elucidation of these mechanisms is of high clinical importance and may lead to improved therapeutics for smoking cessation. To fill in this critical knowledge gap, our current study focused on identifying sex-specific brain-derived extracellular vesicles (BDEV) signatures in male and female rats post nicotine self-administration. Extracellular vesicles (EVs) are comprised of phospholipid nanovesicles such as apoptotic bodies, microvesicles (MVs), and exosomes based on their origin or size. EVs are garnering significant attention as molecules involved in cell–cell communication and thus regulating the pathophysiology of several diseases. Interestingly, females post nicotine self-administration, showed larger BDEV sizes, along with impaired EV biogenesis compared to males. Next, using quantitative mass spectrometry-based proteomics, we identified BDEV signatures, including distinct molecular pathways, impacted between males and females. In summary, this study has identified sex-specific changes in BDEV biogenesis, protein cargo signatures, and molecular pathways associated with long-term nicotine self-administration.

Pre-conception exposure to THC fails to impact nicotine reward in adult offspring

Exposure to environmental stimuli in one generation can produce altered behavioral and neurobiological phenotypes in descendants. Recent work has shown that parental exposure to cannabinoids alters the rewarding properties of other abused drugs in the subsequent generation. However, whether preconception Δ⁹-tetrahydrocannabinol (THC) administration modifies the affective properties of nicotine in offspring is unknown. To address this question, male and female rats (F0) received THC (0 or 1.5 mg/kg) throughout the adolescent window and were bred on PND 65. In Experiment 1, adult F1-THC and F1-Veh progeny (males and females) underwent nicotine locomotor sensitization procedures during which nicotine (0 or 0.4 mg/kg) was administered every other day for five exposures, and locomotor activity was recorded on each exposure followed by a final nicotine challenge. There was no cross-generational effect of THC on nicotine locomotor sensitization, although acute exposure to nicotine produced greater activity in females relative to males independent of THC history. In Experiment 2, adult F1-THC and F1-Veh progeny (males and females) were implanted with jugular catheters and trained to self-administer nicotine (0.03 mg/kg/infusion). Following acquisition, all subjects were allowed to self-administer nicotine on a number of reinforcement schedules, e.g., FR2, FR5 and PR, followed by dose response and extinction procedures. Across all indices, F1-THC and F1-Veh subjects displayed similar IVSA of nicotine with no sex differences. The fact that there was no evidence of cross-generational effects of THC on nicotine suggests that such effects are drug-specific.

Nicotine Effects, Body Weight Concerns and Smoking: A Literature Review

How people become addicted to cigarette smoking and remain addicted despite repeated attempts to quit requires piecing together a rather complex puzzle. The present review contextualizes the role of nicotine and smoking sensory stimulation on maintaining smoking, describes nicotine's effects on feeding behavior and body weight, and explores the impact of smoking outcome expectancies, including the belief that nicotine suppresses appetite and body weight, on the decision to smoke or vape (use of e-cigarettes). The analysis concludes with a review of rat models of human nicotine intake that attempt to isolate the effects of nicotine on appetite and weight gain. Animal research replicates with relative closeness phenomena observed in smokers, but the rat model falls short of replicating the long-term weight gain observed post-smoking cessation.

Using Heterogeneous Stocks for Fine-Mapping Genetically Complex Traits

In this chapter we will review both the rationale and experimental design for using Heterogeneous Stock (HS) populations for fine-mapping of complex traits in mice and rats. We define an HS as an outbred population derived from an intercross between two or more inbred strains. HS have been used to perform genome-wide association studies (GWAS) for multiple behavioral, physiological, and gene expression traits. GWAS using HS require four key steps, which we review: selection of an appropriate HS population, phenotyping, genotyping, and statistical analysis. We provide advice on the selection of an HS, comment on key issues related to phenotyping, discuss genotyping methods relevant to these populations, and describe statistical genetic analyses that are applicable to genetic analyses that use HS.

Social and anxiety-like behaviors contribute to nicotine self-administration in adolescent outbred rats

Both emotional and social traits interact with genetic factors to influence smoking behavior. We previously established a socially acquired nicotine intravenous self-administration model where social learning of a nicotine-associated odor cue reversed conditioned flavor aversion and promoted nicotine intake. In this study, we first phenotyped ~800 adolescent heterogeneous stock rats in open field, novel object interaction, social interaction, elevated plus maze, and marble burying behaviors. These rats were then phenotyped on socially acquired nicotine self-administration. We found 243 significant correlations between different behavioral tests. Principal component regression analysis found that ~10-20% of the variance in nicotine-related measures, such as intake during the first or the last three fixed-ratio sessions, the progressive ratio session, and reinstatement behavior, can be explained by variations in behavioral traits. Factors corresponding to social behavior and anxiety were among the strongest predictors of nicotine intake and reinstatement of nicotine-seeking behavior. We also found many sex differences in behavioral measures. These data indicated that the genetic diversity of this population, in combination with social behaviour and anxiety, are significant contributors to the divergent nicotine self-administration behavior and indicated a high probability of discovering sex-specific genetic mechanisms for nicotine intake in future genome-wide association studies.

MicroRNA cluster miR199a/214 are differentially expressed in female and male rats following nicotine self-administration

Previous research has established sex differences associated with nicotine intake, however a significant gap in knowledge remains regarding the molecular mechanisms that govern these differences at the transcriptional level. One critical regulator of transcription are microRNAs (miRNAs). miRNAs are a family of non-coding RNAs that regulate an array of important biological functions altered in several disease states, including neuroadaptive changes within the brain associated with drug dependence. We examined the prefrontal cortex (PFC) from male and female Sprague-Dawley rats following self-administration (22 days) of nicotine or yoked saline controls using next generation RNA-Sequencing (RNA-Seq) technology and found an array of miRNAs to be significantly and differentially regulated by nicotine self-administration. Of these, we found the expression of miR-199a and 214, which are expressed on the same cluster of chromosome 1, to be upregulated in the female rats exposed to nicotine; upregulation in this group was further validated by real time polymerase chain reaction (RT-PCR). Bioinformatics analysis to assess common targets of miR-199/214 identified Sirtuin 1 (SIRT1), a nicotinamide adenine dinucleotide (NAD)- dependent deacetylase that plays a role in apoptosis, neuron survival, and stress resistance. Using western-blot, we confirmed downregulation of SIRT1 and increased cleaved caspase 3 expression in the brains of nicotine-exposed female rats and no change in expression levels in the other groups. Collectively, our findings highlight a miR-199/214 regulatory network that, through SIRT1, may be associated with nicotine seeking in females which may serve as a potential therapeutic target for sex-specific treatment approaches.

Sex differences in the brain: Implications for behavioral and biomedical research

Biological differences between males and females are found at multiple levels. However, females have too often been under-represented in behavioral neuroscience research, which has stymied the study of potential sex differences in neurobiology and behavior. This review focuses on the study of sex differences in the neurobiology of social behavior, memory, emotions, and recovery from brain injury, with particular emphasis on the role of estrogens in regulating forebrain function. This work, presented by the authors at the 2016 meeting of the International Behavioral Neuroscience Society, emphasizes varying approaches from several mammalian species in which sex differences have not only been documented, but also become the focus of efforts to understand the mechanistic basis underlying them. This information may provide readers with useful experimental tools to successfully address recently introduced regulations by granting agencies that either require (e.g. the National Institutes of Health in the United States and the Canadian Institutes of Health Research in Canada) or recommend (e.g. Horizon 2020 in Europe) the inclusion of both sexes in biomedical research.

Heterogeneous Stock Populations for Analysis of Complex Traits

Heterogeneous Stock (HS) populations allow for fine-resolution genetic mapping of a variety of complex traits. HS mice and rats were created from breeding together eight inbred strains, followed by maintaining the colony in a manner that minimizes inbreeding. After 50 or more generations of breeding, the resulting animals' chromosomes represent a genetic mosaic of the founders' haplotypes, with the average distance between recombination events in the centiMorgan range. This allows for genetic mapping to only a few Mb, a much smaller region than what can be identified using traditional F2 intercross or backcross mapping strategies. HS animals have been used to fine-map a variety of complex traits including anxiety and fear behaviors, diabetes, asthma, and heart disease, among others. Once a quantitative trait locus (QTL) has been identified, founder sequence and expression analysis can be used to identify underlying causal genes. In the following review, we provide an overview of how HS rats and mice have been used to identify genetic loci, and in some cases the causal genes, underlying complex traits. We discuss the creation and breeding strategies for both HS rats and mice. We then discuss the statistical analyses used to identify genetic loci, as well as strategies to identify causal genes underlying these loci. We end the chapter by discussing limitations faced when using HS populations, including several statistical challenges that have not been fully resolved.

Sex differences in nicotine intravenous self-administration: A meta-analytic review

OBJECTIVE

This report reflects a meta-analysis that systematically reviewed the literature on intravenous self-administration (IVSA) of nicotine in female and male rats. The goal was to determine if sex differences in nicotine IVSA exist, estimate the magnitude of the effect, and identify potential moderators of the relationship between sex differences and nicotine consumption.

METHODS

Extensive search procedures identified 20 studies that met the inclusion criteria of employing both female and male rats in nicotine IVSA procedures. The meta-analysis was conducted on effect size values that were calculated from mean total intake or nicotine deliveries using the Hedges' unbiased g^u statistic.

RESULTS

A random effects analysis revealed that overall females self-administered more nicotine than males (weighted g^u=0.18, 95% CI [0.003, 0.34]). Subsequent moderator variable analyses revealed that certain procedural conditions influenced the magnitude of sex differences in nicotine IVSA. Specifically, higher reinforcement requirements (>FR1) and extended-access sessions (23h) were associated with greater nicotine IVSA in females versus males. Females also displayed higher nicotine intake than males when the experiment included a light cue that signaled nicotine delivery. Sex differences were not influenced by the diurnal phase of testing, dose of nicotine, or prior operant training.

CONCLUSION

Overall, the results revealed that female rats display higher levels of nicotine IVSA than males, suggesting that the strong reinforcing effects of nicotine promote tobacco use in women.

Sex differences in nicotine preference

Smoking is the major cause of preventable deaths worldwide, and although there is a decline in overall smoking prevalence in developed countries, the decline in women is less pronounced than in men. Women become dependent faster and experience greater difficulties in quitting. Similar trends have been observed in animal models of nicotine/tobacco addiction. Individual differences in vulnerability to drug abuse are also observed in nicotine/tobacco addiction and point to the importance of sex differences. This Review, summarizes findings from three experimental approaches used to depict nicotine preference in animal models, intravenous and oral nicotine self-administration and nicotine-induced conditioned place preference. Nicotine preference is considered to be reflected in the animal's motivation to administer the drug (intravenously or orally) or to prefer an environment paired with the presence of the drug (conditioned place preference). These approaches all point to the importance of sex and age of the subjects; the preference of females and adolescents appear to be more pronounced than that of males and adults, respectively. A closer look at these factors will help us understand the mechanisms that underlie nicotine addiction and develop strategies to cope. Ignoring sex differences and reaching conclusions based only on studies using male subjects has resulted in erroneous generalizations in the past. Sex differences in nicotine preference have been clearly documented, and awareness on this aspect of nicotine dependence will significantly impact our success in translational research. © 2016 Wiley Periodicals, Inc.

Social learning promotes nicotine self-administration by facilitating the extinction of conditioned aversion in isogenic strains of rats

Both social environment and genetic factors are critical for smoking initiation and nicotine addiction. We reported that rats developed conditioned flavor (i.e., taste and odor) aversion to intravenously self-administered (IVSA) nicotine, and that social learning promoted nicotine IVSA with flavor cues. We thus tested the hypothesis that socially acquired nicotine IVSA is a heritable trait by using female rats of six inbred strains and six F1 hybrids. Each strain was tested for 10 daily IVSA sessions. We found that the intake of nicotine (15 and 30 μg/kg/inf) varied among these strains by 33.7–56.6-fold. The heritability of nicotine intake was estimated to be 0.54–0.65. Further, there was a strong correlation in nicotine intake (R² = 0.85, p < 0.0001) between the two nicotine doses. Another cohort of rats was given three daily IVSA sessions followed by five sessions that tested conditioned flavor aversion. Nicotine intake was highly correlated with the extinction of the conditioned aversion (R² = 0.58, p < 0.005). These data showed that nicotine intake in the socially acquired nicotine self-administration model is controlled by genetic factors and that the role of social learning is likely in facilitating the extinction of conditioned aversive response to nicotine.

Alterations in alpha5* nicotinic acetylcholine receptors result in midbrain- and hippocampus-dependent behavioural and neural impairments

RATIONALE

Evidence links alterations in α5-containing nicotinic receptors (α5*-nAChRs) to nicotine addiction. Notably, the rs16969968 polymorphism in the α5 gene (α5SNP) increases the risk for heavy smoking and impairs nicotine-rewarding properties in mice. Additional work is needed to understand how native and polymorphic α5*-nAChRs contribute to processes associated with the risk for nicotine addiction.

OBJECTIVES

We aimed at understanding the contribution of α5*-nAChRs to endophenotypes like increased responses to novelty and anxiety, known to promote vulnerability to addiction, and to the response of the dopamine and serotonin systems to nicotine.

METHODS

Behavioural phenotypes were investigated in mice lacking the α5 gene (α5(-/-)). Nicotine injections were performed to test the consequences of nicotine exposure on the phenotypes identified. Dopamine and serotonin signalling were assessed using in vivo microdialysis and electrophysiology. We used lentiviral vectors to compare the consequences of re-expressing either the α5 wild-type allele or the α5SNP in specific brain areas of α5(-/-) mice.

RESULTS

α5(-/-) mice did not exhibit high responses to novelty but showed decreased novelty-induced rearing behaviour together with high anxiety. Exposure to high doses of nicotine rescued these phenotypes. We identified altered spontaneous and nicotine-elicited serotonin and dopamine activity in α5(-/-) mice. Re-expression of α5 in the ventral tegmental area and hippocampus rescued rearing and anxiety levels in α5(-/-) mice, respectively. When expressing the α5SNP instead, this resulted in a knockout-like phenotype for both behaviours.

CONCLUSIONS

We propose that altered α5*-nAChR cholinergic signalling contributes to emotional/behavioural impairments that may be alleviated by nicotine consumption.

Sex differences in neurotensin and substance P following nicotine self-administration in rats

Investigator-administered nicotine alters neurotensin and substance P levels in Sprague-Dawley rats. This finding suggested a role of the dopamine-related endogenous neuropeptides in nicotine addiction. We sought to extend this observation by determining the responses of neurotensin and substance P systems (assessed using radioimmunoassay) in male and female rats following nicotine self-administration (SA). Male and female Sprague-Dawley were trained to self-administer nicotine, or receive saline infusions yoked to a nicotine-administering rat during daily sessions (1-h; 21 days). Brains were extracted 3 h after the last SA session. Nicotine SA increased tissue levels of neurotensin in the males in the anterior and posterior caudate, globus pallidus, frontal cortex, nucleus accumbens core and shell, and ventral tegmental area. Nicotine SA also increased tissue levels of neurotensin in the females in the anterior caudate, globus pallidus, nucleus accumbens core and shell, but not in the posterior caudate, frontal cortex, or ventral tegmental area. There were fewer sex differences observed in the substance P systems. Nicotine SA increased tissue levels of substance P in both the males and females in the posterior caudate, globus pallidus, frontal cortex, nucleus accumbens shell, and ventral tegmental area. A sex difference was observed in the nucleus accumbens core, where nicotine SA increased tissue levels of substance P in the males, yet decreased levels in the females. The regulation of neuropeptides following nicotine SA may play a role in the susceptibility to nicotine dependence in females and males. Synapse 70:336-346, 2016. © 2016 Wiley Periodicals, Inc.

Socially acquired nicotine self-administration with an aversive flavor cue in adolescent female rats

RATIONALE

Establishing a behavioral model for the effect of social environment on nicotine intake in rodents facilitates the investigation of molecular mechanisms critical for the interaction between social environment and cigarette smoking.

OBJECTIVES

Our main objective was to test the hypothesis that nicotine is the primary reinforcer in the socially acquired nicotine intravenous self-administration (IVSA) model by using an aversive flavor cue.

METHODS

Adolescent female rats were placed in operant conditioning chambers equipped with two lickometers. Operant licking triggered concurrent deliveries of a flavor (i.e., taste and odor) cue containing either quinine or saccharin and an i.v. infusion (30 μg/kg nicotine or saline). An audiovisual cue was provided for some groups of rats. A second rat that did not receive nicotine was placed in the operant conditioning chambers to provide either a neutral or an inducing (i.e., by consuming the flavored solution) social environment. These two rats were separated by a divider that allowed orofacial interactions.

RESULTS

Rats acquired stable nicotine IVSA with either the aversive or the appetitive flavor cue in the inducing social environment, and obtained similar amounts of infusions. The neutral social environment did not support nicotine IVSA with either cue. The audiovisual cue per se did not support nicotine IVSA but enhanced nicotine intake. Nicotine increased the number of concurrent nose pokes by the two rats into the center divider, a measure of social interaction.

CONCLUSIONS

Despite its aversive effects, nicotine is the primary reinforcer for the operant responses in the socially acquired nicotine IVSA model.

Cognitive Dysfunction, Affective States, and Vulnerability to Nicotine Addiction: A Multifactorial Perspective

Although smoking prevalence has declined in recent years, certain subpopulations continue to smoke at disproportionately high rates and show resistance to cessation treatments. Individuals showing cognitive and affective impairments, including emotional distress and deficits in attention, memory, and inhibitory control, particularly in the context of psychiatric conditions, such as attention-deficit hyperactivity disorder, schizophrenia, and mood disorders, are at higher risk for tobacco addiction. Nicotine has been shown to improve cognitive and emotional processing in some conditions, including during tobacco abstinence. Self-medication of cognitive deficits or negative affect has been proposed to underlie high rates of tobacco smoking among people with psychiatric disorders. However, pre-existing cognitive and mood disorders may also influence the development and maintenance of nicotine dependence, by biasing nicotine-induced alterations in information processing and associative learning, decision-making, and inhibitory control. Here, we discuss the potential forms of contribution of cognitive and affective deficits to nicotine addiction-related processes, by reviewing major clinical and preclinical studies investigating either the procognitive and therapeutic action of nicotine or the putative primary role of cognitive and emotional impairments in addiction-like features.

Estrogen involvement in social behavior in rodents: Rapid and long-term actions

This article is part of a Special Issue ("Estradiol and cognition"). Estrogens have repeatedly been shown to influence a wide array of social behaviors, which in rodents are predominantly olfactory-mediated. Estrogens are involved in social behavior at multiple levels of processing, from the detection and integration of socially relevant olfactory information to more complex social behaviors, including social preferences, aggression and dominance, and learning and memory for social stimuli (e.g. social recognition and social learning). Three estrogen receptors (ERs), ERα, ERβ, and the G protein-coupled ER 1 (GPER1), differently affect these behaviors. Social recognition, territorial aggression, and sexual preferences and mate choice, all requiring the integration of socially related olfactory information, seem to primarily involve ERα, with ERβ playing a lesser, modulatory role. In contrast, social learning consistently responds differently to estrogen manipulations than other social behaviors. This suggests differential ER involvement in brain regions important for specific social behaviors, such as the ventromedial and medial preoptic nuclei of the hypothalamus in social preferences and aggression, the medial amygdala and hippocampus in social recognition, and the prefrontal cortex and hippocampus in social learning. While the long-term effects of ERα and ERβ on social behavior have been extensively investigated, our knowledge of the rapid, non-genomic, effects of estrogens is more limited and suggests that they may mediate some social behaviors (e.g. social learning) differently from long-term effects. Further research is required to compare ER involvement in regulating social behavior in male and female animals, and to further elucidate the roles of the more recently described G protein-coupled ERs, both the GPER1 and the Gq-mER.

Animal models of social contact and drug self-administration

Social learning theories of drug abuse propose that individuals imitate drug use behaviors modeled by social peers, and that these behaviors are selectively reinforced and/or punished depending on group norms. Historically, animal models of social influence have focused on distal factors (i.e., those factors outside the drug-taking context) in drug self-administration studies. Recently, several investigators have developed novel models, or significantly modified existing models, to examine the role of proximal factors (i.e., those factors that are immediately present at the time of drug taking) on measures of drug self-administration. Studies using these newer models have revealed several important conclusions regarding the effects of social learning on drug abuse: 1) the presence of a social partner influences drug self-administration, 2) the behavior of a social partner determines whether social contact will increase or decrease drug intake, and 3) social partners can model and imitate specific patterns of drug self-administration. These findings are congruent with those obtained in the human laboratory, providing support for the cross-species generality and validity of these preclinical models. This mini-review describes in detail some of the preclinical animal models used to study social contact and drug self-administration to guide future research on social learning and drug abuse.

Psychiatric disorders as vulnerability factors for nicotine addiction: what have we learned from animal models?

Epidemiological studies indicate a high prevalence of tobacco smoking in subjects with psychiatric disorders. Notably, there is a high prevalence of smoking among those with dependence to other substances, schizophrenia, mood, or anxiety disorders. It has been difficult to understand how these phenomena interact with clinical populations as it is unclear what preceded what in most of the studies. These comorbidities may be best understood by using experimental approaches in well-controlled conditions. Notably, animal models represent advantageous approaches as the parameters under study can be controlled perfectly. This review will focus on evidence collected so far exploring how behavioral effects of nicotine are modified in animal models of psychiatric conditions. Notably, we will focus on behavioral responses induced by nicotine that are relevant for its addictive potential. Despite the clinical relevance and frequency of the comorbidity between psychiatric issues and tobacco smoking, very few studies have been done to explore this issue in animals. The available data suggest that the behavioral and reinforcing effects of nicotine are enhanced in animal models of these comorbidities, although much more experimental work would be required to provide certainty in this domain.

Carbon disulfide mediates socially-acquired nicotine self-administration

The social environment plays a critical role in smoking initiation as well as relapse. We previously reported that rats acquired nicotine self-administration with an olfactogustatory cue only when another rat consuming the same cue was present during self-administration. Because carbon disulfide (CS₂) mediates social learning of food preference in rodents, we hypothesized that socially acquired nicotine self-administration is also mediated by CS₂. We tested this hypothesis by placing female adolescent Sprague-Dawley rats in operant chambers equipped with two lickometers. Licking on the active spout meeting a fixed-ratio 10 schedule triggered the concurrent delivery of an i.v. infusion (saline, or 30 µg/kg nicotine, free base) and an appetitive olfactogustatory cue containing CS₂ (0–500 ppm). Rats that self-administered nicotine with the olfactogustatory cue alone licked less on the active spout than on the inactive spout. Adding CS₂ to the olfactogustatory cue reversed the preference for the spouts. The group that received 500 ppm CS₂ and the olfactogustatory cue obtained a significantly greater number of nicotine infusions than other groups. After extinction training, the original self-administration context reinstated nicotine-seeking behavior in all nicotine groups. In addition, in rats that received the olfactogustatory cue and 500 ppm CS₂ during SA, a social environment where the nicotine-associated olfactory cue is present, induced much stronger drug-seeking behavior compared to a social environment lacking the olfactogustatory cue. These data established that CS₂ is a critical signal that mediates social learning of nicotine self-administration with olfactogustatory cues in rodents. Additionally, these data showed that the social context can further enhance the drug-seeking behavior induced by the drug-taking environment.

Menthol facilitates the intravenous self-administration of nicotine in rats

Menthol is preferred by ~25% of smokers and is the most common flavoring additive in tobacco and electronic cigarettes. Although some clinical studies have suggested that menthol facilitates the initiation of smoking and enhances the dependence on nicotine, many controversies remain. Using licking as the operant behavior, we found that adolescent rats self-administering nicotine (30μg/kg/infusion, free base, i.v.) with contingent oral menthol (60μl, 0.01% w/v) obtained significantly more infusions than rats receiving a vehicle cue or rats self-administering i.v. saline with a menthol cue. Rats yoked to their menthol-nicotine masters emitted significantly fewer licks on the active spouts, indicating that contingent pairing between nicotine and menthol is required for sustained nicotine intake. Rats that self-administered nicotine with a menthol cue also exhibited a long-lasting extinction burst and robust reinstatement behavior, neither of which were observed in rats that self-administered saline with a menthol cue. The cooling sensation of menthol is induced by activating the transient receptor potential M8 (TRPM8) channel. When WS-23, an odorless agonist of the TRPM8 channel, was used as a contingent cue for nicotine, the rats obtained a similar number of nicotine infusions as the rats that were provided a menthol cue and exhibited a strong preference for the active spout. In contrast, highly appetitive taste and odor cues failed to support nicotine self-administration. These data indicated that menthol, likely by inducing a cooling sensation, becomes a potent conditioned reinforcer when it is contingently delivered with nicotine. Together, these results provide a key behavioral mechanism by which menthol promotes the use of tobacco products or electronic cigarettes.

Genomes and phenomes of a population of outbred rats and its progenitors

Finding genetic variants that contribute to phenotypic variation is one of the main challenges of modern genetics. We used an outbred population of rats (Heterogeneous Stock, HS) in a combined sequence-based and genetic mapping analysis to identify sequence variants and genes contributing to complex traits of biomedical relevance. Here we describe the sequences of the eight inbred progenitors of the HS and the variants that segregate between them. We report the genotyping of 1,407 HS rats, and the collection from 2,006 rats of 195 phenotypic measures that are relevant to models of anxiety, type 2 diabetes, hypertension and osteoporosis. We make available haplotype dosages for the 1,407 genotyped rats, since genetic mapping in the HS is best carried out by reconstructing each HS chromosome as a mosaic of the progenitor genomes. Finally, we have deposited an R object that makes it easy to incorporate our sequence data into any genetic study of HS rats. Our genetic data are available for both Rnor3.4 and Rnor5.0 rat assemblies.