Behavioral Consequences of Repeated Nicotine During Adolescence in Alcohol-Preferring AA and Alcohol-Avoiding ANA Rats

A review on the reciprocal interactions between neuroinflammatory processes and substance use and misuse, with a focus on alcohol misuse

Background: The last decade has witnessed a surge of findings implicating neuroinflammatory processes as pivotal players in substance use disorders. The directionality of effects began with the expectation that the neuroinflammation associated with prolonged substance misuse contributes to long-term neuropathological consequences. As the literature grew, however, it became evident that the interactions between neuroinflammatory processes and alcohol and drug intake were reciprocal and part of a pernicious cycle in which disease-relevant signaling pathways contributed to an escalation of drug intake, provoking further inflammation-signaling and thereby exacerbating the neuropathological effects of drug misuse.Objectives: The goal of this review and its associated special issue is to provide an overview of the emergent findings relevant to understanding these reciprocal interactions. The review highlights the importance of preclinical and clinical studies in testing and validation of immunotherapeutics as viable targets for curtailing substance use and misuse, with a focus on alcohol misuse.Methods: A narrative review of the literature on drug and neuroinflammation was conducted, as well as articles published in this Special Issue on Alcohol- and Drug-induced Neuroinflammation: Insights from Pre-clinical Models and Clinical Research.Results: We argue that (a) demographic variables and genetic background contribute unique sensitivity to drug-related neuroinflammation; (b) co-morbidities between substance use disorders and affect dysfunction may share common inflammation-related signatures that predict the efficacy of immunotherapeutic drugs; and (c) examination of polydrug interactions with neuroinflammation is a critical area where greater research emphasis is needed.Conclusions: This review provides an accessible and example-driven review of the relationship between drug misuse, neuroinflammatory processes, and their resultant neuropathological outcomes.

The incentive amplifying effects of nicotine: Roles in alcohol seeking and consumption

Nicotine has a unique profile among drugs of abuse. To the noninitiated user, nicotine has powerful aversive effects and its relatively weak euphorigenic effects undergo rapid tolerance. Despite this, nicotine is commonly abused despite negative heath consequences, and nicotine users have enormous difficulty quitting. Further, nicotine is one of the most commonly co-abused substances, in that it is often taken in combination with other drugs. One explanation of this polydrug use is that nicotine has multiple appetitive and consummatory conditioning effects. For example, nicotine is a reinforcement enhancer in that it can potently increase the incentive value of other stimuli, including those surrounding drugs of abuse such as alcohol. In addition, nicotine also has a unique profile of neurobiological effects that alter regulation of alcohol intake and interoception. This review discusses the psychological and biological mechanisms surrounding nicotine's appetitive conditioning and consummatory effects, particularly its interactions with alcohol.

Adolescent and adult nicotine exposure differentially impacts oral nicotine and oral saccharin self-administration in mice

Smokers that begin during adolescence are more likely to develop nicotine dependence than those who begin as adults. However, the factors that contribute to this remain largely unknown. Here we utilized a novel operant oral nicotine self-administration procedure in mice to assess the consequences of adolescent nicotine exposure on nicotine and saccharin (non-drug) reinforcement in adults. Animals were given non-contingent exposure to either saline or nicotine using the osmotic minipumps during both adolescence and adulthood for 2 weeks. Reinforcing efficacy for oral nicotine and saccharin was assessed using the progressive ratio schedule 2-weeks following the washout period in adults. Non-contingent nicotine exposure in adolescence drastically increased operant responding for oral nicotine but reduced responding for oral saccharin in the group re-exposed to nicotine in adulthood. Interestingly, adolescent nicotine-exposed mice that received saline exposure as adults exhibited higher preference for oral saccharin. However, breakpoints for oral nicotine in these mice remained comparable to control animals. Surprisingly, both adolescent and adult nicotine exposure increased inactive lever responding during self-administration presumably reflecting impulsive responding. Our data suggest that adolescent nicotine exposure produces an increase in reinforcement sensitivity in adulthood as reflected by increased saccharin self-administration but this sensitivity becomes biased towards nicotine self-administration when re-exposed to nicotine in adulthood. Moreover, nicotine/saccharin reinforcement could be impacted by changes in cognitive control, such as increased impulsivity. These distinct behavioral mechanisms may act in concert to facilitate maladaptive nicotine taking in smokers that initiate nicotine use during adolescence.

Nicotine as a discriminative stimulus for ethanol use

Abused drugs reinforce behavior; i.e., they increase the probability of the behavior preceding their administration. Abused drugs can also act as discriminative stimuli; i.e., they can set the occasion for responding reinforced by another event. Thus, one abused drug could come to set the occasion for the use of another and this functional relationship may play a role in polysubstance abuse, where common patterns of use could result in this relationship. Here we establish nicotine (0.4mg/kg, ip 5-min pre-session) as a discriminative stimulus for behavior reinforced by ethanol (0.1ml 8% w/v po, versus food) and determine the ability of nicotine (0.02-0.4mg/kg), varenicline (0.1-3.0mg/kg), and ethanol (250 and 500mg/kg) to control responding for ethanol. We compare these results to those from rats where nicotine signaled food was available (and ethanol was not). Nicotine came to function as a discriminative stimulus. Nicotine and varenicline produced dose-dependent increases in responding on the nicotine-appropriate lever while ethanol produced responding on the vehicle-appropriate lever. Whether this responding occurred on the lever that produced ethanol or food access depended on the training condition. These results demonstrate that a drug can come to set the occasion for use of another and suggest that this behavioral mechanism could play an important role in the maintenance of and recovery from polysubstance abuse, depending on the pattern of use.

Effects of Nicotine on Alcohol Drinking in Female Mice Selectively Bred for High or Low Alcohol Preference

BACKGROUND

Studies show that repeated nicotine use associates with high alcohol consumption in humans and that nicotine exposure sometimes increases alcohol consumption in animal models. However, the relative roles of genetic predisposition to high alcohol consumption, the alcohol drinking patterns, and the timing of nicotine exposure both with respect to alcohol drinking and developmental stage remain unclear. The studies here manipulated all these variables, using mice selectively bred for differences in free-choice (FC) alcohol consumption to elucidate the role of genetics and nicotine exposure in alcohol consumption behaviors.

METHODS

In Experiments 1 and 2, we assessed the effects of repeated nicotine (0, 0.5, or 1.5 mg/kg) injections immediately before binge-like (drinking-in-the-dark; Experiment 1) or during FC alcohol access (Experiment 2) on these alcohol drinking behaviors (immediately after injections and during re-exposure to alcohol access 14 days later) in adult high- (HAP2) and low-alcohol-preferring (LAP2) female mice (co-exposure model). In Experiments 3 and 4, we assessed the effects of repeated nicotine (0, 0.5, or 1.5 mg/kg) injections 14 days prior to binge-like and FC alcohol access on these alcohol drinking behaviors in adolescent HAP2 and LAP2 female mice (Experiment 3) or adult HAP2 female mice (Experiment 4).

RESULTS

In Experiment 1, we found that repeated nicotine (0.5 and 1.5 mg/kg) and alcohol co-exposure significantly increased binge-like drinking behavior in HAP2 but not LAP2 mice during the re-exposure phase after a 14-day abstinence period. In Experiment 2, 1.5 mg/kg nicotine injections significantly reduced FC alcohol intake and preference in the third hour postinjection in HAP2 but not LAP2 mice. No significant effects of nicotine treatment on binge-like or FC alcohol drinking were observed in Experiments 3 and 4.

CONCLUSIONS

These results show that the temporal parameters of nicotine and alcohol exposure, pattern of alcohol access, and genetic predisposition for alcohol preference influence nicotine's effects on alcohol consumption. These findings in selectively bred mice suggest that humans with a genetic history of alcohol use disorders may be more vulnerable to develop nicotine and alcohol co-use disorders.

Re-exposure to nicotine-associated context from adolescence enhances alcohol intake in adulthood

Alcohol and nicotine are the two most commonly-abused substances and are often used together. Nicotine enhances alcohol-drinking behaviors in humans and in animals, and was suggested to enhance the reinforcing properties of other reinforcers. Here, we show that nicotine-associated environment, rather than nicotine itself, enhances alcohol intake in rats. Adolescent rats received repeated intermittent injections of nicotine (0.4 mg/kg, i.p., 5 injections, every 3^rd day) or saline. The injection was paired with their home cage, or with the subsequent alcohol self-administration context. Rats were then trained to self-administer 20% alcohol. Nicotine given in the home cage did not alter subsequent alcohol intake. However, pairing nicotine with the operant chamber during adolescence led to a long-lasting increased alcohol self-administration in adulthood, compared to nicotine pre-treatment in other contexts. This effect persisted 3 months after nicotine cessation, in a relapse test after abstinence. Furthermore, re-exposure to the nicotine-associated context in adult rats led to a decrease in glial cell line-derived neurotrophic factor (Gdnf) mRNA expression in the ventral tegmental area, an effect that leads to increased alcohol consumption, as we have previously reported. Our findings suggest that retrieval of nicotine-associated contextual memories from adolescence may gate alcohol intake in adulthood, with a possible involvement of GDNF.

Alcohol and nicotine interactions: pre-clinical models of dependence

While the co-morbidity of alcohol (ethanol) and tobacco (nicotine) dependence is well described, the processes that underpin this strong connection are still under debate. With the increasing popularity of electronic cigarettes (e-cigarettes), it is now becoming more important to look to the neurobiological mechanisms involving alcohol and nicotine interactions to effectively treat a new generation of co-dependent individuals. Researchers have already recognized that the neuropathology produced by the combination of nicotine and ethanol is likely to produce an addictive nature very different to that of either one alone, and are employing a mixture of pre-clinical techniques to establish and investigate every stage in the development of both nicotine and ethanol-seeking behaviors. While it is agreed that multiple pathways orchestrate the complex reward profile of alcohol and nicotine co-addiction, several lines of evidence suggest the convergent site of action is within the mesolimbic dopaminergic system, at neuronal nicotinic acetylcholine receptors (nAChRs). A whole host of strategies are currently being employed to discover and unravel previously unknown or ill understood neurobiological processes in the brain, contributing greatly toward the development of novel pharmacotherapies with the aim of improving patient outcomes. This review intends to shed some light on the most influential and most recent pre-clinical work that is leading the charge in modeling this complicated relationship.

Different Hypothalamic Nicotinic α7 Receptor Expression and Response to Low Nicotine Dose in Alcohol-Preferring and Alcohol-Avoiding Rats

BACKGROUND

The aim of this study was to examine possible differences in nicotinic acetylcholine receptors and responses in rats with genetic preference or avoidance for alcohol. This was done by using 2 rat lines with high alcohol preference (Alko Alcohol [AA]) or alcohol avoidance (Alko Non-Alcohol [ANA]).

METHODS

Locomotor activity was measured following nicotine and histamine H3 receptor (H3R) antagonist treatment. In situ hybridization and receptor ligand binding experiments were used in drug-naïve animals to examine the expression of different α nicotinic receptor subunits.

RESULTS

The AA rats were found to be more sensitive to the stimulatory effect of a low dose of nicotine than ANA rats, which were not significantly activated. Combination of histamine H3R antagonist, JNJ-39220675, and nicotine resulted to similar locomotor activation as nicotine alone. To further understand the mechanism underlying the difference in nicotine response in AA and ANA rats, we studied the expression of α5, α6, and α7 nicotinic receptor subunits in specific brain areas of AA and ANA rats. We found no differences in the expression of α5 nicotinic receptor subunits in the medial habenula and hippocampus or in α6 subunit in the ventral tegmental area and substantia nigra. However, the level of α7 nicotinic receptor subunit mRNA was significantly lower in the tuberomamillary nucleus of posterior hypothalamus of alcohol-preferring AA rats than in alcohol-avoiding ANA rats. Also the hypothalamic [125I-α-bungarotoxin binding was lower in AA rats indicating lower levels of α7 nicotinic receptors.

CONCLUSIONS

The lower expression and receptor binding of α7 nicotinic receptors in the tuberomamillary nucleus of AA rats suggest a difference in the regulation of brain histamine neurons between the rat lines since the α7 nicotinic receptors are located in histaminergic neurons. Stronger nicotine-induced locomotor response, mediated partially via α7 receptors, and previously described high alcohol consumption in AA rats could be explained by the found difference in tuberomamillary α7 receptor levels.

Cigarettes and alcohol: The influence of nicotine on operant alcohol self-administration and the mesolimbic dopamine system

Studies in human populations consistently demonstrate an interaction between nicotine and ethanol use, each drug influencing the use of the other. Here we present data and review evidence from animal studies that nicotine influences operant self-administration of ethanol. The operant reinforcement paradigm has proven to be a behaviorally relevant and quantitative model for studying ethanol-seeking behavior. Exposure to nicotine can modify the reinforcing properties of ethanol during different phases of ethanol self-administration, including acquisition, maintenance, and reinstatement. Our data suggest that non-daily intermittent nicotine exposure can trigger a long-lasting increase in ethanol self-administration. The biological basis for interactions between nicotine and ethanol is not well understood but may involve the stress hormone systems and adaptations in the mesolimbic dopamine system. Future studies that combine operant self-administration with techniques for monitoring or manipulating in vivo neurophysiology may provide new insights into the neuronal mechanisms that link nicotine and alcohol use.

The predicted impact of reducing the nicotine content in cigarettes on alcohol use

INTRODUCTION

Product standards reducing the level of nicotine in cigarettes could significantly improve public health by reducing smoking behavior and toxicant exposure. However, relatively little is known about how the regulatory strategy could impact alcohol use, a closely related health behavior that is also a major contributor to morbidity and mortality. The primary objective of this paper is to predict the effect of nicotine reduction on alcohol use, identify priorities for future research, and highlight areas for mitigating any adverse outcomes.

METHODS

We critically reviewed and integrated literatures examining the effects of very low nicotine content (VLNC) cigarettes on smoking-related outcomes (nicotine exposure, nicotine withdrawal, and smoking as a cue to drink) and, in turn, the effects of those outcomes on alcohol use.

RESULTS

Current evidence suggests reducing the nicotine content of cigarettes may benefit public health by reducing alcohol use and problematic drinking over time as a consequence of reduced exposure to nicotine and the smoking cues associated with drinking. Nicotine withdrawal could increase risk of drinking, although these effects should be short-lived and could be mitigated by other sources of nicotine. Gender, hazardous drinking, and psychiatric comorbidities are likely to be important moderators of the effects of VLNC cigarettes.

CONCLUSIONS

It is imperative to broadly assess the public health impact of potential tobacco product regulations by including measures of closely related health behaviors that could be impacted by these interventions. Nicotine reduction in cigarettes may contribute to improved public health through reductions in alcohol use.

Potential substrates for nicotine and alcohol interactions: a focus on the mesocorticolimbic dopamine system

Epidemiological studies consistently find correlations between nicotine and alcohol use, yet the neural mechanisms underlying their interaction remain largely unknown. Nicotine and alcohol (i.e., ethanol) share many common molecular and cellular targets that provide potential substrates for nicotine-alcohol interactions. These targets for interaction often converge upon the mesocorticolimbic dopamine system, where the link to drug self-administration and reinforcement is well documented. Both nicotine and alcohol activate the mesocorticolimbic dopamine system, producing downstream dopamine signals that promote the drug reinforcement process. While nicotine primarily acts via nicotinic acetylcholine receptors, alcohol acts upon a wider range of receptors and molecular substrates. The complex pharmacological profile of these two drugs generates overlapping responses that ultimately intersect within the mesocorticolimbic dopamine system to promote drug use. Here we will examine overlapping targets between nicotine and alcohol and provide evidence for their interaction. Based on the existing literature, we will also propose some potential targets that have yet to be directly tested. Mechanistic studies that examine nicotine-alcohol interactions would ultimately improve our understanding of the factors that contribute to the associations between nicotine and alcohol use.

Development of the motivational system during adolescence, and its sensitivity to disruption by nicotine

The brain continues to develop during adolescence, and exposure to exogenous substances such as nicotine can exert long-lasting adaptations during this vulnerable period. In order to fully understand how nicotine affects the adolescent brain it is important to understand normal adolescent brain development. This review summarizes human and animal data on brain development, with emphasis on the prefrontal cortex, for its important function in executive control over behavior. Moreover, we discuss how nicotine exposure during adolescence can disrupt brain development bearing long-term consequences on executive cognitive function in adulthood.

Exposure to nicotine during periadolescence or early adulthood alters aversive and physiological effects induced by ethanol

The majority of smokers begin their habit during adolescence, which often precedes experimentation with alcohol. Interestingly, very little preclinical work has been done examining how exposure to nicotine during periadolescence impacts the affective properties of alcohol in adulthood. Understanding how periadolescent nicotine exposure influences the aversive effects of alcohol might help to explain why it becomes more acceptable to this preexposed population. Thus, Experiment 1 exposed male Sprague Dawley rats to either saline or nicotine (0.4mg/kg, IP) from postnatal days 34 to 43 (periadolescence) and then examined changes in the aversive effects of alcohol (0, 0.56, 1.0 and 1.8g/kg, IP) in adulthood using the conditioned taste aversion (CTA) design. Changes in blood alcohol concentration (BAC) as well as alcohol-induced hypothermia and locomotor suppression were also assessed. To determine if changes seen were specific to nicotine exposure during periadolescence, the procedures were replicated in adults (Experiment 2). Preexposure to nicotine during periadolescence attenuated the acquisition of the alcohol-induced CTAs (at 1.0g/kg) and the hypothermic effects of alcohol (1.0g/kg). Adult nicotine preexposure produced similar attenuation in alcohol's aversive (at 1.8g/kg) and hypothermic (1.8g/kg) effects. Neither adolescent nor adult nicotine preexposure altered BACs or alcohol-induced locomotor suppression. These results suggest that nicotine may alter the aversive and physiological effects of alcohol, regardless of the age at which exposure occurs, possibly increasing its overall reinforcing value and making it more likely to be consumed.

Expression of the gene encoding the ghrelin receptor in rats selected for differential alcohol preference

The mechanisms involved in alcohol use disorder, a chronic relapsing brain disorder, are complex and involve various signalling systems in the brain. Recently, the orexigenic peptide ghrelin was shown to be required for alcohol-induced reward, an effect mediated via ghrelin receptors, GHS-R1A, at the level of the cholinergic-dopaminergic reward link. Moreover, ghrelin increases and GHR-R1A antagonists reduce moderate alcohol consumption in mice, and a single nucleotide polymorphism in the GHS-R1A gene has been associated with high alcohol consumption in humans. Therefore, GHS-R1A gene expression and alcohol intake were investigated in high, AA (Alko, Alcohol), versus low, ANA (Alko, Non-Alcohol), alcohol consuming rats as well as in Wistar rats. In the AA and ANA rats plasma ghrelin levels were also measured. GHS-R1A gene expression was increased in AA compared to ANA rats in nucleus accumbens, ventral tegmental area, amygdala, prefrontal cortex and hippocampus. A similar trend was observed in the ventral tegmental area of Wistar rats consuming high amounts of alcohol. Furthermore, the AA rats had significantly smaller reduction of plasma ghrelin levels over time, after several weeks of alcohol exposure, than had the ANA rats. The present study provides further evidence for that the ghrelin signalling system, in particular at the level of the mesocortocolimbic dopamine system, is involved in alcohol consumption, and thus possibly contributes to alcohol use disorder. Therefore the GHS-R1A may constitute a novel candidate for development of new treatment strategies for alcohol dependence.