Effects of the CRF receptor antagonist D-Phe CRF(12-41) and the alpha2-adrenergic receptor agonist clonidine on stress-induced reinstatement of nicotine-seeking behavior in rats

Mifepristone decreases nicotine intake in dependent and non-dependent adult rats

BACKGROUND

Addiction to tobacco and nicotine products has adverse health effects and afflicts more than a billion people worldwide. Therefore, there is an urgent need for new treatments to reduce tobacco and nicotine use. Glucocorticoid receptor blockade shows promise as a novel treatment for drug abuse and stress-related disorders.

AIM

These studies aim to investigate whether glucocorticoid receptor blockade with mifepristone diminishes the reinforcing properties of nicotine in rats with intermittent or daily long access to nicotine.

METHODS

The rats self-administered 0.06 mg/kg/inf of nicotine for 6 h per day, with either intermittent or daily access for 4 weeks before treatment with mifepristone. Daily nicotine self-administration models regular smoking, while intermittent nicotine self-administration models occasional smoking. To determine whether the rats were dependent, they were treated with the nicotinic acetylcholine receptor antagonist mecamylamine, and somatic signs were recorded.

RESULTS

The rats with intermittent access to nicotine had a higher level of nicotine intake per session than those with daily access but only the rats with daily access to nicotine showed signs of physical dependence. Furthermore, mecamylamine increased nicotine intake during the first hour of access in rats with daily access but not in those with intermittent access. Mifepristone decreased total nicotine intake in rats with intermittent and daily access to nicotine. Moreover, mifepristone decreased the distance traveled and rearing in the open field test and operant responding for food pellets.

CONCLUSION

These findings indicate that mifepristone decreases nicotine intake but this effect may be partially attributed to the sedative effects of mifepristone.

Neurobiology of Stress-Induced Nicotine Relapse

Tobacco smoking is the leading cause of preventable death and disease. Although there are some FAD-approved medicines for controlling smoking, the relapse rate remains very high. Among the factors that could induce nicotine relapse, stress might be the most important one. In the last decades, preclinical studies have generated many new findings that lead to a better understanding of stress-induced relapse of nicotine-seeking. Several molecules such as α3β4 nicotinic acetylcholine receptor, α2-adrenergic receptors, cannabinoid receptor 1, trace amine-associated receptor 1, and neuropeptide systems (corticotropin-releasing factor and its receptors, dynorphine and kappa opioid receptor) have been linked to stress-induced nicotine relapse. In this review, we discuss recent advances in the neurobiology, treatment targets, and potential therapeutics of stress-induced nicotine relapse. We also discuss some factors that may influence stress-induced nicotine relapse and that should be considered in future studies. In the final section, a perspective on some research directions is provided. Further investigation on the neurobiology of stress-induced nicotine relapse will shed light on the development of new medicines for controlling smoking and will help us understand the interactions between the stress and reward systems in the brain.

Development of Dependence in Smokers and Rodents With Voluntary Nicotine Intake: Similarities and Differences

INTRODUCTION

Smoking and vaping throughout adolescence and early adulthood lead to nicotine dependence. Nicotine withdrawal is associated with somatic and affective withdrawal symptoms that contribute to smoking and relapse. Affective nicotine withdrawal symptoms in humans include craving for cigarettes, depression, anxiety, trouble sleeping, and cognitive deficits.

METHODS

Herein, we review clinical studies that investigated nicotine dependence in people who smoke or vape. We also discuss studies that investigated the development of dependence in animals with oral nicotine intake, nicotine aerosol self-administration, and intravenous nicotine self-administration.

RESULTS

Clinical studies report that adolescents who smoke daily develop nicotine dependence before those who smoke infrequently, but ultimately all smokers become dependent in adulthood. Preclinical studies indicate that rats that self-administer nicotine also become dependent. Rats that self-administer nicotine display somatic withdrawal signs and affective withdrawal signs, including increased anxiety and depressive-like behavior, cognitive deficits, and allodynia. Most nicotine withdrawal signs were observed in rodents with daily (7 days/week) or intermittent long access (23-hour) to nicotine. Clinical smoking studies report symptoms of nicotine dependence in adolescents of both sexes, but virtually all preclinical nicotine self-administration studies have been done with adult male rats.

CONCLUSIONS

The role of sex and age in the development of dependence in nicotine self-administration studies remains under-investigated. However, the role of sex and age in nicotine withdrawal has been thoroughly evaluated in studies in which nicotine was administered noncontingently. We discuss the need for volitional nicotine self-administration studies that explore the gradual development of dependence during adolescence and adulthood in rodents of both sexes.

IMPLICATIONS

The reviewed clinical studies investigated the development of nicotine dependence in male and female adolescent and young adult smokers and vapers. These studies indicate that most adolescent smokers and vapers gradually become nicotine dependent. Preclinical studies with rodents show that nicotine intake in widely used self-administration models also leads to dependence. However, almost all animal studies that investigated the development of nicotine dependence have been conducted with adult male rats. To better model smoking and vaping, it is important that nicotine intake in rats or mice starts during adolescence and that both sexes are included.

Corticotropin releasing factor and drug seeking in substance use disorders: Preclinical evidence and translational limitations

The neuropeptide, corticotropin releasing factor (CRF), has been an enigmatic target for the development of medications aimed at treating stress-related disorders. Despite a large body of evidence from preclinical studies in rodents demonstrating that CRF receptor antagonists prevent stressor-induced drug seeking, medications targeting the CRF-R1 have failed in clinical trials. Here, we provide an overview of the abundant findings from preclinical rodent studies suggesting that CRF signaling is involved in stressor-induced relapse. The scientific literature that has defined the receptors, mechanisms and neurocircuits through which CRF contributes to stressor-induced reinstatement of drug seeking following self-administration and conditioned place preference in rodents is reviewed. Evidence that CRF signaling is recruited with repeated drug use in a manner that heightens susceptibility to stressor-induced drug seeking in rodents is presented. Factors that may determine the influence of CRF signaling in substance use disorders, including developmental windows, biological sex, and genetics are examined. Finally, we discuss the translational failure of medications targeting CRF signaling as interventions for substance use disorders and other stress-related conditions. We conclude that new perspectives and research directions are needed to unravel the mysterious role of CRF in substance use disorders.

Actions of feeding-related peptides on the mesolimbic dopamine system in regulation of natural and drug rewards

The mesolimbic dopamine system is the primary neural circuit mediating motivation, reinforcement, and reward-related behavior. The activity of this system and multiple behaviors controlled by it are affected by changes in feeding and body weight, such as fasting, food restriction, or the development of obesity. Multiple different peptides and hormones that have been implicated in the control of feeding and body weight interact with the mesolimbic dopamine system to regulate many different dopamine-dependent, reward-related behaviors. In this review, we summarize the effects of a selected set of feeding-related peptides and hormones acting within the ventral tegmental area and nucleus accumbens to alter feeding, as well as food, drug, and social reward.

Substance abuse and neurotransmission

The number of people who suffer from a substance abuse disorder has continued to rise over the last decade; particularly, the number of drug-related overdose deaths has sharply increased during the COVID-19 pandemic. Converging lines of clinical observations, supported by imaging and neuropsychological performance testing, have demonstrated that substance abuse-induced dysregulation of neurotransmissions in the brain is critical for development and expression of the addictive properties of abused substances. Recent scientific advances have allowed for better understanding of the neurobiological processes that mediates drugs of abuse and addiction. This chapter presents the past classic concepts and the recent advances in our knowledge about how cocaine, amphetamines, opioids, alcohol, and nicotine alter multiple neurotransmitter systems, which contribute to the behaviors associated with each drug. Additionally, we discuss the interactive effects of HIV-1 or COVID-19 and substance abuse on neurotransmission and neurobiological pathways. Finally, we introduce therapeutic strategies for development of pharmacotherapies for substance abuse disorders.

The impact of adolescent stress on nicotine use and affective disorders in rodent models

Recent findings indicate that stress exposure during adolescence contributes to the development of both nicotine use and affective disorders, suggesting a potential shared biological pathway. One key system that may mediate the association between adolescent stress and nicotine or affective outcomes is the hypothalamic-pituitary-adrenal (HPA) axis. Here we reviewed evidence regarding the effects of adolescent stress on nicotine responses and affective phenotypes and the role of the HPA-axis in these relationships. Literature indicates that stress, possibly via HPA-axis dysfunction, is a risk factor for both nicotine use and affective disorders. In rodent models, adolescent stress modulates behavioural responses to nicotine and increases the likelihood of affective disorders. The exact role that the HPA-axis plays in altering nicotine sensitivity and affective disorder development after adolescent stress remains unclear. However, it appears likely that adolescent stress-induced nicotine use and affective disorders are precipitated by repetitive activation of a hyperactive HPA-axis. Together, these preclinical studies indicate that adolescent stress is a risk factor for nicotine use and anxiety/depression phenotypes. The findings summarized here suggest that the HPA-axis mediates this relationship. Future studies that pharmacologically manipulate the HPA-axis during and after adolescent stress are critical to elucidate the exact role that the HPA-axis plays in the development of nicotine use and affective disorders following adolescent stress.

Neurochemical mechanisms and neurocircuitry underlying the contribution of stress to cocaine seeking

In individuals with substance use disorders, stress is a critical determinant of relapse susceptibility. In some cases, stressors directly trigger cocaine use. In others, stressors interact with other stimuli to promote drug seeking, thereby setting the stage for relapse. Here, we review the mechanisms and neurocircuitry that mediate stress-triggered and stress-potentiated cocaine seeking. Stressors trigger cocaine seeking by activating noradrenergic projections originating in the lateral tegmentum that innervate the bed nucleus of the stria terminalis to produce beta adrenergic receptor-dependent regulation of neurons that release corticotropin releasing factor (CRF) into the ventral tegmental area (VTA). CRF promotes the activation of VTA dopamine neurons that innervate the prelimbic prefrontal cortex resulting in D1 receptor-dependent excitation of a pathway to the nucleus accumbens core that mediates cocaine seeking. The stage-setting effects of stress require glucocorticoids, which exert rapid non-canonical effects at several sites within the mesocorticolimbic system. In the nucleus accumbens, corticosterone attenuates dopamine clearance via the organic cation transporter 3 to promote dopamine signaling. In the prelimbic cortex, corticosterone mobilizes the endocannabinoid, 2-arachidonoylglycerol (2-AG), which produces CB1 receptor-dependent reductions in inhibitory transmission, thereby increasing excitability of neurons which comprise output pathways responsible for cocaine seeking. Factors that influence the role of stress in cocaine seeking, including prior history of drug use, biological sex, chronic stress/co-morbid stress-related disorders, adolescence, social variables, and genetics are discussed. Better understanding when and how stress contributes to drug seeking should guide the development of more effective interventions, particularly for those whose drug use is stress related.

Drug addiction co-morbidity with alcohol: Neurobiological insights

Addiction is a chronic disorder that consists of a three-stage cycle of binge/intoxication, withdrawal/negative affect, and preoccupation/anticipation. These stages involve, respectively, neuroadaptations in brain circuits involved in incentive salience and habit formation, stress surfeit and reward deficit, and executive function. Much research on addiction focuses on the neurobiology underlying single drug use. However, alcohol use disorder (AUD) can be co-morbid with substance use disorder (SUD), called dual dependence. The limited epidemiological data on dual dependence indicates that there is a large population of individuals suffering from addiction who are dependent on more than one drug and/or alcohol, yet dual dependence remains understudied in addiction research. Here, we review neurobiological data on neurotransmitter and neuropeptide systems that are known to contribute to addiction pathology and how the involvement of these systems is consistent or divergent across drug classes. In particular, we highlight the dopamine, opioid, corticotropin-releasing factor, norepinephrine, hypocretin/orexin, glucocorticoid, neuroimmune signaling, endocannabinoid, glutamate, and GABA systems. We also discuss the limited research on these systems in dual dependence. Collectively, these studies demonstrate that the use of multiple drugs can produce neuroadaptations that are distinct from single drug use. Further investigation into the neurobiology of dual dependence is necessary to develop effective treatments for addiction to multiple drugs.

The reinforcement threshold and elasticity of demand for nicotine in an adolescent rat model of depression

BACKGROUND

The Food and Drug Administration (FDA) is considering setting a nicotine standard for tobacco products to reduce their addictiveness. Such a standard should account for the apparent greater vulnerability to nicotine addiction in some subpopulations, such as adolescents with depression. The present study examined whether the reinforcement threshold and elasticity of demand (i.e., reinforcing efficacy) for nicotine in a genetic inbred rat model of depression (Flinders Sensitive Line [FSL]) differs from an outbred control strain.

METHODS

Acquisition of nicotine self-administration (NSA) across a wide range of nicotine doses was measured in both FSL and Sprague-Dawley (SD) control adolescent rats. At the highest dose, elasticity of demand was also measured. Nicotine pharmacokinetics was examined to determine whether it might modulate NSA, as it does smoking in humans.

RESULTS

FSL rats acquired self-administration quicker and showed more inelastic demand (greater reinforcing efficacy) than SDs at the highest unit dose. However, there was no strain difference in the reinforcement threshold of nicotine. FSL rats exhibited faster nicotine clearance, larger volume of distribution, and lower plasma and brain nicotine concentrations. However, these differences were not consistently related to strain differences in NSA measures.

CONCLUSION

These findings are consistent with studies showing greater dependence and reinforcing efficacy of cigarettes in smokers with depression and those with relatively fast nicotine metabolism. However, these findings also suggest that a nicotine standard to reduce initiation of tobacco use should be similarly effective in both the general adolescent population and those with depression.

Evaluation of Sex Differences in the Elasticity of Demand for Nicotine and Food in Rats

INTRODUCTION

Animal studies can inform policy regarding nicotine levels in tobacco products and e-cigarette solutions. Increasing the price of nicotine-containing products decreases their use, but it is unknown how the relationship between price and consumption is affected by both sex and nicotine dose.

METHODS

A behavioral economics procedure was used to determine the demand elasticity for nicotine in male and female rats. Demand elasticity describes the relationship between price and consumption. A high level of elasticity indicates that consumption is relatively sensitive to increases in price. The rats self-administered a low dose (0.01 mg/kg/inf) or a standard dose (0.03 mg/kg/inf) of nicotine for 9 days under a fixed-ratio (FR) 1 schedule. Then the price (FR schedule) of nicotine was increased, and a demand analysis was conducted. A similar study was conducted with palatable food pellets.

RESULTS

There were no sex differences in nicotine or food intake under the FR1 schedule. However, demand for 0.03 mg/kg/inf of nicotine was more elastic in females than males. Demand for 0.01 mg/kg/inf of nicotine and food was more elastic in males than females.

CONCLUSIONS

These findings indicate that there are no differences in nicotine and food intake between males and females when the price is low. When the price of nicotine or food is increased, males maintain their old level of intake longer than females when they have access to a standard dose of nicotine, and females maintain their intake longer when they have access to a low dose of nicotine or food.

IMPLICATIONS

This behavioral economics analysis indicates that there is no sex difference in nicotine intake when the price of nicotine is low. Increasing the price of nicotine decreases nicotine intake in a dose- and sex-specific manner. Males maintain their old level of intake longer when they have access to a standard dose of nicotine and females when they have access to a low dose. This has implications for tobacco regulatory policy. In a regulatory environment where only low nicotine-containing products are allowed, increasing the price of nicotine products may lead to a greater decrease in nicotine use in males than females.

More than Smoke and Patches: The Quest for Pharmacotherapies to Treat Tobacco Use Disorder

Tobacco use is a persistent public health issue. It kills up to half its users and is the cause of nearly 90% of all lung cancers. The main psychoactive component of tobacco is nicotine, primarily responsible for its abuse-related effects. Accordingly, most pharmacotherapies for smoking cessation target nicotinic acetylcholine receptors (nAChRs), nicotine's major site of action in the brain. The goal of the current review is twofold: first, to provide a brief overview of the most commonly used behavioral procedures for evaluating smoking cessation pharmacotherapies and an introduction to pharmacokinetic and pharmacodynamic properties of nicotine important for consideration in the development of new pharmacotherapies; and second, to discuss current and potential future pharmacological interventions aimed at decreasing tobacco use. Attention will focus on the potential for allosteric modulators of nAChRs to offer an improvement over currently approved pharmacotherapies. Additionally, given increasing public concern for the potential health consequences of using electronic nicotine delivery systems, which allow users to inhale aerosolized solutions as an alternative to smoking tobacco, an effort will be made throughout this review to address the implications of this relatively new form of nicotine delivery, specifically as it relates to smoking cessation. SIGNIFICANCE STATEMENT: Despite decades of research that have vastly improved our understanding of nicotine and its effects on the body, only a handful of pharmacotherapies have been successfully developed for use in smoking cessation. Thus, investigation of alternative pharmacological strategies for treating tobacco use disorder remains active; allosteric modulators of nicotinic acetylcholine receptors represent one class of compounds currently under development for this purpose.

Adolescent nicotine and tobacco smoke exposure enhances nicotine self-administration in female rats

Most people start experimenting with tobacco products or e-cigarettes in early adolescence and become habitual smokers in late adolescence or adulthood. These studies investigated if exposure to tobacco smoke or nicotine during early and mid-adolescence affects nicotine intake in late adolescence and early adulthood. Male and female rats were exposed to tobacco smoke from low- and high-nicotine SPECTRUM cigarettes or nicotine (0.3 mg/kg, twice a day) from postnatal day (P) 24-42. The self-administration sessions started at P55. The rats self-administered nicotine for 14-15 days under a fixed-ratio 1 schedule, and on the first day, the maximum number of infusions was twenty. Exposure to smoke from high, but not low, nicotine cigarettes during adolescence increased nicotine self-administration in female but not male rats. Adolescent treatment with nicotine facilitated nicotine self-administration. On the first day of nicotine self-administration, nicotine-treated rats reached the maximum number of infusions before the saline-treated control rats. Nicotine intake was also higher in the nicotine-treated female rats than in the saline-treated females. There was no sex difference in nicotine intake in controls when the data from the studies were combined. Smoke exposure led to a dose-dependent increase in plasma nicotine and cotinine levels in adolescent rats. Exposure to smoke from high-nicotine cigarettes and 0.3 mg/kg of nicotine led to plasma nicotine and cotinine levels that are similar to those in tobacco users. These findings indicate that in females, but not males, exposure to nicotine during adolescence may facilitate smoking and e-cigarette use later in life.

Role of corticotropin-releasing factor in alcohol and nicotine addiction

The two most prevalent substance use disorders involve alcohol and nicotine, which are often co-abused. Robust preclinical and translational evidence indicates that individuals initiate drug use for the acute rewarding effects of the substance. The development of negative emotional states is key for the transition from recreational use to substance use disorders as subjects seek the substance to obtain relief from the negative emotional states of acute withdrawal and protracted abstinence. The neuropeptide corticotropin-releasing factor (CRF) is a major regulator of the brain stress system and key in the development of negative affective states. The present review examines the role of CRF in preclinical models of alcohol and nicotine abuse and explores links between CRF and anxiety-like, dysphoria-like, and other negative affective states. Finally, the present review discusses preclinical models of nicotine and alcohol use with regard to the CRF system, advances in molecular and genetic manipulations of CRF, and the importance of examining both males and females in this field of research.

Variability in nicotine conditioned place preference and stress-induced reinstatement in mice: Effects of sex, initial chamber preference, and guanfacine

Relapse to smoking occurs at higher rates in women compared with men, especially when triggered by stress. Studies suggest that sex-specific interactions between nicotine reward and stress contribute to these sex differences. Accordingly, novel treatment options targeting stress pathways, such as guanfacine, an α2-adrenergic receptor agonist, may provide sex-sensitive therapeutic effects. Preclinical studies are critical for elucidating neurobiological mechanisms of stress-induced relapse and potential therapies, but rodent models of nicotine addiction are often hindered by large behavioral variability. In this study, we used nicotine conditioned place preference to investigate stress-induced reinstatement of nicotine preference in male and female mice, and the effects of guanfacine on this behavior. Our results showed that overall, nicotine induced significant place preference acquisition and swim stress-induced reinstatement in both male and female mice, but with different nicotine dose-response patterns. In addition, we explored the variability in nicotine-dependent behaviors with median split analyses and found that initial chamber preference in each sex differentially accounted for variability in stress-induced reinstatement. In groups that showed significant stress-induced reinstatement, pretreatment with guanfacine attenuated this behavior. Finally, we evaluated neuronal activation by Arc immunoreactivity in the infralimbic cortex, prelimbic cortex, anterior insula, basolateral amygdala, lateral central amygdala and nucleus accumbens core and shell. Guanfacine induced sex-dependent changes in Arc immunoreactivity in the infralimbic cortex and anterior insula. This study demonstrates sex-dependent relationships between initial chamber preference and stress-induced reinstatement of nicotine conditioned place preference, and the effects of guanfacine on both behavior and neurobiological mechanisms.

Overexpression of corticotropin-releasing factor in the nucleus accumbens enhances the reinforcing effects of nicotine in intact female versus male and ovariectomized female rats

This study assessed the role of stress systems in the nucleus accumbens (NAc) in promoting sex differences in the reinforcing effects of nicotine. Intravenous self-administration (IVSA) of various doses of nicotine was compared following overexpression of corticotropin-releasing factor (CRF) in the NAc of female and male rats. Ovariectomized (OVX) females were also included to assess the role of ovarian hormones in promoting nicotine reinforcement. Rats received intra-NAc administration of an adeno-associated vector that overexpressed CRF (AAV2/5-CRF) or green fluorescent protein (AAV2/5-GFP). All rats were then given extended access (23 h/day) to an inactive and an active lever that delivered nicotine. Separate groups of rats received intra-NAc AAV2/5-CRF and saline IVSA. Rats were also allowed to nose-poke for food and water during IVSA testing. At the end of the study, the NAc was dissected and rt-qPCR methods were used to estimate CRF overexpression and changes in CRF receptors (CRFr1, CRFr2) and the CRF receptor internalizing protein, β-arrestin2 (Arrb2). Overexpression of CRF in the NAc increased nicotine IVSA to a larger extent in intact female versus male and OVX females. Food intake was increased to a larger extent in intact and OVX females as compared to males. The increase in CRF gene expression was similar across all groups; however, in females, overexpression of CRF resulted in a larger increase in CRFr1 and CRFr2 relative to males. In males, overexpression of CRF produced a larger increase in Arrb2 than females, suggesting greater CRF receptor internalization. Our results suggest that stress systems in the NAc promote the reinforcing effectiveness of nicotine in female rats in an ovarian hormone-dependent manner.

Neurobiological and Neurophysiological Mechanisms Underlying Nicotine Seeking and Smoking Relapse

Tobacco-related morbidity and mortality continue to be a significant public health concern. Unfortunately, current FDA-approved smoking cessation pharmacotherapies have limited efficacy and are associated with high rates of relapse. Therefore, a better understanding of the neurobiological and neurophysiological mechanisms that promote smoking relapse is needed to develop novel smoking cessation medications. Here, we review preclinical studies focused on identifying the neurotransmitter and neuromodulator systems that mediate nicotine relapse, often modeled in laboratory animals using the reinstatement paradigm, as well as the plasticity-dependent neurophysiological mechanisms that facilitate nicotine reinstatement. Particular emphasis is placed on how these neuroadaptations relate to smoking relapse in humans. We also highlight a number of important gaps in our understanding of the neural mechanisms underlying nicotine reinstatement and critical future directions, which may lead toward the development of novel, target pharmacotherapies for smoking cessation.

Anti-stress neuropharmacological mechanisms and targets for addiction treatment: A translational framework

Stress-related substance use is a major challenge for treating substance use disorders. This selective review focuses on emerging pharmacotherapies with potential for reducing stress-potentiated seeking and consumption of nicotine, alcohol, marijuana, cocaine, and opioids (i.e., key phenotypes for the most commonly abused substances). I evaluate neuropharmacological mechanisms in experimental models of drug-maintenance and relapse, which translate more readily to individuals presenting for treatment (who have initiated and progressed). An affective/motivational systems model (three dimensions: valence, arousal, control) is mapped onto a systems biology of addiction approach for addressing this problem. Based on quality of evidence to date, promising first-tier neurochemical receptor targets include: noradrenergic (α1 and β antagonist, α2 agonist), kappa-opioid antagonist, nociceptin antagonist, orexin-1 antagonist, and endocannabinoid modulation (e.g., cannabidiol, FAAH inhibition); second-tier candidates may include corticotropin releasing factor-1 antagonists, serotonergic agents (e.g., 5-HT reuptake inhibitors, 5-HT3 antagonists), glutamatergic agents (e.g., mGluR2/3 agonist/positive allosteric modulator, mGluR5 antagonist/negative allosteric modulator), GABA-promoters (e.g., pregabalin, tiagabine), vasopressin 1b antagonist, NK-1 antagonist, and PPAR-γ agonist (e.g., pioglitazone). To address affective/motivational mechanisms of stress-related substance use, it may be advisable to combine agents with actions at complementary targets for greater efficacy but systematic studies are lacking except for interactions with the noradrenergic system. I note clinically-relevant factors that could mediate/moderate the efficacy of anti-stress therapeutics and identify research gaps that should be pursued. Finally, progress in developing anti-stress medications will depend on use of reliable CNS biomarkers to validate exposure-response relationships.

Synaptic Plasticity in the Bed Nucleus of the Stria Terminalis: Underlying Mechanisms and Potential Ramifications for Reinstatement of Drug- and Alcohol-Seeking Behaviors

The bed nucleus of the stria terminalis (BNST) is a component of the extended amygdala that shows significant changes in activity and plasticity through chronic exposure to drugs and stress. The region is critical for stress- and cue-induced reinstatement of drug-seeking behaviors and is thus a candidate region for the plastic changes that occur in abstinence that prime addicted patients for reinstatement behaviors. Here, we discuss the various forms of long-term potentiation (LTP) and long-term depression (LTD) in the rodent BNST and highlight the way that these changes in excitatory transmission interact with exposure to alcohol and other drugs of abuse, as well as other stressors. In addition, we highlight potential areas for future research in this area, including investigating input- and cell-specific bidirectional changes in activity. As we continue to accrue foundational knowledge in the mechanisms and effects of plasticity in the BNST, molecular targets and treatment strategies that are relevant to reinstatement behaviors will also begin to emerge. Here, we briefly discuss the effects of catecholamine receptor modulators on synaptic plasticity in the BNST due to the role of norepinephrine in LTD and dopamine on the short-term component of LTP as well as the role that signaling at these receptors plays in reinstatement of drug- and alcohol-seeking behaviors. We hope that insights gained on the specific changes in plasticity that occur within the BNST during abstinence from alcohol and other drugs of abuse will provide insight into the biological underpinnings of relapse behavior in human addicts and inform future treatment modalities for addiction that tackle this complex biological problem.

Basolateral amygdala, nicotinic cholinergic receptors, and nicotine: Pharmacological effects and addiction in animal models and humans

The amygdala is involved in processing incoming information about rewarding stimuli and emotions that denote danger such as anxiety and fear. Bi-directional neural connections between basolateral amygdala (BLA) and brain regions such as nucleus accumbens, prefrontal cortex, hippocampus, and hindbrain regions regulate motivation, cognition, and responses to stress. Altered local regulation of BLA excitability is pivotal to the behavioral disturbances characteristic of posttraumatic stress disorder, and relapse to drug use induced by stress. Herein, we review the physiological regulation of BLA by cholinergic inputs, emphasizing the role of BLA nicotinic receptors. We review BLA-dependent effects of nicotine on cognition, motivated behaviors, and emotional states, including memory, taking and seeking drugs, and anxiety and fear in humans and animal models. The alterations in BLA activity observed in animal studies inform human behavioral and brain imaging research by enabling a more exact understanding of altered BLA function. Converging evidence indicates that cholinergic signaling from basal forebrain projections to local nicotinic receptors is an important physiological regulator of BLA and that nicotine alters BLA function. In essence, BLA is necessary for behavioral responses to stimuli that evoke anxiety and fear; reinstatement of cue-induced drug seeking; responding to second-order cues conditioned to abused drugs; reacquisition of amplified nicotine self-administration due to chronic stress during abstinence; and to promote responding for natural reward.

Self-administration of the synthetic cathinone MDPV enhances reward function via a nicotinic receptor dependent mechanism

Methylenedioxypyrovalerone (MDPV) is an addictive synthetic drug with severe side effects. Previous studies have shown that MDPV has positive reinforcing properties. However, little is known about the effect of MDPV self-administration on the state of the brain reward system and the neuronal mechanisms by which MDPV mediates its effects. The goal of the present studies was to determine the effect of MDPV self-administration on reward function and the role of cholinergic neurotransmission in the reinforcing effects of MDPV. To study the effect of MDPV self-administration on the brain reward system, rats were prepared with intravenous catheters and intracranial self-stimulation electrodes (ICSS). For 10 days, the reward thresholds were assessed immediately before (23 h post prior session) and after 1 h of MDPV self-administration. The reward thresholds were decreased immediately after MDPV self-administration, which is indicative of a potentiation of brain reward function. The reward thresholds 23 h after MDPV intake gradually increased over time, which is indicative of anhedonia. Pretreatment with the nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine decreased the self-administration of MDPV and completely prevented the decrease in reward thresholds. A control study with palatable chocolate pellets showed that responding for a natural reinforcer does not affect the state of the brain reward system. Furthermore, mecamylamine did not affect responding for food pellets. In conclusion, the self-administration of MDPV potentiates reward function and nAChR blockade prevents the reward enhancing effects of MDPV self-administration. Preventing the MDPV-induced increase in cholinergic neurotransmission might be a safe approach to diminish MDPV abuse.

Intersection of stress and gender in association with transitions in past year DSM-5 substance use disorder diagnoses in the United States

Background

Stress contributes to the development and maintenance of substance use disorders (SUD), with some research suggesting that the impact of stress on SUD is greater in women. However, this has yet to be evaluated in a national dataset, across major substances of abuse.

Methods

Using data from the newly available U.S. National Epidemiologic Survey on Alcohol and Related Conditions (NESARC; Wave 3; n=36,309) we evaluated relationships among past year stressful life events (0 or 1 vs. 2+ events, range 0-16) and gender, and their association with transitions (new vs. absent cases; ongoing vs. remitted cases) in Diagnostic and Statistical Manual of Mental Disorders Fifth Edition (DSM-5) alcohol use disorder (AUD), tobacco use disorder (TUD), cannabis use disorder (CUD), and nonmedical prescription opioid use disorder (OUD) diagnoses.

Results

Having 2 or more stressful life events in the past year increased the odds of having a new AUD, TUD, CUD, and OUD (OR=3.14, 2.15, 5.52, and 3.06, respectively) or ongoing AUD, TUD, and CUD (OR=2.39, 2.62, and 2.95, respectively) compared to 0 or 1 stressful life event. A stress by gender interaction for new vs. absent AUD demonstrated that having 2 or more stressful life events was associated with increased odds of new AUD in men (OR=2.51) and even greater odds of new AUD in women (OR=3.94).

Conclusions

Results highlight that stress is a robust factor in both men and women with new or ongoing substance use disorders, and that effective treatments for substance use should consider the role of stress in addiction etiology and maintenance. There was little evidence for gender differences in the role of stress on transitions in substance use disorders, except for the onset of alcohol use disorders. Given that rates of alcohol use disorders are increasing in women; the impact of stress needs to be considered.

Probing for Neuroadaptations to Unpredictable Stressors in Addiction: Translational Methods and Emerging Evidence

Stressors clearly contribute to addiction etiology and relapse in humans, but our understanding of specific mechanisms remains limited. Rodent models of addiction offer the power, flexibility, and precision necessary to delineate the causal role and specific mechanisms through which stressors influence alcohol and other drug use. This review describes a program of research using startle potentiation to unpredictable stressors that is well positioned to translate between animal models and clinical research with humans on stress neuroadaptations in addiction. This research rests on a solid foundation provided by three separate pillars of evidence from (a) rodent behavioral neuroscience on stress neuroadaptations in addiction, (b) rodent affective neuroscience on startle potentiation, and (c) human addiction and affective science with startle potentiation. Rodent stress neuroadaptation models implicate adaptations in corticotropin-releasing factor and norepinephrine circuits within the central extended amygdala following chronic alcohol and other drug use that mediate anxious behaviors and stress-induced reinstatement among drug-dependent rodents. Basic affective neuroscience indicates that these same neural mechanisms are involved in startle potentiation to unpredictable stressors in particular (vs. predictable stressors). We believe that synthesis of these evidence bases should focus us on the role of unpredictable stressors in addiction etiology and relapse. Startle potentiation in unpredictable stressor tasks is proposed to provide an attractive and flexible test bed to encourage tight translation and reverse translation between animal models and human clinical research on stress neuroadaptations. Experimental therapeutics approaches focused on unpredictable stressors hold high promise to identify, repurpose, or refine pharmacological and psychosocial interventions for addiction.

Dissociable Role of Corticotropin Releasing Hormone Receptor Subtype 1 on Dopaminergic and D1 Dopaminoceptive Neurons in Cocaine Seeking Behavior

The ability of many drugs of abuse, including cocaine, to mediate reinforcement and drug-seeking behaviors is in part mediated by the corticotropin-releasing hormone (CRH) system, in which CRH exerts its effects partly via the CRH receptor subtype 1 (CRHR1) in extra-hypothalamic areas. In fact, CRHR1 expressed in regions of the mesolimbic dopamine (DA) system have been demonstrated to modify cocaine-induced DA release and alter cocaine-mediated behaviors. Here we examined the role of neuronal selectivity of CRHR1 within the mesolimbic system on cocaine-induced behaviors. First we used a transgenic mouse line expressing GFP under the control of the Crhr1 promoter for double fluorescence immunohistochemistry to demonstrate the cellular location of CRHR1 in both dopaminergic and D1 dopaminoceptive neurons. We then studied cocaine sensitization, self-administration, and reinstatement in inducible CRHR1 knockouts using the CreERT2/loxP in either dopamine transporter (DAT)-containing neurons (DAT-Crhr1) or dopamine receptor 1 (D1)-containing neurons (D1-Crhr1). For sensitization testing, mice received five daily injections of cocaine (15 mg/kg IP). For self-administration, mice received eight daily 2 h cocaine (0.5 mg/kg per infusion) self-administration sessions followed by extinction and reinstatement testing. There were no differences in the acute or sensitized locomotor response to cocaine in DAT-Crhr1 or D1-Crhr1 mice and their respective controls. Furthermore, both DAT-Crhr1 and D1-Crhr1 mice reliably self-administered cocaine at the level of controls. However, DAT-Crhr1 mice demonstrated a significant increase in cue-induced reinstatement relative to controls, whereas D1-Crhr1 mice demonstrated a significant decrease in cue-induced reinstatement relative to controls. These data demonstrate the involvement of CRHR1 in cue-induced reinstatement following cocaine self-administration, and implicate a bi-directional role of CRHR1 for cocaine craving.

Basolateral amygdala and stress-induced hyperexcitability affect motivated behaviors and addiction

The amygdala integrates and processes incoming information pertinent to reward and to emotions such as fear and anxiety that promote survival by warning of potential danger. Basolateral amygdala (BLA) communicates bi-directionally with brain regions affecting cognition, motivation and stress responses including prefrontal cortex, hippocampus, nucleus accumbens and hindbrain regions that trigger norepinephrine-mediated stress responses. Disruption of intrinsic amygdala and BLA regulatory neurocircuits is often caused by dysfunctional neuroplasticity frequently due to molecular alterations in local GABAergic circuits and principal glutamatergic output neurons. Changes in local regulation of BLA excitability underlie behavioral disturbances characteristic of disorders including post-traumatic stress syndrome (PTSD), autism, attention-deficit hyperactivity disorder (ADHD) and stress-induced relapse to drug use. In this Review, we discuss molecular mechanisms and neural circuits that regulate physiological and stress-induced dysfunction of BLA/amygdala and its principal output neurons. We consider effects of stress on motivated behaviors that depend on BLA; these include drug taking and drug seeking, with emphasis on nicotine-dependent behaviors. Throughout, we take a translational approach by integrating decades of addiction research on animal models and human trials. We show that changes in BLA function identified in animal addiction models illuminate human brain imaging and behavioral studies by more precisely delineating BLA mechanisms. In summary, BLA is required to promote responding for natural reward and respond to second-order drug-conditioned cues; reinstate cue-dependent drug seeking; express stress-enhanced reacquisition of nicotine intake; and drive anxiety and fear. Converging evidence indicates that chronic stress causes BLA principal output neurons to become hyperexcitable.

Effect of doxazosin on stress reactivity and the ability to resist smoking

Preclinical findings support a role for α1-adrenergic antagonists in reducing nicotine-motivated behaviors, but these findings have yet to be translated to humans. The current study evaluated whether doxazosin would attenuate stress-precipitated smoking in the human laboratory. Using a well-validated laboratory analogue of smoking-lapse behavior, this pilot study evaluated whether doxazosin (4 and 8 mg/day) versus placebo attenuated the effect of stress (vs neutral imagery) on tobacco craving, the ability to resist smoking and subsequent ad-libitum smoking in nicotine-deprived smokers ( n=35). Cortisol, adrenocorticotropin, norepinephrine, epinephrine, and physiologic reactivity were assessed. Doxazosin (4 and 8 mg/day vs placebo) decreased cigarettes per day during the 21-day titration period. Following titration, doxazosin (4 and 8 mg/day vs placebo) decreased tobacco craving. During the laboratory session, doxazosin (8 mg/day vs placebo) further decreased tobacco craving following stress versus neutral imagery. Doxazosin increased the latency to start smoking following stress, and reduced the number of cigarettes smoked. Dosage of 8 mg/day doxazosin increased or normalized cortisol levels following stress imagery and decreased cortisol levels following neutral imagery. These preliminary findings support a role for the noradrenergic system in stress-precipitated smoking behavior, and support further development of doxazosin as a novel pharmacotherapeutic treatment strategy for smoking cessation.

Pharmacotherapy in smoking cessation: Corticotropin Releasing Factor receptors as emerging intervention targets

Smoking represents perhaps the single most important health risk factor and a global contributor to mortality that can unquestionably be prevented. Smoking is responsible for many diseases, including various types of cancer, chronic obstructive pulmonary disease, coronary heart disease, peripheral vascular disease and peptic ulcer, while it adversely affects fetal formation and development. Since smoking habit duration is a critical factor for mortality, the goal of treatment should be its timely cessation and relapse prevention. Drug intervention therapy is an important ally in smoking cessation. Significant positive steps have been achieved in the last few years in the development of supportive compounds. In the present review, we analyze reports studying the role of Corticotropin Releasing Factor (CRF), the principle neuroendocrine mediator of the stress response and its two receptors (CRF1 and CRF2) in the withdrawal phase as well as in the abstinence from nicotine use. Although still in pre-clinical evaluation, therapeutic implications of these data were investigated in order to highlight potential pharmaceutical interventions.

Neural stress reactivity relates to smoking outcomes and differentiates between mindfulness and cognitive-behavioral treatments

Stress and negative affect are known contributors to drug use and relapse, and several known treatments for addictions include strategies for managing them. In the current study, we administered a well-established stress provocation during functional magnetic resonance imaging (fMRI) to 23 participants who completed either mindfulness training (MT; N=11) or the American Lung Association's Freedom From Smoking (FFS; N=12), which is a cognitive-behavioral treatment (CBT) for smoking cessation. Across the entire sample, we found that stress reactivity in several brain regions including the amygdala and anterior/mid insula was related to reductions in smoking after treatment, as well as at 3-month post-treatment follow-up. Moreover, conjunction analysis revealed that these same regions also differentiated between treatment groups such that the MT group showed lower stress-reactivity compared to the FFS/CBT group. This suggests that reduction in stress reactivity may be one of the mechanisms that underlie the efficacy of MT in reducing smoking over time. The findings have important implications for our understanding of stress, the neural and psychological mechanisms that underlie mindfulness-based treatments, and for smoking cessation treatments more broadly.

Neuropeptide systems and new treatments for nicotine addiction

RATIONALE

The mildly euphoric and cognitive enhancing effects of nicotine play a role in the initiation of smoking, while dysphoria and anxiety associated with smoking cessation contribute to relapse. After the acute withdrawal phase, smoking cues, a few cigarettes (i.e., lapse), and stressors can cause relapse. Human and animal studies have shown that neuropeptides play a critical role in nicotine addiction.

OBJECTIVES

The goal of this paper is to describe the role of neuropeptide systems in the initiation of nicotine intake, nicotine withdrawal, and the reinstatement of extinguished nicotine seeking.

RESULTS

The reviewed studies indicate that several drugs that target neuropeptide systems diminish the rewarding effects of nicotine by preventing the activation of dopaminergic systems. Other peptide-based drugs diminish the hyperactivity of brain stress systems and diminish withdrawal-associated symptom severity. Blockade of hypocretin-1 and nociceptin receptors and stimulation of galanin and neurotensin receptors diminishes the rewarding effects of nicotine. Both corticotropin-releasing factor type 1 and kappa-opioid receptor antagonists diminish dysphoria and anxiety-like behavior associated with nicotine withdrawal and inhibit stress-induced reinstatement of nicotine seeking. Furthermore, blockade of vasopressin 1b receptors diminishes dysphoria during nicotine withdrawal, and melanocortin 4 receptor blockade prevents stress-induced reinstatement of nicotine seeking. The role of neuropeptide systems in nicotine-primed and cue-induced reinstatement is largely unexplored, but there is evidence for a role of hypocretin-1 receptors in cue-induced reinstatement of nicotine seeking.

CONCLUSION

Drugs that target neuropeptide systems might decrease the euphoric effects of smoking and improve relapse rates by diminishing withdrawal symptoms and improving stress resilience.

Targeting the Brain Stress Systems for the Treatment of Tobacco/Nicotine Dependence: Translating Preclinical and Clinical Findings

Tobacco use is the leading cause of preventable mortality in the United States, and Food and Drug Administration (FDA) approved medications fail to maintain long-term abstinence for the majority of smokers. One of the principal mechanisms associated with the initiation, maintenance of, and relapse to smoking is stress. Targeting the brain stress systems as a potential treatment strategy for tobacco dependence may be of therapeutic benefit. This review explores brain stress systems in tobacco use and dependence. The corticotropin-releasing factor (CRF) system, the hypothalamic-pituitary-adrenal (HPA) axis, and the noradrenergic system are discussed in relation to tobacco use. Preclinical and clinical investigations targeting these stress systems as treatment strategies for stress-induced tobacco use are also discussed. Overall, nicotine-induced activation of the CRF system, and subsequent activation of the HPA axis and noradrenergic system may be related to stress-induced nicotine-motivated behaviors. Pharmacological agents that decrease stress-induced hyperactivation of these brain stress systems may improve smoking-related outcomes.

Nicotine, adolescence, and stress: A review of how stress can modulate the negative consequences of adolescent nicotine abuse

In order to continue the decline of smoking prevalence, it is imperative to identify factors that contribute to the development of nicotine and tobacco addiction, such as adolescent initiation of nicotine use, adolescent stress, and their interaction. This review highlights the biological differences between adolescent and adults in nicotine use and resulting effects, and examines the enduring consequences of adolescent nicotine administration. A review of both clinical and preclinical literature indicates that adolescent, but not adult, nicotine administration leads to increased susceptibility for development of long-lasting impairments in learning and affect. Finally, the role stress plays in normal adolescent development, the deleterious effects stress has on learning and memory, and the negative consequences resulting from the interaction of stress and nicotine during adolescence is reviewed. The review concludes with ways in which future policies could benefit by addressing adolescent stress as a means of reducing adolescent nicotine abuse.

Modulatory effects of the basolateral amygdala α2-adrenoceptors on nicotine-induced anxiogenic-like behaviours of rats in the elevated plus maze

The present study was designed to clarify whether α2-adrenoceptors of the basolateral amygdala (BLA) are involved in nicotine-induced anxiogenic-like behaviours. Adult male Wistar rats were bilaterally cannulated in the BLA and anxiety-like behaviours were assessed in an elevated plus maze (EPM) task. Systemic intraperitoneal (i.p.) administration of nicotine (0.3, 0.5 and 0.7 mg/kg) dose-dependently decreased open arm time (%OAT) and open arm entry (%OAE), indicating the anxiogenic-like effect of nicotine. The activation of the BLA α2-adrenoceptors by the injection of α2-receptor agonist, clonidine (0.1, 0.3 and 0.5 μg/rat) into the BLA (intra-BLA) reversed nicotine-induced anxiogenic-like behaviours. It is important to note that intra-BLA injection of a higher dose of clonidine (0.5 μg/rat) by itself increased %OAT, but not %OAE which showed an anxiolytic effect of the agonist. On the other hand, intra-BLA injection of different doses of α2-adrenoceptor antagonist, yohimbine (1, 3 and 5 μg/rat) in combination with an ineffective dose of nicotine (0.3 mg/kg) decreased %OAT and %OAE, suggesting a potentiative effect of the antagonist on nicotine response. In addition, intra-BLA injection of the same doses of yohimbine did not alter %OAT and %OAE. Interestingly, intra-BLA injection of yohimbine (0.5 and 1 μg/rat) significantly reversed the inhibitory effect of clonidine on nicotine-induced anxiogenic-like behaviours. It should be considered that the drug treatments had no effect on locomotor activity in all experiments. Taken together, it can be concluded that nicotine produces anxiogenic-like behaviours which may be mediated through the BLA α2-adrenoceptor mechanism.

Stress-Induced Reinstatement of Drug Seeking: 20 Years of Progress

In human addicts, drug relapse and craving are often provoked by stress. Since 1995, this clinical scenario has been studied using a rat model of stress-induced reinstatement of drug seeking. Here, we first discuss the generality of stress-induced reinstatement to different drugs of abuse, different stressors, and different behavioral procedures. We also discuss neuropharmacological mechanisms, and brain areas and circuits controlling stress-induced reinstatement of drug seeking. We conclude by discussing results from translational human laboratory studies and clinical trials that were inspired by results from rat studies on stress-induced reinstatement. Our main conclusions are (1) The phenomenon of stress-induced reinstatement, first shown with an intermittent footshock stressor in rats trained to self-administer heroin, generalizes to other abused drugs, including cocaine, methamphetamine, nicotine, and alcohol, and is also observed in the conditioned place preference model in rats and mice. This phenomenon, however, is stressor specific and not all stressors induce reinstatement of drug seeking. (2) Neuropharmacological studies indicate the involvement of corticotropin-releasing factor (CRF), noradrenaline, dopamine, glutamate, kappa/dynorphin, and several other peptide and neurotransmitter systems in stress-induced reinstatement. Neuropharmacology and circuitry studies indicate the involvement of CRF and noradrenaline transmission in bed nucleus of stria terminalis and central amygdala, and dopamine, CRF, kappa/dynorphin, and glutamate transmission in other components of the mesocorticolimbic dopamine system (ventral tegmental area, medial prefrontal cortex, orbitofrontal cortex, and nucleus accumbens). (3) Translational human laboratory studies and a recent clinical trial study show the efficacy of alpha-2 adrenoceptor agonists in decreasing stress-induced drug craving and stress-induced initial heroin lapse.

Enhanced motivation for food reward induced by stress and attenuation by corticotrophin-releasing factor receptor antagonism in rats: implications for overeating and obesity

RATIONALE

Overeating beyond individuals' homeostatic needs critically contributes to obesity. The neurobehavioral mechanisms underlying the motivation to consume excessive foods with high calories are not fully understood.

OBJECTIVE

The present study examined whether a pharmacological stressor, yohimbine, enhances the motivation to procure food reward with an emphasis on comparisons between standard lab chow and high-fat foods. The effects of corticotropin-releasing factor (CRF) receptor blockade by a CRF1-selective antagonist NBI on the stress-enhanced motivation for food reward were also assessed.

METHODS

Male Sprague-Dawley rats with chow available ad libitum in their home cages were trained to press a lever under a progressive ratio schedule for deliveries of either standard or high-fat food pellets. For testing yohimbine stress effects, rats received an intraperitoneal administration of yohimbine 10 min before start of the test sessions. For testing effects of CRF1 receptor blockade on stress responses, NBI was administered 20 min prior to yohimbine challenge.

RESULTS

The rats emitted higher levels of lever responses to procure the high-fat food pellets compared with their counterparts on standard food pellets. Yohimbine challenge facilitated lever responses for the reward in all of the rats, whereas the effect was more robust in the rats on high-fat food pellets compared with their counterparts on standard food pellets. An inhibitory effect of pretreatment with NBI was observed on the enhancing effect of yohimbine challenge but not on the responses under baseline condition without yohimbine administration.

CONCLUSIONS

Stress challenge significantly enhanced the motivation of satiated rats to procure extra food reward, especially the high-fat food pellets. Activation of CRF1 receptors is required for the stress-enhanced motivation for food reward. These results may have implications for our better understanding of the biobehavioral mechanisms of overeating and obesity.

Targeting the noradrenergic system for gender-sensitive medication development for tobacco dependence

INTRODUCTION

Tobacco use remains the leading cause of morbidity and mortality for both women and men in the United States, and women often experience poorer smoking cessation outcomes than men. Preliminary evidence suggests there are sex differences in medication effectiveness for smoking cessation. However, current medications do not take into account gender-sensitive treatment development and efficacy, underscoring the importance of this underdeveloped area of research.

METHODS

We reviewed preclinical and clinical evidence for gender differences in the inability to quit smoking by examining (a) the effect of increased negative affect and stress reactivity on smoking outcomes in women and (b) smoking for nicotine reinforcement in men. We also reviewed the current literature targeting the noradrenergic system as a novel gender-sensitive treatment strategy for tobacco dependence.

RESULTS

We hypothesize that noradrenergic agents that normalize noradrenergic activity may differentially attenuate stress reactivity in women and nicotine-related reinforcement in men, indicating that targeting the noradrenergic system for smoking cessation may be effective for both genders, with benefits operating through sex-specific mechanisms.

CONCLUSIONS

Converging lines of preclinical and clinical evidence suggest that gender-sensitive approaches to medication development for smoking cessation are a critical next step for addressing low quit rates and exacerbated health risks among women. Evidence reviewed indicates that smoking activates different brain systems modulated by noradrenergic activity in women versus men, and noradrenergic compounds may preferentially target these gender-sensitive systems.

Chronic agmatine treatment prevents behavioral manifestations of nicotine withdrawal in mice

Smoking cessation exhibits an aversive withdrawal syndrome characterized by both increases in somatic signs and affective behaviors including anxiety and depression. In present study, abrupt withdrawal of daily nicotine injections (2mg/kg, s.c., four times daily, for 10 days) significantly increased somatic signs viz. rearing, grooming, jumping, genital licking, leg licking, head shakes with associated depression (increased immobility in forced swim test) as well as anxiety (decreased the number of entries and time spent in open arm in elevated plus maze) in nicotine dependent animals. The peak effect was observed at 24h time point of nicotine withdrawal. Repeated administration of agmatine (40-80µg/mouse, i.c.v.) before the first daily dose of nicotine from day 5 to 10 attenuated the elevated scores of somatic signs and abolished the depression and anxiety like behavior induced by nicotine withdrawal in dependent animals. However, in separate groups, its acute administration 30min before behavior analysis of nicotine withdrawal was ineffective. This result clearly shows the role of agmatine in development of nicotine dependence and its withdrawal. In extension to behavioral experiments, brain agmatine analyses, carried out at 24h time point of nicotine withdrawal demonstrated marked decrease in basal brain agmatine concentration as compared to control animals. Taken together, these data support the role of agmatine as common biological substrate for somatic signs and affective symptoms of nicotine withdrawal. This data may project therapies based on agmatine in anxiety, depression and mood changes associated with tobacco withdrawal.

A critical role for the melanocortin 4 receptor in stress-induced relapse to nicotine seeking in rats

Tobacco addiction is characterized by a lack of control over smoking and relapse after periods of abstinence. Smoking cessation leads to a dysphoric state that contributes to relapse to smoking. After the acute withdrawal phase, exposure to stressors increases the risk for relapse. Blockade of melanocortin 4 (MC4 ) receptors has anxiolytic and antidepressant-like effects in animal models. The aim of these studies was to investigate the role of MC4 receptors in the dysphoria associated with nicotine withdrawal and stress-induced reinstatement of nicotine seeking. To study stress-induced reinstatement, rats self-administered nicotine for 16 days and then nicotine seeking was extinguished by substituting saline for nicotine. Nicotine seeking was reinstated by intermittent footshock stress. The intracranial self-stimulation (ICSS) procedure was used to assess the negative mood state associated with nicotine withdrawal. Elevations in the ICSS thresholds are indicative of a dysphoric state. The selective MC4 receptor antagonists HS014 and HS024 prevented stress-induced reinstatement of extinguished nicotine seeking. Drug doses that prevented stress-induced relapse did not affect responding for food pellets, which indicates that the drugs did not induce sedation or motor impairments. In the ICSS experiments, the nicotinic acetylcholine receptor antagonist mecamylamine elevated the ICSS thresholds of the nicotine-dependent rats. Pre-treatment with HS014 or HS024 did not prevent the elevations in ICSS thresholds. These studies indicate that MC4 receptors play a critical role in stress-induced reinstatement of nicotine seeking, but these receptors may not play a role in the dysphoria associated with acute nicotine withdrawal.

Beta-2 adrenergic receptors mediate stress-evoked reinstatement of cocaine-induced conditioned place preference and increases in CRF mRNA in the bed nucleus of the stria terminalis in mice

RATIONALE

Understanding the mechanisms responsible for stress-induced relapse is important for guiding treatment strategies aimed at minimizing the contribution of stress to addiction. Evidence suggests that these mechanisms involve interactions between noradrenergic systems and the neuropeptide corticotropin-releasing factor (CRF).

OBJECTIVES

The interaction between β-adrenergic receptors (ARs) and CRF as it relates to the reinstatement of cocaine-conditioned reward in response to a stressor was examined in mice. We hypothesized that β2-ARs are required for stress-induced activation of CRF pathways responsible for reinstatement.

METHODS

Stress-induced relapse was examined based on the re-establishment of cocaine-induced conditioned place preference (CPP; 4 × 15 mg/kg cocaine, i.p.) after extinction using forced swim (6 min at 22 °C) or an injection of the β2-AR agonist, clenbuterol (4 mg/kg, i.p.). The CRF-R1 antagonist antalarmin (10 mg/kg, i.p.) or the β2-AR antagonist ICI-118,551 (1 mg/kg, i.p.) were given 30 min prior to reinstating stimuli. Quantitative PCR was conducted in dissected bed nucleus of the stria terminalis (BNST) and amygdala, putative sources of CRF that contribute to reinstatement, to examine the effects of ICI-118,551 on swim-induced increases in CRF messenger RNA (mRNA) in mice with a cocaine history.

RESULTS

Pretreatment with ICI-118,551 or antalarmin blocked swim-induced reinstatement of CPP. Reinstatement by clenbuterol was also blocked by antalarmin. ICI-118,551 pretreatment prevented swim-induced increases in CRF mRNA in the BNST. Effects in the amygdala were not observed.

CONCLUSIONS

These findings indicate that, during stress, norepinephrine, via β2-ARs, either directly or indirectly activates CRF-releasing neurons in the BNST that interface with motivational neurocircuitry to induce reinstatement of cocaine-conditioned reward.

Convergent pharmacological mechanisms in impulsivity and addiction: insights from rodent models

Research over the last two decades has widely demonstrated that impulsivity, in its various forms, is antecedent to the development of drug addiction and an important behavioural trait underlying the inability of addicts to refrain from continued drug use. Impulsivity describes a variety of rapidly and prematurely expressed behaviours that span several domains from impaired response inhibition to an intolerance of delayed rewards, and is a core symptom of attention deficit hyperactivity disorder (ADHD) and other brain disorders. Various theories have been advanced to explain how impulsivity interacts with addiction both causally and as a consequence of chronic drug abuse; these acknowledge the strong overlaps in neural circuitry and mechanisms between impulsivity and addiction and the seemingly paradoxical treatment of ADHD with stimulant drugs with high abuse potential. Recent years have witnessed unprecedented progress in the elucidation of pharmacological mechanisms underpinning impulsivity. Collectively, this work has significantly improved the prospect for new therapies in ADHD as well as our understanding of the neural mechanisms underlying the shift from recreational drug use to addiction. In this review, we consider the extent to which pharmacological interventions that target impulsive behaviour are also effective in animal models of addiction. We highlight several promising examples of convergence based on empirical findings in rodent-based studies.

Role of cues and contexts on drug-seeking behaviour

Environmental stimuli are powerful mediators of craving and relapse in substance-abuse disorders. This review examined how animal models have been used to investigate the cognitive mechanisms through which cues are able to affect drug-seeking behaviour. We address how animal models can describe the way drug-associated cues come to facilitate the development and persistence of drug taking, as well as how these cues are critical to the tendency to relapse that characterizes substance-abuse disorders. Drug-associated cues acquire properties of conditioned reinforcement, incentive motivation and discriminative control, which allow them to influence drug-seeking behaviour. Using these models, researchers have been able to investigate the pharmacology subserving the behavioural impact of environmental stimuli, some of which we highlight. Subsequently, we examine whether the impact of drug-associated stimuli can be attenuated via a process of extinction, and how this question is addressed in the laboratory. We discuss how preclinical research has been translated into behavioural therapies targeting substance abuse, as well as highlight potential developments to therapies that might produce more enduring changes in behaviour.

Amplified reacquisition of nicotine self-administration in rats by repeated stress during abstinence

RATIONALE

Quitting smoking is often very challenging, leading to frequent relapse. Exposure to acute and chronic stress during abstinence increases the likelihood of relapse to smoking. In rodents, stress acutely reinstates nicotine seeking after extinction of nicotine self-administration (SA). However, whether reacquisition of nicotine taking is amplified by chronic stress during abstinence from nicotine SA has not been determined in animals.

OBJECTIVES

We sought to determine effects of repeated restraint stress during abstinence on reacquisition of nicotine SA.

METHODS

Rats acquired nicotine SA (23 h/day) under a fixed-ratio (FR) 5 schedule of reinforcement, which was followed by an abstinence phase. Restraint (0, 2, and 4 times) was administered during abstinence. Animals reacquired nicotine SA, first under a progressive ratio (PR) schedule, beginning immediately after the final stress, followed by an FR5 schedule. In another experiment, reacquisition (FR5) began 24 h after the final stress. No PR testing was conducted.

RESULTS

Four restraint stress exposures during abstinence, but not only two, enhanced reacquisition of nicotine SA by increasing nicotine injections under a PR schedule beginning immediately after the final stress (p < 0.05) followed by increasing nicotine intake under an FR5 schedule (p < 0.05). This was observed even when the final stress and reacquisition trial were separated by 24 h. Moreover, repeated stress-induced nicotine taking during the behaviorally inactive phase (i.e., lights on) of the 24-h diurnal cycle.

CONCLUSIONS

Chronic (i.e., repeated) stress during abstinence promotes reacquisition of nicotine SA and affects diurnal pattern of nicotine intake.

Examination of the role of dopamine D₂ and adrenergic α₂ receptors in resurgence of food seeking

Resurgence refers to the reappearance of an extinguished operant behavior when reinforcement for an alternative behavior is also subsequently discontinued. Resurgence has been noted as a source of relapse to problem behavior following interventions involving alternative reinforcement, and has also been recently used as an animal model of relapse to drug seeking induced by reinforcement loss. Existing information about the neuropharmacology of resurgence is scarce, but suggests overlap between relapse observed in the resurgence model and relapse observed in reinstatement and renewal models. In the present experiment rats earned food pellets for pressing a target lever in Phase I. In Phase II lever pressing no longer produced food, but food was delivered for an alterative nose poke response. Finally in Phase III, neither response produced food deliveries. Prior to these Phase III sessions, separate groups of rats were injected with 0, 50, or 100 μg/kg of the dopamine D2 receptor antagonist raclopride or 0, 20, or 40 μg/kg of α 2 agonist clonidine. Both doses of raclopride were effective in blocking resurgence, but there was evidence that the higher dose did so via motor rather than motivational impairment. Only the higher dose of clonidine blocked resurgence, but did so with no evidence of motor impairment. Raclopride significantly impacted extinction of the alternative poke at both doses tested, whereas clonidine had no effect at either dose. The results of the present study provide additional information about the neuropharmacology of resurgence, as well as additional evidence of overlap between resurgence, reinstatement, and renewal.

Restraint stress attenuates nicotine's locomotor stimulant but not discriminative stimulus effects in rats

Stress enhances the locomotor stimulant and discriminative stimulus effects of several addictive drugs (e.g., morphine) in rodents, yet interactions between stress and nicotine's effects in these behavioral models have not been well established. To this end, the current studies examined the effects of restraint stress on nicotine-induced locomotor activity and nicotine discrimination in rats. We used a novel approach in which onset of stress and nicotine administration occurred concurrently (i.e., simultaneous exposure) to simulate effects of stress on ongoing tobacco use, as well as a more traditional approach in which a delay was imposed between stress and nicotine administration (i.e., sequential exposure). Simultaneous exposure to stress reduced the rate of locomotor sensitization induced by daily injections of nicotine (0.4 mg/kg, s.c.). A lower dose of nicotine (0.1mg/kg, s.c.) produced modest effects on activity that were generally unaffected by simultaneous exposure to stress. Sequential exposure to stress and nicotine (0.4 mg/kg, s.c.) slightly suppressed nicotine-induced activity but did not influence rate of locomotor sensitization. Neither simultaneous nor sequential exposure to stress influenced the discriminative stimulus effects of nicotine (0.01-0.2mg/kg, s.c.). These data show that restraint stress reduces nicotine's locomotor stimulant effects, particularly when onset of stress and nicotine exposure occurs simultaneously, but does not influence nicotine discrimination. These findings contrast with the ability of stress to enhance the effects of other drugs in these models. This study also suggests that studying the influence of simultaneous stress exposure on drug effects may be useful for understanding the role of stress in addiction.

Effects of stressors on the reinforcing efficacy of nicotine in adolescent and adult rats

RATIONALE AND OBJECTIVES

Stress increases drug intake. This depends on the stressor, drug, and aspect of drug seeking assessed. The objectives of these experiments done in adolescent and adult male rats were to (1) examine social defeat effects on acquisition of nicotine self-administration (SA) and the reinforcing efficacy of nicotine and (2) determine the effects of acute exposure to intermittent footshock (FS) or yohimbine on the reinforcing efficacy of nicotine.

METHODS

In experiment 1, rats received four defeat exposures prior to nicotine SA acquisition and progressive ratio (PR) SA sessions (30 μg/kg nicotine/infusion). Exposure to an olfactory cue previously paired with defeat was also tested on responding maintained by nicotine on the PR schedule. In experiments 2 and 3, the effects of FS (5 and 10 min) or yohimbine (0.625 and 1.25 mg/kg, i.p.) on PR responding for nicotine (15, 30, or 60 μg/kg/infusion) were assessed. Adolescents were aged PD34-36 and adults PD81-85 at the beginning of nicotine SA training.

RESULTS

Defeat did not affect nicotine SA acquisition. Prior exposure to defeat or a defeat-paired olfactory cue did not affect PR responding for nicotine. FS modestly decreased PR responding in adolescents at the middle nicotine infusion dose. Yohimbine increased PR responding independent of nicotine infusion dose and age.

CONCLUSIONS

Together with previous work with other drugs, our data indicate that the effects of stress on the reinforcing efficacy of nicotine are stressor- and drug-dependent. This suggests that there is heterogeneity among stressors on how they affect neuronal systems underlying drug intake.