Naltrexone attenuation of conditioned but not primary reinforcement of nicotine in rats

A novel tramadol-polycaprolactone implant could palliate heroin conditioned place preference and withdrawal in rats: behavioral and neurochemical study

Drug dependence is a chronic brain disease characterized by craving and recurrent episodes of relapse. Tramadol HCl is a promising agent for withdrawal symptoms management, considering its relatively low abuse potential and safety. Oral administration, however, is not preferred in abstinence maintenance programs. Introducing an implantable, long-lasting formula is suggested to help outpatient abstinence programs achieve higher rates of treatment continuation. Tramadol implants (T350 and T650) were prepared on polycaprolactone polymer ribbons by the wet method. Male Wistar rats were adapted to heroin-conditioned place preference (CPP) at escalating doses (3-30 mg/kg, intraperitoneally, for 14 days). Implants were surgically implanted in the back skin of rats. After 14 days, the CPP score was recorded. Naloxone (1 mg/kg, intraperitoneally) was used to induce withdrawal on day 15, and symptoms were scored. Elevated plus maze and open field tests were performed for anxiety-related symptoms. Striata were analyzed for neurochemical changes reflected in dopamine, 3,4-dihydroxyphenyl acetic acid, gamma-aminobutyric acid, and serotonin levels. Brain oxidative changes including glutathione and lipid peroxides were assessed. The tramadol implants (T350 and T650) reduced heroin CPP and limited naloxone-induced withdrawal symptoms. The striata showed increased levels of 3,4-dihydroxyphenyl acetic acid, and serotonin and decreased levels of gamma-aminobutyric acid and dopamine after heroin withdrawal induction, which were reversed after implanting T350 and T650. Implants restore the brain oxidative state. Nonsignificant low naloxone-induced withdrawal score after the implant was used in naive subjects indicating low abuse potential of the implants. The presented tramadol implants were effective at diminishing heroin CPP and withdrawal in rats, suggesting further investigations for application in the management of opioid withdrawal.

Naltrexone engages a brain reward network in the presence of reward-predictive distractor stimuli in males

The non-selective opioid receptor antagonist, naltrexone is one of the most prescribed medications for treating alcohol and opioid addiction. Despite decades of clinical use, the mechanism(s) by which naltrexone reduces addictive behavior remains unclear. Pharmaco-fMRI studies to date have largely focused on naltrexone's impact on brain and behavioral responses to drug or alcohol cues or on decision-making circuitry. We hypothesized that naltrexone's effects on reward-associated brain regions would associate with reduced attentional bias (AB) to non-drug, reward-conditioned cues. Twenty-three adult males, including heavy and light drinkers, completed a two-session, placebo-controlled, double-blind study testing the effects of acute naltrexone (50 mg) on AB to reward-conditioned cues and neural correlates of such bias measured via fMRI during a reward-driven AB task. While we detected significant AB to reward-conditioned cues, naltrexone did not reduce this bias in all participants. A whole-brain analysis found that naltrexone significantly altered activity in regions associated with visuomotor control regardless of whether a reward-conditioned distractor was present. A region-of-interest analysis of reward-associated areas found that acute naltrexone increased BOLD signal in the striatum and pallidum. Moreover, naltrexone effects in the pallidum and putamen predicted individual reduction in AB to reward-conditioned distractors. These findings suggest that naltrexone's effects on AB primarily reflect not reward processing per se, but rather top-down control of attention. Our results suggest that the therapeutic actions of endogenous opioid blockade may reflect changes in basal ganglia function enabling resistance to distraction by attractive environmental cues, which could explain some variance in naltrexone's therapeutic efficacy.

Opioid Withdrawal Abruptly Disrupts Amygdala Circuit Function by Reducing Peptide Actions

While the physical signs of opioid withdrawal are most readily observable, withdrawal insidiously drives relapse and contributes to compulsive drug use, by disrupting emotional learning circuits. How these circuits become disrupted during withdrawal is poorly understood. Because amygdala neurons mediate relapse, and are highly opioid sensitive, we hypothesized that opioid withdrawal would induce adaptations in these neurons, opening a window of disrupted emotional learning circuit function. Under normal physiological conditions, synaptic transmission between the basolateral amygdala (BLA) and the neighboring main island (Im) of GABAergic intercalated cells (ITCs) is strongly inhibited by endogenous opioids. Using patch-clamp electrophysiology in brain slices prepared from male rats, we reveal that opioid withdrawal abruptly reduces the ability of these peptides to inhibit neurotransmission, a direct consequence of a protein kinase A (PKA)-driven increase in the synaptic activity of peptidases. Reduced peptide control of neurotransmission in the amygdala shifts the excitatory/inhibitory balance of inputs onto accumbens-projecting amygdala cells involved in relapse. These findings provide novel insights into how peptidases control synaptic activity within the amygdala and presents restoration of endogenous peptide activity during withdrawal as a viable option to mitigate withdrawal-induced disruptions in emotional learning circuits and rescue the relapse behaviors exhibited during opioid withdrawal and beyond into abstinence.SIGNIFICANCE STATEMENT We find that opioid withdrawal dials down inhibitory neuropeptide activity in the amygdala. This disrupts both GABAergic and glutamatergic transmission through amygdala circuits, including reward-related outputs to the nucleus accumbens. This likely disrupts peptide-dependent emotional learning processes in the amygdala during withdrawal and may direct behavior toward compulsive drug use.

Elucidating the reinforcing effects of nicotine: a tribute to Nadia Chaudhri

Nadia Chaudhri worked with us as a graduate student in the Center for Neuroscience at the University of Pittsburgh from 1999 until she earned her PhD in 2005, a time that coincided with the discovery in our lab of the dual reinforcing actions of nicotine, a concept that she played an important role in shaping. The research that was described in her doctoral thesis is among the foundational pillars of the now well-accepted notion that nicotine acts as both a primary reinforcer and an amplifier of other reinforcer stimuli. This reinforcement-enhancing action of nicotine is robust and likely to be a powerful driver of nicotine use. Below, we discuss the evidence that these two actions of nicotine - primary reinforcement and reinforcement enhancement - are distinct and dissociable, a finding that Nadia was closely associated with. We go on to address two other topics that greatly interested Nadia during that time, the generalizability of the reinforcement-enhancing action of nicotine to multiple classes of reinforcing stimuli and potential sex differences in the dual reinforcing actions of nicotine. The research has greatly expanded since Nadia's involvement, but the core ideas that she helped to develop remain central to the concept of the dual reinforcing actions of nicotine and its importance for understanding the drivers of nicotine use.

Nicotine and opioid co-dependence: Findings from bench research to clinical trials

Concomitant use of tobacco and opioids represents a growing public health concern. In fact, the mortality rate due to smoking-related illness approaches 50% among SUD patients. Cumulative evidence demonstrates that the vulnerability to drugs of abuse is influenced by behavioral, environmental, and genetic factors. This review explores the contribution of genetics and neural mechanisms influencing nicotine and opioid reward, respiration, and antinociception, emphasizing the interaction of cholinergic and opioid receptor systems. Despite the substantial evidence demonstrating nicotine-opioid interactions within the brain and on behavior, the currently available pharmacotherapies targeting these systems have shown limited efficacy for smoking cessation on opioid-maintained smokers. Thus, further studies designed to identify novel targets modulating both nicotinic and opioid receptor systems may lead to more efficacious approaches for co-morbid nicotine dependence and opioid use disorder.

Targeting the Opioid Receptors: A Promising Therapeutic Avenue for Treatment in “Heavy Drinking Smokers”

Aims

Despite a general decline in tobacco use in the last decades, the prevalence of tobacco smoking in individuals with alcohol use disorder (AUD) remains substantial (45–50%). Importantly, the co-use of both substances potentiates the adverse effects, making it a significant public health problem. Substantial evidence suggests that AUD and Tobacco use disorder (TUD) may share common mechanisms. Targeting these mechanisms may therefore provide more effective therapy. Numerous studies describe a potential role of the endogenous opioid system in both AUD and TUD. Reviewing this literature, we aim to evaluate the efficacy of molecules that target the opioid system as promising therapeutic interventions for treating alcohol and tobacco co-use disorders.

Methods

We provide a synthesis of the current epidemiological knowledge of alcohol and tobacco co-use disorders. We evaluate clinical and preclinical research that focuses on the regulation of the endogenous opioid system in alcohol, nicotine, and their interactions.

Results

The epidemiological data confirm that smoking stimulates heavy drinking and facilitates alcohol craving. Pharmacological findings suggest that treatments that are efficacious in the dual addiction provide a beneficial treatment outcome in comorbid AUD and TUD. In this regard, MOP, DOP and NOP-receptor antagonists show promising results, while the findings prompt caution when considering KOP-receptor antagonists as a treatment option in alcohol and tobacco co-use disorders.

Conclusions

Existing literature suggests a role of the opioid system in sustaining the high comorbidity rates of AUD and TUD. Molecules targeting opioid receptors may therefore represent promising therapeutic interventions in ‘heavy drinking smokers.’

Smokers with opioid use disorder may have worse drug use outcomes after varenicline than nicotine replacement

INTRODUCTION

Smokers with opioid use disorder (OUD) have little success with smoking cessation, possibly due to interactions between nicotine and opioid receptor systems. Smokers with OUD versus non-opioid substance use disorders (NOUD) have not been compared for response to smoking treatment. Data to make this comparison came from our previous study of 12 weeks (plus dose run-up) of varenicline (VAR) versus 12 weeks of nicotine patch (NRT), in a double-placebo design.

METHODS

The current study reports secondary analyses comparing smokers with OUD (n = 47) and NOUD (n = 90) on pretreatment smoking, alcohol and drug use, intolerance of physical discomfort, smoking medication adherence, and 3- and 6-month smoking and substance use outcomes (by VAR versus NRT).

RESULTS

Smokers with OUD did not differ on pretreatment alcohol or smoking measures while reporting significantly more drug use days. Smokers with OUD versus NOUD had significantly fewer days adherent to VAR or placebo capsules but not to patches, and were more tolerant of physical discomfort. While smoking and heavy drinking days at follow-ups did not differ by diagnosis, smokers with OUD had significantly more drug use days in months 4-6 when assigned to VAR (16.4 days) than to NRT (8.1 days).

CONCLUSIONS

NRT might be a better choice than VAR for smokers with OUD due to lower adherence and more drug use days with VAR. However, this novel comparison of smoking pharmacotherapy response in smokers with OUD versus NOUD needs to be confirmed with larger numbers of participants.

Nicotine Gateway Effects on Adolescent Substance Use

Given the rise in teenage use of electronic nicotine delivery systems ("vaping") in congruence with the increasing numbers of drug-related emergencies, it is critical to expand the knowledge of the physical and behavioral risks associated with developmental nicotine exposure. A further understanding of the molecular and neurochemical underpinnings of nicotine's gateway effects allows emergency clinicians to advise patients and families and adjust treatment accordingly, which may minimize the use of tobacco, nicotine, and future substances. Currently, the growing use of tobacco products and electronic cigarettes among teenagers represents a major public health concern. Adolescent exposure to tobacco or nicotine can lead to subsequent abuse of nicotine and other substances, which is known as the gateway hypothesis. Adolescence is a developmentally sensitive time period when risk-taking behaviors, such as sensation seeking and drug experimentation, often begin. These hallmark behaviors of adolescence are largely due to maturational changes in the brain. The developing brain is particularly vulnerable to the harmful effects of drugs of abuse, including tobacco and nicotine products, which activate nicotinic acetylcholine receptors (nAChRs). Disruption of nAChR development with early nicotine use may influence the function and pharmacology of the receptor subunits and alter the release of reward-related neurotransmitters, including acetylcholine, dopamine, GABA, serotonin, and glutamate. In this review, we emphasize that the effects of nicotine are highly dependent on timing of exposure, with a dynamic interaction of nAChRs with dopaminergic, endocannabinoid, and opioidergic systems to enhance general drug reward and reinforcement. We analyzed available literature regarding adolescent substance use and nicotine's impact on the developing brain and behavior using the electronic databases of PubMed and Google Scholar for articles published in English between January 1968 and November 2018. We present a large collection of clinical and preclinical evidence that adolescent nicotine exposure influences long-term molecular, biochemical, and functional changes in the brain that encourage subsequent drug abuse.

Targeting the α4β2- and α7-Subtypes of Nicotinic Acetylcholine Receptors for Smoking Cessation Medication Development

Nicotine exerts its reinforcing actions via activating the nicotinic acetylcholine receptors (nAChRs). Among an increasing number of nAChR subtypes, the α4β2 and α7 nAChRs are the two major ones, accounting for about 95% of the whole nAChR population in brain. Research findings from our own laboratory, together with other reports in the field, suggest critical and differential involvement of the α4β2 and α7 nAChRs in the process of nicotine dependence and tobacco addiction. Specifically, rat models of nicotine consumption and cue-induced relapse were used to examine the effects of selective antagonism of these two nAChR subtypes on the primary reinforcement of nicotine and the conditioned reinforcing actions of nicotine-associated environmental stimuli (cues). Results demonstrated that blockade of the α4β2 but not α7 subtype effectively reduced nicotine intake, whereas α7 but not α4β2 nAChR blockade reversed cue-triggered nicotine relapse behavior. These findings lend support for the continued effort to develop cholinergic agents aiming at the α4β2 nAChRs for reducing or stopping smoking. However, it is suggested that manipulation of α7 nAChR activity would be a promising target for preventing smoking relapse triggered by exposure to environmental cues.

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.

Caenorhabditis elegans Show Preference for Stimulants and Potential as a Model Organism for Medications Screening

The nematode Caenorhabditis elegans (C. elegans) is a popular invertebrate model organism to study neurobiological disease states. This is due in part to the intricate mapping of all neurons and synapses of the entire animal, the wide availability of mutant strains, and the genetic and molecular tools that can be used to manipulate the genome and gene expression. We have shown that, C. elegans develops a conditioned preference for cues that had previously been paired with either cocaine or methamphetamine exposure that is dependent on dopamine neurotransmission, similar to findings using place conditioning with rats and mice. In the current study, we show C. elegans also display a preference for, and self-exposure to, cocaine and nicotine. This substance of abuse (SOA) preference response can be selectively blocked by pretreatment with naltrexone and is consistent with the recent discovery of an opioid receptor system in C. elegans. In addition, pre-exposure to the smoking cessation treatment varenicline also inhibits self-exposure to nicotine. Exposure to concentrations of treatments that inhibit SOA preference/self-exposure did not induce any significant inhibition of locomotor activity or affect food or benzaldehyde chemotaxis. These data provide predictive validity for the development of high-throughput C. elegans behavioral medication screens. These screens could enable fast and accurate generation of data to identify compounds that may be effective in treating human addiction. The successful development and validation of such models would introduce powerful and novel tools in the search for new pharmacological treatments for substance use disorders, and provide a platform to study the mechanisms that underlie addictions.

Endogenous opioid system: a promising target for future smoking cessation medications

BACKGROUND

Nicotine addiction continues to be a health challenge across the world. Despite several approved medications, smokers continue to relapse. Several human and animal studies have evaluated the role of the endogenous opioid system as a potential target for smoking cessation medications.

METHODS

In this review, studies that have elucidated the role of the mu (MORs), delta (DORs), and kappa (KORs) opioid receptors in nicotine reward, nicotine withdrawal, and reinstatement of nicotine seeking will be discussed. Additionally, the review will discuss discrepancies in the literature and therapeutic potential of the endogenous opioid system, and suggest studies to address gaps in knowledge with respect to the role of the opioid receptors in nicotine dependence.

RESULTS

Data available till date suggest that blockade of the MORs and DORs decreased the rewarding effects of nicotine, while activation of the MORs and DORs decreased nicotine withdrawal-induced aversive effects. In contrast, activation of the KORs decreased the rewarding effects of nicotine, while blockade of the KORs decreased nicotine withdrawal-induced aversive effects. Interestingly, blockade of the MORs and KORs attenuated reinstatement of nicotine seeking. In humans, MOR antagonists have shown benefits in select subpopulations of smokers and further investigation is required to realize their full therapeutic potential.

CONCLUSION

Future work must assess the influence of polymorphisms in opioid receptor-linked genes in nicotine dependence, which will help in both identifying individuals vulnerable to nicotine addiction and the development of opioid-based smoking cessation medications. Overall, the endogenous opioid system continues to be a promising target for future smoking cessation medications.

Current insights into the mechanisms and development of treatments for heavy drinking cigarette smokers

There is a strong association between cigarette smoking and alcohol use at the epidemiological, behavioral, and molecular levels, and this co-use creates substantial impediments to smoking cessation among smokers who are also heavy drinkers. Compared with individuals who only smoke, those who both drink and smoke heavily experience more severe health consequences and have greater difficulty in quitting smoking. During smoking abstinence, greater alcohol use is associated with decreased odds of smoking cessation, and smokers are substantially more likely to experience a smoking lapse during drinking episodes. As heavy drinking smokers are less responsive to the currently available pharmacological treatments, this subgroup of high-risk substance users possesses a unique clinical profile and treatment needs. Thus, treatment development for heavy drinking smokers represents a significant and understudied research area within the field of smoking cessation. This review will briefly describe findings from epidemiological, behavioral, and molecular studies illustrating alcohol and tobacco co-use and identify how the behavioral and neurobiological mechanisms underlying the interaction of alcohol and nicotine may inform the development of targeted treatments for this unique population of smokers.

Neuroscience of nicotine for addiction medicine: novel targets for smoking cessation medications

Morbidity and mortality associated with tobacco smoking constitutes a significant burden on healthcare budgets all over the world. Therefore, promoting smoking cessation is an important goal of health professionals and policy makers throughout the world. Nicotine is a major psychoactive component in tobacco that is largely responsible for the widespread addiction to tobacco. A majority of the currently available FDA-approved smoking cessation medications act via neuronal nicotinic receptors. These medications are effective in approximately half of all the smokers, who want to quit and relapse among abstinent smokers continues to be high. In addition to relapse among abstinent smokers, unpleasant effects associated with nicotine withdrawal are a major motivational factor in continued tobacco smoking. Over the last two decades, animal studies have helped in identifying several neural substrates that are involved in nicotine-dependent behaviors including those associated with nicotine withdrawal and relapse to tobacco smoking. In this review, first the role of specific brain regions/circuits that are involved in nicotine dependence will be discussed. Next, the review will describe the role of specific nicotinic receptor subunits in nicotine dependence. Finally, the review will discuss the role of classical neurotransmitters (dopamine, serotonin, noradrenaline, glutamate, and γ-aminobutyric acid) as well as endogenous opioid and endocannabinoid signaling in nicotine dependence. The nicotinic and nonnicotinic neural substrates involved in nicotine-dependent behaviors can serve as possible targets for future smoking cessation medications.

The opioid receptors as targets for drug abuse medication

The endogenous opioid system is largely expressed in the brain, and both endogenous opioid peptides and receptors are present in areas associated with reward and motivation. It is well known that this endogenous system plays a key role in many aspects of addictive behaviours. The present review summarizes the modifications of the opioid system induced by chronic treatment with drugs of abuse reported in preclinical and clinical studies, as well as the action of opioid antagonists and agonists on the reinforcing effects of drugs of abuse, with therapeutic perspectives. We have focused on the effects of chronic psychostimulants, alcohol and nicotine exposure. Taken together, the changes in both opioid peptides and opioid receptors in different brain structures following acute or chronic exposure to these drugs of abuse clearly identify the opioid system as a potential target for the development of effective pharmacotherapy for the treatment of addiction and the prevention of relapse.

Examination of the effects of varenicline, bupropion, lorcaserin, or naltrexone on responding for conditioned reinforcement in nicotine-exposed rats

Smoking tobacco remains one of the leading causes of preventable deaths in North America. Nicotine reinforces smoking behavior, in part, by enhancing the reinforcing properties of reward-related stimuli, or conditioned stimuli (CSs), associated with tobacco intake. To investigate how pharmaceutical interventions may affect this property of nicotine, we examined the effect of four US Food and Drug Administration (FDA) approved drugs on the ability of nicotine to enhance operant responding for a CS as a conditioned reinforcer. Thirsty rats were exposed to 13 Pavlovian sessions where a CS was paired with water delivery. Nicotine (0.4 mg/kg) injections were administered before each Pavlovian session. Then, in separate groups of rats, the effects of varenicline (1 mg/kg), bupropion (10 and 30 mg/kg), lorcaserin (0.6 mg/kg), and naltrexone (2 mg/kg), and their interaction with nicotine on responding for conditioned reinforcement were examined. Varenicline and lorcaserin each reduced nicotine-enhanced responding for conditioned reinforcement, whereas naltrexone had a modest effect of reducing response enhancements by nicotine. In contrast, bupropion enhanced the effect of nicotine on this measure. The results of these studies may inform how pharmaceutical interventions can affect smoking cessation attempts and relapse through diverse mechanisms, either substituting for, or interacting with, the reinforcement-enhancing properties of nicotine.

Neurobiological Bases of Cue- and Nicotine-induced Reinstatement of Nicotine Seeking: Implications for the Development of Smoking Cessation Medications

A better understanding of the neurobiological factors that contribute to relapse to smoking is needed for the development of efficacious smoking cessation medications. Reinstatement procedures allow the preclinical assessment of several factors that contribute to relapse in humans, including re-exposure to nicotine via tobacco smoking and the presentation of stimuli that were previously associated with nicotine administration (i.e., conditioned stimuli). This review provides an integrated discussion of the results of animal studies that used reinstatement procedures to assess the efficacy of pharmacologically targeting various neurotransmitter systems in attenuating the cue- and nicotine-induced reinstatement of nicotine seeking. The results of these animal studies have increased our understanding of the neurobiological processes that mediate the conditioned effects of stimuli that trigger reinstatement to nicotine seeking. Thus, these findings provide important insights into the neurobiological substrates that modulate relapse to tobacco smoking in humans and the ongoing search for novel efficacious smoking cessation medications.

Pharmacological Options for Smoking Cessation in Heavy-Drinking Smokers

There is a high prevalence of comorbid tobacco use and alcohol use disorder (AUD), affecting more than 6 million people in the US. Globally, tobacco and alcohol use rank fourth and fifth, respectively, for disability-adjusted life-years lost. Levels of alcohol use are higher in smokers than nonsmokers, and the prevalence of smoking is higher in heavy drinkers compared with nondrinkers. This relationship is driven by many different factors, including genetics, neurobiological mechanisms, conditioning processes, and psychosocial influences. Although this unique population tends to experience more negative health consequences, more severe AUD, and poorer response to treatment than those with either AUD or tobacco use disorder alone, there are currently no available treatment protocols tailored to this comorbid condition. In this review, we provide a comprehensive review of ongoing clinical research into smoking cessation options for heavy-drinking smokers (HDS) through an evaluation of the effect of promising novel pharmacotherapies as well as combination therapies, including varenicline, naltrexone, the combination of varenicline and naltrexone, and the combination of naltrexone and nicotine replacement therapy (NRT). These treatments are considered in light of the standard of care for smoking cessation, and seek to improve upon the available guidelines for this sizeable subgroup of smokers, namely those smokers who drink heavily.

Operant self-administration of alcohol and nicotine in a preclinical model of co-abuse

RATIONALE AND OBJECTIVES

Alcohol and nicotine are often taken together. In humans, intake of nicotine, via smoked tobacco, increases alcohol drinking, and alcohol increases smoking. Chronic nicotine treatment increases alcohol self-administration (SA) in laboratory animals; the reverse relationship is less clear. Most animal work modeling this has used passive administration, which lacks relevance to human co-abuse. Here, we describe a model based on sequential operant SA of alcohol and nicotine.

METHODS

Animals are first trained on alcohol SA (0.19 ml of 12 % alcohol (w/v)/delivery) and then receive separate alcohol (8 %, w/v) and nicotine (15 μg/kg/infusion) SA sessions on the same day ("daily dual access"). Animals then receive access to alcohol and then to nicotine (or in the reverse order) in alternating 5-min periods in 2-h sessions ("alternating access"). We then determine if alternating access modifies the effects of naltrexone on responding for alcohol and nicotine.

RESULTS

We found that with daily dual access, nicotine significantly increased alcohol SA when alcohol access occurred prior to nicotine access and that nicotine SA significantly decreased when the alcohol SA session preceded it. During alternating access, nicotine also significantly increased alcohol intake. Naltrexone (0.3 or 1 mg/kg) significantly reduced alcohol SA during these alternating access sessions in animals that also received nicotine SA, but had minimal effects on animals receiving alcohol SA alone. Naltrexone did not affect nicotine SA under any condition.

CONCLUSIONS

This sequential access procedure effectively models the effects of nicotine on alcohol intake noted in humans.

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.

Effects of blockade of α4β2 and α7 nicotinic acetylcholine receptors on cue-induced reinstatement of nicotine-seeking behaviour in rats

Exposure to environmental stimuli conditioned to nicotine consumption critically contributes to the high relapse rates of tobacco smoking. Our previous work demonstrated that non-selective blockade of nicotinic acetylcholine receptors (nAChRs) reversed the cue-induced reinstatement of nicotine seeking, indicating a role for cholinergic neurotransmission in the mediation of the conditioned incentive properties of nicotine cues. The present study further examined the relative roles of the two major nAChR subtypes, α4β2 and α7, in the cue-induced reinstatement of nicotine seeking. Male Sprague-Dawley rats were trained to intravenously self-administer nicotine (0.03 mg/kg/infusion, free base) on a fixed-ratio 5 schedule of reinforcement. A nicotine-conditioned cue was established by associating a sensory stimulus with each nicotine infusion. After nicotine-maintained responding was extinguished by withholding the nicotine infusion and its paired cue, reinstatement test sessions were conducted with re-presentation of the cue but without the availability of nicotine. Thirty minutes before the tests, the rats were administered the α4β2-selective antagonist dihydro-β-erythroidine (DHβE) and α7-selective antagonist methyllycaconitine (MLA). Pretreatment with MLA, but not DHβE, significantly reduced the magnitude of the cue-induced reinstatement of responses on the active, previously nicotine-reinforced lever. In different sets of rats, MLA altered neither nicotine self-administration nor cue-induced reinstatement of food seeking. These results demonstrate that activation of α7 nAChRs participates in the mediation of the conditioned incentive properties of nicotine cues and suggest that α7 nAChRs may be a promising target for the development of medications for the prevention of cue-induced smoking relapse.

Opioid receptor types involved in the development of nicotine physical dependence in an invertebrate (Planaria) model

Recent data suggest that opioid receptors are involved in the development of nicotine physical dependence in mammals. Evidence in support of a similar involvement in an invertebrate (Planaria) is presented using the selective opioid receptor antagonist naloxone, and the more receptor subtype-selective antagonists CTAP (D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2) (μ, MOR), naltrindole (δ, DOR), and nor-BNI (norbinaltorphimine) (κ, KOR). Induction of physical dependence was achieved by 60-min pre-exposure of planarians to nicotine and was quantified by abstinence-induced withdrawal (reduction in spontaneous locomotor activity). Known MOR and DOR subtype-selective opioid receptor antagonists attenuated the withdrawal, as did the non-selective antagonist naloxone, but a KOR subtype-selective antagonist did not. An involvement of MOR and DOR, but not KOR, in the development of nicotine physical dependence or in abstinence-induced withdrawal was thus demonstrated in a sensitive and facile invertebrate model.

Attenuation of cocaine and heroin seeking by μ-opioid receptor antagonism

RATIONALE

Evidence has implicated the endogenous opioids, in particular μ-opioid receptors, in emotional behavior and regulation of reward circuits, especially in the context of heroin addiction and hedonic responses to ingestive rewards. The μ-opioid receptor antagonist naltrexone (NTX) has been reported to be effective in preventing relapse to alcoholism and in reducing alcohol and cocaine craving during abstinence.

OBJECTIVES

The aim of the present experiments was to investigate the effects of a novel selective μ-opioid receptor antagonist GSK1521498 on cocaine and heroin seeking and the primary reinforcement of drug self-administration behavior.

METHODS

Rats were first trained to self-administer cocaine or heroin and then to seek the drugs over prolonged periods of time under a second-order schedule of reinforcement, in which responding is maintained by contingent presentation of a drug-associated conditioned reinforcer. On a stable baseline, animals were treated with either GSK1521498 (0.1, 1, 3 mg/kg; IP) or NTX (0.1, 1, 3 mg/kg; SC) before each test session.

RESULTS

Cocaine seeking was dose-dependently decreased following GSK1521498 treatment. However, the same treatment had no effect on cocaine self-administration under a continuous reinforcement schedule. Treatment with NTX had a less pronounced but similar effect. GSK1521498, but not NTX, dose-dependently reduced heroin seeking both before and after infusion of the drug although both increased heroin self-administration under continuous reinforcement.

CONCLUSIONS

These data suggest that GSK1521498, by reducing opioid receptor signaling at the μ-opioid receptor, may have therapeutic potential to reduce the propensity to seek cocaine or heroin and, additionally, to diminish the consequence of an initial relapse to heroin taking.

Association of time-dependent changes in mu opioid receptor mRNA, but not BDNF, TrkB, or MeCP2 mRNA and protein expression in the rat nucleus accumbens with incubation of heroin craving

RATIONALE AND OBJECTIVES

Responding to heroin cues progressively increases after cessation of heroin self-administration (incubation of heroin craving). We investigated whether this incubation is associated with time-dependent changes in brain-derived neurotrophic factor (BDNF) and methyl-CpG binding protein 2 (MeCP2) signaling and mu opioid receptor (MOR) expression in nucleus accumbens (NAc), dorsal striatum (DS), and medial prefrontal cortex (mPFC). We also investigated the effect of the preferential MOR antagonist naloxone on cue-induced heroin seeking during abstinence.

METHODS

We trained rats to self-administer heroin or saline for 9-10 days and then dissected the NAc, DS, and mPFC at different abstinence days and measured mRNA and protein levels of BDNF, TrkB, and MeCP2, as well as MOR mRNA (Oprm1). In other groups, we assessed cue-induced heroin seeking in extinction tests after 1, 11, and 30 abstinence days, and naloxone's (0-1.0 mg/kg) effect on extinction responding after 1 and 15 days.

RESULTS

Cue-induced heroin seeking progressively increased or incubated during abstinence. This incubation was not associated with changes in BDNF, TrkB, or MeCP2 mRNA or protein levels in NAc, DS, or mPFC; additionally, no molecular changes were observed after extinction tests on day 11. In NAc, but not DS or mPFC, MOR mRNA decreased on abstinence day 1 and returned to basal levels over time. Naloxone significantly decreased cue-induced heroin seeking after 15 abstinence days but not 1 day.

CONCLUSIONS

Results suggest a role of MOR in incubation of heroin craving. As previous studies implicated NAc BDNF in incubation of cocaine craving, our data suggest that different mechanisms contribute to incubation of heroin versus cocaine craving.

Recent advances in understanding nicotinic receptor signaling mechanisms that regulate drug self-administration behavior

Tobacco smoking is one of the leading causes of disease and premature death in the United States. Nicotine is considered the major reinforcing component in tobacco smoke responsible for tobacco addiction. Nicotine acts in the brain through the neuronal nicotinic acetylcholine receptors (nAChRs). The predominant nAChR subtypes in mammalian brain are those containing α4 and β2 subunits. The α4β2 nAChRs, particularly those located in the mesoaccumbens dopamine pathway, play a key role in regulating the reinforcing properties of nicotine. Considering that twelve mammalian nAChR subunits have been cloned, it is likely that nAChRs containing subunits in addition to, or other than, α4 and β2 also play a role in the tobacco smoking habit. Consistent with this possibility, human genome-wide association studies have shown that genetic variation in the CHRNA5-CHRNA3-CHRNB4 gene cluster located in chromosome region 15q25, which encode the α5, α3 and β4 nAChR subunits, respectively, increases vulnerability to tobacco addiction and smoking-related diseases. Most recently, α5-containing nAChRs located in the habenulo-interpeduncular tract were shown to limit intravenous nicotine self-administration behavior in rats and mice, suggesting that deficits in α5-containing nAChR signaling in the habenulo-interpeduncular tract increases vulnerability to the motivational properties of nicotine. Finally, evidence suggests that nAChRs may also play a prominent role in controlling consumption of addictive drugs other than nicotine, including cocaine, alcohol, opiates and cannabinoids. The aim of the present review is to discuss recent preclinical findings concerning the identity of the nAChR subtypes that regulate self-administration of nicotine and other drugs of abuse.

Age-dependent and strain-dependent influences of morphine on mouse social investigation behavior

Opioid-coded neural circuits play a substantial role in how individuals respond to drugs of abuse. Most individuals begin using such drugs during adolescence and within a social context. Several studies indicate that adolescent mice exhibit a heightened sensitivity to the effects of morphine, a prototypical opiate drug, but it is unclear whether these developmental differences are related to aspects of motivated behavior. Moreover, exposure to opioids within the rodent brain can alter the expression of social behavior, yet little is known about whether this relationship changes as a function of development or genetic variation. In this study, we conducted a series of experiments to characterize the relationship between genetic background, adolescent development and morphine-induced changes in mouse social investigation (SI). At two time points during adolescent development [postnatal days (PD) 25 and 45], social interactions of test mice of the gregarious C57BL/6J (B6) strain were more tolerant to the suppressive effects of morphine [effective dose 50 (ED50)=0.97 mg/kg and 2.17 mg/kg morphine, respectively] than test mice from the less social BALB/cJ (BALB) strain (ED50=0.61 mg/kg and 0.91 mg/kg morphine, respectively). By contrast, this strain-dependent difference was not evident among adult mice on PD 90 (ED50=1.07 mg/kg and 1.41 mg/kg morphine for BALB and B6 mice, respectively). An additional experiment showed that the ability of morphine to alter social responsiveness was not directly related to drug-induced changes in locomotor behavior. Finally, administration of morphine to stimulus mice on PD 25 reduced social investigation of test mice only when individuals were from the B6 genetic background. Overall, these results indicate that alterations in endogenous opioid systems are related to changes in SI that occur during adolescence, and that morphine administration may mimic rewarding aspects of social encounter.

Activation of the opioid μ1, but not δ or κ, receptors is required for nicotine reinforcement in a rat model of drug self-administration

There has long been an interest in examining the involvement of opioid neurotransmission in nicotine rewarding process and addiction to nicotine. Over the past 3 decades, however, clinical effort to test the effectiveness of nonselective opioid antagonists (mainly naloxone and naltrexone) for smoking cessation has yielded equivocal results. In light of the fact that there are three distinctive types of receptors mediating actions of the endogenous opioid peptides, this study, using a rat model of nicotine self-administration, examined involvement of different opioid receptors in the reinforcement of nicotine by selective blockade of the μ1, the δ, and the κ opioid receptors. Male Sprague-Dawley rats were trained in daily 1h sessions to intravenously self-administer nicotine (0.03 mg/kg/infusion) on a fixed-ratio 5 schedule. After establishment of stable nicotine self-administration behavior, the effects of the opioid antagonists were tested. Separate groups of rats were used to test the effects of naloxanazine (selective for μ1 receptors, 0, 5 and 15 mg/kg), naltrindole (selective for δ receptors, 0, 0.5 and 5mg/kg), and 5'-guanidinonaltrindole (GNTI, selective for κ receptors, 0, 0.25 and 1mg/kg). In each individual drug group, the 3 drug doses were tested by using a within-subject and Latin-Square design. The effects of these antagonists on food self-administering behavior were also examined in the same rats in each respective drug group after retrained for food self-administration. Pretreatment with naloxonazine, but not naltrindole or GNTI, significantly reduced responses on the active lever and correspondingly the number of nicotine infusions. None of these antagonists changed lever-pressing behavior for food reinforcement. These results indicate that activation of the opioid μ1, but not the δ or the κ, receptors is required for the reinforcement of nicotine and suggest that opioid neurotransmission via the μ1 receptors would be a promising target for the development of opioid ligands for smoking cessation.

Translational research in addiction: toward a framework for the development of novel therapeutics

The development of novel substance use disorder (SUD) therapeutics is insufficient to meet the medical needs of a growing SUD patient population. The identification of translatable SUD models and tests is a crucial step in establishing a framework for SUD therapeutic development programs. The present review begins by identifying the clinical features of SUDs and highlights the narrow regulatory end-point required for approval of a novel SUD therapeutic. A conceptual overview of dependence is provided, followed by identification of potential intervention targets in the addiction cycle. The main components of the addiction cycle provide the framework for a discussion of preclinical models and their clinical analogs, all of which are focused on isolated behavioral end-points thought to be relevant to the persistence of compulsive drug use. Thus, the greatest obstacle to successful development is the gap between the multiplicity of preclinical and early clinical end-points and the regulatory end-point of sustained abstinence. This review proposes two pathways to bridging this gap: further development and validation of the preclinical extended access self-administration model; inclusion of secondary end-points comprising all of the measures highlighted in the present discussion in Phase 3 trials. Further, completion of the postdictive validation of analogous preclinical and clinical assays is of high priority. Ultimately, demonstration of the relevance and validity of a variety of end-points to the ultimate goal of abstinence will allow researchers to identify truly relevant therapeutic mechanisms and intervention targets, and establish a framework for SUD therapeutic development that allows optimal decision-making and resource allocation.

Naltrexone attenuates amphetamine-induced locomotor sensitization in the rat

Amphetamine, and other stimulants, readily induces behavioral sensitization, an effect hypothesized to reflect neurobiological changes that may underlie certain aspects of drug addiction. Apart from the effects on the dopamine system, previous studies have also shown that amphetamine interacts with other neurotransmitters, including the endogenous opioid system. The unselective opioid receptor antagonist naltrexone (NTX) modulates amphetamine-induced effects in both laboratory animals and humans. To further examine this interaction, the aim of the present study was to investigate the effect of NTX on the expression of locomotor sensitization and conditioned locomotor response in animals previously conditioned with amphetamine. Sensitization was induced by repeated administration of amphetamine (2 mg/kg) for 10 consecutive days. After a 10-day drug-free period, the rats were administered NTX (3 mg/kg) 30 minutes prior to the administration of a challenge dose of either amphetamine (0.5 mg/kg) (test for drug-induced sensitization) or saline (test for conditioned locomotor response). NTX had no effect on acute amphetamine-induced locomotor activity or on general locomotor activity in animals without a history of amphetamine conditioning. However, animals previously conditioned with amphetamine showed a sensitized locomotor response to the amphetamine challenge following the 10-day drug-free period. This sensitized response was significantly inhibited by NTX pre-treatment. In addition, NTX pre-treatment blocked the conditioned locomotor response when the amphetamine-conditioned animals were placed in the previously amphetamine-paired context. This study showed that NTX attenuates drug- and cue-induced locomotor behavior in amphetamine-conditioned animals, supporting recent clinical findings that indicated a potential role of NTX as a treatment for amphetamine dependence.

Endogenous opiates and behavior: 2009

This paper is the 32nd consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2009 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Alteration of intravenous nicotine self-administration by opioid receptor agonist and antagonists in rats

RATIONALE

The role played by endogenous opioids in mediating the reinforcing properties of nicotine is unclear. As with preclinical studies, clinical trials with naloxone, a prototypic opioid receptor antagonist have yielded equivocal findings with regard to its efficacy in reducing cigarette smoking.

OBJECTIVE

The aim of the present study was to examine the effects of three opioids that exhibit relative selectivity at mu-, kappa- and delta-opioid receptors on nicotine self-administration in male hooded Lister rats.

METHODS

Graded doses (0.3, 1.0, and 3.0 mg/kg IP) of each opioid agonist or antagonist were tested in different groups of rats repeatedly over three consecutive nicotine intravenous nicotine-self administration (0.03 mg/kg/infusion) sessions. The same treatments were tested in parallel groups of rats trained to respond for food reinforcement.

RESULTS

Naloxone was very effective in attenuating the levels of nicotine self-administered across all doses tested. The selective kappa-opioid receptor agonist U50,488, reduced nicotine self-administration in doses of 1 and 3 mg/kg, while the 0.3 mg/kg dose produced a small increase in nicotine intake. Finally, the specific delta-opioid receptor antagonist, naltrindole did not significantly modify nicotine self-administration behaviour. In contrast, all three opioids failed to modify behaviour maintained by food reinforcement.

CONCLUSIONS

These findings suggest endogenous opioids are crucial in mediating the reinforcing effects of nicotine and that the mu-opioid receptor subtype may represent a potential target for selectively reducing nicotine-taking behaviour as part of a pharmacological approach to develop smoking cessation aids.

Tobacco, cocaine, and heroin: Craving and use during daily life

BACKGROUND

Relationships among tobacco smoking, tobacco craving, and other drug use and craving may have treatment implications in polydrug-dependent individuals.

METHODS

We conducted the first ecological momentary assessment (EMA) study to investigate how smoking is related to other drug use and craving during daily life. For up to 20 weeks, 106 methadone-maintained outpatients carried PalmPilots (PDAs). They reported their craving, mood, behaviors, environment, and cigarette-smoking status in 2 to 5 random-prompt entries/day and initiated PDA entries when they used cocaine or heroin or had a discrete episode of craving for cocaine or heroin.

RESULTS

Smoking frequency increased linearly with random-prompt ratings of tobacco craving, cocaine craving, and craving for both cocaine and heroin. Smoking frequency was greater during discrete episodes of cocaine use and craving than during random-prompt reports of low craving for cocaine. This pattern was also significant for dual cocaine and heroin use and craving. Smoking and tobacco craving were each considerably reduced during periods of urine-verified abstinence from cocaine, and there was a (nonsignificant) tendency for morning smoking to be especially reduced during those periods.

CONCLUSIONS

This EMA study confirms that smoking and tobacco craving are strongly associated with the use of and craving for cocaine and heroin. Together with prior findings, our data suggest that tobacco and cocaine may each increase craving for (and likelihood of continued use of) themselves and each other. Treatment for tobacco dependence should probably be offered concurrently with (rather than only after) initiation of treatment for other substance-use disorders.

Opioid receptors in the basolateral amygdala but not dorsal hippocampus mediate context-induced alcohol seeking

Contexts associated with the availability of alcohol can induce craving in humans and alcohol seeking in rats. The opioid antagonist naltrexone attenuates context-induced reinstatement (renewal) of alcohol seeking and suppresses neuronal activation in the basolateral amygdaloid complex and dorsal hippocampus induced by such reinstatement. The objective of this study was to determine whether pharmacological blockade of opioid receptors in the basolateral amygdala or dorsal hippocampus would attenuate the context-induced reinstatement of alcohol seeking. Rats were trained to self-administer alcohol in one context (Context A), extinguished in a distinct context (Context B) and then tested for reinstatement of alcohol seeking in A and B contexts. Prior to the test session, rats were bilaterally microinjected with 0, 333 or 1000ng (total) naloxone methiodide into the basolateral amygdala or dorsal hippocampus. Naloxone methiodide in the amygdala, but not the hippocampus, dose dependently suppressed context-induced reinstatement. This suggests that opioid transmission in the basolateral amygdaloid complex is an important mediator of context-induced alcohol seeking.

The alpha4beta2 nicotinic acetylcholine-receptor partial agonist varenicline inhibits both nicotine self-administration following repeated dosing and reinstatement of nicotine seeking in rats

INTRODUCTION

The alpha4beta2 nicotinic acetylcholine receptor partial agonist varenicline has greater efficacy than other pharmacotherapeutic aids for smoking cessation. This presents an opportunity to evaluate the predictive validity of rat models of nicotine taking and relapse. The aim of this study was to evaluate the ability of varenicline to attenuate nicotine self-administration and relapse, as modelled by the reinstatement model of nicotine relapse in rats.

MATERIALS AND METHODS

Rats were trained to respond for intravenous nicotine under a fixed ratio schedule of reinforcement. The effects of varenicline (0.3-3.0 mg/kg s.c.) on both nicotine and food self-administration and reinstatement of nicotine seeking were evaluated.

RESULTS AND DISCUSSION

Varenicline dose-dependently reduced nicotine self-administration and attenuated both nicotine prime and combined nicotine prime plus nicotine-paired cue-induced reinstatement. Varenicline had no effect on cue-induced reinstatement in the absence of a nicotine prime nor did it induce reinstatement when given alone.

CONCLUSION

The effects of varenicline on nicotine-induced reinstatement of drug-seeking are consistent with the demonstrated clinical efficacy of varenicline for smoking cessation.