Pharmacologic characterization of nicotine-induced conditioned place preference

Sex differences in nicotine intake and relapse behavior in nicotine-dependent adult wistar rats

Introduction

Tobacco use is highly addictive and the leading cause of premature mortality in the world. Long-access nicotine self-administration procedures in rats closely model human smoking behavior. However, significant gaps remain in our understanding of sex differences in the development of dependence and relapse in adult rats.

Methods

In the present study, we investigated operant responding for both nicotine and saline and the development of dependence in adult rats of both sexes. The rats had daily access to nicotine or saline for 6 h per day, 7 days per week. Dependence was assessed by evaluating precipitated and spontaneous somatic withdrawal signs, measuring locomotor activity in the small open field test, and assessing anxiety-like behavior in the large open field and elevated plus maze test. The sucrose preference test was used to determine if cessation of nicotine intake leads to anhedonia. It was also investigated if a period of forced abstinence affects nicotine-seeking behavior.

Results

This study showed that nicotine intake is higher in females than in males when given daily long access to nicotine. Daily nicotine self-administration led to more precipitated and spontaneous somatic withdrawal signs compared to saline self-administration, with no sex differences observed. In addition, cessation of nicotine intake led to a similar increase in activity in both males and females in the small open field test. However, cessation of nicotine intake did not increase anxiety-like behavior or cause anhedonia in either males or females. A time course analysis revealed that the nicotinic acetylcholine receptor antagonist mecamylamine affected nicotine intake differently in males and females, increasing intake in males and decreasing intake in females. Three weeks of forced abstinence led to an increase in nicotine and saline-seeking behavior. The rats exhibited more nicotine than saline seeking, and the females displayed more nicotine seeking than the males.

Discussion

The present findings demonstrate that females self-administer more nicotine and display more nicotine-seeking behavior than males. Furthermore, there were no sex differences in somatic withdrawal signs or activity during abstinence from nicotine. This work underscores the importance of considering sex differences across various aspects of addiction, including intake and relapse, when developing novel treatments for tobacco use disorder.

Nicotine Motivated Behavior in C. elegans

To maximize the advantages offered by Caenorhabditis elegans as a high-throughput (HTP) model for nicotine dependence studies, utilizing its well-defined neuroconnectome as a robust platform, and to unravel the genetic basis of nicotine-motivated behaviors, we established the nicotine conditioned cue preference (CCP) paradigm. Nicotine CCP enables the assessment of nicotine preference and seeking, revealing a parallel to fundamental aspects of nicotine-dependent behaviors observed in mammals. We demonstrated that nicotine-elicited cue preference in worms is mediated by nicotinic acetylcholine receptors and requires dopamine for CCP development. Subsequently, we pinpointed nAChR subunits associated with nicotine preference and validated human GWAS candidates linked to nicotine dependence involved in nAChRs. Functional validation involves assessing the loss-of-function strain of the CACNA2D3 ortholog and the knock-out (KO) strain of the CACNA2D2 ortholog, closely related to CACNA2D3 and sharing human smoking phenotypes. Our orthogonal approach substantiates the functional conservation of the α2δ subunit of the calcium channel in nicotine-motivated behavior. Nicotine CCP in C. elegans serves as a potent affirmation of the cross-species functional relevance of GWAS candidate genes involved in nicotine seeking associated with tobacco abuse, providing a streamlined yet comprehensive system for investigating intricate behavioral paradigms within a simplified and reliable framework.

Strawberry Additive Increases Nicotine Vapor Sampling and Systemic Exposure But Does Not Enhance Pavlovian-Based Nicotine Reward in Mice

Nicotine is an addictive drug whose popularity has recently increased, particularly among adolescents, because of the availability of electronic nicotine devices (i.e., "vaping") and nicotine e-liquids containing additives with rich chemosensory properties. Some efforts to understand the role of these additives in nicotine reward suggest that they increase nicotine reward and reinforcement, but the sensory contributions of additives, especially in their vapor forms, are largely untested. Here, to better understand how a fruit-flavored (i.e., strawberry) additive influences nicotine reward and aversion, we used a conditioned place preference (CPP) procedure in which nicotine and a strawberry additive were delivered as a vapor to male and female adolescent mice. We found that nicotine vapor alone can lead to a dose-dependent CPP when using a biased design. The strawberry additive did not produce CPP on its own, and we did not observe an effect of the strawberry additive on nicotine vapor-induced reward. Nevertheless, mice exposed to nicotine plus strawberry additive vapor had higher plasma cotinine concentrations, which did not appear to reflect altered nicotine metabolism. Instead, by directly measuring vapor sampling through respiration monitoring, we uncovered an increase in the amount of sniffing toward strawberry-containing nicotine vapor compared with nicotine vapor alone. Together these data indicate that chemosensory-rich e-liquid additives may enhance the perceived sensory profile of nicotine vapors rather than the reward value per se, which leads to overall increased nicotine exposure.

An inhibitory brainstem input to dopamine neurons encodes nicotine aversion

Nicotine stimulates the dopamine (DA) system, which is essential for its rewarding effect. Nicotine is also aversive at high doses; yet, our knowledge about nicotine's dose-dependent effects on DA circuits remains limited. Here, we demonstrate that high doses of nicotine, which induce aversion-related behavior in mice, cause biphasic inhibitory and excitatory responses in VTA DA neurons that can be dissociated by distinct projections to lateral and medial nucleus accumben subregions, respectively. Guided by computational modeling, we performed a pharmacological investigation to establish that inhibitory effects of aversive nicotine involve desensitization of α4β2 and activation of α7 nicotinic acetylcholine receptors. We identify α7-dependent activation of upstream GABA neurons in the laterodorsal tegmentum (LDT) as a key regulator of heterogeneous DA release following aversive nicotine. Finally, inhibition of LDT GABA terminals in VTA prevents nicotine aversion. Together, our findings provide a mechanistic circuit-level understanding of nicotine's dose-dependent effects on reward and aversion.

Stimulus functions of nicotine

Behavioral pharmacology has made vital contributions to the concepts and methods used in tobacco and other drug use research, and is largely responsible for the now generally accepted notion that nicotine is the primary component in tobacco that engenders and maintains tobacco use. One of the most important contributions of behavioral pharmacology to the science of drug use is the notion that drugs can act as environmental stimuli that control behavior in many of the same ways as other stimuli (e.g., visual, gustatory, olfactory). The purpose of this chapter is to provide an overview of research that illustrates the respondent and operant stimulus functions of nicotine, using a contemporary taxonomy of stimulus functions as a general framework. Each function is formally defined and examples from research on the behavioral pharmacology of nicotine are presented. Some of the factors that modulate each function are also discussed. The role of nicotine's stimulus functions in operant and respondent theories of tobacco use is examined and some suggestions for future research are presented. The chapter illustrates how a taxonomy of stimulus functions can guide conceptions of tobacco use and direct research and theory accordingly.

Translational Research in Nicotine Addiction

While commendable strides have been made in reducing smoking initiation and improving smoking cessation rates, current available smoking cessation treatment options are still only mildly efficacious and show substantial interindividual variability in their therapeutic responses. Therefore, the primary goal of preclinical research has been to further the understanding of the neural substrates and genetic influences involved in nicotine's effects and reassess potential drug targets. Pronounced advances have been made by investing in new translational approaches and placing more emphasis on bridging the gap between human and rodent models of dependence. Functional neuroimaging studies have identified key brain structures involved with nicotine-dependence phenotypes such as craving, impulsivity, withdrawal symptoms, and smoking cessation outcomes. Following up with these findings, rodent-modeling techniques have made it possible to dissect the neural circuits involved in these motivated behaviors and ascertain mechanisms underlying nicotine's interactive effects on brain structure and function. Likewise, translational studies investigating single-nucleotide polymorphisms (SNPs) within the cholinergic, dopaminergic, and opioid systems have found high levels of involvement of these neurotransmitter systems in regulating the reinforcing aspects of nicotine in both humans and mouse models. These findings and coordinated efforts between human and rodent studies pave the way for future work determining gene by drug interactions and tailoring treatment options to each individual smoker.

Relationship Between Nicotine Intake and Reward Function in Rats With Intermittent Short Versus Long Access to Nicotine

INTRODUCTION

Tobacco use improves mood states and smoking cessation leads to anhedonia, which contributes to relapse. Animal studies have shown that noncontingent nicotine administration enhances brain reward function and leads to dependence. However, little is known about the effects of nicotine self-administration on the state of the reward system.

METHODS

To investigate the relationship between nicotine self-administration and reward function, rats were prepared with intracranial self-stimulation electrodes and intravenous catheters. The rats were trained on the intracranial self-stimulation procedure and allowed to self-administer 0.03 mg/kg/infusion of nicotine. All rats self-administered nicotine daily for 10 days (1 hour/day) and were then switched to an intermittent short access (ShA, 1 hour/day) or long access (LgA, 23 hour/day) schedule (2 days/week, 5 weeks).

RESULTS

During the first 10 daily, 1-hour sessions, nicotine self-administration decreased the reward thresholds, which indicates that nicotine potentiates reward function. After switching to the intermittent LgA or ShA schedule, nicotine intake was lower in the ShA rats than the LgA rats. The LgA rats increased their nicotine intake over time and they gradually consumed a higher percentage of their nicotine during the light phase. The nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine induced a larger increase in reward thresholds (ie, anhedonia) in the LgA rats than the ShA rats. In the LgA rats, nAChR blockade with mecamylamine decreased nicotine intake for 2 hours and this was followed by a rebound increase in nicotine intake.

CONCLUSIONS

A brief period of nicotine self-administration enhances reward function and a high level of nicotine intake leads to dependence.

IMPLICATIONS

These animal studies indicate that there is a strong relationship between the level of nicotine intake and brain reward function. A high level of nicotine intake was more rewarding than a low level of nicotine intake and nicotine dependence was observed after long, but not short, access to nicotine. This powerful combination of nicotine reward and withdrawal makes it difficult to quit smoking. Blockade of nAChRs temporarily decreased nicotine intake, but this was followed by a large rebound increase in nicotine intake. Therefore, nAChR blockade might not decrease the use of combustible cigarettes or electronic cigarettes.

Rewarding Effects of Nicotine in Adolescent and Adult Male and Female Rats as Measured Using Intracranial Self-stimulation

INTRODUCTION

Tobacco is highly addictive, and after the development of dependence, it is difficult to quit smoking. Therefore, it is important to understand the factors that play a role in the initiation of smoking. The rewarding effects of nicotine play a role in the initiation of smoking and the goal of the present study was to determine the rewarding effects of nicotine in adolescent and adult male and female rats.

METHODS

Male and female Wistar rats were prepared with intracranial self-stimulation (ICSS) electrodes between postnatal day (P) 23 and 33. They were then trained on the ICSS procedure and the effect of nicotine (0, 0.03, 0.1, 0.3 mg/kg) on the reward thresholds and response latencies was investigated during adolescence (P40-59) or adulthood (>P75).

RESULTS

Nicotine lowered the brain reward thresholds of the adult and adolescent male and female rats. The nicotine-induced decrease in the reward thresholds was the same in the adult male and adult female rats. However, nicotine induced a greater decrease in the reward thresholds of the adolescent female rats than the adolescent male rats. Nicotine decreased the response latencies of all groups and there was no effect of age or sex.

CONCLUSIONS

Nicotine enhances reward function and psychomotor performance in adolescent and adult male and female rats. Adolescent female rats are more sensitive to the acute rewarding effects of nicotine than adolescent male rats. Therefore, the rewarding effects of nicotine might play a greater role in the initiation of smoking in adolescent females than in adolescent males.

IMPLICATIONS

The great majority of people start smoking during adolescence. The present studies suggest that during this period female rats are more sensitive to the acute rewarding effects of low and intermediate doses of nicotine than male rats. The rewarding properties of nicotine play a role in the initiation of smoking and establishing habitual smoking. Therefore, the present findings might explain why adolescent females are at a higher risk for becoming nicotine dependent than adolescent males.

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.

Discriminative stimulus effects of mecamylamine and nicotine in rhesus monkeys: Central and peripheral mechanisms

Mecamylamine is a non-competitive nicotinic acetylcholine receptor (nAChR) antagonist that has been prescribed for hypertension and as an off-label smoking cessation aid. Here, we examined pharmacological mechanisms underlying the interoceptive effects (i.e., discriminative stimulus effects) of mecamylamine (5.6 mg/kg s.c.) and compared the effects of nAChR antagonists in this discrimination assay to their capacity to block a nicotine discriminative stimulus (1.78 mg/kg s.c.) in rhesus monkeys. Central (pempidine) and peripherally restricted nAChR antagonists (pentolinium and chlorisondamine) dose-dependently substituted for the mecamylamine discriminative stimulus in the following rank order potency (pentolinium > pempidine > chlorisondamine > mecamylamine). In contrast, at equi-effective doses based on substitution for mecamylamine, only mecamylamine antagonized the discriminative stimulus effects of nicotine, i.e., pentolinium, chlorisondamine, and pempidine did not. NMDA receptor antagonists produced dose-dependent substitution for mecamylamine with the following rank order potency (MK-801 > phencyclidine > ketamine). In contrast, behaviorally active doses of smoking cessation aids including nAChR agonists (nicotine, varenicline, and cytisine), the smoking cessation aid and antidepressant bupropion, and the benzodiazepine midazolam did not substitute for the discriminative stimulus effects of mecamylamine. These data suggest that peripheral nAChRs and NMDA receptors may contribute to the interoceptive stimulus effects produced by mecamylamine. Based on the current results, the therapeutic use of mecamylamine (i.e., for smoking or to alleviate green tobacco sickness) should be weighed against the potential for mecamylamine to produce interoceptive effects that overlap with another class of abused drugs (i.e., NMDA receptor agonists).

The effects of nicotine on conditioning, extinction, and reinstatement in humans

Nicotine has been shown to enhance the reinforcement and reward-responsiveness of non-nicotine stimuli. To determine whether nicotine enhances the strength of conditioning to context, undergraduate participants with varying levels of nicotine dependence were recruited for a two-day study and tested on a virtual reality (VR) conditioned place preference (CPP) paradigm. On day one, participants explored two virtual rooms where they received multiple pairings of M&M rewards in one room and no rewards in the other room, followed by a free-access test session with no rewards. On day two, participants received three test sessions to assess extinction. Subsequently, participants received M&Ms. in a novel context and were then tested for reinstatement. Prior to testing on each day, subjects were administered either nicotine (4mg) or placebo lozenges, in a between-subjects, four-group, 2×2 design (nicotine or placebo on days 1 and 2). After conditioning on day one, only participants who received placebo exhibited a CPP by spending significantly more time in the room previously-paired with M&Ms. Contrary to our hypothesis, nicotine-treated participants did not display a significant CPP, and there were no significant differences between treatment groups. However, post hoc analysis indicated that in a subset of participants with greater nicotine dependence, the nicotine group displayed a CPP by rating the M&M-paired room as significantly more enjoyable than those who received placebo. Additionally, while neither treatment group showed significant place preferences during the first two extinction sessions on Day 2, individuals who received nicotine on Day 1 or placebo on Day 2 spent significantly more time in the M&M-paired room during the final extinction session. Finally, those who received nicotine on Day 2 exhibited significantly greater reinstatement compared to placebo-treated participants. These results partially support preclinical evidence that nicotine can affect learning, extinction, and reinstatement.

Tobacco's minor alkaloids: Effects on place conditioning and nucleus accumbens dopamine release in adult and adolescent rats

Tobacco products are some of the most commonly used psychoactive drugs worldwide. Besides nicotine, alkaloids in tobacco include cotinine, myosmine, and anatabine. Scientific investigation of these constituents and their contribution to tobacco dependence is less well developed than for nicotine. The present study evaluated the nucleus accumbens dopamine-releasing properties and rewarding and/or aversive properties of nicotine (0.2-0.8mg/kg), cotinine (0.5-5.0mg/kg), anatabine (0.5-5.0mg/kg), and myosmine (5.0-20.0mg/kg) through in vivo microdialysis and place conditioning, respectively, in adult and adolescent male rats. Nicotine increased dopamine release at both ages, and anatabine and myosmine increased dopamine release in adults, but not adolescents. The dopamine release results were not related to place conditioning, as nicotine and cotinine had no effect on place conditioning, whereas anatabine and myosmine produced aversion in both ages. While the nucleus accumbens shell is hypothesized to play a role in strengthening drug-context associations following initiation of drug use, it may have little involvement in the motivational effects of tobacco constituents once these associations have been acquired. Effects of myosmine and anatabine on dopamine release may require a fully developed dopamine system, since no effects of these tobacco alkaloids were observed during adolescence. In summary, while anatabine and myosmine-induced dopamine release in nucleus accumbens may play a role in tobacco dependence in adults, the nature of that role remains to be elucidated.

Effects of pharmacological manipulation of the kappa opioid receptors on the aversive effects of nicotine

Nicotine, an addictive component of tobacco smoke, produces both rewarding and aversive effects. Increasing the aversive effects of nicotine may help in promoting smoking cessation. However, neural targets mediating the aversive effects of nicotine have not been fully identified. In this study, we evaluated the role of kappa opioid receptors (KORs) in the aversive effects of nicotine (0.4 mg/kg, base; s.c.) using the nicotine-induced conditioned taste aversion (CTA) model in Wistar rats. The KORs were activated using the selective KOR agonist (±)U-50,488H (0, 0.03, 0.15 & 0.3mg/kg; s.c.) and inhibited using the KOR antagonist nor-binaltorphimine (nor-BNI; 0, 15 & 30mg/kg; s.c.) in separate groups of rats using a between-subjects design. Pretreatment with the KOR agonist (±)U-50,488H (0.3mg/kg) significantly increased aversion for the nicotine-associated solution. Additionally, (±)U-50,488H (0.3mg/kg) on its own induced aversion to the flavored solution associated with it even in the absence of nicotine, suggesting that the KOR agonist induced increase in nicotine-induced aversion was an additive effect. Interestingly, administration of the KOR antagonist nor-BNI (30mg/kg) prior to conditioning with nicotine/saline, but not after conditioning with nicotine/saline, attenuated nicotine-induced aversive effects compared to saline controls. Taken together, these data suggest a role for KORs in the aversive effects of nicotine.

Effects of propofol on conditioned place preference in male rats: Involvement of nitrergic system

BACKGROUND

Drug-induced conditioned place preference (CPP) is linked to the addictive properties of the drug used. The number of studies that have investigated the effects of propofol on CPP is limited. Research findings suggest that nitric oxide (NO) might play an important role in substance use disorders.

OBJECTIVES

The present study sought to investigate the role of the nitrergic system on the rewarding effects of propofol by using the CPP protocol in rats.

METHODS

The experiment followed habituation, pre-conditioning, conditioning, and post conditioning sessions. Male Wistar albino rats weighing 240-290 g were divided into eight groups: control (saline), propofol (10, 20, and 40 mg/kg), the NO synthase (NOS) inhibitor N^G-nitro-L-arginine methyl ester (L-NAME) alone (30 and 60 mg/kg), and in combination with propofol (30 and 60 mg/kg L-NAME plus 40 mg/kg propofol) (n = 8 for each group). The CPP effects of propofol, L-NAME, saline, and their combinations were evaluated. All the drug and saline administrations were performed by intraperitoneal (ip) injections.

RESULTS

Propofol (10-40 mg/kg) produced CPP that was statistically significant relative to saline. Propofol-induced CPP was significantly reversed by pretreatment with L-NAME. When administered alone, L-NAME did not produce CPP and also did not produce any significant change on locomotor activity of naïve rats.

CONCLUSION

Our results suggest that propofol produces CPP effects in rats and that NO-related mechanisms may be responsible for propofol-induced CPP. Thus, propofol might have the potential to be addictive, and this possibility should be considered during clinical applications of this drug.

Behavioral and Biochemical Impact of Chronic Unpredictable Mild Stress on the Acquisition of Nicotine Conditioned Place Preference in Rats

Addiction is a chronic psychiatric disease which represents a global problem, and stress can increase drug addiction and relapse. Taking into account frequent concomitance of nicotine dependence and stress, the purpose of the present study was to assess behavioral and biochemical effects of chronic unpredictable mild stress (CUMS) exposure on nicotine reward in rats measured in the conditioned place preference (CPP) paradigm. Rats were submitted to the CUMS for 3 weeks and conditioned with nicotine (0.175 mg/kg) for 2 or 3 days. Our results revealed that only CUMS-exposed animals exhibited the CPP after 2 days of conditioning indicating that stressed rats were more sensitive to the rewarding properties of nicotine and that chronic stress exacerbates nicotine preference. Administration of metyrapone (50 mg/kg), a glucocorticosteroid antagonist, and imipramine (15 mg/kg), an antidepressant, abolished nicotine CPP in stressed rats after 2 days of conditioning. The biochemical experiments showed increased markers of oxidative stress after nicotine conditioning for 2 and 3 days, while the CUMS further potentiated pro-oxidative effects of nicotine. Moreover, metyrapone reversed oxidative changes caused by stress and nicotine, while imipramine was not able to overwhelm nicotine- and stress-induced oxidative damages; however, it could exert antioxidant effect if administered repeatedly. The results suggest that recent exposure to a stressor may augment the rewarding effects of nicotine through anhedonia- and stress-related mechanisms. Our study contributes to the understanding of behavioral and biochemical stress-induced modification of the rewarding effects of nicotine on the basis of the development of nicotine dependence.

Both nicotine reward and withdrawal are enhanced in a rodent model of diabetes

RATIONALE

It is presently unclear whether diabetic rats experience greater rewarding effects of nicotine and/or negative affective states produced by nicotine withdrawal.

OBJECTIVE

The present study utilized a rodent model of diabetes to examine the rewarding effects of nicotine and negative affective states and physical signs produced by withdrawal.

METHODS

Separate groups of rats received systemic administration of either vehicle or streptozotocin (STZ), which destroys insulin-producing beta cells in the pancreas and elevates glucose levels. Place conditioning procedures were utilized to compare the rewarding effects of nicotine (conditioned place preference; CPP) and negative affective states produced by withdrawal (conditioned place aversion; CPA) in vehicle- and STZ-treated rats. CPA and physical signs of withdrawal were compared after administration of the nicotinic receptor antagonist mecamylamine to precipitate withdrawal in nicotine-dependent rats. A subsequent study utilized elevated plus maze (EPM) procedures to compare anxiety-like behavior produced by nicotine withdrawal in vehicle- and STZ-treated rats.

RESULTS

STZ-treated rats displayed greater rewarding effects of nicotine and a larger magnitude of aversive effects and physical signs produced by withdrawal as compared to vehicle-treated controls. STZ-treated rats also displayed higher levels of anxiety-like behavior on the EPM during nicotine withdrawal as compared to controls.

CONCLUSION

The finding that both nicotine reward and withdrawal are enhanced in a rodent model of diabetes implies that the strong behavioral effects of nicotine promote tobacco use in persons with metabolic disorders, such as diabetes.

Effects of drugs of abuse on hippocampal plasticity and hippocampus-dependent learning and memory: contributions to development and maintenance of addiction

It has long been hypothesized that conditioning mechanisms play major roles in addiction. Specifically, the associations between rewarding properties of drugs of abuse and the drug context can contribute to future use and facilitate the transition from initial drug use into drug dependency. On the other hand, the self-medication hypothesis of drug abuse suggests that negative consequences of drug withdrawal result in relapse to drug use as an attempt to alleviate the negative symptoms. In this review, we explored these hypotheses and the involvement of the hippocampus in the development and maintenance of addiction to widely abused drugs such as cocaine, amphetamine, nicotine, alcohol, opiates, and cannabis. Studies suggest that initial exposure to stimulants (i.e., cocaine, nicotine, and amphetamine) and alcohol may enhance hippocampal function and, therefore, the formation of augmented drug-context associations that contribute to the development of addiction. In line with the self-medication hypothesis, withdrawal from stimulants, ethanol, and cannabis results in hippocampus-dependent learning and memory deficits, which suggest that an attempt to alleviate these deficits may contribute to relapse to drug use and maintenance of addiction. Interestingly, opiate withdrawal leads to enhancement of hippocampus-dependent learning and memory. Given that a conditioned aversion to drug context develops during opiate withdrawal, the cognitive enhancement in this case may result in the formation of an augmented association between withdrawal-induced aversion and withdrawal context. Therefore, individuals with opiate addiction may return to opiate use to avoid aversive symptoms triggered by the withdrawal context. Overall, the systematic examination of the role of the hippocampus in drug addiction may help to formulate a better understanding of addiction and underlying neural substrates.

Consumption of Substances of Abuse during Pregnancy Increases Consumption in Offspring: Possible Underlying Mechanisms

Correlative human observational studies on substances of abuse have been highly dependent on the use of rodent models to determine the neuronal and molecular mechanisms that control behavioral outcomes. This is particularly true for gestational exposure to non-illicit substances of abuse, such as excessive dietary fat, ethanol, and nicotine, which are commonly consumed in our society. Exposure to these substances during the prenatal period has been shown in offspring to increase their intake of these substances, induce other behavioral changes, and affect neurochemical systems in several brain areas that are known to control behavior. More importantly, emerging studies are linking the function of the immune system to these neurochemicals and ingestion of these abused substances. This review article will summarize the prenatal rodent models used to study developmental changes in offspring caused by prenatal exposure to dietary fat, ethanol, or nicotine. We will discuss the various techniques used for the administration of these substances into rodents and summarize the published outcomes induced by prenatal exposure to these substances. Finally, this review will cover some of the recent evidence for the role of immune factors in causing these behavioral and neuronal changes.

The discriminative stimulus effects of mecamylamine in nicotine-treated and untreated rhesus monkeys

The extent to which chronic nicotine treatment can alter the effects of the nicotinic acetylcholine receptor antagonist mecamylamine, and whether those effects can be attenuated by nicotine have not been clearly established in the literature. Here, the discriminative stimulus effects of mecamylamine were compared between one group of rhesus monkeys receiving a continuous infusion of nicotine base (5.6 mg/kg/day subcutaneously) and another group of monkeys not receiving nicotine treatment. Both groups responded under a fixed ratio 5 schedule of stimulus-shock termination. Stimulus control was obtained at doses of 1.78 mg/kg mecamylamine in monkeys receiving continuous nicotine and 5.6 mg/kg mecamylamine in monkeys not receiving continuous nicotine treatment. Nicotine did not attenuate the discriminative stimulus effects of mecamylamine in either group. Discontinuation of continuous nicotine produced responding on the mecamylamine lever within 24 h in some but not all monkeys. This may indicate a qualitative difference in the discriminative stimulus effects of mecamylamine between groups, perhaps reflecting antagonism of nicotine and nicotine withdrawal in monkeys receiving continuous nicotine. The failure of nicotine to reverse the effects of mecamylamine is consistent with a noncompetitive interaction at nicotinic acetylcholine receptors and indicates that mecamylamine-induced withdrawal cannot be readily modified by nicotine.

Basolateral amygdala CB1 cannabinoid receptors mediate nicotine-induced place preference

In the present study, the effects of bilateral microinjections of cannabinoid CB1 receptor agonist and antagonist into the basolateral amygdala (intra-BLA) on nicotine-induced place preference were examined in rats. A conditioned place preference (CPP) apparatus was used for the assessment of rewarding effects of the drugs in adult male Wistar rats. Subcutaneous (s.c.) administration of nicotine (0.2mg/kg) induced a significant CPP, without any effect on the locomotor activity during the testing phase. Intra-BLA microinjection of a non-selective cannabinoid CB1/CB2 receptor agonist, WIN 55,212-2 (0.1-0.5 μg/rat) with an ineffective dose of nicotine (0.1mg/kg, s.c.) induced a significant place preference. On the other hand, intra-BLA administration of AM251 (20-60 ng/rat), a selective cannabinoid CB1 receptor antagonist inhibited the acquisition of nicotine-induced place preference. It should be considered that the microinjection of the same doses of WIN 55,212-2 or AM251 into the BLA, by itself had no effect on the CPP score. The administration of a higher dose of AM251 (60 ng/rat) during the acquisition decreased the locomotor activity of animals on the testing phase. Interestingly, the microinjection of AM251 (20 and 40 ng/rat), but not WIN55,212-2 (0.1-0.5 μg/rat), into the BLA inhibited the expression of nicotine-induced place preference without any effect on the locomotor activity. Taken together, these findings support the possible role of endogenous cannabinoid system of the BLA in the acquisition and the expression of nicotine-induced place preference. Furthermore, it seems that there is a functional interaction between the BLA cannabinoid receptors and nicotine in producing the rewarding effects.

Varenicline and cytisine diminish the dysphoric-like state associated with spontaneous nicotine withdrawal in rats

Tobacco addiction is characterized by a negative mood state upon smoking cessation and relapse after periods of abstinence. Clinical studies indicate that negative mood states lead to craving and relapse. The partial α4/α6/β2* nicotinic acetylcholine receptor (nAChR) agonists varenicline and cytisine are widely used as smoking cessation treatments. Varenicline has been approved in the United States for smoking cessation and cytisine is used in Eastern European countries. Despite the widespread use of these compounds, very little is known about their effects on mood states. These studies investigated the effects of varenicline, cytisine, and the cytisine-derivative 3-(pyridin-3'-yl)-cytisine (3-pyr-Cyt) on brain reward function in nicotine-naive and nicotine-withdrawing rats. The cytisine-derivative 3-pyr-Cyt is a very weak α4β2* nAChR partial agonist and like cytisine and varenicline has antidepressant-like effects in animal models. The intracranial self-stimulation (ICSS) procedure was used to investigate the effects of these compounds on brain reward function. Elevations in ICSS thresholds reflect a dysphoric state and a lowering of thresholds is indicative of a potentiation of brain reward function. It was shown that acute administration of nicotine and varenicline lowered ICSS thresholds. Acute administration of cytisine or 3-pyr-Cyt did not affect ICSS thresholds. Discontinuation of chronic, 14 days, nicotine administration led to elevations in ICSS thresholds that lasted for about 2 days. Varenicline and cytisine, but not 3-pyr-Cyt, diminished the nicotine withdrawal-induced elevations in ICSS thresholds. In conclusion, these studies indicate that varenicline and cytisine diminish the dysphoric-like state associated with nicotine withdrawal and may thereby prevent relapse to smoking in humans.

Chronic nicotine attenuates phencyclidine-induced impulsivity in a mouse serial reaction time task

Schizophrenia is a disorder characterized by positive, negative, and cognitive symptoms. While positive symptoms can be effectively treated with typical antipsychotic medication, which generally affects the dopaminergic system, negative and cognitive symptoms, including attentional deficits and impulsive behavior, are less sensitive to standard treatments. It has further been well documented that schizophrenic patients use tobacco products at a rate much higher than the general population, and this persists despite treatment. It has been argued this behavior may be a form of self-medication, to alleviate some symptoms of schizophrenia. It has further been posited that prefrontal glutamatergic hypofunction may underlie some aspects of schizophrenia, and in accordance with this model, systemic phencyclidine has been used to model the disease. We employed a modified 5-choice serial reaction time test, a paradigm that is often used to investigate many of the treatment-resistant symptoms of schizophrenia including impulsivity, selective attention, and sustained attention/cognitive vigilance, to determine the medicinal effects of nicotine. We demonstrate that chronic oral, but not acute injections of nicotine can selectively attenuate phencyclidine-induced increases in impulsivity without affecting other measures of attention. This suggests that nicotine use by schizophrenics may provide some relief of distinct symptoms that involve impulsive behaviors.

Potential therapeutic uses of mecamylamine and its stereoisomers

Mecamylamine (3-methylaminoisocamphane hydrochloride) is a nicotinic parasympathetic ganglionic blocker, originally utilized as a therapeutic agent to treat hypertension. Mecamylamine administration produces several deleterious side effects at therapeutically relevant doses. As such, mecamylamine's use as an antihypertensive agent was phased out, except in severe hypertension. Mecamylamine easily traverses the blood-brain barrier to reach the central nervous system (CNS), where it acts as a nicotinic acetylcholine receptor (nAChR) antagonist, inhibiting all known nAChR subtypes. Since nAChRs play a major role in numerous physiological and pathological processes, it is not surprising that mecamylamine has been evaluated for its potential therapeutic effects in a wide variety of CNS disorders, including addiction. Importantly, mecamylamine produces its therapeutic effects on the CNS at doses 3-fold lower than those used to treat hypertension, which diminishes the probability of peripheral side effects. This review focuses on the pharmacological properties of mecamylamine, the differential effects of its stereoisomers, S(+)- and R(-)-mecamylamine, and the potential for effectiveness in treating CNS disorders, including nicotine and alcohol addiction, mood disorders, cognitive impairment and attention deficit hyperactivity disorder.

Animal models of nicotine exposure: relevance to second-hand smoking, electronic cigarette use, and compulsive smoking

Much evidence indicates that individuals use tobacco primarily to experience the psychopharmacological properties of nicotine and that a large proportion of smokers eventually become dependent on nicotine. In humans, nicotine acutely produces positive reinforcing effects, including mild euphoria, whereas a nicotine abstinence syndrome with both somatic and affective components is observed after chronic nicotine exposure. Animal models of nicotine self-administration and chronic exposure to nicotine have been critical in unveiling the neurobiological substrates that mediate the acute reinforcing effects of nicotine and emergence of a withdrawal syndrome during abstinence. However, important aspects of the transition from nicotine abuse to nicotine dependence, such as the emergence of increased motivation and compulsive nicotine intake following repeated exposure to the drug, have only recently begun to be modeled in animals. Thus, the neurobiological mechanisms that are involved in these important aspects of nicotine addiction remain largely unknown. In this review, we describe the different animal models available to date and discuss recent advances in animal models of nicotine exposure and nicotine dependence. This review demonstrates that novel animal models of nicotine vapor exposure and escalation of nicotine intake provide a unique opportunity to investigate the neurobiological effects of second-hand nicotine exposure, electronic cigarette use, and the mechanisms that underlie the transition from nicotine use to compulsive nicotine intake.

The "stop" and "go" of nicotine dependence: role of GABA and glutamate

Nicotine plays an important role in the initiation and maintenance of tobacco smoking. Importantly, chronic nicotine exposure alters the function of brain reward systems, resulting in the development of a nicotine-dependent state. This nicotine-dependent state is associated with aversive affective and somatic signs upon abstinence from smoking, often leading to relapse in abstinent smokers. This article reviews the role of the major excitatory and inhibitory neurotransmitters glutamate and γ-aminobutyric acid (GABA), respectively, in both the reinforcing effects of nicotine and development of nicotine dependence. Evidence suggests that blockade of glutamatergic neurotransmission attenuates both nicotine intake and nicotine seeking. In contrast, both nicotine intake and nicotine seeking are attenuated when GABA neurotransmission is facilitated. In conclusion, medications that either attenuate/negatively modulate glutamatergic neurotransmission or facilitate/positively modulate GABA neurotransmission may be useful for promoting smoking cessation in humans.

A role for matrix metalloproteinases in nicotine-induced conditioned place preference and relapse in adolescent female rats

Reconfiguration of extracellular matrix proteins appears to be necessary for the synaptic plasticity that underlies memory consolidation. The primary candidates involved in controlling this process are a family of endopeptidases called matrix metalloproteinases (MMPs); however, the potential role of MMPs in nicotine addiction-related memories has not been adequately tested. Present results indicate transient changes in hippocampal MMP-2, -3, and -9 expression following context dependent learning of nicotine-induced conditioned place preference (CPP). Members of a CPP procedural control group also indicated similar MMP changes, suggesting that memory activation occurred in these animals as well. However, hippocampal MMP-9 expression was differentially elevated in members of the nicotine-induced CPP group on days 4 and 5 of training. Inhibition of MMPs using a broad spectrum MMP inhibitor (FN439) during nicotine-induced CPP training blocked the acquisition of CPP. Elevations in hippocampal and prefrontal cortex MMP-3 expression-but not MMP-2 and -9-accompanied reactivation of a previously learned drug related memory. Decreases in the actin regulatory cytoskeletal protein cortactin were measured in the HIP and PFC during the initial two days of acquisition of CPP; however, no changes were seen following re-exposure to the drug related environment. These results suggest that MMP-9 may be involved in facilitating the intracellular and extracellular events required for the synaptic plasticity underlying the acquisition of nicotine-induced CPP. Furthermore, MMP-3 appears to be important during re-exposure to the drug associated environment. However, rats introduced into the CPP apparatus and given injections of vehicle rather than nicotine during training also revealed a pattern of MMP expression similar to nicotine-induced CPP animals.

Effects of the specific α4β2 nAChR antagonist, 2-fluoro-3-(4-nitrophenyl) deschloroepibatidine, on nicotine reward-related behaviors in rats and mice

RATIONALE

Alleviating addiction to tobacco products could prevent millions of deaths. Investigating novel compounds selectively targeting α4β2 nAChRs hypothesized to have a key role in the rewarding effects of nicotine may be a useful approach for future treatment.

OBJECTIVES

The present study was designed to evaluate 2-fluoro-3-(4-nitrophenyl) deschloroepibatidine (4-nitro-PFEB), a potent competitive antagonist of neuronal α4β2 nAChRs, in several animal models related to nicotine reward: drug discrimination, intracranial self-stimulation (ICSS), conditioned place preference, and limited access to self-administration.

METHODS

Long Evans rats were trained in a two-lever discrimination procedure to discriminate 0.4 mg/kg nicotine (s.c.) from saline. Male Sprague-Dawley rats were stereotaxically implanted with electrodes and trained to respond for direct electrical stimulation of the medial forebrain bundle. ICR mice were evaluated using an unbiased place preference paradigm, and finally, male Wistar rats were implanted with intrajugular catheters and tested for nicotine self-administration under limited access (1 h/day).

RESULTS

4-Nitro-PFEB attenuated the discriminative stimulus effects of nicotine, but alone did not produce nicotine-like discriminative stimulus effects. Nicotine-induced facilitation of ICSS reward thresholds was reversed by 4-nitro-PFEB, which alone had no effect on thresholds. 4-Nitro-PFEB also blocked the conditioned place preference produced by nicotine, but alone had no effect on conditioned place preference. Finally, 4-nitro-PFEB dose-dependently decreased nicotine self-administration.

CONCLUSIONS

These results support the hypothesis that neuronal α4β2 nAChRs play a key role in mediating the rewarding effects of nicotine and further suggest that targeting α4β2 nAChRs may yield a potential candidate for the treatment of nicotine dependence.

Affective taste responses in the presence of reward- and aversion-conditioned stimuli and their relationship to psychomotor sensitization and place conditioning

Anecdotal experience and empirical evidence suggest animals approach or avoid conditioned stimuli based on the ability of those stimuli to elicit affective responses or interfere with affective assessments of ongoing stimuli. Thus, this study investigated the relationship between the ability of drug-conditioned environments to induce conditioned place preference or aversion and their ability to influence palatability responses to sucrose and quinine in those same environments. Mice were conditioned to methamphetamine (2mg/kg), morphine (10mg/kg) or naloxone (10mg/kg). Following testing for the expression of place conditioning, palatability responses to sucrose and quinine in the conditioned contexts were assessed. In general, virtually no effects of exposure to drug-conditioned contexts on overall positive or aversive palatability responses were observed. However, in naloxone-conditioned mice, the strength of conditioned place aversion to the naloxone-paired context correlated with aversive taste reactivity responses to quinine in that context. In morphine-conditioned mice, positive reactions to sucrose in the morphine-paired context negatively correlated with positive reactions to sucrose in the vehicle-paired context. Interestingly, the rate of methamphetamine-induced behavioral sensitization during conditioning and positive taste responses to sucrose in the methamphetamine-paired context positively correlated. These studies suggest that conditioned stimuli interact with or modulate the affective experience of ongoing unconditioned stimuli such as tastants, and these may reflect behavioral processes that guide behavior optimally.

CREB involvement in the regulation of striatal prodynorphin by nicotine

RATIONALE

The transcription factor cAMP response element binding (CREB) protein plays a pivotal role in drug-dependent neuronal plasticity. CREB phosphorylation at Ser133 is enhanced by drugs of abuse, including nicotine. Dynorphin (Dyn) contributes to the addictive process and its precursor gene prodynorphin (PD) is regulated by CREB. PD mRNA and Dyn synthesis were enhanced in the striatum following acute nicotine, suggesting genomic regulation.

OBJECTIVE

These studies investigated PD transcription in mice acutely treated with nicotine, determined the role of CREB, and characterized the receptors involved.

RESULTS

Acute nicotine increased adenylyl cyclase activity, cAMP, and pCREB Ser133 levels in striatum and enhanced CREB binding to CRE elements (DynCREs) of the PD promoter, preferentially DynCRE3. DynCRE3 binding was dose dependent with 1 mg of nicotine giving a maximal response. Additionally, DynCRE binding was time dependent, rising by 15 min, reaching a maximum at 1 h, and returning to control by 3 h, a temporal pattern similar to that of cAMP and pCREB. Supershift experiments showed that CREB and pCREB Ser133 were the major contributors to DynCRE3 binding complex. The nAChR antagonist mecamylamine and the dopamine D1-like receptor antagonist SCH 23390 prevented the nicotine-induced increase of pCREB and nuclear protein binding to DynCRE3.

CONCLUSIONS

Our findings suggest that nicotine regulates PD expression in striatum at the transcriptional level and CREB is involved. Dopamine D1 receptor stimulation by nAChR-released dopamine appears to be an underlying mechanism. Altered Dyn synthesis might be relevant for the behavioral actions of nicotine and especially its aversive properties.

Varenicline dose dependently enhances responding for nonpharmacological reinforcers and attenuates the reinforcement-enhancing effects of nicotine

INTRODUCTION

Varenicline (VAR), a partial nicotinic agonist, is one of the most effective smoking cessation pharmacotherapies. The therapeutic efficacy of VAR could be partly the result of substituting for and/or blocking the reinforcement-enhancing effects of nicotine (NIC). We assessed the effects of VAR alone and in combination with NIC (0.4 mg/kg) while rats pressed the lever for a moderately reinforcing visual stimulus (VS).

METHODS

Rats were injected with placebo (0.9% saline), NIC, VAR (0.1-1 mg/kg), or NIC + VAR. A follow-up study was conducted with a broader dose range of VAR-alone dosages (0.01-3.0 mg/kg). All drug manipulations were conducted in a between-subjects design to prevent confounding effects of repeated exposure.

RESULTS

There was a dose-dependent effect of VAR alone. Moderate doses of VAR (0.1 and 1.0 mg/kg) increased the number of VS presentations earned, while lower and higher VAR doses (0.01 and 3.0 mg/kg) did not change responding for the VS. VAR dose dependently attenuated the reinforcement-enhancing effects of NIC, with the highest dose (1.0 mg/kg) exhibiting the greatest antagonist effect.

CONCLUSIONS

The results of these studies support the assertion that the therapeutic efficacy of VAR may be due to the partial agonist characteristics of the drug, specifically, its ability to partially replace the reinforcement-enhancing effects of NIC as well as antagonize these effects.

The dynorphin/κ-opioid receptor system and its role in psychiatric disorders

The dynorphin/κ-opioid receptor system has been implicated in the pathogenesis and pathophysiology of several psychiatric disorders. In the present review, we present evidence indicating a key role for this system in modulating neurotransmission in brain circuits that subserve mood, motivation, and cognitive function. We overview the pharmacology, signaling, post-translational, post-transcriptional, transcriptional, epigenetic and cis regulation of the dynorphin/κ-opioid receptor system, and critically review functional neuroanatomical, neurochemical, and pharmacological evidence, suggesting that alterations in this system may contribute to affective disorders, drug addiction, and schizophrenia. We also overview the dynorphin/κ-opioid receptor system in the genetics of psychiatric disorders and discuss implications of the reviewed material for therapeutics development.

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.

Systemic ghrelin and reward: effect of cholinergic blockade

AIMS

Ghrelin is one of the most potent orexigens known to date. Recent data suggested that ghrelin is involved in reward-mediated processes such as the rewarding value of food. Whereas the neuronal pathways by which ghrelin regulates energy balance are well described, those involved in ghrelin-induced reward are still confusing. Therefore, we attempted to delineate the involvement of physiological and pharmacological rises in plasma ghrelin in the modulation of food reward seeking behaviours, using the classical conditioned place preference (CPP) procedure in C57BL6J mice, as well as in mice lacking the ghrelin receptor (GHSR1a -/-). We also determined whether these effects on reward-related behaviours could be partly mediated by cholinergic pathways by pre-treating mice with mecamylamine.

RESULTS

Upon moderate caloric restriction, systemic ghrelin levels increased from 108 ± 21 to 148 ± 39 pg/ml in C57BL6J mice and from 111 ± 24 to 179 ± 41 pg/ml in GHSR1a-null mice. Short exposure to rewarding food elicited a strong CPP and stimulation of locomotor activity in GHSR1a wild-type and C57BL6J mice. Conversely, the GHSR1a -/- mice did not exhibit such a food CPP, despite a negative energy balance. Pharmacological rise in systemic ghrelin further increased the time spent in the food-paired side with a higher CPP score (+71%) and this effect was blunted after cholinergic blockade by mecamylamine.

CONCLUSIONS

The ghrelin receptor is obligatory to acquire a food-CPP. The level of plasma ghrelin during conditioning determines the strength of food-induced reward seeking behaviours. The cholinergic pathway partly mediates the further enhancement of food reward induced by pharmacological rises in plasma ghrelin, but not that induced by physiological increases in ghrelin.

Nicotine-induced conditioned place preference in rats: sex differences and the role of mGluR5 receptors

To elucidate sex differences in nicotine addiction and the underlying mechanisms of the conditioning aspects of nicotine, nicotine-induced conditioned place preference (CPP) was evaluated in male and female Sprague Dawley rats using a three-chambered CPP apparatus and a biased design. In a series of experiments, the dose-response curve was obtained, pairings between the drug and initially non-preferred versus preferred compartments were compared, and the involvement of mGluR5 receptors in nicotine-induced CPP was evaluated. Modulation of nicotine-induced CPP with mGluR5 inhibition was obtained by MPEP (2-methyl-6-(phenylethynyl)-pyridine hydrochloride). Our results show that nicotine induces CPP dose-dependently in male rats but not in female rats. The comparison of the biased protocol, pairing nicotine with the initially preferred and non-preferred chambers, indicated that nicotine-induced CPP in male rats under both conditions, but the effect was stronger when nicotine was paired with the initially non-preferred side. The selective mGluR5 antagonist MPEP inhibited nicotine-induced CPP in male rats. In conclusion, the results of the current study in rats demonstrate that the conditioning effect of nicotine is more important in males than in females. Furthermore, in line with reported findings, our results suggest that mGluR5 antagonism may be therapeutically useful in smoking cessation during the maintenance of smoking behavior when conditioning plays an important role, notwithstanding the fact that this effect is observed only in male rats, not in females.

Electrolytic lesions of the nucleus accumbens enhance locomotor sensitization to nicotine in rats

Electrolytic lesions of the medial core of the nucleus accumbens (NAc) in male Long-Evans rats increased spontaneous locomotion, enhanced the locomotor stimulating effect of acute 5.0 mg/kg cocaine, enhanced the development and subsequent expression of locomotor sensitization produced by repeated injections of 0.4 mg/kg nicotine but not 7.5 mg/kg cocaine, and enhanced the expression of conditioned locomotion. Given that 6-hydroxydopamine lesions of the NAc typically have effects on locomotor-related processes that are opposite of those produced by electrolytic and excitotoxic lesions, these data are consistent with a hypothesis that the NAc output, especially from the core, inhibits a variety of such processes and that the DA input to the NAc enhances these processes by inhibiting this inhibitory output.

Measurement of affective state during chronic nicotine treatment and withdrawal by affective taste reactivity in mice: the role of endocannabinoids

Despite tobacco being highly addictive, it is unclear if nicotine has significant affective properties. To address this, we studied taste reactions to gustatory stimuli, palatable sucrose and unpalatable quinine, which are believed to reflect ongoing affective state. Taste reactivity was assessed during chronic nicotine administration and spontaneous withdrawal and the role of the endogenous cannabinoids was also investigated. C57BL6J mice were implanted with intraoral fistula to allow passive administration of solutions. In the first study, taste reactivity was tracked throughout chronic vehicle or nicotine (12 mg/kg/day) infusion via osmotic minipumps and spontaneous withdrawal following removal of minipumps. In the second study, the endocannabinoid CB1-receptor antagonist AM251 (1, 3 and 10mg/kg, intraperitoneal) or vehicle was acutely administered before taste reactivity measurement during chronic nicotine administration. Chronic nicotine treatment and spontaneous withdrawal did not influence taste reactions to sucrose or quinine. AM251 decreased positive reactions to sucrose and increased negative reactions to quinine. The effects of AM251 were respectively attenuated and enhanced in nicotine infused mice. These results suggest chronic nicotine exposure and withdrawal has no apparent affective sequelae, as probed by taste reactivity, and thus may not explain the difficulty tobacco-users have in achieving abstinence. In contrast, endocannabinoids elevate affective state in drug-naïve animals and changes in endogenous endocannabinoid tone may underlie compensations in affective state during chronic nicotine exposure.

Synergistic interaction between nicotine and social rewards in adolescent male rats

RATIONALE

Smoking typically begins during adolescence and is largely reinforced by social cues. During adolescence in rats, sensitivity to both social cues and drugs of abuse is enhanced.

OBJECTIVES

We have previously demonstrated in adolescent male rats that a low dose of cocaine interacts with social reward to produce an enhanced conditioned place preference (CPP) relative to either reward given alone. The present study further examined the nature of drug-social reward interactions using nicotine.

METHODS

Dose-effect functions for nicotine-CPP were established using two different routes of administration (vehicle, 0.1, 0.3, and 0.6 mg/kg, SC and vehicle, 0.01, 0.03, and 0.06 mg/kg, IV). The effects of nicotine on social reward-CPP and social play behavior were next examined using parameters presumed to be sub-threshold for establishing social reward- and nicotine-CPP.

RESULTS

Dose-dependent nicotine-CPP was observed using both routes of administration. Two pairings of the initially non-preferred side of the apparatus with either SC nicotine or another adolescent rat failed to produce CPP when examined alone, but together produced a robust CPP despite nicotine reducing social play. This interaction effect was not observed with the IV nicotine. A final experiment demonstrated that the enhancement of CPP with the combination of rewards was not due to additive effects of weak, sub-threshold conditioning.

CONCLUSIONS

These findings suggest that nicotine and social rewards interact synergistically in adolescent rats resulting in a greater, perhaps qualitatively different, reward than either reward given alone. Understanding drug-social reward interactions may provide new directions for development of preventions and interventions of adolescent smoking.

Mechanism-based medication development for the treatment of nicotine dependence

Tobacco use is a global problem with serious health consequences. Though some treatment options exist, there remains a great need for new effective pharmacotherapies to aid smokers in maintaining long-term abstinence. In the present article, we first discuss the neural mechanisms underlying nicotine reward, and then review various mechanism-based pharmacological agents for the treatment of nicotine dependence. An oversimplified hypothesis of addiction to tobacco is that nicotine is the major addictive component of tobacco. Nicotine binds to alpha4beta2 and alpha7 nicotinic acetylcholine receptors (nAChRs) located on dopaminergic, glutamatergic and GABAergic neurons in the mesolimbic dopamine (DA) system, which causes an increase in extracellular DA in the nucleus accumbens (NAc). That increase in DA reinforces tobacco use, particularly during the acquisition phase. Enhanced glutamate transmission to DA neurons in the ventral tegmental area appears to play an important role in this process. In addition, chronic nicotine treatment increases endocannabinoid levels in the mesolimbic DA system, which indirectly modulates NAc DA release and nicotine reward. Accordingly, pharmacological agents that target brain acetylcholine, DA, glutamate, GABA, or endocannabonoid signaling systems have been proposed to interrupt nicotine action. Furthermore, pharmacokinetic strategies that alter plasma nicotine availability, metabolism and clearance also significantly alter nicotine's action in the brain. Progress using these pharmacodynamic and pharmacokinetic agents is reviewed. For drugs in each category, we discuss the mechanistic rationale for their potential anti-nicotine efficacy, major findings in preclinical and clinical studies, and future research directions.

Varenicline attenuates nicotine-enhanced brain-stimulation reward by activation of alpha4beta2 nicotinic receptors in rats

Varenicline, a partial alpha4beta2 and full alpha7 nicotinic receptor agonist, has been shown to inhibit nicotine self-administration and nicotine-induced increases in extracellular dopamine in the nucleus accumbens. In the present study, we investigated whether varenicline inhibits nicotine-enhanced electrical brain-stimulation reward (BSR), and if so, which receptor subtypes are involved. Systemic administration of nicotine (0.25-1.0 mg/kg, i.p.) or varenicline (0.03-3 mg/kg, i.p.) produced biphasic effects, with low doses producing enhancement (e.g., decreased BSR threshold), and high doses inhibiting BSR. Pretreatment with low dose (0.03-1.0 mg/kg) varenicline dose-dependently attenuated nicotine (0.25 or 0.5 mg/kg)-enhanced BSR. The BSR-enhancing effect produced by varenicline was blocked by mecamylamine (a high affinity nicotinic receptor antagonist) or dihydro-beta-erythroidine (a relatively selective nicotinic alpha4-containing receptor antagonist), but not methyllycaconitine (a selective alpha7 receptor antagonist), suggesting an effect mediated by activation of alpha4beta2 receptors. This suggestion is supported by findings that the alpha4beta2 receptor agonist SIB-1765F produced a dose-dependent enhancement of BSR, while pretreatment with SIB-1765F attenuated nicotine (0.5 mg/kg)-enhanced BSR. In contrast, the selective alpha7 receptor agonist ARR-17779, altered neither BSR itself nor nicotine-enhanced BSR, at any dose tested. These findings suggest that: 1) varenicline inhibits nicotine-enhanced BSR, supporting its use as a smoking cessation aid; and 2) varenicline-enhanced BSR by itself and varenicline's anti-nicotine effects are mediated by activation of alpha4beta2, but not alpha7, receptors.

Effects of mecamylamine on nicotine-induced conditioned hyperactivity and sensitization in differentially reared rats

Rats reared in an enriched condition (EC) display less sensitization to nicotine than rats reared in an impoverished condition (IC). However, it is unknown what effect differential rearing has on nicotine-induced conditioned hyperactivity. The present study determined whether differential rearing affects nicotine-induced conditioned hyperactivity. This study also examined the effects of mecamylamine on conditioned hyperactivity and sensitization. EC, IC, and social condition (SC) rats were reared from 21 to 51 days of age before receiving repeated nicotine injections (.4 mg/kg) prior to 1-h locomotor sessions. Following the conditioned-hyperactivity test, rats received additional training sessions followed by a drug-free rest period before the sensitization test. Mecamylamine (1.0 mg/kg) was administered prior to the conditioned-hyperactivity test and sensitization test. Nicotine treatment resulted in sensitization and conditioned hyperactivity in all differential rearing groups. EC rats displayed less locomotor activity in response to nicotine than both IC and SC rats. Pretreatment with mecamylamine blocked the expression of conditioned hyperactivity only in EC and SC rats and attenuated sensitization in all three rearing groups. These findings suggest that environmental enrichment may alter nicotinic acetylcholine receptors during development and may be a protective factor in the initiation and relapse of smoking behavior.

Exposure to acute restraint stress reinstates nicotine-induced place preference in rats

Tobacco addiction is associated with high rates of relapse to drug use even after prolonged periods of abstinence. Relapse can occur upon reexposure to the drug of abuse, exposure to stress or to stimuli associated with drug consumption. The reinstatement of conditioning place preference (CPP) provides a simple and easy approach to investigate the mechanisms for drug relapse. We evaluated whether exposure to restraint stress could reinstate nicotine-induced CPP 1 or 15 days after its extinction. Nicotine produced place preference to the compartment paired with its injections during conditioning (0.16 mg/kg, subcutaneous; four drug sessions). Once established, nicotine CPP was extinguished by alternate exposure to each compartment after a saline injection (four exposures to each compartment). After this extinction phase, the reinstatement of place conditioning was investigated. For this purpose, rats were exposed to 30-min restraint stress 1 or 15 days after the extinction test, then immediately tested for reinstatement of CPP. Our results show that exposure to restraint stress reinstated CPP 1 and 15 days after extinction. Our study indicates for the first time that the vulnerability to stress-induced reinstatement of nicotine CPP is long-lasting, corroborating clinical studies showing that stress is positively associated with relapse to tobacco use even after a long period of nicotine withdrawal.

Nicotine enhances context learning but not context-shock associative learning

Nicotine has been found to enhance learning in a variety of tasks, including contextual fear conditioning. During contextual fear conditioning animals have to learn the context and associate the context with an unconditioned stimulus (footshock). As both of these types of learning co-occur during fear conditioning, it is not clear whether nicotine enhances one or both of these types of learning. To tease these two forms of learning apart, the authors made use of the context preexposure facilitation effect (CPFE). Acquisition of the CPFE requires that contextual and context-shock learning occurs on separate days, allowing for their individual manipulation. Nicotine (0.09 mg/kg) administered prior to contextual learning and retrieval enhanced the CPFE whereas administration prior to context-shock learning and retrieval had no effect. Thus, nicotine enhances contextual learning but not context-shock associative learning. Finally, the results are discussed in terms of a theory of how nicotine could alter hippocampal-cortical-amygdala interactions to facilitate contextual learning.

Neuropeptide S inhibits the acquisition and the expression of conditioned place preference to morphine in mice

Neuropeptide S (NPS), a recently identified bioactive peptide, was reported to regulate arousal, anxiety, motoring and feeding behaviors. NPS precursor and NPS receptor mRNA were found in the amygdala, the ventral tegmental area (VTA) and the substantia nigra, the area thought to modulate rewarding properties of drugs. In the present study, we examined the influence of NPS on the rewarding action of morphine, using the unbiased conditioned place preference (CPP) paradigm. Morphine (1, 3 and 6 nmol, i.c.v.) induced a significant place preference. For testing the effect of NPS on the acquisition of morphine CPP, mice were given the combination of NPS and morphine on the conditioning days, and without drug treatment on the followed test day. To study the effect of NPS on the expression of morphine CPP, mice received the treatment of saline/morphine on the conditioning days, and NPS on the test day, 15 min before the placement in the CPP apparatus. Our results showed that NPS (0.3-10 nmol) alone neither induced place preference nor aversion, however, NPS (1 and 3 nmol) blocked the acquisition of CPP induced by 3 nmol morphine, and acquisition of 6 nmol morphine-induced CPP was also reduced by NPS (6 and 10 nmol). Moreover, the expression of CPP induced by 6 nmol morphine was also inhibited by NPS (0.1, 1 and 10 nmol). These results revealed the involvement of NPS in rewarding activities of morphine, and demonstrated the interaction between NPS system and opioid system for the first time.

Acute nicotine changes dynorphin and prodynorphin mRNA in the striatum

RATIONALE

Nicotine displays rewarding and aversive effects, and while dopamine has been linked with nicotine's reward, the neurotransmitter(s) involved with aversion remains speculative. The kappa-dynorphinergic system has been associated with negative motivational and affective states, and whether dynorphin (Dyn) contributes to the behavioral pharmacology of nicotine is a pertinent question.

OBJECTIVE

We determined whether administration of a single dose of nicotine alters the biosynthesis of Dyn in the striatum of mice.

RESULTS

Nicotine free base, 1 mg/kg, sc, induced a biphasic, protracted increase of striatal Dyn, an initial rise by 1 h, which declined to control levels by 2 h, and a subsequent increase, between 6 and 12 h, lasting over 24 h. At 1 h, the nicotine effect was dose dependent, with doses>or=0.5 mg/kg inducing a response. Prodynorphin mRNA increased by 30 min for over 24 h, and in situ hybridization demonstrated elevated signal in caudate/putamen and nucleus accumbens. The nicotinic antagonist mecamylamine prevented the Dyn response, and a similar effect was observed with D1- and D2-like dopamine receptor antagonists, SCH 23390, sulpiride, and haloperidol. The glutamate NMDA receptor antagonist MK-801 reversed the nicotine-induced increase of Dyn, while the AMPA antagonist NBQX had a marginal effect.

CONCLUSIONS

We interpret our findings to indicate that acute nicotine enhances the synthesis and release of striatal Dyn. We propose that nicotine influences Dyn primarily through dopamine release and that glutamate plays a modulatory role. A heightened dynorphinergic tone may contribute to the aversive effects of nicotine in naive animals and first-time tobacco smokers.

Enhanced vulnerability to the rewarding effects of nicotine during the adolescent period of development

This study compared the rewarding and aversive effects of nicotine in adolescent, adult, and adult rats preexposed to nicotine during adolescence. Prior to conditioning, the rats were tested for their initial preference for either of 2 distinct compartments. Adolescent and adult rats then received various nicotine doses in their initially non-preferred side on one day and saline in the other side on alternate days. This 2-day procedure was repeated over 8 consecutive days. Following conditioning, rats were re-tested for their preference. Another cohort of adolescent and adult rats were conditioned with various doses of D-amphetamine. Nicotine produced CPP in an inverted U-shaped manner in both age groups. However, adolescents displayed a larger upward shift in CPP that was significant across a wider dose range relative to adults. There were no developmental differences to CPP produced by D-amphetamine. In a final study, adolescents were prepared with pumps that delivered nicotine for 14 days. These rats were conditioned later as adults using the same procedures used previously. Pre-exposure to nicotine during adolescence diminished the aversive effects produced by the highest nicotine dose in naive adults. Taken together, these studies provide a basis for enhanced vulnerability to nicotine during adolescence.

Nicotine withdrawal disrupts new contextual learning

Interactions between nicotine and learning could contribute to nicotine addiction. Although previous research indicates that nicotine withdrawal disrupts contextual learning, the effects of nicotine withdrawal on contextual memories acquired before withdrawal are unknown. The present study investigated whether nicotine withdrawal disrupted recall of prior contextual memories by examining the effects of nicotine withdrawal on recall of nicotine conditioned place preference (CPP) and contextual fear conditioning. C57BL/6J mice trained in CPP exhibited a significant preference for an initially non-preferred chamber that was paired with 0.35 mg/kg nicotine. Following CPP, mice were implanted with mini-osmotic pumps containing 6.3 mg/kg/d nicotine or saline. Pumps were removed twelve days later and nicotine CPP was retested 24 h later. Mice withdrawn from chronic nicotine exhibited CPP, suggesting that older drug-context associations are not disrupted by nicotine withdrawal. One hour later, the same mice were trained in contextual and cued fear conditioning; nicotine withdrawal disrupted contextual but not cued fear conditioning. A subsequent experiment demonstrated that nicotine withdrawal did not disrupt recall of contextual or cued fear conditioning when acquisition occurred before nicotine withdrawal. These data suggest that nicotine withdrawal disrupts new contextual learning, but does not alter contextual learning that occurred before withdrawal.