Nicotine as a typical drug of abuse in experimental animals and humans

The effect of mixed tobacco monoamine oxidase inhibitors in animal models relevant to tobacco dependence

RATIONALE

Tobacco monoamine oxidase (MAO) inhibitors have long been suspected of influencing tobacco dependence, but direct evidence of their effects has been difficult to obtain. Recently we have identified two new groups of monoamine oxidase inhibitors, hydroquinones and polyunsaturated fatty acids (linoleic and linolenic acid), abundant in tobacco smoke.

OBJECTIVES

To test, in relevant animal models, whether the combined effect of these inhibitors is sufficient to affect addictive responses to nicotine.

METHODS

Here we report the first tests of the effects of mixed tobacco MAO inhibitors in three animal behavioural tests relevant to nicotine addiction, conditioned place preference, locomotor sensitisation and nicotine self-administration. Inhibitors used were the aforementioned linoleic and linolenic acid, and catechol, 4-ethylcatechol, 4-methyl catechol and hydroquinone, together with the already known inhibitors harman and norharman. They were administered together in the ratios found in tobacco smoke.

RESULTS

In conditioned place preference and in self-administration tests the addition of these tobacco MAO inhibitors significantly increased responding to nicotine and motivation to self-administer nicotine, supporting the hypothesis that inhibition of MAO enzymes in the brain enhances addictive responses such as that for nicotine. The combined MAO inhibitors without nicotine did not cause increased locomotor activity and did not induce a place conditioned response.

CONCLUSIONS

Our results show that the combined effect of three groups of major MAO inhibitors present in tobacco smoke can enhance the addictive responses to nicotine in rats. There is no evidence from this study that these MAO inhibitors are addictive in themselves.

Cofilin linked to GluN2B subunits of NMDA receptors is required for behavioral sensitization by changing the dendritic spines of neurons in the caudate and putamen after repeated nicotine exposure

BACKGROUND

Nicotine dependence is associated with glutamatergic neurotransmission in the caudate and putamen (CPu) of the forebrain which includes alterations in the structure of dendritic spines at glutamate synapses. These changes after nicotine exposure can lead to the development of habitual behaviors such as smoking. The present study investigated the hypothesis that cofilin, an actin-binding protein that is linked to the GluN2B subunits of N-methyl-D-aspartate (NMDA) receptors regulates the morphology of dendritic spines in the neurons of the CPu after repeated exposure to nicotine.

RESULTS

Adult male rats received subcutaneous injections of nicotine (0.3 mg/kg/day) or vehicle for seven consecutive days. DiI staining was conducted to observe changes in dendritic spine morphology. Repeated subcutaneous injections of nicotine decreased the phosphorylation of cofilin while increasing the formation of thin spines and filopodia in the dendrites of medium spiny neurons (MSN) in the CPu of rats. Bilateral intra-CPu infusion of the cofilin inhibitor, cytochalasin D (12.5 µg/µL/side), restored the thin spines and filopodia from mushroom types after repeated exposure to nicotine. Similar results were obtained from the bilateral intra-CPu infusion of the selective GluN2B subunit antagonist, Ro 25-6981 (4 µM/µL/side). Bilateral intra-CPu infusion of cytochalasin D that interferes with the actin-cofilin interaction attenuated the repeated nicotine-induced increase in locomotor sensitization in rats.

CONCLUSIONS

These findings suggest that active cofilin alters the structure of spine heads from mushroom to thin spine/filopodia by potentiating actin turnover, contributing to behavioral sensitization after nicotine exposure.

Nicotine use disorder and Neuregulin 3: Opportunities for precision medicine

Nicotine use disorder remains a major public health emergency despite years of trumpeting the consequences of smoking. This is likely due to the complex interplay of genetics and nicotine exposure across the lifespan of these individuals. Genetics influence all aspects of life, including complex disorders such as nicotine use disorder. This review first highlights the critical neurocircuitry underlying nicotine dependence and withdrawal, and then describes the cellular signaling mechanisms involved. Finally, current genetic, genomic, and transcriptomic evidence for new drug development of smoking cessation aids is discussed, with a focus on the Neuregulin 3 Signaling Pathway.

Double dissociation of perirhinal nicotinic acetylcholine receptors and dopamine D2 receptors in modulation of object memory consolidation by nicotine, cocaine and their conditioned stimuli

There are indications that drug conditioned stimuli (CS) may activate neurochemical systems of memory modulation that are activated by the drugs themselves. To directly test this hypothesis, a cholinergic nicotinic receptor antagonist (mecamylamine; MEC: 0, 10 or 30 µg/side) and a dopamine D2 receptor antagonist (l-741,626: 0, 0.63, 2.5 µg/side) were infused in the perirhinal cortex (PRh) to block modulation of object recognition memory consolidation induced by 0.4 mg/kg nicotine, 20 mg/kg cocaine, or their CSs. To establish these CSs, male Sprague-Dawley rats were confined for 2 h in a chamber, the CS+, after injections of 0.4 mg/kg nicotine, or 20 mg/kg cocaine, and in another chamber, the CS-, after injections of vehicle. This was repeated over 10 days (5 drug/CS+ and 5 vehicle/CS- pairings in total). It was found that the memory enhancing action of post-sample nicotine was blocked by intra-PRh infusions of both MEC doses, and 30 µg/side MEC also blocked the memory enhancing action of the nicotine CS. Interestingly, intra-PRh MEC did not block the memory enhancing effect of cocaine, nor that of the cocaine CS. In contrast, the memory enhancing action of post-sample cocaine administration was blocked by both l-741,626 doses, and 2.5 µg/side also blocked the effect of the cocaine CS, but not the memory effects of nicotine or of the nicotine CS. This functional double dissociation strongly indicates that drug CSs modulate memory consolidation by activating neural systems that are activated by the drugs themselves.

Development of a nicotine aerosol self-administration model in rats and the effects of e-liquid flavors

Electronic nicotine delivery system (ENDS) use is maintained by the positive reinforcement associated with preferred flavors. These flavors become conditioned reinforcers through pairings with primary reinforcers. This study sought to extend prior research with intravenous nicotine self-administration and develop a more ecologically valid preclinical model of aerosol self-administration in rats that incorporated flavors paired with sucrose. Rats were first trained to respond for oral sucrose with or without raspberry flavor to establish the flavor as a conditioned reinforcer for some groups. Rats were then exposed to aerosol self-administration. All groups responded for raspberry-flavored aerosol with or without nicotine. Rats responded more for raspberry flavored sucrose than unflavored sucrose. Despite raspberry increasing responding for sucrose, the flavor did not function as a conditioned reinforcer during aerosol self-administration and did not increase responding for nicotine. Throughout the aerosol self-administration phase, most groups responded more on the active than inactive lever, and some groups increased their response when the fixed ratio value was increased. At the end of the study, rats in nicotine groups earned similar or fewer aerosol deliveries than rats in vehicle groups. Aerosolized nicotine did not function as a reinforcer in this study, whereas aerosolized raspberry flavor may have maintained self-administration. Further preclinical investigation is needed to articulate the impact of flavors on ENDS use and whether they offset some aversive effects of nicotine or maintain responding on their own. If flavors reduce some aversive effects of self-administered nicotine, then policies to regulate flavors in e-liquids are prudent.

Phosphorylation of GluA1-Ser831 by CaMKII Activation in the Caudate and Putamen Is Required for Behavioral Sensitization After Challenge Nicotine in Rats

BACKGROUND

Phosphorylation of the glutamate receptor (GluA1) subunit of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor plays a crucial role in behavioral sensitization after exposure to psychostimulants. The present study determined the potential role of serine 831 (Ser831) phosphorylation in the GluA1 subunit of the caudate and putamen (CPu) in behavioral sensitization after challenge nicotine.

METHODS

Challenge nicotine (0.4 mg/kg) was administered subcutaneously (s.c.) after 7 days of repeated exposure to nicotine (0.4 mg/kg, s.c.) followed by 3 days of withdrawal in rats. Bilateral intra-CPu infusions of drugs were mainly performed to test this hypothesis.

RESULTS

Challenge nicotine increased both phosphorylated (p)Ser831 immunoreactivity (IR) and pCa2+/calmodulin-dependentprotein kinases II (pCaMKII)-IR in the medium spiny neurons (MSNs) of the CPu. These increases were prevented by bilateral intra-CPu infusion of the metabotropic glutamate receptor 5 (mGluR5) antagonist MPEP (0.5 nmol/side) and the N-methyl-D-aspartate (NMDA) receptor antagonist MK801 (2 nmol/side). However, the dopamine D1 receptor (D1R) antagonist SCH23390 (7.5 nmol/side) prevented only pSer831-IR alone. Bilateral intra-CPu infusion of the Tat-GluA1D peptide (25 pmol/side), which interferes with the binding of pCaMKII to GluA1-Ser831, decreased the challenge nicotine-induced increase in locomotor activity.

CONCLUSIONS

These findings suggest that the GluA1-Ser831 phosphorylation in the MSNs of the CPu is required for the challenge nicotine-induced behavioral sensitization in rats. CaMKII activation linked to mGluR5 and NMDA receptors, but not to D1R, is essential for inducing the CaMKII-Ser831 interaction.

The multitarget FAAH inhibitor/D3 partial agonist ARN15381 decreases nicotine self-administration in male rats

Tobacco use disorder is a worldwide health problem for which available medications show limited efficacy. Nicotine is the psychoactive component of tobacco responsible for its addictive liability. Similar to other addictive drugs, nicotine enhances mesolimbic dopamine transmission. Inhibition of the fatty acid amide hydrolase (FAAH), the enzyme responsible for the degradation of the endocannabinoid anandamide (AEA), palmitoylethanolamide (PEA) and oleoylethanolamide (OEA), reduces nicotine-enhanced dopamine transmission and acquisition of nicotine self-administration in rats. Down-regulation of dopamine transmission by antagonists or partial agonists of the dopamine D3 receptor (DRD3) also reduced nicotine self-administration and conditioned place preference. Based on these premises, we evaluated the effect of ARN15381, a multitarget compound showing FAAH inhibition and DRD3 partial agonist activity in the low nanomolar range, on nicotine self-administration in rats. Pretreatment with ARN15381 dose dependently decreased self-administration of a nicotine dose at the top of the nicotine dose/response (D/R) curve, while it did not affect self-administration of a nicotine dose laying on the descending limb of the D/R curve. Conversely, pretreatment with the selective FAAH inhibitor URB597 and the DRD3 partial agonist CJB090 failed to modify nicotine self-administration independent of the nicotine dose self-administered. Our data indicates that the concomitant FAAH inhibition and DRD3 partial agonism produced by ARN15381 is key to the observed reduction of nicotine self-administration, demonstrating that a multitarget approach may hold clinical importance for the treatment of tobacco use disorder.

Cannabinoid receptor GPR55 activation blocks nicotine use disorder by regulation of AMPAR phosphorylation

RATIONALE

Nicotine use disorder can alter synaptic plasticity correlated with learning and memory process. G protein-coupled receptor 55 (GPR55), a novel cannabinoid receptor, which is highly expressed in the central nervous system, plays a prominent role in learning and memory. However, the role of GPR55 in nicotine use disorder remains unclear.

METHODS

In this study, we used the conditioned place preference (CPP) paradigm, a standard and well-established model for evaluating the rewarding effect of drug abuse, to investigate nicotine use disorder behavior in mice. After behavioral tests, the effect of GPR55 on nicotine response was evaluated using Western blotting, immunofluorescence staining, whole-cell patch-clamp recordings, and ELISA.

RESULTS

GPR55 activation significantly reduced nicotine-CPP behavior by decreasing the spontaneous excitatory postsynaptic currents frequency in the nucleus accumbens (NAc) and the release of dopamine in serum. Furthermore, we found that the inhibition effects of nicotine response were mediated by phosphorylation of AMPAR. The PI3K-Akt signaling was involved in nicotine-CPP via GPR55 activation.

CONCLUSION

Our findings showed that GPR55 in the NAc plays a specific role in blocking nicotine-CPP behavior and might be a potential target for the treatment of nicotine use disorder.

Evaluating a school-based science program that teaches the physiological effects of nicotine

School-based drug prevention programs represent a widely endorsed public health goal, with an important aspect of knowledge-based curricula being education about the physiological effects of drugs. Nicotine is one of the world's most addictive substances and in this program we have used nicotine-induced mammalian-like behaviors in flatworms called planarians to successfully teach students (4th-12th grade; n = 1,616 students) about the physiological and addictive effects of nicotine. An initial study tested the change in knowledge about addictive substances in 6th-12th grade students after they completed a lab examining the effects of two concentrations of nicotine on the number of stereotypies (C-shaped spasms) planarians demonstrate in a 5-minute period of time. Lab discussion focused on developing and testing hypotheses, measurement reliability, and mechanisms of nicotine action. Surveys given pre- and post-lab experience showed that 6th grade students have significantly lower knowledge about nicotine than 7th-12th grade students (6th grade: 40.65 ± 0.78% correct, 7th-12th grade: 59.29 ± 1.71%, p < 0.001) pre-lab, but that students in all grades showed a significant increase in knowledge post-lab (p < 0.001). In 6th grade the lab was effective in improving knowledge about nicotine in urban, suburban and rural schools, p < 0.001, with students in suburban schools showing significantly greater knowledge both pre-test (urban: 37.62 ± 1.45%; suburban: 48.78 ± 1.62%; rural: 37.33 ± 0.99%; p < 0.001) and post-test (urban:60.60 ± 1.85%; suburban: 67.54 ± 1.82%; urban: 61.66 ± 1.18%; p < 0.001). A second study, modifying the lab so that the time spent observing the planarians is reduced to a 1-minute period, showed that students in both 4th and 5th grades had a significant increase in knowledge about the physiological and addictive effects of nicotine post-lab (p < 0.001).

The importance of acquisition learning on nicotine and varenicline drug substitution in a drug-discriminated goal-tracking task

Nicotine and varenicline (Chantix®; the leading non-nicotine cessation pharmacotherapy) can come to control appetitive behaviors such as goal-tracking. We tested rats (N = 48) in a drug-discriminated goal-tracking (DGT) task where each rat received daily subcutaneous injections of either nicotine (0.4 mg/kg) or saline (0.9% [w/v]) interspersed across the acquisition phase (Phase 1). On saline days, sucrose was intermittently available. On nicotine days, sucrose was withheld. All rats acquired the discrimination with increased goal-tracking rates on saline days relative to nicotine days. Following acquisition, rats were separated into four groups to assess drug-substitution and discrimination reversal in Phase 2. The first group maintained the stimulus-reinforcer relation from acquisition (NIC-). The reversal group was now given access to sucrose on nicotine days (NIC+). The substitution group replaced nicotine with varenicline (1 mg/kg) while maintaining the acquisition stimulus-reinforcer relation (VAR-). The substitution and reversal group had nicotine replaced by varenicline and the stimulus-reinforcer relation reversed (VAR+). Rats in all groups learned or maintained their Phase 1 discriminations. For Phase 2, the reversal groups (+ conditions) acquired their discriminations within 10 sessions. The VAR- group displayed a pattern of disrupted discrimination at the outset of Phase 2 but was reestablished after continued training. In substitution testing, VAR groups received nicotine and NIC groups received varenicline. The NIC- and VAR- groups displayed full substitution of the test stimulus whereas the NIC+ and VAR+ groups displayed partial substitution of the test stimulus. Rats underwent nicotine extinction in Phase 3. Initial responding for each group mimicked Phase 2 training (i.e., higher responding by the reversal groups). All rats maintained similarly low levels of responding after six sessions. In conclusion, initial learning history with nicotine (i.e., + or -) influenced drug-stimulus substitution and the rate at which new learning (e.g., reversal) occurs with the varenicline and nicotine interoceptive stimuli.

Drug instrumentalization

Psychoactive drugs with addiction potential are widely used by people of virtually all cultures in a non-addictive way. In order to understand this behaviour, its population penetrance, and its persistence, drug instrumentalization was suggested as a driving force for this consumption. Drug instrumentalization theory holds that psychoactive drugs are consumed in a very systematic way in order to make other, non-drug-related behaviours more efficient. Here, we review the evolutionary origin of this behaviour and its psychological mechanisms and explore the neurobiological and neuropharmacological mechanisms underlying them. Instrumentalization goals are discussed, for which an environmentally selective and mental state-dependent consumption of psychoactive drugs can be learned and maintained in a non-addictive way. A small percentage of people who regularly instrumentalize psychoactive drugs make a transition to addiction, which often starts with qualitative and quantitative changes in the instrumentalization goals. As such, addiction is proposed to develop from previously established long-term drug instrumentalization. Thus, preventing and treating drug addiction in an individualized medicine approach may essentially require understanding and supporting personal instrumentalization goals.

Non-pharmacological factors that determine drug use and addiction

Based on their pharmacological properties, psychoactive drugs are supposed to take control of the natural reward system to finally drive compulsory drug seeking and consumption. However, psychoactive drugs are not used in an arbitrary way as pure pharmacological reinforcement would suggest, but rather in a highly specific manner depending on non-pharmacological factors. While pharmacological effects of psychoactive drugs are well studied, neurobiological mechanisms of non-pharmacological factors are less well understood. Here we review the emerging neurobiological mechanisms beyond pharmacological reinforcement which determine drug effects and use frequency. Important progress was made on the understanding of how the character of an environment and social stress determine drug self-administration. This is expanded by new evidence on how behavioral alternatives and opportunities for drug instrumentalization generate different patterns of drug choice. Emerging evidence suggests that the neurobiology of non-pharmacological factors strongly determines pharmacological and behavioral drug action and may, thus, give rise for an expanded system's approach of psychoactive drug use and addiction.

Randomized Clinical Trials Investigating Innovative Interventions for Smoking Cessation in the Last Decade

Every year, billions of dollars are spent treating smoking and related conditions, yet smoking-related morbidity and mortality continue to rise. There are currently only three FDA-approved medications for smoking cessation: nicotine replacement therapy, bupropion, and varenicline. Although these medications increase abstinence rates, most individuals relapse following treatment. This chapter reviews clinical trials published within the past 10 years investigating novel smoking cessation pharmacotherapies. Among these pharmacotherapies, some showed promising results, such as cytisine and endocannabinoid modulators, whereas others failed to produce significant effects. More research is needed to develop drugs that produce higher rates of long-term abstinence and to determine which subgroups of patients benefit from a given treatment.

Intranasal mesenchymal stem cell secretome administration markedly inhibits alcohol and nicotine self-administration and blocks relapse-intake: mechanism and translational options

BACKGROUND

Chronic consumption of most drugs of abuse leads to brain oxidative stress and neuroinflammation, which inhibit the glutamate transporter GLT-1, proposed to perpetuate drug intake. The present study aimed at inhibiting chronic ethanol and nicotine self-administration and relapse by the non-invasive intranasal administration of antioxidant and anti-inflammatory secretome generated by adipose tissue-derived activated mesenchymal stem cells. The anti-addiction mechanism of stem cell secretome is also addressed.

METHODS

Rats bred for their alcohol preference ingested alcohol chronically or were trained to self-administer nicotine. Secretome of human adipose tissue-derived activated mesenchymal stem cells was administered intranasally to animals, both (i) chronically consuming alcohol or nicotine and (ii) during a protracted deprivation before a drug re-access leading to relapse intake.

RESULTS

The intranasal administration of secretome derived from activated mesenchymal stem cells inhibited chronic self-administration of ethanol or nicotine by 85% and 75%, respectively. Secretome administration further inhibited by 85-90% the relapse "binge" intake that occurs after a protracted drug deprivation followed by a 60-min drug re-access. Secretome administration fully abolished the oxidative stress induced by chronic ethanol or nicotine self-administration, shown by the normalization of the hippocampal oxidized/reduced glutathione ratio, and the neuroinflammation determined by astrocyte and microglial immunofluorescence. Knockdown of the glutamate transporter GLT-1 by the intracerebral administration of an antisense oligonucleotide fully abolished the inhibitory effect of the secretome on ethanol and nicotine intake.

CONCLUSIONS

The non-invasive intranasal administration of secretome generated by human adipose tissue-derived activated mesenchymal stem cells markedly inhibits alcohol and nicotine self-administration, an effect mediated by the glutamate GLT-1 transporter. Translational implications are envisioned.

Cocaine, nicotine, and their conditioned contexts enhance consolidation of object memory in rats

To test the hypothesis that drugs of abuse and their conditioned stimuli (CSs) enhance memory consolidation, the effects of post-training exposure to cocaine and nicotine were compared to the effects of post-training exposure to contextual stimuli that were paired with the effects of these drugs. Using the object recognition (OR) task, it was first demonstrated that both 10 and 20 mg/kg cocaine, and 0.2 and 0.4 mg/kg nicotine, enhanced recognition memory when administered immediately after, but not 6 h after the sample phase. To establish the drug CSs, rats were confined for 2 h in a chamber (the CS+) after injections of 20 mg/kg cocaine, or 0.4 mg/kg nicotine, and in another chamber (the CS−) after injections of vehicle. This was repeated over 10 d (5 drug/CS+ and 5 vehicle/CS− pairings in total). At the end of this conditioning period, when tested in a drug-free state, rats displayed conditioned hyperactivity in the CS+ relative to the CS−. More important, immediate, but not delayed, post-sample exposure to the cocaine CS+, or nicotine CS+, enhanced OR memory. Therefore, this study reports for the first time that contextual stimuli paired with cocaine and nicotine, like the drugs themselves, have the ability to enhance memory consolidation.

Nicotine drug discrimination and nicotinic acetylcholine receptors in differentially reared rats

RATIONALE

Individuals vary in sensitivity to the behavioral effects of nicotine, resulting in differences in vulnerability to nicotine addiction. The role of rearing environment in determining individual sensitivity to nicotine is unclear. The neuropharmacological mechanisms mediating the effect of rearing environment on the behavioral actions of nicotine are also poorly understood.

OBJECTIVES

The contribution of rearing environment in determining the sensitivity to the interoceptive effects of nicotine was determined in rats reared in isolated conditions (IC) or enriched conditions (EC). The role of dopamine receptors and α4β2*-nicotinic acetylcholine (nACh) receptors in mediating the differential effect of IC and EC on the interoceptive action of nicotine was determined.

METHODS

The interoceptive action of nicotine was measured as the discriminative stimulus effect of nicotine. Mecamylamine- and eticlopride-inhibition of the nicotine stimulus were used to examine nACh and dopamine receptors, respectively. α4β2*-nACh receptor expression in the mesolimbic dopamine pathway was determined by quantitative autoradiography of [¹²⁵I]-epibatidine binding.

RESULTS

EC-reared rats are less sensitive than IC-reared rats to the discriminative stimulus effects of nicotine at all but maximally effective doses. Mecamylamine inhibited the nicotine stimulus threefold more potently in EC-reared rats (IC₅₀ = 0.25 mg/kg) compared to IC-reared rats (IC₅₀ = 0.75 mg/kg); eticlopride inhibition was not different. [¹²⁵I]-epibatidine binding in the ventral tegmental area of EC-reared rats was reduced (2.8 ± 0.3 fmol) compared to that of IC-reared rats (4.0 ± 0.4 fmol); there was no difference in the nucleus accumbens.

CONCLUSIONS

Rearing environment regulates the sensitivity to the interoceptive effects of nicotine and α4β2*-nACh receptor expression in the mesolimbic dopamine pathway.

Sex differences in nicotine preference

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

Altered nicotine reward-associated behavior following α4 nAChR subunit deletion in ventral midbrain

Nicotinic acetylcholine receptors containing α4 subunits (α4β2* nAChRs) are critical for nicotinic cholinergic transmission and the addictive action of nicotine. To identify specific activities of these receptors in the adult mouse brain, we coupled targeted deletion of α4 nAChR subunits with behavioral and and electrophysiological measures of nicotine sensitivity. A viral-mediated Cre/lox approach allowed us to delete α4 from ventral midbrain (vMB) neurons. We used two behavioral assays commonly used to assess the motivational effects of drugs of abuse: home-cage oral self-administration, and place conditioning. Mice lacking α4 subunits in vMB consumed significantly more nicotine at the highest offered nicotine concentration (200 μg/mL) compared to control mice. Deletion of α4 subunits in vMB blocked nicotine-induced conditioned place preference (CPP) without affecting locomotor activity. Acetylcholine-evoked currents as well as nicotine-mediated increases in synaptic potentiation were reduced in mice lacking α4 in vMB. Immunostaining verified that α4 subunits were deleted from both dopamine and non-dopamine neurons in the ventral tegmental area (VTA). These results reveal that attenuation of α4* nAChR function in reward-related brain circuitry of adult animals may increase nicotine intake by enhancing the rewarding effects and/or reducing the aversive effects of nicotine.

Nicotine Acutely Enhances Reinforcement from Non-Drug Rewards in Humans

Preclinical research documents that, aside from the primary and secondary reinforcing effects of nicotine intake itself, nicotine also acutely enhances the reinforcing efficacy of non-drug reinforcers ("rewards"). Study of these effects in humans has largely been overlooked, but very recent findings suggest they may have clinical implications for more fully understanding the persistence of tobacco dependence. This overview first outlines the topic and notes some recent human studies indirectly addressing nicotine effects on related responses (e.g., subjective ratings), explaining why those findings do not directly confirm enhancement of behavioral reinforcement per se due to nicotine. Then, the methodology used in the subsequently presented studies is described, demonstrating how those studies specifically did demonstrate enhancement of reinforced responding for non-drug rewards. The main section focuses on the limited controlled research to date directly assessing nicotine's acute reinforcement-enhancing effects in humans, particularly as it relates to reinforced behavioral responding for non-drug rewards in non-human animal models. After detailing those few existing human studies, we address potential consequences of these effects for dependence and tobacco cessation efforts and then suggest directions for future research. This research indicates that nicotine per se increases responding in humans that is reinforced by some rewards (auditory stimuli via music, visual stimuli via video), but perhaps not by others (e.g., money). These reinforcement-enhancing effects in smokers are not due to dependence or withdrawal relief and can be restored by a small amount of nicotine (similar to a smoking lapse), including from e-cigarettes, a non-tobacco nicotine product. Future clinical research should examine factors determining which types of rewards are (or are not) enhanced by nicotine, consequences of the loss of these nicotine effects after quitting smoking, potential individual differences in these effects, and the possibility that nicotine via nicotine replacement therapy and non-nicotine quit medications may attenuate loss of these effects upon quitting. Further study with humans of nicotine's reinforcement-enhancing effects may provide a more complete understanding of smoking persistence and added mechanisms of cessation medication efficacy.

Drugs as instruments: a new framework for non-addictive psychoactive drug use

Most people who are regular consumers of psychoactive drugs are not drug addicts, nor will they ever become addicts. In neurobiological theories, non-addictive drug consumption is acknowledged only as a "necessary" prerequisite for addiction, but not as a stable and widespread behavior in its own right. This target article proposes a new neurobiological framework theory for non-addictive psychoactive drug consumption, introducing the concept of "drug instrumentalization." Psychoactive drugs are consumed for their effects on mental states. Humans are able to learn that mental states can be changed on purpose by drugs, in order to facilitate other, non-drug-related behaviors. We discuss specific "instrumentalization goals" and outline neurobiological mechanisms of how major classes of psychoactive drugs change mental states and serve non-drug-related behaviors. We argue that drug instrumentalization behavior may provide a functional adaptation to modern environments based on a historical selection for learning mechanisms that allow the dynamic modification of consummatory behavior. It is assumed that in order to effectively instrumentalize psychoactive drugs, the establishment of and retrieval from a drug memory is required. Here, we propose a new classification of different drug memory subtypes and discuss how they interact during drug instrumentalization learning and retrieval. Understanding the everyday utility and the learning mechanisms of non-addictive psychotropic drug use may help to prevent abuse and the transition to drug addiction in the future.

Attenuation of cue-induced reinstatement of nicotine seeking by URB597 through cannabinoid CB1 receptor in rats

RATIONALE

The endocannabinoid system is composed of endocannabinoids (such as anandamide), their target receptors (CB1 and CB2 receptors, CB1Rs and CB2Rs), the enzymes that degrade them (fatty-acid-amide-hydrolase (FAAH) for anandamide), and an endocannabinoid transporter. FAAH inhibition has been recently identified as having a critical involvement in behaviors related to nicotine addiction and has been shown to reduce the effect of nicotine on the mesolimbic dopaminergic system via CB1R and peroxisome proliferator-activated receptor alpha (PPARα). Thus, inhibition of FAAH may represent a novel strategy for smoking cessation, but its mechanism of action on relapse to nicotine seeking is still unknown.

OBJECTIVE

The study aims to explore the mechanism of action of the inhibitor of FAAH activity, URB597, on relapse to nicotine seeking by evaluating the effect of the CB1R, CB2R, and PPARα antagonists on the attenuating effect of URB597 on cue-induced reinstatement of nicotine seeking in rats.

RESULTS

URB597 reduced cue-induced reinstatement of nicotine seeking, an effect that was reversed by the CB1R antagonist rimonabant, but not by the CB2R or PPARα antagonists AM630 and MK886, respectively.

CONCLUSIONS

These results indicate that URB597 reduces cue-induced reinstatement in rats through a CB1 receptor-dependent mechanism, and not via CB2R or PPARα. Since FAAH inhibition represent a novel and promising strategy for tobacco smoking cessation, dissecting how it produces its action may lead to a better understanding of the neurobiological mechanisms underlying nicotine addiction.

An Inventory of Methods for the Assessment of Additive Increased Addictiveness of Tobacco Products

BACKGROUND

Cigarettes and other forms of tobacco contain the addictive drug nicotine. Other components, either naturally occurring in tobacco or additives that are intentionally added during the manufacturing process, may add to the addictiveness of tobacco products. As such, these components can make cigarette smokers more easily and heavily dependent.Efforts to regulate tobacco product dependence are emerging globally. Additives that increase tobacco dependence will be prohibited under the new European Tobacco Product Directive.

OBJECTIVE

This article provides guidelines and recommendations for developing a regulatory strategy for assessment of increase in tobacco dependence due to additives. Relevant scientific literature is summarized and criteria and experimental studies that can define increased dependence of tobacco products are described.

CONCLUSIONS

Natural tobacco smoke is a very complex matrix of components, therefore analysis of the contribution of an additive or a combination of additives to the level of dependence on this product is challenging. We propose to combine different type of studies analyzing overall tobacco product dependence potential and the functioning of additives in relation to nicotine. By using a combination of techniques, changes associated with nicotine dependence such as behavioral, physiological, and neurochemical alterations can be examined to provide sufficient information.Research needs and knowledge gaps will be discussed and recommendations will be made to translate current knowledge into legislation. As such, this article aids in implementation of the Tobacco Product Directive, as well as help enable regulators and researchers worldwide to develop standards to reduce dependence on tobacco products.

IMPLICATIONS

This article provides an overall view on how to assess tobacco product constituents for their potential contribution to use and dependence. It provides guidelines that help enable regulators worldwide to develop standards to reduce dependence on tobacco products and guide researches to set research priorities on this topic.

Individual variation in the motivational properties of a nicotine cue: sign-trackers vs. goal-trackers

RATIONALE

Individuals vary in the extent to which they attribute incentive salience to reward cues. Discrete food and drug (cocaine and opioid) cues become more attractive, eliciting approach toward them, and more "wanted," in that they serve as more effective conditioned reinforcers, in some rats (sign-trackers, STs), than in others (goal-trackers, GTs).

OBJECTIVES

We asked whether there is similar variation in the extent to which a cue associated with a drug from another class, nicotine, acquires incentive motivational properties.

METHODS

First, a Pavlovian conditioned approach procedure was used to identify rats that attribute incentive salience to a food cue (i.e., STs and GTs). We then measured the extent to which a cue (a light) paired with intravenous nicotine injections acquired two properties of an incentive stimulus: (1) the ability to elicit approach toward it, and (2) the ability to act as a conditioned reinforcer.

RESULTS

In contrast to previous findings with food, cocaine, and opioid cues, we found that the nicotine cue was equally attractive in STs and GTs, eliciting dose-dependent approach behavior in both. However, the nicotine cue was a more effective conditioned reinforcer in STs than in GTs.

CONCLUSIONS

We suggest the dissociation between these two measures of incentive salience attribution may be related to the fact that when present (as in the test of Pavlovian approach), nicotine can act as a potent "incentive amplifier," and by this action, nicotine may render cues especially salient for all animals.

Prenatal stress induces vulnerability to nicotine addiction and alters D2 receptors' expression in the nucleus accumbens in adult rats

Prenatal stress (PS) can induce several long-lasting behavioral and molecular abnormalities in rats. It can also be considered as a risk factor for many psychiatric diseases like schizophrenia, depression or PTSD and predispose to addiction. In this study, we investigated the effect of prenatal stress on the reinforcing properties of nicotine in the CPP paradigm. Then, we examined the mRNA expression of the D2 dopaminergic receptors using the quantitative real-time PCR technique in the nucleus accumbens (NAcc). We found that prenatally stressed rats exhibited a greater place preference for the nicotine-paired compartment than the control rats. Moreover, we observed an overexpression of the DRD2 gene in adult offspring stressed in utero and a downregulation in the PS NIC group (PS rats treated with nicotine) compared with their control counterparts (C NIC). These data suggest that maternal stress can permanently alter the offspring's addictive behavior and D2 receptors' expression.

Concurrent access to nicotine and sucrose in rats

BACKGROUND

Animal models that allow concurrent access to drug and nondrug reinforcers provide unique insight into the etiology, maintenance, and treatment of drug use.

OBJECTIVES

We sought to develop and utilize a concurrent access procedure with nicotine and sucrose in rats.

METHODS

Pressing one lever delivered intravenous nicotine, and pressing another lever delivered sucrose pellets, with both reinforcers freely available throughout daily sessions.

RESULTS

Rats that had been pretrained with nicotine on some days and sucrose on other days responded on both levers when subsequently given concurrent access, but almost all responded at substantially higher rates on the sucrose lever. In contrast, rats pretrained exclusively with nicotine before being given concurrent access showed individual differences, with about half responding more on the nicotine lever. Treatment with the nicotinic receptor partial agonist varenicline selectively decreased nicotine self-administration. Food restriction and removal of the sucrose lever both increased nicotine self-administration.

CONCLUSIONS

The finding that rats continue to take nicotine when sucrose is concurrently available-and in many cases take it more frequently than sucrose-demonstrates that nicotine self-administration does not only occur in the absence of alternative reinforcement options. As a model of human nicotine use, concurrent access is more naturalistic and has higher face validity than procedures in which only one reinforcer is available or choosing one reinforcer precludes access to other reinforcers. As such, this procedure could be useful for evaluating therapeutic agents and improving our understanding of environmental conditions that promote or discourage nicotine use.

Sensory reinforcement-enhancing effects of nicotine via smoking

As has been found in nicotine research on animals, research on humans has shown that acute nicotine enhances reinforcement from rewards unrelated to nicotine intake, but this effect may be specific to rewards from stimuli that are "sensory" in nature. We assessed acute effects of nicotine via smoking on responding for music or video rewards (sensory), for monetary reward (nonsensory), or for no reward (control), to gauge the generalizability of nicotine's reinforcement-enhancing effects. Using a fully within-subjects design, dependent smokers (N = 20) participated in 3 similar experimental sessions, each following overnight abstinence (verified by carbon monoxide <10 ppm) and varying only in the smoking condition. Sessions involved no smoking or smoking "denicotinized" ("denic;" 0.05 mg) or nicotine (0.6 mg) Quest brand cigarettes in controlled fashion prior to responding on a simple operant computer task for each reward separately using a progressive ratio schedule. The reinforcing effects of music and video rewards, but not money, were significantly greater due to the nicotine versus denic cigarette (i.e., nicotine per se), whereas there were no differences between denic cigarette smoking and no smoking (i.e., smoking behavior per se), except for no reward. These effects were not influenced by withdrawal relief from either cigarette. Results that generalize from an auditory to a visual reward confirm that acute nicotine intake per se enhances the reinforcing value of sensory rewards, but its effects on the value of other (perhaps nonsensory) types of rewards may be more modest.

Clarifying the neural basis for incentive salience of tobacco cues in smokers

In functional magnetic resonance imaging (fMRI) studies, smoking cues have been found to elicit increases in brain activity in regions associated with processing rewarding and emotional stimuli. However, most smoking cue studies to date have reported effects relative to neutral control stimuli with no incentive properties, making it unclear whether the observed activation pertains to value in general or the value of cigarettes in particular. The current fMRI study sought to clarify the neural activity reflecting tobacco-specific incentive value versus domain-general incentive value by examining smoking cues, neutral cues, and a third set of cues, monetary cues, which served as an active control condition. Participants were 42 male daily smokers. Compared to neutral cues, significantly greater activation was found in the left ventral striatum in response to tobacco and money cues. Monetary cues also elicited significantly increased activation in the right inferior frontal gyrus and cuneus compared to the other two cue types. Overall, the results suggest that the salience of monetary cues was the highest and, as a result, might have reduced the incentive salience of tobacco cues.

Administration of the nicotinic acetylcholine receptor agonists ABT-089 and ABT-107 attenuates the reinstatement of nicotine-seeking behavior in rats

Current smoking cessation pharmacotherapies have modest efficacy, and most smokers relapse within the first few days after a quit attempt. Nicotine withdrawal-induced craving and cognitive impairments predict smoking relapse during abstinence and suggest that cognitive-enhancing drugs may prevent relapse. ABT-089 and ABT-107 are subtype-selective nAChR agonists that improve cognitive performance in laboratory animals. However, there are no studies examining the effects of ABT-089 and ABT-107 on nicotine self-administration and the reinstatement of nicotine-seeking behavior, an animal model of relapse in human smokers. The goal of the present study was to determine the effects of the α4β2*/α6β2* nAChR agonist ABT-089 and the α7 nAChR agonist ABT-107 on nicotine taking and seeking in rats. The effects of acute ABT-089 and ABT-107 pretreatment on nicotine self-administration and reinstatement were tested in male Sprague Dawley rats. Parallel studies of ABT-089 and ABT-107 on sucrose self-administration and reinstatement were tested in separate groups of rats to determine if the effects of these drug treatments generalized to other reinforced behaviors. Nicotine and sucrose self-administration behaviors were not altered following acute administration of ABT-089 (0, 0.12, 1.2 and 12.0mg/kg) or ABT-107 (0, 0.03 and 0.3mg/kg). In contrast, both ABT-089 and ABT-107 pretreatment dose-dependently attenuated nicotine reinstatement. These effects were reinforcer-specific as no effects of ABT-089 or ABT-107 pretreatment on sucrose seeking were noted. Taken together, these findings suggest that ABT-089 and ABT-107 do not affect nicotine consumption, but may reduce the likelihood that a smoking lapse will lead to relapse.

What is the role of the D3 receptor in addiction? A mini review of PET studies with [(11)C]-(+)-PHNO

The chronic use of drugs, including psychostimulants such as cocaine and amphetamine, has been associated with low D2/3 dopamine receptor availability, which in turn has been linked to poor clinical outcome. In contrast, recent studies focused on the D3 receptor (a member of the D2-like receptor family) suggest that chronic exposure to stimulant drugs can up-regulate this receptor subtype, which, in preclinical models, is linked to dopamine system sensitization - a process hypothesized to contribute to relapse in addiction. In this mini review we present recent human data suggesting that the D3 receptor may contribute to core features of addiction, and discuss the usefulness of the PET imaging probe [(11)C]-(+)-PHNO in investigating this question.

Nicotine-induced increase of dopaminergic mesoaccumbal neuron activity is prevented by acute restraint stress. In vivo electrophysiology in rats

Stress is well known to affect responsiveness to drugs of abuse and influencing approaching and drug-taking behaviour in both animals and humans. Consistently, in nicotine addicted subjects both negative events and perceived stress levels are reported to increase drug use and facilitate relapse to smoke even after long periods of abstinence. It has been suggested that stressful stimuli may influence the rewarding properties of abused drugs by acting on the dopaminergic mesolimbic system. In line with this hypothesis, a recent microdialysis study in rats has shown that acute restraint stress exposure prevents the nicotine-induced mesolimbic dopaminergic activation in the nucleus accumbens (NAC) shell via a corticosterone-mediated mechanism. In the present study we sought to evaluate the impact of acute restraint stress on nicotine-induced activation of the mesoaccumbal dopaminergic system by extracellular single unit recordings of antidromically-identified NAC shell projecting dopaminergic neurons within the ventral tegmental area (VTA). Nicotine intravenous administration dose-dependently (0.05-0.4mg/kg) stimulated the spontaneous firing and bursting of mesoaccumbal dopaminergic neurons in unstressed rats, as previously reported. By contrast, nicotine failed to increase mesoaccumbal dopaminergic neuron activity in rats previously exposed to 1-h immobilisation stress. Our observations show that acute restraint stress inhibits the response of the mesoaccumbal dopaminergic system to the stimulating properties of nicotine. These findings corroborate the notion that stress reduces the sensitivity to nicotine and suggest that the decreased dopaminergic release in the NAC shell is due to a reduced firing and bursting activity in the VTA.

Optimizing treatments for nicotine dependence by increasing cognitive performance during withdrawal

INTRODUCTION

Current FDA-approved smoking cessation pharmacotherapies have limited efficacy and are associated with high rates of relapse. Therefore, there is a clear need to develop novel antismoking medications. Nicotine withdrawal is associated with cognitive impairments that predict smoking relapse. It has been proposed that these cognitive deficits are a hallmark of nicotine withdrawal that could be targeted in order to prevent smoking relapse. Thus, pharmacotherapies that increase cognitive performance during nicotine withdrawal may represent potential smoking cessation agents.

AREAS COVERED

The authors review the clinical literature demonstrating that nicotine withdrawal is associated with deficits in working memory, attention and response inhibition. They then briefly summarize different classes of compounds and strategies to increase cognitive performance during nicotine withdrawal. Particular emphasis has been placed on translational research in order to highlight areas for which there is strong rationale for pilot clinical trials of potential smoking cessation medications.

EXPERT OPINION

There is emerging evidence that supports deficits in cognitive function as a plausible nicotine withdrawal phenotype. The authors furthermore believe that the translational paradigms presented here may represent efficient and valid means for the evaluation of cognitive-enhancing medications as possible treatments for nicotine dependence.

Activation of α4β2*/α6β2* nicotinic receptors alleviates anxiety during nicotine withdrawal without upregulating nicotinic receptors

Although nicotine mediates its effects through several nicotinic acetylcholine receptor (nAChR) subtypes, it remains to be determined which nAChR subtypes directly mediate heightened anxiety during withdrawal. Relative success in abstinence has been found with the nAChR partial agonist varenicline (Chantix; Pfizer, Groton, CT); however, treatment with this drug fails to alleviate anxiety in individuals during nicotine withdrawal. Therefore, it is hypothesized that success can be found by the repurposing of other nAChR partial agonists for cessation therapies that target anxiety. It is noteworthy that the selective partial agonists for α4β2, ABT-089 [2-methyl-3-[2(S)-pyrrolidinylmethoxy]pyridine], and α7, ABT-107 [5-(6-[(3R)-1-azabicyclo[2.2.2]oct-3-yloxy] pyridazin-3-yl)-1H-indole] (AbbVie, North Chicago, IL), have not been evaluated as possible therapeutics for nicotine cessation. Therefore, we examined the effect of ABT-089 and ABT-107 on anxiety during withdrawal from nicotine in the novelty-induced hypophagia (NIH) paradigm. We found that short-term administration of ABT-089 and ABT-107 alleviate anxiety-like behavior during withdrawal from nicotine while long-term administration of ABT-089 but not ABT-107 reduces anxiety-like behavior during withdrawal. After behavioral testing, brains were harvested and β2-containing nAChRs were measured using [(3)H]epibaditine. ABT-089 and ABT-107 do not upregulate nAChRs, which is in contrast to the upregulation of nAChRs observed after nicotine. Furthermore, ABT-089 is anxiogenic in nicotine naive animals, suggesting that the effects on anxiety are specifically related to the nicotine-dependent state. Together, these studies identify additional nAChR partial agonists that may aid in the rational development of smoking cessation aids.

1H-Nuclear magnetic resonance-based metabolomic analysis of brain in mice with nicotine treatment

Background

Nicotine is rapidly absorbed from cigarette smoke and therefore induces a number of chronic illnesses with the widespread use of tobacco products. Studies have shown a few cerebral metabolites modified by nicotine; however, endogenous metabolic profiling in brain has not been well explored.

Results

H NMR-based on metabonomics was applied to investigate the endogenous metabolic profiling of brain hippocampus, nucleus acumens (NAc), prefrontal cortex (PFC) and striatum. We found that nicotine significantly increased CPP in mice, and some specific cerebral metabolites differentially changed in nicotine-treated mice. These modified metabolites included glutamate, acetylcholine, tryptamine, glucose, lactate, creatine, 3-hydroxybutyrate and nicotinamide-adenine dinucleotide (NAD), which was closely associated with neurotransmitter and energy source. Additionally, glutathione and taurine in hippocampus and striatum, phosphocholine in PFC and glycerol in NAc were significantly modified by nicotine, implying the dysregulation of anti-oxidative stress response and membrane metabolism.

Conclusions

Nicotine induces significant metabonomic alterations in brain, which are involved in neurotransmitter disturbance, energy metabolism dysregulation, anti-oxidation and membrane function disruptions, as well as amino acid metabolism imbalance. These findings provide a new insight into rewarding effects of nicotine and the underlying mechanism.

Peroxisome proliferator-activated receptor (PPAR) agonists as promising new medications for drug addiction: preclinical evidence

This review examines the growing literature on the role of peroxisome proliferator-activated receptors (PPARs) in addiction. There are two subtypes of PPAR receptors that have been studied in addiction: PPAR-α and PPAR-γ. The role of each PPAR subtype in common models of addictive behavior, mainly pre-clinical models, is summarized. In particular, studies are reviewed that investigated the effects of PPAR-α agonists on relapse, sensitization, conditioned place preference, withdrawal and drug intake, and effects of PPAR-γ agonists on relapse, withdrawal and drug intake. Finally, studies that investigated the effects of PPAR agonists on neural pathways of addiction are reviewed. Taken together these preclinical data indicate that PPAR agonists are promising new medications for drug addiction treatment.

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

Social and genetic factors can influence smoking behavior. Using olfactogustatory stimuli as the sensory cue for intravenous nicotine self-administration (SA), we previously showed that social learning of nicotine contingent odor cue prevented rats from developing conditioned taste aversion and allowed them to instead establish stable nicotine SA. We hypothesized that genetic factors influenced socially acquired nicotine SA. A heterogeneous stock (HS; N/NIH) of outbred rats was trained to self-administer nicotine using the social learning protocol. Both male and female HS rats acquired nicotine SA, but females self-administered more nicotine than males. After extinction, the context previously paired with nicotine SA, in conjunction with socially transmitted drug cues, was sufficient to cause reinstatement of drug-seeking behavior. Wide variation in both nicotine intake and reinstatement was observed. Using multiple regression analysis, we found that measures of social interaction were significant predictors of nicotine intake and reinstatement of drug seeking in both males and females. Furthermore, measures of depression were predictors of nicotine intake in both males and females, anxiety was a predictor only in males and response to novelty was a predictor only in females. In males, measures of both depression and anxiety predicted nicotine reinstatement. Together, these data supported the ideas that genetically determined propensities for emotional and social phenotypes are significant determinants for nicotine-reinforced behavior, and that the HS rat is a suitable tool for dissecting genetic mechanisms that may underlie the interaction between social behavior, anxiety, depression and smoking.

Reinforcement enhancing effects of nicotine via smoking

RATIONALE

In animals, nicotine enhances reinforcement from stimuli unrelated to nicotine intake. Human research is suggestive but has not clearly shown a similar influence of nicotine.

OBJECTIVES

We assessed acute effects of nicotine via smoking on enhancement of positive (money, music) or negative (termination of noise) reinforcers, or no "reward" (control). These different rewards determined the generalizability of nicotine effects.

MATERIALS AND METHODS

Dependent (n = 25) and nondependent (n = 27) smokers participated in three sessions, each after overnight abstinence. Using a within-subjects design, sessions involved no smoking or smoking denicotinized (0.05 mg) or nicotine (0.6 mg) Quest(R) brand cigarettes. For comparison, a fourth session involved no abstinence prior to smoking one's own brand to gauge responses under typical nicotine satiation. Reinforcement was assessed by responses on a simple operant computer task for the rewards, each available singly on a progressive ratio schedule during separate trials.

RESULTS

The reinforcing effect of music, but not other rewards, was greater due to the nicotine cigarette, compared to the denicotinized cigarette or no smoking. Reinforcement enhancing effects of nicotine did not differ between dependent and nondependent groups, indicating no influence of withdrawal relief. Responding due to acute nicotine after abstinence was very similar to responding to one's own brand after no abstinence.

CONCLUSIONS

Acute nicotine intake per se from smoking after abstinence enhances the reinforcing value of rewards unassociated with smoking, perhaps in a manner comparable to ad lib smoking after no abstinence. Nicotine's reinforcement enhancing effects may be specific to certain rewards, perhaps those sensory in nature.

Translational research in nicotine dependence

Nicotine addiction accounts for 4.9 million deaths each year. Furthermore, although smoking represents a significant health burden in the United States, at present there are only three FDA-approved pharmacotherapies currently on the market: (1) nicotine replacement therapy, (2) bupropion, and (3) varenicline. Despite this obvious gap in the market, the complexity of nicotine addiction in addition to the increasing cost of drug development makes targeted drug development prohibitive. Furthermore, using combinations of mouse and human studies, additional treatments could be developed from off-the-shelf, currently approved medication lists. This article reviews translational studies targeting manipulations of the cholinergic system as a viable therapeutic target for nicotine addiction.

The effect of nicotine on sign-tracking and goal-tracking in a Pavlovian conditioned approach paradigm in rats

RATIONALE

Nicotine (NIC) potently increases operant responding for non-NIC reinforcers, and this effect may depend on drug-mediated increases in incentive motivation. According to this hypothesis, NIC should also potently increase approach to Pavlovian-conditioned stimuli associated with rewards.

OBJECTIVE

The present studies explored the effects of NIC on Pavlovian-conditioned approach responses.

METHOD

To do so, liquid dippers were used to deliver an unconditioned stimulus (US; 0.1 ml sucrose) after presentation of a conditioned stimulus (CS; 30 s illumination of a stimulus light)-both the CS and US were presented in receptacles equipped to monitor head entries.

RESULTS

In experiment 1, the CS and US were presented in the same receptacle, but NIC pretreatment (0.4 mg/kg base) did not increase conditioned approach responses. Delivery of the sucrose US was then shifted to receptacle in a different location. All rats learned to approach the new US location (goal-tracking) at similar rates. Approach to the CS receptacle (sign-tracking) declined for saline-pretreated rats, but NIC pretreatment increased sign-tracking. In experiment 2, NIC pretreatment increased sign-tracking when the CS and US were spatially separated during acquisition. In experiment 3, NIC pretreatments were replaced with saline, but the effect of NIC persisted for an additional 24 test sessions.

CONCLUSION

The findings suggest that NIC increases incentive motivation and that this effect is long-lasting, persisting beyond the pharmacological effects of NIC.

Effects of the combination of metyrapone and oxazepam on intravenous nicotine self-administration in rats

RATIONALE

Despite increased education regarding its dangers, cigarette smoking remains a significant public health concern due to serious associated health consequences such as cancer and respiratory and cardiovascular diseases. Most smokers fail in their attempts to quit smoking, and current pharmacological interventions have relatively low levels of efficacy and are associated with significant adverse events. We have previously reported that combinations of metyrapone and oxazepam, administered at doses that were ineffective when delivered singly, resulted in dose-related decreases in cocaine self-administration in rats while not affecting food-maintained responding during the same sessions.

OBJECTIVES

The current study was designed to test the effects of the administration of a metyrapone:oxazepam combination on nicotine self-administration in rats.

METHODS

Several dose combinations of metyrapone (12.5, 25 or 50 mg/kg) and oxazepam (5 or 10 mg/kg) were tested in rats trained to intravenously (IV) self-administer nicotine (0.03 mg/kg/infusion) during 1-h self-administration sessions using both fixed-ratio and progressive-ratio (PR) schedules of reinforcement.

RESULTS

The administration of low doses of metyrapone and oxazepam in combination significantly decreased IV nicotine self-administration in rats. At the lowest doses of 12.5 mg/kg of metyrapone and 5 mg/kg of oxazepam, the drugs alone did not decrease IV nicotine self-administration, but the combination was effective. Varenicline was also tested using the fixed-ratio schedule, and reductions in nicotine intake were similar to those seen with the moderate dose of the combination.

CONCLUSIONS

The results of this study suggest a potential utility of the combination of metyrapone and oxazepam for smoking cessation in humans.

Evaluation of the effect of ammonia on nicotine pharmacokinetics using rapid arterial sampling

INTRODUCTION

The nicotine bolus theory states that the dependence-producing potential of cigarettes relates to a rapid increase in nicotine at brain receptor sites. It has been suggested that ammonia, a compound typically found in tobacco products, further increases the amount of nicotine absorbed and its absorption rate. The aim of this study was to determine whether different ammonia yields in cigarettes affected the rate or amount of nicotine absorption from the lungs to arterial circulation.

METHODS

34 adult smokers received 3 separate puffs from each of 2 test cigarettes with different ammonia yields (ammonia in smoke: 10.1 μg per cigarette vs. 18.9 μg per cigarette), followed by rapid radial arterial blood sampling (maximum one sample per second) with 30 min between puffs. Arterial blood samples were assayed for nicotine by liquid chromatography tandem mass spectrometry. Pharmacokinetic modeling was performed and the two test cigarettes were assessed for bioequivalence.

RESULTS

No significant differences were found in area under the curve, C(max), or T((max)) and the 2 test cigarettes were found to be bioequivalent based on 2 one-sided tests at a significance level of 5%. In addition, the zero-order rate constant (k(0)) obtained from the initial slope of the curves and the model-dependent first-order rate constant (k(a)) were not significantly different.

CONCLUSIONS

This study provides strong evidence that the different ammonia yields of the test cigarettes had no impact on nicotine pharmacokinetics; thus, the ammonia did not increase the rate or amount of nicotine absorption from a puff of cigarette smoke.

Enhanced attenuation of nicotine discrimination in rats by combining nicotine-specific antibodies with a nicotinic receptor antagonist

Tobacco addiction requires activation by nicotine of a variety of central nicotinic acetylcholine receptors (nAChRs). In animals, both nAChR antagonists and immunization against nicotine can reduce nAChR activation by nicotine and block a variety of addiction-relevant behaviors. However, clinical use of nAChR antagonists for smoking cessation is limited by dose-related side effects, and immunization does not reliably produce sufficient antibody levels in smokers to enhance smoking cessation rates. Combining these approaches may be one way of addressing the limitations of each while enhancing overall efficacy. This study examined the individual and combined effects of passive immunization with the monoclonal nicotine-specific antibody Nic311 and the nicotinic receptor antagonist mecamylamine (MEC) on nicotine's discriminative stimulus effects. Rats were trained to discriminate 0.4 mg/kg of nicotine from saline using a two-lever operant discrimination procedure. Antagonism of nicotine discrimination by Nic311 (160 mg/kg i.v.) and ascending doses of MEC (0.03, 0.1, 0.3, and 1.0 mg/kg s.c.) was assessed across four consecutive daily 2-min extinction test sessions using a 2×2 design. Nic311 alone produced a 24-48% reduction in % nicotine-lever responding (%NLR) across all four test sessions. MEC produced a dose-dependent decrease in %NLR, with no effect at the two lowest doses and 80-93% attenuation at the two highest doses. Nic311 combined with MEC significantly suppressed %NLR at every MEC dose (85-92% reduction across all four test sessions). Very low doses of MEC that were ineffective alone completely blocked nicotine discrimination when combined with Nic311. These data demonstrate that nicotine-specific antibodies and MEC can work synergistically to suppress the subjective effects of nicotine and suggest that low doses of MEC may significantly enhance the efficacy of immunotherapy.

Nicotinic acetylcholine receptor variants are related to smoking habits, but not directly to COPD

Genome-wide association studies identified single nucleotide polymorphisms (SNPs) in the nicotinic acetylcholine receptors (nAChRs) cluster as a risk factor for nicotine dependency and COPD. We investigated whether SNPs in the nAChR cluster are associated with smoking habits and lung function decline, and if these potential associations are independent of each other. The SNPs rs569207, rs1051730 and rs8034191 in the nAChR cluster were analyzed in the Vlagtwedde-Vlaardingen cohort (n = 1,390) that was followed for 25 years. We used GEE and LME models to analyze the associations of the SNPs with quitting or restarting smoking and with the annual FEV(1) decline respectively. Individuals homozygote (CC) for rs569207 were more likely to quit smoking (OR (95%CI) = 1.58 (1.05-2.38)) compared to wild-type (TT) individuals. Individuals homozygote (TT) for rs1051730 were less likely to quit smoking (0.64 (0.42; 0.97)) compared to wild-type (CC) individuals. None of the SNPs was significantly associated with the annual FEV(1) decline in smokers and ex-smokers. We show that SNPs in the nAChR region are associated with smoking habits such as quitting smoking, but have no significant effect on the annual FEV(1) decline in smokers and ex-smokers, suggesting a potential role of these SNPs in COPD development via smoking habits rather than via direct effects on lung function.

Nicotine-taking and nicotine-seeking in C57Bl/6J mice without prior operant training or food restriction

The ability to examine genetically engineered mice in a chronic intravenous (IV) nicotine self-administration paradigm will be a powerful tool for investigating the contribution of specific genes to nicotine reinforcement and more importantly, to relapse behavior. Here we describe a reliable model of nicotine-taking and -seeking behavior in male C57BL/6J mice without prior operant training or food restriction. Mice were allowed to self-administer either nicotine (0.03mg/kg/infusion) or saline in 2-h daily sessions under fixed ratio 1 (FR1) followed by FR2 schedules of reinforcement. In the nicotine group, a dose-response curve was measured after the nose-poke behavior stabilized. Subsequently, nose-poke behavior was extinguished and ability of cue presentations, priming injections of nicotine, or intermittent footshock to reinstate responding was assessed in both groups. C57BL/6J mice given access to nicotine exhibited high levels of nose-poke behavior and self-administered a high number of infusions as compared to mice given access to saline. After this acquisition phase, changing the unit-dose of nicotine resulted in a flat dose-response curve for nicotine-taking and subsequently reinstatement of nicotine-seeking behavior was achieved by both nicotine-associated light cue presentation and intermittent footshock. Nicotine priming injections only triggered significant reinstatement on the second consecutive day of priming. In contrast, mice previously trained to self-administer saline did not increase their responding under those conditions. These results demonstrate the ability to produce nicotine-taking and nicotine-seeking behavior in naive C57BL/6J mice without both prior operant training and food restriction. Future work will utilize these models to evaluate nicotine-taking and relapsing behavior in genetically-altered mice.

Blockade of dopamine d4 receptors attenuates reinstatement of extinguished nicotine-seeking behavior in rats

Since cloning of the dopamine receptor D4 (DRD4), its role in the brain has remained unclear. It has been reported that polymorphism of the DRD4 gene in humans is associated with reactivity to cues related to tobacco smoking. However, the role of DRD4 in animal models of nicotine addiction has seldom been explored. In our study, male Long-Evans rats learned to intravenously self-administer nicotine under a fixed-ratio (FR) schedule of reinforcement. Effects of the selective DRD4 antagonist L-745,870 were evaluated on nicotine self-administration behavior and on reinstatement of extinguished nicotine-seeking behavior induced by nicotine-associated cues or by priming injections of nicotine. L-745,870 was also tested on reinstatement of extinguished food-seeking behavior as a control. In addition, the selective DRD4 agonist PD 168,077 was tested for its ability to reinstate extinguished nicotine-seeking behavior. Finally, L-745,870 was tested in Sprague Dawley rats trained to discriminate administration of 0.4 mg/kg nicotine from vehicle under an FR schedule of food delivery. L-745,870 significantly attenuated reinstatement of nicotine-seeking induced by both nicotine-associated cues and nicotine priming. In contrast, L-745,870 did not affect established nicotine self-administration behavior or reinstatement of food-seeking behavior induced by food cues or food priming. L-745,870 did not produce nicotine-like discriminative-stimulus effects and did not alter discriminative-stimulus effects of nicotine. PD 168,077 did not reinstate extinguished nicotine-seeking behavior. As DRD4 blockade by L-745,870 selectively attenuated both cue- and nicotine-induced reinstatement of nicotine-seeking behavior, without affecting cue- or food-induced reinstatement of food-seeking behavior, DRD4 antagonists are potential therapeutic agents against tobacco smoking relapse.

Electrophysiological correlates of associative learning in smokers: a higher-order conditioning experiment

BACKGROUND

Classical conditioning has been suggested to play an important role in the development, maintenance, and relapse of tobacco smoking. Several studies have shown that initially neutral stimuli that are directly paired with smoking are able to elicit conditioned responses. However, there have been few human studies that demonstrate the contribution of higher-order conditioning to smoking addiction, although it is assumed that higher-order conditioning predominates learning in the outside world. In the present study a higher-order conditioning task was designed in which brain responses of smokers and non-smokers were conditioned by pairing smoking-related and neutral stimuli (CS1smoke and CS1neutral) with two geometrical figures (CS2smoke and CS2neutral). ERPs were recorded to all CSs.

RESULTS

Data showed that the geometrical figure that was paired with smoking stimuli elicited significantly larger P2 and P3 waves than the geometrical figure that was paired with neutral stimuli. During the first half of the experiment this effect was only present in smokers whereas non-smokers displayed no significant differences between both stimuli, indicating that neutral cues paired with motivationally relevant smoking-related stimuli gain more motivational significance even though they were never paired directly with smoking. These conclusions are underscored by self-reported evidence of enhanced second-order conditioning in smokers.

CONCLUSIONS

It can be concluded that smokers show associative learning for higher-order smoking-related stimuli. The present study directly shows the contribution of higher-order conditioning to smoking addiction and is the first to reveal its electrophysiological correlates. Although results are preliminary, they may help in understanding the etiology of smoking addiction and its persistence.

Therapeutic vaccines for chronic diseases: successes and technical challenges

Chronic, non-communicable diseases are the major cause of death and disability worldwide and have replaced infectious diseases as the major burden of society in large parts of the world. Despite the complexity of chronic diseases, relatively few predisposing risk factors have been identified by the World Health Organization. Those include smoking, alcohol abuse, obesity, high cholesterol and high blood pressure as the cause of many of these chronic conditions. Here, we discuss several examples of vaccines that target these risk factors with the aim of preventing the associated diseases and some of the challenges they face.