Route of nicotine administration influences in vivo dopamine neuron activity: habituation, needle injection, and cannula infusion

Mesolimbic dopamine (DA) systems play a critical role in tobacco addiction driven by nicotine. Nicotine activates midbrain DA neurons and, consequently, elevates DA concentrations in targets, especially in the nucleus accumbens (NAc) of the ventral striatum. The route of drug administration influences the impact of addictive drugs. Here, we examine whether the nature of the administration alters DA neuron activity and DA concentrations in the NAc. Using unhabituated naïve freely moving rats, microdialysis measurements showed that nicotine administered via needle injection caused greater DA release in the NAc than the same dose administered via an implanted chronic cannula. After habituation to the needle injections, however, there was no significant difference in DA signaling between the needle and cannula routes of administration. Consistent with these microdialysis results after habituation, our in vivo tetrode unit recordings showed no significant difference in midbrain DA neuron activity in response to nicotine delivered by needle or cannula as long as predictive cues were avoided

Tobacco addiction: a biochemical model of nicotine dependence

Nicotine is the main psychoactive substance present in tobacco, targeting in the CNS the nicotinic acetylcholine receptors (nAChR). The main effects of nicotine associated with smoking are nAChR upregulation, nAChR desensitization and modulation of the dopaminergic system. However, there is a lack of a comprehensive explanation of their roles that effectively makes clear how nicotine dependence might be established on those grounds. Receptor upregulation is an unusual effect for a drug of abuse, because theoretically this implies less need for drug consumption. Receptor upregulation and receptor desensitization are commonly viewed as opposite, homeostatic mechanisms. We here analyze the available information under a model in which both receptor upregulation and receptor desensitization are responsible for establishing a mechanism of nicotine dependence, consequently having an important role in starting and maintaining tobacco addiction. We propose that negative feedbacks on dopamine release regulated by alpha4beta2 nAChRs are disrupted by nicotine. nAChR desensitization is the disrupting mechanism, while nAChR upregulation is the reinforcing process of nicotine dependence, which eventually initiates tobacco addiction. A conclusion of the model is that drugs used for smoking cessation should inhibit preferentially alpha4beta2 nAChRs and to have a low or null ability to upregulate nAChRs, as this characteristic allows the smoker to achieve downregulation without abstinence symptoms. A relationship between this hypothesis and smoking and schizophrenia is also discussed.

Effect of a history of major depressive disorder on smoking-induced dopamine release

BACKGROUND

Dopamine (DA) system dysfunction is implicated in the pathophysiology of major depressive disorder (MDD). We sought to determine if cigarette smokers with a history of MDD and current mild depressive symptoms have abnormal smoking-induced DA release (measured indirectly as change in (11)C-raclopride binding potential [BP(ND)]).

METHODS

Fifty-six cigarette smokers either with (n = 10) or without (n = 46) a history of MDD (MDD+ and MDD-, respectively) underwent bolus-plus-continuous-infusion (11)C-raclopride positron emission tomography, during which they smoked a regular cigarette. Presmoking to postsmoking changes in (11)C-raclopride BP(ND) were compared between groups. Also, correlations were determined between change in BP(ND) and depression, anxiety, and withdrawal rating scale scores for the MDD+ group.

RESULTS

The MDD+ group had a significantly greater reduction in (11)C-raclopride BP(ND) (-16.3%) than the MDD- group (-8.4%) (analysis of covariance [ANCOVA], p = .03). Significant negative correlations were found between depression/anxiety and change in (11)C-raclopride BP(ND) (r = -.77, p < .01 and r = -.74, p = .01, respectively).

CONCLUSIONS

MDD+ smokers have greater smoking-induced DA release than MDD- smokers, and higher depression/anxiety levels are associated with greater smoking-induced DA release. These findings support the theory that MDD+ smokers have DA system dysfunction, including heightened smoking-induced DA release.

Nicotinic receptor-based therapeutics and candidates for smoking cessation

Tobacco dependence is the most preventable cause of death and is a chronic, relapsing disorder in which compulsive tobacco use persists despite known negative health consequences. All currently available cessation agents (nicotine, varenicline and bupropion) have limited efficacy and are associated with high relapse rates, revealing a need for more efficacious, alternative pharmacotherapies. The major alkaloid in tobacco, nicotine, activates nicotinic receptors (nAChRs) which increase brain extracellular dopamine producing nicotine reward leading to addiction. nAChRs are located primarily presynaptically and modulate synaptic activity by regulating neurotransmitter release. Subtype-selective nAChR antagonists that block reward-relevant mesocorticolimbic and nigrostriatal dopamine release induced by nicotine may offer advantages over current therapies. An innovative approach is to provide pharmacotherapies which are antagonists at nAChR subtypes mediating nicotine evoked dopamine release. In addition, providing multiple medications with a wider array of targets and mechanisms should provide more treatment options for individuals who are not responsive to the currently available pharmacotherapies. This review summarizes the currently available smoking cessation therapies and discusses emerging potential therapeutic approaches employing pharmacological agents which act as antagonists at nicotinic receptors.

D-Cycloserine attenuates reactivity to smoking cues in nicotine dependent smokers: a pilot investigation

Increasing evidence indicates that smoking cues contribute to nicotine self-administration and attenuating conditioned reactivity to smoking cues may aid abstinence of smoking and prevention of smoking relapse in individuals with nicotine dependence. Based on prior studies showing that the partial N-methyl-D-aspartate (NMDA) agonist D-cycloserine (DCS) facilitates extinction of learned fear during behavioral exposure therapy in humans and facilitates extinction of cocaine-induced conditioned place preference in animals, we evaluated whether DCS would have potential for reducing reactivity to smoking cues when combined with cue exposure treatment in humans with nicotine dependence. In this double-blind placebo-controlled pilot laboratory study, 25 smokers were recruited from the general community and randomized to DCS or placebo, plus cue exposure therapy. DCS significantly attenuated smoking cue reactivity in response to in-vivo smoking cues based on physiological reactivity and subjective urge-to-smoke ratings and led to a significantly smaller expired carbon monoxide (CO) level at the one-week follow-up compared to placebo, although exploratory analyses indicated no effect on smoking behavior overall. These findings provide promising support for DCS combined with cue exposure therapy in attenuating conditioned reactivity to smoking cues.

Exposure to smoking cues during an emotion recognition task can modulate limbic fMRI activation in cigarette smokers

Smoking cues (SCs) refer to smoking-associated environmental stimuli that may trigger craving and withdrawal symptoms, and predispose to relapse in smokers. Although previous brain imaging studies have explored neural responses to SCs, no study has characterized the effects of SCs on cerebral activity in smokers engaged in an attention-demanding cognitive task that is unrelated to smoking. Thirteen tobacco smokers and a demographically matched group of 13 healthy non-smokers participated in a fast event-related functional magnetic resonance imaging (fMRI) study that involved a visual task integrating SCs and neutral cues (NCs) during emotion recognition trials requiring a high level of attention. No significant SC-induced alterations were detected in smokers' behavioural performance. fMRI results show that non-smokers exhibited no difference between SC and NC trials; in contrast, smokers showed SC-induced widespread deactivations in a limbic, paralimbic and striatal network classically involved in addiction, and activation in the right dorsolateral prefrontal cortex. In addition, a correlation between deactivation of the right insula and the severity of smoking dependence (Fagerström test) was detected in smokers. These results suggest that the neural reactivity of smokers to SCs can be modified in the context of a cognitive challenge. This could reflect smokers' ability to inhibit cue-induced craving and may help in smoking cessation.

Hypocretin mechanisms in nicotine addiction: evidence and speculation

BACKGROUND

The hypocretin/orexin system has been implicated in arousal mechanisms, sleep, and sleep disorders, including narcolepsy, and more recently in drug addiction. Theoretically, hypocretin (hcrt) mechanisms appear to be potential substrates for nicotine addiction: arousal and attentional mechanisms influence use and withdrawal symptoms, and hcrt systems overlap anatomically with a number of brain regions associated with nicotine addiction.

OBJECTIVE

This review summarizes the studies that have examined hcrt mechanisms in the effects of nicotine and describes hcrt innervation of, and effects in, several brain regions implicated in nicotine addiction. The review speculates on the possible mechanisms by which hcrt may contribute to nicotine addiction in these regions, with the objective of encouraging research in this area.

RESULTS

In a small literature, both experimenter-administered and self-administered nicotine have been shown to elicit or depend on hcrt signaling. However, although untested in experimental designs, there is compelling evidence that hcrt mechanisms in the ventral tegmental area, the pontine region, thalamocortical circuits, the prefrontal cortex, and the amygdala could have a broad influence on nicotine addiction.

CONCLUSIONS

Evidence reviewed leads to the conclusion that hcrt mechanisms could mediate several dimensions of nicotine addiction, including a multi-faceted regulation of mesocorticolimbic dopaminergic function, but beyond dopaminergic mechanisms, hcrt could influence nicotine use and relapse during abstinence through broadly based arousal/attentional effects. These speculative ideas need to be examined experimentally; the potential gains are a more thorough understanding of the pathophysiology of nicotine addiction, and the discovery of novel targets for the development of pharmacotherapeutics.

Tobacco dependence and withdrawal: science base, challenges and opportunities for pharmacotherapy

Several pharmacotherapies for tobacco dependence and withdrawal have been approved by the Food and Drug Administration to aid smoking cessation. These medicines double to triple the odds of cessation compared to placebo, with the diversity in chemical entity (e.g., nicotine, varenicline, bupropion) and route (e.g., nicotine gum and transdermal patch) providing options for people who find a given medication unacceptable or ineffective. Treatments in development include vaccines, combinations of existing products, and new indications, such as reduced tobacco use and exposure. These therapies have been developed on the foundation of research on the neuropharmacology of tobacco dependence and withdrawal. Ongoing research is expected to contribute to more efficacious use of existing therapies and the development of new approaches. This article addresses these developments as well as the challenges to medication development. Challenges include understanding the population-based and individual differences in the vulnerability to dependence and responsiveness to various treatment options, which could contribute to effective treatment to patient matching. Research on the CNS effects of administration and withdrawal of nicotine and other tobacco product constituents is expanding, providing the basis for more effective therapeutic approaches and new medications development. Additionally, whereas medications are approved on the basis of standardized assessments of efficacy and safety in clinical trials, the public health impact of medications depends also on their appeal to smokers and their effectiveness in actual use settings. Research on more effective medication use along with policies that support improved access and utilization are vital to conquering the tobacco epidemic.

Positron emission tomographic measure of brain dopamine dependence to nicotine as a model of drugs of abuse

RATIONALE

Nicotine/tobacco are prototypic substances used throughout the world. Nicotine abstinence produces some depressive-like effects which are treated by the dopamine (DA) and norepinephrine reuptake inhibitor bupropion. A quantitative measure of the regional brain utilization of these catecholamines (CA) during nicotine dependence and withdrawal is important.

OBJECTIVE

The aim of this study was to prove that regional brain DA utilization by nicotine can be quantified by positron emission tomography (PET) using L-[beta-(11)C]DOPA.

MATERIALS AND METHODS

Eight young Macaca mulatta monkeys were given 0.9% NaCl or nicotine in doses of 32 or 100 microg/kg i.m. bid for 9 days to produce minimal dependence. On the tenth day, PET measurements were repeated before and after i.v. nicotine administration. PET studies were done in habituated, trained, and fully conscious animals.

RESULTS

Compared to a 0.9% NaCl control, acute i.v. nicotine as a bolus plus infusion for 30 min in similar doses to maintain a steady-state level for 30 min did not affect the utilization rate constant (k (3)) in dorsal or ventral striatum as measured by L-[beta-(11)C]DOPA. When monkeys were given nicotine bid repeatedly after overnight nicotine abstinence, CA utilization was reduced. A subsequent nicotine dose normalized utilization to slightly above control levels. Changes in ventral striatum were similar to those in dorsal striatum. The reduced rate of utilization demonstrated with L-[beta-(11)C]DOPA after overnight nicotine abstinence and its reversal by nicotine the next day provides an important PET measure of brain nicotine dependence and withdrawal. This method can be applied to other substances of abuse that release DA.

Subjective responses to intravenous nicotine: greater sensitivity in women than in men

Although approximately 45% of smokers in the United States are women, the influence of sex on nicotine dependence remains incompletely understood. Evidence from preclinical and clinical studies has indicated that there are significant sex differences in nicotine's effects. The authors' goal in this report was to determine whether men and women differ in their acute response to intravenous nicotine, which has not been examined in previous studies. Twelve male and 12 female smokers received saline followed by 0.5 mg/70 kg and 1.0 mg /70 kg nicotine intravenously. In response to nicotine, women, as compared with men, had enhanced ratings for drug strength, head rush, and bad effects. Women and men experienced similar suppression of smoking urges by nicotine as assessed by the Brief Questionnaire on Smoking Urges. Nicotine-induced heart rate and systolic and diastolic blood pressure increases were also similar in magnitude in men and women. The findings, consistent with those of several previous studies, support greater sensitivity of female smokers to some but not all of the subjective effects of nicotine. Further studies are warranted to examine the role of this differential nicotine sensitivity to development of nicotine dependence and response to nicotine replacement treatments in men and women.

Biological models for studying and assessing tobacco use

The purpose of this chapter on biological models for studying and assessing tobacco use is to provide an introduction to some of the common concepts and biomarkers in this arena to ultimately inform intervention research by nurse scientists. An overview of selected biomarkers of tobacco exposure in individuals includes exhaled carbon monoxide, cotinine (the proximate metabolite of nicotine), and measurement of an individual's puffing pattern termed smoking topography. Common tobacco contents discussed include tobacco specific nitrosamines (TSNA) and polycyclic aromatic hydrocarbons (PAH) some of which increase disease risk including cancer. Exemplars of additives to cigarettes by the tobacco industry will be described including menthol, one additive marketed by the industry. Genetics and tobacco addiction has emerged as a rapidly expanding field. Illustrative of this area are twin studies, nicotinic receptors, CYP2A6 polymorphisms, and genes that impact dopamine receptors. The cadre of nurse scientists conducting research in this much needed area is small. The opportunity for nurse scientists educated in biological inquiry in tobacco-related research is great. Nurse scientists actively involved in multidisciplinary translational teams to address nicotine addition are needed.

[The influence of tobacco smoking on the occurrence of copper in hydroxiapatites of gallstones]

The studies were carried out on the changes in copper content in concretions of gallstones which were taken from 146 women (49 smoking and 97 no smoking) and 24 men (11 smoking and 13 no smoking), respectively. The elemental composition of gallstones was determined with ICP-AES method (Inductively Coupled Plasma - Atomic Emission Spectroscopy). Studies on copper occurrence in concretions of gallstones showed the presence of its higher concentrations in case of smoking women (9.59 microg/g) and men (8.99 microg/g) in comparison with no smoking women (7.02 microg/g) and men (7.66 microg/ g). It was shown differences in pattern of changes of copper concentration in gallstones in case of smoking and no smoking women and men.

Varenicline for tobacco dependence

A 57-year-old man with a 60-pack-year history visits his primary care provider because he wants to quit smoking. He has a history of stable coronary artery disease, peripheral vascular disease, and hypertension. He also has severe obstructive lung disease (forced expiratory volume in 1 second, 39% of the predicted value) with a progressive increase in dyspnea. He has quit smoking and has had numerous relapses; the longest duration of abstinence from smoking was 7 months. Each relapse involved casual smoking to “test himself.” During previous attempts to quit, he has used a nicotine patch, a nicotine inhaler, and sustained-release bupropion. He feels motivated to quit smoking because he recently heard about a new medication to aid in quitting, and he wants to improve his health. The patient and his physician discuss the therapeutic options and agree that varenicline (Chantix) may be an option.

The endocannabinoid system: a new molecular target for the treatment of tobacco addiction

Tobacco addiction is one of the leading preventable causes of mortality in the world and nicotine appears to be the main critical psychoactive component in establishing and maintaining tobacco dependence. Several lines of evidence suggest that the rewarding effects of nicotine, which underlie its abuse potential, can be modulated by manipulating the endocannabinoid system. For example, pharmacological blockade or genetic deletion of cannabinoid CB(1) receptors reduces or eliminates many behavioral and neurochemical effects of nicotine that are related to its addictive potential. This review will focus on the recently published literature about the role of the endocannabinoid system in nicotine addiction and on the endocannabinoid system as a novel molecular target for the discovery of medications for tobacco dependence.

Disrupting nicotine reinforcement: from cigarette to brain

Cigarette smoking is a tenacious addiction that is maintained to a significant extent by the reinforcing effects of nicotine. An emerging theme in smoking cessation treatment is the development of methods for interfering with these reinforcing effects. By attenuating nicotine reinforcement, treatments may enhance a smoker's chances of successfully remaining abstinent. Several treatment approaches will be described, including the use of denicotinized cigarettes, nicotine vaccines, nicotinic receptor agonists and antagonists, and modulators of brain reinforcement processes. These techniques highlight the numerous sites along the path between the cigarette and the brain that can be targeted for intervention. In addition to unimodal therapies, treatment combinations will be discussed that might more effectively block cigarette reward and thereby further enhance smoking abstinence.

Measuring nicotine intake among highly-dependent adolescent smokers: comparability of saliva and plasma cotinine concentrations

Cotinine is the most common biomarker used to assess nicotine exposure and abstinence. It can be measured in various matrices including saliva, plasma, and urine. Previous research with adults has shown high correlations between saliva and plasma cotinine concentrations. However, the research has not examined this relationship in adolescents. Additionally, variability in saliva flow and metabolism across gender, ethnicity, and age may impact the relationship between saliva and plasma cotinine concentration. Our aim was to examine the relationship between saliva and plasma cotinine concentration in a group of nicotine-dependent adolescent smokers. Additionally, we examined these correlations across gender, ethnicity and age. The sample consisted of 66 adolescent smokers (age 15.1+/-1.3, 63.6% girls, 66.7% European American, CPD 18.3+/-8.5, FTND 7.1+/-1.3). Saliva and plasma specimens were collected before the treatment phase of a nicotine replacement therapy trial and analyzed. The relationship between saliva and plasma cotinine concentration was analyzed using Pearson's correlation coefficients. We performed a secondary analysis using multiple regressions to compare correlations across race, gender and age. Results indicated a positive correlation between saliva cotinine and plasma cotinine concentration (r=0.84, p<0.001). Differences in correlations across age were significant (t=3.03, p<0.01). Differences across ethnicity approached significance (t=-1.93, p=0.058). Future research should seek to further validate saliva-to-plasma cotinine concentration ratios in adolescents as well as characterize saliva-to-plasma concentration differences and their underlying mechanisms.

Smoking kills (alcoholics)! shouldn't we do something about it?

In general, 'drinkers smoke', and a high proportion of the alcohol-dependent population is also nicotine-dependent. Statistically, the majority of alcoholics will die of smoking-related, rather than alcohol-related, disease. This co-dependent sub-population may have higher levels of nicotine dependence, and find smoking cessation more difficult. Major reasons are that concurrent alcohol use, and/or prior alcohol exposure, may change the reinforcing effects of nicotine, and that each drug becomes a pharmacological cue for the expectation of the other. If so, then smokers whose nicotine dependence is impacted by alcohol, represent a large and distinct sub-population in which both the therapeutic and molecular targets for smoking cessation are altered. This, in turn, has implications for the validity of animal models of nicotine reinforcement, and for the development of novel smoking cessation medications. It is no longer possible to ignore the fact that the two most prevalent and damaging addictive drugs in our society are very commonly used by the same individuals. Without a better understanding of the psychological and pharmacological interactions between alcohol and nicotine that impact dependence, we cannot hope to provide appropriate medications for this large and problematic patient group. Our intention in this opinion overview is to use the current literature to provide a framework for future studies into the impact of alcohol use on the reinforcing effects of nicotine.

Secretion of total solids (solutes) in the saliva of long-term tobacco users

BACKGROUND

It is generally believed that repeated exposure of a receptor to a stimulus results in inactivation (suppression or adaptation) of the receptor. Most of the methods of tobacco use are linked to the oral cavity where the taste receptors, a primary site for stimulation of salivary secretion, are constantly exposed to tobacco for long time. The present study was designed to document changes in salivary concentration of total solids in response to this effect in chronic tobacco users, if any.

METHODS

Subjects of the study were divided into smokers, pan (tobacco-betel-lime quid) chewers, niswar (moist oral snuff) dippers and non tobacco users as controls. The saliva of each subject was collected under resting condition and following application of crude nicotine and citric acid solutions to the tip of his tongue.

RESULTS

After stimulation with nicotine all groups showed a decrease in the total solids concentration but the decrease was not significant in any group. After stimulation with citric acid, further decrease was seen in all groups but it was also insignificant statistically.

CONCLUSION

We conclude that the total solids concentration decreases with the increase in salivary flow rates (and vice versa) in long-term tobacco users, salivary reflex is not adversely affected by long-term use of tobacco and this observation is not much different from that in non users.

Regional brain activity correlates of nicotine dependence

Fifteen smokers participated in a study investigating brain correlates of nicotine dependence. Dependence was reduced by having subjects switch to denicotinized cigarettes for 2 weeks while wearing nicotine skin patches. Positron emission tomography (PET) scans assessed regional cerebral metabolic rate for glucose (rCMRglc) after overnight nicotine abstinence on three occasions: (1) at baseline; (2) after 2 weeks of exposure to denicotinized cigarettes+nicotine patches; and (3) 2 weeks after returning to smoking the usual brands of cigarettes. Craving for cigarettes and scores on the Fagerström Test of Nicotine Dependence (FTND) questionnaire decreased at the second session relative to the first and last sessions. Regional brain metabolic activity (normalized to whole brain values) at session 2 also showed a significant decrease in the right hemisphere anterior cingulate cortex. Exploratory post hoc analyses showed that the change in craving across sessions was negatively correlated with the change in rCMRglc in several structures within the brain reward system, including the ventral striatum, orbitofrontal cortex and pons. The between-session difference in thalamus activity (right hemisphere) was positively correlated with the difference in FTND scores. Correlational analyses also revealed that reported smoking for calming effects was associated with a decrease (at session 2) in thalamus activity (bilaterally) and with an increase in amygdala activity (left hemisphere). Reported smoking to enhance pleasurable relaxation was associated with an increase in metabolic activity of the dorsal striatum (caudate, putamen) at session 2. These findings suggest that reversible changes in regional brain metabolic activity occur in conjunction with alterations in nicotine dependence. The results also highlight the likely role of thalamic gating processes as well as striatal reward and corticolimbic regulatory pathways in the maintenance of cigarette addiction.

Higher nicotine and carbon monoxide levels in menthol cigarette smokers with and without schizophrenia

This study examined whether smoking menthol cigarettes was associated with increased biochemical measures of smoke intake. Expired carbon monoxide (CO) and serum nicotine and cotinine were measured in 89 smokers with schizophrenia and 53 control smokers immediately after smoking an afternoon cigarette. Serum nicotine levels (27 vs. 22 ng/ml, p = .010), serum cotinine levels (294 vs. 240 ng/ml, p = .041), and expired CO (25 vs. 21 ppm, p = .029) were higher in smokers of menthol compared with nonmenthol cigarettes, with no differences in 3-hydroxycotinine/cotinine ratios between groups when controlling for race. Backward stepwise linear regression models showed that, in addition to having a diagnosis of schizophrenia, smoking menthol cigarettes was a significant predictor of nicotine and cotinine levels. Individuals with schizophrenia or schizoaffective disorder smoked more generic or discount value brands (Basic, Doral, Monarch, USA, Wave, others) compared with control smokers (28% vs. 6%, p = .002) but did not smoke more brands with high nicotine delivery as estimated by the U.S. Federal Trade Commission method. Although rates of mentholated cigarette smoking were not higher in smokers with schizophrenia overall, they were significantly higher in non-Hispanic White people with schizophrenia compared with controls of the same ethnic/racial subgroup (51% vs. 28%, p<.0001). The higher exhaled CO in menthol smokers suggests that the higher nicotine levels are at least partly related to increased intake of smoke from menthol cigarettes, although menthol-mediated inhibition of nicotine metabolism also may be a factor. Menthol is an important cigarette additive that may help explain why some groups have lower quit rates and more smoking-caused disease.

Role of acetaldehyde in tobacco smoke addiction

This review evaluates the presumed contribution of acetaldehyde to tobacco smoke addiction. In rodents, acetaldehyde induces reinforcing effects, and acts in concert with nicotine. Harman and salsolinol, condensation products of acetaldehyde and biogenic amines, may be responsible for the observed reinforcing effect of acetaldehyde. Harman and salsolinol inhibit monoamine oxidase (MAO), and some MAO-inhibitors are known to increase nicotine self-administration and maintain behavioural sensitization to nicotine. Harman is formed in cigarette smoke, and blood harman levels appear to be 2-10 times higher compared to non-smokers. Since harman readily passes the blood-brain barrier and has sufficient MAO-inhibiting potency, it may contribute to the lower MAO-activity observed in the brain of smokers. In contrast, the minor amounts of salsolinol that can be formed in vivo most likely do not contribute to tobacco addiction. Thus, acetaldehyde may increase the addictive potential of tobacco products via the formation of acetaldehyde-biogenic amine adducts in cigarette smoke and/or in vivo, but further research is necessary to substantiate this hypothesis.

Negative Allosteric Modulation of Nicotinic Acetylcholine Receptors Blocks Nicotine Self-Administration in Rats

Drugs that antagonize nicotinic acetylcholine receptors (nAChRs) can be used to inhibit nicotine-induced behavior in both humans and animals. The aim of our experiments is to establish a proof-of-principle that antagonism of nAChRs by negative allosteric modulation can alter behavior in a relevant animal model of addiction, nicotine self-administration. We have identified a novel, negative allosteric modulator of nAChRs, UCI-30002 [N-(1,2,3,4-tetrahydro-1-naphthyl)-4-nitroaniline], with selectivity for the major neuronal nAChR subtypes over muscle-type nAChRs. After systemic administration, UCI-30002 significantly reduces nicotine self-administration in rats on both fixed ratio and progressive ratio schedules of reinforcement. The minimum effective dose that significantly alters nicotine self-administration corresponds to brain concentrations of UCI-30002 that produce at least 30% inhibition of the major neuronal nAChR subtypes measured in vitro. UCI-30002 has no effect on responding for food reinforcement in rats on either type of schedule, indicating that there is no effect on general responding or natural reward. UCI-30002 represents validation of the concept that negative allosteric modulators may have significant benefits as a strategy for treating nicotine addiction and encourages the development of subtype-selective modulators.

Chronic bupropion attenuated the anhedonic component of nicotine withdrawal in rats via inhibition of dopamine reuptake in the nucleus accumbens shell

Bupropion, a dopamine reuptake inhibitor, is an effective therapy for smoking cessation, but the behavioral and neurochemical mechanisms mediating its antismoking properties are relatively unknown. To explore the hypothesis that bupropion ameliorates nicotine withdrawal partly by a dopamine-dependent mechanism, we investigated the effects of chronic bupropion on potassium-stimulated dopamine overflow in the nucleus accumbens shell in nicotine-withdrawing rats. We also assessed the effects of chronic bupropion on behavioral aspects of nicotine withdrawal measured by elevations in brain reward thresholds and somatic signs of withdrawal. Rats were treated with nicotine or saline for 7 days and then coadministration of bupropion or saline was initiated. After 14 days of coadministration of bupropion/saline and nicotine/saline, nicotine/saline administration was terminated, whereas bupropion/saline administration continued. These conditions mimic bupropion administration in human smokers. Cessation of nicotine administration in non-bupropion-treated rats elevated reward thresholds reflecting a reward deficit, increased somatic signs and diminished potassium-evoked dopamine overflow in the nucleus accumbens shell. Chronic bupropion lowered reward thresholds and increased potassium-evoked dopamine release regardless of previous nicotine exposure, possibly by inhibition of dopamine reuptake, and thus attenuated the anhedonic and neurochemical effects of nicotine withdrawal. Chronic bupropion blocked withdrawal-associated increased somatic signs. Finally, acute experimenter-administered nicotine enhanced brain reward function equally in all groups, indicating that bupropion does not alter the reward-facilitating effects of experimenter-administered nicotine. In conclusion, the bupropion-induced increase in extracellular dopamine in the nucleus accumbens shell may ameliorate the anhedonia associated with nicotine withdrawal, which in turn may facilitate smoking cessation.

Individual differences in response to novelty predict prefrontal cortex dopamine transporter function and cell surface expression

Previous research has shown that individual differences in response to novelty predict self-administration and the locomotor response to psychostimulant drugs of abuse. The aim of the present study was to determine if individual differences in response to novelty based on inescapable or free-choice novelty tests predict dopamine transporter (DAT) function and trafficking as well as nicotine-induced modulation of DAT. Results show that the maximal velocity (Vmax) of [3H]dopamine uptake into prefrontal cortex (PFC) synaptosomes correlated negatively with the locomotor response to inescapable novelty. In contrast, Vmax correlated positively with novelty place preference in the free-choice novelty test. The divergent correlations between DAT and the two behavioral phenotypes suggest a differential contribution of DAT in these phenotypes, which are known not to be isomorphic. Furthermore, rats categorized as high responders to inescapable novelty had lower Vmax values, which were accompanied by less DAT expression at the cell surface in PFC compared with low responders, suggesting that inherent individual differences in DAT cellular localization may underlie the differential response to novelty. Compared with the saline control, nicotine increased Vmax and cell surface DAT immunoreactivity in PFC from high responders but not from low responders. Similarly, nicotine increased Vmax and cell surface DAT in PFC in rats classified as low in novelty place preference but not in rats classified as high in novelty place preference. Thus, despite the different behavioral phenotypes, the pharmacological effect of nicotine to increase DAT function and cell surface expression was apparent, such that rats with inherently lower DAT function show a greater sensitivity to the neurochemical effect of nicotine.

The neurobiological bases for the pharmacotherapy of nicotine addiction

Nicotine, the major psychoactive agent present in tobacco, acts as a potent addictive drug both in humans and laboratory animals, whose locomotor activity is also stimulated. A large body of evidence indicates that the locomotor activation and the reinforcing effects of nicotine may be related to its stimulatory effects on the mesolimbic dopaminergic function. Thus, it is now well established that nicotine can increase in vivo DA outflow in the nucleus accumbens and the corpus striatum. The stimulatory effect of nicotine on DA release most probably results from its ability to excite the neuronal firing rate and to increase the bursting activity of DA neurons in the substantia nigra pars compacta (SNc) and the ventral tegmental area (VTA), and from its stimulatory action on DA terminals in the corpus striatum and the nucleus accumbens. The neurochemical data are consistent with neuroanatomical findings showing the presence of nicotinic acetylcholine receptors (nAChRs) in the SNc, the VTA, and in projection areas of the central dopaminergic system such as the corpus striatum and the nucleus accumbens. Several lines of evidence indicate that the reinforcing properties of drugs of abuse, including nicotine, can be affected by a number of transmitter systems which may act by modulating central dopaminergic function. In this paper, the neurobiological mechanisms underlying nicotine addiction will be reviewed, and the possible strategies for new pharmacological treatments of nicotine dependence will be examined.