Monoamine oxidase and cigarette smoking

Nicotine and brain development

Preclinical studies, using primarily rodent models, have shown acetylcholine to have a critical role in brain maturation via activation of nicotinic acetylcholine receptors (nAChRs), a structurally diverse family of ligand-gated ion channels. nAChRs are widely expressed in fetal central nervous system, with transient upregulation in numerous brain regions during critical developmental periods. Activation of nAChRs can have varied developmental influences that are dependent on the pharmacologic properties and localization of the receptor. These include regulation of transmitter release, gene expression, neurite outgrowth, cell survival, and synapse formation and maturation. Aberrant exposure of fetal and neonatal brain to nicotine, through maternal smoking or nicotine replacement therapy (NRT), has been shown to have detrimental effects on cholinergic modulation of brain development. These include alterations in sexual differentiation of the brain, and in cell survival and synaptogenesis. Long-term alterations in the functional status and pharmacologic properties of nAChRs may also occur, which result in modifications of specific neural circuitry such as the brainstem cardiorespiratory network and sensory thalamocortical gating. Such alterations in brain structure and function may contribute to clinically characterized deficits that result from maternal smoking, such as sudden infant death syndrome and auditory-cognitive dysfunction. Although not the only constituent of tobacco smoke, there is now abundant evidence that nicotine is a neural teratogen. Thus, alternatives to NRT should be sought as tobacco cessation treatments in pregnant women.

Methylphenidate enhances the abuse-related behavioral effects of nicotine in rats: intravenous self-administration, drug discrimination, and locomotor cross-sensitization

Stimulant drugs, including D-amphetamine, cocaine, and methylphenidate, increase cigarette smoking in controlled human laboratory experiments. Although the mechanism(s) underlying this effect are unknown, it is possible that stimulants may enhance directly the abuse-related effects of nicotine. In the present study, we characterized the behavioral pharmacological interactions between methylphenidate and nicotine in the intravenous self-administration, drug discrimination, and locomotor cross-sensitization procedures. Adult male Sprague-Dawley rats were trained to respond for intravenous nicotine (0.01 or 0.03 mg/kg/infusion) or sucrose, and the acute effects of methylphenidate (1.25-10 mg/kg) were determined; in addition, separate groups of rats were treated with methylphenidate (2.5 mg/kg) or saline before 12 consecutive nicotine (0.03 mg/kg/infusion) self-administration sessions. Next, the discriminative stimulus effects of nicotine (0.03-0.3 mg/kg) and methylphenidate (1.25-10 mg/kg), alone and in combination with a low nicotine dose (0.056 mg/kg), were tested in nicotine-trained rats. Finally, the locomotor effect of repeated methylphenidate (2.5 mg/kg) was tested in rats previously treated with nicotine (0.2-0.8 mg/kg). Results indicated that acute methylphenidate increased the rate of nicotine self-administration at doses that reduced sucrose-maintained responding; furthermore, tolerance to this effect was not apparent following repeated methylphenidate. Methylphenidate, while not substituting for nicotine alone, dose-dependently enhanced the discriminative stimulus effect of a low nicotine dose. In addition, repeated nicotine exposure promoted the development of locomotor sensitization to methylphenidate. Taken together with recent clinical findings, these results suggest that methylphenidate may enhance the abuse-related behavioral effects of nicotine, perhaps increasing vulnerability to tobacco dependence.

Smoking predicts posttraumatic stress symptoms among rescue workers: a prospective study of ambulance personnel involved in the Enschede Fireworks Disaster

BACKGROUND

Examining whether smoking is a risk factor for posttraumatic stress disorder (PTSD) symptoms among rescue workers affected by a disaster.

METHODS

Ambulance personnel (N=66) participated in surveys 2-3 weeks (T1) and 18 months after a fireworks disaster (T2). Hierarchical multiple regression analyses were conducted with cigarette consumption at T1 as a predictor of PTSD symptoms at T2. Demographic characteristics, disaster experiences, peritraumatic dissociation, intrusions and avoidance, psychological distress and alcohol consumption assessed at T1 were included as covariates.

RESULTS

Regression analyses showed that smoking at T1 independently predicted intrusions, avoidance, hostility, and depression symptoms at T2. Results were not affected by controlling for post-disaster critical incidents at work.

CONCLUSIONS

This is the first prospective study among rescue workers demonstrating that smoking soon after a disaster predicts PTSD symptoms in the intermediate term. Findings substantiate results of previous studies indicating that smoking is a relevant risk factor. Future research on how changes in cigarettes consumption post-trauma affect risk of PTSD is required.

The monoamine oxidase inhibitor phenelzine enhances the discriminative stimulus effect of nicotine in rats

In addition to delivering nicotine, tobacco smoke also inhibits monoamine oxidase (MAO). Although MAO inhibitors (MAOIs) can increase nicotine self-administration in rodents, the effects of MAOIs on the discriminative stimulus effect of nicotine are not known. This study examined the effects of three MAOIs (phenelzine, clorgyline and pargyline) with varying selectivity for MAOA and MAOB in the nicotine drug discrimination procedure in rats. Adult male Sprague-Dawley rats were trained to discriminate nicotine (0.3 mg/kg, subcutaneously) from saline in a standard, two-lever food-reinforced operant task. Once the discrimination was acquired, the ability of each MAOI to substitute for or alter the discriminative stimulus effect of nicotine was determined. In substitution tests, nicotine (0.03-0.3 mg/kg) produced full, dose-dependent substitution. Although the selective MAOA inhibitor clorgyline (3-56 mg/kg) and the selective MAOB inhibitor pargyline (3-56 mg/kg) did not elicit any nicotine-appropriate responding, partial substitution was obtained with the nonselective MAO inhibitor phenelzine (1-17 mg/kg). Phenelzine (10 mg/kg) also enhanced the discriminative stimulus effect of a low dose of nicotine (0.056 mg/kg) and prolonged the time course effect of the nicotine-training dose. These findings indicate that concomitant inhibition of MAOA and MAOB can enhance the discriminative stimulus effect of nicotine in rats.

Imaging the addicted human brain

Modern imaging techniques enable researchers to observe drug actions and consequences as they occur and persist in the brains of abusing and addicted individuals. This article presents the five most commonly used techniques, explains how each produces images, and describes how researchers interpret them. The authors give examples of key findings illustrating how each technique has extended and deepened our knowledge of the neurobiological bases of drug abuse and addiction, and they address potential clinical and therapeutic applications.

Nicotinic Receptors: Role in Addiction and Other Disorders of the Brain

Nicotine, the addictive component of cigarette smoke has profound effects on the brain. Activation of its receptors by nicotine has complex consequences for network activity throughout the brain, potentially contributing to the addictive property of the drug. Nicotinic receptors have been implicated in psychiatric illnesses like schizophrenia and are also neuroprotective, potentially beneficial for neurodegenerative diseases. These effects of nicotine serve to emphasize the multifarious roles the drug, acting through multiple nicotinic acetylcholine receptor subtypes. The findings also remind us of the complexity of signaling mechanisms and stress the risks of unintended consequences of drugs designed to combat nicotine addiction.

Is MAO-B activity in platelets associated with the occurrence of suicidality and behavioural personality traits in depressed patients?

OBJECTIVE

Low platelet monoaminoxidase B (MAO-B) activity has been associated with various forms of impulsive behaviour and suicidality. The present study investigated the relationship between MAO-B activity in platelets and aspects of suicidality in depressed patients and controls.

METHOD

In 87 patients with affective spectrum disorders (58% suffering from a major depressive episode - MDE) the potential association between platelet MAO-B activity and suicidality was examined. Fifty-nine of the patients had committed suicide attempt recently (SA -'suicide attempters'), 28 patients were acutely depressed without having shown suicidal thoughts or suicidal behaviour in the past (NA -'non-suicide attempters').

RESULTS

The SA and NA were comparable as to their diagnoses and general demographic and psychopathological parameters. MAO-B activity did not differ between SA and NA. No systematic correlations existed between MAO-B activity and any dimensions of suicidal behaviour or psychopathology. As a single finding only a weak positive association of higher MAO-B activity in SA with a fatal intention of the SA was observed.

CONCLUSION

Our findings do not support a consistent association of platelet MAO-B activity and suicidal behaviour in general, but specific facts of suicidality might be associated.

Effects of progression to cigarette smoking on depressed mood in adolescents: evidence from the National Longitudinal Study of Adolescent Health

AIMS

To investigate the relationship between smoking status and continuously distributed depressed mood among a cohort of adolescents.

DESIGN

Quasi-experimental design, selecting the subset of adolescents who reported never having smoked a cigarette at baseline, some of whom progressed subsequently to smoking at follow-up approximately 1 year later.

SETTING

Data were drawn from the National Longitudinal Study of Adolescent Health, an ongoing study designed to assess the health status of adolescents, and explore the causes of adolescent health-related behaviours.

PARTICIPANTS

Nationally representative sample of adolescents from the USA (n = 12 149), including a subsample who reported never having smoked a cigarette at baseline (n = 5475), aged on average 15 years at baseline and of predominantly European ancestry.

MEASUREMENTS

Logistic and linear regression models controlling for potential confounders to explore the relationship between smoking status and depressed mood measured using the Centers for Epidemiological Studies Depression Scale (CES-D).

FINDINGS

Various relationships between smoking status and depressed mood were observed, with a general trend for these effects to be greater among females. Smoking status at baseline did not significantly predict CES-D score at follow-up, although this effect approached significance in females (P = 0.077). Among never smokers at baseline, level of depressed mood at baseline predicted subsequent progression to smoking initiation (P = 0.022) but not progression to regular smoking (P = 0.229). Among never smokers at baseline, progression to smoking initiation during the follow-up period was associated with higher CES-D scores at follow-up, even after adjusting for baseline depressed mood (P < 0.001), with this effect greater for females than for males. Among those who initiated smoking, progression to regular smoking was associated with higher CES-D score at follow-up among females (P = 0.001), but not males (P = 0.966).

CONCLUSIONS

These data appear to support a complex model of the relationship between depressed mood and smoking status which includes elements of both confounding and causal models. The relationship between cigarette smoking and depression may be a factor in the development of subsequent dependence.

Monoamine oxidase inhibition for tobacco pharmacotherapy

Tobacco addiction is the most significant preventable cause of morbidity and mortality in the Western world, with >430,000 deaths annually from tobacco-related diseases being reported in the United States. Although effective treatments are available for cessation of smoking (e.g., nicotine replacement therapies, sustained-release bupropion and varenicline), they do not work for all smokers. Therefore the development of more effective medications for treating tobacco dependence, based on novel mechanisms, is a high priority. This article reviews the links between smoking and monoamine oxidase (MAO) inhibition, which could lead to the development of novel pharmacotherapies to treat tobacco dependence.

Introducing oral tobacco for tobacco harm reduction: what are the main obstacles?

With the number of smokers worldwide currently on the rise, the regular failure of smokers to give up their tobacco addiction, the direct role of smoke (and, to a much lesser extent, nicotine) in most tobacco-related diseases, and the availability of less toxic (but still addictive) oral tobacco products, the use of oral tobacco in lieu of smoking for tobacco harm reduction (HR) merits assessment.

Instead of focusing on the activity itself, HR focuses on the risks related to the activity. Currently, tobacco HR is controversial, generally not discussed, and consequently, poorly evaluated.

In this paper, we try to pinpoint some of the main reasons for this lack of interest or reluctance to carry out or fund this type of research. In this paper we deal with the following issues: the status of nicotine in society, the reluctance of the mainstream anti-tobacco lobby toward the HR approach, the absence of smokers from the debate, the lack of information disseminated to the general population and politicians, the need to protect young people, the role of physicians, the future of HR research, and the role of tobacco companies.

Epidemiologic studies of environmental exposures in Parkinson's disease

Parkinson's disease (PD) is the most common cause of the parkinsonian syndromes and the most frequent neurodegenerative disease after Alzheimer's disease. Because of the ageing of Western populations, an increasing number of persons will be affected with PD in the future and neither curative treatments nor preventive measures have been identified. PD is considered as a multifactorial disease, resulting from the effect of environmental factors and genetic susceptibility. Increasing age and male sex appear to be associated with an increased risk of PD. In addition, recent epidemiological studies have identified environmental exposures that influence the risk of PD. This review provides an overview of the epidemiologic evidence for environmental etiologies in PD; we will focus on two environmental exposures that have been quite consistently associated with PD -- cigarette smoking and pesticide exposure -- and will summarize briefly the findings for other exposures. Understanding the mechanisms underlying these epidemiological associations is an essential step for the understanding of the etiology of this neurodegenerative condition and, ideally, to develop neuroprotective drugs.

Monoamine oxidase (MAO) intron 13 polymorphism and platelet MAO-B activity in combat-related posttraumatic stress disorder

BACKGROUND

The neurobiology of posttraumatic stress disorder (PTSD) involves alterations in multiple neuroendocrine and neurotransmitter systems. Platelet monoamine oxidase (MAO-B) has been associated with susceptibility to various psychiatric disorders, personality traits and behaviors.

METHODS

Platelet MAO-B activity and MAO-B intron 13 polymorphism (a G/A substitution) were determined in male war veterans (n=106) with DSM-IV diagnosed current and chronic PTSD, divided into subgroups of PTSD patients with (n=28) or without (n=78) psychotic features, combat exposed veterans (n=41) who did not develop PTSD, and healthy control men (n=242).

RESULTS

Two-way ANOVAs revealed a significant effect of diagnosis and smoking, a significant effect of smoking, no significant effect of genotype, and no significant interaction between genotype, smoking or diagnosis, on platelet MAO-B activity. One-way ANOVAs showed significantly lower platelet MAO-B activity in smokers than in nonsmokers. After controlling for smoking, veterans with psychotic PTSD had significantly higher platelet MAO-B activity than veterans with or without PTSD, or healthy subjects.

LIMITATIONS

The results were obtained on peripheral biochemical marker, i.e. platelet MAO activity.

CONCLUSIONS

The MAO-B intron 13 polymorphism was not functional, and did not affect platelet MAO-B activity. The allele frequencies of the MAO-B genotype were similarly distributed among healthy controls and veterans with or without PTSD and/or psychotic symptoms. The results suggest that platelet MAO-B activity, controlled for smoking status, might be used as a peripheral marker of the psychotic symptoms in PTSD.

Gestational exposure to nicotine and monoamine oxidase inhibitors influences cocaine-induced locomotion in adolescent rats

RATIONALE

Many pregnant women continue to smoke, despite a strong association between maternal smoking and neurobehavioral deficits in the offspring. Although gestational nicotine (GN) treatment in rodents is used as the primary animal model of maternal smoking, tobacco smoke contains more than 4,000 constituents, including monoamine oxidase inhibitors (MAOIs).

OBJECTIVES

The aim of this study was to determine whether there are interactions between the effects of gestational exposure to nicotine and MAOIs on cocaine-induced locomotor sensitization in adolescent rats.

MATERIALS AND METHODS

Pregnant rats were implanted on day 4 of gestation with osmotic minipumps delivering saline, nicotine (3 mg/kg per day), the MAOIs clorgyline and deprenyl (1 and 0.25 mg/kg per day, respectively), or nicotine/clorgyline/deprenyl (GMN). Adolescent female offspring were tested for cocaine-induced locomotor sensitization. Animals were treated with saline or cocaine (5 or 15 mg/kg, intraperitoneally) daily from postnatal (P) days 32-36 and challenged with cocaine (15 mg/kg) on P51 (day 20).

RESULTS

Group differences were observed in chronic but not acute effects of cocaine. Whereas gestational MAOI treatment, with or without nicotine, increased ambulatory response to cocaine on day 5, the opposite was found for vertical activity. Different adaptive responses were observed on cocaine challenge day. GNM animals exhibited enhanced locomotor activity in the cocaine-associated environment before cocaine challenge on day 20. In contrast, only GN animals exhibited significant locomotor sensitization to the cocaine challenge.

CONCLUSIONS

Gestational nicotine and MAOIs both influence brain development. Such interactions may sensitize adolescents to drug abuse and should be considered in animal models of maternal smoking.

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.

Genetics of nicotine dependence and pharmacotherapy

Nicotine dependence is substantially heritable. Several regions across the genome have been implicated in containing genes that confer liability to nicotine dependence and variation in individual genes has been associated with nicotine dependence. Smoking cessation measures are also heritable, and measured genetic variation is associated with nicotine dependence treatment efficacy. Despite significant strides in the understanding of the relative contribution of genetic and environmental factors to nicotine dependence and treatment, emergent challenges necessitate interdisciplinary coordinated effort for effective problem solving. These challenges include refinement of the nicotine dependence phenotype, better understanding of the dynamic interplay between genes and environment in nicotine dependence etiology, application and development of molecular and statistical methodology that can adequately address vast amounts of data, and continuous translational cross-talk.

Smoking Cessation Pharmacotherapy?Nicotine and Non-Nicotine Preparations

Cigarette smokers tend to die prematurely from a number of diseases. Cigarette smoking is an important modifiable risk factor for cardiovascular morbidity and mortality. Despite the clear health benefits of smoking cessation, smokers usually find it difficult to stop and behavioral therapies often prove insufficient. Pharmacologic intervention may aid the process because of the addictive nature of nicotine. Nicotine replacement therapy, which is regarded as first-line therapy, was developed to overcome the symptoms of nicotine withdrawal that many patients find distressing. Different modes of administration include inhalation and buccal or transdermal absorption. The orally administered non-nicotine drugs varenicline and bupropion are also regarded as first-line treatments, either used alone or as an adjunct to nicotine replacement therapy. Second-line treatments include clonidine and nortriptyline. Other treatment strategies that have been examined include monoamine oxidase inhibitors and selective serotonin reuptake inhibitors; efficacy has yet to be proven definitively. A novel approach to treatment using the cannabinoid-1 receptor antagonist rimonabant is also under investigation.

Non-Nicotinic Therapies for Smoking Cessation

Cigarette smoking is the primary cause of numerous preventable diseases; as such, the goals of smoking cessation are both to reduce health risks and to improve the quality of life. Currently, the first-line smoking cessation therapies include nicotine replacement products and bupropion. The nicotinic receptor partial agonist varenicline has recently been approved by the FDA for smoking cessation. A newer product currently under development and seeking approval by the FDA are nicotine vaccines. Clonidine and nortriptyline have demonstrated some efficacy but side effects may limit their use to second-line therapeutic products. Other therapeutic drugs that are under development include rimonabant, mecamylamine, monoamine oxidase inhibitors, and dopamine receptor D3 antagonists. Inhibitors of nicotine metabolism are also promising candidates for smoking reduction and cessation. In conclusion, promising new therapeutic products are emerging and they will provide smokers additional options to assist in achieving smoking cessation.

New pyrrole inhibitors of monoamine oxidase: synthesis, biological evaluation, and structural determinants of MAO-A and MAO-B selectivity

A series of new pyrrole derivatives have been synthesized and evaluated for their monoamine oxidase (MAO) A and B inhibitory activity and selectivity. N-Methyl,N-(benzyl),N-(pyrrol-2-ylmethyl)amine (7) and N-(2-benzyl),N-(1-methylpyrrol-2-ylmethyl)amine (18) were the most selective MAO-B (7, SI = 0.0057) and MAO-A (18, SI = 12500) inhibitors, respectively. Docking and molecular dynamics simulations gave structural insights into the MAO-A and MAO-B selectivity. Compound 18 forms an H-bond with Gln215 through its protonated amino group into the MAO-A binding site. This H-bond is absent in the 7/MAO-A complex. In contrast, compound 7 places its phenyl ring into an aromatic cage of the MAO-B binding pocket, where it forms charge-transfer interactions. The slightly different binding pose of 18 into the MAO-B active site seems to be forced by a bulkier Tyr residue, which replaces a smaller Ile residue present in MAO-A.

Effects of high dose transdermal nicotine replacement in cigarette smokers

RATIONALE

Nicotine replacement therapies (NRT) have been evaluated to facilitate cigarette smoking reduction in smokers unwilling or unable to quit. In most of these studies, only conventional doses of NRT have been tested and higher doses may be required to result in significant reductions in smoking and in biomarkers of exposure.

OBJECTIVE

To determine if higher NRT doses in conjunction with smoking are safe and may promote significant reductions in cigarette smoking and biomarkers of exposure.

METHODS

A dose-ranging, within-subject design was implemented to evaluate the effects of 15, 30 and 45 mg nicotine-patch treatment on measures of safety and the extent of smoking reduction and biomarker exposure per cigarette in smokers (N=20 completers) not immediately interested in quitting.

RESULTS

Concurrent smoking and NRT were generally tolerated and resulted in no changes in blood pressure or heart rate. Slightly less than 10% of the study sample was not given the highest dose of NRT due to side effects. Self-reported cigarette smoking decreased with increasing doses of nicotine replacement and significant reductions were observed for total NNAL (a carcinogen biomarker) and carbon monoxide. However, even at the 45 mg dose, increased carbon monoxide and total NNAL per cigarette occurred, even though cotinine levels increased on average, 69.3% from baseline.

CONCLUSIONS

The present results suggest that the use of high dose NRT is safe, leads to significant reductions in smoking (-49%), significant but less reductions in total NNAL (-24%) and carbon monoxide (-37%) due to compensatory smoking.

Monoamine oxidase and tobacco dependence

Tobacco smoking is a leading cause of preventable death around the world, and there are major public health and research efforts in many countries aimed at reducing its usage. However, the molecular mechanisms underlying tobacco dependence are still not completely understood. Nicotine's action on nicotinic acetylcholine receptors, and the downstream release of dopamine, is believed to be the major pathway underlying tobacco dependence. However there is mounting evidence indicating that non-nicotinic components of tobacco smoke also play a role by inhibiting monoamine oxidase (MAO) and subsequently altering neurotransmitter levels. This article provides a review of the current knowledge of the association between MAO and tobacco dependence and suggests that further research into this topic is likely to lead to more effective pharmacotherapies for smoking cessation.

Differential nicotinic regulation of the nigrostriatal and mesolimbic dopaminergic pathways: implications for drug development

Neuronal nicotinic acetylcholine receptors (nAChRs) modulate dopaminergic function. Discovery of their multiplicity has lead to the search for subtype-selective nAChR agonists that might be therapeutically beneficial in diseases linked to brain dopaminergic pathways. The regulation and responses of the nigrostriatal and mesolimbic dopaminergic pathways are often similar, but some differences do exist. The cerebral distribution and characteristics of various nAChR subtypes differ between nigrostriatal and mesolimbic dopaminergic pathways. Comparison of nicotine and epibatidine, two nAChR agonists whose relative affinities for various nAChR subtypes differ, revealed differences in the nAChR-mediated regulation of dopaminergic activation between these dopamine systems. Nicotine preferentially stimulates the mesolimbic pathway, whereas epibatidine's stimulatory effect falls on the nigrostriatal pathway. Thus, it may be possible to stimulate the nigrostriatal pathway with selective nAChR agonists that do not significantly affect the mesolimbic pathway, and thus lack addictive properties. Furthermore, dopamine uptake inhibition revealed a novel inhibitory effect of epibatidine on accumbal dopamine release, which could form a basis for novel antipsychotics that could alleviate the elevated accumbal dopaminergic tone found in schizophrenia during the active psychotic state. Different regulation of nigrostriatal and mesolimbic dopaminergic pathways by nAChRs could be an important basis for developing novel drugs for treatment of Parkinson's disease and schizophrenia.

Contribution of monoamine oxidase (MAO) inhibition to tobacco and alcohol addiction

Whole-body PET-scan studies in brains of tobacco smokers have shown a decrease in monoamine oxidase (MAO) activity, which reverts to control level when they quit smoking. The observed decrease in MAO activity in smokers is presumably due to their exposure to tobacco constituents that possess MAO-inhibiting properties. The inhibition of MAO activity seems, however, not to be a unique feature of tobacco smoking as subjects with Type II alcoholism have been reported to show a similar decrease in MAO activity that reverses when they cease to use alcohol. The present review summarizes the data on MAO-inhibiting tobacco constituents and explains that the decrease in MAO activity observed in alcoholics is probably due to concomitant tobacco use. It is concluded that the inhibition of MAO by constituents contained in tobacco and tobacco smoke, enhances the addiction induced by tobacco smoking.

Smoking cessation and depression: current knowledge and future directions

This paper reviews the literature on comorbid smoking and depression. Current models used to explain this co-occurrence are examined, as are treatment options (both psychological and pharmacological). This paper surmises that treatment planning should consider factors that potentially confound treatment efficacy, including the nature of the depressive illness and the patient's smoking profile. Although there is limited research examining the benefits of a stepped-care framework, a tiered treatment format appears to work well, assisting those who require minimal treatment, as well as those who prolonged difficulties. Further research examining a stepped-care framework for smokers at risk of depression is required, as is appropriate training for health practitioners using this model. Further directions for research and practice are also discussed.

Functional brain imaging of tobacco use and dependence

While most cigarette smokers endorse a desire to quit smoking, only about 14% to 49% will achieve abstinence after 6 months or more of treatment. A greater understanding of the effects of smoking on brain function may (in conjunction with other lines of research) result in improved pharmacological (and behavioral) interventions. Many research groups have examined the effects of acute and chronic nicotine/cigarette exposure on brain activity using functional imaging; the purpose of this paper is to synthesize findings from such studies and present a coherent model of brain function in smokers. Responses to acute administration of nicotine/smoking include: a reduction in global brain activity; activation of the prefrontal cortex, thalamus, and visual system; activation of the thalamus and visual cortex during visual cognitive tasks; and increased dopamine (DA) concentration in the ventral striatum/nucleus accumbens. Responses to chronic nicotine/cigarette exposure include decreased monoamine oxidase (MAO) A and B activity in the basal ganglia and a reduction in alpha4beta2 nicotinic acetylcholine receptor (nAChR) availability in the thalamus and putamen. Taken together, these findings indicate that smoking enhances neurotransmission through cortico-basal ganglia-thalamic circuits either by direct stimulation of nAChRs, indirect stimulation via DA release or MAO inhibition, or a combination of these factors. Activation of this circuitry may be responsible for the effects of smoking seen in tobacco dependent subjects, such as improvements in attentional performance, mood, anxiety, and irritability.

Translational neuroimaging: positron emission tomography studies of monoamine oxidase

Positron emission tomography (PET) using radiotracers with high molecular specificity is an important scientific tool in studies of monoamine oxidase (MAO), an important enzyme in the regulation of the neurotransmitters dopamine, norepinephrine, and serotonin as well as the dietary amine, tyramine. MAO occurs in two different subtypes, MAO A and MAO B, which have different substrate and inhibitor specificity and which are different gene products. The highly variable subtype distribution with different species makes human studies of special value. MAO A and B can be imaged in the human brain and certain peripheral organs using PET and carbon-11 (half-life 20.4 minutes) labeled mechanism-based irreversible inhibitors, clorgyline and L -deprenyl, respectively. In this article we introduce MAO and describe the development of these radiotracers and their translation from preclinical studies to the investigation of variables affecting MAO in the human brain and peripheral organs.

The lack of association between monoamine oxidase (MAO) intron 13 polymorphism and platelet MAO-B activity among men

Monoamine oxidase (MAO), a mitochondrial flavine containing enzyme, exists in two isoenzymes, MAO-A and MAO-B. Platelets contain MAO-B subtype, proposed to be a biomarker for different personality characteristics and vulnerability for substance abuse. The most common polymorphism of MAO-B gene, a single base change (A or G) occurs in intron 13. It has been proposed to be a functional polymorphism, controlling the activity of MAO-B in platelets. The aim of the study was to determine the association between platelet MAO-B activity and MAO-B intron 13 polymorphism in 225 racially and ethnically uniform healthy Caucasian men of the Croatian origin. Our results showed that platelet MAO-B activity did not differ between subjects subdivided into those with <<A-allele>> or <<G-allele>>. This polymorphism of the MAO-B gene did not control the activity of the MAO-B in platelets. Platelet MAO-B activity was associated only with the smoking status, and it was significantly decreased in smokers when compared to nonsmokers. No significant association was found between MAO-B polymorphism and smoking status. In healthy individuals of the Croatian origin, the studied MAO-B polymorphism showed a lack of functional importance in regulating MAO-B activity in platelets. Since different populations may vary in the association between functional polymorphism and the MAO-B activity, and the genotype of transcription factor AP-2 beta was reported to be associated with altered platelet MAO-B activity, and with specific personality traits, further studies on different populations should be conducted to elucidate the molecular mechanism/s regulating platelet MAO-B activity.

Human monoamine oxidase enzyme inhibition by coffee and β-carbolines norharman and harman isolated from coffee

Monoamine oxidase (MAO) is a mitochondrial outer-membrane flavoenzyme involved in brain and peripheral oxidative catabolism of neurotransmitters and xenobiotic amines, including neurotoxic amines, and a well-known target for antidepressant and neuroprotective drugs. Recent epidemiological studies have consistently shown that coffee drinkers have an apparently lower incidence of Parkinson's disease (PD), suggesting that coffee might somehow act as a purported neuroprotectant. In this paper, "ready to drink" coffee brews exhibited inhibitory properties on recombinant human MAO A and B isozymes catalyzing the oxidative deamination of kynuramine, suggesting that coffee contains compounds acting as MAO inhibitors. MAO inhibition was reversible and competitive for MAO A and MAO B. Subsequently, the pyrido-indole (beta-carboline) alkaloids, norharman and harman, were identified and isolated from MAO-inhibiting coffee, and were good inhibitors on MAO A (harman and norharman) and MAO B (norharman) isozymes. beta-carbolines isolated from ready-to-drink coffee were competitive and reversible inhibitors and appeared up to 210 microg/L, confirming that coffee is the most important exogenous source of these alkaloids in addition to cigarette smoking. Inhibition of MAO enzymes by coffee and the presence of MAO inhibitors that are also neuroactive, such as beta-carbolines and eventually others, might play a role in the neuroactive actions including a purported neuroprotection associated with coffee consumption.

Analysis of monoamine oxidase enzymatic activity by reversed-phase high performance liquid chromatography and inhibition by beta-carboline alkaloids occurring in foods and plants

Monoamine oxidase (MAO) is a flavin adenine dinucleotide (FAD)-containing enzyme located at the outer membranes of mitochondria that catalyzes the oxidative deamination of biogenic and xenobiotic amines. We have used a chromatographic method to measure MAO-enzymatic activity by using kynuramine as a non-selective substrate with its MAO-oxidation product subsequently analyzed by RP-HPLC-DAD and HPLC-mass spectrometry (MS). This method was applied to study the kinetic parameters, inhibition and reaction products of MAO recombinant enzymes in presence of tetrahydro-beta-carboline and beta-carboline alkaloids occurring in foods, plants and mammals. Analysis by HPLC showed that tetrahydro-beta-carbolines or beta-carbolines were not modified by MAO. Several beta-carbolines such as tryptoline (1,2,3,4-tetrahydro-beta-carboline) and 1-methyltryptoline (1-methyl-1,2,3,4-tetrahydro-beta-carboline) were inhibitors of MAO-A; instead their corresponding 6-hydroxy-derivatives (6-hydroxytryptoline and 6-hydroxy-1-methyltryptoline) lacked this activity. Tetrahydro-beta-carboline-3-carboxylic acids were unable to inhibit MAO enzymes. In contrast, their oxidation products, i.e. the fully aromatic beta-carbolines (norharman and harman), acted as good inhibitors of MAO. Two tetrahydro-beta-carbolines (i.e. tryptoline and 1-methyltryptoline) occurring in foods were isolated by solid-phase extraction (SPE) and RP-HPLC from selected samples of sausages and the corresponding extracts exhibited good inhibition properties over MAO-A. These results suggest that beta-carbolines from foods, plants, and mammals may exert inhibitory actions on MAO enzymes.

Imbalance between drug and non-drug reward availability: A major risk factor for addiction

Laboratory animals self-administer most, though not all, drugs of abuse. Recent evidence shows that with increased drug availability, most laboratory rats develop all the major behavioral signs of addiction, including: 1) drug intake escalation, 2) increased motivation for the drug, 3) difficulty to abstain, 4) decreased reward function, and 5) inflexible drug use. The large prevalence of addicted rats may suggest that they are particularly vulnerable to develop compulsive drug use. I review evidence showing that this apparent vulnerability results in large part from the lack of positive (i.e., alternative non-drug rewards) and negative (i.e., costs) incentives capable of turning animals away from the pursuit of drugs. In particular, most animals seem to take drugs and eventually become addicted, not because drugs are intrinsically addictive, but more likely because drugs are the only significant sources of reward available in the laboratory. Laboratory animals would therefore represent more of a model of high-risk human groups than of the general population. Consequently, they should be more suited for searching factors that protect from, rather than predispose to, drug addiction. Reconsidering the environmental background of drug self-administration experiments in laboratory animals raises intriguing implications for understanding the initial demand for drug consumption and the transition to drug addiction, and for extrapolation from laboratory animals to humans.

Genotype combinations for monoamine oxidase-B intron 13 polymorphism and dopamine D2 receptor TaqIB polymorphism are associated with ever-smoking status among men

Tobacco smoke inhibits monoamine oxidase-B (MAO-B) activity in vitro and in vivo, suggesting that MAO-B inhibition is a possible contributing factor to tobacco smoke addiction. Thus, MAO-B is a possible candidate gene for predisposition to smoking. The TaqIB polymorphism for the Dopamine D2 Receptor gene (DRD2) has been previously associated with smoking status, although with some contradictory results. We investigated whether genetic variants of MAO-B intron 13 and DRD2 TaqIB polymorphism could be associated with smoking status among control subjects. There was no association of the intron 13 polymorphism itself with smoking status in either men or women. Similarly, no association with smoking status was observed for the TaqIB polymorphism of DRD2 itself. However, among men, there was an interaction between MAO-B intron 13 polymorphism and the DRD2 TaqIB polymorphisms, in which subjects carrying MAO-B allele A and genotype B12 of DRD2 were 2.50 times (95% CI=1.05-5.95) more likely to be ever-smokers than the pool of men carrying all other genotype combinations. These results demonstrate that particular combinations of genotypes for MAO-B and DRD2 genes are associated with significantly higher risk for smoking behavior in men, but not in women.

Biomonitoring of inhaled complex mixtures--ambient air, diesel exhaust and cigarette smoke

Human biomonitoring comprises the determination of biomarkers in body-fluids, cells and tissues. Biomarkers are generally assigned to one of three classes, namely, biomarkers of exposure, effect or susceptibility. Since biomarkers represent steps in an exposure-disease continuum, their application in epidemiological studies ('molecular epidemiology') shows promise. However, to be a predictor of disease, a biomarker has to be validated. Validation criteria for a biomarker include intrinsic qualities such as specificity, sensitivity, knowledge of background in the population, existence of dose-response relationships, degree of inter- and intra-individual variability, knowledge of the kinetics, confounding and modifying factors. In addition, properties of the sampling and analytical procedures are of relevance, including constraints and non-invasiveness of sampling, stability of sample as well as simplicity, high sensitivity, specificity and speed of the analytical method. It is of particular importance to prove by suitable studies that the biomarker of exposure indicates the actual exposure, the biomarker of effect strongly predicts the actual risk of disease and the biomarker of susceptibility actually modifies the risk. Biomonitoring of the exposure to complex mixtures such as polluted ambient air, diesel exhaust or tobacco smoke is a particular challenge since these exposures have many constituents in common and many people were exposed to more than one of these mixtures. Data on the exposure to polycyclic aromatic hydrocarbons (PAH) and benzene from ambient air, diesel exhaust and tobacco smoke will be presented. In addition, some source-specific biomarkers such as nitro-arenes and nicotine metabolites as well as their application in population groups will be discussed. The second part of the presentation addresses the application of biomarkers for assessing so called 'potentially reduced exposure products' (PREPs). According to a recent report of the Institute of Medicine (USA), "reducing risk of disease by reducing exposure to tobacco toxicants is feasible" and "surrogate biological markers that are associated with tobacco-related diseases could be used to offer guidance as to whether or not PREPs are likely to be risk-reducing." In general, the same validation criteria apply as discussed above. In addition, it is suggested that a panel of biomarkers should be used, representing both smoke phases (gas and particulate phase) and the various chemical classes of smoke constituents (e.g., carbonyls, benzene, PAH, tobacco-specific nitrosamines, aromatic amines). Also, a panel of biomarkers of effect should cover the major known adverse effects of smoking (e.g., oxidative stress, inflammatory processes, lipid peroxidation, lipometabolic disorders, mutagenic effects). Biomarkers of nicotine and carbon monoxide uptake are of interest for evaluating the smoking and inhalation behavior, respectively. Finally, suitable study designs for evaluating PREPs are discussed. It is concluded that suitable biomarkers for assessing the exposure to complex mixtures such as ambient air, diesel exhaust and tobacco smoke as well as for evaluating the exposure-reducing properties of PREPs are already available. Future efforts should focus on the development and validation of biomarkers of effect.

Evidence for reduced arterial plasma input, prolonged lung retention and reduced lung monoamine oxidase in smokers

We have previously found that smokers have reduced brain monoamine oxidase (MAO) A and B using positron emission tomography (PET) and the irreversible mechanism-based radiotracers [(11)C]-labeled clorgyline (CLG) and deprenyl (DEP) and their deuterated analogs (D CLG, D DEP). More recently, we have estimated MAO A and B activity in other organs using the deuterium isotope effect to determine binding specificity for MAO and a three-compartment model to estimate k(3), the model term proportional to MAO A activity. Here, we have investigated the robustness of the model term k(3) for estimating lung MAO A and B in light of our unexpected finding that lung MAO activity (k(3)) was reduced for smokers relative to nonsmokers, although radiotracer uptake in the lungs was similar at peak and plateau for the two groups.

METHODS

Time-activity data from lung and arterial plasma were used from seven nonsmokers and seven smokers scanned previously with CLG and D CLG, and five nonsmokers and nine smokers scanned previously with DEP and D DEP. The measured time-activity curves for lung and plasma and the integrals for the arterial plasma time-activity curves were compared at an early time point (2.5 min) and at the end of the study (55 min). A three-compartment irreversible model was used to estimate the differences between smokers and nonsmokers, and the stability of the parameter (k(3)) while varying model assumptions for the relative fractions of lung tissue, blood and air in the PET voxel.

RESULTS

The peak in the arterial plasma input function and the integral of the arterial plasma time-activity curve over the first 2.5 min after radiotracer injection were significantly lower for smokers relative to nonsmokers for all four tracers. However, although the peak and plateau of the lung time-activity curves were similar for smokers and nonsmokers, the decline in radioactivity from peak to plateau was slower for smokers for all tracers. Using a three-compartment irreversible model, we estimated the ratio of MAO subtypes A and B in normal lung tissue to be on the order of 3 to 1 (MAO A to B) and that smokers have reduced MAO levels for both subtypes as measured by the model parameter, k(3). The values of k(3) are insensitive to model assumptions of variations in air and tissue fraction in the PET voxel. Most of the effects of changes in these fractions are absorbed into the parameter K(1), which governs the plasma-to-tissue transfer of tracer and is a function of blood flow. K(1) was found to be larger in smokers, although the values depend upon model assumptions of air and tissue fractions. k(3) was found to be significantly lower in smokers; for CLG, a 50% reduction in MAO A for both CLG and D CLG was observed. For DEP, k(3) was also significantly lower in smokers with a reduction of approximately 80% in lung MAO B, although there was a very large coefficient of variation in the smoker's k(3). We also found larger values of lambda (K(1)/k(2)) for smokers relative to nonsmokers for all tracers consistent with a longer lung retention of the nonenzyme-bound tracer, which explains the slower decline in uptake from peak radioactivity for smokers.

CONCLUSIONS

The measured arterial input function values for smokers and nonsmokers are significantly different for these two tracer pairs for nonsmokers and smokers particularly for the first few minutes after radiotracer injection. Model estimates of k(3) that indicate that smokers have lower lung MAO A and B activity than nonsmokers are robust and insensitive to variations in model assumptions for relative fractions of lung tissue, blood and air in the PET voxel. Although we have only investigated the behavior of [(11)C]clorgyline and [(11)C]l-deprenyl and their deuterium-substituted analogs in this report, the extent to which reduced arterial input and longer lung retention also hold for other tracers for subjects who smoke merits investigation.

The nicotine market: An attempt to estimate the nicotine intake from various sources and the total nicotine consumption in some countries

Tobacco--particularly smoked products--has been associated with great harm and growing public disapproval and can be expected to suffer in the marketplace. This situation has created opportunities for other less harmful nicotine-containing products such as smokeless tobacco and nicotine replacement products, which are gaining public support. Little is known about the level of nicotine intake in our society. Tobacco sales are known, but how much nicotine is extracted and actually absorbed by users is largely unknown. The present study is a first attempt to estimate uptake of nicotine from tobacco and nicotine replacement products and to map nicotine consumption in a few countries, with special emphasis on Sweden. Relevant pharmacokinetic studies for three types of nicotine-containing products (cigarettes, smokeless tobacco, and nicotine replacement products) were analyzed for bioavailable nicotine. Estimates of nicotine intake from each category were made. These were then multiplied by the amount consumed in the respective countries. Tobacco consumption statistics were usually from official records of taxed sales. In Sweden about 54% of all nicotine intake comes from smoked sources, 45% from nonsmoked tobacco, and 1.3% from nicotine replacement products. For men, 63% of the nicotine consumed comes from nonsmoked tobacco. Per-capita nicotine intake per year for adults aged 15 years or older is 3,321 mg for Austria, 3,043 mg for Sweden, 3,014 mg for Denmark, 2,955 mg for the United States, 2,244 mg for Norway, and 2,023 mg for Finland. Compared with cigarette smokers, snus users seem to have a somewhat higher daily intake (34 mg vs. 25 mg). The cleanest nicotine products, nicotine replacement products, represent a negligible part (about 1%) of the total nicotine consumption in most countries.

Human monoamine oxidase is inhibited by tobacco smoke: beta-carboline alkaloids act as potent and reversible inhibitors

Monoamine oxidase (MAO) is a mitochondrial outer-membrane flavoenzyme involved in brain and peripheral oxidative catabolism of neurotransmitters and xenobiotic amines, including neurotoxic amines, and a well-known target for antidepressant and neuroprotective drugs. Recently, positron emission tomography imaging has shown that smokers have a much lower activity of peripheral and brain MAO-A (30%) and -B (40%) isozymes compared to non-smokers. This MAO inhibition results from a pharmacological effect of smoke, but little is known about its mechanism. Working with mainstream smoke collected from commercial cigarettes we confirmed that cigarette smoke is a potent inhibitor of human MAO-A and -B isozymes. MAO inhibition was partly reversible, competitive for MAO-A, and a mixed-type inhibition for MAO-B. Two beta-carboline alkaloids, norharman (beta-carboline) and harman (1-methyl-beta-carboline), were identified by GC-MS, quantified, and isolated from the mainstream smoke by solid phase extraction and HPLC. Kinetics analysis revealed that beta-carbolines from cigarette smoke were competitive, reversible, and potent inhibitors of MAO enzymes. Norharman was an inhibitor of MAO-A (K(i)=1.2+/-0.18 microM) and MAO-B (K(i)=1.12+/-0.19 microM), and harman of MAO-A (K(i)=55.54+/-5.3nM). Beta-carboline alkaloids are psychopharmacologically active compounds that may occur endogenously in human tissues, including the brain. These results suggest that beta-carboline alkaloids from cigarette smoke acting as potent reversible inhibitors of MAO enzymes may contribute to the MAO-reduced activity produced by tobacco smoke in smokers. The presence of MAO inhibitors in smoke like beta-carbolines and others may help us to understand some of the purported neuropharmacological effects associated with smoking.

Drug addiction: the neurobiology of behaviour gone awry

Drug addiction manifests as a compulsive drive to take a drug despite serious adverse consequences. This aberrant behaviour has traditionally been viewed as bad "choices" that are made voluntarily by the addict. However, recent studies have shown that repeated drug use leads to long-lasting changes in the brain that undermine voluntary control. This, combined with new knowledge of how environmental, genetic and developmental factors contribute to addiction, should bring about changes in our approach to the prevention and treatment of addiction.

Oxygen as a therapy for reducing nicotine withdrawal symptoms

Nicotine addiction leads to withdrawal symptoms in many persons who quit smoking. In addition to craving, the most commonly experienced symptoms are: depression; difficulty sleeping; irritability, frustration, or anger; anxiety; difficulty concentrating; restlessness; decreased heart rate; and increased appetite or weight gain. The severity of withdrawal symptoms are variable, and often lead to relapse. By altering the course of withdrawal symptoms it is possible to improve the chances of the quit attempt and reduce the risk of relapse. There is strong evidence that dopamine is the primary neurotransmitter in the reward pathway in addiction. The use of nicotine replacement therapies and Zyban (sustained release bupropion hydrochloride), both of which are suspected of exploiting dopamine's role in addiction, enhance the likelihood of long-term smoking cessation. Anecdotal reports of oxygen's capacity to reduce both the desire for nicotine and withdrawal symptoms suggest that this may be a therapeutic possibility for those who do not experience success with more traditional cessation approaches. Oxygen may have a favorable effect on nicotine withdrawal, as it appears to alter the balance of central neurotransmitters such as dopamine.

Comparison of the binding of the irreversible monoamine oxidase tracers, [(11)C]clorgyline and [(11)C]l-deprenyl in brain and peripheral organs in humans

The monoamine oxidase A and B (MAO A and B) radiotracers [(11)C]clorgyline (CLG) and [(11)C]L-deprenyl (DEP) and their deuterium labeled counterparts (CLG-D and DEP-D) were compared to determine whether their distribution and kinetics in humans are consistent with their physical, chemical and pharmacological properties and the reported ratios of MAO A:MAO B in post-mortem human tissues. Irreversible binding was consistently higher for DEP in brain, heart, kidneys and spleen but not lung where CLG >DEP and not in thyroid where there is no DEP binding. The generally higher DEP binding is consistent with its higher enzyme affinity and larger free fraction in plasma while differences in regional distribution for CLG and DEP in brain, heart, thyroid and lungs are consistent with different relative ratios of MAO A and B in humans.

Electrochemical investigation into the redox activity of Fe(II)/Fe(III) in the presence of nicotine and possible relations to neurodegenerative diseases

The biological relevance of Fe(II)/Fe(III) is becoming evermore apparent, especially in relation to its potential role in the progression of neurodegenerative diseases such as Parkinson's and Alzheimer's disease. The reported relationship between smoking and a reduced incidence of neurodegenerative disorders prompted this work. In order to investigate whether nicotine can interact with iron, we have studied the electrochemical behaviour of a Fe(II)/Fe(III) redox couple in the presence of nicotine. Solubility issues and lack of available nonreacting salts of nicotine necessitated studies being conducted at low pH values. Cyclic voltammetry experiments revealed a definite alteration in the electrochemical behaviour of the Fe(II)/Fe(III) redox couple suggesting the capability of nicotine to complex with free iron and, hence, reduce its reactivity. This is evident from a slower rate of heterogeneous electron transfer, ks, and a shift from reversible to quasi-reversible behaviour, as characterised from the diffusion coefficient (D), the full width half maximum (FWHM), DeltaEp and Ef. Additional complexation titrations, pH ranging from 1 to 7, confirm a weak complexation reaction occurring between Fe(III) and nicotine.

Perspectives on MAO-B in aging and neurological disease: where do we go from here?

The catecholamine-oxidizing enzyme monoamine oxidase-B (MAO-B) has been hypothesized to be an important determining factor in the etiology of both normal aging and age-related neurological disorders such as Parkinson's disease (PD). Catalysis of substrate by the enzyme produces H2O2 which is a primary originator of oxidative stress which in turn can lead to cellular damage. MAO-B increases with age as does predisposition towards PD which has also been linked to increased oxidative stress. Inhibition of MAO-B, along with supplementation of lost dopamine via L-DOPA, is one of the major antiparkinsonian therapies currently in use. In this review, we address several factors contributing to a possible role for MAO-B in normal brain aging and neurological disease and also discuss the use of MAO-B inhibitors as drug therapy for these conditions.

MAO-B knockout mice exhibit deficient habituation of locomotor activity but normal nicotine intake

Low levels of monoamine oxidase-B (MAO-B) activity, such as those observed in smokers, are also associated with behavioral traits such as a heightened responsiveness to novelty. However, the exact mechanism by which low MAO-B activity influences smoking and heightened responsiveness to novelty is still poorly understood. We used MAO-B knockout (KO) mice to test the hypothesis that MAO-B concomitantly affects locomotor responses in a novel inescapable open field and nicotine intake. Male wild-type (WT) and MAO-B KO mice were placed in an inescapable open field and their horizontal locomotor activity was measured for 30 min per day for 5 days. MAO-B KO mice exhibited impaired within-session habituation of locomotor activity, as compared to WT mice. Separate groups of male WT and MAO-B KO mice were individually housed in their home cages with two water bottles. One of the bottles contained tap water and the other contained nicotine (0, 3.125, 6.25, 12.5, 25, 50 or 100 micro g/ml). The total amount of water and nicotine solution consumed was measured every three days for 16 days. MAO-B KO mice and WT mice consumed equal amounts of nicotine and exhibited comparable concentration-dependent nicotine preference and aversion over a period of 16 days. The data suggest that the absence of MAO-B impairs the ability of mice to habituate in the inescapable environment, but does not alter their nicotine intake.

Genetics and drug use as a complex phenotype

Drug use is a complex behavior influenced by multiple biological, family, and sociocultural factors. The concurrent use/misuse of multiple drugs is often seen and drug use also co-occurs with other psychiatric conditions. Behavior and molecular genetic studies support an important posited role of genes in drug use. This posited genetic risk does not appear to be conferred by one or two major genes manifesting large effects, but rather by a number of genes manifesting smaller effects. Genetic factors explain, on average, only about half of the total variability in drug use, with the remaining variability influenced by environmental factors. Also, genetic risk may be differentially expressed in the presence vs. absence of particular environmental conditions. Thus, investigation of environmental factors and their interaction with genetic risk is a necessary component of genetic research. While the full potential of genetic investigations for the prevention of drug misuse has yet to be realized, an example of the impact of risk factor modification under various conditions of gene-environment interaction is provided, and the implications for use of genetic information in drug-misuse prevention are discussed. The multifactorial nature of drug use necessitates coordinated investigation from multiple disciplines and timely dissemination of scientific findings. In addition, this work demands adherence to the highest standards of confidentiality and ethical use of genetic information to best inform future prevention efforts.