An acute dose of nicotine does not inhibit MAO B in baboon brain in vivo

Monoamine oxidase: radiotracer chemistry and human studies

Monoamine oxidase (MAO) oxidizes amines from both endogenous and exogenous sources thereby regulating the concentration of neurotransmitter amines such as serotonin, norepinephrine, and dopamine as well as many xenobiotics. MAO inhibitor drugs are used in the treatment of Parkinson's disease and in depression stimulating the development of radiotracer tools to probe the role of MAO in normal human biology and in disease. Over the past 30 years since the first radiotracers were developed and the first positron emission tomography (PET) images of MAO in humans were carried out, PET studies of brain MAO in healthy volunteers and in patients have identified different variables that have contributed to different MAO levels in brain and in peripheral organs. MAO radiotracers and PET have also been used to study the current and developing MAO inhibitor drugs including the selection of doses for clinical trials. In this article, we describe the following: (1) the development of MAO radiotracers; (2) human studies including the relationship of brain MAO levels to genotype, personality, neurological, and psychiatric disorders; and (3) examples of the use of MAO radiotracers in drug research and development. We will conclude with outstanding needs to improve the radiotracers that are currently used and possible new applications.

Whole tobacco smoke extracts to model tobacco dependence in animals

Smoking tobacco is highly addictive and a leading preventable cause of death. The main addictive constituent is nicotine; consequently it has been administered to laboratory animals to model tobacco dependence. Despite extensive use, this model might not best reflect the powerful nature of tobacco dependence because nicotine is a weak reinforcer, the pharmacology of smoke is complex and non-pharmacological factors have a critical role. These limitations have led researchers to expose animals to smoke via the inhalative route, or to administer aqueous smoke extracts to produce more representative models. The aim was to review the findings from molecular/behavioural studies comparing the effects of nicotine to tobacco/smoke extracts to determine whether the extracts produce a distinct model. Indeed, nicotine and tobacco extracts yielded differential effects, supporting the initiative to use extracts as a complement to nicotine. Of the behavioural tests, intravenous self-administration experiments most clearly revealed behavioural differences between nicotine and extracts. Thus, future applications for use of this behavioural model were proposed that could offer new insights into tobacco dependence.

Nicotine-, tobacco particulate matter- and methamphetamine-produced locomotor sensitisation in rats

RATIONALE

Repeated nicotine exposure produces a weak and transient sensitised locomotor response in rats. Since tobacco smoke contains thousands of non-nicotine chemical constituents, these could alter the sensitised response.

OBJECTIVES

This study aims to compare the magnitude, persistence and spatial distribution of locomotor sensitisation produced by repeated doses of nicotine, aqueous tobacco particulate matter (TPM) and a positive methamphetamine control.

METHODS

Male Sprague-Dawley rats received five nicotine (0.0, 0.2 or 0.4 mg/kg), TPM (containing 0.2 or 0.4 mg/kg nicotine) or methamphetamine (0.5 mg/kg) injections every second day, followed by a 4-day withdrawal before the first challenge (Challenge 1, C1). The animals were re-challenged again at 15 days post C1 to test for the persistence of sensitisation (Challenge 2, C2).

RESULTS

There were no major differences in sensitisation profile between nicotine and TPM. At the lowest 0.2 mg/kg nicotine/TPM dose, however, small differences emerged on select test days.

CONCLUSIONS

The results indicated that the non-nicotinic agents in TPM did not greatly impact the nicotine-produced locomotor-sensitised response. These findings might suggest that the differential pharmacological properties of TPM do not have major clinical significance. Alternatively, the locomotor model might not expose effects of non-nicotinic constituents, and furthermore, might not closely relate to human tobacco dependence. Different reward-related behavioural models should also be utilised to assess potential effects of non-nicotinic constituents before a role in dependence is discounted.

Reduced monoamine oxidase A activity in pregnant smokers and in their newborns

BACKGROUND

Tobacco smoking is associated with reduced monoamine oxidase A (MAOA) activity. Smoking-associated low MAOA activities in pregnancy and in newborns may have negative perinatal and postnatal consequences. We aimed to compare, in everyday clinical conditions, biomarkers of MAOA activity in smoking (SPW) and lifetime nonsmoking pregnant women (NSPW) and in cord blood and to assess the newborns' behavior during the first 48 hours of life.

METHODS

Thirty SPW and 29 NSPW in their second trimester of pregnancy were included. Plasma MAOA dependent metabolites of norepinephrine: dihydroxyphenylglycol; dopamine: homovanillic and dihydroxyphenylacetic acid; and serotonin: 5-hydroxy-indol acetic acid were measured at the end of the second trimester, at delivery, and in arterial cord blood along with plasma cotinine. The newborns' discomfort was evaluated every 8 hours by a standardized questionnaire.

RESULTS

The SPW smoked, on average, 73 cigarettes per week at the end of second trimester and 80 cigarettes per week at delivery. Mean plasma cotinine was 84 ng/mL, 105 ng/mL, and 95 ng/mL at the end of second trimester, at delivery, and in cord blood, respectively (NSPW < 10 ng/mL). Plasma markers of MAOA activity, in particular those reflecting dopamine's catabolism, were significantly lower in SPW and in the arterial cord blood of their newborns than in NSPW and their newborns. Newborns of SPW showed significantly more facial discomfort than those of NSPW.

CONCLUSIONS

Smoking is associated with MAOA inhibition in pregnant women and in their newborns at birth. Further studies are needed to estimate the behavioral significance of these findings.

In vivo brain imaging of human exposure to nicotine and tobacco

While most cigarette smokers endorse a desire to quit smoking, only 14-49% will achieve abstinence after 6 months or more of treatment. A greater understanding of the effects of smoking on brain function may result in improved pharmacological and behavioral interventions for this condition. Research groups have examined the effects of acute and chronic nicotine/cigarette exposure on brain activity using functional imaging; the purpose of this chapter 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 reduced 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 (accompanied by an overall upregulation of these receptors). These findings indicate that smoking enhances neurotransmission through cortico-basal ganglia-thalamic circuits by direct stimulation of nAChRs, indirect stimulation via DA release or MAO inhibition, or a combination of these and possibly other factors. Activation of this circuitry may be responsible for the effects of smoking seen in tobacco-dependent smokers, such as improvements in attentional performance, mood, anxiety, and irritability.

Guidelines on nicotine dose selection for in vivo research

RATIONALE

This review provides insight for the judicious selection of nicotine dose ranges and routes of administration for in vivo studies. The literature is replete with reports in which a dosaging regimen chosen for a specific nicotine-mediated response was suboptimal for the species used. In many cases, such discrepancies could be attributed to the complex variables comprising species-specific in vivo responses to acute or chronic nicotine exposure.

OBJECTIVES

This review capitalizes on the authors' collective decades of in vivo nicotine experimentation to clarify the issues and to identify the variables to be considered in choosing a dosaging regimen. Nicotine dose ranges tolerated by humans and their animal models provide guidelines for experiments intended to extrapolate to human tobacco exposure through cigarette smoking or nicotine replacement therapies. Just as important are the nicotine dosaging regimens used to provide a mechanistic framework for acquisition of drug-taking behavior, dependence, tolerance, or withdrawal in animal models.

RESULTS

Seven species are addressed: humans, nonhuman primates, rats, mice, Drosophila, Caenorhabditis elegans, and zebrafish. After an overview on nicotine metabolism, each section focuses on an individual species, addressing issues related to genetic background, age, acute vs chronic exposure, route of administration, and behavioral responses.

CONCLUSIONS

The selected examples of successful dosaging ranges are provided, while emphasizing the necessity of empirically determined dose-response relationships based on the precise parameters and conditions inherent to a specific hypothesis. This review provides a new, experimentally based compilation of species-specific dose selection for studies on the in vivo effects of nicotine.

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.

Isolation and characterization of a monoamine oxidase B selective inhibitor from tobacco smoke

It is well established that tobacco smokers have reduced levels of monoamine oxidase activities both in the brain and peripheral organs. Furthermore, extensive evidence suggests that smokers are less prone to develop Parkinson's disease. These facts, plus the observation that inhibition of monoamine oxidase B protects against the parkinsonian inducing effects of the nigrostriatal neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, have prompted studies to identify monoamine oxidase inhibitors in the tobacco plant and tobacco cigarette smoke. Our previous efforts on cured tobacco leaf extracts have led to the characterization of 2,3,6-trimethyl-1,4-naphthoquinone, a non-selective monoamine oxidase inhibitor, and farnesylacetone, a selective monoamine oxidase B inhibitor. We now have extended these studies to tobacco smoke constituents. Fractionation of the smoke extracts has confirmed and extended the qualitative results of an earlier report [J. Korean Soc. Tob. Sci.1997, 19, 136] demonstrating the inhibitory activity of the terpene trans,trans-farnesol on rat brain MAO-B. In the present study, K(i) values for the inhibition of human, baboon, monkey, dog, rat, and mouse liver MAO-B have been determined. Noteworthy is the absence of inhibitory effects on human placental MAO-A and beef liver MAO-B. A limited structure-activity relationship study of analogs of trans,trans-farnesol is reported. Although the health hazards associated with the use of tobacco products preclude any therapeutic opportunities linked to smoking, these results suggest the possibility of identifying novel structures of compounds that could lead to the development of neuroprotective agents.

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.

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.

2-deoxy-2-[18F]fluoro-d-glucose and alternative radiotracers for positron emission tomography imaging using the human brain as a model

2-deoxy-2-[18F]fluoro-D-glucose (18FDG) is now routinely available in many hospitals and other institutions either via on-site production or from one of the dozens of regional radiopharmacies worldwide. Its reliable production has opened the possibility for use in both basic and clinical investigations and also in pairing it with other more biologically specific positron emission tomography tracers to provide an important functional perspective to the measurement. In this article, we highlight examples in which 18FDG is paired with another carbon-11- or fluorine-18-labeled radiotracer in the same subject to correlate neurotransmitter-specific effects with regional metabolic effects using the human brain as a model. We describe studies that fall into three major areas: normal aging, neuropsychiatric disorders, and drug action.

Low monoamine oxidase B in peripheral organs in smokers

One of the major mechanisms for terminating the actions of catecholamines and vasoactive dietary amines is oxidation by monoamine oxidase (MAO). Smokers have been shown to have reduced levels of brain MAO, leading to speculation that MAO inhibition by tobacco smoke may underlie some of the behavioral and epidemiological features of smoking. Because smoking exposes peripheral organs as well as the brain to MAO-inhibitory compounds, we questioned whether smokers would also have reduced MAO levels in peripheral organs. Here we compared MAO B in peripheral organs in nonsmokers and smokers by using positron emission tomography and serial scans with the MAO B-specific radiotracers,l-[11C]deprenyl and deuterium-substituted l-[11C]deprenyl (l-[11C]deprenyl-D2). Binding specificity was assessed by using the deuterium isotope effect. We found that smokers have significantly reduced MAO B in peripheral organs, particularly in the heart, lungs, and kidneys, when compared with nonsmokers. Reductions ranged from 33% to 46%. Because MAO B breaks down catecholamines and other physiologically active amines, including those released by nicotine, its inhibition may alter sympathetic tone as well as central neurotransmitter activity, which could contribute to the medical consequences of smoking. In addition, although most of the emphases on the carcinogenic properties of smoke have been placed on the lungs and the upper airways, this finding highlights the fact that multiple organs in the body are also exposed to pharmacologically significant quantities of chemical compounds in tobacco smoke.

Cigarette smoke and nicotine protect dopaminergic neurons against the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine Parkinsonian toxin

Epidemiological studies have found a negative association between cigarette smoking and Parkinson's disease (PD). In order to analyze the putative neuroprotective effect of cigarette smoke and nicotine, one of its major constituents, we examined their effects in an animal model of PD provoked by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) intoxication. Two groups of mice were chronically exposed to cigarette smoke (a low exposure subgroup and a high exposure subgroup; 5 exposures per day at 2-h intervals), two other groups received nicotine treatment (two doses tested 0.2 and 2 mg/kg, 5 injections i.p. per day at 2-h intervals) and one group placebo. On day 8 after the beginning of the treatment, 4 injections of MPTP hydrochloride (15 mg/kg, i.p., at 2-h intervals) or saline were administered to these animals. Nicotine and cotinine plasmatic concentration was quantified by the HPLC method, and degeneration of the nigrostriatal system was assessed by tyrosine hydroxylase (TH) immunohistochemistry. The loss of dopaminergic neurons induced by MPTP in the substantia nigra was significantly less severe in the chronic nicotine treatment groups (at 0.2 and 2 mg/kg) and the low exposure to cigarette smoke group than in the high exposure to cigarette smoke subgroup and the placebo treated subgroup. In contrast, no preservation of TH immunostaining of nerve terminals was observed in the striatum in any group. This suggests that nicotine and low exposure to cigarette smoke may have a neuroprotective effect on the dopaminergic nigrostriatal system by an as yet unknown mechanism.

Monoamine oxidase and cigarette smoking

Current cigarette smokers have reduced monoamine oxidase (MAO) and there is evidence that this is a pharmacological effect of tobacco smoke exposure rather than a biological characteristic of smokers. This article summarizes human and animal studies documenting the inhibitory effects of tobacco smoke on MAO and discusses MAO inhibition in the context of smoking epidemiology, MAO inhibitor compounds in tobacco, reinvestigations of low platelet MAO in psychiatric disorders and smoking cessation.

Lazabemide, a selective, reversible monoamine oxidase B inhibitor, as an aid to smoking cessation

BACKGROUND

Previous research has shown that smokers have reduced brain and platelet monoamine oxidase B (MAOB) activity. This is probably due to some components of tobacco smoke. When smokers quit, MAOB activity returns to normal. Reduced MAO activity may increase nicotine's addictive potential.

AIMS

To assess whether lazabemide, a reversible selective MAOB inhibitor, promotes smoking cessation.

STUDY DESIGN

Double-blind, randomized, placebo-controlled, multicenter phase II study. Placebo, lazabemide 100 mg/day and 200 mg/day were administered for 8 weeks. This was a dose finding, proof-of-concept, exploratory study.

SETTING

General practices and anti-smoking clinics in France and Belgium.

PARTICIPANTS

Smokers smoking > or=15 cigarettes per day and motivated to quit.

MAIN OUTCOME MEASURE

Sustained abstinence during the last 4 weeks of the study.

FINDINGS

The study was discontinued prematurely by the sponsor before randomization of the planned 420 smokers because of liver toxicity observed in other indications. Data of 330 randomized subjects could be analysed. Sustained abstinence during the last 4 weeks of treatment was 9%, 11% and 17% in the intent-to-treat population [P for trend: 0.036 (one-sided)]; 11%, 14% and 21% in the intent-to-treat population of smokers without those excluded because of discontinuation of the study [n = 262, P for trend: 0.02 (one-sided)], and 19%, 27% and 35% in completers [P for trend: 0.03 (one-sided)], in the placebo, lazabemide 100 mg/day and lazabemide 200 mg/day groups, respectively. Point prevalence abstinence (intent-to-treat population) at the end of treatment (week 8) was 17%, 19% and 30% in the placebo, lazabemide 100 mg/day and lazabemide 200 mg/day groups, respectively (placebo vs. lazabemide 200 mg/day: P = 0.01, one-sided). No treatment emergent major adverse event occurred. More nausea and insomnia were reported with lazabemide than with placebo.

CONCLUSIONS

MAOB inhibitors are promising treatments as an aid in smoking cessation. There may be an interest to develop MAOB inhibitors with an acceptable toxicity profile. Further studies may associate MAOB inhibitors with nicotine replacement therapies to increase therapeutic efficacy.

Imaging the neurochemistry of nicotine actions: studies with positron emission tomography

Although the effects of nicotine in the brains of laboratory animals have been investigated extensively, very little is known about its effects in the human brain. With positron emission tomography (PET), a non-invasive imaging technology that allows measurement of the concentration of positron-labeled compounds that are of physiological and pharmacological relevance, it has become possible to investigate the effects of nicotine in the human brain. These imaging studies have shown that nicotine has very fast pharmacokinetics in the human brain, that it changes cerebral blood flow (CBF) and brain metabolism, and that at least some of these effects show acute tolerance. PET studies have also shown that, in addition to nicotine, cigarettes possess other pharmacological actions that may contribute to their reinforcing effects, that cigarettes inhibit monoamine oxidase (MAO) A and B in the brain, and that this inhibition recovers with cigarette discontinuation. Although the nicotine receptors have not yet been imaged in the living human brain, PET studies in the primate brain have shown very high concentration of receptors in the thalamus and a high rate of blockade by doses of nicotine that approximate plasma levels achieved by humans when smoking cigarettes. However, further studies are required to determine the levels of nicotine receptor occupancies achieved when smoking a cigarette and those required for the nicotine patch to be therapeutically effective, to measure the half-life for MAO inhibition by cigarettes and the mechanisms underlying this inhibition, and to evaluate the effects of smoking on nicotine receptors and on other neurotransmitter systems in the human brain.

Studies on the interactions of tobacco leaf and tobacco smoke constituents and monoamine oxidase

Studies have demonstrated that smokers have lower levels of brain monoamine oxidase (MAO) A and B activity and lower MAO-B platelet activity than non-smokers. Recent speculations suggest that in addition to nicotine, tobacco components which are MAO inhibitors, may contribute to some tobacco related psychopharmacological effects. Furthermore, epidemiological evidence indicates a lower incidence of Parkinson's disease in smokers than in non-smokers. This relationship also might be linked to MAO inhibition. These intriguing observations prompted studies on the effects of tobacco leaf and tobacco smoke constituents on MAO activity. Studies reported here demonstrate that crude hexane tobacco leaf and hexane and aqueous leaf extracts have MAO inhibitory properties. Rat brain mitochondrial MAO-A and MAO-B activity are not altered following continuous 28 day exposure to (osmotic minipump) to two tobacco alkaloids, (S)-nicotine or (R,S)-N-methylanatabine. However, earlier studies in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treated parkinsonian C57BL/6 mouse model have provided indirect evidence that the tobacco derived 2,3,6-trimethyl-1,4-naphthoquinone (an MAO-A and B inhibitor) is effective in inhibiting MAO-B in vivo and is neuroprotective. Results reported here from more extensive tobacco leaf extractions provide evidence for three additional compounds with MAO-B inhibitory properties. One contains a chromone system, another a polyunsaturated macro-cycle and the third we have identified as farnesylacetone. These findings provide support to the thesis that components of tobacco smoke may be responsible for the inhibition of brain MAO-A and brain and platelet MAO-B in human smokers.

The effects of nicotine on neural pathways implicated in depression: a factor in nicotine addiction?

The prevalence of tobacco smoking varies considerably between different groups within the community, tobacco smoking being particularly prevalent in patients with depressive disorder. This review will focus on results, derived from animal studies, which suggest that, in addition to its primary reinforcing properties, nicotine also exerts effects in stressful environments, which may account for its enhanced addictive potential in depressed patients. It focuses on the evidence that depression sensitises patients to the adverse effects of stressful stimuli, and that this can be relieved by drugs that stimulate dopamine release in the forebrain. This mechanism, it is proposed, contributes to the increased craving to smoke in abstinent smokers exposed to such stimuli, because they become conditioned to use this property of nicotine to produce rapid alleviation of the adverse effects of the stress. The review also explores the possibility that chronic exposure to nicotine elicits changes in 5-HT formation and release in the hippocampus which are depressogenic. It is postulated that smokers are protected from the consequences of these changes, while they continue to smoke, by the antidepressant properties of nicotine. However, they contribute to the symptoms of depression experienced by many smokers when they first quit the habit.

Isolation and characterization of a monoamine oxidase inhibitor from tobacco leaves

Recent positron emission tomography imaging studies have demonstrated a significant decrease in both monoamine oxidase A and B (MAO-A and MAO-B) activities in the brains of smokers. Normal levels of activity are observed in former smokers, suggesting the presence of one or more compounds in tobacco smoke that may inhibit these enzymes. In this paper, we report the results of efforts to identify compounds present in flue-cured tobacco leaves that inhibit MAO. The isolation procedure was guided by estimating the inhibitory properties of tobacco leaf extracts on the liver mitochondrial MAO-B-catalyzed oxidation of 1-methyl-4-(1-methylpyrrol-2-yl)-1,2,3, 6-tetrahydropyridine to the corresponding dihydropyridinium metabolite. Fractionation of extracts from flue-cured tobacco leaves led to the isolation of a competitive inhibitor of human MAO-A (K(i) = 3 microM) and MAO-B (K(i) = 6 microM), the structure of which could be assigned by classical spectroscopic analysis and confirmed by synthesis. This information may help to provide insights into some aspects of the pharmacology and toxicology of tobacco products.

Smoking a single cigarette does not produce a measurable reduction in brain MAO B in non-smokers

Positron emission tomography (PET) studies with [11C]L-deprenyl-D2 have shown that brain monoamine oxidase (MAO) B is 40% lower in smokers than in non-smokers. Here we investigated whether MAO B inhibition can be detected after smoking a single cigarette. Eight normal healthy non-smokers (35 +/- 11 years) received two PET studies 2 h apart with [11C]L-deprenyl-D2, one at baseline and the second 5-10 min after the subject had smoked a single cigarette. Plasma nicotine and expired carbon monoxide (CO) were measured prior to smoking and 10 min after smoking completion as an index of tobacco smoke exposure. A three-compartment model was used to calculate lambda k3, a model term which is proportional to MAO B activity and which is derived from the time course of carbon-11 in the brain and the time course of the radiotracer in the plasma and K1, the plasma-to-brain transfer constant (for [11C]L-deprenyl-D2) which is related to brain blood flow. Subjects experienced difficulty inhaling and became dizzy and/or nauseous after smoking. Plasma nicotine averaged 11.6 +/- 5.5 ng/ml and expired CO averaged 8 +/- 10 ppm after smoking. The average lambda k3 and K1 for 11 different brain regions did not differ significantly between baseline and smoking. These results indicate that the reduction in MAO B in smokers probably occurs gradually and requires chronic tobacco smoke exposure.