Inhibition of monoamine oxidase B in the brains of smokers

Platelet MAO activity in subtypes of alcoholics and controls in a homogenous population

The level of platelet monoamine oxidase (MAO) activity has been found to vary between alcoholics and healthy controls and between subtypes of alcoholics, in different populations. This study measured the MAO activity in a group of 46 abstinent alcohol dependent subjects and 22 healthy non-alcoholic controls, male and female, in an ethnically homogenous Irish population. The healthy control subjects had a mean MAO activity of 0.27 pmol/hr/10(5) platelets (+/- SEM 0.03), while the alcoholics overall had a mean activity of 0.34 pmol/hr/10(5) platelets (+/- 0.03) and this difference was not significant, P = 0.2. When the alcoholic group was divided into subtypes of alcoholism, (type 1 and type 2) there was no difference between subtypes of alcoholics nor between each subtype and the controls in their MAO activity. There was no correlation between MAO activity and the age of the subjects, the duration of abstinence from alcohol, the severity of alcoholism, smoking status or the gender of the subjects. These data are discussed in the context of the current literature on alcoholism and platelet MAO in different ethnic populations and the status of platelet MAO as a potential marker for alcoholism.

Cocaine abusers do not show loss of dopamine transporters with age

Cocaine blocks dopamine transporters (DAT) and this effect is crucial to its reinforcing properties. To assess the effects of chronic cocaine on DAT we evaluated 20 current cocaine abusers and 20 age matched controls using PET and [C-11]cocaine as a DAT ligand. Though there were no differences in DAT availability between groups, current cocaine abusers (and 12 detoxified cocaine abusers studied previously) did not show the typical age-related decline in DAT seen in controls. Though further studies are required to rule out sampling effects and to control for confounding variables (i.e. smoking), one could speculate that chronic DAT blockade by cocaine has a protective effect on the loss of DAT with age.

Serotonergic dysfunction in addiction: effects of alcohol, cigarette smoking and heroin on platelet 5-HT content

The impact of ethanol, cigarette smoking and heroin on serotonin function was evaluated, first in alcoholics during chronic ethanol intoxication and in opiate addicts after long-term heroin consumption, and secondly in both patient groups after detoxification treatment (i.e. a short-term abstinence of 8 days). Our results showed that the 5-hydroxytryptamine (5-HT) content in platelets was: (1) increased in the subgroup of anti-social alcoholics; (2) transiently and differently altered in alcoholics compared to opiate addicts; and (3) lowered in drinking alcoholics and normal in alcoholics who were drinking as well as smoking (that may occur via MAO-B inhibition by smoke). The findings indicate that alterations of the peripheral and possibly the central serotonin system may occur as predisposing factors for alcoholism in individuals with anti-social traits; they may also have some impact on the progression of alcoholism due to its lowered function during chronic ethanol intoxication that is substantially modified by smoking.

Increased stress response and beta-phenylethylamine in MAOB-deficient mice

MAOA and MAOB are key iso-enzymes that degrade biogenic and dietary amines. MAOA preferentially oxidizes serotonin (5-hydroxytryptamine, or 5-HT) and norepinephrine (NE), whereas MAOB preferentially oxidizes beta-phenylethylamine (PEA). Both forms can oxidize dopamine (DA). A mutation in MAOA results in a clinical phenotype characterized by borderline mental retardation and impaired impulse control. X-chromosomal deletions which include MAOB were found in patients suffering from atypical Norrie's disease, which is characterized by blindness and impaired hearing. Reduced MAOB activity has been found in type-II alcoholism and in cigarette smokers. Because most alcoholics smoke, the effects of alcohol on MAOB activity remain to be determined. Here we show that targetted inactivation of MAOB in mice increases levels of PEA but not those of 5-HT, NE and DA, demonstrating a primary role for MAOB in the metabolism of PEA. PEA has been implicated in modulating mood and affect. Indeed, MAOB-deficient mice showed an increased reactivity to stress. In addition, mutant mice were resistant to the neurodegenerative effects of MPTP, a toxin that induces a condition reminiscent of Parkinson's disease.

Pathological mutations of nicotinic receptors and nicotine-based therapies for brain disorders

Nicotinic acetylcholine receptors are allosteric ligand-gated ion channels present in muscle and brain. Recent studies suggest that mutations altering their functional properties may produce congenital myasthenia and familial frontal lobe epilepsy. Current research also indicates that although nicotinic ligands often possess addictive properties, they could serve as therapeutic agents for Alzheimer's disease and Tourette's syndrome, as well as for schizophrenia.

Age-related increases in brain monoamine oxidase B in living healthy human subjects

Several studies of human brain postmortem report that monoamine oxidase B (MAO B) increases with age and it has been proposed that this increase reflects age-associated increases in glial cells. We measured brain MAO B in a group of normal healthy human subjects (n = 21; age range 23-86; 9 females and 12 males; nonsmokers) using [11C]L-deprenyl-D2 and positron emission tomography. Brain glucose metabolism was also measured with 18FDG in 15 of the subjects. MAO B increased (p < 0.004) in all brain regions examined except the cingulate gyrus. In contrast, subjects showed the expected regional age-related decreases in blood flow and metabolism. In the 15 subjects in whom both MAO B and LCMRglu was measured, there was a trend (p < 0.03) toward an inverse association between brain glucose metabolism and MAO B activity in the frontal and parietal cortices. Although the age-related increase in brain MAO B in living subjects is consistent with postmortem reports, the degree of increase is generally lower.

A key amino acid responsible for substrate selectivity of monoamine oxidase A and B

Monoamine oxidase (MAO) oxidizes biologically important amines including neurotransmitters and plays a central role in the regulation of intracellular level of these amines. Two distinct forms of MAO (MAO A and MAO B) were defined based on differences in substrate and inhibitor specificities. We earlier reported that the region between about residues 120 and 220 of rat MAO is responsible for determination of the substrate selectivity of MAO A and B (Tsugeno, Y. Hirashiki, I., Ogata, F., and Ito, A. (1995) J. Biochem. (Tokyo) 118, 974-980). To determine the essential amino acids in this region that participate in substrate recognition, a series of mutant enzymes in which amino acid residues that are conserved among various species but are different between the two forms of the enzyme were replaced with the corresponding amino acids of the counterpart and were engineered from the cDNAs of rat liver MAO A and B, and affinities for several substrates were examined. A single mutation in which Phe-208 in MAO A was substituted by the corresponding residue of Ile in MAO B was sufficient to convert the A-type substrate selectivity, and the reverse was exactly the case. Phe at this position was replaceable with Tyr for the A-type specificity and Ile was replaceable with Val and Ala for the B-type. Thus, aromatic and aliphatic residues seem to contribute to render substrate selectivity of MAO A and MAO B, respectively.

Effects of past history of major depression on smoking characteristics, monoamine oxidase-A and -B activities and withdrawal symptoms in dependent smokers

Past history of major depression is more common in smokers than in non-smokers. We have shown in a previous study that lifetime prevalence of major depression is higher in dependent smokers and they have lower monoamine oxidase-A and -B activities than non-smokers. Because several studies have found an association between MAO-B activity and depression we analysed data of these smokers to assess whether past history of major depression is associated with reduced monoamine oxidase activities (A and B) or not. Further, we tried to characterize smokers with past history of major depression and its effect on withdrawal symptoms. The data of 88 dependent smokers (Fagerström Tolerance Questionnaire score > or = 6 and smoking > or = 20 cigarettes/day) who participated in a smoking cessation study were analysed. Smokers with past history of major depression but without current illness did not differ in demographic and smoking characteristics from smokers without past history of major depression. Smokers with past history of major depression were mainly women and had lower body mass index. Adjusted for gender and body mass index dependent smokers with or without past history of depression had similar MAO-A and MAO-B activities but smokers with past history of major depression had significantly lower resting plasma norepinephrine levels. Smokers with past history of depression had not significantly higher ratings for depression (Montgomery-Asberg Depression Rating Scales) and anxiety (Hamilton Anxiety Scales) and smoking cessation did not exacerbate these ratings (assessed up to 3 months) and none had depressive episode during the postcessation period up to one year. Past history of depression was associated with higher scores on 'expressed sadness' and 'depressive mood'. Abstinent smokers with past history of depression had significantly higher ratings in one of the seven ratings of a 6 months period for craving (day 28), anxiety (day 7) and total withdrawal symptom score (day 7) when compared to those who had no past history of major depression. It is concluded that (i) past history of major depression is more frequent in female smokers; (ii) smokers with past history of depression may have more intense withdrawal symptoms (craving and anxiety) at some time after cessation: and (iii) past history of depression does not affect monoamine oxidase activities, therefore, reduced monoamine oxidase activities found in previous studies are possibly characteristic features of smoking.

Platelet monoamine oxidase (MAO) activity and alcoholism: is there a genuine association?

Monoamine oxidase (MAO) is a particle-bound flavoenzyme principally located in the outer mitochondrial membrane, which catalyzes the oxidative deamination of a variety of the amine transmitters in the central nervous system. There have been many reports from different groups around the world over the last 20 years that indicated a lower platelet MAO activity was associated with alcoholism. As there are multiple influences upon measured MAO activity, this article attempts to address each of the issues in turn for potential confounding of the reported association. These issues include: the duration of abstinence from alcohol, the substrate used for analysis, gender issues, association with different subtypes of alcoholics, the influence of other psychopathology, personality traits of alcoholics, the question of smoking, and proposed modes of transmission of MAO activity. The initial promise about MAO activity being a possible marker for alcoholism or a least a genetically predisposed subgroup has receded as the number of negative typology-based studies increases. The failure to account for a major confounding variable, e.g. smoking and other minor but possibly vital confounds in almost all these studies leaves the whole hypothesis open to criticism. However, the possible association between personality traits that might predispose to some if not all substances of addiction to low MAO activity may make it possible to reframe the hypothesized association in the setting of a generalized predisposition to addictions in particular, and to psychopathology in general. Nonetheless the association remains of great interest and may yet yield vital genetic and clinical information.

Psychosocial and pharmacologic explanations of nicotine's "gateway drug" function

Research has shown that adolescent users of tobacco are much more likely to progress to use of illicit drugs than are nonusers of tobacco. This article suggests potential psychosocial reasons for the progression based on principles of learning theory, Theory of Reasoned Action, Health Belief Model, and Cognitive Dissonance. In addition, a neuropharmacologic causal mechanism is discussed. The existence of tobacco's gateway function has important implications in the nation's efforts to reduce illicit drug use and adolescent smoking.

Nigrostriatal monoamine oxidase A and B in aging squirrel monkeys and C57BL/6 mice

In this study we assessed the two forms of monoamine oxidase (MAO) in the caudate, putamen, and substantia nigra of young (4-year-old), intermediate-aged (11-year-old), and aged (20-year-old) squirrel monkeys and in the striata of young (2-month-old) and older (10-month-old) C57Bl/6 mice. MAO A and B activities were determined by measuring the rate of oxidation of the specific substrates phenethylamine and serotonin. In squirrel monkey, the vast majority of MAO activity was MAO B with activity of this isoform 10 times greater than of MAO A, while in mice the activity of the two forms was approximately equivalent. Although mice demonstrated nearly twofold selective increases in striatal MAO B between 2 and 10 months of age, neither MAO B nor A showed statistically significant changes with age in squirrel monkeys. These results document the marked differences between nonhuman primates and rodents with respect to the relative activities and the effects of age on MAO A and B, and indicate that increased MAO is not an inevitable feature of aging.

Inhibition of brain monoamine oxidase by adducts of 1,2,3,4-tetrahydroisoquinoline with components of cigarette smoke

A series of adducts of 1,2,3,4-tetrahydroisoquinoline (TIQ) and some components of tobacco smoke were investigated for their ability to inhibit rat brain monoamine oxidase. 1-Cyano-TIQ (1CTIQ), N-(1'-cyanoethyl)-TIQ (CETIQ), N-(1'-cyanopropyl)-TIQ (CPTIQ), and N-(1'-cyanobutyl)-TIQ (CBTIQ) were found to act as competitive inhibitors for both MAO-A and MAO-B. Ki values ranged from 16.4 to 37.6 microM. N-(Cyanomethyl)-TIQ (CMTIQ) was not found to be an inhibitor (Ki > 100 microM). These findings may help to explain the in vivo inhibitory effects of tobacco smoke on MAO activity and the suggested protective effect of tobacco smoking against Parkinson's disease. They also appear to reinforce the usefulness of reversible MAO inhibitors in smoking cessation and abstinence. However, different results must be expected between Burley and Bright tobacco.

Brain monoamine oxidase A inhibition in cigarette smokers

Several studies have documented a strong association between smoking and depression. Because cigarette smoke has been reported to inhibit monoamine oxidase (MAO) A in vitro and in animals and because MAO A inhibitors are effective antidepressants, we tested the hypothesis that MAO A would be reduced in the brain of cigarette smokers. We compared brain MAO A in 15 nonsmokers and 16 current smokers with [11C]clorgyline and positron emission tomography (PET). Four of the nonsmokers were also treated with the antidepressant MAO inhibitor drug, tranylcypromine (10 mg/day for 3 days) after the baseline PET scan and then rescanned to assess the sensitivity of [11C]clorgyline binding to MAO inhibition. MAO A levels were quantified by using the model term lambda k3 which is a function of brain MAO A concentration. Smokers had significantly lower brain MAO A than nonsmokers in all brain regions examined (average reduction, 28%). The mean lambda k3 values for the whole brain were 0.18 +/- 0.04 and 0.13 +/- 0.03 ccbrain (mlplasma)-1 min-1 for nonsmokers and smokers, respectively; P < 0.0003). Tranyl-cypromine treatment reduced lambda k3 by an average of 58% for the different brain regions. Our results show that tobacco smoke exposure is associated with a marked reduction in brain MAO A, and this reduction is about half of that produced by a brief treatment with tranylcypromine. This suggests that MAO A inhibition needs to be considered as a potential contributing variable in the high rate of smoking in depression and in the development of more effective strategies for smoking cessation.

Understanding drug addiction: implications for treatment

The addicted brain is qualitatively different from the nonaddicted brain, in ways that include glucose use, gene expression, and responsiveness to environmental cues. Such discoveries place researchers in the early but hopeful stages of translating fundamental findings into new treatments that address the neurobiologic basis of drug craving--even for cocaine, against which there are currently no pharmacologic interventions.