On the substrate specificities of the two forms of monoamine oxidase

Age-Related Changes in Central Nervous System 5-Hydroxytryptamine Signalling and Its Potential Effects on the Regulation of Lifespan

Serotonin or 5-hydroxytryptamine (5-HT) is an important neurotransmitter in the central nervous system and the periphery. Most 5-HT (~99%) is found in the periphery where it regulates the function of the gastrointestinal (GI) tract and is an important regulator of platelet aggregation. However, the remaining 1% that is found in the central nervous system (CNS) can regulate a range of physiological processes such as learning and memory formation, mood, food intake, sleep, temperature and pain perception. More recent work on the CNS of invertebrate model systems has shown that 5-HT can directly regulate lifespan.This chapter will focus on detailing how CNS 5-HT signalling is altered with increasing age and the potential consequences this has on its ability to regulate lifespan.

Phenethylamine is a substrate of monoamine oxidase B in the paraventricular thalamic nucleus

Monoamine oxidase (MAO) is a key enzyme responsible for the degradation of neurotransmitters and trace amines. MAO has two subtypes (MAO-A and MAO-B) that are encoded by different genes. In the brain, MAO-B is highly expressed in the paraventricular thalamic nucleus (PVT); however, its substrate in PVT remains unclear. To identify the MAO-B substrate in PVT, we generated Maob knockout (KO) mice and measured five candidate substrates (i.e., noradrenaline, dopamine, 3-methoxytyramine, serotonin, and phenethylamine [PEA]) by liquid chromatography tandem mass spectrometry. We showed that only PEA levels were markedly elevated in the PVT of Maob KO mice. To exclude the influence of peripheral MAO-B deficiency, we developed brain-specific Maob KO mice, finding that PEA in the PVT was increased in brain-specific Maob KO mice, whereas the extent of PEA increase was less than that in global Maob KO mice. Given that plasma PEA levels were elevated in global KO mice, but not in brain–specific KO mice, and that PEA passes across the blood–brain barrier, the substantial accumulation of PEA in the PVT of Maob KO mice was likely due to the increase in plasma PEA. These data suggest that PEA is a substrate of MAO-B in the PVT as well as other tissues.

The neurobiology of human aggressive behavior: Neuroimaging, genetic, and neurochemical aspects

In modern societies, there is a strive to improve the quality of life related to risk of crimes which inevitably requires a better understanding of brain determinants and mediators of aggression. Neurobiology provides powerful tools to achieve this end. Pre-clinical and clinical studies show that changes in regional volumes, metabolism-function and connectivity within specific neural networks are related to aggression. Subregions of prefrontal cortex, insula, amygdala, basal ganglia and hippocampus play a major role within these circuits and have been consistently implicated in biology of aggression. Genetic variations in proteins regulating the synthesis, degradation, and transport of serotonin and dopamine as well as their signal transduction have been found to mediate behavioral variability observed in aggression. Gene-gene and gene-environment interactions represent additional important risk factors for aggressiveness. Considering the social burden of pathological forms of aggression, more basic and translational studies should be conducted to accelerate applications to clinical practice, justice courts, and policy making.

Betel quid dependence mechanism and potential cessation therapy

BACKGROUND

Global reports estimate the number of betel quid (BQ) chewers up to 600 million. The proportion of betel quid dependence (BQD) is 20%-90% among current users. BQD mechanisms are not fully understood, and no pharmacological solution exists for its cessation therapy.

METHODS

We present a systematic review on BQD mechanisms and examine potential cessation therapeutic drugs. We conducted a systematic literature search in PubMed and Web of Science databases and identified the latest 10 years' relevant articles for reviews.

RESULTS

Functional magnetic resonance imaging results demonstrate that neurological mechanisms link the brain reward, cognitive, and impulsive systems in BQ or BQD users. The use of the areca nut increases both brain serotonin and noradrenaline levels, whereas arecoline, a potentially addictive areca nut component, has monoamine oxidase-A (MAO-A) inhibitor-like properties. MAO-A inhibitors prevent neurotransmitter breakdown and increase dopamine and serotonin concentrations in the brain. A reduction of daily BQ use was observed among patients with depression after antidepressant therapy, including MAO-A inhibitor and selective serotonin reuptake inhibitor (SSRI). Arecoline is a nicotinic acetylcholine receptor agonist expressed in Xenopus oocytes. However, relatively negligible amounts of nicotine are detected in the areca nut.

CONCLUSION

In conclusion, the current evidence provides a better understanding of the neurological and pharmacological mechanisms behind BQD. Arecoline, an MAO-A inhibitor, may account for BQD. Future translational studies are needed to verify the efficacy of potential BQD cessation drugs. MAO-A inhibitor and SSRI would thus be potentially promising targets for clinical trials.

Hydride Abstraction as the Rate-Limiting Step of the Irreversible Inhibition of Monoamine Oxidase B by Rasagiline and Selegiline: A Computational Empirical Valence Bond Study

Monoamine oxidases (MAOs) catalyze the degradation of a very broad range of biogenic and dietary amines including many neurotransmitters in the brain, whose imbalance is extensively linked with the biochemical pathology of various neurological disorders, and are, accordingly, used as primary pharmacological targets to treat these debilitating cognitive diseases. Still, despite this practical significance, the precise molecular mechanism underlying the irreversible MAO inhibition with clinically used propargylamine inhibitors rasagiline and selegiline is still not unambiguously determined, which hinders the rational design of improved inhibitors devoid of side effects current drugs are experiencing. To address this challenge, we present empirical valence bond QM/MM simulations of the rate-limiting step of the MAO inhibition involving the hydride anion transfer from the inhibitor α-carbon onto the N5 atom of the flavin adenin dinucleotide (FAD) cofactor. The proposed mechanism is strongly supported by the obtained free energy profiles, which confirm a higher reactivity of selegiline over rasagiline, while the calculated difference in the activation Gibbs energies of ΔΔG^‡ = 3.1 kcal mol^-1 is found to be in very good agreement with that from the measured literature k_inact values that predict a 1.7 kcal mol^-1 higher selegiline reactivity. Given the similarity with the hydride transfer mechanism during the MAO catalytic activity, these results verify that both rasagiline and selegiline are mechanism-based irreversible inhibitors and offer guidelines in designing new and improved inhibitors, which are all clinically employed in treating a variety of neuropsychiatric and neurodegenerative conditions.

Effect of antidepressants for cessation therapy in betel-quid use disorder: a randomised, double-blind, placebo-controlled trial

AIMS

More than one-half of betel-quid (BQ) chewers have betel-quid use disorder (BUD). However, no medication has been approved. We performed a randomised clinical trial to test the efficacy of taking escitalopram and moclobemide antidepressants on betel-quid chewing cessation (BQ-CC) treatment.

METHODS

We enrolled 111 eligible male BUD patients. They were double-blinded, placebo-controlled and randomised into three treatment groups: escitalopram 10 mg/tab daily, moclobemide 150 mg/tab daily and placebo. Patients were followed-up every 2 weeks and the length of the trial was 8 weeks. The primary outcome was BQ-CC, defined as BUD patients who continuously stopped BQ use for ⩾6 weeks. The secondary outcomes were the frequency and amount of BQ intake, and two psychological rating scales. Several clinical adverse effects were measured during the 8-week treatment.

RESULTS

Intention-to-treat analysis shows that after 8 weeks, two (5.4%), 13 (34.2%) and 12 (33.3%) of BUD patients continuously quit BQ chewing for ⩾6 weeks among placebo, escitalopram, moclobemide groups, respectively. The adjusted proportion ratio of BQ-CC was 6.3 (95% CI 1.5-26.1) and 6.8 (95% CI 1.6-28.0) for BUD patients who used escitalopram and moclobemide, respectively, as compared with those who used placebo. BUD patients with escitalopram and moclobemide treatments both exhibited a significantly lower frequency and amount of BQ intake at the 8th week than those with placebo.

CONCLUSIONS

Prescribing a fixed dose of moclobemide and escitalopram to BUD patients over 8 weeks demonstrated treatment benefits to BQ-CC. Given a relatively small sample, this study provides preliminary evidence and requires replication in larger trials.

Computational Insight into the Mechanism of the Irreversible Inhibition of Monoamine Oxidase Enzymes by the Antiparkinsonian Propargylamine Inhibitors Rasagiline and Selegiline

Monoamine oxidases (MAOs) are flavin adenine dinucleotide containing flavoenzymes that catalyze the degradation of a range of brain neurotransmitters, whose imbalance is extensively linked with the pathology of various neurological disorders. This is why MAOs have been the central pharmacological targets in treating neurodegeneration for more than 60 years. Still, despite this practical importance, the precise chemical mechanisms underlying the irreversible inhibition of the MAO B isoform with clinical drugs rasagiline (RAS) and selegiline (SEL) remained unknown. Here we employed a combination of MD simulations, MM-GBSA binding free energy evaluations, and QM cluster calculations to show the MAO inactivation proceeds in three steps, where, in the rate-limiting first step, FAD utilizes its N5 atom to abstracts a hydride anion from the inhibitor α-CH₂ group to ultimately give the final inhibitor-FAD adduct matching crystallographic data. The obtained free energy profiles reveal a lower activation energy for SEL by 1.2 kcal mol^-1 and a higher reaction exergonicity by 0.8 kcal mol^-1, with the former being in excellent agreement with experimental ΔΔG^‡_EXP = 1.7 kcal mol^-1, thus rationalizing its higher in vivo reactivity over RAS. The calculated ΔG_BIND energies confirm SEL binds better due to its bigger size and flexibility allowing it to optimize hydrophobic C-H···π and π···π interactions with residues throughout both of enzyme's cavities, particularly with FAD, Gln206 and four active site tyrosines, thus overcoming a larger ability of RAS to form hydrogen bonds that only position it in less reactive orientations for the hydride abstraction. Offered results elucidate structural determinants affecting the affinity and rates of the inhibition reaction that should be considered to cooperate when designing more effective compounds devoid of untoward effects, which are of utmost significance and urgency with the growing prevalence of brain diseases.

Discovery of monoamine oxidase inhibitors by medicinal chemistry approaches

Neuropsychiatric disorders, such as Alzheimer's disease (AD), Parkinson's disease (PD) and depression, have seriously inconvenienced the lives of patients. Growing evidence indicates that these diseases are closely related to the monoamine oxidase (MAO) enzyme, making it an attractive target for the exploitation of potent MAO inhibitors (MAOIs) with high selectivity and low side effects. Although various MAOIs have been discovered, the discovery of an ideal MAOI is not an easy task. In this review, we discuss the currently available rational design strategies for obtaining ideal MAOIs, including ligand-based and receptor-based design strategies, and these strategies were further illustrated with the aid of specific examples from the recent literature. To better understanding the biological activity of MAO, we also highlight the binding modes of typical inhibitors against MAO. Besides, advanced strategies for finding upcoming potent MAOIs were prospected.

Androgen metabolites impact CSF amines and axonal serotonin via MAO-A and -B in male macaques

A number of studies have shown that mutations or deletions of the monoamine oxidase-A (MAO-A) gene cause elevated CNS serotonin and elevated impulsive aggression in humans and animal models. In addition, low cerebrospinal fluid (CSF) 5-hydroxyindole acetic acid (5HIAA) has been documented in a limited number of violent criminal populations and in macaques that exhibit impulsive aggression. To reconcile these different analyses, we hypothesized that CSF 5HIAA reflected degradation of serotonin by the activity of MAO-A; and that low MAO-A activity would result in lower CSF 5HIAA, but overall higher serotonin in the CNS. To test this hypothesis, male Japanese macaques (Macaca fuscata) were castrated, rested for 5-7months, and then treated for 3months with [1] placebo, [2] testosterone (T), [3] dihydrotestosterone (DHT; non-aromatizable androgen) and 1,4,6-androstatriene-3,17-dione (ATD) (steroidal aromatase inhibitor), or [4] flutamide (FLUT; androgen antagonist) and ATD (n=5/group). These treatments enable isolation of androgen and estrogen activities. In the dorsal raphe, MAO-A and MAO-B expressions were determined with in situ hybridization (ISH) and protein expression of aromatase was determined with immunohistochemistry (IHC). CSF concentrations of 5HIAA, 3-methoxy-4-hydroxyphenylglycol (MHPG), and homovanillic acid (HVA) were determined with liquid chromatography/mass spectrometry (LC/MS). From the same animals, previously published data on serotonin axon density were used as a proxy for CNS serotonin. Aromatase conversion of T to estrogen (E) suppressed MAO-A (positive pixel area, p=0.0045), but androgens increased MAO-B (positive pixel area, p=0.014). CSF 5HIAA was suppressed by conversion of T to E (Cohen's d=0.6). CSF 5HIAA was positively correlated with MAO-A-positive pixel area (r(2)=0.78). CSF 5HIAA was inversely correlated with serotonin axon-positive pixel area (r(2)=0.69). In summary, CSF 5HIAA reflects MAO-A activity rather than global serotonin. Low CSF 5HIAA may, in this paradigm, reflect higher serotonin activity. Androgens lower MAO-A activity via metabolism to E, thus elevating CNS serotonin and decreasing CSF 5HIAA. Since androgens increase certain types of aggression, these data are consistent with studies demonstrating that lower MAO-A activity is associated with elevated serotonin and increased aggression.

Kinetic and structural analysis of the irreversible inhibition of human monoamine oxidases by ASS234, a multi-target compound designed for use in Alzheimer's disease

Monoamine oxidases (MAO) and cholinesterases are validated targets in the design of drugs for the treatment of Alzheimer's disease. The multi-target compound N-((5-(3-(1-benzylpiperidin-4-yl)propoxy)-1-methyl-1H-indol-2-yl)methyl)-N-methylprop-2-yn-1-amine (ASS234), bearing the MAO-inhibiting propargyl group attached to a donepezil moiety that inhibits cholinesterases, retained activity against human acetyl- and butyryl-cholinesterases. The inhibition of MAO A and MAO B by ASS234 was characterized and compared to other known MAO inhibitors. ASS234 was almost as effective as clorgyline (kinact/KI=3×10(6) min(-1)M(-1)) and was shown by structural studies to form the same N5 covalent adduct with the FAD cofactor.

Psychobiology of borderline personality traits related to subtypes of eating disorders: a study of platelet MAO activity

Increased and decreased levers of platelet monoamine oxidase (MAO) activity have been reported in patients with eating disorders, indicating abnormalities of the serotonin turnover. However, whether these findings are related to eating disorders or are rather reflecting the pathophysiology of borderline personality traits in these patients is still unknown. Platelet MAO activity and comorbid personality disorders were investigated in 72 patients with different subtypes of eating disorders (ED) and in a group of 28 healthy controls. ED patients comprised the following subtypes: 25 anorexia nervosa (AN) restrictive, 14 AN binge eating-purging (AN b-p), 3 anorexia nervosa not otherwise specified (AN NOS) and 30 bulimia nervosa (BN). Personality disorders and traits were assessed with the Structured Interview for Personality Disorders (SCID-II), the Zanarini Rating Scale for Borderline Personality Disorder, and the Barrat Impulsiveness Scale. Platelet MAO activity was significantly lower in ED patients with comorbid borderline personality disorder (BPD) than in ED without Borderline personality disorder (BDP). Platelet MAO activity was significantly and inversely correlated with the number and severity of BPD clinical features. In the subsample of patients with binge eating-purging symptoms (AN b-p, AN NOS and BN), platelet MAO activity was significantly lower in binge-purge patients with comorbid BPD than in binge-purge patients without BPD. The whole group of eating disorders had a significantly reduced lever of platelet MAO activity compared with the control group. The results suggest that low platelet MAO activity might characterize eating disorders with comorbid borderline personality traits, reflecting greater serotonin dysfunction in these patients. The role of decreased platelet MAO as an endophenotype with specific clinical manifestations should be explored in future studies.

Efficacy, safety, and patient preference of monoamine oxidase B inhibitors in the treatment of Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disease and the most treatable. Treatment of PD is symptomatic and generally focuses on the replacement or augmentation of levodopa. A number of options are available for treatment, both in monotherapy of early PD and to treat complications of advanced PD. This review focuses on rasagiline and selegiline, two medications that belong to a class of antiparkinsonian drugs called monoamine oxidase B (MAO-B) inhibitors. Topics covered in the review include mechanism of action, efficacy in early and advanced PD, effects on disability, the controversy regarding disease modification, safety, and patient preference for MAO-B inhibitors.

Monoamine oxidase inhibitors: a modern guide to an unrequited class of antidepressants

Monoamine oxidase inhibitors (MAOIs) currently have a "bad rap" and are thus infrequently used in psychopharmacology, even by experienced clinicians. Misinformation about the dietary and drug interactions of MAOIs is widespread, whereas pragmatic tips for utilizing MAOIs to minimize risks and to maximize therapeutic actions are largely lacking in the contemporary literature. While clearly not first-line treatments, MAOIs, in the hands of experienced and well-informed clinicians, can be a powerful therapeutic intervention for patients with depression, panic disorder, and other anxiety disorders who have failed first-line treatments. This article focuses on the pharmacologic mechanisms of MAOIs, since an understanding of these mechanisms may provide a rationale to empower experts to expand their use of these agents. Discussed here are not only the mechanisms of therapeutic action of MAOIs, but also the mechanisms explaining their side effects, including hypertensive interactions with dietary tyramine (so-called "cheese reactions") and drug interactions that can lead to hypertensive reactions with some drugs and serotonin toxicities with others. This article also provides practical tips on how to use MAOIs, including debunking certain myths and giving specific guidance about which foods and drugs to avoid. Those with no previous interest in MAOIs may discover in this article a new "secret weapon" to add to their therapeutic armamentarium for patients who fail to respond to the better-known agents.

Monoamine oxidase-A modulates apoptotic cell death induced by staurosporine in human neuroblastoma cells

Monoamine oxidases (MAOs) are mitochondrial enzymes which control the levels of neurotransmitters in the brain and dietary amines in peripheral tissues via oxidative deamination. MAO has also been implicated in cell signalling. In this study, we describe the MAO-A isoform as functional in apoptosis induced by staurosporine (STS) in human dopaminergic neuroblastoma cells (SH-SY5Y). Increased levels of MAO-A activity were induced by STS, accompanied by increased MAO-A protein and activation of the initiator of the intrinsic pathway, caspase 9, and the executioner caspase 3. MAO-A mRNA levels were unaffected by STS, suggesting that changes in MAO-A protein are due to post-transcriptional events. Two unrelated MAO-A inhibitors reduced caspase activation. STS treatment resulted in sustained activation of the mitogen-activated protein kinase pathway enzymes extracellular regulated kinase, c-jun terminal kinase and p38, and depletion of the anti-apoptotic protein Bcl-2. These changes were significantly reversed by MAO inhibition. Production of reactive oxygen species was increased following STS exposure, which was blocked by both MAO inhibition and the antioxidant N-acetylcysteine. Therefore our data provide evidence that MAO-A, through its production of reactive oxygen species as a by-product of its catalytic activity on the mitochondrial surface, is recruited by the cell to enhance apoptotic signalling.

Eugenol and its structural analogs inhibit monoamine oxidase A and exhibit antidepressant-like activity

Eugenol (1) is an active principle of Rhizoma acori graminei, a medicinal herb used in Asia for the treatment of symptoms reminiscent of Alzheimer's disease (AD). It has been shown to protect neuronal cells from the cytotoxic effect of amyloid beta peptides (Abetas) in cell cultures and exhibit antidepressant-like activity in mice. Results from this study show that eugenol inhibits monoamine oxidase A (MAOA) preferentially with a K(i)=26 microM. It also inhibits MAOB but at much higher concentrations (K(i)=211 microM). In both cases, inhibition is competitive with respect to the monoamine substrate. Survey of compounds structurally related to eugenol has identified a few that inhibit MAOs more potently. Structure activity relationship reveals structural features important for MAOA and MAOB inhibition. Molecular docking experiments were performed to help explain the SAR outcomes. Four of these compounds, two (1, 24) inhibiting MAOA selectively and the other two (19, 21) inhibiting neither MAOA nor MAOB, were tested for antidepressant-like activity using the forced swim test in mice. Results suggest a potential link between the antidepressant activity of eugenol and its MAOA inhibitory activity.

Monoamine oxidase inhibitory activities of the scorpion Mesobuthus gibbosus (Buthidae) venom peptides

In the present study, crude venom of Mesobuthus gibbosus (Buthidae), a scorpion distributed all over Anatolia was isolated and purified by the Sephadex G-50 gel filtration and high pressure liquid chromatographic (HPLC) separation. Two of the five fractions (fractions 4 and 5) obtained from the Sephadex G-50 filtration and detected as lethal on mice and Musca domestica larvae in in vivo toxicity tests, were independently subjected to the HPLC separation. Only one of seven fractions (fraction 5.5*) obtained from the HPLC separation of the fraction 5 was found to be extremely lethal. Sodium dodecylsulfate polyacrylamide gel electrophoretic (SDS-PAGE) analysis of the crude venom and its chromatographic fractions demonstrated that crude venom consisted of peptides with molecular weights of 6500-210,000 Da. The neurotoxic fraction 5.5* appeared as a single band of 28,000 Da and two bands of 6200 and 22,000 Da in SDS-PAGE under non-reducing and reducing conditions, respectively, suggesting that it might consist of two chains attached by a disulfide bridge. Fractions 5 and 5.5* inhibited monoamine oxidase A (MAO-A) of rat liver reversibly and non-competitively, in a concentration-dependent manner. Fraction 5.5* appeared as a potent and specific MAO-A inhibitor with a Ki value of 0.12 mg venom proteinml(-1). The inhibitory effect of venom peptide 5.5* on MAO-A was found to be dependent on the preincubation time suggesting that the peptide binds to some site other than the substrate-binding site. Results of the present study demonstrated that M. gibbosus venom contains a peptide with specific MAO-A inhibitory activity which may be responsible for the anxiogenic effects of the scorpion venoms on animals and humans.

Effects of ovarian steroids and raloxifene on proteins that synthesize, transport, and degrade serotonin in the raphe region of macaques

In the monkey dorsal raphe, we reported that 1-month (mo) of estrogen replacement, with or without progesterone supplementation for 14 days, significantly increased tryptophan hydroxylase-1 (TPH-1) mRNA; decreased serotonin reuptake transporter (SERT) mRNA and decreased monoamine oxidase (MAO)-A mRNA, but had no effect on MAO-B mRNA. Here, we questioned what effect would 1 or 5 mo of ovarian hormones or the selective estrogen receptor modulator (SERM), raloxifene, have on TPH protein and phosphorylation, and on protein expression of SERT, MAO-A or MAO-B? Raloxifene antagonizes estrogen in breast or uterus, but estrogen-like activities in the brain have been reported. Cytoplasmic and membrane extracts of the dorsal raphe region were processed for Western blotting. TPH, phosphoserine, SERT, MAO-A, and MAO-B were detected with specific antibodies. The optical densities of the signals were measured with NIH image and analyzed by ANOVA. Both 1 and 5 mo of estrogen, with or without progesterone, and 5 mo of raloxifene significantly increased TPH protein. Administration for 5 mo of estrogen plus progesterone and raloxifene also increased TPH phosphorylation. Estrogen, with or without progesterone, for 1 mo had no effect on SERT protein. However, 5 mo of estrogen and 5 mo of raloxifene increased SERT protein. Estrogen alone or combined with progesterone for 1 mo caused a significant reduction in MAO-A. Yet, after 5 mo of the same treatments, MAO-A was not different from spayed controls. Estrogen alone had no effect on MAO-B. However, the addition of progesterone significantly increased MAO-B. Raloxifene for 5 mo had no effect on MAO-A or MAO-B. Thus, to various extents, estrogen, progesterone, and raloxifene may increase serotonin production and transport. The expression of the degradative enzymes suggests a complex combination of gene transcription, post-transcriptional processing, and substrate feedback mechanisms.

Cardiopulmonary effects of medetomidine, oxymorphone, or butorphanol in selegiline-treated dogs

OBJECTIVES

To determine if chronic selegiline HCl administration affects the cardiopulmonary response to medetomidine, oxymorphone, or butorphanol in dogs.

STUDY DESIGN

Prospective randomized experimental study.

ANIMALS

Twenty-eight adult, random source, hound dogs weighing 21-33 kg.

METHODS

Dogs were assigned to the following treatment groups: selegiline + medetomidine (MED; n = 6); placebo + MED (n = 6), selegiline + oxymorphone (OXY; n = 6); placebo + OXY (n = 6); selegiline + butorphanol (BUT; n = 7) or placebo + BUT (n = 6). Nine dogs were treated with two of the three pre-medicants. Dogs were treated with selegiline (1 mg kg(-1) PO, q 24 hours) or placebo for at least 44 days prior to pre-medicant administration. On the day of the experiment, arterial blood for blood gas analysis, blood pressure measurements, ECG, cardiac ultrasound (mM-mode, 2-D, and continuous wave Doppler), and behavioral observations were obtained by blinded observers. An IV injection of MED (750 micro g m(-2)), OXY (0.1 mg kg(-1)) or BUT (0.4 mg kg(-1)) was given. Cardiopulmonary and behavioral data were collected at 1, 2, 5, 15, 30, and 60 minutes after injection.

RESULTS

Selegiline did not modify responses to any of the pre-medicant drugs. Medetomidine caused a significant decrease in heart rate (HR), cardiac output (CO), and fractional shortening (FS). Mean arterial pressure (MAP), systemic vascular resistance (SVR), and central venous pressure (CVP) were increased. Level of consciousness and resistance to restraint were both decreased. Oxymorphone did not affect MAP, CO, CVP, or SVR, but RR and PaCO(2) were increased. Level of consciousness and resistance to restraint were decreased. BUT decreased heart rate at 1 and 5 minutes. All other cardiovascular parameters were unchanged. BUT administration was associated with decreased arterial pH and increased PaCO(2). BUT decreased level of consciousness and resistance to restraint.

CONCLUSIONS AND CLINICAL RELEVANCE

Although pre-medicants themselves altered cardiopulmonary and behavioral function, selegiline did not affect the response to medetomidine, oxymorphone, or butorphanol in this group of normal dogs.

Effects of selegiline, phenylpropanolamine, or a combination of both on physiologic and behavioral variables in healthy dogs

OBJECTIVE

To determine effects of selegiline hydrochloride, phenylpropanolamine (PPA), or a combination of both on physiologic and behavioral variables in dogs.

ANIMALS

40 adult hound-type dogs.

PROCEDURE

Dogs were assigned to 4 groups. One group received selegiline (1 mg/kg, PO, q 24 h) and PPA (1.1 mg/kg, PO, q 8 h), a second group received selegiline alone, a third group received PPA alone, and a fourth group received neither drug. Dogs were observed 3 times/d throughout the 30-day study (daily during the first week, on alternate days during the next 2 weeks, and again daily during the final week). Observers recorded rectal temperature, pulse, respiratory rate, oscillometric blood pressure, and lead-II ECG and assessed 4 behaviors, using an analogue scale. Variables were compared among treatment groups by use of a 2-factor ANOVA with data categorized into three 10-day treatment periods. A similar comparison was made among treatment groups with data categorized by time of observation (morning, afternoon, or evening) for all study days.

RESULTS

Variables did not differ among groups at study initiation. Pulse rate was the only variable that differed significantly among treatment groups during the study. During the first 10 days of treatment, dogs receiving PPA had a lower pulse rate than dogs that did not. Although signs of illness were apparent in a few dogs, illness did not appear to be related to treatment.

CONCLUSIONS AND CLINICAL RELEVANCE

Adverse effects were not detected after administration of selegiline, PPA, or a combination of the drugs in healthy dogs.

Decreased platelet monoamine oxidase activity in female bulimia nervosa

The involvement of brain serotonin systems in the pathophysiology of eating disorders has been repeatedly demonstrated in recent studies. Platelet MAO activity is an index of brain serotonin activity and lowered platelet MAO levels have been found in association with impulsive behaviors. In addition, some preliminary reports indicate that platelet MAO could be lowered in eating disorder patients.

METHODS

47 patients with DSM-IV eating disorders were studied, including 30 with bulimia nervosa and 17 with anorexia nervosa binge eating-purging type. Platelet MAO activity was measured by isotopic methods using C-14 benzylamine and compared with a control group of 30 healthy subjects. Impulsive personality features were studied with specific rating scales.

RESULTS

Platelet MAO activity was significantly lower (4.4+/-2.4 nmol/h/10(8) platelets) in the bulimic patients than in the control group (6.9+/-2.5) (p<0.001). No significant differences were found between pure bulimics and binge eating-purging anorectics. Platelet MAO was inversely and significantly correlated with scores on impulsivity scales and with borderline personality disorder characteristics.

CONCLUSIONS

Platelet MAO activity is lowered in patients with bulimia, which may reflect dysfunction in impulse control mechanisms. Since platelet MAO has a predominant genetic component, there is need for studies on the association of low platelet MAO and higher risk for developing eating disorders.

Monoamine oxidase A inhibition by fluoxetine: an in vitro and in vivo study

Monoamine oxidase A (MAO-A) inhibition was investigated both in vitro and in vivo in rat brains by using the radioligand, 18F-fluoroclorgyline (N-[3-(2',4'-dichlorophenoxy)-2-18F-fluoropropyl]-N-methylpropa rgylamine). In vitro binding affinities of six compounds, clorgyline, Ro 41-1049, deprenyl, fluoxetine, norfluoxetine and citalopram, were studied. Fluoxetine and norfluoxetine showed in vitro affinities of 36.5 and 68 microM for MAO-A, respectively. Fluoxetine and norfluoxetine also significantly inhibited (more than 20%) the binding of the radioligand in vivo while citalopram and deprenyl showed very poor affinities in vitro for MAO-A and had no effect in vivo. The in vivo effects of the various drugs were directly comparable to their in vitro affinities for binding to MAO-A as seen in the correlation plot of percent control in vivo binding of 18F-fluoroclorgyline and binding affinity, -log IC50 (R2 = 0.979). An acute dose of 20 mg/kg of fluoxetine inhibited binding of 18F-fluoroclorgyline by more than 20%, while lower doses had some significant effects. These results provide evidence on the in vitro and in vivo inhibition of monoamine oxidase A by fluoxetine.

Localization of monoamine oxidase mRNA in human placenta

Monoamine oxidase (MAO) oxidatively deaminates vasoactive and biogenic amines and exists in two distinct forms (A and B), coded for by separate genes, which exhibit distinct substrate specificities and inhibitor sensitivities. Using specific primers for MAO-A and MAO-B mRNA in a reverse transcription-polymerase chain reaction (RT-PCR) on RNA from human liver, the predicted products for both enzymes were detected. Furthermore, RT-PCR on RNA from human placenta, believed to contain predominantly (or only) MAO-A protein, also indicated the presence of both A and B gene transcripts. The cellular distribution of MAO mRNA in placental tissue was analyzed by in situ hybridization of MAO-A and MAO-B mRNA-specific cRNA probes on paraffin sections. MAO-A mRNA was mainly evident in the syncytiotrophoblastic layer. None was detected in the vascular endothelium/smooth muscles. Significantly, MAO-B mRNA signal was also evident in the placental villi, notably in the syncytiotrophoblasts, intermediate trophoblasts, cytotrophoblasts, and the vascular endothelium. To our knowledge, this is the first demonstration of the cellular distribution of MAO mRNA in human placenta via in situ hybridization. The expression of MAO-B in placental tissue rather than in blood elements within placenta is also unequivocally demonstrated. These highly specific cRNA probes can now be used to study the distribution of MAO-A and MAO-B expression in other tissues.

Role of monoamine oxidase type A and B on the dopamine metabolism in discrete regions of the primate brain

The role of monoamine oxidase (MAO) type A and B on the metabolism of dopamine (DA) in discrete regions of the monkey brain was studied. Monkeys were administered (-)-deprenyl (0.25 mg/kg) or clorgyline (1.0 mg/kg) or deprenyl and clorgyline together by intramuscular injections for 8 days. Levels of DA and its metabolites, dihydroxy phenylacetic acid (DOPAC) and homovanillic acid (HVA) were estimated in frontal cortex (FC), motor cortex (MC), occipital cortex (OC), entorhinal cortex (EC), hippocampus (HI), hypothalamus (HY), caudate nucleus (CN), globus pallidus (GP) and substantia nigra (SN). (-)-Deprenyl administration significantly increased DA levels in FC, HY, CN, GP and SN (39-87%). This was accompanied by a reduction in the levels of DOPAC (37-66%) and HVA (27-79%). Clorgyline administration resulted in MAO-A inhibition by more than 87% but failed to increase DA levels in any of the brain regions studied. Combined treatment of (-)-deprenyl and clorgyline inhibited both types of MAO by more than 90% and DA levels were increased (57-245%) in all brain regions studied with a corresponding decrease in the DOPAC (49-83%) and HVA (54-88%) levels. Our results suggest that DA is metabolized preferentially, if not exclusively by MAO-B in some regions of the monkey brain.

The oxidation of dopamine and epinine by the two forms of monoamine oxidase from rat liver

Information on the "in vitro" oxidation of epinine by monoamine oxidase (MAO) compared to dopamine is very poor. The aim of this work was to study the oxidative deamination of epinine and dopamine by rat liver MAO-A and MAO-B. The contributions of MAO-A and B to the metabolism of dopamine (55% and 45%, respectively) and epinine (70% and 30%, respectively) were similar. The results of this study show that epinine is a substrate for both forms of MAO in rat liver, although the contribution of MAO A to the deamination of this secondary amine appears to be slightly more important than that of MAO B.

Differential trace amine alterations in individuals receiving acetylenic inhibitors of MAO-A (clorgyline) or MAO-B (selegiline and pargyline)

Marked, dose-dependent elevations in the urinary excretion of phenylethylamine, para-tyramine, and meta-tyramine were observed in depressed patients treated for three or more weeks with 10, 30, or 60 mg/day of the partially-selective inhibitor of MAO-B, selegiline (l-deprenyl). In comparative studies with other, structurally similar acetylenic inhibitors of MAO, pargyline, an MAO-B > MAO-A inhibitor used in doses of 90 mg/day for three or more weeks, produced elevations in these trace amines which were similar to those found with the highest dose of selegiline studied. Clorgyline, a selective inhibitor of MAO-A used in doses of 30 mg/day for three or more weeks (a dose/time regimen previously reported to reduce urinary, plasma, and cerebrospinal fluid 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG) > 80%, indicating a marked inhibitory effect on MAO-A in humans in vivo) produced negligible changes in trace amine excretion. In comparison to recent studies of individuals lacking the genes for MAO-A, MAO-B, or both MAO-A and MAO-B, the lack of change in trace amine excretion in individuals with a mutation affecting only MAO-A is in agreement with the observed lack of effect of clorgyline in the present study. Selegiline produced larger changes in trace amines--at least at the higher doses studied--than found in individuals lacking the gene for MAO-B, in agreement with other data suggesting a lesser selectivity for MAO-B inhibition when selegiline was given in doses higher than 10 mg/day. Overall, trace amine elevations in individuals receiving the highest dose of deprenyl or receiving pargyline were approximately three to five-fold lower than the elevations observed in individuals lacking the genes for both MAO-A and MAO-B, suggesting that these drug doses yield incomplete inhibition of MAO-A and MAO-B.

Effect of monoamine oxidase A and B and of catechol-O-methyltransferase inhibition on L-DOPA-induced circling behavior

The effect of enzyme-inhibiting adjuvants on L-DOPA + benserazide-induced contralateral turning in unilateral 6-hydroxydopamine (6-OHDA)-lesioned rats was studied. Both the number of turns and the duration of turning were examined. Inhibition of MAO-A with 10 mg/kg Ro 41-1049 increased both parameters; inhibition of COMT with 30 mg/kg Ro 40-7592 had a similar effect. In contrast, inhibition of MAO-B with 10 mg/kg Ro 19-6327 did not change turning behavior. A further potentiation of turning behavior was observed after the combined administration of both the MAO-A and COMT inhibitor. MAO-A inhibition in conjunction with MAO-B inhibition prolonged the duration of L-DOPA-induced turning but had no effect on the number of turns. However, in conjunction with COMT inhibition, 10 mg/kg of the MAO-B inhibitor, Ro 19-6327, significantly affected both the number and duration of turning behavior. An even further potentiation of turning behavior was observed after the combined administration of all three enzyme-inhibitors.

Molecular neuroanatomy of human monoamine oxidases A and B revealed by quantitative enzyme radioautography and in situ hybridization histochemistry

Monoamine oxidases are key enzymes in the metabolism of amine neurotransmitters and neuromodulators and are targets for drug therapy in depression, Parkinson's and Alzheimer's diseases. Knowledge of their distribution in the brain is essential to understand their physiological role. To study the regional distribution and abundance of monoamine oxidases A and B in human brain, pituitary and superior cervical ganglion, we used quantitative enzyme radioautography with radioligands [3H]Ro41-1049 and [3H]lazabemide, respectively. Furthermore, 35S-labelled oligonucleotides complementary to isoenzyme messengerRNAs were used to map the cellular location of the respective transcripts in adjacent sections by in situ hybridization histochemistry. A markedly different pattern of distribution of the isoenzymes was observed. Highest levels of monoamine oxidase A were measured in the superior cervical ganglion, locus coeruleus, interpeduncular nucleus and ventromedial hypothalamic nucleus. The corresponding messengerRNA was detected only in the noradrenergic neurons of the superior cervical ganglion and locus coeruleus. In contrast to rat brain, monoamine oxidase B was much more abundant in most human brain regions investigated. Highest levels were measured in the ependyma of ventricles, stria terminalis and in individual hypothalamic neurons. Monoamine oxidase B transcripts were detected in serotoninergic raphe neurons, histaminergic hypothalamic neurons and in dentate gyrus granule cells of the hippocampal formation. We conclude that [3H]Ro41-1049 and [3H]azabemide are extremely useful radioligands for high-resolution analyses of the abundance and distribution of catalytic sites of monoamine oxidases A and B, respectively, in human brain sections. From levels of messenger RNA detected, the cellular sites of synthesis of the isoenzymes are the noradrenergic neurons of the locus coeruleus (for monoamine oxidase A) and the serotoninergic and histaminergic neurons of the raphe and posterior hypothalamus, respectively (for monoamine oxidase B). The combination of quantitative enzyme radioautography with in situ hybridization histochemistry is a useful approach to study, with high resolution, both the physiology and pathophysiology of monoamine oxidases in human brain.

Monoamine oxidase-B-positive granular structures in the hippocampus of aged senescence-accelerated mouse (SAMP8)

We examined the histochemical localization of monoamine oxidase in the hippocampus of young and old senescence-accelerated mouse (SAM). We found a monoamine oxidase-B-positive granular structure (MGS) in the hippocampus of old SAMP8, an accelerated senescence-prone line of SAM. The MGS was a round-shaped granular structure of 0.5 to 5 microns diameter and usually formed a cluster, the largest diameter of which ranged from 50 to 150 microns. No MGS were found in the hippocampus of young SAMP8 or of young SAMR1, an accelerated senescence resistant line of SAM, and only few, if any, were seen in old SAMR1. A monoamine oxidase-positive astrocyte was usually observed in the central area of each cluster of MGS. Furthermore, the MGS was in close anatomical relationship with monoamine oxidase-positive astrocytic processes. The enzyme inhibition experiments showed that monoamine oxidase activities localized in the MGS and astrocytes were both predominantly of type B. These findings suggest MGS occurs at least partly in monoamine oxidase-B-positive astrocytes. Furthermore, the MGS was similar to a periodic acid-Schiff-positive granular structure, a polyglucosan body previously documented in the brains of old SAMP8 and some other aged mice strains including C57BL/6 and nude mice, in terms of their size, morphological appearances and topographical distribution in the hippocampus. Thus, the present results suggest that monoamine oxidase type B is a proteinaceous component of the periodic acid-Schiff-positive granular structure in aged mice brains, and might provide some clues for clarifying the mechanisms of age-related occurrence of periodic acid-Schiff-positive granular structures in mice brains.

Aminium cation radical mechanism proposed for monoamine oxidase B catalysis: are there alternatives?

1. The interaction of bovine liver mitochondrial monoamine oxidase B (MAO B) with a series of benzylamine analogues was investigated to provide mechanistic information relative to the proposed cation radical mechanism and to provide information on the structural requirements of the substrate binding site. 2. Steady-state kinetic analysis of MAO B with 11 ring-substituted benzylamine analogues showed substitution does not alter the reaction pathway. All amine analogues tested exhibit sizeable deuterium kinetic isotope effects. 3. Anaerobic stopped-flow kinetic studies showed (1) C-H bond cleavage is rate-limiting in enzyme-bound flavin reduction and (2) that no specially detectable flavin radicals are observed. 4. The binding affinity of para-substituted benzylamine analogues to MAO B increased as the hydrophobicity of the substituent increased. In contrast, meta-substitution of the ring showed reduced affinity with an increase in the van der Waals volume of the substituent. 5. The rate of enzyme reduction by para-substitution exhibited a strong negative dependence with the Taft (Es) steric value of the substituent. In contrast, the rate of enzyme reduction by meta-substituted benzylamines is independent of the nature of the substituent. 6. para-Substituted N,N-dimethylbenzylamine analogues are not substrates for MAO B but are competitive inhibitors of benzylamine oxidation with a weaker affinity with increasing van der Waals volume of the substituent. In contrast, meta-substituted N,N-dimethyl benzylamine analogues are weak substrates for MAO B with oxidation occurring exclusively at the benzyl carbon. 7. The consequences of these results on the possible mechanisms (aminium cation radical, H abstraction, and nucleophilic mechanism) for C-H bond cleavage proposed for MAO B are discussed.

Structure activity studies of the substrate binding site in monoamine oxidase B

The influence of para and meta substitution of benzylamine analogues on their interaction with bovine liver monoamine oxidase B has been investigated to provide insights into the nature of the substrate binding site. Binding data with para-substituted benzylamine analogues show the area of the binding site about the para position to be hydrophobic and exhibiting some steric constraints. Alkylation of the benzylamine nitrogen with methyl groups results in a dominance of steric constraints about the para-position as an influence on binding. meta-Substitution of the benzylamine ring results in a decreased binding affinity which exhibits a dependence on the van der Waals volume of the substituent indicating steric constraints also occur about this area of the bound substrate. The independence of the rate of enzyme reduction with the nature of the meta-substituent suggests these benzylamine analogues are bound in the substrate site in a manner which optimizes overlap of the pro-R benzyl C-H bond with the lone pair orbital on the nitrogen. In contrast, the observed rates of enzyme reduction by para-substituted benzylamine analogues exhibit a dominant steric dependence which suggests the mode of binding of this class of analogues does not provide this optimal overlap for efficient C-H bond cleavage. Support for this conclusion also comes from the observation that para-substituted N,N-dimethylbenzylamine analogues are competitive inhibitors and not substrates for monoamine oxidase B while the meta-substituted analogues are substrates, albeit poor ones. The demonstration of a tunneling contribution to the C-H bond cleavage step demonstrates the absence of any motion or changes in solvation coupled with that catalytic event and the close proximity of the enzyme group accepting the H to the pro-R position of the bound substrate. Little or no influence of meta or para benzylamine substituent on the rate of O2 reaction with the reduced flavin-protonated imine complex is observed which suggests alterations in the configuration of the bound substrate do not influence the reactivity of the reduced flavin.

Monoamine oxidase inhibitory effects of some 4-aminophenethylamine derivatives

The in vitro and ex vivo monoamine oxidase (MAO) inhibitory effects of (+/-)4-dimethylamino-alpha-methyl-phenethylamine (4-DMAA) and (+/-)4-methylamino-alpha-methyl-phenethylamine (4-MAA) were reassessed, in comparison with the previously unstudied achiral parent compound, 4-dimethyl-aminophenethylamine (4-DMAPEA) and with a salt of 4-DMAA enriched in the levo isomer, ("-")-4-DMAA, using amiflamine [S-(+)-4-dimethylamino-alpha,2-dimethylphenethylamine] as positive control. The in vitro studies confirmed that 4-amino-alpha-methylphenethylamine derivatives are highly selective and reversible MAO-A inhibitors. Furthermore, ("-")-4DMAA was less active than the racemic mixture. The side chain-unsubstituted compound, 4-DMAPEA, proved to be a nonselective and reversible MAO inhibitor. The ex vivo results, in which catecholamines, serotonin (5-HT) and their metabolites were measured in two brain regions after i.p. administration, confirmed the results obtained in vitro. These results are consistent with the suggestion that the 4-amino group contributes to MAO inhibitory effects of alpha-methyl-phenethylamines, and show that the presence and orientation of an alpha-methyl side chain substituent may be important when determining the potency and selectivity of these compounds. All compounds tested could be quantified by HPLC with electrochemical detection.

Dopamine metabolism in the guinea pig striatum: role of monoamine oxidase A and B

These studies were carried out to determine whether the greater abundance of monoamine oxidase B in the guinea pig affects the actions of (-)-deprenyl on dopamine metabolism in whole tissue or in extracellular fluid. The administration of (-)-deprenyl in doses that do not affect monoamine oxidase A activity (1-4 mg kg-1, 2 h) increases striatal 2-phenylethylamine and dopamine concentrations and reduces 3,4-dihydroxyphenylacetic acid. No effects were observed on striatal homovanillic acid, 5-HT and 5-hydroxyindole acetic acid. Inhibition of monoamine oxidase A with clorgyline with doses up to 8 mg kg-1 (2 h) does not affect striatal 2-phenylethylamine but increases dopamine and 5-HT concentrations and reduces 3,4-dihydroxyphenyl-acetic acid and 5-hydroxyindole acetic acid. (-)-Deprenyl (2-4 mg kg-1) did not change the extracellular concentrations of dopamine but the higher dose produced a limited reduction in extracellular 3,4-dihydroxyphenylacetic acid. Inhibition of monoamine oxidase A and monoamine oxidase B with pargyline (75 mg kg-1, 2 h) significantly increased the levels of extracellular dopamine and reduced those of their acid metabolites. These results show that in the guinea pig striatum inhibition of monoamine oxidase B by (-)-deprenyl impairs the metabolism of dopamine in the whole tissue but does not produce a marked increase in extracellular dopamine.

4-Dimethylaminophenethylamine, a sensitive, specific, electrochemically detectable monoamine oxidase-B substrate

4-Dimethylaminophenethylamine (DMAPEA) was characterized as an MAO substrate. This compound was unaffected by MAO-A, while its oxidation by MAO-B was linear as a function of both time and enzyme concentration, with Km = 5.8 microM and Vmax = 21.2 pmol/min/mg protein, using a crude rat brain mitochondrial suspension as source of MAO. Both DMAPEA and its oxidation product, 4-dimethylaminophenylacetic acid (DMAPAA), can be detected electrochemically at 0.85 V. The high MAO-B affinity and selectivity of DMAPEA, together with its low oxidation potential, make this molecule a unique tool to determine MAO-B activity in a wide variety of tissue preparations using HPLC-ED.

Monoamine oxidase inhibition by L-deprenyl depends on both sex and route of administration in the rat

The monoamine oxidase B (MAO-B) inhibitor L-deprenyl, widely used to treat Parkinson's disease, has frequently been studied in animal models. We have examined the effects of several variables on activity levels of MAO-A and B in rat brain and liver following chronic (3 wks) treatment with L-deprenyl. Significant effects were observed for sex (females showed lower overall MAO-B activity in the liver), dose (MAO-A and B inhibition increased with dose, with females exhibiting greater sensitivity), route of administration (subcutaneous injection was more efficient than oral dosing), and dosing interval (MAO-B was significantly inhibited when dosing interval was increased to as long as 168 hours). Our results thus indicate that the effectiveness of L-deprenyl in vivo is dependent on several factors and that these must be taken into account in studies involving the benefits or risks of this drug.

A subpopulation of mouse striatal cholinergic neurons show monoamine oxidase activity

We examined the histochemical localization of monoamine oxidase (MAO) in the mouse striatum. MAO activity was observed in glial cells and vessels in accordance with previous reports in the rat brain. In addition to these previously documented structures, we found MAO-containing neurons in the mouse striatum. Enzyme inhibition experiments showed that MAO in the positive neurons was predominantly of type B. Double-labeling studies showed that in this region these neurons were cholinergic. The functional role of MAO activity in the mouse striatal cholinergic neurons is presently unclear.