Dopamine metabolism and neurotransmission in primate brain in relationship to monoamine oxidase A and B inhibition

Inhibiting Monoamine Oxidase in CNS and CVS would be a Promising Approach to Mitigating Cardiovascular Complications in Neurodegenerative Disorders

The flavoenzyme monoamine oxidases (MAOs) are present in the mitochondrial outer membrane and are responsible for the metabolism of biogenic amines. MAO deamination of biological amines produces toxic byproducts such as amines, aldehydes, and hydrogen peroxide, which are significant in the pathophysiology of multiple neurodegenerative illnesses. In the cardiovascular system (CVS), these by-products target the mitochondria of cardiac cells leading to their dysfunction and producing redox imbalance in the endothelium of the blood vessels. This brings up the biological relationship between the susceptibility of getting cardiovascular disorders in neural patients. In the current scenario, MAO inhibitors are highly recommended by physicians worldwide for the therapy and management of various neurodegenerative disorders. Many interventional studies reveal the benefit of MAO inhibitors in CVS. Drug candidates who can target both the central and peripheral MAO could be a better to compensate for the cardiovascular comorbidities observed in neurodegenerative patients.

Virus infection participates in the occurrence and development of human diseases through monoamine oxidase

Monoamine oxidase (MAO) is a membrane-bound mitochondrial enzyme that maintains the steady state of neurotransmitters and other biogenic amines in biological systems through catalytic oxidation and deamination. MAO dysfunction is closely related to human neurological and psychiatric diseases and cancers. However, little is known about the relationship between MAO and viral infections in humans. This review summarises current research on how viral infections participate in the occurrence and development of human diseases through MAO. The viruses discussed in this review include hepatitis C virus, dengue virus, severe acute respiratory syndrome coronavirus 2, human immunodeficiency virus, Japanese encephalitis virus, Epstein-Barr virus, and human papillomavirus. This review also describes the effects of MAO inhibitors such as phenelzine, clorgyline, selegiline, M-30, and isatin on viral infectious diseases. This information will not only help us to better understand the role of MAO in the pathogenesis of viruses but will also provide new insights into the treatment and diagnosis of these viral diseases.

Hydroxypyrone derivatives in drug discovery: from chelation therapy to rational design of metalloenzyme inhibitors

The versatile structural motif of hydroxypyrone is found in natural products and can be easily converted into hydroxypyridone and hydroxythiopyridone analogues. The favourable toxicity profile and ease of functionalization to access a vast library of compounds make them an ideal structural scaffold for drug design and discovery. This versatile scaffold possesses excellent metal chelating properties that can be exploited for chelation therapy in clinics. Deferiprone [1,2-dimethyl-3-hydroxy-4(1H)-one] was the first orally active chelator to treat iron overload in thalassemia major. Metal complexes of hydroxy-(thio)pyr(id)ones have been investigated as magnetic resonance imaging contrast agents, and anticancer and antidiabetic agents. In recent years, this compound class has demonstrated potential in discovering and developing metalloenzyme inhibitors. This review article summarizes recent literature on hydroxy-(thio)pyr(id)ones as inhibitors for metalloenzymes such as histone deacetylases, tyrosinase and metallo-β-lactamase. Different approaches to the design of hydroxy-(thio)pyr(id)ones and their biological properties against selected metalloenzymes are discussed.

Kinetic isotope effects and synthetic strategies for deuterated carbon-11 and fluorine-18 labelled PET radiopharmaceuticals

The deuterium labelling of pharmaceuticals is a useful strategy for altering pharmacokinetic properties, particularly for improving metabolic resistance. The pharmacological effects of such metabolites are often assumed to be negligible during standard drug discovery and are factored in later at the clinical phases of development, where the risks and benefits of the treatment and side-effects can be wholly assessed. This paradigm does not translate to the discovery of radiopharmaceuticals, however, as the confounding effects of radiometabolites can inevitably show in preliminary positron emission tomography (PET) scans and thus complicate interpretation. Consequently, the formation of radiometabolites is crucial to take into consideration, compared to non-radioactive metabolites, and the application of deuterium labelling is a particularly attractive approach to minimise radiometabolite formation. Herein, we provide a comprehensive overview of the deuterated carbon-11 and fluorine-18 radiopharmaceuticals employed in PET imaging experiments. Specifically, we explore six categories of deuterated radiopharmaceuticals used to investigate the activities of monoamine oxygenase (MAO), choline, translocator protein (TSPO), vesicular monoamine transporter 2 (VMAT2), neurotransmission and the diagnosis of Alzheimer's disease; from which we derive four prominent deuteration strategies giving rise to a kinetic isotope effect (KIE) for reducing the rate of metabolism. Synthetic approaches for over thirty of these deuterated radiopharmaceuticals are discussed from the perspective of deuterium and radioisotope incorporation, alongside an evaluation of the deuterium labelling and radiolabelling efficacies across these independent studies. Clinical and manufacturing implications are also discussed to provide a more comprehensive overview of how deuterated radiopharmaceuticals may be introduced to routine practice.

A comprehensive review of monoamine oxidase inhibitors as Anti-Alzheimer's disease agents: A review

Monoamine oxidases (MAO-A and MAO-B) are mammalian flavoenzyme, which catalyze the oxidative deamination of several neurotransmitters like norepinephrine, dopamine, tyramine, serotonin, and some other amines. The oxidative deamination produces several harmful side products like ammonia, peroxides, and aldehydes during the biochemical reaction. The concentration of biochemical neurotransmitter alteration in the brain by MAO is directly related with several neurological disorders like Alzheimer's disease and Parkinson's disease (PD). Activated MAO also contributes to the amyloid beta (Aβ) aggregation by two successive cleft β-secretase and γ-secretase of amyloid precursor protein (APP). Additionally, activated MAO is also involved in aggregation of neurofibrillary tangles and cognitive destruction through the cholinergic neuronal damage and disorder of the cholinergic system. MAO inhibition has general anti-Alzheimer's disease effect as a consequence of oxidative stress reduction prompted by MAO enzymes. In this review, we outlined and addressed recent understanding on MAO enzymes such as their structure, physiological function, catalytic mechanism, and possible therapeutic goals in AD. In addition, it also highlights the current development and discovery of potential MAO inhibitors (MAOIs) from various chemical scaffolds.

Synthetic melanin nanoparticles as peroxynitrite scavengers, photothermal anticancer and heavy metals removal platforms

Melanin is a ubiquitous natural polyphenolic pigment with versatile applications including physiological functions. This polymeric material is found in a diversity of living organisms from bacteria to mammals. The biocompatibility and thermal stability of melanin nanoparticles make them good candidates to work as free radical scavengers and photothermal anticancer substrates. Research studies have identified melanin as an antioxidative therapeutic agent and/or reactive oxygen species (ROS) scavenger that includes neutralization of peroxynitrite. In addition, melanin nanoparticles have emerged as an anticancer photothermal platform that has the capability to kill cancer cells. Recently, melanin nanoparticles have been successfully used as chelating agents to purify water from heavy metals, such as hexavalent chromium. This review article highlights some selected aspects of cutting-edge melanin applications. Herein, we will refer to the recent literature that addresses melanin nanoparticles and its useful physicochemical properties as a hot topic in biomaterial science. It is expected that the techniques of Dynamic Light Scattering (DLS), Scanning Electron Microscopy (SEM), and time-resolved Electron Paramagnetic Resonance (EPR) will have a strong impact on the full characterization of melanin nanoparticles and the subsequent exploration of their physiological and chemical mechanisms.

Alpha1-Adrenergic Receptor Mediated Long-Term Depression at CA3-CA1 Synapses Can Be Induced via Accumulation of Endogenous Norepinephrine and Is Preserved Following Noradrenergic Denervation

Locus coeruleus (LC) provides the sole source of noradrenergic (NA) innervation to hippocampus, and it undergoes significant degeneration early in Alzheimer's disease (AD). Norepinephrine (NE) modulates synaptic transmission and plasticity at hippocampal synapses which likely contributes to hippocampus-dependent learning and memory. We previously reported that pharmacological activation of α1 adrenergic receptors (α1ARs) induces long-term depression (LTD) at CA3-CA1 synapses. Here, we investigated whether accumulation of endogenous NE via pharmacological blockade of norepinephrine transporters (NETs) and the NE degradative enzyme, monoamine oxidase (MAO), can induce α1AR LTD, as these inhibitors are used clinically. Further, we sought to determine how degeneration of hippocampal NA innervation, as occurs in AD, impacts α1AR function and α1AR LTD. Bath application of NET and MAO inhibitors in slices from control rats reliably induced α1AR LTD when β adrenergic receptors were inhibited. To induce degeneration of LC-NA innervation, rats were treated with the specific NA neurotoxin DSP-4 and recordings performed 1-3 weeks later when NA axon degeneration had stabilized. Even with 85% loss of hippocampal NA innervation, α1AR LTD was successfully induced using either the α1AR agonist phenylephrine or the combined NET and MAO inhibitors, and importantly, the LTD magnitude was not different from saline-treated control. These data suggest that despite significant decreases in NA input to hippocampus, the mechanisms necessary for the induction of α1AR LTD remain functional. Furthermore, we posit that α1AR activation could be a viable therapeutic target for pharmacological intervention in AD and other diseases involving malfunctions of NA neurotransmission.

Monoamine oxidase-B inhibitors as potential neurotherapeutic agents: An overview and update

Monoamine oxidase (MAO) inhibitors have made significant contributions and remain an indispensable approach of molecular and mechanistic diversity for the discovery of antineurodegenerative drugs. However, their usage has been hampered by nonselective and/or irreversible action which resulted in drawbacks like liver toxicity, cheese effect, and so forth. Hence, the search for selective MAO inhibitors (MAOIs) has become a substantial focus in current drug discovery. This review summarizes our current understanding on MAO-A/MAO-B including their structure, catalytic mechanism, and biological functions with emphases on the role of MAO-B as a potential therapeutic target for the development of medications treating neurodegenerative disorders. It also highlights the recent developments in the discovery of potential MAO-B inhibitors (MAO-BIs) belonging to diverse chemical scaffolds, arising from intensive chemical-mechanistic and computational studies documented during past 3 years (2015-2018), with emphases on their potency and selectivity. Importantly, readers will gain knowledge of various newly established MAO-BI scaffolds and their development potentials. The comprehensive information provided herein will hopefully accelerate ideas for designing novel selective MAO-BIs with superior activity profiles and critical discussions will inflict more caution in the decision-making process in the MAOIs discovery.

Monoamine oxidase inhibitors, and iron chelators in depressive illness and neurodegenerative diseases

In early 1920s, tyramine oxidase was discovered that metabolized tyramine and in 1933 Blaschko demonstrated that this enzyme also metabolized adrenaline, noradrenaline and dopamine. Zeller gave it the name monoamine oxidase (MAO) to distinguish it from the enzyme that oxidatively deaminated diamines. MAO was recognized as an enzyme of crucial interest to pharmacologists because it catalyzed the major inactivation pathway for the catecholamines (and, later, 5-hydroxytryptamine, as well). Within the few decade, the inhibitors of MAO were discovered and introduced for the treatment of depressive illness which was established clinically. However, the first clinical use exposed serious side effects, pharmacological interest in, and investigation of, MAO continued, resulting in the characterization of two forms, distinct forms, MAO-A and -B, and selective inhibitors for them. Selective inhibitors of MAO-B (selegiline, rasagiline and safinamide) have found a therapeutic role in the treatment of Parkinson's disease and reversible inhibitors of MAO-A offered antidepressant activity without the serious side effects of the earlier nonselective MAO inhibitors. Subsequent molecular pharmacological have also generated the concept of neuroprotection, reflecting the possibility of slowing, halting and maybe reversing, neurodegeneration in Parkinson's or Alzheimer's diseases. Increased levels of oxidative stress through the accumulation of iron in the Parkinsonian and Alzheimer brains has been suggested to be critical for the initiation and progress of neurodegeneration. Selective inhibition of brain MAO could contribute importantly to lowering such stress, preventing the formation of hydrogen peroxide. Interaction of Iron with hydrogen peroxide and lead to Fenton reaction and production of the most reactive radical, namely hydroxyl radical. There are complex interactions between free iron levels in brain and MAO, and cascade of neurotoxic events may have practical outcomes for depressive disorders and neurodegenerative diseases. As consequence recent novel therapeutic drugs for neurodegenerative diseases has led to the development of multi target drugs, that possess selective brain MAO A and B inhibitory moiety, iron chelating and antioxidant activities and the ability to increase brain levels of endogenous neurotrophins, such as BDNF, GDNF VEGF and erythropoietin and induce mitochondrial biogenesis.

Chlorogenic Acid from Hawthorn Berry (Crataegus pinnatifida Fruit) Prevents Stress Hormone-Induced Depressive Behavior, through Monoamine Oxidase B-Reactive Oxygen Species Signaling in Hippocampal Astrocytes of Mice

SCOPE

Oxidative stress has been implicated in mental disorders, including depression. Chlorogenic acid (CGA), one of the abundant phenolic compounds in herbs and fruits, has the properties of a natural antioxidant and free-radical scavenger. Therfore, we investigated the antidepressant-like effects and active mechanisms of CGA from the extract of Crataegus pinnatifida (CP) fruit.

METHODS AND RESULTS

Depression-like phenotypes were induced in mice by daily injection of stress hormone for 1-2 weeks. The brains of these animals exhibited reduced brain-derived neurotrophic factor expression and increased astrocytic hypertrophy, which are typical markers of depression in animal models. Stress hormone injection 1) upregulated monoamine oxidase B (MAOB) expression and 2) reduced spine numbers along neuronal dendrites, which indicates synaptic depression. The oral administration of CGA (30 mg kg^-1 ) or CP (300 mg kg^-1 ) prevented MAOB activation following reactive oxygen species (ROS) production and had an ameliorative effect on depressive behavioral tests (e.g., tail suspension and forced swim tests). In vitro assays performed on cultured C8-D1A cells revealed that CGA and CP inhibited MAOB activity and ROS production.

CONCLUSION

Our study indicates that CGA and CP extracts prevented depressive behavior and thereby have potential as natural antidepressants.

Pharmacological aspects of the neuroprotective effects of irreversible MAO-B inhibitors, selegiline and rasagiline, in Parkinson's disease

The era of MAO-B inhibitors dates back more than 50 years. It began with Kálmán Magyar's outstanding discovery of the selective inhibitor, selegiline. This compound is still regarded as the gold standard of MAO-B inhibition, although newer drugs have also been introduced to the field. It was revealed early on that selective, even irreversible inhibition of MAO-B is free from the severe side effect of the non-selective MAO inhibitors, the potentiation of tyramine, resulting in the so-called 'cheese effect'. Since MAO-B is involved mainly in the degradation of dopamine, the inhibitors lack any antidepressant effect; however, they became first-line medications for the therapy of Parkinson's disease based on their dopamine-sparing activity. Extensive studies with selegiline indicated its complex pharmacological activity profile with MAO-B-independent mechanisms involved. Some of these beneficial effects, such as neuroprotective and antiapoptotic properties, were connected to its propargylamine structure. The second MAO-B inhibitor approved for the treatment of Parkinson's disease, rasagiline also possesses this structural element and shows similar pharmacological characteristics. The preclinical studies performed with selegiline and rasagiline are summarized in this review.

Cellular uptake of quercetin and luteolin and their effects on monoamine oxidase-A in human neuroblastoma SH-SY5Y cells

Monoamine oxidase-A (MAO-A) is the main enzyme in the metabolism of the neurotransmitter serotonin (5-hydroxytryptamine). Elevated activity of MAO-A in the brain may contribute to the pathogenesis of depressive disorders. Plant flavonoids, such as flavonol quercetin and flavone luteolin, have been suggested to be potential antidepressant compounds because they exert a suppressive effect on the MAO-A reaction. We evaluated the effects of these flavonoids on MAO-A activity and protein level using SH-SY5Y as model serotoninergic nerve cells. Quercetin and luteolin were incorporated into SH-SY5Y cells rapidly and converted to O-methylated derivatives. Luteolin accumulated in cells after 24-h incubation, whereas quercetin disappeared completely from cell fractions and culture medium. Addition of ascorbic acid prevented the disappearance of quercetin and allowed it to exert its cytotoxicity (similar to luteolin) at >10 μM. Luteolin and quercetin were incorporated into mitochondria fractions within 1-h incubation and attenuated MAO-A activity slightly but significantly. After 24-h incubation, luteolin attenuated MAO-A activity, but quercetin needed ascorbic acid for its attenuation. Neither luteolin nor quercetin significantly affected MAO-A protein level. These data suggest that luteolin and quercetin can be direct inhibitors of MAO-A in nerve cells by targeting mitochondria.

Radiochemistry devoted to the production of monoamine oxidase (MAO-A and MAO-B) ligands for brain imaging with positron emission tomography

Monoamine oxidase (MAO) belongs to a family of flavin-containing integral enzymes that are present in the outer mitochondrial membrane in neurons and glial cells in the central nervous system. These enzymes catalyze the oxidative deamination of various neurotransmitters, biogenic amines, and xenobiotics, thereby influencing their availability and physiological activity in brain and body. Over the past decades, many potential positron emission tomography tracers have been put forward to visualize MAO in the brain with varying success, and recent publications on the topic illustrate the continuing interest in the field. The present review gives an overview of the compounds that have been put forward as possible MAO tracers in the brain and focuses on the radiochemical procedures that have been developed to produce them up till now. Relevant radioligands are grouped by the main radiochemical strategies that have been employed to synthesize them, and some interesting details and findings that are crucial to the radiosyntheses are provided.

Elevated brain monoamine oxidase activity in SIV- and HIV-associated neurological disease

We recently demonstrated direct evidence of increased monoamine oxidase (MAO) activity in the brain of a simian immunodeficiency virus (SIV) model of human immunodeficiency virus (HIV)-associated central nervous system (CNS) disease, consistent with previously reported dopamine deficits in both SIV and HIV infection. In this study, we explored potential mechanisms behind this elevated activity. MAO B messenger RNA was highest in macaques with the most severe SIV-associated CNS lesions and was positively correlated with levels of CD68 and GFAP transcripts in the striatum. MAO B messenger RNA also correlated with viral loads in the CNS of SIV-infected macaques and with oxidative stress. Furthermore, in humans, striatal MAO activity was elevated in individuals with HIV encephalitis, compared with activity in HIV-seronegative controls. These data suggest that the neuroinflammation and oxidative stress caused by SIV infection in the CNS may provide the impetus for increased transcription of MAO B and that MAO, and more broadly, oxidative stress, have significant potential as therapeutic targets in CNS disease due to HIV.

Orally disintegrating selegiline for the treatment of Parkinson's disease

The selective monoamine oxidase type B inhibitor selegiline is commonly administered as medical treatment to patients suffering from Parkinson's disease. The clinical value of conventional selegiline is, however, compromised by extensive first-pass metabolism, which reduces its bioavailability and leads to the production of possibly harmful methamfetamine metabolites. This review aims to evaluate a novel, orally disintegrating formulation of selegiline by examining scientific evidence from previous pharmacological and clinical studies. As a result of improved bioavailability, orally disintegrating selegiline can be administered at lower doses than conventional selegiline with similar clinical effect. It also leads to less variable selegiline blood concentrations and produces significantly less methamfetamine metabolites. We conclude that this novel formulation offers an interesting treatment option, especially for patients who report adverse events after initial treatment with conventional selegiline or who suffer from swallowing difficulties.

Multifunctional neuroprotective derivatives of rasagiline as anti-Alzheimer's disease drugs

The recent therapeutic approach in which drug candidates are designed to possess diverse pharmacological properties and act on multiple targets has stimulated the development of the multimodal drugs, ladostigil (TV3326) [(N-propargyl-(3R) aminoindan-5yl)-ethyl methyl carbamate] and the newly designed multifunctional antioxidant iron chelator, M-30 (5-[N-methyl-N-propargylaminomethyl]-8-hydroxyquinoline). Ladostigil combines, in a single molecule, the neuroprotective/neurorestorative effects of the novel anti-Parkinsonian drug and selective monoamine oxidase (MAO)-B inhibitor, rasagiline (Azilect, Teva Pharmaceutical Co.) with the cholinesterase (ChE) inhibitory activity of rivastigmine. A second derivative of rasagiline, M-30 was developed by amalgamating the propargyl moiety of rasagiline into the skeleton of our novel brain permeable neuroprotective iron chelator, VK-28. Preclinical experiments showed that both compounds have anti-Alzheimer's disease activities and thus, the clinical development is oriented toward treatment of this type of dementia. This review discusses the multimodal effects of two rasagiline-containing hybrid molecules, namely ladostigil and M-30, concerning their neuroprotective molecular mechanisms in vivo and in vitro, including regulation of amyloid precursor protein processing, activation of protein kinase C, and mitogen-activated protein kinase signaling pathways, inhibition of cell death markers and upregulation of neurotrophic factors. Altogether, these scientific findings make these multifunctional compounds potentially valuable drugs for the treatment of Alzheimer's disease.

Current understanding of the binding sites, capacity, affinity, and biological significance of metals in melanin

Metal chelation is often invoked as one of the main biological functions of melanin. In order to understand the interaction between metals and melanin, extensive studies have been carried out to determine the nature of the metal binding sites, binding capacity, and affinity. These data are central to efforts aimed at elucidating the role metal binding plays in determining the physical, structural, biological, and photochemical properties of melanin. This article examines the current state of understanding of this field.

Inhibitory Effects of Neurotransmitters and Steroids on Human CYP2A6

Human CYP2A6 catalyzes the metabolism of nicotine, cotinine, and coumarin as well as some pharmaceutical drugs. CYP2A6 is highly expressed in liver and, also, in brain and steroid-related tissues. In this study, we investigated the inhibitory effects of neurotransmitters and steroid hormones on CYP2A6 activity. We found that coumarin 7-hydroxylation and cotinine 3'-hydroxylation by recombinant CYP2A6 expressed in baculovirus-infected insect cells were competitively inhibited by tryptamine (both K(i) = 0.2 microM), serotonin (K(i) = 252 microM and 167 microM), dopamine (K(i) = 49 microM and 22 microM), and histamine (K(i) = 428 microM and 359 microM). Cotinine formation from nicotine was inhibited by tryptamine (K(i) = 0.7 microM, competitive), serotonin (K(i) = 272 microM, noncompetitive), dopamine, noradrenaline, and adrenaline (K(i) = 11 microM, 54 microM, and 81 microM, uncompetitive). Estrogens (K(i) = 0.6-3.8 microM), androgens (K(i) = 60-149 microM), and corticosterone (K(i) = 36 microM) also inhibited cotinine formation, but coumarin 7-hydroxylation and cotinine 3'-hydroxylation did not. Nicotine-Delta(5'(1'))-iminium ion formation from nicotine was not affected by these steroid hormones, indicating that the inhibition of cotinine formation was due to the inhibitory effects on aldehyde oxidase. The nicotine-Delta(5'(1'))-iminium ion formation was competitively inhibited by tryptamine (K(i) = 0.3 microM), serotonin (K(i) = 316 microM), dopamine (K(i) = 66 microM), and histamine (K(i) = 209 microM). Thus, we found that some neurotransmitters inhibit CYP2A6 activity, being related with inter- and intraindividual differences in CYP2A6-dependent metabolism. The inhibitory effects of steroid hormones on aldehyde oxidase may also contribute to interindividual differences in nicotine metabolism.

Implications of co-morbidity for etiology and treatment of neurodegenerative diseases with multifunctional neuroprotective-neurorescue drugs; ladostigil

The recent therapeutic approach in which drug candidates are designed to possess diverse pharmacological properties and act on multiple targets has stimulated the development of several multifunction drugs. These include ladostigil (TV3326) [(N-propargyl-(3R) aminoindan-5yl)-ethyl methyl carbamate], which combines the pharmacophore-neuroprotective effects of rasagiline, a selective monoamine oxidase (MAO)-B inhibitor, with the cholinesterase (ChE) inhibitory activity of rivastigmine or iron chelating moiety such as M30. In the case of M30 the pharmacophore of brain permeable iron chelator VK-28 plus the MAO inhibitor-neuroprotective propargylamine moiety of rasagiline are combined in a single molecule as a potential treatment for Alzheimer's disease, Lewy body disease, and Parkinson's disease with dementia. Here, we discuss the activities of ladostigil in terms of its cholinesterase cognitive enhancing potential, antiParkinson, antidepressant, neuroprotection and APP (amyloid precursor protein) processing potential. One major attribute of ladostigil is its neuroprotective activity in neuronal cell cultures and in vivo. Employing an apoptotic model of neuroblastoma SK-N-SH cells, the molecular mechanism of its neuroprotective activity has been determined. The current studies show that ladostigil significantly decreased apoptosis via inhibition of the cleavage and prevention of caspase-3 activation through a mechanism related to regulation of the Bcl-2 family proteins, resulting in reduced levels of Bad and Bax and induced levels of Bcl-2. In addition, ladostigil elevated the levels of pPKC(pan). We have also followed the regulation of APP processing and found that ladostigil markedly decreased apoptotic-induced levels of holo-APP, as well as stimulated the release of the non-amyloidogenic soluble APP (sAPPalpha) into the conditioned medium via a established protein kinsae C-MAPkinase dependent pathway. Similar to ladostigil, its S-isomer, TV3279, which is a ChE inhibitor lacking MAO inhibitory activity, exerted similar neuroprotective properties and APP processing, suggesting that the mode of action is independent of MAO inhibition. These effects were shown to reside in the propargylamine moiety. These findings indicate that the dual actions of the anti-apoptotic-neuroprotective activity and the ability to modulate APP processing, could make ladostigil a potentially valuable drug for the treatment of Alzheimer's disease.

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.

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

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

Zydis selegiline reduces off time in Parkinson's disease patients with motor fluctuations: a 3-month, randomized, placebo-controlled study

Zydis selegiline dissolves on contact with saliva and undergoes pregastric absorption. This minimizes first-pass metabolism and provides high plasma concentrations of selegiline. In this study, the efficacy and safety of Zydis selegiline was assessed in Parkinson's disease (PD) patients who were experiencing motor fluctuations with levodopa. Patients were randomly assigned to either drug or placebo in a 2:1 ratio in this double-blind, multicenter trial. Significant reductions in daily off time occurred at 4 to 6 weeks with the 1.25 mg dose (9.9%, P = 0.003) and 10 to 12 weeks with the 2.5 mg dose (13.2%, P < 0.001). The total number of off hours was reduced by 2.2 hours at Week 12 from baseline (compared with 0.6 hours in the placebo group). The average number of dyskinesia-free on hours for the Zydis selegiline patients increased by 1.8 hours at Week 12. There was no change in mean percentage of "Asleep" time throughout the study. No apparent differences were detected in the occurrence of drug-related adverse events between the Zydis selegiline group and placebo-treated groups. Adverse events were consistent with known effects of levodopa therapy. Zydis selegiline safely reduces daily off time when used as adjunctive therapy with levodopa in patients with PD.

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.

Monoamine oxidase A imaging in peripheral organs in healthy human subjects

Monoamine oxidase (MAO) catalyzes the oxidative deamination of many biogenic and dietary amines. Though studies of MAO have focused mainly on its regulatory role in the brain, MAO in peripheral organs also represents a vast mechanism for detoxifying vasoactive compounds as well as for terminating the action of physiologically active amines, which can cross the blood brain barrier. Indeed, robust central and peripheral MAO activity is a major requirement in the safe use of many CNS drugs, particularly antidepressants, and thus an awareness of the MAO inhibitory potential of drugs is essential in therapeutics. In this study, we examined the feasibility of quantifying MAO A in peripheral organs in healthy human subjects using comparative positron emission tomography (PET) imaging with carbon-11 (t(1/2): 20.4 min) labeled clorgyline ([(11)C]clorgyline) a suicide inactivator of MAO A and its deuterium labeled counterpart ([(11)C]clorgyline-D2). Heart, lungs, kidneys, thyroid, and spleen showed a robust deuterium isotope effect characteristic of MAO and the magnitude of the effect was similar to that of trancylcypromine, an irreversible MAO inhibitor used in the treatment of depression. Liver time-activity curves were not affected by deuterium substitution precluding the estimation of liver MAO in vivo. In organs showing an isotope effect, MAO A is greatest in the lungs and kidneys followed by the thyroid and heart. This method, which has been previously applied in the human brain, opens the possibility to also directly assess the effects of different variables including smoking, dietary substances, drugs, disease, and genetics on peripheral MAO A in humans.

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.

Novel monoamine oxidase inhibitors, 3-(2-aminoethoxy)-1,2-benzisoxazole derivatives, and their differential reversibility

Although possible usefulness of non-selective monoamine oxidase (MAO) inhibitors for Parkinson's disease therapy has been suggested in the literature, MAO inhibitors whose inhibition is reversible and have dual action to both MAO-A and -B subtypes is not available yet. Subtype selectivity and reversibility of a series of novel MAO inhibitors, 3-(2-aminoethoxy)-1,2-benzisoxazole derivatives, were studied. Several dual MAO inhibitors, which inhibit both MAO-A and -B, were obtained. When administered to mice, their effects were generally reversible. Among the derivatives, RS-1636 and RS-1653 had much longer duration of brain MAO-B inhibition than that of MAO-A. In vitro, the inhibited MAO-A activity by these compounds was partially recovered by buffer change at 4 degrees C, while little MAO-B activity was recovered. Although it is not fully elucidated yet, the reversibility of these inhibitors is probably determined primarily by this dissociation profile. This unique differential reversibility indicates that optimization of the balance of actions can be achieved by differentiating reversibility to each target molecule.

Deprenyl stimulates the efflux of monoamines from the rat hypothalamus in vitro

The direct effects of L-deprenyl, a monoamine oxidase inhibitor, on the hypothalamus of male Sprague-Dawley rats was investigated by measuring the efflux of norepinephrine (NE), dopamine (DA), serotonin (5-HT), dihydroxyphenylacetic acid (DOPAC), and 5-hydroxyindoleacetic acid (5-HIAA) using a combination of high performance liquid chromatography with electrochemical detection and an in vitro incubation system. After measuring basal efflux by incubating the hypothalami with Krebs-Ringers Henseleit (KRH) alone during the first incubation period, hypothalami were incubated either with the medium, KRH alone (0 mM), or KRH containing 0.1, 1, and 10 mM L-deprenyl. During the third incubation period, hypothalami were again incubated with KRH alone to measure the residual effects if any. During the final incubation period, the hypothalami were stimulated with high K(+) KRH. Deprenyl produced a dose-dependent increase in the efflux of NE, DA, and 5-HT from the hypothalami. Neurotransmitter efflux returned to pretreatment levels when L-deprenyl was removed from the medium. In contrast to NE, DA, and 5-HT, the efflux of the metabolites DOPAC and 5-HIAA was inhibited in a dose-dependent fashion after incubation with L-deprenyl. Results from this study demonstrate that L-deprenyl is capable of stimulating the efflux of neurotransmitters in vitro by a direct action on the hypothalamus.

Nontricyclic antidepressants: current trends in children and adolescents

OBJECTIVES

First, to review the extant data on the safety and efficacy of the use of nontricyclic antidepressants in children and adolescents; second, to identify the main limitations of our current knowledge in this area; and third, to point to future research directions.

METHOD

A Medline search and a review of previous scientific meetings were conducted; all available reports on the efficacy and safety of nontricyclic antidepressants in children and adolescents were critically reviewed.

RESULTS

As in adults, also in children nontricyclic antidepressants are potentially useful in treating a variety of psychiatric disorders. The data supporting their efficacy, however, are quite limited. Obsessive-compulsive disorder is the only psychiatric diagnosis for which pediatric use of selective serotonin reuptake inhibitors has been approved. One placebo-controlled study in children and adolescents with major depression supports the efficacy of fluoxetine. Other clinical trials of nontricyclic antidepressants in depressed adolescents are in progress. Available data indicate that the safety of these medications is good, at least in the short term.

CONCLUSIONS

The potential usefulness of nontricyclic antidepressants for children and adolescents suffering from a range of disorders is considerable. While information from adults can suggest potential areas of possible efficacy in pediatric patients suffering from similar psychopathology, further research is essential to provide the necessary information on the efficacy, safety, and pharmacokinetics of these medications in children and adolescents.

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.

Selective reduction of monoamine oxidase A and B in the frontal cortex of subordinate rats

We have previously shown that subordination causes a reduction in the levels of 5-hydroxytryptamine and dopamine selectively in the frontal cortex [6]. These monoamines are catabolised mainly by the enzyme monoamine oxidase (MAO) which exists in two isoforms; MAO-A and MAO-B. The present study was carried out to determine whether there is any change in the activity of these two iso-enzymes induced by subordination and if any such alteration is confined to the frontal cortex. The animal model of dominance-subordination used was a worker-parasite paradigm in male Wistar rats. The enzyme activities were measured in five brain regions, the frontal cortex, entorhinal cortex, hippocampus, hypothalamus and striatum, using kynuramine as the substrate. Clorgyline and L-deprenyl were used in vitro to block the activities of MAO-A and MAO-B, respectively. There was a significant (P < 0.001) reduction in the activity of MAO-A as well as MAO-B selectively in the frontal cortex of the subordinate animals. This finding may suggest a reduced neurotransmitter turnover in the serotonergic and dopaminergic neurons terminating in the frontal cortex.

Dopamine-induced apoptosis in human neuronal cells: inhibition by nucleic acids antisense to the dopamine transporter

Human neuroblastoma NMB cells take up [3H]dopamine in a selective manner indicating that dopamine transporters are responsible for this uptake. These cells were therefore used as a model to study dopamine neurotoxicity, and to elucidate the role of dopamine transporters in controlling cell death. Treatment with 0.05 0.4 mM dopamine changed cells' morphology within 4 h, accompanied by retraction of processes, shrinkage, apoptosis-like atrophy, accumulation of apoptotic particles, DNA fragmentation and cell death. Cycloheximide inhibited dopamine's effect suggesting that induction of apoptosis by dopamine was dependent upon protein synthesis. Dopamine cytotoxicity, monitored morphologically by flow cytometric analysis, and by lactate dehydrogenase released, was blocked by cocaine but not by the noradrenaline and serotonin uptake blockers desimipramine and imipramine, respectively. Attempting to inhibit dopamine transport and toxicity in a drug-free and highly selective way, three 18-mer dopamine transporter antisense phosphorothioate oligonucleotides (numbers 1, 2 and 3) and a new plasmid vector expressing the entire rat dopamine transporter complementary DNA in the antisense orientation were prepared and tested. Antisense phosphorothioate oligonucleotide 3 inhibited [3H]dopamine uptake in a time- and dose-dependent manner. Likewise, transient transfection of NMB cells with the plasmid expressing dopamine transporter complementary DNA in the antisense orientation partially blocked [3H]dopamine uptake. Antisense phosphorothioate oligonucleotide 3 also decreased, dose-dependently, the toxic effect of dopamine and 6-hydroxydopamine. Western blot analysis with newly prepared anti-human dopamine transporter antibodies showed that antisense phosphorothioate oligonucleotide 3 decreased the transporter protein level. These studies contribute to better understand the mechanism of dopamine-induced apoptosis and neurotoxicity.

The therapeutic potential of moclobemide, a reversible selective monoamine oxidase A inhibitor in Parkinson's disease

Dopamine is equally well deaminated oxidatively by monoamine oxidase (MAO) A and B types. Selegiline (L-deprenyl), a selective inhibitor of MAO-B, ameliorates the "wearing off" akinesia and delays the need for levodopa in mild, previously untreated Parkinson's disease. The therapeutic potential of selective inhibition of MAO-A in Parkinson's disease has not been examined in detail. MAO-A accounts for only about 20% of total MAO activity in the human basal ganglia, and it differs from MAO-B in distribution. In contrast to MAO-B, which is confined to the extraneuronal compartment, MAO-A is found both extraneuronally and within the presynaptic dopaminergic terminals. The inhibition of MAO-A might alter the intraneuronal handling of dopamine reuptaken from synaptic clefts and thereby prolong oral levodopa benefit. We have given moclobemide, a selective, reversible inhibitor of MAO-A, to nondepressed patients with Parkinson's disease receiving standard levodopa/peripheral decarboxylase inhibitor or levodopa with dopaminergic agonist (bromocriptine, pergolide). Selegiline was discontinued at least 8 weeks earlier. A standard oral levodopa challenge was performed at the patient's entry to the study and repeated on the 22nd day of moclobemide treatment (150 mg thrice daily). The overall time spent "on" and "off" before the onset of treatment and during the last week on the drug was estimated from the patients' diaries. Neuropsychological assessments were also made before and after 3 weeks of moclobemide to measure possible effects on cognitive performance and mood. In acute levodopa challenge, the latency of motor response was significantly shortened and its duration was prolonged during moclobemide treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective MAO-A and B inhibitors, radical scavengers and nitric oxide synthase inhibitors in Parkinson's disease

In the absence of identification of either an endogenously or an exogenously derived dopaminergic neurotoxin, the most valid hypothesis currently envisaged for etiopathology of Parkinson's disease (PD) is selective oxidative stress (OS) in substantia nigra (SN). Although OS is not proven, a significant body of evidence from studies on animal and Parkinsonian brain neurochemistry supports it. This hypothesis is based on excessive formation of reactive oxygen species (O2 and OH.) and demise of systems involved with scavenging or preventing the formation of such radicals from H2O2, generated as a consequence of dopamine oxidation (autoxidation and deamination). Since MAO (monoamine oxidase A and B are the major H2O2 generating enzymes in the SN much attention has been paid to their selective inhibitors as symptomatic and neuroprotective agents in PD. Attention should also be given to radical scavengers (e.g. iron chelators, lipid peroxidative inhibitors and Vitamin E derivatives) as therapeutic neuroprotective agents in PD. This is considered valid since a significant elevation of iron is known to occur selectively in SN zone compacta and within the remaining melanized dopamine neurons of Parkinsonian brains. Although all the mechanism of iron induced oxygen free radical formation is not fully known there is no doubt that it participates with H2O2 (Fenton chemistry) to generate cytotoxic hydroxyl radical (OH.) and induce tissue OS and neurodegeneration in 6-hydroxydopamine model of PD. The dramatic proliferation of reactive amoeboid macrophages and microglia seen in SN of PD brains together with OS is highly compatible with an inflammatory process, similar to what has been observed in Alzheimer's disease and multiple sclerosis brains. This has led us to examine the ability of reactive macrophages to produce oxygen free radicals in response to nitric oxide (NO) production. The latter radical has been implicated in the excitotoxicity of glutaminergic neurons innervating the striatum and SN. Indeed we have now observed that in reactive macrophages NO acts as a signal transducer of O2 production which can synergize with dopamine oxidation.

The relevance of glial monoamine oxidase-B and polyamines to the action of selegiline in Parkinson's disease

Dopamine and 2-phenylethylamine levels in striatal tissue are known to be increased after administration of selegiline (L-deprenyl), but it is still difficult to explain why this treatment induces longevity or dopaminergic neuroprotection in Parkinson's disease. In the absence of significant polyamine or diamine oxidase activities in human brain, polyamines and histamine are detoxified by N-acetylation and methylation, respectively. Methylhistamine as well as N-acetylated polyamine derivatives are selective substrates for monoamine oxidase type B (MAO-B). Theoretically at least, MAO-B inhibition by selegiline could result in the increase in the levels of polyamines and their N-acetyl derivatives. This could have significance for the action of selegiline in Parkinson's disease, as overactive corticostriatal glutaminergic function has been implicated in the degeneration of nigrostriatal dopamine neurons, and polyamines are potent modulators of the excitotoxic NMDA (N-methyl-D-aspartate)-glutamate subtype receptor.