Biochemistry and physiology of monoamine oxidase (MAO) activity in the ring dove (Streptopelia risoria). II. Distribution of MAO in different tissues

Monoamine oxidase (MAO) activity was measured fluorometrically in liver, kidney, intestine and brain of adult male and female ring doves. Liver MAO was inhibited in a concentration-related fashion by clorgyline and harmaline (MAO type A inhibitors) where a plateau in the inhibition curve occurred with about 15% activity remaining, and also by the type B inhibitor deprenyl, which produced a plateau when about 85% activity remained. Kidney, intestine and brain MAO were inhibited in a biphasic manner by harmaline. Results with inhibitors suggest that 85% of liver MAO, 86% of kidney MAO, 88% of intestine and 75% of brain MAO is type A. Using 10(-6) M harmaline to differentiate between MAO-A and MAO-B type activities, the apparent maximal velocities (Vmax) and Michaelis constants (Km) were determined in different tissues. Most activity occurred in the intestine, with proportionally lesser amounts of kidney, liver and brain. The majority of MAO present was in the A form. Except for kidney, Km of MAO-B was higher than that of MAO-A. Both MAO-A and -B activities were higher in the intestines of male birds, although sex differences in content and type of MAO activity were not observed in other tissues of the ring dove.     

Effectiveness of polyene antibiotics in treatment of transmissible spongiform encephalopathy in transgenic mice expressing Syrian hamster PrP only in neurons

To date very few drugs have favorably influenced the course of transmissible spongiform encephalopathies. In previous studies, the polyene antibiotics amphotericin B (AmB) and MS-8209 prolonged the incubation time in Syrian hamsters of the 263K strain of scrapie, but AmB had no effect against other scrapie strains in Syrian hamsters. In the present experiments using transgenic mice expressing Syrian hamster PrP in neurons only, MS-8209 extended the life spans of animals infected with the 263K strain but not the DY strain. AmB was effective against both 263K and DY and prevented death in 18% of DY-infected animals. The AmB effect against strain 263K was more prominent in mice whose endogenous PrP gene had been inactivated by homologous recombination. It was unclear whether this difference was due to a change in the duration of the disease or to possible interactive effects between the mouse PrP gene and the drugs themselves. The effectiveness of treatment after intracerebral scrapie infection in transgenic mice expressing PrP only in neurons suggested that neurons are important sites of action for these drugs.     

Multiple forms of monoamine oxidase in rabbit platelets.

Rabbit platelets were found to contain both types A and B MAO activities. The specific enzymatic activity of rabbit platelet MAO was higher for the substrate serotonin than for phenylethylamine. The K_m's for rabbit platelet MAO indicated that the MAO-B enzyme was similar to human platelet MAO and that both MAO-A and MAO-B enzymes in the rabbit platelet are similar to the corresponding forms in the rabbit brain. The drugs clorgyline and deprenyl confirmed the existence of types A and B MAO in the platelet and furthermore indicated that the type A form accounted for approximately 90% of the total enzymatic activity. Amitriptyline at low (micromolar) concentrations selectively inhibited MAO-B activity in both rabbit platelets and brain.     

Novel multifunctional neuroprotective iron chelator-monoamine oxidase inhibitor drugs for neurodegenerative diseases: in vitro studies on antioxidant activity, prevention of lipid peroxide formation and monoamine oxidase inhibition

Iron-dependent oxidative stress, elevated levels of iron and of monoamine oxidase (MAO)-B activity, and depletion of antioxidants in the brain may be major pathogenic factors in Parkinson's disease, Alzheimer's disease and related neurodegenerative diseases. Accordingly, iron chelators, antioxidants and MAO-B inhibitors have shown efficacy in a variety of cellular and animal models of CNS injury. In searching for novel antioxidant iron chelators with potential MAO-B inhibitory activity, a series of new iron chelators has been designed, synthesized and investigated. In this study, the novel chelators were further examined for their activity as antioxidants, MAO-B inhibitors and neuroprotective agents in vitro. Three of the selected chelators (M30, HLA20 and M32) were the most effective in inhibiting iron-dependent lipid peroxidation in rat brain homogenates with IC50 values (12-16 microM), which is comparable with that of desferal, a prototype iron chelator that is not has orally active. Their antioxidant activities were further confirmed using electron paramagnetic resonance spectroscopy. In PC12 cell culture, the three novel chelators at 0.1 microM were able to attenuate cell death induced by serum deprivation and by 6-hydroxydopamine. M30 possessing propargyl, the MAO inhibitory moiety of the anti-Parkinson drug rasagiline, displayed greater neuroprotective potency than that of rasagiline. In addition, in vitro, M30 was a highly potent non-selective MAO-A and MAO-B inhibitor (IC50 < 0.1 microM). However, HLA20 was more selective for MAO-B but had poor MAO inhibition, with an IC50 value of 64.2 microM. The data suggest that M30 and HLA20 might serve as leads in developing drugs with multifunctional activities for the treatment of various neurodegenerative disorders.     

Inhibition of brain monoamine oxidase activity by the generation of hydroxyl radicals: potential implications in relation to oxidative stress

Monoamine oxidase (MAO) is an enzyme involved in brain catabolism of monoamine neurotransmitters whose oxidative deamination results in the production of hydrogen peroxide. It has been documented that hydrogen peroxide derived from MAO activity represents a special source of oxidative stress in the brain. In this study we investigated the potential effects of the production of hydroxyl radicals (*OH) on MAO-A and MAO-B activities using mitochondrial preparations obtained from rat brain. Ascorbic acid (100 microM) and Fe2+ (0.2, 0.4, 0.8, and 1.6 microM) were used to induce the production of *OH. Results showed that the generation of *OH significantly reduced both MAO-A (85-53%) and MAO-B (77-39%) activities, exhibiting a linear correlation between both MAO-A and MAO-B activities and the amount of *OH produced. The reported inhibition was found to be irreversible for both MAO-A and MAO-B. Assuming the proven contribution of MAO activity to brain oxidative stress, this inhibition appears to reduce this contribution when an overproduction of *OH occurs.     

Monoamine Oxidase Activity and Monoamine Metabolism in Brains of Parkinsonian Patients Treated with l-Deprenyl

Monoamine oxidase (MAO) type A and type B were measured using kynuramine, 3,4-dihydroxyphenylethylamine (dopamine, DA), and 5-hydroxytryptamine (5-HT, serotonin) in 20 brain areas. The highest activities were found in the striatum (caudate nucleus, putamen, globus pallidus, and substantia nigra), hypothalamus, and c-mammilare. The ratio of DA to 5-HT deamination varied in the different regions, being in favor of DA in the striatum. With kynuramine as the substrate IC50 values of a number of inhibitors indicated that l-deprenyl was far more potent an inhibitor of human brain MAO than clorgyline or harmaline. N-Desmethylpropargylindane hydrochloride (AGN 1135) was also shown to have MAO-B inhibitory selectivity similar to that of l-deprenyl. Brains obtained at autopsy from l-deprenyl-treated Parkinsonian patients showed that, whereas MAO-B was fully inhibited by the therapeutic doses of l-deprenyl, substantial MAO-A activity was still evident. These results are matched by the significant increases of DA noted in caudate nucleus, globus pallidus, putamen, and substantia nigra and the unaltered 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) in the same regions. These data indicate that the therapeutic actions of l-deprenyl may lie in its selective inhibition of MAO-B resulting in increased brain levels of DA formed from L-dihydroxyphenylacetic acid (L-DOPA).     

大白鼠下丘、桥脑B型单胺氧化酶(MAO-B)的研究

脊椎动物听觉皮层下该团的功能性研究被揭示时尚很有限,(?)报导用神经生化的方法,研究了大白鼠中枢神经系统中,与听觉有关的下丘,桥脑比全脑MAO-B monoamme oxidase E C 1.23.4的活性,及在药物作用下的变化,以探讨单胺类神经递质在支因下听觉中枢作用的可能性,结果表明:1.与听觉有关的下丘,桥脑MAO-B活性明显高于全脑的平均水平.2.药物制首乌对下丘、桥脑及全脑的MAO-B活性均有显著的抑制作用,结果提示:下丘、桥脑MAO-B对单胺类神经递质的氧化脱氨作用高于全脑的平均水平,单胺类神经递质有可能作为皮层下听觉中枢的神经递质参与听觉的活动.     

Inhibition of Monoamine Oxidase by Phenyl Azides

We had previously shown that 4-fluoro-3-nitrophenyl azide (FNPA) is a competitive inhibitor of both types of monoamine oxidase (MAO) in the dark, but it is a preferential photoaffinity label for only the type B MAO (MAO-B). Recently we synthesized a number of arylazido compounds with structures related to FNPA and determined the effects of these compounds on the two types of MAO in rat brain cortex. We found that the fluoro group of FNPA was not required for the inhibition of MAO activities because neither the presence nor the position of the fluoro group affected its inhibition of MAO. On the other hand, both the nitro and the azido groups of FNPA were shown to be important for FNPA inactivation of two types of MAO. The inhibitory potency was significantly lower for compounds without either group. Furthermore, we found that all nitrophenyl azide isomers except 2-nitrophenyl azide were photodependent inhibitors of MAO-B. Under the same experimental conditions none of the compounds photoinactivated MAO-A. On the basis of these findings, mechanisms for FNPA inhibition of the two types of MAO are discussed.     

Targeting imidazoline site on monoamine oxidase B through molecular docking simulations

Monoamine oxidase (MAO) is an enzyme of major importance in neurochemistry, because it catalyzes the inactivation pathway for the catecholamine neurotransmitters, noradrenaline, adrenaline and dopamine. In the last decade it was demonstrated that imidazoline derivatives were able to inhibit MAO activity. Furthermore, crystallographic studies identified the imidazoline-binding domain on monoamine oxidase B (MAO-B), which opens the possibility of molecular docking studies focused on this binding site. The goal of the present study is to identify new potential inhibitors for MAO-B. In addition, we are also interested in establishing a fast and reliable computation methodology to pave the way for future molecular docking simulations focused on the imidazoline-binding site of this enzyme. We used the program 'molegro virtual docker' (MVD) in all simulations described here. All results indicate that simplex evolution algorithm is able to succesfully simulate the protein-ligand interactions for MAO-B. In addition, a scoring function implemented in the program MVD presents high correlation coefficient with experimental activity of MAO-B inhibitors. Taken together, our results identified a new family of potential MAO-B inhibitors and mapped important residues for intermolecular interactions between this enzyme and ligands.     

Attenuation of MPTP-induced dopaminergic neurotoxicity by TV3326, a cholinesterase-monoamine oxidase inhibitor

(R)-[(N-propargyl-(3R) aminoindan-5-yl) ethyl methyl carbamate] (TV3326) is a novel cholinesterase and brain-selective monoamine oxidase (MAO)-A/-B inhibitor. It was developed for the treatment of dementia co-morbid with extra pyramidal disorders (parkinsonism), and depression. On chronic treatment in mice it attenuated striatal dopamine depletion induced by MPTP and prevented the reduction in striatal tyrosine hydroxylase activity, like selective B and non-selective MAO inhibitors. TV3326 preferentially inhibits MAO-B in the striatum and hippocampus, and the degree of MAO-B inhibition correlates with the prevention of MPTP-induced dopamine depletion. Complete inhibition of MAO-B is not necessary for full protection from MPTP neurotoxicity. Unlike that seen after treatment with other MAO-A and -B inhibitors, recovery of striatal and hippocampal MAO-A and -B activities from inhibition by TV3326 did not show first-order kinetics. This has been attributed to the generation of a number of metabolites by TV3326 that cause differential inhibition of these enzymes. Inhibition of brain MAO-A and -B by TV3326 resulted in significant elevations of dopamine, noradrenaline and serotonin in the striatum and hippocampus. This may explain its antidepressant-like activity, resembling that of moclobemide in the forced-swim test in rats.     

Modification of substrate-inhibitor affinities of human platelet monoamine oxidase B in vitro

The rate of benzylamine utilization by monoamine oxidase (MAO)-B from human blood platelets was 2-4 times higher than that for octopamine. Both activities were inhibited 100% by 10(-7) M deprenyl (a specific MAO-B inhibitor) and were not affected by clorgyline (a specific MAO-A inhibitor) or by polyclonal antibodies to MAO-A. The preincubation of platelet MAO-B with purified MAO-A from mitochondrial membranes of human placenta resulted in appearance of excess octopamine activity. This additional activity was not precipitated by antibodies to MAO-A or inhibited by deprenyl but was inhibited by clorgyline. Incubation of the MAO-A preparation from placenta at 45 degrees C for 15 min before its preincubation with MAO-B caused 50% loss of both activities. Protease inhibitors had no effect on the modification of MAO. These data indicate that MAO-A or a factor tightly bound to it can modify MAO-B yielding a form of the enzyme with both MAO-A and MAO-B substrate and inhibitor affinities and MAO-B immunospecificity.     

Type A monoamine oxidase is the target of an endogenous dopaminergic neurotoxin, N-methyl(R)salsolinol, leading to apoptosis in SH-SY5Y cells

Mitochondrial monoamine oxidase (MAO) has been considered to be involved in neuronal degeneration either by increased oxidative stress or protection with the inhibitors of type B MAO (MAO-B). In this paper, the role of type A MAO (MAO-A) in apoptosis was studied using human neuroblastoma SH-SY5Y cells, where only MAO-A is expressed. An endogenous dopaminergic neurotoxin, N-methyl(R)salsolinol, an MAO-A inhibitor, reduced membrane potential, DeltaPsim, in isolated mitochondria, and induced apoptosis in the cells, which 5-hydroxytryptamine, an MAO-A substrate, prevented. In contrast, beta-phenylethylamine, an MAO-B substrate, did not suppress the DeltaPsim decline by N-methyl(R)salsolinol. The binding of N-methyl(R)salsolinol to mitochondria was inhibited by clorgyline, a MOA-A inhibitor, but not by (-)deprenyl, an MAO-B inhibitor. RNA interference targeting MAO-A significantly reduced the binding of N-methyl(R)salsolinol with simultaneous reduction in the MAO activity. To examine the intervention of MAO-B in the apoptotic process, human MAO-B was transfected to SH-SY5Y cells, but the sensitivity to N-methyl(R)salsolinol was not affected, even although the activity and protein of MAO increased markedly. These results demonstrate a novel function of MAO-A in the binding of neurotoxins and the induction of apoptosis, which may account for neuronal cell death in neurodegenerative disorders, including Parkinson's disease.     

Inhibitory effects of cis- and trans-resveratrol on noradrenaline and 5-hydroxytryptamine uptake and on monoamine oxidase activity

This study investigated for the first time the potential effects of cis- and trans-resveratrol (c-RESV and t-RESV) on noradrenaline (NA) and 5-hydroxytryptamine (5-HT) uptake by synaptosomes from rat brain, on 5-HT uptake by human platelets, and on monoamine oxidase (MAO) isoform activity. Both c-RESV and t-RESV (5-200 microM) concentration-dependently inhibited the uptake of [3H]NA and [3H]5-HT by synaptosomes from rat brain and the uptake of [3H]5-HT by human platelets. In both experimental models, t-RESV was slightly more efficient than c-RESV. Furthermore, in synaptosomes from rat brain, the RESV isomers were less selective against [3H]5-HT uptake than the reference drug fluoxetine (0.1-30 microM). On the other hand, both c-RESV and t-RESV (5-200 microM) concentration-dependently inhibited the enzymatic activity of commercial (human recombinant) MAO isoform (MAO-A and MAO-B) activity, c-RESV being slightly less effective than t-RESV. In addition, both RESV isomers were slight but significantly more selective against MAO-A than against MAO-B. Since the principal groups of drugs used in the treatment of depressive disorders are NA/5-HT uptake or MAO inhibitors, under the assumption that the RESV isomers exhibit a similar behaviour in humans in vivo, our results suggest that these natural polyphenols may be of value as structural templates for the design and development of new antidepressant drugs with two important biochemical activities combined in the same chemical structure: NA/5-HT uptake and MAO inhibitory activity.     

The monoamine oxidase inhibition properties of C6-mono- and N3/C6-disubstituted derivatives of 4(3H)-quinazolinone

Parkinson’s disease is characterised by the death of the nigrostriatal neurons and depletion of striatal dopamine. The standard symptomatic therapy consists of dopamine replacement with l-dopa, the metabolic precursor of dopamine, which represents the most effective treatment. Since monoamine oxidase (MAO) B is a key dopamine metabolising enzyme in the brain, MAO-B inhibitors are often used as adjuvants to l-dopa. In addition to the symptomatic benefits offered by MAO-B inhibitors, these drugs may also possess neuroprotective properties and possibly delay the progression of Parkinson’s disease. Based on the therapeutic use of MAO-B inhibitors, the present study evaluates a series of mono- and disubstituted derivatives of 4(3H)-quinazolinone as potential inhibitors of recombinant human MAO-A and MAO-B. Twelve C6-monosubstituted and nine N3/C6-disubstituted 4(3H)-quinazolinone derivatives were synthesised, which led to the discovery of novel quinazolinone derivatives with micromolar and submicromolar activities as inhibitors of MAO-B. The most potent mono- and disubstituted derivatives exhibited IC₅₀ values of 6.35 μM (7f) and 0.685 μM (8b), respectively. This study identifies suitable substitution patterns for the design of 4(3H)-quinazolinone derivatives as MAO-B inhibitors.     

A new therapeutic approach in Alzheimer disease: some novel pyrazole derivatives as dual MAO-B inhibitors and antiinflammatory analgesics

The increasing life expectancy in our population makes Alzheimer's disease (AD) a growing public health problem. There is a great need to find a way to prevent and delay the disease. It was shown that monoamine oxidase-B (MAO-B) inhibitors and antiinflammatory agents might be effective in treating AD. Therefore, a novel series of 1-thiocarbamoyl-3-substituted phenyl-5-(2-pyrrolyl)-4,5-dihydro-(1H)-pyrazole derivatives as promising MAO-B inhibitors was synthesized and investigated for the ability to inhibit selectively the activity of the A and B isoforms of monoamine oxidase (MAO). Most of the synthesized compounds showed high activity against both the MAO-A (compounds 3e-3h) and the MAO-B (compounds 3i-3l) isoforms. All the synthesized compounds were also tested for their in vivo antiinflammatory activity by two different bioassays namely, carrageenan-induced oedema and acetic acid-induced increase in capillary permeability in mice. In addition, analgesic and ulcerogenic activities were determined. The combined antiinflammatory data from in vivo animal models showed that compound 3k exhibited anti-inflammatory activity comparable to that of indomethacin with no ulcerogenic effects. Since compound 3k exhibits both antiinflammatory-analgesic activity and MAO-B inhibition, it needs further detailed studies.     

Promoting effect and recovery activity from physical stress of the fruit of Morus alba

We examined the effects of the fruit of M. alba extracts on the changes of the monoamine oxidase (MAO) activities during and after the physical exercise in rat. Each activity was measured by used serotonin(5-HT) and benzylamine as substrate. Lactate dehydrogenase(LDH) activity and the concentrations of lactate in blood which were clinical indexes of physical exercise were also determined to compare with the relation of MAO activities. Those activities during and after the physical exercise have different tendency in each other enzyme. MAO-A activity was sharply decreased with stress by physical activities compared to the normal group, whereas MAO-B activity was increased for 60 minutes after exercise. All of these indexes were recovered to normal state by oral administration of extract of M. alba. These results of this study suggested M. alba may modulate the MAO activities during exercise and promote the capability of physical activities and show anti-stress effect. In general, MAO inhibitors have been used drugs for the purpose of treatment Parkinson's disease, dementia, depression. These results can apply to produce the health and functional foods that have modulating effects for these diseases.     

Different effects of monoamine oxidase inhibition on MPTP depletion of heart and brain catecholamines in mice

Pargyline, an inhibitor of monoamine oxidase type B (MAO-B), did not prevent the depletion of heart norepinephrine 24 hr after a single dose of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) in mice. In mice killed 24 hr after the last of 4 daily doses of MPTP, the depletion of dopamine in the striatum and of norepinephrine in the frontal cortex was completely prevented by pargyline, but the depletion of heart norepinephrine was not prevented. These results with pargyline are the same as results obtained earlier with deprenyl, another selective inhibitor of MAO-B. The doses of pargyline and of deprenyl that were used resulted in almost complete inhibition of MAO-B activity (phenylethylamine as substrate) in brain, heart and liver of mice. Deprenyl did not inhibit MAO-A activity (serotonin as substrate) in brain, but pargyline caused some inhibition of MAO-A in brain. In heart and liver, serotonin was oxidized only at about 1/10 the rate of phenylethylamine oxidation, suggesting that MAO-B predominates in these tissues. Both pargyline and deprenyl caused some inhibition of serotonin deamination in heart and liver, suggesting that the oxidation may have been due partly to MAO-B. Experiments with selective MAO inhibitors in vitro showed that only about 20% of the oxidation of serotonin was occurring via MAO-B in heart and liver. The in vitro oxidation of MPTP by MAO in mouse brain, heart and liver was almost completely inhibited by pretreatment with either pargyline or deprenyl. Neither pargyline nor deprenyl had any significant effect on the concentrations of MPTP in brain or heart one-half hr after injection of MPTP into mice. The concentrations of the metabolite, MPP+ (1-methyl-4-phenyl-pyridinium), were markedly reduced in brain and in heart by pretreatment with either pargyline or deprenyl. The data suggest that MPP+ formation, which is necessary for the depletion of brain catecholamines after MPTP injection, may not be necessary for depletion of norepinephrine in heart. Since the oxidation of MPTP in vitro was inhibited more by pargyline or deprenyl pretreatment than was the appearance of MPP+ in vivo, the possibility exists that some MPP+ formation might occur by an enzyme other than MAO.     

Inhibition of Type A Monoamine Oxidase by Methylquinolines and Structurally Related Compounds

Abstract: A series of methylquinolines (MQ) were found to inhibit markedly type A monoamine oxidase (MAO) in human brain synaptosomal mitochondria. 4-MQ and 6-MQ inhibited type A MAO (MAO-A) competitively and 7- and 8-MQ inhibited MAO-A noncompetitively. Among these four isomers of MQ, 6-MQ was the most potent inhibitor; the K _i value toward MAO-A was 23.4 ± 1.8 μ M , which was smaller than the K _m value toward kynuramine, ± amine substrate, 46.2 ± 2.8 μ M . On the other hand, MQ were very weak inhibitors of type B MAO (MAO-B) and 8-MQ did not inhibit MAO-B in brain synaptosomal mitochondria. The inhibition of MAO-A proved to be reversible; by dialysis the inhibition of MQ was completely reversible. The affinity of these isomers of MQ toward MAO-A or -B was confirmed further with human liver mitochondria as sources of MAO-A and -B and with human placental mitochondria and rat pheochromocytoma PC12h cell line as sources of MAO-A. The relationship of the chemical structure of structurally related quinoline and isoquinoline derivatives to inhibition of the activity of type A or B MAO was examined.     

Inhibition of monoamine oxidase by 8-phenoxymethylcaffeine derivatives

A recent study has reported that a series of 8-benzyloxycaffeines are potent and reversible inhibitors of both human monoamine oxidase (MAO) isoforms, MAO-A and -B. In an attempt to discover additional caffeine derivatives with potent MAO inhibitory activities, and to contribute to the known structure-activity relationships of MAO inhibition by caffeine derived compounds, the present study investigates the MAO inhibitory potencies of series of 8-phenoxymethylcaffeine and 8-[(phenylsulfanyl)methyl]caffeine derivatives. The results document that the 8-phenoxymethylcaffeine derivatives act as potent reversible inhibitors of MAO-B, with IC(50) values ranging from 0.148 to 5.78 μM. In contrast, the 8-[(phenylsulfanyl)methyl]caffeine derivatives were found to be weak inhibitors of MAO-B, with IC(50) values ranging from 4.05 to 124 μM. Neither the 8-phenoxymethylcaffeine nor the 8-[(phenylsulfanyl)methyl]caffeine derivatives exhibited high binding affinities for MAO-A. While less potent than the 8-benzyloxycaffeines as MAO-B inhibitors, this study concludes that 8-phenoxymethylcaffeines may act as useful leads for the design of MAO-B selective inhibitors. Such compounds may find application in the therapy of neurodegenerative disorders such as Parkinson's disease. Using molecular docking experiments, this study also proposes possible binding orientations of selected caffeine derivatives in the active sites of MAO-A and -B.     

Neurochemical and Neuroprotective Effects of Some Aliphatic Propargylamines: New Selective Nonamphetamine-Like Monoamine Oxidase B Inhibitors

Abstract: Aliphatic N -propargylamines have recently been discovered to be highly potent, selective, and irreversible monoamine oxidase B (MAO-B) inhibitors. N -Methyl- N -(2-pentyl)propargylamine (M-2-PP) and N -methyl- N -(2-hexyl) propargylamine (2-HxMP), for example, are approximately fivefold more potent than I -deprenyl at inhibiting mouse brain MAO-B activity following oral administration. These inhibitors are nonaromatic compounds and are chemically quite different from other known MAO-B inhibitors. Some of their neurochemical and neuroprotective properties have been evaluated and compared with those of I -deprenyl. We have confirmed that these new inhibitors selectively inhibit MAO-B activity both in vitro and in vivo. 2-Phenylethylamine levels were substantially increased following administration of M-2-PP, but the levels of dopamine, 3,4-dihydroxyphenylacetic acid, homovanillic acid, 5-hydroxytryptamine, and 5-hydroxyindoleacetic acid were not affected except at high, nonselective doses. Chronic oral administration of I -deprenyl and M-2-PP causes selective inhibition of MAO-B activity and increases dopamine levels in mouse caudate. M-2-PP, like I -deprenyl, has been shown to be potent in protecting against MPTP-induced damage in the mouse. N -(2-Chloroethyl)- N -ethyl-2-bromobenzylamine (DSP-4), a noradrenaline neurotoxin, is not an MAO substrate. Its noradrenaline-depleting effects were substantially mitigated by I -deprenyl as well as by M-2-PP and 2-HxMP in the mouse hippocampus. Administration of 2-phenylethylamine, however, failed to reverse the effect of DSP-4. The neuroprotective effect of M-2-PP and 2-HxMP is apparently unrelated to the uptake of DSP-4.