In vivo comparison of the effects of inhibition of MAO-A versus MAO-B on striatal L-DOPA and dopamine metabolism

Enzyme Inhibition Assays for Monoamine Oxidase

Monoamine oxidase (MAO) catalyzes the oxidative deamination of monoamines with two isoforms, namely, MAO-A and MAO-B, in mitochondrial outer membranes. These two types of MAO-A and MAO-B participate in changes in levels of neurotransmitter such as serotonin (5-hydroxytryptamine) and dopamine. Selective MAO-A inhibitors have been targeted for anti-depression treatment, while selective MAO-B inhibitors are targets of therapeutic agents for Alzheimer's disease and Parkinson's disease. For this reason, study on the development of MAO inhibitors has recently become important. Here, we describe methods of MAO activity assay, especially continuous spectrophotometric methods, which give relatively high accuracy. MAO-A and MAO-B can be assayed using kynuramine and benzylamine as substrates, respectively, at 316 nm and 250 nm, respectively, to measure their respective products, 4-hydroxyquinoline and benzaldehyde. Inhibition degree and pattern can be analyzed by using the Lineweaver-Burk and secondary plots in the presence of inhibitor, and reversibility of inhibitor can be determined by using the dialysis method.

Revisiting the Role of Astrocytic MAOB in Parkinson's Disease

Monoamine oxidase-B (MAOB) has been believed to mediate the degradation of monoamine neurotransmitters such as dopamine. However, this traditional belief has been challenged by demonstrating that it is not MAOB but MAOA which mediates dopamine degradation. Instead, MAOB mediates the aberrant synthesis of GABA and hydrogen peroxide (H₂O₂) in reactive astrocytes of Parkinson’s disease (PD). Astrocytic GABA tonically suppresses the dopaminergic neuronal activity, whereas H₂O₂ aggravates astrocytic reactivity and dopaminergic neuronal death. Recently discovered reversible MAOB inhibitors reduce reactive astrogliosis and restore dopaminergic neuronal activity to alleviate PD symptoms in rodents. In this perspective, we redefine the role of MAOB for the aberrant suppression and deterioration of dopaminergic neurons through excessive GABA and H₂O₂ synthesis of reactive astrocytes in PD.

Redefining differential roles of MAO-A in dopamine degradation and MAO-B in tonic GABA synthesis

Monoamine oxidase (MAO) is believed to mediate the degradation of monoamine neurotransmitters, including dopamine, in the brain. Between the two types of MAO, MAO-B has been believed to be involved in dopamine degradation, which supports the idea that the therapeutic efficacy of MAO-B inhibitors in Parkinson's disease can be attributed to an increase in extracellular dopamine concentration. However, this belief has been controversial. Here, by utilizing in vivo phasic and basal electrochemical monitoring of extracellular dopamine with fast-scan cyclic voltammetry and multiple-cyclic square wave voltammetry and ex vivo fluorescence imaging of dopamine with GRABDA2m, we demonstrate that MAO-A, but not MAO-B, mainly contributes to striatal dopamine degradation. In contrast, our whole-cell patch-clamp results demonstrated that MAO-B, but not MAO-A, was responsible for astrocytic GABA-mediated tonic inhibitory currents in the rat striatum. We conclude that, in contrast to the traditional belief, MAO-A and MAO-B have profoundly different roles: MAO-A regulates dopamine levels, whereas MAO-B controls tonic GABA levels.

Interactions between monoamine oxidase A rs1137070 and smoking on brain structure and function in male smokers

The monoamine oxidase A (MAOA) enzyme metabolizes monoamine neurotransmitters such as dopamine, serotonin and norepinephrine, and its genetic polymorphism (rs1137070) influences its activity level and is associated with smoking behaviors. However, the underlying neural mechanisms of the gene × environment interactions remain largely unknown. In this study, we aimed to explore the interactive effects of the rs1137070 and cigarette smoking on gray matter volume (GMV) and functional connectivity strength (FCS). A total of 81 smokers and 42 nonsmokers were enrolled in the present study. Voxel-based morphometry analysis showed a significant rs1137070 genotype × smoking effect on the GMV of the left orbitofrontal cortex (OFC), such that individuals with risk allele had greater GMV among nonsmokers but not smokers. Meanwhile, rs1137070 variant and nicotine dependence interactively altered the FCS of the right hippocampus, the left inferior parietal lobule (IPL), the left dorsolateral prefrontal cortex and bilateral OFC. In addition, the FCS in the left IPL was correlated with smoking initiation and smoking years in smokers with the risk allele. These findings suggest that MAOA rs1137070 contributes to the susceptibility to nicotine dependence through its influence on brain circuits involved in reward and attention, and interacts with smoking in the progression.

Comparative Analysis of the Neurochemical Profile and MAO Inhibition Properties of N-(Furan-2-ylmethyl)-N-methylprop-2-yn-1-amine

The regulation of brain monoamine levels is paramount for cognitive functions, and the monoamine oxidase (MAO A and B) enzymes play a central role in these processes. The aim of this study was to evaluate whether the procognitive properties exerted by propargylamine N-(furan-2-ylmethyl)-N-methylprop-2-yn-1-amine (F2MPA) are related to changes in monoamine content via MAO inhibition. In vivo microdialysis and ex vivo amine metabolite measurement demonstrated region-specific alterations in monoamine metabolism that differ from both of the classic MAO A and MAO B inhibitors, clorgyline and l-deprenyl, respectively. Although all the inhibitors (1 and 4 mg/kg) increased cortical serotonin tissue content, only F2MPA increased the levels of cortical noradrenaline. In the striatum, clorgyline (1 mg/kg), but not F2MPA (1 mg/kg), reduced extracellular levels of dopamine metabolites at rest or stimulated by the intrastriatal application of the MAO substrate 3-methoxytyramine. In vitro, F2MPA exhibited a low affinity toward MAO B and MAO A. Nonetheless, it modified the B form of MAO, forming a flavin adduct structurally similar to that with deprenyl. F2MPA was rapidly metabolized in the presence of rat but not human microsomes, producing a hydroxylated derivative. In conclusion, the effect of F2MPA on cognition may arise from monoaminergic changes in the cortex, but the role of MAO in this process is likely to be negligible, consistent with the poor affinity of F2MPA for MAO.

Restricted diffusion of dopamine in the rat dorsal striatum

Recent evidence has shown that the dorsal striatum of the rat is arranged as a patchwork of domains that exhibit distinct dopamine kinetics and concentrations. This raises the pressing question of how these distinct domains are maintained, especially if dopamine is able to diffuse through the extracellular space. Diffusion between the domains would eliminate the concentration differences and, thereby, the domains themselves. The present study is a closer examination of dopamine's ability to diffuse in the extracellular space. We used voltammetry to record dopamine overflow in dorsal striatum while stimulating the medial forebrain bundle over a range of stimulus currents and frequencies. We also examined the effects of drugs that modulated the dopamine release (raclopride and quinpirole) and uptake (nomifensine). Examining the details of the temporal features of the evoked profiles reveals no clear evidence for long-distance diffusion of dopamine between fast and slow domains, even though uptake inhibition by nomifensine clearly prolongs the time that dopamine resides in the extracellular space. Our observations support the conclusion that striatal tissue has the capacity to retain dopamine molecules, thereby limiting its tendency to diffuse through the extracellular space.

Modification of L-DOPA pharmacological activity by MAO inhibitors

Dopamine behaves mainly as a MAO-A substrate in rodent brain, but selective inhibition of MAO-B results in an increased turning activity following L-DOPA administration in hemi-Parkinsonian rodents. Unilateral substantia nigra dopaminergic denervation results in serotonergic hyper-innervation which may increase the contribution of MAO-A in the denervated striatum. Possibly as a result of this, there was no change in striatal MAO-A activity when 95% of dopaminergic innervation was reduced by 6-hydroxydopamine, as assessed by apomorphine-induced turning activity. MAO-B as well as MAO-A may contribute to deamination of dopamine produced from L-DOPA.

PET and SPECT imaging in psychiatric disorders

OBJECTIVES

To review recent findings from positron emission tomography (PET) and single photon emission computed tomography (SPECT) studies that investigate the pathophysiology and treatment of schizophrenia, depression, and dementia.

METHODS

We carried out a review of the literature.

RESULTS

PET and SPECT studies have provided evidence of dopamine system dysregulation in patients with schizophrenia and variable loss of monoamines in patients with depression. Antipsychotic response has been demonstrated to be associated with blockade of dopamine D2 receptors, and antidepressant response has now been linked to blockade of serotonin transporter receptors. PET and SPECT have been extensively evaluated as diagnostic procedures for dementia. Substantial progress has been made in developing radioligands that bind to amyloid deposits in the brain, which should provide new opportunities for early diagnosis and treatment monitoring in Alzheimer's disease.

CONCLUSION

Advances in PET and SPECT imaging have provided new insights into the biology of major psychiatric disorders and their treatment. In the future, we can expect that these imaging techniques will become more central to the management of psychiatric disorders.

Effects of subchronic exposure to styrene on the extracellular and tissue levels of dopamine, serotonin and their metabolites in rat brain

At present, there is controversy over the neurotoxic potential of styrene. Several epidemiological and clinical studies have shown that styrene exposure causes alterations of central nervous system functions in humans. Neurotransmitters have been implicated in the pathogenesis of styrene neurotoxicity in rodents. Several studies carried out on postmortem brain tissue suggest that styrene may alter dopaminergic neurotransmission in rabbit or rat brain. Moreover, in vitro studies suggest that both styrene and styrene oxide inhibit the uptake of dopamine (DA) in purified synaptic vesicles prepared from rat brain striata. To date, biochemical studies on animals have explored global tissue levels of neurotransmitters with sub-acute exposures to styrene. However, extracellular levels of neurotransmitters are more closely related to behaviour than are global tissue levels. The present study determined changes in the extracellular concentrations of DA, serotonin (5-HT) and their acid metabolites, dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindolacetic acid (5-HIAA), in striatal dialysates from freely moving adult male rats after exposure to 750 and 1,000 ppm styrene, 6 h per day, 5 days per week for 4 weeks. We also determined the concentrations of DA, 5-HT and their acid metabolites in striatum, nucleus accumbens and prefrontal cortex obtained postmortem from similarly exposed rats. Exposure to 1,000 ppm of styrene caused a significant decrease in extracellular acid metabolite concentrations. Tissue levels of acid metabolites were also decreased to a lesser extent. The effects were observed 72 h after discontinuing exposure but had vanished 17 days later. There was no change in DA or 5-HT concentrations either in the dialysates or tissues. Exposure to 750 ppm styrene caused no changes in the concentrations of DA, 5-HT and their acid metabolites either in the dialysates or tissues. The possibility that the effect of styrene is mediated by monoamine oxidase (MAO) inhibition is discussed.

Effects of Subchronic Treatment With Selegiline on L-DOPA-Induced Increase in Extracellular Dopamine Level in Rat Striatum

Selegiline is used an adjunct to L-DOPA therapy. We investigated extracellular striatal dopamine (DA) level in awake rats treated with L-DOPA and/or selegiline using a microdialysis method. Rats given 10 mg/kg, i.p. per day selegiline for 7 days were administered with a single dose of 100 mg/kg, i.p. L-DOPA 0 (3 h), 1, 3, 7, 14, 21, or 28 days after the last selegiline treatment. Carbidopa was administered 0.5 h before L-DOPA administration. The significant increase in basal DA level before L-DOPA treatment persisted until 1 day after the last selegiline treatment, and the significant decrease in basal DOPAC level persisted for more than 28 days. Thus, selegiline affected DA catabolism for more than 28 days. Total monoamine oxidase (MAO) and MAO-B activities at day 0 decreased by 22% and 5.7%, respectively. The significant enhancement of L-DOPA-induced increase in DA level was observed until 3 days after the last selegiline treatment. Next, the effects of reducing L-DOPA dose by 25% were examined 3 h after the last selegiline treatment. A dose-dependent decrease in DA level was observed, indicating that DA level in selegiline-treated rats can be controlled by L-DOPA dose.

Effect of monoamine oxidase inhibition on amphetamine-evoked changes in dopamine receptor availability in the living pig: A dual tracer PET study with [11C]harmine and [11C]raclopride

The activity of both isozymes of monoamine oxidase (MAO) is reduced by 50% in the brain of human smokers. We hypothesized that this is not an epiphenomenon, but should bring about potentiation of the action of psychostimulant drugs. To test this hypothesis, we carried out serial positron emission tomography (PET) studies in Göttingen miniature pigs to measure the binding of the MAO-A ligand [11C]harmine and to measure the changes in [11C]raclopride binding evoked by a low dose of amphetamine (0.7 mg/kg as free base, i.v.), first in a baseline condition, and, one month later, after acute treatment with pargyline (2 x 3 mg/kg as free base, i.m.). In the baseline, the distribution volume of [11C]harmine relative to the arterial input (V(d), ml g(-1)) ranged from 74 ml g(-1) in cerebellum to 139 ml g(-1) in the medial hypothalamus. Pargyline treatment reduced the magnitude of V(d) globally to 34-54 ml g(-1). Nearly complete displacement of [11C]harmine binding was detected in neocortex and striatum, but there was evidence for pargyline-resistant binding in the pituitary gland and diencephalon. In the baseline condition, the low dose of amphetamine evoked a 14% decline in the binding potential (BP) (pB) of [11C]raclopride in striatum (P = 0.026). After pargyline treatment, the amphetamine effect was of similar magnitude (-11%), although not statistically significant (P = 0.054). However, the second amphetamine challenge evoked a 24% reduction in [11C]raclopride pB relative to the original baseline condition (P = 0.018). Present results do not strongly support our hypothesis that MAO inhibition should potentiate the amphetamine-evoked dopamine release as measured in the [11C]raclopride competition paradigm.

Effects of isatin on rotational behavior and DA levels in caudate putamen in Parkinsonian rats

Isatin was a potent endogenous monoamine oxidase (MAO) inhibitor that is more active against MAO-B than MAO-A. The acute effects of isatin on apomorphine (APO)-induced rotations were evaluated in Parkinsonian rats induced by 6-hydroxydopamine (6-OHDA) lesion. Furthermore, the effects of isatin on DA release in caudate putamen (CPu) of model and normal rats were monitored using fast cyclic voltammetry (FCV). The contents of monoamine transmitters and their metabolites in CPu of model and normal rats were also analyzed by high performance liquid chromatography with electrochemical detection after administration of isatin. Here we show that isatin (100 mg/kg, i.p.) apparently inhibited APO-induced rotations of Parkinsonian rats to 39.1+/-3.7% of the control (n=12), while it had no apparent effects on electrical stimuli-induced DA release either in normal rats or in model rats. In addition, the content of 5-hydroxytryptamine but not DA was increased in both normal rats and model rats after isatin (100 mg/kg, i.p.) was administered (P<0.01, n=6). The content of 5-hydroxyindole acetic acid was not changed. These results suggest that isatin cannot increase DA levels in rat CPu. Therefore, the effects of isatin on APO-induced rotations of our Parkinsonian rats could not attribute to its inhibition of DA catabolism as a MAO inhibitor.

Selective inhibition of MAO-B through chronic low-dose (R)-deprenyl treatment in C57BL/6 mice has no effect on basal neostriatal dopamine levels

C. Thiffault, L. Lamarre-Théroux, R. Quirion, and J. Poirier (1997, Mol. Brain Res. 44: 238-244) recently reported that chronic treatment of young (12 week old) C57BL/6 mice with (R)-deprenyl, a mechanism-based inactivator of monoamine oxidase B (MAO-B), leads to a more than fourfold increase in neostriatal dopamine levels. Such an effect could complicate the interpretation of results obtained from mechanistic studies designed to evaluate the putative neuroprotective effects of (R)-deprenyl in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned mice. In contrast to the results of Thiffault et al., we have found that neostriatal dopamine levels in mature (32 week old) C57BL/6 mice were unaltered by chronic (R)-deprenyl treatment even though brain monoamine oxidase B activity was reduced by more than 80%. Neostriatal dopamine levels also were unaltered in both young and mature mice when the (R)-deprenyl treatment period was doubled compared to that reported by Thiffault et al. Consequently, studies on the putative neuroprotective properties of (R)-deprenyl in MPTP-lesioned mice are unlikely to be complicated by the possibility that inhibition of MAO-B alone will lead to an increase in neostriatal dopamine levels.

Pharmacological analysis of extracellular dopamine and metabolites in the striatum of conscious as/agu rats, mutants with locomotor disorder

The as/agu rat is a spontaneously occurring mutation which exhibits locomotor abnormalities, reduced tyrosine hydroxylase levels in the substantia nigra and lower extracellular levels of dopamine. The animal could represent a model of some human locomotor disorders. High-potassium medium evoked a 460% rise of dopamine levels in control rats but double this in mutants. Amphetamine increased extracellular dopamine by 710% in controls and 1480% in mutants. Clorgyline produced a small increase of dopamine levels in controls but an 1170% increase in mutants. The uptake inhibitor nomifensine increased dopamine levels by 910% in controls but only 270% in mutants. After treatment with benserazide plus L-DOPA, an acute injection of L-DOPA evoked a release of dopamine which was twice as large in the as/agu rats compared with controls. The results show reduced extracellular dopamine in as/agu rats when the locomotor disorder is apparent, but there has been little loss of tyrosine hydroxylase. The responses to drugs are qualitatively different from those obtained using 6-hydroxydopamine.Overall, the effects of compounds affecting aminergic neurons suggest that one possible mechanism for the neuronal abnormality in as/agu rats is a defective regulation of dopamine release from striatal terminals.

Tranylcypromine, but not moclobemide, prolongs the inhibitory action of dopamine on midbrain dopaminergic neurons: an in vitro electrophysiological study

The degradation of dopamine by monoamine oxidase (MAO) enzymes plays an important role in the function of dopamine receptors in the central nervous system. Accordingly, it has already been reported that the blockade of MAO by specific inhibitors prolongs the effects of dopamine on its receptors. By using intracellular electrophysiological recordings, here we report that the irreversible MAO A and B inhibitor tranylcypromine, but not the reversible MAO A inhibitor moclobemide, potentiates DA responses in rat midbrain dopaminergic neurones maintained in vitro. Moclobemide was not effective even when the MAO B enzymes were additionally blocked by the MAOI deprenyl. Thus, our electrophysiological findings confirm that the degradation DA is very important to control the effects of this catecholamine at a cellular level. Furthermore, they demonstrate that tranylcypromine potentiates DA neurotransmission while moclobemide is devoid of dopaminergic action in an in vitro condition. The phenomena reported above support the hypothesis that part of the antidepressant and antiparkinsonian effects of tranylcypromine depend on an action on DA transmission.

Adaptive changes in postsynaptic dopamine receptors despite unaltered dopamine dynamics in mice lacking monoamine oxidase B

Monoamine oxidase (MAO) B is considered a key enzyme in dopamine metabolism. The present studies, conducted in MAO B knockout mice, show that lack of MAO B does not alter extracellular levels of dopamine in striatum. Similarly, the synthesis, storage, uptake, and release of dopamine are also unaltered. However, autoradiography revealed a significant up-regulation of the D2-like dopamine receptors in the striatum of MAO B knockout mice. Mutant mice also exhibit a functional supersensitivity of D1-dopamine receptors in the nucleus accumbens. Thus, the agonist SKF 38,393-induced c-Fos immunoreactivity was significantly increased in knockout mice as compared with wild-type controls. In view of the apparently normal basal dopamine dynamics observed in MAO B knockout mice, we hypothesize that a dopamine-independent mechanism underlies adaptations in dopamine receptor function that occur as a consequence of MAO B depletion. Finally, these findings suggest that chronic administration of MAO inhibitors, as occurs in the treatment of Parkinson's disease and depression, may be associated with an increased responsiveness of CNS neurons to dopamine receptor ligands.

Is phentermine an inhibitor of monoamine oxidase? A critical appraisal

Phentermine produces a spectrum of concentration-dependent biochemical effects. It interacts with NE transporters at 0.1 microM, DA transporters at about 1 microM, 5-HT transporters at 15 microM and MAO-A at about 100 microM. When administered at typical anorectic doses, phentermine primarily interacts with DA and NE transporters and does not produce biochemical or neurochemical effects which would occur if it were inhibiting MAO-A. Some other explanation other than MAO inhibition must be sought to explain how oral phentermine increases platelet 5-HT, since platelet MAO-B does not metabolize platelet 5-HT, and since amphetamine-type drugs are even weaker inhibitors of MAO-B than MAO-A. Clinical studies in humans have shown that amphetamine, which is a more potent inhibitor of MAO-A than phentermine, does not inhibit MAO-A at therapeutic doses. Neither phentermine alone, fluoxetine alone or their combined use have been associated with cardiac valvulopathy, and clinical experience has shown their combined use to be free of significant adverse effects. Viewed collectively, there appears to be no data to support the hypothesis that phentermine inhibits MAO at typical therapeutic doses.

Effect of selegiline on dopamine concentration in the striatum of a primate

Monoamine oxidase (MAO) has two subtypes, A and B, that have different distributions between the rodent and the human. In the striatum, dopamine (DA) of the rat seems to be metabolized by MAO A, and DA of the human is largely deaminated by MAO B. MAO in the striatum of common marmosets is also type B. Using in vivo microdialysis, we investigated the pharmacological activity of selegiline, a selective irreversible inhibitor of MAO B, in the striatum of marmosets. Intraperitoneal co-administration of selegiline (1 mg kg-1, i.p.) with levodopa/carbidopa (10/2.5 mg kg-1, i.p.) did not significantly increase extracellular concentration of DA in the striatum of common marmosets compared with control animals receiving levodopa/carbidopa alone. Daily pretreatment with 0.1 mg kg-1 (i.p.) selegiline for two weeks, however, dramatically increased extracellular concentration of DA to about seven times that of control animals treated with levodopa/carbidopa alone in marmosets. Such an increase in extracellular concentrations of DA could not be observed in a similar study with Wistar rats. This study showed that chronic administration of a small dose of selegiline caused a marked increase in extracellular DA concentration in the striatum of primates, but not in the rodents.

Modification of levodopa responses by deprenyl (selegiline): an electrophysiological and behavioral study in the rat relevant to Parkinson's disease

From using in vitro intracellular recordings from mesencephalic neurons and monoamine-depleted rats, we report that the functions of levodopa in the brain are greatly enhanced and prolonged by high doses of the monoamine oxidase (MAO) inhibitor deprenyl. Dopaminergic neurons were hyperpolarized and inhibited by levodopa application. These effects of levodopa were largely potentiated by pretreatment with nonselective doses of deprenyl. Furthermore, when locomotor activity induced by levodopa was examined on a rodent model of Parkinson's disease, pretreatment of the animals with nonselective doses of deprenyl caused an enhancement of the antiparkinsonian action of levodopa. The great increase in levodopa responses by deprenyl suggests a likely therapeutic use of this dopamine precursor with a higher dosage of the MAO inhibitor, to reduce effectively the daily levodopa requirements in Parkinson's disease patients.