Effect of long-term treatment with selective monoamine oxidase A and B inhibitors on dopamine release from rat striatum in vivo

Effects of rasagiline combined with levodopa and benserazide hydrochloride on motor function and homocysteine and IGF-1 levels in elderly patients with Parkinson's disease

BACKGROUND

During the course of their illness, people with Parkinson's disease may see changes in their insulin-like growth factor (IGF-1) and serum homocysteine (Hcy) indices. In this study, patients with intermediate to severe Parkinson's disease were examined for how Resagiline and levodopa and benserazide hydrochloride affected their motor performance, serum levels of homocysteine (Hcy), and insulin-like growth factor (IGF-1).

METHODS

From June 2020 to December 2021, a total of 100+ cases of Parkinson's patients over 60 years old in the middle and late stages of Parkinson's were seen in the outpatient and inpatient departments of the Third People's Hospital of Chengdu City and had a detailed observation record, and according to the inclusion criteria, the patients who met the criteria were randomly grouped into a clinical observation group and a control group. The subjects in the control group received only levodopa and benserazide hydrochloride treatment, while the observation group was treated with Resagiline in combination with the clinical control group. The total treatment observation period was 1 year for both groups, and the motor function and serum Hcy and IGF-1 indexes of both groups were compared after the end of treatment.

RESULTS

We randomly and evenly grouped 64 patients who met the requirements of the inclusion criteria into a clinical observation group and a control group, each with 32 patients, from among 168 patients over 60 years of age with detailed observation records in the middle and late stages of Parkinson's. After the 1-year observation period, we found that the total effective rate after treatment in the clinical observation group (93.75%) and significantly higher than that in the control group (68.75%) (P < 0.05); after 1 year of treatment, the UPDRS score decreased in both groups, and the observation group was significantly lower than the control group (P < 0.05); after treatment, serum Hcy decreased and IGF-1 increased in both groups, and the observation group was higher than the control group mean values (P < 0.05).

CONCLUSIONS

In patients with Parkinson's disease who are in the middle and late stages of the disease, the administration of Resagiline combined with levodopa and benserazide hydrochloride can significantly lower the body's serum Hcy level, significantly raise IGF-1 levels, and significantly improve motor function in patients with Parkinson's disease. It can also have significant therapeutic effects.

Monoamine Oxidase B (MAO-B): A Target for Rational Drug Development in Schizophrenia Using PET Imaging as an Example

Monoamine oxidase B (MAO-B) is an important high-density enzyme involved in the generation of oxidative stress and central in the catabolism of dopamine, particularly in brain subcortical regions with putative implications in the pathophysiology of schizophrenia. In this chapter, we review postmortem studies, preclinical models, and peripheral and genetic studies implicating MAO-B in psychosis. A literature search in PubMed was conducted and 64 studies were found to be eligible for systematic review. We found that MAO-B could be identified as a potential target in schizophrenia. Evidence comes mostly from studies of peripheral markers, showing reduced platelet MAO-B activity in schizophrenia, together with preclinical results from MAO-B knock-out mice resulting in a hyperdopaminergic state and behavioral disinhibition. However, whether brain MAO-B is altered in vivo in patients with schizophrenia remains unknown. We therefore review methodological studies involving MAO-B positron emission tomography (PET) radioligands used to quantify MAO-B in vivo in the human brain. Given the limitations of currently available treatments for schizophrenia, elucidating whether MAO-B could be used as a target for risk stratification or clinical staging in schizophrenia could allow for a rational search for newer antipsychotics and the development of new treatments.

P2B001 (Extended Release Pramipexole and Rasagiline): A New Treatment Option in Development for Parkinson's Disease

Despite levodopa’s superior efficacy in reducing the motor symptoms of Parkinson’s disease (PD), its risk to induce motor complications requires consideration of the pros and cons of initiating treatment with levodopa-sparing strategies. The current drive toward early levodopa monotherapy is primarily driven by safety and tolerability concerns with dopamine agonists and only mild efficacy of other available approaches. Recently, P2B001, a novel once-daily combination of low-dose, extended-release formulations of pramipexole and rasagiline (0.6 mg and 0.75 mg respectively), has entered clinical development. In this drug evaluation, we review the preclinical and current clinical data for P2B001 and its components. The P2B001 combination has the potential to provide greater efficacy than either pramipexole or rasagiline alone and a better tolerability profile compared to higher dosage dopamine agonist monotherapy, while maintaining the advantage of lower motor complication risk than levodopa.

Parkinson’s disease is the fastest growing neurologic disorder across the globe. Once diagnosed, it is now generally agreed that there is no clinical rationale to postpone symptomatic treatment in people who develop Parkinson’s-related disability. There are three main treatment options available for use in early Parkinson’s disease: levodopa, dopamine agonists and monoamine oxidase type B (MAO-B) inhibitors. Of these, there is a current push toward using levodopa as the main first-line therapy. This is primarily because of the significant safety and tolerability concerns with dopamine agonists and only mild efficacy of MAO-B inhibitors. Recently, P2B001, a novel drug formulation combining once-daily, extended-release, low dosages of the dopamine agonist pramipexole and the MAO-B inhibitor rasagiline (0.6 mg and 0.75 mg respectively), has entered clinical development. In this article, the authors review the preclinical and current clinical data on P2B001 and its components. The P2B001 combination has the potential to provide greater efficacy than either pramipexole or rasagiline alone and a better tolerability profile compared to higher dosage dopamine agonist monotherapy, while maintaining the advantage of lower motor complication risk than levodopa.

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

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

Differential Susceptibilities of Catecholamines to Metabolism by Monoamine Oxidases

The endogenous catecholamines dopamine (DA), norepinephrine (NE), and epinephrine (EPI) play key roles in neurobehavioral, cardiovascular, and metabolic processes; various clinical disorders; and effects of numerous drugs. Steps in intracellular catecholamine synthesis and metabolism were delineated long ago, but there remains a knowledge gap. Catecholamines are metabolized by two isoforms of monoamine oxidase (MAO), MAO-A and MAO-B, and although the anatomic localization of MAO-A and MAO-B and substrate specificities of enzyme inhibitors are well characterized, relative susceptibilities of the endogenous catecholamines to enzymatic oxidation by MAO-A and MAO-B have not been studied systematically. MAOs catalyze the conversion of catecholamines to catecholaldehydes-3,4-dihydroxyphenylacetaldehyde (DOPAL) from DA and 3,4-dihydroxyphenylglycolaldehyde (DOPEGAL) from NE and EPI. In this study we exploited the technical ability to assay DOPAL and DOPEGAL simultaneously with the substrate catecholamines to compare DA, NE, and EPI in their metabolism by MAO-A and MAO-B. For both MAO isoforms, DA was the better substrate compared to NE or EPI, which were metabolized equally. Since catecholaminergic neurons express mainly MAO-A, the finding that MAO-A is more efficient than MAO-B in metabolizing endogenous catecholamines reinforces the view that the predominant route of intraneuronal enzymatic oxidation of catecholamines is via MAO-A. The results have implications for clinical neurochemistry, experimental therapeutics, and computational models of catecholaminergic neurodegeneration. For instance, the greater susceptibility of DA than the other catecholamines to both MAO isoforms can help explain relatively high concentrations of the deaminated DA metabolite 3,4-dihydroxyphenylacetic acid than of the NE metabolite 3,4-dihydroxyphenylglycol in human plasma and urine. SIGNIFICANCE STATEMENT: Endogenous catecholamines are metabolized by monoamine oxidase (MAO)-A and -B, yielding the catecholaldehydes 3,4-dihydroxyphenylacetaldehyde (DOPAL) from dopamine (DA) and 3,4-dihydroxyphenylglycolaldehyde (DOPEGAL) from norepinephrine (NE) and epinephrine (EPI). Based on measurements of DOPAL and DOPEGAL production, DA is a better substrate than NE or EPI for both MAO isoforms, and MAO-A is more efficient than MAO-B in metabolizing DA, NE, and EPI. MAO-A is the main route of intraneuronal metabolism of endogenous catecholamines.

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.

KDS2010, a Newly Developed Reversible MAO-B Inhibitor, as an Effective Therapeutic Candidate for Parkinson's Disease

Monoamine oxidase-B (MAO-B) is a well-established therapeutic target for Parkinson's disease (PD); however, previous clinical studies on currently available irreversible MAO-B inhibitors have yielded disappointing neuroprotective effects. Here, we tested the therapeutic potential of KDS2010, a recently synthesized potent, selective, and reversible MAO-B inhibitor in multiple animal models of PD. We designed and synthesized a series of α-aminoamide derivatives and found that derivative KDS2010 exhibited the highest potency, specificity, reversibility, and bioavailability (> 100%). In addition, KDS2010 demonstrated significant neuroprotective and anti-neuroinflammatory efficacy against nigrostriatal pathway destruction in the mouse MPTP model of parkinsonism. Treatment with KDS2010 also alleviated parkinsonian motor dysfunction in 6-hydroxydopamine-induced and A53T mutant α-synuclein overexpression rat models of PD. Moreover, KDS2010 showed virtually no toxicity or side effects in non-human primates. KDS2010 could be a next-generation therapeutic candidate for PD.

Excessive Astrocytic GABA Causes Cortical Hypometabolism and Impedes Functional Recovery after Subcortical Stroke

Glucose hypometabolism in cortical structures after functional disconnection is frequently reported in patients with white matter diseases such as subcortical stroke. However, the molecular and cellular mechanisms have been poorly elucidated. Here we show, in an animal model of internal capsular infarct, that GABA-synthesizing reactive astrocytes in distant cortical areas cause glucose hypometabolism via tonic inhibition of neighboring neurons. We find that reversal of aberrant astrocytic GABA synthesis, by pharmacological inhibition and astrocyte-specific gene silencing of MAO-B, reverses the reduction in cortical glucose metabolism. Moreover, induction of aberrant astrocytic GABA synthesis by cortical injection of putrescine or adenovirus recapitulates cortical hypometabolism. Furthermore, MAO-B inhibition causes a remarkable recovery from post-stroke motor deficits when combined with a rehabilitation regimen. Collectively, our data indicate that cortical glucose hypometabolism in subcortical stroke is caused by aberrant astrocytic GABA and MAO-B inhibition and that attenuating cortical hypometabolism can be a therapeutic approach in subcortical stroke.

[Pharmacological properties and clinical efficacy of rasagiline mesylate (Azilect®)]

Parkinson's disease is a neurodegenerative disorder that manifests as motor deficits, tremors, rigidity, and postural instability. The most prominent pathological feature of this disease is reduced striatal dopamine concentration due to the loss of nigrodopaminergic neurons. Symptomatic dopamine replacement therapy is the standard management approach for Parkinson's disease. Treatment with monoamine oxidase B (MAO-B) inhibitors also improves Parkinson's disease symptoms by inhibiting the striatal dopamine metabolism and increasing the intracerebral dopamine concentration. Rasagiline is a potent and specific MAO-B inhibitor and is currently approved as an antiparkinsonian drug in more than 50 countries, including the United States and European countries. Clinical trials conducted in Japan to evaluate the efficacy of rasagiline monotherapy in patients with early-stage Parkinson's disease using the Movement Disorder Society-Unified Parkinson's Disease Rating Scale Part II (patient motor experience of daily living) and Part III (clinician motor examination) have demonstrated the antiparkinsonian action of rasagiline. Furthermore, in patients with advanced Parkinson's disease receiving levodopa, concomitant rasagiline administration reduced the duration of "wearing-off". Based on these favorable results, rasagiline mesilate (Azilect^® tablets) was approved for manufacture and sales in Japan in March 2018. Here, we provide a comprehensive overview of the pharmacological properties and clinical effects of rasagiline based on the results of domestic trials, with the aim of increasing the understanding of rasagiline use in Japan.

Possible involvement of PI3K/AKT/mTOR signaling pathway in the protective effect of selegiline (deprenyl) against memory impairment following ischemia reperfusion in rat

Short-term cerebral ischemia led to memory dysfunction. There is a pressing need to introduce effective agents to reduce complications of the ischemia. Involvement of PI3K/AKT/mTOR signaling pathway has been determined in the neuroprotective effect of various agents. Selegiline (deprenyl) possessed neuroprotective properties. In this study global ischemia/reperfusion was established in rats. Selegiline (5 mg/kg for 7 consecutive days) administrated via intraperitoneal route. Possible involvement of PI3K/AKT/mTOR signaling pathway was evaluated using qRT-PCR, immunohistochemistry and histophatologic evaluations in the hippocampus. Spatial memory was evaluated by morris water maze (MWM). Results showed that ischemia impaired the memory and ischemic rats spent more time to find hidden platform in the MWM. Ischemia significantly decreased levels of PI3K, AKT and mTOR in the hippocampus. Histopathologic assessment revealed that the percent of dark neurons significantly increased in the CA1 area of the hippocampus of ischemic rats. Selegiline improved the memory as ischemic rats spent fewer time to find hidden platform in the MWM. Findings showed that selegiline increased the level and expression of PI3K, AKT and mTOR as well as decreased the proportion of dark neurons in the CA1 area of the pyramidal layer of the hippocampus. We concluded that selegiline, partially at least, through increases the expression of PI3K, AKT and mTOR as well as decreases the percent of dark neurons in the hippocampus could improve the memory impairment following the ischemia in rats.

Inhibitors of MAO-B and COMT: their effects on brain dopamine levels and uses in Parkinson's disease

MAO-B and COMT are both enzymes involved in dopamine breakdown and metabolism. Inhibitors of these enzymes are used in the treatment of Parkinson's disease. This review article describes the scientific background to the localization and function of the enzymes, the physiological changes resulting from their inhibition, and the basic and clinical pharmacology of the various inhibitors and their role in treatment of Parkinson's disease.

Behavioral, Biochemical and Molecular Characterization of a Parkinson's Disease Mouse Model Using the Neurotoxin 2'-CH3-MPTP: A Novel Approach

The neurotoxin MPTP has long been used to create a mouse model of Parkinson's disease (PD). Indeed, several MPTP analogues have been developed, including 2'-CH₃-MPTP, which was shown to induce nigrostriatal DA neuronal depletion more potently than MPTP. However, no study on behavioral and molecular alterations in response to 2'-CH₃-MPTP has been carried out so far. In the present work, 2'-CH₃-MPTP was administered to mice (2.5, 5.0 and 10 mg/kg per injection, once a day, 5 days) and histological, biochemical, molecular and behavioral alterations were evaluated. We show that, despite a dose-dependent-like pattern observed for nigrostriatal dopaminergic neuronal death and dopamine depletion, dose-specific alterations in dopamine metabolism and in the expression of dopaminergic neurotransmission-associated genes could be related to specific motor deficits elicited by the different doses tested. Interestingly, 2'-CH₃-MPTP leads to increased DAT and MAO-B transcription, which could explain, respectively, its higher potency and the requirement of higher doses of MAO inhibitors to prevent nigrostriatal neuronal death when compared to MPTP. Also, perturbations in dopamine metabolism as well as possible alterations in dopamine bioavailability in the synaptic cleft were also identified and correlated with strength and ambulation deficits in response to specific doses. Overall, the present work brings new evidence supporting the distinct effects of 2'-CH₃-MPTP when compared to its analogue MPTP. Moreover, our data highlight the utmost importance of a precise experimental design, as different administration regimens and doses yield different biochemical, molecular and behavioral alterations, which can be explored to study specific aspects of PD.

Selegiline induces a wake promoting effect in rats which is related to formation of its active metabolites

The goal of the present work was to characterise the effects of selegiline on the rat sleep pattern. Furthermore, for comparative purposes, the pharmacokinetics of selegiline and its metabolites in brain and plasma were investigated, and microdialysis experiments were performed to examine the resulting effect on dopamine, noradrenaline and serotonin levels. Selegiline (1, 5, 10 and 30mg/kg) was found to dose-dependently increase the time spent awake following acute dosing. The pharmacokinetic assessment of selegiline showed that, following an oral dose of 5mg/kg, low circulating levels of the parent compound were found relative to those of biotransformed l-methamphetamine and l-amphetamine. The time course of selegiline-induced wakefulness was shown to follow the time course of l-methamphetamine and l-amphetamine in brain, suggesting that these metabolites are responsible for the modulation of sleep architecture. Furthermore, selegiline (5mg/kg) caused a significant increase of extracellular levels of DA (250%) and NA (200%), but not of 5-HT, in the rat prefrontal cortex. In summary, an integrated experimental approach was undertaken here to evaluate selegiline's effect on sleep architecture in rats in relation to its pharmacokinetics and changes in monoaminergic neurotransmitter levels in the brain. The effect of selegiline on sleep was likely mediated by an increase of dopamine and noradrenaline levels in the brain caused by the formed metabolites.

Inhibitors of MAO-A and MAO-B in Psychiatry and Neurology

Inhibitors of MAO-A and MAO-B are in clinical use for the treatment of psychiatric and neurological disorders respectively. Elucidation of the molecular structure of the active sites of the enzymes has enabled a precise determination of the way in which substrates and inhibitor molecules are metabolized, or inhibit metabolism of substrates, respectively. Despite the knowledge of the strong antidepressant efficacy of irreversible MAO inhibitors, their clinical use has been limited by their side effect of potentiation of the cardiovascular effects of dietary amines (“cheese effect”). A number of reversible MAO-A inhibitors which are devoid of cheese effect have been described in the literature, but only one, moclobemide, is currently in clinical use. The irreversible inhibitors of MAO-B, selegiline and rasagiline, are used clinically in treatment of Parkinson's disease, and a recently introduced reversible MAO-B inhibitor, safinamide, has also been found efficacious. Modification of the pharmacokinetic characteristics of selegiline by transdermal administration has led to the development of a new drug form for treatment of depression. The clinical potential of MAO inhibitors together with detailed knowledge of the enzyme's binding site structure should lead to future developments with these drugs.

Molecular aspects of monoamine oxidase B

Monoamine oxidases (MAO) influence the monoamine levels in brain by virtue of their role in neurotransmitter breakdown. MAO B is the predominant form in glial cells and in platelets. MAO B structure, function and kinetics are described as a background for the effect of alterations in its activity on behavior. The need to inhibit MAO B to combat decreased brain amines continues to drive the search for new drugs. Reversible and irreversible inhibitors are now designed using data-mining, computational screening, docking and molecular dynamics. Multi-target ligands designed to combat the elevated activity of MAO B in Alzheimer's and Parkinson's Diseases incorporate MAO inhibition (usually irreversible) as well as iron chelation, antioxidant or neuroprotective properties. The main focus of drug design is the catalytic activity of MAO, but the imidazoline I2 site in the entrance cavity of MAO B is also a pharmacological target. Endogenous regulation of MAO B expression is discussed briefly in light of new studies measuring mRNA, protein, or activity in healthy and degenerative samples, including the effect of DNA methylation on the expression. Overall, this review focuses on examples of recent research on the molecular aspects of the expression, activity, and inhibition of MAO B.

Response to Transdermal Selegiline Smoking Cessation Therapy and Markers in the 15q24 Chromosomal Region

INTRODUCTION

Current treatments for smoking cessation have limited efficacy. A potential pharmaceutical treatment for smoking cessation is selegiline, a selective and irreversible monoamine oxidase B inhibitor. A few clinical trials have been carried out using selegiline but the results have been mixed. We sought to determine if genetic markers in cholinergic loci in the 15q24 chromosomal region predict response to smoking cessation therapy with selegiline.

METHODS

We performed an 8-week double-blind, placebo-controlled clinical trial of the selegiline transdermal system in heavy smokers, with follow-up at weeks 25 and 52. Eight single nucleotide polymorphisms (SNPs) in the 15q24 region, which contains the genes for the nicotinic acetylcholine receptor subunits CHRNA5, CHRNA3, and CHRNB4, were investigated for association with treatment response.

RESULTS

The CHRNB4 promoter SNP rs3813567 was associated with both point prevalence abstinence and post-quit craving. Carriers of the minor C allele treated with selegiline showed lower rates of abstinence and higher levels of craving than selegiline-treated non-carriers, indicating that the rs3813567 C allele adversely affects abstinence in selegiline-treated smokers. This effect was not present among placebo-treated smokers. Selegiline-treated smokers with the CHRNA5 rs680244 GG genotype had lower post-quit craving, and unlike placebo-treated GG-carrying smokers, did not experience a post-quit increase in depressive symptoms.

CONCLUSIONS

Variants in genes encoding cholinergic receptors affect abstinence, craving and mood in selegiline-treated smokers. Selegiline primarily affects dopamine levels in the brain, but cholinergic input affects nicotine-induced dopaminergic activity. These markers may have value in identifying those likely to respond to selegiline for smoking cessation.

Modulation of monoamine oxidase (MAO) expression in neuropsychiatric disorders: genetic and environmental factors involved in type A MAO expression

Monoamine oxidase types A and B (MAO-A, MAO-B) regulate the levels of monoamine neurotransmitters in the brain, and their dysfunction may be involved in the pathogenesis and influence the clinical phenotypes of neuropsychiatric disorders. Reversible MAO-A inhibitors, such as moclobemide and befloxatone, are currently employed in the treatment of emotional disorders by inhibiting the enzymatic degradation of dopamine, serotonin and norepinephrine in the central nervous system (CNS). It has been suggested that the irreversible MAO-B inhibitors selegiline and rasagiline exert a neuroprotective effect in Parkinson's and Alzheimer's diseases. This effect, however, is not related to their inhibition of MAO activity; in animal and cellular models, selegiline and rasagiline protect neuronal cells through their anti-apoptotic activity and induction of pro-survival genes. There is increasing evidence that MAO-A activity, but not that of MAO-B, is implicated in the pathophysiology of neurodegenerative disorders, but also in gene induction by MAO-B inhibitors; on the other hand, selegiline and rasagiline increase MAO-A mRNA, protein, and enzyme activity levels. Taken together, these results suggest that each MAO subtype exerts effects that modulate the expression and activity of the other isoenzyme. The roles of MAO-A and -B in the CNS should therefore be re-evaluated with respect to the "type-specificity" of their inhibitors, which may not be unconditional during chronic treatment. Mao-a expression, in particular, may be implicated in pathogenesis and phenotypes in neuropsychiatric disorders. MAO-A expression is modified by mao polymorphisms affecting its transcriptional efficiency, as well as by mutations and polymorphism of parkin, Sirt1, FOXO, microRNA, presenilin-1, and other regulatory proteins. In addition, childhood maltreatment has been shown to have an impact upon adolescent social behavior in children with mao-a polymorphisms of low transcriptional activity. Low MAO-A activity may increase the levels of serotonin and norepinephrine, resulting in disturbed neurotransmitter system development and behavior. This review discusses genetic and environmental factors involved in the regulation of MAO-A expression, in the contexts of neuropsychiatric function and of the regulation of neuronal survival and death.

Glial GABA, synthesized by monoamine oxidase B, mediates tonic inhibition

GABA is the major inhibitory transmitter in the brain and is released not only from a subset of neurons but also from glia. Although neuronal GABA is well known to be synthesized by glutamic acid decarboxylase (GAD), the source of glial GABA is unknown. After estimating the concentration of GABA in Bergmann glia to be around 5-10 mM by immunogold electron microscopy, we demonstrate that GABA production in glia requires MAOB, a key enzyme in the putrescine degradation pathway. In cultured cerebellar glia, both Ca(2+)-induced and tonic GABA release are significantly reduced by both gene silencing of MAOB and the MAOB inhibitor selegiline. In the cerebellum and striatum of adult mice, general gene silencing, knock out of MAOB or selegiline treatment resulted in elimination of tonic GABA currents recorded from granule neurons and medium spiny neurons. Glial-specific rescue of MAOB resulted in complete rescue of tonic GABA currents. Our results identify MAOB as a key synthesizing enzyme of glial GABA, which is released via bestrophin 1 (Best1) channel to mediate tonic inhibition in the brain.

Increased L-DOPA-derived dopamine following selective MAO-A or -B inhibition in rat striatum depleted of dopaminergic and serotonergic innervation

BACKGROUND AND PURPOSE

Selective MAO type B (MAO-B) inhibitors are effective in potentiation of the clinical effect of L-DOPA in Parkinson's disease, but dopamine (DA) is deaminated mainly by MAO type A (MAO-A) in rat brain. We sought to clarify the roles of MAO-A and MAO-B in deamination of DA formed from exogenous L-DOPA in rat striatum depleted of dopaminergic, or both dopaminergic and serotonergic innervations. We also studied the effect of organic cation transporter-3 (OCT-3) inhibition by decinium-22 on extracellular DA levels following L-DOPA.

EXPERIMENTAL APPROACH

Striatal dopaminergic and/or serotonergic neuronal innervations were lesioned by 6-hydroxydopamine or 5,7-dihydroxytryptamine respectively. Microdialysate DA levels after systemic L-DOPA were measured after inhibition of MAO-A or MAO-B by clorgyline or rasagiline respectively. MAO subtype localization in the striatum was determined by immunofluorescence.

KEY RESULTS

Rasagiline increased DA extracellular levels following L-DOPA to a greater extent in double- than in single-lesioned rats (2.8- and 1.8-fold increase, respectively, relative to saline treatment); however, clorgyline elevated DA levels in both models over 10-fold. MAO-A was strongly expressed in medium spiny neurons (MSNs) in intact and lesioned striata, while MAO-B was localized in glia and to a small extent in MSNs. Inhibition of OCT-3 increased DA levels in the double- more than the single-lesion animals.

CONCLUSIONS AND IMPLICATIONS

In striatum devoid of dopaminergic and serotonergic inputs, most deamination of L-DOPA-derived DA is mediated by MAO-A in MSN and a smaller amount by MAO-B in both MSN and glia. OCT-3 plays a significant role in uptake of DA from extracellular space. Inhibitors of OCT-3 are potential future targets for anti-Parkinsonian treatments.

Effects of smoking on D₂/D₃ striatal receptor availability in alcoholics and social drinkers

Studies have reported lower striatal D₂/D₃ receptor availability in both alcoholics and cigarette smokers relative to healthy controls. These substances are commonly co-abused, yet the relationship between comorbid alcohol/tobacco abuse and striatal D₂/D₃ receptor availability has not been examined. We sought to determine the degree to which dual abuse of alcohol and tobacco is associated with lower D₂/D₃ receptor availability. Eighty-one subjects (34 nontreatment-seeking alcoholic smokers [NTS-S], 21 social-drinking smokers [SD-S], and 26 social-drinking non-smokers [SD-NS]) received baseline [(11)C]raclopride scans. D₂/D₃ binding potential (BPND ≡ Bavail/KD) was estimated for ten anatomically defined striatal regions of interest (ROIs). Significant group effects were detected in bilateral pre-commissural dorsal putamen, bilateral pre-commissural dorsal caudate; and bilateral post-commissural dorsal putamen. Post-hoc testing revealed that, regardless of drinking status, smokers had lower D₂/D₃ receptor availability than non-smoking controls. Chronic tobacco smokers have lower striatal D₂/D₃ receptor availability than non-smokers, independent of alcohol use. Additional studies are needed to identify the mechanisms by which chronic tobacco smoking is associated with striatal dopamine receptor availability.

Neuroprotective effects of the MAO-B inhibitor, PF9601N, in an in vivo model of excitotoxicity

BACKGROUND

PF9601N [N-(2-propynyl)-2-(5-benzyloxy-indolyl) methylamine] is an inhibitor of monoamine oxidase B (MAO-B), which has shown to possess neuroprotective properties in several in vitro and in vivo models of Parkinson's disease (PD). As there is evidence that excitotoxicity may be implicated in the pathophysiology of several neurodegenerative diseases, the aim of the present work was to investigate the effects of PF9601N in an acute in vivo model of excitotoxicity induced by the local administration of kainic acid during striatal microdialysis in adult rats.

METHODS

The basal and evoked release of neurotransmitters was monitored by HPLC analysis of microdialysate samples and tissue damage was evaluated histologically "ex vivo."

RESULTS

PF9601N (40 mg/kg, single i.p. administration) reduced the kainate-evoked release of glutamate and aspartate and increased taurine release, but it had no effect on the release of dopamine, DOPAC, and HVA. PF9601N pretreatment also resulted in a significant reduction in the kainate-induced astrocytosis, microgliosis, and apoptosis.

CONCLUSIONS

The results suggest PF9601N to be a good candidate for the treatment of neurodegenerative diseases mediated by excitotoxicity.

Update on the pharmacology of selective inhibitors of MAO-A and MAO-B: focus on modulation of CNS monoamine neurotransmitter release

Inhibitors of monoamine oxidase (MAO) were initially used in medicine following the discovery of their antidepressant action. Subsequently their ability to potentiate the effects of an indirectly-acting sympathomimetic amine such as tyramine was discovered, leading to their limitation in clinical use, except for cases of treatment-resistant depression. More recently, the understanding that: a) potentiation of indirectly-acting sympathomimetic amines is caused by inhibitors of MAO-A but not by inhibitors of MAO-B, and b) that reversible inhibitors of MAO-A cause minimal tyramine potentiation, has led to their re-introduction to clinical use for treatment of depression (reversible MAO-A inhibitors and new dose form MAO-B inhibitor) and treatment of Parkinson's disease (MAO-B inhibitors). The profound neuroprotective properties of propargyl-based inhibitors of MAO-B in preclinical experiments have drawn attention to the possibility of employing these drugs for their neuroprotective effect in neurodegenerative diseases, and have raised the question of the involvement of the MAO-mediated reaction as a source of reactive free radicals. Despite the long-standing history of MAO inhibitors in medicine, the way in which they affect neuronal release of monoamine neurotransmitters is still poorly understood. In recent years, the detailed chemical structure of MAO-B and MAO-A has become available, providing new possibilities for synthesis of mechanism-based inhibitors. This review describes the latest advances in understanding the way in which MAO inhibitors affect the release of the monoamine neurotransmitters dopamine, noradrenaline and serotonin (5-HT) in the CNS, with an accent on the importance of these effects for the clinical actions of the drugs.

Protective effect of rasagiline in aminoglycoside ototoxicity

Sensorineural hearing losses (SNHLs; e.g., ototoxicant- and noise-induced hearing loss or presbycusis) are among the most frequent sensory deficits, but they lack effective drug therapies. The majority of recent therapeutic approaches focused on the trials of antioxidants and reactive oxygen species (ROS) scavengers in SNHLs. The rationale for these studies was the prominent role of disturbed redox homeostasis and the consequent ROS elevation. Although the antioxidant therapies in several animal studies seemed to be promising, clinical trials have failed to fulfill expectations. We investigated the potential of rasagiline, an FDA-approved monomanine oxidase type B inhibitor (MAO-B) inhibitor type anti-parkinsonian drug, as an otoprotectant. We showed a dose-dependent alleviation of the kanamycin-induced threshold shifts measured by auditory brainstem response (ABR) in an ototoxicant aminoglycoside antibiotic-based hearing loss model in mice. This effect proved to be statistically significant at a 6-mg/kg (s.c.) dose. The most prominent effect appeared at 16kHz, which is the hearing sensitivity optimum for mice. The neuroprotective, antiapoptotic and antioxidant effects of rasagiline in animal models, all targeting a specific mechanism of aminoglycoside injury, may explain this otoprotection. The dopaminergic neurotransmission enhancer effect of rasagiline might also contribute to the protection. Dopamine (DA), released from lateral olivocochlear (LOC) fibers, was shown to exert a protective action against excitotoxicity, a pathological factor in the aminoglycoside-induced SNHL. We have shown that rasagiline enhanced the electric stimulation-evoked release of DA from an acute mouse cochlea preparation in a dose-dependent manner. Using inhibitors of voltage-gated Na(+)-, Ca(2+) channels and DA transporters, we revealed that rasagiline potentiated the action potential-evoked release of DA by inhibiting the reuptake. The complex, multifactorial pathomechanism of SNHLs most likely requires drugs acting on multiple targets for effective therapy. Rasagiline, with its multi-target action and favorable adverse effects profile, might be a good candidate for a clinical trial testing the otoprotective indication.

Recent advances in Parkinson’s disease therapy: use of monoamine oxidase inhibitors

Monoamine oxidase inhibitors inhibit dopamine metabolism and are therefore effective in treating Parkinson's disease, a condition associated with progressive striatal dopamine deficiency secondary to degeneration of dopaminergic neurons in the substantia nigra. Selegiline is currently the most widely used monoamine oxidase-B inhibitor for Parkinson's disease, but has a low and variable bioavailability, and is metabolized to L-methamphetamine and L-amphetamine that carry a risk for potential neurotoxicity. There are two new approaches that circumvent these potential disadvantages. First, selegiline orally disintegrating tablets provide a novel delivery form of selegiline, avoiding first pass metabolism by rapid absorption through the oral mucosa, thus leading to significantly lower plasma concentrations of L-metamphetamine and L-amphetamine. Selegiline orally disintegrating tablets prove to be clinically effective and safe in patients with moderately advanced Parkinson's disease. Second, rasagiline is a new monoamine oxidase inhibitor, without known neurotoxic metabolites. In large clinical trials, rasagiline proves effective as monotherapy in early Parkinson's disease, as well as adjunctive therapy to levodopa in advanced disease. Clinical data suggest, in addition, a disease-modifying effect of rasagiline that may correlate with neuroprotective activity of monoamine oxidase-B inhibitors in animal models of Parkinson's disease.

Rasagiline prevents apoptosis induced by PK11195, a ligand of the outer membrane translocator protein (18 kDa), in SH-SY5Y cells through suppression of cytochrome c release from mitochondria

Rasagiline protects neuronal cells from cell death caused by various lines of insults. Its neuroprotective function is due to suppression of mitochondrial apoptosis signaling and induction of neuroprotective genes, including Bcl-2 and neurotrophic factors. Rasagiline inhibits the mitochondrial membrane permeabilization, an initial stage in apoptosis, but the mechanism has been elusive. In this paper, it was investigated how rasagiline regulates mitochondrial death cascade in apoptosis induced in SH-SY5Y cells by PK11195, a ligand of the outer membrane translocator protein of 18 kDa. Rasagiline prevented release of cytochrome c (Cyt-c), and the following caspase 3 activation, ATP depletion and apoptosis, but did not inhibit the mitochondrial membrane potential collapse, in contrast to Bcl-2 overexpression. Rasagiline stabilized the mitochondrial contact site and suppressed Cyt-c release into cytoplasm, which should be the critical point for the regulation of apoptosis. Monoamine oxidase was not associated with anti-apoptotic activity of rasagiline in PK11195-induced apoptosis.

Striatal D(2)/D(3) receptor availability is inversely correlated with cannabis consumption in chronic marijuana users

BACKGROUND

Although the incidence of cannabis abuse/dependence in Americans is rising, the neurobiology of cannabis addiction is not well understood. Imaging studies have demonstrated deficits in striatal D(2)/D(3) receptor availability in several substance-dependent populations. However, this has not been studied in currently using chronic cannabis users.

OBJECTIVE

The purpose of this study was to compare striatal D(2)/D(3) receptor availability between currently using chronic cannabis users and healthy controls.

METHODS

Eighteen right-handed males age 18-34 were studied. Ten subjects were chronic cannabis users; eight were demographically matched controls. Subjects underwent a [(11)C]raclopride (RAC) PET scan. Striatal RAC binding potential (BP(ND)) was calculated on a voxel-wise basis. Prior to scanning, urine samples were obtained from cannabis users for quantification of urine Δ-9-tetrahydrocannabinol (THC) and THC metabolites (11-nor-Δ-9-THC-9-carboxylic acid; THC-COOH and 11-hydroxy-THC;OH-THC).

RESULTS

There were no differences in D(2)/D(3) receptor availability between cannabis users and controls. Voxel-wise analyses revealed that RAC BP(ND) values were negatively associated with both urine levels of cannabis metabolites and self-report of recent cannabis consumption.

CONCLUSIONS

In this sample, current cannabis use was not associated with deficits in striatal D(2)/D(3) receptor availability. There was an inverse relationship between chronic cannabis use and striatal RAC BP(ND). Additional studies are needed to identify the neurochemical consequences of chronic cannabis use on the dopamine system.

MAO-B inhibitors: multiple roles in the therapy of neurodegenerative disorders?

Monoamine oxidases play a central role in catecholamine catabolism in the central nervous system. The biochemical and pharmacological properties of inhibitors of the monoamine oxidase type B are reviewed. The evidence for biochemical activities distinct from their ability to inhibit MAO-B is discussed, including possible antioxidative and antiapoptotic activities of these agents. The significance of these properties for the pharmacological management of Parkinson's disease and the evidence for a neuroprotective effect of one such agent (selegiline) is also discussed.

Rasagiline and selegiline, inhibitors of type B monoamine oxidase, induce type A monoamine oxidase in human SH-SY5Y cells

Type B monoamine oxidase (MAO-B) is proposed to be involved in the pathogenesis of neurodegenerative disorders, such as Parkinson's disease, through oxidative stress and synthesis of neurotoxins. MAO-B inhibitors, rasagiline and selegiline [(-)deprenyl], protect neuronal cells by direct intervention in mitochondrial death signaling and induction of pro-survival Bcl-2 and neurotrophic factors. Recently, type A MAO (MAO-A) was found to mediate the induction of anti-apoptotic Bcl-2 by rasagiline, whereas MAO-A increases in neuronal death and also serves as a target of neurotoxins. These controversial results suggest that MAO-A may play a decisive role in neuronal survival and death. This paper reports that rasagiline and selegiline increased the mRNA, protein and catalytic activity of MAO-A in SH-SY5Y cells. Silencing MAO-A expression with small interfering (si)RNA suppressed rasagiline-dependent MAO-A expression, but MAO-B overexpression in SH-SY5Y cells did not affect, suggesting that MAO-A, not MAO-B, might be associated with MAO-A upregulation. Rasagiline reduced R1, a MAO-A specific repressor, but selegiline did not. Mithramycin-A, an inhibitor of Sp1 binding, and actinomycin-D, a transcriptional inhibitor, reduced the rasagiline-dependent upregulation of MAO-A mRNA, indicating that rasagiline induced MAO-A transcriptionally through R1-Sp1 pathway, whereas selegiline by another non-defined pathway. These results are discussed in relation to the role of MAO-A and these MAO-B inhibitors in neuronal death and neuroprotection.

Selective inhibition of monoamine oxidase A or B reduces striatal oxidative stress in rats with partial depletion of the nigro-striatal dopaminergic pathway

Partial lesion (50%) of the nigro-striatal dopaminergic pathway induces compensatory increase in dopamine release from the remaining neurons and increased extracellular oxidative stress (OS(-ec)) in the striatum. The present study was designed to explore the role of monoamine oxidase types A and B (MAO-A, MAO-B) in producing this increased oxidative stress. Lesion of the dopaminergic pathways in the CNS was produced in rats by intra-cerebroventricular injection of 6-hydroxydopamine (6-OHDA; 250 μg) and striatal microdialysis was carried out 5 weeks later. Striatal OS(ec) was determined by measurement of oxidized derivatives of the marker molecule N-linoleyl-tyrosine. Striatal tissue MAO-A activity was unchanged by 6-OHDA lesion but MAO-B activity was increased by 16%, together with a 45% increase in glial cell content. The selective MAO-B inhibitor rasagiline (0.05 mg/kg s.c. daily for 14 days) did not affect microdialysate dopamine concentration [DA(ec)] in sham-operated rats, but decreased OS(ec) by 30%. In lesioned rats, rasagiline decreased [DA(ec)] by 42% with a 49% reduction in OS(ec). The decrease in [DA(ec)] was reversed by the dopamine D2 receptor antagonist sulpiride (10 mg/kg s.c.). The selective MAO-A inhibitor clorgyline (0.2 mg/kg s.c. daily for 14 days) increased striatal [DA(ec)] by 72% in sham-operated rats with no change in OS(ec). In lesioned rats clorgyline increased [DA(ec)] by 66% and decreased OS(ec) by 44%. Rasagiline and clorgyline were effective to a similar extent in reduction of tissue levels of 7-ketocholesterol and the ratio GSSG/GSH, indicative of reduced intracellular oxidative stress level. This data implies that gliosis in our 6-OHDA animals together with inhibition of glial cell MAO-B by rasagiline causes an increase in local levels of dopamine at the presynaptic receptors, and a reduction in dopamine release (and in [DA(ec)]) by presynaptic inhibition. Moreover, inhibition of MAO-A or MAO-B reduces the enhanced level of oxidative stress in the lesioned striatum, and while both clorgyline and rasagiline reduced DA oxidative metabolism, rasagiline possesses an additional antioxidant property, not only that resulting from MAO inhibition.

Cardiovascular baroreceptor activity and selective inhibition of monoamine oxidase

Cardiovascular baroreceptor responsiveness of conscious rats treated with selective inhibitors of monoamine oxidase (MAO) types A and B was determined by measurement of blood pressure (BP) and heart rate (HR) responses to intravenous injection of phenylephrine and sodium nitroprusside. Treatment with selegiline (1 or 5 mg/kg p.o. daily for 7 days) did not significantly modify resting levels of BP and HR, lower or upper HR plateau levels, or HR/BP gain. Treatment with clorgyline (2 mg/kg p.o. daily for 7 days) increased HR/BP gain but also did not modify resting BP or HR, or lower and upper plateau levels of HR. The results are compatible with an effect of MAO-A inhibition to modify monoamine levels in medullary areas participating in CNS control of blood pressure.

Explaining ADAGIO: a critical review of the biological basis for the clinical effects of rasagiline

The ADAGIO study demonstrated a symptomatic benefit for rasagiline in early Parkinson's disease (PD) and suggested a disease-modifying effect. Evidence indicates that mitochondrial dysfunction plays a role in the pathogenesis of PD and that this may be the site of effect for rasagiline. In this systematic review, evidence for the role of mitochondria in the pathogenesis of PD are reviewed in light of other proposed mechanisms of neuronal degeneration and the actions of rasagiline and its component parts, namely propargylamine and the metabolite, aminoindan. Evidence for the role of mitochondria in the pathogenesis and treatment of PD are reviewed in light of other proposed mechanisms of neuronal degeneration and clinical actions of rasagiline. Monoamine oxidase B (MAO-B) located in the outer mitochondrial membrane controls dopamine metabolism in early PD, and this is the likely location for the symptomatic action of rasagiline. Accumulating evidence indicates that mitochondrial impairment contributes to dopaminergic neuronal loss in PD, either directly or through other mechanisms such as oxidative stress or protein misfolding. Further rasagiline affects numerous mitochondrial mechanisms that prevent apoptotic cell death including prevention of opening of the mitochondrial transition pore, decreased release of cytochrome C, alterations in pro-antiapoptotic genes and proteins, and the nuclear translocation of glyceraldehyde 3-phosphate dehydrogenase (GAPDH). Thus, the functional neuroprotective actions of rasagiline may not be dependent on MAO-B inhibition, but rather may involve actions of the propargylamine moiety and the aminoindan metabolite. An accumulating body of literature indicates a mitochondrial site of action for rasagiline and highlights the neuroprotective action of the drug, providing strong biological plausibility for disease-modifying effects of the drug such as those observed in ADAGIO.

Alterations in striatal oxidative stress level produced by pharmacological manipulation of dopamine as shown by a novel synthetic marker molecule

Oxidative stress (OS) is thought to participate in neurodegenerative diseases such as Parkinson's disease, but the contribution of dopamine metabolism and auto-oxidation to OS in Parkinson's and other diseases is not clear. Oxidative stress in rat striatum was measured by microdialysis using a novel synthetic compound composed of tyrosine and linoleic acid (LT), and determination of the oxidation products LT-OOH and LT-epoxy by HPLC-MS. Since LT is non-diffusible through the microdialysis membrane, the oxidized products formed in microdialyzate reflect oxidation state in the extracellular compartment. The extracellular oxidative stress (OS(ec)) was compared with intracellular oxidative stress (OS(ic)) as measured by tissue levels of oxidized and reduced glutathione and 7-ketocholesterol. Reserpinization caused an increase in OS(ic) but a reduction in OS(ec). Inhibition of both subtypes of monoamine oxidase (MAO-A and MAO-B) with tranylcypromine caused a reduction in both OS(ic) and OS(ec) whereas selective inhibition of MAO-A with clorgyline caused a reduction in Os(ic) but no change in OS(ec). A high dose (10 mg/kg) of amphetamine caused an increase in OS(ec) whereas a smaller dose (4 mg/kg) caused a reduction in OS(ec). Both doses of amphetamine reduced OS(ic). The present findings are consistent with a role of monoamine oxidase as well as dopamine auto-oxidation in production of striatal OS.

Pharmacology of Rasagiline, a New MAO-B Inhibitor Drug for the Treatment of Parkinson's Disease with Neuroprotective Potential

Rasagiline (Azilect) is a highly selective and potent propargylamine inhibitor of monoamine oxidase (MAO) type B. Like other similar propargylamine inhibitors, rasagiline binds covalently to the N5 nitrogen of the flavin residue of MAO, resulting in irreversible inactivation of the enzyme. Therapeutic doses of the drug which inhibit brain MAO-B by 95% or more cause minimal inhibition of MAO-A, and do not potentiate the pressor or other pharmacological effects of tyramine. Metabolic conversion of the compound in vivo is by hepatic cytochrome P450-1A2, with generation of 1-aminoindan as the major metabolite. Rasagiline possesses no amphetamine-like properties, by contrast with the related compound selegiline (Deprenyl, Jumex, Eldepryl). Although the exact distribution of MAO isoforms in different neurons and tissues is not known, dopamine behaves largely as a MAO-A substrate in vivo, but following loss of dopaminergic axonal varicosities from the striatum, metabolism by glial MAO-B becomes increasingly important. Following subchronic administration to normal rats, rasagiline increases levels of dopamine in striatal microdialysate, possibly by the build-up of β-phenylethylamine, which is an excellent substrate for MAO-B, and is an effective inhibitor of the plasma membrane dopamine transporter (DAT). Both of these mechanisms may participate in the anti-Parkinsonian effect of rasagiline in humans. Rasagiline possesses neuroprotective properties in a variety of primary neuronal preparations and neuron-like cell lines, which is not due to MAO inhibition. Recent clinical studies have also demonstrated possible neuroprotective properties of the drug in human Parkinsonian patients, as shown by a reduced rate of decline of symptoms over time.