MAOIs Research Articles

The monoamine oxidase inhibitor phenelzine enhances the discriminative stimulus effect of nicotine in rats

In addition to delivering nicotine, tobacco smoke also inhibits monoamine oxidase (MAO). Although MAO inhibitors (MAOIs) can increase nicotine self-administration in rodents, the effects of MAOIs on the discriminative stimulus effect of nicotine are not known. This study examined the effects of three MAOIs (phenelzine, clorgyline and pargyline) with varying selectivity for MAOA and MAOB in the nicotine drug discrimination procedure in rats. Adult male Sprague-Dawley rats were trained to discriminate nicotine (0.3 mg/kg, subcutaneously) from saline in a standard, two-lever food-reinforced operant task. Once the discrimination was acquired, the ability of each MAOI to substitute for or alter the discriminative stimulus effect of nicotine was determined. In substitution tests, nicotine (0.03-0.3 mg/kg) produced full, dose-dependent substitution. Although the selective MAOA inhibitor clorgyline (3-56 mg/kg) and the selective MAOB inhibitor pargyline (3-56 mg/kg) did not elicit any nicotine-appropriate responding, partial substitution was obtained with the nonselective MAO inhibitor phenelzine (1-17 mg/kg). Phenelzine (10 mg/kg) also enhanced the discriminative stimulus effect of a low dose of nicotine (0.056 mg/kg) and prolonged the time course effect of the nicotine-training dose. These findings indicate that concomitant inhibition of MAOA and MAOB can enhance the discriminative stimulus effect of nicotine in rats.

Efficacy of selegiline add on therapy to risperidone in the treatment of the negative symptoms of schizophrenia: a double‐blind randomized placebo‐controlled study

It has been reported that selegiline, a Selective Monoamine Oxidase Inhibitor B (MAOI-B), at low doses would be helpful for treating negative symptoms in schizophrenia. Nevertheless, the results are contradictory so far. This study was designed to investigate the effect of selegiline added to risperidone as augmentation therapy in patients with chronic schizophrenia and prominent negative symptoms in an 8 week, double blind and randomized clinical trial. Eligible participants in this study were 40 patients with chronic schizophrenia. All patients were inpatients and were in the active phase of the illness, and met DSM-IV-TR criteria for schizophrenia. Patients were allocated in a random fashion, 20 to risperidone 6 mg/day plus selegiline 10 mg/day (5 mg bid) and 20 to risperidone 6 mg/day plus placebo. The principal measure of the outcome was Positive and Negative Syndrome Scale (PANSS). Although both protocols significantly decreased the score of the positive, negative, and general psychopathological symptoms over the trial period, the combination of risperidone and selegiline showed a significant superiority over risperidone alone in decreasing negative symptoms and PANSS total scores. The present study indicates selegiline as a potential adjunctive treatment strategy for the negative symptoms of schizophrenia. Nevertheless, results of larger controlled trials are needed before recommendation for a broad clinical application can be made.

Selegiline transdermal system in the treatment of depressive disorders

Selegiline is a selective irreversible inhibitor of monoamine oxidase (MAO)-B during low-dose oral treatment. Additional MAO-A inhibition occurs in higher dose ranges. Selegiline transdermal system (STS) shows a more potent MAO inhibition in comparison with orally administered selegiline, and at minimum doses of 20 mg/20 cm2 patch delivering 6 mg/24 h, no dietary restrictions, including low-tyramine food, are necessary. While oral selegiline has been used for years in the treatment of Parkinson’s disease, the US FDA recently approved the STS for use in treating major depression. Data from three randomized controlled trials showed a significantly better efficacy of STS in the treatment of unipolar major depression in comparison with placebo during 6–8 weeks of treatment. In addition, one long-term randomized controlled trial demonstrated the efficacy of the STS in relapse prevention of unipolar depression over 1 year. A total number of 515 depressed patients received STS during these studies. The tolerability profile of STS without dietary restrictions was excellent. The only side effects that were more frequent than with placebo were application-site reactions. No change in vital parameters and no hypertensive crisis have been recorded. Comparative studies including treatment with established antidepressants and the investigation of bipolar depressed patients and of specific patient subgroups (e.g., anergic and atypical depression) have to follow. The combination of the well-known effectiveness of irreversible MAO inhibitors with a good tolerability profile and a new pharmaceutical form, possibly enhancing compliance, represents a promising further expansion of the pharmacotherapeutic repertoire in the treatment of depression.

Differential behavioral syndrome evoked in the rats after multiple doses of SSRI fluoxetine with selective MAO inhibitors rasagiline or selegiline

This study investigated whether rasagiline and selegiline (MAO-B inhibitors) induce serotonin syndrome in fluoxetine-treated rats. Rats received rasagiline (0.1, 0.5, 2.0 mg/kg), or selegiline (0.8, 4.0, 16.0 mg/kg) (doses reflecting the clinical ratio of 1:8 base) in drinking water for 28 days. During the last 21 days, they received injections of fluoxetine 10 mg/kg (controls received water only, then saline injections; a fluoxetine only group received water only then fluoxetine). Serotonin syndrome was assessed using neurological severity score (NSS), food intake and weight gain. Mean NSS significantly increased, and weight and food consumption significantly decreased in rats receiving fluoxetine alone compared with controls. Selegiline 16 mg/kg but not rasagiline (regardless of dose) exacerbated these effects. We concluded that selegiline's amphetamine-like metabolites may increase synaptic cathecholamines and possibly serotonin, aggravating fluoxetine's effect. Rasagiline is devoid of this effect and may therefore be safer for use with serotonergic drugs in parkinsonian patients.

Three Dimensional Pharmacophore Modelling of Monoamine oxidase-A (MAO-A) inhibitors

Flavoprotein monoamine oxidase is located on the outer membrane ofmitochondria. It catalyzes oxidative deamination of monoamine neurotransmitters such asserotonin, norepinephrine and dopamine and hence is a target enzyme for antidepressantdrugs. MAO (mono amine oxidase) has two isoforms, namely MAO-A and MAO-B.MAO-A isoform has higher affinity for serotonin and norepinephrine, while; MAO-Bpreferentially deaminates phenylethylamine and benzylamine. These important propertiesdetermine the clinical importance of MAO inhibitors. Selective MAO-A inhibitors are usedin the treatment of neurological disorders such as depression. In this article we havedeveloped a Hypogen pharmacophore for a set of 64 coumarin analogs and tried to analyzethe intermolecular H-bonds with receptor structure.

Differential effects of selegiline on glucose synthesis in rabbit kidney-cortex tubules and hepatocytes: In vitro and in vivo studies

The action of selegiline, a selective and irreversible inhibitor of monoamine oxidase B, commonly applied in the therapy of Parkinson's disease, on glucose formation was investigated in isolated rabbit hepatocytes and kidney-cortex tubules, maintaining the whole body glucose homeostasis via gluconeogenic pathway activity. An intensive hepatic metabolism of selegiline resulted in formation of selegiline- N-oxide, desmethylselegiline, methamphetamine and amphetamine, whereas during slow degradation of the drug in freshly isolated renal tubules selegiline- N-oxide was mainly produced. At 100 μM concentration selegiline markedly diminished glucose synthesis in isolated renal tubules incubated with dihydroxyacetone or alanine + glycerol + octanoate (by about 60 and 30%, respectively), while at 5 μM concentration a similar degree of inhibition was achieved in renal tubules grown in primary culture under the same conditions (about 40 and 60%, respectively). Moreover, desmethylselegiline and selegiline- N-oxide considerably diminished glucose production in renal tubules whereas selegiline and its metabolites did not affect gluconeogenesis in hepatocytes. Contrary to control animals, following selegiline administration to alloxan-diabetic rabbits for 8 days (10 mg kg −1 body wt. daily) the blood glucose and serum creatinine levels were significantly diminished, suggesting a decrease in renal gluconeogenesis and improvement of kidney functions. Since in renal tubules selegiline induced a decline in the intracellular levels of gluconeogenic intermediates and ATP content accompanied by a decrease in oxygen consumption in both kidney-cortex and hepatic mitochondria it seems possible that its inhibitory action on renal gluconeogenesis might result from an impairment of mitochondrial function, while an intensive selegiline metabolism in hepatocytes causes decrease of its concentration and in consequence no inhibition of gluconeogenesis. In view of these observations it is likely that an increased risk of selegiline-induced hypoglycemia might be expected particularly in patients exhibiting an impairment of liver function and following transdermal administration of this drug, i.e. under conditions of increased serum selegiline concentrations.

Human and rat monoamine oxidase-A are differentially inhibited by ( S)-4-alkylthioamphetamine derivatives: Insights from molecular modeling studies

Four enantiomerically pure ( S)-4-alkylthioamphetamine derivatives were evaluated as monoamine oxidase (MAO) inhibitors using the human and rat isoforms of the enzyme. Molecular dockings were performed in order to gain insights regarding the binding mode of these inhibitors. All compounds were potent and selective MAO-A inhibitors although different rank orders of potencies were observed against the enzymes from different species. This behavior can be rationalized on the basis of different binding modes to each enzyme, as determined in silico. These findings further support the concept that MAO inhibitory activity of novel compounds, determined with enzymes from diverse mammalian species, should be considered with caution if human MAO is the final target to be addressed.

The effect of l-deprenyl on tissue mRNA expressions of NOS isoforms and NO levels in an experimental diabetes mellitus model

Diabetes and aging share some common mechanisms in their pathogenesis and diabetics are more prone to diseases of the elderly. Seeking for therapies likely to be proposed in the synchronised treatment of aging and diabetes is of great interest and l-deprenyl, a selective monoamine oxidase (MAO-B) inhibitor, is a possible candidate with its antioxidant, antiapoptotic and neuroprotective properties. Tissue MAO, NO and mRNA expression of nitric oxide (NO) synthase (NOS) isoforms were assessed in streptozotocin (STZ)-induced diabetic rats to evaluate the effect of l-deprenyl treatment. Twelve weeks of treatment had no significant effect on NO levels. Four-weeks treatment decreased tissue MAO activities and caused a decrease in expression of NOS-2 and NOS-3 in heart tissue of both controls and diabetics, and a decrease of liver NOS-3 expression in controls (p < 0.05). l-Deprenyl, causing a decrease in tissue NOS expressions, might be of benefit by protecting the organism from the toxic radical effects of NO.

Species Differences in the Selective Inhibition of Monoamine Oxidase (1-methyl-2-phenylethyl)hydrazine and its Potentiation by Cyanide

The potentiating effects of cyanide on the inhibition of rat liver mitochondrial monoamine oxidase-A & B and of ox liver mitochondrial MAO-B by pheniprazine [(1-methyl-2-phenylethyl)hydrazine] has been studied. Pheniprazine was shown to behave as a mechanism-based MAO inhibitor. For rat liver MAO-B, the initial non-covalent step was characterized by dissociation constant (K (i)) of 2450 nM and the first-order rate constant (k (+2)) for the covalent adduct formation was 0.16 min(-1). As a reversible inhibitor it was selective towards rat liver MAO-A (K (i) = 420 nM) but the rate of irreversible inhibition of that enzyme was considerably slower (k (+2) = 0.06 min(-1)). MAO-B from ox liver more closely resembled MAO-A from the rat in sensitivity to reversible inhibition by pheniprazine (K (i) = 450 nm) but it was closer to rat liver MAO-B in rate of irreversible inhibition (k (+2) = 0.29 min(-1)). The K (i) values were significantly decreased in the presence of KCN but there was little effect on the k (+2) values. However, sensitivities of the different enzymes to KCN varied widely and considerably higher concentrations of KCN were required for this effect to be apparent with the rat liver mitochondrial MAO-A than with MAO-B from rat and ox liver. The kinetic behaviour of cyanide activation was consistent with partial (non-essential) competitive activation in all cases.

Effects of co-administration of antidepressants and monoamine oxidase inhibitors on 5-HT-related behavior in rats

5-hydroxytryptamine (5-HT) syndrome is a dangerous condition of 5-HT excess that can occur during co-administration of a monoamine oxidase (MAO) inhibitor and an antidepressant. We investigated the effects of acute administration of MAO inhibitors and subchronic administration of tricyclic and heterocyclic antidepressants, and a serotonin-noradrenaline reuptake inhibitor (SNRI) on 5-HT syndrome in rats treated with 5-hydroxytryptophan (5-HTP). The irreversible and non-selective MAO inhibitor pargyline, and the reversible and selective MAO-A inhibitor clorgyline, produced increases in 5-HT syndrome in the 5-HTP-treated rats, while subchronic co-administration of imipramine partly intensified and partly attenuated the syndrome, whereas milnacipran only attenuated the syndrome. Co-administration of mianserin partly intensified and partly attenuated the syndrome but the attenuating effect was dominant. Administration of the irreversible and selective MAO-B inhibitor selegiline did not produce any increase in 5-HT syndrome in the 5-HTP-treated rats, compared with the saline control. These data suggest that non-selective MAO and selective MAO-A inhibitors can induce 5-HT syndrome in humans when co-administered with antidepressants. Furthermore, the risk of 5-HT syndrome may be lower with the selective MAO-B inhibitor selegiline.

Tranylcypromine enhancement of nicotine self-administration

Tobacco use has one of the highest rates of addiction of any abused drug. Paradoxically, in animal models, nicotine appears to be a weak reinforcer. We report here that the inhibition of monoamine oxidase (MAO), a major effect of tobacco smoke, increases the reinforcing effect of nicotine. Rats (aged postnatal day 27 and 90) were tested for self-administration, without prior response training, in five daily 3-h sessions. Whereas control rats did not self-administer nicotine, low doses of nicotine (2.5 to 21 μg/kg/injection) were avidly self-administered following a pretreatment with tranylcypromine (3 mg/kg), an irreversible and non-selective MAO inhibitor. Tranylcypromine-enhanced nicotine (10 μg/kg/injection, i.v.) self-administration was reduced by systemic injection of a D1-dopaminergic receptor antagonist, SCH23390 (0.02 mg/kg). Moreover, an increase in extracellular dopamine in the nucleus accumbens was detected, using microdialysis, following nicotine (60 μg/kg) injection in tranylcypromine pre-treated rats. Depending on the time of tranylcypromine pretreatment (20 or 1 h), MAO activity was decreased by 72% and 99% and nicotine intake at day 5 was increased by 619 and 997%, respectively. Taken together, these results indicate that in a stringent self-administration acquisition test, MAO inhibition increases the rewarding effect of low doses of nicotine, possibly via a dopamine-dependent mechanism.

Pharmacological treatment of anxiety disorders - is there a state of the art?

Selection of appropriate treatment for anxiety disorders is influenced by several considerations, including psychiatric comorbidity. Emerging data suggest that anxiety disorders have a chronic course and a high comorbidity with depression. Successful treatment can be facilitated by first establishing treatment goals, which include managing acute anxiety and following through to remission. Prevention of recurrence of anxiety disorders should be the ultimate objective.Various treatment options exist for the treatment of anxiety disorders, including selective serotonin reuptake inhibitors (SSRIs), serotonin-noradrenaline reuptake inhibitors (SNRIs), tricyclic antidepressants, pregabalin, benzodiazepines, buspirone, and reversible and irreversible MAO inhibitors. Some SSRIs have been demonstrated efficacy in both acute and long-term trials. Regarding their risk-benefit ratio, they are established as first-line therapies. The combination of drug treatment with cognitive behaviour therapy (CBT) is also recommended.The chronic nature of anxiety disorders, different treatment response among different anxiety disorders and the recognition of their frequent comorbidity with depression requires an informed and evidence based choice of the best pharmacological approach to the individual patient. The presentation will present the most recent data from randomised clinical trials of newer generation agents and put them into perspective, to help the physicians to appropriately diagnose anxiety disorders and achieve the goal of bringing patients to full remission.

New Pyrrole Inhibitors of Monoamine Oxidase: Synthesis, Biological Evaluation, and Structural Determinants of MAO-A and MAO-B Selectivity

A series of new pyrrole derivatives have been synthesized and evaluated for their monoamine oxidase (MAO) A and B inhibitory activity and selectivity. N-Methyl,N-(benzyl),N-(pyrrol-2-ylmethyl)amine (7) and N-(2-benzyl),N-(1-methylpyrrol-2-ylmethyl)amine (18) were the most selective MAO-B (7, SI = 0.0057) and MAO-A (18, SI = 12500) inhibitors, respectively. Docking and molecular dynamics simulations gave structural insights into the MAO-A and MAO-B selectivity. Compound 18 forms an H-bond with Gln215 through its protonated amino group into the MAO-A binding site. This H-bond is absent in the 7/MAO-A complex. In contrast, compound 7 places its phenyl ring into an aromatic cage of the MAO-B binding pocket, where it forms charge-transfer interactions. The slightly different binding pose of 18 into the MAO-B active site seems to be forced by a bulkier Tyr residue, which replaces a smaller Ile residue present in MAO-A.

Selegiline Transdermal System: In the Treatment of Major Depressive Disorder

Selegiline is a selective irreversible inhibitor of monoamine oxidase (MAO)-B during low-dose oral treatment. Additional MAO-A inhibition occurs in higher dose ranges. Selegiline transdermal system (STS) shows a more potent MAO inhibition in comparison with orally administered selegiline, and at minimum doses of 20 mg/20 cm2 patch delivering 6 mg/24 h, no dietary restrictions, including low-tyramine food, are necessary. While oral selegiline has been used for years in the treatment of Parkinson’s disease, the US FDA recently approved the STS for use in treating major depression. Data from three randomized controlled trials showed a significantly better efficacy of STS in the treatment of unipolar major depression in comparison with placebo during 6–8 weeks of treatment. In addition, one long-term randomized controlled trial demonstrated the efficacy of the STS in relapse prevention of unipolar depression over 1 year. A total number of 515 depressed patients received STS during these studies. The tolerabil...

Absence of tissue-bound semicarbazide-sensitive amine oxidase activity in carp tissues

We have previously reported that carp ( Cyprinus carpio) tissue mitochondria contain a novel form of monoamine oxidase (MAO), which belongs neither to MAO-A nor to MAO-B of the mammalian enzyme. This conclusion results from the findings that the carp MAO was equally sensitive to a selective MAO-A inhibitor clorgyline and to the MAO-B selective inhibitor l-deprenyl, when tyramine, a substrate for both forms, serotonin or β-phenylethylamine, a substrate for either A or B-form of mammalian MAO, was used. In the present study, we tried to detect another amine oxidase, termed tissue-bound semicarbazide-sensitive amine oxidase (SSAO), activity in carp tissues. As definition of SSAO was used, such as insensitivity to inhibition of the kynuramine oxidizing activity by an MAO inhibitor pargyline and high sensitivity to the SSAO inhibitor semicarbazide. The results indicated that the oxidizing activity was selectively and almost completely inhibited by 0.1 mM pargyline alone or a combination of 0.1 mM pargyline plus 0.1 mM semicarbazide, but not by 0.1 mM semicarbazide alone. We also tried to detect any SSAO activity by changing experimental conditions, such as lower incubation temperature, higher enzyme protein concentration, a lower substrate concentration and different pH's in the reaction, as the enzyme source. However, still no SSAO activity could be detected in the tissues. These results conclusively indicate that carp tissues so far examined do not contain SSAO activity.

L-Methamphetamine and selective MAO inhibitors decrease morphine-reinforced and non-reinforced behavior in rats; Insights towards selegiline's mechanism of action

Selegiline is an inhibitor of type B monoamine oxidase (MAO) with psychostimulant effects that can decrease morphine-reinforced and non-reinforced responding. The present study was undertaken to compare the effects of MAO inhibition and treatment with l-methamphetamine, the major psychostimulant metabolite of selegiline, on these behaviors. After rats acquired a stable pattern of morphine self-administration under a progressive ratio schedule, chronic treatment was initiated with vehicle, l-methamphetamine, clorgyline (a selective inhibitor of MAO-A), or rasagiline (a selective inhibitor of MAO-B); with both MAO inhibitors administered at a dose selective for one MAO isoform and a higher dose that inhibited both isoforms. Rats were evaluated for up to four cycles of opiate dependence maintained by morphine self-administration and withdrawal during which extinction responding was recorded. Most behavioral measures (92.4%) did not differ in animals evaluated during an initial and subsequent cycles of dependence and withdrawal. All active treatments attenuated non-reinforced responding during extinction. Morphine reinforcement was also decreased by each of the three active treatments, but greater and more prolonged effects were observed following inhibition of MAO-B with rasagiline. Responding during either cue- or morphine-induced reinstatement was attenuated by either clorgyline or rasagiline administered at nonselective doses, but not by either compound administered at selective dose levels. Treatment with l-methamphetamine did not produce significant effects on cue-induced reinstatement, but decreased non-reinforced responding during morphine-induced reinstatement. These findings indicate that morphine reinforcement and different non-reinforced behaviors differ greatly in their susceptibility to modification by psychostimulant treatment or MAO inhibition.

Antidepressants in the treatment of fibromyalgia

Fibromyalgia syndrome is a chronic disease of widespread and debilitating pain whose cause is unknown and whose risk factors are poorly understood. It is often comorbid with rheumatoid and other pain disorders as well as psychiatric disorders such as anxiety and depression. Although they are not officially approved for this indication, antiepileptics and antidepressants are often used to treat fibromyalgia. The tricyclic antidepressants (TCAs), particularly amitriptyline, are among the most common treatment strategies. Because of the poor tolerability of the tricyclics, the newer antidepressants have been widely tested in fibromyalgia. The selective serotonin reuptake inhibitors (SSRIs) and the reversible monoamine oxidase inhibitors do not seem to be particularly helpful. The serotonin and norepinephrine reuptake inhibitors (SNRIs), duloxetine and milnacipran, on the other hand, have been shown in placebo-controlled trials to offer significant relief to patients suffering from fibromyalgia. Although no direct comparative studies have been performed, these compounds appear to be as effective as the TCAs but much better tolerated. The effectiveness of the SNRIs as well as other dual acting antidepressants, such as mirtazapine, but not the SSRIs, implies that a dysfunction of both serotonin and norepinephrine neurotransmission probably exists in fibromyalgia. The effectiveness of antidepressants appears to be independent of their effect on comorbid depression.

The Path from Anti Parkinson Drug Selegiline and Rasagiline to Multifunctional Neuroprotective Anti Alzheimer Drugs Ladostigil and M30

The therapeutic use of enzyme inhibitors in treatment of neurodegenerative diseases has its origin in the anti Parkinson action of the selective monoamine oxidase (MAO) B inhibitor, l-deprenyl (selegiline ), a failed anti depressant in 1975. This led to further development of MAO- A and B, catechol-O-methyltansferase and cholinestrerase inhibitors as anti Parkinson and Alzheimer drugs. One of the main reasons for the cognitive deficit in dementia of the Alzheimer' type (AD) and in dementia with Lewy bodies (DLB) is degeneration of cholinergic cortical neurones and synaptic plasticity. This led to a correlation that similar to Parkinson's Disease (PD), cholinesterase inhibitors (ChEI) may also have therapeutic activity in AD. Significant percentage of AD and DLB subjects also nigrostriatal dopaminergic, locus ceruleous noradrenergic and raphe nucleus serotoninergic neurones. The present ChEI anti AD drugs have limited symptomatic activity and devoid of neuroprotective property that is needed for disease modifying action. It is becoming clear that there are no magic bullets for neurodegenerative disorders and shut gun approach is needed either as polypharmacology or drugs with multiple activity at different target sites in the CNS. The complex pathology of AD as well as cascade of events that leads to the neurodegenerative process has led us to develop several multifunctional neuroprotective drugs with several CNS targets with possible disease modifying activity. Employing the pharamcophore of our antiparkinson drug rasagiline (Azilect, Agilect, N-propagrgyl-1R-aminoindan) we have developed a novel multifunctional neuroprotective drug, ladostigil [TV-3326 (N-propargyl-3R-aminoindan-5yl)-ethyl methylcarbamate)], with both cholinesterase-butyrylesterase (Ch-BuE) and brain selective monoamine-oxidase (MAO) AB inhibitory activities possessing the neuroprotective-neurescue propargyl moiety, as potential treatment of AD and DLB and PD with dementias. Since brain MAO and iron increase in AD, PD and ageing, that could lead to iron dependent oxidative stress neurodegeneration, we have developed another series of multifunctional drugs (M30 HLA-20 series) which are brain permeable iron chelators- brain selective MAO inhibitors and possess the propargyl neuroprotective moiety. These series of drugs have the ability of regulating and processing APP (amyloid precursor protein) and reducing Abeta peptide, since APP is a metaloprotein, with an iron responsive element 5d'UTR similar to transferring and ferritin.

Cardiovascular responses to combined treatment with selective monoamine oxidase type B inhibitors and L-DOPA in the rat.

Postural hypotension is a common side-effect of L-DOPA treatment of Parkinson's disease, and may be potentiated when L-DOPA is combined with selegiline, a selective inhibitor of monoamine oxidase B (MAO-B). Rasagiline is a new, potent and selective MAO-B inhibitor, which does not possess the sympathomimetic effects of selegiline. We have studied the effects of these selective MAO inhibitors, L-DOPA and dopamine on the cardiovascular system of the rat. Blood pressure and heart rate was measured in conscious rats following acute or chronic administration of rasagiline, selegiline and L-DOPA, by comparison with the selective MAO-A inhibitor clorgyline, or the MAO-A/B inhibitor tranylcypromine. Cardiovascular responses, catecholamine release, and modification of pressor response to dopamine were studied in pithed rats. In conscious rats neither rasagiline nor selegiline caused significant potentiation of the effects of L-DOPA (50, 100, 150 mg.kg(-1)) on blood pressure or heart rate at doses which selectively inhibited MAO-B, but L-DOPA responses were potentiated by clorgyline and tranylcypromine. In rats treated twice daily for 8 days with L-DOPA and carbidopa, selegiline (5 mg.kg(-1)) but not rasagiline (0.2 mg.kg(-1)) caused a significant hypotensive response to L-DOPA and carbidopa, although both drugs caused similar inhibition of MAO-A and MAO-B. In pithed rats, selegiline but not rasagiline increased catecholamine release and heart rate, and potentiated dopamine pressor response at MAO-B selective dose. The different responses to the two MAO-B inhibitors may be explained by the amine releasing effect of amphetamine metabolites formed from selegiline.

Monoamine Oxidase Assays

Monoamine oxidase catalyzes the oxidative deamination of primary aliphatic and aromatic amines and some secondary and tertiary amines, including the hormone and neurotransmitter amines epinephrine, dopamine, norepinephrine and serotonin. The two isoenzymes, MAO-A and MAO-B, differ according to substrate and inhibitor specificities. For example, selective inhibitors of MAO-A have been shown to be effective antidepressants whereas some MAO-B inhibitors have been reported to be beneficial in the treatment of Parkinson's and perhaps Alzheimer's diseases. Thus, it is important to have accurate procedures for determining the activities of each MAO isoenzyme in tissues that may contain both. In this unit, several MAO assay procedures are described, along with support protocols for equilibrating ion-exchange resin, preparing aldehyde dehydrogenase, preparing dose-response curves for determining the relative proportions of isoenzyme activities, determining concentrations of each isoenzyme, assessing new compounds as putative MAO inhibitors, and determining protein concentration of membrane-bound proteins.