Effects of different monoamine oxidase inhibitors on the metabolism of L-dopa in the rat brain

(13)C/(15)N-Enriched l-Dopa as a Triple-Resonance NMR Probe to Monitor Neurotransmitter Dopamine in the Brain and Liver Extracts of Mice

In an attempt to monitor μm-level trace constituents, we applied here (1)H-{(13)C-(15)N} triple-resonance nuclear magnetic resonance (NMR) to (13)C/(15)N-enriched l-Dopa as the inevitable precursor of the neurotransmitter dopamine in the brain. The perfect selectivity (to render endogenous components silent) and μm-level sensitivity (700 MHz spectrometer equipped with a cryogenic probe) of triple-resonance allowed the unambiguous and quantitative metabolic and pharmacokinetic analyses of administered l-Dopa/dopamine in the brain and liver of mice. The level of dopamine generated in the brain (within the range 7-76 μm, which covers the typical stimulated level of ∼30 μm) could be clearly monitored ex vivo, but was slightly short of the detection limit of a 7 T MR machine for small animals. This work suggests that μm-level trace constituents are potential targets of ex vivo monitoring as long as they contain N atom(s) and their appropriate (13)C/(15)N-enrichment is synthetically accessible.

Dopamine modulating drugs influence striatal (+)-[11C]DTBZ binding in rats: VMAT2 binding is sensitive to changes in vesicular dopamine concentration

Binding of (+)-[11C]DTBZ (dihydrotetrabenazine) to the striatal vesicular monoamine transporter (VMAT2) is widely considered to be a stable marker of dopamine neurone integrity. However, we now find that specific binding of a tracer dose of (+)-[11C]DTBZ is modestly increased in rat striatum following dopamine depletion with alpha-methyl-p-tyrosine (alpha-MPT, +14%) or d-amphetamine (d-AMPH, 20 mg/kg, +12%) and decreased following dopamine elevation with gamma-hydroxybutyrate (GHB, -16%) or levodopa (-20%). We suggest that in vivo (+)-[11C]DTBZ binding in imaging studies is subject to competition by vesicular dopamine and, in this respect, is not a "stable" dopamine biomarker as is generally assumed.

Effect of adenosine A(2A) receptor antagonists on L-DOPA-induced hydroxyl radical formation in rat striatum

A(2A) adenosine receptor antagonists have been proposed as a new therapy for Parkinson's disease (PD). Since oxidative stress plays an important role in the pathogenesis of PD, we studied the effect of the selective A(2A) adenosine receptor antagonists 8-(3-chlorostyryl)caffeine (CSC) and 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385) on L: -3,4-dihydroxyphenylalanine (L: -DOPA)-induced hydroxyl radical generation using in vivo microdialysis in the striatum of freely moving rats. L: -DOPA (100 mg/kg; in the presence of benserazide, 50 mg/kg) given acutely or repeatedly for 14 days generated a high level of hydroxyl radicals, measured by HPLC with electrochemical detection, as the product of their reaction with p-hydroxybenzoic acid (PBA). CSC (1 mg/kg) and ZM 241385 (3 mg/kg) decreased haloperidol (0.5 mg/kg)-induced catalepsy, while at low doses of 0.1 and 0.3 mg/kg, respectively, they did not display an effect. CSC (1 and 5 mg/kg) and ZM 241385 (3 and 9 mg/kg) given acutely, or CSC (1 mg/kg) and ZM 241385 (3 mg/kg) given repeatedly, increased the production of hydroxyl radicals in dialysates from rat striatum. Both acute and repeated administration of CSC (0.1 and 1 mg/kg) and ZM 241385 (3 mg/kg) decreased L: -DOPA-induced generation of hydroxyl radicals. However, a high single dose of either CSC (5 mg/kg) and ZM 241385 (9 mg/kg) markedly potentiated the effect of L: -DOPA on hydroxyl radical production. The increase in hydroxyl radical production by acute and chronic injection of CSC and ZM 241385 may be related to the increased release of dopamine (DA) and its metabolism in striatal dialysates. Similarly, increased DA release following a single high dose of CSC or ZM 241385 appears to be responsible for augmentation of L: -DOPA-induced hydroxyl radical formation. Conversely, the inhibition of L: -DOPA-induced production of hydroxyl radical by single and repeated low doses of CSC or repeated low doses of ZM 241385 may be related to reduced DA metabolism. Summing up, A(2A) antagonists, used as a supplement of L: -DOPA therapy, depending on the dose used, may have a beneficial or adverse effect on ongoing neurodegenerative processes and accompanying oxidative stress.

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.

Clozapine Modulates Aromatic l-Amino Acid Decarboxylase Activity in Mouse Striatum

Clozapine is efficacious for treating dopaminergic psychosis in Parkinson's disease and ameliorates l-DOPA-induced motor complications. Based on its pharmacology and reported enhancing effects on dopamine metabolism and tyrosine hydroxylase activity, we investigated whether it could modulate the activity of aromatic l-amino acid decarboxylase (AAAD), the second enzyme for the biosynthesis of catecholamines and indoleamines. A single dose of clozapine increased AAAD activity of striatum in a dose- and time-dependent manner. At 1 h, enhanced enzyme activity was characterized by an increased V(max) for substrate and cofactor and was accompanied by elevated levels of protein in striatum and mRNA in substantia nigra, ventral tegmental area, locus coeruleus, and raphe nuclei. Acute clozapine increased tyrosine hydroxylase activity in striatum but with differing temporal patterns from AAAD and heightened dopamine metabolism. Interestingly, the response of the dopaminergic markers to clozapine was greater following a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) lesion. Chronically administered clozapine increased AAAD activity and protein and dopamine metabolism in striatum without affecting tyrosine hydroxylase. Exogenous l-DOPA decarboxylation was accelerated in the striatum of intact and MPTP-lesioned mice following acute clozapine, and the effect was exaggerated in the MPTP mice. To identify receptors involved, antagonists of receptors occupied by clozapine were employed. D4, 5-HT1(A), and 5-HT2(A), in addition to D1, D2, and D3, antagonists, augmented AAAD activity in striatum, whereas 5-HT2(C), 5-HT3, muscarinic, and alpha-1 and alpha-2 adrenergic antagonists were ineffective. For the first time, these studies provide evidence that clozapine modulates AAAD activity in the brain and suggests that dopamine and serotonin receptors are involved.

Discriminative stimulus and reinforcing effects of p-fluoro-L-deprenyl in monkeys

RATIONALE

para-Fluoro-L-deprenyl (Fludepryl), a halogenated derivative of L-deprenyl, shares structural similarities with amphetamine and may have potential as a medication for psychostimulant abuse.

OBJECTIVES

p-Fluoro-L-deprenyl was evaluated for psychomotor stimulant, discriminative stimulus, and reinforcing effects in squirrel monkeys.

METHODS

One group of monkeys was trained under a ten-response fixed-ratio (FR10) schedule of stimulus termination to discriminate between methamphetamine (0.32 mg/kg, i.m.) and saline. Other monkeys were trained to self-administer i.v. cocaine under either a simple FR10 schedule or a second-order fixed-interval 5-min schedule with FR10 components.

RESULTS

Full generalization to the methamphetamine-training stimulus was produced by an i.m. dose of 10.0 mg/kg p-fluoro-L-deprenyl. L-Deprenyl and the metabolites of p-fluoro-L-deprenyl, p-fluoro-L-amphetamine, and p-fluoro-L-methylamphetamine were more potent, producing full generalization at doses of 1.0-3.2 mg/kg. Under the FR10 schedule of drug injection, persistent self-administration behavior was maintained by i.v. cocaine injections but not by injections of vehicle or injection doses of p-fluoro-L-deprenyl up to 1.0 mg/kg. However, p-fluoro-L-deprenyl did maintain moderate levels of i.v. self-administration responding under the second-order schedule of drug injection. Peak response rates maintained by 0.1-mg/kg injections of p-fluoro-L-deprenyl were significantly greater than those associated with saline substitution, yet significantly lower than those maintained by cocaine or D-amphetamine.

CONCLUSIONS

p-Fluoro-L-deprenyl has methamphetamine-like discriminative-stimulus properties in squirrel monkeys that appear at higher doses than for its parent compound, L-deprenyl. It also appears to function as a relatively limited reinforcer of intravenous self-administration behavior in monkeys trained to self-administer i.v. cocaine.

A meta-analysis of the response to chronic L-dopa in patients with schizophrenia: therapeutic and heuristic implications

RATIONALE

While it is generally believed that administration of the dopamine precursor levodopa ( L-dopa) exacerbates symptoms of schizophrenia, numerous reports suggest that adjunctive L-dopa may be beneficial. This body of literature has not been critically reviewed.

OBJECTIVES

On the basis of published studies, to determine whether L-dopa administered concomitantly with antipsychotic drugs provides a beneficial response in patients with schizophrenia.

METHODS

This review examined 30 studies involving 716 patients. Due to wide methodological variability and limited statistical information, only five studies encompassing 160 patients could be included in a meta-analysis. The others were evaluated qualitatively.

RESULTS

When L-dopa was added to antipsychotic drugs, the overall improvement was moderate ( d=0.71) and highly significant ( P<0.0001). There were 16 other studies in which L-dopa was added to antipsychotic drugs, but which did not meet criteria for inclusion in the meta-analysis. In these, worsening occurred in less than 20% of patients; the percentage of improved patients varied widely but had a central tendency around 50%.

CONCLUSIONS

. In patients already on antipsychotic drugs, the addition of L-dopa can be beneficial. Dopamine agonists merit further consideration as adjuncts to antipsychotic drugs in the treatment of schizophrenia.

Modulation of dihydroxyphenylacetaldehyde extracellular levels in vivo in the rat striatum after different kinds of pharmacological treatment

We recently identified the direct product of dopamine (DA) by monoamine-oxidase (MAO) activity, dihydroxyphenylacetaldehyde (DOPALD) in the trans-striatal dialysate. Based on these findings, in this work, we directly measured the variations in DOPALD levels after various kinds of pharmacological treatment in rat striatal extracellular fluid. Using both reversible and irreversible MAO inhibitors, we found that MAO-A inhibition suppressed, whereas MAO-B inhibition did not modify DOPALD levels in the dialysate. The vesicular DA uptake blocker Ro 4-1284 led to an increase in extracellular DA and DOPALD, whereas the increase in extracellular DA obtained after administration of the plasma membrane DA uptake blocker GBR-12909 occurred without concomitant changes in DOPALD extracellular levels. Microinfusions of DA through the dialysis probe or systemic administration of L-DOPA increased striatal DOPALD to a greater extent compared with other DA metabolites, both in intact and in 6-hydroxydopamine (6-OHDA)-lesioned striatum. This study indicates that the direct product of MAO activity within the rat striatum derives from the activity of the isoenzyme MAO-A. The assay of DOPALD, together with DOPAC, represents a reliable tool to measure directly, in freely moving animals, DA oxidative metabolism. As recent studies have shown that microinfusions of exogenous DOPALD might induce cell death, pharmacological modulation of DOPALD levels might also be relevant for an understanding of the mechanisms involved in DA neurotoxicity.

Striatal dopamine metabolism in monoamine oxidase B-deficient mice: a brain dialysis study

We have studied striatal dopamine (DA) metabolism in monoamine oxidase (MAO) B-deficient mice using brain microdialysis. Baseline DA levels were similar in wild-type and knock-out (KO) mice. Administration of a selective MAO A inhibitor, clorgyline (2 mg/kg), increased DA levels and decreased levels of its metabolites in all mice, but a selective MAO B inhibitor, l-deprenyl (1 mg/ kg), had no effect. Administration of 10 and 50 mg/kg L-DOPA, the precursor of DA, increased the levels of DA similarly in wild-type and KO mice. The highest dose of L-DOPA (100 mg/kg) produced a larger increase in DA in KO than wild-type mice. This difference was abolished by pretreating wild-type mice with l-deprenyl. These results suggest that in mice, DA is only metabolized by MAO A under basal conditions and by both MAO A and B at high concentrations. This is in contrast to the rat, where DA is always metabolized by MAO A regardless of concentration.

Effect of monoamine oxidase A and B and of catechol-O-methyltransferase inhibition on L-DOPA-induced circling behavior

The effect of enzyme-inhibiting adjuvants on L-DOPA + benserazide-induced contralateral turning in unilateral 6-hydroxydopamine (6-OHDA)-lesioned rats was studied. Both the number of turns and the duration of turning were examined. Inhibition of MAO-A with 10 mg/kg Ro 41-1049 increased both parameters; inhibition of COMT with 30 mg/kg Ro 40-7592 had a similar effect. In contrast, inhibition of MAO-B with 10 mg/kg Ro 19-6327 did not change turning behavior. A further potentiation of turning behavior was observed after the combined administration of both the MAO-A and COMT inhibitor. MAO-A inhibition in conjunction with MAO-B inhibition prolonged the duration of L-DOPA-induced turning but had no effect on the number of turns. However, in conjunction with COMT inhibition, 10 mg/kg of the MAO-B inhibitor, Ro 19-6327, significantly affected both the number and duration of turning behavior. An even further potentiation of turning behavior was observed after the combined administration of all three enzyme-inhibitors.

An isocratic assay for norepinephrine, dopamine, and 5-hydroxytryptamine using their native fluorescence by high-performance liquid chromatography with fluorescence detection in discrete brain areas of rat

A rapid and simple isocratic chromatographic procedure for simultaneous determination of norepinephrine (NE), dopamine (DA), and 5-hydroxytryptamine (5-HT) using their native fluorescence by high-performance liquid chromatography coupled to fluorescence detection (HPLC-FD) is described. Since the present procedure does not involve sample prepurification, the recovery of monoamines was more than 97% (n = 12) and within a given run, coefficient of variation was less than 3.1% (n = 12). Accordingly, use of an internal standard is not mandatory. In a single chromatographic run, levels of NE, DA, and 5-HT can be determined in less than 30 min. The minimum concentration of monoamines which could be detected by this method was found to be 250 pg for NE and DA and 100 pg for 5-HT. The validity of the method was confirmed by the estimation of levels of monoamines in the hypothalamus and striatum of rat brain following treatment with clorgyline, a monoamine oxidase inhibitor.

Monoamine oxidase-dependent metabolism of dopamine in the striatum and substantia nigra of L-DOPA-treated monkeys

The effects of monoamine oxidase (MAO) inhibitors on the metabolism of dopamine synthesized from exogenous L-DOPA were investigated in the striatum and substantia nigra of squirrel monkeys. Administration of a single dose of L-DOPA (methyl ester, 40 mg/kg, i.p.) caused a significant increase in the levels of dopamine, 3-4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) and in the DOPAC/dopamine ratio in the putamen, caudate and substantia nigra. These changes were more pronounced in the substantia nigra than in the striatum and within the striatum of L-DOPA-treated monkeys, levels of dopamine and its metabolites were higher in the putamen than in the caudate nucleus. When L-DOPA treatment was preceded by the injection of clorgyline or deprenyl at a concentration (1 mg/kg) which selectively inhibited MAO A or MAO B, respectively, striatal dopamine was increased while the striatal DOPAC and HVA levels and DOPAC/dopamine ratio were significantly reduced as compared to the values obtained with 1-DOPA alone. The two MAO inhibitors also counteracted the increase in the DOPAC and HVA levels and DOPAC/dopamine ratio induced by L-DOPA in the substantia nigra. Thus, both MAO A and MAO B contribute to the metabolism of dopamine when higher levels of this neurotransmitter are generated from L-DOPA in the squirrel monkey. The extent of reduction of dopamine catabolism (as assessed by the decrease in DOPAC and HVA levels) in the striatum and substantia nigra was similar with clorgyline and deprenyl even if the ratio MAO A/MAO B was approximately 1 to 10. This indicates that, though catalyzed by both MAO A and MAO B, dopamine deamination following treatment with L-DOPA preferentially involves MAO A.

Minor effect of tolcapone, a catechol-O-methyltransferase inhibitor, on extracellular dopamine levels modified by amphetamine or pargyline: a microdialysis study in anaesthetized rats

In vivo microdialysis was used to examine the effects of tolcapone (30 mg/kg) on dopamine metabolism in amphetamine (5 mg/kg) and pargyline (75 mg/kg) treated rats. Amphetamine- or pargyline-induced decreases in the extracellular dihydroxyphenyl acetic acid (DOPAC) levels were counteracted by additional tolcapone. Tolcapone also decreased homovanillic acid effluxes below those caused by amphetamine or pargyline. However, dopamine effluxes were not further enhanced by additional tolcapone. These results show that central metabolism of dopamine can be modulated by catechol-O-methyltransferase (COMT) inhibition also without exogenous L-DOPA. However, extracellular dopamine levels are not easily increased.

Pharmacology and neuroprotective properties of rasagiline

Rasagiline [R(+)-N-propargyl-1-aminoindane] is a selective irreversible inhibitor of MAO-B which is not metabolised to amphetamine-like derivatives. Like deprenyl, when given to rats in a dose selective for inhibition of MAO-B, it does not affect striatal extracellular fluid dopamine levels, but when administered chronically (21 days) it increased striatal microdialysate dopamine without reduction in deaminated metabolites. Similarly to deprenyl, rasagiline (10(-6)M) increased the percentage of tyrosine hydroxylase positive cells in a primary culture of rat fetal mesencephalic cells (6 days in culture). Rasagiline, but not deprenyl, also increased the number of neurons per field in this organotypic culture.

Are metabolites of l-deprenyl (selegiline) useful or harmful? Indications from preclinical research

A frequent topic of controversy has been whether metabolism of l-deprenyl (selegiline) to active metabolites is a detriment to clinical use. This paper reviews possible roles of the metabolites of l-deprenyl in producing unwanted adverse side effects or in augmenting or mediating its clinically useful actions. Levels of l-amphetamine and l-methamphetamine likely to be reached, even with excessive intake of l-deprenyl, would be unlikely to produce neurotoxicity and there is no preclinical or clinical evidence of abuse liability of l-deprenyl. In contrast, there is evidence that l-amphetamine and l-methamphetamine have some qualitatively different actions than their d-isomer counterparts on EEG and cognitive functioning which might result in beneficial clinical effects and complement beneficial clinical actions of l-deprenyl itself.

The effect of selegiline on the peripheral pharmacokinetics of levodopa in young volunteers

The plasma pharmacokinetics of levodopa were studied in eight healthy young subjects following an i.v. infusion of 50 mg over 5 min. Subjects were studied on two occasions in random order following treatment with carbidopa; on one occasion they were pretreated with selegiline (four doses of 10 mg over the preceding 3 days) and on the other with a placebo. The mean plasma concentration-time curves on each occasion were essentially superimposable and there were no significant differences in any calculated pharmacokinetic parameter. Selegiline does not significantly alter the distribution or elimination of levodopa from plasma.

Pharmacology of reversible and selective inhibitors of monoamine oxidase type A

The concept of reversible type A monoamine oxidase (MAO-A) inhibitors as effective antidepressant drugs with a minimal side effect profile has been vindicated in practice. Despite this, the pharmacological basis for their actions is unclear. Studies with the irreversible inhibitor clorgyline have shown that chronic but not acute treatment of rats leads to a significant enhancement of noradrenaline release from peripheral sympathetic nerves and cerebral cortex together with a more effective inhibition of MAO-A, as shown by reduction in levels of deaminated metabolites in cortical microdialysis fluid. Reversible inhibitors, however, do not have a cumulative effect on MAO inhibition and may have different effects on noradrenaline release. Reversible inhibitors did not produce the acute reduction in sympathetic nerve activity seen with clorgyline, which may be one factor in explaining their milder side effect profile. Other aspects of the pharmacology of reversible and irreversible selective inhibitors of MAO-A are reviewed.

Evaluation of deprenyl for cocaine-like discriminative stimulus effects in rats

The antiparkinsonian agent l-deprenyl is metabolized to l-methamphetamine and l-amphetamine and, at higher doses, can facilitate the release and inhibit the reuptake of dopamine. Since l-deprenyl can affect dopamine release and reuptake it was important to evaluate it for cocaine-like discriminative stimulus effects. Male Fisher rats were trained to discriminate cocaine (10 mg/kg, i.p.) from saline in a two-lever, operant-conditioning procedure using schedules of food-delivery or stimulus-shock termination. l-Deprenyl (17 mg/kg, i.p.) produced full generalization to cocaine under the food-delivery schedule but this or higher doses produced only partial generalization to cocaine under the stimulus-shock termination schedule. d-Deprenyl produced full generalization to cocaine under both schedules at i.p. doses of 5.6 to 10 mg/kg. These cocaine-like discriminative stimulus effects occur only at doses that are well above the clinically relevant dose range for l-deprenyl.

Influence of monoamine oxidase inhibitors on striatonigral dynorphin system: a study with deprenyl and clorgyline

This study examined the influence of selected monoamine oxidase (MAO) inhibitors on basal ganglia neurotransmitters (dopamine and 5-hydroxytryptamine) and neuropeptide (dynorphin) systems of Sprague-Dawley rats. The striatum or substantia nigra or both were used for biochemical determinations. The striatal concentrations of DA, 5-hydroxytryptamine (5-HT) and their metabolites were determined by HPLC. The levels of striatal and nigral dynorphin A (1-8) (DYN) were determined by radioimmunoassay. The abundance of striatal prodynorphin (PD) mRNA was determined by Northern blot analysis using a cRNA probe. Deprenyl, a MAO-B selective inhibitor (0.25, 0.5, 5, 10 or 20 mg/kg/day, subcutaneously (s.c.) for 4 d) and clorgyline, a MAO-A inhibitor (0.5, 5, 10 or 20 mg/kg/day, s.c. for 4 d) produced a dose-related increase in DA and 5-HT and a decrease in their metabolites in the striatum. Only high doses (20 mg/kg) of deprenyl or clorgyline induced an increase in DYN levels in the striatum and substantia nigra (DYN terminal region); the increased level of DYN was accompanied by an increase in PD-mRNA levels in striatum (DYN cell-body region). Co-administration of low doses (2.5 mg/kg/day, s.c. for 4 d) of deprenyl and clorgyline, that would selectively inhibit MAO-B and MAO-A respectively, produced a marked increase in DA and 5-HT, a decrease in DOPAC and 5-HIAA, an increase in DYN levels in the striatum and substantia nigra and an increase in PD-mRNA levels in the striatum. The results indicate that concurrent inhibition of MAO-B and MAO-A, that results in markedly elevated levels of DA and 5-HT in the striatum, is associated with an increase in dynorphin biosynthesis in the striatonigral neurons.

Effects of monoamine oxidase inhibitors on the acid metabolites of some trace amines and of dopamine in the rat striatum

The effects of the administration of selective and non-selective inhibitors of monoamine oxidase (MAO) on the concentrations of three trace acid metabolites [phenylacetic acid (PAA); m-hydroxyphenylacetic acid (mHPAA); and p-hydroxyphenylacetic acid (pHPAA)] and of an acid metabolite of dopamine [3,4-dihydroxyphenylacetic acid (DOPAC)] in the rat striatum were determined. Administration of brofaromine (1-100 mg/kg, s.c.) a type AMAO inhibitor, dose-dependently decreased DOPAC and mHPAA levels. pHPAA levels were decreased by 100 mg/kg brofaromine, but PAA levels were unaffected. Doses of deprenyl of less than 100 mg/kg, i.p., had no effect on any of the acids, while 100 mg/kg decreased DOPAC, mHPAA and pHPAA but not PAA levels. Clorgyline, pargyline and tranylcypromine treatment decreased the levels of DOPAC, mHPAA and pHPAA but not PAA. Administration of alpha-monofluoromethyldopa, an inhibitor of aromatic amino acid decarboxylase, decreased the levels of all four acids. It was concluded that deamination of the respective parent amine by type A MAO is primarily responsible for the synthesis of DOPAC and mHPAA, but that another pathway contributes to pHPAA synthesis. It appears that either PAA arises predominantly independently from the actions of MAO or that is removal via transport or further metabolism regulates its concentration.

Monoamine oxidase, dopamine and Parkinson's disease

Four aspects about monoamine oxidase (MAO; E.C. 1.4.3.4) are of obvious interest in relation to Parkinson's disease and its treatment with the irreversible and selective MAO-B inhibitor L-deprenyl and are discussed in this review: 1) To what extent the two forms of MAO are of importance for the deamination of dopamine and to what degree MAO localised inside and outside of dopaminergic nerve terminals contributes 2) The kinetics of the MAO-protein, i.e. the rate of recovery of MAO after irreversible inhibition. 3) To what extent MAO may be changed as a consequence of the pathophysiological processes. 4) To what extent MAO may be involved as a force in the pathophysiological processes.

A review of the pharmacology of selegiline

Selegiline (1-deprenyl) is an irreversible inhibitor of monoamine oxidase (MAO) type B. Because in the human brain, dopamine is metabolised mainly by MAO-B, selegiline increases dopamine content in the central nervous system. Besides the inhibition of MAO-B, selegiline also inhibits the uptake of dopamine and noradrenaline into presynaptic nerve and increases the turnover of dopamine. Thanks to these properties, selegiline significantly potentiates the pharmacological effects of levodopa. These favourable characteristics have been applied in the treatment of Parkinson's disease using selegiline both with levodopa and alone. Unlike earlier MAO-inhibitors, selegiline does not potentiate the hypertensive effects of tyramine. This is due to the selectivity to MAO-B, leaving intestinal MAO-A intact, and also due to the fact that selegiline inhibits the uptake of tyramine into neurons. Selegiline can prevent the parkinsonism caused by MPTP in animals; similar findings have been reported with other toxins like 6-OHDA and DSP-4, that destroys noradrenergic nuclei. Furthermore, selegiline reduces oxidative stress caused by degradation of dopamine and increases free radical elimination by enhancing superoxide dismutase and catalase activity. These findings may be important when considering the possible neuroprotective effects of selegiline. Besides the basic pharmacology also the interactions and pharmacokinetics of selegiline are reviewed in this article.

Effect of high single doses of levodopa and carbidopa on brain dopamine and its metabolites: modulation by selective inhibitors of monoamine oxidase and/or catechol-O-methyltransferase in the male rat

The upper limits of striatal and hypothalamic dopamine formation and metabolism in the rat were defined after acute levodopa/carbidopa (100/100 mg/kg) in combination with MAO (clorgyline; 32 mg/kg or pargyline; 100 mg/kg) and/or COMT inhibitors (OR-462, OR-611, Ro 41-0960, 30 mg/kg). Striatal and hypothalamic dopa and 3-OMD levels increased several hundred times after levodopa/carbidopa treatment alone. Dopamine, DOPAC, HVA and 3-MT levels elevated also but noradrenaline and 5-HT did not. Clorgyline further increased 3-OMD, dopamine and 3-MT concentrations while DOPAC and HVA levels decreased. These changes were even more pronounced after pargyline. In the striatum, all COMT inhibitors (with levodopa/carbidopa) blocked 3-OMD formation but elevated neither dopamine nor DOPAC levels. OR-462 increased dopa levels. Only Ro 41-0960, the brain penetrating compound, blunted HVA levels. All three COMT inhibitors decreased high 3-OMD levels evoked by MAO inhibitors (+ levodopa/carbidopa). In pargyline-treated rats, COMT inhibitors did not alter dopamine, DOPAC or HVA levels but all of them decreased significantly 3-MT levels, particularly Ro 41-0960. Striatal dopamine levels increased maximally 6 times compared to those in the saline-treated controls. In the hypothalamus, COMT inhibitors decreased 3-OMD levels to 1/5-1/30 of those after levodopa/carbidopa alone. COMT inhibitors suppressed 3-OMD formation also in clorgyline and pargyline (+ levodopa/carbidopa) treated rats. After clorgyline, OR-611 and Ro 41-0960 increased high dopamine levels but only Ro 41-0960 suppressed HVA and 3-MT levels. None of the COMT inhibitors changed the high dopamine and low DOPAC levels after pargyline.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of L-dopa loading on 5-HTP decarboxylation in rat brain areas

The time course of 5-hydroxytryptophan (5-HTP), serotonin (5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) concentrations in four rat brain areas (hypothalamus, hippocampus, striatum and olfactory bulbs) were investigated after treatment with L-dopa (125 mg/kg, ip) + benserazide (50 mg/kg, ip). 5-HTP levels increased as early as 0.5 h, showed maximum accumulation at 1.5 h and returned to control levels within 4 h, while 5-HT was markedly decreased in all four structures, with a maximum effect at 1.5 h (approximately -70%) in the four areas. The decrease in 5-HT was not accompanied by changes in 5-HIAA levels. In agreement with previous studies, these data demonstrate that L-dopa loading interferes with serotonin metabolism in the rat brain. However, in addition to the releasing action of newly-synthesized dopamine, the accumulation of 5-HTP and the parallel decrease in 5-HT indicate a reduction in 5-HT synthesis. This inhibition could be explained by a competitive effect of L-dopa for aromatic aminoacid decarboxylase activity.

Effect of acute levodopa on brain catecholamines after selective MAO and COMT inhibition in male rats

Interactions between a selective catechol-O-methyltransferase (COMT) inhibitor OR-462 and a monoamine oxidase (MAO)-A inhibitor clorgyline were studied measuring concentrations of L-dopa, dopamine and their metabolites in the rat hypothalamus and striatum after administration of levodopa/carbidopa (15/30 mg/kg i.p.). Part of the experiments were performed in rats pretreated with 6-OH-dopamine (6-OHDA) intracerebroventricularly (i.c.v.) to determine whether changes in dopamine metabolism occurred inside or outside catecholaminergic neurons. OR-462 was an effective COMT inhibitor at the doses 3 and 30 mg/kg i.p. Inhibition of 3-O-methyldopa (3-OMD) formation from L-dopa was reflected in the hypothalamus (45-81% decrease) and striatum (87-88% decrease), since 3-OMD penetrates the blood-brain barrier. Homovanillic acid (HVA) was decreased only in the striatum at 30 mg/kg of OR-462. Clorgyline (8 and 32 mg/kg i.p.) decreased 3,4-dihydroxyphenylacetic acid (DOPAC) formation in the hypothalamus and striatum by 61-91%. When given together, OR-462 and clorgyline elevated hypothalamic dopamine levels 3.2-4.6-fold, but striatal dopamine only 1.3-1.9-fold. The formation of 3-OMD and DOPAC remained suppressed and even brain HVA levels were decreased by 51-97%. 6-OHDA treatment decreased striatal and hypothalamic dopamine by 50% and noradrenaline by 75%. In these animals levodopa/carbidopa increased brain L-dopa 2.4-4-fold, those of 3-OMD 1.2-1.7-fold compared to intact animals, but the synthesis and metabolism of dopamine and the effects of COMT and MAO inhibitors were not significantly changed. Levodopa/carbidopa treatment decreased significantly prolactin and thyrotropin levels in serum but none of the additional treatments changed this action.

Modification of vesicular dopamine and norepinephrine by monoamine oxidase inhibitors

The possible effects of inhibitors of the two forms of monoamine oxidase (types A and B) on dopamine (DA) and norepinephrine (NE) accumulation and metabolism in the cytoplasmic and microsomal (vesicular) fractions of the rat brain have been examined. It was found that, while L-DOPA treatment raised only cytoplasmic DA without affecting vesicular DA and NE, clorgyline and pargyline treatments caused significant increases in DA and NE concentrations in both cytoplasmic and vesicular fractions. The DA increase in the synaptic vesicles (200-600%) was much more pronounced than that (150%) in the cytoplasm. In contrast, deprenyl treatment increased vesicular DA only slightly without any effect on either vesicular or cytoplasmic NE. L-DOPA administration to rats pretreated with clorgyline and pargyline, but not with deprenyl, further increased cytoplasmic and vesicular DA and NE concentrations. However, excessive increases in vesicular DA lowered vesicular NE. Reserpine drastically reduced vesicular and cytoplasmic DA and NE, and L-DOPA administration to the reserpine-treated rats caused a DA increase only in the cytoplasmic fraction without affecting vesicular DA or NE. The effect of reserpine was abolished by pargyline treatment, which suggests that pargyline may interact with the reserpine-sensitive vesicular uptake. There was a significant correlation between vesicular DA and NE increase.

Studies on the formation of 6-hydroxydopamine in mouse brain after administration of 2,4,5-trihydroxyphenylalanine (6-hydroxyDOPA)

2,4,5-Trihydroxyphenylalanine (6-OH-DOPA) destroys central and peripheral noradrenergic neurons, while sparing dopaminergic neurons. Previous studies indicate that 6-OH-DOPA toxicity is mediated by the formation of 6-hydroxydopamine. However, levels of 6-hydroxydopamine in brain following peripheral administration of 6-OH-DOPA have not been documented. In the current study, 6-OH-DOPA and 6-hydroxydopamine were measured in brain by HPLC with electrochemical detection after intraperitoneal injection of 6-OH-DOPA. When mice were injected with 100 mg 6-OH-DOPA/kg, 6-hydroxydopamine levels in the striatum were highest (1.9 microgram/g) at 15 min and fell slowly to 24% of the peak value at 4 h. Experiments with reserpine indicated that the relatively stability of 6-hydroxydopamine was largely dependent upon storage in synaptic vesicles. Reserpine (10 mg/kg) lowered striatal 6-hydroxydopamine levels to 21.6% of control (non-reserpine-treated) values at 1 h, and to 8.9% of control values at 4 h. Levels of 6-hydroxydopamine in the striatum at 1 h were increased 113% by pargyline (100 mg/kg), 145% by alpha-methyldopahydrazine (carbidopa; 25 mg/kg), and 261% by pargyline and carbidopa together. Levels of dopamine in the striatum were unchanged at 2.5 h after 200 mg 6-OH-DOPA/kg (with pargyline and 50 mg carbidopa/kg), whereas levels of norepinephrine in the frontal cortex fell by 77%. At the same time, 6-hydroxydopamine levels were 8.8-fold higher in the striatum (5.54 micrograms/g) than in the cortex (0.63 micrograms/g).(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine turnover and glutathione oxidation: implications for Parkinson disease

Parkinson disease is characterized by a major loss (approximately 80% or more) of dopaminergic nigrostriatal neurons and by an increased turnover of neurotransmitter by surviving neurons of the nigrostriatal tract. In theory, increased turnover of dopamine should be associated with an oxidative stress derived from increased production of hydrogen peroxide. The peroxide is formed during the oxidative deamination of dopamine by monoamine oxidase. In experiments with mice, increased presynaptic turnover of dopamine was evoked by injection of reserpine, which interferes with the storage of dopamine in synaptic vesicles. Loss of dopamine and formation of deaminated metabolites were accompanied by a significant rise (87.8%) in the level of oxidized glutathione in brain. This change was observed in the striatum, which is richly innervated by dopamine terminals, but not in the frontal cortex, which receives a much sparser innervation by catecholamine nerve terminals. The rise in oxidized glutathione was seen even though dopamine terminals constitute only 1% or less of the mass of the striatum. Clorgyline, an inhibitor of monoamine oxidase type A, blocked the formation of oxidized glutathione. These observations confirm that a selective increase in neurotransmitter turnover within nigrostriatal nerve terminals can evoke a change in cellular redox status. We suggest that an oxidative stress may play a role in the natural history of Parkinson disease.

Tremorogenesis by LON-954 [N-carbamoyl-2-(2,6-dichlorophenyl) acetamidine hydrochloride]: evidence for the involvement of 5-hydroxytryptamine

LON-954 [N-carbamoyl-2-(2,6-dichlorophenyl) acetamidine hydrochloride], a novel tremorogen known to affect the central dopaminergic system, has been investigated in rats for tremor and 5-hydroxytryptamine (5-HT) metabolism. Five, 10 and 20 mg/kg of LON-954 IP caused a reproducible and consistent tremor with a high frequency (16 Hz) within 2 minutes and lasting 30-45 minutes. 5-HT content of the tuberculum olfactorium and basal ganglia was found to be increased significantly at a time when 5-hydroxyindoleacetic acid (5-HIAA) content showed a decrease. 5-HT and 5-HIAA of medulla oblongata showed significant changes only after 15 minutes. The alterations in the levels of the indoleamine in tuberculum olfactorium and its relationship with dosage as well as duration and intensity of LON-954 tremor indicate the involvement of the mesolimbic system in its action. A direct role of 5-HT in LON-tremor was evidenced since the drug failed to produce tremor in rats pretreated with p-chlorophenylalanine (300 mg/kg IP) for 3 days.