Possible mechanisms of action of (-)deprenyl and other MAO-B inhibitors in some neurologic and psychiatric disorders

Effects of prenatal and postnatal exposure to amitraz on norepinephrine, serotonin and dopamine levels in brain regions of male and female rats

The effects of maternal exposure to amitraz on brain region monoamine levels of male and female offspring rats at 60 days of age were observed. Maternal and offspring body weight, physical and general activity development were unaffected by the exposure of dams to amitraz (20mg/kgbw, orally on days 6-21 of pregnancy and 1-10 of lactation). Male and female offspring were sacrificed at 60 days of age and possible alterations in the content and metabolism of NE, DA and 5-HT were determined in brain regions by HPLC. The results showed that all these neurotransmitter systems were altered in a brain regional-related manner. In male and female offspring, amitraz induced a significant decrease in the prefrontal cortex 5-HT and its metabolite 5-HIAA and DA and its metabolites DOPAC and HVA levels with interaction of sex. Nevertheless, we verified that striatum DA and 5-HT and corresponding metabolite contents decreased in male and female offspring without statistical distinction of sex. In contrast, amitraz did not modify 5-HT content, but caused an increase in 5-HIAA content in the medulla oblongata and hippocampus in male and female offspring. Alterations in the hippocampus DA, DOPAC and HVA levels after amitraz exposure were also observed displaying a sex interaction. NE levels also showed a decrease after amitraz treatment in the prefrontal cortex and striatum without statistical sex interaction, but MHPG levels decreased in both regions with a sex interaction. Amitraz evoked increases in 5-HT turnover in the prefrontal cortex as well as in DA turnover in the striatum and hippocampus but decreases in NE turnover in the hypothalamus, prefrontal cortex and striatum. The present findings indicated that maternal exposure to amitraz altered noradrenergic, serotonergic and dopaminergic neurochemistry in their offspring in the prefrontal cortex, striatum and hippocampus, and those variations could be related to several alterations in the functions in which these brain regions are involved.

A proteomics study reveals a predominant change in MaoB expression in platelets of healthy volunteers after high protein meat diet: relationship to the methylation cycle

Studies investigating the impact of high meat intake on cognition have yielded contradictory results as some show improved cognitive performance, whereas others report an increase of risk factors for dementia. However, few studies were designed to directly assess the effect of a high protein (HP) diet on both cognitive performance and corresponding biochemical parameters. A randomised intervention study was conducted with 23 healthy males (aged 19-31 years) to investigate the effects of a usual (UP) versus a HP diet on cognitive function and on the platelet proteome a well-established model for neurons. The study individuals were assigned to either a UP diet (15% energy) or a HP diet (30% energy) for 3 weeks with controlled intake of food and beverages. Blood samples were taken along with measurements of cognitive functions at the beginning and at the end of the intervention period. Among 908 reproducibly studied platelet proteins only the level of monoamine oxidase B (MaoB), a neurotransmitter degrading enzyme, decreased by 26% significantly (adjusted P value < 0.05) due to the HP diet. In addition, we found a correlation (r = 0.477; P < 0.02) between the decrease of MaoB expression and the shortened reaction time (cognitive function) which is in accordance with reports that dementia patients show increased MaoB activity. Plasma vitamin B(12) concentration was increased by the HP diet and correlates inversely with platelet MaoB expression (r = -0.35; P < 0.02). Healthy young males on a HP diet showed improved cognitive function and counteract well-known dementia biomarkers such as platelet MaoB and components of the methylation cycle such as vitamin B(12) and homocysteine.

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.

Metabolism of N-methyl, N-propargylphenylethylamine: studies with human liver microsomes and cDNA expressed cytochrome P450 (CYP) enzymes

1. We used an in vitro screening procedure and studies with individual human liver microsomes and cDNA-expressed CYP enzymes to investigate the metabolism of the putative neuroprotective drug N-methyl,N-propargyl-2-phenylethylamine (MPPE) to N-methylphenylethylamine (N-methylPEA) and N-propargylphenylethylamine (N-propargylPEA). 2. An electron-capture gas chromatographic procedure previously developed in our laboratories was used to measure the quantities of N-methylPEA and N-propargylPEA formed in the experiments with a single donor human liver microsome panel and cDNA expressed single CYP enzyme systems. The data were fitted to nonlinear regressions using Prism to determine kinetic constants. The results from a fluorogenic screen determined which cDNA-expressed single CYP enzymes were investigated. 3. CYP2B6, CYP2C19, and CYP2D6 all contributed to the formation of N-methylPEA, while only CYP2B6 catalyzed the formation of N-propargylPEA. The K (M) and V (max) values for N-propargylPEA formation were 290 +/- 70 microM and 139+/-16 ng/mL/min. The values for formation of N-methylPEA were not determined from these experiments due to the complexity of fitting the data to a three-variable equation, but data on the time course of N-methylPEA formation are presented. 4. Catabolism of MPPE to N-methylPEA and N-propargylPEA is catalyzed by CYP enzymes. CYP2B6, 2C19 and 2D6 all contribute to the depropargylation of the parent compound, but only CYP2B6 also catalyzes demethylation. CYP2C19 was found to be the most active with respect to generation of N-methylPEA.

A systematic review on clinical management of antipsychotic-induced sexual dysfunction in schizophrenia

INTRODUCTION

Sexual dysfunction frequently occurs in patients with schizophrenia under antipsychotic therapy, and the presence of sexual side effects may affect compliance. The aim of this study was to review and describe clinical findings relating to the appropriate management of such dysfunctions.

MATERIAL AND METHODS

The research was carried out through Medline (from 1966 to March 2005), PsycInfo (from 1974 to March 2005), and Cochrane Library (from 1965 to March 2005) and included any kind of study, from case reports to randomized trials.

RESULTS

The most common sexual dysfunctions found in the literature were libido decrease, difficulties in achieving and maintaining erection, ejaculatory dysfunction, orgasmic dysfunction, and menstrual irregularities. Thirteen papers were found: eight of them were open-label studies, four were descriptions of cases, and only one was a randomized clinical trial. All of them were short-term and had small sample sizes. The agents used were: bromocriptine, cabergoline, cyproheptadine, amantadine, shakuyaku-kanzo-to, sildenafil and selegiline.

DISCUSSION

There was no evidence that those agents had proper efficacy in treating the antipsychotic-induced sexual dysfunction. An algorithm for managing sexual dysfunction induced by antipsychotics is suggested as a support for clinical decisions. Since the outcome from schizophrenia treatment is strongly related to compliance with the antipsychotics, prevention of sexual dysfunction is better than its treatment, since there is a scarcity of data available regarding the efficacy of intervention to deal with these problems.

Effects of co-administration of a selective serotonin reuptake inhibitor 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 in the case of co-administration of a monoamine oxidase (MAO) inhibitor and a serotonin reuptake inhibitor (SSRI). The goal of the present study was to investigate the effects of acute administration of MAO inhibitors and subchronic administration of fluvoxamine on 5-HT-related behaviors (head shaking and 5-HT syndrome) in rats treated with 5-hydroxytryptophan (5-HTP). Administration of the non-selective MAO inhibitor, pargyline, and the selective MAO-A inhibitor, clorgyline, resulted in 5-HT syndrome in 5-HTP-treated rats, and subchronic co-administration of fluvoxamine intensified the syndrome. However, administration of the selective MAO-B inhibitor, selegiline, did not induce 5-HT syndrome with or without subchronic fluvoxamine co-administration. These data suggest that non-selective MAO and selective MAO-A inhibitors can induce 5-HT syndrome in humans when co-administered with SSRI. Further, the risk of 5-HT syndrome may be lower with the selective MAO-B inhibitor, selegiline.

Isatin, an endogenous MAO inhibitor, and a rat model of Parkinson's disease induced by the Japanese encephalitis virus

A single dose of isatin (indole-2,3-dione)(i.p.), an endogenous MAO inhibitor, significantly increased norepinephrine and 5-hydroxytryptamine concentrations in the rat brain and also significantly increased acetylcholine and dopamine (DA) levels in the rat striatum. Urinary isatin concentrations in patients with Parkinson's disease tend to increase according to the severity of disease. We have developed a rat model of Parkinson's disease induced by the Japanese encephalitis virus (JEV). The distribution of the pathological lesions of JEV-rats resemble those found in Parkinson's disease. Significant behavioral improvement was observed in JEV-rats after isatin, L-DOPA and selegiline administration using a pole test. Both isatin and selegiline prevented the decrease in striatum DA levels of JEV-rats. The increased turnover of DA (DOPAC/DA) induced by JEV was significantly inhibited by isatin, but not selegiline. These findings suggest that JEV-infected rats may serve as a model of Parkinson's disease and that exogenously administered isatin and selegiline can improve JEV-induced parkinsonism by increasing DA concentrations in the striatum.

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.

Disclosure of a pro-apoptotic glyceraldehyde-3-phosphate dehydrogenase promoter: anti-dementia drugs depress its activation in apoptosis

Overexpression and subsequent nuclear accumulation of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is involved in neuronal apoptosis induced by several stimuli in which GAPDH antisense oligonucleotides specifically block the increment (2 approximately 3 fold) of GAPDH mRNA contents occurring prior to neuronal death. However, these agents do not affect the basal, constitutive mRNA contents. This suggests that there may be distinct gene regulations for GAPDH mRNA expression. Herein, we cloned two types of promoter regions upstream of this gene; viz., #104 (1.02-kb) and #302 (2.46-kb). These fragments were inserted into the pGL3 luciferase reporter system and transiently transfected into cultured cerebellar neurons undergoing cytosine arabinonucleoside-induced apoptosis. The functional analysis of these constructs revealed that #104, but not #302, increased luciferase activity in response to the apoptotic stimulus. Deletion and replacement mutation analysis of the #104 fragment disclosed the promoter core harbored between the 154-bp and 84-bp domains (3.5-fold activity of the control). Furthermore, anti-dementia drugs (such as Cognex and Aricept) markedly depress the expression of this pro-apoptotic GAPDH promoter activity. Interestingly, immunocytochemical examination of human post-mortem materials from patients with Alzheimer's disease revealed nuclear aggregated GAPDH in neurons of the affected brain regions, implying an association with apoptotic cell death. The current findings indicate that induction of the pro-apoptotic protein GAPDH is genetically regulated at the level of promoter activation, and this protein may be an important molecular target for developing anti-apoptotic therapeutic agents in certain neurological illnesses.

Comparative Study of the Effects of Isatin, an Endogenous MAO-Inhibitor, and Selegiline on Bradykinesia and Dopamine Levels in a Rat Model of Parkinson's Disease Induced by the Japanese Encephalitis Virus

We previously reported that exogenously administered isatin, an endogenous monoamine oxidase (MAO) inhibitor, significantly increased acetylcholine (ACh) and dopamine (DA) levels in the rat striatum. Selegiline [(-)-deprenil] was developed as a MAO-B inhibitor more than 30 years ago and widely used in the treatment of Parkinson's disease. Effects of isatin or selegiline were investigated in Japanese encephalitis virus (JEV)-induced post-encephalitic parkinsonism rats by a pole test for detecting motor activity and by the determination of biogenic amine levels. Motor activity of JEV-induced rats receiving isatin (100 mg/kg per day for 1 week, i.p.) or selegiline (0.2 mg/kg per day for 1 week, i.p.) was significantly improved compared with that of untreated JEV-infected rats. Both isatin and selegiline prevented the decrease in striatal DA levels in JEV-rats. The increased turnover of DA (DOPAC/DA) induced by JEV was significantly inhibited by isatin, but not by selegiline. These results suggested that exogenously administered isatin and selegiline can improve JEV-induced parkinsonism by increasing DA concentrations in the striatum.

Transdermal selegiline: targeted effects on monoamine oxidases in the brain

BACKGROUND

The oral administration of monoamine oxidase (MAO) inhibitors has the potential to cause a hypertensive reaction after the ingestion of tyramine-containing compounds. Because transdermal drug administration bypasses gastrointestinal absorption, it is possible that inhibition of MAO-A in brain may be achieved without enzyme inhibition in the gastrointestinal system, thereby eliminating the possibility of this drug interaction. These studies determined whether the transdermal administration of selegiline has differential effects on MAOs in brain versus the gastrointestinal system.

METHODS

Rats were exposed to various doses of selegiline via a transdermal patch for up to 30 days, and MAO-A and MAO-B activities were determined in brain regions and gastrointestinal tissue.

RESULTS

In all brain regions, transdermal selegiline, at doses that produced maximal MAO-B inhibition, led to a dose- and time-dependent MAO-A inhibition. The inhibition of MAOs in gastrointestinal tissue was less than that in brain, and doses that produced maximal MAO-A inhibition in brain inhibited MAO-A in gastrointestinal tissue by only 30%-40%.

CONCLUSIONS

Results suggest that transdermal selegiline preferentially inhibits MAO-A in brain relative to the gastrointestinal system. As a consequence, transdermal selegiline should be devoid of the potential to cause a hypertensive reaction after the ingestion of tyramine-containing compounds.

Selegiline in the treatment of sexual dysfunction in schizophrenic patients maintained on neuroleptics: a pilot study

A double-blind, placebo-controlled crossover study was undertaken in 10 neuroleptic-treated male schizophrenic outpatients to assess the effect of coadministration of selegiline 15 mg/day for 3 weeks on their sexual dysfunction. Selegiline was not found to be effective in improving any domain of sexual functioning despite a significant decrease in prolactin levels (P < 0.05). This study emphasizes the complex nature of sexual dysfunction in schizophrenic-treated patients and the need for placebo-controlled trials for this condition.

SELEDO: a 5-year long-term trial on the effect of selegiline in early Parkinsonian patients treated with levodopa

The SELEDO (from selegiline plus levodopa) study was carried out as a randomized, prospective, placebo-controlled, double- blind, multicenter long-term, 5-year trial to evaluate the possible advantages of combining selegiline and levodopa in the early treatment of Parkinson's disease. One-hundred-and-sixteen patients were randomized either to selegiline or placebo. Before starting the study medication, the levodopa dose was titrated to the individual requirements of each patient. The primary study end point (time when levodopa had to be increased by >50% of the titrated dose) was reached in 23 of 59 patients in the selegiline group and 26 of 48 patients in the placebo group. At the end of the 5 years' treatment period the rates derived from a life-table analysis were 50.4% in the selegiline group and 74.1% in the placebo group (P = 0.027, log-rank test). The median time to reach the primary end point was 4.9 years in the selegiline group and 2.6 years in the placebo group. In patients treated with selegiline, the mean levodopa dose changed only slightly over the 5 years of treatment compared to the initially titrated dose, but rose markedly in the placebo group, where the dose of levodopa had to be adjusted earlier than in the selegiline group. At the same time, the lower levodopa dosage in the selegiline group was accompanied by at least equal therapeutic efficacy (which is necessary for an unambiguous interpretation). Subgroup analyses showed greater benefit for selegiline treated) patients in the earlier stages. Long-term side effects appeared later in the selegiline group, although the difference was not significant. The early combination of selegiline and levodopa proved to be clearly superior to levodopa monotherapy.

N8-acetyl spermidine protects rat cerebellar granule cells from low K+-induced apoptosis

The endogenous polyamines have been extensively studied with respect to their role in cellular death mechanisms, although the results are contradictory. In contrast, their primary metabolites, the N-acetyl polyamines, have not been much studied. It has been hypothesized that the N-acetyl metabolites may play a role in cellular death mechanisms, and some of the variability between different reports may be due to altered polyamine metabolic capacities. Using primary cultures of rat cerebellar granule cells, the effects of N-acetyl metabolites have been examined on basal, cytosine beta-D-arabinofuranoside (Ara-C)-induced and low K+-induced apoptosis. None of the compounds affected either basal or Ara-C-induced apoptosis at low doses. At higher doses, all compounds were toxic. Two compounds, N8-acetyl spermidine and N1-acetyl spermine, were found to protect cells from low K+-induced apoptosis, which has been shown to be p53-independent. In contrast, the parent polyamines were devoid of protective activity at subtoxic doses. This represents the first time that an antiapoptotic effect of N-acetyl polyamines has been demonstrated. These results raise the possibility that these compounds may act as endogenous neuroprotectants. The lack of effect on basal apoptosis provides evidence of at least two forms of p53-independent apoptosis that can be regulated independently.

Prolongation of life in an experimental model of aging in Drosophila melanogaster

(R)-Deprenyl, the archetypical monoamine oxidase-B inhibitor, has been shown to increase life-span in a number of species. Although many theories for this effect have been suggested, for example, an increase in superoxide dismutase (SOD) activity, the mechanism of action has yet to be elucidated. To investigate this phenomenon, we have examined the effects of (R)-deprenyl, and some aliphatic propargylamines, in an experimental aging model in Drosophila melanogaster. Both wild-type Oregon-R type flies, as well as a SOD knock-out mutant strain were used. Flies obtained from a series of paired mates were divided equally among treatment groups. In all studies, flies were treated for the duration of life following adult emergence. The aging model consists of substitution of sucrose with galactose in the regular food media of the flies. Initial experiments confirmed that such a substitution resulted in a significant (p < 0.01, Breslow test) reduction in mean and maximal life-span of flies, an effect not due to nutrient deprivation. Inclusion of (R)-deprenyl and the aliphatic propargylamines in the media, at average daily doses in the range 0.5-1 ng/fly/day, led to a significant increase in mean and maximal life-span of galactose-treated, but not control flies. This effect was seen in both wild-type and mutant flies.

Neuronal sparing and behavioral effects of the antiapoptotic drug, (-)deprenyl, following kainic acid administration

(-)Deprenyl is an irreversible inhibitor of monoamine oxidase B (MAO-B) frequently used as an adjunct therapy in the treatment of Parkinson's Disease. Recent evidence, however, has found that deprenyl's metabolites are associated with an antiapoptotic action within certain neuronal populations. Interestingly, deprenyl's antiapoptotic actions appear not to depend upon the inhibition of MAO-B. Due to a paucity of information surrounding (-)deprenyl's ability to spare neurons in vivo, a series of studies was conducted to further investigate this phenomenon within an apoptotic neuronal death model: kainic acid induced excitotoxicity. Results indicated that (-)deprenyl increased hippocampal neuronal survival compared to saline-matched controls following kainic acid insult. Furthermore, it was discovered that (-)deprenyl treatment could be stopped 14 days following CNS insult by kainate, with evidence of neuronal sparing still present by day 28. In open-field locomotor activity testing of kainate-treated animals, those given subsequent (-)deprenyl treatment showed habituation curves similar to control subjects, while saline-treated animals did not. Given deprenyl's antiapoptotic actions, it is proposed that (-)deprenyl may be beneficial in the treatment of a variety of neurodegenerative diseases where evidence of apoptosis exists, such as Parkinson's and Alzheimer's Disease, by slowing the disease process itself.

Anticonvulsant efficacy of L-deprenyl (selegiline) during chronic treatment in mice: continuous versus discontinuous administration

We have previously reported that L-deprenyl (selegiline), an irreversible inhibitor of monoamine oxidase type B (MAO-B), exerts anticonvulsant activity against different seizure types in mice and rats. The anticonvulsant effect of L-deprenyl was rapid in onset but short lasting, arguing in favor of other, reversible mechanisms of L-deprenyl as a basis for the anti-seizure activity. For further evaluation, we administered L-deprenyl continuously via subcutaneously implanted osmotic minipumps in mice and determined the threshold for myoclonic seizures induced by i.v. infusion of pentylenetetrazol repeatedly during prolonged treatment, with treatment periods lasting from 2 to 4 weeks. For comparison with continuous administration via minipumps, L-deprenyl was injected once daily at a dose (10 mg/kg) known to produce complete and irreversible inhibition of MAO-B and anticonvulsant effects after acute administration in rodents. Continuous administration of L-deprenyl, 50 or 100 mg/kg per day, led to a progressive increase in seizure threshold in the absence of any observable adverse effects, while administration of 10 mg/kg per day via minipumps was devoid of any significant anticonvulsant effect. When 10 mg/kg were administered once daily in the afternoon for 4 weeks and the seizure threshold was determined repeatedly in the morning, no significant anticonvulsant effect was observed. The data argue against a critical role of MAO-B inhibition in the anticonvulsant activity of L-deprenyl but suggest that other, reversible biochemical and cellular effects known to occur at higher doses of this drug are involved in this respect. In view of the short half-life of L-deprenyl, these reversible effects can only be maintained during chronic treatment when the drug is given continuously such as via implanted minipumps.

Dopamine-degrading activity of monoamine oxidase in locus coeruleus and dorsal raphe nucleus neurons. A histochemical study in the rat

Dopamine-degrading activity of monoamine oxidase (MAO) was detected in the rat using a new histochemical method, with dopamine as the substrate. Our new method, designed to minimise the non-enzymatic oxidation of dopamine, was applied in combination with tyrosine hydroxylase (TH) and serotonin immunohistochemistry. We showed that the distribution pattern of MAO neurons was similar to that of TH-immunoreactive neurons (i.e. noradrenergic neurons) in the locus coeruleus (LC) and to that of serotonergic neurons in the dorsal raphe nucleus (DR). Since LC neurons form dopamine during noradrenaline biosynthesis, and DR neurons produce dopamine from exogenously administered L-dopa, our results indicate that dopamine produced in LC and DR neurons may be degraded, at least in part, by MAO.

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

Utilizing the cerebral microdialysis technique, we have compared in vivo the effects of selective MAO-A, MAO-B, and nonselective MAO inhibitors on striatal extracellular levels of dopamine (DA) and DA metabolites (DOPAC and HVA). The measurements were made in rats both under basal conditions and following L-DOPA administration. Extracellular levels of dopamine were enhanced and DA metabolite levels strongly inhibited both under basal conditions and following L-DOPA administration by pretreatment with the nonselective MAO inhibitor pargyline and the MAO-A selective inhibitors clorgyline and Ro 41-1049. The MAO-B inhibitor deprenyl had no effect on basal DA, HVA, or DOPAC levels. Nevertheless, deprenyl significantly increased DA and decreased DOPAC levels following exogenous L-DOPA administration, a finding compatible with a significant glial metabolism of DA formed from exogenous L-DOPA. We conclude that DA metabolism under basal conditions is primarily mediated by MAO-A. In contrast, both MAO-A and MAO-B mediate DA formation when L-DOPA is administered exogenously. The efficacy of newer, reversible agents which lack the "cheese effect" such as Ro 41-1049 are comparable to the irreversible MAO-A inhibitor clorgyline. The possible relevance of these findings for the treatment of Parkinson's disease is discussed.

The renal monoamine oxidases: pathophysiology and targets for therapeutic intervention

The mitochondrial enzyme monoamine oxidases A and B are among the major metabolic agents for the degradation of the biogenic amines adrenaline, noradrenaline, dopamine and serotonin. The fact that the kidney contains a large amount of monoamine oxidase suggests that the renal effects of biogenic amines might depend in part on the activity of these enzymes.

The effect of L-deprenyl, D-deprenyl and MDL72974 on mitochondrial respiration: a possible mechanism leading to an adaptive increase in superoxide dismutase activity

L-Deprenyl is an irreversible monoamine oxidase-B inhibitor with a complex pharmacological profile. For instance, L-deprenyl administration to rat and mice increases cytosolic CuZn- and mitochondrial Mn-superoxide dismutase activities in the striatum. CuZn- and Mn-superoxide dismutase are enzymes involved in defense against superoxide (O2.) radicals. Hence, an increase in CuZn- and Mn-superoxide dismutase activities is suggestive of oxidative stress. The major intracellular site of O2. radicals formation is the mitochondrial respiratory chain. Several reports indicated that alterations in mitochondrial respiratory functions enhances O2. production. We observed that L-deprenyl induced a dose-dependent inhibition of oxygen (O2) consumption (state 3) during ATP synthesis in presence of complex I (pyruvate and malate) and complex II (succinate) substrates in fresh mitochondrial preparations. D-Deprenyl produced a similar inhibitory profile whereas MDL72974, a selective monoamine oxidase-B inhibitor, was less effective. Administration of D-deprenyl or MDL72974 to mice resulted in an increase in both striatal CuZn- and -Mn-superoxide dismutase activities. Accordingly, we propose that the impairment of mitochondrial respiratory functions which stimulates O2. formation could modulate CuZn- and Mn-superoxide dismutase activities, through a mechanism that appears to be independent of monoamine oxidase-B inhibition.

Increased activity and expression of tyrosine hydroxylase in the rat substantia nigra after chronic treatment with nomifensine

We have studied the effect of chronic treatment with nomifensine on dopaminergic functioning in the nigrostriatal system. The striatal dopaminergic system was not altered by chronic nomifensine treatment. In contrast, there were overall decreases of different dopamine (DA) metabolites in the cell body region in the substantia nigra after nomifensine treatment, which clearly indicates a diminished DA turnover. These results suggest that long-lasting inhibition of the high affinity DA uptake system triggers long term regulatory, compensatory mechanisms in the cell body region to preserve normal dopaminergic function in the terminal field in striatum. We also tested whether transcriptional regulatory mechanisms were altered. We studied the cellular expression of tyrosine hydroxylase (TH) mRNA in substantia nigra by in situ hybridization, and the amount and activity of TH enzyme in the cell body and terminal field regions. Our results indicate that nomifensine treatment increased TH mRNA levels within individual nigral cells, which paralleled the changes in TH enzyme amount and activity in this brain area. Our data confirm the important role of the high affinity DA uptake system in regulating dopaminergic transmission in the nigrostriatal system.

Selegiline percutaneous absorption in various species and metabolism by human skin

PURPOSE

A Selegiline Transdermal System (STS) is under development for indications which may not be optimally or safely treated with oral selegiline. Studies were conducted to evaluate the in vitro penetration and skin metabolism of selegiline in order to better understand the toxicological findings and the observed plasma levels of selegiline and its metabolites in animals and man.

METHODS

In vitro penetration studies were conducted in four different species (male hairless mice, male and female rats, female dog and male Micropig) and compared to human skin. In another study, viable human skin was used to estimate the extent of metabolism of selegiline by human skin using Franz diffusion cells.

RESULTS

Results indicated that female dog and male Micropig skin were reasonable animal models for 24 hour in vitro selegiline penetration through human skin. Penetration of selegiline through rat skin and hairless mouse skin was 2-fold and 3-fold higher than through human skin, respectively. Metabolism was negligible in human skin. Selegiline metabolites (L-methamphetamine and N-desmethylselegiline but not L-amphetamine) were detected at all time points but the extent of selegiline metabolism was negligible. The cumulative 24 hour in vitro selegiline permeation from a 1.83 mg/cm2 STS through human skin was 5.0 mg. This was similar to the in vivo permeation in humans as assessed by residual patch analysis.

CONCLUSIONS

The similarity of dog and human skin permeation results support the use of the dog as a species for evaluating the toxicology of transdermally-administered selegiline. Selegiline is not metabolized cutaneously and hence the metabolic profile following STS administration is likely due to hepatic metabolism only.

Aromatic L-amino acid decarboxylase: a neglected and misunderstood enzyme

Classically, aromatic L-amino acid decarboxylase (AADC) has been regarded as an unregulated, rather uninteresting enzyme. In this review, we describe advances made during the past 10 years, demonstrating that AADC is regulated both pre- and post-translation. The significance of such regulatory mechanisms is poorly understood at present, but the presence of tissue specific control of expression raises the real possibility of AADC being involved in processes other than neuro-transmitter synthesis. We further discuss clinical and physiological situations in which such regulatory mechanisms may be important, including the intriguing possibility of AADC gene regulation being linked to that of factors thought to have a role in apoptosis and its prevention.

Dopamine produced from L-DOPA is degraded by endogenous monoamine oxidase in neurons of the dorsal raphe nucleus of the rat: an immunohistochemical study

The aim of the present study is to examine by immunohistochemistry whether dopamine produced from L-DOPA in serotonin neurons of the rat brain is degraded by endogenous monoamine oxidase (MAO). In rats that received intraperitoneally L-DOPA plus a peripheral decarboxylase inhibitor, carbidopa, a cluster of dopamine-immunoreactive neurons was found in the dorsal raphe nucleus (DR). In L-DOPA/carbidopa-injected rats that were pretreated with an intraperitoneal injection of a MAO inhibitor, pargyline, when compared with the L-DOPA/carbidopa-injected rats without the pargyline pretreatment, neurons of the cluster of the DR became much darker in dopamine staining. The distribution of dopamine-stained neurons in the DR of these rats corresponded very closely to the previously reported distribution of serotonin-immunoreactive neurons of normal rats. In normal or only pargyline-injected rats, dopamine-stained neurons were scarcely observed in the DR. We previously showed that serotonin neurons of the rat DR were induced to contain dopamine by the injection of L-DOPA plus carbidopa. These findings suggest that the newly produced dopamine from L-DOPA in serotonin neurons of the rat DR is degraded by endogenous MAO.

The effects of L-deprenyl on spatial short term memory in young and aged dogs

1. Young and aged dogs were tested on a spatial memory task using a delayed non matching to sample technique. Dogs were tested with 20, 70 and 110 second delay intervals. Animals were pretrained to a stable level of performance prior to treatment. 2. During treatment periods, dogs were orally administered a placebo or I-deprenyl in doses of 0.5 and 1.0 mg/kg in a repeated measures design. 3. Young dogs did not show any significant effects of I-deprenyl, however the sample size was limited. 4. L-deprenyl administration improved spatial memory in aged dogs. 5. The optimal dose or length of treatment time of I-deprenyl varied among individual dogs.

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.