Inhibition Of Aromatic Amino Acid Decarboxylase Research Articles

Carbidopa/levodopa/entacapone: the evidence for its place in the treatment of Parkinson’s disease

Parkinson's disease (PD) is a common neurodegenerative disease. In the 1960s, it was shown that the degeneration of dopamine producing neurons in the substantia nigra (SN) caused the motor features of PD. Dopamine replacement with levodopa, a dopamine precursor, resulted in remarkable benefit. Yet, the intermittent administration of levodopa is a major cause of motor complications, such as "wearing-off" of levodopa's benefit and involuntary movements, known as dyskinesia. Therefore, agents that prolong levodopa's half-life were employed, such as carbidopa, an aromatic amino acid decarboxylase (AADC) inhibitor, and entacapone, a catechol-O-methyltransferase (COMT) inhibitor. The combination product carbidopa/levodopa/entacapone (CLE) was approved in 2003 for the treatment of PD patients. To assess the evidence for the place of CLE in the treatment of PD. CLE has a good efficacy, safety and tolerability profile, similar to that of entacapone taken separately with carbidopa/levodopa (CL). Compared to CL alone, it prolongs levodopa's benefit, and improves the quality of life but not the motor performance in PD patients with nondebilitating "wearing-off" or dyskinesia. However, it increases the dyskinesia rate in early PD patients, and has adverse events in advanced patients with significant motor complications. There is insufficient evidence regarding cost-effectiveness. CLE is an attractive alternative for patients with nondisabling "wearing-off" or dyskinesia taking CL with or without entacapone. It cannot be recommended for early PD patients, as it can induce more dyskinesia than CL alone, or in any patients who seem to have more adverse events.

Lack of differential serotonin biosynthesis capacity in genetically selected low and high aggressive mice

Reduced brain serotonin (5-HT) activity has been linked to impulsive and violent forms of aggression for decades. Despite a vast accumulation of data pertinent to the above observation, information about the possible mechanisms underlying such a decreased 5-HT functioning is virtually absent. Amongst many, reduced 5-HT biosynthetic capacity is a likely possibility in violent individuals and/or in high-aggressive animals. In order to examine this hypothesis, the current study principally aimed at the determination and comparison of the 5-HT biosynthetic capacity in three different strains of high- and low-aggressive mice obtained by artificial genetic selection. While low Tryptophan Hydroxylase (TPH) activity can be expected to lead to low 5-HT levels and pathological aggression, high TPH activity can be expected to increase 5-HT levels and normal territorial aggression. The above hypothesis was assessed by estimating the in-vivo synthesis rate and synthesis rate constant of 5-HT biochemically by measuring the accumulation of 5-hydroxytryptophan (5-HTP) following treatment with the central aromatic amino-acid decarboxylase inhibitor 3-hydroxybenzylhydrazine (NSD-1015). Surprisingly, we found no differences in the 5-HT biosynthetic capacity between the high- and low-aggressive selection lines in their prefrontal cortices and raphe nuclei, two main brain regions closely involved in aggression control. Thus, the underlying inherent genetic differences in aggressiveness observed in these artificially selected mouse strains are not due to constitutive functional differences in their TPH activity in these brain regions.

The Pharmacokinetics and Pharmacodynamics of Levodopa in the Treatment of Parkinsons Disease

Levodopa, a prodrug of dopamine, remains to be one of the main drugs in the treatment of Parkinson's disease. All current levodopa products are formulated with aromatic amino acid decarboxylase inhibitors such as carbidopa or benserazide to prevent the metabolism of levodopa in the gastrointestinal tract and systemic circulation. Levodopa pharmacokinetic profiles remain unchanged after multiple doses, and are similar between healthy volunteers and patients and among patients at different stages of disease. Entacapone inhibits the metabolism of levodopa therefore increases the area under the plasma concentration-time profile of levodopa; however, it may decrease the initial absorption rate of levodopa in some patients probably due to competitive absorption. Food appears to affect the absorption of levodopa, but its effects vary with formulations. The results of positron emission tomography study suggest that a high protein diet may compete with the uptake of levodopa into the brain, therefore, may result in reduced levodopa effects. Since infusion studies demonstrated that it is beneficial to maintain stable plasma concentrations of levodopa, controlled-release formulations have been designed to provide prolonged absorption of levodopa. However, subsequent pharmacokinetic and pharmacodynamic studies demonstrated that a threshold concentration of levodopa appears to be necessary to switch patients "on". Once patients are turned "on", the duration of levodopa effects may be correlated with plasma concentration of levodopa. As such, more recent studies have demonstrated significant clinical benefits such as shorter time to "on" and longer duration of "on" when combining the immediate- and controlled-release levodopa products as compared to controlled-release levodopa products. Given these findings, it is important for physicians to understand the relationship between the pharmacokinetics and pharmacodynamics of levodopa in order to provide dosage regimens that meet patient needs. The pharmacokinetics and pharmacodynamics data of levodopa reported in the literature are reviewed here.

Anxiolytic effect of serotonin depletion in the novelty-induced hypophagia test

Relatively little is known about the neural mechanisms underlying anxiety in the novelty-induced hypophagia test, the only known anxiety test that is responsive to chronic but not acute or subchronic antidepressant treatment. The goal of the present experiment was to characterize the role of serotonin in the ability of novelty to suppress feeding. Pair-housed male Sprague-Dawley rats were trained to eat graham cracker crumbs individually in their home cage (15 min/day). After stable daily intakes were obtained, the animals were depleted of serotonin using 4-chloro-DL -phenylalanine (150 mg kg(-1) day(-1) x 2 days). Forty-eight hours later, central serotonin was restored by the administration of the peripheral L -aromatic amino acid decarboxylase inhibitor, benserazide (10 mg/kg), followed 15 min later with the immediate precursor of serotonin, 5-hydroxy-L -tryptophan (30 mg/kg). Thirty minutes later, the animals were given access to graham cracker crumbs in a novel environment. The animals demonstrated increased latencies to approach the food and reduced food intake in the novel environment. This effect was attenuated by serotonin depletion. Repletion of central serotonin restored the inhibitory response to novelty. The analysis of serotonin content in different brain regions confirmed that serotonin was depleted by greater than 90%, whereas the repletion treatment resulted in serotonin levels similar to nondepleted animals. Acute depletion of serotonin acts to reduce anxiety behavior as measured by an inhibitory anxiety response during exposure to novel stimuli. These findings are in agreement with the proposed general role for serotonin in behavioral inhibition and that reductions of serotonin facilitate the adoption of more active coping responses to stress.

Metabolism of 3-Nitrotyrosine Induces Apoptotic Death in Dopaminergic Cells

Intrastriatal injection of 3-nitrotyrosine, which is a biomarker for nitrating oxidants, provokes dopaminergic neuronal death in rats by unknown mechanisms. Herein, we show that extracellular 3-nitrotyrosine is transported via the l-aromatic amino acid transporter in nondopaminergic NT2 cells, whereas in dopaminergic PC12 cells, it is transported by both the l-aromatic amino acid and the dopamine transporters. In both cell lines, 3-nitrotyrosine is a substrate for tyrosine tubulin ligase, resulting in its incorporation into the C terminus of alpha-tubulin. In NT2 cells, incorporation of 3-nitrotyrosine into alpha-tubulin induces a progressive, reversible reorganization of the microtubule architecture. In PC12 cells, 3-nitrotyrosine decreases intracellular dopamine levels and is metabolized by the concerted action of the aromatic amino acid decarboxylase and monoamine oxidase. Intracellular levels of 133 micromol of 3-nitrotyrosine per mole of tyrosine did not alter NT2 viability but induced PC12 apoptosis. The cell death was reversed by caspases and aromatic amino acid decarboxylase and monoamine oxidase inhibitors. 3-Nitrotyrosine induced loss of tyrosine hydroxylase-positive primary rat neurons, which was also prevented by an aromatic amino acid decarboxylase inhibitor. These findings provide a novel mechanism by which products generated by reactive nitrogen species induce dopaminergic neuron death and thus may contribute to the selective neurodegeneration in Parkinson's disease.

Effects of Manganese on Extracellular Levels of Dopamine in Rat Striatum: An Analysis In vivo by Brain Microdialysis

The aim of this study is to determine the effects of intrastriatal administration of MnCl2, on the extracellular levels of dopamine (DA) and metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in basal conditions and stimulated by depolarization with KCl and pargyline administration. Also, we studied the effect of MnCl2 on extracellular levels of l-Dopa in the presence of aromatic amino acid decarboxylase (AADC) inhibitor 3-hydroxybencilhydracine-HCl (NSD 1015). This study concluded that MnCl2, reduced the basal and K+-stimulated DA-release in striatum, without notably affecting the DOPAC and HVA levels. Intraperitoneal injection of pargyline increased striatal DA levels, decreasing DOPAC and HVA levels. The infusion of MnCl2 removed the increase in DA levels, without affecting DOPAC and HVA levels. Perfusion of NSD 1015 increased the extracellular levels of L-DOPA in striatum, and MnCl2 increased the effect of NSD1015 on L-Dopa.

Commonly used L-amino acid decarboxylase inhibitors block monoamine oxidase activity in the rat.

The effects of the peripheral aromatic amino acid decarboxylase (AADC) inhibitors, carbidopa and benserazide, and the central AADC inhibitor, 3-hydroxybenzylhydrazine (NSD-1015) on peripheral and brain monoamine oxidase (MAO) A and B activity were investigated in the rat. In vitro, carbidopa, benserazide and NSD-1015 all potently inhibited hepatic MAO A and B activity (IC(50) 10-50 micro M). In ex vivo studies following systemic drug administration, NSD-1015 (100 mg/kg ip) produced 88% and 96% inhibition of hepatic and striatal MAO A and B activity respectively. Carbidopa (12.5 mg/kg i.p.) and benserazide (50 mg/kg i.p.) had no effect on striatal MAO A activity or hepatic MAO B activity. However, they inhibited striatal MAO B activity by 45 +/- 10% and 36 +/- 10% respectively. In conclusion, carbidopa and benserazide may not only protect L-DOPA from peripheral decarboxylation, but also increase striatal dopamine content through MAO inhibition. NSD-1015 should not be used to investigate the neuromodulatory role of L-DOPA as it potently inhibits rat striatal MAO.

The central catechol-O-methyltransferase inhibitor tolcapone increases striatal hydroxyl radical production in L-DOPA/carbidopa treated rats.

Inhibition of catechol catechol-O-methyltransferase (COMT) in the brains of subjects treated with L-DOPA (L-3,4-dihydroxylphenylalanine) and an aromatic amino acid decarboxylase (AADC) inhibitor is suggested to cause an increase of L-DOPA, which might lead to oxidative damage through enhanced formation of free radicals. To investigate this hypothesis, the acute effects of two doses of the systemically administered COMT inhibitors entacapone (peripheral) and tolcapone (peripheral and central) on the extracellular formation of hydroxyl radicals in vivo following treatment with L-DOPA and the AADC inhibitor carbidopa were examined. The formation of extracellular hydroxyl radicals were determined by the measurement of 2,3-dihydroxybenzoic acid (2,3-DHBA), a reaction product of hydroxyl radicals with sodium salicylate, using microdialysis in the striatum of anesthetised rats. The COMT inhibitors were administered together with 50 mg/kg i.p. carbidopa as 5% gum arabic suspensions intraperitoneally (i.p.) at doses of 0, 1.0, and 10 mg/kg body weight to a total of 36 male HAN-Wistars rats. L-DOPA was injected i.p. 40 min after drugs of interest. Microdialysis samples were collected every 20 min for 400 min at a perfusion rate of 1 microl/min. Systemically administered 10 mg/kg tolcapone, but not entacapone, induced an increase in hydroxyl radical formation in the striatum of anesthetised rats following treatment with L-DOPA/carbidopa. The increase in hydroxyl radical formation was reflected by higher extracellular concentrations of the hydroxylate product of salicylate, 2,3-DHBA, peaking at 192% of baseline at the end of the observation period. Similar results were also found using the AUC (area under the curve) value estimated for the observation period. We conclude that the increase in hydroxyl radical formation is likely to result from an increased rate of monoamine oxidase-mediated and non-enzymatic (autoxidation) dopamine metabolism following increased central availability caused by reduction in COMT-mediated metabolism. We cannot, however, exclude the possibility that hydroxyl radicals are produced by tolcapone as a result of uncoupling mitochondrial oxidative phosphorylation.

BIA 3-202, a novel catechol- O-methyltransferase inhibitor, enhances the availability of l-DOPA to the brain and reduces its O-methylation

1-[3,4-Dihydroxy-5-nitrophenyl]-2-phenyl-ethanone (BIA 3-202) is a new long-acting catechol- O-methyltransferase (COMT) inhibitor with limited access to the brain. The present study evaluated the interference of BIA 3-202 upon levels of l-3,4-dihydroxyphenylalanine ( l-DOPA) and metabolites in plasma (3- O-methyl- l-DOPA) and brain [3- O-methyl- l-DOPA, dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA)] in rats orally treated with l-DOPA (20 mg/kg) plus benserazide (30 mg/kg). At different time points (1, 3 and 6 h) after the administration of BIA 3-202 (0, 3, 10 and 30 mg/kg) or l-DOPA plus benserazide, rats were sacrificed and the right striatum was quickly dissected out and stored for the assay of l-DOPA, 3- O-methyl- l-DOPA, dopamine and amine metabolites. Levels of l-DOPA, 3- O-methyl- l-DOPA, dopamine, DOPAC and HVA in the striatum in l-DOPA plus benserazide-treated rats were higher than in vehicle-treated rats. However, this increase in striatal l-DOPA, dopamine, DOPAC and HVA was, in a dose- and time-dependent manner, even higher ( P<0.05) in rats given BIA 3-202 (3, 10 and 30 mg/kg). This effect was accompanied by a marked decrease in 3- O-methyl- l-DOPA levels in the striatum of l-DOPA plus benserazide-treated rats. Increases in levels of l-DOPA and decreases in 3- O-methyl- l-DOPA levels in plasma also accompanied the administration of BIA 3-202. BIA 3-202 did not significantly affect levels of DOPAC and HVA in the striatum in vehicle-treated rats. It is concluded that administration of BIA 3-202 enhances the availability of l-DOPA to the brain by reducing its O-methylation in the periphery, which may prove beneficial in parkinsonian patients treated with l-DOPA plus an aromatic amino acid decarboxylase inhibitor.

Inhibition by l-3,4-dihydroxyphenylalanine of hippocampal CA1 neurons with facilitation of noradrenaline and γ-aminobutyric acid release

Electrophysiological studies were performed to elucidate whether l-3,4-dihydroxyphenylalanine ( l-DOPA) acted on hippocampal CA1 neurons, since this drug has been reported to act as a neurotransmitter in the hypothalamus and striatum. Hippocampal slices (450 μm thick) obtained from male Wistar rats (4–7 weeks of age) were placed in a bath (maintained at 30±1°C) continuously perfused with artificial cerebrospinal fluid. The population spikes elicited by electrical stimuli applied to the Schaffer collateral/commissural fibers were recorded in the hippocampal CA1 region, using a glass micropipette filled with 3 M NaCl. Drugs were applied in the bath through a perfusion system. The population spikes were inhibited by l-DOPA (1 nM–10 μM) with a bell-shaped concentration–response curve ( n=7–15). Maximum inhibitory effects were obtained at 100 nM. l-DOPA cyclohexyl ester, a putative l-DOPA recognition site antagonist, antagonized the l-DOPA-induced inhibition of population spike. However, the inhibition remained unaffected in the presence of 3-hydroxybenzylhydrazine, an aromatic amino acid decarboxylase inhibitor. Furthermore, bath application of either phentolamine, an α-adrenoceptor antagonist, or bicuculline, a GABA A receptor antagonist, antagonized the inhibitory effects of l-DOPA on population spikes. In addition, bicuculline (1 μM) antagonized the inhibition of population spike induced by 6-fluoronorepinephrine (10 μM), an α-adrenoceptor agonist, while phentolamine (10 μM) did not affect the muscimol (1 μM)-induced inhibition. These results suggested that l-DOPA itself acted on l-DOPA recognition sites to release noradrenaline, and that the latter facilitates γ-aminobutyric acid (GABA) release via α-adrenoceptors located on the GABA-containing cells and/or their nerve terminals, thereby inhibiting the population spikes in the hippocampal CA1 field.

Serum leptin levels after central and systemic injection of a serotonin precursor, 5-hydroxytryptophan, in mice

Effects of peripheral and central injections of a serotonin (5-hydroxytryptamine, 5-HT) precursor, 5-hydroxytryptophan (5-HTP), on serum leptin levels were studied in mice. Intracerebroventricular (i.c.v.) injection of 5-HTP or 5-HT did not increase serum leptin levels, although the peripheral injection of 5-HTP elicited an apparent hyperleptinemia. The elevation of serum leptin levels in mice induced by the peripheral injection of 5-HTP was inhibited by pretreatment with the peripheral aromatic amino acid decarboxylase inhibitor, benserazide. Furthermore, the peripheral injection of 5-HT increased serum leptin levels. These results suggest that the hyperleptinemia following systemic injection of 5-HTP is elicited by 5-HT formed in the peripheral system.

Increasing CNS norepinephrine levels by the precursor l-DOPS facilitate beam-walking recovery after sensorimotor cortex ablation in rats

The present investigation was conducted to document a role of l-threo-3,4-dihydroxyphenylserine ( l-DOPS), precursor of l-norepinephrine (NE), in the functional recovery from beam-walking performance deficits in rats after unilateral sensorimotor cortex ablation. l-DOPS was administered simultaneously with benserazide (BSZ; a peripheral aromatic amino acid decarboxylase inhibitor), and the regional contents of NE in the cerebral cortex, hippocampus, and cerebellum were assayed. Behavioral recovery was demonstrated by the rats treated with l-DOPS and BSZ, and the rate of recovery was significantly different from that of either BSZ-treated or vehicle-treated control rats. The NE tissue levels in the three discrete regions of the rat brain were significantly elevated in the experimental rats receiving both l-DOPS and BSZ. The present studies indicate that increasing NE levels by the precursor l-DOPS may be responsible for facilitating behavioral recovery from beam-walking performance deficits in rats, and further suggest that l-DOPS may become one of the candidate compounds for further clinical human trials promoting functional recovery after injuries to the cerebral cortex.

The serotonin precursor 5-hydroxytryptophan elevates serum leptin levels in mice

The effects of a serotonin (5-HT) precursor 5-hydroxytryptophan (5-HTP) on serum leptin levels were investigated in mice. 5-HTP dose dependently increased serum leptin levels in mice. Pretreatment of the peripheral aromatic amino acid decarboxylase inhibitor carbidopa suppressed 5-HTP-induced hyperleptinemia. These results suggest that the secretion of leptin may be modified by serotonergic mechanisms.

L-DOPS-Accelerated recovery of locomotor function in rats subjected to sensorimotor cortex ablation injury: pharmacobehavioral studies.

Central norepinephrine (NE) has been shown to play a beneficial role in amphetamine-facilitated recovery of behavior. To give insight into understanding the mechanism, the present studies were conducted to examine (a) the effects of L-threo-3,4-dihydroxyphenylserine (L-DOPS) combined with benserazide (BSZ; a peripheral aromatic amino acid decarboxylase inhibitor) and L-3,4-dihydroxyphenylalanine (L-DOPA), precursors of NE and dopamine (DA), respectively, on the recovery from beam-walking performance deficits in rats subjected to unilateral sensorimotor cortex ablation injury, and (b) the relationships between the behavioral recovery and the frequency of postoperative training and the size of ablation injury. It was found that the combined treatments with L-DOPS and BSZ promoted the recovery of locomotor function as early as 24 hours after injury. L-DOPA alone, however, did not facilitate behavioral recovery. The results of assay for the tissue levels of NE and its major metabolite (3-methoxy-4-hydoxyphenylethylene glycol; MHPG) in the brain using high-pressure liquid chromotography showed MHPG, but not NE, significantly increased in the cerebellum and the hippocampus. The behavioral recovery was also significantly correlated with the frequency of training subsequent to injury, but inversely with the size of cortex ablation. These results suggest that NE is likely to modulate functional recovery in this rodent model.

Retrovirus-mediated double transduction of the GTPCH and PTPS genes allows 6-pyruvoyltetrahydropterin synthase-deficient human fibroblasts to synthesize and release tetrahydrobiopterin.

The tetrahydrobiopterin (BH4) cofactor is essential for the aromatic amino acid hydroxylases that are involved in phenylalanine degradation and catecholamine and serotonin biosynthesis. Furthermore, BH4 is an essential and limiting cofactor for all types of nitric oxide synthases. BH4 deficiency results in hyperphenylalaninemia and monoamine neurotransmitter depletion associated with progressive mental retardation and is most commonly due to autosomal recessive mutations in 6-pyruvoyltetrahydropterin synthase (PTPS), the second enzyme for cofactor biosynthesis. Due to the relatively poor blood-brain barrier penetration of the cofactor, conventional therapy requires, besides oral doses of synthetic BH4, administration of neurotransmitter precursors and an aromatic amino acid decarboxylase inhibitor. The outcome of this therapy is not always beneficial. In this study we transduced into primary patient fibroblasts the human cDNAs for the BH4 biosynthetic enzymes GTP cyclohydrolase I and PTPS, expressed from different retroviral vectors. This allowed BH4 biosynthesis in originally PTPS-deficient cells. Moreover, the double-transduced fibroblasts released between 200 and 800 pmol of BH4/10(6) cells/day. Such engineered fibroblasts may be grafted into the central nervous system and used as depository cells for constitutive delivery of BH4.

Neurotrophic effects of L-DOPA in postnatal midbrain dopamine neuron/cortical astrocyte cocultures.

L-DOPA is toxic to catecholamine neurons in culture, but the toxicity is reduced by exposure to astrocytes. We tested the effect of L-DOPA on dopamine neurons using postnatal ventral midbrain neuron/cortical astrocyte cocultures in serum-free, glia-conditioned medium. L-DOPA (50 microM) protected against dopamine neuronal cell death and increased the number and branching of dopamine processes. In contrast to embryonically derived glia-free cultures, where L-DOPA is toxic, postnatal midbrain cultures did not show toxicity at 200 microM L-DOPA. The stereoisomer D-DOPA (50-400 microM) was not neurotrophic. The aromatic amino acid decarboxylase inhibitor carbidopa (25 microM) did not block the neurotrophic effect. These data suggest that the neurotrophic effect of L-DOPA is stereospecific but independent of the production of dopamine. However, L-DOPA increased the level of glutathione. Inhibition of glutathione peroxidase by L-buthionine sulfoximine (3 microM for 24 h) blocked the neurotrophic action of L-DOPA. N-Acetyl-L-cysteine (250 microM for 48 h), which promotes glutathione synthesis, had a neurotrophic effect similar to that of L-DOPA. These data suggest that the neurotrophic effect of L-DOPA may be mediated, at least in part, by elevation of glutathione content.

Reduced striatal tyrosine hydroxylase activity is not accompanied by change in responsiveness of dopaminergic receptors following chronic treatment with deprenyl

Deprenyl is the only selective monoamine oxidase B (MAO-B) inhibitor that is in clinical use for the treatment of Parkinson's disease. Our previous studies showed that chronic treatment of rats with low (MAO-B selective) doses of deprenyl inhibited dopamine (DA) re-uptake and enhanced DA release in the striatum. These changes could affect DA synthesis rate by activation of negative feedback loops. Chronic deprenyl treatment has also been suggested to cause down-regulation of release-modulating DA receptors. The effects of chronic and acute treatment with deprenyl on ex vivo striatal tyrosine hydroxylase activity were therefore studied, by determination of steady-state tissue level of DOPA following administration of NSD-1015 (100 mg/kg i.p.). In addition, we assessed changes in the in vivo sensitivity of dopaminergic receptors from the reduction in DOPA extracellular level after systemic apomorphine administration (2.5 mg/kg s.c.), following elevation of microdialysate DOPA by systemic or local aromatic amino acid decarboxylase inhibition with NSD-1015. Chronic treatment with deprenyl (0.25 mg/kg s.c. daily for 21 days) caused a significant reduction in tyrosine hydroxylase activity to 60% of control, with no change in the apomorphine-induced reduction of microdialysate DOPA and DOP AC. The reduction in tyrosine hydroxylase activity is compatible with our previous results showing an increase in striatal DA extracellular level following chronic treatment with deprenyl. The increased extracellular striatal DA level could reduce tyrosine hydroxylase activity through activation of a negative feedback loop, by activation of either presynaptic or postsynaptic DA receptors.

Influence of an Aromatic Amino Acid Decarboxylase Inhibitor on GTP Cyclohydrolase I Activity in the Rat Brain

Summary Based on the hypothesis that catecholamine decreases may cause increases in biopterin, we measured the GTP cyclohydrolase I(GTPCH-I) activity and biopterin content in the cerebral cortex, midbrain/diencephalon, cerebeilum, pons/medulla oblongata, striatum , and hippocampus of brains from rats given NSDlOIS, an inhibitor for aromatic amino acid decarboxylase . The catecholamine contents decreased in all the regions tested. The changes in the GTPCH -I activity and bioptcrin content were different among the regions; they decreased in some regions contrary to expectations and increased in other regions. However, either the enzyme activity or biopterin content increased in the midbrain/ diencephalon and pons/ medulla oblongata, in which similar levels of catecholamines, GTPCH -I activity and biopterin were detected. These results suggest that for the midbrain/diencephalon and pons/medulla oblongata the surmise that a decrease in catecholamines causes an increase in the biopterin synthesis may be correct.

Cerebral 6-[ 18F]fluoro- l-DOPA uptake in rhesus monkey: pharmacological influence of aromatic amino acid decarboxylase (AAAD) and catechol- O-methyltransferase (COMT) inhibition

FDOPA/PET scans were performed in one rhesus monkey to study the influence of three catechol- O-methyltransferase (COMT) inhibitors (CGP 28014, OR-611 and Ro 40-7592) on FDOPA pharmacokinetics. COMT inhibitors were administered in combination with carbidopa, a peripherally acting inhibitor of the aromatic amino acid decarboxylase (AAAD). FDOPA was administered intravenously and its metabolic fate in plasma was determined using an HPLC system with an on-line γ- γ coincidence detector. Cerebral tracer uptake was assessed in the striatum and in a non-dopaminergic brain region (occipital cortex). In the periphery, the pharmacokinetic efficiency of FDOPA was increased due to the combined inhibition of COMT and AAAD activity. All three COMT inhibitors reduced the FDOPA methylation rate constant in plasma, with complete suppression obtained in the case of Ro 40-7592. In the brain, specific 18F radioactivity (striatal minus brain reference radioactivity) increased as a result of the increase in FDOPA plasma availability following the administration of COMT and AAAD inhibitors. We established a significant linear correlation between striatal radioactivity and FDOPA plasma levels ( r=0.924±0.048, P<0.0001 for total striatal and r=0.948±0.054, P<0.0001 for specific striatal radioactivity). Using plasma FDOPA radioactivity as input, we found that the striatal FDOPA uptake rate constant K i FD was not changed by any of the inhibitors. Thus, the enhancement of striatal radioactivity after application of enzyme inhibitors is a consequence of the increase in plasma FDOPA that becomes available for conversion to fluorodopamine in the striatal dopaminergic nerve terminals. By contrast, using the radioactivity in a non-dopaminergic region (cortex) as input, we found that the striatal FDOPA uptake rate constant K i ref was significantly ( P<0.0001) increased following pretreatment with COMT inhibitors. Our analysis demonstrated that K i ref and the 3-OMFD contribution to the cerebral radioactivity were inversely correlated.

Catechol O-Methyltransferase Inhibition and the Treatment of Parkinson's Disease

Publisher Summary Catechol O-methyltransferase (COMT) plays an important role in the metabolism of the dopamine precursor, l -dopa, and of dopamine. Inhibitors of COMT could alter the metabolism of L-dopa and dopamine to therapeutic advantage by a number of mechanisms in L-dopa-treated patients with Parkinson's disease (PD). The introduction of entacapone and tolcapone, reversible inhibitors of COMT that are well absorbed and well tolerated, has permitted investigation of the clinical importance of COMT in Parkinson's disease. Entacapone is primarily a peripheral inhibitor of COMT; tolcapone penetrates the blood brain barrier and inhibits brain COMT as well as peripheral COMT. This chapter discusses l -dopa absorption. COMT is in high concentration in the gut and liver and may contribute to first-pass metabolism of levodopa. Inhibition of gut and liver COMT would then be expected to increase the bioavailability of orally administered l -dopa. Pharmacokinetically, this should be manifest by an increase in peak concentration of plasma l -dopa and a decrease in the time-to-peak concentration. Neither entacapone nor tolcapone increases the peak concentration of levodopa or shortens the time-to-peak concentration in subjects with PD. These results are in contrast to the effect of inhibition of first-pass decarboxylation of l -dopa; the aromatic amino acid decarboxylase inhibitors, carbidopa and benserazide, markedly increase the peak plasma L-dopa concentrations. These observations argue that first-pass O-methylation of l -dopa is not quantitatively important. To the extent that dyskinesia and, perhaps, some mental effects are related to peak concentrations of l -dopa, this lack of effect on peak concentrations by COMT inhibitors may be beneficial. The chapter also discusses L-dopa elimination, Antiparkinsonian effects of COMT inhibition, l -dopa transport into brain and inhibition of brain COMT. Potential benefit of inhibition of brain COMT is sparing of S- adenosylmethionine, the methyl donor for O-methylation reactions. Low cerebrospinal fluid S-adenosylmethionine concentrations have been linked to depression and dementia.