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

The present study aims at determining the effects of the catechol-O-methyltransferase (COMT) inhibitor BIA 3-202 [1-(3,4-dihydroxy-5-nitrophenyl)-2-phenyl-ethanone] upon levels of L-3,4-dihydroxyphenylalanine (L-DOPA) and metabolites in peripheral circulation (jugular vein), whole brain, and striatal microdialysates in rats orally treated with L-DOPA plus benserazide. A low dose (3 mg/kg) of BIA 3-202 was relatively selective to inhibit liver COMT, being devoid of major significant inhibitory effects upon brain COMT. By contrast, a high dose (30 mg/kg) of BIA 3-202 markedly inhibited liver and brain COMT. BIA 3-202 (3 and 30 mg/kg) reduced the L-DOPA-induced rise of 3-O-methyl-L-DOPA in the peripheral circulation (jugular vein), brain tissue, and striatal dialysate, but failed to increase the levels of dopamine in striatal dialysates despite the increase in dopamine brain levels. However, the changes in brain levels of L-DOPA, 3-O-methyl-L-DOPA, and dopamine and in striatal dialysate levels of L-DOPA and 3-O-methyl-L-DOPA, obtained with 3 mg/kg BIA 3-202, were not different from those obtained with 30 mg/kg BIA 3-202. In conclusion, inhibition of peripheral COMT by BIA 3-202 may suffice to improve the availability of L-DOPA to the brain.     

Catechol-O-methyltransferase inhibition in erythrocytes and liver by BIA 3-202 (1-[3,4-dibydroxy-5-nitrophenyl]-2-phenyl-ethanone)

The present study evaluated the relationship between the degree of catechol-O-methyltransferase (COMT) inhibition in erythrocytes and liver by BIA 3-202 (1-[3,4-dihydroxy-5-nitrophenyl]-2-phenyl-ethanone) and determined its effects upon the O-methylation of L-DOPA in rats orally treated with L-DOPA plus benserazide. The soluble form of COMT (S-COMT) in erythrocytes was endowed with the same affinity as liver S-COMT for the substrate adrenaline. BIA 3-202 inhibited erythrocytes and liver S-COMT with ED50's of 1.9 (0.7, 3.1) and 1.9 (0.5, 3.2) (95% confidence limits) mg kg(-1), respectively. BIA 3-202 reduced the L-DOPA-induced rise of 3-O-methyl-L-DOPA in the peripheral circulation, striatal dialysate levels and striatum, and increased dopamine striatal levels. In BIA 3-202-treated rats the increase in L-DOPA in peripheral blood and striatal dialysates was significantly greater than in vehicle-treated rats. It is concluded that S-COMT activity in erythrocytes may provide important information on the pharmacodynamic profile of COMT inhibitors. The novel COMT inhibitor BIA 3-202 is a potent COMT inhibitor that enhances the availability of L-DOPA to the brain by reducing its O-methylation, which may prove beneficial in patients with Parkinson's disease treated with L-DOPA.     

The effect of benserazide on the peripheral and central distribution and metabolism of levodopa after acute and chronic administration in the rat.

1. The effects of levodopa alone (50 mg kg-1) and levodopa (10 mg kg-1) plus benserazide (50 mg kg-1) were tested on the levels of dopa, dopamine, 3-methoxytyrosine (3-MT), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), measured by h.p.l.c. with electrochemical detection, in samples of plasma, CSF, urine, striatum and hypothalamus of rats taken 30 min after injection. Levodopa plus benserazide produced significantly higher levels of dopa in plasma and brain than levodopa alone and reduced the peripheral synthesis and metabolism of dopamine. 2. When given chronically over 6 weeks the advantages of adding benserazide (50 mg kg-1 day-1) to levodopa (40 mg kg-1 day-1) were less marked and although more dopamine was present in the striatum than with levodopa given alone (200 mg kg-1 day-1) there was no evidence of any increase in its metabolites (HVA and DOPAC) and therefore of its turnover and utilisation. 3. The most striking effect of chronic treatment with levodopa plus benserazide was the appearance of large quantities of 3-MT in plasma, CSF and brain. 4. When levodopa alone, or levodopa plus benserazide, was given as an acute challenge to animals receiving the same treatment chronically, it was found that levodopa alone still produced increases in striatal dopamine, DOPAC and HVA in those animals dosed chronically on levodopa, but it was less effective in this respect when given with benserazide to the animals dosed with levodopa plus benserazide.(ABSTRACT TRUNCATED AT 250 WORDS)     

Regulation of the synthesis and metabolism of striatal dopamine after disruption of nerve conduction in the medial forebrain bundle.

1. After physical (knife-cut) or chemically-mediated (tetrodotoxin 300 nM, 1.5 microliters; 1.0 microliters min-1) interruption of nerve conduction in the nigrostriatal tract, there was a marked increase in the synthesis and metabolism of dopamine in the isolated dopaminergic nerve terminals of the striatum. The effect peaked at 4 h post-transection, at which time 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were increased by 300% and 700% respectively (DOPAC: 27 +/- 13 vs 80 +/- 17 nmol g-1; HVA: 6.66 +/- 3.57 vs 54 +/- 18 nmol g-1). The increases in dopamine content and metabolism are secondary to an increase in the rate of synthesis on the lesioned side, versus the intact, control side. 2. In both experimental situations, haloperidol (1.0 mg kg-1, i.p.) retained its known ability to induce a significant increase in DOPAC and HVA in the striatum, despite the interruption of nerve conduction in the nigrostriatal tract. 3. Six days after cutting the left nigrostriatal tract, dopamine in the left striatum was reduced to less than 5% of the control value, and DOPAC and HVA were not detectable. In the denervated, left striatum, the synthesis of dopamine (from injected L-DOPA), and its metabolism to DOPAC and HVA, occurred to the same degree as in the intact right side. In these DOPA-treated rats, haloperidol (1.0 mg kg-1, i.p.) caused a further increase in DOPAC and HVA in the intact striatum, but not in the denervated striatum.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of apomorphine on the in vivo release of dopamine and its metabolites, studied by brain dialysis

The effect of apomorphine (0.05-0.5 mg/kg s.c.) on the release of endogenous dopamine and extracellular levels of the metabolites 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5HIAA) were examined in vivo by intracerebral dialysis. A dialysis tube was implanted stereotaxically through both caudate nuclei of rats and perfused with Ringer solution at a rate of 2 microliters/min. The amount of dopamine, DOPAC, HVA and 5HIAA in the perfusates was measured by HPLC with electrochemical detection. With the dialysis tube implanted into the striatum of anaesthetized rats it was possible to measure basal levels of dopamine and the metabolites in the perfusates; dopamine, 0.27 +/- 0.05 pmol/20 min (n = 15), DOPAC 43.3 +/- 2.57 pmol/20 min (n = 15), HVA 24.5 +/- 1.89 pmol/20 min (n = 15) and 5HIAA 13.9 +/- 1.77 pmol/20 min (n = 15). The % recoveries of the monoamines through the membrane were estimated to be 12% (dopamine), 21% (DOPAC), 23% (HVA) and 25% (5HIAA). Apomorphine 0.05-0.2 mg/kg decreased the spontaneous release of dopamine by a maximum of approximately 50%. When the dose of apomorphine was raised up to 0.5 mg/kg there was a 100% inhibition of dopamine release. Also, the extracellular levels of the metabolites DOPAC and HVA decreased following apomorphine administration; however there was no consistent change in 5HIAA. These findings indicate that the dopamine autoreceptors decrease dopamine release in vivo by 0-50% while larger decreases probably involve postsynaptic neurons engaging short- as well as long-loop reflexes.     

Assessment of renal dopaminergic system activity during cyclosporine A administration in the rat.

1. Administration of cyclosporine A (CsA; 50 mg kg-1 day-1, s.c.) for 14 days produced an increase in both systolic (SBP) and diastolic (DBP) blood pressure by 60 and 25 mmHg, respectively. The urinary excretion of dopamine, DOPAC and HVA was reduced from day 5-6 of CsA administration onwards (dopamine from 19 to 46%, DOPAC from 16 to 48%; HVA from 18 to 42%). In vehicle-treated rats, the urinary excretion of dopamine and DOPAC increased (from 7 to 60%) from day 5 onwards; by contrast, the urinary excretion of HVA was reduced (from 27 to 60%) during the second week. 2. No significant difference was observed between the Vmax and Km values of renal aromatic L-amino acid decarboxylase (AAAD) in rats treated with CsA for 7 and 14 days or with vehicle. 3. Km and Vmax of monoamine oxidase types A and B did not differ significantly between rats treated with CsA for 7 and 14 days or with vehicle. 4. Maximal catechol-O-methyltransferase activity (Vmax) in homogenates of renal tissues obtained from rats treated with CsA for 7 or 14 days was significantly higher than that in vehicle-treated rats; Km (22.3 +/- 1.5 microM) values for COMT did not differ between the three groups of rats. 5. The accumulation of newly-formed dopamine and DOPAC in cortical tissues of rats treated with CsA for 14 days was three to four times higher than in controls. The outflow of both dopamine and DOPAC declined progressively with time and reflected the amine and amine metabolite tissue contents. No significant difference was observed between the DOPAC/dopamine ratios in the perifusate of renal tissues obtained from CsA- and vehicle-treated rats. In addition, no significant differences were observed in k values or in the slope of decline of both DA and DOPAC between experiments performed with CsA and vehicle-treated animals. 6. The Vmax for the saturable component of L-3,4-dihydroxyphenylalanine (L-DOPA) uptake in renal tubules from rats treated with CsA was twice that of vehicle-treated animals. Km in CsA- and vehicle-treated rats did not differ. 7. The decrease in the urinary excretion of sodium and an increase in blood pressure during CsA treatment was accompanied by a reduction in daily urinary excretion of dopamine. This appears to result from a reduction in the amount of L-DOPA made available to the kidney and does not involve changes in tubular AAAD, the availability of dopamine to leave the renal cells and dopamine metabolism.(ABSTRACT TRUNCATED AT 400 WORDS)     

Enantiomeric differences in the effects of 3,4-methylenedioxymethamphetamine on extracellular monoamines and metabolites in the striatum of freely-moving rats: an in vivo microdialysis study.

The effects of (+) and (-) 3,4-methylenedioxymethamphetamine (MDMA) and racemic p-chloroamphetamine (PCA) on extracellular dopamine and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), as well as the metabolite of 5-hydroxytryptamine (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), were determined in dialysates of the striatum conscious rats by using intracerebral dialysis and high performance liquid chromatography with electrochemical detection (HPLC-EC). The (+) and (-)MDMA isomers (5, 10 mg/kg, s.c.) and PCA (2.5, 5 mg/kg, s.c.) caused a rapid increase of extracellular levels of dopamine and decreased extracellular levels of DOPAC and HVA immediately after administration in dialysates of striatum. The order of potency for this effect was PCA greater than (+)MDMA greater than (-)MDMA. The levels of 5-HIAA also decreased after the administration of drugs, but the effect had a slower time course than DOPAC and HVA and did not exhibit an enantiomeric difference. The data indicate that, although these drugs are thought to affect the 5-HT neuronal system preferentially, they also affect dopamine systems and by a mechanism in which the (+) isomer was more potent than the (-).     

The metabolism of systemically-administered L-dihydroxyphenylalanine, by intact and dopamine-denervated striata, as revealed by brain microdialysis

Idiopathic Parkinson's Disease arises from the progressive loss of dopamine (DA)-utilizing neurons of the nigrostriatum and responds to the replacement of DA with L-dihydroxyphenylalanine (L-DOPA). In awake rats, with unilateral lesions induced with 6-hydroxydopamine (6-OHDA) of the DA-utilizing nigrostriatal pathway, treatment with L-DOPA causes the rapid onset of brisk contralateral turning behaviour. In urethane-anesthetized rats with identical unilateral lesions of the nigrostriatum, dialysis of the striatum, performed before and after the systemic administration of L-DOPA (25 mg/kg i.p.), did not demonstrate any alteration in extracellular DA in the striatum which was DA-deprived compared to intact striata. After treatment with L-DOPA extracellular levels of the metabolites of DA. DOPAC and HVA increased several fold. These results suggest: (a) DA neurons surviving after extensive lesions with 6-OHDA can compensate for loss of DA in the striatum and maintain extracellular fluid (and presumably synaptic) concentrations of DA; (b) in striata with extensive depletion of DA L-DOPA undergoes rapid decarboxylation to DA, followed by catabolism to DOPAC and HVA; and (c) in urethane-anesthetized animals, DA formed from DOPA does not appear to enter a releasable pool.     

Comparison of the effects of deramciclane, ritanserin and buspirone on extracellular dopamine and its metabolites in striatum and nucleus accumbens of freely moving rats

In the present study, we assessed the effect of single graded doses of a putative anxiolytic compound, the 5-HT(2A/C )antagonist, deramciclane fumarate (EGIS-3886), on the dopamine efflux and metabolism in nucleus accumbens and striatum and thus evaluated the dose window for deramciclane to cause adverse effects related to the brain dopaminergic system. Dual probe in vivo microdialysis in freely moving rats was used to compare the effects of graded doses of deramciclane fumarate (3, 10 and 30 mg/kg), 5-HT(2A/C )antagonist ritanserin (1 mg/kg) and a partial 5-HT(1A) agonist buspirone hydrochloride (5 mg/kg) on the extracellular levels of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in nucleus accumbens and striatum assayed by high performance liquid chromatography with electrochemical detection. The indirect dopamine agonist, D-amphetamine sulfate (2 mg/kg), was used as a positive control. Ritanserin, buspirone and deramciclane 3 and 10 mg/kg had no significant effects on the extracellular dopamine levels in either brain area but deramciclane 30 mg/kg significantly increased accumbal dopamine as well as DOPAC and HVA in both brain areas. As expected, the positive control D-amphetamine significantly increased both striatal and accumbal dopamine levels. The effects of buspirone or the highest deramciclane dose and D-amphetamine on DOPAC and HVA levels were opposite; buspirone and deramciclane increased while D-amphetamine decreased the metabolite levels in both brain areas. The results indicate that a single high dose of deramciclane has the neuroleptic- or buspirone-like effect, particularly in mesolimbic regions. There is at least a 5-fold margin between the anxiolytic and neuroleptic doses of deramciclane in the rat.     

Release of dopamine is reduced by diazepam more in the nucleus accumbens than in the caudate nucleus of conscious rats

The effects of 1-20 mg/kg diazepam were studied on the extracellular concentrations of dopamine (DA), dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the nucleus accumbens and striatum of conscious rats, using intracerebral microdialysis. Five, but not 1 mg/kg diazepam significantly reduced extracellular DA, DOPAC and HVA in the nucleus accumbens. Twenty mg/kg diazepam significantly reduced extracellular DA, DOPAC and HVA in the striatum. A significant effect on striatal DOPAC, but not on DA and HVA, was seen with 10 mg/kg diazepam, while no changes were found with 5 mg/kg diazepam. The results suggest that diazepam reduces the release and metabolism of DA in the nucleus accumbens more than in the striatum.     

MK801 influences L-DOPA-induced dopamine release in intact and hemi-parkinson rats

In vivo microdialysis was used to investigate the influence of dizocilpine (MK801) on basal and levodopa (L-DOPA)-induced extracellular dopamine levels in striatum and substantia nigra of intact and 6-hydroxydopamine-lesioned rats. In lesioned rats, extracellular dopamine was decreased in striatum but not in substantia nigra. L-DOPA (25 mg/kg i.p. after benserazide 10 mg/kg i. p.) increased the dopamine levels in striatum and substantia nigra of intact and dopamine-depleted rats. This increase was significantly higher in dopamine-depleted compared to intact striatum. Pretreatment with MK801 (0.1 and 1.0 mg/kg i.p.) dose-dependently attenuated the L-DOPA-induced dopamine release in substantia nigra of intact rats. In dopamine-depleted striatum, MK801 enhanced L-DOPA-induced dopamine release. The present results indicate that the influence of MK801 on L-DOPA-induced dopamine release in striatum and substantia nigra depends on the integrity of the nigrostriatal pathway. In Parkinson's disease, NMDA receptor antagonists could be beneficial by enhancing the therapeutic efficacy of L-DOPA at the level of the striatum.     

Does brain 3,4-dihydroxyphenylacetic acid reflect dopamine release?

After sulpiride (75 mg kg-1) administration, a significant increase in dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) occurred in the rat striatum. No attenuation in the sulpiride-induced dopamine and DOPAC increase was seen in the striatum from rats previously treated with benztropine (25 mg kg-1). On the other hand, in the benztropine + sulpiride-treated group, HVA levels were significantly lower compared with those of rats to which only sulpiride had been given. The results presented suggest that under appropriate conditions, as when sulpiride is used, dopamine can be deaminated before its release has occurred.     

Behavioral and neurochemical effects induced by chronic L-DOPA administration

L-DOPA, in combination with benserazide, in the ratio 4:1 (w/w), was administered orally to rats. In the staircase maze test a low dose of L-DOPA (3 mg/kg/day) reduced the increase in errors caused by 20 days interruption of daily training, while a higher dose (30 mg/kg/day) was ineffective. A decrease in levels of dopamine in the olfactory system and DOPAC in the striatum was seen at all tested doses of L-DOPA, while an increase in 5-HT levels was seen in the hippocampus and in the striatum. 5-HIAA levels did not change. Levels of ACh in the olfactory system were reduced at all doses of L-DOPA, while in the hippocampus this effect was seen only at the dose of 90 mg/kg/day. The density of muscarinic receptors was not altered. AI1 tested doses of L-DOPA caused norepinephrine levels to fall in the hippocampus and increase in the striatum. The density of α₁-adrenoceptors was reduced only at the two lower doses of L-DOPA. A comparison of the neurochemical results with the behavioral modifications seen in the staircase maze test suggests that the catecolaminergic systems are implicated in the memory process.     

Mexidol potentiates antiparkinsonian effect of L-DOPA in MPTP-induced parkinsonism model]

Preliminary peroral administration of mexidol into mice with (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine)-induced parkinsonism model prevented the formation of malonaldehyde in striatum. In combination with small doses of L-DOPA (2-mg/kg), mexidol decreased oligokinesia and muscular rigidity manifestations and increased the levels of dopamine and its metabolites--dioxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in striatum. The combined (mexidol + L-DOPA) drug administration produced a 3-5 h increase in the time of the parkinsonism syndrome inhibition as compared to that upon the administration of a greater L-DOPA dose (100 mg/kg) without mexidol. It is suggested that mexidol, possessing antioxidant properties, is capable of preventing the damage and loss of the nigrostriatial dopaminergic neurons.     

Catechol‐O‐methyltransferase Inhibition in Erythrocytes and Liver by BIA 3‐202 (1‐[3,4‐dibydroxy‐5‐nitrophenyl]‐2‐phenyl‐ethanone)

Abstract: The present study evaluated the relationship between the degree of catechol‐O‐methyltransferase (COMT) inhibition in erythrocytes and liver by BIA 3‐202 (1‐[3,4‐dihydroxy‐5‐nitrophenyl]‐2‐phenyl‐ethanone) and determined its effects upon the O‐methylation of L‐DOPA in rats orally treated with L‐DOPA plus benserazide. The soluble form of COMT (S‐COMT) in erythrocytes was endowed with the same affinity as liver S‐COMT for the substrate adrenaline. BIA 3‐202 inhibited erythrocytes and liver S‐COMT with ED50's of 1.9 (0.7, 3.1) and 1.9 (0.5, 3.2) (95% confidence limits) mg kg^?1, respectively. BIA 3‐202 reduced the L‐DOPA‐induced rise of 3‐O‐methyl‐L‐DOPA in the peripheral circulation, striatal dialysate levels and striatum, and increased dopamine striatal levels. In BIA 3‐202‐treated rats the increase in L‐DOPA in peripheral blood and striatal dialysates was significantly greater than in vehicle‐treated rats. It is concluded that S‐COMT activity in erythrocytes may provide important information on the pharmacodynamic profile of COMT inhibitors. The novel COMT inhibitor BIA 3‐202 is a potent COMT inhibitor that enhances the availability of L‐DOPA to the brain by reducing its O‐methylation, which may prove beneficial in patients with Parkinson's disease treated with L‐DOPA.     

Deamination of norepinephrine, dopamine, and serotonin by type A monoamine oxidase in discrete regions of the rat brain and inhibition by RS-8359

Levels of monoamines and their metabolites were determined in the cortex, hippocampus, and striatum of rats killed by microwave irradiation. Moclobemide (20 mg/kg, p.o.) and clorgyline (10 mg/kg, p.o.), type A monoamine oxidase (MAO-A) inhibitors, increased the levels of normetanephrine (NM) and 3-methoxytyramine (3MT) and decreased those of 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5HIAA) in almost all three regions. Deprenyl (10 mg/kg, p.o.), a type B monoamine oxidase inhibitor, however, little affected monoamine and metabolite levels in all regions. The maximum effects of RS-8359 (10 mg/kg, p.o.) were obtained at 2 to 6 hr after administration, when the levels of norepinephrine (NE), NM, 3MT, and serotonin (5HT) in all regions and dopamine (DA) in the striatum increased, while DOPAC and HVA levels decreased. The levels of monoamines and metabolites had returned to normal by 20 hr after administration. Dose-dependency of the effects of RS-8359 on monoamine metabolites was observed at doses up to 30 mg/kg (p.o.) at 1 and 6 hr after administration. In conclusion, NE, DA, and 5HT are exclusively or preferentially deaminated by MAO-A in the cortex, hippocampus, and striatum of rats, and RS-8359 exhibits a reversible MAO-A inhibitory action in all three regions tested in vivo.     

A kinetic study of the rate of formation of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the brain of the rat: implications for the origin of DOPAC

The rates of disappearance of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the brain of rats treated with the tyrosine hydroxylase inhibitor alpha-methyl-p-tyrosine were measured. Levels of dopamine. DOPAC and HVA in tissue were quantified in the striatum, nucleus accumbens and frontal cortex of the rat by means of high pressure liquid chromatography with electrochemical detection. Rats were sacrificed 1, 2 or 3 hr after the administration of alpha-methyl-p-tyrosine (200 mg/kg, i.p.). Levels of dopamine, DOPAC and HVA in tissue were logarithmically transformed, plotted against the duration of inhibition of tyrosine hydroxylase and the rate constant of the decline of dopamine and of its metabolites calculated. The rate constant of decline of DOPAC in each of brain studied was found to be greater than that for dopamine. The rate constant for the decline of dopamine was found to be greater in areas of the brain presenting the largest DOPAC/dopamine tissue ratios (frontal cortex greater than nucleus accumbens greater than striatum). The present results suggest that a substantial amount of DOPAC in brain derives from a newly formed pool of dopamine.     

Role of type A and B monoamine oxidase on the formation of 3,4-dihydroxyphenylacetic acid (DOPAC) in tissues from the brain of the rat

The role of monamine oxidase (MAO), type A and B, on the deamination of dopamine in the striatum, nucleus accumbens and frontal cortex of the rat was studied. Levels of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) tissue were quantified by means of high pressure liquid chromatography with electrochemical detection. Rats were given pargyline (75 mg/kg), selegyline (5 mg/kg) or clorgyline (2 mg/kg) by the intraperitoneal route, 60 min before sacrifice; in another set of experiments, clorgyline (2 mg/kg, i.p.) was given 15 or 30 min before sacrifice. Only clorgyline and pargyline were found to reduce significantly the formation of DOPAC and HVA in all the three areas of brain under study (83-97% reduction). The inhibition of deamination of dopamine by clorgyline and pargyline was accompanied by an increase in levels of dopamine in tissue. The increase of the levels of amine in tissue, as a result of inhibition of MAOA was more marked in the frontal cortex (52% increase) and the accumbens (39% increase), than in the striatum (25% increase). The results suggest that a substantial amount of DOPAC in brain derives from the deamination of dopamine by MAOA.     

Manganese increases L-DOPA auto-oxidation in the striatum of the freely moving rat: potential implications to L-DOPA long-term therapy of Parkinson's disease

We have previously shown that manganese enhances L-dihydroxyphenylanine (L-DOPA) toxicity to PC12 cells in vitro. The supposed mechanism of manganese enhancing effect [an increase in L-DOPA and dopamine (DA) auto-oxidation] was studied using microdialysis in the striatum of freely moving rats. Systemic L-DOPA [25 mg kg(-1) intraperitoneally (i.p.) twice in a 12 h interval] significantly increased baseline dialysate concentrations of L-DOPA, dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and uric acid, compared to controls. Conversely, DA and ascorbic acid concentrations were significantly decreased. A L-DOPA oxidation product, presumptively identified as L-DOPA semiquinone, was detected in the dialysate. The L-DOPA semiquinone was detected also following intrastriatal infusion of L-DOPA. In rats given L-DOPA i.p. , intrastriatal infusion of N-acetylcysteine (NAC) significantly increased DA and L-DOPA dialysate concentrations and lowered those of L-DOPA semiquinone; in addition, NAC decreased DOPAC+HVA and uric acid dialysate concentrations. In rats given L-DOPA either systemically or intrastriatally, intrastriatal infusion of manganese decreased L-DOPA dialysate concentrations and greatly increased those of L-DOPA semiquinone. These changes were inhibited by NAC infusion. These findings demonstrate that auto-oxidation of exogenous L-DOPA occurs in vivo in the rat striatum. The consequent reactive oxygen species generation may account for the decrease in dialysate DA and ascorbic acid concentrations and increase in enzymatic oxidation of xanthine and DA. L-DOPA auto-oxidation is inhibited by NAC and enhanced by manganese. These results may be of relevance to the L-DOPA long-term therapy of Parkinson's disease.     

Noradrenaline synthesis from L-DOPA in rodents and its relationship to motor activity

Evidence has been obtained for an increase in noradrenaline (NA) turnover after administration of I-DOPA to rodents. Normal mice, and those pre-treated with either reserpine or α-methyl-p-tyrosine (AMPT) were given L-DOPA (200 mg/kg) plus MK 486 (α-methyldopahydrazine; 25 mg/kg). In all cases L-DOPA produced a rise in cerebral dopamine (DA) levels. Cerebral NA levels were increased by L-DOPA in reserpinised and AMPT-treated mice. The same dose of L-DOPA produced no change in NA in normal mice, although pre-treatment with the monoamine oxidase inhibitor pargyline (200 mg/kg) resulted in a greater rise in NA 1 hr after L-DOPA compared to animals receiving pargyline alone. This evidence suggests that NA is synthesized from L-DOPA in all these situations. But whole brain 3-methoxy-4-hydroxyphenylglcol sulphate (MOPEG-SO4), a major metabolite of NA, measured after administration of the same dose of L-DOPA plus MK 486, was unaltered in normal and AMPT-treated rats, and was significantly decreased in reserpinised rats. However, an elevation of whole brain MOPEG-SO4 was found in reserpinised and AMPT-treated rats after a lower dose of L-DOPA (50 mg/kg). This discrepancy may be explained by high doses of L-DOPA causing inhibition of catechol-O-methyl transferase (COMT), which is suggested by the observation that the forebrain homovanillic acid (HVA): 3,4-dihydroxyphenylacetic acid (DOPAC) ratio was significantly lower after the high dose of L-DOPA than in untreated mice. Such an inhibition would prevent formation of MOPEG-SO4. Pretreatment with the dopamine-β-hydroxylase inhibitor FLA (63(bis-(1-methyl-4-monopiperazinyl-thiocarbonyl)disulphide) prevented the increase in NA and MOPEG-SO4 formation observed following L-DOPA induced motor activity in these groups of animals suggesting the involvement of NA in the production of such behaviour.