Effects of L-DOPA and L-5-HTP on the visual evoked response

Small doses (10 and 20 mg/kg) of L-DOPA inhibited the amplitude of the visual evoked response (VER), while large doses (40 and 80 mg/kg) enhanced it. Though low doses (12.5 and 25 mg/kg) of L-5-HTP caused a slight increase in amplitude of the VER, the simultaneous administration of 12.5 mg/kg of L-5-HTP and 10 mg/kg of L-DOPA produced a marked enhancement. The peak latency was prolonged after the injection of any doses of L-DOPA, L-5-HTP, or both.     

L-DOPA modulates striatal dopaminergic function in vivo: Evidence from PET investigations in nonhuman primates

Significant increases in striatal L-[¹¹C]DOPA retention were observed in adult female rhesus monkeys with positron emission tomography (PET) following administration of drugs that increase cerebral L-DOPA concentrations. The monkeys were scanned twice: at baseline (using 10–50 μg of tracer substance) and during continuous administration of L-DOPA (3 or 15 mg/kg/h) and 6-R-Erythro-4,5,6,7-tetrahydrobiopterin (6R-BH₄) (5 mg/kg/h) and during combined administration of both drugs. PET scans of L-[¹¹C]DOPA distribution were obtained in GE2048-15B or GE4096-15WB Plus positron tomographs. In all studies the specific striatal L-[¹¹C]DOPA influx rate increased by an average of 17–20%. These increases were significantly higher than the retest variability obtained with saline infusions under identical experimental conditions. In individual monkeys the magnitude of increase in the striatal L-[¹¹C]DOPA influx rate varied from no effect of the drug infusion to a 45% increase. Taken together, the results of this study demonstrate that L-DOPA in itself can affect dopaminergic neurotransmission in vivo and also adds further evidence that the neuromodulatory effects of the amino acid are predominantly autoreceptor antagonist-like. The findings most likely have importance for the further understanding of the dopaminergic system in neurodegenerative and psychiatric disorders. Synapse 25:56–61, 1997. © 1997 Wiley-Liss, Inc.     

Mucuna pruriens proves more effective than L-DOPA in Parkinson's disease animal model

Seeds of the Mucuna pruriens plant, now known to contain L -DOPA, have long been used for the treatment of Parkinson's disease patients in ancient Eastern Indian ethnotherapeutics. Following validation of the intrastraital 6-OHDA injection with amphetamine in the parkinsonian rat model, the animals were fed synthetic L -DOPA (125 or 250 mg/kg) or Mucuna pruriens endocarp (MPE, 2.5 or 5.0 g/kg) mixed with rat chow (n =6, for each dose and drug). Controls received no drug. An additional dose of L -DOPA or MPE in the same doses plus carbidopa (50 mg/kg) were administered via gavage (controls received only carbidopa 50 mg/kg) 1 h prior to testing with rotometer. Contralateral rotation (to the side of the 6-OHDA lesion) (CLR) was recorded for 240 min as a measure of antiparkinsonian activity. Results indicated that dose for dose, MPE showed twice the antiparkinsonian activity compared with synthetic L -DOPA in inducing CLR in the parkinsonian animal model. This study suggests that MPE may contain unidentified antiparkinsonian compounds in addition to L -DOPA, or it may have adjuvants that enhance the efficacy of L -DOPA. © 1997 John Wiley & Sons, Ltd.     

TRH on rat conditioned avoidance behavior: interaction with brain catecholamines

TRH (10 micrograms) intracerebroventricularly injected improves the acquisition of a two-day avoidance conditioning. This effect is partially antagonized by pretreatment IP with alpha-methyltyrosine (60 mg/kg) or disulfiram (300 mg/kg). L-DOPA (100 mg/kg) administered IP 2 hr after alpha-MT partially restores the facilitatory effect of the hormone. The possible roles of brain catecholamines on the behavioral effect of TRH are analysed. Other tentative mechanisms of action are also discussed.     

Catecholamine synthesis and metabolism in the central nervous system of mice lacking α2-adrenoceptor subtypes

Background and purpose:

This study investigates the role of α₂-adrenoceptor subtypes, α_2A, α_2B and α_2C, on catecholamine synthesis and catabolism in the central nervous system of mice.

Experimental approach:

Activities of the main catecholamine synthetic and catabolic enzymes were determined in whole brains obtained from α_2A-, α_2B- and α_2C-adrenoceptor knockout (KO) and C56Bl\7 wild-type (WT) mice.

Key results:

Although no significant differences were found in tyrosine hydroxylase activity and expression, brain tissue levels of 3,4-dihydroxyphenylalanine were threefold higher in α_2A- and α_2C-adrenoceptor KO mice. Brain tissue levels of dopamine and noradrenaline were significantly higher in α_2A and α_2CKOs compared with WT [WT: 2.8 ± 0.5, 1.1 ± 0.1; α_2AKO: 6.9 ± 0.7, 1.9 ± 0.1; α_2BKO: 2.3 ± 0.2, 1.0 ± 0.1; α_2CKO: 4.6 ± 0.8, 1.5 ± 0.2 nmol·(g tissue)⁻¹, for dopamine and noradrenaline respectively]. Aromatic L-amino acid decarboxylase activity was significantly higher in α_2A and α_2CKO [WT: 40 ± 1; α_2A: 77 ± 2; α_2B: 40 ± 1; α_2C: 50 ± 1, maximum velocity (V_max) in nmol·(mg protein)⁻¹·h⁻¹], but no significant differences were found in dopamine β-hydroxylase. Of the catabolic enzymes, catechol-O-methyltransferase enzyme activity was significantly higher in all three α₂KO mice [WT: 2.0 ± 0.0; α_2A: 2.4 ± 0.1; α_2B: 2.2 ± 0.0; α_2C: 2.2 ± 0.0 nmol·(mg protein)⁻¹·h⁻¹], but no significant differences were found in monoamine oxidase activity between all α₂KOs and WT mice.

Conclusions and implications:

In mouse brain, deletion of α_2A- or α_2C-adrenoceptors increased cerebral aromatic L-amino acid decarboxylase activity and catecholamine tissue levels. Deletion of any α₂-adrenoceptor subtypes resulted in increased activity of catechol-O-methyltransferase. Higher 3,4-dihydroxyphenylalanine tissue levels in α_2A and α_2CKO mice could be explained by increased 3,4-dihydroxyphenylalanine transport.     

Synergistic effects of adenosine A2A antagonist and L-DOPA on rotational behaviors in 6-hydroxydopamine-induced hemi-Parkinsonian mouse model

In this study, we examined the combination effects of L-DOPA and adenosine receptor antagonists on rotational behaviors in a hemi-Parkinsonian mouse model induced by unilateral 6-hydroxydopamine (6-OHDA) injection. The adenosine A(2A) antagonist SCH-58261, but not the A(1)-receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine or A(2B)-receptor antagonist alloxazine, synergistically potentiated the L-DOPA-induced rotational behaviors in the 6-OHDA-lesioned mice. In addtion, the 6-OHDA-induced lesions of the dopaminergic system did not affect the in vivo binding of an adenosine A(2A)-receptor tracer [(11)C]SCH-442416 in the striatatum. These findings suggest that adenosine A(2A) antagonists are extremely useful for pharmacotherapy of L-DOPA in Parkinson's disease patients.     

Survival of Neonatal Rats during Hypoxia after Monoamine Stimulating Agents

Abstract: Survival of 4 days old rats exposed to 6% O₂-94% N₂ was studied. Administration of L-DOPA (100 mg/kg) or L-5-HTP (100 mg/kg) reduced survival during hypoxia to about 30% of controls. A further reduction of survival time was noted after combined administration of L-DOPA and L-5-HTP. Administration of increasing doses of L-DOPA or L-5-HTP resulted in a dose-related decrease in neonatal survival time. After inhibition of the peripheral L-aminoacid decarboxylase with MK-486, L-DOPA caused the same reduction of survival time during hypoxia as after L-DOPA alone. Clonidine (2 mg/kg) was found to reduce hypoxic survival time to about 60%, while apomorphine had no effect compared to controls. Clonidine and apomorphine together had the same effect as clonidine alone. It is suggested that central monoamine neurotransmitters are involved in the mechanisms determining survival during neonatal oxygen deprivation.     

Quantitative Determination of L-DOPA in Seeds of Mucuna Pruriens Germplasm by High Performance Thin Layer Chromatography

Mucuna pruriens Linn. is an important medicinal plant used for treatment of Parkinson's disease and many others in ancient Indian medical system. L-DOPA extracted from seeds of Mucuna is a constituent of more than 200 indigenous drug formulations and is more effective as drug than the synthetic counterpart. A densitometric high performance thin-layer chromatographic (HPTLC) method was developed for quantification of L-DOPA content present in the seeds extract. The method involves separation of L-DOPA on precoated silica gel 60 GF(254) HPTLC plates using a solvent system of n-butanol-acetic-acid-water (4:1:1, v/v) as the mobile phase. Quantification was done at 280 nm using absorbance reflectance mode. Linearity was found in the concentration range of 100 to 1000 ng/spot with the correlation coefficient value of 0.9980. The method was validated for accuracy, precision and repeatability. Mean recovery was 100.89%. The LOD and LOQ for L-DOPA determination were found to be 3.41 ng/spot and 10.35 ng/spot respectively. The proposed HPTLC method was found to be precise, specific and accurate for quantitative determination of L-DOPA. It can be used for rapid screening of large germplasm collections of Mucuna pruriens for L-DOPA content. The method was used to study variation in fifteen accessions of Mucuna germplasm collected from different geographical regions.     

Past, present and future of A(2A) adenosine receptor antagonists in the therapy of Parkinson's disease

Several selective antagonists for adenosine A_2A receptors (A_2AR) are currently under evaluation in clinical trials (phases I to III) to treat Parkinson's disease, and they will probably soon reach the market. The usefulness of these antagonists has been deduced from studies demonstrating functional interactions between dopamine D₂ and adenosine A_2A receptors in the basal ganglia. At present it is believed that A_2AR antagonists can be used in combination with the dopamine precursor L-DOPA to minimize the motor symptoms of Parkinson's patients. However, a considerable body of data indicates that in addition to ameliorating motor symptoms, adenosine A_2AR antagonists may also prevent neurodegeneration. Despite these promising indications, one further issue must be considered in order to develop fully optimized antiparkinsonian drug therapy, namely the existence of (hetero)dimers/oligomers of G protein-coupled receptors, a topic that is currently the focus of intense debate within the scientific community. Dopamine D₂ receptors (D₂Rs) expressed in the striatum are known to form heteromers with A_2A adenosine receptors. Thus, the development of heteromer-specific A_2A receptor antagonists represents a promising strategy for the identification of more selective and safer drugs.     

Deuterium substitutions in the L-DOPA molecule improve its anti-akinetic potency without increasing dyskinesias

Treatment of Parkinson's disease is complicated by a high incidence of L-DOPA-induced dyskinesias (LID). Strategies to prevent the development of LID aim at providing more stable dopaminergic stimulation. We have previously shown that deuterium substitutions in the L-DOPA molecule (D3-L-DOPA) yield dopamine that appears more resistant to enzymatic breakdown. We here investigated the effects of D3-L-DOPA on motor performance and development of dyskinesias in a rodent model of Parkinson's disease. Through acute experiments, monitoring rotational behavior, dose–effect curves were established for D3-L-DOPA and L-DOPA. The equipotent dose of D3-L-DOPA was estimated to be 60% of L-DOPA. Subsequently, animals were treated with either the equipotent dose of D3-L-DOPA (5 mg/kg), the equivalent dose of D3-L-DOPA (8 mg/kg), L-DOPA (8 mg/kg) or vehicle. The equivalent dose of D3-L-DOPA produced superior anti-akinetic effects compared to L-DOPA in the cylinder test (p < 0.05), whereas the equipotent dose of D3-L-DOPA produced an anti-akinetic effect similar to L-DOPA. Dyskinesias developed to the same degree in the groups treated with equivalent doses of D3-L-DOPA and L-DOPA. The equipotent dose of D3-L-DOPA induced fewer dyskinesias than L-DOPA (p < 0.05). In conclusion, our study provides evidence for improved potency and reduced side-effects of L-DOPA by deuterium substitutions in the molecule. These results are of clinical interest since the occurrence of LID is related to the total L-DOPA dose administered. D3-L-DOPA may thus represent a novel strategy to reduce the total dose requirement and yet achieve an effective control of parkinsonian symptoms.