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PubMed1998

[Is endogenously released DOPA itself an upstream factor for increase in glutamate release and delayed neuronal cell death induced by transient ischemia in rats?].

DOPA itself is a neuromodulator in striata. In rat striata, DOPA by itself released neuronal glutamate in slices and caused cell death via glutamate release in cultured fetal neurons, suggesting involvement of DOPA in an upstream process of mechanisms for in vivo neuronal cell death. We attempted to clarify whether or not this idea is truly the case in conscious Wistar rats. Four vessels were occluded for 10 min during microdialysis of striata. DOPA, dopamine and glutamate in perfusates collected every 10 min were measured by HPLC-ECD and spectrophotometer. Delayed neuronal cell death in striata and hippocampus was evaluated 96 hr after ischemia. DOPA was indeed evoked with dopamine and glutamate during and after ischemia, and peak increases by respective 6-, 210- and 8-fold of a basal level were seen at the fraction immediately after ischemia. Delayed neuronal cell death was slight to moderate in striata and severe in hippocampus. Intrastriatal perfusion of NSD-1015, a central DOPA decarboxylase inhibitor, at 30 microM 10 min before ischemia, markedly increased DOPA and glutamate release by ischemia with slight inhibition of dopamine release and exaggerated delayed neuronal cell death in striata. Meanwhile, intrastriatal perfusion of DOPA cyclohexyl ester (DOPA CHE) at 10-100 nM, a novel stable potent competitive DOPA antagonist, antagonized dose-dependently increases in glutamate release by ischemia without modification of dopamine release. DOPA CHE at 100 nM protected striatal neurons from delayed cell death. Hippocampal neuronal cell death was neither affected by NSD-1015 nor by DOPA CHE. Endogenously released DOPA itself seems to act on its recognition site and to be a causal factor for increase in glutamate release and resultant delayed neuronal cell death by transient ischemia in rats.

Animals↗
PubMed1996

Characterization of L-DOPA transport in cultured rat and mouse astrocytes.

The present work studied the transport of L-DOPA in cultured rat and mouse astrocytes. Results indicated that the uptake of L-[14C]DOPA in both rat and mouse astrocytes was Na+ independent and temperature sensitive. It was mediated by a carrier-mediated mechanism with Km values of 36 and 60.3 microM, and V(max) values of 2.4 and 1.9 nmol/min/mg protein for rat and mouse cells, respectively. L-DOPA uptake was potently inhibited by aromatic or branched-chain amino acids. Interestingly, the release of intracellular L-[14C]DOPA was also trans stimulated by competitors that affect L-DOPA uptake. The 14C recovered in the 2-min uptake process was identified as L-[14C]DOPA. However, [14C]dopamine (DA) was also detected in both rat and mouse astrocytes after 30 min L-DOPA incubation, indicating the existence of aromatic L-amino acid decarboxylase (AADC). Taken together, L-DOPA was uphill transported into rat and mouse astrocytes by a (Na+) -independent exchanger with preference for aromatic and branched-chain amino acids. Rat astrocytes possessed higher affinity for L-DOPA uptake than mouse astrocytes. Both cells synthesized DA from exogenous L-DOPA. Thus, astrocytes serve not only as a temporary storage site for L-DOPA but also as a DA-producing machinery. These results may be of particular importance to parkinsonian patients under L-DOPA medication.

Amino Acids↗
PubMed1995

The effects of cisapride on plasma L-dopa levels and clinical response in Parkinson's disease.

Cisapride (CIS) is a prokinetic agent that increases gastrointestinal motility in normal individuals and improves constipation in Parkinson's disease (PD). We studied the effects of CIS on the clinical response and the peripheral pharmacokinetics of orally administered L-dopa given to patients with PD. Twenty patients with idiopathic PD and chronic constipation, whose response to L-dopa was suboptimal or characterized by fluctuations, agreed to participate in an open study that lasted for 2 weeks. Fourteen patients completed the study (mean age 65 +/- 9.3 years, mean duration of treatment 5.7 +/- 4.2 years, mean L-dopa daily doses 658.9 +/- 269.9 mg); six patients were excluded due to lack of compliance or changes in medication during the study. The end points of the study included the mean levels of L-dopa, the height of the peak of L-dopa in plasma, mean plasma levels of 3-OM-dopa, and the speed and quality of gait and visuomanual coordination before and during treatment with CIS. CIS increased peak plasma levels of L-dopa by 37% and the mean plasma levels of L-dopa by 13% with respect to those obtained with the same dose of L-dopa before the addition of CIS. Therefore, CIS appears to increase early absorption of L-dopa through acceleration of gastric emptying. CIS also increased plasma 3-OM-dopa levels, improved visuomanual coordination, and reduced gait disability. CIS improves gastrointestinal function and response to L-dopa in patients with PD and could be a helpful add-on medication in these patients.

Aged↗
PubMed2004

Bilateral subthalamic nucleus lesion reverses L-dopa-induced motor fluctuations and facilitates dyskinetic movements in hemiparkinsonian rats.

Glutamatergic overactivity might be involved in L-dopa-induced motor complications since glutamate antagonists reverse and prevent L-dopa-induced shortening in motor response duration in 6-hydroxydopamine-lesioned (6-OHDA) rats and improve L-dopa-induced dyskinesias in parkinsonian monkeys and in patients with Parkinson's disease (PD). An increase in the subthalamic nucleus (STN) glutamatergic activity is believed to contribute to the pathophysiology of PD. However, the role of STN activity in L-dopa-induced motor complications is not so clear. In this study, the effect of STN lesions on L-dopa-induced motor response complications was investigated in rats with a nigrostriatal pathway lesion induced by 6-OHDA. Animals were injected with 6-OHDA in the medial forebrain bundle and treated with L-dopa or saline for 22 days. On day 16, animals were randomly distributed in groups that underwent surgery in the STN ipsilateral or contralateral to 6-OHDA lesion, or bilateral. Rotational behavior was measured on days 1, 15, and 22. Attenuation of STN activity by contralateral and bilateral, but not ipsilateral, STN lesion reversed the shortening in motor response duration induced by L-dopa. L-dopa administration, but not saline, induced prominent dyskinesias in 6-OHDA-lesioned rats with additional bilateral STN lesions. The results indicate that bilateral lesions of STN potentiate the duration of L-dopa-induced motor response and facilitate chronic L-dopa-induced abnormal involuntary movements in 6-OHDA-lesioned rats. The characteristics of the abnormal involuntary movements observed in these animals are similar to L-dopa-induced dyskinesias in parkinsonian patients and might be useful as an experimental model for the study of L-dopa-induced dyskinesia.

Animals↗
PubMed1984

"On-off" phenomenon in Parkinson's disease: correlation to the concentration of dopa in plasma.

To investigate the relation between "on-off" fluctuations in symptomatology and bioavailability of dopa in patients with Parkinson's disease, five Parkinsonian patients with pronounced "on-off" symptoms were studied. Continuously during the study the degree of disability in the patients was registered. Every one hour, and in addition, whenever there was a change from "on" to "off" or vice versa, a blood sample was collected for dopa determination. Since dopa is transported from plasma into the brain by a saturable carrier for which it has to compete with endogenous large neutral amino acids (LNAA), the concentrations of these competitors were measured too. In four of the patients there were considerable oscillations in the plasma dopa concentration during the day; in one of these patients the highest value was as much as 12 times higher than the lowest value. These dramatic fluctuations in the absolute concentration of dopa in plasma had a major influence on the relative dopa concentrations (calculated as the ratio dopa/sum of LNAA) as the fluctuations in the concentrations of LNAA in plasma were much less pronounced. Consequently, the absolute and the relative concentrations of dopa in plasma were highly parallelled. In four of the five patients "on"-periods began within one hour after a peak in the concentration of dopa in plasma and in the fifth patient five out of seven "on"-periods were preceded by a rise in plasma dopa concentration within the same time interval. From the present data it could be concluded that the "on-off" phenomenon in Parkinson's disease, at least partly, is due to oscillations in the concentration of dopa in plasma. A reduction in the variations of the concentration of dopa in plasma seems to be necessary to overcome the "on-off" problem. The introduction of a slow release preparation of dopa is therefore urgently warranted. The concentration of LNAA in plasma must, however, also be considered in this context.

Aged↗
PubMed1989

Endogenous L-dopa in the rat dorsal vagal complex: an immunocytochemical study by light and electron microscopy.

The aim of this work was to examine L-DOPA immunoreactivity (L-DOPA-IR) in the dorsal vagal complex (DVC) of the rat medulla oblongata containing A2/C2 catecholaminergic cell groups, in order to further evaluate the previously proposed hypothesis that various pools of endogenous L-DOPA could be immunocytochemically demonstrated in the mammalian brain. For this purpose, L-DOPA-IR was studied in DVC in comparison with both some other catecholaminergic areas and dopamine immunoreactivity (DA-IR) on adjacent sections of the same brain, by using specific antibodies against glutaraldehyde conjugated L-DOPA and DA. Also, the first preliminary observations of L-DOPA-IR in DVC neurons at the ultrastructural level are reported. The following main results were obtained: (1) bright, intense and homogeneous L-DOPA staining was found in perikarya and proximal neuronal processes situated within the rostrocaudal extension of the DVC; (2) this staining pattern was readily distinct from weak and heterogeneous DA staining; (3) an inverse L-DOPA/DA staining pattern ratio was identified between the DVC and the mesencephalon; (4) L-DOPA-IR at electron microscopic level was roughly similar to that previously observed for DA-IR in mesencephalic cells and their presumptive projections. Although some discrepancies were noticed between L-DOPA staining and data from the literature on tyrosine hydroxylase labeling, our results could not invalidate the hypothesis that, among high L-DOPA/DA ratio containing neurons, some cells in the DVC may contain only L-DOPA.

Animals↗
PubMed1988

Determination of 3,4-dihydroxyphenylalanine (DOPA) in plasma and cerebrospinal fluid by high performance liquid chromatography with electrochemical detection.

We report a reliable method for determining DOPA levels in plasma and cerebrospinal fluid. The method is based on complete conversion of DOPA to dopamine and quantification by HPLC-ECD of the dopamine formed. Lower limit of detection was 0.5 nmol/l. No differences in plasma DOPA levels were found between normal children (0-15 yr, n = 60), normal adults (n = 39) and patients with essential hypertension (n = 40) or Parkinson's disease (no DOPA therapy, n = 30). In normal individuals and in patients with essential hypertension venous plasma levels were higher than arterial levels (10.2 vs 9.3 nmol/l, p less than 0.001, V/A ratio 1.11 (SD 0.08), n = 15). Sympathetic stimuli (standing, tilting, bicycle exercise, tyramine) did not influence DOPA levels. In untreated depressed patients (n = 10) and in non-parkinsonian neurological patients (n = 12) cerebrospinal fluid levels of DOPA were 4.5 (SD 2.4) and 5.2 (SD 1.3) nmol/l respectively. A direct method for the measurement of DOPA by HPLC-ECD after deproteinization of plasma is also described and compared with the conversion method. Good agreement was found when plasma DOPA levels exceeded 0.25 mumol/l (y(conversion method) = 0.943x (direct method) + 0.118; n = 60; r = 0.985). The direct method, because of greater simplicity and the possibility of simultaneous measurement of the DOPA metabolite 3-O-methyldopa, is the method of choice with plasma samples from DOPA-treated patients. In non-DOPA treated individuals the conversion method is superior and has proved to be an accurate and sensitive method for the determination of DOPA levels in plasma and cerebrospinal fluid.

Adolescent↗
PubMed1995

L-DOPA metabolism in cortical and striatal tissues in an animal model of parkinsonism.

Rats with unilateral 6-hydroxydopamine lesions of the midbrain tegmentum were treated with 25 mg/kg L-DOPA methyl ester/2 mg/kg carbidopa. The effects of the L-DOPA treatment upon serum, neocortical, and striatal L-DOPA and 3-O-methyl dopa (3-OMD) concentrations were measured. The highest L-DOPA and 3-OMD concentrations were obtained in the serum and in a ratio of approximately 2:1. In the brain, there was a uniform distribution of 3-OMD but L-DOPA concentrations were highly nonhomogeneous. Regression line equations for the statistically significant correlation coefficients between L-DOPA and tissue dopamine concentrations suggested that L-DOPA generated 50-60 times as much dopamine in the intact striatum as in cortex. The regional variation of L-DOPA concentration appears related to the capability of the brain tissue to generate and store dopamine from L-DOPA. In addition, the findings suggest that the behavioral ineffectiveness of L-DOPA in intact animals is related to its capacity to transform L-DOPA to tissue bound dopamine.

Animals↗
PubMed2004

Biosynthesis and turnover of DOPA-containing proteins by human cells.

Protein-bound 3,4-dihydroxyphenylalanine (PB-DOPA) is a major product of hydroxyl radical attack on tyrosine residues of proteins. Levels of PB-DOPA in cells and tissues have been shown to be greatly elevated in age-related diseases. We demonstrate for the first time that l-DOPA (levodopa) can be biosynthetically incorporated into cell proteins by human cells (THP-1 monocytes and monocyte-derived macrophages). The DOPA-containing proteins generated were selectively visualized on PVDF membranes using a redox-cycling staining method. Many cell proteins contained DOPA and seemed to be synthesized as their full-length forms. The cellular removal of DOPA-containing proteins by THP-1 cells was by proteolysis involving both the proteasomal and the lysosomal systems. The rate of cellular proteolysis of DOPA-containing proteins increased at lower levels of DOPA incorporation but decreased at higher levels of DOPA incorporation. The decreased rate of degradation was accompanied by an increase in the activity of cathepsins B and L but the activity of cathepsin S increased only at lower levels of DOPA incorporation. These data raise the possibility that PB-DOPA could be generated in vivo from l-DOPA, which is the most widely used treatment for Parkinson disease.

Cathepsins↗
PubMed2005

A serotonin 5-HT1A receptor agonist prevents behavioral sensitization to L-DOPA in a rodent model of Parkinson's disease.

Marked fluctuation of dopamine concentration in the striatum following long-term L-DOPA administration contributes to the development of L-DOPA-induced motor complications including L-DOPA-induced dyskinesias and wearing-off in patients with Parkinson's disease. We have shown that pretreatment with 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A (5-hydroxytryptamine) receptor agonist, alleviates fluctuation of dopamine levels in the dopamine-denervated striatum of 6-hydroxydopamine-lesioned (hemiparkinsonian) rats after L-DOPA treatment. To determine whether co-administration of 8-OH-DPAT with L-DOPA prevents L-DOPA-induced motor complications, we examined rotation behavior and levels of messenger RNAs coding for dynorphin and glutamic acid decarboxylase in the striatum of 6-hydroxydopamine-lesioned rats treated with L-DOPA alone or L-DOPA + 8-OH-DPAT, twice daily, for 2 weeks. Co-administration of 8-OH-DPAT inhibited an increase of rotation behavior to L-DOPA and L-DOPA-induced increases in levels of messenger RNAs coding for dynorphin and glutamic acid decarboxylase in the dopamine-denervated striatum, both of which are established indices of L-DOPA-induced motor complications. These results suggest that pharmaceutical products that stimulate 5-HT1A receptors could prove useful in prevention of the development of L-DOPA-induced motor complications in patients with Parkinson's disease.

8-Hydroxy-2-(di-n-propylamino)tetralin↗
PubMed2000

Involvement of rBAT in Na(+)-dependent and -independent transport of the neurotransmitter candidate L-DOPA in Xenopus laevis oocytes injected with rabbit small intestinal epithelium poly A(+) RNA.

Although L-3,4-dihydroxyphenylalanine (L-DOPA) is claimed to be a neurotransmitter in the central nervous system (CNS), receptor or transporter molecules for L-DOPA have not been determined. In an attempt to identify a transporter for L-DOPA, we examined whether or not an active and high affinity L-DOPA transport system is expressed in Xenopus laevis oocytes injected with poly A(+) RNA prepared from several tissues. Among the poly A(+) RNAs tested, rabbit intestinal epithelium poly A(+) RNA gave the highest transport activity for L-[(14)C]DOPA in the oocytes. The uptake was approximately five times higher than that of water-injected oocytes, and was partially Na(+)-dependent. L-Tyrosine, L-phenylalanine, L-leucine and L-lysine inhibited this transport activity, whereas D-DOPA, dopamine, glutamate and L-DOPA cyclohexylester, an L-DOPA antagonist did not affect this transport. Coinjection of an antisense cRNA, as well as oligonucleotide complementary to rabbit rBAT (NBAT) cDNA almost completely inhibited the uptake of L-[(14)C]DOPA in the oocytes. On the other hand, an antisense cRNA of rabbit 4F2hc barely affected this L-[(14)C]DOPA uptake activity. rBAT was thus responsible for the L-[(14)C]DOPA uptake activity expressed in X. laevis oocytes injected with poly A(+) RNA from rabbit intestinal epithelium. As rBAT is localized at the target regions of L-DOPA in the CNS, rBAT might be one of the components involved in L-DOPAergic neurotransmission.

Amino Acid Transport Systems, Basic↗
PubMed1999

Changes in the regional and compartmental distribution of FosB- and JunB-like immunoreactivity induced in the dopamine-denervated rat striatum by acute or chronic L-dopa treatment.

This study was carried out in order to examine the effects of acute or chronic L-DOPA treatment on striatally expressed FosB- and JunB-like proteins in a rat model of Parkinson's disease. Rats with a unilateral, near-total 6-hydroxydopamine lesion of the ascending mesostriatal projection received either an acute challenge or a one-week treatment with 10 mg/kg/day methyl L-DOPA (combined with 15 mg/mg benserazide), and were killed at either 3 h or two days post-injection. Both acute and chronic L-DOPA treatment caused a pronounced, persistent increase in the number of FosB-like immunoreactive cells in the dopamine-denervated striata (five- and seven-fold increase, respectively, above the levels found in lesioned but non-drug-treated controls), but the two treatment groups differed markedly with respect to both the average amount of staining per cell, which was two-fold larger in the chronic L-DOPA cases, and the anatomical distribution of the labeled cells. After an acute injection of L-DOPA, FosB-positive cells were distributed rather uniformly across all striatal subregions, whereas chronic L-DOPA treatment induced discrete clusters of strongly FosB-like immunoreactive cells within medial and central striatal subregions, as well as in a large, yet sharply defined portion of the lateral caudate-putamen. Strongly labeled cell clusters that appeared in the medial and central caudate-putamen were preferentially located within calbindin-poor, mu-opioid receptor-rich striosomes, whereas the lateral area displaying FosB activation encompassed both striosomal and matrix domains. In both the medial and the lateral striatum a near-total overlap was found between strongly FosB-like immunoreactive cell groups and areas showing pronounced dynorphin expression. NADPH-diaphorase-positive striatal interneurons did not express FosB-like immunoreactivity after a 6-hydroxydopamine lesion alone, a negligible proportion of them did after an acute L-DOPA challenge, but about 8% of these interneurons were FosB positive following chronic L-DOPA treatment. Like FosB, JunB was induced in the DA-denervated striatum by both acute and chronic L-DOPA treatment, and exhibited similar distribution patterns. However, JunB did not exhibit prolonged expression kinetics, and was somewhat down-regulated in the chronically compared with the acutely L-DOPA-treated rats. The present results show that L-DOPA administration produces a long-lasting increase in the levels of FosB-, but not JunB-like immunoreactivity in the dopamine-denervated striatum. More importantly, these data show that striatal induction of FosB- and JunB-like proteins by chronic L-DOPA treatment exhibits both regional and compartmental specificity.

Animals↗
PubMed1998

Improvement of L-dopa absorption by dipeptidyl derivation, utilizing peptide transporter PepT1.

In the present study, possible enhancement of intestinal absorption of L-dopa by utilizing intestinal peptide transporter was examined using Caco-2 cells and Xenopus oocytes expressing human peptide transporter (hPepT1). To see whether this peptide transporter could be utilized for the improvement of L-dopa absorption, we employed a dipeptide-mimetic derivative of L-dopa, L-dopa-L-Phe. L-Dopa-L-Phe inhibited the uptake of [14C]Gly-Sar, but not that of L-[3H]-dopa by Caco-2 cells. Uptake of L-dopa-L-Phe was increased by expression of hPepT1 in Xenopus oocytes. The appearance of L-dopa and its metabolite, dopamine, on the basolateral side of Caco-2 cells was significantly higher after addition of L-dopa-L-Phe than after that of L-dopa and was reduced by the presence of Gly-Sar on the apical side. These results indicate that the L-dopa-L-Phe is absorbed more efficiently than L-dopa and is taken up via the peptide transporter, but not via the amino acid transporter, demonstrating the possibility of targeting the peptide transporter as a means for improving intestinal absorption of peptide-like drugs.

Amino Acid Transport Systems↗
PubMed2001

A pharmacokinetic model to predict the PK interaction of L-dopa and benserazide in rats.

PURPOSE: To study the PK interaction of L-dopa/benserazide in rats. METHODS: Male rats received a single oral dose of 80 mg/kg L-dopa or 20 mg/kg benserazide or 80/20 mg/kg L-dopa/benserazide. Based on plasma concentrations the kinetics of L-dopa, 3-O-methyldopa (3-OMD), benserazide, and its metabolite Ro 04-5127 were characterized by noncompartmental analysis and a compartmental model where total L-dopa clearance was the sum of the clearances mediated by amino-acid-decarboxylase (AADC), catechol-O-methyltransferase and other enzymes. In the model Ro 04-5127 inhibited competitively the L-dopa clearance by AADC. RESULTS: The coadministration of L-dopa/benserazide resulted in a major increase in systemic exposure to L-dopa and 3-OMD and a decrease in L-dopa clearance. The compartmental model allowed an adequate description of the observed L-dopa and 3-OMD concentrations in the absence and presence of benserazide. It had an advantage over noncompartmental analysis because it could describe the temporal change of inhibition and recovery of AADC. CONCLUSIONS: Our study is the first investigation where the kinetics of benserazide and Ro 04-5127 have been described by a compartmental model. The L-dopa/benserazide model allowed a mechanism-based view of the L-dopa/benserazide interaction and supports the hypothesis that Ro 04-5127 is the primary active metabolite of benserazide.

Algorithms↗
PubMed1988

The effects of carbidopa dose and time and route of administration on systemic L-dopa levels in rats.

The effects of carbidopa dose and time and route of administration on systemic plasma levels of parenterally and nonparenterally administered L-dopa were examined in rats. Intravenous coadministration of L-dopa + carbidopa resulted in significant (P less than 0.05) carbidopa-dependent increases in both the area under the plasma L-dopa concentration versus time profile (AUC; +27%) and the plasma L-dopa half-life (t1/2; +35%). Simultaneous duodenal or rectal carbidopa administration did not alter the L-dopa i.v. pharmacokinetic profile. Carbidopa pretreatment significantly increased the i.v. L-dopa AUC (+38 and +82% for i.v. and duodenal pretreatments, respectively) compared to simultaneous administration. Both i.v. and duodenal carbidopa increased duodenal L-dopa AUC to a similar extent (+282 and +239% for i.v. and duodenal administration, respectively). Rectal studies indicated poor absorption of both L-dopa and carbidopa, with no demonstrable effect on plasma L-dopa. The results indicate that the timing and route of carbidopa and L-dopa administration are important in determining the extent of i.v. or duodenal L-dopa systemic availability. The rat model affords results similar to those reported in human studies and may be useful for more extensive evaluation of L-dopa and carbidopa interactions.

Animals↗
PubMed1999

Role of high-affinity dopamine uptake and impulse activity in the appearance of extracellular dopamine in striatum after administration of exogenous L-DOPA: studies in intact and 6-hydroxydopamine-treated rats.

The differential behavioral and neurochemical effects of exogenous L-DOPA in animals with intact versus dopamine (DA)-denervated striata raise questions regarding the role of DA terminals in the regulation of dopaminergic neurotransmission after administration of exogenous L-DOPA. In vivo microdialysis was used to monitor the effect of exogenous L-DOPA on extracellular DA in intact and DA-denervated striata of awake rats. In intact striatum, a small increase in extracellular DA was observed after administration of L-DOPA (50 mg/kg i.p.) but in DA-denervated striatum a much larger increase in extracellular DA was elicited. Additional experiments assessed the role of high-affinity DA uptake and impulse-dependent neurotransmitter release in the effect of exogenous L-DOPA on extracellular DA in striatum. Pretreatment with GBR-12909 (20 mg/kg i.p.), a selective DA uptake inhibitor, enhanced the ability of L-DOPA to increase extracellular DA in intact striatum. However, in DA-denervated striatum, inhibition of DA uptake did not alter the extracellular DA response to L-DOPA. Impulse-dependent neurotransmitter release was blocked by the infusion of tetrodotoxin (TTX; 1 microM), an inhibitor of fast sodium channels, through the dialysis probe. Application of TTX significantly attenuated the L-DOPA-induced increase in extracellular DA observed in striatum of intact rats pretreated with GBR-12909. In a similar manner, TTX infusion significantly attenuated the increase in extracellular DA typically observed in striatum of 6-OHDA-lesioned rats after the administration of L-DOPA. The present results indicate that DA terminals, via high-affinity uptake, play a crucial role in the clearance of extracellular DA formed from exogenous L-DOPA in intact striatum. This regulatory mechanism is absent in the DA-denervated striatum. In addition, this study has shown that DA synthesized from exogenous L-DOPA primarily is released by an impulse-dependent mechanism in both intact and DA-denervated striatum. The latter result suggests an important role for a nondopaminergic neuronal element in striatum that serves as the primary source of extracellular DA formed from exogenous L-DOPA.

Animals↗
PubMed2003

Cellular and behavioural effects of the adenosine A2a receptor antagonist KW-6002 in a rat model of l-DOPA-induced dyskinesia.

We have examined the ability of KW-6002, an adenosine A2a antagonist, to modulate the dyskinetic effects of L-DOPA in 6-hydroxydopamine-lesioned rats. In animals rendered dyskinetic by a previous course of L-DOPA treatment, KW-6002 did not elicit any abnormal involuntary movements on its own, but failed to reduce the severity of dyskinesia when coadministered with L-DOPA. A second experiment was undertaken in order to study the effects of KW-6002 in L-DOPA-naive rats. Thirty-five animals were allotted to four groups to receive a 21-day treatment with: (i) KW-6002 (10 mg/kg/day); (ii) L-DOPA (6 mg/kg/day) i.p.; (iii) KW-6002 plus L-DOPA (same doses as above) or (iv) vehicle. Chronic treatment with KW-6002-only produced a significant relief of motor disability in the rotarod test in the absence of any abnormal involuntary movements. Combined treatment with L-DOPA and KW-6002 improved rotarod performance to a significantly higher degree than did each of the two drugs alone. However, this combined treatment induced dyskinesia to about the same degree as did L-DOPA alone. In situ hybridization histochemistry showed that KW-6002 treatment alone caused an approximately 20% reduction in the striatal levels of preproenkephalin mRNA, whereas neither the coadministration of KW-6002 and L-DOPA nor L-DOPA alone significantly altered the expression of this transcript in the dopamine-denervated striatum. Either alone or in combination with L-DOPA, KW-6002 did not have any modulatory effect on prodynorphin mRNA expression or FosB/DeltaFosB-like immunoreactivity in the dopamine-denervated striatum. These results show that monotreatment with an adenosine A2a receptor antagonist can relieve motor disability without inducing behavioural and cellular signs of dyskinesia in rats with 6-hydroxydopamine lesions. Cotreatment with KW-6002 and L-DOPA potentiates the therapeutic effect but not the dyskinesiogenic potential of the latter drug.

Animals↗
PubMed2003

Free and peptide-bound DOPA can inhibit initiation of low density lipoprotein oxidation.

Hydroxyl radicals have been shown to convert free tyrosine to 3,4-dihydroxyphenyl-alanine (DOPA) which has reducing properties. During protein or peptide oxidation such reducing species are also formed from tyrosine residues. Free DOPA or peptide-bound DOPA (PB-DOPA) is able to promote radical-generating events, facilitating the damage of biomolecules such as nucleic acids. Radical induced lipid oxidation in low density lipoprotein (LDL) transforms the lipoprotein into an atherogenic particle. As PB-DOPA has been found in atherosclerotic plaques, we tested the ability of free and PB-DOPA to influence LDL oxidation. Free DOPA, in contrast to tyrosine had strong inhibitory action on both, the copper-ion initiated and metal ion independent (AAPH-induced) lipid oxidation. Free DOPA also inhibited LDL oxidation induced by the copper transport protein ceruloplasmin. To test if PB-DOPA was also able to inhibit LDL oxidation, DOPA residues were generated enzymatically in the model peptides insulin and tyr-tyr-tyr, respectively. PB-DOPA formation substantially increased the ability of both molecules to inhibit LDL oxidation by copper or AAPH. We hypothesize that DOPA-peptides and -proteins may have the potential to act as efficacious antioxidants in the atherosclerotic plaque.

Antioxidants↗