Characterization of the tyraminergic system in the central nervous system of the locust,Locusta migratoria migratoides

Tyramine occurs in the central nervous system (CNS) of the migratory locust,Locusta migratoria migratoides. The distribution of tyramine within the CNS does not parallel that of octopamine. Tyramine is synthesised from tyrosine in the presence of tyrosine decarboxylase. A second decarboxylase in the CNS is active against 5HTP and DOPA. The locust ganglia incorporate tyramine by high- and low-affinity uptake processes that appear to be independent of dopamine and octopamine. Depolarisation of the locust ganglia by high potassium concentration results in calcium-dependent release of incorporated [³H]tyramine.     

Microbial preparation of L-[15N]tyrosine and [15N]tyramine and their gas chromatographic-mass spectrometric analyses

The preparation of L-[15N]tyrosine and [15N]tyramine by microbial synthesis is described. Immobilized Erwinia herbicola cells were added to a reaction mixture containing phenol, pyruvic acid, and 15NH4Cl. The reaction was driven by excess nonlabeled pyruvate and phenol. Under these denaturing concentrations of phenol, immobilized cells were more effective than free ones. Gram quantities of L-[15N]tyrosine were obtained without label dilution. The conversion of this L-[15N]tyrosine into [15N]tyramine by Streptococcus faecalis was performed at maximal efficiency. Gas chromatographic-mass spectrometric studies and 1H and 15N NMR analyses of the labeled compounds are reported.     

A subacute treatment of L-methionine induces an increase in the number of [3H]spiperone binding sites in the striatum of the rat

A subacute treatment, 500 mg/kg I.P. twice daily during 5 days, by L-methionine provoked an increase in the Bmax of [3H]-spiperone binding in the striatum of the rat. This increase was associated to a decrease in membrane microviscosity. However in these conditions no changes were found in the [3H]-DHA, [3H]QNB bindings or in the brain dopamine sensitive adenylate cyclase activity. L-methionine treatment reduced the accumulation of Dopa after NSD 1015 and antagonized the decrease in striatal acetylcholine provoked by haloperidol. Thus L-methionine might be a new potential drug for Parkinson's disease treatment.     

Modulation of dopaminergic neurotransmission in rat striatum upon in vitro and in vivo diclofenac treatment

Diclofenac (DCF) is a widely used non-steroidal anti-inflammatory drug, which also act as a mitochondrial toxin. As it is known that selective mitochondrial complex I inhibition combined with mild oxidative stress causes striatal dopaminergic dysfunction, we tested whether DCF also compromise dopaminergic function in the striatum. [3H]Dopamine ([3H]DA) release was measured from rat striatal slices after in vitro (2 h, 10-25 micromol/L) or in vivo (3 mg/kg i.v. for 28 days) DCF treatment. In vitro treatment significantly decreased [3H]DA uptake and dopamine (DA) content of the slices. H2O2 (0.1 mmol/L)-evoked DA release was enhanced. Intracellular reactive oxygen species production was not significantly changed in the presence of DCF. After in vivo DCF treatment no apparent decrease in striatal DA content was observed and the uptake of [3H]DA into slices was increased. The intensity of tyrosine hydroxylase immunoreactivity in the striatum was highly variable, and both decrease and increase were observed in individual rats. The H2O2-evoked [3H]DA release was significantly decreased and the effluent contained a significant amount of [3H]octopamine, [3H]tyramine, and [3H]beta-phenylethylamine. The ATP content and adenylate energy charge were decreased. In conclusion, whereas in vitro DCF pre-treatment resembles the effect of the mitochondrial toxin rotenone, in vivo it rather counteracts than aggravates dopaminergic dysfunction.     

effect of stress on levels of octopamine,dopamine and serotonin in the American cockroach (Periplaneta americana L.)

1. Various biogenic amines including octopamine, dopamine and serotonin, and their precursors and metabolites in haemolymph and the central nervous system from American cockroaches (Periplaneta americana L.) were measured using electrochemical detection.2. Octopamine was found in similar high relative abundances in haemolymph and the central nervous system.3. The amount of octopamine was much higher than that of tyramine and synephrine in haemolymph and thoracic nerve cord, whereas tyramine was at the highest level followed by octopamine and synephrine in the brain.4. Insects were stressed by vibrating at 100 or 1000 Hz, visually by flashing light at 4 Hz for 15 min or by immersing the insect in water at 60°C for 30 sec, which resulted in the elevation of octopamine, tyramine, synephrine and tyrosine levels in thoracic nerve cord.     

Changes in the Content of Brain Biogenic Amine Associated with Early Colony Establishment in the Queen of the Ant, Formica japonica

We examined changes in the content of biogenic amines in the brains of ant queen associated with early colony establishment. In ants, including Formica japonica, winged virgin queens lose their wings following copulation, and then start establishing a colony. Significant changes in brain biogenic amine content in the queen are associated with transition from winged virgin queen to wingless mated queen. The levels of serotonin (5HT), octopamine (OA) and dopamine (DA) decreased significantly in the brain of the queen after starting a colony. On the other hand, tyramine (TA) increased significantly in the brain following colony establishment. Catabolized substances of the biogenic amines in the brain were also measured. The levels of N-acetyloctopamine (Nac-OA) and N-acetyltyramine (Nac-TA) in the brain did not show a significant change after the queen established a colony. However, the levels of N-acetylserotonin (Nac-5HT) in the brain were significantly higher in wingless mated queens than in winged virgin queens, whereas levels of N-acetyldopamine (Nac-DA) in the brain were significantly lower in wingless mated queens than winged virgin queens. These results suggest that serotonergic and octopaminergic systems in the brain of the queen change when the mated queen starts to establish a new colony.     

Biosynthesis of bisbenzylisoquinoline alkaloids in cultured roots of Stephania cepharantha

Cultured roots of Stephania cepharantha, which are rich sources of bisbenzylisoquinoline alkaloids, were fed 14C-labelled tyrosine, tyramine or dopamine. While tyrosine was well incorporated into the bisbenzylisoquinolines, tyramine and dopamine were poorly incorporated. Incorporated tyrosine was shown to be decarboxylated and stored as tyramine in the roots, then gradually converted to the bisbenzylisoquinolines. Tracer experiments using [3-13C]tyrosine demonstrated that tyrosine was specifically incorporated into the corresponding sites of aromoline, which verified that aromoline was composed of four molecules of tyrosine. The ratio of 13C-enrichments of C-4 and C-alpha in (R) and (S) halves of aromoline was the same within experimental limits. This indicated that the two coclaurine units must have one and the same biogenetic origin.     

Biogenic amines and their metabolites in Trichostrongylus colubriformis, a nematode parasite of ruminants

The following biogenic amines (BA) and BA metabolites were identified via HPLC in homogenates prepared from adults of Trichostrongylus colubriformis (Nematoda) recovered from the intestines of goats: N-acetyldopamine, DOPA, dopamine, epinephrine, epinine, 5-hydroxyindoleacetic acid, 4-hydroxy-3-methoxy-phenylglycol, 3-(p-hydroxyphenyl) proprionic acid, metanephrine, O-methyl-DOPA, 3-methoxytyramine, norepinephrine, normetanephrine, octopamine, p-hydroxymandelic acid, serotonin, synephrine, tyramine and vanillylmandelic acid. The mean concentrations of these compounds in groups of worms collected from different goats did not differ significantly with sex, but between the groups variance was high with probable components of both host and nematode origin.     

The uptake and subcellular distribution of aromatic amines in the brain of the rat

The hydroxylated phenylethylamines p-tyramine, m-tyramine, octopamine, metaraminol and norepinephrine were accumulated by homogenates of rat brain much more vigorously than β-phenethylamine or amphetamine. The affinity concentrations (K_m) for initial (5-min) uptake by homogenates of whole brain were 0.5, 3 and 6 μM for DL-norepine-phrine, p-tyramine and DL-octopamine, respectively. The uptake of these three hydroxylated compounds was much more vigorous in striatal tissue than in cortical tissue, and in both tissues the rate of uptake decreased in the sequence: norepinephrine > tyramine > octopamine. The uptake of these three substances was inhibited by reduced temperature, by lack of glucose, by CN- and DNP, and by desmethylimipramine, cocaine and ouabain. The uptake of norepinephrine and octopamine appeared to require Na⁺. Pretreatment of rats with reserpine or 6-hydroxydopamine decreased the ability of brain to take up norepinephrine or octopamine. Previously accumulated labelled phenylethylamines migrated in sucrose density gradients with a peak of radioactivity corresponding to an equilibrium position of catecholamine-containing nerve endings. The magnitude of the retention of [³H]amine in this synaptosornal peak decreased in the order: norepinephrine > octopamine > tyramine. The accumulated amines were released by sonic, osmotic and thermal stresses which disrupt neuronal membranes. The presence of a β-hydroxyl group appeared to protect amines from destruction by monoamine oxidase, presumably by virtue of uptake in presynaptic storage vesicles. During superfusion, tyramine and metaraminol appeared to displace [³H]norepinephrine from binding sites in brain slices.     

Quinone methides—and not dehydrodopamine derivatives—as reactive intermediates of β-sclerotization in the puparia of flesh fly Sarcophaga bullata

Cuticular phenoloxidase(s) from Sarcophaga bullata larvae oxidized a variety of o-diphenolic compounds. While catechol, 3,4-dihydroxybenzoic acid, dopa, dopamine, and norepinephrine were converted to their corresponding quinone derivatives, other catechols such as 3,4-dihydroxyphenylacetic acid, 3,4-dihydroxyphenethyl alcohol, 3,4-dihydroxyphenyl glycol, 3,4-dihy-droxymandelic acid, and N-acetyldopamine were oxidized to their side-chain oxygenated products. In addition, the enzyme-catalyzed oxidation of the latter group of compounds accompanied the formation of colorless catecholcuticle adducts consistent with the operation of β-sclerotization. Radioactive trapping experiments failed to support the participation of 1,2-dehydro-N-acetyldopamine as a freely formed intermediate during phenoloxidase-mediated oxidation of N-acetyldopamine. When specifically tritiated substrates were provided, cuticular enzyme selectively removed tritium from [7-³H]N-acetyldopamine and not from either [8-³H] or [ring-³H]N-acetyldopamine during the initial phase of oxidation. The above results are consistent with the generation and subsequent reactions of quinone methides as the initial products of enzyme-catalyzed N-acetyldopamine oxidation and confirm our hypothesis that quinone methides and not 1,2-dehydro-N-acetyldopamine are the reactive intermediate of β-sclerotization of sarcophagid cuticle. Quinone methide sclerotization resolves a number of conflicting observations made by previous workers in this field.     

Effect of gonadal steroids and gamma-aminobutyric acid on LH release and dopamine expression and activity in the zona incerta in rats

A dopaminergic system in the zona incerta stimulates LH release and may mediate the positive feedback effects of the gonadal steroids on LH release. In this study the mechanisms by which steroids might increase dopamine activity in the zona incerta were investigated. In addition, experiments were conducted to determine whether the inhibitory effects of gamma-aminobutyric acid (GABA) on LH release in the zona incerta are due to suppression of dopamine activity in this area or conversely whether the stimulatory effects of dopamine on LH release are due to suppression of a tonic inhibitory GABAergic system. Ovariectomized rats were treated s.c. with oil, 5 micrograms oestradiol benzoate or 5 micrograms oestradiol benzoate followed 48 h later by 0.5 mg progesterone, and killed 54 h after the oestradiol benzoate injection. At this time the LH concentrations were suppressed in the oestradiol benzoate group and increased in the group treated with oestradiol benzoate and progesterone. The ratio of tyrosine hydroxylase:beta-actin mRNA in the zona incerta was significantly increased by the oestradiol benzoate treatment, but the addition of progesterone resulted in values similar to those in the control group. At the same time, the progesterone treatment increased tyrosine hydroxylase activity in the zona incerta as indicated by an increase in L-dihydroxyphenylalanine (L-DOPA) accumulation after 100 mg 3-hydroxybenzylhydrazine hydrochloric acid (NSD1015) kg-1 and an increase in dopamine release as indicated by a increase in dihydroxyphenylacetic acid (DOPAC) concentrations (one of the major metabolites of dopamine). Ovariectomized rats treated with oestradiol benzoate plus progesterone were also injected i.p. with 75 mg gamma-acetylenic GABA kg-1 (a GABA transaminase inhibitor) to increase GABA concentrations in the brain. This treatment had no effect on the ratio of tyrosine hydroxylase:beta-actin mRNA but decreased L-DOPA accumulation and DOPAC concentrations in the zona incerta, indicating a post-translational inhibition of dopamine synthesis and release. Treatment of ovariectomized rats with oestradiol benzoate followed by 100 mg L-DOPA i.p. to increase dopamine concentrations in the whole brain had no effect on glutamic acid decarboxylase mRNA expression in the zona incerta, although it increased the glutamic acid decarboxylase:beta-actin mRNA ratio in other hypothalamic areas (that is, the medical preoptic area, ventromedial nucleus and arcuate nucleus). In conclusion, the steroids act to increase dopamine activity in different ways: oestrogen increases tyrosine hydroxylase mRNA expression and progesterone acts after translation to increase tyrosine hydroxylase activity and dopamine release (as indicated by increases in DOPAC concentrations). This latter effect may be due to progesterone removing a tonic GABAergic inhibition from the dopaminergic system.     

Incorporation of leucine into phospholipids of Bacteroides thetaiotaomicron.

L-[4,5-3H]- or L-[U-14C]leucine was incorporated by Bacteroides thetaiotaomicron into acid-precipitable material even when the bacteria were treated with concentrations of tetracycline high enough to prevent growth. Similar results were obtained when L-[2,3,4-3H]valine or L-[4,5-3H]isoleucine was used instead of leucine. In bacteria which had been treated with tetracycline, the acid-precipitable label was not solubilized by treatment with protease, lysozyme, or deoxyribonuclease. However, virtually all of the label was extractable with chloroform-methanol, indicating that the label had been incorporated into membrane lipids. Since L-[1-14C]leucine was not incorporated into lipids, leucine was probably decarboxylated before incorporation. When a chloroform extract from bacteria which had been labeled with both [32P]phosphate and [3H]leucine was resolved into component phospholipids by two-dimensional thin-layer chromatography, 3H was incorporated into all of the phospholipids. When these phospholipids were deacylated, the 3H from leucine was associated with released fatty acids rather than with the head groups. Thus, it appears that B. thetaiotaomicron can utilize leucine and similar amino acids not only by incorporating them into protein but also by incorporating portions of these amino acids into membrane phospholipids.     

Inhibition of dihydropteridine reductase by catecholamines and related compounds

Catecholamines and related compounds, such as dopamine, 5- or 6-hydroxydopamine, N-methyldopamine, tyramine, octopamine, norepinephrine and epinephrine, inhibit human liver dihydropteridine reductase (NADH:6,7-dihydropteridine oxidoreductase, EC 1.6.99.10) noncompetitively with Ki values ranging from 7.0 X 10(-6) - 1.9 X 10(-4)M (I50 values = 2.0 X 10(-5) - 2.0 X 10(-4)M). The tyrosine analogs alpha-methyltyrosine and 3-iodotyrosine are weak inhibitors of this enzyme (I50 greater than 10(-3)M). The inhibitory effect of catecholamines is slightly decreased by O-methylation of one hydroxyl group, but is essentially abolished by total methylation. The inhibitory strength of the catecholamines and related compounds tested against this enzyme can be arranged in the following order: dopamine, 6-hydroxydopamine, 5-hydroxydopamine, N-methyldopamine greater than tyramine, 3-O-methyldopamine, 4-O-methyldopamine much greater than epinephrine, 3-O-methylepinephrine, norepinephrine, octopamine less than tyrosine much less than alpha-methyltyrosine, 3-iodotyrosine much less than homoveratrylamine. These results suggest that dopamine, norepinephrine and epinephrine may serve as physiological regulators of mammalian dihydropteridine reductase.     

Initial catabolism of aromatic biogenic amines by Pseudomonas aeruginosa PAO: pathway description, mapping of mutations, and cloning of essential genes

Pseudomonas aeruginosa PAO1 was able to utilize several aromatic biogenic amines as sole sources of carbon or nitrogen. These included the phenethylamines tyramine and dopamine and the phenethanolamines octopamine, synephrine, and norepinephrine. Initial catabolism of the phenethylamines was mediated by a membrane-bound tyramine dehydrogenase which produced 4-hydroxyphenylacetaldehyde (4HPAL) with tyramine as the substrate. The enzyme was induced by growth with both classes of amines. Initial catabolism of octopamine (except when present as the sole source of carbon and nitrogen) was mediated by a soluble enzyme with activity against the phenethanolamines but not against tyramine or dopamine. The product of the reaction with octopamine as substrate was also 4HPAL. Addition of NAD to reaction mixtures yielded 4-hydroxyphenylacetic acid and NADH. These activities, octopamine hydrolyase and 4-HPAL dehydrogenase (measured as a combined activity, OCAH-4HPALDH), were only induced by growth with phenethanolamines. However, the combined activities were not observed in extracts from cells grown with octopamine as the sole source of carbon and nitrogen, suggesting that an alternate pathway is used under this growth condition. Two independently isolated mutant strains were unable to utilize tyramine as a sole source of carbon or nitrogen. These mutants were also unable to utilize dopamine but grew at wild-type rates on the phenethanolamines. The mutations were mapped at about 70 min on the PAO1 chromosome with the chromosome-mobilizing plasmid R68.45, and both were linked to the catA1, mtu-9002, tyu-9009, and puuE mutations. DNA complementing both of the mutations was cloned on a single BamHI fragment approximately 13.8 kilobase pairs in length. Analysis of a subcloned fragment showed that the two mutations were in different genes.     

Amphetamine-induced release of dopamine from the substantia nigra in vitro

Incubation of chopped tissue from the substantia nigra of the rat brain with d-amphetamine resulted in a significant release of [³H]dopamine into the incubation medium. This effect was observed with both exogenous [³H]dopamine previously taken up by the tissue and [³H]dopamine endogenously synthesized from L-[3,5-³H]tyrosine. The observed release was greater in magnitude when the apparent conversion of released dopamine to 3-methoxytyramine was taken into account. The relevance of the present results to the previously postulated self-inhibition by dopaminergic neurons of the substantia nigra pars compacta is discussed. The present data also provide support for the concept that catechol-O-methyltransferase (E.C.2.1.1.6.) is located primarily extraneuronally in brain.     

Properties of tyrosine hydroxylation in living mouse neuroblastoma clone N1E-115

Tyrosine hydroxylation was studied in intact cells of mouse neuroblastoma clone N1E-115 which have high levels of tyrosine 3-monooxygenase (EC 1.14.16.2) and which have been fully characterized for tyrosine transport. Measurement of [3H]OH formed from L-[3,5(-3)H]tyrosine in the medium was the method of assay and [3H]OH formed was stoichiometric with the formation of L-[3H]3,4-dihydroxyphenylalanine. Tyrosine hydroxylation was dependent on time of incubation, cell number, and the concentration of [3H]tyrosine in the medium. From velocity vs. [3H]tyrosine concentration experiments, two apparent Km values were obtained: Km1 = 10 +/- 2 microM; Km2 = 140 +/- 10 microM. Substrate inhibition occurred with tyrosine concentrations between 20 and 50 microM. The reaction was twice as fast at pH 5.5 as at pH 7.4. alpha,alpha'-Dipyridyl (1 mM) caused major inhibition (75%) when [3H]tyrosine concentration was 10 microM. L-3-Iodotyrosine was a competitive inhibitor with Ki = 0.3 microM. Dopamine was a non-competitive inhibitor with Ki = 500 microM. 1-Norepinephrine had no effect. These results show that the hydroxylation of tyrosine by living N1E-115 cells has many of the properties of the reaction catalyzed by purified tyrosine 3-monooxygenase from normal tissue.     

The effects of 5-hydroxytryptophan and 5-hydroxytryptamine on dopamine synthesis and release in rat brain striatal synaptosomes.

The effects of 5-hydroxytryptophan (5-HTP) and serotonin (5-HT) on dopamine synthesis and release in rat brain striatal synaptosomes have been examined and compared to the effects of tyramine and dopamine. Serotonin inhibited dopamine synthesis from tyrosine, with 25% inhibition occurring at 3 μM-5-HT and 60% inhibition at 200 μM. Dopamine synthesis from DOPA was also inhibited by 5-HT, with 30% inhibition occurring at 200 μ. At 200 μM-5-HTP, dopamine synthesis from both tyrosine and DOPA was inhibited about 70%. When just the tyrosine hydroxylation step was measured in the intact synaptosome, 5-HT, 5-HTP, tyramine and dopamine all caused significant inhibition, but only dopamine inhibited soluble tyrosine hydroxylase [L-tyrosine 3-monooxygenase; L-tyrosine, tetrahydropteridine oxygen oxidoreductase (3-hydroxylating); EC 1.14.16.2] prepared from lysed synaptosomes. Particulate tyrosine hydroxylase was not inhibited by 10 μM-5-HT, but was about 20% inhibited by 200 μM-5-HT and 5-HTP. At 200 μM both 5-HT and 5-HTP stimulated endogenous dopamine release. These experiments suggest that exposure of dopaminergic neurons to 5-HT or 5-HTP leads to an inhibition of dopamine synthesis, mediated in part by an intraneuronal displacement of dopamine from vesicle storage sites, leading to an increase in dopamine-induced feedback inhibition of tyrosine hydroxylase, and in part by a direct inhibition of DOPA decarboxylation.     

Evidence for an ascorbate shuttle for the transfer of reducing equivalents across chromaffin granule membranes

Adrenal chromaffin granules must shuttle reducing equivalents from the cytosol inward to reduce ascorbic acid oxidized during norepinephrine biosynthesis by intragranular dopamine-beta-hydroxylase. A transmembrane electron shuttle between the external (cytosolic) and intragranular ascorbate pools was demonstrated in vitro in intact bovine chromaffin granules undergoing tyramine- or dopamine-stimulated dopamine-beta-hydroxylase turnover. Incubation of intact chromaffin granules with tyramine results in a time-dependent decrease in reduced intragranular ascorbate and production of octopamine. The rate of ascorbate oxidation is a function of the extragranular concentrations of tyramine over the range 50 microM to 2 mM and is 95% inhibited by addition of the dopamine-beta-hydroxylase inhibitor disulfiram. The stoichiometry of octopamine synthesized/ascorbate oxidized closely approximates unity. The presence of extragranular dopamine also induces oxidation of intragranular ascorbate which is inhibited by blocking dopamine transport with reserpine. On the other hand, incubation with octopamine, which is also transported by the granules, causes no net decrease in reduced intragranular ascorbate. The presence of 400 microM extragranular ascorbate abolishes the observed tyramine-induced intragranular ascorbate oxidation. The addition of ascorbate extragranularly 30 min after addition of tyramine reverses the oxidation of intragranular ascorbate. The measurement of [14C]ascorbate distribution ratios in granule pellets and supernatants indicates that there is no transmembrane transport of ascorbate. Extravesicular NADH had no significant effect on matrix ascorbate levels during beta-hydroxylation. These data provide new in vitro evidence that chromaffin granules shuttle reducing equivalents inwardly from an extra- to an intravesicular ascorbate pool and that cytosolic ascorbate is the source of the intragranular reducing equivalents required during norepinephrine biosynthesis.     

Release of aromatic amines from brain tissues of the rat in vitro

Radioactive β-hydroxylated phenylethylamines, DL-octopamine and DL-metara-minol, as well as DL-norepinephrine, were released from slices or minces of rat brain in vitro by electrical or ionic depolarizing stimuli. In contrast, labelled D-amphetamine and p-tyramine were poorly released and the release of urea was barely perceptible. Addition of unlabelled L-norepinephrine to an incubating medium led to displacement of previously bound [³H]octopamine and [³H]metaraminol from a synaptosomal preparation. Pretreat-ment of rats with intrathecal 6-hydroxydopamine markedly decreased the ability of slices of their brains to release [³H]norepinephrine and [³H]octopamine. We suggest that certain phenolic β-hydroxylated phenylethylamines may enter central, norepinephrine-containing nerveendings, may be stored in presynaptic vesicles and may be released by nerve stimulation. Under certain circumstances such compounds may act as neurotransmitters with unknown post-synaptic effects.     

Dopamine Transporter Loss in 6-OHDA Parkinson’s Model Is Unmet by Parallel Reduction in Dopamine Uptake

The dopamine transporter (DAT) regulates synaptic dopamine (DA) in striatum and modulation of DAT can affect locomotor activity. Thus, in Parkinson’s disease (PD), DAT loss could affect DA clearance and locomotor activity. The locomotor benefits of L-DOPA may be mediated by transport through monoamine transporters and conversion to DA. However, its impact upon DA reuptake is unknown and may modulate synaptic DA. Using the unilateral 6-OHDA rat PD model, we examined [³H]DA uptake dynamics in relation to striatal DAT and tyrosine hydroxylase (TH) protein loss compared with contralateral intact striatum. Despite >70% striatal DAT loss, DA uptake decreased only ∼25% and increased as DAT loss approached 99%. As other monoamine transporters can transport DA, we determined if norepinephrine (NE) and serotonin (5-HT) differentially modulated DA uptake in lesioned striatum. Unlabeled DA, NE, and 5-HT were used, at a concentration that differentially inhibited DA uptake in intact striatum, to compete against [³H]DA uptake. In 6-OHDA lesioned striatum, DA was less effective, whereas NE was more effective, at inhibiting [³H]DA uptake. Furthermore, norepinephrine transporter (NET) protein levels increased and desipramine was ∼two-fold more effective at inhibiting NE uptake. Serotonin inhibited [³H]DA uptake, but without significant difference between lesioned and contralateral striatum. L-DOPA inhibited [³H]DA uptake two-fold more in lesioned striatum and inhibited NE uptake ∼five-fold more than DA uptake in naïve striatum. Consequently, DA uptake may be mediated by NET when DAT loss is at PD levels. Increased inhibition of DA uptake by L-DOPA and its preferential inhibition of NE over DA uptake, indicates that NET-mediated DA uptake may be modulated by L-DOPA when DAT loss exceeds 70%. These results indicate a novel mechanism for DA uptake during PD progression and provide new insight into how L-DOPA affects DA uptake, revealing possible mechanisms of its therapeutic and side effect potential.