Specific Activity Of Dopamine Research Articles

Computational simulations of dopaminergic varicosities suggest two sources of DOPAC rather than two populations of dopamine storage vesicles

Results from several different experimental paradigms have been interpreted as supporting the concept that two populations of dopamine storage vesicles exist in the nerve terminals of dopaminergic neurons. The goal of this work is to develop a computer simulation model of a dopaminergic varicosity that provides a plausible quantitative description of these populations and a possible set of rules for dopamine movement between two populations of vesicles. We first looked at how well a one compartment model provides accurate simulations of published experimental data. The model allocates dopamine among three compartments: vesicles, cytosol, and extracellular. Dopamine moves from vesicles to extracellular (exocytosis), extracellular to cytosol (dopamine transporter), and from cytosol to vesicles (vesicular monoamine transporter). Synthesis of new dopamine molecules occurs in the cytosolic compartment, with new dopamine entering that compartment. Metabolism of dopamine also occurs in the cytosolic compartment, with a fraction of the dopamine in that compartment being metabolized to DOPAC. With appropriate values for all rate constants, this model successfully explains all data purportedly supporting two populations of storage vesicles in paradigms that stimulate dopaminergic neurons at rates much faster than physiological. However, this model does not explain other data that support the two populations of vesicles concept. Models with two storage vesicle compartments were evaluated for ability to explain these data; however, none were successful. An alternate model was developed from the one storage compartment model but with the addition that the dopamine synthetic process has a branch point where newly synthesized dopamine is either secreted to the extracellular space or converted to DOPAC, which is deposited into the cytosolic compartment. This model successfully explains data regarding the specific activity of dopamine and metabolites after injection of labeled tyrosine into the varicosity, dopamine metabolite kinetics after inhibition of dopamine synthesis, and preferential secretion of newly synthesized dopamine. Thus, our model suggests that dopaminergic varicosities have two sources of DOPAC, one likely associated with mitochondria and the other associated with the dopamine synthetic complex.

Monitoring the specific activities of dopamine and its metabolites in striatum and olfactory tubercle after intravenous administration of l-[ 3H]tyrosine: Complex relations, indicating more than two compartments

It is still a matter of debate whether in dopaminergic nerve endings dopamine (DA) is present in different functional and/or metabolic compartments. To investigate this, DA metabolism was studied in vivo by measuring the specific activity of DA and its metabolites after intravenous administration of l-[3,5- 3H]tyrosine (200 μCi/rat) to freely moving animals. The incorporation of 3H into DA and metabolites was determined in striatum and olfactory tubercle at 5, 10, 20, 40, 60 and 80 min after [ 3H]tyrosine administration. In both structures the level of [ 3H]tyrosine initially declined monoexponentially, but deviated from that pattern later on. The curves representing the formation in time of [ 3H]DA and [ 3H]metabolites were very similar in both structures, although as a whole, the levels in the olfactory tubercle were higher. The relative patterns of the specific activities of DA and those of its metabolites, a possible clue to DA compartmentation, neither indicated a clearcut metabolic one-compartment, nor a two-compartment system. The flow of radioactivity through DA metabolism could in fact only be explained by assuming more complex metabolic relations.

Mobilization of storage pool dopamine and late ipsilateral augmentation of striatal dopamine synthesis in the trained circling rat.

Rats were infused intraventricularly with [3H]tyrosine over a 20-min period during various times while circling. 3,4-Dihydroxyphenylethylamine (dopamine) and dihydroxyphenylacetic acid (DOPAC) levels were measured using HPLC with electrochemical detection and fractions were collected for tritium monitoring. During the first 20 min of circling, the specific activity of dopamine was increased by 290% in striatum contralateral to the circling direction whereas DOPAC specific activity was increased 50% on the same side. This differential change in relative specific activity suggests that unlabeled storage pool dopamine was mobilized to DOPAC during circling. Synthesis of dopamine and DOPAC in contralateral striatum returned to baseline levels as turning slowed (50-70 min). When turning ceased, there was an increase in ipsilateral striatal dopamine synthesis during the 20-min period following circling. We hypothesize that this ipsilateral increase represents either a "stop" signal following circling or a release of inhibition of ipsilateral nigral neurons.

Disappearance of newly synthesized and total dopamine from the striatum of the rat after inhibition of synthesis: evidence for a homogeneous kinetic compartment.

Abstract— The hypothesis that the biphasic disappearance of dopamine (DA) from the rat striatum following inhibition of synthesis with α‐methyl‐p‐tyrosine (AMPT) represents catabolism from separate‘functional’and 'storage’compartments (Javoy & Glowinski, 1971) was tested by administering 3H‐tyrosine i.v. 10 min before 400 mg per kg AMPT. The levels of newly synthesized 3H‐DA and total DA were then determined at 5‐min intervals. While both declined biphasically, the rate of decay of 3H‐DA was significantly less than that predicted by the two‐pool hypothesis. In fact, the patterns of change of 3H‐DA and total DA were identical and the specific activity of DA did not vary following AMPT. To determine if an increase in DA synthesis and release would result in the preferential catabolism of newly synthesized 3H‐DA, the experiment was repeated fol!owing treatment with 0.1 mg per kg haloperidol i.v. 3H‐DA and total DA still exhibited identical biphasic declines and there was no change in the specific activity of DA after AMPT even though 3H‐DA levels were increased three‐fold by haloperidol. Thus (a) no evidence for the preferential catabolism of newly synthesized 3H‐DA was obtained and (b) newly synthesized and total striatal DA behaved as if localized in a homogeneous kinetic compartment under all conditions employed.Rates of striatal DA metabolism were estimated in both experiments from changes in total DA after AMPT and by the formation of 3H‐DA in the initial 10 min after 3H‐tyrosine injection. Results of both approaches were consistent and indicated that the rates of striatal DA synthesis and catabolism may vary rapidly between approx. 20 and 90 nmol per g per h without violating single compartment kinetics. It is proposed that any labile pool contains no more than a few per cent of the DA in the striatum and the implications of this hypothesis are discussed.

Peptides and the conversion of [ 3H]tyrosine to catecholamines: Effects of ACTH-analogs, melanocyte-stimulating hormones and lysine-vasopressin

ACTH 4–10, [ D-Phe 7]ACTH 4–10, ACTH 1–24, α-MSH and lysine-vasopressin (LVP) were tested for their effects on the cerebral content and metabolism of dopamine (DA) and norepinephrine (NE). CD-1 mice were injected daily for 3 days with equimolar concentrations of long-acting preparations of the peptides. Twenty-four hours after the third peptide treatment, mice were injected with [ 3H]tyrosine and the specific activities of tyrosine, DA and NE were measured 10 min later. ACTH 1–24, ACTH 4–10 or [ D-Phe 7]ACTH 4–10 stimulated the conversion of [ 3H]tyrosine to DA but did not alter the cerebral dopamine content, suggesting that the turnover of dopamine was increased. α-MSH, β-MSH or LVP did not show this effect. Since ACTH 4–10 has low steroidogenic activity it appears that the effect on [ 3H]DA metabolism was a direct effect of the peptide and not mediated by increased corticosterone secretion. None of the peptides had any detectable effect on the metabolism of [ 3H]NE. ACTH 1–24 but no other peptide decreased the cerebral concentration of NE. In mice adrenalectomized one day before the first peptide injection, ACTH 4–10 did not alter the specific activity of DA or NE.

Factors modifying the synthesis of dopamine from tyrosine in pedal ganglia of Mercenaria mercenaria (mollusca)

1. 1. The dopamine (DA) and 5-hydroxytryptamine (5-HT) contents of the ganglia of Mercenaria mercenaria expressed as ng. per ganglion were as follows: Pedal ganglion: 236±16 ng. DA 101± NG. 5-HT. Visceral ganglion: 76±3 ng. DA 116±7 NG. 5-HT. Cerebral ganglion: 37±5 ng. DA 107±15 ng. 5-HT. 1. 2. Pedal ganglia synthesize [ 14C]DA from [ 14C]tyrosine but do not form significant amounts of [ 14C]noradrenaline (NA). [ 14C]5-HT is also formed from [ 14C]tryptophan in pedal ganglia. 2. 3. Incubation of pedal ganglia in KCl-rich sea water accelerates synthesis of [ 14C]DA from [ 14C]tyrosine two- to threefold compared with controls. The specific activity of DA isolated from the K +-stimulation ganglia is corresponding increased. 3. 4. The acceleration of DA synthesis appears to take place at the tyrosine hydroxylation step. 4. 5. Synthesis of DA from tyrosine in pedal ganglia is inhibited up to 60 per cent by 10 −4 M DA in the medium.