Simultaneous determination of the formation rate of dopamine and its metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in various rat brain areas.

Administration of the MAO-inhibitor pargyline resulted in an increase of dopamine (DA) and an exponential decrease of the metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in various rat brain areas. From these curves the rate of formation of DA and the rate of removal of DOPAC were calculated, within one experiment, for the striatum, tuberculum olfactorium and frontal cortex. When the accumulation rate of 3-methoxytyramine was taken into account it appeared that the rate of DA formation was in good agreement with the rate of DOPAC removal in the brain areas studied. The contribution of the DA formation to the synthesis of noradrenaline in the frontal cortex appeared negligible. The earlier reported rapid rise of striatal DA levels after administration of aminotetralin-derived DA agonists suggests that the synthesis rate of the biogenic amine is higher than its rate of metabolism. It appeared from the present study that an increased rate of synthesis of DA induced by the DA agonists was responsible for this discrepancy.

Effects of drugs interfering with dopamine and noradrenaline biosynthesis on the endogenous 3,4-dihydroxyphenylalanine levels in rat brain.

A chemical assay of 3,4-dihydroxyphenylalanine (DOPA) in nervous tissue is described. The method is based on a rapidly performed isolation of DOPA on small Sephadex G-10 columns, followed by reverse-phase HPLC with a trichloroacetic acid-containing eluent, in conjunction with a rotating disk electrochemical detector. The detection limit of the assay (about 100 pg/tissue sample) permits a detailed investigation of the regional distribution of endogenous DOPA levels in the rat brain. DOPA as well as dopamine (DA) could be quantified in the same chromatographic run. The assay was applied to a study of the effects of alpha-methyl-p-tyrosine, apomorphine, chlorpromazine, clonidine, gamma-butyrolactone, haloperidol, morphine, oxotremorine, pargyline, reserpine, and tyrosine methylester on the concentration of DOPA in the striatum, hypothalamus, frontal cortex, and cerebellum of the rat brain. Drugs known to interact with DA biosynthesis all caused characteristic changes of the DOPA content in the striatum and not in nondopaminergic brain areas. A close correlation existed between drug-induced changes in tyrosine hydroxylase activity and changes in the DOPA content in the striatum. Tyrosine methylester increased DOPA concentrations in all brain areas studied.

On the significance of endogenous 3-methoxytyramine for the effects of centrally acting drugs on dopamine release in the rat brain.

3-Methoxytyramine (3-MT) was measured in the striata of rats killed by microwave radiation. Apomorphine, gamma-butyrolactone (GBL), and reserpine decreased the 3-MT content. A slight but transient increase in 3-MT was observed after haloperidol. The turnover rate of 3-MT was unchanged 60 min after haloperidol treatment. (+)-Amphetamine induced a pronounced rise in the 3-MT content, which was potentiated after combined treatment with haloperidol. The increased 3-MT turnover rate that was observed after amphetamine treatment suggests that monoamine oxidase (MAO) inhibition is no explanation for the mechanism of interaction of this drug with dopamine (DA) metabolism. The central stimulants amphonelic acid and nomifensine increased 3-MT levels; no substantial change was seen after benztropine, morphine, or oxotremorine. It is concluded that a decreased release of DA is closely and rapidly reflected by decreased formation of 3-MT. 3-MT seems to be a much better indicator for decreased DA release than 3,4-dihydroxyphenylacetic acid or homovanillic acid.

Estimation of the turnover of 3-methoxytyramine in the rat striatum by HPLC with electrochemical detection: implications for the sequence in the cerebral metabolism of dopamine.

A highly sensitive method for the determination of 3-methoxytyramine (3-MT) in nervous tissue is described. The method is based on a rapidly performed isolation of 3-MT on small columns of Sephadex G 10, followed by reverse-phase high-performance liquid chromatography in conjunction with a rotating disk electrochemical detector. The detection limit of the assay (0.5-1 pmol/tissue sample) is about 10% of control value for microwave-killed rats. 3-MT as well as dopamine could be quantified in the same chromatographic run. Inhibition of catechol-O-methyl transferase with tropolone resulted in an exponential decline of 3-MT. From this exponential decline a turnover rate for 3-MT of 1.9 nmol/gh/h was calculated. In the same group of rats the turnover rate of homovanillic acid was 9.1 nmol/g/h. From these data it is concluded that in the rat striatum about 80% of homovanillic acid is formed from 3,4-dihydroxyphenylacetic acid and 20% from 3-MT.