ESTIMATION OF NORADRENALINE AND ITS MAJOR METABOLITES SYNTHESIZED FROM [³H]TYROSINE IN THE RAT BRAIN

Three major metabolites of [3H]noradrenaline and its major metabolites free and conjugated 3‐methoxy‐4‐hydroxyphenylglycol (MOPEG) and free andconjugated 3,4‐dihydroxyphensyl Glycol (DOPEGI) are estimated in the rat brain using adsorption on to alumina, cation exchange chromatography and enzymatic hydrolysis of conjugates and thin

Abstract

Abstract— A new procedure is described for the estimation of [ 3 H]noradrenaline (NA) and its major metabolites free and conjugated 3‐methoxy‐4‐hydroxyphenylglycol (MOPEG) and free and conjugated 3,4‐dihydroxyphenylglycol (DOPEGI in the rat brain. The procedure involves adsorption on to alumina, cation exchange chromatography. enzymatic hydrolysis of conjugates and thin‐layer‐chromatography after intraventricular (IVT) or intravenous injection of [ 3 H]tyrosine. In a time‐course study the formation and accumulation of the metabolites have been measured from 15min to 23h after IVT injection of [ 3 H]tyrosine. [ 3 H]MOPEG and [ 3 H]DOPEG were found in almost equal amounts during the synthesis phase of [ 3 H]NA as well as during the storage and disappearance phase of [ 3 H]NA. The maximum levels of conjugated [ 3 H]MOPEG and conjugated [ 3 H]DOPEG were found 2 h after IVT [ 3 H]tyrosine. At this time interval the levels of free [ 3 H]MOPEG and free [ 3 H]DOPEG amounted to 25% and 11%, respectively of the corresponding conjugates. Increasing doses of IVT injected [ 3 H]tyrosine (10‐90 °Ci) revealed that the accumulation of [ 3 H]NA and metabolites was linear up to about 50 °Ci. Following intravenous instead of IVT injection of [ 3 H]tyrosine. much higher doses (325 °Ci) were needed to obtain measurable amounts of total [ 3 H]MOPEG and [ 3 H]DOPEG‐SO 4 in the rat brain. The formation of labelled NA metabolites from [ 3 H]NA in the rat brain in vim measured as total [ 3 H]MOPEG and [ 3 H]DOPEG‐SO 4 was influenced by drugs affecting [ 3 H]NA synthesis, release and metabolism. Synthesis inhibition with a‐methyltyrosine (250mg‐kg −1 ) or FLA‐63 (30mg‐kg −1 ) and inhibition of monoamine oxidase with pargyline (75mg‐kg −1 ) or clorgyline (2mg‐kg −1 ) strongly decreased the accumulation of total [ 3 H]MOPEG and [ 3 H]DOPEG‐SO 4 . Noradrenaline receptor blockade with phenoxybenzamine (20mg‐kg −1 ) increased both total [ 3 H]MOPEG and [ 3 H]DOPEG‐SO 4 to about 160% of the control values. NA release and uptake inhibition induced by d‐amphetamine (10mg‐k −1 ) or phenylethylamine (two doses of 80mg‐kg −1 ) decrease strongly the levels of [ 3 H]NA and [ 3 H]DOPEG‐SO 4 . whereas total [ 3 H]MOPEG was only very slightly decreased or even increased as compared to controls.