ACCUMULATION AND DISAPPEARANCE OF NORADRENALINE AND ITS MAJOR METABOLITES SYNTHESIZED FROM INTRAVENTRICULARLY INJECTED [ 3 H]DOPAMINE IN THE RAT BRAIN

— A new combined ion‐exchange and thin‐layer‐chromatographic procedure is described which separates and measures quantitatively, after intraventricular injection of [ 3 H]dopamine (DA), the rat brain content of labelled noradrenaline (NA) and the following labelled noradrenaline metabolites: free 3‐methoxy‐4‐hydroxyphenylethyleneglycol (MOPEG), conjugated MOPEG, free plus conjugated dihydroxyphenylethyleneglycol (DOPEG), vanillic mandelic acid (VMA) and normetanephrine (NM). Labelled dopamine and its metabolites were also measured. The time‐course study performed from 5 min to 24 h after [ 3 H]DA showed that MOPEG and DOPEG, mainly as conjugates, are major NA metabolites whereas VMA is a very insignificant NA metabolite in the rat brain. A very rapid initial increase of [ 3 H]NM, free MOPEG and conjugated MOPEG was found during the time interval where the [ 3 H]NA biosynthesis is very high (0–15 min). This combined with the finding that these metabolites stabilize at lower levels during the [ 3 H]NA ‘storage phase’ (9–24 h) provides a strong indication that newly synthesized NA preferentially is metabolized. Our measurements of endogenous NA, free MOPEG and conjugated MOPEG provide additional support. The injections of various decreasing doses of [ 3 H]DA (3·08–0·0010 μg) showed that the proportions of total [ 3 H]MOPEG and total [ 3 H]DOPEG to [ 3 H]NA were constant after all [ 3 H]DA doses investigated. This finding indicates that the [ 3 H]NA synthesized in situ behaves as a tracer, even after injections of non‐tracer doses of [ 3 H]DA. The results seem thus to indicate that the present technique provides a powerful tool for the investigations on central noradrenaline metabolism.