Measurement of tritiated norepinephrine metabolism in intact rat brain

A procedure for the study of NE metabolism in the intact rat brain is described. The method involves ventriculocisternal perfusion of the adult male rat with artificial CSF containing [3H]NE. Radioactivity in the perfusate associated with NE and its metabolites 3,4-dihydroxymandelic acid (DOMA), 3,4-dihydroxyphenylethyleneglycol (DHPG), 3-methoxy-4-hydroxymandelic acid (VMA), 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG), and normetanephrine (NMN) is separated using high-performance liquid chromatography (HPLC). After 80 min the radioactivity in the perfusate reaches an apparent steady-state. Analysis of the steady-state samples shows higher activity in the fractions corresponding to DHPG and MHPG than in those corresponding to DOMA and VMA, confirming glycol formation as the major pathway of NE metabolism in rat brain. Pretreatment with an MAO inhibitor (tranylcypromine) results in a marked decrease in the deaminated metabolites DHPG and MHPG and a concurrent increase in NMN. The results indicate this to be a sensitive procedure for the in vivo determination of changes in NE metabolism.