Effects of L-Dopa on Norepinephrine Meta

Rats received intracisternal (3H)norepi- nephrine and, after 5 min, intraperitoneal L-dopa (100 mg/kg); they were killed at intervals during the subse- quent 24 hr. Their brains were assayed for norepinephrine, L-dopa, S-adenosylmethionine, and (3Hlnorepinephrine and its metabolites. Rats that received L-dopa had mark- edly lower brain concentrations of S-adenosylmethionine and of O-methylated metabolites of (3H)norepinephrine than controls. An increase (15-40%) in brain norepineph- rine content and accelerated turnover of brain norepi- nephrine was also observed in the animals receiving L-dopa. These changes were all transient, lasting about 1 hr and coinciding with the period when appreciable amounts of L-dopa were detectable in the brain. The initial enzymatic reaction in the biosynthesis of brain catecholamines involves the formation of the catechol amino acid L-dihydroxyphenylalanine (L-dopa) from L-tyrosine.