Effects of L-Dopa on Norepinephrine Metabolism in the Brain

Rats received intracisternal [(3)H]norepinephrine and, after 5 min, intraperitoneal L-dopa (100 mg/kg); they were killed at intervals during the subsequent 24 hr. Their brains were assayed for norepinephrine, L-dopa, S-adenosylmethionine, and [(3)H]norepinephrine and its metabolites. Rats that received L-dopa had markedly lower brain concentrations of S-adenosylmethionine and of O-methylated metabolites of [(3)H]norepinephrine than controls. An increase (15-40%) in brain norepinephrine content and accelerated turnover of brain norepinephrine 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.