Studies on the depletion of brain amines by m-tyrosine

The biochemical basis of the well-known physiological and pharmacological actions of m-tyrosine was examined by a detailed study of its effect on the brain biogenic amines. m-Tyrosine was injected i.p. and rat brain monoamine levels were measured. Endogenous levels of dopamine, norepinephrine and serotonin all showed approximately 50% reductions 1 h after the administration of L-m-tyrosine at 150 mg/kg. These actions of L-m-tyrosine could be blocked by the inhibition of the central dopa decarboxylase. Depletion of brain monoamines was also observed with the D-isomer of m-tyrosine, although this effect was less pronounced than that of the L-isomer. In vitro experiments with rat brain homogenates showed that L-m-tyrosine, m-tyramine and m-octopamine enhanced in efflux of exogenous labeled monamines from brain particles, whereas D-m-tyrosine was completely ineffective. From these results it is concluded that the observed decreased in brain monamine levels by L-m-tyrosine may be due to a m-tyramine-enhanced release of the amines which are quickly metabolized in vivo.