Uptake of 5-hydroxy[14C]tryptophan by rat and dog brain slices

Slices of rat brain take up and concentrate 5-hydroxy[(14)C]tryptophan when incubated at 37 degrees C in vitro. Uptake is inhibited by methyldopa and L-phenylalanine in a competitive manner. Only a small proportion of the 5-hydroxy[(14)C]tryptophan taken up is decarboxylated. This observation, and the lack of correlation between the effects of various amino-acid inhibitors on uptake and on decarboxylase, suggest that the two processes are not linked. In the cold some passive diffusion of the amino-acid occurs, as is thought to take place also at 37 degrees C. Experiments with dog brain show that uptake varies from one part of the brain to another, being greatest in the caudate nucleus. The possible implications of the uptake mechanism in the treatment of animals or patients with large doses of amino-acids are discussed.