Pyridoxine deficiency in post-weanling rats caused a marked decrease in body weight and a small but significant decrease in brain weight. Although the concentration of circulating 5-hydroxytryptamine was markedly decreased, the concentrations of 5-hydroxytryptamine and noradrenaline in the brain were not affected. p-Chlorophenylalanine, an inhibitor of 5-hydroxytryptamine synthesis, decreased the 5-hydroxytryptamine content of brain to very low values in both the deficient and control animals, whereas the noradrenaline contents were not appreciably affected. The concentration of 5-hydroxytryptamine in blood, the origin of which is primarily gastrointestinal, was decreased only in the controls but not in the deficient animals after p-chlorophenylalanine treatment. These results suggest that whereas l-tryptophan hydroxylase (EC is rate-limiting in the brain as has been reported by others, the pyridoxal 5′-phosphate-dependent enzyme 5-hydroxytryptophan decarboxylase (EC may be more important in the gastrointestinal tract in the regulation of 5-hydroxytryptamine synthesis.

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