Total and free plasma tryptophan levels in patients with affective disorders: effects of a peripheral decarboxylase inhibitor.

Previous reports of decreased cerebrospinal fluid tryptophan levels and decreased free plasma tryptophan levels, as well as a reduction in the volume of distribution of tryptophan, suggest that alterations in the disposition of plasma tryptophan may occur in depressives. We examined the disposition of plasma tryptophan in ten normal controls and ten depressed patients. These measures were made on two drug-free baseline days and on two days when the subjects had been receiving the peripheral decarboxylase inhibitor, carbidopa, which inhibits tryptophan metabolism via extracerebral indoleamine pathways.