• 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.
During the baseline days no statistically significant differences were found between the patients and controls in either total or free plasma tryptophan levels. For controls, there was no change in total tryptophan, but a significant decrease occurred in free plasma tryptophan concentrations while receiving carbidopa (P <.05). In patients, the perturbing effects of carbidopa resulted in an increase in both total (P <.05) and free (P <.05) plasma tryptophan levels. These results suggest that an altered flux of tryptophan metabolism may exist in depressed patients that is uncovered by the administration of an extracerebral decarboxylase inhibitor.