Cerebrospinal fluid 5-hydroxyindoleacetic acid (5-HIAA), the major metabolite of serotonin, was assayed in a group of five depressed patients before and during treatment with L-tryptophan, with and without probenecid. L-tryptophan administration was associated with a slight increase in the baseline levels of 5-HIAA, but caused a twofold to threefold increase in the accumulation of 5-HIAA in the cerebrospinal fluid following probenecid administration. The apparent increase in brain serotonin turnover produced by L-tryptophan in these patients was not accompanied by an improvement in depression.
Dunner DL, Goodwin FK. Effect of L-Tryptophan on Brain Serotonin Metabolism in Depressed Patients. Arch Gen Psychiatry. 1972;26(4):364-366. doi:10.1001/archpsyc.1972.01750220074014