The acute tryptophan depletion and loading tests: Specificity issues

The acute tryptophan (Trp) depletion (ATD) and loading tests are a powerful tool for examining the role of serotonin in normal subjects and in those with behavioral and other disorders. The question of how specific to serotonin the tests are, has, however, received little attention. In particular, two questions arise regarding the potential effects of the tests on catecholamine synthesis, which may confound interpretation of behavioral data, namely whether: (1) the absence of Trp or its addition alters the [Phe + Tyr]/[LNAA + Trp] ratio, thereby influencing the entry of tyrosine and phenylalanine into the brain; (2) the relatively larger amounts of the 3 large neutral amino acids (LNAA), namely Val, Leu and Ile, in the amino acid formulation, compared with those of Phe and Tyr, could actually decrease the above ratio. Furthermore, the [Phe + Tyr]/[LNAA + Trp] and/or the [Trp]/[CAA] ratio may also be altered in control balanced and/or in Trp-loaded amino acid formulations, given the disproportionate contents of these 6 competitors in the different formulations. We addressed these questions in 114 normal US subjects divided into 5 groups of 20–25 each receiving Trp-depleted, -loaded or -balanced amino acid formulations at two dose levels: 50 g and the traditional 100 g. As expected, the [Trp]/[CAA] ratio was decreased by ATD and increased after Trp loading. However, the [Phe + Tyr]/[LNAA + Trp] ratio was also decreased by both ATD and Trp loading at both dose levels of each formulation. In subjects receiving a 50 g balanced control amino acid formulation, both the [Trp]/[CAA] and the [Phe + Tyr]/[LNAA + Trp] ratios were decreased. In all cases, the above unwanted decreases were due to the relatively larger contents in the various formulations of the above 3 LNAA, compared with Phe and Tyr. Based on these results and on theoretical considerations, we suggest that by either decreasing the contents of the LNAA by up to ∼ 30% or increasing those of Phe and Tyr by up to ∼ 50%, the [Phe + Tyr]/[LNAA + Trp] ratio can be maintained at normal baseline levels after consumption of the depletion, loading or balanced formulation and the [Trp]/[CAA] ratio can also be kept unaltered in the latter control formulation. We believe that this approach could achieve a greater specificity for these Trp manipulations and thus enhance the validity of the ATD and loading tests and improve interpretation of behavioral and related data.