Effect of an oral tryptophan/carbohydrate load on tryptophan, large neutral amino acid, and serotonin and 5-hydroxyindoleacetic acid levels in monkey brain.

Plasma and brain levels of tryptophan and other large neutral amino acids, and brain levels of serotonin and 5-hydroxyindoleacetic acid (5 HIAA) were measured in groups of adult cynomolgus monkeys 1 hr after they ingested one of four doses of a tryptophan-carbohydrate mixture. The doses had been administered once daily for 13 weeks. Dose-related increments occurred in plasma tryptophan, the plasma ratio of tryptophan to the sum of other large neutral amino acids, and in brain tryptophan levels. In contrast, the plasma ratios and brain levels of the other neutral amino acids each declined. Serotonin and 5 HIAA levels increased significantly, and in a dose-related manner in the brainstem and striatum, but not in cortex or hypothalamus. The results suggest that while tryptophan administration can stimulate serotonin production in primate brain, the effect may be restricted to certain brain regions. They also suggest that the transport of the large neutral amino acids into brain occurs via a competitive mechanism similar to that for other mammals.