Tryptophan Availability and Serotonin Synthesis in Brain

Abstract

One of the most important factors governing the rate of serotonin (5HT) synthesis in the mammalian brain is the local concentration of the substrate amino acid, L-tryptophan (see (1)). As a result, a variety of factors that influence brain tryptophan levels are also found to modify 5HT synthesis. These include such phenomenon as the ingestion of single meals, stress, the development of diabetes, and certain forms of malnutrition (see (1)). Where studied, it has been possible to show that when 5HT synthesis has been modified by one of the above phenomenon, 5HT release is also changed, as evidenced by effects on particular brain functions thought to be regulated in part by 5HT neurons. This article reviews data that have established connections between brain tryptophan levels and 5HT synthesis, and between tryptophan-related changes in 5HT synthesis and 5HT release at the synapse. It also considers evidence linking meal ingestion to alterations in brain tryptophan levels and 5HT synthesis, and food-induced changes in 5HT synthesis to appetites for a specific macronutrient (carbohydrate). It should be evident from the dis­cussion that while a substantial amount of evidence can now be assembled to support many of these relationships, several require additional experimental validation before they can be fully evaluated or accepted.