Acute reduction of brain serotonin and 5-HIAA following food consumption: correlation with the ratio of serum tryptophan to the sum of competing amino acids.

In a series of experiments designed to test further our hypothesis that brain tryptophan and 5-hydroxyindole levels vary post-prandially as a function of the serum tryptophan to competitor ratio (i.e., ratio of serum tryptophan concentration to the sum of the concentrations of competing neutral amino acids), rats fasted overnight were allosed to consume for two hours single meals containing carbohydrates, fats, and various amino acids: Diet 1 contained no amino acids; Diet 2 contained five neutral amino acids known to compete with tryptophan for uptake into the brain; Diet 3 lacked tryptophan and the five competitors, but contained twelve non-neutral amino acids found in dietary protein; Diet 4 contained tryptophan and these same twelve non-neutral amino acids. The diet-induced modifications in brain tryptophan and 5-hydroxyindoles followed closely the changes in the serum tryptophan/competitor ratio, but not in serum tryptophan alone: The ingestion of Diet 4 raised serum tryptophan, but not the serum ratio, brain tryptophan or 5-hydroxyindoles; Diet 3 did not raise serum tryptophan, but did elevate the serum ratio, brain tryptophan and 5-hydroxyindoles. Diet 2 caused major reductions in serum tryptophan, the serum ratio, and brain tryptophan and the 5-hydroxyindoles. This diet might provide a simple, attraumatic means for lowering brain serotonin in studies of the physiology or serotonergin neurons. These studies provide further evidence that the effect of insulin on brain tryptophan uptake is not direct, but instead mediated via insulin-induced changes in the serum tryptophan/competitor ratio.