Dietary disproportions of amino acids in the rat: effects on food intake, plasma and brain amino acids and brain serotonin.

Food intake, growth, plasma and brain amino acid, and brain serotonin and 5-hydroxyindole-3-acetic acid (5-HIAA) concentrations were measured in rats fed low protein diets containing disproportionate amounts of large neutral amino acids (LNAA) devoid of tryptophan or histidine (tryptophan or histidine imbalance). Five-day food intakes and weight gains of rats fed the imbalanced diets were depressed. The concentration of the limiting amino acid was low in brains of rats fed diets containing LNAA that compete with either tryptophan or histidine for entry into brain. Correlations were observed between the brain concentrations of most individual LNAA and either the ratios of the plasma concentration of that LNAA to the sum of the other LNAA, or the predicted rates of influx of that LNAA. Cumulative food intakes were correlated with brain concentrations of the limiting amino acid, tryptophan or histidine. Food intakes were not consistently correlated with concentrations of serotonin and 5-HIAA because these compounds were altered only in brains of rats in the tryptophan study. Competition among amino acids for uptake into brain appears to be involved in the feeding response of the rat to dietary disproportions of amino acids, but this response is not directly related to changes in brain concentrations of serotonin and 5-HIAA.