Food Intake Regulation: Amino Acid Toxicity and Changes in Rat Brain and Plasma Amino Acids

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Abstract
Food intake and changes of brain and plasma amino acid concentrations of young rats fed diets high in individual amino acid (6% casein diet plus 5% of L-methionine, L-tryptophan, L-histidine, L-leucine, L-phenylalanine, DL-threonine, L-lysine or L-glutamic acid) were investigated. Inclusion of methionine or tryptophan in the diet produced the most severe depressions in food intake and growth, followed in decreasing order by histidine, leucine and phenylalanine, lysine and threonine, and glutamic acid. Brain and plasma concentrations of the amino acids included in diets to create the amino acid toxicity were elevated to varying degrees. The elevation was less in brain than in plasma, indicative of a blood–brain barrier. Ingestion of high glutamic acid diet had little effect on concentrations of other brain amino acids. However, ingestion of diets containing methionine, tryptophan, histidine, leucine or phenylalanine in excess depressed the concentrations of indispensable amino acids other than threonine, lysine and arginine. Ingestion of high threonine or high lysine diet depleted brain isoleucine and leucine and/or arginine. The patterns of depletions agree with evidence that there is more than one amino acid transport system in the brain. The importance of relative concentrations of blood amino acids which share a single transport system is discussed in relation to food intake depression.