Oral Administration of L-serine Modifies Amino Acid Metabolism in the Brain of Rats

Objective: Whether orally administered L-serine influences amino acid metabolism in the brain was investigated. Methods: L-Serine (6 mmol/10 ml/kg) was orally administered to rats after a 9 hr fast and plasma and brain samples were obtained at 30, 60 and 120 min in Experiment 1. In Experiment 2, the effect of L-serine and glycine on brain amino acid metabolism was compared. Results: In Experiment 1, plasma serine rapidly increased 30 min after administration and then decreased, but serine in both the cerebral cortex and hippocampus gradually increased with time. Plasma taurine and glutamine were positively and leucine, phenylalanine and lysine were negatively correlated with plasma serine concentration. Cerebral cortex glutamine, citrulline, tyrosine, β-alanine, and tryptophan were negatively and α-amino adipic acid and glycine were positively correlated with serine concentration. In the hippocampus, glutamine and tyrosine were negatively correlated with L-serine. In Experiment 2, threonine concentrations were significantly decreased by L-serine treatment compared with the control (distilled water) and glycine in both brain regions. Serine concentrations in both brain regions were greatly increased by L-serine treatment, but were also moderately increased by glycine. Similarly, glycine in both regions was increased by glycine treatment and by L-serine treatment. Cysteine in both regions was increased by glycine treatment. Tyrosine in both regions was decreased by both L-serine and glycine treatments. Conclusion: Orally administered L-serine greatly influenced brain amino acid metabolism. L-Serine and glycine influenced each other, but the effect on brain amino acid metabolism was somewhat different between the two amino acids.