Soy peptide ingestion augments the synthesis and metabolism of noradrenaline in the mouse brain

To examine whether edible peptide intake affects neurotransmitter metabolism in the brain, we evaluated the effect of peptides derived from soy proteins or fish collagen on free amino acids and monoamines in the mouse brain. Ingestion of soy peptides led to markedly higher levels of tyrosine, a catecholamine precursor, in the serum, and cerebral cortex compared to those following ingestion of vehicle alone or collagen peptides. Soy peptide ingestion also effectively increased 3-methoxy-4-hydroxyphenylethyleneglycol and normetanephrine, the principal metabolites of noradrenaline, in the cerebral cortex, hippocampus, and brainstem, whereas collagen peptides did not exert such effects. Further, soy peptide ingestion led to a significant increase in noradrenaline itself in the brainstem, where noradrenergic neurons are present. Noradrenergic turnover was also markedly stimulated in these regions after soy peptide ingestion. These in vivo observations suggest that soy peptide ingestion can maintain and promote the synthesis and metabolism of noradrenaline in the brain.