Effect of chronic corn consumption on serotonin content of rat brain.

SEROTONIN, a putative neurotransmitter in the mammalian central nervous system, is synthesized in the brain by the 5-hydroxylation and decarboxylation of the essential aminoacid L-tryptophan1 ' 2. The control of serotonin biosynthesis seems to involve a different mechanism from that responsible for catecholamine biosynthesis3' 4 in its dependence on the availability of the amino-acid precursor', 6. Thus, small doses of tryptophan that do not increase brain or plasma tryptophan concentrations beyond their normal daily ranges cause significant increases in the serotonin concentration of rat brain. Conversely, the chronic ingestion of diets lacking in tryptophan (with casein hydrolysates or amino-acid mixtures substituted for natural proteins) depresses brain serotonin levels8-10. The dependence of serotonin biosynthesis on tryptophan availability probably arises from the unusually high substrate KM that characterizes tryptophan hydroxylasel. It seems likely that this enzyme normally functions in an unsaturated state; hence physiological increases in intraneuronal tryptophan could drive the hydroxylation of the amino-acid and, ultimately, its conversion to serotonin. Mammals are unable to synthesize tryptophan, so that all of the tryptophan available for conversion to serotonin in their brains must ultimately be derived from dietary proteins. The proportion of tryptophan within most such proteins is usually very low1l; in corn protein, the concentrations of tryptophan and lysine are so low that they may limit the ability of the body to utilize the other amino-acids for protein synthesist2 . To determine whether the tryptophan deprivation resulting from the chronic ingestion of a naturally occurring tryptophan-poor protein can also interfere with serotonin biosynthesis, we examined brain serotonin levels in young adult rats given a diet with corn as the sole protein source. Our results indicate that