Rapid repletion of brain serotonin in malnourished corn-fed rats following L-tryptophan injection.

The concentration of tryptophan in serum, and the levels of tryptophan, serotonin (5-HT), and 5-hydroxyindole-acetic acid (5-HIAA) in brain are substantially reduced in rats that consume for 6 weeks a diet in which corn is the only source of protein. Single injections of L-tryptophan (25, 50, or 100 mg/kg) cause dose-related increases in brain tryptophan, 5-HT, and 5-HIAA in corn-fed animals. At each dose, brain tryptophan content rises to a proportionately greater extent in corn-fed rats than in well-nourished controls, even though serum tryptophan concentrations attain higher levels in controls. This difference may reflect the greatly reduced serum concentrations in corn-fed rats of other large neutral amino acids that compete with tryptophan for uptake into the brain (tyrosine, phenylalanine, leucine, isoleucine, and valine). However, the substantial decrease in serum albumin levels also diminishes the binding of tryptophan to serum albumin; thus it is not yet possible to state which of these changes is responsible for the much greater increments in brain tryptophan observed in corn-fed rats after tryptophan injection. The fact that tryptophan administration rapidly restores brain 5-hydroxyindole levels in corn-fed animals suggests that the reductions in 5-HT and 5-HIAA levels associated with this type of malnutrition may be largely caused by inadequate availability of substrate.