Effects of Long-Term Low Dietary Tryptophan Intake on Determinants of 5-Hydroxytryptamine Metabolisn in the Brains of Young Rats

Abstract

The relationship between plasma and brain tryptophan (TRP) concentrations and brain 5-hydroxytryptamine (5-HT) metabolism was studied in weanling rats fed diets containing either 0.4 g or 1.45 g TRP/100 g casein hydrolysate. Both groups gained weight comparably, although food intakes were generally higher in the low-TRP group. Severe depletion of plasma total and free TRP and of brain TRP, 5-HT and 5-hydroxyIAA (5-HIAA) occurred within 1 day of feeding the 0.4% TRP diet. Levels became stable after 7 days. The decreased brain TRP concentration of the rats on the 0.4% TRP diet did not cause a compensatory rise of the tryptophan hydroxylase (TRP OHase) activity in vitro. In the low-TRP group, neither plasma free TRP nor total TRP correlated significantly with brain TRP, and although plasma TRP/large neutral amino acid ratios correlated significantly with the time course of brain TRP, this statistical relationship depended almost completely on variation of TRP values alone. In the higher TRP group none of these correlations were significant. A plot of the mean plasma free TRP vs. brain TRP gave 2 distinct regression lines with similar slopes and corresponding to values before and after 7 days on the diet. The time course of brain 5-hydroxyIAA concentrations did not parallel those of brain TRP and suggested that changes of TRP OHase activity influenced 5-HT synthesis.