Chronic effects of dietary protein quality and quantity on tryptophan levels and serotonin synthesis in rat hypothalamus

Abstract

Tryptophan (TRP) levels in rat brain control serotonin (5HT) synthesis, and diet influences brain TRP and thus 5HT. But few data exist on the effect of protein quality or quantity on this relationship. We fed male rats (200 g) ad lib for 14 d diets containing zein, gluten, casein, soy or lactalbumin at 2%, 9% or 15% (% energy), & killed them 2.5 hr into the daily dark period, 30 min after injecting m‐hydroxybenzylhydrazine (100 mg/kg ip; to estimate 5HT synthesis). Blood & hypothalamus were obtained; TRP, tyrosine (TYR) & other amino acids, & 5‐hydroxytryptophan (5HTP) and dihydroxyphenylalanine (DOPA), indices of 5HT & catecholamine synthesis, were measured in hypothalamus by HPLC & electrochemical detection. At 10% & 15% protein, brain TRP and hypothalamic 5HTP were lowest in rats ingesting zein (≈2 nmol/g;≈75 ng/g/30 min), highest in rats ingesting lactalbumin (≈13 nmol/g;≈300 ng/g/30 min), & intermediate in rats ingesting the other proteins. The correlation coefficients (TRP vs. 5HTP) were ≈0.90. At 2% protein, brain TRP & 5HTP showed no clear differences among dietary proteins. Brain TYR increased with dietary protein content, but showed no clear effect of protein quality; Effects on DOPA synthesis were relatively small. Hence, in a range of protein intake bracketing the rat's protein requirement, brain TRP level & 5HT synthesis reflect the quality of the dietary protein; TYR level & catecholamine synthesis do not.