Cerebrospinal fluid concentrations of tryptophan and 5-hydroxyindoleaceticacid in Macaca mulatta: diurnal variations and response to chronic changesin dietary protein intake

In rats, dietary protein is known to influence brain tryptophan (TRP) conce ntrations and serotonin (5HT) synthesis. However, few studies have examined this relationship in primates (including humans). We therefore studied the effect in monkeys of changes in chronic protein intake on plasma and cereb rospinal fluid (CSF) concentrations of TRP and 5-hydroxyindoleacetic acid ( 5HIAA), the principal 5HT metabolite. Juvenile male monkeys (Macacca mulatt a) consumed for sequential 4-week periods diets differing in protein conten t (similar to 23% --> similar to 16% --> similar to 10% --> similar to 6% p rotein [%-energy/day]). Each day, food was presented as a morning meal of f ruit, and an afternoon meal consisting of a pelleted, commercial diet and f ruit. During week 4 on each diet, blood and CSF were sampled diurnally via indwelling catheters. Plasma and CSF TRP varied diurnally and with dietary protein content. On all diets, CSF TRP declined modestly in the morning, an d increased in the afternoon; the magnitude of the increments varied direct ly with dietary protein content. Diurnal variations were absent for CSF 5HI AA; however, CSF 5HIAA varied directly with chronic dietary protein content . We conclude that dietary protein content can chronically influence CSF TR P concentrations in monkeys. The variation in CSF 5HIAA suggests chronic pr otein intake may influence serotonin synthesis and turnover, perhaps via ch anges in TRP concentrations.