Tryptophan and serotonin metabolism after sustained tryptophan infusion

In an attempt to elucidate the effects of sustained administration of tryptophan on serotonin synthesis and turnover in mammalian brain, mini-osmotic pumps containing tryptophan or vehicle were implanted in albino mice for 24 and 96 h. Despite the extremely low dose of tryptophan administered by these pumps (8–12 mg/kg-day) statistically significant treatment effects were apparent with both treatment durations. Plasma and brain tryptophan concentrations varied in unison, and were inversely related to the tryptophan degradative capabilities of the liver as reflected in tryptophan pyrrolase activity. After 24 h of tryptophan infusion the hepatic enzyme activity was elevated and tryptophan values were no different from controls, and after 96 h the hepatic enzyme activity was reduced and tryptophan values in treated animals were greater than controls. Serotonin was elevated in treated animals after 24 h, but not after 96 h despite the elevated tryptophan concentration at this time. The turnover of serotonin, as evidenced by 5-hydroxyindoleacetic acid concentrations, was not significantly affected by either treatment. Hepatic degradation of tryptophan thus seemed to be an important determinant of total plasma tryptophan, and brain tryptophan values paralleled plasma tryptophan. It appears that serotonin biosynthesis is regulated by factors other than tryptophan availability when the latter is chronically elevated.