High doses of vitamin B6 in the rat are associated with inhibition of hepatic tryptophan metabolism and increased uptake of tryptophan into the brain.

Acute administration of vitamin B6 to rats (10 mg/kg body weight) led to reduced urinary excretion of N1-methyl nicotinamide and methyl pyridone carboxamide, indicating inhibition of the oxidative metabolism of tryptophan. There was a considerable reduction in the production of 14CO2 from [ring-2-14C]tryptophan, and a significant inhibition of hepatic tryptophan oxygenase when measured in liver homogenates, together with an increase in the concentration of tryptophan in plasma. There was an increase in both the concentration of tryptophan in the brain and the uptake into the brain of peripherally administered [3H]tryptophan, accompanied by a small increase in the rate of synthesis of 5-hydroxytryptamine in the brain. It is suggested that this increase in the uptake of tryptophan into the brain following a relatively large dose of vitamin B6 may explain the beneficial action of the vitamin in some cases of depressive illness.