MECHANISM OF DECLINE IN RAT BRAIN 5‐HYDROXYTRYPTAMINE AFTER INDUCTION OF LIVER TRYPTOPHAN PYRROLASE BY HYDROCORTISONE: ROLES OF TRYPTOPHAN CATABOLISM AND KYNURENINE SYNTHESIS

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Abstract
1 Two mechanisms have been proposed to explain the decline in brain tryptophan and 5-hydroxytryptamine (5-HT) after administration of hydrocortisone and the subsequent induction of liver pyrrolase. These are depletion of tryptophan by high rates of tryptophan catabolism and inhibition of tryptophan uptake by elevated levels of the tryptophan catabolite, kynurenine. 2 The increase in plasma kynurenine after hydrocortisone injection (25 mg/kg) was small, and kynurenine, at a concentration ten fold greater, did not inhibit tryptophan uptake by brain as measured by the Oldendorf technique. Thus, inhibition of tryptophan uptake by kynurenine is not an important mechanism in the control of brain tryptophan and 5-HT. 3 The decline in brain tryptophan after hydrocortisone was comparable to that seen in other tissues, which comprise more than half of the body weight of a rat. 4 The total decline in free tryptophan stores in whole animals treated with hydrocortisone was estimated to be about 450 μg. This amount of tryptophan would be catabolized by tryptophan pyrrolase in about 20 min, when the enzyme is induced, according to an earlier estimate of the rate of tryptophan catabolism in vivo. 5 Tryptophan pyrrolase activity remains high for much longer than 20 min, suggesting that there is net protein catabolism, which releases tryptophan and prevents non-protein tryptophan levels falling very far. 6 These results demonstrate that the decline in brain tryptophan and 5-HT after hydrocortisone is caused by depletion of tryptophan stores due to the high activity of tryptophan pyrrolase. However, our data suggest that this effect is diminished by release of tryptophan from proteins. Thus, peripheral protein metabolism may be an important factor in the control of brain tryptophan levels and 5-HT synthesis.