Involvement of plasma leptin, insulin and free tryptophan in cytokine-induced anorexia

Background: Hypothalamic serotonin, the synthesis of which parallels plasma free tryptophan, contributes to satiety. Plasma free tryptophan, insulin and leptin, all of which can also decrease food intake partly via the hypothalamic serotonergic system, are modulated by cytokines, which decrease food intake. The mechanisms of anorexia induced by cytokines, as related to plasma tryptophan, leptin and insulin, have not been fully determined. Objective: We determined the plasma concentrations of free as well as total tryptophan, leptin and insulin, and correlations to those of food intake and body weight change after cytokines or tryptophan injection. Design: Interleukin-1α and/or tumor necrosis factor-α, or tryptophan was injected subcutaneously into male rats for 2 days. Daily food intake, body weight, carcass adiposity, plasma total as well as free tryptophan, plasma leptin and insulin were measured. Results:Interleukin-1α injection decreased food intake, body weight, carcass adiposity and plasma leptin, but increased plasma free tryptophan and insulin. Tryptophan injection increased both free and total tryptophan, but did not change food intake, body weight, carcass adiposity or leptin. Plasma free tryptophan, but not total tryptophan, was significantly negatively correlated with food intake. There was a negative correlation between plasma insulin and food intake. Conclusions: Increased plasma free tryptophan may contribute to synthesis of brain serotonin but anorexia may be due to stimulation of its release induced by interleukin-1α. Plasma insulin, but not leptin, may partly contribute to anorexia of cytokines.