Abstract
Serotonin and melatonin inhibit phytohemagglutinin- (PHA) induced interferon-γ (IFN-γ) production by lymphocytes. In this paper, it is shown that IFN-γ-increased tryptophan uptake by lymphocytes and macrophages led to an enhanced production of serotonin. When IFN-γ and serotonin were added together to a lymphocyte culture, N-acetyl serotonin and melatonin production was increased, whereas the path to 5-hydroxyindoleacetic acid remained unchanged. Therefore, the stimulated IFN-γ production of serotonin and melatonin by lymphocytes and macrophages and the inhibition of IFN-γ synthesis by these indoleamines suggest a hypothesis for an immunoregulatory circuit.
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