Antioxidative properties of serotonin and the bactericidal function of polymorphonuclear phagocytes

We investigated the antioxidative properties of platelet-released serotonin on the bactericidal function of polymorphonuclear neutrophils (PMN) since there is a surprising co-incidence of low blood serotonin and an increased rate of infections. The antioxidative properties of serotonin were demonstrated by its suppressive effects on phagocytosis-associated, luminol-enhanced chemiluminescence (CL). The bactericidal activity of PMN was determined by a microbiological assay using opsonized Staphylococcus aureus. Serotonin suppresses luminol-enhanced chemiluminescence in a dose-dependent manner indicating an interaction with reactive oxygen species, which are responsible for effective bacterial killing during the phagocytosis-associated "respiratory burst". The modulation of the bactericidal function of PMN by serotonin is complex and depends upon the amount of serotonin: at concentrations normally present at sites of tissue injury and consecutive thrombus formation (10(-6) to 10(-5) M) bacterial killing increases by about 50%. In contrast, at pharmacological concentrations (10(-3) to 10(-2) M) an adverse effect can be observed: the elimination of opsonized S. aureus is reduced by 30 to 90%. Exogenous serotonin is capable of modulating important biological functions of human PMN in vitro. At appropriate concentrations, the antibacterial defence improves significantly probably due to reduced autooxidation, whereas higher concentrations counteract an efficient bacterial killing.