Melatonin effect on serotonin uptake and release in rat platelets: diurnal variation in responsiveness

The present study was conducted to examine whether melatonin impairs serotonin (5HT) release and uptake in rat platelets. Exposure of platelet-rich plasma samples (PRP) to melatonin induced a concentration-dependent inhibition of 5HT uptake and the value of IC50 was 1.3 x 10(-3) M. We have also investigated the melatonin effect on the kinetic parameters of platelet 5HT uptake. Transport capacity was inhibited (Vmax;

2.28 +/- 0.52, Melatonin: 0.74 +/- 0.13 pmol/10(7) platelet.min; p < 0.05) while the affinity of 5HT for its uptake carriers remained unaltered, thus indicating a non-competitive effect. Studies carried out to determine the existence of a differential morning (8:00h)-evening (21:00h) melatonin effect showed a higher platelet uptake sensitivity at 8:00h (two-way ANOVA, p < 0.001). Spontaneous 5HT release was not impared by the hormone and no daily variation in sensitivity was detected. The possible mechanism of action of melatonin on platelet transport is discussed, and the results support the suitability of the platelet model for studying sensitivity changes in target cells to the hormone.