A comparison of the inhibitory effects of melatonin and indomethacin on platelet aggregation and thromboxane release.

Collagen-induced platelet aggregation and thromboxane release is inhibited, in a concentration response relationship, by preincubation of gel-filtered platelets with melatonin in the concentration range 430 nM - 4.3 mM. Inhibition of platelet aggregation and thromboxane release also occurs in the presence of indomethacin (4.3 nM - 4.3 mM), a known potent inhibitor of prostaglandin synthesis. Arachidonic acid-induced platelet aggregation and thromboxane release was inhibited in the presence of 4.0 mM melatonin. We therefore propose that inhibition of prostaglandin synthesis maybe the mechanism by which melatonin expresses its activity. Its antigonadotropic activity may result from inhibition of PGE2 synthesis in the hypothalamus and median eminence.