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Abstract
Blood levels of melatonin, serotonin, cortisol, prolactin, and serotonin uptake by
platelets were measured at 08:00, 14:00, 20:00, 02:00, and 08:00 hours in 10 healthy
men who ranged in age from 27 to 35 years. The Km values of serotonin active transport
by platelets were significantly correlated with melatonin blood levels. There were
no other significant correlations. The secretion of steroid hormones and prolactin
showed an increase at 02:00 hours; levels of prolactin decreased at 08:00 hours, but
steroid levels continued to rise. This finding suggests either a direct effect of
melatonin on serotonin active transport or the influence of the suprachiasmatic nucleus
on serotonin uptake by platelets. It is also possible that there is a simultaneous
decrease in serotonin uptake and decline from peak levels of melatonin due to the
rise in steroid secretion.