Suppression of sleep-related prolactin secretion and enhancement of sleep-related growth hormone secretion.

Methysergide, a clinically-used blocker of serotonin receptors, was administered to 10 normal young men at a dose of 2 mg every 6 h for 48 h. After drug treatment, serum levels of growth hormone during sleep were 41.9% higher than placebo values (less than 0.001). In contrast, drug treatment was associated with a 36.4% decrease in stimulated growth hormone secretion during insulin tolerance testing (P less than 0.01). These opposite effects of methysergide suggest that different mechanisms are responsible for sleep-related and insulin-induced growth hormone secretion. Accordingly, data obtained with pharmacologic stimuli may lead to erroneous inferences regarding physiologic growth hormone control mechanisms. Administration of methysergide profoundly suppressed sleep-related prolactin secretion; overall nocturnal mean prolactin fell by 70.3% from 4.30+/-0.19 to 1.28+/-0.06 ng/ml (P less than 0.0001). It appears that serotonin may be significant modulating neurotransmitter for the control of growth hormone secretion, limiting sleep-related release, and enhancing insulin-induced release. It seems likely from these data that the role of serotonin in the control of prolactin secretion is relatively more important, since serotonin receptor blockade dramatically reduced sleep-related prolactin secretion.