Inhibitory effect of serotonin antagonists on growth hormone release in acromegalic patients

We evaluated the effect of the serotonin antagonists cyproheptadine (Cypro) and methysergide (Methy) on growth hormone secretion in six patients with acromegaly. Two days administration of Cypro decreased the plasma GH concentration during oral glucose tolerance tests in four of the six patients evaluated; 2 days administration of Methy reduced the plasma GH levels of only one of the four patients evaluated. The one patient whose plasma GH concentration was lowered by Methy, did not have a decrease in plasma GH concentration after Cypro administration. Acromegalic patients have normal serum serotonin concentration and normal 5-hydroxyindoleacetic acid excretion. If Cypro lowers plasma GH by antagonizing serotonin, our data would suggest that serotoninergic neuronal pathways are important in the regulation of pituitary GH secretion in some patients with acromegaly.