Serotonin Receptor Antagonists Block a Natural, Short Term Surge in Serum Growth Hormone Levels*

Abstract

Serum GH [growth hormone] levels in untreated rats increased spontaneously to over 300 ng/ml around the onset of darkness (1900 h) and then decreased to under 100 ng/ml by 2000 h. Pretreatment with the serotonin receptor blockers, metergoline (0.2, 0.5, 1 or 2 mg/kg) or cyproheptadine (1, 2 or 5 mg/kg), at 1800 h significantly blunted this physiological rise in serum GH. In rats bearing chronic right atrial cannulae from which blood samples were drawn every 15 min for 5 h, the administration of metergoline (1 mg/kg) also reduced plasma GH levels over a 3.25 h period. Serotonin-containing neurons in the brain seem to be involved in physiological GH secretion.