Serotonin receptor antagonists block a natural, short term surge in serum growth hormone levels

Serum GH levels in untreated rats were found to increase spontaneously to over 300 ng/ml around the onset of darkness (1900 h) and then decrease 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) was also observed to reduce plasma GH levels over a 3.25-h period. Serotonin-containing neurons in the brain thus seem to be involved in the physiological secretion of GH.