Effect of the Serotonin Precursor, Tryptophan, on Pituitary Hormone Secretion1

L-tryptophan (TP) availability has been shown to be a major factor regulating CNS serotonin concentration. Consequently, this essential amino acid was administered to healthy men and its effect on the pituitary hormone secretion investigated. The oral administration of 10 g of L-TP significantly increased L-TP levels by 15 min (19.1 ±2.0 to 55.2 ±.5 μg/ml, P < 0.01) and caused drowsiness in all subjects. In 11 men plasma cortisol decreased from a baseline of 8.1 ±0.9 /xg/dl to 5.6 ±.7 /μtg/dl by 45 min. The maximal nadir of 4.7 ±0.8 Atg/dl, a decrease of 43.2% (range 33-87%), occurred at 15-75 min and was followed by a rebound above baseline to 11.2 ±1.5 fig/dl. A significant inverse relationship existed during the first 45 min between TP and cortisol concentrations. TP treatment 15 min before the iv administration of regular insulin (0.1 U/kg) significantly blunted the peak corticotropin (ACTH) and cortisol response to hypoglycemia. The infusion of L-TP was followed by a significantly greater fall in ACT...