Stimulation of human prolactin secretion by intravenous infusion of L-tryptophan

Previous studies have demonstrated that the secretion of human prolactin is regulated primarily by factors that influence catecholamines of the hypothalamus. In an effort to identify other factors that may regulate prolactin secretion, the amino acid L-tryptophan, a precursor in the synthesis of serotonin, was infused into normal human volunteers. Intravenous infusion of L-tryptophan, 5-10 g over a 20 min period, but not equivalent amounts of 17 other amino acids, induced marked increases in serum prolactin concentrations in eight normal human volunteers. Increases of 20-200 ng/ml above the control level were observed with peak values at 20-45 min after initiation of the infusion. In addition, infusion of L-tryptophan was associated with decreases in serum concentrations of follicle-stimulating hormone (FSH), luteinizing hormone (LH), and thyrotropin in those subjects in whom the base-line serum hormone concentration was above the lower limits of assay detectability. No consistent change was observed in serum concentrations of growth hormone, cortisol, or glucose. Four subjects with juvenile diabetes demonstrated increases in serum prolactin values comparable with those observed in healthy individuals in response to infusions of L-tryptophan. Serum prolactin values in patients with surgically induced hypopituitarism were undetectable or deficient after infusion of 10 g of L-tryptophan. In this respect, infusion of L-tryptophan was equally effective in these subjects as the standard chlorpromazine stimulation test in identifying patients with hypopituitarism, indicating that the infusion of L-tryptophan may serve as a sensitive and reliable clinical test of prolactin secretory reserve. Further studies relating to the possible mechanism of action of L-tryptophan indicated that infusion of 5-hydroxytryptophan represents a much more potent stimulus for the secretion of prolactin and that premedication with the serotonin antagonist, methysergide maleate, serves to blunt the effect of L-tryptophan on prolactin secretion. These results support the concept that the effect of L-tryptophan on the secretion of human prolactin is mediated through its conversion to serotonin and are consistent with reported experimental observations that serotonin may participate in the reciprocal regulation of prolactin and gonadotropins.