Neuroendocrine effects of L-tryptophan and dexamethasone.

An infusion of L-tryptophan was administered twice to five healthy male volunteers, once after pretreatment with dexamethasone 1 mg the previous evening and once after no dexamethasone. Cortisol, prolactin, and growth hormone levels were measured, and the responses to L-tryptophan were compared with those seen after an infusion of L-threonine. L-tryptophan did not produce cortisol secretion after dexamethasone, but prolactin and growth hormone responses were noticed. The results demonstrate a stimulatory effect of L-tryptophan on prolactin and growth hormone secretion, and the former is facilitated by pretreatment with dexamethasone.