Transient Effect of L-5-Hydroxytryptophan on Pituitary Function in Men and Women*

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Abstract
To further elucidate the role of serotonin in the secretion of pituitary hormones, a loading dose of 0.8 mg/kg for 1 h, followed by a maintenance dose of 0.1 mg/kg-h of L-5-hydroxytryptophan (5-HTP) for 23 h were administered iv to five normal men and five normal women. Five additional men received repeated loading doses of 5-HTP 4 and 6 h after the initial one, with or without the maintenance dose. The initial studies demonstrated a significant, but transient, increase in plasma PRL, GH, cortisol (P < 0.01) and TSH (P < 0.05) in the five men and a consistent and significant transient increase only in PRL and cortisol in the five women. Plasma LH and FSH values were not affected by 5-HTP administration. The constant administration of 5-HTP revealed a blunting effect on the nocturnal rise of TSH in men. The continuous administration of 5-HTP failed to maintain the rise induced by the loading dose. Individuals receiving additional loading doses of 5-HTP demonstrated a subsequent increase in GH and cortisol, but not in serum PRL. This study suggests that endogenous serotonin may exert its stimulatory effect on pituitary hormone secretion primarily by sporadic release, rather than by continuous secretion.