Neuroendocrine Responses to Intravenous Tryptophan in Major Depression

• The increases in plasma levels of prolactin (PRL) and growth hormone (GH) following intravenous administration of the 5-hydroxytryptamine precursor tryptophan (100 mg/kg) were assessed in 30 depressed patients and 30 control subjects. In depressed patients who lost less than 10 lb, PRL responses were significantly reduced compared with controls. In contrast, the PRL responses of patients with weight loss exceeding 10 lb were significantly greater than those of either controls or the other depressed patients. Growth hormone responses to tryptophan were significantly decreased in patients who lost less than 10 lb. Prolactin, but not GH, responses correlated significantly with the postdexamethasone plasma cortisol concentration; however, an apparent relationship between GH and PRL responses and suicidal behavior was probably due to the common factor of weight loss. The results suggest that depressed patients have different types of abnormal 5-hydroxytryptamine—mediated neuroendocrine responses that correlate with the presence or absence of severe weight loss and cortisol hypersecretion. Further investigations are needed to establish if these abnormalities are central to the depressive disorder or have implications for treatment response.