The tryptophan (TRP) depletion paradigm has been employed to investigate mood and behavioral effects of acutely lowering plasma TRP, and presumably brain serotonin (5-hydroxytryptamine [5-HT]) levels through administration of a special diet and/or amino acid drink. Our goal was to test the assumption that a corresponding fall in central levels of TRP and 5-HT (measured by its major metabolite, 5-hydroxyindoleacetic acid [5-HIAA]) occurs during the standard execution of this method in healthy adult subjects. Three males and two females completed the protocol, which included a one-day low-TRP diet and a TRP-free amino acid drink. Lumbar puncture was performed, with placement of an indwelling catheter connected to a peristaltic pump and fraction collector.Cerebrospinal fluid (CSF) was sampled continuously for a 13.5-hour period (before, during, and The role of serotonin (5-hydroxytryptamine, 5-HT) in the pathogenesis and treatment of major neuropsychiatric disorders, especially depression, continues to be the subject of intensive research (Maes and Meltzer 1995). One paradigm for studying the role of 5-HT involves acutely manipulating levels of its amino acid precursor, tryptophan (TRP), and measuring corresponding behavioral effects. Because the synthesis of 5-HT is dependent on the availability of TRP, rapidly-induced shifts in the central level of TRP should produce transient alterations in the amount of 5-HT available for neurotransmission (Schaechter and Wurtman 1990;Young 1991). Observed or reported changes in behavior or psychiatric symptoms occurring during acute TRP manipulation have provided clues about the mechanism of action of serotonergic drugs, and the biology of several psychiatric disorders.Beginning in the 1970s, researchers have developed several strategies for altering TRP levels, in efforts to explore the relationship between the precursor amino acid, serotonergic function, and behavior. Preclinical studies demonstrated that dietary restriction of TRP could evoke modest reductions in plasma and brain TRP (Fernstrom 1977), with corresponding behavioral indices reflecting diminished 5-HT function (Lytle et al. 1975;Messing et al. 1976;Gibbons et al. 1979; Moja et al. Walters et al. 1979). Dietary TRP restriction in both animals and humans has subsequently been associated with enhanced response to neuroendocrine challenges in a manner consistent with compensatory postsynaptic supersensitivity (Clemens et al. 1980;Delgado et al. 1989), again suggesting that the depletion is diminishing 5-HT function at a central nervous system (CNS) level. More rapid decreases in plasma and brain TRP levels can be achieved by the administration of a TRP-free amino acid mixture Young et al. 1995;Moja et al. 1988). Here the mechanism involves providing substrate for induction of protein synthesis to such a degree that available peripheral TRP stores are largely used up, resulting in a drop in the ratio of TRP to other large neutral amino acids in plasma (Harper et al. 1970). Because TRP competes with the other lar...