Tryptophan-depletion challenge in depressed patients treated with desipramine or fluoxetine: Implications for the role of serotonin in the mechanism of antidepressant action

Background: Brain serotonin (5-HT) content is dependent on plasma levels of the essential amino acid, tryptophan (TRP). Ne hale previously reported that rapid TRP depletion more frequently reversed the antidepressant response to monoamine oxidase inhibitors and 5-HT I reuptake inhibitors than to desipramine (DMI), This study further investigates the relationship of relapse during TRP depletion to antidepressant type in nonrefractory, depressed patients randomly assigned to treatment with either DMI or fluoxetine (FLU). Methods: Fifty-five drug-free depressed (DSM-III-R) patients were randomly assigned to antidepressant treatment with either DMI or FLU. All patients,were either treatment naive (n = 34) or had previously received successful antidepressant treatment (n = 21), During the treatment phase, 35 patients had therapeutic responses by predetermined criteria (DMI 18/25; FLU 17/23) and 30 of these (15 DMI responders and 15 FLU responders) went ent on to TRP depletion resting. Patients received two 2-day test sessions involving administration of similar amino acid drinks. One session led to rapid TRP depletion and the other did not. Behavioral ratings [Hamilton Depression Scale (HDRS)] and plasma for TRP levels were obtained prior to, during, and after testing. Relapse,was defined as a 50% increase ill HDRS with total less than or equal to 17 Results: Total and free TRP decreased 70% to 80% 5 hours after the TRP-free drink. While 8/15 FLU responders relapsed, only, 1/15 of the DMI responders relapsed No patient experienced significant depressive symptoms during control testing, Conclusions: Rapid depletion of plasma TRP transiently reverses the antidepressant response in many patients on FLU but not DMI. Depressive relapse during;TRP depletion appears to be more related to antidepressant type than to patient variables since patients were randomly assigned to the two treatments, Antidepressant response to FLU appears to be more dependent on 5-HT availability than that of DMI, suggesting that antidepressants mediate their therapeutic effects through different mechanisms. (C) 1999 Society of Biological Psychiatry.