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Alexander Neumeister, MD; Erick H. Turner, MD; Jefferey R. Matthews, MD; Teodor T. Postolache, MD; Ronald L. Barnett, PhD; Manfred Rauh, MD; Rina G. Vetticad; Siegfried Kasper, MD; Norman E. Rosenthal, MD

Arch Gen Psychiatry. 1998;55:524-530.

Background  Although hypotheses about the therapeutic mechanism of action of light therapy have focused on serotonergic mechanisms, the potential role, if any, of catecholaminergic pathways has not been fully explored.

Methods  Sixteen patients with seasonal affective disorder who had responded to a standard regimen of daily 10000-lux light therapy were enrolled in a double-blind, placebo-controlled, randomized crossover study. We compared the effects of tryptophan depletion with catecholamine depletion and sham depletion. Ingestion of a tryptophan-free amino acid beverage plus amino acid capsules was used to deplete tryptophan. Administration of the tyrosine hydroxylase inhibitor -methyl-para-tyrosine was used to deplete catecholamines. Diphenhydramine hydrochloride was used as an active placebo during sham depletion. The effects of these interventions were evaluated with measures of depression, plasma tryptophan levels, and plasma catecholamine metabolites.

Results  Tryptophan depletion significantly decreased plasma total and free tryptophan levels. Catecholamine depletion significantly decreased plasma 3-methoxy-4-hydroxyphenylethyleneglycol and homovanillic acid levels. Both tryptophan depletion and catecholamine depletion, compared with sham depletion, induced a robust increase (P<.001, repeated-measures analysis of variance) in depressive symptoms as measured with the Hamilton Depression Rating Scale, Seasonal Affective Disorder Version.

Conclusions  The beneficial effects of light therapy in the treatment of seasonal affective disorder are reversed by both tryptophan depletion and catecholamine depletion. These findings confirm previous work showing that serotonin plays an important role in the mechanism of action of light therapy and provide new evidence that brain catecholaminergic systems may also be involved.

From the National Institute of Mental Health, Clinical Psychobiology Branch, Bethesda, Md (Drs Neumeister, Turner, Matthews, Postolache, Barnett, and Rosenthal and Ms Vetticad); Department of General Psychiatry, Vienna University, Vienna, Austria (Drs Neumeister and Kasper); and Department of Pediatrics, University of Erlangen, Erlangen, Germany (Dr Rauh).

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