Extrapineal melatonin and exogenous serotonin in seasonal affective disorder.

Visible light inhibits the binding of melatonin and serotonin to cultured human peripheral blood mononuclear leukocytes (PBMLs) in winter. The decreased binding switches the metabolism in PBMLs towards serotonin synthesis, resulting in the reduced production of melatonin. The ingestion of L-tryptophan during the day is hypothesized to increase the levels of melatonin, released from the gastrointestinal tract, in patients with winter seasonal affective disorder (SAD). Due to the relative shortage of light, coincident with a predisposed metabolic error, there would be no switch towards serotonin synthesis among winter SAD patients in winter. The rate of serotonin synthesis could thus remain inadequately low to maintain optimal mood in winter SAD patients.