Effect of light therapy on biopterin, neopterin and tryptophan in patients with seasonal affective disorder.

The serotonergic system is believed to play a key role in the pathophysiology of seasonal affective disorder (SAD). Tetrahydrobiopterin is an essential cofactor in the hydroxylation of tryptophan and, therefore, in the synthesis of serotonin, while neopterin is known as a marker of cell-mediated immune activity. The present study was designed to measure levels of biopterin, neopterin and tryptophan in plasma of 19 depressed patients with a history of SAD, before and after light therapy as well as in a control group. In the group of patients a significantly lower plasma biopterin and tryptophan level and a higher neopterin level was demonstrated. After light therapy, the level of biopterin increased to that of the controls but lowered again in summer. Neopterin concentrations remained on the same level after light therapy, whereas tryptophan levels increased slightly after light therapy and reached normal values in summer. It is concluded that the vulnerability for a depressive episode is enhanced by lowered levels of biopterin that, however, in SAD becomes symptomatically manifest in the presence of increased immune activity at the same time.