Normal levels of tryptophan hydroxylase immunoreactivity in the dorsal raphe of depressed suicide victims

A variety of evidence suggests that serotonin neurotransmission is altered in the brain of suicide victims and depressed patients. While numerous post-mortem studies have investigated serotonin transporters and receptors, few studies have examined the biosynthetic integrity of the rate-limiting enzyme, tryptophan hydroxylase (TPH), in post-mortem specimens of depressed suicide subjects. Therefore, the aim of the present study was to test the hypothesis that the levels of TPH immunoreactivity (IR) are altered in specific subnuclei of the dorsal raphe (DR) in depressed suicide victims. Suicide victims with a confirmed diagnosis of major depression were matched with non-psychiatric controls based on age, gender and post-mortem interval. Frozen tissue sections containing the DR were selected from two anatomical levels and processed for TPH radioimmunocytochemistry. The optical density corresponding to the regional levels of TPH-IR was quantified in specific subnuclei of the DR from the film autoradiographic images. No significant differences in the levels of TPH-IR were found in any DR subnuclei between depressed suicide victims and control subjects. The lack of change in TPH-IR levels does not necessarily imply that serotonin synthesis or neurotransmission is not altered in the brain of depressed subjects. Many factors influence and regulate serotonin synthesis, and it is conceivable that alterations exist at other levels of regulation of serotonin biosynthesis in depression. Our findings indicate that TPH biosynthesis, at least at the protein level, is not significantly altered in the DR of depressed suicide victims.