Imaging the serotonergic system in depression

Positron emission tomography (PET) and single photon emission computed tomography (SPECT) allow for the in vivo visualisation and measurement of, e. g. the serotonergic system in the brain of depressed patients. Currently, the available ligands permit the investigation of 5-HT(2A) and 5-HT(1A) receptors, the serotonin transporter and serotonin synthesis. 5-HT(2A) receptors have most extensively been investigated and increases, decreases or no differences in ligand binding have been found. Previous treatment and suicidality could be major confounding variables. Tricyclics seem to decrease ligand binding, while SSRIs in most studies increase ligand binding. A few studies have looked at the 5-H(T1A) receptor and demonstrated decreases in binding. The one study which looked at the effect of an SSRI treatment did not find any effect. The serotonin transporter availability seems to be reduced in depression. Tryptophane depletion studies have demonstrated effects on brain metabolism in serotonin related regions and on 5-HT(2A) receptors. Finally, serotonin synthesis studies have shown interesting differences between males and females.