Serotonin and its place in the pathogenesis of depression

The authors review the literature in an attempt to evaluate the relationship between serotonin and depression. Animal studies show that the administration of tryptophan (the precursor of serotonin) increased serotonin synthesis and influenced behavior. Low plasma tryptophan levels have been found in patients with endogeneous depression. Postmortem studies have shown an association between lowered hindbrain serotonin levels and suicide among depressed persons. The decreased serotonin levels in blood platelets during depression mirrored the neuronal changes. Tricyclic antidepressants inhibited platelet serotonin uptake and reduced imipramine binding sites on the platelets. A positive correlation between depression rating scores and platelet aggregatory response has been reported. Serotonin stimulated release of prolactin and growth hormone, although the prolactin response was less marked in depression. A marker for depressive illness is still sought, but it is likely to be related in some way to serotonin.