The double life of serotonin metabolites: in the mood for joining neuronal and immune systems

Neuroimmune system is nowadays considered as one complex, but unique example of coordination between cellular and molecular networks, only apparently segregated, but strictly collaborating for the maintenance of body integrity. Too often, serotonin and its metabolites have been considered merely as neurotransmitters, when they have multiple effects spreading from the modulation of mood and behavioral processes to the regulation of a wide range of physiologic and pathophysiologic processes in most human organs, not least the immune response. The purpose of this review is to highlight the importance of metabolites generated along the serotonin pathway in the constant dialogue between neuroendocrine and immune systems; moreover, we would like to point out that the molecules produced in the two main routes of tryptophan metabolism are involved in a loop of self-regulation aimed at maintaining the equilibrium between these two metabolic pathways in the neuroimmune system, in both physiologic and pathologic conditions.