Role of Melatonin in Viral, Bacterial and Parasitic Infections

Abstract

In all mammals, the circulating pool of MLT is synthesized in the pineal gland, during the night darkness hours. Its main function is that of synchronizing the organism in the photoperiod. In contrast, extra-pineal MLT is synthesized in peripheral organs, does not follow any circadian rhythm nor circulates and plays a detoxifying and cytoprotective action. Circulating MLT may stimulate both innate and acquired immune responses by its circadian action and by activating high affinity receptors on immunocompetent cells. Extra-pineal MLT may have anti-oxidant and anti-inflammatory effects dampening the innate immune response. These two seemingly divergent roles may be considered as two faces of the same coin. In fact, the integration of both circulating and extra-pineal MLT functions might generate a balanced and effective immune response against microbial pathogens. The studies described in this review investigated the effect of exogenous MLT in various models of infectious diseases using extremely different doses and treatment schedules. None of them evaluated the possibility to integrate the non-circadian anti-inflammatory effect with the circadian immunoenhancing action of MLT. As a consequence, in spite that most studies agree that MLT has a beneficial effect against infections, it seems difficult to draw any definite conclusion about its possible therapeutic use.