Melatonin, the main secretory product of the pineal gland, is known to collaborate against oxidative stress within cells, but its mechanism of action in terms of stimulating antioxidant enzymes remains unclear. Herein, we propose that melatonin modulates antioxidant enzyme activities via its interaction with calmodulin, which in turn inhibits downstream
processes that lead to the inactivation of nuclear RORα melatonin receptor. Eventually, this nuclear transcription factor downregulates NF-kB-induced antioxidant enzyme expression. Therefore, the increment in antioxidant enzyme activities induced by melatonin involves the inhibition of the RORα pathway. Thus, in addition to its direct free radical scavenging activities, melatonin has important actions in oxidative defense by stimulating enzymes which metabolize free radicals and radical products to innocuous metabolites.