Melatonin is an indoleamine neurohormone found across plants and animals, produced endogenously from serotonin (5-HT) and secreted in animals as a regulatory signal for synchronization of the circadian rhythm and the sleep-wake cycle. The melatonin receptor system, consisting of the MEL-1A-R, MEL-1B-R, and MT3 subtypes, demonstrates particular plasticity and modularity - antagonists such as Luzindole (sc-202700) and 2-Phenylmelatonin (sc-203466) show modification of systemic responses to the Melatonin signal without impeding activation of the receptors by Melatonin. Powerful antioxidant activity is associated with Melatonin, and it is known to provide protection to lipids, proteins, and DNA against oxidative damage. Several antioxidant enzymes are shown to be upregulated by Melatonin, including glutathione peroxidase, superoxide dismutases, and catalase. Melatonin also scavenges free radicals as a terminal antioxidant, reacting to generate stable end products and terminate radical chain reactions. Free movement through the blood-brain barrier positions Melatonin as a particularly significant endogenous antioxidant. Melatonin is a rat NOS1 (nNOS) inhibitor. Melatonin is an activator of MEL-1A-R and MEL-1B-R.