Invited Speaker

Antioxidant and Anti-inflammatory Activity of the Brain Melatonin Metabolites and Their Synthetic Analogs. Evaluation in Neurodegenerative Diseases
Dario Acuña-Castroviejo, Germaine Escames, Luis C López
Spain

Melatonin (aMT) exerts a variety of physiological functions, some of them linked to specific receptors and cytosolic proteins. Specifically, melatonin binds to calcium-calmodulin (CaCaM) blocking CaCaM-dependent multiple cytosolic pathways, including the activation of neuronal nitric oxide synthase (nNOS). It is currently suggested that some of the melatonin's effects depend on its physiological active metabolites. Melatonin is mainly metabolized to N¹-acetyl- N²-formyl-5-methoxykynuramine (AFMK) and N¹-acetyl-5-methoxykynuramine (AMK). The cytosolic indoleamine-2,3-dioxygenase (IDO) transforms aMT to AFMK, although aMT may also be metabolized to AFMK after scavenging free radicals. Subsequently, AFMK can be metabolized to AMK through arylamine formamidase and catalase pathways.

Electrophysiological and biochemical experiments have shown that the neuroprotective properties of aMT involving the inhibition of nNOS/iNOS depend on AMK rather than aMT itself. These data supported the synthesis of new compounds structurally analogs to melatonin's metabolites for its use in clinical antioxidant and anti-inflammatory therapy.

Here, we compared the effects of aMT, AFMK, AMK, and some of these sunthetic active compounds on nNOS/iNOS activities in vitro and also in vivo in a model of Parkinson's disease in MPTP-treated mice. Melatonin and AMK (10-11-10-3 M), but not AFMK, inhibited nNOS activity in vitro in a dose-response manner. The IC50 value for AMK (70 µM) was significantly lower than for aMT (>1 mM). A 20% nNOS inhibition was reached with either 10-9 M aMT or 10-11 M AMK. AMK inhibits nNOS by a non-competitive mechanism through its binding to CaCaM. The inhibition of nNOS elicited by melatonin, but not by AMK, was blocked with 0.05 mM norharmane, an indoleamine-2,3-dioxygenase inhibitor. In vivo, the potency of AMK to inhibit nNOS activity was higher than that of melatonin, as a 25% reduction in rat striatal nNOS activity was found after the administration of either 10 mg/kg of AMK or 20 mg/kg of melatonin. Also, in vivo, the administration of norharmane blocked the inhibition of nNOS produced by melatonin administration, but not the inhibition produced by AMK.

Moreover, AMK, but not AFMK, inhibits iNOS induction and nitric oxide production in the substantia nigra of mice treated with MPTP. Some synthetic compounds assayed yielded significant inhibitory rates on both nNOS and iNOS activities. These data reveal that aMT metabolites may be used as templates for the design of new antioxidant and/or anti-inflammatory drugs.