Comparative effects of melatonin, l‐deprenyl, Trolox and ascorbate in the suppression of hydroxyl radical formation during dopamine autoxidation in vitro

Abstract

Degeneration of nigrostriatal dopaminergic neurons is the major pathogenic substrate of Parkinson's disease (PD). Inhibitors of monoamine oxidase B (MAO-B) have been used in the treatment of PD and at least one of them, i.e., deprenyl, also displays antioxidant activity. Dopamine (DA) autoxidation produces reactive oxygen species implicated in the loss of dopaminergic neurons in the nigrostriatal pathway. In this study we compared the effects of melatonin with those of deprenyl and vitamins E and C in preventing the hydroxyl radical (8OH) generation during DA oxidation. The rate of production of 2,3-dihydroxybenzoate (2,3-DHBA) in the presence of salicylate, an *OH scavenger, was used to detect the in vitro generation of *OH during iron-catalyzed oxidation of DA. The results showed a dose-dependent effect of melatonin, deprenyl and vitamin E in counteracting DA autoxidation, whereas vitamin C had no effect. Comparative analyses between the effect of these antioxidants showed that the protective effect of melatonin against DA autoxidation was significantly higher than that of the other compounds tested. Also, when melatonin plus deprenyl were added to the incubation medium, a potentiation of the antioxidant effect was found. These findings suggest that antioxidants may be useful in brain protection against toxicity of reactive oxygen species produced during DA oxidation, and melatonin, alone or in combination with deprenyl, may be an important component of the brain's antioxidant defenses to protect it from dopaminergic neurodegeneration.

Synergistic effects of melatonin and deprenyl against MPTP-induced mitochondrial damage and DA depletion

“…Melatonin also regulates the expression and activity of the redox enzymes [9,16]. Deprenyl shares with melatonin common features such as neuroprotective and antioxidant effects, and we recently reported a synergistic effects between both compounds against DA autoxidation in vitro [32]. Thus, we asked if these compounds may also collaborate in vivo against MPTP-induced neuronal damage.…”

“…The neuroprotection by deprenyl has been associated with several intracellular mechanisms including inhibition of DA uptake, increase of mRNA trophic factors, and increase TH enzyme [40,49]. Other actions such as an antioxidant role, enhancement of the antioxidant enzymes superoxide dismutase and catalase, and preservation of mitochondrial membrane potential are directly related with the neuroprotective actions of deprenyl [32,34]. To study whether melatonin potentiates the effects of deprenyl, a low dose of the drug was used.…”

“…Anyway, the synergy between melatonin and deprenyl may depend on several factors. One, the protective effect of both compounds against DA autoxidation [32], since DA accumulates extraneuronally mainly due to a massive DA efflux induced by the MPP + [50] plus the deprenyl-dependent DA uptake inhibition [26]. In addition, deprenyl-induced TH enzyme may be favored by the increase of TH activity due to melatonin [7,31].…”

“…The homogenates were centrifuged at 15,000 × g for 20 min at 4 • C and the supernatant were passed through a 0.2 m filter and analyzed by HPLC-ED [32]. Separations were done in a C-18 reversed-phase 5 m sphere analytical column.…”

“…Deprenyl, a MAO B inhibitor, blocks MPP + formation from MPTP in the glial cells protecting neurons against the devastating effect of the neurotoxin [26]. Other actions of deprenyl such as the increase in superoxide dismutase activity [34], the reduction of free radical formation [32], and the antiapoptotic activity [55], are also involved in its neuroprotective effect. Melatonin is a strong scavenger of free radicals [46,48] and regulates the activity and expression of antioxidant enzymes [9,37].…”