Oxidative stress is defined as a disturbance in the prooxidant-antioxidant balance, leading to potential cell damage. Reactive oxygen species such as superoxide radicals, hydroxyl radicals and hydrogen peroxide may act also as secondary intermediaries in intracellular signaling leading to cell death. The neuroprotective effect of melatonin has been observed both in vivo and in vitro. The objective of this research, therefore, was to better understand the cellular mechanisms of neuronal cell degeneration induced via oxidative stress and the protective roles of melatonin on this cell death. In the present study, the effects of melatonin on H(2)O(2)-induced neuronal cell degeneration in human dopaminergic neuroblastoma SH-SY5Y cultured cells were investigated. The results showed that H(2)O(2) significantly decreased cell viability and melatonin reversed the toxic effects of H(2)O(2). An inhibition of caspase enzyme activity by Ac-DEVD-CHO, a caspase-3 inhibitor, significantly increased cell viability in H(2)O(2)-treated cells. The phosphorylation of transcription factors, nuclear factor kappa B (NF-kappaB) was increased in H(2)O(2)-treated cells and this effect was abolished by melatonin. Translocation of phosphorylated NF-kappaB to perinuclear and nuclear sites, estimated using immunofluorescence, occurred to a greater extent in H(2)O(2)-treated cells than in untreated control cells and again this effect was abolished by melatonin. In addition, induction of Bcl-2 and Bax proteins was demonstrated in SH-SY5Y cultured cells treated with H(2)O(2), whereas the induction of Bax but not Bcl-2 was diminished by melatonin. In light of these finding, it is possible that the antioxidative stress effect of melatonin associated with inhibition of Bax expression, may offer a means of treating neuronal degeneration and disease.

Melatonin plays a key role in a variety of important physiological functions including influencing cerebral blood vessels. Therefore, in the present study, we have identified the existence of melatonin receptors and test the effect of melatonin on hydrogen peroxide-induced endothelial nitric oxide synthase (eNOS) expression in bovine cerebral arteries. The results indicate that mt1A melatonin receptor mRNA is expressed in bovine cerebral arteries. The relative levels of mt1A melatonin receptor mRNA expression in anterior, posterior, middle and vertebral cerebral arteries were compared. The data show the highest and lowest levels of mRNA expressions in the middle and vertebral cerebral arteries, respectively. The maximal number (B(max)) of different types of melatonin receptors in various regions of cerebral arteries were identified and further characterized by using the selective 2-[(125)I] iodomelatonin binding assay. Saturation studies revealed that the binding represented a single site of high affinity binding for the melatonin receptor with the highest and lowest binding capacities in the middle and vertebral arteries, respectively. In order to elaborate the functional significance of melatonin in cerebral blood vessels, hydrogen peroxide- induced induction in eNOS protein level and phosphorylation of calcium/calmodulain-dependent protein kinase II (phospho-CaMKII) were demonstrated in the bovine isolated cerebral arteries with these effect being abolished by melatonin. This is the first evidence showing expression of mt1A melatonin receptor in the bovine cerebral arteries. However, further studies are necessary to delineate the role of melatonin and its receptors in regulating physiology of the cerebral vessels.