Neuroprotective effect of melatonin against ischemia is partially mediated by alpha-7 nicotinic receptor modulation and HO-1 overexpression

Melatonin has been widely studied as a protective agent against oxidative stress. However, the molecular mechanisms underlying neuroprotection in neurodegeneration and ischemic stroke are not yet well understood. In this study, we evaluated the neuroprotective/antioxidant mechanism of action of melatonin in organotypic hippocampal cultures (OHCs) as well as in photothrombotic stroke model in vivo. Melatonin (0.1, 1, and 10m) incubated postoxygen and glucose deprivation (OGD) showed a concentration-dependent protection; maximum protection was achieved at 10m (90% protection). Next, OHCs were exposed to 10m melatonin at different post-OGD times; the protective effect of melatonin was maintained at 0, 1, and 2hr post-OGD treatment, but it was lost at 6hr post-OGD. The protective effect of melatonin and the reduction in OGD-induced ROS were prevented by luzindole (melatonin antagonist) and -bungarotoxin (-Bgt, a selective 7 nAChR antagonist). In Nrf2 knockout mice, the protective effect of melatonin was reduced by 40% compared with controls. Melatonin, incubated 0, 1, and 2hr post-OGD, increased the expression of heme oxygenase-1 (HO-1), and this overexpression was prevented by luzindole and -bungarotoxin. Finally, administration of 15mg/kg melatonin following the induction of photothrombotic stroke in vivo, reduced infarct size (50%), and improved motor skills; this effect was partially lost in 0.1mg/kg methyllycaconitine (MLA, selective 7 nAChR antagonist)-treated mice. Taken together, these results demonstrate that postincubation of melatonin provides a protective effect that, at least in part, depends on nicotinic receptor activation and overexpression of HO-1.