The neuroprotective activities of melatonin against the Alzheimer β-protein are not mediated by melatonin membrane receptors

Exposure of neuronal cells to the Alzheimer's amyloid β protein (Aβ) resuhs in extensive oxidative damage of bio-molecules that are profoundly harmful to neuronal homeostasis. It bas been demonstrated that melatonin protects neurons against Aβ-mediated neurotoxicity, including cell death and a spectrum of oxidative lesions. We undertook the current study to determine whether melatonin membrane receptors are involved in the mechanism of neuroprotection against Aβ neurotoxicity. For this purpose, we characterized the free-radical scavenging potency of several compounds exhibiting various affinities for melatonin membrane receptors (MLT 1a and 1b), Aβ-mediated neurotoxicity was assessed in human neuroblastoma cells and in primary hippocampal neurons. In sharp contrast with melatonin, no neuroprotection against Aβ toxicity was observed when we used melatonin membrane receptor agonists that were devoid of antioxidant activity. In contrast, the cells were fully protected in parallel control experiments when either melatonin, or the structurally unrelated free-radical scavenger phenyl-N-t-butyl nitrone (PBN), were added to Aβ-containing culture media. This study demonstrates that the neuroprotective properties of melatonin against Aβ-mediated toxicity does not require binding of melatonin to a membrane receptor and is likely the result of the antioxidant and antiamyloidogenic features of the agent.