Melatonin prevents the delayed death of hippocampal neurons induced by enhanced excitatory neurotransmission and the nitridergic pathway

The mechanisms by which neurons die after stroke and status epilepticus and related neuropathological conditions are unclear, but may involve voltage-dependent Na+ channels, glutamate receptors, and nitric oxide (NO.). These questions were investigated using an in vitro primary cell culture model in which hippocampal pyramidal neurons undergo a gradual and delayed neurodegeneration induced by enhanced excitatory neurotransmission. When cells were treated with Mg2+-free, glycine-supplemented medium for a brief period (15 nain) and examined 24 h later, similar to 30-40% of the neurons had died. Cell death could be inhibited by blockers of voltage-sensitive Na+ channels and by N-methyl-D-aspartate receptor antagonists. Application of either the endogenous antioxidant melatonin (EC50: 19.2+/-2.8 mu M) or the NO. synthase inhibitor N-omega-nitro-L-arginine after, but not during, Mg2+-free exposure protected against delayed neuronal death; significant neuroprotection was observed when the addition was delayed for up to 4 h, This operational time window suggests that an enduring production of NO. and reactive oxygen species from neuronal sources is responsible for delayed cell death. A role fbr reactive oxygen species in this injury process was strengthened by the finding that, whereas neurons cocultured with astroglia were more resistant to killing, agents capable of lowering intracellular glutathione negated this protection. Because secretion levels of melatonin are decreased with aging, reductions in this pineal hormone may place neurons at a heightened risk for damage by excitatory synaptic transmission.