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 epilepticu s and related neuropathological conditions are unclear, but may involv e voltage-dependent Na+ channels, glutamate receptors, and nitric oxid e (NO.). These questions were investigated using an in vitro primary c ell culture model in which hippocampal pyramidal neurons undergo a gra dual and delayed neurodegeneration induced by enhanced excitatory neur otransmission. When cells were treated with Mg2+-free, glycine-supplem ented medium for a brief period (15 nain) and examined 24 h later, sim ilar 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-aspart ate receptor antagonists. Application of either the endogenous antioxi dant 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 prot ected 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 d eath. A role fbr reactive oxygen species in this injury process was st rengthened by the finding that, whereas neurons cocultured with astrog lia were more resistant to killing, agents capable of lowering intrace llular glutathione negated this protection. Because secretion levels o f melatonin are decreased with aging, reductions in this pineal hormon e may place neurons at a heightened risk for damage by excitatory syna ptic transmission.