| Glutamate is the major excitatory neurotransmitter of the brain as well as a mediator of neuronal excitotoxicity in ischemia, epilepsy, trauma, and neurodegeneration. Metabotropic glutamate receptors (mGluRs) are G-protein-coupled receptors that modulate functions in neurons and glia. To study the significance of mGluRs in neuronal survival, we have used Semliki Forest Virus vector to transfer genes coding for mGluR subtypes. Overexpression of mGluR1α and mGluR5 exacerbated, while mGluR2, mGluR3 attenuated, neuronal death caused by glutamate as determined by lactate dehydrogenase (LDH) assay. However, in cultures with higher percentage of astrocytes, overexpression of all four subtypes attenuated neuronal death caused by glutamate as shown by LDH assay. By combining the viral vector overexpression of mGluR subtypes and agonists and antagonists of these receptors, we found that group I mGluRs is involved in an astrocyte-dependent neuroprotective signaling pathway. The exacerbation of neuronal death by group I mGluRs may enhance neuronal necrotic death.; PAF is a messenger that mediates multiple events in the nervous system under physiologic and pathologic conditions. Superoxide, hydrogen peroxide, and peroxynitrite are three free oxidative radicals involved in glutamate-mediated neuronal injury. In the present study, PAF (500 nM) was found to potentiate hydrogen peroxide-mediated neuronal death but not that by superoxide nor peroxynitrite as determined by LDH assay, Hoechst staining, and TUNEL. PAF by itself did not trigger neuronal injury at this concentration. The PAF potentiation was blocked by the PAF membrane receptor antagonist, BN52021, but not by the intracellular antagonist, BN50730. The potentiation was eliminated by the agonist of group I mGluRs, (S)-3,5-dihydroxyphenylglycine (DHPG; 250μM).; In summary, PAF action potentiated neurotoxicity caused by added hydrogen peroxide. This potentiation promotes neuronal injury, probably by apoptotic signaling. Group I mGluRs activation was found to potentiate NMDA-mediated neurotoxicity, most likely through a necrotic mechanism. However, group I mGluRs activation blocked the synergistic effect of PAF and hydrogen peroxide in neuronal death, probably by inhibiting pro-apoptotic mechanism. |
