Glutamate-Mediated Excitotoxic Death of Cultured Striatal Neurons Is Mediated by Non-NMDA Receptors

Considerable interest has focused on the role of glutamate-mediated excitotoxicity in neurodegenerative disorders of the basal ganglia. The in vitro data on the receptor mechanisms involved in this process, however, have been inconclusive. Some studies have indicated that excitotoxins acting at NMDA receptors kill striatal neurons and others have indicated that NMDA receptor-mediated excitotoxic death of striatal neurons is minimal in the absence of cortex. In the present study, we used a pharmacological approach to carefully reexamine this issue in 2-week-old cultures of striatal neurons dissociated from E17 rat embryos. The sensitivity of these neurons to glutamate agonists and antagonists was determined by monitoring cell loss in identified regions of the growth dishes. We found that glutamate killed striatal neurons with an EC₅₀ of 100 μM. This loss was not mediated by NMDA receptors, since it was not reduced by the NMDA receptor antagonist APV (0.1-1.0 mM). Consistent with this result, up to 50 mM NMDA receptor-specific excitotoxin quinolinic acid (QA) did not affect neuronal survival. Depolarizing the QA-exposed neurons with 35 mM potassium chloride to enhance NMDA receptor activation by QA also did not produce neuron loss. The metabotropic glutamate receptor antagonist AP3 (500 μM) also had no effect on the striatal neuron loss produced by 100 μM glutamate. In contrast, the non-NMDA antagonist GYKI 52466 (100 μM) did block the excitotoxic effect of glutamate (100 μM). Specific AMPA and KA receptor agonists and the non-NMDA antagonist GYKI 52466 revealed that the non-NMDA receptor-mediated excitotoxic effect of glutamate was mediated by KA receptors. These results suggest that cultured striatal neurons are directly vulnerable to non-NMDA glutamate agonists, but not to NMDA and metabotropic glutamate agonists. Thus, non-NMDA receptors may play a greater role in the excitotoxic death of striatal neurons in disease and experimental animal models than previously realized.