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Overstimulation of ionotropic glutamate receptors leads to excitotoxic neuronal death, which has been implicated in the neurodegeneration of neurological diseases. The present study examined the role of putative low-affinity kainate receptor subtype (GluR5-7) agonists in excitotoxicity in cultured murine cortical neurons. The concentration-dependent decrease in cell viability induced by the agonists kainate (1-1,000 microM) and (RS)-2-amino-3-(hydroxy-5-tert-butylisoxazol-4-yl) propanoic acid (ATPA; 1-1,000 microM) was only attenuated by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM) and 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI 52466; 20 microM). (S)-5-iodowillardiine (1-1,000 microM)-induced toxicity was attenuated by CNQX (20 microM), GYKI 52466 (20 microM) and MK-801 (10 microM); however, (2S, 4R)-4-methylglutamate (1-120 microM)-induced toxicity was not attenuated by the antagonists. None of the agonists possessed selective actions at GluR5-7. Morphological observations (phase-contrast and fluorescence microscopy) revealed that the agonists induced two distinct patterns of neuronal injury. After 24 hr of treatment, low concentrations of agonists (1-30 microM) produced cellular shrinkage and nuclear granulation consistent with slow, apoptotic-like neuronal death. Pyknotic labeling with the DNA binding dye Sytox green confirmed these apoptotic characteristics, which significantly decreased with increasing concentrations. After 4 hr, increasing concentrations of agonists (100-1,000 microM) induced cellular swelling, with subsequent extracellular debris; labeling with propidium iodide revealed isolated nuclei consistent with the increased involvement of rapid necrosis. Thus, all putative GluR5-7 agonists produced excitotoxicity across a necrotic-apoptotic continuum in murine cortical neuron cultures.

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