The effect of excitatory amino acids (EAAs) on c-fos mRNA expression was studied in primary cultures of mouse cerebellar granule cells and in neocortical neurons after 2 and 7 days in vitro (div). In cultured granule cells at 2 and 7 div, and in cortical neurons at 2 div, exposure to low levels (I10 mM) of a variety of
EAAs (viz. glutamate [Glu], S-sulpho-L-cysteine [SC],
N-methyl-D-aspartate [NMDA], a-amino-3-hydroxy-5methyl-4-isoxazole [AMPA], and kainate [KA]) resulted in a transient increase in the level of c-fos mRNA which peaked at 30 min but returned to a basal level by 120 min. However, exposure of granule cells (7 div) to high levels (250 mM) of Glu, NMDA, KA, SC and of cortical neurons (7 div) to high levels (250 mM) of Glu, NMDA, KA, SC, or AMPA and to low levels (I10 mM) of Glu and AMPA resulted in a delay in c-fos mRNA induction but a subsequent, progressive increase that was sustained for at least 240 min. Furthermore, this effect was accompanied by a doserelated increase in the release of the cytosolic enzyme, lactate dehydrogenase, used as an indicator of excitotoxicity. A ratio (Q 240/30 ) for the steady-state levels of c-fos mRNA after 30 min and 240 min of exposure to EAAs was determined which showed that Q 240/30 G2 correlated reproducibly with excitotoxic cell death, whereas a ratio of I1 correlated with a nonexcitotoxic event. In both cell types at 7 div, coadministration of the selective NMDA receptor antagonist, DL(6)-2amino-5-phosphonopentanoic acid (APV) with cytotoxic levels of Glu 1) protected against EAA-induced neurotoxicity and 2) exhibited a transient c-fos mRNA expression (Q 240/30 values D1). In contrast, the AMPA/KA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), provided no protection against excitotoxicity and had no significant effect on the Glu-induced delay in c-fos mRNA expression. These results suggest that the Q 240/30 c-fos mRNA ratio may 1) be used as a predictive index for excitotoxic neuronal death, 2) provide information on the identity of the receptor subtype mediating excitotoxicity in different brain cell types, and 3) aid in establishing the role of excitotoxicity during the development of neurons in vitro.