NMDA receptor-mediated excitoprotection of cultured cerebellar granule neurons fails to alter glutamate-induced expression of c-fos and c-jun mRNAs

Exposure of cultured cerebellar granule neurons to subtoxic concentrations of N-methyl-d-aspartate (NMDA) induces a state of excitoprotection when measured by subsequent exposure to toxic concentrations of glutamate. This NMDA-induced excitoprotective state is prevented by inhibitors of new RNA and protein synthesis. Since the neurotrophic and excitoprotective effects of NMDA in cerebellar granule neurons may involve changes in the expression of the immediate early genes c-fos and c-jun, we measured c-fos and c-jun mRNAs in cerebellar granule neurons after exposure to either toxic concentrations of glutamate or excitoprotective (subtoxic) concentrations of NMDA. Exposure of cerebellar granule neurons to toxic concentrations of glutamate induced a dramatic increase in c-fos and c-jun mRNAs which was not associated with a corresponding increase in c-fos and c-jun proteins as measured immunocytochemically. However, the increase in c-fos and c-jun mRNAs induced by toxic concentrations of glutamate was not altered by preexposing cerebellar granule neurons to NMDA, suggesting that increased expression of c-fos and c-jun mRNAs is not sufficient for glutamate toxicity of these neurons. Preexposure of cerebellar granule neurons to NMDA for 24 h, which induced a maximal excitoprotective state, resulted in a transient increase in c-fos, and to a lesser degree c-jun, mRNAs similar to that induced by toxic concentrations of glutamate. The induction of c-fos, but not that of c-jun, mRNA both by excitoprotective concentrations of NMDA and by neurotoxic concentrations of glutamate was blocked by the non-competitive NMDA receptor antagonist, MK-801. These data show that c-fos and c-jun may play a role in NMDA receptor-mediated excitoprotection as well as in NMDA receptor-mediated neurotoxicity. NMDA receptor-mediated excitoprotection is likely to interfere with the intracellular cascade of glutamate toxicity by acting on an element downstream of c-fos and c-jun-dependent transcriptional regulation.