Characterization of the excitoprotective actions of N-methyl-D-aspartate in cultured cerebellar granule neurons.

Exposure of cultured cerebellar granule neurons to subtoxic concentrations of N-methyl-D-aspartate (NMDA) has been shown previously to result in a neuroprotective state, as measured by subsequent exposure to toxic concentrations of glutamate. In the present study, we have further characterized the excitoprotective actions of NMDA in these neurons. NMDA-induced excitoprotection was concentration dependent (EC50 approximately 30 microM) and time dependent, with maximal protection observed following 16 h of preexposure to NMDA. NMDA-induced excitoprotection did not require continuous exposure to NMDA, as a 4-h preincubation was sufficient to induce full excitoprotection when measured 8 h later. Maximal protection was manifest as a "right shift" in the concentration-response relationship for glutamate toxicity of approximately three orders of magnitude (EC50 approximately 30 microM in untreated neurons compared with > or = 50 mM in NMDA-treated neurons). After removal of NMDA, complete reversal of the excitoprotective state was observed by 48 h (t1/2 approximately 24 h). The ability of NMDA to induce excitoprotection was observed in neurons maintained for up to 14 days in vitro (DIV) [postnatal day (PND) 22], but was absent at 21 and 32 DIV (PND 29-40), despite little to no difference in the toxicity of glutamate at any DIV examined. Preexposure of cerebellar granule neurons to a maximally excitoprotective concentration of NMDA (50 microM) failed to alter the density of NMDA receptors measured by the specific binding of [3H]MK-801.(ABSTRACT TRUNCATED AT 250 WORDS)