N-Methyl-?-aspartate receptor activation inhibits protein synthesis in cortical neurons independently of its ionic permeability properties

Transient cerebral ischemia, which is accompanied by a sustained release of glutamate, strongly depresses protein synthesis. We have previously demonstrated in cortical neurons that a glutamate-induced increase in intracellular Ca2+ is likely responsible for the blockade of the elongation step of protein synthesis. In this study, we provide evidence indicating that NMDA mobilizes a thapsigargin-sensitive pool of intracellular Ca2+. Exposure of cortical neurons to NMDA, in the absence of external Ca2+, produced a transient rise in intracellular Ca2+ that was suppressed by pretreatment with thapsigargin. This rise in intracellular Ca2+ did not result from an influx of Na+ via reversal of the mitochondrial Na+/Ca2+ exchanger since it persisted in a Na+-free medium or in the presence of CGP 37157, an inhibitor of the exchanger. Moreover, the NMDA-induced increase in intracellular Ca2+ required the presence of D-serine, was blocked by D(−)-2-amino-5-phosphonopentanoic acid, but was not reduced in the presence of external Mg2+. This unexpected non-ionotropic effect of NMDA was associated with an inhibition of protein synthesis that was also insensitive to the absence of external Ca2+ or Na+, or presence of Mg2+. NMDA treatment resulted in an increase in the phosphorylation of eEF-2 in the absence or presence of external Ca2+. The initiation step of protein synthesis was not blocked by NMDA since the phosphorylation of initiation factor eIF-2α subunit was not altered by NMDA treatment. In conclusion, we provide evidence indicating that NMDA can inhibit protein synthesis in cortical neurons through a process that involves the mobilization of intracellular Ca2+ stores via a mechanism that is not linked to the ionic properties of NMDA receptors.