Is the AMPA receptor subunit GluR2 mRNA an early indicator of cell fate after ischemia? A quantitative single cell RT-PCR study.

After a moderate global cerebral ischemia, two hypothetical populations of pyramidal neurons are present among the hippocampal CA1 pyramidal neurons: one that will die and another one that will survive. Prior analysis of dissected hippocampal CA1 regions has shown a reduction of the GluR1-3 mRNA following ischemia. In order to identify these changes in single neurons, quantitative single cell RT-PCR was used to analyze the expression of GluR1-4 mRNA in rats 24 h after ischemia and also in rats after tolerance inducing ischemia. Control CA1 cells had a median copy-number of 290, 247, 207 and 16 GluR1-4, respectively. The tolerant cells showed small significant up-regulations of GluR1, 3 and 4 mRNA, while the GluR2 mRNA showed a more than 4-fold up-regulation compared to control cells. All the cells from ischemic animals displayed down-regulations of GluR1-3 mRNA. The GluR4 mRNA was not detectable in the ischemic animals. Our results thus show that the CA1 neurons react uniformly 24 h after a moderate ischemia independent of the fate of the neuron: thus two neuron populations with different GluR2 profiles cannot be identified in post-ischemic animals at 24 h. It seems however that an increased level of GluR2 can be used as an indicator of tolerance to ischemia.