Alterations of NMDA receptor subunits NR1, NR2A and NR2B mRNA expression and their relationship to apoptosis following transient forebrain ischemia

Abstract

Glutamate excitotoxicity mediated by NMDA receptor activation plays a key role in many aspects of ischemic brain injury, but the expression of NMDA receptor subunits NR1, NR2A and NR2B mRNA and their relationship to apoptosis is still unclear. In this study, we applied in situ hybridization and TUNEL staining to investigate the expression of NMDA receptor subunit mRNA and apoptosis in hippocampus of rats after transient forebrain ischemia. The results showed that in the CA1 region, NR1 mRNA expression was significantly increased following ischemia–reperfusion (IR), reaching peak levels at IR 24 h, and then gradually decreasing until IR 7 days. NR2A and NR2B mRNA expression dropped to lowest levels at IR 6 h and IR 12 h, respectively, and then started to recover. The mRNA expression of both NR2A and NR2B then increased to peak levels at IR 48 h, followed by a sustained decline until IR 7 days. In the CA3 region and dentate gyrus the range of variation in mRNA expression was significantly reduced gradually. At IR 24 h, apoptosis-positive cells were observed mainly in the CA1 region. The number of apoptosis-positive cells continuously grew and showed a dramatic increase at IR 48 h and peaked at IR 72 h. Then, the number of apoptosis-positive cells started to decrease, but at IR 7 days the apoptosis-positive cells still remained. These results indicate that the alterations of NMDA receptor subunit mRNA expression may contribute to the ischemic apoptosis of hippocampus after transient forebrain ischemia.