Rapid hypoxia preconditioning protects cortical neurons from glutamate toxicity through δ-opioid receptor

Background and Purpose - Hypoxia preconditioning (HPC), rapid or delayed, has been reported to induce neuroprotection against subsequent severe stress. Because δ-opioid receptor (DOR) plays an important role in delayed HPC-induced neuroprotection against severe hypoxic injury, we asked whether DOR is also involved in the rapid HPC-induced neuroprotection. Methods - Cultured rat cortical neurons at culture days 8 to 9 were exposed to a short-term hypoxia (1% O₂ for 30 minutes) to induce HPC followed by 30-minute normoxia before exposing to glutamate toxicity (100 μmol/L; 4 hours). Neuronal viability was assessed by lactate dehydrogenase leakage and morphological assessment. Protein and mRNA levels of DOR were detected by receptor binding and RT-PCR, respectively. Naltrindole was used to block DOR. Developmental changes in NMDA receptor expression was measured by Western blots. Results - HPC significantly reduced the glutamate-induced neuronal injury. Receptor binding showed that HPC increased DADLE (a DOR ligand) binding density in the cultured cortical neurons by >90% over control level (P<0.05), although RT-PCR did not detect any appreciable change in DOR mRNA. DOR inhibition with naltrindole had no effect on neuronal injury and completely abolished the HPC-induced neuroprotection. In contrast to HPC-induced increase in DADLE binding density, prolonged hypoxia caused severe neuronal injury with a significant decrease in DADLE binding density and DOR mRNA level. Conclusions - DOR is involved in neuroprotection induced by rapid HPC in cortical neurons.