Early response of neurons and glial cells to hypoxia in the retina

PURPOSE. The present study was undertaken to examine the involvement of nitric oxide (NO) and excitotoxicity in the development of hypoxia-induced retinopathy in adult rats. METHODS. Retinas of adult rats were examined at 3 hours to 14 days after hypoxia. The mRNA and protein expression of endothelial, neuronal, and inducible nitric oxide synthase (eNOS, nNOS, and iNOS, respectively), hypoxia-inducible factor-la (HIF-1 alpha), vascular endothelial growth factor (VEGF), N-methyl-D-aspartate receptor subunit 1 (NMDAR1), and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid glutamate (AMPA GluR2 and GluR3) receptors in the retina was determined by real-time RT-PCR, Western blot analysis, and immunohistochemistry. The response of retinal microglial cells to hypoxia was also studied by immumohistochemistry. RESULTS. Hemorrhages were observed in the retina after hypoxia. Upregulated mRNA and protein expression of HIF-1 alpha, NMDAR1, GluR2, GluR3, VEGF, eNOS, nNOS, and iNOS in the retina was observed in response to hypoxia. Complement type 3 (CR3) receptors and major histocompatibility complex (MHC) class I and II antigen expression on the microglial cells was increased after exposure to hypoxia. CONCLUSIONS. The findings of this study indicate that NO and excitotoxicity may produce damage to retina in response to hypoxia. Increased expressions of eNOS and VEGF in response to hypoxia are indicative of vasodilatation and increased permeability of retinal blood vessels. Increased phagocytosis by retinal microglial cells evidenced by increased expression of CR3 receptors may occur for the removal of hemorrhagic debris. Upregulation of MHC antigens indicates the readiness of these cells to participate in an immune response.