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Localization of Adenosine A2a Receptor in Retinal Development and Oxygen-Induced Retinopathy

From the Wilmer Ophthalmological Institute, Johns Hopkins University School of Medicine, Baltimore, Maryland.

PURPOSE. To investigate the association of adenosine A2a receptors (A2aR) with retinal vasculogenesis and angiogenesis that occurs in the canine model of oxygen-induced retinopathy (OIR).

METHODS. One-day-old dogs were exposed to 100% oxygen for 4 days and killed in oxygen (5 days old) and at 3, 10, 17, and 23 days after exposure to hyperoxia. Room air control animals were killed at 1, 5, 8, 15, 22, and 28 days of age. Immunolocalization of A2aR was performed on frozen sections, and reaction product density was quantified using microdensitometry. Cell types were identified in serial sections using antibodies against von Willebrand factor (endothelial cells) and GFAP (astrocytes), and enzyme histochemistry for menadione-dependent -glycerophosphate dehydrogenase (M--GPDH) (to label angioblasts and developing blood vessels).

RESULTS. A2aR immunoreactivity was associated with forming blood vessels and angioblasts in the nerve fiber layer (NFL) of peripheral retina. As development progressed, vascular labeling decreased, whereas labeling of neuronal elements increased. In OIR, A2aR immunoreactivity in the NFL was reduced after exposure to hyperoxia and significantly elevated in the inner retina throughout vascularized retina and in advance of forming vasculature in all oxygen-treated animals returned to room air. A2aR immunoreactivity was also prominent in fronds of intravitreal neovascularization.

CONCLUSIONS. A2aR immunoreactivity was associated with developing retinal vessels. As development progressed, vascular-associated A2aR labeling decreased and, concomitantly, labeling of neuronal elements increased. A2aR immunoreactivity was significantly elevated at the edge of forming vasculature in all animals returned to room air after hyperoxia and in intravitreal neovascular formations. These results provide additional evidence for the importance of A2aR and its ligand adenosine in retinal vascular development and in the vasoproliferative stage of canine OIR.

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