During hypoxia, extracellular adenosine levels are increased to prevent cell damage, playing a neuroprotective role mainly through adenosine A1 receptors. The aim of the present study was to analyze the effect of hypoxia in both adenosine A1 and A2A receptors endogenously expressed in C6 glioma cells. Two hours of hypoxia (5% O2) caused a significant decrease in adenosine A1 receptors. The same effect was observed at 6 h and 24 h of hypoxia. However, adenosine A2A receptors were significantly increased at the same times. These effects were not due to hypoxia-induced alterations in cells number or viability. Changes in receptor density were not associated with variations in the rate of gene expression. Furthermore, hypoxia did not alter HIF-1α expression in C6 cells. However, HIF-3α, CREB and CREM were decreased. Adenosine A1 and A2A receptor density in normoxic C6 cells treated with adenosine for 2, 6 and 24 h was similar to that observed in cells after oxygen deprivation. When C6 cells were subjected to hypoxia in the presence of adenosine deaminase, the density of receptors was not significantly modulated. Moreover, DPCPX, an A1 receptor antagonist, blocked the effects of hypoxia on these receptors, while ZM241385, an A2A receptor antagonist, was unable to prevent these changes. These results suggest that moderate hypoxia modulates adenosine receptors and cAMP response elements in glial cells, through a mechanism in which endogenous adenosine and tonic A1 receptor activation is involved.
J. Neurochem. (2008) 105, 2315–2329.