Adenosine (ADO) is produced by cultured neurons and astrocytes, albeit by different pathways, during in vitro stroke models (Parkinson and Xiong [2004] J. Neurochem. 88:1305-1312). Expression of ecto-5' nucleotidase (e-N), the enzyme responsible for extracellular dephosphorylation of AMP to ADO, is more abundant in astrocytes than neurons. Therefore, we tested the hypothesis that N-methyl-D-aspartate (NMDA) evokes ADO release per se from neurons, whereas dephosphorylation of extracellular adenine nucleotides contributes to NMDA-evoked ADO production in the presence of astrocytes. We used four different cell preparations-cortical rat neurons, cortical rat astrocytes, cocultures of neurons and astrocytes, and transient cocultures of neurons with astrocytes on transwell filters-to show that astrocytes contribute to NMDA-evoked increases in extracellular ADO. NMDA significantly increased ADO and inosine (INO) production from cultured cortical neurons but only increased extracellular INO production from cocultures. In neurons, the equilibrative nucleoside transport (ENT) inhibitor dipyridamole (DPR) prevented NMDA-evoked ADO and INO production, whereas the e-N inhibitor alpha,beta-methylene ADP (AOPCP) had no effect. Conversely, from both cocultures and transient cocultures DPR significantly decreased NMDA-evoked INO but not ADO generation. AOPCP inhibited NMDA-evoked production of both ADO and INO from transient cocultures. In the absence of astrocytes, NMDA evoked release of intracellular ADO and INO from cultured cortical neurons through ENT. However, in the presence of astrocytes, extracellular conversion of adenine nucleotides to ADO contributed significantly to NMDA-evoked production of this purine.
(c) 2008 Wiley-Liss, Inc.