Adenosine is a potent anticonvulsant acting on excitatory synapses through A1 receptors. Cellular release of ATP, and its subsequent extracellular enzymatic degradation to adenosine, could provide a powerful mechanism for astrocytes to control the activity of neural networks during high-intensity activity. Despite adenosine's importance, the cellular source of adenosine remains unclear. We report here that multiple enzymes degrade extracellular ATP in brain tissue, whereas only Nt5e degrades AMP to adenosine. However, endogenous A1 receptor activation during cortical seizures in vivo or heterosynaptic depression in situ is independent of Nt5e activity, and activation of astrocytic ATP release via Ca 2+ photolysis does not trigger synaptic depression. In contrast, selective activation of postsynaptic CA1 neurons leads to release of adenosine and synaptic depression. This study shows that adenosine-mediated synaptic depression is not a consequence of astrocytic ATP release, but is instead an autonomic feedback mechanism that suppresses excitatory transmission during prolonged activity.purinergic signaling | purine | glia | calcium signaling S everal lines of work over the past three decades have documented that adenosine acts as an endogenous anticonvulsant (1-3). The extracellular concentration of adenosine increases during seizures, and it has been proposed that status epilepticus is a result of loss of adenosine signaling (4). Conversely, mice lacking A1 receptors exhibit a decreased threshold for seizure propagation (5-7). Adenosine can be generated in the cytosol of neurons as a consequence of the metabolic exhaustion and released directly, as adenosine, via the equilibrative nucleoside transporters (ENTs)-e.g., the ubiquitously expressed ENT1 and ENT2 (8, 9). Alternatively, adenosine is released indirectly, as ATP followed by extracellular enzymatic catabolism to adenosine. The CNS expresses several ectoenzymes, including nucleoside triphosphate diphosphohydrolases (NTPDases; e.g., CD39, NTPDase-2), ectonucleotide pyrophosphatase/phosphodiesterases (e.g., autotaxin), and ecto-5′-nucleotidases (CD73/Nt5e, prostatic acid phosphatase, and alkaline phosphatase) (10-12). Although the regional and cellular activity patterns for each of these enzymes have not been fully explored, exogenous addition of ATP to ex vivo preparations has shown that all regions of the mammalian brain can dephosphorylate ATP to AMP through an ADP intermediate, whereas dephosphorylation of AMP to adenosine occurs primarily in the striatum and olfactory bulb (11). However, the presence of ectoenzymes does not prove that extracellular conversion of ATP to adenosine plays a physiological role, because extracellular adenosine is rapidly recirculated back into the cytosol by ENT1 and ENT2 (13). Further, ATP is released in small quantities during cell-cell signaling, and the effective diffusion of adenosine is limited because of the potent reuptake mechanisms (14). Thus, it is possible that extracellularly generated adenosine is transported back...