Ecto-Nucleotidases and Nucleoside Transporters Mediate Activation of Adenosine Receptors on Hippocampal Mossy Fibers by P2X7Receptor Agonist 2′-3′-O-(4-Benzoylbenzoyl)-ATP

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Abstract
The ionotropic and cytolytic P2X7 receptor is typically found on immune cells, where it is involved in the release of cytokines. Recently, P2X7 receptors were reported to be localized to presynaptic nerve terminals and to modulate transmitter release. In the present study, we reassessed this unexpected role of P2X7 receptors at hippocampal mossy fiber-CA3 synapses. In agreement with previous findings, the widely used P2X7 agonist 2′-3′-O-(4-benzoylbenzoyl)-adenosine-5′-triphosphate (BzATP) clearly depressed field potentials (fEPSPs); however, no evidence for an involvement of P2X7 receptors could be obtained. First, depression of fEPSPs by BzATP was unchanged in P2X7-/- mice. Second, experiments using P2X7-/- mice, immunohistochemistry, and electron microscopy showed that the antigen detected by frequently used P2X7 antibodies is not compatible with a plasmalemmal P2X7 receptor. Third, BzATP did not alter Ca2+ levels in synaptic terminals. In contrast, the depression of fEPSPs by BzATP was fully blocked by adenosine (A1) receptor antagonists. Furthermore, the application of BzATP also activated postsynaptic A1 receptor-coupled K+ channels. This effect of BzATP was mimicked by ATP and adenosine and was completely prevented by enzymes specifically degrading adenosine. Activation of A1-coupled K+ channels by BzATP was dependent on ecto-nucleotidases, extracellular enzymes that convert ATP to adenosine. Moreover, the opening of A1-coupled K+ channels by BzATP was dependent on nucleoside transporters. Taken together, our results indicate that BzATP is extracellularly catabolized to Bz-adenosine and subsequently hetero-exchanged for intracellular adenosine and then depresses mossy fiber fEPSPs through presynaptic A1 receptors rather than through P2X7 receptors. Thus, the present study casts doubts on the neuronal localization of P2X7 receptors in rodent hippocampus.