No abstract

The ionotropic and cytolytic P2X 7 receptor is typically found on immune cells, where it is involved in the release of cytokines. Recently, P2X 7 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 P2X 7 receptors at hippocampal mossy fiber-CA3 synapses. In agreement with previous findings, the widely used P2X 7 agonist 2Ј-3Ј-O-(4-benzoylbenzoyl)-adenosine-5Ј-triphosphate (BzATP) clearly depressed field potentials (fEPSPs); however, no evidence for an involvement of P2X 7 receptors could be obtained. First, depression of fEPSPs by BzATP was unchanged in P2X 7 Ϫ/Ϫ mice. Second, experiments using P2X 7 Ϫ/Ϫ mice, immunohistochemistry, and electron microscopy showed that the antigen detected by frequently used P2X 7 antibodies is not compatible with a plasmalemmal P2X 7 receptor. Third, BzATP did not alter Ca 2ϩ levels in synaptic terminals. In contrast, the depression of fEPSPs by BzATP was fully blocked by adenosine (A 1 ) receptor antagonists. Furthermore, the application of BzATP also activated postsynaptic A 1 receptor-coupled K ϩ channels. This effect of BzATP was mimicked by ATP and adenosine and was completely prevented by enzymes specifically degrading adenosine. Activation of A 1 -coupled K ϩ channels by BzATP was dependent on ecto-nucleotidases, extracellular enzymes that convert ATP to adenosine. Moreover, the opening of A 1 -coupled K ϩ 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 A 1 receptors rather than through P2X 7 receptors. Thus, the present study casts doubts on the neuronal localization of P2X 7 receptors in rodent hippocampus.