Activation of Presynaptic P2X7-Like Receptors Depresses Mossy Fiber–CA3 Synaptic Transmission through p38 Mitogen-Activated Protein Kinase

Abstract

P2X₇receptor subunits form homomeric ATP-gated, calcium-permeable cation channels. In this study, we used Western blots and immunocytochemistry to demonstrate that P2X₇receptors are abundant on presynaptic terminals of mossy fiber synapses in the rat hippocampus. P2X₇-immunoreactive protein was detected using a specific P2X₇antibody in Western blots of protein isolated from whole hippocampus and from a subcellular fraction containing mossy fiber synaptosomes. P2X₇immunoreactivity was colocalized with syntaxin 1A/B-immunoreactivity in mossy fiber terminals in the dentate hilus and stratum lucidum of CA3. Extracellular and whole-cell voltage-clamp recordings in CA3 revealed that bath application of the potent P2X₇agonist 2′,3′-O-(4-benzoylbenzoyl)-ATP (Bz-ATP) caused a long-lasting inhibition of neurotransmission at mossy fiber–CA3 synapses. Consistent with a presynaptic action at mossy fiber synapses, Bz-ATP had no significant effect on neurotransmission at associational–commissural synapses in CA3 but increased paired-pulse facilitation during depression of mossy fiber evoked currents. In addition, Bz-ATP had no postsynaptic effect on holding current or conductance of CA3 neurons. Bz-ATP-induced mossy fiber synaptic depression was blocked by the P2X₇antagonist oxidized ATP but not by the P2X_1–3,5,6antagonist pyridoxalphosphate-6-azophenyl-2′,4′-disulfonic acid or the P2Y antagonist reactive blue 2. Finally, an antagonist of p38 MAP kinase activation [4-(4-fluorophenyl)2-(4-methylsulfinylphenyl)5-(4-pyridyl)imidazole] but not extracellular signal-regulated kinase 1/2 MAP kinase (2′-amino-3′-methoxyflavone) blocked the synaptic depression mediated by Bz-ATP, suggesting that this presynaptic inhibition was mediated by activation of p38 MAP kinase. The results of the present study demonstrate that activation of presynaptic P2X₇receptors depresses mossy fiber–CA3 synaptic transmission through activation of p38 MAP kinase.