Characteristics of extracellular ATP-evoked electrical responses in rat hippocampal neurons were investigated. Extracellular ATP (100 μM) induced a rapid depolarization followed by repetitive firings of spikes in these cells under whole-cell current-clamp. In whole-cell voltage-clamp experiments, ATP activated 2 types of inward currents that were inhibited by P2-purinoceptor blocker suramin (300 μM). One is a small (about -20 pA) sustained current which is insensitive to tetrodotoxin (TTX), and the other is a large (-100 to -300 pA) transient current which abolished in the presence of 3 μM TTX. The ATP-induced transient current was blocked by 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX; 30 μM), a non-N-methyl-d-aspartate (non-NMDA) receptor antagonist. ATP failed to induce the transient current in the cell which showed the desensitization to quisqualic acid (QA; 10 μM), a non-NMDA receptor agonist. These findings suggest that ATP directly activates small sustained currents, and indirectly induces the transient currents by evoking glutamate release.
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