Extracellular ATP evoked two excitatory responses in hippocampal neuroblastoma cells (HN2). The first, an opening of a receptor-operated non-selective cation channel and the second was a leftward shift in Na + channel activation. Both ATP (5–1000 μM) and 2′,3′-(4-benzoyl)-benzoyl-ATP (Bb-ATP, 50 μM) activated a non-selective cation current reversing near 0 mV and shifted the Na + activation and inactivation curves to the left. Based on a comparison of a series of agonists and antagonists, the inward current appeared to be partially mediated by activation of a P2X 7 receptor, although hybrid channels cannot be ruled out. The shift in Na + channel gating could be separated from the opening of the cation channel, as application of the P2Y antagonist Reactive Blue-2 and GTP shifted the Na + current activation to the left but failed to elicit the inward cation current. Both responses to ATP and Bb-ATP were insensitive to block by the P2X antagonist suramin (300 μM) but were prevented by incubation in oxidized ATP (200 μM); a putative P2X 7 receptor antagonist. Prior screening of the surface negative charge of the membrane with a high concentration of divalent cations prevented both responses. We suggest that ATP 4− activates a P2X receptor and becomes trapped on a site, on or near the Na + channel. Activation of the P2X receptor leads to the opening of a non-specific cation channel, while the binding of ATP 4− leads to a modified charge sensed by the Na + channel, similar to what occurs in the presence of charged amphiphiles as well as a number of β-scorpion toxins.