APPS November 2002 Meeting Abstract 2412

ATP and glutamate are excitatory neurotransmitters in the central nervous system acting through multimeric ligand-gated P2X and NMDA receptors, respectively. Increased levels of glutamate result in hypoxic-ischaemic neuronal cell death, thus inhibitors of current through NMDA receptors have neuroprotective potential. Whereas glutamate does not gate or modulate P2X receptors, ATP has been reported to rescue cultured hippocampal neurons from glutamate-mediated neurotoxicity. In the present study, we investigated the effect of ATP on recombinant heteromeric NR1A/NR2B NMDA receptors expressed in Xenopus oocytes using the two-electrode voltage clamp recording technique. ATP was found to directly inhibit glutamate-induced current through NMDA receptors. The amplitude of current activated by 1 然 of glutamate (in the presence of 10 然 glycine) was decreased by ATP (disodium salt) in a concentration-dependent manner and was completely abolished by 3 mM ATP. The concentration of ATP that produced 50% inhibition (IC₅₀) was ~500 然 for currents evoked by either 1 然 or 10 然 glutamate. The rate of onset of ATP block was significantly faster than recovery from block. The non-hydrolyzable analogs, ATPγS and α,β-meythylene ATP (0.1-1 mM) also produced a concentration-dependent inhibition of glutamate-evoked currents. The inhibitory effect of ATP was not dependent on membrane potential and ATP did not alter the reversal (zero-current) potential of glutamate-activated currents. The lack of voltage dependence of the block indicates that ATP acts outside of the NMDA receptor pore. Given that block by ATP (≥ 1 mM) was not overcome by increasing concentration of glutamate indicates that ATP acts at a site remote from the glutamate binding site. The results suggest that ATP can attenuate glutamate-mediated excitotoxicity at the NMDA receptor.