Eosine-induced blockade of N-methyl-D-aspartate channels in acutely isolated rat hippocampal neurons

Acutely isolated rat hippocampal neurons were voltage-clamped in the whole-cell configuration. The currents through N-methyl-D-aspartate (NMDA) channels were elicited by fast application of aspartate in a Mg2+-free 3 mu M glycine-containing solution. Eosine, known as a potent reversible inhibitor of the plasma membrane Ca2+ pump, proved to be able to induce a blockade of NMDA channels. The eosine-induced inhibition of NMDA-mediated currents enhanced with eosine concentration (IC50 = 248 mu M) but did not depend on the membrane potential, agonist (aspartate) or coagonist (glycine) concentrations, pH, or the presence of spermine, ethanol, and the disulfide-reducing agents dithiothreitol and glutathione. Zn2+ inhibited NMDA channels with equal efficiency both in the presence and absence of eosine. These results suggest that eosine interacts with a new, previously unknown NMDA receptor regulatory site.