Mechanisms underlying H2O2-mediated inhibition of synaptic transmission in rat hippocampal slices

Hydrogen peroxide (H2O2) inhibits the population spike (PS) evoked by Schaffer collateral stimulation in hippocampal slices. Proposed mechanisms underlying this effect include generation of hydroxyl radicals (. OH) and inhibition of presynaptic Ca2+ entry. We have examined these possible mechanisms in rat hippocampal slices. Inhibition of the evoked PS by H2O2 was sharply concentration-dependent: 1.2 mM H2O2 had no effect, whereas 1.5 and 2.0 mM H2O2 reversibly depressed PS amplitude by roughly 80%. The iron chelator, deferoxamine (1 mM), and the endogenous . OH scavenger, ascorbate (400 muM), prevented PS inhibition, confirming . OH involvement. Isoascorbate (400 muM), which unlike ascorbate is not taken up by brain cells, also prevented PS inhibition, indicating an extracellular site of . OH generation or action. We then investigated whether H2O2-induced PS depression could be overcome by prolonged stimulation, which enhances Ca2+ entry. During 5-s, 10-Hz trains under control conditions, PS amplitude increased to over 200% during the first three-four pulses, then stabilized. In the presence of H2O2. PS amplitude was initially depressed, but began to recover after 2.5 s of stimulation, finally reaching 80% of the control maximum. In companion experiments, we assessed the effect of H2O2 on presynaptic Ca2+ entry by monitoring extracellular Ca2+ concentration ([Ca2+](o)) during train stimulation in the presence of postsynaptic receptor blockers. Evoked [Ca2+](o) shifts were apparently unaltered by H2O2, suggesting a lack of effect on Ca2+ entry. Taken together, these findings suggest new ways in which reactive oxygen species (ROS) might act as signaling agents, specifically as modulators of synaptic transmission. (C) 2000 Elsevier Science BN. All rights reserved.