Mechanisms underlying H O - mediated inhibition of synaptic 2 2 transmission in rat hippocampal slices *

Hydrogen peroxide (H O ) inhibits the population spike (PS) evoked by Schaffer collateral stimulation in hippocampal slices. 2 2 21 Proposed mechanisms underlying this effect include generation of hydroxyl radicals (?OH) and inhibition of presynaptic Ca entry. We have examined these possible mechanisms in rat hippocampal slices. Inhibition of the evoked PS by H O was sharply concentration2 2 dependent: 1.2 mM H O had no effect, whereas 1.5 and 2.0 mM H O reversibly depressed PS amplitude by roughly 80%. The iron 2 2 2 2 chelator, deferoxamine (1 mM), and the endogenous ?OH scavenger, ascorbate (400 mM), prevented PS inhibition, confirming ?OH involvement. Isoascorbate (400 mM), 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 H O -induced PS depression could be overcome by 2 2 21 prolonged stimulation, which enhances Ca 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 H O , PS amplitude was initially depressed, but began to 2 2 recover after 2.5 s of stimulation, finally reaching 80% of the control maximum. In companion experiments, we assessed the effect of 21 21 21 H O on presynaptic Ca entry by monitoring extracellular Ca concentration ([Ca ] ) during train stimulation in the presence of 2 2 o 21 21 postsynaptic receptor blockers. Evoked [Ca ] shifts were apparently unaltered by H O , suggesting a lack of effect on Ca entry. o 2 2 Taken together, these findings suggest new ways in which reactive oxygen species (ROS) might act as signaling agents, specifically as modulators of synaptic transmission.  2000 Elsevier Science B.V. All rights reserved. Theme: Excitable membranes and synaptic transmission Topic: Presynaptic mechanisms 21