THE EFFECTS OF THIOL REDUCTION AND OXIDATION ON THE INHIBITION OF NMDA-STIMULATED NEUROTRANSMITTER RELEASE BY ETHANOL

The N-methyl-D-aspartate (NMDA) receptor is sensitive to inhibition by pharmacologically relevant concentrations of ethanol and may be an important target for the actions of ethanol in the brain. The mechanisms responsible for ethanol's action are unknown but may involve perturbation of one of the multiple regulatory sites that have been identified on the receptor. Previous studies have shown that thiol reducing and oxidizing agents selectively alter the activity of the NMDA receptor via a redox modulatory site. In this study, these agents were tested for their ability to modify the inhibitory actions of ethanol on NMDA-stimulated neurotransmitter release from rat brain slices and the potentiation of NMDA-stimulated release by the glycine agonist D-serine. Treatment of hippocampal slices with the thiol reducing agent dithiothreitol (DTT) significantly potentiated (>2-fold) the NMDA-stimulated release of tritiated norepinephrine ([H-3]NE) from rat hippocampal slices. Slices treated with the thiol oxidizing agent dithionitrobenzoic acid (DTNB) had slightly lower (10-15%) levels of NMDA-stimulated neurotransmitter release. Ethanol (25-200 mM) dose-dependently inhibited the NMDA-stimulated release of [H-3]NE from control slices with a calculated IC50 value of 73.1 mM. The inhibitory potency for ethanol was increased following DTNB treatment (IC50 of 40.9 mM) while DTT treatment slightly reduced ethanol's potency (IC50 of 85.6 mM). The ability of D-serine to augment NMDA-stimulated neurotransmitter release was tested in the presence and absence of ethanol following treatment of slices with either DTT or DTNB to examine any potential interaction between these modulatory sites. D-serine (1-30 mu M) potentiated the NMDA-stimulated release of neurotransmitter from both control and DTNB treated slices but did not appear to alter the inhibition produced by 50 mM ethanol in either case. In contrast, D-serine did not potentiate NMDA-stimulated release in DTT treated slices under control conditions nor did it significantly alter the inhibition of neurotransmitter release produced by 50 mM ethanol. These results suggest that changes in the redox state of the NMDA receptor may alter the sensitivity of the receptor to the inhibitory actions of ethanol. These results also suggest that the inhibitory effects of ethanol are not mediated via a selective interaction with the glycine site on the receptor.