Both ethanol and ifenprodil inhibit NMDA-evoked release of various neurotransmitters at different, yet proportional potency: potential relation to NMDA receptor subunit composition

Superfused slices of the rat cerebral cortex and corpus striatum were used to investigate the effect of ethanol on the N-methyl-D-aspartate (NMDA)-evoked release of various [3H]neurotransmitters. To obtain information on the subunit composition of the NMDA receptors involved, the influence of ifenprodil (a drug which preferentially inhibits NMDA receptors containing the NMDAR2B subunit) on the NMDA-evoked [3H]neurotransmitter release was also determined. Ethanol inhibited the NMDA-evoked release of all neurotransmitters investigated in a concentration-dependent manner. Irrespective of the brain region, ethanol was 2.6-5.1 times more potent in inhibiting the release of [3H]noradrenaline, [3H]5-hydroxytryptamine and [3H]GABA than the release of [3H]acetylcholine and [3H]dopamine. Ifenprodil inhibited the NMDA-evoked release of these [3H]neurotransmitters in both brain regions at virtually the same potency order as ethanol. Comparison of the potencies of ethanol and ifenprodil in inhibiting NMDA-evoked [3H]neurotransmitter release revealed an excellent correlation (r = 0.96; P < 0.001). After blockade of the highly ifenprodil-sensitive fraction of the NMDA receptors by 100 nM ifenprodil, the potency of ethanol in inhibiting NMDA-evoked [3H]noradrenaline release in the cerebral cortex was decreased by a factor of 6. In contrast, 100 nM ifenprodil did not affect the potency of ethanol in inhibiting [3H]dopamine and [3H]acetylcholine release in the corpus striatum. It is concluded that ethanol predominantly inhibits NMDA receptors containing a high proportion of the NMDAR2B subunit (as reflected by high sensitivity to ifenprodil), i.e. the NMDA receptors involved in stimulation of noradrenaline, 5-hydroxytryptamine and GABA release. In view of the ability of ifenprodil to interact with ethanol at NMDA receptors, which are important sites of action of ethanol, ifenprodil may be assumed to influence neuronal effects of ethanol in vivo.