Enhancement of NMDA-induced functional responses without concomitant NMDA receptor changes following chronic ethanol exposure in cerebellar granule cells

Primary cultures of rat cerebellar granule cells were used to investigate the effects of chronic ethanol exposure (50-100 mM for 3 days) on NMDA receptor functions (Ca2+ fluxes and neurotoxicity), binding parameters of the non-competitive NMDA receptor antagonist [H-3]MK-801, relative abundance of mRNAs coding for NMDA receptor subunits, and expression of NMDA receptor subunit proteins. Ethanol exposure caused a marked increase in NMDA-produced neurotoxicity but produced a differential pattern of effects on NMDA-induced Ca2+ fluxes with a marked enhancement of NMDA-stimulated free cytoplasmic Ca2+ concentrations ([Ca2+](i)), but no changes in NMDA-induced Ca-45(2+) uptake. As shown by [H-3]MK-801 binding experiments, chronic ethanol had no effect on affinity or number of the NMDA receptors. Furthermore, ethanol exposure had no effect on the relative abundance of the mRNAs for any of the NMDA receptor subunits (four splice variants of NR i, or NR2A-C), or on the expression of NMDA receptor subunit proteins. Our data confirm previous observations that chronic ethanol exposure enhances NMDA receptor-mediated neurotoxicity and elevation of [Ca2+](i), but also suggest that the increased responsiveness of NMDA receptors is not necessarily associated with alterations in the subunit composition or the ligand binding properties of NMDA receptors.