Differential ethanol sensitivity of recombinant N-methyl-D-aspartate receptor subunits

The recombinant N-methyl-D-aspartate (NMDA) receptor subunit zeta 1 and the heteromeric subunit combinations epsilon 1/zeta 1, epsilon 2/zeta 1, and epsilon 3/zeta 1 were expressed in Xenopus oocytes and their sensitivities to ethanol were investigated using the two-electrode voltage-clamp technique. NMDA-activated currents in oocytes expressing subunit combinations epsilon 1/zeta 1 or epsilon 2/zeta 1 were significantly inhibited by 50 mM ethanol, whereas NMDA-activated currents associated with the homomeric expression of zeta 1 or the heteromeric epsilon 3/zeta 1 combination were not significantly affected by 50 mM ethanol. Ethanol decreased the maximal amplitude (Emax) of the concentration-response curve for NMDA-activated current, without significantly affecting the EC50. The values of percentage inhibition by ethanol were not significantly different, regardless of the amplitude of current activated by NMDA concentrations from 10 to 250 microM. Different NMDA receptor subunits and subunit combinations exhibited differences in the concentration-response curves for ethanol. NMDA-activated current associated with the epsilon 1/zeta 1 subunit combination was increasingly inhibited by increasing concentrations of ethanol from 25 to 100 mM, whereas 25 mM ethanol elicited nearly maximal inhibition of NMDA-activated current associated with the epsilon 2/zeta 1 subunits, i.e., the inhibition by 50 or 100 nM ethanol was not significantly different. NMDA-activated current associated with the epsilon 3/zeta 1 subunit combination, on the other hand, was significantly inhibited only by 100 mM ethanol, and NMDA-activated current associated with the homomeric zeta 1 subunit was not significantly affected by ethanol concentrations of < or = 100 mM. Because NMDA receptor subunits are differentially distributed throughout the brain, the observations suggest that the differential sensitivity of NMDA receptor subunits to ethanol may contribute to the differences in ethanol sensitivity observed in different types of neurons.