Deletion of the N-Terminal Domain Alters the Ethanol Inhibition of N-Methyl-d-Aspartate Receptors in a Subunit-Dependent Manner
Ethanol (EtOH) inhibition of N-methyl-d-aspartate (NMDA) receptors is poorly understood due in part to the organizational complexity of the receptor that provides ample locations for sites of action. Among these, the N-terminal domain (NTD) of NMDA receptor subunits contains binding sites for a variety of modulatory agents including zinc, protons, and GluN2B selective antagonists such as ifenprodil or Ro-25-6981. EtOH inhibition of neuronal NMDA receptors expressed in some brain areas has been reported to be occluded by the presence of ifenprodil or similar compounds suggesting that the NTD may be important in regulating the EtOH sensitivity of NMDA receptors.
Wild-type GluN1 and GluN2 subunits and those in which the coding sequence for the NTD was deleted were expressed in HEK293 cells. Whole-cell voltage-clamp recording was used to assess EtOH inhibition of wild-type and mutant receptors lacking the NTD.
As compared to wild-type GluN1/GluN2A receptors, EtOH inhibition was slightly greater in cells expressing GluN2A subunits lacking the NTD. In contrast, GluN2B N-terminal deletion mutants showed normal EtOH inhibition while those lacking the NTD in both GluN1 and GluN2B subunits had decreased EtOH inhibition as compared to wild-type receptors. NTD lacking GluN2B receptors were insensitive to ifenprodil but retained normal sensitivity to EtOH.
These findings indicate that the NTD modestly influences the EtOH sensitivity of NMDA receptors in a subunit-dependent manner. They also show that ifenprodil's actions on GluN2B-containing receptors can be dissociated from those of EtOH. These results suggest that while the NTD is not a primary site of action for EtOH on NMDA receptors, it likely affects sensitivity via actions on intrinsic channel properties.