Acute effects of ethanol on an NMDA receptor-like complex in the mammalian CNS

N-Methyl-D-Aspartate (NMDA) receptors are a subtype of excitatory amino acid receptors in the central nervous system that are implicated in learning and memory processes, and also play a role in excitotoxicity and neurodegenerative diseases such as Alzheimer's and Parkinson's disease. Functional NMDA receptors are composed of NMDAR1, NMDAR2, and NMDAR3 protein subunits. However, a purified and reconstituted complex of proteins found in synaptic membranes composed of the glutamate-binding protein (GBP) subunit, the glycinebinding protein (GlyBP) subunit, the CPP-binding protein (CBP) subunit and the TCP-binding protein (TCPBP) subunit has been found to conduct current in artificial bilayers upon activation by L-glutamate and glycine or NMDA and glycine. Binding studies on the isolated NMDA receptor-like complex also indicate that this complex is sensitive to the NMDA receptor antagonists 2-APS, 5,7-DCKA, MK-801 and ketamine.; Physiologically relevant concentrations of ethanol [EtOH] inhibit the activation of NMDA receptors up to half-maximally in neuronal cultures or brain slices. Acute EtOH treatment of cells expressing only the NMDAR1/R2 complex also leads to inhibition of NMDA-induced ionic currents. On the other hand, chronic ethanol exposure of cultured neurons leads to enhanced responsiveness to NMDA. Such enhancement in receptor sensitivity is correlated with increases in the expression of both the NMDAR1 subunit of NMDA receptors and the GBP, GlyBP and CBP subunits of the NMDA receptor-like complex. The effects of ethanol on the NMDA receptor-like complex was found to show a trend towards inhibition at 25 mM EtOH yet showed recovery upon increasing the EtOH concentration to 50–100 mM EtOH.; The main objective in the present study was to determine the contribution of the NMDA receptor-like complex in NMDA-induced currents in cultured rat hippocampal neurons and the effects of acute EtOH treatment on the NMDA receptor-like complex and not NMDAR1/NMDAR2/NMDAR3 receptors. The application of an antibody raised to the GlyBP subunit (Anti-GlyBP) was found to terminate current through the isolated NMDA receptorlike complex activated by L-glutamate and glycine in artificial bilayers. This polyclonal antibody was able to reduce NMDA-activated currents by ∼35% in individual hippocampal neurons and reduce the sensitivity on NMDA-activated currents to low concentrations of EtOH (<25 mM). Anti-GIyBP may be a specific tool to selectively target the GlyBP subunit of the NMDA receptor-like complex and may demonstrate that the NMDA receptor-like complex contributes to signal transduction.