Ethanol-Induced Inhibition of [3H]Thienylcyclohexylpiperidine (TCP) Binding to NMDA Receptors in Brain Synaptic Membranes and to a Purified Protein Complex

Abstract: N -Methyl- d -asparate receptors (NMDARs) are a major target of ethanol effects in the nervous system. Haloperidol-insensitive, but dizocilpine (MK-801)-sensitive, binding of N -1-(2-³H]thienyl)cyclohexyl]piperidine (³H]TCP) to synaptic membranes has the characteristics of ligand interaction with the ion channel of NMDARs. In the present studies, ethanol produced a concentration-dependent decrease in the maximal activation of ³H]TCP binding to synaptic membranes by NMDA and Gly, but a moderate change in the activation by l -Glu when l -Glu was present at concentrations < 100 µ M . However, ethanol (100 m M ) inhibited completely the activation of ³H]TCP binding produced by high concentrations of l -Glu (200–400 µ M ). It also inhibited strongly the activation of ³H]TCP binding by spermidine or spermidine plus Gly. In a purified complex of proteins that has l -Glu-, Gly-, and ³H]TCP-binding sites, ethanol (100 m M ) decreased significantly the maximal activation of ³H]TCP binding produced by either l -Glu or Gly. Activation constants ( K _act) for l -Glu and Gly acting on the purified complex were 12 and 28 µ M, respectively. Ethanol had no significant effect on the K _act of l -Glu but caused an increase in the K _act of Gly. These studies have identified at least one protein complex in neuronal membranes whose response to both l -Glu and Gly is inhibited by ethanol. These findings may explain some of the effects of acute and chronic ethanol treatment on the function and expression of the subunits of this complex in brain neurons.