³H-labeled MK-801 binding to the excitatory amino acid receptor complex from rat brain is enhanced by glycine

We have studied the binding of the excitatory amino acid antagonist ³H-labeled MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohept-5,10-imine maleate] to extensively washed rat brain membranes. Binding of ³H-labeled MK-801 was inhibited by phencyclidine, Mg²⁺, and 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid in a manner dependent on the presence of L-glutamate or N-methyl-D-aspartate, suggesting that it labeled a site linked to the N-methyl-D-aspartate subtype of the glutamate receptor. Glycine also regulated ³H-labeled MK-801 binding, enhancing it in the concentration range of 0.01-10 μM. The actions of glutamate and glycine involved increases in binding affinity, without altering the number of ³H-labeled MK-801 binding sites. The effects of glycine in this system were mimicked by L- and D-alanine and L- an D-serine. However, β-alanine and taurine were much less effective, and strychnine did not block the actions of glycine indicating that they were not mediated by the classical glycine receptors. Glycine also enhanced the ability of N-methyl-D-aspartate to increase Ca²⁺ influx into primary cultures of mouse striatal neurons measured using the Ca²⁺-sensitive fluorescent dye fura-2. These results support the suggestion that glycine may be an important regulator of the physiological actions of glutamate in vivo.