Modification of the glycine (co-activator) binding site of the N-methyl- d-aspartate receptor in the guinea pig fetus brain during development following hypoxia

Abstract

The present study was designed to investigate the mechanism of NMDA receptor activation as a function of brain maturation by studying the development of the glycine binding site of the NMDA receptor and its modification by in-utero hypoxia in the guinea pig fetus brain during gestation. Measurements of B max (number of functional receptors) and K d (apparent receptor affinity) of glycine binding sites of the NMDA receptor were performed in eleven (45 days, n = 5: 60 days, n = 6) synaptosomal membranes from normoxic (control) fetuses and ten (45 days, n = 4; 60 days, n = 6) synaptosomal membranes constituted the hypoxic (experimental) group. In the experimental group, fetuses were exposed to maternal hypoxia (FiO 2 0.07) for 1 h. Synaptosomal membranes were prepared and strychnine-insensitive specific [ 3Hlglycine binding was determined. During development, the number of glycine binding sites increased ( B max : 392 ± 30 vs. 583 ± 30 fmol/mg protein at 45 and 60 days respectively, P < 0.05) where as the affinity remained unchanged ( K d : 190 ± 9 vs. 211 ± 30 nM at 45 and 60 days'respectively). Following hypoxia, glycine binding sites increased at 45 days ( B max : 392 ± 30 vs. 561 ± 96 fmol/mg protein, P < 0.005) but decreased at 60 days ( B maX : 583 ± 85 vs. 411 65 fmol/mg protein, P < 0.005) with change in K d only at 60 days ( K d : 211 ± 30 vs. 149 ± 52 nM, P < 0.05). The data show that there are alterations in the characteristics of the glycine binding site during development and following hypoxia. We conclude that developmental changes in the glycine binding site might modulate NMDA receptor activation, as a function of brain maturation. Furthermore, hypoxia-induced modification of the glycine binding site might be a potential mechanism of neurotoxicity and might increase susceptibility of the fetal brain to excitotoxicity at term.