Effect of graded hypoxia on the high-affinity CPP binding site of the NMDAreceptor in the cerebral cortex of newborn piglets

Previous studies have shown that the N-methyl-D-aspartate (NMDA) receptor i s modified during hypoxia in the cerebral cortex of newborn piglets. The pr esent study tests the hypothesis that the NMDA receptor 3-(2-carbaxypiperaz in-4-yl)propyl-1-phosphonic acid (CPP) high-affinity binding site is modifi ed during hypoxia and that the degree of modification correlates with the p rogressive decrease in cerebral cellular energy metabolism and increase in lipid peroxidation induced by hypoxia. Studies were conducted in twelve ane sthetized, ventilated newborn piglets, five normoxic and seven hypoxic whic h were exposed to decreased fraction of inspired oxygen (FiO(2)) to achieve varying phosphocreatine (PCr) levels. (3)[H]-CPP binding was performed wit h CPP concentrations ranging from 0.5 to 1500 nM at 23 degreesC for 40 min in P-2 membrane fractions. Brain tissue PCr levels were determined biochemi cally. Conjugated dienes (CDs) were measured as an index of lipid peroxidat ion. Ln the normoxic group, B-max (receptor number) fur the CPP binding sir e was 329+/-93 fmol/mg protein and K-d (dissociation constant) 137+/-44 nM, the mean PCr value was 2.5+/-0.4 mu mol/g brain and the CD level was 0.0 n mol/g brain. As tissue hypoxia worsened, there was a gradual decline in tis sue PCr as well as receptor B-max and K-d values, and there was an increase in conjugated dienes. Both the receptor B-max (r=0.90) and K-d (r=0.72) de creased in a linear relationship as PCr decreased. As the levels of CDs inc reased both the receptor B-max (r=0.88) and K-d (r=0.68) decreased in a lin ear fashion. The data show that there is not a critical hypoxic threshold f or modification of the CPP binding site of the NMDA receptor, but that modi fication is coupled to a gradual decrease in brain cell energy metabolism a nd increase in lipid peroxidation. We speculate that hypoxia-induced modifi cation of the NMDA receptor is mediated not only by changes in the receptor recognition site but also by an alteration of brain cell membrane structur e secondary to conjugated diene formation. (C) 2001 Elsevier Science B.V. A ll rights reserved.