Previous studies have shown that the N-methyl-D-aspartate (NMDA) receptor is modified during hypoxia in the cerebral cortex of newborn piglets. The present study tests the hypothesis that the NMDA receptor 3-(2-carboxypiperazin-4-yl)-1-phosphonic acid (CPP) binding site is altered in response to the progressive decrease in cerebral cellular energy metabolism induced by hypoxia. Studies were conducted in 17 anesthetized, ventilated newborn piglets, 8 normoxic and 9 exposed to decreased FiO2 at different concentrations and durations to achieve varying phosphocreatine(PCr) levels. 3H-CPP binding was performed at concentrations ranging from 2.5 to 1500 nM at 23°C for 40 minutes in P2 membrane fractions. Brain tissue ATP and PCr levels were determined biochemically. In the normoxic group the mean PCr value was 2.7 ± 0.3 μmol/g brain, Bmax (receptor number) 343 ± 80 fmoles/mg protein and Kd (dissociation constant) 130 ± 46 nM. For the group exposed to varying degrees of hypoxia, PCr (μmol/g brain), Bmax (fmoles/mg protein) and Kd (nM) were as follows: (0.04, 50, 58), (0.5, 166, 156), (0.8, 61, 65), (0.9, 59, 28), (0.9, 97, 47), (1.5, 142, 80), (2.7, 193, 188), (3.2, 261, 119), and (4.4, 326, 112). In contrast to PCr, tissue ATP levels decreased significantly only when PCr values were less than 1.0 μmol/g brain (4.8 ± 1.0 versus 1.5± 1.9 μmol/g brain, p<0.001). The results demonstrate that both the receptor Bmax (r = 0.9) and Kd (r = 0.83) decrease in a linear relationship as PCr decreases. The data show that there is not a critical hypoxic threshold for modification of the CPP binding site of the NMDA receptor, but that modification is coupled to a gradual decrease in brain cell energy metabolism as reflected by brain tissue PCr levels. We speculate that NMDA receptor modulation may be mediated by changes in phosphorylation of the receptor recognition site and may be initiated by subtle decreases in tissue oxygenation in the newborn brain. (Funded by NIH-HD-20337, MOD #6-FY94-0135, UCPR 506-93)