Modification of the NMDA Receptor Ion Channel During Graded Hypoxia in the Cerebral Cortex of Newborn Piglets. 1723

Previous studies have shown that the N-methyl-D-aspartate (NMDA) receptor ion channel has an increased affinity for MK-801 during hypoxia in newborn piglets. The present study tests the hypothesis that modification of the NMDA receptor ion channel during hypoxia correlates with the progressive decrease in cerebral cellular energy metabolism induced by hypoxia. Studies were conducted in ventilated piglets - 7 normoxic and 10 hypoxic. Different degrees of hypoxia were attained by different levels of FiO₂ (5-9%) and were confirmed by brain tissue ATP and phosphocreatine (PCr) levels.³[H]MK-801 binding was performed with 100μM glutamate, 100μM glycine and ³[H]MK-801 from 0.5 - 50 nM in P₂ membrane fractions. Nonspecific binding was determined with 10μM unlabelled MK-801³[H]MK-801 receptor affinity (Kd) and number (Bmax) were determined. In the normoxic group mean (± SD) ATP was 4.45±0.5 μmoles/g brain, PCr 3.54±0.4 μmoles/g brain, Kd 5.14±0.3 nM and Bmax 1.19±0.08 pmoles/mg protein. For the group exposed to varying degrees of hypoxia ATP (μmoles/g brain), PCr (μmoles/g brain) and Kd (nM) were:(5.31, 2.80, 5.08), (4.92, 2.80, 5.50), (3.67, 0.50, 2.57), (3.19, 1.40, 3.17), (2.80, 1.14, 3.15), (2.61, 2.38, 5.13), (2.55, 1.02, 2.45), (2.50, 1.76, 3.50), (2.30, 1.48, 2.90), (1.78, 1.08, 4.76), (1.78, 2.80, 4.76) and(0.12, 0.42, 3.21). Receptor Kd decreased in a linear relationship as both ATP(r=0.65) and PCr decreased (r = 0.70), but Bmax did not. The data show that NMDA receptor ion channel opening measured by ³[H]MK-801 binding is directly correlated with the energy state of the cell and increases as oxidative phosphorylation decreases during hypoxia. We speculate that dephosphorylation of the receptor ion channel site during hypoxia enhances MK-801 binding through an allosteric or conformational change in the receptor allowing increased accessibility of MK-801 to its binding site within the ion channel and that NMDA receptor modification may be initiated by subtle decreases in tissue oxygenation in the newborn brain. (NIH HD 20337, MOD 6-FY94-0135)