Glutamate Activation of the NMDA Receptor Ion Channel Complex During Graded Hypoxia in the Cerebral Cortex of Newborn Piglets. † 1008

Previous studies have shown that N-methyl-D-aspartate (NMDA) receptor activation as measured by ³[H]-MK-801 binding is enhanced by the excitatory neurotransmitter glutamate during normoxia and gradually increases during the progressive decrease in cerebral energy metabolism induced by hypoxia in the cerebral cortex of newborn piglets. The present study tests the hypothesis that the degree of glutamate activation of the NMDA receptor ion channel during hypoxia is dependent on the decrease in cerebral energy metabolism induced by hypoxia. Studies were performed in 4 normoxic (Nx) and 10 hypoxic (Hx) ventilated newborn piglets. In the Hx group varying degrees of cerebral energy metabolism were achieved by administration of different concentrations of O₂ (5-9%) and were documented biochemically by tissue ATP and phosphocreatine (PCr) levels. ³[H]MK-801 binding was performed with 100mM glycine, 12.5nM ³[H]MK-801 and increasing concentrations of glutamate (0.01-15 mM) in P₂ membrane fractions. Nonspecific binding was determined in the presence of 10mM unlabeled MK-801. Percent activation by glutamate was determined. For the Hx group tissue ATP (mmol/g brain), PCr(mmol/g brain), the concentration of glutamate resulting in 100% activation(mM) and% receptor activation at 0.1m M glutamate were as follows: (5.3, 2.8, 4, 65), (4.9, 2.8, 6, 24), (3.2, 1.4, 3, 85), (3.0, 0.3, 1, 22), (2.8, 1.1, 2, 52), (2.6, 2.5, 4, 70), (2.6, 2.4, 4.8, 46), (1.8, 1.1, 3, 57), (0.1, 0.4, 1, 35) and (0.1, 0.1, 2, 85), respectively. Glutamate concentrations resulting in 100% activation of the NMDA receptor correlated linearly with brain tissue levels of PCr, with concentrations of glutamate resulting in 100% activation of the receptor decreasing as PCr levels decreased (r=0.6). Percent NMDA receptor activation at 0.1mM glutamate also correlated with tissue ATP levels, with% receptor activation at 0.1mM glutamate increasing as tissue ATP levels decreased (r=0.5). The data demonstrate that as cerebral energy metabolism decreases, maximal receptor activation by glutamate increases in a linear relationship, indicating that during hypoxia there is an increased affinity of the NMDA receptor for glutamate which correlates with decreased cerebral energy metabolism. We speculate that with subtle decreases in cerebral energy metabolism, the NMDA receptor glutamate recognition site is dephosphorylated, leading to increased receptor affinity for glutamate and potentiating neurotoxicity.