Deleterious brain cell membrane effects after NMDA receptor antagonist administration to newborn piglets

Previous studies have shown that administration of the N-methyl--aspartate (NMDA) receptor antagonist 3-(2-carboxypiperazin-4-yl)-1-phosphonic acid (CPP) reduces NMDA-mediated neurotoxicity in animal models of hypoxia/ischemia but also may induce brain tissue vacuolization and alter glucose metabolism. The present study tests the hypothesis that CPP administration alters brain cell membrane structure and function in the cerebral cortex of normoxic newborn piglets through the generation of oxygen free radicals and induction of lipid peroxidation. Twenty six anesthetized, ventilated newborn piglets—13 treated with 2 mg/kg i.v. CPP and 13 untreated controls—were studied. ATP and phosphocreatine (PCr) levels were measured as an index of cellular energy metabolism and tissue glucose levels determined. Na⁺,K⁺-ATPase activity was measured as an index of brain cell membrane function and the lipid peroxidation products conjugated dienes (CD) and fluorescent compounds (FC) measured. Free radical generation was detected on cortical biopsies homogenized with α-phenyl-N-tert-butyl-nitrone (PBN) through electron spin resonance spectroscopy. Signal height of spectrum was divided by dry tissue weight and expressed as mm/g tissue. In the two groups brain tissue ATP and PCr levels were not different. Tissue glucose levels were higher in the CPP group (24±5 mg/dl) than in controls (14±3 mg/dl), p<0.05, whereas Na⁺,K⁺-ATPase activity was lower in the CPP group than in controls (34±4 vs. 43±6 μmol Pi/mg protein/h), p<0.05. Lipid peroxidation products were higher in the CPP group (CD: 57±19 nmol/g brain, FC: 1.5±0.3 μg/g brain) than in controls (CD: 0±0 nmol/g brain, FC: 0.9±0.2 μg/g brain), p<0.05. Free radical intensity was higher in the CPP group (493±397 mm/g tissue) than in controls (51±83 mm/g tissue), p<0.05. In vitro administration of CPP to brain cell membranes did not change Na⁺,K⁺-ATPase activity or the generation of lipid peroxidation products. The data demonstrate that administration of CPP induces lipid peroxidation, results in free radical generation, decreases brain cell membrane Na⁺,K⁺-ATPase activity and alters glucose metabolism in the cerebral cortex of newborn piglets. Since CPP is a potent antagonist of the NMDA receptor, we speculate that CPP generates free radicals through a pathway independent of the NMDA receptor by altering cellular metabolism and possibly glucose utilization during normoxia in newborn piglets.