EFFECT OF CEREBRAL HYPOXIA ON NMDA RECEPTOR-BINDING CHARACTERISTICS AFTER TREATMENT WITH 3-(2-CARBOXYPIPERAZIN-4-YL)PROPYL-1-PHOSPHONIC ACID (CPP) IN NEWBORN PIGLETS

Previous studies have shown that hypoxia modifies the NMDA receptor/io n channel complex in cortical brain cell membranes of newborn piglets, The present study tests the hypothesis that blockade of the glutamate recognition site of the NMDA receptor with the competitive antagonist 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) prevents mo dification of the receptor during hypoxia. Twenty seven anesthetized, ventilated newborn piglets were randomized into four groups: 7 normoxi c (Nx), 6 CPP-treated normoxic (CPP-Nx), 8 hypoxic (Hx) and 6 CPP-trea ted hypoxic (CPP-Hx). Treatment groups received CPP 2 mg/kg i.v. The C PP-Hx group received CPP 30 min prior to hypoxia, which was induced by lowering the FiO(2) to 5-7% for 1 h. Physiologic data showed no chang e in heart rate, blood pressure, arterial blood gas values, glucose or lactate following CPP administration, During hypoxia there was a sign ificant decrease in PaO2, pH and an increase in lactate compared to ba seline values. The CPP-Hx group had significantly higher lactate level s than the Hx group during hypoxia. P-2 membrane fractions were prepar ed and thoroughly washed. Characteristics of the NMDA receptor ion cha nnel were determined by [H-3]MK-801 binding assays and characteristics of the glutamate recognition site by specific NMDA-displaceable [H-3] glutamate binding assays. Brain tissue ATP and PCr levels confirmed ti ssue hypoxia, and were not preserved by CPP administration. [H-3]MK-80 1 binding assays revealed that CPP treatment attenuated the hypoxia-in duced decrease in the number of receptors (B-max) and receptor binding affinity (K-d) during hypoxia. CPP treatment also decrease receptor a ffinity (increased K-d) for [H-3]MK-801 binding during normoxia and hy poxia. Assays of [H-3]glutamate binding revealed that hypoxia decrease d both the B-max and the K-d of the NMDA receptor for [H-3]glutamate a nd both were preserved by CPP treatment prior to hypoxia. CPP had no e ffect on [H-3]glutamate B-max or K-d during normoxia. We conclude that hypoxia decreases the B-max and K-d of the NMDA receptor glutamate re cognition site for [H-3]glutamate and the ion channel site for [H-3]MK -801 in newborn piglets. These changes are prevented by CPP administra tion prior to hypoxia. The different effects of CPP binding during nor moxia and hypoxia suggest a use-dependent mechanism for CPP binding du ring hypoxia, possibly through an hypoxia-induced alteration of the hi gh-affinity binding site for CPP. During both normoxia and hypoxia CPP binding appeared to induce a conformational change in the receptor ca using a decrease in binding affinity for [H-3]MK-801. CPP administrati on did not preserve brain tissue ATP or PCr levels during hypoxia and may alter cellular metabolism in addition to its action at the NMDA re ceptor, However, even with depletion of the energy precursors, ATP and PCr, and with higher lactate levels in the CPP-Hx group, CPP was able mo maintain NMDA receptor binding characteristics during hypoxia and may decrease excitotoxic cellular damage from hypoxia.