THE IN-VIVO EFFECT OF BILIRUBIN ON THE N-METHYL-D-ASPARTATE RECEPTOR ION CHANNEL COMPLEX IN THE BRAINS OF NEWBORN PIGLETS/

Bilirubin neurotoxicity can be mediated by numerous mechanisms due to its increased permeability in neuronal membranes. The present study te sts the hypothesis that a prolonged bilirubin infusion modifies the N- methyl-D-aspartate (NMDA) receptor/ion channel complex in the cerebral cortex of newborn piglets. Studies were performed in seven control an d six bilirubin-exposed piglets, 2-4 d of age. Piglets in the bilirubi n group received a 35 mg/kg bolus of bilirubin followed by a 4-h infus ion (25 mg/kg/h) of a buffer solution containing 0.1 N NaOH, 5% human albumin, and 0.055 Na2HPO4 with 3 mg/dL bilirubin. The final mean bili rubin concentration in the bilirubin group was 495.9 +/- 85.5 mu mol/L (29.0 +/- 5.0 mg/dL). The control group received a bilirubin-free buf fer solution. Sulfisoxazole was administered to animals in both groups . P-2 membrane fractions were prepared from the cerebral cortex. [H-3] MK-801 binding assays were performed to study NMDA receptor modificati on. The B-max in the control and bilirubin groups were 1.20 +/- 0.10 ( mean +/- SD) and 1.32 +/- 0.14 pmol/mg protein, respectively. The valu e for K-d in the control brains was 6.97 +/- 0.80 nM compared with 4.8 0 +/- 0.28 nM in the bilirubin-exposed brains (p < 0.001). [H-3]Glutam ate binding studies did not show a significant difference in the B-max and K-d for the NMDA-specific glutamate site in the two groups. The r esults show that in vivo exposure to bilirubin increases the affinity of the receptor (decreased K-d) for [H-3]MK-801, indicating that bilir ubin modifies the function of the NMDA receptor/ion channel complex in the brain of the newborn piglet. We speculate that the affinity of bi lirubin for neuronal membranes leads to bilirubin-mediated neurotoxici ty, resulting in either short- or long-term disruption of neuronal fun ction.