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 tests 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 and six bilirubin-exposed piglets, 2-4 d of age. Piglets in the bilirubin group received a 35 mg/kg bolus of bilirubin followed by a 4-h infusion (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 bilirubin 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 buffer 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 modification. 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 value for K-d in the control brains was 6.97 +/- 0.80 nM compared with 4.80 +/- 0.28 nM in the bilirubin-exposed brains (p < 0.001). [H-3]Glutamate 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 results show that in vivo exposure to bilirubin increases the affinity of the receptor (decreased K-d) for [H-3]MK-801, indicating that bilirubin modifies the function of the NMDA receptor/ion channel complex in the brain of the newborn piglet. We speculate that the affinity of bilirubin for neuronal membranes leads to bilirubin-mediated neurotoxicity, resulting in either short- or long-term disruption of neuronal function.