Sex-specific regional brain bilirubin content in hyperbilirubinemic Gunn rat pups

BACKGROUND: The hyperbilirubinemic j/j Gunn rat is frequently used to study the effects of neonatal hyperbilirubinemia on the developing central nervous system (CNS). Despite evidence that the cerebellar region and males are predisposed to bilirubin-induced brain injury in this animal model, there are limited regional and no sex-specific brain bilirubin content data. OBJECTIVE/METHODS: To characterize and contrast the regional (cortex, brainstem, cerebellum) and sex-specific CNS bilirubin contents of hyperbilirubinemic j/j Gunn rat pups and their age-matched (15-19 days) nonjaundiced J/j counterparts. Pups were injected 24 h prior to sacrifice with sulfadimethoxine (200 mg/kg i.p.) to enhance the CNS bilirubin content. RESULTS: The CNS bilirubin contents in each region and total serum bilirubin levels were significantly greater in jaundiced j/j pups versus nonjaundiced J/j pups. Within the sulfadimethoxine-treated male j/j cohort, the mean brain bilirubin content was highest in the cerebellum (18.9 +/- 7.8 microg/g), intermediate in the brainstem (10.7 +/- 8.0 microg/g), and lowest in the cortex (4.7 +/- 3.0 microg/g) (F = 11.31, p < 0.001 by ANOVA), and the cerebellar bilirubin level was significantly higher than in the littermate-matched sulfadimthoxine-treated j/j female pups (p < 0.02). The serum albumin levels were not different between j/j male and j/j female pups. CONCLUSIONS: We conclude that the brain bilirubin content of hyperbilirubinemic j/j Gunn rat pups is greater than in nonjaundiced J/j pups and varies as a function of CNS region and sex. We speculate that the higher cerebellar bilirubin content may preferentially predispose male j/j Gunn rat pups to bilirubin-induced neurotoxicity.     

Auditory brainstem response and unbound bilirubin in jaundiced (jj) Gunn rat pups

The role of plasma bilirubin-albumin binding in the pathogenesis of kernicterus in human newborns is controversial. Kernicterus in the jaundiced (jj) Gunn rat pup, an animal model for kernicterus, prolongs interwave intervals and decreases wave amplitude in the auditory brainstem response (ABR). Plasma total bilirubin concentration (TBC), albumin concentration, and unbound bilirubin concentration (UBC), a measure of bilirubin-albumin binding, were measured in 16-day-old jj Gunn rat pups (n = 21) and compared with ABR wave latencies, interwave intervals, and wave amplitudes by linear correlation. The UBC, but not the TBC or TBC/albumin ratio, correlated positively and significantly with ABR I-II and I-III interwave intervals (r = 0.55, p = 0.009, and r = 0.60, p = 0.004, respectively). The UBC, but not the TBC or TBC/albumin ratio, predicts bilirubin toxicity, as measured by bilirubin-induced ABR changes in jj Gunn rat pups.     

The toxicity of bilirubin to the central nervous system]

Whatever the causes of its accumulation, excessive production and/or insufficient elimination, unconjugated bilirubin when it reaches a certain concentration threshold, is responsible for neurones and astrocytes death. In this paper the mechanisms involved in this process of cellular death, from hemolysis to oxydation in neurons and neuroglial cells, are reviewed.     

Somatosensory and brainstem auditory evoked potentials in the Gunn rat model of acute bilirubin neurotoxicity

Brainstem auditory evoked potentials (BAEPs) are a sensitive indicator of bilirubin neurotoxicity. Somatosensory evoked potentials (SEPs) have been proposed as another measure of toxicity, though the lemniscal pathways that generate the SEP are not involved in kernicterus. In 16 to 17-d-old jaundiced (jj) Gunn rats, serial BAEPs and SEPs were obtained up to 8 h after acute bilirubin toxicity. jjs were injected with 150 mg/kg sulfadimethoxine to displace bilirubin into brain tissue (n = 8); littermate controls were jjs given saline (n = 4) and nonjaundiced given sulfadimethoxine or saline (n = 7). After anesthesia, BAEP and SEP recordings were obtained at baseline and serially after injection. SEPs to median nerve stimulation were recorded from surface electrodes over the brachial plexus (Erb's) and contralateral parietal cortex, and subtracted to obtain central conduction time (CCT). There were no statistically significant different baseline values between groups. Baseline BAEP I, I-II, and I-III were 1.22 +/- 0.13, 1.11 +/- 0.12, and 2.10 +/- 0.15 ms, and SEP Erb's and CCT were 1.48 +/- 0.20 and 5.59 +/- 0.50 ms, respectively (n = 19). At 6.8 +/- 1.5 h after injection BAEP I, I-II, and I-III increased 0.10 +/- 0.08, 0.18 +/- 0.17, and 0.56 +/- 0.33 ms over baseline, respectively (p = 0.005, 0.01, and 0.001, respectively, paired, 1-tailed t-tests), in experimental but not control groups. SEP Erb's decreased slightly, -0.06 +/- 0.04 ms in experimental and -0.08 +/- 0.08 ms in control groups, while CCT did not change significantly. BAEPs were completely abolished in two jjs with no SEP changes. When injection of sulfonamide induced significant peripheral and central BAEP abnormalities in jaundiced rats, no SEP abnormalities occurred. SEPs assess proprioception but not other somatosensory function or sensory integration. The results demonstrate the selectivity of acute bilirubin toxicity for the auditory nervous system.     

Deposition of bilirubin acid in the central nervous system--a hypothesis for the development of kernicterus

On the basis of the concentration of unconjugated bilirubin and available albumin for the binding of bilirubin it is possible to calculate the level of unbound bilirubin in a serum sample. The solubility of bilirubin can further be calculated when the pH is known. In cases of threatened kernicterus the free bilirubin concentration in serum samples from newborn infants surpasses the solubility by a factor close to one hundred. It is hypothesized that deposition of bilirubin in tissues takes place as an ongoing event, the deposited pigment being eliminated by bilirubin oxidase in healthy infants. Kernicterus results when the rate of deposition becomes overwhelming as a result of high bilirubin concentration, low albumin reserve, low pH, after administration of a displacing drug, or if the bilirubin oxidase system has been compromised by preceding birth asphyxia or other forms of central nervous system injury.     

Activated charcoal decreases plasma bilirubin levels in the hyperbilirubinemic rat

The effectiveness of phototherapy and activated charcoal feeding for reducing plasma bilirubin concentrations was studied in the hyperbilirubinemic Gunn rat. The feeding of charcoal alone was just as effective in reducing plasma bilirubin concentrations as continuous phototherapy. An additive effect on reducing plasma bilirubin concentration was observed when phototherapy and the feeding of charcoal were administered together. The feeding of a 5% charcoal diet to weanling Gunn rats for 8 wk had no effect on growth rate.